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.

The present work reports the effect of lamination of biocompatible lithium sodium potassium niobate [Li_0.06(Na_0.5K_0.5)_0.94NbO₃, LNKN] multilayered tapes between hydroxyapatite (HA) layers on the dielectric and electrical properties of HA. The LNKN tapes were laminated between HA layers via various buffer interlayers. It has been found that for the optimal molar ratio X = 3–7 of LNKN in the LNKN : HA (X : 1) buffer interlayer, good adhesion between HA and LNKN layers were observed. The effect of lamination of buffer and HA layers and subsequent sintering on the dielectric and electrical properties of inserted ferroelectric LNKN has been evaluated after removing these laminated layers from the composite (HB-LNKN). The crystal structure of HB-LNKN has been changed from the tetragonal to the orthorhombic phase. In addition, the dielectric measurement suggests that the tetragonal region, i.e., the range between T_O–T and T_C for HB-LNKN has also been reduced as compared to that of as-sintered LNKN. The variation in the tetragonal region in HB-LNKN has been found to depend on the composition (X) of the buffer layer in the parent laminated composite. For the buffer layer composition X = 7, the improved piezoelectric (d₃₃ = 104 pC N⁻¹) as well as ferroelectric (E_c = 11 kV cm⁻¹, P_max = 23 μC cm⁻²) response of HB-LNKN was observed. The associated polarization mechanisms in the context of ferroelectric LNKN have also been explored. An approximate 6-times increase in the polarizability of HA (for X = 7) is obtained without affecting its biocompatibility by the proposed concept of the laminated composite. In addition, the developed laminated composite is piezoelectric (d₃₃ = 2 pC N⁻¹) in nature, like living bone. Further, the effect of increased polarizability of hydroxyapatite (for X = 7) on the in vitro bioactivity has been examined after immersion in simulated body fluid (SBF) for 3, 7 and 14 days, respectively. The augmented polarizability of HA almost doubled the apatite formation rate in SBF.