Direct Thermal-To-Pneumatic Body Heat Harvesting to Power Soft Actuators

Wearable assistive devices have emerged as a viable aid in performing activities of daily living for people with physical disabilities, a group representing up to 27% of the U.S. adult population [1]. In particular, soft wearable assistive devices—most of which are pneumatically actuated [2]—can help those with limited mobility and dexterity caused by physical disabilities. A common limitation of these pneumatic assistive devices, however, is that they typically require a tether to an external pressure source, limiting adoption. Researchers have attempted to address this limitation by investigating methods to use the human body itself as a power source through various energy harvesting techniques, including implementation of piezoelectric, triboelectric, and thermoelectric devices [3,4]. However, these electronic energy harvesting approaches require multiple energy conversion steps for operation of pneumatic assistive devices (i.e., a mechanical or thermal input is converted first to electrical energy and then finally to pneumatic energy).<br/><br/>To address these energy conversion limitations, mitigate portability concerns caused by tethers to external power sources, and appeal to a larger percentage of those with physical disabilities, we developed a wearable textile-based system that bypasses the intermediate electrical conversion step by directly converting heat harvested from the body to pneumatic energy using a low-boiling-point fluid (LBPF). Our system consists of two pouches—a “warm” and “cool” pouch, thermally insulated from one another—where heat emitted from the body vaporizes a LBPF in the warm pouch to drive pneumatic actuation. After actuation, the vapor phase of the LBPF is recovered via condensation in the cool pouch, which is exposed to the lower temperature of the ambient compared to that of the body. Following eventual depletion of the LPBF in the warm pouch after a number of actuation cycles, the device can be flipped, placing the now-fluid-filled cool pouch in contact with the user to continue to vaporize the LBPF in a repeatable manner.<br/><br/>We characterized the system performance by modeling the transient pressure response, power output, efficiency, and life cycle of the device and compared modeling results with experiments, as well as with comparable thermal-to-electronic energy harvesting devices used in wearables in prior work. Our model uses fundamental thermodynamic and heat transfer principles to show how the frequency of operation and geometry of the device affect the overall system performance. We also demonstrated the practicality of this system with a user applying the device to pneumatically actuate an assistive actuator for grasping. This work enables the use of body heat as a viable power source for pneumatically actuated assistive devices that can be used to enhance mobility assistance.<br/><br/>[1] CDC, “Disability Impacts All of Us Infographic | CDC,” Centers for Disease Control and Prevention. Accessed: Oct. 15, 2023.<br/>[2] B. Jumet, M. D. Bell, V. Sanchez, and D. J. Preston, “A Data-Driven Review of Soft Robotics,” <i>Adv. Intell. Syst.</i>, vol. 4, no. 4, p. 2100163, 2022.<br/>[3] Z. Zhou <i>et al.</i>, “Smart Insole for Robust Wearable Biomechanical Energy Harvesting in Harsh Environments,” <i>ACS Nano</i>, vol. 14, no. 10, pp. 14126–14133, Oct. 2020.<br/>[4] Y. Liu <i>et al.</i>, “Advanced Wearable Thermocells for Body Heat Harvesting,” <i>Adv. Energy Mater.</i>, vol. 10, no. 48, p. 2002539, 2020.