Crosslinked LCPs (CLCPs) are stimuli-responsive materials. Common stimulus sources include light, electricity, heat, and magnetic fields. Based on the degree of backbone flexibility, crosslinking, and glass transition temperature (T_g), CLCPs can be divided into liquid crystalline elastomers (LCEs) and liquid crystalline networks (LCNs) [24,25].

Due to the highly flexible backbone, LCEs exhibit both the properties of LC materials and elastomers. LCEs consisted of crosslinked LC side-chain and/or main-chain mesogenic units, but the polymer backbones have flexibility and a low overall crosslink density. These materials possess the ability to undergo large mechanical deformations while maintaining their LC order and can be programmed into different shapes by external stimuli. Moreover, LCNs are formed by a higher degree of crosslinking, resulting in a more rigid polymer network. This increased crosslinking restricts the backbone flexibility, leading to enhanced mechanical strength and stability. Compared to LCEs, LCNs typically exhibit a higher T_g and achieve a transition of glassy state with higher temperatures.

The division of CLCPs into LCEs and LCNs allows for a classification that considers the varying degrees of backbone flexibility, crosslinking density, and thermal properties, enabling researchers to tailor the material’s characteristics for specific applications in areas such as soft robotics, actuators, and smart materials. It will be summarized by introducing their background, manufacturing methods, advantages, and latest work progress.

LLCPs do not require chemical crosslinking to have good mechanical properties as well as LC orientation. Most LCPs are polymerized by chemical crosslinking. However, LLCPs are composed of high molecular weight that use ring-opening metathesis polymerization (ROMP), which breaks through a complex manufacturing process to solve the messy synthesis of LCPs. LLCPs can be processed using traditional melting methods and solution treatments, allowing for efficient fabrication of intricate designs and tailored properties, thus overcoming the challenges associated with the complex synthesis of conventional LCPs.

Flexible microtubes are an assembled, optional modular component for microfluidic devices. A 2-layer photocontrollable flexible microtube (PFM) with a commercially available ethylene-vinyl acetate (EVA) layer as a support layer and a 5 wt% dichloromethane copolymer (PABBP) solution coated internally to form a deformed PABBP layer was fabricated (Fig. 4A and B) [44]. This PFM tube can be molded into any shape and can be knotted or suspended from a weight of 200 g without damage. Slugs were induced to move in a controlled direction and climb an 11° slope at 0.4 mm s⁻¹ by irradiating one end of an isopropanol bullet confined in microtubes with a 470 nm spot. Light-controlled deformation of the PABBP layer geometry from a cylindrical to a conical region in a PFM tube was observed by super-resolution microscopy, where asymmetric expansion occurred along the long axis of the microtubule, resulting in asymmetric capillary forced driving fluid motion. The article conducts experiments on fluid motion on the finger, and a PFM attached to the index finger changes its trajectory with different hand gestures.

In 2016, the study introduced the first design of a phototropic tubular microactuator (TMA) capable of easily manipulating liquid motion. This TMA exhibited asymmetric deformation when stimulated by 470-nm light, thereby generating uneven capillary forces that drive the liquid forward (Fig. 4C and D) [45]. Previously, TMAs made from CLCPs exhibited poor processability during chemical crosslinking. However, this TMA was prepared using a new type of LLCP, which is more robust. It had a strength of up to 20 MPa and an elastic modulus of 96 ± 19 MPa. These values indicated that, when combined with a well-designed LLCP, TMAs could enhance their mechanical properties. Yu et al. [46] synthesized 3 spacers with different lengths and intermediates with spontaneously oriented photodeformable LCPs to form TMA annealed films by CH₂CI₂ solution processing and cutting to length and explored the relationship between different intermediate lengths and phase transition, bending behavior, and fluid transport (Fig. 4E). DSC determined that LLCPs gradually stabilized the LC phase with increasing spacer length. Under UV/visible light irradiation, PC-AB6 films bent toward the light source in 6 s, while PC-3AB6 and PC-7AB6 bent away from the light source direction in 2.5 and 2 s and recovered rapidly under visible light. This was because the anisotropic shrinkage caused by the photoisomerization of PC-AB6 was mainly produced in the intermediate orientation direction on the film surface, the photoisomerization caused the surface swelling of PA-nAB6, and the swelling increased with growth of spacer lengths. In the liquid transport of PC-nAB6 TMA with an inner diameter of 500 μm, the light intensity gradually decayed from left to right, the isopropanol slug with the uneven concave surface was pushed to the right, the intensity decay was reversed, the slug moved reverse, the length of the intermediate was longer, the angle of inclination was larger, and it moved faster. When the inclination angle was 90°, the coaxial orientation and the speed were at their maximum.

Yu et al. [47] used a simple ROMP to polymerize 2 photosensitive lateral LLCPs with different spacer lengths, polymer SLLCPs, with azobenzene as the side chain, and processed them into thin films by a melt shear-induced orientation method. Flexible spacers were investigated on the photodeformation behavior and deformation behavior of the films. The molecular weight and polymer dispersion indices of SLLCPs show that the polymers synthesized in this work have an order of magnitude higher molecular weight than other LCPs and that higher molecular weight improves the mechanical processing properties of materials. The higher T_g of PNb-AB4 than PNb6AB4 is due to the decoupling and reduced hindrance to the movement of the main side chains by the flexible spacers of PNb6AB4. The high T_g of PNb-AB4 and PNb6AB4 reached 60° and 72° in 3 and 2 s under UV light irradiation, respectively. Due to the low coupling effect between the main chain and the azo phenyl group, films with long spacers bent faster and at larger angles. Moreover, the films bent under visible light, indicating that the SLLCPs have good shape stability.

Combining azobenzene and benzoate to form LLCP fiber based on the principle of shape memory effect and photochemical phase change and introducing benzoate as a eutectic intermediate is beneficial to activate chain mobility to a greater extent (Fig. 4F) [48]. The fibers were stretched above T_LC-iso to self-assemble the azobenzene and benzoate intermediates in the side chains into a sheet-like structure, and after cooling to room temperature, the fibers kept their stretched shape without a rebound. The highly ordered structure of the LC phase in this process inhibits the skeleton movement and locks the strain energy in the fiber. Under the light, the azobenzene LC lamellar structure was disrupted by isomerization, allowing the release of prestored strain energy, leading to shrinkage of the fiber, which is 81% (without applied load, stretched up to 400%, shrinkage reaching 81% after light, constant with increasing stretch ratio; in the presence of load, the greater the applied load, the smaller the shrinkage, 900% of the studies for the stretch ratio) (Fig. 4G). This strategy drives the development of research on light-dependent deformation. At the same time, a new well-stabilized LLCP with a wide range of intermediate phase temperatures and photosensitive properties, which contain azo esters by ring-opening translocation polymerization called PC11AE6, was synthesized [49]. Spontaneous orientation of PC11AE6, photo-induced deformation, and the relationship between light intensity, film thickness, and temperature were investigated. The highly ordered rigid azobenzene mesogen structure contributed to the flexibility of PC11AE6. Through 2D-wide angle x-ray diffraction characterization, it was observed that PC11AE6 spontaneously forms a uniform mesophase orientation in both low-angle and high-angle regions. The film possessed an anisotropic structure, which was smectic A phase at room temperature and in the bonded smectic C phase had an anisotropic structure above the phase transition temperature. The mesogens became the zigzag tilting in the lamella of smectic C phase, which showed the ability of PC11AE6 to spontaneously orient. Because the internal resistance of thick films was greater than that of thin films, films and fibers bent in the direction away from the light source, and thin films (10 μm/68°) bent more than thick films (20 μm/21°) under 365-nm UV irradiation. The higher the intensity of the radiated light, the more the film bent, as the higher photon energy drove more isomerization. The photo-induced deformation was performed in the coating A stage at a high temperature of 60 °C. The film was exposed to the light source for 5 s, and upon removal of the light source, it quickly recovered. This behavior was attributed to the instability of the cis structure at high temperatures. The results of the above investigations provide insight into the design of LLCP with a wide range of intermediate-phase temperatures. An LLCP with an azobenzene structure as a side chain was designed in 2021 (Fig. 4H) [50]. Under UV irradiation, the glass transition temperature on the surface of the LLCP film on the polymethyl methacrylate (PMMA) substrate was reduced to −39 °C, increasing viscous flow dynamics, “convex” shape, and channel cross-sectional area. Under 470-nm blue light radiation, the liquid slug moved away from the irradiated light source, but it passed through many complex pathways and could also perform operations such as liquid merging (Fig. 4I). The cross-sectional area of the channel is isosceles to achieve integrated operations such as movement, separation, and merging, which is difficult to ignore. Finally, this microfluidic chip was used for the first time in catalytic reactions and biological protein detection, opening the idea of portable testing.

Yu et al. [51] used a bottom-up 2-step synthesis and a single crystal-single crystal approach to fabricate large-sized platinum-based linear polymer crystals, in which ligand-dependent self-assembly was used to confirm that anthracene could achieve head-to-toe and head-to-tail formation by [4+4] cycloaddition, thereby forming the desired polymer crystals. Major investigations were carried out to follow its structural evolution during polymerization and depolymerization using x-ray diffraction techniques, and the depolymerization properties of polymer Pt-BA2DA slowly progressed with temperature. By single-crystal x-ray diffraction analysis, Pt-BA2DA single crystals were irradiated with white light at room temperature for 7 days to get Pt-BA2DA-i, which is 44% polymerized in the anthracene group, and longer irradiation time could not increase the degree. Therefore, a lower temperature of 265 K was used and 100% polymerization was obtained in 7 days for poly(Pt-BA2DA). The polymer depolymerization process is closely related to the heating temperature, and the 2 depolymerization processes require different energy barriers. Poly(Pt-BA2DA) to Pt-BA2DA-i has a lower exchange barrier, so the depolymerization rate is faster. To study the photodeformation behavior of the polymer, it was compounded with poly(vinylidene fluoride) (PVDF) to prepare thin films that bent toward the light source under light illumination. The synthetic strategy for preparing large Pt-based polymer crystals reported in this study has broad implications. Atom transfer radical polymerization was used to prepare polyacrylate with a flexible spacer, and azo-phenyl group as the main chain in the side chain of the polymer and cyclic gel permeation chromatography was used to prepare polymers of different molecular weights to be investigated (Fig.4J) [52]. They explored the photo-induced solid–liquid transition of polymers and photo-induced photomechanical response, mechanical properties, and healing ability of nitrogen-containing polymers. Since the polymer in this work is an entangled linear azide polymer, they are tougher and more mechanical and machineable compared to small-molecule polymers. The films exhibited photo-induced reversible bending behavior under UV illumination with or without stretching of the azobenzene intermediate at 90 °C to obtain aligned alignment. Photo-induced reversible solid–liquid exchange softens the freestanding films (photoisomerization causes the conversion of the light-emitting surface into a cis-rich state and the cis state into a viscous polymer melt) and bends more easily. Photo-induced reversible solid–liquid conversion enables the repair and reprocessing of azide polymer photoactuators with light at ambient temperature and in the absence of solvent. This polymer-entangled nitrogen-containing benzene polymer with photo-induced reversible solid–liquid conversion could help in the future design of other curable actuators. In 2022, a heterogeneous bead–spring model based on the photoreversible molecule azobenzene as the tailing group was developed. The kinetic relaxation behavior of this model in cis-trans isomerization is clearly described and analyzed in terms of photophysical calculations [53]. Furthermore, both models of cis-trans isomerism exhibited good photochromism and exhibited different states at room temperature (trans-hard solid, cis-soft solid). The tail-end relaxation time decreased with increasing cis content. However, at high temperatures, this conclusion is difficult to detect.

Photoinduced LCP materials with azobenzene photosensitive groups are attractive, are precisely and remotely operable in specific regions, and offer some opportunities for applications such as actuators. However, there are certain drawbacks to consider, notably the inherent thermodynamic instability of cis-azobenzene, which can undergo spontaneous conversion to trans-azobenzene. As a result, the transformation is not maintained for a long time even at room temperature, which limits the application of photo-deformable polymers to some extent. Therefore, in addition to the photosensitive group azobenzene, many other structures have been used to produce photodeformable polymers.

Some types of photosensitive groups can be used directly to construct LCEs with photosensitivity. A physically crosslinked photosensitive LCE (LCE-PM10PVPCm) was developed using α-cyanostilbene as an intermediate with different alkoxy tail lengths [54]. Characterization of monomer and polymer revealed that the higher the temperature, the stronger the hydrogen bond interactions of polymer chains. And the thermal stability of polymer proved by thermal decomposition experiments. With the increase of the alkoxy tail length, the T_gdecreased, while the LC phase was more stable. The uniaxially oriented LC elastic fibers prepared by the melt stretching method bend in the light emission direction under light irradiation because the polymer undergoes cis-trans isomerization when excited by UV light, the maximum bending angle of m = 4 is 113°, and the experiment also proves that more alkoxy groups are not favorable for a molecular motion to hinder isomerization.

Cis-α-cyanostilbene has good thermal stability. Regardless of the cis-trans conformation, they are not uniformly distributed in the radial direction of the fiber and can exhibit shape memory properties. Zhang et al. [55] synthesized a series of multi-hydrogen-bonded crosslinked α-cyanostilbene-based LCEs with different flexible spacer lengths (LCE-mCS, m = 0, 2, 4, 6, 8 and 10) (Fig. 5A and B). The issues of polymer thermal/liquid crystal properties, the effect of flexible spacer length on photodeformation behaviors, and the applications of LCE-mCS were investigated. The experimental analysis presented that T_g of all LCE-mCS decreased with increasing spacer length (Fig. 5C and D). Under UV irradiation, the uniaxially oriented LCE-mCS fibers bend along the fiber axis toward the light source, and both the bending speed and the bending angle increase with increasing spacer length. This is the result of the influence of T_g. The maximum tilt angle of LCE-mCS reaches 130°. Compared with uniaxially oriented photochemically reactive LCE fibers with azobenzene, LCE-mCS shows great advantages in bending angle and bending speed. The deformation of LCE-mCS fibers is attributed to the trans-cis isomerization of α-cyanostilbene, and its photothermal effect and the plasticization effect of cis are greater than those of trans. In addition, the application of LCE-mCS in information encryption materials is explored, which provides a new idea for the preparation of new efficient photosensitive LCEs. Liao et al. [3] used poly(4-vinylpyridine) (P4VP) as the main chain and α-cyanostilbene derivative (Z-TCS) as the side chain, and both formed a series of supramolecular polymers of P4VP (Z-TCS)x in different molar ratios. This polymer undergoes photodeformation behavior under UV light irradiation, and the rate of bending increases with increasing x. The Z-TCS intermediate undergoes an order–disorder phase transition, and partial absorption of light energy by the α-cyanostilbene intermediate in the polymer for Z- and E-isomerization causes the P4VP backbone to contract, which together cause asymmetric contraction. At the same time, the T_g of the polymer decreases as the plasticizing effect of Z-TCS increases. x = 1.0 fibers bend to 90° in just 8 s, and the maximum bending angle of all P4VP(Z-TCS)x fibers is 135° or more. In addition, P4VP(Z-TCS)x fibers can be used with a photothermal effect for applications in information encryption and other smart manufacturing materials.

Diarylethene is one of classic photochromic chromophores [56]. The units of diarylethene undergo ring-closing and ring-opening via a 6π electrocyclization. LCN based on diarylethene (DAE-LCN) was designed [57]. The diarylethene unit and LCN were photopolymerized through 2 acrylate groups (Fig. 5E). The covalent bond between them ensures the stability of connections (Fig. 5F). The ring-closed diarylethene unit provided the large conjugated aromatic structure increasing the rigidity of the molecule. The bending angle varied for different UV intensities, and small bending is possible under a 550-nm lamp without using UV light (Fig. 5G and H). The increase of intensity made the diarylethene unit capture more light energy and thus the heat energy converted increases, which is macroscopically shown as a large bending deformation.

A complex maneuver was used to synthesize a cyclic, linear polymer with 9,10-dithiophenanthracene as the head group (Fig. 5I) [58]. This group can exhibit a unique conformation under UV, and the introduction of 3-hexylthiophene onto anthracene greatly inhibits the photodimerization of the anthracene group. These 2 polymers were prepared as polyethylene and polyamide films, respectively, and both exhibited good photodeformation behaviors. This leads to swelling mainly due to the distortion of the molecular structure of the thienyl-anthracenyl-thiophene group under light. Although there is some cis-trans structure that affects deformation, this type of isomerization is too rapid to produce a small effect.

Two fluorescence-tunable groups, tetraphenylene and spiropyran (SP), were used to bind to LCE (Fig. 5J) [59]. The photodeformation behavior of LCE is controlled by NIR initiation. Deformation and luminescence phenomena were studied by adjusting the presence of photothermal dye (YHD796)/tetraphenylethene (TPE)/SP. The group alternated between 530-nm visible light and UV light and found that the color of fluorescence changed between blue–green and red (Fig. 5K). Blue–green fluorescence is controlled by TPE, and red is controlled by merocyanine (MC). Irradiation with visible light for 90 s caused MC-SP to heterodimerize, during which excitation with UV radiation showed blue–green fluorescence. In addition, changing the MC structure of SP causes the fluorescence color to slowly change from blue–green to red under UV irradiation.

In addition to LC crosslinking, direct copolymerization can also achieve photodeformable behavior. Zhang et al. [60] synthesized a polyimide (azo-PI) polymer containing azobenzene as a photodeformable polymer material and explored its deformation behavior under unpolarized light. Since light-induced molecular alignment causes the azo-benzene network to expand or contract concerning polarization and ultimately conform to macroscopic deformation, it is necessary to induce molecular alignment by stretching and introducing flexible linker molecules into it. Thermally stretched azo-PI films were able to demonstrate their reversible bending behavior under alternating UV and visible light irradiation. In addition to this phenomenon that could occur in air, it could also occur in hot water (silicone oil) at 80 °C. It bent toward the light source within 8 s (12 s) after irradiation, reached a bending angle of 45°, and returned to its original position under green light. This showed that PI had good thermal stability and mechanical properties. This work motivated the design of promising actuators for azo PI for high-temperature applications. A polymer was formed with the hydrophilic monomer hydroxyethyl acrylate copolymerized with the monomer azobenzene, and another triple copolymer based on N-vinyl-2-pyrrolidone was formed [61]. The light response time of both copolymers was 30 s, which is higher than the light response time of other literature, and the recovery time of 110 min under white light irradiation was also slow. But the reason for their slow recovery was that this group used light intensity, solvent properties, and ambient temperature to influence the time faster or slower. The higher the light intensity, the shorter the recovery time.

He et al. [62] showed for the first time a functional polymer single crystal obtained by self-inoculation with polyethylene glycol (PEG) using azobenzene as a capping group. It was confirmed that the trans-cis isomerization of azobenzene still occurs well in light even with a single crystal structure. mPEG-Azo flaked rapidly isomerized as in a solution. This indicates that the environment around the azo-phenyl groups attached to single crystals is relatively loose. Although the synthesis of these polymer single crystals is relatively simple, they cannot have a high degree of freedom compared to polymer materials, such as bending and twisting actions.

The natural world has produced sophisticated, complex, and intelligent living systems that have served as inspiration for material design. Flexible LCP materials offer adaptive advantages in mimicking soft biological structures, and the combination of photoresponsive driving enables the design of intelligent microsystems that mimic biological behavior.

Bimorph composites fabricated using polyamide (Kapton) substrates and LCP layers copolymerized with 2 monomers M6ABOC2/M6AzPy, and bionic sleep of Albizia julibrissin leaves were explored (Fig. 7A and B) [70]. Chemically crosslinked films were found to reach photo-equilibrium within 5 s. Under UV irradiation, the maximum displacement angle was reached in 2 s and the films bent toward the Kapton layer regardless of the direction of irradiation (Fig. 7C and D). In the theoretical model, 2 relationships were observed while keeping the Kapton thickness constant. Firstly, the light driving force was found to be proportional to the radiated light intensity. Second, the displacement angle was also found to be proportional to the radiated light intensity. It was further observed that as the LCP layer thickness increased, both the light driving force and displacement angle also increased. In addition, under gradient UV radiation, artificial leaves showed circadian rhythmic motion according to the light changed from “dawn” to “dusk” (Fig. 7E). This intelligent light sensing and motion execution system pave the way to mimic the fine motor control system in natural plants.

Nowadays, many photoresponsive LCPs are used to design bionic functional applications. When driven by light, they regulate the circadian rhythm of plants, the opening of fingers, the movement of caterpillars, the blooming of flowers, the swinging forward of swimmers, and other shape changes or movements (Fig. 7F) [33,41,59,71,72]. The results of these studies have provided creative inspiration for functionalized applications of light-responsive LCPs.

Photothermally responsive polymer materials are heated locally by light control. If the temperature of the heated region is higher than the T_LC-iso, the LC radicals randomly lose their previous orientation direction, thereby restoring the polymer chains to a random curl conformation. During the phase transition, the polymer undergoes contraction parallel to the initial direction of the LC and expansion in the perpendicular direction. Once cooled below the transition temperature, it spontaneously reverts back to its original shape due to self-alignment.

In 2017, Yu et al. [73] fabricated LC post-crosslinked random copolymers (AZ-CLCPs) with azobenzene as a side chain (Fig. 7G). The effect of copolymers containing different AZ concentrations and crosslinking densities on the photoresponsive behavior was investigated. The amorphous LC copolymer became soft when the temperature reached above 80 °C, and it rapidly melt under UV irradiation. The photo-softening and melting behavior of the amorphous LC copolymer were attributed to the photo-induced phase transition from the LC phase to isotropic phase when the temperature exceeded T_g. However, after crosslinking, the AZ-CLCP fibers exhibited highly ordered interfacial morphology, and under the combined influence of light and temperature, they could achieve light-induced bending. Below the T_g temperature, the chain segments were frozen and both crosslinked and uncrosslinked fibers remained unchanged. When the temperature reached 80 °C, the photodeformation behavior appeared rapidly (Fig. 7H). The bending rate of fibers increased linearly with increasing temperature. At higher temperatures, the recovery rate of fiber surface expansion induced by AZ photoisomerization became faster (Fig. 7I). Below T_LC-iso, fiber programming could be achieved by controlling the temperature to shape the fibers and achieve an encrypted effect.

A variety of photothermal reagents capable of forming composites with LCPs have been reported, including carbon nanotubes, graphene, metal nanoparticles, organic molecular dyes, conjugated polymers, and polydopamine. All of them can absorb light in a specific wavelength range, convert light energy into heat energy, and release heat into the polymer matrix, thereby driving shape change. Zhang et al. [71] fabricated an LC mixture containing azobenzene mixed with GO doped into PU to obtain shape memory PU (SMPU) films. This composite film has excellent response properties to UV–visible (Vis)–NIR, and bending behavior occurs due to various factors when irradiated by different light sources. The LC composite was composed of different ratios of 5CAZ/5CB, and experiments showed that the content of 5CAZ affected the photodeformation response under UV, while the corresponding GO played an important role under NIR. 5CAZ/5CB/GO (3 wt‰)/SMPU films could recover bending by high-intensity NIR light. The film achieves a continuous shape change through continuous thermal stretching and light irradiation. The biomimetic bending and opening of a finger could be achieved reversibly through a combination of continuous thermal stretching and light irradiation (Fig. 8F). When GO was introduced into the 5CAZ/5CB/SMPU thin film, a parameter of bending degree (F = 0.31) of the elongated 5CAZ/5CB/GO (3wt%)/SMPU film significantly increased after visible light exposure. During the visible light exposure process, the surface temperature of the stretched 5CAZ/5CB/GO (3 wt%)/SMPU film rose to 45.1 °C, higher than T_i (39.5 °C). This indicated that the visible light-induced bending deformation of the elongated 5CAZ/5CB/GO (3wt%)/SMPU film was attributed to the cis-trans photoisomerization of 5CAZ, while the photothermal-induced LC-to-I phase transition was originated from the photothermal effect of GO. However, this doping is doped with many things that affect its performance and the doping limit is unknown.

Microactuators are commonly used in biomedicine, chemical analysis, and other fields and have potential application opportunities in bionic blood vessels, minimally invasive surgery, etc. The use of light to precisely control the advance, retreat, merging, and separation of fluids is now the pursuit of microfluidic technology. In addition, the microfluidic system can realize noncontact control of the fluid. The first article on LLCP integrated TMAs for optically driven manipulation of liquids was published in 2016 [45]. In contrast to previous reports that could only manipulate specific liquids, this work demonstrates that each type of TMA (straight, Y-shaped, serpentine, helical) could manipulate liquids ranging from polar to nonpolar, including silicone oil, hexane, ethyl acetate, ethanol, and water (Fig. 8A). Furthermore, it has potential applications in the field of biology. The research team also demonstrated the optically driven manipulation of commonly used biological liquids such as bovine serum albumin solution, phosphate buffer solution, cell culture media, and cell suspension. This simple TMA greatly simplifies the device for microfluidics and provides significant support in the field of microactuator technology. Furthermore, this work also breaks through linear motion within the plane, as the droplets could climb slopes at a speed of 0.05 mm s⁻¹ with an inclination angle of 17°, providing assistance in the design of microfluidic reactors. Yu et al. [50] have mostly reported on microfluidic technology, which mainly used photodeformable LLCPs to fabricate the micropipe and PMMA as the substrate to achieve this fluid transport. Among them, the separation and merging of the liquid was mainly affected by the cross-sectional area, and the length of the radiant side was reduced so that the surface tension on both sides was unequal, resulting in fluid movement. Through research and exploration, the most suitable cross-sectional area should be considered as an isosceles trapezoid (Fig. 8B and C). However, such dynamic LLCPs can only be driven under UV lamps, whose deep penetration greatly hinders their biological application.

Because of the current problems of how to eliminate light stimulation operation, configuration limitations, etc., it is imperative to improve the directionality of movement accuracy and control soft actuators and fast response mode since actuators guided by UV light have precise scanning direction constraints. Therefore, it is important to adopt a light source that can replace UV light and thus guide actuators with multiple motion patterns. In this respect, NIR light has an advantage due to its fewer side effects and less toxicity in biological tissues. NIR light has a high selectivity in the wavelength region and low light penetration. This allows NIR light to be effectively separated from other wavelengths, avoiding interference and false triggering. This selectivity is critical in the precise control and manipulation of photodeformable materials [74]. Besides, NIR photodeformable materials typically have good repeatability and long lifetime.

Organic absorbers in the NIR include compounds such as benzobisthiadiazoles, dinuclear ruthenium complexes of 1,2-dicarbohydrazide ligands, and metal dithiones, but the synthesis of these materials is more complex. Based on this, Chen et al. prepared a synthetically simple NIR-absorbing diol and applied it to the crosslinked elastic polymer of PU to obtain PU-T [75]. To compare the effect of the structure of the diol on the deformation behavior, a diol-free polymer PU-BL was prepared. Under the irradiation of NIR light at 808 nm, the expanded PU-T restored its initial state within 60 s. In addition, it was able to lift 160 times its weight, indicating that the material with this structure has good photodeformation behavior and photothermal stability and that it has a wide range of absorption capabilities (700 to 900 nm).

A bidirectional switch, a bidirectional walker, and a visible/infrared triple wavelength modulated walking robot, all of which can effectively control the gait and deformation of the actuator, providing ideas for multistimulus responsive soft actuators [30]. The actuators exhibit strong macroscopic motion and fast response speed and can be easily operated using stimuli. Zhou et al. [76] made a dual-responsive jumping frog, providing a new design strategy for actuators (Fig. 8D).

Designing simple molecules and precisely modulating the molecular structure is key to the development of NIR photodeformable materials for future applications. Modulation of actuators using NIR and visible light enables fast response over a wide range, while the multistep response increases the applicability of flexible actuators. Combining NIR photodeformable materials with other functional materials can realize multifunctional integration. This multifunctional integration will promote the expansion of the application of NIR photodeformable materials. At the same time, by regulating the optical, electrical, magnetic, and other properties of the materials, it has photoelectric conversion, sensing, manipulation, and memory effects and other characteristics. It will open up new possibilities for implementing more complex functional systems.

Inspired by chameleons and color-changing insects in nature, we can change the appearance and color of materials through multiple stimuli. In the LC materials, the azo phenyl group is of wide interest as it can be isomerized upon light exposure, and in addition to this group, SP is also a potential isomerization group, which is also responsible for color change. In general, the isomerization group induces a color response to temperature, pH, light, and stress. A hydrogel composed of N-isopropylacrylamide (NIPAAm), acryloyl 6-aminohexanoic acid (A6ACA), and spiro-acrylamide (SP) derivatives was developed [77]. This hydrogel can change its color and deformation with the presence of Fe³⁺. In the absence of Fe³⁺, the incorporation of Fe³⁺ caused anisotropic swelling in selected regions of the hydrogel, leading to macroscopic deformation of the hydrogel (Fig. 8E to G). Not only that, the absorption peak at 540 nm of red hydrogel after UV irradiation disappeared to yellow color after soaking by Fe³⁺. This provides a new idea for the dual response of color-changing deformation.

Our group explored a new isomerization group, the tristyryl group, which has the advantages of simple structure, convenient synthesis, longer cycles, and lower cost compared to other groups since 2016 [78–80]. We applied it to the polymer material PMMA to form thin films to explore the properties of data encryption, rewritability, and others. Based on this, digital light-responsive 3D structures are made of methyl methacrylate containing triphenylene (TrPEF₂-MA) combined with 3D printing technology, which is expected to combine shape memory materials and LC materials to achieve the dual effect of color change and deformation (Fig. 8H) [81]. The hydrogel is also achieving this dual effect through 3D printing as shown in Fig. 8I [82]. A predesigned hydrogel model was printed by an LD-001 3D printer. The slice thickness of each layer was 50 μm during the printing. The exposure time of each layer was set to 3 s, except for the first layer, which was set to approximately 180 s. The obtained structure (PANT hydrogels) was rinsed in a sonicator bath for 1 min to remove the monomers/PI residues, and it needed to be stored in a sealed environment. The PANT hydrogels were immersed in a 0.1 M solution of Eu(NO₃)₃ and Tb(NO₃)₃ for 15 min to get the Eu/Tb-PANT hydrogels. With different molar ratios of Tb³⁺/Eu³⁺, they have dynamic fluorescent colors under UV@254 nm. Based on this, dual effect was realized.

In addition to these bionic, photo-thermal synergistic encrypted storage, microfluidic, and color-changing deformation dual-action applications, LCPs can also be applied to sensors, autonomous healing, and drug transport [40,83,84].

Self-healing is a way to delay the life and life cycle of materials and improve their reliability. Smart materials with this capability have far-reaching implications for engineering manufacturing, medical implants, and other applications where damage to materials is irreversible and prohibitively expensive. The article by Patrick et al. [84] reviewed 3 different mechanisms of self-healing at 3 different levels: At the molecular level, broken bonds are repaired by supramolecular interactions such as ion pairing; at the microscopic scale, microfractures can be bridged by adding effective additives or a mixture of repair agents and catalysts; and even more destructive, only vascular network delivery methods are used to achieve the healing function. Due to the demanding environmental requirements in terms of pH, temperature, humidity, and oxygen concentration, it is a significant challenge to improve the self-healing properties of materials. A simple 2-step method to synthesize photomechanical elastomers (PMEs) for optical drivers was presented [85]. Three different azobenzenes were used to produce the PME break film. Azobenzene molecules were covalently linked to the PU backbone in 2 phases and crosslinked by hydrogen bonding. The PME has unique autonomous self-healing properties, which exhibited a maximum absorption peak at 385 nm when measured in tetrahydrofuran. However, under UV irradiation, the absorption peak gradually decreased, while the absorption value in the visible region increased. After 30 s of irradiation, the curve returned to its original state. A solid PME film under UV irradiation had a complete bending motion time of 50 s. The highly dynamic hydrogen bonding conducted noncovalent crosslinking and formed a network that enabled autonomous self-healing. After cutting the arbitrarily aligned film into 2 pieces, the newly cut edges were exposed to room temperature for 5 min. Remarkably, the healed film exhibited the ability to stretch without breaking, reaching a strain of γ = 100%. Then, this image was applied to a light-driven soft robot gripper.

When the porous LCN acting as a substrate is stimulated, the fluid lubricant is preprogrammed to acquire fluid secretion sites and the capillary bridges are programmed for ligand adhesion, enabling self-regulated drug transport or chemical reagent release in response to stimulation. Under UV light irradiation, the secreted sensitive dye reagent is visualized by pH and its secretion can be detected by UV–Vis absorbance applied to the release of medical drugs.

Photodeformable polymer materials have obtained an increasing interest as an emerging field of research, due to their advantages of cleanliness, remoteness, tunable intensity and wavelength, and nondestructiveness under light excitation. In this review, we have summarized the classification of photodeformable polymer materials, from molecular design preparation to applications of photodeformable materials in various challenging fields. Additionally, we also described the challenges of their current development. For the photodeformable polymer materials, LCPs are regarded as the largest class, which have excellent reversibility and accuracy under light stimuli. Besides, other photodeformable polymer systems including hydrogels, SMC polymers, and carbon-based materials are also summarized.

In the past years, the development of photodeformable polymer materials has undergone remarkable advancement, particularly in the development based on the molecular design, group selection, and applications. However, there are still some concerning limitations impeding the achievement of high-performance photodeformable polymer materials. First, the kinds of photo-responsive substances that have been used in photodeformable polymer materials are relatively few. Apart from the widely studied azobenzene molecules, stilbene, cyclic photosensitive groups, transition metal complexes, conjugated polymers, and other groups can be applied in photodeformable polymer materials in the future, which is helpful to improve maneuverability and beneficial to enhance service lifetime for applications. Subsequently, most photodeformable polymer materials only respond to UV light, which limits their applications. By combining or doping substitutional groups that respond to visible or NIR light, it is possible to expand the range of dynamic modulation of light irradiation. Then, the stability of photodeformable polymer materials still needs to be strengthened to circumvent the light absorption dilemma. The choice of various types of crosslinking agents (photosensitive crosslinking agents, thermal crosslinking agents, chemical crosslinking agents, ionic crosslinking agents, and so on) plays a crucial role in crosslinking process. In terms of crosslinking mechanism, these crosslinkers vary in nature, which can achieve a selectively crosslinking process to allow for flexible shape morphing. In addition, it was found that not only photodeformable polymers are found in the form of film or fiber but also the absorption peaks of light sources are similar to the changing photosensitive groups, which limits the preparation of the actuator and makes it difficult to commercialize. To develop more high-performance and diverse applications, the study of photodeformable polymer materials could move toward materials for multiple stimulus response integration and robots with self-adjusting behaviors depending on environmental conditions. At the same time, developing some new applications, for instance, microtechnology, nanotechnology, photonics, and sensors, is also indispensable for future development. We hope that this review could provide new directions to solve problems and have further development in the field of photodeformable polymer materials.