Super-water-retention and underwater anti-swelling hydrogel flexible sensor prepared through coaxial 3D printing and method of super-water-retention and underwater anti-swelling hydrogel flexible sensor

Abstract

The invention discloses a super-water-retention and underwater anti-swelling hydrogel flexible sensor prepared through coaxial 3D printing and a method thereof, and particularly relates to a super-water-retention and underwater anti-swelling hydrogel flexible sensor prepared through coaxial 3D printing assisted by ultraviolet light and a method thereof. The preparation method of the super-water-retention and underwater anti-swelling hydrogel flexible sensor comprises the following steps: (1) taking hydrogel as a core layer material, and taking photo-curable polydimethylsiloxane resin added with fumed silica as a shell layer material; and (2) integrally extruding the shell layer organic silicon resin and the core layer hydrogel through an ultraviolet light-assisted coaxial 3D printing technology to form a three-dimensional structure, and applying the three-dimensional structure to a flexible preparation sensor. The shell layer light-cured resin material and the core layer light-cured hydrogel material are simultaneously extruded, deposited and molded through a coaxial needle head, and ultraviolet light is used for assisting in curing in the extrusion process. According to the coaxial structure printed in the mode, the hydrogel can be completely packaged in the shell layer resin, and meanwhile the water-retaining property and the swelling resistance of the hydrogel are guaranteed.

Description

Technical Field

[0001] The present invention relates to a method for preparing a super water-retaining and underwater anti-swelling hydrogel flexible sensor by coaxial 3D printing and a method thereof. Specifically, it relates to a method for forming a three-dimensional structure by combining hydrogel and light-curing silicone resin through ultraviolet light-assisted coaxial 3D printing and applying it to a flexible sensor. It belongs to the field of polymer material technology. Background Art

[0002] With the development of science and technology and society, electronic products are becoming more and more inseparable from people's social life. As a member of electronic equipment, flexible electronic sensors make our lives more and more comfortable and convenient. Flexible sensors have attracted widespread attention due to their portability, mechanical robustness, high flexibility, strong adaptability, and ability to convert external stimuli (such as strain, pressure, temperature, and chemical pH) into detectable electrical signals (such as resistance, current, voltage, and capacitance).

[0003] Hydrogel is a three-dimensional cross-linked network composed of hydrophilic polymer chains in a water-rich environment. The cross-linking methods of hydrogel are divided into physical cross-linking and chemical cross-linking. It has the advantages of simple preparation process, good biocompatibility, adjustable performance, high viscoelasticity and unique stimulus response characteristics. Based on these advantages, hydrogel materials have shown great potential in flexible electronics fields such as remote health monitoring, environmental monitoring, human-computer interface interaction, and intelligent robots. However, the large-scale application of hydrogel materials also puts forward higher requirements on its functional reliability, shape diversity, molding method, and long-term applicability in multiple scenarios.

[0004] The main components of hydrogels are water and hydrophilic polymers. The hydrogel system contains a large amount of water. In a normal temperature and humidity environment, due to the natural evaporation effect, water molecules overflow uncontrollably, causing the hydrogel mass to decrease and the performance to be unstable and not lasting. In addition, when the hydrogel is in a humid or underwater environment, because the osmotic pressure of the polymer chain network inside the gel is much higher than the surrounding watery environment, water molecules diffuse into the gel driven by the osmotic pressure difference, causing the hydrogel volume to expand, thereby reducing the mechanical and conductive properties of the hydrogel.

[0005] At present, physical methods such as adding a large amount of inorganic salt ions to form hydrated ions with water molecules and adding other solvents such as dimethyl sulfoxide to reduce the water content have been used to retain water in hydrogel systems. In addition, the addition of zwitterions such as methyl methacryloylethyl sulfobetaine can form electrostatic effects to remove water molecules, and the construction of ionic / covalent multiple cross-linked networks has been used to resist swelling and retain water in hydrogels. The above methods can only ensure that the performance of the hydrogel system does not decrease due to water loss and swelling in a short period of time, and the improvement method is not universal.

[0006] Multi-material encapsulation is a universal way to effectively solve the problem of water loss and swelling of hydrogels. This method uses a variety of materials such as resins and polydimethylsiloxane (PDMS) to encapsulate hydrogels to form a completely or incompletely closed structure, thereby preventing the evaporation of water in the hydrogel system and the entry of external water molecules into the hydrogel system. Coaxial 3D printing is a very effective encapsulation method. This method integrates the core layer hydrogel and the shell layer material, which can achieve complete encapsulation of the hydrogel and form different three-dimensional structures to meet customized needs. The preparation efficiency of hydrogel flexible sensors is improved by UV-assisted curing printing.

[0007] This method for preparing water-retaining and anti-swelling hydrogel flexible sensors can promote the larger-scale, long-term and stable application of hydrogels in the field of flexible electronics. Summary of the invention

[0008] Purpose of the invention: In order to overcome the deficiencies in the prior art, the present invention provides a coaxial 3D printing method for preparing a flexible sensor with super water retention and underwater anti-swelling hydrogel, wherein the shell layer photocurable resin material and the core layer photocurable hydrogel material are simultaneously extruded and deposited by a coaxial needle, and ultraviolet light is used to assist curing during the extrusion process. The coaxial structure printed in this way can completely encapsulate the hydrogel in the shell layer resin, while ensuring the water retention and anti-swelling properties of the hydrogel.

[0009] Technical solution: To achieve the above objectives, the first objective of the present invention is to provide a hydrogel flexible sensor, which is a core-shell material with a coaxial structure, wherein the core-shell material uses hydrogel as the core layer material and polydimethylsiloxane resin with added fumed silica as the shell layer material.

[0010] The second object of the present invention is to provide a method for preparing a hydrogel flexible sensor, comprising the following steps:

[0011] S1. Preparation of shell material

[0012] (1) adding a photoinitiator to a polydimethylsiloxane resin having a mercapto group on the side chain and vinyl groups at both ends in a molar ratio of mercapto to vinyl of 2:1, and dispersing and mixing once to obtain a milky white light-curable silicone ink;

[0013] (2) adding fumed silica to the milky white photocurable silicone ink, performing secondary dispersion and mixing to obtain a shell paste slurry;

[0014] S2. Preparation of core layer material

[0015] Dissolving acrylamide in water, adding polyethylene glycol diacrylate, a photoinitiator, and lithium chloride, heating and stirring to obtain an acrylamide hydrogel precursor solution, pouring the acrylamide hydrogel precursor solution into a mold, and irradiating with ultraviolet light to obtain an acrylamide solid hydrogel;

[0016] S3. Preparation of hydrogel flexible sensors

[0017] The shell paste slurry is extruded and fed from the side of the coaxial needle by air pressure control, and the acrylamide liquid hydrogel is extruded and fed from the top of the coaxial needle by mechanical pressure control. The two materials are extruded and deposited simultaneously from a discharge port. After co-extrusion, they are cured in real time by an ultraviolet light source located at the side of the coaxial needle to form a hydrogel flexible sensor with a coaxial core-shell structure.

[0018] The third object of the present invention is to provide an application of any of the above-described hydrogel flexible sensors or those prepared according to any of the above-described methods in the field of flexible sensors.

[0019] The inventive points of the present invention are specifically as follows:

[0020] (1) Polydimethylsiloxane resin (PDMS) with thiol groups on the side chains and vinyl groups on both ends was selected as the shell resin, and the rheological and mechanical properties were adjusted by adding fumed silica. The shell resin was rapidly formed during the UV-curing coaxial 3D printing process through the photocuring click reaction of thiol-ene.

[0021] (2) Photocurable acrylamide hydrogel is selected as the core layer hydrogel material, and is integrally formed with the shell layer resin material during the UV light-curing coaxial 3D printing process.

[0022] (3) The shell layer photocurable resin material and the core layer photocurable hydrogel material are simultaneously extruded and deposited through a coaxial needle, and ultraviolet light is used to assist curing during the extrusion process. The coaxial structure printed in this way can completely encapsulate the hydrogel in the shell layer resin while ensuring the water retention and anti-swelling properties of the hydrogel.

[0023] (4) This UV-curable coaxial printing method is combined with hydrogels of various cross-linking systems to further highlight the universality of the printing method.

[0024] Beneficial effects:

[0025] (1) Compared with the existing research on water retention and anti-swelling of hydrogels, the present invention can solve the problems of water loss and swelling of hydrogels at the same time.

[0026] (2) Compared with the water retention and anti-swelling effects of existing hydrogels, the hydrogel flexible sensor prepared by the present invention loses only 3.6% of its mass after being placed at room temperature for 20 days, and increases only 2% of its mass after being immersed in water for 20 days.

[0027] (3) The printing method of the present invention has universal applicability to hydrogel systems and can be applied to a variety of hydrogel systems.

[0028] (4) The hydrogel flexible sensor prepared by the present invention can maintain stable resistance after being placed at room temperature for up to 14 days. At the same time, it also has excellent corrosion resistance in acidic and alkaline environments and still has excellent thermal stability at 120°C. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 A schematic diagram of UV-curing coaxial 3D printing provided by the present invention, including the feeding direction of core layer and shell layer materials and the UV-assisted curing method.

[0030] Figure 2 This is a shear modulus variation curve of the shell layer organic silicon photocurable resin provided in Example 1 of the present invention.

[0031] Figure 3 This is a real-time photocuring modulus change curve of the shell layer organic silicon photocurable resin provided in Example 1 of the present invention.

[0032] Figure 4 This is a real-time photocuring modulus change curve of the core layer photocurable acrylamide hydrogel provided in Example 2 of the present invention.

[0033] Figure 5 This is a tensile stress-strain curve diagram of the shell layer silicone photocurable resin and the core layer acrylamide hydrogel provided in Examples 1 and 2 of the present invention.

[0034] Figure 6 This is a digital photo of the organic silicon photocurable resin and acrylamide hydrogel provided in Example 1 and Example 2 of the present invention, which are immersed and coated.

[0035] Figure 7 This is a digital photo of coaxial 3D printing using a shell layer silicone photocurable resin and a core layer acrylamide hydrogel provided in Examples 1 and 2 of the present invention.

[0036] Figure 8 This is an electron microscope image of a coaxial sample cross section obtained by coaxial 3D printing using a shell layer silicone photocurable resin and a core layer acrylamide hydrogel, as provided in Examples 1 and 2 of the present invention.

[0037] Fig. 9 Digital photos of single-layer spiral structures provided in Example 1 and Example 4 of the present invention that were coaxially 3D printed using a shell layer of organosilicon photocurable resin and a core layer of polyvinyl alcohol hydrogel.

[0038] Fig.10Digital photos of multi-layer spiral structures provided in Example 1 and Example 5 of the present invention that were coaxially 3D printed using a shell layer of organosilicon photocurable resin and a core layer of sodium alginate / acrylamide hydrogel.

[0039] Fig.11 This is a comparative curve diagram of the water loss effects of the PDMS-immersed and coated acrylamide hydrogel provided in Example 6 of the present invention, the acrylamide hydrogel provided in Example 2, and the coaxial structure sample.

[0040] Fig.12 This is a comparative curve diagram of the water loss effects of the core layer acrylamide hydrogel and the coaxial structure provided in Examples 2 and 3 of the present invention.

[0041] Fig.13 This is a comparative curve diagram of the swelling effects of the core layer acrylamide hydrogel and the coaxial structure provided in Examples 2 and 3 of the present invention.

[0042] Fig.14 A resistance change curve of the coaxial structure sample provided in Example 1 and Example 2 of the present invention, which is coaxial 3D printed using a shell layer silicone photocurable resin and a core layer acrylamide hydrogel, before and after being placed at room temperature for 14 days.

[0043] Fig.15 This is a schematic diagram of the coaxial structure hydrogel flexible sensor provided in Example 3 of the present invention.

[0044] Fig.16 This is a water loss curve diagram of the polyvinyl alcohol system hydrogel and sodium alginate hydrogel provided in Examples 4 and 5 of the present invention.

[0045] Fig.17 A resistance change curve of a coaxial sample in different chemical reagents obtained by coaxial 3D printing using a shell layer silicone photocurable resin and a core layer acrylamide hydrogel provided in Examples 1 and 2 of the present invention.

[0046] Fig.18 The resistance change curve of the acrylamide hydrogel and the coaxial 3D printed coaxial structure sample during the heating process provided in Example 2 of the present invention. DETAILED DESCRIPTION

[0047] Multi-material encapsulation is a universal way to effectively solve the problem of water loss and swelling of hydrogels. This method uses a variety of materials such as resins and polydimethylsiloxane (PDMS) to encapsulate hydrogels to form a completely or incomplete closed structure, thereby preventing the evaporation of water in the hydrogel system and the entry of external water molecules into the hydrogel system. Coaxial 3D printing is a very effective encapsulation method. This method integrates the core layer hydrogel and the shell layer material, which can achieve complete encapsulation of the hydrogel and form different three-dimensional structures to meet customized needs. The preparation efficiency of hydrogel flexible sensors is improved by ultraviolet light-assisted curing printing. This method for preparing water-retaining and anti-swelling hydrogel flexible sensors can promote the larger-scale, long-term and stable application of hydrogels in the field of flexible electronics.

[0048] Based on this, the present invention discloses a coaxial 3D printing method for preparing a super water-retaining and underwater anti-swelling hydrogel flexible sensor and a method thereof, specifically a method for preparing a super water-retaining and underwater anti-swelling hydrogel flexible sensor using ultraviolet light-assisted coaxial 3D printing. The method for preparing a super water-retaining and underwater anti-swelling hydrogel flexible sensor comprises: (1) using hydrogel as the core layer material and adding a photocurable polydimethylsiloxane resin with a gas phase silica as the shell layer material. (2) using ultraviolet light-assisted coaxial 3D printing technology to integrally extrude the shell layer silicone resin and the core layer hydrogel to form a three-dimensional structure and apply it to the preparation of a flexible sensor. The above-mentioned shell layer photocurable resin material and the core layer photocurable hydrogel material are simultaneously extruded and deposited by a coaxial needle, and ultraviolet light is assisted in curing during the extrusion process. The coaxial structure printed in this way can completely encapsulate the hydrogel in the shell layer resin while ensuring the water retention and anti-swelling properties of the hydrogel.

[0049] Specifically, the coaxial 3D printing method for preparing ultra-water-retaining and underwater anti-swelling hydrogel flexible sensors of the present invention is as shown in the following embodiments.

[0050] Example

[0051] A first aspect of the present invention is a hydrogel flexible sensor, which is a core-shell material with a coaxial structure, wherein the core-shell material uses hydrogel as a core layer material and polydimethylsiloxane resin with added fumed silica as a shell layer material.

[0052] Optionally, in some embodiments of the present invention, the water retention of the hydrogel flexible sensor is not less than 99% of the mass retention rate after being placed for 36 hours. Preferably, the mass retention rate is not less than 99.5% after being placed for 36 hours.

[0053] Optionally, in some embodiments of the present invention, the water retention of the hydrogel flexible sensor is a mass retention rate of not less than 98% within 6 days, preferably a mass retention rate of not less than 97% for more than 6 days.

[0054] Optionally, in some embodiments of the present invention, the water retention of the hydrogel flexible sensor is such that after being placed for 20 days, the mass retention rate is not less than 96%, preferably, after being placed for 20 days, the mass retention rate is not less than 96.4%.

[0055] Optionally, in some embodiments of the present invention, the hydrogel flexible sensor has an anti-swelling property in water of within 2% swelling in more than 2 days, preferably within 2% swelling in more than 10 days, and preferably within 2% swelling in more than 20 days.

[0056] Optionally, in some embodiments of the present invention, the performance stability period of the hydrogel flexible sensor at room temperature is up to 14 days.

[0057] Optionally, in some embodiments of the present invention, the heat resistance of the hydrogel flexible sensor is as high as 120°C.

[0058] Optionally, in some embodiments of the present invention, in the hydrogel flexible sensor, the core-shell material is integrally formed.

[0059] Optionally, in some embodiments of the present invention, the ratio of the core layer material to the shell layer material is a mass ratio of (1-3): (1-5).

[0060] Optionally, in some embodiments of the present invention, in the shell material, the ratio of fumed silica added to the polydimethylsiloxane resin is 5wt%-12wt%.

[0061] Optionally, in some embodiments of the present invention, in the core-shell material, the diameter ratio of the core layer material to the shell layer material is (1:2)-(1:3).

[0062] The second aspect of the present invention is a method for preparing a hydrogel flexible sensor, such as Figure 1 As shown, the following steps are included:

[0063] S1. Preparation of shell material

[0064] (1) adding a photoinitiator to a polydimethylsiloxane resin having a mercapto group on the side chain and vinyl groups at both ends in a molar ratio of mercapto to vinyl of 2:1, and dispersing and mixing once to obtain a milky white light-curable silicone ink;

[0065] (2) adding fumed silica to the milky white photocurable silicone ink, performing secondary dispersion and mixing to obtain a shell paste slurry;

[0066] S2. Preparation of core layer material

[0067] Dissolving acrylamide in water, adding polyethylene glycol diacrylate, a photoinitiator, and an inorganic salt, heating and stirring to obtain an acrylamide hydrogel precursor solution, pouring the acrylamide hydrogel precursor solution into a mold, and irradiating with ultraviolet light to obtain an acrylamide solid hydrogel;

[0068] S3. Preparation of hydrogel flexible sensors

[0069] The shell paste slurry is extruded and fed from the side of the coaxial needle by air pressure control, and the acrylamide liquid hydrogel is extruded and fed from the top of the coaxial needle by mechanical pressure control. The two materials are extruded and deposited simultaneously from a discharge port. After co-extrusion, they are cured in real time by an ultraviolet light source located at the side of the coaxial needle to form a hydrogel flexible sensor with a coaxial core-shell structure.

[0070] like Figure 1 As shown in the reaction equation, in the present invention, the reaction principle of the polydimethylsiloxane resin with a thiol group on the side chain and a vinyl group on both ends is as follows: the thiol group of the silicone generates a thiol free radical under ultraviolet light irradiation and the initiation of a photoinitiator, and reacts with the vinyl group to undergo a free radical addition reaction. When the molar ratio of the thiol group to the vinyl group is 2:1, it can ensure that all the vinyl groups are polymerized, and the reaction rate is relatively fast. The main molecular chain of polydimethylsiloxane is a silicon-oxygen bond, which has a higher bond energy and better thermal stability, and the molecular chain is flexible, and the side chain group can rotate freely around the silicon-oxygen bond, and has good flexibility.

[0071] Optionally, in some embodiments of the present invention, in step S1(1), the molecular weight of the polyethylene glycol diacrylate is 400-800.

[0072] Optionally, in some embodiments of the present invention, in step S1(1), the amount of the photoinitiator added is 0.5wt%-3wt%. Preferably, the amount of the photoinitiator added is 2wt%.

[0073] Optionally, in some embodiments of the present invention, in step S1(1), the photoinitiator includes ethyl 2,4,6-trimethylbenzoylphenylphosphonate, phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide or 2-hydroxy-2-methyl-1-phenyl-1-propanone.

[0074] Optionally, in some embodiments of the present invention, in step S1(1), the inorganic salt includes at least one of sodium chloride, ferric chloride, and lithium chloride.

[0075] Optionally, in some embodiments of the present invention, in step S1(1), the primary dispersion mixing comprises using a high-speed ball mill to uniformly disperse at a rotation speed of 2000 rpm / min-3000 rpm / min for 3-5 minutes.

[0076] Optionally, in some embodiments of the present invention, in step S1(2), the amount of fumed silica added is 5wt%-12wt% of the total mass of the shell material.

[0077] Optionally, in some embodiments of the present invention, in step S1(2), the secondary dispersion mixing includes using a high-speed ball mill to uniformly disperse at a rotation speed of 2000 rpm / min-3000 rpm / min for 3-5 minutes.

[0078] Optionally, in some embodiments of the present invention, in step S2, the concentration of acrylamide dissolved in water is 20wt%-40wt% of the total mass of the core layer material.

[0079] Optionally, in some embodiments of the present invention, in step S2, the mass ratio of acrylamide, polyethylene glycol diacrylate, photoinitiator and lithium chloride is (20wt%-40wt%): (0.2wt%-1wt%): (0.2wt%-2wt%): (1wt%-5wt%).

[0080] Optionally, in some embodiments of the present invention, in step S2, the photoinitiator includes 4-(2-hydroxyethoxy)phenyl 2-hydroxy-2-dipropyl ketone;.

[0081] Optionally, in some embodiments of the present invention, in step S2, the heating and stirring time is 15-30 min, and the temperature is 30-60°C.

[0082] Optionally, in some embodiments of the present invention, in step S2, the ultraviolet irradiation time is 1-5 minutes.

[0083] Optionally, in some embodiments of the present invention, in step S3, the air pressure is 500 kPa-700 kPa.

[0084] Optionally, in some embodiments of the present invention, in step S3, the mechanical pressure is 0.1 mL / min-0.5 mL / min.

[0085] The third aspect of the present invention is the application of any of the above described or hydrogel flexible sensors prepared according to any of the above described methods in the field.

[0086] The present invention will be further described below in conjunction with the accompanying drawings and Examples. According to the following examples, the present invention can be better understood. However, it is easy for those skilled in the art to understand that the specific material ratios, process conditions and results described in the examples are only used to illustrate the present invention, and should not and will not limit the present invention described in detail in the claims.

[0087] Example 1

[0088] Preparation of Shell-layer Photocurable Polydimethylsiloxane Silicone Resin

[0089] (a) A polydimethylsiloxane resin with a mercapto group on the side chain and a vinyl group at both ends was added to a ball mill at a molar ratio of 2:1, and then a photoinitiator 2,4,6-trimethylbenzoylphenylphosphonic acid ethyl ester (TPO-L) was added at 2 wt% of the total mass of PDMS. A high-speed ball mill was used to uniformly disperse the resin at a speed of 2500 rpm / min for 5 min to obtain a milky white photocurable silicone ink.

[0090] (b) adding fumed silica having a mass fraction of 10 wt % of the silicone ink to the prepared milky white photocurable silicone ink, and then uniformly dispersing the mixture using a high-speed ball mill (rotation speed 3000 rpm / min, 5 minutes).

[0091] A shell paste slurry that can be used for UV-curing coaxial 3D printing is obtained.

[0092] Example 2 Preparation of acrylamide AAm solid hydrogel

[0093] 14.9 g acrylamide (AAm) was dissolved in 35 g deionized water, followed by the addition of 0.1 g polyethylene glycol diacrylate, 0.5 g photoinitiator 4-(2-hydroxyethoxy)phenyl 2-hydroxy-2-dipropyl ketone, and 1 g lithium chloride, and heated and stirred for 15 min to obtain an AAm hydrogel precursor solution. The precursor solution was poured into a mold and irradiated under ultraviolet light for 2 min to obtain an AAm solid hydrogel.

[0094] Example 3 Preparation of hydrogel flexible sensor

[0095] See attached Figure 1 , which is a schematic diagram of the UV-curable coaxial 3D printing needle used in the present invention. The shell paste slurry obtained in Example 1 is extruded and fed from the side of the coaxial needle by 500kPa air pressure control, and the AAm liquid hydrogel obtained in Example 2 is extruded and fed from the top by mechanical pressure at a speed of 0.2mL / min. The two materials are extruded and deposited simultaneously from a discharge port, and after co-extrusion, they are cured in real time by an ultraviolet light source located at the side of the needle. The two materials together form a hydrogel flexible sensor with a coaxial core-shell structure.

[0096] Example 4 Preparation of polyvinyl alcohol (PVA) hydrogel

[0097] 2.5g of polyvinyl alcohol (PVA) was added to 22.5g of deionized water, and stirred in a 90℃ oil bath for 1h until PVA was completely dissolved, followed by adding 15g of dimethyl sulfoxide (DMSO) solution (the mass ratio of DMSO to deionized water was 3:2) and 0.5g of lithium chloride, and stirred at 40℃ for 30min to obtain a PVA / DMSO solution. The solution was poured into a mold and placed in a -24℃ refrigerator for freezing. After 12h, it was taken out and thawed at room temperature for 3h. After 3 freeze-thaw cycles, a solid polyvinyl alcohol PVA hydrogel was obtained.

[0098] Example 5 Preparation of SA / AAm hydrogel

[0099] 2g sodium alginate (SA) was added to 100ml deionized water, stirred at 60℃ for 1h to obtain SA solution, then 15g acrylamide (AAm) was added, and stirring was continued for 30min until acrylamide was dissolved, followed by sequential addition of 10ml N,N-methylenebisacrylamide aqueous solution (1000ml / g), 5ml ammonium persulfate aqueous solution (100ml / g), and 0.2ml tetramethylammonium hydroxide solution, and stirred to obtain SA / AAm hydrogel precursor solution. The solution was allowed to stand at room temperature 25℃ for 24h to obtain SA / AM hydrogel.

[0100] Example 6 Immersion coating of AAm hydrogel

[0101] The AAm hydrogel prepared in Example 2 was first irradiated under ultraviolet light for 2 minutes to be cured and then immersed in the silicone resin prepared in Example 1. Finally, it was taken out and irradiated under ultraviolet light again for 2 minutes to shape the photocurable silicone resin and achieve coating.

[0102] Test Case

[0103] See attached Figure 2 , which is a shear modulus variation curve of the shell layer organic silicon photocurable resin provided in Example 1 of the present invention. As can be seen from the figure, the storage modulus and loss modulus of the shell layer resin gradually decrease during the dynamic shear process, but the shell layer organic silicon photocurable resin is in the linear viscoelastic region under low shear stress, and the storage modulus is greater than the loss modulus, while under high shear stress, the storage modulus of the ink is less than the loss modulus, indicating that the shell layer slurry can be smoothly extruded under high shear stress and can maintain its shape when the shear stops.

[0104] See attached Figure 3 , which is a real-time photocuring modulus change curve of the shell layer organic silicon photocurable resin provided in Example 1 of the present invention. Before and after ultraviolet light irradiation, the shell layer organic silicon photocurable resin is always in a state where the storage modulus is greater than the loss modulus. After ultraviolet light irradiation, the storage modulus of the shell layer organic silicon photocurable resin rises rapidly within a few seconds and reaches a steady state within 5 seconds, meeting the extrusion printing requirements.

[0105] See attached Figure 4 , a real-time photocuring modulus change curve of the core layer photocurable acrylamide hydrogel provided in Example 2 of the present invention. After ultraviolet light irradiation, the storage modulus of the core layer photocurable acrylamide hydrogel is greater than the loss modulus in about 30 seconds, and reaches a steady state within 100 seconds, having a faster photocuring speed.

[0106] See attached Figure 5 , which is a tensile stress-strain curve of the shell layer organic silicon photocurable resin and the core layer acrylamide hydrogel provided in Examples 1 and 2 of the present invention. As can be seen from the figure, the elongation at break of the shell layer organic silicon photocurable resin of Example 1 and the core layer acrylamide hydrogel of Example 2 are both greater than 400%.

[0107] See attached Figure 6 , which is prepared by using the organic silicon photocurable resin and acrylamide hydrogel provided in Example 1 and Example 2 by using the soaking and coating method in Example 6 of the present invention. The acrylamide hydrogel is first photocured and then soaked in the organic silicon photocurable resin and cured again to coat the hydrogel.

[0108] See attached Figure 7 , which is a digital photo of coaxial 3D printing using a shell layer of organic silicon photocurable resin and a core layer of acrylamide hydrogel provided in Examples 1 and 2 of the present invention. A clear coaxial structure can be seen from the interface at the end of the line.

[0109] See attached Figure 8 , which is a cross-sectional electron microscope image of a coaxial sample using a shell layer of organic silicon photocurable resin and a core layer of acrylamide hydrogel for coaxial 3D printing provided in Examples 1 and 2 of the present invention. From the cross section, it can be seen that the shell layer resin has no defects and can completely wrap the core layer of hydrogel, and the core layer of hydrogel can form a complete circle.

[0110] See attached Fig. 9 , which is a digital photo of a single-layer spiral structure coaxially 3D printed using a shell layer organic silicon photocurable resin and a core layer polyvinyl alcohol hydrogel provided in Examples 1 and 4 of the present invention. The printing method can be applied to a freeze-thaw-cycle hydrogel system.

[0111] See attached Fig.10 , which is a digital photo of a multi-layer spiral structure coaxially 3D printed using a shell layer of organic silicon photocurable resin and a core layer of sodium alginate / acrylamide hydrogel provided in Examples 1 and 5 of the present invention. The printing method can be applied to a room temperature ion cross-linked hydrogel system.

[0112] See attached Fig.11, which is a comparative curve of the water loss effect of the PDMS soaked and coated acrylamide hydrogel provided in Example 6 of the present invention, the acrylamide hydrogel provided in Example 2, and the coaxial structure sample. After the coaxial structure sample was placed for 36 hours, the mass retention rate was 99.6%, the mass retention rate of the soaked and coated sample was about 98%, and the mass retention rate of the bare hydrogel was about 87%. The coaxial structure has a better water retention effect than the soaking and coating method.

[0113] See attached Fig.12 , which is a comparative curve diagram of the water loss effect of the core layer acrylamide hydrogel and the coaxial structure provided by Examples 2 and 3 of the present invention. It can be seen from the figure that the coaxial structure hydrogel of Example 3 loses only less than 3% of its mass after being placed at room temperature for 14 days, while the acrylamide hydrogel of Example 2 loses more than 70% of its mass after being placed at room temperature for 14 days. The coaxial structure has an excellent water retention effect.

[0114] See attached Fig.13 , which is a comparison curve of the swelling effects of the core acrylamide hydrogel and the coaxial structure provided by Examples 2 and 3 of the present invention. It can be seen from the figure that the acrylamide hydrogel of Example 2 swells by more than 200% after being placed in water for 20 days, while the coaxial structure hydrogel of Example 3 only swells by less than 2% after being immersed in water for 20 days. The coaxial structure has an excellent anti-swelling effect.

[0115] See attached Fig.14 , which is a resistance change curve of the coaxial structure sample provided by Example 1 and Example 2 of the present invention using the shell layer organic silicon photocurable resin and the core layer acrylamide hydrogel for coaxial 3D printing before and after being placed at room temperature for 14 days. It can be seen from the figure that the sensing performance of the coaxial structure sample is still stable after being placed for 14 days.

[0116] See attached Fig.15 , which is a schematic diagram of the coaxial structure hydrogel flexible sensor provided by Example 3 of the present invention. According to different bending angles of the finger, the coaxial structure hydrogel flexible sensor of Example 3 can obtain different resistance change signals.

[0117] See attached Fig.16 , which is a water loss curve of the polyvinyl alcohol hydrogel and the sodium alginate hydrogel provided in Examples 4 and 5 of the present invention. The physical cross-linked system hydrogel represented by the polyvinyl alcohol hydrogel and the ionic / covalent double cross-linked network system hydrogel represented by the sodium alginate hydrogel both have a certain water loss phenomenon.

[0118] See attached Fig.17, which is a resistance change curve of the coaxial sample in different chemical reagents for coaxial 3D printing using shell layer organic silicon photocurable resin and core layer acrylamide hydrogel provided in Examples 1 and 2 of the present invention. The change curve of the coaxial structure sample in sodium hydroxide aqueous solution (NaOH), hydrochloric acid aqueous solution (HCl), dichloromethane (CH2Cl2), and dimethyl sulfoxide aqueous solution (DMSO) is basically consistent with the change curve in deionized water. The coaxial can be used for underwater sensing and has excellent corrosion resistance.

[0119] See attached Fig.18 , which is the resistance change curve of the acrylamide hydrogel and the coaxial sample of coaxial 3D printing during heating provided by Example 2 of the present invention. The results show that the coaxial structure sample can remain stable after the temperature reaches 120°C; while the exposed hydrogel gradually changes its resistance due to evaporation at 120°C and cannot remain stable. The coaxial structure sample has excellent thermal stability.

[0120] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principle of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A hydrogel flexible sensor, characterized in that: The hydrogel flexible sensor is a core-shell material with a coaxial structure. The core-shell material uses hydrogel as a core layer material and polydimethylsiloxane resin with added fumed silica as a shell layer material.

2. The hydrogel flexible sensor according to claim 1, characterized in that: The water retention of the hydrogel flexible sensor is not less than 99% after being placed for 36 hours, preferably, not less than 99.5% after being placed for 36 hours; and / or Preferably, the water retention of the hydrogel flexible sensor is not less than 98% of the mass retention rate within 6 days, preferably not less than 97% of the mass retention rate for more than 6 days; and / or Preferably, the water retention of the hydrogel flexible sensor is that after being placed for 20 days, the mass retention rate is not less than 96%, preferably, after being placed for 20 days, the mass retention rate is not less than 96.4%; and / or preferably, the swelling resistance of the hydrogel flexible sensor in water is within 2% swelling for more than 2 days, preferably within 2% swelling for more than 10 days, and preferably within 2% swelling for more than 20 days; and / or Preferably, the performance stability period of the hydrogel flexible sensor at room temperature is up to 14 days; and / or Preferably, the heat resistance of the hydrogel flexible sensor is as high as 120°C.

3. The hydrogel flexible sensor according to claim 1, characterized in that: In the hydrogel flexible sensor, the core-shell material is integrally formed; and / or Preferably, the ratio of the core layer material to the shell layer material is (1-3): (1-5) by mass; and / or Preferably, in the shell material, the mass fraction of fumed silica is 5wt%-12wt%.

4. The hydrogel flexible sensor according to claim 1, characterized in that: In the core-shell material, the diameter ratio of the core layer material to the shell layer material is (1:2)-(1:3).

5. A method for preparing a hydrogel flexible sensor, characterized in that: The following steps are involved: S1. Preparation of shell material (1) adding a photoinitiator to a polydimethylsiloxane resin having a mercapto group on the side chain and vinyl groups at both ends in a molar ratio of mercapto to vinyl of 2:1, and dispersing and mixing once to obtain a milky white light-curable silicone ink; (2) adding fumed silica to the milky white photocurable silicone ink, performing secondary dispersion and mixing to obtain a shell paste slurry; S2. Preparation of core layer material Dissolving acrylamide in water, adding polyethylene glycol diacrylate, a photoinitiator, and an inorganic salt, heating and stirring to obtain an acrylamide hydrogel precursor solution, pouring the acrylamide hydrogel precursor solution into a mold, and irradiating with ultraviolet light to obtain an acrylamide solid hydrogel; S3. Preparation of hydrogel flexible sensors The shell paste slurry is extruded and fed from the side of the coaxial needle by air pressure control, and the acrylamide liquid hydrogel is extruded and fed from the top of the coaxial needle by mechanical pressure control. The two materials are extruded and deposited simultaneously from a discharge port. After co-extrusion, they are cured in real time by an ultraviolet light source located at the side of the coaxial needle to form a hydrogel flexible sensor with a coaxial core-shell structure.

6. The method according to claim 5, characterized in that In step S1(1), the molecular weight of the polyethylene glycol diacrylate is 400-800; and / or Preferably, in step S1(1), the amount of the photoinitiator added is 0.5wt%-3wt% of the total mass of the shell resin, preferably 2wt%; and / or Preferably, the photoinitiator comprises ethyl 2,4,6-trimethylbenzoylphenylphosphonate, phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide or 2-hydroxy-2-methyl-1-phenyl-1-propanone; and / or Preferably, the inorganic salt comprises at least one of sodium chloride, ferric chloride and lithium chloride; and / or Preferably, the primary dispersing and mixing comprises using a high-speed ball mill to uniformly disperse at a rotation speed of 2000 rpm / min-3000 rpm / min for 3-5 min.

7. The method according to claim 5, characterized in that In step S1(2), the amount of fumed silica added is 5wt%-12wt% of the total mass of the shell material. Preferably, the secondary dispersion mixing includes using a high-speed ball mill to uniformly disperse at a speed of 2000 rpm / min-3000 rpm / min for 3-5 min.

8. The method according to claim 5, characterized in that In step S2, the concentration of acrylamide dissolved in water is 20wt%-40wt% of the total mass of the core layer material; and / or Preferably, the mass ratio of acrylamide, polyethylene glycol diacrylate, photoinitiator and lithium chloride is (20wt%-40wt%): (0.2wt%-1wt%): (0.2wt% 2wt%): (1wt%-5wt%); and / or Preferably, the photoinitiator comprises 4-(2-hydroxyethoxy)phenyl 2-hydroxy-2-dipropyl ketone, phenyl bis(2,4,6-trimethylbenzoyl)phosphine oxide or 2-hydroxy-2-methyl-1-phenyl-1-propanone; and / or Preferably, the heating and stirring time is 15-30 min, and the temperature is 30-60° C.; and / or Preferably, the ultraviolet irradiation time is 1-5 min.

9. The method according to claim 5, characterized in that In step S3, the air pressure is 500 kPa-700 kPa; and / or Preferably, the mechanical pressure is 0.1 mL / min-0.5 mL / min.

10. Application of the hydrogel flexible sensor according to any one of claims 1 to 4 or prepared according to the method according to any one of claims 5 to 9 in the field.

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