Osteointegration of titanium and its alloys by anodic spark deposition and other electrochemical techniques: a review.

Abstract

Direct osteointegration of titanium and titanium alloy implants is one of the main goals of biomaterial research for den- tal and orthopedic applications. Chemical, mechanical or biological treatments have been investigated to obtain a fast and durable implant to bone bonding. In recent years, to improve osteointegration scientists have focussed their attention on the interface intere- actions between the implant surface and the bone. In particular, many efforts have been concentrated on the modification of the thin oxide layer spontaneously formed on the surface of titanium. This stable oxide layer has been considered responsible for the titanium biocompatibility and for the ability of this material to osteointegrate. The stability of the oxide layer in air is well known. However, when titanium is placed in vivo the titanium oxide layer changes its properties because of the electrolytic nature of the body fluids, whose components are able to interact and bind to the metal surface ox- ide. In particular, titanium dioxide exhibits hydroxyl group formation which, in their deprotonated form, can bind calcium thus promoting osteointegration. Therefore, the rate of osteointegration can be modulated tailoring the oxide layer properties before im- plantation. The introduction of new techniques capable of changing the physicochemical and morphological properties of the oxide film can be a step forward towards developing a new generation of highly compatible surfaces suitable to induce a strong and durable bonding to the bone. Recently, an electrochemical technique, discovered in the early 1930s and widely investigated in the 1960s, has been applied to titanium and its alloys to enhance implant osteointegration. This technique, initially employed to produce a corro- sion-resistant thick oxide film on valve metals, is known as Anodic Spark Deposition or Anodic Spark Discharge (ASD). It consists of a high voltage anodization of titanium in electrolyte solutions, whose ions become embedded in the thickened dioxide layer as a consequence of the melting process at the surface induced by the high plasma temperature. The high temperature is generated by ran- domly produced sparks originated during the process by the dielectric breakdown of the thickened oxide layer. By this technique it is possible to obtain relatively thick oxide layers enriched with the electrolytes originally dissolved in the medium and bringing a micro- porous and oxygen-rich surface. These properties can be finely modulated controlling each parameter of the reaction. Several research groups studied ASD and explored its potential, sometimes by coupling this technique with further treatments such as hydrothermal, thermal and chemical treatments, with the goal of obtaining a new generation of biocompatible and osteoinductive or osteoconduc- tive surfaces. The first studies on ASD were performed by Kurze's research group, who paved the way for the first really effective sur- face treatment developed by Ishizawa et al. Ishizawa patented a new method to coat titanium with thydroxyapatite, through the en- richment of titanium oxide layer with calcium and phosphorus ions via ASD process. In this method, after ASD process titanium oxide layer is treated to a hydrothermal process, which introduces on the surface a thin layer of hydroxyapatite crystals. Kokubo et al considered the ASD technique to improve the titanium dioxide bioactivity, and demonstrated the capability of this modified oxide lay- er to enhance calcium-phosphate nucleation after soaking in SBF. Recently, the same authors of this paper functionalized the Ca and P enriched titanium dioxide by a final alkali treatment, achieving a nano-structured surface that proved to exhibit high mineraliz- ing potential and selective adsorption of protein (fibronectin). ASD can now be considered an effective method for the modification of titanium surfaces. By this method a new generation of implantable surfaces can be therefore designed, which may improve the per- formances of orthopedic and dental implants. (Journal of Applied Biomaterials & Biomechanics 2003; 1: 91-107)