Biomedical Engineering Reference
In-Depth Information
(a)
(b)
(c)
10 μm
10 μm
10 μm
FIGURE 2.19
Morphology of adherent platelets on (a) Si, (b) a-C:H, and (c) a-C:H(F) surfaces observed using SEM. (Reprinted
with permission from Saito et al.,
Diamond Rel. Mater.
, 14, 1116, 2005.)
Liu et al. [73] evaluated the hemocompatibility of Ti alloys with and without phosphorus
incorporated tetrahedral amorphous carbon (ta-C(P)) films. (These films may be hydroge-
nated as PH
3
was used as the precursor gas.) Platelet spreading and morphology are numer-
ically estimated using the size and circularity indexes of platelets [74]. Low-phosphorus
ta-C(P) film (<6at.% P) with the lowest indexes exhibits the slightest adhesion, spreading,
and activation of platelets. However, a high content of phosphorus (~8 at.%) increases the
fraction of sp
2
-hybridized carbon atoms in ta-C(P) film and the ordering of structure, leading
to a loss in blood compatibility (refer to Figure 2.20). They suggest the graphitized structure
decreases the proportion of the polar component in surface energy, thus making the activa-
tion of platelets on the surface stronger.
In another study [75], they varied the deposition bias voltage of ta-C(P) (presumably
<8at.% P) and found that all ta-C(P) films exhibit more hydrophilic surfaces than undoped
ta-C film. Platelet adhesion tests show that ta-C(P) films with less spreading area and cir-
cularity index of platelets represent lower adhesion and activation of platelets in terms of
the higher polar-dispersive ratio (as reviewed by the determined surface energy).
Kwok et al. [76] also observed a drastic increase in the polar component of the surface
energy (which is also increased) of their phosphorus-doped a-C:H deposited by a plasma
immersion ion implantation and deposition system as compared to undoped a-C:H. It is
highly possible that this is due to the various PO
x
and CP
x
O
x
species near the surface of the
film. Platelet adhesion and activation are suppressed on the a-C:H(P) sample compared to
the undoped a-C:H film. Their results suggest that one of the reasons for the good hemo-
compatibility is that the a-C:H(P) coating significantly minimizes the interactions with
plasma protein, giving rise to slight changes in the conformation of adsorbed plasma pro-
teins and preferentially adsorbed albumin.
Effects of Post-Deposition Treatment
Zhang et al. [77] studied the effect of post-deposition annealing temperature on the hemo-
compatibility of silicon-incorporated amorphous carbon. The number of adhered plate-
lets and the fraction of inactivated platelets are presented in Figure 2.21 as a function of
the annealing temperature. The number of adherent platelets increased when the anneal-
ing temperature was increased. Although this may be viewed as a degradation of blood
