Flexible hydrogel based on cellulose framework as well as preparation method and application of flexible hydrogel

Abstract

The invention relates to flexible hydrogel based on a cellulose framework as well as a preparation method and application of the flexible hydrogel, and belongs to the technical field of flexible hydrogel. In order to solve the problems that an existing flexible wood-based hydrogel process is low in efficiency, the microstructure is difficult to regulate and control and the performance is unbalanced, the invention provides a preparation method of a flexible hydrogel based on a cellulose framework, which comprises the following steps: delignifying natural wood chips to obtain delignified wood chips, and then carrying out alkali treatment under a time gradient to obtain the cellulose framework; the preparation method comprises the following steps: immersing the cellulose framework in a PAM precursor solution prepared from an acrylamide monomer, a cross-linking agent and an initiator in vacuum, and finally performing in-situ polymerization under ultraviolet light to obtain the flexible hydrogel based on the cellulose framework. Precise regulation and control of a supramolecular structure of the cellulose framework are realized through alkali treatment time gradient optimization, and the prepared cellulose framework has toughness and strength; structural integrity and function synergy of the ionic conductive hydrogel are guaranteed through an in-situ polymerization technology, and the application prospect of the flexible hydrogel is widened.

Description

Technical Field

[0001] This invention belongs to the field of flexible hydrogel technology, and particularly relates to a flexible hydrogel based on a cellulose framework, its preparation method and application. Background Technology

[0002] Wood-based hydrogels are novel composite materials prepared using a cellulose skeleton derived from natural wood as the core and combined with functional polymers. The unique anisotropic porous structure of wood provides it with significant mechanical property advantages, such as directional transport capacity and high compressive strength, superior to traditional hydrogels. Furthermore, the natural source of wood endows it with sustainability and biodegradability, aligning with the development trend of green materials.

[0003] Hydrogel sensors are flexible sensing devices based on hydrophilic polymer networks, capable of converting external physical or chemical stimuli into detectable electrical signals. These sensors are composed of a three-dimensional cross-linked polymer network, possessing high water content, excellent biocompatibility, and flexibility and elastic modulus similar to human tissue. Therefore, they are widely used in wearable health monitoring devices, human-computer interfaces, and intelligent medical diagnostics.

[0004] However, existing processes for preparing wood-based hydrogels often require multiple chemical treatments, such as alternating acid-base delignination and oxidative bleaching, resulting in cumbersome procedures, high costs, and a tendency to induce cellulose chain degradation, significantly weakening the material's mechanical integrity. Some methods require combination with high-temperature, high-pressure treatment or prolonged ultrasonic treatment, further increasing energy consumption and equipment requirements, making it difficult to meet the needs of industrial-scale production.

[0005] Current technologies for controlling the microstructure of cellulose frameworks remain relatively crude. After lignin removal, the crystallinity and porous structure arrangement of cellulose are difficult to precisely control, leading to material collapse or insufficient flexibility under high pressure, thus failing to balance high sensitivity and cycle durability. This structure-performance imbalance further limits applications; increased flexibility often comes with the risk of conductive network breakage, while high cross-linking, although enhancing strength, leads to increased brittleness, making it difficult to adapt to long-term use in dynamic environments. Therefore, the shortcomings of existing wood-based hydrogel sensors in terms of process efficiency, structural controllability, and performance balance urgently need to be addressed to promote the practical application of wood-based hydrogel sensors. Summary of the Invention

[0006] To address the problems of low process efficiency, difficulty in controlling microstructure, and uneven performance of existing flexible wood-based hydrogels, this invention provides a flexible hydrogel based on a cellulose framework, its preparation method, and its application.

[0007] The technical solution of the present invention: A method for preparing a flexible hydrogel based on a cellulose framework includes the following steps: Step 1: Preparation of the cellulose framework: Natural wood was made into thin slices, which were then soaked in a delignification aqueous solution to remove lignin. The resulting sample A was then washed, solvent replaced, and freeze-dried to obtain delignified wood chips. The delignified wood chips were soaked in an alkaline solution for time-gradient alkaline treatment. The resulting sample B was then washed, solvent replaced, and freeze-dried to obtain a cellulose framework. Step 2: Preparation of flexible hydrogel: Acrylamide monomer, crosslinking agent and initiator were dissolved in deionized water and thoroughly mixed to obtain PAM precursor solution; the cellulose framework obtained in step one was immersed in PAM precursor solution, and after being fully impregnated under vacuum, the obtained sample C was placed between two glass plates and subjected to polymerization reaction under ultraviolet light to obtain flexible hydrogel based on cellulose framework.

[0008] Furthermore, the natural wood mentioned in step one is balsa wood, and the thickness of the thin slice is 1.0~1.2 mm; the delignification aqueous solution is prepared by deionized water, sodium chlorite and acetic acid in a volume-to-mass ratio of 195mL:2.25g:1.5mL, the temperature of the delignification aqueous solution is 75 ºC, and the soaking time is 6 h.

[0009] Furthermore, in step one, the solvent replacement is tert-butanol replacement, the freeze-drying temperature is -50℃, the vacuum degree is 100-200Pa, and the processing time is 24h.

[0010] Furthermore, the alkaline solution in step one is a 15 wt% sodium hydroxide solution, and the time gradient is 1 h, 2 h, 3 h, 4 h, and 5 h.

[0011] Furthermore, in step two, the mass ratio of acrylamide monomer, crosslinking agent, initiator, and deionized water is 20:0.04:0.2:80; the crosslinking agent is N,N-methylenebisacrylamide, and the initiator is α-ketoglutaric acid.

[0012] Furthermore, in step two, the wavelength of the ultraviolet light is 365nm, the power is 200W, and the irradiation time is 40min.

[0013] Furthermore, LiCl solution is also added to the PAM precursor solution described in step two.

[0014] Furthermore, the concentration of the LiCl solution is 0.5M, and the amount of LiCl solution added is 2.1 g / 100mL.

[0015] A flexible hydrogel based on a cellulose framework prepared by the method of the present invention, wherein the thickness of the hydrogel is 1.0~1.3 mm.

[0016] Application of a flexible hydrogel based on a cellulose framework prepared by the method of the present invention in flexible hydrogel sensors and flexible smart writing boards.

[0017] The beneficial effects of this invention are: This invention achieves precise control over the supramolecular structure of cellulose frameworks through optimized alkali treatment time gradients. After removing lignin from balsa wood using a glacial acetic acid / sodium chlorite system, the effects of alkali treatment on the crystal form, crystallinity, and supramolecular structure of cellulose were systematically investigated using sodium hydroxide time gradient treatment. XRD and SAXS tests show that alkali treatment can regulate the intermolecular spacing of cellulose and optimize its crystalline structure, thereby enhancing flexibility while maintaining high mechanical strength. Unlike traditional high-temperature alkali treatment processes, the method of this invention is simple to operate, requires no fixed high-temperature conditions, and produces a cellulose framework that combines toughness and strength, providing an ideal reinforcing skeleton for subsequent hydrogel preparation.

[0018] This invention ensures the structural integrity and functional synergy of ion-conducting hydrogels through in-situ polymerization technology. The invention employs a photo-initiated in-situ polymerization method to polymerize acrylamide (AM) monomers, preparing a cellulose framework-reinforced ion-conducting hydrogel. This process allows the polyacrylamide network to uniformly penetrate into the cellulose pores, forming strong interfacial bonds, while fully preserving the pre-designed supramolecular structure. Through multi-level pore confinement and the synergistic effect of polar functional groups, Li... + The uniform spatial distribution and efficient migration of the ion-conducting hydrogel endow it with both high ionic conductivity and excellent mechanical properties. This synergistic enhancement of structure and function enables the ion-conducting hydrogel to stably detect human motion signals (such as joint flexion and pulse), showing broad prospects in wearable health monitoring, human-computer interaction interfaces, and electronic skin. Furthermore, the ion-conducting hydrogel prepared in this invention exhibits high sensitivity in detecting minute deformations and recognizing specific characters, making it suitable for interactive devices such as smart writing tablets.

[0019] The present invention provides a simple preparation process for flexible hydrogels based on cellulose frameworks, using renewable raw materials, which is both environmentally friendly and has the potential for large-scale production, providing an innovative material basis for flexible electronic devices and biosensors. Attached Figure Description

[0020] Figure 1 SEM images of cross sections of the cellulose framework, natural wood chips, and delignified wood chips prepared in Examples 1-5; Figure 2 SEM images of longitudinal sections of the cellulose framework, natural wood chips, and delignified wood chips prepared in Examples 1-5; Figure 3 X-ray diffraction patterns of the cellulose framework, natural wood chips and delignified wood chips prepared in Examples 1-5; Figure 4 Log-scale strength diagrams of the cellulose framework, natural wood chips, and delignified wood chips prepared in Examples 1-5; Figure 5 The following are the semi-logarithmic scale strength maps of the cellulose framework, natural wood chips, and delignified wood chips prepared in Examples 1-5, with the strength multiplied by q. 2 ; Figure 6 Typical 2D small-angle X-ray scattering images of the undried cellulose framework, natural wood chips and delignified wood chips prepared in Examples 1-5; Figure 7 The images show the cellulose framework, natural wood chips, and delignified wood chips prepared in Examples 1-5 bent at 180°. Figure 8 Stress-strain curves along the growth direction for the cellulose framework, natural wood chips, and delignified wood chips prepared in Examples 1-5; Figure 9 Comparison of tensile toughness along the growth direction for the cellulose framework, natural wood chips, and delignified wood chips prepared in Examples 1-5; Figure 10 The image shows the bending flexibility of the flexible ion-conducting hydrogel based on a cellulose framework prepared in Example 6. Figure 11 The image shows the adhesion of the flexible ion-conducting hydrogel based on a cellulose framework prepared in Example 6. Figure 12 Comparison of stress-strain curves for the flexible hydrogels prepared in Examples 4 and 6, the undried cellulose framework prepared in Example 4, the PAM precursor solution, and the LiPAM-containing precursor solution. Figure 13 The following are real-time current change monitoring results of the flexible ion-conducting hydrogel sensor sample based on cellulose framework prepared in Example 6 under different states: a is wrist bending, b is finger bending, c is pulse, d is vocalization, e is swallowing, and f is nodding. Figure 14 Example 6 shows the effect of monitoring real-time current changes when the flexible ion-conducting hydrogel sensor sample based on a cellulose framework simulates writing on a smart writing board. Detailed Implementation

[0021] The technical solution of the present invention will be further described below with reference to embodiments, but it is not limited thereto. Any modifications or equivalent substitutions to the technical solution of the present invention without departing from the spirit and scope of the technical solution of the present invention should be covered within the protection scope of the present invention. In the following embodiments, the process equipment or apparatus not specifically specified are all conventional equipment or apparatus in the art. Unless otherwise specified, the raw materials used in the embodiments of the present invention are all commercially available; unless otherwise specified, the technical means used in the embodiments of the present invention are all conventional means well known to those skilled in the art.

[0022] Example 1 This embodiment provides a method for preparing a flexible hydrogel based on a cellulose framework, comprising the following steps: Step 1: Preparation of the cellulose framework: Natural balsa wood was cut into 80mm × 80mm chips with a thickness of 1mm. A delignified aqueous solution was prepared by mixing 195mL of deionized water, 2.25g of sodium chlorite, and 1.5mL of acetic acid. The balsa wood chips were then immersed in the delignified aqueous solution at 75℃ for 6 hours to remove lignin. Sample A was then subjected to multiple washing processes, solvent replacement with tert-butanol, and freeze-dried at -50℃ under a vacuum of 100-200Pa for 24 hours to obtain delignified wood chips.

[0023] The obtained delignified wood chips were immersed in a 15wt% sodium hydroxide solution and kept immersed at room temperature (25ºC) for 1 hour. To prevent the wood chips from floating during immersion, a petri dish was placed on top of the wood chips to ensure that the wood chips were completely submerged in the sodium hydroxide solution. The resulting sample B was repeatedly washed, replaced with tert-butanol solvent, and freeze-dried at -50℃ under a vacuum of 100-200 Pa for 24 hours to obtain the cellulose framework ADW1. Step 2: Preparation of flexible hydrogel: 20g of acrylamide monomer (AM), 0.04g of crosslinking agent N,N-methylenebisacrylamide (MBA), and 0.2g of initiator α-ketoglutaric acid were dissolved in 80g of deionized water and thoroughly mixed to obtain a PAM precursor solution. The cellulose framework obtained in step one was immersed in the PAM precursor solution and thoroughly impregnated under vacuum. The resulting sample C was placed between two glass plates and polymerized under 365nm, 200W ultraviolet light for 40min to obtain a 1.0mm thick flexible hydrogel PADW1 based on a cellulose framework.

[0024] Example 2 This embodiment provides a method for preparing a flexible hydrogel based on a cellulose framework, comprising the following steps: Step 1: Preparation of the cellulose framework: Natural balsa wood was cut into 80mm × 80mm chips with a thickness of 1mm. A delignified aqueous solution was prepared by mixing 195mL of deionized water, 2.25g of sodium chlorite, and 1.5mL of acetic acid. The balsa wood chips were then immersed in the delignified aqueous solution at 75℃ for 6 hours to remove lignin. Sample A was then subjected to multiple washing processes, solvent replacement with tert-butanol, and freeze-dried at -50℃ under a vacuum of 100-200Pa for 24 hours to obtain delignified wood chips.

[0025] The obtained delignified wood chips were soaked in a 15wt% sodium hydroxide solution and kept soaked at room temperature (25ºC) for 2 hours. To prevent the wood chips from floating during soaking, a petri dish was placed on top of the wood chips to ensure that the wood chips were completely submerged in the sodium hydroxide solution. The resulting sample B was repeatedly washed, replaced with tert-butanol solvent, and freeze-dried at -50℃ under a vacuum of 100-200 Pa for 24 hours to obtain the cellulose framework ADW2. Step 2: Preparation of flexible hydrogel: 20g of acrylamide monomer (AM), 0.04g of crosslinking agent N,N-methylenebisacrylamide (MBA), and 0.2g of initiator α-ketoglutaric acid were dissolved in 80g of deionized water and thoroughly mixed to obtain a PAM precursor solution. The cellulose framework obtained in step one was immersed in the PAM precursor solution and thoroughly impregnated under vacuum. The resulting sample C was placed between two glass plates and polymerized under 365nm, 200W ultraviolet light for 40min to obtain a 1.0mm thick flexible hydrogel PADW2 based on a cellulose framework.

[0026] Example 3 This embodiment provides a method for preparing a flexible hydrogel based on a cellulose framework, comprising the following steps: Step 1: Preparation of the cellulose framework: Natural balsa wood was cut into 80mm × 80mm chips with a thickness of 1mm. A delignified aqueous solution was prepared by mixing 195mL of deionized water, 2.25g of sodium chlorite, and 1.5mL of acetic acid. The balsa wood chips were then immersed in the delignified aqueous solution at 75℃ for 6 hours to remove lignin. Sample A was then subjected to multiple washing processes, solvent replacement with tert-butanol, and freeze-dried at -50℃ under a vacuum of 100-200Pa for 24 hours to obtain delignified wood chips.

[0027] The obtained delignified wood chips were soaked in a 15wt% sodium hydroxide solution and kept soaked at room temperature (25ºC) for 3 hours. To prevent the wood chips from floating during soaking, a petri dish was placed on top of the wood chips to ensure that the wood chips were completely submerged in the sodium hydroxide solution. The resulting sample B was repeatedly washed, replaced with tert-butanol solvent, and freeze-dried at -50℃ under a vacuum of 100-200 Pa for 24 hours to obtain the cellulose framework ADW3. Step 2: Preparation of flexible hydrogel: 20g of acrylamide monomer (AM), 0.04g of crosslinking agent N,N-methylenebisacrylamide (MBA), and 0.2g of initiator α-ketoglutaric acid were dissolved in 80g of deionized water and thoroughly mixed to obtain a PAM precursor solution. The cellulose framework obtained in step one was immersed in the PAM precursor solution and thoroughly impregnated under vacuum. The resulting sample C was placed between two glass plates and polymerized under 365nm, 200W ultraviolet light for 40min to obtain a 1.0mm thick flexible hydrogel based on a cellulose framework, PADW3.

[0028] Example 4 This embodiment provides a method for preparing a flexible hydrogel based on a cellulose framework, comprising the following steps: Step 1: Preparation of the cellulose framework: Natural balsa wood was cut into 80mm × 80mm chips with a thickness of 1mm. A delignified aqueous solution was prepared by mixing 195mL of deionized water, 2.25g of sodium chlorite, and 1.5mL of acetic acid. The balsa wood chips were then immersed in the delignified aqueous solution at 75℃ for 6 hours to remove lignin. Sample A was then subjected to multiple washing processes, solvent replacement with tert-butanol, and freeze-dried at -50℃ under a vacuum of 100-200Pa for 24 hours to obtain delignified wood chips.

[0029] The obtained delignified wood chips were soaked in a 15wt% sodium hydroxide solution and kept soaked at room temperature (25ºC) for 4 hours. To prevent the wood chips from floating during soaking, a petri dish was placed on top of the wood chips to ensure that the wood chips were completely submerged in the sodium hydroxide solution. The resulting sample B was repeatedly washed, replaced with tert-butanol solvent, and freeze-dried at -50℃ under a vacuum of 100-200 Pa for 24 hours to obtain the cellulose framework ADW4. Step 2: Preparation of flexible hydrogel: 20g of acrylamide monomer (AM), 0.04g of crosslinking agent N,N-methylenebisacrylamide (MBA), and 0.2g of initiator α-ketoglutaric acid were dissolved in 80g of deionized water and thoroughly mixed to obtain a PAM precursor solution. The cellulose framework obtained in step one was immersed in the PAM precursor solution and thoroughly impregnated under vacuum. The resulting sample C was placed between two glass plates and polymerized under 365nm, 200W ultraviolet light for 40min to obtain a 1.0mm thick flexible hydrogel based on a cellulose framework, PADW4.

[0030] Example 5 This embodiment provides a method for preparing a flexible hydrogel based on a cellulose framework, comprising the following steps: Step 1: Preparation of the cellulose framework: Natural balsa wood was cut into 80mm × 80mm chips with a thickness of 1mm. A delignified aqueous solution was prepared by mixing 195mL of deionized water, 2.25g of sodium chlorite, and 1.5mL of acetic acid. The balsa wood chips were then immersed in the delignified aqueous solution at 75℃ for 6 hours to remove lignin. Sample A was then subjected to multiple washing processes, solvent replacement with tert-butanol, and freeze-dried at -50℃ under a vacuum of 100-200Pa for 24 hours to obtain delignified wood chips.

[0031] The obtained delignified wood chips were soaked in a 15wt% sodium hydroxide solution and kept soaked at room temperature (25ºC) for 5 hours. To prevent the wood chips from floating during soaking, a petri dish was placed on top of the wood chips to ensure that the wood chips were completely submerged in the sodium hydroxide solution. The resulting sample B was repeatedly washed, replaced with tert-butanol solvent, and freeze-dried at -50℃ under a vacuum of 100-200 Pa for 24 hours to obtain the cellulose framework ADW5. Step 2: Preparation of flexible hydrogel: 20g of acrylamide monomer (AM), 0.04g of crosslinking agent N,N-methylenebisacrylamide (MBA), and 0.2g of initiator α-ketoglutaric acid were dissolved in 80g of deionized water and thoroughly mixed to obtain a PAM precursor solution. The cellulose framework obtained in step one was immersed in the PAM precursor solution and thoroughly impregnated under vacuum. The resulting sample C was placed between two glass plates and polymerized under 365nm, 200W ultraviolet light for 40min to obtain a 1.0mm thick flexible hydrogel based on a cellulose framework, PADW5.

[0032] Example 6 This embodiment provides a method for preparing a flexible hydrogel based on a cellulose framework, comprising the following steps: Step 1: Preparation of the cellulose framework: Natural balsa wood was cut into 80mm × 80mm chips with a thickness of 1mm. A delignified aqueous solution was prepared by mixing 195mL of deionized water, 2.25g of sodium chlorite, and 1.5mL of acetic acid. The balsa wood chips were then immersed in the delignified aqueous solution at 75℃ for 6 hours to remove lignin. Sample A was then subjected to multiple washing processes, solvent replacement with tert-butanol, and freeze-dried at -50℃ under a vacuum of 100-200Pa for 24 hours to obtain delignified wood chips.

[0033] The obtained delignified wood chips were soaked in a 15wt% sodium hydroxide solution and kept soaked at room temperature (25ºC) for 4 hours. To prevent the wood chips from floating during soaking, a petri dish was placed on top of the wood chips to ensure that the wood chips were completely submerged in the sodium hydroxide solution. The resulting sample B was repeatedly washed, replaced with tert-butanol solvent, and freeze-dried at -50℃ under a vacuum of 100-200 Pa for 24 hours to obtain the cellulose framework ADW4. Step 2: Preparation of flexible hydrogel: 20g of acrylamide monomer (AM), 0.04g of crosslinking agent N,N-methylenebisacrylamide (MBA), and 0.2g of initiator α-ketoglutaric acid were dissolved in 80g of deionized water. 5mL of 0.5M LiCl solution was added, and the mixture was stirred thoroughly to obtain PAM-Li precursor solution. The cellulose framework obtained in step one was immersed in PAM-Li precursor solution and thoroughly impregnated under vacuum. The resulting sample C was placed between two glass plates and polymerized under 365nm, 200W ultraviolet light for 40min to obtain a flexible ion-conductive hydrogel PADW4-Li based on a cellulose framework with a thickness of 1.0mm.

[0034] I. The microstructure of cellulose frameworks, natural wood chips and head lignin wood chips prepared under different alkali treatment time gradients in Examples 1-5 was investigated.

[0035] Figure 1 and Figure 2 SEM images of cross-sections and longitudinal sections of the cellulose framework, natural wood chips, and delignified wood chips prepared in Examples 1-5; as shown. Figure 1 and Figure 2 As shown in the SEM images, the microstructure of the sample underwent a significant and regular evolution with increasing processing intensity, which directly determines its macroscopic properties. Natural balsa wood (W): exhibits the typical multi-level structure of natural wood. Cross section ( Figure 1 The visible structure is a 3D porous framework composed of regular polygonal cell cavities; longitudinal section ( Figure 2 This clearly shows the directional arrangement of cellulose microfibrils along the growth direction.

[0036] Delignified wood (DW): After most of the lignin is removed, the cell walls become thinner and microcracks appear: Due to the loss of lignin, a key crusting substance, the rigid support of the cell walls weakens. The interfiber bonding force is lost: This results in parallel gaps and cracks appearing on the longitudinal section. Although this stage opens up the structure, the overall integrity is compromised, leading to a general decrease in mechanical properties (such as strength).

[0037] Alkali-treated cellulose framework (ADW1-5): Subsequent alkali treatment (NaOH) further removes residual hemicellulose and lignin, triggering physicochemical reorganization of cellulose: cell walls continue to thin, and pit membranes between adjacent cells are partially opened, forming a more interconnected hierarchical channel structure. The treatment removes the amorphous components surrounding the cellulose microfibrils, fully exposing their surface and revealing a rich nanoscale filament structure. Unlike the "structural destruction" of DW, moderate alkali treatment (such as ADW4), while removing components, promotes the rearrangement and stacking of remaining cellulose molecular chains, forming a denser network structure with stronger hydrogen bonds. This is why the mechanical properties of ADW4 can be restored and even surpass those of natural wood. However, excessive treatment (ADW5) leads to the degradation of the cellulose itself, further disrupting this network.

[0038] The time gradient design (ADW1 to ADW5) visualizes the entire structural evolution process, clearly capturing the complete sequence of structural changes from component removal, cell wall thinning, and nanofiber exposure to potential cellulose degradation due to overtreatment. This is something that single-condition comparative experiments cannot provide. Through the systematic time gradient, not only was the optimal-performing ADW4 identified, but more importantly, the critical inflection point between performance improvement and decline (i.e., between ADW4 and ADW5) was determined. The continuous gradient changes provide multiple intermediate state samples, enabling a one-to-one correspondence between quantitative data such as crystallinity, porosity, and mechanical strength and structural features in SEM images. This provides a continuous chain of evidence for the core conclusion that "alkali treatment time determines macroscopic performance by regulating microstructure."

[0039] Figures 3-6X-ray diffraction (XRD) patterns, logarithmic-scale intensity maps, corresponding semi-logarithmic-scale intensity maps, and typical 2D small-angle X-ray scattering (XRD) patterns of the cellulose framework, natural wood chips, and delignified wood chips prepared in Examples 1-5 are shown. The XRD patterns reveal significant peak position changes associated with the transformation of the cellulose I structure to the cellulose II structure during NaOH treatment. W and DW exhibit typical XRD characteristics of cellulose I, with a composite peak at approximately 15.5º, corresponding to the overlapping (1-10) and (110) lattice planes, and a strong peak at approximately 22.5º, attributed to the (200) crystal plane. After alkali treatment, these peaks shifted, and new characteristic peaks appeared at 12.5º (1-10), 20.2º (110), and 21.5º (020)—characteristic peaks of cellulose II. Alkali treatment both induced the transformation to cellulose II and increased crystallinity.

[0040] 2D SAXS images revealed that W exhibited significant anisotropic scattering, reflecting the highly ordered arrangement of its cellulose microfibrils supported by lignin and hemicellulose matrices. Although the macroscopic structure of the cell wall remained highly anisotropic after lignin removal, the scattering arc of DW became wider in orientation compared to natural wood (W). This widening indicates a degree of local disorder and increased mobility of the cellulose microfibrils after the removal of the constraining lignin matrix. A significant increase in scattering intensity was also observed, attributed to the formation of nanoscale pores within the cell wall after lignin removal. After alkali treatment, the NaOH swelling effect promoted continuous pore enlargement, while the scattering anisotropy gradually disappeared, and the cellulose microfibrils underwent lateral deaggregation and disordered arrangement.

[0041] W, DW, and ADW (1-5) exhibit intensity peaks at 0.180, 0.155, 0.052, 0.048, 0.052, 0.048, and 0.052 Å⁻¹, respectively. The interfibrillation length d describes the average distance between the centers of two microfibrils and can be derived using Bragg's law (d = 2π / q), as shown in Table 1. Compared to W, the interfibrillation length d of DW is increased due to the removal of most of the lignin, but the increase is limited. After alkali treatment, d is significantly increased due to the removal of lignin, hemicellulose, and amorphous cellulose, but the difference in d under different alkali treatment conditions is not significant.

[0042] Table 1

[0043] The aforementioned structural changes work together to create ideal conditions for fabricating high-performance hydrogel sensors: well-defined crystalline regions, acting as physical cross-linking points, effectively disperse and transfer stress, serving as the fundamental source of high mechanical strength (strength, modulus). Furthermore, the presence of pores allows the material to deform under stress without fracturing, contributing to its toughness. Simultaneously, the open and interconnected pore structure provides a "highway" network for the rapid penetration and diffusion of Li⁺ ions, a prerequisite for achieving high ionic conductivity. Moreover, the exposed nanofibers and porous structure result in a large specific surface area, providing numerous sites for Li⁺ loading and distribution, ensuring uniform Li⁺ distribution and avoiding excessively high or low local concentrations.

[0044] High crystallinity and oriented structure provide a mechanical framework, ensuring the material's mechanical integrity. An interconnected porous network permeates this rigid framework, forming continuous ion transport channels. Ultimately, the material forms an integrated "steel-channel" structure: crystalline cellulose microfibrils act as "steel bars," providing strength; while the interwoven pores act as "pipes," responsible for ion transport. This seamless spatial integration achieves synergistic optimization of mechanical properties and ionic conductivity.

[0045] II. The mechanical properties of cellulose frameworks, natural wood chips, and head lignin wood chips prepared under different alkali treatment time gradients in Examples 1-5 were investigated.

[0046] Figure 7 The actual samples of the cellulose framework, natural wood chips, and delignified wood chips prepared in Examples 1-5, bent at 180°, are shown below. Figure 7 As shown, natural balsa wood (W) fractures brittlely due to the rigid support of lignin. While delignified balsa wood (DW) gains some flexibility due to the removal of crusting material, its loose cellulose network leads to stress concentration and cracking during bending. Short-term alkali treatment (ADW1-ADW3) increases porosity, but fiber remodeling is incomplete, resulting in microcracks. In contrast, samples treated for 4-5 hours (ADW4-ADW5) exhibit superior flexibility, completely withstanding 180° bends without damage. This indicates that sufficient alkali treatment induces optimization of the cellulose supramolecular structure: strong hydrogen bond reconstruction provides strength, while interfibrillation acts as an effective energy dissipation mechanism, endowing the material with extremely high toughness and achieving a balance between rigidity and flexibility.

[0047] Figure 8 and Figure 9 Stress-strain curves and tensile toughness comparison diagrams along the growth direction of the cellulose framework, natural wood chips and delignified wood chips prepared in Examples 1-5.

[0048] like Figure 8As shown in the stress-strain curves, the curve for W is short and steep, with minimal fracture strain, exhibiting typical brittle fracture. This is because the rigid matrix formed by lignin and hemicellulose restricts the movement of cellulose microfibrils. The strength of DW decreases significantly, but the fracture strain increases. This indicates that the material strength weakens after lignin removal, but the degrees of freedom of the cellulose network increase, allowing it to begin to possess some deformation capacity. The curves from ADW1 to ADW4 show a trend of being higher and longer. The peak stress of the curves is significantly higher than that of DW, indicating that the recrystallization of cellulose and hydrogen bond reconstruction induced by alkali treatment significantly enhance the material's load-bearing capacity. The fracture strain increases significantly at each level, indicating that the material transitions from brittle to tough. This is attributed to the alkali treatment optimizing the interface between microfibrils, allowing them to slip under stress, thereby effectively dissipating energy. The curve for ADW5 is slightly lower than that for ADW4, indicating that excessively long treatment times begin to lead to degradation of the cellulose molecular chains, thus impairing its mechanical properties. This demonstrates the existence of an optimal treatment time (4 hours).

[0049] like Figure 9 The tensile toughness comparison chart shows that toughness is typically the area under the stress-strain curve, directly quantifying a material's ability to absorb energy before fracture. From W to ADW4, tensile toughness increases by orders of magnitude, with ADW4 exhibiting the highest toughness value. The bar chart clearly shows a trend of first increasing and then slightly decreasing (ADW4>ADW3>ADW5), demonstrating the gradient effect of alkali treatment time on material toughness and clarifying that ADW4 is the optimal preparation condition for performance.

[0050] all in all, Figure 8 and Figure 9 All of these studies demonstrate that the alkali treatment strategy developed in this study can greatly optimize the mechanical properties of the cellulose framework, transforming it from a brittle material into a high-strength, high-toughness flexible material, and the optimal process conditions for achieving this performance leap have been determined.

[0051] III. The properties of flexible ion-conducting hydrogels based on cellulose frameworks were investigated.

[0052] Figure 10 The image shows the bending flexibility of the flexible ion-conducting hydrogel based on a cellulose framework prepared in Example 6; as shown. Figure 10 As shown, the flexible ion-conducting hydrogel based on the cellulose framework has excellent, recoverable flexibility, and can withstand extreme deformation while maintaining structural integrity and functional stability.

[0053] Figure 10 This visually demonstrates that the hydrogel sample can be folded completely in half, and possibly even folded to smaller angles. This contrasts with the previously observed brittle fracture of natural balsa wood. Figure 7This contrasts sharply with previous findings, demonstrating that the material has transformed from rigid to flexible after treatment. Furthermore, the sample rapidly rebounded to its original flat state upon release, without permanent creases or damage. This indicates that the material is not only soft but also possesses good elasticity, rather than undergoing plastic deformation. This reversible bending behavior testifies to the high stability and resilience of its internal three-dimensional network structure. The cellulose framework provides a robust skeleton, while the dynamic bonds (such as hydrogen bonds) in the hydrogel network undergo reversible breakage and reconstruction under stress, thereby dissipating energy and allowing for recovery of deformation.

[0054] Figure 11 The image shows a physical example of the adhesion properties of the flexible ion-conducting hydrogel based on a cellulose framework prepared in Example 6; as shown. Figure 11 As shown, flexible ion-conductive hydrogels possess universal and robust adhesion properties, enabling them to form tight and stable interfacial bonds with the surfaces of various common materials. Figure 11 This indicates that hydrogels can adhere firmly to substrates with vastly different properties, such as paper, glass, rubber, wood chips, and plastics. This demonstrates that their adhesion mechanism is universal and does not depend on any specific surface chemical property.

[0055] Figure 12 The stress-strain curves are compared for the flexible hydrogels prepared in Examples 4 and 6, the undried cellulose framework prepared in Example 4, the PAM precursor solution, and the Li-containing PAM precursor solution. To verify the reinforcing effect of the wood hydrogel, tensile strength tests were performed on samples cut along the longitudinal direction (L). The wet cellulose framework (ADW4-Wet) served as a control group. The tensile strength at break of the wood-based hydrogel reinforced with the Li-ion-free cellulose framework (PADW4) was 4.99 MPa, an increase of 760.34% compared to ADW4-Wet's 0.58 MPa. The tensile strength at break of the Li-containing PADW4-Li was 7.16 MPa, an increase of 1134.48% compared to ADW4-Wet. The tensile strength at break of Li-free polyacrylamide (PAM) hydrogel was 0.02 MPa, while that of Li-containing PAM-Li was 0.03 MPa. PADW4 showed increases of 24850% and 16533.33% respectively compared to PAM-Li, and PADW4-Li showed increases of 35700% and 23766.67% respectively. Because the CNF bundles arranged in the cellulose framework possess high tensile strength along the growth direction, the cellulose framework-reinforced wood-based hydrogel exhibits significantly improved tensile strength. Furthermore, the added Li ions do not react with the hydrogel network or cellulose framework; they are simply added as inorganic ions to the system. Therefore, the strength of the Li-containing sample is higher than that of the Li-free sample.

[0056] Figure 13The following are real-time current monitoring results of the flexible ion-conducting hydrogel sensor sample based on a cellulose framework prepared in Example 6 under different states: a) wrist flexion, b) finger flexion, c) pulse, d) vocalization, e) swallowing, and f) head nodding. Figure 13 As shown, the sensor is attached to the wrist, and during repeated flexion and extension of the wrist, the current signal exhibits a periodic and highly consistent waveform. During flexion, the hydrogel undergoes stretching deformation, the conductive path is lengthened or the cross-section is reduced, leading to an increase in resistance; upon straightening, the material springs back, and the resistance returns to its initial value. This response is clear and repeatable, indicating that the sensor can reliably capture large-radius joint movements. Similar to wrist flexion, repeated finger flexion also causes periodic changes in resistance. Due to the large amplitude and high speed of finger joint movement, the sensor can still output clear and sharp response peaks, indicating that it has good dynamic response capabilities and can meet the monitoring needs of fine hand movements.

[0057] The sensor can capture weak and regular pulse fluctuation signals, with waveforms exhibiting typical pulse characteristics. This result highlights the sensor's extremely high sensitivity, enabling it to effectively detect minute mechanical vibrations on the human body surface caused by blood flow pulsation, and shows potential applications in wearable health monitoring.

[0058] When the volunteers said words such as "hi," their throat muscles and skin produced specific vibration patterns, and the sensor successfully collected the corresponding unique electrical change signals. The fact that different pronunciations elicited distinguishable signal patterns demonstrates the sensor's potential applications in speech recognition and human-computer interaction.

[0059] The swallowing action triggers a noticeable but brief physical displacement of the larynx, and the current response curve exhibits a brief but intense pulse-like peak. This indicates that the sensor can effectively detect instantaneous physiological activity and is suitable for swallowing function assessment or daily activity monitoring.

[0060] The head nodding motion triggered a relatively large-amplitude, slowly changing current response. The waveform was smooth and highly repeatable, indicating that the sensor could maintain reliable signal output stability even under low-frequency, large-amplitude motion scenarios.

[0061] In summary, Figure 13 Through a series of real-time monitoring examples, the flexible ion-conductive hydrogel sensor has been fully verified as a high-performance wearable sensing platform, demonstrating broad application prospects in human motion capture and physiological signal monitoring.

[0062] Figure 14Example 6 shows the real-time current monitoring effect of the flexible ion-conducting hydrogel sensor sample based on a cellulose framework during writing on a simulated smart writing board. The results demonstrate that the sensor can clearly and stably capture and distinguish complex and subtle changes. When used as a smart writing board, writing different content (numbers 1 and 2) elicited very significant and consistent current change signals. Each number writing corresponds to a sharp, high signal-to-noise ratio response peak, indicating that the sensor is extremely sensitive to minute deformations. As shown in the figure, writing the numbers "1" and "2" produces distinctly different signal characteristics due to their different stroke paths. The number "1" is typically written as a single stroke—a straight line or a simple bend—and its corresponding signal presents as a single peak or a relatively simple pulse. The number "2" usually contains more complex strokes such as turns and curves, and its corresponding signal exhibits a typical "double peak" or a broader and more complex waveform (possibly corresponding to the start, turn, and end of the stroke). This one-to-one correspondence and distinguishable signal pattern proves that the sensor can not only sense "whether there is movement" but also analyze "what kind of movement," demonstrating broad application potential. Meanwhile, the amplitude and shape of the response signal remained highly consistent when "1" and "2" were written repeatedly, indicating that the sensor has good mechanical stability and signal reproducibility, avoiding signal drift or attenuation.

Claims

1. A method for preparing a flexible hydrogel based on a cellulose framework, characterized in that, Includes the following steps: Step 1: Preparation of the cellulose framework: Natural wood was made into thin slices, which were then soaked in a delignification aqueous solution to remove lignin. The resulting sample A was then washed, solvent replaced, and freeze-dried to obtain delignified wood chips. The delignified wood chips were soaked in an alkaline solution for time-gradient alkaline treatment. The resulting sample B was then washed, solvent replaced, and freeze-dried to obtain a cellulose framework. Step 2: Preparation of flexible hydrogel: Acrylamide monomer, crosslinking agent and initiator were dissolved in deionized water and thoroughly mixed to obtain PAM precursor solution; the cellulose framework obtained in step one was immersed in PAM precursor solution, and after being fully impregnated under vacuum, the obtained sample C was placed between two glass plates and subjected to polymerization reaction under ultraviolet light to obtain flexible hydrogel based on cellulose framework.

2. The method for preparing a flexible hydrogel based on a cellulose framework according to claim 1, characterized in that, The natural wood mentioned in step one is balsa wood, and the thickness of the thin slice is 1.0~1.2 mm; the delignification aqueous solution is prepared by deionized water, sodium chlorite and acetic acid in a volume-to-mass ratio of 195mL:2.25g:1.5mL, the temperature of the delignification aqueous solution is 75℃, and the soaking time is 6 h.

3. The method for preparing a flexible hydrogel based on a cellulose framework according to claim 2, characterized in that, In step one, the solvent replacement was tert-butanol, the freeze-drying temperature was -50℃, the vacuum degree was 100-200Pa, and the processing time was 24h.

4. The method for preparing a flexible hydrogel based on a cellulose framework according to claim 3, characterized in that, The alkaline solution in step one is a 15 wt% sodium hydroxide solution, and the time gradient is 1 h, 2 h, 3 h, 4 h and 5 h.

5. The method for preparing a flexible hydrogel based on a cellulose framework according to claim 4, characterized in that, In step two, the mass ratio of acrylamide monomer, crosslinking agent, initiator, and deionized water is 20:0.04:0.2:80; the crosslinking agent is N,N-methylenebisacrylamide, and the initiator is α-ketoglutaric acid.

6. The method for preparing a flexible hydrogel based on a cellulose framework according to claim 5, characterized in that, The ultraviolet light used in step two has a wavelength of 365nm, a power of 200W, and an irradiation time of 40min.

7. A method for preparing a flexible hydrogel based on a cellulose framework according to any one of claims 1-6, characterized in that, The PAM precursor solution described in step two also contains LiCl solution.

8. The method for preparing a flexible hydrogel based on a cellulose framework according to claim 7, characterized in that, The concentration of the LiCl solution is 0.5M, and the amount of LiCl solution added is 2.1 g / 100mL.

9. A flexible hydrogel based on a cellulose framework prepared by the preparation method according to any one of claims 1-8, characterized in that, The thickness of the hydrogel is 1.0~1.3 mm.

10. The application of a flexible hydrogel based on a cellulose framework prepared by the preparation method as described in claim 7 or 8 in flexible hydrogel sensors and flexible smart writing boards.

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