The glass / PDMS plasma bonding

The plasma bonding step enables to finish your microfluidic chip fabrication. To permanently bond the PDMS chip to the glass slide researchers use a plasma cleaner to change the surface properties of glass and PDMS. The plasma treatment will modify the surface chemicals and allows to stick the PDMS with the channels against other substrates (PDMS or glass).

If this step is going wrong, your chip will leak and will not be up to use normally. You will have to be careful on different points in the choice of the plasma cleaner but in the protocol too.

Surface properties

The plasma is a device that influences the surface; all contamination will highly affect the final results of the treatment. Contrary to popular thinking, for a glass PDMS plasma bonding, a longer treatment will not improve the surface except for really particular cases. For example the presence of fat such as a fingerprint will lead to a treatment failure on the concerned surface.

The air pollution inside the plasma cleaner chamber

The gas composition inside the plasma room will change the kind of chemical links that are created on the surface of your glass or PDMS. Some impurities even in very low quantity will pollute your surface. The most common polluting is oil coming from the vacuum pump or from the compressors around. Because of the oil inside the plasma cleaner chamber, you will probably see the same plasma that you used to see but the chemical will be different and the PDMS will not bond.

One indicator of the plasma success: its stability and color

A good indicator of the quality of the plasma is generally its color/luminosity (which depends on the pressure and the gas used). Since the color may change, what you have to remember is, if your color plasma change with the same parameters, a problem probably has happened.

The plasma time

The timing is a good key of the success of your treatment and thus of your bonding. A too short plasma treatment will not functionalized the whole surface and a plasma treatment too long will too strongly modify your PDMS surface. The longer the plasma is activated, the rougher is your surface and the bonding properties will be affected. With plasma usually used for soft lithography, the perfect time to have the most powerful bond is generally between 20 and 60 seconds.

The timing after the plasma

Just after the plasma treatment the chemical bonds of the surface begin to recombine and after a few minutes the surface becomes not enough activated for glass PDMS plasma bonding. For this statement, you have to do the bonding just after the treatment, do not let your sample inside the plasma room after venting, and put them together rapidly.

A bake to improve the glass PDMS plasma bonding quality

To make the chemical link easier after the contact between the PDMS and the glass or PDMS, it is recommended to heat the set. The time, the temperature and the device can change between the laboratory and the user. A baking at 80-90°C during 15-30 minutes is usually enough to have a great bonding.

The choice of the gas

The surface state will depend on the gas used. A room air plasma will work really well in most cases. Some researchers will prefer pure O2 to have a total controlled atmosphere but it requires more equipment and strictness during the process.

The dusts

The presence of dust on the surface will prevent the glass PDMS plasma bonding where the dust is but also on a disc around and the size will depend on the rigidity of the PDMS. A first cleaning with, at least, a clean dry air jet is required. There are other methods to remove dust, you can use 3M Scotch tape to remove particles on the surface or more efficiently you can dive your chip in Isopropanol and use sound wave to detach all unwanted particles on the surface and inside the holes in the PDMS.
To clean the glass use successively acetone, isopropanol, water and dry it.

You need to press strongly on the PDMS chip to correct the glass PDMS plasma bonding

Press strongly on the PDMS to urge a bonding is tempting to correct a bad plasma treatment. However it is not working, what you really risk to do is to collapse and deform your channel irreversibly. Keep in mind that the bonding have to be fast and easy. If the contact is not good between your two parts it is perhaps because of dust. In order to correct the bonding, the only try to do, is to heat your set and press gently again the chip.

We can move the PDMS chip a second time after a first contact

If you put your chip at the wrong place the first time or if the plasma treatment is not working everywhere it’s useless to try to reposition your chip a second time. The best thing to do is to throw out the chip or to do the plasma treatment again but it can’t be certain to work and it’s certainly not reproducible.

First test: the angle contact measure

The plasma treatment modifies the surface properties and namely the hydrophobicity of the glass and PDMS. A good treatment makes the surface hydrophilic. A first test consists in putting a water droplet (about 20µl) on each surface and measure the angle contact with the surface. A contact angle below 20° will generally lead to an adhesion strength upper than 2.5bar.

Second test: the spreading of the sticking front

When you bond your chip, at the contact the part become darker so you can follow the contact. After the plasma treatment drop off gently the PDMS on the glass or on an other piece of PDMS the contact front should progress of itself quickly and easily.

Third test: no leaking at high pressure

A third test consists in injecting a liquid inside your device and test its behaviour with high pressure. You can use a simple syringe and push with fingers it will be enough to have several bar.

Fourth test: the wrenching

This last test is destructive, and consists in wrenching the chip from its shelf. The chip should break and not be removable, it will remain PDMS on both parts. Normally a good bonding will enable to use your device for pressure until around 3 to 5 bars.