Additive Manufacture of Small-Scale Metamaterial Structures for Acoustic and Ultrasonic Applications

micromachines

Review Additive Manufacture of Small-Scale Metamaterial Structures for Acoustic and Ultrasonic Applications

Alicia Gardiner 1,2,*, Paul Daly 1 , Roger Domingo-Roca 1 , James F. C. Windmill 1 , Andrew Feeney 2 and Joseph C. Jackson-Camargo 1

1 Centre for Ultrasonic Engineering, Department of Electronic & Electrical Engineering, University of Strathclyde, Glasgow G1 1XW, UK; [email protected] (P.D.); [email protected] (R.D.-R.); [email protected] (J.F.C.W.); [email protected] (J.C.J.-C.) 2 Centre for Medical and Industrial Ultrasonics, James Watt School of Engineering, University of Glasgow, Glasgow G12 8QQ, UK; [email protected] * Correspondence: [email protected]

Abstract: Acoustic metamaterials are large-scale materials with small-scale structures. These structures allow for unusual interaction with propagating sound and endow the large-scale material with exceptional acoustic properties not found in normal materials. However, their multi-scale nature means that the manufacture of these materials is not trivial, often requiring micron-scale resolution over centimetre length scales. In this review, we bring together a variety of acoustic metamaterial designs and separately discuss ways to create them using the latest trends in additive manufacturing. We highlight the advantages and disadvantages of different techniques that act as barriers towards the development of realisable acoustic metamaterials for practical audio and ultrasonic applications and speculate on potential future developments. Citation: Gardiner, A.; Daly, P.; Domingo-Roca, R.; Windmill, J.F.C.; Keywords: acoustic metamaterials; additive manufacturing; acoustics; ultrasonics Feeney, A.; Jackson-Camargo, J.C. Additive Manufacture of Small-Scale Metamaterial Structures for Acoustic and Ultrasonic Applications. Micromachines 2021, 12, 634. 1. Introduction https://doi.org/10.3390/mi12060634 1.1. Overview The progression of research concerning the acoustic, vibrational and mechanical Academic Editor: Alireza Daman Pak behaviour of small-scale structures is closely linked to the status of microfabrication/3D printing technologies [1,2]. As these manufacturing methods become more developed, Received: 22 April 2021 increasingly complex smaller-scale structures can be printed with finer detail and accuracy. Accepted: 19 May 2021 Therefore, the potential exists for opening up significant opportunities to create high- Published: 29 May 2021 performance devices for sound propagation and sensing across audio and ultrasonic frequency regimes. One such beneficiary of recent progress in microscale manufacturing Publisher’s Note: MDPI stays neutral is the field of acoustic metamaterials (AMM)—artificial materials (and devices built from with regard to jurisdictional claims in those materials) that manipulate acoustic fields in a variety of ways [3]. Despite extensive published maps and institutional affil- research, AMMs have yet to be fully realized in commercial applications, with few examples iations. available of their current deployment in industry (primarily based in sound control) [4–6]. This is in part due to the prohibitive and logistical restrictions in manufacturing these structures; as such, experimental prototypes of AMMs are typically 3D-printed to achieve the required level of precision and size [7]. Hence, they are rapidly prototyped but difficult Copyright: © 2021 by the authors. to manufacture in bulk. Additive manufacturing technology with the ability to print Licensee MDPI, Basel, Switzerland. microscopic structures is still advancing; and there are now several viable methods to realize This article is an open access article small-scale metamaterial structures. In this review, the range of AMMs under current distributed under the terms and investigation are explored, demonstrating the mechanisms by which their fabrication can conditions of the Creative Commons be achieved, and their potential for practical application in the future for both acoustic Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ and ultrasonic applications. Microscale printing techniques are discussed at length, with 4.0/). reference to significant printed small-scale examples documented in the literature. In this

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review, small-scale refers to structures with a minimum feature size of a few millimetres or lower, although optimum small features can be within the micro or nanoscales.

1.2. Additive Manufacturing Microscale printing has only recently been demonstrated to any significant scope [8–10], despite early additive manufacturing technology emerging in the 1980s [11,12]. While there were methods proposed and patented earlier, Charles Hull is generally credited with creating the first 3D printer. This system, patented in 1986 [13], used stereolithography (SLA) and reportedly required several months to fabricate its first print. Other printing methods followed in quick succession, including fused deposition modelling (FDM), selective laser sintering (SLS), and laminated object manufacturing (LOM), all of which were successfully patented by the end of the 1980s. Since then, additive manufacture technology has progressed exponentially, and by the 2000s had established itself as a popular, cost-effective option for academia and industry alike for rapid prototyping [14], and 3D printing is now popular even at home for amateur enthusiasts. Nowadays, additive printing methods can produce 3D structures with resolution reaching the nanoscale while maintaining a high print quality, for example, exhibiting no visible defects under an electron microscope [15]. The elimination of visible defects at these scales is a vital enabler for printing complex structures for acoustic and ultrasonic applications, devices for which precise control of geometry is required to tune dynamic properties. Key barriers to the manufacture of microstructures, and therefore AMMs, for use in industry are (i) the ability to mass-produce; (ii) the minimum printable feature size; and (iii) the ability to print multi-material structures. Addressing these challenges is imperative to fully realize the potential of AMMs across multiple applications, including underwater acoustics [16], medical imaging and NDT [17], acoustic cloaking [18] and sound control/attenuation [19].

1.3. Acoustic Metamaterials AMMs have been studied extensively [3,20–24] and can be described as a structure composed of (often) periodically repeated acoustic elements that can manipulate sound waves to produce novel and interesting effects. For example, they can achieve a negative effective bulk modulus and negative effective density [3,21]. Unlike natural materials, AMMs derive their unique properties from their structure rather than their innate material composition. The size of their periodic resonant units, also called “meta-atoms”, are closely linked to the operational wavelength. There is substantial interest in creating AMM devices at sub-wavelength scales [20], as when the resonant units are significantly smaller than the wavelength (typically 1/10 the size) [24], the elements can behave as one effective continuous medium—waves do not see the granularity of the medium. Early forms of AMMs have dimensions similar to their operational wavelength [25], and their properties derive from scattering. As such, the upper wavelength limit, and corresponding largest possible meta-atom size, in the ultrasonic regime (i.e., >20 kHz in air) is 17.25 millimetres, demonstrating the need for the development of smaller-scale AMMs. Acoustic wave- lengths, with air as the transmission medium, range from 17.25 millimetres (at 20 kHz) to 17.25 metres (at 20 Hz)—this poses an issue to commercializing audio-range technologies with wavelength equivalent sizing, due to their unfeasible dimensions. Devices operating in a fluid medium [16] or mechanical capacity [26] also present similar issues. AMMs using local resonances can be subwavelength and are thus more attractive and applicable. Developing 3D printing technology with finer resolutions is vital to miniaturize AMM designs to a practical size and enable their use in potential applications. To manufacture AMMs that can operate at higher ultrasonic frequencies, or sub-wavelength devices at audio frequencies, there is a demand for printing technologies with resolution in the micro (or even nano) scale. A noteworthy area of AMM research is active structures, whose material properties and/or physical geometries can be changed by external stimuli. Parameters of particular interest are tuneable bulk modulus and effective density [27–32]. In the context of this review, Micromachines 2021, 12, 634 3 of 45

an active AMM indicates an AMM system whose acoustic properties and/or geometric structure can be modified using an external interaction. This external input could involve an applied magnetic field [33], electric current, mechanical energy [34], temperature, chem- ical energy, fluid filling [35], hydration [36], or various other energy inputs/outputs [35]. Active AMMs can be broadly separated into non-Hermitian (involving acoustic gain) or externally biased (where there is no transfer of energy with acoustic waves) [22]. A common trend in active AMMs is to incorporate piezo-electric elements that allow tuning via an applied electric field [37,38]. In traditional (or passive) AMMs, the material properties cannot be altered after fabrication, and the operational bandwidth is fixed and often narrow. One limitation of this is their unsuitability for potentially transformative applications requiring a broader range of frequencies. However, AMM structures with a tuneable bandwidth present a potential opportunity to surpass this limitation. Active AMMs often incorporate many materials, due to their novel modulation systems; therefore, progressing multi-material 3D printing would greatly benefit the production of active AMMs.

1.4. Scope of Review This review explores the variety of small-scale AMMs currently in development, along with the 3D printing methods that show potential to manufacture them. The outlook and potential applications for these AMMs are discussed critically, highlighting the barriers to overcome to enable widespread use of AMMs in industry, with recommendations for promising manufacturing methods to progress the field of microscale acoustic metamaterials.

2. Acoustic Metamaterials 2.1. Background AMMs are artificial structures designed to achieve novel material properties that can manipulate acoustic waves in ways beyond the scope of natural materials. For example, AMMs have been documented with the following unusual features: negative bulk modulus; negative effective density; negative refractive index; and sub-wavelength diffrac- tion/focusing [3,20]. While AMMs have yet to be implemented widely in industry, they show promise for use in several potential applications, such as those requiring sound attenuation [39] or acoustic signal processing [40], and widely-used applications including non-destructive testing [17], and acoustic and ultrasound imaging [23]. Long before the term “metamaterials” was coined, there existed optical devices that demonstrated these extraordinary material properties. Arguably, the earliest conceived “metamaterial” was created in 1898 and was composed of jute fibres twisted in opposing directions [41]. This structure produced an optical twist in the plane of polarization of incident light waves, which is an effect not observed in any naturally-occurring materials. In 1968, Veselago theorized a material with a simultaneously negative dielectric constant and magnetic permeability [42], thereby beginning the research of metamaterials as a scientific field. These two properties are fundamental to the propagation of electromagnetic waves in matter, meaning this discovery offered new insight into the manipulation of waveform matter. Metamaterial research was largely theoretical until 1996, when Pendry proposed an artificial microstructure capable of manipulating very low frequency plasmons [43]. This design, as shown in Figure1, consisted of a periodic lattice of thin metallic wires, arranged at increments close to the plasmon wavelength. The quantum band theory of solids used in Pendry’s design would also inspire the development of phononic crystals, an example of an early AMM. Given that optical phenomena can often find analogies in other wave phenomena such as acoustics, it is not surprising that by the late 1990s, acoustic band gaps in phononic crystals was demonstrated both in theory [44] and experimentally [45]. Micromachines 2021, 12, x FOR PEER REVIEW 4 of 57

acoustic band gaps in phononic crystals was demonstrated both in theory [44] and exper- Micromachines 2021, 12, 634 imentally [45]. 4 of 45

Figure 1. Pendry’s artificial microstructure design for manipulating low-frequency plasmons. FigureReprinted 1. with Pendry’s permission artificial from [43] (As microstruc follows: [J. B.ture Pendry design et al., Physical for manipulating Review Letters, low-frequency plasmons. Re- printedVolume 76, with Pages 4773–4776, permission 1996.], copyrightfrom [43] (1996) (As by the follows: American [J. Physical B. Pendry Society). et al., Physical Review Letters, Volume 76, PagesJust prior 4773–4776, to Pendry’s artificial1996.], microstructure copyright (1996) research, by Helmholtz the American resonator arraysPhysical Society). were among the first AMMs with potential applications discussed. In 1992, Sugimoto proposed using Helmholtz resonators embedded in a train tunnel to suppress shockwaves [46]. PhononicJust crystals, prior defined to Pendry’s as a heterogeneous artificial media microstr composed ofucture elastic inclusions research, in a Helmholtz resonator arrays wereperiodic among matrix, are the capable first of AMMs impeding acousticwith potential wave propagation applications at specific frequen- discussed. In 1992, Sugimoto pro- cies [47]. By 2010, these artificial materials were researched and developed in one [48], posedtwo [49] andusing three Helmholtz [50] dimensions, resonators allowing their designembedded complexity in to a be train tailored tunnel for to suppress shockwaves [46].the situational Phononic requirements. crystals, A key defined function of as phononic a heteroge crystals, whichneous is the media control ofcomposed of elastic inclusions sound transmission spectra, has been explored for use in several potential applications, inincluding a periodic sound filtering matrix, [1], wave are guiding capable [51], radio-frequency of impeding communications acoustic [52 wave] and propagation at specific fre- quenciesacoustic sensors [47]. [53]. By 2010, these artificial materials were researched and developed in one A significant breakthrough in AMMs is the development of acoustic cloaks. Working [48],similarly two to an [49] optical and cloak, three an acoustic [50] cloak dimensions, can shield an object allowing from the their surrounding design complexity to be tailored acoustic field. A feasible 2D acoustic cloak prototype was first published in 2008 [54], forbuilding the onsituational a theoretical design requirements. proposed the previous A key year function [55]. Following of phononic this, a 2D ul- crystals, which is the control oftrasonic sound cloak transmission for underwater operation spectra, was manufactured, has been consistingexplored of a for planar use network in several potential applications, of acoustic circuits machined into a circular aluminium plate [56]. This device successfully includingprevented a small sound test object filtering from scattering [1], wave oncoming guiding pressure waves, [51], demonstrating radio-frequency communications [52] andthe feasibility acoustic of this sensors technology [53]. to be used for sonar and other underwater applications. Omni-directional 3D cloaks are more complex to produce, but despite this, a working prototypeA significant was created in 2014breakthrough via 3D printing, andin AMMs is shown in is Figure the2 development[ 57]. The device of acoustic cloaks. Working similarlyproved effective to at an reducing optical pressure-wave cloak, scatteringan acoustic in air at cloak 3 kHz, and can it is shield thus a viable an object from the surrounding option to meet the unrealized potential of acoustic cloaking. As additive manufacturing acoustictechnology develops,field. A this feasible will allow 3D2D cloaks acoustic with fewer cloak loss/scattering prototype effects was to be first published in 2008 [54], buildingproduced, and on with a functionalitytheoretical at broaderdesign frequency proposed ranges. the Current previous cloak designs year [55]. Following this, a 2D are already showing an improvement in operational bandwidth [58]. This extraordinary ultrasonicinvention has yet cloak to be commercially for underwater available, but operation it is of interest wa fors several manufactured, airborne and consisting of a planar net- workunderwater of acoustic applications. circuits machined into a circular aluminium plate [56]. This device successfully prevented a small test object from scattering oncoming pressure waves, demonstrating the feasibility of this technology to be used for sonar and other underwater applications. Omni-directional 3D cloaks are more complex to produce, but despite this, a working prototype was created in 2014 via 3D printing, and is shown in Figure 2 [57]. The device proved effective at reducing pressure-wave scattering in air at 3 kHz, and it is thus a viable option to meet the unrealized potential of acoustic cloaking. As additive manufacturing technology develops, this will allow 3D cloaks with fewer loss/scattering effects to be produced, and with functionality at broader frequency ranges. Current cloak designs are already showing an improvement in operational bandwidth [58]. This extraordinary invention has yet to be commercially available, but it is of interest for several airborne and underwater applications.

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FigureFigure 2.2. Omni-directional 3D 3D acoustic acoustic cloak cloak manufactured manufactured using using 3D 3D printing printing (cloak (cloak operational operational for foracoustic acoustic waves waves at a atfrequency a frequency of 3 kHz) of 3 kHz)[57]. (Rep [57].rinted (Reprinted by permission by permission from Springer from Springer Nature: [Na- Na- ture:ture Materials, [Nature Materials, ‘Three-dimensional ‘Three-dimensional broadband broadband omnidirectional omnidirectional acoustic acoustic ground ground cloak’, cloak’,L. Zi- goneanu et al., 2014]). L. Zigoneanu et al., 2014]).

2.2.2.2. TypesTypes ofof AMMAMM AMMsAMMs have been separated separated into into three three distinct distinct categories categories for for the the purposes purposes of ofthis this re- review.view. These These categories categories are are (i) (i) periodic/lattice metamaterials; metamaterials; (ii) (ii) coiled structuresstructures andand metasurfaces;metasurfaces; andand (iii)(iii) activeactive metamaterials.metamaterials. TheseThese classiﬁcationsclassifications areare notnot exclusiveexclusive andand therethere maymay bebe somesome overlap,overlap, withwith somesome AMMAMM examplesexamples belongingbelonging toto multiplemultiple categories.categories.

2.2.1.2.2.1. Periodic/LatticePeriodic/Lattice Metamaterials Metamaterials ThisThis typetype ofof AMMAMM consistsconsists ofof periodicperiodic structuresstructures ofof repeatingrepeating cellscells oror meta-atoms.meta-atoms. PhononicPhononic crystalscrystals describedescribe aa varietyvariety ofof latticelattice structuresstructures withwith ﬂuid,fluid, elasticelastic oror combinationcombination inclusionsinclusions in a different different material material matrix matrix [21]. [21]. However, However, there there is a is distinct a distinct difference difference be- betweentween these these crystals crystals and and periodic periodic AMMs, AMMs, as as the the latter latter utilizes locally resonant unit-cellsunit-cells toto manipulatemanipulate acoustic acoustic waveforms. waveforms. Phononic Phononic crystals crystals possess possess lattice lattice sizes sizes on the on same the scalesame asscale their as operational their operational wavelength wavelength and function and func viation scattering via scattering and interference and interference of the incident of the wavesincident globally waves throughgloballythe through lattice. the In contrast,lattice. In periodic contrast, AMMs periodic use AMMs local resonance use local withreso- minimalnance with cross-talk minimal between cross-talk cells, between meaning cells, the meaning wavelength the wavelength and resonant and frequencies resonant arefre- noquencies longer are reliant no longer on the reliant cell dimensions on the cell or dimensions global lattice or parameters.global lattice It parameters. is the independent It is the behaviourindependent of thesebehaviour meta-atoms of these that meta-atoms allows sub-wavelength that allows sub-wavelength geometry to be geometry achieved. to be achieved.Phononic crystals, despite their dimensional limitations, are still effective at sound attenuation applications. This is presented in a known phononic crystal design [59], which Phononic crystals, despite their dimensional limitations, are still effective at sound has been subsequently replicated in FEA to demonstrate its band gap properties. Figure3 attenuation applications. This is presented in a known phononic crystal design [59], which displays the resulting FEA sound pressure plot of the crystal at two key frequencies. The has been subsequently replicated in FEA to demonstrate its band gap properties. Figure 3 waves have frequencies of approximately 2 kHz and 4 kHz and lie without and within the displays the resulting FEA sound pressure plot of the crystal at two key frequencies. The band gap, respectively. In the former case, sound pressure levels (SPLs) remain relatively waves have frequencies of approximately 2 kHz and 4 kHz and lie without and within the consistent throughout the whole domain, while in the latter, levels drop considerably band gap, respectively. In the former case, sound pressure levels (SPLs) remain relatively through and after the crystal. The effect that causes this transmission loss is also apparent consistent throughout the whole domain, while in the latter, levels drop considerably in the high pressures inside the resonating inclusions. In a phononic crystal (and other through and after the crystal. The effect that causes this transmission loss is also apparent lattice types) the band gap is caused by Bragg scattering, whereby waves of a particular in the high pressures inside the resonating inclusions. In a phononic crystal (and other wavelength (determined by the lattice constant) are scattered from the inclusions and interferelattice types) destructively the band [gap60]. Acousticis caused meta-atomsby Bragg scattering, that are smaller whereby than waves the acousticof a particular wave- lengthwavelength utilise (determined gaps due to resonanceby the lattice effects constant [24]. If,) are in ascattered metamaterial, from the both inclusions mechanisms and are in- possibleterfere destructively and the Bragg [60]. and Acoustic resonance meta-atoms gaps can be that coupled, are smaller the gap than is called the aacoustic hybridisation wave- gaplength [61 ],utilise which gaps can due be deeper to resonance than the effects gaps individually.[24]. If, in a metamaterial, both mechanisms are possible and the Bragg and resonance gaps can be coupled, the gap is called a hybridisation gap [61], which can be deeper than the gaps individually.

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FigureFigure 3.3. AnAn FEA FEA plot plot of of sound sound pressure pressure level level of a of Ma atryoshka Matryoshka locally locally resonant resonant sonic sonic crystal, crystal, mod- modelledelled using using COMSOL COMSOL Multiphysics. Multiphysics. Sound Sound pressure pressure levels levels are are relatively relatively consistent throughoutthroughout whenwhen thethe incident wave wave has has a afrequency frequency of of2 kHz 2 kHz (top (top) and) and lies liesoutside outside the band the band gap but gap are but atten- are Periodic,attenuateduated considerablylocally-resonant considerably at 4 kHz atAMMs 4 kHz(bottom (canbottom) beinside )produced inside the gap. the gap.in a variety of shapes and sizes, and from different materials, manufactured using both 3D printing and conventional fab- Periodic, locally-resonant AMMs can be produced in a variety of shapes and sizes, rication techniques. The local resonances can be created using two different kinds of meta- and from different materials, manufactured using both 3D printing and conventional atom: intrinsic and inertial [21,62]. Intrinsic AMMs consist of a single inclusion (with one fabrication techniques. The local resonances can be created using two different kinds of bulk material) per unit-cell, with the phase speed of the inclusion much slower than that meta-atom: intrinsic and inertial [21,62]. Intrinsic AMMs consist of a single inclusion (with of the surrounding fluid matrix. Intrinsic AMMs are usually simpler, both in design and one bulk material) per unit-cell, with the phase speed of the inclusion much slower than ability to manufacture without 3D printing. These materials were investigated in early that of the surrounding fluid matrix. Intrinsic AMMs are usually simpler, both in design AMM research [63–66], and as manufacturing methods became more sophisticated, this and ability to manufacture without 3D printing. These materials were investigated in early allowed the production of more complex periodic metamaterials. AMM research [63–66], and as manufacturing methods became more sophisticated, this Inertial meta-atoms operate as independent mechanical oscillators and can be math- allowed the production of more complex periodic metamaterials. ematically modelledInertial using meta-atoms a dynamic operate system as independent of masses, springs mechanical and dampers. oscillators This and allows can be mathe- accuratematically resonant modelledbehaviour using prediction a dynamic of increasingly system of masses,complex, springs periodic and metamaterials. dampers. This allows Some inertialaccurate examples resonant built behaviour upon existing prediction intrinsic of increasingly designs by adding complex, coatings periodic to metamaterials.spherical inclusionsSome inertial[67], while examples others built involve upon arrays existing of intrinsicHelmholtz designs resonators by adding [68,69], coatings periodic to spherical cavities inclusionswithin a material [67], while [70] others and novel involve configurations arrays of Helmholtz of other resonators mass–spring–damper [68,69], periodic cavi- (MSD) configurations,ties within a material such as [70 membranes] and novel configurations[71,72], perforated of other plates mass–spring–damper [73] and internal (MSD) masses [74].configurations, Examples of such these as are membranes shown in [ 71Figures,72], perforated 4 and 5. As plates an extension [73] and internalto this type masses [74]. of AMM,Examples holey-structured of these meta are shownmaterials in Figureshave been4 and used5. for As acoustic an extension imaging to this beyond type the of AMM, diffractionholey-structured limit. An anisotropic metamaterials AMM lens have comprised been used of for periodic acoustic air imaging channels beyond with dif- the diffrac- ferent modulatedtion limit. diameters An anisotropic has achieved AMM lensthis through comprised the of coupling periodic of air evanescent channels waves with different [75]. modulated diameters has achieved this through the coupling of evanescent waves [75].

(a) (b)

Figure 4. ExamplesFigure of inertial 4. Examples meta-atom of inertial structures: meta-atom with (a structures:) showing awith matrix (a) showing of coated a spherical matrix of inclusions coated spherical (From [67]. Reprinted with permissioninclusions from (From AAAS). [67]. Reprinted Displayed with in ( bpermission) are various from conﬁgurations AAAS). Displayed of Helmholtz in (b) are resonator various arrayscon- [68] figurations of Helmholtz resonator arrays [68] (Reprinted from Applied Acoustics, Volume 155, (Reprinted from Applied Acoustics, Volume 155, D.P.Jena, J. Dandsena, V.G.Jayakumari, ‘Demonstration of effective acoustic D.P. Jena, J. Dandsena, V.G. Jayakumari, ‘Demonstration of effective acoustic properties of differ- properties of different conﬁgurations of Helmholtz’, Pages 371-382, Copyright (2019), with permission from Elsevier). ent configurations of Helmholtz’, Pages 371-382, Copyright (2019), with permission from Elsevier).

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(a) (b)

FigureFigure 5. 5.Examples Examples of of AMM AMM devicesdevices incorporatingincorporating me membranesmbranes and and perforated perforated plates, plates, where where (a) ( adisplays) displays a uniform a uniform metastructure of hexagonal unit-cells arranged in parallel and sealed by membranes at each open end (Reprinted from [71], metastructure of hexagonal unit-cells arranged in parallel and sealed by membranes at each open end (Reprinted from [71], with the permission of AIP Publishing). (b) shows a perforated thin-plate design with holes drilled at the cross points of with the permission of AIP Publishing). (b) shows a perforated thin-plate design with holes drilled at the cross points of the the resin frame to introduce anti-resonant behaviour [73] (Republished with permission of IOP Publishing Ltd., from resin[Journal frame of to introducePhysics D: anti-resonant Applied Physics, behaviour Y. Xu, [73 et] (Republishedal., Volume 52, with Edition permission No. 40, of 2019.]; IOP Publishing permission Ltd., conveyed from [Journal through of PhysicsCopyright D: Clearance Applied Physics, Center, Y.Inc., Xu, Danvers, et al., Volume MA, USA). 52, Edition No. 40, 2019.]; permission conveyed through Copyright Clearance Center, Inc., Danvers, MA, USA). Membrane-based acoustic metamaterials are capable of producing near complete re- Membrane-based acoustic metamaterials are capable of producing near complete flection in a narrowband at low frequencies [76]. In their first configurations, a single mass reflection in a narrowband at low frequencies [76]. In their first configurations, a single was attached to an elastic rubber membrane that was held under tension and fixed to a mass was attached to an elastic rubber membrane that was held under tension and fixed to relatively rigid frame [77]. Such a system has two resonant eigenmodes and, between a relatively rigid frame [77]. Such a system has two resonant eigenmodes and, between them, an anti-resonant frequency where acceleration is out of phase with an incident pres- them, an anti-resonant frequency where acceleration is out of phase with an incident sure wave [78]. Consequently, membrane displacement and energy transmission are min- pressure wave [78]. Consequently, membrane displacement and energy transmission imised. With an aim to increase the sound attenuating bandwidth, the effects of annular are minimised. With an aim to increase the sound attenuating bandwidth, the effects mass loadings and eccentric positioning have been investigated [79]. Some designs have of annular mass loadings and eccentric positioning have been investigated [79]. Some incorporated arrays of membrane meta-units while others have stacked them in series designs have incorporated arrays of membrane meta-units while others have stacked [80]. The field remains active, with some emerging examples of 3D-printed membrane them in series [80]. The field remains active, with some emerging examples of 3D-printed membranetype metamaterials type metamaterials [81]. As the [81]. designs As the have designs become have becomeinexorably inexorably more complex, more complex, the man- theufacturing manufacturing difficult difficult has increased, has increased, and and3D printing 3D printing is in is our in our opinion opinion both both an anenabler enabler and anda barrier a barrier to tothe the development development of ofmore more complex complex metamaterials. metamaterials.

2.2.2.2.2.2. Coiled Coiled Structures Structures and and Metasurfaces Metasurfaces ThisThis sub-section sub-section is concernedis concerned with with AMMs AMMs with with chiral chiral and and coiled coiled structures. structures. These These structuresstructures often, often, but but not not always, always, coincide coincide with with metasurfaces—defined metasurfaces—defined as planar as planar metama- met- terialamaterial and hence and operatinghence operating in 2D only. in 2D Given only. that Given these that coiled these structures coiled structures are asymmetric, are asym- usingmetric, two using dimensions two dimensions considerably considerably simplifies the simplifies design. Thethe coileddesign. nature The coiled of these nature mate- of rialsthese is inherently materials suitableis inherently for subwavelength suitable for subwavelength devices, as propagating devices, as waves propagating are made waves to travelare made much to further travel than much the further global than length the of global the structure. length of the structure. AnAn example example of of this this type type includes includes labyrinthian labyrinthian channels channels of of air, air, and and there there is evidenceis evidence thatthat such such a designa design proved proved capable capable of of directing directin ag phasea phase shift shift in in acoustic acoustic wavefronts wavefronts [82 [82].]. FigureFigure6 shows 6 shows an an example example of of a a 2D 2D coiledcoiled design. In In examples examples such such as asthis, this, discrete discrete incre- incrementsments of ofphase phase shift shift can can be be induced induced by modulatingmodulating thethe widthwidth of of the the coiled coiled structures. structures. AnotherAnother sub-wavelength sub-wavelength metasurface metasurface employed employed Helmholtz Helmholtz resonators resonators of of various various neck neck lengthslengths to to induce induce a phasea phase change change of of up up to to 2π 2radiansπ radians [83 [83].]. This This structure structure is composedis composed of of eighteight periodic periodic supercells supercells of of resonators resonators with with neck neck length length tuned tuned for for phase phase changes changes in πin/4 π/4 radianradian increments. increments. Experimental Experimental testing testing of of a (vata (vat polymerization polymerization method) method) 3D-printed 3D-printed modelmodel proved proved the the ability ability to to steer steer an an incoming incoming acoustic acoustic wave, wave, as as well well as as converting converting it toit ato a surfacesurface wave, wave, thereby thereby introducing introducing negative negative reflection. reflection. An An extraordinary extraordinary attribute attribute of of this this device is its deep sub-wavelength thickness, which was a factor of 1/30 smaller than the operational wavelength.

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Micromachines 2021, 12, 634 device is its deep sub-wavelength thickness, which was a factor of 1/30 smaller than 8the of 45 operational wavelength.

Figure 6. A 2D coiled structure acoustic metasurface [82]. Figure 6. A 2D coiled structure acoustic metasurface [82].

AnotherAnother quality quality achievable achievable by by this this AMM AMM type type is isnegative negative refraction refraction [84]. [84 ].This This can can provideprovide superior superior wave wave focusing focusing and and attenuation attenuation at atsub-wavelength sub-wavelength scales. scales. Figure Figure 7 dis-7 dis- playsplays labyrinthine labyrinthine structures structures using using negative negative refraction refraction to toattenuate attenuate sound, sound, with with designs designs Micromachines 2021, 12, x FOR PEERimplemented REVIEW in both 2D and 3D [85]. Due to the coiled geometry and an impedance mis-10 of 57 implemented in both 2D and 3D [85]. Due to the coiled geometry and an impedance matchmismatch between between the structure the structure material material and andmedium, medium, negative negative effective effective density density and and bulk bulk modulusmodulus present present simultaneously simultaneously at atspecific speciﬁc frequencies. frequencies. Hence, Hence, the the refractive refractive index index is is negative, and superior sound dispersion is achieved within very narrow frequency bands. negative, and superior sound dispersion is achieved within very narrow frequency bands.

FigureFigure 7.7. CoiledCoiled acousticacoustic metamaterialmetamaterial structuresstructures inin ( a(a)) 2D 2D and and ( b(b)) 3D, 3D, showing showing the the equivalent equivalent cells incells each in caseeach [85case]. [85].

CoiledCoiled structuresstructures havehave alsoalso exhibitedexhibited significantsignificant benefitsbenefits inin enhancingenhancing thethe broadbandbroadband abilitiesabilities ofof AMMsAMMs [[86].86]. AA gradientgradient coiledcoiled metamaterial,metamaterial, printedprinted withwith aa vatvat polymerizationpolymerization 3D3D printingprinting method, method, successfully successfully magnified magnified acoustic acoustic pressure pressure amplitudes amplitudes by approximately by approxi- 80mately times 80 their times original their magnitude,original magnitude, compared co tompared its non-coiled to its non-coiled counterpart counterpart (straight channels (straight withchannels a gradient), with a gradient), which magnified which magnified pressures pressures by a factor by of a 50.factor This of gradient 50. This AMMgradient is displayedAMM is displayed in Figure8 in. Additionally, Figure 8. Additionally, propagation propagation depth varies depth with frequency,varies with where frequency, high frequencywhere high components frequency components are dissipated are in dissipat shallowered in areas shallower and low areas frequencies and low frequencies in deeper areasin deeper of the areas coiled of structure. the coiled This structure. acoustic This phenomenon acoustic phenomenon is called rainbow is called trapping, rainbow with trap- the AMMping, with structure the AMM acting structure a means ofacting sensing a means and filteringof sensing different and filtering signals different by their frequency.signals by Hilberttheir frequency. fractals have Hilbert also fractals been considered have also been for broadband considered sound for broadband absorption, sound specifically absorp- fortion, low specifically frequencies for [ 87low]. Tofrequencies illustrate, [87]. a fractal To illustrate, metasurface a fractal device metasurface is shown in deviceFigure8 is. Byshown selectively in Figure arranging 8. By selectively different-order arranging fractal different-order meta-atoms, fractal this meta-atoms, device achieved this device wide spectrumachieved absorptionwide spectrum of soundwaves absorption of between soundwaves 225 Hz between and 1175 225 Hz. Hz This and prototype 1175 Hz. wasThis constructedprototype was with constructed photosensitive with resin photosensitive (via 3D printing) resin (via and 3D incorporated printing) and a wall incorporated thickness ofa wall 400 µthicknessm. As features of 400 μ inm. metasurface As features designsin metasurface approach designs the microscale, approach the 3D printingmicroscale, is increasingly3D printing is used increasingly as the primary used as manufacturing the primary manufacturing method, allowing method, the allowing complexity the tocom- be withinplexity structures to be within more structures easily. more easily.

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(A) (B)

FigureFigure 8. 8. BroadbandBroadband enhancement enhancement examples examples using using acoustic acoustic metasurfaces. metasurfaces. (A (A) )displays displays a agradient gradient coiled coiled metamaterial metamaterial with with (a(a––cc) )presenting presenting the the design design at at various various angles angles and and cross-sections cross-sections and and (d (d) )showing showing a a diagram diagram of of the the acoustic acoustic boundaries boundaries [86] [86 ] (Republished with permission of IOP Publishing Ltd., from [Journal of Physics D: Applied Physics, T. Chen, et al., Volume (Republished with permission of IOP Publishing Ltd., from [Journal of Physics D: Applied Physics, T. Chen, et al., Volume 54, 2020.]; permission conveyed through Copyright Clearance Center, Inc). (B) shows a 3D-printed fractal metamaterial 54, 2020.]; permission conveyed through Copyright Clearance Center, Inc.). (B) shows a 3D-printed fractal metamaterial design (Reprinted from [87], with the permission of AIP Publishing). design (Reprinted from [87], with the permission of AIP Publishing). 2.2.3. Active Metamaterials 2.2.3. Active Metamaterials This category includes controlled, time-variant AMMs, meaning their material prop- This category includes controlled, time-variant AMMs, meaning their material proper- erties, physical structure or acoustic effects can be altered during use and are not constant ties, physical structure or acoustic effects can be altered during use and are not constant variables. These materials can be modulated using a variety of external inputs and can variables. These materials can be modulated using a variety of external inputs and can function either with or without exchanging energy gain from the external acoustic field function either with or without exchanging energy gain from the external acoustic field [22]. [22]. This branch of AMMs addresses some key issues hindering the use of AMMs in ap- This branch of AMMs addresses some key issues hindering the use of AMMs in applica- plications, because of their versatility in design and unprecedented tuneable properties. tions, because of their versatility in design and unprecedented tuneable properties. A common feedback method in active AMMs involves using piezoelectric material A common feedback method in active AMMs involves using piezoelectric material to toconvert convert acoustic acoustic pressure pressure into into electric electric current current and and vice vice versa. versa. Initial Initial active active AMM AMM designs de- signsconsisted consisted of rudimentary of rudimentary fluid fluid cells cells separated separated by piezoelectric by piezoelectric boundaries boundaries [30]. [30].Figure Fig-9a uredisplays 9a displays an experimental an experimental model model of this of initialthis initial active active cell [cell38], [38], which which was was tested tested and and con- confirmedfirmed as aas viable a viable device device for tuningfor tuning the effectivethe effective density. density. To achieve To achieve this, an this, electric an electric charge chargeis applied is applied to the to piezoelectric the piezoelectric diaphragms, diaphragms, which which varies varies the cell the stiffness, cell stiffness, and the and active the activematerial material is coupled is coupled with the with fluid the medium. fluid medium. The resulting The resulting outcome isoutcome a relatively is a broadrelatively tune- broadable effectivetuneable density. effective This density. variation This of variation active AMMs of active has AMMs been developed has been furtherdeveloped in recent fur- theryears, in recent including years, implementing including implementing closed-loop controlclosed-loop over control the effective over the density effective of multi-cell density ofsystems multi-cell [37] systems and demonstrating [37] and demonstrating it experimentally it experimentally [88]. This experimental [88]. This meta-cell, experimental shown meta-cell,in Figure 9shownb, achieved in Figure a wide 9b, achieved range of effectivea wide range density of effective values ( 0.35–13density timesvaluesthat (0.35–13 of the timescontained that of fluid the contained medium). fluid Another medium). novel proposal,Another novel building proposal, on the building aforementioned on the afore- active mentionedAMM research, active employsAMM research, a gyroscopic employs AMM a gyroscopic cell to control AMM the cell magnitude to control and the direction magni- tudeof propagating and direction acoustic of propagating waves [89 acoustic]. Developing waves [89]. this Developing technology this further technology shows promisefurther showsfor the promise future construction for the future of construction practical acoustic of practical cloaks [acoustic90] and high-performancecloaks [90] and high-per- tuneable formanceacoustic filterstuneable [91 ].acoustic filters [91]. Other actuation methods for active AMMs include mechanical energy, temperature and applied magnetic fields. Helmholtz resonator-based devices can incorporate mechanical plungers to change their internal cavity volume, resulting in a shift in acoustic properties. To illustrate, Figure 10 displays a metamaterial comprised of pneumatically- actuated Helmholtz resonators, with each unit-cell acting in one direction and arranged periodically across a 2D plane [92]. Both the plunger and resonator walls were manufactured using fused deposition modelling (FDM), with the device scale in the order of centimetres. The pistons are moved by inflatable balloons, with some variation in cavity

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depth between each periodic resonator cell (cavity depth across the device was regulated within an error margin of 3 mm).

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Other actuation methods for active AMMs include mechanical energy, temperature (a) and applied magnetic fields. Helmholtz resonator-based(b) devices can incorporate mechan-

FigureFigure 9. 9(a. )(a Initial) Initial experimental experimentalical plungers prototype prototype ofto of an change an active active AMM theirAMM cell internalcell comprised comprised cavity of of an an acrylicvolume, acrylic cylinder cylinder resulting with with piezoelectric piezoelectricin a shift in acoustic prop- bimorphsbimorphs sealing sealing the the ends. ends. Theerties. The volume volume To of illustrate, theof the cell cell is filledis filledFigure with with water 10 water displays to actto act as as the athe transmission me transmissiontamaterial medium. medium. comprised The The piezoelectric piezoelectric of pneumatically-actu- membranesmembranes act act as as both both actuators actuatorsated Helmholtz and and sensing sensing apparatus; apparatus;resonators, hencehenc ewith they are each connected connected unit-cell via via electrodesacting electrodes in to to onea acontroller controller direction and and am- and arranged peri- plifier (Reprinted from [38], with the permission of AIP Publishing). (b) Experimental set-up for an active AMM cell using amplifier (Reprinted from [38], with the permission of AIP Publishing). (b) Experimental set-up for an active AMM cell piezoelectric membranes.odically A similar but across more refia 2Dned planeversion [92].of the experimentalBoth the plunger prototype and in (a), resonator with this example walls being were manufactured using piezoelectric membranes. A similar but more refined version of the experimental prototype in (a), with this example more compact with additionalusing embedded fused deposition pressure sensors modelling and closed-loop (FDM), control with [88] the (Reprinted device from scale Applied in the Acous- order of centimetres. being more compact with additional embedded pressure sensors and closed-loop control [88] (Reprinted from Applied tics, Volume 178, W. Akl andThe A. pistons Baz., ‘Active are controlmoved of theby dynamicinflatable density balloons, of acoustic with metamaterials’, some variation Copyright in (2021), cavity depth between Acoustics,with permission Volume 178,from W. Elsevier). Akl and A. Baz., ‘Active control of the dynamic density of acoustic metamaterials’, Copyright (2021), with permission fromeach Elsevier). periodic resonator cell (cavity depth across the device was regulated within an error margin of 3 mm).

(a) (b) (c)

Figure 10. 10.Tuneable Tuneable Helmholtz Helmholtz resonator resonator showing showing (a) a rendering (a) a of rendering an experimental of an metamaterial experimental metamaterial design, composed composed of tuneable of tuneable Helmholtz Helmholtz resonators resonators that are actuated that are by inflatableactuated balloons, by inflatable (b) the balloons, (b) the variations of of the the plunger plunger clearance clearance depicted depicted with the balloonwith the inflated balloon and inflated deflated and and (c )deflated clearances and (c) clearances as shownshown in in the the experimental experimental prototype prototype [92]. [92].

The spectral effects of temperature have been analysed and applied to create a tuneable array of water-ﬁlled Helmholtz resonators [29]. By adjusting the water temperature between 0 and 75 ◦C, the negative bulk modulus band and the local resonant band gap can both shifted by a maximum of 11% from their initial values. This highlights the future possibilities of tuning AMMs via temperature. A promising actuation method uses electromagnets to change acoustic parameters, with an experimental design capable of a 40% resonant band gap shift [33]. The meta-unit design, as shown in Figure 11, consists of an acoustic cavity with a membrane attached to electromagnetic clamps. These clamps,

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The spectral effects of temperature have been analysed and applied to create a tuneable array of water-filled Helmholtz resonators [29]. By adjusting the water temperature between 0 and 75 °C, the negative bulk modulus band and the local resonant band gap can both shifted by a maximum of 11% from their initial values. This highlights the future possibilities of tuning AMMs via temperature. A promising actuation method uses electromagnets to change acoustic parameters, with an experimental design capable of a 40%

Micromachines 2021, 12, 634 resonant band gap shift [33]. The meta-unit design, as shown in Figure11 of 45 11, consists of an acoustic cavity with a membrane attached to electromagnetic clamps. These clamps, operated with an applied electromagnetic field, manipulate the tension and stiffness of the membrane,operated with consequently an applied electromagnetic changing ﬁeld, the manipulate acoustic the properties tension and of stiffness the entire of the meta-unit. membrane, consequently changing the acoustic properties of the entire meta-unit.

FigureFigure 11. Structure Structure of the of active the meta-unitactive meta-unit with an electromagnetic with an electromagnetic tuning mechanism. Atuning membrane mechanism. A mem- braneis tensioned is tensioned using electromagnetically-operated using electromagnetically-operate clamps attached tod the clamps pipe exterior attached [33]. to the pipe exterior [33]. Hydrogels are prevalent materials in the literature surrounding active AMMs, broad- eningHydrogels the field by are introducing prevalent soft, materials flexible capabilities in the literature that allow surrounding for easier physical active AMMs, broad- eningactuation. the Thesefield malleable,by introducing hydrogel-based soft, flexible AMMs are capabilities referred to as that metagels. allow A for novel easier physical actu- hydrogel-composite design, shown in Figure 12, includes fillable channels within the ation.hydrogel These that malleable, can tailor the hydrogel-based metagel’s acoustic properties AMMs are in order referred to create to broadbandas metagels. A novel hydrogel-compositetuneable transmission/reflection design, shown [93 in]. The Figure internal 12, metagel includes microstructure fillable channels can be filled within the hydrogel thatwith can any fluidtailor material the metagel’s to change the acoustic acoustic impedance, properties resulting in order in a variety to create of effects broadband tuneable Micromachines 2021, 12, x FOR PEER REVIEW 15 of 57 on incident sound waves, as shown in the sub-figures of Figure 12. Mismatched acoustic transmission/reflectionimpedance is a significant challenge [93]. The to overcome internal in medical metagel and underwatermicrostructure imaging, can and be filled with any fluidtherefore material the ability to tochange adapt these the propertiesacoustic is impedanc a valuable developmente, resulting for in the a potential variety of effects on inci- dentapplications sound ofwaves, metamaterials. as shown in the sub-figures of Figure 12. Mismatched acoustic impedance is a significant challenge to overcome in medical and underwater imaging, and therefore the ability to adapt these properties is a valuable development for the potential applications of metamaterials.

Figure 12. Metagel design consisting of microstructure channels in a tough hydrogel matrix. The Figure 12. Metagel design consisting of microstructure channels in a tough hydrogel matrix. The acousticacoustic impedance impedance of of the the hydrogel hydrogel matches matches well well wi withth water, water, resulting resulting in in almost almost total total transmission transmission ofof incident incident waves. waves. These These channels channels can can be be filled filled with with various various liquids liquids to to modulate modulate the the presenting presenting transmissiontransmission characteristics characteristics of of the the metagel. metagel. In In the the figure, figure, ( (AA)) shows shows “no “no channels”, channels”, with with almost almost total total acousticacoustic transmission; transmission; (B (B) )shows shows “water-filled “water-filled channels”, channels”, with with almost almost total total acoustic acoustic transmission; transmission; ((CC)) shows shows “air-filled “air-filled channels”, channels”, with with almost almost total total acoustic acoustic reflection; reflection; and and (D (D) )shows shows “liquid “liquid gallium- gallium- filledfilled channels”, channels”, with with combined combined transmission transmission and and re reflectionflection characteristics. characteristics. (Reprinted (Reprinted from from [93], [93], withwith the the permission permission of of John John Wiley Wiley & & Sons Sons Inc.). Inc.).

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Micromachines 2021, 12, 634 By using hydrogels as a primary design material, this also allows12 of 45for movement control through changes in hydration. A novel hydrogel-composite metamaterial uses a horseshoe-shaped unit design to facilitate the unusual behaviour of shrinking with water absorption,By using hydrogelsand vice asversa a primary [36]. designThe microstruc material, thisture, also shown allows forin Figure movement 13 at various stages control through changes in hydration. A novel hydrogel-composite metamaterial uses a horseshoe-shapedof hydration, works unit design by conver to facilitateting hydraulic the unusual swelling behaviour into of shrinking bending with deformation, wa- resulting terin anisotropic absorption, and negative vice versa swelling [36]. The on microstructure, the macro-scale. shown Though in Figure 13hydration, at various this metamaterial stageshas fully of hydration, tuneable works stress–strain by converting curves hydraulic and ca swellingn present into bending specific deformation, anisotropic expansion/con- resultingtraction inbehaviours anisotropic negativebased on swelling its geometry. on the macro-scale. A multi-material Though hydration, polyjet thisprinter was used for metamaterial has fully tuneable stress–strain curves and can present speciﬁc anisotropic expansion/contractionfabrication with a minimum behaviours based layer on thickness its geometry. of A90 multi-material μm. In addition, polyjet printerhydrogels are low-cost, wasenvironmentally used for fabrication friendly with a minimum and compatible layer thickness with of 90humanµm. In addition,tissue. hydrogelsManufacturing hydrogel- arecomposite low-cost, environmentallystructures with friendly finer detail and compatible would withcreate human more tissue. versatility Manufactur- in the achievable ma- ing hydrogel-composite structures with ﬁner detail would create more versatility in the achievableterial properties material propertiesand improve and improve tunability tunability across across a wider a wider broadband.

Figure 13. Lattice comprised of hydrogel-composite meta-units, with successive stages of hydration displayed.Figure 13. Here, Lattice the scale comprised bar length equatesof hydrogel-composite to 5 mm. The initial state meta-units, is shown at ambientwith successive conditions, stages of hydration ◦ beforedisplayed. immersion Here, in waterthe scale for 45 bar min, length followed equates by dehydration to 5 mm. via aThe drying initial oven state set at is 75 shownC for at ambient condi- 35tions, min [before36]. immersion in water for 45 min, followed by dehydration via a drying oven set at 75 °C for 35A min relevant [36]. lattice-type structure here is tensegrity metamaterials, which, through their geometrically non-linear response, can tune global elastic characteristics and hence modulateA relevant the structures lattice-type effect on acoustic structure waves. here These is structures tensegrity consist metamateri of an assemblyals, which, through of rigid, compressive elements and deformable, tensile elements [94] that result in complex, non-lineartheir geometrically mechanical behaviour non-linear determined response, by the can geometric, tune global mechanical elastic and pre-stresscharacteristics and hence variablesmodulate [95 the]. Tensegrity structures metamaterials effect on typicallyacoustic consist waves. of repeated These structures units of tensegrity consist of an assembly prismsof rigid, within compressive an array or lattice. elements Small and changes deformable in the pre-stress, tensile of components elements making [94] that result in com- upplex, the non-linear prism unit-cell mechanical result in drastically behaviour different determ acousticined and by mechanical the geometric, properties, mechanical and pre- which is useful for application in engineering, architecture and acoustic industries. Novel applicationstress variables examples [95]. include Tensegrity acoustic lenses metamateri [96], impact protectionals typically devices consist [94], mechani- of repeated units of caltensegrity actuators prisms and sensors within [97], temporaryan array sheltersor lattice. and Small deployable changes antennas in the [98], pre-stress most of of components whichmaking require up the units prism to be scaledunit-cell down result to be adequatein drastically size for different feasible use. acoustic For example, and mechanical prop- tensegrity prisms have been utilized to design a tuneable impact-absorption device that, shoulderties, thewhich prism is unit-cells useful befor scaled application down to 10in µengim, couldneering, maintain architecture an effective an impactd acoustic industries. protectionNovel application barrier with aexamples feasible total include length requirement acoustic lenses of 10 mm [96], [95]. impact protection devices [94], mechanicalActive AMMs actuators are designed and sensors to have modifiable [97], temporary effects on shelters the surrounding and deployable acoustic antennas [98], field;most therefore, of which the changeablerequire units parameters to be arescaled usually down directly to related be adequate to the equations size for feasible use. For acoustic wave propagation. Bulk modulus and effective density are of specific interest, with aexample, significant tensegrity portion of active prisms AMM have research been focused utiliz oned allowing to design these a parameterstuneable impact-absorption to be de- tuneablevice that, [27 should,28,30,32, 88the,99 prism,100]. Many unit-cells new tuneable be scaled devices down build uponto 10 early μm, AMMs, could such maintain an effective asimpact Helmholtz protection resonator barrier arrays and with periodic a feasible lattice structures,total length by incorporating requirement active of 10 ele- mm [95]. ments.Active A meta-unit AMMs design are was designed proposed using to have an actively-controlled modifiable effects system on of symmetric, the surrounding acoustic double Helmholtz resonators [101], which successfully tuned the bulk modulus (including negativefield; therefore, values) over the a changeable broad frequency parameters range by using are piezoelectricusually directly diaphragms related with to the equations for activeacoustic feedback wave control. propagation. A theoretical Bulk acoustic modulus cloak withand tuneableeffective parameters density isare then of specific interest, with a significant portion of active AMM research focused on allowing these parameters to be tuneable [27,28,30,32,88,99,100]. Many new tuneable devices build upon early AMMs, such as Helmholtz resonator arrays and periodic lattice structures, by incorporating active elements. A meta-unit design was proposed using an actively-controlled system of symmetric, double Helmholtz resonators [101], which successfully tuned the bulk mod-

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discussed with multiple layers composed of these periodic meta-units. The acoustic parameters of each layer can be changed by tuning the gain of the diaphragms, resulting in a fully-modiﬁable cloak over the frequency range of 3000–6000 Hz. The upper frequency limit is restricted by the subwavelength size the Helmholtz resonators, and so further developing microscale manufacturing methods could help realize the potential of a broadband, fully-tuneable acoustic cloak.

3. Additive Manufacturing Methods The previous section demonstrates the wide range of AMM designs in the literature but is by no means exhaustive. However, it is clear that the concepts and theoretical approaches are being pushed towards greater complexity, and it is our opinion that the greatest barrier to increased complexity will be the ease of manufacture. In this section, we will discuss additive manufacturing methods currently used, which, within the scope of this review, include vat polymerization techniques, powder bed fusion techniques, extrusion and deposition methods, and hybrid techniques.

3.1. Vat Polymerization Techniques The distinctive feature of vat polymerization is that targeted photopolymerization is employed to construct custom 3D parts, as opposed to other methods which use heat-based fusion or material extrusion. These additive manufacturing techniques can be broadly deﬁned as the curing of photo-reactive polymers via an electromagnetic radiation source (for example, lasers, visible light and UV light) [11]. The techniques within the vat polymerization category which are discussed comprise stereolithography (SLA), digital light processing (DLP), and multi-photon polymerization (MPP). Due to the fabrication process, material options are primarily restricted to liquid photopolymer resins; however, small particulates of various materials, such as ceramics [102], metals [103] and live cells [104], can be added to inﬂuence material properties.

3.1.1. Stereolithography (SLA) This 3D printing technique was among the ﬁrst developed and involves the scanning of a photo-curable liquid, layer by layer, to create a 3D part [2]. Traditional SLA cures the 2D cross-section using a laser beam moving in the X–Y plane across the vat of liquid material, before submerging the part one layer-depth and creating the next layer. The laser can use UV or visible light; in some cases infrared light has been used to cure the resin [105]. The exposure time of the laser directly relates to the layer thickness, calculated using the Beer–Lambert law, and microscale cure depths as low as 25 µm can easily achieved by shortening this time [106], where the citation provides a detailed explanation of the SLA printing process. Depending on the material and geometry, lengthy post-processing can be required to maintain mechanical properties. For example, a thermal post-cure increased the compressive modulus of thin polymer layers from 0.6 MPa to 25 MPa, including a 24-h ethanol soak of both samples, which further increased the modulus. The absolute minimum resolution for SLA generally does not exceed 10 µm; however, recent experimental methods employed an optimization algorithm to control the laser beam exposure and consequently achieved resolutions lower than a single micron [107], as illustrated in Figure 14. Micromachines 2021, 12, 634 14 of 45 Micromachines 2021, 12, x FOR PEER REVIEW 18 of 57

Figure 14. Electron microscope image of a seahorse feature printed using an optimized SLA tech- Micromachines 2021, 12, x FOR PEER REVIEWFigure 14. Electron microscope image of a seahorse feature printed using an optimized SLA technique.19 of 57 Thenique. right-side The right-side image has image the predicted has the predicted shape superimposed. shape superimposed. The scale The bar herescale equates bar here to equates 1 µm [107 to]. 1 μm [107]. (Reprinted from IFAC-PapersOnLine, Volume 50, Issue 1, A. Fleming, et al., ‘Experimental (Reprinted from IFAC-PapersOnLine, Volume 50, Issue 1, A. Fleming, et al., ‘Experimental Scanning Scanning Laser Lithography with Exposure Optimization’, Pages 8662-8667, Copyright (2017), with Laser Lithography with Exposure Optimization’, Pages 8662-8667, Copyright (2017), with permission permission from IFAC). from IFAC).

InIn another another example example of of SLA, SLA, a a small-scale small-scale Helmholtz Helmholtz resonator resonator (total (total length length 5.1 5.1 mm mm withwith a a wall wall thickness thickness 0.5 0.5 mm) was successfully fabricated and and validated experimentally withwith a a commercially commercially available available SLA SLA device device [108 [108],], the the minimum minimum resolution resolution length length of of which which waswas 27 27 μµmm [109]. [109]. The The resolution resolution limits limits of of SLA SLA methods methods are are enforced enforced from from several several factors, factors, includingincluding the the absorption absorption spectra, spectra, the the focal focal point point size/resolving size/resolving power power of of the the laser laser and and the the levellevel of of inhibitor inhibitor molecules molecules present present (usually (usually diffused diffused oxygen) oxygen) [2]. [2]. FurtherFurther SLA SLA restrictions restrictions exist exist in in the the printa printableble material material options, options, which which generally generally are are aa form form of of photocurable photocurable liquid; liquid; however, however, there there is is still still versatility, versatility, with ceramic inclusions beingbeing used used to to improve improve mechanical, mechanical, thermal, thermal, elec electricaltrical and and magnetic magnetic properties properties of of the the base base resinresin [102]. [102]. Additionally, Additionally, to to print print pure, pure, non- non-compositecomposite ceramic ceramic material, material, there there must must be be a a processprocess to to debind debind and and remove remove the the photocurable photocurable resin. resin. Despite Despite these these restrictions, restrictions, there there are are aa variety variety of of materials materials printable printable using using SLA. SLA. A A novel novel example example is is a a millimetre-scale millimetre-scale bio-bot bio-bot comprisedcomprised of of hydrogel hydrogel and and actuated actuated by by skeletal skeletal muscle muscle [104], [104], as as shown shown in in Figure Figure 15.15.

FigureFigure 15 15.. AssemblyAssembly process process of a hydrogel micro-SLA-printedmicro-SLA-printed bio-bot,bio-bot,actuated actuated with with skeletal skeletal muscle. muscle.The The biobot biobot (i) ( isi) placedis placed within within the the holder holder (ii (),ii), and and the the assembly assembly ( iii(iii)) is is ﬁlled filled withwith thethe cell-matrixcell-matrix solution. This material is compacted around the biobot pillars to form a solid muscle strip (v) solution. This material is compacted around the biobot pillars to form a solid muscle strip (v) and and with immunostaining (vi). The final device is shown in the bottom-right. All scale bars here with immunostaining (vi). The ﬁnal device is shown in the bottom-right. All scale bars here are are 1 mm [104]. 1 mm [104].

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SLA printers have been utilized to fabricate devices for acoustic and ultrasonic applications.SLA printersNotably, have an in-air been ultrasonic utilized to beam fabricate shifter devices was produced for acoustic from and clear ultrasonic resin, using appli- acations. commercially Notably, available an in-air 3D ultrasonic SLA printer beam (Asiga shifter PICO2 was produced HD), where from the clear prototype resin, using was experimentallya commercially verified available to confirm 3D SLA its printer beam (Asigashift performance PICO2 HD), on where acoustic the fields prototype [110]. The was acousticexperimentally response verified of the device to confirm was tested its beam and compared shift performance with a simulated on acoustic model fields in COM- [110]. SOLThe Multiphysics, acoustic response where of the the results device of was both tested were and largely compared in agreement with a simulatedwith the analyti- model cally-predictedin COMSOL Multiphysics, behaviour. whereThis metamateri the resultsal of device both were was largely composed in agreement of periodically- with the spacedanalytically-predicted plates, with a thickness behaviour. of This500 μ metamaterialm, and was appropriately device was composed scaled to of operate periodically- at 40 kHz.spaced plates, with a thickness of 500 µm, and was appropriately scaled to operate at 40 kHz.Multi-material micro-SLA has also recently been demonstrated, with several col- ouredMulti-material resins used in micro-SLAa single print has and also a recentlylayer thickness been demonstrated, of 30 μm [111], with illustrated several coloured in Fig- ureresins 16 for used the in multi-colour a single print print and at a 200 layer μm thickness scale. An of active 30 µm AMM [111 ],has illustrated also been in fabricatedFigure 16 withfor the ferroelectric multi-colour material, print atallowing 200 µm tuneable scale. An st activeiffness AMM via an has external also been electric fabricated field [112]. with Theferroelectric efficiency material, of this method allowing is limited tuneable due stiffness to the scanning via an external mechanism, electric which field is [ 112required]. The toefficiency move across of this the method vat incrementally is limited dueto cure to the a layer. scanning Other mechanism, vat polymerization which is methods required addressto move this across drawback, the vat namely incrementally digital tolight cure pr aocessing layer. Other (DLP), vat by polymerization curing the entire methods layer inaddress one exposure. this drawback, Alternatively, namely digital better light reso processinglutions approaching (DLP), by curing the nanoscale the entire can layer be in achievedone exposure. with multi-photon Alternatively, polymerization, better resolutions another approaching vat-polymerization the nanoscale technique. can be achieved with multi-photon polymerization, another vat-polymerization technique.

FigureFigure 16. 16. Micro-SLA-fabricatedMicro-SLA-fabricated 3D 3D cross-shape, cross-shape, prin printedted with with multicolour multicolour resins. resins. The The modelled modelled CADCAD geometrygeometry is is shown shown from from orthogonal orthogonal ( (aa)) and and top-view top-view ( (bb)) angles angles alongside alongside the the finished ﬁnished part part at atorthogonal orthogonal (c ()c and) and top top view view (d ()d angles.) angles. Here, Here, scale scale bars bars are are 200 200µ mμm [111 [111].].

3.1.2.3.1.2. Digital Digital Light Light Processing Processing (DLP) (DLP) FromFrom SLA SLA technology, technology, another another microscale microscale printing printing method method emerged—digital emerged—digital light light processing.processing. This This is is similar similar to SLA; however, DLPDLP curescures an an entire entire layer layer at at once once by by projecting project- ingpatterns patterns of lightof light [113 [113].]. This This method method features features both both high high resolution resolution and quickand quick print times,print times,with resolution with resolution lengths lengths as small as assmall 0.6 µasm 0.6 and μm a layerand a printinglayer printing speed speed of 1000 of mm/h. 1000 mm/h. Some SomeDLP printersDLP printers are capable are capable of printing of printing large la volumerge volume parts parts in short in short timeframes, timeframes, where where for forexample example a part a part with with dimensions dimensions 38 38 cm cm by by 61 61 cm cm by by 76 76 cm cm was was reportedly reportedly printed printed in in 1 1 h hand and 45 45 min min with with DLP.DLP. However, However, fast fast printingprinting atat suchsuch sizesize scales incurs the cost cost of of lower lower precisionprecision and and resolution. resolution. µ Notably,Notably, a a micro-beam micro-beam was was fabricated fabricated using using DLP DLP with with a a layer layer thickness thickness of of 30 30 μmm and and anan exposure exposure time of 3 ss (per(per layer)layer) [ 114[114].]. The The printed printed micro-beam micro-beam is is displayed displayed in inFigure Figure 17 , 17,taken taken from from a microscopea microscope image. image. To To prevent prevent breakage,breakage, structuralstructural supports were were added, added, with a diameter of 3 µm and a height of 5 µm. The same beam design was also fabricated using SLA, but the latter method was not able to print the supports, so instead the beam was printed in 2D. This direct comparison demonstrates DLP is not only the faster and

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Micromachines 2021, 12, 634 16 of 45 with a diameter of 3 μm and a height of 5 μm. The same beam design was also fabricated using SLA, but the latter method was not able to print the supports, so instead the beam was printed in 2D. This direct comparison demonstrates DLP is not only the faster and moremore precise method, but the micro-resolution allowed supports to be added that vastly improvedimproved part part quality. quality. DLP thrived at printing complex, microscale 3D structures, while micro-SLAmicro-SLA performed better wi withth simpler 2D structures.

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Figure 17. A micro-beam fabricated with DLP using clear IP-S resin photopolymer, where the smallest Figure 17. A micro-beam fabricated with DLP using clear IP-S resin photopolymer, where the small- non-support feature is 86.7 µm. Twenty supports with a diameter of 3 µm were used during the est non-support feature is 86.7 μm. Twenty supports with a diameter of 3 μm were used during the building process. Here, the scale bar is 200 µm [114]. buildingIn process.the literature, Here, the SLAscale barand is DLP200 μ mare [114]. further contrasted, with SLA’s printing speed (in the verticalIn the literature, Z direction) SLA andrestricted DLP are to furthersevera contrasted,l mm/h due with to its SLA’s stepwise printing regime, speed where (in each theindividual vertical Z layer direction) takes restricted seconds toto several manufacture mm/h dueand to is its 50–100 stepwise μm regime, thick. Unlike where each SLA, DLP’s individualprinting is layer generally takes seconds continuous to manufacture and restricted and is 50–100insteadµ mby thick. resin Unlike cure rates SLA, DLP’sand viscosity, printingand hence is generally can achieve continuous speeds at and least restricted two orders instead of bymagnitude resin cure faster rates [115]. and viscosity, A gyroid struc- andturehence printed can at achieve 500 mm/h speeds using at leastDLPtwo can ordersbe seen of in magnitude Figure 18B, faster with [115 a diagram]. A gyroid of the DLP structure printed at 500 mm/h using DLP can be seen in Figure 18B, with a diagram of process. An important benefit of this manufacturing technique is that increasing resolu- the DLP process. An important beneﬁt of this manufacturing technique is that increasing resolution/layertion/layer thickness thickness has hasless less effect effect on onthe the print print time—this time—this was was testedtested with with the the ramp ramp struc- structurestures shown shown in Figure in Figure 18C 18 Cprinted printed with with slice slice thicknesses thicknesses of 100100 µμm,m, 2525µ μmm and and 1 μm. 1Thereµm. There are additional are additional factors factors to consider to consider for for viable viable printing, printing, such such as as modulating the the “dead- “dead-zone”zone” and oxygen and oxygen flux ﬂuxthrough through the the permeable permeable window. window.

FigureFigure 18. 18Gyroid. Gyroid structure structure printed printed in microscale in microscale with DLP with showing DLP showing (A) a diagram (A) a of diagram the DLP set-up;of the DLP set- (up;B) the (B) gyroid the gyroid part, printed part, printed at 500 mm/h; at 500 andmm/h; (C) rampand (C test) ramp prints test with prints slice thicknesseswith slice thicknesses of 100 µm, of 100 25μm,µm 25 and μm 1 µandm, successively.1 μm, successively. (From [115 (From], Reprinted [115], Repr withinted permission with frompermission AAAS). from AAAS).

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Micromachines 2021, 12, 634 17 of 45 The printable material is limited by similar restrictions to SLA; additionally, multi- material prints are more difficult to achieve with DLP because of the holistic nature of the curingThe printablemechanism. material One is limited of the by similarearliest restrictions DLP multi-material to SLA; additionally, prints, multi- in 2011, required the materialmaterial prints vat are to more be removed difﬁcult to achieve and replaced with DLP becauseto print of thea different holistic nature material of the [116]. This method curing mechanism. One of the earliest DLP multi-material prints, in 2011, required the materialwas particularly vat to be removed susceptible and replaced to material to print a different waste materialand material [116]. This contamination method between vats, waswhich particularly was countered susceptible toby material using wastea bottom-up and material pr contaminationojection method between and vats, implementing a two- whichstage was cleaning countered process by using between a bottom-up vat projection changes. method This and adapted implementing DLP meth a two-od managed to print stage cleaning process between vat changes. This adapted DLP method managed to print multi-materialmulti-material parts parts with a with layer thickness a layer ofthickness 100 µm, as of illustrated 100 μm, in Figureas illustrated 19. in Figure 19.

FigureFigure 19. 19Test. Test print print for a multi-materialfor a multi-material DLP method DLP using method red and using yellow red photocurable and yellow resin photocurable resin (Reprinted from [116], with author permission). (Reprinted from [116], with author permission). A subsequent multi-material DLP method, with a top-down approach, employed an air jet-basedA subsequent cleaning step multi-material to minimize resin DLP waste method, and cross-contamination with a top-down [117]. Thisapproach, employed an novel method forewent the use of a vat, instead utilizing a glass plate with different photocurableair jet-based liquids cleaning applied step in “puddles” to minimize using electronically-controlled resin waste and syringes.cross-contamination Two [117]. This materialsnovel method can be printed forewent simultaneously, the use withof a avat, wide inst varietyead of utilizing resins able a to glass be utilized. plate with different pho- Atocurable layer thickness liquids of 25 appliedµm was used,in “puddles” and multilateral using layers electronically-controlled reportedly took (in ideal syringes. Two ma- conditions) 2.2 times longer to print than a single material equivalent. To shorten the time takenterials to switchcan be materials printed in multi-material simultaneously, DLP printing, with another a wide (bottom-up) variety methodof resins was able to be utilized. A proposedlayer thickness that uses dynamic of 25 μ fluidm was control used, to exchange and multilateral resins [118]. Photo-polymerization layers reportedly took (in ideal con- takesditions) place in2.2 a sealedtimes fluidic longer cell, withto print UV light than exposed a single through material a transparent equivalent. top window, To shorten the time in which different liquid materials are pumped through and directed using a series of valves.taken Resinsto switch with polymer,materials ceramic in multi-material and metallic composites DLP canprinting, be used, another as well as (bottom-up) method activewas proposed hydrogels, allowing that uses thermally-actuated dynamic fluid soft control robots to to be printed.exchange A full resins material [118]. Photo-polymeri- changezation was takes reportedly place 30in times a sealed quicker fluidic than manually cell, wi changingth UV light material exposed vats, with through four a transparent top measured materials exchanging 95% cell volume in 3.2–12.8 s. Furthermore, this method demonstratedwindow, in high which resolution, different with a minimumliquid materials feature size are of 5 µpumpedm. through and directed using a seriesThere of are valves. currently Resins promising with advancements polymer, in ceramic printing metallic and metallic materials withcomposites DLP can be used, as bywell using as silveractive nanoparticles hydrogels, incorporated allowing in thermally-ac a polymer matrixtuated [103]. Anothersoft robots novel to ma- be printed. A full ma- terialterial is change hydrogel, was often reportedly used in active 30 metamaterial times quicker designs. than A temperature-responsive manually changing material vats, with hydrogel was successively printed in the microscale using DLP [119]. Some multi-material structuresfour measured have been materials experimentally exchanging produced by exposing95% cell the volume same base in resin 3.2–12.8 to either s. Furthermore, this UVmethod or visible demonstrated light, hence creating high regions resolution, with different with a material minimum properties feature [120]. size The of 5 μm. geometryThere of an are example currently part printed promising using DLP advancements is shown in Figure in 20 printing, where the metallic purple materials with DLP areas are rendered with UV light and clear areas with visible light. Another proposed multi-materialby using silver DLP approachnanoparticles achieves aincorporated minimum feature in size a polymer of 200–300 µmatrixm [121]. [103]. Over- Another novel ma- all,terial the resolutionis hydrogel, seems often to be significantly used in active reduced me whentamaterial using multi-material designs. capableA temperature-responsive technologies,hydrogel was with successively printed designs strugglingprinted in to reachthe microscale the microscale. using DLP [119]. Some multi-material structures have been experimentally produced by exposing the same base resin to either UV or visible light, hence creating regions with different material properties [120]. The geometry of an example part printed using DLP is shown in Figure 20, where the purple areas are rendered with UV light and clear areas with visible light. Another proposed multi-material DLP approach achieves a minimum feature size of 200–300 μm [121]. Over- all, the resolution seems to be significantly reduced when using multi-material capable technologies, with printed designs struggling to reach the microscale.

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Micromachines 2021, 12, x FOR PEER REVIEW 25 of 57 FigureFigure 20.20.Multi-material Multi-material printed printed part part using using DLP DLP with with UV andUV visibleand visible light exposure,light exposure, where (wherea) shows (a) theshows geometry the geometry of the part of withthe part UV irradiatedwith UV irra sectionsdiated highlighted sections highlighted in purple, and in purple, (b) shows and the (b ﬁnished) shows the finished print with (left) no thermal post-cure and (right) 3-h thermal post-cure at 60 °C. Here, print with (left) no thermal post-cure and (right) 3-h thermal post-cure at 60 ◦C. Here, the scale bar the scale bar represents 12 mm [120]. represents 12 mm [120].

AnAn emerging emerging tomographic tomographic technique technique buildi buildingng on on DLP DLP technology technology employs employs a asingle- single- stepstep multi-beam multi-beam exposure, exposure, omitting omitting the the typical typical stepwise stepwise protocol protocol of ofmost most additive additive manu- man- facturingufacturing methods methods [122]. [122 This]. This unique unique polymerization polymerization approach approach requires requires the the resin resin to to be be transparenttransparent and and the subsequentsubsequent absorptionabsorption length length to to be be tuned tuned to ensureto ensure that that light light can reachcan reachthe necessary the necessary cure cure depths. depths. The transparentThe transpar vatent ofvat resin of resin is rotated, is rotated, and aand DLP a DLP modulator mod- ulatordisplays displays low-energy low-energy light patternslight patterns in sync in with sync this with rotation, this rotation, and after and a after full revolution,a full revo- a lution,3D rendered a 3D rendered part is produced. part is produced. The intensity The ofintensity a single of illumination a single illumination pattern is insufﬁcientpattern is insufficientto cure the to resin; cure hence, the resin; the summation hence, the ofsu everymmation light of pattern every islight required pattern to solidifyis required the ﬁn-to solidifyished part. the finished This new part. method This has new a theoreticalmethod has resolution a theoretical of 23–33 resolutionµm and of has23–33 successfully μm and hasprinted successfully structures printed with structures microscopic with features. microscopic Figure features. 21 shows Figure a miniature 21 shows “Notre a miniature Dame “Notrearchway” Dame printed archway” in acrylic printed resin, in acrylic with a resin, smallest with feature a smallest size feature of 80 µ sizem and of 80 a relativelyμm and avery relatively short very print short time ofprint 19.5 time s. of 19.5 s.

FigureFigure 21. 21. “Notre“Notre Dame” Dame” printed printed part part using using the the novel novel tomographic tomographic multi-beam multi-beam method. method. The The scale scale barsbars are are 5 5mm mm in in the the main main image, image, and and 1 1mm mm for for the the inset inset [122]. [122].

3.1.3. Multi-Photon Polymerization (MPP) First proposed in 1997 [123], multi-photon polymerization (or two-photon polymerization) is an advancement on the original SLA fabrication method, the key distinction being that while SLA cures a layer with incremental laser exposure, MPP cures a layer point by point. The MPP fabrication process involves directing a femtosecond laser pulse through a lens, so it converges into a single focal point within the vat. This focused energy triggers the

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3.1.3. Multi-Photon Polymerization (MPP) First proposed in 1997 [123], multi-photon polymerization (or two-photon polymerization) is an advancement on the original SLA fabrication method, the key distinction being that while SLA cures a layer with incremental laser exposure, MPP cures a layer point by point. The MPP fabrication process involves directing a femtosecond laser pulse through a lens, so it converges into a single focal point within the vat. This focused energy triggers the two-photon absorption process, and a small volume of resin is polymerized as a result. The volume pixel (voxel) of cured material can be several orders below the Micromachines 2021, 12, 634 19 of 45 laser wavelength, which is typically about 810 nm [2]. This method allows for an extremely precise curing of resin and hence offers a very high resolution, with a minimum resolution size in the order of 100 nm [113]. This does come at the cost of print size, with two-photon absorption process, and a small volume of resin is polymerized as a result. The volumeX–Y maximum pixel (voxel) print of cured sizes material ranging can from be several 10 μ ordersm by below10 μm, the to laser 2.2 wavelength,mm by 2.2 mm. Similarly whichto DLP, is typically MPP is about not limited 810 nm [2 by]. This supporting method allows materials for an extremelyor layer-by-layer precise curing fabrication of regimes, resinas polymerization and hence offers acan very occur high resolution,in any 3D with spatial a minimum point, resolutionand not sizejust inthe the current order build surface of[7]. 100 These nm [113 characteristics]. This does come make at the costMPP of of print significant size, with X–Yinterest maximum as a printfabrication sizes method for µ µ rangingdrug-delivery from 10 mdevices, by 10 m, implantable to 2.2 mm by microelectromechanical 2.2 mm. Similarly to DLP, MPP systems is not limited (MEMS) and tissue by supporting materials or layer-by-layer fabrication regimes, as polymerization can occur inscaffolding any 3D spatial applications. point, and not just the current build surface [7]. These characteristics make MPP ofMPP signiﬁcant in prior interest studies as a fabrication has repeatedly method fordemonstrated drug-delivery devices,nanoscale implantable print size, succeeding microelectromechanicalthe aforementioned systems vat polymerization (MEMS) and tissue methods scaffolding significantly applications. in ultra-high resolution [124,125].MPP in priorAn example studies has of repeatedly this is a demonstratedsynthetic nanoporous nanoscale print geomaterial size, succeeding fabricated the with MPP, aforementioned vat polymerization methods signiﬁcantly in ultra-high resolution [124,125]. 3 Anas exampleillustrated of this in is Figure a synthetic 22, nanoporous which shows geomaterial an electron fabricated microscope with MPP, as image illustrated of a 150 μm cube inof Figure replica 22, shale which rock shows with an electron a minimum microscope feature image size of a 150of 160µm 3nmcube [15]. of replica The print shale was evaluated rockwith with the amicroscope minimum feature and has size no of external 160 nm [15 defects,]. The printvalidating was evaluated the quality with of the this 3D printing microscopemethod. Feature and has nowidths external as defects, small as validating 65 (±5) thenm quality were ofconsistently this 3D printing achieved method. by lowering the Feature widths as small as 65 (±5) nm were consistently achieved by lowering the laser laser wavelength to 520 nm. wavelength to 520 nm.

FigureFigure 22. 22MPP-printed. MPP-printed 150 µ m1503 cube μm of3 cube replica of shale replica rock withshale nanoscale rock with features, nanoscale where the features, image where the im- isage taken is taken with an with electron an microscopeelectron microscope [15]. [15].

In another example of MPP fabrication, Figure 23 displays two woodpile structures with beamIn another thicknesses example of 72 nm of andMPP 60 fabrication, nm, respectively Figure [126 ].23 Due displays to the high two spatial woodpile structures resolutionwith beam of MPP, thicknesses there are limitations of 72 nm with and regards 60 nm, to print respectively speed. MMP [126]. printing Due speeds to the high spatial generallyresolution range of betweenMPP, there 0.5 mm/s are limitations to >100 mm/s with in the regards X–Y plane to print [127– 131speed.], depending MMP printing speeds ongenerally the resolution range required between and technology0.5 mm/s used.to >100 Novel mm/s microscale in the methods X–Y plane incorporating [127–131], depending high frequency resonant-imaging mirrors can reach speeds as high as 8000 mm/s [132]. It ison important the resolution to note that required these speeds and technology are in the X–Y used. plane and,Novel as thismicroscale method cures methods each incorporating voxelhigh individually, frequency itsresonant-imaging speed in the Z-direction mirrors is severely can reach limited, speeds making as this high method as only8000 mm/s [132]. It suitableis important for small to or note 2D prints. that these speeds are in the X–Y plane and, as this method cures each

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stable tensegrity lattices with nanoscale features [97]. The process used a diffusion-assisted femtosecond laser for high-precision polymerisation and enabled the effective fabrication of unit-cells and arrays comprised of struts with a 250 nm radius. Microscopic imaging was used to assess the deformation phenomena at nanoscales and their effect on macroscopic force–displacement curves. The analysed nanolattices successfully demonstrated a bi-stable response, viscoelastic behaviour and softening and stiffening defor-

Micromachines 2021, 12, 634 mation mechanisms. Therefore, MPP was shown to be a 20viable of 45 fabrication method for tensegrity metamaterials.

(a) (b)

Figure 23. Nanoscale woodpile structures printed with MPP showing (a) laser excitation of 0.6 µW andFigure a resultant 23. Nano beam thicknessscale woodpile of 72 nm, and structures (b) laser excitation printed of 0.45 withµW andMPP a resultant showing beam (a) laser excitation of 0.6 μW thicknessand a resultant of 60 nm [126 beam]. thickness of 72 nm, and (b) laser excitation of 0.45 μW and a resultant beam

thicknessThe manufacture of 60 nm of [126] small-scale. lattice structures is a significant area of interest for tensegrity metamaterial research. Discussed earlier in Section 2.2.3, tensegrity structures employWhile compressive the andprintable tensile elements resin to needs produce to devices be photocurable with tuneable and, non-linearnanocomposite inclusions can be mechanical and acoustic behaviours. These structures require very small feature sizes to maintainadded complexthat allow dynamic for behaviours a variety and toof be material deployed in properties, various applications. such MPPas conductivity, magnetism, offerspiezoelectricity, a solution to these mechanical manufacturing properties, limitations and high has been refractive used to build index bi-stable [128] and biodegradability tensegrity lattices with nanoscale features [97]. The process used a diffusion-assisted femtosecond[133], which laser is for also high-precision true for polymerisation other vat polymerization and enabled the effective methods. fabrication Soft materials, like hydro- ofgel, unit-cells can also and arrays be printed comprised for of struts use with in active a 250 nm metamaterial radius. Microscopic micro imaging-devices was [134]. There is signif- usedicant to assessinterest the deformation in MPP printing phenomena of at nanoscaleshydrogels and theirfor drug effect on delivery macroscopic and tissue scaffolding, due force–displacement curves. The analysed nanolattices successfully demonstrated a bi-stable response,to the viscoelastic method’s behaviour high spatial and softening resolution and stiffening and deformation composite mechanisms.-printing capabilities [135,136]. Therefore,Other medical MPP was shownapplications to be a viable for fabrication this printing method for method tensegrity ha metamaterials.ve been realized [137], for example While the printable resin needs to be photocurable, nanocomposite inclusions can bewith added an that MPP allow-printed, for a variety porous of material filter properties, being such used as conductivity, to filter magnetism,blood cells in a microfluidic chip piezoelectricity,[138]. A microscope mechanical properties,image of high red refractive blood index cells [128 filtered] and biodegradabil- from plasma with a filter printed ity [133], which is also true for other vat polymerization methods. Soft materials, like hydrogel,using MPP can also is beshown printed forin useFigure in active 24. metamaterial micro-devices [134]. There is significant interest in MPP printing of hydrogels for drug delivery and tissue scaffolding, due to the method’s high spatial resolution and composite-printing capabilities [135,136]. Other medical applications for this printing method have been realized [137], for example with an MPP-printed, porous filter being used to filter blood cells in a microfluidic chip [138]. A microscope image of red blood cells filtered from plasma with a filter printed using MPP is shown in Figure 24.

Figure 24. Microscope image of red blood cells filtered from plasma with a micro-porous filter, printed with MPP [138]. (Republished with permission of Royal Society of Chemistry, from [Lab

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FigureFigure 24. 24Microscope. Microscope image image of red blood of red cells blood filtered cells from filtered plasma withfrom a micro-porousplasma with filter, a micro-porous filter, printedprinted with with MPP MPP [138]. (Republished[138]. (Republished with permission with of Royalpermissi Societyon of of Chemistry, Royal Society from [Lab of on Chemistry, from [Lab aon chip, a chip, L. Amato, L. Amato, et al., Volume et al., 12, Volume Issue 6, Pages 12, 1135-1142,Issue 6, Pages 2012]; permission 1135-1142, conveyed 2012]; through permission conveyed Copyright Clearance Center, Inc). through Copyright Clearance Center, Inc). 3.1.4. Summary/Manufacture of AMMs 3.1.4.Vat Summary/Manufacture polymerization methods are prolific of AMMs for constructing AMM prototypes, with several examples presented in the previous AMM section in Figures6 and8a,b. In this section, a varietyVat of polymerization novel fabrication examples methods were are discussed, prolific demonstrating for constructing the capabilities AMM prototypes, with sev- anderal limitations examples of presented three vat polymerization in the previous methods: AMM SLA, se DLPction and in MPP. Figures Below, 6 theand 8a,b. In this section, variables pertaining to the additive manufacture of AMMs for use in widespread industry area variety discussed, of with novel the aforementionedfabrication examples techniques compared were discussed, and contrasted demonstrating for suitability the capabilities and inlimitations each category. of In three general vat vat polymerization polymerization is rapid methods: and accurate SLA, with DLP cm-scale and MPP. build Below, the variables volumes.pertaining The use to ofthe a liquid additive to cure manufacture to solid means that of certainAMMs features, for use such in as inclusions,widespread industry are dis- can be difficult to print, as liquid can remain trapped if a means of exit is not included, whichcussed, increases with post-processing. the aforementioned techniques compared and contrasted for suitability in eachAbility category. to mass-produce: In general vat polymerization is rapid and accurate with cm-scale build volumes.In terms The of vat use polymerisation of a liquid techniques, to cure to DLP solid is the means fastest production that certain method features, by a such as inclusions, significant margin. Additionally, its simultaneous layer curing regime also allows for the creationcan be of difficult several printed to print, parts withoutas liquid additional can remain printing time.trapped SLA isif the a nextmeans fastest, of exit is not included, duewhich to its increases layerwise scanning, post-processing. with MPP being the slowest fabrication process because ofAbility its precise to point-by-pointmass-produce: curing mechanism. Unlike DLP, printing extra parts adds proportionally to the printing time for SLA and MPP. Considering this, DLP is the most promisingIn terms vat polymerisation of vat polymerisation method for mass-producing techniques, AMMs. DLP is the fastest production method by Minimum printable feature size: a significantMPP has the highestmargin. resolution Additionally, of all the additive its simultaneous manufacturing techniques layer curing discussed regime also allows for inthe this creation review, with of aseveral minimum printed feature size parts well without within the a nanoscaledditional (as printing low as 60 nm). time. It SLA is the next fast- alsoest, is due not restricted to its layerwise by the use ofscanning, support structures, with MPP as the being point-by-point the slowest polymerization fabrication process because regime can fabricate at any 3D spatial point, unlike DLP and SLP, which are limited to aof 2D its build precise surface. point-by-point DLP performs better curing than SLAmechanis at printingm. smallUnlike features, DLP, but printing with extra parts adds theproportionally resolution of both to generally the printing restricted time to the for microscale. SLA and DLP MPP. has beenConsidering recorded to this, DLP is the most reachpromising minimum vat features polymerisation sizes as small as method 600 nm. With for allmass-producing systems, the build volumesAMMs. are typically inversely correlated with build volume, meaning higher resolution comes at the costMinimum of smaller finishedprintable products. feature size: Multi-material printing: Multi-materialMPP has the printing highest is inherently resolution difficult of with all vat-polymerisationthe additive manufactur techniques ing techniques dis- duecussed to the in possibility this review, of cross-contamination with a minimum of the differentfeature liquid size resinswell usedwithin as build the nanoscale (as low as material.60 nm). Despite It also this, is several not restricted multi-material by prints the wereuse presentedof support in this structures, section, with as the point-by-point SLA presenting the best multi-material capabilities in Figure 16. DLP also presented multi-materialpolymerization methods regime requiring can either fabricate intensive at cleaningany 3D stepsspatial between point, vat unlike changes DLP and SLP, which are limited to a 2D build surface. DLP performs better than SLA at printing small features, but with the resolution of both generally restricted to the microscale. DLP has been recorded to reach minimum features sizes as small as 600 nm. With all systems, the build volumes are typically inversely correlated with build volume, meaning higher resolution comes at the cost of smaller finished products. Multi-material printing: Multi-material printing is inherently difficult with vat-polymerisation techniques due to the possibility of cross-contamination of the different liquid resins used as build material. Despite this, several multi-material prints were presented in this section, with SLA presenting the best multi-material capabilities in Figure 16. DLP also presented multi- material methods requiring either intensive cleaning steps between vat changes to ensure

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to ensure no cross-contamination of resin, or complex and novel implemented systems for resin transfer (see references [117,118]). Overall, SLA provides the simplest and most effective multi-material printing process out of the vat polymerisation techniques; however, extrusion/deposition techniques perform signiﬁcantly better in this category. Active metamaterial fabrication: The construction of active metamaterials is closely linked to the range of build materials available to the applicable fabrication methods. Printed active metamaterial examples using vat polymerisation methods are sparse in the literature, with no AMM-speciﬁc examples mentioned in this text. The material restrictions of the photocurable liquid resin hinder the fabrication of actuatable metamaterials, making it generally unfeasible to produce these with any vat polymerization techniques. The exception to this is hydrogel-based active designs, as these can be quite easily fabricated using SLA, DLP and MMP, with an SLA-printed example shown in Figure 15.

3.2. Powder Bed Fusion Techniques Powder bed fusion is an additive manufacturing method wherein a heat source is applied to a bed of ﬁne particles in order to bind that construction material together and form highly-detailed 3D parts [139]. The scope of this review is to consider fusion methods using lasers (SLS/SLM), electron beams (EBM) and infrared projections (MJF). Interest in this type of additive manufacturing has grown in recent years due to its low cost, material variety, and its requirement for no or minimal printing supports. Due to the powder form of the construction material and heat based bonding method, a wide range of material types can be used, including plastics, glass, metals and alloys, with the unused powder also being recyclable.

3.2.1. Selected Laser Sintering/Melting (SLS/SLM) These two powder bed fusion methods use laser beams to bond powder into 3D parts. However, there is a key distinction between SLS and SLM. The former works solely with plastics, whereas the latter primarily with metals [139]. The manufacturing process is of a step-wise configuration, with a layer coating of powder sprayed on after every previous bonded layer. For SLS, the fabrication process begins similarly to other printing techniques, where the 3D CAD part is divided into 2D layers with incremental thicknesses, and the printing chamber is prepared with the desired powder base. Next, a CO2 laser heats the layer, point by point, until the desired pattern is sintered together. The platform descends in the Z direction, and a new layer of powder is spread over evenly and the process repeats until the part is finished. SLM operates a similar way but with some important differences. In order to melt the material, the laser must be higher energy, also requiring inert gas to function. Overall, SLM has several benefits over SLS, namely it is faster and has a lower minimum layer thickness (20 µm for SLM compared to 80 µm for SLS). SLS can print with nylon, thermoplastic and polystyrene, while SLM can print with metallic powders. Recent developments that have incorporated optical fibres with high-powered lasers have enabled further material options for SLM, with composite and ceramic printing now within reach. Both SLS and SLM can yield high quality prints; however, post-processing is required to eliminate potential issues such as porosity, surface roughness and crack defects. SLM can incur welding defects, so it requires thermal post processes to assure part quality. SLS can print with relatively high spatial resolution and short print duration; however, SLS generally requires one of these qualities to be prioritized over the other. An example of this is perforated plates for tuneable AMMs [140], as shown in Figure 25, fabricated with a relatively fast printing speed (4000 mm/s in the X–Y plane) and a comparatively large minimum resolution length (0.12 mm layer thickness). Micromachines 20212021,, 1212,, 634x FOR PEER REVIEW 3023 of of 4557

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Figure 25.25. Geometry of a micro-perforated plate fabricated with SLS [[140].140].

An exampleAn example of an SLS-fabricated of an SLS-fabricated AMM in AMM the literature in the literature is a locally-resonant is a locally-resonant panel panel

structurestructure that demonstrates that demonstrates acoustic acoustic band gaps band [141 gaps]. This [141]. structure This structure exploits exploits an isolated an isolated bendingbending mode thatmode can that be can easily be easily modelled modelled with anwith MSD an MSD system system for straightforwardfor straightforward cal- calculationculation of the of band the gapband frequency. gap frequency. A design A design was fabricated was fabricated and tested and to tested demonstrate to demonstrate proof ofproof concept, of concept, with SLS with speciﬁcally SLS specifically chosen aschos theen manufacturing as the manufacturing method method due to itsdue to its ability toability construct to construct complex complex topology topology without withou the needt the of need support of support structures. structures. The main The main limitationlimitation of this and of this other and powder other powder bed fusion bed methods fusion methods is that the is that design the must design not must have not have sealed cavities,sealed cavities, otherwise otherwise the unused the unused powder powder cannot cannot be removed. be removed. Shown Shown in Figure in Figure 26, 26, the the AMMAMM prototype prototype contains contains 8 by 8 8by unit-cells 8 unit-cells with with a maximum a maximum component component thickness thickness of of 4 mm 4 mm (minimum(minimum possible possible thickness thickness of 80ofµ 80m). μ Them). prototypeThe prototype is lightweight, is lightweight, with only with 30% only 30% weight ofweight each unit-cellof each unit-cell being the being resonant the resonant structure st andructure the and rest the being rest the being solid the enclosure, solid enclosure, and modular,and modular, meaning meaning the cell the conﬁguration cell configuration of the panels of the canpanels be changedcan be changed to produce to produce differentdifferent acoustic acoustic responses. responses.

(a) (b) (c)

Figure 26.FigureA lightweight 26. A lightweight panel-based panel-based AMM structure,AMM structure, with each with panel each comprisedpanel comprised of 8 by of 8 8 locally-resonantby 8 locally-resonant unit-cells unit-cells and fabricatedand fabricated using SLS, using showing SLS, showing (a) the schematic (a) the schematic of a single of panel,a single with panel, an insetwith an section inset displayingsection displaying the geometry the geometry of a of a single unit-cell,single unit-cell, (b) a diagram (b) a diagram of the prototype, of the prototype, where the where panels the are panels arranged are arranged in a box shapein a box and shape then and housed then byhoused a solid by a solid material border,material and border, (c) the and experimental (c) the experimental setup to measure setup to the measure acoustic the response acoustic of response the printed of the prototype printed [prototype141]. (Reprinted [141]. (Re- from Mechanicalprinted from Systems Mechanical and Signal System Processing,s and Signal Volume Processing, 70, C. Claeys, Volume et al.,70, ‘AC. lightweightClaeys, et al., vibro-acoustic ‘A lightweight metamaterial vibro-acoustic met- demonstrator:amaterial Numerical demonstrator: and experimental Numerical investigation’, and experimental Pages invest 853-880,igation’, Copyright Pages (2016), 853-880, with Copyri permissionght (2016), from with Elsevier). permission from Elsevier). SLS can also be used to print soft AMMs with mechanical actuation capabilities. Several configurations of a 3D soft auxetic metamaterial were fabricated using SLS, as shown in Figure 27, and their resulting mechanical characteristics were investigated [142]. The printed lattice was constructed from thermoplastic polyurethane (TPU) powder, with the material evaluated based on its flowability, size distribution, thermal transition iden- tification and melting and decomposition energy analysis. The final TPU powder was characterized in order to optimize the printing of complex 3D geometries for a specific laser calibration. These lattice structures, known as Bucklicrystals, are designed to exhibit

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SLS can also be used to print soft AMMs with mechanical actuation capabilities. Sev- eral configurations of a 3D soft auxetic metamaterial were fabricated using SLS, as shown in Figure 27, and their resulting mechanical characteristics were investigated [142]. The printed lattice was constructed from thermoplastic polyurethane (TPU) powder, with the material evaluated based on its flowability, size distribution, thermal transition identifi- Micromachines 2021, 12, 634 cation and melting and decomposition energy analysis. The final TPU powder was24 of 45char- acterized in order to optimize the printing of complex 3D geometries for a specific laser calibration. These lattice structures, known as Bucklicrystals, are designed to exhibit buck- bucklingling behaviour behaviour under under compressive compressive loading. loading. This This results results in in volume volume shrinkage shrinkage across across two twotransverse transverse directions, directions, and and hence, hence, the the Bucklicryst Bucklicrystalsals possess possess a negative a negative Poison’s Poison’s ratio. ra- Such tio.structures Such structures could couldbe used be in used applications in applications requiring requiring mechanical mechanical damping, damping, energy energy absorp- absorptiontion andand mechanical mechanical actuation. actuation.

FigureFigure 27. 27.Three Three distinct distinct printed printed designs designs of aof 3D a 3D soft, soft, auxetic auxetic lattice lattice structure, structure, commonly commonly referred referred to to as as a a Bucklicrystal. Bucklicrystal. TheseThese crystal crystal lattices lattices all wereall were SLS-printed SLS-printed using using TPU TPU powder powder and and with with 60% 60% porosity porosity and and show show (a )(a a) crystala crystal lattice lattice with with body-centred-cubicbody-centred-cubic unit-cells unit-cells with with 6 holes, 6 holes, (b) ( ab crystal) a crystal lattice lattice with with cubic cubic solid-centred solid-centred unit-cells unit-cells with with 12 12 holes, holes, and and (c) (c a) a crystalcrystal lattice lattice with with body-centred-cubic body-centred-cubic unit-cells unit-cells with with 12holes. 12 holes. Here, Here, the the scale scale bar ba equatesr equates to 1 to cm 1 cm [142 [142].]. (Reprinted (Reprinted from from MaterialsMaterials & Design, & Design, Volume Volume 120, S.120, Yuan, S. Yuan, et al., et ‘3D al., soft ‘3D auxetic soft auxe latticetic lattice structures structures fabricated fabricated by selective by selective laser sintering: laser sintering: TPU powderTPU evaluationpowder evaluation and process and optimization’, process optimization’, Pages 317-327, Pages Copyright317-327, Copyright (2017), with (2017) permission, with permission from Elsevier). from Elsevier).

SLMSLM is one is one of theof the most most promising promising 3D 3D printing printing methods methods for for fabricating fabricating metallic metallic small- small- scalescale parts parts without without the the use use of compositeof composite materials, material resinss, resins or binders or binders [143 [143].]. Key Key parameters parameters currentlycurrently restricting restricting SLM SLM to primarily to primarily macro-scale macro-scale fabrication fabrication are laserare laser calibration calibration (particu- (partic- larlyularly laser laser type, type, intensity intensity and spotand size),spot size), scanning scanning regime regime (including (including strategy, strategy, hatch hatch spacing spac- anding scanning and scanning speed) andspeed) powder and powder bed characteristics bed characteristics (such as (such recoating as recoating mechanism mechanism and powderand powder properties). properties). Commercial Commercial SLM systems SLM system use powders use powder particle particle sizes in sizes the 20–50 in theµ 20–50m range,μm whichrange, resultswhich inresults minimum in minimum layer thicknesses layer thicknesses ranging ranging between between 20 µm and20 μ 100m andµm. 100 Typicallyμm. Typically for microscale for microscale SLM printing, SLM theprinting, powder the gain powder sizes mustgain besizes less must than be 10 lessµm, than and 10 theμ laserm, and focal the diameter laser focal lower diameter than lower 40 µm than [144 40]. Oneμm [144]. of the One early of potential the early applicationspotential appli- of microscalecations of microscale SLM was manufacturingSLM was manufacturing microneedles microneedles for drug delivery.for drug delivery. A stainless A stainless steel needlesteel produced needle produced with an with inner an hollow inner hollow diameter diameter 160 µm, 160 achieved μm, achieved using ausing femtosecond a femtosec- laser, is shown in Figure 28 as an example [145]. As observed in the ﬁgure, the largest Micromachines 2021, 12, x FOR PEER REVIEWond laser, is shown in Figure 28 as an example [145]. As observed in the figure, the33 oflargest 57 detrimentdetriment of thisof this print print is the is the outer outer surface surfac roughnesse roughness and and residue residue powder, powder, which which was was not not improvedimproved after after 35 min35 min of ultrasonic of ultrasonic cleaning. cleaning.

FigureFigure 28.28.An An SLM-fabricatedSLM-fabricated micro-needlemicro-needle forfor drugdrug deliverydelivery applications,applications, wherewhere thethe needle needle was was constructedconstructed fromfrom 316L316L stainlessstainless steelsteel powder,powder, withwith aa lengthlength ofof 550550µ μmm andand anan interiorinterior apertureaperture measuring 160 μm across [145]. measuring 160 µm across [145].

Recent studies have been conducted on the design and testing of lattices fabricated by SLM [146,147]. In particular, a cubic lattice design was topologically optimized to show increased mechanical properties and energy absorption capabilities [148]. Along with enhanced topology, the implemented improvements include optimizing the printing parameters to limit the prevalence of metallurgical defects, which affect mechanical properties and part quality. The following parameters were used to fabricate a titanium-alloy lattice with periodic spacings of 1 mm and 1.74 mm: (i) a laser power of 280 W; (ii) a scanning speed of 1200 mm/s; (iii) a layer thickness of 30 μm; (iv) a hatch distance of 140 μm; and (v) a scanning direction rotated 67° between alternate layers. This optimized lattice is shown in Figure 29, with no macro defects occurring during fabrication and good surface quality. The final printed result proved to be a high-performance and lightweight matrix structure, fabricated with an estimated dimensional accuracy of +55 μm/–49 μm.

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Recent studies have been conducted on the design and testing of lattices fabricated by SLM [146,147]. In particular, a cubic lattice design was topologically optimized to show increased mechanical properties and energy absorption capabilities [148]. Along with enhanced topology, the implemented improvements include optimizing the printing parameters to limit the prevalence of metallurgical defects, which affect mechanical properties and part quality. The following parameters were used to fabricate a titanium-alloy lattice with periodic spacings of 1 mm and 1.74 mm: (i) a laser power of 280 W; (ii) a scanning speed of 1200 mm/s; (iii) a layer thickness of 30 µm; (iv) a hatch distance of 140 µm; and (v) Micromachines 2021, 12, x FOR PEER REVIEW 34 of 57 a scanning direction rotated 67◦ between alternate layers. This optimized lattice is shown in Figure 29, with no macro defects occurring during fabrication and good surface quality. The ﬁnal printed result proved to be a high-performance and lightweight matrix structure, fabricated with an estimated dimensional accuracy of +55 µm/−49 µm.

FigureFigure 29.29. Cross-sectionCross-section of of optimized optimized lattice lattice design design printed printed using using SLM SLM with with Ti-6Al-4V Ti-6Al-4V powder. powder. TheThe bottombottom middle panel panel displays displays the the pattern pattern geom geometryetry with with two two alternating alternating periodic periodic spacings. spacings. The remaining panels (a–e) respectively show labelled and magnified images of the lattice in the The remaining panels (a–e) respectively show labelled and magnified images of the lattice in the microscale, where the scale bar is 500 μm [148]. (Reprinted from Journal of Manufacturing Pro- microscale, where the scale bar is 500 µm [148]. (Reprinted from Journal of Manufacturing Processes, cesses, Volume 56, L. Zhang, et al., ‘Topology-optimized lattice structures with simultaneously Volumehigh stiffness 56, L. Zhang,and light et al.,weight ‘Topology-optimized fabricated by selective lattice laser structures melting: with Design, simultaneously manufacturing high stiffness and andcharacterization’, light weight fabricated Pages 1166-1177, by selective Copyright laser melting: (2020), with Design, permission manufacturing from Elsevier). and characterization’, Pages 1166–1177, Copyright (2020), with permission from Elsevier). A novel application of printed AMMs is ultrasound cloaking technology, designed to maskA novel the presence application of objects of printed from AMMs the su isrrounding ultrasound acoustic cloaking field. technology, SLM has been designed used to mask the presence of objects from the surrounding acoustic field. SLM has been used to construct an AMM underwater cloak with an operational bandwidth of 100–900 kHz, to construct an AMM underwater cloak with an operational bandwidth of 100–900 kHz, frequencies commonly used in medical ultrasound and non-destructive testing (NDT) frequencies commonly used in medical ultrasound and non-destructive testing (NDT) [149]. [149]. This broadband ultrasonic cloak uses metagrating—defined as a rigid surface with This broadband ultrasonic cloak uses metagrating—defined as a rigid surface with pe- periodically modulated grooves—to enable its wave-redirection mechanism. The design riodically modulated grooves—to enable its wave-redirection mechanism. The design is simplified so that performance is dependent on a single geometric variable, therefore is simplified so that performance is dependent on a single geometric variable, therefore eliminating the complexity found in conventional underwater cloak designs. SLM was eliminating the complexity found in conventional underwater cloak designs. SLM was used to construct the cloak out of stainless steel with a periodic spacing of 3.5 mm and a used to construct the cloak out of stainless steel with a periodic spacing of 3.5 mm and a minimum feature size of 1.2 mm. The metagrating proved extremely capable of concealing minimum feature size of 1.2 mm. The metagrating proved extremely capable of concealing objects of arbitrary shape and considerable size from incident ultrasonic wavefronts. objects of arbitrary shape and considerable size from incident ultrasonic wavefronts. While thisWhile is significantthis is significant progress progress in the field in ofthe underwater field of underwater cloaking, furthercloaking, research further needed research to realiseneeded omni-directional to realise omni-directional characteristics. characteri Furtherstics. reducing Further the reducing minimum the minimum printable feature printa- sizeble feature for the SLMsize for process the SLM would process allow would for future allow innovation for future in innovation underwater in cloaksunderwater such ascloaks this. such For example,as this. For reducing example, the reducing periodic the spacing periodic would spacing allow would for ultra-broadband allow for ultra- capabilitiesbroadband atcapabilities even higher at even frequencies. higher frequencies.

3.2.2.3.2.2. MultiMulti JetJet FusionFusion (MJF)(MJF) ThisThis innovativeinnovative technique technique builds builds on on the the traditional traditional SLS SLS method method and and shows shows promising promis- improvementsing improvements in some in some key key printing printing variables variab [les150 [150].]. While While the initialthe initial powder powder bed bed set upset andup and core core construction construction materials materials are are similar similar to to SLS, SLS, the the method method diverges diverges after after thisthis point.point. Multi jet fusion (MJF) uses an array of jets to dispense a fusing agent on top of the powder bed in the shape of the layer cross section. Following this, instead of lasers, MJF uses projections of infrared light to simultaneously heat and bind an entire layer. This method has many benefits over SLS, with MJF-printed parts demonstrating faster print times and more dimensional accuracy while using effectively-identical polyamide powder. How- ever, the SLS-printed parts exhibit superior mechanical properties, exceeding the MJF- printed parts in Young’s modulus, impact toughness and tensile strength. This is likely due to SLS being more developed. SLS techniques have been improved over decades and the optimal laser parameters for commonly-used materials are well understood. MJF is an emerging technique and will take more time in development before its printing abilities

Micromachines 2021, 12, 634 26 of 45

Multi jet fusion (MJF) uses an array of jets to dispense a fusing agent on top of the powder bed in the shape of the layer cross section. Following this, instead of lasers, MJF uses projections of infrared light to simultaneously heat and bind an entire layer. This method has many beneﬁts over SLS, with MJF-printed parts demonstrating faster print times and more dimensional accuracy while using effectively-identical polyamide powder. However, the SLS-printed parts exhibit superior mechanical properties, exceeding the MJF-printed Micromachines 2021, 12, x FOR PEER REVIEW 35 of 57 parts in Young’s modulus, impact toughness and tensile strength. This is likely due to SLS being more developed. SLS techniques have been improved over decades and the optimal laser parameters for commonly-used materials are well understood. MJF is an areemerging optimized, technique in which and case will it take shows more potential time in to development surpass SLS beforeand several its printing other printing abilities methodsare optimized, in the future. in which case it shows potential to surpass SLS and several other printing methodsMJF was in the used future. to print an acoustic metamaterial panel capable of attenuating sound at a specificMJF was frequency used to print range an acoustic[151]. The metamaterial MJF-printed panel metamaterial capable of attenuatingpanels, as soundshown at in a Figurespecific 30a, frequency were compared range [151 experimentally]. The MJF-printed to solid metamaterial panels of panels, the same as shown material in Figure (polyam- 30a, idewere 11) compared with similar experimentally dimensions. toThe solid AMM panels panels of thedemonstrat same materialed superior (polyamide sound 11attenu-) with ationsimilar over dimensions. their homogeneous The AMM counterparts. panels demonstrated Aside from superior the attenuation sound attenuation peak at over1050 theirHz, thehomogeneous metamaterial counterparts. showed sound Aside transmission from the attenuation peaks at 550 peak Hz at and 1050 1250 Hz, Hz, the metamaterialwhich mark theshowed limits sound of the transmission band gap region. peaks Finite at 550 elem Hz andent 1250analysis Hz, which(FEA) markmodels the of limits the attenuation of the band gap region. Finite element analysis (FEA) models of the attenuation and transmission peaks and transmission peaks are shown in Figure 30b,c, respectively. The resonant modes were are shown in Figure 30b,c, respectively. The resonant modes were skewed by up to 200 Hz due skewed by up to 200 Hz due to the geometric tolerances of the fabrication method, with to the geometric tolerances of the fabrication method, with 0.1 mm deviations in the meta-cell 0.1 mm deviations in the meta-cell diameter affecting the frequency response. diameter affecting the frequency response.

FigureFigure 30. 30. TheThe design of an acoustic metamaterial panel panel that that us useses local local resonance resonance to to attenuate attenuate sound, sound, where where the the panel panel consistsconsists of of a a 2D 2D array array of of unit-cells unit-cells held held in in place place by by a a solid solid border. border. These These unit-cells unit-cells consist consist of of a a clover-shaped clover-shaped mass mass inclusio inclusionn connected to thin structural supports. The figure shows (a) the schematics of the panel design, with dimensions shown in connected to thin structural supports. The figure shows (a) the schematics of the panel design, with dimensions shown in mm, (b) the displacement plot of a single meta-cell during attenuation peak at 1050 Hz, and (c) the displacement plot of a mm, (b) the displacement plot of a single meta-cell during attenuation peak at 1050 Hz, and (c) the displacement plot of single meta-cell during transmission peak at 1380 Hz [151]. (Used with permission of American Society of Mechanical Engineers,a single meta-cell from [151] during [‘Experimental transmission an peakd Numerical at 1380 HzAssessment [151]. (Used of Local with Resona permissionnce Phenomena of American in Society 3D-Printed of Mechanical Acoustic Metamaterials’,Engineers, from Journal [151] [‘Experimental of vibration and Numericalacoustics, D. Assessment Roca, et al., of LocalVolume Resonance 142, Issue Phenomena 2, 2020.]; inpermission 3D-Printed conveyed Acoustic throughMetamaterials’, Copyright Journal Clearance of vibration Center, and Inc.). acoustics, D. Roca, et al., Volume 142, Issue 2, 2020.]; permission conveyed through Copyright Clearance Center, Inc.). The internal resonance of the system is very sensitive to small deviations in geometry, whichThe is a internal common resonance problem ofin thethe system fabrication is very of sensitiveresonant toAMMs. small Ther deviationsefore, refi in geometry,ning the accuracywhich is aof common powder problembed fusion in printing the fabrication methods of resonantcould be AMMs.a significant Therefore, step towards refining the the large-scaleaccuracy of production powder bed of fusionfeasible printing locally-resonant methodscould metamaterials be a significant for industrial step towards applica- the tion.large-scale production of feasible locally-resonant metamaterials for industrial application. 3.2.3. Electron Beam Melting (EBM) 3.2.3. Electron Beam Melting (EBM) This fusion method uses an electron beam as the heat source in order to fabricate This fusion method uses an electron beam as the heat source in order to fabricate both both metal and plastic material formulas [139]. The powder bed is prepared similarly to metal and plastic material formulas [139]. The powder bed is prepared similarly to the the previous methods, and an electron beam is deployed from a tungsten filament and previous methods, and an electron beam is deployed from a tungsten filament and con- controlled via a computerized coil. While there is more variety in the possible fabrication trolledmaterials, via a this computerized method does coil. require While a there vacuum-sealed is more variety build in chamber the possible to function. fabrication materials, this method does require a vacuum-sealed build chamber to function. A recent example shows EBM used to fabricate a metallic micro-lattice with mechanical capabilities that can replicate bone [152]. Both EBM and SLM are of interest in biomedical applications due to their ability to fabricate biocompatible metallic structures with high complexity; however, EBM has more material options than other powder bed methods. The lattice design incorporates large and small size pores heterogeneously throughout the structure for the purposes of nutrient flow and cell seeding, respectively. A compressive strength of 169.5–250.9 MPa was achieved by the printed structures, along with a Young’s modulus of 14.7–25.3 GPa and high porosity (up to 60%). This successfully imitates the strength of bone, which has a compressive strength in the region of 188–222

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A recent example shows EBM used to fabricate a metallic micro-lattice with mechanical capabilities that can replicate bone [152]. Both EBM and SLM are of interest in biomedical applications due to their ability to fabricate biocompatible metallic structures with high complexity; however, EBM has more material options than other powder bed methods. The lattice design incorporates large and small size pores heterogeneously throughout the Micromachines 2021, 12, x FOR PEER REVIEWstructure for the purposes of nutrient ﬂow and cell seeding, respectively. A compressive36 of 57

strength of 169.5–250.9 MPa was achieved by the printed structures, along with a Young’s modulus of 14.7–25.3 GPa and high porosity (up to 60%). This successfully imitates the MPastrength and ofa Young’s bone, which modulus has ain compressive the order of 15–35 strength GPa. in The the regionaddition of of 188–222 solid edges MPa to and the a micro-latticeYoung’s modulus resulted in thein an order increase of 15–35 of 45% GPa. in compressive The addition strength of solid and edges 25% to in the Young’s micro- modulus.lattice resulted Several in anprinted increase samples of 45% are in shown compressive in Figure strength 31 with and different 25% in dimensions, Young’s modulus. with theSeveral poreprinted architecture samples highlighted are shown inin Figu Figurere 31c. 31 withA matrix different wall dimensions, thickness of with 500 μ them porewas µ usedarchitecture with a high highlighted relative indensity Figure of 31 50%c. A to matrix ensure wall no thicknessbuckling ofunder 500 lowm was strains. used Ti-6Al- with a 4Vhigh powder relative was density used to of construct 50% to ensure the lattices, no buckling with an under average low particle strains. size Ti-6Al-4V around powder 70 μm, was used to construct the lattices, with an average particle size around 70 µm, and the and the following parameters were implemented in the EBM setup: (i) 60 kV accelerating following parameters were implemented in the EBM setup: (i) 60 kV accelerating voltage; voltage; (ii) 50 μm layer thickness; (iii) a line offset of 100 μm; (iv) a hatch depth of 50 μm; (ii) 50 µm layer thickness; (iii) a line offset of 100 µm; (iv) a hatch depth of 50 µm; and and (v) the vacuum chamber maintained at 2 μBar. Some challenges with geometry dis- (v) the vacuum chamber maintained at 2 µBar. Some challenges with geometry distortion tortion occurred when feature sizes reached 400 μm, as this is near the resolving power of occurred when feature sizes reached 400 µm, as this is near the resolving power of EBM. EBM.

FigureFigure 31. 31. DigitalDigital images images of ofthree three different different micro-lattice micro-lattice designs designs (a) with (a) withand ( andb) without (b) without solid outer solid supportsouter supports on the onedges. the The edges. unit-cells The unit-cells of Models of A Models to C are A progressively to C are progressively smaller in smaller size, with in size,cell lengths ranging from 3.33 mm to 2 mm. The supports for models A+ to C+ also range in thicknesses with cell lengths ranging from 3.33 mm to 2 mm. The supports for models A+ to C+ also range from 670 μm to 400 μm. Sub-figure (c) shows magnified sections of the models displayed in ( , ), in thicknesses from 670 µm to 400 µm. Sub-ﬁgure (c) shows magniﬁed sections of the modelsa b with focus on the smaller (c)—(i) and larger (c)—(ii) pore structures. Here, the scale bar is 3 mm displayed in (a,b), with focus on the smaller (c)—(i) and larger (c)—(ii) pore structures. Here, [152]. (Reprinted from Additive Manufacturing, Volume 36, P. Wang, et al., ‘Electron beam melted heterogeneouslythe scale bar is 3 porous mm [152 microlattices]. (Reprinted for from metallic Additive bone Manufacturing,applications: Design Volume and 36, investigations P. Wang, et al.,of boundary‘Electron beam and edge melted effects’, heterogeneously Copyright (2020), porous with microlattices permission for from metallic Elsevier). bone applications: Design and investigations of boundary and edge effects’, Copyright (2020), with permission from Elsevier). Given the powder bed can only contain one fusible material at a time, multi-material Given the powder bed can only contain one fusible material at a time, multi-material parts are usually comprised of one base powder (typically metallic) and another non-fu- parts are usually comprised of one base powder (typically metallic) and another non-fusible sible support powder/material. A novel multi-material design was proposed consisting of support powder/material. A novel multi-material design was proposed consisting of an an auxetic chiral meta-cell that is permeated with silicon to make a hybrid material [153]. auxetic chiral meta-cell that is permeated with silicon to make a hybrid material [153]. The chiral shape used corresponds to the tenth eigenmode of the regular cubic unit-cell, the behaviour of which is well known in the literature. Uniform structures of the unit-cell are fabricated via EBM using copper powder, with some prototypes kept in this condition (without the silicon composite) as a point of comparison for the performance of the hybrid structures. An example of this is shown in Figure 32. The hybrid design was found to reach much higher values of plateau stress during compressive testing to failure. In summary, the introduction of the silicon support material enhanced the mechanical properties of auxetic cellular structures and resulted in improved performance for energy absorption applications.

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The chiral shape used corresponds to the tenth eigenmode of the regular cubic unit-cell, the behaviour of which is well known in the literature. Uniform structures of the unit- cell are fabricated via EBM using copper powder, with some prototypes kept in this condition (without the silicon composite) as a point of comparison for the performance of the hybrid structures. An example of this is shown in Figure 32. The hybrid design was found to reach much higher values of plateau stress during compressive testing to failure. In summary, the introduction of the silicon support material enhanced the mechanical Micromachines 2021, 12, x FOR PEER REVIEW 37 of 57 properties of auxetic cellular structures and resulted in improved performance for energy absorption applications.

FigureFigure 32.32. AA uniform,uniform, chiralchiral structurestructure mademade ofof coppercopper shownshown ((aa)) withoutwithout andand (b(b)) withwith aa rubber rubber encasing.encasing. TheThe hybridhybrid compositecomposite structurestructure (b(b)) combines combines the the initial initial EBM-bonded EBM-bonded copper copper lattice lattice with with a silicon polymer filler while maintaining a vacuum seal of 0.5 MPa in the build chamber during a silicon polymer filler while maintaining a vacuum seal of 0.5 MPa in the build chamber during fabrication [153]. (Reprinted from Composite Structures, Volume 234, N. Novak, et al., ‘Mechanical fabricationproperties [of153 hybrid]. (Reprinted metamaterial from Composite with auxetic Structures, chiral cellular Volume structure 234, N. and Novak, silicon et filler’, al., ‘Mechanical Copyright properties(2020), with of permission hybrid metamaterial from Elsevier). with auxetic chiral cellular structure and silicon filler’, Copyright (2020), with permission from Elsevier). 3.2.4. Summary/Manufacture of AMMs 3.2.4. Summary/Manufacture of AMMs Powder bed fusion methods are less prevalent in AMM research than that of vat Powder bed fusion methods are less prevalent in AMM research than that of vat polymerization;polymerization; however, however, there there is is potential potential in in the the discussed discussed methods methods (SLS, (SLS, SLM, SLM, MJF MJF and and EBM)EBM) toto contributecontribute toto thethe improvedimproved fabricationfabricationof of AMMs.AMMs. Namely,Namely, MJF MJFshows showssignificant significant capabilitycapability inin thethe fastfast productionproduction ofof polymer-basedpolymer-based AMMsAMMs withwith improvedimproved mechanicalmechanical propertiesproperties and and resolution resolution on on par par with with SLS. SLS. WithWith further further development, development, MJP MJP could could not not only only surpasssurpass otherother powderpowder bedbed methodsmethods inin mechanicalmechanical part part quality quality and and minimum minimum feature feature size size butbut showsshows potentialpotential toto exceedexceed manymany vatvat polymerizationpolymerization andand extrusionextrusion techniquestechniques asas well.well. Below,Below, the the variables variables pertaining pertaining to theto the additive additive manufacture manufacture of AMMs of AMMs for use for in use widespread in wide- industryspread industry are discussed, are discussed, with the aforementionedwith the aforementioned techniques techniques compared compared and contrasted and con- for suitabilitytrasted for in suitability each category. in each category. AbilityAbility to mass-produce: to mass-produce: MJF is the fastest production method presented in this sub-section, as it employs MJF is the fastest production method presented in this sub-section, as it employs in- infrared projections to render an entire layer at once. This feature is not shared by the frared projections to render an entire layer at once. This feature is not shared by the other other powder bed fusion methods listed and is very useful for the batch production of powder bed fusion methods listed and is very useful for the batch production of AMM AMM devices. The other methods employ a scanning regime for heating parts, meaning devices. The other methods employ a scanning regime for heating parts, meaning decreas- decreasing the layer thickness to achieve finer features will have a detrimental impact on ing the layer thickness to achieve finer features will have a detrimental impact on the the printing time. SLS and SLM examples are shown with printing speeds in the X–Y plane printing time. SLS and SLM examples are shown with printing speeds in the X–Y plane of of 4000 mm/s and 1200 mm/s, respectively. An added benefit for AMM manufacture is 4000 mm/s and 1200 mm/s, respectively. An added benefit for AMM manufacture is these these techniques require minimal support structures. techniquesMinimum require printable minimal feature support size: structures. MinimumThe minimum printable feature feature sizes size: of all powder bed methods are roughly equivalent to each other,The ranging minimum from feature >400 µ sizesm (EBM) of all topowder 100–20 beµdm methods (SLM). Theare roughly minimum equivalent feature sizeto each is heavilyother, ranging restricted from by the>400 particle μm (EBM) size ofto the100–20 build μm powder, (SLM). withThe minimum the particle feature size always size is heavily restricted by the particle size of the build powder, with the particle size always being smaller than the resulting layer thickness. Refining powder to nanoscale particle sizes would be expensive and not necessarily successful in increasing method resolution (due to other restrictive variables like laser intensity/focal diameter or scanning regime). Overall, powder bed methods are limited to microscale features—SLM presents the smallest feature size of 20 μm. Multi-material printing:

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being smaller than the resulting layer thickness. Reﬁning powder to nanoscale particle sizes would be expensive and not necessarily successful in increasing method resolution (due to other restrictive variables like laser intensity/focal diameter or scanning regime). Overall, powder bed methods are limited to microscale features—SLM presents the smallest feature size of 20 µm. Multi-material printing: In general, powder bed methods cannot yet create multi-material parts without additional processing/material addition after printing, with an example if this shown in Figure 32. Currently no method accounts for the cross contamination of powder that would occur, meaning the initial process can only use a single base powder. EBM has the strongest potential to be developed into a multi-material technique as it has the widest range of applicable materials, including both plastics and metals. However, EBM requires a vacuum-sealed build chamber to function, making it a costlier manufacturing process. Active metamaterial fabrication: SLS and MJF utilize plastics as the build material, which is generally more appropriate for fabricating standard AMMs, whereas SLM uses metal particles. Only SLM and EBM have metal-printing capabilities, which could be used to fabricate active metamaterials controlled by electromagnetic ﬁelds. Hydrogels are not compatible with any powder bed techniques, limiting the actuation possibilities of printed metamaterials.

3.3. Extrusion/Deposition Techniques Extrusion-based techniques were among the ﬁrst additive manufacturing techniques developed, with a signiﬁcant literature surrounding this form of 3D printing in macro-scale. This technology is capable of multi-material and multicolour prints with a wide range of materials, such as plastics, ceramics [154], metals [155], food and even living cells [11]. Developing competitively smaller print sizes for these techniques is a newer concept, with the full potential of micro-extrusion not yet practically realized [156]. The techniques discussed below can be collectively described as layer-wise additive manufacturing methods, wherein deposited material builds a 3D part from the bottom-up. Down-scaling traditional extrusion methods, like fused deposition modelling (FDM), would have a broad range of industry applications, from biomedical devices to microelectronics and sensors. Addition- ally, micro-extrusion could provide greater design freedom, fewer processing steps, lower cost for small batch sizes and a variety of compatible build materials. This section also encompasses other deposition methods, with a focus on droplet-based based techniques, including direct ink writing (DIW) and jetting systems.

3.3.1. Fused Deposition Modelling (FDM) This method extrudes build material through a nozzle via a system using pneumatic or mechanical actuation [156]. The key difference for FDM is it uses a more continuous deposition of solid filament, whereas the other discussed additive methods are droplet- based (with liquid build material) and are therefore generally non-continuous. During its initial development in the 1990s, FDM primarily used thermoplastic polymer filament due to its low melt temperature, formability, and low price [11]. Nowadays, a variety of materials can be used, including through the addition of fibre inclusions, nanoparticles and other additives to improve the properties of the base material. A study into FDM printable materials used test specimens made with carbon composite filaments, along with their standard base materials (PLA and ABS). Their mechanical, rheological and thermal properties were measured and compared [157]. All composite samples showed improved mechanical properties from their pure base materials, with the tensile modulus of ABS samples increasing from 919.25 MPa to 2193.37 MPa, and from 1538.05 MPa to 2637.29 MPa for PLA samples, because of the addition of carbon fibres. Overall, FDM-printed parts with carbon fibre composite PLA performed the best mechanically. A novel freeform extrusion technique derived from the principles of FDM has been used to fabricate high-density ceramic lattices [158]. The part was printed at low tempera- Micromachines 2021, 12, 634 30 of 45

tures and achieved feature sizes well within the microscale, as shown in Figure 33. The Micromachines 2021, 12, x FOR PEER REVIEWlattice “woodpile” structure used is a well-known geometry in the AMM literature,39 of and 57

the acoustic response of the design was tested experimentally, confirming the band gap capabilities of the sonic lattice. Some defects/acoustic inaccuracies were present in the thisstructure, solvent-based likely due method to microporosity proved efficient and non-optimized at producing printing microscale parameters. zirconia In general,lattices, this solvent-based method proved efficient at producing microscale zirconia lattices, which which could be applied in several potential AMM applications. Another freeform extru- could be applied in several potential AMM applications. Another freeform extrusion sion example produced fine ceramic lattices with spatial resolution lower than 100 μm, example produced fine ceramic lattices with spatial resolution lower than 100 µm, and and it is shown in Figure 34 [154]. This method can be used with any ceramic powder, a it is shown in Figure 34[ 154]. This method can be used with any ceramic powder, a feature that could be utilized in a range of AMM applications. The multi-size pores used feature that could be utilized in a range of AMM applications. The multi-size pores used in in this design (from macro to submicron dimensions) and the biocompatible build mate- this design (from macro to submicron dimensions) and the biocompatible build materials rials are particularly useful for medical applications, such as drug delivery and hard tissue are particularly useful for medical applications, such as drug delivery and hard tissue scaffolding. Fully realizing this extrusion technology would allow the rapid prototyping scaffolding. Fully realizing this extrusion technology would allow the rapid prototyping of of 1–3 ceramic–polymer composites for piezoelectric bio-sensors. 1–3 ceramic–polymer composites for piezoelectric bio-sensors.

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(a) (b)

FigureFigure 33. 33. MicroscopicMicroscopic images images of of zirconia zirconia lattices lattices printed printed via via freeform freeform extrusion, extrusion, with with (a ()a 300) 300 μmµm andand ( (bb)) 70 70 μµm size pores. Here, the scale bar is 200 μµm [158]. [158].

Figure 34.34. AA fine ﬁne ceramic ceramic (calcium (calcium phosphate) phosphate) lattic latticee printed printed via via freeform freeform extrusion extrusion and and with with a a mesh spacing/porespacing/pore size of 70 µμm. Calcium phosphate isis biocompatiblebiocompatible andand commonlycommonly used used as as bone substitutionbone substitution material. material. Here, He there, scale the barscale size bar is size 500 isµ m.500 (Reprinted μm. (Reprinted from [from154], [154], with the with permission the per- ofmission John Wiley of John & Wiley Sons Inc). & Sons Inc).

Another emerging FDM method involves the liquid deposition of a conductive

nanocomposite, comprised of multiwall carbon nanotubes suspended in PLA, using a high volatility solvent as the dispersion medium [159]. This method not only has promising micro-fabrication abilities, but the printing setup requires only minor modifications to a commercially available, low-cost FDM printer to function. These modifications include adding a syringe dispenser to enable fluid deposition of the composite and solvent mixture.

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Another emerging FDM method involves the liquid deposition of a conductive nanocomposite, comprised of multiwall carbon nanotubes suspended in PLA, using a high volatility solvent as the dispersion medium [159]. This method not only has promising Micromachines 2021, 12, 634 micro-fabrication abilities, but the printing setup requires only minor modifications31 ofto 45a commercially available, low-cost FDM printer to function. These modifications include adding a syringe dispenser to enable fluid deposition of the composite and solvent mixture.The solvent The solvent dispersion dispersion medium medium is important, is import asant, it ensures as it ensures quick evaporation quick evaporation of the wet of ﬁla-the wetment filament and therefore and therefore rapid construction rapid construction of rigid of microstructures. rigid microstructures. Printed featuresPrinted features as small as small100 µ mas can 100 be μ reproduciblym can be reproducibly obtained with obtained the conductive with the nanocomposite, conductive nanocomposite, demonstrating demonstratingthe future potential the future of this potential novel technique of this no tovel be developedtechnique intoto be a developed viable micro-fabrication into a viable micro-fabricationmethod for several method key materials. for several A successfully-printed key materials. A successfully-printed nanocarbon woven nanocarbon structure is wovenshown instructure Figure 35is .shown The prototype in Figure demonstrated 35. The prototype its conductive demonstrated capabilities its conductive by lighting capa- up bilitiesan LED by in lighting a simple up electric an LED circuit, in a assimple exhibited electric in Figurecircuit, 35asc, exhibited and has anin averageFigure 35c, feature and hasthickness an average of 100 featureµm, as shownthickness in furtherof 100 detailμm, as by shown Figure in 35 furthera,b. With detail further by development,Figure 35a,b. Withthis technology further development, could allow this for technology the cheap and could reliable allow fabricationfor the cheap of microstructuresand reliable fabrica- in a tionvariety of microstructures of materials. in a variety of materials.

Figure 35. Carbon nanotube prototype with woven structure printed via liquid deposition deposition modelling modelling (a (a variant variant of of FDM). FDM). The microstructure is displayed from (a) a top view view and and ( (bb)) a a side side view, view, with with scale scale bars bars of of 100 100 μµmm and and 30 30 μµm,m, respectively, respectively, and (c) shows the experimental confirmation of the prototypes electrical properties by connecting it to a simple electric and (c) shows the experimental conﬁrmation of the prototypes electrical properties by connecting it to a simple electric circuit that lights up an LED [159]. (Reprinted from Composites Part A: Applied Science and Manufacturing, Volume 76, circuit that lights up an LED [159]. (Reprinted from Composites Part A: Applied Science and Manufacturing, Volume 76, G. Postiglione, et al., ‘Conductive 3D microstructures by direct 3D printing of polymer/carbon nanotube nanocomposites viaG. Postiglioneliquid deposition, et al., ‘Conductivemodeling’, Pages 3D microstructures 110-114, Copyright by direct (2015), 3D with printing permission of polymer/carbon from Elsevier). nanotube nanocomposites via liquid deposition modeling’, Pages 110–114, Copyright (2015), with permission from Elsevier).

3.3.2. Direct Ink Writing (DIW) This method uses similar principles to FDM; however, the key difference is this method employs inks as the construction material [160]. Unlike FDM, time spent drying and solidifying the part is not necessary, as the shape retention comes from the rheological properties of the inks. This branch of additive manufacturing has extremely diverse material options, including ceramics, metal alloys, polymers, food, biomaterials, colloidal gels, hydrogels and electronically-functional matter. Additionally, DIW can construct complex shapes at low cost and without the use of masks or dies. DIW can be split into two definitive types: continuous filament/ink writing and inkjet printing. The former was originally developed as a technology for 2D image formation and has since progressed into a robust fabrication method wherein 3D parts are built up by the sequential selective deposition of ink droplets onto a substrate [2]. DIW is known to have a wide variety of ceramic and polymer material options, whereas current metallic material options are largely restricted to ink with silver particles [155]. Small-scale metallic parts can be DIW-formed by using concentrated metal nanoparticle inks with micrometre-sized (1–10 µm) nozzles. Figure 36 shows a metal microstructure printed using ink with a concentration of 75 wt% of 20 ± 5 nm silver nanoparticles. Metal-based DIW requires a post-printing annealing step to ensure structure stability. A novel modification to the DIW process countered this limitation by adding an in situ laser annealing system that anneals the ink as it leaves the nozzle. This alteration improved the minimum feature size of conventional DIW processes for metallic ink from 2 µm (for a 1 µm diameter nozzle) to as low as 600 nm. Additionally, conventional DIW has a printing rate of 20–500 µm/s, which increases to 0.5–1 mm/s once the laser annealing system is integrated. Simultaneous annealing and printing processes allow for almost arbitrary geometry to be created without the use of supports. This technology has been Micromachines 2021, 12, x FOR PEER REVIEW 42 of 57

3.3.2. Direct Ink Writing (DIW) This method uses similar principles to FDM; however, the key difference is this method employs inks as the construction material [160]. Unlike FDM, time spent drying and solidifying the part is not necessary, as the shape retention comes from the rheological properties of the inks. This branch of additive manufacturing has extremely diverse material options, including ceramics, metal alloys, polymers, food, biomaterials, colloidal gels, hydrogels and electronically-functional matter. Additionally, DIW can construct complex shapes at low cost and without the use of masks or dies. DIW can be split into two definitive types: continuous filament/ink writing and inkjet printing. The former was originally developed as a technology for 2D image formation and has since progressed into a robust fabrication method wherein 3D parts are built up by the sequential selective deposition of ink droplets onto a substrate [2]. DIW is known to have a wide variety of ceramic and polymer material options, whereas current metallic material options are largely restricted to ink with silver particles [155]. Small-scale metallic parts can be DIW-formed by using concentrated metal nanoparticle inks with micrometre-sized (1–10 μm) nozzles. Figure 36 shows a metal microstructure printed using ink with a concentration of 75 wt% of 20 ± 5 nm silver nanoparticles. Metal-based DIW requires a post-printing annealing step to ensure structure stability. A novel modification to the DIW process countered this limitation by adding an in situ laser annealing system that anneals the ink as it leaves the nozzle. This alteration improved the minimum feature size of conventional DIW processes for metallic ink from 2 μm (for a 1 μm diameter nozzle) to as low as 600 nm. Additionally, conventional DIW has Micromachines 2021, 12, 634 a printing rate of 20–500 μm/s, which increases to 0.5–1 mm/s once the laser annealing32 of 45 system is integrated. Simultaneous annealing and printing processes allow for almost arbitrary geometry to be created without the use of supports. This technology has been ap- appliedplied to tomake make connecting connecting wires wires for for an an array array of of micro-solar micro-solar cells, cells, demonstrating potential forfor variousvarious small-scalesmall-scale electronicelectronic applications.applications.

Figure 36.36. AA metallic metallic circular circular microstructure microstructure made made usin usingg DIW DIW with with highly highly concentrated concentrated silver silver na- Micromachines 2021, 12, x FOR PEER nanoparticleREVIEWnoparticle ink. ink. The The structure structure was was fabricated fabricated layer- layer-by-layerby-layer with with continuous continuous ink inkdeposition deposition and43 of and has 57

hasa minimum a minimum feature feature size size of 2 of μ 2m.µm. Here, Here, the the scale scale bar bar is is 200 200 μµm.m. (Reprinted (Reprinted from from [155], [155], with thethe permission of John Wiley & Sons Inc.). permission of John Wiley & Sons Inc.). Typically, DIW techniques have a resolution strongly limited by their nozzle diame- Typically, DIW techniques have a resolution strongly limited by their nozzle diameter; ter; however, submicron feature sizes have been achieved using support structures and however, submicron feature sizes have been achieved using support structures and micro- microcapillary nozzles of varying diameters [7]. A hollow silicon microlattice was con- capillary nozzles of varying diameters [7]. A hollow silicon microlattice was constructed, structed, as shown in Figure 37, using a DIW-printed polyelectrolyte ink scaffolding that as shown in Figure 37, using a DIW-printed polyelectrolyte ink scaffolding that enabled the enabled the structure to have a wall thickness of <0.5 μm [161]. The production method structure to have a wall thickness of <0.5 µm [161]. The production method first requires a first requires a solid polymer lattice with a rod diameter of 1μm to be constructed via DIW solid polymer lattice with a rod diameter of 1µm to be constructed via DIW (this does not (this does not represent the minimum feature size of DIW—rods with diameters as low as represent the minimum feature size of DIW—rods with diameters as low as 600 nm can be made).600 nm Then,can be a thinmade). (100 Then, nm) silica a thin coating (100 nm) is added, silica andcoating the scaffoldis added, removed. and the The scaffold thermal re- propertiesmoved. The of thermal the polyelectrolyte properties of ink the mean polyel it canectrolyte be melted ink mean and subsequentlyit can be melted removed and sub- at sequently removed at temperatures easily withstood by solid silica. Next a final silicon temperatures easily withstood by solid silica. Next a final silicon coating is added for a totalcoating wall is thicknessadded for of a roughlytotal wall 300 thickness nm. The of spectral roughly response 300 nm. and The optical spectral properties response of and the “woodpile”optical properties structure of dependsthe “woodpile” on the geometry structure ofdepends the original on the polymer geometry lattice; of asthe such, original the dimensionspolymer lattice; can beas tailoredsuch, the for dimensions the desired can photonic be tailored effects. for Thisthe desired method photonic shows promise effects. forThis the method quick andshows simple promise fabrication for the of quick photonic and crystals,simple fabrication as well as otherof photonic novel devices crystals, like as silicon-basedwell as other novel microfluidic devices networks like silicon-based and low-cost microfluidic MEMS. networks and low-cost MEMS.

A B

Figure 37. MagnificationMagnification of hollow “woodpile” microstructure.microstructure. TheThe latticelattice designdesign isis mademade ofof siliconsilicon and formedformed usingusing aa directdirect inkink writtenwritten polymerpolymer scaffoldscaffold thatthat waswas thenthen silicon-coatedsilicon-coated andand meltedmelted toto produce the hollow structure. The lattice is 8 by 8 with lateral dimensions of 250 μm by 250 μm. (A) produce the hollow structure. The lattice is 8 by 8 with lateral dimensions of 250 µm by 250 µm. Magnified image of lattice architecture with a scale bar of 5 μm. (B) Magnified image with a scale (A) Magnified image of lattice architecture with a scale bar of 5 µm. (B) Magnified image with a scale bar of 500 nm]. (Reprinted from [161], with the permission of John Wiley & Sons Inc.). bar of 500 nm]. (Reprinted from [161], with the permission of John Wiley & Sons Inc.).

DIW technology can use a variety of ink formulations with different materials, including live cells and human tissue mimics. A novel printer set-up bio-printed thick (>1cm)

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Micromachines 2021, 12, 634 DIW technology can use a variety of ink formulations with different materials,33 of in- 45 cluding live cells and human tissue mimics. A novel printer set-up bio-printed thick (>1cm) vascularized tissues within 3D perfusion chips, as displayed in Figure 38 [162]. This complex fabrication involved the co-printing of several inks comprised of human stemvascularized cells to form tissues an within extracellular 3D perfusion matrix chips, with asembedded displayed vascular in Figure structures. 38[ 162]. ThisNotably, complex fabrication involved the co-printing of several inks comprised of human stem cells to this distinctive method can print tissues of arbitrary thickness, as the final product does form an extracellular matrix with embedded vascular structures. Notably, this distinctive not need to be UV cured. This means the method can be easily expanded to print other method can print tissues of arbitrary thickness, as the ﬁnal product does not need to be UV complex biomaterials. cured. This means the method can be easily expanded to print other complex biomaterials.

FigureFigure 38 38.. An DIW-fabricated vascularised tissue, tissue, with with thickness exceeding exceeding 1 1 cm, printed in a 3D perfusion chip, showing ( a)) the the chip chip that that is printed in silicon and that enables perfusion within the tissuetissue to occur forfor longlong timetime periods periods (greater (greater than than 6 weeks),6 weeks), and and (b )(b the) the tissue, tissue, which which is a is multicellular a multicellular matrix with embedded vascular structures (coloured red in the image). This tissue was con- matrix with embedded vascular structures (coloured red in the image). This tissue was constructed structed by co-printing several inks, which were composed of live cells (such as human stem cells by co-printing several inks, which were composed of live cells (such as human stem cells and dermal and dermal fibroblasts) [162]. ﬁbroblasts) [162].

DIWDIW has has been been established established as as a a high-resolutio high-resolutionn technique for a broad array of material material optionsoptions and and with with minimal cost [7]. [7]. Its Its liquid liquid deposition regime regime coupled with the many availableavailable ink ink formulations formulations are are particularly particularly us usefuleful to to medical medical and and bioprinting bioprinting applications. applications. OtherOther breakthroughs inin DIWDIW technologytechnology include include microﬂuidic microfluidic multi-nozzle multi-nozzle arrays, arrays, hydrogel hydro- geland and viscoelastic viscoelastic inks, inks, printing printing soft-robotics, soft-robotics, simultaneous simultaneous multi-material multi-material printingprinting and multi-coremulti-core shell printing.

3.3.3.3.3.3. Jetting Jetting Systems Systems IntroducedIntroduced in in the the previous previous DIW section, inkjet printing ejects ink onto a substrate usingusing either controlled thermal or piezoelectric/mechanicalpiezoelectric/mechanical actuation actuation [2]. [2]. In In the thermal regime,regime, a a printhead mounted mounted above above the the nozzle se sendsnds electric pulses to heat the ejection site,site, which which causes causes the the ink ink to be expelled by local evaporation and air bubbles, whereas piezoelectric actuation actuation involves involves the the deformation deformation of of a piezocrystal, which pushes the ink volume through a nozzle. Thermal-based Thermal-based inkjet inkjet printing printing can can propel propel droplets at pressures inin excess of 125125 atmatm andand work work with with an an operational operational efﬁciency efficiency of of 160,000 160,000 droplets droplets per per second. second.The surfaceThe surface tension, tension, inertia inertia and viscosity and viscosit of they of ink the must ink be must carefully be carefully modulated modulated to control to controlthe droplet the droplet deposition, deposition, as too high as too a viscosity high a viscosity can obstruct can theobstruct ﬂuid streamthe fluid and stream too low and a toosurface low tensiona surface can tension cause can nozzle cause leakage nozzle and leakage affect and printing affect resolution.printing resolution. Droplet sizeDroplet and sizenozzle and diameter nozzle arediameter the key are parameters the key parameters affecting the affecting minimum the print minimum size for inkjetprint printing,size for inkjetwhich printing, is typically which below is typically 10 µm but below canbe 10 muchμm but smaller. can be much smaller. AnAn application of of interest for inkjet manufacturing methods is the production of acousticacoustic metamaterial metamaterial sound absorbers. An An ad additiveditive polyjet polyjet system was used to fabricate aa thin-walled (1 (1 mm mm thick), periodic AMM, with resonance exploited in the unit-cells by coupledcoupled tubes tubes [163]. [163]. The The geometry geometry of of the the unit-c unit-cell,ell, shown shown in in Figure Figure 39,39, is designed to achieveachieve high high sound absorption within a band bandwidthwidth of 300–600 Hz. Experimental testing of a printed prototype of 4 by 4 arranged unit-cells successfully demonstrated superior sound attenuation within the given frequency ranges, with twice the acoustic power absorbed compared to standard absorbers (such as wool-covered perforated panels of similar size). The polyjet setup has a dimensional accuracy of 20 µm in-plane and 16 µm out of plane and uses polymers as the construction medium, meaning the part requires a UV curing post-process. Micromachines 2021, 12, x FOR PEER REVIEW 45 of 57

of a printed prototype of 4 by 4 arranged unit-cells successfully demonstrated superior sound attenuation within the given frequency ranges, with twice the acoustic power absorbed compared to standard absorbers (such as wool-covered perforated panels of simi-

Micromachines 2021, 12, 634 lar size). The polyjet setup has a dimensional accuracy of 20 μm in-plane and 16 μm34 out of 45 of plane and uses polymers as the construction medium, meaning the part requires a UV curing post-process.

Figure 39. An AMM unit-cell for superior sound attenuation, fabricated using polyjet technology Figurewith 39 polymer. An AMM ink. unit-cell The unit-cell for superior is contained sound within attenuation, a 50 mm fabricated wide cubic using volume, polyjet with technology the printed withprototype polymer being ink. The 200 unit-cell mm by is 200 contained mm by with 50 mmin a and50 mm containing wide cubic 16 volume, unit-cells with [163 the]. printed (Reprinted prototypefrom Procedia being 200 Engineering, mm by 200 Volume mm by 176, 50 mm R. Vdovin, and contai et al.,ning ‘Implementation 16 unit-cells [163]. of the (Reprinted Additive from PolyJet Procedia Engineering, Volume 176, R. Vdovin, et al., ‘Implementation of the Additive PolyJet Technology to the Development and Fabricating the Samples of the Acoustic Metamaterials’, Pages Technology to the Development and Fabricating the Samples of the Acoustic Metamaterials’, 595–599, Copyright (2017), with permission from Elsevier). Pages 595-599, Copyright (2017), with permission from Elsevier). An innovative manufacturing method uses precisely-controlled electrohydrodynamic An innovative manufacturing method uses precisely-controlled electrohydrody- (EHD) jets to achieve submicron feature sizes with extremely fast printing speeds [164]. namic (EHD) jets to achieve submicron feature sizes with extremely fast printing speeds Figure 40 shows a microscale part printed using this method, with a water-based ink con- [164]. Figure 40 shows a microscale part printed using this method, with a water-based taining polyethylene oxide (PEO) nanofibers. The EHD jets can reach speeds above 1 m/s, inkwith containing ink propelled polyethylene from the oxide nozzle (PEO) by nano a powerfulfibers. The applied EHD jets electric can reach field. speeds By employing above 1 m/s,electrostatic with ink deflection propelled viafrom controlled the nozzle voltage by a powerful input to applied several directingelectric field. electrodes, By employ- the jet ingtrajectory electrostatic can deflection be carefully via modulated controlled voltage with lateral input accelerations to several directing of up to electrodes, 106 m/s2. the The 6 2 jetlayerwise trajectory regime can be of carefully this method modulated can stack with up lateral to 2000 accelerations sub-micrometre of up layers to 10 in onem/s second,. The layerwiseresulting regime in a printing of this speedmethod three can to stack four up orders to 2000 of magnitude sub-micrometre faster layers than comparable in one second, meth- resultingods with in equivalenta printing minimumspeed three feature to four sizes. orders An of added magnitude benefit faster of electrohydrodynamic than comparable methodsjetting iswith that equivalent nozzle clogging minimum at the feature submicron sizes. scale An isadded not an benefit issue, unlikeof electrohydrody- FDM, enabling namic jetting is that nozzle clogging at the submicron scale is not an issue, unlike FDM, Micromachines 2021, 12, x FOR PEER nanometre-sizedREVIEW jets (less than 100 nm) to be used. The speed limits of this jetting technique46 of 57 enablingare currently nanometre-sized 0.5 m/s in-plane jets (less and than 0.4 100 mm/s nm) out to be of used. plane, The and speed the resolution limits of this is currently jetting techniquelimited to are 100 currently nm (dependent 0.5 m/s onin-plane fibre thicknessand 0.4 mm/s and jet out apertures). of plane, Thisand the technology resolution can is be currentlyexpanded limited to print to 100 with nm different (dependent ink compositions, on fibre thickness including and jet other apertures). polymers, This biomaterials technol- ogyand can living be expanded cells. to print with different ink compositions, including other polymers, biomaterials and living cells.

Figure 40.40. A cylindrical microstructuremicrostructure printedprinted usingusing EHDEHD jetting jetting with with a a novel novel electrostatic electrostatic deﬂection deflec- system.tion system. The partThe waspart fabricatedwas fabricated from from polyethylene polyethylene oxide oxide (PEO) (PEO) ink and ink achieved and achieved a submicron a submicron feature sizefeature without size without the use the of supports.use of supports. Here, Here, a scale a scale bar of bar (a )of 5 (µa)m 5 andμm and (b) 1(bµ) m1 μ arem are used used to imageto image the the microstructure [164]. microstructure [164].

3.3.4. Summary/Manufacture of of AMMs AMMs Extrusion/deposition methods are anan extremelyextremely robustrobust and and versatile versatile branch branch of of additive addi- manufacturing.tive manufacturing. This isThis reflected is reflected in its application in its application in AMM in manufacture AMM manufacture(Figures 10(Figures, 13 and 10, 39 ) 13 and 39) and across several fields: from conductive nanocomposites (Figure 35) for use in MEMs, to tissue fabrication (Figure 38) for medical applications. Discussed in this section are FDM, DIW and jetting systems—notably, all these techniques excel at the range of printable materials available. In particular, ink-based methods (DIW and jetting systems) have an extremely diverse set of material options beyond the scope of most other printing techniques. Below, the variables pertaining to the additive manufacture of AMMs for use in widespread industry are discussed, with the aforementioned techniques compared and contrasted for suitability in each category. Ability to mass produce: FDM has the cheapest build material (solid filament) and likely the fastest production method due to the continuous material deposition regime. With DIW, small volumes of ink droplets are added discontinuously, leading to a slower print rate; however, jetting systems partially addresses this limitation. This is evident in the print rate of thermally- actuated jetting systems, which can be as high as 160,000 droplets per second. Some electrohydrodynamic setting systems can reach quick print rates of 0.5 m/s in-plane and 0.4 mm/s out of plane. Minimum printable feature size: The ink droplet-based methods offer extremely small printable feature capabilities, with resolutions of 2 μm to 600 nm for DIW and 10 μm to 100 nm for jetting systems. The minimum feature size has been greatly reduced by recent innovations in both these methods, such as integrated laser annealing for metallic DIW printing (Figure 36), and employing electrostatic jet deflection to direct ink placement (Figure 40). Aside from MPP, these methods offer the greatest resolution currently possible with additive manufacturing. In addition, FDM is by no means inadequate in terms of resolution—its minimum feature size of 100 μm puts its printing capabilities well into the microscale. Multi-material printing: In general, extrusion/deposition techniques have strong multi-material printing capabilities because instead of reacting with a large volume of build material, in a vat or bed, the part is built up using controlled addition of material. FDM has particularly strong multi-material printing capabilities because its solid filament has minimal risk of cross contamination and can easily be interchanged during printing. Ink-droplet methods also demonstrate these abilities, with a complex multi-material example presented in Figure

Micromachines 2021, 12, 634 35 of 45

and across several fields: from conductive nanocomposites (Figure 35) for use in MEMs, to tissue fabrication (Figure 38) for medical applications. Discussed in this section are FDM, DIW and jetting systems—notably, all these techniques excel at the range of printable materials available. In particular, ink-based methods (DIW and jetting systems) have an extremely diverse set of material options beyond the scope of most other printing techniques. Below, the variables pertaining to the additive manufacture of AMMs for use in widespread industry are discussed, with the aforementioned techniques compared and contrasted for suitability in each category. Ability to mass produce: FDM has the cheapest build material (solid filament) and likely the fastest production method due to the continuous material deposition regime. With DIW, small volumes of ink droplets are added discontinuously, leading to a slower print rate; however, jetting systems partially addresses this limitation. This is evident in the print rate of thermally- actuated jetting systems, which can be as high as 160,000 droplets per second. Some electrohydrodynamic setting systems can reach quick print rates of 0.5 m/s in-plane and 0.4 mm/s out of plane. Minimum printable feature size: The ink droplet-based methods offer extremely small printable feature capabilities, with resolutions of 2 µm to 600 nm for DIW and 10 µm to 100 nm for jetting systems. The minimum feature size has been greatly reduced by recent innovations in both these methods, such as integrated laser annealing for metallic DIW printing (Figure 36), and employing electrostatic jet deflection to direct ink placement (Figure 40). Aside from MPP, these methods offer the greatest resolution currently possible with additive manufacturing. In addition, FDM is by no means inadequate in terms of resolution—its minimum feature size of 100 µm puts its printing capabilities well into the microscale. Multi-material printing: In general, extrusion/deposition techniques have strong multi-material printing capabilities because instead of reacting with a large volume of build material, in a vat or bed, the part is built up using controlled addition of material. FDM has particularly strong multi-material printing capabilities because its solid filament has minimal risk of cross contamination and can easily be interchanged during printing. Ink-droplet methods also demonstrate these abilities, with a complex multi-material example presented in Figure 38. Coupled with the wide range of materials available (plastics, metals, ceramics, hydrogels, nanocarbons and live cells), these methods are very promising for the further production of multi-material parts. Active metamaterial fabrication: Although no active examples were provided in this section, the potential of these methods to achieve this is substantial. The printed examples consisting of ceramic (Figures 33 and 34) or metallic (Figure 36) materials demonstrate the ability to fabricate metamaterials with piezoelectric or electromagnetic actuation. Carbon nanotube composites show particular promise for active metamaterials, as they are lightweight, flexible and electrically conductive. An example is the fabrication of a carbon nanotube microstructure, as seen in Figure 35, using a low-cost FDM printer with only minor modifications.

38. Coupled with the wide range of materials available (plastics, metals, ceramics, hydrogels, nanocarbons and live cells), these methods are very promising for the further production of multi-material parts. Active metamaterial fabrication: Although no active examples were provided in this section, the potential of these methods to achieve this is substantial. The printed examples consisting of ceramic (Figures 33 and 34) or metallic (Figure 36) materials demonstrate the ability to fabricate metamaterials with piezoelectric or electromagnetic actuation. Carbon nanotube composites show particular promise for active metamaterials, as they are lightweight, flexible and electrically conductive. An example is the fabrication of a carbon nanotube microstructure, as seen in Figure 35, using a low-cost FDM printer with only minor modifications.

3.4. Hybrid Techniques A novel hybrid technique combined digital light processing (DLP) and direct ink writing (DIW) to produce multi-material prints with functional inks, including liquid crystal elastomers (LCEs) and conductive silver inks [165]. This unique method combines both vat polymerization and ink deposition into one printing device capable of printing with both techniques simultaneously. The top-down DLP component had a resolution of Micromachines 2021, 12, 634 30–100 μm (dependent on focal length/frame adjustment), whereas the DIW syringe res- 36 of 45 olution range was from 100 μm to 1.54 mm (dependent on nozzle size used). This method shows promise for soft robotic applications, with examples presented using LCE fibres as the DIW print material and active component. Shown in Figure 41, this active structure structureconsists of consists a DLP-printed of a DLP-printed soft elastomer soft matr elastomerix with the LCE matrix fibres with embedded the LCE using ﬁbres DIW, embedded usingwith layer DIW, thicknesses with layer of thicknesses 50 μm and 700 of 50μm,µ mrespectively. and 700 µ m,The respectively. soft robots are The reversibly soft robots are reversiblyactuated using actuated temperature, using temperature, with the fibres with shrinking the ﬁbres as a shrinkingreaction to asheat, a reaction hence causing to heat, hence causingthe robot the to bend/close. robot to bend/close. While this example While is this not example in the microscopic is not in therange, microscopic the minimum range, the minimumlayer thickness layer of thickness 100 μm for of 100thisµ methodm for this shows method promise shows for promiseprinting formicron-size printing soft micron-size softrobots. robots.

FigureFigure 41.41. SeveralSeveral soft soft robot robot prototypes prototypes printed printed with witha DLP–DIW a DLP–DIW hybrid hybridmethod, method, with the withfour the four structure designs shown in (b–e) and a colour-coded key for the different materials in (a). These structure designs shown in (b–e) and a colour-coded key for the different materials in (a). These active structures are shown after printing (left), during heating/actuation (middle), and after cool- activeing/relaxation structures (right are). The shown structures after printing are printed (left with), during layer thicknesses heating/actuation of 50 μm (for (middle the DLP), andpoly- after cool- ing/relaxationmer matrix) and (700right μm). (for The the structures DIW LCE fibres), are printed and the with scale layerbar is 10 thicknesses mm [165]. (Reprinted of 50 µm from (for the DLP polymerAdditive matrix)Manufacturing, and 700 Volumeµm (for 40, the X. Peng, DIW et LCE al., ﬁbres),‘Integrating and digital the scale light bar processing is 10 mm with [165 direct]. (Reprinted ink writing for hybrid 3D printing of functional structures and devices’, Copyright (2021), with per- from Additive Manufacturing, Volume 40, X. Peng, et al., ‘Integrating digital light processing with mission from Elsevier). Micromachines 2021, 12, x FOR PEER REVIEWdirect ink writing for hybrid 3D printing of functional structures and devices’, Copyright48 of 57 (2021), with

permission from Elsevier).

AnotherAnother DLP-based DLP-based process process allows allows microscale microscale features features to be applied to be applied to macro-scale to macro-scale 3D objects, objects, pre-printed pre-printed withwith any suitable any suitable manufacturing manufacturing method [166]. method This process [166]. mod- This process ifiesmodiﬁes the surface the surface functionality functionality of an inserted of an object inserted to produce object a tohydrophobic produce aeffect. hydrophobic Figure effect. 42Figure displays 42 displays an example an exampleof the aforementi of the aforementionedoned hydrophobic hydrophobic microstructure, microstructure, which can which controlcan control the surface the surface adhesion adhesion of water droplets of water by droplets modulating by the modulating number of the“egg number beater” of “egg structuresbeater” structures present. present.

Figure 42 42.. AnAn example example of ofa hydrophilic a hydrophilic “egg-beater” “egg-beater” microstructure microstructure printed printedusing a DLP-based using a DLP-based printing method, showing (a) the CAD model, and (b) an electron microscope image of the printed printing method, showing (a) the CAD model, and (b) an electron microscope image of the printed part, where the scale bar is 100 μm (top) and 200 μm (bottom). (Reprinted from [166], with the permissionpart, where of theJohn scale Wiley bar & Sons is 100 Inc.).µm (top) and 200 µm (bottom). (Reprinted from [166], with the permission of John Wiley & Sons Inc.). While this example only demonstrates hydrophobic effects, a variety of surface properties could be induced with an appropriately designed microstructure. This DLP-based process can be used alongside virtually every additive manufacturing method and has the potential to revolutionize how surface effects are added to 3D-printed parts. Manipulating microdroplets in this fashion shows potential for use in several important applications, such as oil spill clean-up, micro reactors, 3D cell cultures and self-cleaning devices. Adding DLP principles to the SLS/SLM manufacturing process has enabled simultaneous laser beam melting (SLBM) to be developed [167]. This technique allows several polymer powders to be melted and additively manufactured within one building process, therefore allowing the seamless production of multi-material parts. The process involves the polymer powders being deposited on the build platform and heated to just below melting temperature by infrared light emitters. Following this, CO2 and thulium lasers are used in conjunction with a DLP chip to achieve the simultaneous melting of both pre- heated polymers in the desired layer shape. Figure 43 displays a cross-section of the microstructure of a multi-material print after 10 s of thulium laser exposure, with a resulting layer thickness of 175 μm. Polypropylene (PP) and polyamide 12 (PA12) powders were used in Figure 43, which are both thermoplastic elastomers. SLBM presents a new approach to constructing multi-material polymer parts with microscale accuracy and short irradiation times; however, more research is required before it is a fully-realized additive manufacturing technique.

Micromachines 2021, 12, 634 37 of 45

While this example only demonstrates hydrophobic effects, a variety of surface properties could be induced with an appropriately designed microstructure. This DLP-based process can be used alongside virtually every additive manufacturing method and has the potential to revolutionize how surface effects are added to 3D-printed parts. Manipulating microdroplets in this fashion shows potential for use in several important applications, such as oil spill clean-up, micro reactors, 3D cell cultures and self-cleaning devices. Adding DLP principles to the SLS/SLM manufacturing process has enabled simultaneous laser beam melting (SLBM) to be developed [167]. This technique allows several polymer powders to be melted and additively manufactured within one building process, therefore allowing the seamless production of multi-material parts. The process involves the polymer powders being deposited on the build platform and heated to just below melting temperature by infrared light emitters. Following this, CO2 and thulium lasers are used in conjunction with a DLP chip to achieve the simultaneous melting of both pre-heated polymers in the desired layer shape. Figure 43 displays a cross-section of the microstructure of a multi-material print after 10 s of thulium laser exposure, with a resulting layer thickness of 175 µm. Polypropylene (PP) and polyamide 12 (PA12) powders were used in Figure 43, which are both thermoplastic elastomers. SLBM presents a new

Micromachines 2021, 12, x FOR PEER approachREVIEW to constructing multi-material polymer parts with microscale accuracy and49 short of 57 irradiation times; however, more research is required before it is a fully-realized additive manufacturing technique.

FigureFigure 43.43. Cross-sectionCross-section ofof molten molten PA12 PA12 after after 10 10 s simultaneouss simultaneous melting melting by by a thuliuma thulium laser, laser, which which is is an integral step in the SLBM fabrication process. This study uses polypropylene (PP) and poly- an integral step in the SLBM fabrication process. This study uses polypropylene (PP) and polyamide amide 12 (PA12) powder to print a multi-material part, where the resulting layer thickness is 175 12 (PA12) powder to print a multi-material part, where the resulting layer thickness is 175 µm. μm. (Reprinted with permission from [167]. Copyright (2015), Laser Institute of America). (Reprinted with permission from [167]. Copyright (2015), Laser Institute of America). A polyjet approach incorporating inkjet technology alongside the photocuring of A polyjet approach incorporating inkjet technology alongside the photocuring of resin, resin, a distinctive feature of vat polymerization methods, has been applied to manufac- a distinctive feature of vat polymerization methods, has been applied to manufacture novel Luneburgture novellenses Luneburg for the lenses purposes for the of purposes ultrasound of ultrasound beam steering beam [168 steering]. The smallest[168]. The feature small- ofest the feature meta-unit of the used meta-unit in the used lens, in shown the lens, in Figure shown 44 ina, Figure is 400 µ 44a,m and is 400 designed μm and to designed operate atto anoperate ultrasonic at an frequencyultrasonic offrequency 40 kHz. Theof 40 potential kHz. The exists, potential however, exists, to however, scale the to meta-unit scale the downmeta-unit to function down to at function higher ultrasonic at higher frequencies,ultrasonic frequencies, because the because polyjet printerthe polyjet used printer has a maximumused has a possible maximum resolution possible of resolution 16 µm. The of orthogonal 16 μm. The truss orthogonal meta-cells truss are interconnected meta-cells are tointerconnected form a uniform to form 3D lattice a uniform that is 3D both lattice lightweight that is both and lightweight self-supporting and self-supporting with minimal riskwith of minimal deformation. risk of Thedeformation. manufactured The manufactured designs consist designs of a 2D consist and 3D of flatteneda 2D and Luneb-3D flat- urgtened lens, Luneburg illustrated lens, in illustrated Figure 44 b,c,in Figure which 44b,c, are arranged which are in arranged 5 and 19 in stacked 5 and 19 meta-cell stacked layers,meta-cell respectively. layers, respectively. This fabrication This fabricatio technologyn technology offers a new offers avenue a ne tow fabricateavenue to ultrasonic fabricate beam-formingultrasonic beam-forming devices that devices is both that efficient is both and efficient precise inand microscale precise in resolutions. microscale Polyjetresolu- printingtions. Polyjet is currently printing commercially is currentlyavailable commercially and already available established and already as a established robust printing as a methodrobust printing at marginally method larger at marginally scales. As larger the demands scales. As for the finer demands lattice geometryfor finer lattice increase geom- to etry increase to surpass current bandwidth and frequency restrictions, polyjet printers show promise to maintain their continuous development to meet these ever-changing requirements. Should their consistent progress persevere, this technology demonstrates a clear feasibility in manufacturing AMMs for beam forming and other important industrial applications.

Micromachines 2021, 12, 634 38 of 45

Micromachines 2021, 12, x FOR PEER REVIEW 50 of 57 surpass current bandwidth and frequency restrictions, polyjet printers show promise to maintain their continuous development to meet these ever-changing requirements. Should their consistent progress persevere, this technology demonstrates a clear feasibility in

manufacturing AMMs for beam forming and other important industrial applications.

FigureFigure 44.44. The geometry of of the the flattened flattened Luneburg Luneburg lens lens designs, designs, fabricated fabricated using using polyjet polyjet technology, technology,showing showing (a) the (a periodic) the periodic unit-cell unit-cell of both of lenses both with lenses dimensional with dimensional variables variablesappropriately appropriately labelled— labelled—thisthis proposedproposed building block building can block form cana uniform form a 3D uniform lattice 3D once lattice connected once connected to adjacent, toadjacent, identical unit-cells; and (b) the 2D and (c) 3D lens designs printed with 5 and 19 stacked layers, respectively. identical unit-cells; and (b) the 2D and (c) 3D lens designs printed with 5 and 19 stacked layers, (Reprinted from [168], with the permission of AIP Publishing). respectively. (Reprinted from [168], with the permission of AIP Publishing). 4.4. DiscussionDiscussion andand OutlookOutlook TheThe fieldfield of of acoustic acoustic metamaterials metamaterials holds holds much much promise promise for for the the future, future, with with novel novel so- lutionssolutions to acousticto acoustic problems problems becoming becoming increasingly increasingly close toclose real-world to real-world applications. applications. While theWhile field the of AMMsfield of coversAMMs such covers problems such prob as earthquakelems as earthquake engineering, engineering, building acoustics building andacoustics underwater and underwater applications, applications, we have chosenwe have to chosen focus on to thefocus potential on the usespotential of acoustic uses of metamaterialsacoustic metamaterials on airborne on systemsairborne for systems audio for and audio ultrasonic and ultrasonic applications. applications. WithWith airborneairborne soundsound coveringcovering wavelengthswavelengths fromfrom metresmetres toto millimetres,millimetres, thethe scalescale ofof materialsmaterials thatthat interactinteract withwith thesethese soundssounds isis similar,similar, soso notnot onlyonlyare are novel novel subwavelength subwavelength meta-atommeta-atom designsdesigns important,important, butbut alsoalso thethe abilityability toto createcreate themthem isis crucial.crucial. AdditiveAdditive man-man- ufacturingufacturing hashas proved to be be the the most most promisin promisingg out out of of existing existing technologies, technologies, exceeding exceeding its itssubtractive subtractive counterparts, counterparts, to tobuild build up up comple complexx materials materials from from their their constituent constituent meta-at- meta- atoms.oms. However, However, the the outlook outlook is challenging is challenging for translating for translating this academic this academic research research into fully- into fully-realizedrealized devices devices for forindustrial industrial and and consumer consumer applications. applications. Rapid Rapid prototyping, whilewhile cheapcheap forfor everydayeveryday use,use, stillstill becomesbecomes expensiveexpensive withwith scale,scale, particularly techniquestechniques requir-requir- inging customcustom materials.materials. AdditiveAdditive technologiestechnologies workingworking atat multiplemultiple scalesscales isis notnot feasible,feasible, so so obtainingobtaining micron micron resolution resolution over over scales scales in excessin excess of a of centimetre a centimetre is close is toclose impossible. to impossible. Com- plexComplex shapes shapes are challenging, are challenging, and aspects and aspect of temperatures of temperature dependence dependence or curing/annealing or curing/an- mechanismsnealing mechanisms of raw materials of raw materials can mean can that mean it is difficultthat it is to difficult print with to print adequate with accuracyadequate andaccuracy reliability. and reliability. For example, For example, the printing the ofprinting membranes—commonly of membranes—commonly used to provideused to mechanicalprovide mechanical behaviour behaviour to a meta-atom—is to a meta-atom—is challenging, challenging, where where at a small at a small scale thescale raw the materialsraw materials can have can post-productionhave post-production tension tension and warping and warping that is that difficult is difficult to compensate to compen- for. Insate addition, for. In addition, each technique each technique has production has prod requirementsuction requirements unique unique to itself. to As itself. mentioned, As men- vattioned, polymerization vat polymerization techniques techniques cannot print cannot voids print or inclusionsvoids or inclusions directly, instead directly, requiring instead ventsrequiring or other vents ways or other to leak ways out to the leak original out the liquid original polymer. liquid polymer. Designs ofDesigns AMMs of need AMMs to accountneed to foraccount that, providingfor that, providing another barrieranother to barrier production. to production. Clearly, for a practical and realisable AMMs, we need to be able to create large-scale Clearly, for a practical and realisable AMMs, we need to be able to create large-scale materials (and ultimately devices) with small-scale features. Additive manufacturing is the materials (and ultimately devices) with small-scale features. Additive manufacturing is best candidate for both rapid prototyping and bulk manufacture, but limitations on specific the best candidate for both rapid prototyping and bulk manufacture, but limitations on fabrication characteristics act as a barrier to AMM research, constraining printable ideas specific fabrication characteristics act as a barrier to AMM research, constraining printable and restricting its potential. Given that metamaterials typically derive their properties from ideas and restricting its potential. Given that metamaterials typically derive their proper- repeating units of local resonators or geometrical features, a key issue in their manufacture ties from repeating units of local resonators or geometrical features, a key issue in their is consistency and reproducibility across length scales. A material in which some of manufacture is consistency and reproducibility across length scales. A material in which the meta-atoms failed to print correctly could alter the acoustic properties and potentially some of the meta-atoms failed to print correctly could alter the acoustic properties and disrupt the predicted acoustic behaviour. This high dependency on uniformity, or reliability potentially disrupt the predicted acoustic behaviour. This high dependency on uni- of shape, across meta-atoms means that AMMs are still very challenging to create, even with formity, or reliability of shape, across meta-atoms means that AMMs are still very challenging to create, even with additive manufacturing, and especially at smaller scales. The

Micromachines 2021, 12, 634 39 of 45

additive manufacturing, and especially at smaller scales. The robustness of metamaterials to meta-atom failure could be a key area of research to progress AMMs to commonplace use. This review discussed many viable additive manufacturing methods for small structures, and it discussed techniques best suited to address some key fabrication parameters for AMMs. MPP was found to have the smallest printable feature size, followed by DIW, jetting systems and DLP, with the highest resolution of these methods well within the nanoscale. This result is supported by similar findings in the literature [113,169]. For structures requiring varied material options, like active AMMs, DIW and jetting systems were found to have the widest material range—from ceramics and metals to live cells—and a minimal risk of cross-contamination of materials, unlike vat polymerisation and powder bed methods. Additionally, extrusion/deposition methods are regarded as comparatively fast additive manufacture techniques, further demonstrating the significant potential of DIW and jetting systems as extremely effective and well-rounded techniques for AMM manufacture. Other noteworthy techniques include SLM and EBM, which excel at fabricating metallic parts with microscale resolution without the use of support structures. Vat polymerisation and ink deposition methods are the set of techniques best suited to printing hydrogels—a key material for the fabrication of some active metamaterial designs. Vat polymerisation shows strong potential for AMM fabrication, with SLA, DLP and MPP all being robust techniques with individual benefits; however, their main limitation lies in their material restrictions and post-processing requirements. Further development to overcome these challenges would be beneficial for their application in AMM construction. Over the following 10 years, the next generation of practical AMMs will need to come from either significant progress in rapid prototyping and manufacturing technology, or the discovery of meta-atom designs that have complex physical behaviour while being sufficiently simple enough to be printed. Active acoustic metamaterials promise to push development forward, compensating for manufacturing challenges by adding complexity through controlled actuation rather than static structure, but they are also difficult to produce beyond very basic systems at the smallest scale. Nevertheless, it will be exciting to monitor which of these two strands of research will succeed first.

Funding: This research received no external funding. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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