Tendon-Driven Stiffness-Tunable Soft Actuator via Thermoelectric-based Bidirectional Temperature Control

Soft robots have excellent spatial adaptability and high flexibility, but they are limited by the low stiffness of their constituent materials when faced with high-load tasks. In recent years, there have been many works on the development of stiffness-tunable soft actuators by introducing variable stiffness materials into soft actuators, but the existing solutions usually suffer from the problems of slow response, complex structure, and the need of many auxiliary devices to support the completion of the stiffness tuning cycle. This paper proposes a tendon-driven stiffness-tunable soft actuator that addresses these issues. Benefiting from the bidirectional temperature control of thermoelectric modules and the excellent in-plane thermal conductivity of graphene, the actuator is capable of achieving the heating and cooling process by transferring the heat flow through the graphene structure into and out of the shape-memory polymer (SMP) layer of the tendon-driven actuator. This enables stiffness tuning via a single device, reducing the dependence on complex external cooling systems. The use of tendon-driven actuators further eliminates the complex bellow structure of conventional pneumatic actuators and dramatically reduces the size and manufacturing difficulty of individual actuators. Finally, the high load capacity and shape adaptability of the actuator are demonstrated by a gripper equipped with three actuators, which successfully grips objects of various shapes and weights, ranging from less than 10 g to up to 1.6 kg.