Ca-P-O thin film preparation, modification and characterisation

Abstract

Bioceramics are biocompatible ceramic materials, that interact with biological systems of the body to treat, strengthen or replace body functions. Most conventional bioceramics are oxide ceramics, glass ceramics and calcium phosphate ceramics. Among these calcium phosphates, synthetic hydroxyapatite has been extensively studied due to its biomimetic properties similar to that of natural bone. One approach to prepare synthetic hydroxyapatite is to first deposit amorphous Ca-P-O thin film and then by means of post deposition annealing initiate the formation of hydroxyapatite crystals to the film. In addition to the correct film composition and crystalline structure, other surface properties such as wettability and surface nano- and microtopography are very significant for the biocompatibility. In this study Ca-P-O thin films were deposited using two different techniques, ion beam sputtering and atomic layer deposition (ALD). Films deposited using both techniques were amorphous after deposition. For sputter deposited films, the Ca/P atomic ratio determined by means of ion beam analysis approached stoichiometric hydroxyapatite when hydroxyapatite powder doped with extra phosphorous was used as the sputtering target. Though the sputtered films showed good biocompatibility in cell attachment studies, the dissolution of as-deposited films in cell culture medium is a disadvantage. The surface of as-deposited and annealed thin films deposited using ALD were locally modified by high and low energy irradiation. After low energy ion irradiation the as-deposited films showed an increase in hydrophilicity, which was determined using contact angle measurements, and also greater spreading of mouse pre-osteoblast cells. The effect of surface topography was studied by abrading Ti metal substrates to different roughnesses before depositing 2–50 nm thick ALD Ca-P-O films on them. Ti was selected in order to combine the biocompatibility of Ca-P-O thin-films with the mechanical strength of the Ti metal substrate. Ca-P-O films deposited on smoother substrates showed higher Ca/P atomic ratios than those on the rough substrates, demonstrating the influence of roughness on the film growth during the deposition. As-deposited Ca-P-O films acted as equally good cell culture substrate as untreated Ti samples used as a control. After post deposition annealing at 800 C, Ca-P-O films were crystallized on Si substrates and showed filopodic morphology of pre-osteoblast cells. Films on Ti showed also filopodic morphology of cells after 700 C annealing but at 800 C the number of cells substantially dropped, most likely due to formation of a less biocompatible TiO2 rutile phase on the surface.