I fabricate PDMS micropillars with LIGA-made casting mold (Nickel Metal Sheet) consisting sinking holes of 240µm depth and around 18µm diameter. My casting setup is as the figure attached. A cannula with a diameter 5mm is clipped tightly together with the casting sheet with one or two clips. Sylgard 182 Silicone for casting the pillars is moved into the cannula by a pipette. Then after the degassing process and the baking process the silicone becomes cured.
My question is concerning the peel-off process. Currently I first remove the clips and then put the whole thing into a vibrating bath (the bath is filled with isopropanol) and wait for the cannula with pillars and the casting sheet to become apart. The pillars are expected to be 240µm high (same as the hole depth). However, most pillars I get suffer from not enough height, that is they are torn apart after the silicone get cured.
May I ask if anyone has similar experiences and do you have any suggestions concerning the peeling off process? Or maybe anyone has some other ways for peeling them off?
Thanks very much!
Hallo Mr. Jones,
Thanks for your answer!
However my current casting mold has no anti-adhesion coating. The mold is ordered from a company. Do you have some recommendations that maybe I can add one such layer myself? Our institute concentrates on aerodynamics but maybe I can borrow such devices from other institutes.
Besides, it is already immersed into isopropanol when the whole thing is put into the vibration bath.
Yes, I checked under microscope that the ends of the pillars appear to be torn.
Hi Mr. Jones,
Thank you again for your answer.
But my casting mold is made of Nickel, would the treatment of oxygen plasma turn its surface into Nickel oxide? Is it OK that I use vapour silanization without treating with oxygen plasma?
By the way, may I ask if I have to treat the surface every time before I pour the PDMS or I only have to do it once? Sorry that I am no expert in it.
It may sound silly, but you have to make it hydrophobic some how.
I am generally using a small mixture of detergent and water to polish the mold before pouring the PDMS and curing agent. Therefore after baking stage it easly peel off.
Again it seem as silly method :D
Hi Mr. Bahrieh,
Thank you for your answer.
Usually I clean the mold with isopropanol and then water, then dry it with pressured air. Then I put it into fridge for about 10minutes. Afterwards I pour the PDMS. May I ask if you find this process not that OK? Thank you very much!
Collapsing probably takes place due to high aspect ratio (low stiffness). I don't know how to prevent that exactly but you can save them by sonication in ethanol 1-2 minutes. If you need them dry, you can use point critical drying.
There could be two reason for the problem
First, it could be that you need to vacuum longer to ensure the uncured PDMS reaches the bottom of the pillar mold, i.e. to make no bubble is trapped.
Second, for the release, you could consider using a surface treatment of the old with a fluorinated silane to ease the release.
Finally, you could play with the stiffness of the PDMS by varying the amount of curing agent.
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