Investigations On The Coating Materials Of The Drug Eluting Stent

Coronary stenting is largely used in percutaneous transluminal coronary interventions.Bare metal stents present excellent mechanical performance,but may lead to inflammation or thrombosis.A number of drug-eluting stents(DES) have been developed,using non-resorbable polymers as coating materials.Bioresorbable aliphatic polyesters have received growing attention for biomedical applications due to their outstanding biocompatibility and variable degradability.This work aims to develop bioresorbable DES coating materials with suitable mechanical properties, degradability,drug release behavior and appropriate biocompatibility.A series of bioresorbable polymers were prepared by ring-opening polymerization of L-Lactide(LLA),D,L-Lactide(DLLA),e-caprolactone(CL) and 1,3-trimethylene carbonate(TMC),using low toxic zinc lactate as catalyst.The various polymers were characterized by using various analytical techniques such as proton nuclear magnetic resonance(~1H NMR),size-exclusion chromatography,(SEC), differential scanning calorimetry(DSC) and mechanical tests.The mechanical properties of a series of D,L-LA/TMC copolymers were evaluated. In vitro hydrolytic degradation and drug release experiments were also realized.The results show that the copolymers exhibit appropriate tensile strength and elongation at break.The degradation and paclitaxel release rates depend on the copolymer composition.The higher the TMC content,the lower the degradation rate,and the higher the drug release rate.The haemo- and cyto-compatibility of PLLA,PTMC,PCL homopolymers and D,L-LA/TMC,L-LA/TMC,CL/TMC copolymers with 50/50 molar ratios were investigated in order to evaluate the potential of the polymers as coating material in DES.Haemolysis tests show that all the homo- and copolymers present very low haemolytic ratios,indicating good haemolytic properties.Adhesion and activation of platelets were observed on the surface of PLLA,PCL,and D,L-LA/TMC and L-LA/TMC copolymers,while less platelets and lower activation were found on PTMC and CL/TMC copolymer.Both indirect and direct cytocompatibility studies were performed on the polymers.The relative growth ratio data obtained from the liquid extracts during the 6-day cell culture period indicate that all the polymers present very low cytotoxicity.Microscopic observations demonstrate adhesion, spreading and proliferation of human umbilical vein endothelial cells ECV304 cells.It is concluded that these bioresorbable polymers,in particular D,L-LA/TMC, L-LA/TMC and CL/TMC copolymers,are promising candidate materials as coating material in drug eluting stents.