Soft-bodied aquatic invertebrates, such as sea slugs and snails, are capable of diverse locomotion modes under water. Recapitulation of such multimodal aquatic locomotion in small-scale soft robots is challenging, due to difficulties in precise spatiotemporal control of deformations and inefficient underwater actuation of existing stimuli-responsive materials. Solving this challenge and devising efficient untethered manipulation of soft stimuli-responsive materials in the aquatic environment would significantly broaden their application potential in biomedical devices. We mimic locomotion modes common to sea invertebrates using monolithic liquid crystal gels (LCGs) with inherent light responsiveness and molecular anisotropy. We elicit diverse underwater locomotion modes, such as crawling, walking, jumping, and swimming, by local deformations induced by selective spatiotemporal light illumination. Our results underpin the pivotal role of the physicomechanical properties of LCGs in the realization of diverse modes of light-driven robotic underwater locomotion. We envisage that our results will introduce a toolbox for designing efficient untethered soft robots for fluidic environments.

Macroscale robotic systems have demonstrated great capabilities of high speed, precise, and agile functions. However, the ability of soft robots to perform complex tasks, especially in centimeter and millimeter scale, remains limited due to the unavailability of fast, energy-efficient soft actuators that can programmably change shape. Here, we combine desirable characteristics from two distinct active materials: fast and efficient actuation from dielectric elastomers and facile shape programmability from liquid crystal elastomers into a single shape changing electrical actuator. Uniaxially aligned monoliths achieve strain rates over 120%/s with energy conversion efficiency of 20% while moving loads over 700 times the actuator weight. The combined actuator technology offers unprecedented opportunities towards miniaturization with pre-1 arXiv:1904.09606v1 [cond-mat.soft] 21 Apr 2019 cision, efficiency, and more degrees of freedom for applications in soft robotics and beyond.Force generation, efficiency, strength-to-weight ratio, work capacity, and shape programmability will be key for the next generation of soft robots to perform complex functions. Despite significant advances in robots, including gymnastic feats (1), the underlying rigid actuation mechanisms, use of electric motors or hydraulic and pneumatic actuators, remain relatively unchanged and potentially hinder their miniaturization and, more importantly, their use in human collaborative environments (2). Efficient and programmable soft actuators, like an artificial muscle, would significantly increase the capabilities and potential applications of soft robotic systems in aerospace, industrial, or medical technologies (3-5). Among many soft actuation mechanisms that have been explored, dielectric elastomer (DE) actuators appear promising and even outperform skeletal muscle in some aspects (6-9). Separately, liquid crystal elastomers (LCEs) have demonstrated reversible large mechanical deformation by thermal and optical actuation. Recent advances in photoalignment and top-down microfabrication techniques have enabled pre-programming of LC alignment in microdomains for complex shape morphing (10-12). However, both actuator types have their drawbacks: DE films need to be prestretched, making it difficult to program local actuation behaviors microscopically (13). Meanwhile, directly converting electrical energy to mechanical work utilizing LCEs has, until now, remained limited due to the small strain generated (14-19). Typically, DE actuators function by electrostatic attraction between two compliant electrodes coated on opposing sides of an isotropic DE to form a variable resistor-capacitor (20).High voltage applied to the compliant electrodes induces an electrostatic pressure called Maxwell stress. The electrical actuation mechanism can result in much higher operating efficiency (ratio of mechanical work to input electrical energy) and faster actuation speed than those of LCEs (8,9). Besides functioning as soft linear actuators, DE actuators could be applied...

Self-peeling of gecko toes is mimicked by integration of film-terminated fibrillar adhesives to hybrid nematic liquid crystal network (LCN) cantilevers. A soft gripper is developed based on the gecko-inspired attachment/detachment mechanism. Performance of the fabricated gripper for transportation of thin delicate objects is evaluated by the optimum mechanical strength of the LCN and the maximum size of the adhesive patch.

The shape‐shifting behavior of liquid crystal networks (LCNs) and elastomers (LCEs) is a result of an interplay between their initial geometrical shape and their molecular alignment. For years, reliance on either one‐step in situ or two‐step film processing techniques has limited the shape‐change transformations from 2D to 3D geometries. The combination of various fabrication techniques, alignment methods, and chemical formulations developed in recent years has introduced new opportunities to achieve 3D‐to‐3D shape‐transformations in large scales, albeit the precise control of local molecular alignment in microscale 3D constructs remains a challenge. Here, the voxel‐by‐voxel encoding of nematic alignment in 3D microstructures of LCNs produced by two‐photon polymerization using high‐resolution topographical features is demonstrated. 3D LCN microstructures (suspended films, coils, and rings) with designable 2D and 3D director fields with a resolution of 5 µm are achieved. Different shape transformations of LCN microstructures with the same geometry but dissimilar molecular alignments upon actuation are elicited. This strategy offers higher freedom in the shape‐change programming of 3D LCN microstructures and expands their applicability in emerging technologies, such as small‐scale soft robots and devices and responsive surfaces.

Muscle-driven actuation of biomimetic microfibrillar structures is achieved using integrative soft-lithography on a backing splayed liquid-crystal elastomer (LCE). Variation in the backing LCE layer thickness yields different modes of thermal deformation from a pure bend to a twist-bend. Muscular motion and dynamic self-cleaning of gecko toe pads are mimicked via this mechanism.

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Inspired by the superior adhesive ability of the gecko foot pad, we report an experimental study of conformal adhesion of a soft elastomer thin film on biomimetic micropatterned surfaces (micropillars), showing a remarkable adhesion enhancement due to the surface patterning. The adhesion of a low-surface-energy poly(dimethylsiloxane) tape to a SU-8 micropatterned surface was found be able to increase by 550-fold as the aspect ratio increases from 0 to 6. The dependency of the adhesion enhancement on the aspect ratio is highly nonlinear. A series of peeling experiment coupled with optical interference imaging were performed to investigate the adhesion enhancement as a function of the height of the micropillars and the associated delamination mechanisms. Local elastic energy dissipation, side-wall friction, and plastic deformations were analyzed and discussed in terms of their contributions to the adhesion enhancement. We conclude that the local adhesion and friction events of pulling micropillars out of the embedded polymer film play a primary role in the observed adhesion enhancement. The technical implications of this local friction-based adhesion enhancement mechanism were discussed for the effective assembly of similar or dissimilar material components at small scales. The combined use of the micro/nanostructured surfaces with the van der Waals interactions seem to be a potentially more universal solution than the conventional adhesive bonding technology, which depends on the chemical and viscoelastic properties of the materials.

Poly(acrylic acid-co-N,N'-methylenebisacrylamide) hydrogel films were synthesized by copolymerizing acrylic acid (AAc) with N,N'-methylenebisacrylamide (MBA) as a cross-linker via photo polymerization in the spacing confined between two glass plates. NMR spectroscopy was utilized to determine the cross-linking density. We found that the cross-linking density determined by NMR is higher than that expected from the feed concentrations of cross-linkers, suggesting that MBA is more reactive than AAc and the heterogeneous nature of the cross-linking. In addition to the swelling tests, indentation tests were performed on the hydrogel films under water to investigate effects of the cross-linking density on the adhesion and mechanical properties of the hydrogel films in terms of adhesive pull-off force and Hertz-type elastic modulus. As the cross-linker concentration increased, the effective elastic modulus of the hydrogel films increased dramatically at low cross-linking densities and reached a high steady-state value at higher cross-linking densities. The pull-off force decreased with increasing cross-linker concentration and reached a lower force plateau at high cross-linking densities. An optimal "trade-off" cross-linking density was determined to be 0.02 mol fraction of MBA in the hydrogel, where balanced elastic modulus and adhesive pull-off force can be obtained.