W. Richard Bowen and Nidal Hilal 4
W. Richard Bowen and Nidal Hilal 4
W. Richard Bowen and Nidal Hilal 4
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7.2 ENgINEERINg THE ECM FOR PRObINg CELL SENSINg 199<br />
because of these challenges that intense effort from many fields has been<br />
attracted, leading to the combination of micro- <strong>and</strong> nanotechnology with<br />
biological sciences <strong>and</strong> the formation of the interdisciplinary ‘bionanotechnology’<br />
field.<br />
Advances in micro- <strong>and</strong> nanofabrication can produce precisely controlled<br />
model systems at a single molecule level, <strong>and</strong> provide a systematic<br />
approach to dissect the roles of intertwined parameters in the ECM. In<br />
combination with other approaches (including optical microscopy, AFM<br />
<strong>and</strong> scanning electron microscopy [SEM]), these fabrication methods have<br />
proven to be powerful in the elucidation of complex cell behaviour. Here<br />
we will discuss the development of engineered substrates for the investigation<br />
of cell interactions with the ECM. Specifically, we will review the<br />
achievements of these substrates in mimicking chemical <strong>and</strong> physical cues<br />
in the ECM. Although not explicitly stated in the review below, many of<br />
these studies described have been underpinned by AFM measurements of<br />
surface interactions, topography or materials compliance.<br />
7.2.1 Surface Patterning (Chemical Signals)<br />
Micropatterning<br />
Micropatterning is based on photolithography [19], which produces<br />
features with dimensions over 1 �m. As illustrated in the schematic representation<br />
in Figure 7.2(A), this process involves the UV irradiation of a<br />
spin-coated photosensitive polymer layer (photoresist) through a mask.<br />
UV irradiation through the mask causes the photoresist polymer chains<br />
to either break up (positive resist) or crosslink (negative resist), leading<br />
to a difference in solubility between the exposed <strong>and</strong> unexposed regions<br />
when immersed in a “developer” solution. After developing, the patterns<br />
from the mask have been effectively transferred onto the substrate. The<br />
patterned photoresist can then serve as a protecting layer in subsequent<br />
processes, such as lift-off to produce metal patterns, or etching to generate<br />
a relief on the substrate.<br />
Micropatterning has been extensively used for surface patterning of<br />
biological molecules. Its main purpose is to allow fine control over the size<br />
<strong>and</strong> spatial arrangement of regions that can be specifically functionalised<br />
for the attachment of ECM proteins. For this, selective immobilisation of<br />
adhesive molecules on the patterned area is required. It should be noted<br />
that preventing the physisorption of biomolecules (especially proteins)<br />
on both the patterned <strong>and</strong> non-patterned surfaces is equally important, as<br />
non-specifically adsorbed proteins could also serve as an adhesive region<br />
for cell attachment.<br />
A key development in the generation of chemically patterned substrates<br />
has exploited the formation of self-assembly monolayers (SAM) from<br />
heterobifunctional organic molecules that bear specific functional groups