Influence of plasma-forming gas and plasma source driving frequency on surface properties of silicone catheters for reducing bacterial adhesion

The present study investigates the influence of 40 kHz and 13.56 MHz driving frequencies on the surface properties of silicone catheter surfaces when oxygen/argon gas is used as plasma-forming gas. In this study, the primary focus is to examine the effect of different plasma discharge conditions on plasma properties such as plasma density, electron temperature and formation of reactive species and their linkage with surface and bacterial adhesion properties of silicone catheters. Experiments are conducted using capacitively coupled electrode configurations at low pressure. Plasma treatment was given for 10 min at 150 W fixed power. Langmuir probe analysis conveys that oxygen plasma exhibits higher plasma density compared to argon plasma regardless of the choice of driving frequency. Oxygen plasma is also found to be rich in terms of various types of gas phase reactive species, as reflected in optical emission spectroscopy (OES) analysis. Both 13.56 MHz and 40 kHz plasmas can induce significant physico-chemical changes on the silicone catheter surface. However, the 13.56 MHz plasma-treated catheter surface exhibited higher average surface roughness compared to surfaces treated by 40 kHz plasma discharge, as confirmed by atomic force microscopy (AFM) results. Plasma treatment on the catheter surface has resulted in a significant reduction in bacterial adhesion by gram-negative species. The comparative study of kHz and MHz range driving frequencies for the surface modification of a silicone catheter is a novel work and the outcome of this study is useful in selecting appropriate driving frequency and plasma-forming gas for the development of any biomedical/industrial application.