Demonstration of biological piezoelectricity in a spherical protein

Direct piezoelectric materials convert mechanical energy to electrical energy. There is a lack of characterization of biological piezoelectricity and, until now, piezoelectricity had not been proven in globular proteins. A cross-disciplinary team of scientists from Ireland and Portugal examined lysozyme as a model protein to show that physical principles of solid state physics can be applied to biological materials. Their findings are reported in Applied Physics Letters.

The symmetries of the monoclinic and tetragonal crystals formed by the anti-bacterial protein lysozyme allowed the researchers to predict piezoelectric properties. Tofail Syed explained that preparing fragile lysozyme crystals in a format amenable for conventional measurements was challenging. Glycerol was added to increase viscosity for drop-casting onto specially designed electrodes. The electrodes were glass backed with a film coated in indium thin oxide for conductivity. The counter electrode was identical and Tofail Syed emphasized that preventing the electrodes from touching was the trickiest part of the technique.

The researchers showed reversal of polarity on inversion in a single lysozyme crystal using a commercial and custom-made technique. The piezoelectric voltage increased linearly with mechanical stress, a behavior described by classical piezoelectricity. Tofail Syed pointed out the longitudinal piezoelectricity generated in tetragonal aggregate films of lysozyme, a surprising occurrence given the crystal symmetry. However, this could be explained by an averaging or symmetry lowering effect.

Physiological relevance of piezoelectricity in biological molecules is unclear, but some speculation is provided in the paper. Lead author Aimee Stapleton and Tofail Syed think that studying single biomolecule crystals similar to those found in nature will provide a better understanding of their electro-mechanical properties and develop applications, for example, controlling the release of drugs in vivo.

Source: “The direct piezoelectric effect in the globular protein lysozyme,” by A. Stapleton, M. R. Noor, J. Sweeney, V. Casey, A. L. Kholkin, C. Silien, A. A. Gandhi, T. Soulimane, and S. A. M. Tofail, Applied Physics Letters (2017). The article can be accessed at https://doi.org/10.1063/1.4997446 .