Shape Memory Alloy-Based Soft Finger with Changeable Bending Length Using Targeted Variable Stiffness

This work described a bioinspired soft robotic finger with variable bending length to conform objects with different sizes by means of selectively varying the structural stiffness of its segments. The basic design is a shape memory alloy-based soft actuator with embedded stiffness-varying structures serving as modifiable endoskeletons. The stiffness-varying structure is composed of shape memory polymer (SMP) embedded with Nichrome (Ni-Cr) wires as heating elements. Joule heating of SMP through Ni-Cr wire inducing its phase change from the glassy state to the rubbery state enables the actuator structure change from the stiff state (E = 125.65 MPa) to the compliant state (E = 3.33 MPa). The Ni-Cr wire was designed with multiple solder tabs to enable the SMP that can be heated segmentally leading to the stiffness reduction of desired segments of finger to obtain different bendable lengths. A finger with three segments was fabricated, and its deformation and actuation force were measured based on different bendable lengths. A gripper was then assembled using two identical fingers where the angle between them can be manually adjusted. The angles for the two fingers with specific bending length were determined to enable them to form a closed configuration when maximum bending is reached. The grasping force of the gripper was then measured, and it was used to grip different objects. Results show that the performance of the gripper in gripping the size and weight of the object was markedly improved compared with the gripper that cannot vary its gripping length.