Stability of Poly(ethylene terephthalate) Endovascular Prostheses to Hydrolytic and Enzymatic Degradation

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Title: Stability of Poly(ethylene terephthalate) Endovascular Prostheses to Hydrolytic and Enzymatic Degradation
Author: Sumanasinghe, Ruwan Deepal
Advisors: Dr. Martin W. King, Committee Chair
Dr. Behnam Pourdeyhimi, Committee Co-Chair
Dr. Alan E. Tonelli, Committee Member
Abstract: Poly(ethylene terephthalate) (PET) is a thermoplastic polymer which is widely used in the construction of woven and knitted endoprostheses for reconstructive arterial surgery. Analysis of many explanted PET vascular prostheses has given evidence of the existence of a long term in-vivo degradation of the PET material. Due to the increasing demand for less invasive methods of abdominal aortic aneurysm (AAA) repair using PET endoprostheses, there is a significant need for predicting the biostability of these materials by means of an accelerated in-vitro test. The purpose of this research has been to develop an in-vitro accelerated test protocol based on hydrolytic/enzymatic degradation of PET to predict the biostability of PET endovascular stent grafts. In addition, this study involves assessing whether the presence of nitinol stent material influences the rate of degradation of the PET graft. Non-sterile warp knitted prosthetic fabric specimens from Type 56 Dacron&174; PET fibers were subjected to degradation in neutral enzymatic and alkaline environments for periods of up to 9 weeks and 10 days, respectively. The alkali degradation resulted in a maximum weight loss of 80%, while degradation in enzyme showed a weight gain. Both, alkali at pH 13.4 and papain at pH 7.2 caused a change in crystallinity of the PET specimens. The behavior of the melt endotherms especially in alkali degraded specimens, indicated the presence of two types of crystalline areas in the fiber. These results correlated well with the observation made on explanted vascular prostheses. The degradation of PET in alkali was well confirmed by the exponential growth of the concentration of terephthalate anions in the degradation solutions. Examination of the change in conformation of the ethylene glycol segments in PET fibers using FTIR gave evidence of a significant conformational change in alkali degraded specimens. Similar observations were made with specimens degraded in saline without nitinol stents after 3 and 6 weeks. A significant change in the conformation of -CH₂-CH₂- bonds occurred in enzyme degraded specimens after 3 and 6 weeks. With the alkali degraded specimens a linear correlation was shown between the loss in probe bursting strength and the duration of degradation which correlated well with the observations on explanted prostheses. SEM observations revealed the formation of pits elongated in the direction of the fiber axis after 2 days of alkali degradation. The enzyme particles were observed to be trapped and bound within the interstices and on the surface of fibers in specimens degraded with enzyme at 50°C. Except for a minor discoloration due to alkali degradation and deposition of enzymes, the surface of the nitinol stent elements appeared to be unchanged. Based on the post-degradation bursting strength measurements, 2.6 days of in-vitro degradation of PET endoprostheses in pH 13.4 and 65°C produced results which correlated well with 162 ± 23 months of in-vivo degradation. The nitinol stents did not appear to influence degradation of PET in any of the three environments.
Date: 2003-11-11
Degree: MS
Discipline: Textile Management and Technology

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