Hydroxyapatite formed on titanium via a self-assembled monolayer and its in-vitro behaviour

Abstract

Hydroxyapatite ( HA ) coatings are widely used on metal implant devices to improve biocompatibility, enhance bonding strength and to shorten bonding aging between the implant and natural bones. Current coating methods share a common drawback : coatings produced by these processes are not crystalline and further heat treatment must be performed at high temperature ( 700 ° C ). Unfortunately, this treatment often reduces the bonding strength between the coating layer and the metal substrates and may cause chemical degradation of the HA. Recently, a biomimetic coating method has been developed. The strategy adopted in this method is to induce formation of the HA layer by coating implant surfaces with biologically inspired functional groups. This attempts to simulate the natural mineralization process occurring in the human body and it is hoped that the low crystallinity problem may be overcome. Unfortunately, industrial - scale production is not attractive due to the slow growth rate of the crystalline layer. Several weeks are required to produce a detectable thickness of HA and the deposited layer ' s in - vivo performance has not been quantified. As a consequence, further studies elucidating the key factors influencing formation of the HA layers and its in - vivo performance are desirable. In this study, self - assembled monolayers ( SAM layers ) with different functional groups were produced on titanium substrates and characterized. The titanium substrates were immersed in simulated body fluid ( SBF ) to synthesize HA coatings. After measuring the chemical compositions, crystallinity, morphology and growth rates of those coatings, the optimal SAM for HA formation was determined. In addition, the influences of key variables such as temperature, pH, ionic concentrations and functional groups on HA formation were investigated. The goal of this work was to accelerate the growth of the HA coatings for industrial scale production. Finally, human bone cells behaviors on HA coated titanium were observed to confirm an improvement in the biocompatibility and bioconductivity. It was found that SAMs significantly enhanced the formation of HA coatings on titanium surface. The optimum functional group for the SAM was - COOH. This functional group produced the fastest rate of formation and a HA coating with morphological attributes and crystallinity most like natural bone. Significant factors affecting HA formation on - COOH SAM were temperature, calcium concentration and Ca / P ratio in SBF. Significantly, it was discovered that higher temperatures and calcium concentration in SBF substantially increased rate of growth of the HA coating whilst appearing not to adversely affect crystallinity. This could reduce time for producing SAM induced HA coatings on titanium implants from a month to a week. Finally, the HA coating formed at 37 ° C on - COOH SAM substantially enhanced the growth of human bone cells on the titanium surface.