of responsive materials as machines is in robotics. The vision, leadership, and research of Bar-Cohen is seminal in both invigorating and focusing current-day research activities to develop responsive materials [2] and implement [3] them as lightweight, dexterous, and gentle (e.g., soft) robotic elements. In this spirit, this review exhaustively details the materials, the nature of their stimuli-response, and discusses considerations for their implementation in robotic systems and subsystems.Robotics is a well-established but growing field of research. Sustained progress in both performance and functionality continue to be realized in commercial robotic systems largely based on conventional materials and their integration with mechanisms. A recent example is the Atlas robot from Boston Dynamics [4] (Figure 1a). The incorporation of stimuli-responsive materials in robotics has largely focused on component-level demonstrations to extend the performance of a subsystem (such as a hand or gripper).Stimuli-responsive materials, spanning nearly all classes of materials and size scales, are currently subject to widespread examination in corporate, government, and academic research laboratories (Figure 1b-d). [5][6][7] In some cases, these materials have already found widespread commercial implementation in end use and are comparatively mature. The materials and fundamentals of their responses are summarized in Figure 2. Shape memory alloys (SMAs) and ceramic piezoelectric materials are distinctive in that they are hard, stimuli-responsive materials. Deformation of these materials can produce large energy densities due to their inherent stiffness. Electroactive polymers (EAPs) remain a topic of considerable interest, particularly dielectric elastomer actuators (DEAs). Engineered systems are rapidly emerging and enabling performance gains in robotics, largely based on pneumatic or fluidic (such as HASEL [8] actuators) transport processes that localize deformation to generate force or produce motion. Soft materials, such as shape memory polymers (SMPs), hydrogels, and liquid crystalline polymer networks (LCNs) and elastomers (LCEs) may offer distinctive functional performance to robotic systems in allowing local control of deformation without the need for complex interfacing with mechanisms. As will be evident, each of these materials has inherent advantages and performance tradeoffs that must be considered in functional implementations. However, responsive material systems have a common obstacle to widespread use: the performance and Machines are systems that harness input power to extend or advance function. Fundamentally, machines are based on the integration of materials with mechanisms to accomplish tasks-such as generating motion or lifting an object. An emerging research paradigm is the design, synthesis, and integration of responsive materials within or as machines. Herein, a particular focus is the integration of responsive materials to enable robotic (machine) functions such as gripping, lifting, or motility (walk...

Liquid crystalline elastomers (LCE) are stimuli‐responsive materials with a distinguished mechanical response. LCE have been subject to numerous recent functional examinations in robotics, health sciences, and optics. The liquid crystallinity of the elastomeric polymer networks of LCE are largely derived from liquid crystalline monomer precursors. Recent reports have utilized commercially available liquid crystalline diacrylate monomers in chain extension reactions to prepare LCE. These reactions have been largely based on monomeric precursors originally to enhance the and thermal stability of optical films. Here, it is demonstrated that preparing LCE via a liquid crystalline diacrylate with reduced mesogen–mesogen interaction enhances and sharpens the thermotropic actuation of these materials. Robust composition‐response correlations are demonstrated in LCE prepared by three common synthetic methods. The enhanced thermotropic response of LCE prepared from this precursor increases the thermomechanical efficiency by sixfold. Accordingly, this work addresses important limitations in utilizing the thermal response of LCE in robotics, health care, and consumer goods.

distinctive due to the salient features of light that can allow remote and wireless activation, spatial and temporal control, and regulation via wavelength, intensity, and polarization. To date, photomechanical effects in photochromic polymeric materials have been predominately examined in azobenzene-functionalized compositions typified by a short-lived deformation (<24 h). [8] In end use implementations, the short-lived deformation will limit utility or require constant energy expenditure to maintain the desired shape reconfiguration. As such, there is an unmet need to develop photoresponsive polymeric materials capable of long-lived energy efficient deformations.The azobenzene moiety can be functionalized to exhibit liquid crystalline phases that undergo order-decreasing phototropic phase transitions with light irradiation. [9] By functionalizing azobenzene molecules with polymerizable groups such as acrylates, azobenzene-functionalized liquid crystalline polymer networks (LCNs; T g above room temperature) and elastomers have been synthesized and examined. For azobenzenefunctionalized LCNs and LCEs, light irradiation can induce isothermal disorganization. Specifically, the photoinduced trans→cis isomerization of azobenzene alters the geometric configuration of the chromophore in the macromolecular network. The result is photoinduced directional strain wherein the network contracts parallel to the nematic director and expands in the orthogonal directions. [9,10] Azobenzene-functionalized LCNs and LCEs have been demonstrated to exhibit photoresponsive shape reconfiguration, [11] responsive surfaces, [12] and motility. [13] Azobenzene-functionalized LCNs and LCEs have primarily been based on the copolymerization of acrylate monomers consisting of 2-30 wt% of the classical azobenzene chromophore. The short-lived photomechanical deformations of these materials are governed by the thermal relaxation of the azobenzene cis-isomer. Substitution of the azobenzene chromophore in the para-or ortho-positions of the molecule has been long known to influence the kinetics of the cis→trans isomerization. [10a,14] Of particular relevance to our interest in realizing long-lived and all-optical control of shape are prior examinations detailing substitutions that increase the lifetime of the cis-isomer. [15] Recently, ortho-fluorination of azobenzene has been presented as a method to produce stable cis-isomers with a dramatic half-life of approximately 700 days. [16] The small size of the fluorine substituents leads to minimal distortion of the planar azobenzene structure, making this an ideal approach for the preparation of azobenzene-functionalized LCNs and LCEs with long-lived macroscopic shape changes. Photoresponsive liquid crystal elastomers (LCEs) are a unique class of anisotropic materials capable of undergoing large-scale, macroscopic deformations when exposed to light. Here, surface-aligned, azobenzene-functionalized LCEs are prepared via a radical-mediated, thiol-acrylate chain transfer reaction. A long-lived, macrosc...

Liquid crystal elastomers (LCEs) are soft materials that undergo large anisotropic shape change in response to stimuli. Rational organization of the local director field can impart spatial control of the strain profile, enabling stretch-based deformation capable of nearly 20 J kg −1 of output force. LCEs are increasingly being considered in end-use applications in robotics, therapeutics, and optics. Here, a new synthetic approach is introduced to prepare LCEs composed of main chain mesogens via the cationic photopolymerization of the epoxy liquid crystal monomer (LCM). This examination details the optical, mechanical, and thermal properties of epoxide-based LCEs as a function of spacer length (3, 6, or 11 carbons). The oxygen insensitivity of the cationic photopolymerization of these monomers makes this approach particularly attractive for implementation with emerging additive manufacturing techniques. This contribution focuses on microstructuring LCEs via 2-photon direct laser writing (2P-DLW). A custom heated cell facilitated 2P-DLW of the aligned LCE epoxy resin melts to fabricate diverse geometric arrays. Enabled by the orthogonality of the reaction chemistry, hybrid and microstructured material compositions are prepared via the encapsulation of LCE epoxy micropatterns with free-radical polymerization of acrylate-based LCEs. The distinct thermomechanical response of the hybridized and microstructured LCE composites enables local and spatially controlled actuation.

Liquid crystalline elastomers (LCEs) are well known for their stimuli-responsive behavior. Of interest to the work presented here is the distinctive, nonlinear deformation of these materials to load. Here, we assess the cyclic deformation and elastic recovery of acrylate-based LCEs synthesized by chain-transfer reactions. Mechanical deformation of the LCEs (prepared with this synthetic approach) beyond a threshold strain value does not elastically recover, unless heat-treated. The thermomechanical actuation of these materials exhibits limited hysteresis over five cycles. Exploration of the deformation mechanics and elastic recovery extends the understanding of this material composition and informs its potential use in applications.

Thiol-yne photopolymerization in miniemulsion is demonstrated as a simple, rapid, and one-pot synthetic approach to polythioether nanoparticles with tuneable particle size and clickable functionality. The strategy is also useful in the synthesis of composite polymer-inorganic nanoparticles.

Liquid crystal elastomers (LCEs) are anisotropic polymeric materials. When subjected to an applied stress, liquid crystalline (LC) mesogens within the elastomeric polymer network (re)orient to the loading direction. The (re)orientation during deformation results in nonlinear stress‐strain dependence (referred to as soft elasticity). Here, we uniquely explore mechanotropic phase transitions in elastomers with appreciable mesogenic content and compare these responses to LCEs in the polydomain orientation. The isotropic (amorphous) elastomers undergo significant directional orientation upon loading, evident in strong birefringence and x‐ray diffraction. Functionally, the mechanotropic displacement of the elastomers to load is also nonlinear. However, unlike the analogous polydomain LCE compositions examined here, the isotropic elastomers rapidly recover after deformation. The mechanotropic orientation of the mesogens in these materials increase the toughness of these thiol‐ene photopolymers by nearly 1300 % relative to a chemically similar elastomer prepared from wholly isotropic precursors.

This paper reports the synthesis of catechol-functionalized thiol–ene networks as photocurable adhesives, where adhesive interactions are derived from 4-allylpyrocatechol – an alkene readily obtained from Syzygium aromaticum flower buds (clove oil).