Metal ion release from metallic materials, e.g. stainless steel, cobalt–chromium alloy, titanium, and titanium alloys, implanted into human body was reviewed in this paper. Surface oxide films on metallic materials play an important role as an inhibitor of ion release and they change with the release in vivo. Low concentration of dissolved oxygen, inorganic ions, proteins, and cells may accelerate the metal ion release. The regeneration time of the surface oxide film after disruption also governs the amount of released ion. In addition, preferential release of specific elements during wear and fretting of metallic materials occurs. The behavior of metal ion release into biofluid is governed by the electrochemical rule. Released metal ions do not always combine with biomolecules to appear toxicity because active ion immediately combine with a water molecule or an anion near the ion to form an oxide, hydroxide, or inorganic salt. Thus, there is only a small chance that the ion will combine with biomolecules to cause cytotoxicity, allergy, and other biological influences.
