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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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