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Decreasing size and power requirements of wearable microelectronics make it possible to replace batteries with systems that capture energy from the user's environment. Unobtrusive devices developed at the MIT Media Lab scavenge electricity from the forces exerted on a shoe during walking: a flexible piezoelectric foil stave to harness sole-bending energy and a reinforced PZT dimorph to capture heel-strike energy.
Shenck et al. (Mon,) studied this question.