The main parts of the realized instrumentation system are the probe, the electronics interface and the computation unit. The whole system can be modelled using the simple diagram illustrated in Figure 1.

Figure 1: Block diagram of the system.

The instrument must interface with the skin tissue through the direct contact of the non-invasive probe.
Main purpose of the probe is the detection of bioelectrical currents providing at the same time a safe interface with the skin. The electronics interface matches the electrical characteristics of the measurement with the computation unit, preserving the efficiency of the probe and providing secondary signal processing functions. Finally, the computation unit provides primary user interface and control for the overall system thanks to a Graphical User Interface and to specific software. It also provides data storage, primary signal processing functions and
maintains safe operation of the instrument. The software implementation combines a variety of graphical techniques to create a powerful system that will enable users to perform an accurate and reliable analysis of the emitted currents and to easily go on to further applications and research. Figure 2 shows the non invasive probe that has been designed in order to realize the measurements. The electrode of the probe is made of casting of Ag-AgCl powder and it is actually a transducer, converting ionic currents in the body into electronic currents in the probe. This need to be done avoiding the formation of spurious potentials that would contaminate the measurements (overpotentials). In particular, the measurement technique must not depend on the skin ph and on the internal resistance of the characteristic point.
 

Figure 2: The non invasive probe.

The overpotential of the electrode is given by the sum of three polarization mechanisms: ohmic, concentration, and activation overpotentials:

 Vp = Vr + Vc + Va (1)


where Vr is the ohmic overpotential, Vc is the concentration overpotential, and Va is the activation overpotential [13]. These overpotentials impede current flow across the interface and need to be minimised. A way to minimise Vp is to use nonpolarizable electrodes. These allow conduction current to flow across the interface with no energy exchange and there are no overpotentials for this type of electrode. The best electrode to use for all possibilities for biological electrode system is the silver/silver chloride (Ag/AgCl) electrode. This is made of a silver metal base with attached insulated lead wire coated with a layer of the ionic compound AgCl. Two different shapes have been analyzed for
the electrode; initial experiments have been performed using a 2mm diameter cylindrical tip. It has been found out that this shape could injure the skin during the measurements at some probe-skin angles and a second semispherical tip, also with a 2 mm diameter, has been then used to minimise the pain of the patient.
As it can be seen in Figure 3, the contact area between the semispherical tip and the skin changes according to the force exerted by the operator. In a) the tip is leaning on the skin and the contact surface is just a single point; in b) a certain force is exerted on the tip and the contact surface increases; in c) the tip is pushed on the skin with a value of the force that causes the maximum contact surface. In the figure the probe is always kept vertically to the skin; changing the inclination of the probe leads to changing in the contact surfaces. The contact resistance is then function of the pressure and of the contact surface. If the pressure of application exceeds a certain value, traumatisation of test points could occurs through repeated application of the test electrode. Thanks to the pressure compensation system the value of the pressure is always kept to a well defined and safe value.
 


Figure 3: Contact areas according to the exerted
forces.