Formulation and testing of biodegradable polymeric coating on zinc wires in cardiovascular stent application

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

Effect of PLLA coating on corrosion and biocompatibility of zinc in vascular environment

Zinc (Zn) has recently been introduced as a promising new metal candidate for biodegradable vascular stent applications with a favorable degradation rate and biocompatibility. Corrosion-resistant metal stents are often coated with drug-eluting polymer layers to inhibit harmful biological responses. Here, the authors aimed to investigate the interaction between biodegradable zinc metal and a conventional biodegradable polymer coating. Zinc wires with a diameter of 0·25 mm were surface-modified using 3-(trimethoxysilyl)propyl methacrylate (MPS) and then coated with a 1–12 μm film of poly(l-lactic acid) (PLLA). The corrosion behavior of PLLA/MPS-coated zinc wires was studied in simulated body fluid using electrochemical impedance spectroscopy. An increase in the impedance from \u3c1000 to \u3e15 000 Ω cm2 was recorded for the zinc wires after being coated with PLLA. The PLLA/MPS-coated zinc specimens were implanted into the abdominal rat aorta to assess their biodegradation and biocompatibility compared to uncoated zinc wires. PLLA/MPS-coated wires corroded at approximately half the rate of unmodified zinc during the first 4·5 months. A histological analysis of the biological tissue surrounding the zinc implants revealed a reduction in the biocompatibility of the polymer-coated samples, as indicated by increasing cell toxicity and neointimal hyperplasia