Fabrication and Performance of a Multi-Discharge Cold Atmospheric Pressure Plasma Array

Abstract

Cold atmospheric pressure plasma (CAP) has been shown to kill bacteria and remove biofilms. Here we report the development of a unique CAP array device consisting of a parallel stack of eight linear-discharge plasma elements that create a ~ 5 cm² (2.4 cm × 2 cm) treatment area. The CAP device is fabricated from Low Temperature Co-fired Ceramic (LTCC) layers to create 24 mm long linear-discharge channels (500 μm gap) with embedded opposing silver metal electrodes. A 20 kHz AC voltage (0.5-5 kV) applied to the electrodes generates an Ar/O₂ plasma between the plates, with the gas flow directing the reactive species toward the biological sample (biofilms, etc.) to affect the antimicrobial treatment. External ballast resistors were used to study discharge uniformity in the stacked array elements and internal thick film ballast resistors (≈150 kΩ) were developed to create a fully integrated device. Typical element discharge currents were 1-2.5 mA with the total array current tested at 20 mA to provide optimal device uniformity. The plasma discharge was further shown to produce reactive hydrogen peroxide and exert antimicrobial effects on Pseudomonas biofilms and Salmonella contaminated eggshell samples, with >99% of the bacterial cells killed with less than 60 seconds of plasma exposure.

Keywords: Cold atmospheric pressure plasma; antimicrobial; biofilms; reactive oxygen species.

Fig. 1.

Pictorial of CAP System Configuration showing two mass flow controllers, a HV probe, a current transformer, and the AC high voltage source.

Fig. 2.

Layout of the CAP Electrode substrate showing device dimensions. (a) electrode substrate structural dimensions with gas feed center hole and 4 mounting holes, (b) embedded ballast resistor trace dimensions in combination with the connections in 2(d), (c) electrode substrate layout for a direct-connected electrode, and (d) electrode substrate layout for an electrode connected through ballast resistors (e) spacer that fixes the distance (gap) of the discharge. Gas flows in at the rounded part of the V structure and down toward the target allowing the gas to expand across the element width. The four small holes are for mounting.

Fig. 3.

Exploded view of the plasma array assembly showing a 4-element array. The electrode substrates are separated by LTCC spacer plates. There are high voltage and ground (embedded resistor) plates. Ballast resistors can be embedded in the ceramic material and covered with a thin layer of LTCC. Electrical vias attach to the internal electrode circuit, and silver tape is compressed between the plates to electrically connect electrodes on either side (high voltage and ground).

Fig. 4.

Circuit diagram of plasma electrical configurations showing the plasma resistance (R_p), capacitance (C_p), and the ballast resistance (R_b) for the array parallel electrical configurations with 3 plasma elements.

Fig. 5.

IV curves of a CAP array using external ballast resistors. The current was determined by measuring the voltage across each resistor while the entire array was operating. Measurements were made for 1 kΩ, 100 kΩ, and 200 kΩ resistors.

Fig. 6.

(Left) Photograph of the entire CAP array operating with 8 elements and internal ballast resistors showing the reflection of the discharge in a mirror. (Right) magnified photograph of the array discharges in a darkened room. Total discharge current is 9.5 mA at 0.9 kV and 9 LPM Ar flow.

Fig. 7.

(Left) CAP treatment of 2-day Ps. fluorescens biofilms on steel coupons. (Right) CAP treatment of eggshells inoculated with Salmonella enterica. Data points are the mean of 3 experiments (± standard error).

Fig. 8.

(a) Standard curve of fluorescence (ex 488 nm/em 525 nm) response to hydrogen peroxide concentration. (b) Accumulation of hydrogen peroxide concentration with 8-element plasma array exposure using Ar/O₂ (13/0.5 slm) feed gas and a discharge voltage of 1.45 kV as a function of plasma array exposure time. Data is the mean of 3 experiments (± standard error).