Fiber‐Based Flexible Ionic Diode with High Robustness and Rectifying Performance: Toward Electronic Textile Circuits

In response to the growing demand for wearable devices designed for seamless integration with 3D bio‐surfaces, fiber‐based devices have gained prominence in textile‐based wearable electronics due to their flexibility and unique structure. In particular, though diodes with rectifying properties are crucial for a range of electronic systems, there has been scant reporting on diodes with a fiber structure. This study introduces a fiber‐based flexible ionic diode that exhibits a rectification ratio of 2773 and an output current of 28.2 mA at 3 V. The diode is composed of a double helical Zn‐based fiber anode, a Ti‐based fiber cathode on Au nanoparticle‐based flexible fiber electrodes, and a LiCl hydrogel electrolyte. By modulating the double helical design and the ionic conductivity of the hydrogel, the electrical performance of the diode to achieve varying rectification ratios and output currents can be tailored. Owing to the remarkable flexibility and stability of the fiber electrodes, the fiber‐based ionic diode consistently upholds its rectifying capabilities, even under washing procedures and significant bending deformation. Furthermore, this diode seamlessly integrates into various electronic circuits, including half‐wave rectifiers, capacitor–diode filters, and logic gate systems.


Introduction
[3] In particular, fiberbased soft electronic devices, which feature a 1D structure, have been intensively developed for wearable electronic textiles due to their high mechanical compliance and the ability to be seamlessly integrated into a fiber-based textile platform. [4,5]Leveraging their unique structural advantage, a variety of fiber-based devices, DOI: 10.1002/aelm.202300653such as fiber transistors, [6,7] lightemitting diodes, [8,9] energy harvesting and storage devices, [10][11][12][13][14][15][16] mechanical sensors, [5,[17][18][19][20][21][22][23] electrostimulation devices, [24][25][26] drug-releasing systems, [27,28] and optoelectronic devices, [29][30][31][32] have been reported, achieving significant contributions to the development of in-textile-embedded devices.In order to further advance wearable electronic textile systems, it is fundamentally essential to develop fiber-based passive electronic components, such as fiber resistors, transistors, capacitors, inductors, and diodes, for achieving textile-based integrated circuit systems.Particularly, diodes, with their asymmetric current rectifying function, stand out as indispensable elements in integrated circuits due to their roles in rectification, voltage regulation, and device protection from overvoltage.42] Among the various types of flexible diodes, ionic diodes, which rely on the unidirectional transportation of ions in ionic active materials, offer several promising advantages such as easy fabrication, high mechanical compliance, and low power consumption. [41,43]In these ionic diodes, a forward bias applied to the diodes generally induces dissolution of the surface of the metal electrode, resulting in a forward electrical current and switching the diodes to the "on" state.In contrast, a backward bias forms an oxide layer on the surface of the electrode, effectively blocking the electrical current.Guo et al. developed a flexible ionic diode with high rectifying and temperature-tolerant performance by using an ionic hydrogel in an ethylene glycolwater binary solvent system. [44]The developed flexible diode exhibited a high current output and rectifying ratio across a wide working temperature range.Nevertheless, the planar structure of the diode could be potentially limited in its mechanical deformability and structural adaptability to the complex morphologies of typical textiles.Building upon the developments, Choi et al. reported a fibriform organic electrochemical diode that can be used for rectification (≈123 of rectification ratio), logic circuits, and transient voltage suppression circuits in electronic textiles. [45]Composed of two metal wire electrodes, a polymeric semiconductor layer, and an ionic gel, the developed fibriform diode showed promising integration capabilities within textile structures.However, the inherent rigidity of the metal wire electrodes could significantly hinder the desired mechanical flexibility of the fibriform diode for real-world electronic textile applications, despite some degree of pliability.Furthermore, there exists a clear demand to enhance the rectification ratio of the fibriform diode to accommodate a broad range of electronic applications.
Herein, we report a fiber-based flexible ionic diode that incorporates a hydrogel ionic diode system onto a fiber electrode.This ionic diode employs two metal fiber electrodes, Zn anode, and Ti cathode, on Au nanoparticles (AuNPs)-based fiber electrodes.The AuNPs-based fiber electrodes were fabricated by absorbing a large amount of Au + ions into a polyurethane (PU) layer uniformly coated on a Nylon fiber, and subsequently reducing the absorbed Au + ions into AuNPs chemically.Benefiting from its elastomeric matrix, the AuNPs-based fiber electrodes not only exhibit a low electrical resistance of ≈2 Ω cm −1 but also impressive stability.The two fiber electrodes were organized in a double helical structure with a uniform distance, fully covered with a LiCl hydrogel in the fiber-based diode.The fiber-based ionic diodes achieve a high rectification ratio of 2773 and an output current of 28.2 mA, offering excellent performance over previous flexible ionic diodes.The diode's functionality can be adeptly tuned by modulating the double helical configuration and the ionic conductivity of the gel.Demonstrating robust flexibility and stability, the diode upholds its rectifying capability across diverse bending deformation.Moreover, this fiber-based ionic diode has been seamlessly incorporated into diode-centric electronic frameworks, including half-wave rectifiers, capacitor-diode filters, and logic gates.

Design and Working Mechanism of Fiber-Based Ionic Hydrogel Diode
We designed a fiber-based ionic hydrogel diode with high rectification performance, utilizing AuNPs-based flexible fiber electrodes and an ionic hydrogel that facilitates ion movement between the fiber electrodes.Figure 1a presents schematic illustrations for the structure of the fiber-based ionic hydrogel diode.The fiber-based ionic diode comprises two fiber electrodes integrated on a core supporting fiber, with an ionic hydrogel covering both fiber electrodes.The anode and cathode for the fiber diode were constructed by depositing Zn and Ti layers, respectively, onto AuNPs-based conductive and flexible fiber electrodes.These AuNPs-based fiber electrodes were produced by incorporating a large quantity of AuNPs into a PU elastomeric layer dip-coated on a nylon-based fiber scaffold (Figure S1, Supporting Information).The AuNPs were incorporated into the PU layer, which was dip-coated on the Nylon core fiber, using a chemical reduction method.This approach involved absorbing a substantial amount of Au + ions into the PU layer and subsequently reducing the absorbed Au + ions to AuNPs within the PU layer through a mild reducing agent, as previously reported. [46]The thickness and electrical resistance of the AuNPs-based fiber electrode could be effectively modulated by adjusting fabrication parameters such as the drawing rate during the dip-coating of the PU layer and the number of repetitions for the AuNPs chemical reduction process on a single fiber electrode (Figure S2, Supporting Information).The fabricated AuNPs-based fiber electrode exhibited a low electrical resistance of ≈2 Ω cm −1 , attributed to the conductive path formed by the densely packed AuNPs within the electrode.Subsequently, the fiber anode was coated with an ≈8 μm Zn layer using the electroplating deposition method, while the fiber cathode was coated with a ≈3 μm Ti layer using the sputtering deposition method, both on the AuNPs-based fiber electrodes (Figure S3, Supporting Information).Figure S4 (Supporting Information) presents typical scanning electron microscope (SEM) images showing that the Ti layer is uniformly deposited on the surface of the fiber electrode via the sputtering method.The incorporation of AuNPs into the PU layer and the deposition of the Zn and Ti layers on the AuNPs-based fiber electrodes were further characterized using energy dispersive spectroscopy (EDS) and Xray photoelectron spectroscopy (XPS), as shown in Figures S5,S6 (Supporting Information).The Zn/AuNPs-based fiber anode and Ti/AuNPs-based fiber cathode were aligned on a core supporting fiber in a double helical configuration, maintaining a consistent inter-distance (Figure 1a).To achieve unidirectional rectifying capability between the two electrodes, an ionic hydrogel made of polyacrylamide (PAAm) infused with a LiCl solution was applied over the fiber electrodes wound around the core supporting fiber, acting as the electrolyte for the ionic diode.Figure 1b displays a photograph of the flexible fiber-based ionic diode, constructed with two double-helical electrodes at three turns per cm and a core fiber with a diameter of 500 μm.To prevent the ionic hydrogel covering the electrodes from evaporating over time, it was securely encapsulated within a thin Ecoflex layer, as shown in Figure 1c.
As depicted in Figure 1d, the fabricated fiber-based ionic diode achieves asymmetric rectifying characteristics, rooted in aqueous ion transport that permits current flow under forward bias between the fiber anode and cathode.In particular, under a positive potential applied to the Zn/AuNPs-based fiber anode (the forward bias), anions (Cl − , OH − ) within the hydrogel electrolyte gravitate toward the fiber anode.Simultaneously, several cations (Zn 2+ , Li + , and H + ) migrate to the fiber cathode due to the induced electric field.As the applied potential exceeds a specific threshold (the threshold voltage of the diode), redox reactions are markedly activated at both fiber electrodes: 2H + + 2e = H 2 on the cathode surface and Zn = Zn 2+ + 2e on the anode surface.Due to the oxidation reaction occurring in the Tibased cathode, electrons flow into the outer circuit toward the Zn-based anode, which causes current to flow through the diode.Owing to these pronounced redox reactions combined with ion transportation through the hydrogel electrolyte, the current significantly increased with an increasing forward bias, transitioning the fiber diode to the "on" state (Figure 1e).Conversely, upon the imposition of a negative potential on the Zn/AuNPsbased fiber anode (reverse bias), cations are drawn to the surface of the Zn fiber anode while anions are attracted to the surface of the Ti fiber cathode (Figure S7, Supporting Information).However, an oxidation reaction on the cathode surface (Ti = Ti 2+ +2e) is restricted due to the passive behavior of the Ti-based electrode under the reverse bias, [47] constraining ion transportation within the fiber diode and resulting in its "off" state.

Basic Properties of the Fiber-Based Ionic Diode
Figure 2a displays the current-voltage (I-V) characteristic curve of the fiber-based ionic diode, fabricated using double helical electrodes with a density of three turns per cm, a core supporting fiber with a 500 μm diameter, and a length of 3 cm, as well as a 5 m LiCl hydrogel electrolyte.The fiber diode exhibited an apparent asymmetry in current flow, thus demonstrating its current rectifying capabilities.With a forward bias exceeding the threshold voltage of 0.38 V, the current rapidly increased, transitioning the diode into the "on" state.Under reverse bias conditions, the fiber diode exhibits a negligible "off" current (<0.8 mV) until reaching a specific level known as the breakdown voltage.The rectification ratio for the fabricated fiber diode was measured to 67.37 for a ±1 V bias and 27.19 for a ±3 V bias.To further understand the threshold voltage and breakdown voltage of the fiber diode, the cell potential (E Cell ) for the diode system was calculated using the Nernst equation.The electrochemical behavior of the fiber diode under forward bias depends primarily on the redox reaction Zn(s) + 2H + (aq) → Zn 2+ (aq) + H 2 (g), which takes place on the surfaces of the Zn and Ti-based two fiber electrodes.In nonstandard conditions, the cell potential can be changed by the concentration of any substance involved in the redox reaction, on a per-electron basis, as described by the following equation: where n represents the number of electrons transferred during the redox reaction, [C Ox ] * and [C Red ] * denote the concentrations of the oxidized and reduced species, respectively.The standard cell potential (E °Cell ) is commonly defined as the difference between the standard reduction potentials (E °Red ) of the redox reactions involved.To determine the E °Red values for the fiber anode and cathode under nonstandard conditions, Tafel plots for both electrodes were measured in a 5 m LiCl aqueous solution, as shown in Figure 2b.From the Tafel plots, the reduction potentials of the electrodes relative to the Ag/AgCl reference electrode were measured to be ≈−1.1767V for the Zn-based fiber anode and ≈−0.3806V for the Ti-based fiber cathode.Using these parameters, the equilibrium cell potential, E Cell , of the fiber diode was theoretically calculated to be −0.3823V, meaning that the threshold voltage of the fiber diode is 0.3823 V.The calculated threshold voltage aligns closely with the experimentally measured threshold voltage, as shown in Figure 2a.Detailed calculations of the equilibrium cell potential of the fiber diode are described in the Supporting Information.Under the reverse bias, the electrical behavior of the fiber diode is primarily governed by the following redox reaction: Ti(s) + 2H + (aq) → Ti 2+ (aq) + H 2 (g).Utilizing the Tafel plot for the Ti-based fiber cathode depicted in Figure 2b, the breakdown voltage of the fiber diode was theoretically calculated to be −3.3435V.The methodology behind this calculation is further elaborated in the Supporting Information.Based on the distinct asymmetric rectifying behavior of the fiber diode, the repeated switching performance of the diode was also investigated.Figure 2c illustrates the output current response of the fiber diode under repeated square-wave input voltage of ±1 V, highlighting its robust rectification capability and stable switching performance.In addition, the fiber diode displayed a rapid "on" response time of ≈0.11 s and an "off" response time of ≈0.09 s, as shown in Figures S9,S10 (Supporting Information).

Performance Characterization of Fiber-Based Ionic Diode
In electrochemical systems, the electrochemical current relies mainly on the charge transfer at the surface of two electrodes governed by redox reaction and ion movement through the bulk electrolyte.The electrical current flowing through the electrode, which arises from the charge transfer reaction occurring at the electrode surface, can be expressed mathematically by the Butler-Volmer equation: where n means the number of electrons transferred in the redox reaction, F is Faraday's constant, equivalent to the charge of one mole of electrons, v net indicates the net rate of the redox reaction, and A means the surface area of the electrode. [11,48]Furthermore, the current produced by ion movement through the bulk electrolyte is largely determined by two factors: the ionic conductivity of the electrolyte and the distance separating the two electrodes.
To investigate these effects, fiber-based ionic diodes with different densities of the two double helically wound fiber electrodes were prepared, as shown in Figure 3a.The density of the fiber electrodes in the diode has a direct correlation with their length, and by extension, their surface area.Therefore, a higher density leads to a greater surface area available for redox reactions when a potential is applied.According to Equation (2), the increased surface area of the electrodes can contribute to enhanced charge transfer current, improving the overall output current of the fiber diode.In particular, the length (l) of the fiber electrodes, which is intrinsically related to the electrode's surface area participating in the redox reaction, in the diode can be mathematically described using the density (n) of the double-helical turns of the fiber electrodes per centimeter and the diameter (d) of the core supporting fiber in the fiber diode, as follows: Detailed calculation for the length of the fiber electrode in the diode is described in the Supporting Information.Figure 3b shows that the output current of the fiber diode increases in correlation with a higher density of helical turns of the fiber electrodes in the diode.This relationship is mainly attributed to the longer fiber electrodes in the fiber diode with higher density, which consequently enlarges the surface area of the fiber electrodes involved in the redox reaction within the fiber diode (Figure S11, Supporting Information).In addition, the decreased distance between the two fiber electrodes with a higher density is beneficial for enhancing the output current of the fiber diode by reducing the electrical resistance associated with ion movement within the electrolyte hydrogel.Despite the increased output current, the rectification ratio of the fiber diode declined with the increased density of helical turns of the fiber electrodes, due to a corresponding elevation in "off" current in the diode (Figure S12, Supporting Information).
Similarly, the rectifying performance of the fiber diode can also be tuned by adjusting the diameter of its core supporting fiber.To investigate this effect, fiber diodes with different core diameters of 250, 500, and 750 μm were prepared, as shown in Figure 3c.The fiber diode including a core supporting fiber with a larger diameter contains longer fiber electrodes within the fiber diode according to Equation (3), thereby having a larger surface area of the fiber electrodes within the diode (Figure S11, Supporting Information).Figure 3d shows that the output current of the fiber diode increases with a larger diameter of the core supporting fiber in the diode.Although the rectification ratio of the fiber diode decreased with a larger diameter of the core supporting fiber, the fiber diode fabricated with a double helical structure of three turns per cm and a core supporting fiber with 250 μm diameter exhibited a high rectification ratio of 2773 (Figure S12, Supporting Information).
The electrical performance of the fiber-based ionic diode can also be modulated by the concentration of the LiCl electrolyte hydrogel in the diode.As shown in Figure 3e, the output current of the fiber diode effectively increased according to increasing LiCl concentration.This relationship is attributed to the increase in the ionic conductivity of the electrolyte with higher ion concentration.The increased ionic conductivity enhances the ion transportation through the electrolyte, thereby increasing the output current of the diode.Based on the double-helical structure of the fiber electrodes, the presented fiber-based ionic diode exhibited a higher rectification ratio and output current in comparison with the previously reported planar and fibrous flexible diodes (Figure 3f and detailed information in Table S1, Supporting Information).

Durability Demonstration of Fiber-Based Ionic Diode
To address the need for integration into practical electronic wearable systems, evaluating the stability of these diodes through washing processes becomes crucial.To assess their resistance against washing procedures, the fiber-based ionic diodes were immersed in an aqueous detergent solution and agitated at 500 rpm for durations ranging from 1 to 4 h, as shown in Figure 4a. Figure 4b,c notably illustrates that the electrical robustness and rectification ratio of the diodes remained durable regardless of the number of washing cycles performed.Table S2 (Supporting Information) provides comprehensive rectification ratio data for the fiber-based ionic diode across various stages of the washing process.
To assess the suitability of the fiber-based ionic diode for diverse wearable applications, it is essential to evaluate its robustness against physical deformations, particularly bending.We  and c) rectification ratios at ±2 V bias of the fiber diode corresponding to different washing cycles.d) Photograph of the fiber diodes subjected to bending deformation at various bending radii.e) I-V characteristic curves and f) rectification ratios at ±2 V bias of the fiber diode corresponding to different bending radii.g) I-V characteristic curves and h) rectification ratios at ±2 V bias of the fiber diode corresponding to repetitive bending deformations with a bending radius of 4.1 mm.i) Performance stability of the fiber-based ionic diode under square-wave input voltage at ±1 V bias with 0.5 Hz.".investigated the stability of the fiber diode under varying bending curvatures, as illustrated in Figure 4d. Figure 4e presents the I-V characteristic curves for the fiber diode subjected to multiple bending conditions.Remarkably, the fiber diode maintained stable rectifying performance across various bending deformations, demonstrating its high stability and robustness against mechanical deformation.The rectification ratio of the fiber diode was effectively retained without any considerable degradation, even when subjected to a bending deformation with a radius as small as 4.1 mm (See Figure 4f; Table S3, Supporting Information).In exploring the diodes' enduring performance, a series of repetitive bending deformations were conducted at a consistent bending radius of 4.1 mm.Remarkably, even after subjecting the diodes to 100 such repetitive bending deformations, their robustness remained virtually unchanged from their initial state, as evidenced in Figure 4g.Furthermore, the rectification ratio at a ±2 bias exhibited a consistent level throughout the 100 bending cy-cles (see Figure 4h; Table S4, Supporting Information).To more closely evaluate the impact of bending deformation on device performance, the Zn and Ti layers deposited on the fiber electrode were investigated upon repeated bending deformation (Figure S13, Supporting Information).The Zn and Ti layers on the surface of the electrode were intact maintained with no significant damage and delamination even under 100 bending cycles with a bending radius of 5 mm, demonstrating high stability of the Zn and Ti layer on the fiber electrodes.These results firmly establish the durability of the fiber-based ionic diodes against both washing procedures and bending deformations, thereby confirming the potential applicability of fiber-based ionic diodes in real-world wearable textile systems.
Additionally, to demonstrate long-term stability, the endurance of the fiber-based diode was rigorously tested.Subjected to 100 cycles of a square-wave input voltage with a ±1 V bias at a frequency of 0.5 Hz, the diode displayed consistent robustness, with negligible changes in performance.This demonstration of enduring stability is depicted in Figure 4i.The experiment comparing the robustness of the fiber-based ionic diode for 24 h can be confirmed in Figure S14 (Supporting Information).Data measured at 3 h intervals show almost similar robustness for 1 day.

Integrated Circuits of Fiber-Based Ionic Diode
Given the impressive performance and robustness of the fiberbased ionic diode, we demonstrated its potential application in half-wave rectifiers, capacitor-diode filters, and logic gate circuits.The fiber diode effectively rectified input sinusoidal signals at varying frequencies, 50, 100, and 200 mHz, converting them into half-waves, which affirms its rectifying capability (Figure 5a; Figure S15, Supporting Information).Leveraging this rectification performance, the fiber diode was integrated into a capacitordiode filter, complemented by a resistor and capacitor in parallel (Figure 5b).For AC inputs at frequencies of 1 and 2 Hz, the repeated charge and discharge cycles of the rectified AC were distinctly observed, highlighting the successful operation of the fiber diode in the capacitor-diode filter configuration (Figure 5c; Figure S16, Supporting Information).
Logic gates, in their various forms, support digital integrated circuits by orchestrating decisions derived from input signal combinations.Typically, logic gates come with dual inputs and a singular output.The output signal depends on both the logic gate type and the input signals.When categorizing input signals, "off" is labeled as "FALSE (0)" and "on" as "TRUE (1)".The outcome at each logic gate terminal is thus determined as TRUE or FALSE by the input combinations.In particular, an OR gate is activated with just one signal engaged, whereas an AND gate necessitates the activation of both signals.The intrinsic switching capabilities of diodes can be employed to architect these logic gate circuits.Given that diodes conduct current in a singular direction, current will only traverse when the applied voltage upstream exceeds that downstream of the diode.Figure 5d and Figure S17 (Supporting Information) provide circuit diagrams for AND and OR logic gates, tailored using two fiber-based ionic diodes and a resistor.Figure S18 (Supporting Information) depicts a circuit configuration within a textile comprising two fiber diodes and an AuNPs-based fiber resistor.In the OR gate configuration, the output signal was unequivocally set to TRUE when either input signal was TRUE, as illustrated in Figure 5e.Nevertheless, there was a slight attenuation in the OR gate's output voltage amplitude relative to the input, attributed to the diodes' forward voltage drop (V F ). Figure 5f displays the AND gate's operation: the output is TRUE only when both input signals are TRUE.The seamless functioning of the OR and AND gates, organized by using the fiber-based ionic diode, underscores the diode's remarkable rectifying and switching capabilities.Such findings suggest the vast potential for its incorporation into diverse digital circuitry, where logic gates compute the amalgamation of input signals.

Conclusion
In this study, we introduce a fiber-based flexible ionic diode characterized by high rectifying capabilities and exceptional flexibility.This fiber diode is fabricated by coating two fiber electrodes, a Zn-based anode and a Ti-based cathode, which are doublehelically wound around a central supporting fiber, with a LiCl hydrogel.To ensure enhanced flexibility and stability, we employed AuNPs-based flexible fiber electrodes as core electrodes for both the Zn anode and Ti cathode.By altering the doublehelical configuration of these fiber electrodes, we could adeptly regulate the diode's rectifying performance and output current.Based on the unique structure, our fiber diode achieves a high rectification ratio of 2773 and an output current of 28.2 mA at 3 V.Leveraging the structural advantage of fiber-based electronic devices, this fiber-based ionic diode can be seamlessly integrated into advanced textile-based electronic systems.This integration achieved without any soldering points by utilizing the fiber electrodes in the devices as interconnects.Even under washing procedures and diverse bending stresses, the fiber diode consistently maintained its rectifying performance, demonstrating its stability and flexibility.However, further work is required to achieve higher mechanical flexibility of the fiber diode before it can be applied to practical applications.To this end, reducing the diameters of both the fiber electrodes and the core fiber scaffold could be a viable approach.This reduction would decrease the overall diameter of the fiber diode, consequently leading to lower bending stiffness.Nevertheless, the fiber electrodes with reduced diameter can also compromise electrical conductivity.Therefore, it is necessary to systematically examine the effects of varying the diameters of both the fiber electrodes and the core fiber scaffold on the electrical performance and mechanical flexibility of the diode.This fiber-based ionic diode can be integrated into closed circuit devices fabricated with fiber electrodes, and is compatible with various electronic components, facilitating its incorporation into a range of diode-centric electronic systems, from half-wave rectifiers, capacitor-diode filters, and logic gate circuits.

Experimental Section
Preparation of the Flexible Conductive Fiber Electrode: Commercial nylon fibers (CAPOS Fluoro Nylon monofilament 0.6, Shinyang) were prepared and cleaned by mild sonication in DI water for 5 min.The cleaned nylon fibers were coated with PU layer through the dip-coating method at a rate of 0.2 cm s −1 with a 20 wt.% PU (UR30-GL-000101, Polyurethane Granule 3-5 mm particle size, Goodfellow Cambridge Ltd.) /THF solution.The flexible conductive fiber electrodes were fabricated using a chemical reduction method.To this end, the PU-coated nylon fibers were immersed in a 20 wt.% solution of Au precursors (Gold(III) chloride trihydrate, 520 918, ≥99.9% trace metals basis, Sigma-Aldrich) in ethanol for 25 min to absorb a large number of Au + ions into the PU-coated nylon fibers.Subsequently, the alcoholic solvent within the fibers was vaporized entirely in the air for 10 min.To convert Au + ions within the fibers into AuNPs, the fibers containing Au + ions were immersed in a chemical-reducing solution with 10 wt.% L-ascorbic acid (A5960, BioXtra, ≥99.0%, crystalline, Sigma-Aldrich) in DI water.After 60 min, the reducing agent solution was thoroughly washed from the fibers with DI water several times, and any remaining DI water was evaporated in the air for 20 min.All the steps involving the absorption and reduction of AuNPs in the fibers were repeated for four cycles to increase the concentration of the AuNPs.To fabricate the fiber anode, the Zn layer was electroplated onto the AuNPs-based fiber electrodes by applying −2 V in 1 m ZnSO4 solution for 40 s.Zn foil was used for the counter electrode, and an Ag/AgCl electrode was employed as the reference electrode.All electroplating steps were carried out using amperometry in a potentiostat (Multi Autolab M204).Using the sputtering method, the Ti layer was deposited onto the AuNPs-based fiber electrode for the fiber cathode.During the deposition process with a 5 min pre-deposition time and a 5 min deposition time, the sputtering power was set at 200 W, and the Ar gas conditions were adjusted to 20 ppm concentration and 5 mtorr pressure.
Fabrication of the Fiber-Based Ionic Diode: The electrolyte hydrogel layer was prepared using the following steps.A mixture of 25 wt.% acrylamide powder (A3553, suitable for electrophoresis, ≥99% (HPLC), powder, Sigma-Aldrich) and 0.15 wt.% N,Nʹ-methylenebisacrylamide (M7279, powder, for molecular biology, suitable for electrophoresis, ≥99.5%, Sigma-Aldrich) was dissolved in a 5 m LiCl aqueous solution.Additionally, 0.001 g of ammonium persulfate (A3678, for molecular biology, suitable for electrophoresis, ≥98%, Sigma-Aldrich) and 2 μl of N,N,Nʹ,Nʹtetramethylethylenediamine (T7024, BioReagent, for molecular biology, ≥99% (GC), Sigma-Aldrich) were dissolved in 1 mL of the LiCl/PAAm aqueous solution.The fiber electrodes, wound around the core supporting fiber, were set into a round mold containing the solution designated for hydrogel formation.Subsequently, an Ecoflex layer was applied over the hydrogel electrolyte on the fiber, serving as an encapsulation layer that enveloped the entire fiber diode.
Characterization of Fiber-Based Electrode: The surface morphologies of the fiber-based electrodes and the cross-section of the fiber-based ionic diode were examined using a field emission scanning electron microscope (Hitachi Inc., S-4800) coupled with an EDS module (Hitachi Inc.) to obtain the SEM images and EDS mapping images.The SEM images containing ionic hydrogel were obtained using the freeze-drying method.The components of the fiber-based electrodes were investigated using X-ray photoelectron spectroscopy (Thermo Scientific Inc., ESCALAB 250Xi).The Tafel plots of the fiber electrodes were obtained using a potentiostat to measure the electrode reduction potential in a LiCl aqueous solution.
Measurements of the Fiber-Based Ionic Diode: The I-V characteristic curve of the fiber-based ionic diodes was measured using a sourcemeter (Keithley 2450).To assess the durability of the fiber-based ionic diodes during washing procedures, an aqueous detergent solution was formulated by combining 3 mL of sodium laureth ether sulfate (SLES)-based detergent with 500 mL of water.Subsequently, the fiber-based ionic diodes were submerged in the solution and agitated at 500 rpm at room temperature for 1 h.Following this, the diodes underwent a 1 h drying process under a bench-top fume hood.This washing cycle was repeated four times for the evaluation.To assess the bending stability of the fiber electrodes, a custom-made bending stage was employed to apply bending deformation to the fiber electrodes and diodes, and their electrical performance was simultaneously measured using a sourcemeter.Half-wave rectifier, capacitor-diode filter, and logic gate circuits were constructed on breadboards.The half-wave rectifier circuit was constructed with a 330 Ω resistor and a fiber diode, and ±2 V bias AC signals with frequencies ranging from 0.05 to 0.2 Hz were applied.In the capacitor-diode filter, a 5 kΩ resistor, and a 470 μF capacitor were connected in parallel with the fiber-based ionic diode, and ±2 V bias AC input signals with frequencies ranging from 0.

Figure 1 .
Figure 1.a) Schematic illustration depicting the structure of the fiber-based flexible ionic diode.The high-magnification inset reveals the two doublehelical fiber electrodes on the core-supporting fiber, the LiCl ionic hydrogel layer, and the Ecoflex-based encapsulation layer.b) Photograph of the fabricated fiber-based flexible ionic diode.c) SEM image illustrating the cross-section of the fiber-based ionic diode.d) Simplified circuit diagram of the fiber-based flexible ionic diode.e) Ion transportation within the hydrogel electrolyte of the fiber diode under forward bias.

Figure 2 .
Figure 2. a) I-V characteristic curve of the fiber-based ionic diode.b) Tafel plots of the Zn/AuNPs-based fiber anode and Ti/AuNPs-based fiber cathode, measured in a 5 m LiCl aqueous solution.c) Output current response of the fiber diode subjected to a repeated square-wave input voltage of ±1 V at a frequency of 0.05 Hz.

Figure 3 .
Figure 3. a) Microscopic images and b) I-V characteristic curves of fiber-based ionic diodes with different double helical turn densities of the fiber electrodes.c) Microscopic images and d) I-V characteristic curves of fiber-based ionic diodes with different core fiber diameters.e) I-V characteristic curves of fiber diodes with different LiCl concentrations of electrolyte hydrogel (Conditions except independent variable are three turns per cm, 500 μm of core diameter, 5 m LiCl in electrolyte hydrogel, and 3 cm of length).f) Maximum rectification ratio and output current of recently reported film/fiber-based flexible diodes and the fiber-based ionic diode in this work.

Figure 4 .
Figure 4. a) Schematic diagram of washing processes of textile-embedded fiber-based ionic diode.b) I-V characteristic curves andc) rectification ratios at ±2 V bias of the fiber diode corresponding to different washing cycles.d) Photograph of the fiber diodes subjected to bending deformation at various bending radii.e) I-V characteristic curves and f) rectification ratios at ±2 V bias of the fiber diode corresponding to different bending radii.g) I-V characteristic curves and h) rectification ratios at ±2 V bias of the fiber diode corresponding to repetitive bending deformations with a bending radius of 4.1 mm.i) Performance stability of the fiber-based ionic diode under square-wave input voltage at ±1 V bias with 0.5 Hz.".

Figure 5 .
Figure 5. a) AC input (50 mHz) and the resultant output signals of the fiber-based ionic diode in the half-wave rectifier.b) Circuit diagram of a capacitordiode filter incorporating the fiber diode.c) AC input signals with frequencies of 1 and 2 Hz applied to the capacitor-diode filter, alongside the associated output signals.d) Schematic diagrams of OR and AND logic gates.e) Input and output signals for the OR gate and f) for the AND gate, both fabricated using the fiber diodes.

5 to 2
Hz were applied.The logic gate circuits consisted of two fiber-based ionic diodes and a 330 Ω resistor.Arduino relay modules were used to apply two input signals simultaneously.All input voltages were generated using a waveform generator (Keysight 33500B), and the output signals were measured using a digital storage oscilloscope (Keysight DSOX 1202A).government (MSIT, No. NRF-2022M3E5E9017837), by the Industrial Fundamental Technology Development Program (20018274, Development of gripper system for various production processes and multi-modal flexible tactile sensor system) funded by the Ministry of Trade, Industry & Energy (MOTIE) of Korea, and by the Korea Medical Device Development Fund grant funded by the Korea government (the Ministry of Science and ICT, the Ministry of Trade, Industry and Energy, the Ministry of Health & Welfare, the Ministry of Food and Drug Safety) (Project Number: 1711196794, RS-2023-00243310).