PEO-like films deposited by means of a Dielectric Barrier Discharge
fed with an aerosol precursor
G. Da Ponte1, E. Sardella2, F. Fanelli1, M. Nardulli1, R. Gristina2, R. d’Agostino1,2,3, P. Favia1,2,3
1 Department of Chemistry, University of Bari, via Orabona 4, Bari 70126, Italy
2 Institute of Inorganic Methodologies and Plasma (IMIP) CNR, via Orabona 4, Bari 70126, Italy
3 Plasma Solution Srl, Spin off of the University of Bari, via Orabona 4, 70126, Italy
Abstract: Atmospheric pressure dielectric barrier discharge are becoming a new promising frontier in
material processing also for biomedical applications. In this work, we have combined the advantages
of working at atmospheric pressure mode with an aerosol precursor for plasma-depositing Poly
ethylene oxide (PEO)-like coatings, as it is already done at low pressure. The use of an aerosol
precursor permits higher monomer structure retention in the coating and higher deposition rate with
respect to low pressure systems. Depending on their structure such coatings can be synthesized with
unique resistance to protein adsorption and cell-adhesion in water media, or with swelling properties
for drug delivery systems. The effect of the aerosol concentration, of the total He flow and of the
applied voltage (Va) was evaluated on the chemical composition of the PEO-like films tuning in a
proper way the PEO-character of the deposited film.
Keywords: PEO-like film, non-fouling, dielectric barrier discharges, aerosol
low pressure processes, the interest toward the
1. Introduction
DBDs is growing up also for thin deposition
Plasma technology finds broad applications
from industrial fields to materials processing.
Major of the industrial processes have been
carried out
for
several years
close
to
atmospheric pressure by means of equilibrium
thermal plasmas as arc and plasma torch
(cutting,
spark
erosion,
destruction
of
hazardous waste, plasma spraying), as it is
exhaustive reported in previous reviews [1-3].
For many applications, like the materials
processing, the characteristic high temperature
of the equilibrium plasma is not required and
often even prohibitive. Dielectric barrier
discharges (DBDs), combining the advantages
of atmospheric operational mode with those of
non-thermal plasma, are an useful instrument
in the processing of out-gassing materials as
textiles, membranes, scaffolds, etc. Hence,
films and treatments with biomedical purposes
[4-7], research field traditionally dominated by
low pressure plasma [8, 9]. Inspired by Heyse
et al [4], we have combined the advantages of
working at atmospheric pressure mode with an
aerosol
precursor
for
plasma-depositing
Polyethylene oxide (PEO)-like coatings, as it
has been already done at low pressure [8, 9].
Depending on their structure such coatings can
be synthesized with unique resistance to
protein adsorption and cell-adhesion in water
media, or with swelling properties for drug
delivery
systems.
Tetra
ethylene
glycol
dimethyl ether (TEGDME) was chosen as
precursor, fed in the DBD system as aerosol
with He, used as carrier and as aerosol gas.
The effect of the concentration of the
TEGDME aerosol, of the total He flow, and of
despite the well-established knowledge of the
1
the applied voltage (Va) was evaluated on the
investigated in order to tailor the chemical
chemical composition of the PEO-like films.
composition of PEO-like film deposited in our
The process is aimed to find experimental
DBD system. The effect of the concentration
conditions where a high monomer structure
of the aerosol, i.e. of the active species, was
retention (high density of the CH2CH2-O ether
evaluated by changing the total flow (aerosol
functionalities) and the related non fouling
flow+ carrier flow) from 8 to 10 slm; while the
character [8, 9] could be found in the coatings
TEGDME aerosol was let in the discharge with
a constant He flow rate of 3.15 slm. The input
2. Experiment
Va was varied between 6.5 and 8.5 kVpp, at a
Plasma processes were carried out in an home-
constant
made DBD reactor, schematically shown in
deposition time was of 5 min in each
Figure 1 [10]. The experimental apparatus
condition. Water contact angle (WCA, CAM
consisted of a parallel plate electrode system
200), profilometry (Alpha-step 500 KLA
(4mm interelectrodic distance), enclosed in an
TENCOR), x-rays photoelectron spectroscopy
airtight Plexiglas chamber. Each electrode (50
(XPS,
x 50 cm2 area ) was covered by 1.9 mm thick
performed to characterize PEO-like coatings.
frequency
Theta
of
Probe
26.6
kHz.
Spectrometer)
The
were
70 x 70 mm2 Al2O3 plate (CoorsTek, 96%
purity).
3. Result and discussion
A systematic evaluation of the electrical
Baratron
Carrier F
HV
Aerosol F
characterization
Electrode
Dielectric
Gas inlet
the
discharge
was
performed in each experimental condition
Gas outlet
showing that not significant alteration of the
Needle
valve
Rotary
pump
Atomizer, 3076 TSI
of
current signal occurred if a mixtures of He and
TEGDME is used as feed gas (data not
shown).
Figure 1- Schematic of the experimental apparatus
An
aerosol
(CH3O(CH2CH2O)4CH3,
of
Aldrich,
Table 1: Chemical composition of PEO-like films deposited as
TEGDME
≥
99%
purity) was used as organic precursor. Two
a function of the Va (total flow constant at 9 slm) and as a
function of the total flow (Va constant at 8.5 kVpp ), at a fixed
frequency of 26.6 kHz, for 5 min.
Va
(kVpp)
6.5
Total
Flow(slm)
9
%C-C/H
285eV
24±2
%CO-R
286.5eV
65±1
%C=O
287.9eV
8±2
%COOH/R
289.2eV
1±1
7
9
24±2
61±2
11±2
4±1
7.6
9
34±2
60±1
5±1
1±1
8.5
9
35±2
60±1
4±1
1±1
used as carrier gas (carrier flow) to transport
8.5
8
24±1
70±1
5±1
1±1
the organic precursor into the discharge cell.
8.5
10
37±4
54±1
8±2
1±1
different He flows were used. One constant He
flow (3.15 slm- aerosol flow) allowed to
generate the TEGDME aerosol by a constant
output atomizer (TSI, 3076). The other was
Several
experimental
conditions
were
2
The effect on the chemical composition of the
increasing the total flow rate of the feed (i.e.,
applied voltage and of the relative TEGDME/
by decreasing the concentration of the aerosol
He ratio was evaluated by XPS measurements.
in the plasma) at Va constant (8.5 kVpp).
In our deposition conditions, it seems that the
effect of the Va is almost negligible when the
total flow is kept constant at 9slm. In fact, the
ether content slightly increased from 60 to
65% of the C1s signal as decrease the voltage
from 8.5 to 6.5 kVpp, i.e. as decrease the
applied power from 13 to 8 W, as shown in
table I. A O/C surface ratio just slightly
298
lower than 0.5 was obtained, that is the
O/C ratio in TEGDME, for the coating
deposited at 6.5 kVpp. These results attest for
a
certain
cross-linking
of
the
coatings
deposited. The thickness of the film was
instead significantly affected due to the high
monomer fragmentation as the Va is increased,
resulting in a variation of the deposition rate
from 18±3 nm/min to 37±2. The influence of
the total flow was evaluated at a fixed Va of
8.5 kVpp, by changing the flow rate of the
carrier gas (+/- 1 slm) at a fixed aerosol flow of
3.15
slm.
In
this
case,
the
chemical
composition of the coating have been nicely
tuned by varying the aerosol concentration in
the discharge with the flow rate of He carrier
gas. A decrease of the total flow rate from 10
to 8 slm leads to a deposition of PEO-like
coatings with a PEO-character of 70%, as
shown by the best fitting of C1s XPS spectrum
of figure 2 and table I, confirming that these
coatings are less cross-linked with respect to
those deposited at higher flow rate. The
296
294
292 290 288 286
Binding Energy (eV)
284
282
Figure 2: XPS C1s signal of the PEO-like film (He total flow=
8 slm, Va= 8.5 kVpp, f=26.6 kHz, td= 5min)
PEO-like coatings with PEO character of 70%
and higher are known to be non fouling when
in contact with proteins and cells in water
media [8, 9]. The higher PEO character
obtained when the TEGDME concentration is
increased in the discharge (lower He total flow
rate),
is
clearly
due
to
the
reduced
fragmentation of TEGDME. Considering the
experimental conditions used in this work and
both the effect of Va and of the total feed flow,
it
can
be
reported
that
the
chemical
composition of the DBD deposited PEO-like
coating is much more influenced by the
chemical composition of the feed rather than
by the applied power, at least in our system.
Tuning the aerosol concentration and the
resident time of active species in the discharge,
thus resulted in tuning the chemical properties
of the film, and the behaviour of cells in
contact with them. Further experiments are
deposition rate was decreased, instead, due to
being performed to better optimize the
the dilution, from 36±4 to 18±2 nm/min by
deposition process, and to test relevant
3
properties of the coatings such as stability
in
various
media
and
non
fouling
properties toward proteins and cells.
Acknowledgement
Dr Sabine Paulussen (VITO, Expert centre for
materials technology, Flemish Institute of
Technological
Research)
is
gratefully
acknowledged for the scientific support and for
the
knowledge
on
atmospheric
pressure
dielectric barrier discharges combined with
aerosol precursors.
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4