ABSTRACT
A mathematical description of pH excursions produced in interstitial fluid by a spherical stimulation electrode is presented. The pH is calculated as a function of current density, electrode radius, distance, time, and pulsing regimen for an electrode driven by biphasic current pulses. Calculations indicate that large pH excursions occur around electrodes pulsed at current densities used for neural stimulation. For an electrode with a radius of about 1 micron these transient pH changes extend only a few micrometers from the electrode surface. The practical importance of these pH changes remains to be determined.
Subject(s)
Electric Stimulation/instrumentation , Electrodes , Extracellular Space/metabolism , Biomedical Engineering , Biometry , Humans , Hydrogen-Ion Concentration , Models, BiologicalABSTRACT
Two estimates of temperature rise produced in body tissue when a spherical electrode passes current have been calculated. The estimates bracket the expected temperature rise. Time-transient and steady-state results have been obtained. The effects of heat transfer through the highly conductive metal electrode and irreversible Faradaic reactions have been considered. The calculations indicate that electrodes smaller than about 2 micron in radius produce a peak temperature rise of about 1 degree C when driven by typical square current pulses of 25 microA intensity and 200 microseconds duration. The results are presented in a graphic form allowing for quick estimation of the expected peak temperature rise around electrodes of a specific radius, which are driven with a pulse of known current density and duration.