In situ FTIR/ATR study of brushite electrodeposition and its transformation to hydroxyapatite.

摘要

Hydroxyapatite (HA, Ca₅(PO₄)₃OH) has proven to be a good coating material for metallic orthopedic implants, which can improve their biocompatibility. It has been found that a relatively reactive calcium phosphate ceramic (CPC) phase of brushite (CaHPO₄·2H ₂O) can be easily coated on metallic substrates. This brushite coating can subsequently transform to HA when immersed in Hank's balanced salt solution (HBSS). In this study, an attenuated total reflection (ATR) flow cell was successfully modified into a spectroelectrochemical cell, where the internal reflection element (IRE) crystal was sputter-coated with various types of thin metal layers and used as a cathode to study the electrodeposition of brushite in situ. It was found that the metal oxide on the substrate surface played an important role in the bonding between the brushite coating and the substrate. It is proposed that hydrogen bonding provides the adhesion between brushite and substrate. The transformation of brushite to HA in HBSS with different additives was also studied using in situ ATR together with external reflectance FTIR and other techniques such as SEM, EDS, and XRD. The results indicated that brushite dissolves and reprecipitates as HA without going through other intermediate phases. Calcium ion retards the transformation process by slowing down the dissolution of the brushite. Magnesium ion retards the transformation process by blocking the active growth sites of HA. Bone morphogenic protein 4 (BMP4) was found to accelerate the transformation process by adsorbing on the CPC surface, providing extra active growth sites for HA precipitation.