Although magnetic particles have proven exceedingly useful in nanotherapy protocols, applications making use of external magnets have been limited to surface or shallow targets. The ability to use external magnets to focus on deeper targets has remained a challenge. To this end, a collaboration between researchers from the University of Maryland and Weinberg Medical Physics LLC has devised a way to use pulsed magnetic fields to direct particles to hard-to-reach targets.
Overcoming theoretical limitations
The authors liken the principles behind their work to those guiding the behavior of bar magnets. When the bars are placed near a larger magnet, the poles nearest the magnet will determine their behavior. If the nearest poles are opposite, the bars will be attracted by the magnet. Conversely, if the nearest poles are the same, the bars will be repelled.
The present study makes use of this repulsion of magnetic rods antialigned with a magnet. An initial magnetic field serves to orient the nanorods. Following this, a magnetic field gradient antialigned with the rods is applied. The result is movement away from the magnet. Because the rods eventually begin to rotate to match the field’s alignment, it is necessary to pulse the magnetic forces, applying transient magnetic gradients that are removed before the rods can realign with the magnetic field. This sequential pulsing allows the particles to be focused to precise targets between the electromagnets.
Putting theory to practice
Proof of principle was provided in vitro, utilizing commercially obtained cobalt rods measuring 200 μm long × 200 nm. The rods were suspended and dispersed by gentle shaking. Upon application of the magnetic pulse and gradient elements, the ferromagnetic rods were successfully focused to a central target. Additional experiments using nickel rods with or without a PEG coating were carried out with similar results. Suspension solutions included tap water, 1X buffered saline, and a 1:5 hexane:isopropanol solution.
The research team acknowledges that there is still much work to be done before the results can be applied in a clinical setting. Currently, the research is focusing on an in vivo demonstration of the approach. Used in combination with existing methods, the study provides a number of possibilities for nanotherapy of deep tissue targets.
The study, entitled “Dynamic Inversion Enables External Magnets To Concentrate Ferromagnetic Rods to a Central Target” is presented in this week’s issue of “Nano Letters.”
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