Digital microfluidics

Digital microfluidics is an alternative technology for lab-on-a-chip systems based upon micromanipulation of discrete droplets. Microfluidic processing is performed on unit-sized packets of fluid which are transported, stored, mixed, reacted, or analyzed in a discrete manner using a standard set of basic instructions.

Basics
In analogy to digital microelectronics, these basic instructions can be combined and reused within hierarchical design structures so that complex procedures (e.g. chemical synthesis or biological assays) can be built up step-by-step. And in contrast to continuous-flow microfluidics, digital microfluidics works much the same way as traditional bench-top protocols, only with much smaller volumes and much higher automation. Thus a wide range of established chemistries and protocols can be seamlessly transferred to a nanoliter droplet format. Electrowetting, dielectrophoresis, and immiscible-fluid flows are the three most commonly used principles, which have been used to generate and manipulate microdroplets in a digital microfluidic device.

Working Principle
In brief, the droplets are formed using the surface tension properties of liquid. For example, water placed on a hydrophobic surface will lower its contact with the surface by creating drops whose contact angle with the substrate will increase as the hydrophobicity increases. However, in some cases it is possible to control the hydrophobicity of the substrate by using electrical fields. This is referred to as electrowetting on dielectric or EWOD. In thin layers of Teflon for example, while no field is applied the surface will be extremely hydrophobic, but when a field is applied electrons will be driven out therefore creating a polarized surface thus increasing its affinity to water. A droplet would therefore spread in that region. By controlling the localization of this polarisation it is possible to control the displacement of the droplet.

Implementation
The basic unit cell of a EWOD-based microfluidic biochip consists of two parallel glass plates. The bottom plate contains a patterned array of individually controllable electrodes, and the top plate is coated with a continuous grounding electrode. A dielectric insulator coated with a hydrophobic is added to the plates to decrease the wettability of the surface and to add capacitance between the droplet and the control electrode. The droplet containing biochemical samples and the filler medium, such as the silicone oil, are sandwiched between the plates; the droplets travel inside the filler medium. In order to move a droplet, a control voltage is applied to an electrode adjacent to the droplet, and at the same time, the electrode just under the droplet is deactivated. By varying the electric potential along a linear array of electrodes, electrowetting can be used to move droplets along this line of electrodes.