Silicone rubber is currently the main material for medical devices. Its main component is polysiloxane, with a structural formula of - OsiR2-. The R-groups in medical silicone rubber are mostly - CH3 and vinyl groups. Silicone rubber has excellent biocompatibility, high temperature resistance, softness, easy deformation, aging resistance, high transparency, and good mechanical properties. Due to its superior mechanical properties and good biocompatibility, it is widely used in the field of biomedical devices. In order to improve the biocompatibility and antibacterial performance of silicone rubber medical catheters, it is necessary to prepare a coating with antibacterial function on their surface. However, due to the superhydrophobic properties and high surface inertness of silicone rubber, water-soluble coatings are difficult to fully infiltrate and spread on the surface of silicone rubber, resulting in poor adhesion between the coating and the substrate. In order to expand the application of silicone rubber, different surface activation methods can be used to modify the surface of silicone rubber polymer materials, such as light radiation, plasma treatment, ozone induction, etc. After modification, reactive groups are introduced into the surface of silicone rubber, and then water-soluble coating substances are chemically grafted onto the material surface. Plasma, as the fourth state of material existence, has the advantages of simplicity, efficiency, and environmental friendliness for surface modification, and can be widely applied to various polymer materials.

O2 plasma treatment of silicone rubber medical catheter surface

Surface contact angle analysis

The static contact angle method is an important method for evaluating the surface hydrophilicity of samples. The smaller the contact angle with water, the better the hydrophilicity, and the larger the contact angle, the worse the hydrophilicity. The surface activation degree of O2 plasma treatment under different powers was evaluated by measuring the contact angle method, as shown in Figure 1: with the increase of treatment power, the static contact angle showed a trend of first decreasing and then increasing. When the power was 200W, the static contact angle was the smallest.

Figure 1 Contact angle of silicone rubber surface treated with plasma of different powers

Surface roughness of silicone rubber after plasma treatment

White light interference was used to detect the surface morphology changes of O2 plasma pretreated silicone rubber, and the surface roughness of the pretreated silicone rubber was measured with a test area of 25 μ m × 25 μ m. Figure 2 shows the surface morphology of the sample measured by white light interferometry. Figure 2-6 (a) shows the surface morphology of untreated silicone rubber, which is flat with a roughness of 267nm. Figure 2 (b) shows the surface morphology of the pre treated silicone rubber, which has an increased surface roughness compared to the untreated sample. The surface roughness of the pre treated sample is 373nm. The data shows that after plasma pretreatment of the silicone rubber surface, the surface roughness of the silicone rubber material increases, and the plasma has an etching effect on the material. Moreover, the increase in roughness enhances the contact area between the coating and the substrate, thereby improving the adhesion between the coating and the substrate. After plasma treatment, the surface roughness of polymer materials increases to varying degrees due to the etching effect of plasma on the material surface, which affects the adsorption of liquid on the surface and ultimately changes the surface wettability.

Figure 2 Surface morphology of silicone rubber before and after white light interference plasma treatment

Infrared spectroscopy analysis

Normally, after being treated with gas plasma such as Ar, O2, N2, etc., polymer materials will introduce functional groups such as COOH, C=O, NH2 on the surface, increasing surface hydrophilicity. Through infrared analysis of O2 plasma silicone rubber samples, it is known from the infrared spectrum analysis that: as shown in Figure 3, 3000cm-1 is the C-H vibration absorption peak, 1280cm-1 is the Si-CH3 vibration absorption peak, 1100cm-1 is the Si-O-Si vibration absorption peak, and 750cm-1 is the Si-CH3 absorption peak. There is a C=O bond absorption peak at 1636cm-1, and an O-H bending vibration absorption peak of carboxylic acid at 1419cm-1. So the O2 plasma treatment of the sample increases the hydrophilicity of the silicone rubber sample due to the generation of carbonyl and carboxyl groups.

Figure 3 Infrared Spectral Analysis of Plasma treated Silicone Rubber

By treating the surface of silicone rubber medical catheters with O2 plasma, active functional groups can be introduced to increase the hydrophilicity and roughness of the silicone rubber surface, providing an active surface for the subsequent preparation of functional coatings. When preparing coatings on the substrate surface, the coating is more likely to form mechanical interlocking with the substrate, significantly improving the bonding strength between the coating and the substrate.