Recyclable, conductive alginate-based hydrogels with high stretchability and low electrical hysteresis for wireless wearable sensors

Abstract

Soft electronics based on conductive hydrogels hold considerable promise for advanced wearable technologies. However, these systems face critical challenges, particularly in mitigating electronic waste and ensuring electrical stability. In this study, we present a highly stretchable and recyclable hydrogel composed of the natural polymer alginate (ALG) as the matrix and the lab-synthesized poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) as a conductive filler, ionically crosslinked using calcium chloride (CaCl₂). The CaCl₂-incorporating ALG/PEDOT:PSS hydrogel exhibited a stretchability of 138% and a low hysteresis of 2.95% (50% strain) while retaining stable electrochemical properties over 400 stretching cycles achieved without relying on a synthetic polymer matrix. Integrating ALG with conductive PEDOT:PSS established robust conductive pathways and reinforced intermolecular interactions, yielding a relative resistance change of 0.58 and a gauge factor of 0.58 at 100% strain. Skin-adaptable sensors fabricated from this hydrogel effectively detected both large-scale and subtle human movements in real-time. Furthermore, the integration of the hydrogel into wireless sensor systems afforded a consistent and reliable performance for real-time movement monitoring. These findings highlight the potential of the fabricated hydrogel for high-performance, stretchable electronic devices, particularly due to its excellent recyclability.

---

Introduction

In the rapidly evolving field of wearable technology, it is crucial to develop materials that meet high functional demands and adhere to environmental sustainability standards. Recyclable and conductive hydrogels are emerging as superior candidates that offer a promising solution to the challenges of eco-friendliness and performance in wearable sensors.^1,2 These hydrogels, which are known for their ability to conduct electricity and be recycled, align wearable technologies with green manufacturing practices. The significance of recyclable hydrogels lies in their ability to mitigate the environmental impact of electronic waste, which is a growing concern for the development of disposable wearable devices.³ These hydrogels are designed to break down into their constituent substances under controlled conditions, enabling them to be reused in producing new hydrogel products. This process minimizes waste generation and reduces reliance on raw, unprocessed materials, contributing to resource conservation and reduction in production costs.⁴

Recent advancements in hydrogel-based materials have significantly expanded their applicability in flexible sensors, bioelectronics, and wearable technology. Multifunctional hydrogels that integrate conductive fillers and dynamic crosslinking strategies have enabled enhanced electromechanical performance, self-healing capabilities, and environmental stability, thereby broadening their use in next-generation electronic devices.^5,6 Nanocomposite hydrogels embedded with carbon-based materials, such as graphene and carbon nanotubes, have demonstrated improved conductivity and sensitivity, making them ideal candidates for strain and pressure sensors.⁷ Additionally, recent efforts to develop stretchable and transparent ionic hydrogels have facilitated their integration into bio-integrated electronics and soft robotics.⁸ Furthermore, bioinspired hydrogels with high adhesion and self-repairing properties have improved the durability of wearable sensors, ensuring long-term performance in dynamic environments.⁹ These advancements highlight the growing potential of hydrogels as sustainable and multifunctional materials for future soft electronics and biomedical applications.

Among various polymers, alginate (ALG) has emerged as a particularly promising matrix polymer for recyclable and conductive hydrogels. ALG hydrogels derived from brown seaweeds are renowned for their ease of gelation under mild conditions and excellent biocompatibility.¹⁰ These properties render ALG ideal for wearable applications, where materials must retain their performance under mechanical and biochemical stresses in the human body. However, despite these benefits, pure ALG exhibits insulating properties, which limits its use in electronic applications.¹¹

Conductive fillers, such as carbon-based nanoparticles,^12,13 metallic elements,^14,15 or conductive polymers,^16,17 can be combined with other materials to significantly enhance the electrical properties, flexibility, biocompatibility, environmental stability, mechanical properties, and ease of integration of hydrogels. These benefits make conductive polymers versatile and effective choices for advanced hydrogel-based applications. The versatility of hydrogels with conductive polymers enables customization of the electrical properties of conductive inclusions, providing tailored solutions for specific sensing applications.¹⁸ The dual functionality of recyclability and conductivity renders these hydrogels particularly appealing as wearable sensors. Furthermore, they are practical owing to their environmental benefits, as well as technically proficient and capable of performing complex monitoring tasks.¹⁹ The mechanical properties of these hydrogels, such as their stretchability and stability, can be finely tuned to ensure comfort and durability when worn directly on the skin. Moreover, their biocompatibility ensures compatibility with biological tissues, enabling continuous noninvasive monitoring.²⁰

In particular, the conductive polymer poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) is notable for its high electrical conductivity, mechanical flexibility, and environmental stability.^21,22 The PEDOT:PSS complex has been widely applied in diverse fields—ranging from solar cells to bioelectronics—because of its high electrical conductivity and mechanical flexibility.^23–25 Additionally, its intrinsic conductivity and robust mechanical properties make it a promising candidate for neural electrode coatings, which demand stable and biocompatible bioelectronic interfaces.²⁶ Recent research has also broadened the role of PEDOT:PSS-based conductive systems in soft robotics, enabling highly stretchable electronic circuits that can dynamically respond to mechanical stimuli.²⁷ These varied applications highlight the multifunctionality of PEDOT:PSS as a key component in emerging flexible, stretchable, and recyclable hydrogel-based electronic devices.

Moreover, the PEDOT:PSS composite hydrogels reported by Badawi et al. were able to self-heal and revert to their original composition, rendering them suitable for smart wearable electronics and supercapacitor applications with high ionic conductivity requirements.²⁸ Wang et al. reported that core–shell sodium ALG/PEDOT:PSS composite fibers manufactured using a single-nozzle process are promising candidates for wearable sensors.²⁹ Lee et al. developed ionically conductive hydrogel composites with triple-network structures made of PEDOT:PSS, ALG, polyacrylamide, and carbon cellulose as adaptive conformal brain interfaces.³⁰ Hydrogels based on ALG, polyacrylamide, and poly(3,4-ethylene dioxythiophene)-co-(3-thienylboronic acid) exhibited self-healing properties, with a toughness of 36 209 J m⁻² and strain capabilities of up to 1300%.³¹ However, ALG-based conductive hydrogels exhibiting recyclability, high stretchability, and low electrical hysteresis for wearable sensors have not been extensively studied.

Low electrical hysteresis is a critical requirement for stretchable sensors, essential for accurate and reliable signal detection. A low hysteresis ensures that the output of the sensors remains consistent and repeatable under cyclic loading and unloading conditions, which is crucial for applications requiring the precise monitoring of electromechanical signals and human movement.³² For example, in electronic skin applications, a low hysteresis enables the precise detection of large muscle actions and joint movements, thereby enhancing the effectiveness of sensors in real-time monitoring and control systems.³³ In addition, a low hysteresis contributes to the long-term stability and durability of the sensors, rendering them suitable for continuous use in wearable applications.

The development of hydrogels with low hysteresis involves innovative material design and fabrication techniques, such as the incorporation of hybrid networks and dynamic crosslinking, which improve the mechanical properties and resilience of hydrogels.³⁴ Furthermore, the combination of stretchable sensors with wireless technology has expanded the possibilities of their applications. Wireless sensors can send real-time data to a monitoring system without requiring physical connections, thereby increasing the mobility and comfort of wearables.³⁵ Wireless properties are particularly beneficial for health-monitoring applications that require continuous and remote tracking of human physical activity information. The combination of low hysteresis hydrogels and wireless technology enables the development of advanced wearable systems capable of providing precise, dependable, and long-term monitoring of human movements and other critical parameters, paving the way for novel solutions in areas such as healthcare and sports.³⁶

In this study, we developed a highly stretchable and recyclable ALG/PEDOT:PSS hydrogel, which was ionically crosslinked with CaCl₂ and exhibited high stretchability and electrical properties with low electrical hysteresis. The low electrical hysteresis and superior electromechanical stability of the hydrogels were attributed to the improved conductive pathways and robust physical contact between the ALG matrix and conductive PEDOT domains. Furthermore, reversible interactions significantly enhanced the recyclability of the hydrogels, thereby aligning with the requirements for reusable electronics that reduce environmental impact and costs. Few studies have addressed the development of recyclable conductive hydrogels that retain high stretchability and low electrical hysteresis, using ALG as the natural polymer matrix and PEDOT:PSS as the conductive filler, all without incorporating an additional synthetic polymer network.³⁷ Skin-adaptable sensors made from these hydrogels could successfully detect large human movements and small activities in real-time, even after recycling, with high electrical responses, excellent sensitivity, and rapid response and recovery times. These findings show that these recyclable, flexible, and low-hysteresis hydrogels have the potential for use in long-lasting stretchable electronic devices. We believe that the fabricated high-performance ALG/PEDOT:PSS hydrogels can facilitate the development of highly stretchable, low-hysteresis wearable sensors and aid in reducing electronic waste.

Results and discussion

The ALG/PEDOT:PSS hydrogels were fabricated using an ionic crosslinking approach (Fig. 1). The pure ALG hydrogel demonstrates moderate stretchability (117%), but its low toughness (0.49 MJ m⁻³) and high Young's modulus (0.40 MPa) indicate limited energy dissipation, leading to brittle failure under stress (Fig. S1c†). To enhance the stretchability, elasticity, and tensile strength of the ALG films, we introduced 10 vol% glycerol into the alginate matrix. Glycerol, a common polyhydric plasticizer, forms multiple hydrogen bonds with the hydroxyl and carboxylate groups of ALG (and with the sulfonate groups of PSS), thereby weakening the direct alginate–alginate and alginate–PSS interactions.³⁸ This reduction in intermolecular attractions increases chain mobility, enabling the glycerol-plasticized alginate film to deform significantly more before fracture. As a result, the film achieves a high elongation at break (282% strain) and improved toughness (1.12 MJ m⁻³), while retaining a comparatively low Young's modulus (0.08 MPa) (Fig. S1c†). Principally, strong polar interactions of glycerol loosen the alginate network, yielding a softer, more flexible matrix that markedly improves the stretchability and durability of film. We subsequently combined PEDOT:PSS with the glycerol-containing ALG to fabricate conductive ALG/PEDOT:PSS hydrogels, which inherit the enhanced flexibility and mechanical robustness imparted by glycerol (Fig. S1d–f†).

Fig. 1 (a) Schematic of the molecular mechanism of stretching and relaxation in the ALG/PEDOT:PSS hydrogels and (b) chemical structures of the alginate (ALG)/poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) hydrogels including the “egg box” structure for alginate gelation.

The mechanical behavior of ALG/PEDOT:PSS hydrogels is governed by the synergy of hydrogen bonding and electrostatic interactions, with hydrogen bonding playing a primary role in enabling large stretchability and facilitating rapid recovery. Hydrogen bonds form between hydroxyl groups in alginate and sulfonic acid groups in PSS, creating a reversible, dynamic network that accommodates large deformations and quickly reforms upon unloading—thus reducing hysteresis. Meanwhile, electrostatic interactions between carboxylate ions (COO⁻) in alginate and positively charged PEDOT chains or Ca²⁺ ions provide essential network stability and mechanical reinforcement.³⁹

By retaining structural integrity while hydrogen bonds break and reform under strain, these ionic crosslinks further promote cyclic resilience and minimize permanent deformation. Furthermore, ethylene glycol (EG) and the nonionic surfactant Capstone FS-31 were incorporated into PEDOT:PSS to further enhance its stretchability, stability, and conductivity. The incorporation of EG improves the mechanical properties of the PEDOT:PSS system. EG acts as a plasticizing agent, intercalating between PEDOT:PSS chains and forming hydrogen bonds with the PSS sulfonate groups. This interaction reduces inter-chain attractions and increases free volume, yielding a softer and more pliable network.⁴⁰ Consequently, the EG-induced ALG/PEDOT:PSS hydrogels can deform more easily under stress, exhibiting enhanced flexibility compared to EG-free controls. In addition, EG functions as a secondary dopant in the PEDOT:PSS system. Its high dielectric constant screens the coulombic interactions between the positively charged PEDOT chains and negatively charged PSS, prompting partial phase separation of excess PSS from PEDOT-rich regions, which facilitates charge transport by creating more continuous conductive pathways.⁴¹ As a result, the EG-incorporating hydrogels retain excellent electrical conductivity. By simultaneously plasticizing the polymer network and optimizing its microstructure for charge percolation, EG significantly contributes to the overall performance of the hydrogel. Moreover, Capstone FS-31 stabilized the dispersion, refined the morphology, and increased the adhesion, optimizing both mechanical and electrical properties (Fig. S1g†).⁴²

The Raman spectra of the pure ALG and ALG/PEDOT:PSS hydrogels are shown in Fig. 2a and b, respectively. Pure ALG exhibited symmetric carboxylate stretching and C–O single bond stretching vibrations at 1427 and 1325 cm⁻¹, respectively. In addition, skeletal vibrations at 961, 885, and 809 cm⁻¹ were observed, along with the glycosidic ring breathing mode at 920 cm⁻¹.⁴³ The ALG/PEDOT:PSS hydrogels demonstrated typical PEDOT fingerprints, with bands appearing at 975, 1253, 1368, and 1421 cm⁻¹, which correspond to the vibration modes of the thiophene C–S bonds, C–C inter-ring stretching, C–C stretching, and C=C symmetrical stretching, respectively. The PSS peak appeared at 1517 cm⁻¹.⁴⁴ The positively charged PEDOT facilitated carrier conduction along its chains and through resonance structures. In contrast, PSS with negatively charged sulfonate groups connected to PEDOT via coulombic attraction and exhibited insulating properties that deteriorated the conductivity of PEDOT:PSS.⁴⁵ The thermal degradation behavior of pure ALG and ALG/PEDOT:PSS hydrogels was investigated via thermogravimetric analysis (TGA, Fig. 2c). Pure ALG exhibited a lower onset temperature of degradation (below 150 °C), corresponding to the evaporation of water and volatiles. In contrast, ALG/PEDOT:PSS demonstrated improved thermal stability, shifting degradation transitions to higher temperatures. The maximum degradation rate occurred at 260 °C for pure ALG and 270 °C for the composite hydrogel (inset DTG curve, Fig. 2c), confirming the stabilizing effect of PEDOT:PSS. The total weight loss was 92% for pure ALG and 89% for ALG/PEDOT:PSS, indicating a reduced thermal degradation in the composite hydrogel. This enhancement is attributed to the thermally stable PEDOT:PSS, which strengthens the ALG matrix by delaying the breakdown of its organic components.

Fig. 2 (a and b) Raman spectra and (c) thermogravimetric analysis curves of (a) pure ALG and (b) the ALG/PEDOT:PSS hydrogels.

These findings are consistent with composite material theory, which suggests that a stable filler (PEDOT:PSS) increases the overall thermal resistance, making ALG/PEDOT:PSS hydrogels more suitable for heat-sensitive applications.^46,47

To further improve the electrical and mechanical properties of the ALG/PEDOT:PSS hydrogels, a CaCl₂ solution was introduced. The impact of varying the CaCl₂ content on the mechanical characteristics of the hydrogels is shown in Fig. 3. Before crosslinking with CaCl₂, the non-crosslinked ALG/PEDOT:PSS hydrogel (control sample) exhibited a stretchability of 70.79%, a tensile strength of 0.66 MPa, a Young's modulus of 0.92 MPa, and a toughness of 0.27 MJ m⁻³ (Fig. 3b and c). Additionally, water stability experiments show that the CaCl₂-crosslinked hydrogel retains its structure for at least 30 minutes in deionized water, whereas the non-crosslinked sample rapidly disintegrates (Fig. S3†). The absence of Ca²⁺ resulted in lower mechanical strength and increased deformability, indicating that although the hydrogel retained flexibility, its structural integrity was weaker and its resistance to mechanical stress was reduced. The CaCl₂-incorporating ALG/PEDOT:PSS hydrogel exhibited mechanical robustness, high stretchability, and deformability. The hydrogel containing 0.25 vol% CaCl₂ exhibited a high stretchability of 138%, tensile strength of 0.18 MPa, Young's modulus of 0.12 MPa, and toughness of 0.32 MJ m⁻³ (Fig. 3b and c). Increasing the CaCl₂ amount reduced the elongation at break, while simultaneously increasing Young's modulus and toughness.⁴⁸ Hydrogels containing 1 vol% CaCl₂ showed a reduced stretchability of 74.8% and increased tensile strength and Young's modulus of 1.71 and 2.28 MPa, respectively, with a high toughness of 0.67 MJ m⁻³. The maximum tensile strength observed at 1 vol% CaCl₂ reflects an optimal cross-linking density, where the electrostatic interactions between the Ca²⁺ ions and ALG chains form a robust three-dimensional (3D) network. This balanced cross-linking enhances both the strength and toughness of the hydrogel while preserving its flexibility. At concentrations higher than 1 vol%, excessive cross-linking occurs, leading to increased rigidity, reduced elongation, embrittlement, and the formation of microfractures. These effects compromise the material's ability to deform under stress, highlighting the importance of maintaining an optimal CaCl₂ concentration for a desirable mechanical performance.⁴⁹ The strong noncovalent interactions in the hydrogels included hydrogen bonds between PEDOT:PSS and ALG and electrostatic interactions between ALG and PEDOT or ALG and Ca²⁺ ions, effectively producing a 3D conductive network and considerably improving the strength and toughness of the hydrogels. In addition, CaCl₂ in the ALG matrix acted as a gelling agent that reduced the elongation at break and deformability, while enhancing the mechanical strength. The Ca²⁺ ions interacted with the carboxylate groups on the ALG chains, leading to the formation of crosslinks between the polymer chains. This crosslinking created a 3D network that provided high strength and toughness.⁵⁰

Fig. 3 (a) Photographs of the as-prepared, stretched, bent, and twisted hydrogels. (b) Tensile stress–strain curves. (c) Young's moduli and toughness values of the hydrogels. (d) Relative resistances of the ALG/PEDOT:PSS hydrogels according to CaCl₂ contents. (e) Loading–unloading cycles of the ALG/PEDOT:PSS hydrogels incorporated with CaCl₂ (0.25 vol%) under different strains. (f) Comparison of the mechanical performances of ALG/PEDOT:PSS and other reported ALG-based hydrogels.

Fig. 3d presents the relative resistance change (R/R₀) of the ALG/PEDOT:PSS hydrogels with varying CaCl₂ concentrations during loading–unloading cycles at 50% strain. The results demonstrate a low hysteresis compared to the initial state, indicating excellent mechanical properties and a strong interfacial adhesion between the ALG matrix and PEDOT:PSS, even when stretched to 50%. The observed electrical hysteresis is attributed to either the adhesion between the conductive filler and the matrix or the intrinsic mechanical properties of the materials (such as elasticity and toughness), which influence their sensing accuracy.⁵¹ The degree of hysteresis (DH) is calculated using the formula, DH = (|A_L – A_U|/A_L) × 100%, where A_L and A_U represent the areas under the loading and unloading curves,⁵² respectively. Notably, hydrogels containing 0.25 vol% CaCl₂ exhibited the lowest DH of only 2.95% and an R/R₀ of 1.24 under 50% strain. These characteristics highlight the potential of hydrogel as an excellent candidate for low-hysteresis sensor applications, supported by its outstanding mechanical properties, including a stretchability of 138% and toughness of 0.32 MJ m⁻³. The enhanced physical interactions between ALG and PEDOT:PSS, combined with an increase in localized charges within the hydrogel networks contributed to the low electrical hysteresis observed in the CaCl₂-incorporating hydrogels.

We performed a cyclic tensile loading–unloading test on the hydrogels (0.25 vol% CaCl₂) while increasing the strain up to 100% to study the mechanical reversibility of the hydrogels (Fig. 3e). The stable resistance (R/R₀) during strain cycling in the CaCl₂-added ALG/PEDOT:PSS hydrogels is attributed to the ionic crosslinking of alginate by the Ca²⁺ ions and the dynamic reorganization of the PEDOT domains. The “egg box” structure provides a reversible, elastic network that retains its structural integrity under strain (Fig. 1a). Simultaneously, the PEDOT domains realign to preserve the conductivity pathways, supported by the interactions between the Ca²⁺ ions and the negatively charged PSS and alginate. This energy minimizes conductive network disruption, ensuring a low hysteresis and consistent electrical performance over repeated cycles.⁵³ The hysteresis loops were associated with the partial disruption of extensive noncovalent bonds, such as hydrogen bonds and hydrophobic interactions. The large hysteresis loop, mainly due to disrupted hydrogen bonds, offered benefits such as high energy dissipation and extensibility. However, it reduced the sensing accuracy in some applications.

The developed hydrogels exhibited a noticeable strain relaxation during the cyclic loading–unloading process, indicating that the broken physical crosslinked network was restored upon the removal of external forces. This self-healing ability helped the hydrogel retain its structure and performance over multiple loading and unloading cycles. Although the hydrogels exhibited mechanical hysteresis, the electrical hysteresis remained minimal. Our CaCl₂-added ALG/PEDOT:PSS hydrogels exhibited a superior stretchability and decreased toughness compared with those of other ALG-based hydrogels, owing to the abundance of noncovalent interactions and strong interfacial adhesion between the ALG matrix and PEDOT:PSS (Fig. 3f).^54–64 The ability of these conductive hydrogels to stretch up to 138% with minimal electrical hysteresis offers significant advantages in the development of highly stretchable and electrically stable wearable sensors.

Based on the excellent mechanical and electrical properties of the hydrogels, we fabricated strain sensors using ALG/PEDOT:PSS with CaCl₂ (0.25 vol%) and explored the sensing characteristics of the hydrogels subjected to tensile strain (Fig. 4a and b). The hydrogels gradually extended to 100% strain and then returned to their initial state, with R/R₀ values recorded at each strain. The hydrogels exhibited outstanding stability and continuous performance during loading–unloading cycles under various strain conditions. The nearly identical resistance changes during both the stretching and releasing phases indicate the superior mechanical stability of the hydrogels (Fig. 4b). In addition, the CaCl₂-added ALG/PEDOT:PSS hydrogels showed fast response and recovery times of 0.59 and 0.57 s, respectively, when strained at 100%, indicating that the conductive PEDOT:PSS fillers effectively created percolative pathways in the hydrogels that facilitated rapid electron transport and enabled quick sensing of changes (Fig. 4c).

Fig. 4 (a) Plot of the relative resistances of the ALG/PEDOT:PSS hydrogels treated with CaCl₂ (0.25 vol%) under cyclic strain-releasing. (b) Resistance changes of the hydrogel during stepwise stretching from 0% to 100% strain and subsequent release to 0%. (c) Response and recovery times of the hydrogel under a strain of 100%. (d) Sensitivity of the hydrogel under loading/unloading, showing gauge factors (GF) and near-linear strain dependence. (e) Cyclic stretching–releasing of the ALG/PEDOT:PSS hydrogels for 450 cycles at a strain of 50%.

The sensitivity of the sensor is quantified using the gauge factor (GF), defined as (ΔR/R₀)/ε, where ΔR is the resistance change at a specific strain, R₀ is the initial resistance, and ε is the applied strain (Fig. 4d). The CaCl₂-incorporating ALG/PEDOT:PSS hydrogels achieved a ΔR/R₀ of 0.58 (corresponding to R/R₀ of 1.58) and a GF of 0.58 at 100% strain, while retaining a low DH of 6.65%. The high linearity between the loading and unloading phases (R² = ∼0.9931) confirms the accuracy and reliability of the sensor response to varying degrees of mechanical strain. The moderate GF of 0.58 is consistent with previously reported results, presenting similar values ranging between 0.5 and 1 at 100% strain and highlighting the potential of the hydrogel for applications in wearable technology.^65–68 Its outstanding stretchability, recyclability, and stability under cyclic loading make it ideal for use in flexible electronics, health monitoring systems, and soft robotics. Furthermore, the hydrogels showed high resistance stability, even after 450 stretching cycles at 50% strain (Fig. 4e), due to the stable conductive network formed by PEDOT:PSS. These results highlight the hydrogel's ability to effectively absorb and disperse energy while retaining structural integrity and electromechanical performance over multiple stretching cycles.

In cyclic tensile tests, the peak resistance of the ALG/PEDOT:PSS hydrogel varies from cycle to cycle rather than reaching a stable steady-state. This is caused by ongoing microstructural changes in the material during deformation. ALG-based hydrogels are viscoelastic, meaning that after each stretch, they relax and recover only partially over the time scale of the experiment. As a result, the internal configuration of the polymer chains and the distribution of the conducting PEDOT:PSS filler evolve with each cycle. The conductive pathways are not identical every time: some connections may strain or temporarily break and then reform differently upon release. This dynamic reorganization of the network effectively self-optimizes the conductivity of the material under repeated deformation, a behavior that has also been noted in other soft conductive hydrogels.^69,70 Small fluctuations in the peak resistance of our ALG/PEDOT:PSS hydrogels do not indicate sensor failure but rather reflect the adaptability of material; the conductive network adapts and heals during each cycle, helping retain functionality over long-term use. This result highlights the advantage of a dynamically percolating network in soft sensors, as it can withstand deformation through continuous internal reconfiguration.

To investigate the sensing capability of the CaCl₂-incorporating ALG/PEDOT:PSS hydrogels for detecting human motion, the hydrogels were attached to human joints (Fig. 5a–e). These sensors enabled real-time monitoring of both subtle and extensive human movements. The sensors attached to the finger to detect bending and relaxation motions exhibited notably increased resistance changes with increasing angles of finger bending, showing R/R₀ values of approximately 1.15 and 1.19 at 45° and 90°, respectively (Fig. 5a). While bending and relaxing, the sensor demonstrated response and recovery times of 0.52 and 0.70 s, respectively (Fig. S2a†). Similarly, when attached to the wrist, elbow, and back of the hand, the sensors detected real-time movements with fast response and recovery times (Fig. 5b–e). Moreover, the sensors effectively monitored small-scale human activities, including facial expression changes, such as blinking, smiling, and mouth opening (Fig. 5f, g and S2e†). These results suggested that the CaCl₂-incorporated ALG/PEDOT:PSS sensors have substantial potential for use in health and motion-monitoring applications. Furthermore, the sensors could detect pressure changes from actions such as the loading and unloading of objects (2, 5, 10, and 20 g), producing distinct and easily identifiable characteristic signals (Fig. 5h). Similarly, the signals generated by touching the hydrogel surfaces were successfully measured (Fig. 5i).

Fig. 5 Sensing responses of the sensors for (a) finger bending, (b) wrist bending, (c) elbow bending, (d) back of hand, (e) knee bending, (f) blinking, and (g) smiling. Pressure sensor testing for (h) load pressures and (i) finger tapping.

The CaCl₂-added ALG/PEDOT:PSS hydrogels exhibited an outstanding recycling performance because of their reprocessable nature, which enabled repeated recycling without significant degradation of their properties. Recycling experiments involved cutting the waste from the CaCl₂-incorporated ALG/PEDOT:PSS hydrogels into small pieces and dissolving it in deionized water at 60 °C. The solution was degassed to remove bubbles and recast in a Petri dish to form the hydrogels (Fig. 6a). This process ensured that the CaCl₂-added ALG/PEDOT:PSS hydrogel was completely recycled. The fabrication method proved to be environmentally friendly, enabling the material to be reused in new sensors. The recycled CaCl₂-added ALG/PEDOT:PSS hydrogel effectively regained its original performance and functionality. The signals produced by gradually applying strain were distinctly clear, and they returned fully to their original state (Fig. 6b). The recycled CaCl₂-added ALG/PEDOT:PSS hydrogels were strained at 70% and exhibited response and recovery times of 0.70 and 0.52 s, respectively, which were comparable to those of the original hydrogels (Fig. 6c). The strain sensor with the recycled CaCl₂-added ALG/PEDOT:PSS hydrogel exhibited a better performance than that of the original sensors. The R/R₀ clearly increased along with the strain, with a DH of approximately 8.44% (Fig. 6d). Furthermore, a high linearity was observed between the strain-releasing processes (R² = ∼0.9816).

Fig. 6 (a) Recycling process of hydrogels. (b) Relative resistances of the ALG/PEDOT:PSS recycled hydrogel strain sensors under consecutive step-and-hold testing. (c) Response and recovery times of the sensor applied under a strain of 70%. (d) Sensitivity responses of the ALG/PEDOT:PSS recycled hydrogels during loading and unloading cycles with a GF and linear curves of the strain sensor. (e) Resistance changes upon repeated loading and unloading for 300 cycles at a strain of 50%. (f) Cyclic stretching–releasing of the ALG/PEDOT:PSS recycled hydrogels at different strain levels. A demonstration of the recycled sensor for (g) finger bending, (h) wrist bending, and (i) mouth opening/closing.

The resistance change of the recycled hydrogels was stable under continuous stretching for 300 cycles at a strain of 50% (Fig. 6e). In addition, stable electrical responses were observed under repeated strain-releasing processes at various strains (Fig. 6f). The recycled CaCl₂-added ALG/PEDOT:PSS hydrogels attached to several body parts successfully monitored signals from human movements, including the finger, wrist, and face (Fig. 6g–i). These findings suggest that the recycled hydrogels retained the sensing capabilities of the original hydrogels without significant degradation. The inherent recovery properties of our hydrogels enabled them to regain their original form and distribution of conductive pathways, ensuring that the electromechanical properties remained effective. Additionally, we evaluated the recyclability of CaCl₂-added ALG/PEDOT:PSS hydrogels over five cycles to examine whether mechanical integrity and sensing performance are retained. The mechanical properties showed gradual changes consistent with network densification: tensile strength increased from 0.18 MPa (original) to 1.60 MPa after five cycles, and Young's modulus rose from 0.12 MPa to 1.96 MPa (Fig. S4d–f†). Conversely, the strain at break decreased from 138% to 81.8%, reflecting a stiffer, less extensible network. Despite this increase in stiffness, the hydrogels retained acceptable mechanical integrity for strain-sensing applications through at least the third cycle. Electromechanical performance also remained stable during the initial recycling cycles. For example, R/R₀ at 50% strain changed only slightly, from 1.24 in the original hydrogel to 1.12 after three cycles. Similarly, the GF leveled off at approximately 0.23 after an initial decrease. A weight loss analysis showed that the percentage of water loss during drying increased with each cycle (Fig. S5†), indicating progressively higher water content in the recycled gels. However, these changes in water content may not appear to significantly affect the solid polymer material (ALG and PEDOT:PSS). Thus, the conductive network remained intact, and the strain-sensing function was preserved across cycles. These results suggest that the CaCl₂-added ALG/PEDOT:PSS hydrogel can be recycled up to three times with minimal loss of performance, showing its suitability for sustainable and reusable electronic devices.

Wireless transmission sensors offer real-time signal acquisition and remote transmission, eliminating cable and distance limitations.⁷¹Fig. 7a shows a wireless motion sensor based on the CaCl₂-incorporating ALG/PEDOT:PSS hydrogel attached to an index finger for motion sensing. At the system level, the wireless motion sensing (WMS100) device integrated a robust processing unit, sensing module, and effective power management. ESP-32-S3 was selected for its integrated ADC, MCU capabilities, and BLE 5 support, ensuring stable wireless data transfer. The raw signal from the hydrogel sensor was processed using a sensing module to measure the voltage change and reduce unwanted signals. The clean signal was digitized using a 12 bit ADC at a sampling rate of 100 Hz. MCU converted the voltage level to the resistance value of the hydrogel sensor, which was wirelessly transmitted to a PC via BLE communication. A user-friendly interface software was developed to display and record the sensing data on a PC. The power management unit included a 3.3 V voltage regulator to supply the entire circuit board, which was powered by a 500-mAh lithium-ion battery and controlled by a battery charger (Fig. 7b).

Fig. 7 (a) Schematic of the wireless motion-sensing device worn on a finger. (b) System-level diagram of the wireless motion sensing for real-time sensing, data processing, and wireless transmission. A demonstration of the wireless sensor for (c) finger bending and (d) writing the word “KOREA”.

The fully integrated wireless motion sensor comprised signal transduction, conditioning, processing, and Bluetooth transmission capabilities to transmit the electromechanical signals to a user interface, enabling real-time monitoring of movement.⁷² After the finger was bent from 0° to 90°, the portable computer received the wireless signal and displayed the finger-bending signals on a screen (Fig. 7c and Movie S1†). A wireless sensor was attached to the thumb to detect handwriting movements and the electrical response curves for the handwriting movements were recorded with high sensitivity and stability (Fig. 7d). These wireless sensors demonstrated high sensitivity and reliability, which rendered them suitable for monitoring physical activity. Furthermore, the robust performance and versatility of these hydrogel-based sensors indicated their significant promise for integration into advanced wearable technologies for real-time health monitoring and movement analysis.

Experimental

Materials

ALG, with a molecular weight (MW) of 220 000 g mol⁻¹, was obtained from Junsei Chemical Co., Ltd. Glycerol (MW: 92.09 g mol⁻¹) and anhydrous calcium chloride (CaCl_2, MW: 110.98 g mol⁻¹) were purchased from Sigma-Aldrich. The synthesis of PEDOT:PSS was conducted in the lab using 3,4-ethylenedioxythiophene (EDOT), polystyrene sulfonate (PSS, MW: 75 000 g mol⁻¹), sodium persulfate (Na₂S₂O₈), and iron(III) sulfate (Fe₂(SO₄)₃). Ethylene glycol (C₂H₆O₂, MW: 62.07 g mol⁻¹) was also procured from Sigma-Aldrich, while Capstone FS-31 was purchased from Chemours.

Preparation of PEDOT:PSS

The lab-synthesized PEDOT:PSS solution was prepared by mixing poly(styrene sulfonate) (4.0 wt% in deionized water), sodium persulfate (Na₂S₂O₈) (with an EDOT-to-Na₂S₂O₈ molar ratio of 1 : 0.9), ferric sulfate Fe₂(SO₄)₃ (with an EDOT-to-Fe₂(SO₄)₃ molar ratio of 1 : 0.02), and deionized water (400 g). The mixture was stirred for 1 h at room temperature. Subsequently, the EDOT monomer and enhancer CSE100 (1 wt%, AH Materials, Korea), which was added to improve the conductivity and stretchability of the hydrogels, were introduced. The PEDOT:PSS weight ratio was maintained at 1 : 2. The mixture was stirred continuously at room temperature for 20 h to ensure homogeneity. This step was performed at room temperature to maintain consistent reaction conditions throughout the process.⁷³

Preparation of the ALG/PEDOT:PSS hydrogels

The ALG/PEDOT:PSS hydrogels were fabricated using a mixed evaporation process. To enhance hydrogel formation and surface properties, 10 mL of PEDOT:PSS was mixed with 600 μL of ethylene glycol and 10 μL of the Capstone FS-31 nonionic surfactant. Separately, 0.4 g ALG was dissolved in deionized water to make a 2 wt% solution. To improve the stretchability and conductivity of hydrogel, 10 vol% of glycerol and 2.5 vol% of the produced PEDOT:PSS solution were added to the ALG solution. To optimize the ALG/PEDOT:PSS solution, various amounts of a 0.5 M CaCl₂ solution in deionized water were applied. The concentrations used were 0.25, 0.5, 0.75, and 1 vol%, and the mixtures were stirred until homogeneous. After mixing thoroughly, the solution was degassed to remove bubbles and ensure a smooth hydrogel texture. Then, the homogenized solution was dried at a controlled temperature of 60 °C for 10 h to form hydrogels. Once dried, the hydrogels were characterized to assess their mechanical and electrical properties and subsequently applied to test their practical usability.

Recycling process of the ALG/PEDOT:PSS hydrogels

For the recycling test, 2 g of the original CaCl₂-treated ALG/PEDOT:PSS hydrogel was dissolved in 20 mL of deionized water at 60 °C for 6 h under continuous stirring. The solution was then vacuum degassed to remove bubbles and moisture before being reshaped by solution casting at 60 °C for 10 h.

Characterization of devices

The sheet resistances of the hydrogels were measured using the van der Pauw method with a Keithley 2401 source-measurement device. The mechanical properties were characterized using a universal testing machine (ST-1000). The change in resistance under tensile tension was recorded on a custom-made stage equipped with a Keithley 2401 source meter. A chemical structural analysis was conducted using Raman spectroscopy with a LabRAM HR Evolution device (Horiba) operating at 633 nm. The thermal stability of the hydrogels was determined via TGA (55, TA Instruments). The performance of the ALG/PEDOT:PSS hydrogel sensor was thoroughly characterized on a custom-designed stage or directly on the human body using either a source-measuring unit (Keithley 2401) or a wireless system, enabling its application in multifunctional sensors. Wireless motion-sensing (WMS1000) devices were developed by combining the fabricated ALG/PEDOT:PSS hydrogel, a sensing module, a rechargeable lithium battery (500 mA h), and an ES32 module. The system included a microcontroller unit (MCU), 12-bit analog-to-digital converter (ADC), and Bluetooth low-energy (BLE)/WiFi module for wireless communication. Analog signals from the motion sensor were digitized at 100 samples per second and transmitted via BLE to a personal computer (PC). The user interface software displayed and recorded the data, enabling effective characterization of the wireless performance of the sensor. The wearable sensor tests were conducted with the approval of the Institutional Review Board of Pukyong National University. All participants voluntarily provided informed consent for their data to be included and published in this manuscript.

Conclusions

In this study, we developed high-performance, stretchable, and recyclable ALG/PEDOT:PSS hydrogels with excellent mechanical properties and low electrical hysteresis. These hydrogels, which were fabricated using a physical crosslinking approach with the addition of glycerol and CaCl₂, exhibited outstanding mechanical and electrical properties. The incorporation of glycerol enhanced the stretchability and toughness of the hydrogels, while CaCl₂ acted as a gelling agent that further improved their mechanical properties. The hydrogels exhibited a high stretchability of 138%, while demonstrating a limited resistance change (1.6-fold at 100% strain). In addition, they displayed low hysteresis characteristics, with a DH of only 2.95% (50% strain). The hydrogels retained their electrical and mechanical performance through multiple stretch-release cycles, demonstrating outstanding stability and minimal electrical hysteresis. Utilizing their excellent mechanical and electrical properties, we fabricated strain sensors using the CaCl₂-added ALG/PEDOT:PSS hydrogels. These sensors exhibited a high sensitivity and stability, rendering them ideal for applications in physical activity tracking. The sensors demonstrated low electrical hysteresis and retained a consistent performance over extended cycles, highlighting their suitability for real-time monitoring applications. Moreover, the hydrogels exhibited excellent recyclability and retained their original performance after reprocessing. This attribute, along with their environmentally friendly fabrication process, make these hydrogels sustainable options for recyclable wearable sensors. In addition, the integration of these hydrogels into wireless sensor systems has the potential for advanced wearable technology. Wireless sensors are highly sensitive and reliable, enabling real-time movement monitoring. Their robust performance and adaptability demonstrate the significant potential of these hydrogel-based sensors for incorporation into wearable technologies, enabling reliable real-time health monitoring and comprehensive movement analyses.

Data availability

The data supporting this article have been included as part of the ESI.†

Author contributions

Yulia Shara br Sembiring: conceptualization, investigation, writing – original draft. Truong Tien Vo: investigation, writing – original draft. Siti Aisyah Nurmaulia Entifar: investigation. Anky Fitrian Wibowo: investigation. Jung Ha Kim: investigation. Nisa Aqilla Ellenahaya Entifar: investigation. Jang Hyeok Lee: formal analysis. Si Won Baek: formal analysis. Soo In Lee: formal analysis. Min Seong Kim: investigation, Sang Min Jeon: investigation. Jincheol Kim: formal analysis. Junghwan Oh: project administration, writing – review & editing. Yong Hyun Kim: project administration, writing – review & editing. All authors commented on the paper.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors acknowledge the financial support from Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (No. 2021R1A2C1094308 and No. 2022R1A5A8023404). This study was also supported by the Global Joint Research Program, which was funded by Pukyong National University (202411800001).

References

1. S. Sharma, G. B. Pradhan, S. Jeong, S. Zhang, H. Song and J. Y. Park, ACS Nano, 2023, 17, 8355–8366.
2. W. B. Han, G.-J. Ko, K.-G. Lee, D. Kim, J. H. Lee, S. M. Yang, D.-J. Kim, J.-W. Shin, T.-M. Jang, S. Han, H. Zhou, H. Kang, J. H. Lim, K. Rajaram, H. Cheng, Y.-D. Park, S. H. Kim and S.-W. Hwang, Nat. Commun., 2023, 14, 2263.
3. W. Zhang, L. Dai, T. Sun, C. Qin, J. Wang, J. Sun and L. Dai, Polymer, 2023, 283, 126281.
4. C. Barone, P. Maccagnani, F. Dinelli, M. Bertoldo, R. Capelli, M. Cocchi, M. Seri and S. Pagano, Sci. Rep., 2022, 12, 9861.
5. F. Huang, X. Sun, Y. Shi and L. Pan, FlexMat, 2024, 9, 1.
6. N. Liu, H. Ma, M. Li, R. Qin and P. Li, FlexMat, 2024, 1, 269–301.
7. T. Cheng, F. Wang, Y.-Z. Zhang, L. Li, S.-Y. Gao, X.-L. Yang, S. Wang, P.-F. Chen and W.-Y. Lai, Chem. Eng. J., 2022, 450, 138311.
8. T. Cheng, Z.-T. Liu, J. Qu, C.-F. Meng, L.-J. He, L. Li, X.-L. Yang, Y.-J. Cao, K. Han, Y.-Z. Zhang and W.-Y. Lai, Adv. Sci., 2024, 11, 2403358.
9. G. Li, Y. Gong, S. Fang, T. You, R. Shao, L. Yao, C. Liu, C. Wu, J. Niu and W.-Y. Lai, Sci. China Mater., 2024, 67, 1481–1490.
10. Y. Wang and Y. Lu, Ind. Eng. Chem. Res., 2023, 62, 11279–11304.
11. Z. Qin, X. Sun, H. Zhang, Q. Yu, X. Wang, S. He, F. Yao and J. Li, J. Mater. Chem. A, 2020, 8, 4447–4456.
12. L. Mottet, D. Le Cornec, J.-M. Noël, F. Kanoufi, B. Delord, P. Poulin, J. Bibette and N. Bremond, Soft Matter, 2018, 14, 1434–1441.
13. N. Qureshi, V. Dhand, S. Subhani, R. S. Kumar, N. Raghavan, S. Kim and J. Doh, Adv. Mater. Technol., 2024, 9, 2400250.
14. K. K. Kim, S. Hong, H. M. Cho, J. Lee, Y. D. Suh, J. Ham and S. H. Ko, Nano Lett., 2015, 15, 5240–5247.
15. M. Li, J. Fang, L. Weng, J. X. Gao, Y. Wan, H. Yan and J. Liu, ACS Appl. Electron. Mater., 2024, 6, 4057–4065.
16. K. Namsheer and C. S. Rout, RSC Adv., 2021, 11, 5659–5697.
17. A. F. Wibowo, J. W. Han, J. H. Kim, A. Prameswati, S. A. N. Entifar, J. Park, J. Lee, S. Kim, D. C. Lim, Y. Eom, M. W. Moon, M. S. Kim and Y. H. Kim, ACS Appl. Mater. Interfaces, 2023, 15, 18134–18143.
18. H. Ji, H. He, J. Sun, W. Lu, H. Yu, J. Zhu, Y. Liu, S. Chen and H. Cao, ACS Appl. Electron. Mater., 2024, 6, 4619–4629.
19. A. F. Wibowo, S. Nagappan, S. A. Nurmaulia Entifar, J. H. Kim, Y. S. br Sembiring, J. W. Han, J. Oh, G. Xie, J. Lee, J. Kim, D. Chan Lim, M. W. Moon, M. S. Kim, S. Kim and Y. H. Kim, J. Mater. Chem. A, 2024, 12, 19403–19413.
20. A. Roy, S. Zenker, S. Jain, R. Afshari, Y. Oz, Y. Zheng and N. Annabi, Adv. Mater., 2024, 36, 2404225.
21. X. Fan, N. E. Stott, J. Zeng, Y. Li, J. Ouyang, L. Chu and W. Song, J. Mater. Chem. A, 2023, 11, 18561–18591.
22. H. Li, R. Luo, J. Hu, S. Zhou, X. Zhou and B. Du, J. Mater. Chem. A, 2024, 12, 15290–15299.
23. H. Li, J. Cao, R. Wan, V. R. Feig, C. M. Tringides, J. Xu, H. Yuk and B. Lu, Adv. Mater., 2025, 37, 2415151.
24. X. Xie, Z. Xu, X. Yu, H. Jiang, H. Li and W. Feng, Nat. Commun., 2023, 14, 4289.
25. J. Yu, F. Tian, W. Wang, R. Wan, J. Cao, C. Chen, D. Zhao, J. Liu, J. Zhong, F. Wang, Q. Liu, J. Xu and B. Lu, Chem. Mater., 2023, 35, 5936–5944.
26. Y. Xue, X. Chen, F. Wang, J. Lin and J. Liu, Adv. Mater., 2023, 35, 2304095.
27. J. Li, J. Cao, B. Lu and G. Gu, Nat. Rev. Mater., 2023, 8, 604–622.
28. N. M. Badawi, M. Bhatia, S. Ramesh, K. Ramesh, M. Kuniyil, M. R. Shaik, M. Khan, B. Shaik and S. F. Adil, Polymers, 2023, 15, 571.
29. M. Wang, Q. Gao, J. Gao, C. Zhu and K. Chen, J. Mater. Chem. C, 2020, 8, 4564–4571.
30. S. Lee, K. Park, J. Kum, S. An, K. J. Yu, H. Kim, M. Shin and D. Son, Polymers, 2023, 15, 84.
31. K. Park, K. Kang, J. Kim, S. D. Kim, S. Jin, M. Shin and D. Son, ACS Appl. Mater. Interfaces, 2022, 14, 56395–56406.
32. C. Wei, Y. Wang, Y. Liang, J. Wu, F. Li, Q. Luo, Y. Lu, C. Liu, R. Zhang, Z. Lu, B. Xu, N. Qing and L. Tang, J. Mater. Chem. A, 2024, 12, 10392–10402.
33. Y. Wang, X. Ma, Y. Jiang, W. Zang, P. Cao, M. Tian, N. Ning and L. Zhang, Resour. Chem. Mater., 2022, 1, 308–324.
34. J. Wu, W. Huang, Z. Wu, X. Yang, A. G. P. Kottapalli, X. Xie, Y. Zhou and K. Tao, ACS Mater. Lett., 2022, 4, 1616–1629.
35. Y. Gong, Y. Z. Zhang, S. Fang, Y. Sun, J. Niu and W. Y. Lai, ACS Appl. Mater. Interfaces, 2022, 14, 47300–47309.
36. S. Z. Homayounfar and T. L. Andrew, SLAS Technol., 2020, 25, 9–24.
37. J. García-Torres, S. Colombi, L. P. Macor and C. Alemán, Int. J. Biol. Macromol., 2022, 219, 312–332.
38. X. Wang, H. Zhang, X. Zhang, C. Shen, M. Liu, S. Liu, Y. Han and T. He, Int. J. Biol. Macromol., 2024, 254, 127879.
39. S. Santhanam, V. R. Feig, K. W. McConnell, S. Song, E. E. Gardner, J. J. Patel, D. Shan, Z. Bao and P. M. George, Adv. Mater. Technol., 2023, 8, 2201724.
40. S.-S. Yoon and D.-Y. Khang, J. Phys. Chem. C, 2016, 120, 29525–29532.
41. S. Masoumi, K. Zhussupbekov, N. Prochukhan, M. A. Morris and A. Pakdel, J. Mater. Chem. C, 2024, 12, 14314–14329.
42. Y. T. Tseng, Y. C. Lin, C. C. Shih, H. C. Hsieh, W. Y. Lee, Y. C. Chiu and W. C. Chen, J. Mater. Chem. C, 2020, 8, 6013–6024.
43. R. Kadri, S. Bresson and T. Aussenac, Appl. Sci., 2023, 13, 8846.
44. Y. Zhu, D. Yao, X. Gao, J. Chen, H. Wang, T. You, C. Lu and X. Pang, ACS Appl. Mater. Interfaces, 2024, 16, 32466–32480.
45. S. Choi, W. Kim, W. Shin, H. J. Han, C. Park, H. Oh, S. Jung, S. Park and H. Lee, Appl. Surf. Sci., 2023, 610, 155609.
46. S. U. Khan, M. Sultan, A. Islam, A. Sabir, S. Hafeez, I. Bibi, M. N. Ahmed, S. M. Khan, R. U. Khan and M. Iqbal, Int. J. Biol. Macromol., 2021, 182, 72–81.
47. L. Bahsis, E. H. Ablouh, H. Anane, M. Taourirte, M. Julve and S. E. Stiriba, RSC Adv., 2020, 10, 32821–32832.
48. H. Zhou, N. Yang, J. Hou, C. Yu, Z. Jin, P. Zeng, L. Yang, Y. Fu, Y. Shen and S. Guo, Food Packag. Shelf Life, 2022, 34, 100935.
49. A. S. Giz, M. Berberoglu, S. Bener, S. Aydelik-Ayazoglu, H. Bayraktar, B. E. Alaca and H. Catalgil-Giz, Int. J. Biol. Macromol., 2020, 148, 49–55.
50. I. Donati and B. E. Christensen, Carbohydr. Polym., 2023, 321, 121280.
51. M. Li, J. Pu, Q. Cao, W. Zhao, Y. Gao, T. Meng, J. Chen and C. Guan, Chem. Sci., 2024, 15, 17799–17822.
52. Z. Shen, Z. Zhang, N. Zhang, J. Li, P. Zhou, F. Hu, Y. Rong, B. Lu and G. Gu, Adv. Mater., 2022, 34, 2203650.
53. A. Saeed, S. F. A. Zaidi, C. G. Park and J. H. Lee, Adv. Mater. Technol., 2024, 9, 2400355.
54. X. Yan, F. Li, K.-D. Hu, J. Xue, X.-F. Pan, T. He, L. Dong, X.-Y. Wang, Y.-D. Wu, Y.-H. Song, W.-P. Xu and Y. Lu, Sci. Rep., 2017, 7, 13851.
55. Y. Fu, Z. Wan, S. Zhao, Y. Tian, Z. Zhao and Z. Wei, Smart Mater. Struct., 2023, 32, 015001.
56. H. Wang, J. Guo, Y. Lei, W. Chen, Z. Zhang, M. Zhu and H. Xu, ACS Appl. Nano Mater., 2024, 7, 2100–2109.
57. A. Kannan, M. Dheeptha and Y. S. Sistla, Eng. Proc., 2023, 37, 80.
58. H. Li, C. Hu, Y. Jiang, J. Zhu, Y. He, X. Hu and D. Tang, ACS Sustain. Chem. Eng., 2023, 11, 10620–10630.
59. N. Kaur, A. Arya, R. Kumar, J. Kaur, Savita, N. Khattar, P. K. Diwan and A. Sharma, Mater. Sci. Eng. B, 2023, 293, 116495.
60. C. Zhao, B. Xia, A. Hu, M. Hou, T. Li, S. Wang, M. Chen and W. Dong, Int. J. Biol. Macromol., 2023, 236, 123939.
61. N. Sharmin, C. Pang, I. Sone, J. L. Walsh, C. G. Fernández, M. Sivertsvik and E. N. Fernández, Nanomaterials, 2021, 11, 2306.
62. S. Wei, X. Zhang, K. Zhao, Y. Fu, Z. Li, B. Lin and J. Wei, Polym. Compos., 2016, 37, 1292–1301.
63. S. Bhatia, Y. A. Shah, A. Al-Harrasi, S. Ullah, M. K. Anwer, E. Koca, L. Y. Aydemir and M. R. Khan, Food Sci. Nutr., 2024, 12, 1056–1066.
64. M. M. Hasan, M. S. Hossain, M. D. Islam, M. M. Rahman, A. S. Ratna and S. Mukhopadhyay, ACS Appl. Mater. Interfaces, 2023, 15, 32011–32023.
65. S. Guan, C. Xu, X. Dong and M. Qi, J. Mater. Chem. A, 2023, 11, 15404–15415.
66. S. Hong, T. Park, J. Lee, Y. Ji, J. Walsh, T. Yu, J. Y. Park, J. Lim, C. Benito Alston, L. Solorio, H. Lee, Y. L. Kim, D. R. Kim and C. H. Lee, ACS Sensors, 2024, 9, 662–673.
67. W. Huang, X. Wang, F. Luo, X. Zhao, K. Chen and Y. Qin, ACS Appl. Mater. Interfaces, 2024, 16, 49834–49844.
68. S. Azadi, S. Peng, S. A. Moshizi, M. Asadnia, J. Xu, I. Park, C. H. Wang and S. Wu, Adv. Mater. Technol., 2020, 5, 2000426.
69. Z. Fan, D. Ji and J. Kim, Adv. Intell. Syst., 2023, 5, 2300194.
70. Y. Wang, Y. Jia, H. Ren, C. Lao, W. Peng, B. Feng and J. Wang, Mater. Today Bio, 2021, 12, 100138.
71. L. Kong, W. Li, T. Zhang, H. Ma, Y. Cao, K. Wang, Y. Zhou, A. Shamim, L. Zheng, X. Wang and W. Huang, Adv. Mater., 2024, 36, 2400333.
72. F. Gao, C. Liu, L. Zhang, T. Liu, Z. Wang, Z. Song, H. Cai, Z. Fang, J. Chen, J. Wang, M. Han, J. Wang, K. Lin, R. Wang, M. Li, Q. Mei, X. Ma, S. Liang, G. Gou and N. Xue, Microsystems Nanoeng., 2023, 9, 1.
73. J. W. Han, J. Park, J. H. Kim, S. A. Entifar, A. Prameswati, A. F. Wibowo, S. Kim, D. C. Lim, J. Lee, M.-W. Moon, M.-S. Kim and Y. H. Kim, Materials, 2022, 15, 5009.

---

Footnotes

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4ta08909j

‡ These authors contributed equally to this work.

---