In this paper, we present a method for fabricating one-dimensional flexible conductive materials using cotton fibers as the substrate. A one-dimensional conductor with outstanding mechanical strength was obtained by combining the unique characteristics of cotton fibers with the properties of silver nanoparticles (Ag-NWs) through the integration of the traditional spinning techniques with aerogel preparation methods. The details of this procedure are illustrated in Fig. 1a. Firstly, based on previous research, to achieve a thorough microscale mixing of Ag-NWs and cotton fibers, we prepared a well-dispersed cotton fiber suspension using the Mechanical Crushing Method. In the suspension, the length distribution of cotton fibers is primarily concentrated in the range of 0 to 400 µm, and the diameter distribution is mainly concentrated in the range of 0 to 2 µm.(Li et al. 2022) Subsequently, the prepared cotton fiber suspension was mixed with the silver nanowire (Ag-NWs) suspension and stirred to ensure effective coating of Ag-NWs onto the cotton fibers.
Next, the functionalized cotton fiber suspension was poured into a designed container for freeze molding. Freeze-drying was then employed to maintain the spatial structure of the functionalized cotton fibers and prevent the generation of large amount of hydrogen bonds between cotton fibers during the dehydration process, which could affect the material's flexibility at low temperatures.
After freeze-drying, the aerogel block was cut into strips. These strip-shaped aerogels were then molded with silk press rotating molds. After molding, the uniformly long lines with elliptical cross-sections were obtained, as shown in Fig. 1b. The cross-sectional structure of these long lines show they still retained the porous layered structure of the aerogel, with cotton fibers primarily overlapping and stacking with each other. At this stage, the long lines exhibited a combination of excellent flexibility but lower tensile strength at low temperatures, because of the relatively week interaction among cotton fibers. To enhance the mechanical strength of the one-dimensional conductor, the obtained cotton fiber aerogel long lines were twisted to intertwine the cotton fibers internally, by emulating the spinning process of natural fibers, and thus the mechanical strength of the one-dimensional conductor was improved. The cross-section after twisting is showed in Fig. 1c. Ultimately, we obtained a one-dimensional cotton fiber-based flexible conductive material that combines good mechanical strength with low-temperature flexibility, as shown in Fig. 1d. Comparing the cross-sectional structure before and after twisting, the layered structure disappeared after twisting, replaced by intertwining cotton fibers which led to the improvement of mechanical strength. Additionally, after twisting, difference in the intertwining state between the central core and the outer shell cotton fibers can be observed. For the outer cotton fibers, they exhibit a layered arrangement, held together in a rolled-up manner layer by layer. In contrast, the central cotton fibers are intricately interwoven, forming a denser "core". The emergence of this "core" structure is a key factor contributing to the increased mechanical strength of the one-dimensional conductor.
In one-dimensional cotton fiber-based flexible conductive materials, the primary influencing factor on electrical performance is the content of silver nanowires (Ag-NWs). Ag-NWs are prepared with the ethylene glycol reduction method of silver nitrate, with a size distribution ranging from diameters of 10 to 20 nm and lengths primarily concentrated between 10 and 20 µm. Ag-NWs coat on the cotton fibers, and forming an interconnected conductive network as the cotton fibers overlap with each other, as shown in Figs. 2a and 2b. The changes in electrical performance of the aerogel long lines during the preparation process with increasing Ag-NWs content were measured and shown in Fig. 2c. With the relative mass ratio of Ag-NWs to cotton fibers increases, the line resistance continuously decreases. In other words, the overall electrical conductivity increases with an increase in the relative mass of Ag-NWs. However, at a mass ratio of 1:3, a decrease in calculated conductivity occurs due to the increase of the conductor's cross-sectional dimension. When the mass ratio of cotton fibers to Ag-NWs is 1:2, the electrical conductivity reaches 231,500 S/m. Comparing the electrical conductivity with other conductive fillers (PPy(Huang et al. 2016), PEDOT:PSS(Alhashmi Alamer 2017, Shakeri Siavashani et al. 2021), graphene (GA)(Liu et al. 2022, Hernandez et al. 2008), carbon nanotubes (CNT)(Ilanchezhiyan et al. 2015, Zhao et al. 2018, Alhashmi Alamer et al. 2020), metal nanomaterials (Ag(Ahmed et al. 2020, Zhao et al. 2019, Wang et al. 2022), Cu(Hassan, Kalaoglu and Atalay 2020))) in cotton fiber-based flexible conductive materials, as shown in Fig. 2d, the one-dimensional conductor in this paper exhibits excellent electrical performance. In terms of electrical conductivity, it outperforms most conductors using polymer materials, graphene, and carbon nanotubes as conductive fillers. Furthermore, when compared to flexible conductors that utilize metal nanomaterials as conductive fillers, its electrical conductivity is significantly higher than those prepared through top-down methods, showing the advantage of the bottom-up approach in electrical performance. However, with the increase of Ag-NWs, when the mass ratio of cotton fibers to Ag-NWs reaches 1:4, the relative volume of Ag-NWs inside the aerogel long wires increases. This alteration affects the movement and rotation of cotton fibers during the intertwining process. As a result, when the mass ratio is 1:4, it is hard to further twist the aerogel long wire, and the strength of the obtained one-dimensional conductor is too low to meet the requirements. Therefore, the two options with the best mass ratio of cotton fiber to Ag-NWs are 1:2 and 1:3.
Twisting was applied to the aerogel long wires with mass ratios of cotton fibers to Ag-NWs at 1:2 and 1:3 to investigate the impact of Ag-NWs' relative mass and twist on the mechanical properties of the resulting one-dimensional conductor. The aerogel long wires with mass ratios of 1:2 and 1:3 was twisted to 7 r/cm, 8 r/cm, 9 r/cm, 10 r/cm, 11 r/cm, respectively. During the twisting process, the long wires with Ag-NWs mass ratio of 1:3 could not achieve the twist of 11 r/cm, because the lines broke over 10 r/cm of twisting due to the large internal stress.
Stretch tests were conducted on the twisted one-dimensional conductor, and the typical stress-strain curve during the stretching process is shown in Fig. 3a. Figure 3b illustrates the variation in tensile strength of the conductor with twisting. At a mass ratio of 1:2, with increasing twist, the tensile strength initially increased and then exhibited a decreasing trend. The reason for this trend is as follows: First, before the twist reached 9 r/cm, the tensile strength was mainly provided by the friction between cotton fibers. As the twist increased, the intertwining of cotton fibers became tighter, leading to an increase in friction between them and, consequently, results an increase in tensile strength. Continuing to twist, the maximum friction exceeded the strength of the cotton fibers, and the tensile strength reached its maximum at a twist of 9 r/cm. Further twisting resulted in increase of internal stress and decrease in modulus, as shown in Fig. 3c, and led to a significant reduction in tensile strength at a twist of 11 r/cm compared to 9 r/cm and 10 r/cm. Continuing to twist beyond this point would cause the break of the conductor. At a mass ratio of 1:3, the tensile strength showed no significant increase with increasing twist, neither the modulus. This is because the high relative volume of Ag-NWs and their stiffness hindered the twisting and bending of functionalized cotton fibers. As a result, they couldn't achieve high twisting level, leading to lower tensile strength after twisting. As shown in Fig. 3d, the fracture elongations were nearly the same for different twisting levels and mass ratios. This is because during the stretching process, the elongation of this one-dimensional conductor is mainly due to the straightening of the twisted cotton fibers, which result in similar elongation rates under different mass ratios and twisting levels.
Then the low temperature mechanical properties of the 1D CFC were tested. The CFC with a mass ratio of 1:2 and a twist level of 10 r/cm was selected due to its good mechanical property at room temperature. Tensile tests were conducted on this conductor in liquid nitrogen environment (77K). The tensile strength, modulus, and fracture elongation at 77K are shown in Figs. 3e and 3f. Comparing its mechanical properties at room temperature, the tensile strength remained relatively unchanged at 77K, but the modulus significantly increased. This indicates a substantial increase in the modulus of cotton fibers at low temperature. Besides, the elongation rate decreased significantly at low temperatures compared to room temperature. This is due to the reduced flexibility of cotton fibers at low temperatures. Examining the cross-sectional structures of the fractures at room temperature and low temperature, as shown in Figs. 3h and 3i, the fracture surface at low temperatures appears flatter. This confirms that the conductor's fracture during the tensile process is caused by the fracture of cotton fibers at low temperature. The structure of the cotton fiber fracture surface is shown in Fig. S1. As described in Fig. 3g, during the stretching process of this one-dimensional conductor material, the internal state of the cotton fibers undergoes the following changes: At room temperature, as stretching progresses, cotton fibers go through a process of first being straightened and then being pulled to the point of breaking. And at low temperatures, due to the increased stiffness of the cotton fibers, as tension applied, the cotton fibers are directly broken in a bent state because their degree of free bending is reduced. Comparing the mechanical performance of the material at a mass ratio of 1:3 with that at a mass ratio of 1:2, the latter exhibits superior mechanical properties, and it maintains good mechanical strength during low-temperature testing. Therefore, a mass ratio of 1:2 for cotton fibers to Ag-NWs is selected as the optimal ratio for preparing one-dimensional conductors and for further research.
By observing the conductor cross-sections with a mass ratio of 1:2 and twist of 7 r/cm, 9 r/cm, and 11 r/cm using SEM microscopy, as shown in Figs. 4a, 4b, and 4c, it is evident that the conductor's radius gradually decreases with increasing twist. When comparing the 7 r/cm and 9 r/cm twists, as shown in Figs. 4d and 4e, at a twist of 7 r/cm, the conductor's cross-section exhibits a layered "concentric-coil" structure with low intertwining between cotton fibers, resulting in low strength. As shown in Fig. 4e, with an increase of twist to 9 r/cm, the wire radius decreases compared to 7 r/cm, and the layered structure almost disappears, with the central layer of cotton fibers intertwining predominantly. When comparing the arrangement of cotton fibers at different depths on the conductor surface, the outer layer of cotton fibers were removed, and the arrangements of outer and central layer cotton fibers are compared under SEM microscopy. In the outer layer, the arrangement of cotton fibers forms a larger angle with the axial direction, while in the central layer, the cotton fibers align almost parallel to the axial direction. This further indicates that the outer layer's layered structure rolls together, and the increase of the central layer's structure is the significant reason for the noticeable increase in strength at 9 r/cm compared to 7 r/cm. When further twisting is carried out to reach 11 r/cm, as shown in Figs. 4c and 4f, the wire radius further decreases, and lateral cotton fiber alignment begins to appear in the central layer structure. This suggests that excessive twisting leads to the generation of internal stress within the material, causing the cotton fiber bundles to curl and resulting in a significant reduction in material strength. The trend of decreasing conductor diameter with increasing twist level is illustrated in Fig. 4i.
Conductors with twist of 9 r/cm, 10 r/cm, and 11 r/cm, which exhibited good mechanical performance, were chosen for the subsequent low-temperature flexibility tests. In order to accurately evaluate its low-temperature flexibility, we have developed an innovative low-temperature flexibility testing system, as illustrated in Figs. 5a and S2. Video 1 demonstrates the deformation of the conductor during the testing process. As shown in the video, during the testing process, translational motion of the testing rod is employed to induce relative translational displacement in the conductor wound around the testing rod. This, in turn, leads to bending occurring sequentially at different positions along the conductor, allowing us to assess its low-temperature flexibility. Compared to the traditional flexural tests where bending is induced by the rotation of the material itself, in the video, the conductor remains stationary during the flexibility testing process. This prevents issues like oscillations and false connections caused by material movement, ensuring more stable test results that better reflect the changes in electrical resistance during conductor flexibility testing. The test results, illustrated in Fig. 5, show that after subjecting samples with twist of 9 r/cm, 10 r/cm, and 11 r/cm to 1000 cycles of testing in liquid nitrogen at 77K, none of the conductors experienced any fractures, as illustrated in Fig. 5b, which confirms the excellent low-temperature flexibility of this type of conductors. Additionally, when examining the resistance change rate for each cycle, as shown in Fig. 5c, it is evident that conductors with a twist of 10 r/cm exhibit the most stable resistance variation during a single flexing cycle. Comparing the resistance change rates after 1000 cycles, the conductor with a twist of 10 r/cm exhibited a minimal relative resistance increase of only 0.27%. In contrast, the conductors with twists of 9 r/cm and 11 r/cm experienced relative resistance increases of 1.47% and 8.42%, respectively. This demonstrates the superior electrical stability of the 10 r/cm conductor during low-temperature flexibility tests compared to the 9 r/cm and 11 r/cm conductors. Consequently, we chose 10 r/cm as the final twist for the preparation of the 1D CFC.
Subsequently, extreme low-temperature flexibility tests were performed on the prepared 1D CFC with a mass ratio of 1:2 and a twist of 10 r/cm. This involved subjecting it to ten thousand cycles of testing in liquid nitrogen at 77K while observing changes in its electrical performance and material structure. As illustrated in Fig. 5d, over the course of ten thousand tests, the 1D CFC displayed overall stability in resistance with minor fluctuations, indicating excellent material stability. Examining the structural changes, as shown in Fig. 5e and 5f, only minimal surface frayed can be observed, the overall material structure remained intact with no cracks or defects emerging after testing.
To explore the feasibility of using this 1D CFC in the field of flexible electrical system, it was sewn onto fabric, and relevant tests for wear resistance and washability were conducted. First, in the wear resistance test, as shown in Fig. S3, repetitive friction was used to simulate the wear that occurs during physical activity. The test results, as shown in Fig. 6c, indicate that this 1D CFC maintained a highly stable resistance throughout 18,000 cycles of wear testing. The structures before and after testing are shown in Figs. 6a and 6b, respectively. The conductor exhibited no damage or breakage in its structure before and after testing, demonstrating excellent wear resistance for this material. Subsequently, in the washability test, as shown in Fig. S4, magnetic stirring was employed to simulate the washing conditions encountered by clothing in a washing machine. Comparing the structure of the conductor before and after washing, as shown in Fig. S5, the material did not exhibit any looseness, damage, or breakage during the washing process. The resistance variation during washing, as shown in Fig. 6d, remained stable, indicating excellent washability for the 1D CFC.
Subsequently, to further assess the potential application of this one-dimensional conductor within an integrated electronic system, the conductor was fixed to an automated robotic arm and integrated into an actual circuit, as shown in Figs. S6 and S7. By utilizing the bending and straightening motion of the robotic arm in liquid nitrogen (77K), deformation process of the one-dimensional conductor was simulated, such as robot pivots and spacesuit joints, where high low-temperature flexibility is required. The circuit diagrams are illustrated in Figs. 6e and 6h. During testing, as the robotic arm bending and straightening, the 1D CFC exhibited corresponding bending and straightening deformations, as shown in Figs. 6f and 6i.
The testing process is demonstrated in Video 2, where the LED light strip remained consistently bright throughout the test, indicating that the material's resistance remained stable during bending and straightening. The LED strip and rotating display screen responded rapidly to input commands, demonstrating quick switching and content display changes, without any noticeable delays in current passing through the conductor. Real-time monitoring of the conductor's resistance variations, as shown in Fig. 6j, revealed that while the resistance change rate at low temperatures was slightly higher than at room temperature, overall resistance fluctuations were minimal, demonstrating excellent flexibility, electrical performance, and stability for this 1D CFC in low-temperature environments (77K).