According solvents cannot wet PDMS. Therefore hydrophobic

According to the European
Centre for Ecotoxicology and Toxicology for Chemicals “PDMS is not hazardous
and has not been classified under the European Commision for Dangerous
Substances Directive (67/548/EEC) and its subsequent amendments.” 7

PDMS is, at ambient
temperature and pressure, a clear and colourless and highly viscous liquid. It
is also odourless and barely changes its physical properties for higher degrees
of polymerisation 7. When PDMS is obtained commercially it usually
has a purity of 99.5% by weight. The common impurities are a mixture of other
siloxanes depending on the production process 7. By adding a cross-linking agent, PDMS turns
into a hydrophobic elastomer after some time depending on the temperature. The
hydrophobic surface means, that water and other polar solvents cannot wet PDMS.
Therefore hydrophobic molecules that are solved in the water are absorbed by
the PDMS surface.

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PDMS commonly finds use across a variety of everyday
products. It can be found in shampoos and other cosmetics as a defoamer 2. It is also used in food due
to its biocompatibility. Examples are cooking oils. The function in these
cooking oils is to prevent oil splatter by functioning as an antifoaming agent 20, 32. Another mostly unknown use,
is the use as part of the lubricant for condoms 7. In the framework of this
this project, the most important use of PDMS is the use as stamp resin for soft
lithography and microfluidic bulk material, making it the most widely adapted
material for microfluidic chips.

The widespread use of
PDMS can be awarded to its many favourable properties. It is transparent in the
visible spectrum, which allows the visual observation of fluids in microfluidic
channels. It is also mostly not auto-fluorescent 26. As mentioned before it has a
high degree of bio-compatibility and is safe to use or ingest for humans.
Regarding the fabrication of microfluidic devices, it is highly advantageous,
that PDMS bonds tightly to glass or other PDMS layers after a plasma treatment.
Due to this, devices can be multilayered or functional layers (metal, oxide,
surface functionalization) can be deposited on glass, which would be very hard
to do on PDMS itself. PDMS is also deformable, which allows applications like
the microfluidic valves, explained in chapter 2.2..
The cost of PDMS is very low, which makes it suitable for rapid prototyping or
academic trainings. Another point, that makes it great for training purposes,
is that after mixing with cross-linking agent the PDMS needs a very long time
to solidify at room temperature. This allows for easy mould fabrication.
Another important advantage, is that it is gas- permeable. This allows air
bubbles to escape the PDMS under pressure. 25

But there are also some disadvantages to PDMS. One is, that
the direct deposition of metal is almost impossible. This makes it hard to
integrate electrical components directly into microfluidic systems. The common
workaround is to bond the PDMS to a glass substrate. Furthermore, the material
ages, so that the mechanical properties of PDMS change over the years. Also,
PDMS is permeable to water vapor, which can lead to a loss of liquid in the
channel in some applications. Another disadvantage is, that PDMS can easily be
attacked by strong acids, bases or organic solvents. 25

While PDMS is by far the most commonly used material in
microfluidics, there are other materials, that can be found. One is
Polymethylmethacrylate (PMMA). PMMA is more rigid and less gas permeable than
PDMS, which can be advantageous for some applications. This material has some
important drawbacks though. Its properties are very dependent on the producer,
which makes it hard to reproduce results. For some applications it can also be
disadvantageous, that PMMA absorbs radiation and is fluorescent. The moulding
with PMMA requires trained personnel due to the fact, that temperatures have to
be very precise, so that stamps don’t get too hard while also not being partly
liquid 21. Another material, that is
described for future applications is Polyfluoropolyethers (PFPEs) 28. PFPEs are liquid at room
temperature and can be photocured to microfluidic devices. They have low
surface energy, low modulus, high gas permeability and low toxicity. As a big
advantage over PDMS they cannot be attacked by strong acids, bases or other
organic solvents 28. At this point it is
important to clarify, that there are many more materials with specific advantages,
that find application for special user cases. Therefore before choosing PDMS
because it is “just at hand” may not be the best way to go in microfluidics as
found by Mukhopadhyay et al. 21.

To allow work with
PDMS, the RMIT’s Micro- Nano Research Facility (MNRF) is equipped with state of
the art equipment to obtain the best possible results. It is classified as ISO
5 Class 100 laboratory, which allows fabrication of novel PDMS devices without
any risk for cross-contamination from any external sources 24. The PDMS lab allows access to an evacuated chamber for surface salination.
There is also equipment for plasma treatment, as well as vacuum chambers.

Regarding the
commercial use of PDMS created devices, it has to be stated that as of right
now, it is mostly used in academia due to its low price and easy handling, so
that students can use it to learn while mistakes are relatively uncostly. PDMS
has not yet found access in widespread commercial use in microfluidics. One
reason seems to be the still very small microfluidics industry as well as the general
lack of commercial applications for PDMS devices. 21