A Versatile Kit Based on Digital Microfluidics Droplet Actuation for Science Education

We present a digital microfluidics educational kit based on a commercially-sourced printed circuit board that allows the user to get hands-on experience with digital microfluidics. This is a viable, low cost solution for education provided that digital PCB design files can be shared. We propose to use digital microfluidics as an educational tool because droplets are manipulated on generic electrode array platforms.

Users can leverage on an extended set of electronic components now highly accessible for do-it-yourself applications to electronically interface with the droplets. Demonstrating the procedure will be Yu Hao Guo, a graduate student from the Yang Laboratory. Begin by soldering the surface mount resistors, transistors, and light emitting diodes onto the PCB board.

Connect the output of the high voltage power supply board to the PCB board with the soldered components. Then connect the battery to the voltage booster board to boost the voltage from six volts to 12 volts. Connect the humidity sensor, the ultrasonic piezo atomizer, and the atomizer driver board to the microcontroller board.

Turn on the microcontroller using the provided supplementary code. Adjust the variable resistor of the high voltage board and use the digital multi-meter to measure the voltage of the EWOD electrode. Wear clean nitrile gloves and apply 10 microliters of five centistoke silicone oil on the electrode area using a micropipette.

Spread the oil evenly on the electrode area using a finger. Cut a piece of food wrap with dimensions 2.5 by four centimeters and place it on top of the electrode. Apply silicone oil on the electrode area using a micropipette and spread it evenly.

To perform a chemiluminescence experiment, place two to five microliters of luminol solution on the target electrode using a micropipette. Place 10 microliters of 0.1%potassium ferrocyanide on the electrode which can be moved as a droplet for electrowetting. Turn on the microcontroller so that the potassium ferrocyanide droplet merges with the luminol.

For fluorescent imaging, cut a one square centimeter piece of semi-transparent tape and place it between the excitation light emitting diode and EWOD electrodes. Attach the emission glass filter on the camera of the smartphone with tape and place 10 microliters of the potassium ferrocyanide solution on the electrodes. Record the video of the droplet actuation using a smartphone.

For long-term droplet actuation, place one milliliter of water onto the ultrasonic atomizer. Place a droplet of potassium ferrocyanide and turn on the microcontroller. Then immediately close the lid of the enclosure.

Check the droplet actuation after an hour. A representative droplet movement is shown here. For the chemiluminescence experiment, the droplet of ferrocyanide is actuated to move and mix with the pre-deposited luminol droplet on the target electrode at 12 seconds.

A schematic setup of an LED serving as the light source for excitation, a semi-transparent clear office tape as a light diffuser, and the emission filter directly attached to the smartphone camera is shown here. Fluorescent imaging of the droplet containing fluorescein isothiocyanate in the dark is seen as a result of the semi-transparent tape serving as the diffuser to evenly distribute the excitation light. For a long-term experiment, successful droplet actuation can be observed.

Representative humidity data under the action of an ultrasonic atomizer is shown here. This protocol can be used to develop an educational kit based on digital microfluidics. A luminol-based chemiluminescence experiment is reported as a specific example.

The kit can be assembled within a short period of time and with minimal training in electronics. The simplified experiment described here can be extended to other experiments. For example, a paper test kit can be used by moving the droplet to the paper to be absorbed.

A microcontroller with interface logic circuit can also be added to provide more sophisticated digital control and programmability. This protocol can benefit non-professional enthusiasts to learn and apply electronics to further advance their knowledge of the field.