Ring-Opening Cryo-polymerization Of NCA And Oriented Cryogels As Bio-scaffolds

Cryogel has being attracted more and more attention in the last two decades due to its interconnecting macro-porous structure,adjustable mechanical properties and pore alignment.However,the polymerization technique for preparing cryogel is developed to a relatively low level.What is commonly used is free radical polymerization under freezing condition,while the study for cryo-polymerizations with other mechanisms is seldom reported.The ring opening polymerization(ROP)of N-carboxy-a-amino acid anhydride(NCA)initiated by primary amine has been an efficient and facile method to synthesize polypeptides.Since the normal ROP of NCA exhibits bad control over molecular weight and broad molecular weight distribution,it is urgent to improve the livingness of this polymerization.Inspired by that low temperature contributes to reducing side reactions,ROP of NCA was conducted under freezing conditions to study the kinetics of this novel cryo-polymerization for the first time.In the other respects,cryogel has gained researchers9 interests in biomedical application especially in bio-scaffold materials since its macro-porous structure can promote the transport of nutrients and metabolite of cells.As for some specific structure of tissues,the corresponding cells such as smooth muscle cells,myocardial cells,neurocyte and vascular endothelial cells tend to grow along an oriented direction.Hence,a series of three dimensional scaffolds with hierarchical pores were prepared,then seeded with NIH 3T3 to observe their effects on the growth,proliferation and morphology of cells.Two related projects are summarized as below.(1)Ring-opening polymerization(ROP)of N-carboxy-?-amino acid anhydride(NCA)of y-benzyl L-glutamate(BLG)was investigated under frozen condition for the first time.This cryo-ROP was performed in DMSO or 1,4-dioxane below the freezing point of polymerization mixture.The kinetics was monitored with proton nuclear magnetic resonance spectroscopy and gel permeation chromatography,disclosing the well control over molecular weight,living polymerization features and considerable polymerization rate.Compared with the polymerization proceeding at 30 0? which showed weak linearity regarding the reaction kinetics and relatively broad distribution of molecular weight,the cryo-ROP exhibited excellent first-order kinetics as well as narrow molecular distribution.(2)Gelatin and chitosan are ideal biomaterials for scaffolds since they are naturally derived polymers with excellent biocompatibility,mechanical properties as well as degradability.The gelatin-chitosan scaffold with regular pores was prepared by unidirectional freezing technique.Next,different methods of post crosslinking was utilized to enhance mechanical properties of scaffold while it would not cause damage to the structure of pores.NIH 3T3 cells were treated with cell counting kit-8(CCK-8)and fluorescent dyes after being cultured on scaffold for a period of time.Finally,under the observation of fluorescence microscope,we discovered that NIH 3T3 presented oriented growth guided by the physical cue(regular morphology in the scaffold).Meanwhile,other types of synthetic polymer scaffolds with ordered pores were prepared through cryo-polymerization under UV light and successfully guided the oriented growth of cells seeded in them.