How Do We Make Small Things? - Introduction to BioMEMS - page 26

5. For some applications in which surface purity is paramount, there is the concern that,
at least under certain conditions, small amounts of dimethylsiloxane monomer seem
to remain on the areas where the PDMS stamp has contacted the surface.
6. Fabrication of the stamp is a very low-throughput process because it takes hours to pro-
duce a single PDMS replica and peeling it of the master is not easily automated. his
limitation has, no doubt, slowed down the adoption of sot lithography by the industry.
In summary, microstamping is inferior to photolithography in pattern idelity (the stamp
deforms—a severe problem because it is pattern-dependent) and in fabrication throughput
(molding is time-consuming—a severe problem because it is diicult to automate), but clearly
superior in cost-efectiveness and materials versatility.
1.6.4 Microluidic Patterning
An interesting variation of microstamping takes advantage of the fact that the PDMS stamp can
be designed to form a network of microchannels on the areas where the stamp does not contact
the surface (see Figure 1.18 ). he microchannels can thus be used to deliver luids onto selected
areas of a substrate, a strategy we refer to as microluidic patterning . he luid can, depending
on its nature, be either cured into a solid itself, used as a vehicle to deposit a material that remains
when the microchannels are peeled of, or used to remove (i.e., dissolve or etch) underlying mate-
rial. herefore, microluidic patterning can be an additive as well as a subtractive technique.
As opposed to microstamping, in (additive) microluidic patterning, the material is added
where the PDMS does not come into contact with the surface. he luids are blocked from wet-
ting the substrate in the areas where the PDMS contacts the surface thanks to the unique prop-
erty of PDMS that it self-seals reversibly against another smooth, dry surface (see properties of
PDMS mentioned in Section 1.6.2). his is probably because of a combination of its elastomeric
nature allowing for a highly conformal contact and its hydrophobic surface, which impedes
wetting of liquids into the PDMS-substrate interface. On the other hand, solvents easily wet
PDMS, which can be used to quickly ill microchannels with a liquid precursor to the polymer
(“prepolymer”) by capillary action—a technique developed by Whitesides' group and dubbed
micromolding in capillaries or MIMIC ( Figure 1.23 ). Other materials such as crystals and
beds of packed beads have also been patterned by MIMIC.
Microchannels featuring deeper and wider proiles can be illed by pressure-driven low over
larger areas than shallower microchannels. Shown in Figure 1.24 are computer tomography
images of polyurethane microstructures. Two sets of straight microchannels were sealed facing
each other at 90 degrees, illed with the polyurethane precursor, then exposed to UV to cure the
precursor into polyurethane ( Figure 1.24a ); at these dimensions, manual stacking (and bonding
with polyurethane precursor again) produces reproducible 3-D structures ( Figure 1.24b ).
In 1987, Friedrich Bonhoefer's group, then at the Max Planck Institute in Tübingen (Germany),
was the irst to use micromolded PDMS microchannels to microluidically deliver biomolecules such
as cell membrane fragments for axon guidance experiments (which came to be known as the “stripe
assay” in the axon guidance community; see Figure 6.41 in Section 6.5.1.1); Bonhoefer's technique
required a porous substrate to capture the biological material. Hans Biebuyck and colleagues at IBM
Zurich have been credited for being the irst to demonstrate, in 1997, microluidic patterning of puri-
ied proteins on solid substrates (a somewhat incremental claim with respect to Bonhoefer's, consid-
ering the IBM study was published in Science ; see Figure 2.18 in Section 2.4.3). he most attractive
aspects of microluidic patterning are:
1. he experimental protocols traditionally used for coating the surface of an open
petri dish with any given biomolecule are translatable directly to the microluidic
format: the solution of interest is introduced in a microchannel and allowed to deposit
onto the surface under the same conditions as the traditional protocol that has been
known, oten for decades, to work well for that surface.