# Anti-Bacterial Action of Plasma Multi-Jets in the Context of Chronic Wound Healing
## 4 Discussion  
This work reports on the design and operation of multi jets devices based on the plasma gun technology [42; 43]. The first device developed as a \"multi-spot plasma applicator\" and a slightly modified derived version labeled as a \"diffuse mode plasma applicator\" allows for the delivery of 52 mm-sized helium jets in ambient air at a pulse repetition rate of 2 kHz. In the diffuse mode operation, additional plasma volume is generated in between the 52 multi jets. These two applicators have been developed and used to plasma treat bacteria contaminated substrates either 55 mm in diameter or consisting in large 12 \(\times\) 12 cm\({}^{2}\) samples. The main objective was to assess the antibacterial action of such multi jets for various bacteria, including drug resistant strains. Such plasma potentiality was already reported in the literature, but this work addresses simultaneously various plasma delivery strategies for decontamination applications in the perspective of infected and chronic wounds treatment in hospital centers. Indeed, four main plasma exposure protocols have been experienced and demonstrated: plasma treatment of agar samples right after their inoculation with various single bacteria strain, plasma treatment of mature bacterial lawn grown on agar substrates, plasma treatment of multi-contaminated with three bacteria strains samples, and multi jet treatment in static and scanning modes over extended surface samples. Figure 12: Photo of the delivery of a single jet generated with the low voltage applicator on a patient’s wound included in the pilot tolerance study performed at CHRO, (right) the treatment monitored by means of an IR camera. Figure 13: Normalized colony reduction versus plasma treatment time with the diffuse mode plasma applicator for (**left**) _Staphylococcus aureus ATCC9144_ and (**right**) _Pseudomonas aeruginosa ATCC9027_ strains. The experiments were performed in triplicate with three different initial colony concentrations for each strain. The second class of multi jet device assessed in this work consisted of a bunch of individual millimeter-sized helium jets generated at the lower voltage of 2.5 kV peak amplitude and a higher repetition rate of 5 to 20 kHz. It has been developed to deliver one single jet, and 4, 9 and 13 secondary helium jets. It is reported that for this low voltage applicator, the power and current are quite equally distributed in between each secondary jet (see Figure 6), and that this \"low voltage applicator\" allows for safe and painless multi jet exposure even on uneven, non-flat surfaces (see Figure 2). A single jet low voltage plasma jet representative of this second class of plasma device has been used in a pilot clinical tolerance study performed at the Orleans Hospital (CHRO) on superinfected wound bearing patients. The bacterial decontamination study indicates first that the plasma action is localized in the region impacted by the plasma jets, and it does not diffuse strongly in between the secondary jets regions during plasma jet exposure (see Figure 7). This result is a good indication in the perspective of wound treatment, as the plasma treatment can be restricted to the zone where bacterial decontamination is required, while not exposing healthy tissues. A time exposure dependent inhibition surface is measured on contaminated samples exposed to multi jets. As the diameter of the multi jets is almost constant, when emerging from same diameter channels drilled in the plasma applicators, two main conclusions result from this measurement. First, since the plasma light emission volume remains constant throughout any exposure time, it is concluded that the plasma light emission cross section observed on the sample surface is not the best indicator to predict the inhibition zone diameter. It is proposed that both plasma generated reactive species accumulation and the combination of these reactive species with transient electric fields inherent with the multi-jets delivery may play a role in the bacteria decontamination.