Intelligent Soft Actuators and Flexible Devices

Abstract

Soft actuators and flexible devices such as stretchable sensors, with their inherent compliance, can exhibit large deformation and high adaptability to complex environments. By utilizing various actuation mechanisms, soft actuators have been applied to versatile manipulation, bionic soft robots, energy harvesting, and medical surgery across scales. Moreover, to endow soft robots with abilities of perception and human-machine interaction, stretchable and wearable sensors have been introduced in recent years. These flexible sensors are capable of detecting external physiological signals, e.g., pressure, temperature, and provide feedback for the control system. To date, owing to advances in material science, various types of smart materials have been developed for soft robots and sensors, such as pH-responsive hydrogels used for 4D printing, dielectric elastomers, liquid crystal elastomers, and liquid metal. These smart materials enable soft structures to achieve programmable shape-morphing and perceive changes in environments. On the other hand, the development of manufacturing techniques has revolutionized the construction of soft actuators and flexible devices. For instance, multi-materials printing techniques and hybrid assembly , i.e. combination of top-down and bottom-up approaches, result in new types of 2-D and 3-D flexible devices with control-lable material distribution. Also, progress in control strategies and systems, such as built-in feedback control, has led to the realization of high-performance soft robotic systems. With the growing interest in soft actuators and flexible devices, considerable achievements have been made in this interdisciplinary field. To summarize recent advances and updates in this field, we organized this special issue of Advanced Intelligent Systems which focuses on "Intelligent soft actuators and flexible devices". This issue brings a collection of papers covering a wide range of materials, fabrication techniques , actuation, and potential applications for soft actuators and flexible devices, which can be basically divided into the following three key aspects: 1. Novel materials technologies for flexible devices Soft materials with stimuli-responsive or conductive properties have demonstrated great potential in constructing flexible devices and robots. For instance, liquid crystal polymers actuated by UV or visible light have been used for intelligent actuators. To enhance the capability of liquid crystal polymers, Yanlei Yu and co-workers developed a facile strategy to synthesize reactive azobenzene-containing liquid crystal polymers (azo-LCPs) and photo-deformable fibers (article number 2000254). These reac-tive azo-LCPs exhibit stable liquid crystallinity and reversible photochemical properties. In addition, metal oxides/hydroxides materials such as cobalt oxides/hydroxides and manganese oxides can undergo electrochemical actuation behavior through volume-changing redox reactions. Alfonso H.W. Ngan et al. reviewed a class of stimuli-responsive oxides/hydroxides with turbostratic crystal structures (article number 2000215). Liquid metal alloys are widely used in stretchable strain sensors due to their high electrical conductivity under stretch conditions. Run-wei Li and co-workers used selective wetting and transferring process to fabricate a liquid metal-based resistive strain sensor that exhibits broad strain sensing range, ultralow detection limit, high mechanical robustness, and good repeatability (article number 2000235). The flexible sensor exhibits minimal hysteresis and fast response and demonstrates great potential in human health and motion monitoring, and virtual reality applications. 2. Novel actuation and control strategies for intelligent soft actuators and robots To improve practical outcomes of soft actuators and robots under complex environments, developing suitable and reliable actu-ation and control strategies is indispensable. Due to the capabilities of generating large deformation and high force output, pneumatic actuation becomes one of the most promising actu-ation strategies for soft robots. Guoying Gu and co-workers presented a planar laser cutting and stacking fabrication (PLCSF) approach to create multimaterial pneumatic soft actuators and robots with complex structures (article number 2000257). Multiple degrees of freedom pneumatic soft robots including bioinspired soft hand and crawling robots are fabricated to verify