Study on the Development of Calcium Phosphate based Machinable Bioceramics

Abstract

Calcium phosphates (CaP) are widely used in various fields of biomedical applications as implant materials, mainly due to their chemical similarity to the inorganic component of bone. In many of the applications the dimensional accuracy is critical and implants need to be shaped, sized and machined as per requirement. But, being a ceramic material with strong atomic bonding, they have high hardness, brittleness and thus a limited ability to be machined and shaped to an accurate dimension required for many critical surgical applications. Hence, these ceramic based materials cannot compete with the other metal or polymers based systems and are less popular in such applications with dimensional accuracy and strictness.

In the present work, such a study has been undertaken to make CaP based ceramics machinable. Rare earth phosphates (REP) are known to impart machinable property to conventional oxide ceramics, which act as weak interface material, preventing crack growth by inducing interfacial debonding and crack deflection during machining. Again REPs are also known in the biomedical field and have been shown to be non-toxic and biocompatible in different biomedical applications, primarily as a biosynthetic phosphor / luminescent marker for bio-imaging. Introduction of REPs in CaPs has been planned in the work to make a sintered composite composition for the introduction of machinability in the CaPs.

In the proposed experimental investigation, CaP based ceramics, namely, hydroxyapatite (HAp) and beta tri-calcium phosphate (β-TCP) and REP, namely, lanthanum phosphate (LP) and yttrium phosphate (YP) are prepared by wet chemical synthesis. Effects of calcination temperature on the phase formation of the individual phosphates are studied. Each of the synthesized phosphates is also characterized for various property evaluations. Synthesis route for preparing the CaPs is also optimized by characterizing the calcined powders and evaluating their sintered properties. Then CaP-REP composites are prepared with varying amount of REP, from 10-50 wt. % and the shapes are prepared by uniaxial pressing and sintering. Sintered composites are characterized for phase analysis, densification studies, strength evaluation and microstructural evaluation. Machinability study was done by evaluating the drillablity of the sintered composites using conventional cemented carbide and solid carbide tools. Conventional radial drilling machine aided with piezoelectric drill dynamometer was used to evaluate the drillability with varying speed and feed rate and measuring the thrust force and torque for drilling. Composites are also studied for biocompatibility and bio-activity characterization by MTT assay method and apatite deposition in simulated body fluid (SBF) solution respectively. Microstructural characterization before and after SBF treatment is done for understanding the bioactivity study of the composites. Introduction of REPs is found to impart machinability in the composites and increasing amount of the REP is found to ease the drilling process with reduced force and torque requirement. However, increasing the amount of REPs has shown a gradual decrease in densification, strength and biological properties for both the HAp and β-TCP. β-TCP-30 wt. % LP composite sintered at 1250°C is found to have relatively good physical and mechanical properties along with improved machinability character, requiring a minimum thrust force to drill, and also has good bioactivity and biocompatibility property; hence the composite may be selected as the optimized composition.