The movement of a liquid or droplets through microfluidic channels usually takes place according to physical principles such as the capillary effect, in which the diameter and wettability of the channels are crucial. In other cases, external or internal pump systems are used to move the fluids. But not surprisingly, there is also the possibility to fully digitize this process. Welcome to the world of digital microfluidics.
In digital microfluidics, the bottom layer of the device contains an array of individually controllable electrodes. The size and position of each droplet can be determined with electrical signals. Droplets are moved by sequentially switching the voltage on and off across adjacent electrodes. Using the same technique, fluids can also be separated or mixed.
There are two ways to create new droplets with a digital microfluidic device. Either an existing droplet can be split in half, or a new droplet can be made from external material. An aquatic droplet can be split in two when it sits on an uncharged electrode. By charging two electrodes on either side of this neutral electrode, the droplet will move to both active electrodes, causing a break in the middle. This way, two droplets have formed.
Just as a droplet can be split using electrodes, droplets can also be joined in this way. On a charged electrode, droplets from surrounding uncharged electrodes will come together and merge into one droplet. Although water-based droplets respond best to electrical signals, experiments are also being conducted with water-in-oil and oil-in-oil options.
Digital microfluidic platforms often make use of the principle of electrowetting. This means that the wettability of a surface can be adjusted using voltage. On a hydrophobic surface, droplets normally have a large contact angle, but an electrical current can cause the droplet to spread atypically across the hydrophobic surface.
Another procedure that can be performed using digital microfluidics, is the separation and extraction of target analytes. This can be done for instance with a magnetic field that can be switched on and off, and by introducing magnetically charged particles that bind to the analyte of interest into the sample material. Next to magnetism, digital microfluidics devices can also be fitted with thermal and optical components, which paves the way to perform processes such as PCR, beads based separation, and quantitative analysis.
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