All piezoelectric materials produce electric fields when subjected to strain, they also all produce strain when subjected to electric fields. This is how ultrasonic transducers produce the ultrasonic vibrations, they are piezoelectric materials, and when excited at ultrasonic frequencies produce acoustic vibrations at the same frequency.

The major bone stiffness regulating mechanism is known to be NO generated via shear in fluid in bone porosity due to strain of bone. This NO activates the cells that cause deposition of bone mineral and inhibit the cells that cause resorption of bone mineral.

Aqueous biological fluids are pretty conductive. Lipids are virtually non-conductive. For conduction you need mobile charge carriers. In metal those charge carriers are electrons, in aqueous systems the carriers are hydrated ions (which are many orders of magnitude less mobile). The effects of electric fields can be mediated through the field (electrostatics) or via charge and charge carrier reactions (electrolytic effects). It is difficult to have electric field effects in conductive materials.

I suspect that the effect of electrical stimulation of bones is not mediated through electrical effects per se, but rather through electric field induced strain which activates NO via shear and so upregulates the normal NO mediated regulation of bone healing.

Another common example of piezoelectric effects that everyone is familiar with is the quartz clock. A piezoelectric quartz crystal is cut such that it has a mechanical resonance at a certain frequency that mechanical resonance interacts with its piezoelectric properties to cause a resonant electrical frequency, such that the quartz crystal generates a constant frequency when used as a component tin an oscillating circuit. That precise frequency is used as the basis of timing circuits.

There are ways of extracting energy from motion that actually increase efficiency. Biological tissues that covert chemical energy into mechanical energy are not effective at doing the reverse, converting mechanical energy into chemical energy. In some cyclical movements, there are instances where kinetic energy has to be dissipated by using mechanical energy generated by muscle. If that kinetic energy could be regenerated into electricity, then the body wouldn’t need to exert muscle power to do it, and muscle exertion would be reduced while generating positive power. It isn’t a first or second law violation, it is simply using a different mechanism for generating force used at the right time to reduce muscle work requirements.

Lcd t-shirts are probably the most high tech clothing I’ve seen to date. I can imagine that piezoelectronic equipment could be very useful in powering the batteries.

Magnus –
My thought was that the first commercial uses would be for entertainment purposes. Perhaps piezoelectrics will energize lights on clothing, or something of that nature. I recall that apple already has an electronic pedometer that inserts into special Nike running shoes and sends data to your ipod. So one application of electronic clothing could be to provide status updates about body conditions: heat generation, sweat, what-have-you. At first this will be novel and expensive, of course, but overtime should become cheap and commonplace.

I tend to think that if something can be done, somebody will do it … if it proves practical or popular is another story.

Or if, as you suggested, install one of these devices in the human body to capture energy from a rotating heart, will it put too much strain on the heart and with ~7% efficiency it seems hardly worth it.

I guess you could apply this argument to any or all modern energy devices, but considering the increased probability of anthropogenic global warming, it’s important we seriously investigate these arguments.