Study On The Key Factors Affecting Radial Mechanical Properties Of Poly(L-Lactic Acid) Bioresorbable Vascular Stents

Poly(L-lactic acid)(PLLA)is currently the major bioresorbable polymer material for vascular stents with its superior biocompatibility and mechanical properties.However,comparing to conventional metal stents,PLLA stents have to increase the strut thickness to maintain the radial supporting capacity,which may cause inflammation and in-stent restenosis.Therefore,this dissertation discusses the key factors affecting the radial mechanical properties of PLLA bioresorbable stents,and aims to improve the radial supporting capacity of PLLA stents without increasing the strut thickness.Main research results of this dissertation are as below:1.Introduction of the development direction and main problems of biodegradable PLLA stents.By comparing the structure and performance of commercial stents and combining with the properties of PLLA materials,it is pointed out that the current research focus is to reduce the strut thickness while maintaining the radial supporting performance of bioresorbable vascular stents.2.Failure analysis of the radial supporting performance of PLLA stents.From the view of macro mechanics,the failure of PLLA scaffold radial supporting capacity could mainly result from plastic strain and surface cracks on the material.And from the view of microstructure,the effects of molecular weight,molecular orientation and structural defects of PLLA on the radial support properties of scaffolds are discussed.The analysis enlightens the experimental research as a theoretical basis.3.Investigation of two strategies to decrease strut thickness of PLLA tubes,and mechanical tests of the thin-strut stents.Two groups of thin-wall PLLA tubes are constructed respectively by longitudinally stretching(Group 1)and directly rubbing(Group 2)the original 150-?m-thick tubes.Three stages of mechanical tests are conducted in this study,which are defined as tensile tests of dogbone specimens,radial loading tests of tubes and radial loading tests of stents.The results demonstrate that Group 2 has higher radial supporting capacity than Group 1 with the same wall thickness.4.Discussion of a new method to manufacture elastomer coated PLLA stents and the effect of elastomer coating on mechanical properties of PLLA stent.On the basis of the thin-wall PLLA stents,the whole surface of PLLA stent is coated with elastomer polymer using ultrasonic atomization and spraying.The coating layer acts as a buffer in case of large deformation of the stents,and reduces structural defects or surface cracks caused by stress concentration,thus improving the radial supporting performance of the stent.In summary,the dissertation achieves to improve the comprehensive mechanical properties of PLLA stents by optimizing the fabrication of thin-wall tubes and coating an auxiliary elastic layer on the surface of stents.The work provides an inspiration for manufacturing thin-strut stents with sufficient radial supporting capacity.