Biomedical Engineering Reference
In-Depth Information
et al
.
[58,59] also show better hemocompatibility of a-C:H deposited at a floating condition
by studying the HSA/Fib ratio on the films.
The hemocompatibility of ta-C and low temperature isotropic pyrolytic carbon (LTI car-
bon) (a material used clinically used for artificial heart valves) was compared [60]. By evalu-
ating the hemolysis ratio (measuring absorbencies of samples immersed in different diluted
blood solutions) and observing the platelet morphology, their results indicate ta-C's having
a better anticoagulation property than LTI carbon. The hemolysis ratio of ta-C film is much
lower compared to LTI carbon, and there are significantly much lower platelet adhesion,
aggregation, and activated platelets as well. In order to understand the observed hemo-
compatibility results, the surface energy of the films was evaluated to analyze surface
adsorption of HSA and Fib. However, it was found that the ta-C films are identical to LTI
carbon with respect to protein adsorption. It is reported that the formation of thrombus
on the biomaterial surface is correlated with charge transferring from the inactive state of
the protein (e.g., fibrinogen) to the material surface [61]. During this process, fibrinogen is
oxidized and transforms to fibrin monomer, which then cross-links to give an irreversible
thrombus. Fibrinogen has an electronic structure similar to a semiconductor with a band
gap of 1.8 eV [62]. The effective work function of the ta-C films is approximately 1.0 eV,
and LTI carbon's is a graphitic material with an effective work function of approximately
5.5 eV [60]. Therefore, the charge transferring from the inactive state of fibrinogen to the
ta-C films is much more difficult than that to the surface of LTI carbon. Consequently, the
platelet adhesion, which is related to fibrinogen adsorption and decomposing processes, is
prevented, resulting in improved anticoagulation property.
Nurdin et al
.
[63] also show that amorphous carbon coating prolonged the clotting time
and tended to suppress the platelet and complement convertase activation. Fedel et al
.
[64] conducted an extensive study on several commercially available amorphous carbon
coated substrates. They showed that a low platelet activation and aggregation degree on
the carbon-coated surfaces generally related to a low fibrinogen/albumin adsorption ratio.
It was proposed that human serum albumin rapidly adsorbs on and strongly binds to
the coated surfaces, causing amorphous carbon surface passivation and preventing high
adsorption of fibrinogen and other platelet adhesive proteins.
Effects of Doping
Ong et al
.
[65] studied the effect of silicon incorporation on the hemocompatibility of mag-
netron sputter deposited amorphous carbon films. The number of adhered platelets and
the fraction of inactivated platelets are presented in Figure 2.14. From a
t
-test, there is
no significant difference between the count on a-C(Si7.4at.%) and a-C, while the rest are
significantly different (having a
p
value smaller than 0.05, especially a-C(Si37.6at.%) with
p
< 0.001). In addition, the result from ANOVA shows that Si concentration in a-C indeed
influenced the number of adhering platelets. The results clearly show a decrease in the
number of adhering platelets as well as in the fraction of activated platelets as the Si con-
centration in the film is increased. The decrement is comparable to Raman spectroscopy
results (refer to Figure 2.15), suggesting the increasing Si concentration and sp
3
/sp
2
ratio
improve the hemocompatibility of the film. However, the evolution of the surface proper-
ties of these films is not directly related to the platelet count. There is a decrease in the total
surface energy until the Si concentration of 16.6 at.%. The decrease in the platelet count
with decreasing total surface energy in the low Si concentration region is also shown by
Okpalugo et al
.
[66] on hydrogenated a-C films. The results of films with higher Si con-
centration cannot be compared, as there is currently no hemocompatibility study done on
