Biomedical Engineering Reference
In-Depth Information
of time to induce platelet adhesion and aggregation.
98
Fibrinogen undergoes
a time-dependent loss in α helical content and gain in β structures upon
adsorption on non-modifi ed polyurethane (PU).
25
However, these changes
are not observed when PU is coated with heparin.
26
Similar results in the
changes of α helical and β sheet content are revealed by circular dichroism
when fi brinogen adsorbs on titanium oxide and hydroxyapatite.
104
6.10 Competitive adsorption
Competitive adsorption of fi brinogen with other blood plasma protein has
been extensively studied. In general, other proteins have very little effect
on fi brinogen adsorption. Additionally, fi brinogen displaces other surface-
bound proteins.
10
Recent research on competitive fi brinogen adsorption was
carried out on a variety of surfaces.
32,35,93,95,96,99,105,106
In general, adsorption of
fi brinogen from undiluted blood plasma onto material surfaces, such as glass
and polyethylene, shows a rapid increase in the total amount of adsorbed
proteins within minutes and then a gradual decrease.
32
Additionally, max-
imum fi brinogen adsorption on polymers like polyurethanes occurs at an
intermediate dilution of plasma.
35
The amount of fi brinogen adsorbed on
any given surface is generally higher than that of many proteins. Also, fi brin-
ogen adsorbs slightly more on many hydrophobic materials than on other
surfaces.
10,34,95
In particular, competing against albumin, fi brinogen shows
preferential adsorption on hydrophobic surfaces.
107
However, the extent of
fi brinogen adsorption can be substantially reduced when the surface is fi rst
passivated with albumin.
99,106
This observation is explained by the ability
of albumin to exert a relatively high degree of interfacial relaxation on a
hydrophobic surface, which reduces available adsorption sites for fi brino-
gen.
95
In contrast, after considerable residence time, adsorbed fi brinogen on
a hydrophobic surface is unable to be displaced by other plasma proteins.
This is attributed to the post-adsorptive structural changes of fi brinogen.
38
6.11 Atomic force microscopy (AFM)
Atomic force microscopy (AFM) is a relatively young, versatile microscopic
technique that allows the examination of structures and/or functions of
biomolecules at their natural, physiological-like conditions. The operational
principles of AFM and various modes of operation are described briefl y in
the following.
6.11.1 Operation principles
In AFM, a probe tip or stylus with a radius of curvature on the order of
tens of nanometers is in contact with the sample surface. Owing to the fi nite
