Making PDMS (polydimethylsiloxane) is straightforward: you mix a silicone base with a curing agent at a 10:1 ratio by weight, remove air bubbles, pour the mixture into a mold, and heat it until it solidifies into a flexible, transparent rubber. The whole process takes anywhere from 10 minutes to two days depending on your curing temperature. Here’s how to do it right, step by step.
What You Need Before You Start
The most common PDMS kit is Sylgard 184 from Dow, which comes as two matched components: a base (Part A) and a curing agent (Part B). You’ll also need a scale accurate to at least 0.1 grams, a mixing container (disposable plastic cups work), a stirring rod or spatula, and access to a vacuum desiccator and an oven or hot plate. Safety glasses are recommended, though the material safety data sheet notes that gloves and skin protection aren’t strictly necessary for room-temperature handling. If you’re spraying or creating any aerosol, you’ll need a respirator with a particulate filter.
Mixing the Base and Curing Agent
Place your mixing container on the scale and tare it to zero. Pour in the base, note the weight, then add the curing agent at a 10:1 ratio. For example, if you measure 30 grams of base, add 3 grams of curing agent. The ratio works by either weight or volume, so you don’t need to worry about density conversions.
Stir thoroughly for at least three to four minutes. The mixture will look cloudy and full of tiny bubbles, which is normal. Incomplete mixing is one of the most common reasons PDMS fails to cure properly, leaving you with a sticky, partially solidified mess. Make sure you scrape the sides and bottom of your container while stirring so no pockets of unmixed material remain.
Removing Air Bubbles
After mixing, the PDMS is loaded with trapped air. Place the container in a vacuum desiccator and pull it down to roughly 100 millitorr (a strong vacuum, but standard for most lab desiccators). You’ll see the mixture foam up dramatically as dissolved gas expands out, sometimes nearly overflowing the cup. If it rises too high, briefly release the vacuum to collapse the foam, then reapply it.
For a typical batch, 30 to 45 minutes of degassing is enough to pull out all the trapped air. Research from Biomicrofluidics found that 45 minutes was sufficient for complete degassing in devices several millimeters thick, and going longer provided no additional benefit. If you’re working with a very thick pour or a mold with fine features, you may want to extend the time to be safe. The PDMS should look completely clear with no visible bubbles when it’s ready.
Pouring and Curing
Pour the degassed PDMS slowly into your mold or onto your master pattern, tilting gently to minimize new bubble formation. If a few small bubbles appear, you can pop surface bubbles with a needle or briefly return the filled mold to the vacuum chamber.
Curing temperature determines how long you’ll wait:
- Room temperature (~25°C): approximately 48 hours
- 60°C: about 3 hours
- 100°C: roughly 45 minutes
- 150°C: as little as 10 minutes
Lower temperatures produce less internal stress in the cured material, which matters if you’re making something dimensionally precise like a microfluidic chip. Higher temperatures are convenient when speed matters and slight thermal expansion of your mold won’t cause problems. Most researchers default to 60 to 80°C for a few hours as a practical compromise. Once cured, the PDMS should feel firm and dry to the touch, not tacky.
Preventing PDMS From Sticking to Molds
Cured PDMS bonds well to many surfaces, which is a problem when you need to peel it off a master mold cleanly. Treating the mold surface with a silane coating before pouring prevents adhesion. The most commonly used treatment is trimethylchlorosilane (TMCS), which creates a thin hydrophobic monolayer through a process called silanization. You typically place the mold in a sealed container with a few drops of liquid TMCS and let the vapor deposit onto the surface for 15 to 30 minutes.
Other options include fluorinated silanes like tridecafluoro-1,1,2,2-tetrahydrooctyl trichlorosilane (sometimes called TFOCS or PFOS in shorthand), which are especially useful for molds with deep, narrow features where PDMS tends to grip tightly. For repeated PDMS-to-PDMS casting, where you’re using a PDMS mold to make another PDMS part, silanization is essential. Without it, the two layers will chemically crosslink together and become inseparable.
Making the Surface Hydrophilic
Freshly cured PDMS is naturally hydrophobic, meaning water beads up on it rather than spreading out. This is a problem for microfluidic applications where you need liquid to flow through tiny channels. Oxygen plasma treatment temporarily converts the surface to hydrophilic by introducing oxygen-containing groups onto the silicone.
At 70 watts of plasma power for at least 300 seconds (5 minutes), the surface contact angle drops to about 17 degrees, meaning water spreads almost flat across it. This hydrophilic state isn’t permanent. After about 6 hours of exposure to air, the contact angle climbs back up to 50 to 60 degrees as the silicone chains migrate back to the surface. That recovery window means you should bond or use your plasma-treated PDMS relatively quickly.
Plasma treatment also enables permanent bonding between PDMS and glass, or between two PDMS layers. Treat both surfaces, press them together within a few minutes, and the bond becomes irreversible.
Troubleshooting Common Problems
If your PDMS remains sticky or gooey after the expected cure time, the most likely cause is an incorrect mixing ratio. Too little curing agent leaves excess uncrosslinked base, and too much curing agent can also cause issues. Weigh carefully rather than estimating by eye.
Certain chemicals inhibit the platinum catalyst in PDMS and prevent curing entirely. Sulfur-containing compounds are notorious for this, so latex gloves (which contain sulfur-based accelerators) can poison your batch if residue transfers from your hands. Use nitrile gloves instead if you choose to wear gloves. Amine-containing compounds and some photoresist residues can also cause cure inhibition, so make sure your mold is thoroughly cleaned before pouring.
Bubbles trapped in fine mold features are another common frustration. If vacuum degassing after pouring doesn’t clear them, try pouring a very thin initial layer, degassing it in the mold, then adding the remaining PDMS and degassing again. For molds with especially deep or narrow channels, casting under positive pressure rather than vacuum can force PDMS into features more effectively.
Adjusting Stiffness and Thickness
The 10:1 ratio produces PDMS with a standard stiffness suitable for most applications. You can make softer, more flexible material by increasing the ratio (for instance, 20:1 or even 30:1), though this increases tackiness and cure time. Stiffer material results from ratios closer to 5:1, which adds more crosslinks. These modified ratios are common in biomechanics research where matching the stiffness of biological tissues matters.
Thickness is simply a matter of how much PDMS you pour. For thin membranes, spin-coating the degassed mixture onto a flat substrate at controlled speeds lets you achieve uniform layers from tens to hundreds of micrometers thick. For bulk casting, pour enough to fill your mold to the desired depth. A helpful rule: 1 gram of uncured PDMS occupies roughly 1 cubic centimeter, so you can estimate your pour volume from the mold dimensions.