Soft materials and actuators enable applications that are not possible using traditional rigid robots and actuators. Owing to their flexibility and compliance, soft actuators can adapt to complex and dynamic environments, which makes them exceptionally advantageous for physical interaction with fragile objects or living organisms. However, despite great advances in the field of soft robotics, both replicating the performance of biological actuators and the implementation of soft robots in industrial applications remain a challenge owing to the high-level complexity of biological actuators. Biological muscle and animal limbs have long served as inspiration for the design of soft actuators featuring a range of complex actuations (such as reversible contraction, expansion and rotation) and motions (such as bending and twisting), as well as specific strain, force, energy and power performance metrics 1-3 (Box 1). However, biological muscles have evolved to operate robustly in dynamic environments with high performance (efficiency, speed and power) and complex actions (including sensing, tunable stiffness or repair mechanisms), all of which are challenging to replicate in synthetic systems.Individual performance metrics can be particularly useful for specific tasks and are thus usually designed for specific applications. However, combining the performance and physical intelligence of biological systems (such as self-healing and self-adaptive behaviours) would greatly advance the integration of soft actuator and robot technology in multiple fields (for example, health care 4 , agriculture 5,6 , industry 7,8 and mobility 9 ).In this Review, we discuss materials and soft actuation methods that can enable advanced physical intelligence 10 , function and performance in soft actuator technology. We first examine promising soft actuation methods, including tethered, untethered and biohybrid actuation, and then discuss actuation strategies based on programmable soft materials that improve the performance and multifunctionality of soft actuators. We highlight state-of-the-art demonstrations of soft actuators towards real-world applications, including soft grippers, artificial muscles, sensor-integrated soft robots, haptic displays and biomedical applications. Lastly, we discuss challenges and opportunities for next-generation soft actuators, including integrating physical intelligence, encoding adaptability, manufacturing scalability and reproducibility, and improving the durability of soft robots. Promising soft actuation methodsMaterial selection, structural design and fabrication techniques are key factors in designing soft actuators that can convert multiple energy inputs into mechanical energy outputs using tethered, untethered or biohybrid approaches. Tethered synthetic soft actuationTethered soft actuators are usually actuated through a change of fluidic pressure in hollow channels of the soft actuator body, by electrically driven shape change or through passive deformation produced by motion

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.

Untethered synthetic microrobots have significant potential to revolutionize minimally invasive medical interventions in the future. However, their relatively slow speed and low controllability near surfaces typically are some of the barriers standing in the way of their medical applications. Here, we introduce acoustically powered microrobots with a fast, unidirectional surface-slipping locomotion on both flat and curved surfaces. The proposed threedimensionally printed, bullet-shaped microrobot contains a spherical air bubble trapped inside its internal body cavity, where the bubble is resonated using acoustic waves. The net fluidic flow due to the bubble oscillation orients the microrobot's axisymmetric axis perpendicular to the wall and then propels it laterally at very high speeds (up to 90 body lengths per second with a body length of 25 μm) while inducing an attractive force toward the wall. To achieve unidirectional locomotion, a small fin is added to the microrobot's cylindrical body surface, which biases the propulsion direction. For motion direction control, the microrobots are coated anisotropically with a soft magnetic nanofilm layer, allowing steering under a uniform magnetic field. Finally, surface locomotion capability of the microrobots is demonstrated inside a threedimensional circular cross-sectional microchannel under acoustic actuation. Overall, the combination of acoustic powering and magnetic steering can be effectively utilized to actuate and navigate these microrobots in confined and hard-to-reach body location areas in a minimally invasive fashion. microrobots | acoustic actuation | magnetic control | microswimmers | bubble oscillation U ntethered synthetic microrobots have been recently investigated for their potential applications in targeted drug delivery, detoxification, and noninvasive surgeries (1-4). The existing microswimmers are powered by different external energy sources, such as light (5-7), electrical (8,9), magnetic (10, 11), and acoustic (12, 13) fields, or fueled by chemicals in the environment (14, 15). Among these actuation schemes, magnetic and acoustic field-based powering methods are the most prevalent in the biomedical context thanks to their deep-tissue penetration and high-energy-density capabilities. While the acoustic waves can deliver strong propulsion forces (16), the magnetic field can provide controlled steering of the microswimmers (17, 18). For example, acoustically excited bubbles can generate high streaming forces (16,19), and when employed in the robot's body they can act as an engine for the propulsion (12,20).Up to now, a few studies have used two-dimensional (2D) microfabrication (20, 21) and ultraviolet light-based polymerization techniques (12,22) to fabricate around 100-to 300-μm microrobots with cylindrical or conical cavities for bubble entrapment. As the bubble diameter scales down to the 10-to 30-μm range, the cylindrical cavity geometry would require advanced hydrophobic treatment to hold a microbubble stable due to the increased surface tens...

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...

Surface microrollers are promising microrobotic systems for controlled navigation in the circulatory system thanks to their fast speeds and decreased flow velocities at the vessel walls. While surface propulsion on the vessel walls helps minimize the effect of strong fluidic forces, three-dimensional (3D) surface microtopography, comparable to the size scale of a microrobot, due to cellular morphology and organization emerges as a major challenge. Here, we show that microroller shape anisotropy determines the surface locomotion capability of microrollers on vessel-like 3D surface microtopographies against physiological flow conditions. The isotropic (single, 8.5 µm diameter spherical particle) and anisotropic (doublet, two 4 µm diameter spherical particle chain) magnetic microrollers generated similar translational velocities on flat surfaces, whereas the isotropic microrollers failed to translate on most of the 3D-printed vessel-like microtopographies. The computational fluid dynamics analyses revealed larger flow fields generated around isotropic microrollers causing larger resistive forces near the microtopographies, in comparison to anisotropic microrollers, and impairing their translation. The superior surface-rolling capability of the anisotropic doublet microrollers on microtopographical surfaces against the fluid flow was further validated in a vessel-on-a-chip system mimicking microvasculature. The findings reported here establish the design principles of surface microrollers for robust locomotion on vessel walls against physiological flows.

Untethered microrobots offer a great promise for localized targeted therapy in hard-to-access spaces in our body. Despite recent advancements, most microrobot propulsion capabilities have been limited to homogenous Newtonian fluids. However, the biological fluids present in our body are heterogeneous and have shear rate–dependent rheological properties, which limit the propulsion of microrobots using conventional designs and actuation methods. We propose an acoustically powered microrobotic system, consisting of a three-dimensionally printed 30-micrometer-diameter hollow body with an oscillatory microbubble, to generate high shear rate fluidic flow for propulsion in complex biofluids. The acoustically induced microstreaming flow leads to distinct surface-slipping and puller-type propulsion modes in Newtonian and non-Newtonian fluids, respectively. We demonstrate efficient propulsion of the microrobots in diverse biological fluids, including in vitro navigation through mucus layers on biologically relevant three-dimensional surfaces. The microrobot design and high shear rate propulsion mechanism discussed herein could open new possibilities to deploy microrobots in complex biofluids toward minimally invasive targeted therapy.

Mobile microrobots hold remarkable potential to revolutionize health care by enabling unprecedented active medical interventions and theranostics, such as active cargo delivery and microsurgical manipulations in hard-to-reach body sites. High-resolution imaging and control of cell-sized microrobots in the in vivo vascular system remains an unsolved challenge toward their clinical use. To overcome this limitation, we propose noninvasive real-time detection and tracking of circulating microrobots using optoacoustic imaging. We devised cell-sized nickel-based spherical Janus magnetic microrobots whose near-infrared optoacoustic signature is enhanced via gold conjugation. The 5-, 10-, and 20-μm-diameter microrobots are detected volumetrically both in bloodless ex vivo tissues and under real-life conditions with a strongly light-absorbing blood background. We further demonstrate real-time three-dimensional tracking and magnetic manipulation of the microrobots circulating in murine cerebral vasculature, thus paving the way toward effective and safe operation of cell-sized microrobots in challenging and clinically relevant intravascular environments.

Controlled microrobotic navigation in the vascular system can revolutionize minimally invasive medical applications, such as targeted drug and gene delivery. Magnetically controlled surface microrollers have emerged as a promising microrobotic platform for controlled navigation in the circulatory system. Locomotion of micrororollers in strong flow velocities is a highly challenging task, which requires magnetic materials having strong magnetic actuation properties while being biocompatible. The L10‐FePt magnetic coating can achieve such requirements. Therefore, such coating has been integrated into 8 µm‐diameter surface microrollers and investigated the medical potential of the system from magnetic locomotion performance, biocompatibility, and medical imaging perspectives. The FePt coating significantly advanced the magnetic performance and biocompatibility of the microrollers compared to a previously used magnetic material, nickel. The FePt coating also allowed multimodal imaging of microrollers in magnetic resonance and photoacoustic imaging in ex vivo settings without additional contrast agents. Finally, FePt‐coated microrollers showed upstream locomotion ability against 4.5 cm s−1 average flow velocity with real‐time photoacoustic imaging, demonstrating the navigation control potential of microrollers in the circulatory system for future in vivo applications. Overall, L10‐FePt is conceived as the key material for image‐guided propulsion in the vascular system to perform future targeted medical interventions.