W. Richard Bowen and Nidal Hilal 4

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