Sustaining a long-term but viable power source for underwater electronics has been an engineering conundrum due to a lack of feasible physical mechanisms to harvest energy in the underwater environment. In this regard, thermoelectricity, which converts heat into electricity, can suggest a potent solution, since the ocean and other water bodies maintain the constant water temperature and therefore serve as a permanent thermal differential. In addition to energy harvesting, the same thermoelectric device can be also utilized to both heat and cool, thereby providing an effective means of controlling the temperature of the arbitrary subject in the underwater environment. In this light, we present a soft and stretchable multi-modal thermoelectric skin (TES) that can both (i) generate electricity across the temperature differential between the ocean water and the human body and (ii) thermoregulate the body temperature in the underwater environment. The soft and elastic nature of TES enables an intimate and thorough contact with the deformable and irregular surfaces of the human skin, therefore maximizing the heat conduction at the human-device interface that the rigid or flexible thermoelectric devices can not fully attain. To the authors’ best knowledge, TES produces the highest electrical power density when compared to the stretchable thermoelectric devices reported so far, mainly owing to its optimum design factors. Along with the outstanding device performance, the underwater environment further boosts the thermoelectric efficiency of TES both in energy harvesting and thermoregulatory perspectives due to the much more favorable thermal properties of water than those of air. Furthermore, to verify the practical usage of TES and demonstrate its high wearability, we incorporated multiple TES units into the neoprene dry-suit. The TES units can self-power multiple embedded sensors that wirelessly monitor the physiological condition and further provide the spatial information of the tactical diver, such that the TES units and embedded system can function as a wearable underwater rescue platform. Lastly, the temperature feedback loop algorithm embedded in the thermoregulatory system allows the TES units to constantly regulate the temperature of the human body and thus prevent underwater hypo-/hyperthermia.

Conclusion

We developed a skin-like multi-modal thermoelectric device that can harvest energy or regulate the human body temperature for underwater applications. Based on the optimized design of the serpentine circuit array and a highly stretchable backbone of the thermally conductive elastomer, the device can stretch up to 230% and withstand a variety of cyclic mechanical deformations while exhibiting better performance than previously reported stretchable thermoelectric devices. Furthermore, benefitting from the highly advantageous thermal properties of water, TES demonstrated outstanding thermoregulatory performance and generated V_oc of 0.971 V with the maximum power density of 3.42 mV cm⁻² at a temperature difference of 50 K, which is more than 4.48 times greater than the highest stretchable TEG power density record reported previously. Then, to display the characterized strengths of TES and further validate the practical usage in real-life applications, the multiple TES units were incorporated into the deformable neoprene textile of the diving dry-suit as a proof-of-concept. The TES suit demonstrated significant capabilities to self-power the wireless embedded sensors and maintain the human body temperature in the underwater environment with the PID feedback loop regardless of the surrounding water temperature, thereby creating the novel wearable multi-modal platform that can switch between dual modes depending on the user’s need. However, we do not wish to limit the perimeter of potential TES applications to only human subject applications since the TES platform can be used in a variety of underwater applications as TES can conform to any type of surface, owing to its soft and elastic backbone. For this reason, we expect that the proposed design concept, engineering principles, and device integration into functional electronics will make a significant contribution to underwater electronics in the future.

More Information : Soft multi-modal thermoelectric skin for dual functionality of underwater energy harvesting and thermoregulation - ScienceDirect