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Research Papers

The Effect of Static and Dynamic Loading on Degradation of PLLA Stent Fibers

[+] Author and Article Information
Danika Hayman

Department of Bioengineering,
Imperial College of London,
South Kensington Campus,
London SW7 2AZ, UK
e-mail: danika.hayman@imperial.ac.uk

Christie Bergerson

Department of Biomedical Engineering,
Texas A&M,
College Station, TX 77843
e-mail: cmbergerson@gmail.com

Samantha Miller

Department of Biomedical Engineering,
Texas A&M,
College Station, TX 77843

Michael Moreno

Department of Biomedical Engineering,
Texas A&M,
College Station, TX 77843
e-mail: mmoreno@bme.tamu.edu

James E. Moore

Department of Bioengineering,
Imperial College of London,
South Kensington Campus,
London SW7 2AZ, UK
e-mail: james.moore.jr@imperial.ac.uk

1Corresponding author.

Manuscript received November 14, 2013; final manuscript received April 11, 2014; accepted manuscript posted May 8, 2014; published online June 3, 2014. Assoc. Editor: Sean S. Kohles.

J Biomech Eng 136(8), 081006 (Jun 03, 2014) (9 pages) Paper No: BIO-13-1533; doi: 10.1115/1.4027614 History: Received November 14, 2013; Revised April 11, 2014; Accepted May 08, 2014

Understanding how polymers such as PLLA degrade in vivo will enhance biodegradable stent design. This study examined the effect of static and dynamic loads on PLLA stent fibers in vitro. The stent fibers (generously provided by TissueGen, Inc.) were loaded axially with 0 N, 0.5 N, 1 N, or 0.125–0.25 N (dynamic group, 1 Hz) and degraded in PBS at 45 °C for an equivalent degradation time of 15 months. Degradation was quantified through changes in tensile mechanical properties. The mechanical behavior was characterized using the Knowles strain energy function and a degradation model. A nonsignificant increase in fiber stiffness was observed between 0 and 6 months followed by fiber softening thereafter. A marker of fiber softening, β, increased between 9 and 15 months in all groups. At 15 months, the β values in the dynamic group were significantly higher compared to the other groups. In addition, the model indicated that the degradation rate constant was smaller in the 1-N (0.257) and dynamic (0.283) groups compared to the 0.5-N (0.516) and 0-N (0.406) groups. While the shear modulus fluctuated throughout degradation, no significant differences were observed. Our results indicate that an increase in static load increased the degradation of mechanical properties and that the application of dynamic load further accelerated this degradation.

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Figures

Grahic Jump Location
Fig. 1

Illustration of the device used to apply a dynamic axial load to fibers. The load fluctuated between 0.125 N and 0.25 N at a frequency of 1 Hz while the fibers were submerged in PBS maintained at 45 °C

Grahic Jump Location
Fig. 2

Average stress versus strain curves for the elastic regions of fibers exposed to (a) dynamic load, (b) no load, (c) small static load, and (d) large static load at 0, 6, 9, 12, and 15 months

Grahic Jump Location
Fig. 3

Illustration of the effect of varying β while keeping μ and m constant. An increase in β indicates a softer material at higher strains.

Grahic Jump Location
Fig. 4

β, a Knowles model constant, increases with degradation time. *p < 0.05 compared to the no load group at 15 months. #p < 0.05 compared to the same loading group at 9 and 12 months.

Grahic Jump Location
Fig. 5

The degradation model is fitted to the values of β to obtain values for the degradation constants K(F)

Grahic Jump Location
Fig. 6

The model parameter Kβ for each static loading group was fitted to a linear model. The values for the dynamic load are also shown (red square) but not included when determining the linear relationship between Kβ and degradation stress.

Grahic Jump Location
Fig. 7

The percent crystallinity (Xc) changes with equivalent degradation time. A significant interaction between load and time was observed. *p < 0.05 compared to the dynamic load group within the same time point. #p < 0.05 compared to the small load group within the same time point. +p < 0.05 compared to the 6-month time point of that loading group. &p < 0.05 compared to the 9-month time point of that loading group. p < 0.05 compared to the 15-month time point of that loading group.

Grahic Jump Location
Fig. 8

As the fiber degrades and percent crystallinity increases, the rigidity of the fiber also decreases. There is an approximate linear relationship between Xc and β.

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