Biomedical Engineering Reference
In-Depth Information
bombardment promotes fi lm graphitization, as it has been shown that such a graphitization process
is thermally driven [158].
The surface energy (
γ
s
) and its polar and dispersive components (
γ
s
p
and
γ
s
d
) are shown in
Figure 19.35 together with the contact angle of water. The results show that all the surfaces have
a hydrophobic nature with much higher
γ
s
d
than
γ
s
p
. As the absolute value of the bias voltage
increases, the polar part of the surface energy decreases by nearly 60% from 6.4 to 3.8 mJ/m
2
.
The contact angle of water on the fi lms is about 75°-85° and decreases with higher
V
b
. The trends
are consistent with the change of the
I
D
/
I
G
ratio as displayed in Figure 19.36. The surface energy
thus appears to be affected mainly by the sp
3
content in the fi lms, and Pinzari's study on the wet-
tability of diamond fi lms in fact has shown similar effects [160].
Figure 19.37 exhibits the statistical results of the platelets adhered on the a-C:H fi lm surfaces
from PRP, and expresses as a percentage of platelets adhered on the stainless steel in the same
test. After incubation in PRP for 15 min, the number of adherent platelets slightly decreases with
increasing
V
b
(absolute value) and is less than that on the stainless steel surface. The percentage of
unactivated platelets drops steeply from 30% to 8% when
V
b
is changed from
-
75 to
-
900 V. The
percentage of unactivated platelets on stainless steel is in between.
Platelets are strongly surface activated by the a-C:H fi lm deposited at high
V
b
(absolute value)
compared to those at low
V
b
(absolute value). It is consistent with the variation of the polar compo-
nent
γ
s
p
of the surface energy. The platelet attachment studies suggest that the adhesion behavior
of the platelets is related to the surface energy of the fi lm. The higher the absolute value of
V
b
,
the lower is the value of
γ
s
p
, and accordingly, the higher is the activation of adherent platelets.
Besides, the results show that when the bias
V
b
(absolute value) is increased, the interfacial energy
of albumin and fi brinogen increases from 10.8 and 15.0 mJ/m
2
to 15.9 and 20.9 mJ/m
2
, respec-
tively. It is expected because protein molecules will undergo a conformational transformation when
plasma protein adsorbs onto an artifi cial surface (higher interfacial energy) from its aqueous phase
(lower interfacial energy). Thus, it suggests that the higher the interfacial energy
γ
sp
, the larger the
conformation changes. Moreover, exacerbation of activation of the platelets adhered on the a-C:H
fi lm deposited at high
V
b
is probably due to the changes of fi brinogen conformation.
60
90
s
d
s
p
50
w
85
40
30
80
20
75
10
0
70
a-C:H-1
a-C:H-2
a-C:H-4
FIGURE 19.35
Surface energies (
γ
s
) and their polar and dispersive components (
γ
s
p
and
γ
s
d
) of the a-C:H
fi lms. (From Yang, P. et al.,
Biomaterials
, 24, 2821, 2003. With permission.)
