Modeling of Biodegradable Polyesters With Applications to Coronary Stents

Abstract

The interest in biodegradable polymers for clinical and biomedical engineering applications has seen a dramatic increase in the last 10 years. Recent innovations include bioresorbable polymeric stents (BPS), which are temporary vascular scaffolds designed to restore patency and provide short-term support to a blocked blood vessel, before becoming naturally resorbed over time. BPS offer possibilities to overcome the long-term complications often observed with the permanent metallic stents, well established in the treatment of coronary and peripheral artery disease. From the perspective of designing next generation BPS, the bulk degradation behavior of the polymer material adds considerable complications. Computational modeling offers an efficient framework to predict and provide understanding into the behavior of medical devices and implants. Current computational modeling techniques for the degradation of BPS are either phenomenologically or physically based. In this work, a physically based polymer degradation model is implemented into a number of different computational frameworks to investigate the degradation of a number of polymeric structures. A thermal analogy is presented to implement the degradation model into the commercially available finite-element code, abaqus/standard. This approach is then applied to the degradation of BPS, and the effects of material, boundary condition, and design on the degradation rates of the stents are examined. The results indicate that there is a notable difference in the molecular weight trends predicted for the different materials and boundary condition assumptions investigated, with autocatalysis emerging as a dominant mechanism controlling the degradation behavior. Insights into the scaffolding ability of the various BPS examined are then obtained using a suggested general relationship between Young's modulus and molecular weight.

Manufacturing, characterization, and degradation of a poly(lactic acid) warp-knitted spacer fabric scaffold as a candidate for tissue engineering applications

“…Polylactide, or poly(lactic acid), is a biodegradable polyester which has been widely employed in drug delivery systems, sutures, vascular grafts, surgical implants together with porous scaffolds for soft and hard tissue regeneration. 22 The mechanism of bulk degradation of bioabsorbable polyesters is well known, with a number of numerical models currently under investigation for predicting the degradation of 3D structures with different geometries 25,60 as well as their mechanical properties evolution during hydrolysis.…”

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

“…Model 1 uses the outputs of the amorphous degradation model (Appendix A), in which the molecular weight is assumed to change in proportion to the changes in the concentration of ester bond in the polymer, C e (relative to the initial concentration, C e0 ). The assumed relationship between C e and M w is further discussed in [33]. Therefore, the damage for Model 1 is:…”

“…BPS degradation and crystallisation are simulated for 52 weeks (approximately 1 year) using the equations in Sect. 2.1 and implemented through a user material subroutine for a thermal analysis (UMATHT), which captures the reaction-diffusion equations by analogy to the heat equation [33]. The resulting molecular weight, M n , and degree of crystallinity, X c , for three different stent thicknesses (in the radial direction) are examined (78 µm, 156 µm and 234 µm).…”

“…Previously, the implementation of a physio-chemical degradation model for amorphous polymers [19] into the FEA software package Abaqus/Standard by the authors [33] allowed for predictions in the degradation rates of various BPS to be made. In the present study, to build on this, a degradation theory applicable to semi-crystalline polymers [34] is incorporated into the user material thermal subroutine (UMATHT) in Abaqus/Standard.…”

“…The theoretical framework originally developed by Gleadall et al [34] for semi-crystalline polymers is presented here for clarity and the simpler theory for amorphous polymers [19] (which was previously implemented in Abaqus by the authors [33]) is summarised in Appendix A. The constitutive formulation used for the PLLA material is presented followed by the coupling between the degradation and mechanical components of the framework.…”