Biomedical Engineering Reference
In-Depth Information
6.12.4 Fibrin formation and assembly
AFM is used to observe, in real time, the process of fi brin assembly on a
hydrophobic graphite surface under aqueous solution.
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In accordance with
the general mechanism of fi brin formation described above (Section 6.7.2),
surface-bound fi brinogen assembles into short fi brin strands during the ini-
tial phase, followed by the formation of long fi bers as more fi brin monomers
are available on the surface. Linear two-stranded protofi brils are predom-
inant at the beginning of fi brin assembly. Branch points are observed as
more fi brin strands are formed, leading to trimolecular branch points
which are encountered more frequently than the tetramolecular counter-
parts. Eventually, the surface is almost fully covered by interconnected
fi brin strands. A characteristic eyelet structure with various aspect ratios is
observed within the fi brin network. The dimensions of fi brin strands suggest
that fi brin monomers are arranged in a half-staggered structure, which is
consistent with EM studies. In contrast, the same assembly process was not
observed when the experiment was repeated on hydrophilic mica. Instead,
all the protein species (fi brinogen, thrombin, soluble fi brin) appear to main-
tain their monomeric state. This observation suggests that fi brin assembly
depends on the properties of the underlying surfaces. Specifi cally, hydro-
phobic and electrostatic interactions are important factors for fi brin(ogen)
interacting with graphite and mica, respectively. On the other hand, the
time-lapse fi brinolytic process is also observed by AFM under aqueous con-
ditions. A fully formed clot is dissolved proteolytically and the process is
completed in <15 min.
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6.12.5 Adhesion strength
A fi brinogen-coated AFM tip is pulled from a silica surface after various
contact times.
188,189
The rupture force increases proportionally to the dura-
tion of contact. It is found that the rupture force increases in proportion to
the loading time and it can vary from 0.3 to 1.4 nN depending on the dura-
tion of contact. The minimal time needed for fi brinogen to bind strongly
to the surface is between 50 and 200 ms. The inter-rupture distance shows
a maximum at around 20-25 nm, which is half of the molecular length of
fi brinogen. This implies that fi brinogen adsorbs on the surface through the
D and E domains. Further analysis demonstrates that the rupture forces are
multiple integers of 180-200 pN (Table 6.3).
By reversing the experimental setup, a colloid-probe modifi ed AFM tip
is used to probe the interaction of fi brinogen adsorbed on glass surface.
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Different colloidal probes are used to examine the adhesion force between
adsorbed fi brinogen and surfaces with different degrees of wettability. The
