DLC/Hydroxyapatite Nanocomposites
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DLC/Hydroxyapatite Nanocomposites Roger J. Narayan School of Materials Science and Engineering Georgia Institute of Technology Atlanta, GA 30332-0245 USA ABSTRACT Studies of orthopaedic implant failures have shown that mechanical failure of an implant almost exclusively occurs at the biomaterial-tissue interface. Hydroxyapatite (HA) mimics the behavior of natural bone, and provides a strong, long-lasting adhesive interface between a bone replacement implant and the surrounding tissue. Currently, thin film HA is not commonly used because it contains defects, porosity, and cracks. Delamination of the hydroxyapatite film and formation of hydroxyapatite particles may lead to implant wear and loosening. Furthermore, the coating also acts only as a temporary barrier to ion release from the bulk biomaterial. One method to improve the tribological properties of a bioactive coating is to strengthen the microstructure of the coating through the placement of a DLC (hydrogen-free diamondlike carbon) interlayer. DLC coatings possess properties close to diamond in terms of hardness, atomic smoothness, and chemical inertness. We have developed a diamondlike carbon/HA bilayer, in which the bilayer surface (HA) is bioactive and the interlayer (diamondlike carbon) is biocompatible, wear resistant, and corrosion resistant. We have successfully deposited nanocrystalline hydroxyapatite and DLC films by ablating a hydroxyapatite target and a graphite target using a KrF laser. A novel target design was adopted to incorporate alloying atoms into the films during pulsed laser deposition. These alloying elements possess unique biological properties. Surface morphology was studied using SEM, interfacial structure was studied using TEM, and HA phase microstructure was studied using XRD. The DLC/nanocrystalline HA bilayer material is potentially useful for several orthopedic implant designs. INTRODUCTION Studies of implant failures have shown that mechanical failure of an implant almost exclusively occurs at the biomaterial-tissue interface. When biomaterials are nearly inert, there is no bonding at the biomaterial-tissue interface. A fibrous capsule develops to surround the implant. This fibrous interface does not allow good adhesion of the implant to the biological site. If movement occurs at the biomaterial-tissue interface, a decline in implant function may occur. The best replacements for bone have characteristics that approximate those of natural bone. The most desirable mechanism to repair damaged bone is to regrow natural bone by means of tissue engineering. For a relatively large weight bearing joint, tissue engineering of bulk biomaterials is not feasible. One approach to providing a strong, long-lasting adhesive interface between a bone replacement implant and the surrounding tissue involves the use of bioactive materials. These bioactive materials mimic the behavior of natural bone. Bioactive materials have properties so similar to natural bone that the osteoclasts (bone-dissolving cells) tear down these materials and repla
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