In spite of the success of surgical implants such as artificial hip joints, the materials used to make them are not always quite up to the job. Even stainless steel and titanium alloys can break under the enormous stress on load-bearing joints and corrode in the salty environment of the body. Deposits of inorganic salts can scratch weight-bearing surfaces, making artificial joints stiff and awkward. As a result, the lifetime of an implant is, at most, 10–15 years.
Metallurgists and engineers often treat the surfaces of metal parts to improve their properties. The use of advanced surface-treatment techniques such as glow-discharge ion implantation, plasma deposition, and plasma coating can significantly improve the strength, hardness, and corrosion resistance of metal implants. At the same time, these methods should also improve the biocompati-bility of the implanted devices.
Cobalt-based alloys are widely used for joint replacements. However, other compounds, such as titanium alloys, have excellent potential biocompatibility and interesting but imperfect mechanical properties. Stainless steel, namely 316L, has a good price-to-mechanical-properties ratio, but has the lowest corrosion resistance of the most commonly used metallic biomaterials.