Synthesis and control of microstructure, mechanical properties, and bioactivity in biphasic and preferentially oriented calcium phosphate bioceramics

Abstract

Calcium phosphate bioceramics are recognized as one of the most biocompatible materials and are widely used in systems for bone repair and regeneration. In this work, the pulsed laser deposition technique was chosen to produce calcium phosphate materials with different crystallographic texture and chemical composition for investigation of their structural, mechanical, as well as biological properties. Various calcium phosphate coatings were obtained on Ti6Al4V substrates using a KrF excimer laser (248 nm) with an energy density of 4–8 J/cm² and substrate temperature of 625–730°C. A method was developed that enables the deposition of highly textured hydroxyapatite by controlling the laser fluence and angle of incidence of the plume. Nanoindentation results showed enhancement of hardness and Young’s modulus in c-axis-oriented hydroxyapatite coatings, compared to randomly oriented coatings. Human mesenchymal stem cells attached in greater numbers to the hydroxyapatite with c-axis texture, as compared with randomly-oriented coatings. These results indicate that calcium phosphates with surfaces exhibiting controlled crystallographic texture achieved improved mechanical properties and bioactivity and may be a promising system for clinical applications requiring superior biological and biomechanical performance. In addition, highly crystalline, biphasic hydroxyapatite/tetracalcium phosphate coatings were produced by laser ablation of targets of pure crystalline hydroxyapatite. The fraction of tetracalcium phosphate phase in the coatings was controlled either by varying substrate temperature or partial pressure of water vapor in the chamber during deposition. Systematic studies of phase composition in the hydroxyapatite/tetracalcium phosphate biphasic coatings were performed using X-ray diffraction. Tetracalcium phosphate in the coatings seems to be formed not by decomposition of hydroxyapatite but by local nucleation during deposition. In-vitro dissolution studies revealed that the hydroxyapatite phase in the coatings remain essentially unaltered for periods of up to one week after immersion in a physiological solution while all the tetracalcium phosphate phase in the coating dissolves within 12 h. In-vivo studies showed a higher percentage of mineralized bone contiguous to the bone-coating interface for the biphasic coatings, suggesting enhanced osteoconduction in comparison to pure hydroxyapatite. Results suggest that biphasic coatings with well-controlled tetracalcium phosphate content may represent a route for triggering optimized biological response shortly after implant insertion, followed by a period of osteointegration on a still mechanically robust nonresorbable coating.