Biomedical Engineering Reference
In-Depth Information
with a metal such as gold, silver, or platinum. SAMs are commonly formed on a
thin gold layer coated on a glass plate, due to its easy preparation and its stability in
the ambient environment. Furthermore, gold is compatible with sensitive methods
for analyzing surface phenomena, including surface plasmon resonance (SPR) [
12
],
ellipsometry [
13
], Fourier-transformed infrared-reflection adsorption spectroscopy
(FTIR-RAS) [
13
], and quartz crystal microbalance (QCM) [
14
]. The gold-sulfur
bond is relatively stable, with a
D
H
28 kcal/mol [
15
,
16
]. In addition,
alkanethiols self-assemble through van der Waals interactions between alkyl
chains. Alkanethiols with long alkyl chains (
n
>
11) form closely packed SAMs,
with approximately 21.4
˚
2
of occupied area per molecule [
17
,
18
]. Due to the thiol
anchoring to the gold and the close packing of the alkyl chain, another terminal
group, X, can be effectively displayed on the surface of the SAM. Alkanethiols with
various functional groups are commercially available, and SAMs with different
functional groups are easily prepared.
The surface properties of SAMs can be finely controlled by coadsorbing a
mixture of alkanethiols with different functional groups. The composition of a
SAM can be determined by spectroscopic methods like FTIR-RAS and X-ray
photoelectron spectroscopy (XPS). The fraction of given alkanethiol on the
mixed SAM surface reflects its mole fraction in the mixed solution, but it is not a
linear relationship. The adsorbed fraction is highly dependent on the chain length of
alkanethiol [
19
] and its terminal functional group [
20
,
21
]. The water contact angle
measurement of mixed SAMs gradually changes with the fraction of mixed SAMs
on the surface, and the relationship can be approximately expressed with Cassie's
equation. Thus, the preparation of mixed SAMs with different alkanethiols allows
us to systematically change the surface properties to produce a variety of different
model surfaces.
A micropattern can be printed onto the SAM surface. This is achieved by UV
light irradiation through a photomask to cause photodegradation of alkanethiols
[
22
-
24
] or by microcontact printing with a pattern stamp made from poly(dimethyl-
siloxane) [
25
,
26
]. Micropatterned SAMs have been employed as a high-throughput
platform for studies on biomolecular interactions that included arrays of DNAs
[
27
-
29
], proteins [
30
-
33
], and cells [
34
-
37
]. Micropatterned SAMs have also been
used to examine cell fate after controlling the geometry of cell adhesion on a
micrometer scale [
38
-
41
].
2.2 Cell Adhesion on Material Surfaces
When a cell suspension is applied to a surface, the events that occur can be
conceptually classified into three stages: (1) a cell approaches the surface, (2) the
cell attaches to the surface, and (3) the cell adheres, and thus, spreads out on
the surface. Most studies of cell adhesion on artificial materials measure the number
of adherent cells, the cell morphology, and changes in protein expression. To
gain more detailed insight into the biophysical mechanism of cell adhesion requires
