Variable Stiffness and Selective Compliance of Thermally Activated Architectured Structures for Soft Robots

Variable stiffness technologies are widely recognized as essential for many soft robotics applications. In this paper, variable stiffness structures relying on temperature changes to achieve global or selective variations in stiffness are presented. Our design leverages the glass transition point of Polylactic Acid (PLA), where the material transitions from a rigid to a rubbery state. Our first design exploits a helical structure, and demonstrates how the glass transition phenomenon can be exploited to design modules for soft robotic arms. The structures exhibits substantial relative changes in axial and bending stiffness, with factors of 167 and 13.6, respectively. By integrating an additional helical structure made of elastic Thermoplaytic Polyurethane (TPU) in a skin-core configuration, we illustrate the potential of these structures for applications in soft robotics. Additionnaly, we propose a novel slider structure that enables the decoupling of axial and bending stiffness, ensuring that changes in one degree of freedom do not compromise the stiffness of the other. This design highlights how independent stiffness control in different axes can enhance adaptabvility for various tasks.