Enhanced polarization of hydroxyapatite using the design concept of functionally graded materials with sodium potassium niobate

Abstract

The present work aims to enhance the electrical activities of hydroxyapatite (HA) without affecting its bioactivity through the development of functionally graded materials (FGM) using biocompatible sodium potassium niobate (NKN) piezoelectrics as an intermediary layer. The NKN layer was sandwiched between HA layers via buffer interlayers (abbreviated as HA–NKN–HA) and optimally processed using the spark plasma sintering route. The dielectric and electrical properties were studied over a wide range of temperatures (25–500 °C) and frequencies (10⁻¹ to 10⁶ Hz). In vitro cellular response in terms of initial cell adhesion and proliferation on the FGM as well as the corresponding monoliths was assessed using human osteoblast-like SaOS2 cells. A reasonably good combination of dielectric and electrical properties, such as dielectric constant (38), AC conductivity [5.5 × 10⁻⁹ (ohm cm)⁻¹], piezoelectric strain coefficient (4.2 pC N⁻¹), electromechanical coupling coefficient (0.17), mechanical quality factor (81) and remnant polarization (0.06 μC cm⁻²) in reference to natural bone has been achieved with the developed FGM. The mechanism of conduction remains similar in the FGM to that in pure HA. Impedance analyses suggest the occurrence of two polarization processes in HA and NKN monoliths, whereas more than two polarization processes are observed in the FGM. The significant increase in cell proliferation with culture duration of up to 5 days suggests that the developed FGM favor the cell growth and proliferation. In addition, the present study also establishes the superior cytocompatibility of the perovskite NKN phase. The developed FGM can be a potential substitute for electro-active orthopedic prosthetic implant applications.