Strain-Based Degradation Model With Application to Poly-L-Lactide Acid (PLLA) Artery Stent

The permanent implantation of stents with and without carrying drugs led to the unresolvable long-term complications such as restenosis and thrombosis. There are increasing interests in bioresorbable stents, which have the potential to resume the vessel tone and mitigate complications. The bioresorbable stent served as a temporary scaffold for an expected period of 6 to 12 months. The design of both polymeric material and stent structure is acknowledged to impact the mechanical integrity of bioresorbable stent. Poly-l-lactide (PLLA), a polymer-based material, is the most adopted bioresorbable stent materials due to its sufficient strength and degradation resistance. The mechanical performances of the PLLA stents need to be evaluated before it is considered to serve in the clinical application. Because of difficulty and time-consuming of physical tests, finite element method (FEM) has become an efficient way to solve the problem. However, the mechanical performances of PLLA stents during the degradation process could not be fully captured using the existing numerical models. A strain-based degradation numerical model with consideration of stent-artery interaction was proposed based on the previous published experimental data. In this model, the elongation fracture of the PLLA material was correlated to the degradation degree, a scalar factor controlled by the time and local strain. The degradation evolution process was then fully captured after the stent implantation using the derived material model. With continuously loss of mass in material, the degradation rates were not uniform in different locations along the stent structure. Severe degradation was observed at the higher strain regions of the stent, which locates at the outer surface of the stent, near the crowns of the ring stent strut. At first stage of degradation, the stent strut thinning was observed in the model which was also found in the previous experimental study. At second stage of degradation, the degradation happened at the connection region between the link strut and ring struts which resulted in the break of mechanical integrity. The diameter of the vessel has minor change during the first stage of the degradation process, while at the second stage, with the breakdown of the ring structure, the vessel recoiled to its original diameter in one month time. The two-staged degradation process showed a vision for the ideal stent design. The developed computational model provided more insights into the degradation process, which could complement the discrete experimental data for improving the design and clinical management of the bioresorbable vascular scaffold.