A Stiffness-Tunable Composite with Wide Versatility and Applicability Based on Low-Melting-Point Alloys

Flexible materials are essential in bionic fields such as soft robots.However,the lack of stiffness limits the mechanical performance of soft robots and makes them difficult to develop in many extreme working conditions,such as lifting and excavation operations.To address this issue,we prepared a stiffness-tunable composite by dispersing low-melting-point alloy into thermosetting epoxy resin.A dramatic and rapid change in stiffness was achieved by changing the state of matter at lower temperatures,and accurate control of the composite modulus was achieved by controlling the temperature.When the alloy content is at 30vol%,the tensile modulus changes 41.6 times,while the compressive modulus changes 58.9 times.By applying the composite to a flexible actuator,the initial stiffness of the actuator was improved by 124 times,reaching 332 mN/mm.In addition,the use of stiffness-tunable materials in the wheel allowed for timely changes in the grounding area to improve friction.These flexible materials with manageable mechanical properties have wide applicability in fields including bionics,robotics,and sensing.Our findings provide a new approach to designing and developing flexible materials with improved stiffness and controllability.

Biomimetic soft robotic wrist with 3-DOF motion and stiffness tunability based on ring-reinforced pneumatic actuators and a particle jamming joint

The human wrist, a complex articulation of skeletal muscles and two-carpal rows, substantially contributes to improvements in maneuverability by agilely performing three-degree-of-freedom(3-DOF) orienting tasks and regulating stiffness according to variations in interaction forces. However, few soft robotic wrists simultaneously demonstrate dexterous 3-DOF motion and variable stiffness;in addition, they do not fully consider a soft-rigid hybrid structure of integrated muscles and two carpal rows.In this study, we developed a soft-rigid hybrid structure to design a biomimetic soft robotic wrist(BSRW) that is capable of rotating in the x and y directions, twisting around the z-axis, and possessing stiffness-tunable capacity. To actuate the BSRW, a lightweight soft-ring-reinforced bellows-type pneumatic actuator(SRBPA) with large axial, linear deformation(η_(lcmax)=70.6%,η_(lemax)=54.3%) and small radial expansion(η_(demax)=3.7%) is designed to mimic the motion of skeletal muscles. To represent the function of two-carpal rows, a compact particle-jamming joint(PJJ) that combines particles with a membrane-covered ballsocket mechanism is developed to achieve various 3-DOF motions and high axial load-carrying capacity(>60 N). By varying the jamming pressure, the stiffness of the PJJ can be adjusted. Finally, a centrally positioned PJJ and six independently actuated SRBPAs, which are in an inclined and antagonistic arrangement, are sandwiched between two rigid plates to form a flexible,stable, and compact BSRW. Such a structure enables the BSRW to have a dexterous 3-DOF motion, high load-carrying ability,and stiffness tunability. Experimental analysis verify 3-DOF motion of BSRW, producing force of 29.6 N and 36 N and torque of2.2 Nm in corresponding rotations. Moreover, the range of rotational angle and stiffness-tuning properties of BSRW are studied by applying jamming pressure to the PJJ. Finally, a system combining a BSRW and a soft enclosing gripper is proposed to demonstrate outstanding manipulation capability in potential applications.