Research Papers

Modeling of Deformation-Accelerated Breakdown of Polylactic Acid Biodegradable Stents

[+] Author and Article Information
João S. Soares

Department of Mathematics, Center for Mathematics and Its Applications (CEMAT), Instituto Superior Tecnico, Avenida Rovisto Pais 1, Lisboa 1049-001, Portugaljoao.soares@math.ist.utl.pt

James E. Moore

Department of Biomedical Engineering, Texas A&M University, 337 Zachry Engineering Center, 3120 TAMU, College Station, TX 77843jmoorejr@tamu.edu

Kumbakonam R. Rajagopal

Department of Biomedical Engineering, Department of Mechanical Engineering, Texas A&M University, College Station, TX 77843krajagopal@tamu.edu

J. Med. Devices 4(4), 041007 (Dec 06, 2010) (10 pages) doi:10.1115/1.4002759 History: Received August 31, 2009; Revised May 17, 2010; Published December 06, 2010; Online December 06, 2010

The use of biodegradable polymers in biomedical applications has been successful in nonload bearing applications, such as biodegradable implants for local drug delivery, and in simple load bearing situations such as surgical sutures and orthopedic fixation screws. The desire to incorporate these materials in more complex load bearing situations, such as tissue engineering scaffolds and endovascular or urethral stents, is strong, but the lack of constitutive models describing the evolution of biodegradable polymers over the course of degradation has severely hampered the rational design process for these more complex biodegradable medical applications. With the objective of predicting biodegradable stent behavior, we incorporated constitutive models of biodegradable polymeric materials in a computational setting and the mechanical response of three different stent designs were analyzed as degradation progressed. A thermodynamically consistent constitutive model for materials undergoing deformation-induced degradation was applied to a commonly employed biodegradable polymer system, poly(L-lactic acid), and its specific form was determined by corroboration against experimental data. Depreciation of mechanical properties due to degradation confers time-dependent characteristics to the response of the biodegradable material: the deformation imparted by a constant load increases over time, i.e. the body creeps, and the stress necessary to keep a fixed deformation decreases, i.e. the body relaxes. Biodegradable stents, when subjected to constant pressure in its exterior, deflect inwards and ultimately fail as the structure loses its mechanical integrity. The complex geometry of endovascular stents and their physiological loading conditions lead to inhomogeneous deformations, and consequently, inhomogeneous degradation ensues. Degradation is mostly confined to the bends of the stent rings and junction points, which are the locations that carry most of the deformation, whereas mostly undeformed connector bars remain less degraded. If failure occurs, it will occur most likely at those sensitive locations and large, nondegraded pieces can provoke severe embolic problems. Highly nonuniform degradation indicates that some stent designs are at higher risk for complications. Deformation patterns of stents made of a material that loses its integrity are different than those of permanent stents. Blind adaptation of permanent stent design concepts is ill-suited for biodegradable stent design. The time-dependent aspect of the implant not only must be taken into account but should also be used to interact with the body’s reaction and to enhance healing.

Copyright © 2010 by American Society of Mechanical Engineers
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Figure 1

Semicrystalline poly(L-lactic acid). Monofilament fibers were approximately 5 cm long and had an approximate diameter of 1 mm. Samples of 2 cm in length were obtained from the PLLA fibers and their tensile properties evaluated with uniaxial extension at constant strain rates.

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Figure 2

Instantaneous elastic response of nondegraded poly(L-lactic acid). Strain is defined as the ratio of the increase in length over the initial length and experimentally measured stress was defined as force over undeformed cross-sectional area (Piola stress). The specific form of Helmholtz potential (cf. Eq. 10) leads to a very good description of the response of the nondegraded material when subjected to uniaxial extension. The family of responses of the degrading polymer at different levels of degradation predicted by the model are also shown as dashed lines. Note that increasing degradation leads to a change in the overall stiffness of the material.

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Figure 3

Degradation of the 2B3 stent (cf. Refs. 53-54) subjected to outer pressurization at several time points. As time and degradation progress, the stent degrades mostly at the bends of the rings, where a state of bending prevails, and at their junction points with the connector bars. These remain mostly nondegraded. As the material softens, the constant outer pressurization is responsible for the creep of the stent inwards. Reprint with permission from Ref. 31 in Cardiovascular Engineering Technology,© 2010 Springer.

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Figure 4

Degradation of the 1Z1 stent (cf. Refs. 53-54) subjected to outer pressurization at several time points. This stent is much stiffer than the 2B3 and much of the deformation and degradation are confined to the junction points and sharp crowns of the stent rings. If failure occurs at these sensible locations, struts that are mostly nondegraded can provoke embolic complications downstream. As the material loses its integrity, buckling of the cylindrical structure is observed to initiate, even though the outer pressurization is kept constant. Reprint with permission from Ref. 31 in Cardiovascular Engineering Technology,© 2010 Springer.

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Figure 5

Degradation of a biodegradable version of the Cypher stent subjected to outer pressurization at several time points. Although this real stent geometry is characterized by being somewhat stiff at its nondegraded stage (constant outer pressurization initially does not promote a large deflection), as degradation occurs dramatically at the junction points due to their peculiar complex shape, the stent loses its stiffness and creeps inwards to a great extent.

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Figure 6

Average relative recoil versus time of the different stent designs subjected to constant outer pressurization. Average relative recoil was measured as the ratio of average current diameter ϕ(t) and average expanded diameter ϕref. Stent design 2B3 is the most compliant, as a recoil of approximately 25% occurs initially. Stent design 1Z1 and Cypher are much stiffer initially but the latter shows a smooth loss of stent performance as degradation proceeds. The end point of each line represents the breakdown of the numeric scheme (as ψ→0 at critical locations of the stent), which is, at least, related with physical breakdown of the structure.



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