PDMS: A review

Introduction to poly-di-methyl-siloxane (PDMS)

PDMS

Polydimethylsiloxane called PDMS or dimethicone is a polymer widely used for the manufacture 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). Outside of microfluidics, it is used as a food additive (E900), in shampoos, as an anti-foaming agent in beverages or in lubricating oils.

For the manufacture of microfluidic devices, PDMS (liquid) mixed with a cross-linking agent is poured into a microstructured mold and heated to obtain a replica of the elastomer mold (PDMS cross-linked).

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Some chemistry about PDMS

A little chemistry

A little chemistry will help us better understand the advantages and drawbacks of PDMS for microfluidic applications. The empirical formula of PDMS is (C2H6OSi)n and Fragmented formula is CH3[Si(CH3)2O]nSi(CH3)3, n being the number of repetitions of the monomer. Depending on the size of the string of monomer, the non-cross-linked PDMS may be almost liquid (poor n) or semi-solid (large n). The siloxane bonds allow a flexible polymer chain with a high level of viscoelasticity to be obtained.

After “cross-linking”

PDMS becomes a hydrophobic elastomer. Polar solvents such as water struggle to wet the PDMS (the water beads and does not spread) and this leads to the adsorption of hydrophobic contaminants contained in the water.

Oxidation of PDMS

Oxidation of PDMS using plasma, changes the surface chemistry of PDMS and produces silanol terminations (SiOH) on its surface. This helps make PDMS hydrophilic for thirty minutes. This process also makes the surface resistant to the adsorption of hydrophobic and negatively-charged molecules. In addition, plasma oxidation of PDMS is used to functionalise the surface of PDMS with the trichlorosilane or to covalently bond PDMS (on an atomic scale) on an oxidized glass surface by the creation of a Si-O-Si bond.

Whether the surface of PDMS is oxidized in plasma or not, it does not allow the water, glycerol, methanol or ethanol to infiltrate and deform it. Thus, it is possible to use PDMS with these fluids without fear of micro-structure deformation. However, PDMS deforms and swells in the presence of diisopropylamine, chloroform and ether and to a lesser extent in the presence of acetone, propanol and pyridine.

The PDMS in microfluidics

PDMS is a preferred material for molding microfluidic devices. We describe here the production by molding of a PDMS microfluidic chip. The molding step allows mass production of microfluidic chips from a mold.

  • Step 1 and 2: A mixture of PDMS (liquid) and cross-linking agent (to harden the PDMS) is poured into the mold and placed in a furnace.
  • Step 3: Once the PDMS is hardened, it can be taken out of the mold. We obtain a replica of micro-channels in PDMS. The completion of the microfluidic device.
  • Step 4: To allow the injection of fluids for future experiments, the inputs and outputs of the microfluidic device are drilled with a needle or a punch of the size of future outer tubes.
  • Step 5 and 6: Finally, the face of the PDMS block with micro-channels is bonded to a glass slide using a plasma treatment to close the microfluidic chip.

Why using PDMS for microfluidic device fabrication?

PDMS was chosen for manufacturing microfluidic chips primarily for those reasons:

  • The PDMS is transparent at optical frequencies (240nm-1100nm), which facilitates the observation of the contents of micro-channels visually or under the microscope.
  • The PDMS has a low autofluorescence [1] and is considered as biocompatible (with some restrictions). The PDMS sticks tightly to glass or another PDMS layer with a simple plasma treatment. This allows the production of multilayer PDMS devices and enables taking advantage of technological possibilities of glass substrate as the use of metal deposition, oxide deposition or surface functionalisation.
  • The  PDMS, during the cross-linking, can be coated with a controlled thickness on a substrate using a simple spincoat. This allows manufacturing multilayer devices and integrating micro valves.
  • The PDMS is deformable, which allows the integration of microfluidic valves using the deformation of the channels.
  • The PDMS is inexpensive Compared to previously used materials (like silicon). The PDMS is easy to mould, because even when mixed with the cross-linking agent, the PDMS remains liquid at room temperature for many hours. The PDMS can mold structures at high resolutions. With some optimization  it is possible to mold the structures of a few nanometers [2].
  • The PDMS is gas permeable this allows cell culture by controlling the amount of gas through PDMS or fill channels without any outputs (residual air bubbles under the pressure of the liquid may escape through PDMS to be balanced with atmospheric pressure).

PDMS issues for microfluidic applications are:

It is almost impossible to perform metal deposition 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 attaches to a glass slide using a plasma treatment. Thus, the various thin layer metal or dielectric depositions can be performed on the glass slide.

  • The PDMS is a material that ages, therefore after a few years the mechanical properties of this material can be changed.
  • The PDMS adsorbs hydrophobic molecules and can release some molecules from a bad cross-linking into the liquid and this can be awkward for some biological study in PDMS microfluidic devices. The PDMS is permeable to water vapor which makes evaporation in PDMS device difficult to control. The PDMS is sensitive to exposure to some chemicals (see below).

Different PDMS used in microfluidic

glass PDMS ( poly-di-methyl-siloxane ) microfluidic chip

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PDMS is used for the manufacture of 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, the experience of researchers can help choose the most suitable PDMS for an application [3]:

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 for being dirty. (For example, Fluidigm has discarded 90% of the RTV-615 they received). There are variability in plasma bond strength according to batch. This makes it necessary to adjust the bonding parameters with each purchase.

2) PDMS Sylgard 184 (Dow Corning)

The cleaner of the two PDMS. This PDMS is less often used for multilayers. It makes the bonding more difficult between two layers of PDMS. It generate more failures during the manufacture of devices. This PDMS is most often used in mammalian cell cultures in microfluidic chips.

Chemical resistance of PDMS

You will find below an immersion study of microstructure PDMS (h: 11μm, L: 45μm) in a variety of chemicals [4], this study was performed with PDMS Sylgard 184.

pdms-chemical-resistance

(Legend: No: no effect on microstructures, Total: complete destruction of microstructures)

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Article written by Guilhem Velvé Casquillas

[1] Piruska, A.; Nikcevic, I.; Lee, S. H.; Ahn, C.; Heineman, W. R.; Limbach, P. A.; Seliskar, C. J. Lab Chip (2005), 5, 1348-54.

[2] Hua, F.; Sun, Y.; Gaur, A.; Meitl, M. A.; Bilhaut, L.; Rotkina, L.; Wang, J.; Geil, P.; Shim, M.; Rogers, J. A. Nano letters (2004), 4, 2467-2472.

[3] From James M. Spotts 2008 Microfluidics Course Institute for Systems Biology November 17, 2008

[4] Characterization of Polydimethylsiloxane (PDMS) Properties for Biomedical Micro/Nanosystems, Biomedical Microdevices 7:4, 281–293, 2005