Biomedical Engineering Reference
In-Depth Information
TABLE 2.3
Base (−) and Acid (+) Contributions to Polar Component of a-C(Si37.6at.%) with Increasing
Annealing Temperatures
As-deposited
200°C
400°C
600°C
21.3
20.5
17.7
14.9
γ
− (mN/m)
0.30
0.32
0.36
0.08
γ
+ (mN/m)
Source:
Zhang et al.,
Thin Solid Films
, 515, 66, 2006. With permission.
value, no significant influence is expected for the adsorption of platelets since the platelets'
size is about 2
µ
m. Proteins have a similar size of a few nanometers. For instance, albu-
min globule has a triangular shape of about 8 nm on the sides and 3 nm in thickness [80].
Therefore, the roughness (
R
rms
~0.76 nm), though low, might have a certain effect on the
adsorption of the proteins through mechanical interlocking. The changes in the number of
adherent platelets and activation fraction are not likely to be affected by the changes in the
internal bonding structure of the films as the films are thermally stable up to 600°C.
Table 2.3 shows that the base components for all the films are again much larger than the
acid components, indicating that the polar component of the surface is predominantly nega-
tively charged. Therefore, as discussed earlier, there will not be any preferential adsorption
of any of the proteins and platelets due to electrostatic interaction if the polar component is
increased. However, computing the relative contribution of the respective surface energy
component reveals that the polar-to-dispersive ratio decreased as the annealing tempera-
ture was increased (see Figure 2.23). Therefore, the repulsion between the film surface
and proteins decreases as the annealing temperature increases. The adsorption of both
albumin and fibrinogen will now increase, with a slightly stronger effect on fibrinogen
since it has a higher polar component. The result is a higher count of adherent platelets on
the a-C(Si) annealed at higher temperatures. On the other hand, since the dispersive com-
ponent of the films is much larger than the polar component, the interaction is predomi-
nantly based on dispersive forces. Since albumin has a higher dispersive component, it
12
10
8
6
4
2
0
200
400
600
Annealing temperature (°C)
FIGURE 2.23
Ratio of polar to dispersive surface energy component on a-C(Si37.6at.%) film annealed at different tempera-
tures. Ratio decrease at high annealing temperatures is mainly due to decrease in polar component. Since there
is a decrease in surface and subsurface O level after thermal treatment, polar bonds such as Si-O and C-O also
decrease, thereby causing decrement of the polar component of surface energy. (Reprinted with permission
from Zhang et al.,
Thin Solid Films
, 515, 66, 2006.)
