The Preparation Of A Novel Biodegradable Polymer External Reinforcement And The Impact On Intimal Hyperplasia Of Vein Graft

Although the wide use of autologous saphenous vein graft in bypass procedures, failure of arterial vein grafts(AVGs) remains a major problem.Many attempts,such as drugs and external mechanical support at the anastomoses,have been made to inhibit or counteract the AVG failure modes of early thrombosis and intimal hyperplasia(IH).Intimal hyperplasia is one of the major causes of vein graft occlusion,and the increase of circumferential wall stress(CWS) has been shown to promote IH formation.The CWS can be increased dramatically in a distended vein transposed to the arterial circulation compared to that in a vein under normal circumstances.Indeed,it has been shown that increased levels of shear stress tend to modulate IH,but the increased circumferential stretch plays a more significant role in promoting intimal thickening than the increased shear stress does in preventing it.In particular,the law of Laplace states that the CWS in a thin-walled blood vessel is directly proportional to its diameter and acting intraluminal pressure.Thus,preventing acute distension and hence an abrupt increase in CWS of AVGs by adding an external reinforcement or sheath has seemingly improved various pathologic responses. According to the recent studies,an desirable external reinforcement should match the following characteristics:(ⅰ) The biodegradation rate of the external reinforcement should be tunable;(ⅱ) The support intensity should be enough for the AVGs;(ⅲ) The external reinforcement must not affect tissue viability or functionality.In this study,a new type of biodegradable polymer has been synthesized to produce taxol containing AVGs external reinforcements by electrospinning textile technologies to meet the above requirements.After that,the impact of AVGs external reinforcement on intimal hyperplasia was evaluated.PartⅠPreparation and Performance Testing of Degradable Copolymer of Ethylene Carbonate andε-CaprolactoneObjective:To synthesize a new type of biodegradable copolymer poly(EC-CL) at room temperature.Methods:Ring-opening copolymerization of ethylene carbonate (EC) withε-caprolactone(CL) was carried out using neodymium tris(2,6-di-tert-butyl-4-methylphenolate) as a single component catalyst.1H NMR, XRD and DMA was performed to detect the chemical structure,crystallinity and mechanical properties of the copolymer.Results:The poly(EC-CL) was found as a random copolymer.The crystallinities of the copolymers decreased with the increasing EC molar percentage in the products.Compared with homopoly(ε-caprolactone),the copolymers with EC units exhibited increased glass transition temperatures(-35.6℃),reduced melting temperatures(44.5℃) and greatly enhanced elongation percentage at break(2383%) based on dynamic mechanic analysis.Conclusion:Poly(EC-CL) is a new type degradable polymer with tunable mechanical properties and biomedical application prospect.PartⅡBioeompatibility of the Degradable Poly(EC-CL)Objective:To evaluate the biocompatibility of theε-Caprolactone and Ethylene Carbonate Copolymer.Methods:Biocompatibilities of Poly(EC-CL)s was evaluated in vivo and in vitro by the means of cell adhesion experiments,cell proliferation experiments,hemolysis test,LDH release assay,MTS cytotoxicity test and muscle implantation test,subcutaneous implantation test,systemic toxicity testing and beagle aortic implantation experiment were performed.Results:Biocompatibility tests of the Poly(EC-CL) Copolymer provided evidences of good cell adhesion,growth viability, morphology and mitochondrial activity of L929 cells.Hemolysis test,muscle transplantation,systemic toxicity testing and beagle aortic implantation experiment approved Poly(EC-CL)s as biocompatible copolymer.Conclusion:Poly(EC-CL) is suitable biomaterial with acceptable cell compatibility,tissue compatibility and blood compatibility.PartⅢPreparation and In Vitro Test of Taxol Containing Biodegradable Poly(EC-CL) External ReinforcementObjective:To prepare taxol containing biodegradable poly(EC-CL) anastomotic external reinforcement for counteracting vein graft failure.Methods:Different taxol concentration biodegradable external reinforcement had been prepared by electrospinning textile techniques.The drug release patterns of Poly(EC-CL) electrospinning membrane were evaluated by HPLC,and cytotoxicity of the Poly (EC-CL) film were investigated in vitro.Results:Poly(EC-CL) electrospinning films with 5%and 10%taxol content could be able to continue to release taxol more than three month.The Poly(EC-CL) film can inhibit the proliferation of the vascular endothelialium,vascular smooth muscle cells and fibroblast with a positive correlation with the concentration of taxol.The apoptosis rate of Jurkat cells could be significantly induced by10%of taxol Poly(EC-CL),P＜0.05;While the apoptosis rate of Jurkat cells induced by 1%and 5%taxol Poly(EC-CL) seems no significant difference with the negative control group,P＞0.05.Conclusion:5%taxol electrospinning copolymer film with the impact of inhibiting cell proliferation and non-inductive effect of apoptosis,are suitable for produce vein graft external reinforcement.PartⅣThe Impact of Taxol Containing Poly(EC-CL) External Reinforcement On Intimal Hyperplasia of Vein GraftObjective:To evaluate the impact of taxol containing poly(EC-CL) external reinforcement on intimal hyperplasia of vein graft.Methods:Beagle vein graft model had been established by transplanting forelimb superficial veint to the hindlimb superficial femoral artery,and the vein graft anastomosis had been wrapped by 5% taxol containing Poly(EC-CL) external reinforcement.Imaging examination, histology and scanning electron microscopy detection inspection were performed 6 month after operation to evaluate the AVGs patency and venous wail remodeling.Results:6-month postoperative color Doppler ultrasound showed that six vein graft patency of experimental group with no significant expansion of the wall.After 3 months,scanning electron microscopy and factorⅧimmunohistochemical staining prompted vascular endothelial repair in experimental group and control group have been completed.After 6 months,the vein graft intima of control group was much thicker than the experimental group.Histological examination revealed smooth muscle cells,fibroblasts and elastic fibers were involved in vein graft wall remodeling.Conclusion:Anastomotic external reinforcement could inhibit intimal hyperplasia and improve the patency rate of vein graft.