Influence of Thermal Annealing on the Mechanical Properties of PLLA Coiled Stents
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To investigate the influence of elevated annealing temperature (70–90 °C) on the mechanical properties of coiled helical PLLA stents. PLLA 0.10 mm fibers and Ø3 mm × 12 mm stents were fabricated and annealed at 70, 80 and 90 °C for 25 min. The mechanical properties of the fibers and the functional characteristics of the stents were measured and compared to a control group processed at 21 °C. The stents were mounted, expanded and relaxed using an Ø3 mm × 2 cm balloon catheter to a maximum balloon pressure of 12 atm. Measurements of stent diameter, length, and the balloon pressure were used to determine the effective circumferential strain, incremental stiffness, elastic recoil, and lengthening of the stents. Stents exhibited progressively higher incremental stiffness with annealing temperature, higher collapse resistance and a reduction in elastic recoil vs. controls. Single fiber mechanical properties decreased as annealing temperature increased. Differential scanning calorimetry revealed crystallinity increased within thermally annealed stent fibers compared with controls. SEM examination indicated thermally annealed stents underwent less twisting than controls during balloon-induced unfurling. Thermal annealing of PLLA fibers and stents between 70 and 90 °C induced changes in crystalline structure, thereby favorably influencing fiber stress–strain behavior and stent expansion characteristics.
KeywordsThermal annealing PLLA stent Bioresorbable polymer Crystallinity
- D12 atm
Stent diameter at a balloon pressure of 12 atm
- D0 atm
Stent diameter when the balloon pressure is at 0 atm
Instantaneous stent diameter at each pressure level
Stent diameter for the unfurled state
- Xc (%)
We thank Ms. Martha Gracey and Dr. Howard Arnott of the Biology Department, UT Arlington, for their help in SEM image generation.
Conflict of Interest
Dr. Tré R. Welch, Dr. Robert C. Eberhart, Dr. Joan Reisch, and Dr. Cheng-Jen Chuong has no conflict of interest. No human and or animal studies were carried out by the authors for this article.
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