Photodeformable liquid crystal polymers (LCPs) that adapt their shapes in response to light have aroused a dramatic growth of interest in the past decades, since light as a stimulus enables the remote control and diverse deformations of materials. This review focuses on the growing research on photodeformable LCPs, including their basic actuation mechanisms, the various deformation modes, the newly designed molecular structures, and the improvement of processing techniques. Special attention is devoted to the novel molecular structures of LCPs, which allow for easy processing and alignment. The soft actuators with various deformation modes such as bending, twisting, and rolling in response to light are also covered with the emphasis on their photo-induced bionic functions. Potential applications in energy harvesting, self-cleaning surfaces, sensors, and photo-controlled microfluidics are further illustrated. The existing challenges and future directions are discussed at the end of this review.

Photodeformable Azobenzene-Containing Liquid Crystal Polymers and Soft Actuators.

It is widely appreciated that surface tension can dominate the behavior of liquids at small scales. Solids also have surface stresses of a similar magnitude, but they are usually overlooked. However, recent work has shown that these can play a central role in the mechanics of soft solids such as gels. Here, we review this emerging field. We outline the theory of surface stresses, from both mechanical and thermodynamic perspectives, emphasizing the relationship between surface stress and surface energy. We describe a wide range of phenomena at interfaces and contact lines where surface stresses play an important role. We highlight how surface stresses cause dramatic departures from classic theories for wetting (Young–Dupre), adhesion (Johnson–Kendall–Roberts), and composites (Eshelby). A common thread is the importance of the ratio of surface stress to an elastic modulus, which defines a length scale below which surface stresses can dominate.

Elastocapillarity: Surface Tension and the Mechanics of Soft Solids

Soft tubular actuators can be widely found both in nature and in engineering applications. The benefits of tubular actuators include (i) multiple actuation modes such as contraction, bending, and expansion; (ii) facile fabrication from a planar sheet; and (iii) a large interior space for accommodating additional components or for transporting fluids. Most recently developed soft tubular actuators are driven by pneumatics, hydraulics, or tendons. Each of these actuation modes has limitations including complex fabrication, integration, and nonuniform strain. Here, we design and construct soft tubular actuators using an emerging artificial muscle material that can be easily patterned with programmable strain: liquid crystal elastomer. Controlled by an externally applied electrical potential, the tubular actuator can exhibit multidirectional bending as well as large (~40%) homogenous contraction. Using multiple tubular actuators, we build a multifunctional soft gripper and an untethered soft robot.

Electrically controlled liquid crystal elastomer–based soft tubular actuator with multimodal actuation

There is growing interest in creating untethered soft robotic matter that can repeatedly shape-morph and self-propel in response to external stimuli. Toward this goal, we printed soft robotic matter composed of liquid crystal elastomer (LCE) bilayers with orthogonal director alignment and different nematic-to-isotropic transition temperatures (T_(NI)) to form active hinges that interconnect polymeric tiles. When heated above their respective actuation temperatures, the printed LCE hinges exhibit a large, reversible bending response. Their actuation response is programmed by varying their chemistry and printed architecture. Through an integrated design and additive manufacturing approach, we created passively controlled, untethered soft robotic matter that adopts task-specific configurations on demand, including a self-twisting origami polyhedron that exhibits three stable configurations and a “rollbot” that assembles into a pentagonal prism and self-rolls in programmed responses to thermal stimuli.

https://robotics.sciencemag.org/content/robotics/4/33/eaax7044.full-text.pdf

Untethered soft robotic matter with passive control of shape morphing and propulsion.

Hydrogels possess magnificent properties which may be harnessed for novel applications. However, this is not achievable if the mechanical behaviors of hydrogels are not well understood. This paper aims to provide the reader with a bird's eye view of the mechanics of hydrogels, in particular the theories associated with deformation of hydrogels, the phenomena that are commonly observed, and recent developments in applications of hydrogels. Besides theoretical analyses and experimental observations, another feature of this paper is to provide an overview of how mechanics can be applied.

Advances in Mechanics of Soft Materials: A Review of Large Deformation Behavior of Hydrogels

Traditional hard robots often require complex motion-control systems to accomplish various tasks, while applications of soft-bodied robots are limited by their low load-carrying capability. Herein, a hybrid tensegrity robot composed of both hard and soft materials is constructed, mimicking the musculoskeletal system of animals. Employing liquid crystal elastomer-carbon nanotube composites as artificial muscles in the tensegrity robot, it is demonstrated that the robot is extremely deformable, and its multidirectional locomotion can be entirely powered by light. The tensegrity robot is ultralight, highly scalable, has high load capacity, and can be precisely controlled to move along different paths on multiterrains. In addition, the robot also shows excellent resilience, deployability, and impact-mitigation capability, making it an ideal platform for robotics for a wide range of applications.

A Light-Powered Ultralight Tensegrity Robot with High Deformability and Load Capacity.

Specially arranged external stimuli or carefully designed geometry are often essential for realizing continuous autonomous motion of active structures without self-carried power. As a consequence, ...

Light or Thermally Powered Autonomous Rolling of an Elastomer Rod.

In this paper, we study light-driven bending vibration of a liquid crystal elastomer (LCE) beam. Inhomogeneous and time-dependent number fraction of photochromic liquid crystal molecules in cis state in an LCE beam is considered in our model. Using mode superposition method, we obtain semi-analytic form of light-driven bending vibration of the LCE beam. Our results show that periodic vibration or a statically deformed state can be induced by a static light source in the LCE beam, which depends on the light intensity and position of the light source. We also demonstrate that the amplitude of the bending vibration of the LCE beam can be regulated by tuning light intensity, damping factor of the beam, and thermal relaxation time from cis to trans state, while the frequency of the vibration in the beam mainly depends on the thermal relaxation time. The method developed in the paper can be important for designing light-driven motion structures and photomechanical energy conversion systems.

Modeling of Light-Driven Bending Vibration of a Liquid Crystal Elastomer Beam

Two solids can adhere to each other in the presence of a liquid bridge between them, which is called wet adhesion. When the solid is soft, the liquid bridge can cause deformation in the material, and in turn, the deformation may have dramatic effects on the wet adhesion. To investigate the effect, in this article, we calculate the deformation in two soft layers with different separations and connected by a liquid bridge. We illustrate the effect of deformation in the soft layers on the adhesive force. For a given liquid volume and separation between the two layers, the adhesive force increases dramatically by decreasing the elastic moduli of the soft layers. We also discuss the contact between the two soft layers due to the deformation caused by the liquid bridge. Depending on the volume of the liquid bridge, the two layers may be in contact with each other at the center of the wetting area or some other locations between the center and the contact line. The results may improve current understanding of wet adhesion between soft materials and have potential applications in designing and fabricating soft devices and structures.

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Kai Li

Papers

A Light-Powered Ultralight Tensegrity Robot with High Deformability and Load Capacity.

Light or Thermally Powered Autonomous Rolling of an Elastomer Rod.

Modeling of Light-Driven Bending Vibration of a Liquid Crystal Elastomer Beam

Wet adhesion between two soft layers

Light-powered self-excited oscillation of a liquid crystal elastomer pendulum