Abstract
Bio-physical signal measurement tools embedded in clothing are becoming a viable alternative in mobile health monitoring systems, particularly Wearable Electronic Textile-based Systems (WETS). To assure clinical viability, utilizing flexible and inconspicuous conductive media that can acquire and transmit reliable signals while assuring signal durability and biocompatibility are particularly important when developing WETS for medical applications. To accomplish this task, conductive threads are emerging as an appropriate electrical medium for health monitoring garments. However, little has been studied on the behavior of these conductive threads under various conditions. We report here the electrical conductive properties of specific conductive threads under two conditions: (i) as sewn configurations onto a textile substrate with different stitch types and (ii) as independent strands under controlled extension independent from a sewing machine. Statistical results showed that the stitch class and thread location significantly influenced the electrical resistance of the conductive thread, revealing the chain stitch to provide resistance even lower than the un-stitched conductive thread. In addition, under controlled extension all three of the conductive threads exhibited both a hysteresis and a stress-relaxation effect. These are important phenomena to examine when conductive threads are incorporated into WETS because the choice of stitch type will influence the strength of the signals received and transmitted, while the wearers’ body movements will cause the threads to encounter multi-axial stretch. Knowing the influence of stitch type, stretch, and relaxation on conductive thread resistance will inform objective design and manufacturing decisions for developing clinical-grade textile-based electrical circuits for medical applications.
The field of wearable technology is growing exponentially, and while some wearable electronics, such as wrist bands and eyeglasses, do not incorporate textiles, the market for smart textiles shows strong promise. On 2 June 2016, the PR Newswire announced that the market for wearable electronics will grow from US$20 billion in 2015 to almost US$70 billion in 2025 and the article further determines that the principal industry segment is, and will continue to be, healthcare. The term smart textiles describes textiles with a range of functions from fiber-level phase-change polymers where products change shape when exposed to environmental fluctuations, to integrating conductive properties at the textile manufacturing stage, such as adding an external coating of conductive polymers to the finished textile. Electrical properties of conductive media are reported in terms of conductance, the ability of electrical current to pass through an object. 1 A third type of smart textiles, the focus of our study, involves applying conductant paths to the surface of regular textiles and integrating sensors and electronic components with those paths into a finished garment; we are naming this last category of smart textiles Wearable Electronic Textile-based Systems (WETS). Conductive paths on WETS are commonly applied through printing or sewing. To render the design and manufacturing of WETS more efficient and effective, the purpose of this study was to investigate the influence of various types of sewing machine stitches on the electrical conductance of three different kinds of conductive yarns, when sewn onto a non-conductive textile.
Acquiring flexible and inconspicuous highly conductive media is an important requirement for WETS, and particularly challenging when creating these systems for healthcare. The primary functional requirements of WETS for health monitoring are (a) acquiring reliable sensing of signals, (b) effective signal transmission, (c) signal durability, (d) bio-compatibility, (e) comfort, and (f) aesthetic properties. Acquiring and transmitting reliable signals and assuring signal durability and biocompatibility are particularly important when developing WETS for medical applications in order to assure clinical viability. 2 Comfort and aesthetic properties are essential factors to address in order for users to continuously and correctly adopt WETS. 3
Development of textile-based conductive media
To accomplish electronic signal transmission, a variety of textile friendly conductive media in the form of fibers, thread, yarn, rubber, and printable ink are being developed.2,3 Conductance can be integrated into the fiber through nanotechnology, for example, and then directly incorporated into the fabricated textile structure during fabrication or post-fabrication with specialized finishes. Conductive yarns and threads have several potential applications in physical and electrophysiological (electrocardiogram, electroencephalogram, electromyogram) signal monitoring and other bio-sensing techniques 4 and are less rigid and bulky than wires, making them particularly suitable for the comfort and aesthetic functions of WETS.
Indeed, conductive threads and yarns are becoming an integral part of WETS. Weaving, knitting, bonding, and stitching are several ways to integrate conductive yarn into a textile. The integration of conductive yarn through weaving or knitting can be desirable when striving to provide an inconspicuous integration of conductance throughout the textile. However, another important mode of integrating conductance is by stitching conductive thread onto the surface of the textile in a specific, limited area. Thread, a yarn that has been processed to make it thin, smooth, and continuous, can be used to assemble soft goods and/or applied on the surface of textiles either by hand or by machine. Stitching is a way to secure threads from free falling and to direct a small, efficient electrical path as required by the electric circuit. Stitching provides the freedom to adjust the electrical path easily and allows for a wide variety of textile substrates to be considered when developing WETS products. When evaluating materials for textile-based soft goods design and manufacturing, conductive threads are an excellent choice from a workflow standpoint as well. Conductive threads are particularly compatible with the soft goods manufacturing arena because soft good machinery is already set up to employ thread for textile assembly.
Several prior studies have employed conductive yarns through weaving and knitting because of conductive yarn’s flexibility and conformability. 5 However, the influence of various kinds of stitches on the conductance of thread when applied to the surface of a non-conductive textile has not been found in the literature. The focus of this paper is an alternative to fabricating an entire textile with conductive properties: applying conductive threads to the surface of a non-conductive textile by machine stitching.
Conductive threads applied to the surface of a non-conductive textile can be composed of non-conductive fibers combined with conductive particles. The manufacture of such conductive threads can be accomplished at the fiber stage during extrusion by forming a bi-component fiber with the conductive component and a synthetic fiber, or by adding an external coating of conductive polymers (PEDOT, polyaniline, polypyrrole, etc.) after the fiber has been extruded. Conductive metals such as ferrous alloys, nickel, stainless steel, titanium, aluminum, copper, silver, and carbon have also been used to incorporate conductance into fibers, yarns, and threads. 6 Comingling of textile fiber strands with metallic fibers has also been reported. 7 Manufacturers of commercially available conductive yarns and threads provide specification sheets providing resistance values (the level of difficulty of passing an electric current through a material, the opposite of conductance); however, the performance of conductive threads under stress and when applied to a textile through a sewing machine have not been reported.
Measuring and reporting electrical properties
The electric conductance of threads composed of conductive and non-conductive materials behaves differently than electric conductance of pure metals. 5 The level of conductance of thread is reported by manufacturers as resistance and is influenced by both the length of the thread and its diameter. When sewing with conductive thread on a textile, thread length and diameter are directly affected by both the extension of the thread and the stitch class used. A stitch is the result of a threaded needle piercing a textile and looping with another thread on the reverse side, leaving a trail of thread on both surfaces of the textile. Stitch class refers to a variety of configurations of this movement. Interestingly, there is a gap in the literature regarding the effect of various stitch classes on the fundamental property of resistance of thread. The effective signal transmission ability of conductive threads under various stitch classes needs to be understood in order to assure reliable signal transmission. The purpose of our study is to report the performance of the stitch types sewn with conductive thread in a specific distance on a textile substrate as a single unit, analogous to a thread, conforming to prior studies.8–10 Thus, we report resistance values (Ω) and not the resistivity, which is a calculation resulting from inputs related to the mass and resistance value.
To examine the influence of sewing with conductive thread onto a textile, we report here both the electrical resistance properties of specific thread under controlled extension independent from a sewing machine as well as the linear electrical properties of conductive thread when sewn onto a textile substrate using a sewing machine to execute five different stitch types, as defined by the ASTM standard D-6193. 11
Typically, the resistance per unit length is reported by the manufacturers as a performance metric of the conductive thread. However, translating this performance metric to stitches on garments required further analysis because engaging the thread in a sewing machine where it forms diverse stitch classes or types results in different resistance values. Considering the total influence of these stitches, we provide a comparison of resistance values for a unit length of stitch classes that will be valuable information for WETS product design and manufacturing, particularly when choosing components, defining conductive paths, and determining stitching methods.
In the context of the above-stated background, this study explored two hypotheses and two research questions as follows. H1: Stitch class will influence a conductive thread’s resistance. H2: Thread location on the sewing machine (spool or bobbin or both) will influence conductive thread’s resistance. RQ1: Does a stitched conductive thread provide less electrical resistance when compared to non-stitched, flat conductive thread? RQ2: Does thread elongation influence resistance in conductive threads?
Experimental procedure
To establish the methods to evaluate the conductance of threads when used in a sewing machine, choices were made on the materials, stitch classes, testing techniques, and approaches for data collection and analysis. The following section describes the methods employed.
Materials
Materials for this study included non-conductive and conductive thread, a non-conductive textile substrate, sewing machines, a tensile testing machine, a polarizing microscope, and a multi-meter. All non-conductive thread was 50% cotton/50% polyester. Three different kinds of commercially available conductive threads (A, B, and C) were used to investigate in this study. Threads A and B were bought from Shieldex.
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Thread A (Shieldex 117/17) is a silver-coated two-ply nylon yarn with an average resistance of 98.5 Ω/10 cm. Thread B (Shieldex 234/34) is a four-ply silver-coated nylon yarn with an average resistance of 8 Ω/10 cm. Thread C, purchased from Lamé Lifesaver,
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is a three-ply silver-coated yarn with an average resistance of 6.5 Ω/10 cm. The resistance values reported here were furnished by the thread manufacturers. Microscopic images of the conductive threads are shown in Figure 1, taken with a Nikon Eclipse E600 POL microscope with an Infinity 1 computerized camera attachment.
Microscopic images of threads. Threads are in the order A (two-ply silver-coated nylon yarn), B (four-ply silver-coated nylon yarn), and C (three-ply silver-coated nylon yarn) from left to right.
Non-conductive textile substrates in knit and woven fabrications were considered. A plain woven unfinished, 100% cotton fabric – commonly known as muslin – was chosen. The simple weave and natural cellulosic fiber provided a neutral background for the stitching. Using the same cotton muslin fabric provided consistency and normalized the thread tension adjustments. The rigid hand of the cotton muslin controlled for textile stretch.
The sewing machines used were a Singer 791 D200A, a Consew 199R-3A, and a Bernina 1005. The Bernina 1005 was used for the 301 Stitch class and the couch stitch configuration. The Singer 791 D200A is an industrial chain stitch machine, while the Consew is an industrial multi zig-zag machine. This combination of sewing machines was required in order to include the range of stitch classes chosen. A Thwing Albert model number 1350-42 tensile testing machine was used to evaluate the resistance of the thread when under tension and a Fluke 87 V True RMS multi-meter (a volt-ohm meter) was used to measure resistance.
Stitch classes
To provide a diverse and thorough analysis, we chose five different stitch classes: 301, straight lock stitch; 304, zig-zag lock stitch; 315, three-step zig-zag lock stitch; 401, two-thread chain stitch; and an adapted 304 with a flat conductive thread inside a closely sewn zig-zag, known as the couching stitch (see Figure 2). The 301 straight lockstitch was important to include because it is a commonly used stitch for woven apparel manufacturing. The second common woven apparel manufacturing stitch is the 401 two-thread chain stitch. The 304 zig-zag and 315 multi zig-zag stitches are commonly used for knit apparel assembly and for woven apparel embellishment. The couch stitch was chosen because of the potential to avoid putting the conductive thread into the machine and to maintain the shortest conductive thread path-length possible. Understanding the process required for the formation of the stitches for each of the stitch classes is important when considering conductive thread performance, because the formation influences the amount of tension required on the machine, the thread length, and the number of contact points, all components relevant to conductance. The straight, zig-zag, and the multi zig-zag stitches are formed when an upper thread meets a lower thread underneath the bed of the machine. The upper thread, or top, passes through a series of tension bars and spring releases and, finally, the eye of a needle. The lower thread, or bobbin, is wound around a small spool that fits into a bobbin case that is then secured into a looping mechanism under the bed of the machine. When the needle pierces the material from the top of the machine, the upper thread is pulled around the bobbin case to create a loop with the bobbin thread, unifying the two on either side of the material. As Figure 2 indicates, each time the top and bottom stitches intersect, a pattern appears on the surface. For the straight stitch and the chain stitch, the pattern makes a straight line, for the zig-zag stitch the path moves from left to right at an angle, and the multi zig-zag makes three straight stitches in one direction and then changes the direction at a 45 degree angle for three more straight stitches, continuing in this manner. The two-thread chain stitch requires both the top and bottom threads to feed into the machine from spools; no bobbin is employed. The couch stitch, for this study, involves a special process in which a conductive thread is held in place by a non-conductive, cotton/polyester thread used for the top and bobbin. By employing a closely sewn zig-zag stitch formation called a satin stitch, the conductive thread is positioned on top of the material within the left and right extremes of the zig-zag. Laying flat and straight, the conductive thread is encased within the satin stitch formation, and it is not engaged with the sewing machine at any time. The stitch length for all stitch formation groups was kept constant: 3.2 stitches per cm or (8 spi) for the lock stitch and chain stitch; 5.6 stitches per cm or (14 spi) for the multi zig-zag; 3.6 stitches per cm or (9 spi) for the zig-zag; and 20 stitches per cm or (50 spi) for the couch stitch.
Schematic of the stitch classes employed. (Source: ASTM D-6193-11 Standard Practice for Stitches and Seams.)
Sample preparation and measuring electrical resistance
Classes of stitch and conductive thread positioning
The lengths of the sewn paths were marked at 30.5 cm. Resistance values were taken according to an adapted AATCC Test Method 84-2011 Electrical Resistance of Yarns procedure. Before measuring the resistance, the sample was placed on an anti-static mat, made of vinyl with a snap-on grounding cord. In order to dissipate any static charge, a Staticmaster ionizing bar was mounted on a customized wooden instrument (see Figure 3) that enabled passing the ionizing bar over the sample at the specified 2 cm height over the sample. This neutralization of static charge was performed in a controlled lab environment.
Schematic diagram of neutralization of the sample using an ionizer and electrical resistance measurement.
The resistance value of each 30.5 cm sewn path was measured with a two-probe Fluke Multi-meter, as shown in Figure 3. The described procedure was followed to measure all of the conductive path configurations.
Elongation/relaxation and thread resistance
To estimate the influence of thread tension on the resistance of conductive threads, a hysteresis test was performed. Using a Thwing Albert model number 1350-42 tensile tester, conductive threads of 30.5 cm (12 inches) sample length were subjected to a constant rate of elongation (15 mm/minute) and the resistance was measured at intermediate periodic intervals. Each 30.5 cm thread was clamped into two non-conductive heads. When the machine was engaged, one of the heads moved to a specified distance, thus elongating the thread (see Figure 4). The resistance value for the elongated thread was measured on the 30.5 cm sections at elongation intervals of 0.25 cm until a final 5 cm elongation was reached. After elongation, the samples were allowed to relax at intervals of 0.25 cm from 35.5 to 30.5 cm and the resistance values were measured at those relaxation intervals.
Schematic of elongation/relaxation testing.
Statistical analysis
The various factors for analysis of variance tests
Similarly, the influence of thread tension on resistance was reported for the three samples of each thread, A, B, and C. The average of those values per thread was plotted in order to realize the hysteresis effect.
Results and discussion
Stitch classes and resistance values
Mean resistance values (Ω per unit length of 30.5 cm) and standard deviation (SD) of the stitch samples
Mean resistance per unit length of 30.5 cm for the three thread types used in various stitch types.
A comparison of the resistance per unit length of the thread with the resistance per unit length of the stitch is shown in Figure 6. Keeping the resistance of the flat thread as the base value, the change in the resistance of each stitch class was assessed, as given in Equation (1)
Change in the resistance of stitches when compared to resistance per unit length (30.5 cm) of raw conductive thread.
In support of H1 and H2, the statistical results showed that the stitch class and location of the threads significantly influenced the resistance of the conductive thread. The resistance values among different stitch types and different locations were significantly different. The least resistance values were found in the following ascending order (for thread A): lock stitch, multi zig-zag, and zig-zag.
Conductive thread resistance of the lock stitch seems to be affected by the amount of thread consumed, that is, when the conductive thread is only in the bobbin resistance is higher and when the conductive thread is both from the top spool and the bobbin resistance decreases due to an increase in points of contact. The zig-zag stitch drastically increases the resistance while the multi-zig-zag provides comparatively less resistance than the zig-zag, a phenomenon that once again may be explained by an increase in points of contact through the multiple stitches required by the multi zig-zag. In cases when thread B was employed, the lock stitch-II where conductive thread is used in both top and bottom seems to have the least resistance. The zig-zag stitch holds the second best option for thread B. In the case of thread C, the lock stitch-II shows the least resistance, followed by zig-zag.
Overall, the resistance value is less when the thread is used in both the bobbin and the spool than when it is only used in the bobbin. For all three threads, the resistance value decreased from its base value if the lock stitch was used with conductive threads in both the bobbin and the top spool. The chain stitch and couching stitch classes were not included in the ANOVA, but their mean resistance values are given in Figure 5 to compare with the rest. In particular, for thread A, the chain stitch showed the least resistance. Although it may not be of statistical significance, it can provide an informed decision to designers for a choice of selection.
As demonstrated in Figure 6, the chain stitch sewn with thread A actually decreases the resistance of the thread beyond the base value of the flat, un-stitched conductive thread. Similarly, for threads B and C, lock stitch-II showed less resistance than the measured resistance for the non-stitched base value. This addresses the research question RQ1, that the resistance value decreases for stitch types such as the chain and lock stitch-II when compared to a flat thread. This phenomenon could possibly be due to the effect of sustained strain and stretch on the thread caused by sewing. Previous studies have mentioned that with increased strain on conductive yarns, the number of contact points increases and the overall resistance drops.14,15 Likewise, with sewing, it is probable that the number of contact points would increase as a combined resultant of both stitch structure and thread tension. Despite the results partially supporting this phenomenon, it was not observed in every stitch type, motivating further exploration.
Elongation/relaxation and thread resistance
To explore further the relationship between strain and resistance, a series of tests were conducted to estimate any hysteresis that may be present in the conductive threads. When examining the effect of elongation/relaxation on thread resistance, the results (see Figure 7) show that thread A exhibits a decrease in resistance, supporting the concept that strain increases the number of contact points, for up to 4% elongation, followed by an increase in resistance. Such a phenomenon is not observed in threads B and C. Thus, thread tension alone need not necessarily decrease the resistance, but can show such a decrease when combined with specific stitch classes.
Results of hysteresis tests on the three threads (A, B, and C) from left to right. Threads B and C showed a permanent increase in resistance.
After elongation, the samples were allowed to relax from 35.5 to 30.5 cm and the resistance was once again measured at 0.25 cm relaxation intervals. All three threads exhibited a hysteresis effect, as shown in Figure 7, and had a stress-relaxation effect. Among them, thread A showed better hysteresis than the others.
Limitations and future study
To accomplish our study, we limited the variations in relation to the type of conductive thread, non-conductive textile, and stitch type. For example, the three threads with five sets of stitch samples for this study were placed on a woven cotton textile, yet future study could analyze a wider range of conductive threads on a wider range of substrates, including stretchable fabrications and synthetic fibers. In addition, the resistance values were taken at ambient temperature and humidity, with the intent to replicate the apparel production environment. Further study duplicating our method in an environmentally controlled chamber could provide further insight.
Because analysis of the conductive thread behavior when employed in a sewing machine is a relatively new field of study, the potential for future study is vast. Looking at the influence of diverse sewing machine brands, tension settings, stitch density, conductive threads made from diverse materials, and even testing resistance of threads after being sewn into a garment are all areas for future exploration.
The stitch type influences thread conductance in terms of the length of the thread consumed by the stitch conformation, the number of contact points that a stitch creates, the position of the thread on the sewing machine, the amount of tension the thread undergoes, and friction points of the conductive thread on the sewing machine. Further study could examine each of these elements independently and as a whole to provide insight for thread manufacturers as well as designers and manufacturers of WETS.
Conclusion and implications
In conclusion, when choosing a method for developing the conductive path on WETS, it is important to consider the option to stitch onto the surface of the textile with conductive thread. Stitching onto the surface provides the designer with opportunities to meet the aesthetic requirements of the consumer by allowing for an open range of garment colors, fabrics, and seaming options. By providing designers with the prospect of more choices while maintaining and even possibly enhancing conductance and performance of the WETS product, consumer acceptance of textile-based electronics may be enhanced. Stitching the path onto a textile will also enrich the comfort and bio-compatibility of the WETS product, because the conductive thread will be flexible and the path can be positioned in a variety of locations around the body. Furthermore, embedding the conductive thread through sewing is able to be accomplished later in the product development stage than weaving or knitting them into the garment, allowing for flexibility in the manufacturing process and potentially reducing costs.
Electrical circuits are often designed with resistors that control the amount of electrical current that passes through each component. Designers of circuits for WETS can engineer the resistance into the textile-based circuit by intentionally specifying different stitch formations using the same thread, eventually eliminating the need for resistors; designers of WETS will be able to choose the stitch type in relation to the resistance values desired.
A significant finding of this study is that the most commonly used stitch for woven apparel, the straight lock stitch, for all three types of conductive thread, increased resistance when conductive thread was used in the bottom, but decreased resistance when conductive thread was used in both the top and bottom. Another significant finding is that the chain stitch decreases resistance, most likely because of the multiple contact points created by the chain stitch, a phenomenon mentioned in the literature. Similarly, by employing the zig-zag stitch where the conductive thread was in both the top and bottom, resistance was reduced. Applying this knowledge of conductive stitch type to the development of wearable electronic garments can create more efficient electronic systems. For example, instead of using 30.5 cm of flat conductive thread in the garment in place of regular wire, a designer could choose the 301 lock stitch with the conductive thread in both the top and bottom for 30.5 cm, and the conductance of the stitched path will be better than 30.5 cm of raw conductive thread.
In addition, the effect of elongation and relaxation on thread resistance is an important phenomenon to consider when conductive threads are incorporated into WETS. Garments tend to encounter stretch in multiple directions due to wearers’ body movement. Thus, conductive thread embedded in the garment may be elongated or relaxed. Knowing the influence of stretch and relaxation on threads’ resistance will be beneficial when troubleshooting changes in signals due to movement or diverse body shapes.
Most importantly, by selecting the correct stitch formation and understanding the influence of stretch on the conductive thread, textile-based clinically viable remote medical monitoring systems may be able to help millions of people around the world improve their health and well-being. Indeed, the discoveries from this study are particularly salient to the development of WETS for medical applications, where sensors collect biofeedback that can be utilized by medical professionals to diagnose and treat illnesses and disorders and where it is essential to assure maximum signal quality.
Footnotes
Declaration of conflicting interests
The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.
Funding
The authors received no financial support for the research, authorship, and/or publication of this article.