Sustainable, Robust, and Electrothermal Metal Yarns

with Fully Recycled Polystyrene/Polyethylene Terephthalate for Smart Textiles

Authors: Na Kyoung Kim, Seok Hee Han, Suhyeon Kim, Geon Hwee Kim

Abstract

In this study, we report a sustainable fabrication strategy for electrothermally active metal yarns using fully recycled polystyrene (re-PS) and polyethylene terephthalate (re-PET) as core materials. Hybrid nanofiber yarns were produced via dual-nozzle electrospinning, combining the processability of re-PS and the mechanical reinforcement of re-PET. Electroless copper plating was subsequently performed under ambient conditions following surfactant-assisted activation and palladium seeding, resulting in uniform and continuous metallic coatings. The Cu-plated hybrid yarns exhibited high electrical conductivity with a resistance of 2.84 Ω and showed efficient Joule heating, reaching 153.3 °C at a low applied voltage of 1.2 V. Stable temperature output (∼96 °C) was maintained during continuous 1 h operation, and rapid heating–cooling response was retained over 300 cycles under both flat and bent configurations, confirming mechanical and thermal durability. This approach presents a scalable method for converting plastic waste into high-performance functional textiles. The fabricated metal yarns are lightweight, flexible, and conductive, showing strong potential for integration into wearable heaters and next-generation smart textile systems.

Introduction

The evolution of textile technology has closely paralleled the advancement of human civilization. The transition from natural fibers to synthetic fibers enabled the mass production of uniform, low-cost fabrics and accelerated the development of functional textiles capable of providing thermal insulation, water resistance, and other advanced properties. , Recently, textiles have progressed beyond traditional protective roles to become smart textiles that actively respond to environmental stimuli or physiological signals. , Among these emerging technologies, thermally functional textiles have attracted growing interest for applications in medical garments, military uniforms, and wearable electronic devices. −

Conventional textile-based thermal protection primarily relies on passive approaches, such as modifying fabric thickness or structure to control heat transfer. In contrast, smart heating textiles based on conductive yarns provide active, electrically driven heat generation, enabling precise and energy-efficient thermal management.

A widely used strategy for introducing electrical conductivity into textiles involves incorporating metal-based conductive components. Representative approaches include blending conductive polymers, incorporating metal nanowires or carbon-based fillers, and depositing metal layers onto polymer fibers. −

Among these methods, metal deposition on nonconductive fibers is particularly attractive because it maintains the mechanical flexibility of the original textile while providing stable and high electrical conductivity. In particular, electroless plating has emerged as a key technique in smart textile research. Unlike electroplating, which requires conductive substrates and external power supplies, electroless plating allows metal deposition on insulating polymer fibers under ambient conditions through relatively simple chemical reactions. In addition, the solution-based nature of electroless plating enables simple, scalable, and cost-efficient mass production, allowing uniform metal coatings to be applied continuously to large areas of textile substrates.

However, forming uniform and robust electroless metal layers on conventional woven textiles or fiber mats remains challenging. Electroless plating requires sufficient nucleation sites on the substrate surface and rapid infiltration of the plating solution into the fiber network. Traditional textiles often exhibit irregular pore sizes, multiscale hierarchical structures, and complex three-dimensional networks, which hinder uniform metal deposition and limit mechanical durability. To address these challenges, electrospinning has emerged as a promising manufacturing method for producing highly uniform, high-surface-area nanofibers that are well suited for electroless metallization. ,

Electrospinning generates ultrafine polymer fiberstypically tens to hundreds of nanometers in diameterby ejecting a polymer solution through a nozzle under a strong electric field. The resulting nanofibers have been widely used in applications such as tissue engineering, , filtration, , and high-performance sensors. , Electrospun nanofiber mats possess high porosity, narrow fiber diameter distributions, and interconnected structures that facilitate rapid penetration of plating solutions and enhance the uniformity of metal deposition. Furthermore, catalysts for metal nucleation can be incorporated directly into the electrospinning solution, enabling the formation of optimized surfaces for subsequent electroless plating. Owing to these advantages, electrospinning is considered one of the most effective approaches for preparing conductive smart textiles via electroless metallization.

For broader textile applications, however, the fabrication of aligned nanofiber yarns is essential. Conventional electrospinning typically produces random nonwoven mats, which are difficult to weave into desired textile patterns. To overcome this limitation, various alignment-controlled electrospinning techniques have been developed, including rotating cylindrical collectors, parallel electrode systems, and cone-shaped rotating collectors. −

By appropriately combining a rotating conical collector with a winding drum, nanofibers can be continuously twisted and drawn into yarn structures that resemble traditional textile yarns. , Electrospun yarns exhibit enhanced mechanical strength and are more suitable for subsequent functionalization, including metal plating. Their structural characteristicscontinuous fiber orientation, tunable porosity, and controllable mechanical robustnessmake them excellent candidates for high-performance conductive yarn development. Numerous studies have demonstrated the successful fabrication of electrospun yarns from a wide range of polymer systems. −

In addition to performance advantages, electrospinning-based metal yarn fabrication also offers an attractive route for plastic-waste upcycling. Growing environmental concerns have intensified interest in converting waste plastics into value-added functional materials. , Because electrospinning requires polymer dissolution rather than melting, plastic waste of various shapes and forms can readily be dissolved in appropriate solvents and repurposed as electrospinning feedstock. In this context, polystyrene (PS) and polyethylene terephthalate (PET) are particularly promising candidates, as they constitute major fractions of global plastic waste from disposable packaging and beverage containers. Recent studies have demonstrated the fabrication of electrospun fibers from waste PS and PET dissolved in solvents such as tetrahydrofuran (THF) and dimethylformamide (DMF). , However, most previous works have been limited to producing planar mats or simple stacked structures, which remain closer to conventional recycling rather than true upcycling. In contrast, converting waste polymers into functional nanofiber yarns and further integrating them with electroless metallization provides both material circularity and added functional value.

In this study, we present a sustainable electrothermal textile by fabricating hybrid electrospun nanofiber yarns composed of recycled polystyrene (re-PS) and recycled polyethylene terephthalate (re-PET), followed by electroless copper (Cu) plating. First, we optimized the yarn-forming electrospinning conditions using re-PS, which exhibits favorable spinnability, and then explored four candidate polymer combinations to enhance the mechanical properties of the resulting yarns. A dual-nozzle electrospinning system was employed to produce composite yarns composed of two distinct recycled polymers. Based on material characteristics and yarn performance, the re-PS/re-PET hybrid was identified as the optimal combination, providing superior mechanical stability. Prior to electroless plating, the electrospun yarns underwent surfactant-assisted surface activation and palladium (Pd) seeding to ensure uniform copper deposition.

The Cu-plated hybrid yarns were systematically evaluated for their morphology, electrical conductivity, mechanical strength, and Joule heating performance. The resulting structures exhibited excellent electrothermal responsiveness, reaching 153.3 °C under a low applied voltage of 1.2 V, along with outstanding thermal and mechanical durability. These findings highlight the strong potential of the proposed hybrid yarns as sustainable, high-performance materials for next-generation wearable heating textiles.

Experimental Section

Materials

N,N-Dimethylformamide (DMF; special grade, 99.5%, 68-12-2), tetrahydrofuran (THF; stabilized, 99.5%, 109-99-10), and dichloromethane (DCM; 99.0%, 75-09-02), buffer solution pH 4.0 (877-24-7), and formaldehyde solution (35%, 50-00-0) were purchased from Samchun Chemicals (Korea). Sodium dodecyl benzenesulfonate (SDBS; technical grade, 25155-30-0), bovine serum albumin (BSA; 9048-46-8), potassium sodium tartrate tetrahydrate (ACS reagent, 81-59-5), sodium hydroxide (NaOH; 1310-73-2), and palladium­(II) chloride (PdCl2; 7647-10-01) were purchased from Sigma-Aldrich (USA). Copper­(II) sulfate pentahydrate (CuSO4·5H2O; 7758-99-8) was purchased from Junsei Chemical (Japan). All reagents were used as received, without any purification. Deionized (DI) water was prepared in the laboratory and used for all aqueous solutions. Polystyrene (PS), polyurethane (PU), polyvinyl chloride (PVC), polycarbonate (PC), and polyethylene terephthalate (PET) were all obtained from waste polymeric products. The waste products were washed with DI water and ethanol, cut into small flakes, and stirred with organic solvents to prepare electrospinning solutions.

Fabrication Process

Preparation of Electrospinning Solutions

Polystyrene (PS), polyurethane (PU), and polyvinyl chloride (PVC) were each dissolved in a DMF/THF mixture (1:1 mass ratio) at 20 wt %. Polycarbonate (PC) was dissolved in dichloromethane (DCM) at 10 wt %, and polyethylene terephthalate (PET) was dissolved in a THF/DCM mixture (7:3 mass ratio) at 20 wt %. All polymer solutions were stirred thoroughly at room temperature until fully dissolved before use.

Electrospinning Machine Setup and Fabrication

of Nanofiber Yarns

Nanofiber yarns were fabricated using a lab-made modular electrospinning machine consisting of a stainless-steel rotating collector (funnel shape, 45 mm diameter, 25 mm height) and a cylindrical winding drum (Figure A,B). The rotating collector was positioned 7 cm below the syringe needle at a 60° angle to the ground, and slip rings were used to maintain electrical contact during rotation. The fiber collector and winding drum were controlled through Arduino UNO boards, and the housing components were all fabricated by fused deposition modeling (FDM)-type additive manufacturing machine. The flow rate of polymer solution was 0.05 mL/min with industrial luer-lock plastic needles (25 G, inner 0.26 mm) and the applied voltage ranged from 9 kV to 17 kV. The rotation speed of collector varied from 250 to 750 rpm, and the winding drum was placed away from collector and rotated at 1 rpm, achieving a yarn production rate of approximately 500 mm/min. For bicomponent yarn fabrication, dual nozzles were used at a fixed spacing (0.5 cm), with flow rates of 0.04 mL/min per nozzle and other electrospinning conditions were same as described above.

Copper Electroless Plating of Electrospun

Yarn

The electrospun nanofiber yarns were first immersed in a 5 wt % sodium dodecylbenzenesulfonate (SDBS) aqueous solution for 20 min to remove surface impurities and introduce uniform negative charges on the fiber surface. As an anionic surfactant, SDBS enhances surface activation and facilitates subsequent adsorption of positively charged metal ions. After rinsing with deionized (DI) water, the yarns were immersed in a 0.25 wt % bovine serum albumin (BSA) solution for 10 min. BSA acts as a surface-modifying agent that converts the inherently hydrophobic polymer surface into a more hydrophilic and reactive interface by introducing various functional groups (–NH2, –COOH, –OH), thereby improving the adhesion of palladium seed particles. Subsequently, the yarns were soaked in a palladium (Pd) seeding solution composed of 50 vol % buffer solution (pH 4.0) and 0.05 wt % palladium­(II) chloride (PdCl2) in DI water for 10 min to deposit catalytic Pd nuclei. The Pd^2+^ ions are reduced in situ on the fiber surface, forming catalytic Pd^0^ nuclei that initiate the copper plating process. The chemical equation is as followsPd2++HCHO+H2O→Pd0+HCOOH+2H+

Finally, the yarns were immersed in an electroless copper plating bath prepared by dissolving 11.57 wt % potassium sodium tartrate tetrahydrate, 3.3 wt % sodium hydroxide, and 2.5 wt % copper­(II) sulfate pentahydrate in DI water. The plating process was conducted at room temperature (25 °C) with a consistent yarn length-to-bath ratio of 20 cm per 10 mL of solution, ensuring minimal and precise chemical consumption. The plating time varied from 4 to 14 min to optimize the electrical properties, and the 14 min plated samples were primarily used for the subsequent electrothermal and mechanical characterizations. Continuous agitation was maintained at 70 rpm using a digital shaker to ensure uniform ion diffusion and coating consistency. Immediately before use, the Cu plating solution was mixed with formaldehyde in a 1 (v/v) ratio to initiate the reduction of Cu^2+^ ions and enable autocatalytic Cu deposition on the Pd-seeded fiber surface. The chemical equation is as followsCu2++2HCHO+4OH−→Cu0+2HCOO−+2H2O

Characterization

The morphological characteristics of the nanofiber yarn were analyzed using field emission scanning electron microscopy (FE-SEM; Ultra Plus; ZEISS, Germany) and optical microscopy (OM; ECLIPSE LV150N; Nikon, Japan). The diameter of yarn and individual nanofibers consisting of yarn were measured from FE-SEM and OM images using ImageJ software (NIH, USA). All measurements were performed five times per sample, and the mean value was used for analysis. The properties of the nanofibers were analyzed with an energy dispersive X-ray spectrometer (EDS; FlatQUAD; Bruker, USA) and Fourier transform infrared spectrometer (FTIR; Cary670; Agilent Technologies, USA). Constant agitation during the plating process was provided by a digital shaker (S04-02-205; LK Lab Korea, Korea). The mechanical properties of the nanofiber yarn were evaluated by a tensile test using a universal testing machine (QM100S; QMESYS, Korea). Yarn samples of approximately 30 mm in length were prepared and tested at room temperature. Tensile tests were performed at a constant extension rate, and the ultimate tensile strength was calculated as the average of five measurements. Electrical conductivity of the Cu-plated yarns was measured using a digital multimeter (DAQ6510; KEITHLEY, USA). To evaluate the heating performance of the conductive yarns, a DC power supply (2230-30-6; KEITHLEY, USA) was used, and temperature changes were monitored using a precision high-resolution infrared camera (PI 640i; Optris, Germany).

Results and Discussion

Effect of Collector Rotation Speed and Electrospinning

Voltage on Yarn Morphology

The morphology of electrospun nanofiber yarns is predominantly governed by the collector rotation speed and the applied electrospinning voltage. ,

Figure summarizes the relationships between these parameters and the resulting yarn diameter as well as the diameter of the constituent nanofibers. Figure A–C present SEM images of yarns produced at collector speeds of 250, 500, and 750 rpm, respectively. In the funnel-type collector system, fibers ejected from the nozzle are continuously twisted as the funnel rotates, while the drum located downstream pulls the fiber bundle to form a continuous yarn. , During this process, the fiber bundle develops a conical structure, which must remain stable for successful yarn formation, as previously reported. Accordingly, the rotation speed of the funnel collector plays a critical role in determining the structural characteristics of the resulting yarn.

At the lowest rotation speed (Figure A), the twisting rate of the fiber bundle is relatively slow compared with the pulling speed of the drum, leading to the formation of yarns with the smallest twisting angle (α). As shown in Figure B,C, the twisting angle progressively increases with increasing collector rotation speed, reaching 24.96°, 36.44°, and 43.03° at 250, 500, and 750 rpm, respectively. The corresponding high-magnification SEM images (Figure A′–C′) confirm that most nanofibers are aligned along the twisting direction of the yarn. A higher twisting angle indicates that the fibers experience stronger winding action at elevated rotation speeds, which in turn influences both the packing density and the overall yarn diameter.

As illustrated in Figure D, both yarn and fiber diameters exhibit a decreasing trend as the collector speed increases. The yarn diameter decreases from approximately 576 μm at 250 rpm to 373 μm at 750 rpm, while the average fiber diameter decreases from roughly 1.3 to 0.79 μm. The reduction in fiber diameter results from the additional elongational forces imposed on the fibers during twisting, whereas the decrease in yarn diameter arises from both fiber thinning and the denser fiber packing achieved at higher rotation speeds. This is also visually evident in Figure A–C, A′–C′, where the yarn produced at lower rotation speed exhibits a comparatively loose fiber arrangement, whereas that produced at higher rotation speed forms a more compact structure with fewer voids between fibers.

To quantitatively assess the geometric compactness of the nanofiber yarns as a function of collector rotation speed, the length-to-volume ratio (L/V) and porosity were evaluated (Figure S1 and Table S1). The L/V ratio was calculated from ten locally measured diameters for each yarn and averaged, resulting in values of 4.31, 7.13, and 12.80 mm^–2^ at 250, 500, and 750 rpm, respectively. The increase in L/V indicates a progressive reduction in the effective cross-sectional area with increasing twisting rate. The porosity of the electrospun yarns was determined using the envelope volume method, calculated from the mass and external dimensions of a 15 cm yarn segment based on its area-averaged diameter. , The porosity was calculated using the following equationP=1−VfiberVenvelopewhere V envelope is the total volume of the yarn segment estimated from its area-averaged diameter and length, assuming a cylindrical geometry, and V fiber is the actual volume of the polymer fibers derived from the measured mass and the density of the polymer.

Due to the large interstitial air gaps between the ultrafine nanofibers (0.79–1.3 μm) and the overall microscale dimensions of the yarn bundle (373–576 μm), the resulting porosity values were inherently high, ranging from 98.03% to 97.32%. As summarized in Table S1, the porosity decreased slightly with increasing rotation speed, confirming the gradual densification of the yarn structure through twisting-induced lateral compaction. Despite this high porosity, the yarns maintained robust mechanical integrity due to enhanced interfiber friction from the high degree of alignment. The applied voltage also significantly affects the continuity of yarn production and the resulting fiber dimensions. Figure E shows the change in fiber and yarn diameters as a function of applied voltage. As the voltage increases, the fiber diameter gradually decreases from an average of ∼1 μm to ∼0.78 μm. The yarn diameter exhibits its maximum value of 605 μm at 13 kV. At voltages below 9 kV, the system fails to reach the threshold required for stable electrospinning, making continuous yarn formation difficult. Once the applied voltage exceeds 9 kV, stable fiber ejection is achieved, enabling the consistent formation of nanofiber yarns. The voltage range of 13–15 kV produces the most stable yarn formation.

It is important to note that not all electrospun fibers are incorporated into the yarn; some fibers disperse into the surrounding air or adhere to the inner surface of the funnel collector. At voltages higher than 13 kV, the yarn diameter begins to decrease again, and above 17 kV, excessive fiber deposition on the funnel’s inner walls disrupts the yarn formation process. This instability is attributed to the increased whipping and dispersion of fibers at high voltages, causing a substantial fraction of them to deviate from the yarn formation path.

Mechanical Properties of Electrospun Nanofiber

Yarns

Figure presents the mechanical properties of the electrospun re-PS nanofiber yarns. Figure A,B show the stress–strain behavior and the corresponding ultimate tensile strengths of yarns fabricated at collector rotation speeds of 250, 500, and 750 rpm. Both the tensile strength and elongation at break increase with increasing collector speed. This enhancement is attributed to the formation of denser yarn structures at higher rotation speeds.

The stress–strain curves exhibit a characteristic serrated profile rather than a smooth nonlinear increase typically observed in conventional tensile specimens. This jagged pattern results from the sequential fracture of individual nanofibers within the yarn during stretching, accompanying the overall deformation of the yarn bundle. The mechanical strength of nanofiber yarns is closely related to the twisting yarns with tighter and more compact twisting exhibit increased interfiber contact and frictional interactions, enabling them to withstand higher tensile loads. As the collector rotates faster, the fibers experience stronger twisting and are compressed more tightly toward the center of the yarn. This reduces the spacing between individual fibers and increases overall packing density, which enhances tensile strength. This enhanced lateral compaction allows the yarn to sustain greater tensile forces before failure. Consequently, the yarn produced at 750 rpm exhibited the highest elongation (32.3%) and ultimate tensile strength (4.6 MPa).

It should be noted, however, that there exists an optimal twisting angle; excessive twisting can deteriorate mechanical performance by inducing structural defects or overstressing individual fibers.

The influence of electrospinning voltage on the mechanical performance of the yarns is shown in Figure C,D. In general, higher voltages increase the cohesive interactions within the fiber network, which can enhance tensile strength. In this study, stable yarn formation was achieved at applied voltages of 13–15 kV, and the maximum tensile strength of 3.36 MPa was obtained at 15 kV. At excessively high voltages, however, the instability of fiber trajectory and incomplete incorporation of fiber into the yarn hindered the formation of a robust yarn structure, leading to reduced mechanical performance.

Effect of Polymer Blending on the Chemical/Mechanical

Properties of Nanofiber Yarns

Figure summarizes the mechanical behavior of electrospun yarns fabricated from pure PS, recycled PS (re-PS), and polymer–blended hybrid systems, along with corresponding FTIR analyses. Figure A compares the tensile strength of yarns produced from pure PS and re-PS under identical electrospinning conditions. The re-PS yarn exhibited a tensile strength of 3.36 MPa, which is comparable to that of the pure-PS yarn (3.45 MPa), indicating that mechanical degradation due to polymer recycling was minimal. FTIR spectra further confirmed the chemical equivalence of the two materials. As shown in Figure B, both pure PS and re-PS yarns displayed identical characteristic absorption bands of aromatic C–H stretching at 3025 cm^–1^, CH2 stretching at 2919 cm^–1^, aromatic CC stretching near 1600 cm^–1^, and aromatic C–H bending at 1490 and 1450 cm^–1^. Strong out-of-plane C–H bending peaks at 754 and 696 cm^–1^typical signatures of PSwere also present. , These results demonstrate that re-PS can be effectively electrospun into yarns with physicochemical characteristics and mechanical performance comparable to those of pure PS. This chemical and mechanical equivalence is a significant finding, as it validates the technical feasibility of utilizing plastic waste as a direct replacement for virgin polymers in high-precision nanofiber manufacturing. Demonstrating such performance parity provides a robust foundation for resource circularity, proving that recycled materials can achieve the necessary functional standards without the performance trade-offs often expected in upcycled systems.

Figure C,D presents the results of polymer blending to enhance the mechanical robustness of nanofiber yarns. Prior to blending, we attempted to fabricate single-polymer yarns using commonly recycled polymers such as PU, PVC, PC, and PET. Although these materials are widely used in conventional electrospinning, they did not successfully form continuous yarns under the yarn-spinning conditions employed in this study, where fiber collection and twisting occur simultaneously. In our experiments, PS exhibited excellent spinnability due to favorable solution rheologyproviding sufficient chain entanglement, appropriate viscosity, and suitable solvent evaporation kinetics. In contrast, polymers such as PET and PC required restrictive solvent systems and undergo rapid solidification, which reduces interfiber tackiness and cohesion during simultaneous collection and twistingthereby hindering continuous yarn formation. , Co-electrospinning these polymers with PS enabled successful yarn formation by allowing PS to function as a carrier matrix that stabilized the electrospinning jet, improved interfiber cohesion, and enhanced twistability. These findings highlight that solution viscosity, molecular entanglement density, and solvent-induced drying behavior are critical determinants of yarn-forming capability in electrospinning.

Based on these insights, we fabricated hybrid yarns by simultaneously electrospinning re-PS with each recycled polymer using a dual-nozzle configuration. As shown in Figure C, all hybrid yarns exhibited enhanced tensile strength compared to single-component re-PS yarns. The tensile strengths of re-PS blended with re-PU, re-PVC, re-PC, and re-PET were 5.49, 5.64, 8.20, and 8.83 MPa, respectively. Among these, the re-PS/re-PET hybrid yarn showed the highest tensile strengthapproximately 2.64 times higher than that of re-PS alonedemonstrating a clear reinforcing effect from PET. FTIR analysis of the re-PS/re-PET hybrid yarn (Figure D) revealed characteristic PET absorption bands, including CO stretching at 1714 cm^–1^ and C–O–C stretching at 1245 and 1097 cm^–1^, which were absent in the pure PS spectra. The appearance of these peaks confirms the successful incorporation of PET within the hybrid yarn. Taken together, these results demonstrate that dual-nozzle coelectrospinning enables the fabrication of structurally integrated hybrid yarns with significantly improved mechanical properties.

Morphology and Electrical Conductivity of

Electroless Cu-Plated Nanofiber Yarns

Figure summarizes the morphological evolution and electrical performance of the electrospun re-PS/re-PET hybrid yarns before and after electroless copper plating. Optical microscopy, FE-SEM imaging, and EDS elemental analysis were used to evaluate the uniformity of the deposited metal layer, and resistance measurements were conducted to assess the electrical properties of the plated yarns.

Figure A presents optical images of the yarn bobbin before and after plating. After electroless copper deposition, the yarns exhibit a uniform reddish-metallic color over the entire ∼2 m length wound on the bobbin, indicating homogeneous deposition from the starting point at the upper right to the end point at the lower left of the spool. Figure B shows magnified optical micrographs of individual yarn segments, clearly demonstrating that the copper layer fully covers the yarn surface without noticeable gaps or uncoated areas.

Figure C shows the evolution of resistance as a function of plating time. During electroless copper growth, isolated copper nuclei initially form and gradually grow laterally and vertically until they coalesce into a continuous metallic film. This behavior is reflected in the high resistance values observed at short plating times (908 Ω at 4 min and 420 Ω at 6 min). As the plating time increases beyond 8 min, the resistance drops sharply to 7.05, 5.04, and 3.56 Ω (in 2 min increments), eventually reaching 2.84 Ω at 14 min. The sudden decrease in resistance after approximately 8 min indicates the formation of a percolated and highly interconnected copper network that provides efficient conductive pathways along the yarn. To examine the internal deposition behavior, cross-sectional SEM and EDS mapping were additionally performed (Figure S2). At 4 min, the Cu signal is sparse and discontinuous, appearing mainly near the edge of nanofibers. At 8 and 12 min, the Cu-rich region becomes more continuous and more broadly distributed around the yarn periphery, indicating increased copper coverage and improved interfiber connectivity. This qualitative trend is consistent with the rapid resistance decrease from 420 Ω at 6 min to 7.05 Ω at 8 min. Because the yarn has a porous bundle structure and the fractured cross sections are not sufficiently uniform, the present SEM/EDS analysis is used here as qualitative evidence of deposition evolution rather than as a rigorous thickness quantification.

The elemental mapping results in Figure D further confirm that copper is uniformly distributed across the entire surface of the plated yarn. FE-SEM images in Figure E and the high-magnification view in Figure E′ reveal the detailed microstructure of the metal coating. At lower magnification (Figure E), the copper layer forms a dense and continuous sheath around the fiber bundle. At higher magnification (Figure E′), each individual nanofiber is shown to be fully coated with compact submicron-sized copper particles, demonstrating conformal and uniform deposition along the entire yarn structure.

Together, these analyses confirm that the electroless copper plating process effectively produces a continuous, homogeneous metallic layer on the nanofiber yarn, thereby imparting excellent electrical conductivity suitable for functional textile applications.

Electrothermal Properties of Electroless Cu-Plated

Nanofiber Yarns

To evaluate the performance of the fabricated re-PS/re-PET hybrid metal yarn as a smart-textile heater, its Joule-heating characteristics were investigated. First, Figure A shows the heating behavior of the yarn under stepwise voltage increments. The straight yarn was subjected to voltages ranging from 0 to 1.2 V with 0.2 V intervals, and its temperature evolution was monitored using an infrared camera. Before voltage application, the entire yarn surface maintained a uniform temperature of approximately 21.8 °C. Once the voltage was applied, the temperature increased instantaneously and remained stable at each voltage step. Notably, the Cu-plated hybrid yarn reached a maximum center temperature of 153.3 °C even at a low operating voltage of 1.2 V. However, fracture occurred approximately 10 s after reaching the maximum temperature. Given that the thermal decomposition temperature of PS and the melting point of PET are ∼240 °C and ∼280 °C, respectively, and their glass transition temperatures are ∼100 °C and ∼75 °C, the observed high temperature of 153.3 °C is sufficient to induce thermal softening of the polymeric body. Thus, the subsequent fracture is attributed to the inability of the polymer fibers to withstand sustained thermal loading.

Figure B presents the long-term heating stability of the Cu-plated hybrid yarn under a constant voltage of 1 V for 1 h. After a rapid temperature rise upon voltage application, the yarn maintained a stable average temperature of approximately 96.21 °C (standard 0.86 °C) throughout the entire operation period. No thermal drift, degradation in heating performance, or mechanical failure was observed, demonstrating the excellent electrical continuity and thermal stability of the plating layer. A comparable long-term heating stability was also observed when the yarn was operated in a bent configuration, as shown in Figure S3, indicating that mechanical deformation does not compromise the electrothermal performance.

Figure C,D show the cyclic heating response of the yarn under repetitive on/off switching at 1 V with a 1 s interval for 300 cycles. Both the flat and bent yarns exhibited instantaneous heating within 1 s upon voltage application and immediate cooling upon voltage removal. The temperature profiles were nearly identical in all cycles, confirming consistent heating/cooling behavior, high responsiveness, and strong cycle-to-cycle reproducibility of the Cu-plated yarn.

Figure E shows the power efficiency (η) of the yarn, calculated from the saturated temperature (T Sat.) increase and the input power (P Area) normalized by the effective heating area. The Cu-plated hybrid yarn exhibits a high-power efficiency of 36.95 °C·cm^2^/W, indicating efficient conversion of electrical energy into heat under low-voltage operation. This high efficiency is attributed to the one-dimensional yarn geometry combined with the uniformly plated copper layer, which enables continuous electron transport and efficient Joule heat generation while minimizing heat dissipation through the surrounding air.P=I·V PArea=PA

Figure F compares the electrothermal performance of this work with recently reported yarn-based heaters. For a morphology-consistent comparison, only fiber or yarn type heaters were considered. As summarized in the comparison plot, the Cu-plated hybrid yarn demonstrates superior heating efficiency and a markedly lower operating voltage than most previously reported yarn-based heaters. −

Notably, a maximum temperature of 153.3 °C was achieved at an ultralow voltage of 1.2 V, outperforming many conductive polymer- or nanomaterial-based yarn heaters that typically require higher driving voltages (Table S2).

To further assess the practical durability, the mechanical and adhesive robustness of the Cu-plated hybrid yarn was evaluated. The electrothermal stability under extreme deformation was tested by winding the yarn around a 1 cm diameter cylinder. Remarkably, the yarn achieved a steady-state temperature of 118 °C at an applied voltage of 1 V. Postbending SEM analysis revealed no structural failure or remarkable cracks in the Cu plating layer (Figure S4). High-magnification SEM images confirm that the Cu layer consists of a dense granular morphology even in its initial state. This granular structure effectively dissipates the mechanical strain during bending, allowing the metallic sheath to maintain its integrity without observable delamination. Additionally, the interfacial adhesion of the Cu sheath was verified through a tape-peeling test using 3 M Scotch tape. The resistance showed only a minimal increase to 4.07 Ω after three peeling cycles, with EDS mapping confirming the integrity of the metallic layer (Figure S5). These results indicate that the BSA-assisted activation provides strong chemical anchoring between the polymer core and the conductive sheath. Furthermore, the mechanical properties of the Cu-plated hybrid yarns were evaluated through tensile testing (Figure S6). The formation of the rigid metallic sheath resulted in a reduction in tensile strength and elongation at break compared with the as-spun hybrid yarns, which can be attributed to the inherent brittleness of the electrolessly deposited copper layer. Specifically, the yarn fabricated at 750 rpm exhibited a tensile strength of 2.32 MPa and an elongation at break of 8.5%. Although these values are lower than those of the as-spun state, they remain sufficient to accommodate the typical deformation levels encountered in smart textile applications. Overall, these results demonstrate that the Cu-plated hybrid yarn exhibits excellent heating efficiency, thermal stability, and mechanical durability even under low-voltage operation. Such performance highlights its strong potential as a promising candidate material for next-generation smart-textile applications, including flexible textile-based heaters and wearable heating systems.

Application Demonstration: Electrothermal

Drying of Wet Fabric

To further demonstrate the practical applicability of the Cu-plated hybrid yarn as a smart-textile heater, an electrothermal fabric-drying experiment was conducted using a water-soaked textile. A commercial polyester fabric (4 cm × 4 cm, thickness ≈60 μm) was uniformly wetted with deionized water and subjected to two different drying electrothermal drying induced by the Cu-plated hybrid yarn and natural drying under ambient conditions. For the electrothermal drying test, the wet fabric was placed in direct contact with the Cu-plated hybrid yarn, and Joule heating was induced by applying a constant voltage of 0.4 V in 30 s heating intervals. The mass of the fabric was measured after each heating step to quantitatively evaluate the moisture removal during drying. For comparison, an identical wet fabric sample was allowed to dry naturally at room temperature without electrical heating, and its mass was recorded at the same time intervals.

Figure A shows the fabric weight as a function of drying time under electrothermal and ambient conditions. Upon voltage application, the electrothermally heated fabric exhibited a rapid mass reduction, reaching its dry-state weight within approximately 1.5 min, whereas natural drying required more than 7 min to achieve a comparable mass. The identical final dry weights indicate complete moisture removal without material degradation.

Figure B presents infrared thermal images captured during electrothermal drying. Upon electrical heating, the low-temperature (wet) region gradually diminished, and the temperature distribution became increasingly uniform. After 90 s, the wet region was no longer observable, confirming complete drying. These IR results are consistent with the weight-loss measurements and demonstrate the effectiveness of localized Joule heating enabled by the Cu-plated hybrid yarn. Overall, this application demonstration confirms that the Cu-plated hybrid yarn can function as an effective electrothermal element for rapid textile drying under low-voltage operation. Compared to ambient drying, the localized Joule heating significantly accelerates moisture removal while maintaining uniform thermal distribution, highlighting the potential of the yarn for practical smart-textile and wearable thermal management systems.

Also, the washability of the Cu-plated hybrid yarn was rigorously tested to simulate real-world maintenance. The yarns underwent three full cycles of washing in various mediaDI water, 0.4 wt % SDBS solution, and commercial detergentunder vigorous agitation (800 rpm at 40 °C for 30 min), followed by consecutive drying in an oven at 60 °C. As shown in Figure S7, the resistance remained exceptionally stable across all conditions, with the detergent-washed sample maintaining a low resistance of 3.75 Ω after the final cycle. The structural and elemental preservation of the Cu coating after repeated mechanical and chemical stress demonstrates the long-term reliability of the hybrid yarns for integrated smart garments.

Conclusion

In this study, hybrid nanofiber yarns were fabricated using a dual-nozzle electrospinning process based on recycled polystyrene (re-PS) and recycled polyethylene terephthalate (re-PET), followed by electroless copper plating to realize conductive and electrothermally functional metal yarns. The twisting angle, diameter, and fiber alignment of the yarns were quantitatively tuned by the collector rotation speed and applied voltage during electrospinning. Notably, the yarns exhibited the highest degree of fiber alignment and a minimum diameter of 373 μm at 750 rpm, while stable yarn formation was achieved within the voltage range of 13–15 kV.

Mechanical characterization revealed that the twisting structure directly influenced the tensile properties, with the yarn spun at 750 rpm showing a maximum tensile strength of 4.6 MPa and an elongation of 32.3%. Hybrid yarns produced by blending re-PS with other recycled polymers exhibited further improvements in tensile performance compared with single-component re-PS yarns (3.36 MPa). In particular, the re-PET/re-PS hybrid yarn achieved a tensile strength of 8.83 MPa, representing a 2.64-fold enhancement attributable to the reinforcing effect of re-PET. This confirms the effectiveness of polymer blending as a strategy for structural reinforcement in recycled fiber systems.

Electroless copper plating produced a uniform and continuous metallic layer throughout the hybrid yarns, as confirmed by high-resolution SEM and EDS analyses. A low resistance of 2.84 Ω was obtained after 14 min of plating. The Cu-plated yarns also demonstrated excellent electrothermal performance, reaching a central temperature of 153.3 °C under a low input voltage of 1.2 V and maintaining a stable average temperature of 96.21 °C during 1 h of continuous operation. Furthermore, the yarns preserved consistent heating response over 300 on/off cycles, indicating high thermal stability and mechanical durability. Beyond material-level characterization, the Cu-plated hybrid yarn enabled rapid and uniform electrothermal drying of a wet polyester fabric, achieving complete drying within 1.5 min at 0.4 V, compared to 7 min under ambient conditions.

Overall, the fabrication strategy presented here offers a low-cost, nonvacuum, and environmentally friendly alternative to conventional metallized textile manufacturing routes that typically require high-temperature processing, vacuum environments, or conductive substrates. While the process involves the use of Pd and formaldehyde, their environmental impact was strictly minimized through trace-amount consumptionsspecifically requiring only 10 mL of PdCl2 solution (0.05 wt %) per 20 cm of yarn and a minimal 1 volume ratio for formaldehyde. This targeted use of chemicals ensures superior electrothermal efficiency, such as reaching 153.3 °C at 1.2 V, which justifies the environmental trade-off by enabling high-performance upcycling. In future studies, these reducing agents are expected to be replaced with nontoxic alternatives, such as glucose, to further enhance sustainability. The Cu-plated hybrid yarns developed in this work combine flexibility, electrical conductivity, and thermal robustness, positioning them as promising next-generation wearable heating materials for smart textile applications.

Supplementary Material