Biomedical Engineering Reference
In-Depth Information
C h a p t e r 2
Microlows
2.1 Introduction
2.1.1 On the Importance of Microfluidics in Biotechnology
Biotechnology is closely linked to microfluidics. Biological targets are nearly always
transported by a buffer fluid or carrier fluid, in vitro and in vivo. In the human
body, any bioMEMS has to deal with body fluids. With in vitro microsystems, the
target molecules/particles are nearly always transported by a buffer fluid for several
reasons. First, the target molecules/particles are, most of the time, extracted from a
liquid (e.g., DNA and cells); second, the biochemical reactions on these targets are
performed in an aqueous environment; and third, the confinement of the targets is
easier in a liquid than in a gas. Very few examples of biotechnological microsystems
exist that do not require the use of microfluidics. One counterexample might be the
electronic nose, in which the detection of target molecules transported by ambient
air is done directly on a dry contact surface by mass spectrometry. The standard
procedure to detect bacteria carried by air, like
legionella
, is
to capture and concen-
trate them in a water-based solution.
In this chapter, we discuss liquid microflows, but we will not consider gas flows. For
the reader whose concern is gas microflows, useful information can be found in [1].
If liquids are the most frequent carrier of microparticles, the flow pattern can be
very different. In Section 2.2, we present the different types of microflows currently
used in biotechnology.
2.1.2 From Single Continuous Flow to Droplets
In biotechnology, microfluidics is present under various forms depending on the
different applications. The most usual form is single-phase microflow in channels
and capillary tubes (Figure 2.1). This is the general case of buffer liquids carrying
biological targets and circulating in microchannel networks where different biologi-
cal processes successively take place. Different reactions, biochemical analysis, and
detection can be done in microchambers placed alongside the flow. Usually, the fluid
is moved under the effect of pressure (from a syringe or a micropump), sometimes
by electric forces (electro-osmotic flow).
In order to accelerate the speed of biorecognition (for high throughput screen-
ing, for example) samples of different buffer solutions have to be treated simultane-
ously and continuously. In such a case, the solution is to convey successive buffer
fluid plugs in capillary tubes; the plugs are separated by a nonmiscible, biocompat-
ible liquid (Figure 2.2). Such a type of flow is commonly called the multiphase flow,
or, if there are only two liquids, a two-phase flow. One of the fluids may be a gas.
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