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Applied Surface Science 255 (2009) 6686–6690
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Applied Surface Science
journal homepage: www.elsevier.com/locate/apsusc
Research on micro-structure and hemo-compatibility of the artificial
heart valve surface
Xia Ye a,b, Yun-liang Shao a, Ming Zhou a,*, Jian Li a, Lan Cai a
a
b
Center of Photonics Fabrication, Jiangsu University, Zhenjiang 212013, PR China
School of Mechanical Engineering, Jiangsu Teachers University of Technology, Changzhou 213015, PR China
A R T I C L E I N F O
A B S T R A C T
Article history:
Received 29 October 2008
Received in revised form 22 January 2009
Accepted 22 February 2009
Available online 4 March 2009
In order to seek the method to improve the hemo-compatibility of artificial mechanical heart valve, the
surface of rabbit’s heart valve was observed using the scanning electron microscopy (SEM). The results
showed that the dual-scale structure which consists of cobblestones-like structure of 8 mm in underside
diameter and 3 mm in height, and the fine cilia of about 150 nm in diameter, was helpful to the hemocompatibility of the heart valve. Therefore, the polydimethylsiloxane (PDMS) surface with hierarchical
micro-structure was fabricated using femtosecond laser fabrication technique and soft lithography. At
the same time, the tests of apparent contact angle and platelet adhesion on both smooth and textured
PDMS surfaces were carried out to study their wettability and hemo-compatibility. The results
demonstrated that the surface with textured structure displayed more excellent wettabililty and anticoagulation property than that of smooth surface. The apparent contact angle of textured surface
enhanced from 113.18 to 163.68 and the amount of adsorbed platelet on such surface was fewer, no
distortion and no activation were found.
ß 2009 Elsevier B.V. All rights reserved.
PACS:
81.40.z
87.19.R
87.85.jj
Keywords:
Heart valve
PDMS
Micro-structure
Superhydrophobicity
Hemo-compatibility
1. Introduction
The replacement of artificial heart valve is the only way to save
the life of the patient whose heart valve is badly pathologically
changed by rheumatic heart disease, bacterium caused endocarditis disease and so on. The artificial heart valves used in clinic are
classified into bioprosthetic valve and mechanical valve. The
biocompatibility and anti-thrombus property of the bioprosthetic
valve are quite good, but its durability remains a problem, because
it is easy to calcify and to be torn. The average lifetime of
bioprosthetic valve is only 5–8 years [1]. So far, there is no effective
way to improve its durability. In contrast, the main part of
mechanical valve is made of non-biological materials, such as
titanium alloy, stainless steel, low temperature isotropic carbon
(LTIC) and polyurethane and so on [2], therefore the mechanical
valve has a relatively long durability, which can be used as long as
30–50 years. However, the mechanical valve has poor hemocompatibility. In order to decrease the risk of thrombosis
complication, the patient who is implanted the mechanical valve
should be treated with anticoagulant every day [3]. Even if they are
* Corresponding author. Tel.: +86 511 88791458; fax: +86 511 88791288.
E-mail address: [email protected] (M. Zhou).
0169-4332/$ – see front matter ß 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.apsusc.2009.02.068
treated with sufficient anticoagulant, thromboembolism is still
founded in 1–2% patients every year [4]. It has been more than 40
years for the clinical application since Harken et al. [5] first
replaced the aortic valve with artificial caged ball valve and Starr
and Edwards [6] replaced the mitral valve with ball valve at the
same year in 1960. However, up to now, there is no satisfying valve
which enjoys good hemo-compatibility and long durability in
clinic. Accordingly, it is an urgently key problem to be solved to
improve the hemo-compatibility of the mechanical valve. In recent
years, all researches about improving the hemo-compatibility of
the mechanical valve focus on two aspects: one is to seek new
materials with good hemo-compatibility, and another is to modify
the artificial surface by using coating or film. The most important
studies related to the second aspect are as follows: Li et al. [7]
implanted nitrogen ion on the LTIC by ion beam; Dion et al. [8]
deposited diamond-like carbon (DLC) film on valve materials; Ali
et al. [9] prepared the Cr-modified diamond-like carbon (Cr-DLC)
films using the magnetron sputtering technique and Grill et al. [10]
prepared Si-DLC films using the same technique; Huang et al. [11]
synthesized the Ti-coating by ion beam; Leng et al. [12] deposited
the duplex films, consisting of layers of Ti–O and Ti–N, using
plasma immersion ion implantation and deposition method; Chen
et al. [13] deposited Ti–O coatings doped with Ta and synthesized
Ti(Ta5+)O2 thin films using magnetron sputtering and thermal
X. Ye et al. / Applied Surface Science 255 (2009) 6686–6690
oxidation procedures; Bolz and Schaldach [14] deposited a coating
of amorphous hydrogenated silicon carbide by plasma-enhanced
chemical vapor deposition (PE-CVD). All the above studies show
that the blood compatibility of the artificial heart valve can be
increased by the surface modification technologies such as surface
coating and deposition of thin film. However, due to the curved
surface of the artificial heart valve, it is difficult to ensure the
uniformity of the coating or film. Furthermore, the adhesive force
between the coating layer and matrix also affect the working life of
the artificial heart valve.
Polydimethylsiloxane (PDMS) is a curable inorganic polymer
with desirable properties. It is transparent, non-fluorescent,
biocompatible and nontoxic, and has been used extensively as a
biomaterial in catheters, drainage tubing, insulation for pacemakers, membrane oxygenators, and ear and nose implants [15],
since its initial use for topographic cell substrates [16]. In addition,
PDMS is chemically inert, thermally stable, permeable to gases,
simple to handle and manipulate, exhibits isotropic and homogeneous properties, and can conform to submicron features to
develop micro-structures [17–19].
In this paper, the micro-structure on the surface of the rabbit’s
heart valve was studied using scanning electron microscopy (SEM).
And the hierarchical structure on PDMS surface was prepared by
femtosecond laser fabrication technique and soft lithography
technology. The hemo-compatibility of such surface was also
examined. The purpose of the present study is to explore the effect
of surface micro-structure on the blood compatibility and provide
a basis for the research of the artificial heart valve.
2. Experimental
2.1. Observation on the micro-structure of biological prototype
The heart of the rabbit was took out and irrigated with normal
saline. Then the left atrium and ventricle were cut open with
scissors, the mitral valve was removed and fixed in solution of
glutaraldehyde (Res Group Co., Ltd. chemical reagents). After low
temperature freeze-drying (ES-2030 vacuum freeze-drying device,
Hitachi Japan) in the 10 8C for 2–3 h, the valve was treated by
spray-gold (E-1010 ion sputtering device, Hitachi Japan) and
observed using the scanning electron microscope (S-3000N
scanning electron microscope SEM, Hitachi Japan).
2.2. Preparation of the bionic valve surface
The sample with a micro-structure on the surface, which is
similar to the surface of biological prototype, was fabricated using
soft lithography. In order to obtain the patterned mold, pit structure
on the K9 glass surface was fabricated using femtosecond laser. After
pattering, the mold was exposed to Trichloro(1H,1H,2H,2Hperfluorooctyl)silane(CF3(CF2)5CH2CH2SiCl3, Aldrich) for 2 h in a
low-pressure chamber, allowing an easy removal of the PDMS
membrane from it. PDMS gel was generally prepared from a 10:1
mixture (by weight exactly) of silicone elastomer and catalyst
(Sylgard 184, Dow Corning, Co.), poured onto the patterned K9 glass,
and left to settle for 10 min, so that the trapped air bubbles could
emerge to the surface. After the removal of all the air bubbles, the
patterned K9 glass with PDMS gel was put into a vacuum chamber
(100 8C) for 30 min. After curing, the PDMS sample was gently
peeled from the mold. The sample was observed using scanning
electron microscope (SEM, JSM-840A, JEOL LTD., Japan)
2.3. Contact angle measurements
Contact angle was used to determine the wettability of regular
arrays micro-structure surface. The static contact angle (CA) of
6687
water on the sample surface was measured in laboratory atmosphere at room temperature using the sessile drop method of a
contact angle goniometer (OCAH200, Dataphysics, Germany). A
deionized water drop of 1 mL was deposited on the surface, and
each reported angle was calculated as the average of five
measurements in different points. In order to compare the
wettability of textured surface with that of smooth surface, the
CA of the water on the flat PDMS surface was also measured.
2.4. Platelet adhesion in vitro
In order to test the potential hemo-compatibility of the
materials, platelet adhesion studies were conducted since platelet
adhesion was one of the most important steps during blood
coagulation on artificial surface [20–22]. Textured PDMS surface
and smooth PDMS surface which was used to show a contrast were
washed in the ultrasonic wave cleaning machine (Kunshan
Ultrasound Machines Ltd.) with deionized water for three times
and then equilibrated in phosphate buffer solution (PBS, pH 7.4) for
120 min. In the meantime, fresh human blood added with
anticoagulant citrate dextrose (9:1) (provided by Jiangbin Hospital
in Zhenjiang) was centrifuged at 1000 rpm for 10 min to obtain
platelet-rich plasma (PRP). Then two kinds of PDMS surface were
incubated in it at 37 8C for 120 min. After being washed three times
with PBS to remove non-adherent platelets, the samples were
soaked in 2.5 vol% glutaraldehyde for 60 min to fix the adhered
platelets. Then they were washed with PBS (PH 7.4) again, and
dehydrated in a series of gradual ethanol/distilled water mixtures
from 10% to 100% (20% ethanol increment) for 30 min each. After
drying at room temperature, the samples surface were treated by
spary-gold using magnetron sputtering technique and tested by
SEM. The results of SEM experiments were shown with representative micrographs. Eight to 10 fields of view were chosen
randomly to obtain good statistical density of the platelet
adhesion. The flat PDMS sample was taken as a contrast. The final
number of platelets adhering to the surface was expressed as a
percentage of the control.
3. Results and discussion
3.1. Characterization of biological prototype surface
The SEM image of the surface of the rabbit’s mitral valve was
shown in Fig. 1. It could be evidently seen from images that there
were some regular cobblestones-like on the heart valve surface
which seemed very smooth by unaided eye. The underside
diameter of each cobblestone-like was about 8 mm, and the height
was about 3 mm. Fig1c showed a high-resolution SEM image of a
single cobblestone-like, where hierarchical structure was in the
forms of tenuous villus in average diameters of 150 nm and height
of 500 nm on each cobblestone-like. According to the Wenzel
theory [23] and Cassie and Baxter theory [24], this hierarchical
structure on the surface of the heart valve, which is similar to that
of lotus leaf surface [25], may be very useful to modify the
wettability of heart valve surface and reduce the amount of
adsorbed platelet on the surface, thus decreases the form of
thrombus on the heart valve surface.
3.2. Characterization of artificial surface
According to the structure of the biological prototype, the
regular array mastoid structure was designed on the sample
surface. And the geometry size of mastoid was as follows: basal
diameter a, spacing b and height h, as in sketched in Fig. 2. In this
experiment, the size a was 60 mm, b was 40 mm and h was 30 mm.
Fig. 3a showed the SEM image of the mastoid of the textured PDMS
X. Ye et al. / Applied Surface Science 255 (2009) 6686–6690
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Fig. 1. The heart valve of rabbit (a) mitral valve; (b) large-scale SEM image of the micro-structure of valve surface and (c) magnified image on a single cobblestone of b, and
some nano tenuous villus on it.
Table 1
Contact angle of surface of the sample (8).
Contact angle
Mastoid structure surface
Smooth PDMS surface
Fig. 2. Sketch of mastoid micro-structure: a is the basal diameter of a single mastoid,
b is the space between two mastoids and h is the height of mastoid.
surface. It was seen from the images that the surface quality of
PDMS surface with mastoid array structure was very good. There
were almost no surface distortion and damnification on the
surface. Moreover, in the enlarged SEM image of a single mastoid
(Fig. 3b), the fine structure could be observed on the mastoid. In
fact, the protrusion with sizes from 1 to 4 mm, which provided the
hierarchical structure on the artificial PDMS surface, enhanced the
hydrophobicity of the surface.
3.3. Contact angle measurements
To compare the wettability of smooth PDMS surface with that of
PDMS surface with mastoid micro-structure, the static contact
angle of water on both surfaces was measured using the sessile
droplet method and the results were shown in Table 1. The results
showed that the smooth surface was hydrophobic (CA = 113.18)
and the micro-structure surface was superhydrophobic
u1
u2
u3
u4
u5
uAV
163.6
112.3
166
112.1
163.4
113.5
160.8
113.5
164.4
114.0
163.6
113.1
(CA = 163.68). The shape of water droplet on the both surfaces
showed in Fig. 4. It was clearly seen that the droplet sit on the peak
of the roughness (Fig. 4b) and the light could pass through the
space in the depressions of the rough surface, which indicated that
a composite surface (Cassie state) was formed on the textured PDM
surface. It is indicated that the hierarchical mastoid microstructure on the surface enhanced the wettability of the surface,
although the sizes of the mastoid structure on the PDM surface
(50 mm in diameter and 30 mm in height) were greater than that of
the cobblestone-like structure (8 mm in diameter and 3 mm in
height) on the rabbit’s mitral valve. According to the Cassie–Baxter
equation [24],
cos uCB ¼ 1 þ f ð1 þ cos u Y Þ
where f is an area fraction of solid in contact with the liquid and uY
is Young’s contact angle, the geometric parameters of the mastoid
structure greatly affect the value of apparent contact angle uCB,
namely affect the wettability of the rough surface. The detail of the
effect of the different sizes of mastoids on the wettability and
hemo-compatibility of the artificial heart valve will be further
studied theoretically and experimentally in the future researches.
3.4. Platelets adhesion
Adhesion of platelets to foreign materials is one of their most
important functions. When blood contacts with the surface for a
period of time, there are a certain number of platelets adhesion to
Fig. 3. SEM images of the textured PDMS surface (a) SEM of mastoid micro-structure and (b) SEM of a single mastoid, some fine structure on it.
X. Ye et al. / Applied Surface Science 255 (2009) 6686–6690
6689
Fig. 4. Photographs of water droplets on the sample surface (a) droplet on the smooth PDMS surface and (b) droplet on the mastoid micro-structure PDMS surface.
Fig. 5. Platelets adhesion on the smooth surface: (a) low magnification and (b) high magnification.
the surface. Under the surface tension force, platelets are activated.
Then platelets are interacted with the material surface, lead to a
series of complex reaction, join together, release the coagulation
factor, and activate coagulation reaction finally.
Platelet–surface interaction is a very complex and dynamic
series of events. Platelet adhesion on the surface is invariably
followed by the appearance of platelet aggregates and platelets
spreading and subsequent thrombus formation that causes
potential danger of using artificial heart valve in vivo [25]. Platelet
adhesion and activation are the indicators of thrombosis on a
biomaterial surface. Utilizing the platelet adhesion test in vitro, the
preliminary evaluation on hemo-compatibility of the microstructure surface can be obtained through measuring the number
of platelet adhesion.
Fig. 5 showed the SEM photograph of the smooth PDMS surface
on which platelet adhesion experiment had been tested. There
were a large number of blood platelets attached to the smooth
surface (Fig. 5a). It showed the morphology of platelets on flat
PDMS surface flattened and platelets aggregated. Part of adhered
platelets had produced distortion and extended long pseudopod
(Fig. 5b). Compared with that of the smooth surface, the number of
the attached platelets on the mastoid micro-structure surface was
fewer (Fig. 6a). It was seen from the enlarged SEM image (Fig. 6b) of
the rough surface, the shape of attached platelets was whole round,
without deformation and aggregation. It showed that there was no
platelet activation on the mastoid micro-structure surface.
Why do different platelet adhesion phenomena respond to the
two different surfaces in this way? Possibly, the different
hydrophobility is affected in some way. Although the chemical
compositions of both surfaces are the same, the wettability of
surface is different. That is, the smooth PDMS surface is
hydrophobic and the mastoid micro-structure surface is super-
Fig. 6. Platelets adhesion on the mastoid micro-structure PDMS surface: (a) low magnification and (b) high magnification.
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X. Ye et al. / Applied Surface Science 255 (2009) 6686–6690
hydrophobic. It means that the surface energy of the patterned
surface is lower than that of the flat surface. So the amount of the
adhered platelets on the patterned surface is less than that of the
smooth surface. The agglomerate phenomenon and the level of
activation apparent of the smooth surface are more obvious than
that of the patterned surface. So the clotting reaction of the microstructure surface is slower than that of the smooth surface, which
makes better hemo-compatibility of the surface with mastoid
micro-structure. Moreover, the nano-structure on the mastoid
surface may extremely contribute to improve the hydrophobility
of the patterned surface and reduces the number of adhered
platelets.
4. Conclusion
In this paper, we investigate the micro-structure on the rabbit’s
mitral valve surface. It is shown that the dual-scaled structure
which consists of both micro cobblestones-like structure and nano
tenuous villus, is helpful in reducing the formation of thrombus.
Therefore, the PDMS surface with hierarchical regular microstructure is prepared using femtosecond laser fabrication technique and soft lithography. And the mastoid micro-structure surface
displays more excellent superhydrophobicity and blood compatibility than that of the smooth surface. Therefore, both surface
wettability and surface morphology play important roles in hemocompatibility. An excellent anticoagulation material should have a
superhydrophobic merit in order to adsorb platelets as fewer as
possible. Meanwhile, it should also have a relatively fine structure.
In this way, the coagulation reaction can be delayed. In addition,
the present study provides a basic investigation for the design and
fabrication of the artificial heart valve.
Acknowledgements
This research is supported by the National Natural Science
foundation of China (50435030, 50775104), the Program for New
Century Excellent Talents in Chinese University and Excellent
Young Scholars foundation of Jiangsu (BK 200607).
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