Figure 1.
Plasma device (sawtooth electrode) used for earlier experiments in this paper.
Figure 2.
Schematic showing the difference between (A) Volume Plasma Configuration (B) Surface Plasma Configuration.
In both (A) and (B), the powered electrode is the dashed, black surface on top. The grounded electrode is the solid, black surface on the bottom. The grey surface in between is the dielectric barrier. In (A), a small scalpel that needs to be sterilized would be placed in the discharge gap between the dielectric barrier and grounded electrode since this is where plasma is generated. In (B), the same small scalpel would be placed on top of the dashed black surface, since this is where the plasma would be generated.
Figure 3.
Schematic of the Experimental Setup used.
Figure 4.
The plasma device used for sterilization experiments in this paper (A) un-powered (B) powered (generating plasma).
Figure 5.
Sterilization plots obtained using S. cerevisiae (yeast) with (A) sawtooth electrode (B) comb-like electrode.
Earlier sterilization trials were conducted using the sawtooth configuration. However, with extended usage, the sawtooth configuration began posing a problem, which is why a new comb-like electrode was designed and implemented.
Figure 6.
Sterilization plots obtained using E. coli as the test pathogen using (A) FR4 dielectric (B) semi-ceramic (SC) dielectric.
The former achieves complete sterilization, starting from an initial concentration of 107 cfu in 90s, while the latter achieves the same in 120s.
Figure 7.
Sterilization plots using G. stearothermophilus as the test pathogen.
FR4 dielectric was used for these tests. Complete sterilization, starting from an initial concentration of 106 cfu was obtained in 20 min.
Figure 8.
Spectral signature of (A) a clean FR4 device (B) an inoculated FR4 device i.e. a device on which 20 µl of E. coli was applied uniformly.
Spectral signature was recorded every 10s. Devices were powered for a total of 2 minutes. Only spectra at 30s, 60s, 90s and 120s are shown. Y-axis lists emission intensity in arbitrary unit.
Figure 9.
Expanded version of the spectral signature of (A) a clean FR4 device (B) an inoculated FR4 device.
This is an expanded image of Figure 8, wherein, the intensity peak in the wavelength range 330–350 nm is depicted to highlight the intensity difference between the clean and inoculated case at different sampling times (30s, 60s, 90s, and 120s).
Figure 10.
Variation of the maximum ozone levels w.r.t the volume of the acrylic enclosure.
Four plots are shown. Each plot shows the maximum ozone level noted in each enclosure for the respective device. “FR4-clean” shows this plot for a clean FR4 device, generating plasma for 1 minute. “FR4-inoc” shows this plot for an inoculated FR4 device, generating plasma for 1 minute. “SC-clean” shows this plot for a clean semi-ceramic (SC) device, generating plasma for 1 minute. “SC-inoc” shows this plot for an inoculated semi-ceramic (SC) device generating plasma for 1 minute.
Figure 11.
Depicting the trend of ozone production and dissipation for four different acrylic enclosures of different volumes using A) Clean FR4 devices (B) Inoculated FR4 devices.
These devices were powered for 1 minute. Ozone data is sampled every 10s.
Figure 12.
Variation of the input power over time for clean and inoculated FR4 and SC devices.
Input power was sampled every 15s over a 2 minute interval for all four cases. Devices were powered during the entire 2-minute interval.
Figure 13.
Comparison of surface temperatures during plasma generation for clean and inoculated FR4 and SC devices, measured using an infrared camera.
Devices were powered for 2 minutes.
Figure 14.
Preliminary SEM studies depicting the appearance of the dielectric substrate in (A) a fresh unused plasma device (B) a plasma device that has been powered continuously for 30 minutes.
The devices shown have been imaged at a magnification of 500×. Comparing (A) and (B), it is evident that while (A) shows a fresh dielectric surface, (B) shows a degraded dielectric surface, wherein it appears that the top-layer seems to have eroded away, thus displaying the underlying fibers. A more detailed study of this degradation is ongoing and will be reported later.
Figure 15.
Sterilization plots for inoculation volume = 40 µl of E. coli.
Figure 16.
Plot of pH values, obtained by rinsing devices with Millipore water after plasma generation and measuring the pH value of this water in each case.
Both FR4 and SC dielectrics are compared. pH values do not change as much in the case of SC, as compared to the case of FR4.