Biomedical Engineering Reference
In-Depth Information
interest [
19
,
62
,
76
], or radiolabeled proteins can be added to the serum [
45
,
78
].
Lestelius et al. used ellipsometry to examine antibody binding to 12 different
plasma proteins that adhered to SAMs exposed to human plasma. Those SAMs
carried terminal methyl
(
CH
3
),
trifluoromethyl ester
(
OCOCF
3
), sulfate
(
OH) groups [
62
]. Adherence of
proteins relevant to coagulation and complement activation depended on the sur-
face functionalities of SAMs. The sulfate and the carboxyl surfaces captured
coagulation proteins, like high molecular weight kininogen, factor XII, and
prekallikrein; the hydroxyl surface captured low amounts of complement protein,
C3c. Tidwell et al. reported the adsorption of radiolabeled Fn to SAMs with
terminal methyl, hydroxyl , carboxyl , and methyl ester (
OSO
3
H), carboxyl (
COOH), or hydroxyl (
COOCH
3
) groups in
10% bovine serum [
45
]. COOH-SAM exhibited high levels of Fn adsorption,
which correlated well with the adhesion of endothelial cells.
Figure
4c
shows that the amount of adsorbed proteins is rapidly saturated
within several minutes of exposing serum-containing medium to a surface.
Albumin, the most abundant serum protein, was expected to preferentially
adsorb onto the surfaces during early time points. Then, adsorbed albumin
was expected to be displaced by cell adhesion proteins. To investigate the effect
of preadsorbed albumin displacement on cell adhesion, SAMs were first
exposed to albumin; then, HUVECs suspended in a serum-supplemented
medium were added [
21
,
42
]. Very few cells adhered to hydrophobic SAMs
that had been pretreated with albumin, due to the large interfacial tension
between water and the hydrophobic surfactant-like surface. Albumin was infre-
quently displaced by the cell adhesive proteins Fn and Vn. One the other hand,
HUVECs adhered well to hydrophilic SAM surfaces that had been preadsorbed
with albumin. In that case, the preadsorbed albumin was readily displaced by
cell adhesive proteins.
2.3.3 Protein Conformation and Orientation on a Surface
Proteins undergo conformational (or orientational) changes after adsorption to a
material surface, which can influence their subsequent biological functions. Cells
adhere to a surface through an interaction between integrin receptors on the cell
membrane and the specific amino acid sequences, RGD and PHSRN, of cell
adhesion proteins. When cell adhesion proteins undergo a conformational (or
orientational) change that hinders the integrin binding site, cells cannot adhere to
the surface.
The conformation and orientation of adsorbed proteins has been examined with
monoclonal antibodies that recognize a specific site in a protein of interest.
Keselowsky et al. examined the conformation of Fn adsorbed to SAMs that carried
methyl, hydroxyl, carboxyl, and amine groups [
79
]. They used monoclonal antibodies
that recognized the central cell-binding domain of Fn near the RGD motif. Different
SAM functionalities differentially modulated the binding affinities of the monoclonal
antibodies (OH
COOH
ΒΌ
NH
2
>
CH
3
). The strength of cell adhesion to these
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