Renewal of electrodes in Sudoscan
The Sudoscan technology measures the electrochemical skin conductance (ESC). This is achieved by placing 4 electrodes on the palms of hands and soles of feet and by applying at the electrode called anode, incremental DC voltages less than 4V. By alternating anode and cathode, each electrode: either left or right and feet or hands, participates in the electrical set-up as both anode and cathode (but of course not simultaneously).
The Smart Electrode replacement process:
Step 1: Unsecure the lock
Step 2: Remove Smart Electrode
Step 3: Dispose in receptacle; EPA friendly
Below shows an example of an electrode that does not get replace(top) and our Smart Electrodes that get replace regularly (bottom).
Material, lifetime of the electrodes
All the electrodes are made of the 304L austenitic stainless steel (C < 0.03%, 18%< Cr <20%, 8%< Ni < 12%). This steel alloy exhibits excellent corrosion resistance and is widely used in chemical and food industry, kitchen and hospital equipment and some medical devices .
The surface finish used in the technology is either “recuit brilliant” (“glossy/shiny” annealing) or “poli miroir” (mirror-grade polish) with thicknesses of 0.6mm to 1.5mm. This high-quality surface finish is important for reproducibility and cleaning. Such stainless steel is a material known to be healthy  and perennial, indeed it has an asymptotic lifetime i. e. a shelf life in normal conditions greater than 15 years . However, the actual lifetime usually depends on three degradation mechanisms: fatigue, wear and corrosion.
Here we will focus on corrosion which is the main mechanism. The effect of the corrosion and the ageing of the material of electrodes on their electrochemical behavior was thoroughly evaluated - .
Corrosion at the anode
At the anode, after use, an oxide film is formed at the surface of the electrode. In-vitro studies have shown that this film is thickening with increasing the number of electrochemical measurements. This leads to a surface degradation. We found precisely that a film of 4 nm was formed after 12 In-vitro measurements - .
At high anodic potential, a pitting corrosion can take place due to the adsorption of chloride ions and breakdown of the oxide film.
According to our in-vitro experiments and the Sudoscan measurements, the in-vitro currents are at least 10 times higher than that of the in-vivo currents. Accordingly, the thickness increase of the oxide film, generated at the surface of the electrode during the in-vivo (technology) measurements, is slower than that during the in-vitro measurements. When taking into consideration the level of currents, the in-vivo measurements can result in slower thickness increase of the oxide layer. However, in the long run, this may lead to a high degradation and surface modification of the electrodes surface.
Furthermore, a non-appropriate cleaning or storage of the electrodes, not conformed to the user or install guides, can accelerate the surface corrosion and degradation of the electrode.
Consequently, the life time of the electrode should be evaluated.
One of the parameters, that is impacted by the corrosion and degradation of the electrode surface, is the offset that can change with increasing the number of tests. In fact, this offset depends on the nature of the material and its freshness (wear and corrosion), and also on the scanned person (the sweat composition and the level of his electrochemical skin conductance). The offset corresponds to the equilibrium thermodynamic voltage augmented with the overpotential at the electrode (i. e. an electromotive force or also to a typical diode-like behavior: the anode consumes voltage), see figure 1.
Figure 1 : Offset, oxidative wall and measured signal.
Partial regeneration at the cathode
At the cathode, the proton is the main electro-active ion. It participates in the partial reduction of the oxides formed in the anode stage. This regenerates the electrode, but never completely.
On the surface of the electrode, unlike sediments of physical origin: major culprits include fats, salts, components of sweat, germs, bacteria, etc., which are almost or completely chemically inactive and which are removed by the use of a suitable low detergent level cleaning product; the unavoidable chemical layer of oxides cannot be removed.
Thus, due to the use, the wear progresses: the oxide layer thickens very slowly but surely. This ultimately alters the electro-chemical behavior of the electrodes.
This alteration is particularly visible through the growth of the offset at the anode. The following figure displays the evolution: a significant growth of the offset as function of the number of scans, on a population of more than 4000 tests, extracted from the study . A very high offset is to avoid entirely as it can lead to the entry of the signal measured in the oxidation wall, which pollutes the signal and completely distorts the measurements.
In fact, an oxidative wall at the anode electrode, due to the oxidation reaction(s) occurring between the electrode and the electrolyte (here sweat) leading to the electron transfer, it is represented by a voltage-current curve. Hence it reflects the intrinsic current capacity of the electrode for a given voltage and any physiological signal (current) to be measured should range below this wall. See figure 1.
Otherwise, when the offset is high enough, the measured signal, shifted by this offset, will cross the oxidative wall and interact with it. This results in electrochemical distortion of the physiological signal, see figure 3.
Figure 2 : Evolution of the offset on the first 1000 tests.
Figure 3 : High offset, entry in the wall and distorted signal.
Replacement after a number of tests
To conclude, in order to ensure the quality of the measure, the complete set of electrodes should be replaced after a number of tests.
Our in vitro and in vivo experiments allow us to ensure the execution of 100 scans (thickness of the electrodes 0.6mm). And for standard electrodes (thickness 1.5mm), it is strongly recommended not to exceed xxx scans.
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