Functionalization of an Inner-Wall of Diamond-Like Carbon Coated Small-Diameter Long-Sized Tube by Oxygen Plasma Treatment

1. Introduction: Medical small-diameter long tube such as artificial vascular grafts have issue of stenosis and occlusion due to thrombus caused by thrombocyte aggregation and adhesion to the lumen. In order to bring the lumen of the medical tube closer to the biological environment, to create the biomimetic structure by oxygen plasma treatment after DLC (Diamond-like Carbon) deposition, developed an AC high-voltage plasma CVD (Chemical Vapor Deposition) method. Observed the formation of a thin Hydrogenated Amorphous Carbon (a-C:H) film on the lumen by Raman analysis by using methane as the source gas. Achieved the glow discharge inside the silicone tube with an aspect ratio of 750 inner diameter of 2 mm and an overall length of 1,500 mm [1]. The introduction of the functional group depends on <i>sp³</i> C-C ratio because the bond energies of the <i>sp³</i> C-C bond is lower than the <i>sp²</i> C-C bond [2]. The surface zeta potential can be controlled to a negative charged [3]. In this study, evaluate <i>sp²</i> / <i>sp³</i> ratio by the method of Near-Edge X-ray Absorption Fine-Structure (NEXAFS) spectroscopy to know the easiness to introduce carboxyl group on an inner-wall of small-diameter long-sized silicone tube by using oxygen plasma treatment after DLC deposition. Also confirm the surface zeta potential will be controlled to a negative charged by carring out dissociation of protons also to know the introducing carboxyl group. Furthermore, in order to bring the inner-wall of the medical tube closer to the biological environment, to perform DLC deposition and oxygen plasma treatment to create the biomimetic structure and to confirm the adhesion characteristic of thrombocyte.<br/>2. Experimental Method: The discharge conditions for DLC deposition and oxygen plasma treatment are AC voltage 5 kV, offset voltage 2 kV, frequency 10 kHz, pulse frequency 10 pps, combined pressure 39 Pa, and methane gas and oxygen gas are introduced as the source material gas at 96 ccm. In the experiment, the DLC was deposited for 20 minutes and the oxygen plasma was treated for 1-5 seconds after the film was formed.The sample size was inner diameter of 4 mm and length of 350 mm. However, it is difficult to measure the insulator by NEXAFS. By this reason, placed a piece of silicon wafer 5 mm away from the exit of the tube, irradiated the plasma at 90° angle, and evaluated the film deposited on the silicon wafer. At this experiment, silicone sheet inserted on a inner-wall to evaluate zeta potential and water contact angle. As a biological evaluation, human blood was used.<br/>3. Results and Discussion: By NEXAFS observation <i>sp³</i> / (<i>sp³</i>+<i> sp²</i>) = 43.9% calculated by comparison with HOPG(Highly oriented pyrolytic graphite). It seems enough ratio to introduce carboxyl group on an inner-wall. Zeta potential has been controlled to a negative potential by oxygen plasma treatment after DLC deposition. The water contact angle of Normal silicone tube was 115.1 °; the DLC-coated silicone tube film deposited for 20 minutes was 104.6 °, the DLC-coated silicone tube treated for 5 seconds with oxygen plasma, after film deposited for 20 minutes, was 79.1 °. The more the contact angle decreases the more easily compatible with water it becomes. It was found that the hydrophobic inner wall of the tube surface can be hydrophilized by oxygen plasma-treated DLC coating. Futhermore, the counts (/2,150,000 μm2) of the luminescence of autofluorescent derived from thrombocytes were 30.8 ± 26.2 for normal graft mean ± SD, 8.2 ± 5.4 for DLC coated graft, and 5.5 ± 4.0 for O2 treated DLC graft. Therefore, it can be made biomimetic DLC coating on a lumen of tube functionalized by oxygen plasma treatment.<br/>References<br/>[1] T. Nakatani, Y. Imai, Y. Fujii, T. Goyama, S. Ozawa, J. Photopolym. Sci. Technol., 31, 373, (2018).<br/>[2] Y. Nitta, K. Okamoto, T. Nakatani, M. Shinohara, IEEE Trans. plasma Sci., 40 (8), 2073, (2012)<br/>[3] Y. Nitta, K. Okamoto, T. Nakatani, H. Hoshi, A. Honma, E. Tatsumi, Y. Taenaka, Diam. Relat. Mat.,17, 1972, (2008).