Polydimethylsiloxane, called PDMS or dimethicone, is a polymer widely used for the fabrication and prototyping of microfluidic chips.
It is a mineral-organic polymer (a structure containing carbon and silicon) of the siloxane family (word derived from silicon, oxygen and alkane). Apart from microfluidics, it is used as a food additive (E900), in shampoos, and as an anti-foaming agent in beverages or in lubricating oils.
For the fabrication of microfluidic devices, Polydimethylsiloxane (liquid) mixed with a cross-linking agent is poured into a microstructured mold and heated to obtain a elastomeric replica of the mold (cross-linked).
A little bit of chemistry will help us better understand the advantages and drawbacks of PDMS for microfluidic applications.
The Polydimethylsiloxane empirical formula is (C2H6OSi)n and its fragmented formula is CH3[Si(CH3)2O]nSi(CH3)3, n being the number of monomers repetitions.
Depending on the size of monomers chain, the non-cross-linked PDMS may be almost liquid (low n) or semi-solid (high n). The siloxane bonds result in a flexible polymer chain with a high level of viscoelasticity.
PDMS becomes a hydrophobic elastomer. Polar solvents, such as water, struggle to wet the PDMS (water beads and does not spread) and this leads to the adsorption of hydrophobic contaminants from water on the material’s surface.
PDMS oxidation using plasma changes the surface chemistry, and produces silanol terminations (SiOH) on its surface. This helps making the material hydrophilic for thirty minutes or so. This process also makes the surface resistant to the adsorption of hydrophobic and negatively-charged molecules. In addition, its plasma oxidation is used to functionalize the surface with trichlorosilane or to covalently bond PDMS (at the atomic scale) on an oxidized glass surface by the creation of a Si-O-Si bonds.
Whether the surface is plasma oxidized or not, it does not allow water, glycerol, methanol or ethanol infiltration and consecutive deformation. Thus, it is possible to use PDMS with these fluids without fear of micro-structure deformation. However, it deforms and swells in the presence of diisopropylamine, chloroform and ether, and also, to a lesser extent, in the presence of acetone, propanol and pyridine – therefore, PDMS is not ideal for many organic chemistry applications.
PDMS is one of the most employed materials to mold microfluidic devices.
We describe here the fabrication of a microfluidic chip by soft-lithography methods [1].
(1) The molding step allows mass-production of microfluidic chips from a mold.
(2) A mixture of PDMS (liquid) and crosslinking agent (to cure it) is poured into the mold and heated at high temperature.
(3) Once it has hardened, it can be taken off the mold. We obtain a replica of the micro-channels on the block.
Microfluidic device completion:
(4) To allow the injection of fluids for future experiments, the inputs and outputs of the microfluidic device are punched with a PDMS puncher the size of the future connection tubes.
(5) Finally, the face of the block of PDMS with micro-channels and the glass slide are treated with plasma.
(6) The plasma treatment allows PDMS and glass bonding to close the microfluidic chip.
The chip is now ready to be connected to microfluidic reservoirs and pumps using microfluidic tubing. Tygon tubing and Teflon tubing are the most commonly used tubings on microfluidic setups.
If you are unsure about choosing the appropriate tubing for your setup, see our dedicated tutorial pages : Basics About Microfluidic Tubings & Sleeves and How to choose microfluidic Tubing ?
Human alveolar epithelial and pulmonary microvascular endothelial cells cultivated in a PDMS chip to mimick lung functions
It is transparent at optical frequencies (240 nM – 1100 nM), which facilitates the observation of contents in micro-channels visually or through a microscope.
It has a low autofluorescence [2]
It is considered as bio-compatible (with some restrictions).
The PDMS bonds tightly to glass or another PDMS layer with a simple plasma treatment. This allows the production of multilayer PDMS devices to take advantage of the technological possibilities offered by glass substrates, such as the use of metal deposition, oxide deposition or surface functionalization.
PDMS, during cross-linking, can be coated with a controlled thickness on a substrate using a simple spincoat. This allows the fabrication of multilayer devices and the integration of micro valves.
It is deformable, which allows the integration of microfluidic valves using the deformation of PDMS micro-channels, the easy connection of leak-proof fluidic connections and its use to detect very low forces like biomechanics interactions from cells.
It is inexpensive compared to previously used materials (e.g. silicon).
It is also easy to mold, because, even when mixed with the cross-linking agent, it remains liquid at room temperature for many hours. The PDMS can mold structures at high resolutions. With some optimization, it is possible to mold structures of a few nanometers [3].
It is gas permeable. It enables cell culture by controlling the amount of gas through the material or dead-end channels filling (residual air bubbles under liquid pressure may escape through PDMS to balance atmospheric pressure).
Electrodes deposited on glass to be integrated in the PDMS microfluidic chip
It is almost impossible to perform metal and dielectric deposition on PDMS. This severely limits the integration of electrodes and resistors. Nevertheless, this problem is minimized by the fact that this material easily bonds to glass slides using a plasma treatment, even if large metal areas can prevent a good bonding. Thus, the various thin metal layers or dielectric depositions can be performed on glass slides.
PDMS ages, therefore after a few years the mechanical properties of this material can change.
It adsorbs hydrophobic molecules and can release some molecules from a bad cross-linking into the liquid and this can be a problem for some biological studies in PDMS microfluidic devices.
This material is permeable to water vapor which makes evaporation in such devices hard to control.
It is also sensitive to the exposure to some chemicals (see below).
PDMS microfluidic chip
PDMS is used to fabricate microfluidic devices (single layer and bilayer) and micro-imprint stamps. Two different types are commonly used by researchers for these applications: PDMS RTV-615 and PDMS Sylgard 184. The exact composition of these two is… kept secret. However, with experience, researchers can help choosing the most suitable PDMS for an application [4]:
The preferred PDMS of S. Quake (Co-inventor of the microfluidic valve).
The most robust and convenient to bond bilayer microfluidic devices.
It has the reputation of being dirty. (For example, Fluidigm has discarded 90% of the RTV-615 they received).
There are variabilities in plasma bond strength between different batches. This makes it necessary to adjust the bonding parameters with each purchase.
You will find below an immersion study of microstructured PDMS (h: 11µm, L: 45µm) in a variety of chemicals [5], this study was performed with PDMS Sylgard 184. Find more information about chemical resistance of different materials for microfluidics.
(Legend: No: no effect on microstructures, Total: complete destruction of microstructures)
For more reviews about microfluidics, you can have a look here: «Microfluidics reviews». The photos in this article come from the Elveflow® data bank, Wikipedia or elsewhere if precised. Article written by Guilhem Velvé Casquillas and Timothée Houssin.
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