Chapter 1
Application of Stimuli-Sensitive Materials in Smart Textiles

Ali Akbar Merati

Advanced Textile Materials and Technology Research Institute and Textile Engineering Department, Amirkabir University of Technology, Tehran, Iran

Email: merati@aut.ac.ir

Abstract

Stimuli-sensitive materials have the ability to sense and respond to various kinds of physical and chemical or biochemical stimuli in their environment. These materials are a convergence of different sciences such as material sciences, physics, chemistry, electrical engineering, wireless and mobile telecommunications, and nanotechnologies. They have many potential applications in smart textiles in the fields of medicine, protection, security communication, and textile electronics. Smart textiles are an interesting class of materials that can be prepared by a variety of methods. The functionality of smart textiles consists of many fields such as informing, protecting, and relaxing the wearer. The objective of this chapter is to present the latest research results together with basic concepts related to the preparation methods, characterizations, and applications of stimuli-sensitive materials in smart textiles and their importance in clothing. Future trends in this area of research are presented and issues regarding technology development and its uptake are highlighted.

Keywords: Smart textile, chromic materials, conductive materials, electronic textiles, phase change materials, shape memory materials

1.1 Introduction

Processability and flexibility are usually the two most important parameters of fine and elastic fibers used in order to make comfortable fabric and clothing. The wearable and comfortable fibrous materials such as yarn, fabric, and garments should be able to withstand handling in processing and end use without damaging functionality. The smart wearable textiles are fibrous materials made of smart materials such as shape memory materials (SMMs), phase change materials (PCMs), chromic materials, optic fibers, conductive materials, mechanical responsive materials, hydrogels, intelligent coating/membranes, micro and nanomaterials, and piezoelectric materials able to sense both the human body and external environment thanks to the presence of various kinds of sensors in their structure [1-4]. In other words, a smart textile allows the user to wear functionalized common clothes in which the user can access information about his personal biophysical data and/or environmental data. The potential of smart textile is enormous. One could think of smart clothing that makes us feel comfortable at all times, during any activity and in any environmental condition. A suit that protects and monitors, that warns in case of danger and even helps to treat diseases and injuries, is an example of smart clothing. Such clothing could be used from the moment we are born till the end of our life. These clothes should be like ordinary clothes providing special functions in various situations according to their design and application [5].

All smart materials involve an energy transfer from the stimuli to response given out by the material. They have the ability to do some sort of processing, analyzing, and responding. The amount of energy transferred to the response is determined by the properties of the material. For example, a material's specific heat (property) will determine how much heat (energy) is needed in order to change its temperature by a specified amount. The smart materials can be incorporated into the textile substrates at any of the levels, namely, fiber spinning level, yarn/fabric formation level, and finishing level [6]. Numerous scientists are researching to develop products with the emerging demand of smart textiles in various phases of life.

This chapter highlights all the main fields of applications of stimuli-responsive smart materials in textiles in various fields of applications such as healthcare, health monitoring, medicine, personal protective equipment, personal communication, textile antennas, garments for motion capture, and sensors (Table 1.1).

Table 1.1 Examples of application of stimuli-responsive materials in textile.

Stimuli-sensitive materials Benefits of treating textile Examples of potential applications Phase change materials (PCMs) Cooling, insulation, thermoregulating Blankets, bed sheets, dress shirts, T-shirts, jackets, vests, undergarments, socks, gloves, helmets, shoes and boots, earmuffs, hats and rainwear, seat covers in cars and chairs in offices, firefighters protective clothing, bulletproof fabrics, space suits, sailor suits, and other textile products Shape memory materials (SMMs) Insulation, shape forming, protection, compression, moisture management Shoes, breathable fabrics, thermal insulating clothes, crease- and shrink-resistant fabrics, fishing yarn, shirt neck bands, cap edges, casual clothing and sportswear, shape-formed dresses, protective clothing, flame-retardant fabrics, compression stocking, aesthetic effects, etc. Chromic materials Color change Fancy clothes, sports garments, workwear, soldier and weapons camouflage fabrics, technical and medical textiles Conjugated polymers Sensing Sensors for various biologically and chemically important target molecules, scaffolds for nerve tissue engineering Conductive materials Electrically conducting Electrically conductive textiles (fibers, yarns, fabrics), wearable electronics and fashion for healthcare, safety, homeland security, computation, thermal purposes, protective clothing, child monitoring, health monitoring, space programs, interior design Piezoelectric materials Energy harvesting, energy conversion, sensing, electricity generating E-textiles and wearable computing, electricity generation for various device applications, motion sensor Optic fibers Sensing, illumination, radiation, signal transmission Flexible flat panel displays, optic fiber fabric display Hydrogels Swelling/shrinkage change Water vapor-permeable fabrics, thermal-responsive hygroscopic fabrics

1.2 Phase Change Materials

Phase change materials (PCMs) are theoretically able to change state at nearly a constant temperature and therefore to store a large quantity of energy to regulate temperature fluctuations [4, 7]. PCMs can exist in at least two different phases (an amorphous and one or more crystalline phases), and they can be switched repeatedly between these phases. The thermal energy storage in PCMs occurs when they change from solid to liquid and the energy dissipates when they change back from liquid to solid. The different phases of PCMs have distinctly different physical properties such as electrical conductivity, optical reflectivity, mass density, or thermal conductivity. PCMs keep people comfortable through the absorbing, storage, and releasing of the heat. Without PCMs, the thermal insulation capacity of clothing depends on the thickness and density of the fabric. Incorporating microcapsules of PCMs into textile structures improves the thermal performance of the textiles [4]. There are many thermal benefits of treating textile structures with PCM microcapsules such as cooling, insulation, and the thermoregulating effect. PCMs are applicable in blankets and comforters, bed sheets, dress shirts, T-shirts, undergarments, swaddling blankets, and other textile products. There are several factors that need to be considered when selecting a PCM. An ideal PCM will have high heat of fusion, high thermal conductivity, high specific heat and density, long-term reliability during repeated cycling, and dependable freezing behavior.

Paraffin waxes are the most common PCMs, which can be microencapsulated and then either integrated into fiber or used as a coating in textiles that have a high heat of fusion per unit weight, large melting point selection, and a low thermal conductivity; provide dependable cycling; are noncorrosive; and are chemically inert. When designing with paraffin PCM, void management is important due to the volume change from solid to liquid. Hydrated salts are another category of PCMs. These PCMs have a high heat of fusion per unit weight and volume, have a relatively high thermal conductivity for non-metals, and show small volume changes between solid and liquid phases. There are many other classes of PCMs. PCMs that have a melting point from 15 to 35°C are the most effective useful PCMs in textile fields. Other required properties for a PCM for a high-efficiency cooling system in textile fields are the slight temperature difference between the melting point and the solidification point, having low toxicity and being harmless to the environment, being non-flammable, ease of availability, and low price. The specified roles of PCMs in outdoor and protective smart textiles are the absorption of body heat surplus, insulation effect caused by heat emission of the PCM into the fibrous structure, and thermoregulating effect, which maintains the microclimate temperature to nearly constant [8].

The incorporation of PCMs within a fiber in the spinning process, coating, and laminating on the fabric are various methods of using PCMs in...