Materials and approaches for on-body energy harvesting
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troduction There has been a recent proliferation of wearable electronics, including health and wellness monitors,1 and sensing systems embedded into clothing.2 The vast majority of these devices are battery powered. In some cases, this is not a concern, as regular recharging or replacement is not a major inconvenience. However, in other cases, for example, in the case of 24/7 wellness monitoring, it is critical that the sensing systems not have breaks in operation due to lack of power. Breaks in operation can lead to a situation in which health-critical parameters are not being monitored, which can present safety issues for the user. Furthermore, in most cases, removing the need to replace batteries improves the user experience. In their seminal study, Starner and Paradiso3 reviewed human processes that might be tapped for powering wearable or implantable electronics. In the intervening years, there have been many demonstrations of on-body energy harvesters. The most prevalent targeted sources of energy are upper body motion,4–6 thermal gradients,7–9 and heel strike (or shoeintegrated harvesters).10–12 This article synthesizes recent work on energy harvesting for wearables, focusing on a discussion of system considerations and enabling advances in materials. Given the practical limitations of reviewing such a broad area, this review will focus primarily on three approaches to energy harvesting— thermal energy harvesting, mechanical-inertial mass-based
harvesters, and clothing integrated harvesters. Both systemlevel approaches and the relevant materials considerations are discussed. For all body-worn harvesting approaches, the problem of energy harvesting can be broken down into three pieces— capturing energy from the environment, transducing that energy to electricity, and conditioning the electrical energy for use. (Note, some systems also contain an energy-storage function such as a rechargeable battery or a supercapacitor.) This process is illustrated in Figure 1. Thermal energy harvesting utilizes the temperature difference between the human body and the ambient. The “capture mechanism” would be a heat spreader that touches the skin and a heatsink in contact with the ambient. The heat spreader and sink direct heat flow through the thermoelectric (TE) elements and should be designed to ensure optimal temperature drop across the TE elements, which are the transducing material. This article discusses the design of capture mechanisms and transducer materials, while devoting minimal attention to conditioning electronics.
Thermal energy harvesting System considerations The transduction material for thermal energy harvesting for on-body applications is often a TE material. TE materials function by converting a temperature difference into an electrical potential. The primary system consideration for thermal
Shad Roundy, The University of Utah, USA; [email protected] Susan Trolier-McKinstry, The Pennsylvania State University, USA; [email protected] doi:10.1557/mrs.2018.33
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• VOLUME 43 • MARCH 2018 • www.mrs.o
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