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Bone and tooth enamel surface are made up of different crystallographically hydroxyapatite (HA), whereby bone comprises a(b)-planes of HA whilst tooth enamel comprises c-planes of HA. The reason behind the difference between HA orientations in both tissues are still yet to be discovered. We report the fabrication of anisotropic coatings of a-plane oriented HA according to the following process: (i) synthesis of a-plane oriented HA powders via a hydrothermal reaction and (ii) fabrication of a-plane oriented HA coatings via a Drop-on-Demand (DoD) micro-dispensing technique. Two types of HAs with high a-plane orientation degree of 72.2 and 75.6% (HA10 and HA84, respectively) was synthesised for the DoD coatings. The HA84 coating demonstrated more uniform and flatter surface than HA10 coating. The heat treatment of HA10 and HA84 coatings prepared with mesh-sieved particles led to a low adhesion strength for the substrates. However, the orientation degrees of those coatings (96.0 and 94.6%, respectively) were higher than the coatings without mesh-sieving. [ABSTRACT FROM AUTHOR]

Abstract: In vertebrate bones and tooth enamel surfaces, the respective a,b-planes and c-planes of hydroxyapatite (HAp) crystals are preferentially exposed. However, the reason why the HAp crystals show different orientations depending on the type of hard tissues is not yet understood. To clarify this question, appropriate ceramic models with highly preferred orientation are necessary. In the present study, dense HAp ceramic models which have the same orientation as living bones were fabricated using composite powders of c-axis-oriented single-crystal apatite fibers (AF) and wet-synthesized apatite gels (AG). The results of crystalline identification and ultrastructural observation showed that the resulting HAp ceramics maintained the c-axis orientation of the AF particles, and their high a,b-plane orientation degrees could be maintained with small additive amounts of AG; however, when the AG content was over 30 mass%, this value decreased. The influence of orientation degree on the surface characteristics was investigated by evaluating the surface zeta-potential and wettability. These results show that increasing the a,b-plane orientation degree shifted the surface charge from negative to positive, and decreased the surface wettability. Initial cell-attachment assays were performed on these resulting ceramics using MC3T3-E1 cells as models of osteoblasts. The results show that the cell-attachment efficiency decreased with increasing a,b-plane orientation degree. [Copyright &y& Elsevier]