Investigating the Thermo-Physiological Comfort Properties of Weft-Knitted Smart Structures Having a Negative Poisson’s Ratio

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Introduction
An efect in which the materials or structures extend perpendicular to the directions of applied force is demarcated as Poisson's efect [1,2]. Typically, Poisson's ratio of isotropic homogeneous structures ranges from −1 to 0.5. Poisson's ratio describes how much structure contracts or expands when we apply force on them. Te phenomenon of auxeticity results from the internal structure of the molecules and the way they deform themselves [3]. Due to the axial load, auxetic structures become thicker. On the application of compression load, it becomes thinner because of its hingelike structure that causes fex when it is stretched. Tese materials/structures worked well during shear modulus, energy absorption, vibration damping, and sound absorption testing [4]. Auxetic materials have numerous desirable characteristics due to their special quality, including good body ft, better shear resistance, improved indentation resistance, increased plane strain fracture toughness, increased resistance to shock, soundproofng, and other features. Tere are several possible uses for auxetic materials, such as knee pads, bulletproof vests, shockproof gloves, explosionproof drapes, medical dressings, and areas of tissue engineering, for example, artifcial blood vessels and artifcial folding mucosa [5][6][7].
Available literature has witnessed the work on helical yarns, woven and composite structures, and warp-knitted and weft-knitted auxetic structures [8,9]. Knitting technology allows several mesh planar structures and foldable 3D structures with auxetic potential, and this study feld is expanding day by day. A warp-knitted fabric is developed using principles of the helical yarn structures with inlay yarn and pillar stitches. Wales are knitted from softer, thicker flaments, and the stifer yarn is used as inlay. On the application of force, it expands in a widthwise direction on the application of longitudinal force and behaves as an auxetic fabric [10]. NPR of warp-knitted hexagonal structure made from spandex and polyester relies on adopting spandex polyester arrangements in wales. Te auxetic behavior of such a structure is more evident in a 45-degree wales-wise direction [11]. A spacer structure is made of two parallelograms mutually aligned in a V shape. Results showed that NPR value is afected by yarn types, the direction of stretching, and the angle between two parallelograms. It delivers better results in the diagonal direction than in the warp and weft direction [12].
In weft knitting, auxetic fabrics are formed based on the reentrant model, rotating triangles, folded structures, and hexagons and can be formed using a computerized fat-knitting machine [13,14]. Te separate rectangle units can be knitted and then connected on certain vertexes. Te results show that the auxetic efect mainly depends on knitted structures. Flatknitting technology could provide a simple but highly efective way of fabricating auxetic fabrics from conventional yarns [15]. Changing the geometry of the weft-knitted fabric folded structure can be achieved, and these closely folded fabrics typically have more excellent negative Poisson's ratio [16]. Weft-knitted auxetic fabrics were made with high-performance yarns (polyamides). NPR of the developed structures was strongly infuenced by the material parameter (fber type), structural parameter (loop length, cover factor, and yarn density), and machine parameters (takedown tension) [13]. An increase in loop length resulted in a signifcant improvement in NPR due to the availability of longer yarn lengths in the stitches for structural rearrangement [17]. Another study reveals the infuence of stitch length and raw material on the auxetic efect. Samples were made with cotton and acrylic yarns at three diferent stitch length values, making loose, medium, and tight fabrics [18]. Decreasing values of the stitch length, initial angles of foldable auxetic fabrics also decrease and make fabric tighter, which retain their shape and subsequently show a higher auxetic efect. Regarding the efect of diferent yarns, acrylic shows a higher auxetic efect [19].
Developing smart and novel structures is one of the most focused research areas in the last three decades [12]. A lot of research has been done in to develop such structures by varying the forming element and their combinations [20][21][22]. Te researchers have developed auxetic fbers [9], yarns [23], fabrics [24], and woven and knitted structures [25]. Some of the knitted structures have a specifc application for clothing [26,27], especially kids wear and maternity clothes, where the comfort of fabric and structure is of high importance. None of the literature is available on the comfort properties of auxetic weft-knitted structures, as per the author's knowledge. Te current study focuses on the evaluation of the thermo-physiological comfort of three diferent auxetic weft-knitted structures made by commonly used yarns for the purpose of clothing. Te study also evaluates the auxetic behavior of these knitted structures. Te thermo-physiological comfort includes air permeability, overall moisture management capacity (OMMC) and thermal resistance. Te fndings of this study will be helpful for textile technologies and designers to develop auxetic structures with improved comfort properties for kids' wear and maternity clothes.

Materials and Methods
Te materials used in this research work were polyester, nylon, acrylic, and cotton yarns having a linear density of 16 tex. Polyester and nylon were used in an intermingled flament form, while cotton and acrylic were in the spun form. Foldable diferent auxetic structures were made having a combination of face and reverse loops by using a Shima Seiki fat-knitting machine (SVR123SP, Japan), with a gauge of 14, and equipped with the Apex 3 design system. Te stitch length was constant (0.42 cm) for all structures. Tree diferent auxetic knitted structures (Figures 1-3) were made using polyester, nylon, acrylic, and cotton yarns.
Te factors and their levels are described in Table 1. Te design of the experiment (DOE) of the current research work was decided after applying full factorial through the Minitab 17 software on the above two input variables, as given in Table 2.

Stitch Transfer Process.
In the development of auxetic fabrics, a transfer stitch technique was used to make a foldable structure. In a fat-knitting machine, a transfer needle is used for this purpose (Figure 4). For the stitch transfer, a delivering needle is raised above the clearing height with the cams in the carriage, and the loop stretches over the transfer spring ( Figure 5). Ten, the receiving needle is raised by the cam on the other bed, enters the transfer spring of the delivering needle, and penetrates the loop which will be transferred ( Figure 6). Te delivering needle moves back, leaving the loop on the receiving needles, and in this way, the loop is transferred to the other loop, as shown in Figure 7.
In single jersey knitting, after transforming the straight yarn into a knitted loop, a loop is joined with bending and twisting forces during the knitting of fabric, and the residual inner force in the produced fabric is stored. Tese forces can cause the edge to get out of the fabric plane and cause deformation and curling [28]. Weft-knitted auxetic structures can be made by front and back knitting on the fat-knitting machine (Figure 8), and due to the curling efect of the single jersey structure, after relaxation, these structures become foldable, front-knitted loops curls at the back and back knitted loops curls at the front side making the structure foldable. Developed structures are available in Figure 9.

Characterization
2.2.1. Physical Characteristics. Diferent physical parameters of all auxetic structures were calculated after knitting. Wales per inch and course per inch were checked using pick glass from 5 diferent places and calculated the average per standard ASTM D8007-15. Stitch length and stitch density are calculated. Te areal density of all auxetic structures was measured according to standard ASTM D3776. Te coefcient of friction of yarns to solid material has been checked using ASTM D: 3108-2013.

Negative Poisson's Ratio.
A lab-developed standard was followed for the characterization of auxetic behavior. Te test was performed on a Universal Testing Machine LLOYD, LRX PLUS. First, samples were placed for 24 hours for conditioning under 65 ± 2% relative humidity and 20 ± 2°C temperature. Ten, samples are cut to size to ft into the jaws of UTM (100 mm × 150 mm). Te efective length of the sample was 100 mm × 100 mm. Samples were clamped,      Advances in Materials Science and Engineering 5 and a scale was placed behind the specimen to physically take the value of extension or contraction and photographed from the front side by using a camera fxed on a stand. Te jaws were moved in a step of 10 mm and then stopped. After each 10 mm movement, a photograph is taken, as shown in Figure 10. Tis process continues until the extension of 100 mm is reached or the sample shows signifcant resistance to more stretching. A total of 120 images were taken, 11 images for each sample. Te images are then analyzed physically to calculate the extension, and the NPR was calculated using Equations (1)-(3). Te lateral strain (εx), axial strain (εy), and Poisson's ratio (ϑ) were calculated using the following equations:

Advances in Materials Science and Engineering
D 0 is the width of the fabric when no force is applied to it, and D is the width of the fabric under force. Tus, L 0 indicates the length of fabric under no force, and L indicates the length of fabric under force.

Termo-Physiological Comfort.
Te air permeability of fabric is determined as per ASTM D737-18. Tis test measures the rate of airfow perpendicular through the fabric under prescribed air pressure. It is an essential feature of the fabric's breathability and is measured in mm/sec. Fabric is placed in the circular test area of 20 cm 2 at an air pressure of 100 Pa. Te thermal insulation test is performed according to ISO-11092 standard using the sweating guard hot plate. Specimen of 10/10 inches' dimensions of the fabric placed in the chamber. Te temperature and humidity of the air are 20°C and 65% ± 2. Te airspeed is specifed at 1 m/s at a point 15 mm above the center of the plate surface. Te air velocity coefcient of variation due to turbulence is between 5% and 10%. Te amount of heat that passes through the specimen is used to calculate the conductivity/insulation value of the specimen and is measured in Km 2 /W (Table 3). Liquid moisture management of samples was analyzed on a moisture management tester (SDL-ATLAS, M-290) against standard AATCC 195-2011. Fabric samples are placed between upper and lower concentric moisture sensors to test the ability of the fabric to control liquid moisture. Te upper side of the fabric is exposed to a predetermined amount of test solution (synthetic sweat), which subsequently transfers onto the cloth in three directions: (1) extending outward on the fabric's top surface; (2) moving from the top to the bottom of the fabric; (3) moving laterally across the fabric's lower surface before drying up and disappearing. A material's dynamic liquid moisture transport behaviors in these various directions within the material can be evaluated using the measured liquid moisture content. Te overall moisture management capacities (OMMC) of all samples were compared. Te mean of up to fve measurements was calculated [29].

Analysis of Variance (ANOVA).
Te analysis of variance technique is a statistical method and is employed to recognize the prominent process parameter in the multiresponse model. Te signifcance of model phrases is determined in terms of F value or p value. Te F value is the ratio of the factor mean square to the mean square error. Usually, a factor with a larger F value has a signifcant efect on the dependent. Te signifcance of the model parameters can also be identifed using p value which is less than α level of signifcance (5% theoretically). ANOVA is employed to the values of input parameters to identify the signifcant infuence on all the dependent parameters of thermophysiological comfort for which the p value is less than the level of signifcance, 5% in this study. Te statistical analysis of data was carried out using Minitab 17 software. Te p value (Table 4) shows whether the efect of the input factor on responses is signifcant or not. It ranges from 0 to 1, and its lower value suggests that the efect of input on output is more signifcant. Any term with p value >0.05 shows a lack of signifcance at a confdence level of 95%. Based on the p values in Table 4, the infuence of knitted structures is signifcant in terms of their thermal resistance, but it is insignifcant in terms of their air permeability and OMMC value. Te material type has a considerable impact on the thermo-physiological properties of smart structures. Te p value demonstrates that the efect of material's efect is signifcant in AP and thermal resistance; however, material type has no impact on the OMMC value. Te choice of material is a key consideration for thermo-physiological properties. From

Results and Discussion
3.1. Physical Characteristics. Te yarn's mechanical properties have been given in Table 5 and the diferent physical parameters of all specimens were measured after knitting. Te detail of the physical parameters of knitted fabrics is given in Table 6. Stitch length and yarn count are constant in all samples. Tere is a diference in the WPI and CPI of all samples. All samples are made by the front and reverse knitting on the fat-knitting machine. After front knitting, a stitch is transferred to the back bed for a joint and vice versa. When the fabric relaxes, these folded auxetic structures are formed    because of structural disequilibrium. Te diference in the WPI and CPI is due to the design's geometry change. Star structure shrinks more than lines and zigzag structure, increasing the WPI and CPI of the fabric [30], as given in Table 6. GSM is the weigh-in gram per meter of fabric. Te more the fabric shrinks, the more WPI and CPI, causing an increase in the GSM of the folded structure. Star structure shrinks more than lines and zigzag structure and has more GSM than the other two structures [31]. Te thickness of the samples is given in Figure 11. Results of friction of materials have been given in Figure 12, and it is clear from the results that polyester has the highest coefcient of friction and nylon has the least. Figure 13 represents the extension and contraction in the structure when stretching in a course direction in a step of 10 mm. Folded weft-knitted structures can be produced by a combination of front and back loops. Due to the structural disequilibrium and interaction of face and back loops, the fabric shrinks and, after relaxation, becomes a folded weft-knitted fabric [3]. When the fabric is stretched in the lateral direction, a gradual widthwise expansion shows the negative Poisson's ratio (NPR) efect on the opening of the folded structures in both course and wale directions. It is found that the structure made from nylon yarn does not have an extension in the course direction. Samples made with nylon yarn do not fold, and when the force is applied, no extension has been observed, and the structure behaves like normal fabric. While the structures produced by polyester, cotton, and acrylic show expansion in course direction and Figure 14 shows that due to the fabric's unfolding nature, an increasing auxetic efect is achieved when stretching from 20 to 50 mm in a lateral direction. Te fabric opens to its fullest after it starts contracting and    Advances in Materials Science and Engineering behaves like a conventional and nonauxetic structure [23]. Yarn-to-yarn entanglement plays a signifcant role in the NPR efect [32]. From Figure 14, we can observe that polyester does have the best NPR value due to its rough nature and twist in the yarn. Te twisted yarn has less yarn-to-yarn slippage in a knitted structure. When this fabric is stretched in a course direction, it delivers better NPR values, and after a 50 mm extension, it tends to decrease the negative Poisson's efect. Nylon does not show NPR efect due to the zero-twist multiflament in yarn and its soft nature. Te coefcient of friction of the yarn surface decreases, which afects the folding of a knitted structure after relaxation [33]. In weftknitted fabrics, the NPR efect is due to the folding of the fabric [16]. Both acrylic and cotton are spun yarn, but cotton has a lower NPR efect than acrylic due to the soft nature of cotton. Figure 15 shows the expansion/contraction in star structure due to the strain applied in course direction, folded Star structure made by the combination of face and back loops, after relaxation due to structural changes in fabric. Folded fabric gains a 3D shape with a thickness mentioned in Figure 11. Te same trend of expansion can be seen in this structure. Te sample made with nylon showed contraction from the start when the force was applied, and the structure behaves like normal fabric. While the other three structures, polyester, cotton, and acrylic, show extension in course direction.

Auxeticity.
From Figure 16, we can observe that when the fabric is stretched up to 40 mm an increasing auxetic efect is achieved in cotton, polyester, and acrylic samples, due to the unfolding behavior of the loops in structure. After that fabric starts contracting, the fabric opens to its fullest, and no more expansion is possible, and it begins to behave like a conventional, nonauxetic structure. Polyester ofers the best NPR value in star structure. It ofers the maximum value at a 10 mm stretch and decreases gradually due to the uneven surface and twist in the yarn. Each structure has diferent geometry and exhibits accordingly. Yarn with a twist has less yarn-to-yarn slippage in a knitted structure [34]. When the fabric is stretched in a course direction, it exhibits better NPR values. Nylon shows the same NPR efect as star structure due to the zero-twist multiflament of nylon yarn and its soft nature. Cotton shows a less NPR efect than acrylic because of the soft nature of cotton than acrylic and less fber-to-fber entanglement with other loop yarn [25]. Figure 17 describes the expansion/reduction in the line structure. Sample made with nylon yarn showed contraction after showing extension at the start i.e., 10 mm when the force was applied, while polyester, cotton, and acrylic structures showed extension up to 50 mm extension in the course direction. Nylon shows the minor auxetic efect only in star structure and can be seen in Figure 18, polyester does the best NPR value at 20 mm stretch and then decreases gradually. Nylon structure shows a minor NPR efect at the start and then its behavior is like conventional fabrics. Cotton shows less NPR efect than acrylic. Figure 19 describes the structure-wise comparison and the maximum value of NPR of a specifc yarn. Te arrangement of stitches is the main factor in making a structure auxetic. Polyester ofers the maximum value of NPR −0.4 in line structure at 20 mm extension. In lines, structure lines of the front and back stitches are aligned at 90°t o each other [35]. On application of force, the folding of the structure opens. At the start, the fabric is fully relaxed, and yarn-to-yarn surface friction is more when a force applied loops open without yarn slippage. As more force is applied, NPR produces a structure, yarn slips from the surface of other yarn, causing a reduction in NPR, and gradually fabric behaves like conventional material. In the polyester case, the unsmooth surface and twist in the yarn take more energy at the start to open the folding, causing a better NPR value than others. Using the appropriate structure and knit pattern makes it possible to produce knitted fabrics with an NPR on a fat-knitting machine [36].

Air Permeability.
Te air permeability of fabric is a very sensitive display in deriving the comfort properties of any textile material. Air permeability can infuence the comfort behaviors of the fabric in several ways. Te higher air permeability rates are normally attributed to the quickest heat loss obtained from textile materials [37]. If we compare the samples structure-wise (Figure 20), the zigzag structure shows the maximum air permeability than the lines and star structure, because of its structural nature, the zigzag structure shrinks less making the structure less compact and causing increased air permeability of the fabric. Te air permeability of the nylon zigzag structure is 540 mm/sec while the other structures' star and lines have less value of AP 510 mm/sec and 530 mm/sec, respectively. In the case of materials, samples made with nylon yarn show promising results of air permeability than acrylic, cotton, and polyester, respectively. From Figure 12, the value of COF of nylon yarn is less than the other materials. Due to this property, structures made with nylon shrinks less and make the structure less compact. Structures made with polyester and acrylic show the least value of AP, i.e., 320 and 290, respectively. If we see the main efect plot of air permeability, a similar trend can be seen in Figure 21.

Termal Resistance.
Termal resistance, which is defned as a material's capacity to encapsulate heat rather than convey it, is the most signifcant factor in determining the thermal comfort of any garment. Air is less heat conductive than any textile fber, hence the number of dead air pockets has a signifcant impact on the overall thermal resistance of    the fabric. For textile materials, the most essential component for conductivity value is still air in the fabric structure, since still air has the lowest thermal conductivity value of all fbers (λ air � 0.025 W·mK −1 ) [38]. If we compare the samples structure-wise (Figure 22), compared to lines and zigzag structures, the star structure shows the highest thermal resistance 0.75 Km 2 /W. Tis is because, as seen in Figure 8, the more the structure shrinks, the thicker it becomes, creating tight air pockets and so enhancing its heat resistance. In the case of materials, structures made with acrylic yarn have more thermal resistance than cotton, polyester, and nylon. Wet-processed acrylic fbers are extremely porous, including numerous 0.1-1 m microvoids which allows the acrylic fbers to develop wool-like bulkiness [39]. If we see the main efect plot of thermal resistance, a similar trend can be seen in Figure 23.

Overall Moisture Management Capacity (OMMC).
Moisture plays an important role in the cooling mechanism of humans. Transferring moisture from the surface of the skin to the atmosphere is an important property for next-toskin use. OMMC shows the overall management performance of liquid moisture of fabric, and the higher this value, the better the liquid transport performance of fabric [40]. Te one-way transport capability property denotes the ability of the fabric to transfer liquid moisture from one side of the fabric to the other side. In other words, a higher oneway transport capability means that sweat can be easily and quickly transferred from the skin to the outer [41]. If we compare the samples structure-wise (Figure 24), the zigzag structure shows the maximum OMMC value than the lines and star structure. Structures made with polyester yarn manage moisture better than the other materials due to the smooth surface of polyester. If we see the main efect plot of OMMC, a similar trend can be seen in Figure 25.

. Conclusion
Diferent auxetic weft-knitted fabrics have been developed on Shima Seiki's computerized fat-knitting machines using four types of yarns polyester, nylon, acrylic, and cotton. Tis research concludes that NPR in weft-knitted auxetic textiles is afected by loop arrangement, material type, yarn surface friction, yarn-to-yarn surface slippage, and knitted structure geometry. By using a fat-knitting machine, these structures can be made due to the possibility of stitch transfer from one needle bed to another. Te results also reveal that the NPR exists in all fabrics made from polyester, acrylic, and cotton yarn, but structures made with nylon yarn behave like a conventional knitted fabric. Termo-physiological properties show that nylon in zigzag structures provides the best AP value to other specimens. Te thermal resistance of acrylic is best in star structures than all other samples and polyester gives the best results of OMMC in zigzag structures. Flat-knitting technology, due to the possibility of stitch transfer from one needle bed to another, can provide a simple but highly efective way of fabricating these fabrics by using conventional yarns as a new structure with nonconventional properties. NPR weft-knitted fabrics will fnd multiple potential applications in diferent felds, such as protective wear, medical textile, and sportswear. Further research is needed to explore this area for the potential application of these fabrics.

Data Availability
Te data will be made available on request.

Conflicts of Interest
Te authors declare that there are no conficts of interest with respect to the research, authorship, and/or publication of this article.