An ionically conductive hydrogel, sensors, and methods of making and using the same

Abstract

The application belongs to the technical field of high polymer materials, and particularly relates to an ion conductive hydrogel, a sensor and a preparation method and application thereof. The ion conductive hydrogel comprises a double-network hydrogel and ion salts and alcohol dispersed in the double-network hydrogel; the first network of the double-network hydrogel is formed by cross-linking of sodium alginate, and the second network is formed by radical polymerization of unsaturated monomers. The second network is formed by radical polymerization of unsaturated monomers, and the double-network hydrogel is formed by cooperation of sodium alginate, so that the ion conductive hydrogel has excellent mechanical properties. The ion salts are added in the double-network hydrogel, and especially, an alcohol / water binary solvent system is introduced. Due to strong ion hydration and strong hydrogen bond interaction between water molecules and alcohol, ice crystallization is inhibited, so that the hydrogel still has good flexibility and conductivity below zero degrees.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of high polymer materials, and particularly relates to an ion-conducting hydrogel, a sensor and a preparation method and application thereof. BACKGROUND

[0002] In recent years, flexible wearable sensors have shown unique advantages in the fields of electronic skin, medical health monitoring and human-computer interaction. The development of flexible substrates has attracted widespread attention from researchers. In order to obtain high-performance flexible sensors, people try to combine metal nanomaterials, carbon nanotubes, graphene, MXene and other conductive materials with polydimethylsiloxane and other flexible substrates to prepare various flexible conductive materials. Due to the poor compatibility of these conductive materials with the flexible substrates, the conductive paths are broken under large strain, which limits their application in a large strain range.

[0003] Among them, the conductive hydrogel is a polymer material with a three-dimensional cross-linked network with a high water content. Due to its adjustable mechanical properties and good conductivity, it is gradually becoming the most promising substrate material in flexible wearable sensors. At present, in addition to being used as a strain sensor, the conductive hydrogel is also increasingly valued in temperature sensors due to its change in conductivity with temperature. The abundant water in the hydrogel can provide good conductivity and efficient ion migration, but in extreme environmental conditions, the water in the hydrogel often leads to insufficient flexibility and poor conductivity. This is because the water inside the hydrogel freezes below zero degrees and evaporates at room temperature or higher, resulting in a decrease in the elasticity and conductivity of the hydrogel, which limits the application of flexible sensors in extreme environments. Therefore, the development of anti-freezing conductive hydrogels for flexible sensors and their application in health monitoring in various environments have become the current research hotspot. SUMMARY

[0004] The present application aims to at least solve one of the technical problems existing in the prior art. To this end, the present application proposes an ion-conducting hydrogel with excellent anti-freezing and mechanical properties, a sensor based on the ion-conducting hydrogel with stress-temperature dual response function and super-high sensitivity.

[0005] The present application also proposes a preparation method and application of the ion-conducting hydrogel.

[0006] The present application also proposes a sensor with the above ion-conducting hydrogel and a preparation method and application thereof.

[0007] In the first aspect of the present application, an ion-conducting hydrogel is proposed, which comprises a double-network hydrogel and ion salts and alcohol dispersed in the double-network hydrogel; the first network of the double-network hydrogel is formed by cross-linking sodium alginate, and the second network is formed by free radical polymerization of unsaturated monomers.

[0008] According to the first aspect of the present application, at least the following advantages are achieved:

[0009] The present application forms a second network by free radical polymerization of unsaturated monomers, and forms a double network structure hydrogel with sodium alginate, and the ionic conductive hydrogel has excellent mechanical properties. Adding ionic salt in the double network hydrogel, especially introducing alcohol / water binary solvent system, due to strong ionic hydration and strong hydrogen bond interaction between water molecules and alcohol, the formation of ice crystals is inhibited, so that the hydrogel still has good flexibility and conductivity below zero, and has ultra-high sensitivity to temperature and strain.

[0010] Preferably, the alcohol includes at least one of ethylene glycol, 1,2-propanediol, glycerol, and more preferably glycerol.

[0011] Preferably, the ionic conductive hydrogel further includes water, and the mass / volume ratio of the sodium alginate to the water is 1g:10-30mL, more preferably the mass / volume ratio of the sodium alginate to the water is 1g:10-20mL; further preferably the mass / volume ratio of the sodium alginate to the water is about 1g:10mL.

[0012] Preferably, the mass of the alcohol accounts for 1-40% of the mass of the ionic conductive hydrogel, and more preferably 10-20%.

[0013] Preferably, the ionic salt includes at least one of calcium chloride, magnesium chloride, lithium chloride, sodium chloride, potassium chloride, lithium bromide, sodium citrate, and sodium nitrate.

[0014] Preferably, the mass of the ionic salt accounts for 0.1-5% of the mass of the ionic conductive hydrogel, more preferably 0.1-3%, and further preferably about 2%.

[0015] Preferably, the unsaturated monomer includes at least one of acrylamide, acrylic acid, sodium acrylate, methacrylic acid, and sodium methacrylate.

[0016] Preferably, the mass ratio of the unsaturated monomer to the sodium alginate is 1-6:1, more preferably 1-5:1, and further preferably about 2-3:1.

[0017] Preferably, the free radical polymerization of the unsaturated monomer further adds a crosslinking agent and a photoinitiator.

[0018] Preferably, the crosslinking agent includes at least one of N,N'-methylenebisacrylamide, pentaerythritol triacrylate, pentaerythritol triethylate, and polyethylene glycol diacrylate; and the molar amount of the crosslinking agent accounts for 0.05-0.4% of the total molar amount of the unsaturated monomers, and more preferably 0.1-0.2%.

[0019] Preferably, the photoinitiator comprises at least one of 2-ketoglutaric acid, 2-hydroxy-2-methylpropiophenone; the molar amount of the photoinitiator accounts for 0.01-1% of the total molar amount of the unsaturated monomers, more preferably 0.01%-0.08%, and further preferably 0.01-0.03%.

[0020] In a second aspect of the present application, a method for preparing the ion-conducting hydrogel comprises the following steps:

[0021] The sodium alginate, the unsaturated monomers, the ionic salt and the alcohol are mixed, and the ion-conducting hydrogel is obtained through cross-linking and solidification.

[0022] Preferably, water is further added during the preparation process, and the mass-volume ratio of the sodium alginate to water is 1-3 g:10 mL, and more preferably 1-2 g:10 mL.

[0023] Preferably, the cross-linking and solidification is performed through ultraviolet curing, and the curing time is 5-30 min, and more preferably 10-20 min; and the wavelength of the ultraviolet light is 350-400 nm, and more preferably about 365 nm.

[0024] Preferably, the method for preparing the ion-conducting hydrogel comprises the following steps:

[0025] S1. Dissolving sodium alginate in water to obtain a sodium alginate solution;

[0026] S2. Adding unsaturated monomers and ionic salt to the sodium alginate solution and stirring to obtain a mixed solution;

[0027] S3. Adding alcohol to the mixed solution and stirring;

[0028] S4. Adding a cross-linking agent and a photoinitiator to the solution obtained in step S3, and performing ultraviolet curing to obtain the ion-conducting hydrogel.

[0029] In a third aspect of the present application, a sensor is provided, and the sensor comprises the ion-conducting hydrogel.

[0030] In a fourth aspect of the present application, the ion-conducting hydrogel and the sensor are applied to environmental temperature monitoring, human body temperature and motion state monitoring.

[0031] Compared with the prior art, the present application has at least the following excellent effects:

[0032] 1. Through radical polymerization of unsaturated monomers, formation of a double-network hydrogel with sodium alginate, introduction of ionic salt and alcohol (such as glycerol) / water binary solvent, and utilization of strong ionic hydration and strong hydrogen bond interaction between water molecules and alcohol, ice crystal formation is inhibited, and an ion-conducting hydrogel with good anti-freezing performance and mechanical properties is obtained.

[0033] 2. The conductivity of the ion-conductive hydrogel of the present invention has ultra-high sensitivity to temperature and strain. It can not only be used to monitor different ambient temperatures, but also as a wearable strain sensor to monitor human movement.

[0034] 3. The method for preparing ion-conductive hydrogels of the present invention is simple and can be mass-produced. Attached Figure Description

[0035] The present invention will be further described below with reference to the accompanying drawings and embodiments, wherein:

[0036] Figure 1 The stress-strain comparison diagrams are for the ion-conducting hydrogels of Comparative Example 1 and Comparative Example 2.

[0037] Figure 2 This is a stress-strain comparison diagram of the ion-conductive hydrogels in Comparative Example 1, Example 1, and Example 2;

[0038] Figure 3 The following are DSC test results for the ion-conductive hydrogels of Example 1 and Comparative Example 3.

[0039] Figure 4 The conductivity change of the ion-conductive hydrogel in Example 1 within the temperature range of -30 to 80°C;

[0040] Figure 5 The sensor corresponding to Example 1 has (a) the relative resistance change rate in the temperature range of 0 to 80°C, and (b) the cyclic stability of the temperature rising from 20°C to 40°C and then to 60°C.

[0041] Figure 6 The sensor corresponding to Example 1 has (a) the relative resistance change rate in the human body temperature range of 36.5 to 40°C, and (b) the corresponding temperature sensitivity in the range of 36.5 to 40°C.

[0042] Figure 7 The sensor corresponding to Example 1 (a) has a relative resistance change rate and corresponding sensitivity within a strain range of 1973%, and (b) has undergone a load-unload cycle test within a strain range of 1% to 5%. Detailed Implementation

[0043] The following will describe the concept and technical effects of the present invention clearly and completely with reference to embodiments, so as to fully understand the purpose, features and effects of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are all within the scope of protection of the present invention.

[0044] Example 1

[0045] (I) Preparation of hydrogels:

[0046] (1) Weigh 1.0g sodium alginate and dissolve it in 10mL of deionized water, stirring thoroughly to dissolve; (2) Then add 3.0g acrylamide and 2% lithium chloride, stirring to form a uniform and transparent solution; (3) Add 10% glycerol to the above solution; (4) Add 0.2% crosslinking agent N,N'-methylenebisacrylamide and 0.03% photoinitiator 2-ketoglutaric acid to the above solution in sequence, stir quickly to remove bubbles, pour the solution into a mold, and irradiate under a UV lamp for 10min to obtain an ion-conductive hydrogel through free radical polymerization. The percentages of lithium chloride and glycerol here are the mass percentages of lithium chloride and glycerol in the aqueous solution (solution of sodium alginate, water, and acrylamide); while the percentages of crosslinking agent and initiator are the molar percentages of crosslinking agent and initiator in the unsaturated monomer (i.e., acrylamide).

[0047] (II) Fabrication of a temperature-strain dual-response sensor:

[0048] A hydrogel temperature sensor was assembled using hydrogel as the conductor and encapsulated in a polyethylene film to prevent moisture evaporation during temperature sensing tests. Different heating and cooling experiments were conducted on the hydrogel sensor using a semiconductor refrigeration controller, and the sensor's response signal was recorded using an electrochemical workstation.

[0049] Example 2

[0050] The main difference compared to Example 1 is the different glycerol ratio, specifically:

[0051] (1) Weigh 1.0g of sodium alginate and dissolve it in 10mL of deionized water, stirring thoroughly until dissolved; (2) Then add 3.0g of acrylamide and 2% lithium chloride, stirring to form a uniform and transparent solution; (3) Add 20% glycerol to the above solution; (4) Add 0.2% of the crosslinking agent N,N'-methylenebisacrylamide and 0.03% of the photoinitiator 2-ketoglutaric acid to the above solution in sequence, stir rapidly to remove bubbles, pour the solution into a mold, and irradiate under a UV lamp for 10min to obtain an ion-conductive hydrogel through free radical polymerization. The percentages of lithium chloride and glycerol here are the mass percentages of lithium chloride and glycerol in the aqueous solution, respectively; while the percentages of crosslinking agent and initiator are the molar percentages of crosslinking agent and initiator in the unsaturated monomer (i.e., acrylamide).

[0052] Comparative Example 1

[0053] Compared to Example 1, the difference is that it does not contain glycerin, specifically:

[0054] (1) Weigh 1.0g of sodium alginate and dissolve it in 10mL of deionized water, stirring thoroughly to dissolve; (2) Then add 3.0g of acrylamide and 2% lithium chloride, stirring to form a uniform and transparent solution; (3) Add 0.2% of the crosslinking agent N,N'-methylenebisacrylamide and 0.03% of the photoinitiator 2-ketoglutaric acid to the above solution in sequence, stir quickly to remove bubbles, pour the solution into a mold, and irradiate under a UV lamp for 10min to obtain an ion-conductive hydrogel through free radical polymerization. The percentage content of lithium chloride here refers to the mass percentage of lithium chloride in the aqueous solution; while the percentage content of crosslinking agent and initiator refers to the molar percentage of crosslinking agent and initiator in the unsaturated monomer (i.e., acrylamide).

[0055] Comparative Example 2

[0056] Compared with Comparative Example 1, the difference lies in the proportion of crosslinking agent, specifically:

[0057] (1) Weigh 1.0g of sodium alginate and dissolve it in 10mL of deionized water, stirring thoroughly to dissolve; (2) Then add 3.0g of acrylamide and 2% lithium chloride, stirring to form a uniform and transparent solution; (3) Add 0.1% of the crosslinking agent N,N'-methylenebisacrylamide and 0.03% of the photoinitiator 2-ketoglutaric acid to the above solution in sequence, stir quickly to remove bubbles, pour the solution into a mold, and irradiate under a UV lamp for 10min to obtain an ion-conductive hydrogel through free radical polymerization. The percentage of lithium chloride here is the mass percentage of lithium chloride in the aqueous solution; while the percentage of crosslinking agent and initiator is the molar percentage of crosslinking agent and initiator in the unsaturated monomer (i.e., acrylamide).

[0058] Comparative Example 3

[0059] Compared to Example 1, the difference lies in the absence of glycerol and ionic salts, specifically:

[0060] (1) Weigh 1.0g of sodium alginate and dissolve it in 10mL of deionized water, stirring thoroughly until dissolved; (2) Then add 3.0g of acrylamide and stir to form a uniform and transparent solution; (3) Add 0.2% of the crosslinking agent N,N'-methylenebisacrylamide and 0.03% of the photoinitiator 2-ketoglutarate to the above solution in sequence, stir rapidly to remove bubbles, pour the solution into a mold, and irradiate under a UV lamp for 10min to obtain an ion-conductive hydrogel through free radical polymerization. The percentage content of crosslinking agent and initiator here refers to the molar percentage of crosslinking agent and initiator to unsaturated monomer (i.e., acrylamide).

[0061] Test case

[0062] This experimental example tested the properties of the hydrogels prepared in the examples and comparative examples.

[0063] 1. Properties of ion-conducting hydrogels

[0064] (1) Mechanical properties

[0065] Depend on Figure 1 The tensile strength of Comparative Example 1 is greater than that of Comparative Example 2, while the elongation at break is less than that of Comparative Example 2. This indicates that the tensile strength of the hydrogel increases while the elongation at break decreases with the increase of the crosslinking agent content. This is because with the increase of the crosslinking agent content, the ion-conductive hydrogel forms a higher crosslinking density, resulting in greater tensile strength, while reducing the stretchability of the chain segments.

[0066] Figure 2 Comparative Example 1, Example 1, and Example 2 illustrate that increasing the glycerol content significantly reduces tensile strength and Young's modulus. This is because the plasticizing behavior of glycerol weakens the van der Waals forces between polymer molecules (i.e., the hydrogen bonds between glycerol and the polymer chain), reducing the formation of hydrogen bonds between polymer molecules, thereby leading to a decrease in the tensile properties of the hydrogel.

[0067] (2) DSC

[0068] The antifreeze ability of the hydrogels prepared in Example 1 and Comparative Example 3 was demonstrated by DSC testing, and the results are as follows: Figure 3 As shown, the hydrogel of Comparative Example 3 exhibited a crystallization peak at -5.5℃, while the hydrogel of Example 1 showed no crystallization peak after the addition of 2wt% LiCl and 10% glycerol. The results indicate that the introduction of LiCl and glycerol effectively improves the low-temperature stability of the hydrogel and inhibits ice crystal formation, demonstrating that the hydrogel of Example 1 has good antifreeze properties.

[0069] (3) Conductivity

[0070] like Figure 4 As shown, the conductivity of the hydrogel prepared in Example 1 increases with increasing temperature. This is because the hydrogel is an ionic conductor, and its conductivity is mainly related to the directional movement of anions and cations. On the one hand, the ion migration rate increases with increasing temperature; on the other hand, the polymer chain segments move faster, forming cavities and providing more channels for ion movement. Therefore, ions move faster at higher temperatures, thus increasing conductivity. Furthermore, the addition of LiCl and glycerol gives Example 1 good antifreeze properties, ensuring ion migration at -30°C. The results indicate that the hydrogel obtained in Example 1 is suitable for monitoring a wide range of ambient temperatures.

[0071] 2. Temperature sensor

[0072] (1) Rate of change of resistivity in the range of 0~80℃

[0073] like Figure 5As shown in (a), within the temperature range of 0–80°C, during the heating process, the relative resistance change rate of the temperature sensor corresponding to Example 1 gradually increases and remains stable at a fixed temperature, indicating that it has good sensitivity to temperature response. This is because the temperature sensor promotes the movement of polymer chain segments and ions during the heating process, leading to an increase in the relative resistance change rate. The temperature response of the sensor in Example 1 during the heating-cooling cycle in the temperature range of 20°C–40°C–60°C is as follows: Figure 5 As shown in (b), it demonstrates good cyclic stability.

[0074] (2) Rate of change of resistivity within the human body temperature range

[0075] like Figure 6 As shown, within the human body temperature range of 36.5–40℃, the resistance change rate of the temperature sensor increases with increasing temperature, and its temperature sensitivity is as high as 5.51% / ℃. This indicates that the temperature sensor can be effectively used to monitor changes in human body temperature to characterize human health status.

[0076] (3) Strain sensitivity

[0077] like Figure 7 As shown in (a), the hydrogel sensor exhibits a gradually increasing rate of resistance change with increasing strain over a wide strain range of 0–1973%, achieving a sensitivity as high as 17.3 within the large strain range of 1000%. Figure 7 As shown in (b), it can detect strains as low as 1% and exhibits excellent cyclic stability within the 1%–5% strain range. This demonstrates that the hydrogel sensor possesses high sensitivity and excellent cyclic stability, making it well-suited for detecting human motion.

[0078] The embodiments of the present invention have been described in detail above with reference to the accompanying drawings. However, the present invention is not limited to the above embodiments, and various changes can be made within the scope of knowledge possessed by those skilled in the art without departing from the spirit of the present invention. Furthermore, the embodiments of the present invention and the features thereof can be combined with each other unless otherwise specified.

Claims

1. An ion-conducting hydrogel, characterized in that, The ion-conducting hydrogel comprises a dual-network hydrogel and ion salts and alcohols dispersed in the dual-network hydrogel; the first network of the dual-network hydrogel is formed by cross-linking sodium alginate, and the second network is formed by unsaturated monomer free radical polymerization. The alcohol accounts for 10% of the mass of the ion-conducting hydrogel; the mass ratio of the unsaturated monomer to sodium alginate is 2-3:1; the ion salt accounts for 2-3% of the mass of the ion-conducting hydrogel; the unsaturated monomer free radical polymerization also includes a crosslinking agent and a photoinitiator; the molar amount of the crosslinking agent accounts for 0.1-0.2% of the total molar amount of the unsaturated monomer; the molar amount of the photoinitiator accounts for 0.01-1% of the total molar amount of the unsaturated monomer. The ion-conducting hydrogel is prepared by a method comprising the following steps: S1, Dissolve sodium alginate in water to obtain sodium alginate solution; S2, add unsaturated monomers and ionic salts to the sodium alginate solution and stir to obtain a mixed solution; S3, Add the alcohol to the mixture and stir; S4, add a crosslinking agent and a photoinitiator to the solution in step S3, and cure with ultraviolet light to obtain the ion-conductive hydrogel; The alcohol is glycerol; the ionic salt is lithium chloride; the unsaturated monomer is acrylamide; The crosslinking agent is selected from N,N' At least one of methylene bisacrylamide, pentaerythritol triacrylate, pentaerythritol triethyl acrylate, and polyethylene glycol diacrylate; The photoinitiator is selected from 2 Ketoglutaric acid, 2 hydroxyl 2 At least one of methyl acetone.

2. A sensor, characterized in that, The sensor comprises the ion-conducting hydrogel as described in claim 1.

3. The application of the ion-conductive hydrogel of claim 1 or the sensor of claim 2 in environmental temperature monitoring.

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