Biomedical Engineering Reference
In-Depth Information
0.2
0.1
(b)
0.0
3
2
1
0
0
200
400
600
800
1000
(a)
Negative bias voltage (v)
FIGURE 2.12
Effects of bias voltage on: (a) intensity ratio of D band to G band and (b) ratio of polar/dispersive components of
surface energy. (Reprinted with permission from Yang et al.,
Biomaterials
, 24, 2821, 2003.)
layers, such as the protein layers [54,55], that are formed after dipping the carbon-based
surfaces into the protein solutions and drying them afterward. The sp
3
content was found
to control the hemocompatibility of the a-C films. That is, the HSA/Fib ratio increases
with the sp
3
fraction. Remarkable enhancement in the HSA/Fib ratio was observed for
the a-C:H films grown with floating biased substrates and with 10 at.% H
2
concentration
in plasma. The lowest HSA/Fib ratio was deduced for the ultrathin ta-C film examined in
this work. This can be justified either by the surface properties of this type of film and/or
its thickness, which is the lowest among the examined films.
A follow-up study by Logothetidis [56] with additional platelet morphology studies by
AFM on hydrogenated a-C was carried out. On biased a-C:H films, platelets become aggre-
gated and develop pseudopodia, as derived by AFM measurements, while on floating
a-C:H thin films, platelets are round without pseudopodia. These morphological changes
indicate platelets activation and the beginning of thrombus formation on biased films.
The ratio HSA/Fib has a smaller value on biased a-C:H thin films compared with floating
films, when protein adsorption is studied by SE. Therefore, hemocompatibility evalua-
tion through both protein adsorption and platelet adhesion, by both techniques, verifies
that the floating films are more hemocompatible than the biased ones. Protein adsorption
mechanisms were also studied by SE and AFM. It was revealed that protein adsorption
takes place very quickly. The fibrinogen adsorption rate is slower on the floating a-C:H
film. Conformation of the proteins' changes during adsorption even after the equilibrium
was observed. Based on the AFM measurements, a possible model of protein adsorption is
suggested: protein molecules initially form islands and then coalesce until they fully cover
the film's surface. The characteristics of the FIB molecule were preserved on the biased
a-C:H film, even after a long incubation time (~70 min). Modification of FIB's conformation
and interactions between molecules were also observed.
In another work by Karagkiozaki et al. [57], a study of platelets' response to a-C:H by
the implementation of AFM enhanced by IR SE method was made in order to provide a
more detailed description of nanoscale mechanisms of bio and non-bio interactions. The
nondestructive FTIRSE technique was used for investigation of the vibrational properties
of the bonding structure of the adsorbed platelets that preserve their viability, providing
