High‐Sensitivity and Flexible Motion Sensing Enabled by Robust, Self‐Healing Wood‐Based Anisotropic Hydrogel Composites

Abstract

By integrating polyvinyl alcohol (PVA)‐borate‐tannic acid (TA)‐sodium sulfate into cellulosic wood matrices, a novel wood‐basedPVA‐borate‐TA‐sodium sulfate (WPBTS) hydrogel is successfully synthesized. Through a multicomponent synergistic design combining natural lignocellulose, PVA, borax, TA, and sodium sulfate, multiple dynamic cross‐linking mechanisms—dynamic borate bonding, hydrogen bonding, and metal‐ligand interactions—are established, resulting in WPBTS hydrogels with exceptional mechanical properties and self‐healing capabilities. The mechanical strength of the WPBTS hydrogel reached an impressive 19.8 MPa, a 45‐fold increase compared to PVA‐borax‐tannic acid (PBTS) hydrogels. Furthermore, the assembled WPBTS hydrogel‐based flexible sensor demonstrates a remarkably fast response time of just 20 ms and maintains excellent performance in challenging simulated saline environments. This innovation represents a significant advancement in sensor technology and highlights the potential for transformative applications in complex and demanding scenarios.

This study develops a WPBTS hydrogel with exceptional mechanical strength, self‐healing capability, and high electrical conductivity. The hydrogel‐enabled flexible sensor demonstrates stable sensing performance in complex environments, advancing the development of wearable electronic devices and multifunctional sensing technologies.

1. Introduction

The rapid advancements in flexible electronics are driven by the growing demand for devices that seamlessly combine flexibility, portability, and adaptability.^{[ 1 ]} At the core of this technological evolution are flexible motion sensors, which are pivotal in the development of wearable electronics.^{[ 2} , ³ , ^{4 ]} These sensors have an extraordinary ability to adapt to the dynamic movements of the human body, including bending, folding, and stretching, enabling precise detection and monitoring of human motion under diverse conditions.^{[ 5 ]} Despite these advances, the commonly used substrates ranging from synthetic polymers such as rubber and polyethylene terephthalate (PET)^{[ 6 ]} to various elastomers, although increasing flexibility, often lack biocompatibility and environmental sustainability thus causing significant challenges in integrating them into human‐centered applications.

Hydrogels have emerged as a revolutionary material for flexible electronics due to their inherent flexibility, stretchability, and biocompatibility.^{[ 7} , ^{8 ]} However, conventional hydrogels often exhibit poor mechanical properties and limited reusability due to their high‐water content, which restricts their application in long‐term motion monitoring.^{[ 9 ]} While cellulose‐based hydrogels have shown significant potential as wearable sensors, their application has been hindered by complex preparation processes and isotropic structural designs, which fail to meet the demand for direction‐specific mechanical properties.^{[ 10} , ¹¹ , ^{12 ]}

In response to these challenges, wood‐based hydrogels have emerged as a novel class of composite materials that synergistically combine the mechanical strength of wood with the multifunctional properties of hydrogels. By leveraging the natural cellular structure of wood, composed primarily of cellulose, hemicellulose, and lignin, these hydrogels take full advantage of the structural support provided by cellulose while strategically removing lignin and hemicellulose. This process preserves the directional alignment of cellulose fibers, enabling the development of anisotropic hydrogels with directional mechanical properties and tailored functionalities. Such materials uniquely embody the flexibility and biocompatibility of hydrogels while inheriting the solid mechanical strength of wood, representing a significant breakthrough for the long‐term application of hydrogels in flexible electronics.

In this study, we present an innovative approach to synthesize WPBTS hydrogel by integrating PVA‐borate‐TA‐sodium sulfate into cellulosic wood matrices. PVA forms the primary crosslinking network, while borax introduces dynamic borate bonding, which enhances the hydrogel's self‐healing capabilities. The addition of sodium sulfate further improves electrical conductivity, making the hydrogel suitable for sensing applications. Borax also serves as a crosslinking agent between cellulose chains, forming transient coordination bonds with hydroxyl groups and creating a robust 3D network that significantly enhances the hydrogel's mechanical strength and structural integrity.^{[ 13 ]} The mechanical strength of the WPBTS hydrogel reached an impressive 19.8 MPa, a 45‐fold increase compared to PVA‐borax‐TA (PBTS) hydrogels. Furthermore, the assembled WPBTS hydrogel‐based flexible sensor demonstrated a remarkably fast response time of only 20 ms, making it a promising candidate for real‐time motion detection in wearable sensors and extends to underwater signaling applications, such as transmitting Morse code. Furthermore, hydrogel maintains its functionality in challenging environments, including saline conditions, over extended periods, showcasing its robustness and adaptability. With this pioneering work, we have not only addressed the limitations surrounding the mechanical properties of hydrogels, but also paved the way for sustainable, biocompatible, and mechanically robust materials, thus enriching the ongoing debate in the field of materials science and engineering. The combination of anisotropic mechanical properties, self‐healing capabilities, and environmental adaptability positions our WPBTS hydrogel as a multifunctional material that bridges the gap between high‐performance hydrogels and sustainable materials science.

2. Results and Discussion

In nature, wood is characterized by a complex 3D layered framework composed of hollow fibers. These fibers have cell walls consisting of robust cellulose fibrils embedded within a softer, amorphous matrix of hemicellulose and lignin.^{[ 14 ]} In almost all types of wood, these fibers are primarily aligned along the tree's growth direction, forming a highly anisotropic structure (Figure  1 ). Each wood fiber contains numerous microfibers, which can be further broken down into nanofibers, including cellulose nanofibers (CNFs). Inspired by this unique structural phenomenon, a process known as delignification (Figure 1; Figure S1, Supporting Information) can be employed to remove lignin from wood, releasing the tight connections between CNFs bundles while preserving the inherent structure of the aligned cellulose backbone. By constructing a microstructure combining CNFs, PVA‐borax‐TA, and sodium sulfate, a flexible WPBTS hydrogel was synthesized. Through this process, we achieved a transformation from macroscopic natural wood to nanomaterials.

Figure 1.

Schematics for the delignification and synthesis of WPBTS hydrogel.

In this system, the 3D cross‐linked network formed by the interactions between cellulose, PVA, borax, and TA through chemical bonds and non‐covalent interactions. The cellulose chains, rich in hydroxyl groups (─OH), form a robust hydrogen bonding network with the hydroxyl groups of PVA chains. Meanwhile, the borax dissociates into borate ions (B(OH)₄ ⁻), which can form reversible borate ester bonds with the hydroxyl groups on both the cellulose and PVA chains, enhancing the network's strength and flexibility. Specifically, the borate ions coordinate with adjacent hydroxyl groups at the C₂ and C₃ positions of the glucose units in the cellulose chains, forming a bidentate coordination structure. This reversible chemical cross‐linking enables the cellulose network to exhibit excellent elasticity and self‐healing properties by breaking and reforming under mechanical stress or environmental changes. Additionally, A, a polyphenol molecule, forms hydrogen bonds with the hydroxyl groups on cellulose and PVA, further densifying and stabilizing the network. It can also interact synergistically with borate ions, enhancing the network's dynamic self‐healing performance. Through the synergistic effects of multiple hydrogen bonds and reversible chemical cross‐linking, this structure demonstrates superior mechanical properties, elasticity, and self‐healing capability, making it highly suitable for the development of multifunctional soft materials (Figures S1, Supporting Information).

A Scanning Electron Microscope (SEM) was used to examine the microstructures of natural wood (basswood) and WPBTS hydrogels. Small pine wood blocks measuring 1 cm in length and width and 1 mm in thickness were prepared for characterization. Natural wood exhibited a uniformly distributed pore structure of varying sizes in the radial (R) direction (Figure  2a) and a neatly oriented, aligned structure in the longitudinal (L) direction (Figure 2b). A significant difference was observed when comparing the natural wood to WPBTS hydrogels. In the radial (R) direction of the wood within the WPBTS hydrogel, the honeycomb structure of the wood showed substantial contraction. This phenomenon is attributed to the use of sodium hydroxide, which softens the cellulose cell wall structure.^{[ 15 ]} Furthermore, as shown in Figure 2c,d,g, hydrogel condensation was observed both within and around the wood pores in the L and R directions, indicating successful adhesion of the hydrogel to the wood cellulose. Elemental mapping revealed the presence of four elements: boron (B), carbon (C), sodium (Na), and oxygen (O) (Figure 2e,f,h,i). These elemental maps further confirmed the successful integration of the WPBTS hydrogel into the wood structure.

Figure 2.

Morphological characterization of WPBTS hydrogel. a) SEM image of natural wood in R direction. b) SEM image of natural wood in L direction. c) SEM image of WPBTS in R direction. d) Enlarged image of c. e) The elemental maps of C for WPBTS hydrogel. f) The elemental maps of O for WPBTS hydrogel. g) SEM image of WPBTS in L direction. h) The elemental maps of B for WPBTS hydrogel. i) The elemental maps of Na for WPBTS hydrogel.

As depicted in Figure S2 (Supporting Information), after the removal of lignin, the color of natural wood undergoes a transformation from yellow to white, accompanied by a substantial mass loss of 60 wt.%. This color change is attributed to the fact that the color‐contributing elements in natural wood primarily stem from lignin. Eliminating lignin and hemicellulose results in a softer “white wood” appearance. The results of the compositional measurements are shown in Table S1 (Supporting Information). After acid chemical treatment of the wood, the cellulose content in the wood skeleton increased to 71.41%, while the lignin content decreased to 3.29% and the hemicellulose content decreased to 25.30%. After alkaline chemical treatment, the cellulose content in the wood skeleton further increased significantly to 91.35%, while the lignin and hemicellulose contents decreased to 3.09% and 5.56% (Table S1, Supporting Information), respectively, providing an excellent base material for the preparation of composite hydrogels. This can be attributed to the fact that a significant portion of lignin and hemicellulose in the wood skeleton treated with acid and alkaline salt solutions was solubilized and removed. In contrast, the cellulose percentage increased due to its crystalline structure and high degree of polymerization, which make it more resistant to chemical degradation, resulting in minimal cellulose loss during the treatment process. The weight ratios of natural wood, delignified wood, and WPBTS show the same trend (Figure 3a).

Figure 3.

Structure properties of WPBTS and PBTS. a) Weight ratio of natural wood, delignified wood, and WPBTS. b) XRD pattern of natural wood, delignified wood, PBTS hydrogel, and WPBTS hydrogel. c) FTIR of WPBTS hydrogel, PBTS hydrogel, and natural wood. d)TGA of WPBTS hydrogel. e) Changes in weight of WPBTS hydrogel and PBTS hydrogel mass in the swelling rate test. f) Transmittance of WPBTS hydrogel in R direction and L direction.

We analyzed X‐ray diffraction (Figure 3b) on natural wood, delignified wood, PBTS hydrogel, and WPBTS hydrogel. Our findings revealed the presence of two broad crystal reflections within the 2θ range of 10–40°, occurring at ≈16.4°, 22.8°, and 35.1, corresponding to the (1 0 1), (0 0 2), and (0 0 4) crystal planes of cellulose.^{[ 15 ]} These reflections are indicative of the characteristic peaks associated with cellulose crystal structures. This XRD analysis underscores that the delignification process did not alter the crystal structure of cellulose. The intensity of these two diffraction peaks was higher in delignified wood compared to natural wood, indicating a higher crystallinity in the dignified sample. This phenomenon is attributed to the increased relative cellulose concentration within the wood composition following the removal of hemicellulose and lignin, resulting in heightened diffraction intensity.^{[ 16 ]} Compared with PBTS hydrogels, WPBTS hydrogels showed a new peak of cellulose crystals at 22.8°corresponding to (0 0 2) crystal reflection. This indicates that cellulose did not undergo significant structural damage during the hydrogel preparation process, and its crystal structure was preserved, which can serve as a directional framework in WPBTS.

In addition, Fourier transform infrared spectroscopy (FTIR) characterization of WPBTS hydrogels was used to determine the successful binding of the hydrogel to the wood. In Figure 3c for WPBTS hydrogels, the first is the feature in the region 3315 cm⁻¹ which is concerned with ─OH group stretching vibrations.^{[ 17 ]} C─H bond stretches ≈2960 cm⁻¹ and the C─O bond stretches at 1086 cm⁻¹, both characteristic peaks for PVA. The spectral peak at 1705 cm⁻¹ is due to C═O stretching vibrations in TA.^{[ 18 ]} The peaks at 1426 and 1338 cm⁻¹ are related to the asymmetric stretching vibration of B─O, which proves the presence of borax.^{[ 19 ]} These characteristic peaks successfully demonstrate the successful binding of PVA‐Borax‐TA hydrogels to cellulosic wood matrices. The FTIR of nature wood and delignified wood is shown in Figure S3 (Supporting Information), the peaks at 3416, 2933, and 1736 cm⁻¹ show O─H, C─H, and C═O stretching vibrations which represent cellulose, lignin, and hemicellulose.^{[ 20 ]}

Thermogravimetric Analysis (TGA) on both WPBTS and PBTS hydrogels are shown in Figure 3d and Figure S4 (Supporting Information). Specifically, for the WPBTS hydrogel, we observed distinct thermal degradation patterns. The mass loss attributed to the evaporation of water occurred in the temperature range of 0–200 °C (Figure 3d). Subsequently, between 200–300 °C (Figure 3d), the primary degradation process was associated with PVA.^{[ 21 ]} The most rapid degradation, occurring between 300–500 °C (Figure 3d), was ascribed to cellulose within the hydrogel. For the PBTS hydrogel, as shown in Figure S4, since water makes up most of the mass of the PBTS hydrogel (≈80%), the TGA curve decreases dramatically from 0 to 150 °C, mainly due to the evaporation of water from the hydrogel. This process is due to the volatilization of water from the hydrogel. After 150 °C, the TGA curve decreases slowly and stabilizes. TGA analysis provides valuable information about the thermal stability and decomposition behavior of the hydrogel constituents.

Subsequently, the water retention capacity of WPBTS and PBTS hydrogels was evaluated, as shown in Figure 3e. In our work, the water content of WPBTS hydrogels was as high as 63.3%. Water retention in the hydrogel was assessed by placing the samples at room temperature and measuring the change in their mass over time. Both hydrogels were subjected to the same conditions, and changes in mass were monitored every 12 h. Compared to the PBTS hydrogel, the WPBTS hydrogel demonstrated superior water retention. This enhanced water retention capability can be attributed to the following reasons. Hydrophilic groups in the multi‐component hydrogel (PVA, CNFs, borax, and TA) interact through hydrogen bonding and electrostatic interactions, effectively locking in water molecules. The fibrous and porous structure of the cellulosic wood framework aids in absorbing and storing moisture. Additionally, the sandwich structure formed between the wood and hydrogel further slows down water loss.^{[ 22 , 23 ]}

The WPBTS hydrogel also demonstrated excellent optical properties, as depicted in Figure 3f. It exhibited an impressive transmittance of 40% across the wavelength range of 400 to 1000 nm. This underscores the hydrogel's transparency and its ability to allow the passage of light, which makes it suitable for various optical applications. When the transmittance along the L direction and the R direction were examined separately, different results were observed due to the oriented structure of the cellulose backbone. WPBTS hydrogel has a slightly higher light transmission in the R direction than in the L direction.

As shown in Figure  4a,b, the longitudinal (L) direction of wood aligns with the primary growth direction of cellulose, while the radial (R) direction is perpendicular to the annual rings, reflecting the arrangement of cells formed during radial division. To illustrate the exceptional flexibility of the WPBTS hydrogel, in Figure 4c it is shown to be easily folded along the diagonal. It was folded in the L and R directions as shown in Figure 4d,e. No cracking or damage was observed upon returning the hydrogel to its original shape. Furthermore, the WPBTS hydrogel could be rolled up and recycled along either the L or R direction. It could be folded from all sides without compromising its surface structure as shown in Figure 4f. These demonstrations unequivocally highlight the outstanding flexibility and resilience of the WPBTS hydrogel, making it a robust and adaptable material for various applications. Whether in the rigid longitudinal (L) direction or the more pliable radial (R) direction, the WPBTS hydrogel demonstrates exceptional flexibility. It can be bent freely without incurring any damage to the hydrogel, displaying its robust and resilient nature in any direction.

Figure 4.

Physical picture of WPBTS hydrogel. a) Schematic 3D model of WPBTS hydrogel in L and R directions b) Optical image of WPBTS hydrogel in L and R directions. c) WPBTS hydrogel folded in half. d) WPBTS hydrogel bends in the L direction. e) WPBTS hydrogel bends in the R direction. f) WPBTS hydrogel folds in four different directions simultaneously to the back.

SAXS (Small‐Angle X‐ray Scattering) and WAXS (Wide‐Angle X‐ray Scattering) tests conducted on natural wood, delignified wood, and WPBTS hydrogels revealed that all three materials exhibited significant orientation (Figure  5 ). This orientation can be attributed to the stable structure of the cellulose backbone inherent in wood. The SAXS azimuthal intensity map (Figure 5d) shows the orientation distribution of the three samples: natural wood, delignified wood, and WPBTS hydrogel. The x‐axis represents the azimuthal angle (φ°) from 0° to 360°, and the y‐axis represents the normalized scattering intensity. Delignified wood shows distinct peaks at 90° and 270°, indicating a highly anisotropic structure with strong preferential alignment of CNFs due to the removal of lignin and hemicellulose. Natural wood shows weaker peaks, indicating a moderately anisotropic cellulose structure that is more randomly distributed due to the presence of lignin and hemicellulose. WPBTS hydrogel shows a relatively flat curve, suggesting that the modification process involving hydrogel formation decreases the directional alignment of the CNFs, resulting in reduced orientation. The calculated orientation degrees for the three materials are 91.6%, 80.1%, and 79.6%, respectively. The WAXS azimuthal intensity map (Figure 5h) shows a similar orientation trend among the samples. The calculated orientation degrees for natural wood, delignified wood, and WPBTS hydrogel are 90.6%, 88.7%, and 85.0%, respectively. According to SAXS and WAXS analysis highlights how delignification enhances the crystalline alignment of CNFs, while the introduction of a hydrogel network partially reduces this orientation. These findings are consistent with observations from the SEM images.

Figure 5.

SAXS and WAXS of natural wood, delignified wood, and WPBTS. a) SAXS of natural wood. b) SAXS of delignified wood. c) SAXS of WPBTS. d) Azimuthal intensity plot of natural wood, delignified wood, and WPBTS SAXS data. e) WAXS of natural wood. f) WAXS of delignified wood. g) WAXS of WPBTS. h) Azimuthal intensity plot of natural wood, delignified wood, and WPBTS WAXS data.

To quantify the effect of PVA content on WPBTS hydrogels, the tensile strength of WPBTS hydrogels with varying PVA concentrations (5, 10, and 15 wt.%) was assessed, as depicted in Figure  6a. The PVA concentration significantly influenced the hydrogel's mechanical properties. For WPBTS with 5 wt.% PVA, the low PVA content resulted in insufficient solid content within the hydrogel, hindering the formation of effective crosslinking. Consequently, this hydrogel exhibited relatively low breaking strength. In contrast, WPBTS with 10 wt.% PVA demonstrated remarkable mechanical performance, with an elongation rate of 13% and an impressive breaking strength of 19.8 MPa (Figure 6c), representing a 45‐fold improvement compared to PBTS hydrogels (Figure 6b). This combination of enhanced elongation and higher breaking strength highlights its exceptional mechanical properties. However, WPBTS with 15 wt.% PVA exhibited reduced elongation, registering only 7% (Figure 6a). This reduction can be attributed to the excessively high PVA concentration, which increased the viscosity of the hydrogel solution, making it difficult for the solution to fully penetrate the wood structure. In summary, for WPBTS hydrogels, the optimal PVA concentration is 10 wt.%, as it strikes an excellent balance between elongation and breaking strength, significantly outperforming PBTS hydrogels in mechanical properties.

Figure 6.

Mechanical properties of WPBTS and PBTS. a) Tensile strain of 5wt%,10wt%.15wt% PVA WPBTS hydrogel in L direction. b) Tensile strain of 10wt% WPBTS hydrogel in L direction, R direction, and PBTS hydrogel. c) Breaking strength of 5wt%,10wt%.15wt% PVA WPBTS in L direction and R direction. d) Elastic modulus and toughness of WPBTS hydrogel in L direction and R direction and PBTS hydrogel. e) Changes in the orientation of WPBTS hydrogel during stretching. f) Our work compares elastic modulus and fracture strength with other material.

When comparing the breaking strength in the longitudinal (L) direction with that in the radial (R) direction, it is apparent that the breaking strength in the R direction is relatively lower, as shown in Figure 6b. However, the R direction demonstrates better elongation at the point of rupture. This phenomenon can be attributed to the structural differences between the two directions. In the R direction, the hydrogel forms individual connections with the cellulose skeleton, as observed in SEM images. The exceptional stretchability of the hydrogel in this direction contributes to a higher fracture strain during stretching. Consequently, while the breaking strength may be lower, the hydrogel's ability to elongate before rupture is notably enhanced (Figure 6b).

In contrast, in the L direction, the cellulose backbone bears most of the load during stretching. Although this direction exhibits higher breaking strength, it shows lower strain at the point of rupture. In summary, the differences in mechanical properties between the L and R directions arise from the distinct roles played by the cellulose skeleton within the hydrogel. The R direction benefits from the hydrogel's stretchability, leading to improved elongation, while the L direction depends more on the strength of the cellulose backbone. This outcome highlights the hydrogel's remarkable stability and resilience, maintaining its compressibility and structural integrity even under repetitive compression cycles (Figure 6c).

In addition to that, as shown in Figure 6d, the WPBTS hydrogel possesses an elastic modulus of up to 200 MPa in the L‐direction, which is much higher than its elastic modulus in the R‐direction and PBTS hydrogel. Additionally, the corresponding material toughness can be obtained by calculating the integral area of the stress–strain curves of the WPBTS and PBTS hydrogels from the single‐notch samples in the L and R directions. This is due to the orientation of WPBTS hydrogels in the L‐direction being much higher than in the R‐direction and PBTS hydrogels. The oriented structure allows stresses to be distributed more evenly within the hydrogel, absorbing and dispersing energy more efficiently under tension, which helps to prevent stress concentration and localized fracture, resulting in improved toughness.

Subsequently, in situ SAXS characterization (Figure 6e) was performed by stretching the WPBTS hydrogel on a customized tensile testing holder. The orientation of the WPBTS hydrogel progressively increased with strain, attributed to the improved alignment of the cellulose backbone during stretching.^{[ 24 ]} This enhancement is evident in the material's increased longitudinal orientation under tension.^{[ 25 ]} These experiments provided valuable insights into the relationship between the mechanical behavior and structural changes of WPBTS under varying stretching conditions. As shown in Figure 6f and Table S2 (Supporting Information), WPBTS hydrogels exhibit excellent mechanical properties compared to other hydrogels or natural wood tissue structures.^{[ 24} , ²⁵ , ²⁶ , ²⁷ , ²⁸ , ²⁹ , ³⁰ , ³¹ , ³² , ³³ , ³⁴ , ^{35 ]} Not only does it possess excellent fracture strength, but it also has a very high modulus of elasticity.

The self‐healing ability of hydrogels is critical for the development of long‐lasting, flexible, and reliable wearable devices. Dynamic hydrogen bonding between cellulose and PVA, along with dynamic borate diester bonding between PVA and borax, provides an ideal foundation for the self‐healing properties of WPBTS hydrogels. Borate esters enhance the network structure through crosslinking, while borate ions and cellulose contribute to bond reformation, promoting improved self‐healing performance when damage occurs.^{[ 36 ]} We tested longitudinal (L) and radial (R) cuts of WPBTS hydrogels to investigate their self‐healing properties (Figure S5, Supporting Information). As shown in Figure  7a,b, the self‐healing fracture strength of WPBTS hydrogel in the R direction was determined to be 0.4 MPa, representing 30% of its initial fracture strength (Figure 7b). In contrast, the self‐healing performance in the L direction was more effective, with the self‐healing fracture strength amounting to 11.19 MPa, which is 56.5% of its initial fracture strength (Figure 7a). The variation in self‐healing performance is attributed to the hydrogel's distribution around the cellulose backbone. Cutting in the L direction, it only affects a small portion of the cellulose, allowing the remaining hydrogel to successfully self‐heal. Conversely, cutting in the R direction severs a substantial portion of the cellulose backbone, significantly compromising the mechanical properties of the self‐healing hydrogel. After self‐healing, the elastic modulus of WPBTS in the L direction recovers to 37.5% of its original state, while its toughness is restored to 50.8% (Figure 7c).

Figure 7.

Self‐healing properties of hydrogels. a) Tensile strain of original and self‐healing WPBTS in L direction. b) Tensile strain of original and self‐healing WPBTS in R direction. c) Elastic modulus and toughness of original and self‐healing WPBTS in L direction.

WPBTS hydrogel is an ideal candidate for flexible sensors due to its excellent mechanical properties and self‐healing ability. Therefore, when preparing the hydrogel precursor solution, we added sodium sulfate as an ionic conductive agent to investigate the effect of different concentrations of sodium sulfate solution on the electrical conductivity of WPBTS hydrogel and to select the WPBTS hydrogel with the best conductivity properties as a flexible sensor to monitor the change of electrical signals during the movement of the human body. As the concentration of sodium sulphate increased, so did the hydrogel's conductivity. However, once the sodium sulfate content exceeded 0.9wt%, the hydrogel's conductivity remained stable and did not increase further with higher ion concentrations (Figure  8a). The maximum conductivity of WPBTS hydrogel is ≈3.67 mS m⁻¹. Meanwhile, in terms of self‐healing, the conductivities of the hydrogel in the L and R directions after self‐healing were 3.7 and 2.44 mS m⁻¹, which were close to the initial condition (Figure S6, Supporting Information).

Figure 8.

Sensing performance of WPBTS as flexible sensors. a) Effect of different concentrations of Na₂SO₄ on conductivity. b) Response time of WPBTS. c) The change of current signal when stretching WPBTS hydrogel with different strain. d) Current signals of finger bending with different amplitudes. e) Current signals of elbow bending. f) Current signals of pulse.

To conduct real‐time monitoring of human motion, we affixed WPBTS hydrogel to different joints in the human body using ordinary adhesive tape and recorded changes in electrical current through an electrochemical workstation (Gamry Instrument Inc., USA). The WPBTS hydrogel exhibited an impressive response time of only 20 milliseconds (Figure 8b). When stretched along the R direction, the WPBTS hydrogel displayed different current changes at varying strains, with the magnitude of the current change increasing proportionally to the strain, as shown in Figure 8c. We observed that the current fluctuations corresponded to movements such as finger and elbow bending (Figure 8d,e). This phenomenon occurs because bodily motion compresses and deforms the hydrogel, altering the ion concentration per unit volume, which in turn affects conductivity and current flow. These results indicate that WPBTS hydrogels can effectively detect human motion in real time. Furthermore, the WPBTS hydrogels continued to detect stable signals during activities such as monitoring the pulse (Figure 8f), indicating their ability to perform reliably even under slight deformations. This versatility suggests that WPBTS hydrogels have potential applications in monitoring various human activities and physiological processes.

To explore the sensitivity limits of WPBTS hydrogels, we immersed the hydrogel in ultrapure water for varying durations to investigate changes in sensitivity. When the WPBTS hydrogel was submerged, different weights (10, 20, 50, and 100 g) generated distinct electrical currents upon contact with the hydrogel (Figure S7, Supporting Information). Additionally, the WPBTS hydrogel demonstrated high sensitivity to water wave changes. When different weights were placed into the same water container, even without direct human interaction, the hydrogel generated varying currents depending on the size of the resulting water waves (Figure S8, Supporting Information).

The swelling properties of hydrogels have an important influence on their ionic conductivity and electrochemical properties. As shown in Figure  9a, WPBTS hydrogels exhibit excellent resistance to swelling due to the steel‐like framework structure of the cellulose skeleton. The conductivity of the hydrogel showed excellent stability and did not change significantly after eight days in the water entry (Figure 9b). This increase in water content enhances the rate of ion flow within the hydrogel, subsequently influencing its sensitivity (Figure 9c). The generated different current signals were recorded when weights of different masses contacted the hydrogel. The data is then fitted and processed, and the slope of the straight line obtained is the sensitivity of the hydrogel (Figure S9, Supporting Information). Notably, the highest sensitivity of WPBTS was achieved after a 4h immersion. However, beyond this duration, the hydrogel's sensitivity began to decline due to the excessive water content, which resulted in a reduction in the relative ion concentration. Subsequently, the hydrogel reached a state of swelling equilibrium, and its sensitivity remained stable. Throughout the process, the maximum sensitivity of the hydrogel increased to 1.52 times the initial sensitivity. This demonstrates the capability of adjusting the water content of the hydrogel as an effective means to enhance its sensitivity, offering potential applications where sensitivity adjustments are critical.

Figure 9.

Performance of hydrogels in water versus saline water. a) Swelling rate of WPBTS hydrogel in water. b) Conductivity of WPBTS hydrogel in water within 8 days. c) Sensitivity of WPBTS hydrogel in water. d) Changes in mass of WPBTS hydrogel in saline water. e) TGA data of WPBTS hydrogel before and after immersion in saline water. f) Changes in conductivity of WPBTS hydrogels before and after immersion in saline water.

However, in practice, hydrogels often face more complex environments than pure water, especially saline and alkaline environments, which impose higher requirements on the stability and functionality of the materials. Therefore, exploring the performance of hydrogels under saline and alkaline conditions can not only further prove their broad applicability, but also provide an essential basis for designing more robust hydrogel materials. The focus will be on the ability of hydrogels to maintain their structure and function in the face of saline and alkaline environments, especially under high salt and high pH conditions, which are often damaging to many materials. To assess the suitability of WPBTS hydrogel for deployment in extreme environments, we subjected it to immersion in a salt solution simulating a saline environment. After 21 days, we removed the hydrogel and observed that its morphology remained unchanged. However, the color gradually transitioned from light yellow to dark yellow as shown in Figure S10 (Supporting Information), and its overall quality declined (Figure 9d). This transformation occurs because the salt ions in the saline water have a higher concentration in the hydrogel, causing water from within the hydrogel to migrate toward the external salt solution to establish concentration equilibrium. Subsequently, we conducted Thermogravimetric Analysis (TGA) tests and determined that the hydrogel's structure remained unaltered (Figure 9e). The conductivity of the WPBTS hydrogels after saline water immersion also showed only a small decrease (Figure 9f). This observation suggests that the hydrogel exhibits notable resistance to both salt and alkali, making it a promising candidate for applications in challenging environments, including saline conditions.

Due to its excellent sensitivity to small deformations, WPBTS hydrogel can be effectively used to convey information. Morse code, as a concise and efficient way to communicate, uses only dots and scratches to represent letters and numbers, making messages extremely simple and quick. The WPBTS hydrogel is placed in the water to simulate the underwater environment. Then long and short presses are used to represent the horizontal lines and dots in Morse code, which can clearly and effectively convey the information, whether it is a relatively simple “OK” or a relatively complex word “water” can be accurately expressed, which is important for underwater signaling (Figure  10a–d). Demonstrates great potential and application value for used for underwater sensing.

Figure 10.

WPBTS Hydrogel for Underwater Sensing. a–d) WPBTS hydrogel conveys messages such as “OK”, “SOS”, “help” and “Water” underwater.

3. Conclusion

By integrating PVA‐borate‐TA‐sodium sulfate into cellulosic wood matrices, we successfully synthesized a novel WPBTS hydrogel. These hydrogels exhibit exceptional flexibility, impressive mechanical strength, high electrical conductivity, and rapid response times. Additionally, they demonstrate remarkable sensitivity and self‐healing capabilities while maintaining their functionality in high‐saline environments over extended periods. This research breakthrough holds great promise for advancing the development of next‐generation wearable devices and multifunctional sensors with biomimetic properties. The synergistic integration of wood‐based materials and hydrogels opens exciting possibilities for innovative applications in various fields, offering solutions that effectively harness the advantages of both natural and synthetic materials.

4. Experimental Section

Chemicals

Basswood purchased from Amazon. PVA (Mw 89 000–98 000, 99+% hydrolyzed), Sodium tetraborate decahydrate (ACS reagent, ≥99.5%), Tannic acid (ACS reagent), Sodium sulfate (ACS reagent, ≥99.0%, anhydrous, granular), Sodium chlorite (technical grade, 80%), Sodium hydroxide (ACS reagent, ≥97.0%, pellets), Acetic acid (glacial, ReagentPlus, ≥99%), and other chemical reagents were all purchased from Sigma‐Aldrich Corporation (Canada). 3D hydrogel molds were printed by Fortus 360 mc at the Rapid Prototyping Centre at the University of Waterloo.

Removal of Lignin from Natural Wood

Natural wood was precisely cut into small pieces measuring 1×1 cm with a thickness of 1 mm (Figure S1, Supporting Information). Subsequently, 5 g of sodium chlorite (NaClO₂) were dissolved in water to make a 2 wt% solution and subjected to sonication for 30 min until the solution was clarified. After this, acetic acid was gradually added dropwise to the sodium chlorite (NaClO₂) solution until its pH reached ≈4.6. The NaClO₂ solution was then heated in an oil bath, raising its temperature to 130 °C until it reached a boiling point. Following this, the prepared wood slices were immersed in this solution for 2 h, transforming their color from yellow to white. That notably; the NaClO₂ solution may undergo evaporation during this process, necessitating timely replenishment with NaClO₂ solution and acetic acid. Once the wood chips had completely transitioned to a white color (≈2 h), they were removed from the solution and subsequently washed three times with ethanol and deionized (DI) water, respectively, to ensure the complete removal of all chemicals. Following this, the wood chips were dried in an oven set at 80 °C for 4 h. To prepare a 15 wt% sodium hydroxide (NaOH) solution, 15 g of sodium hydroxide pellets were introduced into 85 mL of DI water. The NaOH solution was then subjected to sonication for 1 h until it achieved a clear and clarified state. The previously dried “white wood” was immersed in this NaOH solution for 2 h, after which it was removed and subjected to several washes with ethanol and DI water to ensure the complete removal of all chemicals. Finally, the “white wood” was again placed in an oven set at 80 °C for 4 h to complete the drying process.

Synthesis of WPBTS and PBTS Hydrogels

First, 6 g of PVA powder was dissolved in 54 mL of UP water, the temperature was maintained at 90 °C during the dissolution process while the solution was stirred at 400 r min⁻¹ until a clear and clarified PVA solution was formed. After that, 0.6 g of sodium sulfate powder and 0.6 g of TA powder were added to the solution respectively and stirred continuously until completely dissolved. In parallel, 0.4 g of Borax powder were dissolved in 9.6 mL of UP water, and the solution was sonicated for 1 h to ensure complete dissolution. Subsequently, 4 mL of the Borax solution was added to the PVA solution. The mixture was stirred at 600 r min⁻¹ for 1 h to yield the clarified hydrogel precursor solution. The prepared “white wood” was immersed in this precursor solution and subsequently placed in an oven at 90 °C for half an hour to evacuate air bubbles from the solution. Following this step, the beaker containing the hydrogel precursor solution with the “white wood” was placed in an oil bath at 90 °C and heated for 12 h to allow complete immersion of the hydrogel solution into the wood chips. Subsequently, the wood was removed from the hydrogel solution and positioned between two glass pieces. It was then transferred to a −20 °C refrigerator and frozen for 12 h. Finally, the wood was removed from the refrigerator and allowed to thaw at room temperature for 1 h, forming the Wood‐PVA‐Borax‐Tannic Acid‐Sodium Sulfate (WPBTS) hydrogel. A similar process was followed to prepare PVA‐Borax‐Tannic Acid‐Sodium Sulfate (PBTS) hydrogels. The hydrogel precursor solution was synthesized and poured into pre‐prepared 3D printed molds, followed by freezing in a refrigerator for 12 h and subsequent thawing at room temperature. The synthesis process of WPBTS hydrogel is shown in Figure 1. By removing lignin and hemicellulose from natural wood. After that, the cellulose skeleton is combined with the PBTS hydrogel solution and finally frozen in a refrigerator at −20 °C and thawed to obtain the WPBTS hydrogel.

Characterization

Measurement of the content of each component inside the WPBTS hydrogel: The masses of natural wood, delignified wood, and WPBTS hydrogel were weighed separately using a balance to determine the content of each component of the hydrogel.

Morphological observation: Optical images of natural wood, delignified wood and hydrogel wood are taken by Mobile phones. Their microstructures are photographed by scanning electron microscopy (SEM, Hitachi SU5000 FESEM) with an accelerating voltage 20 kV.

Crystalline structure characterization: The crystal structures of natural wood and delignified wood were obtained by X‐ray diffraction patterns (MPD Powder XRD, K‐Alpha1 wavelength is 1.540598 and K‐Alpha2 wavelength is 1.544426).

Material composition mapping: The Energy Dispersive X‐ray Spectroscopy (EDS, Hitachi SU5000 FESEM) was used for materials composition mapping.

Attenuated total reflection Fourier transform infrared (ATR‐FTIR) characterization: Fourier transform infrared (FTIR) spectroscopy was performed using a Nicolet itable0 FTIR spectrometer (Thermo Scientific Inc., America) equipped with a reflectance attenuated total reflection (ATR) system. All samples were analyzed at 500–4000 cm⁻¹ using 16 cumulative scans with a resolution of 2 cm⁻¹.

Water Content Measurement

Prepared multiple WPBTS hydrogels and weighed separately. Then freeze‐dried for 3 days and weighed. Lastly, the water content was measured. The water content (Wc) was defined as follows:

$$w_c=\frac{\left(w_i-w_d\right)}{w_i}\times 100\%$$ (1)

where W _i and W _d were the mass of initial hydrogels and dry hydrogels.

Measurement of water retention: The water retention of WPBTS and PBTS hydrogels in the same environment was obtained by weighing the masses at different times.

Measurement of Swelling rate: The swelling rate of WPBTS and PBTS hydrogel was calculated by weighing the mass of the hydrogel when taken out at different times after soaking in water at room temperature.

Thermo‐Gravimetric Analysis: Thermo‐Gravimetric Analysis of WPBTS hydrogel, PBTS hydrogel, nature wood and delignified wood was measured by TA Instruments (TGA Q500).

Composition Content Measurements

The natural wood, delignified wood, alkali‐treated delignified wood, and WPBTS hydrogels were calculated based on the original weight of the wood sample and weight of the prepared composite hydrogel sample.

Measurement of light transmittance: The light transmittance of WPBTS hydrogel and PBTS hydrogel was measured by UV–vis (UV‐2600i, SHIMADZU).

Small‐angle X‐ray scattering and Wide‐angle X‐ray scattering: SAXS‐WAXS experiments were performed at the Canadian Light Source (CLS) synchrotron using the Brockhouse X‐ray Diffraction and Scattering (BXDS) sector Wiggler Low Energy (WLE) beamline.^{[ 37 ]} The photon energy was 9846.6 eV selected using a Si(111) monochromator. Diffraction patterns were collected with a Rayonix MX300 CCD detector (300 mm × 300 mm active area with 73.242 µm × 73.242 µm pixels). The sample to detector distance was 2337 mm for SAXS and 293 mm for WAXS, with both configurations using a 4.0 mm diameter beamstop. The data were calibrated using silver behenate (SAXS) and NIST 660b LaB6 (WAXS) standards and reduced to 1D plots using GSAS‐II version 5789.^{[ 38 ]}

Orientation Calculation

The intensity‐azimuth plot is obtained by doing a loop integration on the SAXS and WAXS 2D plots. For the SAXS data, the integration process was performed by going to q = 0.53 nm⁻¹. For the WAXS data, the (200) reflection of cellulose I was analyzed for orientation. According to the formula:

$$\mathrm{Orientation}\mathrm{degree}=\left(\frac{180-FWHM}{180}\right)\times 100\%$$ (2)

where FWHM is full width at half maximum, indicates the width of the diffraction peak at half maximum intensity, reflecting the sharpness of the peak.

Measurement of sensitivity: To perform the sensitivity of the hydrogel, the hydrogel was placed on a PET substrate wrapped with two pieces of copper tape at the top and bottom, connected to a Gamry electrochemical workstation, and fed with a steady 1 V voltage. Afterward, different masses of weights were used to press the hydrogel separately to detect the change in sensitivity.

$$\mathrm{Sensitivity}\mathrm{is}\mathrm{defined}\mathrm{as}S=\mathrm{\Delta }I/m\mathrm{and}\mathrm{\Delta }\mathrm{I}=\left(I-I_0\right)/I_0$$ (3)

I ₀ is the initial current, I is the current after the weight is touched, and m is the mass of the weight

Salt and alkali resistance of WPBTS hydrogel: WPBTS hydrogels were immersed in a salt solution of simulated saline soil for 14 days, and the changes in morphology and quality were examined to determine their salinity resistance.

Tensile test measurement: The hydrogels were cut into strips of equal size (3 × 1 × 1 mm) and clamped on a mechanics machine (ADMET, USA) at a stretching rate of 20 mm min⁻¹ using Instron (ADMET, Inc, USA). Each hydrogel of different ratios was measured three times to determine its mechanical properties.

Sensing performance measurement: The hydrogel was placed on the PET substrate and the top and bottom ends were wrapped with copper tape, respectively. Afterward, it was connected to the Gamry electrochemical workstation with a constant voltage 1 V. The hydrogel was then attached to each joint of the human body with double‐sided tape to detect changes in the current signal before and after human movement.

Conflict of Interest

The authors declare no conflict of interest.

Author Contributions

Y.T., Z.Z., and Y.C. contributed equally to this work. Y.T. and Y.A.W. conceived the research. Y.A.W. supervised the research. Y.T. and Z.Z. synthesized and characterized the samples. Y.C. data processing and drawing. A.L. assisted with SAXS and WAXS measurements. T.C., Z.S., M.G. and Y.Z. and I.M.S. assisted with materials characterizations. K.C.T. was involved in the discussion. Z.Z., Y.T., and Y.A.W. co‐wrote the manuscript. All the co‐authors read and approved the final version of the manuscript.

Supporting information

Supporting Information

Teng Y., Zhang Z., Cui Y., Su Z., Godwin M., Chung T., Zhou Y., Leontowich A. F. G., Islam M. S., Tam K. C., Wu Y. A., High‐Sensitivity and Flexible Motion Sensing Enabled by Robust, Self‐Healing Wood‐Based Anisotropic Hydrogel Composites. Small 2025, 21, 2500944. 10.1002/smll.202500944

Data Availability Statement

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