Hydrogel-matrix encapsulated Nitinol actuation with self-cooling mechanism†
Shape-memory Nitinol holds burgeoning promise as smart actuators due to its effective resilience, high energy density, and scalability for a myriad of mesoscale machines and robotic applications. However, the higher actuation temperature and prolonged cooling time for a cyclic response make Nitinol precarious and less appealing for commercial use. On the contrary, hydrogels belong to the three dimensional (3D) polymer family where the bulk of the matrix encapsulates water (≈80–90 wt%) constituting a compelling heat-trapping medium. In this paper, we demonstrate a novel self-cooling mechanism comprising a Hydrogel-matrix Encapsulated Nitinol Actuator (HENA) where the heat emitted due to the high temperature (200–400 °C) of Nitinol is trapped in the hydrogel-matrix, maintaining a surface temperature of 20–22 °C. For quantitative analysis, we performed control tests with the state-of-the-art Silicone Elastomer Nitinol Actuator (SENA) which maintained a three times or higher temperature profile (65–90 °C) than its HENA counterpart. HENA is able to entrap 85% heat for actuation of 200 cycles while SENA dissipates the same amount in the first cycle. For impending biomedical applications, HENA with a single Nitinol wire shows a bending displacement up to 45% of its length for trans-oral navigation purposes. A HENA soft robotic gripper with two Nitinol wires can carry delicate, low-melting-point food items (e.g. cheese, chocolate, tofu etc.) with different morphologies that weigh up to 450% of its own weight.