The overall goal of this procedure is to present different options for microfluidic rapid prototyping platforms. This is accomplished by first fabricating a removable interconnector using PDMS. The second step is to secure micro tubes in the interconnector or to the microfluidic chip directly.
Next, the microfluidic chips are assembled and connected to an electrical interface. Ultimately, fluid is flowed through the interconnector and into the microfluidic chip at different speeds. To evaluate the strength of the PDMS Interconnector, I first had the idea for this method when designing a high electrical input output microfluidic chip, and needed an easily removable interconnector to test different pattern of ENC channel electrical field propagation with a large number of ENC channel electrodes.
To begin prepare the PDMS by mixing the PDMS and curing agent in a 10 to one ratio, and then degas the mixture by placing it under a low vacuum for 60 minutes. Next, preheat and oven temperature to 80 degrees Celsius and wait for the temperature to stabilize while the oven heats up. Clean the surface of the microfluidic chip where the PDMS interconnector will be placed using ethanol.
Allow it to dry in a dust-free area. Transfer the dish of PDMS to the preheated oven and let the PDMS slightly polymerize at 80 degrees Celsius for 12 to 15 minutes, place the microfluidic chip to which the connection will be made in the oven with the PDMS layer so that it will be at the same temperature after 15 minutes. Remove the PDMS and the microfluidic chip from the curing oven.
Then use a razor blade to cut out a five by five millimeter piece of PDMS. Then punch two two millimeter holes through the PDMS layer. Remove the PDMS from the Petri dishes and place it over the microfluidic chip.
Take care to precisely align the holes and then gently press the PDMS into place. With the interconnector in place, trim two millimeter diameter Teflon tubes with a 100 micron inner diameter, using a new blade so that the edge of the micro tube ends up cut perpendicular to its length. The tubing should be long enough to be connected to a syringe pump without applying any stress on the interconnector.
Using a micro positioner insert tubes through the PDMS interconnector layer if necessary, add epoxy between the tube and the PDMS for better adherence. Then connect the other end of the tubes to the liquid injection system. As an alternative to the PDMS based interconnector epoxy may be used to permanently secure the micro tubes into place.
To accomplish this, use the micro positioner to carefully place the micro tube in the access hole of the microfluidic chip. Then apply a UV curable or standard epoxy around the access holes and cure it according to manufacturer specifications. To begin solder surface mount multip pin electrical connectors to a printed circuit board.
Then inspect the electrical pads of the microfluidic chip and clean any surface contamination by exposing to plasma oxygen for about 10 minutes with more or less time, depending on the required amount of cleaning. If no surface contamination is found, simply clean the pads with ethanol. Next, use a sharp blade to cut conductive tape into small two millimeter by one millimeter pieces so that they are the same dimensions as the microfluidic chips.
Electrical pads. Then use clean forceps to place the conductive tape onto the printed circuit board Over the electrical pads. Align the microfluidic chip with the interconnector already in place above the printed circuit board.
Using an alignment machine, place an insulation layer such as paper on the printed circuit board to avoid short circuits, and then attach the metal bracket using machine screws and nuts with the assembly complete. Begin to test the individual setups by flowing deionized water through the syringe pump. Increase the pump speed slowly until the system begins to leak, at which point the max pressure has been reached.
Following your experiment, remove the machine screws nuts and the metal brackets. Then gently separate the microfluidic chip and the printed circuit board. Finally, use ethanol to remove the conductive tape adhesive from the electrical pads on the printed circuit board and microfluidic chip prior to reassembly.
Three different interconnector tube attachment techniques are compared in the table shown here. The PDMS interconnector is suggested for use with low pressure applications when the microfluidic design must be uncovered and requires fast prototyping. Standard and UV epoxy provide a strong interconnection and can be used for both low and high pressure applications, but creates a permanent seal with the device, which may be unwanted.
The PDMS interconnectors are shown here compared with some of the more common interconnector setups. The one millimeter thick PDMS interconnector can withstand three times the pressure of the 0.45 millimeter thick interconnector. This relates to five times the flow rate for the one millimeter thick interconnector compared to the 0.45 millimeter interconnector Once mastered, this technique can be completed in 10 to 20 minutes if it is done properly.
