Nano-cellulose reinforced composite hydrogel as well as preparation method and application thereof

Abstract

The invention discloses nano-cellulose reinforced composite hydrogel and a preparation method and application thereof, and belongs to the technical field of functional polymer material preparation and bionic skin. Acrylamide and polyvinyl alcohol (AM and PVA) are used as hydrogel base materials, a first cross-linked network is generated through thermal polymerization of acrylamide, and the first cross-linked network is used as a first cross-linked network; the polyvinyl alcohol is frozen and thawed once to form a second crystal network, and the CNF penetrates through and is tightly connected with the first cross-linked network and the second crystal network like a'skeleton ', so that a unique and stable three-network interpenetrating framework is formed; the nanocellulose reinforced composite hydrogel integrates excellent mechanical properties (high stretch rate, high toughness and compression resistance), stable conductivity, quick response and recovery capability and good biocompatibility.

Description

Technical Field

[0001] This invention relates to the fields of functional polymer material preparation and biomimetic skin technology, specifically to a nanocellulose-reinforced composite hydrogel and its preparation method and application. Background Technology

[0002] Hydrogels, as flexible and water-rich three-dimensional materials, have been widely used in tissue engineering, drug delivery, artificial intelligence materials, and biomimetic electronic skin due to their excellent biocompatibility, unique flexibility, extensibility, conductivity, and similarity to natural tissue structures. Among these applications, the use of hydrogels as biocompatible scaffolds and functional component carriers for biomimetic skin has attracted significant research interest. The scaffold materials selected for constructing tissue-engineered skin should possess reasonable mechanical strength and biocompatibility, providing a site for seed cell adhesion, proliferation, and functional expression. This enhances the integration of tissue-engineered skin with local normal skin tissue and creates a specific microenvironment to control and guide the formation of new tissue morphologies by seed cells and host skin tissue, thereby accelerating wound regeneration and repair.

[0003] However, traditional hydrogel materials suffer from poor durability, low mechanical strength, low strain elongation, poor electrical conductivity, and a lack of effective energy dissipation mechanisms. These shortcomings severely limit their application in tissue-engineered skin, electronic skin, and load-bearing materials. Cellulose, a renewable and abundant polymer composed of glucose, possesses excellent biocompatibility and biodegradability. Therefore, cellulose can serve as a material for skin tissue engineering scaffolds, effectively healing wounds. Furthermore, the abundant hydroxyl and amino functional groups on the cellulose molecular chain facilitate hybridization or functionalization, making it an ideal material for constructing hydrogels. However, using only cellulose as a reinforcing phase to prepare hydrogels, relying on its hydrogen bonding and other physical interactions to improve mechanical properties, still results in unsatisfactory mechanical properties. Patent application CN120554669A discloses a loofah nanocellulose hydrogel, its preparation method, and its applications. This invention utilizes nanocellulose to enhance the mechanical properties of the hydrogel, but it does not consider the potential coordination mechanism between the carboxyl groups on the CNF surface and metal ions. Consequently, the tensile strength of the hydrogel in this invention is only 65.33 kPa, and the electrical conductivity is only 5.45 S / m. The conductivity of hydrogels usually requires the addition of conductive polymers or salts. Therefore, the mechanical properties and conductivity are often simply the sum of the effects of these technical characteristics.

[0004] To enhance the mechanical properties of hydrogels, existing technologies offer various enhancement methods, including nano-reinforced hydrogels, multi-hydrogen-bonded hydrogels, supramolecular hydrogels, and dual-network (DN) hydrogels. DN hydrogels are widely used due to their simple preparation process and strong mechanical properties and high elongation. DN hydrogels generally have two networks: a tough and tightly cross-linked first network and a flexible and weakly cross-linked second network. These two networks interweave to form an energy dissipation network, thereby enhancing the hydrogel's properties. In recent years, numerous DN hydrogels have been developed for applications in artificial tissues, smart materials, 3D printers, and biosensors. However, existing methods for constructing multi-network hydrogels are typically cumbersome. To achieve higher mechanical strength, researchers often require multiple, step-by-step polymerization or freeze-thaw cycles to form each network separately. This multi-step preparation process is not only time-consuming and energy-intensive, but multiple polymerization or freeze-thaw cycles may also create excessive crystalline regions, reducing the hydrogel's elasticity and extensibility, increasing the risk of mechanical damage, and severely limiting its large-scale production and practical application in the field of biomimetic skin. For example, patent application CN120718300A discloses a high-strength biodegradable self-healing hydrogel and its preparation method. This technology enhances the hydrogel's tensile strength and elongation at break through the synergistic reinforcement of the dual network structure. However, its elasticity is poor, which limits its application in biomimetic skin. Furthermore, the preparation of this hydrogel requires a series of processes such as polymerization, catalysis, and post-treatment, which takes at least 75 hours. Therefore, the preparation steps are complex and the preparation cycle is long.

[0005] Patent application CN111303452A discloses a biomimetic antibacterial high-adhesion dual-network hydrogel, its preparation method and application. This technology retains the antibacterial properties while improving the adhesion performance of the hydrogel. It also enhances the mechanical properties and toughness of the hydrogel through enzymatic covalent crosslinking and dynamic imine bonds. However, the combination of strong and weak bonds in this hydrogel will cause it to have a certain lag effect, making it unable to respond to external changes in a timely manner and resulting in low sensitivity. Summary of the Invention

[0006] This invention provides a nanocellulose-reinforced composite hydrogel, its preparation method, and its application. It effectively solves the technical problems of existing hydrogels with nanocellulose as the reinforcing phase having single function, complex preparation process of double-network hydrogels, poor response to external changes, and low sensitivity. This invention provides a nanocellulose-reinforced three-network interpenetrating composite hydrogel with synergistic improvement in mechanical strength, toughness, and conductivity.

[0007] The first objective of this invention is to provide a method for preparing a nanocellulose-reinforced composite hydrogel, comprising the following steps: Using cellulose nanofibers as the backbone, acrylamide as the polymerizing monomer, polyvinyl alcohol and Ca were added. 2+ The source, under the action of an initiator and a crosslinking agent, undergoes a polymerization reaction, and acrylamide polymerizes to obtain polyacrylamide, forming the first crosslinking network. Simultaneously, the carboxyl groups on the surface of cellulose nanofibers react with Ca... 2+ Dynamically reversible metal-carboxyl coordination bonds are formed to obtain a primary hydrogel; The primary hydrogel is subjected to a freeze-thaw cycle, during which polyvinyl alcohol in the primary hydrogel forms a second crystalline network, and cellulose nanofibers penetrate and connect the first cross-linked network and the second crystalline network. Furthermore, the hydroxyl groups on the surface of the cellulose nanofibers form hydrogen bonds with polyvinyl alcohol and polyacrylamide, resulting in a nanocellulose-reinforced composite hydrogel.

[0008] As a preferred embodiment, the cellulose nanofibers and Ca 2+ The ratio of the source used is 1g:0.03mol~0.09mol; the Ca 2+ The source is selected from calcium chloride or calcium nitrate.

[0009] As a preferred embodiment, the freeze-thaw cycle specifically involves freezing the primary hydrogel at -25℃ to -20℃ for 16 to 20 hours and then thawing it at room temperature for 4 to 5 hours.

[0010] In a preferred embodiment, the mass ratio of the cellulose nanofibers, polyvinyl alcohol and acrylamide is 0.04-0.12:1-4:7.2.

[0011] In a preferred embodiment, the mass ratio of acrylamide, initiator and crosslinking agent is 1.2:0.01:0.0015.

[0012] In a preferred embodiment, the polymerization reaction is specifically carried out at 65℃~68℃ for 3h~5h.

[0013] In a preferred embodiment, the cellulose nanofibers have a length of 1 μm to 2 μm and a diameter of approximately 5 nm to 20 nm.

[0014] In a preferred embodiment, the initiator is ammonium persulfate and the crosslinking agent is N,N'-methylenebisacrylamide.

[0015] A second objective of this invention is to provide a nanocellulose-reinforced composite hydrogel, prepared by any of the preparation methods described above.

[0016] A third objective of this invention is to provide an application of the above-mentioned nanocellulose-reinforced composite hydrogel as a support layer and / or sensing layer in biomimetic skin.

[0017] Compared with the prior art, the beneficial effects of the present invention are as follows: This invention provides a method for preparing a nanocellulose-reinforced composite hydrogel, using cellulose nanofibers, acrylamide, polyvinyl alcohol, and Ca... 2+ Using raw materials, a primary hydrogel is obtained through a one-pot process with the aid of initiators and crosslinking agents. This is followed by a freeze-thaw cycle to form a composite hydrogel with a three-network structure. The interpenetrating three-network structure of the composite hydrogel enhances its mechanical properties. Acrylamide and polyvinyl alcohol (AM and PVA) are used as the hydrogel matrix materials. Acrylamide is thermally polymerized to generate the first crosslinking network, and polyvinyl alcohol undergoes a freeze-thaw cycle to form the second crystalline network. CNF fibers act as a "skeleton," penetrating and tightly connecting the first crosslinking network and the second crystalline network, forming a unique and stable interpenetrating three-network architecture.

[0018] This invention combines the nano-reinforcing effect of cellulose nanofibers with Ca 2+ The combination of ionic conductivity and coordination crosslinking effect produces a significant synergistic enhancement effect. The hydroxyl groups on the CNF surface form a large number of hydrogen bonds with polyvinyl alcohol and polyacrylamide; at the same time, the carboxyl groups on the CNF surface interact with Ca... 2+ Dynamically reversible metal-coordination bonds are formed. These two strong non-covalent bonds together serve as an efficient energy dissipation mechanism, preferentially breaking to absorb energy when the material is under stress, and reversibly recovering after unloading. This significantly improves the strength, toughness, and elastic recovery rate of the hydrogel, achieving mechanical synergy. This invention uses Ca... 2+ The free ions provided by the source ensure basic conductivity; while CNF-Ca 2+ The formation of coordination bonds not only enhances the network but also provides a stable and reversible ion transport pathway. This enables the composite hydrogel to maintain the stability and high sensitivity of the conductive signal even under large deformation, thereby achieving electrical synergy.

[0019] The composite hydrogel reinforced with nanocellulose prepared by this invention has significantly improved mechanical properties compared with single-network hydrogels under optimal ratios. It overcomes the difficulty of traditional hydrogels having high strength but low elasticity and toughness due to repeated freeze-thaw cycles. Moreover, the preparation steps are simple, time-saving and energy-saving, saving about 65% of the time compared with traditional preparation methods, making it suitable for large-scale production.

[0020] The nanocellulose-reinforced composite hydrogel prepared by this invention integrates excellent mechanical properties (high tensile strength, high toughness, and compression resistance), stable electrical conductivity, rapid response and recovery ability, and good biocompatibility. It can be used as a support layer for biomimetic skin, providing a scaffold for seed cell adhesion, proliferation, and function, promoting rapid wound repair. It can also be used as a sensing layer for biomimetic skin, serving as a sensing device to monitor skin condition in real time. This provides a new material basis for the development of biomimetic skin and achieves the integrated integration of support and sensing functions. Attached Figure Description

[0021] Figure 1 This is a schematic diagram of the preparation process of the nanocellulose-reinforced composite hydrogel in Example 1 of the present invention.

[0022] Figure 2 The images shown are scanning electron microscope (SEM) images of the hydrogels prepared in the embodiments and comparative examples of the present invention, wherein (a) is a comparative example, (b) is a comparative example 2, (c) is example 1, and (d) is example 2.

[0023] Figure 3 The nanocellulose-reinforced composite hydrogel (PP) of Example 5 of this invention 10 C 0.2 C) Sensitivity test results, of which (a) is the GF curve of the composite hydrogel resistance as a function of tensile strain; (b) is the response diagram of the composite hydrogel under step strain.

[0024] Figure 4 This is a comparison chart showing the effects of Example 5 of the present invention and the negative control group on the activity of mouse L929 cells.

[0025] Figure 5 The nanocellulose-reinforced composite hydrogel (PP) prepared in Example 5 of this invention 10 C 0.2 Sample image of C). Detailed Implementation

[0026] To enable those skilled in the art to better understand and implement the technical solutions of this invention, the invention is further described below with reference to specific embodiments. However, the embodiments are not intended to limit the invention. Unless otherwise specified, the following test methods and detection methods are conventional methods; unless otherwise specified, the reagents and raw materials are commercially available.

[0027] Regarding existing hydrogel materials, firstly, the preparation of hydrogels using a single CNF as a reinforcing phase relies on its hydrogen bonds and other physical interactions to improve mechanical properties, resulting in a relatively limited function. For the conductivity of hydrogels, additional conductive polymers or salts are usually required, thus the mechanical and conductivity effects are often simply a superposition of technical characteristics. Secondly, existing methods for constructing multi-network hydrogels are typically cumbersome. To achieve high mechanical strength, multiple, stepwise polymerization or freeze-thaw cycles are needed to form each network separately. This multi-step preparation process is not only time-consuming and energy-intensive, but multiple polymerization or freeze-thaw cycles may also create excessive crystalline regions, reducing the elasticity and ductility of the hydrogel, increasing the risk of mechanical damage, and severely restricting its large-scale production and practical application in the field of biomimetic skin. Thirdly, the combination of strong and weak bonds in hydrogels can cause a certain degree of hysteresis, making it difficult to respond promptly to external changes. Its low sensitivity means it only has signal sensing capabilities and lacks real-time feedback capabilities for interactive states. Furthermore, the temperature sensitivity of existing hydrogels affects the stability and accuracy of strain response. To address the aforementioned technical problems, this invention provides a nanocellulose-reinforced composite hydrogel, its preparation method, and its application.

[0028] The technical solution of the present invention will be described in detail below.

[0029] This invention first provides a method for preparing a nanocellulose-reinforced composite hydrogel, comprising the following steps: Using cellulose nanofibers as the backbone, acrylamide as the polymerizing monomer, polyvinyl alcohol and Ca were added. 2+ The source, under the action of an initiator and a crosslinking agent, undergoes a polymerization reaction, and acrylamide polymerizes to obtain polyacrylamide, forming the first crosslinking network. Simultaneously, the carboxyl groups on the surface of cellulose nanofibers react with Ca... 2+ Metal-carboxyl coordination bonds are formed to obtain a primary hydrogel.

[0030] The primary hydrogel is subjected to a freeze-thaw cycle, during which the polyvinyl alcohol in the primary hydrogel forms a second crystalline network. Hydroxyl groups on the surface of the cellulose nanofibers form hydrogen bonds with the polyvinyl alcohol and polyacrylamide. The cellulose nanofibers penetrate and connect the first cross-linked network and the second crystalline network, resulting in a nanocellulose-reinforced composite hydrogel. The nanocellulose-reinforced composite hydrogel prepared by this invention exhibits excellent mechanical properties (high tensile strength, high toughness, and compression resistance), stable electrical conductivity, rapid response recovery, and good biocompatibility.

[0031] In the above technical solution, acrylamide and polyvinyl alcohol (AM and PVA) are used as hydrogel matrix materials. The first cross-linked network is generated by the thermal polymerization of acrylamide, and the second crystalline network is formed by the freeze-thaw cycle of polyvinyl alcohol. CNF fibers, like a "skeleton", run through and tightly connect the first cross-linked network and the second crystalline network, forming a unique and stable three-network interpenetrating structure.

[0032] As a preferred embodiment, the cellulose nanofibers and Ca 2+ The ratio of the source used is 1g:0.03mol~0.09mol; the Ca 2+ The source is selected from calcium chloride or calcium nitrate. If Ca 2+ If the concentration is too low, there will be insufficient conductive ions in the system, affecting conductivity; while when Ca... 2+ When the concentration is too high, it affects cell survival rate and damages biocompatibility.

[0033] As a preferred embodiment, the single freeze-thaw cycle specifically involves freezing the primary hydrogel at -25℃ to -20℃ for 16 to 20 hours, followed by thawing at room temperature for 4 to 5 hours. The CNF (Crystal-Freezing Network) used in this invention provides a pre-existing reinforcing framework, reducing dependence on high-density PVA crystallization. Therefore, a strong and tough PVA network can be obtained with a single freeze-thaw cycle. Furthermore, multiple freeze-thaw cycles do indeed form more and more complete PVA crystals, resulting in a stronger but more brittle network. Thus, due to the compensation provided by CNF, a balance between toughness and strength can be achieved with just one freeze-thaw cycle. Regarding the aforementioned freeze-thaw parameters, excessively high freezing temperatures and slow freezing rates will form large ice crystals, leading to a coarse and uneven PVA network structure, ultimately resulting in weak mechanical strength of the hydrogel. Conversely, excessively fast freezing rates will form numerous small and unstable crystal nuclei, resulting in imperfect crystals and similarly weakening the reinforcing effect of the PVA network. If the freezing time is too short, complete and uniform freezing cannot be achieved, resulting in insufficient PVA crystallization, inadequate secondary crystal network strength, and a decline in the overall mechanical properties of the hydrogel. Conversely, if the freezing time is too long, the benefit of increasing the number of crystals is no longer significant, constituting a waste of time and failing to significantly improve performance; instead, it reduces production efficiency. Regarding the thawing process, both excessively high thawing temperatures and excessively short thawing times accelerate thawing. Rapid thawing causes the PVA network to loosen instantly, preventing the formation of an ideal tough structure, resulting in a softer hydrogel with reduced elastic modulus and strength. Conversely, excessively low thawing temperatures and insufficient thawing times lead to incomplete thawing, leaving residual internal stress within the hydrogel and causing unstable performance.

[0034] In a preferred embodiment, the mass ratio of cellulose nanofibers, polyvinyl alcohol, and acrylamide is 0.04–0.12:1–4:7.2. Since CNF fibers act like a "skeleton," penetrating and tightly connecting the first cross-linked network and the second crystalline network, forming a unique and stable three-network interpenetrating structure, insufficient cellulose nanofibers will affect the formation of the three-dimensional network, and there will be insufficient carboxyl groups to form coordination bonds with calcium ions, preventing the hydrogel from achieving the expected mechanical strength. Conversely, excessive CNF will lead to excessive physical cross-linking and stress concentration, also causing a decline in mechanical properties. Regarding the amount of polyvinyl alcohol, insufficient amounts will result in an incomplete second crystalline network and insufficient toughness of the composite hydrogel; while excessive polyvinyl alcohol will affect the formation of the first cross-linked network, limiting the overall strength of the three-network hydrogel.

[0035] In a preferred embodiment, the mass ratio of acrylamide, initiator and crosslinking agent is 1.2:0.01:0.0015.

[0036] In a preferred embodiment, the polymerization reaction is specifically carried out at 65℃~68℃ for 3h~5h. For the polymerization temperature and time, too low a polymerization temperature or insufficient polymerization time will lead to incomplete reaction, a weak first network backbone, and fundamentally damage to the mechanical strength of the hydrogel; too high a polymerization temperature or too long a polymerization time will lead to an excessively rapid and vigorous reaction, forming a heterogeneous network structure and making the hydrogel brittle.

[0037] In a preferred embodiment, the cellulose nanofibers have a length of 1 μm to 2 μm and a diameter of approximately 5 nm to 20 nm.

[0038] In a preferred embodiment, the initiator is ammonium persulfate, and the crosslinking agent is N,N'-methylenebisacrylamide. The initiator used in this invention can also be replaced with other water-soluble thermally decomposable initiators that can decompose to generate anionic free radicals under heating (65°C); the crosslinking agent used in this invention can be replaced with water-soluble bifunctional acrylamide or acrylate covalent crosslinking agents, which can undergo efficient copolymerization with AM monomers.

[0039] The technical effects of the present invention will be described below through specific embodiments and comparative examples.

[0040] The preparation method of the cellulose nanofiber suspension used in this invention includes the following steps: 5.0 g of softwood pulp is chopped, soaked in 0.1 mol / L NaOH solution for 12 hours, and then washed until neutral to obtain purified cellulose fibers; the purified cellulose fibers are dispersed in 1000 mL of deionized water, 0.08 g of TEMPO (2,2,6,6-tetramethylpiperidine-1-oxy radical) and 0.8 g of NaBr are added, and 5.0 mmol / g NaClO solution is slowly added dropwise in a 4°C water bath, and the process is further refined by dropwise addition. The NaOH solution was used to maintain the pH at 10.0–10.5 until the reaction was complete (the pH no longer decreased). The mixture was then filtered and repeatedly washed with water until no bromide ions remained, yielding purified oxidized cellulose. The purified oxidized cellulose was prepared into a 1.0 wt% dispersion and homogenized under high pressure at 700 bar for 8 cycles. It was then freeze-dried, its mass was accurately weighed, and the corresponding mass of deionized water was added. The mixture was redispersed and prepared into a 1.0 wt% CNF suspension. The suspension was then sealed and stored in an 8°C refrigerator for later use.

[0041] Example 1 A method for preparing a nanocellulose-reinforced composite hydrogel, such as... Figure 1 As shown, it includes the following steps: S1. Take 4 mL of the CNF suspension prepared in this invention, add 1 g of polyvinyl alcohol (PVA) particles and 28 g of H2O and stir continuously. Dissolve at 98°C for 2 h to form a mixed solution. Under normal temperature conditions, add 7.2 g of acrylamide (AM), 0.06 g of initiator ammonium persulfate (APS), 0.009 g of N,N'-methylenebisacrylamide and 0.4 g of CaCl2 to the mixed solution and stir to mix them evenly to form a precursor solution.

[0042] S2, the precursor solution was sonicated in an ice bath for 40 minutes to remove air bubbles, then injected into a polytetrafluoroethylene mold and sealed. The sealed mold was then placed in an oven at 65°C for 3 hours, frozen at -20°C for 16 hours, and thawed at room temperature for 4 hours to form a nanofiber-reinforced three-network hydrogel, thus obtaining a nanocellulose-reinforced composite hydrogel, denoted as PP. 2.5 C 0.1 C indicates that the mass fraction of PVA in the prepared nanocellulose-reinforced composite hydrogel is 2.5%, and the mass fraction of CNF is 0.1%.

[0043] Example 2 A method for preparing a nanocellulose-reinforced composite hydrogel includes the following steps: S1. Take 4 mL of the CNF suspension prepared in this invention, add 2 g of polyvinyl alcohol (PVA) particles and 27 g of H2O and stir continuously. Dissolve at 98°C for 2 h to form a mixed solution. Under normal temperature conditions, add 7.2 g of acrylamide, 0.06 g of initiator ammonium persulfate (APS), 0.009 g of N,N'-methylenebisacrylamide and 0.4 g of CaCl2 to the mixed solution and stir to mix them evenly to form a precursor solution.

[0044] S2, the precursor solution was sonicated in an ice bath for 40 minutes to remove air bubbles, then injected into a polytetrafluoroethylene mold and sealed. The sealed mold was then placed in an oven at 65°C for 3 hours, frozen at -20°C for 16 hours, and thawed at room temperature for 4 hours to form a nanofiber-reinforced three-network hydrogel, thus obtaining a nanocellulose-reinforced composite hydrogel, denoted as PP. 5.0 C 0.1 C indicates that the mass fraction of PVA in the prepared nanocellulose-reinforced composite hydrogel is 5.0%, and the mass fraction of CNF is 0.1%.

[0045] Example 3 A method for preparing a nanocellulose-reinforced composite hydrogel includes the following steps: S1. Take 4 mL of the CNF suspension prepared in this invention, add 3 g of polyvinyl alcohol (PVA) particles and 26 g of H2O and stir continuously. Dissolve at 98°C for 2 h to form a mixed solution. Under normal temperature conditions, add 7.2 g of acrylamide, 0.06 g of initiator ammonium persulfate (APS), 0.009 g of N,N'-methylenebisacrylamide and 0.4 g of CaCl2 to the mixed solution and stir to mix them evenly to form a precursor solution.

[0046] S2, the precursor solution was sonicated in an ice bath for 40 minutes to remove air bubbles, then injected into a polytetrafluoroethylene mold and sealed. The sealed mold was then placed in an oven at 65°C for 3 hours, frozen at -20°C for 16 hours, and thawed at room temperature for 4 hours to form a nanofiber-reinforced three-network hydrogel, thus obtaining a nanocellulose-reinforced composite hydrogel, denoted as PP. 7.5 C 0.1 C indicates that the mass fraction of PVA in the prepared nanocellulose-reinforced composite hydrogel is 7.5%, and the mass fraction of CNF is 0.1%.

[0047] Example 4 A method for preparing a nanocellulose-reinforced composite hydrogel includes the following steps: S1. Take 4 mL of the CNF suspension prepared in this invention, add 4 g of polyvinyl alcohol (PVA) particles and 25 g of H2O and stir continuously. Dissolve at 98°C for 2 h to form a mixed solution. Under normal temperature conditions, add 7.2 g of acrylamide, 0.06 g of initiator ammonium persulfate (APS), 0.009 g of N,N'-methylenebisacrylamide and 0.4 g of CaCl2 to the mixed solution and stir to mix them evenly to form a precursor solution.

[0048] S2, the precursor solution was sonicated in an ice bath for 40 minutes to remove air bubbles, then injected into a polytetrafluoroethylene mold and sealed. The sealed mold was then placed in an oven at 65°C for 3 hours, frozen at -20°C for 16 hours, and thawed at room temperature for 4 hours to form a nanofiber-reinforced three-network hydrogel, thus obtaining a nanocellulose-reinforced composite hydrogel, denoted as PP. 10 C 0.1 C indicates that the mass fraction of PVA in the prepared nanocellulose-reinforced composite hydrogel is 10%, and the mass fraction of CNF is 0.1%.

[0049] Example 5 A method for preparing a nanocellulose-reinforced composite hydrogel includes the following steps: S1. Take 8 mL of the CNF suspension prepared in this invention, add 4 g of polyvinyl alcohol (PVA) particles and 21 g of H2O and stir continuously. Dissolve at 98°C for 2 h to form a mixed solution. Under normal temperature conditions, add 7.2 g of acrylamide, 0.06 g of initiator ammonium persulfate (APS), 0.009 g of N,N'-methylenebisacrylamide and 0.4 g of CaCl2 to the mixed solution and stir to mix them evenly to form a precursor solution.

[0050] S2, the precursor solution was sonicated in an ice bath for 40 minutes to remove air bubbles, then injected into a polytetrafluoroethylene mold and sealed. The sealed mold was then placed in an oven at 65°C for 3 hours, then frozen at -20°C for 16 hours, and finally thawed at room temperature for 4 hours to form a nanofiber-reinforced three-network hydrogel, thus obtaining a nanocellulose-reinforced composite hydrogel. Figure 5 As shown, denoted as PP 10 C 0.2 C indicates that the mass fraction of PVA in the prepared nanocellulose-reinforced composite hydrogel is 10%, and the mass fraction of CNF is 0.2%.

[0051] Example 6 A method for preparing a nanocellulose-reinforced composite hydrogel includes the following steps: S1. Take 12 mL of the CNF suspension prepared in this invention, add 4 g of polyvinyl alcohol (PVA) particles and 17 g of H2O and stir continuously. Dissolve at 98°C for 2 h to form a mixed solution. Under normal temperature conditions, add 7.2 g of acrylamide, 0.06 g of initiator ammonium persulfate (APS), 0.009 g of N,N'-methylenebisacrylamide and 0.4 g of CaCl2 to the mixed solution and stir to mix them evenly to form a precursor solution.

[0052] S2, the precursor solution was sonicated in an ice bath for 40 minutes to remove air bubbles, then injected into a polytetrafluoroethylene mold and sealed. The sealed mold was then placed in an oven at 65°C for 3 hours, frozen at -20°C for 16 hours, and thawed at room temperature for 4 hours to form a nanofiber-reinforced three-network hydrogel, thus obtaining a nanocellulose-reinforced composite hydrogel, denoted as PP. 10 C 0.3 C indicates that the mass fraction of PVA in the prepared nanocellulose-reinforced composite hydrogel is 10%, and the mass fraction of CNF is 0.3%.

[0053] To further illustrate the technical effects of the present invention, a comparative example is provided, as follows.

[0054] Comparative Example 1 The difference from Example 1 is that only polyacrylamide gel was prepared, without adding PVA particles and CNF suspension.

[0055] A method for preparing a hydrogel includes the following steps: S1, at room temperature, 7.2 g AM, 0.06 g APS, 0.009 g N,N'-methylenebisacrylamide and 0.4 g CaCl2 were directly added to 33 g H2O and stirred to mix them evenly to form a precursor solution.

[0056] S2, the precursor solution was sonicated in an ice bath for 40 min to remove air bubbles, and then injected into a polytetrafluoroethylene mold and sealed. Subsequently, the sealed mold was placed in an oven at 65°C for 3 h, then frozen at -20°C for 16 h, and then thawed at room temperature for 4 h to form a hydrogel, which was denoted as pure PAM hydrogel.

[0057] Comparative Example 2 The difference from Example 1 is that CNF suspension is not added.

[0058] A method for preparing a dual-network hydrogel includes the following steps: S1. Add 1g of PVA particles to 32g of water and stir continuously. Dissolve the PVA particles at 98℃ for 2 hours to form a PVA solution. Then, at room temperature, add 7.2g of AM, 0.06g of APS, 0.009g of N,N'-methylenebisacrylamide and 0.4g of CaCl2 to the PVA solution and stir to mix them evenly to form a precursor solution.

[0059] S2, the precursor solution was sonicated in an ice bath for 40 min to remove air bubbles, and then injected into a polytetrafluoroethylene mold and sealed. Subsequently, the sealed mold was placed in an oven at 65℃ for 3 h, then frozen at -20℃ for 16 h, and then thawed at room temperature for 4 h to form a double network hydrogel, denoted as PP2.5C, indicating that the mass fraction of PVA in the prepared double network hydrogel is 2.5%.

[0060] The properties of the hydrogels provided in the embodiments and comparative examples of the present invention were characterized, and the results are as follows.

[0061] Test Example 1: The pore size and microstructure of the hydrogel samples were analyzed using a Tescan Vega4 scanning electron microscope (Tescan, Czech Republic). The hydrogel samples prepared in Examples 1-2 and Comparative Examples 1-2 were frozen at -80°C and then freeze-dried for 24 hours using a freeze dryer (TianFeng, Shanghai). Furthermore, the freeze-dried samples were broken in half, and the fracture surfaces were sputtered with gold for observation.

[0062] Figure 2 The porous and dense three-dimensional network structure of the nanocellulose-reinforced composite hydrogel prepared according to embodiments of the present invention is shown. Compared with the pure PAM hydrogel of Comparative Example 1, the pores become denser with the addition of components, indicating that these components permeate each other and form a multi-layered network structure.

[0063] Test Example 2: Tensile tests were performed on the hydrogels using a universal testing machine to determine the influence of each component of the composite material on the tensile properties of the hydrogels. The hydrogels prepared in Examples 1-6 and Comparative Example 1 were fabricated into dumbbell-shaped specimens with a width of 4 mm, a thickness of 2 mm, and a length of 25 mm. The specimens were fixed at both ends with clamps, and tensile tests were conducted at a speed of 30 mm / min. The tensile cyclic test only increased the strain condition and the number of cycles, while keeping other conditions unchanged.

[0064] Through a series of mechanical deformation tests, including bending, twisting, knotting, and stretching, on the hydrogel samples prepared in Examples 1-6 and Comparative Example 1, it was observed that they were not damaged under repeated loading, demonstrating the excellent toughness of the nanocellulose-reinforced composite hydrogel prepared in this invention. Comparing the stress-strain curves of the composite hydrogels with different PVA contents in Examples 1-4 during the stretching process, it can be found that the pure PAM hydrogel of Comparative Example 1 has weaker mechanical properties, with lower tensile stress (0.07 MPa) and tensile strain (50.5%) than the other prepared hydrogels. Compared with pure PAM hydrogel, the three-network composite hydrogel prepared in this invention has significantly stronger mechanical properties; even with the addition of small amounts of PVA and CaCl2, it enhances the PP of Example 1. 2.5 C 0.1 The tensile stress (0.12 MPa) and tensile strain (559%) of the C hydrogel indicate that the addition of the three-network interpenetration and CaCl2 salt solution makes the hydrogel network more compact, thereby enhancing the mechanical properties of the composite hydrogel. Notably, the hydrogel stress-strain curve shows a positive correlation with PVA content; both the tensile stress and tensile strain of the hydrogel sample increase with increasing PVA content. With the PVA content increasing to 10 wt%, the PP in Example 4... 10 C 0.1 The C hydrogel reached its maximum tensile stress (0.53 MPa), and the tensile strain increased to 619%. This indicates that the increased PVA content introduced more hydroxyl groups, promoted the formation of the PVA crystal network, and enhanced the hydrogen bonds with the PAM polymer network, thereby significantly improving the mechanical properties of the hydrogel.

[0065] The addition of CNF (cellulose nanoparticles) further improved the mechanical properties of the composite hydrogel. With increasing CNF content, the mechanical properties of the nanocellulose-reinforced composite hydrogel of this invention were significantly improved. At the optimal CNF content of 0.2 wt%, the PP in Example 5 showed improved mechanical properties. 10 C 0.2 The C-cellulose hydrogel exhibited the highest tensile strength (1.41 MPa) and excellent tensile strain (1332%). This enhancement can be attributed to the interaction between CNF and the dual-penetration network. CNF fibers not only participate in the formation of the PAM network and reinforce the polymer network, but also form hydrogen bonds with the hydroxyl groups on the PVA molecular chains after freeze-thaw cycles, thereby enhancing the PVA freeze-thaw network. These strong fibers, acting as "fiber ropes," tightly connect the two networks as a third network, further enhancing the toughness of the internal network of the nanocellulose-reinforced composite hydrogel. When the CNF content reaches 0.3 wt%, the tensile strength and strain of the composite hydrogel decrease slightly. This is due to excessive physical cross-linking and stress concentration caused by excessively high CNF concentration, which reduces the hydrogel's performance. Meanwhile, for PP...10 C 0.2 The C-hydrogel was tested under different strains and subjected to multiple cyclic stretching under the same strain. It was found that the stress-strain trends of the hydrogel were the same at 10%, 30%, 50%, 80%, and 100%, and the stress-strain curves basically overlapped under cyclic stretching. This indicates that the nanocellulose-reinforced composite hydrogel prepared in this invention can effectively dissipate energy from the load and has excellent toughness.

[0066] Test Example 3: The hydrogels were further characterized for their compressive strength using a universal testing machine. The hydrogels prepared in Examples 1-6 and Comparative Example 1 were fabricated into cylindrical samples with a height of 10 mm and a diameter of 11 mm. The compression test speed was 10 mm / min. For the compression cycle test, the strain was controlled at 80% and the number of cycles was determined, while other conditions remained unchanged.

[0067] For PP in Example 5 10 C 0.2 Compression tests on the C-composite hydrogel revealed that it immediately recovered after reaching 95% compressive strain and being unloaded, indicating excellent compression recovery performance. Local compression of the hydrogel using a blade showed that it returned to its original state without being cut after the load was released, demonstrating that PP... 10 C 0.2 C-type hydrogels exhibit excellent toughness, resisting blade cutting. Comparison of stress-strain curves of hydrogels with different PVA and CNF contents during compression revealed a positive correlation between the mechanical properties of the hydrogels and the PVA and CNF contents, similar to tensile tests. Compression performance increased with increasing PVA content, and was further enhanced by CNF incorporation through hydrogen bonding, ionic coordination, and triple-network interactions. Specifically, PP... 10 C 0.2 C reaches the maximum compressive stress of 21.99 MPa (90% compressive strain). Meanwhile, for PP... 10 C 0.2 The cellulose nanoparticle-reinforced hydrogel underwent 10 cycles of cyclic compression testing at 80% compressive strain. The stress-strain curve of the first cycle showed excellent compressive strength, while the curve of subsequent cycles showed a slight decrease. This phenomenon is attributed to the incomplete recovery of broken bonds and cross-linked networks in the hydrogel after the first cycle. The recovery rate of compressive performance was calculated based on the highest compressive stress of each cycle. Statistical analysis showed that the recovery rate of the hydrogel stabilized at approximately 90% after multiple cycles. The energy density of the hydrogel gradually decreased with increasing compression cycles, but eventually reached a stable value. These results indicate that the multiple networks and abundant hydrogen bonds in the nanocellulose-reinforced composite hydrogel provide an effective energy dissipation mechanism, resulting in its excellent mechanical properties.

[0068] Test Example 4: The rheological characterization of the hydrogels was conducted using a Netzsch rheometer. The hydrogels prepared in Examples 1-6 and Comparative Examples 1-2 were cut into circular slices with a diameter of 20 mm and a thickness of 1 mm using a cutter. All samples were tested using circular flat plates. The rheological properties of various hydrogel samples were tested using a rheometer to further analyze the viscoelastic behavior of the hydrogels.

[0069] The storage modulus G' of various hydrogels initially remained stable within a certain range, then began to decrease at a certain point, representing the linear elastic region of the material. Subsequent rheological tests were based on this linear elastic region. Frequency scanning of the hydrogels at a constant strain of 1% and a frequency range of 0.1–100 Hz revealed that the storage modulus G' of all hydrogels was significantly greater than the dissipation modulus G''. This indicates that all hydrogel samples could stably maintain their gel state and possess excellent elastic properties from low to high frequencies. The storage modulus reflects the strength of the hydrogel. Frequency scanning curves showed that the composite hydrogel reinforced with three-network nanocellulose was significantly stronger than the pure PAM hydrogel with a single network. The addition of CNF and CaCl2 further enhanced G', attributed to the functional groups in the CNF molecular chain providing additional physical crosslinking points. When the CNF content reached 0.2%, both G' and G'' reached their maximum values, consistent with the mechanical property results.

[0070] To further test the rheological properties of the hydrogel, PP from Example 5 was used. 10 C 0.2 Taking C as an example, a rheometer was used to observe the recovery properties of the hydrogel by switching between large and small strains. The hydrogel sample was first subjected to a 1% strain for 5 minutes, then a 20% strain was applied and held for the same time, followed by a 1% strain recovery. It was clearly observed that G' and G'' of the hydrogel changed with the strain, but when the strain recovered, G' and G'' were almost the same as the original state, which means that the PPCC hydrogel has excellent recovery properties. According to the viscosity-frequency relationship curves of each hydrogel sample, it can be found that the viscosity of the hydrogel gradually decreases with increasing frequency. This is because there are a large number of hydrogen bonds in the nanocellulose-reinforced composite hydrogel, and the internal hydrogen bond cross-linking structure of the hydrogel is destroyed with increasing frequency. Since the increased PVA and CNF content introduces abundant functional groups and enhances the intermolecular interactions, the viscosity of the nanocellulose-reinforced composite hydrogel is always greater than that of the pure PAM hydrogel.

[0071] Test Example 5: The conductivity of the hydrogels was tested using a DMM6500 digital measuring instrument (Keithley, America). The hydrogels prepared in Examples 4-6 and Comparative Examples 1-2 were cut into standard samples as shown in Test Example 2 and then connected to a small light bulb circuit. The change in conductivity of the hydrogel with tensile strain was directly observed through the change in the brightness of the light bulb. With increasing tensile strain, the ion transport distance of the nanocellulose-reinforced composite hydrogel increased, and the ion concentration per unit volume decreased rapidly, resulting in increased resistance and decreased current in the circuit, causing the light bulb to dim. Comparing the conductivity of each hydrogel standard sample, it was found that pure PAM hydrogel had poor conductivity, only 32.78 mS / m. However, the incorporation of CaCl2 introduced a large number of conductive ions, making the conductivity of the PP hydrogel in Comparative Example 2 significantly higher. 2.5 The conductivity of the C-type hydrogel increased to 147.03 mS / m. Compared with other composite hydrogels, the conductivity varied slightly with composition, but the difference was not significant. This is because the conductivity of the composite hydrogel mainly depends on the conductive ions introduced by the incorporation of CaCl2, and the amount of CaCl2 incorporated results in minimal change in conductivity. The conductivity of the composite hydrogel gradually increased with increasing CNF content. This is because CNF prepared by the TEMPO oxidation method introduces some carboxylates during the reaction to produce carboxyl groups, thereby enhancing the conductivity of the composite hydrogel.

[0072] A digital measuring instrument was used to monitor the change in relative electrical resistance of the hydrogel in real time during the tensile process, while simultaneously using a universal testing machine to perform tensile testing on the hydrogel. and using sensitivity factor The relationship between relative resistance change and tensile strain was evaluated to assess the potential of this hydrogel as a strain sensor. Figure 3As shown in Figure (a), the rate of change of resistance gradually increases with tensile strain and can be divided into three regions by the change of slope: GF=1.135 (0%~400%), GF=4.092 (400%~900%), and GF=8.883 (900%~1380%). This is because the conductivity of the nanocellulose-reinforced composite hydrogel is mainly due to ion movement, and its conductivity depends on the change of resistance caused by the change in the geometry of the hydrogel. During tensile strain, the ion transport path increases and the channel narrows, which slows down the ion movement speed. This causes the GF value to increase from 1.135 at low strain to 8.883 at high strain with the change of tension, which also shows that the nanocellulose-reinforced composite hydrogel has excellent sensitivity in a large strain range (0%~1380%). Furthermore, the tensile strain was set to 10%, 30%, 50%, 80%, and 100%, and each cycle was repeated 5 times to monitor the change of resistance of the hydrogel in real time. It can be observed that this hydrogel exhibits a highly consistent resistive response within the 10%–100% strain range, and remains stable after multiple load-unload cycles at 100% strain, demonstrating its excellent durability. The nanocellulose-reinforced composite hydrogel also possesses rapid electrical response properties, such as… Figure 3 As shown in Figure (b), for PP 10 C 0.2 After C hydrogel is energized, a 50% strain load is suddenly applied for 1.5 seconds, and then unloaded to 0%. PP 10 C 0.2 The C-type hydrogel responded to a load in 108 ms and recovered to its initial state in 140 ms. Even when the hydrogel was cut and reconnected while energized, the resistance recovered to its initial value within 1.1 seconds. These test results demonstrate that the nanocellulose-reinforced composite hydrogel combines high conductivity, high sensitivity, fast response, and stability, highlighting its significant application potential in the field of electronic skin. A comparison of the mechanical and electrical properties of the various hydrogel samples of this invention is shown in Table 1.

[0073] Table 1. Mechanical and electrical properties of hydrogel samples from embodiments and comparative examples of the present invention. " / " indicates that it cannot be measured.

[0074] Test Example 6: The nanocellulose-reinforced composite hydrogel of this invention does not contain harmful additives during synthesis. Calcium ions are introduced to provide coordination bonds and bactericidal activity to the hydrogel; however, the introduction of salt solution may affect cell growth. Therefore, the cytotoxicity of the composite hydrogel prepared by this invention was tested using mouse L929 fibroblasts via the CCK-8 assay.

[0075] 1 g of nanocellulose-reinforced composite hydrogel was placed in a glass culture dish and sterilized under ultraviolet radiation for 4 hours. The hydrogel sample was then immersed in 5 mL of fresh culture medium (DMEM, 10% FBS, 1% PS) and incubated at 37°C for 24 hours. The supernatant was collected and sterilized using a filter head to obtain the nanocellulose-reinforced composite hydrogel extract. The cell density was set at 2 × 10⁻⁶ cells / mL. 3 L929 cells were seeded into 96-well plates and cultured at 37°C for 24 hours. Then, the medium was replaced with 180 μL of fresh medium along with hydrogel extract. After 24 hours of incubation, 20 μL of CCK-8 solution was added to each well, and the plates were incubated again for 2 hours (37°C, 5% CO2). Cell viability was calculated by measuring the absorbance of each well at 450 nm using a microplate reader (Multiskan Go, Thermal Fisher, USA). A negative control group consisted of equal volumes of complete medium with CCK-8 but no cells, while a positive control group consisted of wells containing cells but no hydrogel extract; both were cultured using the same procedure. Unused wells surrounding the experimental groups were kept in a liquid environment with 200 μL of PBS. Figure 4 As shown, compared with the control group, the survival rate of L929 cells cultured in the hydrogel extract was greater than 100% after 24 hours. Even after co-incubation with the 48-hour hydrogel extract for 24 hours, the L929 cells still maintained over 100% cell viability. This indicates that the nanocellulose-reinforced composite hydrogel has excellent cell compatibility, and an appropriate amount of CaCl2 does not affect cell viability. This means that CNF and CaCl2 incorporation into the PAM / PVA tri-network enhances their interaction through physical connection. In summary, the nanocellulose-reinforced composite hydrogel prepared in this invention possesses excellent mechanical properties and good biocompatibility, showing great potential in the fields of biomedicine and biomimetic skin.

[0076] Obviously, those skilled in the art can make various modifications and variations to this invention without departing from its spirit and scope. Therefore, if these modifications and variations fall within the scope of the claims of this invention and their equivalents, this invention also intends to include these modifications and variations.

Claims

1. A method for the preparation of nanocellulose-reinforced composite hydrogels, characterized by, The method comprises the following steps: Cellulose nanofiber is used as the skeleton, acrylamide is used as the polymerization monomer, polyvinyl alcohol and Ca 2+ Source are added, and polymerization reaction occurs under the action of initiator and crosslinking agent, acrylamide is polymerized to obtain polyacrylamide, a first crosslinking network is formed, and the carboxyl groups on the surface of the cellulose nanofiber form dynamic reversible metal-carboxyl coordination bonds with Ca 2+ Source, and a primary hydrogel is obtained; The primary hydrogel is subjected to a freeze-thaw cycle, polyvinyl alcohol in the primary hydrogel forms a second crystalline network, the cellulose nanofiber penetrates and connects the first crosslinked network and the second crystalline network, and the hydroxyl groups on the surface of the cellulose nanofiber form hydrogen bonds with the polyvinyl alcohol and the polyacrylamide, to obtain a nanocellulose-reinforced composite hydrogel.

2. The method of producing a nanocellulose-reinforced composite hydrogel according to claim 1, characterized in that, The cellulose nanofiber is used in an amount of 1 g per 0.03 to 0.09 mol of the Ca 2+ source. The Ca 2+ source is selected from calcium chloride or calcium nitrate.

3. The method of producing a nanocellulose-reinforced composite hydrogel according to claim 1, characterized in that, The freeze-thaw cycle specifically comprises freezing the primary hydrogel at-25℃ to-20℃ for 16h to 20h and thawing at room temperature for 4h to 5h.

4. The method of producing a nanocellulose-reinforced composite hydrogel according to claim 1, characterized in that, The mass ratio of the cellulose nanofiber, the polyvinyl alcohol and the acrylamide is 0.04 to 0.12:1 to 4:7.

2.

5. The method of producing a nanocellulose-reinforced composite hydrogel according to claim 4, characterized in that, The mass ratio of the acrylamide, the initiator and the crosslinking agent is 1.2:0.01:0.0015.

6. The method of producing a nanocellulose-reinforced, composite hydrogel according to claim 1, characterized in that, The polymerization reaction specifically comprises polymerization at 65℃ to 68℃ for 3h to 5h.

7. The method of producing a nanocellulose-reinforced, composite hydrogel according to claim 1, characterized in that, The cellulose nanofiber has a length of 1μm to 2μm and a diameter of about 5nm to 20nm.

8. The method of producing a nanocellulose-reinforced, composite hydrogel according to claim 1, characterized in that, The initiator is ammonium persulfate, and the crosslinking agent is N,N'-methylenebisacrylamide.

9. A nanocellulose-reinforced composite hydrogel, characterized in that, Prepared by the preparation method in any one of claims 1 to 8. 10.A nanocellulose-reinforced composite hydrogel according to claim 8, used as a support layer or / and a sensing layer in a biomimetic skin.

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