Micromechanical modelling of poly-l-lactide degradation in bioresorbable polymeric stents

Biodegradable semi-crystalline polymers, such as poly (L-lactic acid) (PLLA), have shown initially promising results as a replacement for permanent implants such as vascular stents, but their detailed mechanical response before and during degradation is not yet fully understood or easily predicted. There are many factors that could affect mechanical behaviour of polymers, such as crystallinity (𝑋𝑐 ), molecular orientation, molecular weight (𝑀𝑛), processing condition and temperature. This thesis aims to develop a computational modelling framework for biodegradable polymers. Our approach involves analysis of the microstructure, which evolves based on the degradation behaviour. The degradation results, which are required by the micromechanical model, include the molecular weight of the amorphous region, the crystalline volume fraction and the porosity (𝜙). First, a microscale finite element model of a semi-crystalline polymeric material, using a representative volume element (RVE) approach is created; amorphous, crystalline, and porous regions are considered. Crystalline regions are anisotropic and randomly oriented. Periodic boundary conditions are applied to the RVE and effective modulus (𝐸̅) is determined. Second, numerical results are validated against theoretical bounds and simulations reveal stress concentrations for some microstructures. A parameter space of 𝑀𝑛 and 𝑋𝑐 is created with a microstructure for each combination to map the mechanical behaviour. This database builds a framework to predict the modulus during degradation instead of running a simulation every time. This database is expanded to add third parameters (porosity) and investigate how this contributes to changes in Young’s modulus during degradation. In the third phase of the thesis, an integrated degradation framework is developed using user subroutines to predict the effective modulus at each integration point by the finite element solver. This degradation framework couples two frameworks: (i) a micromechanical model that investigates the effect of changes in crystallinity and molecular weight over degradation on evolving mechanical response, as described earlier; (ii) a physically-based model that previously implemented in Abaqus/Standard (Shine et al., 2021, 2017). The integrated degradation framework is used to predict the degradation response of a deployed PLLA stent into a mock vessel. Moreover, two diffusion boundary conditions are considered. This degradation framework is expanded to investigate the influence of heterogeneity in crystallinity after the deployment process at stent hinges. Finally, using a micromechanical model of a two-phase material, the RVE framework is expanded to explore changes in ductility before degradation and ductility and tensile strength during degradation. In particular, the role of plasticity in the amorphous phase in the post-yield behaviour of the semi crystalline polymer is explored

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NUI Galway

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Attribution-NonCommercial-NoDerivs 3.0 Ireland
CC BY-NC-ND 3.0 IE

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University of Galway Theses (PhD Theses)