Fabrication of stretchable circuits with horseshoe-shaped double-layer composite conductive structures via direct ink writing and electrochemical 3D printing technologies

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Among these, the stretchable circuit, serving as the core structure that connects various components, plays a pivotal role in determining the performance of the entire flexible and stretchable electronic system. However, there exists an inherent trade-off relationship between the electrical performance and the stretchability of stretchable circuits. In this study, a horseshoe-shaped double-layer composite conductor structure was designed for the fabrication of stretchable circuits using direct writing technology and electrochemical 3D printing technology. This double-layer horseshoe-shaped geometric structure not only improves the electrical performance of the wir but also ensures its mechanical stability during the stretching process. Compared to single-structure stretchable circuits, the stretchable circuits featuring a horseshoe-shaped double-layer composite conductive structure exhibit an enhancement in electrical performance by 2.86 times in the unstretched state and by 4.05 times at a 70% stretch rate. Furthermore, after 1000 cycles of stretching and bending, the resistance change rate remains below 15%. The fabricated stretchable circuits not only maintain superior electrical performance but also demonstrate remarkable stretchability and durability, thereby offering a novel approach for the development of flexible stretchable circuits. double-layer stretchable conductor structural design extrusion-based 3D printing electrochemical 3D printing Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 1. Introduction Conventional electronic systems based on silicon or copper exhibit significant limitations when subjected to mechanical deformations such as bending, stretching, or folding. These systems are prone to functional failure or fracture and demonstrate considerable drawbacks in biocompatibility, conformability to curved surfaces, and dynamic adaptability[ 1 ]. For instance, in wearable health monitoring devices, rigid circuits cannot conformally adhere to the human skin, often causing discomfort or device failure. In the field of soft robotics, they severely restrict the range of motion and natural morphing capabilities. In contrast,stretchable circuits leverage a synergistic combination of materials and structural design to maintain stable electrical performance and mechanical reliability under deformation. As a result, they show broad application potential in areas such as stretchable electrodes [ 2 ], displays[ 3 , 4 ], sensors[ 5 – 7 ], soft robots [ 8 , 9 ], and electronic skin [ 10 – 12 ]. Consequently, there is a pressing need to develop stretchable circuits that integrate high electrical conductivity, superior mechanical stretchability, and environmental stability. Current design methodologies for fabricating stretchable circuits are primarily divided into two categories: structural film design and intrinsically stretchable materials. The fundamental principle of structural film design involves patterning specific geometric configurations (e.g., wavy, U-shaped, or serpentine)[ 13 – 16 ] into rigid conductive films, enabling conductivity maintenance through reversible structural deformation rather than material elongation [ 17 ]. Khang [ 18 ] transferred flat silicon films onto pre-strained substrates and released the strain to induce in-plane buckling, forming wavy structures that granted stretchability to silicon, allowing devices to function at up to 20% tensile strain. Xu [ 19 ] deposited copper layers on polyimide (PI) films via electron-beam evaporation and fabricated rectangular stretchable interconnects using selective wet etching, achieving stable performance at 30% tensile strain. A lower-cost alternative employs mask printing to fill pre-fabricated grooves with metal pastes, which are then dried to form stretchable interconnects. However, this method offers limited structural customization due to dependence on the mask template. The second category employs intrinsically stretchable conductors, which are directly made from ductile conductive materials, including ionic liquids, liquid metals, and conductive composites. Ionic liquids, composed entirely of ions, are often used as conductive materials in flexible devices owing to their high electrical conductivity and low volatility[ 20 ]. However, their electrical resistance is highly susceptible to environmental factors like humidity, resulting in poor stability and limiting their broader application. Liquid metals [ 21 – 23 ], another common class, combine metal-like conductivity with fluid-like deformability, making them promising for flexible electronics. For example, Dickey [ 24 ] and Kim [ 25 ] encapsulated gallium-based liquid metals within elastic microchannels to create stretchable conductors that maintained nearly constant resistance under tensile strains up to 60%. Conductive composites have also attracted extensive research interest. These are typically prepared by blending conductive fillers (e.g., silver nanowires, nanoparticles, or microflakes) with an elastic polymer matrix [ 26 – 29 ]. Wu[ 30 ] mixed silver flakes with silicone rubber and methyl isobutyl ketone (MIBK) to formulate a stretchable conductive ink, which was screen-printed onto silicone rubber substrates to form conductors. Feng [ 31 ] synthesized silver nanoparticles via a redox method, followed by mechanical stirring and ultrasonic dispersion into liquid PDMS, forming a stretchable conductive adhesive (CA) that was subsequently sintered into conductive structures. Nevertheless, such composites exhibit a critical drawback: achieving high electrical conductivity requires a high volume fraction of conductive fillers, which severely compromises the elasticity of the material. This leads to significant resistance changes during stretching, a propensity for fracture[ 32 , 33 ], and electrical conductivity that remains substantially lower than that of bulk metals. Although significant progress has been made in the material and structural design of flexible stretchable circuits, stretchable circuit fabrication strategies still face significant challenges. On the one hand, structural film designs, though capable of imparting a degree of stretchability to conventional metals through specific geometries (e.g., wavy or serpentine structures), rely heavily on complex processes such as photolithography, vacuum deposition, and precision etching. These processes lead to high manufacturing costs, complexity, and limited structural customization capabilities. On the other hand, while liquid metals—as intrinsically stretchable materials-possess excellent deformability and high conductivity, they suffer from poor adhesion to polymer substrates, fatigue-induced leakage of encapsulation structures, and surface oxidation leading to increased interfacial resistance. Conductive composites are hampered by an inherent trade-off between conductivity and stretchability: high filler loading required for high conductivity severely degrades material elasticity, resulting in dramatic resistance changes during stretching, susceptibility to fracture, and conductivity that is still far inferior to conventional metal wires. In this paper, A horseshoe-shaped double-layer stretchable circuit was fabricated through direct ink writing and electrochemical 3D printing technologies. First, a stretchable conductive silver paste was printed using direct ink writing and printed into a horseshoe-shaped seed layer on a PDMS substrate. Subsequently, a copper thin film was electrochemically deposited onto the surface of the seed layer using electrochemical 3D printing. Finally, the entire circuit was encapsulated with PDMS, yielding a double-layer, horseshoe-shaped, stretchable conductive structure. This manufacturing process facilitates the fabrication of metal-film-based stretchable wires by circumventing the high costs typically associated with complex techniques such as photolithography. The developed horseshoe-shaped, double-layer composite conductive structure exhibits a 2.86-fold improvement in electrical performance compared to single-layer silver paste circuits in the unstretched state, increasing to a 4.05-fold enhancement under 70% tensile strain. Furthermore, after 1000 stretching-bending cycles, the resistance change rate remained below 15%, demonstrating that the fabricated stretchable circuits possess not only excellent electrical performance but also robust long-term operational stability. This innovative approach not only overcomes the drawbacks of existing methods but also opens up new possibilities for the development of high - performance stretchable circuits in a wide range of applications. 2. Materials and methods 2.1. Materials The copper plating solution (Copper sulfate pentahydrate (CuSO₄·5H₂O), sodium chloride (NaCl), and deionized water) was purchased from Sinopharm Chemical Reagent Co., Ltd., China. PDMS and curing agent (Sylgard 184) were both from Dow Corning. The stretchable conductive silver paste was from Shandong Jiahui Material Technology Co., Ltd. 2.2. Methods The surface morphology of the stretchable wires was characterized by an optical microscope (DXS510, Olympus, Japan). Bending, stretching, and twisting tests were conducted using a tensile-bending testing machine. Infrared thermal imaging was performed with a C5 from FLIR (USA). The PDMS was degassed under vacuum conditions using a DZF-6092 vacuum drying oven manufactured by Shanghai Yiheng Scientific Instruments Co., Ltd. 3. Results and Discussion 3.1 Manufacturing of horseshoe-shaped double-layer stretchable circuits Figure 1 shows a horseshoe-shaped double-layer stretchable circuit fabricated by direct ink writing (DIW) and electrochemical 3D printing techniques. First, according to the preset printing path, the corresponding parameters were configured, and the PDMS base layer was printed using the DIW process. After printing, the structure was thermally cured at 75℃for 15 minutes. Next, a stretchable conductive silver paste circuit was prepared on the PDMS substrate via DIW and cured at 150℃ for 60 minutes. Then, a copper sulfate solution was injected into the material storage cylinder, with the copper wire inside the cylinder connected to the positive electrode and the stretchable conductive silver paste connected to the negative electrode. Upon energization, the copper anode undergoes an oxidation reaction, releasing Cu²⁺ions into the solution, while Cu²⁺ ions in the electrolyte are reduced and deposited as a metallic copper film on the surface of the silver paste cathode. To avoid residual waste liquid contaminating electronic components or interfering with subsequent processes, a custom printhead with integrated waste liquid recycling capability was adopted (Fig. 1 c). The printhead includes an inner liquid outlet nozzle and an outer liquid suction nozzle: the inner nozzle is used to eject electrolyte for electrochemical 3D printing, while the outer nozzle simultaneously aspirates waste liquid, thereby maintaining circuit integrity; the process can be repeated to achieve the desired copper film thickness. Finally, a PDMS encapsulation layer was printed on the deposited structure using DIW and cured under conditions similar to those for the base layer (80 ℃ for 15 minutes). The resulting horseshoe-shaped double-layer structure is shown in Fig. 1 f: the deposited copper film effectively improves the conductivity of the underlying conductive silver paste; its special horseshoe geometry can effectively accommodate strain through arc segment unfolding during stretching, thereby achieving stable current transmission. The cross-sectional view in Fig. 1 f shows that the stretchable conductive silver paste and copper film are well bonded, mainly due to the tight adhesion formed between the polymer matrix in the silver paste and the copper surface through van der Waals forces, which prevents the copper film from easily peeling off during stretching or bending. Notably, even if the copper layer undergoes local fracture due to significant stretching, the underlying stretchable conductive silver paste can still form alternative conductive paths in the fractured regions, ensuring the circuit maintains stable electrical performance under large deformation. 3.2 Simulation Research and Optimization of Horseshoe-shaped Wire Structure This study numerically investigated the influence of varying curvature-to-spacing ratios( CSR = D/R , D is the distance between adjacent arcs, and R is the radius of the arc) on tensile conductive performance based on a horseshoe-shaped wire geometry configuration using ANSYS software. The simulation results are shown in Fig. 2 . The simulation results show that under the same tensile rate, the larger the curvature spacing ratio, the lower the maximum stress value of the manufactured wire and the better the tensile performance (Fig. 2 a). However, when the curvature spacing ratio exceeds 0.8, the maximum stress value increases, which is due to the proximity of the wires between two adjacent cycles, resulting in the superposition of the stress of adjacent wires. According to the stress simulation analysis, the horseshoe-shaped conductor structure with a curvature-to-spacing ratio of 0.8 exhibits optimal tensile performance. Furthermore, under identical base area, the effects of varying number of horseshoe-shaped interconnection wires on tensile properties were investigated at a consistent elongation rate (10%). The simulation results are shown in Fig. 2 b. The findings demonstrate that as the number of cycles increases, the maximum stress in horseshoe-shaped wires under equivalent elongation rates rises(Fig. 2 d). This occurs because the increased number of cycles leads to a larger wire area per unit substrate, making the substrate's pulling effect more pronounced during stretching. Additionally, the reduced spacing between adjacent wires with increasing cycle numbers results in stress superposition between neighboring wires, thereby amplifying the overall stress level. Considering that different application scenarios have different requirements on wire performance, it is necessary to comprehensively balance the relationship between the number of cycles and tensile performance in practical application. In addition, the stress change should be closely monitored to ensure the stability and reliability of the wire. (a)Simulation Analysis of the CSR; (b)Simulation Diagram of Horseshoe Turn Count; (c)Influence of the CSR; (d)Influence of the Horseshoe Turn Count 3.3 Exploration on the Manufacturing Rules of Horseshoe-shaped Stretchable Conductors Since the line width was determined by the stretchable conductive silver paste, researchers conducted a systematic investigation into the influence of key process parameters—such as printing speed, printing pressure, and printing height—in ink direct-writing 3D printing to achieve precise control over printed circuit line widths. Under constant conditions, including a printing platform temperature of 30°C, a fixed printing height of 0.2 mm, and a nozzle inner diameter of 0.36 mm, the effects of printing speed and pressure on the line width of the stretchable conductive silver paste were specifically examined. As shown in Fig. 3 a, variations in printing pressure (0.1–0.4 MPa) significantly influenced line width across the printing speed range of 1–6 mm/s. Specifically, continuous lines could not be formed at a pressure of 0.1 MPa when the printing speed exceeded 2 mm/s; however, stable line printing was achieved within the pressure range of 0.15–0.4 MPa. With other parameters held constant, the line width decreased monotonically as printing speed increased. This trend is attributed to the reduced volume of paste deposited per unit length, which leads to lateral shrinkage of the printed line. In contrast, an increase in printing pressure resulted in wider lines due to enhanced paste extrusion per unit time. Furthermore, Fig. 3 b illustrates that increasing the printing height from 0.1 mm to 0.5 mm gradually reduced the line width from 570 µm to 212 µm. Beyond a height of 0.5 mm, continuous filament formation was no longer achievable. This behavior can be explained by the increased distance between the nozzle and the substrate, which allows gravitational forces to stretch the extruded paste, thereby reducing its cross-sectional area and yielding finer lines. In summary, stable and uniform printing of stretchable conductive silver paste lines can be achieved through careful optimization of printing pressure, height, and speed. Since residual waste solution remaining on the printing platform after electrochemical 3D printing can adversely affect subsequent printing processes, a printhead integrated with waste recovery functionality was developed. The configuration is shown in Fig. 4 (a). The electrode nozzle has an inner diameter (ID) of 100 µm and an outer diameter (OD) of 400 µm, whereas the suction nozzle features an ID of 720 µm. During electrochemical 3D printing, copper sulfate (CuSO₄) solution is delivered through the electrode nozzle, and the resulting waste solution is subsequently aspirated through the suction nozzle for effective recovery. Jetting pressure and suction pressure are recognized as critical parameters that influence printing quality, the amount of residual waste solution on the platform, and ultimately, the performance of printed circuits. Therefore, a systematic investigation was conducted to evaluate the effects of these two pressure parameters. An orthogonal array experiment was designed using combinations of jetting pressure (10 kPa, 15 kPa, 20 kPa) and suction pressure (-5 kPa, -10 kPa, -15 kPa) to identify the optimal conditions for electrochemical 3D printing using CuSO₄ solution. The experimental results are summarized in Table 1 . All experiments were performed under constant conditions: an initial circuit resistance (without electrochemical deposition) of 4.8 Ω, a printing voltage of 160 V, and a printing speed of 4 mm/s. The results show that, at a fixed suction pressure, increasing the jetting pressure leads to a significant improvement in circuit conductivity. However, when the jetting pressure reaches 20 kPa, the rate of conductivity enhancement diminishes. Concurrently, an increase in the volume of residual waste solution is observed. Balancing the improvement in conductivity against the accumulation of residual waste, the optimal process parameters were determined to be a jetting pressure of 15 kPa and a suction pressure of -15 kPa. To enhance the overall conductivity of printed circuits, the influence of key parameters in electrochemical 3D printing was systematically investigated, with particular focus on printing velocity and applied voltage. As shown in Fig. 4 (b), resistance variations were measured across trace widths of 0.2 mm, 0.4 mm, and 0.6 mm at printing velocities ranging from 2 to 12 mm/s. Wider traces (0.6 mm) exhibit significantly improved conductivity after deposition compared to narrower ones (0.2 mm). This improvement is primarily attributed to cross-sectional flattening: at a constant conductor volume, increasing trace width expands the effective deposition area of copper films, thereby enhancing current-carrying capacity. Notably, resistance remains stable within the velocity range of 2–4 mm/s. However, beyond 4 mm/s at a trace width of 0.4 mm, significant exposure of the underlying silver paste substrate is observed (inset in Fig. 4 c), accompanied by a sharp increase in resistance and degraded print quality. This deterioration in conductivity is mainly due to insufficient electrolyte supply per unit length at higher velocities, which limits metal ion availability and reduces deposition efficiency. Considering both deposition consistency and electrical performance, a printing velocity of 4 mm/s was determined to be optimal. The printing voltage controls the electrochemical deposition kinetics by modulating the mobility of metal ions. Under otherwise identical conditions, the DC voltage was systematically varied from 100 V to 200 V to identify the optimal processing parameters. As shown in Fig. 4 (f), cross-sectional morphologies were observed at 120 V, 160 V, and 200 V, respectively. Figure 4 (d) demonstrates that within the 100–160 V range, conductivity steadily increases with rising voltage. Concurrently, the cross-sectional images in Fig. 4 f(i–ii) reveal a structural evolution from a loose, porous copper film to a dense, compact deposition layer. However, beyond 160 V, electrical resistance begins to rise. Mechanistic analysis indicates that below 160 V, higher voltage enhances current density, thereby accelerating the deposition rate. In contrast, above 160 V, ion diffusion becomes the rate-limiting step, and excessive voltage promotes dendritic growth, increases film porosity, and compromises layer densification, as evidenced in Fig. 4 f(iii). Taking into account both deposition uniformity and electrical conductivity, 160 V is established as the optimal printing voltage. Table 1 Law exploration of printing air pressure and extraction air pressure on resistance value and waste liquid residue amount. Experimental groups Printing pressure (kPa) Extraction pressure (kPa) Resistance (Ω) Residue amount of waste liquid(ml) 1 10 -5 3.3 3.5 2 10 -10 3.48 1.9 3 10 -15 3.72 0 4 15 -5 2.16 4.7 5 15 -10 2.22 1.5 6 15 -15 2.22 0 7 20 -5 2.13 8.9 8 20 -10 2.1 5.4 9 20 -15 2.13 3.4 3.4 Performance testing and application of stretchable wires To evaluate the performance advantages of the horseshoe-shaped structure in stretchable circuit design, tensile tests were conducted on four circuits with distinct geometric configurations. By measuring the initial resistance (R₀) and the resistance (R) during stretching, the relative resistance change (R/R₀) was calculated to assess the stretchability of each circuit structure, as illustrated in Fig. 5 a. The results show that the linear circuit exhibits the poorest conductivity performance under strain. At 50% strain, its resistance increases sharply; at 80% strain, the circuit fails completely. In contrast, the sinusoidal circuit experiences an 11.2-fold increase in resistance when stretched to 80% strain. Both U-shaped and horseshoe-shaped circuits maintain stable conductivity up to 60% strain. However, at 80% strain, the resistance of the U-shaped circuit rises to 3.9 times its initial value, while that of the horseshoe-shaped circuit increases by only 1.06 times. This superior performance is attributed to the unique geometry of the horseshoe structure, which effectively redistributes tensile stress and minimizes material deformation during stretching, thereby preserving electrical continuity. Furthermore, increasing the number of deposited layers within a fixed unit length enlarges the cross-sectional area of the copper traces, enhancing overall electrical conductivity. However, excessive layer thickness may lead to cracking during stretching, thereby compromising conductivity under strain. Consequently, the influence of the number of deposited layers on both electrical conductivity and mechanical stretchability was systematically investigated, as illustrated in Figs. 5 b and 5 c. Figure 5 demonstrates how varying the number of deposition layers affects the electrical performance of the circuit. When the number of deposition layers reaches seven, the conductivity increases threefold compared to the baseline before copper deposition. However, as the number of layers increases, so does the overall rigidity of the circuit, which restricts its deformation capacity and increases the likelihood of brittle fracture in the copper layer during stretching, as shown in Fig. 5 c. When the layer count increased to five, although the initial resistance was relatively low, it rose to 4.6 times its original value at a stretch ratio of 70%. In contrast, with only three deposited layers, the resistance increased by just 1.7 Ω when the circuit was stretched to 70%. Considering the trade-off between conductivity and stretch-induced resistance change, the optimal number of copper deposition layers is determined to be between one and three. Furthermore, the resistance variations (R/R₀) of the fabricated horseshoe-shaped silver-copper bilayer stretchable circuits were evaluated under 1000 cycles of cyclic bending (bending radius: 10 mm) and stretching (strain: 20%), as shown in Figs. 5 d and 5 e. The results indicate that after 1000 cycles, the conductivity variation remains within 15%, demonstrating excellent electromechanical stability. In contrast, the conductivity of the straight circuit deteriorates significantly after the same number of stretching cycles. This performance disparity can be attributed to the curved geometry of the horseshoe-shaped circuit, which effectively absorbs external mechanical stress during deformation, mitigates localized stress concentration, and suppresses crack initiation, thereby enhancing structural durability and operational lifespan. Furthermore, Fig. 5 f illustrates the LED brightness states of the fabricated horseshoe-shaped silver-copper bilayer stretchable circuit under four distinct conditions: natural, bent, stretched, and twisted. The results show that the LEDs maintain stable light emission across all conditions, demonstrating that the stretchable circuit retains reliable electrical performance even under complex and multimodal mechanical deformations. The uniform deposition of copper and silver layers ensured reliable electrical conductivity, while the horseshoe geometry prevented cracking or delamination during stretching. This scalability supports the integration of stretchable circuits into larger electronic systems, such as flexible displays or smart textiles, paving the way for next-generation flexible electronics. As a demonstration, by leveraging the integrated direct ink writing and electrochemical 3D printing technology developed in this study, along with optimized process parameters, a thermotherapy wristband measuring 150 mm × 60 mm × 1.5 mm was successfully fabricated. The device features a mesh-structured heating coil connected through bilateral horseshoe-shaped circuits, with a centrally integrated indicator light to display the operational status. As shown in Fig. 6 a, the wristband is depicted before and after electrochemical 3D printing (i.e., copper deposition). During operation, connecting a 4.5 V power supply to the circuit terminals closes the circuit, resulting in the illumination of the indicator light and the generation of therapeutic heat by the coil. To evaluate the effect of the deposited copper layer, electrothermal performance was compared before and after electrochemical 3D printing, revealing a significant enhancement in heat output following the treatment (Fig. 6 b). Specifically, the pre-deposition wristband required 260 seconds to reach its peak temperature of 45.5°C, whereas the post-deposition wristband achieved a higher peak temperature of 46.8°C in just 140 seconds. Detailed thermal images captured at 30, 60, 90, and 120 seconds during heating after copper deposition are presented in Fig. 6 d(I)–(IV). The observed enhancement is attributed to intensified Joule heating caused by reduced electrical resistance—under constant voltage and duration, lower resistance leads to greater heat generation, thereby enhancing therapeutic efficacy. Concurrently, the wristband underwent tensile cycling tests under 30% strain (Fig. 6 c), exhibiting negligible resistance change after 1,000 cycles. This indicates that the structure incorporating stretchable conductors possesses excellent fatigue resistance. Collectively, these results demonstrate that stretchable circuits fabricated using this method not only achieve improved electrical conductivity but also exhibit superior anti-fatigue performance. Overall, the successful fabrication and evaluation of the thermotherapy wristband validate the potential of the integrated direct ink writing and electrochemical 3D printing technology in producing high-performance stretchable circuits for flexible electronics applications. 4. Conclusion In this study, a manufacturing method for stretchable circuits featuring a horseshoe-shaped, double-layer composite structure was proposed. Using polydimethylsiloxane (PDMS) as the substrate, a horseshoe-shaped, double-layer stretchable circuit is fabricated via a combination of extrusion-based and electrochemical 3D printing techniques. The inner layer consists of a stretchable conductive silver paste, while the outer layer comprises a copper film, forming a dual-conductive pathway. The horseshoe geometry enhances structural stability during mechanical deformation. Superior conductivity is achieved through synergistic conduction in the composite channels. Tensile tests demonstrate that, under identical strain conditions, the horseshoe configuration exhibits better electrical performance than alternative geometries. Compared to a single-layer stretchable silver paste circuit, the double-layer circuit exhibits 2.86 times higher conductivity in the unstretched state, 3.12 times at 50% strain, and 4.05 times at 70% strain. Furthermore, the resistance remains nearly unchanged after 1,000 cycles of stretching and bending, confirming the excellent electromechanical reliability of the double-layer design. As a demonstration, an electrothermal heating therapy wristband based on a horseshoe-shaped circuit structure is fabricated. Experimental results indicate that the wristband incorporating a deposited copper layer reduces the time required to reach therapeutic temperature by nearly 50% compared to the non-deposited counterpart, thereby significantly enhancing heating efficiency. 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J Mater Chem C 2:1298–1305 Kim D-H, Yu K-C, Kim Y, Kim J-W (2015) Highly stretchable and mechanically stable transparent electrode based on composite of silver nanowires and polyurethane–urea. ACS Appl Mater Interfaces 7:15214–15222 Additional Declarations No competing interests reported. Cite Share Download PDF Status: Published Journal Publication published 05 Feb, 2026 Read the published version in Progress in Additive Manufacturing → Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-8078759","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":555464145,"identity":"9990a574-82ea-4d36-bd0b-9de43e9d4969","order_by":0,"name":"Rongxiao Ma","email":"","orcid":"","institution":"Qingdao University of Technology","correspondingAuthor":false,"prefix":"","firstName":"Rongxiao","middleName":"","lastName":"Ma","suffix":""},{"id":555464148,"identity":"a36b8351-e49c-498b-a054-8bafcd19a9de","order_by":1,"name":"Hongke Li","email":"","orcid":"","institution":"Qingdao University of 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12:02:22","extension":"html","order_by":16,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":96730,"visible":true,"origin":"","legend":"","description":"","filename":"earlyproof.html","url":"https://assets-eu.researchsquare.com/files/rs-8078759/v1/f610f269b4073b52cd18e9eb.html"},{"id":97524356,"identity":"0079abc4-71be-4c1b-a82c-532ea5364bb5","added_by":"auto","created_at":"2025-12-05 12:02:21","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":635109,"visible":true,"origin":"","legend":"\u003cp\u003ePreparation process and structure of the horseshoe-shaped double-layer stretchable circuit. (a) Printing of the PDMS base layer. (b) Printing of the stretchable silver paste. (c) Preparation of the copper film. (d) Printing of the PDMS encapsulation layer. (e) Horseshoe-shaped double-layer stretchable circuit.(f) Structure of the horseshoe-shaped double-layer stretchable circuit.(g)Horseshoe-shaped double-layer stretchable circuit element analysis diagram\u003c/p\u003e","description":"","filename":"floatimage1.png","url":"https://assets-eu.researchsquare.com/files/rs-8078759/v1/f6ca74ba970f8696ebe487df.png"},{"id":97524360,"identity":"06a7e2e8-40dc-4fe0-b170-ade70a09af6e","added_by":"auto","created_at":"2025-12-05 12:02:21","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":1782322,"visible":true,"origin":"","legend":"\u003cp\u003eSimulation analysis of the horseshoe-shaped double-layer stretchable circuit.\u003c/p\u003e\n\u003cp\u003e(a)Simulation Analysis of the CSR; (b)Simulation Diagram of Horseshoe Turn Count; (c)Influence of the CSR; (d)Influence of the Horseshoe Turn Count\u003c/p\u003e","description":"","filename":"floatimage2.png","url":"https://assets-eu.researchsquare.com/files/rs-8078759/v1/bd93a7f6f61d1e3cbc50d378.png"},{"id":97670761,"identity":"4a090084-9402-463d-a085-98870bae10d0","added_by":"auto","created_at":"2025-12-08 09:31:18","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":206064,"visible":true,"origin":"","legend":"\u003cp\u003eLaw research and demonstration of extruded 3D-printed stretchable silver paste. (a) Printing speed and printing air pressure. (b) Printing height.\u003c/p\u003e","description":"","filename":"floatimage3.png","url":"https://assets-eu.researchsquare.com/files/rs-8078759/v1/7b509e9519221811ae7b77f4.png"},{"id":97669885,"identity":"f1dcc936-4852-4881-81c2-6d59b90294a7","added_by":"auto","created_at":"2025-12-08 09:29:16","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":371693,"visible":true,"origin":"","legend":"\u003cp\u003eLaw research and demonstration of electrochemical 3D printing.(a) Working principle of the printhead (b) The influence of speed on printing effect under different line widths.(c) The influence of speed on printing effect under the line width of 0.4 mm. (d) The influence of voltage on printing effect under different line widths. (e) The influence of voltage on printing effect under the line width of 0.4 mm. (f)Cross-sectional morphology images at different voltages\u003c/p\u003e","description":"","filename":"floatimage4.png","url":"https://assets-eu.researchsquare.com/files/rs-8078759/v1/6dd17c0f5b99d3d832380361.png"},{"id":97524367,"identity":"c3cf7189-1c4f-41ff-8231-09ecbf79f28c","added_by":"auto","created_at":"2025-12-05 12:02:22","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":337001,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Influence of different shapes on conductivity during stretching. (b) Number of deposition layers. (c) Resistance changes during stretching with different numbers of layers. (d) Bending fatigue test. (e) Tensile fatigue test. (f) Small lamp under various working conditions.\u003c/p\u003e","description":"","filename":"floatimage5.png","url":"https://assets-eu.researchsquare.com/files/rs-8078759/v1/1d33c4ff4642940256b47159.png"},{"id":97670436,"identity":"2c8e7a0d-4915-4e3a-b008-9c0a41d13809","added_by":"auto","created_at":"2025-12-08 09:30:41","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":564346,"visible":true,"origin":"","legend":"\u003cp\u003eHeat therapy wristband. (a) Physical image of the heat therapy wristband. (b) Heating time. (c) tensile cycle test(d)Temperatures at different times.\u003c/p\u003e","description":"","filename":"floatimage6.png","url":"https://assets-eu.researchsquare.com/files/rs-8078759/v1/7a5ad61bfabaf8043189bd36.png"},{"id":102234017,"identity":"54368fb6-3968-4c65-bcbc-5cda984147d3","added_by":"auto","created_at":"2026-02-09 16:03:50","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":4835929,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-8078759/v1/d95910f6-c8d0-4caf-8b4c-1a2283b7fda7.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Fabrication of stretchable circuits with horseshoe-shaped double-layer composite conductive structures via direct ink writing and electrochemical 3D printing technologies","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eConventional electronic systems based on silicon or copper exhibit significant limitations when subjected to mechanical deformations such as bending, stretching, or folding. These systems are prone to functional failure or fracture and demonstrate considerable drawbacks in biocompatibility, conformability to curved surfaces, and dynamic adaptability[\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e]. For instance, in wearable health monitoring devices, rigid circuits cannot conformally adhere to the human skin, often causing discomfort or device failure. In the field of soft robotics, they severely restrict the range of motion and natural morphing capabilities. In contrast,stretchable circuits leverage a synergistic combination of materials and structural design to maintain stable electrical performance and mechanical reliability under deformation. As a result, they show broad application potential in areas such as stretchable electrodes [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e], displays[\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e, \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e], sensors[\u003cspan additionalcitationids=\"CR6\" citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e], soft robots [\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e, \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e], and electronic skin [\u003cspan additionalcitationids=\"CR11\" citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e]. Consequently, there is a pressing need to develop stretchable circuits that integrate high electrical conductivity, superior mechanical stretchability, and environmental stability.\u003c/p\u003e\u003cp\u003eCurrent design methodologies for fabricating stretchable circuits are primarily divided into two categories: structural film design and intrinsically stretchable materials. The fundamental principle of structural film design involves patterning specific geometric configurations (e.g., wavy, U-shaped, or serpentine)[\u003cspan additionalcitationids=\"CR14 CR15\" citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e] into rigid conductive films, enabling conductivity maintenance through reversible structural deformation rather than material elongation [\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e]. Khang [\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e] transferred flat silicon films onto pre-strained substrates and released the strain to induce in-plane buckling, forming wavy structures that granted stretchability to silicon, allowing devices to function at up to 20% tensile strain. Xu [\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e] deposited copper layers on polyimide (PI) films via electron-beam evaporation and fabricated rectangular stretchable interconnects using selective wet etching, achieving stable performance at 30% tensile strain. A lower-cost alternative employs mask printing to fill pre-fabricated grooves with metal pastes, which are then dried to form stretchable interconnects. However, this method offers limited structural customization due to dependence on the mask template. The second category employs intrinsically stretchable conductors, which are directly made from ductile conductive materials, including ionic liquids, liquid metals, and conductive composites. Ionic liquids, composed entirely of ions, are often used as conductive materials in flexible devices owing to their high electrical conductivity and low volatility[\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e]. However, their electrical resistance is highly susceptible to environmental factors like humidity, resulting in poor stability and limiting their broader application. Liquid metals [\u003cspan additionalcitationids=\"CR22\" citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e], another common class, combine metal-like conductivity with fluid-like deformability, making them promising for flexible electronics. For example, Dickey [\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e] and Kim [\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e] encapsulated gallium-based liquid metals within elastic microchannels to create stretchable conductors that maintained nearly constant resistance under tensile strains up to 60%. Conductive composites have also attracted extensive research interest. These are typically prepared by blending conductive fillers (e.g., silver nanowires, nanoparticles, or microflakes) with an elastic polymer matrix [\u003cspan additionalcitationids=\"CR27 CR28\" citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e]. Wu[\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e] mixed silver flakes with silicone rubber and methyl isobutyl ketone (MIBK) to formulate a stretchable conductive ink, which was screen-printed onto silicone rubber substrates to form conductors. Feng [\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e] synthesized silver nanoparticles via a redox method, followed by mechanical stirring and ultrasonic dispersion into liquid PDMS, forming a stretchable conductive adhesive (CA) that was subsequently sintered into conductive structures. Nevertheless, such composites exhibit a critical drawback: achieving high electrical conductivity requires a high volume fraction of conductive fillers, which severely compromises the elasticity of the material. This leads to significant resistance changes during stretching, a propensity for fracture[\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e, \u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e], and electrical conductivity that remains substantially lower than that of bulk metals.\u003c/p\u003e\u003cp\u003eAlthough significant progress has been made in the material and structural design of flexible stretchable circuits, stretchable circuit fabrication strategies still face significant challenges. On the one hand, structural film designs, though capable of imparting a degree of stretchability to conventional metals through specific geometries (e.g., wavy or serpentine structures), rely heavily on complex processes such as photolithography, vacuum deposition, and precision etching. These processes lead to high manufacturing costs, complexity, and limited structural customization capabilities. On the other hand, while liquid metals\u0026mdash;as intrinsically stretchable materials-possess excellent deformability and high conductivity, they suffer from poor adhesion to polymer substrates, fatigue-induced leakage of encapsulation structures, and surface oxidation leading to increased interfacial resistance. Conductive composites are hampered by an inherent trade-off between conductivity and stretchability: high filler loading required for high conductivity severely degrades material elasticity, resulting in dramatic resistance changes during stretching, susceptibility to fracture, and conductivity that is still far inferior to conventional metal wires.\u003c/p\u003e\u003cp\u003eIn this paper, A horseshoe-shaped double-layer stretchable circuit was fabricated through direct ink writing and electrochemical 3D printing technologies. First, a stretchable conductive silver paste was printed using direct ink writing and printed into a horseshoe-shaped seed layer on a PDMS substrate. Subsequently, a copper thin film was electrochemically deposited onto the surface of the seed layer using electrochemical 3D printing. Finally, the entire circuit was encapsulated with PDMS, yielding a double-layer, horseshoe-shaped, stretchable conductive structure. This manufacturing process facilitates the fabrication of metal-film-based stretchable wires by circumventing the high costs typically associated with complex techniques such as photolithography. The developed horseshoe-shaped, double-layer composite conductive structure exhibits a 2.86-fold improvement in electrical performance compared to single-layer silver paste circuits in the unstretched state, increasing to a 4.05-fold enhancement under 70% tensile strain. Furthermore, after 1000 stretching-bending cycles, the resistance change rate remained below 15%, demonstrating that the fabricated stretchable circuits possess not only excellent electrical performance but also robust long-term operational stability. This innovative approach not only overcomes the drawbacks of existing methods but also opens up new possibilities for the development of high - performance stretchable circuits in a wide range of applications.\u003c/p\u003e"},{"header":"2. Materials and methods","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e\u003ch2\u003e2.1. Materials\u003c/h2\u003e\u003cp\u003eThe copper plating solution (Copper sulfate pentahydrate (CuSO₄\u0026middot;5H₂O), sodium chloride (NaCl), and deionized water) was purchased from Sinopharm Chemical Reagent Co., Ltd., China. PDMS and curing agent (Sylgard 184) were both from Dow Corning. The stretchable conductive silver paste was from Shandong Jiahui Material Technology Co., Ltd.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec4\" class=\"Section2\"\u003e\u003ch2\u003e2.2. Methods\u003c/h2\u003e\u003cp\u003eThe surface morphology of the stretchable wires was characterized by an optical microscope (DXS510, Olympus, Japan). Bending, stretching, and twisting tests were conducted using a tensile-bending testing machine. Infrared thermal imaging was performed with a C5 from FLIR (USA). The PDMS was degassed under vacuum conditions using a DZF-6092 vacuum drying oven manufactured by Shanghai Yiheng Scientific Instruments Co., Ltd.\u003c/p\u003e\u003c/div\u003e"},{"header":"3. Results and Discussion","content":"\u003cdiv id=\"Sec6\" class=\"Section2\"\u003e\u003ch2\u003e3.1 Manufacturing of horseshoe-shaped double-layer stretchable circuits\u003c/h2\u003e\u003cp\u003eFigure \u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e shows a horseshoe-shaped double-layer stretchable circuit fabricated by direct ink writing (DIW) and electrochemical 3D printing techniques. First, according to the preset printing path, the corresponding parameters were configured, and the PDMS base layer was printed using the DIW process. After printing, the structure was thermally cured at 75℃for 15 minutes. Next, a stretchable conductive silver paste circuit was prepared on the PDMS substrate via DIW and cured at 150℃ for 60 minutes. Then, a copper sulfate solution was injected into the material storage cylinder, with the copper wire inside the cylinder connected to the positive electrode and the stretchable conductive silver paste connected to the negative electrode. Upon energization, the copper anode undergoes an oxidation reaction, releasing Cu\u0026sup2;⁺ions into the solution, while Cu\u0026sup2;⁺ ions in the electrolyte are reduced and deposited as a metallic copper film on the surface of the silver paste cathode. To avoid residual waste liquid contaminating electronic components or interfering with subsequent processes, a custom printhead with integrated waste liquid recycling capability was adopted (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ec). The printhead includes an inner liquid outlet nozzle and an outer liquid suction nozzle: the inner nozzle is used to eject electrolyte for electrochemical 3D printing, while the outer nozzle simultaneously aspirates waste liquid, thereby maintaining circuit integrity; the process can be repeated to achieve the desired copper film thickness. Finally, a PDMS encapsulation layer was printed on the deposited structure using DIW and cured under conditions similar to those for the base layer (80 ℃ for 15 minutes). The resulting horseshoe-shaped double-layer structure is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ef: the deposited copper film effectively improves the conductivity of the underlying conductive silver paste; its special horseshoe geometry can effectively accommodate strain through arc segment unfolding during stretching, thereby achieving stable current transmission. The cross-sectional view in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ef shows that the stretchable conductive silver paste and copper film are well bonded, mainly due to the tight adhesion formed between the polymer matrix in the silver paste and the copper surface through van der Waals forces, which prevents the copper film from easily peeling off during stretching or bending. Notably, even if the copper layer undergoes local fracture due to significant stretching, the underlying stretchable conductive silver paste can still form alternative conductive paths in the fractured regions, ensuring the circuit maintains stable electrical performance under large deformation.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec7\" class=\"Section2\"\u003e\u003ch2\u003e3.2 Simulation Research and Optimization of Horseshoe-shaped Wire Structure\u003c/h2\u003e\u003cp\u003eThis study numerically investigated the influence of varying curvature-to-spacing ratios(\u003cem\u003eCSR\u0026thinsp;=\u0026thinsp;D/R\u003c/em\u003e, \u003cem\u003eD\u003c/em\u003e is the distance between adjacent arcs, and \u003cem\u003eR\u003c/em\u003e is the radius of the arc) on tensile conductive performance based on a horseshoe-shaped wire geometry configuration using ANSYS software. The simulation results are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e. The simulation results show that under the same tensile rate, the larger the curvature spacing ratio, the lower the maximum stress value of the manufactured wire and the better the tensile performance (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ea). However, when the curvature spacing ratio exceeds 0.8, the maximum stress value increases, which is due to the proximity of the wires between two adjacent cycles, resulting in the superposition of the stress of adjacent wires. According to the stress simulation analysis, the horseshoe-shaped conductor structure with a curvature-to-spacing ratio of 0.8 exhibits optimal tensile performance. Furthermore, under identical base area, the effects of varying number of horseshoe-shaped interconnection wires on tensile properties were investigated at a consistent elongation rate (10%). The simulation results are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eb. The findings demonstrate that as the number of cycles increases, the maximum stress in horseshoe-shaped wires under equivalent elongation rates rises(Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ed). This occurs because the increased number of cycles leads to a larger wire area per unit substrate, making the substrate's pulling effect more pronounced during stretching. Additionally, the reduced spacing between adjacent wires with increasing cycle numbers results in stress superposition between neighboring wires, thereby amplifying the overall stress level. Considering that different application scenarios have different requirements on wire performance, it is necessary to comprehensively balance the relationship between the number of cycles and tensile performance in practical application. In addition, the stress change should be closely monitored to ensure the stability and reliability of the wire.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003e(a)Simulation Analysis of the CSR; (b)Simulation Diagram of Horseshoe Turn Count; (c)Influence of the CSR; (d)Influence of the Horseshoe Turn Count\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec8\" class=\"Section2\"\u003e\u003ch2\u003e3.3 Exploration on the Manufacturing Rules of Horseshoe-shaped Stretchable Conductors\u003c/h2\u003e\u003cp\u003eSince the line width was determined by the stretchable conductive silver paste, researchers conducted a systematic investigation into the influence of key process parameters\u0026mdash;such as printing speed, printing pressure, and printing height\u0026mdash;in ink direct-writing 3D printing to achieve precise control over printed circuit line widths. Under constant conditions, including a printing platform temperature of 30\u0026deg;C, a fixed printing height of 0.2 mm, and a nozzle inner diameter of 0.36 mm, the effects of printing speed and pressure on the line width of the stretchable conductive silver paste were specifically examined. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ea, variations in printing pressure (0.1\u0026ndash;0.4 MPa) significantly influenced line width across the printing speed range of 1\u0026ndash;6 mm/s. Specifically, continuous lines could not be formed at a pressure of 0.1 MPa when the printing speed exceeded 2 mm/s; however, stable line printing was achieved within the pressure range of 0.15\u0026ndash;0.4 MPa. With other parameters held constant, the line width decreased monotonically as printing speed increased. This trend is attributed to the reduced volume of paste deposited per unit length, which leads to lateral shrinkage of the printed line. In contrast, an increase in printing pressure resulted in wider lines due to enhanced paste extrusion per unit time. Furthermore, Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eb illustrates that increasing the printing height from 0.1 mm to 0.5 mm gradually reduced the line width from 570 \u0026micro;m to 212 \u0026micro;m. Beyond a height of 0.5 mm, continuous filament formation was no longer achievable. This behavior can be explained by the increased distance between the nozzle and the substrate, which allows gravitational forces to stretch the extruded paste, thereby reducing its cross-sectional area and yielding finer lines. In summary, stable and uniform printing of stretchable conductive silver paste lines can be achieved through careful optimization of printing pressure, height, and speed.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eSince residual waste solution remaining on the printing platform after electrochemical 3D printing can adversely affect subsequent printing processes, a printhead integrated with waste recovery functionality was developed. The configuration is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e(a). The electrode nozzle has an inner diameter (ID) of 100 \u0026micro;m and an outer diameter (OD) of 400 \u0026micro;m, whereas the suction nozzle features an ID of 720 \u0026micro;m. During electrochemical 3D printing, copper sulfate (CuSO₄) solution is delivered through the electrode nozzle, and the resulting waste solution is subsequently aspirated through the suction nozzle for effective recovery. Jetting pressure and suction pressure are recognized as critical parameters that influence printing quality, the amount of residual waste solution on the platform, and ultimately, the performance of printed circuits. Therefore, a systematic investigation was conducted to evaluate the effects of these two pressure parameters. An orthogonal array experiment was designed using combinations of jetting pressure (10 kPa, 15 kPa, 20 kPa) and suction pressure (-5 kPa, -10 kPa, -15 kPa) to identify the optimal conditions for electrochemical 3D printing using CuSO₄ solution. The experimental results are summarized in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. All experiments were performed under constant conditions: an initial circuit resistance (without electrochemical deposition) of 4.8 Ω, a printing voltage of 160 V, and a printing speed of 4 mm/s. The results show that, at a fixed suction pressure, increasing the jetting pressure leads to a significant improvement in circuit conductivity. However, when the jetting pressure reaches 20 kPa, the rate of conductivity enhancement diminishes. Concurrently, an increase in the volume of residual waste solution is observed. Balancing the improvement in conductivity against the accumulation of residual waste, the optimal process parameters were determined to be a jetting pressure of 15 kPa and a suction pressure of -15 kPa.\u003c/p\u003e\u003cp\u003eTo enhance the overall conductivity of printed circuits, the influence of key parameters in electrochemical 3D printing was systematically investigated, with particular focus on printing velocity and applied voltage. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e(b), resistance variations were measured across trace widths of 0.2 mm, 0.4 mm, and 0.6 mm at printing velocities ranging from 2 to 12 mm/s. Wider traces (0.6 mm) exhibit significantly improved conductivity after deposition compared to narrower ones (0.2 mm). This improvement is primarily attributed to cross-sectional flattening: at a constant conductor volume, increasing trace width expands the effective deposition area of copper films, thereby enhancing current-carrying capacity. Notably, resistance remains stable within the velocity range of 2\u0026ndash;4 mm/s. However, beyond 4 mm/s at a trace width of 0.4 mm, significant exposure of the underlying silver paste substrate is observed (inset in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ec), accompanied by a sharp increase in resistance and degraded print quality. This deterioration in conductivity is mainly due to insufficient electrolyte supply per unit length at higher velocities, which limits metal ion availability and reduces deposition efficiency. Considering both deposition consistency and electrical performance, a printing velocity of 4 mm/s was determined to be optimal. The printing voltage controls the electrochemical deposition kinetics by modulating the mobility of metal ions. Under otherwise identical conditions, the DC voltage was systematically varied from 100 V to 200 V to identify the optimal processing parameters. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e(f), cross-sectional morphologies were observed at 120 V, 160 V, and 200 V, respectively. Figure\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e(d) demonstrates that within the 100\u0026ndash;160 V range, conductivity steadily increases with rising voltage. Concurrently, the cross-sectional images in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ef(i\u0026ndash;ii) reveal a structural evolution from a loose, porous copper film to a dense, compact deposition layer. However, beyond 160 V, electrical resistance begins to rise. Mechanistic analysis indicates that below 160 V, higher voltage enhances current density, thereby accelerating the deposition rate. In contrast, above 160 V, ion diffusion becomes the rate-limiting step, and excessive voltage promotes dendritic growth, increases film porosity, and compromises layer densification, as evidenced in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ef(iii). Taking into account both deposition uniformity and electrical conductivity, 160 V is established as the optimal printing voltage.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003e\u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e\u003ccaption language=\"En\"\u003e\u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e\u003cdiv class=\"CaptionContent\"\u003e\u003cp\u003eLaw exploration of printing air pressure and extraction air pressure on resistance value and waste liquid residue amount.\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"5\"\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\"\u003e\u003cp\u003eExperimental groups\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c2\"\u003e\u003cp\u003ePrinting pressure (kPa)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c3\"\u003e\u003cp\u003eExtraction pressure (kPa)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c4\"\u003e\u003cp\u003eResistance (Ω)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c5\"\u003e\u003cp\u003eResidue amount of waste liquid(ml)\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e1\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e10\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e-5\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e3.3\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e3.5\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e2\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e10\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e-10\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e3.48\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e1.9\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e3\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e10\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e-15\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e3.72\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e0\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e4\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e15\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e-5\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e2.16\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e4.7\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e5\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e15\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e-10\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e2.22\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e1.5\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e6\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e15\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e-15\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e2.22\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e0\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e7\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e20\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e-5\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e2.13\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e8.9\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e8\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e20\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e-10\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e2.1\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e5.4\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e9\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e20\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e-15\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e2.13\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e3.4\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/colgroup\u003e\u003c/table\u003e\u003c/div\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec9\" class=\"Section2\"\u003e\u003ch2\u003e3.4 Performance testing and application of stretchable wires\u003c/h2\u003e\u003cp\u003eTo evaluate the performance advantages of the horseshoe-shaped structure in stretchable circuit design, tensile tests were conducted on four circuits with distinct geometric configurations. By measuring the initial resistance (R₀) and the resistance (R) during stretching, the relative resistance change (R/R₀) was calculated to assess the stretchability of each circuit structure, as illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ea. The results show that the linear circuit exhibits the poorest conductivity performance under strain. At 50% strain, its resistance increases sharply; at 80% strain, the circuit fails completely. In contrast, the sinusoidal circuit experiences an 11.2-fold increase in resistance when stretched to 80% strain. Both U-shaped and horseshoe-shaped circuits maintain stable conductivity up to 60% strain. However, at 80% strain, the resistance of the U-shaped circuit rises to 3.9 times its initial value, while that of the horseshoe-shaped circuit increases by only 1.06 times. This superior performance is attributed to the unique geometry of the horseshoe structure, which effectively redistributes tensile stress and minimizes material deformation during stretching, thereby preserving electrical continuity. Furthermore, increasing the number of deposited layers within a fixed unit length enlarges the cross-sectional area of the copper traces, enhancing overall electrical conductivity. However, excessive layer thickness may lead to cracking during stretching, thereby compromising conductivity under strain. Consequently, the influence of the number of deposited layers on both electrical conductivity and mechanical stretchability was systematically investigated, as illustrated in Figs.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eb and \u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ec. Figure\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e demonstrates how varying the number of deposition layers affects the electrical performance of the circuit. When the number of deposition layers reaches seven, the conductivity increases threefold compared to the baseline before copper deposition. However, as the number of layers increases, so does the overall rigidity of the circuit, which restricts its deformation capacity and increases the likelihood of brittle fracture in the copper layer during stretching, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ec. When the layer count increased to five, although the initial resistance was relatively low, it rose to 4.6 times its original value at a stretch ratio of 70%. In contrast, with only three deposited layers, the resistance increased by just 1.7 Ω when the circuit was stretched to 70%. Considering the trade-off between conductivity and stretch-induced resistance change, the optimal number of copper deposition layers is determined to be between one and three. Furthermore, the resistance variations (R/R₀) of the fabricated horseshoe-shaped silver-copper bilayer stretchable circuits were evaluated under 1000 cycles of cyclic bending (bending radius: 10 mm) and stretching (strain: 20%), as shown in Figs.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ed and \u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ee. The results indicate that after 1000 cycles, the conductivity variation remains within 15%, demonstrating excellent electromechanical stability. In contrast, the conductivity of the straight circuit deteriorates significantly after the same number of stretching cycles. This performance disparity can be attributed to the curved geometry of the horseshoe-shaped circuit, which effectively absorbs external mechanical stress during deformation, mitigates localized stress concentration, and suppresses crack initiation, thereby enhancing structural durability and operational lifespan. Furthermore, Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ef illustrates the LED brightness states of the fabricated horseshoe-shaped silver-copper bilayer stretchable circuit under four distinct conditions: natural, bent, stretched, and twisted. The results show that the LEDs maintain stable light emission across all conditions, demonstrating that the stretchable circuit retains reliable electrical performance even under complex and multimodal mechanical deformations. The uniform deposition of copper and silver layers ensured reliable electrical conductivity, while the horseshoe geometry prevented cracking or delamination during stretching. This scalability supports the integration of stretchable circuits into larger electronic systems, such as flexible displays or smart textiles, paving the way for next-generation flexible electronics.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eAs a demonstration, by leveraging the integrated direct ink writing and electrochemical 3D printing technology developed in this study, along with optimized process parameters, a thermotherapy wristband measuring 150 mm \u0026times; 60 mm \u0026times; 1.5 mm was successfully fabricated. The device features a mesh-structured heating coil connected through bilateral horseshoe-shaped circuits, with a centrally integrated indicator light to display the operational status. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ea, the wristband is depicted before and after electrochemical 3D printing (i.e., copper deposition). During operation, connecting a 4.5 V power supply to the circuit terminals closes the circuit, resulting in the illumination of the indicator light and the generation of therapeutic heat by the coil. To evaluate the effect of the deposited copper layer, electrothermal performance was compared before and after electrochemical 3D printing, revealing a significant enhancement in heat output following the treatment (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003eb). Specifically, the pre-deposition wristband required 260 seconds to reach its peak temperature of 45.5\u0026deg;C, whereas the post-deposition wristband achieved a higher peak temperature of 46.8\u0026deg;C in just 140 seconds. Detailed thermal images captured at 30, 60, 90, and 120 seconds during heating after copper deposition are presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ed(I)\u0026ndash;(IV). The observed enhancement is attributed to intensified Joule heating caused by reduced electrical resistance\u0026mdash;under constant voltage and duration, lower resistance leads to greater heat generation, thereby enhancing therapeutic efficacy. Concurrently, the wristband underwent tensile cycling tests under 30% strain (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ec), exhibiting negligible resistance change after 1,000 cycles. This indicates that the structure incorporating stretchable conductors possesses excellent fatigue resistance. Collectively, these results demonstrate that stretchable circuits fabricated using this method not only achieve improved electrical conductivity but also exhibit superior anti-fatigue performance. Overall, the successful fabrication and evaluation of the thermotherapy wristband validate the potential of the integrated direct ink writing and electrochemical 3D printing technology in producing high-performance stretchable circuits for flexible electronics applications.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e"},{"header":"4. Conclusion","content":"\u003cp\u003eIn this study, a manufacturing method for stretchable circuits featuring a horseshoe-shaped, double-layer composite structure was proposed. Using polydimethylsiloxane (PDMS) as the substrate, a horseshoe-shaped, double-layer stretchable circuit is fabricated via a combination of extrusion-based and electrochemical 3D printing techniques. The inner layer consists of a stretchable conductive silver paste, while the outer layer comprises a copper film, forming a dual-conductive pathway. The horseshoe geometry enhances structural stability during mechanical deformation. Superior conductivity is achieved through synergistic conduction in the composite channels. Tensile tests demonstrate that, under identical strain conditions, the horseshoe configuration exhibits better electrical performance than alternative geometries. Compared to a single-layer stretchable silver paste circuit, the double-layer circuit exhibits 2.86 times higher conductivity in the unstretched state, 3.12 times at 50% strain, and 4.05 times at 70% strain. Furthermore, the resistance remains nearly unchanged after 1,000 cycles of stretching and bending, confirming the excellent electromechanical reliability of the double-layer design. As a demonstration, an electrothermal heating therapy wristband based on a horseshoe-shaped circuit structure is fabricated. Experimental results indicate that the wristband incorporating a deposited copper layer reduces the time required to reach therapeutic temperature by nearly 50% compared to the non-deposited counterpart, thereby significantly enhancing heating efficiency. With ongoing advancements in materials and structural design of stretchable circuits, highly flexible and highly integrated multifunctional stretchable electronic systems are expected to play an increasingly critical role in applications such as smart wearables, healthcare monitoring, and flexible displays, ultimately reshaping the future landscape of the flexible electronics industry.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eDeclaration of Competing Interest\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eWu W (2019) Stretchable electronics: functional materials, fabrication strategies and applications. 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ACS Appl Mater Interfaces 7:15214\u0026ndash;15222\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"double-layer stretchable conductor, structural design, extrusion-based 3D printing, electrochemical 3D printing","lastPublishedDoi":"10.21203/rs.3.rs-8078759/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-8078759/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eFlexible, stretchable electronics, which possess the remarkable capability to adapt to various deformations, including bending, twisting, and stretching, exhibit extensive potential applications in areas such as flexible sensing and wearable devices. Among these, the stretchable circuit, serving as the core structure that connects various components, plays a pivotal role in determining the performance of the entire flexible and stretchable electronic system. However, there exists an inherent trade-off relationship between the electrical performance and the stretchability of stretchable circuits. In this study, a horseshoe-shaped double-layer composite conductor structure was designed for the fabrication of stretchable circuits using direct writing technology and electrochemical 3D printing technology. This double-layer horseshoe-shaped geometric structure not only improves the electrical performance of the wir but also ensures its mechanical stability during the stretching process. Compared to single-structure stretchable circuits, the stretchable circuits featuring a horseshoe-shaped double-layer composite conductive structure exhibit an enhancement in electrical performance by 2.86 times in the unstretched state and by 4.05 times at a 70% stretch rate. Furthermore, after 1000 cycles of stretching and bending, the resistance change rate remains below 15%. The fabricated stretchable circuits not only maintain superior electrical performance but also demonstrate remarkable stretchability and durability, thereby offering a novel approach for the development of flexible stretchable circuits.\u003c/p\u003e","manuscriptTitle":"Fabrication of stretchable circuits with horseshoe-shaped double-layer composite conductive structures via direct ink writing and electrochemical 3D printing technologies","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-12-05 12:02:17","doi":"10.21203/rs.3.rs-8078759/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"d35d6d6c-c9b8-4dc1-99f8-877d1dab8fa3","owner":[],"postedDate":"December 5th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[],"tags":[],"updatedAt":"2026-02-09T16:01:03+00:00","versionOfRecord":{"articleIdentity":"rs-8078759","link":"https://doi.org/10.1007/s40964-025-01521-7","journal":{"identity":"progress-in-additive-manufacturing","isVorOnly":false,"title":"Progress in Additive Manufacturing"},"publishedOn":"2026-02-05 15:57:42","publishedOnDateReadable":"February 5th, 2026"},"versionCreatedAt":"2025-12-05 12:02:17","video":"","vorDoi":"10.1007/s40964-025-01521-7","vorDoiUrl":"https://doi.org/10.1007/s40964-025-01521-7","workflowStages":[]},"version":"v1","identity":"rs-8078759","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-8078759","identity":"rs-8078759","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}