A method of constructing highly durable conductive materials by growing metal particles inside and outside �bers through solvation and multivalent bonding forces

Metallized textiles have shown promising applications in the �elds of electrical conductivity, Joule heating and electromagnetic shielding. Poor durabilities, especially washability, which caused by the signi�cant mechanical mismatch between rigid metal parts and textiles have hindered commercialization process of these functional textiles. This work constructed ultra-durable conductive cotton fabrics by growing copper nanoparticles with amorphous region-controlled swelling and multivalent bonding forces to complex the metal particles. The enlarged �ber amorphous zone and phen-amine molecules are used as templates to provide further possibilities for the internal and external enrichment growth of copper nanoparticles, providing good conductivity and high durability of the processed cotton fabric. The constructed fabric exhibits excellent electrical conductivity (6.09±0.36×10 -3 Ω /sq), electrothermal conversion (60 s, 1 V, ~140 °C) and electromagnetic shielding e�ciency (65.32 dB). Notably, the electrical conductivity of the fabric remains essentially unchanged (Rs/R 0 =1.106) after 100 standard washing tests. This is attributed to the increase in metal particle loading and the enhancement of metal-ber bonding fastness. Therefore, this work might provide a novel insight for constructing ultra-washable conductive clothing textiles with heating and EMI shielding performance.


Introduction
In recent years, wearable electronics have penetrated into all works of life, from medical (Chu, et al. 2023;Zhao, et al. 2022), military (Cai, et al. 2017) and monitoring to aerospace (Lu, et al. 2021;Oh, et al. 2022;Wu, et al. 2019).Textiles are considered as one of the most ideal choices of wearable electronic substrates owing to their high mechanical exibility (Xu, et al. 2023;Candadai, et al. 2021), lightweight (Sun, et al. 2016;Huang, et al. 2019) and high breathability (Nasreldin, et al. 2020;Afroj, et al. 2020;Li, et al. 2017;Zeng, et al. 2014;Agcayazi, et al. 2018).The fabric itself is a non-conductive polymer, which needs to be lled with conductive components such as conductive polymers, carbon materials, metal particles, etc. to give the fabric conductive properties (Du, et al. 2022;Wu, et al. 2018;Jin, et al. 2020;Xiong, et al. 2017).At the present stage, the metallization techniques for textiles mainly include blending metal wires or metal bers with textile bers, coating method, chemical plating and so on (Uzun, et al. 2019;Guo, et al. 2019;Liu, et al. 2014).Metal bers are di cult to prepare and have low production e ciency.The conductive materials prepared by the coating method have certain limitations, such as easy to fall off under friction and bending deformation, resulting in a sharp decline in conductivity (Allison, et al. 2017;Malaki, et al. 2019;Johnson, et al. 2022).The conductive textiles prepared by chemical plating do not have a strong bond between the conductive component and the substrate, such as chemical bonding.Which makes it tend to fall off under washing, rubbing and other conditions, resulting in poor durability performance during use (Zhu, et al. 2018;Liu, et al. 2021;Khair, et al. 2019).
Therefore, there is still a bottleneck in how to enhance the bonding ability between particles and bers in metallized conductive fabrics.There are two ways to enhance the adhesion of the particles to the bers (Wang, et al. 2021): (1) increasing the loading of the conductive particles and (2) enhancing the bonding fastness between the conductive particles and the bers.
Load capacity of fabrics for metal particles is in uenced by their own ber morphology and supramolecular structure.Cotton fabrics are economical (Kert and Skoko 2023), with porous structure (Liu, et al. 2010;Zhang, et al. 2023) and a large number of hydroxyl groups in the cellulose molecular chain (Wang, et al. 2019), which is conducive to their functionalization as a substrate material for conductive fabrics.However, hydroxyl group forms intramolecular and intermolecular hydrogen bonding easily, enabling the cellulose molecular chain to easily aggregate together, forming a supramolecular structure with regularly arranged crystalline regions and irregular amorphous regions (Keraani, et al. 2022;Gupta, et al. 2020).In addition, the reaction between chemical reagents and bers must go through the process of gradual transition from surface to inside, from amorphous zone to crystalline zone (Kärger and Ruthven 2016).Therefore, to allow the reaction to be more than con ned to the surface of the amorphous and crystalline zones of the ber, the supramolecular structure of the ber needs to be disrupted.In general, the amorphous regions of the cellulose bers can be swollen effectively in alkaline solutions (Hossain, et al. 2020).Moreover, there are studies illustrated that the microporous structure of cotton is in uenced by the swelling state, as for example the pore size is about 6 Å in the dry state and about 15-30 Å in the wet swollen state (Sun, et al. 2016).The swollen bers assist the growth of metal particles into the interior of the bers (Zhou, et al. 2022), raising the electrical conductivity as well as the washing stability.
However, the enhancement of the durability of conductive fabrics depends not only on the loading of conductive particles but also on the bonding fastness between the particles and bers.The metal-organic coordination reaction is a promising strategy to design conductive materials by enhancing the bonding between bers and metal particles.Recently, polydopamine (PDA) involved one-step co-deposition has emerged as facile approach to fabricate conductive materials (Waite 2008;Danner, et al. 2012).However, the high costs of dopamine and the dark color of PDA coatings are undesirable in practical applications.Fortunately, as a plant polyphenol, caffeic acid (CA) has also been recently reported to possess many advantages of PDA such as the ability to adhere rmly to various substrates, spontaneous polymerization under mild conditions, and covalent bonding to thiol and amine groups (Lee, et al. 2007;Liu, et al. 2016;Crisp, et al. 1985).Additionally, polyphenols are low-cost, accessible, environmentally friendly (Liu, et al. 2022).As polyethyleneimine (PEI), an amino-rich polymer, has been reported to be able to accelerate the deposition process of dopamine and catechol via Michael addition and Schiff-base reactions (Gu, et al. 2017).The above substances have been demonstrated for fabric interface functionalization applications and metal absorption (Du, et al. 2022).
In this work, the innovative combination of swelling and polyvalent bonding forces of polyphenols allows metal particles to grow inside and outside the ber, effectively improving the durability of conductive fabric.The fabricated fabric exhibits excellent electrical conductivity (6.09 ± 0.36×10 − 3 Ω/sq) with a variation in sheet resistance less than 0.001 after 100 washes.In addition, it shows great and stable electro-thermal performance and remarkable electromagnetic shielding effect while maintaining its exibility and breathability.All these results indicate the fabricated conductive cotton fabric in this work is a promising E-textile.
2. Materials and methods

Pretreatment of cotton fabrics
First, cotton fabrics with size of 4 cm × 4 cm were immersed in a 10 g/L NaOH solution and boiled for 30 minutes.Then these processed fabrics were rinsed with deionized (DI) water several times to eliminate the residual NaOH solution, and dried in an oven under 70 ºC for 10 min.Next, the prepared fabrics were pipped in a mixture of CA and PEI and reacted under certain conditions for a period time.Finally, the CA and PEI coated cotton fabrics were washed with DI water, and dried in an oven at 70 ºC for 10 min.

Preparation of conductive Cu/Ag/CA-PEI@Cotton
The prepared CA-PEI composite fabrics were soaked in 2 g/L AgNO 3 for 10 min at room temperature of 25 ºC, followed by copper plating treatment using the plating solution for 4 h.The composition of the copper plating solution was 15 g/L CuSO 4 •5H 2 O, 14 g/L NaOH, 19 g/L EDTA, 0.1 g/L K 4 Fe(CN) 6 , and 10 ml/L HCHO.Then the nal composite fabric named Cu/Ag/CA-PEI@Cotton were washed with DI water and dried in an oven at 70 ºC for 10 min.The whole construction of Cu/Ag/CA-PEI@Cotton is illustrated in Fig. 1.For comparison, Cu/Ag/CA-PEI@Cotton without pre-swelling cotton was reacted under the same conditions.

Characterization
Micrographs were taken by Ultra Deep Field Microscope (VHX-6000).The tensile strength was measured on an Instron universal test instrument (model 5576, Instron Instruments, USA), with a crosshead speed of 10 mm/min and the gauge length of 15 mm.Morphology of samples was observed by Field Emission Scanning Electron Microscope (FESEM) (S-4800).Energy dispersive spectroscopy (EDS) mapping was conducted at a magni cation of 10000 times with an acceleration voltage of 10.0 kV.The X-ray diffraction (XRD) mineralogical analysis was performed by a Bruker D8 X-ray diffractometer using Cu Kα radiation (λ ~ 1.54 Å) with 2θ range of 5°-80° and a scanning rate of 5°/min.The chemical reactive groups of the reagents were analyzed by variable temperature Fourier transform infrared spectroscopy (FTIR), and data were recorded in the range of 500-4000 cm − 1 .The electrical resistance was measured with a low resistivity meter (Loresta-AX MCP-T370, Mitsubishi Chemical Analytech Co., Ltd.) using the four-point probe method.The weight of the samples after reaction were weighed using an analytical balance.The thermal degradation behaviors were determined with the thermogravimetric analyzer (TGA 8000) under a dynamic nitrogen atmosphere at a heating rate of 20°C/min over a test temperature range of 30-800°C.Capillary ow ori ce analyzer (Porolux 100) was used to determine the permeability and pore-size distribution of different fabrics.Joule heating was powered by a power supply (RXN-305D) at different voltage.Thermal images were collected with a FLIR ONE Pro Thermal camera, and then used to evaluate the electro-thermal performance, cyclic performance and heat distributions of the composites with sample size of 1 cm×4 cm.The EMI shielding effectiveness (EMI SE) was measured by a vector network analyzer (Keysight PNA-L N5234B network analyzer) in the frequency range of 8.2-12.4GHz.
Electromagnetic shielding effect was performed on the conductive fabrics using a vector network analyzer.Standard washing was tested according to GB/T 3921.3 (Du, et al. 2022) and the wash was recorded for 30 min as a wash cycle to test its wash resistance.The bending cycle test done on the samples up to 1000 times to show the durability and exibility.Cu/Ag/CA-PEI@Cotton was placed in air about 20 ºC to study the air stability.The abrasion resistance of the fabric samples (15 cm ×6 cm) was measured by textile abrasion tester (Y571B), recorded 10 rounds as one cycle.

Fabrication of Cu/Ag/CA-PEI@Cotton
Cellulose is a polydisperse linear polyglucan that readily forms hydrogen bonds and constitutes an aggregated structure containing crystalline and amorphous regions (Liu, et al. 2019).The contact of the NaOH solutions starts the dissolution of the randomly deformed molecular chains, which causes the relaxation of the ber and the formation of a locally expansion structure.This is manifested in the microstructure as an increase in the amorphous regions (Fig. 2c, f).The apparent morphology (Fig. 2a-e) shows a signi cant increase in the diameter of the swollen cotton bers.It changed from 9.595 ± 1.44 µm to 16.049 ± 1.53 µm, an increase of approximately 67.26% (Fig. 2g).The strength retention of the bers after swelling treatment is approximately 94% (Fig. S2), still maintaining good mechanical properties.As shown in Fig. 2h, i, cotton fabric samples exhibited similar peaks at 2θ = 14.98°, 16.7°, 22.8°, 34.49°( French 2014), which are attributed to cellulose I (Xu, et al. 2023).This indicates that the swelling process did not destroy the crystal structure of the cotton, which explains the no signi cant deterioration in mechanical properties of the fabric.The crystallinity of the cotton bers changed as NaOH penetrated in the ber, accompanied by the breaking and recombination of inter-and intra-molecular hydrogen bonds within the ber.The crystallinity of the cellulose decreased from 61.13-52.89%after the NaOH swelling treatment, obtained by calculating the peak area.The reduction in ber crystallinity leads to an increase of the amorphous zone, allowing solute molecules to easily enter the interior of the ber.This is further con rmed in the Fig. S1, where the cross-sectional area of the bers with the Cu NPs loading increases signi cantly after swelling.Moreover, Cu NPs entered more into the interior of the ber contributing to the formation of a continuous conductive pathway.
CA features a catechol structure that also allows for the formation of hydrogen bonds, π-π stacking, metal bonds with chemicals (Bharath, et al. 2018) as shown in Fig. 3a.Taking advantage of the high diffusivity of small molecules of phenol-amine substances that can enter the interior of the bers to provide adsorption sites for metals, we chose CA and PEI, whose molecular structure and possible reaction mechanisms are shown in Fig. S3. Figure 3b shows that the phenol-amine particles are uniformly distributed on the fabric.In addition, the treatment of phenol-amine substance to the smooth PET lm generated uniformly sized brown patches (Fig. 3c), which provided the prerequisite for the uniform adsorption of metal particles.IR spectra and XPS results veri ed the successful reaction of CA with PEI, speci cally analyzed in SI.4.
The reaction of CA with PEI can in uence the adsorption sites of metal particles and has a signi cant impact on the electrical conductivity of the fabric.In this experiment, fabric with high electrical conductivity was fabricated by optimizing the reaction conditions of CA and PEI.Sheet resistance value of 6.09 ± 0.36×10 − 3 Ω/sq can be reached in a 0.1 mol/L mixture of CA: PEI = 2:1 at 40°C for 4 h (Fig. S5).
A large quantity of CA-PEI particles were attached to the surface of the cotton fabric after the modi cation, which provided conditions for the subsequent deposition of metal particles (Fig. 4b).Further, the yarns drawn from the conductive fabric were cut into sections with a Haar slicer to observe the loading of metal particles.The metal particles were primarily loaded only on the edges of the conductive yarns without swelling (Fig. 4c).In contrast, the metallic lusters of the cross-section in Fig. 4d are distinct.Particles of copper can be noticed lling even the interior of single bers, indicating a considerable boost in the loading of metallic copper particles.This coincides with the experimental assumption of solubilization-enhanced metal particles loading.The physical photograph of the sample is shown in Fig. S6.The Cu elements covered the fabric and the silver seeds, resulting in low elemental content of C, O and Ag (Fig. S7).Moreover, the XRD, TGA, DTG spectra con rmed the loading of Cu NPs (Fig. S8-10).
3.2 Durability performance of Cu/Ag/CA-PEI@Cotton Durability, which is closely related to the loading and binding fastness of the metal particles, is a major constraint to the commercialization of conductive fabrics.The durability of Cu/Ag/CA-PEI@Cotton was evaluated by washing, rubbing, bending and air environmental stability tests.Notably, Cu/Ag/CA-PEI@Cotton exhibits excellent resistance to washing compared to recent literature reporting on conductive materials (Table.S1), with a change in sheet resistance of R s /R 0 = 1.1 for 100 washes under standard washing conditions (Fig. 5a).When Cu/Ag/CA-PEI@Cotton was treated with friction up to 1000 times, the sheet resistance of it gradually decreased and then stabilized at about 1.6×10 − 3 Ω/sq (Fig. S11), which was caused by the dislodging of poorly attached metal particles on the fabric surface during friction.Additionally, Cu/Ag/CA-PEI@Cotton also possesses excellent resistance to bending and environmental stability as shown in Fig. 5b, c.The pore size distribution of CA and PEI modi ed cotton was approximately the same as that of the original cotton fabric due to the uniform penetration of CA and PEI into the pores of the fabric (Fig. 5e).The small pore size distribution of the conductive fabric increased after in-situ growth of metal particles, with the 0.27 µm pore size content increasing from 15-53%, indicating that the metal particles were adequately grown in the fabric voids.The suitable combination of micropores and mesopores allowed the preparation of conductive fabrics with fast ionic mobility, favorable conductive properties and durability properties (Khair, et al. 2019).Nevertheless, the reduced pore size of the fabric resulted in a slight decrease in breathability (Fig. 5d, j).Moreover, the process is universal and can be applied to different substrates, such as lyocell, silk, polyester, nylon, aramid fabrics (Fig. 5f-i).
3.3 Practical application of Cu/Ag/CA-PEI@Cotton Materials with high electrical conductivity often have electrical heating and EMI shielding properties.The maximum heating temperature of conductive fabric is an important criterion to evaluate the performance of the electrothermal fabric.The electrical heating performance of Cu/Ag/CA-PEI@Cotton were tested under different voltages.The maximum heating temperature of Cu/Ag/CA-PEI@Cotton increased to 96°C, 143°C, 176°C and 210°C with voltages of 0.8 V, 1.0 V, 1.2 V and 1.4 V respectively (Fig. 6a), allowing controlled temperatures to be generated in a short period of time by applying low voltages.It can be observed that at room temperature 25 ºC, Cu/Ag/CA-PEI@Cotton can be heated to saturation temperature within 60 s and can maintain a stable temperature until the voltage is switched off.This provides excellent electrical heating performance compared to recently reported electrical heated materials (Table.S1).The surface temperature of Cu/Ag/CA-PEI@Cotton increased continuously with the increase in voltage.And it can be assumed that U 2 -T conforms to a linear correlation with R 2 = 0.97367 (Fig. 6b).
The electric heating performance such as maximum temperature and temperature response rapidity remained unchanged during the continuously applied voltage (Fig. 6c).Additionally, the fabric electrothermal effect was not affected by the bending of the positive and negative sides during the electrothermal process (Fig. 6d).These results indicate conductive fabrics prepared in this study also have great potential for application in electrothermal textiles.
Generally, high electrical conductivity is of great importance for EMI shielding performance.Cu/Ag/CA-PEI@Cotton with excellent electrical properties can be a candidate for EMI shielding materials.The EMI SE (SE T ) was the total sum of microwave re ection (SE R ) and microwave absorption (SE A ) (He, et al. 2023) when SE T exceeded 10 dB, and the average of SE T , calculated SE A and SE R are shown in Fig. 6f.It appears the EMI shielding performance of original cotton of 0 dB while Cu/Ag/CA-PEI@Cotton can reach up to 65.32 dB at a frequency of 8 GHz.Moreover, Fig. 6e indicates the EMI shielding performance of Cu/Ag/CA-PEI@Cotton in the frequency range from 8.2 to 12.4 GHz.Notably, the lowest EMI effectiveness (38.7 dB at frequency of 12.4 GHz) is still higher than that required commercial applications (20 dB) (Li, et al. 2021).This exible high EMI SE material makes it having broad application prospects in highperformance EMI shielding as well as wearable and portable electronics.

Conclusion
In this work, the innovative combination of swelling and multivalent bonding forces of phenol-amine substances allowed metal particles to grow inside and outside the bric, effectively improving the durability of conductive fabrics.The conductive textiles constructed exhibit excellent electrical conductivity (6.09 ± 0.36×10 − 3 Ω/sq) and the electrical conductivity of the fabric remained essentially unchanged after 100 standard washing tests (Rs/R 0 = 1.106).This con rmed the textiles constructed with high durability.In addition, it also possesses excellent Joule heating performance (reaching about 140 ºC in 60 s at 1 V) and remarkably high EMI SE (65.32 dB).Overall, this study provided a novel method for the preparation of multifunctional conductive fabrics, which has broad application prospects in advanced thermal management, energy conversion and EMI shielding elds.

Declarations Figures
Page 13/17 Schematic illustration of the fabrication process of durable conductive textiles.
Comparison of ber morphology and crystallinity before and after swelling.
(a-f) Longitudinal, transverse morphology and cross-sectional microstructure of bers before and after swelling, (g) Effect of swelling treatment on ber diameter changes, (g, i) Changes in crystallinity of bers before and after swelling.

Figure 3 The
Figure 3