Impact of Degradation and Material Crystallinity on the Mechanical Performance of a Bioresorbable Polymeric Stent

Bioresorbable polymeric stents (BPS) offer possibilities to help address the long-term complications associated with permanent vascular implants, however in-vivo degradation behaviour is not yet fully understood. Here, finite element analysis (FEA) techniques based on physio-chemical reaction diffusion equations are used to predict and analyse BPS degradation behaviour. Physio-chemical degradation models for polymers, both amorphous and semi-crystalline, are incorporated into the FEA software package Abaqus/Standard allowing for BPS degradation rate predictions to be made, with a focus on poly-L-lactide (PLLA). The outputs of the degradation models are linked to mechanical behaviour via three different damage models which couple the changes in molecular weight and crystallinity with a hyperelastic constitutive model for PLLA mechanical behaviour. A simplified representation of a PLLA BPS in an artery is used as a demonstration case. The effects of applied degradation product diffusion boundary conditions on the molecular weight and crystallinity of PLLA BPS under simulated degradation are examined, and the impact of material heterogeneities and mechanical load boundary condition on the scaffolding performance and elastic properties of the degrading stent are investigated. The results suggest that the BPS performance are strongly dependent on the assumed boundary conditions, both in terms of degradation product diffusion and mechanical loading.