The Interaction of Atmospheric Pressure Plasma DBDs and Jets with Liquid
Covered Tissues: Fluxes of Reactants to Underlying Cells
Wei Tian, Seth Norberg, Natalia Yu. Babaeva and Mark J. Kushner
University of Michigan, 1301 Beal Ave, Ann Arbor, MI 48109-2122 USA
[email protected], [email protected], [email protected], [email protected]
Abstract: When treating tissue in biomedical applications, atmospheric pressure plasmas
are typically not in direct contact with the tissue. Rather the plasma first contacts an overlying thin layer of liquid, typically water having dissolved gases and organic matter, such
as blood-serum. The plasma generated reactivity is filtered by the liquid prior to reaching
the tissue. We discuss results from a computational investigation of dielectric barrier discharge and plasma jet treatment of tissue through thin water layers.
Keywords: atmospheric plasmas, DBDs, jets, liquids, plasma medicine, modeling
1. Introduction
The use of atmospheric pressure plasmas for treating
human tissue often involves cells and tissue covered by a
hundreds of micron thick layer of liquid. This layer
could be, for example, blood serum consisting of water
with dissolved proteins and gases. The reactive species
produced by the plasma (radicals, ions, photons) must
penetrate through the liquid layer or react with the liquid
before this reactivity reaches the underlying cells. The
liquid has a finite vapor pressure and so evaporates into
the adjacent air, which likely saturates the air directly
above the liquid with water vapor. The plasma then interacts with this high humidity region prior to reaching
the liquid. Biological liquids are not pristine water –
they contain organic matter and dissolved gases. The
reactivity of plasma produced radicals and ions with the
liquid, and the transmission of reactivity to the underlying tissue, can be a sensitive function of these dissolved
materials and gases.
In this paper, we discuss a computational investigation of the interaction of atmospheric pressure dielectric
barrier discharges (DBDs) sustained in air and plasma
jets with liquid layers in contact with tissue. The liquid
layer is water with dissolved gases (here, O2) and hydrocarbons. We find that liquid layers of only a few hundred microns are sufficient to dramatically change the
character of the reactive species produced in the gas
phase prior to reaching the underlying tissue. The
fluences of the reactive species to the tissue are sensitive
functions of the UV/VUV radiation produced by the
plasma and which is incident onto the liquid layer, the
organic content within the liquid and dissolved gases.
2. Description of the Model.
In this investigation, we used the modeling platform
nonPDPSIM, which addresses plasma, photon and neutral transport on a 2-dimensional unstructured mesh.[1]
Poisson’s equation is solved for the electric potential
coincident with the electron energy equation using rate
coefficients obtained from stationary solutions of Boltzmann’s equation. Navier-Stokes equations are solved for
neutral flow. In this model, the liquid is conceptually
treated identically to a gas phase plasma – it merely has
a neutral component with a liquid density. The transport
of gas phase species into the liquid is governed by Henry’s law that specifies the saturated density of the gas
component at the surface of the liquid. We also allow
evaporation of liquid vapor into the gas phase. This is
accomplished by holding the gas phase density of the
evaporating species at the saturated vapor pressure at the
surface of the liquid, and allowing diffusion to naturally
transport the species into the gas phase. The reaction
mechanism in water was developed from many sources,
including the environmental chemistry literature.[2]
Radiation transport in nonPDPSIM is addressed
using a propagator – Green’s function technique. In this

technique, the rate of absorption at r for by species j of

photons emitted by species i at site r is

Rj r

Ai N i r e

Nj r
ji
i

r

r

4 r

Nk r
k

r
2
jk

dr

d 3r
where ij is the photo-absorption cross section and Ai is
the Einstein coefficient. The integral along the propaga
tion path accounts for absorption prior to reaching r .
This technique is directly applied to photons generated in
the gas phase incident onto and into the liquid. Photon
transport from the gas phase into the liquid is naturally
accounted for by the line-of-sight integral for absorption
along the mean-free-path. In these results, we do not
account for reflection or refraction at the gas-liquid
boundary. We are only including deep UV or VUV radiation in our transport model, which has a mean free
path for absorption in the water of only a few microns,
and so refraction effects will be small.
The base case consists of a DBD sustained in atmospheric pressure air which is initially dry having a
gap of 1.5 mm between the dielectric covered electrode
and the surface of the water. The water layer is 200 m
thick on top of a tissue like dielectric. The effective
ground plane is 2 mm under the tissue. The water has 3
ppm dissolved O2. The water evaporates into the dry air
with a vapor pressure of 27 Torr at the surface of the
water. The applied voltage is -18 kV. The water is first
allowed to evaporate for 10 ms. Three discharge pulses
are then simulated at 100 Hz (10 ms separation) followed by a 1 s afterglow. In select cases, there is 30
ppm of an alkane-like hydrocarbon, designated as RH.
Figure 1 – Positive ion densities during and after the
third discharge pulse. (left) Total positive ion density
in the gas phase at 6 ns into the pulse. (top enlargement) H2OL+ at 10 ns in the liquid layer. (bottom enlargement) H3OL+ at 40 ms.
3. DBDs Incident onto Liquid Covered Tissue
Positive ions during and after the third discharge
pulse are shown in Fig. 1. (The enlargements in the figure show the water layer above the tissue.) The total
positive ion density in the gas phase is shown near its
peak density (5 1014 cm-3) at about 6 ns into the discharge pulse. H2OL+ is shown near the peak of its density (3 1015 cm-3) at 10 ns. (The subscript L denotes a
liquid species.) Positive ions which diffuse into the water produce H2OL+ by charge exchange with the equivalent of gas kinetic rate coefficients. This corresponds to
a mean free path of at most only a few microns. A major
contribution to H2OL+ is photoionization by UV/VUV
radiation produced in the gas phase. The cross section
for photolysis and photoionization of H2OL is near 10-17
cm2, and so at liquid densities, the photolysis also occurs
within microns of the surface. The H2OL+ then quickly
reacts with H2OL (again, with nearly the equivalent of
gas kinetic rate coefficients) to form hydronium, H3OL+
and OHL. H3OL+ is a relatively stable species which then
diffuses through the water layer to the tissue below. Its
peak density at 40 ms in the afterglow (2 1015 cm-3) is
nearly the same as for H2OL+.
Figure 2 – OH, OHL and H2O2L during and after the
third discharge pulse. (left) OH density in the gas
phase at 10 ns into the pulse. (top) OHL at 10 ns in the
liquid layer. (bottom) H2O2L at 70 ms.
Hydrogen-peroxide is an important biocidal species
in plasma activated water. H2O2L is dominantly a product of reactions between two OHL radicals. The densities of OH, OHL and H2O2L are shown in Fig. 2 during
and after the 3rd discharge pulse. The gas phase distribution of OH is quite broad due to the accumulation and
transport of unreacted OH from the previous discharge
pulses and afterglows. The density of OH increases as
the water surface is approached due to the higher density
of H2O vapor resulting from evaporation. The density of
OHL is maximum at the end of the discharge pulse at the
surface of the water. This early peak is due to the photodissociation of H2OL by plasma produced UV/VUV
fluxes and the charge exchange of H2OL+ with H2OL
which forms H3OL+ and OHL. The lifetime of OHL is
relatively short as it is rapidly consumed by formation of
H2O2L. Since in the absence of organic matter H2O2L is
relatively stable in water, its density accumulates from
pulse to pulse as the H2O2L slowly diffuses through the
water to the underlying tissue.
The possible importance of photolysis in producing
reactive species in the water is shown in Fig. 3 where the
densities of H2O2L and O3L are plotted as a function of
time 25 m below the surface of the water. Values are
shown with and without photolysis reactions during the
three discharge pulses and up to 0.15 s into the afterglow. The formation of H2O2L is dominated by OHL
whose production is in turn significantly enhanced by
photolysis. In the absence of photolysis, H2O2L is
formed by gas phase OH or H2O2 that diffuses into the
water and is 100 times lower in density than with photolysis. This is a general trend for any species whose
origin can be traced to either OHL or HL. In contrast,
O3L is formed either by diffusion of O3 from the gas
phase into the water or reactions of OL with O2L in the
water. Since neither of these processes is sensitive to the
Figure 3 – Densities H2O2L and O3L during and after
the three discharge pulses. Values are shown 25 m
below the surface of the water with and without photolysis reactions. The times of the 3 pulses are noted
by arrows.
production of OHL or HL by photolysis, the density of
O3L is essentially the same with or without photolysis.
The presence of organic material in the water can
significantly alter the reactive species. The reaction of
potentially highly oxidizing species such as OH, H2O2
and O3 with organic molecules in the gas phase is nearly
gas kinetic. In most cases, the reaction sequence begins
by hydrogen abstraction from the organic species by the
oxidizing species – for example,
OH
RH
R
H 2O .
Here, RH represents an alkane-like hydrocarbon and R
represents an alkyl-like radical. Note that similar reactions with aromatic or cyclic organic compounds also
have nearly gas kinetic rate coefficients. The analogous
reactions in water are also the equivalent of being nearly
gas kinetic.[2] These reactions are the basis of many
water purification systems.
The densities of O3L are shown in Fig. 4 as a function of time 25 m below the surface of the water and 25
m above the tissue. Values are shown with and without
30 ppm of RH through the 1 second afterglow. In the
absence of RH, O3L is relatively unreactive. After the
discharge pulses and the diffusion of O3 into the water
from the gas phase, the density of O3L becomes fairly
uniform in the water layer. This uniformity is indicated
by the densities of O3L at the top and bottom of the liquid
layer converging. With RH, the density of O3L at the
tissue is significantly reduced due to reactions with RH
as the O3L diffuses through the water layer. Note that
O3L is relatively slow reacting compared to other reactive
oxygen species (ROS).
Ultimately, it is the reactants that survive traversing
the liquid layer or are produced during the traversal that
Figure 4 – Densities O3L during and after the three
discharge pulses and 1 s into the afterglow. Values
are shown 25 m below the surface of the water and
25 m above the tissue; with and without 30 ppm RH.
Figure 5 – Fluences of (top) ions and (bottom) neutrals to the underlying tissue after 1 s exposure following the three discharge pulses. Values are shown for
the (blue) base case, (red) without UV/VUV photolysis and (green) with 30 ppm RH.
are responsible for treating the underlying tissue. As a
measure of this treatment potential, the fluxes of reactants to the tissue and cells underlying the liquid were
integrated for 1 s after the discharge pulses. The resulting fluences (cm-2) for the base case, without UV/VUV
fluxes and with RH are shown in Fig. 5.
The fluences of positive ions to the tissue are dominated by hydronium (H3OL+) in all cases. It is likely that
all positive ions striking the surface of the water produce
H2OL+ which quickly forms H3OL+ in addition to the
photoionization of water. H3OL+ is essentially a terminal
species once formed. It is possible that H3OL+ may
charge exchange to low ionization potential RH species,
a process not included here. The fluences of negative
ions are more sensitive to the details of the reaction
mechanism than positive ions. For example, in the absence of UV/VUV fluxes the fluence of O3L- decreases
by a factor of nearly 300. This decrease is ultimately a
consequence of the decrease in the production of OHL in
the absence of photolysis. The reaction sequence is:
e
H 2 OL
eL
H 2 O2 L
OH L
OH L
OL
O2 L
O3 L .
Here eL is a solvated electron (an electron loosely bound
to water molecules). In this particular example, the production of O3L- is facilitated by the presence of dissolved
O2L in the water. In the absence of dissolved O2L the
fluence of O3L- further diminishes. The fluence of neutral species that trace their origin to production of OHL
are generally reduced in the absence of the photolysis of
H2OL, the most important of these species being H2O2L.
Species that are not sensitive to the production of OH L,
including the majority of reactive nitrogen species
(RNS), change little in the absence of UV/VUV photolysis. An exception is NO2L- which increases in the absence of OHL production as it charge exchanges with
OHL to form OHL-.
With 30 ppm of RH, the fluences of ROS to the underlying tissue are significantly changed. Nearly all of
the ROS with the exception of O3L and H3OL+ significantly decrease due to the rapid reaction with RH, beginning with simple hydrogen abstraction. The fluence
of O3L is less diminished only because it is less reactive
with RH however over many seconds its fluence would
also decrease relative to non-ROS species. Since O3L is
primarily formed by diffusion of O3 from the gas phase,
it is naturally less sensitive to the liquid kinetics. The
fluences of most RNS species, particularly HNO3L, are
nearly insensitive to the presence of RH. The exception
is NO2L-. The fluence of O3L- increases due to the reduction in the density of H2O2L, which would otherwise react with OL-, a precursor of O3L-.
The lost reactivity to the tissue due to the reduction
in the fluence of ROS is replaced by a large fluence of
R L. We expect that the initially produced R L by reaction of ROS with RH undergoes a series of quasithermo-neutral reactions with other RH species. These
reactions produce other forms of alkyl like radicals or
selectively degrade the RH through chain breaking reactions. The terminal reaction of R L is likely with the
underlying tissue.
4. Extension to plasma jets
The interaction of plasma jets with thin water layers
potentially and significantly differs from DBD treatment.
The acidification of the liquid is likely dominated by direct injection of positive ions (or production of positive
ions by photoionization) in DBDs, and so the acidification
likely more rapid in DBDs than in plasma jets. The remote nature of the plasma jets diminishes production of
OHL in at least two ways – reduced rates of photolysis
and reduced production of H3OL+ through charge exchange of its precursor H2OL+. Reactive species which do
not trace their production to OHL will therefore be more
important, on a relative basis, in jets compared to DBD
treatment. These species include RNS and O3L. The role
of OHL is further diminished by the more rapid gas flow
in jets which disperses the water vapor evaporated from
the liquid surface. These results are sensitive to whether
the plasma bullet produced by the jet extends to the surface of the liquid. If that is the case, the differences between DBD and plasma jet treatment of liquid covered
tissue diminish.
5. Acknowledgements
This work was supported by US Dept. of Energy Office of Fusion Energy Science and the US National Science Foundation.
6. References
[1] Z. Xiong, E. Robert, V. Sarron, J.-M. Pouvesle and
M. J. Kushner, J. Phys. D: Appl. Phys. 45, 275201
(2012).
[2] G. V. Buxton, C. L. Greenstock, W. P. Helman, A.
B. Ross, J. Phys. Chem. Ref. Data 17, 512 (1988).