Titanium's propensity to form an oxide layer on its surface prevents corrosion of surfaces that are in contact with human tissues because the surface oxides minimize diffusion of metal ions from the bulk material to the surface. When titanium gains a coating to make it more bioactive, it can turn the already biocompatible titanium surface into an interface able to enhance osteoblast adhesion and able to promote osseointegration. Today, research is heavily focused on improving the success rate of integration and uses an understanding of the natural process of bone growth and repair to create coatings that will enhance the surface finish and surface properties of the implant. These adjustments allow the artificial structure to mimic biological materials and to gain acceptance into the body with fewer negative side effects.
A 3-year clinical and radiographic study found implants in humans coated by nanocrystalline hydroxylapatite (HA) to support osseointegration. The nanocrystalline HA was developed with a large rough surface of interconnecting pores between 10 and 20 nm of the silica matrix gel, resulting in a porous bone structure. Mean rates of marginal bone loss were insignificant and the periotest values were indicative of a solid osseointegration.
In effect, the pores are structured in such a way that they are able hold onto the proteins on the biomaterial's surface. Ideally, this allows the body to engage in self-repair in that the synthetic HA is recognized as a like-nanomaterial in which live tissues may develop
Titanium foams can be coated with HA through various methods including plasma spraying, sol-gel and electrophoretic deposition. It has been shown that HA-coated titanium exhibits increased interfacial strength in comparison to titanium foams without the coating. In an effort to enhance bone in-growth, Spoerke et al. developed a method for growing organoapatites on titanium implants. Organoapatites may assist in-bone in-growth at the implant interface. The foams were manufactured using a modified HIP process, which exploits the allotropic nature of titanium to create higher porosity foams. Previous in vitro experimentation with the organoapatite-titanium foam held promising results including the possibility that ingrown tissue within these coated pores will improve the lifetime use of the foam through reduction of stress-shielding effects.