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Research Papers

Modeling of Biodegradable Polyesters With Applications to Coronary Stents

[+] Author and Article Information
Rosa Shine

Biomechanics Research Centre (BMEC),
Biomedical Engineering,
National University of Ireland Galway,
University Road,
Galway H91 HX31, Ireland
e-mail: r.shine2@nuigalway.ie

Reyhaneh Neghabat Shirazi

Biomechanics Research Centre (BMEC),
Biomedical Engineering,
National University of Ireland Galway,
University Road,
Galway H91 HX31, Ireland
e-mail: reyhaneh.shirazi@nuigalway.ie

William Ronan

Biomechanics Research Centre (BMEC),
Biomedical Engineering,
National University of Ireland Galway,
University Road,
Galway H91 HX31, Ireland
e-mail: william.ronan@nuigalway.ie

Caoimhe A. Sweeney

Biomechanics Research Centre (BMEC),
Biomedical Engineering,
National University of Ireland Galway,
University Road,
Galway H91 HX31, Ireland
e-mail: caoimhe.a.sweeney@gmail.com

Nicola Kelly

Biomechanics Research Centre (BMEC),
Biomedical Engineering,
National University of Ireland Galway,
University Road,
Galway H91 HX31, Ireland
e-mail: nicola.kelly@nuigalway.ie

Yury A. Rochev

National Centre for Biomedical
Engineering Science (NCBES),
National University of Ireland Galway,
University Road,
Galway H91 HX31, Ireland
e-mail: yury.rochev@nuigalway.ie

Peter E. McHugh

Professor
Biomechanics Research Centre (BMEC),
Biomedical Engineering,
National University of Ireland Galway,
University Road,
Galway H91 HX31, Ireland
e-mail: peter.mchugh@nuigalway.ie

1Corresponding author.

Manuscript received July 5, 2016; final manuscript received January 5, 2017; published online May 3, 2017. Assoc. Editor: Marc Horner.

J. Med. Devices 11(2), 021007 (May 03, 2017) (12 pages) Paper No: MED-16-1261; doi: 10.1115/1.4035723 History: Received July 05, 2016; Revised January 05, 2017

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.

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Figures

Grahic Jump Location
Fig. 1

For the semi-analytical model, the dimension L of the films investigated each time is represented using the one-dimensional unit shown in (a) and (b). Cm is assumed to have a value of zero at the assigned free surface, and a monomer flux of zero is imposed at the opposite boundary (b). The geometry used in each of the finite-element investigations for the PLA and PLGA materials is shown in (c) as a representative unit of material of thickness L. The finite-element mesh and boundary conditions applied are also shown. Cm = 0 is applied atthe free surface on the left-hand boundary of the model, and all the other surfaces are set to impermeable surfaces (c).

Grahic Jump Location
Fig. 2

The open-cell and zig-zag BPS geometries examined are shown in (a)–(e) and (f), respectively. For the open-cell geometry, the thickness and width of the stent strut are varied uniformly in two dimensions to create three different stent struts sizes. (a) and (c) The stent when the struts are of dimensions 156 μm thick × 150 μm wide, (d) for struts 78 μm thick × 75 μm wide, and (e) struts 234 μm thick × 225 μm wide. The two boundary conditions investigated, SFS and SFE, are also highlighted. The surfaces at which Cm=0 is applied are shown in (c), when the outer surface of the stent is set to an impermeable surface (i.e., (∂Cm/∂n)=0), which is the boundary applied to all of the surfaces in (a).

Grahic Jump Location
Fig. 3

The predicted changes in the volume averaged M¯n of the PLA plate and film investigated by Grizzi et al. [11] for a degradation period of 30 weeks are shown. The predictions of the matlab, comsol, and abaqus/standard simulations are compared to those obtained by Wang et al. [21] and also to the experimental data [11].

Grahic Jump Location
Fig. 4

The results predicted for the range of PLGA films investigated using the matlab and abaqus/standard simulations are compared to the comsol predictions of Shirazi et al. [13]. The predictions for thinner PLGA films (0.25, 2.5, and 25 μm) are shown in (a), whereas the predictions for the 10, 250, and 2500 μm films are compared in (b). The differences predicted by the semi-analytical and FE models for the thicker PLGA film cases are highlighted in (c).

Grahic Jump Location
Fig. 5

Predictions in the volume averaged M¯n for the different materials, strut size, and boundary conditions assumed are compared for a degradation period of 104 weeks

Grahic Jump Location
Fig. 6

The contour plots shown depict M¯n and Cm in the stent (156 × 150 μm struts, open-cell geometry) for when BC1 was applied for the PLA, PLLA, and PLGA materials. The predictions for M¯n and Cm are shown for the PLA material when BC2 was applied to the same stent geometry.

Grahic Jump Location
Fig. 7

The predictions in M¯nv versus time for the two stent geometries (open-cell and zig-zag) are compared for 104 weeks

Grahic Jump Location
Fig. 8

The predictions in the Young's modulus, E, for the open-cell 156 μm stents are shown for a time of 90 weeks, when the initial moduli, E0, are assigned as 3500 MPa, 4800 MPa, and 2000 MPa for the PLA, PLLA, and PLGA materials and where the value of km is 1400 MPa [28]

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