Microfluidics for Biologists Fundamentals and Applications

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incubated at temperature that exceeds the evaporation temperature of the solvent.
This allows the extra solvent to evaporate from substrate surface while a bond is
formed. However, the incubation temperature should not exceed the Tg of the
plastic as this can bring distortion of the existing features and formation of artefacts.
The plastic substrates are typically exposed to solvent vapours to minimize feature
loss at bulk solvent treatment. Critical parameter for this process is the exposure
time that can vary depending on the critical dimensions. A risk exists that miniscule
features are damaged when the exposure times are long; too short exposure times
can lead to poor bonding quality.


3.3 Bonding Through Pressure Sensitive Adhesive


Bonding through pressure sensitive adhesive (PSA) is a method that requires
minimum investment but time, effort and manual operations. The method has
been reported for sealing laser-ablated channels on a disc bonded through pres-
sure-sensitive adhesive (86μm, PSA, Adhesive Research, Inc.) to another PMMA
disc, which hosted loading, collection chambers, as well as milled channels and
valve beds [ 18 , 19 ]. The method involves cutting out the silhouette of these
structures from the PSA using a precision knife cutter from Graphtec. The extra
material is removed manually which can be time consuming and even prohibitive
for small dimensions (< 20 μm) restricted by the stylus dimensions of the knife
cutter. Alignment is achieved using pins or rig to facilitate the adjustment of the
PSA relative to the features on the substrate. For designs that include suspended,
flimsy, features a transport film needs to be used in order to handle the PSA more
easily. After alignment, the protective layers from the foil are removed and the
plastic layers are brought together. Pressure is applied to finalize the bonding
process. Ideally, a hot/cold press or laminator provide enough pressure; rubber,
manual rollers can be used in case resources are limited.


3.4 Plasma Bonding of PDMS


After casting of PDMS structures they must be de-moulded and sealed either with a
piece of plastic, glass or another piece from the elastomer. Well-established method
is plasma activated bonding of PDMS. In the process RF-generated plasma is used
to activate the surfaces of both substrate and PDMS. A radio frequency voltage is
applied between two electrodes that cause the free electrons to oscillate and to
collide with gas molecules leading to RF plasma generation. Electrons pick-up
enough energy during oscillation in the RF field to cause ionization of the gas,
which is entrapped in the reactor chamber. Steady flow of gas (e.g., oxygen,
nitrogen or air) into the reactor chamber of the plasma generator provides popula-
tion of molecules that can be ionised. Very importantly the pressure inside the


100 N. Dimov

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