Electrophoretic Deposition of Hydroxyapatite Coatings and Corrosion Aspects of Metallic Implants

Abstract

Metallic materials such as stainless steels, cobalt-chromium alloys, titanium and its alloys, have been used extensively for centuries in restoration of anatomical structures due to their superior mechanical properties. The most important considerations for their selection in the human body are their biocompatibility, corrosion resistance, tissue reactions, surface conditions, and osseointegration (a bone bed formed through direct attachment to bone). Metallic implants are unique in that they are exposed to living cells, tissues and biological fluids, which are not only dynamic but also form a hostile environment for the implant. Clinical experience has shown that metallic implants are susceptible to localized corrosion in the human body, releasing metal ions into the surrounding tissues. Numerous failures of such implants have led to the application of biocompatible and corrosion resistant coatings, as well as to surface modification of the alloys. Ceramic materials that are based on hydroxyapatite (HAP), the principal constituent of dentine, bone and other hard tissues, are considered promising for osteo-implants and as a means of aiding the regeneration of bone. There are indications that chemical bonding may occur between HAP and bone. However, the poor mechanical properties of HAP limit its use in implantation. Bioceramic coatings have been intensively studied in order to obtain implant materials which have both biological affinity and high mechanical strength. This article reviews various aspects of corrosion resistance and biocompatibility of metallic implants. Surface modification of metallic materials by electrophoretic deposition (EPD) of HAP and the corrosion performance of these coatings in simulated body fluid conditions are also discussed.