# Deposition of Cell Culture Coatings Using a Cold Plasma Deposition Method
## 3 Results  
### Characterization of the Plasma Discharge  
The current and voltage measurements were consistent with previous descriptions of nonthermal helium plasma discharges at atmospheric pressure [24; 27]. As shown in Figure 3, the applied power was provided as a pulsed discharge with a damped pseudo-sinusoidal wave form. The peak voltage was 5.28 kV (peak-peak) and the corresponding current was 0.10 A. The low current flow confirms that the plasma was not arcing to ground through the formation of high energy streamers. Instead, the system produced a low level glow and the current values observed are typical of glow like discharges as described by [PERSON] [28]. Using Equation (1):  
\[W=F\int_{t}^{t+T}I(t)V(t)dt \tag{1}\]  
the plasma discharge power was determined to be 4.25 W. Previous studies of similar systems have established that these conditions generate a cold atmospheric pressure plasma discharge capable of depositing thin film coatings [14; 19; 25; 29]. Upon addition of the collagen spray through the nebulizer, the plasma was visibly quenched, and the power delivered was reduced to 4.01 W. This was mainly driven by a reduction in the delivered current to a maximum of 0.08 A.  
Figure 3: Typical voltage and current traces collected from the plasma deposition device without collagen aerosol. Optical emission spectroscopy was used to examine discharge properties with and without precursor flow. A typical spectrum for a helium plasma in the spectral range of 200-850 nm is shown in Figure 4 and produced a spectrum that is typical of atmospheric pressure helium discharges with a mixture of helium and nitrogen derived features. The helium plasma produced a series of intense emission lines which can be ascribed to both nitrogen molecular (N\({}_{2}\) at 300-450 nm), nitrogen molecular ion lines (N\({}_{2}\)\({}^{+}\) at 358, 391, 419, and 427 nm) and helium (588, 667, 706, and 727 nm) species in the plasma. Addition of the collagen spray did not produce any additional peaks in the spectrum. The emission intensity dropped upon addition of the collagen, as seen in Figure 4. The decrease in emission intensity is primarily around the N\({}_{2}\) and N\({}_{2}\)\({}^{+}\) peaks between 300 and 435 nm. These peaks are normally assigned to the reaction associated with Penning ionization of the nitrogen molecules due to helium metastables [30]. The decrease in emission intensity is therefore most likely due to consumption of the helium metastables caused by plasma quenching due to the interaction with the water droplets and the nitrogen species as the collagen solution is introduced. This decrease in ionization rate will reduce the production of free electrons [31] and in turn reduce the possible damage caused to the collagen by the plasma. _Analysis of Collagen Films._ Gravimetric analysis was used to measure the deposition rate of the plasma coating. In order to remove any potential variability arising from ablation or cross-linking of the polystyrene substrate, a series of coatings were deposited onto double polished four-inch silicon wafers and the weight gain was measured. Figure 4: Typical optical emission spectra collected from pure helium plasma (**top**) and plasma with collagen aerosol (**lower**). The result was plotted against a theoretical deposition rate of a process with 100% efficiency, shown in Figure 5. There was an average deposition efficiency of 40-50%, with a relatively linear increase in deposition mass with time. If the plasma process had been 100% efficient, then the plasma coated microplates would generate a collagen loading of 3.5 ug/cm\({}^{2}\). This is significantly less than the 76 ug/cm\({}^{2}\) produced using the traditional wet chemical method, representing a more protein efficient process. The physical structure of the collagen films deposited via wet chemical deposition and via the plasma deposition method were compared using standard surface analytical techniques. None of the films were visible via optical microscopy, indicating that both coatings were optically clear.