ABSTRACT
184Bone is a natural functionally graded material, which exhibits two types of structures. One is a dense and stiff structure and the other is a porous, soft load-bearing structure known as cortical and cancellous bone, respectively. This property has inspired the development of functionally graded bioceramic resources as implant materials with the original biotissue. 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). Surgical grade stainless steel and titanium and its alloys are widely used in orthopedic and dental restorations. The human body is a very hostile environment and metals and alloy implants are unique that they are exposed to this dynamic environment containing living cells, tissues, and biological fluids. Clinical experience has shown that metallic implants are susceptible to localized corrosion in the human body, releasing metal ions into the surrounding tissues. Common failures of metallic implants have led to the application of biocompatible and corrosion resistant coatings, as well as to surface modification of the alloys.
A bioactive material is one that elicits a specific biological response at the interface, which results in the formation of a bond between the tissues and the material. Hydroxyapiatite ceramics offer attractive properties, such as lack of toxicity, absence of intervening fibrous tissue, the possibility of forming a direct contact with bone, and the possibility to stimulate bone growth. Deposition of layers of nanobioceramic coatings on 316L SS, titanium, and magnesium alloys are a viable solution. Surface engineering, nanobioceramics, and functionally graded coatings are the promising techniques to battle corrosion of biomaterials. Nanobioceramics-based orthopedic implants as scaffolds and coatings combined with tissue engineering would lead to the development of new hybrid biomedical devices with the better understanding of the structure–property relationship. Modification of biomaterial surface properties through control of the characteristic length scale is one of the promising approaches to modulate select cell functions.
