Preparation Of HA-SiO₂ Porous Ceramic Scaffolds By Freeze Casting

Bioceramics have been considered as the most common bone substitute materials for bone repair or replacement in tissue engineering.As the main inorganic component of natural bone,hydroxyapatite?HA?has received increasing attention due to its excellent biocompatibility,osteoconductivity and bone bonding ability to surrounding tissue.The addition of silica?SiO₂?can result in partial decomposition of HA into tricalcium phosphate?TCP?with a better degradation performance at high temperatures,in order to improve its bone conduction effect.The ideal bone substitutes should not only have good biological properties,but also contact as much as possible with the surrounding tissues so that they can be closely integrated.Therefore,the HA-SiO₂ porous scaffolds with different SiO₂ content were prepared by freeze casting in this paper.And the structure of the scaffold can be regulated by the process parameters.Subsequently,the effect of SiO₂ on the microstructure of the scaffold,the growth of the bone-like layer and the pH value of 1.5×SBF were investigated by SEM,XRD,FT-IR and pH measurements.Finally,the growth mechanism of apatite bone-like layer was analyzed by TEM and Ca²⁺concentration.In this paper,the main research results are as follows:?1?The HA-SiO₂ porous ceramic scaffolds prepared by freeze casting shows a gradient change.The dense structure,the cellular structure and the lamellar structur e were presented from the bottom to up.The closer to the initial cold finger,the denser the structure was.The farther from the initial cold finger,the larger the lamellar structure was.It is found that the freezing temperature,the sintering temperature and the HA content were the most influential factors to the interlamellar space,the shrinkage and the porosity,respectively.The effect of freezing temperature,HA content,SiO₂ content and sintering temperature on the interlamellar space,shrinkage and porosity were defined.?2?20vol%HA mixed 0wt%,2wt%,5wt%and 10wt%SiO₂ were frozen under-40 ^oC and sintered at 1200^oC for 3h,and the effect on microstructure and properties of the porous scaffold were analyzed.HA was gradually decomposed into TCP??-TCP and?-TCP?with increasing content of SiO₂,and HA decomposition products could react with SiO₂ to form some Ca₂SiO₄ and bioactive glass.The HA-SiO₂ scaffolds with different SiO₂ content had various surface microstructures.The porosity first increased and then decreased as a function of the SiO₂ content,whereas the SiO₂ content had an opposite effect on the shrinkage and compressive strength.And the HA-5SiO₂ scaffold possessed a unique microstructure resulting in a minimum shrinkage and 81%maximum porosity.Furthermore,The addition of SiO₂ had effect on the growth rate and microstructure of the bone-like layer on the surface of the scaffold.Especially,HA-5SiO₂ scaffold exhibited faster growth rate and denser structure of bone-like layer than the other scaffolds.The results of the characterization confirmed that the bone-like layer material was a carbonate-containing hydroxyapatite.And the pH value of1.5×SBF immersed in the HA-5SiO₂ ceramic scaffold consistently maintained the lowest.?3?The growth mechanism of apatite bone-like layer on the surface of HA-SiO₂scaffold was presented.During the formation of apatite bone-like layer,the Ca/P ratio was initially 1.67,first increased up to 1.82,decreased to 1.48 and then increased again to converge at 1.69.It can be concluded that Ca-rich ACP,Ca-poor ACP and apatite were formed on the surface of scaffold in turn.Ca²⁺concentration measurement results show first increasing and then decreasing behavior.Ca²⁺concentration increasing stage is known as the bone-like layer formation stage,and Ca²⁺concentration reduction stage is also as the bone-like layer growth stage.