The acquisition of small dc current using the realized probe requires some precautions in order to minimise the error on the measured data. The characteristic point is considered as a small localized bio-source emitting a small dc current and it can be modeled as a current generator I and its internal resistance Ri . Figure 4 shows a simplified schematic of the acquisition system. The main components of the
scheme are the current generator, the probe and the electronic interface.
Figure 4: Simplified schematic of the acquisition circuit.
The current generator is in series with the skin and connected to a current operational amplifier through the contact tip of the probe P, being v0 the output voltage of the amplifier. The component in the red dashed square in figure 4 represents the skin/electrode interface and the biopotential electrode. To describe the electrical behaviour of the skin/electrode interaction it can be used the equivalent model of Figure 5. The right part of the model is the equivalent circuit of the biopotential electrode. Ehc is the halfcell potential, Re and Ce make up the impedance associated with the electrode-electrolyte interface and polarization effects.
Figure 5: Equivalent model used to describe the electrical behaviour of the skin/electrode interaction.
Figure 6 shows the variation of the impedance of the electrode-electrolyte interface with the frequency.
Figure 6: Variation of the impedance of the electrode-electrolyte interface with the frequency.
The epidermis is the outermost layer and plays the most important role in the electrode-skin interface. Its equivalent circuit is similar to the one of the electrode (associated impedance Rep, Cep and overpotential Eep). It is a constantly changing layer, the outer surface of which consists of dead material on the skin’s surface with different electrical characteristics from live tissue. The deeper layers contain the vascular and nervous components of the skin as well as the sweat glands, ducts, and hair follicles. Those are taken into account also by a parallel RgCg combination with a potential Eg representing the wall of the sweat gland and duct. The dermis and subcutaneous layer under it behave in general as pure resistance (Rde) and generate negligible dc potentials. The characteristic resistance of the skin varies from about 100K to 1 M. Acupuncture points present a lower resistance (10-50 K). Thanks to the semispherical shaped tip of the electrode, the probe can directly enter in contact with the skin within a certain pressure range and the equivalent resistance of the contact spot can be considered in series to the current generator. As illustrated in the previous figure 3, different values of pressure on the skin lead to
different equivalent contact surfaces and to a different equivalent contact resistance. The contact surface will increase or decrease according to the pressure and pressure unbalance will introduce an error at a rate proportional to the unbalance itself. It can be easily seen that an error of more than 100% of the measured current can be introduced for a pressure variation of about 300 grams. Assuming that the reliability of the measure must be as high as possible, the above mentioned error cannot be tolerated and a pressure compensation system must be introduced to make the measure almost insensitive to pressure changes for a wide span of values.