Characteristics and anticoagulation behavior of polyethylene terephthalate
modified by C2 H2 plasma immersion ion implantation-deposition
J. Wanga)
Department of Physics and Materials Science, City University of Hong Kong, Tat Chee Avenue,
Kowloon, Hong Kong
C. J. Pan
School of Material Science and Engineering, Southwest Jiaotong University, Chengdu 610031, China
S. C. H. Kwok
Department of Physics and Materials Science, City University of Hong Kong, Tat Chee Avenue,
Kowloon, Hong Kong
P. Yang, J. Y. Chen, G. J. Wan, and N. Huang
School of Material Science and Engineering, Southwest Jiaotong University, Chengdu 610031, China
P. K. Chub)
Department of Physics and Materials Science, City University of Hong Kong, Tat Chee Avenue,
Kowloon, Hong Kong
共Received 18 June 2003; accepted 20 October 2003; published 8 January 2004兲
Acetylene (C2 H2 ) plasma immersion ion implantation-deposition 共PIII-D兲 is conducted on
polyethylene terephthalate 共PET兲 to improve its blood compatibility. The structural and
physicochemical properties of the modified surface are characterized by, Raman spectroscopy, x-ray
photoelectron spectroscopy 共XPS兲, and static contact angle measurement. Atomic force microscopy
discloses that the average roughness (R a ) of film surface decreases from 58.9 nm to 11.4 nm after
C2 H2 PIII-D treats PET. Attenuated total reflection Fourier transform infrared spectroscopy shows
that the specfic adsorption peaks for PET decrease after ion implantation and deposition. Raman
spectroscopy indicates that a thin amorphous polymerlike carbon 共PLC兲 film is formed in the PET.
The effects of the surface modification on the chemical bonding of C, H, and O are examined by
XPS and the results show that the ratio of sp 3 CuC to sp 2 CvC is 0.25. After C2 H2 PIII-D, the
polar component ␥ p of surface energy increases from 2.4 mN/m to 12.3 mN/m and ␥ p / ␥ d increases
from 0.06 to 0.35. The wettability of the modified surfaces is improved. Scanning electron
microscopy and optical microscopy reveal that the amounts of adhered, aggregated and
morphologically changed platelets are reduced by the deposition of an amorphous polymer-like
carbon film. The thrombin time, prothrombin time, and activated partial thromboplastin time of the
modified PET are longer than those of the untreated PET. Our result thus shows that the amorphous
PLC film deposited on the PET surface by C2 H2 PIII-D improves platelet adhesion and
activation. © 2004 American Vacuum Society. 关DOI: 10.1116/1.1633569兴
I. INTRODUCTION
Poly共ethylene terephthalate兲 共PET兲 is one of the most important polymeric materials used in the biomedical field because of its excellent mechanical properties and moderate
biocompatibility. In particular, it is widely adopted in cardiovascular implants, such as artificial heart valve sewing
rings1,2 and artificial blood vessels.3–5 However, the blood
compatibility of PET is insufficient for the long-term antithrombogenic demand for in vivo applications and attempts
must be made to improve its blood compatibility.
Surface modification techniques, such as low-temperature
plasma treatment6 – 8 or ion implantation,9–11 have been ata兲
Also at: School of Material Science and Engineering, Southwest Jiaotong
University, Chengdu 610031, China.
b兲
Author to whom correspondence should be addressed; electronic mail:
[email protected]
170
J. Vac. Sci. Technol. A 22„1…, JanÕFeb 2004
tempted to improve the blood compatibility of polymers.
Plasma immersion ion implantation-deposition 共PIII-D兲, an
effective approach for the surface modification of materials,
incorporates both ion implantation and low-temperature
plasma processing. This burgeoning technique has been applied to polymers to improve the mechanical properties,12
electrical conductivity,13 and gas barrier properties.14 –16
However, relatively little work has been reported on the surface modification of polymers by PIII-D to enhance the
blood compatibility properties.
In this article, we report our study on the surface modification of PET by acetylene (C2 H2 ) PIII-D. The surface
structure and properties of the modified PET are characterized using laser Raman spectroscopy, attenuated total reflection Fourier transform infrared spectroscopy 共ATR-FTIR兲,
x-ray photoelectron spectroscopy 共XPS兲, and contact angle
measurements. The blood compatibility in vitro is evaluated
by platelet adhesion observation employing optical micros-
0734-2101Õ2004Õ22„1…Õ170Õ6Õ$19.00
©2004 American Vacuum Society
170
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Wang et al.: Characteristics and anticoagulation behavior
171
FIG. 3. ATR-FTIR spectra of 共a兲 PET control and 共b兲 and PIII-PET.
FIG. 1. Schematic diagram of the PIII equipment.
copy and scanning electron microscopy 共SEM兲 as well as
coagulation factor experiments.
placed on the PET film laid on the stainless substrate to make
the same electric contact as the PET specimen.
C2 H2 PIII was performed at a sample voltage of 5 kV for
40 min. The base pressure in the vacuum chamber was 2.4
⫻10⫺3 Pa. The pressure in the vacuum chamber rose to
1.0⫻10⫺1 Pa followed by the introduction of C2 H2 at a flow
rate of 30 standard cubic centimeters per minute. The sample
II. EXPERIMENT
A. Plasma immersion ion implantation-D treatment
The 10 ␮m thick PET films purchased from 3M 共USA兲
were washed ultrasonically in methanol, acetone, and doubly
distilled water for 10 min and dried in a purified biological
lab. The PET films were put on a stainless substrate attached
to an insulated stainless-steel electrode in the center of the
vacuum chamber as shown in Fig. 1. A negative voltage was
then applied to the electrode. For some samples, the carbon
layer was deposited on Si 共100兲 wafers. These wafers were
FIG. 2. Raman spectra of the untreated PET, PIII-PET, and amorphous carbon film on Si wafer.
JVST A - Vacuum, Surfaces, and Films
FIG. 4. XPS survey spectra of 共a兲 PET control and 共b兲 PIII-PET.
172
Wang et al.: Characteristics and anticoagulation behavior
172
TABLE I. Chemical composition of the untreated and the modified PET calculated from the XPS spectra.
Atomic percent 共%兲
Substrate
C
O
PET
PIII-PET
75.0
85.0
25.0
15.0
high voltage pulse width was 20 ␮s and frequency was 100
Hz. These samples are designated as PIII-PET in this work.
B. Surface characterization
Static contact angles were measured using the sessile drop
method with a contact angle geniometer 共JY-82, China兲 at
ambient humidity and temperature. All liquid drop contact
angles reported here are the mean values of six measurements on different parts of the film. ATR-FTIR spectra were
acquired using a Perkin–Elmer 16PC spectrometer 共20 cu-
FIG. 6. AFM photomicrographs of 共a兲 PET control and 共b兲 PIII-PET.
mulative scans兲 with an ATR attachment composed of a
KRS-5 crystal prism at an incident angle of 45°. XPS measurements were performed using the PHI-5802 equipped
with a monochromatic Al K ␣ source. To investigate the induced surface chemical changes, the C 1s and O 1s corelevel spectra were recorded. Raman spectra were obtained by
a Raman spectrometer 共JobinYi, T64000兲 using the 514.5 nm
resonant line of an Ar⫹ laser. Atomic force microscopy
共AFM兲 studies were carried out on a Autoprobe Research
System 共Park Scientific Instrument兲 in the noncontact mode
employing the PSI Image Processing and Data Analysis software.
C. In vitro platelet adhesion experiment
The samples were immersed into human fresh plateletrich plasma 共PRP兲 and incubated at 37 °C for 20 and 120
min. After rinsing, fixing, and critical point drying, the specimens were coated with gold palladium and examined using
SEM and optical microscopy. Six fields were chosen at random to obtain averages of the quantity of the adherent platelets.
TABLE II. Contact angle and surface energy of the untreated and the modified PET film.
Surface energy 共mN/m兲
Contact angle
FIG. 5. 共a兲 C 1s spectra of the PET control and the PIII-PET films and 共b兲
C 1s spectra of the PIII-treated PET film fitted for various chemical states.
J. Vac. Sci. Technol. A, Vol. 22, No. 1, JanÕFeb 2004
Substrate
Water
Diiodomethane
␥s
␥d
␥p
␥p /␥d
PET
PIII-PET
83.5
64.8
30.2
32.4
43.3
47.1
40.9
34.8
2.4
12.3
0.06
0.35
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Wang et al.: Characteristics and anticoagulation behavior
173
FIG. 7. Quantity of platelets adhered on the surface of the PET control and
PIII-PET.
D. In vitro coagulation time tests
The antithrombogenicity of the modified films was evaluated by in vitro coagulation time tests, including activated
partial thromboplastin time 共APTT兲, prothrombin time 共PT兲,
and thrombin time 共TT兲. In the measurement, the samples
(1⫻1 cm2 ) were attached to a silcaned tube 共diameter: 3 cm;
height: 2 mm兲 and incubated with fresh human platelet-poorplasma 共PPP兲 for 3 min at 37 °C. The reagents for each
coagulation time test were added to the tube, respectively.
The clotting time of the plasma solution was measured by a
coagulometer 共Clot 1A, Innova Co, Ita.兲. The experiment
was repeated in quadruplicate and a mean value was calculated.
III. RESULTS AND DISCUSSION
A. Surface spectroscopic and morphological analysis
The Raman spectrum of the untreated PET film exhibits
the characteristic sharp peaks at 1292, 1616, and 1727 cm⫺1
关Fig. 2共a兲兴. The peak at 1292 cm⫺1 is attributed to the ring
and the CvO stretch, whereas the 1616 and 1727 cm⫺1
peaks are due to CvC ring and carbonyl stretch, respectively. The Raman spectrum of the PIII-treated PET reveals a
strong carbon layer band on which the PET Raman bands is
superimposed 关Fig. 2共b兲兴. The Raman spectrum of carbon
film deposited on the Si wafer shows two broad peaks at
1300– 1450 cm⫺1 and 1500– 1650 cm⫺1 as shown in Fig.
2共c兲. Such a spectrum with an asymmetric broad peak is
often seen in a carbon deposit that is called amorphous polymerlike carbon 共PLC兲 film.17
Figure 3 shows the ATR-FTIR spectra of the original PET
served as a control and PIII-PET. The creation of alkyne end
groups (RuCwCuH) is evidenced by the appearance of a
band at 3300 cm⫺1 , which is assigned to CuH stretching
vibration mode of the alkyne end group. Large peaks are
seen at 1630 cm⫺1 due to CvC (s p 2 bonding兲 of fatty hydrocarbons. The intensity of the peaks assigned to sp 3 CH2
JVST A - Vacuum, Surfaces, and Films
FIG. 8. Morphology of adherent platelets on 共a兲 PET control and 共b兲 PIIIPET after 120 min of incubation in PRP observed by SEM.
and sp 3 CH3 at around 2850– 2950 cm⫺1 is increased. The
result reveals that this surface coating is mostly comprised of
a mixture of sp 2 , sp 3 , and a few sp 1 coordinated carbon
atoms in a disordered network.
Figure 4 depicts the XPS survey scan spectra of 关Fig.
4共a兲兴 PET and 关Fig. 4共b兲兴 PIII-PET surfaces. The chemical
composition calculated from the XPS survey scan spectra is
shown in Table I. The carbon content of PET surface increases from 75.0% to 85.0% by C2 H2 PIII treating. In Fig.
5共a兲, the XPS spectrum for C 1s is resolved into three peaks
which represent the interatomic bonds of carbon: CuC
(sp 3 ), CvC (sp 2 ), and CvO peak. The dominant features
in the spectrum acquired from the treated PET film are sp 2
共284.5 eV兲 and CuH bonding (285⫾0.2 eV). The spectrum
is analyzed in more details by fitting Gaussian functions for
the proportions of sp 3 , sp 2 , and CvO as shown in Fig.
5共b兲. The ratio of sp 3 CuC to sp 2 CvC is 0.25. This
indicates that the PLC layer is dominantly graphite carbon.
Figures 6共a兲 and 6共b兲 contrast the 5⫻5 ␮ m2 surface topography as measured by AFM without any filtering. The
surface of the untreated PET sample exhibits a rectangular
structure. Pinnaclelike structure can be observed in the image
of PIII-treated PET. Each AFM image is analyzed in terms of
surface average roughness. The average roughness (R a ) of
the PET surface modified by acetylene PIII-D decreases from
58.9 nm to 11.4 nm. This result indicates that the surface
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Wang et al.: Characteristics and anticoagulation behavior
174
TABLE III. Interfacial energy parameters of PET and PIII-PET with plasma protein system.
PET surfacea
(dyn/cm) 1/2
Protein
Polar/nonpolar
ratio
Fibrinogen
␥-globulin
Albumin
1.626
1.205
1.072
PIII-PET surfaceb
␣i
␤i
␥ij
共dyn/cm兲
共␤i⫺␤ j 兲2
共␣i⫺␣ j 兲2
␥ij
共dyn/cm兲
共␤i⫺␤ j 兲2
共␣i⫺␣ j 兲2
4.972
5.428
5.602
6.346
5.961
5.798
25.0
20.4
18.8
11.4
20.8
22.8
11.6
8.1
6.8
2.2
2.9
3.3
For values of ␣ i ⫽6.4 (dyn/cm) 1/2 and ␤ i ⫽1.6 (dyn/cm) 1/2.
For values of ␣ i ⫽6.9 (dyn/cm) 1/2 and ␤ i ⫽3.5 (dyn/cm) 1/2.
a
b
morphology of the PET film is significantly affected by C2 H2
PIII-D and a carbon layer is deposited on the PET film.
B. Surface energy and wettability
The total solid surface free energy and its components are
determined using double-distilled water and diiodomethane
as testing liquids. This determination is based on Young’s
equation18 that relates the surface tension of a liquid in equilibrium with its vapor. Surface energies together with the
contact angle of water and diiodomethane are listed in Table
II. After C2 H2 PIII-D, the polar component ␥ p of surface
energy increases from 2.4 mN/m to 12.3 mN/m and ␥ p / ␥ d
increases from 0.06 to 0.35. The wettability of the PET with
the PLC coating is improved by C2 H2 PIII-D.
C. Platelet adhesion and activation
Platelet adhesion and activation is an indicator of the antithrombogenicity of surface modified PET films. The platelet adhesion test shows significant difference in the behavior
of platelet adhesion among different PET surfaces. Figure 7
displays the numbers of adherent platelets on the surface of
the PET control and PIII-PET after 20 min incubation. The
number of adherent platelets on PIII-PET is lower than that
on the untreated surface. On the PIII-PET sample, an average
of 60 contact-adherent platelets are present, compared to 120
contact-adherent platelets present on the PET control. This
suggests that adhesion of platelets is significantly suppressed
by the amorphous carbon film deposited on PET.
SEM is used to study the morphology of adhered platelets
and to compare the platelets shape changes on the different
surfaces after 120 min incubation. As shown in Fig. 8共b兲, the
adherent platelets on the surface of PIII-PET are relatively
isolated and round. In comparison, most of the adherent
platelets on the PET control are in aggregation and spreading
pseudopodium 关Fig. 8共b兲兴.
It is believed that the initial adsorption of protein from
blood onto a material surface has a great influence on platelet
adhesion and activation.19 Therefore, a good understanding
of the relationships between the material surface and plasma
proteins 共fibrinogen, g-globulin, and albumin兲 is very important for surface design of biomedical materials. For interfacial tension ( ␥ i j ) between condensed phase i and j, a relative evaluation can be made:
␥ i j ⫽ 共 ␣ i ⫺ ␣ j 兲 2 ⫹ 共 ␤ i ⫺ ␤ j 兲 2 ⫹⌬ i j ,
J. Vac. Sci. Technol. A, Vol. 22, No. 1, JanÕFeb 2004
共1兲
where ␣ S and ␤ S defined by Kaelble and Moacanin20 are
dispersion ␣ S ⫽( ␥ dS ) 1/2 and polar ␤ S ⫽( ␥ Sp ) 1/2 components of
polymer surface, respectively. In this work, interfaces dominated by van der Waals interactions and the term ⌬ i j describe
the ion–covalent interaction that can be considered negligible. According to Ref. 21, ␣ i and ␤ i of plasma proteins are
listed in Table III. The interfacial energy parameters for different proteins can be calculated as given in Table III for
PET surface 共1兲 ␣ i ⫽6.4 (dyn/cm) 1/2, ␤ i ⫽1.6 (dyn/cm) 1/2,
and an amorphous carbon film deposited by C2 H2 PIII on
PET 共2兲 ␣ i ⫽6.9 (dyn/cm) 1/2, and ␤ i ⫽3.5 (dyn/cm) 1/2. It is
expected that the conformation of adsorbed plasma protein
will change. Several studies22,23 have indicated that this conformational change plays an important role in platelet adhesion and activation. Our previous results24 suggest that the
higher the interfacial energy, the larger the conformation
changes. From Table III, it appears that the interfacial energy
is lowest with albumin in both surfaces compared to fibrinogen and ␥-globulin. However, both the interfacial energy
with albumin and fibrinogen or ␥-globulin in the case of
PIII-PET surface is lower than those on the untreated PET
surface. Hence, the possibility of weak adsorption and less
conformation of these proteins is suggested on the PLC
films.
As the evaluation criteria of the abnormality of blood
plasma in clinical tests, the APTT, PT, and TT have recently
been used in the evaluation of the in vitro antithrombogenicity of biomaterials.25,26 Table IV shows the APTT, PT, and
TT of the PPP in contact with the test materials. In this test,
glass and silcanzed glass are used as positive and negative
controls, respectively. PT and APTT are generally used to
detect the degree of activation of the exogenic and endogenic
clotting system.27 TT represents the duration from the addition of thrombin to the conformation of the insoluble fibrin.
The PT of PPP incubated with various materials are not altered much, whereas the APTT and TT values of the PPP in
TABLE IV. In vitro coagulation time 共APTT, PT, and TT兲 of PPP incubated
with various materials.
Samples
APTT 共s兲
PT 共s兲
TT 共s兲
PPP⫹glass
PPP⫹silanized glass
PPP⫹PET
PPP⫹PIII-PET
50.7⫾2.0
57.4⫾0.9
51.4⫾1.1
56.4⫾1.2
13.2⫾0.4
13.9⫾0.3
13.1⫾0.2
13.4⫾0.2
17.4⫾0.8
20.6⫾1.1
18.6⫾0.7
21.2⫾0.3
175
Wang et al.: Characteristics and anticoagulation behavior
contact with the PIII-PET surface are longer than those of the
untreated PET as well as positive and negative controls.
Hence, the antithrombogenic effect of the amorphous PLC
film deposited on PET is to interdict the endogenic conformation of thrombin.
According to the aforementioned discussion, the suppression of the platelet adhesion and activation on the amorphous
PLC film deposited by C2 H2 PIII-D depends not only on the
presence of adsorbed plasma proteins but also on the conformation change of adsorbed plasma proteins. Another plausible reason for good blood compatibility of the amorphous
PLC film is that this surface can interdict the contact activation path of the clotting process.
IV. CONCLUSION
Amorphous PLC coatings are fabricated on PET films by
acetylene PIII-D and characterized by Raman, XPS, ATRFTIR, and AFM. Platelet adhesion and activation are suppressed on the PLC coatings compared with the untreated
PET film. Our results suggest that one reason for the good
hemocompatibility is that the PLC coating minimizes its interactions with plasma protein and slightly changes the conformation of adsorbed plasma protein. The other reason is
that this coating suppresses the endogenic clotting system of
blood.
ACKNOWLEDGMENTS
This research was jointly and financially supported by
City University of Hong Kong SRG No. 7001447, Natural
Science Foundation of China 共No. 50203011兲, China Key
Basic research 共No. G 1999064706兲, and Southwest Jiaotong
University Foundation 共No. 2002B02兲.
JVST A - Vacuum, Surfaces, and Films
175
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