A bionic robotic trunk with tensegrity-enabled elephant-comparable stiffness variability for assisted daily living

Elephant trunks can rapidly vary their stiffness over a broad range, seamlessly switching between soft states for dexterous operation and rigid states for load-bearing tasks. Despite extensive efforts to mimic this stiffness variability using various approaches, such as jamming structures and phase-change materials, existing bionic robots are limited to narrow tunable stiffness ranges and/or slow switching frequencies. In this work, we present a bionic robotic trunk with a cable-driven tensegrity skeleton, leveraging synergistic and antagonistic muscle-mimicking mechanisms to achieve dynamic stiffness regulation. Through coordinated contraction of motor-actuated cables (i.e., antagonistic action), the robotic trunk achieves a stiffness range of 23.94 to 542.47 N/m and a switching frequency of 1.06 Hz, matching the adaptability of elephant trunks. This rapid and large-scale stiffness variation enables dexterous navigation in unstructured environments and powerful manipulation of heavy objects. Incorporated into an electric wheelchair with the human-machine interface, the robotic trunk assists a post-stroke individual with daily activities, such as opening cabinet doors, retrieving milk from refrigerators, and watering flowers. This work advances bio-inspired robotics and highlights the potential of stiffness-tunable robotic trunks in assistive applications.