3.1. Electrode Impedance Characterization
The characteristics of the embroidered polyamide hybrid conductive thread-based dry EMG electrodes were investigated by measuring skin impedance without the use of conductive gel and skin preparations. The experimental setup is shown in
Figure 5. The impedance characteristics of the developed electrode for different IEDs are shown in
Figure 8a. It was observed that the inter-electrode distance (IED) of 25 mm provided the best comparable results with the gelled electrode, followed by 20 mm. Whereas, 15 mm IED showed higher impedance characteristics that negatively affected the quality of the acquired EMG signals. Since non-optimized inter-electrode distance can distort the target muscle signal and lead to incorrect interpretation of muscle activation, the inter-electrode distance is optimized prior to recording the sEMG signals using wearable sensors [
20]. Even though the IED of 25 mm yielded a lower impedance value compared to 20 mm for all the frequencies studied, the correlation coefficient of 20 mm and 25 mm were comparable to that of the reference functional Ag/AgCl, whose IED was kept at 25 mm (refer
Table 1). Considering the fact that the textile electrodes on usage could fold or shrink with a decrease in IED, we therefore chose to use the sub-optimal IED of 20 mm for further experiments. Further, it has been reported earlier that the lesser IED the lower the crosstalk contamination while recording sEMG experiments [
20].
The effect of the double and single embroidered electrodes is shown in
Figure 8b. The double embroidered electrodes showed lower impedance compared to the single layer, with improved signal characteristics. The increased conductive thread density is attributed to the enhancement in the performance of the electrode through reduced impedance [
21]. To ensure the reproducibility of the recorded EMG signals, a reliability experiment was performed using three sets of similar electrodes. It is a well-known fact that textile electrodes show high variability when repeatedly measured on human skin. In earlier studies, such variations are explained through differences observed in the skin property of diverse subjects [
22]. In this work, we used three identical electrodes, EM01, EM02, and EM03 made of the same material and dimension (20 mm in diameter) [
23], see
Figure 8c. In a reproducibility experiment, even though the standard Ag/AgCl electrodes with conductive gel showed a lower impedance compared to embroidered dry textile electrodes the correlation coefficients (i.e., covariance/standard deviation) of impedance among the triplicates were comparable and differed only marginally (refer
Table 2).
3.2. EMG Signal Acquired Using Textile Electrodes
Initially, we measured the EMG data with different holding pressures of 5 mmHg, 10 mmHg, and 20 mmHg so as to optimize the signal with respect to the contact pressure as shown in
Figure 9 [
24].
Table 3 shows that the SNR value increased with pressure and reached an optimal value at 10 mM and started to decline at 20 mmHg. We identified that the maximum pressure of 20 mmHg used in the experiments caused more discomfort to the subjects during data collection.
We also examined the effects of loads of 0 kg, 2 kg, 4 kg, and 6 kg on the EMG signals obtained using the embroidered textile electrodes made from polyamide conductive hybrid threads, as shown
Figure 10. In the load effect experiments, we fixed the pressure and inter-electrode distances at 10 mmHg and 20 mm, respectively. With increased load, the time features of the EMG signal such as RMS, ARV, and SNR improved significantly, see
Table 4. The results suggest that the load primarily decreased the contact impedance mediated by sweating of the skin during exercises. In fact, sweat is known to facilitate the flow of current at the skin–electrode interface and improve the signal quality [
24,
25].
The performance of the developed dry textile sensor was evaluated based on the average RMS and ARV values for the nine subjects who participated in the study as shown in
Table 5. The results suggest that the intra and inter subject variation for these parameters is insignificant and the developed textile electrodes are considered to be suitable for recording high bio-signal quality required of medical devices.
The evaluation of RMS, and ARV of sEMG signals recorded by the designed textile electrodes before and after washes is listed in
Table 6. After repeated washing, the electrode started to lose its initial conductivity. Though an increase in the absolute surface resistance was observed for repeated wash cycles, the electrodes still retained good electrical conductivity and time domain features, see
Figure 11. The electrical properties of the designed textile electrodes were assessed through resistance measurements using a two point-probe methods after multiple wash cycles. The electrical resistance increased slightly after the first wash but was relatively stable until the fifth wash,
Figure 12. Thereafter (in 6th wash cycle), the surface resistance increased four-fold. The mechanical action of laundry physically affect the conductive track of the samples leading to increased electrical resistance. The derived RMS, ARV and SNR values also indicated a marginal decrease for the Hybrid thread based-Embroidered electrode after five wash cycles, see
Table 6.
The stretchability test studies the effects of tensile strain on electrical resistance of the EMG electrode as presented in
Figure 13. The results of the stretchability test for the electrode show that the resistance of the developed electrode increased relatively by 0.25 times compared to its initial surface resistance, for an increase of 12% in the strain. The strain (
) is defined as the ratio of the change in length (ΔL) with respect to its initial length (L
0), for the stretch or stress (σ) applied to the knitted elastic band. Beyond a threshold value for stress applied the strain experienced by the material remains constant and indicates its maximum stretchable length (ΔL
max). When the base fabric is stretched, the cross sectional area (
) of the individual fiber decreases with a concomitant increase in its length (
) along the elongation axis. This leads to an increase in the conductive resistance (
) until a threshold strain value for the fabric is reached. Beyond this threshold value the stain of the fiber remains constant (i.e. its
), therefore the corresponding resistance also doesn’t vary. In practice, the wearable e-textile fabrics are not stretched significantly or subjected to high strains. Rather the fabrics will be subjected to repeated stretches of lower strains and hence its electrical resistance is bound to change less significantly over time. Our experiments show that the designed embroidered electrode is not affected significantly either by repeated washes or by strain resulting from stretches, indicating its suitability for sEMG measurements in biomedical applications.