# Deposition of Cell Culture Coatings Using a Cold Plasma Deposition Method
## 4 Discussion  
The electrical and optical emission data demonstrates the formation of a nonthermal helium plasma discharge. There was no evidence of high energy streamers or arcs detected and the current measurements were consistent with a glow like discharge [28]. Given the absence of high energy arcs, the delivered power within the system was significantly less than 10 W. This low power regime limited the potential for the plasma discharge to fragment or disrupt any molecules it may contact. Such systems have previously been shown to be capable of producing controlled soft polymerization [13; 14; 15; 16]. As evidenced by OES, the reactive nitrogen species present within the plasma were quenched by the introduction of the aqueous aerosol mist, which minimized any destructive species within the plasma. Numerous publications have now determined that such atmospheric helium plasma systems are capable of depositing functionally intact coatings on a range of surfaces [21]. Recent studies have shown that such plasma systems are also capable of depositing functional biological coatings [22] and in particular collagen coatings [23; 36]. This may be in part limited to the transient nature of the contact with the helium plasma, which minimizes the potential for process mediated structural modifications. Previous studies of how proteins interact following prolonged exposure to helium plasma jets have suggested that the plasma can induce reactions involving the thiol group of cysteine residues [37]. However, no evidence of such reactions has been detected at this point in this deposition system. As the structure and biological activity of the protein has been maintained without measurable denaturation, it is unlikely that the hydrogen bonding within the molecule has been degraded. Instead, the coagulation may simply involve hydrogen bond formation between adjacent native protein molecules in the deposited layer [38]. Although still speculative, such a mechanism would result in a thin film coating in which the biological activity of the protein is maintained, and the collagen layers deposited by the plasma systems have been shown to promote biological responses and are biocompatible in a manner expected from the native protein. The surface analysis strongly supports the creation of a continuous nanolayer of collagen on the surface and this layer is comparable to the collagen layer produced through traditional bench top wet chemical techniques. No evidence of adverse altered structure, porosity, coverage, chemical composition, or protein integrity was detected as a function of the plasma deposition process. The main differences between the two coating approaches are the processing time and coating thickness. The plasma process was capable of coating a microplate in under 2 min, whereas the wet chemical technique required overnight incubation under aseptic environment to produce a coating. Preliminary cell culture experiments suggest that the plasma coating will promote the proliferation of a number of cell lines and seems to outperform the wet chemical process. Figure 13: MTT assay at 24 h from HepG2 cells. Results are average of six readings. Switching from a wet chemical approach to a plasma coating process offers the opportunity to replace a complex, multistep process with a single step manufacturing process with significantly higher throughput, reduced biologic usage and potential to radically reduce costs. The coating applied herein took less than 100 s per plate and used less than 5% of the collagen solution when compared to the traditional approach. This opens the possibility for large area, aseptic manufacturing systems to be replaced with a controllable, cost effective, compact plasma device with flexibility for biologic coatings. This work was limited to collagen, however, further validation with other proteins and molecular structures are required to challenge the full flexibility of this low power deposition process. Significantly, this approach may also be applicable to wound care.