**3.2 Color-changing textiles**

*Advanced Functional Materials*

**3. Applications of smart textiles**

effect at temperatures close to body temperature.

**3.1 Shape memory textiles**

Although these developments are considered unquestionable, it is thought that these studies can only be implemented in the future with more advanced technology due to the limited information that can be obtained from the human body and the lack of

Materials capable of remembering the original shape are called shape memory materials. Materials are shaped out of its original shape as the temperature change returns to its original shape with a chemical, mechanical, magnetic, or electrical external effect. There are many classes of shape memory materials such as alloys, polymers, gels, and ceramics. Shape memory alloys and shape memory polymers are the types of shape memory materials with applications in textiles. The important point in these applications is that the material used exhibits the shape memory

Shape memory alloys are composed of a combination of two or more elements with the properties of hardness and elasticity that vary considerably at certain temperatures. An example of the application of shape memory alloys in textiles is a nickel-titanium alloy, which is used in protective clothing against fire and high temperatures and provides different levels of protection according to temperature. Under the degree of activation, the easily deformable alloy becomes more rigid at the degree of activation, taking its original shape. Alloy applied to the fabric in the form of a flat surface takes the form of spring with the effect of temperature, increasing the air gap in the fabric, thus increasing the protection of the garment

and the formation of second-degree burns under the same conditions.

another example of the application of shape memory alloys in textiles.

The degree of activation can be adjusted by changing the ratio of nickel to titanium in the alloy. T-shirts developed by an Italian company Corpo Nove shortens the sleeves with the increase in temperature and do not require ironing, which is

Shape memory polymers can be used in fiber production or can be applied to the fabric by finishing, coating, or lamination processes. Polymers have different water vapor permeability, air permeability, modulus of elasticity, refractive index, and expansion properties below and above the glass transition temperature (Tg). The shape-memory polymer, placed between two layers of fabric, has a tight structure below a certain temperature and prevents heat, water and wind circulation around the body. By the increase in temperature it starts the molecules motion and becomes a porous structure resulted with the expulsion in body heat. This flexible barrier function makes it possible to adjust the insulation properties of the garment to temperature changes and to provide optimum comfort in any environment. The crystal structure of a material at a given temperature determines its many physical properties. During the phase change, besides microscopic changes, macroscopic changes such as modulus of elasticity, coefficient of friction, electrical conductivity, and hardness occur. One of the important applications using these

Self-tangled surgical threads are designed for endoscopic surgery, and implants that are small in normal ambient conditions are designed for use in endoscopic surgery. Thus, it will be possible to perform operations with small incisions, shortening the healing time and reducing the risk of infection. Shape memory textiles can also be used for esthetic and decorative purposes. Textile materials that deform with the

materials and concepts for the systems to process this information [8, 9].

**248**

changes is surgical yarns.

stimulating effect acquire a third dimension [1, 8, 9].

They are intelligent textile materials that have the ability to change color with an external stimulus effect. They are obtained by incorporating color-changing materials into the structure of textile materials. Color-changing materials are chromic materials or chameleon materials. There are many different color-changing mechanisms, but mostly the electron density or molecular structure of the material changes due to the external stimulus effect and the color change occurs; when the stimulus effect disappears, they return to their initial state where they are more stable and get their first color.

Color changing materials are specified according to the effect mechanism. Light, heat, pH change, solution, friction and pressure are basic effect parameters. They are also called by the effect type as photochromic (light effected), thermochromic (heat effected), electrochromic (electric effected), solventchromic (solution effected), halochromic (pH effected), tribochromic (friction effected), mechanochromic (pressure effected).

The application of chromic materials to textile materials can be done by different methods at different stages. For example, a chromic dyestuff can be used for dyeing fibers by conventional dyeing methods; the fibers can be added to the fiber structure at the polymer stage; color-changing fibers can be obtained by melt spinning or wet spinning; they can be mixed with resin and coated onto the fabric surface, thereby using them for fabric printing or dyeing.

Smart textiles change colors depending on environmental factors; they are important because of their esthetic advantages. It is thought that the use of colorchanging textiles will become more widespread in the future in the field of fashion and will change the color depending on many other effects besides the existing ones. Photochromic, thermochromic, electrochromic, and solventchromic textile applications can be seen in fashion and decoration. They are available for T-shirts, bags, and hats.

The reversible color-changing property of thermochromic dyes indirectly changes the heat absorbing property of the textile material. While light reflection increases, darker colors increase heat absorption. Because of these properties, thermochromic dyestuffs are used to coat the uniforms of firefighters who turn white under very high temperatures and reflect the heat in this way and also in building coatings. The fact that thermochromic dyes accelerate the dimensional change of fibers provides another thermoregulation effect. At high temperatures, fibers containing thermochromic dyestuff shorten. The pores of the fabric are enlarged so that a large amount of air is introduced in and consequently the body temperature decreases. At low temperatures, the fibers are elongated, the pores are closed, and the fabric maintains the body's temperature [1, 2, 10, 11].
