Date of Award

Spring 5-4-2024

Degree Type

Thesis

Degree Name

Master of Science in Materials Science and Engineering (MSMSE)

Department

Materials Science

Committee Chair

Dr. Sudhakar Vadiraja

First Advisor

Dr. Richard LaDouceur

Second Advisor

Dr. Blaine Berrington

Abstract

Several clinical studies since 2008 have reported stent failure through mechanical means from inadequate strength, fatigue failure, coating failure via delamination and cracking, and pathological failures. Enhancement of the mechanical performance and stability of the polymer coating has the potential to minimize coating failure, aiding in the prevention of the other failure modes. The present study fabricated a polymer matrix composite coating from poly(D,L-lactide-co-glycolic) acid (PLGA) and hydroxyapatite (HA) nanoparticles on nitinol wires. Five weight fractions of HA (1.0, 2.5, 5.0, 7.5, and 10.0 wt.% HA) were studied for their influence on the mechanical performance of nitinol. The coatings were applied through dip coating, in which concentration of PLGA solution, dip speed, number of coatings, dwell time, and drainage time effects on coating thickness and uniformity were evaluated. Increasing the solution concentration enhanced the coating uniformity. Coating thickness increased with increased dip speed, decreased number of coats, increased dwell time, and decreased drainage time. Dip coating was performed in a 10 w/v % PLGA solution matrix at 100 mm/min, 30 minute dwell time, 1 minute drainage time, and 1 coating to achieve a coating thickness of 18.7 microns. There was a clear improvement in the mechanical behavior through the addition of the coating and up to 2.5 wt.% HA. Increase in the properties was caused by load transfer of particulates in the PMC coating. The properties decreased with further addition of HA from 5.0 to 10.0 wt.% due to the agglomeration of nanoparticles in the PLGA matrix, which served as stress concentrators. The trend in the data indicates there was an optimal HA at 2.5 wt.% to observe a maximum ultimate tensile strength improvement to 1686 MPa, associated with a 13% change.

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