Date of Award

2017

Document Type

Open Access Master's Thesis

Degree Name

Master of Science in Materials Science and Engineering (MS)

Administrative Home Department

Department of Materials Science and Engineering

Advisor 1

Jaroslaw Drelich

Committee Member 1

Jeremy Goldman

Committee Member 2

Daniel Seguin

Abstract

Biodegradable and biocompatible poly (L-lactic-acid) (PLLA) coating was applied on a modified zinc (Zn) substrate by dip coating, with the intent to delay the bio-corrosion and slow the degradation rate of zinc substrate. 3-(Trimethoxysilyl) propyl methacrylate (MPS) was used for modification of the zinc substrate for promoting the adhesion between the metallic substrate and the polymer coating. It is hypothesized that the delay in Zn biodegradation could be useful in the first several weeks to prevent the early loss of mechanical integrity of the endovascular stent and to improve the healing process of the diseased vascular site. The PLLA coating was used in this study because of its biodegradability, favorable degradation rate, hydrophobicity and favorable mechanical properties. Static immersion, electrochemical and inductively coupled plasma (ICP) tests were used to investigate the degradation behavior of a polymer coated modified Zn substrate. Two uniform polymer layers with thickness of 1 and 3 µm were coated on the Zn substrate. The potentiodynamic polarization test indicated that the 1 µm polymer coated specimen has higher corrosion potentials (Ecorr) and lower corrosion cur rents (icorr) in the simulated body fluid (SBF) compared to the uncoated Zn. AC impedance measurement in EIS test also demonstrated a significant improvement in the impedance and polarization resistance of the coated Zn substrate. However, after 10 days of immersion in the SBF, the impedance reduced drastically which is indicative of a coating degradation and penetration of the electrolyte to the zinc substrate. Immersion degradation studies showed that the cross-sectional area (CSA) reduction and penetration rate (PR) for polymer coated samples are 5 times smaller than for uncoated samples after 14 days of immersion in SBF solution. Results of the ICP method indicated an increase in the release of the Zn2+ to the solution for the uncoated Zn, while the 1 µm PLLA coated sample demonstrated much slower release rate of Zn2+ and the concentration of Zn ion during the 14 days’ immersion in SBF was almost the same. In in vivo studies, the polymer-coated Zn and uncoated Zn samples were implanted into the abdominal aorta of the rats and then directed into the lumen. The explants were extracted after 0.5 to 6 months. The results of in vivo study indicated that the uncoated samples have approximately two times higher CSA reduction and PR in comparison to the coated samples during first 4.5 months. After 4.5 months, the CSA reduction and PR increased significantly. However, the histological analysis of the biological tissue surrounding samples showed a reduction in biocompatibility of the polymer coated samples indicated by increasing cell toxicity and neointimal hyperplasia.

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