Biomedical Engineering Reference
In-Depth Information
and trapping. It may also be very easy to separate and purify target materials
such as mammalian cells, red blood cell s ( RBC s) and carbon nanotube s ( CNT s)
with high precision by arranging the ferromagnetic materials in a variety of
structures [34 - 37] .
Because a microchannel in which the fl uid containing the cells or microbeads
fl ows is located beside the wire, attention will be focused on the magnetic fi eld
gradient alongside the ferromagnetic wire. When applied in a perpendicular direc-
tion to the ferromagnetic wire, the magnetic fi eld is relatively more sparse along-
side the wire than at more distant regions. As shown in Figure 3.1a, the RBCs will
be repelled from a magnetized wire, despite their having natural magnetic proper-
ties. In contrast, the white blood cells (WBCs), which have diamagnetic properties,
are attracted to the magnetized wire. When the magnetic fi eld is applied in a hori-
zontal direction to the ferromagnetic wire, the fi eld will be more dense alongside
the wire; consequently, any paramagnetic particles (RBCs) will be attracted to the
magnetized wire, whereas any diamagnetic particles (WBCs) will be forced away
to the sparse magnetic fi eld (Figure 3.1b ).
3.3
Magnetophoresis in Microfl uidic Devices
3.3.1
Design and Microfabrication Processes
The microfl uidic device for magnetophoresis generally requires several compo-
nents for the microchannels and the magnetic energy source. The most popular
fabrication methods for microfl uidic channels include the poly(dimethylsiloxane)
(PDMS) micromolding process and Si-wafer micromachining. Other fabrication
processes, such as hot embossing and injection molding, can provide low- cost and
single-use plastic chips for magnetophoresis. However, when considering the
applications and integration of microchannels with a magnetic energy source,
there will be a restriction in the choice of available microfabrication process. In
general, magnetic energy sources are provided either by an electromagnet or a
permanent magnet. As shown in Table 3.1, the electromagnetic system [38] is
advantageous for controlling magnetic forces on substances in the microchannel,
although several problems may arise due to Joule heating and complicated fabrica-
tion processes.
A permanent magnet system is preferable in microfl uidic magnetophoresis,
because present-day, laboratory-based research investigations require simple
and easy accessibility rather than the complicated, on-chip integration used
in a commercial set-up [39]. In the case of the permanent magnetic system,
several approaches have been shown to improve the magnetic fl ux density gradient
across the microfl uidic channel, using ferromagnetic microstructures [33, 40-42].
As described in Section 3.2.2, the magnetic force acting on a particle is propor-
tional to the magnetic fl ux density gradient (d
B
/d
x
), and this is rapidly reduced
