Electroconductive textiles and textile-based electromechanical sensors—integration in as an approach for smart textiles

Electroconductive textiles and textile-based electromechanical sensorsdintegration in as an approach for smart textiles

28

L. Guo, T. Bashir, E. Bresky, N.-K. Persson University of Borås, Borås, Sweden

28.1

Introduction

Both as a concept and as a term smart textile has two sides: the smartness and the textile-ness. Smartness is often realized by electronics, the textile-ness by fabrics and garments. Merging clothes with electronics is the archetypical instance of smart textiles. But soon enough when thinking further, and in accordance with the drive of science itself, it is recognized that generalizations are possible. Beyond electronics in all its already wide aspects one could introduce enabling technologies, ie, any addition from other domains of the universe of technology such as photonics, chemistry, biomedical engineering, nanotechnology, etc. And beyond garments, also interior as well as technical textiles can be endowed with smartness. The artefact that then results from the unification of the enabling technology and the textile shows a wider spectrum of functions than what a garment on its own expressda sock becomes both a garment with usual protective and comfort aspects and a medical measuring device for walking patterns. From a manufacturing point of view the adding of enabling technologies to textiles could be described in terms of integration in and integration on. “Integration on” means that the enabling technology in its physical manifestation is placed on the textiles. Examples are sewn pockets for sensor boxes, woven bands for smartphone holders and glued-on photochromic sheets. Thus, here smartness and textile-ness are relatively separateddin many cases, in fact, easily spatially detachable. The textile is more or less only a carrier. “Integration in” means that the fact that textiles are around is taken in account. Textile manufacturing processes are used for creating the physical manifestation of the enabling technology. Textile properties are used for enabling the smartness functions. Examples are textile fibers that are electrically conductive. Knitted together they form a stretchable, foldable electrode. Smartness and textile-ness are intertwined and not easily spatially detachable. The textile is far from working just as a carrier. In the following discussion we will emphasize the integration “in” for smart textiles.

Smart Textiles and Their Applications. http://dx.doi.org/10.1016/B978-0-08-100574-3.00028-X Copyright © 2016 Elsevier Ltd. All rights reserved.

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Textile systems Garment Fabrics Fabric repeats (loops) Yarns Multifilaments Filaments Fibrills Polymer Monomer

Figure 28.1 The hierarchy of textiles.

A simple but important observation is that textiles are hierarchical; textile yarns consist of filaments, filaments of polymera1 chains, and polymer chains of monomers (Fig. 28.1). And yarns are put together in weft and warp systems, knitwear and braiding; fabrics thereof are sewn, laminated, and pressed together into textile products. Hierarchy increases the degree of freedom for how to create functions because enabling technologies could be added on a polymer level, on filaments, fibers, yarns, fabrics, or levels above. A similar example is dyeing, which could be carried out in many ways; spin coloring, yarn dyeing, fabric or garment color treatments, each connected to a specific level. All these conceptsdenabling technologies, physical manifestations, functions, integration in and integration on, textiles as hierarchical materials, as well as stimulus and response from and to the textile artefact vis-
a-vis the surroundingsdform the fundament on which the text is based. Out of all the imaginable enabling technologies, the field of electronics stands in a class of its own. This is due to a number of reasons, for example,

1

Occasionally metal or ceramic filaments.

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•

•

659

In contemporary technology electrics is a means for transporting energy, by this creating physical work, from one spatial position, A, to another, B. The distance between A and B, jA  Bj, could be on the order of nanometers as inside integrated circuits (ICs) or hundreds of kilometers in high power transmission lines in air or in sea cables. Competing means include mechanical transmission as in cars. In contemporary technology electrics is a means for transporting information from one spatial position, A, to another, B. The distance between A and B could be on the order of nanometers as in a transistor. The medium between A and B could be air or atmosphere or the inside of a signal cable. Then jA  Bj could be thousands of kilometers. Competing enabling technologies include photonics for which fiberoptics make it possible to send minimally distorted signals very long distances and radio technology. If sensors are described in a stimuluseresponse fashion, taking a wide spectrum of input (chemically, electrical, physiological, etc.) and leaving an output, it is a fact that most sensors have an electrical signal as output.

By this it is clear that unifying textiles and electrics opens up many interesting possibilities for energy transport, energy storage, information transport, actuation, and sensorics close to humans as human life and society are intertwined with textiles, in short, for smart textiles. In this chapter we will focus on conductive structures (by which we always will mean electroconductive, rather than, for example, thermoconductive, and skip the electro prefix) and conductive structures for sensoring the perhaps most fundamental of stimuli, namely mechanical ones. We will go from the general to the specific, ending up in a discussion of a number of directions and applications in detail, hopefully in a directly useful way for the reader.

28.2

Fundamental sensorics

Most often smart textile artefacts are defined in a sensor fashion, ie, applying a stimuluseresponse perspective with a wide spectrum of input possibilities, of chemical, electrical, mechanical, physiological, etc. nature, and giving some relevant output (most often limited to an electrical signal). Stimulus thus says something about the surrounding environment around the artefact. Perhaps the most fundamental class of properties that is of interest is the mechanical. This is because it includes such basic quantities as position, movement, speed, acceleration, elongation, impact forces, static and dynamic pressure, vibrations, torsions, and bending. In Table 28.1 a number of mechanical properties are listed in column 2. Mechanical sensors are those sensors that measure these mechanical properties. In order to get an output, it is necessary to take advantage of some mechanism, ie, some physical (technical, chemical, etc.) property or phenomenon or law that technologically is employed (column 4 in Table 28.1). For example, pressure can be measured by a spring arrangement that uses the (linear) elasticity in a Hooke’s law where elongation is proportional to the exerted pressure. There can be many mechanisms used in a given artefact and there can be many different mechanisms that can be used for measuring a given stimulus. Of course, within the realms of present technology there is a myriad of possible devices for performing different measurements of

Stimulus mechanical measurands

Examples of device types

Examples of mechanisms

Position and displacement (dimension)

Absolute (for position) and relative (for displacement), odometers, strain and displacement sensors

Optical, capacitive, resistive, inductive, Hall, magnetostrictive effect

Proximity and presence

Proximity sensors, motion detectors

Optical, capacitive, resistive, inductive, Hall, Doppler, magnetostrictive effect. Infrared sensitivity, microwave, ultrasonics

Mass and weight

Balances, scales

Balancing of the gravitation force, Hooke’s law, springs deflection, strain gauge, ie, length-sensitive electrical resistance

Speed (velocity, rotation, angular velocity)

Speedometers, pit logs, odometers, tachometers RPM gauge

Magnetic effect, torsion springs

Acceleration

Accelerometer, gyroscopes

Suspended spring, capacitance, resonant piezoresistive effect, diaphragm

Impacted force (static and inertial)

Load cells, elastic elements, force gauge, balances, strain gauge, bending elements

Hooke’s law, transforming force to displacement by an elastic element, piezoresistive effect, capacitive effect, direct or inverse magnetostrictive and magnetoelastic effects

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Mechanical properties

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Table 28.1 A number of mechanical properties as stimulus also called measurands, device types for mechanical sensors and some mechanisms often employed for the measurand in question. Piezoresistance is discussed further in the text. Magnetostriction is deformation induced by field, and inverse magnetostrictive effect is when magnetic properties are modified by the application of stress, resonant sensors have a vibrating solid that is sensitive for a given measurand

Manometers, barometers, pressure taps, pressure probes. Microphones, pitot probes, bourdon tube

The effect on the height of a column of liquid, deformations in flexible membranes amplified and made visible, resistance change strain gauge, resonant phenomena, piezoresistivity, diaphragm and plate capacitance, electrets

Flow

Orifice systems, pitot probes, hydrometer, anemometers

Floating: Archimedes principle, as for pressure, cylinder pistons, spring-loaded plug, diaphragm, Doppler, Coriolis

Level (of liquid in a vessel)

Level sensors,

Resonance, rotating paddle, capacitance, microwave/ radar, diaphragm, laser and photocell, magnetoresistance, air bubbling

Torque

Torque sensors, strain gauge

Hall effect, induction, resistance change strain gauge, inverse magnetostrictive effect to detect torque through the magnetization change induced by a torsional stress, resonant effect

Mechanical stress

Load cells, strain gauge

Hooke’s law, capacitance, resonant effect, resistance

Mechanical strain

Strain gauge

Deformed foil and change in resistance, piezoresistance, optical effects

Density and (dynamic shear) viscosity

Viscometers, densitometers, pycnometers, hydrometers

Balances/scales and volume measurements, inertial mass, gravitational mass, buoyance, Coriolis force, electrical induction

Sound and vibrations

Microphone, hydrophone, accelerometers

Induction, capacitance change, piezoelectricity, spring-mounted magnetic mass moving inside a wire coil to generate an electrical current, piezoelectricity

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Pressure (static and dynamic)

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stimulus as in Table 28.1. Some of these could be integrated with textiles. Most often this is done in an integration on fashion, sometimes creating a product that works and is practical but sometimes something that is clumsy, irrational to produce, and of single use character. Instead it is interesting to look at possibilities where the mechanism is realized by the textile itself, ie, integration in. We will describe a number of cases where this approach is taken. As textiles are per definition made of fibers, it is important to start looking at the different variants of these building blocks. Thus, conductive fibers are discussed in Section 28.3. Mechanical sensors that give an electrical output that are realized in textiles are then discussed in Section 28.4. The physical phenomenon of piezoelectricity is studied as well as piezophenomena in general. Piezoelectricity is the interplay of mechanical work (compression or elongation) and electrical voltage and acts as a mechanism. We also look at how capacitance can be integrated in textiles for mechanical sensorics.

28.3 28.3.1

Electroconductive textile structures Definitions

In the introduction electronics was put forward as an enabling technology. A key concept here is electrical current. Electrical current is the same as charges in motion. Charges are by no means rare. On the contrary they are always around in a physical matter, although not evenly distributed but concentrated on certain species, charge carriers. These charge carriers are then related to other species of many different types: (positive and negative) polarons, (positive and negative) bipolarons, solutions, etc. For our purposes it will be enough to simplify and discuss charge carriers as negative-charged electrons, e, and positive-charged holes, hþ. Holes are the same as the lack of electron at a position in an atom. Simplified, electrons are the charge carriers in metals and electrons and holes are in what are called semiconductors. To come into motion charge carriers have to be mobile. Mobility is a highly material-dependent property and is different for different charge carriers in the same material. For motion the charge carriers also need to experience an electrical field, in turn due to voltage, an electrical potential difference. It is the maintenance of this that needs energy and is realized in the form of batteries or power generators. A number of formulas can be found in Table 28.2. As conductivity, in accordance with formula [28.9] in Table 28.2 relates to the number (per volume) of charges, and charges as said are always around, the phenomenon of conductivity is also in principle always around. But it is just levels that need moderate voltages (in turn, moderate energies) to create current that are of any practical use. A coarse classification is to divide materials into insulators (below 103 S/m), conductors (above 106 S/m) and semiconductors in between. Unfortunately, the term semiconductor refers not just to “half-good conductors,” which might be confusing. Semiconductors most often refers to those materials that are the basis for devices that are steerable in a certain fashion such as diodes, transistors, etc. ie, the basics for computers. We leave the details aside.

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Table 28.2 Some simple formulas illustrating relationships between a number of basic electrical quantities DQ Dt where I (Ampere, A) is electric current and DQ (Coulomb, C) is the number of electrical charges per time Dt (s). Ih

The electron mobility is defined by the equation: vd ¼ mE where vd is the charge carrier drift speed and E is the magnitude of the electric field applied to the material, and m is the charge carrier mobility ((m/s)/(V/m) ¼ m2/(V$s)). Both electron and hole mobilities are positive by definition. A simple relationship between the voltage (volt, V) U and E is U E¼ d for an arrangement where there is a voltage U between two electrodes the distance d apart.

[28.1]

[28.2]

[28.3]

Further: I¼

1 U R

[28.4]

which is the Ohm’s law where U is the voltage that is “trying” to drive a current, which is counteracted by the resistance (ohm, U). A case where there is a more complicated relationship between I and U is the diode equation:  Ue  I ¼ I0 ehkT  1

[28.5]

with I0 a constant in a given arrangement, h, ideality factor dependent on material, k Boltzmann constant, 1.38$1027 J/K, T absolute temperature, e electron charge 1.602$1019 C. P ¼ UI ¼ RI 2 Relationship between the power production/consumption, P, and quantities in Ohm law. l A Sometimes called Pouillet’s formula, which applies to simple, uniform, homogenous conductor of length l (m) and constant cross section, A (m2). For complicated, composed materials as textiles this formula is too simplified. r is resistivity (Um) a material parameter. R¼r

1 r where s is conductivity ((Um)1 ¼ S/m, Siemens per meter) s¼

s ¼ neme þ pemp

[28.6]

[28.7]

[28.8]

[28.9]

n is number of electrons per volume, n is number of holes per volume, me is electron mobility, mp is hole mobility. Continued

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Table 28.2

Smart Textiles and Their Applications

Continued

Capacitance, C: Q V In special case of parallel plate capacitor, Ch

A d A distance, d, between plats of area, A (neglecting rand effects) with εr relative permittivity, ε0 permittivity 8.854$1012 As/Vm C ¼ εr ε0

Between the mechanical force, F, and the elongation, Dx it is common to apply Hooke’s law F ¼ kDx where k is the spring constant characteristic for each elastic system considered.

28.3.2

[28.10]

[28.11]

[28.12]

Types of electroconductive textiles

There are now many different types of textiles fibers, filaments, yarns, and fabrics that are electrically conductive. We focus here on electrons and holes as charge carriers, leaving for example, polymeric ion conductors such as poly (ethylene oxide) and others aside. Accordingly, there are also many possible ways of classifying, according to conduction mechanisms, charge carriers, type of textile structure, material, conductivity level, etc. We will follow a somewhat mixed approach starting from a material perspectivedinorganic-organic types of substances together with conductor substance application (in, or on or around the (classical) textile part). We propose the following seven classes of conductive fibers and yarns for textile use: 1. Metal(mono)filaments. Mono- and multifilaments completely made of metal (alloys) such as copper, stainless steel, etc. Multifilaments here are often referred to as wires. 2. Co-spun polymer-metal yarns. These consist of a polymeric textile fiber and a metal filament part united by a yarn spinning method. 3. Metal-coated polymeric filaments and yarns. Hybrids so that traditional polymeric fibers and yarns are coated by a metal. 4. Metal-filled polymeric filaments. In the filament production process metal flakes or metal powders are added. 5. Carbon allotropes as the conductive agent such as for carbon fibers, or when in the synthetic polymeric filament production step conductive particles such as carbon black, graphite, graphene, or carbon nanotubes are used. 6. Conductive polymer such as polyaniline (PANI), polypyrrole (PPy), poly (3,4-ethylenedioxythiophene) (PEDOT) as the conductive agent, most often blended with a common fiber forming polymer (PA, PES, etc.) in the extrusion. 7. A mechanically stable common textile fiber is coated by a conductive polymer.

28.3.2.1 Class I: metal filament The history of metallic fibers is about 3000 years old. In fact, the first man-made fibers used in textiles were not nylon or rayon but silver and gold. When the highest level of

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electrical conductivity is needed, the use of metallic fibers or filaments in the yarn production process should be considered. One has to be aware that different metals (and alloys) behave differently and not all are useful as conductors. Threads of stainless (Fig. 28.2, left) have moderate values (1.3 $ 106 S/m for intrinsic materialsdvalues for a certain yarn or textile component might here and in what follows be much less, due to protected and insulating chromic oxide layer on the surface as well as due to morphology. Also, for copper (Fig. 28.2, right)doften an excellent conductor (58$106 S/m)done has to be aware of the copper oxide protective layer developed in air. Also important to observe is if the copper wire is polymer coated or not. Both types exist. Although the conductivity of silver (62$106 S/m) and gold (44$106 S/m) are very high, wires thereof are almost never used outside labs due to the high price. Aluminum (37$106 S/m), and alloys thereof, exist as textile processable filaments but conductivity is in practical applications low due to the passivation layer, ie, aluminum oxide layer, developed on the surface. One has to be aware not to confuse these given conductivity values with the linear resistance (see Section 28.3.3.1 below). Very thin metallic filaments with diameter ranging from 1 to 80 mm up to some mm can be produced by wire drawing, bundle wire drawing, melt spinning, or shaving process [1,2]. These are being widely used as textile transmission lines, heating elements, antimicrobial agents, antistatic interfaces, electromagnetic interference (EMI) shielding, cutting resistance, and sensors in apparel applications [3]. Despite the fact that metallic fibers have good electroconductive properties, they have sometimes limited textile applications because of their relatively high weight, low flexibility, thus low comfort quality, stiffness, low compatibility with other (polymeric) materials, higher cost, and sometimes tricky weaving and knitting properties [4]. Apart from these drawbacks, almost all metals would tend to tarnish with the passage of time due to corrosion if not protected. In open environment, the metal oxides formed on the surface of all metals, except gold, create a product that has worse electroconductive properties as compared to pure metal alloys. Due to the development of

Figure 28.2 Commercially available metallic monofilament, stainless steel (left), copper (right).

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Smart Textiles and Their Applications

the modern surface protection techniques, cheaper metallic fibers could be used in textile applications. The surface-protected metallic fibers, such as anodized and dyed aluminum filaments that are manufactured not only have colorful appearance but also corrosion resistance.

28.3.2.2 Class II: co-spun polymeric-metal yarns The mechanical properties of a conductive material play an important role when selection is performed for any particular textile application. Metallic filaments usually compromise good electroconductive characteristics with poorer mechanical properties. A very high stiffness and lower stretchability of metallic fibers not only makes the woven or knitting process difficult but also reduces their service life. On the other hand, polymeric fibers or yarns exhibit good elongation and recovery properties. The combination of nonstretchable metallic fibers with stretchable polymeric yarns creates a new class of metal-based electroconductive fibers, which is known as co-spun polymeric-metal yarns (Fig. 28.3). In this class, the highly conductive thin filaments or yarns are either twisted or wrapped around or incorporated in the conventional textile yarns. The hybrid yarn obtained after this combination should have enhanced elastic behavior, which is important not only for comfort but also for processability. In order to spin the metallic fibers or filaments with other textile fibers, a number of methods, such as friction spinning, ring spinning, and hollow spinning, are being used [5]. The most extensively used metal yarns in these kinds of combinations are 12 mm/91 filaments and 25 mm/91 filaments. The produced hybrid yarns can be used for cut resistance apparel, antistatic brushes for industrial machines, antistatic filter bags, and lightning strike protection, and for signal and power transmission in smart and interactive applications [3,6].

Figure 28.3 Co-spun metallic-polymer yarns made in-house at Swedish School of Textiles. (left) cotton Ne 16/1 þ Cu 0.08 mm gives 100 Sm (right) PET 600 dtexdstainless steel 0.07 mm.

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28.3.2.3 Class III: metal-coated polymeric filaments and yarns In order to unite the electrical properties of metals and mechanical properties of textile materials, there is another approach to produce the conductive yarns that is called “textile metallization.” In textile metallization process, in its general sense, metallic particles are deposited on the surface of conventional textile materials [7]. Depending on the type of textile substrates and coated materials, various coating techniques can be used. The more commonly used coating technologies are vacuum deposition, electroplating, ion plating, and electroless plating [7,8]. By applying these highly conductive coatings of metals and metallic alloys on “normal” textile fibers and filaments, conductive textile structures can be obtained. The metallized conductive fibers/yarns usually exhibit good, useful levels of conductivity along with good mechanical properties. Silver (62$106 S/m) is the choice here in spite of being expensive and with environmental issues [9], and different types of silver-coated yarns are commercially available (see Fig. 28.4). Conductive coatings either can be applied on a fabric structure or a fabric structure can be produced from coated yarns. The metallic coatings on textile materials not only introduce a similar level of electrical conductivity as pure metals but also produce decorative and protective effects. The metal-coated textile fibers can withstand temperatures up to 700 C for longer periods of time [10]. The aluminum and gold coatings on textile fibers can reflect radiant heat up to 90% and 100%, respectively [6]. Despite of all these advantages, there are

Figure 28.4 Commercially available silver-coated polymeric yarns with high conductivity.

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also drawbacks: relatively high weight, stiffness, high cost, brittleness, adhesion problems, sometimes poor corrosion resistance, and lower comfort level. The metallized textile fibers are being widely used in a broad range of application areas such as military, decorative wall coverings, medical, EMI, and fabric design applications.

28.3.2.4 Class IV: metal-filled polymeric filaments Another way to introduce conductivity in nonconducting materials is to blend good fiber-forming materials but inherently insulating ones, such as polyethylene, polypropylene, and polystyrene, with highly conductive materials, such as metals in the form of flakes, spheres, etc. resulting in a conductive system that can be spun into textile fibers. The most common and economical method used for the production of conductive composite fibers is melt spinning. The electrical and mechanical properties of produced fibers depend on many factors and the most important one is concentration and volume loading of metallic fillers. Metals could be added as particles that have a high aspect ratio (ie, the ratio between the longest length and the typical breadth) as this supports percolation, a phenomenon where conductive parts are able to connect via the network they form throughout the material. The metal-filled conductive fibers could have better electrical conductivity values with higher concentration of fillers, but then one has to compromise on mechanical properties. So, there will always be an optimum concentration of conductive fillers added to get the better mechanical properties of produced fibers. The metal-based blended fibers are suitable to make protective fabrics that could protect individuals from the hazardous effects of electrostatic discharge and electromagnetic radiation. However, due to the higher stiffness, lower flexibility, lower comfort level, and higher weight, they still have limited use in apparel applications.

28.3.2.5 Class V: carbon allotropes There are many different forms, allotropes, of carbon, where the carbon atoms are bonded differently to each other: diamond, graphene, graphite, fullerenes, carbon nanotubes, etc. It is fascinating that some of these are among the best electrical insulators (diamond) and some best conductors (graphene) known. For textiles, most well known are carbon fibers. Due to orientation of the carbon structure, mechanical properties are very good along the fiber, Carbon fibers are also thermal stable up above 100 C. Pure carbon fibers having graphite structure exhibit very high conductivity values similar to metals, ie, 104e106 [11]. The textile structure from pure carbon fibers could be possibly used to make composite materials. Mostly, carbon fiber composites are used in structural applications where high strength, stiffness, lower weight, and extraordinary fatigue characteristic are needed [12,13]. However, in clothing industry, the use of carbon fibers is limited because of their stiffness and brittleness, which makes especially knitting processing difficult. Weaving is more easily performed. Not only this, their esthetic considerations and health-related issues are also strong reasons for restricted use in apparel applications.

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Conductive composites or conductive-filled fibers can also be produced by adding conductive fillers such as carbon black, carbon nanotubes, graphite, and graphene to nonconductive materials like polyethylene, polystyrene, or polypropylene [14e16]. The electrical conductivity level of composite fibers strongly depends on two factors: volume loading of conductive filler and the filler shape [17]. Comparatively higher electrical conductivity can be achieved by adding fibrous fillers in the spinning process. Usually, melt spinning and solution spinning processes are used to produce the filled conductive fibers. However, in order to obtain good levels of bulk conductivity, the premixing step is necessary before the actual spinning process. Among the spinning techniques, melt spinning is the most economical and least complex process for the production of filled conductive fibers. Along with other conductive fillers, carbon nanotubes (CNTs) have exceptional properties such as unique one-dimensional structure, high aspect ratio, and superior mechanical, thermal, and electrical characteristics. The CNTs-based nanocomposites have been used in many sophisticated applications such as nanoelectronic devices, nanoscale structural materials, actuators and sensors, and functional fabrics for smart textile applications [18e20]. For textile applications, CNT-composite fibers can be produced by several methods such as electrospinning, electrophoretic spinning, solution spinning, melt spinning, and direct yarn spinning [5]. However, the lower electrical and mechanical properties of produced conductive fibers limit their use in smart and interactive textile applications. The reason for this is most often aggregation of the carbon particles. Dispersion is very critical for obtaining a good conducting carbon-containing textile fiber (Fig. 28.5).

28.3.2.6 Class VI: conductive polymers Polymers are usually considered as insulators because of their higher electrical resistance values, ie, 1018 S/cm [21]. However, there is a new class of polymers that is known as intrinsically or inherently conductive polymers (ICPs) and also as conjugated polymers or unluckily as organic metals and electroactive polymers, which should be reserved for actuating materials. They are still in a developing phase,

Figure 28.5 Polyester (PES) carbon fiber co-spun (left) and two types of carbon fibers (right).

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Smart Textiles and Their Applications

O

N H

O

S

n

n

S

n Polyethylenedioxythiophene (PEDOT)

Polypyrrole (PPy)

NH

N

Polythiophene (PT)

N

H N n

Polyaniline (PANI)

Figure 28.6 Chemical structures of some commonly used inherently conductive polymers.

even if the area of research was rewarded with the Nobel Prize in Chemistry in 2000 (Alan J. Heeger, Alan G. MacDiarmid, and Hideki Shirakawa). The most commonly used ICPs are polyaniline, polypyrrole, polythiophene, and poly (3,4-ethylenedioxythiophene) (PEDOT) and derivatives and copolymers thereof. The chemical structures of these conjugated polymers are shown in Fig. 28.6. During the last decade, they have attracted considerable attention due to their rather high conductivity, low weight, and environmental stability. They have a wide range of conductivity values ranging from 108 to 105 S/cm [21] where the higher values seldom are obtained outside labs. Due to their advantageous nature, they are considered as an efficient alternative to conventional conductive materials in the field of functional textiles. However, the production of all-organic fibers from ICPs is still complicated and expensive. Several attempts have been made to obtain conductive fibers from ICPs such as polyaniline, PEDOT:PSS, and pure PEDOT with conductivity values from 150 to 250 S/cm [22,23]. However, due to poor mechanical strength, microscale size, a low production rate, brittleness, and difficult processing, useful commercial applications are still limited. On the other hand, combination of other textile materials with ICPs could enhance their application areas. It could be done in two ways, either by mixing ICPs with insulating polymers such as polypropylene, polyethylene, and polystyrene, or by coating conventional textiles with ICPs. The coating method is explained in the next section. All polymer electroconductive fibers can be obtained by blending ICPs with common polymers. At our in-house Smart Textile facilities we produced PP/PANI and PP/ PA/PANI composite fibers, Fig. 28.7. The production of these fibers involved two steps: compounding and spinning. For fiber spinning, the melt spinning process is the most suitable method. The ICP-based electroconductive fibers we have produced

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Figure 28.7 Conductive polymer PANI added to PP made in-house at Smart Textiles, Swedish School of Textiles.

exhibit 102 S/cm conductivity value and acceptable mechanical properties to integrate them in some textile applications [24,25].

28.3.2.7 Class VII: nonmetallic conductive coated fibers The second way to enhance the usability of ICPs is to apply coatings thereof on textile materials. A very thin layer of conductive polymers can be applied on the surface of textile substrates by solution casting, inkjet printing, in situ polymerization, vapor phase polymerization, and chemical vapor deposition techniques [26e29]. The nano-microscale conductive coatings not only provide high level of conductivity but also preserve the flexibility and elasticity of substrate fibers. However, due to the health-related issues of some carbon-based materials one has to be observant about what is possible and what is not in apparel applications. On the other hand, intrinsically conductive polymers with high conductivity, good environmental stability, lower weight, and high compliance are good alternatives to conventionally used conductive coatings. However, difficult processing, poor adhesion with substrates, and durability issues limit the application areas of ICPs. The direct deposition of ICPs on the surface of textile substrates not only provides high electrical conductivity but also preserves the parent properties of substrate materials. In our lab we transformed the conventional textiles into electrically conductive materials by applying very thin layers of conjugated polymer. For this purpose, an efficient coating technique called chemical vapor deposition (CVD) was used to coat the textile substrates with conjugated polymer, poly(3,4-ethylenedioxythiophene) (PEDOT), in the presence of a suitable oxidant. A schematic diagram of all steps involved in this coating process is shown in Fig. 28.8. The direct deposition of PEDOT on the surface of textile substrates produced the relatively highest level of electrical conductivity. We successfully coated different textile materials, such as viscose, polyester, aramid, and polyamide fibers, with this method and achieved approximately 15 S/cm conductivity values [31e34].

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Smart Textiles and Their Applications

4 3

PEDOT + substrate before doping FeCl3

Substrate + FeCl3

2

solution EDOT recovery

FeCl3 solution

Vacuum N2 gas EDOT vapors

EDOT

1

Heating

PEDOT + substrate after doping CVD reactor

5 Methanol

Textile substrate

PEDOT coated textile substrate

Figure 28.8 Schematic diagram of chemical vapor deposition process and all steps involved in this method [30].

From this also fabrics could be made. An all-polymeric textile stretch sensor was prepared from the produced PEDOT-coated yarns. The knitted structure, shown in Fig. 28.9, was made along with pure polyester yarn. The stretch sensing properties of this knitted patch were determined on a cyclic tester. The stretch sensing behavior of PEDOT-coated yarns is shown in Fig. 28.10. It was concluded that even after several extensionecontraction cycles, the knitted structures preserved their stretch recovery and electrical properties [35].

Figure 28.9 The PEDOT-coated yarn (black) knitted together with polyester yarn (white), together making an all-polymeric textile stretch sensor.

Electroconductive textiles and textile-based electromechanical sensorsdintegration

95

0

100

200

85

5% extension

85

200 25% extension

65

10% extension

75 65

15% extension

75 65

20% extension

75

Resistance (kΩ)

Resistance (kΩ)

100

75

75

65

0

673

30% extension

75 65

35% extension

70 60

40% extension

70 0

100 Time (s)

200

60

0

100 Time (s)

200

Figure 28.10 Stretch sensing behavior of PEDOT-coated yarn at different extension (%) [35].

28.3.3 Measurement of electroconductive textiles Characterization of textiles that have both a classical textile dimension and an extra electrical one needs consideration. Compared to classical electrical devices, textiles are soft, flexible, and of poor dimension stability. Using conventional electrical measurement methods and devices may result in imprecise or even wrong measurement results. In this chapter, the suitable resistance measurement methods for both conductive fibers and yarns and textiles are introduced. Special measurement devices are demonstrated. In addition, the measurement dynamic electrical properties, ie, the electromechanical properties, are also presented.

28.3.3.1 Resistance measurement of conductive yarns The central measure for characterization and comparison of conductive materials is of course the electrical conductivitydor, given the same amount of information, electrical resistivity is used. We rewrite formula [28.7] from Table 28.2 as: The electrical resistivity r is defined as r ¼ R

A l

where, the electrical resistivity is r, l is the length of the piece of material (measured in meters, m), A is the (assumed uniform) cross-sectional area of the specimen (measured in square meters, m2), and R is the measured electrical resistance of the materials (measured in ohm, U). However, most often, electroconductive textiles are in principle composites combining insulating materials (polyester, polyamide, etc.) and conductive

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materials (stainless steel, silver, copper, etc.). Neither A nor l is well determined. The overall resistivity of the material is much different from that of the conductive components. Therefore, the resistivity most often becomes complicated and meaningless in an electroconductive textiles context. When choosing electroconductive yarns, the linear resistance should be used as the critical parameter rather than the overall resistivity. The linear resistance is calculated as the resistance per unit length (measured in U/m). Linear resistance can be simply measured by an ohmmeter (Fig. 28.11, top), a so-called two-points measurement method. The equivalent circuit of this measurement setup is shown in Fig. 28.12. The ohmmeter is actually giving the total resistance of the subject and the measurement wires. This method is applicable when the resistance of the measured subject is much greater than the wires. However, conductive yarns are usually expected to be highly conductive, and the measurement error introduced by the wire resistance will then be substantial.

2 points measurement

Ohm-meter R

Conductive yarn 4 points measurement

A V

Conductive yarn

Figure 28.11 The linear resistance measurement of a yarn using two-point measurement (top) and four-point measurement (bottom) methods.

Ω

R_wire R_subject

R_wire

Figure 28.12 The equivalent electrical circuit of two-point measurement setup.

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A R_wire

+ –

V

R_subject

R_wire

Figure 28.13 The equivalent electrical circuit of four-point measurement setup.

A better method of measuring yarn resistance is using a four-point measurement setup. This method involves the use of both an amperemeter and a voltmeter; the resistance is possible to determine from Ohm’s law (Fig. 28.13). The current is the same at all points in the circuit. In this method, voltage drop across the subject resistance is measured together with the current, therefore, the calculated resistance is closer to the real value of the subject component (the conductive yarn). Four-point measurement is preferable when resistance of the conductive yarns is low. In practical terms, the electrical resistance measurements on conductive fibers/yarns is always problematic due to their soft, flexible, and poor dimensional stabilities. Very few publications have reported research regarding electrical measurements on fibrous structures. Usually, the conventional method, in which crocodile clips are attached with a voltmeter, is used for this purpose. Crocodile clips hold the conductive fibers/yarns of specific length and then electrical resistance is measured on particular voltage values. However, the hard grip of crocodile clips damages any conductive coatings or creates internal cracks in the fibrous structures, which cause the permanent loss in electrical properties of conductive threads. Consequently, consistent results with crocodile clips cannot be obtained. We have developed a novel electrical resistance measuring setup, which can not only be used for fibrous structures but also for fabric samples with particular dimensions. The complete setup is shown in Fig. 28.14. This setup basically has two parts: (a) a Kiethley picoammeter 6000 with a voltage source including a high-impedance voltmeter; these two instruments are connected to a computer (right-hand side of Fig. 28.14); and (b) a sample holder (at the left of Fig. 28.14). The sample holder is manufactured in-house and can be seen in Fig. 28.15(a). The test object, of a certain specific length, is fastened at one end into a holder (7 in Fig. 28.15(a)). The other end of the test object is pulled down by a weight, (8) for example 50 g, to ensure that the test object is stretched and also in contact with two or four wheels, as shown in Fig. 28.15(a). This setup can also be used to measure the electrical resistance values of fibers during their production. The sample holder should be placed just before the final winding of fiber on the bobbin. With the movement of the fibrous sample, the wheels would also rotate; thus, the surface morphology of the object would not be affected because of its friction with the surface of the wheels.

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Figure 28.14 Electrical resistance measuring setup used for fibrous structures.

Figure 28.15 Manufacture sample holder (a). It has four essentially identical units to support the test object, which could be a single thread or a piece of fabric. The wheel holders (2) are made of aluminum, and the rods (3) that hold them are made of steel. The intermediate rods (4) are made of Teflon to assure electrical insulation from the common holder below. The Teflon holder is attached to a steel rod that sits on an aluminum plate (5) that could be loosened and moved in the slot of the aluminum profile (6). (b) A single brass wheel. It has four essentially identical units to support the test object, which could be a single thread or a piece of fabric. The wheels (1) are made of brass, with dimensions R1 ¼ 30 mm, R2 ¼ 40 mm, and L ¼ 50 mm.

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28.3.3.2 Surface resistance measurement of conductive fabrics Surface resistance measurement of fabrics is mostly adapted from the ASTM D-257 standard [36]. Concentric ring probe is used (Fig. 28.16) in this method. In the standard method, a constant voltage is applied across the probe. Current measured during the test is given by an amperemeter. The surface resistivity can be calculated by knowing the resistance outer radius of the center electrode (R1) and the inner radius of the outer ring electrode (R2) and the geometry coefficient of the probe [37].

R2

Sample

R1

Figure 28.16 Surface resistance measurement configuration for concentric ring probe [37].

In contrast to many bulk materials, textiles have high porosity and often show anisotropy. Therefore, the surface resistivity is not a valuable indication in most of the cases; instead, the surface resistance per unit area is more critical. Preferably, the resistance per unit area should be given with direction. An example of a probe [38] specific for textile surface measurement is shown in (Fig. 28.17).

Figure 28.17 The surface resistance measurement probe and the measurement setup.

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Figure 28.18 The linear electromechanical testing setup.

In this setup, the surface resistivity in warp and weft directions or any diagonal directions can be measured. The probe was made of an insulating body onto which four metal electrodes were placed parallel to each other. The two outer electrodes were connected to the current, whereas the two inner electrodes formed a square over which the voltage drop was measured.

28.3.3.3 Measurement of linear electromechanical properties Changes in electrical quantities in a material such as textile when under mechanical strain are the basis for electromechanical sensorics. The electromechanical properties of the conductive yarns/fabrics can be determined by a tensile tester and a multimeter [39] (Fig. 28.18). Resistance can for example act as the interesting electrical quantity. The mechanical quantity is expressed by the resistance change (DR/R) against elongation (ε ¼ DL/L). Fig. 28.19 gives an example of how resistance changes as a function of elongation [39]. Conductive materials with high electromechanical sensitivity can be used as strain sensors; on the other hand materials with stable electromechanical property are ideal for data transmission usage.

28.3.3.4 Measurement of cyclic electromechanical properties Cyclic electromechanical property is another important characteristic of conductive yarn/fabrics utilized as strain sensors. Stable cyclic electromechanical properties

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1.2

Normalized resistance (R/R)

1

Data 1 Linear

y = 0.026*x – 0.029

0.8

0.6 0.4 0.2

0

0

5

10

15

20

25

30

35

40

Elongation (%)

Figure 28.19 Relative resistance change as a function of elongation up to 40%.

indicate low hysteresis. To measure the cyclic electromechanical properties, a cyclic force is applied to the testing specimen by a cyclic tester. The cyclic tester is a home-built device with a fixed end and a moveable end [40]. The speed of the moveable end can vary between 5 and 50 mm/s. The length of the whole testing device is 400 mm, and 64 different steps can be customized by varying the speed and position of the moveable end in order to record the resistance change under the applied cycles. The cyclic tester is operated together with a multimeter. With the help of the LabVIEW program, the multimeter can automatically record a graph between resistance and time, and by varying the time of the stretching in the cyclic tester different graphs can be obtained. Both these devices are connected together through software and the whole setup is shown in Fig. 28.20.

Figure 28.20 Cyclic tester along with attached sample sensor, Kiethly (multimeter) and software.

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28.4 28.4.1

Smart Textiles and Their Applications

Textile-based electromechanical sensors Textile-based piezosensors

As mentioned at the end of Section 28.2 we focus here on sensors measuring mechanical measurands based on the mechanisms of piezo and capacitance (in Section 28.4.2).

28.4.1.1 Piezo phenomena Piezoelectricity, from Greek plεzεln meaning to press, to squeeze [41], is the interplay between mechanical and electrical features of a material or a device. Changing one will impact the other. There are different types of materials that show these properties. Quartz, topaz, and tourmaline minerals; human and animal bone tissue; different proteins; Rochelle salts (sodium, potassium tartrate tetrahydrate); barium titanate, lead zirconate titanate, PZT; and the polymer poly (vinylidene fluoride) (PVDF), are some examples. Piezoelectricity, compared to many other areas of science, was relatively recently discovered, by Jacques and Pierre Curie [42] working with quartz among other minerals. It is obvious that materials with mechanical stresseelectricity interactions are technologically very interesting; sensors can be made that catch mechanical vibrations and transform those to an electrical signal, mechanical tensions can be measured electronically, and so on. Piezoelectric systems are not the only mechanism for mechanical sensorics (see Table 28.1) but are certainly abundant and sometime very practical to work with. Also, within the textile community there is increasing interest in piezoelectricity. As bonds that keep materials together are electrons, and electrons are the basis for electricity, it is not strange that there is a connection between material mechanics and material electronics. Perhaps it is more surprising that the list of piezoelectric materials is not longer, and further more, consists of somewhat odd materials. The reason for this is that there need to be certain possibilities for ordering of the charges. In most materials positive and negative charge densities are counteracting each other and any piezoelectricity is lost. It is then not surprising that many piezoelectric materials are crystals, having a high degree of order. But crystals need to be noncentrosymmetric so that a charge shift is developed. In fact there are many types of piezo phenomena. The (direct) piezoelectric effect (PE) is generation of electric charge and voltage as a result of applied mechanical stress-tension, compression, twist. The reversed piezoelectric effect (RPE) is the opposite, stress in response to an electrical field or voltage. By this, for example, length changes (although small) can be induced by electrical means. Piezoresistivity (PR) is defined as the change of resistivity (not resistance) when deformed. Semiconductors such as (single crystal, amorphous, polycrystalline) silicon and germanium are piezoresistive. Thus there are many piezo devices using these phenomena. Microphone membranes (PE), loud speakers (RPE), sonar (historically the first application; RPE), piezoresistors (PR) for pressure measurements, cigarette lighters (PE), and quartz watches (RPE) are examples.

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Integration of piezotechnology in textiles isdwith the exception of integration on solutions with a piezodevice in a boxdeither in the form of films (ceramic or polymeric) or fibers. Piezofibers are not that common; examples include fibers made of PZT and barium titanate [43] most often of very short length from a textile processing point of view. Polymeric piezofibers and -filaments have been presented by research groups at SwereaIVF and Swedish School of Textiles, Sweden [44]; Institut f€ur Textiltechnik der RWTH Aachen; Universidade do Minho, Portugal [45]; and Laboratory of Textile Physics and Mechanics, France [46].

28.4.1.2 A case study: fabric-based sensors for breathing monitoring A prototype garment shown in Fig. 28.21 was developed to monitor breathing [40]. The two piezoresistive sensors were produced with a coating of conductive silicone (ELASTOSIL® LR3162) on the surface of the garment. By this a device is able to mimic an otherwise denoted material property, piezoresistivity. The sensors were placed on the chest and abdominal positions, respectively. The sensors were coated

Piezoresistive sensor

Reference resistor

Piezoresistive sensor

Reference resistor

DAQ NI USB-6277

+ – Battery

Figure 28.21 The prototype garment and the equivalent testing circuit.

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directly on a front pattern to permit future adjustments and refinement of the sensor performance by adjusting material combinations in the construction. Conductive yarns (Shieldex 235f34 dtex 4-ply HC, Statex) were connected to the sensors and used as data transmission wires. The prototype garment was tested by five subjects, three females and two males, 25e45 years old (average 35 years). The subjects were then requested to manipulate their breathing pattern and keep the breathing rhythm for some minutes. The signal was recorded after a few minutes without the subjects being informed. The subjects were instructed to follow a sequence of breathing movements, such as simultaneous breath, rapid breath, slow breath, and sleeping apnea simulation. In addition, to verify that the sensors were able to distinguish the predominant breathing compartment, the subjects were trained to apply chest-dominated breathing in the upright position and abdomen-dominated breathing in the supine position for 1 min, respectively. The results showed that the breathing in different circumstances can be successfully detected and monitored using fabric-based sensors (Figs. 28.22 and 28.23). The quality 0.5

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Figure 28.22 The recorded normal breathing, rapid breath and slow in a normal subject (left) and the breathing apnea stimulation (right) [39]. Chest dominated breathing-Channel 1 (subject #2) 0.4

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Figure 28.23 Chest-dominated breathing (left) and abdomen-dominated breathing (right) in subject #2. Channel 1: sensor placed in chest position; Channel 2: sensor placed in abdomen position [39].

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of the signals from the garment-based sensors shows very little deviation from the piezoelectric reference sensor. The overall construction of the system provides a good signal quality, combined with comfort and ease of use. In addition, combining two sensors positioned in different places on the garment has improved the signal qualities as well as detected the predominant breathing compartment.

28.4.1.3 A case study: sewn piezoelectric textile devices for walking patterns As an example of integration in, we now describe a case where the “device” is (1) made by a textile production process and (2) made as a textile. Almost all types of electronic devices are in the form of the triad, ie, three thin film layers on top of each other. Examples include the capacitor, the LED, the solar cell. The middle layer in the triad is denoted the active layer where, due to any physical mechanism, something happens (light absorption of a photon, followed by exciton formation and charge carrier separation in case of a solar cell, for example) and two electrodes on each side necessarily accompany for creating a closed circuit. One electrode conveys the electron and the other the positively charged hole. In all its simplicity the triad is a very potent model for devices and can be generalized in many ways, such as adding more layers, allowing for nonhomogenous films, and cutting films normal to the film plane (Fig. 28.24). The question is now whether there is a textile counterpart to the triad. In the semiconductor industry triads are made in highly controlled, clean room facilities in well-defined atmosphere and/or vacuum and with nanometer precision. The contact between the layers is good and the interface is well defined, both geometrically and chemically. Oxygen and moisture, most often devastating factors for electrical performance, are minimized. So it seems that one should be completely at odds mimicking the triad by textile processes and in textiles, ie, “textilizing” it. But, as we will see, it is possible, given that we are allowed to alter the scale and change the performance requirement. Fibers and yarns and threads are typically in the order of micrometer or millimeter. This could be compared with triad layers in the realm of nanometer to 10 of nanometer. Textile processes like yarn spinning, embroidery, etc. for sure make different textiles come together, but this is due to mechanical-physical means, not chemical. Triads are the basis for transistors, the heart of high-performance computation. We cannot expect the same level of efficiency quality for any textile device mimicking the triad of state-of-the art electronics.

1–100 nm

Upmost electrode Active layer Bottommost electrode Substrate

Figure 28.24 Schematic picture of the triad as a pattern for devices beyond the flat geometry paradigm. Dotted areas are insulating, nonactive support. Layers are often shifted for ease of contacting. Junctions are indicated by arrows.

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Figure 28.25 Schematic picture of the cross section of a piezoelectric filament with conducting core and piezoelectric sheath. Due to rheologic, stochastic effects the core is not always centrosymmetric.

The device we will show possible to realize is a piezoelectric tension sensor made by sewing. We have for a number of years developed polymer-based piezoelectric textile filaments and yarns [44]. Filaments are melt spun and bicomponents core-and-sheath type based on a blend of a normal bulk polymer like high-density polyethylene and the piezoelectric material (PVDF), surrounding a core of a conductive polyethylenecarbon black mixture (see Fig. 28.25). The core is electrically conductive and will act like an electrode. PVDF is not necessarily piezoelectric when it comes out from the spinneret. It has been shown that the cold drawing impacts the morphology in such a way that the PVDF chains reordered in what is called an alpha formation [44]. But this is not enough. The PVDF chains need to be in a so-called beta phase in order to show piezoelectric properties. This is achieved by a process called poling. This is to expose the material to a high voltage, typically some kV, meanwhile the material is heated, to 70 C, so that the polymer chains can rearrange. It is shown that the beta phase is stable for a long time [47]. Poling can be performed both directly on a PVDF containing fiber in alpha phase and on any garment where such fiber has been incorporated. This is yarn poling and garment poling, respectively. The content of PVDF, which is chemically close to Teflon, to which not much adheres, makes the piezofiber notoriously difficult as a substrate for further coating, such as an outer electrode, which should have been the obvious choice for creating a triad. Even plasma treatment has in our own experiments been shown to be inadequate for enhancing adherence. Another strategy has to be chosen. This is to let the piezofiber (ie, the conductive core with high-density polyethylene (Aspun 6835A, Dow, USA) and carbon black (Ketjenblack EC-600JD Akzo Nobel, the Netherlands) and the piezoelectric sheath with PVDF (Solef 1006, Solvay Solexis, Italy)) mechanically come into close proximity with a conductive thread. Here we take a thread from Class III above, Shieldex® (Statex, Germany, silver-coated polyamide). By this we form a textile variant of the triad with the conductive carbon black core as the inner electrode, PVDF sheath as the active layer, and the Shieldex as the outer electrode.

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Figure 28.26 Sewn sample. Two connections to inner and outer electrodes are seen.

Initial experiments showed that it is possible to twist the two yarns together by hand, contacting the two electrodes to an oscilloscope, and get a signal when the system is stretched. In fact the piezofiber is very sensitive. Already small movements give a clear signal with still moderate noise levels. More interesting is to use the textile process of sewing. A Brother EXEDRA (DB2-B737-403, Brother, Japan) sewing machine was used. The piezofibre was used as needle thread and the Shieldex as bobbin thread. As different thicknesses of needle- and bobbin threads cause different tensions with the risk of creasing and wrinkling, a Shieldex 235/34  2 was used as this is similar to the thickness of piezofiber. A common lock stitch was used. On 15 single-jersey fabric (polyester, dTex 100/34  2) samples a 15  12 cm sewing of a length of 15 cm was performed with three different stitch lengths: 1.25, 2.25, and 3.25 each on five fabric pieces. Loose ends (10 cm) of both threads were left at the beginning and from the end of the seam for electrical connection; see Fig. 28.26. Poling was then performed. The ends of the piezoyarn where cut sliced in order to expose the inner electrode. AGAR Silver paint G3691 was applied and was left to dry. Then copper tape (3M™ EMI Copper Foil Shielding Tape 1181) was wrapped outside of this. The Shieldex thread was dressed with the same kind of copper tape. On an insulated glass surface each sample was placed in turn and in-house-built heat treatment equipment with a heating lamp and a digital thermometer (Fluke) was used. The piezoyarn and the Shieldex yarn were coupled to a high voltage supply (PHYWE) as well as to a multimeter (Fluke) in order to have control of the current generated. Breakdowns have to be avoided as this destroys the piezoeffect. The sample was heated to 70 C. A voltage of 2.5 kV during 10 s was applied. After the poling each sample was tested for any piezoelectric activity by connecting them to a PC oscilloscope (Picoscope 2204 Pico Technology). In the in-house-built extensiometer, see Fig. 28.27, samples were clamped. Samples were connected via a probe (Tektronix, TPP0101-Voltage Probe 100 MHz with 10 magnification) to the PC oscilloscope (Picoscope 2204 Pico Technology). From a starting position when the sample was horizontal above the table, repeated extension relaxations were performed. Avoiding any transients, after 10 min measurement

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Figure 28.27 Extensiometer. Samples are clamped (red arrows). Moving the right clamp the sample is exposed to oscillatory movement (black, dotted arrow). In-house software controls the instrument.

started of the oscillating sample, which went on for 1 min. The extension amplitude was 4 mm with a clamp speed of 50 mm/s. The same procedure was repeated for all 15 samples, randomized. As clearly seen (Fig. 28.28), the sewn piezoelectric tension device works. A highly repeated pattern is developed synchronized with the movement of the sample. Hysteresis is low. Furthermore, the mean value for the measurement during the probing period for each sample is put into Fig. 28.29. The larger the stitch length the larger the amplitude in the diagram and the larger voltage generated. It is assumed that a large stitch length allows better for any movement in the fabric-and-seam system to be conveyed. Stitch length 3.25 mm shows the least spread. An observation was also that the 3.25 mm samples had a better visual appearance. The other stitches turned out visually to be scattered. By this procedure it is possible to go further and produce a number of devices such as socks for monitoring walking patterns. Diagrams like in Fig. 28.28 result. Hysteresis is low and any features such as walking pace, unbalance in walking pattern between left and right foot, etc. can be followed. So a simple textile device was constructed following the integration in approach. In spite of the poor quality electrical contact between the two fibers, the device works. In fact there are even advantages with the sewing approach. Thanks to the spatial separation of the inner and the outer electrode connections are made much easier than if a tricomponent fiber would have been used. Distances in Fig. 28.24 makes it difficult to address specific layers, but is in our textile case easy; see Fig. 28.26.

28.4.2

Textile-based capacitance sensors

28.4.2.1 Capacitance A capacitor is a device consisting of two electrical conductors that are separated by a dielectric material, ie, an insulatordin principle, any of two conducting materials that sandwich any insulating material form a capacitor. The capacitance of the capacitor is

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500.0 mV DC 400.0

300.0

200.0

100.0

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Figure 28.28 Typical result showing the output as a voltage as the sample is exposed to oscillating movement.

dependent on the area of the conductors, the distance between the two conductors, and the dielectric property of the insulator. The capacitance can be calculated as: C ¼ εr ε0

A d

where C is the capacitance, in farads; A is the area of overlap of the two metal plates, in square meters; εr is the relative static permittivity (dielectric constant) of the material between the plates; ε0 is the electric constant; and d is the distance between the plates, in meters. Capacitors can be used as energy storage elements; the energy stored in a capacitance is proportional to the capacitance. Often the capacitor is used in electrical circuits

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250

Amplitude (mV)

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Stitch length 2.25 cm

Stitch length 3.25 cm

Figure 28.29 The impact of stitch length.

to buffer fluctuations in the power supply. Capacitors can be also used as pressure or deformation sensors as the capacitance is related to the distance between the electrodes. By using several electrodes forming a matrix, the pressure distribution can be measured [48]. Fabric-based capacitors can be fabricated from conductive materials acting as the electrodes and a soft insulator in between. The plates can be made using common textile manufacturing methods, such as weaving [49,50], knitting [51], sewing [52,53], and embroidery [54]. Conductive yarns are used in this case. The plates can also be painted, screen printed, or coated using conductive ink, and ICP materials. The dielectrics used are typically foams and polymers. However, those materials have drawbacks, including hysteresis, poor resilience, signal drift, and lateral movement [55], therefore the capacitance outputs are often nonlinear with the change of distance. Most of the fabric-based capacitors are used as pressure sensors to monitor vital signs of the user or detect the pressure distribution while sitting or walking.

28.4.2.2 A case study: 3D-woven parallel plate capacitor A fabric-based capacitor was made using a novel 3D-weaving technique [56]. As shown in Fig. 28.30, the multilayer woven structure is made of five layers: the conductive layers (A and D), the insulating and stabilizing layers (B and E), and the middle layer (C), which acts as the distancing and insulating layer. The measurements on such a structure were done by constructing a first-order passive high-pass filter and using the fabric sample as the capacitor series connected to a 1 MU resistor. To verify the pressure sensor function, a sine signal with frequency 10 kHz and peak-to-peak amplitude 3 V was sent by a function generator to the textile sample (Fig. 28.31, left). The output signal was read on an oscilloscope. The distance

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A

B C E

D A: conductive layer B: insulating layer C: separating layer D: conductive layer E: insulating layer

Figure 28.30 3D-woven structure as a parallel plate capacitor.

between the two outer layers of the 3D-woven structure and the two metal plates was varied from 15 to 5 mm, output voltage recorded every 1 mm. By using circuit analysis one will find that the output voltage, Vout, will be approximately inversely proportional to d, indicated by the solid curve in Fig. 28.31, right. The theoretical expectations together with two testing results are shown in Fig. 28.31. In the first case the sample was simply supported by two Plexiglas plates, and in the second measurement the sample was glued to the Plexiglas to prevent lateral movement. Results showed that the textile-based sensor behavior is close to the expected one and already at this stage the structure might be used to indicate the presence of a pressure. The deviation was mainly due to the lateral movement of the conductive layers. As the distance decreases the rigidity of the spacer structure produces a shear force, which makes the conductive layers move laterally so that the overlapping area is no longer constant. Future work should aim to resolve the unwanted lateral motion of the conductive layers as well as to make a precise model of the partially filled capacitor in order to predict the effective permittivity of the dielectric. If these issues are taken care of the structure will also be suitable for making absolute measurements of either distance or pressure.

0.5

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0.4 0.3 0.2 0.1 0 0.004 0.006 0.008 0.01 0.012 0.014 0.016 0.018 0.02 Distance between plates (m)

Figure 28.31 Equivalent circuit of the measurement setup of 3D-woven capacitor and the testing result.

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28.5

Future perspectives

Today we see a tendency that what is a material and what is a device merge. A smart textile in itself is an example of this and especially the integration in approach. We could hope that this could re-empower the textile community so that it could take the lead in its own development rather than being dependent on enabling technologies from elsewhere. Smart textiles is an area that is an example of a paradigm shift where the textile processes enable new innovations by bringing together materials, techniques, and components to complete systems and expression. Innovation does not happen by itself; it requires environments where people with different backgrounds and skills come together and work together to create both sought-after solutions as well as unexpected solutions. An important aspect of the development in the field of smart textiles is to create opportunities for multidisciplinary and intersectoral meetings and collaborations to take advantage of the opportunities for smart textiles. This is the exciting future of smart textiles. Despite promising results that have already been obtained, more research is needed, and the development of smart textiles can be further improved in several ways: • •

•

• •

Sensor and system development: New conductive materials and textile manufacturing or finishing technologies are needed to improve sensor sensitivity and to simplify the manufacturing processes. Energy consumption: When an electrical system is working, heating is always generated as a side effect, and this part of energy is usually wasted. The total energy consumption in a system can be reduced by lowering the heating generated in the working system. Further on, the usage of the energy in heat as an energy source is interesting, especially if new generations of low-consumption electronics are coming. Environmental issues: Many times smart textile means adding new materials to a textile. Such blending has to be overcome in order to pave the way for environmentally friendly solutions. Solutions may include the class VI and VII fibers, which have the potential to provide an all-polymer structure. Measurement device: Smart textiles is an area that brings added value to the products in terms of new functionality and performance; however, the complexity of smart textiles being both textile and electronics asks for new ways of testing and evaluations before market launch. Standard for testing: New devices and methods to measure functionality and performance should be standardized to have comparable testing outcomes [57,58].

It would be an interesting exercise to investigate how many of the measurands and the mechanisms in Table 28.1 that in fact already are, and potentially could be, introduced in textiles by the integration in approach. But this we leave for some other occasion to study.

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