How to Do PDMS Lithographic Replication: A Step-by-Step Guide

PDMS is a polymer widely used in the field of microfluidics, and it is often used to manufacture microfluidic devices such as lab on chip. Every year, many labs start doing microfluidics activities, and sometimes, they don’t have good experimental equipment or know good experimental methods, this blog post aims to provide basic knowledge on how to do PDMS lithographic replication, PDMS lithographic replication and Known as the soft lithography process. I’m going to give you some tips and tricks here to make PDMS lithography replication as easy as pushing a button.

PDMS lithography replication can be divided into 8 main steps:

1. Preparation of silanization mold
2. Ratio and mixing of PDMS and curing agent
3. Degassing and removing air bubbles
4. Pouring PDMS on the mold
5. PDMS baking
6. PDMS peeling mold
7. PDMS cutting and drilling
8. PDMS Bond

The following will briefly introduce the above 8 steps.


PDMS replication: preparation of silanized molds

When the mold is used for the first time, you must make it hydrophobic. For molds, you can buy from some companies or process them yourself. In fact the silicon of the SU-8 mold is usually very hydrophilic, so the PDMS has a good affinity to the silicon, this affinity is so great that it is unlikely that the PDMS will be peeled from the silicon, and what you can do is break the mold And back to the beginning of the mold process, so don’t forget about the hydrophobic treatment of the mold.

There are several ways to make silicon hydrophobic. We use the functionality of silanes (which allow for permanent bonding on surfaces) that allow hydrophobic contact angles on surfaces beyond 90°. You can process with gas or liquid, which means you can bond the silane functionality to the surface through a gas or a continuous water bath. We can briefly look at these two solutions.

Silanization is a very dangerous product for humans and must be used under appropriate conditions such as a fume hood. If you have any questions, please communicate with others in time.

If contact angle testing is inconclusive, check your testing protocol and repeat. Please don’t try to put PDMS on the mold, or you will regret it later.

Silane reacts with air, so make sure the TMCS or OTS bottle is really sealed and sealed with parafilm for a good security. The liquid option is effective but takes more time and materials to complete, the gas option is often used and with a little practice you can get the same results as liquid.


PDMS replication: ratio and mixing of PDMS and curing agent

Once your mold is ready, we can process PDMS. PDMS is the name of the polymer. To make it harder, you have to add a curing agent. A commonly used PDMS is apparently Sylgard 184. For this PDMS processing, a common ratio of PDMS to curing agent is 1:10 by weight. For some specific applications, this ratio can be changed to make PDMS softer or stiffer.

Please follow the recipe below to process your PDMS:

(1) Weigh PDMS (eg 60g)
(2) Add curing agent (eg 6g)
(3) Mix PDMS and curing agent thoroughly

It seems easier, but first put the PDMS first, then put the curing agent. If these two steps are reversed, it may result in poor cross-linking of the polymer.


PDMS replication: PDMS degassing to remove air bubbles

Due to the intense mixing, you will have a lot of air bubbles during the preparation of the mixed PDMS and curing agent, but these air bubbles must be removed, because if they are not removed, they will be trapped in the PDMS chip. There are several methods that can be used to remove air bubbles from the PDMS mixture, you can use a centrifuge or a desiccator and a vacuum pump. In fact, a centrifuge is more suitable for debubbling PDMS with a smaller weight.

Use a desiccator to remove air bubbles in the PDMS mixture, cover with a glass cover and seal it well. Use a vacuum pump or vacuum line for vacuum operation. If the vacuum is drawn too strongly or too quickly, be careful not to spill the PDMS. Under vacuum conditions, it usually takes 30 min to remove all air bubbles in the PDMS mixture. Once the air bubbles are removed, you can turn off the vacuum pump , and bake the PDMS. In this step, be careful not to open the desiccator when the pressure is unstable or your PDMS may blow up.


PDMS replication: pouring PDMS on the mold

Always clean the mold prior to use, using nitrogen or clean compressed air to remove all dust and particles from the surface of the SU-8 mold. Once the PDMS is degassed, you can pour the PDMS into the previously silanized SU-8 mold. Put PDMS in a container such as a Petri dish, the size of the container will determine the amount of PDMS you need and the thickness of the device you want to obtain. For a substrate size of 4 inches, 40g of PDMS will yield a device about 5mm thick.

During the pouring process, if there are air bubbles, you can use a steel needle to remove the air bubbles or put the wafer back into the vacuum vessel. If you choose the second option, you must be careful to prevent the wafer from floating and the PDMS entering the bottom of the wafer, because once this happens, it will be more difficult to release the PDMS later.


PDMS replication: PDMS baking

Once the PDMS and curing agent were mixed together, the cross-linking reaction had started, but it took 24 hours to get a sufficiently rigid device. That’s why molds and PDMS need to be baked. Under the use of the same tool, the baking time and temperature vary with the laboratory and the user. I’ll try to give some advice for doing this step successfully.

People usually use a hot plate or an oven to bake PDMS, there are no good tools here, only inside the oven, you can bake multiple molds at the same time. For the setting of temperature and time, it depends on the specific equipment. Through experiments, under different baking conditions, the contact angle between the surface of the PDMS layer and water was measured. Therefore, baking will not affect the chemical composition of PDMS, but will affect the mechanical properties. In fact, after PDMS is baked, PDMS will become more or less soft, so it will be more or less easier to handle. In any case, PDMS will be developed to have the exact same mechanical properties, but it may take a longer time. However, be careful not to bake your PDMS too much, otherwise, the PDMS will become “old”, which means that it will be very difficult to drill holes in the PDMS later. Doing this with old PDMS plates (PDMS plates that are a few months old) also observes the same phenomenon.

The temperature must be selected according to the container of the mold. If you are using a petri dish, the heating temperature should not exceed 90°C. I recommend you to bake in the oven at 80°C for 2h. After baking and cooling, you can manipulate your PDMS.


PDMS Replication: PDMS Stripping Mold

After the mold and PDMS have cooled down, you can cut your substrate and peel off the PDMS to get your device. You can easily cut PDMS with a scalpel. During cutting, be careful to keep the blade tangent to the mold and not break the mold.

During the stripping process, the PDMS can be easily removed by pulling it gently. If the PDMS sticks tightly to the wafer, then it may be because you did not silanize the mold or the PDMS is not made well enough.

Be sure to wear gloves during this step to avoid adding fatty fingerprints or other marks on your PDMS. Because these fingerprints or marks are difficult to remove and cause major problems for subsequent processing.


PDMS replication: PDMS replication and punching

At this point, you have all the PDMS you need for your device, and you have to cut it to get the different chips. Like before, you can use a scalpel to do a little.

In this step, if you want to add or remove liquid later, then you have to machine some small holes in the PDMS, that is, punch the PDMS. To do this, you can choose the appropriate punch according to the size of catheter you are using . Since PDMS is a soft material, the size of the hole needs to be slightly smaller than the size of the catheter when drilling, so that the catheter will be firmly maintained on the chip and the leakage problem will also be reduced.

Make sure you wear gloves during this step and don’t leave fat marks etc on the PDMS as it will be difficult to remove. Fat imprints left on PDMS will have a certain impact on the subsequent processing.


PDMS replication: PDMS bonding

Key PDMS devices use this to obtain sealed channels or trenches. PDMS can be bonded to another piece of PDMS or glass, and the bonding protocol or rules are the same for both. Before the bonding operation, each part must be well cleaned to remove all dust and particles on the surface. You can use isopropyl alcohol (IPA) to clean your PDMS and glass slides, and you can even use an ultrasonic cleaner to clean the surface and inside of PDMS pores.

In order to bond PDMS, you need to activate the surface of PDMS or glass, convert the Si-CH3 functional groups of PDMS to Si-OH functional groups, when you press the same functional groups, you will create strong and permanent Si-O-Si links. A common tool for this is to use a plasma cleaner with oxygen or air function. In order to produce a better plasma, you must control the pressure inside the plasma cleaner chamber. I suggest some equipment that can help you achieve a perfect plasma treatment like Equinox at a pressure of 500mTorr. To functionalize the surface, the plasma treatment does not have to be very long, a treatment time of 2 minutes was enough to form a strong bond with glass or PDMS. However, under certain experimental conditions, the plasma treatment time may also be less than 2 minutes.

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