Biomedical Engineering Reference
In-Depth Information
support perpendicular to two magnets. Between the magnets, the magnetic field is
approximately uniform. If a drop containing paramagnetic particles is deposited
on the plate between the wires, then the microparticles move rapidly towards the
central axis; later they migrate slowly along this central axis.
9.7 Magnetic Beads in EWOD Microsystems
As mentioned in the introduction of this chapter, in biotechnology, magnetic beads
are often used for the transport of macromolecules: superparamagnetic beads are
functionalized or labeled to recognize and bind to a specific molecule. Functional-
ization is achieved by coating the magnetic bead with molecules having a chemical
affinity for the target molecules. Hence, when dispersed in a liquid, the labeled beads
bind to the target molecules, forming a composite macromolecule. In the absence
of a magnetic field, the particles disperse in the liquid phase if they are sufficiently
small. In presence of a magnetic field, they are attracted towards the magnetic pole,
and aggregate together because of the formation of chains of magnetic dipoles.
Techniques based on magnetic capture and concentration, similar to those us-
ing microflows, have been developed in digital microfluidic devices, with the all the
advantages of smaller liquid volumes, allowing more precise and sensitive recogni-
tion and/or bioanalysis. Figure 9.17 shows an open EWOD device (with catena)
and a droplet containing magnetic beads aggregated by a minimagnet placed below
the substrate. On one hand, if the electrowetting forces are sufficient and on the
other hand if the magnetic forces are sufficient, the magnetic aggregate separates
from the droplet.
Figure 9.18 demonstrates the principle of concentration of target molecules by
paramagnetic labeled microbeads. First the beads are dispersed in the droplet and
some bind on the target molecules (a); then the beads are aggregated by using a
minimagnet (b); the droplet is then motioned by electrowetting actuation (c and d);
if the magnetic and electrowetting forces are sufficient, the droplet continues its
motion, leaving behind the aggregate with a small amount of liquid (e). Target mol-
ecules are now concentrated in a “nano” droplet.
Figure 9.17
Combination of magnetic forces exerted on an aggregate of magnetic beads and
electrowetting forces exerted on a conductive liquid droplet. In this case, the magnetic forces are
sufficiently important to pin the aggregate and the electrowetting forces are sufficiently large to
move the droplet, leaving behind the aggregate with a small amount of liquid. (Photo courtesy N.
Chiaruttini, CEA-LETI.)
