The Preparation And Properties Of Modified Poly(Lactic Acid) Microsphere Scaffolds Suitable For Drug Loading

Poly(lactic acid)(PLA),a synthetic aliphatic polymer,has been approved to be one of the most promising polymers by the United States Food and Drug Administration(FDA)and widely used in biomedical and pharmaceutical applications because of its advantages of low cytotoxicity,excellent biocompatibility and biodegradation i.e.It has been confirmed that PLA microspheres are suitable for acting as carriers of drugs and cells in drug delivery and tissue repair.The advantage of using drug-loaded microspheres as tissue engineering scaffolds is that they can be administered relatively easily via simple injection without the necessity of surgery,greatly reducing the difficulty of surgery and alleviating surgical trauma.Meanwhile,The long-term sustained release of drugs at specific bone sites can not only promote tissue repair but also avoid provoking adverse side effects of drugs.Therefore,PLA drug-loaded microspheres have a great potential for the repair and regeneration of irregular shape or small bone defects.However,the further application of PLA microspheres as cell and drug carriers in tissue engineering has been limited due to its poor bioactivity resulting from the high hydrophobicity and lacking of cell specific recognition sites,uncontrollable degradation cycle,noninfectious inflammation around the implanted part due to the influence of acidic products generated in the degradation process and low encapsulation efficiency for hydrophilic drugs owing to lack of active functional groups interacting with drugs,so some modifications are needed.The preparation and biomedical properties(such as bioactivity,drug-loading capacity and drug release performance)of several modified PLA microsphere scaffolds are mainly investigated in this thesis.The main research works and the obtained results are as follows:(1)According to the structural characteristics of PLA molecules riching in ester groups and the mechanism of classical ammonolysis reaction between esters and amines,ethylenediamine with multiple amino groups was selected as a modifier.Ethylenediamine can be chemically bonded to PLA by—C(=O)-NH-formed during the aminolysis reaction between one of its amino groups and ester groups of PLA,with the other amino group remaining free,so the hydrophilic amino group was introduced into PLA molecule and the aminated modified polylactic acid(EPLA)with improved bioactivity was obtained.The results of 1H-NMR,IR,XPS and GPC confirmed the occurrence of the aminolysis reaction between PLLA and ethylenediamine.A stable emulsion was formed through dispersing EPLA/1,4-dioxane solution into glycerol,and then the as-synthesized emulsion was immediately quenched to-15℃ for phase separated to obtain EPLA microspheres.The influences of aminolyzing time,the concentration and amount of ethylenediamine aqueous solution on the morphology,structure and surface wettability of microspheres were investigated by SEM and contact angle meter,and the optimum conditions for the formation of nanofibrous structure were determined.The results of the in vitro apatite-formation ability test revealed that EPLA nanofibrous microspheres had an enhanced bioactivity comparing with unmodified PLA microspheres with smooth surface due to the similarities in structure and chemical composition to that of the natural extracellular matrix(ECM).(2)EPLA nanofibrous microspheres with high porosity,big specific surface area,strong adsorption capacity and riching in active amino groups that can interact with drugs were used to load hydrophilic alendronate by facile immersion-adsorption method.The results of SEM,IR,XRD and TG confirmed that the drug had been successfully adsorbed by EPLA microspheres in a considerable amount.The adsorption amount of EPLA microspheres towards alendronate was calculated according to the concentration of alendronate solution before and after adsorption that were detected by UV spectrophotometer.The influences of several important factors such as contact time,temperature,the dosage of microspheres,pH value and the initial concentration of alendronate solution on the drug loading of microspheres were studied systematically and the optimum conditions for the drug loading were determined.The drug-loaded microspheres were prepared under the optimum conditions for the study on in vitro drug release.It was found that EPLA nanofibrous microspheres had an greatly improved drug loading capacity and sustained release performance comparing with unmodified PLA microspheres with smooth surface due to its strong adsorption capacity and hydrogen bond,electrostatic interaction with alendronate drug molecules.(3)Amino groups can act as active sites to adsorb ca2+ in solution,so EPLA/nHA composite microspheres can be obtained through the in-situ biomimeticly mineralized deposition of nano-hydroxyapatite(nHA)crystals on the surface of EPLA microspheres.The growth mechanism of hydroxyapatite crystals on the surface of microspheres was analyzed and the forming conditions were discussed in detail.It was found that the formation,morphology and structure of hydroxyapatite crystals were affected by the concentrations and pH values of Ca(NO3)2 and K2HPO4 solution and the immersion time of microspheres in the solution.Alendronate was used as a model drug to prepare EPLA/nHA/AL microspheres and the effects of drug concentration and loading time on the drug loading amount of EPLA/nHA microspheres were studied.The result of in vitro drug release experiment revealed that EPLA/nHA composite microspheres had a strong loading capacity and excellent sustained release ability for alendronate because of the strong bonding interaction between HA and alendronate drug molecules.(4)By using NH2 groups as active sites for the further reaction and glutaraldehyde as a crosslinker,gelatin,a biological macromolecule,was immobilized on the surface of EPLA microsphere to prepare Gelatin-g-EPLA composite microsphere.SEM,IR and XPS were used to confirm the successful grafting of gelatin molecules onto the surface ofEPLA microsphere.According to the strong adsorption capacity of EPLA nanofibrous microspheres and the existing of interaction between amino group on the surface of the microspheres and carboxyl group in ibuprofen(IBU)molecules,EPLA/IBU drug-loaded microspheres were prepared by facile immersion-adsorption method,and then Gelatin-g-EPLA/IBU sustained-release microspheres were obtained by coating a layer of gelatin on the surface of EPLA/IBU microspheres.It was found out that Gelatin-g-EPLA composite microspheres had a better bioactivity and more excellent drug sustained release performance than EPLA microspheres.