DESIGN AND DEVELOPMENT OF MEDICAL ELECTRONIC ...

VERY REALISTIC PHYSIOLOGICAL SIGNAL SOURCES 299OscilloscopeIsolationAmplifiersCH120pF1MICH220pF1MIForearm123ReferenceElectrode1M1.6µFI10Vp-pIsolatedSignalGenerator100KFigure 6.34 Rosell’s method applied to measure electrode–skin impedances using carbon-loaded silicone electrodes: 1, stimulating ringelectrode; 2 and 3, voltage-sensing electrodes. The electrodes are embedded at a depth of 1.5 mm within an isolating silicone rubber cast.(with a time constant given by R3 and C2) throughout the duration of the stimulation phase ofthe FNS pulse to emulate the integration process carried out by the excitable tissue membrane.After the stimulation phase, the integrated voltage was read through the computer’s A/D card.The actual position of the probe was measured by digitizing the voltage output from the x-, y-,and z-axis potentiometers linked to the three-dimensional manipulator. The computer reset theintegrator by strobing the RESET line prior to taking a new measurement.VERY REALISTIC PHYSIOLOGICAL SIGNAL SOURCESNo physical model can really replace the biological realism that can be achieved with anintact animal model with implanted instrumentation. However, the intact animal presentsthe problem that body geometry and exact organ configuration are very difficult to integrateas part of an experimental study. A compromise solution is to use an isolated preparationof the organ generating the biopotentials placed in a synthetic volume conductor thatsimulates the human cavity holding the organ.