Hydroxyapatite Thin Films with Giant Electrical Polarization

Abstract

It is demonstrated that hydroxyapatite, a type of calcium phosphate commonly found in bone tissue, retains surprisingly large stored charge when synthesized electrochemically from aqueous solution. Thin films of hydroxyapatite formed on titanium and stainless steel electrodes were found to display giant polarization with quasi-permanent stored charge in excess of 70 000 microcoulombs per square centimeter. The polarization of the hydroxyapatite film develops during synthesis as a result of field-induced changes in concentration of ionic reactants in the electrical double layer near the electrode surface. This novel mechanism of polarization during synthesis provides much larger stored charge than what is possible by postsynthesis poling of ferroelectric or electret materials. The polarized hydroxyapatite films on titanium are shown through in vitro experiments to hold promise in stimulating bone growth and may enable new applications in ion exchange separations, drug delivery, or energy storage.

Synthesis and characterization of hydroxyapatite nanoparticles prepared by a high-gravity precipitation method

“…The carbonate was probably caused by absorption of CO 2 from atmosphere during preparation under alkaline conditions [26]. Such carbonate composition was also observed in recently reported bone apatite that has 4-6% carbonate content [27]. For comparison, the HAP STR were also analyzed by FTIR.…”

Electrochemical growth of composite hydroxyapatite coatings for controlled release

“…Release of these molecules therefore must occur in part through diffusion through the composite film rather than only from the more rapid dissolution from the surface of the film. In addition, it is known that the surface of the HA layer formed in the first stage of the process is rich in negatively charge PO 4 3 − and OH − groups [32].…”

“…The local field strength in the Helmholtz layer can easily exceed 10 6 V/cm, and the high energy density of this layer is the basis for electrical storage in double layer capacitors [30,31]. Our group recently reported that the strong local fields result in an extremely large quantity of trapped ionic charge in electrochemically grown HA coatings [32]. In the present study, we modify the electrochemical synthesis of HA to trap non-reactive ionic species into the coating during synthesis.…”

Self‐poled Efficient Flexible “Ferroelectretic” Nanogenerator: A New Class of Piezoelectric Energy Harvester

“…9 These materials with improved d ij values exhibit square‐shaped hysteresis loops, termed as “ferroelectretic” 9. Recently, several approaches have been undertaken where the high electric poling field was avoided through careful synthesis of the in situ charged materials to serve as electrets, such as a silicon (Si) nanowires electret,10 zinc oxide (ZnO)/zinc sulfide (ZnS) core/shell electret,11 hydroxyapatite thin‐film electret12 etc. However, the preparation of such electret material requires multiple steps, and its inflexibility hindered the fabrication of suitable devices using these materials.…”