Biomimetic Mineralization Of Hydroxyapatite Crystal In The Presence Of Zwitterionic Polymer

Biomimetic mineralization is an effective method to induce the formation of inorganic mineral by utilizing functionalized polymer templates.Biomimetic mineralization of nano-hydroxyapatite（nHA）in vitro applying to the field of bone repair material has aroused widespread attention due to the simple and controllable process.However,it still seems a big challenge to reveal natural biological mineralization mechanism and develop synthetic hydroxyapatite based materials with bionic structure.In this paper,we firstly synthesized two kinds of organic polymers as templates,one was cationic PCBAA-ester with positive quaternary ammonium group（-R₃N⁺）,the other was zwitterionic PCBAA with both positive quaternary ammonium group（-R₃N⁺）and negative carboxyl group（-COO^-）.Their successful synthesis was characterized by proton nuclear magnetic resonance spectroscopy（¹H-NMR）.Then they were as templates to induce the formation of nHA,referred as C-nHA,Z-nHA respectively.The phase,morphology,size of apatite and interactions between nHA and polrmers in the presence of polymers with different concentrations were systemically characterized by means of XRD,TEM,FTIR and TGA.The results indicated that the phase of apatite prepared under the control of two kinds of polymers was nHA.Nevertheless,zwitterionic PCBAA could better play an important role in regulating the morphology and size of nHA compared with PCBAA-ester.Hence,we chose PCBAA as templates in the following research,and then investigated the nucleary and growth of nHA under different conditions（e.g.pH value,mineralization time,etc.）.The results showed that Z-nHA could be formed at all pH ranges in the presence of PCBAA.In addition,the higher pH value was beneficial for the formation of nHA with rod-like shape and smaller size,so the morphology and size variation of Z-nHA along with mineralization time were detailedly investigated in this condition.The results showed that when the pH value was 11.1,Z-nHA exhibited the transitional morphology from fiber-like shape to short rod-like shape.A possible mechanism for the formation of Z-nHA crystals in the presence of PCBAA was proposed based on the experimental results finally:zwitterionic PCBAA could provide nucleation sites for Ca²⁺and PO₄^3-simultaneously,which made nHAs quickly precipitate on the surface of PCBAA.With the increase of reaction time,PCBAA molecules could be adsorbed on the surface of Z-nHAs through electrostatic interactions,then Ca²⁺and PO₄^3-could be absorbed on the new joined PCBAA chain and Z-nHAs could grow in radial direction.The other pathway for the growth of Z-nHAs to larger nanoparticles and transitions to rod-like shape could be achieved through the assembling of Z-nHAs themselves.