Achieving both high compliance and stiffness is a key issue in stiffness-tunable soft robots. A wide-range variable-stiffness method keeping pure soft characteristic is proposed by bioinspired design of deep-sea glass sponges adopting thermoplastic starch. The stiffness-tunable mechanism is designed through force analysis and optimization of its bionic cellular structure. It is fabricated with load-weight ratio exceeding 470. Then, a wide-range stiffness-tunable omnidirectional-bending soft actuator (WOSA) is realized, and the bending stiffness model is established. Comparative experiments of stiffness and deformation are conducted on WOSA and a pure soft actuator (PSA) with the same size. Results show that the WOSA can get 92.3 times initial bending and 70.8 times torsional stiffness variation range, of which the flexibility is even better than PSA. A gripper assembled by three WOSAs is verified through stiffness adjustment that it can grasp different weight fragile, soft items from the unshelled fresh egg, boiled egg yolk to grapes. It can even lift a dumbbell weighting 3.32 kg. Finally, a manipulator demonstrated its potential in future minimally invasive surgical applications due to its wide stiffness range and large deformation capacity.
Keywords: bionic structural design; glass sponge; soft actuator; thermoplastic starch; variable stiffness.