Interferometric characterization of high-frequency piezoelectric effects in hydroxyapatite thin films

Hydroxyapatite (HAp) qualities such as biocompatibility and piezoelectricity make it an excellent material for biomedical applications. Here, HAp thin film samples were fabricated onto silica substrates to study their piezoelectric behavior with optical interference. Thin solid films were deposited using previously synthesized HAp powder, sol-gel, and spin-coating techniques. The samples were characterized by scanning electron microscopy, energy dispersive X-ray, Raman, and UV-Vis spectroscopies. The statical piezoelectric effect of the samples was studied by using a finite element method and computational software. Additionally, an experimental Bode analysis allows computing the internal electrical components of the films using high-frequency AC voltage. The electrical behavior is related to the equivalent circuit of a piezoelectric crystal, which indicates the range of frequencies where the electromechanical effect is maximum. A Sagnac interferometer was used to characterize the strain in the HAp as a function of the electrical input frequency. The displacement of the interferometric fringe pattern allows us to estimate the change in the thickness of the sample and compute the piezoelectric constant. The synthesis of HAp thin films with piezoelectric properties provides an effective way to produce sensors and actuators for technological applications.