Microfluidic fabrication of dual-functional hydrogel optical fibers with controlled swelling for simultaneous light transmission and ionic conductivity

Abstract

Multifunctional hydrogel fibers with integrated optical and electrical properties are essential for bioelectronics and soft robotics. Herein, we report on conductive hydrogel optical fibers (CHOFs) that combine light transmission, ionic conductivity, and mechanical flexibility via microfluidic coaxial extrusion. The fibers feature core-cladding structures composed of double-network hydrogels made of acrylamide and alginate. The strategic incorporation of sodium acrylate (SA) into the cladding enabled controlled differential swelling and maintained refractive index contrast for optical waveguiding under ionic conditions. Systematic SA variation (0–5 mol%) demonstrated precise property control, achieving a numerical aperture of 0.339, an optical attenuation of 0.088 dB cm⁻¹, an ionic conductivity of 26.6 mS cm⁻¹ in 1.0 M NaCl, and a stretchability of up to 321% strain. These CHOFs can function as strain sensors (gauge factor = 1.21) while maintaining optical transmission. Real-time human motion detection validated the dual functionality in wearable applications, demonstrating superior performance under mechanical deformation compared to conventional optical fibers. These CHOFs provide a platform for applications that require integrated optical transmission, electrical conduction, and mechanical sensing.

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1. Introduction

Hydrogels have emerged as transformative materials for biomedical applications owing to their unique combination of biocompatibility, mechanical flexibility, and tunable properties.^1–6 softness and adaptability minimize the immune reactions upon implantation, thereby supporting their long-term integration with biological tissues.⁷ Furthermore, their mechanical properties, ranging from soft to load-bearing tissues, can be precisely tailored through modifications in crosslinking or reinforcement.^8,9 Their responsiveness to stimuli, such as pH, temperature, and biomolecules, further enables the creation of dynamic, intelligent therapeutic systems.^10–12 Moreover, hydrogels can provide unique advantages, such as the ability to incorporate functional materials, excellent tissue adhesion, and outstanding ionic or electronic conductivity.¹³

Owing to these advantages, recent progress in hydrogel optical fiber (HOF) technology has opened new avenues for guiding light and transmitting signals within biological tissues.⁴ Although rigid silica optical fibers offer excellent optical performance, their mechanical incompatibility with soft tissue limits their biomedical applications.^14,15 HOFs naturally conform to dynamic tissue environments while preserving optical performance as a critical feature for emerging fields such as bio-photonics and optogenetics.^16–18

Recent advances in microfluidic fabrication^19–23 have enabled the production of HOFs with enhanced structural control, optical transmission under mechanical deformation, and bio integration.¹⁴ These developments have demonstrated the potential of hydrogel-based optical systems; however, current HOFs remain limited to passive light delivery without integrated sensing capabilities. The fundamental challenge of maximizing the numerical aperture (NA) through controlled refractive-index optimization in swollen hydrogel networks, while simultaneously incorporating functional properties such as electrical conductivity, remains largely unresolved. Optical fibers rely on total internal reflection, which requires a refractive index contrast between the core and cladding.²⁴ HOFs replicate this architecture, but face inherent limitations owing to the relatively small refractive index contrast achievable in hydrated polymer systems compared with that in silica-based fibers.²⁵

In addition to simple optical transmission, the multifunctional nature of hydrogels makes HOFs ideal for next-generation biomedical applications.^15,26,27 Incorporating ionic conductivity into HOFs enables the simultaneous transmission of both optical and electrical signals, thereby simplifying the devices and reducing potential failure points. Such dual-functional fibers allow applications in neural interfaces that require simultaneous optogenetic stimulation and electrical recording or in smart surgical tools that combine phototherapy with electrical strain sensing.^28–32 Incorporating electrical conductivity into the proven HOF technology faces fundamental material science challenges that are directly related to the swelling control strategies used for optical optimization. The direct incorporation of conductive elements often compromises optical transparency and mechanical flexibility.^33,34

Herein, we report the first demonstration of integrating of ionic conductivity into microfluidically fabricated HOFs without compromising their optical performance or mechanical properties. Building upon proven HOF technology and swelling control strategies, we developed a controlled approach that introduces high ionic conductivity while preserving and even enhancing light-guiding efficiency through an optimized numerical aperture design. By precisely controlling the chemical composition, we manipulated the swelling ratio between the core and cladding materials to maximize the refractive index contrast for improved optical performance, while simultaneously forming controlled pathways for ionic conductivity enhancement through an NaCl treatment. The resulting conductive hydrogel optical fibers (CHOFs) simultaneously achieved high optical transmission (0.088 dB cm⁻¹) and ionic conductivity (26.6 mS cm⁻¹), while retaining the mechanical flexibility and deformation resilience that define advanced HOF systems.

2. Experimental section

2.1. Materials

Acrylamide (AAm), sodium alginate (Na–Alg), sodium acrylate (SA), and calcium chloride (CaCl₂) were purchased from Sigma-Aldrich (St. Louis, MO, USA). N,N′-methylenebis(acrylamide) (bisAA) was purchased from Bio Basic Inc. (Markham, Canada). Lithium phenyl-2,4,6-trimethylbenzoylphosphinate (LAP) was purchased from SNVia (Busan, South Korea). Sodium chloride (NaCl) was obtained from Daejung Chemicals and Metals (Gyeonggi-do, South Korea). All purchased chemicals were used without further purification.

2.2. Preparation of microfluidic devices

A glass capillary (inner) and PTFE capillaries (middle and outer) were coaxially aligned and placed on a glass slide. The diameters of the inner, middle, and outer capillaries were 1.1, 2.2, and 6.0 mm, respectively. The glass capillaries (inner) were soaked in 0.2% (v/v) octadecyl trichlorosilane dissolved in toluene for 20 min to form a hydrophobic surface before being assembled into devices. The samples were dried for an hour at 100 °C in an oven.

2.3. Preparation CHOF

The CHOFs were fabricated via two steps. In the first step, two sets of pre-gel solutions were prepared for the core and cladding. The core pre-gel solution (5 mL) was prepared by dissolving AAm (1.75 g, 24.62 mmol), Na–Alg (0.175 g, 3.5 wt%), and bisAA (60.73 mg, 1.6 mol% of AAm) in deionized water. The cladding pre-gel solution (5 mL) contained AAm, Na–Alg (0.075 g, 1.5 wt%), bisAA (1.0 mol% of AAm), and SA at varying compositions (0–5 mol%). The total monomer concentration was kept constant at ∼10.5 mmol, with SA substituting AAm on a molar basis. The bisAA crosslinker was adjusted proportionally to maintain 1.0 mol% relative to AAm in each composition. To initiate free-radical polymerization, LAP (2.5 mg, 0.005 mmol) was added to the degassed pre-gel solutions. The core and cladding pre-gel solutions were injected into the inner and middle capillaries, respectively, and the outer capillary was filled with 0.3 M CaCl₂ in a 0.01 M Tris buffer solution. The flow rate of each solution (inner: 300 µL min⁻¹; middle: 600 µL min⁻¹; outer: 1500 µL min⁻¹) was adjusted using syringe pumps (Legato 100, KD Scientific, USA). The HOFs were collected in a coagulant bath containing 0.3 M CaCl₂ in a 0.01 M Tris solution after being photopolymerized by curing under 365-nm UV light (45 mW cm⁻²) (Omnicure S1500, Lumen Dynamics, Canada) for 20 min. In the second step, the fabricated HOFs were submerged in aqueous 1.0 M NaCl overnight.

2.4. Fabrication of strain sensors

To prepare strain sensors, the CHOFs were cut into pieces of 2–5 cm in length, depending on the specific use. Each piece was placed on a flexible tape and carefully aligned with a copper wire. The fiber and wire were then twisted together to form a firm connection. A copper tape was pressed onto both ends of the sensor to hold the fibers and wires in place. Finally, the connection points were wrapped with an insulating tape to protect them from damage during use.

2.5. Measurements

A universal testing machine (Shimadzu EZ-SX, Japan) was used to obtain the tensile stress–strain curves at a consistent stroke speed of 50 mm min⁻¹. After being sliced to a length of approximately 3 cm, the CHOFs were positioned between two machine grips with an initial grasp no longer than 1 cm. The refractive index (RI) of the hydrogel sheets was measured using a digital refractometer (OFR-3SM; Kern and Sohn, Germany). Hydrogel sheets were prepared with the same cladding and core compositions using a molding method. A power meter (1919-R, Newport, USA) and continuous-wave laser (PL202, λ = 635 nm, 0.9 mW, Thorlabs, USA) were used to quantify light-loss and light-guiding characteristics. Light attenuation was measured using a cut-back method, applying the equation, α (dB) = log₁₀(P₁/P₂), where P₁ and P₂ represent the transmitted light intensities before and after cutting, respectively. The measurements were performed sequentially after removing 1 cm segments from a fiber that was initially 20 cm in length. An LCR meter (Wayne Kerr 4100, UK) was used to measure resistance. An x-axis motorized device (SL2-15, ST1, Korea) was used for repetitive strain testing. A holding duration of one second and pace of two millimeters per second were chosen. A LED was powered using a DC power source (DP30-0.3, Toyotech, China) of 10 V and 0.1 A. A DMI-3000B microscope (Leica, Germany) was used to capture optical microscopy images in the bright mode.

3. Results and discussion

Dual-function hydrogel optical fibers were fabricated using a two-step process. As shown in Fig. 1a, the core pre-gel solution (5 mL) contained AAm (35 mol%), Na–Alg (3.5 wt%), and bisAA (1.6 mol% of AAm), while the cladding pre-gel solution contained AAm (15 mol%), Na–Alg (1.5 wt%), bisAA (1.0 mol%), and SA (varying from to 0–5 mol%). The light-guiding characteristics of the HOFs were established by forming a high-RI core and low-RI cladding. Based on this, the chemical compositions of the core and cladding pre-gel solutions were selected for microfluidic fabrication. A higher Numerical aperture (NA) value helps reduce light loss and enhance light guidance.^24,35 SA was selectively incorporated only into the cladding pre-gel solution to increase the NA by swelling the cladding region. As a highly hydrophilic and ionizable monomer, SA increases water uptake and swelling through electrostatic interactions, enabling controlled refractive-index tuning and swelling-behavior optimization.

Fig. 1 (a) Chemical compositions of core and cladding pre-gel solutions. (b) Schematic illustration of microfluidic coaxial extrusion system for continuous fabrication of dual-functional HOFs and optical microscopy images of the fabricated CHOFs. (c) Schematic representation of network formation stages: (i) initial pre-gel solution, (ii) ionic crosslinking with Ca²⁺, (iii) UV-induced covalent crosslinking forming double-network structure, and (iv) ionic conductivity introduction through NaCl immersion.

Continuous fabrication of CHOFs was achieved using a microfluidic coaxial extrusion system. As illustrated in Fig. 1b, multichannel microfluidic devices were fabricated by the coaxial positioning of capillaries of different diameters: a glass capillary (inner: 1.1 mm) and PTFE capillaries (middle: 2.2 mm and outer: 6.0 mm). Prior to assembly, the glass capillaries were treated with octadecyltrichlorosilane to form hydrophobic surfaces.

Three synchronized fluid streams were injected: core pre-gel solution (300 µL min⁻¹), cladding pre-gel solution (600 µL min⁻¹), and coagulation bath containing 0.3 M CaCl₂ in a 0.01 M Tris buffer (1500 µL min⁻¹). The Ca²⁺ ions in the coagulant solution diffused quickly into the core and cladding solutions, facilitating ionic crosslinking with Alg to form a solid Ca–Alg network. This rapid ionic reaction maintained the shape of the fiber as the solution flowed. Subsequently, UV light irradiation (365 nm, 45 mW cm⁻²) initiated the photopolymerization of the acrylate-functionalized components AAm and bisAA during flow. This resulted in a double-network (DN) structure in which the covalently cross-linked AAm was interlocked with the ionically cross-linked Ca–Alg. The DN system was adopted to combine the advantages of both networks, with the rigid Ca–Alg network providing structural integrity and shape retention during fabrication, while the flexible AAm network ensuring mechanical toughness and stretchability. This dual crosslinking strategy enabled the fibers to maintain their optical performance under mechanical deformation while providing the robustness required for practical applications.

In the second step, the fabricated hydrogel optical fibers were submerged in 1.0 M NaCl overnight to induce ionic conductivity. The selection of NaCl as the ionic medium ensured its biocompatibility for potential biomedical applications.^36,37 Sodium ions diffused throughout the hydrogel network, enabling charge transport and transforming the fiber into a dual-functional system. The Ca–Alg network remained stable during NaCl immersion due to the stronger binding affinity of Ca²⁺ ions compared to Na⁺ ions with alginate chains. The differential swelling between the core and cladding, driven by SA in the cladding region, ensured that the refractive index contrast necessary for light guidance was maintained, even after the NaCl treatment.

The resulting CHOFs exhibited well-defined core-cladding structures with smooth interfaces (Fig. 1c). Optical microscopy revealed clear demarcation between regions, with typical fiber diameters ranging from to 800–1200 µm and core-to-total diameter ratios of approximately 0.5. The flexibility of CHOFs was immediately apparent, as they could be bent, twisted, and stretched without visible damage, demonstrating the successful integration of optical transparency, mechanical compliance, and ionic conductivity into a single platform.

The development of CHOFs requires the maintenance of an adequate refractive index contrast between the core and cladding under ionic conditions. SA was incorporated only into the cladding region at varying compositions (0–5 mol%) to establish controlled differential swelling between the core and cladding.

Hydrogel sheets with a cladding composition of 1.0 M NaCl showed that the linear swelling ratio (λ) increased systematically from 1.05 to 1.19 as the SA content increased from 0 to 5 mol%. The progressive increase in swelling occurred because SA, as a highly hydrophilic and ionizable monomer, introduced anionic carboxylate groups that increased the electrostatic repulsion among the polymer chains and attracted more water molecules into the network structure.

The direct consequence of differential swelling was demonstrated by refractive index measurements of the corresponding hydrogel sheets (Fig. 2b). As the cladding swelled more extensively with increasing SA content, the material density decreased. The calculated density decreases systematically from approximately 1.14 to 1.08 g cm⁻³ as SA content increases (Fig. S1), reflecting the increased water uptake. This density reduction correlates directly with the refractive index decrease, consistent with the Lorentz–Lorenz equation which relates refractive index to material density and composition leading to a corresponding reduction in the refractive index. The core composition maintained a higher refractive index (1.39 in NaCl) than the increasingly swollen SA-rich cladding compositions, establishing a refractive index gradient suitable for optical waveguiding. The systematic relationship between the SA composition, swelling ratio, and refractive index provided precise control over the optical properties of the CHOF system.

Fig. 2 (a) Linear swelling ratio (λ) of cladding hydrogel sheets as a function of SA composition (0–5 mol%) in a 1.0 M NaCl solution. (b) Refractive index of hydrogel sheets with varying SA compositions (0–5 mol%) measured in 1.0 M NaCl. (c) Numerical aperture of CHOFs as a function of SA composition. (d) Light transmission photographs of CHOFs with different SA compositions. (e) Optical attenuation measurements of CHOFs as a function of SA composition.

The effectiveness of the differential swelling strategy was quantified through NA measurements, where represented the light-gathering capability of the optical fiber.³⁸ The controlled refractive-index contrast achieved through SA-induced differential swelling directly translated to enhanced optical performance.

For the structural characterization, we conducted SEM analysis of freeze-dried cladding samples with varying SA contents (Fig. S2). The images reveal porous network structures with similar overall morphology across all SA compositions.

As shown in Fig. 2c, the NA increased progressively from 0.252 for 0 mol% SA to 0.339 for 5 mol% SA, representing a 35% improvement in the light-gathering capability. This systematic improvement directly correlated with the increased refractive-index contrast resulting from increased cladding swelling at higher SA compositions. Optical transmission characterization using a red laser (λ = 635 nm) provided visual and quantitative confirmation of the improved performance. Light propagation photographs showed that fibers with higher SA content exhibit brighter and more uniform light transmission with improved optical confinement (Fig. 2d). The systematic improvement in brightness and uniformity reflects the increased NA and reduced light leakage achieved through the optimized refractive-index contrast.

As shown in Fig. 2e, quantitative optical loss measurements revealed that the optimized CHOFs with 5 mol% SA achieved a remarkably low attenuation of 0.088 dB cm⁻¹. This excellent optical performance demonstrates that the SA-based differential swelling approach successfully optimized the light-guiding efficiency while enabling ionic conductivity. We have also evaluated the wavelength-dependent optical performance by including a green laser source at 532 nm (Fig. S3a). CHOFs successfully transmit light at both wavelengths, demonstrating their capability for multi-wavelength optical transmission. Quantitative measurements (Fig. S3b) reveal that the optical attenuation is slightly higher at 532 nm (0.093 dB cm⁻¹) compared to 635 nm (0.088 dB cm⁻¹), consistent with the expected increase in scattering at shorter wavelengths. Both wavelengths exhibit excellent optical performance with attenuation below 0.1 dB cm⁻¹ confirming the versatility of CHOFs for various bio-optoelectronic applications.

The systematic relationship between the SA composition, differential swelling, refractive index contrast, and optical transmission establishes a robust design principle for developing high-performance dual-functional hydrogel optical fibers.

The incorporation of SA into the cladding region influenced the mechanical properties of CHOFs through changes in the swelling behavior. Tensile testing revealed that the SA content significantly affected the stretchability of the fiber system (Fig. 3a). As the SA content increased from 0 to 5 mol%, the maximum strain capacity increased substantially, reaching over 300% elongation at the highest SA content. The fracture stress remained relatively stable across the different SA compositions, indicating that the overall structural integrity was maintained.

Fig. 3 (a) Tensile stress–strain curves of CHOFs with varying SA compositions (0–5 mol%). (b) Young's modulus as a function of SA content in CHOFs. (c) Normalized light transmittance of 5 mol% SA CHOFs under uniaxial stretching up to 50% strain. (d) Photographs of light transmission through 5 mol% SA CHOFs under progressive stretching from 0% to 50% strain. (e) Normalized light transmittance of 5 mol% SA CHOFs during 30 cycles of 25% stretching and relaxation. (f) Photographs of 5 mol% SA CHOFs under various bending angles from 0° to 180°. (g) Light attenuation of 5 mol% SA CHOFs at different bending angles.

The enhanced stretchability with increasing SA content was attributed to swelling-induced softening of the hydrogel network. A higher SA composition led to increased water uptake and network expansion, which reduced the effective crosslinking density and allowed for greater deformation before failure. As shown in Fig. 3b, the elastic modulus systematically decreased from 420 kPa at 0 mol% SA to 80 kPa at 5 mol% SA, which directly correlated with the increased swelling ratio observed in Fig. 2a. The double-network (DN) structure, consisting of flexible AAm and rigid alginate networks, remained consistent across all compositions, with the sacrificial alginate network effectively dissipating tensile stress and the AAm network maintaining structural integrity.

A key advantage of CHOFs over conventional silica-based optical fibers is their ability to maintain light transmission under substantial mechanical deformation.¹⁴ When the CHOFs were stretched to 50% strain, light transmission continued effectively despite a reduction in intensity (Fig. 3c). T/T₀ represents the normalized transmittance, where T is the transmitted light intensity at a given deformation state and T₀ is the initial transmitted light intensity without deformation. The gradual decrease in the normalized transmittance from 1.0 to ∼0.6 demonstrates that the fibers retained their functional optical performance even under significant elongation. Representative photographs under various elongations ranging from 0% to 50% (Fig. 3d) visually confirmed that the light-guiding capability was preserved throughout the deformation range. The fibers continued to confine light effectively along the fiber axis, demonstrating that the core-cladding structure maintained a sufficient refractive index contrast even under substantial mechanical strain. The ability to operate under such deformation conditions represents a significant advantage over rigid optical fibers, which would fail catastrophically under similar mechanical stress conditions. Their long-term reliability under repeated deformations was demonstrated through cyclic stretching tests (Fig. 3e). The CHOFs subjected to repeated cycles of stretching and relaxation maintained a consistent optical performance over 30 half-cycles, with the normalized transmittance oscillating reversibly between approximately 0.65 and 1.0. We furthermore have evaluated the optical performance after repeated stretching cycles. The optical attenuation of CHOFs increased slightly from 0.088 to 0.098 dB cm⁻¹ after 100 cycles of 50% stretching (Fig. S4), demonstrating excellent stability with less than 12% change. Their stable cyclic response indicated that both the mechanical structure and optical pathway exhibited excellent reversibility and resistance to fatigue damage, enabling reliable operation in dynamic applications.

A bending performance evaluation further demonstrated the superior flexibility of the CHOFs compared to conventional optical fibers. The fibers maintained their light-guiding capability across the full range of bending angles from 0° to 180° (Fig. 3f), forming tight curves while preserving optical transmission. Such extreme bending causes the immediate failure of silica-based optical fibers. Quantitative measurements showed that light attenuation increased progressively but remained at acceptable levels throughout the bending range (Fig. 3g). The attenuation increased from approximately 0.08 dB cm⁻¹ at 0° to about 0.14 dB cm⁻¹ at 180°, representing only a modest increase that maintained functional optical performance. The ability to operate effectively under severe bending conditions demonstrates the unique advantages of hydrogel-based optical systems.

The combination of high stretchability, consistent optical performance under deformation, and excellent bending flexibility enables CHOFs to function in applications unsuitable for conventional optical fibers. The reversible performance of CHOFs under repeated mechanical stress makes them promising candidates for wearable electronics, soft robotics, and biomedical devices requiring both optical functionality and mechanical compliance.^7,39,40

The electrical performances of the CHOFs were evaluated to assess their potential for dual-functional applications requiring both optical transmission and electrical conduction. The CHOFs were immersed in 1.0 M NaCl to induce ionic conductivity through the infiltration of Na⁺ and Cl⁻ ions throughout the hydrogel network. The ionic conductivity was determined by measuring the resistance and applying the conductivity formula σ = l(AR)⁻¹, where R is the resistance, A is the cross-sectional area, and l is the fiber length. As shown in Fig. 4a, the ionic conductivity increased systematically with the SA content, reaching 26.6 mS cm⁻¹ for the 5 mol% SA sample when immersed in 1.0 M NaCl, compared to 9.4 mS cm⁻¹ for the 0 mol% sample. The enhanced conductivity at higher SA compositions resulted from multiple factors related to the swelling behavior and network structure modifications induced by SA incorporation. The ionic conductivity of CHOFs (26.6 mS cm⁻¹ in 1 M NaCl) is comparable to other reported hydrogel ionic conductors (Table S1, SI), while uniquely combining this electrical functionality with optical transmission capability.

Fig. 4 (a) Ionic conductivity of CHOFs as a function of SA composition in 1.0 M NaCl. (b) Nyquist plot from electrochemical impedance spectroscopy of 5 mol% SA CHOF. (c) CHOF functioning as both electrical conductor and optical waveguide to power an LED during stretching. (d) Photographs of CHOFs with 0 and 5 mol% SA swollen in 1.0 M NaCl at 0 and 12 h under ambient conditions. (e) Photographs of 5 mol% SA CHOFs swollen in different NaCl concentrations at 0 and 12 h. (f) Weight change of CHOFs with different SA compositions swollen in 1.0 M NaCl over time. (g) Weight change of 5 mol% SA CHOFs swollen in different NaCl concentrations over time.

The primary mechanism for conductivity enhancement involved an increased water content and expanded pore structure in the SA-rich regions. As shown in Fig. 2a, higher SA contents led to greater swelling ratios, creating larger and more interconnected water-filled channels within the hydrogel matrix. These expanded pathways enhanced the ion mobility and reduced the tortuosity of the ionic transport paths. Additionally, the anionic carboxylate groups introduced by SA provided favorable electrostatic environments for ion solvation and transport, further enhancing the ionic conductivity.⁴¹ The calculated ionic conductivity of 26.6 mS cm⁻¹ represents excellent performance for soft, flexible materials, placing the CHOFs in a range comparable to that of other ionic conductors used in soft electronics.^42–46 The double-network structure enabled both mechanical robustness and ionic mobility, with free ions migrating through the polymer matrix when a potential difference was applied.

Electrochemical impedance spectroscopy (EIS) was performed using a two-probe setup to assess the ionic transport characteristics of the CHOFs. Nyquist plots (Fig. 4b) revealed the electrical behavior over a wide frequency range, confirming low impedance and effective ionic transport within the fiber. The measured bulk resistance of 313 Ω at high frequency corresponds well with the direct current resistance measurements, validating the stable ionic conductivity of the CHOF system.

The EIS results demonstrated that the CHOFs exhibit favorable electrochemical properties with minimal capacitance effects and efficient ionic diffusion characteristics. The low internal resistance confirmed the effectiveness of NaCl treatment in establishing reliable ionic conduction pathways throughout the hydrogel network.

To validate the simultaneous optical and electrical functionality of the CHOFs, a conventional metal wire was replaced with a CHOF to power an LED (Fig. 4c). Upon connection, the LED illuminated successfully, confirming that the CHOFs can serve as effective ionic conductors capable of transporting current between circuit components. Simultaneously, the CHOF transmitted light along its length, demonstrating that both optical waveguiding and electrical conduction operated effectively within the same fiber. This demonstration shows the unique capability of CHOFs to function as multifunctional components, providing electrical conduction, optical transmission, and mechanical flexibility on a single platform. The ability to stretch the fiber while maintaining both optical and electrical functionalities represents a significant advantage over conventional systems that require separate components for each function.

In addition to introducing ionic conductivity, NaCl immersion provides another crucial benefit: it enhances environmental stability through moisture retention. Hydrogel-based materials typically suffer from functionality loss when water evaporates from the network system, leading to shrinkage, brittleness, and the loss of both optical and electrical properties.⁴⁷ As demonstrated in Fig. 4d, the weight change behavior varied significantly with the SA composition and NaCl treatment. CHOFs with higher SA contents showed better moisture retention, with the 5 mol% SA sample exhibiting negligible weight loss compared to the 40% weight loss observed in the 0 mol% SA sample after extended exposure to ambient conditions. The comparison of different NaCl compositions revealed that CHOFs treated with 1.0 M NaCl maintained a stable weight, while untreated hydrogel fibers (0.0 M NaCl) lost approximately 60% of their weight within one day. Visual documentation of the weight change process (Fig. 4e) confirmed that NaCl-treated CHOFs retain their form, transparency, and flexibility even after prolonged ambient exposure, whereas untreated samples rapidly transition from wet to dry states with an accompanying loss of functionality. The ionic environment reduced water vapor pressure through osmotic effects and provided favorable hydration conditions that prevented dehydration-induced performance degradation.

CHOFs were tested for conductivity and stability in PBS over 4 days. They retained 67% of their initial weight in 1.0 M PBS (Fig. S5a), showing effective retention of water content and structural integrity under ambient conditions, indicating minimal water loss or degradation. Ionic conductivity measurements revealed that 1.0 M PBS had a conductivity of 22 mS cm⁻¹ (Fig. S5b), which is adequate for strain sensing. This demonstrates that CHOFs maintain functionality and hydration in physiological buffer environments, supporting their use in biomedical applications.

The ionic conductivity of the CHOFs enables their application as strain sensors through piezoresistive effects. When a mechanical deformation is applied, changes in the hydrogel fiber geometry and internal structure lead to measurable resistance variations.^20,22,44,48 The strain-sensing performance was evaluated using the optimal CHOF composition (5 mol% SA) to establish the sensing capabilities across different strain ranges. As shown in Fig. 5a, the CHOF in the cladding demonstrates clear piezoresistive responses under mechanical deformation up to 200% strain. A CHOF of 1.5 cm in length exhibited a resistance of 462.3 Ω. During stretching, the fiber length increased, whereas the cross-sectional area decreased, leading to a systematic increase in resistance with respect to the geometric changes. The resistance change showed a progressive increase with the applied strain, reflecting the combined effects of length extension and cross-sectional area reduction. The gauge factor, defined as the ratio of the relative resistance change to the strain, was determined to be 1.21, indicating good strain sensitivity for hydrogel-based sensors.

Fig. 5 (a) Resistance–strain relationship for CHOFs with 5 mol% of SA composition. (b) Resistance change of CHOF under cyclic strain from 0.25% to 200%. (c) Resistance ratio (R/R₀) of CHOF during 1000 cycles at 50% strain. (d) Resistance change during finger motion detection using 5 mol% SA CHOF. (e) Resistance response during elbow motion monitoring using 5 mol% SA CHOFs. (f) Photographs of 5 mol% SA CHOF during wrist bending showing optical transmission and corresponding resistance (R/R₀, left axis) and optical transmission (T/T₀, right axis) variation.

To assess the dynamic sensing performance across different strain amplitudes, cyclic testing was performed at strains ranging from 0.25% to 200% (Fig. 5b). The sensor exhibited consistent and reproducible responses over the entire strain range. Even at very small strains of 0.25% and 0.5%, the sensor produced measurable signals, demonstrating its sensitivity in detecting subtle mechanical changes. The amplitude of the resistance change varied systematically with the applied strain, enabling quantitative strain measurements across a wide dynamic range suitable for monitoring both fine motor movements and large-scale body motions. The dynamic response performance of the sensor was evaluated by measuring its response time during cyclic elongation and release at 5% strain. The CHOF sensor exhibits an exceptionally rapid response time of approximately 143 ms. (Fig. S6)

Long-term reliability under repeated deformation is crucial for practical sensor applications. Extended cyclic testing at 50% stretching demonstrated the exceptional mechanical and electrical stabilities of the CHOFs under continuous operation (Fig. 5c). The resistance ratio (R/R₀) remained remarkably stable around 1.3 over 1000 cycles, showing minimal drift or degradation throughout the testing period. The outstanding stability under extended cyclic loading indicates robust network recovery and minimal structural damage under repeated mechanical stress.

To evaluate the practical performance of the CHOFs in real-world applications, strain-sensing experiments were conducted on various human body joints while simultaneously monitoring their optical transmission capability. Experiments demonstrated that both functions could operate simultaneously without mutual interference.

For finger motion detection, the sensor was placed on the index finger over a nitrile glove (Fig. 5d). The resistance increased with finger bending, reaching R/R₀ values of ∼1.2 at 60 degree bending of the finger and returned to the initial value upon straightening. Throughout the bending motion, the fiber continued to effectively transmit light, as clearly shown in the photographs. When positioned on the elbow, the sensor produced continuous signals corresponding to arm motion (Fig. 5e). The resistance ratio shows consistent oscillations, with R/R₀ values reaching 1.17 during bending motions, enabling the monitoring of large-amplitude joint movements. Wrist motion monitoring demonstrated the detection of complex multidirectional movements (Fig. 5f). The relative variation in the resistance during wrist bending reached an R/R₀ value of ∼1.3. To demonstrate the feasibility of this dual-modal operation, we have further conducted simultaneous optical transmission and electrical measurements during dynamic bending, as shown in Fig. 5f. Light transmission is maintained while electrical signals accurately track mechanical deformation, validating that both functionalities operate independently without interference. While the current demonstration uses a simple motion detection setup, the same principle applies to the biomedical scenarios described above, where stable light delivery must be maintained during tissue/device movement.

To assess performance under different environmental conditions, we tested the strain sensing function in 1.0 M NaCl at varying relative humidity levels (10%, 30%, 50%, and 70% RH). The sensor maintains stable and reproducible resistance signals across all humidity conditions during repeated finger bending cycles (Fig. S7). The consistent performance from low (10%) to high (70%) RH demonstrates robust operation in diverse environmental conditions, confirming the suitability of CHOFs for wearable sensor applications.

These demonstrations across multiple body joints revealed that CHOFs can successfully perform dual functions in practical wearable applications, with optical transmission maintained throughout all motion detection processes.

4. Conclusion

We successfully developed CHOFs via a microfluidic fabrication process that integrated optical transmission, ionic conductivity, and mechanical flexibility into a single platform. The strategic incorporation of SA into the cladding region resulted in controlled differential swelling, preserving the optimal refractive index contrast even under ionic conditions. The SA composition enabled precise control over multiple material properties simultaneously. Higher SA content (5 mol%) increased the NA to 0.339 and achieved optical attenuation as low as 0.088 dB cm⁻¹, while enhancing ionic conductivity to 26.6 mS cm⁻¹ and mechanical stretchability up to 321% strain. NaCl treatment provided dual benefits such as ionic conductivity and environmental stability through moisture retention.

Simultaneous optical transmission and strain sensing during human motion detection validated its practical utility in real-world applications. The ability to function under mechanical deformation conditions represents a significant advantage in dynamic environments. The integration of multiple functionalities into a mechanically compliant platform provides opportunities for advanced applications, including optogenetic neural interfaces with real-time position monitoring, soft robotics with proprioceptive sensing and optical communication, and wearable health-monitoring systems with optical biosensing and motion detection. These characteristics make CHOFs enabling materials for next-generation smart textiles, interactive wearable systems, and biomedical devices that require multifunctional performance for a single component.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data that support the findings of this study are available from the corresponding author upon request.

The data supporting this article have been included as part of the supplementary information (SI). Supplementary information is available. See DOI: https://doi.org/10.1039/d5tc03226a.

Acknowledgements

This study was supported by the 2024 Research Fund of the University of Seoul for J. Y. and Ministry of Science and ICT and National Research Foundation of Korea through the Engineering Research Center Program (RS-2025-00512708) for A. S.

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