The relationship between protein adsorption on hydroxyapatite (HAp) particles and their surface structure was investigated. Because the various crystal planes of HAp have been reported to exhibit selective adsorption, numerous studies have focused on developing methods to control HAp morphology for selective adsorption. However, few studies have examined the systematic adsorption of proteins on the HAp particles. We firstly synthesized HAp particles under various aging times and mild reaction conditions intending to obtain HAp particles having various surface structures despite similar morphology, chemical composition, and crystallinity. The aging time affected the pore size distribution of the HAp particles. A peak indicating pores with a diameter of approximately 2.5 nm was observed in the pore size distribution plots of the HAp particles prepared using aging times of 48 h or less. The adsorption of proteins on HAp particles with different surface structures was studied. The bovine serum albumin (BSA) adsorption behavior was influenced by the presence of pores on the HAp surface. The amount of BSA adsorbed on the HAp particles aged 72 h having no pores was nearly 1.5 times that of the other HAp particles having pores. These results indicated that the pore size distribution of HAp particles is one of the most important factors in controlling their protein adsorption behavior.

In this study, we report a simple method for synthesizing HAp particles intended for the selective adsorption of basic proteins. HAp particles for the adsorption of basic proteins were prepared from a mixture of calcium acetate and diammonium hydrogen phosphate solutions, both adjusted at pH 10 (HAp-pH10) and a non-adjusted pH (HAp-none). For HAp-none, the amount of bovine serum albumin (BSA, an acidic protein) adsorbed per unit specific surface area was larger than the amount of lysozyme (LSZ, a basic protein) adsorbed. The ratio of the amounts of protein adsorption, i.e., LSZ/BSA, was 0.32 for HAp-none. The amount of LSZ adsorbed on HAp-pH10 was larger than those for BSA. The ratio of LSZ/BSA was 2.38 on HAp-pH10 aged for 1 h. An increase in the aging time (72 h) caused an increase in LSZ/BSA to 3.17.

The surface structure of Hydroxyapatite (HAp) particles during hydrothermal synthesis and their protein adsorption behavior was investigated. The HAp particles were prepared by mixing calcium acetate solution and diammonium hydrogen phosphate solution by hydrothermal synthesis. When the temperature of mixture were heated up to 120°C, 150°C and 180°C, the HAp particles were collected during hydrothermal synthesis. The adsorption properties of proteins onto HAp were studied using three types of proteins: bovine serum albumin (BSA), myoglobin (MGB) and lysozyme (LSZ). Surface analysis by BET revealed that their pore volumes were decreased by increasing synthesis temperature. The adsorbed amount of BSA and LSZ per unit milligrams showed no obvious difference in all of the HAp particles prepared with synthesis temperature. In contrast, the amount of MGB adsorbed onto HAp particles synthesized at 120°C, 150°, and 180°C shows the decreasing with an increasing synthesis temperature. This result suggests that the adsorbed amount of MGB was decreased by decreasing pore volume of HAp particles.