INTRODUCTION The internal architecture of porous scaffolds at macro- and micro-levels plays a significant role in determining cell behaviours and tissure ingrowth[1]. Surface topography is known to regulate cellular behaviour including adhesion, proliferation and differentiation on 2D surfaces[2][3]. Integrating surface topography into pore surfaces of three- dimensional (3D) scaffolds for bone tissue engineering would be useful to direct the cell behaviour and inducing tissue growth. But how to fabricate such 3D scaffolds with controlled surface topography are remaining challenges. Therefore, the present research focuses on the modulation of macro-, micro- and nano-structures on the pore surfaces of porous scaffolds for bone tissue engineering.
MATERIALS & METHODS Porous hydroxyapatite (HA) ceramic scaffolds were prepared by previously reported method[4][5]. Sugar spheres were used as porogens and the gelation of HA/chitin slurry was controlled by moisture to fabricate stripe structure on pore surfaces. Another approach to grow micro- or nano-structures onto pore surfaces is hydrothermal process. The parameters which control the surface texture were investigated. In addition, inorganic ions such as copper strontium were added in the reaction solution to modulate surface textures of porous scaffolds. Finally, protein absorption/release, cell behaviour and bone formation on micro/nano-structured CaP surface of interconnected scaffolds were investigated in vitro and in vivo.
RESULTS AND DISCUSSION The macroporous structures were effectively modulated by different manufacturing approaches such as sugar leaching, spherulite-accumulating and foam template. The stripe structure on pore surfaces of HA scaffolds were harvested with sugar sphere template pre-treated by the humidity condition. The moisture which existed on the sugar sphere surface plays important rule in gelation of HA slurry so that the strip structure formed during slurry filling the sugar template. In hydrothermal conditions, calcium phosphate was deposited on scaffolds assisted by 1,2,3,4,5,6-cyclohexanehexa-carboxylic acid (H6L) molecules and copper ions. Calcium phosphate crystals on the scaffolds changed from plates to wires, then to flowers according to the concentrations of H6L and copper ions. These results indicated that small organic molecules and inorganic ions had an impact on the morphogenesis of CaP assembly and formation. Meanwhile, substitutions of inorganic ions in CaP bioceramics have been a useful tool to improve the biological performance of CaP materials.
In addition, protein adsorption results demonstrated that the CaP surface texture with nano/micro-structures may have a selective adsorption and controlled release on proteins. It was reported that CaP particles of nano- to micro-meter sizes, with high specific surface areas, can enhance protein adsorption and promote cell growth[6]. Cell experiments showed that micro- and nano-structured surfaces of pores were benefit for cell proliferation and differentiation. In vivo animal experiments showed that the different ectopic bone formation was induced by different macro-pore structures after intramuscular implantation, which demonstrated the significant effect of macro-pore structures of scaffolds on osteoinduction and vascularization.
CONCLUSION It is shown that the different surface micro- or nano-structure can be manufactured by various processes. In the sol-gel sugar leaching process, the moisture is important to form the strip structure on pore surface. In hydrothermal process, the small organic molecules are necessary for calcium phosphate growth on the surface. In addition, micro/nano-structured Ca/P surfaces have a selective protein adsorption and play a key role in controlling protein release so as to regulate cell differentiation behavior. In vivo animal results reveal that the pore architecture of scaffolds can contribute not only to the amount of space available for tissue ingrowth but also to the occurrence of vascularization, osteoinduction and osteogenesis.
National Basic Research Program of China (973 Program, No. 2012CB933600); National Natural Science Foundation of China (No. 51172188); Science & Technology Pillar Project of Sichuan (No. 2010FZ0048)
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