Properties Of Octacalcium Phosphate Controlled By Substituting With Bioactive Ions And Its Effects On The Behaviors Of Mesenchymal Stem Cells

Octacalcium phosphate?OCP?,which is considered as one of the precursors of biological apatite,indicates the potential role as a bone substitute material for the clinical application,owing to its high osteoconductivity and biodegradability.The specific physicochemical properties of OCP could be closely involved in the stimulatory capacity for osteoblastic differentiation and osteoclast formation,as well as the new bone formation.In this study,OCP was selected as a research model for mimicking biological apatite in bone tissue.After incorporated by the biological relevant bioactive iron ions(Fe³⁺)and strontium ions(Sr²⁺),the chemical constitutions,micromorphology and microstructure of ions substituted OCPs were investigated.The regulation mechanisms of the behaviors of protein adsorption and mesenchymal stem cells?MSCs?stimulated by the physicochemical properties of these OCPs were further illuminated.Moreover,the biological activities of OCP-based biomaterials could be also improved by the Fe-/Sr-substitutions for the further application of bone regeneration.OCP crystals were synthesized using chemical precipitation method controlled by the hydrolysis of urea at the temperature above 70 ?.The temperature and pH value in the precipitation process were both the critical factors for regulating the phase composition,morphology and structure of final products.The appropriate synthesis conditions of pure highly crystalline plate-like OCP crystals were urea concentration of 0.5 M,initial pH value of 5.9,reaction temperature of 90 ? and reaction time of 2 h.Compared with other synthetic calcium phosphates,OCP revealed better cellular affinity and adhesion,as well as better or comparable capacities of cellular proliferation and differentiation.Fe³⁺ and Sr²⁺ were successfully introduced into OCP structure,leading to lattice expansion for the incorporation of Fe?OH?2+/Fe?OH?2+ or Sr²⁺.Both ionic substitutions had slight effects on the micromorphology and microstructure of typical plate-like OCP crystals.Particularly,nanosized particles containing rich Fe were deposited on the surface of Fe-OCP crystals.Surface physicochemical properties of OCP crystals,including specific surface area?SSA?,surface electronegativity and roughness,were alerted by the Fe-/Sr-substitutions,which had subsequently resulted in the better adsorption behaviors of bovine serum albumin?BSA?and lysozyme?LSZ?.Fe-/Sr-substituted OCPs could promote the formation of bone-like apatite,along with the sustained stable release of Fe³⁺ and Sr²⁺ during the biomineralization process in vitro.Compared with the undoped OCP,MSCs adhered on Fe-/Sr-substituted OCPs revealed better affinity,spreading shape and area,as well as higher gene expression of integrin ?5?1.In addition,the proliferation and differentiation of MSCs were enhanced by these substituted OCPs,accompanied with better total proteins adsorptive capacity during cell culture.From these results,two mechanisms could be put forward: 1)Fe-/Sr-substituted OCPs stimulated cellular responses through the primary interaction of material-protein,the potential recognition of material-membrane and the activation of surface topography,which were mainly controlled by the physicochemical characteristics.2)The dynamically released bioactive Fe³⁺ and Sr²⁺ had also promoted the cellular calcification along with their positive stimulation roles in the cellular behaviors.The safe and effective doses of Fe³⁺ and Sr²⁺ had been studied by regulating the substitution contents of Fe/Sr in the OCP structure.The contents of Fe should be controlled below 15 mol.%,otherwise,the overproduced Fe³⁺ could cause side effects for the cellular activities.Sr²⁺ revealed better promotive capacity in a large doping range,even at 25 mol.%.Based on the thermal stability and physiologic instability,these ionic substituted OCPs were treated under various conditions for the products with different OCP-HA conversion degrees.Compared with control samples,these treated samples exhibited more active surfaces,which were beneficial for corresponding cellular responses.In addition,the thermal stabilities of Fe-OCP and Sr-OCP were respectively lower and higher than that of undoped OCP,which had influenced the distinct release of both bioactive ions.High-dose released Fe³⁺ would produce toxic side effects,while stable Sr²⁺ release had obviously facilitated cellular behaviors.Hence,the roles of surface properties and bioactive ions on the biocompatibility of treated ionic substituted OCPs should be comprehensively balanced for the designs of highly bioactive OCP-based bone substitute biomaterials at clinic.Moreover,the mechanisms of OCP-HA conversion and its roles in the biological process induced by OCP-based implants could be better understood by mimicking constitutes of biological apatite with the involvements of mineral ions.