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Due to fiber swelling, textile fabrics containing hygroscopic fibers tend to decrease pore size under wet or increasing humid- ity and moisture conditions, the reverse being true. Nevertheless, for personal thermal regulation and comfort, the opposite is desirable, namely, increasing the fabric pore size under increasing humid and sweating conditions for enhanced ventila- tion and cooling, and a decreased pore size under cold and dry conditions for heat retention. This paper describes a novel approach to create such an unconventional fabric by emulating the structure of the plant leaf stomata by designing a water responsive polymer system in which the fabric pores increase in size when wet and decrease in size when dry. The new fabric increases its moisture permeability over 50% under wet conditions. Such a water responsive fabric can find various applications including smart functional clothing and sportswear. Keywords Breathable fabric · Fabric pores · Hydrogel · Leaf stomata · Water responsive Introduction wet conditions due to fiber swelling and increased pore size under dry conditions. Smart responsive textiles that regulate their structure and There have been many attempts to create such an uncon- properties according to external stimuli such as moisture and ventional fabric behavior. For example, Zhong et al.  and temperatures are attracting increasing attention for various Mu et al.  created pre-cut flaps in patterned Nafion™, a applications [1–4]. In such textiles, it is highly desirable, perfluorosulfonic acid ionomer (PFSA)-based films, which for example, to increase the opening of fabric pores under opened in high humidity. More recently, other materials humid and sweating conditions so as to maximize ventilation have been used to construct moisture sensitive flaps for and cooling, while closing the pores under cold and dry con- modulating ventilation in clothing. For example, Wang et al. ditions for increased heat retention and barrier protection.  constructed a heterogeneous biohybrid film, compris- Such a behavior is, however, contrary to that of the conven- ing a humidity-sensitive layer of special living cells and a tional fabric which generally have reduced pore size under humidity-insensitive layer; Kim et al.  developed a bilayer moisture-responsive actuator comprising a hydrophobic pol- yethylene terephthalate (PET) layer and hygroscopic cellu- lose acetate (CA)/polyethylene glycol (PEG) layer; Li et al. * Jintu Fan email@example.com  developed a three-layer moisture-responsive film com- prising nylon, silver (Ag) and polystyrene-poly(ethylene- Department of Fiber Science & Apparel Design, Cornell ran-butylene)-polystyrene (SEBS) nanocomposite layers. University, Ithaca, NY 14853, USA Nevertheless, in all these attempts, all the flapping actions Present Address: Department of Materials Science occurred out of the fabric planes, which adversely affected and Engineering, University of Illinois Urbana-Champaign, the tactile comfort and/or appearance of the clothing. Hence, Urbana, IL 61801, USA Jia, Hu, Khan and Liu et al. [10–12] adopted a different Sibley School of Mechanical and Aerospace Engineering, approach, in which the fabric or textile changed its shape Cornell University, Ithaca, NY 14853, USA and openings with the increase of humidity via twisted and Present Address: Querrey Simpson Institute coiled artificial muscles (e.g., silk fiber and bamboo fiber) so for Bioelectronics, Northwestern University, Evanston, IL 60208, USA as to enlarge the unclothed body area for cooling. Although this approach involved only the in-plane deformation, it School of Fashion and Textiles, Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong, China Vol.:(0123456789) 1 3 Advanced Fiber Materials Fig. 1 Schematic illustration of the design of a stretchable fabric outer layers being composed of highly cross-linked (HC) little swella- simulating the plant leaf stomata. a The opening and closing behav- ble hydrogel and less cross-linked (LC) highly swellable hydrogel, ior of guard cells (pores) in leaf stomata. b The design of a stretch- respectively. These simulated leaf stomata were assumed to open the able fabric simulating the plant leaf stomata where the slit areas have pores/slits under wet conditions, and close the pores/slits under dry been coated with three hydrogel layers, the inner layer and the two conditions could only be used in certain garment parts, e.g., sleeves cross-linked (LC) PAAm hydrogel at the outer layer of the and pants. slit swells 100 times more than that of its highly cross-linked Our present approach is different in that it involves pores (HC) counterpart at the inner layer when wet, the hydrogel (or cut slits) within the fabric enlarging or shrinking in a layers (acting as artificial guard cells) bend outwards to open similar way to that which occurred in the leaf stomata of a the slits (pores) in the fabric as the surrounding wetness plant (Fig. 1a). A stoma is a portal in a plant leaf which con- increases. Unlike the “flap approach” in most previous stud- trols carbon dioxide (C O ) exchange and water transpiration ies [5–9], the fabric deforms within its own plane, thereby for photosynthesis and other metabolic processes . It has not affecting the overall appearance or the tactile comfort. two guard cells, with relatively inextensible thick inner walls and relatively extensible thin outer walls . The stomata open and close in response to changes in the environments, Experiments such as humidity, light and CO levels [15, 16]. Under high humidity (wet) or light intensities, the two guard cells swell Preparation of Polyacrylamide Hydrogel and bend apart, creating an open pore to draw the water in; whereas the reverse is true under low levels of humidity and The polyacrylamide (PAAm) hydrogels were synthe- light, the two cells shorten, leading to closing of the pores sized as follows . 12 wt % acrylamide (AAm; Sigma- so as to prevent a loss of water (Fig. 1a). Aldrich, A8887) aqueous monomer solution was prepared, In the present approach, the actuation of the leaf stomata to which a photoinitiator, Dar ocur 1173 (BASF) and a is emulated in a stretchable knitted nylon fabric by creating cross-linker, N,N′-methylene bisacrylamide (MBA; Sigma, artificial guard cells at the edges of the cut slits using the M7279) were subsequently added. The photoinitiator was water responsive polyacrylamide (PAAm) hydrogel at two 0.2 wt% of the solution and the cross-linker was 0.12 wt% different levels of cross-linking (Fig. 1b). Because the less for the less cross-linked (LC) hydrogel and 1.2 wt% for the 1 3 Advanced Fiber Materials highly cross-linked (HC) hydrogel. To certain solutions, Characterizations 1.2 wt% of PAAm polymer (M ~ 5 × 10 , Sigma-Aldrich, 92560) was added for a comparison . After stirring Morphology A scanning electron microscopy (SEM; Tescan the solutions at room temperature overnight, each homo- Mira3 FESEM) was used to observe the morphology of the geneous solution was poured into a microtome embedding PAAm hydrogels having the different cross-linker loadings mold (15 mm wide, 7 m high and 4 mm deep, Electron as well as the fabrics with and without the hydrogel coat- Microscopy Sciences) and photo-polymerized under ultra- ings in the slit. The different samples were freeze-dried and violet (UV) light at 365 nm (UVP Blak-Ray™ B-100AP coated with gold palladium before the observation. High-Intensity UV Inspection Lamps, Fisher Scientific) Water uptake (swelling) and volume change of individual for 10 min. hydrogels Various PAAm hydrogel samples (4 mm wide, 7 mm high, and 1 mm thick) were immersed into the water Preparation of Three‑Layer Sandwiched Hydrogel for the swelling test. The samples were taken out at different Composite times, surfaces wiped with Kimwipes™ Delicate Task Wip- ers and then weighed (W , g). The water uptake (swelling) A three-layer sandwiched hydrogel composite was synthe- ratio was defined as follows: sized by curing the two hydrogel solutions in the microtome W −W mold step-by-step, the two outer layers being composed of t 0 × 100%, (1) the less cross-linked (LC) hydrogel and the inner layer of the highly cross-linked (HC) hydrogel. For each layer, 400 µL where W was the original weight (g) before the water of the solutions was used and photo-polymerized under UV immersion[20, 21]. The volume change was also determined light (365 nm) for 10 min. The stacking and polymerization by measuring the dimensions of each hydrogel sample at dif- sequence was as follows: the LC solution for the first layer, ferent times during the swelling (V, mm ) with the volume the HC solution for the second layer and another LC solution expansion ratio being defined as follows: for the third (viz. final) layer. V −V t 0 Coating of three‑layer hydrogel on slit areas × 100%, (2) of the fabrics where V was the original volume (mm ) before immersion Knitted nylon fabrics (100% Nylon, NNP 32,003) provided in the water .Three specimens were tested to obtain the by Nanjing Yuyuan Textile Co., Ltd, were first washed, and average value. then pre-activated in a 5 wt% benzophenone (BP, Alfa Aesar, Mechanical tests of hydrogels The mechanical properties A10739)/ethanol solution . To prepare the hydrogel coat- of the two differently cross-linked PAAm hydrogels before ing, the BP pre-treated fabric was covered with a tape mask (as-prepared) and after water swelling (for 24 h) were tested which had a patterned pore/slit groove structure. The slit at 0.1 N/min to a maximum force of 18.0 N on a dynamic area was composed of three windows, with widths of 2 mm mechanical analysis (DMA) tensile tester (Q800, TA Instru- and 2.5 mm for the inner and two outer windows, respec- ments), the hydrogel samples being cut into a rectangle tively, and a same height of 9 mm. Each slit area was spaced shape (~ 4 mm wide, 8 mm high, and 0.2 mm thick). From by 6 mm and 4 mm in horizontal and vertical directions, the obtained stress–strain curves, the Young’s modulus (E, at respectively. A water-based glue (Elmer's School Glue) was 2.5% strain), fracture stress (σ ) and fracture strain (ε ) were f f then pasted into the grooves of the tape mask. After the derived by means of Universal Analysis 2000 software (TA glue had solidified, the mask was peeled off to form a stoma instruments). At least three specimens were tested and their template with three windows on the fabric. After that, 10 µL values were averaged. of each hydrogel solution was poured into the windows and Water response of hydrogels with cut slits Samples (4 mm cured in a sequence, i.e., first the HC hydrogel solution in the wide, 7 mm high, and 1 mm thick) of the LC hydrogel, HC inner window, and then the LC hydrogel solution in the two hydrogel and three-layer sandwiched hydrogel composite outer windows. Afterwards, a 9-mm slit was cut with a blade were cut from the cross-sections of the molded samples, in the central position of the inner HC hydrogel layer and and a 6-mm slit was then cut in the central position of each completely through the fabric. Afterwards, the fabric was sample. The samples were immersed into water for 10 min immersed in a water bath to wash out the glue, and finally at room temperature and tested their water response. dried at room temperature. Water response of fabrics with hydrogel coated slits The fabrics with the slit areas coated with different hydrogel combinations were tested for the water response by 1 3 Advanced Fiber Materials immersing them for 10 min in water at room temperature, 8FTV C = , (4) 2 2 the control fabric with slits only (viz. no coating) also being D P − 1 tested. Optical microscopy (BX51, Olympus Corporation) where C is the Darcy’s Permeability Constant (Darcy or cm , was used to assess the slit behavior and the slit dimensions. –9 2 3 Artificial sweat response of fabrics with hydrogel 1 Darcy equal to 9.87 × 10 cm ), F is the air flow (cm /s), T is the sample thickness (mm, ~ 0.39 mm for the knitted coated slits The slit response to artificial sweat was assessed by immersing the fabric with the optimal hydro- nylon fabric), V is the air viscosity (0.0185 CP), D is the sample diameter (mm), and P is the pressure (atmospheres, gel coating in the slit areas into contact with the artificial sweat for 10 min at room temperature. During the first psi). Prior to testing, all the samples were conditioned in a standard atmosphere (20 ± 2 °C temperature, 65 ± 2% rela- 5 min, the fabric was floated on the liquid, after which it was immersed completely in the liquid for the remain- tive humidity) for 24 h. The results were obtained by averag- ing the values of three specimens . ing 5 min. The fabric with slits only (viz. no coating) was also tested as a control. In both cases, videos were Data analysis The ANOVA was used to analyze the data with the significance level set at p < 0.05, and the results used to record the slit response. The artificial sweat was prepared according to the ISO 3160-2 standard, which being given as mean ± standard deviation. contains 20 g/L NaCl, 17.5 g/L NH Cl, 5 g/L acetic acid and 15 g/L D, L-lactic acid with pH of 4.7 adjusted by Results NaOH . Water vapor transmission rate (WVTR) The WVTR of Three‑Layer Sandwiched Hydrogel various fabrics was measured by means of a cup method (BS 7209 standard) [24, 25], with the temperature raised The polyacrylamide (PAAm) hydrogels were synthesized to 35 °C. Various circular samples with a diameter of ~ 90 mm were cut, and then attached to the edge of by means of UV photo-polymerization, with N, N’-meth- ylene bisacrylamide (MBA) as the cross-linker , two standard aluminum cups via an adhesive plus a tape. Each cup was pre-filled with ~ 60 g water, with a triangular alu- hydrogels with different cross-linker loadings being pre- pared. The less cross-linked (LC) hydrogel, with a cross- minum support being used to prevent the testing sample from sagging into the water. The cups were placed on a linker loading of 0.12 wt%, appeared transparent, whereas the highly cross-linked (HC) hydrogel, with a cross-linker heater with a temperature of 35 °C, and mass of the water lost/evaporated through the fabric was measured after 1 h. of 1.2 wt% appeared white (Fig. 2a), possibly as a result of the non-homogeneous gelation with relatively large The WVTR (g/m /h) was calculated as follows: amount of the cross-linker . The microstructure of WVTR = , both hydrogels displayed an open porous morphology after (3) At freeze-drying with the HC hydrogel network being less porous than that of the LC hydrogel due to the increased where M is the water mass loss (g), A is the area of the fabric cross-linker concentration (Fig. 2b) . exposed to the water in the cup and is the same as the inter- The water uptake (swelling ratio) (solid lines in Fig. 2c) nal area of the cup (m ), and t is the test period (h). Prior to and volume expansion (dashed lines in Fig. 2c) were found testing, all the samples were placed in a standard atmosphere to be different for the two hydrogels (significance level (20 ± 2 °C temperature, 65 ± 2% relative humidity) for 24 h. p < 0.05). The LC hydrogel rapidly absorbed water and The results were obtained by averaging the values of three became swollen with a high deformation speed, reaching specimens. almost 130% of the original weight and 115% the original Air permeability The air permeability of the fabrics volume after 300 min, after which little further water absorp- were tested on a gas permeability module on a Capillary tion or swelling occurred; In contrast to this, the HC hydro- Flow Porometer 7.0 (CFP-1100-AEHXL, Porous Mate- gel showed little swelling (the images can be found in Fig. rials Inc.) . Various circular samples with a diam- S1) and even experienced a slight decrease in weight and eter of ~ 25 mm were cut, and then gently placed in the volume, possibly due to the water loading in the gel already chamber, and fixed by means of an O-ring (18.3 mm and being higher than that the highly cross-linked network could 25 mm inner and outer diameters, respectively). As the accommodate, leading to a rejection of the excessive water fabrics were being tested, air pressure increased, and air when the hydrogel reaches its equilibrium. Xu et al.  pre- flow and pressure drop across the samples were meas- viously reported a similar decrease in the volume and weight ured. The results of Darcy’s Permeability Constant (C) of nanocomposite hydrogels when immersed in water, also was obtained by means of the associated software, calcu- attributing it to the increased cross-linking density of the lated as follows: hydrogels. The volume change showed a similar trend as 1 3 Advanced Fiber Materials Fig. 2 Morphologies, mechanical properties and water behaviors of composite. f–h Water response of the cut slits in the central position differently cross-linked polyacrylamide (PAAm) hydrogels and three- of the LC (left), HC (middle) and three-layer (right) hydrogel sam- ples, all having the same dimension (4 mm wide, 7 mm high, and layer sandwiched hydrogel composite. a Photo and b SEM images of 1 mm thick), at f as-prepared, g wet and h dry conditions, respec- the less cross-linked (LC) (left) and highly cross-linked (HC) (right) tively. Wet refers to the fabrics being immersed in water for 10 min. PAAm hydrogels, respectively. c Water uptake (swelling ratio) and The pictures (diagrams) on the right illustrate the response of the slit volume expansion of the two hydrogels. d Representative stress– in the three-layer sandwiched hydrogel samples strain curves of the two hydrogels before and after swelling. e Top and cross-sectional views of the three-layer sandwiched hydrogel that of the water uptake (weight change) but to a slightly less hydrogel forming the inner layer and the transparent LC hydro- extent, which is in line with the trends observed in previous gel forming the two outer layers, respectively (denoted as LC/ studies [22, 30]. HC/LC sandwiched hydrogel composite), which is in contrast Figure 2d shows representative stress–strain curves of to the single-color appearance of the two pure hydrogel samples the two hydrogels before and after swelling in water. The (Fig. 2a). After this, a 6-mm slit was cut in a central position HC hydrogel, as expected, has a higher tensile strength of each hydrogel sample, with the slit response of the different and stiffness (modulus) than the LC hydrogel (p < 0.05), samples being compared (Fig. 2f). After contacting with water due to its higher cross-linking density [31–33]. When the for 10 min, no change was observed in the slits in the two pure hydrogels were swollen in water for 24 h, the Young’s samples, except in the case of the swelling of the whole LC modulus and facture strain of the LC hydrogel decreased hydrogel sample (Fig. 2g, left sample). The three-layer sand- significantly(p < 0.05) (Fig. S2), which was expected wiched hydrogel composite, on the other hand, showed a differ - since its higher water absorption loosens the hydrogel ent response in that the inner HC hydrogel walls bowed apart, networks [34, 35]; on the other hand, the mechanical resulting in the slit opening (Fig. 2g, right sample). When the properties of the HC hydrogel changed little (p > 0.05) samples were dried for 1 h (not completely dry), the inner HC (Fig. S2) when immersed in the water as a result of its hydrogel walls closed again (Fig. 2h, right). The phenomenon high cross-linking density and a non-swelling property was similar to the pore behavior of leaf stomata (illustrated by (Fig. 2c) [34, 36]. the adjacent diagrams), i.e., pore opens when wet and closes Three-layer sandwiched hydrogel composites were pro- when dry, respectively. duced in a microtome mold (15 mm wide, 7 m high, and 4 mm deep) by means of a step-by-step curing of the two Three‑Layer Hydrogel Coating on the Fabric Slits hydrogel solutions. The cross-sectional view of the com- posite (4 mm wide, 7 mm high, and 1 mm thick) confirmed Simulated leaf stomata pores/slits were created by means the sandwich layer structure (Fig. 2e), with the white HC of a three-layer hydrogel coating on the stretchable knitted 1 3 Advanced Fiber Materials Fig. 3 Fabrication and structure of knitted nylon fabric with slit areas hydrogel, insets illustrate the designed hydrogel coatings and the coated with three-layer polyacrylamide (PAAm) hydrogels. a Sche- dimensions and spacings of the three windows in the slit area. c SEM matic illustration of the procedure of creating simulated stomata morphologies of the control nylon fabric, benzophenone (BP) pre- pores/slits by means of the three-layer hydrogel coating. b A typical tread fabric, the inner and outer windows in the three-layer hydrogel fabric with slit areas coated with the three-layer LC/HC/LC PAAm coated fabric slit areas nylon fabric according to the procedure shown in Fig. 3a was formed on the fabric by pasting and solidifying the glue (also see Fig. S3, for the actual sample procedure). As a through a laser-cut tape mask having a groove pattern. The first step, the clean fabric was pre-activated by means of a HC hydrogel solution was then poured into the inner win- benzophenone (BP)/ethanol solution, in order to create the dow and the LC hydrogel solution into the two outer win- radical sites for bonding with the polyacrylamide hydrogel dows, both then being cured under UV light at 365 nm for . To prevent the wide spread of hydrogel liquid solu- 10 min in a sequential order. After curing, a 9-mm slit was tions on the fabric, a glue patterned template with three cut in the central position of the inner HC hydrogel layer and windows (overall 7 mm wide and 9 mm high, with 2-mm completely through the fabric, and then the glue solid was wide inner window and two 2.5-mm wide outer windows) washed out in a water bath. 1 3 Advanced Fiber Materials Table 1 Summary of water response when the window areas of the slit (simulated stomata) are coated with different hydrogel combinations at room temperature Condition Wet Dry Outer layer Outer layer Inner layer Zero (0) AAm & AAm (LC) AAm & AAm Zero (0) AAm & AAm AAm & AAm (HC) PAAm PAAm (HC) PAAm (LC) PAAm (LCP) (HCP) (LCP) (HCP) Zero (0) – + + – – – – – – – AAm & PAAm – + + – + – (LCP) AAm (LC) – + + – + + a) a) AAm & PAAm – + + + + – + – (HCP) a) a,b) a) a,b) AAm (HC) – + + + + – – – The combinations include zero (0) hydrogel, less cross-linked (LC) hydrogel, highly cross-linked (HC) hydrogel, less cross-linked hydrogel with PAAm (LCP) and highly cross-linked hydrogel with PAAm (HCP) –, + and + + symbols represent slit close, slightly open, and very open, respectively Represents good combinations of hydrogels for slit opening and closing Represents the selected hydrogel combination for the further experiments opened widely when wet and closed completely when dry The water response of the slits coated with different a) (noted as in Table 1). Adding PAAm polymer made little combinations of less cross-linked (LC) and highly cross- difference. In fact, it even retarded the closure of the slits linked (HC) PAAm hydrogels was investigated by immers- for the LCP/HCP/LCP hydrogel coated sample (viz. HCP ing the coated fabrics in water for 10 min at room tempera- hydrogel coated inner window and LCP hydrogel coated two ture (Table 1, and Fig. S4). The aim of soaking the fabrics outer windows). Therefore, the LC/HC/LC combination (viz. was to study the slit response to a situation where the fabric HC for the inner and LC for the two outer windows) was becomes completely wet due to sweating or a wet environ- b) selected for the following experiments, as noted in Table 1. ment, since under certain circumstances it is desirable that Figure 3b showed a typical nylon fabric with the slit win- the fabric pores enlarge so as to increase the transmission of dow areas (7 mm wide and 9 mm high) coated with the moisture and air. Furthermore, the effect of adding PAAm three-layer LC/HC/LC PAAm hydrogel, which maintained polymer to the cross-linked hydrogel solutions (denoted as a flat surface without buckling. The spacings between each LCP for less cross-linked solution with added PAAm poly- slit area were 6 mm and 4 mm in in horizontal and vertical mer and HCP for highly cross-linked solution with added directions, respectively. The hydrogels have fully covered PAAm) was also investigated . It was found the slits the templated window areas of the fabrics, since the areas (viz. simulated stomata) did not respond to being wet when were made completely wet by the hydrogel spreading after it only the inner window area was coated with either LC, LCP, was injected (Fig. S3f). The thickness of the hydrogel-coated HC, or HCP hydrogel. Furthermore, the slits opened slightly area was 0.42 ± 0.01 mm, as compared to 0.39 ± 0.01 mm when only the two outer windows were coated with the LC for the control nylon fabric. Figure 3c shows the SEM mor- or LCP hydrogel but did not open when only the two outer phologies of the knitted nylon fabric before and after being windows were coated with the HC or HCP hydrogel. How- coated with the hydrogel. The fibers of both the control fab- ever, when the inner window and the two outer windows ric and the BP pre-treated fabric had a smooth surface, while were coated with the LC or LCP hydrogel and the HC or the fibers in the window areas coated with hydrogels had a HCP hydrogel, respectively, the slits opened slightly when relatively rough appearance, with the hydrogel creating inte- wet, but remained open when they dried. The possible rea- grated bonds. It was apparent that the hydrogel coating on son for this behavior is that the outer HC or HCP hydrogel the fiber surface was not perfectly smooth and homogenous, takes less time to dry due to lower water uptake (Fig. 2c), but that it had penetrated across the fabric (Fig. S5a), as the and the dried HC or HCP solidifies to fix the shape of the back window areas exhibited a similar rough appearance simulated stomata before the inner LCP becomes dry, lead- (Fig. S5b). The HC hydrogel coated inner window area and ing to only a partial closure of the slit. Only when the inner the LC hydrogel coated outer window area exhibited a simi- window was coated with the HC or HCP hydrogel and the lar fiber surface appearance. two outer windows with the LC or LCP hydrogel, the slits 1 3 Advanced Fiber Materials Apart from water, the slit response to artificial sweat of Performance of Fabrics with Three‑Layer Hydrogel Coated Slits the control fabric with slits only (viz. no coating) and the fabric with the slit window areas coated with the three-layer The water response of the nylon fabrics with the slit window PAAm hydrogels was investigated. The slits of the control fabric did not show any response when in contact with or areas coated with the three-layer LC/HC/LC PAAm hydrogels was investigated in terms of dimensional changes as well as immersed for 10 min in the artificial sweat (Movie S1 and Fig. S7a). From the Movie S2, we can see the hydrogel water vapor transmission and air permeability. When in con- tact with water for 10 min at room temperature, the slits in the coated slits started to open almost instantaneously (in ~ 10 s) when in contact with the artificial sweat, and largely main- sample were found to open widely (Fig. 4a, left). According to the optical images, the slit width increased to an average of tained their open status after being immersed in the artificial sweat for 1 min and 10 min, respectively (Movie S2 and Fig. 0.72 ± 0.26 mm (Fig. S6a, i), the largest slit reaching around 1.30 mm, compared to 0.20 mm for the as-prepared fabric S7c). When completely dry, the slits closed once again (Fig. S7d). The deformation and response of the hydrogel coated slits. When dried for 1 h at room temperature, the slits on the fabric closed to a width (opening) of 0.20 ± 0.07 mm (Fig. 4a, slits to artificial sweat were almost the same as that in pure water (Fig. 4a). It is worth noting that, the above experiment right, and Fig. S6a), which was similar to the original size. The original fabric (viz. control) and fabric with slits only (viz. on the slit response to artificial sweat was conducted after the fabric samples were placed in a dry laboratory envi- no coating) did not respond when contacting with the water (Fig. S6). The opening and closing behavior of the simulated ronment (20 ± 2 °C temperature, 65 ± 2% relative humid- ity) for over 1 year, whereas the slit response to pure water stomata changed very little during 20 wet and dry alternation cycles, indicating excellent durability of its functionalities. was conducted 1 year before. The almost same response of Fig. 4 The effect of the response of the slit areas coated with three- tively; inset illustrates the slit response. b Water vapor transmission layer PAAm hydrogels on the water vapor transmission and air per- rates at 35 °C and c air permeability at room temperature for control meability of the knitted nylon fabrics. a Photo images of a knitted nylon fabric, control fabric with slits only, fabric coated with three- nylon fabric with the slit areas coated with three-layer LC/HC/LC layer hydrogels but without slits, and fabric with both hydrogels and PAAm hydrogels under wet (left) and dry (right) conditions, respec- slits under both dry and wet conditions, respectively 1 3 Advanced Fiber Materials the artificial stomata to artificial sweat (tested 1 year after) slits only, as well as the hydrogel coated fabric but without and to pure water (tested 1 year before) demonstrated the slits (Fig. S9). An average Darcy’s Permeability Constant excellent durability of the "open-close" behavior of artificial for each testing sample was calculated based on the sample stomata to water absorption/desorption. Furthermore, the thickness and diameter, and the air flow and pressure. As response rate of the simulated stomata to water or sweat can can be seen in Fig. 4c, the Darcy’s Permeability Constant be adjusted by changing the formulation of the less cross- showed a similar trend as that of the WVTR results, i.e., the linked (LC) hydrogel and/or that of the highly cross-linked fabric with cut slits coated with the three-layer hydrogels had –9 2 (HC) hydrogel, so that they will exhibit different swelling a higher permeability (> 43 × 10 cm ) compared to that of ratio and volume expansion. both control fabrics and the fabric with hydrogel but without –9 2 The breathability in terms of water vapor transmission slits (< 38 × 10 cm ). This applied to both the dry or wet and air permeability of the designed nylon fabric with the condition (p < 0.05). The wet fabric had a higher air perme- –9 2 –9 2 three-layer hydrogel coated slits was evaluated. Water vapor ability (~ 50 × 10 cm ) than the dry fabric (43 × 10 cm ) transmission rate (WVTR) was evaluated by means of the (p < 0.05), thereby proving that the opening of the pores/ cup method (BS7209 standard) [24, 25] at a temperature of slits greatly improved the air flow through the fabric, and 35 °C to simulate the temperature of the human skin (Fig. consequently also the fabric breathability and heat transfer. S8a) [37, 38]. The control nylon fabrics with and without cut slits and the fabric with hydrogel coating but without slits were also tested (Fig. S8b–d). As shown in Fig. 4b, Discussion the dry fabrics with the cut slits (15 slits cut in a 90-mm circular fabric sample) regardless of hydrogel coating had a While a resting person usually produces ~ 30 g/m /h insen- slightly higher WVTR compared with the dry control sample sible perspiration, it can be as high as ~ 1000 g/m /h sweat 2 2 (300–306 g/m /h vs. 285 g/m /h). This was due to the cut during physical activities . In order to maximize ther- slits in the fabrics, even though the slits were nominated mal comfort and barrier protection, it is therefore highly “closed”, their gap of 0.20 mm (Fig. S6a, f) was still much desirable to design smart fabric materials, the vapor and air larger than the original pores between yarns (Fig. S6d) and permeability of which can change according to the body’s could possibly accelerate the transmission rate. The fabric physiological and external conditions . with the hydrogel coating but without the slits exhibited In this study, a novel fabric was developed which has the lowest transmission rate (261 g/m /h) of all the samples environmental-responsive permeability. This was achieved (p < 0.05), due to the hydrogel coating in the window area by by creating leaf stomata-mimicking water responsive pores blocking the gaps between the yarns and thereby retarding (cut slits) in the fabric, where the pores/slits are coated with or greatly reducing the moisture transmission [39–41]. All a specific pattern of highly cross-linked (HC) and less cross- the fabrics increased the vapor transmission rate when wet, linked (LC) PAAm hydrogels. The hydrogel coated pores/ due to the surface evaporation of the vapor. Nevertheless, slits act like leaf stomata, which open under wet and close compared to the two control fabric samples (372 and 375 g/ under dry conditions. To construct the special hydrogel pat- m /h) and the sample with the hydrogel coating but without tern, the precise attachment of each hydrogel material in its any cut slits (365 g/m /h), the sample with the hydrogel designated position in the fabric is key. To do so, a photo- coating and cut slits exhibited a significant higher (56% initiator, benzophenone was used as a bonding agent, due to higher) vapor transmission (477 g/m /h) (p < 0.05) under dry its oxygen inhibition effect, which provided radical sites on conditions. Clearly, the opening of the slits when wet (Fig. the fabric for robust reaction with the UV-curable hydrogel S8i) greatly increased the transmission of vapor, exceeding monomers . For selective coating in the desired areas, the effect of the hydrogel blocking. Therefore, the results glue templates were used in order to prevent the spreading showed the presence and dimensions of the slits, which are of the hydrogel solution. For a knitted nylon fabric contain- larger than the pores of the fabric, play an important role in ing the designed simulated leaf stomata, the outer highly regulating the moisture transmission properties of the fabric swellable LC hydrogel layer functions as the driver of the when wet. “guard cells”, which swell greatly under wet conditions. Due Finally, the air permeability, the convective heat transfer to the linkage with the inner less swellable HC hydrogel, during thermal regulation, was measured by means of a gas the dimensions of the “guard cells” change asymmetrically permeability module on a Capillary Flow Porometer . during the swelling, i.e., the outer LC layers swell far more As for the WVTR test, an 18.3-mm circular fabric sample than the inner HC layers, resulting in the outward bend- with two cut slits coated with three-layer hydrogels was ing of the two “guard cells”. Since the knitted fabric sub- tested both under dry and wet conditions at room tempera- strate is very stretchable and firmly bonded to the hydrogel ture, and the results were compared to those of 3 control fab- network via BP pre-treatment, the outward bending of the rics, namely the original fabric and the fabric with two cut “guard cells” bowed the walls of the slit apart, leading to its 1 3 Advanced Fiber Materials opening. Though the hydrogel coating could slightly retard dimensions and automating the hydrogel coatings on the the moisture transmission [39–41], the opening/enlarge- surface of the fabric or membrane. If the pore/slit dimen- ment of the pores/slits under wet conditions significantly sion is reduced to a micro- size, the fabric or membrane increased the moisture transmission of the fabric to 477 g/ would have a greater barrier protection and hence the heat m /h at 35 °C, being 56% higher than that under dry con- retention and water repellency when the pores/slits are ditions when the pores/slits are closed, and 100% higher closed , and the size of the opening slits/pores might than that of the reported Nafion™ (perfluorosulfonic acid also be more controllable. While cutting slits in the fabric ionomer) film shirt with semilunar flipping patterns (237 g/ may adversely affect its mechanical properties to some m /h) . This unique property of the fabric with simulated extent, it should be acceptable as laser cut slits are very leaf stomata (viz. hydrogel coated slits) will greatly facilitate common in sportswear and fashion apparel . In future, sweat evaporation under profuse sweating conditions and one can also consider creating the holes or slits during this effect would be further enhanced by body movement the fabric manufacturing process and treat their adjacent and/or by wind. Conversely, when the fabric dries out, the areas with the responsive hydrogel materials in order to water evaporation causes the swollen hydrogels back to their eliminate any potential problems related to the mechanical original unswollen dimensions and hence the pores/slits to properties of the fabric. Finally, to commercialize the new close again. As there is no bending, there is little effect on development, it is also desirable to scale up the coating the air permeability. process, which could possibly be achieved by means of The opening and closing behaviors of the simulated leaf either screen or digital printing, which would also greatly stomata in the developed fabric will be very beneficial to simplify the process by depositing the hydrogels in the personal moisture and thermal management under different desired regions without the need for the pre-templating physiological and environmental conditions. For example, procedure. when a wearer is resting under normal/dry conditions, the simulated stomata remain closed in order to retain the heat and maximize the barrier protection. Conversely, when the wearer is sweating profusely either during active sports or Conclusion exposure to hot and humid conditions, particularly wet con- ditions, the pores/slits of the fabric will be opening in order In conclusion, it can be stated that an environmental respon- to increase the heat and moisture permeability and thereby sive smart fabric has been developed in which simulated thermal comfort. This is also the reason why water immer- leaf stomata pores/slits were created in a commercial knitted sion instead of moisture was used in an attempt to mimic the nylon fabric, with the inner windows of the slit areas being severe sweating conditions where the wearer’s skin could be coated with a highly cross-linked (HC) less swellable PAAm covered by a layer of water (sweat). Liquid water was used hydrogel, and the two outer windows of the slits with a less as the substitute of sweat in most of the experiments since cross-linked (LC) highly swellable hydrogel. Because of the the response of the PAAm hydrogel to water is the same as different swelling behavior of the two hydrogels, the slits that to the artificial sweat (Figs. 4a and S7c, d), as the PAAm act as simulated leaf stomata, which open under wet condi- hydrogel not having any surface charges, hence being unaf- tions and close under normal/dry conditions. The hydro- fected by the electrolytes in the sweat. gel coating in the slit areas was investigated by means of Apart from the nylon fabric, a cotton fabric and a polyure- SEM images, while the opening and closing behavior of the thane fabric were also used to demonstrate the feasibility of slits was studied using optical imaging. The slit response the proposed concept after being pre-activated by benzophe- to artificial sweat was the same as that to water. Due to the none (BP) initiator and coated with the three-layer hydro- opening of the pores/slits under wet conditions, both vapor gel on their cut slits. As shown in Fig. S10, both fabrics transmission and air permeability increased significantly for displayed opening and closing behaviors on the slits under the developed fabric. This smart fabric can have applications wet and dry conditions, respectively, similarly as that on the in functional clothing to maximize thermal comfort under nylon fabric (Fig. 4a), demonstrating our proposed strategy changing and extreme physiological and environmental con- can be applicable in different materials. It is also believed ditions. Furthermore, the concept can be extended to other that the same effect can be achieved with other materials, responsive membranes, such as wound dressings, controlled such as polyester, linen and wool, as long as they are pre- drug and nutrient release, and various other relevant indus- treated by a bonding agent, such as the BP initiator used in trial products. this work to enable the hydrogel coating to adhere to the Supplementary Information The online version contains supplemen- fabric. tary material available at https://doi. or g/10. 1007/ s42765- 023- 00269-5 . The present design of water responsive simulated leaf stomata can be further modified by reducing the stomata 1 3 Advanced Fiber Materials Acknowledgements This work was supported by Prof. Fan’s Faculty H. Harnessing the hygroscopic and biofluorescent behaviors of Startup Fund of the College of Human Ecology, Cornell University. genetically tractable microbial cells to design biohybrid weara- This work made use of the facilities of the Cornell Center for Materi- bles. Sci Adv. 2017;3: e160198. als Research (CCMR) supported by the National Science Foundation 8. Kim G, Gardner C, Park K, Zhong Y, Jin SH. Human-skin- under Award Number DMR-1719875. The authors also acknowledge inspired adaptive smart textiles capable of amplified latent heat Dr. Xia Zeng for equipment guidance and support, Charles V. Beach transfer for thermal comfort. Adv Intell Syst. 2020;2:2000163. and Vincent Chicone for their assistance with the mask fabrication. 9. Li XQ, Ma BR, Dai JY, Sui CX, Pande D, Smith DR, Brinson Finally, the PI, Prof. Fan would like to acknowledge the funding sup- LC, Hsu PC. 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J Mater Chem B. 2016;4:5814. 2021. Previously, she received 36. Li SZ, Zhou D, Pei MJ, Zhou YS, Xu WL, Xiao P. Fast gelling and her B.S. degree from Cornell non-swellable photopolymerized poly (vinyl alcohol) hydrogels University in 2016 in both with high strength. Eur Polym J. 2020;134:109854. Mechanical and Aerospace Engi- 37. Bierman W. The Temperature of the Skin Surface. J Am Med neering, and Operations Assoc. 1936;106:1158. Research and Information Engi- 38. Fan JT, Chen YS. Measurement of clothing thermal insulation and neering. Her research focuses on moisture vapour resistance using a novel perspiring fabric thermal the design and fabrication of soft manikin. Meas Sci Technol. 2002;13:1115. optical sensors, miniature soft 39. Wang XW, Hu HW, Yang ZY, He L, Kong YY, Fei B, Xin JH. robots and bioelectronics. Smart hydrogel-functionalized textile system with moisture management property for skin application. Smart Mater Struct. Prof. Jintu Fan holds Ph.D from 2014;23: 125027. Leeds University (1989) and BSc 40. Li B, Li DP, Yang YN, Zhang L, Xu K, Wang JP. Study of ther- from Donghua University (1985). mal-sensitive alginate-Ca2+/poly(N-isopropylacrylamide) hydro- He is currently Lee family Profes- gels supported by cotton fabric for wound dressing applications. sor in Textiles Technology and Text Res J. 2019;89:801. Chair Professor of Fiber Science 41. Bashari A, Nejad NH, Pourjavadi A. Effect of stimuli-responsive and Apparel Engineering at School nano hydrogel finishing on cotton fabric properties. Indian J Fibre of Fashion and textiles, Hong Text Res. 2015;40:431. Kong Polytechnic University 42. Kamalha E, Zeng YC, Mwasiagi JI, Kyatuheire S. The com- (PolyU). Professor Fan is a Former fort dimension; a review of perception in clothing. J Sens Stud. Head of Institute of Textiles and 2013;28:423. Clothing at PolyU (2018–2022) 43. Mukhopadhyay A, Midha VK. A review on designing the water- and Former Department Chair and proof breathable fabrics part i: fundamental principles and design- Vincent VC Woo Professor in ing aspects of breathable fabrics. J Ind Text. 2008;37:225. Fiber Science and Apparel 44. McQuerry M, Hogans K. Assessment of ventilated athletic uni- Design at Cornell University forms for improved thermal comfort. AATCC J Res. 2018;5:1. (2012–2018). Publisher's Note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. 1 3
Advanced Fiber Materials – Springer Journals
Published: Jun 1, 2023
Keywords: Breathable fabric; Fabric pores; Hydrogel; Leaf stomata; Water responsive
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