Preparation method of hydrogel strain sensor capable of being subjected to 4D printing

Abstract

The invention discloses a preparation method of a hydrogel strain sensor capable of 4D printing. The preparation method comprises the following steps: S1, preparing a mixed solution A and a mixed solution B; s2, taking the mixed solution A as functionalized precursor ink, directly extruding the functionalized precursor ink into the mixed solution B in an extrusion printing mode, and standing for 25-35 minutes to wait for growth of the ordered-structure hydrogel; and S3, packaging the hydrogel with the ordered structure, which is formed by printing. The 4D printing technology of extrusion printing and standing growth forming is adopted, ordered structures of hydrogel with different three-dimensional shapes can be prepared in a customized mode according to people of different ages and different parts of human bodies, and therefore the hydrogel strain sensor is produced on the basis of the hydrogel with the customized ordered structures with the three-dimensional shapes; the customized requirements of people of different ages and different parts of human bodies on the shape and the size of the hydrogel strain sensor are met.

Description

Technical Field

[0001] This invention relates to the field of electronic device engineering technology, and in particular to a method for fabricating a 4D-printable hydrogel strain sensor. Background Technology

[0002] Hydrogel strain sensors are flexible sensor devices with conductive hydrogel as the core sensing material. They can convert externally applied mechanical deformation (such as tension, compression, and bending) into quantifiable changes in electrical signals (such as resistance, capacitance, or voltage fluctuations), thereby enabling precise monitoring of various physical motions.

[0003] However, existing hydrogel strain sensors generally rely on the intrinsic structural characteristics of their homogeneous, random networks. Further performance improvements are limited by a fundamental inherent contradiction: on the one hand, high sensitivity requires rapid and significant changes in the internal conductive pathways during deformation (such as rapid breakage of the conductive network or abrupt changes in contact area); on the other hand, a large strain range and high toughness require the material network to possess efficient energy dissipation capabilities, ensuring structural integrity under large deformation by suppressing crack propagation. However, in homogeneous material systems, these two requirements are difficult to meet simultaneously. Furthermore, the structural disorder of random networks can lead to signal response hysteresis and decreased stability. Simultaneously, existing hydrogel strain sensors are generally fabricated using molds, making it difficult to meet the customized needs of different age groups and different parts of the body for the shape and size of hydrogel strain sensors. Summary of the Invention

[0004] The main objective of this invention is to propose a method for fabricating 4D-printable hydrogel strain sensors. This method aims to address the technical challenges of existing fabrication methods in meeting the customized needs of different age groups and different parts of the human body for the shape and size of hydrogel strain sensors. Furthermore, the improvement of high sensitivity, large strain range, and high toughness of hydrogel strain sensors is limited by the intrinsic structural characteristics of homogeneous and random networks.

[0005] To achieve the above objectives, the present invention proposes a method for fabricating a 4D-printable hydrogel strain sensor, comprising the following steps: S1: Prepare mixed solution A and mixed solution B; S2: Using mixed solution A as a functionalized precursor ink, the functionalized precursor ink is directly extruded into mixed solution B by extrusion printing, and left to stand for 25-35 minutes to wait for the growth of the ordered hydrogel. S3: Encapsulate the printed ordered hydrogel structure.

[0006] Optionally, step S1 includes the following steps: S11: Mix water-soluble olefin monomers, crosslinking agents, two-dimensional conductive nanomaterial dispersions, thickeners, and deionized water until homogeneous to obtain mixed solution A.

[0007] Optionally, the mass ratio of water-soluble olefin monomers, crosslinking agents, two-dimensional conductive nanomaterial dispersions, thickeners, and deionized water in the mixed solution A is 15~25:0.0005~0.0001:0.05~0.006:0.8~1.6:12~18.

[0008] Optionally, the two-dimensional conductive nanomaterial dispersion is an MXene dispersion.

[0009] Optionally, the mass of the MXene dispersion accounts for 0.64‰ of the total mass of the mixed solution A.

[0010] Optionally, the thickener is hydroxypropyl methylcellulose.

[0011] Optionally, step S1 further includes the following steps: S12: Add the initiator to the aqueous solution to obtain mixed solution B.

[0012] Optionally, the mass ratio of initiator to water in the mixed solution B is 20~30:200~500.

[0013] Optionally, step S3 includes the following steps: S31: Immerse the printed ordered hydrogel structure in an electrolyte solution; S32: Encapsulate the ordered hydrogel after soaking.

[0014] Optionally, the extrusion printing method is one of pneumatic, piston, or screw type.

[0015] The technical solution of this invention has the following beneficial effects: The 4D printing technology employed in this invention, combining extrusion printing and static growth molding, enables the customized fabrication of ordered hydrogel structures with varying three-dimensional shapes for different age groups and body parts. This allows for the production of hydrogel strain sensors based on these customized three-dimensional ordered hydrogel structures, meeting the customized shape and size requirements of different age groups and body parts. Furthermore, while the ordered hydrogel with its customized three-dimensional shape appears as a single macroscopic unit, it is internally composed of ordered fibrous structures. When the ordered hydrogel undergoes minute stretching, the fibrous structures first undergo slight changes, rapidly altering the internal conductive pathways and achieving high sensitivity. Further stretching of the ordered hydrogel causes the entire fibrous structure to slip and extend, resulting in a wide strain range and high toughness. Attached Figure Description

[0016] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on the structures shown in these drawings without creative effort.

[0017] Figure 1 This is a schematic diagram of the overall structure of the hydrogel strain sensor prepared by the method for preparing a 4D-printable hydrogel strain sensor according to the present invention. Figure 2 This is a physical image of the hydrogel strain sensor prepared by the method for fabricating a 4D-printable hydrogel strain sensor according to the present invention. Figure 3 The tensile stress-strain curve of the hydrogel strain sensor prepared by the method for preparing a 4D printable hydrogel strain sensor according to the present invention. Figure 4 The hysteresis ratio statistics of the hydrogel strain sensor prepared by the method for preparing a 4D printable hydrogel strain sensor according to the present invention are shown in the figure under various strains. Figure 5 The conductivity statistics of the hydrogel strain sensor prepared by the method for fabricating a 4D-printable hydrogel strain sensor according to the present invention are shown in the figure.

[0018] The reference numerals are as follows: 1. Ordered hydrogel; 2. Polyacrylate tape; 3. Copper electrode wire.

[0019] The realization of the objective, functional features and advantages of the present invention will be further explained in conjunction with the embodiments and with reference to the accompanying drawings. Detailed Implementation

[0020] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0021] It should be noted that all directional indications (such as up, down, left, right, front, back, etc.) in the embodiments of the present invention are only used to explain the relative positional relationship and movement of each component in a certain specific posture (as shown in the figure). If the specific posture changes, the directional indication will also change accordingly.

[0022] Furthermore, the technical solutions of the various embodiments can be combined with each other, but only if they are feasible for those skilled in the art. If the combination of technical solutions is contradictory or cannot be implemented, it should be considered that such combination of technical solutions does not exist and is not within the scope of protection claimed by this invention.

[0023] This invention proposes a method for fabricating a 4D-printable hydrogel strain sensor.

[0024] like Figures 1 to 5 As shown, in Embodiment 1 of the present invention, the method for fabricating the 4D-printable hydrogel strain sensor includes the following steps: S1: Prepare mixed solution A and mixed solution B; S2: Using mixed solution A as a functionalized precursor ink, the functionalized precursor ink is directly extruded into mixed solution B by extrusion printing, and left to stand for 25-35 minutes to wait for the growth of ordered hydrogel 1. S3: Encapsulate the printed ordered structure hydrogel 1.

[0025] In this embodiment, the optimal effect is achieved when the mixed solution A is extruded into the mixed solution B, and then allowed to stand for 30 minutes to allow the ordered hydrogel 1 to grow before removal. Furthermore, the extrusion printing method can be any of pneumatic, piston, or screw type.

[0026] Specifically, this invention uses a mixed solution A as a functionalized precursor ink. The functionalized precursor ink is moved and extruded through a mixed solution B along a preset path, allowing the mixed solution B to act on the extruded functionalized precursor ink. During a 25-35 minute settling period, the functionalized precursor ink undergoes continuous chain polymerization and cross-linking reactions to generate a network structure and enhance its mechanical strength. This transforms the liquid functionalized precursor ink into a solid, stable, and custom-shaped ordered hydrogel 1. Compared to existing methods that prepare homogeneous hydrogels using molds, this invention employs 4D printing technology combining extrusion printing and static growth. This technology can prepare custom-shaped ordered hydrogels 1 with specific three-dimensional shapes for different age groups and body parts. Based on these custom-shaped ordered hydrogels 1, hydrogel strain sensors can be produced, meeting the customized needs of different age groups and body parts for the shape and size of hydrogel strain sensors.

[0027] In this embodiment, step S1 includes the following steps: S11: Mix water-soluble olefin monomers, crosslinking agents, two-dimensional conductive nanomaterial dispersions, thickeners, and deionized water until homogeneous to obtain mixed solution A; S12: Add the initiator to the aqueous solution to obtain mixed solution B.

[0028] Specifically, in the above mixed solution A, the mass ratio of water-soluble olefin monomers, crosslinking agent, two-dimensional conductive nanomaterial dispersion, thickener, and deionized water is 15~25:0.0008~0.0001:0.05~0.006:0.8~1.6:12~18. In mixed solution B, the mass ratio of initiator to water is 20~30:200~500.

[0029] In this embodiment, the water-soluble olefin monomer in mixed solution A is acrylamide monomer, the crosslinking agent is N,N-methylenebisacrylamide, the two-dimensional conductive nanomaterial dispersion is MXene dispersion, and the thickener is hydroxypropyl methylcellulose. The initiator for mixed solution B is ammonium persulfate. In other embodiments, the water-soluble olefin monomer can be acrylic acid or hydroxyethyl methacrylate, the crosslinking agent can be polyethylene glycol diacrylate, the two-dimensional conductive nanomaterial dispersion can be graphene or molybdenum disulfide, the thickener can be sodium alginate, and the initiator can be azobisisobutyramidine hydrochloride, etc.

[0030] Preferably, the optimal ratio of the above mixed solution A is 20 g acrylamide monomer, 0.428 mg N,N-methylenebisacrylamide crosslinking agent, 0.48 g MXene dispersion, 1.2 g hydroxypropyl methylcellulose, and 15 g deionized water. The optimal ratio of mixed solution B is 25 g ammonium persulfate and 400 ml aqueous solution.

[0031] Preferably, the optimal ratio of the MXene dispersion to the total mass of the mixed solution A is 0.64‰. Compared with the ordered structure hydrogels 1 prepared by the ratios of 0.16 wt‰, 0.32 wt‰ and 1.28 wt‰, the ordered structure hydrogel 1 prepared by the 0.64‰ ratio has the best elasticity and mechanical strength.

[0032] The hysteresis, ultimate strength, tensile strength, and conductivity of the ordered hydrogel 1 prepared using MXene dispersions at ratios of 0.16 wt‰, 0.32 wt‰, 0.64 wt‰, and 1.28 wt‰ are shown in the table below at 200% stretching. MXene concentration Hysteresis ratio (%) Strength (MPa) Strength (%) Electrical conductivity (S / m) 0.16 wt‰ 26.36 0.16 818 4.30 0.32 wt‰ 19.59 0.30 664 3.61 0.64 wt‰ 2.93 0.74 588 2.71 1.28 wt‰ 4.75 0.73 410 1.70 As shown in the table, the difference in MXene dispersion content has a significant regulatory effect on the key mechanical properties of hydrogels, such as elasticity, strength, and tensile strength. This is because the MXene dispersion dominates the polymerization reaction of acrylamide monomers. When the MXene dispersion content is too low, the ordered hydrogel 1 prepared by the above method exhibits a loose conductive network, low conductivity, and minimal improvement in mechanical properties. When the MXene dispersion content is too high, the ordered hydrogel 1 prepared by the above method will experience performance degradation because MXene is more prone to aggregation and self-stacking, which disrupts the uniformity of the hydrogel network, introduces stress defects, and leads to further performance degradation.

[0033] It is worth noting that: Mixed solution A is a functionalized precursor ink, designed to both print smoothly and contain all the functional components of the final product. Therefore, mixed solution A, prepared by uniformly mixing the above materials, is rich in reactive monomers and functional materials. Among them, water-soluble olefin monomers are the cornerstone for forming the hydrogel network structure; crosslinking agents are used to connect linear polymer chains into a three-dimensional network, giving the ordered structured hydrogel 1 mechanical strength and elasticity; the two-dimensional conductive nanomaterial dispersion serves both as a conductive filler to provide electrical properties for the sensor and as a polymerization accelerator to promote rapid molding; and thickeners are used to adjust the rheological properties (viscosity and shear thinning behavior) of the functionalized precursor ink, making it easy to extrude from the printhead and maintain its shape without collapsing after extrusion.

[0034] Furthermore, Mixture Solution A also exhibits excellent thixotropic properties: due to the addition of thickeners and two-dimensional conductive nanomaterial dispersions, the functionalized precursor ink has a high viscosity when at rest, capable of supporting its own weight; when subjected to the shear force of printing extrusion (shear thinning), the viscosity decreases, making it easier to flow. This facilitates clear, continuous printing and precise stacking into the predetermined 3D structure. Additionally, because Mixture Solution A lacks an initiator, water-soluble olefin monomers will not undergo polymerization, allowing Mixture Solution A to maintain a stable liquid state for a period of time, facilitating storage and handling before printing.

[0035] In this embodiment, mixed solution B serves as the initiation environment for the polymerization reaction, its core function being to provide the chemical trigger for polymerization. The initiator contained in mixed solution B can decompose in aqueous solution to generate free radicals. These free radicals are the key to opening the double bonds of olefin monomers and initiating a chain polymerization reaction. Because mixed solution B is an aqueous solution with very low viscosity, close to that of water, it facilitates easy movement of the printhead in the solution and also helps to avoid mechanical interference with the functionalized precursor ink extruded from the printhead. When the functionalized precursor ink is extruded into mixed solution B, the initiator molecules rapidly diffuse into the interior of the functionalized precursor ink, initiating the polymerization and crosslinking reaction of water-soluble olefin monomers.

[0036] Specifically, the mixed solution B provides an initiator-filled "water bath" environment for printing the ordered hydrogel 1, ensuring that the reaction starts simultaneously from all surfaces of the ordered hydrogel 1 and penetrates uniformly into the interior, thereby achieving uniform and rapid gelation of the functionalized precursor ink. Furthermore, the buoyancy of the mixed solution B also provides support for the extruded, uncured, soft functionalized precursor ink, preventing it from deforming or collapsing due to gravity, thus enabling the printing of complex, customized three-dimensional structures.

[0037] In this embodiment, step S3 includes the following steps: S31: Immerse the printed ordered structure hydrogel 1 in an electrolyte solution; S32: Encapsulate the ordered structured hydrogel 1 after soaking.

[0038] The electrolyte solution is a sodium chloride aqueous solution with a concentration ranging from 0.1 g / ml to 0.5 g / ml, with a concentration of 0.2 g / ml showing the best effect. In other embodiments, electrolyte solutions such as potassium chloride or lithium chloride can also be used for soaking. After soaking, the ordered hydrogel 1 is connected to the copper electrode wire 3, and polyacrylate tape 2 (VHB) is used to encapsulate the ordered hydrogel 1 and the copper electrode wire 3 to form a hydrogel strain sensor.

[0039] Specifically, the ordered hydrogel 1 printed in step S2 has the disadvantage of insufficient conductivity or instability. Therefore, it is soaked in an electrolyte solution to allow ions in the solution to penetrate into the conductive network of the ordered hydrogel 1, so as to greatly enhance the conductivity of the conductive network through the ion conduction mechanism, thereby improving the sensitivity and signal stability of the ordered hydrogel 1.

[0040] Specifically, the working principle and process of this invention are as follows: This invention uses water-soluble olefin monomers, crosslinking agents, two-dimensional conductive nanomaterial dispersions, thickeners, and deionized water to prepare a mixed solution A, which serves as a functionalized precursor ink. An initiator is then added to the aqueous solution to prepare a mixed solution B, which serves as the initiation environment for the polymerization reaction of the functionalized precursor ink. Next, using mixed solution A as the functionalized precursor ink, the ink is extruded into mixed solution B along a predetermined path via extrusion printing, and allowed to stand for 25-35 minutes to allow the ordered hydrogel 1 to grow and solidify. After the ordered hydrogel 1 has solidified, it is immersed in an electrolyte solution to enhance the conductivity of the conductive network, thereby improving the sensitivity and signal stability of the ordered hydrogel 1. Finally, the ordered hydrogel 1 is encapsulated into a hydrogel strain sensor using copper electrode wires 3 and polyacrylate tape 2 (VHB).

[0041] The 4D printing technology employed in this invention, combining extrusion printing and static growth molding, enables the customized fabrication of ordered hydrogel structures with varying three-dimensional shapes for different age groups and body parts. This allows for the production of hydrogel strain sensors based on the customized three-dimensional ordered hydrogel structure 1, meeting the customized shape and size requirements of different age groups and body parts. Furthermore, while the customized three-dimensional ordered hydrogel structure 1 appears as a single macroscopic unit, it is internally composed of ordered fibrous structures. When the ordered hydrogel structure 1 undergoes minute stretching, the fibrous structure first undergoes slight changes, rapidly altering its internal conductive pathways and achieving high sensitivity. Further stretching of the ordered hydrogel structure 1 causes the entire fibrous structure to slip and extend, resulting in a large strain range and high toughness.

[0042] The above description is merely a preferred embodiment of the present invention and does not limit the patent scope of the present invention. Any equivalent structural transformations made using the contents of the present invention's specification and drawings under the inventive concept of the present invention, or direct / indirect applications in other related technical fields, are included within the patent protection scope of the present invention.

Claims

1. A method for fabricating a 4D-printable hydrogel strain sensor, characterized in that, Includes the following steps: S1: Prepare mixed solution A and mixed solution B; S2: Using mixed solution A as a functionalized precursor ink, the functionalized precursor ink is directly extruded into mixed solution B by extrusion printing, and left to stand for 25-35 minutes to wait for the growth of the ordered hydrogel. S3: Encapsulate the printed ordered hydrogel structure.

2. The method for fabricating a 4D-printable hydrogel strain sensor according to claim 1, characterized in that, Step S1 includes the following steps: S11: Mix water-soluble olefin monomers, crosslinking agents, two-dimensional conductive nanomaterial dispersions, thickeners, and deionized water until homogeneous to obtain mixed solution A.

3. The method for fabricating a 4D-printable hydrogel strain sensor according to claim 2, characterized in that, The mass ratio of water-soluble olefin monomers, crosslinking agents, two-dimensional conductive nanomaterial dispersions, thickeners, and deionized water in the mixed solution A is 15~25:0.0005~0.0001:0.05~0.006:0.8~1.6:12~18.

4. The method for fabricating a 4D-printable hydrogel strain sensor according to claim 3, characterized in that, The two-dimensional conductive nanomaterial dispersion is an MXene dispersion.

5. The method for fabricating a 4D-printable hydrogel strain sensor according to claim 4, characterized in that, The mass of the MXene dispersion accounts for 0.64‰ of the total mass of the mixed solution A.

6. The method for fabricating a 4D-printable hydrogel strain sensor according to claim 3, characterized in that, The thickener is hydroxypropyl methylcellulose.

7. The method for fabricating a 4D-printable hydrogel strain sensor according to claim 2, characterized in that, Step S1 further includes the following steps: S12: Add the initiator to the aqueous solution to obtain mixed solution B.

8. The method for fabricating a 4D-printable hydrogel strain sensor according to claim 7, characterized in that, The mass ratio of initiator to water in the mixed solution B is 20~30:200~500.

9. The method for fabricating a 4D-printable hydrogel strain sensor according to claim 1, characterized in that, Step S3 includes the following steps: S31: Immerse the printed ordered hydrogel structure in an electrolyte solution; S32: Encapsulate the ordered hydrogel after soaking.

10. The method for fabricating a 4D-printable hydrogel strain sensor according to claim 1, characterized in that, The extrusion printing method is one of pneumatic, piston, or screw type.

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