Whether you’ve been learning about microfluidics here at Microfluidic Future or somewhere else, you’ve undoubtedly come across the elastomer poly(dimethylsiloxane) (PDMS). PDMS has radically changed the capabilities of microfluidics (and its price tag) since it was first brought into microfluidics by George Whitesides in 1998. PDMS has effectively replaced glass and silicon which were borrowed from existing micromachining industries. PDMS has great resolution and can contain sub-0.1 µm features. But how is PDMS used, and what makes it so great? Hopefully you’ll have these answers by the end of this post.
Soft lithography is a widely used technique to create PDMS structures. This process first requires a mold, or a master, generally produced by photolithography. The mold must represent the hollow space of the microfluidic chip, where the fluids will eventually flow. PDMS is formed by mixing one component which contains silicon hydride groups and another component which contains a vinyl group. This mixture is then poured on top of the master. PDMS cures to a solid after an hour at 70o C, and can then be peeled away from the master without damage to either part, allowing the master mold to be reused. The earlier steps can be completed within 24 hours, and replicas can be mass produced after designing and creating a master.
After removing the PDMS replica, we have an open space in the structure from the master, but our channels have no bottom! We next have to seal the PDMS to another surface, such as glass, silicon, or itself. More importantly, PDMS can be sealed reversibly or irreversibly.
Van der Waals forces allow reversible sealing to smooth surfaces. This seal is airtight, but can only withstand pressures around 5 psi. Adhesive silicone or cellophane tapes can also be used to create a reversibly seal that is stronger than PDMS alone. Reversible seals allow for reuse or added functionality. Remember that the PDMS used in the SIMBAS lab-on-a-chip was sealed reversibly to the glass, allowing reuse or future analysis.
In some cases, a lab-on-a-chip may require an irreversible seal, especially if operating under higher pressures. PDMS can be irreversibly sealed by exposing it (and possibly the other surface) to oxygen plasma. However, the two pieces of chip must be aligned within a minute or else the oxidized PDMS surface will reconstruct in the air.
Complex structures can be created by “sandwiching” multiple layers of PDMS. Furthermore, additional components like membranes can be included in this sandwich. This can be a bit of a challenge depending on the complexity of the features, and micro-stages have been created to ensure perfect alignment after oxidation. Alternatively, PDMS can be sealed with the aid of polar solvents. A thin film of a polar solvent is placed between PDMS layers and then heated, evaporating the solvent and sealing the layers.
Instead of using photolithography to create the mold, 3D printing can also be used. 3D printing prints small globules of material according to a CAD file to make 3D structures. 3D printing itself has been growing in capabilities due to its extremely quick prototyping speed. It can achieve a resolution between 50 and 100 µm, which is suitable for microfluidic devices. This mold would then be used in the same way to create PDMS replicas.
Microfluidic chips can be very complex, featuring pumps, valves and mixers. These parts can easily be implemented in a PDMS structure. Mixers can be incorporated into the masters, transferring them to the floor of a channel. Other parts that are made separately, like some of the microvalves I discussed previously, can be placed in the PDMS while curing. This allows a single master to be very versatile while moving independent features about the device.
Here are some key features of PDMS adapted from “Poly(dimethylsiloxane) as a Material for Fabricating Microfluidic Devices” by Whitesides.
transparent; UV cutoff, 240 nm
optical detection from 240 to 1100 nm
insulating; breakdown voltage,
2 x 107 V/m
allows embedded circuits; intentional breakdown to open connections
elastomeric; tunable Young's modulus, typical value of
conforms to surfaces; allows actuation by reversible deformation; facilitates release from molds
insulating; thermal conductivity, 0.2 W/(m*K); coefficient of thermal expansion, 310 µm/(m*°C)
can be used to insulate heated solutions; does not allow dissipation of resistive heating from electrophoretic separation
low surface free energy
replicas release easily from molds; can be reversibly sealed to materials
impermeable to liquid water; permeable to gases and nonpolar organic solvents
contains aqueous solutions in channels; allows gas transport through bulk material; incompatible with many organic solvents
inert; can be oxidized by exposure to plasma
unreactive toward most reagents; surface can be etched; can be modified to be hydrophilic and also reactive toward silanes; etching can alter topography of surfaces
can be implanted in vivo; supports mammalian cell growth
Some clear advantages and unique properties can be seen from this table. I would also like to point out some additional advantages and disadvantages.
McDonald, J., & Whitesides, G. (2002). Poly(dimethylsiloxane) as a Material for Fabricating Microfluidic Devices Accounts of Chemical Research, 35 (7), 491-499 DOI: 10.1021/ar010110q