the interplay between different stimuli. When multiple stimuli can be synergistically employed, there is often a significantly larger benefit as compared to the sum of analogous systems using a single stimulus. [38][39][40] Herein, we highlight recent examples of research advances using multiaddressable systems (Scheme 1). This review is broken down into six parts: i) photoand chemical-addressable architectures; ii) photo-and pHaddressable architectures; iii) photo-and thermal-addressable architectures; iv) photo-and redox-addressable architectures; v) multi-photoaddressable architectures; and vi) photo-and less common stimuli-addressable architectures. We focus on multiaddressable systems that leverage photoswitches as one of their key design features. For each section, we highlight applications in soft materials that demonstrate the potential of these multiaddressable photochromic systems. We exclude multiaddressable systems that don't contain a photoswitching molecule as they have been reviewed elsewhere. [38][39][40][41][42][43][44]

studies have focused on programming desired 3D structures of soft materials such as shape-memory polymers [2,3] and gels [4][5][6][7] by introducing spatial variations in thermal expansion/contraction, swelling, or molecular order. [8] A particularly useful class of materials to achieve dynamic 2D to 3D shape transformations are liquid crystal elastomers (LCEs), where the coupling between the orientational ordering of polymerized mesogens and the conformation of a polymer backbone can be leveraged for large, anisotropic deformations that are dictated by the director field. [9,10] Using oriented surface alignment layers [11] or microchannels, [12] director orientation can be patterned with a resolution approaching 10 µm. [13] A subsequent reduction of nematic ordering, usually driven by heating, leads to local contraction along the director and expansion along the transverse directions, driving out-of-plane buckling into 3D shapes that are "blueprinted" by the pattern of director orientation. However, while geometric methods allow for the deduction of the necessary inplane director orientation field to generate a desired profile of Gaussian curvature, [14][15][16][17] there are a number of practical drawbacks to this approach. First, prescription of complex director fields requires significant processing, making high-throughput fabrication, and evaluation of designs challenging. Additionally, the surface alignment methods needed to specify director orientation with high spatial resolution are only amenable to certain chemistries due to the need for high mesogen content, and thus cannot be widely generalized to all LCE systems. For example, while classical LCE systems based on siloxanes, [18,19] as well as recently developed systems that rely on simple and efficient "click" chemistries, [20] offer attractive thermal and mechanical properties for shape-morphing systems, they typically only allow for alignment of the director field with coarse spatial resolution such as through application of shear stress [21][22][23] or magnetic fields. [24] To circumvent the need for a spatially varying director orientation, where the direction of deformation varies but the magnitude is constant, a potential alternative method to drive shape changes is to instead locally prescribe the magnitude of deformation within an otherwise homogeneous director field. While spatial variations in the extent of deformation have been widely employed for shape Liquid crystal elastomers (LCEs) are an attractive platform for dynamic shape-morphing due to their ability to rapidly undergo large deformations. While recent work has focused on patterning the director orientation field to achieve desired target shapes, this strategy cannot be generalized to material systems where high-resolution surface alignment is impractical. Instead of programming the local orientation of anisotropic deformation, an alternative strategy for prescribed shape-morphing by programming the magnitude of stretch ratio in a thin LCE sheet with constant director orientation is...

in temperature-responsive hydrogels. [4,7] Although requiring careful consideration of heat transfer, such photothermal shape changes can provide rapid kinetics (at least for hydrogels with small dimensions, where mass transport is fast), nearly complete reversibility, and large volume changes.Directing the motion of micro-and nanoscale objects at an air/water interface is important from numerous perspectives, ranging from improving fundamental understanding of biological systems [8] to designing synthetic microrobots and swimmers. [9] Capillary and Marangoni forces can drive motion of sub-mm-scale objects, since these forces greatly exceed gravity or thermal energy on this scale. [10,11] Recently, reversible and programmed capillary assembly was demonstrated using temperature-responsive hydrogels with 3D shapes specified by spatial variations in swelling. [12] The shapes of the buckled particles distorted the contact line at the air-water interface, giving rise to capillary attraction between particles with a symmetry specified by the particle shape, and therefore the pattern of swelling. Another method to dynamically control capillary forces relied on the torques applied by rotating magnets. [13] However, these prior examples were limited to global changes in the state of the whole system, i.e., the temperature of water or the orientation of the magnetic field, thereby preventing control of single objects within a larger ensemble. While controlling the interactions and assemblies of such interfacially adsorbed objects with light is attractive for enabling high levels of spatiotemporal control, previous methods to drive motion through shape morphing have relied on patterning light on a length scale substantially smaller than the individual objects, [4,5,14] making this a challenging approach for dynamic or multicomponent systems.Here, we demonstrate spatial patterning of Au NPs as photothermal heaters embedded within temperature-responsive polymer hydrogels situated at an air/water interface, generating materials that exhibit reconfigurable capillary assembly and motion. We show that photochemical reduction [15] of a gold salt, using pendent benzophenone groups in a copolymer, enables high-resolution NP micropatterning within 2D sheets of temperature-responsive hydrogels. The resulting in-plane variations in Au NP concentration produce spatially nonuniform temperature profiles, and therefore hydrogel swelling under Patterning of nanoparticles (NPs) via photochemical reduction within thermally responsive hydrogel films is demonstrated as a versatile platform for programming light-driven shape morphing and materials assembly. Responsive hydrogel disks, containing patterned metal NPs, form characteristic wrinkled structures when illuminated at an air/water interface. The resulting distortion of the three-phase (air/water/hydrogel) contact lines induces capillary interactions between two or more disks, which are either attractive or repulsive depending on the selected pattern of light. By programming the shapes of t...

Photoisomerization of azobenzene in polymer matrices is a powerful method to convert photon energy into mechanical work. While most previous studies have focused on incorporating azobenzene within amorphous or liquid crystalline materials, the limited extents of molecular ordering and correspondingly modest enthalpy changes upon switching in such systems has limited the achievable energy densities. In this work, we introduce a semi-crystalline main-chain poly(azobenzene) where photoisomerization is capable of reversibly triggering melting and re-crystallization under essentially isothermal conditions. These materials can be drawn into aligned fibers, yielding optically-driven two-way shape memory actuators capable of reversible bending. Scheme 1. Preparation of semi-crystalline poly(azobenzene)s ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website.

While most photomechanical materials developed to date have relied on free‐space illumination to drive actuation, this strategy fails when direct line‐of‐site access is precluded. In this study, waveguided light is harnessed by liquid crystal elastomer (LCE) nanocomposites to drive actuation. Using photo‐chemical reduction of gold salts to plasmonic nanoparticles, prescription of photoresponsive regions within fibers of mono‐domain LCEs is demonstrated with control over both the location along the fiber axis, as well as in the azimuthal direction. Due to localized photothermal heating provided by plasmonic absorption of waveguided light and resulting inhomogeneous thermally induced deformation of the LCE, reversible bending along multiple axes is demonstrated.

Shape transformation of thin two-dimensional sheets into three-dimensional structures using light is of great interest for remotely-controlled fabrication, surface modulation, and actuation. Over the last few decades, significant efforts have been made to develop materials systems incorporating photochemical or photothermal elements to drive deformation in response to illumination. However, the full extent of the interplay between chemistry, optics, and mechanics in these materials is poorly understood. In this Review, we introduce principles of shape morphing in these systems by considering the underlying physics of photo-induced stresses and how these have been used in recent literature. In addition, we provide a critical overview of the important design characteristics of both photochemical and photothermal system and offer our view on the open opportunities and challenges in this rapidly growing field.

Photoinduced shape morphing has implications in fields ranging from soft robotics to biomedical devices. Despite considerable effort in this area, it remains a challenge to design materials that can be both rapidly deployed and reconfigured into multiple different three-dimensional forms, particularly in aqueous environments. In this work, we present a simple method to program and rewrite spatial variations in swelling and, therefore, Gaussian curvature in thin sheets of hydrogels using photoswitchable supramolecular complexation of azobenzene pendent groups with dissolved α-cyclodextrin. We show that the extent of swelling can be programmed via the proportion of azobenzene isomers, with a 60% decrease in areal swelling from the all trans to the predominantly cis state near room temperature. The use of thin gel sheets provides fast response times in the range of a few tens of seconds, while the shape change is persistent in the absence of light thanks to the slow rate of thermal cis–trans isomerization. Finally, we demonstrate that a single gel sheet can be programmed with a first swelling pattern via spatially defined illumination with ultraviolet light, then erased with white light, and finally redeployed with a different swelling pattern.

Actuation remains a significant challenge in soft robotics. Actuation by light has important advantages: Objects can be actuated from a distance, distinct frequencies can be used to actuate and control distinct modes with minimal interference, and significant power can be transmitted over long distances through corrosion-free, lightweight fiber optic cables. Photochemical processes that directly convert photons to configurational changes are particularly attractive for actuation. Various works have reported light-induced actuation with liquid crystal elastomers combined with azobenzene photochromes. We present a simple modeling framework and a series of examples that study actuation by light. Of particular interest is the generation of cyclic or periodic motion under steady illumination. We show that this emerges as a result of a coupling between light absorption and deformation. As the structure absorbs light and deforms, the conditions of illumination change, and this, in turn, changes the nature of further deformation. This coupling can be exploited in either closed structures or with structural instabilities to generate cyclic motion.