Atmospheric plasma created in air are known to kill bacteria. The killing effect is not fully understood but it results from the very reactive nature of the plasma. To explain this the typical atmospheric plasma is a non-equilibrium plasma. This means that the electrons are accelerated by the strong electrical field. Because the electron is lighter, it does not exchange energy with the background gas very efficiently. The electrons remain hot, a million degrees is quite possible while the gas temperature remains at room temperature. The chemistry is driven by the higher temperatures and the electron temperature is the important factor. So with a plasma we get the very high chemical reactivity but a low gas temperature.
In this paper the authors apply a 5kV to a range of needles. The power supply is derived from a battery operated system making the apparatus portable. An array of 12 stainless steel needles is used as an electrode. The radius of the needle tips is ∼50μm, the diameter of a human hair. Ballast resistors, 50MOhm are used to limit the discharge current using Ohms law I= V/R. This is made to minimize the plasma heating and electric shock effects on the human body and to make it safe to touch. So, although the voltage at 5kV could be lethal, the ballast resistors prevent the current reaching a dangerous level. The plasma gas temperature is measured within the 20–28 ◦C range, which is very close to room temperature.
The discharge voltage is 5kV and current reaches 5-6mA. The discharges lasts a few hundred nanoseconds with a repeat rate of approximately 20 kHz. It appears that this is due to discharging of a capacitor after the ballast resistors. The capaitor is charged up by the current through the ballast resitors (100uA) and when the voltage on the cpacitor reachs 5kV the plasam is created and discharges the capacitor and quenches the plasma. The peak current reaches 5-6mA. The power dissipated into the plasma was estimated to be ∼60mW. The plasma was able to penetrate 17 layer of a bio film (25.5μm-thick) and exert a bactericidal effect.
This paper shows that a hand wash or wound treatment solution is possible. The system is compact and should be in-expensive. We certainly will see this type of technology appear on the market in the next few years. While the air-plasma will create mostly short-lived, unwanted species such as ozone, the density and toxicity should be much less that of chemical cleaners.
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