To begin, tape the SU8 master silicon wafer for the microfluidic channel design on the inside of a plastic 14-centimeter Petri dish and clean it with nitrogen gas. Weigh appropriate amounts of polydimethylsiloxane, or PDMS base and PDMS curing agent, in a paper cup. Using a wooden spatula, mix the two until the mixture becomes cloudy white in color.
Pour the PDMS mixture into the plastic Petri dish containing the silicon wafer. Then, place the Petri dish into a vacuum desiccator equipped with a three-way stopcock. Turn the valve of the stopcock to connect the vacuum to the desiccator chamber to remove air bubbles from the mixture.
Once all air bubbles are removed from the features of the channels, place the Petri dish inside an oven at 70 degrees Celsius for four hours. After the Petri dish cools to room temperature, place it on a cutting mat. Using a scalpel, cut out the portion of the PDMS above the silicon wafer.
Place the cut-out PDMS between two sheets of the lab wrapping film. The gap between the indent of the microchannel and the film helps to locate the inlet and outlet of the microfluidic channel. Next, using a razor blade, cut out an individual channel from the large PDMS and, with respective biopsy punches, make appropriate inlet and outlet holes in the channel.
Place the hole-punched channel with the channel side facing up onto a clean glass slide. Place a glass substrate containing thin film indium-tin oxide, or ITO interdigitated electrodes, on the same slide with the electrodes facing up. Then, gently place the glass slide into a plasma cleaner.
After closing the gas valve and switching on the pump, wait for two minutes to obtain a sensor reading of 600 to 800 millitorr. Next, turn on the power switch and wait 30 seconds before turning the RF power knob from low to high and waiting for one minute. Then, to switch off the set, follow the reverse sequence.
Immediately after opening the chamber of the plasma cleaner, lift and rotate the PDMS 180 degrees so that the channel side is facing down. Place the channel on the top of the ITO substrate to initiate the bonding process. Using tweezers, gently press down on the corners of the PDMS for about three seconds.
Load the priming medium into a one-milliliter syringe with a 23-gauge needle. Slowly and carefully wet the channel by inserting the needle straight into the inlet well before releasing the medium without introducing air bubbles. After incubating for at least three minutes, remove the prime medium using a 10-microliter pipette tip.
Finally, wash the channel with dielectrophoresis medium three times by inserting the medium into the channel.
