-Based 3D and Surface Texturing A Selimis and M Farsari, Foundation for Research and Technology – Hellas (FO.R.T.H.), Institute of Electronic Structure and Laser (I.E.S.L.), Heraklion, Greece

r 2016 Elsevier Inc. All rights reserved.

1 Introduction 1 2 SLA 1 3 SLS 2 4 Microscale Techniques 4 4.1 Projection Micro-Stereolithography 4 4.2 Multiphoton 5 4.2.1 Multiphoton polymerization 6 4.2.2 The diffraction limit 7 4.2.3 Experimental set-up 7 4.3 Materials for Laser Polymerization 8 4.3.1 Introduction 8 4.3.2 Photoinitiators 9 4.3.3 Organic photopolymers 9 4.3.3.1 SU-8 10 4.3.3.2 Hybrid materials 10 4.4 Applications 11 4.4.1 Metamaterials 11 4.4.2 Biomedical applications 13 5 Laser-Based Surface Texturing Techniques 14 6 Conclusions 18 References 18

1 Introduction

3D printing or additive manufacturing (AM) is described as the process of making a three-dimensional solid object from a digital file using additive processes. A 3D object is created by depositing successive layers of a material according to a computer design. To print a 3D object, first one needs to have the design. A completely new object can be made in CAD (computer-aided design) s s file using a 3D-modeling program, such as SolidWorks or AutoCad . Alternatively, the copy of an existing object can be made by scanning it using a 3D scanner. Using appropriate software, the 3D model is sliced into horizontal layers, and loaded onto a 3D . The 3D printer builds the final object one layer at a time. A large number of 3D printing techniques is now available, the main differences between them being the way layers are deposited and the materials used. Amongst them, laser-based techniques are very popular, as the laser beam can be focused and directed very accurately. In this chapter we will detail and compare the most popular laser-based 3D printing techniques. In Sections 2 and 3 and we will describe Stereolithography (SLA) and Selective Laser Sintering (SLS), the most industrial of the laser-based 3D printing techniques. In Section 4 we will concentrate on microscale 3D printing techniques: projection stereolithography (PSL, Section 4.1) and (MPL, Section 4.2); We will describe extensively the principles and applications of the latter, the most popular and widely used of all microscale 3D printing techniques. Finally, in Section 5, we will describe how are used to structure the surface of polymers, , semiconductors, and metals.

2 SLA

SLA is an AM technology for producing 3D objects in a layer-by-layer fashion, by photopolymerizing a photosensitive material with an ultraviolet (UV) laser. The father of SLA is Charles (Chuck) W. Hull; he first coined the term ‘stereolithography’ and described the apparatus in the 1986 patent “Apparatus for production of three-dimensional objects by stereolithography.”1 There, it is described as “a new and improved system for generating a three-dimensional object by forming successive, adjacent, cross- sectional laminae of that object at the surface of a fluid medium capable of altering its physical state in response to appropriate

Comprehensive Materials Finishing doi:10.1016/B978-0-12-803581-8.09171-2 1 2 Laser-Based 3D Printing and Surface Texturing

Figure 1 SLA design, as described in the original C.W. Hull patent. Hull, C. W. Apparatus for Production of Three-Dimensional Objects by Stereolithography. U.S. Patent 4,575,330, 1986. synergistic stimulation, the successive laminae being automatically integrated as they are formed to define the desired three- dimensional object.” It is basically describing a system for making solid objects by successively curing thin layers of a photo- polymer using UV light. The patent described a focused UV light beam drawing each layer of the object onto the surface of a vat filled with liquid resin, as shown in Figure 1. Also in 1986, C.W. Hull founded 3D Systems Inc.,2 the first company to develop a commercial SLA system. In a commercial SLA machine, a focused UV laser beam – nowadays usually the 3rd harmonic of an yttrium aluminum garnet (YAG) laser – is used to photopolymerize an acrylate or epoxy liquid resin.3 The first (usually sacrificial) layer of the component is built on an elevator platform. At every layer and always according to a CAD design, the beam draws a pattern on the surface of the liquid resin, selectively solidifying it and making it adhere to the layer below. After each layer has been ‘written,’ the elevator platform lowers by a single layer, the built section is re-coated with liquid resin (made flat by employing a re-coating blade), and another layer is ‘written.’ After the completion of the laser writing, the unexposed photopolymer is removed by immersing the structure in a solvent that dissolves the liquid monomer, and not the polymerized material. To save time, each layer is not fully photopolymerized, but instead the laser traces its exterior, and crosshatches in between. This results in a slice with a honeycomb interior and solid skins on top and bottom, encapsulating liquid resin. After removing the unexposed resin, the 3D object is ‘baked’ inaUV‘oven,’ to solidify the encapsulated liquid resin. Very often the objects made by SLA need to have supporting structures, to avoid deformation due to the piston movement, gravity, and the re-coating blade. These supports need to be removed manually after the completion of the building process. SLA can produce almost infinitely complex functional prototypes in a time length from a few hours to more than a day. The resolution can be down to 5 mm, while the overall size depends on the type of the machine and the size of the elevator platform. Typically, an SLA machine will be capable of producing components of maximum size 50 50 60 cm3. However, nowadays there are SLA machines, such as the Mammoth,4 that can produce sizes up to 2 m. On the downside, both photosensitive resins and SLA machines can be very expensive. The main application of SLA is prototyping; as such, it has been employed by a variety of industries, from medical prototyping to the car industry, where it is routinely used to make polymer prototypes of car engines. However, the use of SLA models is not limited to prototypes; modern SLA resins provide strong enough models to be machined and make functional synthetic parts, but they are mostly used as master patterns for injection molding and metal casting. SLA has also been applied in medicine, to fabricate guides for tumor identification and surgery guidance,5,6 but also for the – fabrication of implants and scaffolds for bone and cartilage tissue engineering.6 10 The materials used in this case can be pure photopolymers, or custom materials such as resins mixed with hydroxyapatite.

3 SLS

The SLS technique employs a high-power laser to sinter layers of material in powder form, in order to build 3D structures. A typical experimental setup is shown in Figure 2. A layer of powder is first deposited on a fabrication piston substrate. The laser beam is scanned over the powder surface following a CAD design. The laser beam heats the powder to melting point and causes the powder particles to melt together to form a solid mass. Subsequent layers are built directly on top of the earlier layers, with new layers of powder deposited on the top of the already sintered layers using a roller. The materials used for building components in SLS machines can be single component powders (such as metal powders produced by ball milling) or, more commonly two-component powders, coated powders or powder mixtures. Compared to other Laser-Based 3D Printing and Surface Texturing 3

Figure 2 The SLS process, showing (top) the rolling of a thin layer of powder over the work area, (middle) the sintering of powder by a laser beam to build up the workpiece, and (bottom) the rolling of fresh powder over the workpiece to begin a new layer (http://www.britannica.com/ technology/3D-printing/images-videos – Encyclopedia Britannica).

Figure 3 (a) A dental implant made using SLS34 and (b) poly-e-caprolactone scaffolds made using SLS for cardiac regeneration.82

3D laser printing techniques, SLS can build components from a wide variety of materials. These can be polymers such as nylon and – – – – – nylon-coated metal particles,11 17 polystyrene,15,18 26 metals such as titanium,27 35 steel,36 44 alloys,32,45 49 and several biomaterials – such as hydroxyapatite, polycaprolactone, polylactide, and mixtures of those with other biopolymers.50 77 Depending on the material, up to 100% density can be achieved with material properties comparable to those from conventional milling methods. Due to its material versatility and its potential to make very complex geometries directly from computer models, SLS is widely used nowadays. While its original use was in prototyping, it is now used also in custom or limited-run manufacturing to produce end-use parts. One such area is dental implants Figure 334 and scaffolds for hard and soft tissue engineering. SLS has been extensively used to create hard scaffolds for bone and cartilage tissue engineering,62,78,79 often employing mixtures of poly-e-caprolactone with calcium phosphates as a scaffold material.80 In soft tissue engineering its use has been more limited; it 4 Laser-Based 3D Printing and Surface Texturing has been used to melt together poly-e-caprolactone layers for cardiac regeneration81,82; biodegradable polycaprolactone was also structured in order to grow avidin–biotin human hepatoma Hep G2 cells, for liver tissue engineering. SLS has also been used to make replication masters for biomedical implants.83

4 Microscale 3D Printing Techniques

Progress in technologies such as MEMS and microfluidics has increased the requirement for 3D printing technologies with micro and nano features resolution, using diverse materials, such as ceramics and polymers. In the following paragraphs we describe two laser-based 3D microfabrication techniques: Projection micro-steleolithography and multiphoton lithography.

4.1 Projection Micro-Stereolithography

In Section 2 we described SLA as a technique for fabricating large-scale components and prototypes. The same technique was miniaturized to produce micron scale components,84 and is now called micro-stereolithography (mSLA). The procedure is exactly the same as in classic SLA, except the beam is focused very tightly, to enable high resolution. While mSLA worked well, it never became popular for mass production as it was very slow. To overcome this problem, Alain Bertsch and co-workers proposed a new system based on a dynamic mask pattern being projected on the liquid resin surface, therefore photopolymerizing each layer of the pattern in one shot.85,86 The experimental setup is shown in Figure 4. There, the beam of a green laser (515 nm) was modulated by a liquid crystal display (LCD), to project an image on a platform immersed into the photosensitive resin. As each component layer is made using a single light exposure, the only moving part in the system is the z-axis of the component platform. As the Bertsch system was designed to operate with green light (a wavelength of transparency of the LCD display), it could not use any of the commercially available photosensitive resins designed for classic SLA. This was accomplished by the Chatwin group – (Sussex University, UK), who replaced the LCD with a custom-made spatial light modulator (SLM), transparent to the UV.87 89 – Several materials composed for SLA and high-resolution structures were made using this technique (Figure 5).87 90

Figure 4 The first projection mSLA set-up using an liquid crystal display as a dynamic mask.85,86

Figure 5 3D structures fabricated using an spatial light modulator as a dynamic mask.89 Laser-Based 3D Printing and Surface Texturing 5

Figure 6 3D complex microstructures fabricated employing a DMD as a dynamic mask. (a) Micro matrix with suspended beam diameter of 5 mm; (b) high aspect-ratio micro rod array consists of 21 11 rods with the overall size of 2 mm 1 mm. The rod diameter and height is of 30 mm and 1 mm, respectively; (c) micro coil array with the coil diameter of 100 m and the wire diameter of 25 mm; and (d) suspended ultra fine line with the diameter of 0.6 mm.92

Replacing LCDs with SLMs solved the material availability problem but generated others: the low switching speed and low optical density of the refractive elements of the SLM during the OFF mode reduce the contrast of the transmitted pattern. In addition, the SLM UV transparency is limited, resulting in energy wastage and overheating (therefore limited lifetime) of the device itself. At the same time as the Chatwin team developed SLM-based mSLA, digital micromirror devices (DMDs) became available by Texas Instruments.91 DMD is a reflective array of fast, digital light switches that are monolithically integrated onto a address chip. Used as a projection system, a DMD replaced the SLM in a system implemented by Sun et al.,92 providing high-quality, seamless, all-digital images with exceptional stability and minimal power loss. The quality and resolution of the structures fabricated using this technique was superior to anything presented before (Figure 6). The application of DMDs in mSLA is still fairly limited, and most research has focused on improving the process rather than – developing applications.93 96 Again, however, the main area of research applications has been developing microenvironments for – cell growth.97 100

4.2 Multiphoton Lithography

Multiphoton Lithography based on the Multiphoton Polymerization of photosensitive materials is a direct laser writing technique – that allows the fabrication of three-dimensional structures with submicron resolution.101 105 The polymerization is based on two- photon absorption (TPA); when the beam of an ultrafast laser is tightly focused into the volume of a transparent, photosensitive material, the polymerization process can be initiated by nonlinear absorption within the focal volume. By moving the laser focus three-dimensionally through the material, 3D structures can be fabricated. The technique has been implemented with a variety of acrylate and epoxy materials and several components and devices have been fabricated such as photonic crystal templates,106 mechanical devices,107 and microscopic models.108 Resolution below 100 nm has been achieved using this technique.109 The unique capability of MPL lies in that it allows the fabrication of computer-designed fully 3D structures with resolution beyond the diffraction limit. No other competing technology offers these advantages. Classic 3D prototyping techniques such as UV laser SLA, 3D , and laser sintering can also produce fully three-dimensional structures; however, they cannot provide resolution better than a few microns. On the other hand, lithographic techniques with superior resolution, such as e-beam lithography, cannot produce anything more complicated than high-aspect ratio two-dimensional structures. In this section we summarize the principles of microfabrication by MPL. We discuss the fundamental principles of multiphoton absorption (MPA) and describe a typical MPL experimental setup. Then we concentrate on materials used for MPL micro- fabrication, on recent progress in the functionalization of the surface and the bulk of the 3D-fabricated structures, while at the same time discussing applications of the technology. 6 Laser-Based 3D Printing and Surface Texturing

Figure 7 Sequential (a) and simultaneous absorption (b). In the first case the intermediate energy level is an actual energy level, while in the second case it is virtual.

4.2.1 Multiphoton polymerization The basis of MPL is the phenomenon of MPA. For simplicity, we will discuss TPA and two-photon polymerization (TPP) – the same rules apply to more than two photons. There are two types of TPA: sequential and simultaneous. In sequential, the absorbing species is excited to a real intermediate state, then, a second photon is absorbed. The presence of the intermediate energy state implies that the material absorbs at this specific wavelength; it will therefore be a surface effect and will follow the Beer–Lambert law.3 The simultaneous absorption, on which the MPL technique is based, was originally predicted by Maria Göppert-Mayer in 1931 in her doctoral dissertation.110 It is defined as “an absorption event caused by the collective action of two or more photons, all of which must be present simulta- neously to impart enough energy to drive a transition.” This prediction was not experimentally verified until over 30 years later by Werner Kaiser, when the invention of the laser permitted the first experimental verification of the TPA through the two-photon excited fluorescence detection in a europium-doped crystal.111 In simultaneous absorption, there is no real intermediate energy state, i.e., the material is transparent at that wavelength. Instead, there is a virtual intermediate energy state and TPA happens only if another photon arrives within the virtual state lifetime.112 For this to occur high intensities are required, which can only be provided by a tightly focused femtosecond (fs) laser beam. This is illustrated in Figure 7; as it can be seen, the electron transition in this case is caused by two photons of energy hv/2 rather than one of energy hv. Ti:Sapphire and near- lasers are widely used for this purpose. They have two main advantages; firstly they have very short pulses, in the order of a few tens of femtoseconds, so they do not cause thermal damage. Secondly, their standard wavelength is 780–820 nm, which is twice the wavelength of polymerization of a wide range of photopolymers. In addition, most photo- polymers are transparent at around 800 nm, which allows in-volume focusing of the laser beam with minimal scattering. When the laser is focused tightly into the material, the photoinitiator used to initiate the polymerization will absorb two or more photons and produce radicals. As the material response is proportional to the square of the intensity, this will only happen at the focal point, which, combined with the fact that the two-photon transition rate is very small, will provide very high-spatial resolution. Photopolymerization is a light-induced reaction, which converts a liquid or gel monomer into a solid polymer. These reactions require the use of an appropriate photoinitiator, which is a light sensitive molecule that produces an active species upon irradiation with UV, visible, or infrared light. The photoinitiators that have been most extensively used so far are divided into two main categories depending on the nature of generated active species (radicals or cations). An effective initiator has a high-quantum yield in the generation of the active moieties, high-thermal stability and stability in darkness, and is highly soluble in the polymerization medium. Free-radical polymerizations are chain reactions in which the addition of a monomer molecule to an active chain-end regenerates the active site at the chain-end. – The free-radical photopolymerization mechanism involves at least three different kinds of reactions (Eqn [1a]–[1b]):113 115

• The first step is the initiation during which the free-radical initiator is decomposed with light in the presence of monomer to form an active species (Step 1a). • In the next step, known as the propagation, the initiator fragment reacts with a monomer molecule to form the first active adduct that is capable of being polymerized. Monomers continue to add in the same manner resulting in the formation of macroradicals which are end-active polymers (Step 1b). Laser-Based 3D Printing and Surface Texturing 7

• The final step is the termination during which the growth center is deactivated and the final polymer molecules are formed. This step normally involves the reaction between two polymers bearing active centers and can proceed by two different mechanisms, combination or disproportionation, leading to the formation of one or two polymers chains, respectively (Step 1c).

Equations [1a]–[1b] show the TPP process

hv; hv à d Initiation I-I -R ½1a

d þ - d-M ⋯- d ½ Propagation R M RM RMM RMn 1b

d þ d - ½ Termination RMn RMm RMnþmR 1c Besides the above, other reactions, such as chain transfer and chain inhibition, often take place and complicate the mechanism of free-radical polymerization. Photopolymerizations follow the general scheme for any polymerization, however, the use of light, rather than heat, to drive the reaction has certain advantages, such as the elimination of solvent, the high-reaction rates at room temperature, and the spatial control of the polymerization.

4.2.2 The diffraction limit Theoretically, the highest resolution that can be achieved by a focused laser beam is given by Abbe’s diffraction limit (eqn [2]).116

0:5l Diffraction limit ¼ ½2 N:A:

where l is the laser wavelength and N.A. is the numerical aperture of the focusing objective; this has fueled the race for ever decreasing wavelengths, such as electron wavelengths and for alternative, non-light patterning techniques such as atomic force – – microscopy117 121 and near-field scanning optical microscopy,122 124 however, these techniques only allow surface and not in- volume patterning. To produce 3D structures with in-volume patterning, and produce photopolymerized voxels smaller than that defined by the diffraction limit, materials with well-defined photopolymerization threshold need to be used. As the photoinitiator is excited by the laser process, it produces radicals; these radicals are quenched by oxygen and other quenchers in the system. Quenching is a competing effect to photopolymerization and is usually considered detrimental to the process. In MPL, however, it can be used to circumvent the diffraction limit and produce structures of very high resolution. This can be done by modifying the light intensity at the focal volume, in a manner so that the light-produced radicals exceed the quenchers and initiate polymerization only at a region where exposure energy is larger than the threshold. In this case the diffraction limit becomes just a measure of the focal spot size and it does not really determine the voxel size.

4.2.3 Experimental set-up A typical experimental setup for the fabrication of three-dimensional microstructures by MPL is shown in Figure 8. The laser source typically is a Ti:Sapphire femtosecond oscillator operating at 800 nm; there are also examples in literature where an Optical Parametric Oscillator is used with a Ti:Sapphire laser, to reduce the laser wavelength to visible wavelengths.109,125 The laser will typically have a pulse length of less than 200 fs and a repetition rate of 50–80 MHz. The energy required for the polymerization process will depend on the material, the photoinitiator and the focusing, but is usually in the order of a few nanojoules per pulse. The photopolymerized structure is usually generated in a layer-by-layer format. Each layer is formed either using an x–y galvanometric mirror scanner or x–y piezoelectric stages. The main difference between the two cases being that in the former case, the structure remains immobile and the structure is generated by the laser beam moving, while in the latter case the x–y stages move the structure and the laser beam remains immobile. Movement on the z-axis can be achieved using a piezoelectric or a high resolution linear stage. To achieve the tight focusing conditions required for TPP to occur, a microscope objective needs to be used; when the numerical aperture of the objective is higher than 1, immersion oil is used for index matching. Galvo scanners have to be adapted to accommodate microscope objectives, as usually they are designed to take lenses with long focal lengths. Beam control can be achieved by either using a fast mechanical shutter or an acousto-optic modulator, while beam intensity control can be achieved using neutral-density filters, a variable attenuator, or a combination of a polarizer and a waveplate. For the online monitoring of the photopolymerization process a charge-coupled device camera can be mounted behind a dichroic mirror, as shown in Figure 8. This is possible as the refractive index of most photopolymers changes during poly- merization, so that the illuminated structures become visible during the building process. When the photosensitive polymer is in a liquid form, in order to avoid liquid movement as the samples moves, the samples are prepared in a sandwich format between two thin coverslips; a spacer needs to be used to maintain sample uniformity. When the sample is a solid or a gel, then there is no need for the sandwich format. To avoid the distortion of the focused laser beam by the built structures, they are fabricated layer-by-layer bottom up with the last layer attached to the glass coverslip. 8 Laser-Based 3D Printing and Surface Texturing

Figure 8 A typical setup for multiphoton polymerization, consisting of a fs laser, a galvanometric mirror scanner, moving stages, directional and focusing , and a monitoring camera.

Figure 9 MPL experimental procedure (I) beam focusing, (II) laser writing, (III) development, and (IV) completed structure.

After the completion of the photopolymerization process and in order to remove the unphotopolymerized resin, the samples need to be developed like in any lithographic process. The developer used and the time for development will depend on the material. The experimental procedure for fabricating a 3D structure by MPL is shown in Figure 9: (1) The laser beam is tightly focused into the volume of the material. (2) Either the focused beam or the sample move following a computer-generated pattern. (3) After the laser writing of the structure, the sample is immersed into an appropriate developer. (4) The freestanding structure is revealed.

4.3 Materials for Laser Polymerization 4.3.1 Introduction In general, a material suitable for structuring with MPL includes at least two components: (1) a monomer, or a mixture of monomers/oligomers, which will provide the final polymer and (2) a photoinitiator, which will absorb the laser light and provide the active species that will cause this polymerization. Several monomer/oligomer and photoinitiator combinations have been used for this purpose. These are mostly negative photoresists such as hydrogels, acrylate materials,126,127 the epoxy-based photoresist Laser-Based 3D Printing and Surface Texturing 9

SU-8,102 and hybrid materials.103,128 Recently, redox and Diels–Alder photopolymerization have been also been reported.129,130 In the following sections, we will discuss briefly these photoinitiators and materials.

4.3.2 Photoinitiators During polymerization, a monomer is converted into polymer and this transformation can be induced by light. In classic photolithography, a photoinitiator absorbs the light and produces an active species which causes the photopolymerization. In TPP, however, things are more complicated, and these extra requirements must be fulfilled:101,126

• Both the photoinitiator and the monomer/oligomer are transparent at the laser wavelength used, so that the laser beam can be focused inside the volume of the material without being absorbed at the surface. • The monomer/oligomer needs to be transparent at the TPA wavelength (l/2); if it is not, then the photopolymer is likely to be burnt or ablated. • The photoinitiator needs not only to absorb at the two-photon wavelength, but also to have a high two-photon cross-section, a high radical quantum yield and highly-active radical species generated; typically, if any two of these three are large enough, the initiator will normally be efficient for TPP.

In classic lithography, there are two types of photoinitiators: radical photoinitiators and cationic photoinitators. All the materials developed to date for MPL employ radical photoinitiators; these, upon light irradiation, generate free radicals, which initiate a polymerization process of acrylates or vinyl ethers. Cationic initiators are photo-acid generators that produce cations – upon light irradiation and are used for the polymerization of epoxides or vinyl ethers;113 115 to the best of our knowledge, there are no materials to date, specifically developed for MPL employing cationic polymerization, however, it is employed in the most commonly used photolithography resist, SU-8. An effective MPL photoinitiator has a high quantum yield in the generation of the active moieties, high thermal stability and stability in darkness, and is highly soluble in the polymerization medium.131 The most commonly used free-radical photoinitiator is benzophenone and its derivatives.132,133 Nowadays, there is a lot of concentrated effort to synthesize fast and efficient photoinitiators specifically for multiphoton applications.134 In addition, there is a lot of work toward biocompatible photoinitiators, specifically for bio-applications. To this end, classic dyes such as Bengal Rose, Eosin, Nile Red, biomolecules such as flavin mononucleotide, but also novel, synthetic – photoinitiators have been used.127,131,135 141

4.3.3 Organic photopolymers The first materials used for MPL were acrylate photopolymers142 (Figure 10). These materials have several properties which make them attractive for MPL applications: a wide variety of the full composites or their monomers are commercially available; they are transparent at visible and near-infrared wavelengths, and can therefore be processed by IR and green ultrafast lasers; they can be developed in common, nonaggressive solvents such as isopropanol; they can be polymerized fast and with low shrinkage and, after polymerization, they are mechanically and chemically stable. Due to their versatile chemistry, acrylate photopolymers have been used in their pure form but also doped with other materials

to add them functionality. They have been doped with TiO2 to increase their refractive index for photonic crystal applications;143 with CdS nanoparticles for the fabrication of light-emitting 3D structures (Figure 11);144,145 with metal-binding materials to cover them with metals using electroless plating;146,147 with chitosan to make them suitable for bio-applications;148 with Poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) to make them electroluminescent.149

Figure 10 The first 3D structure fabricated by MPL.142 10 Laser-Based 3D Printing and Surface Texturing

Figure 11 A 3D light-emitting structure.144

Figure 12 Microstructures fabricated using SU-8. (a) A photonic woodpile structure108 and (b) A movable microfluidic structure.158

4.3.3.1 SU-8 SU-8 is a negative, epoxy-based photo-resist commonly used for the fabrication of high-aspect ratio structures, using standard contact lithography. Its absorption maximum is at 365 nm. When exposed to the appropriate light, SU-8's long molecular chains cross-link, causing the material to become insoluble to most common solvents. For this to happen, the SU-8 films require postexposure heat treatment. The possibility to structure SU-8 by MPL polymerization was first demonstrated by Witzgall et al., who employed single shots from Ti:Sapphire femtosecond amplifier to fabricate small dots.150 More complex 2D and 3D structures were built by Belfield et al. and Kuebler et al., respectively, who worked on the development of novel, efficient photoinitiators for cationic polymeriza- – tion.151,152 Since then, SU-8 has been used by a large of the research teams involved in MPL fabrication have used SU-8.153 157 SU-8 is thermally stable, transparent in the visible and highly resistant to solvents, acids, and bases. These properties make it suitable for permanent use application; by itself, SU-8 has been employed for the fabrication of photonic (Figure 12(a)),108 – microfluidic (Figure 12(b)),158 160 and biomedical structures,161 while it has also been covered with metal for metamaterial fabrication.

4.3.3.2 Hybrid materials Over the last few years, MPL materials research has focused on the development of photosensitive hybrid composites. Especially silicate-only based photopolymers have proved to be a very popular choice, as they can be commercially available and they combine properties of silicate glasses such as hardness, chemical and thermal stability, and optical transparency with the laser processing at low temperatures of organic polymers; properties impossible to achieve with just inorganic or polymeric – s materials.125,162 178 The most widely used and studied silicate material is the photopolymer ORMOCER , commercially available from Microresist Technologies, Germany, and it has been used for a variety of mostly photonic applications, like the optically active polymer microdisk of Figure 13. Laser-Based 3D Printing and Surface Texturing 11

Figure 13 A microdisk fabricated using ORMOCERs.173

Figure 14 (a) A three-dimensional photonic crystal with sub-100 nm features187 and (b) an IR polarizer.188

s ORMOCER and other silicate-only based hybrid materials have provided the possibility to fabricate high-resolution 3D structures. They do not allow, however, the optimization and ‘fine-tuning’ of the materials properties for specific applications. The versatile chemistry of hybrid composites allows the copolymerization of more than one metal alkoxides; this has been shown to enhance the material’s mechanical stability and allows the modification of its optical properties. There are a few examples of composite photosensitive hybrid materials used in MPL applications;103,133,179,180 Ovsianikov et al. showed that under specific fluence conditions, specific material combinations can be structured into complex 3D structures without shrinkage.181,182 In addition to metal alkoxides, hybrid materials chemistry provides the possibility of the inclusion of functional groups, such as nonlinear optical molecules,183,184 quantum dots,185 and metal-binding materials.186 Zirconium silicates doped with a monomer containing amine moieties were used for the fabrication, for the first time using MPL, of 3D structures selectively metalized with silver (Figure 14).187,188

4.4 Applications 4.4.1 Metamaterials Metamaterials are artificial materials with properties that do not exist in nature; these properties are due to structure and not material composition. Their name derives from the greek word ‘meta,’ which means beyond, because these materials have properties that extend beyond materials found naturally. In metamaterials, an assembly of structures can replace the role that atoms and molecules have in conventional materials, resulting in a composite structure with electromagnetic properties beyond anything that can be found naturally, or chemically synthesized. An excellent tutorial on metamaterials can be found in the website of Prof. David R. Smith (Duke University).189 Photonic metamaterials consist of nanostructured metalo-dielectric subwavelength building components, and allow the reali- zation of many new and unusual optical properties, such as negative refractive index, magnetism at optical frequencies, perfect absorption, and enhanced optical nonlinearities. Several applications of metamaterials have been proposed, including ultrahigh- resolution imaging systems, compact polarization optics, and cloaking devices.190 The realization of these applications requires the fabrication of large-scale metallo-dielectric structures, a very challenging task. There has been some limited research into the direct 12 Laser-Based 3D Printing and Surface Texturing

Figure 15 MPL of positive photoresists and filling with gold using electroplating. A positive photoresist is spun onto a glass substrate covered with a 25-nm thin film of conductive indium-tin oxide (ITO) shown in green. After MPL and development, an array of air helices in a block of polymer is fabricated. After electroplating the polymer is removed by plasma , leading to an array of freestanding 3D gold helices.197 fabrication of metallic 3D structures using multiphoton reduction of metal ions. The quality of the structures, however, has been compromised by the reduced transparency of the metal ion solutions at the laser wavelengths used (500–800 nm).191,192 Metallic – woodpile structures have also been realized experimentally at micron wavelengths using traditional lithographic techniques.193 195 However, lithographic techniques can accommodate only a very limited number of layers, and aligning each layer with the previous one is difficult. MPL is the only inherently 3D fabrication technique, with the potential to fabricate 3D structures, but the majority of the materials structurable by MPL are dielectrics. A popular approach is to use MPL to make dielectric structures, and subsequently metallize them. The most successful approaches and their advantages and disadvantages are listed in below:

1. MPL of positive photoresists and filling with gold using electroplating.196,197 Here, voids are created in a positive tone pho- toresist using MPL; these are subsequently filled with gold using classic electroplating (Figure 15). The main advantage of this technique is that there is no need to remove the photoresist as the refractive index contrast between the gold and the dielectric material is very high. The main disadvantage is that the number of designs that can be structured is limited, as the right apertures of the material removal and gold filling have to be allowed; therefore, this is fundamentally a 2.5D structuring technique. 2. MPL of dielectric structures and nonselective metallization with electroless plating. Here, a standard photolithographic material is used, such as SU-8, for the fabrication of the structures, and subsequently their surface is covered with silver using classic – electroless plating.198 201 Additional processing, to enable the metal adhesion on the surface, is required and the quality, structural integrity, and resolution of the structures depend on the building material and the surface-processing step. The advantage of this technique is that any photopolymer can be used; the disadvantages are that, as the density of the metal- binding sites on the structure cannot be controlled, the metallization quality can vary. In addition, along with the surface of the structures, the substrate is also activated; the metallization is therefore not selective, often requiring an extra step to remove the structures from the metalized substrate.199 3. MPL of dielectric structures and selective metallization with electroless plating. Here, a composite doped with the metal- binding sites is employed for the structure fabrication.147,186,187 The main advantages in this case are that the metallization is selective, and the density and distribution of the metal-binding sites can be controlled. The main disadvantages are that specific metal-binding materials need to be used, and in most cases these were not able to provide the required resolution and structural integrity required for optical metamaterials. Only very recently it was shown that it is possible to use this method for the fabrication of optical nanophotonic devices.187 4. MPL of dielectric structures and metallization with chemical vapor deposition (CVD). Here MPL is used to fabricate 3D structures by any material, typically SU-8.202 The surface of the structures is subsequently activated using plasma etch. Finally, they are covered with silver using CVD. The main advantage of this technique is that any photopolymer can be used, and the resolution and final quality of the structures will depend on that. The main disadvantages are firstly, CVD can only penetrate a small number of structure layers, allowing only a small number of unit cells, and secondly, there is no selectivity; as with the plasma etch, the substrate is activated as well as the structure.

It should also be noted that, recently, there has been reported some work concerning structuring by MPL of transparent conducting materials, such as ionogels.203,204 However, neither the conductivity nor the resolution of these materials are sufficient for applications in optical metamaterials. Laser-Based 3D Printing and Surface Texturing 13

4.4.2 Biomedical applications The biomedical applications of MPL can be broadly divided into two categories: biomedical microdevices and scaffolds for cell culture. The first category, biomedical microdevices, includes devices such as micro-needles for drug delivery, and micromechanic, microfluidic, or optofluidic functional devices.205,206 Despite the enormous potential of 3D nanostructuring in this field, there are only few examples to date, mostly because there has not been much research into the integration of MPL-made structure into existing micromechanical systems. Only recently, Amato et al. employed MPL to build a cell sorter inside a commercial micro- – fluidic chip.207,208 Most other structures reported are freestanding and have not been integrated into a functional device.209 216 Special mention should be made to some recent research introducing magnetic materials into the photopolymerizable resin, making possible the contact-free controlled movement to the fabricated devices.217,218 The second category, MPL scaffolds for cell culture studies and tissue engineering, has seen an enormous growth over the last few years (Figures 16 and 17). Tissue engineering is the discipline applying the principles of engineering and life sciences toward the development of biological substitutes that restore, maintain, or improve tissue function or a whole organ.219 An important part of the development of artificial tissue is the choice of scaffold, as it can influence the attachment, migration, and proliferation of cells. 3D cell cultures offer a realistic environment where the functional properties of cells can be observed – and manipulated.220 222 Although producing scaffolds using laser-based, user controlled manufacturing techniques such as MPL or classic SLA is time consuming and therefore costly, recent studies have shown that they can provide tissue engineering solutions for aligned and complex tissue growth. An important advantage is that a controlled topological environment for cell growth can be achieved (Figure 18). In addition, ordered 3D scaffolds are ideally suited for exploring the relationship between 3D topology and cell proliferation. The possibility to manipulate particles in an aqueous solution in order to produce composite material scaffolds with different biological properties selectively localized in space is another advantage of MPL.223

Figure 16 SEM pictures of a 3D artificial scaffolds. (a) A cubic porous scaffold consisting of horizontal and vertical ring structures in 170-fold magnification and (b) a scaffold cell unit in 550-fold magnification.226

Figure 17 A freestanding micro-scaffold.227 14 Laser-Based 3D Printing and Surface Texturing

Figure 18 Scanning electron microscopy images illustrating the fabrication of 3D composite polymer scaffolds by sequential MPL. (a) In a first step, 3D frameworks consisting of protein-repellent PEG-DA with 4.8% PETA are polymerized and developed. (b) Frameworks are then cast with Ormocomp and cubes (one cube is highlighted in red) are precisely attached to the PEG-DA beams in a second MPL step.228

Recent reviews by Raimondi et al.,224 Ovsianikov et al.,225 and Selimis et al.104 describe in great detail the use of lasers in tissue engineering applications. In this field, most groups have used permanent scaffolds for hard tissue engineering or investigating cell – (including stem cell) growth.148,161,226 237 More recently, there has been a lot of active research into the synthesis of new, biocompatible and biodegradable materials.132,238,239

5 Laser-Based Surface Texturing Techniques

In general, the assortment of the laser materials processing comprises cutting, drilling, welding, surface hardening, alloying, cladding, rapid prototyping, laser-assisted forming, ablation, and shot peening. In our days, the available laser units can deliver beam with wavelength ranging from the UV to the infrared (IR) spectral region owning continuous or pulsed power density – the laser power may extend from low values (BmW) to extremely high ones as 1–100 W – exhibiting not only both spatial and temporal coherence but low divergence as well.240,241 Upon the laser materials processing, a portion of an either pulsed or laser beam – which incidents on the material surface – is absorbed, while the greater one is reflected. The absorbed photon energy may lead to excitation of valence and/or conduction band electrons, excited electron–phonon interaction within 10 11–10 12 s, electron–electron or electron–plasma interaction and electron–hole recombination in a 10 9–10 10 s timescale.242,243 That is to say, extreme heating and cooling rates (typically 103–1010 Ks 1) are achieved within the laser-treated region and, most importantly, besides the high total deposited energy density (103–104 Jcm 2), the bulk material temperature is not significantly affected242,244,245 allowing, thus, the high controlled and precise processing of the material. In any case, toward the achievement of the desired and effective degree of heating and phase transition for the laser surface-processing gamut, the amount of the deposited laser energy density as well as the laser pulse duration should accordingly be picked out. The laser surface processing can be accomplished, grosso modo, either without melting (hardening, bending) requiring low power density, with melting (cladding, welding, cutting) necessitating high power density, or, with vaporization (cutting, drilling, ablation) calling for significantly high power density being deposited at ultrashort pulse duration. In the present section, we shall focus on techniques aiming to the surface roughness modification relying indicatively on and laser texturing. The tailoring of surface roughness and topography is very promising for a plethora of applications and the various results found at the literature for metals, ceramics, glasses, and polymers are presented. In general, the formation mechanism(s) underlying the laser-induced topographies remain under study because of the laser machining (LM) process complexity. They are interpreted in terms of material laser ablation (Figure 19) based on physical models of molecular dynamic – simulations246 248, but they do not account for the poorly understood laser-generated effects of plasma plume formation and expansion, oxide formation, optical property changes, shielding, incubation, self-organization as well as for the medium in which the laser processing takes place.249 Recently, a detailed theoretical approach concerning the development of various types of self-assembled morphologies, ranging from nano-ripples to periodic microgrooves and quasi-periodic micro-spikes has been reported.250 For more than 20 years, laser surface texturing (LST) has proven an efficient and controllable method for producing the texture of micropores on magnetic disk drives251 and ceramics.252 LST is, nowadays, an efficient and clean to the environment technique which enables the precise shape and dimension control of induced micro-patterns. It is considered to be the most precocious – technique for polymer, metal, ceramic, glass, and semiconductor surface engineering toward the control of wettability,253 261 the Laser-Based 3D Printing and Surface Texturing 15

Figure 19 Spectrum of laser-matter interactions leading to material ablation. color marking for counterfeit protection,262,263 the control and improvement of wear resistance and friction coefficient in tri- – – – bological contact,264 268 biomimetic,269 272 and microfluidics273 275 purposes. LST, relying on material ablation, induces a regular array of a great number of micro-dimples on a surface which meets many challenges in various lubricated applications by friction reduction in conformal lubricated contacts.273 Indeed, these dimples serve either as traps for wear debris, as reservoir for lubricant under starved lubrication, or, they may enhance the hydrodynamic lubrication mechanism.274 There have been performed many theoretical and experimental studies concerning the LST contribution to increase the surface adhesion for bonding275 and to the tribological performance of: (1) lubricating sliding contacts pinpointing at first place the fluid film lubrication – e.g., oil, water, – aqueous bovine serum albumin solution264,265,274,276 286 but, the solid film lubrication287,288 and, the dry one289,290 as well, (2) reciprocating automotive components,291,292 and (3) diamond-like coatings.293,294 The UV lasers, offering the advantage of reduced surrounding surface overheating over the IR ones, they have been adopted in the surface topography modification of ceramic and hard coatings.295,296 In advance, LST has also been applied in the fiber laser microgrooves and micro- 297 holes fabrication on the surface of polycrystalline diamond tools, in the tribological properties improvement of Al2O3/Mo laminated composites at room and high temperatures,298 of gray cast iron,299 of titanium alloy combined with diamond-like carbon films coating technology300 and of hard TiAlSiN coatings combined with high-energy ion implantation.301 Besides the many advantages that LST offers, such as noncontact, good flexibility, and high spatial resolution, its broader application is hindered due to the residual stress and the shrinkage cavity generated by thermal effects during LST process. A surface treatment process for improving several mechanical properties, for example, fatigue, durability, corrosion resistance, and wear performance, of metals is the laser shock peening (LSP) one.302 It is a cold machining process which relies on the laser- generated shock waves able to induce severe plastic deformation of materials surface: a section of the material to be peened is paint coated and a thin film of water runs over the surface, while, a laser beam passes through the water layer being focused onto a particular target location causing the paint layer to vaporize into plasma.303 Upon plasma expansion, high pressure (GPa-TPa), short duration (ns), shock waves propagate into the water layer, which confines the energy and increases the pressure pulse intensity within the base material under texturing (Figure 20). – It is the amplitude of the laser-induced shock waves that causes the plastic deformation at the surface.304 306 LSP has been proposed for the fabrication of micro-dimple arrays on copper surface307 and, it can be applied as a post-processing method for the improvement of surface layers mechanical properties since it induces a compressive residual stress within a depth of more than 1 mm. LSP subsequently to LST has been applied in the case of bionic non-smooth 0Cr18Ni9 stainless steel surfaces fabrication308 and the effects of LSP and groove spacing on the their wear behavior have been investigated.302 The range and variety of the laser-induced microstructures on the surfaces of titanium, stainless steel, aluminum, molybdenum, – – – and copper substrates comprise holes,253,309 313 bumps or spikes,254,270,309,314 321 bulges,322 undulating grooves,323 325 dim- ples,322,326,327 honeycombs,328 linked nanostructure-textured mound-shaped microstructures,329 melt-like312,330 and cauliflower- like331 structures. In addition, the formation of laser-induced periodic surface structures, nano-pillars, and micro-columns has also 332 been reported. Other than that, there has been observed the formation of nano-ripples on TiO2 surface following UV femto- second laser irradiation,333 the fs laser nanostructuring of nitrided alloy steels containing Cr,334 the LST of plasma electrolytically oxidized aluminum 6061 alloy for improved hydrophobicity335 and the IR fs laser nano- and micro-texturing of a stainless steel surface combined with chemisorption of an hydrophobic agent.336 In general, the optimization of the laser settings toward a desired surface texture consists a tediously long procedure given the large breadth of the experimental parameters space:249 laser 16 Laser-Based 3D Printing and Surface Texturing

Figure 20 Laser shock peening principle.

fluence value, number of applied laser pulses, pulse duration, repetition rate, scanning speed, and number of scans – varying, at the same time, from metal to metal.337 In several applications (cutting tools, wearing parts, heat engines, medical implants, construction) for which exceptional physical and mechanical properties are required – such as high hardness, advanced thermal resistance, and chemical stability – the ceramics have established a strong foothold.338,339 LM has proven a promising tool for their texturing, especially if one takes into account the fact that conventional techniques can hardly face the issue of their machining due to the brittle and hard nature the – ceramic materials exhibit.339 341 There are mainly three ceramic LM techniques: the laser-assisted chemical etching, the LM and the laser-assisted machining (LAM) one.339,342 In any case, the challenge in the laser micromachining of ceramics lies in the great scattering of common laser wavelengths from their surface restricting, thus, the localized energy absorption.343 According to the laser-assisted chemical etching, laser radiation excites the material surface and/or the molecules of an employed appropriate etchant acting, thus, upon the reaction between the material and the etchant. In such a way, the etching rate is mostly affected by the laser fluence value.344,345 In LM, the absorbed high-energy density of the incident laser photons causes the breakage of the chemical bonds leading to the material ejection from the irradiated area inducing grooves formation on the ceramic surface.346,347 The mechanism underlying the material removal deals with material melting, decomposition, and evaporation. The crucial parameter for the process effec- tiveness employs different properties of the ceramic under treatment each time, such as, reflectivity – especially the multiple reflections in the laser-induced grooves348 thermal conductivity, specific heat, and latent heats of melting and – evaporation.339,349 351 Alumina owing excellent dielectric strength, thermal stability, and conductivity352 has been laser textured 353–356 357–360 343 357,361 352 by CO2 laser, YAG lasers, , KrF laser, and near infrared femtosecond laser. In addition, alumina has been laser machined in three dimensions (3D) by 3D laser carving; an industrial technique for ceramic manufacturing of complex shapes based on 3D CAD model slices362 and by the laser milling technique.363 In advance, it has been proposed, that, after a focused laser beam scribes two groove-cracks at two intersecting surfaces of a rectangular alumina substrate, a defocused one is applied throughout the length of the groove-cracks, generating, thus, great thermal stress leading to the two groove-cracks link together, requiring much lower energy power compared to the conventional LM technique.364,365 Several research publica- tions366,367 report on the use of two laser beams for laser alumina texturing toward the fracture control upon the procedure.

Recently, a new methodology has been mentioned for the fabrication of complex-shaped Al2O3 ceramic parts by combining LM and gelcasting technique.368 According to this technique, the unwanted ceramic powders parts are selectively removed by LM specified by a computer program, and the gelcast Al2O3 green bodies were machined to a designed shape by a CO2 laser. Bearings, rocker arms, piston rings, and cams can be made by LM of for automotive, semiconductor, and aerospace industries 369–371 372 373 374 purposes. CO2 laser in air or under N2,O2 gas atmospheres, Nd:YAG laser and systems have been employed in the LM of silicon nitride aiming to the micro-cracks minimization. 3D structuring of silicon nitride by employing a relatively new machining process, namely, the laser milling technique relying on CAD data has also been reported,375 based on an ablation operation leading to the material removal in a layer-by-layer fashion. Another broadly laser-machined ceramic is the 70,376,377 373,378,379 372 360,378–381 silicon carbide. It has been treated by CO2 laser in air or in N2,O2 gas atmosphere by Nd:YAG laser Laser-Based 3D Printing and Surface Texturing 17 and by KrF laser beam361 leading to the fabrication of controlled wettability textures.382,383 The high thermal conductivity and small thermal expansion mismatch with silicon of aluminum nitride justify its widely use in microelectronic substrates and packages384 and has been laser machined with near-IR and excimer lasers.359,384,385 The fabrication of pH meters, fuel cells, IR radiators, pressure, and oxygen sensors employs the use of zirconia due to is unique characteristics: low thermal conductivity and friction coefficient, high fracture toughness, and good thermal shock resistance.386 As the previously mentioned ceramics, it has 349,379 370,387 377 74 been textured with Nd:YAG laser, CO2 laser, and porous structures are induced upon its XeF laser treatment. LM of blind holes in glass-ceramic sheets have also been applied for inclusion of thermal sensors in the substrate and for fastening the 388 glass-ceramic to other ceramic or metallic components. The LM has also been applied to Al2O3–34 wt.% TiC -the induced 389,390 textured ceramic can be used for high precision parts, such as magnetic head sliders and to Si3N4/TiC ceramic for wavy grooves fabrication391 using Nd:YAG laser.392 As far as it concerns the recent developments in laser texturing of ceramics, there has been reported the formation of alumina precise micro-channels with surface micro-patterns393 by using LM, imprinting, and a technique of interposing laser-formed sacrificial heat-removable sheets in between ceramic sheets; these features can be used under conditions where chemical and heat resistances are necessary.394 In addition, it has been mentioned, that, when cutting hard- brittle ceramic biomaterials to prepare scaffold and implant, or, when sectioning them for porosity evaluation, it is better to choose KrF excimer LM.395 Unlike LM, in LAM the material removal occurs with no melting of the ceramic, but instead, with a conventional tool following the localized heating of the ceramic by the laser beam. The laser beam deforms the brittle behavior of the ceramic to a ductile – one.339,391 This technique has been applied for the laser texturing of silicon nitride,391,393,396 403 zirconia,404,405 magnesia- 406 407–409 410 411 partially-stabilized zirconia, Al2O3, and Inconel 718 with carbide and ceramic inserts. A recent publication offers a comparison of LAM technique with conventional machining methods as well as the recent developments in the field.412 It seems that laser texturing of ceramics meets the increasing demand for the fabrication of parts owing microscale features for semiconductor, biomedical devices, and optics applications.413 The rapid, noncontact, and highly flexible nature of the laser process leads to the accurate dimensional design of micro-featured products,414 for example, of kerfs with depth and width smaller than 100 mm,339 ensuring the employment of ceramics LM in a great variety of tremendous applications. The Nd:YAG lasers are 341 broadly used compared to the CO2 ones due to their high-energy density and small focused spot size. However, in most cases, it is a combination of short pulse duration and short wavelength that guarantees best results in many ceramics, with no evidence of melting.343 New innovations such as short pulsed or femtosecond lasers have drastically decreased the amount of unwanted damage associated with LM.411 In any case, the conditions of the ceramics laser processing to be selected depend strongly on the material properties and the released constituents upon texturing in order the safety and health requirements to be fulfilled. Other than that, surface structures can be produced traditionally using lithography with photoresist coating, exposure, and transfer processes415 or printing, molding, and embossing methods.416,417 Mask-less laser interference lithography (LIL) enables the large-area periodic structuring with simple equipment.418,419 This technique relies on the interference of coherent light beams to form a horizontal standing wave for a grating pattern able to be recorded on a photoresist. The period of the structures depends – on the laser wavelength and the angle(s)420 between the -two or more420 426 incident beams intersection, whereas the laser fluence,427 the polarization424,427 as well as the exposure time428 strongly influence the procedure. By 901 rotation of the sample, and a second exposure, the grid pattern on the photoresist can be defined.419 LIL has been applied in: the preparation of large-area, long-range ordered ZnO429,430 and Si431 nanowires arrays, the micro-lens array fabrication for super-resolution surface nano- patterning,432 the production of magnetic nano-dot arrays433,434 and AlGaAs microdisks,435 the nanostructures fabrication for surface plasmon resonance tuning by gold and silver bimetallic dot arrays,436 the GaAs solar cells improved efficiency by the induced nanopatterning,437 and the Cr, Co, Ni, and Ge high-density dot arrays fabrication.438 One should also mention that there have been textured sixfold symmetry structures on a two-dimensional polymer (SU-8),439 periodic grating structures on poly (methyl-methacrylate) by soft X-ray laser beam,440 and nanostructured holes of hexagonal symmetry on tungsten oxide sur- faces,441 via LIL. LIL on photoresist or as photofluidization lithography has emerged as a technology capable to fabricate templates by irra- diating an azo-polymer or a photoresist with interference patterns obtained by using two or more coherent – typically UV – laser – beams, mostly operating in continuous wave mode.442 444 In addition, the LIL-induced nano-patterning structure, following the pattern transfer, can be used in: optical devices,445 surface plasmon resonance sensors,446,447 metamaterials,448 electrodes for cross- bar electronic devices like memories as well as transparent electrodes for light-emitting-diodes and photovoltaic cells,449 large-area – broadband photonic crystal membrane reflectors on glass substrates,450 biomimetics,451 micro-sieves for micro-filtration,452 454 template for self-assembly,455 field emission panel displays,456 large-area photonic crystal slabs for visible light,457 and micro- and nano- fluidics.458,459 The potential of the LIL technique wider employment in industry has also been mentioned.460 Alternatively, the direct laser interference patterning (DLIP) technique enables a scalable process for polymer substrates – structuring by direct substrate material ablation, requiring only a single processing step without solvents or heating.461 463 In DLIP, similarly to LIL, two or more coherent laser beams are brought to interference with each other on a substrate and different one- and two-dimensional periodic patterns are induced by direct ablation of the substrate – with spatial periods ranging from some hundreds nanometers up to several micrometers – in the absence of any masks or templates. The produced patterns are attributed to the periodic light intensity variation and the high light absorption by the material.464 18 Laser-Based 3D Printing and Surface Texturing

Silicon rewards a special dedication since it is an important semiconductor material with numerous applications of broad – range, and has erstwhile till recently been laser surface textured for microelectronics, solar cells, or biomedical purposes.465 472 In general, micro-, submicro-, and nano-periodic patterns473 (e.g., grooves, pits, spikes) are induced on the Si surface depending on the laser irradiation parameters, the irradiation gas atmosphere or environment474 affecting the melt material distribution following ablation. In advance, near-field enhanced laser irradiation with micro-nano-sized spheres475,476 and near-field scanning optical microscope477,478 have been used. On the contrary, a non-ablative laser surface structuring technique to form a regular array of micro-bumps on Si surface by a 1090 nm continuous wave fiber laser has been proposed,479 besides the high transparency that Si exhibits at this wavelength. The non-ablative laser-induced micro-bumps formation was attributed to localized Si oxidation.480 Recently, there has been reported that upon 1064 nm picosecond laser irradiation of Si, micro-nano hierarchical pore structures are formed on its surface, which are subsequently precisely chemically etched and, finally, light- trapping structures are fabricated.481 LIL has also been applied for the fabrication of two-dimensional photonic crystals in silicon-on-insulator wafers482 and for Si texturing with thoroughly investigated wetting properties of the induced periodical nano-patterns and hierarchical structures.483 More recently, there has been reported the case of moth-like422 as well as of grating, triangle, and square423 surface structures induced on silicon by multi-beam LIL. Concerning the solar cells applications, sub-micrometer conical and pillar-shaped spikes have been fabricated by irradiating hydrogenated amorphous silicon thin films deposited on glass substrates with hundreds of 800 nm-wavelength, 130 fs-duration laser pulses in air and water environments, respectively.484 Lately, there has been reported the laser-induced formation of 300 nm period pore arrays on Si combined with electrochemical etching.485 – Concerning the LST of polymers, a key parameter of the procedure is the polymer molecular weight.486 489 Low molecular weight polymers with short polymer chains are volatile, whereas, the longer ones of the high molecular weight polymers melt. In this case, the material removal is due to evaporation from the polymer substrate – upon an explosive boiling procedure – in an ablation plume full of molecular fragments, ions, free electrons, neutral particles, and products from the chemical reactions between the plume components and the irradiation atmosphere. Among the laser textured polymers, poly(methyl-methacrylate) – (PMMA) has received the lion’s share of the universal applied research interest,258,486,489 501 for example, for microfluidic devices fabrication, by using mainly UV laser radiation. However, from an application point of view, the thermal damage minimization of the surrounding (the laser irradiated area) material is crucial since it deteriorates both the spatial resolution and the ablation quality.502 Consequently, the photochemical processes which lead to the so called ‘cold’ ablation are most preferable. In general, the energy of the UV lasers photons is high enough to induce the direct bond breakage in a strongly absorbing at the irradiation wavelength organic material. It is of great importance the fact that the bulk properties of the laser textured polymers remain intact and, thus, they can be employed in bioengineering applications since the microscale topography may influence several properties, – such as wetting properties260,503 and cell behavior.504 508 At the end, it is worth mentioning the recent laser texturing developments at the sake of tissue engineering applications, since they prove the potential spreading of the technique in this rapidly growing interdisciplinary field: the micro-channels on medical needles,509 the osteoblastic commitment improvement of human mesenchymal stem cells on Ti alloys,510 the neuronal cells cultivation on Si spikes470 the fibroblasts cell culture on laser-made chitosan scaffolds511 and the osteoblast proliferation and morphology on metallic alloys.512

6 Conclusions

This chapter has attempted to cover the laser-based 3D printing and surface texturing techniques which meet the ever increasing challenges and demands of either existing or emerging scientific fields. The superior precision and reproduction, the high working speed, the wide bandwidth of possible materials, and the exceptional degree of flexibility justify the laser fabrication techniques employment in a smorgasbord of human life domains in the industrialized countries: manufacturing, communication, military aspects, health technologies, metrology, reprography, and entertainment. The laser versatility in the ability to create structures of any desired dimension and geometry guarantees the progress of future structuring procedures. Yet, there is a long trail behind the Holy Grail of laser processing capabilities: the controllable fabrication at nanometer scale, which will provide a broad range of technical opportunities with frame and limits left to human imagination.

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