Analysis of Driven Nanorod Transport Through a Biopolymer Matrix.

Applying magnetic fields to guide and retain drug-loaded magnetic particles in vivo has been proposed as a way of treating illnesses. Largely, these efforts have been targeted at tumors. One significant barrier to long range transport within tumors is the extracellular matrix (ECM). We perform single particle measurements of 18 nm diameter nanorods undergoing magnetophoresis through ECM, and analyze the motion of these nanorods in two dimensions. We observe intra-particle magnetophoresis in this viscoelastic environment and measure the fraction of time these nanorods spend effectively hindered, versus effectively translating.

Increasing the oscillation frequency of strong magnetic fields above 101 kHz significantly raises peripheral nerve excitation thresholds.

PURPOSE: A time-varying magnetic field can cause unpleasant peripheral nerve stimulation (PNS) when the maximum excursion of the magnetic field (ΔB) is above a frequency-dependent threshold level [P. Mansfield and P. R. Harvey, Magn. Reson. Med. 29, 746-758 (1993)]. Clinical and research magnetic resonance imaging (MRI) gradient systems have been designed to avoid such bioeffects by adhering to regulations and guidelines established on the basis of clinical trials. Those trials, generally employing sinusoidal waveforms, tested human responses to magnetic fields at frequencies between 0.5 and 10 kHz [W. Irnich and F. Schmitt, Magn. Reson. Med. 33, 619-623 (1995), T. F. Budinger et al., J. Comput. Assist. Tomogr. 15, 909-914 (1991), and D. J. Schaefer et al., J. Magn. Reson. Imaging 12, 20-29 (2000)]. PNS thresholds for frequencies higher than 10 kHz had been extrapolated, using physiological models [J. P. Reilly et al., IEEE Trans. Biomed. Eng. BME-32(12), 1001-1011 (1985)]. The present study provides experimental data on human PNS thresholds to oscillating magnetic field stimulation from 2 to 183 kHz. Sinusoidal waveforms were employed for several reasons: (1) to facilitate comparison with earlier reports that used sine waves, (2) because prior designers of fast gradient hardware for generalized waveforms (e.g., including trapezoidal pulses) have employed quarter-sine-wave resonant circuits to reduce the rise- and fall-times of pulse waveforms, and (3) because sinusoids are often used in fast pulse sequences (e.g., spiral scans) [S. Nowak, U.S. patent 5,245,287 (14 September 1993) and K. F. King and D. J. Schaefer, J. Magn. Reson. Imaging 12, 164-170 (2000)]. METHODS: An IRB-approved prospective clinical trial was performed, involving 26 adults, in which one wrist was exposed to decaying sinusoidal magnetic field pulses at frequencies from 2 to 183 kHz and amplitudes up to 0.4 T. Sham exposures (i.e., with no magnetic fields) were applied to all subjects. RESULTS: For 0.4 T pulses at 2, 25, 59, 101, and 183 kHz, stimulation was reported by 22 (84.6%), 24 (92.3%), 15 (57.7%), 2 (7.7%), and 1 (3.8%) subjects, respectively. CONCLUSIONS: The probability of PNS due to brief biphasic time-varying sinusoidal magnetic fields with magnetic excursions up to 0.4 T is shown to decrease significantly at and above 101 kHz. This phenomenon may have particular uses in dynamic scenarios (e.g., cardiac imaging) and in studying processes with short decay times (e.g., electron paramagnetic resonance imaging, bone and solids imaging). The study suggests the possibility of new designs for human and preclinical MRI systems that may be useful in clinical practice and scientific research.

Parasitic trap cancellation using multiple frequency dielectrophoresis, demonstrated by loading cells into cages.

This paper presents a method of using multiple frequencies to counteract electric field distortions that interfere with the dielectrophoretic (DEP) manipulation of particles or cells. To demonstrate the technique, simulations were performed for a scenario in which cells were to be loaded into a cage whose walls created parasitic trapping sites that prevented cells from entering it. By employing negative DEP on one electrode in conjunction with positive DEP on another, these traps could be almost completely cancelled. The model predictions were validated experimentally: multiple frequency DEP was used to load many cells into a cage in a matter of seconds in fluid flows of up to 300 microm s(-1), which could not be done with single frequency DEP. Actively cancelling field distortions permits the presence of features that would otherwise be prohibited near regions of dielectrophoretic manipulation, significantly expanding the environments in which dielectrophoresis can be used.

Multiple frequency dielectrophoresis.

A novel method of modeling multiple frequency dielectrophoresis (MFDEP) is introduced based on the concept of an effective Clausius-Mossotti factor, CM(eff), for a particle that is exposed to electrical fields of different frequencies, coming either from one or multiple pairs of electrodes. This analysis clearly illustrates how adding frequencies adds control parameters, up to two additional parameters per frequency. As a result, MFDEP can be used for a wide variety of applications, including separating particles with very similar Clausius-Mossotti spectra, trapping multiple groups of cells simultaneously, and cancelling unwanted dielectrophoretic traps. Illustrating the modeling approach, we determine the CM(eff)s for live and dead yeast cells, and then predict their equilibrium distribution on a three-electrode configuration, with two electrodes at different frequencies and the third electrode at ground. This prediction is validated experimentally, using MFDEP to selectively attract live cells to one location and dead cells to another, trapping both. These results demonstrate that the use of multiple frequencies for the manipulation of particles can enhance the performance of dielectrophoretic devices, not only for sorting, but also for such applications as patterning cells in close proximity for the formation of cell consortia.