Smart polymers and nanocomposites for 3D and 4D printing

Mojtaba Falahati1*, Parvaneh Ahmadvand2*, Shahriar Safaee1, Yu- Chung Chang1, Zhaoyuan Lyu1, Roland Chen1, Lei Li1, Yuehe Lin1**

1.School of Mechanical and Materials Engineering, Washington State University, Pullman, WA, 99164, USA 2.Department of Chemistry, Washington State University, Pullman, WA, 99164, USA * These authors contributed equally to this work. ** Corresponding author: [email protected]

Abstract

Smart materials, also known as intelligent materials, which are responsive to the external stimuli including heat, moisture, stress, pH, and magnetic fields, have found extensive applications in sensors, actuators, soft robots, medical devices and artificial muscles. Using three-dimensional (3D) printing techniques for fabrication of smart devices allows for complex designs and well-controlled manufacturing processes. 4D printing is attributed to the of smart materials that can be significantly transformed over time. Herein the smart materials including hydrogels and polymeric nanocomposites used in 4D printing were reviewed and the fundamental mechanisms responsible for the functionalities were discussed in detail. In this report, 4D printing of smart systems and their applications in sensors, actuators and biomedical devices were reviewed to provide a deeper understanding of the current development and the future outlook. 1. Introduction

Introduction of single laser-based curing system by Kodama of Nagoya Municipal Industrial Research Institute in 1980’s paved a new avenue to create three-dimensional objects [1]. The technology was later improved and commercialize by Dr. Chuck Hull in 1984 [2]. In 1989, S. Scott Crump invented an extrusion-based method for creating three- dimensional polymeric parts later known as fused deposition modeling [3]. This approach set the foundation of layer-by-layer processes and sparked a new generation of manufacturing and processing technology [4]. Additive manufacturing (AM) or commonly known as 3D printing (3DP) is a type of layer-up technique to create complex and near net shape objects as opposed to the traditional manufacturing processes.

AM created a new era of manufacturing due to its nature of customization, rapidity, material tuning capability, no or less assembly steps, and few geometry limitations. Over the years, AM has also evolved from just a mock-up prototyping method to a practical manufacturing process that is scalable and ready to use. This technology was categorized into a couple of significant types such as fused deposition modeling, direct ink writing, 3D ink-jet printing, , selective laser , binder jetting and et al [5]. Nonetheless, these are all a variation of controllable bottom-up processes that are used to manufacture different structures and shapes which have been extensively developed [6]. Due to the practical limitations engineers and scientists have to compromise the product design in order to fit the needs in different stages from a traditional design standpoint. Design for traditional manufacturing greatly reduces the potential on maximizing efficiency. However, the combined AM technologies boost industries such as aerospace [7]–[11], healthcare [12]–[16], sensing [17], [18], [19]–[23], and consumer products [24]–[28] and provide additional choices and possibilities for more intuitive design so that every product can further be tailored to a specific individual need.

Beyond what is created using AM technologies there are applications such as fabrication of parts with multifunction and even multi-shape throughout its life and manufacturing cycle. To reach a higher performance and functionality; this is where 4D printing of smart materials comes in with its unique shapeshifting property that can be initiated with environmental stimuli [29]. Unlike the original AM techniques which create permanent shapes of three-dimensional objects, 4D printing of materials uses similar additive manufacturing techniques but with an extra transformation process or property change over time [30]. The defining feature of 4D printing in this case is the introduction of “time” in the process as the 4th dimension [31]. Even the printing process is finished, the object will continue evolve through passive or active factors until reaching its final operational shape or properties. Therefore, the biggest advantage is not the scale of applications, but rather is the effective transformation as the operational shape can be pre-programmed in the design to further optimize the process in production and operation cycle.

The smart materials are also known as where the as printed shape or properties are not necessarily the final state, and yet when the final form is needed, they can shapeshift into the preprogrammed form when exposed to external stimuli. Some of the common environmental stimuli in this case are moisture [32], pH [33], magnetic field [34], temperature [35], and light [36]. The stimulus induces either physical or chemical reactions such as phase change, stress relaxation, or molecular motion that ultimately causes shape deformation or reformation. The programmed materials attracted a greater attention in multiple fields from large scale construction to micro/nanoscale manufacturing for applications such as biomedical or .

4D printing have been discussed in various perspectives based on shape-shifting behaviors, techniques, materials and applications [37]–[50]. In this review we will thoroughly go over a variety of stimulus reactive 4D polymers and composite materials used in different additive manufacturing methods in their respective fields and applications. Fig. 1 illustrates potential applications of 4D printed materials and devices. The review as an attempt to reflect the latest developments of smart materials for 3D and 4D printing starts with a brief introduction of different additive manufacturing methods, and then discusses materials with stimuli responsive properties, and finally the practical use of the 4D smart materials in full details. With the comprehensive review we hope to provide the general research communities with a clear scope of the current state of technologies and where future might head.

Fig. 1: Potential applications of 4D printed materials and devices

2. Additive manufacturing techniques

Additive manufacturing, also known as 3D printing, has exceedingly reduced the manufacturing cost and time, especially for geometrically complicated and multi-material components. Although conventional manufacturing techniques are still preferred for mass production of regular products due to the high reliability and low average total cost, they are inefficient approaches for low-volume and custom manufacturing due to a high fixed cost. 3D printing, however, has the potential to be an alternative allowing elaborate designs while cutting tooling costs and skipping the assembly processes. This technology minimizes the number of the parts, reduce the total weight and facilitate the product customization. Using multi-material AM reduces the design complications and provides more functionalities for the final products. On-demand 3D printing also obviates the need for inventory space [51]–[53]. This technology has found extensive multi-scale applications in art, architecture, aerospace, civil engineering, optics, , biology and tissue engineering over the recent decades [54]–[61]. Although the conventional manufacturing techniques including molding, casting, forming and machining are well-suited for mass production, they suffer from limits on design and control of complex and multi-material structures [62], [63]. 3D printing has mitigated these constraints using a well-controlled computer-assisted process. This bottom-up technique is based on the layer-by-layer joining of materials to create an object from a three-dimensional computer model [64], [65]. The 3D model is sliced as a STL file (Surface Tessellation Language) using a slicing software and transferred to a 3D printer to be used as the laser scanning path for polymerization of the base monomer or sintering of the base powder or to define the material deposition path [66], [67]. Several techniques have been developed for additive manufacturing of hydrogels and nanocomposite materials [68]–[71]which can be categorized in three main class including extrusion based, powder based, and photo polymerization based. The main 3D printing techniques are illustrated in Fig. 2 and are briefly discussed in the following.

2.1 Extrusion based 3D printing

The most popular additive manufacturing process is material extrusion on a predetermined pathway. In this extrusion-based technique either a thermoplastic filament is fed into a heated printhead to be melted and printed on a 2 or 3 directional moving bed or viscoelastic material is forced through a moving tiny nozzle using an external pressure to directly write a desired object based on a 3D CAD model. The former is named as Fused Deposition Modeling (FDM), also known as Fused Filament Fabrication (FFF) (Figure 2(a)), and the latter is known as Robocasting or Direct Ink Writing (DIW) (Figure 2(b)) [72]–[74]. FDM printed polymeric parts demonstrate anisotropic properties due to their layered structure. For instance, load carrying printed parts must be loaded considering the print orientation [55], [75], [76]. They present a superior performance in compression rather than tension loading. FDM printed parts are susceptible to structural defects owing to imprecise printing process, filament nonhomogeneity, and imperfect interlayer adhesion[77], [78]. Optimization of the nozzle temperature and printing speed, well- controlled production process of the filament and decreasing the layers thickness can improve the mechanical properties and surface quality [77], [79], [80]. Fabrication of a composite product using FDM technique requires a special attention to feedstock development. The filament should be developed of accurate contents and proper composition to give desirable rheological and mechanical properties. Providing a homogeneous dispersion of reinforcements and inhibiting from void formation during fabrication process of the filament are of the most significant challenges of FDM method [81]. Flowability of the melted composite filament also needs to be controlled through a proper selection of viscosity of the extruded material. The filament should be strong enough to play the role of a piston and push the material during the process. [59], [82]. To form 3D structures using DIW technique, also known as Liquid deposition modeling (LDM), a viscoelastic material is extruded from a moving nozzle with a submillimeter typical diameter on a print tray which can be solidified after printing. The printing process is governed by the rheological properties of the ink which exits from the nozzle in a liquid-like state due to the shear thinning while retaining its shape after printing [83], [84]. Thus, precise control of the ink rheology is essential in this technique and introducing rheology modifiers is inevitable most of the time. A wide range of polymeric materials including thermosets can be printed using this method followed by a post- treatment process [85]. UV-assisted DIW allowed for creating 3D composite structures with outstanding mechanical properties and functional materials using photocurable resins [86]–[89]. However, the printed parts have a lower dimensional precision compared to SLA products. 2.2 Photopolymerization based 3D printing In photopolymerization technique, a photo-reactive liquid resin is polymerized layer by layer using a scanning focused light or a light field based on a sliced 3D model. Laser lithography approaches such as stereolithography (SLA) or vat polymerization are the most common photopolymerization technique. In this technique a cross-section of the part in a photo-cure resin is cured while the surrounding area remains uncured. After complete curing of the first cross-section, the platform is lowered so that a new layer of resin covers the previous layer. This process is continued until the final part is created [67], [90]. Photopolymerization processes can create complex microarchitectures with reasonable resolution. Fine features in printed parts using laser-based SLA printers can be as small as the laser beam diameter (1-2 µ). The printed parts using stereolithography do not have superior mechanical properties [91]. That is why this technique is not common in fabrication of load-carrying parts. Nonetheless, mixing fiber reinforcement in the base starting material can noticeably mitigate this weakness and bring the SLA method back into the manufacturing approaches list for functional components [92]. For fabrication of nanocomposite parts using SLA, a photopolymer should be mixed with nanoparticles which either leads to an improvement in the mechanical properties or introduce new properties to the matrix. This composite has a higher viscosity than the initial polymer which hinder the coverage of the new layer on the previous cured one. Homogeneity is a big challenge in the mixture. In most of the cases the particulates need to be coated with an appropriate surfactant to uniformly become dispersed in the liquid resin. Mixing nanoparticles in the resin introduces some bubbles into the liquid which could turn to pores after curing which deteriorate the mechanical properties of the final product. The particles might partially block the light rays and reduce the light energy absorption by the liquid resin. Hence, curing the composite normally takes longer than pure resin. Post-curing or thermal curing alongside photo-curing can help alleviate the problem [93]–[95]. Another technology of vat polymerization is Digital Light Processing (DLP) which uses a tunable light field instead of a scanning focused light beam. In DLP printers, a digital micromirror device (DMD) including an array of millions of independently rotatable micromirrors can be used to generate an image of a cross-sectional layer on the resin vat and all the target points can be simultaneously polymerized. Thus, this technique is typically faster than SLA [96], [97]. Droplets of photo-cure materials can be also selectively deposited through multiple nozzles or a nozzle array on a substrate to build up a 3D structure. In this technique, which is called inkjet printing, each printed layer of ink is solidified on exposure to the UV light immediately following the printing process [90], [98], [99]. Droplets are formed at the nozzle due to single pressure pulses created by piezoelectric, acoustic, hydrostatic or thermal actuators. Resolution of the printed parts highly depends on the effective parameters on the droplet formation and wetting such as ink density, viscosity, surface tension, nozzle diameter and the pressure pulse [98], [99]. Noncontact nature of inkjet printing, unlike extrusion-based approaches such as DIW, enables printing on a broad range of substrates over large areas. Dispensing mechanisms of the Drop-on-demand (DoD) techniques, however, poses some limitations on the ink viscosity [100]. DIW printers by extruding a continuous filament, based on shear-induced deformation, are preferable for printing viscoelastic inks and high-viscosity fluids. Electrohydrodynamic (EHD) inkjet printing that benefits from an electric filed in the ink dispensing system can be also used for printing viscose inks with no nozzle clogging [101], [102]. Complex and detailed geometries with high resolution down to several microns can be developed using inkjet printers. Multi-material and functionally gradient materials can be printed by independently controlled multiple nozzles. Thus, this technology has extensive applications in printing jewelries, electronic devices, medical devices, and pharmaceutical industries. Low build speed and clogging of the nozzle during jetting higher viscosity inks are of disadvantages of the inkjet 3D printers [37], [59]. 2.3 Powder based 3D printing Powder based techniques employ a laser beam or a chemical binder to locally bind the powder materials layer by layer on a powder bed. One of the most common powder- based methods is binder jetting where a chemical binder is sprayed on a powder bed to attach the particles and form a layer. Selective Laser Sintering/ (SLS/SLM) are other members of this family where a laser beam sinters or melts the binder particles on a predefined path to create individual layers and build up a product [103]–[105]. These techniques have gained ground in processing of composite materials due to the ease of mixing different materials to develop a wide range of properties [106]. In liquid phase sintering or partial melting, structural material is in a solid phase, and the binder is the only part which becomes melted with laser power. However, in complete melting both the powder and binder are liquified. This method is widely used for fabrication of polymer matrix composite while using liquid phase sintering [107]. In some cases, the powder particles are coated with an appropriate surfactant to reduce the surface energy and provide a homogeneous dispersion of nanoparticles in the polymer matrix. Besides, inorganic materials need additional processes to improve the laser- particles interaction [82], [108], [109]. For instance, Zheng et al. [109] modified the alumina nanoparticles with a polystyrene coating to form a core-shell polymeric composite which led to a noticeable improvement in the sintering behavior of the mixed powder and an enhancement of the mechanical properties of the printed objects.

Fig. 2: Schematic representation of 3D printing techniques; a) Fused Deposition Modeling (FDM). b) Direct Ink Writing (DIW). c) Stereolithography (SLA). d) Select Laser Sintering (SLS). e) Binder Jetting. f) Inkjet printing

3. Smart polymers and composites for 4D printing Smart materials can chemically or physically change in response to an environmental stimulus such as humidity, environmental pH, magnetic or electrical field, temperature, and light [110]. Environmentally induced changes in shape, size, surface characteristics, physical phase, solubility, and conductivity can be harnessed in various engineering and medical applications [111]. These responses to the external stimuli give rise to macroscale movements such as shrinkage, swelling, stretching, bending, twisting, and rolling of the material, depending on the geometrical designs, which are essential to the smart systems and devices. Incorporating the stimuli-responsive materials in 3D printing results in products with the ability of shapeshifting over the time. Here we review some of the most important printable smart materials and their applications. 3.1 Moisture-responsive hydrogels Hydrophilic natural or synthetic polymer chains are either physically or chemically bound together to form macromolecular polymer gels known as hydrogels. Hydrogels are able to absorb a huge amount of water in their network structure due to the void imperfections [112]–[114]. Accordingly, hydrogel parts can exhibit dynamic shape change in response to the variations of environmental humidity. Soft rubbery structures resembling living tissues and organs can be fabricated with combination of hydrogels and 3D printing techniques [115]. Several designs have been reported where geometric expansion, stretching, folding and bending were managed to develop smart hydrogel structures. Raviv et al [116]simulated and constructed a multi-material structure that could be stretched and bent in different directions. They printed a strip of hydrophilic expanding material (hydrogel) over a strip of rigid material which demonstrated different behavior when exposed to water. The hydrophilic layer expanded in water and forced a shape change such as stretching or folding into the structure. The stretching length was controlled by choosing the ring radius and the final angle of bending was specified by placing spatially distributed rigid discs as stoppers across the printed strips (Fig. 3a). Switching the material printing order resulted in a bending in the opposite direction. Baker et al. [117] 3D printed trilayer origami-inspired structures including a polyurethane hydrogel core and polyurethane elastomer skins. Discrete localized gaps at the elastomeric skin were made to act as active hinges. Shape changes upon hydration resulted in different complex origami fold pattern based on spatial distribution of hinges (Fig. 3b). Cellulose is the most abundant renewable natural polymer in the earth which has been used as a cost-effective programmable smart material [118]. Hydrogen bonding between molecules results in a crystalline and an amorphous phase in cellulose structure. The water diffusion into a cellulosic material weaken the hydrogen bonding by forming competitive hydrogen bonds which leads to the material swelling and a reversible shape transformation [119], [120]. Gladman et al. [121] harnessed a localized anisotropic swelling behavior to precisely control the dynamic configuration in bilayer structures. Inspired by nastic movements of plants they 3D printed composite hydrogel architectures with aligned cellulose fibrils along prescribed printing pathway. The composite hydrogel were composed of nanofibrillated cellulose (NFC) embedded in an acrylamide matrix such as N,N-dimethylacrylamide (or N-isopropylacrylamide for reversible systems). Shear induced alignment of fibrils during DIW printing led to an anisotropic stiffness of filament and different swelling behaviors in longitudinal and transvers directions (Fig. 3c). Accordingly, by controlling parameters such as filament size, fibrils orientation, printing path and interfilament spacing, programmable shape morphing of the printed architectures can be achieved upon hydration. Mulakkal et al. [120] developed a cellulose-hydrogel composite ink for 3D printing by incorporating cellulose pulp fibers into carboxymethylcellulose (CMC) hydrocolloid. High total volume fractions of cellulose fibers, up to 50% for the dehydrated composite, were achieved and perfect dispersion of fibers in the hydrogel matrix allows for a smooth extrusion during 3D printing process. To impede hydrogen bonding between cellulose pulps, CMC and water, montmorillonite clay platelets were added to the ink. The clay inclusion stabilized the ink formulation for long term storage and also enhanced both rheological properties of the ink for more smooth extrusion and mechanical robustness for shape stability after printing. As-printed objects underwent reversibly programmed transformation upon hydration and dehydration. Zhao et al. [122] benefitted from digital light processing technique and desolvation- induced self-folding to develop three dimensional electronic devices. They printed a photo curable resin (poly (ethylene glycol) diacrylate (PEGDA)) in the form of a flat sheet using a DLP printer and provided a programmed gradient of cross-link density in printed sheet using grayscale light patterns and adding photo-absorber (Sudan I) to the resin. The printed parts were desolved by immersing in water where the uncured oligomers dissolved away, and as-cured flat pattern transformed to an origami shape. 3D origami structures were swollen and recovered to the flat sheet in an appropriate solution such as acetone [123]. A theory for finite deformation describing desolvation and swelling behavior of partially crosslinked components was developed which correlated the structural nonuniformity with the mechanical deformation [124]. This technique was employed to fabricate self-folded 3D LED boxes through direct writing of silver nanoparticles ink on a precursory printed flat sheet (Fig. 3d). Table 1 summarizes the moisture-responsive materials used in 3D printing and their proposed applications.

Table 1: 4D printing of moisture-responsive materials and their applications Printing Materials Printing Method Applications Ref. Vinyl Caprolactam, Polyethylene, Epoxy diacrylate Inkjet printing Self-folding grids [116] oligomer Polyurethane Hydrogel, Polyurethane elastomer FDM Origami Structures [117] N,N-dimethylacrylamide, glucose oxidase, glucose, DIW Biomimetic [121] nanofibrillated cellulose, nanoclay architectures Sodium carboxymethyl cellulose (CMC-Na/CMC), Cotton DIW Shape morphing [120] derived pulp linters, montmorillonite structure poly(ethylene glycol)diacrylate (PEGDA) DLP Gripper, Origami, [122]– 3D Electronic [124] device

Fig. 3: 3D printed moisture-responsive hydrogels. a) 4D printed smart joint that can be folded in water (left) and a self-evolving grid (right) that can deform in convex and concave shapes. The red parts are the swellable hydrogel and the white parts are the rigid polymer [116]. b) Origami-inspired trilayer structure folding upon hydration (core from polyurethane hydrogel and skins from polyurethane elastomer) [117]. C) DIW printing of composite hydrogel with the shear-induced alignment of cellulose fibrils to create anisotropic swelling (left) 3D printed bilayer orchids with different print path in each petal (Right) [121]. d) Box LED device formed after desolving as-print flat structures with DIW printed conducting ink (left). As-print, desolved and swelled flat origami structures (right). [122].

3.2 PH-responsive hydrogels pH-responsive polymers constitute another significant group of smart materials that undergo transitional changes in shape, volume or solubility with variation of pH value [111], [125], [126]. These polymers are polyelectrolytes containing ionizable side groups in their structure that can reversibly receive or release protons in response to pH change. Polyelectrolytes include polyacids or polyanions which contain ionizable acid groups such as carboxylic or sulphonic groups and polybases or polycations with functional base groups such as ammonium salt. Acid groups in polyacids release protons in higher pH levels which cause polymer chains to stretch due to the electrostatic repulsion of the negatively charged groups. By decreasing the pH level, the charged functional groups accept protons and become neutralized. In contrast, basic functional groups like pyridine groups undergo protonation at higher pH levels and are deprotonated in lower pH levels [110], [127]–[132]. pH-responsive polymers have found various applications in soft robots, drug delivery [129], [133], [134], biocatalysts [33], actuators [111], valves [33], bioseperation [135], colloid stabilization[128]. Several extrusion-based AM techniques can be used to print soft materials like hydrogels. A flow regulating valves exhibiting reversible pH-dependent swelling were developed through FDM 3Dprinting of poly(2-vinylpyridine) (P2VP). The filament with optimal mechanical properties and satisfactory printability was produced by adding a thermoplastic like acrylonitrile−butadiene−styrene (ABS) to P2VP. The 3D printed objects turned to a pH-responsive hydrogel through a postprocessing to develop a quaternary compound. The 3D printed P2VP objects were also utilized as catalytic objects using coordinating ability of pyridyl groups to immobilize a metal precursor [33]. Okwuosa et al. [136] engineered and fabricated an enteric-coated gastric-resistant tablet using a dual extrusion-based 3D printer (Fig. 4a). Two different filaments were produced using a hot-melt extruder to feed two nozzles printing core and shell. The core was polyvinylpyrrolidone (PVP) containing drugs like theophylline, budesonide and diclofenac sodium, and shell was composed of a pH-responsive methacrylic acid co- polymer. Insolubility of the shell at low pH values inhibits the drug from releasing in the gastric environment and the solubility at higher pH values allows for delayed release of the drug in the intestinal phase (pH 6.8). A minimum shell thickness of 0.52 mm was required to guarantee the full drug protection from early releasing at acidic environments. Similar idea was used by Larush et al. [137]to 3D print a pH-responsive hydrogel using acrylic acid monomer by a digital light processing to develop a drug- loaded tablet exhibiting swelling behavior and a quick drug release in high pH values. Recently Odent et al. [138] developed a hydrogel-based actuator with a compositional gradient across the thickness from passive (poly(N-isopropylacrylamide) (PNIPAAm)) to active (poly(2-carboxyethylacrylate) (PCEA)) layers towards environmental pH changes. They used resin vat exchange in an SLA printer to reach the different chemical composition of discrete layers which resulted in an anisotropic swelling behavior of the 3D-printed actuator. PCEA (upper layer) swelled in high pH values due to deprotonation of the acid groups while PNIPAAm (lower layer) slightly swelled in an acidic pH. Thus, different volume expansion of discrete layers led to reversible bending actuations (Fig. 4b). 3D printing of natural polymers such as collagens and keratins have recently attracted increasing attention. Collagen has been used as a bioink for 3D bioprinting either alone or combined with different polymers such as alginate [139] and gelatin [140] due to its outstanding biocompatibility [141]. Collagens as a major extracellular matrix protein have several ionizable groups such as hydroxyl and amine groups in its molecular chains. Thus, collagens can exhibit swelling behavior in response to pH changes [39], [131], [142]. Type I Collagen is widely utilized to coat 2D substrates and to construct 3D scaffolds. It holds a gel form at neutral pH and 37℃ and can be dissolved in acidic environment [143]–[146]. Lee et al. [147]used collagen gel as a primary matrix to construct skin tissue through a bioprinting technique. The gelation process was managed with regulating the pH value. They printed protein encapsulated in a cultural medium and used nebulized sodium hydrogen carbonate (NaHCO3) vapor as a cross-linker for collagen to provide a homogeneous gelation. Table 2 summarizes the pH-responsive materials used in 3D printing and their proposed applications.

Table 2: 3D printing of pH-responsive materials and their applications Printing materials Printing method Applications Ref. poly(2-vinylpyridine) (P2VP), ABS FDM Flow-regulating [33] valves Eudragit L100-55, Polyvinylpyrrolidone (PVP, FDM Delayed drug release [136] Theophylline poly(N-isopropylacrylamide) (PNIPAAm), poly(2- SLA Gripper, Actuator [138] carboxyethylacrylate) (PCEA) Collagen, Alginate DIW Tissue engineering [139] Collagen, methacrylated Gelatin (GelMA) Inkjet printing Tissue engineering [140]

Fig. 4: 3D printing of pH-responsive hydrogels. a) Dual extrusion technique to fabricate a core-shell enteric tablet. The core is PVP containing a drug and the shell is composed of Methacrylic polymer which serves as a gastric protector [136]. B) SLA 3D printing of a functionally graded hydrogel-based structure. The chemical composition of discrete layers varies from 100% PCEA at the top layer to 100% PNIPAAm at the bottom layer which exhibit different swelling behavior in response to environmental pH changes. Bidirectional shape shifting of the printed structure over the time in different pH values was observed (Right) [138].

3.3 Electro-magnetic functional materials Metallic materials and structures have always attracted considerable attention in fabrication of smart devices due to their exceptional thermal, electrical and magnetic properties [148]. However, additive manufacturing techniques for metallic materials are consistent with specific alloys and requires costly and complicated equipment[149]. Magnetic polymer nanocomposites have been an alternative over the past few years owing to the outstanding properties such as stability, elasticity, light weight, good biocompatibility, ease of synthesis and flexible processing alongside a spatially and temporally controllable magnetic properties [150]–[156]. Magnetic nanocomposites are developed through embedding the magnetic nanoparticles including metallic and metal oxides in a polymeric matrix [157]–[162]. Magnetic functional materials have applications in shapeshifting systems, and custom reinforced composites. Electric field can also be used for actuation in 3D printed smart systems. Conductive hydrogels and nanocomposites are of most popular materials in soft robotics, tissue engineering and sensors design. Electro-responsive nanocomposite can be developed by embedding conductive additives such as metal nanoparticles [163] and carbon nanomaterials (CNT , , carbon black) [164], [165] in polymeric matrices. Homogeneity of the electrical properties in nanocomposites highly depends on uniform dispersion of the conductive nanoparticles in the matrix. chains such as polypyrrole, polythiophene, polyaniline can also be introduced into hydrogel networks to form electroactive composite hydrogels which provide more homogeneous electrical properties due to the interpenetrating network [166], [167]. Polyelectrolyte hydrogels such as gelatin and chitosan are of other printable electro-responsive materials which are used for 3D printing of soft actuators. These materials are able to present mechanical responses to electrical stimuli in an electrolyte environment [168]–[171]. 3.3.1 Magnetically induced shapeshifting Tunable and rapid remote actuation in different applications such as biological systems is highly favored. Magnetic stimuli can be used to temporally and spatially control the shapeshifting through a physical non-invasive process and without material phase change [172]. Zhu et al. [173] fabricated magnetically responsive 3D structures using a direct writing process which presented a quick response time to an external magnetic field. They incorporated soft iron magnetic nanoparticles into an elastomeric matrix such as poly(dimethyl siloxane) (PDMS) to develop a composite ink with low magnetic coercivity for 4D printing. This technique was employed to create a remotely tunable three-dimensional terahertz (3D-TPC) with a fast response time. Wei et al. [174] developed a photocurable poly (lactic acid) based ink containing magnetic nanoparticles which was able to exhibit a shape-memory behavior. They printed several active shape-changing structures with multifunctional properties using direct ink writing. For instance, alternating magnetic fields were utilized for remote guiding and self-heating of a 3D-printed self-expandable intervascular stent that have a potential application in minimally invasive medical procedures (Fig. 5a). Kim et al. [175] reported direct writing of soft structures with multiple magnetic domains using an elastomer composite ink containing ferromagnetic particles(neodymium-iron-boron). The printed structures were able to demonstrate fast programmed transformations in response to an external magnetic field. The ferromagnetic particles aligned themselves along with the magnetic field, applied by a tunable electromagnet around the printer nozzle, while printing (Fig. 5b (up)). Changing the magnetic field direction over the course of printing resulted in 3D-printed multi-domain magnetic objects. Various 2D and 3D structures with patterned magnetic domains were printed which offered complex predictable shape changes (Fig. 5b (down)). Multidomain shape-shifting soft materials can find potential applications in flexible electronics, soft robotics and biomedical devices. Shinoda et al. [176] adopted a similar approach to fabricate biomimetic actuators such as worm-type- actuator and artificial cilia which exhibited a metachronal wave movement on exposure to a rotational magnetic field. They prepared a magnetic resin by embedding ferromagnetic particles in urethane acrylate monomer to be used in a photolithography based conventional 3D printer. An ordered magnetic anisotropy was incorporated in the printed objects by changing the direction of the applied magnetic field during the printing process. Distinct magnetic domains in the 3D printed object were aligned with the external field which resulted in a prescribed movement (Fig. 5c). A continuous optical 3D printer equipped with a digital micromirror device were employed by Zhu et al. [177] to fabricate a chemically propelled and magnetically steerable detoxifier microswimmer. They printed an artificial microfish with a biomimetic structure and locomotive capabilities using a biocompatible hydrogel such as poly (ethylene glycol) diacrylate (PEGDA) containing functional nanoparticles. They embedded functionalized iron oxide nanoparticles in the head of the hydrogel microfish for magnetic steering and platinum nanoparticles in the tail for chemical propulsion. Hydrogen peroxide was utilized as a power source acting based on a catalytic decomposition. The catalytically produced oxygen bubble at the tail propelled the fish. By incorporating polydiacetylene (PDA) nanoparticles, the hydrogel swimmer was also used for toxin neutralization. Given its technical flexibility and versatile functionality this technique can find applications in functional soft robotics, sensing, actuations and targeted delivery.

Fig. 5: 4D printed magnetic responsive structures a) Intervascular stent printed using DIW expands based on a restricted shape recovery upon heating through an alternating magnetic field, [174]. b) Orientation of ferromagnetic particles in the composite ink by applying an external magnetic field around the nozzle during printing process (up) and two-dimensional printed structures with programmed multiple magnetic domains exhibiting 3D shape change on exposure to an applied magnetic field (down), [175]. c) Setting magnetic anisotropy during the printing process to fabricate a multidomain structure acting as a worm-like soft actuator under the influence of a rotational magnetic field, [176].

3.3.2 Magnetic metamaterials and custom composites 3D printing techniques have been also used for creating metamaterials in various scales featuring light weight and outstanding mechanical, thermal and functional properties. Metamaterials in the form of complex architectures can be found in the nature either in stochastic structures such as bones, teeth, and shells of abalones or in ordered cellular structures such as mollusk’s protective shell and mantis shrimp dactyl club [178], [179]. Functional heterogeneous architectures were developed using multi-material direct writing by Kokkinis et al. [180]. Photocurable resins containing various concentrations of magnetized alumina platelets were 3D printed with local control of composition and magnetic particles orientation by applying an external magnetic field on the deposited ink. Fumed silica nanoparticles were incorporated in the resin solution as a rheological modifier to provide an ink with a rheological behavior ranging from pure Newtonian for particles orientation to viscoelastic for shape control. Another research group utilized an SLA-based magnetic 3D printing to develop customized reinforced architectures using magnetized discontinuous fibers [161], [181]. Ceramic particles like alumina, silica, and calcium phosphate magnetized by adsorption of iron oxide nanoparticles were embedded in a light sensitive resin and were aligned under the influence of an external magnetic field during printing. patterned orientation of the particles is accomplished by selective polymerization of the active voxels in each layer and then shifting the magnetic field for a different particle orientation in the next active voxels. Various complex microarchitectures with different load carrying capacities can be attained using this technique which, for instance, hold a potential application in crack steering (Fig. 6a). A tunable mechanical reversibly responsive to the applied magnetic fields were presented by Jackson et al. [182]. They 3D printed vascular lattices with a photocurable resin using a photochemical AM technique such as large area projection micro-stereolithography (LAPmSL). A magnetorheological (MR) fluid composed of magnetic microparticles dispersed in a nonmagnetic liquid was injected in the core of the printed vasculature (Fig. 6b). The rheological properties of MR fluid could be tuned through variation of the remotely applied magnetic field. The suspended magnetic particles within the polymeric tubes aligned themselves along the magnetic field and formed a needle-like structure which led to an increase in the fluid viscosity and therefore, a rise in the global stiffness of the printed structure filled with MR fluid (Fig. 6c-d). Field responsive mechanical metamaterials (FRMM) can find applications in soft robotics, smart helmet and other wearables. Bastola et al. [183], [184] employed a multi-material extrusion-based printer to develop an MR hybrid elastomer. A UV-curable silicon-based resin and an MR fluid containing carbonyl iron particles were layer by layer printed through two distinctive nozzles. Using this technique, a tunable elastic behavior and damping capacity with the aid of a controllable external magnetic field were achieved. The compression stiffness of MR samples increased by applying a stronger external magnetic field due to the enhanced magnetic particle interaction. Moreover, samples with line-patterned MR fluid exhibited an anisotropic behavior by changing the magnetic field direction. 3D printed MR hybrid elastomer can find application in active vibration control systems. A summary of printable materials responsive to the magnetic stimuli and their applications were provided in Table 3.

Table 3: 3D printing of magnetic-responsive materials and their applications Printing materials Printing method Applications Ref. Matrix: poly(dimethyl siloxane) (PDMS) , DIW Terahertz photonic [173] Additive: soft Fe magnetic nanoparticle (MNP) crystal (3D-TPC) Matrix: Poly(lactic acid) (PLA), Benzophenone (BP), DIW Intravascular stent [174] Additive: Fe3O4 NP Matrix: SE 1700, Ecoflex 00-30 Part B, DIW Magnetic Soft robot, [175] Additive: Fumed silica NP, NdFeB microparticles Soft electronic device Matrix: Urethane acrylate monomer, SLA Biomimetic actuators [176] Additive: Strontium-ferrite and carbonyl iron powder Matrix: Polyurethane acrylate (PUA), DIW Anisotropic [180] Additive: modified alumina platelets composites Matrix: Urethane diacrylate, isobornyl acrylate SLA Bioinspired [161] Additive: modified alumina platelets composites 1,6-hexanediol diacrylate (HDDA) LAPmSL smart mechanical [182] metamaterials Matrix: SS-155 and SS-3006T silicone, DIW Vibration control [184] Additive: Carbonyl iron powders systems Matrix: poly (ethylene glycol) diacrylate, DLP Steerable locomotive [177] Additive: iron oxide NPs

Fig. 6: a) 3D printing of reinforced composites by programmed orientation of magnetic particles in different voxels [161]. b) 3D printed FRMM structure; Filling a printed vasculature with magnetorheological fluid. c) Dependence of the effective stiffness on the magnetic field intensity. d) Loading the FRMM lattice and alignment of the magnetic particles under the influence of the magnetic field [182].

3.4 Temperature responsive polymers and composites 3.4.1 Thermo-responsive shape memory polymers and composites Shape memory polymers (SMPs) are a significant part of stimuli-responsive materials which can restore their original shape in response to an external stimulus such as pressure, light or heat [185]–[188]. This type of smart materials has attracted great attention in design and development of smart devices for sensing, actuation and biotechnology. The synthesis, characterization, application and behavioral mechanisms of SMPs have been elaborately studied over the past few years [189]–[194]. Polymers can inherently exhibit shape memory behavior depending on their rubber elasticity but with various strain recovery rate [195]. Shape memory behavior includes two main steps of programing and recovery. Programing step for temperature responsive SMPs begins with heating above a transition temperature (Tt), which is either a glass transition temperature for amorphous materials or a melting temperature for semi crystalline ones, followed by a deformation process. By cooling down below the Tt the induced shape is maintained while the strain energy of deformation is stored. Increasing the temperature above Tt triggers restoration of the configurational entropy and in turn the primary shape is recovered (recovery step) [191], [192], [195]. Remarkable advances in 3D printing techniques allow printing of various SMPs such as polycaprolactone (PCL) [196]–[198], polyurethane (PU) [199]–[201], polylactic acid (PLA) [202]–[204], acrylate-based and epoxy-based resins [205]–[209] to develop intelligent structures such as actuators, robots and medical devices. Zarek et al. [196] 3D printed several models with complex structures and submillimeter thickness with methacrylated PCL using a commercial SLA 3D printer (Fig. 7a). The dependence of the thermomechanical properties of the resin on the degree of methacrylation were explored on a couple of printed specimens with different degrees of methacrylation. The higher crosslinking densities led to a lower and broader melting transition due to a lower degree of crystallinity. Flexible electrical devices such as temperature sensors were developed by inkjet printing of a conductive ink (silver nanoparticles or carbon nanotubes) onto a printed SMP object which was able to close a circuit through the shape recovery upon heating. Multi-materials with different transition temperatures can be used to print active composite structures with multi-shape memory effects. Wu et al. [210] reported an approach to print a designed multilayer structure composed various digital SMP fibers (DM8530, DM9895) and an elastomeric matrix which underwent multiple thermomechanically programmed shape changes. The CAD design and shape memory behavior of a printed trestle is shown in Fig. 7b. Ge et al. [211], [212] fabricated an intelligent hinge by 3D printing of SMP fibers (termed Gray 60) in an elastomeric matrix (TangoBlack) based on the concept of self-assembling origami. The active composite was printed in the form of a multilayers flat sheet which can be folded into a desired final shape through a thermomechanical programing (Fig. 7c). Various complex configuration including bending, coiling, twisting with spatially varied curvature can be programed in this technique. Shape changes of the printed structures can be regulated by managing the main parameters including material properties of the active composite, 3D printing parameters and thermomechanical loading profile. Functionally graded SMPs which are able to memorize more than one temporary shape can also be used for 3D printing of structures exhibiting multiple shape memory behavior with tuning the environmental stimulus. This kind of SMPs present either a broad range of shape memory transition temperature [213]–[217], or multiple discrete reversible thermomechanical transitions [218]–[224]. Different methods to develop multiple SMPs have been reported including patterned UV polymerization , gradual photo-degradation, and compositional gradient through interdiffusion and co-extrusion [209]. Versatility of multi-material AM techniques have extremely facilitated creating structures with multi- shape memory effect. Materials with spatially varied thermomechanical properties can be 3D printed by control of local compositions and configurations which results in a functional gradient of property distribution and consequently a multi-shape memory effect. Yu et al. [209] utilized this approach to fabricate a hinge-based helical SMP components with sequential shape changes (Fig. 7d). Different UV-curable epoxy resins with distinct glassy transition temperatures were developed through tuning weight ratio of two constituent liquid monomers and were printed using a multi-materials 3D printer based on a predefined distribution of material property. Due to the different transition temperatures of the hinge sections the shape recovery of the hinges occurred in different times. Li et al. [213] combined spatio-assembly of thermoplastic building blocks and a 3D printing technique to develop a multi-shape memory component. Using methyl acrylate- co-styrene (MA-co-St) copolymers they prepared several tri-block chains of St-block-(St- random-MA)-block-St with various compositions of the mid-block. Multiple shape memory effects were achieved through patterned printing of SMPs of different weight ratio of St:MA in the mid-block which exhibited different transition temperatures. Programmed incorporation of one-dimensional fibers in SMPs is another approach to effectively achieve multiple shapeshifting [225]. Inspired by biological materials, Ren et al. [226] recently proposed a novel shape memory composite with a tunable shape memory behavior through incorporating high aspect ratio stiff fibers in the SPM matrix. They utilized an external magnetic field to align the steel fibers in a slurry photosensitive resin during an SLA based 3D printing process. Permanent shape of the printed components was programmed by regulating the angle of fiber orientations. The strategy was based on the orientation-dependent resistance of fibers against an induced shrinkage during a post-curing process either in a UV chamber or a hot water bath. The printed strips with fibers of smaller angles of orientation exhibited milder shrinkages due to the stronger restriction of the fibers. Both shape recovery ratio and shape recovery rate were affected by introducing the fibers to SMP depending on the angle of orientation. Fibers with smaller angle further decreased the recovery rates. This technique was employed for fabrication of a sequentially deployed robotic hand which can found applications in soft robotics and sequential drug delivery (Fig. 7e). Manipulation of 3D printing parameters (such as direction of filament in FDM process) and multilayer architectures to program the shape memory characteristics also reported in other researches [227], [228]. Ding et al. [229] proposed a new 4D printing technique where the printed shape of SMP was temporary and could evolve into a second permanent shape in response to an environmental stimulus. Several predesigned structural elements with complex geometries and a high resolution were printed using an inkjet printer with two different materials including a commercial glassy SMP (VeroClear) and an elastomer (TangoBlack+). After removing the printed parts from the print tray and softening the SMP sections by heating the built-in compressive strain, induced to the elastomeric sections during photopolymerization process, was released and led to a permanent stable shape. The design of the architectures and the control of 3D printing parameters of the composite such as layer printing time was the strategy exploited to integrate the programming phase into the 3D printing process. Multiple shapes can be programmed base on the shape memory properties of the glassy SMP by thermomechanical loadings (Fig. 7f). Based on a similar idea, Wang et al. [230] 3D printed Polyester (PE) fibers on a paper substrate to develop a bilayer soft self-bending mechanism. The stored strain in the printed PE layer was released upon heating above transition temperature which led to a shrinkage in PE fibers in the form of an inward bending. Permanent shape was attained where the contracting force within the PE layer and the resistance of paper layer reached an equilibrium. Different grippers fabricated using this technique were demonstrated in Fig. 7g. 4D printing concept was also utilized to fabricate leaf springs out of multilayer composites with long continuous fibers [231]. Long fibers were embedded in an epoxy resin with a different orientation in each individual layer. The thermal curing process were completed in an autoclave and followed by a cooling process down to the room temperature. Due to the different thermal contractions of printed layers upon cooling, the composite structure underwent a transformation to a curved composite beam which exhibited an acceptable stiffness as a leaf spring. A list of printable SMP-based materials was presented in Table 4. Thermally induced reversible shape changes can be also attained using liquid crystal elastomers (LCEs). LECs undergo contraction/expansion along the mesogen direction through a phase change at the nematic-isotropic temperature (TNI) [232], [233]. The mesogen units alignment can be manipulated during the printing process to program different complex deformation modes. A high operating temperature DIW technique was employed to orient the mesogen units along the print path. Using this approach various 2D architectures were printed which were able to undergo a 2D-to-3D shape morphing in response to heating above TNI [232]. Temperature gradient across the thickness of the printed layers can be used to provide an orientation gradient of the mesogen unites within the printed parts which enables more complicated shape shifting [234].

Fig.7: 4D printing of thermo-responsive SMPs; a) SLA printed shape memory structures with macromethacrylate [196]. b) Design for 4D printing of a multi-material trestle and its shape memory behavior including programming and recovery steps [210]. c) SMP fibers in intelligent hinges of a self-folding cube [212]. d) Sequential folding of a 3D printed helical and interlocking SMP structure with graded hinges [209]. e) Graded bending of the fingers of a printed robotic hand upon curing and the shape recovery in 60℃ water [226]. f) Direct 4D printing of structures and programing into several different configurations [229]. g) Self-bending gripers formed by printing PE on a paper substrate [230].

Table4: 3D printing of shape memory polymers and composites and their applications Printing Materials Printing Method Applications Ref. PCL DLP Electronic devices. [196] Commercial epoxy polymers inkjet Self-adopting structures. [209] Matrix: TangoBlack plus, and PolyJet Soft robots, biomedical devices. [210] Verowhite, Fibers: DM8530, DM9895 Tangoblack, and Verowhite PolyJet Self-assembly structures. [211], [212] poly(St-b-(MA-random-St)-b-St), FDM Multi-stage shape changing [213] poly(St-random-MA) copolymers devices. Clear FLGPCL04, Flexible FLFLGR02 SLA Soft robotics, drug delivery. [226] Steel fibers, fumed silica Polyester printed on paper FDM Soft and printable robots. [227] Tangoblack+ and VeroClear PolyJet Soft robots. [229] Polytester printed on paper FDM Self-adopting robots. [230]

3.4.2 Thermo-responsive hydrogels Temperature is widely used to stimulate programmed shape change in smart mechanisms owing to its convenient application and process control [110]. Thermo- responsive polymers exhibiting volume change or phase transition in response to variations of temperature have found various applications in remote control systems, drug delivery vehicles and on-off switches [235]. Critical solution temperature is an essential feature of thermo-responsive hydrogels [127] which categorize them in two main groups. Hydrogels with lower critical solution temperature (LCST) undergo a phase separation upon heating and Hydrogels with Upper critical solution temperature which are not much common show an opposite behavior and phase-separate upon cooling [236]. PNIPAAm as one of the most popular thermo-responsive hydrogels has been widely used in fabrication of shape-morphing objects and thermally driven actuators. This hydrogel presents a large thermo-reversible volume change in water at LCST (around 33℃). Its molecular behavior in an aqueous environment is highly hydrophilic at the temperatures lower than LCST. The hydrophobic groups outbalance upon heating above LCST and result in a coil-globule transition [237], [238]. Bakarich et al [32] designed a hydrogel using alginate and PNIPAAm to be used in an extrusion-based 3D printer. Increasing the PNIPAAm content in the hydrogel network up to 20% led to a dramatic volume change upon temperature changes. This approach was utilized for fabrication of a smart valve to control the water flow. The 3D printed hydrogel valve automatically closed the hot water flow due to the volume contraction and reopened on the cold-water flow. In another work a hydrophilic long chain polyether-based polyurethane (PEO-PU) was combined with NIPAAm monomer to provide rheological properties required for extrusion-based 3D printing and appropriate mechanical robustness for 3D printed objects. Phase separation during polymerization is a significant challenge which might disrupt the printing process or compromise the mechanical properties. To inhibit phase separation, the printing and curing process are required to be performed at subambient temperature [32]. However, using a different solvent such as ethanol can be a more effective strategy to suppress the LCST phase separation [239].

Han et al. [240] employed a projection micro-stereolithography (PµSL) to print high resolution objects with PNIPAAm combined with N,N'-Methylene-bis(acrylamide) as a cross-linker (Fig. 8a). Effective parameters of the material and process were manipulated to independently control the hydrogel swelling at low temperatures and shrinking at high temperatures. The high molar ratio of the cross-linker to the monomer resulted in a shorter polymer chain length (more crosslinking sites) which entailed a lower degree of swelling. The volume of the shrunken hydrogel above LCST was dependent on the monomer concentration. The swelling behavior of the hydrogel were spatially controlled within printed layers through regulating the curing light intensity using grayscale patterns. An anisotropic swelling behavior (different thermo-responsive behavior in lateral and vertical directions) can be achieved by tuning the layer thickness and providing a gradient of crosslinking density throughout the thickness. Adding a positively charged ionic co-monomer, Methacrylamidopropyltrimethyl-ammonium Chloride (MAPTAC), to the resin led to an increase in hydrophilicity of the polymer chain and consequently the transition temperature was increased. Chen et al. [241] combined 3D printing with electrospinning to provide rapid responses in hydrogel actuators. They prepared a highly porous membrane using electrospinning of an ink composed of NIPPAm and a crosslinker such as acryloylben-zophenone which offered a fast swelling- deswelling in response to the temperature changes. Rigid patterns of PNIPAAm/nanoclay composite were then printed on the membrane that gave rise to an anisotropic swelling behavior of the printed structure. The printed patterns steered the thermally induced shape morphing due to the imposed interlayer and in-plane stresses stemmed from the swelling mismatch.

Incorporating multiple active and passive materials into 3D printing allows for fabrication of components with complex and locally controllable shape shifting. Naficy et al. [242] printed a hybrid bilayer hydrogel to fabricate a self-folding box using NIPAAm and another UV-curable monomer such as 2-hydroxyethyl methacrylate (HEMA) to provide structural integrity (Fig. 8b). The thermo-responsive NIPAAm-PEO-PU ink dissolved in ethanol was printed in the first layer followed by a second layer of the nonactive HEMA-PEO-PU. The printed flat object was automatically folded to a closed box when became hydrated in 20℃ owing to the structural swelling of PNIPAAm below its LCST. Recently, Liu et al. [243] 3D-printed Dual hydrogels with symmetric and alternating segmented tubular structures which exhibited spatially programmed swelling behavior in response to temperature in an aqueous environment (Fig. 8c). They developed two inks, of which one containing PNIPAAm as an active thermo-responsive gel and one containing polyacrylamide (PAAm) as a passive thermally nonresponsive gel to be used in a DIW printer. Laponite nano-clay was introduced into the inks to modify the rheological properties and to provide shear thinning for direct writing process. A cross- linker such as N,N′-methylenebis(acrylamide) was also added to AAm ink to inhibit it from dissolving in water after curing. Various shape changes including axial elongation, radial expansion, and bending were achieved by bioinspired patterned printing of dual hydrogels based on numerical designs and simulations which could find applications in soft robotics and biomedical engineering. Jin et al. [244] added Graphene oxide (GO) to the PNIPAAm-Laponite composite to enhance the temperature responsivity of the hydrogel and to program the shape change (Fig. 8d). GO particles are highly responsive to near- infrared light and act as nano-heaters owing to their photothermal properties and the excellent thermal conductivity. 2D and 3D patterned structures with different shape- shifting behavior were printed using a multi-material extrusion-based printer and a thermal microfluidic valve was developed using this technique.

Chen et al [245] blended NIPAAm with a sheer-thinning carbomer colloid and printed a bioinspired bilayer hydrogel leaf using a DIW printer with two different inks containing NIPAAm and PAAm. Carbomer is an extremely efficient rheology modifier which serves as a support frame material and enable printing multifunctional hydrogels in the air. Using this cytocompatible modifier in 3D printing inks considerably enhances the mechanical properties and the structural fidelity of the printed objects. The bilayer leaves in an aqueous environment at the temperature above LCST exhibited various shape morphing behavior based on their anisotropic structures that stemmed from the different printing paths (Fig. 8e). Shrinkage of PNIPAAm layer and swelling of PAAm layer made as-printed leaf curl in 50℃ water. The rheology modifier of Carbomer enables a wide range of hydrogels to be used as an ink in extrusion-based printers. In another work, viscous polymer solutions containing poly(acrylic acid-co-acrylamide) (P(AAc-co-AAm)) or poly(acrylic acid-co-N-isopropyl acrylamide) (P(AAc-co-NIPAAm)) or both of them were printed using a multi-material DIW printer. The as-printed structures were crosslinked by forming strong metal-coordination complexes in a Ferric solution. The P(AAc-co-NIPAAm) gel contracted within a concentrated saline solution whereas the P(AAc-co-AAm) demonstrated a trivial responsiveness. Patterned stacking of passive and active layers induced internal stresses which allowed for various rapid and programmed shape changes such as bending, twisting and rolling. In the meanwhile, the excellent mechanical properties of the hydrogels and the strong interlayer bonds provided the printed structures with a high loading capacity [246].

Synthesis and printing process of other different thermo-responsive hydrogels have been recently reported in the literatures. Zhang et al. [247] synthesized a dual stimuli- responsive hydrogel that responded to both shear forces and temperature. They used a controlled ring-opening polymerization to develop Poly (isopropyl glycidyl ether)-block- poly (ethylene oxide)-block-poly (isopropyl glycidyl ether) ABA triblock copolymers. This polymer formed a hydrogel in an aqueous environment which exhibited a thermo- reversible shape transition upon LCST of poly (isopropyl glycidyl ether) block. Using a similar approach, ABA triblock copolymers composed of poly(allyl glycidyl ether)-stat- poly(alkyl glycidyl ether)-block-poly(ethylene glycol)-block-poly(allyl glycidyl ether)- stat-poly(alkyl glycidyl ether) were synthesized. Triple stimuli-responsive hydrogels for direct-write 3D printing were attained using these copolymers which responded to temperature, UV light and pressure depending on the composition and the molecular weight of the ‘A’ blocks [248].

Kobayashi et al. [249]reported a strategy to develop a multi-temperature responsive hydrogel-based structure based on copolymerization level and the pendent group chain length. The LCST of poly[oligo (ethylene glycol) methyl ether methacrylate] (POEGMA) gels can be tuned by adjusting the ethylene glycol chain length (Fig. 8f). 3D printed multi- gel structures with multiple prescribed volume transition temperatures have potential applications in biological systems. Lei et al. [250] developed a multifunctional and mechanically compliant skin-inspired sensor by incorporating a 3D printed thermo- responsive hydrogel film with submillimeter resolution into a capacitor circuit. The double network hydrogels were synthesized using a micellar copolymerization process of hydrophobic n-octadecyl acrylate (C18) and N,N-dimethylacrylamide (DMA) in NaCl aqueous solution which resulted in composite networks consisting of physically crosslinked crystalline domains embedded in a covalent network. Biocompatible thermo- responsive hydrogels such as Pluronic F127 [251] and PolyIsoCyanide (PIC) [252] have been also processed for biological application using 3D printing techniques.

Table 5 summarizes the thermo-responsive hydrogels used in and the proposed applications.

Table 5: 3D printing of Thermo-responsive hydrogels and their applications Printing Materials Printing Method Applications Ref. PNIPAAm, Alginate DIW Smart valves and actuators. [32] PNIPAAm, N,N'-Methylene-bis(acrylamide) PµSL Grippers [240] NIPPAm, N,N′- Methylenebisacrylamide, DIW Actuators [241] Laponite, NIPAAm-PEO-PU ink Inkjet Printing Shape morphing [242] structures. Laponite nano-clay, N,N′-methylenebis DIW Grippers [243] (acrylamide), PNIPAAm and PAAm Graphene oxide, PNIPAAm-Laponite DIW Microfluidic valves [244] (P(AAc-co-AAm)), (P(AAc-co-NIPAAm)) DIW Grippers, Actuators [246] n-octadecyl acrylate (C18) and N,N- DIW Wearable sensors. [250] dimethylacrylamide (DMA) Pluronic F127 DIW Tissue Engineering. [251]

Fig. 8: Shape shifting process of thermo-responsive hydrogels; a) Swelling behavior of 3D-Printed part using projection micro-stereolithography [240]. b) 3D-printed bilayer cube including active and passive layers and the thermal self-folding process [242]. c) Various thermally induced shape changes of 3D-printed segmented dual- gel structures based on the swelling of the active segments (red) [243]. d) Incorporation of GO into the hydrogel- based ink to spatially tune the thermal responsiveness in heterogeneous printed structures [244]. e) shape shifting of a bilayer hydrogel-based leaf upon heating according to the printing pathway [245]. f) Tuning the volume transition temperature of POEGMA gels as a function of ethylene glycol chain length and a multi- temperature responsive soft gripper [249].

3.5 Light-responsive materials

Smart systems using light-responsive materials for actuation benefit from several distinctive advantages such as selective ability to pause and resume the actuation, temporal and spatial remote and precise control, and the ability to be precisely tuned [253]. Light is deemed as an effective trigger for smart mechanisms due to three main reasons. First, light can provide a high resolution down to microscale. Second, compared to other stimuli, light can be used in a non-contact mode. Third, intrinsic characteristics of light such as wavelength and intensity can be tuned to control the actuation process. Light is a clean and easy-to-access energy resource. Moreover, light can be used as an indirect and non-contact controllable heating resource only by adjusting the light source parameters [254].

Light responsiveness can be induced with the aid of photo-triggered units by either being chemically bound to polymer chains, being dispersed in composite matrices, or being used for photothermal effects. In general, two different mechanisms of photochemical and photothermal actuation in polymers can be exploited to achieve light-actuated material systems [253]. Both mechanisms are based on reversible actuation which happens in the form of mechanical shape changes. In photothermal systems, the light energy converts to the heat which triggers a mechanical energy. Through dispersion of absorbing fillers or dyes in a polymeric matrix, coating the polymer surface with absorbent materials, or using intrinsic absorption of the base material, photothermal actuation can be initiated [255]. In photochemical mechanisms, light-sensitive groups get involved into reactions such as photodimerization or photoisomerization and generate a mechanical force to drive the shape changing phenomena.

Remarkable versatility of the 3D printing technology in creating freeform structures encouraged researchers to employ these techniques in development of smart photo- actuated systems. The most popular materials in light-actuated polymeric structures can be categorized as hydrogels, shape memory polymers, and liquid crystal polymers.

3.5.1 Photothermal Light-Actuated materials

Plenty of researches in the field of light-actuated polymers involved photothermal effects owing to their simple mechanisms. One of the common light-responsive materials is polystyrene (PS) which can reach its glass transition state upon light absorption and experience a shape shifting process. Lee et al. [256] used this material to fabricate self- folding origami structures by printing black-colored patterns on a PS sheet to spatially control the light absorption. Based on the gradient of the light absorption in thickness they could tune the rate and sequence of deformation in different lines by controlling the line width and darkness (Fig. 9a). Continuum model analyses on deformation of polystyrene sheet upon light exposure can be used for design of various self-folding structures [257] [258].

Light absorbers have been included in other shape-morphing light-actuated structures for local light absorption. Liu et al. [259] employed a desktop printer to pattern a black ink on either side of a prestressed polystyrene shrink sheet (Graffix Shrink film) followed by irradiation of infrared (IR) light. The patterned areas that have a higher light absorbability generated more heat on exposure to the light. Hence, the generated heat resulted in locally varying shrinkage across the Graffix which induced folding in the ink- covered areas as a programmable hinge. In a more advanced work of the same research team, the concept of differential light absorption was utilized to build sequentially self- folding polymeric 2D structures [260].They used a color-based light absorption property in 3D printed structures to induce sequential folding reactions. A LaserJet printer was used to print color patterns on a 2D polymeric substrate (Graffix Shrink film). The printed area acted as programmed hinges due to the localized shrinkage on exposure to the various wavelengths of the light. Different areas reacted to different wavelengths at different time-steps and as a result, they sequentially be deformed as schematically shown in Fig. 9b.

PLA is another popular material in light-responsive structures due to its thermoplastic properties, mechanical strength, and shape recovery abilities. Hua et al. [261] extruded a composite of PLA on a paper substrate using FDM to form light responsive shape morphing structures (Fig. 9c).

Embedding particles with high thermal conductivity [261], [262] or surface plasmon resonance (SRP) effect [263], [264] in polymeric matrices are of the most convenient ways to develop photothermal properties. Hu et al. [265] combined reduced graphite oxide and carbon nanotubes with PDMS to take advantage of the photothermal effect in a bimorph- based rolled robot which crawled when exposed to the light. Their light driven crawler- type robot started expanding as exposed to a lateral illumination. The side facing the light became uncurled and triggered a rolling movement which exposed a new side to the light and the process continued in the same way. Zhang et al. [266] developed composites using PNIPAAm and single-walled carbon nanotubes (SWNT) that could provide a tunable ultrafast photo-responsiveness. Carbon black has been also included in light-actuated systems due to its efficient photo absorbing properties. PU SMPs loaded with carbon black (PUCB) were 3D printed by FDM to form objects with different 3D geometries and thicknesses [267]. For instance, a photo-responsive sunflower was printed using PUCB which showed “blooming” behavior due to its photothermal properties when exposed to the IR light (Fig. 9d).

Hauser et al. [263] introduced gold nanospheres into a liquid crystal polymer and illuminated different patterns of white light on the nanocomposite films to exploit the photothermal effect triggered by SRP of nanoparticles and therefore, induce an out of plane buckling. In another work, Gupta [264]used this technique for selective rupturing of a hydrogel-based 3D printed core/shell capsule.

Fig. 9: a) Thermographic image of light-induced photothermal icosahedral shape deformation of PS sheets [256]. b) Sequential photo-responsive structures; folding of yellow hinge (responded to blue LED) and cyan hinge (responded to red LED) [260]. c) Reversible photo-triggered shape morphing of a 3D printed PLA structure; The flower reversibly changes its shape from bud to bloom upon illumination [261]. d) Transformation of a PUCB sunflower from bud to bloom over the times in response to the light [267]. 3.5.2 Photochemical Light-Actuated materials

Trans-cis isomerization is one of the significant mechanisms in photo-triggered transformation systems. Incorporation of photochromic molecules such as azobenzene in liquid crystal polymers is a common way to trigger light-actuated transformation from flat trans state to bent cis state which results in a macroscopic shapeshifting. Oosten et al. [268] reported an approach in fabrication of light-responsive actuators in the form of microactuators using inkjet printing process followed by post-photopolymerization. They used an ink containing reactive liquid crystal mixed with azobenzene moieties in their inkjet process to build actuators that mimic the natural cilia. Different ink compositions were used to prepare actuators sensitive to different wavelengths of the light as shown in Fig. 10a. Gelebart et al. [269] successfully fabricated a rewritable and reprogrammable dual photo-responsive actuator with liquid crystalline polymers containing a pH-responsive zomerocyanine dye. Upon exposure to acid vapor, this dye can be chemically converted to a new dye responsive to a light of different wavelength. Thus, by using an acidic patterning and based on sensitivity of the different dyes to the different light wavelengths, the programmable actuators are achieved (Fig. 10b). As the acidic patterning is reversible, these actuators can be re-used in different shapes and patterns. Gritsai et al. [270] explored the response of azobenzene-containing polymers to the polarized light both along and perpendicular to the polarization direction. Complicated microstructures, including arrays of posts and rectangular gratings, were fabricated by soft lithography. These structures were deformed into blazed asymmetric patterns when exposed to the polarized light. Based on the polarization direction, the deformation was different, which proved the ability of the proposed process in fabrication of self-adopting optical micro-devices.

Self-assembly of block copolymers to form periodic nanostructures during polymer processing was reported by Boyle [271]. Thermoplastics composed of dendritic block copolymers were used for FDM 3D printing of photonic crystals with different geometries which were able to reflect the lights of visible spectrum. The printed parts exhibited various structural colors which were originated from the periodic nanostructures formed during polymerization. The wavelength of the reflected light tuned by adjusting the molecular weight of the block copolymer. Fig. 10c displays a 3D printed U-shape hallow photonic crystal that act as a light guide.

Frontal photopolymerization (FPP) can be utilized to harvest the mechanical energy of nonuniform polymerization for shapeshifting. In this method, light with strong attenuation can be used to initiate the curing front from one side of a thick polymer film [272]. Once the solidification front is initiated, photocuring in upcoming areas is triggered and propagates all over the polymeric thick film. Zhao et al. [273] exploited FPP and harnessed the shrinkage that induced during the photopolymerization to create origami structures as shown in Fig. 10d. One-side or two-side temporally and spatially controlled light was illuminated on flat layer of acrylate resin along pre-defined grayscale patterns to initiate hinges that underwent shrinkage-induced bending and formed 3D structures. Table 6 summarizes the light-responsive materials, the employed 3D printing techniques and the proposed applications.

Fig. 10: a) Cilia structure made of A3MA polymer floated in water is flapping in response to UV light [268]. b) various light-induced bending of a patterned film in response to lights of different wavelengths. [269]. c) 3D printed photonic crystal acts as a light guide [271]. b) Schematic of fabrication process of origami structures by prescribed two-side illumination and examples with different geometries [273].

Table6: 3D printing of light-responsive materials and their applications Printing Materials Printing Method Applications Ref. polylactic acid (PLA) and multi-walled FDM Biomimetic actuators [261] carbon nanotubes (MWCNTs) and soft robots. Polyurethane (PU), carbon black FDM Biomimetic smart [267] particles devices, soft robots C6M, C6BP, C6BPN, A3MA and DR1A inkjet printing Micro-actuators [268] PEGDA DLP Origami structures [273] PLGA/Gold nanorods LDM Programmable drug [264] delivery devices. Dendritic block copolymer composed of FDM Smart self-adopting [271] a benzyl and alkyl wedge type monomer optical devices.

4. 4D printing multifunctionality Integrating multiple functions in a single material can provide higher levels of processability, tailorability and functionality for the smart systems. This type of materials, known as multifunctional materials, has increasingly attracted attention due to their tremendous impacts on the engineered systems by boosting the efficiency, versatility and flexibility while reducing cost, weight, size and complexity [274], [275]. Various types of multifunctional materials can be found in the nature such as biological materials or can be engineered by architectural design and integration of new functionalities such as healing, sensing, heating into some traditional materials [276]. 3D printing is one of the most promising technology to impart multiple functionalities in geometrically complex products. AM polyvalent techniques allow for architecturally induced functionalities and enables spatially programmed properties through manipulating the additives gradient throughout the printed parts. Although shapeshifting in 3D printed products can develop or enhance special functionalities over time to evoke the concept of 4D printing [277], introduction of time-dependent functionalities such as self-healing, energy harvesting, and color generation in additively manufactured products have also opened a new season in 4D printing multifunctionalities. A number of innovative ideas in 3D and 4D printing of multifunctional materials are reviewed in the following. 4.1 Polymer-based photovoltaic materials Increasing drawbacks of fossil energy such as environmental degradation, rising cost and health issues have turned the world’s attention to a new form of energy coming from natural resources. Among all renewable energies from wind and hydropower to geothermal, solar energy is the most accessible, fastest growing and cleanest energy with a low operating cost [278]. One of the ever-evolving technologies for harnessing this abundant energy is to use photovoltaic (PV) solar system. PV technology utilizes semiconducting materials to convert the sunlight energy to the electricity. The most popular PV material is silicon (Si) in different forms of monocrystalline (ultra-high efficiency), polycrystalline (high efficiency) and amorphous (low efficiency) materials due to its durability and reliability [279]. Perovskite materials being endowed with a large absorption coefficient and a direct band gap are also known as highly efficient PV absorbers [280], [281]. Lower-efficiency materials such as polymer organic materials [282] and quantum dots materials [283]are other materials that can be used in PV solar devices. High cost is still a brake in development of solar cell technology. The main strategies of cost reduction in PV systems center on to improve the system efficiency and to decrease the production cost including the starting materials and the manufacturing process. More than 80% of flat and rigid conventional solar cells are composed of crystalline silicon (c- Si) which is costly to extract [278]. Using thin film solar cells can considerably reduce the cost (due to a drop in material consumption) and facilitate the solar cells application with a higher adaptability. Amorphous silicon (a-Si), perovskites, organic polymers and some mineral compounds consisting of copper, indium cadmium and gallium (such as CIS, CIGS and CdTe) are of commonly used PV thin films. More efficient and less expensive solar panel can be developed using printing techniques which enable using new lightweight materials and innovative designs. Noncontact and selective ink deposition, roll-to-roll process and high resolution of inkjet printing enable production of large flexible sheets of solar cells that can be used on various irregular surfaces [278], [279]. Using inkjet printing process substantially decreases the cost production of thin film solar cells while enhancing their efficiencies. Photoactive materials with a thickness of less than 100 microns can be deposited on either rigid or flexible substrates using the inkjet printers which can be barely achieved with conventional screen printers that use silicon wafers (due to the indirect bandgap of c-Si) [279], [284]. Si-based thin film solar cells are typically fabricated using vacuum deposition and thermal evaporation techniques. Printing Silicon thin films under atmospheric conditions is still a challenge due to the air sensitivity of the precursors. The National Renewable Energy Lab inkjet printed a liquid organic ink, cyclopentasilane (CPS), on a glass substrate in an inert controlled ambient to form an a-Si film using a UV-polymerization process. An optical postprocessing was applied to reach a crystallized silicon film solar cell [285]. Thanks to low-temperatures solution processability, inkjet printing method, however, is widely used to fabricate organic polymer and perovskite solar cells. Polymer- bulk heterojunction (BHJ) solar cells are an attractive group of organic solar cells due to their low manufacturing cost, low weight, low environmental impact, nontoxicity, flexibility, and short energy payback time. Power conversion efficiency (PCE) of BHJ solar cells has substantially increased to more than 17% during the past decade [286]. This layered-structure PV device can be easily produced in large areas and different colors on flexible substrates via roll-to-roll inkjet processes. Conventional BHJ solar cell include a layer of photoactive blend of electron donor and acceptor sandwiched between two hole- collecting and electron-collecting electrodes (Fig. 11). The most common photoactive material is a mixture of a fullerene derivative acceptor such as [6,6]-phenyl-C 61 butyric acid methyl ester (PCBM), and a polymer donor such as poly(3-hexylthiophene-2,5-diyl) (P3HT) [287], [288]. The photoactive ink can be attained using an appropriate solvent to be inkjet printed on different substrates including glass, plastic, and paper. The ink printability, wetting and drying behavior of the printed film can be controlled through the solvent formulation and the print table temperature. The morphology and the interfacial properties of the inkjet printed PV film are also highly dependent on the solution formulation and the inkjet latency time (gelation effect). The inkjet latency time of the photoactive blend can be controlled by adjusting the regioregularity of the polymer donor (P3HT) [282], [284]. Inkjet printing is also vastly used in production of organic-inorganic perovskite solar cells that have been featured as a promising candidate in self-power systems such as wearable electronics due to the rapid growth of their PCE (up to 23%) [281], [289].

Fig.11: a) Inkjet printing of the organic photoactive film, b) Typical structure of an organic PV solar cell device [288].

4.2 Polymeric Self-healing materials Material degradation in harsh environment, fatigue and other unintended failure mechanisms can initiate microscopic damages which undermine the system functionalities and, if remained unattended, would shorten the operational lifetime. In long-term engineering applications where the damage detection and repair demand complex and expensive systems, using self-healing materials can be a viable survival strategy without any external intervention [290], [291]. These intelligent materials are able to autonomously repair the mechanical damages and restore the material functionalities. Healing mechanisms in polymeric self-healing materials can be categorized as intrinsic repair where the material repeatedly recover the damages by itself and extrinsic repair where a healing agent needs to be intentionally embedded in the materials. Self-healing process in some polymer materials can be activated in response to external stimuli such as heat or light [290], [292]. The dynamic linkages in intrinsic healing mode arise from either supramolecular interactions such as hydrogen bonding, metal coordination, π-π stacking, and host-guest interaction or reversible covalent bonds such as Diels–Alder reactions, imine, desulfide and acylhydrazone bonds [293]. In extrinsic mechanisms, however, the healing agents in the form of microcapsules or vascular networks are surrounded by the polymer matrix. In the event of the material damages the healing agent would be released into the crack planes and prompt the healing process as shown in Fig. 12 [294].

Fig. 12: Self-healing mechanisms in polymeric materials: a) Capsule-based extrinsic self-healing, b) Vascular- based extrinsic self-healing, c) Intrinsic self-healing [294].

Although self-healing materials are highly desired due to the autonomic healing capability their synthesis and design still remained a major challenge. 3D printing technology has recently played a significant role in bridging this gap by enabling the sophisticated designs and controllable fabrication processes. For instance, 3D printing techniques including DIW, SLA, and inkjet printing were effectively utilized to develop more efficient vascular networks within the polymeric structures using sacrificial scaffolds to facilitate the healing agents flow over the intended areas [295], [296]. Extrinsic graded healing functionality could be induced in the complex structures with a spatial gradient of susceptibility by control of size and distribution of vascular network throughout the material during printing process [297]. Various approaches of topology optimization, evolutionary algorithm and genetic algorithm can be used to design the flow network based on the available 3D printing technology [298]. Process design and simulation capabilities of AM approaches allow for prediction and control of the mechanical properties and the material functionalities before damage occurrence and after healing process. 3D printing has been recently employed to form capsule-based extrinsically self-healing structures. Sanders et al synthesized urea-formaldehyde microcapsules containing anisole and 5% PMMA as a healing agent and integrated into a UV-curable resin to be used in an SLA printer. PMMA participated in a solvent-welding based process and improved the healing functionality of the material. Damage recovery at room temperature was achieved up to 87% using this approach [299]. UV-assisted DIW method was employed to print an elastomeric interpenetrating polymer network with intrinsic self-healing property through embedding a semi-crystalline polycaprolactone (PCL). PCL has a great miscibility with (meth)acrylate-based resins and imparts thermally triggered shape memory and self-healing functionalities to the 4D- printed structures while enhancing the mechanical properties [87]. A similar idea was adopted in a DLP process to 4D print high resolution self-healing SMP structures which were able to recover up to 90% of the mechanical properties in a damage-healing process (Fig. 13) [300]. Invernizzi et al [301] prepared a UV curable ink by co-crosslinking PCL with 2-ureido-4[1H]-pyrimidinone (UPy) units to impart an intrinsic self-healing property, based on reversible supramolecular hydrogen bonding between UPy units, to the DLP- based 4D printed structures. Although supramolecular interaction provides a superior processability and rheological properties desired for 3D printing fails to endow required stability and strength in load- carrying and long-term applications. Design of printable Self-healing materials based on dynamic covalent bonds is highly demanded and holds a great potential for the future researches. Nadgorny et al synthesized a covalently cross-linked gel exhibiting self- healing property due to the reversible imine bonds. Benzaldehyde-functionalized poly(2- hydroxyethyl methacrylate) (PHEMA) was crosslinked with ethylenediamine (EDA) resulting in an extrudable ink with tunable rheological properties. The 4D printed objects with this gel recovered from induce mechanical damages up to 98% [302]. Li et al recently reported preparation of a PU-based photo-curable resin containing dynamic disulfide bonds. This resin was endowed with an excellent fluidity, processability and high curing rate consistent with DLP printers and the healing efficiency of 95% [303]. Table 7 summarizes some of the self-healing materials created using 3D printing techniques. Table7: Self-healing materials in 3D printing, healing mechanism and their applications Printing materials Printing Healing mechanism Healing Application Ref method efficiency Wax, Pluronic F127 DIW Extrinsic (vascular) Up to 100 Coating protection [295] Urea-formaldehyde SLA Extrinsic (capsule-based) 87 Composite structures [299] microcapsules and PUHC resin Aliphatic urethane DIW Intrinsic (Thermoplastic ~30 N/A [87] diacrylate, n-butyl dispersion) acrylate, PCL BMA, PEGDMA, TPO DLP Intrinsic (Thermoplastic 90 Gripper, Stent [300] PCL dispersion) PCL/UPy-based DLP Intrinsic (hydrogen ~50 Soft actuators [301] polymer bonding) functionalized DIW Intrinsic (imine bonds) 98 Self-rolling objects [302] PHEMA, EDA PUSA, PUHA, HEA DLP Intrinsic (Disulfide bonds) 95 Honeycomb structures [303]

Fig. 13: 4D printed self-healing SMP structures using a DLP printer: (a) Chemical structures of the self-healing ink components. (b)Double-network formation by UV light and heating followed by cooling down to the room temperature. (c) High-resolution 4D printed structures, (d) shape memory effect and (e) self-healing process at 80°C in 5 min [300].

4.3 4D Bioprinting AM technology can be utilized for printing biocompatible materials or living cells to fabricate biological structures capable of shape morphing or functionality transformation in response to external stimuli such as humidity, temperature, light and pH level [304], [305]. 4D bioprinting has found various applications in drug delivery [136], wound therapy [306], tissue engineering [307], and organ regeneration [42] by overcoming the limitations of the traditional manufacturing techniques. The 4th dimension is attributed to either shape shifting for biomedical applications or cell maturation on 3D printed tissues. For instance, A self-folding bilayers (PNIPAAm and PCL) construct was prepared for temperature-induced encapsulation/release of yeast cells [308]. pH-induced dissolution of a 3D printed drug-loaded capsule resulted in a delayed drug release in the intestinal phase [136]. Mirani et al [306] used 4D bioprinting for controlled drug delivery and wound therapy by printing drug-loaded and pH-responsive alginate scaffold arrays on a hydrogel-based dressing as shown in Fig. 14 (a). This multifunctional dressing was able to monitor the infection level of the wound and to release antibiotics at the wound site based on pH-induced color change and swelling (pH level variation posed by the bacterial infection.). 3D printed cell-laden tissue constructs such as bones or blood vessels need some maturation postprocesses including cell self-organization, cellular coating and matrix deposition to be suited for the real applications [49]. Integration of self-maturation capability in such constructs gave birth to the cellular 4D bioprinting. Cell viability and functionality conservation during the printing and postprinting are significant challenges of cellular bioprinting. Rheological properties of the bioink need to be precisely selected to assure a homogenous cell distribution (which could be undermined with a low viscosity) and cell viability (which can be affected by high viscosities). The bioink composition must be able to provide required mechanical properties for the tissue construct and support the cell proliferation and maturation [309]. Hydrogels such as alginate and hyaluronic acid are of the most popular materials in cellular bioprinting which can effectively mimic the natural extracellular matrix (ECM) and support the cell viability [43], [251], [310]. Kirrilova et al [310] synthesized photo-curable hydrogels using methacrylated alginate and hyaluronic acid and prepared a bioink by suspending mouse bone marrow stromal cells into the aqueous solution of synthesized polymers to be used in a DIW process. Cell- laden self-folding films were printed, with a good cell viability, which were responsive to humidity and PBS (phosphate-buffered saline). Gradient of light absorbance during the curing process across the film thickness at the presence of living cells led to a crosslinking gradient. Thus, the printed film underwent an ununiform lateral swelling in cell culture media and transformed to a tube shape while saving the cells for 7 days (Fig. 14 (b)). Bioactive stiff particles such as mesoporous bioactive glass (MBG) can be incorporated in hydrogels for development of hierarchical scaffolds for bone regeneration with enhanced mechanical properties, cell infiltration and activity [311]. Although noticeable advances have been reported on drug delivery systems and tissue engineering using 4D bioprinting so far, this technology is still far from meeting the clinical requirements due to the high sensitivity and selectivity of the clinical applications.

Fig. 14: a) 3D printed pH-responsive sensor and drug-loaded scaffold for wound infection monitoring and controlled drug delivery [306]. b) 4D bioprinting of cell-laden self-folding vessel methacrylated alginate and hyaluronic acid [310].

5. Design for 4D printing Although conventional methodologies of design for manufacturing are well stablished, they are not well-suited for shapeshifting products. Design for 4D printing needs to be considered in two different perspectives: design for AM and design for smart materials. Effective designs for AM center on two main goals of maximizing manufacturability and functionality of the products. The versatility of the AM techniques allows more complexities in design in terms of geometry, material and functionality. Taking advantage of these design possibilities for 4D printing, based on the optimized geometry and the minimum weight, enables manufacturing of more efficient products with fast and optimal stimuli-induced responses. The capabilities and limitations of AM techniques were elaborately reviewed in the literature on which the most appropriate technique can be selected given the material and product requirements.

Smart materials are the essential constituents of every 4D printing process and the main source of the printed parts functionality. However, material distribution, either spatial distribution of the smart material or the combination of smart and conventional materials, plays the major role in attaining the prescribed functionalities. Distribution design requires a topology optimization and computational modeling to reach a programed behavior of the final product. Various frameworks for modeling and optimization of materials distribution such as multiple-material topology optimization [312], structural lattice optimization with optimality criteria [313], and active rod structures optimization [314] were reported. A voxel-based modeling strategy can also be adopted to predict the behavior of a given smart material or to define the appropriate material distribution corresponding to a desired response [315], [316]. In this technique, the design space is first discretized into small equal voxels which are connected by 3D beams. Each voxel is defined with a homogeneous isotropic material with fully characterized mechanical properties. To compute the voxelized model deformations a frame model is generated using a skinning technique in which the material properties of each beam member are assigned as composite values of two end voxels properties. Voxel- based modeling and simulation of smart materials is illustrated in Fig. 15. A design tool, known as VoxSmart, based on a graphical algorithm editor, Grasshopper® (GH) and a CAD modeling software, Rhinoceros© (RH), was introduced that can be helpful in simulation of the material distribution and modeling of the object deformation although it still needs improvements in voxelization process and the stimuli simulation [316].

Pattern-based design is another approach that can be used to obtain a prescribed transformation in a 4D printing process. In this explorative technique, different patterns of materials are simulated of which one specific material distribution will fulfill the desired shape change. As an instance, the heat absorbance of Shrink Film substrates with different printed patterns of light absorbing materials were simulated to predict the heat- induced deformation [317]. Different patterns of the light absorbent, which acts as an actuator hinge, result in distinct heat distributions causing different bending behaviors as shown in Fig. 16.

Fig. 15: Voxel-based modeling and simulation of smart materials for deformation prediction [315].

Fig. 16: Simulation of temperature distributions after 40 seconds of IR irradiation, and thermal images of the film deformation with different hinge patterns of honeycomb, zigzag, and square [317].

6. Outlook and perspective 4D printing technology currently suffers from substantial limitations in different areas of design, process, and materials. Research and development of manufacturing processes and techniques, design of stimuli-responsive materials, design and modeling of smart structures are of the significant challenges that need a greater attention. Fig. 17 summarizes the research gaps in 4D printing that need to be addressed. In the following, some of the main concerns and the future opportunities are discussed.

Although 3D printing technology opened a new season in manufacturing of multifunctional devices and structures, there are limitations among the currently available 3D printing processes. The extrusion-based techniques which are widely used for multiscale and multi-material 4D printing of hydrogels and polymeric composites have low production rates, limited structural fidelity, and anisotropic mechanical properties of the printed components. PolyJet as an alternative is restricted to a small number of resins while requiring high cost equipment[210]. Photopolymerization techniques such as SLA, DLP and Continuous Liquid Interface Production (CLIP) are capable of rapid production of high-resolution structures in an efficient process [177], [182], [196], [240], [273] but for a limited number of materials and smaller volume of print.

By tuning the photopolymerization process a prescribed shapeshifting can be induced in as-printed parts. However, fabricating multi-material structures using vat polymerization techniques is still a challenge.

Fig. 17: Summary of research gaps in 4D printing

Multi-material AM processes are still suffering from a couple of significant challenges. STL files as the input data for 3D printers define only the surface geometry and lack materials information. New file formats, such as AMF (Additive Manufacturing File Format) and 3MF (3D Manufacturing Format), are being developed and will enable the storage and communication of comprehensive information including the spatial materials description and the mix ratio of functionally graded materials. However, incorporation of these new file formats to 3D printing processes is not fully supported yet. Bonding strength of dissimilar materials in multi-material 3D printing which is essential for the functionality of the printed parts also poses irritating limitations. Development of hybrid 3D printing systems can help improvement of layers bonding and materials delivery in the build process at the same time [51].

Not only a nonstop optimization and improvement of the current techniques but development of new sophisticated AM methods is inevitable to fulfill the increasing demands on multifunctional systems. Researcher’s continuous efforts promises outstanding developments of modern printing technologies that enable users to overcome the limitations of the traditional printers.

Development of stimuli-responsive polymers and composites have been targeted in numerous researches over the past decades. Despite of remarkable progresses made to date, existing functional materials are still far from satisfying all 4D printing functional demands. Most of the stimuli responsiveness are limited to a singular movement such as swelling, shrinkage, bending and twisting. Multiple shape changes which are desired in automatic multifunctional systems with an effective control over deformation rate and internal stress need more explorations. For instance, multiple compound reversible folding and twisting events are often required for soft robots to accomplish their missions. Environmental stability of the material and repeatability of the actuation process are crucial for different applications. Chemical or thermal stimuli can gradually destabilize the materials functionalities and therefore undermine the process repeatability. In addition, physical factors such as shape, size and the geometrical complexities can affect the material responsivity which require some considerations for distinct applications [318]. Mechanical robustness and rapid response are of the most significant issues in printed smart structures. SMPs and hydrogels that constitute a major portion of smart printable materials demand additional complementary processes to allow for tailoring desired properties for a given application. For instance, using multi- materials or reinforced composites can result in a superior robustness as long as the additives do not disrupt the material responsiveness. The programing step in most SMP composites require triggering mechanical stresses which is a significant challenge in some applications such as biomedical devices. Reversible shape memory properties obviate the need for the programing step which enhance the functionality of the 4D printed products. Including the programing step in the printed SMP parts during printing process can be an effective strategy which require a sophisticated process design and an in-depth understanding of the stimulation mechanisms. Reversibility can be also attained by utilizing more than one stimulus, for instance hydration and heating [242], [319], in printed structures composed of multiple materials responsive to different stimuli. Incorporating different shapeshifting mechanisms in printed structures enables a reversible self-switching between desired configurations.

Shapeshifting in smart structures highly relies on an accurate 4D designs and modeling of as-printed parts [175]. The shape change prediction, target shape optimization and transformation energy control require a thorough understanding of the materials’ characteristics so that the smart structures can be analyzed or numerically modeled. Development of integrated design approaches that include material characteristics, modeling of the material response, as well as the manufacturing process (which may have an impact on the resulting material responses) is imperative for the future advancements. Besides, complex shape morphing well suited to the intricate smart systems such as robots and biomedical devices need more innovative designs with the aid of computer software. Exploring and characterizing the natural organisms broaden designers’ horizon in design and development of versatile smart systems. Biomimetic or bioinspired designs make it possible for engineers to include mutually exclusive properties in the printed structures and provide with more stable and high-performance functional systems [226], [245].

7. Conclusion 4D printing paved an avenue for development of smart devices in diverse areas including aerospace, engineering, medical science, and biology. Using 3D printing technology alongside smart materials such as functional nanocomposites and hydrogels provided on-demand fabrication of complex-architecture smart devices with customized designs. Several extrusion-based, photopolymerization-based and powder-based AM techniques applicable to 4D printing were reviewed in this paper. Functional materials responsive to different external stimuli such as moisture, environmental pH, magnetic field, temperature, and light were discussed in terms of printing process, design and potential applications.

Although 4D printing benefits from state-of-the-art 3D printing techniques it still suffers from several challenges which are still targets of many scientific researches. Limitations of 3D printing techniques, narrow range of printable stimuli-responsive materials and 4D-oriented applicable design and modeling can be named as main challenges demanding more researches and developments. This novel technology is still in its infancy and away from fulfilment of all commercial and industrial requirements considering increasing demands of smart systems in various applications. Fortunately, numerous research groups have focused on this embryonic technology and promising progression has been achieved. Increasing advancement of 3D printing technology and development of novel smart materials and sophisticated modeling techniques will resolve most of current challenges in the not too distant future.

References [1] T. Wohlers and T. Gornet, “History of additive manufacturing,” p. 34, 2014. [2] C. W. Hull, “Apparatus for production of three-dimensional objects by stereolithography,” US4575330A, Mar. 11, 1986. [3] S. S. Crump, “Apparatus and method for creating three-dimensional objects,” US5121329A, Jun. 09, 1992. [4] K. V. Wong and A. Hernandez, “A Review of Additive Manufacturing,” International Scholarly Research Notices, 2012. https://www.hindawi.com/journals/isrn/2012/208760/ (accessed Jul. 29, 2019). [5] F42 Committee, “Terminology for Additive Manufacturing Technologies,” ASTM International. doi: 10.1520/F2792-12A. [6] J. Edgar and S. Tint, “‘Additive Manufacturing Technologies: 3D Printing, , and Direct Digital Manufacturing’, 2nd Edition,” Johns. Matthey Technol. Rev., vol. 59, no. 3, pp. 193–198, Jul. 2015, doi: 10.1595/205651315X688406. [7] L. Brett, “Additive Manufacturing in Aerospace Examples and Research Outlook,” Bridge Front. Eng., vol. 42, no. 1, pp. 13–19, Spring 2012. [8] A. Uriondo, M. Esperon-Miguez, and S. Perinpanayagam, “The present and future of additive manufacturing in the aerospace sector: A review of important aspects,” Proc. Inst. Mech. Eng. Part G J. Aerosp. Eng., vol. 229, no. 11, pp. 2132–2147, Sep. 2015, doi: 10.1177/0954410014568797. [9] R. Dehoff, C. Duty, W. Peter, Y. Yamamoto, W. Chen, and C. Blue, “Case Study: Additive Manufacturing of Aerospace Brackets,” Adv. Mater. Process., pp. 19–22, Mar. 2013. [10] R. Huang et al., “Energy and emissions saving potential of additive manufacturing: the case of lightweight aircraft components,” J. Clean. Prod., vol. 135, pp. 1559–1570, Nov. 2016, doi: 10.1016/j.jclepro.2015.04.109. [11] R. Liu, Z. Wang, T. Sparks, F. Liou, and J. Newkirk, “13 - Aerospace applications of laser additive manufacturing,” in Laser Additive Manufacturing, M. Brandt, Ed. Woodhead Publishing, 2017, pp. 351–371. [12] M. Vaezi and S. Yang, “Extrusion-based additive manufacturing of PEEK for biomedical applications,” Virtual Phys. Prototyp., vol. 10, no. 3, pp. 123–135, Jul. 2015, doi: 10.1080/17452759.2015.1097053. [13] Y.-L. Hao, S.-J. Li, and R. Yang, “Biomedical titanium alloys and their additive manufacturing,” Rare Met., vol. 35, no. 9, pp. 661–671, Sep. 2016, doi: 10.1007/s12598-016-0793-5. [14] J. Parthasarathy, B. Starly, and S. Raman, “A design for the additive manufacture of functionally graded porous structures with tailored mechanical properties for biomedical applications,” J. Manuf. Process., vol. 13, no. 2, pp. 160–170, Aug. 2011, doi: 10.1016/j.jmapro.2011.01.004. [15] S. Singh and S. Ramakrishna, “Biomedical applications of additive manufacturing: Present and future,” Curr. Opin. Biomed. Eng., vol. 2, pp. 105–115, Jun. 2017, doi: 10.1016/j.cobme.2017.05.006. [16] S. Nesaei et al., “Micro additive manufacturing of glucose biosensors: A feasibility study,” Anal. Chim. Acta, vol. 1043, pp. 142–149, Dec. 2018, doi: 10.1016/j.aca.2018.09.012. [17] Y. Wang et al., “A 3D-Printed, Portable, Optical-Sensing Platform for Smartphones Capable of Detecting the Herbicide 2,4-Dichlorophenoxyacetic Acid,” Anal. Chem., vol. 89, no. 17, pp. 9339–9346, Sep. 2017, doi: 10.1021/acs.analchem.7b02139. [18] H. Yang, T. Rahman, D. Du, R. Panat, and Y. Lin, “3-D Printed Adjustable Microelectrode Arrays for Electrochemical Sensing and Biosensing,” Sens. Actuators B Chem., vol. 230, pp. 600–606, 2016, doi: 10.1016/j.snb.2016.02.113. [19] G. D. Goh, S. Agarwala, G. L. Goh, V. Dikshit, S. L. Sing, and W. Y. Yeong, “Additive manufacturing in unmanned aerial vehicles (UAVs): Challenges and potential,” Aerosp. Sci. Technol., vol. 63, pp. 140–151, Apr. 2017, doi: 10.1016/j.ast.2016.12.019. [20] M. Saari, B. Cox, E. Richer, P. S. Krueger, and A. L. Cohen, “Fiber Encapsulation Additive Manufacturing: An Enabling Technology for 3D Printing of Electromechanical Devices and Robotic Components,” 3D Print. Addit. Manuf., vol. 2, no. 1, pp. 32–39, Mar. 2015, doi: 10.1089/3dp.2015.0003. [21] D.-A. Türk, L. Triebe, and M. Meboldt, “Combining Additive Manufacturing with Advanced Composites for Highly Integrated Robotic Structures,” Procedia CIRP, vol. 50, pp. 402–407, Jan. 2016, doi: 10.1016/j.procir.2016.04.202. [22] A. Sadeghi, A. Mondini, and B. Mazzolai, “Toward Self-Growing Soft Robots Inspired by Plant Roots and Based on Additive Manufacturing Technologies,” Soft Robot., vol. 4, no. 3, pp. 211– 223, May 2017, doi: 10.1089/soro.2016.0080. [23] D. Manfredi et al., “Direct Metal Laser Sintering: an additive manufacturing technology ready to produce lightweight structural parts for robotic applications,” no. 10, p. 10. [24] M. Bogers, R. Hadar, and A. Bilberg, “Additive manufacturing for consumer-centric business models: Implications for supply chains in consumer goods manufacturing,” Technol. Forecast. Soc. Change, vol. 102, pp. 225–239, Jan. 2016, doi: 10.1016/j.techfore.2015.07.024. [25] C. Weller, R. Kleer, and F. T. Piller, “Economic implications of 3D printing: Market structure models in light of additive manufacturing revisited,” Int. J. Prod. Econ., vol. 164, pp. 43–56, Jun. 2015, doi: 10.1016/j.ijpe.2015.02.020. [26] J. I. Lipton, M. Cutler, F. Nigl, D. Cohen, and H. Lipson, “Additive manufacturing for the food industry,” Trends Food Sci. Technol., vol. 43, no. 1, pp. 114–123, May 2015, doi: 10.1016/j.tifs.2015.02.004. [27] B. P. Conner et al., “Making sense of 3-D printing: Creating a map of additive manufacturing products and services,” Addit. Manuf., vol. 1–4, pp. 64–76, Oct. 2014, doi: 10.1016/j.addma.2014.08.005. [28] H. G. Kia, N. Huang, J. P. Spicer, and J. F. Arinez, “Additive manufacturing of a unibody vehicle,” US10022912B2, Jul. 17, 2018. [29] S. Tibbits, The emergence of “4D printing.” TED conference, 2013. [30] M. D. Monzón et al., “4D printing: processability and measurement of recovery force in shape memory polymers,” Int. J. Adv. Manuf. Technol., vol. 89, no. 5, pp. 1827–1836, Mar. 2017, doi: 10.1007/s00170-016-9233-9. [31] S. Tibbits, “4D Printing: Multi-Material Shape Change,” Archit. Des., vol. 84, no. 1, pp. 116– 121, Jan. 2014, doi: 10.1002/ad.1710. [32] S. E. Bakarich, R. Gorkin, M. in het Panhuis, and G. M. Spinks, “4D Printing with Mechanically Robust, Thermally Actuating Hydrogels,” Macromol. Rapid Commun., vol. 36, no. 12, pp. 1211–1217, Jun. 2015, doi: 10.1002/marc.201500079. [33] M. Nadgorny, Z. Xiao, C. Chen, and L. A. Connal, “Three-Dimensional Printing of pH- Responsive and Functional Polymers on an Affordable Desktop Printer,” ACS Appl. Mater. Interfaces, vol. 8, no. 42, pp. 28946–28954, Oct. 2016, doi: 10.1021/acsami.6b07388. [34] A. M. Schmidt, “Electromagnetic Activation of Shape Memory Polymer Networks Containing Magnetic Nanoparticles,” Macromol. Rapid Commun., vol. 27, no. 14, pp. 1168–1172, Jul. 2006, doi: 10.1002/marc.200600225. [35] S. T. Ly and J. Y. Kim, “4D printing – fused deposition modeling printing with thermal- responsive shape memory polymers,” Int. J. Precis. Eng. Manuf.-Green Technol., vol. 4, no. 3, pp. 267–272, Jul. 2017, doi: 10.1007/s40684-017-0032-z. [36] H. Yang et al., “3D Printed Photoresponsive Devices Based on Shape Memory Composites,” Adv. Mater., vol. 29, no. 33, p. 1701627, 2017, doi: 10.1002/adma.201701627. [37] J. Z. Gul et al., “3D printing for soft robotics – a review,” Sci. Technol. Adv. Mater., vol. 19, no. 1, pp. 243–262, Dec. 2018, doi: 10.1080/14686996.2018.1431862. [38] M. Nadgorny and A. Ameli, “Functional Polymers and Nanocomposites for 3D Printing of Smart Structures and Devices,” ACS Appl. Mater. Interfaces, vol. 10, no. 21, pp. 17489–17507, May 2018, doi: 10.1021/acsami.8b01786. [39] S. Wang, J. M. Lee, and W. Y. Yeong, “Smart hydrogels for 3D bioprinting,” 2015, doi: 10.18063/IJB.2015.01.005. [40] X. Kuang et al., “Advances in 4D Printing: Materials and Applications,” Adv. Funct. Mater., vol. 29, no. 2, p. 1805290, Jan. 2019, doi: 10.1002/adfm.201805290. [41] A. Mitchell, U. Lafont, M. Hołyńska, and C. Semprimoschnig, “Additive manufacturing — A review of 4D printing and future applications,” Addit. Manuf., vol. 24, pp. 606–626, Dec. 2018, doi: 10.1016/j.addma.2018.10.038. [42] S. Miao et al., “4D printing of polymeric materials for tissue and organ regeneration,” Mater. Today, vol. 20, no. 10, pp. 577–591, Dec. 2017, doi: 10.1016/j.mattod.2017.06.005. [43] Y.-C. Li, Y. S. Zhang, A. Akpek, S. R. Shin, and A. Khademhosseini, “4D bioprinting: the next- generation technology for biofabrication enabled by stimuli-responsive materials,” Biofabrication, vol. 9, no. 1, p. 012001, Dec. 2016, doi: 10.1088/1758-5090/9/1/012001. [44] Y. S. Lui, W. T. Sow, L. P. Tan, Y. Wu, Y. Lai, and H. Li, “4D printing and stimuli-responsive materials in biomedical aspects,” Acta Biomater., vol. 92, pp. 19–36, Jul. 2019, doi: 10.1016/j.actbio.2019.05.005. [45] P. Rastogi and B. Kandasubramanian, “Breakthrough in the printing tactics for stimuli- responsive materials: 4D printing,” Chem. Eng. J., vol. 366, pp. 264–304, Jun. 2019, doi: 10.1016/j.cej.2019.02.085. [46] T. J. Esworthy et al., “Advanced 4D-bioprinting technologies for brain tissue modeling and study,” Int. J. Smart Nano Mater., vol. 0, no. 0, pp. 1–28, Jul. 2019, doi: 10.1080/19475411.2019.1631899. [47] Z. Zhang, K. G. Demir, and G. X. Gu, “Developments in 4D-printing: a review on current smart materials, technologies, and applications,” Int. J. Smart Nano Mater., vol. 0, no. 0, pp. 1–20, Mar. 2019, doi: 10.1080/19475411.2019.1591541. [48] M. Javaid and A. Haleem, “4D printing applications in medical field: A brief review,” Clin. Epidemiol. Glob. Health, Sep. 2018, doi: 10.1016/j.cegh.2018.09.007. [49] B. Gao, Q. Yang, X. Zhao, G. Jin, Y. Ma, and F. Xu, “4D Bioprinting for Biomedical Applications,” Trends Biotechnol., vol. 34, no. 9, pp. 746–756, Sep. 2016, doi: 10.1016/j.tibtech.2016.03.004. [50] C. M. González-Henríquez, M. A. Sarabia-Vallejos, and J. Rodriguez-Hernandez, “Polymers for additive manufacturing and 4D-printing: Materials, methodologies, and biomedical applications,” Prog. Polym. Sci., vol. 94, pp. 57–116, Jul. 2019, doi: 10.1016/j.progpolymsci.2019.03.001. [51] M. Vaezi, S. Chianrabutra, B. Mellor, and S. Yang, “Multiple material additive manufacturing – Part 1: a review: This review paper covers a decade of research on multiple material additive manufacturing technologies which can produce complex geometry parts with different materials,” Virtual Phys. Prototyp., vol. 8, no. 1, pp. 19–50, Mar. 2013, doi: 10.1080/17452759.2013.778175. [52] S. A. M. Tofail, E. P. Koumoulos, A. Bandyopadhyay, S. Bose, L. O’Donoghue, and C. Charitidis, “Additive manufacturing: scientific and technological challenges, market uptake and opportunities,” Mater. Today, vol. 21, no. 1, pp. 22–37, Jan. 2018, doi: 10.1016/j.mattod.2017.07.001. [53] H. Hingorani, Y.-F. Zhang, B. Zhang, A. Serjouei, and Q. Ge, “Modified commercial UV curable elastomers for passive 4D printing,” Int. J. Smart Nano Mater., vol. 0, no. 0, pp. 1–12, Mar. 2019, doi: 10.1080/19475411.2019.1591540. [54] R. Sun, M. Falahati, and L. Li, “Numerical and experimental study on multiphase printing of polymeric biconvex micro lenses,” J. Micromechanics Microengineering, vol. 28, no. 11, p. 115005, Nov. 2018, doi: 10.1088/1361-6439/aada1b. [55] S. C. Ligon, R. Liska, J. Stampfl, M. Gurr, and R. Mülhaupt, “Polymers for 3D Printing and Customized Additive Manufacturing,” Chem. Rev., vol. 117, no. 15, pp. 10212–10290, Aug. 2017, doi: 10.1021/acs.chemrev.7b00074. [56] P. Parandoush and D. Lin, “A review on additive manufacturing of polymer-fiber composites,” Compos. Struct., vol. 182, pp. 36–53, Dec. 2017, doi: 10.1016/j.compstruct.2017.08.088. [57] R. Singh, R. Kumar, N. Ranjan, R. Penna, and F. Fraternali, “On the recyclability of polyamide for sustainable composite structures in civil engineering,” Compos. Struct., vol. 184, pp. 704– 713, Jan. 2018, doi: 10.1016/j.compstruct.2017.10.036. [58] D. Chen and X. Zheng, “Multi-material Additive Manufacturing of Metamaterials with Giant, Tailorable Negative Poisson’s Ratios,” Sci. Rep., vol. 8, no. 1, Dec. 2018, doi: 10.1038/s41598- 018-26980-7. [59] X. Wang, M. Jiang, Z. Zhou, J. Gou, and D. Hui, “3D printing of polymer matrix composites: A review and prospective,” Compos. Part B Eng., vol. 110, pp. 442–458, Feb. 2017, doi: 10.1016/j.compositesb.2016.11.034. [60] A. S. K. Kiran et al., “Additive manufacturing technologies: an overview of challenges and perspective of using electrospraying,” Nanocomposites, vol. 4, no. 4, pp. 190–214, Oct. 2018, doi: 10.1080/20550324.2018.1558499. [61] Y. Weng, M. Li, M. J. Tan, and S. Qian, “Design 3D printing cementitious materials via Fuller Thompson theory and Marson-Percy model,” Constr. Build. Mater., vol. 163, pp. 600–610, Feb. 2018, doi: 10.1016/j.conbuildmat.2017.12.112. [62] A. Bandyopadhyay and B. Heer, “Additive manufacturing of multi-material structures,” Mater. Sci. Eng. R Rep., vol. 129, pp. 1–16, Jul. 2018, doi: 10.1016/j.mser.2018.04.001. [63] B. Bhushan and M. Caspers, “An overview of additive manufacturing (3D printing) for microfabrication,” Microsyst. Technol., vol. 23, no. 4, pp. 1117–1124, Apr. 2017, doi: 10.1007/s00542-017-3342-8. [64] K. Hildebrand, B. Bickel, and M. Alexa, “Orthogonal slicing for additive manufacturing,” Comput. Graph., vol. 37, no. 6, pp. 669–675, Oct. 2013, doi: 10.1016/j.cag.2013.05.011. [65] R. Singh, R. Kumar, I. Farina, F. Colangelo, L. Feo, and F. Fraternali, “Multi-Material Additive Manufacturing of Sustainable Innovative Materials and Structures,” Polymers, vol. 11, no. 1, p. 62, Jan. 2019, doi: 10.3390/polym11010062. [66] P. M. Pandey, N. V. Reddy, and S. G. Dhande, “Slicing procedures in layered manufacturing: a review,” 2003, doi: 10.1108/13552540310502185. [67] B. C. Gross, J. L. Erkal, S. Y. Lockwood, C. Chen, and D. M. Spence, “Evaluation of 3D printing and its potential impact on biotechnology and the chemical sciences,” Anal. Chem., vol. 86, no. 7, pp. 3240–3253, Apr. 2014, doi: 10.1021/ac403397r. [68] V. Francis and P. K. Jain, “Advances in nanocomposite materials for additive manufacturing,” Int. J. Rapid Manuf., vol. 5, no. 3/4, p. 215, 2015, doi: 10.1504/IJRAPIDM.2015.074804. [69] B. E. Yamamoto, A. Z. Trimble, B. Minei, and M. N. Ghasemi Nejhad, “Development of multifunctional nanocomposites with 3-D printing additive manufacturing and low graphene loading,” J. Thermoplast. Compos. Mater., vol. 32, no. 3, pp. 383–408, Mar. 2019, doi: 10.1177/0892705718759390. [70] A. C. de Leon, Q. Chen, N. B. Palaganas, J. O. Palaganas, J. Manapat, and R. C. Advincula, “High performance polymer nanocomposites for additive manufacturing applications,” React. Funct. Polym., vol. 103, pp. 141–155, Jun. 2016, doi: 10.1016/j.reactfunctpolym.2016.04.010. [71] Z.-X. Low, Y. T. Chua, B. M. Ray, D. Mattia, I. S. Metcalfe, and D. A. Patterson, “Perspective on 3D printing of separation membranes and comparison to related unconventional fabrication techniques,” J. Membr. Sci., vol. 523, pp. 596–613, Feb. 2017, doi: 10.1016/j.memsci.2016.10.006. [72] D. Pranzo, P. Larizza, D. Filippini, and G. Percoco, “Extrusion-Based 3D Printing of Microfluidic Devices for Chemical and Biomedical Applications: A Topical Review,” Micromachines, vol. 9, no. 8, p. 374, Jul. 2018, doi: 10.3390/mi9080374. [73] J. K. Placone and A. J. Engler, “Recent advances in extrusion-based 3D printing for biomedical applications,” Adv. Healthc. Mater., vol. 7, no. 8, p. e1701161, Apr. 2018, doi: 10.1002/adhm.201701161. [74] S. Singamneni, D. Smith, M.-J. LeGuen, and D. Truong, “Extrusion 3D Printing of Polybutyrate- Adipate-Terephthalate-Polymer Composites in the Pellet Form,” Polymers, vol. 10, no. 8, p. 922, Aug. 2018, doi: 10.3390/polym10080922. [75] J. M. Chacón, M. A. Caminero, E. García-Plaza, and P. J. Núñez, “Additive manufacturing of PLA structures using fused deposition modelling: Effect of process parameters on mechanical properties and their optimal selection,” Mater. Des., vol. 124, pp. 143–157, Jun. 2017, doi: 10.1016/j.matdes.2017.03.065. [76] W. Wu, P. Geng, G. Li, D. Zhao, H. Zhang, and J. Zhao, “Influence of Layer Thickness and Raster Angle on the Mechanical Properties of 3D-Printed PEEK and a Comparative Mechanical Study between PEEK and ABS,” Materials, vol. 8, no. 9, pp. 5834–5846, Sep. 2015, doi: 10.3390/ma8095271. [77] A. K. Sood, R. K. Ohdar, and S. S. Mahapatra, “Parametric appraisal of mechanical property of fused deposition modelling processed parts,” Mater. Des., vol. 31, no. 1, pp. 287–295, Jan. 2010, doi: 10.1016/j.matdes.2009.06.016. [78] E. Cuan-Urquizo, E. Barocio, V. Tejada-Ortigoza, R. B. Pipes, C. A. Rodriguez, and A. Roman- Flores, “Characterization of the Mechanical Properties of FFF Structures and Materials: A Review on the Experimental, Computational and Theoretical Approaches,” Materials, vol. 12, no. 6, Mar. 2019, doi: 10.3390/ma12060895. [79] O. A. Mohamed, S. H. Masood, and J. L. Bhowmik, “Optimization of fused deposition modeling process parameters: a review of current research and future prospects,” Adv. Manuf., vol. 3, no. 1, pp. 42–53, Mar. 2015, doi: 10.1007/s40436-014-0097-7. [80] J. S. Chohan, R. Singh, K. S. Boparai, R. Penna, and F. Fraternali, “Dimensional accuracy analysis of coupled fused deposition modeling and vapour smoothing operations for biomedical applications,” Compos. Part B Eng., vol. 117, pp. 138–149, May 2017, doi: 10.1016/j.compositesb.2017.02.045. [81] Y.-C. Chang et al., “No Such Thing as Trash: A 3D-Printable Polymer Composite Composed of Oil-Extracted Spent Coffee Grounds and Polylactic Acid with Enhanced Impact Toughness,” ACS Sustain. Chem. Eng., vol. 7, no. 18, pp. 15304–15310, Sep. 2019, doi: 10.1021/acssuschemeng.9b02527. [82] S. Kumar and J.-P. Kruth, “Composites by rapid prototyping technology,” Mater. Des., vol. 31, no. 2, pp. 850–856, Feb. 2010, doi: 10.1016/j.matdes.2009.07.045. [83] E. Feilden, E. G.-T. Blanca, F. Giuliani, E. Saiz, and L. Vandeperre, “Robocasting of structural ceramic parts with hydrogel inks,” J. Eur. Ceram. Soc., vol. 36, no. 10, pp. 2525–2533, Aug. 2016, doi: 10.1016/j.jeurceramsoc.2016.03.001. [84] R. D. Farahani, M. Dubé, and D. Therriault, “Three-Dimensional Printing of Multifunctional Nanocomposites: Manufacturing Techniques and Applications,” Adv. Mater., vol. 28, no. 28, pp. 5794–5821, 2016, doi: 10.1002/adma.201506215. [85] M. Invernizzi, G. Natale, M. Levi, S. Turri, and G. Griffini, “UV-Assisted 3D Printing of Glass and Carbon Fiber-Reinforced Dual-Cure Polymer Composites,” Materials, vol. 9, no. 7, p. 583, Jul. 2016, doi: 10.3390/ma9070583. [86] K. Chen, X. Kuang, V. Li, G. Kang, and H. J. Qi, “Fabrication of tough epoxy with shape memory effects by UV-assisted direct-ink write printing,” Soft Matter, vol. 14, no. 10, pp. 1879– 1886, Mar. 2018, doi: 10.1039/C7SM02362F. [87] X. Kuang, K. Chen, C. K. Dunn, J. Wu, V. C. F. Li, and H. J. Qi, “3D Printing of Highly Stretchable, Shape-Memory, and Self-Healing Elastomer toward Novel 4D Printing,” ACS Appl. Mater. Interfaces, vol. 10, no. 8, pp. 7381–7388, Feb. 2018, doi: 10.1021/acsami.7b18265. [88] M. Layani, X. Wang, and S. Magdassi, “Novel Materials for 3D Printing by Photopolymerization,” Adv. Mater., vol. 30, no. 41, p. 1706344, 2018, doi: 10.1002/adma.201706344. [89] G. Postiglione, G. Natale, G. Griffini, M. Levi, and S. Turri, “UV-assisted three-dimensional printing of polymer nanocomposites based on inorganic fillers,” Polym. Compos., vol. 38, no. 8, pp. 1662–1670, 2017, doi: 10.1002/pc.23735. [90] A. Ambrosi and M. Pumera, “3D-printing technologies for electrochemical applications,” Chem. Soc. Rev., vol. 45, no. 10, pp. 2740–2755, 2016, doi: 10.1039/C5CS00714C. [91] H. N. Chia and B. M. Wu, “Recent advances in 3D printing of biomaterials,” J. Biol. Eng., vol. 9, no. 1, p. 4, Mar. 2015, doi: 10.1186/s13036-015-0001-4. [92] L. Lü, J. Fuh, and Y.-S. Wong, Laser-Induced Materials and Processes for Rapid Prototyping. Springer US, 2001. [93] A. Gupta and A. A. Ogale, “Dual curing of carbon fiber reinforced photoresins for rapid prototyping,” Polym. Compos., vol. 23, no. 6, pp. 1162–1170, 2002, doi: 10.1002/pc.10509. [94] J. Z. Manapat, J. D. Mangadlao, B. D. B. Tiu, G. C. Tritchler, and R. C. Advincula, “High- Strength Stereolithographic 3D Printed Nanocomposites: Graphene Oxide Metastability,” ACS Appl. Mater. Interfaces, vol. 9, no. 11, pp. 10085–10093, Mar. 2017, doi: 10.1021/acsami.6b16174. [95] J. Z. Manapat, Q. Chen, P. Ye, and R. C. Advincula, “3D Printing of Polymer Nanocomposites via Stereolithography,” Macromol. Mater. Eng., vol. 302, no. 9, p. 1600553, 2017, doi: 10.1002/mame.201600553. [96] F. P. W. Melchels, J. Feijen, and D. W. Grijpma, “A review on stereolithography and its applications in biomedical engineering,” Biomaterials, vol. 31, no. 24, pp. 6121–6130, Aug. 2010, doi: 10.1016/j.biomaterials.2010.04.050. [97] G. W. Hadipoespito, Y. Yang, H. Choi, G. Ning, and X. Li, “Digital Micromirror Device Based Microstereolithography for Micro Structures of Transparent Photopolymer and Nanocomposites,” p. 12. [98] L. Nayak, S. Mohanty, S. Kumar Nayak, and A. Ramadoss, “A review on inkjet printing of nanoparticle inks for flexible electronics,” J. Mater. Chem. C, vol. 7, no. 29, pp. 8771–8795, 2019, doi: 10.1039/C9TC01630A. [99] M. Singh, H. M. Haverinen, P. Dhagat, and G. E. Jabbour, “Inkjet Printing—Process and Its Applications,” Adv. Mater., vol. 22, no. 6, pp. 673–685, 2010, doi: 10.1002/adma.200901141. [100] Z. Zhan, J. An, Y. Wei, V. Thai Tran, and H. Du, “Inkjet-printed optoelectronics,” Nanoscale, vol. 9, no. 3, pp. 965–993, 2017, doi: 10.1039/C6NR08220C. [101] J.-U. Park et al., “High-resolution electrohydrodynamic jet printing,” Nat. Mater., vol. 6, no. 10, pp. 782–789, Oct. 2007, doi: 10.1038/nmat1974. [102] W. Zou, H. Yu, P. Zhou, and L. Liu, “Tip-assisted electrohydrodynamic jet printing for high- resolution microdroplet deposition,” Mater. Des., vol. 166, p. 107609, Mar. 2019, doi: 10.1016/j.matdes.2019.107609. [103] S. F. S. Shirazi et al., “A review on powder-based additive manufacturing for tissue engineering: selective laser sintering and inkjet 3D printing,” Sci. Technol. Adv. Mater., vol. 16, no. 3, p. 033502, Jun. 2015, doi: 10.1088/1468-6996/16/3/033502. [104] A. Butscher, M. Bohner, S. Hofmann, L. Gauckler, and R. Müller, “Structural and material approaches to bone tissue engineering in powder-based three-dimensional printing,” Acta Biomater., vol. 7, no. 3, pp. 907–920, Mar. 2011, doi: 10.1016/j.actbio.2010.09.039. [105] M. Hofmann, “3D Printing Gets a Boost and Opportunities with Polymer Materials,” ACS Macro Lett., vol. 3, no. 4, pp. 382–386, Apr. 2014, doi: 10.1021/mz4006556. [106] J.-P. Kruth, G. Levy, F. Klocke, and T. H. C. Childs, “Consolidation phenomena in laser and powder-bed based layered manufacturing,” CIRP Ann., vol. 56, no. 2, pp. 730–759, Jan. 2007, doi: 10.1016/j.cirp.2007.10.004. [107] J. Kruth, P. Mercelis, J. Van Vaerenbergh, L. Froyen, and M. Rombouts, “Binding mechanisms in selective laser sintering and selective laser melting,” Rapid Prototyp. J., vol. 11, no. 1, pp. 26–36, Feb. 2005, doi: 10.1108/13552540510573365. [108] S. Greiner, K. Wudy, L. Lanzl, and D. Drummer, “Selective laser sintering of polymer blends: Bulk properties and process behavior,” Polym. Test., vol. 64, pp. 136–144, Dec. 2017, doi: 10.1016/j.polymertesting.2017.09.039. [109] H. Zheng, J. Zhang, S. Lu, G. Wang, and Z. Xu, “Effect of core–shell composite particles on the sintering behavior and properties of nano-Al2O3/polystyrene composite prepared by SLS,” Mater. Lett., vol. 60, no. 9, pp. 1219–1223, May 2006, doi: 10.1016/j.matlet.2005.11.003. [110] E. Gil and S. Hudson, “Stimuli-reponsive polymers and their bioconjugates,” Prog. Polym. Sci., vol. 29, no. 12, pp. 1173–1222, Dec. 2004, doi: 10.1016/j.progpolymsci.2004.08.003. [111] B. Jeong and A. Gutowska, “Lessons from nature: stimuli-responsive polymers and their biomedical applications,” Trends Biotechnol., vol. 20, no. 7, pp. 305–311, Jul. 2002, doi: 10.1016/S0167-7799(02)01962-5. [112] E. M. Ahmed, “Hydrogel: Preparation, characterization, and applications: A review,” J. Adv. Res., vol. 6, no. 2, pp. 105–121, Mar. 2015, doi: 10.1016/j.jare.2013.07.006. [113] F. Ullah, M. B. H. Othman, F. Javed, Z. Ahmad, and H. Md. Akil, “Classification, processing and application of hydrogels: A review,” Mater. Sci. Eng. C, vol. 57, pp. 414–433, Dec. 2015, doi: 10.1016/j.msec.2015.07.053. [114] F. Ganji, S. Vasheghani-Farahani, and E. Vasheghani-Farahani, “Theoretical Description of Hydrogel Swelling: A Review,” p. 24. [115] T. Billiet, M. Vandenhaute, J. Schelfhout, S. Van Vlierberghe, and P. Dubruel, “A review of trends and limitations in hydrogel-rapid prototyping for tissue engineering,” Biomaterials, vol. 33, no. 26, pp. 6020–6041, Sep. 2012, doi: 10.1016/j.biomaterials.2012.04.050. [116] D. Raviv et al., “Active Printed Materials for Complex Self-Evolving Deformations,” Sci. Rep., vol. 4, p. 7422, Dec. 2014, doi: 10.1038/srep07422. [117] A. B. Baker, S. R. G. Bates, T. M. Llewellyn-Jones, L. P. B. Valori, M. P. M. Dicker, and R. S. Trask, “4D printing with robust thermoplastic polyurethane hydrogel-elastomer trilayers,” Mater. Des., vol. 163, p. 107544, Feb. 2019, doi: 10.1016/j.matdes.2018.107544. [118] M. C. Mulakkal, A. M. Seddon, G. Whittell, I. Manners, and R. S. Trask, “4D fibrous materials: characterising the deployment of paper architectures,” Smart Mater. Struct., vol. 25, no. 9, p. 095052, Aug. 2016, doi: 10.1088/0964-1726/25/9/095052. [119] Y. Habibi, L. A. Lucia, and O. J. Rojas, “Cellulose Nanocrystals: Chemistry, Self-Assembly, and Applications,” Chem. Rev., vol. 110, no. 6, pp. 3479–3500, Jun. 2010, doi: 10.1021/cr900339w. [120] M. C. Mulakkal, R. S. Trask, V. P. Ting, and A. M. Seddon, “Responsive cellulose-hydrogel composite ink for 4D printing,” Mater. Des., vol. 160, pp. 108–118, Dec. 2018, doi: 10.1016/j.matdes.2018.09.009. [121] A. S. Gladman, E. A. Matsumoto, R. G. Nuzzo, L. Mahadevan, and J. A. Lewis, “Biomimetic 4D printing,” Nat. Mater., vol. 15, no. 4, pp. 413–418, Apr. 2016, doi: 10.1038/nmat4544. [122] Z. Zhao, J. Wu, X. Mu, H. Chen, H. J. Qi, and D. Fang, “Desolvation Induced Origami of Photocurable Polymers by Digit Light Processing,” Macromol. Rapid Commun., vol. 38, no. 13, p. 1600625, Jul. 2017, doi: 10.1002/marc.201600625. [123] J. Wu, Z. Zhao, X. Kuang, C. M. Hamel, D. Fang, and H. J. Qi, “Reversible shape change structures by grayscale pattern 4D printing,” Multifunct. Mater., vol. 1, no. 1, p. 015002, Aug. 2018, doi: 10.1088/2399-7532/aac322. [124] Z. Zhao, H. Jerry Qi, and D. Fang, “A finite deformation theory of desolvation and swelling in partially photo-cross-linked polymer networks for 3D/4D printing applications,” Soft Matter, vol. 15, no. 5, pp. 1005–1016, 2019, doi: 10.1039/C8SM02427H. [125] F. Liu and M. W. Urban, “Recent advances and challenges in designing stimuli-responsive polymers,” Prog. Polym. Sci., vol. 35, no. 1–2, pp. 3–23, Jan. 2010, doi: 10.1016/j.progpolymsci.2009.10.002. [126] I. Roy and M. N. Gupta, “Smart Polymeric Materials: Emerging Biochemical Applications,” Chem. Biol., vol. 10, no. 12, pp. 1161–1171, Dec. 2003, doi: 10.1016/j.chembiol.2003.12.004. [127] H. R. Almeida, M. H. do Amaral, and P. A. L. Lobão, “Temperature and pH stimuli-responsive polymers and their applications in controlled and self- regulated drug delivery,” 2012. [128] S. Dai, P. Ravi, and K. C. Tam, “pH-Responsive polymers: synthesis, properties and applications,” Soft Matter, vol. 4, no. 3, p. 435, 2008, doi: 10.1039/b714741d. [129] D. Schmaljohann, “Thermo- and pH-responsive polymers in drug delivery☆,” Adv. Drug Deliv. Rev., vol. 58, no. 15, pp. 1655–1670, Dec. 2006, doi: 10.1016/j.addr.2006.09.020. [130] J. Kost and R. Langer, “Responsive polymeric delivery systemsq,” Adv. Drug Deliv. Rev., p. 24, 2001. [131] Q. Zhang, L. Liu, H. Zhou, X. Wu, and K. D. Yao, “Ph-Responsive Swelling Behavior of Collagen Complex Materials,” Artif. Cells Blood Substit. Biotechnol., vol. 28, no. 3, pp. 255– 262, Jan. 2000, doi: 10.3109/10731190009119356. [132] B. Hilmi, Z. A. A. Hamid, H. M. Akil, and B. H. Yahaya, “The Characteristics of the Smart Polymeras Temperature or pH-responsive Hydrogel,” Procedia Chem., vol. 19, pp. 406–409, 2016, doi: 10.1016/j.proche.2016.03.031. [133] K. S. Soppimath, T. M. Aminabhavi, A. M. Dave, S. G. Kumbar, and W. E. Rudzinski, “Stimulus-Responsive ‘Smart’ Hydrogels as Novel Drug Delivery Systems,” Drug Dev. Ind. Pharm., vol. 28, no. 8, pp. 957–974, Jan. 2002, doi: 10.1081/DDC-120006428. [134] Y. Qiu and K. Park, “Environment-sensitive hydrogels for drug delivery,” Adv. Drug Deliv. Rev., p. 19, 2001. [135] I. Galaev, “‘Smart’ polymers and what they could do in biotechnology and medicine,” Trends Biotechnol., vol. 17, no. 8, pp. 335–340, Aug. 1999, doi: 10.1016/S0167-7799(99)01345-1. [136] T. C. Okwuosa, B. C. Pereira, B. Arafat, M. Cieszynska, A. Isreb, and M. A. Alhnan, “Fabricating a Shell-Core Delayed Release Tablet Using Dual FDM 3D Printing for Patient- Centred Therapy,” Pharm. Res., vol. 34, no. 2, pp. 427–437, Feb. 2017, doi: 10.1007/s11095- 016-2073-3. [137] L. Larush et al., “3D printing of responsive hydrogels for drug-delivery systems,” J. 3D Print. Med., vol. 1, no. 4, pp. 219–229, Oct. 2017, doi: 10.2217/3dp-2017-0009. [138] J. Odent et al., “Hierarchical chemomechanical encoding of multi-responsive hydrogel actuators via 3D printing,” J. Mater. Chem. A, vol. 7, no. 25, pp. 15395–15403, 2019, doi: 10.1039/C9TA03547H. [139] X. Yang, Z. Lu, H. Wu, W. Li, L. Zheng, and J. Zhao, “Collagen-alginate as bioink for three- dimensional (3D) cell printing based cartilage tissue engineering,” Mater. Sci. Eng. C Mater. Biol. Appl., vol. 83, pp. 195–201, Feb. 2018, doi: 10.1016/j.msec.2017.09.002. [140] H. Stratesteffen, M. Köpf, F. Kreimendahl, A. Blaeser, S. Jockenhoevel, and H. Fischer, “GelMA-collagen blends enable drop-on-demand 3D printablility and promote angiogenesis,” Biofabrication, vol. 9, no. 4, p. 045002, Sep. 2017, doi: 10.1088/1758-5090/aa857c. [141] J. Gopinathan and I. Noh, “Recent trends in bioinks for 3D printing,” Biomater. Res., vol. 22, Apr. 2018, doi: 10.1186/s40824-018-0122-1. [142] C. Chen et al., “Collagen/heparin sulfate scaffolds fabricated by a 3D bioprinter improved mechanical properties and neurological function after spinal cord injury in rats,” J. Biomed. Mater. Res. A, vol. 105, no. 5, pp. 1324–1332, 2017, doi: 10.1002/jbm.a.36011. [143] E. A. Roth, T. Xu, M. Das, C. Gregory, J. J. Hickman, and T. Boland, “Inkjet printing for high- throughput cell patterning,” Biomaterials, vol. 25, no. 17, pp. 3707–3715, Aug. 2004, doi: 10.1016/j.biomaterials.2003.10.052. [144] A. D. Nocera, R. Comín, N. A. Salvatierra, and M. P. Cid, “Development of 3D printed fibrillar collagen scaffold for tissue engineering,” Biomed. Microdevices, vol. 20, no. 2, p. 26, 27 2018, doi: 10.1007/s10544-018-0270-z. [145] R. Gibney, S. Matthyssen, J. Steele, J. Patterson, N. Zakaria, and E. Ferraris, “High resolution 3d printing of collagen,” Orthop. Proc., vol. 100-B, no. SUPP_15, pp. 95–95, Nov. 2018, doi: 10.1302/1358-992X.2018.15.095. [146] K. E. Drzewiecki, J. N. Malavade, I. Ahmed, C. J. Lowe, and D. I. Shreiber, “A thermoreversible, photocrosslinkable collagen bio-ink for free-form fabrication of scaffolds for regenerative medicine,” Technology, vol. 5, no. 4, pp. 185–195, Dec. 2017, doi: 10.1142/S2339547817500091. [147] V. Lee et al., “Design and Fabrication of Human Skin by Three-Dimensional Bioprinting,” Tissue Eng. Part C Methods, vol. 20, no. 6, pp. 473–484, Jun. 2014, doi: 10.1089/ten.tec.2013.0335. [148] H. Sirringhaus and T. Shimoda, “Inkjet Printing of Functional Materials,” MRS Bull., vol. 28, no. 11, pp. 802–806, Nov. 2003, doi: 10.1557/mrs2003.228. [149] X. Wang, Q. Guo, X. Cai, S. Zhou, B. Kobe, and J. Yang, “Initiator-Integrated 3D Printing Enables the Formation of Complex Metallic Architectures,” ACS Appl. Mater. Interfaces, vol. 6, no. 4, pp. 2583–2587, Feb. 2014, doi: 10.1021/am4050822. [150] M. Falahati, Y.-C. Chang, P. Ahmadvand, W. Zhou, A. Yi, and L. Li, “Fabrication of aspherical polymeric lenses using tunable ferrogel molds,” Appl. Opt., vol. 59, no. 8, p. 2632, Mar. 2020, doi: 10.1364/AO.386292. [151] M. Falahati, W. Zhou, A. Yi, and L. Li, “Development of an adjustable-focus ferrogel mirror,” Opt. Laser Technol., vol. 125, p. 106021, May 2020, doi: 10.1016/j.optlastec.2019.106021. [152] S. Anandhan and S. Bandyopadhyay, “Polymer Nanocomposites: From Synthesis to Applications,” in Nanocomposites and Polymers with Analytical Methods, J. Cuppoletti, Ed. InTech, 2011. [153] A. Muñoz-Bonilla, J. Sánchez-Marcos, and P. Herrasti, “Magnetic Nanoparticles-Based Conducting Polymer Nanocomposites,” in Conducting Polymer Hybrids, V. Kumar, S. Kalia, and H. C. Swart, Eds. Cham: Springer International Publishing, 2017, pp. 45–80. [154] S. Kalia, S. Kango, A. Kumar, Y. Haldorai, B. Kumari, and R. Kumar, “Magnetic polymer nanocomposites for environmental and biomedical applications,” Colloid Polym. Sci., vol. 292, no. 9, pp. 2025–2052, Sep. 2014, doi: 10.1007/s00396-014-3357-y. [155] S. Behrens, “Preparation of functional magnetic nanocomposites and hybrid materials: recent progress and future directions,” Nanoscale, vol. 3, no. 3, pp. 877–892, 2011, doi: 10.1039/C0NR00634C. [156] J. Thévenot, H. Oliveira, O. Sandre, and S. Lecommandoux, “Magnetic responsive polymer composite materials,” Chem. Soc. Rev., vol. 42, no. 17, p. 7099, 2013, doi: 10.1039/c3cs60058k. [157] Y. Wang, F. Castles, and P. S. Grant, “3D Printing of NiZn ferrite/ABS Magnetic Composites for Electromagnetic Devices,” MRS Online Proc. Libr. Arch., vol. 1788, pp. 29–35, ed 2015, doi: 10.1557/opl.2015.661. [158] L. M. Bollig, P. J. Hilpisch, G. S. Mowry, and B. B. Nelson-Cheeseman, “3D printed magnetic polymer composite transformers,” J. Magn. Magn. Mater., vol. 442, pp. 97–101, Nov. 2017, doi: 10.1016/j.jmmm.2017.06.070. [159] D. Sindersberger, A. Diermeier, N. Prem, and G. J. Monkman, “Printing of hybrid magneto active polymers with 6 degrees of freedom,” Mater. Today Commun., vol. 15, pp. 269–274, Jun. 2018, doi: 10.1016/j.mtcomm.2018.02.032. [160] J. Zhang et al., “3D-printed magnetic Fe 3 O 4 /MBG/PCL composite scaffolds with multifunctionality of bone regeneration, local anticancer drug delivery and hyperthermia,” J Mater Chem B, vol. 2, no. 43, pp. 7583–7595, 2014, doi: 10.1039/C4TB01063A. [161] J. J. Martin, B. E. Fiore, and R. M. Erb, “Designing bioinspired composite reinforcement architectures via 3D magnetic printing,” Nat. Commun., vol. 6, no. 1, Dec. 2015, doi: 10.1038/ncomms9641. [162] S. Safaee and R. Chen, “Investigation of a Magnetic Field-Assisted Digital-Light-Processing Stereolithography for Functionally Graded Materials,” Procedia Manuf., vol. 34, pp. 731–737, Jan. 2019, doi: 10.1016/j.promfg.2019.06.229. [163] M. Shin, K. H. Song, J. C. Burrell, D. K. Cullen, and J. A. Burdick, “Injectable and Conductive Granular Hydrogels for 3D Printing and Electroactive Tissue Support,” Adv. Sci., vol. 6, no. 20, p. 1901229, 2019, doi: 10.1002/advs.201901229. [164] C. A. G. Rosales et al., “3D printing of shape memory polymer (SMP)/carbon black (CB) nanocomposites with electro-responsive toughness enhancement,” Mater. Res. Express, vol. 5, no. 6, p. 065704, Jun. 2018, doi: 10.1088/2053-1591/aacd53. [165] S. Sayyar et al., “UV Cross-Linkable Graphene/Poly(trimethylene Carbonate) Composites for 3D Printing of Electrically Conductive Scaffolds,” ACS Appl. Mater. Interfaces, vol. 8, no. 46, pp. 31916–31925, Nov. 2016, doi: 10.1021/acsami.6b09962. [166] S.-L. Dong, L. Han, C.-X. Du, X.-Y. Wang, L.-H. Li, and Y. Wei, “3D Printing of Aniline Tetramer-Grafted-Polyethylenimine and Pluronic F127 Composites for Electroactive Scaffolds,” Macromol. Rapid Commun., vol. 38, no. 4, p. 1600551, 2017, doi: 10.1002/marc.201600551. [167] L. Jiang et al., “Three-Dimensional Printing and Injectable Conductive Hydrogels for Tissue Engineering Application,” Tissue Eng. Part B Rev., vol. 25, no. 5, pp. 398–411, May 2019, doi: 10.1089/ten.teb.2019.0100. [168] A. Zolfagharian, M. Denk, M. Bodaghi, A. Z. Kouzani, and A. Kaynak, “Topology-Optimized 4D Printing of a Soft Actuator,” Acta Mech. Solida Sin., Sep. 2019, doi: 10.1007/s10338-019- 00137-z. [169] A. Zolfagharian, A. Kaynak, S. Y. Khoo, and A. Z. Kouzani, “Polyelectrolyte Soft Actuators: 3D Printed Chitosan and Cast Gelatin,” 3D Print. Addit. Manuf., vol. 5, no. 2, pp. 138–150, Apr. 2018, doi: 10.1089/3dp.2017.0054. [170] D. Han et al., “Soft Robotic Manipulation and Locomotion with a 3D Printed Electroactive Hydrogel,” ACS Appl. Mater. Interfaces, vol. 10, no. 21, pp. 17512–17518, May 2018, doi: 10.1021/acsami.8b04250. [171] Y.-W. Kang, J. Woo, H.-R. Lee, and J.-Y. Sun, “A mechanically enhanced electroactive hydrogel for 3D printing using a multileg long chain crosslinker,” Smart Mater. Struct., vol. 28, no. 9, p. 095016, Aug. 2019, doi: 10.1088/1361-665X/ab325d. [172] M. Falahati, W. Zhou, A. Yi, and L. Li, “Fabrication of polymeric lenses using magnetic liquid molds,” Appl. Phys. Lett., vol. 114, no. 20, p. 203701, May 2019, doi: 10.1063/1.5090511. [173] P. Zhu, W. Yang, R. Wang, S. Gao, B. Li, and Q. Li, “4D Printing of Complex Structures with a Fast Response Time to Magnetic Stimulus,” ACS Appl. Mater. Interfaces, vol. 10, no. 42, pp. 36435–36442, Oct. 2018, doi: 10.1021/acsami.8b12853. [174] H. Wei, Q. Zhang, Y. Yao, L. Liu, Y. Liu, and J. Leng, “Direct-Write Fabrication of 4D Active Shape-Changing Structures Based on a Shape Memory Polymer and Its Nanocomposite,” ACS Appl. Mater. Interfaces, vol. 9, no. 1, pp. 876–883, Jan. 2017, doi: 10.1021/acsami.6b12824. [175] Y. Kim, H. Yuk, R. Zhao, S. A. Chester, and X. Zhao, “Printing ferromagnetic domains for untethered fast-transforming soft materials,” Nature, vol. 558, no. 7709, pp. 274–279, Jun. 2018, doi: 10.1038/s41586-018-0185-0. [176] H. Shinoda, S. Azukizawa, K. Maeda, and F. Tsumori, “Bio-Mimic Motion of 3D-Printed Gel Structures Dispersed with Magnetic Particles,” J. Electrochem. Soc., vol. 166, no. 9, pp. B3235– B3239, 2019, doi: 10.1149/2.0361909jes. [177] W. Zhu et al., “3D-Printed Artificial Microfish,” Adv. Mater., vol. 27, no. 30, pp. 4411–4417, 2015, doi: 10.1002/adma.201501372. [178] U. G. K. Wegst, H. Bai, E. Saiz, A. P. Tomsia, and R. O. Ritchie, “Bioinspired structural materials,” Nat. Mater., vol. 14, no. 1, pp. 23–36, Jan. 2015, doi: 10.1038/nmat4089. [179] J. C. Weaver et al., “The Stomatopod Dactyl Club: A Formidable Damage-Tolerant Biological Hammer,” Science, vol. 336, no. 6086, pp. 1275–1280, Jun. 2012, doi: 10.1126/science.1218764. [180] D. Kokkinis, M. Schaffner, and A. R. Studart, “Multimaterial magnetically assisted 3D printing of composite materials,” Nat. Commun., vol. 6, no. 1, Dec. 2015, doi: 10.1038/ncomms9643. [181] R. M. Erb, R. Libanori, N. Rothfuchs, and A. R. Studart, “Composites Reinforced in Three Dimensions by Using Low Magnetic Fields,” Science, vol. 335, no. 6065, pp. 199–204, Jan. 2012, doi: 10.1126/science.1210822. [182] J. A. Jackson et al., “Field responsive mechanical metamaterials,” Sci. Adv., vol. 4, no. 12, p. eaau6419, Dec. 2018, doi: 10.1126/sciadv.aau6419. [183] A. K. Bastola, V. T. Hoang, and L. Li, “A novel hybrid magnetorheological elastomer developed by 3D printing,” Mater. Des., vol. 114, pp. 391–397, Jan. 2017, doi: 10.1016/j.matdes.2016.11.006. [184] A. K. Bastola, M. Paudel, and L. Li, “Development of hybrid magnetorheological elastomers by 3D printing,” Polymer, vol. 149, pp. 213–228, Aug. 2018, doi: 10.1016/j.polymer.2018.06.076. [185] W. M. Huang, B. Yang, Y. Zhao, and Z. Ding, “Thermo-moisture responsive polyurethane shape-memory polymer and composites: a review,” J. Mater. Chem., vol. 20, no. 17, pp. 3367– 3381, Apr. 2010, doi: 10.1039/B922943D. [186] J. Hu and S. Chen, “A review of actively moving polymers in textile applications,” J. Mater. Chem., vol. 20, no. 17, pp. 3346–3355, Apr. 2010, doi: 10.1039/B922872A. [187] H. Meng and G. Li, “A review of stimuli-responsive shape memory polymer composites,” Polymer, vol. 54, no. 9, pp. 2199–2221, Apr. 2013, doi: 10.1016/j.polymer.2013.02.023. [188] W. Wang, Y. Liu, and J. Leng, “Recent developments in shape memory polymer nanocomposites: Actuation methods and mechanisms,” Coord. Chem. Rev., vol. 320–321, pp. 38–52, Aug. 2016, doi: 10.1016/j.ccr.2016.03.007. [189] T. Xie, “Recent advances in polymer shape memory,” Polymer, vol. 52, no. 22, pp. 4985–5000, Oct. 2011, doi: 10.1016/j.polymer.2011.08.003. [190] T. Liu et al., “Stimulus methods of multi-functional shape memory polymer nanocomposites: A review,” Compos. Part Appl. Sci. Manuf., vol. 100, pp. 20–30, Sep. 2017, doi: 10.1016/j.compositesa.2017.04.022. [191] F. Pilate, A. Toncheva, P. Dubois, and J.-M. Raquez, “Shape-memory polymers for multiple applications in the materials world,” Eur. Polym. J., vol. 80, pp. 268–294, Jul. 2016, doi: 10.1016/j.eurpolymj.2016.05.004. [192] M. D. Hager, S. Bode, C. Weber, and U. S. Schubert, “Shape memory polymers: Past, present and future developments,” Prog. Polym. Sci., vol. 49–50, pp. 3–33, Oct. 2015, doi: 10.1016/j.progpolymsci.2015.04.002. [193] L. Sun et al., “Stimulus-responsive shape memory materials: A review,” Mater. Des., vol. 33, pp. 577–640, Jan. 2012, doi: 10.1016/j.matdes.2011.04.065. [194] J. Leng, X. Lan, Y. Liu, and S. Du, “Shape-memory polymers and their composites: Stimulus methods and applications,” Prog. Mater. Sci., vol. 56, no. 7, pp. 1077–1135, Sep. 2011, doi: 10.1016/j.pmatsci.2011.03.001. [195] C. Liu, S. B. Chun, P. T. Mather, L. Zheng, E. H. Haley, and E. B. Coughlin, “Chemically Cross- Linked Polycyclooctene: Synthesis, Characterization, and Shape Memory Behavior,” Macromolecules, vol. 35, no. 27, pp. 9868–9874, Dec. 2002, doi: 10.1021/ma021141j. [196] M. Zarek, M. Layani, I. Cooperstein, E. Sachyani, D. Cohn, and S. Magdassi, “3D Printing of Shape Memory Polymers for Flexible Electronic Devices,” Adv. Mater., vol. 28, no. 22, pp. 4449–4454, 2016, doi: 10.1002/adma.201503132. [197] K. Estelle, D. Blair, K. Evans, and B. A. Gozen, “Manufacturing of smart composites with hyperelastic property gradients and shape memory using fused deposition,” J. Manuf. Process., vol. 28, pp. 500–507, Aug. 2017, doi: 10.1016/j.jmapro.2017.04.018. [198] M. Zarek, N. Mansour, S. Shapira, and D. Cohn, “4D Printing of Shape Memory-Based Personalized Endoluminal Medical Devices,” Macromol. Rapid Commun., vol. 38, no. 2, Jan. 2017, doi: 10.1002/marc.201600628. [199] M. Bodaghi, A. R. Damanpack, and W. H. Liao, “Triple shape memory polymers by 4D printing,” Smart Mater. Struct., vol. 27, no. 6, p. 065010, May 2018, doi: 10.1088/1361- 665X/aabc2a. [200] W. J. Hendrikson, J. Rouwkema, F. Clementi, C. A. van Blitterswijk, S. Farè, and L. Moroni, “Towards 4D printed scaffolds for tissue engineering: exploiting 3D shape memory polymers to deliver time-controlled stimulus on cultured cells,” Biofabrication, vol. 9, no. 3, p. 031001, Aug. 2017, doi: 10.1088/1758-5090/aa8114. [201] J.-W. Su et al., “4D printing of polyurethane paint-based composites,” Int. J. Smart Nano Mater., pp. 1–12, May 2019, doi: 10.1080/19475411.2019.1618409. [202] W. Zhang et al., “Shape memory behavior and recovery force of 4D printed textile functional composites,” Compos. Sci. Technol., vol. 160, pp. 224–230, May 2018, doi: 10.1016/j.compscitech.2018.03.037. [203] F. S. Senatov et al., “Shape memory effect in 3D-printed scaffolds for self-fitting implants,” Eur. Polym. J., vol. 93, pp. 222–231, Aug. 2017, doi: 10.1016/j.eurpolymj.2017.06.011. [204] Y. Liu et al., “Shape memory behavior and recovery force of 4D printed laminated Miura- origami structures subjected to compressive loading,” Compos. Part B Eng., vol. 153, pp. 233– 242, Nov. 2018, doi: 10.1016/j.compositesb.2018.07.053. [205] Q. Ge, A. H. Sakhaei, H. Lee, C. K. Dunn, N. X. Fang, and M. L. Dunn, “Multimaterial 4D Printing with Tailorable Shape Memory Polymers,” Sci. Rep., vol. 6, no. 1, Nov. 2016, doi: 10.1038/srep31110. [206] H. Wu, P. Chen, C. Yan, C. Cai, and Y. Shi, “Four-dimensional printing of a novel acrylate- based shape memory polymer using digital light processing,” Mater. Des., vol. 171, p. 107704, Jun. 2019, doi: 10.1016/j.matdes.2019.107704. [207] R. Yu et al., “Three-Dimensional Printing of Shape Memory Composites with Epoxy-Acrylate Hybrid Photopolymer,” ACS Appl. Mater. Interfaces, vol. 9, no. 2, pp. 1820–1829, Jan. 2017, doi: 10.1021/acsami.6b13531. [208] J. Wang et al., “A UV-curable epoxy with ‘soft’ segments for 3D-printable shape-memory materials,” J. Mater. Sci., vol. 53, no. 17, pp. 12650–12661, Sep. 2018, doi: 10.1007/s10853- 018-2520-0. [209] K. Yu, A. Ritchie, Y. Mao, M. L. Dunn, and H. J. Qi, “Controlled Sequential Shape Changing Components by 3D Printing of Shape Memory Polymer Multimaterials,” Procedia IUTAM, vol. 12, pp. 193–203, 2015, doi: 10.1016/j.piutam.2014.12.021. [210] J. Wu et al., “Multi-shape active composites by 3D printing of digital shape memory polymers,” Sci. Rep., vol. 6, no. 1, Jul. 2016, doi: 10.1038/srep24224. [211] Q. Ge, C. K. Dunn, H. J. Qi, and M. L. Dunn, “Active origami by 4D printing,” Smart Mater. Struct., vol. 23, no. 9, p. 094007, Sep. 2014, doi: 10.1088/0964-1726/23/9/094007. [212] Q. Ge, H. J. Qi, and M. L. Dunn, “Active materials by four-dimension printing,” Appl. Phys. Lett., vol. 103, no. 13, p. 131901, Sep. 2013, doi: 10.1063/1.4819837. [213] H. Li, X. Gao, and Y. Luo, “Multi-shape memory polymers achieved by the spatio-assembly of 3D printable thermoplastic building blocks,” Soft Matter, vol. 12, no. 13, pp. 3226–3233, Mar. 2016, doi: 10.1039/C6SM00185H. [214] T. Xie, “Tunable polymer multi-shape memory effect,” Nature, vol. 464, no. 7286, pp. 267–270, Mar. 2010, doi: 10.1038/nature08863. [215] J. Wang, Q. Zhao, H. Cui, Y. Wang, H. Chen, and X. Du, “Tunable shape memory polymer mold for multiple microarray replications,” J. Mater. Chem. A, vol. 6, no. 48, pp. 24748–24755, Dec. 2018, doi: 10.1039/C8TA04763D. [216] G. I. Peterson, E. P. Childers, H. Li, A. V. Dobrynin, and M. L. Becker, “Tunable Shape Memory Polymers from α-Amino Acid-Based Poly(ester urea)s,” Macromolecules, vol. 50, no. 11, pp. 4300–4308, Jun. 2017, doi: 10.1021/acs.macromol.7b00680. [217] A. M. DiOrio, X. Luo, K. M. Lee, and P. T. Mather, “A functionally graded shape memory polymer,” Soft Matter, vol. 7, no. 1, pp. 68–74, Dec. 2010, doi: 10.1039/C0SM00487A. [218] N. Erden and S. C. Jana, “Synthesis and Characterization of Shape-Memory Polyurethane– Polybenzoxazine Compounds,” Macromol. Chem. Phys., vol. 214, no. 11, pp. 1225–1237, 2013, doi: 10.1002/macp.201200315. [219] S. Rimdusit, W. Bangsen, and P. Kasemsiri, “Chemorheology and thermomechanical characteristics of benzoxazine-urethane copolymers,” J. Appl. Polym. Sci., vol. 121, no. 6, pp. 3669–3678, 2011, doi: 10.1002/app.34170. [220] P. Prathumrat, S. Tiptipakorn, and S. Rimdusit, “Multiple-shape memory polymers from benzoxazine–urethane copolymers,” Smart Mater. Struct., vol. 26, no. 6, p. 065025, May 2017, doi: 10.1088/1361-665X/aa6d47. [221] Q. Ge, X. Luo, C. B. Iversen, P. T. Mather, M. L. Dunn, and H. J. Qi, “Mechanisms of triple- shape polymeric composites due to dual thermal transitions,” Soft Matter, vol. 9, no. 7, p. 2212, 2013, doi: 10.1039/c2sm27063c. [222] J. Du, Z. Zhang, D. Liu, T. Ren, D. Wan, and H. Pu, “Triple-stimuli responsive shape memory effect of novel polyolefin elastomer/lauric acid/carbon black nanocomposites,” Compos. Sci. Technol., vol. 169, pp. 45–51, Jan. 2019, doi: 10.1016/j.compscitech.2018.10.029. [223] X. Luo and P. T. Mather, “Triple-Shape Polymeric Composites (TSPCs),” Adv. Funct. Mater., vol. 20, no. 16, pp. 2649–2656, 2010, doi: 10.1002/adfm.201000052. [224] M. R. Ramdas, K. S. S. Kumar, and C. P. R. Nair, “Synthesis, structure and tunable shape memory properties of polytriazoles: dual-trigger temperature and repeatable shape recovery,” J. Mater. Chem. A, vol. 3, no. 21, pp. 11596–11606, May 2015, doi: 10.1039/C5TA00616C. [225] J. Deng et al., “Tunable Photothermal Actuators Based on a Pre-programmed Aligned Nanostructure,” J. Am. Chem. Soc., vol. 138, no. 1, pp. 225–230, Jan. 2016, doi: 10.1021/jacs.5b10131. [226] L. Ren, B. Li, Z. Song, Q. Liu, L. Ren, and X. Zhou, “Bioinspired fiber-regulated composite with tunable permanent shape and shape memory properties via 3d magnetic printing,” Compos. Part B Eng., vol. 164, pp. 458–466, May 2019, doi: 10.1016/j.compositesb.2019.01.061. [227] W. Wang, C. Li, M. Cho, and S.-H. Ahn, “Soft Tendril-Inspired Grippers: Shape Morphing of Programmable Polymer–Paper Bilayer Composites,” ACS Appl. Mater. Interfaces, vol. 10, no. 12, pp. 10419–10427, Mar. 2018, doi: 10.1021/acsami.7b18079. [228] C. Yang, B. Wang, D. Li, and X. Tian, “Modelling and characterisation for the responsive performance of CF/PLA and CF/PEEK smart materials fabricated by 4D printing,” Virtual Phys. Prototyp., vol. 12, no. 1, pp. 69–76, Jan. 2017, doi: 10.1080/17452759.2016.1265992. [229] Z. Ding, C. Yuan, X. Peng, T. Wang, H. J. Qi, and M. L. Dunn, “Direct 4D printing via active composite materials,” Sci. Adv., vol. 3, no. 4, p. e1602890, Apr. 2017, doi: 10.1126/sciadv.1602890. [230] W. Wang, C. Y. Yu, P. A. Abrego Serrano, and S.-H. Ahn, “Soft grasping mechanisms composed of shape memory polymer based self-bending units,” Compos. Part B Eng., vol. 164, pp. 198–204, May 2019, doi: 10.1016/j.compositesb.2018.10.081. [231] S. V. Hoa, “Development of composite springs using 4D printing method,” Compos. Struct., vol. 210, pp. 869–876, Feb. 2019, doi: 10.1016/j.compstruct.2018.12.003. [232] A. Kotikian, R. L. Truby, J. W. Boley, T. J. White, and J. A. Lewis, “3D Printing of Liquid Crystal Elastomeric Actuators with Spatially Programed Nematic Order,” Adv. Mater., vol. 30, no. 10, p. 1706164, 2018, doi: 10.1002/adma.201706164. [233] A. Kotikian et al., “Untethered soft robotic matter with passive control of shape morphing and propulsion,” Sci. Robot., vol. 4, no. 33, Art. no. 33, Aug. 2019. [234] C. Zhang, X. Lu, G. Fei, Z. Wang, H. Xia, and Y. Zhao, “4D Printing of a Liquid Crystal Elastomer with a Controllable Orientation Gradient,” ACS Appl. Mater. Interfaces, vol. 11, no. 47, pp. 44774–44782, Nov. 2019, doi: 10.1021/acsami.9b18037. [235] Y.-J. Kim and Y. T. Matsunaga, “Thermo-responsive polymers and their application as smart biomaterials,” J. Mater. Chem. B, vol. 5, no. 23, pp. 4307–4321, Jun. 2017, doi: 10.1039/C7TB00157F. [236] Y. Zhu, R. Batchelor, A. B. Lowe, and P. J. Roth, “Design of Thermoresponsive Polymers with Aqueous LCST, UCST, or Both: Modification of a Reactive Poly(2-vinyl-4,4- dimethylazlactone) Scaffold,” Macromolecules, vol. 49, no. 2, pp. 672–680, Jan. 2016, doi: 10.1021/acs.macromol.5b02056. [237] X.-Z. Zhang, X.-D. Xu, S.-X. Cheng, and R.-X. Zhuo, “Strategies to improve the response rate of thermosensitive PNIPAAm hydrogels,” Soft Matter, vol. 4, no. 3, p. 385, 2008, doi: 10.1039/b713803m. [238] C. Wu and X. Wang, “Globule-to-Coil Transition of a Single Homopolymer Chain in Solution,” Phys. Rev. Lett., vol. 80, no. 18, pp. 4092–4094, May 1998, doi: 10.1103/PhysRevLett.80.4092. [239] I. Bischofberger, D. C. E. Calzolari, and V. Trappe, “Co-nonsolvency of PNiPAM at the transition between solvation mechanisms,” Soft Matter, vol. 10, no. 41, pp. 8288–8295, Oct. 2014, doi: 10.1039/C4SM01345J. [240] D. Han, Z. Lu, S. A. Chester, and H. Lee, “Micro 3D Printing of a Temperature-Responsive Hydrogel Using Projection Micro-Stereolithography,” Sci. Rep., vol. 8, no. 1, p. 1963, 31 2018, doi: 10.1038/s41598-018-20385-2. [241] T. Chen, H. Bakhshi, L. Liu, J. Ji, and S. Agarwal, “Combining 3D Printing with Electrospinning for Rapid Response and Enhanced Designability of Hydrogel Actuators,” Advanced Functional Materials, May 01, 2018. https://onlinelibrary.wiley.com/doi/abs/10.1002/adfm.201800514 (accessed Jul. 24, 2019). [242] S. Naficy, R. Gately, R. Gorkin, H. Xin, and G. M. Spinks, “4D Printing of Reversible Shape Morphing Hydrogel Structures,” Macromol. Mater. Eng., vol. 302, no. 1, p. 1600212, 2017, doi: 10.1002/mame.201600212. [243] J. Liu et al., “Dual-Gel 4D Printing of Bioinspired Tubes,” ACS Appl. Mater. Interfaces, vol. 11, no. 8, pp. 8492–8498, Feb. 2019, doi: 10.1021/acsami.8b17218. [244] Y. Jin, Y. Shen, J. Yin, J. Qian, and Y. Huang, “Nanoclay-Based Self-Supporting Responsive Nanocomposite Hydrogels for Printing Applications,” ACS Appl. Mater. Interfaces, vol. 10, no. 12, pp. 10461–10470, Mar. 2018, doi: 10.1021/acsami.8b00806. [245] Z. Chen et al., “3D Printing of Multifunctional Hydrogels,” Advanced Functional Materials, May 01, 2019. https://onlinelibrary.wiley.com/doi/abs/10.1002/adfm.201900971 (accessed Jul. 10, 2019). [246] S. Y. Zheng et al., “Programmed Deformations of 3D‐Printed Tough Physical Hydrogels with High Response Speed and Large Output Force,” Advanced Functional Materials, Sep. 01, 2018. https://onlinelibrary.wiley.com/doi/abs/10.1002/adfm.201803366 (accessed Jul. 24, 2019). [247] M. Zhang et al., “Dual-Responsive Hydrogels for Direct-Write 3D Printing,” Macromolecules, vol. 48, no. 18, pp. 6482–6488, Sep. 2015, doi: 10.1021/acs.macromol.5b01550. [248] D. G. Karis et al., “Cross-linkable multi-stimuli responsive hydrogel inks for direct-write 3D printing,” Polym. Chem., vol. 8, no. 29, pp. 4199–4206, Jul. 2017, doi: 10.1039/C7PY00831G. [249] K. Kobayashi, S. H. Oh, C. Yoon, and D. H. Gracias, “Multitemperature Responsive Self- Folding Soft Biomimetic Structures,” Macromol. Rapid Commun., vol. 39, no. 4, p. 1700692, Feb. 2018, doi: 10.1002/marc.201700692. [250] Z. Lei, Q. Wang, and P. Wu, “A multifunctional skin-like sensor based on a 3D printed thermo- responsive hydrogel,” Mater. Horiz., vol. 4, no. 4, pp. 694–700, 2017, doi: 10.1039/C7MH00262A. [251] D. B. Kolesky, R. L. Truby, A. S. Gladman, T. A. Busbee, K. A. Homan, and J. A. Lewis, “3D bioprinting of vascularized, heterogeneous cell-laden tissue constructs,” Adv. Mater. Deerfield Beach Fla, vol. 26, no. 19, pp. 3124–3130, May 2014, doi: 10.1002/adma.201305506. [252] N. Celikkin et al., “3D Printing of Thermoresponsive Polyisocyanide (PIC) Hydrogels as Bioink and Fugitive Material for Tissue Engineering,” 2018, doi: 10.3390/polym10050555. [253] G. Stoychev, A. Kirillova, and L. Ionov, “Light-Responsive Shape-Changing Polymers,” Adv. Opt. Mater., vol. 0, no. 0, p. 1900067, doi: 10.1002/adom.201900067. [254] L. Zhu, Y.-Y. Gao, B. Han, Y.-L. Zhang, and H.-B. Sun, “Laser fabrication of graphene-based electrothermal actuators enabling predicable deformation,” Opt. Lett., vol. 44, no. 6, pp. 1363– 1366, Mar. 2019, doi: 10.1364/OL.44.001363. [255] T. H. Ware, “Photoinduced Shape Programming,” in Photomechanical Materials, Composites, and Systems, John Wiley & Sons, Ltd, 2017, pp. 327–368. [256] Y. Lee, H. Lee, T. Hwang, J.-G. Lee, and M. Cho, “Sequential Folding using Light-activated Polystyrene Sheet,” Sci. Rep., vol. 5, p. 16544, Nov. 2015, doi: 10.1038/srep16544. [257] Y. Lee and M. Cho, “Self-folding structure using light-absorption of polystyrene sheet,” in Behavior and Mechanics of Multifunctional Materials and Composites 2017, Apr. 2017, vol. 10165, p. 101650L, doi: 10.1117/12.2259911. [258] Y. Lee, J. Moon, J. Choi, and M. Cho, “Self-folding Structural Design Using Multiscale Analysis on the Light-absorption Folding Behaviour of Polystyrene Sheet,” Sci. Rep., vol. 7, no. 1, p. 14277, Oct. 2017, doi: 10.1038/s41598-017-14599-z. [259] Y. Liu, J. K. Boyles, J. Genzer, and M. D. Dickey, “Self-folding of polymer sheets using local light absorption,” Soft Matter, vol. 8, no. 6, pp. 1764–1769, 2012, doi: 10.1039/C1SM06564E. [260] Y. Liu, B. Shaw, M. D. Dickey, and J. Genzer, “Sequential self-folding of polymer sheets,” Sci. Adv., vol. 3, no. 3, p. e1602417, Mar. 2017, doi: 10.1126/sciadv.1602417. [261] D. Hua et al., “3D printing of shape changing composites for constructing flexible paper-based photothermal bilayer actuators,” J. Mater. Chem. C, vol. 6, no. 8, pp. 2123–2131, Feb. 2018, doi: 10.1039/C7TC05710E. [262] Y. Hu, G. Wu, T. Lan, J. Zhao, Y. Liu, and W. Chen, “A Graphene-Based Bimorph Structure for Design of High Performance Photoactuators,” Adv. Mater., vol. 27, no. 47, pp. 7867–7873, 2015, doi: 10.1002/adma.201502777. [263] A. W. Hauser, D. Liu, K. C. Bryson, R. C. Hayward, and D. J. Broer, “Reconfiguring Nanocomposite Liquid Crystal Polymer Films with Visible Light,” Macromolecules, vol. 49, no. 5, pp. 1575–1581, Mar. 2016, doi: 10.1021/acs.macromol.6b00165. [264] M. K. Gupta et al., “3D Printed Programmable Release Capsules,” Nano Lett., vol. 15, no. 8, pp. 5321–5329, Aug. 2015, doi: 10.1021/acs.nanolett.5b01688. [265] Y. Hu, G. Wu, T. Lan, J. Zhao, Y. Liu, and W. Chen, “A Graphene-Based Bimorph Structure for Design of High Performance Photoactuators,” Adv. Mater., vol. 27, no. 47, pp. 7867–7873, 2015, doi: 10.1002/adma.201502777. [266] X. Zhang et al., “Optically- and Thermally-Responsive Programmable Materials Based on -Hydrogel Polymer Composites,” Nano Lett., vol. 11, no. 8, pp. 3239–3244, Aug. 2011, doi: 10.1021/nl201503e. [267] H. Yang et al., “3D Printed Photoresponsive Devices Based on Shape Memory Composites,” Adv. Mater., vol. 29, no. 33, p. 1701627, 2017, doi: 10.1002/adma.201701627. [268] C. L. van Oosten, C. W. M. Bastiaansen, and D. J. Broer, “Printed artificial cilia from liquid- crystal network actuators modularly driven by light,” Nat. Mater., vol. 8, no. 8, pp. 677–682, Aug. 2009, doi: 10.1038/nmat2487. [269] A. H. Gelebart, D. J. Mulder, G. Vantomme, A. P. H. J. Schenning, and D. J. Broer, “A Rewritable, Reprogrammable, Dual Light-Responsive Polymer Actuator,” Angew. Chem., vol. 129, no. 43, pp. 13621–13624, 2017, doi: 10.1002/ange.201706793. [270] Y. Gritsai, L. M. Goldenberg, and J. Stumpe, “Efficient single-beam light manipulation of 3D microstructures in azobenzene-containing materials,” Opt. Express, vol. 19, no. 19, pp. 18687– 18695, Sep. 2011, doi: 10.1364/OE.19.018687. [271] B. M. Boyle, T. A. French, R. M. Pearson, B. G. McCarthy, and G. M. Miyake, “Structural Color for Additive Manufacturing: 3D-Printed Photonic Crystals from Block Copolymers,” ACS Nano, vol. 11, no. 3, pp. 3052–3058, Mar. 2017, doi: 10.1021/acsnano.7b00032. [272] J. T. Cabral, S. D. Hudson, C. Harrison, and J. F. Douglas, “Frontal Photopolymerization for Microfluidic Applications,” Langmuir, vol. 20, no. 23, pp. 10020–10029, Nov. 2004, doi: 10.1021/la049501e. [273] Z. Zhao, J. Wu, X. Mu, H. Chen, H. J. Qi, and D. Fang, “Origami by frontal photopolymerization,” Sci. Adv., vol. 3, no. 4, p. e1602326, Apr. 2017, doi: 10.1126/sciadv.1602326. [274] J. P. Thomas and M. A. Qidwai, “Mechanical design and performance of composite multifunctional materials,” Acta Mater., vol. 52, no. 8, pp. 2155–2164, May 2004, doi: 10.1016/j.actamat.2004.01.007. [275] A. D. B. L. Ferreira, P. R. O. Nóvoa, and A. T. Marques, “Multifunctional Material Systems: A state-of-the-art review,” Compos. Struct., vol. 151, pp. 3–35, Sep. 2016, doi: 10.1016/j.compstruct.2016.01.028. [276] A. Vakil Amirkhizi, S. Nemat-Nasser, A. Starr, T. Plaisted, and S. Nemat-Nasser, “Multifunctional Materials,” in Biomimetics, CRC Press, 2005, pp. 309–340. [277] F. Momeni and J. Ni, “Nature-inspired smart solar concentrators by 4D printing,” Renew. Energy, vol. 122, pp. 35–44, Jul. 2018, doi: 10.1016/j.renene.2018.01.062. [278] T. E. Rardin and R. Xu, “Printing Processes Used to Manufacture Photovoltaic Solar Cells,” J. Technol. Stud., vol. 37, no. 2, May 2011, doi: 10.21061/jots.v37i2.a.1. [279] A. Polman, M. Knight, E. C. Garnett, B. Ehrler, and W. C. Sinke, “Photovoltaic materials: Present efficiencies and future challenges,” Science, vol. 352, no. 6283, pp. aad4424–aad4424, Apr. 2016, doi: 10.1126/science.aad4424. [280] W. Xiang and W. Tress, “Review on Recent Progress of All-Inorganic Metal Halide Perovskites and Solar Cells,” Adv. Mater., vol. 31, no. 44, p. 1902851, 2019, doi: 10.1002/adma.201902851. [281] P. Li et al., “Inkjet manipulated homogeneous large size perovskite grains for efficient and large- area perovskite solar cells,” Nano Energy, vol. 46, pp. 203–211, Apr. 2018, doi: 10.1016/j.nanoen.2018.01.049. [282] C. N. Hoth, P. Schilinsky, S. A. Choulis, and C. J. Brabec, “Printing Highly Efficient Organic Solar Cells,” Nano Lett., vol. 8, no. 9, pp. 2806–2813, Sep. 2008, doi: 10.1021/nl801365k. [283] N. Lin, Y. Ye, Q. Guo, J. Yu, and T. Guo, “Effect of using ink containing polyacrylate and silicone surfactant on the inkjet printing of quantum dot films,” J. Inf. Disp., vol. 0, no. 0, pp. 1– 9, Dec. 2019, doi: 10.1080/15980316.2019.1707310. [284] S. K. Karunakaran et al., “Recent progress in inkjet-printed solar cells,” J. Mater. Chem. A, vol. 7, no. 23, pp. 13873–13902, 2019, doi: 10.1039/C9TA03155C. [285] B. Sopori, “Development of Thin Film Silicon Solar Cell Using Inkjet Printed Silicon and Other Inkjet Processes: Cooperative Research and Development Final Report, CRADA Number CRD- 07-260,” NREL/TP-7A10-53846, 1043746, Apr. 2012. doi: 10.2172/1043746. [286] A. Gusain, R. M. Faria, and P. B. Miranda, “Polymer Solar Cells—Interfacial Processes Related to Performance Issues,” Front. Chem., vol. 7, 2019, doi: 10.3389/fchem.2019.00061. [287] A. Hübler et al., “Printed Paper Photovoltaic Cells,” Adv. Energy Mater., vol. 1, no. 6, pp. 1018– 1022, 2011, doi: 10.1002/aenm.201100394. [288] C. N. Hoth, S. A. Choulis, P. Schilinsky, and C. J. Brabec, “High Photovoltaic Performance of Inkjet Printed Polymer:Fullerene Blends,” Adv. Mater., vol. 19, no. 22, pp. 3973–3978, 2007, doi: 10.1002/adma.200700911. [289] X. Hu, F. Li, and Y. Song, “Wearable Power Source: A Newfangled Feasibility for Perovskite Photovoltaics,” ACS Energy Lett., vol. 4, no. 5, pp. 1065–1072, May 2019, doi: 10.1021/acsenergylett.9b00503. [290] Y. C. Yuan, T. Yin, M. Z. Rong, and M. Q. Zhang, “Self healing in polymers and polymer composites. Concepts, realization and outlook: A review,” Express Polym. Lett., vol. 2, no. 4, pp. 238–250, 2008, doi: 10.3144/expresspolymlett.2008.29. [291] Y. Yang and M. W. Urban, “Self-healing polymeric materials,” Chem. Soc. Rev., vol. 42, no. 17, p. 7446, 2013, doi: 10.1039/c3cs60109a. [292] Z. Wei et al., “Self-healing gels based on constitutional dynamic chemistry and their potential applications,” Chem. Soc. Rev., vol. 43, no. 23, pp. 8114–8131, Nov. 2014, doi: 10.1039/C4CS00219A. [293] J. Jin, L. Cai, Y.-G. Jia, S. Liu, Y. Chen, and L. Ren, “Progress in self-healing hydrogels assembled by host–guest interactions: preparation and biomedical applications,” J. Mater. Chem. B, vol. 7, no. 10, pp. 1637–1651, 2019, doi: 10.1039/C8TB02547A. [294] K. Urdl, A. Kandelbauer, W. Kern, U. Müller, M. Thebault, and E. Zikulnig-Rusch, “Self- healing of densely crosslinked thermoset polymers—a critical review,” Prog. Org. Coat., vol. 104, pp. 232–249, Mar. 2017, doi: 10.1016/j.porgcoat.2016.11.010. [295] C. J. Hansen, W. Wu, K. S. Toohey, N. R. Sottos, S. R. White, and J. A. Lewis, “Self-Healing Materials with Interpenetrating Microvascular Networks,” Adv. Mater., vol. 21, no. 41, pp. 4143–4147, 2009, doi: 10.1002/adma.200900588. [296] A. R. Hamilton, N. R. Sottos, and S. R. White, “Self-Healing of Internal Damage in Synthetic Vascular Materials,” Adv. Mater., vol. 22, no. 45, pp. 5159–5163, 2010, doi: 10.1002/adma.201002561. [297] I. P. S. Qamar, N. R. Sottos, and R. S. Trask, “Grand challenges in the design and manufacture of vascular self-healing,” Multifunct. Mater., vol. 3, no. 1, p. 013001, Feb. 2020, doi: 10.1088/2399-7532/ab69e2. [298] A. M. Aragón, J. K. Wayer, P. H. Geubelle, D. E. Goldberg, and S. R. White, “Design of microvascular flow networks using multi-objective genetic algorithms,” Comput. Methods Appl. Mech. Eng., vol. 197, no. 49, pp. 4399–4410, Sep. 2008, doi: 10.1016/j.cma.2008.05.025. [299] P. Sanders et al., “Stereolithographic 3D printing of extrinsically self-healing composites,” Sci. Rep., vol. 9, no. 1, p. 388, Jan. 2019, doi: 10.1038/s41598-018-36828-9. [300] B. Zhang et al., “Self-Healing Four-Dimensional Printing with an Ultraviolet Curable Double- Network Shape Memory Polymer System,” ACS Appl. Mater. Interfaces, vol. 11, no. 10, pp. 10328–10336, Mar. 2019, doi: 10.1021/acsami.9b00359. [301] M. Invernizzi, S. Turri, M. Levi, and R. Suriano, “4D printed thermally activated self-healing and shape memory polycaprolactone-based polymers,” Eur. Polym. J., vol. 101, pp. 169–176, Apr. 2018, doi: 10.1016/j.eurpolymj.2018.02.023. [302] M. Nadgorny, Z. Xiao, and L. A. Connal, “2D and 3D-printing of self-healing gels: design and extrusion of self-rolling objects,” Mol. Syst. Des. Eng., vol. 2, no. 3, pp. 283–292, Aug. 2017, doi: 10.1039/C7ME00023E. [303] X. Li et al., “Self-Healing Polyurethane Elastomers Based on a Disulfide Bond by Digital Light Processing 3D Printing,” ACS Macro Lett., vol. 8, no. 11, pp. 1511–1516, Nov. 2019, doi: 10.1021/acsmacrolett.9b00766. [304] G. H. Yang, M. Yeo, Y. W. Koo, and G. H. Kim, “4D Bioprinting: Technological Advances in Biofabrication,” Macromol. Biosci., vol. 19, no. 5, p. 1800441, 2019, doi: 10.1002/mabi.201800441. [305] Q. Yang, B. Gao, and F. Xu, “Recent Advances in 4D Bioprinting,” Biotechnol. J., vol. 15, no. 1, p. 1900086, 2020, doi: 10.1002/biot.201900086. [306] B. Mirani et al., “An Advanced Multifunctional Hydrogel-Based Dressing for Wound Monitoring and Drug Delivery,” Adv. Healthc. Mater., vol. 6, no. 19, p. 1700718, 2017, doi: 10.1002/adhm.201700718. [307] P. Morouço, W. Lattanzi, and N. Alves, “Four-Dimensional Bioprinting As a New Era for Tissue Engineering and Regenerative Medicine,” Front. Bioeng. Biotechnol., vol. 5, 2017, doi: 10.3389/fbioe.2017.00061. [308] G. Stoychev, N. Puretskiy, and L. Ionov, “Self-folding all-polymer thermoresponsive microcapsules,” Soft Matter, vol. 7, no. 7, pp. 3277–3279, Mar. 2011, doi: 10.1039/C1SM05109A. [309] C. M. Piard, Y. Chen, and J. P. Fisher, “Cell-Laden 3D Printed Scaffolds for Bone Tissue Engineering,” Clin. Rev. Bone Miner. Metab., vol. 13, no. 4, pp. 245–255, Dec. 2015, doi: 10.1007/s12018-015-9198-5. [310] A. Kirillova, R. Maxson, G. Stoychev, C. T. Gomillion, and L. Ionov, “4D Biofabrication Using Shape-Morphing Hydrogels,” Adv. Mater., vol. 29, no. 46, p. 1703443, 2017, doi: 10.1002/adma.201703443. [311] Y. Luo, C. Wu, A. Lode, and M. Gelinsky, “Hierarchical mesoporous bioactive glass/alginate composite scaffolds fabricated by three-dimensional plotting for bone tissue engineering,” Biofabrication, vol. 5, no. 1, p. 015005, Dec. 2012, doi: 10.1088/1758-5082/5/1/015005. [312] A. T. Gaynor, N. A. Meisel, C. B. Williams, and J. K. Guest, “Multiple-Material Topology Optimization of Compliant Mechanisms Created Via PolyJet Three-Dimensional Printing,” J. Manuf. Sci. Eng. Trans. ASME, vol. 136, no. 6, p. 061015, Dec. 2014, doi: 10.1115/1.4028439. [313] T. Stankovic, J. Mueller, P. Egan, and K. Shea, “Optimization of Additively Manufactured Multi-Material Lattice Structures Using Generalized Optimality Criteria,” presented at the ASME 2015 International Design Engineering Technical Conferences and Computers and Information in Engineering Conference, Jan. 2016, doi: 10.1115/DETC2015-47403. [314] O. Weeger, Y. S. B. Kang, S.-K. Yeung, and M. L. Dunn, “Optimal Design and Manufacture of Active Rod Structures with Spatially Variable Materials,” 3D Print. Addit. Manuf., vol. 3, no. 4, pp. 204–215, Dec. 2016, doi: 10.1089/3dp.2016.0039. [315] G. Sossou, F. Demoly, H. Belkebir, H. J. Qi, S. Gomes, and G. Montavon, “Design for 4D printing: A voxel-based modeling and simulation of smart materials,” Mater. Des., vol. 175, p. 107798, Aug. 2019, doi: 10.1016/j.matdes.2019.107798. [316] G. Sossou, F. Demoly, H. Belkebir, H. J. Qi, S. Gomes, and G. Montavon, “Design for 4D printing: Modeling and computation of smart materials distributions,” Mater. Des., vol. 181, p. 108074, Nov. 2019, doi: 10.1016/j.matdes.2019.108074. [317] A. Zolfagharian, A. Kaynak, S. Y. Khoo, and A. Kouzani, “Pattern-driven 4D printing,” Sens. Actuators Phys., vol. 274, pp. 231–243, May 2018, doi: 10.1016/j.sna.2018.03.034. [318] H. Meng and Jinlian Hu, “A Brief Review of Stimulus-active Polymers Responsive to Thermal, Light, Magnetic, Electric, and Water/Solvent Stimuli,” J. Intell. Mater. Syst. Struct., vol. 21, no. 9, pp. 859–885, Jun. 2010, doi: 10.1177/1045389X10369718. [319] Y. Mao et al., “3D Printed Reversible Shape Changing Components with Stimuli Responsive Materials,” Sci. Rep., vol. 6, no. 1, Apr. 2016, doi: 10.1038/srep24761.