Introduction

Fiber materials have driven human civilization forward for thousands of years1,2,3,4,5. Smart yarns, the materials of the future, are capable of enabling communication and interaction between humans and their surroundings, and can adapt autonomously to changes6,7,8. This includes innovations like display fibers9,10 and optoelectronic sensing fibers11,12. Conductive fibers, including carbon nanotube fibers13,14, graphene fibers15,16, and metal-coated fibers17,18, form the foundational elements of smart yarns and have been the subject of extensive research. However, the currently developed conductive fibers often have a high modulus, posing challenges in achieving the necessary elastic deformation for wearable applications19. Developing fiber materials that can reliably transmit electrical signals under various severe mechanical deformations without significant loss of conductivity is crucial for the realization of future smart textiles.

Liquid metal (LM) exhibits excellent fluidity and can adapt its volume freely in response to the substrate’s changes during three-dimensional compressive deformations, thus maintaining conductive pathways20,21. This adaptability holds promise for the production of highly elastic conductive fibers. The fabrication is primarily achieved through methods such as core-shell encapsulation22,23,24,25 and surface coating26,27,28. In core-shell fibers, the cavity contracts during stretching. Owing to the differing rates of volume contraction between the LM and the elastic fiber sheath, instability in the physical structure can occur. For instance, a coaxial PU/LM fiber was prepared using a wet spinning method. Despite the high elasticity of the PU, which allowed for 200% strain, the resistance change (ΔR/R0) was notably high at 4.06529. Surface-coated LM provides more freedom for volume change during deformation, achieving lower resistivity changes (ΔR/R0 approximately 3.08) even at higher strains (240%)30. However, without three-dimensional control, the LM can easily form droplets (a dewetting effect) on the fiber surface during stretching. This not only disrupts the conductive pathways but also risks LM leakage31,32,33. Consequently, managing the free flow of LM on the fiber surface to prevent dewetting and leakage presents a significant challenge.

Herein, we present an adhesion—channel strategy that synergistically integrates silver nanoparticles (Ag NPs) grown on a yarn substrate with the capillary wicking effect of microfibers on the yarn surface, enabling the spontaneous infiltration of LM into the Ag-modified microfiber channels while maintaining strong interfacial adhesion. The design attains robust LM adhesion through multiscale structural interlocking: the Ag NPs are embedded within the microfibers, generating a physical anchoring effect; under capillary action, the LM penetrates to the surface of the Ag-modified microfibers, forming a continuous conductive network and creating a dynamic self-adapting interface during stretching via the metallurgical bonding between Ag NPs and LM (through In-Ag intermetallic phases); the synergistic and macroscopic deformation of the fibers and LM promotes the dynamic rearrangement of the LM. The resulting elastic conductive yarns exhibited high conductivity (0.082 Ω/cm) and stability over a wide range of strains, with a minimal resistance change of 0.703 at 600% strain. Additionally, the yarn demonstrates high conductivity stability under extreme conditions, including large-strain tensile cycling (500%), as well as during twisting, bending, pressing, and washing (Supplementary Table 1). These favorable properties make our yarn an ideal candidate for smart textiles, further evidenced by its successful application in stretchable heaters and electrochromic devices.

Results

Fabrication, characterization, and the enhancing electrical conductivity of the SBS/LM/Ag-SBS (SLMAS) Yarns

The SLMAS yarn was fabricated via a layer-by-layer assembly strategy. Specifically, it can be divided into four steps as illustrated in Fig. 1a. First, poly(styrene-block-butadiene-block-styrene) (SBS) microfibers were wrapped onto commercial spandex (PU) yarn via conjugated electrospinning technology to construct a base yarn (SBS-PU yarn) with three-dimensional microchannel, with a fiber diameter of 1.483 μm (Supplementary Fig. 1, Supplementary Movie 1). The SBS-PU yarn was selected as the core material based on its optimized structural configuration that synergistically combines the advantageous properties of both components. Compared with pure electrospun SBS microfiber yarns (pSBS yarns), this composite design strategically integrates the SBS microfiber shell’s flexibility—which enables interfacial compatibility and maintains a porous architecture conducive to conductive material integration—with the PU core’s superior mechanical durability. Comparative analysis of elastic recovery characteristics among pSBS yarns, commercial PU yarns, and SBS-PU yarns (Supplementary Figs. 2 and 3) further confirms the well-fitting properties of our composite structure. Secondly, Ag was chosen to metalize the microfibers to construct super-adhesive microchannels, which form the first layer to the base yarn due to better wettability between LM and Ag compared to Cu and Ni34,35,36, called Ag-SBS yarn. Thirdly, the Ag-SBS yarns were immersed into the LM bath for LM adsorption to obtain LM/Ag-SBS (LMAS) yarns (forming a second layer) with higher conductivity. Finally, a layer of waterproof electrospun SBS microfibers was wrapped on LMAS yarns for encapsulation to obtain SLMAS yarns (Supplementary Fig. 4).

Fig. 1: SBS/LM/Ag-SBS (SLMAS) yarns.
Fig. 1: SBS/LM/Ag-SBS (SLMAS) yarns.
Full size image

a Schematic illustration of the fabrication of SLMAS yarns. The process involves four steps: construction of channels by electrospun microfibers (SBS-PU yarns), growth of Ag adhesive layer on the surface of the microchannels (Ag-SBS yarns), adsorption of LM (LMAS yarns) and hydrophobic SBS microfibers encapsulation (SLMAS yarns). b–e SEM images of the yarns corresponding to the four steps described above (the upper right corner scale bar: 10 μm). f XRD characterization of SLMAS yarns (red line) and Ag-SBS yarns (blue line). LM alloyed with Ag deposited on SBS microfibers, forming intermetallic compounds. g SEM images of SBS-PU yarns treated by different concentrations of silver precursor solutions (0 wt%, 10 wt%) and the corresponding CA between yarns and LM. h Schematic diagram of the mechanism of LM penetration into the fiber channel and maintaining adhesion. i–l Optical images of SLMAS yarns in their initial state, winding, knotting, and stretching (about 550% strain).

Figure 1b–e illustrates the microstructure changes of the yarn surface during the preparation process. The prepared SBS-PU yarn exhibits a well-defined three-dimensional microchannel structure (Fig. 1b), and after the growth of the Ag layer, this microchannel structure can still be maintained (Fig. 1c), which ensures the complete absorption of LM (Fig. 1d). The uniform formation of hydrophobic SBS microfibers in Fig. 1e implies the final successful preparation of SLMAS yarns. It is worth noting that the prepared conductive yarn has good structural stability and can be made into flat and stable fabrics through classic weaving techniques (Supplementary Fig. 5).

It is worth noting that the fabrication process of SLMAS yarns not only enhances their conductive properties but also ensures the robust adhesion of LM through multiscale interlocking. In the fabrication step 2, SBS-PU yarns were immersed in a silver trifluoroacetate (AgCF3COO) solution in ethanol to absorb the Ag precursor. In this process, an ion-dipole interaction occurred between the hydroxyl groups (–OH) of the ethanol and the trifluoroacetate anions (CF3COO−), which enables the quick and efficient uptake of the Ag precursor into the SBS microfibers. The absorbed Ag precursor was then reduced with a hydrazine hydrate solution, resulting in Ag NPs being embedded and tightly attached to the SBS fibers, thereby imparting conductivity to the yarn17,37. Ag NPs were firmly anchored to the substrate fibers, providing a solid foundation for effective LM adhesion. In addition, the loading of Ag NPs on SBS microfibers could be tuned by controlling the concentration of silver trifluoroacetate (Supplementary Fig. 6). The energy dispersive X-ray spectroscopy (EDS) mapping illustrates that the absorbed Ag is uniformly distributed within the SBS microfiber layer. The amount of Ag absorbed by the yarn increases with the concentration of the silver precursor, as evidenced by the increasing density of Ag elements (Supplementary Fig. 7). Furthermore, the diameter of the Ag NPs formed on the fiber surface increases with the concentration of the silver precursor solution. When the concentration increased from 2 wt% to 10 wt%, the diameter of the Ag NPs grew from 139 nm to 241 nm (Supplementary Fig. 8).

In the fabrication step 3, Ag-SBS yarns were immersed in a LM bath to absorb LM. Typically, the wettability between LM and fibers tends to be poor and there is a gap between them, causing LM to outflow and leading to leakage under mechanical force38. The LM exhibited poor-wetting properties on SBS-PU yarns, with a contact angle (CA) as high as 115.780°. Nevertheless, when the silver trifluoroacetate concentration was raised to 10 wt%, the CA of the LM dropped to 28.504°, which achieved super-wetting of the LM on the yarn substrate and effectively promoted the spreading and adhesion of the LM (Fig. 1g; Supplementary Fig. 6). It is important to note that the variation in LM wettability of the yarn can regulate LM loading. At a silver precursor solution concentration of 10 wt%, the wettability between the LM and fibers was extremely high, allowing the yarn to quickly absorb saturated LM (6.88 mg/cm) upon immersion in the LM bath (Supplementary Fig. 9). Furthermore, compared to conventional SBS coatings, SBS microfibers formed a three-dimensional porous network structure through the electrospinning process. This structure provided space for the three-dimensional distribution of LM, enhanced the wetting effect between the LM and substrate, and reduced the risk of interfacial delamination (Supplementary Fig. 10). An alloying reaction occurs when Ag and LM come into contact, in which In reacts with Ag before Ga and Sn to form Ag9In4 (Eq. 1), and then excess LM reacts with Ag9In4 to eventually produce AgIn2 (Eq. 2)39.

$$9 \; {{\rm{Ag}}}+4 \; {{\rm{In}}}\to {{\rm{A}}}{{{\rm{g}}}}_{9}{{{\rm{In}}}}_{4}$$
(1)
$${{\rm{A}}}{{{\rm{g}}}}_{9}{{{\rm{In}}}}_{4}+14 \; {{\rm{In}}}\to 9 \; {{\rm{Ag}}}{{{\rm{In}}}}_{2}$$
(2)

This is in perfect accordance with our experimental results, as shown in the X-ray diffraction (XRD) spectra, where Ag9In4 and AgIn2 phases were generated by immersing the Ag-SBS yarn in the LM (Fig. 1f). This synergy between the metal bonding mechanism and the capillary wicking effect of microfibers on the yarn surface is the fundamental reason for the excellent adhesion properties of our yarns to LM. When LM contacts the super-wettable Ag-SBS fiber, the capillary force instantly penetrates LM into the fiber pores and spreads it out40, while Ag NPs on the fiber surface anchor LM to the surface through alloying reaction, which eventually forms a stable penetration structure of LM in the fiber pores (Fig. 1h). Compared with the ungrown silver yarns (SLMS yarns), it can be seen from the SEM images (Supplementary Figs. 11a, b and  12) that the LM can spontaneously penetrate into the microfiber pores and tightly adhere to the microfibers in SLMAS yarns. In addition, the cross-sectional SEM maps and corresponding elemental mapping images of SLMAS yarns were investigated to verify the distribution of LM in the yarns (Supplementary Fig. 13). Comparing the elemental images of C with those of Ga, In, and Sn reveals that the distribution of Ga, In, and Sn partially overlaps with the C elements in the core layer of the SLMAS yarns. It indicates that LM is firmly adhered into the Ag-SBS microchannels constructed on the surface of the core yarns. More importantly, the fiber surface maintains complete LM coverage even after 5000 cycles of high-strain cyclic stretching of the SLMAS yarns. This demonstrates the dynamic bonding mechanism between our structure and LM, indicating stable and long-lasting adhesion between the yarns and LM (Supplementary Fig. 11c). Furthermore, SLMAS yarns exhibit a high level of flexibility and stretchability under external mechanical forces (Fig. 1i–l).

Electromechanical properties of SLMAS yarns

Stable conductive properties against mechanical deformation are a primary characteristic of stretchable conductive yarns. To this end, the electrical conductivity and electromechanical properties of SLMAS yarns with different LM loading contents were first investigated. As shown in Fig. 2a, the SLMAS yarns’ resistance is 1.600 Ω/cm in the absence of LM; when the LM loading reaches its peak at 6.88 mg/cm, the yarns’ resistance is only 0.082 Ω/cm. The increase in LM loading content also can significantly improve its electromechanical properties, and compared with the loading content of 1.00 mg/cm, 6.88 mg/cm exhibits a lower resistance change (ΔR/R0) under higher strain (Fig. 2b). It can be attributed to the SBS microfiber encapsulation layer of the yarns forcing more LM into the pores of the highly adherent Ag-SBS microchannel layer to maintain a highly connected 3D conductive pathway during stretching (Supplementary Figs. 14 and 15)41. This also explains why encapsulated yarns experience less resistance change when stretched compared to unencapsulated yarns (Supplementary Fig. 16). This dynamic rearrangement of the LM, driven by the macroscopic deformation of the fibers, provides the yarn with enhanced adhesion and better control over the LM. Besides, SLMAS yarn has a near-constant resistance under extreme strains without going through a pre-stretching process. As shown in Fig. 2c and Supplementary Movie 2, the resistance change (ΔR/R0) of SLMAS yarns is as low as 0.703 even at 600% strain. Our yarns’ resistance change is remarkably lower than the theoretical prediction for a constant-conductivity bulk conductor by Pouillet’s law, which demonstrates sharp increases in resistance with strains (ΔR/R0 = (1 + ε)2 − 1, where ε is the applied strain; Fig. 2c, dashed line)42,43.

Fig. 2: The simultaneous deformation of both the LM and the fiber due to adhesion channels.
Fig. 2: The simultaneous deformation of both the LM and the fiber due to adhesion channels.
Full size image

a Comparison of the initial resistance of Ag-SBS yarns and SLMAS yarns with different LM loading (SLM1.00AS yarns, SLM3.03AS yarns, and SLM6.88AS yarns indicate LM loading of 1.00 mg/cm, 3.03 mg/cm, and 6.88 mg/cm, respectively). b Resistance change of SLMAS yarns with different LM loading during stretching. c Resistance change of SLMAS yarns during stretching (mass loading of LM: 6.88 mg/cm), and theoretical prediction based on bulk conductor assumptions (dashed line). d SEM images of LMAS yarns with different LM loading in the initial state. e SEM images of LMAS yarns with different LM loading at 350% strain. f Schematic illustration of the manipulation for LM flow during stretching through adhesion channels formed by metal bonds.

To better understand its electromechanical performance, morphologies of LMAS yarns in the initial and stretched states under different LM loading content were further studied. As shown in Fig. 2d, e, in the initial state, the LM tightly coated individual tortuous microfibers or bridged several microfibers when the LM loading was 1.00 mg/cm. This is clearly visualized by the high overlap of the C, Ga, In, and Sn elemental images (Supplementary Fig. 17). When the LM loading reaches 3.03 mg/cm, it fills the surface pores. As the LM content further increases, part of the LM penetrates into the pores of the fibers, while the rest remains free in the surface layer of the yarn. This is consistent with the findings in our supplementary longitudinal cross-section SEM images (Fig. 2d I–III, Supplementary Fig. 15a–d). In the stretched state (350% strain), the LM still firmly adhered to the microfibers and stretched simultaneously with the microfibers, keeping the conductive pathways straight and highly connected (Fig. 2e I–III; Supplementary Fig. 18). Consequently, it further confirms the successful preparation of highly adhesion microchannels to provide effective three-dimensional control of LM, which is the key to maintaining strain insensitivity of SLMAS yarns under extreme strains. Figure 2f describes the flow of LM through the yarns during stretching. The strong adhesion of LM to the microchannels through metallic bonding results in an excellent co-deformation between LM and the highly tortuous fiber, which contributes to the superior electrical stability of the yarns.

Due to excellent strain-insensitivity, SLMAS yarns can be utilized as stretchable interconnections. In the following performance tests, the LM loading of the SLMAS yarns, unless otherwise specified, was 6.88 mg/cm. As shown in Fig. 3a, the LED screen displays normally while stretching the yarns to ~550% strain. We also compared the mechanical properties of the yarns obtained at each step, including SBS-PU yarns, Ag-SBS yarns, LM/Ag-SBS yarns, and SLMAS yarns (Fig. 3b, Supplementary Fig. 19). The entirely flexible material system with a highly elastic yarn substrate and LM of excellent fluidity makes the SLMAS yarns highly flexible and stretchable, with a maximum strain of up to 1035%. In addition, cycling tensile tests at different strains showed good elastic recovery of SLMAS yarns (Supplementary Fig. 20). As a result, SLMAS yarns exhibit high-level electrical stability under various mechanical stresses, including twisting, bending, and pressing (Fig. 3c–e). It indicates that there is little effect on the resistance applying different strain rates (20-35 mm/min) for stretching (Supplementary Fig. 21). Notably, a negative resistance change is observed during the stretching cycle. This phenomenon in SLMAS yarns is attributed to a slight decrease in resistance caused by stretch-induced rearrangement of the LM. The compression of the surface SBS encapsulation layer forces the LM to penetrate deeper into the pores of the core fibers, connecting stretch-induced cracks between the Ag NPs on the fiber surface44, leading to a decrease in resistance during the stretching cycle (Supplementary Fig. 15). This stretch-induced directional penetration mechanism of LM allows it to flow within the three-dimensional channel without leakage.

Fig. 3: Electrical and mechanical characterization.
Fig. 3: Electrical and mechanical characterization.
Full size image

a Optical images demonstrating the stretchable operation of an LED screen attached to the SLMAS yarn in the initial state (left) and stretching (right). b Stress–strain curves of SBS-PU yarns, Ag-SBS yarns, LM/Ag-SBS yarns and SLMAS yarns. c–f Resistance change of SLMAS yarns under twisting, bending, pressing, and washing. g Resistance change of the SLMAS yarns during the stretching-releasing cycling test (200% strain and 500% strain). h Quality factor Q of SLMAS yarns compared to previously reported LM-based conductive yarns.

The wrinkled structure formed by the LM oxide layer during cyclic stretching plays a crucial role in maintaining the yarn’s stable electrical properties. This is because the strain during stretching is effectively released through the extension of the LM wrinkles (Supplementary Fig. 22)45. This structural evolution modifies the deformation mechanism of the surface oxide layer, inducing a gradual transition from a stretch-dominated deformation mode to an unfolding-folding dominated mechanical response. Remarkably, the newly generated oxide layer from the altered surface microstructure exerts minimal influence on the electrical properties of bulk LM due to their abundant flowable phases46. Additionally, our yarns exhibit strong dynamic adhesion to LM, which remains fully adhered to the Ag-SBS fiber surface. Even after repeated stretching over long-term cycles, the LM flows with the fibers and does not detach from the surface (Supplementary Fig. 23). Therefore, cycling tests at 200% and 500% strain show SLMAS yarns’ stable conductive properties, proving the reliable capability to operate under repetitive strain (Fig. 3g). It is noteworthy that the yarns maintain essentially unchanged electrical properties after more than 77.5 h of uninterrupted extreme deformation during cyclic stretching at 500% strain, fully demonstrating the excellent long-term stability of our yarns. In comparison, conductive yarns without LM (Ag-SBS yarns) and Ag nanoparticles (LM/SBS yarns) were also fabricated under the same experimental conditions. A substantial increase in the resistance of Ag-SBS yarns and LM/SBS yarns was found during stretching (Supplementary Fig. 24), which could be attributed to the disruption of the penetration path of the rigid conductive filler Ag particles and the LM dewetting phenomenon caused by poor wettability44,47. It is very challenging to keep stretchable conductive yarns steady in humid environments (moisture, perspiration, foreign water) in which the devices are located. Effective hydrophobicity has been shown to contribute to the durability of electronic devices48. As a consequence, the high hydrophobicity of SBS encapsulated microfibers layer ensured the durability of stretchable conductive yarns for underwater work (Supplementary Fig. 25, Supplementary Movie 3). Besides, the resistance of the SLMAS yarns changed only 0.05 Ω after 12 h of water washing (Fig. 3f).

The quality factor Q is widely used to quantitatively analyze electromechanical stability49. The Q value can usually be calculated as:

$$Q=\frac{\varepsilon }{\Delta R/{R}_{0}}$$
(3)

Compared with LM-based conductive yarns prepared by other methods, the SLMAS yarns maintain favorable conductivity stability during stretching, resulting in the high Q value of 8.535 under large deformation, as shown in Fig. 3h23,26,29,30,50,51,52,53,54,55,56.

Joule heating performance of SLMAS yarns

The high conductivity of SLMAS yarns provides a rapid and sensitive response to voltage so that these yarns can be utilized for a low-voltage driven electric heater. The Joule heating performance can be fine-tuned by adjusting the load content of LM or increasing the voltage owing to the high compliance of Ohm’s law and Joule’s law (R2 ≥ 0.990) with the increased LM content and voltage (Fig. 4a–c). For example, when the LM loading was increased from 1.00 mg/cm to 6.88 mg/cm, the yarns’ temperature rose from 86.4 °C to 122.7 °C at a voltage of 1.0 V. At the same time, the temperature of the SLMAS yarns increased gradually with the increase of the applied voltage (Supplementary Figs. 26 and 27). Therefore, the temperature of the SLMAS yarns can be high-precision regulated by adjusting the input voltage, and SLMAS yarns with different LM loading can be selected according to actual temperature needs.

Fig. 4: Joule heating properties.
Fig. 4: Joule heating properties.
Full size image

a I–V linear curve of SLMAS yarns with different LM loading. b Surface temperature changes of SLMAS yarns with different LM loading and varied input voltages (0.2–1.0 V, yarn length: 15 cm). c T-U2 curve of SLMAS yarns with different LM loading. d Temperature variations of SLMAS yarns during stretching. e Temperature variations when switching voltage between 0 and 0.4 V (100 cycles). f The enlarged image of the dashed area in (e) shows the temperature changes in one on-off cycle.

More importantly, due to the high conductive stability of the SLMAS yarns, the Joule heating effect is not greatly affected during stretching. When the SLMAS yarn was stretched to 100% strain, the surface temperature changes by only about 5.445% (Fig. 4d). In addition, the SLMAS yarns exhibit excellent reliability and durability as Joule heating devices. Cycling heating/cooling tests at 0.4 V (Fig. 4e) show highly reproducible temperature changes, proving the long-range stability of the SLMAS heaters. The surface temperature of yarns rapidly rises to around 50 °C within 30 s and drops to 32 °C within 30 s after the voltage is switched off, demonstrating a fast and reliable electro-thermal conversion performance (Fig. 4f). As a result, SLMAS yarns are convincing candidates for low-voltage-driven stretchable heaters.

Electro-thermochromic yarns

SLMAS yarns can easily achieve extension for functional applications. As a proof of concept, adding thermochromic microcapsule to modify the outermost layer of electrospun fibers (thermochromic microcapsule was uniformly doped between the fibers, Supplementary Fig. 28), yarns with electro-thermochromic properties can be achieved (Fig. 5a). Four different thermochromic microcapsules, including light green to deep green, light purple to deep purple, light pink to deep pink and purple to white were selected to demonstrate (Supplementary Fig. 29). As we can see, when the yarns were knitted into fabrics with the letters “LMY”, the color of yarns changed from light color to deep color apparently (Fig. 5b). In this process, the yarns with light purple, light green and light pink were applied with voltages of 0.4 V, 0.5 V, and 0.6 V respectively. And the yarn’s discoloration performance also can be maintained stable under winding, changing from purple to white (Supplementary Fig. 30). The reproducibility and durability of electro-thermochromic yarns were evaluated by turning cyclic on/off voltage (0.4 V). Within 100 cycles, the peak and lowest color were maintained with negligible changes for each cycle (Fig. 5c), ensuring the reliability of electro-thermochromic yarns for long-term applications. Besides, the highly conductivity stability makes these yarns steadily discoloration during stretching. For example, as shown in Fig. 5d, the yarn’s color changed from purple to white after turning on the voltage and remained white without significant change when stretched at 650% strain. These performances indicate electro-thermochromic yarns have great potential for the advanced design of wearable smart display57.

Fig. 5: Characterization of electro-thermochromic properties.
Fig. 5: Characterization of electro-thermochromic properties.
Full size image

a Schematic illustration of the fabrication of electro-thermochromic yarns. b Optical images of the yarns sewn onto a woven fabric by switching the voltage on and off (scale bar: 10 mm). c Hue value during switching voltage on and off (100 cycles). d The electro-thermochromic yarn’s discoloration during stretching (I) turn off the voltage; (II) turn on the voltage; (III–V) keep the voltage on for stretching.

Discussion

Effectively integrating stretchable conductive fillers with highly elastic yarns is crucial for developing materials that can reliably transmit electrical signals under a wide range of severe mechanical deformations without significant loss of conductivity, thereby enabling the smart textiles of the future. Here, we present an adhesion-channeling strategy to achieve stretchable conductive yarns with intrinsically proven conductivity and conductivity stability. This electrical and electromechanical performance is attributed to the three-dimensional control of LM, enabled by the strong adhesion from the metallic bonding between LM and Ag. By enhancing and controlling adhesion, the LM co-deforms effectively with the microfibers. As a result, SLMAS yarns exhibit minimal resistance change under large strains, twisting, bending, pressing, and washing. Furthermore, we demonstrate the applications of SLMAS yarns in stretchable interconnects, Joule heating, and electrothermochromism.

Methods

Preparation of SLMAS yarns

Initially, SBS (13 wt%, Huaxing Co. Ltd.) was dissolved in a tetrahydrofuran (AR, ≥99.5%, Shanghai Titan Technology Co.) and N, N Dimethylformamide (AR, 99.5%, Shanghai Aladdin Biochemical Technology Co.) mixture with a mass ratio of 3:1 and stirred at room temperature for 12 h to form a transparent solution. This solution was then injected into two syringes, and SBS fibers were spun onto commercial spandex yarn (Kangpu Co. Ltd.) through conjugate electrospinning to obtain SBS-PU yarns. During the electrospinning process, the temperature was maintained at 22 °C with a humidity level of around 45%. The SBS-PU yarns were subsequently immersed in an AgCF3COO (RG, 99.9%, Shanghai Titan Technology Co.) solution in ethanol for 10 min and then dried in a vacuum oven at 30 °C for 10 min. Next, the samples were immersed in an ethanol solution of N2H4·H2O (AR, 80%, Shanghai Titan Technology Co., 5 vol%) for 30 min, rinsed with ethanol for 10 min, and dried in a vacuum oven at 60 °C for 30 min. The concentration of AgCF3COO in ethanol (0 wt%, 2 wt%, 5 wt%, and 10 wt%) was varied to regulate the loading of Ag particles on the yarn surface. The obtained Ag-SBS yarns were then dipped into an LM bath (Ga68.5In21.5Sn10, ChuanMao Metal Co. Ltd.) in a glovebox to achieve loadings of 1.00 mg/cm, 3.03 mg/cm, and 6.88 mg/cm, respectively. Finally, SBS electrospun microfibers were deposited onto the LMAS yarns for encapsulation under the same electrospinning conditions to produce the SLMAS yarns.

Characterization of SLMAS yarns

Field Emission Scanning Electron Microscopy (SU8010) was used to study the morphology of the yarns. The yarns were frozen in liquid nitrogen for 20 s and then cut with a sharp blade to obtain cross-sectional samples. The diameter of the yarns was measured by a benchtop scanning electron microscope. The elemental distribution was determined by field emission scanning electron microscopy (SU8010) equipped with an energy-dispersive X-ray spectroscopy (EDS) module. The crystal structure was characterized by XRD (Bruker D8 X-ray diffractometer). Contact angles were all tested by a contact angle meter (KINO). The mechanical properties of yarns were measured using a CTM 2050 universal material testing machine (Xieqiang). The length of the test sample was set to 2 cm and the test was conducted at a speed of 20 mm/min. The electrical properties of yarns were measured by Keithley 2450 and Uni-t UT61E. The electromechanical properties of the yarns, including bending, twisting, and stretching, were investigated using a multimodal testing system (Shengte). In this case, during the bending cycling test, the initial clamping length of the sample was 40 mm, the distance between the collets after bending was 20 mm, and the resistance value during the bending process was recorded. In washing testing, the resistance values were measured by immersing the yarns in deionized water with magnetic stirring at 1000 rpm/min for 30 min. During the testing process, exposed conductive yarns ends are connected to the measuring unit via copper leads, conductive adhesive, and copper tape to ensure that the yarns maintain its connection during all types of deformations while avoiding LM leakage.

Preparation of electro-thermochromic yarns

The preparation of spinning solution involved 3 wt% thermochromic microcapsules (light pink to deep pink, light green to deep green, light purple to deep purple, purple to white; color change temperature: 60 °C, 40 °C, 35 °C, 35 °C; capsule size, 3–9 μm) and 9 wt% SBS were added into a mixture of THF and DMF with a mass fraction of 3:1 and stirred at room temperature for 12 h. It was then injected into two syringes, and the electro-thermochromic yarn was obtained by spinning the thermochromic fibers onto the LMAS yarn by conjugated electrospinning.

Characterization of electric heaters and electrothermochromic yarns

A DC power supply (Keysight E3634A) was connected to both ends of the SLMAS yarn and regulated the voltage. The temperature change at different voltages was monitored using a Fotric IR camera. The Joule heating properties of SLMAS yarns with different LM loadings were investigated. The first law of Joule is:

$${{\rm{Q}}}=\frac{{U}^{2}}{R}\times t$$

where U, R, and t represent voltage, resistance, and heating time, respectively. The color change of the electrothermochromic yarns was measured by Datacolor 850 (Datacolor Trading Co., Ltd, USA).