3.1. Morphology of the Types of Textile-Based Dry Electrodes
lists the morphology and consumption of conductive yarn of the textile-based dry electrode. The surface characteristics of laminating using a conductive sheet, embroidery using conductive yarn, and knit electrode, which are representative dry electrode-manufacturing methods, were compared. First, since the two conductive sheets, CS_1 and CS_2, were manufactured in a film type, they showed a smooth surface without irregularities compared to embroidery or knit electrodes using conductive yarn. In addition, it was confirmed that the conductive sheet-type electrode is significant attached to the fabric without space, it shows the flat surface. Accordingly, the peel strength between the fabric and the electrode made by conductive sheet was about 615 cN/mm, and it was confirmed that it showed better adhesion than the peel strength (440 cN/mm) of the moisture-permeable waterproof fabric for clothing manufactured by the laminating process [18
]. At ×35 magnification, a comparison of CS_1 and CS_2 confirmed that the surface of CS_2 has irregularities compared to CS_1. These irregularities are likely related to the degree of skin contact during EMG collection.
In embroidery electrodes, the lock stitch technique is generally used for durability. On the other hand, as research on textile electrodes progresses, the moss stitch technique has been applied to improve the tactile of the textile electrode in close contact with the skin for collecting biological signals [7
]. Accordingly, two representative embroidery techniques were carried out based on the lock stitch (Em_LS) and moss stitch (Em_MS). The Em_LS is produced in 2D form with two conductive yarns, the lower and upper threads, so that the front and back are the same, whereas Em_MS has a single upper thread passing through the fabric to form a loop, so the face side has a pile-shaped 3D structure and the back side shows the same shape as Em_LS. In addition, through the digital image, the Em_MS showed a larger size than the other samples, which was confirmed since the moss stitch in the form of a loop has a spreading shape. When comparing the use of conductive yarn of the embroidery electrodes of the two techniques, 624.0 cm for Em_LS and 878.4 cm for Em_MS, respectively, confirming that the amount of Em_MS used is slightly higher. Since it is manufactured by forming a 3 mm loop, it was confirmed that a large amount of conductive wire is consumed compared to the same area. Nevertheless, when manufactured with smart wear, Em_MS appears to affect the impedance or SNR by increasing the contact area and improving the comfort due to the loop.
In the case of the knit electrode, the unevenness of the surface exposed to the outside is different according to the plain(Kn_PL), purl(Kn_PU), and interlock(Kn_IN) structures.
Kn_PL was knitted the flattest among the three structures, and the face and back sides were in the wale and course directions, respectively. The loop was knitted most densely within the same magnification. Kn_PU has the same face and back side shape, and it was confirmed that the path inserted in the course direction protrudes compared to the loop in the course direction. In addition, Kn_IN was confirmed in the form, in which the loop in the wale direction protrudes to the face and backside. Accordingly, it was confirmed that the amount of use of the conductive yarn was different, and the Kn_PL, Kn_PU, and Kn_IN were 643.5 cm, 756.5 cm, and 724.5 cm, respectively. Knit electrode could be fabricated of a fabric structure consisting of loops with one row of loops constructed with one continuous yarn and each row of loops connected to the row above and below [8
]. Therefore, the shape of the loop being formed according to the knitting structure and the length of the conductive yarn are expected to affect the skin-electrode impedance and EMG acquirement.
3.3. sEMG Signal and Average Rectified sEMG of the Types of Textile-Based Dry Electrodes
represents the raw sEMG signals of the various textile-based dry electrodes. The raw sEMG signal was intended to confirm the noise of each textile-based dry electrode and the magnitude of the signal obtained during muscle construction. EMG was collected five times by one subject. As shown in Figure 6
, including Ag/AgCl electrode, all textile-based dry electrodes were confirmed to be capable of measuring signals during muscle activation. In addition, the noise signal which is supine position of all electrodes were near 0. Nevertheless, each sample showed different noise signal values. When looking at each electrode, the noise of the reference electrode was ± 0.04 mV and the signal acquisition during muscle activation was ± 2.83 mV (Figure 6
a). Among the textile-based dry electrodes, the raw EMG signals of CS_1 and CS_2, which are conductive sheets, were similar to the reference. The noise of CS_1 and CS_2 were ± 0.02 mV and ± 0.05 mV, respectively, and signal amplitude during muscle activation were ± 2.85 mV and ± 2.41 mV, respectively. Accordingly, it was found that the textile-based electrode of the conductive sheet type that is in close contact with the skin has sEMG signal acquisition performance similar to that of the Ag/AgCl hydrogel electrode. In addition, the embroidered and knitted electrodes made of conductive yarn showed increased noise and amplitude compared to the conductive sheet electrodes.
In the case of the embroidery electrode and the knit electrode, different results were shown depending on the manufactured shape and knitting structure. The embroidery electrodes confirmed that Em_MS and Em_LS were ± 2.07 mV and ± 1.98 mV, respectively, which were similar signal acquisition sizes during muscle activation (albeit with a difference in noise). Overall, the knit electrode had larger noise than the embroidery electrode, and the noise was reduced when it was an interlock structure. In the signal amplitude during muscle activation, Kn_PL < Kn_PU < Kn_IN. The shape and structure of the loop, which changes depending on the knitting type during stretching due to elasticity and flexibility, were different [8
shows the sEMG signal analysis results for each electrode after filtering raw data and converting it to root mean square (RMS). The baseline noise and muscle activation of the reference electrode and textile-based dry electrodes were compared through RMS EMG signal analysis. In the case of baseline noise, the reference electrode was 0.017 mV, it is similar to zero. The conductive sheet type electrodes CS_1 and CS_2 were 0.016 mV and 0.025 mV, respectively, those were more stable than the reference electrode. As previously confirmed in the raw EMG signal, the baseline noise of the embroidered and knitted electrodes was relatively high, the noise of RMS value of Em_LS, Em_MS is twice higher than Kn_PL, Kn_PU, and Kn_IN. On the other hand, it showed different results depending on the manufacturing method and knitting structure, and showed an opposite trend to some skin-electrode contact impedance results. Em_LS showed higher baseline noise than Em_MS and was the same even after five muscle contraction movements. In the case of Em_MS, the initial baseline showed lower and more stable noise than the reference electrode, but it increased as the muscle contraction motion progressed. When measuring the impedance, it proceeded in a static motion and was less affected by noise, but the EMG signal was measured while muscle contraction motion was being performed. Thus, the noise was affected by movement. Therefore, the conductors in the 3D type Em_MS manufactured with loops, rather than the 2D type Em_LS manufactured with stitches, can move along with the muscle contraction motion, confirming that they were affected.
As shown in Figure 7
f–h, the baseline noise of three types of knit electrodes was higher than that of both embroidered electrodes. In addition, the EMG signal appeared differently in the order of Kn_PL < Kn_PU < Kn_IN from the result of skin-electrode contact impedance during muscular activation. In relation to the morphology, the degree of irregularities in the three tissues was observed in the order of Plain < Purl < Interlock. As mentioned above, in the case of skin-electrode contact impedance, the flattest plain as measured in a static state shows a low value and is stable. On the other hand, when the leg sleeve made of the plain structure was worn, the electrode showed a form that expanded in the lateral direction and contracted in the longitudinal direction (Figure S2
). Accordingly, as the lateral length of the electrode becomes longer than that of the rectus femoris to be measured, the contact area of the electrode for sensing the corresponding muscle decreases, and sensing adjacent muscles appears to show baseline noise and lower active EMG signal amplitude [6
]. The purl and interlock structures were similarly expanded in the lateral and longitudinal directions, confirming that the baseline noise was reduced and the active EMG signal amplitude increased since the adjacent muscles were relatively less affected (Figure 5
Based on the results of raw EMG and filtered RMS EMG confirmed earlier, Figure 8
a,b shows the average rectified sEMG of the baseline and muscle activation for different electrode types. Figure 8
a,b show the calculated mean and standard deviation of the three acquired signals out of five (excluding the first and last signal). As shown in Figure 8
a, the baseline noise of CS_1 and CS_2 was near 0.00 mV, which is lower and more stable than the commercial electrode. Em_LS and Em_MS showed between 0.01 and 0.02 mV and similar baseline noise compared to the reference electrode. On the other hand, three knitted electrodes indicate more than 0.05 mV since they have higher baseline noise than the textile-based dry electrodes.
As shown in Figure 8
b, the active signal of the reference electrode was 0.54 ± 0.05 mV. CS_1 and CS_2 show 0.58 ± 0.03 mV and 0.53 ± 0.04 mV, respectively. Therefore, those were a higher signal amplitude than the commercial electrode. In the case of embroidered and knitted electrodes, although it showed a relatively lower signal than the reference electrode, an EMG signal value of 0.30 mV or more was observed. Hence, the quality of the sEMG signal was evaluated by comparing the SNR value of the developed electrodes with those of the commercial electrode (Figure 8
c). The SNR values of the reference, CS_1, CS_2, Em_LS, Em_MS, Kn_PL, Kn_PU, and Kn_IN were 31.10, 55.82, 68.28, 15.81, 15.85, 3.61, 4.73, and 7.03, respectively. On the other hand, when looking at the types of textile-based dry electrodes, the SNR values of the two types of electrodes made from conductive sheet were approximately twice that of the commercial electrodes. The entire electrode area acts as a contact area due to the excellent adhesion to the skin. In the case of Em_LS and Em_MS, the SNRs of similar values were shown, which was attributed to the difference in the embroidery shape of the 2D and 3D shapes depending on the manufacturing method. Em_MS confirmed that the area that can come into contact with the skin in 3D is wider than that of the Em_LS, but it has a similar result since the baseline noise increased due to movement. As mentioned earlier, the knit electrode exhibited a morphological change that was different from that of the conductive and embroidered electrode, that maintains the shape of the electrode when the sleeve is worn. Due to the shape of the deformed and expanded electrode, the noise was formed by the influence of adjacent muscles, and the muscle activity signal was reduced.
Accordingly, since the reference electrode is a hydrogel-type electrode, it could be used in close contact with the skin, thus it was found to be stable against noise. Among the textile-type electrodes, the conductive sheet type produced less noise than the reference electrode, and it was confirmed that Em_LS and Em_MS showed similar levels. It was shown that textile-based dry electrode also possible to replace that wet electrode. In addition, during the muscle activation, CS_1 and CS_2 were able to acquire signal similarly to the reference electrode, and through structural improvement, through structural improvement, it was confirmed that textile based electrodes of embroidery and knitted type could be used as electromyography electrodes to replace Ag/AgCl electrodes.
In this study, to manufacture smart wear for measuring EMG signals in the future, the EMG signals were measured by combining a textile electrode with a commercially available wearable device, and the results are shown in Figure 9
. The measurement method was carried out by connecting a commercial EMG wearable device to the electrode embedded in the leg sleeve in the same manner as the EMG signal acquisition method. Each sample was measured on the same scale. The developed electrodes were capable of obtaining an EMG signal. On the other hand, some different results were obtained in the same way as the results performed with the biopack. In the case of two types of conductive sheet-type electrodes and embroidery electrodes, the baseline noise was similar to that of the reference electrode, and the magnitude of the signal was also measured similarly. In the case of the knit electrode, however, the baseline noise of Kn_PU and IN was close to 0, but it was large in the case of Kn_PL. This was related to the shape of the electrode that changes after wearing according to the knit structure, similar to the one analyzed above. Accordingly, the EMG signal acquirement performance of the textile electrode was confirmed to see if it could be applied as a replaceable electrode for a commercial gel-type electrode. The optimal textile type can enable stable EMG signal acquirement by adjusting various parameters of the embroidery electrode and knit electrode later. Hence, dry electrodes can be developed.