Fully Stretchable Hydrovoltaic Cells Based on Winding-Locked Double-Helical Carbon Nanotube Fibers

Fully Stretchable Hydrovoltaic Cells Based on Winding-Locked Double-Helical Carbon Nanotube Fibers | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Article Fully Stretchable Hydrovoltaic Cells Based on Winding-Locked Double-Helical Carbon Nanotube Fibers Wonkyeong Son, Jae Myeong Lee, Hyunji Seo, Sung Beom Cho, Sungwoo Chun, and 1 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-6608314/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 18 Nov, 2025 Read the published version in npj Flexible Electronics → Version 1 posted 9 You are reading this latest preprint version Abstract Hydrovoltaic power generators that convert water-nanomaterial interactions into electricity represent a promising route for sustainable energy harvesting. However, many previous studies have relied upon conventional two-dimensional planar designs with rigid, non-stretchable materials, typically operating in environments that require continuous water flow or specially designed ionic solutions. These stringent conditions restrict their practical applications, particularly in flexible and wearable systems. Hence, the present study introduces a fully stretchable hydrovoltaic cell (FSHC) that features a parallel double-helix configuration in which neat and oxidized carbon nanotube (CNT) fibers are spirally wound around an elastomeric core. This winding-locked double-helix architecture ensures robust mechanical integrity and stable electrical performance under large deformations. When immersed in quiescent deionized water, the FSHC generates an open-circuit voltage of ~ 0.31 V and a short-circuit current of ~ 2.24 µA/cm 2 . Notably, the FSHC maintains consistent performance under 200% tensile strain. To demonstrate its potential in wearable applications, the FSHC is integrated into a fabric glove. Moreover, multiple FSHCs connected in series or parallel generate sufficient power to drive a twisted CNT fiber-based torsional actuator, suggesting a pathway toward self-powered actuation systems. This study offers a deformable hydrovoltaic platform for fiber-based energy harvesters, broadening their applicability in wearable electronics and autonomous actuation. Physical sciences/Energy science and technology Physical sciences/Engineering Stretchable Hydrovoltaic Double-helix Carbon nanotubes Electrochemical Figures Figure 1 Figure 2 Figure 3 Figure 4 Introduction Water, which accounts for approximately 71% of the Earth's surface area, holds considerable energy in various forms that dominate energy transfer in many natural phenomena 1,2 . Converting the mechanical and chemical energy associated with water into electricity has become increasingly important for sustainable and eco-friendly power generation 3–5 . Hydrovoltaic technologies 6,7 , which leverage the interfacial interactions between water and nanostructured materials, have emerged as a promising approach for harvesting energy from various forms of water such as flows 8–10 , waves 11–13 , droplets 14–19 , and moisture 20–24 . To date, a variety of strategies based on different mechanisms, including electrokinetic effects, alternating potentials, and ionic diffusion, have been explored. For instance, in 2014, Guo et al. discovered that a single water droplet interacting with graphene could generate an electrical output, thus revealing the electrokinetic potential at the liquid-solid interface 15 . In 2015, Qu et al. demonstrated that a graphene oxide framework sandwiched between two grid-patterned gold electrodes was capable of producing intermittent electrical signals through moisture diffusion 20 . In 2017, Guo et al. reported that natural water evaporation could be harnessed using a porous nanostructured graphite film to achieve continuous power generation 25 . More recently, in 2023, Li et al. significantly enhanced droplet-based hydrovoltaics, achieving potentials over 1200 V by optimizing electric double-layer dynamics at interfaces 26 . Building upon these advancements, Yuan et al. in 2024 introduced a hermetic hydrovoltaic cell with internal water circulation driven by temperature fluctuations, achieving stable, long-term electricity generation without water loss 27 . Despite the significant progress achieved in these pioneering studies, current hydrovoltaic devices still face two critical challenges that hinder their further potential and practical applicability. First, their electrical output inherently depends on either continuous charge carrier movement or steady ionic concentration gradients, meaning that any interruption or equilibrium in these conditions rapidly terminates electricity generation. Second, most previously reported hydrovoltaic devices are typically composed of rigid or non-stretchable substrates and functional materials, limiting their adaptability to dynamic and mechanically active environments. In this regard, developing hydrovoltaic devices capable of reliably generating electricity from the environment with few constraints and maintaining stable electrical performance under mechanical deformation remains an urgent challenge for practical and sustainable applications. To address these limitations, a yarn-based flexible carbon:water device that converts chemical energy in the nanotube yarn into electrical form has been reported, enabling water-based electricity generation without requiring flowing water or ionic solutions 28 . Such demonstrations have opened up new possibilities for designing advanced hydrovoltaic devices with exceptional flexibility and adaptability, thereby enabling their integration into practical and versatile systems. In this work, we propose a fully stretchable hydrovoltaic cell (FSHC) with a winding-locked double-helix design, wherein two carbon nanotube (CNT) fibers (neat and oxidized) electrochemically interact with water molecules. Specifically, the wettability difference between the hydrophobic neat CNT (NCNT) fiber and the hydrophilic oxidized CNT (OCNT) fiber induces asymmetric charge interactions at the CNT–water interfaces, establishing a potential difference. To achieve structural stretchability, these CNT fiber electrodes are spirally wound around an elastomeric core that is prestrained to 200% of its original length, maintaining a constant gap between them. Upon release of the prestrain, the elastomer—which had been stretched and thinned—returns to its original dimensions via radial recovery governed by Poisson’s ratio. During this relaxation, the double-helix structure becomes self-locked to form an intimate contact interface between the CNT fibers and the core. This simple and reliable assembly process eliminates the need for an additional separator. The resulting winding-locked architecture not only ensures robust mechanical integrity but also provides exceptional durability under significant mechanical deformations. When brought into contact with quiescent deionized water, the FSHC delivers a high electrical output with an open-circuit voltage of ~0.31 V and a short-circuit current of ~2.24 µA/cm 2 . More importantly, owing to the rational double-helix design and winding-lock effect, the FSHC demonstrates remarkable mechanical robustness, withstanding repeated stretching up to 200% strain without notable degradation in electrical performance. To the best of our knowledge, such a high degree of stretchability has not been previously reported in fiber or yarn-based hydrovoltaic devices, clearly validating the unique advantages of our winding-locked architecture. To further examine its practical applicability, the FSHC fibers were sewn onto a glove, confirming their feasibility for integration into textile-based wearable systems. Moreover, by connecting multiple FSHCs in series or parallel as an energy-harvesting module, the enhanced power output was sufficient to directly drive fiber-type electrochemical torsional actuators consisting of twisted CNT fibers in an electrolyte. This study provides an effective strategy for fabricating stretchable hydrovoltaic devices suitable for wearable electronics, significantly broadening their potential applications in self-powered actuation systems. Results Fabrication and morphological information of the FSHC As shown in Fig. 1a and Supplementary Fig. 1 , the stepwise fabrication of the FSHC involves the following three key steps: (i) prestrain application, (ii) electrode winding, and (iii) strain relaxation. To construct the stretchable hydrovoltaic architecture, a commercially available silicone elastomer was molded into a cylindrical shape using a needle-tip template, as detailed in the Experimental section. This elastomeric core serves as the flexible substrate, providing mechanical compliance and elasticity under repeated deformations. In the first step, the as-fabricated elastomeric core (length: 20 mm, diameter: 0.6 mm) was stretched uniaxially to 200% of its original length. This prestrain application was essential for ensuring tight integration of the CNT fiber electrodes with the elastomeric core after strain relaxation. By maintaining the elastomer in a highly elongated state, the subsequent CNT fiber winding process ensured that the fibers were securely positioned around the core without unintended displacement or detachment. In the second step, two CNT fibers were tightly wound around the elastomeric core in a parallel configuration. These fibers were fabricated by twisting sheets of aligned multiwalled CNTs drawn from forests 29 . Then, ethanol was repeatedly applied to the elastomer surface to densify the fibers during evaporation 30–32 . In the third step, strain relaxation was induced by releasing the prestrain in the elastomeric core, allowing it to contract back to its original dimensions. This contraction, governed by Poisson’s effect, generated a radial expansion that effectively secured the CNT fibers onto the elastomeric core to form a self-locking structure. Due to ethanol densification, the adhesion between the CNT fibers and elastomer core was strong, and no noticeable detachment was observed during tensile strain relaxation. As a result, the CNT fibers became tightly secured onto the elastomeric core without the need for additional adhesives. This winding-locked configuration not only enhances the mechanical robustness of the FSHC but also ensures a stable electrode-substrate interface, thus preventing fiber slippage or delamination under mechanical deformation. When the FSHC is exposed to water (inset, Fig. 1a ), a potential difference is generated between the two CNT fiber electrodes, thus leading to electricity generation. This phenomenon arises from the distinct surface properties of the NCNT and OCNT fibers, which lead to asymmetric interactions with water molecules. Specifically, the NCNT fiber remains hydrophobic, repelling water and minimizing direct charge transfer at the solid–liquid interface. In contrast, the OCNT fiber exhibits strong hydrophilicity due to the presence of oxygen-containing functional groups, thereby allowing enhanced water adsorption. This asymmetric wettability between the two electrodes induces different charge-transfer interactions, thus facilitating hydrovoltaic energy conversion 28 . First, these differences are elucidated by the SEM images in Supplementary Fig. 2 . There, the NCNT fiber exhibits a smooth and compact structure ( Supplementary Fig. 2a and c ), whereas the OCNT fiber displays a rougher and more disordered morphology ( Supplementary Fig. 2b and d ). This structural difference originates from the presence of oxygen-containing functional groups, which disrupt the graphitic stacking of the CNT bundles. Moreover, the successful oxidation of the CNT fiber to obtain the OCNT is confirmed by the XPS results in Supplementary Fig. 3 . Notably, the O/C ratio of the OCNT is seen to have increased to 0.27, compared to 0.11 for the NCNT, thereby indicating a significant incorporation of oxygen-containing functional groups. This chemical modification alters the surface charge distribution of the OCNT, enhancing its ability to interact with water molecules and facilitating charge transfer at the electrode-water interface. In addition, the Raman spectra of the two CNT fibers are presented in Supplementary Fig. 4 . There, the intensity ratio between the D and G bands ( I D / I G ), which represents the degree of atomic defects in the CNT structure, is seen to have increased from 0.52 for the NCNT to 0.82 for the OCNT, thus supporting the abovementioned XPS results. Also, the mechanical properties of the NCNT and OCNT fibers are revealed by the stress-strain curves in Supplementary Fig. 5 , where the tensile strength of the OCNT fibers is seen to have decreased by 25.8% (109.5 MPa) compared to that of the NCNT fibers (147.5 MPa). This can be attributed to the partial disruption of sp²-hybridized bonds in the CNT network caused by oxidation-induced defects. Despite this reduction in mechanical strength, the introduction of oxygen functionalities facilitates the formation of intermolecular hydrogen bonds, thus leading to a slight increase in the maximum strain from 5.1% to 5.5% 33,34 . The parallel double-helix configuration of the CNT fiber electrodes in the FSHC is revealed by the optical micrographs in Fig. 1b . This uniform helical arrangement ensures a well-defined electrode alignment, preserving both structural stability and mechanical flexibility. The precisely controlled fiber winding process minimizes any irregularities that could otherwise compromise the device performance. A magnified view of the electrode arrangement is presented in Fig. 1c, indicating the presence of a well-defined and consistent interstitial spacing between the helical CNT fibers. This is essential for preventing electrical shorting while simultaneously facilitating efficient charge separation, both of which are crucial for stable hydrovoltaic energy conversion. The uniformity of this spacing also suggests a high degree of reproducibility in the fabrication process. The intimate contact between the CNT fibers and elastomeric core after pre-strain relaxation is highlighted in Fig. 1d , where the CNT electrodes appear embedded in the elastomer surface. This observation confirms the effectiveness of the winding-lock mechanism, in which the radial expansion of the elastomer during strain relaxation enhances the interfacial adhesion. As a result, the CNT fibers are securely anchored to the core, thus preventing slippage or delamination and significantly improving the mechanical robustness of the FSHC under repeated deformations. With its uniform and precisely organized parallel double-helix configuration, the as-fabricated FSHC (10 cm in length) exhibits good structural stability, maintaining a moderately straight profile in the free-standing state without the need for additional support ( Fig. 1e ). Moreover, the inset shows that the structure remains intact, without any signs of entanglement or deformation, even in the Fermat spiral configuration. Moreover, as shown in Fig. 1f , the FSHC exhibits excellent elastic recoverability under 200% strain. The cyclic loading-unloading curve (inset) demonstrates that the FSHC returns to its original state with negligible hysteresis even after 100 cycles. Further, the time-dependent strain and resistance variations of the FSHC under cyclic stretching and releasing at 200% strain are shown in Fig. 1g . Here, the resistance remains highly stable throughout the repeated deformation cycles, indicating excellent electromechanical durability. This stable resistance response under large mechanical deformation is a critical requirement for practical applications in stretchable and wearable electronics. Electrical output performance of the nonstrained FSHC Based on the above considerations, the stable voltage output of the FSHC due to asymmetric charge transfer at the CNT–water interface upon immersion is demonstrated in this section. As shown in Fig. 2a , when the OCNT (as the positive electrode) and NCNT (as the negative electrode) are connected to a digital source meter, the FSHC generates an open-circuit voltage of approximately 0.31 V, which remains stable for up to 5000 s in water. Further, as shown in Supplementary Fig. 6 , a short-circuit current is detected when the cell is electrically connected in water. Meanwhile, the voltage distribution exhibits a narrow window centered around 0.30 V ( Fig. 2b ), thereby indicating the high reproducibility of the hydrovoltaic effect. Furthermore, even under partial immersion in deionized water ( Supplementary Fig. 7 ), the generated voltage remains almost identical to that observed under full immersion; in this case, water evaporation has no observable effect on electricity generation. Further, the proposed mechanism underlying the hydrovoltaic effect in the FSHC is confirmed by comparing its voltage output in various electrolyte solutions with differing pH levels ( Supplementary Fig. 8 ). Here, the FSHC exhibits distinct voltage responses depending on the electrolyte composition, which suggests that ion adsorption and interfacial charge transfer at the CNT–water interface are key factors in electricity generation. Specifically, the voltage is highest in 0.1 M HCl, followed by 0.1 M Na 2 SO 4 and 0.1 M KOH, thereby indicating a strong dependence on the proton concentration. This suggests that the availability of protons and their interactions with the CNT surface significantly influence the hydrovoltaic performance. In Fig. 2c , the cell’s operation is examined by studying its electricity generation as a function of the linear helix density and immersion speed. Notably, while the voltage output remains nearly unchanged, the short-circuit current is found to increase with the increase in linear helix density, thereby suggesting that the current is affected by the active surface area. This increase can be attributed to the extended CNT fiber length in higher-density configurations, which provides a larger effective contact area with water. The FSHC’s performance is also unaffected by changes in immersion speed ( Supplementary Fig. 9 ), thereby indicating that its operation differs from that of previously reported electrokinetic or fluidic devices. In addition, the dependence of voltage generation on the presence of asymmetric charge interactions is further evidenced in Fig. 2d , where the use of symmetric electrode configurations with identical surface properties ( i.e., NCNT/NCNT or OCNT/OCNT) results in a negligible voltage output. This demonstrates that differences in surface functionalization are critical for preserving charge separation and sustaining hydrovoltaic energy conversion. Indeed, the role of electrode asymmetry is confirmed by the polarity-switching test results in Fig. 2e . Thus, when the OCNT fiber is connected to the positive terminal, a positive voltage is observed (blue curve), whereas reversing the connections produces an inverted voltage (red curve), thereby confirming the formation of a stable potential difference between the two electrodes. These findings show that voltage generation is governed by asymmetric charge interactions at the CNT–water interface, rather than by external ionic concentration gradients or electrokinetic effects. The electrical power density of the FSHC is demonstrated in Fig. 2f , where the voltage is seen to increase with the increase in load resistance (100 Ω to 10 MΩ), reaching a peak power density of 8.5 μW/cm 2 at 1 MΩ. Finally, to evaluate the cycling stability of the FSHC, a charge-discharge test was performed using an external circuit in which a switch was periodically toggled to control discharge and autonomous recharge ( Fig. 2g ). As shown in Fig. 2h , the FSHC exhibits a self-replenishing voltage after each discharge event and maintains stable charge-discharge behavior across multiple cycles, thus confirming its robustness under continuous operation. Moreover, a closer look at an individual cycle ( Fig. 2i ) underscores the reproducibility of the charge-discharge process, demonstrating that the FSHC can sustain long-term performance without significant degradation. The effects of tensile deformation on the FSHC performance Hydrovoltaic devices that are intended for use in stretchable and deformable electronics must maintain a stable electrical performance under mechanical strain. Unlike conventional fiber-based energy systems, which often suffer from electrical degradation due to mechanical failure, the double-helix configuration of the FSHC provides a unique structural advantage. By employing a winding-locked CNT fiber network, the FSHC can accommodate large tensile deformations while preserving its conductive pathways. However, despite the widespread adoption of helical structures in stretchable electronics, the precise structural changes that occur during elongation remain largely unexplored. Hence, this section presents a detailed structural analysis of the FSHC under mechanical strain in order to demonstrate how its helical architecture adapts to stretching without compromising its electrical output. First, the optical micrographs of the FSHC under increasing tensile strain (0–200%) are presented in Fig. 3a . As the strain increases, the helical CNT fibers are seen to gradually reorient themselves along the stretching direction while maintaining a uniform distribution, with no noticeable detachment or misalignment. This suggests that the double-helix structure undergoes a systematic geometric transformation to accommodate strain without structural failure. Further, these changes are quantitatively analyzed by adopting an “unwound” model of a single helical turn, as shown in Fig. 3b . In this approach, each turn is conceptually unfolded into a straight segment, thereby enabling a mathematical description of the effective fiber length under varying strain levels. This transformation is governed by three key geometric parameters: (i) the thickness ( t ) of the elastomeric core, (ii) the bias angle ( θ ) of the CNT windings, and (iii) the distance ( d ) between the two CNT fiber electrodes (NCNT and OCNT). As shown in Supplementary Fig. 10 , the variations in these experimentally measured parameters according to the increase in tensile strain reveal a systematic structural adaptation. First, the thickness of the elastomeric core decreases due to Poisson’s effect, which induces lateral contraction during uniaxial stretching and influences the winding geometry. Second, the bias angle decreases, thereby indicating progressive alignment of the CNT fibers with the stretching direction. Lastly, the inter-fiber spacing increases as the CNT fibers move farther apart along the axial direction, thereby accommodating the applied strain while preserving the helical configuration. As a result, the calculated unwound fiber length remains nearly constant across all strain levels, with only minor deviations ( Fig. 3c ). This highlights how the double-helix structure efficiently distributes mechanical stress and prevents excessive elongation of individual CNT fibers. Consequently, the electrical resistance exhibits negligible variation, thus ensuring continuous electrical connectivity despite large deformations. These findings validate the strain-adaptive nature of the FSHC, demonstrating that the double-helix configuration preserves both mechanical resilience and electrical integrity under tensile strain. Next, the influence of the strain-adaptive nature of the double-helix structure on the electrochemical performance of the FSHC is examined. Because hydrovoltaic energy generation relies on interfacial charge interactions at the CNT–water interface, any mechanical deformation that compromises the conductive pathways or alters the electrode-electrolyte interactions could degrade the performance. Hence, the intrinsic electrochemical properties of the individual CNT fiber electrodes were first elucidated by CV measurements in a three-electrode system with an aqueous electrolyte (0.1 M Na 2 SO 4 ). As shown in Supplementary Fig. 11 , the OCNT fiber exhibits a 226.1 times larger CV area than that of the NCNT fiber, thereby indicating markedly enhanced surface electrochemical activity due to the presence of oxygen-containing functional groups 35–38 . The electrochemical performance of the fully assembled FSHC device is then demonstrated in its two-electrode configuration, with scan rates ranging from 100 to 1000 mV/s, in Supplementary Fig. 12 . Here, the observed rectangular CV curves without distinct Faradaic redox peaks are consistent with the energy storage capacity of the CNT-induced electric double-layer capacitance and the pseudocapacitance derived from oxygen-containing functional groups. The effect of mechanical strain on the electrochemical response of the FSHC is then evaluated by examining the CV curves obtained at different strain levels (0%, 50%, 100%, and 200%) ( Fig. 3d ). As shown in Supplementary Fig. 13 , the corresponding capacitance remains almost unchanged under all strain conditions, thereby indicating that the FSHC maintains stable electrochemical behavior despite mechanical deformation. The preservation of CV characteristics implies that the structural adaptability of the helical CNT network effectively prevents degradation of the charge-transfer dynamics. Further, the effect of tensile strain on the electrical output of the FSHC is evaluated by measuring the voltage under various static elongations in Fig. 3e . Here, the voltage remains stable at all strain levels, thereby demonstrating that the double-helix structure accommodates mechanical deformation without disrupting the electrode-water interface. Moreover, the inset of Fig. 3e confirms a stable performance over multiple stretching cycles at 200% strain. Finally, the durability of the FSHC under dynamic deformation was examined by cyclic stretching and releasing at strain rates of 2, 4, and 6%/s. As shown in Fig. 3f , the voltage output remains consistent regardless of the strain rate, thereby indicating that the hydrovoltaic performance is independent of the stretching speed. Moreover, no performance degradation is observed over repeated cycles, thereby confirming both the mechanical resilience and electrical stability of the FSHC. Taken together, these results demonstrate that the FSHC effectively maintains its structural and electrical integrity under both static and dynamic strains, thus making it a promising candidate for stretchable hydrovoltaic energy systems. Fabric integration of the FSHC and its application in self-powered actuation In this section, the feasibility of integrating the FSHC into a fabric-based system is investigated. Given its fiber-based architecture, the FSHC can be incorporated into textiles without compromising the structural integrity, which makes it a promising candidate for applications that require mechanical durability and electrical stability. Therefore, a 10 cm-long FSHC was incorporated into a commercially available fabric glove ( Fig. 4a ). The high-resolution images obtained before and after applying 50% tensile strain ( Fig. 4b ) confirm that the FSHC maintains its structural integrity without any visible damage or delamination. This demonstrates the mechanical robustness of the FSHC and its suitability for seamless integration into fabric. Moreover, electrical performance under mechanical strain was evaluated by monitoring the voltage response while applying static stretching up to 200%. The results in Fig. 4c indicate that the output voltage remains stable across all strain levels and is fully recovered upon relaxation, thereby confirming the mechanical resilience and ability of the FSHC to sustain hydrovoltaic energy generation under deformation. In addition, the feasibility of utilizing the FSHC as a power source for a self-powered actuation system was explored. Given that the CNT fibers used in the FSHC are intrinsically twisted, they share structural similarities with twisted CNT actuators, which are known to exhibit untwisting behavior upon electrochemical volumetric expansion 39 . This property enables electrically driven torsional actuation when sufficient voltage is applied. Hence, a self-powered actuation system was designed in which the FSHC directly supplies energy to a twisted CNT fiber-based actuator ( Fig. 4d ). To provide adequate electrical input for actuation, the output voltage and current of the FSHC were increased by connecting multiple cells in series and parallel, respectively. As shown in Fig. 4e , the output voltage increases proportionally with the number of FSHCs connected in series, reaching 1.2 V with four units. Meanwhile, parallel connections effectively amplify the output current, ensuring sufficient electrical stimulation for actuation ( Fig. 4f ). As shown in Fig. 4g , this approach was successfully used to demonstrate the self-powered torsional actuation of a twisted CNT fiber tethered at both ends. For this experiment, ten FSHCs were connected in series to generate an output voltage of approximately 3 V, which is sufficient to drive the actuation. Upon activation, the fiber exhibits a rapid untwisting motion, leading to the rotation of an attached paddle. The actuation is fully reversible, with the fiber returning to its initial state after power removal. This proof-of-concept demonstration highlights the potential of the FSHC as a scalable, sustainable energy source for autonomous, electrochemically driven actuation systems. Discussion Herein, a FSHC with a parallel double-helix structure was developed, wherein neat and oxidized CNT fibers (designated as NCNT and OCNT, respectively) were helically wound around an elastomeric core. The self-locking helical configuration of the CNT fibers ensured its mechanical robustness, thereby allowing the FSHC to withstand large tensile deformations while maintaining its structural integrity. When immersed in water, the FSHC generated a stable open-circuit voltage and short-circuit current of ~0.31 V and 2.24 µA/cm², respectively, without requiring an ionic solution or flow of water. Benefiting from its strain-adaptive design, the FSHC was able to withstand up to 200% tensile strain without any noticeable performance degradation. Moreover, the practical applicability of the FSHC was demonstrated by incorporating it into a fabric glove. Furthermore, multiple FSHCs were connected in series and parallel to increase the power output and successfully drive a twisted CNT fiber-based torsional actuator. This proof-of-concept self-powered actuation system highlights the feasibility of the FSHC as a scalable and deformable energy harvester for electrochemically driven actuation. These findings establish a promising platform for stretchable hydrovoltaic energy systems, paving the way for their integration into wearable electronics, autonomous actuation technologies, and next-generation self-powered devices. Methods Preparation of the CNT fiber electrodes Five layers of CNT sheets (A-Tech system Co., Korea), each having a width of 20 mm and a length of 20 cm, were sequentially stacked. Each end of the CNT stack was then attached to adhesive carbon tape and rolled into a cylindrical structure. The resulting cylinder was mounted on the motor tip of a custom-built twisting machine, and a gradual twisting of 1000 turns per meter was introduced under a tensile load of 4 g. Oxidized CNT fibers were prepared by applying a potentiostatic voltage (5 V vs. Ag/AgCl for 30 s) in a 0.1 M Na 2 SO 4 aqueous electrolyte, using a Pt mesh as the counter electrode and an Ag/AgCl electrode as the reference. Preparation of the elastomeric core substrate The elastomeric core was fabricated using commercially available dielectric silicone rubber (Ecoflex 0030, Smooth-On Inc., USA). The base and curing agent were mixed in a 1:1 ratio and degassed in a vacuum chamber for 30 min to remove any trapped air bubbles. The mixture was then injected into a 20-gauge syringe needle and cured in an oven at 80 °C for 100 min to form an elastomeric fiber. Formation of the parallel double-helix structure with a self-locking effect Opposite ends of the as-prepared elastomeric core were attached to opposite tips of a two-stepping motor, and a prestrain of 200% was applied to the core. The neat CNT (NCNT) fibers and oxidized CNT (OCNT) fibers were then helically wound around the stretched elastomeric core at a specific bias angle along its longitudinal axis. Upon release of the pre-applied strain, the elastic core contracted to form a winding-locked double-helix configuration, thus ensuring robust mechanical stability. Each CNT fiber electrode was electrically connected to a 200-μm diameter soft conductive thread (Soitex, Korea) using flexible silver/graphene paste (Graphene Supermarket, USA) to ensure stable electrical contact. To prevent unintended electrical leakage, all exposed silver/graphene paste regions were encapsulated in a thin layer of Ecoflex, thus providing both insulation and mechanical protection. Characterization Photographs and optical micrographs were captured using a digital camera (D750, Nikon, Japan) and an optical microscope (SMZ1270, Nikon, Japan), respectively. The microscopic surface morphology and structural characterizations were performed using a field-emission scanning electron microscope (FE-SEM, S-4600, Hitachi, Japan) operated at an accelerating voltage of 15–20 kV with a working distance of approximately 40 mm. The mechanical properties were evaluated using a universal testing machine (Instron 5966, Instron, USA) with a strain rate of 1 mm/min. The electrical resistance was measured using a digital multimeter (15+, Fluke, USA). All the electrochemical experiments were performed using an electrochemical analyzer (Vertex EIS, Ivium, Netherlands). The chemical composition of the CNT fiber surface after electrochemical oxidation was analyzed using X-ray photoelectron spectroscopy (XPS, ESCALAB 250XI, Thermo Scientific, USA). Electrical and e lectrochemical measurements The generated open-circuit voltage and short-circuit current signals were recorded using a Keithley 2450 (Keithley Instruments, USA). All samples were short-circuited before testing to avoid any interference from static electricity. The electrochemical performance of the FSHC was evaluated using a two-electrode system, and the capacitance of a single electrode was calculated from the cyclic voltammetry (CV) curves using the equation C = I /( dV / dt ), where I and dV / dt denote the average discharge current and scan rate, respectively. Construction of the self-powered actuation system The self-powered actuation system was assembled in a two-electrode configuration, where the twisted CNT fiber actuator was used as the working electrode, a Pt mesh acted as the counter electrode, and 0.1 M Na 2 SO 4 was used as the electrolyte. Both ends of the fiber actuator were securely fixed to prevent unintended untwisting, thereby ensuring that only the lower half remained immersed in the electrolyte. A 1.8 mg paddle (35 times the weight of the CNT fiber) was affixed at the midpoint of the fiber actuator as an optical marker, thereby facilitating clear visualization of torsional motion. The FSHC functioned as the direct power source, supplying electrical energy to drive charge injection-induced mechanical actuation. The paddle movement was recorded in slow motion at 480 frames per second using a high-speed camera (RX10 IV, SONY, Japan). Torsional actuation was quantitatively analyzed by tracking frame-by-frame variations in the projected paddle width, thus enabling precise measurement of the angular displacement as a function of time. Declarations Data availability The data that support the findings of this study are available from the corresponding author upon reasonable request. Acknowledgments This work was supported by the National Research Foundation of Korea(NRF) grant funded by the Korea government(MSIT) (No. NRF-RS-2021-NR060086). Author contributions W.S. was involved in the investigation and original draft writing. J.M.L. was involved in investigation and methodology. H.S. was involved in performing the experiments. S.B.C. was involved in analyzing the experimental results. S.C. was involved in the formal analysis. C.C. was involved in conceptualization, review, and supervision. All authors read and approved the final manuscript. Co mpeting interests The authors declare no competing interests. Additional information Supplementary information The online version contains supplementary material available at Correspondence and requests for materials should be addressed to Changsoon Choi. 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Supplementary Files SupportingInformationnpjflexible.docx Cite Share Download PDF Status: Published Journal Publication published 18 Nov, 2025 Read the published version in npj Flexible Electronics → Version 1 posted Editorial decision: Revision requested 01 Aug, 2025 Reviews received at journal 03 Jul, 2025 Reviews received at journal 22 Jun, 2025 Reviewers agreed at journal 11 Jun, 2025 Reviewers agreed at journal 11 Jun, 2025 Reviewers invited by journal 11 Jun, 2025 Editor assigned by journal 19 May, 2025 Submission checks completed at journal 08 May, 2025 First submitted to journal 07 May, 2025 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. 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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-6608314","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":470076291,"identity":"2e88c583-d93b-4009-8949-0c7186344f43","order_by":0,"name":"Wonkyeong Son","email":"","orcid":"","institution":"Hanyang University","correspondingAuthor":false,"prefix":"","firstName":"Wonkyeong","middleName":"","lastName":"Son","suffix":""},{"id":470076293,"identity":"fe7e826a-27ec-4bc7-887d-02ff960ba4fa","order_by":1,"name":"Jae Myeong Lee","email":"","orcid":"","institution":"Hanyang University","correspondingAuthor":false,"prefix":"","firstName":"Jae","middleName":"Myeong","lastName":"Lee","suffix":""},{"id":470076296,"identity":"0bd1fb6a-0387-43f3-b060-343b47d85250","order_by":2,"name":"Hyunji Seo","email":"","orcid":"","institution":"Hanyang University","correspondingAuthor":false,"prefix":"","firstName":"Hyunji","middleName":"","lastName":"Seo","suffix":""},{"id":470076297,"identity":"5f1d802b-ae2c-4fb4-a97a-02c881184297","order_by":3,"name":"Sung Beom Cho","email":"","orcid":"","institution":"Ajou University","correspondingAuthor":false,"prefix":"","firstName":"Sung","middleName":"Beom","lastName":"Cho","suffix":""},{"id":470076298,"identity":"b8f49d2e-9485-4387-9c79-2b6a83ac11ff","order_by":4,"name":"Sungwoo Chun","email":"","orcid":"","institution":"Korea University","correspondingAuthor":false,"prefix":"","firstName":"Sungwoo","middleName":"","lastName":"Chun","suffix":""},{"id":470076299,"identity":"6ac15ac5-9331-4e1b-b5e4-40d863dc1ed9","order_by":5,"name":"Changsoon Choi","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA/klEQVRIiWNgGAWjYLCCDwYSzPwSSAKMDQR0MM6osGGXnEGKFmaeM2n8BjeI1WLOfvjxB962w9LGt5ufPeapuWPXwH74AePMPbi1WPakmUlIth02NrtzzNyY59iz5AaeNAPGDc9wazG4wcPGYNh2ONnsRoKZNA/b4WQGhhwGxgcH8Gph/pDYdrh+84z0b9I8/4Ba+N8Q1MIgceBMGrOBRI6ZNNBTdgwSQFs24NEC8otkQ4UNs8SNnDLJuX2HE9gknhkcnIFHCyjEPv8BReWM9G0Sb74dtufnT374sAefw5A5TDwMDIltQAYeDWhaGH8wMNjjUz0KRsEoGAUjEwAAf6VTgpWK5oUAAAAASUVORK5CYII=","orcid":"","institution":"Hanyang University","correspondingAuthor":true,"prefix":"","firstName":"Changsoon","middleName":"","lastName":"Choi","suffix":""}],"badges":[],"createdAt":"2025-05-07 05:38:13","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-6608314/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-6608314/v1","draftVersion":[],"editorialEvents":[{"content":"https://doi.org/10.1038/s41528-025-00493-6","type":"published","date":"2025-11-18T15:59:02+00:00"}],"editorialNote":"","failedWorkflow":false,"files":[{"id":84553401,"identity":"aabddc2f-ff70-4902-bd05-7e9074b33b9a","added_by":"auto","created_at":"2025-06-13 10:55:23","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":4464257,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eThe design and structural characteristics of the fully stretchable hydrovoltaic cell (FSHC) based on carbon nanotube (CNT) fibers. a\u003c/strong\u003e A schematic illustration of the FSHC fabrication process, including: (i) prestrain application to an elastomeric core (200% elongation), (ii) helical winding of the NCNT and OCNT fiber electrodes around the prestrained core, and (iii) strain relaxation, during which the elastomeric core returns to its original dimensions to induce self-locking of the CNT fibers onto the elastomeric core and form a double-helix configuration. When exposed to water, the FSHC generates electricity through hydrovoltaic effects, where interactions between water molecules and the CNT fiber surfaces create an electric potential difference (inset). Water image from the Freepik website, which provides free stock photos licensed. Optical micrographs showing \u003cstrong\u003eb\u003c/strong\u003e the uniform parallel double-helix configuration of the FSHC (scale bar = 500 µm), \u003cstrong\u003ec\u003c/strong\u003e the constant interstitial gap between the helical CNT fiber electrodes (scale bar = 150 µm), and \u003cstrong\u003ed\u003c/strong\u003e the winding-locking-induced intimate contact interface between the CNT fibers and the elastomeric core (scale bar = 100 µm). Here and elsewhere, red and blue shades are used to differentiate the individual CNT fiber electrodes. \u003cstrong\u003ee\u003c/strong\u003e A photographic image of the completed FSHC demonstrating its flexibility, along with an inset showing the Fermat spiral configuration with structural stability. \u003cstrong\u003ef\u003c/strong\u003e The cyclic loading-unloading curves of the FSHC under 200% strain, demonstrating excellent elastic recovery, along with an inset showing the stress-strain curves before (black) and after (blue) the 100\u003csup\u003eth\u003c/sup\u003e loading-unloading cycle. \u003cstrong\u003eg \u003c/strong\u003eThe time-dependent strain and resistance variations of the FSHC during cyclic stretching and releasing at 200% strain.\u003c/p\u003e","description":"","filename":"floatimage1.png","url":"https://assets-eu.researchsquare.com/files/rs-6608314/v1/ec595c590617bb045f26ed90.png"},{"id":84553398,"identity":"16040271-10e8-4784-ace2-1d389f735495","added_by":"auto","created_at":"2025-06-13 10:55:23","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":1346973,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eThe electrical performance of the nonstrained FSHC in deionized water. a\u003c/strong\u003e The voltage output, which remains stable at approximately 0.3 V. \u003cstrong\u003eb\u003c/strong\u003e A histogram of the voltage distribution. \u003cstrong\u003ec \u003c/strong\u003eThe dependence of voltage and current density on linear helix density. \u003cstrong\u003ed\u003c/strong\u003e The negligible voltage generation of FSHCs with symmetric configurations (NCNT/NCNT or OCNT/OCNT). \u003cstrong\u003ee\u003c/strong\u003e The voltage response during polarity switching, along with insets showing the circuit connection. \u003cstrong\u003ef\u003c/strong\u003e The generated voltage and electrical power density as a function of external resistance (10\u003csup\u003e2\u003c/sup\u003e to 10\u003csup\u003e7\u003c/sup\u003e Ω), along with an inset showing the equivalent test circuit diagram. \u003cstrong\u003eg\u003c/strong\u003e A schematic illustration of the charge-discharge test setup. \u003cstrong\u003eh\u003c/strong\u003e The cyclic performance of the FSHC during repeated discharge-autonomous charge, and \u003cstrong\u003ei\u003c/strong\u003e an enlarged view of the purple shaded region in (h), showing a typical discharge-charge cycle.\u003c/p\u003e","description":"","filename":"floatimage2.png","url":"https://assets-eu.researchsquare.com/files/rs-6608314/v1/0ce33e855bf3d7d9d011f208.png"},{"id":84553397,"identity":"c41740af-61a2-4842-80fd-0e72164a0982","added_by":"auto","created_at":"2025-06-13 10:55:23","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":2192696,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eThe stretching principle and electrical output of the FSHC under different tensile strain levels. a\u003c/strong\u003e Optical micrographs of the FSHC under applied tensile strains of 0–200% (scale bar = 500 µm). \u003cstrong\u003eb\u003c/strong\u003e Schematic diagrams showing a single turn of the helical CNT fiber (left) and its unwound version (right) in the FSHC, along with the key structural parameters (core thickness (\u003cem\u003et\u003c/em\u003e), inter-fiber distance (\u003cem\u003ed\u003c/em\u003e), and bias angle (\u003cem\u003eθ\u003c/em\u003e)). \u003cstrong\u003ec\u003c/strong\u003e The determined length of the unwound CNT fiber and its resistance retention as a function of strain. The CV curves \u003cstrong\u003ed\u003c/strong\u003eand voltage outputs \u003cstrong\u003ee\u003c/strong\u003e of the FSHC in the initial (\u003cem\u003eε\u003c/em\u003e = 0%) and statically stretched states, along with an inset showing the voltage stability during cyclic stretching at 200% strain. \u003cstrong\u003ef\u003c/strong\u003e The real-time voltage signals of the FSHC during stretching (S) and releasing (R) at strain rates of 2%/s (yellow), 4%/s (red), and 6%/s (blue).\u003c/p\u003e","description":"","filename":"floatimage3.png","url":"https://assets-eu.researchsquare.com/files/rs-6608314/v1/57a506d5f6768ea47b8b1bf9.png"},{"id":84554779,"identity":"40d1f305-34e8-42da-9e9a-849aa59b4776","added_by":"auto","created_at":"2025-06-13 11:11:23","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":3294069,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eIntegration of the FSHC into a fabric glove and its application in a self-powered actuation system. a\u003c/strong\u003ea photographic image of a 10 cm-long FSHC integrated into a commercial glove. \u003cstrong\u003eb\u003c/strong\u003ethe corresponding high-resolution images before and after applying 50% tensile strain (scale bar = 2 mm). \u003cstrong\u003ec\u003c/strong\u003e the voltage signals recorded under static strains of 0–200%, followed by the recovery phase, demonstrating the electrical stability of the FSHC. \u003cstrong\u003ed\u003c/strong\u003e a schematic illustration of the self-powered actuation system, where the FSHC supplies electrical energy to a twisted CNT fiber-based torsional actuator. \u003cstrong\u003ee\u003c/strong\u003e the voltage outputs of serially connected FSHCs with 1, 2, 3, and 4 units (labeled i, ii, iii, and iv), showing an incremental increase in voltage. \u003cstrong\u003ef\u003c/strong\u003e the current outputs of parallel-connected FSHCs with 1 to 4 units (labeled I, II, III, and IV), demonstrating enhanced current through the parallel configuration. \u003cstrong\u003eg\u003c/strong\u003e the paddle rotation angle (in degrees) over time for a two-end-tethered twisted CNT fiber driven by the FSHC, along with inset photographic images of the fiber actuator in its initial state, during self-powered actuation, and after recovery.\u003c/p\u003e","description":"","filename":"floatimage4.png","url":"https://assets-eu.researchsquare.com/files/rs-6608314/v1/fdfb269a67c566a0d5d3d9b7.png"},{"id":96650411,"identity":"68d2904b-cfe5-4879-9263-490a915d8621","added_by":"auto","created_at":"2025-11-24 16:12:01","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":12756619,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-6608314/v1/0d79a917-b08f-4784-84b7-f098e5013430.pdf"},{"id":84553746,"identity":"0504fb5d-21eb-415b-b74d-c17c32f8d994","added_by":"auto","created_at":"2025-06-13 11:03:23","extension":"docx","order_by":0,"title":"","display":"","copyAsset":false,"role":"supplement","size":2212817,"visible":true,"origin":"","legend":"","description":"","filename":"SupportingInformationnpjflexible.docx","url":"https://assets-eu.researchsquare.com/files/rs-6608314/v1/41161757518e3e360a4bbf72.docx"}],"financialInterests":"No competing interests reported.","formattedTitle":"Fully Stretchable Hydrovoltaic Cells Based on Winding-Locked Double-Helical Carbon Nanotube Fibers","fulltext":[{"header":"Introduction","content":"\u003cp\u003eWater, which accounts for approximately 71% of the Earth\u0026apos;s surface area, holds considerable energy in various forms that dominate energy transfer in many natural phenomena\u003csup\u003e1,2\u003c/sup\u003e. Converting the mechanical and chemical energy associated with water into electricity has become increasingly important for sustainable and eco-friendly power generation\u003csup\u003e3\u0026ndash;5\u003c/sup\u003e. Hydrovoltaic technologies\u003csup\u003e6,7\u003c/sup\u003e, which leverage the interfacial interactions between water and nanostructured materials, have emerged as a promising approach for harvesting energy from various forms of water such as flows\u003csup\u003e8\u0026ndash;10\u003c/sup\u003e, waves\u003csup\u003e11\u0026ndash;13\u003c/sup\u003e, droplets\u003csup\u003e14\u0026ndash;19\u003c/sup\u003e, and moisture\u003csup\u003e20\u0026ndash;24\u003c/sup\u003e. To date, a variety of strategies based on different mechanisms, including electrokinetic effects, alternating potentials, and ionic diffusion, have been explored. For instance, in 2014, Guo et al. discovered that a single water droplet interacting with graphene could generate an electrical output, thus revealing the electrokinetic potential at the liquid-solid interface\u003csup\u003e15\u003c/sup\u003e. In 2015, Qu et al. demonstrated that a graphene oxide framework sandwiched between two grid-patterned gold electrodes was capable of producing intermittent electrical signals through moisture diffusion\u003csup\u003e20\u003c/sup\u003e. In 2017, Guo et al. reported that natural water evaporation could be harnessed using a porous nanostructured graphite film to achieve continuous power generation\u003csup\u003e25\u003c/sup\u003e. More recently, in 2023, Li et al. significantly enhanced droplet-based hydrovoltaics, achieving potentials over 1200 V by optimizing electric double-layer dynamics at interfaces\u003csup\u003e26\u003c/sup\u003e. Building upon these advancements, Yuan et al. in 2024 introduced a hermetic hydrovoltaic cell with internal water circulation driven by temperature fluctuations, achieving stable, long-term electricity generation without water loss\u003csup\u003e27\u003c/sup\u003e.\u003c/p\u003e\n\u003cp\u003eDespite the significant progress achieved in these pioneering studies, current hydrovoltaic devices still face two critical challenges that hinder their further potential and practical applicability. First, their electrical output inherently depends on either continuous charge carrier movement or steady ionic concentration gradients, meaning that any interruption or equilibrium in these conditions rapidly terminates electricity generation. Second, most previously reported hydrovoltaic devices are typically composed of rigid or non-stretchable substrates and functional materials, limiting their adaptability to dynamic and mechanically active environments. In this regard, developing hydrovoltaic devices capable of reliably generating electricity from the environment with few constraints and maintaining stable electrical performance under mechanical deformation remains an urgent challenge for practical and sustainable applications. To address these limitations, a yarn-based flexible carbon:water device that converts chemical energy in the nanotube yarn into electrical form has been reported, enabling water-based electricity generation without requiring flowing water or ionic solutions\u003csup\u003e28\u003c/sup\u003e. Such demonstrations have opened up new possibilities for designing advanced hydrovoltaic devices with exceptional flexibility and adaptability, thereby enabling their integration into practical and versatile systems.\u003c/p\u003e\n\u003cp\u003eIn this work, we propose a fully stretchable hydrovoltaic cell (FSHC) with a winding-locked double-helix design, wherein two carbon nanotube (CNT) fibers (neat and oxidized) electrochemically interact with water molecules. Specifically, the wettability difference between the hydrophobic neat CNT (NCNT) fiber and the hydrophilic oxidized CNT (OCNT) fiber induces asymmetric charge interactions at the CNT\u0026ndash;water interfaces, establishing a potential difference. To achieve structural stretchability, these CNT fiber electrodes are spirally wound around an elastomeric core that is prestrained to 200% of its original length, maintaining a constant gap between them. Upon release of the prestrain, the elastomer\u0026mdash;which had been stretched and thinned\u0026mdash;returns to its original dimensions \u003cem\u003evia\u003c/em\u003e radial recovery governed by Poisson\u0026rsquo;s ratio. During this relaxation, the double-helix structure becomes self-locked to form an intimate contact interface between the CNT fibers and the core. This simple and reliable assembly process eliminates the need for an additional separator. The resulting winding-locked architecture not only ensures robust mechanical integrity but also provides exceptional durability under significant mechanical deformations. When brought into contact with quiescent deionized water, the FSHC delivers a high electrical output with an open-circuit voltage of ~0.31 V and a short-circuit current of ~2.24 \u0026micro;A/cm\u003csup\u003e2\u003c/sup\u003e. More importantly, owing to the rational double-helix design and winding-lock effect, the FSHC demonstrates remarkable mechanical robustness, withstanding repeated stretching up to 200% strain without notable degradation in electrical performance. To the best of our knowledge, such a high degree of stretchability has not been previously reported in fiber or yarn-based hydrovoltaic devices, clearly validating the unique advantages of our winding-locked architecture. To further examine its practical applicability, the FSHC fibers were sewn onto a glove, confirming their feasibility for integration into textile-based wearable systems. Moreover, by connecting multiple FSHCs in series or parallel as an energy-harvesting module, the enhanced power output was sufficient to directly drive fiber-type electrochemical torsional actuators consisting of twisted CNT fibers in an electrolyte. This study provides an effective strategy for fabricating stretchable hydrovoltaic devices suitable for wearable electronics, significantly broadening their potential applications in self-powered actuation systems.\u003c/p\u003e"},{"header":"Results","content":"\u003cp\u003e\u003cstrong\u003eFabrication and morphological information of the FSHC\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eAs shown in \u003cstrong\u003eFig. 1a\u0026nbsp;\u003c/strong\u003eand \u003cstrong\u003eSupplementary\u003c/strong\u003e \u003cstrong\u003eFig. 1\u003c/strong\u003e, the stepwise fabrication of the FSHC involves the following three key steps: (i) prestrain application, (ii) electrode winding, and (iii) strain relaxation. To construct the stretchable hydrovoltaic architecture, a commercially available silicone elastomer was molded into a cylindrical shape using a needle-tip template, as detailed in the Experimental section. This elastomeric core serves as the flexible substrate, providing mechanical compliance and elasticity under repeated deformations. In the first step, the as-fabricated elastomeric core (length: 20 mm, diameter: 0.6 mm) was stretched uniaxially to 200% of its original length. This prestrain application was essential for ensuring tight integration of the CNT fiber electrodes with the elastomeric core after strain relaxation. By maintaining the elastomer in a highly elongated state, the subsequent CNT fiber winding process ensured that the fibers were securely positioned around the core without unintended displacement or detachment. In the second step, two CNT fibers were tightly wound around the elastomeric core in a parallel configuration. These fibers were fabricated by twisting sheets of aligned multiwalled CNTs drawn from forests\u003csup\u003e29\u003c/sup\u003e. Then, ethanol was repeatedly applied to the elastomer surface to densify the fibers during evaporation\u003csup\u003e30\u0026ndash;32\u003c/sup\u003e. In the third step, strain relaxation was induced by releasing the prestrain in the elastomeric core, allowing it to contract back to its original dimensions. This contraction, governed by Poisson\u0026rsquo;s effect, generated a radial expansion that effectively secured the CNT fibers onto the elastomeric core to form a self-locking structure. Due to ethanol densification, the adhesion between the CNT fibers and elastomer core was strong, and no noticeable detachment was observed during tensile strain relaxation. As a result, the CNT fibers became tightly secured onto the elastomeric core without the need for additional adhesives. This winding-locked configuration not only enhances the mechanical robustness of the FSHC but also ensures a stable electrode-substrate interface, thus preventing fiber slippage or delamination under mechanical deformation. When the FSHC is exposed to water (inset, \u003cstrong\u003eFig. 1a\u003c/strong\u003e), a potential difference is generated between the two CNT fiber electrodes, thus leading to electricity generation. This phenomenon arises from the distinct surface properties of the NCNT and OCNT fibers, which lead to asymmetric interactions with water molecules. Specifically, the NCNT fiber remains hydrophobic, repelling water and minimizing direct charge transfer at the solid\u0026ndash;liquid interface. In contrast, the OCNT fiber exhibits strong hydrophilicity due to the presence of oxygen-containing functional groups, thereby allowing enhanced water adsorption. This asymmetric wettability between the two electrodes induces different charge-transfer interactions, thus facilitating hydrovoltaic energy conversion\u003csup\u003e28\u003c/sup\u003e. First, these differences are elucidated by the SEM images in \u003cstrong\u003eSupplementary\u003c/strong\u003e \u003cstrong\u003eFig. 2\u003c/strong\u003e. There, the NCNT fiber exhibits a smooth and compact structure (\u003cstrong\u003eSupplementary\u003c/strong\u003e \u003cstrong\u003eFig. 2a and c\u003c/strong\u003e), whereas the OCNT fiber displays a rougher and more disordered morphology (\u003cstrong\u003eSupplementary\u003c/strong\u003e \u003cstrong\u003eFig. 2b and d\u003c/strong\u003e). This structural difference originates from the presence of oxygen-containing functional groups, which disrupt the graphitic stacking of the CNT bundles. Moreover, the successful oxidation of the CNT fiber to obtain the OCNT is confirmed by the XPS results in \u003cstrong\u003eSupplementary\u003c/strong\u003e \u003cstrong\u003eFig. 3\u003c/strong\u003e. Notably, the O/C ratio of the OCNT is seen to have increased to 0.27, compared to 0.11 for the NCNT, thereby indicating a significant incorporation of oxygen-containing functional groups. This chemical modification alters the surface charge distribution of the OCNT, enhancing its ability to interact with water molecules and facilitating charge transfer at the electrode-water interface. In addition, the Raman spectra of the two CNT fibers are presented in \u003cstrong\u003eSupplementary\u003c/strong\u003e \u003cstrong\u003eFig. 4\u003c/strong\u003e. There, the intensity ratio between the D and G bands (\u003cem\u003eI\u003c/em\u003e\u003csub\u003eD\u003c/sub\u003e/\u003cem\u003eI\u003c/em\u003e\u003csub\u003eG\u003c/sub\u003e), which represents the degree of atomic defects in the CNT structure, is seen to have increased from 0.52 for the NCNT to 0.82 for the OCNT, thus supporting the abovementioned XPS results. Also, the mechanical properties of the NCNT and OCNT fibers are revealed by the stress-strain curves in \u003cstrong\u003eSupplementary\u003c/strong\u003e \u003cstrong\u003eFig. 5\u003c/strong\u003e, where the tensile strength of the OCNT fibers is seen to have decreased by 25.8% (109.5 MPa) compared to that of the NCNT fibers (147.5 MPa). This can be attributed to the partial disruption of sp\u0026sup2;-hybridized bonds in the CNT network caused by oxidation-induced defects. Despite this reduction in mechanical strength, the introduction of oxygen functionalities facilitates the formation of intermolecular hydrogen bonds, thus leading to a slight increase in the maximum strain from 5.1% to 5.5%\u003csup\u003e33,34\u003c/sup\u003e.\u003c/p\u003e\n\u003cp\u003eThe parallel double-helix configuration of the CNT fiber electrodes in the FSHC is revealed by the optical micrographs in \u003cstrong\u003eFig. 1b\u003c/strong\u003e. This uniform helical arrangement ensures a well-defined electrode alignment, preserving both structural stability and mechanical flexibility. The precisely controlled fiber winding process minimizes any irregularities that could otherwise compromise the device performance. A magnified view of the electrode arrangement is presented in \u003cstrong\u003eFig. 1c,\u003c/strong\u003e indicating the presence of a well-defined and consistent interstitial spacing between the helical CNT fibers. This is essential for preventing electrical shorting while simultaneously facilitating efficient charge separation, both of which are crucial for stable hydrovoltaic energy conversion. The uniformity of this spacing also suggests a high degree of reproducibility in the fabrication process. The intimate contact between the CNT fibers and elastomeric core after pre-strain relaxation is highlighted in \u003cstrong\u003eFig. 1d\u003c/strong\u003e, where the CNT electrodes appear embedded in the elastomer surface. This observation confirms the effectiveness of the winding-lock mechanism, in which the radial expansion of the elastomer during strain relaxation enhances the interfacial adhesion. As a result, the CNT fibers are securely anchored to the core, thus preventing slippage or delamination and significantly improving the mechanical robustness of the FSHC under repeated deformations.\u003c/p\u003e\n\u003cp\u003eWith its uniform and precisely organized parallel double-helix configuration, the as-fabricated FSHC (10 cm in length) exhibits good structural stability, maintaining a moderately straight profile in the free-standing state without the need for additional support (\u003cstrong\u003eFig. 1e\u003c/strong\u003e). Moreover, the inset shows that the structure remains intact, without any signs of entanglement or deformation, even in the Fermat spiral configuration. Moreover, as shown in \u003cstrong\u003eFig. 1f\u003c/strong\u003e, the FSHC exhibits excellent elastic recoverability under 200% strain. The cyclic loading-unloading curve (inset) demonstrates that the FSHC returns to its original state with negligible hysteresis even after 100 cycles. Further, the time-dependent strain and resistance variations of the FSHC under cyclic stretching and releasing at 200% strain are shown in \u003cstrong\u003eFig. 1g\u003c/strong\u003e. Here, the resistance remains highly stable throughout the repeated deformation cycles, indicating excellent electromechanical durability. This stable resistance response under large mechanical deformation is a critical requirement for practical applications in stretchable and wearable electronics.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eElectrical output performance of the nonstrained FSHC\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eBased on the above considerations, the stable voltage output of the FSHC due to asymmetric charge transfer at the CNT\u0026ndash;water interface upon immersion is demonstrated in this section. As shown in \u003cstrong\u003eFig. 2a\u003c/strong\u003e, when the OCNT (as the positive electrode) and NCNT (as the negative electrode) are connected to a digital source meter, the FSHC generates an open-circuit voltage of approximately 0.31 V, which remains stable for up to 5000 s in water. Further, as shown in \u003cstrong\u003eSupplementary\u003c/strong\u003e \u003cstrong\u003eFig. 6\u003c/strong\u003e, a short-circuit current is detected when the cell is electrically connected in water. Meanwhile, the voltage distribution exhibits a narrow window centered around 0.30 V (\u003cstrong\u003eFig. 2b\u003c/strong\u003e), thereby indicating the high reproducibility of the hydrovoltaic effect. Furthermore, even under partial immersion in deionized water (\u003cstrong\u003eSupplementary\u003c/strong\u003e \u003cstrong\u003eFig. 7\u003c/strong\u003e), the generated voltage remains almost identical to that observed under full immersion; in this case, water evaporation has no observable effect on electricity generation. Further, the proposed mechanism underlying the hydrovoltaic effect in the FSHC is confirmed by comparing its voltage output in various electrolyte solutions with differing pH levels (\u003cstrong\u003eSupplementary\u003c/strong\u003e \u003cstrong\u003eFig. 8\u003c/strong\u003e). Here, the FSHC exhibits distinct voltage responses depending on the electrolyte composition, which suggests that ion adsorption and interfacial charge transfer at the CNT\u0026ndash;water interface are key factors in electricity generation. Specifically, the voltage is highest in 0.1 M HCl, followed by 0.1 M Na\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e and 0.1 M KOH, thereby indicating a strong dependence on the proton concentration. This suggests that the availability of protons and their interactions with the CNT surface significantly influence the hydrovoltaic performance.\u003c/p\u003e\n\u003cp\u003eIn \u003cstrong\u003eFig. 2c\u003c/strong\u003e, the cell\u0026rsquo;s operation is examined by studying its electricity generation as a function of the linear helix density and immersion speed. Notably, while the voltage output remains nearly unchanged, the short-circuit current is found to increase with the increase in linear helix density, thereby suggesting that the current is affected by the active surface area. This increase can be attributed to the extended CNT fiber length in higher-density configurations, which provides a larger effective contact area with water. The FSHC\u0026rsquo;s performance is also unaffected by changes in immersion speed (\u003cstrong\u003eSupplementary\u003c/strong\u003e \u003cstrong\u003eFig. 9\u003c/strong\u003e), thereby indicating that its operation differs from that of previously reported electrokinetic or fluidic devices. In addition, the dependence of voltage generation on the presence of asymmetric charge interactions is further evidenced in \u003cstrong\u003eFig. 2d\u003c/strong\u003e, where the use of symmetric electrode configurations with identical surface properties (\u003cem\u003ei.e.,\u003c/em\u003e NCNT/NCNT or OCNT/OCNT) results in a negligible voltage output. This demonstrates that differences in surface functionalization are critical for preserving charge separation and sustaining hydrovoltaic energy conversion. Indeed, the role of electrode asymmetry is confirmed by the polarity-switching test results in \u003cstrong\u003eFig. 2e\u003c/strong\u003e. Thus, when the OCNT fiber is connected to the positive terminal, a positive voltage is observed (blue curve), whereas reversing the connections produces an inverted voltage (red curve), thereby confirming the formation of a stable potential difference between the two electrodes. These findings show that voltage generation is governed by asymmetric charge interactions at the CNT\u0026ndash;water interface, rather than by external ionic concentration gradients or electrokinetic effects.\u003c/p\u003e\n\u003cp\u003eThe electrical power density of the FSHC is demonstrated in \u003cstrong\u003eFig. 2f\u003c/strong\u003e, where the voltage is seen to increase with the increase in load resistance (100 \u0026Omega; to 10 M\u0026Omega;), reaching a peak power density of 8.5 \u0026mu;W/cm\u003csup\u003e2\u003c/sup\u003e at 1 M\u0026Omega;. Finally, to evaluate the cycling stability of the FSHC, a charge-discharge test was performed using an external circuit in which a switch was periodically toggled to control discharge and autonomous recharge (\u003cstrong\u003eFig. 2g\u003c/strong\u003e). As shown in \u003cstrong\u003eFig. 2h\u003c/strong\u003e, the FSHC exhibits a self-replenishing voltage after each discharge event and maintains stable charge-discharge behavior across multiple cycles, thus confirming its robustness under continuous operation. Moreover, a closer look at an individual cycle (\u003cstrong\u003eFig. 2i\u003c/strong\u003e) underscores the reproducibility of the charge-discharge process, demonstrating that the FSHC can sustain long-term performance without significant degradation.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eThe effects of tensile deformation on the FSHC performance\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eHydrovoltaic devices that are intended for use in stretchable and deformable electronics must maintain a stable electrical performance under mechanical strain. Unlike conventional fiber-based energy systems, which often suffer from electrical degradation due to mechanical failure, the double-helix configuration of the FSHC provides a unique structural advantage. By employing a winding-locked CNT fiber network, the FSHC can accommodate large tensile deformations while preserving its conductive pathways. However, despite the widespread adoption of helical structures in stretchable electronics, the precise structural changes that occur during elongation remain largely unexplored. Hence, this section presents a detailed structural analysis of the FSHC under mechanical strain in order to demonstrate how its helical architecture adapts to stretching without compromising its electrical output.\u003c/p\u003e\n\u003cp\u003eFirst, the optical micrographs of the FSHC under increasing tensile strain (0\u0026ndash;200%) are presented in \u003cstrong\u003eFig. 3a\u003c/strong\u003e. As the strain increases, the helical CNT fibers are seen to gradually reorient themselves along the stretching direction while maintaining a uniform distribution, with no noticeable detachment or misalignment. This suggests that the double-helix structure undergoes a systematic geometric transformation to accommodate strain without structural failure. Further, these changes are quantitatively analyzed by adopting an \u0026ldquo;unwound\u0026rdquo; model of a single helical turn, as shown in \u003cstrong\u003eFig. 3b\u003c/strong\u003e. In this approach, each turn is conceptually unfolded into a straight segment, thereby enabling a mathematical description of the effective fiber length under varying strain levels. This transformation is governed by three key geometric parameters: (i) the thickness (\u003cem\u003et\u003c/em\u003e) of the elastomeric core, (ii) the bias angle (\u003cem\u003e\u0026theta;\u003c/em\u003e) of the CNT windings, and (iii) the distance (\u003cem\u003ed\u003c/em\u003e) between the two CNT fiber electrodes (NCNT and OCNT). As shown in \u003cstrong\u003eSupplementary\u003c/strong\u003e \u003cstrong\u003eFig. 10\u003c/strong\u003e, the variations in these experimentally measured parameters according to the increase in tensile strain reveal a systematic structural adaptation. First, the thickness of the elastomeric core decreases due to Poisson\u0026rsquo;s effect, which induces lateral contraction during uniaxial stretching and influences the winding geometry. Second, the bias angle decreases, thereby indicating progressive alignment of the CNT fibers with the stretching direction. Lastly, the inter-fiber spacing increases as the CNT fibers move farther apart along the axial direction, thereby accommodating the applied strain while preserving the helical configuration. As a result, the calculated unwound fiber length remains nearly constant across all strain levels, with only minor deviations (\u003cstrong\u003eFig. 3c\u003c/strong\u003e). This highlights how the double-helix structure efficiently distributes mechanical stress and prevents excessive elongation of individual CNT fibers. Consequently, the electrical resistance exhibits negligible variation, thus ensuring continuous electrical connectivity despite large deformations. These findings validate the strain-adaptive nature of the FSHC, demonstrating that the double-helix configuration preserves both mechanical resilience and electrical integrity under tensile strain.\u003c/p\u003e\n\u003cp\u003eNext, the influence of the strain-adaptive nature of the double-helix structure on the electrochemical performance of the FSHC is examined. Because hydrovoltaic energy generation relies on interfacial charge interactions at the CNT\u0026ndash;water interface, any mechanical deformation that compromises the conductive pathways or alters the electrode-electrolyte interactions could degrade the performance. Hence, the intrinsic electrochemical properties of the individual CNT fiber electrodes were first elucidated by CV measurements in a three-electrode system with an aqueous electrolyte (0.1 M Na\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e). As shown in \u003cstrong\u003eSupplementary\u003c/strong\u003e \u003cstrong\u003eFig. 11\u003c/strong\u003e, the OCNT fiber exhibits a 226.1 times larger CV area than that of the NCNT fiber, thereby indicating markedly enhanced surface electrochemical activity due to the presence of oxygen-containing functional groups\u003csup\u003e35\u0026ndash;38\u003c/sup\u003e. The electrochemical performance of the fully assembled FSHC device is then demonstrated in its two-electrode configuration, with scan rates ranging from 100 to 1000 mV/s, in \u003cstrong\u003eSupplementary\u003c/strong\u003e \u003cstrong\u003eFig. 12\u003c/strong\u003e. Here, the observed rectangular CV curves without distinct Faradaic redox peaks are consistent with the energy storage capacity of the CNT-induced electric double-layer capacitance and the pseudocapacitance derived from oxygen-containing functional groups. The effect of mechanical strain on the electrochemical response of the FSHC is then evaluated by examining the CV curves obtained at different strain levels (0%, 50%, 100%, and 200%) (\u003cstrong\u003eFig. 3d\u003c/strong\u003e). As shown in \u003cstrong\u003eSupplementary\u003c/strong\u003e \u003cstrong\u003eFig. 13\u003c/strong\u003e, the corresponding capacitance remains almost unchanged under all strain conditions, thereby indicating that the FSHC maintains stable electrochemical behavior despite mechanical deformation. The preservation of CV characteristics implies that the structural adaptability of the helical CNT network effectively prevents degradation of the charge-transfer dynamics. Further, the effect of tensile strain on the electrical output of the FSHC is evaluated by measuring the voltage under various static elongations in \u003cstrong\u003eFig. 3e\u003c/strong\u003e. Here, the voltage remains stable at all strain levels, thereby demonstrating that the double-helix structure accommodates mechanical deformation without disrupting the electrode-water interface. Moreover, the inset of Fig. 3e confirms a stable performance over multiple stretching cycles at 200% strain. Finally, the durability of the FSHC under dynamic deformation was examined by cyclic stretching and releasing at strain rates of 2, 4, and 6%/s. As shown in \u003cstrong\u003eFig. 3f\u003c/strong\u003e, the voltage output remains consistent regardless of the strain rate, thereby indicating that the hydrovoltaic performance is independent of the stretching speed. Moreover, no performance degradation is observed over repeated cycles, thereby confirming both the mechanical resilience and electrical stability of the FSHC. Taken together, these results demonstrate that the FSHC effectively maintains its structural and electrical integrity under both static and dynamic strains, thus making it a promising candidate for stretchable hydrovoltaic energy systems.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFabric integration of the FSHC and its application in self-powered actuation\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eIn this section, the feasibility of integrating the FSHC into a fabric-based system is investigated. Given its fiber-based architecture, the FSHC can be incorporated into textiles without compromising the structural integrity, which makes it a promising candidate for applications that require mechanical durability and electrical stability. Therefore, a 10 cm-long FSHC was incorporated into a commercially available fabric glove (\u003cstrong\u003eFig. 4a\u003c/strong\u003e). The high-resolution images obtained before and after applying 50% tensile strain (\u003cstrong\u003eFig. 4b\u003c/strong\u003e) confirm that the FSHC maintains its structural integrity without any visible damage or delamination. This demonstrates the mechanical robustness of the FSHC and its suitability for seamless integration into fabric. Moreover, electrical performance under mechanical strain was evaluated by monitoring the voltage response while applying static stretching up to 200%. The results in \u003cstrong\u003eFig. 4c\u003c/strong\u003e indicate that the output voltage remains stable across all strain levels and is fully recovered upon relaxation, thereby confirming the mechanical resilience and ability of the FSHC to sustain hydrovoltaic energy generation under deformation.\u003c/p\u003e\n\u003cp\u003eIn addition, the feasibility of utilizing the FSHC as a power source for a self-powered actuation system was explored. Given that the CNT fibers used in the FSHC are intrinsically twisted, they share structural similarities with twisted CNT actuators, which are known to exhibit untwisting behavior upon electrochemical volumetric expansion\u003csup\u003e39\u003c/sup\u003e. This property enables electrically driven torsional actuation when sufficient voltage is applied. Hence, a self-powered actuation system was designed in which the FSHC directly supplies energy to a twisted CNT fiber-based actuator (\u003cstrong\u003eFig. 4d\u003c/strong\u003e). To provide adequate electrical input for actuation, the output voltage and current of the FSHC were increased by connecting multiple cells in series and parallel, respectively. As shown in \u003cstrong\u003eFig. 4e\u003c/strong\u003e, the output voltage increases proportionally with the number of FSHCs connected in series, reaching 1.2 V with four units. Meanwhile, parallel connections effectively amplify the output current, ensuring sufficient electrical stimulation for actuation (\u003cstrong\u003eFig. 4f\u003c/strong\u003e). As shown in \u003cstrong\u003eFig. 4g\u003c/strong\u003e, this approach was successfully used to demonstrate the self-powered torsional actuation of a twisted CNT fiber tethered at both ends. For this experiment, ten FSHCs were connected in series to generate an output voltage of approximately 3 V, which is sufficient to drive the actuation. Upon activation, the fiber exhibits a rapid untwisting motion, leading to the rotation of an attached paddle. The actuation is fully reversible, with the fiber returning to its initial state after power removal. This proof-of-concept demonstration highlights the potential of the FSHC as a scalable, sustainable energy source for autonomous, electrochemically driven actuation systems.\u003c/p\u003e"},{"header":"Discussion","content":"\u003cp\u003eHerein, a FSHC with a parallel double-helix structure was developed, wherein neat and oxidized CNT fibers (designated as NCNT and OCNT, respectively) were helically wound around an elastomeric core. The self-locking helical configuration of the CNT fibers ensured its mechanical robustness, thereby allowing the FSHC to withstand large tensile deformations while maintaining its structural integrity. When immersed in water, the FSHC generated a stable open-circuit voltage and short-circuit current of ~0.31 V and 2.24 \u0026micro;A/cm\u0026sup2;, respectively, without requiring an ionic solution or flow of water. Benefiting from its strain-adaptive design, the FSHC was able to withstand up to 200% tensile strain without any noticeable performance degradation. Moreover, the practical applicability of the FSHC was demonstrated by incorporating it into a fabric glove. Furthermore, multiple FSHCs were connected in series and parallel to increase the power output and successfully drive a twisted CNT fiber-based torsional actuator. This proof-of-concept self-powered actuation system highlights the feasibility of the FSHC as a scalable and deformable energy harvester for electrochemically driven actuation. These findings establish a promising platform for stretchable hydrovoltaic energy systems, paving the way for their integration into wearable electronics, autonomous actuation technologies, and next-generation self-powered devices.\u003c/p\u003e"},{"header":"Methods","content":"\u003cp\u003e\u003cstrong\u003ePreparation of the CNT fiber electrodes\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eFive layers of CNT sheets (A-Tech system Co., Korea), each having a width of 20 mm and a length of 20 cm, were sequentially stacked. Each end of the CNT stack was then attached to adhesive carbon tape and rolled into a cylindrical structure. The resulting cylinder was mounted on the motor tip of a custom-built twisting machine, and a gradual twisting of 1000 turns per meter was introduced under a tensile load of 4 g. Oxidized CNT fibers were prepared by applying a potentiostatic voltage (5 V \u003cem\u003evs.\u003c/em\u003e Ag/AgCl for 30 s) in a 0.1 M Na\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e aqueous electrolyte, using a Pt mesh as the counter electrode and an Ag/AgCl electrode as the reference.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003ePreparation of the elastomeric core substrate\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe elastomeric core was fabricated using commercially available dielectric silicone rubber (Ecoflex 0030, Smooth-On Inc., USA). The base and curing agent were mixed in a 1:1 ratio and degassed in a vacuum chamber for 30 min to remove any trapped air bubbles. The mixture was then injected into a 20-gauge syringe needle and cured in an oven at 80 \u0026deg;C for 100 min to form an elastomeric fiber.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFormation of the parallel double-helix structure with a self-locking effect\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eOpposite ends of the as-prepared elastomeric core were attached to opposite tips of a two-stepping motor, and a prestrain of 200% was applied to the core. The neat CNT (NCNT) fibers and oxidized CNT (OCNT) fibers were then helically wound around the stretched elastomeric core at a specific bias angle along its longitudinal axis. Upon release of the pre-applied strain, the elastic core contracted to form a winding-locked double-helix configuration, thus ensuring robust mechanical stability. Each CNT fiber electrode was electrically connected to a 200-\u0026mu;m diameter soft conductive thread (Soitex, Korea) using flexible silver/graphene paste (Graphene Supermarket, USA) to ensure stable electrical contact. To prevent unintended electrical leakage, all exposed silver/graphene paste regions were encapsulated in a thin layer of Ecoflex, thus providing both insulation and mechanical protection.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCharacterization\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003ePhotographs and optical micrographs were captured using a digital camera (D750, Nikon, Japan) and an optical microscope (SMZ1270, Nikon, Japan), respectively. The microscopic surface morphology and structural characterizations were performed using a field-emission scanning electron microscope (FE-SEM, S-4600, Hitachi, Japan) operated at an accelerating voltage of 15\u0026ndash;20 kV with a working distance of approximately 40 mm. The mechanical properties were evaluated using a universal testing machine (Instron 5966, Instron, USA) with a strain rate of 1 mm/min. The electrical resistance was measured using a digital multimeter (15+, Fluke, USA). All the electrochemical experiments were performed using an electrochemical analyzer (Vertex EIS, Ivium, Netherlands). The chemical composition of the CNT fiber surface after electrochemical oxidation was analyzed using X-ray photoelectron spectroscopy (XPS, ESCALAB 250XI, Thermo Scientific, USA).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eElectrical and e\u003c/strong\u003e\u003cstrong\u003electrochemical\u0026nbsp;\u003c/strong\u003e\u003cstrong\u003emeasurements\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe generated open-circuit voltage and short-circuit current signals were recorded using a Keithley 2450 (Keithley Instruments, USA). All samples were short-circuited before testing to avoid any interference from static electricity. The electrochemical performance of the FSHC was evaluated using a two-electrode system, and the capacitance of a single electrode was calculated from the cyclic voltammetry (CV) curves using the equation \u003cem\u003eC\u003c/em\u003e = \u003cem\u003eI\u003c/em\u003e/(\u003cem\u003edV\u003c/em\u003e/\u003cem\u003edt\u003c/em\u003e), where \u003cem\u003eI\u003c/em\u003e and \u003cem\u003edV\u003c/em\u003e/\u003cem\u003edt\u003c/em\u003e denote the average discharge current and scan rate, respectively.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConstruction of the self-powered actuation system\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe self-powered actuation system was assembled in a two-electrode configuration, where the twisted CNT fiber actuator was used as the working electrode, a Pt mesh acted as the counter electrode, and 0.1 M Na\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e was used as the electrolyte. Both ends of the fiber actuator were securely fixed to prevent unintended untwisting, thereby ensuring that only the lower half remained immersed in the electrolyte. A 1.8 mg paddle (35 times the weight of the CNT fiber) was affixed at the midpoint of the fiber actuator as an optical marker, thereby facilitating clear visualization of torsional motion. The FSHC functioned as the direct power source, supplying electrical energy to drive charge injection-induced mechanical actuation. The paddle movement was recorded in slow motion at 480 frames per second using a high-speed camera (RX10 IV, SONY, Japan). Torsional actuation was quantitatively analyzed by tracking frame-by-frame variations in the projected paddle width, thus enabling precise measurement of the angular displacement as a function of time.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eData availability\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe data that support the findings of this study are available from the corresponding author upon reasonable request.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAcknowledgments\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis work was supported by the National Research Foundation of Korea(NRF) grant funded by the Korea government(MSIT) (No. NRF-RS-2021-NR060086).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthor contributions\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eW.S. was involved in the investigation and original draft writing. J.M.L. was involved in investigation and methodology. H.S. was involved in performing the experiments. S.B.C. was involved in analyzing the experimental results. S.C. was involved in the formal analysis. C.C. was involved in conceptualization, review, and supervision. All authors read and approved the final manuscript.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCo\u003c/strong\u003e\u003cstrong\u003empeting interests\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors declare no competing interests.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAdditional information\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eSupplementary information\u0026nbsp;\u003c/strong\u003eThe online version contains supplementary material available at\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCorrespondence\u0026nbsp;\u003c/strong\u003eand requests for materials should be addressed to Changsoon Choi.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eReprints and permissions information\u003c/strong\u003e is available at\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003ePublisher\u0026rsquo;s Note\u0026nbsp;\u003c/strong\u003eSpringer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n \u003cli\u003eVisbeck, Z. 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Torsional Carbon Nanotube Artificial Muscles. \u003cem\u003eScience\u003c/em\u003e, \u003cstrong\u003e334\u003c/strong\u003e, 6055, 494\u0026ndash;497 (2011).\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"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":"npj-flexible-electronics","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"npjflexelectron","sideBox":"Learn more about [npj Flexible Electronics](http://www.nature.com/npjflexelectron/)","snPcode":"41528","submissionUrl":"https://submission.springernature.com/new-submission/41528/3","title":"npj Flexible Electronics","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"NPJ","inReviewEnabled":true,"inReviewRevisionsEnabled":true},"keywords":"Stretchable, Hydrovoltaic, Double-helix, Carbon nanotubes, Electrochemical","lastPublishedDoi":"10.21203/rs.3.rs-6608314/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-6608314/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eHydrovoltaic power generators that convert water-nanomaterial interactions into electricity represent a promising route for sustainable energy harvesting. However, many previous studies have relied upon conventional two-dimensional planar designs with rigid, non-stretchable materials, typically operating in environments that require continuous water flow or specially designed ionic solutions. These stringent conditions restrict their practical applications, particularly in flexible and wearable systems. Hence, the present study introduces a fully stretchable hydrovoltaic cell (FSHC) that features a parallel double-helix configuration in which neat and oxidized carbon nanotube (CNT) fibers are spirally wound around an elastomeric core. This winding-locked double-helix architecture ensures robust mechanical integrity and stable electrical performance under large deformations. When immersed in quiescent deionized water, the FSHC generates an open-circuit voltage of ~\u0026thinsp;0.31 V and a short-circuit current of ~\u0026thinsp;2.24 \u0026micro;A/cm\u003csup\u003e2\u003c/sup\u003e. Notably, the FSHC maintains consistent performance under 200% tensile strain. To demonstrate its potential in wearable applications, the FSHC is integrated into a fabric glove. Moreover, multiple FSHCs connected in series or parallel generate sufficient power to drive a twisted CNT fiber-based torsional actuator, suggesting a pathway toward self-powered actuation systems. This study offers a deformable hydrovoltaic platform for fiber-based energy harvesters, broadening their applicability in wearable electronics and autonomous actuation.\u003c/p\u003e","manuscriptTitle":"Fully Stretchable Hydrovoltaic Cells Based on Winding-Locked Double-Helical Carbon Nanotube Fibers","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-06-13 10:55:18","doi":"10.21203/rs.3.rs-6608314/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2025-08-01T04:21:51+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-07-03T07:33:16+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-06-23T01:49:37+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"168086321885229510686000810648591552851","date":"2025-06-12T01:54:17+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"301892216413387776297193499772657622768","date":"2025-06-12T01:26:53+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2025-06-11T15:51:18+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2025-05-19T21:52:14+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2025-05-08T10:31:09+00:00","index":"","fulltext":""},{"type":"submitted","content":"npj Flexible Electronics","date":"2025-05-07T05:25:11+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"npj-flexible-electronics","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"npjflexelectron","sideBox":"Learn more about [npj Flexible Electronics](http://www.nature.com/npjflexelectron/)","snPcode":"41528","submissionUrl":"https://submission.springernature.com/new-submission/41528/3","title":"npj Flexible Electronics","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"NPJ","inReviewEnabled":true,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"f25c08f0-9ecd-4fb3-912d-19f272d4932a","owner":[],"postedDate":"June 13th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[{"id":49915496,"name":"Physical sciences/Energy science and technology"},{"id":49915497,"name":"Physical sciences/Engineering"}],"tags":[],"updatedAt":"2025-11-24T16:07:53+00:00","versionOfRecord":{"articleIdentity":"rs-6608314","link":"https://doi.org/10.1038/s41528-025-00493-6","journal":{"identity":"npj-flexible-electronics","isVorOnly":false,"title":"npj Flexible Electronics"},"publishedOn":"2025-11-18 15:59:02","publishedOnDateReadable":"November 18th, 2025"},"versionCreatedAt":"2025-06-13 10:55:18","video":"","vorDoi":"10.1038/s41528-025-00493-6","vorDoiUrl":"https://doi.org/10.1038/s41528-025-00493-6","workflowStages":[]},"version":"v1","identity":"rs-6608314","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-6608314","identity":"rs-6608314","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}