Hydroxyapatite/Lead-free Ferroelectric Composites Synthesized by Mechanochemically Assisted Solid State Reaction Route for Orthopaedic Applications

Abstract

With the ageing population and rise of accidents and several bone-related diseases, human beings require a substitute material to act as an artificial bone. Recent trends in the healthcare industry over worldwide suggest that joint replacement surgery is likely to gain much attention. From the application point of view, biomaterials have been used to replace many body parts like hip, knee, teeth, ankle, elbow and other joint damages. Hydroxyapatite (HAp) is a bioactive and biocompatible material having a crystal structure similar to that of bone minerals. Due to having poor electrical and mechanical properties of HAp, its use is limited for non-load-bearing applications. But, several studies show that for load-bearing applications, a dense form of HAp can exhibit appropriate mechanical properties. Well-defined particle morphology of HAp powder makes it quite effective in biomedical applications. It has been reported that composites of HAp with suitable leadfree piezoelectric materials can enhance mechanical, electrical, and biological properties. Synthesis of HAp/lead-free piezoelectric nano-composites is highly promising since it can enhance mechanical, electrical properties and can also promote cell attachment. Bone tissue has a very low piezoelectric co-efficient of 0.7pC/N, and it responds to mechanical stress like different movements of the body, resulting in the formation of electrical dipoles. These properties lead to the development of a new field of bone remodeling research. In this work, potential lead-free piezoelectrics are selected to make composites with HAp for better cell-material interactions for orthopaedic applications. From the available literatures, Ca10(PO4)6(OH)2, BaTiO3, 0.5Ba(Zr0.2Ti0.8)O3-0.5(Ba0.7Ca0.3)TiO3, and 85.4(Bi0.5Na0.5)TiO3-2.6(BaTiO3)-12(Bi0.5K0.5)TiO3 are taken as parent materials for composite formation. Following is the list of materials selected for the present study. (a) Ca10(PO4)6(OH)2/(HAp) (b) BaTiO3(BT),0.5Ba(Zr0.2Ti0.8)O3-0.5(Ba0.7Ca0.3)TiO3(BZT-BCT), 85.4(Bi0.5Na0.5)TiO3-2.6(BaTiO3)-12(Bi0.5K0.5)TiO3 (BNT-BT-BKT) (c) xHAp/(1-x) BT (x = 5, 10, 15, 20 wt.%) (d) xHAp/(1-x) BZT-BCT (x = 5, 10, 15, 20 wt.%) (e) xHAp/(1-x) BNT-BT-BKT (x = 5, 10, 15, 20 wt.%) Effect of sintering temperature on densification and resultant mechanical, electrical, and biological properties of mechanochemically processed HAp samples was investigated. HAp samples were sintered at 1200, 1250, and 1300 oC for 4 h. Hap samples sintered at 1250 oC showed better mechanical properties, which was attributed to their having smaller grain size compared to HAp samples sintered at higher temperatures. Nearly same value of dielectric constant (εr) and better cell proliferation were exhibited by HAp samples sintered at 1250 oC and 1300 oC, respectively. At ~210 oC, in all the samples sintered at different temperatures, a dielectric anomaly was obtained, which was attributed to the phase transition temperature of the HAp system. Dielectric properties near phase transition temperature showed a dielectric relaxation type of behaviour attributed to the reorientational motion of OH- ions in the HAp system. Higher cell proliferation and viability were exhibited in HAp1300 samples, whereas comparatively equivalent cell growth and higher mechanical strength were observed in HAp1250 samples. BaTiO3 (BT), 0.5Ba(Zr0.2Ti0.8)O3-0.5(Ba0.7Ca0.3)TiO3 (BZT-BCT), 85.4(Bi0.5Na0.5)TiO3-2.6(BaTiO3)-12(Bi0.5K0.5)TiO3 (BNT-BT-BKT) lead free ferroelectric ceramics were synthesized by high energy ball milling (HEBM) assisted solid-state reaction route. Crystallite size of the HEBM powders was found to be in the nano-range. XRD, SEM, dielectric, polarization vs electric field (P~E) loop and mechanical characterizations and analysis of these sintered ceramics were carried out. Lower calcination and sintering temperatures, along with better densification and having smaller average grain size compared to the same systems synthesized by the solid-state reaction route, were optimised. Enhanced RT values of εr~2875, 5133, and 1701 and lower values of tanδ~0.013, 0.015, and 0.058 were obtained in BT, BZT-BCT, and BNTBT- BKT sintered ceramics, respectively. Well saturated P~E hysteresis loops were obtained for all the ceramics, which confirmed their ferroelectric behaviour. Hardness value ~7.23GPa, 5.53 GPa, and 5.16 GPa was obtained for BT, BZT-BCT, and BNT-BTBKT sintered ceramics, respectively. As bone is a piezoelectric material; therefore, fabricating a composite of HAp with lead-free piezoelectric can lead to the development of a new field in bone remodeling process. From literature survey, BT, BZT-BCT, and BNT-BT-BKT ferroelectrics were selected to synthesize composites with HAp to further increase its mechanical, electrical and biocompatibility properties. All three composite systems showed the formation of intermediate phases during sintering. Inhibition of grain growth in the sintered body of the composites can be attributed to the formation of secondary phases, which provide more surface area to volume ratio with some small pores and favour cell-material interaction. Reduced value of εr of the composite system was related to the isolation of ferroelectric particles by Hap particles, which was supported by different theoretical models. Improved mechanical properties of composite systems were well correlated with the microstructures. Nano sized grains were retained in the microstructure, which is due to the processing of starting powders with HEBM method. Among all of the composites, HAp/BT and HAp/BZT-BCT showed better mechanical properties compared to HAp/BNT-BT-BKT composites. For load-bearing applications, implants with enhanced mechanical properties are required. Whereas biological properties were studied for better cell growth and healthy implantation ascertainment. Therefore, biological properties such as cell proliferation, cell attachment, cytotoxicity, live/dead assay, and hemocompatibility of HAp/BT and HAp/BZT-BCT composite samples were carried out. Among different composite compositions, better mechanical, electrical, and cell culture properties were obtained in 10HAp/90BT and 10HAp/90BZT-BCT composite samples. Hardness of value ~4.61 GPa and ~4.12 GPa, fracture toughness of value ~3.64 MPa.m1/2 and ~3 MPa.m1/2 were obtained in 10HAp/90BT and 10HAp/90BZT-BCT composite samples, respectively. Effect of poling on ALP activity was carried out on poled HAp/BT and HAp/BZT-BCT composite samples.