| Summary: | Percutaneous transluminal balloon angioplasty followed by drug-eluting 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 to
accommodate the increase in caliber and there is no purpose for the stent to remain,
potentially provoking inflammation and foreign body reaction. Thus, a fully
biodegradable polymeric 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. Degradation refers to bond scission process that breaks
polymeric chains down to oligomers and monomers. Extensive degradation leads to
erosion, which is the process of mass loss from the polymer bulk. The prevailing
mechanism of biodegradation of aliphatic polyesters (the main class of biodegradable
polymers used in biomedical applications) is random scission by passive hydrolysis and
results in molecular weight reduction and softening.
In order to understand the applicability and efficacy of biodegradable polymers, a
two pronged approach involving experiments and theory is necessary. 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 and depreciates the
mechanical properties. A rate equation describing the chain scission process confers
characteristics of stress relaxation, creep and hysteresis to the material, arising due to the entropy-producing nature of degradation and markedly different from their viscoelastic
counterparts.
Several initial and boundary value problems such as inflation and extension of
cylinders were solved and the impacts of the constitutive model analyzed. In vitro
degradation of poly(L-lactic acid) fibers under tensile load was performed and
degradation and reduction in mechanical properties was dependent on the mechanical
environment. Mechanical testing of degraded fibers allowed the proper choice of
constitutive model and its evolution. Analysis of real stent geometries was made possible
with the constitutive model integration into finite element setting and stent deformation
patterns in response to pressurization changed dramatically as degradation proceeded.
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