HTML
-
TPP technology empowers the fabrication of multifunctional micro/nanostructures by selecting appropriate photoresist materials tailored to the desired functions of the target application [83]. Currently, a diverse range of photoresists is available for 4D printing [34, 84], with an increasing number satisfying the five fundamental requirements for TPP outlined in the previous section. These specialized photoresists enable the creation of micro/nanostructures exhibiting dynamic properties such as stimulus responsiveness, biomimetic self-actuation, color-changing, and shape-morphing capabilities, which are beyond the reach of commercial photoresists. This section emphasizes the use of 4D printing materials suitable for TPP, and introduces them according to four categories: magnetic materials, SMPs, hydrogels, and LCEs. Table 1 presents the different material types in detail, including the specific materials, their microstructures, corresponding stimulus responses, and transformation types.
Materials Structures Stimuli Transformation References Photoresist SU-8, IP-L Helical microswimmer Magnetic-driven Self-propulsion [39] Photoresist SU-8, PEG Microtransporters, Archimedean screw-pump Magnetic-driven Corkscrew motion and translation [40] Photoresist SU-8, Ferrofluid Helical microswimmer Magnetic-driven Cork-screw propulsion, rotation [85] Photoresist IPL-780, ZIF-8 Helical microswimmer Magnetic-driven Wobbling and corkscrew motion, step-out [86] Photoresist CB/CBX, SB/SBX Helical microswimmer Magnetic-driven Corkscrew motion and translation [53] Photoresist OrmoComp Helical microstructures Magnetic-driven Swimming behaviors [87] Photoresist OrmoComp Helical corkscrew propeller and spiral-shaped micropropeller Magnetic-driven Corkscrew propulsion and rolling motion [88] Photoresist SZ2080 Microtubes Magnetic-driven Rotation and propelling [89] Photoresist SZ2080, IESL-FORTH Conical hollow microhelices Magnetic-driven Forward swimming and lateral drift [90] Photoresist IPL-780 Micromanipulation Light-driven Axial motion [91] Photoresist IP-S Microbutterfly, microsheets, micro-origami Capillary-force-driven Reversible bending [92] Photoresist SZ2080 Micro-actuator, micro-sensor, deformable DOE Solvent-driven (2-propanol, acetone, ethanol, and PEN) Swelling and shrinkage [93] Photoresist IP-S, IP-Visio Dumb-bell shaped fiber structures Solvent-driven (water) Shape morphing [94] Photoresist FemtoBond 4B Ridges, multilayer systems Solvent-driven (isopropanol, toluene) Optical properties changing [95] SMP Vero Clear, HPPA, BPA, TPO Upright grids/color palette Thermo-driven Geometry and optical properties changing [51] SMP IsobA, PEGDA 575, TcddA Double platform, infinity ring, frame Thermo-driven Shape morphing [58] SMP Benzyl methacrylate-based SMP Flowers, cubic lattices Thermo-driven Shape morphing [96] SMP AAc, HPPA, PVP, DPEPA Nanopillars Thermo-driven Geometry and optical properties changing [97] Hydrogel PEGDA, Irgacure 369 Helical microswimmer Magnetic-driven Self-propulsion [52] Hydrogel GelMA, lithium phenyl(2,4,6-trimethylbenzoyl) phosphinate, iron oxide Helical microswimmer Magnetic-driven Wobbling behavior, cork-screw motion [98] Hydrogel PEGDA, PETA Helical microswimmer Magnetic-driven Corkscrewing motion [99] Hydrogel PEGDA, PETA, MNPs, 5-FU, Irgacure-369 Helical microstructure Magnetic-driven Corkscrewing motion, rolling and yawing, nonreciprocal motion [100] Hydrogel GelMA, P2CK, PBS Helical microstructure Magnetic-driven Wobbling and cork-screw motion [101] Hydrogel ChMA, LAP, PEG Helical microstructure Magnetic-driven and light-driven Cork-screw propulsion, rotation [102] Hydrogel NIPAAm, AAc, PVP, EL, DPEPA, TEA, EMK/DMF Lantern, microball, microstent, microcage, micro-umbrella pH-driven Shrinking and swelling behaviors [103] Hydrogel NIPAAm, AAc, EL, PVP, DPEPA, TEA, EMK, DMF Microflower, articulated building blocks, micro-race car pH-driven Swelling/shrinking, bending deformations [104] Hydrogel NIPAAm, AAc, DPEPA, EMK, DMF, TEOA Microcrawlers pH-driven Swelling/shrinking, bending deformations [105] Hydrogel BSA, methylene blue Micro-spider, arm-muscle, claw-muscle pH-driven Shrinking and swelling behaviors [106] Hydrogel DMAEMA, PETA, PEGDA, TPO, HMPP Bionic flytrap microactuator pH-driven Swelling/shrinking, bending deformations [107] Hydrogel AAc, PVP, ethyl lactate, NIPAM Blade structures, flowers, microcage pH-driven Swelling and deswelling [108] Hydrogel BSA, RB, NaOH, HCl 3D reliefs of sculptures, microsieves pH-driven Swelling behavior [109] Hydrogel NIPAAm, AAc, PVP, EL, DPEHA, TEA, EMK, DMF Microtubes, multivalve torsional chiral structure, micro gripper pH-driven Swelling/shrinking, bending and upright [110] Hydrogel AAC, NIPAAm, EL, PVP, DPEPA, DPEPA, EMK, DMF Bilayer heterostructures, microcrawler pH-driven Swelling, bending deformation [111] Hydrogel NIPAM, MBA, LAP, SWNTs, EG, TEOA Hollow buckyball, micropillar, microclamp, artificial aortic valve Light-driven Swelling/shrinking [112] Hydrogel pNIPAM, PETA Microchannel, hetero-structures Light-driven and thermo-driven Shrinking and swelling, bending [113] Hydrogel pNIPAM Micropillar, microvalves Thermo-driven Shrinking and swelling [114] Hydrogel IPAM, MBA, PNIPAM, IP-L Flower structure Thermo-driven Shrinking and swelling [115] Hydrogel PPG-DA, [P4,4,4,6][SPA], DEATC Micro-pillar arrays, micro-spiral arrays, micro-grids, maple leaf Thermo-driven and solvent-driven (water) Swelling/contraction [116] Hydrogel 2,4,6-trimethylbenzoyl, BMA, TMPTA Microflower, microvalve, microclaw Solvent-driven (acetone) Reversible actuation [117] Hydrogel Am-PBA, MBIS, DEATC Vase structure, bilayer beams Solvent-driven (sugar) Expansion and shrinkage [118] Hydrogel PEG-DA-575, PETA, PI Micro-actuator, micro-sensor, deformable DOE Solvent-driven (2-propanol, acetone, ethanol, and PEN) Swelling and shrinkage [93] Hydrogel PEG-DA, MB Microflower, micropillar arrays, joint-like cantilever Humidity-driven Swelling/shrinking [119] LCE Difunctional and monofunctional mesogenic acrylates, photoinitiator Interwoven fabric structure, woodpile, spiral disk Thermo-driven Contraction and expansion, color change [50] LCE ST3021, ST3866, Irgacure 369, dye DR1A Individual voxels, 3D line, rectangular frame Thermo-driven Expansion and shrinkage [120] LCE Monomers, chiral dopant, monofunctional acrylate, and carboxylic acid mesogens Flower, butterfly Humidity-driven and thermo-driven Expansion, color change [121] LCE LC monomer, LC crosslinker, azo dye, Irgacure 369 Microscopic walker Light-driven Contraction [54] LCE LC monomer, LC crosslinker, azo dye, Irgacure 369 Microhand, microfinger Light-driven Bending [122] LCE LC monomer, LC crosslinker, E7 mixture, Irgacure 369 Chiral elastic metamaterial Light-driven (blue LED) Anisotropic shrinkage [123] LCE C6BP, RM257, Irgacure 369 Woodpile, hexagonal crystal structures, frame, microclamp Photothermal-driven Shrinkage [124] LCE Acrylic monomer, bifunctional-acrylate, unreactive E7 mixture Microstructures Photo-driven Shrinkage, bending [125] aAbbreviation: DOE: diffractive optical elements, PEN: 4-methyl-2-pentanone, PEG: Polyethylene glycol; CB: Carboxybetaine methacrylate; CBX: Carboxybetaine dimethacrylate; SB: Sulfobetaine methacrylate; SBX: Sulfobetaine dimethacrylate; HPPA: 2-hydroxy-3-phenoxypropyl acrylate; BPA: Bisphenol A ethoxylate dimethacrylate; TPO: Diphenyl (2,4,6-trimethylbenzoyl) phosphine oxide; IsobA: Isobornyl acrylate; PEGDA: Poly(ethylene glycol) diacrylate; TcddA: Tricyclo[5.2.1.02,6]decanedimethanol diacrylate; PETA: Pentaerythritol triacrylate; MNPs: Magnetic Fe3O4 nanoparticles; 5-FU: 5-fluorouracil; GelMA: Gelatin methacryloyl; P2CK: Synthesis of cyclopentanone and benzaldehyde 3-[(4-formyl-phenyl)-methyl-amino]-propionic acid; ChMA: Methacrylamide chitosan; LAP: Phenyl-2,4,6-trimethylbenzoylphosphinate; NIPAAm: N-isopropylacrylamide; AAc: Acrylic acid; PVP: Polyvinylpyrrolidone; EL: Ethyl lactate; DPEPA: Dipentaerythritol pentaacrylate; TEA: Triethanolamine; EMK: 4,4-bis(diethylamino)benzophenone; DMF: N,N-dimethylformamide; TEOA: Triethanolamine; BSA: Bovine serum albumin; MB: Methylene blue; DMAEMA: 2-(dimethylamino)ethyl methacrylate; HMPP: 2-hydroxy-2-methylpropiophenone; RB: Rose Bengal; HCl: Hydrochloric acid; DPEHA: Dipentaerythritol hexaacrylate; SWNTs: Single-walled carbon nanotubes; EG: Ethylene glycol; MBA: N,N-methylenebis(acrylamide); [P4,4,4,6][SPA]: Tributylhexyl sulfopropyl acrylate; DEATC: 7-diethylamino-3-thenoylcoumarin; BMA: Butyl methacrylate; TMPTA: Propoxylated trimethylolpropane triacrylate; AAm-PBA: 3-(acrylamido)phenylboronic acid; ST3866: 4-methoxybenzoic acid 4-(6-acryloyloxyhexyloxy) phenyl ester; ST3021: 1,4-bis[4-(3-acryloyloxypropyloxy)benzoyloxy]-2-methylbenzene; DR1A: Disperse red 1 acrylate; LC: Liquid crystal; C6BP: 4-methoxybenzoic acid 4-(6-acryloyloxy-hexyloxy) phenyl ester; RM257: 1,4-bis-[4-(3-acryloyloxypropyloxy)benzoyloxy]-2-methylbenzene. Table 1. Summary of the optional materials for TPP-based 4D printing.
-
Magnetic materials, also known as magnetically responsive materials, belong to a subset of materials that exhibit a response or change in their properties when exposed to a magnetic field [126]. These materials have gained prominence due to their rapid, precise, and remote response in various environments [127]. A particularly intriguing application lies in magnetic microswimmers, where rotating magnetic fields induce rotational forces and torques, translating into the rotational motion of the magnetic components. Inspired by the structure of bacterial flagella, helical microswimmers are typically designed with corkscrew-like or spiral shapes. Utilizing a rotating magnetic field, magnetic microswimmers convert rotational motion into translational motion through self-helical propulsion, a phenomenon observed in low Reynolds number regimes [128]. The evolution of TPP-printed magnetic field-driven micro/nanostructures has progressed from simple helical designs to complex motion structures with flexible links and rigid segments [129], demonstrating significant potential in biomedical applications [130]. Given that many TPP-suitable materials (e.g. photoresists, hydrogels, etc) lack inherent magnetic properties, achieving magnetically responsive micro/nanostructures through TPP involves integrating light-curable soft materials with magnetic components. There are two approaches to achieving magnetically responsive micro/nanostructures through TPP incorporating magnetic materials: (1) the sequential post-processing approach: coating the magnetic material on the TPP-printed structures surface; (2) the direct TPP printing approach: mixing magnetic nanoparticles with photoresist, and directly printing them via TPP technology.
-
Coating magnetic materials process involves depositing a magnetic material onto the desired substrate using various techniques. The magnetic layer adds an additional magnetic response to the coated surface, enhancing the performance in applications that require magnetic functionality. The process of selectively coating magnetic materials typically involves two steps: (1) utilizing TPP technology to create microstructures, and (2) depositing magnetic material selectively through physical vapor deposition (PVD). Tottori et al developed a straightforward fabrication approach for creating spiral micromachines by 3D DLW and PVD, enabling the creation of helical devices in versatile shapes [39]. The manufacturing process is outlined in figure 4(a). First, a negative-tone photoresist (such as SU-8 or IP-L) was employed with DLW to construct helical microswimmers. In the second step, the unpolymerized photoresist was removed through development. Finally, thin bilayers of Ni/Ti were deposited onto the surface of the polymer helical micromachine using electron beam evaporation. This coating not only facilitated magnetic actuation but also enhanced the biocompatibility of the surface. Similarly, Huang et al presented a deposit-based method for fabricating micro transporters [40]. As depicted in figure 4(b), utilizing SU-8 photoresist with TPP, these micromachines were fabricated horizontally. After the development, only the outer propeller was coated with Ni/Ti bilayers (300/5 nm thickness) via PVD. Meanwhile, non-actuated components were shielded by printing a sacrificial structure post-assembly. Notably, microfluidic channels and helical microswimmers were independently printed using this approach. Yasa et al employed TPP from a prepolymer solution containing PEGDA and a photoinitiator to create 3D microswimmers (figure 4(c)) [52]. After printing, the structures underwent magnetization through sequential sputter coating with nickel and gold. Subsequently, the structures were modified with thiol-modified PEG. These devices exhibit corkscrew motion when exposed to a sufficiently high input frequency from a rotating field.
Figure 4. Printing process of magnetic materials for TPP 4D printing. (a) Manufacturing process of helical microswimmers. [39] John Wiley & Sons. © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. (b) Schematic description of the fabrication process of the micro transporter. [40] John Wiley & Sons. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. (c) Illustration of the TPP microprinting procedure of PEGDA. From [52]. Reprinted with permission from AAAS. (d) Fabrication of swimming microrobots with engineered magnets. [85] John Wiley & Sons. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. (e) 3D fabrication of the microswimmers using TPP. Reprinted with permission from [98]. Copyright (2019) American Chemical Society.
-
Introducing magnetic properties into materials can also be achieved by mixing photoresists with magnetic nanoparticles. This technique involves incorporating magnetic nanoparticles, usually at the nanoscale, into the formulation of the photoresist. These magnetic nanoparticles become dispersed within the photoresist matrix, resulting in a composite material endowed with magnetic characteristics. This strategy of combining photoresists with magnetic nanoparticles broadens the spectrum of materials capable of exhibiting magnetic behavior. It offers a versatile means to imbue photoresist-based systems with magnetic functionality, facilitating the production of magnetic microstructures with tailored attributes. The integration of magnetite nanoparticles with TPP has enabled the creation of intricate 3D soft-magnetic microdevices boasting shape-independent magnetic features. Peters et al illustrated the fabrication of single- and double-twist microswimmers featuring controlled magnetic anisotropy utilizing a superparamagnetic polymer composite comprised of Fe3O4 nanoparticles embedded in commercial SU-8 negative tone photoresist [85]. The fabrication process involves spin coating, particle alignment, and TPP (figure 4(d)). Additionally, Ceylan et al harnessed TPP to craft a structure with dual helical configurations designed for cargo loading and responsive swimming under the influence of a rotating magnetic field [98]. As displayed in figure 4(e), these microswimmers are 3D printed from a magnetic precursor mixture containing GelMA and biofunctionalized superparamagnetic Fe3O4 nanoparticles. A continuous magnetic field is applied throughout the fabrication process to maintain nanoparticle alignment.
-
SMPs represent a class of smart materials that undergo controlled shape changes in response to external stimuli [131]. Among these, thermos-responsive SMPs exhibit a thermally induced shape memory effect (SME) when the temperature surpasses their glass transition temperature (Tg). This remarkable property facilitates controlled transformations between permanent and temporary shapes, with the ability to return to the original permanent state via temperature adjustments. The exploration of shape transformation within SMPs has ventured into complex 3D geometries with nanoscale attributes, supporting diverse applications ranging from miniaturized deployable biomedical devices to stimuli-responsive mechanical metamaterials.
An innovative breakthrough by Zhang et al introduced a novel shape memory photoresist that achieves an impressive 300 nm half-pitch resolution in printed features through TPP lithography [51]. Their work involved the development and characterization of an SMP photoresist based on Vero Clear, formulated by combining Vero Clear with an elastomeric resist composed of HPPA, BPA, and TPO at varying mass fractions. Figure 5(a) illustrates their approach, where submicron-scale grids were programmed atop a foundation layer. These structures, featuring vertical grids, acted as architectural color filters, selectively transmitting specific visible light wavelengths. Upon heating to elevated temperatures, the structures underwent deformation, flattening the nanostructures and eliminating coloration. Cooling them to room temperature maintained the structures in an invisible state. Reheating the structure restored the original geometry and color of the nanostructures, demonstrating 4D printing at the submicron scale.
Figure 5. TPP-based 4D printing shape memory polymer. (a) Illustration of color and shape changes in invisible ink nanostructured elements. Reproduced from [51], with permission from Springer Nature. (b) SMP ink system for 4D printing at the macro/microscale. [58] John Wiley & Sons. © 2022 The Authors. Advanced Functional Materials published by Wiley-VCH GmbH. (c) A diagram and SEM images of the cylinders shape memory programming cycle, Scale bar 5
m. [96] John Wiley & Sons. © 2020 Wiley-VCH GmbH. Furthermore, researchers have investigated the TPP-printed SMPs at the microscale. Spiegel et al devised a versatile ink system with a shape memory effect suitable for 4D printing at both the macro- and micro-scales, utilizing DLP and DLW technologies, respectively (figure 5(b)) [58]. They identified a common functional core system, comprising appropriate monomers, crosslinkers (IsobA, PEGDA 575, and TcddA), and the photoinitiator (Irg819), that exhibited an SME and was adaptable to both printing technologies. A variety of 3D structures were printed using TPP, including basic strips, frames, and more intricate designs such as infinity rings, double platform geometries, and cubic grids at the centimeter scale. These printing endeavors achieved efficient high-resolution microscale results, with a layer height of 820
m. Moreover, both the large- and small-scale printed structures demonstrated a remarkable SME. Elliott et al introduced an ink composed of acrylate/methacrylate-based compounds [96]. Their ink formulation featured BMA as the first chain builder, a functionalized amine-methacrylate as the second chain builder, and synthetically converted pentaerythritol triacrylate as the crosslinker. The authors successfully demonstrated the SME in printed microstructures employing this particular material (figure 5(c)). -
Hydrogels are 3D crosslinked polymer networks with softness, biocompatibility, and multifunctionality, making them highly valuable in various biomedical applications [132]. In the realm of 4D printing, the use of smart hydrogels is particularly advantageous due to their responsiveness to external stimuli such as temperature, humidity, pH, light, and solutions with varying ionic strengths or concentrations [133]. With their swelling properties, hydrogels can give rise to deformable structures capable of intricate shape changes such as bending, folding, twisting, and multiple deformations. It is noteworthy, however, that achieving complex shape transformations requires a swelling mismatch within the actuating region of the hydrogel to generate internal stress. This is because isotropic hydrogels expand uniformly, leading to structural linear expansion [134]. Moreover, in addition to the hydrogel application mentioned in section 3.1, where it serves as a matrix for magnetic materials in response to magnetic fields, this section will delve into the detailed design and fabrication of hydrogels responsive to other stimuli for TPP-based 4D printing.
Among these, pH-responsive hydrogels merit attention for their ability to undergo structural or chemical changes in response to shifts in pH levels within the surrounding environment, leading to altered swelling behaviors. This characteristic renders them suitable for TPP-based 4D printing due to their ease of preparation [103110]. An advanced strategy for 4D microprinting was introduced by Jin et al, involving the creation of shape-morphing micromachines utilizing stimulus-responsive hydrogels [103]. The key polymerization reaction driving the printing process is depicted in figure 6(a), while the fabrication process is illustrated in figure 6(b). By controlling parameters such as the exposure dose of the femtosecond laser, crosslinking densities, stiffness, swelling/shrinking degrees, and other properties can be finely tuned. Finite-element methods were utilized to predict characterization and shape-changing behaviors (figure 6(b)) [104]. Furthermore, Chen et al demonstrated that bilayer-based microbeams crafted from pH-responsive smart materials via TPP could induce nonmonotonic bending deformations through a sequential size-dependent layer-by-layer swelling effect [105].
Figure 6. TPP-based 4D printing hydrogels. (a) The main materials and polymerization reactions involved in the TPP process. (b) Illustration of the 4D-DLW process of pH-responsive hydrogel. Reprinted from [103], Copyright (2020), with permission from Elsevier. (c) Microcolumnar cilia have different bending elongations under different light stimulation powers. (d) The transformation process of the printed microclamp with different light stimulation powers in an aqueous environment. [112] John Wiley & Sons. © 2023 Wiley-VCH GmbH. (e) Thermal shrinkage of pNIPAM hydrogel microstructures with different structures. [114] John Wiley & Sons. © 2022 The Authors. Advanced Materials Technologies published by Wiley-VCH GmbH. (f) Chemical structures of photoresist components and illustration of the fabrication procedure of the sugar-triggered hydrogel. (g) Swelling mechanism of a sugar-responsive hydrogel. [118] John Wiley & Sons. © 2023 The Authors. Advanced Functional Materials published by Wiley-VCH GmbH.
Light-responsive hydrogels [112, 113], thermo-responsive hydrogels [113116], and solvent-responsive hydrogels [117119] are also pivotal components in two-photon 4D printing. Light-responsive hydrogels respond to light, enabling remote and precise spatial and temporal control. Deng et al devised a 4D printing methodology to create light-triggered micromachines with unique 3D shape-changing capabilities [112]. Through an investigation of carbon nanotube-doped NIPAM composite smart hydrogels, they highlighted distinct responsiveness degrees among structural units to light under a constant laser power (figure 6(c)). Additionally, they designed micropillars with a predetermined 45 bending angle, arranged them circularly, and thereby fabricated a smart microclamp device (figure 6(d)). Thermo-responsive hydrogels offer ease of operation and tunable temperature range responses. An innovative approach by Spratte et al employed pNIPAM-based microactuator systems designed with precision via DLW technology [114]. Their study comprehensively explored the shrinkage and swelling properties of these systems, placing emphasis on actuator size, design variations, and DLW parameter influence on material actuation. Figure 6(e) illustrates the thermally driven shrinkage of pNIPAM hydrogel microstructures with different architectures. Solvent-responsive hydrogels, while allowing reversible shape and size changes, can exhibit slow response times unsuitable for certain applications. Ennis and colleagues leveraged the flexibility of 2PP to develop a novel photoresist based on phenylboronic acid that responds to sugar [118]. As shown in figure 6(f), successful microstructure creation from sugar-responsive hydrogels using TPP is demonstrated, while figure 6(g) elucidates the mechanism behind hydrogel swelling.
-
LCEs stand out as a class of smart materials that exhibit substantial, anisotropic, and reversible shape changes in response to multiple stimuli. With their lightly crosslinked networks boasting oriented mesogenic orientation, LCEs have become a favored choice for realizing responsive structures [135]. Notably, LCEs have gained momentum in research due to their capacity to function without reliance on an aqueous environment or external loads [136]. These materials hold promise across diverse applications, including soft actuators and robots, artificial muscles, active structures, adaptive optics, and energy dissipators. Since the advent of TPP-based 4D printing technology, LCE actuators with more intricate 3D geometries, higher resolution, and an extended size range [137] have come to fruition.
Among the various stimuli, heat has emerged as a widely employed method, rendering thermos-responsive LCEs an optimal candidate for diverse applications. Del Pozo et al pioneered the development of a liquid crystalline photoresist, yielding a densely crosslinked polymer network that facilitates the creation of 4D-fabricated microactuators with predetermined shape changes (figure 7(a)) [50]. These 3D microstructures respond to temperature fluctuations by demonstrating reversible, anisotropic shape expansions and distinct polarization colors that hinge on the structures geometry rather than the DLWTPP parameters (figure 7(b)). Guo et al introduced a method for crafting microscale heterogeneous LCEs with controllable and uncoupled 3D structures through the assembly of microscale LCE voxel building blocks [120]. The anisotropic properties of LCEs in distinct directions enable the creation of devices with diverse physical properties, facilitated by employing 3D initial structures and director fields (figure 7(c)). Moreover, Del Pozo et al detailed the use of a supramolecular cholesteric LCE photonic photoresist to fabricate 4D photonic microactuators [121]. The integration of self-ordering smart materials with DLWTPP facilitates the realization of dual-responsive 3D microstructures, exhibiting controlled expansion and corresponding color changes in response to variations in humidity and temperature.
Figure 7. TPP-based 4D printing Liquid crystal elastomers. (a) Illustration of the fabrication procedure for thermal-driven uniaxially aligned 3D microstructures. (b) Hexagonal plate array in 3D profiles at 20 C and 220 C. [50] John Wiley & Sons. © 2021 The Authors. Small Structures published by Wiley-VCH GmbH. (c) Thermal actuation of a cuboid LCE frame with 80 voxels. Reproduced from [120], with permission from Springer Nature. (d) Schematic of a microhand and mesogen alignment before and after stimulus applied. [122] John Wiley & Sons. © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. (e) Mechanism of the reversible NIR-driven shape morphing behavior of the AuNR/LCE. Reprinted with permission from [124]. Copyright (2019) American Chemical Society. (f) Fabrication of the microscopic walker. (g) SEM image of a microwalker and their behavior of light trigger on and off. The scale bar is 10
m. [54] John Wiley & Sons. © 2015 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. (h) Fabrication process of multiphoto-responsive actuators. [125] John Wiley & Sons. © 2022 The Authors. Advanced Materials Technologies published by Wiley-VCH GmbH. Light serves as a popular stimulus due to its precise spatial and temporal control. TPP enables the intricate design of 3D robots with nanometer-scale precision. Martella et al introduced a light-driven microhand capable of remote control or autonomous action based on optical properties (figure 7(d)) [122]. The chosen splayed alignment prompts the bending of the four orthogonal fingers upon stimulus application (figure 7(e)). Chen et al developed a direct laser printable photoresist incorporating gold nanorods to enhance mechanical properties and achieve NIR-responsive mechanical deformation [124]. Zeng et al fabricated an artificial microwalker equipped with LCE muscle (figures 7(f) and (g)) using acrylic resin (IP-Dip) for the limbs due to its high Youngs modulus. The LCE muscle exhibits a fully reversible mechanical response to light, contracting by approximately 20% along the direction of the nematic LCE network [54]. Hsu et al proposed a straightforward strategy (figure 7(h) for crafting multi-photoresponsive 3D microstructures activated by different light wavelengths, leveraging an aligned LC ink formulation and dyes with orthogonal absorptions [125]. By successfully integrating five distinct dyes into the LC microstructure, they demonstrated the versatility of their approach, enabling actuation across varying wavelength ranges. The combination of different dyes allows the fabrication of multi-response LC microactuators, offering the flexibility to tailor responses as needed.