Biomedical Engineering Reference
In-Depth Information
exaggerated due to the potential offered by the presence of a strong magnetic
moment, associated with the super-spin of the nanoparticle. Thus, it is possible
to imagine, in a suggested order of complexity: (i) magnetic-fl uorescent nanopar-
ticles for dual MRI-optical detection; (ii) magnetic nanoparticles or nanoparticle
clusters for MRI imaging and RF hyperthermia; and (iii) drug- loaded, magnetic -
polymer nanocomposites and magnetoliposomes, or nanoparticles grafted with
drug molecules. These have potentially favorable biodistribution and pharmacoki-
netics, which could be enhanced by magnetic positioning at the site of action on
the application of a static magnetic fi eld.
Recent progress in both optical and magnetic detection has suggested a
strong future for this type of particulate agent in tracking experiments that
may improve our understanding of cellular recognition events, and may even
help develop new methods for assessing biodistribution and even screening for
future therapeutics. For instance, magneto-optical nanocomposites have been
used for tracking stem cells [321, 322], for imaging the macrophage infi ltration of
tissue [323], and for detecting specifi c molecular targets that are weakly expressed
[324] .
A related fi eld of interest is in the development of multiple, simultaneous detec-
tion of different MRI stimuli, or analytes. This is analogous to the recent develop-
ment of ranges of nanoparticles (and quantum dots) with optically distinguishable
signatures which enable the tracking of many biological indicators. Thus, much
interest has been expressed in developing multiplexed MRI sensors, which could
be fabricated to be small enough for use as contrast agents in man [118]. A good
recent example of this has been provided by Zabow and coworkers [325], who
reported the fabrication of structures where
1
H frequency shifts and sensitivity
enhancements were determined by the local geometry within a magnetic micro-
structure. This approach immediately offers multiplexing capability and may
become the method of choice, given the advanced state of modern fabrication
technology, once these structures have been scaled down towards, or even below,
the 100 nm range.
A signifi cant number of reports have also been made where the potential of
magnetic nanoparticle suspensions for dual diagnostic and therapeutic (termed
“theranostic”) applications has been realized. The development of magnetic
nanoparticle suspensions as dual contrast agents and hyperthermia mediators has
been described. These applications are feasible, as nanoparticulate iron oxide has
favorable MRI properties (as discussed earlier), and also has good RF energy-
specifi c absorption rate (SAR) values [5]. Hyperthermia using superparamagnetic
iron oxide has been investigated for many years as a potential treatment for solid
tumors in particular [326].
A second, indirect, form of hyperthermia involves the use of alternate current
fi eld heating to deliver a therapeutic molecule from a carrier, such as a liposome
or a polymer/nanoparticle composite. Examples of this approach have been
reported for a range of carriers, including magnetic iron oxide liposomes for
delivering 5 - fl uorouracil [327] and dextran-stabilized contrast agents combined
