Make VersaLive chips

Protocol for the soft lithography fabrication of VersaLive chips

Materials

  • Master mold
  • VersaLive Starter Kit
  • Chloro-perfluorosilane (1H,1H,2H,2H-perfluorooctyl-trichlorosilane)
  • PDMS kit
  • Desiccator
  • Scalpel or razor blade
  • Glass slides, #1 thickness

Passivation of the master mold

[ ! ] Note: This step of the fabrication protocol needs to be carried out only at the first use of the master or if the previous PDMS peeling off turned out to be more difficult than usual
  1. Using wafer tweezers, place the master mold into the desiccator
  2. Place 20 μL of perfluorosilane into a small container (e.g. vial cap, cut out of a plastic vial, aluminum foil)
  3. Place the container with the perfluorosilane into the desiccator together with the master mold. Note: the container needs to be at the opposite side of the vacuum nozzle otherwise the resulting deposition will be of poor quality
  4. Close the desiccator and connect it to the vacuum line. Institute vacuum line: overnight deposition; direct vacuum pump: 30 - 120 minutes
  5. When the deposition ends, turn off the vacuum and gently vent the desiccator avoiding its content to fly around with the risk of being damaged
  6. Dispose the remaining perfluorosilane traces (e.g. ecobox)
  7. Test the outcome of the deposition by squirting some DI water onto the master mold. The water should slip away or stay on the surface with a contact angle > 90˚C. If the water spreads onto the wafer surface, the deposition was not successful and needs to be repeated
  8. Clean the desiccator chamber first with water and then with IPA. Use a cleanroom wipe to help with this step.

PDMS casting

  1. Mix PDMS base and catalyst in a 10:1 ratio
    * To reach a final height of 5 mm in a 12 cm Petri dish, use 50 g of PDMS base and 5 g of catalyst
    * Use a large weighing boat to mix the PDMS components
    * Mix the components until you are sure it could not be further mixed. Then, mix some more.
  2. Place the PDMS mix in the desiccator and connect it to the vacuum line to degas it. Between 30 to 60 minutes are usually enough to remove most of the bubbles
  3. Coat the 12 cm glass Petri dish with a double layer of aluminum foil and place the master mold in it. Make sure that the aluminum foil is wrinkle-free and the master mold sits flat on it. If the master mold is tilted, the height of the PDMS chips will result uneven compromising the volume capacity of the reservoirs
  4. Pour the degassed PDMS onto the master mold to form a uniform casting
  5. If required, remove the remaining bubbles placing the master into the desiccator again or popping them by hand being careful in not damaging the master mold
  6. Cure the PDMS for at least 2 hours in oven at 80˚C
  7. Remove the cured slab and master from the Petri dish. Free the contour of the master mold by trimming the excess of the PDMS using a sharp scalpel
  8. Peel the PDMS off the master mold with a gentle but steady movement. Ideally, one should prevent a second contact of the PDMS slab with the master because this is likely to damage the delicate features on both sides of the casting elements

Chip cutting, opening of the ports, plasma bonding

  1. Place the PDMS slab with features up and using a single-edge razor blade cut the chips in octagonal shape following the embedded guidelines

This is a good pause point.
PDMS chips can be stored in a clean and sealed dish for weeks before use.

  1. Open all the required ports by using a sharp biopsy punch of the required size (usually, 3 mm in diameter)
  2. Make sure the bonding sides of the PDMS chips and the glass slides are clean and free from dust particles. Adhesive tape is usually enough to clean these surfaces but ultra-sonication in isopropanol could be used for a more aggressive cleaning

[ ! ] Note: To have an easier wetting procedure, bond the chips right before the beginning of the experiment.
  1. The chip is bonded to a glass slide using air plasma. The bonding can be permanent (30 s activation) or reversible (10 s activation)
    * Place both the chip and the cover glass in the plasma chamber. Make sure that the channels of the chip are facing up
    * Close the plasma chamber and turn on the vacuum pump
    * When the pressure gauge is below 0.4 mbar, generate the plasma for the required time (10 or 30 seconds)
    * Vent the chamber and put the two surfaces in contact. Look for the characteristic change in appearance that ensures contact
  2. The chip is kept at room temperature and it is ready for the wetting of the channels.