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Kinetic mismatch between photochemistry and mechanical response. (a) Change in absorbance during thermal relaxation in dark of the cis to the trans state of azobenzene in an LCN* measured at room temperature. The insert shows the change in absorption with time fitted by a single exponential decay of the cis state. (b) Mechanical response measured as a height change of the thin film versus time upon actuation by 365 nm light of intensity 78 mW cm−2 and a subsequent relaxation in dark. The film thickness is 4 μm, the corresponding density decrease is 1.3%.
Source publication
Photoactivated generation of disorder in a liquid crystal network produces free volume that leads to the controlled formation of dynamic corrugations at its surface. The liquid crystal order amplifies the deformation of copolymerized azobenzene, which takes place on molecular length scales, to a micrometre-sized macroscopic phenomenon based on chan...
Contexts in source publication
Context 1
... As soon as the light source is switched off, the film sinks again within 10 s. According to current theory, this fast restoration (in seconds) to the original high density should imply a fast cis-to-trans back isomerization process (relaxation). However, when we record the cis-to-trans relaxation in the LCN* by taking ultraviolet-visible spectra( Fig. 1a; vide infra), we find that relaxation is in the order of hours. The relaxation can be described by a single exponential decay of the cis state with a half time value of 4.0 h. The large mismatch between the slow cis-to-trans-reversed isomerization of azobenzene molecules and the fast density change of the polymer challenge the present ...
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... The fast relaxation of volume is confirmed by mechanical experiments at light-induced volume-expanded surface topographies. For instance, transient friction coefficient measurements demonstrated relaxation within 10 s after the actuating light source is switched off 18 . Also, a reference kinetic measurement of the height change as provided in Fig. 1b shows that both the appearance and the disappearance of additional volume evolves within 10 s. ...
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... studied the conversion of the azobenzene by ultraviolet illumination under various light and intensity conditions, as well as its relaxation at room temperature after switching off the light source. Figure 1a shows a sample that is actuated by an LED lamp emitting at 365 nm light with an intensity of 200 mW cm À 2 . The conversion of the covalently embedded azobenzene to the cis state exceeds 90 mol% at equilibrium. ...
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... room temperature and using LEDs as near- monochromatic light sources. The half-intensity bandwidth of the LEDs used is 10 nm. The results are presented in Fig. 2 where the equilibrium photostationary state is plotted versus the ratio of the intensities of 455 and 365 nm. The percentage cis is calculated from the absorbance extremes as observed in Fig. 1a, assuming 1% in the dark and 96% conversion upon high-intensity 365-nm exposure. Reference measurements by nuclear magnetic resonance and ultraviolet-visible can be found in the Supplementary Figs 1-3 and Supplementary Note 2. For the measurement absorbance, we took a film of LCN* of exactly the same thickness but without azobenzene ...
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... percentage cis is calculated from the absorbance extremes as observed in Fig. 1a, assuming 1% in the dark and 96% conversion upon high-intensity 365-nm exposure. Reference measurements by nuclear magnetic resonance and ultraviolet-visible can be found in the Supplementary Figs 1-3 and Supplementary Note 2. For the measurement absorbance, we took a film of LCN* of exactly the same thickness but without azobenzene modification as baseline. The conversion in the photostationary state of 365 nm exposure alone is almost the same and close to full conversion. ...
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... time of the measured deformation is in the order of 0.1 Hz. An estimate of the cycle rate of the stimulated trans-cis conversion based on the number of absorbed photons per unit of volume 22 is of the order of 1 kHz. These four orders of magnitude difference further support the assumption that the response rate, as for instance, demonstrated in Fig. 1b is dominated by the viscoelastic deformation of the polymer network rather than by the photochemistry. Recently published athermal photofluidization phenomena in azobenzene polymer glasses might further support the possibility of deformation of the glassy network at room temperature 22 ...
Citations
... Built upon this, one of the methods to create surface topographies on a confined LCN is based on the free volume generation. 10,22 Due to the restriction from the substrate, the lateral deformations are largely prohibited and the extra volume can only escape at the coating surfaces to form the topographies. The formed dynamic surface topographies have shown new applications such as self-cleaning action, 23 and controlled adhesion and release. ...
... Unlike LCNs, which create surface topographies through free volume generation, 22 in this work, we investigate the use of the anisotropic deformation properties to achieve both high actuation strain and complex surface topographies in a coating. For this purpose, we develop liquid crystal oligomer networks (LCONs) by crosslinking the liquid crystal oligomers. ...
... The magnitude of the topographical deformation produced by this method is 6 times larger than the ones developed in LCNs. 10,[22][23][24][25][26] ...
We create high-aspect-ratio dynamic poly-regional surface topographies in a coating of a main-chain liquid crystal oligomer network (LCON). The topographies form at the topological defects in the director pattern organized...
... [23][24][25][26][27][28] However, recent works suggest a new mode of generating deformations in liquid crystal polymers. 29,30 In particular, these studies demonstrate that continuous, dynamic trans-cis-trans isomerizations collectively generate significant deformations far greater than those a) Corresponding author: Email: n.swaminathan@iitm.ac.in b) Corresponding author: Email: ratna@iitm.ac.in caused solely by the statistical accumulation of cis isomers. 29 Furthermore, there is a considerable reduction in the density of the polymer due to the free volume generated by the azomolecules undergoing dynamic trans-cis-trans isomerization cycles. ...
... 29,30 In particular, these studies demonstrate that continuous, dynamic trans-cis-trans isomerizations collectively generate significant deformations far greater than those a) Corresponding author: Email: n.swaminathan@iitm.ac.in b) Corresponding author: Email: ratna@iitm.ac.in caused solely by the statistical accumulation of cis isomers. 29 Furthermore, there is a considerable reduction in the density of the polymer due to the free volume generated by the azomolecules undergoing dynamic trans-cis-trans isomerization cycles. 29,30 Additionally, it is important to note that only 2 wt% of azo-molecules within the entire ALCN caused a significant 12% reduction in density. ...
... 29 Furthermore, there is a considerable reduction in the density of the polymer due to the free volume generated by the azomolecules undergoing dynamic trans-cis-trans isomerization cycles. 29,30 Additionally, it is important to note that only 2 wt% of azo-molecules within the entire ALCN caused a significant 12% reduction in density. 29 The azo-mesogens usually occur in the stable trans state and upon illuminating with Ultra-Violet wavelength (365 nm), the trans isomers isomerize to a meta-stable cis state. ...
We use molecular dynamics simulations to unravel the physics underpinning the light-induced density changes caused by the dynamic trans–cis–trans isomerization cycles of azo-mesogens embedded in a liquid crystal polymer network, an intriguing experimental observation reported in the literature. We employ two approaches, cyclic and probabilistic switching of isomers, to simulate dynamic isomerization. The cyclic switching of isomers confirms that dynamic isomerization can lead to density changes at specific switch-time intervals. The probabilistic switching approach further deciphers the physics behind the non-monotonous relation between density reduction and light intensities observed in experiments. Light intensity variations in experiments are accounted for in simulations by varying the trans–cis and cis–trans isomerization probabilities. The simulations show that an optimal combination of these two probabilities results in a maximum density reduction, corroborating the experimental observations. At such an optimal combination of probabilities, the dynamic trans–cis–trans isomerization cycles occur at a specific frequency, causing significant distortion in the polymer network, resulting in a maximum density reduction.
... In contrast, TEGABS/chitosan films exhibit an opposite effect, where irradiation causes an increase in stiffness (photostiffening) and a concurrent bending movement away from the light source. Despite the material differences, both actuation processes are triggered by photomechanical effects (such as free-volume formation [31] ) and photothermal mechanisms [29] (such as photosoftening [25,32] ). As a result, our hypothesis is that photostiffening primarily influences the actuation direction of the film. ...
A sophisticated comprehension of the impacts of photoisomerization and photothermal phenomena on biogenic and responsive materials can provide a guiding framework for future applications. Herein, the procedure to manufacture homogeneous chitosan‐based smart thin films are reported by incorporating the light‐responsive azobenzene‐derivative Sodium‐4‐[(4‐(2‐(2‐(2‐methoxyethoxy)ethoxy)ethoxy)phenyl)diazen‐yl]‐benzenesulfonate (TEGABS) in the biopolymer through electrostatic interactions. When irradiated with UV‐light the TEGABS/chitosan films show a biresponse, comprising the E→Z photoisomerization with a half‐life of 13 – 20 h and the light‐induced evaporation of residual moisture leading to an increase in the reduced indentation modulus (up to 49%) and hardness. Freestanding films of TEGABS/chitosan show actuation up to 13° while irradiated with UV‐light. This work shows the potential of biogenic polysaccharides in the design of biresponsive materials with photomodulated mechanical properties and unveils the link between the humidity of the environment, residual moisture, and the photomodulation of the mechanical properties.
... It would be worthy to note that the local free volume described above does not directly related to the macroscopic volume of polymer network near the surface of the film, in contrast with photoactivated volume generation reported for liquid crystalline networks. [41] In the current case, cis → trans isomerization leads to unbending motion through the expansion of the film surface, which can be explained by increase in end-to-end distance of the crosslink chains. ...
Polymer materials that show macroscopic deformation in response to external stimuli are feasible for novel soft actuators including microactuators. Incorporation of photochromic moieties, such as azobenzenes, into polymer networks enables macroscopic deformation under irradiation with light through photoisomerization. Under cryogenic conditions, however, it has been difficult to induce macroscopic deformation as polymers lose their soft nature due to the severe restrictions of molecular motions. Here, activation of molecular motions and macroscopic deformation in liquid nitrogen only with light for polymers containing photochromic moieties is reported. Photoinduced bending of polymer networks with normal azobenzenes in liquid nitrogen is enabled by preliminary UV irradiation at room temperature to produce cis‐isomers. To realize photoinduced deformation directly in liquid nitrogen, polymer networks are functionalized with bridged azobenzenes, which exist as cis‐isomers in thermodynamic equilibrium. The films with bridged azobenzenes exhibit reversible photoisomerization and bending upon irradiation with light in liquid nitrogen without the need of preliminary irradiation, implying that the change in conformation of polymer chains can be isothermally induced even under cryogenic conditions. Achievement of flexible motions under cryogenic conditions through isothermal processes will greatly expand the operating temperature range of soft actuators.
... 51,52 Upon light-exposure, these molecules either exert a photomechanical force, pulling on the polymer backbone and disrupting the molecular order, 53 act as photothermal heaters, 54 facilitating the thermal phase transition, 27 or act as photoplasticizers to introduce free volume from the continuous oscillating isomerizations. 55 Similarly, use of mesogens with a strong dipole or embedding mesogens in a dielectric matrix allows deformation of the LCE within a changing electric field. 56 Further means of activation include humidity, 57 chemicals, 58 and magnetic fields 59 (for LCEs with incorporated superparamagnetic nanoparticles). ...
Conspectus
Synthetic structures that undergo controlled movement are crucial building blocks for developing new technologies applicable to robotics, healthcare, and sustainable self-regulated materials. Yet, programming motion is nontrivial, and particularly at the microscale it remains a fundamental challenge. At the macroscale, movement can be controlled by conventional electric, pneumatic, or combustion-based machinery. At the nanoscale, chemistry has taken strides in enabling molecularly fueled movement. Yet in between, at the microscale, top-down fabrication becomes cumbersome and expensive, while bottom-up chemical self-assembly and amplified molecular motion does not reach the necessary sophistication. Hence, new approaches that converge top-down and bottom-up methods and enable motional complexity at the microscale are urgently needed.
Synthetic anisotropic materials (e.g., liquid crystalline elastomers, LCEs) with encoded molecular anisotropy that are shaped into arbitrary geometries by top-down fabrication promise new opportunities to implement controlled actuation at the microscale. In such materials, motional complexity is directly linked to the built-in molecular anisotropy that can be “activated” by external stimuli. So far, encoding the desired patterns of molecular directionality has relied mostly on either mechanical or surface alignment techniques, which do not allow the decoupling of molecular and geometric features, severely restricting achievable material shapes and thus limiting attainable actuation patterns, unless complex multimaterial constructs are fabricated. Electromagnetic fields have recently emerged as possible alternatives to provide 3D control over local anisotropy, independent of the geometry of a given 3D object.
The combination of magnetic alignment and soft lithography, in particular, provides a powerful platform for the rapid, practical, and facile production of microscale soft actuators with field-defined local anisotropy. Recent work has established the feasibility of this approach with low magnetic field strengths (in the lower mT range) and comparably simple setups used for the fabrication of the microactuators, in which magnetic fields can be engineered through arrangement of permanent magnets. This workflow gives access to microstructures with unusual spatial patterning of molecular alignment and has enabled a multitude of nontrivial deformation types that would not be possible to program by any other means at the micron scale. A range of “activating” stimuli can be used to put these structures in motion, and the type of the trigger plays a key role too: directional and dynamic stimuli (such as light) make it possible to activate the patterned anisotropic material locally and transiently, which enables one to achieve and further program motional complexity and communication in microactuators.
In this Account, we will discuss recent advances in magnetic alignment of molecular anisotropy and its use in soft lithography and related fabrication approaches to create LCE microactuators. We will examine how design choices—from the molecular to the fabrication and the operational levels—control and define the achievable LCE deformations. We then address the role of stimuli in realizing the motional complexity and how one can engineer feedback within and communication between microactuator arrays fabricated by soft lithography. Overall, we outline emerging strategies that make possible a completely new approach to designing for desired sets of motions of active, microscale objects.
... 51,52 Upon light-exposure, these molecules either exert a photomechanical force, pulling on the polymer backbone and disrupting the molecular order, 53 act as photothermal heaters, 54 facilitating the thermal phase transition, 27 or act as photoplasticizers to introduce free volume from the continuous oscillating isomerizations. 55 Similarly, use of mesogens with a strong dipole or embedding mesogens in a dielectric matrix allows deformation of the LCE within a changing electric field. 56 Further means of activation include humidity, 57 chemicals, 58 and magnetic fields 59 (for LCEs with incorporated superparamagnetic nanoparticles). ...
Synthetic anisotropic materials (e.g., liquid crystalline elastomers, LCEs) with encoded molecular anisotropy that are shaped into arbitrary geometries by top-down fabrication promise new opportunities to implement controlled actuation at the microscale. In such materials, motional complexity is directly linked to the built-in molecular anisotropy that can be “activated” by external stimuli. So far, encoding the desired patterns of molecular directionality has relied mostly on either mechanical or surface alignment techniques, which do not allow the decoupling of molecular and geometric features, severely restricting achievable material shapes and thus limiting attainable actuation patterns, unless complex multimaterial constructs are fabricated. Electromagnetic fields have recently emerged as possible alternatives to provide 3D control over local anisotropy, independent of the geometry of a given 3D object.
The combination of magnetic alignment and soft lithography, in particular, provides a powerful platform for the rapid, practical, and facile production of microscale soft actuators with field-defined local anisotropy. Recent work has established the feasibility of this approach with low magnetic field strengths (in the lower mT range) and comparably simple setups used for the fabrication of the microactuators, in which magnetic fields can be engineered through arrangement of permanent magnets. This workflow gives access to microstructures with unusual spatial patterning of molecular alignment and has enabled a multitude of nontrivial deformation types that would not be possible to program by any other means at the micron scale. A range of “activating” stimuli can be used to put these structures in motion, and the type of the trigger plays a key role too: directional and dynamic stimuli (such as light) make it possible to activate the patterned anisotropic material locally and transiently, which enables one to achieve and further program motional complexity and communication in microactuators.
In this Account, we will discuss recent advances in magnetic alignment of molecular anisotropy and its use in soft lithography and related fabrication approaches to create LCE microactuators. We will examine how design choices─from the molecular to the fabrication and the operational levels─control and define the achievable LCE deformations. We then address the role of stimuli in realizing the motional complexity and how one can engineer feedback within and communication between microactuator arrays fabricated by soft lithography. Overall, we outline emerging strategies that make possible a completely new approach to designing for desired sets of motions of active, microscale objects.
... However, later experiments suggested that the mechanism might be more complex: [15] the basic hypothesis that the geometrical change leads to a change in volume (or density) in the illuminated layers still holds. However, not only is the E to Z isomerization thought to increase the volume, but also the reverse, Z to E isomerization. ...
... It is known that the macroscopically observed relaxation of an azobenzene containing LCN can be very different from the actual half-life time of the azobenzenes. [15] Therefore, the thermal relaxation of the Z-tetrafluoroazobenzene in the polymer film P planar,31 was measured by UV-vis spectroscopy (For the spectra see the Supporting Information). Because the films consist solely of the azobenzene monomer, they are highly absorbing and only the thinnest film could be measured (for photographic images of the films, as well as the effect of a polarizer see Supporting Information). ...
... In this respect, this new LCN is unusual: It has been reported that the thermal Z to E isomerization is much slower than the mechanical deformation (hours vs seconds). [15] However, in our case, thermal relaxation appears to be somewhat faster, although not by orders of magnitude. It also has to be considered that the macroscopic relaxation was observed with the P splay,60 film, whereas the UV-vis measurement was performed with the P planar,31 film. ...
Liquid crystalline polymer networks (LCN) with azobenzene monomers bend reversibly under UV‐light irradiation, combining photomechanical and photothermal effects. However, the harmful nature of UV‐light limits their use in biology and soft robotics. Although visible light‐absorbing tetra‐ortho‐fluoro‐substituted azobenzenes exist, liquid crystalline monomers have never been prepared. Previously, such azobenzenes were added as photoactive additives (up to 10%) to otherwise passive liquid crystalline polymer networks. In this work, a molecular design of a liquid crystalline, polymerizable azobenzene switchable by visible light is presented. The monomer assembles in a highly fluid nematic phase, but polymerizes in a layered smectic C phase. The films are produced solely from the monomer without additional liquid crystalline components and are actuated with visible light. Bending experiments in air and under water differentiate photomechanical and photothermal effects. Remarkably, a 60 µm splay aligned film maintains its deformation for hours, slowly reverting over days. Monomer liquid crystallinity is characterized using differential scanning calorimetry (DSC), wide‐angle X‐ray scattering (WAXS), and polarized optical microscopy (POM); polymer films are analyzed using WAXS and DSC on a homogeneously aligned film. The synthetic procedure is high yielding and polymer film fabrication is scalable, which enables the use of safe and efficient photomechanical LCNs in soft robotics, engineering and biology.
... 20 Such materials can exhibit significant elastic deformations upon phase transitions between the orientationally ordered liquid crystalline state and the disordered isotropic phase and can thus act as photoactuators converting light energy into mechanical work. 20−23 In view of this, photoresponsive LCEs are promising candidates for a variety of applications, 24 including sensors and actuators, 1,25−27 photomechanical devices, 19 morphing surfaces, 22 soft robotics, 28,29 and more, also thanks to advances in chemical synthesis. 30 The photophysics process of switching between the trans and cis states (see Figure 1) of an individual azobenzene molecule has been modeled in many studies in vacuum 4,5 or, at the atomistic level, for azobenzene in isotropic solvents, 31,32 but simulations performed at this level of detail can be considered only for systems with a small number of molecules. ...
... Note that the ρ* values employed are taken to be relaxed equilibrium values that are assumed constant during a simulation run. At variance with this, experiments 22 have shown a reduction in sample density upon sample irradiation, which, however, is a transient phenomenon beyond the scope of the present study. To find equilibrium configurations at a given w c and temperature T, we performed MC simulations following the standard Metropolis algorithm. ...
We investigate main-chain liquid crystal elastomers (LCEs) formed by photoresponsive azobenzene units with different populations of trans and cis conformers (from fully trans to fully cis). We study their macroscopic properties as well as their molecular organization using extensive Monte Carlo simulations of a simple coarse-grained model where the trans and cis conformers are represented by soft-core biaxial Gay-Berne particles with size and interaction energy parameters obtained by fitting a bare bone azobenzene moiety represented at atomistic level. We find that increasing the fraction of cis conformers, as could be obtained by near-UV irradiation, shifts the nematic-isotropic transition to a lower temperature, consistently with experiment, while generating internal stress in a clamped sample. An analysis of pair distributions shows that the immediate surroundings of a bent cis molecule are slightly less dense and more orientationally disordered in comparison with that of a trans conformer. Comparing nematic and smectic LCEs, actuation in the smectic phase proved less effective, disrupting the smectic layers to some extent but preserving orientational order of the azobenzene moieties.
... The magnitude of the deformation was believed to be proportional to the average mass-fraction of the cis isomers [10][11][12][13][14]. However, recent works [15,16] suggest a different physics. In particular, they demonstrate that a continuous, dynamic trans-cistrans isomerizations collectively contribute to a significant increase in the magnitudes of the deformation. ...
... In particular, they demonstrate that a continuous, dynamic trans-cistrans isomerizations collectively contribute to a significant increase in the magnitudes of the deformation. The deformation induced by the dynamic trans-cis-trans isomerizations was found to be much greater than that induced only by the statistical build-up of cis isomers [15]. ...
... However, when the ALCN is illuminated with both 365 nm and 455 nm illuminations, both the trans-cis and cis-trans reactions are promoted resulting in continuous dynamic trans−cis−trans isomerization cycles. If the density reduction (or volume expansion) seen in literature [15] is due to such dynamic isomerizations under the influence of a combined UV and Visible illumination, then the density change should continuously increase with intensities (I) of the two light sources (I 365 and I 455 ). To elucidate the aforementioned point, consider the following simplified probabilistic approach. ...
Recent experimental results [Liu and Broer, Nat. Commun. 6 8334 (2015)] reveal that light-responsive azo-doped liquid crystal polymers under dual-wavelength illumination exhibit a significant reduction in density. This reduction in density was attributed to dynamic trans-cis-trans isomerization cycles. The light-induced isomerization kinetics suggest that the fraction of isomers undergoing dynamic isomerization increases with the light sources' intensity. However, experiments have shown that such an increase in intensity does not result in a monotonic decrease in density. Further, it was observed that there exists an optimal combination of the intensities of the dual-wavelength illumination that results in a maximum density reduction. The exact reason for the existence of such an optimal combination remains elusive. In this work, we have performed atomistic simulations to confirm the hypothesis that the density reduction is caused by the dynamic trans-cis-trans isomerization cycles. Subsequently, the atomistic simulations are used to decipher the underlying physics responsible for the counter-intuitive relation between density reduction and intensities. Intensity variations are simulated by varying the forward and backward isomerization probabilities. The simulations show that an optimal combination of these two probabilities will exhibit a maximum density reduction corroborating experimental observations. Consequently, we discovered that a specific frequency of the dynamic trans-cis-trans isomerization cycles would induce maximum distortion in the polymer network resulting in significant density reduction.
... On the one hand, the photochemical effect has its origin in the fact that irradiation of azobenzene molecules with light at the absorption peak of the trans isomer (365 nm) leads to isomerization from the elongated trans to the bulky cis isomer, causing a decrease in the molecular order of the network. 10,15,20,21 The photothermal contribution, on the other hand, arises from the photon energy absorbed by the chromophore that is transformed into thermal energy, contributing to the order disruption of the LCN. 10,22 For both mechanisms, it needs to be considered that light intensity throughout the sample thickness is not uniform, as dictated by the Beer's law, due to the absorption of light at the illuminated side of the sample. ...
... 10,25 For the employed azobenzene unit, substituted at the 4 0position with oxygen atoms, the cis isomer has a lifetime of several hours at RT. 43 However, isomerization back from the cis to the trans state for this chromophore can also be induced by irradiation with blue light. 21 Therefore, subsequent irradiation with blue light was carried out for 30 minutes, leading to the recovery of the initial length (within 1% of error), as cis isomers are isomerized back to the thermodynamically stable trans form as in the original film (Fig. S5, ESI †). ...
... The observed photoinduced behaviour in air can be explained by the photothermal and photochemical contributions to the contraction of the sample, previously described in the literature. 9,10,15,16,[20][21][22]27 The important photoinduced temperature increment of the sample during UV irradiation contributes to its contraction due to a heat-induced disorder of the mesogens, being especially important in magnitude for the highintensity regime used in our experiments. On the other hand, the photoinduced generation of cis isomers also contributes to this disorder. ...
Soft and mechanically responsive actuators hold the promise to revolutionize the design and manufacturing of devices in the areas of microfluidics, soft robotics and biomedical engineering. In many of these applications, the actuators need to operate in a wet environment that can strongly affect their performance. In this paper, we report on the photomechanical response in a biological buffer of azobenzene-containing liquid crystal elastomer (LCE)-based actuators, prepared by four-dimensional (4D) printing. Although the photothermal contribution to the photoresponse is largely cancelled by the heat withdrawing capacity of the employed buffer, a significant photoinduced reversible contraction, in the range of 7% of its initial length, has been achieved under load, taking just a few seconds to reach half of the maximum contraction. Effective photomechanical work performance under physiological conditions has, therefore, been demonstrated in the 4D-printed actuators. Advantageously, the photomechanical response is not sensitive to salts present in the buffer differently to hydrogels with responses highly dependent on the fluid composition. Our work highlights the capabilities of photomechanical actuators, created using 4D printing, when operating under physiological conditions, thus showing their potential for application in the microfluidics and biomedical fields.
