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and -OH-containing groups on another surface, e.g. PDMS, glass, or silicon,
which form covalent -O-Si-O bonds when brought into contact, creating
a tight, irreversible seal that can withstand pressures up to 30−50 psi.
8
The
ease of bonding also allows for the creation of 3D devices.
10
Additionally,
PDMS makes reversible van der Waals contact to smooth surfaces which
can also be used to seal devices and has been used in patterning.
6
The elasticity of PDMS offers several advantages. First, it is easy to
remove the PDMS mold from the master which prolongs the lifetime of the
master. Second, interfacing with external components for, e.g. sample intro-
duction, is simply performed using press-fit connections.
11
Third, it allows
for the on-chip incorporation of components, such as pumps or valves.
12
However, the elasticity limits the achievable aspect ratio since shrinking or
sagging of features can occur.
6
Furthermore, bulging of PDMS microchan-
nels under pressure-driven flow has been reported.
13
These problems may
be overcome by using PDMS with a high density of crosslink.
The chemical structure of PDMS has repeating -O-Si(CH
3
) units
which render the surface hydrophobic. However, as mentioned, exposure to
oxygen plasma or a corona, generates silanol groups and thus creates a
hydrophilic surface. If the surface is kept in contact with water or polar
solvents, the hydrophilicity is maintained. Otherwise, over time uncross-
linked PDMS chains migrate to the surface.
6
The extraction of uncross-
linked PDMS by organic solvents can delay the return to hydrophobicity.
14
The hydrophobic nature of the surface can cause several different prob-
lems, e.g. nonspecific adsorption
8,15
and absorption,
16
swelling by non-
polar solvents,
14
and difficulty in filling the channels. Numerous methods
of surface modification have been proposed,
17
including self-assembly of
charged surfactants or polyelectrolyte layers, chemical vapor deposition,
or formation of a phospholipid bilayer, to either maintain a hydrophilic
surface or to pattern the surface for, e.g. immunoassays.
18
However, the
hydrophobic properties can also be exploited to control the fluid flow
within devices.
19,20
10.1.2. Fluid mechanics
Fluid flow behaves very differently in microfluidic systems than one would
expect from everyday macroscopic observations. Characteristics of micro-
fluidic systems include viscous dominated, turbulence-free laminar flow
and high surface-to-volume ratios. To be able to design and understand
microfluidic systems, it is crucial to understand the fluid mechanics in small
spaces.
1
Fluid mechanics deals with the behavior of fluids, e.g. liquids or
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