JOURNAL OF MATERIALS SCIENCE: MATERIALS IN MEDICINE 10 (1999) 465±469 Bioactive nanocrystalline sol-gel hydroxyapatite coatings C. S. CHAI, B. BEN-NISSAN* Department of Chemistry, Materials and Forensic Science, University of Technology, Sydney, P.O. Box 123, Broadway, New South Wales, 2007, Australia Sol-gel technology offers an alternative technique for producing bioactive surfaces for improved bone attachment. Previous work indicated that monophasic hydroxyapatite coatings were dif®cult to produce. In the present work hydroxyapatite was synthesized using the sol-gel technique with alkoxide precursors and the solution was allowed to age up to seven days prior to coating. It was found that, similar to the wet-chemical method of hydroxyapatite powder synthesis, an aging time is required to produce a pure hydroxyapatite phase. A methodology that has been successfully used to produce nanocrystalline hydroxyapatite thin ®lm coatings via the sol-gel route on various substrates including alumina, Vycor glass, partially stabilized zirconia, Ti±6Al±4V alloy and single crystal MgO is described. Coatings produced on MgO substrates were characterized by X-ray diffraction and atomic force microscopy, while the analogous gels were examined with thermogravimetric and differential thermal analyses. The coatings were crack free and the surface was covered with small grains, of approximately 200 nm in size for samples ®red to 1000  C. Coating thickness varied between 70 and 1000 nm depending on the number of applied layers. # 1999 Kluwer Academic Publishers 1. Introduction Calcium phosphate ceramics were ®rst proposed by Albee and Morrison [1] in 1920 for biomedical applications. They observed that tricalcium phosphate, injected into defects, demonstrated more rapid bone growth and union than the untreated defects. Hydroxyapatite (HAp) was ®rst identi®ed as being the mineral component of bone in 1926 by DeJong [2]. However, it was not until about 25 years ago that synthetic hydroxyapatite Ca 10 PO 4  6 OH 2  was accepted as a potential biomaterial for use in orthopae- dics, bone grafts and dentistry. It is one of a limited number of materials that will form strong chemical bonds with bone in vivo, while remaining stable under the harsh conditions encountered in the human body. These properties place hydroxyapatite into the class of biomaterials known as surface active or bioactive materials. The only other materials that fall into this highly specialized classi®cation are the biocompatible glasses and glass ceramics [3]. Most metallic orthopaedic and dental implants are bioinert and do not bond chemically to bone as does hydroxyapatite. Consequently they can become encapsu- lated by ®brous tissue. Thus, the only means of bio®xation is mechanical interlock, whereby the implant must be manufactured in such a way that it possesses surface porosity with interconnections of 100 mm and pore sizes of 250 mm or larger in diameter [4], or be suitably surface macrotextured [5] so that hard tissue can grow into the implant and anchor it in place. Other methods available at present to ®x implants ®rmly in place are the use of screws or bone cements [6]. If the implant does not integrate well with the surrounding bone, or is not held rigidly with a fastening device, the implant will be subjected to micromovement, and surrounding bone will remodel. This may lead to implant loosening over a period of time. Brittleness, poor fatigue resistance and strength precludes monolithic HAp from use in load bearing situations. It is presently restricted to applications that involve non-weight-bearing conditions in service, such as bone ®llers and bone graft substitutes in orthopaedics as well as ossicular bone replacements and materials for maxillofacial reconstruction [7]. Coating a load bearing substrate, such as titanium metal, with HAp overcomes the physical inadequacies of HAp. At the same time it combines the bene®cial properties of both materials. During the last two decades various coating methods were proposed to increase the bioactivity and hence accelerate and improve early bone±implant bonding. Methods that have been used to apply HAp coatings include: dip coating into a powder suspension [8], pulsed- laser deposition, electron beam evaporation combined with ion beam mixing, electrophoretic deposition [9], sputter coating [10] and plasma spraying [11]. Of these processes, only plasma spraying is used commercially. *Author to whom correspondence should be addressed. 0957±4530 # 1999 Kluwer Academic Publishers 465