"Jaw movement is the most promising muscle activity from which energy could be harvested in the region of the head."
Are you sick and tired of wasting potentially tens of calories suboptimally chewing? If so, a pair of researchers from Montreal, Aidin Delnavaz and Jérémie Voix, have a solution: the piezoelectric chin-strap.
All you have to do is chew and talk and grind your jaw as per your usual doses of coffee and 5-Hour Energy, and the device provides tiny bits of juice that might be used to power hearing aids, wearable electronics, or some so-far unrealized implantable device of the cyborg future. And if you're already a cyborg, why not just go for it, eh?
The device, described in this week's edition of Smart Materials and Structures, is perhaps only a little less ridiculous than it sounds. It uses a newish technology called piezoelectric fiber composites (PFC)—a fibrous, stretchable version of your standard buck-fifty piezo element. The same technology, using wind energy instead of jaw energy, has already been used to power Arduino sensor networks.
Piezoelectricity in its basic form is a concept that takes advantage of the proclivity of some types of materials (ceramics, crystals, human bones, DNA) to build up electric charge under mechanical stress. The atoms in these materials are arranged so precisely that if you deform the material, bringing the atoms closer together or farther apart, they are forced to either give up or acquire electrons. It's an old idea finding new applications in smart materials, like our newly-unveiled chin-strap.
The prototype is crude, consisting of a pair of earmuffs connected by a strap of the PFC material running under the user's chin. Chewing causes the strap to stretch, which builds up electric charge that's then carried away by integrated electrodes. In the experiments behind the current study, subjects were asked to chew gum for 60 seconds at a time while the energy output of the device was recorded via several different parameters, such as maximum strap elongation and jaw movement frequency.
In the optimum set-up of the current device, the researchers were able to harvest about 10 microwatts (µW), a tiny fraction of the seven milliwatts possible in a completely ideal chewing situation, where all of the mechanical energy is converted to electricity. At the very most, they were able to squeeze out 18 µW. (For reference, a laptop might consume 50 W or 50,000,000 µW in the same period.)
"Kinetic energy (limb motion), strain energy (muscle force), and thermal energy (body heat) are three main forms of energy that can be harvested from the human body," the authors explain. "The jaw movement that normally occurs when chewing, eating and speaking is the most promising muscle activity from which energy could be harvested in the region of the head. For instance, one can obtain approximately 580 [Joules] only from daily chewing, which is equivalent to an average power of approximately 7 mW."
But that 10 µW isn't enough to power an actual device. In a related summary, the authors note that it would take about 20 layers of the PFC material to make up the 200 µW needed to power an intelligent hearing aid. That would make the strap about six millimetres thick and, worse, would send its total cost through the roof to around $400 (the PFC pieces are $20 a pop). The team estimates that with the original single-layer version, it would take about three years of use to make the device cost-effective.
"Future designs may focus on increasing the number of piezoelectric elements to supply the demanded power of small-scale and wearable electronic devices," the study concludes. "Moreover, an appropriate power management circuit would be required to charge a tiny rechargeable battery integrated into the device. Finally the entire strap assembly should be encased in such a way that would make it totally biocompatible." At least the future product shouldn't shock its user in the rain.