Smart Textiles

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MDT COURSE ON SMART TEXTILES

Prof. Dr. ir. Lieva Van Langenhove, dr. h.c.1

Dr. Carla Hertleer, Dr. Lina Rambausek and ir. Sheilla Odhiambo

1Technologiepark 907 Gent 9052 Belgium

corresponding.author Lieva.vanlangenhove@ugent.be

Keywords: smart textiles, sensors, actuators, energy devices, communication devices, textile integration

Abstract

Smart textiles are under development for nearly 20 years now.

This course gives an overview of smart textile materials, processes and products as well as markets. In addition it describes how to design smart textile products in generic terms.

Contents 1 SMART TEXTILES? ......................................................................................................... 3

1.1 Introduction ................................................................................................................. 3

1.2 Definition ..................................................................................................................... 3

1.3 Functions of smart textiles ........................................................................................... 5

1.4 Integration level of smart textiles ................................................................................ 7

2 Why smart textiles? ............................................................................................................. 9

2.1 Versatility .................................................................................................................... 9

2.2 Textiles are part of our lives ...................................................................................... 10

3 How to design smart textiles? ........................................................................................... 10

3.1 Introduction ............................................................................................................... 10

3.2 Design concepts ......................................................................................................... 11

3.3 Case 1: Resistivity based textile sensors ................................................................... 12

3.4 Case 2: Optical systems ............................................................................................. 16

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3.5 Selection of concepts ................................................................................................. 19

4 Materials ........................................................................................................................... 20

4.1 Mechanisms of conductivity ...................................................................................... 20

4.1.1 General ............................................................................................................... 20

4.2 Materials .................................................................................................................... 23

4.2.1 Metallic materials ............................................................................................... 23

4.2.2 Polymers ............................................................................................................. 23

4.2.3 Carbon fibres ...................................................................................................... 25

4.2.4 Polymers with conductive particles .................................................................... 25

4.2.5 Coated fibres, yarns and fabrics ......................................................................... 26

4.2.6 Elastic materials ................................................................................................. 26

4.3 Comparison of conductive materials ......................................................................... 27

5 Smart textile components .................................................................................................. 28

5.1 sensors ....................................................................................................................... 28

5.2 Smart textile actuators ............................................................................................... 30

5.3 Communication ......................................................................................................... 31

5.4 Energy supply ............................................................................................................ 32

5.4.1 Energy storage .................................................................................................... 32

5.4.2 Energy scavenging ............................................................................................. 35

5.5 Data processing .......................................................................................................... 36

5.6 Interconnections: ....................................................................................................... 38

6 Market perspective ............................................................................................................ 39

6.1 A practical case: the smart fire fighter suit ................................................................ 39

6.2 Potential ..................................................................................................................... 41

6.3 Products on the market .............................................................................................. 41

7 CONCLUSIONS ............................................................................................................... 42

8 REFERENCES ................................................................................................................. 42

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1 SMART TEXTILES?

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1.1 Introduction

The first generation of intelligent clothing uses conventional materials and components and tries to adapt the textile design in order to fit in the external elements. They can be considered as e-apparel, where electronics are added to the textile. A first successful step towards wearability was the ICD+ line at the end of the 90ies, which was the result of co-operation between Levi’s and Philips 1 2. This line’s coat architecture was adapted in such a way that existing apparatuses could be put away in the coat: a microphone, an earphone, a remote control, a mobile phone and an MP3 player. The coat construction at that time did require that all these components, including the wiring, were carefully removed from the coat before it went into the washing machine. The limitation as to maintenance caused a high need for further integration.

Further evolution has included 3 trends:

• Search for new concepts and technologies for a wide range of applications • Making electronic components compatible with the textile substrate

• Transforming electronic components into true textile structures

1.2 Definition

Smart textiles have been around for about 15 years now.

Smart textiles can be described as textiles that are able to sense stimuli from the environment, to react to them and adapt to them by integration of functionalities in the textile structure.

Advanced materials, such as breathing, fire-resistant or ultrastrong fabrics, are according to this definition not considered as intelligent, no matter how high-technological they might be.

The extent of intelligence can be divided in three subgroups [3]:

• passive smart textiles can only sense the environment, they are sensors; • active smart textiles can sense the stimuli from the environment and also

react to them, besides the sensor function, they also have an actuator function;

• finally, very smart textiles take a step further, having the gift to adapt their behaviour to the circumstances.

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So two components need to be present in the textile structure in order to bear the full mark of smart textiles: a sensor and an actuator, possibly completed with a processing unit which drives the actuator on the basis of the signals from the sensor.

Sometimes, the change in the material is clearly visible, but sometimes it takes place on a molecular level, completely invisible to the human eye.

The application possibilities offered by these materials are only limited by human imagination.

Processing these intelligent materials (in the form of fibres, threads, gels, liquids, …) into textiles or producing textiles from these intelligent materials results into an intelligent textile. The resulting textile will have self-regulating properties on the basis of changes that occur in the surroundings.

Although smart textiles find and will find applications in numerous fields, most of the attention is focused on health related applications, including sports, health care and protection. These kind of textiles include for example wearable smart textiles (biomedical clothing), designed to fulfil certain functions, but apart from that without any fringes. More casual applications are possible as well, which are expected to be functional as well as fashionable. It also can go as far as daily skin care, where the comfort factor and aesthetics are even more critical. But also smart wound dresses, bandages and hygiene applications are envisaged.

The European standardisation committee CEN has established a technical paper4 that gives a definition of smart textiles.

The document distinguishes between:

• Functional textile material: o Can be components of intelligent textile systems and hence functional textile

materials which are relevant for intelligent textile systems o It has a specific (passive) function o examples are: electrically, thermally or optically conductive materials,

materials that release substances, fluorescent materials

• Intelligent (smart) textile material:

o Functional textile material, which interacts actively with its environment, i.e. it responds or adapts to changes in the environment

o Examples are: chromic, piezo electric, phase change, electroluminescent, shape change, auxetic, electrolytic, dilating or shear thickening, capacitive materials

• Smart textile systems: o A textile system which exhibits an intended and exploitable response as a

reaction either to changes in its surroundings/environment or to an external signal/input

o Comprises actuators, possibly completed by sensors

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o Contains an information management device that controls/manages the information within the textile system

o characterised by two functions: energy and external communication

This means there are 4 types of smart textile systems:

Energy function

Without With

Com

mun

icat

ion

func

tion

With

out

“NoE-NoCom”: self ironing shirt (shape memory materials5

“E-NoCom”: heating gloves

With

“NoE-Com”: photovoltaic textiles

“E-Com”: health monitoring textiles

Smart textile products obviously are high tech products that combine advanced and/or smart materials in a smart way.

Regardless the definition, smart textiles has become a multidisciplinary and promising research subject for many research groups all over the world.

1.3 Functions of smart textiles

Basically, 5 functions can be distinguished in a smart suit, namely:

• Sensors

• Data processing • Actuators

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• Storage • Communication

The different components all have a clear role, although not all smart suits will contain all functions. The functions may be quite apparent, or may be an intrinsic property of the material or the result of a combination of materials or a specific structure. They all require appropriate materials and structures, and they must be compatible with the function of clothing: comfortable, durable, resistant to regular textile maintenance processes and so on.

Basically the smart textile will interact with the body, the environment or itself.

The textile is in contact with the skin over a large body area. This means that monitoring can take place at several locations at the body.

Some examples of body parameters that have been mentioned in literature are

• Temperature,

• Biopotentials: cardiogram, myography, • Acoustic: heart, lungs, digestion, joints, • Ultrasound: blood flow,

• Motion: respiration, movement, • Chemicals (sweat),

• Electric proterties of the skin, • Mechanical properties of the skin,

• Pressure: blood.

It will be clear to the reader that this list is not tentative. Odour for instance, colour of the skin at different wavelengths could mean something. Indeed, we all see when a person we’re familiar with is not feeling well, while a mother can smell her child has fever. These parameters are measured at the surface or in the outer body layers.

One challenge in this respect is the interpretation of data: how to detect when a parameter is deviating? The functioning of a body is extremely individual and time dependent, so standard algorithms won’t be very effective. Multi parameter analysis, correlations between parameters could reveal unexpected combinations. Also, variation of parameters at different positions at the body, variations in time may provide relevant information. It is often a chicken-or-egg question: no one has been able to do the exercise because of lack of suitable systems, and consequently no one makes systems as the feasibility has been demonstrated.

Another type of parameters concern the environment. Information from the environment is required for comfort and protection. Possible parameters are:

Air properties: temperature, humidity, velocity, pressure Radiation Electromagnetic/static fields Chemicals

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Bio activity Motion

A third class of parameters is the textile itself. It is very important that the textile is capable of evaluating its own functionality, in particular in applications dealing with health and safety. But the textile can also warn when water or heat are migrating through the textile, when excessive strain occurred, when it is subject to abrasion etc..

1.4 Integration level of smart textiles

All functions have to be integrated onto or into the textile for achieving the smart textile system without having any impact on comfort and ease of use. in addition all processes to do so have to be compatible with current manufacturing technologies.

Ultimately the goal is to create clothing with integrated sensor function. To reach this goal three tracks can be followed:

Incorporating is the action of uniting (one thing) with something else already in existence, for example attaching electronics onto textiles. Integration levels 1 to 3 (added-on) are related to incorporation.

Embedding is becoming an integral part of a surrounding whole, for example weaving an electronic tape into the textile structure. Integration Levels 3 (built-in) to 5 refer to the action of embedding [6].

Integrating is the action of bringing all parts together to unify them into a whole. The combination of the electronic function with the textile in generally, is called integration.

The first option is mainly driven by the electronics industry, the last by the textile industry.

Definition: Incorporating, Embedding, Integrating

By analysing the smart textiles landscape, including commercial products, patent applications and research in the field the technical level of the concepts for integration can be assessed.

To allow the comparison of technologies, a set of five integration levels has been established (Figure 1). The levels relate to the integration of the electronic function starting at rigid electronics inserted by design, followed by rigid and flexible microelectronics attached to the textile substrate, and textronics permitting full integration. The latter achieves the highest level of integration from a textile engineering perspective and is referring to textronics [7].

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Figure 1: Smart textile system component integration levels

Textronics are textile substrates with an active electronic function that is generated directly, added-on or built-in. This function can be generated on fabric (planar), on fibres and yarns (cylindrical) or material level. Hence, the electronic function and the textile substrate are inseparable. This is an important point when comparing textronics with rigid or flexible electronics which can be removed from the textile and still function independently. An example for textronics is an organic field effect transistor built on a polyester filament. The main manufacturing techniques for producing textronics are situated in thin film technologies.

As one can imagine, the integration level 5 is the most sophisticated level of integration and is challenging research in invisibly merging the two fields: textiles and electronics into textronics. The final product truly integrates textiles and electronics, making separation impossible. Consequently, if electronics are the basis for generating the electronic function, the integration levels 1-3 have to be taken into consideration. If the textile is the base for integration, level 4 is of concern.

Level 5 can be reached in both ways, in textiles this would be full integration of the entire system, in electronics, something like electronic skin could be an option [8]. Up to date, systems on integration level 5 are none existent for both substrate types. To visualize integration level 5 and with reference to Tanaka (2003), at this point of integration, the electronic function should be integrated so that the electronic device (smart textile) can be handled “like a handkerchief” [9].

It is worth noting, that smart textile products are systems. There is a good chance that the single e-textile components of the system are integrated on different levels. The highest level of integration, level 5 relates clearly to the entire smart textile system and not to the single e-textile component. Therefore, it is evident that integration level 5 only can be reached if all components do fulfil these preconditions. For the single component the highest integration level that can be reached is level 3 or alternatively level 4 (textronics).

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2 WHY SMART TEXTILES?

2.1 Versatility

Textiles show several advantages. They are all around: on our body, on the ground, on the wall, under the ground, on our beds and many more. Textile materials are unique in several aspects.

They are extremely versatile in products as well as processes. The building stones of the textile material are fibres or filaments. Fibres and filaments are made of a broad range of materials: natural or synthetic polymers, ceramic or metallic materials. They can have various properties some of which we could call smart (conductive, responsive).

They are available in many dimensions: with varying lengths (mm up to m), fineness (nanometer up to mm), surface roughness, cross-sectional shape (round, triangular, multilobal, …), etc.:

Hollow Trilobal Ribbon Special shape

They are made of one single material or make a combination of materials in several ways:

Concentric/excentric sheat/core Side-by-side Pie wedge Islands in the sea

This enables achieving a very broad range of properties: from high strength to very elastic, hygroscopic or hydrophobic, biocompatible, biodegradable, solid or porous, optical or electro-conductive, and many more.

In addition to that, innumerable combinations of these source materials result into a whole range of textile materials. Fibres of one or various types can be arranged at random or in a strictly organized way in 1- or 2- dimensional structures (yarns or fabrics). Nowadays, even 3-dimensional structures can be constructed. A high level of order can be achieved, or a very random arrangement such as non-wovens.

After treatments broaden the possibilities of achieving very special properties such as hydrophilic/hydrophobic nature, antimicrobial, selective permeability etc.. Textile materials are able to combine advanced multifunctionality with traditional textile properties.

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2.2 Textiles are part of our lives

Clothes are our own personal house. They can be made to measure, with a perfect fit and high level of comfort. Clothes make contact with a considerable part of the body. They are a common material to all of us, in nearly all of our activities. They look nice and attractive, the design and look being adapted to the actual consumer group. We all know how to use them. Maintaining textile is a daily practice: house as well as industrial laundry are well developed.

And last but not least: textiles and clothes can be produced on fast and productive machinery at reasonable cost.

These characteristics open up a number of applications that were not possible before, especially in the area of monitoring and treatment, such as:

• Long term or permanent contact without skin irritation,

• Home applications, • Applications for children: in a discrete and careless way,

• Applications for the elderly: discretion, comfort and aesthetics are important.

It is clear that the intelligent character of the textile material can be introduced at different levels. It can occur at fibre level, a coating can be applied, other threads can be added to the textile material, it is even possible to closely connect completely independent appliances with the textile.

Full success however will only be achieved achieved when the sensors and all related components are entirely converted into 100% textile materials. This is a big challenge because, apart from technical considerations, concepts, materials, structures and treatments must be focusing on the appropriateness for use in or as a textile material. This includes criteria like flexibility, water (laundry) resistance, durability against deformation, radiation etc.

As for real devices, ultimately most signals are being transformed into electric ones. Electroconductive materials are consequently of utmost importance with respect to intelligent textiles.

3 HOW TO DESIGN SMART TEXTILES?

3.1 Introduction

The development of a new smart textile product includes selection of the basic concept, selection of materials and production tools, designing a prototype, manufacturing of a prototype and testing and validation.

Modelling tools are convenient for running simulations in order to study the optimal configuration and the impact of typical conditions of use of textile products such as deformation and humidity. Such tools can drastically reduce development time and effort.

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Manufacturing should be compatible with processes commonly used in textiles.

Testing is an issue. Test methods often exist for non deformable materials such as silicon based electronics. However textile materials can easily be compressed or stretched often changing its properties, their behaviour is sensitive to humidity and many more. Standard test methods have to be extended in this respect.

Each of these steps will be discussed in the following paragraphs as well as some typical examples.

3.2 Design concepts

Although the functions are quite clear and easily identifiable they often can be considered in a very generic way as transformations. Devices or systems that turn a signal such as pressure, deformation and so on into an electrical signal that can be read out in a quantitative way are considered as sensors. Actuators act in the opposite way: they respond upon an electrical signal generating heat, light etc.. Energy generating devices act like sensors but conversion rates are more critical. Antennas can be considered as sensors and actuators at the same time: they capture and send out EM signals.

The types of transformation being identified the next step is to find appropriate concepts. For designing new concepts of smart textile components looking at transformation principles can be quite helpful. Some transformation principles are illustrated in the following table 1:

From/to Thermal Electrical Chemical Optical Mechanical

Thermal Seebeck Chemical reaction

Thermo luminescence

Shape memory material

Electrical Heating : Joule ; cooling : peltier

Electrochemical reaction

Electro luminescence

Electrostrictive polymers

Chemical Exothermal reaction

Electrochemical reaction

Chemo luminescence

gels

Optical Absorption Photovoltaics UV initiated reactions

Photochromic Dyes

Mechanical Friction Piezo-electric Piezo chromic dyes

Table 1: Transformation mechanisms of smart textiles

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Next step is a literature and patent search. Data bases can provide very useful information. The idea is that very likely solutions have been found in other applications or sectors. So the challenge is to find them. An inspiring website is offered by Creax10. Creax has analysed millions of patents. They have developed a classification system that describes a patent in terms of functions in a very generic way. As such the database enables the user to find how similar problems have been solved in other sectors or applications.

An example is11: how to take water out of a glass without touching the glass. The generic mechanism is how to move water? The generic function is moving and a further element of selection is a liquid. The database finds no less than 48 solutions for moving water.

The idea behind this approach is that it is very likely that someone has already found a solution to your problems. It just takes an efficient methodology to find it. Generic classification is one key to this.

3.3 Case 1: Resistivity based textile sensors

One of the first full textile sensors are piezo resistive. They change conductivity as response to strain. Some concepts are described in this paragraph.

All conductive materials show piezo resistivity: when extended length will increase and diameter will decrease resulting into quadratic increase of resistance:

R = ρ l / S = ρ . l / (V/l) = ρ . l2 / V (1)

R = resistance

l = length

S: surface area of the cross section

V = volume of the fibre

ρ = volumetric resistivity

This however requires considerable extension at the fibre level which would not be very comfortable.

Another mechanism is more suited. Staple yarns consist of fibres of limited length. This means none of the fibres ranges over the full length of the yarn. Consequently current has to be passed on from one fibre to the other. Thus resistance will depend on the number of contacts and contact resistance. Stretching will cause the yarn to be more compact, resulting in more and tighter contact. Consequently the number of contact point will increase while contact resistance will decrease. As a result the overall electrical resistance will decrease significantly. This effect does not require very high extension of the yarn so it can be used in textiles. The effect depends on several factors such as the yarn count, twist (density), fibre length and fineness, fibre conductivity, fabric structure.

One of the first prototypes in this respect is the respibelt, developed in Belgium by UGent and KULeuven (fig. 2)

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Figure 2 - Respibelt: piezoresistive knitted structure for monitoring breathing

This belt is fully knitted. A staple yarn made of stainless steel fibres provided by Bekintex is integrated as a sensor. It is inserted as regular yarn all over the width of the knitted belt. The conductivity is measured using pressure studs as contacts.

The signal is shown in following figure 3:

Figure 3 – piezo resistive respiration sensor

Advantages are that it is fully made of textiles, washable and comfortable. The product can be manufactured on a regular jacquard knitting machine. Read out and interpretation are fairly simple as the response is approximately linear.

Unfortunately it has limitations too.

The mechanism is based on relative displacement of fibres relative to one another. Breathing is a cyclic deformation with a frequency of approximately 5-10 cycles per minute. This multiple deformation leads to slow rearrangement of the fibres slowly altering the piezoresistive effect. This is illustrated in the figure 4:

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Figure 4 – evolution of the piezoresistive effect due to multiple deformation

During the first phase of use a consolidation of the structure takes place. This effect stabilises quite fast. It causes the resistance to increase by approximately 20% which is not positive but not problematic either.

More problematic is the change of the gauge factor, which can be considered as a measure for the sensitivity of the sensor. The gauge factor expresses magnitude of the response relative to the change of the input.

0

0

LL

RR

GF ∆

∆

= (2)

∆R = change of resistance and ∆R= Ri - R0 (R0 and Ri are resistance value initial resp. after deformation)

∆L = change of length and ∆L= Li – L0 (L0 and Li are resistance value initial resp. after deformation)

Fig. shows the gauge factor decreases all along the use of the sensor. The first problem is that it limits the sensor to be a qualitative sensor only. Secondly eventually the sensitivity will go below threshold causing the sensor properties to be lost.

Surprisingly the gauge factor increases again after washing. Further analysis has shown this can be correlated with fibre breakage.

Another problem that occurs in many applications of smart textiles is the compatibility of materials. The modulus of stainless steel fibres is much higher than that of common textile fibres. This will cause both fibre types to behave slightly different when stretched leading to slight irreversible relative movement of both fibre types. Ultimately this causes fibre separation as illustrated in figure 5:

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Figure 5 - Effect of separation of materials due to incompatible mechanical properties

Compatibility of materials in terms of mechanical properties has to be considered. Also for coatings this is important, as incompatibility may lead to cracking of the coating.

The sensor construction addressed in this paragraph may also be used in another way namely as inductive sensor. This is called pelthysmography. In this method, the conductive belt is considered as a coil that is wrapped around the body. Through inductance measurements any change in diameter, dielectricum etc can be recorded. The resulting signal is presented in figure 6.

Figure 6 - Respiration rate calculated from variation of inductance

The disadvantage compared to the piezo resistive approach is that the response is clearly non linear. This causes quantitative interpretation to be more complex. On the other hand it does not require a piezo resistive yarn so multiple deformation should not be an issue.

This principle is also used by VivoMetrics Inc. an American company that commercializes the LifeShirt™ [12]. This shirt makes it possible to continuously monitor vital bodily

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functions, such as heart rate, respiration, blood pressure, ... The LifeShirt™ is a sleeveless undergarment, made of a comfortable and washable stretch-material. The fabric of which the shirt is made, contains one or more elongated bands of elastic material. Each of these bands is stretchable in the longitudinal direction and contains at least one conductive wire [13]. This wire is shaped in a sinusoïdal arrangement in order to ensure elasticity of the garment (the wire in itself being non elastic). For monitoring certain physiological functions where a higher degree of sensitivity and accuracy is required, more wires are used. They form a single continuous conductive circuit that encircles the monitored area as many times as there are wires. The bands of elastic material may be formed in any conventional way, which includes warp knitting, weft knitting, weaving, braiding or a nonwoven construction. Warp knitting however is preferable, because it is easy to create bands having narrow widths. The conductive wire is either incorporated into the elastic fabric structure or is sewn to the surface afterwards. A copper wire is used as conductive wire. The wires are attached to a monitoring unit.

Piezoresistive sensors use change in conductivity as a response to deformation. As mentioned before all conductive materials have piezo resistive properties because of the effect of extension. For some materials this effect is bigger than what is to be expected from geometrical effects. This is the case for instance for polymers charged with conductive particles such as carbon. Deformation causes the distance between conductive particles resulting into significant change in conductivity. A similar effect occurs due to swelling/shrinkage of the fibres for instance as a response to wetting or absorption of chemicals. Consequently any mechanism that causes deformation of such conductive fibres can potentially be used as sensor concept. Detection of chemicals has been studied in the European project Inteltex, using change in conductivity by swelling of CNT charged fibres as a basic mechanism [14].

The sensors described above require an electrical current and therefore are called passive sensors. Active sensors exist too. They use for instance piezo electric materials that generate an electrical field as a response to deformation.

Sensors in general and textile sensors in particular struggle with the following problems:

• The flexibility and deformability required for comfort interfere with sensor stability,

• Bio-signals tend to have relatively low amplitude (e.g. µV), • Long term stability is affected by wear and laundry.

3.4 Case 2: Optical systems

Optical systems are based on interaction with light. They use absorption, emission or reflection of light of specific wavelengths.

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Some concepts are based on the Bragg effect. This is based on interaction between a surface with periodic structure and light with a wavelength corresponding to the period of the surface structure.

Fibre Bragg grating uses optical fibres where a periodic grid is applied (eg. by laser) on the surface of the core fibre. When a light beam is transmitted through the fibre the wavelength corresponding to the period of the grid is reflected. As a result the reflected light has only 1 wavelength, one colour. In the spectrum of the transmitted light this one wavelength is missing. Such sensors are commonly used for measuring deformation inside composite structural materials.

Wings of some butterflies are based on the same principle. The surface has a regular scale structure (fig. 7).

Figure 7 - surface structure of wings of butterflies giving rise to colour effects

The colour one sees depends upon the distance between the scales seen from the position of the observer. Her too any effect that changes the periodicity will change the apparent colour.

In this concept a periodic pattern Is applied on a surface, for instance by making a grid by laser or by applying a surface coating with light is reflected only when it has a wavelength corresponding on the distance of.

A broad range of colour changing mechanisms exist. They are based on physical or chemical effects. One of the oldest textile dyestuffs, Tyrian purple, changes colour as a function of UV intensity: it gets its beautiful intense purple colour only after UV exposure. When applied on clothing it allows to see when UV radiation is high, requiring sun cream or other means of protection [15]. Halochromic dyes indicate pH value, which is a useful tool for monitoring proper healing of burning wounds [16].

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Advantages of such systems are:

• ease of read-out: any person can see colour change. • Because the primary signal is optical, it is not subject to interferences caused by static

electricity, strong magnetic fields or surface potential. • The ease of textile integration. • Remote sensing achieve by use low-loss optical fibres; over distances up to about 1

km which is far beyond textile requirements • Multiple analysis with a single control instrument at a central site. • Coupling of sensors for different analytes in a sensor bundle of small size allows

simultaneous monitoring of various analytes.

Of course they also show some disadvantages:

• interpretation may require a reference scale. • Ambient light can interfere. • Limited long-term stability because of photobleaching or wash-out of the indicator. • Mass transfer of the analyte from the sample into indicator phase is necessary in order

to obtain a steady-state signal. • Limited dynamic range. • Selectivity and specificity of indicators are not always optimal.

One solution to the latter problem is to apply the dyestuff on an optical fibre. The optical fibre can take the colour information to a reading device. In this case the transformation mechanism is from any signal (light, temperature, chemical, …) to optical. The dyestuff thus becomes the smart interface.

The dyestuff may react upon several triggers. This means selectivity and specificity are limited. They can be improved by adding filters, for instance by coating the fibres. Such a coating can shield the dyestuff from specific wavelengths of light, chemicals (semi permeable coating) and so on. This is illustrated in the following figure:

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3.5 Selection of concepts

As indicated in the previous paragraphs, often many solutions can be found. However not all of them are suited for smart textile products.

Some guidelines have already been mentioned above.

Factors concern proper functioning including long term, compatibility with textile materials and processes, health or environmental issues.

Proper functioning:

• Response: o Magnitude of response (eg. expressed as gauge factor) o Speed of response o Interval of response o Reversibility o Conditions of response

• Application

o Ease of application

o Compatibility with textile processes

• Long term effects:

o Number of cycles it withstands

o Stability of response

o Sensitivity to heat, light, moisture, …

• Health and environmental effects:

o Non toxic

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o No skin reaction

o Recyclable

4 MATERIALS

The basis of smart textiles are conductive materials. They include optical as well as electrical conductivity.

Common textile materials are insulating. Yet electroconductive materials are vital for smart textile products. Conductive fibres can be achieved through following approaches:

• Making fibres from conductive materials (polymers or metals) • Coating fibres with a conductive layer (polymers or metals)

• Adding conductive particles to the master batch

Each of these solutions has advantages and limitations in terms of level of conductivity, manufacturability, textile compatibility, technical and usage properties of the fibres, price etc.

Conductive fibres are readlidy available. They are used in a range of commercial products such as anti static and EM shielding. However in smart textile applications conductivity should be very high.

The next paragraphs explain the mechanisms of conductivity, materials and cases.

4.1 Mechanisms of conductivity

4.1.1 General

Electrical conduction is the movement of electrically charged particles (electrons, ions, …) through a transmission medium. The movement can form an electric current in response to an electric field. The underlying mechanism for this movement depends on the material. In solid materials, conductivity is achieved by movement of electrons across atoms.

In atoms electrons are arranged around the core at specified distance. Electrons cannot take any position and in addition their positions are interdependent. The distance from the core determines the energy level of the electron: the larger the distance the higher the energy level. The distance ranges are called bands. In terms of conductivity two bands are essential namely the valence band (highest occupied band) and the conduction band (lowest unoccupied band). In solid state physics and related applied fields, the band gap is the energy difference between the top of the valence band and the bottom of the conduction band in insulators and (semi)conductors (fig. 8) .

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Figure 8 - (Semi)conductor band structure.

In a conductor, the highest occupied band is not at all full. This allows the electrons to move in and out from neighboring atoms and therefore conduct easily. Metals are typical conductors. The vibration of the electrons hinders their mobility causing the conductivity to decrease when temperature increases.

In an insulator the highest occupied band (valence band) is full, so electrons have no mobility in that band. The first unfilled band above the valence band which would enable electrons to move is the conduction band. For an insulator the gap between the valence band and the conduction band is large and at room temperature there is not enough energy available to move electrons from the valence band into the conduction band, where they would be able to contribute to conduction. Normally, there is almost no electrical conduction in an insulator. If the applied voltage is high enough (beyond the breakdown voltage) sufficient electrons can be lifted to the conduction band to allow current to flow. Often this flow of current causes permanent damage.

In a semiconductor the gap between the valence band and the conduction band is smaller, and at room temperature there is sufficient energy available to move some electrons from the valence band into the conduction band, allowing some conduction to take place. An increase in temperature increases the conductivity of a semiconductor as more electrons have enough energy to make the jump to the conduction band. This is the basis of an NTC thermistor, opposite to metals. NTC stands for ‘negative temperature coefficient’, ie increased temperature causes resistance to reduce.

The conductivity for these three types of materials is illustrated by figure 9:

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Figure 9 – conductivity of materials

An intrinsic (pure) (semi)conductor's conductivity is strongly dependent on the band gap. The only available carriers for conduction are the electrons which have enough energy to be excited across the band gap, which is defined as the energy level difference between the conduction band and the valence band. From Fermi-Dirac statistics (to be precise the Boltzmann's approximation is actually used), the probability of these excitations occurring is proportional to:

(3)

where:

e is the exponential function

Eg is the band gap energy

k is Boltzmann's constant

T is temperature

Doping is a process in which impurities (dopants) are introduced into a semi-conductive material. There are two different types of dopants: n-type and p-type dopants. Most n-type dopants for silicon are located in group 15 in the periodic table, p-type dopant mainly in group 13. Dopants have energy levels around the band gap facilitating electrons to become excited.

Fermi level

Insulator

Band gap

Semiconductor Conductor

Valence

band

Valence

band

Valence

band

Conduction

band

Conduction

band

Conduction

band

overlap

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4.2 Materials

4.2.1 Metallic materials

Metals like copper and silver and alloys like stainless steel are available in fibre form.

Metals are good conductors because the valence and conduction band overlap. Metals have unfilled space in the valence energy band. In the absence of an electric field, there exist electrons travelling in all directions and at many different velocities. When an electric field is applied, a slight imbalance develops and mobiles electrons to flow. Electrons in this band can be accelerated by the field because there are plenty of nearby unfilled states in the band (fig. 10).

Figure 10 - Conductivity of metals [17]

4.2.2 Polymers

In polymers the valence band and the conduction band are separated. In most polymers this gap is too high to allow electrons to jump which turns them into insulators. In molecules with π conjugated bonds this gap is smaller so they have semi-conductor properties.

The orbital of a C-atom is illustrated in the fig 11:

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Figure 11 – orbital of C atoms

Interaction of sp² orbitals give σ bonds. Interaction with pz orbitals give π bonds. As each pz

orbital only has 1 electron, only half of the orbital is occupied, the other half (π∗ orbital) remains unoccupied. Let us take ethylene C=C as an example18. (fig. 12)

Figure 12 - π∗ orbital of ethylene

This leads to delocalisation of charges between the two carbon atoms. Polymers have long C-chains. Long C-chains with alternating single and double C-C bonds form conjugated bonds. The delocalisation involves all C atoms involved in the conjugated system giving rise to

macroscopic conductivity. π electrons have higher mobility than σ electrons. Nevertheless the mobility is lower than in inorganic conductors.

Polypyrrole (Ppy), polyaniline (PANI) and polythiophene are conductive polymers used in the textile industry. their structure is givne in fig. 13:

Polypyrrole Polyanilin Polythiophene

Figure 13 – structure of conductive polymers

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As described above conjugated bonds are the basis of conductivity. Conjugated bonds are also the basis of many organic colorants. Consequently most conductive polymers are coloured.

Another problem of conductive polymers is that they are neither thermoplastic nor soluble. Recent techniques are based on grafting or the use of resins where the conductive polymer is dispersed. Alternatively they can be applied as a coating by in situ polymerisation. Different conductive polymers can be coated on the surface of the fibres and fabrics, or directly incorporated into the spinning.

PEDOT or Poly(3,4-ethylenedioxythiophene) (fig. 14) is another conductive polymer that has been introduced in textiles recently. Advantages of this polymer are optical transparency in its conducting state, high stability and moderate band gap and low redox potential. It is transparent but water soluable, which is quite inconvenient for textile applications. PEDOT is usually doped with PSS (Polystyrene sulfonate)

Figure 14 - structure of PEDOT and PSS

PEDOT:PSS is a polymer mixture of two ionomers. One component in this mixture is made up of sodium polystyrene sulfonate which is a sulfonated polystyrene. Part of the sulfonyl groups are deprotonated and carry a negative charge. PEDOT is a conjugated polymer and carries positive charges. It is based on polythiophene. Together the charged macromolecules form a macromolecular salt. It is used as transparent, conductive polymer with high ductility in different applications.

4.2.3 Carbon fibres

Carbon fibres are conductive too. In addition they show good strength. Unfortunately they are black. They can be achieved by carbonising viscose fibres. Carbonisation can be limited to the surface or extend over the whole fibre mass.

4.2.4 Polymers with conductive particles

Adding conductive particles can provide conductivity to non conductive fibres. The main advantage of such composite materials is that any fibre matrix can be used. Disadvantages are the limited conductivity and the impact of the particles on the fibre properties.

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A special effect that is achieved is based on the quantum tunnelling effect. This effect being influenced by deformation sensors can be achieved, for instance pressure sensors for instance [19].

4.2.5 Coated fibres, yarns and fabrics

A conductive layer can be applied at the surface of textile substrates. Metallic layers are applied by redox processes requiring either redox chemicals (electroless plating) or electrical current (electrical plating) [20].

Conductive polymers can be applied as well for instance by dip coating [21] or by in situ polymerisation [22].

4.2.6 Elastic materials

4.2.6.1 Elastic conductive materials at fibre level

Extruding elastic polymeric fibres loaded with electro-conductive particles is a prominent example of the achievement of electro-conductivity on fibre level. Most likely, rubber based [23,24,25] or thermoplastic elastomers [26,27,28] are combined with conductive particles, such as carbon black. These structures usually have relatively high resistance per unit length (for instance 70 KΩm-1) and their electro-conductive properties significantly change with strain. Thus, most likely they find application in motion and occupancy detection.

4.2.6.2 Elastic conductive materials at yarn level

Research and product development of twisting or wrapping electro-conductive filaments and yarns around an elastic core yarn is predominantly industry-driven.

Yarn manufacturer Zimmermann holds patents on the structure of an elastic, electro-conductive yarn as well as on its production technique [29]. The hybrid yarn consists of an elastic core yarn around which electro-conductive yarns are wound in one direction and a non-conductive binding yarn is wound around in the opposing direction (fig. 15). Thus, the elasticity of the hybrid yarn is limited by the binding yarn.

Figure 15 - Elastic and electro-conductive yarn structure of Novonic® yarns [30]

The hybrid yarn is produced via the hollow spindle process. A drafted elastic core yarn is passed through a hollow spindle carrying an electro-conductive winding yarn. The winding

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yarn is drawn off the spindle by the core yarn and wound around the core. Subsequently, the single enwound yarn passes through a second hollow spindle which carries a non-conductive binding yarn. The binding yarn is wrapped around the single enwound hybrid yarn opposite the direction of the conductive member. Appropriate machine parameters allow that the produced hybrid yarn possesses no internal torsion stress.

According to company claims, the electrical resistance of the yarn remains constant when stretched by 15%. Today, these yarns are marketed under the trade name Novonic®.

4.3 Comparison of conductive materials

As stated in the introduction, each material has its strengths and weaknesses. In terms of conductivity, metals are superior. This is illustrated in the table 2:

Fiber Volumetric resistance (ohm.cm)

Silver 1.63 · 10-6

Copper 1.72 · 10-6

Stainless steel 72 · 10-6

Carbon from 2.2 · 10-4 till 10 · 10-3

Polymers 10-2 – 10-3

PANI (polyalanine, panion™) 10-3

PA charged with nanoparticles 6.5 . 10-4

Table 2-conductivity of materials

The conductivity and price of a series of materials is illustrated in the figure 16:

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Table 3- resistivity of materials [31]

5 SMART TEXTILE COMPONENTS

5.1 sensors

A list of signals to be measured has been given already in paragraph 1.3. A wide range of textile sensors are already available, at least at the prototype stage.

The firsnt sensors were based on electromagnetic measurements. Conductive textiles were and still are used for monitoring heart rate. This was a major aspect of the EU project MyHeart [32]. Measuring heart rate is fairly easy: 3 conductive electrodes suffice for recording the biopotential from which the ECG and heart rate can be calculated. Conductive electrodes can be knitted or woven into the fabric using jacquard technology. One problem however is the skin contact: body movement causes the contact resistance to change leading to motion artefacts.

Within the European project Context [33] a contactless sensor has been developed. It records biopotential of muscles (myography) in general through changes in capacity instead of

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resistance. The researchers use embroidery and lamination technology to produce EMG sensors for monitoring stress in professional situations. Permanent stress, even at low level, causes false posture and pain in shoudlers and neck. If stress persists the system gives a signal to relax.

Piezo resistive strain sensors indicate respiration rate [34] or motion [35]. Optical systems are used as well [36]. The principles have been described in paragraph 3.3.

For pressure too several principles have been exploited:

• Contact/no contact structures: conductive yarns have been inserted in both sides of this double layer knitted fabric; pressure causes the conductive yarns to contact. As such they can be considered as switches. Fig shows a piano tie designed by Plug&Wear [37].

Figure 16 – piano tie by Plug & Wear

The black areas represent the keys of a piano. They consist of pressure switches. Depending on the “key” that is hit a different circuit is closed leading to a different tone will be produced.

• Capacitive sensors providing a measure of average pressure or a quantitative value of local pressure [38, 39]: capacitive effects occur between two conducting materials, conductive yarns for instance. Because of their open structure the capacitive effect will change when the yarns are compressed. As such each intersection between warp and weft yarn in a woven fabric has the potential of becoming a capacitive pressure sensor.

• Quantum tunnelling effect [40]: a composite material consisting of an elastomeric material and metallic filler particles shows insulating properties in undeformed conditions whereas it becomes an excellent conductor under pressure.

Detection and analysis of sweat has been the topic of Biotex [41]. These principles can be used for sensing humidity and chemicals in solution.

As mentioned before pH can be measured using halochromic dyes. Such dyes change colour as a function of pH, as illustrated by fig. 17:

Figure 17 - colour change of halochromic dyes [42]

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One application is monitoring the healing process of burning injuries. When healing properly a specific pH will follow a specific pattern. Any deviation from this pattern indicates potential problems such as infection. Halochromic dyes allow follow up of the healing process without removing the dressing from the wound.

Textile temperature sensors are configured as thermo couple devices. They are available in the form of wires and as such already compatible with textiles. Work has been done to make them in a true fabric structure by sputter deposition (Fig. 18) [43] or by coating at the fibre level (Prospie project 44).

Figure 18 - Thermocouple based textile temperature sensor

5.2 Smart textile actuators

Although basically the opposite from sensors, actuators are an important challenge. They mostly require quite some energy.

Optical actuators emit light.

The first concepts used optical fibres. Optical fibres have 3 layers namely

• the core which is basically transporting the light, • a cladding layer with lower refractive index in order to reflect as much as possible the

light at the interface with the core (total internal reflection) • an outer layer which serves as external protection

Optical firebs have been designed for full internal reflection. However an optical fibre of which the cladding layer has been damaged will emit light. When properly designed (ie damage of the cladding layer in the right places and correct insertion of the fibre in the textile structure, such fibres can make the textile to become illuminating in dedicated areas creating simple patterns [45]. The optical fibres are lit by LED’s. The system is quite stable but the pattern cannot be changed.

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Figure 19 - France Telecom prototypes of communicating textiles

The Philips programme Lumalive has intended the integration of LED’s in combination with textiles, such as clothes and furniture [46]. Further work has targeted miniaturisation and textile integration [47]. On step further is making the textile itself light emitting [48]. Challenges at this moment are the limited efficiency of organic active layers and their sensitivity to oxidation.

Electrical actuators use textile electrodes for providing electrostimulation [49].

Thermal actuators can provide heating or cooling [50]. One of the first examples of heating textiles as the E-CT fabric from Gorix [51]. Other of commercially available active heating garments are Polartec®Heat® panels of Malden Mills [52], the Bekinox® heating elements by Bekaert [53] and the Novonic Heat System by Zimmerman. Cooling is a very challenging issue. Passive cooling systems use endothermal processes such as evaporation of water, recrystallization, melting etc [54]. They work well, but their capacity is limited.

The Italian company Grado Zero has embedded ultrathin tubes in textile structures through which a cooling liquid can be circulated. An F1 pilot racer suit has been manufactured [55]. The liquid is cooled by a small Peltier element that is fixed at the back side of the suit.

Chemical actuators release chemicals in a controlled way. Several concepts are available such as microcapsules, cyclodextrines, hydrogels and nanofibre structures.

Mechanical actuators do exist, but all of them have major drawbacks. They are either slow, require high voltage, cannot exhibit high forces or are not reversible. They include shape memory materials [56], multilayer textiles [57], electrostrictive materials or diffusion based electro active polymers.

5.3 Communication

BAN1 communication is fairly easy, electro conductive and optical fibres and yarns can be used to this end. Wireless communication is the main challenge. Two approaches have been studied, for medium and short distance, as illustrated in fig. 20. The microstrip patch antenna (left picture) is active in the SIM band, which also includes Bluetooth [58]. This allows

1 Body Area Network

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communication with for instance a smart phone or laptop. Further data processing and wide area communication does not need to be handled by the textile in this way. The antenna is screen printed. The picture on the right illustrates an inductive link [59]. It is embroidered on the textile substrate. Such a connection only works well when the second antenna is nearby and parallel. The electrical field decreases exponentially with distance and should not exceed a few millimetres. This is an appropriate solution for instance for monitoring a person during sleep, where the second antenna can be integrated in the mattress.

a. Antenna operating in the ISM band b. Inductive link

Figure 20 – wireless textile link

In the Proetex project a GPS system embedded in a rescue suit allows to determine the location of a person.

5.4 Energy supply

Energy supply can be achieved through 2 approaches: energy storage and energy scavenging.

5.4.1 Energy storage

Energy storage can be achieved through batteries. Capacitive or electrochemical batteries are currently in use.

An electrochemical battery stores energy in the form of chemical bonds. It involves liquids, which is not the case for a capacitive battery. The chemical reactions produce ions that moves from one electrode to the other, Each reaction in a battery is associated to a specific potential.

A capacitor stores energy in electric field. The advantages of absence of chemical reaction in capacitors are that they can be charged in minutes or seconds, they deliver energy quickly and with unlimited life cycle. The disadvantage of the latter is that the voltage supplied is not constant.

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Capacitors have high charge-discharge rate (with high power delivery rate that lasts for a short time) while the batteries have a lower charge and discharge rate with high power densities, hence can last for several hours. The capacitors have longer cycle lives than the batteries.

For both density of storage is limited, so capacity requires volume. For flat batteries volume means surface.

Hybrid systems combine battery like electrode for energy source and capacitor like electrode for power source in the same cell.

The main physical properties one should care about a battery are the energy it can give, its power, mass and volume.

Batteries are rated according to their voltage and capacitance.

Energy = joules, Power = Energy (j)/time(s) in Watts therefore 1Wh =3600J.

Specific energy is for energy per unit mass,

Energy density is energy per unit volume.

Capacity is the measure of amount of electric charge in coulombs.

Amp-hours is oftenly used instead of coulombs in battery literature.

1Ah = 3600C.

Volts (V) = Joules(J)/Coulomb(C)

Figure 21 - Comparison of power (rate) and energy (capacity) of different energy storage devices

Flexible batteries are commercially available. Although they are flexible they are not breathable so comfort is limited. Research is going on on textile based batteries but they are far from being commercial [60].

Some researchers are working in the direction of textile supercapacitors (ultracapacitors). These supercapacitors use conventional textile material as the base, which are modified by adding conductive polymers or metal particles by various techniques (coating, printing, deposition, dispersion ,on site polymerization, e.t.c). For example by use of high porous, conductive carbon nanotubes, conducting polymers among others to maximize on the

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supercapacitor electrodes functionality. In other scenario the textile substrate can be used as a separator of the electrodes applied on both sides, and as the holder of the electrolyte.

On the other hand some researchers are working on the textile batteries. The main idea is to produce a functional battery incorporated in the fabric [61, 62] or yarn [63] itself. At the fabric level, this is achieved by assembling the battery components (anode, cathode, separators and electrolyte) by either of conventional textile process, i.e. weaving, knitting or printing [64] or paintable [65]. At yarn level, all the battery components are assembled in a bi-component yarn (cable battery) which can later on be processed into fabric.

Some examples:

• Lithium ion textile battery in which they weaved a conductive porous 3D structure from conductive yarns made of pure PET dispersed with carbon nanotubes. These is then filled in with battery electrode material and electrolyte. The assembly is then stuck on flat metallic piece which is the current collector.

• Bhattacharya et al. [66] produce an energy storage device, they call it a battery, where they use a jacquard woven textile substrate. The textile base is woven with inclusion of three silver coated polyamide yarns, which are placed very close to each other. PEDOT/PSS is then systematically coated on to the substrate at a defined small area within the conductive yarns.

• Odhiambo S. et al. [67, 68] used pure stainless steel filament yarns as electrodes which are sewn on textile substrate at a close distance to each other. The textile substrate is a three layer laminate of twill weave fabric. PEDOT:PSS is used as active medium. It performs like a capacitor battery (fig. 22).

Figure 22 – PEDOT:PSS battery

• Flexible supercapacitors were made from PEDOT:PSS nanofibers and PAN nanofibres by Larfogue et al. [69] the PEDOT nanofibres were produced by combination of electro spinning and vapor-phase polymerization, this was used as the active material (electrodes), separated by a sheet of PAN nanofibres. Carbon clothes were used as the current collectors.

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In other type of flexible supercapacitor, textile electrode for the supercapacitor is made from conductive cotton textile sheet that is produced by dipping the cotton material severally in MWCNT (to Increase the MWCNT loading on the fabric) Pseodocapacitive cobalt hydroxide is well dispersed into the conductive cotton textile sheet, that it is able to produce large area specific capacitance of 11.22Fcm-2 [70]

5.4.2 Energy scavenging

Energy is available in the environment under the form of heat, light, motion. Several mechanisms are known to harvest them.

Infineon has developed a device that harvests electricity from body heat. It was one of the first washable smart textile components [71]. The demonstrator developed by Infineon has the dimensions of a eurocoin and produces enough energy for a small sensor (fig 23).

Figure 23 – thermogenerator by Infineon

This means that a standalone sensor can be achieved by integrating a local thermogenerator together with the sensor.

Energy from light is a technology that is commonly used as photovoltaics. Today flexible PV foils become commercially available. It is one of the highlights of the European network on organic large area electronics COLAE [72]. Application of PV cells on textile substrates is the subject of the European project Dephotex [73] (fig. 24).

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Figure 24 – Prototype textile PV device from the Dephotex project

Energy from motion can be captured in 2 ways, namely using piezo electric materials [74] and by EM induction [75].

A typical piezo electric material is PVDF, polyvinylidene difluorid. It can be applied as a coating on fibres or fabrics.

EM induction requires a permanent magnet and a coil in which the moving magnet induces a current. One can be inserted for instance in the clothes, the other in the sleeve. Movement of the arm induces a current in the coil (fig. 24).

Figure 25 – energy from motion through EM induction

5.5 Data processing

Data processing concerns soft and hardware.

Fucntions of software are:

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• System organization (read out of sensors, identification of data, communication etc) • Preprocessing of signals

• Analysis of data • Interpretation of data (data statistics, identification of deviating conditions, …

• Steering of actuators • Steering of communication devices

One challenge is the interpretation of data: how to detect when a parameter is deviating? The functioning of a body is extremely individual and time dependent, so standard algorithms won’t be very effective. Multi parameter analysis, correlations between parameters could reveal unexpected combinations. Also, variation of parameters at different positions at the body, variations in time may provide relevant information. It is often a chicken-or-egg question: no one has been able to do the exercise because of lack of suitable systems, and consequently no one makes systems as the feasibility has been demonstrated.

Steering of actuators is a similar challenge. For instance: how to steer heating or cooling devices for achieving optimal thermal comfort of a person (how much, how long)?

Data processing requires electronic components. Initial research targeted miniaturization and encapsulation. Later on flexible boards have been developed. However they can resist bending but they still are not drapable or stretchable. A major step forward is the development of stretchable electronics [76]. This has largely increased the textile compatibility (fig. 25).

Figure 26 – stretchable electronics

Follow up projects are further elaborating on this (eg. fp7 projects PASTA [77] and PLACE IT [78]).

The ultimate challenge is to develop fibre based electronics. Several projects target fibre transistors [79, 80] (fig. 26)

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Figure 27 – fibre based OFET transistor [80]

5.6 Interconnections:

The last challenge is integrating and interconnecting all active components. All components need to be integrated seamlessly in the textile structure. The Wearable Motherboard that has been discussed previously is the first development in this respect. It consists of a grid of optical fibres embedded in a woven or knitted structure; at several connectors active components such as sensors can be attached.

Based on the work of ETH Zurich [81], the Swiss company Sefar has designed a woven textile structure with a grid of electroconductive yarns onto which electronic components can be attached so that the appropriate connections are achieved with a good level of durability [82].

Embroidery is considered as a textile technology suitable for interconnecting electronics to textiles. T. Linz [83] explored the potential and limitations in his PhD study “Analysis of failure mechanisms of machine enbroidered electrical contacts and solutions for improved reliability”.

Failure happens due to breakage at the transition zone from soft to hard materials, and due to relaxation of the materials.

The first cause will get less important as active elements are being transformed into true textile structures, such as with smart fibres (transistor fibres) or smart yarns [84]. However, electrical contacts remain a major challenge, particular in view of rearrangement of fibres causing the structure to relax.

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6 MARKET PERSPECTIVE

6.1 A practical case: the smart fire fighter suit

The fire fighter suit is an excellent case for illustrating the status of smart textile products. It is a piece of high tech textiles with many active components, offering a good platform for further exploitation in other applications.

Today’s fire fighter suit offers an extreme level of thermal protection. This level of protection is so high that the fire fighter loses perception of the real danger in and around the fire. By the time he (or she) feels the heat or excessive body stress it may be too late to get out and take off equipment and clothes. So monitoring of vital signs and effective intervention are needed.

The European Commission has funded a number of research projects on smart thermal protection for professional use. Most of them target monitoring of vital signs and endangering ambient conditions, as well as communication tools. In addition each project several markets are addressed (rescue workers, security staff, chemical operators, workers at sea, mine workers, …). They develop new sensors, materials, meeting specific challenges in their own way [85,86, 87, 88, 89]

The most advanced project is PROETEX (protective e-textiles) [90]. This FP6 funded European project targeted research and development on materials for smart textiles for improved performance, textile sensors, communication through textiles, development of prototypes including the electronic platform, feasibility study of fibre based smart textiles (such as piezoelectric textiles for energy scavenging and fibre transistors). The suit consists of an inner garment (underwear) containing sensors that have to be in direct contact or near to the body such as temperature, heart rate, respiration rate and composition of sweat (for detecting dehydration).

Figure 28 – Proetex underwear

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The outer jacket measures:

• activities and posture of the wearer (standing still, walking, running; upright or lying down),

• risk of breakthrough of heat through the jacket, • location (in open field, for instance when fighting fires in the forest)

As for data processing the system has following components:

• textile antennae that operate in the ISM band (Bluetooth), enabling communication with a base station (PC or smart phone) within a range of 10 to 100 meters

• an electronic box collects and processes data • LED indicating when a person is in trouble • flexible battery for power supply

Figure 29 – Proetex outer jacket

Gas sensors are integrated in the boots.

A spin off product is the victim patch. This patch is to be put on the arm of victims in case of major disasters, enabling fast evaluation of urgent need for care. It uses a combination of components integrated in the underwear and outer jacket: heart and respiration rate and body temperature.

This is a nice illustration of how concrete products can reach the market before the full system has matured.

A non technological aspect of the fire fighter application is the specific situation of acquisition. Acquisition is often paid by public authorities, be it directly or indirectly. Procurement procedures need to be followed. Often procurement procedures cannot cope with

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innovative and complex systems such as smart fire fighter suits. Within the EU project Smart@fire novel procurement procedures are being established to this end [91].

6.2 Potential

The world annual market for smart textiles is estimated around 1000M$ [92] .

The market for smart textiles is relatively small for now, but it is growing quickly, according to Smart and Interactive Textiles, in August 2007 a report was published by US-based research firm BCC Research. In 2007, the US market for smart textiles was worth about $79m (€53.7m), says BCC, but sales of conductive fabric products are expected to more than double each year through 2012, when the market is expected to reach $392m - a compound annual growth rate of 38%.

The smart fabrics and interactive textiles (SFIT) market is estimated to reach US$1.8 Billion by 2015, according to new report by the Global Industry Analysts, Inc. consulting firm published in 2011. Challenges

6.3 Products on the market

Today some products are on the market. This offers a good range of cases with different specifications and requirements.

The price of the products has been studied in the SYSTEX project. The price depends on the complexity of the system. Simple monitoring products can be as cheap as some tens of euros. When advanced and centralized data processing tools are needed, prices increase to hundreds of euros. High end products for protection against harsh conditions may cost more than 1000€. Some examples:

• Low price products addressing consumer markets: o Adidas-Polar Smart shirt [93] for monitoring heart rate for sports applications

can cost approximately 25€; this price includes only the shirt with electrodes, not the data processing unit; the latter can cost from 40€ (small processor just indicating heart rate) up to 200€ (full connection to smart phone with apps).

o Twirkle shirt is a T-shirt from the London designers CuteCircuit [94]. It blinks as a function of the wearers movements. The LEDs are integrated in the textile whereas the sensors (accelerometers), electronics and battery are embedded in a small pack that is attached to the T-shirt. Its price starts from 150€.

• Medium price range advanced products: Smart carpet tiles SenseFloor [95] from Future Shape follow a person’s behavior in a house. From this a series of conclusions can be drawn such as whether a person has fallen, where a person is, presence of a burglar, etc. The price consists of the tiles, the central data processing unit and communication devices. The tiles are the cheaper part. The data processing unit is more expensive but it can serve up to 10 rooms. Prices start at 600-700€.

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• High end professional products: Smart fire fighter suits are high end complex systems which must be totally reliable even in extreme conditions; all of its functions are demanding. Prices of around 1500€ per suit have been mentioned.

Prices are expected to evolve in time. Prices will drop as market grows and rise when new technologies and additional are added.

7 CONCLUSIONS

There are great potential markets for smart textiles, such as healthcare and PPE, but not for every product. The added value of the textile solution has to be demonstrated, competitive non Smart-textile solutions need to be assessed.

High-performance products do not always result to a commercial success, because many non technological aspects need to be addressed as well, such as directives and standards, societal drivers, marketing and sales.

Many technologies are available at prototype stage. They have to be taken one step further in terms of robustness and large scale manufacturing.

New business models need to be addressed too.

8 REFERENCES

[1] http://worldwide.espacenet.com/publicationDetails/biblio?adjacent=true&KC=A1& date=20011220&NR=0195751A1&DB=EPODOC&locale=en_EP&CC=WO&FT=D

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[3] X ZHANG and X TAO, Smart textiles: Passive smart, June 2001 p 45-49, Smart textiles: Active smart, July 2001 p 49-52, Smart textiles: Very smart, August 2001, p 35-37, Textile Asia

[4] CEN TC 248 WG 31

[5] oricalco shirt https://www.youtube.com/watch?v=_oGQz-eSIOQ

[6] thefreedictionary.com. The free dictionary by farlex. 2014 [cited 2014 12. April]; Available from: http://www.thefreedictionary.com/].

[7] Rambausek, L. 2014. Textronics Definition, Development and Characterization of Fibrous Organic Field Effect Transistors. PhD Dissertation, Ghent University, Belgium, 375 p.]

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[9] Tanaka, K., A basic concept of novel hybrid opto-elctronic integrated systems with textile structures, in Flexible Electronics-Materials and Device Technology, N. Fruehauf, et al., Editors. 2003. p. 265-269

[10] www. Creax.com, patent inspiration, http://www.patentinspiration.com/#tour, accessed September 2014

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[12] www.vivometrics.com - www.lifeshirt.com

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