da Silva Soares, Joao Filipe (2008-05). Constitutive modeling for biodegradable polymers for application in endovascular stents. Doctoral Dissertation. Thesis uri icon

abstract

  • 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.
  • 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.

publication date

  • May 2008