Thesis for the Degree of Master in Science with a major in Textile Engineering
The Swedish School of Textiles 2020-09-11
Report no. 2020.14.05
Impedance and Stimulation Comfort of Knitted Electrodes for Neuromuscular Electrical Stimulation (NMES)
- Influence of electrode construction and pressure application to the electrode
Luisa Katharina Euler
A BSTRACT
Neuromuscular electrical stimulation (NMES) is a modality of electrotherapy which is aiming to restore and improve muscle function by injecting small levels of current into the muscle using different types of electrodes. Advantages are seen in using textile electrodes which can be integrated into wearables. Previous research has been done for the development of textile stimulation electrodes. However, the focus has not been on the electrode construction itself. Therefore, the influence of electrode construction parameters of knitted electrodes as well as of the electrode condition, i.e. wet or dry, on the skin-electrode impedance and on the perceived stimulation comfort were analysed. Further, the application of pressure to the electrode was in- vestigated. It was found that the electrode condition is the most important parameter for the electrode performance as a wet electrode showed a lower impedance and an improved stimulation comfort with a better skin contact. Followed by this, the pres- sure was the second most important factor, especially for dry electrodes. A higher pressure reduced the skin-electrode impedance and improved the skin contact in dry condition. Nevertheless, dry electrodes with a high applied pressure still performed worse than wet electrodes. Regarding the electrode design, the most important factor was the electrode size. A bigger size reduced the impedance. Nevertheless, for the application in NMES, a smaller electrode size is to be preferred as it improved the stimulation selectivity and thus, a lower NMES level was required to induce a plantarflexion without affecting the stimulation comfort. The other investigated con- struction parameters (binding, yarn density, shape) only showed minor influences on the electrode performance. Therefore, the possibilities of applying pressure to the electrode to improve the performance of dry textile electrodes should be further in- vestigated.
Keywords: textile electrodes, impedance, pressure application, electrode construc-
tion, stimulation comfort, NMES
P OPULAR A BSTRACT
Smart Textiles offer possibilities to be used in various application fields within the healthcare sector. One aspect is the integration of textile-based electrodes into wear- ables to be used in electrotherapy. Neuromuscular electrical stimulation (NMES) is a modality of electrotherapy which is aiming to restore and improve muscle function by injecting small levels of current into the muscle using different types of elec- trodes. Most commonly, disposable hydrogel electrodes are used for this purpose.
However, skin irritation and allergic reactions might be caused by the hydrogel.
Therefore, advantages are seen in using textile electrodes to address these problems.
Previous research has been done for the development of textile stimulation elec- trodes. However, the focus has not been on the electrode construction itself. There- fore, in this thesis work, the topic was approached from a textile point of view. Dif- ferent electrode constructions were investigated regarding their electrode perfor- mance in dry and wet condition. Further, a possibility is seen to integrate the elec- trodes into compression stockings. Therefore, the influence of a pressure application to the electrode was analysed. It was found that the electrode performance was greatly improved by wetting the electrodes with tap water or a salt solution. Further, a higher pressure application improved the performance of dry electrodes. Neverthe- less, the dry electrodes still performed worse than the wet electrodes. The electrode construction, on the other hand, only showed minor influences on the performance.
A smaller electrode with a higher yarn density of conductive yarns was found to be
preferable for a better stimulation comfort. Further, in combination with the pressure
application, an uneven surface performed better than a smooth one. The electrode
shape on the other hand did not significantly change the behaviour during electrical
stimulation. In future work, the possibilities of applying pressure to the textile elec-
trode to improve the performance in dry condition should be further investigated.
A CKNOWLEDGEMENTS
First of all, I want to thank my supervisor Li Guo who initiated this project together with Nils-Krister Persson and who helped me tirelessly on the way. You were the best supervisor I could have asked for. You had an open ear whenever I had questions and your input was always helpful and motivating. Thank you for guiding me through this thesis work!
I want to thank everyone who helped me out with their skills and knowledge along the way. Without you, this project would not have been possible. Thank you to all the professors and technicians at the Swedish School of Textiles, especially Lars Brandin for helping me to knit my electrodes. Further, the warmest thank you to Mathias Bräck and Björn Dahlstrand from the DoTank Center for making my exper- imental setup possible by building a great testing rig for me. In addition, I would like to thank Robin Juthberg and Johanna Flodin from the Karolinska Institutes for per- forming the comfort study for me.
Finally, I want to thank my family and friends for always supporting me and being there for me. Special thanks to my wonderful classmates who made this experience so enjoyable. Thank you for all the fun lunch breaks, the helpful advice and all the encouragement and support.
Luisa
T ABLE OF C ONTENTS
Abstract ... ii
Popular Abstract ... iv
Acknowledgements ... vi
Table of Contents ... viii
List of abbreviations and acronyms ... xi
1 Introduction... 1
1.1 Problem description ... 2
1.2 Research questions ... 3
1.3 Limitation of the work ... 3
1.4 Literature review ... 3
1.4.1 Electrode performance and requirements ... 3
1.4.2 Textile-based electrodes for electrotherapy ... 9
2 Materials and methods ... 14
2.1 Materials and chemicals ... 14
2.1.1 Textile electrodes ... 14
2.1.2 Agar dummy... 14
2.2 Equipment ... 15
2.3 Fabrication methods ... 16
2.3.1 Knitting of electrodes ... 16
2.3.2 Dummy production ... 20
2.4 Characterisation methods ... 21
2.4.1 Electrode sample preparation ... 21
2.4.2 Electrical impedance spectroscopy (EIS) ... 22
2.4.3 Comfort study... 26
2.5 Statistical methods ... 28
3 Results – Electrode characterisation ... 29
3.1 Series I: Skin-electrode impedance on arm ... 29
3.1.1 Dry electrodes ... 29
3.1.2 Wet electrodes ... 34
3.2 Series II: Pressure-dependent impedance ... 39
3.2.1 Influence of size and pressure ... 39
3.2.2 Influence of shape and pressure ... 42
3.2.3 Influence of density and pressure ... 45
3.2.4 Influence of topography and pressure ... 47
3.3 Comfort study ... 50
4 Discussion – Electrode characterisation ... 53
4.1 Series I: Skin-electrode impedance on arm... 53
4.1.1 Influence of size ... 55
4.1.2 Influence of shape ... 59
4.1.3 Influence of density... 59
4.1.4 Influence of topography ... 60
4.2 Series II: Pressure-dependent impedance ... 62
4.3 Comfort study ... 64
4.4 Ethical considerations ... 66
4.5 Sustainability aspects ... 67
5 Conclusions ... 69
6 Future work ... 72
References ... 74
Appendix I. Knitting files ... xi
Appendix II. Series I dry – Stabilisation time ... xiv
Appendix III. Series I dry – Size influence ... xvii
Appendix IV. Series I wet – Size influence GLM ... xviii
Appendix V. Series I wet – Size influence Zarea ... xix
Appendix VI. Series I wet – Construction influence ... xx
Appendix X. Series II – Topography influence ... xxvi
L IST OF ABBREVIATIONS AND ACRONYMS
2E, 3E, 4E 2-/ 3-/ 4-electrode configuration
AC Alternating current
Ag/AgCl Silver/ silver chloride (electrode type)
CE Counter electrode
Cl
-Chloride ion
DC Direct current
ECG Electrocardiography
EIS Electrical impedance spectroscopy/ spectroscope EIT Electrical impedance tomography
f Frequency
FRA Frequency response analysis/ analyser
GLM General linear model
Mg
2+Magnesium cation
Na
+Sodium cation
NaCl Sodium chloride
NMES Neuromuscular electrical stimulation NRS Numerical rating scale
PA Polyamide
PEDOT:PSS Poly(3,4-ethylenedioxythiophene) polystyrene sulfonate
PET Polyester
PPy Polypyrrole
RE Reference electrode
Rel dZ Relative impedance change
RT Room temperature
S Sensing electrode
SD Standard deviation
VAS Visual analogue scale
1 I NTRODUCTION
Smart Textiles are gaining importance as they offer opportunities to simplify daily life. The term includes textiles which can actively or passively react to specific situ- ations or their surroundings. This can be realized in various forms and offers ad- vantages in numerous application fields. Among other areas smart textiles find use in the healthcare sector. Here, one important research object is the development of wearables, which means the integration of electronics into textiles to create a wear- able system performing a specific function. This can, for example, be a wearable sensor to track various body functions or to analyse the bioelectrical impedance to estimate the body composition. (Beckmann et al., 2010) Another application are tex- tile electrodes for the use in electrotherapy. Different therapy modalities are included in this domain, which all aim to stimulate muscles or nerves by introducing a low- level electrical current with the help of electrodes. Thereby, the respective muscles or nerves get activated. (Trovato et al., 2014) The goal is to enhance nerve regener- ation and build up muscle strength (Dodla et al., 2019) most often for improving movement of the extremities.
One form of electrotherapy is called Neuromuscular Electrical Stimulation (NMES).
In this treatment, a skeletal muscle, i.e. a muscle which can be contracted voluntarily, is activated by triggering the intramuscular nerve branches with a series of intermit- tent stimuli to generate a visible muscle contraction. Thereby, muscle strength should be improved, restored or preserved. (Maffiuletti, 2010) NMES can for exam- ple be used for facilitating motor relearning or overcoming motor deficits when the patient is suffering from muscle immobility after a stroke (Fu und Knutson 2019) or as an effect of “ageing related to reduced exercise participation” (O'Connor et al.
2018).
To apply a current to the targeted tissue, different kinds of electrodes can be used.
Commonly chosen electrodes are non-polarisable silver/silver chloride (Ag/AgCl) electrodes. They are combined with a hydrogel to create an electrically conductive interface between skin and electrode as well as to improve the skin-electrode contact.
Skin irregularities are evened out and a current flow from the electrode to the pa- tient’s skin can be achieved. (Neuman, 2010) Furthermore, in case of self-adhesive electrodes, a multi-layered hydrogel structure is used with the outermost hydrogel layer acting as adhesive to attach the electrode to the body (Keller and Kuhn, 2008).
However, some problems are related to the use of hydrogels. Skin irritation as well
as allergic reactions were experienced by some patients (Golparvar and Yapici,
2017) and the electrodes can cause discomfort related to the self-adhesive properties
both during use and when removing the electrode (Kusche et al. 2018). Therefore,
an overall discomfort was noted resulting from pain and limited movement (Fu and
Knutson, 2019). Furthermore, the hydrogel dries out during use which degrades the
electrode performance and leads to the electrodes being disposables. (Acar et al.,
tend to peel off and wrinkle when the skin gets stretched during motion (Moineau et al., 2019). This might lead to fixation problems as most electrotherapy modalities include movement of the stimulated limb.
As response to these problems, the development of electroconductive textiles for the use as electrodes has gained interest in the last years. Opportunities are seen to re- duce the discomfort caused by the electrodes and the hydrogel (Zhou et al., 2015) and to improve hygiene conditions for long-term use (Erdem et al., 2016). Further- more, textile electrodes can be integrated in wearable systems which is expected to simplify the electrode placement and improve the wearing comfort. However, due to the lack of a hydrogel, the skin contact of textile electrodes may be impaired (Márquez Ruiz, 2013) and as metal-coated yarns are slightly less conductive than solid metals, problems could arise if their conductivity gets further degraded during a production process. (Beckmann et al., 2010)
1.1 P ROBLEM DESCRIPTION
The research gap for this thesis project is based on the need for an alternative to conventional hydrogel electrodes as they lead to several disadvantages during usage.
An opportunity is seen for textile electrodes to address the experienced problems arising from the hydrogel and the inflexibility of the material. Research was already done in the area of textile electrodes developed for electrostimulation. However, there is still a lack of knowledge about how the textile construction of the electrode influences the electrode performance. This is where the research gap for this thesis was found.
A lot of research was conducted in the field of textile electrodes for monitoring ap- plications, such as electrodes for electrocardiography or electromyography. How- ever, less studies were performed with a focus on textile electrodes for electrostim- ulation. (Stewart et al., 2017) Even though many similarities exist between those two electrode types, still some requirements are varying due to the different working principle (Erdem et al., 2016, Keller and Kuhn, 2008). For monitoring purposes, the most important aspect is a good signal quality to be able to interpret the measured potentials. In electrotherapy on the other hand, the focus is on the stimulation current which is injected to the muscle or nerve.
Within the studied research area of textile stimulation electrodes, great variations can be found regarding the construction and configuration of the chosen electrodes.
This is due to different application fields within the area of electrotherapy. Depend- ing which nerve or muscle should be targeted, varying electrode shapes and sizes are needed because body areas have different thicknesses of underlying fat layers which means that there are varying depths where the nerve is located (Kuhn et al., 2010).
Furthermore, several concepts can be found on how to integrate the electrodes into
garments to create wearable systems. (Moineau et al., 2019, Zięba et al., 2012, Li et
al., 2010) Another reason for the big variations regarding electrode constructions is
that the research done so far is often not approaching from a textile point of view,
because many researchers in the area do not have a textile background. As a result,
the effect of using different textile structures is not well known. No general recom- mendations exist regarding the construction and shape of textile stimulation elec- trodes and a variety of production methods, materials and designs can be found.
1.2 R ESEARCH QUESTIONS
The electrode performance can be evaluated by considering different aspects. In this thesis work, it was determined by examining the skin-electrode impedance as well as the perceived stimulation comfort. Those two aspects were expected to be influ- enced by various textile construction parameters. To define a framework for the the- sis project, several research questions have been established, which are presented below. Thereby, the topic was specified and boundaries for the investigated aspects were set.
1. How does the construction of knitted textile electrodes influence the skin- electrode impedance?
2. How is the impedance affected by applying pressure to the electrode?
3. How do the changes in electrode construction and applied pressure influence the perceived stimulation comfort during NMES?
1.3 L IMITATION OF THE WORK
The topic was approached from a textile perspective meaning that different electrode designs were developed and characterized to find the best suitable textile construc- tion. The work was limited to the investigation of knitted electrodes. This was chosen because of the possibility for a seamless integration of the textile electrodes into a garment, e.g. a sock, bandage or compression stocking. The future aim is to apply pressure to the skin-electrode interface created by the textile structure. Therefore, a knitted construction is favoured. Parameters to investigate regarding their influence on impedance and comfort were limited to knitting parameters, more specifically the binding, number of yarns, shape and size of the electrode, as well as the pressure which was applied to the electrode during measurements. Other influencing factors, such as stimulation site or stimulation settings, were not investigated.
1.4 L ITERATURE REVIEW
The following sections present electrode performance criteria and requirements and give insight to the mechanisms on which the electrode impedance behaviour is based as well as possible electrode characterisation methods. Afterwards, existing textile electrode constructions are presented and compared.
1.4.1 E
LECTRODE PERFORMANCE AND REQUIREMENTSThe overall requirement for electrodes used in electrostimulation is providing an op-
fers to the amplitude when the stimulation is sensed first, e.g. in form of a comfort- able tingling (Sluka and Walsh, 2003). This is usually followed by the motor thresh- old which is reached when a motion is (visibly) generated (Zhou et al., 2015). The third threshold is the so-called pain threshold referring to the intensity at which the sensation is perceived as painful. An additional characteristic value is the maximum tolerable intensity. (Kuhn et al., 2010)
Within these terms, stimulation efficiency for NMES describes how much current must be applied to reach the motor threshold and is mainly influenced by the skin- electrode impedance. An optimum stimulation efficiency can be achieved when maintaining following aspects. A low impedance is desired as this results in a lower intensity required for the stimulation thereby improving the safety and a lower power consumption (Stewart et al., 2017). Furthermore, a too high resistance could cause high temperatures arising from the current flow which can cause skin burns (Zieba et al., 2012). Additionally, a good skin-electrode contact results in the largest contact area possible which in turn reduces the current density peaks caused by too small contact areas (Keller and Kuhn, 2008). Therefore, it is favoured to develop a flexible electrode to maintain good contact with the curved, uneven skin surface (Curteza et al., 2016). If the pain threshold is reached at a lower intensity than the motor thresh- old, the electrode performance is insufficient.
Stimulation selectivity describes the selective activation of the targeted muscle group.
The less selective an electrode performs, the more muscle groups, which are bundled closely together, get activated upon current injection. It is mainly influenced by the electrode dimensions. The limiting factor is the current density, which increases with a decreasing electrode size and can lead to painful stinging sensations. This painful sensation is included in the stimulation comfort. It refers to how the user perceives the stimulation in terms of pain leading to discomfort. (Keller and Kuhn, 2008) Another aspect regarding skin contact is the biocompatibility of the materials used.
The materials must not be toxic to the human body or cause skin irritation, especially for long-term applications. (Xu et al., 2008) Furthermore, the electrode should be easy to apply and detach from the skin (Poboroniuc et al., 2016).
Electrode impedance behaviour
The electrical properties of textile electrodes are frequency dependent. As this fre-
quency dependency is nonlinear, the electrical resistance or conductivity are not suf-
ficient to describe the electrode behaviour. This results from the formation of a dou-
ble layer at the electrode/electrolyte interface. (Hao and Tao, 2015) Therefore, the
concept of electrical impedance must be used. Impedance is a complex quantity
which is determined from the conversion of time-domain input and output signals
into a function of frequency. This transfer function is referred to as impedance,
which is a ratio of potential over current. (Orazem and Tribollet, 2017b) Electrical
impedance in general describes “the effective resistance to the flow of electric cur-
rent at a given frequency in an alternating current circuit” (Morris, 2014) and its SI
unit is ohm (Ω). It can be divided into a real (Z’ or R) and an imaginary part (Z’’ or
X), which are called resistance and reactance or reactive impedance (Orazem and
Tribollet, 2017a). This means that aside from the resistance against a steady current,
also dynamic effects are included. Analogously to the definition of resistance in DC, the complex impedance in AC is defined as:
𝑍 = 𝑣
𝑖 or in the exponential form as 𝑍 = 𝑍 ∗ 𝑒
𝑗𝜑𝑍Where φ
Zrepresents the “phase shift of voltage with respect to the current flowing through the AC impedance”. In the Cartesian form, the complex impedance is ex- pressed as:
Z = R + jX with 𝑍 = √𝑅
2+ 𝑋
2The absolute impedance Z therefore can be expressed as:
𝑍 = 𝑉
𝐼 𝑤ℎ𝑒𝑟𝑒 R = Z ∗ cos 𝜑
𝑍𝑎𝑛𝑑 𝑋 = 𝑍 ∗ sin 𝜑
𝑍(Schmidt-Walter, 2007)
When it comes to the skin-electrode impedance, there are several factors which need to be taken into account to estimate and explain the behaviour. These factors can be summarized into material, shape and size, and construction parameters of the elec- trode, such as surface structure or yarn density. (An and Stylios, 2018) Other factors determining the skin-electrode impedance are caused by the properties of the skin area which is in contact with the electrode (Beckmann, 2008) as well as the skin contact which can for example be influenced by the holding pressure (An et al. 2018) or the skin humidity (Medrano et al., 2007).
Electrical impedance spectroscopy
The investigation of the frequency response of a system under study is the aim when
performing impedance measurements (Orazem and Tribollet, 2017b). The fre-
quency-dependent impedance can be measured using electrical impedance spectro-
scopes (EIS). To investigate a specific frequency range, the EIS measurements can
either be based on impedance analysers to implement single impedance spectrum
frequency measurements or the exiting frequency can be swept with linear or loga-
rithmic steps. When performing a frequency response analysis (FRA), a small AC
wave, commonly a single-frequency sinusoidal current with about 5 to 15 mV, is
applied to the working electrode of the system under investigation and the respond-
ing AC current is measured. This procedure is repeated, and the desired frequency
range is scanned. Thereby, the frequency-dependent impedances can be calculated
from the AC voltage and the current data. The main advantage of using a frequency
sweep is the high signal-to-noise ratio. However, resulting from that, a comparably
long measuring time is needed. Therefore, time-varying processes cannot be ana-
lysed with an instantaneous spectrum because the complete spectrum cannot be de-
tected within a short time, especially for investigations at very low frequencies. This
impedance spectrums. However, the disadvantage is the limited accuracy of the im- pedance spectrum which leads to a high variance of impedance model estimates.
Furthermore, the sensitivity is reduced which means that very small impedance changes cannot be detected. (Sanchez et al., 2012)
EIS can be performed in two different modes, referred to as potentiostatic and gal- vanostatic. Potentiostatic regulation describes that the potential has a defined value with a fixed amplitude, often in form of a sinusoidal wave. The advantage is that linearity of a system is controlled by potential and the linearity is important for elec- trochemical systems. The second mode is called galvanostatic regulation. In this case, the system under study is investigated under constant current density. (Orazem and Tribollet, 2017b)
Further, the impedance spectroscope can be used with several electrode configura- tions. If a 4-electrode configuration (4E) is chosen, all four electrode leads are used individually, i.e. in four different positions. In the potentiostatic mode, a defined potential is applied and the resulting current response and potential difference are measured between specific electrodes. The voltage is applied over the two outer electrodes, which are called working and counter electrode (WE and CE, respec- tively). The working electrode is the electrode under study. (Chang and Park, 2010) The CE is needed to complete the current path and can either be used as current source or sink. Two sense leads are located at the inner electrodes, the so-called reference and sensing electrode (RE and S, respectively). The current response is measured between the CE and WE and the potential difference between the RE and S. (Autolab, 2018)
When performing 2- or 3-electrode measurements, the leads get combined thereby
reducing the number of electrode positions. More concrete, for a 3-electrode setup
(3E), the WE and S are in one position whereas the CE and RE are still separate. For
the 2-electrode configuration (2E) finally, the CE and RE get combined in one posi-
tion as well so that the current response and potential difference are measured in the
same position. (Autolab, 2018) A comparison is shown in Table 1 below.
TABLE 1.OVERVIEW ELECTRODE CONFIGURATIONS.
Measured response Measurement positions
2E Response of complete cell/ setup meas- ured; interfaces at WE and CE meas- ured together
3E Impedance at CE not measured; selec- tive measurement of interface at WE
4E Contribution from all electrodes and in- terfaces removed; solely effect of ma- terial, i.e. the material impedance, measured
Pictures according to Hao and Tao (2015)
Dummies for impedance measurements
When investigating the impedance of a skin-electrode system, the measurements can
either be performed on a human subject or, as electrical body properties are subject
to inter-day and inter-subject variations, measurements can also be performed on a
dummy. (Beckmann, 2008) A dummy mimics a human body part on which specific
measurements can be performed. It is a physical system which has the defined prop-
erties that are needed for the planned measurement, meaning it must be developed
for a specific measurement to be suitable and to be able to lead to representable
results. This is caused by not mimicking all properties of living tissue but only the
ones relevant for the test method. (Medina and Grill, 2015) They are commonly used
for research purposes and simulate electrical or mechanical properties of the human
body with less variations of these properties compared to living tissue. (Yu et al.,
2019) Dummies are used in various biomedical fields, most often for imaging appli-
cations. These can be bio-impedance imaging systems such as electrical impedance
tomography (EIT) or also ultrasound imaging. (Bennett, 2011) Furthermore, other
possible purposes are investigations of electrodes for ECG monitoring (Xu et al.,
When it comes to dummy composition, many different materials can be used. Water- based materials are often chosen when developing tissue dummies. This is due to water being the main component of living soft tissue. Therefore, water-based mate- rials appear to be a good choice for tissue-mimicking dummies. (Cao et al., 2017) Regarding the mechanical properties, the dummy should have similar shape and size to the mimicked body part (Kao et al., 2008). Therefore, a gelling agent is needed together with optional additives to reach a certain property. Common gelling agents for human dummies are agar (Tallgren et al., 2005, Macías et al., 2013) or gelatine (Delgado-Arenas et al., 2019). Agar is a hydrophilic colloid which is insoluble in cold water but becomes soluble in hot water. When cooling down after being dis- solved in water, it forms a resilient gel which is temperature stable up to 85 °C.
(Selby and Whistler, 1993) Doping with graphite (Medina and Grill, 2015, Kao et al., 2008) or addition of NaCl (Bennett, 2011, Kandadai et al., 2012) are common ways to increase the electrical conductivity. Additionally, a multi-layered structure can be chosen instead of a homogeneous dummy depending on the application field (Medina and Grill, 2015).
Stimulation comfort and comfort evaluation
The perceived stimulation comfort is an important aspect when it comes to electrode performance of stimulation electrodes. The NMES settings are dependent on the per- ceived comfort because the patient’s maximum tolerance should not be exceeded.
This means the therapeutic success is affected by the stimulation comfort.
Known influences on the stimulation comfort are the electrode dimensions and shape on one hand and the chosen electrolyte on the other hand. It was found that smaller areas increase the pain sensation perceived for a defined stimulation amplitude. This in turn can limit or reduce the effectiveness of NMES as only lower intensities can be used. (Keller and Kuhn, 2008) However, the chosen electrode size is also the main factor for the stimulation selectivity (Malešević et al., 2012). This means that the comfort-selectivity relation must be considered when choosing an appropriate elec- trode size, as these performance aspects behave contradictory when it comes to the desired contact area. The used electrolyte has an influence on the stimulation comfort in terms of pain and other discomfort triggering aspects such as stickiness and skin irritations.
As comfort and pain are subjective concepts, the experience differs for every human which makes it hard to assess it objectively (Hayashi et al., 2015). However, some methods were developed to find general tendencies for stimulation comfort evalua- tion of electrodes. Quantitative as well as qualitative methods can be used. The ad- vantage of quantitative methods is the comparability of the results whereas qualita- tive methods such as interviews can give more detailed information and new per- spectives to avoid bias.
The most-frequently used methods to assess subjective pain intensities are generic
unidimensional pain questionnaires, such as the visual analog scale (VAS) and the
numeric rating scale (NRS). These are self-report methods which means that the
subject assesses his or her pain perception. (Jensen and Karoly, 2001)
For the VAS method, the pain is evaluated using a continuous visual scale which usually consists of a line of 10 cm length where one end represents “no pain” and the other end means “pain as bad as could be” or “worst imaginable pain”. (Jensen and Karoly, 2001) For this scale, intermediate points or descriptors are not recom- mended to avoid clustering of scores around these points. (Hawker et al., 2011) The subjects are asked to place a mark on the line in the position which best represents the perceived pain intensity. The distance between the “no pain” end and the set mark then represents the pain scoring. (Jensen and Karoly, 2001) The VAS is a widely used method in clinical applications and commonly accepted, although it must be mentioned that very low ratings were found to offer less objectivity, which means poor repeatability, whereas very high ratings offer a higher degree of objec- tivity. (Hayashi et al., 2015) Advantages of VAS are the simplicity and adaptability to many investigations as well as the very high number of response categories be- cause no defined steps are given. Therefore, scores can be seen as ratio data. Disad- vantages are the comparably long evaluation time due to the need to measure the scores by hand and the higher complexity which limits the use for subjects with suf- ficient cognitive abilities. (Jensen and Karoly, 2001, Hawker et al., 2011)
Another option is employing an NRS. In this case, a segmented numeric scale is given where 0 refers to “no pain” and the maximum number to “as bad as could be”.
The NRS can be an 11-point scale, where scores are ranging from 0 to 10, a 21- points scale, from 0 to 20, or a 101-point scale, from 0 to 100. (Jensen and Karoly, 2001) Most commonly, the 11-point system is chosen (Hawker et al., 2011). It can be performed in a written or verbal assessment, where the subject evaluates the per- ceived pain (Jensen and Karoly, 2001). NRS is also regarded as a valid and reliable scale for pain assessment. Advantages are the easy scoring and administration. Dis- advantages are the limited number of response categories, especially for the 11-point scale, and that scores cannot necessarily be regarded as ratio data, which means that differences between two scores cannot be treated as always corresponding to the same magnitude of pain difference. (Jensen and Karoly, 2001, Hawker et al., 2011) The presented scales can also be combined with observation of the stimulation thresholds. Zhou et al. (2015) for example evaluated the comfort at the stimulation amplitudes related to the subject’s pain threshold instead of at a constant intensity for all subjects. The subjects then rated their sensation using a questionnaire and VAS scale.
1.4.2 T
EXTILE-
BASED ELECTRODES FOR ELECTROTHERAPYThe term textile electrode refers to all electrodes produced by textile production
techniques. Most commonly, conventional production methods are used together
with conductive materials to integrate electrical functionality into the textile. (Erdem
et al., 2016) Flexibility is one of the distinguishing properties of textile electrodes
compared to conventional ones. Textiles are easy to deform while having a small
electrodes can be easier attached to the curvature of the human body as they can adapt to the body shape and an intimate contact is enabled. (Gniotek et al., 2011)
Production methods and integration levels
Many different production methods can be found in literature to manufacture textile- based electrodes. In principal, all common textile production methods can be em- ployed, meaning knitting, weaving and nonwoven production. Additionally, non- conductive textile substrates can be used and functionalized on different levels, e.g.
by printing or embroidery. The integration of the conductive material can therefore be classified into different levels, which were defined by Guo et al. (2019): Level 1 means adding-on conductive material onto a substrate. This includes all electrode constructions which are manufactured by producing a fully conductive fabric which is then cut and sewn to design the electrode. Often, these fabrics, which can be pro- duced by weaving (Gniotek et al., 2011), knitting (Stewart et al., 2017) or nonwoven methods (Gniotek et al., 2011, Zieba et al., 2012), are used to produce multilayer structures with additional filling material to increase the height of the electrode. The conductive fabric is then integrated in a system using embroidery (Zieba et al., 2012), glue (Crema et al., 2018, Popović-Bijelić et al., 2005) or detachable fixtures like snap buttons (Zhou et al., 2015, Li et al., 2010) or hook-and-loop fasteners (Stewart et al., 2017). Level 2 is the integration of conductive material onto a textile substrate, e.g. using a conductive ink or paste and applying it by coating (Ali et al., 2018, Oh et al., 2003, Papaiordanidou et al., 2016) or printing (Gniotek et al., 2011, Frydrysiak et al., 2016, Popović-Bijelić et al., 2005, Yang et al., 2014). Level 3 includes all methods where the conductive elements are integrated into the textile, for example by embroidery (Goncu Berk, 2018, Keller and Kuhn, 2008) or machine stitching (Erdem et al., 2016) with conductive yarns. Finally, for Level 4, the conductive ma- terial is integrated seamlessly in a defined shape of local conductivity into a non- conductive textile body in one process step. Examples found in literature are intarsia (Li et al., 2010) or circular knitting (Moineau et al., 2019, Curteza et al., 2016) with conductive yarns.
Conductive materials
Conductive materials can be integrated in different forms, such as particles, coatings, or yarns. Materials found in literature for textile electrodes are mainly inorganic metal conductors, especially silver. Silver can be used as ink (Gniotek et al., 2011, Frydrysiak et al., 2016, Popović-Bijelić et al., 2005, Crema et al., 2018), particles or flakes in coatings (Ali et al., 2018) or a printing paste (Yang et al., 2018), or most often to metallize PA yarns. Another not so commonly used hybrid material is a carbon-loaded silicon-rubber (Yang et al., 2018). Organic polymers, such as PE- DOT:PSS (Papaiordanidou et al., 2016) and PPy (Oh et al., 2003), are also possibil- ities as conductive material for textile electrodes as well as graphite integrated in e.g.
a woven fabric (Gniotek et al., 2011).
The reason, that silver is favoured for textile electrodes in medical applications, is
its antibacterial properties. When silver gets in contact with water, silver ions, which
are chemically active, are released to the aqueous solution. These ions are assumed
to be the reason for the microbial killing effect of silver. (Tomacheski et al., 2016)
This is found beneficial to improve the hygiene of textile products which, for exam- ple, are used in applications when sweating is expected. Further, it offers good elec- trical properties regarding conductivity as it has a low resistivity (Cochrane et al., 2016).
Shape and dimensions
Regarding the shape of stimulation electrodes, some recommendations can be found in literature. In general, round electrodes seem to be preferable when used as single electrodes, meaning they are not arranged in an array. Sharp corners and elongated shapes can lead to needle-like stimulation sensations due to uneven charge distribu- tions at the corners. (Crema et al., 2018) Tjelta and Sunde (2015) investigated the shape influence for ideally polarized electrodes. They found that a circular electrode behaved differently than a rectangular one, and the length-to-width ratio of the rec- tangle had an influence on the impedance spectra as well. According to Newman (1970), this is caused by non-uniform current and potential distributions causing a frequency dispersion at high frequencies. They, in turn, are arising from the bound- ary between the electrode and the non-conductive surroundings (Davis et al., 2018).
For electrode arrays, however, rectangular shapes can better cover the skin surface and reduce the risk of a non-sufficient selectivity for localized motor points some- times observed for round single electrodes. (Crema et al., 2018)
When comparing dimensions of textile stimulation electrodes chosen in different studies, rather big variations can be found. The sizes are ranging from 0.09 cm² (Papaiordanidou et al., 2016) to 36 cm² (Gniotek et al., 2011, Zhou et al., 2015) for square and circular (Goncu Berk) shaped textile electrodes. Differences mainly arise from the planned application regarding the use as single electrodes or in an electrode array, as well as the size of the targeted muscle, nerve depth and the thickness of the fat layer (Keller and Kuhn, 2008). Therefore, Majkowski and Gill (2008) concluded that larger electrodes are found to be better tolerated by the user, but if stimulation selectivity is needed to isolate a specific muscle, small electrodes cannot be avoided.
Another aspect of the electrode shape is the three-dimensional shaping meaning the height of the electrode. A convex shape was evaluated to be beneficial in several of the presented studies, as it was found to improve the skin-electrode contact. This additional height can be achieved by different means. When multilayer structures are employed, an additional filling is included (Zięba et al., 2012, Zhou et al., 2015).
Furthermore, for embroidered electrodes, an additional under-stitching layer can be used to add height to the electrode (Goncu Berk, 2018). Another option, which was not found to be used for textile electrodes in the reviewed studies, would be design- ing a 3D knit structure using a computerized flat-bed knitting machine (Araujo et al., 2011). Additionally, in one study it was found that a rough surface structure, e.g.
small loops in terrycloth, is preferable as it could improve the skin contact while a
flat surface showed a less uniform contact of electrode and skin (Márquez Ruiz,
Electrolyte
Textile electrodes can be used as dry or wet electrodes. Both options offer ad- vantages and disadvantages which need to be considered. When choosing wet textile electrodes, the electrolyte can be water or a saline solution, hydrogel or an electrode cream.
Some researchers successfully showed that the developed textile electrodes could be used in a dry condition without exceeding the pain threshold of the user (Zhou et al., 2015, Yang et al., 2014). In this case, most researchers found that the electrodes should be applied to the skin a few minutes before starting the planned procedure as a stabilisation time is recommended so that the interface has time to settle (An et al., 2019). Related to that, a higher fabric density is to be favoured for dry electrodes as it is expected to improve the hydration of the skin arising from an increased perspi- ration as well as the moisture being retained to a bigger extent (Xu et al., 2008).
However, most researchers in the area concluded in their studies that wet electrodes work more reliably and the risk for painful sensations is minimised. It was found that when the same electrode is used in wet or dry condition differences in performance and stimulation comfort could be observed (Zhou et al., 2015). Even though Yang et al. (2014) did not observe problems with dry electrodes, the majority of studies reports that dry electrodes offer less stimulation comfort and a worse performance in terms of charge transfer resistance and motor threshold compared to wet textile or conventional hydrogel electrodes (Zhou et al., 2015). Therefore, it is concluded, that it is preferred to use textile electrodes in a wet condition.
However, one advantage of textile electrodes even when used as wet electrodes is that the electrolyte does not need to be a hydrogel. Regular tap water or saline solu- tion was found to be sufficient for textile-based electrodes to get a comparable per- formance as conventional hydrogel electrodes (Zhou et al., 2015, Poboroniuc et al., 2016, Moineau et al., 2019). Therefore, problems related to the use of a hydrogel such as skin irritation and limited reusability can be eliminated when replacing con- ventional by textile electrodes. This is further supported by the textile structure which provides good ventilation and flexibility, therefore causing almost no skin irritation (Zhou et al., 2015). Furthermore, tap water is easier accessible and cheaper than a hydrogel and textile electrodes are often washable, which improves the hy- giene and reusability (Erdem et al., 2016).
Wetting can be achieved by pouring a defined amount of water onto the electrodes (Zhou et al., 2015) in the case of detachable electrodes or by using a sponge (Moineau et al., 2019) or tapping with water (Poboroniuc et al., 2016) for wearable electrodes. Nevertheless, the problem of the electrolyte drying out is still present for wet textile electrodes (Zhou et al., 2015).
Wiring and conductive tracks
Conductive tracks can also be produced by textile methods, such as embroidered
(Zieba et al., 2012, Goncu Berk, 2018) or knitted (Keller et al., 2006, Li et al., 2010)
leads for wearable electrode systems. Advantages are that no problems arise from
cables hanging from the garment which could possibly entangle and inhibit move-
ment, and the stigma of using an electrotherapeutic device in the daily life is reduced
as it can be worn under normal clothes and can therefore easier be hidden (Goncu Berk, 2018).
One important aspect is the insulation of these tracks so that the lead wire, which is
making the connection to the instrument trunk cable, cannot contact ground or other
dangerous potential (Xu et al., 2008). This can for example be realized by laminating
with a thermoplastic polyurethane membrane (Goncu Berk, 2018) or by using an
additional non-conductive fabric layer in-between (Crema et al., 2018). Another re-
ported method is using a conductive glue to attach a wire to the textile (Poboroniuc
et al., 2016, Gniotek et al., 2011).
2 M ATERIALS AND METHODS
The following sections give an overview over used materials and equipment. After- wards, the applied methods are described, and the choice of parameters is justified.
If relevant, a discussion for alternative approaches is given and the chosen method is motivated.
2.1 M ATERIALS AND CHEMICALS
In the following sections, used materials and chemicals are presented and the mate- rial choice is motivated.
2.1.1 T
EXTILE ELECTRODESThe knitted electrode samples consist of two different yarns. To construct the elec- trode itself, a conductive yarn was needed. As discussed in section 1.4.2.2, silver in various forms is the most used materials for textile electrodes. This is due to the good electrical conductivity of silver and additionally it offers antibacterial properties (Damerchely et al., 2011). For these reasons, a silver-plated PA yarn was chosen to knit the electrodes. The advantage of a coated yarn compared to a full metal yarn is its flexibility arising from the core material, in this case from the PA (Pani et al.
2015) while still offering sufficient durability and even washability of the knitted fabric (Rantanen and Marko, 2005). The chosen yarn is a multifilament yarn from Statex Produktions- und Vertriebs GmbH (Bremen, Germany) and in the following will be referred to as Shieldex®, which is its trade name. For the surrounding fabric, a non-conductive material was needed. Therefore, a PET multifilament yarn was chosen. To colour code the samples, different yarn colours were used for each ver- sion. The exact specifications of the used materials are listed in Table 2.
TABLE 2.YARN SPECIFICATIONS.
Material Specifications
Shieldex® multifilament yarn
177/17/1 dtex Z100 high conductivity < 500 Ω/m, anti-tarnish coating (Statex GmbH, 2020a) PET multifilament yarns 167/32/1 dtex
2.1.2 A
GAR DUMMYA forearm dummy was needed for experimental Series II (see section 2.4.2.2). The
main requirements were an electrical impedance in the range of human forearm tis-
sue as well as a sufficient mechanical stability to be able to apply pressure without
the dummy breaking. To produce the dummy, deionised water was gelled using a
gelling agent. Water was chosen as base material because it is the main component
of living tissue. This makes it a favoured material for human tissue dummies. As
gelling agent, agar was chosen because it could offer a suitable gel strength. Agar
powder from Sigma Aldrich (St. Louis, Missouri, USA) was used with a gel strength
of > 1000g/cm² to ensure the required mechanical stability. An overview over used chemicals is shown in Table 3.
TABLE 3.OVERVIEW CHEMICALS.
Chemical Specifications Amount
Agar, lab-grade (C12H18O9)n
CAS 9002-18-0
gel strength > 1000g/cm² Sigma Aldrich A9799
17.5 g per dummy
Deionised water H
2O
CAS 7732-18-5 200 ml per dummy
2.2 E QUIPMENT
For knitting the electrodes, the electronic flat knitting machine CMS 330 TC by H. Stoll AG & Co. KG (Reutlingen, Germany) was used. It is a multigauge machine with a working width of maximum 1270 mm (50’). All samples were knitted with a gauge of 12 and constant yarn tension regulated by storage feeders.
To make the agar dummies, a heating plate (800 Watt, MR3001K by Heidolph In- struments GmbH & Co. KG, Schwabach, Germany) with an electronic contact ther- mometer (-50 °C - 450 °C, ETS-D5 by IKA-Werke GmbH & Co. KG, Staufen, Ger- many) and magnetic stirrers as well as a laboratory balance (max. 3100 g and d = 0.01 g, Sartorius AG, Göttingen, Germany) were used.
The electrical impedance spectroscopy was performed using the PGSTAT 204 by
Metrohm Autolab (Utrecht, Netherlands) together with the FRA 32M module, which
is a function generator and transfer function analyser. It was combined with a cus-
tom-built testing rig to be able to apply different pressure levels using a stamp on
top of the electrode, shown in Figure 1. A similar testing rig can be found in literature
where it was used by Beckmann et al. (2010) to investigate the pressure-dependent
impedance of electrodes on an agar dummy. For this thesis work, a balance was
integrated in the bottom of the rig to be able to measure and control the applied
weight. A motor was used to move the stamp up and down with which an adaptive
pressure could be applied, which means the weight was checked constantly through-
out the measurement and adjusted if needed to reduce the impact of a potential
dummy deformation upon pressure. The applied weight and the stamp movement
could be controlled using the developed software. On the bottom of the rig, a 3D-
printed box was used as dummy holder to make sure that the stamp only pressed on
the dummy and did not touch the bottom plate.
FIGURE 1.PRESSURE-APPLICATION TESTING RIG.
To evaluate the perceived pain during electrical stimulation, a conventional battery- driven, medically approved NMES device by CefarCompex (Guildford, United Kingdom) was used. It can reach a maximum intensity of 120 mA. The intensity can be adjusted in ‘energy intervals’ from 0 - 999. In this report, the stimulation intensity will therefore be presented in form of NMES levels given by the device. For the motor point search, the motor point pen from CefarCompex was used.
2.3 F ABRICATION METHODS
This section describes the fabrication of the knitted electrode samples as well as how the agar dummy was made.
2.3.1 K
NITTING OF ELECTRODESIn this project seamless knitting was chosen as favoured production technique, as the seamless integration of the electrodes into a wearable system offers the possibility to produce a garment with integrated electrodes in one step, which leads to less pro- duction time and waste. The investigated electrode constructions can be divided into several factors which were changed individually. The ‘comparison version’ is a cir- cle with two yarns and plain knit. From this, one parameter was varied respectively to create a new version which either has a different binding, a different yarn density or a different shape. An overview is shown in Figure 2. Additionally, all four ver- sions were produced in three sizes. Therefore, twelve different electrodes were knit- ted in the first sample production.
Circle 2 yarns Plain knit
Circle 2 yarns Tuck stitch
Square 2 yarns Plain knit
Circle 3 yarns Plain knit Shape changed
Comparison version
Tuck stitch version
Square version
High density version
FIGURE 2.VERSION OVERVIEW.
In Table 4, a sample plan with varied factors, planned sizes and actual sizes after steaming is shown.
For the knitting pattern, a regular plain knit was chosen as binding for the ‘compar- ison version’ (version name: p2_c) and an uneven surface structure was created for the ‘tuck stitch version’ (version name: t2_c) by combining a tuck stitch with a plain knit to create loops standing out of the surface. Number of yarns refers to how many strands of Shieldex® yarn were used in one yarn carrier and equals the number of cones being used. Thereby, the yarn density in the electrode area was varied for one electrode version as 3 yarns (version name: p3_c) were used instead of 2 yarns.
When it comes to the electrode shape, a circle was chosen as ‘comparison version’
and a square with slightly rounded corners (version name: p2_s) for the shape com- parison. All four versions were produced in three different sizes which refers to the area of the electrode after steaming, shown in Table 4. Close-ups of the electrode versions are shown in Figure 3.
Note: Because of shrinkage after steaming, the electrode shapes were slightly oval or rectangular instead of precise circles and squares.
FIGURE 3.CLOSE-UP OF ELECTRODE SAMPLES (FROM LEFT TO RIGHT): SQUARE VERSION,
PLAIN KNIT CIRCLE, TUCK STITCH, BACKSIDE WITH LEAD AND TAIL.
In addition, a textile lead was knitted on the backside of the samples and the end of the lead was designed to be standing out of the surface so that it would be possible to attach crocodile clips for the following impedance measurements and the comfort study (Figure 3). The lead has dimensions of 5 cm length and 0.5 cm width and a tail of 0.5 cm x 1 cm for all samples.
To knit the different electrodes, knitting patterns were made using the knit design software from Stoll. The technical drawings can be found in Appendix I. For the surrounding fabric around and behind the electrode, a plain knit was chosen with three PET yarns in one yarn carrier, knitted together on the second needle bed.
Thereby, the electrodes were insulated on the back side with the PET fabric. The
Shieldex
®yarn was used to create the different investigated electrode constructions
on the face side of the fabric. The connection of electrode and surrounding fabric
that, the electrodes shrunk differently than expected. Therefore, the planned elec- trode areas were not matched precisely. Consequently, in the table below, the planned area and the actual electrode area are presented.
TABLE 4.ELECTRODE VERSION OVERVIEW.
Version name
Knitting pattern
Number
of yarns Shape
Planned area in cm²
Actual area in cm²
Sample name
Com- parison version
plain knit 2 circle
19.6 17.4 p2_c19
28.3 26.1 p2_c28
38.5 35.6 p2_c38
Square
version plain knit 2 square
19.4 17.4 p2_s19
28.1 26.2 p2_s28
38.4 39.4 p2_s38
High density version
plain knit 3 circle
19.6 19.0 p3_c19
28.3 28.3 p3_c28
38.5 38.5 p3_c38
Tuck stitch version
plain knit with tuck stitch variations
2 circle
19.6 20.9 t2_c19
28.3 31.7 t2_c28
38.5 43.9 t2_c38
A second sample production was necessary to produce the samples for the comfort
study (see section 2.4.3) as here, more samples per version were needed. Addition-
ally, the samples from the first sample production did not match the planned sizes
and therefore had differing electrode areas. Thus, the same versions were chosen for
the comfort study, but the electrode areas were adjusted and less different sizes were
produced. The planned electrode area was changed so that some of the samples from
the previous production could be used. An overview over produced samples with
electrode areas after steaming is shown in Table 5 below.
TABLE 5.OVERVIEW SAMPLES (SECOND SAMPLE PRODUCTION).
Version
Knitting pattern
No. of
yarns Shape
Actual area
in cm² Name
Comparison
version Plain knit 2 circle
16.1 p2_c16 32.4 p2_c32
Square version Plain knit 2 rectangle 16.0 p2_s16
High density
version Plain knit 3 circle 13.9 p3_c14
Tuck stitch version
plain knit with tuck stitch variations
2 circle 31.7 t2_c32
For a consistent wording throughout this report, following terms were defined as presented in Table 6.
TABLE 6.TERM OVERVIEW.
Term Definition Variations/
names
Note
Sample physical sample One sample per version
and size; 12 in total (4 ver- sions X 3 sizes) for the first sample production Version refers to the electrode
construction (binding, no. of yarns, shape)
p2_c p2_s p3_c t2_c
4 versions in total
Electrode size
refers to the conduc- tive area on the sample
sizes 19, 28 and 38
3 sizes in total
2.3.2 D
UMMY PRODUCTIONThe impedance of human body tissue varies between subjects, body location and time. These variations can have a big influence on the impedance of a skin-electrode system on a living subject and therefore can affect the results of electrode compari- sons, especially if the impedance variations are bigger than the expected differences arising from the changed electrode constructions. (Jin et al., 2016) Therefore, a dummy was to be developed to mimic the electrical properties of body tissue while having more homogeneous electrical properties than actual living tissue. For that, two properties are of importance. The first requirement is that the electrical imped- ance should match the range of human tissue impedance. As reference value, an im- pedance range of 50 – 100 Ω for a frequency of 200 kHz – 1 MHz was found in literature (Beckmann et al., 2010). The second requirement is a sufficient mechanical stability so that the dummy could resist pressures being applied during the test of the pressure-dependent impedance (see Series II, section 2.4.2.2) without the dummy breaking or being permanently deformed.
In several studies found in literature, the desired electrical impedance of an agar dummy was reached by adding sodium chloride (NaCl) to increase the electrical conductivity, see for example Macías et al. (2013) and Tallgren et al. (2005). How- ever, as the agar powder also increases the conductivity of the mixture, adding NaCl was not needed in the present study. A comparably high amount of agar was chosen to create the desired mechanical stability. Therefore, this amount of agar was found to be sufficient to reach a dummy impedance in the range of the human body.
To produce the dummy, 17.5 g of agar powder were mixed with 200 ml of deionised water (82 °C) for each dummy. The dummy was shaped by a 3D printed mould.
After removing the dummy from the mould, bumps and other irregularities on the bottom were cut off to make the surface flatter. Afterwards, the dummy was stored in an air-tight plastic bag until it was used for the experiments. In total, six dummies were produced for the experiments in Series II as the dummies tended to change their properties over a longer time period. A dummy and the mould are shown in Figure 4.
FIGURE 4.AGAR DUMMY, DUMMY IN MOULD AND INNER DIMENSIONS OF THE MOULD.
The mould for the dummy had inner dimensions of 12.9 cm length, 5.9 cm width
and a height of 5 cm, as shown in Figure 4. However, the height of the dummy itself
was dependent on the volume of the water-agar solution as the mould was not filled
to the top. The bottom of the mould was curved so that the dummy would have a
curved top, similar to the shape of a human arm. This curve had the same curvature
as the stamp of the testing rig with an inner radius of 3.5 cm. Thereby, it could be
ensured that the stamp would apply pressure on the complete dummy surface and not only punctually.
2.4 C HARACTERISATION METHODS
This section describes all characterisation methods which were used to evaluate the textile electrodes. Two different impedance measurements were performed as well as a comfort study on human subjects.
2.4.1 E
LECTRODE SAMPLE PREPARATIONTo prepare the samples for all experiments, the textile leads were insulated on the face side of the electrode so that they would not touch the skin or the agar dummy during the performed impedance measurements. For that, electrical insulation tape was hand stitched onto the samples, as shown in Figure 5, to fully cover the conduc- tive threads of the leads on the face side of the electrode.
Further, the electrodes with their surrounding fabrics were sewn into sleeves which would be used in experimental Series I (see section 2.4.2.1). For all samples, the same sleeve template was used with dimensions as shown in Figure 6.
FIGURE 5.INSULATED LEAD.
FIGURE 6.SLEEVE TEMPLATE.
Additionally, the dimensions of the produced electrodes were measured. Because knitted fabrics get stretched easily and are not dimensionally stable, a cardboard shape with the same dimensions as the sleeve template was made where the sleeves could be put onto to flatten out the electrodes without actually stretching them. This way, the electrode area was measured with the same low stretch for all samples. The setup is shown in Figure 7. A conventional ruler was used to measure the dimensions in horizontal and vertical direction and three replicates were made per measurement.
Afterwards, the average was taken over all measurements, and the electrode area was calculated for each sample.
For the comfort study, it was necessary to partly remove the surrounding PET fabric
FIGURE 7.CARDBOARD SHAPE WITH ELECTRODE SLEEVE ON IT.
FIGURE 8.SAMPLES FOR COMFORT STUDY.
2.4.2 E
LECTRICAL IMPEDANCE SPECTROSCOPY(EIS)
Common methods for electrode characterisation and performance evaluation are im- pedance measurements, as described in section 1.4.1. When measuring the imped- ance on human subjects, there is the advantage of measuring in a similar setup as the planned application. However, the electrical properties of a human body vary with several factors, such as time or subject. (Hao and Tao, 2015) Therefore, the electrical impedance spectroscopy (EIS) was divided into two experimental series, as pre- sented below in Figure 9. First, the impedance was measured on a living subject in dry and wet electrode condition. Series II on the other hand was performed on a dummy made from agar, which was expected to be more stable. Here, the influence of applying pressure to the system was investigated.
FIGURE 9.OVERVIEW EXPERIMENTAL SERIES I AND II.
All experiments were performed in a conditioned lab in which the electrodes were stored at least 12 hours before starting the experiments. Further, both series were performed using the same instrumental settings for the EIS. The frequency range of the planned application in NMES is comparably low. There, frequencies from 20 Hz to 100 Hz are commonly used. However, when choosing the frequency range to be analysed in impedance measurements, the range should be wide enough to capture both “asymptotic limits in which the imaginary impedance tends toward zero”
(Orazem and Tribollet, 2017b). Therefore, relatively large and small frequencies were needed to cover a wide frequency range. Nevertheless, the asymptotic limit in the low frequency area does not always exist, e.g. for blocking electrodes, or “a true DC limit is not achievable due to nonstationary [behaviour] of the system” (Orazem and Tribollet, 2017b) whereas a high frequency limit can sometimes not be captured due to instrument limitations. Furthermore, low frequency measurements tend to in- clude a lot of noise and are rather instable. Thus, data from the low frequency range might not be reliable in some experiments. Because of these considerations, a fre- quency range of 1 MHz to 0.1 Hz was chosen and the scans were done from high to low frequency so that the system has time to stabilise until the low frequencies are reached. Within the chosen frequency range, 10 frequencies per decade were meas- ured and a sinusoidal signal with an amplitude of 0.01 V was applied.
Following terms were defined, as presented in Table 7.
TABLE 7.TERM DEFINITIONS.
