Design of a wearable pain management system with embroidered TENS electrodes

Gozde Goncu Berk (Department of Textile Engineering, Istanbul Technical University, Istanbul, Turkey)

International Journal of Clothing Science and Technology

ISSN: 0955-6222

Publication date: 5 March 2018

Abstract

Purpose

The purpose of this paper is to develop a user friendly, wearable pain management system by optimizing CAD embroidery parameters for manufacturing high performance dry transcutaneous electrical neural stimulation (TENS) electrodes.

Design/methodology/approach

User-centered design methodology is employed to identify user needs related to TENS devices. Optimization of CAD embroidery parameters was done by measuring and calculating resistance and signal-to-noise values for electrodes manufactured with different conductive thread, stitch pattern, and stitch density types.

Findings

Characteristics of the conductive thread such as thickness and irregularity, embroidery stitch pattern, stitch density therefore the amount of conductive thread used all effect resistance values and signal-to-noise values of TENS electrodes. Low resistance of TENS electrode surface does not mean high signal-to-noise ratio and high TENS signal quality. Satin stitch type with low stitch density provides the best resistance and signal-to-noise ratio for a TENS electrode.

Originality/value

This study reported the design process of a wearable pain management system with a focus on optimization of embroidery manufacturing parameters for development of TENS electrodes. The design process not only required technical optimization but also understanding user problems related to use of conventional TENS devices. Proposed end product is a user friendly, electronic textile based, wireless wearable pain management system in different forms suitable for major pain areas such as knee, elbow and neck.

Keywords

Citation

Goncu Berk, G. (2018), "Design of a wearable pain management system with embroidered TENS electrodes", International Journal of Clothing Science and Technology, Vol. 30 No. 1, pp. 38-48. https://doi.org/10.1108/IJCST-04-2017-0047

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Publisher

:

Emerald Publishing Limited

Copyright © 2018, Emerald Publishing Limited


Introduction

Transcutaneous electrical neural stimulation (TENS) is the electrical stimulation of nerves to block pain sensation for treatment of different acute and chronic pain conditions including labor pain, surgical pain, neck and back pain, arthritis, neuropathic pain and menstrual pain (Hansson and Lundeberg, 1999). The origins of TENS therapy relate back to Shealy et al. (1967) research on neuro-modulation techniques and Melzack and Wall’s (1967) Gate Control Theory which suggests that pain signals need to encounter certain “neurological gates” at the spinal cord level and are not free to reach the brain as soon as they are generated at the injured tissues or sites.

This study aimed to develop a wearable TENS system for pain management that eliminates user problems associated with conventional TENS device use. Commercially available TENS devices are at least the size of a mobile phone, composed of a control unit and electrodes hooked to this unit via cables. The low intensity current created by the TENS control unit is delivered to skin through conventional hydrogel Ag/AgCl electrodes that stick to skin. The proposed product is composed of electrodes created by embroidering conductive thread on non-conductive textile surface and a transferable circuit board and power supply snapped on these surfaces. Electrodes of the wearable TENS system are developed by optimizing computer-aided (CAD) embroidery parameters such as stitch pattern and stitch density. Embroidered TENS electrodes are analyzed in comparison to one another and conventional Ag/AgCl hydrogel electrodes in terms of resistance and signal-to-noise values.

Literature offers a number of studies on development of textile electrodes and sensors for different purposes using embroidery technique. Post et al. (2000) introduced e-broidery (machine controlled embroidery using conductive thread) as an e-textile fabrication method in addition to sewing, nonwoven textiles, knitting, weaving, coating/laminating and printing technologies. CAD embroidery is used to stitch patterns that define circuit traces, component connection pads or sensing surfaces (Castano and Flatau, 2014). According to the review on technical embroidery for smart textiles by Mecnika et al. (2015), advantageous characteristics of embroidery are dimensional stability of the developed structures and good reproducibility of the manufacturing process as well as rapid prototyping via pattern creation with CAD software. According to Stoppa and Chiolerio (2014) embroidery offers advantages over knitting and weaving since it allows control and integration of conductive yarns with different electrical properties such as different resistances on single or multiple layers of fabric or apparel products in one step.

Embroidered sensors such as strain sensors (Vena et al., 2013), pressure sensors (Weinberg et al., 2000), temperature sensors (Elsner, 2010 ) humidity sensors (Pereira et al., 2011) have been studied in the literature. Embroidered electrodes which register electric potential generated by muscles, electric activity of neurons of the brain or the heart have been studied. Kannaian et al. (2013) fabricated textile ECG electrodes by embroidering conductive yarns on polyester fabric in a square form with under layered satin stitches. Results showed that the impedance of the embroidered electrode had an acceptable value and the ECG signals were similar to conventional Ag/AgCl electrodes. Bystricky et al. (2016) compared embroidered and knitted textile electrodes for measurement of ECG signals and reported more readable ECG curves with embroidered electrodes which have better contact with skin. Pola and Vanhala (2007) compared performance of industrially available knitted and woven textile electrodes and a self-made embroidered electrode for ECG monitoring, and concluded that embroidered electrode performed better than industrially available ones since it offers large contact area with skin. Linz et al. (2006) developed a T-shirt that can measure ECG signals using an interconnection technology based on embroidery of conductive yarn. The ECG module was equipped with metallized contact areas which are penetrated by the embroidery machine to make an electrical contact with a conductive thread. Both the conductors as well as the electrodes were embroidered with conductive yarn in one step which eliminated an unnecessary contact and thus additional contact resistance and contact noise. Oliveira et al. (2014) measured electrode-skin impedance and EMG signal quality obtained from embroidered dry electrodes and concluded that impedance magnitude measurement is not enough to monitor quality of EMG signals. Shafti et al. (2016) developed textile electrodes of varying sizes and stitch densities using steel conductive thread to measure EMG signals. Researchers concluded conductive thread-based electrodes have been effective in providing similar results to those of gel based electrodes. Löfhede et al. (2012) developed a monitoring system for neonatal intensive care units by testing embroidered textile electrodes and comparing them to standard high quality electrodes. The acquired signals were compared with respect to morphology, frequency distribution, spectral coherence, correlation and power line interference sensitivity, and the signals were found to be similar in most respects.

In addition to embroidered ECG, EMG and EEG sensors, there exists limited number of research on development of e-textile-based TENS electrodes using different fabrication methods. Gniotek et al. (2011), realized textile electrodes using conductive silver nano-ink and printing valve jet system on different textile substrate. Silver woven electrode, silver nonwoven and graphite on woven electrodes were compared with the printed TENS electrode and changes in the resistance values were measured. Keller and Kuhn (2008) discussed carbon, hydrogel and textile electrodes and summarized important factors that influence efficient TENS. Li et al. (2010) attempted to develop TENS electrode integrated T-shirt using intarsia knitting. Frydrysiak et al. (2012) studied woven, nonwoven and knitted electrodes for muscle electrostimulation. Recently, Erdem et al. (2016) developed textile-based TENS electrodes by sewing conductive thread on a base fabric with different stitch densities and measured resistance and perception of the human subjects. It is concluded that textile electrodes have low resistance values compared to gel electrodes and do not cause any discomfort on human subjects. However, there exists no research that optimizes embroidery parameters for fabrication of dry, textile electrodes to design a user friendly, wearable pain management system as an alternative to commercial TENS devices. This manuscript documents the design process of a wearable pain management system and development of embroidered TENS electrodes and their electrical characterization.

Material and methods

Wearable pain management system design

Wearable pain management system is developed using a user-centered design approach. Users of conventional TENS devices are interviewed to explore limitations of current design and to develop an in-depth understanding of user needs. A total of five face-to-face interviews are conducted with users who apply TENS therapy to different areas of their bodies.

TENS electrode design

Multiple silver and steel conductive thread are tested with the CAD-based ZSK Sprint embroidery machine using different needle sizes and power settings to determine the best available alternative in terms of manufacturability. Two types of conductive silver thread, Shieldex 235/34 dtex (Silver 1) and LessEMF (Silver 2) showed superior performance in terms of continuous manufacturability without jamming or breaking during the process.

In order to optimize the embroidery parameters for a TENS electrode, different embroidery stitches and stitch densities were employed. Using the Tajima DG15 by Pulse embroidery software, textile electrodes with a diameter of 30 mm were modeled using satin stitch, fill stitch, zigzag stitch and star stitch options using two different stitch densities (Table I).

As a result, 16 TENS electrode alternatives were realized by embroidering Silver 1 and Silver 2 on a 100 percent polyester knit fabric using four different stitch types and two different stich densities (Figure 1).

With embroidery on a stretch spacer fabric, it was important to ensure that the thread is not under mechanical tension when the TENS electrodes are worn around elbows or knees. Therefore, it was necessary to stretch the fabric and stabilize it on an embroidery interfacing and also to use under-stitching with non-conductive thread to create a stabilized surface. The under-stitching also helped to add extra raised surface for the electrodes which is critical for better electrode-skin impedance.

Electrical characterization of TENS electrodes

The manufactured TENS electrodes are tested for their electrical characterization. Surface resistance of the electrodes is measured with triple method from 16 different points using a multimeter. The data are analyzed using IBM SPSS Statistics 22 software and the resistance values and their distributions for each electrode are compared according to embroidery stitch type and stitch density (Figure 2).

In addition, signal-to-noise (SNRdb) values of each electrode are calculated and compared with conventional hydrogel Ag/AgCl electrodes. A signal generator is used to create a 10 V, 60 Hz, square wave form signal which is similar to a widely used TENS signal for pain management. A digital oscilloscope is employed to collect the signals from the textile electrodes and conventional hydrogel Ag/AgCl electrode using triple method. The data acquisition for SNRdb value of each electrode is conducted using the MATLAB software by calculating the ratio of the mean value of noise and standard deviation and then converting it to decibels.

Results and discussion

Wearable pain management system design

TENS therapy is delivered via over the counter, commercially available devices as well as more complicated machines controlled by medical professionals. These devices create electrical pulses that disrupt the pain signals created by body parts affected from pain and thus decrease the pain sensation. Compared to pharmaceuticals used for pain management, use of a TENS device does not cause any side effects and provide instant pain relief while in use. However, interviews with users of TENS devices yielded many problems related to their effective use. Most common user problems related to conventional TENS devices are tangling of cables, blanking off electrodes from their sockets during movement and finally stigma of using a TENS device outside home because of its bulky structure. In addition, conventional TENS devises rely on disposable hydrogel Ag/AgCl electrodes which usually require skin preparation such as shaving and cleansing with alcohol prior to application. Long term use of these electrodes were reported be irritating to skin by the interviewees. Moreover, the ability of hydrogel electrodes to reliably perform their functions depends on their ability to retain moisture and remain wet or hydrated. Since these electrodes lose their moisture after a few applications, they also lack reusability and show ineffective adherence to skin after a few uses.

As solution to these user problems, this research proposed a user friendly, electronic textile based, wireless wearable pain management system in different forms suitable for major pain areas such as knee, elbow, and neck. The textile component of the wearable TENS system comes in as a kit that includes an elbow sleeve, knee sleeve and a patch that can stick to body with double sided adhesives. Electrodes that deliver the TENS signal are created by embroidering conductive thread on non-conductive textile surface after optimization of embroidering parameters. The hard component housing circuit board and power supply is snapped on the textile component and is transferrable between different textile components for treatment of different pain areas.

The TENS patch includes two electrodes while elbow and knee sleeves have four electrodes. Conductive tracks are designed and embroidered on the textile surface connecting the TENS electrodes to the control unit. The signal transmission lines are manufactured using embroidery together with TENS electrodes at once and without any interruption which means no connector is necessary between the TENS electrodes and the conductive track. Electrodes and conductive tracks on the textile surface are insulated to eliminate any signal loss between the control unit and the TENS electrodes. The insulation is realized by laminating the textile surface using thermoplastic polyurethane membranes. Textile electrodes are laminated on the outer side of the textile surface while conductive tracks are fully insulated on both sides. The control unit which delivers the TENS signal and holds the power supply are encased in a plastic structure. The control unit can be attached to and detached from the textile TENS system using metal snaps (Figure 3).

The wearable pain management system allows users to move freely while wearing the TENS device and eliminates problems of carrying a bulky device with tangling cables. Since it is textile based, it can come in different colors and patterns and improve the negative perception of using a TENS device with an esthetically appealing look. Additionally, developed textile TENS electrodes are embedded in a wearable form and can be used repeatedly and do not require replacement (Figure 4).

Electrical resistance of embroidered TENS electrodes

Table II illustrates the resistance values for each parameter of embroidery stitch type, stitch density and conductive thread type for textile electrodes and the conventional hydrogel Ag/AgCl electrode. Based on the results, for Silver 1 thread, the lowest electrical resistance was measured for star stitch electrode (M:0.4212, SD:0.1046) while the highest electrical resistance was measured for satin stitch electrode (M:1.0250, SD: 0.3605). When electrical resistance values of electrodes are compared in terms of stitch density for Silver 1 thread, it is observed that electrodes with low stitch density also have lower resistance. Similarly, for Silver 2 thread the lowest electrical resistance value belongs to star stitch electrode (M:9.187, SD:5.671) and the highest electrical resistance for fill stitch electrode (M:711.812, SD:601.972). However, in contrast to electrodes manufactured with Silver 1, electrodes with low stitch density have higher resistance values. Finally, the conventional hydrogel Ag/AgCl electrode (M:649.25, SD:328.76) has higher electrical resistance compared to embroidered electrodes except for fill stitch type using Silver 2 thread.

An important parameter for TENS electrodes is the homogenous distribution of resistance values on the surface of the electrode. Fluctuating resistance values cause delivery of different amounts of voltage to the skin. Especially surface areas with low resistance values on a TENS electrode result in high voltages delivered to skin and stinging pain sensations. Therefore, it is important to analyze distribution of resistance values on different spots of the textile electrode. Distribution of resistance values with minimal standard deviation and no outliers would be ideal for a textile TENS electrode. The following boxplots illustrate the distribution of resistance values for each of the textile electrodes manufactured and the conventional hydrogel Ag/AgCl electrode (Figure 5). Star stitch type with both high and low stitch densities for Silver 1 electrodes demonstrate short boxplots which means these electrodes have relatively low resistance and resistance values on the electrode surface are fairly similar. Although star stitch pattern displays a short boxplot for Silver 2, existence of outlier values indicates that resistance values fluctuate on the electrode surface. Zigzag stitch type with both high and low stitch densities for Silver 1 and Silver 2 electrodes have the second lowest resistance and resistance distribution values. Satin stitch and fill stitch patterns for Silver 2 electrodes show taller boxplots which means that resistance values on the electrodes surface differ as it is also displayed in Table II. On the other hand, for Silver 1 while fill stitch pattern displays tall boxplots and outliers, satin stitch pattern displays relatively even distribution of resistance values on electrode surface. Conventional hydrogel Ag/AgCl electrode displays a right skewed distribution with outlier values which makes it comparable to Silver 2, fill stitch and satin stitch pattern electrodes. The relationship between stitch densities and resistance values do not show a consistent result between Silver 1 and Silver 2 threads and different stitch patterns.

Based on this discussion, it is seen that Silver 1, satin, star and zigzag stitch pattern electrodes could be better options compared to the conventional hydrogel Ag/AgCl electrode. However, it is not possible to come to a conclusion only by exploring resistance values for TENS electrodes. TENS therapy requires signals with specific characteristics in order to be effective, therefore signal-to-noise values for each electrode type should also be explored.

Signal-to-noise values of embroidered TENS electrodes

Signal-to-noise value is another critical electrical parameter in TENS electrode design. TENS therapy requires specific signal characteristics to alleviate pain in affected areas. A typical TENS device functions with asymmetrical biphasic square wave form, 2-150 Hertz pulse frequency, 50-250 micro second pulse width and 0-80 mA current. The signal quality of manufactured textile electrodes and conventional hydrogel Ag/AgCl electrodes are comparatively analyzed by calculating SNRdb values for each electrode. According to the results displayed in Table III, when conductive threads are compared Silver 2 has high SNRdb values, when stitch patterns are compared satin stitch has high SNRdb value for Silver 1 and Silver 2, and when stitch densities are compared high stitch densities show lower SNRdb values for Silver 1 and higher SNRdb values for Silver 2 except the zigzag stitch pattern. The conventional Hydrogel Ag/AgCl electrode has a relatively low or in some situations similar SNRdb value when compared to different types of embroidered electrodes. Selection of the optimum embroidered TENS electrode parameters requires a thorough understanding of both resistance and SNRdb values since low resistance values do not correspond to low SNRdb values. While Silver 2 thread and satin stitch combination has superior SNRdb value, the resistance distribution for same combination varies a lot. Silver 2 thread and zigzag stitch combination has the second highest SNRdb value, however, the distribution of resistance values are right skewed or displays outliers. Silver 1 thread and satin stitch combination with low stitch density can be concluded as the optimum option with high SNRdb values and homogenous distribution of resistance values among the alternatives.

Conclusion

Chronic pain conditions affect everyday life of people at multiple scales and require continuous medications and often times physical therapy. Wearable technologies could offer promising benefits in management of chronic pain conditions as an alternative to pharmaceuticals and physical therapy. This study reported the design process of a wearable pain management system with a focus on optimization of embroidery manufacturing parameters for development of TENS electrodes. Results showed that embroidery stitch pattern and stitch density have effect on surface resistance and signal-to-noise values of a textile-based TENS electrodes. Results also showed that low resistance values do not necessarily mean high quality TENS signal and signal-to-noise value and resistance value should be evaluated at the same time to determine the highest quality TENS electrode.

The design process not only required technical optimization but also understanding user problems related to use of conventional TENS devices. Proposed wearable pain management system allows flexibility for users to choose based on specific pain condition such as the elbows or the knee. Moreover, the wearable pain management system eliminates problems of carrying a bulky device with tangling cables and stigma of using such a device outside home environment with its textile structure that can easily be worn under garments and while in mobile conditions.

Future clinical research is required for exploring effectiveness of the wearable pain management system on patients with chronic pain conditions. Also, future research can be conducted for detecting life span of textile TENS electrodes after multiple use over a period of time and after exposure to washing.

Figures

Examples of TENS electrodes embroidered using satin, fill, star and zigzag stitches

Figure 1

Examples of TENS electrodes embroidered using satin, fill, star and zigzag stitches

Representation of resistance measurement points on TENS electrode surface

Figure 2

Representation of resistance measurement points on TENS electrode surface

Visual schema of wearable TENS management system illustrating the control system and its transferability between patch, elbow and knee sleeves

Figure 3

Visual schema of wearable TENS management system illustrating the control system and its transferability between patch, elbow and knee sleeves

Representation of embroidered TENS electrodes’ placement on the wearable pain management system

Figure 4

Representation of embroidered TENS electrodes’ placement on the wearable pain management system

Boxplots of resistance values for textile TENS electrodes manufactured with Silver 1 and Silver 2, and the conventional Ag/AgCl electrode

Figure 5

Boxplots of resistance values for textile TENS electrodes manufactured with Silver 1 and Silver 2, and the conventional Ag/AgCl electrode

Stitch densities and stitch types used to embroider TENS electrodes

Embroidery Type Satin stitch Fill stitch Start stitch Zigzag stich
High stitch density 1.98 m 1.50 m 1.43 m 1.63 m
Low stitch density 1.54 m 1.05 m 1.02 m 1.27 m

Resistance values for each textile electrode using different conductive thread, embroidery stitch type, stitch density and the conventional hydrogel Ag/AgCl electrode

Conductive thread Embroidery stitch Thread length (m) Min. resistance (Ω) Max. resistance (Ω) Mean SD
Silver 1 Fill stitch High stitch density 0.30 5.70 1.80
1.56
Low stitch density 0.30 3.10 1.11 0.85
Satin stitch High stitch density 0.40 1.60 1.02 0.36
Low stitch density 0.30 0.90 0.63 0.20
Star stitch High stitch density 0.30 0.60 0.44 0.10
Low Stitch Density 0.30 0.50 0.42 0.10
Zigzag stitch High stitch density 0.80 1.10 0.93 0.10
Low stitch density 0.40 0.70 0.53 0.11
Silver 2 Fill stitch High stitch density 7.00 1416.00 666.06 506.86
Low stitch density 12.00 1921.00 711.81 601.97
Satin stitch High stitch density 11.00 1118.00 463.93 331.45
Low stitch density 13.00 1683.00 567.69 489.72
Star stitch High stitch density 5.00 46.00 16.00 13.06
Low Stitch Density 4.00 24.00 9.18 5.67
Zigzag stitch High stitch density 34.00 214.00 92.37 57.35
Low stitch density 48.00 362.00 136.81 86.71
Hydrogel Ag/AgCl electrode 237 1537.00 649.25 328.17

Signal-to-noise ratios for textile TENS electrodes manufactured with Silver 1 and Silver 2, and the conventional Ag/AgCl electrode

Satin stitch Zigzag Stitch Fill stitch Star stitch
Low stitch density High stitch density Low stitch density High stitch density Low stitch density High stitch density Low stitch density High stitch density
Silver 1 15.33 14.40 11.39 6.08 7.89 4.10 10.89 8.66
Silver 2 33.30 34.34 16.44 15.997 12.89 13.10 11.36 13.10
Hydrogel Ag/AgCl electrode 7.42

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Acknowledgements

The author would like to thank Nese Topcuoglu for her help in embroidering the TENS electrodes. This research is funded by TUBITAK (The Scientific and Technological Research Council of Turkey) under the grant number 115M710.

Corresponding author

Gozde Goncu Berk can be contacted at: goncug@itu.edu.tr

About the author

Gozde Goncu Berk holds a PhD Degree in Design from University of Minnesota, USA. Currently, Dr Berk is an Assistant Professor at Istanbul Technical University, College of Textile Technologies and Design. Her research interests are new product development processes, functional apparel design, and wearable products for healthcare applications.