Guimin XU (許桂敏),Yue GENG (耿悅),Xinzhe LI (李昕哲),Xingmin SHI (石興民) and Guanjun ZHANG (張冠軍)

1 School of Electronics and Control Engineering,Chang’an University,Xi’an 710064,People’s Republic of China

2 State Key Laboratory of Electrical Insulation and Power Equipment,Xi’an Jiaotong University,Xi’an 710049,People’s Republic of China

3 School of Public Health,Xi’an Jiaotong University,Xi’an 710064,People’s Republic of China

Abstract This study investigates the influence of two types of target,skin tissue and cell culture medium,with different permittivities on a kHz helium atmospheric pressure plasma jet(APPJ)during its application for wound healing.The basic optical–electrical characteristics,the initiation and propagation and the emission spectra of the He APPJ under different working conditions are explored.The experimental results show that,compared with a jet freely expanding in air,the diameter and intensity of the plasma plume outside the nozzle increase when it interacts with the pigskin and cell culture medium targets,and the mean velocity of the plasma bullet from the tube nozzle to a distance of 15 mm is also significantly increased.There are also multiple increases in the relative intensity of OH (A2Σ→X2Π) and O (3p5S–3s5S) at a position 15 mm away from nozzle when the He APPJ interacts with cell culture medium compared with the air and pigskin targets.Taking the surface charging of the low permittivity material capacitance and the strengthened electric field intensity into account,they make the various characteristics of He APPJ interacting with two different targets together.

Keywords:atmospheric pressure plasma jet,interaction,cell culture medium,skin tissue,wound healing

1.Introduction

Over the last few years,atmospheric pressure plasma jets(APPJs) have emerged as a promising tool and have been successfully used for different applications in many fields,such as agriculture [1],material surface modification [2,3]and medicine [4–9].Many devices have been developed to generate APPJs,differing in structure (number of electrodes,gas flow geometry,etc),driving voltage type (sinusoidal,rectangular,pulsed,etc) and working gas (Ar,He,air,mixture,etc) [10–13].After several experimental and numerical investigations,the physico-chemical mechanisms involved in the initiation and propagation of APPJs expanding freely in the surrounding air,together with the relevant parameters such as gas temperature,electron density,electric field and the distribution of reactive species generated in the plasma plume,are now well understood [14–16].

However,some researchers have observed several differences that occur when APPJs impinge on a target substrate.The presence of a target surface not only influences the flow structure of APPJs,but also significantly affects the plasma properties [17,18].In order to promote the application of APPJs,the complex phenomena when plasma plumes interact with various materials have been studied and the underlying mechanisms explored.Among those experimental investigations,Robertet al[19] suggested that the introduction of a metallic target played a key role in the Ne or He gas flow at the tube outlet when the plasma was switched on through schlieren visualization,which might have an important impact on the application of APPJs in biomedicine.Kovacevicet al[11] carried out a complex study of discharge development,electric field measurement,spatial distribution of the excited species and schlieren imaging,and these experimental results showed that the APPJ plasma–liquid interaction could be prolonged when the gas composition between the jet nozzle and the target was chosen properly.Sobotaet al[20]revealed that when a dielectric target was placed closer than the maximum length of the freely expanding jet,the electric field strength was only enhanced in the vicinity of the target,typically between 0.3 mm and 2 mm above its surface.By means of a computational model,Bredenet al[21]studied the role of proximity (from the gas exit tube to the target distance),target thickness and dielectric properties on APPJ plasma–surface interactions.They revealed that if the thickness of the target (grounded from the back side) was small enough,the streamer would contact and then propagate adjacent to the surface until the fraction of ambient air was great enough to quench the ionization wave.Wanget al[22]used a two-dimensional time-dependent fluid model including 53 reactions to study the discharge dynamics of an APPJ impinging on a dielectric surface.They found that the major ionization reactions in the streamer head were Penning ionization and electron impact ionization of helium atoms,while Penning ionization was the only dominant contributor along the streamer body.In addition,the plasma bullet velocity along the dielectric surface was 10–100 times lower than that in the plasma column.

In the most prominent applications of plasma medicine,including bacterial inactivation,tumor inhibition,wound healing and so on,researchers have found that targets such as bacteria,cells and tissue are always in a humid environment.For example,bacteria and cellsin vitrogrow in a specific culture medium,human tissue contains plenty of tissue fluid and the surface of wounds is also covered by a liquid film[23,24].As a result,it is inevitable that a plasma first interacts with liquid or moist tissue and then performs its function when cells and tissue are treated with an APPJ.Due to the totally different parameters (permittivity,conductivity,material or state)from ambient air,APPJs will exhibit completely different characteristics when they interact with liquid or tissue[25,26].Particularly for the application of APPJs in the promotion of skin wound healing,including in bothin vivoandin vitrostudies,the solid and liquid targets always interact with the cold plasma directly.In these studies,either agarose or gelatin is chosen to model the biological tissues[27,28].However,due to the large variation in structure and constituents,these two models could be different from real biological tissue.Considering its easy availability as well as its similarity to human skin tissue,real biological tissue such as pigskin could be used directly in plasma medicine [29].In addition,Dulbecco’s modified Eagle’s medium (DMEM),which is widely used for culturing cellsin vitro,is now employed to generate the plasma-activated media (PAM)[30].Up to now,most publications have focussed on the type of liquid or solid target treated by APPJs,and there are few studies reporting two different kinds of target treated by APPJs under the same experimental conditions [17].In order to preferably control and manipulate APPJs in wound healing applications in medicine it is necessary to study the characteristics of the plasma jet and underlying mechanisms when it interacts with two kinds of widely used targets including the skin tissue of living animals and the cell culture medium employed to grow cells.

In this present study,a kHz He APPJ is used to treat two kinds of target,and the basic optical–electrical characteristics,initiation and propagation together with the emission spectrum of the He APPJ as a freestanding jet,during interaction with skin tissue and cell culture medium are investigated.Furthermore,the differences in the characteristics of the APPJ under the same experimental conditions are compared,and the underlying mechanisms are revealed.This work aims to provide a further understanding of APPJ plasma–solid/liquid interaction,and also to provide some supporting information for controlling the working parameters of a He APPJ when used for wound healing applications.

2.Experimental setup

2.1.He APPJ device and treated targets

The APPJ device used in this work is the same as in references [31,32] and is shown in figure 1.As indicated,it consists of a quartz tube with an inner radius of 1 mm and an outer radius of 2 mm.Transparent indium tin oxide-polyethylene terephthalate (ITO-PET),1 cm wide,is wrapped outside the tube as the electrodes,and the distance between the electrodes is fixed at 16.5 mm.The distance between the powered electrode and the tube nozzle is 10 mm.The flow rate of the working gas (helium,99.9999%) is adjusted through a mass flow controller (D08-4F,Sevenstar Electronics Co.Ltd).The plasma jet is operated vertically for treating different targets,which are put in the same culture dish(inner diameter 34 mm,height 10 mm) on a grounded metal plate support in order to diminish the impact on the characteristics of the APPJ.

The electrodes are connected to a resonant high-voltage power supply(CTP-2000 K,Suman Electronics Co.Ltd),and the discharge takes place with a sinusoidal waveform at 39.5 kHz under various voltage amplitudes.In order to reduce the influence of excessive gas flow and discharge formation on the shaking liquid surface to obtain a stable working plasma jet,the helium flow rate has a constant value of 3 l min?1and the distance from the tube nozzle to target surface is fixed at 15 mm.

In this study,DMEM and fresh pigskin were chosen as the liquid and solid targets,respectively.The moisture content of fresh pigskin was determined to be about 33% using a digital skin moisture tester (SK-08,Ruierbabi Armarium Co.Ltd),which is the same as that of normal human skin tissue[33].Compared with a freestanding jet,the dielectric properties of the treated target are vital for APPJ characteristics and applications,and the permittivities of pigskin and DMEM were measured as mentioned in our previous study [24].Because the pigskin mainly consists of epidermis and dermis with plenty of cells,keratin,collagenous fiber and so on,its ε′(symbol of permittivity) ranges from 5.5 to 6.6 when the testing frequency is 39.5 kHz.This result coincides with that in Gabrielet al[34],and the ε′ of DMEM is about 27 500 under a testing frequency of 39.5 kHz.This is mainly because the medium is a kind of conductive liquid containing water,amino acids,carbohydrates,inorganic salts,vitamins and other substances.In general,pigskin and DMEM are composite dielectric materials,and their dielectric properties may be different from those of common materials.

2.2.Systems for measuring the electrical and optical characteristics of an APPJ

As depicted in figure 1,the applied voltage between the two outer electrodes is measured using a high-voltage probe(P6015A,Tektronix Inc.) with different peak–peak valuesUp-p.The current through the grounded electrode is obtained via a current probe (Pearson 2877,Pearson Electronic Inc.).The two signals are recorded by a digital oscilloscope(104MXs-B,Lecroy WaveSurfer).Then the power of He APPJ inside the tube is calculated from the measured applied voltage and current.

In order to compare the different discharge patterns,pictures of the the He APPJ are captured using a digital camera (Nikon D7000,Nikon Corp..) with an exposure time of 0.1 s.The length of the jet is visually measured with a mounted ruler from the exit of the quartz tube to the visible tip of the plasma jet.In order to explore the initiation and propagation characteristics of the APPJ,an intensified chargecoupled device(ICCD)camera (iStar334,Andor Technology Ltd) is used with an exposure time of 100 ns and a gain of 4095 to observe the discharge dynamics.Finally,the emission spectra are acquired via an optical fiber placed perpendicular to the jet flux at a distance of 10 mm and coupled with an optical spectrometer (Mechelle 5000,Andor Technology Ltd).The range of the spectrometer allows measurements from 200 nm to 900 nm with a resolution of 0.06 nm.In order to obtain the spatial evolution of the optical emission along the plasma plume,the optical fiber is mounted on a translation rail with a displacement step of 10 mm.Experimental measurements were carried out in triplicate and the standard deviations calculated.

3.Results and discussion

3.1.Photographic observations of a He APPJ

Figure 2 show pictures and the length of the He APPJ as a freestanding jet under different applied voltage amplitudes.In figure 2(a),a reverted conical type plasma plume with a purple color is visible outside the tube.Due to Penning ionization between the excited He atoms and nitrogen in the air,the outside plasma jet is enhanced and moves forward.However,the jet then ceases because the excess nitrogen in air quenches the plasma discharge [14,18].WhenUp-pis adjusted from 5 kV to 7.5 kV,the jet outside tube becomes brighter and its length increases from 13.03±0.047 mm to 23.1±0.047 mm gradually and is finally maintained at 24.04±0.045 mm (figure 2(b)).

Figure 2.Pictures and length of the He APPJ as a freestanding jet under different applied voltages:(a)APPJ pictures,(b)outside jet length.

The underlying mechanisms have been revealed as follows [35].The jet length mainly depends on the diffusion distance of metastable particles and electrons in air,which are generated in the discharge space and gain kinetic energy in an electric field.When the applied voltage increases to a certain degree,the electric field intensity between the two electrodes is enhanced to generate more active particles.As a result,there are more diffuse particles through the working gas flow which spread further in the outside air,making the length of the jet longer.However,as the applied voltage increases further,the energy of particles including He ions,metastable He atoms and electrons obtained from the electric field is also enhanced.These particles will continually strike the internal surface of tube and some impurities will be generated which can react with the metastable particles and decrease their effective diffusion distance.The excess nitrogen has a strong quenching effect on metastable He atoms and limits their propagation length in the air.The jet length will therefore remain unchanged in this case.

On the other hand,from figure 2(a)it can be seen that the dielectric barrier discharge (DBD) between two transparent electrodes inside the tube is intense and plasma fills the area with a white color,which results from the collisional ionization among the He atoms.A pink plasma jet with length of about 3 mm can be observed above the grounded electrode inside tube under aUp-pof 5 kV.As the applied voltage increases,the DBD is more severe and the upper jet length becomes longer.

Figure 3 show the different behaviors of the He APPJ when it interacts with pigskin and DMEM targets.In figure 3(a),the jet outside the tube has reached and spreads on the surface of the pigskin at 5 kV.WhenUp-pincreases to 7.5 kV,the jet length is always 15 mm whereas the jet becomes thicker and has a larger contact area with the target surface.This behavior is completely different when the APPJ interacts with a liquid target (figure 3(b)).The jet has also reached the DMEM surface at 5 kV but looks like a luminous fixed tip where the intensity increases mainly in the region close to the liquid surface.Under different voltages,the length of the jet remains unchanged whereas the color changes from purple to white.Because the dielectric constants of the two targets are greater than that of air,the electric field between the electrode and the target surface is enhanced.The enhanced energy of electrons and other species increases the probability of elastic collisions between them and molecules in the air,making the length and light intensity increase when the He APPJ interacts with pigskin and DMEM.Similar observations were made by Koneet al[36].Note that both the DBD between two transparent electrodes and the upper plasma jet inside the tube become more intense whenUp-pincreases,which seems to be the same as for a freestanding jet.

3.2.Electrical characteristics of a He APPJ

The voltage–current waveforms and dissipated power of a He APPJ interacting with different targets were obtained and are displayed in figures 4–6,respectively.These measured waveforms indicate the typical voltage–current waveforms of the DBD between the two electrodes.The current consists of a discharge current and a displacement current,and the capacitive structure of the APPJ makes the applied voltage lag in phase by 90°behind the displacement current.Standing for the effective gas breakdown,the discharge current has a much shorter duration than the applied voltage period(25 μs)and is observed once during every voltage rising (primary) and falling period (secondary).It is known that the discharge current is caused by charge accumulation on the surface of the dielectric tube,creating an electric potential opposite to the applied voltage.Therefore,it limits the increase in the discharge current and prevents the transition from glow to arc.Meanwhile,the surface charges which are accumulated during one half period will be conducive to gas discharge breakdown in the next half period [37].

Figure 3.Pictures of a He APPJ interacting with pigskin and DMEM (0.1 s exposure time): (a) interaction with the pigskin surface,(b) interaction with the DMEM surface.

Figure 4.The voltage–current waveforms and power of a He APPJ as a freestanding jet: (a) Up-p=5.5 kV,(b) Up-p=6.5 kV,(c) Up-p=7.5 kV,(d) discharge power of an APPJ inside the tube under different voltages.

Figure 5.The voltage–current waveforms and power of a He APPJ interacting with pigskin: (a) Up-p=5.5 kV,(b) Up-p=6.5 kV,(c) Up-p=7.5 kV,(d) discharge power of an APPJ inside the tube under different voltages.

From figure 4(a),it can be inferred that the discharge is unstable with a peak current of 2.3 mA under aUp-pof 5.5 kV.WhenUp-pincreases to 6.5 kV and 7.5 kV,the discharge becomes stable and the peak current decreases,as shown in figures 4(b)and(c),indicating that the mode in the DBD area has changed.The dissipated power of He APPJ gradually increases from 0.453±0.004 W to 1.34±0.007 W whenUp-pincreases from 5 kV to 7.5 kV (figure 4(d)).When the He APPJ interacts with the pigskin,there are no obvious changes in current under the three voltages compared with a freestanding jet (figures 5(a)–(c)).Also,the dissipated power of the APPJ increases from 0.454±0.003 W to 1.44±0.008 W under different values ofUp-p,as plotted in figure 5(d).

As the APPJ interacts with DMEM it presents some different electrical characteristics from the other two working states.The DBD is stable and the peak current is 2.9 mA under aUp-pof 5.5 kV (figure 6(a)); the discharge current increases and the peak value reaches 3.2 mA whenUp-pincreases to 7.5 kV.As shown in figure 6(d),the APPJ power ranges from 0.532±0.004 W to 1.577±0.002 W whenUp-pincreases from 5 kV to 7.5 kV; these values are greater than those under the other two working states.These results indicate that the liquid target causes some change in the discharge mode in the DBD area.

3.3.Initiation and propagation of a He APPJ

Furthermore,in order to better understand the preliminary observations in section 3.2,the plasma jet dynamics both inside and outside the tube are obtained.Considering the discharge stability and current repeatability of a He APPJ,Up-pis set to 6.5 kV when the jet interacts with different targets.The ICCD pictures shown in figures 7–9 are taken with an exposure time and gate width of 100 ns;the gate time of the ICCD is synchronized to the applied voltage via a pulsed signal from the oscillator at the very beginning of the positive half period of voltage[38].For convenience,positive discharge and negative discharge refer to the positive discharge current pulse and the negative discharge current pulse in figures 4(b)–6(b),respectively.

Figure 6.The voltage–current waveforms and power of a He APPJ interacting with DMEM: (a) Up-p=5.5 kV,(b) Up-p=6.5 kV,(c) Up-p=7.5 kV,(d) discharge power of APPJ inside the tube under different voltages.

The dynamics of positive and negative discharges in a He APPJ as a freestanding jet are displayed in figures 7(a) and(b),and two different initiation and propagation processes can be seen.At about 23.9 μs,the positive discharge starts from the bottom of the powered electrode in the form of an ionization wave,which looks like a luminous plasma head and moves downstream.At 25.9 μs,the plasma head reaches the tube nozzle,and a slight discharge appears in the lower edge of the grounded electrode together with the DBD area.Then,this ionization wave moves away from the tube nozzle like a ‘plasma bullet’ and finally disappears [14].During the process of its development,the variation in shape and velocity of the plasma head can be observed.For example,the plasma head moves the first 15 mm in 1.6 μs starting from the nozzle,corresponding to an average velocity of 9.4×103m s–1.Then,both the luminous intensity and velocity decrease until the plasma is invisible through the ICCD camera.During this process,the plasma head moves about 4 mm in 0.8 μs,which corresponds to an average velocity of 5×103m s–1.Meanwhile,another‘plasma bullet’starts from the lower edge of the grounded electrode and moves upstream,and an obvious streamer is observed in the area of the DBD.After that,the discharges both outside tube and inside the DBD weaken and almost quench at 28.7 μs.It can be inferred that during the course of the positive discharge the downstream plasma jet starts at the lower edge of the powered electrode and is independent of the DBD inside the tube,whereas the upstream plasma jet originates from inside the DBD at the upper edge of powered electrode.For the negative discharge in figure 7(b),it is composed of a continuous discharge channel,which starts from the upper edge of the powered electrode at 34.9 μs and then spreads towards two sides.Compared with the positive discharge,the negative discharge emissions are less intense and no obvious pictures of a plasma jet outside tube are captured by the ICCD camera.These differences indicate the polar effect of the He APPJ between the positive and negative discharges,which corresponds to the discharge currents in figure 4(b).

In order to investigate the influence of a target on the properties of the plasma jet,either the solid or the liquid target is placed 15 mm away from the tube outlet and similar experiments are performed.Figure 8 shows the initiation and propagation of a He APPJ interacting with pigskin,and its characteristics are quite different according to the polarity.For the positive discharge in figure 8(a),the plasma head passes through the 15 mm between the tube nozzle and the pigskin surface in 1 μs with an average velocity of 15×103m s–1,which is faster than that in free jet mode.After touching the pigskin,the plasma head starts to spread on its surface.On the other hand,the initiation and propagation of a discharge inside the tube are similar in the free jet mode.Also,as displayed in figure 8(b),the results for a negative discharge are the same as those in a freestanding jet.

Figure 7.The initiation and propagation of a He APPJ as a freestanding jet:(a)ICCD images of the positive discharge,(b)ICCD images of the negative discharge.

Figure 8.The initiation and propagation of a He APPJ interacting with pigskin:(a)ICCD images of a positive discharge,(b)ICCD images of a negative discharge.

Figure 9.The initiation and propagation of a He APPJ interacting with DMEM:(a)ICCD images of a positive discharge,(b)ICCD images of a negative discharge.

Next,the initiation and propagation of a He APPJ interacting with DMEM were recorded and are shown in figure 9.For a positive discharge,the plasma head exited from tube nozzle at 27.1 μs and reached the DMEM surface at 27.9 μs with a mean velocity of 18.8×103m s–1,which is faster than in a freestanding jet and the same as for interaction with pigskin.Due to the conductivity and mobility of the liquid target,the plasma head does not spread but converges on the DMEM surface (figure 9(a)).Similarly,the initiation and propagation of the discharge inside the tube are the same as in a freestanding jet and for interaction with pigskin.For the negative discharge shown in figure 9(b),the initiation and propagation of plasma inside the tube change a little,with the conductive and luminous channel in the area of the DBD lasting for a long time.This observation is in accord with the results in figure 6,where the greatest negative discharge current peak value occurs when the APPJ interacts with DMEM.Besides,at 37.7 μs,another conductive and faint channel can be observed between the tube nozzle and the DMEM surface; this also exists at 41.3 μs.Furthermore,it can be seen that,when the APPJ interacts with DMEM,a secondary bullet travels upstream and occupies two-thirds of the gap between the nozzle exit and liquid surface.One possible source for the ignition of this secondary bullet is the local electric field created by the charge deposited on the gas–liquid interface from the first plasma bullet.With the arrival of the falling edge of the applied voltage,the charge deposited on the water surface can reignite the plasma and lead to a new breakdown in the gap [39].These results indicate that,compared with a freestanding jet and interaction with a solid target,an APPJ can have a greater interaction with a liquid target during a period of applied voltage.A target with a lower permittivity is beneficial for generating a horizontal surface ionization wave(SIW),whereas a target with a higher permittivity will make discharge shrink and be helpful for generation of a conductive channel [40].

3.4.Emission spectra of a He APPJ

With the aim of revealing the presence and modification of reactive oxygen species (ROS) and reactive nitrogen species(RNS) generated in the plasma plume outside the dielectric tube,the emission spectra of a He APPJ interacting with different targets are shown.Here,Up-pis also set at 6.5 kV when the APPJ is under different working states.Figure 10 shows the typical emission spectra of a freestanding jet in the position of the tube nozzle,and one can see that besides plenty of excited state He (He I),other emission lines including OH (A2Σ→X2Π),N2second positive band(C3Πu→B3Πg),N2+first negative (B2Σu→X2Σg),Balmer Hαand O (3p5S–3s5S) are also observed [36,41].It is well known that these ROS and RNS have strong oxidizability and play a very important role in the application of APPJs in dermatology.Note that the excited particles and reactive species of a He APPJ interacting with the other two targets(not presented here) are basically the same as those in a freestanding jet but the relative intensities of the spectral lines are different.

The spatial evolution of relative intensity for the different species in a He APPJ interacting with different targets is presented in figure 11.This study emphasizes the spatial evolution of five wavelengths including OH (A2Σ→X2Π,309 nm),N2(C3Πu→C3Πg,337.13 nm),N2+(B2Σu→X2Σg,391.44 nm),He (33S–23P,706.50 nm) and O(3p5S–3s5S,777.53 nm).For the freestanding jet shown in figure 11(a),the relative intensity of N2+increases from the nozzle and is maximum at 9 mm.For N2,a similar trend can be found,with the maximum relative intensity at 12 mm.The intensities decrease in other positions.As for OH,He and O,their relative intensities decrease from the nozzle to the end of the plasma plume.This spatial evolution of reactive species emission for a freestanding jet is in accord with the results of other studies [36,42].

When pigskin is used as the target,no significant changes in relative intensity were observed compared with a freestanding jet.However,the presence of a solid target modifies their evolutionary trend (figure 11(b)).The intensities of all the emission spectra decrease from the tube nozzle to the 12 mm position except for N2.The relative intensity of N2increases from tube nozzle and is maximum at 9 mm; it then decreases until 12 mm.Above 12 mm,the intensities of the five species increase up to the pigskin surface.Compared with the results mentioned above,the use of DMEM as the target increases the spectral intensities of five species from tube nozzle to the target surface(figure 11(c)).The increase in the intensities of five species can be partially associated with the counter-propagating secondary plasma bullet captured by the ICCD.Among these spectra,the OH emission intensity increases gradually from the tube nozzle to the target surface due to the water in DMEM.The intensities of He and O emission spectra decrease from the nozzle to the target surface.But the emission intensities for N2and N2+increase from tube nozzle and reach their maximum at 9 mm and then decrease up to the liquid surface.These experimental results are also in accord with the spectroscopic study by Kovacevicet alinvestigating the effect of a liquid target on a nonthermal helium plasma jet [11].It needs to be emphasized that the emission spectra in this study can only reflect the excited state of a species qualitatively;quantitative measurement of species density such as OH and O can be obtained via the laserinduced fluorescence diagnostics [39].

Figure 10.The emission spectra of a He APPJ as a freestanding jet.

Using a numerical model,Norberget alinvestigated the interaction between a helium jet and materials with different permittivities,including biological tissue and liquids.The dielectric constants of these materials ranged from 1.5 to 80(and essentially infinite for metals) and it was found that the plasma properties including speed of the ionization wave,electron temperature (Te),electron density (ne),ionization source term (Se),magnitude of the electric field (E) and reactive species density (OH,O and N2*) all increased between the electrode and target surface.Our experimental results correspond with their study [16].That is because the enhancedEwill increase the energy of electrons and speed up the streamer moving forward.Besides,the enhanced energy of electrons and species increases the probability of elastic collisions between them and H2O,O2,making the intensities of OH and O emissions increase when He APPJ interacts with pigskin and DMEM.

Finally,we can make the following conclusions regarding the experimental results in this study and the influences and underlying mechanisms of two targets on the characteristics of a He APPJ compared with the free jet mode.On the one hand the size of plasma head increases horizontally when the APPJ interacts with pigskin,which results from surface charging of the capacitance of this low-permittivity material.This capacitance charging can generate a lateral electric field to sustain ionization on the pigskin surface and form a SIW.DMEM,is similar to a metal and its surface cannot accumulate charge easily.As a result,no capacitance charging occurs when the plasma head reaches the surface,which means that a horizontal component of the electric field and a SIW cannot be established [35].Moreover,when interacting with the targets,the molar fraction of the working gas (He)may become more horizontal,which strengthens the relative intensity of the reactive species in the plasma jet head together with the enhanced electric field.On the other hand,as a liquid,when DMEM is treated by a APPJ,shaking of its surface will occur due to the gas flow,ionic wind and acoustic wave [11].Then this random vibration wave reacts with the gas flow and causes turbulence,which makes the plasma jet outside the tube different from the freestanding jet and from interaction with the pigskin.

Figure 11.Spatial distributions of different emissions for a He APPJ:(a)freestanding jet,(b)interaction with a pigskin target,(c)interaction with a DMEM target.

4.Conclusion

In summary,this study presents the characteristics of a kHz Ne APPJ when it interacts with two kinds of targets widely used in wound healing.From the photographic observations,it can be seen that the diameter and intensity of the plasma plume outside the nozzle are increased when an APPJ interacts with pigskin and DMEM,and the mean velocity of plasma bullet from the tube nozzle to a distance of 15 mm is also significantly increased according to ICCD camera images.The modifications of these mechanisms by introducing targets in front of the plasma jet were investigated according to the nature of target.Moreover,emission spectroscopy studies were also carried out in order to identify the reactive species in the APPJ under different working conditions,with the highest relative intensity of OH (A2Σ→X2Π) and O

(3p5S–3s5S) being observed at a position 15 mm away from nozzle when a He APPJ interacts with DMEM.Finally,the mechanisms of the influence of the two targets on the characteristics of a He APPJ were analyzed through the surface charging of materials with different permittivities.This study could be beneficial for adjusting and controlling the properties of He APPJs in wound healing applications.

Acknowledgments

This work was supported in part by the Scientific Innovation Practice Project of Postgraduates of Chang’an University(No.300103714007),the Fundamental Research Funds for the Central Universities (No.300102329301) and National Natural Science Foundation of China (No.51677146).

ORCID iDs