Biomedical Engineering Reference
In-Depth Information
C h a p t e r 4
Digital, Two-Phase, and
Droplet Microluidics
4.1 Introduction
Due to the miniaturization trend and the need for handling smaller volumes of liquids,
new types of microfluidics have emerged, like digital and droplet microfluidics. These
fields have seen remarkable developments during the last few years. Digital microfluid-
ics is used to move, merge, and mix droplets on a paved grid of a solid planar substrate.
This is a powerful tool for extremely precise droplet handling with applications in the
domain of DNA recognition and analysis. On the other hand, droplet microfluidics is
particularly suited for cell encapsulation, and this is the engine driving today's medical
replacement of defective organs in the body. It has become so popular that Hübner et
al. [1] have published a paper in the journal
Lab-on-a-Chip
entitled: “Microdroplets:
A Sea of Applications.” One could categorize these two fields by two-dimensional and
three-dimensional microfluidics, respectively, for digital and droplet microfluidics. In
this chapter we present the basics of each approach and indicate their applications.
4.2 Digital Microfluidics
4.2.1 Introduction
Digital microfluidics is sometimes called planar microfluidics because it consist in
moving, merging, and dividing droplets on a planar—or at least locally planar—
surface. There are two means of actuation: acoustic and electric. In this chapter we
only deal with the electric actuation, called electrowetting, and more specifically
electrowetting on dielectrics (EWOD) because it is the principle of most digital
microfluidics systems. Acoustic actuation has also seen some interesting develop-
ments, and the reader can refer to the publications of Augsburg University and to
[2]. The advantages of such microsystems are very small liquid samples (less than
100 nl) and extremely precise control of the droplets due to the “digital” actuation.
A sketch of such systems is shown in Figure 4.1.
4.2.2 Theory of Electrowetting
4.2.2.1 Berge-Lippmann-Young (BLY) Equation
In the presence of an electric field, electric charges gather at the interface between
conductive and nonconductive (dielectric) materials. Theses electric charges exert
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