Harvard-MIT Division of Health Sciences and Technology
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In Silico Bioelectromagnetics
Electroporation Theory
Magnetic Field Effects
Weak Field Effects
Microconduit Creation
Skin Electroporation

Weak Field Effects

Key features of the model for biological detection of small magnetic field differences

Fractional change in the reaction rate of the radical-pair reaction vs the magnitude of the magnetic field.

Understanding exposure thresholds for the response of biological systems to extremely low frequency (ELF) electric and magnetic fields is a fundamental problem of long-standing interest. We consider a two-state model for voltage-gated channels in the membrane of an isolated elongated cell (L-cell = 1 mm; r(cell) = 25 mu m) and use a previously described process of ionic and molecular flux rectification to set lower bounds for a threshold exposure. A key assumption is that it is the ability of weak physical fields to alter biochemistry that is limiting, not the ability of a small number of molecules to alter biological systems. Moreover, molecular shot noise, not thermal voltage noise, is the basis of threshold estimates. Models with and without stochastic resonance are used, with a long exposure time, t(exp) = 10(4) s. We also determined the dependence of the threshold on the basal transport rate. By considering both spherical and elongated cells, we find that the lowest bound for the threshold is E-min approximate to 9 x 10(-3) V m(-1) (9 x 10(-5) V cm(-1)). Using a conservative value for the loop radius r(loop) = 0.3 m for induced current, the corresponding lower bound in the human body for a magnetic field exposure is B-min approximate to 6 x 10(-4) T (6 G). Unless large, organized, and electrically amplifying multicellular systems such as the ampullae of Lorenzini of elasmobranch fish are involved, these results strongly suggest that the biophysical mechanism of voltage-gated macromolecules in the membranes of cells can be ruled out as a basis of possible effects of weak ELF electric and magnetic fields in humans.

There is evidence that animals can detect small changes in the Earth's magnetic field by two distinct mechanisms, one using the mineral magnetite as the primary sensor and one using magnetically sensitive chemical reactions. Magnetite responds by physically twisting, or even reorienting the whole organism in the case of some bacteria, but the magnetic dipoles of individual molecules are too small to respond in the same way. Here we assess whether reactions whose rates are affected by the orientation of reactants in magnetic fields could form the basis of a biological compass. We use a general model, incorporating biological components and design criteria, to calculate realistic constraints for such a compass. This model compares a chemical signal produced owing to magnetic field effects with stochastic noise and with changes due to physiological temperature variation. Our analysis shows that a chemically based biological compass is feasible with its size, for any given detection limit, being dependent on the magnetic sensitivity of the rate constant of the chemical reaction.