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Biomedical subjects

F X Hart

Publications and source records attributed to F X Hart.

15 recordsLinked to original sources
PubMed1994

Spreadsheet method for calculating the induced currents in bone-fracture healing by a low-frequency magnetic field.

A commercially available spreadsheet program is used on a microcomputer to calculate the induced current density and electric field patterns produced in a nonhomogeneous, anisotropic model of tissue by a localized, low-frequency magnetic field source. Specific application is made to coils used to promote the healing of bone fractures in limbs. The variation of the conductivity of the fracture gap during healing causes the induced current density pattern to change correspondingly, whereas the induced electric field remains relatively unchanged. Use of more simplified, isotropic models for the bone and for the soft tissue leads to results that differ significantly from those obtained from the full model. The magnetic field beyond the region of the coils contributes little to the induced currents in the fracture gap if the gap is located near the center of the coils.

Bone Remodeling↗
PubMed1993

In vivo measurement of the low-frequency dielectric spectra of frog skeletal muscle.

Capacitance, conductance and dielectric loss spectra are obtained, in vivo, for a number of electrode separations in the gastrocnemius muscle of a frog. At each frequency the reciprocals of these parameters are plotted versus electrode separation. From the slopes of the resulting lines the complex permittivity and the conductivity of the muscle can be determined, with electrode effects eliminated. The sequence of power-law responses which is found is consistent with the fractal model proposed by Dissado. The electrical properties measured in vivo with needle electrodes are similar to those measured with surface electrodes for frequencies between 1 kHz and 1 MHz.

Animals↗
PubMed1992

Numerical and analytical methods to determine the current density distributions produced in human and rat models by electric and magnetic fields.

Some numerical and analytical methods used to estimate the internal electric fields and current densities produced within human and animal models by low-frequency electric and magnetic fields are surveyed. A major goal of such modeling is the design of laboratory experiments on cellular systems or animal models to produce a dosage comparable to that experienced by humans in a particular situation. Specific comparisons are made between the results of ellipsoidal approximations and finite-difference methods applied to irregularly-shaped, homogeneous, human and rat models for applied 60 Hz electric (10 kV/m) and magnetic (10(-4) T) fields. For scaling purposes, the induced current densities in various parts of the body are compared for rat and human models for both types of field. In addition, the current density distribution induced in rectangular culture dishes by applied magnetic fields is also described. The extension of these methods to inhomogeneous models and localized sources may not be simple.

Animals↗
PubMed1990

Use of a spread sheet to calculate the current-density distribution produced in human and rat models by low-frequency electric fields.

The current-density distribution produced inside irregularly shaped, homogeneous human and rat models by low-frequency electric fields is obtained by a two-stage finite-difference procedure. In the first stage the model is assumed to be equipotential. Laplace's equation is solved by iteration in the external region to obtain the capacitive-current densities at the model's surface elements. These values then provide the boundary conditions for the second-stage relaxation solution, which yields the internal current-density distribution. Calculations were performed with the Excel spread-sheet program on a Macintosh-II microcomputer. A spread sheet is a two-dimensional array of cells. Each cell of the sheet can represent a square element of space. Equations relating the values of the cells can represent the relationships between the potentials in the corresponding spatial elements. Extension to three dimensions is readily made. Good agreement was obtained with current densities measured on human models with both, one, or no legs grounded and on rat models in four different grounding configurations. The results also compared well with predictions of more sophisticated numerical analyses. Spread sheets can provide an inexpensive and relatively simple means to perform good, approximate dosimetric calculations on irregularly shaped objects.

Animals↗
PubMed1985

The extremely low frequency electrical properties of plant stems.

The electrical properties (variation of capacitance and conductance with frequency) of a plant stem can be conveniently measured in vivo by time domain dielectric spectroscopy. In this technique a voltage step is applied to a stem. The resulting polarization current is sampled by a microprocessor and Fourier-transformed to yield these properties. Spectra were obtained for seven electrode separations along a Poinsettia stem. The inverse capacitance and conductance were plotted vs separation for 50 frequencies from .35 to 350 Hz. Least-square fits yielded the effective dielectric constant and conductivity of the stem over this frequency range. In this way electrode effects were eliminated. A similar procedure was carried out for Coleus. A log-log plot of dielectric constant vs frequency shows a two-stage linear decrease for both plants. The conductivity is primarily DC. The dielectric loss decreases smoothly with frequency for Coleus. These results are compared to those for bone and the inorganic material hollandite. The dielectric properties seem best described by a cooperative, many-body approach.

Electric Conductivity↗
PubMed1980

Sensitivity to change in electrical environment: a new bioelectric effect.

The action of a 60-Hz, 5 kV/m electric field on erythrocyte parameters in mice was determined. No effects attributable to the magnitude of the field were found, but a transition either from or to an environment containing the field caused decreased red blood cell concentrations and decreased hematocrits. The failure of others to observe effects on erythrocyte parameters following exposure to low-frequency electric fields may have been due to an inappropriate choice of duration of exposure.

Animals↗
PubMed1979

Space osteoporosis: an electromagnetic hypothesis.

Loss of body calcium during spaceflight is a potential problem in long voyages. This loss does not appear to be caused by a deficiency in diet or exercise. The idea is advanced that the altered electromagnetic environment experienced in space may be at least partially responsible. We show that the electric field induced inside astronauts because of their motion in the geomagnetic field is greater than that which has produced a wide variety of biological effects in earth-bound experiments.

Calcium↗
PubMed1977

In vivo bioelectrochemical changes associated with exposure to extremely low frequency electric fields.

One hundred seventy-four 21- to 24-day-old Sprague-Dawley rats were continuously exposed to a 60 Hz electric field of 150 V/cm for one month in ten separate experiments. Biological effects observed included depressed body weights, serum corticoids, and water consumption. The findings are tentatively in terpreted as indicating that a power frequency electric field is a biological stressor. The observed effects cannot be a consequence of Joule heating and therefore indicate that electric fields can influence biological systems either at the systemic level, or at the cellular level via electrochemical alteration of the microenvironment.

Adaptation, Physiological↗