Bioinspired soft actuators with highly ordered skeletal muscle structures

Mammals such as humans develop skeletal muscles composed of muscle fibers and connective tissue,which have mechanical properties that enable power output with three-dimensional motion when activated.Artificial muscle-like actuators developed to date,such as the McKibben artificial muscle,often focus sole contractile elements and have rarely addressed the contribution of flexible connective tissue that forms an integral part of the structure and morphology of biological muscle.Herein,we present a class of pneumatic muscle-like actuators,termed highly mimetic skeletal muscle(HimiSK)actuator,that consist of parallelly arranged contractile units in a flexible matrix inspired by ultrasonic measurements on skeletal muscle.The contractile units act as a muscle fiber to produce active shortening force,and the flexible matrix functions as connective tissue to generate passive deformation.The application of positive pressure to the contractile units can produce a linear contraction and force.In this actuator,we assign different flexible materials as contractile units and a flexible matrix,thus forming five mold actuators.These actuators feature three-dimensional motion on activation and present both intrinsic force-velocity and force-length characteristics that closely resebmle those of a biological muscle.High output and tetanic force produced by harder contractile units improve the maximum output force by up to about 41.3%and the tetanic force by up to about 168%.Moreover,high displacement and velocity can be generated by a softer flexible matrix,with the improvement of maximum displacement up to about 33.3%and velocity up to about 73%.The results demonstrate that contractile units play a crucial role in force generation,while the flexible matrix has a significant impact on force transmission and deformation;the final force,velocity,displacement,and three-dimensional motion results from the interplay of contractile units,fluid and flexible matrix.Our approach introduces a model of the presented HimiSK actuators to better understand the mechanical behaviors,force generation,and transmission in bioinspired soft actuators,and highlights the importance of using flexible connective tissue to form a structure and configuration similar to that of skeletal muscle,which has potential usefulness in the design of effective artificial muscle.