Porous hydroxyapatite bioceramics in bone tissue engineering: current uses and perspectives

The goals of bone tissue engineering are to apply biomaterial scaffolds with adhered cells, such as osteoblasts, bone marrow stromal stem cells, or chondrocytes, to repair, regenerate, and restore the functions of damaged bone tissue or to replace those tissues with porous engineered biomaterials. Over recent decades, a diverse class of biomaterials has been applied in bone tissue engineering field. Porous hydroxyapatite bioceramic is currently receiving significant attention as a bone tissue engineering substitute because of its biological characteristics, including biocompatibility, bioactivity, osteoconduction, and vasculogenesis. This biomaterial has a three-dimensional structure with interconnected spherical pores of uniform size, which encourages bone ingrowth and achieves good integration of the material and the host bone over time. However, the compressive strength and elastic modulus of porous hydroxyapatite scaffolds generally weaken as the porosity increases, in both in vitro and in vivo testing. Zirconia can be used to toughen hydroxyapatite materials for bone repair and replacement because of its unique biomechanical properties, including compressive strength and fracture toughness. Additionally, a zirconia chitosan hybrid containing bone morphogenetic protein-2 and mesenchymal stem cells derived from induced pluripotent stem cells has been used as a coating material adhered to surface of scaffolds to promote bone regeneration and repair. Here, we provide a succinct review of zirconia toughened hydroxyapatite biomaterial scaffolds that incorporate bone morphogenetic protein-2 and mesenchymal stem cells for bone tissue engineering and describe the biomaterials that are currently being investigated based on the recent literature and our own data.