Constitutive Modeling of Biodegradable Polymers for Application in Endovascular Stents

Balloon angioplasty followed by stent implantation has been of great benefit in coronary applications, whereas in peripheral applications, success rates remain low. Analysis of healing patterns in successful deployments shows that six months after implantation the artery has reorganized itself and there is no purpose for the stent to remain, potentially provoking inflammation and foreign body reaction. Thus, a fully absorbable stent that fulfills the mission and steps away is of great benefit. Biodegradable polymers have a widespread usage in the biomedical field, such as sutures, scaffolds and implants. Aliphatic polyesters (the main class of biodegradable polymers used in biomedical applications) degrade by random scission due to passive hydrolysis which results in molecular weight reduction, loss of strength, and ultimately, loss of mass. A constitutive model involving degradation and its impact on mechanical properties was developed through an extension of a material which response depends on the history of the motion and on a scalar parameter reflecting the local extent of degradation. The material properties decrease with degradation and a rate equation describing the chain scission process confers characteristics of stress relaxation, creep and hysteresis. These phenomena arise due to the entropy-producing nature of degradation and are markedly different from their viscoelastic counterparts.