Biomedical Engineering Reference
In-Depth Information
on COOH- and NH 2 -SAMs showed that cell adhesion reached equilibrium after
60 min of incubation, but the adhesion area increased up to 180 min. In contrast, no
spot was observed on SAMs with CH 3 and OH functionalities. The number and total
area of bright spots were much lower on OH- and CH 3 -SAMs than on COOH- and
NH 2 -SAMs at all times observed. After 60 min, some bright spots were observed on
OH-SAMs, but not on CH 3 -SAMs. This indicated that HUVECs adhered poorly to
OH-SAM surfaces and worse to CH 3 -SAMs. The effects of surface functional
groups on SAMs have been extensively studied with various cell lines. Those studies
showed that most cell types adhered well to COOH- and NH 2 -SAMs and poorly to
CH 3 - and OH-SAMs [ 45 - 50 ].
Other studies used various polymers to investigate how cell adhesion depended
on the wettability of materials. They showed that cells adhered well to moderately
wettable materials with water contact angles of 40-60 [ 51 - 57 ]. However, the
results were inconclusive. They employed polymeric materials composed of differ-
ent chemicals that changed wettability concomitantly with changes in the SAM
surface functionality. We performed a study with mixed SAMs to determine the
effect of contact angles on cell adhesion [ 21 ]. SAMs with widely varying
wettabilities were prepared by mixing two alkanethiols with different functional
groups (CH 3 /OH, CH 3 /COOH, and CH 3 /NH 2 ). Figure 3 shows that the number of
adherent cells depended on the contact angles of SAMs for both HUVECs and
human cervical carcinoma (HeLa) cells. The maximum number of adherent cells
occurred at different contact angles for all of these mixed SAMs, and it was
different for different cell types. Thus, the design of a surface that promotes cell
adhesion should take into consideration both the type of surface functional groups
and the types of cells targeted.
2.3 Effect of Protein Adsorption on Cell Adhesion
When cells are suspended in a biological fluid or culture medium, both serum proteins
and cells interact with the surface substrate. Serum protein adsorption behavior on
SAMs has been examined with various analytical methods, including SPR [ 58 - 61 ],
ellipsometry [ 13 , 62 , 63 ], and quartz QCM [ 64 - 66 ]. These methods allow in situ,
highly sensitive detection of protein adsorption without any fluorescence or radioiso-
tope labeling. SPR and QCM are compatible with SAMs that comprise alkanethiols.
In our laboratory, we employed SPR to monitor protein adsorption on SAMs.
SPR detects changes in the refractive index near the surface of a metal film. Two
optical configurations, Kretchmann (Fig. 4a ) and Otto, can be applied with SPR.
The former is easily set-up and is suitable for protein adsorption on SAMs. A beam
of p -polarized light is directed, through a prism, to the back of a sample glass plate,
which is coated with a metal thin film. The front of the sample glass plate faces the
solution of interest. When the incident angle,
exceeds the critical angle, total
internal reflection occurs. An evanescent wave is generated on the surface facing
the solution, which has a lower refractive index than glass. At a specific incident
y
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