Effects of plasma treatments on the controlled drug release from poly(ethylene-co-vinyl acetate)

Plasma surface treatment was investigated as a surface modification method for a drug-eluting stent (DES) coated with polymers. Currently, the implantation of the DES is the most efficient way to treat a coronary artery disease. DES elutes anti-proliferative drugs that suppress proliferation of smooth muscle cells in the stented segment of the artery. Despite the impressive reduction in restenosis by DES, it still occasionally has a major disadvantage for not preventing restenosis at an implant site due to the relatively vast drug release from the stent surface in the early stages of the drug release. To solve the problem, we studied plasma treatments on the polymer surface because there would not be a substantial risk of damaging the bulk properties of the polymer and the stent by plasma surface treatments. In this study, argon, oxygen, and nitrogen were selected as working gases and poly(ethylene-co-vinyl acetate) (EVA), a hydrophobic biomedical polymer, was selected as a base drug-reservoir material for DES. Structural analyses were carried out by water contact angle measurements, X-ray photon spectroscopy (XPS), and the evaluation of the crosslinking degree of EVA polymer. It was found that the initial burst-release and the cumulative released amount of the drug were both effectively suppressed by controlling the plasma processing time. Furthermore, less effective control of the drug release was obtained by using nitrogen or oxygen plasma as a processing gas instead of argon plasma. According to the evaluation of the crosslinking degree, it was found that argon plasma could most effectively induce the crosslinking in EVA, while nitrogen and oxygen plasmas came in second and third, respectively, which corresponded to the results of the drug release experiments. It was expected that the experimental results of the plasma treatments could provide a new and alternative approach to a controllable and sustainable drug release system.