Biomedical Engineering Reference
In-Depth Information
COS7 - 6 h
COS7 - 6 days
100 µm
FIGURE 2.31
Optical micrographs of COS7 on a-C(Si37.6at.%). Cells exhibit long lamellipodes indicating active cell migration.
Long and fine cytoplasmic extensions indicate that there is good adhesion of cells to film, and thus good initial
attachment. On sixth day of culture, complete colonization of cell line is observed. (Reprinted with permission
from Ong et al.,
Thin Solid Films,
516, 5152, 2008.)
Cell adhesion and spreading are influenced by the physicochemical characteristics of
the underlying solid surface. The substrate surface free energy can affect the cell spread-
ing. Poor spreading on hydrophobic surface and good spreading on hydrophilic surface
can be observed in both the absence and presence of preadsorbed serum proteins [108].
Therefore, the substrate characteristics extend through the adsorbed proteins and affect
cell adhesion and spreading. Cells can reach the underlying substrate by pseudopodia pro-
truding through the preadsorbed protein layer, cells can consume preadsorbed proteins
to make direct contact, and/or the substrate characteristics are reflected in the composi-
tion and conformation of adsorbed proteins, thus presenting different molecular groups
to adhering and spreading cells. All proteins have NH
2
and COOH groups at their ends.
The NH
2
tends to be positively charged and the COOH groups negatively charged [109].
a-C(Si) includes C-O and Si-O bonds at the surface that are polarized due to the difference
in electronegativity between each element. Thus, the proteins can electrostatically bond
with the film's surface. Since a-C(Si37.6at.%) has the highest polar component of the surface
energy, it has better affinity with the proteins and in turn attracts more cells to adhere,
spread, and proliferate.
L929 fibroblast adhesion is also improved when nitrogen is doped into hydrogenated
amorphous carbon [110,111]. The cell adhesion percentages to silica substrates and a-C:H
without nitrogen doping were about 28% and 67%, respectively. The cell adhesion per-
centage increased with increasing N concentration, and the best cell adhesion percentage
was about 85%. The Raman I
D
/I
G
ratio is found to be increasing with increasing N con-
centration, that is, sp
3
fraction is decreasing. As can be expected, the film becomes more
hydrophilic when N concentration is increased. It is considered that NH bonds and CN
single and triple bonds exist at the surface in a-C:H(N). It is suggested that the surface of
a-C:H(N) electrostatically bonds with protein molecules because a-C:H(N) polarizes at the
surface due to the difference in electronegativity between carbon and nitrogen, and that
a hydrogen bond is formed between electrically negative oxygen or nitrogen and NH
2
or
NH groups. It is considered that the number of proteins attached on the material's surface
