How to succeed with glass PDMS plasma bonding

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 as well.

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 plasma color changes with the same parameters, a problem probably has occurred.

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 be effective on the whole surface and a plasma treatment too long will too strongly modify your PDMS surface. The longer the plasma is activated, the rougher your surface becomes, 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

Right 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 totally controlled atmosphere but it requires more equipment and strictness during the process.

Dust

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

Pressing strongly on the PDMS to urge a bonding is tempting to correct a bad plasma treatment. However, it will not work. What you really risk to do is to collapse and deform your channel irreversibly. Keep in mind that the bonding has 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 thing 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 point the part becomes 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.