Corrosion of Metallic Biomaterials in Cell Culture Environments

41 Metallic materials are used in biomedical devices for various parts of the human body (Fig. 1). The implanted material is exposed to body fluids, such as intercellular fluid and blood depending on the surrounding tissue. Body fluid consists of inorganic ions (Na+, Cl-, Ca2+, HxPO4 etc.), amino acids, proteins, and organic acids. The pH of body fluid is buffered at 7.15~7.35 but it decreases down to 5.2 during an inflammation reaction.1 The concentration of oxygen depends on the part of body: in the intercellular fluid and arterial blood, it is 1/80~1/4 and 2/3rds of that under ambient conditions, respectively.2 The kind of cell in the surrounding tissue depends on the part of body and varies as time proceeds after implantation. Just after implantation of the material, macrophages assemble around the implanted material as the initial stage in a foreign body reaction. Because macrophages secrete active oxygen species, which chemically attack the surface oxide film of metallic materials, the influence of the presence of macrophages has been investigated.3,4 Mu, et al. reported the importance of the coexistence of foreign bodies based on the result that the acceleration of dissolution of pure Ti was enhanced by the coexistence of polyethylene wear debris.3 This is because the stimulation by a foreign body activates macrophages to secrete active oxygen species. Lin, et al. reported that the presence of macrophages alone does not always accelerate the corrosion of metallic biomaterials.4 Besides active oxygen species, macrophages secrete fibroblast growth factor, inducing fibroblasts, which produce collagen and polysaccharide for encapsulation of the implanted material. The thickness of capsular tissue depends on the biocompatibility and the size of material. With the cure of inflammation, the macrophage disappears within a few weeks and the cells originating from the surrounding tissue adhere to the surface of material. The cells use cell adhesive proteins to adhere to the surface of the material and produce an extracellular matrix (ECM) with a characteristic composition depending on the kind of cell (Fig. 2). For example, osteoblasts produce ECM, encouraging the formation of new bone consisting mainly of collagen and hydroxyapatite (HAp). In the end, the implanted material is surrounded by tissue depending on the part of body. In the case of artificial joints, bone Corrosion of Metallic Biomaterials in Cell Culture Environments