Hydroxyapatite nanocrystals in natural form are a major component of bone- a known piezoelectric material. Synthetic hydroxyapatite is widely used in bone grafts and prosthetic pyroelectric coatings as it binds strongly with natural bone. Nanocrystalline synthetic hydroxyapatite films have recently been found to exhibit strong piezoelectricity and pyroelectricity. While a spontaneous polarization in hydroxyapatite has been predicted since 2005, the reversibility of this polarization (i.e. ferroelectricity) requires experimental evidence. Here we use piezoresponse force microscopy to demonstrate that nanocrystalline hydroxyapatite indeed exhibits ferroelectricity: a reversal of polarization under an electrical field. This finding will strengthen investigations on the role of electrical polarization in biomineralization and bone-density related diseases. As hydroxyapatite is one of the most common biocompatible materials, our findings will also stimulate systematic exploration of lead and rare-metal free ferroelectric devices for potential applications in areas as diverse as in vivo and ex vivo energy harvesting, biosensing and electronics.

Self-assembled peptide nanotubes are unique nanoscale objects that have great potential for a multitude of applications, including biosensors, nanotemplates, tissue engineering, biosurfactants, etc. The discovery of strong piezoactivity and polar properties in aromatic dipeptides [A. Kholkin, N. Amdursky, I. Bdikin, E. Gazit, and G. Rosenman, ACS Nano 4, 610 (2010)] opened up a new perspective for their use as biocompatible nanoactuators, nanomotors, and molecular machines. Another, as yet unexplored functional property is the ability to switch polarization and create artificial polarization patterns useful in various electronic and optical applications. In this work, we demonstrate that diphenylalanine peptide nanotubes are indeed electrically switchable if annealed at a temperature of about 150 °C. The new orthorhombic antipolar structure that appears after annealing allows for the existence of a radial polarization component, which is directly probed by piezoresponse force microscopy (PFM) measurements. Observation of the relatively stable polarization patterns and hysteresis loops via PFM testifies to the local reorientation of molecular dipoles in the radial direction. The experimental results are complemented with rigorous molecular calculations and create a solid background of electric-field induced deformation of aromatic rings and corresponding polarization switching in this emergent material.

Direct current electrical poling of textured hydroxyapatite (HAp) ceramics is reported here to exhibit weak ferroelectricity and pyroelectricity, but a remarkably higher (six orders of magnitude) piezoelectricity. The piezoelectric strain coefficient is an order higher than that of bone and of the same order as that of quartz crystal and uncalcified collagen in tendon. The piezoelectric charge coefficient is an order higher than bone, and three orders higher than that of tendon. Piezoelectric constants of poled HAp ceramics are high enough to have physiological relevance such as in the use of bone grafts for stimulated calcification or in energy harvesting from physiological motions. Piezoelectricity in these ceramics can be tuned by controlling texture, and poling parameters. The density of power that can be harvested from HAp ceramics is currently just one order lower than biocompatible ferroelectric ceramic such as barium titanate.