Practical methods for generating alternating magnetic fields for biomedical research

Abstract

Alternating magnetic fields (AMFs) cause magnetic nanoparticles (MNPs) to dissipate heat while leaving surrounding tissue unharmed, a mechanism that serves as the basis for a variety of emerging biomedical technologies. Unfortunately, the challenges and costs of developing experimental setups commonly used to produce AMFs with suitable field amplitudes and frequencies present a barrier to researchers. This paper first presents a simple, cost-effective, and robust alternative for small AMF working volumes that uses soft ferromagnetic cores to focus the flux into a gap. As the experimental length scale increases to accommodate animal models (working volumes of 100s of cm or greater), poor thermal conductivity and volumetrically scaled core losses render that strategy ineffective. Comparatively feasible strategies for these larger volumes instead use low loss resonant tank circuits to generate circulating currents of 1 kA or greater in order to produce the comparable field amplitudes. These principles can be extended to the problem of identifying practical routes for scaling AMF setups to humans, an infrequently acknowledged challenge that influences the extent to which many applications of MNPs may ever become clinically relevant.

Transgene-free remote magnetothermal regulation of adrenal hormones

“…4F), consistent with the biochemical inertness of iron oxides (30). In the postmortem analysis of the adrenal tissue injected with NPs, we found that 80% of the rats harbored injection sites in the adrenal medulla, 90% had injection sites in the ZF and zona reticularis, and only 25% had injection sites in the zona glomerulosa (n = 12; To deliver AMF conditions sufficient to drive hysteretic heat dissipation in MNPs injected in adrenal glands, we used a custom apparatus (28), capable of producing AMFs with amplitudes up to H = 13.5 ± 1.5 kA/m at a frequency ƒ = 557 kHz (SLP = 620 ± 190 W/g) across a volume of ~200 cm 3 , which accommodates an adult Long-Evans rat (Fig. 5, C and D, and fig.…”

“…Magnetothermal adrenal actuation with an externally applied AMF (15 kA/m, 515 kHz) generated in a custom coil driven by a resonant circuit (28) was observed in 50% of the examined cells (n = 140) in the presence of MNP solutions (5 mg/ml) ( Fig. 3, H to J, and fig.…”

Magnetic Entropy as a Proposed Gating Mechanism for Magnetogenetic Ion Channels

“…For the second experiment, we imaged a new FOV on the same coverslip during stimulation by a magnet or sham magnet. [10][11][12][13][14][15][16][17][18][19][20] FOVs from independent cell cultures were recorded for each condition, yielding R1000 cells per condition. When calculating statistical significance, we considered both individual cells ( Fig.…”

“…Although exogenous chemically synthesized nanoparticles can effectively create magnetically sensitive cells, a method that did not require delivery of synthetic nanoparticles and relied on low-frequency magnetic fields would have a number of advantages. Generating high-frequency AMFs of sufficient amplitude to stimulate cells in vivo requires magnetic field generators that cost thousands of dollars and consume 5-15 kW of power (12). Low-frequency magnetic field generation consumes much less power and can even be achieved using a permanent magnet.…”

Novel test of Lorentz violation in the photon sector with an LC circuit

“…Unfortunately, this approach has a number of disadvantages. High-frequency AC fields -up to ∼ 0.5 MHz -with intensities of O(0.1 T) can only be produced within bore volumes of a few cm 3 [22][23][24][25]. Furthermore, solenoids that produce such high frequencies and field intensities require strong currents and thick, tightly winded coils that would generally experience significant ohmic losses.…”