A nanocomposite for electronic skin

Human skin, the largest organ in our body, is a sensitive detector of both pressure and temperature. Efforts to develop similar sensors for electronics are widespread, and many of the tools are already well known: Piezoelectric materials generate electrical signals in response to changes in applied pressure, and pyroelectrics are sensitive to changes in temperature. Unfortunately, most materials that fall into one of those categories also fall into the other, which makes it difficult to discriminate between pressure and temperature changes. But an international team is reporting a nanocomposite that separates the two sensitivities. The bifunctional material features nanoparticles of the piezoelectric ceramic lead titanate embedded in a ferroelectric polymer that can be pressed into a film 30 µm thick. The polarizations of the two constituents can be configured independently. In particular, an alternating voltage can be used to orient the polymer’s polarization with respect to the ceramic’s. When the polarizations are parallel, the piezoelectric coefficients of the polymer and composite cancel, whereas the pyroelectric response is enhanced; when antiparallel, the material displays only a piezoelectric response. By controlling the phase of the last cycle of the AC voltage applied to different parts of their composite film, the researchers defined regions that were sensitive to either pressure or temperature. The film thus prepared could be mounted to a flexible foil containing silicon or organic transistors; the figure shows a prototype with two sensor regions. Initial results showed linear responses by the pressure- and temperature-sensitive regions with only limited cross-sensitivities. (I. Graz et al. J. Appl. Phys. 106, 034503, 2009 http://dx.doi.org/10.1063/1.3191677 .)