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Abstract

Recently, new developments in the design and performance optimization of smart mechanisms associated with natural and man-made hazards have progressed considerably. This is mainly owing to advances in smart sensing mechanisms including communication and data technologies. This work provides a detailed overview of existing improvements on smart hazard monitoring equipment and materials applied in textile sensing systems. Given that fire is one of the most common disasters in many countries such as Australia, and every year many firefighters are affected by these unfortunate incidents, the focus of this study is on firefighters' protective clothing Fire Fighter Protective clothing. This review provides a unique opportunity to study smart sensing systems in coating technologies, potentially provides more effective techniques for training and better safety protocols of fire fighters. It aims to revisit the existing advances and address recent challenges and opportunities for improvement in the domain of smart coating and fire protective wearables. The goal of this review is to provide information about smart coating in protective clothing for firefighters. The capability of some of these clothing in managing thermal stresses, responding to humid environment, monitoring some critical parameters and adapting to the size of the wearers (clothes fabricated with phase change and shape memory substances) made them attractive choice in adjusting specific design features of industrial textiles. Various types of phase change and shape memory substances are defined and a combination of these substances within the structure of fabrics are presented. This paper also provides a detailed review on the heat exposure and capability of the shape memory substances (SMM) and phase change materials (PCM) to delay the heat transfer through fire fighter protective clothing. Referring to the former research, several issues have been detected using such substances. For instance, combination of phase change and shape memory materials needs fundamental improvements with regards to assessment techniques and testing criteria. Additionally, recent improvements in the domain of PCM and SMM including modifying mechanical features, functionality, and durability under different conditions have been informed. It has been suggested that the major problem in developing fabric-Phase Change Materials (PCMs) and Shape memory material (SMM) systems is their usage methods. At last recent developments on wearable monitoring systems applied in the firefighters’ protective gear. Wearable sensors are usually used directly on the body or located on wearable items to monitor information related to firefighters’ safety.

Keywords

Firefighters' protective clothing phase change materials shape memory material smart sensing mechanisms

Introduction

Smart coatings have gained recognition in material engineering, biomedical and many other fields of science and technology due to their unique intrinsic features. Many of the smart coatings include metal powders provide various exclusive functional features and increase the effectivity of their specific application. These powders are expected to generate the multifunctional coatings. Their distinctiveness is associated with great advances, and improvements in this domain and this expected to continue to grow in extreme conditions such as wildfire.¹ Smart clothing is a smart mechanism that can communicate with the wearers and environmental stimuli and conditions. They use typical technologies to link available electronics and embed them into the fabrics. Smart wearable systems employing non-invasive measurements usually include sensors generating raw information in assessing specific situation. Computers then analyze such data passed by the sensors and monitors by visualizing information given by sensors or smart probes. Smart coating can alter its features in reflection to an environmental incitement.²

The environmental incitements might be variation in moisture level, PH, mechanical harms, temperature and so forth. In a burning structure, firefighters may not face the harms of direct contact with the flame. Many disastrous accidents caused by firefighter’s collapse because of overheating or overextension and sadly the impracticality to rescue the firefighters on time. Particularly smoked-accumulated areas with considerably reduced vision are exceedingly dangerous. To prevent such circumstances, firefighters’ suit plays an essential role. Therefore, improvements in firefighting garments by embedding remote sensing techniques is well justified and may reduce risk of injuries and burns to a great extent. Using smart coatings to clothes is the main technique to produce smart clothing that can increase the protection level and convenient of a clothing system.^3–7

Some of such mechanisms are made more efficiently by combining radio-frequency identification chips in the structure of the fabric not only to sense but also to transfer information wirelessly, called telemedicine.⁸ Among all smart coating techniques, embedding the phase change material (PCM) into Fire Fighter Protective clothing (FFPC) designs is a new technology with potential to enhance the protection provided by the fire fighters clothing.^9,10 PCM usually refers to a substance which has fusion heat. This type of heat gives PCM possibility to save or release a great value of energy within the phase change. Combining PCMs with FFPCs is a novel technique to develop the heat efficiency of FFPCs. This method has been analyzed by many researchers for decreasing the thermal stress.¹¹ Some scientists have investigated the capability of adding PCM to FFPC to develop new thermal protection techniques and heat efficiency methodologies.^12,13

Rossi and Bolli¹⁴ presented an analysis on FFPC combined with PCM exposed to thermal fluxes of 10 kW/m2 and 5 kW/m2. Reference¹⁵ inquired cotton textile combined with two disparate PCMs applied in the multilayer textile mechanism of FFPC. According to their outcomes, the textiles with PCMs can result in a higher level of heat protection. Reference¹⁶ inquired the heat protective efficiency of the textile mechanism facilitated with PCM under two different conditions of fire exposure and low heat radiation. It appeared that such combination can considerably improve the heat protective efficiency of the textile mechanism under lower heat radiation. Shape-memory materials (SMM) are another efficient set of smart materials that can be applied in fabric mechanism. They are intelligent substances which can remember and recover substantial programmed deformation as exposing to an outer stimulus including temperature, a magnetic field, light, etc. SMMs have been applied in numerous fields and fabric application of fire fighters protective clothing.¹⁷ They were used to generate a 7 mm air gap in fabric mechanism by Hendrickson.¹⁸ A radiation heater was applied to model a heat risky exposure within firefighting. It was noted that the use of such materials considerably extended the rescue period to around half minute. Park et al.¹⁹ designed shape memory alloy made into spring shape as the heat liner in FFPC. They observed that the use of Ni-Ti springs into FFPC led to about 30% growth in protection period. Ma et al.²⁰ combined the SMM springs between the heat liner and humidity barrier. The authors observed that such arrangement has a direct effect on the distribution of air layer influencing its protective efficiency.

As FFPC design has made considerable improvements in fire safety technology over the past years, the requirement to explore this domain is evident to create a safer environment for firefighters. Therefore, many studies have been performed to find the most effective technologies and models to protect firefighter’s safety. Nonetheless, the requirement to examine efficient technique in order to assess heat transfer within the gear and provide vision into the effectiveness of the developed techniques of heat protection in FFPC is imperative. This study is aimed to provide a detailed overview of the recent improvements in smart coating used in protective suit design with PCM considering post-fire, exposure and resting phases. Also, the role of SMM in controlling the heat transfer rate in FFPC has been presented. The use of SMM in both heat and cold protective fabric was outlined in terms of human protective clothing mechanisms. In addition, the terms monitored by firefighters’ smart textile are explained. The outcomes of this work present insight into the performances of adding wearable sensors, SMM and PCM to fire fighter protective clothing and helps to improve firefighters’ safety.

Conventional technique of protective wearable systems

Wearable sensing mechanisms is rapidly developing with the improvement in artificial intelligence techniques in the last few years. Investigation and improvement in such technologies are pushing a revolution in presenting new applications in particular the ones used in FFPC design. Wearable mechanisms can be constructed with a goal to provide more control to the firefighters’ health and safety. Using protective wearable mechanisms within our assessment, techniques such as objective digital information can be applied to conduct higher quality clinical services. Sensitive and highly stretchable wearable mechanisms can be combined with the textile or placed on the body for different measurement of the body movement/reactions to the external environmental indices. Additionally, to obtain optimal efficiency, these wearable mechanisms should have some essential features including high flexibility, sufficient stretchability, biocompatibility and lightweight.^21,22 Therefore, opting suitable materials and methods for the wearable sensing textile is of great importance. Protective wearable sensing mechanisms are constructed with different polymers depending on the use of the sensor. Polydimethylsiloxane, polyethylene terephthalate and rubber are some typical polymers used to develop flexible sensor.^23–25 Recently, numerous types of such mechanisms have been proposed by using carbon-based substances combined with flexible and stretchable polymers. Fabric and carbon-based electronics for the recognition of human health and motion controlling are highly advantageous because of their great flexibility and stretchability properties. Carbon-based materials, nanomaterials and electronics textiles can be applied to wearable health sensing mechanisms due to their unique features such as wearability, comfortability and lightweight.^26,27 However, there are many major challenges for practical wearable applications in terms of various physical signals and human activities.

Materials used in in intelligent textile coatings

This section discusses materials used in smart coating, their characteristic, and manufacturing technologies.

Phase change material (PCM)

Phase change materials (PCMs) is a type of latent-thermal storage substance attracting or releasing thermal energy at an approximately fixed temperature. They attract or release thermal energy by altering the state of the substance. The PCM is a solid flake at room temperature and softens as it obtains a particular temperature. Latent heat storage using PCMs finds its way in different research fields such as building energy storage mechanisms, smart textile substances, waste heat recovery mechanisms, heat management of the batteries, photovoltaic thermal uses, space and terrestrial heat energy storage uses, etc.^28–31

Paraffin (CnH2n+2) and fatty acids (CH3(CH2)2nCOOH), Salt hydrates (MnH2O), (Sodium sulphate decahydrate, Na2SO4,10H2O) are some examples of PCMs used in clothing. The main characteristics required for PCMs are indicated in Table 1

Table 1.

The major features of favorable PCM.

Thermal features	Physical features	Kinetic features	Chemical features	Economies
Melting point in favorable working range	Small vapor pressure at working temperature	No supercooling or little in freezing	Chemical stability	Abundant
Great heat conductivity of both phases	Tiny volume change on phase change	Enough rate of crystallization	Compatibility with enclosure substances	Large-scale accessibilities
Great latent fusion heat per unit volume	Great density	—	No flammable, no toxic, no explosive substance	Efficient price
Great specific heat	—	—	Complete reversible melting/freezing cycle	—

In clothing systems, PCMs are used to keep the body heat and maintain the temperature or to avoid getting thermal fluxes to the skin surface for cooling down the body. The main application of thermal storage capacity of such material is to use them as cold-climate outdoor gear, in that PCMs attract and then store the body heat and finally release it as needed. In addition, PCMs can be utilized as a passive technique of heat exposure protection.³² In this state, they attract the external entering great thermal fluxed to protect the body. To calculate the latent thermal energy in the PCM layer, a further parameter is needed in the heat transfer equation. McCarthy³³ used the following equation to calculate the latent energy in the phase change material. The governing heat transfer equation, obtained from the energy conservation, for one dimensional heat transfer with the latent energy in the PCM is given by

ρ C (\frac{\partial T}{\partial t}) = \frac{\partial}{\partial x} (k (\frac{\partial T}{\partial x})) - Q

(1)

In this equation T is the temperature (^oC), and Q is the latent energy attracted in the phase change process. Also $ρ, C, k, θ$ are density, specific thermal capacity, thermal conductivity and temperature.^34,35 The PCMs used in the test were mixed, therefore, there is not an accurate melting temperature; however, their melting point is fluctuated in the range of the substance melted. It is not possible to estimate and understand where and how much of the PCM is melting in the layer. Thus, a function was presented for this volumetric thermal energy absorption value. This function merely used for nodes in the PCM layer and temperature in the rate of melting. To predict the value of latent thermal energy attracted according to the node’s temperature, a Gaussian error function is employed. This accounts the estimation in the finite difference technique, where whole control volume is considered to have equal temperature as the actual temperature in the control volume is considered to have a gradient.³³

Q = \frac{ρ l}{δ t} (\frac{14}{10}) (1 - \erf (\frac{T_{n} - T_{m}}{T_{r}}))

(2)Where erf is the error function and

\begin{array}{l} T_{m} = \frac{T_{1} + T_{2}}{2}; = Approximate melting temperature (° C) \\ T_{r} = T_{2} - T_{1} = Melting temperature range (° C) \\ T_{n} = The temperature of the node \end{array}

As any substance continues to absorb thermal energy, this may be reflected in temperature growth and the state of the material is changed, otherwise, the temperature may not increase. In both mentioned states, the substance absorbs thermal energy. In the first case, the temperature grows due to the thermal energy absorption called “sensible heat”; and in the second case where the temperature does not increase, it is called “latent heat”. In the latent thermal energy absorption, the material alters its state to a lower density and uses the attracted thermal energy to grow its molecules movement. In the phase of “liquid to gas” and “solid to liquid”, a substance requires to attract thermal energy to grow the motion of its molecules and reduce the density while it requires to lose thermal energy to grow density and decreases the molecules movements in the phases of “liquid to solid” and “gas to liquid” (as can be seen in Figure 1).

Figure 1.

Heat transfer and molecules movement in the process of phase change from solid to liquid and gas.

In recent years, carbon black loaded organic PCM is improved to increase thermal conductivity as well as photothermal performance. This was considered as a promising application in latent thermal energy storage.^36,37 Application of PCMs in clothing systems has been widely analyzed as a method of body-heat regulation to increase comfort. Many techniques are presented where PCM is used in fabric substrates at a specified stage of manufacturing, from the early stage of fiber formation to the finished clothes. Generally, there are four techniques to attach PCM in clothing system: (1) by accumulating a hollow fiber using PCM, 2) by adding PCM in fiber spinning;³⁸ (3) by coating the PCMs over the clothes using a crosslinking agent;³⁹ (4) by laminating in which the PCMs first combined in a thin film and then the film is laminated on the surface of the cloth. PCM attached to clothing systems for thermoregulation feature is shown in Figure 2.

Figure 2.

PCM used in smart textile systems as body-heat regulation.

Different kinds of PCMs have been used as a method of body heat regulation for increasing the comfort in different clothes, including space gloves and suit, and sportswear for ice climbing. Such materials attract the thermal energy produced by a human body within the activity and then release the thermal energy as cooled. Recently, microencapsulated PCMs have been applied in fabric system design to attract thermal energy in a risky environment including heat radiation, thermal convection and flash fire.^40,41

PCMs in protective wearable firefighters clothing

The heat protective efficiency of the firefighters’ wearable fabric is a significant concern. Diverse textiles sensing membranes which contain nano/microencapsulated PCMs have been fabricated, showing great thermochromic efficiency and latent thermal storage capacity. PCMs widely used in firefighters’ protective mechanisms, are able to guarantee controlled heat release, developed mechanical and heat features, and have low environmental impacts. As PCMs are type are highly applicable in temperature remote sensing mechanisms of firefighters, analyzing the PCMs with both energy storage and monitoring is regarded as a promising approach with wide and significant commercial value.^42–45 The results of study conducted by McCarthy and Marzo⁴² proved that the latent thermal absorbed within the phase change process decreases temperatures which may be felt by firefighters. Also, the theoretical simulation presented a comparison between the PCM layers to the FFPC fluttering layers and showed that using PCM decreases the gear layer while maintaining analogous heat efficiency. Fonseca et al.¹⁸ provided guidelines for the features of PCM with respect to thermal exposure. They focused on the significance of PCM layers effects on heat load exposed to firefighter, giving the chance to minimize the thermal load on the firefighters, and the cost related to the integration of high loads of PCMs in fabric. Furthermore, the outcomes of reference⁴⁴ indicated that applying PCMs in protective mechanisms can increase the useful period to tolerate thermal load from 5 min (for non-PCM ones) up to 15 min (for PCM ones) under the same conditions.

Types of PCMs

There are different types of PCMs with various thermal storage capacities and melting points. For appropriate use of PCMs in clothing systems, the temperature of the phase change process should be in the range of human body temperature. Generally, the necessary features of PCM to gain a higher performance cooling mechanism for a special use in the fabric system are a melting temperature between 15^oC and 35^oC, a great amount of fusion, small temperature difference between the solidification temperature and the melting temperature, harmless to the environment, being noncombustible, low toxicity, stable repeatability of solidification and melting, great amount of heat conductivity for efficient heat transfer, low cost and ease of accessibility.⁴⁵ Figure 3 Presents a diverse categorization of PCMs.

Figure 3.

A wide category of PCMs.

Solid-solid phase change substances absorb and release thermal energy by reversible phase transitions between a semi-crystalline or crystalline phase and another amorphous, crystalline or semi-crystalline phase. In solid to liquid PCMs, most of inorganic PCMs are incompatible, corrosive with certain substances, prone to segregate in the phase change and experience supercooling under heat cycling. Inorganic PCMs might be nitrate, metallic or salt hydrates. Such materials are more appropriate for great range of temperatures. However, these materials are suitable for their high thermal fusion, easy availability and low price, their clear disadvantages cause them to be unsuitable for clothing systems.⁴⁶

Natural PCMs

Natural PCMs (the oil refining system product) are appropriate for different usage, as they present a wide range of melting points. Their melting point relies on the numeral amount of carbon atoms existed in the chemical structure. Typically, they are economical, nontoxic and they are available in large value. In the paraffin wax, the rise in the chain length illustrated a direct correlation with the rise in their melting temperature.^47–49

Inorganic PCMs

Inorganic salt PCMs which have “n” number of H2O molecules can be applied in the improvement of thermoregulation clothing systems showing the range of phase change temperature from 20^oC to 40^oC. Among these types of PCMs, sodium sulfate is absorbing because of physical and chemical features. Glauber’s salt has a melting point of 32.4 ^oC and 254 J/g of latent thermal storage capacity which is very appropriate for their use in fabric systems.⁵⁰ Using the hydrated salt in thermal energy storage is very useful in comparison to paraffin wax.⁵¹

Techniques of installing PCMs in fabric systems

Lamination

To obtain thermo-physiological wearing convenience of fabric system, PCMs are combined in thin membranes of the polymeric substrate after their use in the internal side of the cloth by lamination procedure. In this method, water-blown polyurethane foam mix is mixed with about 20–60% of PCM capsules after their use to cloth by lamination procedure where the water is dehydrated from the system using drying.⁵² In addition, passive insulation of the cloth is grown by this technique due to wonderful honeycomb structure obtained followed by foam formation rendering the significant value of air trapping in the structure of the foam in that PCM micelles are conveniently differentiated. Figure 4 illustrates a schematic figure of a lamination method for lamination PCM unified foam.

Figure 4.

The lamination process of PCM integrated foam.

The foams were suggested as an appropriate PCM composite in heat insulation devices viz., the internal lining of gloves, shoes, automotive outwears and interiors, medical products upon combination by lamination into clothing system.⁵³ Some of the benefits of the lamination method are low weight of fabric product, greater concentration of PCM per unit surface which is essential for obtaining great storage capacity as well as affordable production price.⁵⁴

Spinning

The spinning technique is one of the most used methods for combining PCMs in textile that can produce a product including different values of PCMs along with different polymer or liquid polymer which can be spun by various methods including dry, electrospinning or melt.⁵³ The main advantage of phase changing spun textile is that there is no need for extra modification in subsequent processing (weaving and knitting). It can be also easily followed by treatment stage (printing, drying, finishing). Nickel addition, the decent durability of PCM capsule shows no negative impact on tough, color, softness and drape of ultimate textile substance. The processibility of substantial diverse type of PCMs and polymers is challenging and hence no typical spinning methods have been used thus far; however, using electrospinning is expressed as a facile, flexible and optimal technique to build many thermoregulating ultrafine phase change textile since many polymer solutions can be used to impregnate PCMs microcapsule.^55–57

Coating

Microcapsules increase the mechanical and thermal efficiency of PCMs utilized in heat energy storage by growing heat transfer field and avoiding the leakage of melting substances, hence, to coat the microcapsules phase change materials in fabric substance, they are suspended and then wetted in water solution suspension embracing thickener, dispersant, polymer combination, anti-foaming agent and surfactant. Following this step, the coating is used in the fabric substrate. Polymers including acrylic and polyurethane-based macromolecule are intensively being used in manufacturing phase transmission cloth. In general, durability, heat stability, tactile comfort and insulating capacity are influenced by numerous coating factors including polymer binder type, PCM/binder mass ratio, mechanical features of shell substance, adhesion amongst clothing substance and PCM, etc.^58–60

Techniques of installing PCMs in fabric systems

There are many different techniques to integrate PCM in textiles. Novel yarns including well-introduced features and particular production procedure (such as polypropylene yarns, fluoropolymers yarns, and polyaniline) for fabric are required to achieve reasonable outcomes. Various levels of the integration methods should be developed and certified. For small scales, a fiber chips (textile grade) into the yarns can be used (for instance, radio-frequency identification chips in the fabric structures to both transmit and sense information wirelessly transferred). Greater and more complicated modules need a particular package with contacts allowing the interconnection to the yarns (see Figure 5(a)). Crimping is the other interconnection method that is promising for electronics integrations in fabrics. There are many shapes of crimping; however, the main stage is the mechanical deformation with great pressure of a metal crimp near the conductors, leading to a conductor’s deformation and then a gas tight reliable contact (see Figure 5(b)).⁶³ The integration of electronics on stretchable circuit boards is the other method. In addition to silicones, thermoplastic elastomers such as polyurethane have prosperously been applied as a base substance. This technique let maintaining the fabric procedures and the electronics procedures detached until the ultimate assembly.⁶¹ Fabric overall size, shape and carrier can be adjusted without changing the stretchable circuit boards (see Figure 5(c)). A novel method to obtain decent electrical and mechanical interconnection in textile remote sensing membranes is laminating with nonconductive thermoplastic elastomers. For instance, in the publication of Le et al.,⁶² silver nanopowders-caoted polyamide yarn applied as the conductive substances. The nanopowders combined with the fabric by weaving the silver nanopowders-coated polyamide yarn into an elastic textile. This procedure does like the nonconductive adhesive bonding. On the fabric, the thermoplastic foil is pre-mounted. After that, the electronic module is located and then pressed on the foil at the temperature upper the melting temperature in the polymer.

Figure 5.

Techniques in embedding remote sensing systems(a) flexible sensor which is embedded through the fibers of a fabricyarn.⁶³ (b) Crimp contacts for a remote sensor,⁶¹ (c) textile circuit board which connects to flexible remote sensors.⁶²

Shape memory materials (SMM)

Shape memory substances can recover their original configurations upon exposure to an external incitement including, electricity, humidity, magnetic field and heat. Some in-stances of such substances are shape memory alloys, shape memory polymer (such as polyester, polyurethane, poly-, polysilamine, and hydroxyproline) and shape memory ceramics. Shape memory alloys are the most notable memory substances presenting excellent features including great strength with wide technical uses. Shape memory alloys like nickel-titanium, copper-zinc-aluminum or gold-cadmium were improved in 1980s whereas shape memory ceramics and polymers were improved on 1990s. In comparison to shape memory ceramics and shape memory alloys, shape memory polymers have some benefits including lightweight, affordable price, decent processability, great shape recoverability, great deformability, tailorable switching temperature and soft handle.^64,65

Shape memory polymers functions in fabrics

Shape memory polymer can be combined with textile mechanism to prepare essential heat protection as accident happens. Such materials present ample water vapor permeability to the textiles, leading to effectively designed temperature-sensitive textiles. Shape memory polymer that is responsive to humidity, is appropriate choice to apply for development of the humidity absorption. Chen et al.⁶⁶ defined a pyridine unit into shape memory polymer to obtain humidity-responsive textile with high resistance. Additionally, Luo et al.⁶⁷ fabricated shape memory polymer composite that showed suitable water and heat sensitivity and can be beneficial for fire fighters’ protective clothing which are exposed high temperature and moisture (due to water spry and sweat).^67,68 The melting point of shape memory polymer is able to be adjusted to the body temperature. Shape memory polymers have various adaptable functions. The following section defines these functions of shape memory polymers used in intelligent textile fabrication.

Humidity and thermal monitoring

The intelligent breathable cloth is one of the most favorable products in the clothing system, fabricated with shape memory polymers. Coating, knitting, weaving and lamination are used to combine shape memory polymers into fabrics systems. This fabric can regulate the humidity and heat transfer to the body of wearers. The water vapor permeability of shape memory polymers regulates with the temperature of the human body. The shape memory polymers molecular free volume soar as the temperature of body is higher than the temperature of glass transition of shape memory polymers. Therefore, the shape memory polymer is a suitable option for keeping the body temperature stable.⁶⁹ The heat efficiency of shape memory polymer is outlined as follow: Most shape memory polymers use thermal energy as their incitements. These types of shape memory polymers can be considered as thermoplastic elastomers in that there is a hard phase with a great temperature of glass transition (T^g) and a second, switching phase with an intermediate or (T_int) temperature which enables the thermally responsive pattern. The temperature surpassing Tg (or T_int) is symbolized as $T_{hight}$ and the temperature being lower than T_g (or T_int) is symbolized as $T_{low}$ . The shape memory polymers are processed into any desired configuration as their permanent configurations. Therefore, as the temperature is greater than T_g or T_int, a temporary configuration can be induced that can be frozen by cooling the deformed phase at the lower temperature condition, $T_{low}$ . In consequent, as heated higher than T_g or T_int, the shape memory transforms back to their permanent configuration. Figure 6 shows the schematic figure of the thermally responsive procedure.^70–73

Figure 6.

The schematic of thermally responsive procedure of shape memory polymers in textiles.

Self-adaptability of configuration

The shape memory polymer fibers fabricated clothes can be appropriately extended to fit into the body of wearers. These fibers are employed to generate self-adaptable fabrics which conveniently regulate the structure with variations in environmental temperature. However, the shape memory impact of textile is regarded as length variation. This shape memory effect can present a range of forms combined into textiles some features such as thickness rise, shrinkage and bending which is restricted by the structures of the fabrics. The cloth made of shape memory polymer fibers can be adapted to the size of the wearers whereas no considerable pressure is being used on the wearers because of the shape memory polymer fibers shape fixity. Also, the shape memory polymer fibers with developed comfort sensation are able to be applied particularly for low-pressure socks and intimate apparel.⁷⁴

Different kinds of shape memory polymers

Morphological structures

The network chains of the shape memory polymers can either be amorphous or crystalline and the thermal transition for triggering the shape memory effect can fall under either glass transition or melting transition category.^75,76 Shape memory polymers can also have either a glass transition temperature (Tg) or a melting temperature (Tm). Based on the research run by Liu et al.,⁷⁷ shape memory polymers depending on their distinctive melting or glass transitions properties can be classified in four groupings: (1) covalently cross-linked glassy thermoset network (2) physically cross-linked glassy copolymer, (3) physically cross-linked semi-crystalline block copolymers, (4) covalently cross-linked semi-crystalline networks. The shape memory features of shape memory polymers are associated with the morphological structures and this relationship is essential for analysis. For instance, the shape memory effect of shape memory polyurethanes, the ability to shape fixing, and the shape recoverability rely on the network structure of polyurethane and its phase transition. Recent studies have shown that physical cross-linking has a significant impact on the features of shape memory polymers. Crystallization of the soft phase leads to the shape fixity of Tm-type shape memory polymers.

Shape memory impact by indirect heating

Usually, the shape memory effect of shape memory polymers is resulted from directly heating the polymer to a temperature upper the temperature of the switching. In numerous practical uses, electrical power is more appropriate to apply to trigger the shape recovery procedure compared to external heating. It was shown that the shape recovery of shape memory polymers can be obtained by Joule heating after the shape memory polymers are filled with conductive fillers including carbon nanotubes and carbon black.⁷⁸

SMP with supramolecular switches

At the molecular level, the shape memory impact of SMPs is a consequence of rapid elastic modulus alteration because of molecule mobility. The dynamic super molecular structure can result in the mechanical feature alterations of polymers; therefore, dynamic super molecular structures can be used as the switches of SMPs. For instance, in some situations, inter-molecular forces can result in the phase separation of polymers. Scientists have successively provided SMPs by taking benefit of the thermal reversibility of hydrogen bonding. By applying hydrogen bonding as heat reversible switches, numerous SMPs have been fabricated.⁶⁹ A kind of supramolecular SMP, is SMP including pyridine moieties. However, many recent analyses have shown the feasibility of fabricating supramolecular SMPs, the analyses on the features of supramolecular SMPs are not systemic. The supramolecular structures considerably influence the thermo-mechanical and mechanical features of the SMPs.

Shape memory materials (SMM)

Using SMM as sensing parts requires further detailed research. There are many research works focused on SMM usual sensor usage⁷⁹ presented that the bending direction/conformation can be templated and leads to show shape memory. Therefore⁷⁹ could produce remote moisture sensors by interfacing them with stretchable strain sensors and improve particularly for the bendable substances.⁸⁰ applied SMM in strain sensors. Their observation showed that the deformed places and the value of the strain can be identified from the acoustic emission signals of SMS wires. The features, fabrication and the design of a new SM polymer with built-in temperature remote sensing abilities were investigated by.⁸¹ A commercial SMM prior to and following heating in 65°C oven for 2 min (to obtain embossed properties) is shown in Figure 7. Three-dimensional scanning shows that the embossed properties, is around 0.1 mm in height that is the same magnitude as that of surface properties in standard bank coins and can be obviously recognized through physical sensation by figures.⁸²

Figure 7.

Usual SMM label (a) Top: prior to heating; bottom: following heating. (b) Three-dimensional scanning outcome, showing that the surface properties following heating for shape recovery is around 100 μm in height.⁸²

The shape memory materials for non-reversible activation; however, the SME cycle is able to be repeated following re-programming. Sensors-based SMMs have the mixed properties of having great security/reliability and being non-reusable/non-repeatability.

Smart textile coatings’ technologies

Textile -based bio-electrodes and sensors

Sensors evaluate and monitor the environmental or biometric information can work as an input interface system. Textile-based electrodes and sensors have been improved from textile optics or conductive textiles.^83,84

i. Biometric data

Fabric sensors are used to record electrocardiogram, heart rate, sweating rate, etc. Typical sensors sometimes pose challenges due to their functional requirements or physical structures. They may face problem including body irritation either by the gel used with typical electrocardiogram electrodes or adhesive part of the electrodes. Fabric sensors, as an alternative to typical sensors, are improved to address such discomforts. This technology can be used to monitor patients in healthcare and clinical conditions, athletes in severe physical activities, firefighters work in hard environmental conditions, etc.^85,86

ii. Body motion/the wearer’s body position

As the textile is stretched, the cotton resistance changes (in correspondence with a decrease and increase of length), which allow monitoring the movement of the wearer. There is increasing interest in intrinsically conductive polymers for usage in intelligent fabric systems as actuators and sensors. Intrinsically conductive polymers are a promising option to be used in wearable mechanisms, as they have electrical and mechanical features, magnetic, optical, and electronic features of metal. Some samples of such polymers are Polaniline and Polypyrrole (Ppy). Intrinsically conductive polymers alone or in combinations with typical polymers can be applied to generate conductive fibers or utilized as coating substances.⁸⁷

Information and energy management technologies

Information management technologies are associated with computation, information processing and memory. Electronic parts are still applied for those tasks in intelligent fabric systems, as there are no fabric substances on hand that can do those tasks by the present. Yet development has been done in the miniaturization direction of the electronic parts and progress of flexible layers so that the electronic components can be combined with the fabric system in the easier path.⁸⁸

Usual wearable electronics are heavy, rigid, and bulky with a short lifetime. These are the main challenges with power supplies applied for typical wearable electronics. Typically, the heaviest and the biggest segment in the wearable electronics is energy storage and supply as well as they require to be recharged with present levels of power consumption. To be used in the design of smart fabrics, power supplies should be light and flexible to be able to combine into the fabric system also do not create not challenge or difficulty for the wearers. In addition, they should be easy and long-lasting to recharge on the move and utilize alternative energy without any requirement for recharging, also they should be resistance to wear and to washing. Many ways have been proposed to decrease the weight and the size of the electronic parts so that they are able to be attached to fabric systems and to improve new technologies of energy supply from different resources such as vibration, thermal sources, sunlight, etc. Power management methods provide the chance for the electronic systems to perform more effectively with a shorter value of power. This leads the battery-powered electronics to have longer lifecycle. Photovoltaic systems, particularly thin membrane technology enabling the generation of flexible structures, has attracted many attentions. In a solar-powered jacket from Scottevest Inc., thin membrane photovoltaic copper indium gallium diselenide, was located on a thin stainless-steel cotton and constitute the separable solar panel. Thin membrane photovoltaic substances including copper indium gallium diselenide or amorphous silicon be able to be layered onto a flexible polymer membrane.^89,90

Parameters monitored by firefighters’ intelligent textile

In this section the applications of smart coating for firefighters are discussed. Two cases of firefighters smart coating applications (health care protective), and (dangerous material) were explained in detail and parameters monitored by firefighters’ intelligent textile are presented. Improvements in protective fabrics and substances can solve a series of challenges including several industrial segments, energy service and hospital sectors that individuals face different kinds of dangers. Protective fabrics can prepare essential protection in fire, heat, chemicals risk etc. The protective coatings performance may be active such as sensory (relating to the senses), self-healing, and adaptive or can be passive including an obstacle against rain, cold, and moisture. A conceptual simulation presenting the human system interaction and the architecture of the intelligent fabric system is shown in Figure 8. Based on this model, the internal fabric keeps the interface between the fabric and the human. This substrate can be attached to external layer regard to insulation target.

Figure 8.

A schematic of human system interaction and the architecture of smart textile.

Intelligent wearable systems can be used in the following fields:

1. Dangerous jobs (including firefighting and construction) to control the vital health signs of the wearer (environmental and activity terms) to avoid potential danger conditions including injuries and accidents and present preventive or corrective actions to stop or minimize further risks.^91,92

2. Sports and lifestyle; to control training activities as well as health terms to avoid potential harms to obtain optimal fitness, evaluate the quality of sleep, contribute to altering lifestyle.^93,94

3. Medical application: to control the health condition of patients (environmental and activity terms) to avoid harms and early detection of disorders/diseases, provide preventive measures to stop or minimize health impacts. Novel improvements in intelligent coatings used for protective fabrics are outlined in the following section.

Intelligent coating for firefighter protective fabric

Protective fabric is imperative for firefighters to protect them in heat exposures and other types of risks. Standard FFPC utilized for structural firefighting containing a multilayer fabric to present the essential radiant and flame thermal protection and also to avoid the penetration of dangerous chemicals and water.^95,96 The FFPC contains a heat liner including a humidity barrier, a lining substance, and a heat barrier, and a flame-resistant outer shell.^97–99

An intelligent clothing based protective mechanism is used to provide more safety to wearers encountering risky conditions. Such a mechanism is installed to a wearer protective suit, and it can control the rate of heart, detect the firefighter movements, combustible and toxic gases, and evaluate the temperature and moisture outside and inside of the cloth. Intelligent clothing has decent features including great durability, washing resistance, convenient and flexibility of fabric and harsh environment resistance. Due to aforesaid reasons, novel technologies for attached sensor modules packing as well as interconnect mechanism, resisted in automatic washing, have been improved. An overview of intelligent wearable systems utilized to control safety factors for firefighters is illustrated in Figure 9.

Figure 9.

The overview of smart wearable systems for firefighters.

FFPCs are significant for firefighters in extreme fire conditions. Sometimes, a flash fire might happen with a thermal flux up to 84 kW/m²,¹⁰⁰ which pose a high risk to firefighter lives and safety. In these conditions, FFPC should prepare decent heat protection for firefighters.^100–102 The heat protective efficiency is an important factor for firefighters’ clothing design. The phase change materials (PCM) clothing was located at different layers of fabrics. A thermal protective performance tester evaluates their heat protective efficiency. PCMs are intensively employed as a method to reduce the inconformity between the demand and the supply of the energy, and they are appropriate tools to control the heat energy. PCMs are latent thermal storage substances with small temperature changes in the phase change process. The phase change materials can be used in clothing systems to regulate thermal energy in them.¹⁰³

Adding PCMs to a typical FFPC can increase the time of second-degree burns and reduce the severity of the burns. Many types of PCMs are available for clothing systems with various melting temperature and different capacities of heat storage.^104,105 The addition of a PCM layer to a FFPC system under various conditions was investigated by Zhu et al.¹⁰⁶ The authors explored a composite fabric system which contains heat liner mixed with PCM, humidity barrier and outer shell, and investigated the influence of PCM temperature as well as its position on the heat protection efficiency of FFPCs using step-cooling technology. Therefore, PX 52 PCMs with a melting point of 47–53°C gave the maximum time of heat protection compared to other types of PCMs. In fact, PCM was situated in a sandwich structure between two flame resistant clothes avoiding flame propagation or burning. A composite fabric including heat liner, humidity barrier, convenient layer and mixed with shape-stabilized phase change material particle can grow the heat capacity.¹⁰⁶

Intelligent coating in systems for dangerous substance protective fabric

Intelligent fabrics have been improved largely. They can detect hazardous heavy metals and protect wearers from harmful chemicals including hydrogen cyanide, benzene, and dioxins, and chemical threats from polluted surfaces. Such coatings contain strippable polymeric compositions including polymers blends, additives and copolymers which are sprayed or brushed onto a surface. As they are employed to a polluted area, such coatings showed to be responsive to chemical stimulations. Chemical incitements including change in PH or the existence of a dangerous substance can lead to an environmental alteration. Intelligent wearable systems employ sensors to collect raw information from sensing chemical threats pollutions.^107,108 Many wearable sensors have been improved largely for real-time non-invasive controlling. This part provides an overview of sensors used to control physiological factors. Using real-time data, such systems provide the wearers some critical information to avoid dangers or minimize exposure to the risky environment. Also, they can be used in a larger domain. An overview of sensors used to minimize the impact of hazardous materials and conditions is shown in Figure 10.

Figure 10.

The overview of smart wearable used against hazardous material.

The novel intelligent coating is perfectly safe substances. They can demonstrate some high level of responsive behavior by showing the polluted surface. This is significant, as polluted part of the coating can be separated from unpolluted surfaces. Moreover, the coatings should be designed in a way that can be purified, reused, and redissolved.

Intelligent coating for health care protective fabric systems

The chemical and biocidal coating may also be used for protective clothing of workers and patients. The vital indications including blood glucose and blood pressure of patients need to be checked frequently. So, the improvement of new wearable smart systems to efficiently control the vital indications 24 h of a day are essential in some cases to understand and monitor the progress of chronic signs. Wearable smart mechanism for health care contains different types of sensors including biochemical, fabric-based sensors and microelectronic. For non-invasive controlling of physiological parameters including electrodiogram, biopotentials, heart rate, blood glucose, blood pressure, blood oxygen saturation, body activities, body sweating and photoplethysmogram. Previously, a wearable mechanism combining an ultrasound sensor has been improved for the cardiopulmonary activity controlling in emergency conditions.^109,110 A highly multifunctional flexible intelligent coating is usually fabricated using spray-coating multiwalled carbon nanotubes (CNTs) suspended in a thermoplastic elastomer solution. A superhydrophobic multiwalled carbon nanotube in thermoplastic polyurethane (MWCNT/TPE) intelligent coating with great-efficiency sensing capability toward bending, torsion and stretching was improved with the capability of being fabricated in ambient conditions without any need for activation of substrate or cleaning. Superhydrophobic intelligent coating can provide different typical objects such as cloth, plastic, metals, and glass with superhydrophobic features for drag decrease, self-cleaning or uses for utilization in human healthcare. Such intelligent coating can be utilized as a high-efficiency, multifunctional and flexible wearable sensor to human health.¹¹¹ Information technologies and modern communication present a cost-effective and effective way that provide convenient conditions instead of using costly healthcare systems for the old to continue to live.¹¹² These mechanisms facilitated with unobtrusive and non-invasive wearable sensors can be promising diagnostic instruments to personal healthcare to control physiological conditions and the patients’ activity in real-time, from a distance facility. These systems might contain various types of flexible sensors which can be attached to clothing systems, elastic bands or installed to the body and textile fiber. These sensors can measure physiological conditions including electrocardiogram, electromyogram, the rate of heart, electrodermal activity, blood pressure, sweating rate, body temperature and arterial oxygen saturation.

An overview of sensors used to control safety factors for firefighter is presented in Figure 11.

Figure 11.

The overview of smart wearable used for health care protective clothing.

Conclusion

Textiles have high potential in achieving fabric substance with active convenience regulation features. Mechanisms for the intelligent coating of fabric substances are based on the usage of various smart polymeric substances. This review presented smart technologies and substances including PCMs, SMM, and wearable monitoring devices utilized in the smart fabrics’ coatings. The PCM presented high-capacity usage in improving smart heat protective clothing mechanisms. The conclusions obtained in this study assist to develop PCMs with higher heat resistance potential and lower thermal release that are highly suitable for the heat protective clothing mechanisms. The main findings can be summarized as follow:

1. It was articulated that the carbon black loaded organic phase change materials were improved to increase the heat conductivity and photothermal performance and to identify a promising application in the latent heat energy storage.

2. The test results indicated that the latent heat attracted within the phase change materials decreases temperature that may be experienced at the fire fighter’s skin surface, advancing the great temperature efficiency of FFPC.

3. Applications of SMM and PCM could be used just to specific fields where fire fighters protective clothing are usually heated to maximize the benefit of having such materials without adding considerable weight.

4. Using multiple substrates of coating into fabric layers to combine multiple functions is straightforward. Multilayer coatings might contribute to addressing significant challenges associated with protective fabric system.

5. Fabrics with shape memory polymers can change or move their shapes, obtaining various three-dimensional forms in clothes, growing their aesthetic appeal. The variation of clothing shape can be employed to protect the wearer in extreme conditions. Due to fast improvement in development of shape memory polymers and phase change materials and new methods to combine them into fabric systems, it is predicted that studies on textile systems will be expanded in multiple directions due to their promising potential usage. It is important to apply multiple layers of coating onto a fabric system to achieve multiple functions. This can contribute to address substantial challenges associated with protective clothing. Multilayer smart coatings to increase protection synergistically require more analysis in the context of protective clothing.

6. In case of wearable monitoring devices, even though, available techniques showed significant improvement in fabricating cloth-based humidity sensors, the electrode durability presented by coating, deposition and printing are remained as a great challenge that should be addressed in the future research.

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

ORCID iD

Maryam Ghodrat

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