During my PhD at the University of Wyoming, I may have made ~400-500 microfluidic devices all from glass substrate. My PhD laboratory uses borosilicate based substrate/cover plate to make the chips for variety of applications such as immunoassay development, separation experiments, fuel cell units. Glass is one of the oldest materials in microfluidic field and has certain advantages (see below) over other materials that are being used in this field of research.
- Inertness to many chemicals
- Optical transparency
- Low fluorescence
- High resistance to mechanical stress
- Well established surface modification procedures
1. Mask design: We create a channel design using CAPopia drawing software. I had never used any drawing softwares before and found this one easy to use. Once the design is complete, we send it out to a printing company (Fineline Imaging, Colorado) to get the mask.
|Mask with channel design|
2. Photolithographic procedure: Once the photo-mask arrives from the printing company, we perform basic photo-lithography procedure to transfer the channel pattern design onto the glass substrate. We buy borosilicate glass substrate (Telic, CA, USA) which is 4” x 4” in dimension and 1.65 mm thick. One side of this glass substrate is coated with a layer of chromium and photoresist on top of each other. Both chromium and photoresist layers are about 100-200 nm in thickness. The photoresist coated on the substrate is polymer based positive photoresist which is sensitive to UV radiation. Therefore the box containing the substrate plates must be opened only in dark room.
In photolithography room turn on the UV light source before at least 15 min you plan to shine it to the substrate plate. We use a custom made box where one can place photo-mask on top of substrate plate. This assembly is then gently pressed by putting a glass plate on top of it. The whole thing is inside a box in which a shutter can be opened when ready to expose to UV radiation.
Typically we expose the substrate to the UV radiation for ~ 30-45sec. During this step the polymer of photoresist breaks down and is removed by soaking the substrate in a photo-developing solution (MF-319, Rohm and Haas) for ~ 5 min. The substrate is then washed with deionized (DI) water (DirectQ Water Purification System, Millipore) and dried by blowing N2 gas. After this washing step, one can clearly see the channel pattern on the glass substrate.
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