Toward Adaptive Grasping: A Flexible Joint with Embedded Stiffness Modulation Using Twisted String Actuation

Soft-rigid robotic grippers offer robust and adaptive object manipulation, but achieving variable stiffness and controlled motion in tendon-driven systems remains challenging. This work presents a 2-degree-of-freedom flexible joint for robotic fingers that enables simultaneous flexion and stiffness modulation within a compact design. The joint is based on a triple wave-shaped compliant structure and actuated by a twisted string actuator (TSA) with four tendons, allowing combined bending and compression. The design is validated through finite element simulations and experimental characterization. The joint is then integrated into a robotic finger and evaluated in grasping tasks. Results show that stiffness can be effectively modulated during motion, significantly influencing both grasping force and fingertip trajectory. These results demonstrate the potential of the proposed joint to enhance control and adaptability in soft-rigid robotic hands.