Progress of twisted and coiled polymer fiber artificial muscles and its application in soft robots

Artificial muscle is a bio-inspired soft actuator that generates large output force and displacement with flexibility and compliance and can interact with humans and biological environments. Due to low load capacity, high hysteresis, and short life cycle of existing artificial muscles, a new kind of twisted and coiled artificial muscles based on polymer fibers, such as nylon, has been developed and called twisted and coiled polymer fiber (TCPF). This artificial muscle is capable of straining more than 40%, which has a load capacity of up to 100 times higher than that of a human muscle, has energy density of up to 5.3 kW/kg, and it is a hot topic in current research. This study reviews the development of state-of-the-art TCPF, focusing on fabrication and actuation mechanism, new materials and structures, improvements on actuation strategy, and applications in soft robotics. Current states are summarized and discussed for future development directions.