Interesting project on Make. Strap this device around your chest, and power a USB device. It uses the expansion and contraction of the chest to drive a generator, converting the mechanical energy into electrical. I could easily see an implantable device like this powering or assisting in powering future implants. Current research is focused on using induction charging, biothermal sequestering, and biomechanical energy from things like walking (in the able bodied).
Even more importantly, and this is something extremely interesting to think about, could we build a power source that mimics automotive hybrids. For instance, a set of stimulating electrodes move a muscle. The small amount of electricity needed to jolt the muscle is recovered, and then some, by a biomechanical generator. Additional charge is provided by the force of gravity pulling the arm back down. In other words, does the energy needed to stimulate the muscles dwarf that generated by their movement? If so, we got us some awesome potential!