Flexible Cold Atmospheric Plasma Jet Sources

Carles Corbella, Sabine Portal and Michael Keidar, Flexible Cold Atmospheric Plasma Jet Sources, Plasma 6, issue 1, pages 72 to 88, 2023. Reproduced under the Creative Commons Attribution 4.0 International licence stated in the article; the published version is at doi.org/10.3390/plasma6010007. The figures and the summary table are described rather than shown, and the reference-number markers have been removed.

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Abstract

The properties of non-thermal atmospheric pressure plasma jets (APPJs) make them suitable for industrial and biomedical applications. They show many advantages when it comes to local and precise surface treatments, and there is interest in upgrading their performance for irradiation on large areas and uneven surfaces. The generation of charged species (electrons and ions) and reactive species (radicals), together with emitted UV photons, enables a rich plasma chemistry that should be uniform on arbitrary sample profiles. Lateral gradients in plasma parameters from multi-jets should, therefore, be minimized and addressed by means of plasma monitoring techniques, such as electrical diagnostics and optical emission spectroscopy analysis (OES). This article briefly reviews the main strategies adopted to build morphing APPJ arrays and ultra-flexible and long tubes to project cold plasma jets. Basic aspects, such as inter-jet interactions and nozzle shape, have also been discussed, as well as potential applications in the fields of polymer processing and plasma medicine.

Keywords: atmospheric pressure plasma jet; multi-jet interaction; flexible nozzles; morphing sources.

1. Introduction

A non-thermal atmospheric pressure plasma jet is the afterglow from a dielectric barrier discharge projected into ambient air. The flow requires a noble gas or an admixture with reactive gases. The elementary parts are streamers, which consist of dipolar charged structures propagating between anode and cathode. The flowing of guided streamers out of the dielectric-barrier-discharge region conform the plasma column, which transits from filamentary to a diffuse homogeneous mode, typically when a set of adjacent streamers overlap.

The portability and cost-effective characteristics of these sources make them very attractive for many uses, from material synthesis, surface modification and functionalization, through to gas treatments and biomedical applications. The latter discipline has been a popular topic in both plasma physics and medical device development. In particular, the adoption of plasma-jet treatments to address cancer therapy via selective killing of cancer cells has been a breakthrough in plasma science and technology, which has enabled strong synergies between electrical engineers and healthcare laboratories. Although plasma cancer therapy is still a young discipline, there is common agreement that ultraviolet radiation and radicals generated in the mixture of a free jet with air, such as reactive oxygen and nitrogen species, are associated with the successful performance of plasma jets in wound healing and tumor degradation.

There has been substantial progress towards the understanding of fundamental mechanisms in plasma-biomaterials interactions. However, a pending issue is the mechanical adaptation of plasma sources to the targeted surface. The classical setup in plasma-jet instrumentation is based on a pen-shape device, which, once fed with electrical power and gas supply, provides a needle-like discharge with millimetric side range. This is the adequate geometry for local treatments, and its use on extended areas demands programming scanning routines and the assembly of several parallel nozzles to provide a multi-jet outcome. Accommodation of plasma sources on surfaces showing uneven topology has been achieved so far by designing large-area sources based on dielectric barrier discharge. Paper-based plasma sanitizers, knitted warfare gadgets, and low-power flexible dielectric-barrier-discharge sources summarize the state-of-the-art equipment in flexible plasma sources for large-area treatments.

It would be desirable to upgrade plasma-jet technology to make it available for uniform treatments over large areas. An important advantage compared with flexible dielectric-barrier-discharge sources is a better control over plasma chemistry and the option of modifying nozzle-sample distances as per required treatment. Figure 1a of the source shows the main configurations of atmospheric plasma sources for surface modification — floating-electrode dielectric barrier discharge, dielectric-barrier surface plasma, and the plasma-jet source — together with a schematic of a jet column and the charged and reactive species it generates. The highlighted jet column shows the main charged and reactive species coexisting with the primary gas source, typically helium or argon. Figure 1b shows images of a plasma multi-jet source aimed at skin treatment. However, this configuration does not guarantee homogeneous influx of plasma species onto the irradiated area. The persisting issue is how to improve the uniformity of plasma-jet treatments over extended surfaces showing arbitrary topography, which is only possible by means of deformable plasma jet arrays or nozzles compliant with target features, such as asperities, trenches, elbows, or holes. This article briefly reviews the main efforts in this respect.

2. Extended and multi-jet devices

The fabrication of flexible plasma-jet devices demands an optimal arrangement of the plasma jet nozzles. Either having sources designed in multi-jet arrays or in special nozzle geometries, one should expect plasma systems operating with important interaction issues and performance depending on the coupling at the plasma-sample interface.

2.1. Multi-jet interactions

An important aspect to consider when designing plasma-jet sources with a multi-jet arrangement is the nature of interactions between adjacent plasma plumes. As concluded from experimental and computational studies, the four key aspects of technological relevance are the discharge voltage waveform, pulsed against sinusoidal alternating current; the timing of each jet; the gas flow rate, one to ten litres per minute; and the inter-nozzle distance, a few millimetres. Two main interactions govern the trajectories of multi-jets: hydrodynamic, which is attractive, and electromagnetic, which is repulsive.

Figure 2 of the source shows a typical setup to study the optical and electrical characteristics of plasma multi-jets, in which both helium and argon jets were studied by measuring signal waveforms and obtaining Schlieren images of the plasma plume fluid dynamics. Here, we only consider free jets. The presence of a target material coupled to the multi-jet requires a separate analysis. Moreover, the memory effect on the surface induced by leftover charges from jet interaction substantially enhances the scenario complexity.

Figure 3 of the source shows optical images of a three-jet array along with the corresponding density contrast images obtained by Schlieren imaging, at different applied alternating voltages and helium flow rates, with an axial jet separation of around 5 millimetres in an asymmetric cross-field, needle-to-ring arrangement. All scenarios evidence multi-jet divergence, whose characteristic angle is larger for the optical emission image due to electrostatic repulsion between the bright plumes. The increase in the divergence angle is observed for lower flow rates and larger voltages. Turbulence phenomena are connected to higher voltages and gas fluxes, which can be explained by the linear dependence of the Reynolds number with the gas linear velocity. The use of heavier gases, for example argon rather than helium, diminishes the divergence angle due to the larger inertia of heavy plasma atoms.

The choice of whether supplying sinusoidal alternating voltage or pulsed rectangular direct voltage waveforms, along with the set amplitude and frequency, determines the spatial charge density of each single plasma jet. Naturally, a larger accumulation of spatial charges enhances repulsive interaction between adjacent plumes due to an increase in the jet potential, thereby contributing to jet deflection. This deflection is ultimately caused by the electrostatic repulsion existing between guided ionization waves or plasma bullets, which are the plasma-jet elementary constituents, and are detectable by high-speed imaging such as the intensified-CCD technique. Such repulsion can be modified by conveniently synchronizing the voltage signal supplied to each jet, which in turn affects the time of flight of the associated bullets. Indeed, Cho and colleagues showed that the interaction potential in a crossed-flow two-plasma beam could be tuned by selecting the same or opposed polarities of the interacting jets, at an incidence angle of ninety degrees.

In contrast with electrical interaction, when the hydrodynamic interaction becomes dominant, the multi-jet system tends to converge into a central jet due to inter-jet attractive forces, which compensate for the electrostatic repulsion barrier. The boundary layer of air formed between freely expanding neighboring jets can become very thin, and, in the limit of close-packed nozzles, such a layer can vanish so that adjacent jets merge into a more intense unified plasma column. Fang and colleagues illustrated this situation by forming one very bright jet out of the merging of all single-participating plasma plumes. In summary, deviations from a straight, collimated multi-jet are expected from the combined hydrodynamic and electrostatic characters of the fluid system. The question of which interaction dominates depends on the specific setup and operating conditions. Hence, source parameters need to be adjusted if the electrostatic and hydrodynamic effects appear unbalanced and a collimated jet bundle is required.

2.2. Plasma-jet nozzle geometries

Besides inter-jet interactions, the shape of the nozzle is a subject of interest in the design of flexible plasma jet sources. In general, the orifice from which a jet flows to open air is cylindrical with a millimetre-sized diameter. This is the case for the channels used to design atmospheric microplasma jet sources. Micro-jets are commonly used in plasma chemistry research, and they serve as inspiration for microfluidic plasmas, which is being consolidated as a research line.

Alternatives to cylindric apertures are nozzles manufactured with a horn-like shape. Although studies on deformable nozzles in operation have not yet been reported, Castro and colleagues have characterized the performance of different horn-like nozzles emitting plasma jets with comparable parameters. In that study, they concluded that the modified area of a polymer surface, in wettability and roughness, systematically exceeds the optically visible interaction area. This result is explained by the presence of non-emitting reactive species in the vicinity of the interaction zone. Figure 4a of the source shows one of the conical jets — a cone-shaped nozzle 64 millimetres in diameter covering a treated vertical object and suppressing air mixing — with the plasma volume adapted to the object being treated.

Another nozzle shape worthy of exploration is the rectangular or slit aperture, which provides plasma afterglows in a two-dimensional laminar form. Planar plasma jets have been explored and show potential applications requiring selected-area plasma processes, sketched in figure 4b of the source. The construction of flexible plasma-jet devices with variable nozzle shapes will be a valuable milestone in cold plasma applications.

3. Morphing plasma-jet arrays

As pointed out in the introduction, many efforts have been made in the construction of flexible plasma source prototypes that operate in ambient air conditions. Some groups mimicked plasma-jet configurations by designing linear arrays of biased pins adaptable to different substrate geometries. Nevertheless, not many authors have reported the design and operation of flexible atmospheric plasma multi-jet sources with an independent control over primary gas and its flow rate.

3.1. Close-packed microplasma multi-jet arrays

Ma and colleagues reported an early approach to a plasma multi-jet source made of a flexible material. The body of the source consisted of a molded silicon block with inserted thin rods acting as electrodes, which were biased by a 20 kilohertz alternating signal at kilovolt-range amplitude. The separation between rods, inter-nozzle distance, and microchannel size were all of the order of 1 millimetre or less. Hence, sub-millimetric nozzles enabled the production of close-packed collimated micro-jets in a square matrix configuration, which ranged from 2 by 2 up to 8 by 8. Up to three rods in parallel arrangement were inserted across the microchannel direction, as shown in figure 5a of the source, whose general and front views give the nozzle arrangement and the relevant distances. This disposition is appropriate to separately modulate the energy and length of different arrays of plasma plumes by applying the appropriate voltage values at selected electrodes. Figure 5b shows the range of discharge voltage and the current associated with this setup. The source characterization was completed by optical emission spectroscopy diagnostics, showing the dominant species in the different positions of the discharge: helium, as a primary gas, was relevant within the discharge source region, while a rich profile of nitrogen lines dominated in open air. A helium back pressure of up to 800 torr was exerted. No jet-to-jet interaction was observable. Finally, this source successfully reduced burn wound size and sterilized drinking water, thereby validating its performance as an efficient healing and bactericide tool.

The production of reactive plasma species based in oxygen and nitrogen formed near the treated sample is enhanced thanks to the interaction of multiple plasma plumes with the surrounding air. Optical emission spectra depicted in figure 6a of the source show how the density of reactive oxygen and nitrogen species dominates over helium atoms and metastables in the afterglow region in the case of a 3 by 3 microplasma array, measured both in the channel and outside in open air. The medical therapy capabilities of the microplasma jet array have been demonstrated by treating burn wounds in animal rat models. The plasma plume array treatment promoted a burn size reduction down to 20 percent of the original size in two weeks, whereas the control sample only decreased to 80 percent of its original size, as plotted in figure 6b for one-minute and two-minute treatments against an untreated control. It was concluded from the tissue histology that the healing process is accelerated through the regulation of anti-inflammatory processing. However, more detailed analysis of gene expression mechanisms, for example via tracking messenger-RNA expressions of inflammatory markers, is pending to complete the study.

3.2. Low-power planar uniform jet arrays

A compact device constituted by six independently actuated discharge cells has been reported by Li and colleagues. The anode-cathode alternating arrangement was operated at an optimized electric field for argon flow. This setup yielded discharges initiated at a breakdown voltage far below the Townsend potential. Such elementary discharges glowed in a non-self-sustained mode at relatively low voltages. Figure 7a and 7b of the source show the basic geometry features of one of the discharge devices of the so-called low-power large-scale uniform laminar plasma jet array, in lateral and top view, with each single discharge controlled by actuating on separate switches. The parallel-connection arrangement of several of these cells was advantageous to send plasma jets with modulated voltage-current characteristics. Although the ceramic housing of the cells is not flexible, the jet striking from each individual anode-cathode segment can be independently sustained by the adequate combination of switches, so that the profile of electromagnetic irradiation and of reactive plasma species can be conveniently adapted to the specific sample topology.

Figure 7c shows flexible arrangements to generate plasma jets using similar power values. The transverse light intensity emitted by the plasma array showed uniform profiles along each cell. In particular, configurations in groups of single, double, or quadruple stable plasma jets could be formed simultaneously by setting the adequate power distribution on an independent switch circuitry. In this way, different zones of a sample surface can be selectively plasma-irradiated with the adequate switching arrangement at a low power cost. Although the impact of the discharge mode variation on the concentration of reactive oxygen and nitrogen species has not been discussed, the basic plasma parameters, such as electron density, are expected to remain constant.

3.3. Deformable plasma-jet platforms

Corbella and colleagues pioneered the use of bendable surfaces as multi-jet emitters. Such pad-like sources are constituted by an array or matrices of one to five nozzles of millimeter size produced onto a silica aerogel foil. Helium plasmas were excited by 15 kilohertz sinusoidal alternating signals of around 10 kilovolts peak-to-peak through copper tape electrodes placed above and below the aerogel region. The spherical shaping of the nozzle aerogel surface provided voltage and current waveforms, and optical emission spectra, qualitatively similar to the flat operation conditions. Therefore, the performance of the plasma multi-jet is not expected to vary upon the changes in the curvature state of the aerogel, which should correspond to the profile of the treated substrate in the final application. Figure 8 of the source shows the multi-jet surface in operation in either concave or convex-shaping modes, at a curvature radius of about 5 millimetres, with the large aperture 1 centimetre in diameter surrounded by a ring of copper tape acting as ground, together with the emission spectrum collected from the flat three-jet configuration. An optical-emission analysis of the multi-jet provided the typical profile of emission lines dominated by nitrogen species, which come from the air mixed with the helium jet column. A jet divergence was clearly observed in flat and bent configurations, and it was attributed to electrostatic interactions between the plasma plumes.

Power values limited to 5 watts were obtained from the analysis of current and voltage waveforms of discharges corresponding to the three-jet utilization. Since the power dissipated by the plasma plumes is a fraction of the total power consumed by the plasma source, that is the dielectric barrier discharge together with the jets, such a configuration may be adequate to build bendable plasma multi-jet prototypes to treat temperature-sensitive samples, such as biopolymers and organic tissues. Furthermore, upscaling the possibilities of this source should be tested by assembling multiple cells managed by independent power supplies.

Following up the study above, identical electric and gas feedings were applied to a low-density polyethylene cylindrical hollow housing to ignite jets in the radial direction. The working principle is schematically shown in figure 9 of the source, whose images show plasma jets ignited from arrays either around the perimeter or along the axial direction of a polyethylene cylinder of 5.4 centimetres outer diameter, with a scheme of the active discharge region and the afterglow region carrying the helium cold atmospheric plasma jets. Briefly, a helium flow was introduced in the axial direction of the cylinder cavity, which had millimetre-size radial channels through the walls. The electric field generated between an axial electrode rod and copper tape glued around the outer perimeter was able to sustain the gas breakdown. The generated jets were produced from the helium flow diverted from axial direction to radial direction in the laminar regime, and they showed electrical and optical behaviors comparable to the aerogel source described above. Although flexing tests of the hollow polyethylene cylinder have not been reported to date, its soft and flexible consistency is promising for applications in which plasma jets need to reach delicate samples placed in locations of difficult access.

There are many possibilities to modify the cylindric source operation. For instance, the number of ejection holes can be increased if required. Simultaneously, the total flow rate should scale with the number of nozzles so that the flow regime does not change. The source can be also rotated around its axis to increase the treatment area and, additionally, to vary the plasma dose on the irradiated sample.

4. Flexible plasma-jet channels

The necessity of treating locations of difficult access, such as in the case of surgical operations or endoscopic applications, motivated the development of plasma jets produced along or at the end of flexible tubes. The basic setup usually consists of a flexible tube or channel coupled to a dielectric-barrier-discharge plasma source.

4.1. Plasma jet at the tube exit

Despite the specific differences in the setups reported in the literature, such as tube length — a few centimetres up to a few metres — and biocompatible tube material, for example polyvinyl chloride and polyurethane, many prototypes involve a plastic tube lodging a thin floating copper wire that prolongs from the discharge source up to a few millimetres before the tube ending. Figure 10 of the source gives the schematic layout of such a flexible tube plasma source for microbial decontamination, a photograph of the polyurethane nasogastric feeding tube used in the experiments, and the jet emerging from the tip of a one-metre plastic tube. In this way, the electrical power can be propagated to the exit of the tube in a transmission line fashion with negligible losses. There is no plasma along the tube, and the jet restricted to the tip can remain near room temperature when operated with the usual parameters, 10 kilovolts of alternating voltage at about 10 kilohertz. Otherwise, without a floating wire, the discharge would occupy the entire tube so that resources to prevent electroshock and plastic overheating would have to be in place.

Kostov and colleagues used an alternating voltage input of 32 kilohertz modulated in 667 hertz power bursts for a better control over the energy delivered to the jet at the tube end. The pulse duty cycle is around 20 percent of the low frequency. Hence, delicate samples, such as human skin or other tissues, may be treated without risk of overheating or burns thanks to a significant decrease in gas temperature. Naturally, the transient states of the discharge will affect the plasma parameters during treatment, and the specific frequency and duty cycle of the periodic signal are selected for each application. This approach has been adopted by other researchers working on long-tube plasma jets for biomedical applications, especially for cancer therapy. Figure 11 of the source shows typical current and voltage waveforms at different timescales, the modulated voltage applied to a flexible capillary with inserted copper wire followed by a detailed view of the voltage and plasma current signals. The asymmetric shape of the current curve is due to the distinct streamer evolution during positive and negative voltage cycles.

A critical issue is the power loss in long dielectric tubes with inserted copper wire. The transmission line model developed by Bastin and colleagues proved excellent to explain the high-frequency electromagnetic energy propagation along this system. Power transmission efficiency is low in open-circuit conditions, that is, when the jet strikes without a target. In this case, leaks occur due to a capacitive coupling between the tube and the atmosphere, as evidenced by the electric current decay measured along the tube. On the contrary, transmission efficiency is maximized in lines loaded with a solid conductive target, whose impedance is significantly lower than the jet impedance. The plume-target gap distance is another important variable for optimal power management.

4.2. Plasma jet along a flexible tube

In the absence of a metallic core wire, as for example in the use of dielectric capillaries with inner diameter below 1 millimetre, the discharge can fully occupy the inner volume of the tube. Such a configuration has been successfully tested in clinical studies, for example in the significant reduction, in living animals, of pancreatic tumour in mice after plasma irradiation via a flexible plasma gun. Figure 12 of the source shows an example of a plasma gun device filled with a neon discharge and a scheme of the discharge generator: the discharge is ignited with a nanosecond pulse driver and leaves the outlet through a 4 millimetre branched glass capillary, which is connected to a silicone capillary acting as a jet guide. In some applications, the tube can be coated with a gold thin film to act as an electrode and to shield ultraviolet radiation from plasma emission as well. Wang and colleagues demonstrated an excellent performance of such a coated channel after bending the capillary two hundred times through minus 180 degrees and plus 180 degrees. Jets operated through capillary tubes must be fed by relatively small gas flow rates, namely of the order of 1 litre per minute or less, to keep a laminar flow regime.

Geng and colleagues explored further geometries of narrow flexible tubes acting as jet sources. Different lengths and diameters of transparent polytetrafluoroethylene tubes were adapted to the tested conditions of high and low values of nitrogen flow rate. Hence, a 10 centimetre tube was appropriate for 6 litres per minute, while a 120 centimetre tube enabled the transport at 18 litres per minute. To prevent gas-heating issues, a vortex tube was added between the electrode and gas supply systems to reduce the plasma jet temperature. Moreover, this system admits several architectural options intended for different applications. Figure 13 of the source shows the flexible plasma jet system operating with different numbers of bifurcations — three bifurcations of differing diameter, then a system with several bifurcations — and as a plasma jet brush with twenty holes in radial mode, the latter being an alternative to the radial source in a hollow polyethylene cylinder discussed above. The enhancement of the wettability of different materials upon plasma irradiation validated the performance of the flexible plasma source to modify surface properties without structural degradation.

It is difficult to predict the maximal length in flexible tubes still enabling an effective jet outcome, given the number of experimental parameters involved, such as tube material and gas flow regime. However, a rough estimation on the order of magnitude can be made. Assuming a longest timescale of about a tenth of a second to a second for plasma chemistry reactions and diffusion processes involving radicals and neutrals, a gas flow rate of 1 litre per minute, and a tube diameter of 1 millimetre — a linear velocity of about 20 metres per second — the theoretical maximal length is the velocity times that time, about 10 metres. This value is consistent with the reported tube devices. Additionally, a gradient in plasma optical emission is expected along the tube due to energy extinction. Therefore, the energy decay rate along the tube length can be monitored by optical emission spectroscopy analysis at different distances from the source.

5. Final remarks

5.1. Summary and discussion

Here, the state-of-the-art research in flexible plasma-jet source design, characterization, and applications has been briefly presented. The discussed prototypes show excellent advantages for many applications, namely room-temperature treatments; adaptation to large-area surfaces containing features such as asperities and microcavities; homogeneous irradiation with reactive neutrals and ions, and ultraviolet photons; and finally, the possibility of working at atmospheric pressure conditions. These properties make flexible jet sources unique for uniform plasma treatments without the inconveniences of vacuum pumping systems and scanning routines to ensure homogeneous plasma fluxes.

Table 1 of the source summarizes the main approaches in the subjects of morphing sources, devices with independent nozzle operation, and plasma jets through flexible tubes. Although the list of references is not exhaustive, the cited works are representative of each type of plasma source. Its four rows read as follows. The close-packed micro-jet source: a silicone polymer body with integrated rod electrodes, 8 by 8 nozzles of 0.35 millimetre diameter, under 5 litres per minute of helium, 20 kilohertz at 1 to 100 milliwatts, aimed at wound healing and drinking-water treatment. The low-power planar discharge cells: ceramic cells activated by independent switching, six cells of 0.5 by 15 square millimetres each, under 15 litres per minute of argon at 8 kilohertz and 5 to 10 watts per cell, aimed at large and uniform plasmas. The flexible plasma-jet platforms: silica aerogel in the flat version and polyethylene in the cylindric one, with external copper tape electrodes, one to five nozzles of 1 millimetre diameter, under 10 litres per minute of helium at 15 kilohertz and 0.1 to 5 watts, aimed at wound healing, surgical margins and surface processing, the last of these proposed rather than demonstrated. The flexible micro-jet tube: silicone or polytetrafluoroethylene, empty or coated, 0.1 to 1 millimetre diameter and a few metres long, under 1 litre per minute of helium, neon, argon or nitrogen with oxygen added to adjust the plasma chemistry, at 1 to 20 kilohertz and 1 to 20 kilovolts, aimed at plasma endoscopy and internal surface processing and machining. A fifth row covers the jet at the end of a flexible tube: polyvinyl chloride or polytetrafluoroethylene with a floating copper wire, above 1 millimetre diameter and under 5 metres long, 1 litre per minute of helium or argon, 5 to 20 kilohertz at 1 to 80 watts, aimed at plasma endoscopy and fungal deactivation.

The above results suggest that a milestone in the design and construction of a flexible multi-jet device could consist of a thin foil enclosing the necessary circuit elements and gas microchannels for an acceptable plasma outcome. Figure 14 of the source sketches the basic concept of a multi-jet plasma shower able to adapt its morphology to any sample shape, drawn against a cross section of skin. The corresponding plasma sources need to prove mechanically flexible, versatile, and efficient performances in surface treatments. For example, in the treatment of skin at body extremities, the plasma-operating parameters should be optimized so that the right dosages of plasma species reach out uniformly to the layer thicknesses of the epidermis, about 40 micrometres to 1 millimetre; the dermis, about 1 millimetre; and the hypodermis, about 10 millimetres. Simultaneously, irradiation doses should lie within the tolerance margins to prevent skin damage.

In fact, human skin constitutes an exceptional laboratory to test the performance of flexible plasma-jet prototypes for gentle healthcare treatments by mimicking complex topologies. The potentials of cold plasma jets in cutaneous biology are numerous and well-adapted to this multiresponse organic tissue. They can indeed exert their activity at different levels of the skin; the length of penetration will promote different types of reaction depending on the treatment. At the superficial level, the epidermis, they promote hydration, acidification, and decontamination of the stratum corneum. Inside the skin, the dermis and hypodermis, via the generated reactive oxygen and nitrogen species, they can penetrate between cells, inside cells, or through skin appendages. They can promote the absorption of other molecules, such as drugs, by loosening the skin barrier. At the molecular and cellular level, plasma jets can influence the oxidation of skin biomolecules, the activation of cellular metabolism, and signaling. Finally, at the tissue level, they can increase cutaneous oxygenation, and stimulate vasculogenesis and the remodeling of the extracellular matrix.

Some of the main technological challenges to be addressed to get uniform plasma treatments of large, uneven areas by compact-morphing sources are mentioned below.

5.2. Outlook

The small number of articles published on the topic of flexible plasma jet sources for room-temperature treatments demonstrates the novelty of such technology. Nevertheless, here we speculate some future directions that research on flexible plasma-jet sources may take in the following years.

The next generation of non-equilibrium flexible plasma jet sources will demand the miniaturization of the driving setup for a more convenient device manipulation. Ongoing efforts involve the manufacturing of small-size piezo transformers to build more compact generators for portable plasma jet devices. Recently, Liu and colleagues fabricated a three-dimensional multi-microhole plasma jet instrument driven with nanosecond-pulsed power for underwater discharges. Its layout could be applied to fabricate a miniaturized version of the soft cylindrical source aimed at plasma-liquid applications.

The development of flexible multi-jet headers for imprinting purposes opens a new field in lithography applications for non-flat surfaces, for instance, in the processing of third-generation photovoltaic systems and other microelectronic architectures. Another interesting approach would be the production of diffuse, and laterally extended and uniform jets through porous flexible structures. Hong and colleagues and Ma and colleagues have studied the formation of diffuse jets through ceramics with open pores, and the next step will involve using a flexible analog to accommodate the porous structure to uneven profiles. An important challenge will be to maintain the processing performance upon the tensile and compressive stresses of the porous structure. Finally, to avoid the degradation of the treated samples, the gas temperature and electron density of the new devices should lie within the tolerances determined by the healthcare and food-processing regulations.

Flexible plasma-jet source prototypes have shown excellent performance so far, and they are promising for industrial steps in which surface modification of soft matter and biomaterials is a must.

Author contributions, funding and data

Conceptualization by M.K. and C.C.; methodology by C.C. and S.P.; investigation by C.C. and S.P.; resources by M.K., S.P. and C.C.; supervision by M.K.; writing of the original draft by C.C.; review and editing by M.K. and S.P.; project administration by M.K. and C.C.; funding acquisition by M.K. All authors have read and agreed to the published version of the manuscript. This research was partially supported by the National Science Foundation through Award number 1919019. No new data were created or analyzed in this study. The authors declare no conflict of interest.