Soft Bionic Sensors and Actuators

Abstract

Nature is an unlimited source of inspiration for soft bionic sensors and actuators, which have been recently spotlighted for applications in soft robotics. Soft bionic sensors are sensing platforms that can bend, stretch, or morph shapes to allow for the detection of external stimuli such as force, displacement, pressure, temperature, or chemicals under slight mechanical deformations. Soft robotics technologies have now evolved beyond tactile sensing, and devices with displacement, temperature, or light detection capabilities are currently under development. Owing to advances in the field of conductive composite materials, various types of soft bionic sensors have been introduced, such as flexible, stretchable, and wearable sensors. With respect to soft bionic actuators, early developments on dielectric elastomers have led to the recent realization of artificial muscles based on smart polymer materials. While pneumatic actuators have opened the field of soft robotics, recent developments include various soft and smart actuating mechanisms using fluids, chemicals, and soft materials. Also, novel manufacturing technologies, such as 3D printing, have been ushering the advancedment of soft bionic actuators for soft robotic fingers inspired by biomimetic structures such as insects, or adopted by architectured solids like auxetic or origami structures. Owing to the increased interest in flexible soft robotics, which is expected to offer alternative ways to build soft and diverse robots, soft bionic sensors are anticipated to consistently grow in importance and become industrially relevant. Before considering the creation of novel soft bionic sensors and actuators, it is highly recommended to consult other fields such as material science to consider existing knowledge on soft and smart concepts. Thus, one of the challenges in soft bionic sensors and actuators is the interdisciplinary collaboration among other engineering fields. To showcase recent advances in this rapidly growing field, we have organized this special issue of Advanced Intelligent Systems focusing on “Soft Bionic Sensors and Actuators”. It is intended to make an open discussion for ongoing interdisciplinary research from various fields and to provide an outline for potential applications. The issue brings contributions together from experts and covers the following three key aspects of soft bionic sensors and actuators: Michael D. Dickey, Tae-il Kim, and co-workers (article number 2000159) reviewed gallium-based liquid metals for the biomedical sensors and soft actuators, which are representative stretchable conductors with metallic conductivity and nearly unlimited extensibility due to their liquid nature. Despite their enormous surface tension, liquid metals can be patterned into non-spherical shapes, such as wires, due to the presence of a native oxide shell. The thin oxide layer also enables the formation of stable liquid colloids and micro/nanosized droplets that do not coalesce so easily. The oxide layer can be exfoliated and chemically modified into semiconductor 2D materials to create and deposit atomically thin materials at room temperature. This review summarizes physical and chemical methods of modifying the surface of liquid metal to tune its properties. The surface modification of liquid metal provides unique applications, including use in soft biomedical sensors and actuators with mechanical properties similar to human tissue. Hyun-Joong Chung and co-workers (article number 2000186) reviewed magnetically controlled soft robotics utilizing elastomers and gels in actuation. A magnetic field has unique advantages in controlling soft robotics inside of an enclosed space, such as surgical catheters or untethered drug-delivering robots operating in the human body. Soft actuators, made of elastomers and gels functionalized with magnetically active materials, are a natural choice to drive magnetically controlled motions of the soft robots. Recent innovations in