Multi-layer fabric using conductive yarn for connection of electronic devices

Excerpt: Consumer demand for more portable and capable electronic devices has driven the development and production of smaller and user-friendlier devices

Department of Textile Engineering, Chemistry and Science, North Carolina State University, Raleigh, NC 27695, USA

Abstract

The paper relates to electronic textiles for smart clothing. Consumer demand for more portable and capable electronic devices has driven the development and production of smaller and user-friendlier devices. Users expect greater functionality out of even smaller devices and carry with them devices that exhibit functionality that was previously not available or only available in non-portable devices. Garments included specialized pockets for phones, GPS devices and music players with built-in sleeves for routing cords for controllers or headsets. In recent years, various products have been introduced such as clothes with functional clothes incorporating Mp3 players, health care, heating system, digital color clothe, underwear for preventing missing children, etc. It is difficult to design electrically conductive textiles so as to correspond to the placement and form of electronic devices maintaining the dynamic wearing characteristics. Electrical circuits were added to fabric after fabric was woven. Attaching digital bands after fabric was woven reduced the flexibility of the fabric and increased post-manufacturing complexity.

Present paper deals with woven article and method to multi layer woven textiles having an electronic circuit woven therein using conductive & nonconductive set of yarns. It deals with method of manufacturing a conductive fabric without compromise in dynamic wearing characteristics & without the complexity of attaching electric yarns in the seams as conductive yarns are used in one set of warp and weft to weave the multi-layer fabric.

Introduction

Currently, research in the field of electronic textiles is very active, and although not a great deal of advanced electronic textile products can be found in the market-place today, it is expected that many new products will find their way to the consumers in the near future [7]. In many fields of endeavor, from military to sport to apparel, a desire exists for electronic circuits to be incorporated into fabric and into articles that may be made of fabric [1]. Bonding electronic device with clothe for making body-mounted wearable computers dates back to 1960s. Wearable computers can be operated at any time because power supply is always turned on and considered as computers combined with clothes[3]. Earlier electrical circuits were added to fabric after fabric is woven. Then came an arrangement for attaching electrical components to woven fabric consisting conductive yarn, such as by connecting the components to the conductive yarn by soldering or embroidery or by applique or by mechanical attachment, thereby adding additional steps and additional complexity to the manufacturing process[1]. In the ubiquitous era, it was demanded to communicate information by accessing a network in real time everywhere and every time. Accordingly, a digital garment worn by human was required to be connected to the near network and perform such function. As a result, digital yarn had been used and connected to seam portions of clothes required to make the clothes [5].

Accordingly, there was a need for a woven textile and article having an electronic circuit function woven therein. Fabric was woven using set of conducting and non-conducting yarn making a completely conductive woven fabric[1]. Extensive studies about high-performance textiles have been made in smart clothes. These textiles such as conductive textile materials, fabric signal line, fabric input devices, optical fibers, and so forth, perform a function to maintain transmit digital signals while producing a tactile feeling and physical properties identical to general textiles [3].

In recent years, various products have been introduced such as clothes with functional clothes incorporating Mp3 players, health care, heating system, digital color clothe, underwear for preventing missing children, etc. It is difficult to design electrically conductive textiles so as to correspond to the placement and form of electronic devices maintaining the dynamic wearing characteristics[3]. One of current approaches regarding textile article incorporating with electric component was to bond it on a flexible printed circuit board to a textile article. Obviously, the flexible printed circuit board could not be excessively bent or stretched due to limited flexibility[6]. The conductive fabric suffered from great limitations in fiber volume, washing characteristics, etc. New researches successfully produced conductive fabrics capable of generating an electric signal and transmitting the generated signal without any restriction to dynamic wearability, and a method for fabricating the electric fabric. Another invention successfully manufactured a was able electronic fabric [3].

Another problem, which aroused was if the circuits were not insulted completely, those electronic components would be easily short with the human body and result in injury to the one who wear the clothing made of those fabrics. Accordingly, a better structure and manufacturing method for conductive fabrics was essentially needed. On account of a layered structure (multi-layer weaving) of the conductive fabric, the circuits of the fabrics could work well without causing any short circuit that an electronic component could be attached onto it and function as well[4]. The conductive fabric had all electrical property, which included resistance, capacitance, inductance, impedance or reactance [8].

There has been a known range of woven electronic textile designing techniques for a flexible electrically conducting fabric. Mainly it is multi-layer textile fabric. It comprises of interwoven electrically conductive yarns, insulating (non-conductive) yarn and one or more electronic component (gadget) connected to the conductive yarns [10].

Background art

The need of body wearable computers has been from the 1960s. Technique of combining apparatus capable of creating electric signals and fabrics were formulated in various patents till the year 2000[3]. Later came printed circuit boards whose base is solid and its integration in textiles reduced quality and comfort properties of a product[10]. Obviously, the flexible printed circuit board utilized could not extensively bent or stretched due to its limited flexibility. Soon, conductive yarn was introduced which was incorporated in the fabric inside the seam or hidden in the background. But, there was a problem that it took a very long process time to make the digital yarn that could conduct electrons. In addition, additional work of connecting digital yarn to seam portions of clothes was required to make the clothes. Accordingly, manufacturing and processing became complicated[5].

Accordingly, there was a need for a fabric having an electronic circuit function woven therein. This was made possible using multilayer technique of weaving[1].

Objectives

The main objective of the inventions was to successfully produce a flexible conductive multilayer woven fabric, comprising conductive & non-conductive weft & warp yarn where the circuits of the fabrics can work well without causing any short circuit so that an electrical component can be attached onto it[4, 8, 9, 10].

Another objective was to fulfill the first objective without any compromise in flexibility, light weight, wearing comfort like general textile articles, rapid manufacture, low cost, ability to achieve larger area of conductivity, aesthetically pleasing overall design, no pollution, easy control of change of light source [6].

In short, it was needed to provide an electric fabric capable of generating an electric signal and transmitting the generated signal without any restriction to dynamic wearability. An electronic fabric circuit could be freely designed, regardless of the form or placement of an electronic device. Most importantly, provide a washable electric fabric and method of fabricating the same[3].

Another objective was to make a layered structure of conductive fabric where the circuits of fabric could work well without causing any short circuit[2,4]. Another object of one invention was to provide an electronic textile that could be cut to measure without losing its functionality[7]. Yet another objective was to provide a digital garment that could provide a high-speed communication path[5].

Method of manufacturing

Conductive yarn

Various methods have been employed in various inventions for manufacturing conducting yarn. The conductive yarn could be coupling yarn constituted by at least one conductive core filament. The conductive yarn can also be a doubled yarn constituted by at least one metal wire with textile fiber like polyester or by at least one metal wire paralleling with another metal wire or at least one conductive yarn coated with non-conductive material [6].

As in figure 1, the digital yarn 1212 may include at least one metal portion1212a that is located at the center about a diameter of the digital yarn, and a coating portion 1212b surrounding the outside of the metal portion so as to shield metal portion from electromagnetic waves. The digital yarn as in Figure 1 includes a metal portion located at the center and a coating portion surrounding the metal portion. In addition, a pore that is, vacant space 1212c may be formed between the coating portion and metal portion. Cover yarn 1212d may be additionally formed to cover the outer circumference edge of the coating portion [5].

The metal portion may be made of any one selected from copper, silver, gold, brass, platinum, aluminum, lead, iron or their alloys or mixture thereof[5, 9]. Conductive threads are also made of any conductive fibers with electrical conductivity, for example, stainless steel fibers, carbon fibers, sputtered silver or their combination[4]. The coating portion 1212b is formed to surround metal portion 1212a, which shields human body from electromagnetic waves generated from the metal portion. For this purpose, the coating portion may be formed of any one selected from ETFE (Ethylene tetra fluoro ethylene), FEP (Fluorinated Ethylene propylene), PVDF (Poly vinylidene fluoride), PFA (Per fluoro alkoxy) and its equivalents. The cover yarn 1212d surrounding the coating portion may constitute the normal warp[5]. It should be flexible & ductile and/or electrically insulative. This layer may be polymeric material, for example, polyethylene, polypropylene, rayon, nylon, acrylic, polyester or aramid[9]. The cover yarn surrounds the digital yarn, thereby protecting the metal portion and coating portion [5].

It is noted that the cored yarn is a basic conductive unit of conductive fabric with a good insulation property, and the cored yarn is flexible and could be easily wound around a shuttle so that the cored yarn could be easily adopted in any conventional textile machinery [4].

The core, which is conductive, may comprise of material, which is solid, liquid, gas or gel. Examples of conductive materials include metals & non-metals such as conductive polymers. The material having suitable degree of conductivity for a given application can be used for the core. The core may also include material, which is piezo functional, which translate pressure or touch into electromagnetic radiations such as a light signal. The piezoelectric material may be organic or inorganic and can include quartz, poly vinylidene fluoride (PVDF), apatite, aluminum nitride etc. [4,5].

Another method of making a conducting yarn as mentioned above is my blending polyester & metal fibers together and making a yarn out of it. This yarn is extremely pliable and soft to the touch so there should be no worry of loss of comfort when incorporating it into a fabric. Each spool of this Nm10/3 conductive yarn is composed of a 80% polyester 20% stainless steel blend, has a breaking load of 8094g, and has a surface resistance of <104Ω. Such a kind of yarn is available in the market and looks like as in _figure 2_. The blending of broken metal fibers, e.g. stainless steel fibers with natural or synthetic textile fibers to produce conductive yarn is the method employed. The conductive fibers are dispersed throughout the cross section of the yarn during spinning. However, since modulus of elasticity of metal and textile fiber differ significantly, it is difficult to reach and retain permanently homogenous metal distribution in the yarn [11].

In yet another method, a textile fiber/polymer is coated with stainless steel or other conductive material. As in figure 3, an SEM photograph of a coupling yarn 142 constituted by many polyester core filaments or nylon filaments 1424 tightly coupling with a rolled copper wire [6].

Conductive fabric

The textile body can be made in a knitting way, warp-knitting way, weft-knitting way, weaving way, or a braiding way. The fabric is manufactured using three warp layers and certain, simplified, weave layouts[7]. The fabric comprises of interwoven electrically conductive yarns, insulated (non-conductive) yarns and one or more electronic component. Lets take an example of simple LEDs, which can be connected to the conductive yarn. A matrix construction is used for the design of an electronic circuit display, so that the conductive yarns in the textile display are arranged in a weft and warp systems[10]

As in figure 4, a three-layer woven fabric 100 is shown having an upper layer 101 and a lower layer 102 which are separated by an intermediate warp layer 103. The upper warp layer comprises of an array of conductive warp yarns 104a-b separated by non-conductive warp yarns 105a-e, and the lower layer 102 have conductive 106a-b and non-conductive 107a-e warp yarns. The intermediate warp layer 103 here consists of all non-conductive warp yarns[7].

In the weft direction, one conductive weft yarn 108 from a first set of conductive weft yarns and two conductive weft yarns 109a-b from a second set of conductive weft yarns can be seen as in Figure 5 & 6. The conductive weft yarn 109a can be seen to alternately go over and under the non-conductive warp yarns in the lower warp layers 102 until crossing the non-conductive warp yarn 107d after which the conductive weft yarn 109a forms a loop 110 around three non-conductive warp yarn 107e, 111 & 105b from all three warp layers. Continuing to follow the same conductive weft yarn 109a, it next crosses the conductive warp yarn 106b in the lower warp layer 102 so that electrical contact is achieved there between [7].

The activation of the display requires a power supply which voltage depends on the amount of devices (light-emitting or other) being used and their technical characteristics [10].

As in figure 5 & 6, the fabric 8 is a cross sectional view of warp yarns 5. There are two sets of weft yarn 3 & 13 travelling across the three sets of warp yarns where 3 is a conductive weft yarn and 13 is a normal weft cotton/polyester yarn. The warp yarn 2 is a conductive warp yarn. 7 is a LED (or any other electronic device), which needs to be connected to the conductive warp 2 and weft 3 yarns. For this, conductive sewing thread 9 is used and LED 7 is connected to conductive weft yarn 3 at 11 and conductive warp 2 at 10[10]. The electronic component 7 as in figure 5 has two leads, which is used for being attached onto the conductive fabric. Similar with sewing buttons onto cloth, the electronic components could be sewn onto the conductive fabric by any conventional sewing machine. The electric component (LED) can function well when the first conductive thread and second conductive thread are electrically connected to the power system [4].

As in figure 7, conductive exposed portions 142̓ are separated from each other by non-conductive portion (yarns) 144 in the fabric 14. The diode-based lighting device 12 is fixed with two contact points 124 thereof to corresponding conductive exposed portions 142̓ to finish the illuminating textile article 1. The conductive yarns 142 also provide terminals for electric connection of a power supply 2 [6].

Primary claims

The primary claims made by patents is to produce a flexible electrically functional woven or nonwoven fabric comprising textile fibers & electrically functional fibers capable of providing energy storage and/ or electrical interconnection to an electrical component. The flexible electrically functional fiber comprises a core surrounded by an insulative coating [9]. A textile, for enabling connection of electrical devices, formed from interwoven electrically conductive and non-conductive yarns into a multi-layer fabric [6, 7]. The base layer composed of a synthetic, regenerated or natural fiber; and a conductive layer formed on the base layer to be capable of being freely formed by a pre-designed electric pattern[3]. The fabric circuit comprising electronic components (devices) and conductive sewing thread [4].

Future trends

As per the discussion till now, we have been successful in manufacturing a multi-layer conductive fabric to which an electronic component could be attached at conductively exposed portions. There is a lot of scope in manufacturing a consumer compatible product by attaching gadgets like keyboards, mp3 players, GPS attached to apparel. Lets take an example of a normal keyboard as in figure 8. It shows a normal hard keyboard with keys 40 which when pressed pushes 30 down connecting the conducting part of 30 to conducting part of 10 and the circuit gets complete and message is sent [3].

Now, as in figure 9 & 10, the inside layer of fabric 120 is a polymer coating to which a conducting area 130 is attached. When a particular portion of yarn gets pressed the conducting portion 130 gets connected to each other and circuit is complete[3]. It can also be incorporated with a piezo functional material, such as a piezoelectric or piezo luminescent material. In general, a piezoelectric material translates pressure or touch into an electrical signal, and a piezo-luminescent material translates pressure or touch into electromagnetic radiation, such as a light signal. The resulting electrical/light signals can be used in various electrical and optical circuits, respectively. The piezo electric material may be organic or inorganic and can include, for example, quartz, poly vinylidene fluoride (PVDF) etc. [9].

As in figure 11, sensors 1300 can be attached to the garment and electrically coupled to the conductive yarn 1200. The sensor can sense various physical signals such as movement, vibration, temperature and pressure of a user or peripheral objects and convert the sensed signals into electrical signals in a shirt 1000. The electrical signals converted from signals sensed by the sensor are inputted to the electronic device 1500 through the digitally conductive yarn 1200. The input device may be a keypad as in figure 10 or a touch screen type 1400 as in figure 11. The operation electronic device is connected to the conductive yarn, which received signals from the input device 1400 and sensors 1300. It performs a series of operations such as analyzing and processing the signals [5].

Concluding remark

Fabrics in modern life are mostly used for being woven into normal clothing. Those fabrics have no additional function except for keeping warm and pursuing fashion. Recently, with the rapid growth of technology, more functions of the fabrics have been developed to increase the convenience of human life. Clothing made of those fabrics with electronic components can be applied to many new fields. Research in the field of electronic textiles is very active and consumer has started expecting new products in this field, which will make their life easier. We found that electrically conductive fabrics are formed during the weaving process by a series of crossing conductive warp and weft fibers of a multi-layer fabric. These fabrics are very flexible, wearable, and washable and have the characteristics just like a normal woven fabric.

The electric component (device) can be attached to the conductive yarn running in the fabric. The conductive yarn is connected to the power source (battery) inside the fabric. Due to coating & multilayer weaving, proper insulating layer is provided to the conductive part. Floats are used to hide the lustrous conductive yarn from being seen on the surface of the fabric and hidden in between the other layers. It is possible to manufacture a keyboard on the sleeve of the fabric and may be, in the near future, we won't have to take mobile phones out of our pockets to operate it.

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