Preparation and characterization of intelligent thermochromic fabric coatings for the detection of fever diseases

Abstract

Real-time monitoring of changes in skin temperature with smart thermochromic fabrics that act as sensors is extremely important in the early diagnosis of febrile diseases such as the COVID-19 epidemic that endanger public health. In this context, the study aims to detect fever, which is the immune response of the body, as a symptom in the diagnosis of various diseases and to prepare a thermochromic functional fabric by coating method to reduce the risk of contamination. For this purpose, a composition containing green pigment and zinc acetate dihydrate as the starting material was prepared using the sol-gel method. The prepared composition was applied to calico and alpaca fabric, and it was provided to show transformation at 37.5 °C with the effect of the pigment, which had a color change feature at 33 °C. The samples were analyzed using scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), and thermogravimetric analysis (TGA) characterization methods. The results showed that it was possible to change the active conversion temperature of the pigment from 33 °C to 37.5 °C, depending on the composition. The use of the compositions developed in this study in alpaca fabric coating provides an area of use as an indicator if the human body temperature reaches 37.5 °C, which is considered the concept of fever.

1. Introduction

Thermochromic fabrics are innovative fabrics that show color change against temperature stimuli by utilizing thermochromic colorants such as dyes and pigments in textile products [1,2]. Thermochromic pigments, on the other hand, are materials that change color with the effect of heat and are reversible or irreversibly separated according to their types [3]. These pigments are mostly based on leuco dyes, which provide ease of use and have a heat-sensitive color-changing feature [[3], [4], [5]]. Leuco dye thermochromic pigments are colored below the melting temperature of the solvent but are colorless above this temperature [1]. When the temperature of thermochromic pigments is increased, the solvent in the pigment microcapsules melts and passes into the liquid phase [3]. Microcapsules in thermochromic pigment can be used as a skin temperature indicator since they include leuco dye that changes the chemical structure [6]. With the use of thermochromic pigments in smart textiles, a new era has entered in innovator fabrics [1].

The physiological skin temperature is between 33 and 38 °C. An increased average skin temperature is also linked to physical fatigue [7]. Thermochromic pigments are used in fabrics to detect this fatigue [6]. The average human body temperature is between 36.5 and 37.5 °C [8]. In studies on the COVID-19 epidemic, a body temperature above 37.5 °C was chosen for the definition of febrile patients [[9], [10], [11], [12], [13]] and it was aimed at maintaining social distance within the scope of preventive measures [14]. In general, the human body keeps its temperature constant at 37 ± 0.5 °C in different climatic conditions. Color change occurs when the fabric temperature rises beyond the activation temperature of the colorant. This thermochromic color change is very evident and takes place over a small temperature range [15]. Smart textiles that monitor this kind of health status are getting more and more attention day by day [16]. Functional fibers are used as the basis for the construction of smart textiles based in the health field [17]. Flexible thermochromic fabrics prepared from thermochromic fibers with excellent reversible color-changing properties by wet-spinning can find use in a range of application areas, from wearable devices to dynamic color displays, from human-machine interfaces to showing temperature changes in healthcare applications [18]. The creation of textile structures that change color in response to changes in temperature by coating the yarns with liquid crystal materials also makes them useable for diabetic hosiery and bandage applications. The production process of these yarns was made with a spool coating apparatus, knitting and weaving techniques were used in the production of breathable textile structures, and the wavy structural change of the coated yarn was revealed with SEM images [19]. Thermochromic fabrics can also be evaluated to provide heat control [20]. Another study investigating tactile binary phase change fibers with thermochromic and triple shape memory functionality is related to human thermal management that adjusts the microclimate of the skin. The fibers made by the melting injection method by adding a thermochromic agent are in the core-shell structure and the flexibility of the wristband, which acts as a continuous heat source for low temperature, provides an effective adaptation to changing thermal conditions as it provides closeness to the body [21]. In a recent study, a layer of small polymer particles has been applied to the textile surface by using colloidal latex and liquid crystals together. In these textiles, which could also be used in medical and biosensing fields, bright color was obtained due to the flattening of cholesteric liquid crystal droplets [22].

Intelligent textiles that can respond to external stimuli such as temperature are mostly produced with the help of methods such as textile finishing and coating [23]. The technological applications of these textiles can offer innovative opportunities for different sectors [24]. The sol-gel method, which is one of the coating methods, is a pioneering and impressive method that can change the properties of recently emerged fabric fibers, is applied to add functionality, and provides homogeneity [25]. In this context, the sol-gel method appears in the literature as a low-cost method that enables thermochromic fabric to be prepared easily and quickly [26]. The problem that arises with the use of thermochromic materials in fabrics is the restricted affinity of these materials for fibers [27]. In this study, it was aimed to prepare thermochromic fabrics with a very rapid thermochromic response when tin (IV) chloride pentahydrate, thermochromic pigment-containing composition was applied to the fabric surface to serve as a sensor in the identification of individuals with fever.

2. Experimental studies

2.1. Preparation of coatings

The coating was formed from a thermochromic pigment and polymer matrix that changes color from green to white with an activation temperature of 33 °C. First, 2.2 g of zinc acetate dihydrate (Merck, EMSURE® ACS) was dissolved in 10 ml of methanol (J.T. Baker). 0.5 ml of pure water and, as a catalyst 10 ml of acetic acid (Merck, EMPROVE® exp 100%) were added to this mixture. In separate beakers, 1 g of tin (IV) chloride pentahydrate (Across Organic, 98+%, extra pure), 5 ml of methanol (J.T. Baker), and 2 g of stearic acid (Merck) were dissolved in 10 ml of methanol and added to this mixture. Finally, 2 g of polyvinylpyrrolidone K 90 (PVP, TCI, average molecular weight: 360,000) was added and mixed for 50 min 0.65 g of green pigment obtained from the Hali pigment company was added to the mixture, which was aged for 1 day. Then it was mixed in the sol IKA-WERKE brand mixer until homogeneous. The flow chart of the prepared sol was shown in Fig. 1 , the chemical amounts used in the composition were shown in Table 1 , and the pH value was measured as 3.138. After the fabric was placed on a flat surface, the solution was applied to the surface of the calico and alpaca fabrics with the help of a brush. When this composition was applied to calico and alpaca fabric, an efficient result was obtained. In this study, only the features of alpaca fabrics were examined and characterized in detail.

Fig. 1.

The flow chart of the prepared sol.

Table 1.

Chemical compositions of solutions.

Amount (g)	Chemical
2.2	Zinc acetate dihydrate
1	Tin(IV) chloride pentahydrate
2	Stearic acid
2	Polyvinylpyrrolidone
0.65	Green thermochromic pigment

2.2. Characterization of coatings

The size distribution percentage values of the created sol in terms of volume were determined by Malvern, Mastersizer 2000 brand device. The thermal stability of the thermochromic coatings was characterized by Q500-TA brand TGA in the temperature range of 25–900 °C. Color transition temperature and color recovery temperature were evaluated by Q2000-TA brand DSC. The chemical bonds formed in the fabrics were evaluated using a Bruker Tensor 27 brand FTIR device. CIE L * a *b * color values of thermochromic fabrics were measured by the Konica Minolta CM 2300D brand spectrophotometer device. Analysis was performed with a Zeiss SupraTM 40 VP brand SEM to confirm the presence of microcapsules in the fabric fibers before and after washing. The color change properties of fabrics with heat were investigated with an HT-02D brand infrared thermal camera.

2.3. Particle size analysis

Particle size is an important factor in the union of phase-change materials into textiles [28]. The particle size distribution graph was given in Fig. 2 . The generated sol showed unimodal particle size distribution. As a result of the measurement of the particle size distribution of the composition, the d_(0.1), d_(0.5), and d_(0.9) values were determined as 10.087 μm, 47.131 μm, and 117.440 μm, respectively.

Fig. 2.

Particle size measurement result of the sol containing tin (IV) chloride pentahydrate and stearic acid.

3. Results & discussion

3.1. Thermal analyzes

The thermal analysis results of the samples were given in Fig. 3 . The first stage of decomposition was up to about 150 °C and was related to the evaporation of water. Similar to the highest mass loss (66.76% in the 200–400 °C range) of the thermochromic pigment, significant weight loss in the thermochromic coating was observed in the temperature range of about 150°C–450 °C.

Fig. 3.

TGA curve of the coating containing tin (IV) chloride pentahydrate and stearic acid.

The phase change temperature from the thermal properties of thermochromic coatings is the most important factor affecting the practical and daily use of the fabric. The phase transition temperatures of the green-colored thermochromic pigment with an activation temperature of 33 °C were determined as 35.07 °C and 24.10 °C for the heating and cooling thermal reactions, respectively (Fig. 4 (a)). This result, which was also consistent with infrared thermometer measurements, was an indication of the color change temperature range for the green thermochromic pigment. That was, the color change temperature of the green thermochromic pigment was in the form of an interval, not a point. The single peak observed during heating corresponds to solid-solid and solid-liquid transitions. The endothermic peak temperature in the coating containing tin (IV) chloride pentahydrate and stearic acid indicates that the transition temperature was 37.5 °C (Fig. 4(b)). During cooling, the exothermic peak was at 32.6 °C, indicating that the thermochromism was reversible. The sharp peaks in the coating's DSC curve indicate that the color transition was clear. Comparing the DSC curves in Fig. 4 (a) and (b), a shift to higher phase transition temperatures was observed in the thermochromic coating. While the reason for the shift of the DSC curve varied due to different reasons, one of these reasons was the variation of the reactions between the core and the shell, together with the sol-gel composition. Thermochromic pigment and coating composition had different particle sizes that could affect their thermal behavior was one of the other factors that cause the shift. Thermochromic pigment and thermochromic coating had different compositions that lead to differences in their thermal properties were among the reasons for the change in the transition temperature. When this composition was changed, the phase transition temperature also changed. In addition, liquid-solid and solid-solid transitions were clearer in cooling since the peak of the thermochromic sample during cooling was narrow and sharp compared to that of the thermochromic pigment.

Fig. 4.

DSC analysis result of (a) green pigment (b) the coating containing tin (IV) chloride pentahydrate and stearic acid. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

3.2. Fourier transform infrared spectroscopy analysis

As a result of the analysis, as shown in Fig. 5 , C–H stretching peaks were observed at 2918 and 2849 cm⁻¹, and C–H bending peaks were observed at 1465 cm⁻¹. The peak at 1741 cm⁻¹ corresponded to the C O stretch, while the peak at 1635 cm⁻¹ was attributed to the C C stretching vibration. The peak in 1541 cm⁻¹ was related to the Zn–O stretching vibration. The peaks seen at 1173 cm⁻¹ and 643 cm⁻¹ were attributed to the Sn–OH and Sn–O–Sn vibrations, respectively. In the FTIR spectrum, the peaks of Sn were located in the fingerprint region.

Fig. 5.

FTIR spectrum of the coating containing tin (IV) chloride pentahydrate and stearic acid.

3.3. Color measurement analysis

The relevant results of the measurement results were given in Fig. 6 and Table 2 a* value of −32.27 on the red-green chromatic axis indicates that the thermochromic fabric color was green. Since two axes were benefited from in the horizontal plane for the true color, the negative a value is perceived as a green color indicator in the measurement, while the b value was detected as 13.16, which indicates that this fabric was also on the yellow color axis. In the color space, the L value, which represents the color point lightness of this fabric and was found as 60.58, was the intersection point of a −32.27 (green), b 13.16 (yellow) values.

Fig. 6.

Color measurement of thermochromic coating. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

Table 2.

Color values of thermochromic coatings.

Sample Name	L	a	b	ΔE*ab
Green fabric	60.58	−32.27	13.16	52.18

3.4. Microstructure analyzes

The microstructural characterizations of samples were given in Fig. 7 (a–d). Broken fibers were observed in the microstructure analysis of the fabric before coating (Fig. 7(a) and (b)). In the SEM analysis, it was seen that the fabric was completely covered with the coating formed by the sol-gel method (Fig. 7(c) and (d)). Collecting and coating the thermochromic capsules on the surface of the fabric fibers confirmed that the capsules were processed into the fabric fibers. The spherical structure of the microcapsules in the green thermochromic pigment was confirmed from the analysis results (Fig. 7(e) and (f)). Despite alpaca fabrics being washed in a washing machine at 40 °C for 1.5 h with plenty of detergent prewash, the density of microcapsules between and on the fabric fibers was remarkable. In addition, the aggregation of microcapsules in the washed fabric was related to the affinity of microcapsules to fabric fibers (Fig. 8 ). While areas circled in red in Fig. 8 show thermochromic microcapsules the yellow arrows indicate fabric fibers. The reason for the formation of agglomerated thermochromic microcapsule particles between the fibers was that these microcapsules are close to each other. This situation between the fibers indicates that the coating was very well bonded to the fabric. It can be seen from Fig. 8 that the microcapsules adhere well to the surface of the alpaca fabric after washing. As a result, the thermochromic coatings were successfully fixed to the fabric fibers. The weight and atomic percentages of the elements in the EDX analysis of coated fabric to detect the chemical compositions were given in Fig. 9 . Element Zn comes from zinc acetate dihydrate, element N comes from polyvinylpyrrolidone, and elements Sn and Cl come from Tin (IV) chloride pentahydrate. The peaks of Cr, Ni, Cu, and Fe elements were related to the composition of green thermochromic pigment. The elements determined in the EDX analysis of the coated washed fabric were C, O, N, Zn, Sn, Ni, Cu, Cr, Si, and Fe (Fig. 10 ). These results once again indicated the presence of Cr, Ni, Cu, and Fe elements in parallel with the EDX results of the green color thermochromic pigment.

Fig. 7.

Scanning electron microscope images of (a–b) uncoated fabric (c–d) coated fabric containing tin (IV) chloride pentahydrate and stearic acid (e–f) spherical structure of green thermochromic pigment microcapsules.

Fig. 8.

Scanning electron microscope images of the washed coated fabric.

Fig. 9.

EDX analysis of coated fabric containing tin (IV) chloride pentahydrate and stearic acid.

Fig. 10.

EDX analysis of the washed coated fabric.

3.5. Color changing properties of fabrics with heat

The image of the thermochromic coating, which was created by applying the prepared sol to the calico fabric with the help of a brush, dated 19 February 2019 was given in Fig. 11 (a). Fig. 11(b) showed the thermochromic coating applied to the alpaca fabric with the brush on February 19, 2019. Fig. 11(c) showed the condition of the coating on this same alpaca fabric on June 30, 2022. The color difference between the two images in Fig. 11(b) and (c) was due to the temperature difference at the time the photos were taken. As a result, although 3 years, 4 months, and 17 days have passed since the formation of the coatings, the fabrics change color sensitively.

Fig. 11.

The image of thermochromic coatings created by the application of sol to (a) calico and (b) alpaca fabric on February 19, 2019 (c) image of the same alpaca fabric on June 30, 2022.

In infrared thermal camera spot measurements, where the colors formed according to temperature were indicated, Fig. 12 (a) showed the point where the temperature is 32.5 °C, while Fig. 12 (c) indicated the point where the temperature is 37.6 °C. In this device, the colors that change according to the temperature of the environment were blue, red, yellow, and white from cold to warm, respectively. Thermochromic fabric, whose color changes were monitored with an infrared thermal camera, was green at 32.5 °C (Fig. 12(b)). It was observed that the color turned completely white as a result of the temperature increased up to 37.6 °C (Fig. 12(d)). The color of the fabrics, which was initially dark green, gradually lightened with the application of heat and turned completely white at 37.6 °C. As the white thermochromic fabrics were cooled again, they became green in color at 32.5 °C. These results were also compatible with the DSC results, and the fabrics produced show reversible color change (Fig. 12(a) and (c)). When the fabric, whose color change performance was examined with an infrared thermal camera, was heated, a gradual lightning was clearly observed (Fig. 13 ).

Fig. 12.

Infrared thermal camera images of the thermochromic fabric.

Fig. 13.

Color change of fabrics with increasing temperature. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

There was no change in the color-changing performance of thermochromic fabrics that were heated and cooled repeatedly. Reversible color change between green and white was observed continuously with the heating and cooling of the fabrics. When the fabric was cooled, the color recovery was good enough to be the same as the original green color, and the sample had excellent color reversibility. The fabrics produced displayed excellent color change and thermochromic performance after multiple heating and cooling cycles. The prepared calico fabrics were washed in the washing machine for 2.5 h at 90 °C, and the alpaca fabrics were washed for 1.5 h at 40 °C with plenty of detergent prewash. After washing, calico and alpaca fabrics showed reversible thermochromic performance. Fabrics washed by using bleach together with detergent also preserved their green color and thermochromic properties (Table 3 ).

Table 3.

Fabrics washed using with bleach and detergent together.

4. Conclusion

In the fabrics formed, the color was reversed as a function of temperature. The thermochromic coating indicated high color contrast and reversible color change. The desired thermochromic functionality, reversibility, and sensitivity were supplied in this composition. Due to the reduction of the softness of the yarns of the fabrics with the coating, the sliding motion of the threads was limited so that the fabric was not prone to tearing. The temperature variation of the made thermochromic coating represented the body temperature range. Thus, with this product, the fever control of people in the little or very old age group who have difficulties in their own care will be able to be easily controlled, and their health status will be monitored. With this study, it was possible to coat and prepare for medical diagnostic purposes reversible color-changing alpaca fabrics depending on temperature change using the sol-gel method. The washing feature of the fabrics showed that the colors were fixed to the fabric as desired. The coating was successfully bonded to the fabric fibers. As a result, even after repeated washings, the thermochromic property of the fabric formed, and its heat-sensitive color-changing properties were preserved. With the studies carried out, manufacturability was realized by using the fabric coating method. In different cases, such as during the COVID-19 pandemic, this product can be used to detect the symptom of a person's fever with a color change at 37.5 °C, which was determined as the critical body temperature.

CRediT authorship contribution statement

Lale Civan: Writing – original draft, Visualization, Investigation. Semra Kurama: Writing – review & editing, Supervision, Validation, Project administration.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Data availability

Data will be made available on request.