Wearable smart monitors (WSMs) applied for the estimation of electrophysiological signals are of utmost interest for a non-stressed life. WSM which records heart muscle activities could signalize timely a life-threatening event. The heart muscle activities are typically recorded across the heart at the surface of the body; hence, a WSM monitor requires high-quality surface electrodes. The electrodes used in the clinical settings [i.e. silver/silver chloride (Ag/AgCl) with the gel] are not practical for the daily out of clinic usage. A practical WSM requires the application of a dry electrode with stable and reproducible electrical characteristics. We compared the characteristics of six types of dry electrodes and one gelled electrode during short-term recordings sessions (≈30 s) in real-life conditions: Orbital, monolithic polymer plated with Ag/AgCl, and five rectangular shaped 10 × 6 × 2 mm electrodes (Orbital, Ag electrode, Ag/AgCl electrode, gold electrode and stainless-steel AISI304). The results of a well-controlled analysis which considered motion artifacts, line noise and junction potentials suggest that among the dry electrodes Ag/AgCl performs the best. The Ag/AgCl electrode is in average three times better compared with the stainless-steel electrode often used in WSMs.
Electrode-skin impedance was measured in the 10 subjects described earlier, with the experiment setup shown in Figure 6. OP177 is an operational amplifier (OP) with a high input impedance (45 MΩ). One electrode was connected to the negative input of OP and the other electrode of the same type was connected to the output of the OP. Electrode pairs were positioned on a plexiglass plane. The subject touched with his/her left and right index finger two electrodes of the same type on different planes.
Vin is a complex low-voltage periodic signal composed of 22 sine waves at different frequencies, described by the term:
Vin was generated as an analog output on NI6363 USB DAQ board, in LabView program (National Instruments, TX, USA). For each subject, Rcurrent was selected to one of the values (500 KΩ, 5 MΩ, 50 MΩ) based on the highest values of impedance in the subjects to avoid saturation of Vout and optimize the resolution of 16-bit AD conversion. Vout was acquired on one analog input by the same LabView program. Each acquisition lasted 40 s. The subject placed the fingers on electrodes after the acquisition started, to ensure the recordings from the instant when the skin touched the electrode (example in Figure 7A). After touching the electrodes, the palms were resting on the plexiglass board to minimize the motion.
From the recorded signals, the program automatically detected the instant when the skin touched the electrodes, as the position of the time window of width fs (fs is sampling rate) in which the equation:
was satisfied for the first time (black line in Figure 7A).
We calculated the fast Fourier transform (FFT) of Vin and Vout in time windows of 2*fs (example in Figure 7B):
Impedance was calculated from the formula:
The impedance values for time windows [0, 2 s], [3 s, 5 s], [15 s, 17 s] and [28 s, 30 s] are shown in Figure 7C (real and imaginary parts) and 7D (absolute values). Transitional time behaviors of impedances for selected frequencies (|Z(j)|, where j corresponds to frequencies of 1, 3, 8, 16, 25 and 81 Hz, are shown in Figure 7E.
The same trends were found in all subjects. The only differences were the maximum and minimum values of the impedance (Figure 8).
Research funding: The work on this project was partly supported by the grants III44008 and III45010 from the Ministry of Education, Science and Technological Development of Serbia, Belgrade.
Conflict of interest: Authors state no conflict of interest.
Informed consent: Informed consent is not applicable.
Ethical approval: The conducted research is not related to either human or animals use.
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