Converse piezoelectricity and ferroelectricity in crystals of lysozyme protein revealed by piezoresponse force microscopy

This work investigates the converse piezoelectric effect in crystals of the protein lysozyme using Piezoresponse Force Microscopy (PFM) in contact and Hybrid modes. The mechanical properties of lysozyme crystals were mapped at the surface by means of Hybrid mode. In addition, ferroelectric loops were measured by the switching-spectroscopy PFM method (SS-PFM). We explore these findings using crystallographic principles and propose that the presence of defects within the crystal may lower the symmetry of lysozyme to a polar one. Our findings point towards the potential of exploiting lysozyme and other proteins in technical applications, especially those in which biocompatibility is critical.