Biomedical Engineering Reference
In-Depth Information
will have a more stable interaction with the film surfaces, thus contributing to the decrease
fraction of activated platelets.
Another study conducted by Okpalugo et al. [66,69] on hydrogenated silicon-incorpo-
rated amorphous carbon also showed that thermal annealing can have a detrimental effect
on the hemocompatibility of the films, but only at higher temperatures (~600ºC and above).
The results of the platelets aggregation study on a-C:H and a-C:H(Si) films for platelet incu-
bation indicated an increase in the level of platelet aggregation at two extremes: when the
films had a higher electrical resistivity and when graphitization had occurred in the films
(see Figure 2.24). Silicon-doped films with lower electrical resistivity than the undoped
films showed a much lower level of platelet aggregation. This trend is consistent with the
report of Bruck on pyrolytic polymers [81,82]. Bruck observed clotting times six to nine
times longer than those observed with nonconducting polymers and also observed little or
no platelet aggregation on electroconducting polymers, when compared to nonconducting
control samples. In the investigation by Okpalugo et al
.
, thermal annealing of a-C:H below
400°C leads to lower levels of platelet aggregation similar to that observed in a-C:H(Si).
This is attributed to the increased electronic conduction before graphitization at the lower
annealing temperature for a-C:H. Zhang et al. [83] also observed more platelet adhesion
and activation when annealing hydrogenated ta-C at 300ºC and above.
The hemocompatibility of plasma-treated, silicon-incorporated hydrogenated amorphous
carbon films was investigated by Roy et al. [84]. a-C:H(Si) films with a Si concentration of 2
at.% were prepared using a capacitively coupled radio frequency plasma assisted chemical
vapor deposition method. The a-C:H(Si) films were then treated with O
2
, CF
4
, or N
2
glow
discharge for surface modification. The aPTT determines the ability of blood to coagulate
through the intrinsic coagulation mechanism. It measures the clotting time from the activa-
tion of factor XII through to the formation of fibrin clot [86]. The aPTT also governs how
a biomaterial affects the coagulation time. The enzymatic activities that lead to clot for-
mation are measured through aPTT measurement. In Figure 2.25, the aPTT of untreated
a-C:H(Si) film is compared with those of the plasma-treated films for an incubation time
of 60 min. It is evident that the O
2
plasma-treated a-C:H(Si) films had a higher aPTT. This
indicates that the O
2
plasma-treated a-C:H(Si) films have a tendency to retard the intrin-
sic coagulation activities of blood compared with the other samples, which is consistent
with the protein adsorption behavior. Similar behavior was observed with N
2
plasma-
treated a-C:H(Si) films. In contrast, CF
4
plasma treatment did not induce a notable change
in aPTT when compared with untreated a-C:H(Si) film. The N
2
and O
2
plasma-treated
Platelet aggregate count for 15 min
incubation on films
2.5
2
1.5
DLC
Si-DLC
1
0.5
0
Annealing temperatures
FIGURE 2.24
Platelet aggregate count on a-C:H and a-C:H:Si(7.6at.%) samples. (Reprinted with permission from Okpalugo
et al.,
Biomaterials
, 25, 239, 2004.)
