Biomedical Engineering Reference
In-Depth Information
according to their chemical compositions, the intensity of the magnetic force - and
its direction - depend on the magnetic properties of the materials and also on their
surroundings, including their diamagnetism and paramagnetism . The principle
of magnetophoresis was recently applied to magnetic biosensors, in which mag-
netic microbeads and nanoparticles were used as a solid support and a labeling,
respectively [11-15]. Originally, the role of magnetic microbeads in magnetic bio-
sensors was linked with the function of separation, because they provided biomol-
ecules of interest which not only had a reaction space but could also be used to
separate the biomolecules. The same strategy has also been adapted to other detec-
tion methods, but based on principles of fl uorescence and electrochemistry [16-
18]. While magnetic microbeads are generally used for the separation of target
materials, magnetic nanoparticles can be used directly for magnetophoretic
sensing in biosensors, these actions being based on the detection of biologically
functionalized magnetic labels of cells or microbeads in a magnetic fi eld - induced
microchannel with high sensitivity. Recently, a number of magnetophoretic bio-
sensors using magnetic nanoparticles as labels have been developed [19, 20] and
applied to the analysis of biomolecule concentrations. This approach employs
the magnetophoretic mobility of a microbead, and depends on the amount of
associated superparamagnetic nanoparticles under a magnetic fi eld gradient in a
microfl uidic channel. By measuring the magnetophoretic defl ection velocity of
microbeads as a signal for the presence of analytes, it was possible to quantify
multiple analytes, in simultaneous fashion, by using conjugated nanoparticles as
labels.
Magnetic microparticles and nanoparticles have long been used for the separa-
tion of biomolecules [21], and among the various separation applications [22] can
be included the immunomagnetic separation of cells. Such separation has been
especially important in cell biology and medicine, where magnetic-activated cell
sorting (MACS) has provided the means to separate cells of interest from mixed-
cell populations [17, 23-27]. In MACS, magnetic microparticles and nanoparticles
are fi rst conjugated with antibodies specifi c to the cell membrane protein of inter-
est. Subsequently, when the magnetic particle-bound cells are maintained in a
strong magnetic fi eld, the target cells will be separated from any untreated samples
containing impurities.
In general, two cell separation methods are used, namely direct and indirect
[28] :
•
In the
direct method
, magnetic nanoparticles with appropriate affi nity ligands are
applied directly to the target cells. When the magnetic nanoparticles have
become bound to the target cells, the solution containing the complexes is
allowed to fl ow through a separating column to which is applied a magnetic
fi eld.
•
In the
indirect method
, a free affi nity ligand (in most cases an appropriate
antibody, which often is biotinylated) is fi rst added to the cell suspension, after
which the labeled cells are captured by magnetic nanoparticles bearing an
affi nity ligand against the primary label (e.g., secondary antibodies or
