Polydimethylsiloxane is commonly known under the abbreviation PDMS. Its full name according to the IUPAC convention is Polyoxy(dimethylsilylene). It is registered under the CAS number: 63148-62-9. 11 The chemical structure of PDMS can be found in Figure 1.
Figure 1 Polydimethylsiloxane chemical structure.
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.” 11
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 11. 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 11. 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.
PDMS commonly finds use across a variety of consumer 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 28, 45. Another mostly unknown use, is the use as lubricant for condoms 11. 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 36. 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. 35
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. 35
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 31. Another material, that is described for future applications is Polyfluoropolyethers (PFPEs) 38. 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 38. 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 easily accessible may not be the best way to go in microfluidics as stated by Mukhopadhyay et al. 31.
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 34. 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. This means that students 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. 31