Realizing high-performance soft robotic grippers is challenging because of the inherent limitations of the soft actuators and artificial muscles that drive them, including low force generation, small actuation range, and poor compactness to name a few. Despite advances in this area, realizing compact soft grippers, which exhibit high dexterity and force output, is still challenging. This article explores using twisted string actuators (TSAs) to drive a soft robotic gripper. TSAs have been widely used in numerous robotic applications, but their inclusion in soft robots has been limited. The proposed design of the gripper was inspired by the human hand, with four fingers and a thumb. Tunable stiffness was implemented in the fingers by using antagonistic TSAs. The fingers' bending angles, actuation speed, blocked force output, and stiffness tuning are experimentally characterized. The gripper achieves a score of 6 on the Kapandji test and recreate 31 of the 33 grasps of the Feix GRASP taxonomy. It exhibits a maximum grasping force of 72 N, which is almost 13 times its own weight. A comparison study reveals that the proposed gripper exhibits equivalent or superior performance compared to other similar soft grippers.

Supercoiled polymer (SCP) artificial muscles exhibit many desirable properties such as large contractions and high power density. However, their full potential as robotic muscles is challenged by insufficient strain or force generation – non-mandrel-coiled SCP actuators produce up to 10-20% strain; mandrel-coiled SCP actuators often lift up to 10-30g of weight. It is strongly desired but difficult to obtain SCP actuators that produce large strain and large force. In this paper, the design, characterization, and modeling of helically wrapped SCP (HW-SCP) actuators are presented, which can produce up to 40-60% strain and lift more than 90g of weight. By adjusting their configuration parameters, their strain and force performance can be changed. Experiments are conducted to characterize the force production, strain, and speed of HW-SCP actuators. A Preisach hysteresis model and a polynomial model are adopted to accurately capture the actuator behaviors. This work contributes to high-performance artificial muscles.

Robotic artificial muscles are a subset of artificial muscles that are capable of producing biologically inspired motions useful for robot systems, i.e., large power-to-weight ratios, inherent compliance, and large range of motions. These actuators, ranging from shape memory alloys to dielectric elastomers, are increasingly popular for biomimetic robots as they may operate without using complex linkage designs or other cumbersome mechanisms. Recent achievements in fabrication, modeling, and control methods have significantly contributed to their potential utilization in a wide range of applications. However, no survey paper has gone into depth regarding considerations pertaining to their selection, design, and usage in generating biomimetic motions. In this paper, we discuss important characteristics and considerations in the selection, design, and implementation of various prominent and unique robotic artificial muscles for biomimetic robots, and provide perspectives on next-generation muscle-powered robots.