For example, researchers at Tohoku University test-produced a functionally gradient cast iron in which there is a gradual transition from graphite particles with a spherical shape on one surface to graphite particles with a flat shape on the opposite surface. One surface of the finished material is strong and tough because of its predominance of spherical graphite particles, and the other has high abrasion resistance and high thermal conductivity because of its predominance of flat particles. Biomaterials Functionally gradient materials also show considerable promise for human implant use, particularly as roots for artificial teeth and replacements for bones and joints. The implants usually consist of apatite and titanium. Apatite has a structure similar to that of natural bone and also has a good affinity with bone and other tissues.
Titanium offers high mechanical strength and is non-toxic.
Researchers at Tokyo Medical and Dental University developed functionally gradient roots for artificial human teeth. Subsequently, in 1993 they announced development of the first functionally gradient material containing titanium and apatite for use as a bone replacement material. Other techniques, such as plasma welding, have been developed to combine titanium and apatite for use as biomaterials. However, they generally have major disadvantages, such as requiring a very high processing temperature and insufficient bonding strength. Earlier this year Kobe Steel Ltd developed an artificial hip joint which actually forms a functionally gradient structure after implantation in the human body.