Development of bioceramics with life functions by harnessing crystallographic anisotropy and their biological evaluations

Abstract

Hydroxyapatite (HAp) is a mineral component of vertebrate hard tissues, thus it assumes a pivot role as essential biomaterials for bone repair. The HAp crystal belongs to a hexagonal system, and has two types of crystal planes with different atomic arrangements: positively-charged calcium ions are packed mainly in the a(b)-plane, while negatively-charged phosphate ions and hydroxyl groups are packed mainly in the c-plane. In vertebrate long bone surfaces, HAp crystals have a c-axis orientation, which leads to the development of the a(b)-plane. On the other hand, in tooth enamel surface, HAp crystals have an a(b)-axis orientation, which results in the c-plane. However, the rationale behind the difference orientations between long bone and tooth enamel in different crystal planes remains poorly understood. Therefore, we would like to elucidate the effect of crystallographic anisotropy of HAp on its osteogenic functions. In particular, single-crystal HAp particles with preferred orientation to a(b)- and c-axes were successfully synthesized as the models to mimic bone and tooth enamel and by investigating the specific adsorption of acidic and basic proteins onto these particles, key understanding of bone regeneration process could be made toward developing bioceramics with life function. In addition, porous HAp ceramics with osteoinductivity and enhanced osteoconductivity properties was also developed using a(b)-plane-exposed HAp fibers to further explore the relationship. In this review, we will describe the development of bioceramics with life functions by harnessing the crystallographic anisotropy of HAp to provide greater insight on the biological properties of the resulting bioceramics, such as cell behavior in vitro and bone formation in vivo.