PDMS: a review

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, PDMS (liquid) mixed with a cross-linking agent is poured into a microstructured mold and heated to obtain an elastomeric replica of the mold (PDMS cross-linked).

A little bit of chemistry

A little bit of chemistry will help us better understand the advantages and drawbacks of PDMS for microfluidic applications.

The PDMS 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.

After “cross-linking”

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 PDMS surface.

PDMS Oxidation

PDMS oxidation using plasma changes the PDMS surface chemistry, and produces silanol terminations (SiOH) on its surface. This helps making the PDMS hydrophilic for thirty minutes or so. This process also makes the surface resistant to the adsorption of hydrophobic and negatively-charged molecules. In addition, PDMS plasma oxidation is used to functionalize the PDMS 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 PDMS 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, the PDMS 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 – thus 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 the PDMS) is poured into the mold and heated at high temperature.

(3) Once the PDMS has hardened, it can be taken off the mold. We obtain a replica of the micro-channels on the PDMS 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.

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 PDMS 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.

PDMS is permeable to water vapor which makes evaporation in PDMS device hard to control.

PDMS is 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 types of PDMS are commonly used by researchers for these applications: PDMS RTV-615 and PDMS Sylgard 184. The exact composition of these two PDMS is… kept secret. However, with experience, researchers can help choosing the most suitable PDMS for an application [4]:

1) PDMS RTV-615

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.