The latest edition of Nano Letters has yet another paper about some sort of piezoelectric fabric that generates electricity when deformed. In Theory, you could wear pants made from this stuff and power, say, your watch just by walking around. Admittedly this isn’t exactly novel. We heard about it already in 2003 (pdf), 2007 and in February last year (at least. I stopped searching after two minutes). Nevertheless, this paper is rather interesting because it moves away from basic materials design and tackles a question that’s closer to applications: How can devices like this be built into everyday clothes?
Organic piezoelectric materials like polyvinylidene difluoride (PVDF) would work well with common fabric, but they are about an order of magnitude less effective than other alternatives. Inorganic materials like lead zirconate titanate (PZT) on the other hand, are far more effective, but their synthesis requires techniques like chemical vapor deposition. And CVD requires temperatures of several hundred degrees, which would likely spoil your average cotton undergarment.
The obvious solution to this is to grow an inorganic piezoelectric material on a normal substrate and transfer it onto a flexible surface afterwards. This sounds a lot easier than it actually is. A piezoelectric nanostructure is usually a few hundred nanometers thick and grown on a hard substrate, to which it sticks. Plus, crystals are brittle. You risk ending up with your stuff in little pieces and all over the place.
Qi et al. tackled this problem by removing the substrate chemically. PZT is grown on magnesium oxide because the lattice constants are a good match. But here they didn’t grow a continuous film, but in ribbons five microns wide. After checking the piezoelectric properties of the material they immersed the wafer in hot phosphoric acid that dissolves the magnesium oxide. Crucially the acid not only etches the material in the gap, but also undercuts the PZT ribbons from the side without dislocating them.
Now the PTZ can simply be transferred by pressing a flexible substrate on the lattice, in this case polydimethylsiloxane. The scientists report that about 95 Percent of all ribbons stick to the surface. Plus, measurements show that they retain their piezoelectric properties. The next step would be to cover them with another polymer film to fix them in place and connect them to an electrode, Of course the authors caution that the mechanic behavior of the combination are not yet understood, but I think modern material science is so good at tweaking material properties that something useful will come from it. The process seems easily scalable, too.
Qi, Y., Jafferis, N., Lyons, K., Lee, C., Ahmad, H., & McAlpine, M. (2010). Piezoelectric Ribbons Printed onto Rubber for Flexible Energy Conversion Nano Letters, 10 (2), 524-528 DOI: 10.1021/nl903377u