The effect of RF plasma treatment on stainless steel surface and polymer electrospraying properties

Vacuum radio frequency (RF) plasma treatment was conducted on electropolished and passivated 316LVM stainless steel (SS) surface by using Ar, O2, Ar/O2 mixture gases. Contact angle measurements revealed that both Ar and O2 plasma treatments significantly increased the wettability of the SS surface by water. Additionally, aging study was carried out for Ar, O2 and Ar/O2 plasma gases to determine the changes in contact angle values on stainless steel surface by time. The possible roles of physical and chemical effects induced by RF plasma treatments, on the wettability of SS surfaces were discussed. The physical and chemical effects induced by Ar and/or O2 plasma on SS were investigated via Atomic Force Microscopy (AFM), surface profilometry and X-ray Photoelectron Spectroscopy (XPS). AFM and surface profilometry analyses revealed that Ar, O2 and Ar/O2 plasma treatments resulted in smoother surface when compared to reference electropolished sample. However, the smoothest topography was obtained by O2 plasma treatment among all other plasma gas options. XPS investigations of the surfaces with respect to Fe 2p, Cr 2p, Mo 3d and O 1s XPS spectra showed that the main role of Ar plasma was thinning of the oxide-hydroxide layer along with the removal of contamination without any definitive difference on the valence states of metal compounds. On the other hand, O2 and Ar/O2 plasma treatments resulted in thickening of oxide layer along with oxidation of the species to their highest valence states (Cr VI, Fe III and Mo VI). Compared to other oxidation methods (electrolytic and high temperature), RF plasma oxidation allows the formation of highest valent oxides (Cr VI, Fe III and Mo VI) in stable forms. The electrospray of PLGA-Sirolimus coating applications on RF plasma treated 316 LVM stainless steel were aligned with the results of characterization studies above. Vacuum RF plasma treatment with Ar, O2 or Ar/O2 gases improved the wettability of PLGA coating via electrospray and stainless steel surface was completely covered by PLGA. Compared to Ar or O2 plasma treatments, the most uniform coating surface was achieved by Ar/O2 plasma treatment. In order to overcome the temporary effect of plasma treatment, PHEMA hydrophilic polymer, which was considered as a permanent hydrophilic layer between PLGA and 316LVM SS substrate, provided a time independent wettability for PLGA coating applications. The best wettabilty of PHEMA was obtained by Ar plasma treatement, so this result showed that oxygen plasma gas negatively influenced on the bonding of polymer chains to the metal surface. Consequently, PLGA was successfully and uniformly coated by electrospray on plasma treated SS surface or PHEMA coating interface layer without any coating defects.