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Surface & Coatings Technology 185 (2004) 120–125
Low pressure plasma immobilization of thin hydrogel films on
polymer surfaces
M. Nitschke*, S. Zschoche, A. Baier, F. Simon, C. Werner
Institute of Polymer Research Dresden and Max Bergmann Center of Biomaterials Dresden, Hohe Straße 6, 01069 Dresden, Germany
Received 8 June 2003; accepted in revised form 6 December 2003
Available online 16 March 2004
Abstract
Thin films of poly(ethylene imine), poly(N-vinyl pyrrolidone) and poly(acrylic acid) were immobilized on poly(tetrafluoroethylene) and
poly(ethylene terephthalate) surfaces using low pressure argon plasma. This technique allows to immobilize polymer films with a thickness
of approximately 10 nm on polymeric substrates while functional groups and the mobility and swelling of the polymer chains are largely
preserved. It is demonstrated, that through the suggested method coatings of very different hydrogel polymers can be obtained on very
different polymeric substrates. Layer formation, plasma-induced anchorage and pH-dependent swelling of the immobilized hydrogels were
analyzed by ellipsometry. X-Ray photoelectron spectroscopy was used to study the degree of degradation of the attached polymers caused by
the plasma treatment. The pH-dependent ionization of the hydrogel layers in aqueous solutions was further characterized by streaming
potential measurements.
D 2003 Elsevier B.V. All rights reserved.
Keywords: Polymer hydrogels; Low pressure plasma; Plasma immobilization
1. Introduction
Polymer hydrogels were proven to be most adequate
materials for many demanding biomedical applications [1].
A preference for hydrogels is often based on their softness,
providing low friction and molecular adaptation of interfaces
to adherent cells and tissues, on the prevention of non-
specific adsorption of biopolymers (non-fouling/protein
resistant surfaces), and on the selective exchange of
dissolved molecules through their solvated molecular
networks [2–4]. Recently, polymer hydrogels were syn-
thesized to implement advanced functions such as struc-
tural transitions upon environmental signals (temperature,
pH, molecular association) [5] or site-specific biodegra-
dation (enzymatic cleavage) [6]. While several medical
technologies make use of hydrogel bulk materials (mem-
branes for hemodialysis, contact lenses, drug delivery
systems, scaffolds for tissue engineering) more and more
emphasis is now also on surface-confined hydrogel layers
0257-8972/$ - see front matter D 2003 Elsevier B.V. All rights reserved.
doi:10.1016/j.surfcoat.2003.12.006
* Corresponding author. Tel.: +49-351-4658-520; fax: +49-351-4658-
533.
E-mail address: nitschke@ipfdd.de (M. Nitschke).
prepared on top of different bulk materials. Examples
include functional coatings for cardiovascular catheters
(reduced friction and/or drug release) [7] and cell carriers
(controlled detachment for harvesting) [8].
There are different approaches to obtain functional
polymer thin films on polymer substrates. Especially
popular are low pressure plasma based techniques like
grafting on plasma activated polymer surfaces or plasma
assisted chemical vapour deposition [9–13]. Beyond this,
pre-adsorbed polymer films with a thickness of a few
nanometers prepared on polymeric substrates can be chem-
ically attached to the substrate material using low pressure
plasma. At appropriate treatment parameters covalent fix-
ation was achieved while important properties of the
immobilized polymer like the presence of functional
groups and the mobility and swelling of the polymer
chains were preserved. In most cases argon discharges
were used for this purpose. Plasma immobilization was
applied to introduce different functional groups [14–16]
and for anchorage of poly(ethylene oxide) containing
surfactants [17–19] on polymer surfaces. In a recent work,
the authors used this technique to immobilize thin films of
different stimuli responsive polymers [20,21]. The immo-
M. Nitschke et al. / Surface & Coatings Technology 185 (2004) 120–125 121
bilization effect depends on the chemical structure of the
substrate as well as the structure of the polymer to be
immobilized. It was found, that radical generation plays an
important role in the mechanism [22]. The efficiency of
immobilization is limited by plasma etching [23].
The aim of this study was to demonstrate, that differ-
ent hydrogel polymers can be immobilized on very
different polymeric substrates using the same plasma
treatment procedure. For that purpose the immobilization
of poly(ethylene imine) (PEI), poly(N-vinyl pyrrolidone)
(PVP) and poly(acrylic acid) (PAAc) on poly (tetrafluoro-
ethylene) (PTFE) and poly(ethylene terephthalate) (PET)
was investigated. Base materials and coatings are well-
established for a wide variety of biomedical applications
[24–26].
2. Experimental
2.1. Materials
Poly(ethylene terephthalate) (Goodfellow) was used for
the preparation of model surfaces. Poly(ethylene imine)
(BASF AG, product name Polymin P, MW 500.000 g/mol,
number of primary, secondary and tertiary amino groups
1:2:1), poly(N-vinyl pyrrolidone) (Fluka, MW 1.270.000 g/
mol) and poly(acrylic acid) (Aldrich, MW 450.000 g/mol)
were used to prepare thin films on the model surfaces (Fig.
1). The solvents hexafluoroisopropanol (99%, Merck) and
methanol (99.5%, Fluka) were used as received. The plasma
apparatus was operated with Argon (99.999%, Messer
Griesheim).
2.2. Preparation of PTFE and PET model surfaces
To allow the ellipsometric investigation of hydrogel
immobilization and swelling, the two substrate polymers
under investigation were prepared as thin films on silicon
wafers 10�20 mm2 with an oxide layer of 50 nm.
1. Non-branched fluorocarbon films with a structure close
to PTFE were kindly provided by the Institute for Energy
Problems of Chemical Physics, Russian Academy of
Sciences (Chernogolovka, Russia). The films were
deposited by plasma polymerization. Tetrafluoroethylene
(C2F4) was introduced downstream into a low pressure
Fig. 1. Structures (repeating units) of hydrogel polymers used throughout
this work.
argon discharge. The silicon wafers were placed further
downstream of the discharge [27].
2. PET films were prepared by spin coating from a 0.4%
wt./wt. PET solution in hexafluoroisopropanol at room
temperature. The spin coater was operated at a
maximum speed of 2000 rev./min and an acceleration
of 1500 rev./min s�1. To provide a hydrophobic surface
for the PET coating the silicon wafers were pre-treated
in a gas-phase reaction with hexamethyldisilazane (99%,
Merck).
2.3. Hydrogel coating
Thin films of the three hydrogel polymers were prepared
on PTFE and PET model surfaces by spin coating from 1%
wt./wt. solutions in methanol. The spin coater was operated
at 3000 rev./min and 3000 rev./min s�1 for PAAc and at
5000 rev./min and 5000 rev./min s�1 for PVP and PEI. To
obtain an appropriate wetting behaviour, PTFE surfaces
were pre-treated in argon plasma as described below for
120 s.
2.4. Plasma immobilization
The hydrogel polymer films were immobilized on the
PTFE and PET model surfaces using low pressure argon
plasma. The samples were prepared as described above.
Plasma treatment was carried out in a computer con-
trolled MicroSys apparatus by Roth&Rau, Germany. The
cylindrical vacuum chamber, made of stainless steel, has
a diameter of 350 mm and a height of 350 mm. The base
pressure obtained with a turbomolecular pump is
<10�7 mbar. On the top of the chamber a 2.46 GHz
electron cyclotron resonance (ECR) plasma source RR160
by Roth&Rau with a diameter of 160 mm and a
maximum power of 800 W is mounted. Argon is intro-
duced into the active volume of the plasma source via a
gas flow control system. When the plasma source is on,
the pressure is measured by a capacitive vacuum gauge.
The samples are introduced by a load-lock-system and
placed on a grounded aluminum holder near the center of
the chamber. The distance between the sample and the
excitation volume of the plasma source is approximately
200 mm. For the experiments of this work the following
parameters were used: effective power 120 W, argon gas
flow 38 standard cubic centimeter per minute, pressure
8�10�3 mbar, treatment time 10 s. After plasma treat-
ment, the samples were rinsed in methanol for 1 h at
room temperature and dried under vacuum.
2.5. Ellipsometry
Spectroscopic ellipsometry was performed by means of a
VASE M44 instrument (Woolam Inc., USA). Ellipsometric
data were collected at 44 wavelengths between 428 and 763
M. Nitschke et al. / Surface & Coatings Technology 185 (2004) 120–125122
nm and three angles of incidence: 68j, 70j and 75j. ForPEI, PVP and PAAc a refractive index of 1.5 was used. A
cell for liquid media with de-ionized water (pH 6.5) was
used for the measurements of the swollen hydrogel layers.
For the variation of the pH value 0.1 mol l�1 HCl or 0.1 mol
l�1 NaOH were added. An effective medium approximation
was applied to model the swelling of hydrogel layers. In
addition, a null ellipsometer ELX02 by DRE Ellipsometer-
bau, Germany was used.
2.6. X-Ray photoelectron spectroscopy
XPS studies were carried out by means of an Axis Ultra
photoelectron spectrometer (Kratos Analytical, Manchester,
UK). The spectrometer was equipped with a monochro-
matic Al Ka X-ray source of 300 W at 15 kV, the radiation
of which was monochromated by a quartz crystal mono-
chromator. The information depth of the XPS method
corresponds with the mean free path of the electrons in
the material under investigation. In the case of polymer
samples the information depth of XPS is not more than
8 nm. The kinetic energy of photoelectrons was deter-
mined using a hemispherical analyzer with a constant pass
energy of 160 eV for survey spectra and 20 eV for high-
resolution spectra. During all measurements, electrostatic
charging of the sample was avoided by means of a low-
energy electron source working in combination with a
magnetic immersion lens. Later, all recorded peaks were
shifted by the same value which was necessary to set the C
1s peak to 285.00 eV [28] (in the case of PEI the C 1s
peak was set to 285.56 eV [28]). Quantitative elemental
compositions were determined from peak areas using
experimentally determined sensitivity factors and the spec-
trometer transmission function. The high-resolution spectra
were analyzed by means of the spectra deconvolution
software (Kratos Analytical, Manchester, UK). Free param-
eters of component peaks were their binding energy (BE),
height, full width at half maximum and the Gaussian–
Lorentzian ratio.
2.7. Electrokinetic measurements
pH-dependent zeta potential data of the substrates and
the hydrogel coatings were determined from streaming
potential measurements (Electrokinetic Analyzer EKA, A.
Paar GmbH, Graz, Austria) using a flat plate measuring
cell. 3�10�4 mol l�1 aqueous KCl solutions were applied
as background electrolyte, solutions of 0.1 mol l�1 HCl
and 0.1 mol l�1 NaOH were added for variation of the pH
value.
Fig. 2. PEI, PVP and PAAc film thickness on PTFE after spin coating (left
bar) and after immobilization and rinsing (right bar).
3. Results and discussion
The immobilization of PEI, PVP and PAAc is demon-
strated by ellipsometry (Figs. 2 and 3). It is shown, that a
part of the material deposited by spin coating remains on
the model surface after plasma treatment and rinsing.
While in the case of PEI approximately 50% of the initial
film thickness are lost, the film thickness is almost
preserved for PVP and PAAc. The results are very similar
for the two substrates, PTFE and PET. However, it is
expected, that the immobilized polymer films are degraded
during plasma treatment. To decide whether or not impor-
tant properties are preserved, different diagnostic techni-
ques were applied.
Table 1 shows the atomic composition of the untreated
and plasma treated hydrogel polymer films on PTFE
substrates compared to the theoretical values. Obviously,
the immobilized polymers are degraded in all cases. In
order to get more information on chemical changes in the
polymer layer which were initiated by the plasma immo-
bilization process the C 1s high-resolution spectra were
analyzed. Fig. 4 compares the C 1s spectra of the unmod-
ified and the plasma treated layers.
The C 1s spectrum of the unmodified PEI film shows
the main component peak B which corresponds with the
CN bonds of the polymer. The small component peak A
appears from hydrocarbon impurities on the sample sur-
face. The plasma treatment causes a partital oxidation of
the PEI structure which is firstly indicated by the intro-
duced high amount of oxygen. The corresponding C 1s
spectrum (Fig. 4) shows different oxygen containing
functional groups. The binding energy of the component
peak U (BE=287.78 eV) excellently corresponds with
OCN bonds typically found for oxazoline structures [28].
Component peak T and S probably represent keto (T),
alcohol and ether (S) groups which can be easily formed
after the abstraction of NH2 or NH groups. Beside the
oxidation reactions a polymer degradation was also ob-
served. The fraction of hydrocarbons CxHy (component
peak A) is strongly increased and the PTFE substrate can
be identified by a small amount of CF2 groups (component
peak W).
Fig. 4. High-resolution C 1s spectra of PEI, PVP and PAAc before (left) and
after plasma treatment (right).
Fig. 3. PEI, PVP and PAAc film thickness on PET after spin coating (left
bar) and after immobilization and rinsing (right bar).
M. Nitschke et al. / Surface & Coatings Technology 185 (2004) 120–125 123
The PVP structure is also partially oxidized in the
plasma process. The C 1s spectrum of the plasma treated
sample shows a sub-spectrum (component peaks A, B, C
and D) which corresponds also with regards on the
stochiometry excellently with the C 1s spectrum of an
unmodified PVP film. Here, A appears from the CH2
groups which have no any functional groups in their
immediate neighbourhood, C shows the CN bonds, and
D represents the amide group NCO. Component peak B
indicates the carbon atom which is in the h position to the
amide group (NC(O)C). The second sub-spectrum is com-
posed of the component peaks S, T, and U. Their
corresponding structure elements can be discussed as
mentioned above.
The C 1s spectrum of PAAc shows rather moderate
changes after the plasma treatment. The shape of the C
1s spectrum of the plasma modified sample is determined
by the three component peaks showing the presence of
carboxyl groups COOH (C), their corresponding carbon
atoms in h position CCOOH (B) and the hydrocarbons
CxHy (A). However, after the plasma treatment the
relative amount of carboxylic groups is smaller than
before. Obviously, the elimination of CO2 (decarboxyl-
ation) is the preferred reaction of the PAAc in the applied
plasma. During this process, some keto groups remain in
the polymer (component peak Y). The component peak X
is found in the untreated and plasma modified PAAc
sample. Its binding energy (BE=286.46 eV) corresponds
Table 1
XPS results for hydrogel polymer films on PTFE substrates before and after plas
PEI
[N]:[C] [O]:[C] [N]:[O]
Theoretical 0.5 – –
Untreated 0.452 0.007 0.016
Plasma treated 0.288 0.195 1.477
with that of the COC groups of PET which was the
substrate material. The corresponding PET ester peak is
overlapped with the component peak of the COOH of
PAAc.
Fig. 5 illustrates the swelling behavior of the immobi-
lized polymers depending on the pH value. PAAc shows the
typical behavior of weakly acidic hydrogels with a strong
increase of swelling at higher pH-values. In this case, we
have immobilized an anionic polyelectrolyte on the sub-
strate. The cationic film of PEI shows the inverse swelling
behavior compared to PAAc. There is a strong increase of
swelling at lower pH-values. This is typical for weakly basic
hydrogels. The absolute degree of swelling is lower for PEI.
ma treatment
PVP PAAc
[O]:[C] [N]:[C] [N]:[O] [O]:[C]
0.166 0.166 1 0.666
0.161 0.161 0.998 0.441
0.143 0.148 1.032 0.323
Fig. 7. Zeta potentials of the PET model surface and the immobilized films
of PEI, PVP and PAAc.
Fig. 5. Swelling behavior of PEI, PVPand PAAc films on PTFE for
different pH-values.
M. Nitschke et al. / Surface & Coatings Technology 185 (2004) 120–125124
The polymer chains are more branched and, as clearly can
be seen in the XPS C 1s spectrum, more degraded after
plasma treatment. The swelling of the PVP film is almost
independent of pH-values. This is expected for a non-ionic
hydrogel. XPS studies do not show significant amounts of
potential charge carriers. However, it cannot be completely
excluded, that ionic groups were formed during plasma
treatment.
Figs. 6 and 7 give zeta potential vs. solution pH for
the unmodified polymers PTFE and PET and for the
immobilized films of PEI, PVP and PAAc on top of
them. The zeta potential data of PTFE (Fig. 6) and PET
(Fig. 7) confirm that the substrates utilized in this study
do not contain significant amounts of dissociating surface
Fig. 6. Zeta potentials of the PTFE model surface and the immobilized films
of PEI, PVP and PAAc.
sites but become charged in aqueous electrolyte solutions
by preferential hydroxide ion adsorption (isoelectric
points close to pH 4, no distinct zeta potential plateau
range) [29].
The immobilized PAAc films show isoelectric points
considerably below pH 3 indicating electrosurface char-
acteristics mainly determined by carboxylic acid groups.
At solution pH values exceeding pH 7 the magnitude of
the zeta potential is reduced due to increased swelling
of the hydrogel film (compare the results of ellipsom-
etry, Fig. 5) affecting both the position of the shear
plane and the ion conductance within the layered sample
(no correction for surface conductivity was applied
here).
The immobilized PVP and PEI films exhibit rather
similar zeta potential vs. solution pH patterns. The iso-
electric points are shifted to higher pH values (compared
to the isoelectric points of the uncoated substrates) and a
zeta potential plateau occurs at acidic pH values indicat-
ing the presence of basic sites. While basic sites were
expected for the PEI hydrogels PVP is intrinsically non-
ionic and apparently gains basic functions during the plas-
ma treatment.
Beyond that, it is demonstrated, that the plasma-immo-
bilized coatings are stable against significant shear forces
appearing during the streaming potential measurements.
4. Conclusions
Low pressure plasma is a useful tool for the immobi-
lization of polymeric thin films of approximately 10 nm on
polymer substrates. It was demonstrated, that different
M. Nitschke et al. / Surface & Coatings Technology 185 (2004) 120–125 125
hydrogel polymers can be immobilized on very different
polymeric substrates using the same plasma treatment
procedure. Despite a number of plasma-induced degrada-
tion effects, important properties of the immobilized
hydrogel polymers are preserved. Therefore, plasma im-
mobilization was confirmed to be an universal and rela-
tively simple tool to tailor the properties of polymer
surfaces for biomedical applications and beyond that.
Recently, thermo-responsive hydrogels coatings for con-
trolled cell adhesion and detachment were prepared by this
technique [30]. Current studies in our group aim at mask-
ing techniques for the manufacturing of coatings with
lateral structures and the formation of functional multi-
layer systems by successive plasma immobilization of
different polymer thin films.
Acknowledgements
The authors thank Christine Arnhold for help with the
streaming potential measurements.
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