Abstract
Introduction
There has been considerable attention for the development and investigation of the electronic textiles (e-textiles) structures for the last two decades.1–4 Since the e-textiles include variety of components such as sensors, actuators, power sources and transmission lines, plethora of the prototypes were already been developed.5,6 One of the main challenges for the e-textile system is to maintain the system reliability when the sub-components come together to build the wearable device. It is common that the components perform well when they are operated independently; however, it is often a difficult task to retain the good performance when they are integrated into the system. There are mainly two system error sources to prevent the wearable device to perform its task, namely connection errors among the system components and reduced power and signal transmission quality. Conductive transmission lines within the e-textile structures are responsible of carrying power from the power source such as flexible batteries to system components and transmitting of the signals received by sensing elements to microcontroller or carrying commands to the actuating unit of the system. Thus, it is inevitable to create reliable transmission lines in the first place for the e-textile system construction. For this aim, researchers have tried different methodologies. Traditional methods such as weaving, knitting, or embroidery can be used to create conductive lines within the structure.7–9 Although the embroidery technique provides great freedom in the creation of conductive lines, conductive yarns should have relatively higher strength and flexibility due to the higher level of stress during the embroidery process. 10 Woven structures enable the creation of multilayer structures; thus, conductive yarns can be hidden in the structure in order to prevent short circuits. 11 However, the main limitation for woven circuits is that the locations of the conductive warp yarns have to be predetermined during the preparation of the warp beam. 12 Since knitted textile structures have a relatively higher level of stretch properties, loop deformation greatly affects the electrical properties of the conductive lines. Thus, when the transmission lines are intended to be built through the knitting route, the conductivity of these lines should be notably higher than that of textile-based sensing areas realized with conductive yarns based upon the applied design. 13 Kursun et al. also revealed the effects of different conductive yarns on the signal quality of an ultrasonic sensor within the woven structure. 11 They used five different kinds of conductive yarn that included stainless steel yarn, insulated copper yarn, and silver-plated polymeric yarn. Experimental results showed that increasing the level of conductivity provided better signal quality. Furthermore, silver-plated polymeric yarn achieved the best compromise between signal quality and retention of textile properties. In a recent study, Choi et al. showed that increasing the number of conductive strands within the yarn structure enhanced the conductivity and the signal quality characteristics of textile-based transmission lines. 14 In addition, conductive polymer–coated polymers or metal-coated fabric strips can be used as electrical interconnections within e-textile structures.15,16 Other novel approaches include the use of screen-printing and fabric etching methods. Locher and Troster used screen-printing of silver ink to create transmission lines and they achieved 50Ω line impedance. 17 They also showed that increasing the number of printed layers provides better electro-mechanical properties.
However, cracks within the printed layers start to occur when the bending radius is less than 1 cm. In another study, Kazani et al. investigated the effect of the two different types of conductive ink on the conductivity properties of screen-printed conductive lines. 18 They also altered the properties of the woven fabrics in terms of the yarns used and the fabric densities created. They concluded that the viscosity and surface tension of the conductive inks affected the conductivity of the printed lines. Also, degradation of printed layers during washing or under abrasive force is one of the major problems for screen-printing of conductive lines. Thus, some studies have demonstrated that the coating of conductive layers and/or conductive yarns without affecting the flexibility and softness of the fabric can be a solution for this problem.19–21 Suh et al. investigated the effect of different types of coating material on the performance of printed conductive lines. 22 They used acrylic, polyurethane, and silicone as coating materials and concluded that silicone coating offers the most promising results among these types of coating materials.
After the careful examination of the literature, polymeric conductive yarns, and stainless steel yarns are more feasible materials to create conductive yarns unlike printed layers they do not degrade easily and have inherent flexibility. However, employing of these yarns to create transmission lines is also not straightforward process using traditional textile production methods. In a previous study, Kursun Bahadir et al. successfully embedded conductive yarns into the two layered fabric structures using hot air welding technology. 23 Unlike traditional integration methodologies, yarns do not experience any stress during the process and they concluded that transmission lines showed superior properties. Here, we propose the construction e-textile transmission lines using ultrasonic welding technology. This method enables bonding of synthetic fabrics owing to generation of heat by vibrations at the applied site. Thus, this method does not require any welding tape for the bonding and offers low cost and fast bonding. In this study, 100% PET fabric was used to create two-layered fabric structure in which the conductive yarns were inserted between those layers, that is, stainless steel yarns and silver-plated nylon yarns to create conducting lines. During the study, the effect of ultrasonic welding technology on the electrical conducting properties of the conductive yarns is investigated under different working parameters.
Materials and methods
Manufacturing of conductive transmission lines
Transmission lines were constructed using ultrasonic welding machine as shown in Figure 1. Conductive yarns were inserted between two layers of navy blue colored polyester fabric structure without any undulation in a straight form.
(a) An image of the ultrasonic welding machine and welded samples and (b) schematic diagram of welded samples.
Table 1 shows the measure properties of the polyester fabric used in the study. Noted here that, amount of the synthetic fiber within the fabric structures should not be less than 60% in order to have successful bonding between two fabric structures. Since the possible application of the proposed structures might be outdoor clothing, we specifically chose the 100% PET fabrics in order to have water repellent properties. Sample dimensions were chosen as 20 cm and 5 cm for the length and width, respectively.
Measured properties of the polyester fabric.
WVP: water vapour permeability.
Conductive yarns for this study are specifically selected based on their conductivity level and inherent flexibilities. We have two different categories of conductive yarns, namely, stainless steel yarns and silver-plated nylon yarns. Table 2 shows the measured properties of the yarns.
Properties of the conductive yarns.
PA: polyamide.
In order to set working parameters for the construction of the transmission lines, preliminary study was conducted for the determination of the upper limit of working parameters. Here, the applied contact force on the samples was varied in order to see how this parameter affects the conductivity of the transmission lines. Here it should be noted that the maximum machine contact force value is 500 N and we applied 8%, 25% and 50% of the maximum force value during manufacturing of transmission lines. However, initial study revealed that while silver-plated yarns maintained their conductivity up the 30 N contact force level, upper contact force level was chosen as 250 N for the stainless steel yarns. Thus, contact force levels were chosen as 10 N, 20 N, and 30 N for silver-plated yarns and 40 N, 125 N, and 250 N for stainless steel yarns, respectively. In addition, samples were manufactured under different applied power value ranging between 230 watt and 400 watt. Those machine parameters were selected considering the polyester fabric structure of which its characteristics are specified in Table 1. Indeed, this study aims to cover the creation of signal transmission lines based on ultrasonic welding technology for being an interconnection between the electronic components of a wearable computing system particularly for outdoor clothing. For this reason, welding the hidden conductive yarns between the waterproof polyester fabrics will additionally enhance the durability of signal lines by protecting them from probable short circuits and water contact. The bonding strength of the fabric was measured as 3.30 N when the applied force was 40 N and the applied power was 400 watts.
Determination of conductivity of the samples
In order to see the affect of welding parameters on the conductivity of the conductive yarns, we measured the conductivity of the samples before and after the ultrasonic welding using Keithley multimeter. For this aim, we used four-wire measurement method that gives the most accurate way to measure electrical resistance of the samples. Lead resistances and contact resistances are automatically reduced using this method. In 4 wire resistance measurement, two leads are used to pass current, while the remaining two are used to measure the voltage drop.
Determination of the bonding strength of the welded samples
It is imperative to determine bonding strength of the samples for the real life applications. It is obvious that if any delamination occurs between the welded samples, conductive lines will be prone to environmental effects and this will deteriorate the signal quality of the system. Thus, we tested the bonding quality of the samples initially before measuring electrical properties of samples. Titan Tensile Test machine was used in order to measure bonding strength, and the bonding quality of the samples was evaluated according to standard DIN 54310. Figure 2 shows the test rig and sample.
Test rig for the determination of the bonding strength of the samples.
Results and discussions
Effect of ultrasonic welding technology on samples produced with stainless steel yarn
The effect of the machine working parameters such as power, contact force, and speed on the electrical conductivity of the samples was studied. Therefore, we alternated the parameters in order to see how these parameters affect the conductivity as well as bonding strength.
Effect of machine contact force on the conductivity of the samples
Figure 3 shows the change in resistance values according to the applied contact force values for the sample types 1, 2, and 3.
The effect of the machine contact force parameter on the conductivity values of the samples.
Herein, maximum machine contact force value is 500 N and we applied 8%, 25% and 50% of the maximum force value during the manufacturing. It is apparent from the graphics that increasing the contact force results in a slight decrease in resistance values. This phenomenon can be explained as when the force applied on the sample, filaments within the yarn structure come closer to each other, thereby reducing the length of the effective conductive pathway that causes decrease in resistance values. However, this decrease is no more than 1.5%, 2.2%, and 2.6% for the sample types 1, 2, and 3, respectively; thus, we can assume that alternating of the contact force values does not adversely affect the conductivıty of the samples for the practical applications.
Effect of ultrasonic welding machine working power on the conductivity of the samples
The power range was set to between 230 watt and 400 watt based on preliminary work. As it can be seen from the graphics shown in Figure 4, increase in power levels merely effect the electrical conductivity values of the each type of the samples.
Graphic shows effect of the machine working power on the conductivıty values of the samples.
However, there is a tendency that electrical resistance values of the samples increase by a small amount. Although it is not clear what really cause this change, we can assume that increase in power level may create some ruptures within the filament of the yarn structure.
Effect of contact force and power on the bonding strength of the samples
Figures 5 and 6 show the effect of the applied contact force and machine working power on the bonding strength of the samples. It is clear from the graphics that increasing both contact force and working power has resulted to obtain samples with higher bonding strengths. This could be attributed to the nature of ultrasonic welding technology since higher contact force together with high power causes melting of the synthetic fibers, thereby creating strong bond between the welded materials.
Graphic shows effect of the contact force on the bonding strength values of the samples.
Graphic shows effect of the working power on the bonding strength values of the samples.
Although the same fabric type and dimensions are used, bonding strength of the samples varies according to the type of the conductive yarn. It is clear from the graphics that samples manufactured with thickest yarn type (type 3) have lowest bonding strength. This can be explained as stainless steel yarn does not melt during ultrasonic welding and thus the areas containing stainless steel yarn do not adhere on top of each other. Since the yarn having large diameter covers the higher surface area, it reduces the bonding strength of the material.
Effect of ultrasonic welding technology on samples produced with silver-plated nylon yarn
The manufacturability of the e-textile transmission line containing conductive polymeric yarn for the ultrasonic welding technology was also investigated. Three different types of silver-plated yarns presented in Table 2 in detail (types 4, 5, and 6) were used. However, the maximum contact force level that could be applied to e-textile samples containing silver-plated yarns was set to 30 N and this force level is far below compared with samples with stainless steel yarn. It is observed that any force level above this point caused the samples to be non-conductive. Figure 7 shows the effect of the contact force on the conductivity of the silver-plated yarns (a) type 4, (b) type 5, and (c) type 6
The effect of the contact force on the conductivity of the samples containing silver-plated yarns: (a) type 4; (b) type 5; (c) type 6.
As it can be seen in Figure 7, highest change and variation were observed in type 6 samples. This can be explained as increase in contact force adversely affects the conductive yarn and finest yarns (type 6) are more prone to rupture. Consequently, rupture within the filaments of the yarns creates discontinuities in conductive pathway, thereby increase the electrical resistance of the samples. Figure 8 shows the images of the samples after increasing the contact force level of the ultrasonic welding and it is clear from the images that yarns have severe ruptures within their filaments with an increase in contact force. Further characterization of the samples was not conducted because of the low performance of the silver-plated nylon yarns that is explained in above statements.
Images show the ruptures within the filaments of the yarns after ultrasonic welding: power in watt; contact force (%, ratio to maximum force level); speed (ft/min).
Effectiveness of the ultrasonic welding technology for e-textile samples while in direct contact with the water
To demonstrate the effectiveness of the ultrasonic welding technology for possible short circuits while the e-textile structures are in direct contact with water, we analyzed the change in electrical resistance of the samples when water droplets were poured on the samples’ surfaces. As it is shown in Figure 9, the electrical resistance of the samples did not change; particularly the linear resistance value of the sample shown in Figure 8 stayed 4 ohms. Thus, it can be concluded that ultrasonic welding technology is suitable for creating e-textile structures by preserving their conductivity levels while e-textile structures are in direct contact with the water.
Images show the electrical resistance of the samples (a) before water droplet (b) after water droplet.
Conclusions
In summary, the usage of the ultrasonic welding technology for the creation of conducting transmission lines for wearable applications is investigated throughout this research work. For this purpose, different stainless steel yarns (with different linear densities and electrical conductivities) and different silver-plated yarns (with different linear densities and electrical conductivities) were combined with the polyester fabric via ultrasonic welding technology in order to create textile-based transmission lines. Ultrasonic welding machine working parameters such as contact force and working power were altered in order to see how these parameters affect the conductivity of the produced samples. It is observed that conductivity of the stainless steel yarns (types 1, 2 and 3) was merely changed for each type; thus, they can be considered suitable candidates for this technology. On the other hand, silver-plated yarns were severely affected by ultrasonic welding treatment due to the ruptures within the filaments of the yarns and their electrical resistance values showed huge variations even in low contact force values; thus, we found that application of silver-plated nylon yarns cannot be considered for the proposed technology.
Transmission lines manufactured using ultrasonic welding technology might be suitable for interconnection of wearable computing applications, smart clothing, and interactive e-textile structures. Welding the hidden conductive yarns between the waterproof polyester fabrics will additionally enhance the durability of signal lines by protecting them from probable short circuits and water contact. Thus, they may have a high potential application in e-textile circuit designs. However, the ultasonic welding parameters need to be carefully controlled according to type of conductive yarn and base fabric structure.