Biomedical Engineering Reference
In-Depth Information
C h a p t e r 9
Magnetic Particles in Biotechnology
9.1 Introduction
In many biotechnological applications, the use of a carrier fluid to transport bio-
logical objects lacks specificity: for example, it is not always possible to bring by
microfluidics transport a biological target to a specific location inside the biochip.
A second complementary carrier is often needed. To this extent, magnetic beads are
one of the most important categories of microparticles. Between 1990 and 2004
they were developed mostly for in vitro applications, principally for biodiagnos-
tic and biorecognition and also for purification and separation operations. More
recently their use has reached the domain of in vivo applications such as cancer
treatment.
In this chapter, we present first the nature of magnetic beads, their magnetic
characteristics, and the force that can be applied on these beads. We then give ex-
amples of trajectory calculation for applications such as separation columns and
magnetic field flow fractionation. (MFFF). Finally, we show how assembly of mag-
netic beads has been used to build new biological tools and we focus on chains of
magnetic beads, ferrofluids, and magnetic membranes.
9.1.1 The Principle of Functional Magnetic Beads
At first sight it might seem strange to consider magnetic actuation of biological
microsystems because neither DNA, proteins, antibodies, cells, nor bacteria (except
just one kind, but that is just anecdotal)
1
are magnetic. However, the principle of
functionalization has totally changed the approach: as soon as it became possible to
bind DNA strands—or other biological or biochemical macromolecules—on mag-
netic microparticles, these microparticles could be used to displace and manipulate
complex biological molecules [1]. The principle of functionalization is schematized
in Figure 9.1.
The principle is to find a chemical linker between the bead surface and the tar-
get in order to attach the target to the bead. There are many types of functionaliza-
tions depending on both the surface of the particle and the target. For example it
has been found that the chemical group streptavidin-biotin is a good linker for the
capture of DNA. It is a very complex task to find the adequate functional coating
of the bead. To facilitate the task, prefunctionalized beads are currently sold by
specialized suppliers.
1
The bacteria
Magnetospirillum magnetoacticum
has magnetic microreceptors to use the Earth's mag-
netic field for orientation.
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