Biomedical Engineering Reference
In-Depth Information
complexed to surface-bound fi brinogen molecules is then indicated by
phase imaging.
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6.13 Future trends
The advent of AFM provides an essential tool to probe molecular level
biological events. Owing to the particular setup of AFM, namely, the
raster-scanning motion of a microfabricated tip on a surface, it is particularly
relevant to examine the interactions of proteins on biomaterial surfaces.
The versatility of AFM to operate in different ways provides the opportu-
nity to determine different interaction parameters using only one piece of
equipment. Fibrinogen is emphasized in this review, but the discussion is rel-
evant to other proteins as well. From the early period when mostly the static
morphology of adsorbed proteins was studied, to the dynamic measurement
of protein spreading, to the functional assays of adsorbed protein and to the
force determination of protein-surface and protein-protein interactions,
the use of AFM to probe protein-surface interactions provides abundantly
new information at molecular level that would be useful in biomaterial sci-
ence and engineering. Through AFM studies, it is now clear that the proper-
ties of the substrate have a major infl uence on the structures and functions
of adsorbed fi brinogen. Since fi brinogen possesses unique characteristics,
distinct responses on surfaces with different physiochemical properties are
observed.
While the initial adsorption rate of fi brinogen on relatively hydropho-
bic and hydrophilic surfaces appears to be comparable, the morphology of
fi brinogen undergoes post-adsorptive time-dependent changes to different
levels. The functional states of adsorbed fi brinogen follow a similar trend.
Generally speaking, more extensive structural rearrangement of adsorbed
fi brinogen molecules is observed on hydrophobic surfaces than on hydro-
philic ones, leading to a tighter binding, less prone to be removed, and a
lower level of surface coverage. A subtle difference is observed, however,
depending on the chemical composition of the underlying substrates that
lead to the resulting degree of surface hydrophobicity.
The number of different AFM functionalities is unmatched as compared
to other surface analysis techniques. The setup of AFM, though, poses limi-
tations for probing certain molecular events. Chief among those is the tem-
poral resolution that is mainly in the minutes for probing protein events at
the molecular level on biomaterial-relevant surfaces. It is thus diffi cult, if not
completely impossible, to have high-level resolution on molecular motions
that usually occur at an order of milliseconds or smaller. Additionally, as the
probe tip interacts with the protein sample through intermolecular interac-
tions, it is unlikely to completely avoid any possible perturbation by the
