JOURNALOF MATERIALS SCIENCE: MATERIALS IN MEDICINE 13 (2002) 281±287 Biological responses of neonatal rat calvarial osteoblasts on plasma-sprayed HA/ZrO 2 composite coating T. M. LEE, R. S. TSAI, E. CHANG Department of Materials Science and Engineering, National Cheng Kung University, Tainan, Taiwan C. Y. YANG*, M. R. YANG Department of Orthopedics, National Cheng Kung University Medical Center, Tainan, Taiwan Plasma-sprayed hydroxyapatite (HA) coating, applied to metal substrates, can induce a direct chemical bond with bone and hence achieve a biological ®xation of the implant. However, the poor bonding strength between the HA coating and the substrate has been a concern for the orthopedists. In a previous study, the zirconia-reinforced hydroxyapatite composite coatings (HA=ZrO 2 ) could signi®cantly improve the mechanical strength before and after soaking in simulated body ¯uid. This study aims to investigate the biological responses of osteoblasts on plasma-sprayed HA=ZrO 2 coating. The osteoblasts derived from neonatal rat calvarial were cultured in Dulbecco's modi®ed Eagle medium (DMEM) with fetal bovine serum (FBS) on the surface of plasma-sprayed HA coating, HA=ZrO 2 coating, and ZrO 2 coating, respectively. The biological responses were investigated by the cell growth (1, 3, 5, and 10 days) and the cell morphology under scanning electron microscopy (SEM) (3, 6, 12, 24 and 48 h). Examination by SEM revealed that osteoblasts on HA coatings exhibit less spreading during the medium phase (6 and 12 h), while, better morphologies were observed at the latter phases (24 and 48 h). This should be derived by the dissolution of HA coating in the culture medium. On HA=ZrO 2 coating, the cells showed the poor morphologies at the latter phases (24 and 48 h). This could be explained by the no apatite formed at the surface HA=ZrO 2 coating after soaking in simulated body ¯uid. The lower contents of ZrO 2 coating in HA coating and the addition of other solid solution (ZrO 2 ±MgO, CaO±ZrO 2 , ZrO 2 ±CeO 2 ) in HA coating are the two possible methods to improve the cytocompatibility of HA=ZrO 2 coating. # 2002 Kluwer Academic Publishers Introduction Hydroxyapatite (HA) has the same chemical and crystal- lographic structure as the apatite of living bone, and can bond physicochemically with bone and promote bone growth onto its surface [1±4]. HA is therefore considered as an excellent bone substitute [5, 6]. However, the bulk form of HA was reported to be susceptible to fatigue failure [7, 8]. The problem can be solved by applying HA coatings on metal substrates; this method combines the biocompatibility and required mechanical properties. Recently, the application of plasma-sprayed HA coatings on a bioinert titanium alloy substrate to promote the ®xation of orthopedic prostheses has attracted worldwide attention [9]. However, the long-term strength and fracture tough- ness of HA and HA coatings have been questioned [10, 11]. Zirconia, having the attributes of high strength and stress-induced phase transformation toughening, is a candidate material to strength the ceramics [12±14]. In a previous study, it was proved that the addition of yttria- stabilized zirconia improved the bonding of HA coatings to titanium, and the bonding degradation of zirconia- reinforced HA composite coatings (HA=ZrO 2 ) was also improved after immersion in simulated body ¯uid [15]. This could be attributed to the factor that the composite coating, with the addition of ZrO 2 in HA, signi®cantly reduced the dissolution rate in simulated body ¯uid. During plasma spraying, less OH  ions were lost in ZrO 2 -containing composite coating. These factors, together with the reduced effective surface of the ZrO 2 - containing HA coating, were attributed to the reduced dissolution rate of the composite coating. The interaction occuring at the bone±biomaterial interface largely determines the success or failure of an *Author to whom all correspondence should be addressed. 0957±4530 # 2002 Kluwer Academic Publishers 281