Light activation of 3D-printed structures: from millimeter to sub-micrometer scale

4.1.1 General photochemical light activation

Among photochemical reactive materials, azobenzene is the most widely studied for light-activated structures (Figure 4) [78], [79], [80], [81], [82], [83], [84], [85]. The molecular structure of azobenzene can be transformed from straight (trans) to bent (cis) upon UV light absorption. This trans-to-cis transformation is reversed upon visible light absorption (Figure 4a). By introducing azobenzene into the smart materials discussed in Section 2, it is possible to fabricate light-activated shape-morphing structures. The isomerization from trans to cis shortens the molecular chain of azobenzene. In this section, we introduce general photochemical design for light activation, which are fabricated by traditional methods, such as hot pressing and molding, and then we discuss the 3D printing of photochemical structures in the next section.

Figure 4:

General photochemical structures based on azobenzene.

Light-activated LCE bending structure: (a) schematic of a reversible bending structure. Helical LC structures can contract upon UV irradiation. The contraction near the top surface can induce bending. The original flat structure is recovered under 532 nm light illumination. Insets are the polarized optical microscopic images. (b) Bending behavior of the fabricated film, which also shows color changes due to the change of the helical pitch. Initially green film becomes red by UV light illumination. Adapted from Ref. [86]. Copyright (2020) Royal Society of Chemistry. Light-activated SMP bending structure: (c) UV–Vis absorption spectra of polyurethane/azobenzene dissolved in DMSO solution. Upon irradiation of UV light, the trans isomer gradually change its form to cis. This transition is reversed with visible light illumination. (d) Bending behavior of the polyurethane/azobenzene film. Adapted from Ref. [87]. Copyright (2017) ACS Publications. Light activated hydrogel structure: (e) schematic of isomerization of azobenzene. With 365 nm light irradiation, the crosslink density decreases (soft state). The transition is reversed with 420 nm light illumination (stiff state). (f) Reversible change of the storage modulus under the light illumination with alternative illumination wavelengths (365 nm, 420 nm). Adapted from Ref. [89]. Copyright (2018) ACS Publications.

A light-activated structure mimicking chameleon’s skin was demonstrated by including azobenzene in cholesteric LCEs [86]. Cholesteric LCEs have a macromolecular helical structure (Figure 4a). This periodic helical structure plays the role of photonic crystals, similar to those in chameleon’s skin. By absorbing 405 nm light, the molecular configuration of azobenzene transforms from trans to cis. The overall volume contraction near the upper surface makes the LCE film bent toward the illumination source. At the same time, photoisomerization causes the helical pitch to change. As a result, the optical reflection peak of the LCE film spectrally shifts; therefore, the film’s color, which is originally green, changes to red. Figure 4b shows the bending behavior of the LCE film, which accompanies a color change from green to red.

A photochemical light-activated film was also demonstrated by including azobenzene groups in liquid crystalline polyurethane networks [87]. Polyurethane is a representative SMP material, and polyurethane films show shape-memory behavior after thermomechanical programming. Because of the liquid crystalline networks, the synthesized liquid crystalline polyurethane structures show a triple shape memory characteristic with two transition temperatures, T _g (44 °C) and T _iso (94 °C). The original shape was deformed at 105 °C and fixed in a first temporary shape at 64 °C. Next, it was re-deformed at 64 °C and re-fixed in a second temporary shape at 25 °C. After this thermomechanical programming, the structure returned to the first temporary shape at 64 °C and to its original shape at 105 °C. Figure 4c presents the UV-visible absorption spectra of polyurethane/azobenzene dissolved in a dimethyl sulfoxide (DMSO) solution. Under 450 nm light illumination, the absorption peak of the trans isomer gradually decreases because photo-isomerization occurs. When the polymer solution is illuminated by 550 nm light again, the trans isomer absorption peak gradually increases back. This demonstrates a reversible photochemical reaction in the polymer solution. Figure 4d shows the photo-induced bending and unbending behavior of the SMP film. After the film is stretched with thermomechanical programming, a fixed temporary shape can shrink by photo-isomerization. Due to the rigidity of the SMP polymer at room temperature, this behavior can be observed only above the T _g of the polymer. SMPs become rubbery above T _g, and the illuminated upper region of the film can shrink. The difference in length between the upper and lower regions bent the structure toward the light source.

Hydrogels can also be light-activated by introducing azobenzene. Xiong et al. demonstrated a changeable modulus of hydrogels by the isomerization of azobenzene in supramolecular structures [88]. The isomerization of azobenzene from trans to cis states makes an originally crosslinked structure into a non-crosslinked structure. Therefore, the modulus decreases while the ductility increases (soft state). This phenomenon is a reversible process; the hydrogel/azobenzene recovers the crosslinked structure (stiff state) under 440 nm light illumination. Rosales et al. utilized a similar supramolecular hydrogel structure to control the release of a drug contained in a hydrogel structure by light [89]. When illuminated with 365 nm light, the hydrogel structure enters the soft state because of the decreased cross-linking density (Figure 4e). The structure can be returned to the stiff state by illuminating with 420 nm light. This transition between stiff and soft states can occur reversibly. In this way, a reversible modulus change of the hydrogel structure was demonstrated, which manifested an approximately 36% change in modulus (Figure 4f). The crosslinking density was correlated with the mesh size of the hydrogel structure. Proteins were encapsulated in the hydrogel structures, and light activation of drug release was demonstrated. The decrease in the crosslinking density by 365 nm light illumination led to an increase in drug release.

4.1.2 Photochemical activation of 3D-printed structures

Recently, the 3D printing of various photochemical structures was demonstrated. For example, DIW was used to 3D print an LCE film that contains azobenzene for light activation [90]. Photopolymerizable inks were prepared, and the LCE mesogens were aligned along the printing direction. The printed structure was polymerized using 530 nm light illumination. Because of the LC alignment, the 3D-printed LCE film shrinks when heated to 100 °C. Azobenzene in the printed film was activated by illumination with 365 nm UV light causing a trans-to-cis transformation. Figure 5a shows the bending and unbending behavior when illuminated by light. The 3D-printed film took 4 min to bend under UV light illumination (50 mW/cm²) (second panel in Figure 5a). Ceasing the illumination, a slight stress relaxation occurred in the film (third panel in Figure 5a). The stress relaxation was caused either by a temperature gradient of the upper and lower parts of the film or by thermally induced isomerization from cis to trans states. The remnant portion of the cis isomers held the structure in the bent shape. After illumination with 455 nm light (4 mW/cm²) for 30 min, the film recovered its initial state.

Figure 5:

3D printing of photochemical structures.

DIW of LCE structures with azobenzene group: (a) real images of LCE structure under UV and visible light illumination. Adapted from Ref. [90]. Copyright (2020) ACS Publications. DIW of non-crosslinked LCs on a Kapton layer: (b) bending behavior under light illumination. Bending occurs in 5 s and recovery occurs in 10 s. Adapted from Ref. [91]. Copyright (2018) ACS Publications. DIW of ADT-included LCE structures: (c) schematic of an LCE network showing the reconfiguration due to the covalent bonding exchange under UV light illumination. (d) Images of printed LCE strip actuation. UV illumination at 125 °C makes the structure to be locked in. The locked-in structure cannot recover the initial state. Instead, its opacity can change under a temperature change. (e) Actuation of a concentric square. It can be morphed to a square cone at high temperature. After UV treatment, it becomes opaque at ambient temperature. (f) Actuation of a bilayer structure. It is composed of ADT-contained upper and ADT-free lower layers. Before UV illumination, the bilayer structure can be actuated similar to a conventional single layer strip. After UV illumination, the lock-in of the upper layer makes the whole structure bent at ambient temperature. Adapted from Ref. [92]. Copyright (2020) John Wiley & Sons.

Light-activated bilayer actuators have also been fabricated using non-crosslinked LC polymers [91]. Unlike conventional LCE structures, non-crosslinked LC polymers were dissolved in an organic solvent and printed using DIW. Azobenzene was attached to the backbone of the linear LC monomers. Non-crosslinked LC polymers were printed on a passive Kapton substrate to form a bilayer structure (Figure 5b). Because the cis state of the azobenzene group increases the volume of the structure in this case, bending occurs toward the Kapton layer side. Figure 5b shows the bending behavior of the bilayer film for the two different orientations of the bilayer; in either case, bending occurs toward the Kapton layer side. Bending can occur within 5 s after a UV source is turned on. When the UV source is turned off, the film takes only 10 s to recover its initial flat structure because of the thermal relaxation from the cis to trans state.

It is also possible to fix the shape of reconfigured structures. Davidson et al. reported 3D-printed LCE structures using DIW, which exhibited light-induced lock-in behavior [92]. Instead of azobenzene, allyl dithiol (ADT) was introduced into the LCE structures. ADT can dynamically exchange bonding upon UV light absorption (Figure 5c). 3D-printed LCE inks containing ADT were photopolymerized with green light. Owing to the alignment of the LC mesogens during printing, the photopolymerized LCEs had an aligned smectic state. When heated above 90 °C, the printed film shrank and recovered its original length when cooled (Figure 5d). The contraction and recovery were fully reversible for at least 20 cycles. When the film is illuminated by UV light and heated to 125 °C, the ADT exchanges its bonds. Dynamic bond exchange disturbs the alignment of the LCE. When cooled back to room temperature, the film became opaque (Figure 5d), and it could not recover its initial state. This state is termed the lock-in state. Diverse active structures can be fabricated by varying the printing path. Figure 5e shows a concentric square that reversibly transforms to a square cone, which is locked-in after UV illumination. Figure 5f shows a bilayer structure consisting of an active layer containing ADT and a passive layer without ADT. By heat treatment, the bilayer structure can only be actuated in the plane. However, after UV illumination, the upper active layer with ADT was locked-in. Only the lower passive layer can recover its length. The length difference between the upper and lower layers bends the entire structure.

4.2.1 General photothermal activation via dyes and colored materials

Photothermal heating is another common method used for remote light activation. Remote light activation can be realized by mixing dyes or nanomaterials (e.g., carbon-based materials, metal nanoparticles, etc.) in a smart material matrix such as SMP, LCE, or hydrogel. In Section 4.2, we first describe general photothermal structures based on nanomaterial composites, which are fabricated by traditional patterning. Then, we discuss how such light-activation mechanisms are employed for 3D-printed active structures reported in the recent literature (Sections 4.3–4.5).

Dye molecules can selectively absorb light in a certain wavelength range. Absorbed light can be converted to heat, leading to an increase in the temperature. Using this photothermal mechanism, heat-responsive smart materials can be actuated by light. For example, Rogóż et al. fabricated a millimeter-scale crawling snail robot using dye-doped LCEs [93]. In their work, glycerin played a role similar to that of the mucus of snails. The fabricated LCE strip underwent repetitive contraction and recovery by laser scanning for light-driven locomotion. Wani et al. fabricated a light-driven artificial flytrap by incorporating dye molecules into splay-aligned LCEs [94]. The light-activated gripper was realized by attaching an LCE strip to an optical fiber tip. Zeng et al. fabricated an LCE iris structure by incorporated dyes. The initially open iris was closed by itself upon light illumination [95]. Shahsavan et al. demonstrated a bioinspired underwater soft robot by incorporating photothermal dye molecules. The fabricated soft robot can walk, crawl, jump, and even swim in water with proper laser irradiation [96].

Ge et al. fabricated a dye-doped LCE bilayer actuator using near-infrared (NIR) absorbing dyes (Figure 6a) [97]. Before crosslinking the LCE strip, it was uniaxially stretched by 330% to align the mesogens. Then, only one side of the LCE bilayer strip was crosslinked in the aligned state with UV light exposure. After crosslinking, the LCE strip was placed in hot water for stress relaxation while the other un-crosslinked side was kept isotropic. In this step, the fabricated strip curled toward the non-crosslinked side. The isotropic un-crosslinked side of the LCE strip was crosslinked with UV light exposure at room temperature. The fabricated LCE strip had a bilayer structure with actuation and non-actuation domains (Figure 6a). The length of the actuation domain can be shortened at high temperatures, leading to flipping of the curved bilayer. Using proper NIR laser irradiation, locomotion and directional changes were demonstrated; the bilayer actuator could switch the moving direction to either left or right under proper laser guidance (Figure 6b). Crawling on a 15° slope was also demonstrated.

Figure 6:

General photothermal method for light activation: dye molecules and colored materials.

NIR activated bilayer LCE structure: (a) schematic of a bilayer LCE structure. NIR dyes are included in an actuation domain. The contraction of the actuation domain can make the structure flip under the NIR irradiation. (b) Locomotion of the fabricated crawler. The first column is a schematic of the crawler actuation under NIR laser scanning. The second column shows the crawler turning left, while the third column shows the crawler tuning right. Adapted from Ref. [97]. Copyright (2018) John Wiley & Sons. Polarization-dependent morphing of LCE structures: (c) absorption spectra of a dichroic dye. α is the angle between the main axis of the dichroic dye and the polarization of light. (d) Schematic of the LCE structure with splay orientation. The parallel alignment (PA) surface is aligned along the main axis of the film, while LC mesogens are aligned vertically on the vertical alignment (VA) surface. θ is the angle between the PA alignment direction and the polarization of the light. (e) Image of the oscillating film under the light polarization of θ = 0°. The film oscillates at 15 Hz. Adapted from Ref. [98]. Copyright (2021) John Wiley & Sons. Light-programmable SMP structures with grayscale printing: (f) images showing the different recovery capacity for varying grayscales. (g) Recovery ratio as a function of the grayscale. (h) Images of a biomimetic leaf actuated by visible light illumination (scale bar: 5 mm). Adapted from Ref. [104]. Copyright (2020) John Wiley & Sons.

The polarization of light can be another control knob for shape morphing. For example, polarization-dependent LCE actuators were demonstrated by introducing dichroic dyes (Figure 6c–e) [98]. The dichroic dye RL202, which has an absorption peak around the wavelength of 488 nm, shows polarization-dependent absorption; light absorption is 3.4 times stronger at 488 nm along the orientation of dichroic dyes, compared to that of the perpendicular orientation (Figure 6c). The LCE film has a splay configuration, where mesogens are aligned in the parallel direction along the long axis at one surface while aligned in the perpendicular direction at the other surface – called parallel alignment (PA) and vertical alignment (VA) surfaces (Figure 6d). Here, θ is defined as the angle between the polarization direction of the incident light and the alignment direction of mesogens at the PA surface. For the polarization of θ = 0°, the PA surface has more shrinkage than the VA surface; this difference leads to the bending of the LCE film. Owing to the splay configuration, the bending angle depends on the polarization angle θ. The bending angle reaches a maximum at θ = 0° and decreases as θ increases.

A polarization-dependent oscillator was demonstrated using the same LCE film (Figure 6e). A threshold laser power exists above which the LCE film can oscillate. The mechanism of this light-driven oscillation can be described as follows: (i) during oscillation, the bent shape of the LCE film deviates from the optimal alignment angle between the polarization angle of light and the direction of the dichroic dye molecules, (ii) the decrease in temperature makes the LCE film recover its shape slightly, and (iii) the re-oriented dichroic dye molecules begin to absorb more light and cause the structure to be bent again. These steps are repeated very quickly, which causes the LCE film to oscillate. At a laser power of 40 mW/cm², the oscillation frequency reached 15 Hz at θ = 0°.

The coating or printing of colored materials can also induce light activation [99], [100], [101], [102]. Li et al. coated polydopamine onto planar SMP sheets [103]. Owing to the polydopamine coating, NIR light energy was converted to thermal energy, and the SMP sheets was actuated. In addition, polydopamine was erasable using NaOH solution. Pre-strained SMP sheets with inkjet-printed patterns can be programmed by light too. For example, shape morphing was demonstrated using poly(L-lactide) (PLLA) as an SMP material [104]. PLLA has crystalline domains at a certain temperature between T _g and the melting temperature. Thermomechanically stretched SMP films with inkjet-printed patterns show different recovery capacities after light illumination, depending on the grayscale patterns. Figure 6f and g compare the recovery capacity of different grayscale samples. As the grayscale is higher, the PLLA reaches a higher temperature under light illumination. This leads to a more crystalline film, which, in turn, disturbs the recovery of SMPs. Using this feature, different structural morphings were realized depending on the grayscale pattern and light illumination. For example, the shape morphing of a bioinspired leaf was demonstrated (Figure 6h).

4.2.2 General photothermal activation via carbon-based materials

Carbon-based nanomaterials such as carbon nanotube (CNT), carbon black (CB), and graphene oxide (GO) can absorb light over a wide frequency range (from optical to microwave frequency region) and can be used for photothermal activation [105], [106], [107]. For example, Li et al. introduced single-walled CNTs (SWCNTs) into LCE structures to utilize them in a solar cell system [108]. Inspired by heliotropism, they placed a solar cell panel on three pillars composed of SWCNT-embedded LCEs. The contraction of LCEs made a solar cell panel directing toward the Sun. Cheng et al. fabricated NIR-Vis-UV responsive actuator films by introducing GOs in an LCE matrix [109]. GOs can absorb NIR-Vis light and convert incident light energy to heat. Dye molecules were also included to absorb UV light. In this manner, they demonstrated actuating-film structures under natural sunlight.

“Healable” SMP films were also fabricated, which were responsive to NIR light by introducing GOs [110]. 3 wt% of poly(acrylic acid) (PAA) grafted GO nanosheets were introduced into a poly(vinyl alcohol) (PVA) matrix. The fabricated SMP film structure was programmed into a temporary W shape (Figure 7a). Upon NIR light illumination, the SMP film recovered its original shape within a few seconds. However, this recovery mechanism suffers from gradual degradation over recovery cycles (red curve in Figure 7b). Degraded SMP films can be healed by water because of reversible hydrogen bonding between PAA and PVA (black curve in the left panel of Figure 7b, PAA/PVA-GO_3%). In contrast, the PVA control sample still shows gradual degradation even after healing (right panel of Figure 7b, PVA). Furthermore, mechanically damaged structures can be repaired completely. Figure 7c shows a healing feature of PAA/PVA-GO_3%. The damaged sample was healed by water. Figure 7d shows the SEM image of the originally fractured boundary in the healed sample.

Figure 7:

General photothermal method for light activation: carbon-based materials.

Self-healable SMP structures activated by NIR light: (a) images of shape memory processes under NIR light illumination. GO effectively absorbs NIR light and produces heat. (b) Recovery ratio of PVA/PAA-GO_3% with or without healing. (c) Images of the healing process. The fractured part is completely repaired by water. (d) SEM image of the healed film. Adapted from Ref. [110]. Copyright (2019) ACS Publications. Reversible shape memory crawling robot: (e) schematic of the crawling robot locomotion under sequential light switching. (f) Optical and IR images of the crawling robot ascending stairs by sequential light switching. Adapted from Ref. [111]. Copyright (2019) ACS Publications.

The photo-thermal approach was used to develop a light-driven crawling robot using carbon-based materials [111]. Ball milling and hot pressing were used to mix CNTs with poly(ethylene-co-octene) (POE). Unlike standard SMPs, POE shows repeated shape morphing owing to the broad melting temperature regime of semicrystalline structures. Inspired by a leopard, the crawling robot was demonstrated by deforming the strip into an M shape (Figure 7e). The folded legs of the M-shape structure can be partially recovered and stretched upon IR irradiation. When the light is turned off, cooling down the structure makes legs contract back. Crawling movement was demonstrated by turning the light on and off repeatedly (Figure 7f). Finely-dispersed CNT/POE materials possess effective electrical paths. The electrical actuation of a gripper was also demonstrated at 36 V.

Shape morphing structures under microwave radiation have also been reported. Patel et al. introduced multi-wall CNTs (MWCNTs) into a polyurethane matrix [112]. A deformed structure, which was initially coil-shaped, can recover its original shape in a microwave oven. As the weight percentage of MWCNTs increases, an increase of shape fixity and modulus was reported.

4.2.3 General photothermal activation via metal nanoparticles

Various photothermal shape morphing structures were demonstrated by introducing metal nanoparticles into LCEs or SMPs. Metal nanoparticles support surface plasmon resonances and can induce strong light absorption. An LCE waveguide actuator, for example, was demonstrated by encasing an Au-containing precursor on the LCE waveguide and precipitating gold nanoparticles (AuNPs) with UV light illumination (Figure 8a) [113]. When 532 nm laser light is coupled to the LCE waveguide, an actuating hinge in the waveguide can be bent toward the AuNP side within a few seconds while light is guided through the waveguide. Figure 8a shows the bending of the LCE waveguide at various laser power. The waveguide was bent more for higher laser power; at 230 mW, the waveguide was bent about 13°. LCE waveguides with two actuating hinges were also demonstrated (Figure 8b). Depending on the hinge location (either top or bottom), the waveguide transformed into different shapes.

Figure 8:

General photothermal method for light activation: metal nanoparticles.

Light-activated LCE waveguide: (a) images of the actuating LCE waveguide with various laser power. The dashed line indicates the original position of the waveguide. (b) Images of LCE waveguides having two actuating hinges. Adapted from Ref. [113]. Copyright (2019) John Wiley & Sons. Light-activated LCE bending structure: (c) images of bending under light illumination. The upper layer of the LCE film transforms to the isotropic state that induces bending. (d) Stress-strain curve with different wt% of AuNRs and directions. Adapted from Ref. [115]. Copyright (2020) John Wiley & Sons. Polarization-dependent shape memory effect: (e) images of recovery characteristics of the SMP strip. (f) Recovery angle of the strip versus the exposure time for vertical and parallel alignments to the polarization of incident light. (g) Recovery angle of different polarization angles. Adapted from Ref. [116]. Copyright (2013) John Wiley & Sons.

Ding et al. fabricated light-induced actuating nanotransducers by coating AuNPs with poly(N-isopropyl acrylamide) (PNIPAm) [114]. A swelling degree of PNIPAM can be controlled in water with temperature. At room temperature, PNIPAM-coated AuNPs were separated by certain distances because of swelled PNIPAM. Laser illumination induced the plasmonic heating of AuNPs; then, the deswelling of coated PNIPAM occurred, and nanoparticles became agglomerated. After cooling, the swelling of PNIPAM made the agglomerated particles apart again.

Surface plasmon resonances in Au nanorods (GNRs) can cause significant photothermal heating. Additionally, they show polarization-dependent behavior because of their elongated shape. For example, AuNRs were added to an LCE matrix for shape morphing [115]. AuNRs with poly(ethylene glycol) (PEG) ligands were infiltrated during the polymerization of the LCE. Well-dispersed PEG-AuNRs could be aligned along the LC oligomers due to capillary force during the infiltration. The fabricated LCE film showed bending when an 800 nm light source illuminated the sample in 5 s (Figure 8c). By introducing AuNRs up to 0.2 wt%, a modulus of the LCE film was improved. Figure 8d compares moduli for different wt% of AuNRs and the LC direction. The pristine LCE film has a modulus of 10.3 MPa, while the LCE film containing AuNRs has a modulus of 20.2 MPa along the aligned axis.

Au nanoparticles have also been introduced into SMP matrices [116, 117]. Zhang et al. demonstrated the SMP composite films that exhibit polarization-dependent recovery behavior (Figure 8e). Thermomechanically deformed SMP strips were recovered under NIR light illumination. When the polarization of incident light was perpendicular to the AuNR direction, no shape recovery occurred. However, a full recovery of the deformed film occurred when the polarization is in parallel to the AuNR direction. Figure 8f compares the recovery of the deformed SMP film for two polarizations. After 7 s of illumination, the parallel polarization showed full recovery while the perpendicular polarization did not show recovery at all. Figure 8g shows how recovery is affected by the polarization angle; as the polarization of incident light is aligned to the AuNR direction, more recovery was obtained. Again, the parallel polarization (0°) shows the full recovery of the deformed SMP film.

4.2.4 General photothermal activation via hybrid structures of metal nanoparticles and carbon materials

Carbon-based materials can absorb light in a wide frequency range. Furthermore, their high thermal conductivity provides rapid heat transfer. On the other hand, metal nanoparticles supporting surface plasmon resonances can absorb light more selectively in a certain wavelength range. The absorption wavelength can be engineered by tuning the size or shape of metal nanoparticles. AuNPs are known to have excellent photothermal energy conversion. Figure 9a shows the UV-visible extinction spectra of CB, Au nanocube, and GNR [118]. CB shows a broad absorption spectrum (black curve), while Au nanocube and GNR exhibit selective extinction peaks (yellow and red curves). However, Au nanoparticles suffer from gradual degradation after a few cycles of heating-up and cooling-down (Figure 9b) and have a drawback in thermal stability [119].

Figure 9:

General photothermal method for light activation: hybrid of metal nanoparticles and carbon nanomaterials.

(a) Extinction spectra of carbon black, gold nanocubes, and gold nanorods. Gold nanoparticles have distinct extinction peaks, while carbon black shows a broad extinction spectrum. Adapted from Ref. [118]. Copyright (2018) ACS Publications. (b) TEM images of gold nanorods before and after laser irradiation. Damage occurs after laser irradiation. Adapted from Ref. [119]. Copyright (2010) OSA Publishing. (c) Temperature increase of GNRs/SiO₂ under laser irradiation. (d) Temperature increase of GNRs/SiO₂/GO-PEG under laser irradiation. It shows a better thermal stability. Adapted from Ref. [124]. Copyright (2019) Elsevier. (e) Images of SMP composites showing shape memory behavior under NIR light illumination. (f) Temperature increase as a function of time under NIR light illumination. (g) Shape recovery ratio as a function of time under NIR light illumination. RGO and AuNP have synergetic effects. Adapted from Ref. [125]. Copyright (2019) Elsevier.

To solve this problem, a hybrid structure of Au nanoparticles and carbon-based nanomaterials was considered [120], [121], [122], [123]. For example, GO-capped GNR structures were fabricated for drug delivery [124]. First, GNRs/mesoporous silica nanoparticles were fabricated and capped by GO-PEG. Figure 9c and d compares temperature changes from the first-to-sixth cycle of GNRs/SiO₂ and GNRs/SiO₂/GO-PEG, respectively. GNRs/SiO₂ (i.e., without carbon materials) shows a gradual deterioration of thermal stability through sequential cycles, as indicated by decreased temperature increases upon light illumination. Meanwhile, GNRs/SiO₂/GO-PEG (i.e., with carbon materials) shows higher thermal stability than GNRs/SiO₂.

A hybrid structure of AuNPs and reduced GO (RGO) was also introduced into SMPs [125]. AuNP-RGO nanohybrids were mixed in a poly(ε-caprolactone) (PCL) solution and hot-pressed. Figure 9e presents the shape-memory behavior of SMPs embedded with various nanomaterials. An 808 nm NIR laser was illuminated to thermomechanically programmed SMP strips. Pristine PCL did not show a shape-memory effect, while PCL/AuNP-RGO exhibited the fastest shape recovery under light illumination. Figure 9f shows temperature changes over time under 808 nm laser illumination. The AuNP-RGO hybrid structure showed synergetic effects on photothermal heating. Figure 9g shows the shape recovery ratio of SMP strips embedded with different nanomaterials. As the weight percentage of AuNPs increased in the hybrid structure, faster shape recovery responses were observed.

4.2.5 General photothermal activation via semiconductor nanoparticles

Copper sulfide (CuS) is a semiconductor nanoparticle showing efficient photothermal conversion. Instead of surface plasmon resonances, the intrinsic energy band transition can make CuS efficiently absorb NIR light. Because of low cost and high photothermal efficiency, CuS nanoparticles have been intensively studied, for example, in cancer therapy [126], [127], [128]. Li et al. introduced CuS nanoparticles into an SMP (polyurethane) matrix [129]. By adding 0.2 wt% of CuS nanoparticles, a deformed polyurethane strip recovered its original shape in 21 s upon the illumination of 808 nm NIR light.

4.3.1 Photothermal activation of 3D-printed SMP structures

More complicated structures can be created using 3D printing. Various 3D printing methods were recently considered for light-activated structures. For example, DIW was used to create self-healing, light-activated structures by integrating aniline trimer (AT) into a polyurethane matrix [130]. When temperature increases, the viscosity of the polyurethane matrix gradually decreases, permitting the DIW of the polyurethane matrix. This viscosity change is induced by thermally dynamic covalent bonding of the polyurethane matrix. Figure 10a shows the shape-memory characteristic of the printed strip. A twisted strip hanging a weight of 5 g showed shape recovery under NIR light illumination. Additionally, thermally dynamic covalent bonding can induce the self-healing of the polyurethane matrix. Figure 10b shows the pictures (upper panel) and microscope images (lower panel) of the printed patterns. It is demonstrated that damaged parts can be repaired under NIR laser irradiation.

Figure 10:

Photothermal activation of 3D-printed structures.

DIW of Light-activated self-healable SMP structures: (a) images of a deformed strip recovering its shape under NIR light illumination. (b) Images of a damaged sample and the healing process using NIR laser irradiation. The SEM images of each step are also shown. Adapted from Ref. [130]. Copyright (2019) Royal Society of Chemistry. FDM of NIR-activated SMP structure: (c) optical and IR images of a butterfly shape, which shows shape recovery by NIR light. Adapted from Ref. [131]. Copyright (2020) Elsevier. DIW of Multi-stimuli induced shape morphing in hydrogel structures: (d) experimental and simulated bending and twisting for different strain values. (e) Images of flower structures under light illumination. Adapted from Ref. [135]. Copyright (2020) John Wiley & Sons.

Dopamine-modified MWCNTs were also introduced into a thermoplastic polyurethane network, where MWCNTs were coated with poly-dopamine (PDA) [131]. Both PDA and MWCNT show significant absorption in the NIR region. The thermal conductive paths in the polyurethane matrix are increased due to the additional hydrogen bonding between PDAs. Therefore, the polyurethane film including PDA-coated MWCNTs have nearly twice higher thermal conductivity. Furthermore, the mechanical properties such as elongation modulus, tensile strength, and elongation-at-break are enhanced, too. Complex structures were fabricated using FDM 3D printing, and shape-memory effects were demonstrated. Figure 10c shows the pictures and thermal images of the printed butterfly structure. The deformed structures were restored to their original, flat state using NIR illumination.

Cui et al. demonstrated the shape-memory behavior of a brain structure under NIR illumination by incorporating graphene into an SMP matrix [132]. Improved cell differentiation was demonstrated, which was attributed to the optoelectronic characteristics of graphene. Keneth et al. fabricated sequential shape morphing structures using the multi-material DLP technique [133]. They incorporated CNTs into an SMP matrix. External and internal lids in a printed box were sequentially opened under the illumination of a 395 nm LED source.

4.3.2 Photothermal activation of 3D-printed hydrogel structures

3D-printed hydrogel structures were also used for photothermal light activation. Han et al. used SLA to fabricate cephalopod-inspired light-activated chromatophores (LACs) [134]. By incorporating PDA nanoparticles (PDA-NPs) into a thermo-sensitive hydrogel matrix, 3D-printed structures were actuated upon illumination with white light (7.7 mW/cm²). They fabricated LACs by printing three different materials. Combining a rigid material for frames and a stretchable material with PDA-NPs-doped hydrogel for muscles, they could fabricate reversibly actuating LACs in water.

Shape-morphing hydrogel structures were also fabricated via DIW, which responded to multiple stimuli such as water, heat, and light [135]. Graphene oxide (GO) flakes were introduced into a sodium alginate (SA) matrix. During the 3D printing, GO flakes were aligned to the printing direction because of the shear force and gravity. Aligned GO flakes produced anisotropic features to the printed structure. Utilizing the bilayer bending mechanism, the fabricate structure could be bent to 0° and 90° printing directions and could be twisted along 45° and −45° printing directions under multiple stimuli. Vapor makes the structure to expand anisotropically. On the other hand, heat and light can make the structure to contract. Figure 10d shows the simulation (left) and sample images (right) of the bending and twisting structures in water for various strain differences. The larger the strain difference was, the more deformation occurred. A flower structure was printed using these features for a shape-morphing demonstration. Figure 10e shows the images of the printed flower, which is actuating under light illumination. Originally flat structures could be morphed into 3D flower structures as petals were twisted under xenon lamp illumination. The deformed structure can be recovered to the original flat shape when water vapor is introduced. This shape morphing happens in a few seconds and occurs reversibly for more than 100 cycles.

4.3.3 Photothermal activation of 3D-printed LCE structures

Ambulo et al. developed 3D-printable inks for DIW by combining an LCE matrix with liquid metal (eutectic gallium indium alloy, EGaIn) [136]. Due to the electrical paths of the liquid metal, the fabricated ink was electrically conductive. LC mesogens were aligned automatically by the shear force during printing. Although the actuation strain was decreased compared to pure LCE inks, there was an increase in the elongation modulus for inks containing liquid metals. With 71 wt% of liquid metals, the fabricated ink showed the photothermal effect in the NIR region. They printed bilayer bending structures by printing a bottom layer at the 0° direction and an upper layer at the 90° direction. Under NIR illumination, the printed bilayer structure was reversibly bent and recovered. With even higher wt% of liquid metals, the printed structure could also be controlled by the electric voltage bias; due to the electrical paths of the liquid metal, the fabricated ink showed electrical conductivity with a resistance range from 0.0038 to 0.035 Ω. The printed structure was deformed by Joule heating at 1.6 V.

4.4.1 Photothermal activation of 3D-printed microstructures

DLW based on multi-photon absorption polymerization can allow sub-micrometer resolutions and can be useful for 3D printing of microstructures. Light-activated hydrogel microstructures, for example, were fabricated using DLW [137], where the resolution of DLW was improved down to 100 nm using an appropriate crosslinker. PNIPAm hydrogels were crosslinked with different resolutions. Controlling slice and hatch distances (Figure 11a), the swelling and deswelling factors of the printed structure were studied. Intriguingly, the printed structure with 100 nm distance showed no deswelling at increased temperature. On the other hands, the printed structures with lower resolution showed deswelling features at increased temperatures. Exploiting this feature, a bilayer helix structure was fabricated (Figure 11b), where the bottom layer was printed with 100 nm resolution and the upper layer with 500 nm resolution. Printed bilayer hydrogel structures were actuated in water at different temperatures. To make the hydrogel structure photo-responsive, AuNRs were dispersed in water. NIR-absorbable AuNRs induced a temperature increase in the bilayer helix structure within 500 ms. Figure 11b shows the schematic of bilayer helices and compares the actuation of bilayer and monolayer layer helices. Under NIR laser illumination, the bilayer helix was collapsed within 500 ms and reversibly actuated. In contrast, the monolayer helix was actuated in an isotropic manner only.

Figure 11:

Photothermal activation of 3D-printed microstructures.

DLW of Hydrogel microstructures: (a) schematic for printing density control via hatching/slicing distance variations from 100 to 500 nm. (b) Schematic of a bilayer helix and phase-contrast images of bilayer and monolayer helices after the laser illumination of 300 and 500 ms. Adapted from Ref. [137]. Copyright (2020) ACS Publications. DLW of LCE microstructures: (c) mechanism of light-induced reversible shape morphing in AuNR/LCE nanocomposite structures. (d) SEM images of AuNR/LCE microstructures (woodpile structure, hexagonal photonic crystal structure, wireframe structure, micro-clamp structure). Adapted from Ref. [138]. Copyright (2019) ACS Publications.

Chen et al. fabricated photothermally actuating LCE microstructure using DLW (Figure 11c and d) [138]. Figure 11c shows the fabrication steps for the LCE microstructure. Unlike the study above, they incorporated AuNRs into un-crosslinked LC monomer solution before conducting two photon polymerization (TPP). Because there is no shear force during the printing in the case of TPP DLW, a rubbed polyimide layer was prepared to align LC mesogens and AuNRs. After alignment, TPP was conducted to create microscale structures. Printed structures can shrink due to the isotropic conversion of the LC mesogens under NIR laser illumination. After the laser is off, it can recover its original state due to the re-alignment of the LC mesogens. They also printed various microstructures including woodpile, hexagonal photonic crystal, wireframe, and micro-clamp structures. Figure 11d shows the SEM images of such microstructures created by TPP DLW.

4.4.2 3D printing of light-driven micro-robots

3D printing was also used to fabricate light-driven micro-robots. Zeng et al. fabricated light-activated microscale walkers using DLW [139]. They introduced dyes in an LC monomer mixture. In this work, the mixture was transparent at the DLW photopolymerization wavelength. Because the absorption peak of the dye is around 530 nm, a chopped 532 nm laser could increase the temperature of the microscopic walker to around 100 °C. Figure 12a shows the SEM image of the fabricated LCE micro-walker. It is composed of a 60 × 30 × 10 μm³ LCE body and four rigid tilted legs. Figure 12b shows the actuation images of the fabricated microscopic walker under 532 nm laser illumination. Due to photothermal heating, its length is contracted by up to 20%. Reversible contraction and recovery through chopped laser illumination generate elastic energy, which is transformed into the kinetic energy for micro-walking. Figure 12c shows the SEM images of the microscopic walker on a nanograting substrate at a velocity of 380 μm/s.

Figure 12:

DLW of light-driven micro-robots. Micro-walker: (a) SEM image of an LCE micro-walker. Tilted four legs induce walking motion (scale bar: 10 μm). (b) Actuation of the micro-walker under 532 nm laser illumination (scale bar: 50 μm). (c) Walking motion of the micro-walker on a nanograting surface. Inset indicates surface morphology. Adapted from Ref. [139]. Copyright (2015) John Wiley & Sons. Micro-gripper: (d) SEM images of a micro gripper. The gripping motion of a micro-cubic structure is demonstrated under 532 nm laser illumination. Adapted from Ref. [140]. Copyright (2017) John Wiley & Sons.

Martella et al. fabricated a light-activated microscale gripper using DLW [140]. They used dye-doped LCEs to actuate the gripper under 532 nm laser illumination using the photothermal method. Splay-aligned LC mesogens were turned into the isotropic state under laser illumination. Figure 12d shows the SEM images of the micro-griper showing a gripping motion. It takes 30 ms to complete the closure of the micro-gripper, and it takes 25 ms for the gripper to be relaxed.