A Convenient and Universal Strategy toward Solvent‐Tolerant Microporous Structure for High‐Performance Wearable Electronics and Smart Textiles

Micro/nanostructures can increase effective surface area and enhance the performance of wearable devices, such as the sensitivity of sensors and output of triboelectric nanogenerators. Empowering commercial fibers and fabrics with durable and robust micro/nanostructures has become a major research concern for sustainable wearables. Many technologies are developed to fabricate micron/nanostructures on fibers and textiles, such as breath figure method, electrospinning, and direct imprinting thermal drawing. However, most of these methods have their own limitations toward mass production and real‐life application, including poor solvent resistance, time assuming, requiring expensive equipment, and limited capacity for post‐adjustment of commercial textiles. Herein, a plasma‐enhanced breath figure (PEBF) technique to fabricate solvent‐tolerant microporous structure on existing fabrics with tailored pore size is developed. By combining the wearable nature of fabrics and the surface engineering power of PEBF, the fabricated solvent‐tolerant microporous fabric offers excellent flexibility, washability, breathability, and suitability for large‐scale production, as well as the advantages of cost effectiveness and fast production. Furthermore, wearable triboelectric nanogenerators are fabricated based on solvent‐resistant microporous structured fabrics, revealing the bright future of the PEBF technology in wearable devices and smart textiles.


Introduction
Enhancing the robustness and durability of existing materials and improving their functionality through proper structural design represents a time and cost-efficient strategy for DOI: 10.1002/admt.202301277[3][4][5][6] It has been proved that micro/nanostructures can increase effective surface area thus enhance the performance of wearable devices in different scenarios. [3,7,8]For example, micro/nanostructures are favorite in the energy harvesting devices as well as gas sensors for enhancement of their performances.[13][14] On the other hand, wearable devices have become part of daily life in such a way that the fabrication of fibers and fabrics with micro/nanostructures has become a major research concern. [7,15,16]here are many technologies to fabricate micron/nanostructures on fibers and textiles, such as Breath Figure (BF) [17,18] method, electrospinning, [10,19,20] and direct imprinting thermal drawing. [4]However, most of these methods have their own limitations toward mass production and real-life application.For example, BF method is a well-studied technique for constructing microporous structures on the surface by polymer coating, with advantages of simplicity, efficiency, operability, and universality.Since its first report in 1994, [21] numerous studies have demonstrated the versatility of the BF method for a variety of polymers and substrates in fields such as membranes, [22,23] templates, [24] responsive surfaces, [25] and catalysts. [18]Although several reports have shown its potential in fabric surfaces and wearable electronic applications, [8,26,27] most of these works face the same challenge that is the rarely mentioned, namely, poor solvent resistance.Solvent resistance is a critical property for materials, especially when dealing with those featuring surface micro/nanostructures, as it significantly influences both their preparation and performance.Throughout various practical applications and subsequent processing stages, materials inevitably come into contact with a wide range of solvents.Surface micro/nanostructures lacking adequate solvent resistance can easily corrode or dissolve when exposed to organic solvents, and this challenge is also encountered in electrospinning.The degradation or dissolution of the material can lead to compromised structural integrity, consequently influencing the product's performance.Such susceptibility has a profound impact on the material's durability, limits the variety of subsequent processing options, and constrains its applicability.To solve this problem, Zhe and wo-workers proposed an in-suit fabrication method to prepare fibers with microstructures using direct imprinting thermal drawing, showing high potential for wearable applications. [4]Although it achieved arbitrarily designed surface patterns on entire fiber surfaces with high resolution in all directions, this method is time assuming and requires expensive equipment.More important, it has limited capacity for post-adjustment of commercial textiles and woven.Therefore, a versatile and effective method for constructing solvent-tolerant micro/nanostructures on the surface of complex woven fibers is highly desired and urgently needed.
As an effective surface modification technique, plasma treatment is widely used to achieve grafting, [28,29] etching [30,31] , and cross-linking [32][33][34][35][36][37] of material surfaces.After plasma treatment, a cross-linking network layer will be formed on the surface, which is extremely difficult to be dissolved in solvents. [32,38]The crosslinked polymers make it difficult for solvents to penetrate the surface, providing excellent solvent resistance and protecting the internal materials from solvent damage. [33,37,39][42][43][44][45][46] However, rare studies have been conducted to assess the possibility of using plasma treatment to enhance the solvent-tolerant properties of existing micro/nanostructures on fiber and textile surfaces.Successful implementation of this approach for fabrication of solventtolerant micro/nanostructures on complex woven and fabrics, could offer a new avenue for the practical application of wearable electronics and smart textiles.Therefore, further exploration of this approach is necessary to realize its full potential.
In this work, we proposed and investigated a novel surface engineering technique to address the long-standing challenge of preparing solvent-tolerant surface microporous structures on fabrics.By combining the BF method and plasma technology for the first time, we developed a cost-effective and universal strategy to fabricate solvent-tolerant microporous structure on existing woven.Unlike traditional methods that suffer from complex, expensive, and manufacturing-dependent processes, our proposed plasma-enhanced BF (PEBF) technique offers a convenient and practical alternative that can be used to modify the surface struc-tures of existing materials.By leveraging molecular-level modifications, we can overcome the practical limitations that have restricted the use of wearable materials in a variety of applications.Furthermore, to reveal this technique's extensive application prospect and good compatibility with commercial fabrics and textiles, wearable triboelectric nanogenerators (TENGs) were fabricated based on nylon cloth with an intermediate layer.In comparison to conventional TENG with flat surfaces, the TENG with surface microporous structured friction layer and intermediate layer had a 15-fold higher output current.Furthermore, the prepared microporous structures exhibited excellent solvent resistance after plasma treatment and possess excellent flexibility, washability, and gas permeability.All these results indicate our technique's promising future in wearable electronics and smart textiles.

Concept and Fabrication of the PEBF-Based Solvent-Tolerant Microporous Fabrics
The fabrication process of the solvent-tolerant microporous surface is illustrated in Figure 1a.The common commercial nylon fabric was chosen as the substrate in this study for its low cost, lightweight, and comfortable wear.To achieve the microporous surface, the nylon fabric was immersed in the polymer solution for 10 min, then moved out to dry under certain environment, while the fabric surface is covered with a layer of polymer solution.After controlled drying, a polymer coating with a microporous structure is eventually formed on the nylon fabric.Due to the inherent solubility of polymers, this surface microporous structure is not solvent-tolerant and can be easily erased.With plasma post-treatment, a cross-linked layer is formed on the polymer surface, which is not soluble to solvent thus protects the inner structure from solvent penetrating, giving the surface microporous structure excellent solvent resistance.This strategy can empower existing materials (eg.fibers and fabrics) with robust and durable microstructures and improve their functional performance, thereby reduce the need for their replacements, showing great potential for smart textiles and wearable electronics (Figure 1b).By combining the wearable nature of fabrics and the surface engineering power of PEBF, the fabricated solvent-tolerant microporous fabric offers excellent flexibility (Figure 1c; Figure S1, Supporting Information), washability (Figure 1d; Figures S2 and S12, Supporting Information), breathability (Figure 1e; Figure S3, Supporting Information), and suitability for large-scale production (Figure 1f), as well as the advantages of cost effectiveness, fast production, and tailored microporous structures.

Tailored Microporous Structures
To achieve tailored microporous structures, we first explored PEBF's ability to construct microporous structures with different pore sizes on the substrate surface.According to the mechanism of the BF process (Figure S4, Supporting Information), [18] the fabric is covered with a layer of polymer solution after being taken out of the polymer solution.During the subsequent drying process, solvent evaporation cooling causes water vapor in the air to condense into water micro-droplets on the surface.As this process proceeds, the water droplets grow and form microphase separations with the polymer solution, eventually self-assembling on the surface to form ordered arrays by the Marangoni convection effect. [47]After thorough drying of the solvent and water droplets, a polymer coating with microporous structures templated by the water droplets array is formed on the fabric surface based on nonsolvent induction effect. [48]Based on the mechanism, the condensation, growth, and self-assembly of water droplets are the key processes affecting the microporous morphology, which are mainly influenced by the types of polymer and solvent, polymer solution concentration, ambient temperature, and relative humidity. [49,50]ere we took the SEBS/chloroform solution as an example to explore the correlation between microporous pore size and the solution concentration, as shown in Figure 2. When SEBS solution was applied to the pure nylon surface at a concentration of 20 mg mL −1 , an excellent microporous structure was formed with an average pore size of ≈2.4 μm (Figure 2a,e).As the concentration decreases, the process of solvent evaporation and water droplet growth last longer, resulting in microporous pores with larger diameters (Figure 2b,f, ≈5.3 μm at 14 mg mL −1 and Figure 2c,g, ≈8.6 μm at 10 mg mL −1 ).As the concentration decreased down to 6 mg mL −1 , it is evident that a continuous microporous structure could not be formed (Figure 2d,h).This might because the polymer concentration was too low to cover the surface of the nylon fabric continuously.When we changed the substrate to polyaniline-coated nylon (Ny-PANi), the microporous morphology with SEBS/chloroform solution showed the same trend (Figure S5a,d, ≈3.5 μm at 40 mg mL −1 , Figure S5b,e, ≈2.3 μm at 80 mg mL −1 , and Figure S5c,f, ≈2.2 μm at 100 mg mL −1 , Supporting Information).These results show that the size of the micropores can be easily adjusted by controlling the polymer concentration within a certain range, which demonstrates the good controllability and practicality of the method.

Versatility to Polymer Materials and Substrates
Besides tailoring microporous structures, we also examined the versatility of this method for various polymers, as shown in Figure 3.Under the same environmental conditions, a series of hydrophobic polymers (SEBS, SIS, PS, PMMA, PCL) in chloroform solutions successfully formed surface coatings with microporous structures on nylon fabrics, while the textile morphology of the nylon fabric itself was well retained (Figure 3a-e).As a comparison, the pristine nylon fabric has a smooth surface and no pore structure (Figure 3f).It shows that the morphology and size of the microporous structures differ from different  polymers.This is mainly due to the different solution viscosities of polymers, which affects the growth and self-assembly of the aqueous microdroplet arrays directly.For hydrophilic polymers like PEO and PAA, it formed almost flat surfaces rather than microporous structures on nylon fabrics (Figure S6, Supporting Information).Similarly, the same flat surface was obtained when the solvent was replaced with tetrahydrofuran.This is because water droplets cannot form stable phase separation with watersoluble polymers or solvents, and therefore cannot form water droplet arrays template for microporous structure.
To further explore the versatility of this PEBF technique, substrates with different characteristics and morphologies were tested.A conductive polyaniline (PANi) layer was grown in situ on the surface of the nylon fabric.Different from the smooth nylon surface used above, the morphology of the PANi layer is composed of vertically aligned nanofibers with a length of ≈200 nm (Figure S7a, Supporting Information).Moreover, PANi has very different physicochemical properties from nylon.The results in Figures S5 and S7 (Supporting Information) showed the successful formation of excellent microporous structures on Ny-PANi fabrics by SEBS/chloroform solution.

Solvent-Resistant Microporous Structure
In order to achieve solvent tolerance, plasma treatment was applied to the surface (Figure 4a).The high-energy particles in the plasma can break and reconnect the chemical bonds on the material surface to achieve surface cross-linking, which can protect materials from solvent damage. [36,39,45]In order to provide a comprehensive understanding of the solvent resistance property of the surface microporous structure prepared by PEBF technique and the impact of this property on the material preparation and application scenarios, comparative experiments were conducted on solvent tolerance.Chloroform, a typical good solvent for SEBS, was chosen as a representative solvent to test the solvent resistance property of the samples and excellent results was obtained.The microporous structure prepared by SEBS/chloroform solution is shown in Figure 4b.After soaking in chloroform for 10 min without plasma treatment, the microporous structure of the sample Figure 4b was almost completely erased by chloroform and the smooth nylon surface exposed, which means nonsolvent tolerance (Figure 4c).On the other hand, as shown in Figure 4d, the microporous structure was well preserved when the sample was first treated with plasma and then soaked in chloroform for 10 min, showing excellent solvent resistance.The same experiments for microporous structure prepared by PS/chloroform solution were performed and showed consistent results (Figure S8, Supporting Information).Besides chloroform, the solvent resistance of the samples to several common solvents was also examined (Figure S11, Supporting Information).As shown in the SEM images, after soaked in solvents for 10 min, the microporous structure prepared by the PEBF technique was well preserved, demonstrating excellent solvent resistance to DDW, ethanol, acetone, and THF (Figure S11e-h, Supporting Information).These results confirm the excellent solvent resistance of the microporous structures prepared by the PEBF technique.Based on earlier reports, [36,37,39] the high-energy particles in the plasma bombard the chemical bonds of polymers, breaking molecular chains of polymers and forming significant amounts of free radicals.Part of the molecular fragments are removed to form the etching effect.While part of the free radicals connects with neighboring molecules, thus causing the polymer molecules crosslinked together to form a cross-linked network layer on the surface.The solubility of the polymers is changed by cross-linking, which is difficult to be dissolved by solvents (chloroform), protecting the inner polymers from solvents and giving the materials good solvent resistance.
The above results demonstrate the successful development of the PEBF technology for the preparation of solvent-resistant microporous structured polymer coatings on nylon fabric surfaces.This technique is applicable to a wide range of hydrophobic polymers and the microporous pore size can be easily adjusted.While the microporous structure is constructed on the surface, the textile morphology of the nylon fabric itself is well retained, ensuring good flexibility and permeability for wearable device applications.To further verify their vaper permeability, we have conducted a careful quantitative study with different materials: Vials containing equal amounts of liquids were sealed tightly with prepared Ny-PANI, Ny-PANI-SEBS, SEBS film, and conventional commercial nylon fabric (Ny), respectively, then placed in the same laboratory environment.Figure S9a,b (Supporting Information) shows the results of the mass loss of the bottles every few hours, with an almost linear relationship between vapor permeation and time.In Figure S9a (Supporting Information), the unsealed sample bottle with black line represents the volatilization rate of water vapor itself, which is 4.5 mg cm −2 h −1 .The purple line reveals that the pure SEBS film has almost no water vapor permeability (0.4 mg cm −2 h −1 ).The water vapor permeability of Ny (red line, 4.6 mg cm −2 h −1 ) and Ny-PANi (blue line, 4.7 mg cm −2 h −1 ) is extremely high, almost the same as the volatilization rate of water vapor.The Ny-PANi-SEBS with microporous structured SEBS coating (green line) also exhibited high water vapor permeability, reaching 2.2 mg cm −2 h −1 .Figure S9b (Supporting Information) shows the permeability for organic vapor, exhibiting a regularity consistent with the water vapor permeability.These results indicate an outstanding water and organic vapor permeability of the prepared microporous structured fabrics under natural condition, providing good comfort for wearable applications.

Wearable TENG Based on Solvent-Resistant Microporous Structured Fabrics
Having excellent compatibility with commercial fibers and textiles and being armed with the solvent-resistant microporous structure, PEBF can be directly applied to wearable devices.To reveal this technique's extensive application prospects in wearables, we design and fabricate wearable TENG based on solventresistant microporous structured fabrics.As an emerging energy harvesting device, TENG can convert mechanical energy (e.g., sound, vibration, tide, and wind) into electricity, making it a promising candidate for powering wearable electronics. [51,52]revious research has demonstrated that micro-and nanostructures can significantly improve the output performance of TENG.Here, we assembled TENG using solvent-resistant microporous structured nylon fabrics fabricated by the PEBF technique (Figure 5).Conductive polyaniline was polymerized in situ on the surface of nylon working as electrode due to its excellent flexibility and stable conductivity.SEBS was chosen as the positive friction material due to its inherent excellent positive tribocharge property and elasticity, and PVDF was chosen as the negative friction material due to its extreme electronegativity.As shown in Figure 5a, TENG with flat SEBS and PVDF friction layer (F-TENG) was fabricated as control group.The microporous structured SEBS coating was constructed on Ny-PANi fabric by the PEBF technique to form Ny-PANi-SEBS as the positive part of the microporous structured TENG (M-TENG).PVDF coating was sprayed onto the microporous structured Ny-SEBS-PANi substrate to form Ny-SEBS-PANi-PVDF, working as the negative part of the M-TENG.Then the positive part (Ny-PANi-SEBS) and the negative part (Ny-SEBS-PANi-PVDF) were assembled into M-TENG with vertical contact-separation mode.The output performance including the open-circuit voltage (Figure 5b) and shortcircuit current (Figure 5c) were collected correspondingly under the same driving parameters with reciprocating motor.The M-TENG with microporous structured friction layers exhibited excellent electrical output performance with an peak open-circuit voltage of 154.7 V and short-circuit current of 3.8 μA.Compared to the conventional F-TENG with flat surface (37.2 V, 0.8 μA), the output voltage and current of M-TENG were 4.2 and 4.75 times better, which is consistent with earlier reports. [53]o further demonstrate the power of PEBF technique with solvent-resistant microporous structure design of fabrics, we fabricated a microporous structured TENG with an intermediate layer (referred to as IM-TENG) for further enhancing the output performance (Figure 5a).As per our prior research, the addition of an intermediate layer between the electrode and the negative friction layers can significantly improve the output performance of TENGs. [54,55]This substantial improvement can be attributed to the interaction between the intermediate layer and the friction layer, which reduces electron decay and increases charge density.It is worth noting that all reported intermediate layers to date have been planar and flat.We propose a 3D intermediate layer with a microporous surface giving a 3D interaction between the intermediate layer and the negative friction layer.This unique structure is expected to a more pronounced enhancing effect on TENG performance.Here, Ny-PANi-SEBS was used as the positive part and Ny-PANi-SEBS-PVDF was used as negative part, in which the SEBS layer with solvent-resistant microporous structure worked as the intermediate layer.The surface morphology of the PVDF layer was characterized by scanning electron micrographs, showing well-defined microporous structure (Figure S7b, Supporting Information).As shown in Figure 5b-d, the output performance of the rational designed IM-TENG was greatly improved with an open-circuit voltage of 437.4 V and a short-circuit current of 12.3 μA, which were 2.8 times (voltage) and 3.2 times (current) higher compared to the M-TENG and 11.8 times (voltage) and 15.4 times (current) higher compared to the F-TENG.This highoutput IM-TENG can be used to light up the signal lamps of "LNBD" (Figure 5e), demonstrating the application prospects for powering warning signals in the dark night.In addition to SEBS, other polymers, such as PS and PCL, can also be fabricated as microstructure intermediate layers for enhancing the performance of TENG (Figure S10, Supporting Information).These results indicate that the microstructures fabricated by PEBF are effective for improving output performance of the TENG and show great potential in wearable applications.
[56][57] Specifically: 1, The additional intermediate layer effectively inhibits the migration of negative charges from the negative friction material to the electrode, reducing charge loss caused by neutralization with positive charges on the electrode.2, The high level of electron traps in the intermediate layer itself functions as a charge storage, thereby increasing both the quantity and storage depth of charge storage.The finely designed 3D intermediate layer features a microporous surface structure that establishes a 3D, cross-interfacial contact interaction with the negative friction layer (as schematically shown in Figure 5a).This configuration offers a larger contact area and a significantly more potent boosting effect compared to 2D planar interactions.

Conclusion
In summary, we have successfully developed a novel method for preparing smart fabrics with solvent-tolerant microporous structures by combining Breathing Figure and plasma technology, and demonstrated this technique's application for high-performance wearable TENGs.Solvent-resistant microporous structured coatings were successfully prepared on commercial nylon fabrics by the PEBF technique, while the textile morphological characteristics of the fabric were well maintained, ensuring good flexibility and permeability for wearable device applications.This method is applicable to various polymers and substrates, and the pore size can be easily adjusted by controlling polymer concentration.We further developed a high output IM-TENG with an peak voltage of 437.4 V and current of 12.3 μA, which was 11.8 times and 15.4 times higher compared to the flat TENG, respectively.Furthermore, this fabric-based TENG possesses excellent flexibility and gas permeability, indicating favorable wearable comfort.More importantly, the PEBF technique we have developed is facile, efficient, low cost, robust, and universal, and avoids expensive equipment and specialized operational processes, bringing new thoughts for large-scale manufacture and the practicality of high-performance wearable electronics and smart textiles.
In Situ Polymerization of Polyaniline on Nylon Woven Fabric: Polyaniline was grown in situ on nylon fabric by the dilute chemical polymerization method. [14]Nylon fabric (4cm × 4 cm) was placed in the solution of aniline monomer (23 mg) and ammonium persulfate (38 mg) in 1 m HClO 4 (25 mL) at 0 °C.After chemical polymerization for 24 h at 0 °C, the nylon fabric was taken out of the solution, and washed with deionized water and 1 m HCl respectively.The conductive NY-PANi fabric was obtained after drying at room temperature.
Fabrication of the Solvent-Tolerant Microporous Surface (PEBF): The nylon substrate was first immersed in a polymer solution at certain concentration and then token out to dry under certain environment, thus obtaining the surface microporous structure.Finally, solvent-resistant property was achieved by glow plasma treatment (Ar, 10 SCCM, 1 min).

Fabrication of M-TENG and IM-TENG:
The positive friction part (Ny-PANi-SEBS) was prepared by the PEBF method using Ny-PANi fabric and SEBS/CHCl 3 solution (100 mg mL −1 ).The negative friction part (Ny-SEBS-PANi-PVDF) was prepared by a spraying coating method (0.3 mL of a 20 mg mL −1 PVDF/DMF solution, sprayed at 80 °C and 14 Psi) based on Ny-SEBS-PANi fabric.Then the positive and negative parts were fixed a Cu wire respectively, assembled into the vertically contact-separation M-TENG.The IM-TENG was prepared in the same procedure with an intermediate layer of SEBS, PS, and PCL respectively.The total effective contact area of the TENGs was 16 cm 2 .
Water and Ethanol Vapor Permeability Test: The testing procedure was based on ASTM E96.The vials were filled with an equal amount of liquid (distilled water, ethanol), and sealed tightly with the corresponding film materials respectively.The sealed bottles were placed in the same laboratory environment.The mass of these bottles was measured every few hours and the reduced mass was considered as the amount of evaporation of the liquid.
Standardized Washing Process: In order to evaluate the durability of the prepared microporous structured fabrics, one or five washing cycles were performed according to ISO 6330 2012 Textiles -Domestic washing and drying procedures for textile testing.The washing test conditions were as follows: washing for 30 min at 60 °C using ECE reference detergent 98 with a concentration of 0.3 g L −1 .After washing, the tested textile was promptly dried in a dryer at 50 °C.Morphological characterization of the samples was performed using scanning electron microscopy (SEM) after drying.
Characterizations and Measurements: Scanning electron microscopy (SEM, su-8010, Japan) and optical microscope (Optika, model: B-350) were used to investigate the microporous morphology.The distribution of pore size was analyzed with ImageJ.In measurement, a reciprocating motor was used to periodically press and release the device to investigate the output performance of the TENG.The output voltage was measured by NI PCI-9215 DAQ card, and the short circuit current was measured by SR570 low noise current amplifier (Stanford Research System, America).All of the experiments were performed at atmospheric pressure and room temperature unless otherwise stated.

Figure 1 .
Figure 1.Overview of plasma-enhanced breath figure technique and solvent-tolerant microporous fabrics.a) Fabrication process of the solvent-tolerant microporous structures, b) Potential applications for wearable electronics and smart textiles.Photographs demonstrating that the solvent-tolerant microporous fabric offers flexibility c), washability d), breathability e), and suitability for large-scale production f).

Figure 2 .
Figure 2. SEM images of the microporous structures on nylon fabrics prepared from SEBS/chloroform solution with different concentrations via PEBF technique: a) 20 mg mL −1 , b) 14 mg mL −1 , c) 10 mg mL −1 , and d) 6 mg mL −1 .The insets of (a-d) are their magnified views correspondingly (scale bar: 10 μm).And e-h) are the corresponding pore size distributions of (a-d).

Figure 4 .
Figure 4. Schematic diagram of the mechanism of solvent-resistant treatment a) and SEM images for solvent-resistant microporous structures: b) the original microporous structure prepared by SEBS/chloroform (20 mg mL −1 ), c) soaking in chloroform for 10 min without plasma treatment and d) soaking in chloroform for 10 min after plasma treatment.The insets of (b-d) are their magnified views correspondingly (scale bar: 10 μm).

Figure 5 .
Figure 5.The effect of microporous structured friction layer and intermediate layer on the TENG output performance.a) The schematic diagram of the different TENG structures: F-TENG: flat TENG; M-TENG: microporous structured TENG; IM-TENG: microporous structured TENG with an intermediate layer.The output voltage b), short-circuit current c), and histogram d) of the TENGs with different structures.e) LEDs were light up by IM-TENG.