A hydrogel and a preparation method and application thereof

Abstract

This invention relates to a hydrogel, its preparation method, and its applications. The preparation method includes the following steps: S1. Mixing polyvinyl alcohol, graphene oxide, and a solvent to obtain a GO-PVA solution; S2. Mixing the GO-PVA solution with a temperature-sensitive polymer solution and freezing to obtain a hydrogel precursor; S3. Immersing the hydrogel precursor in a sodium salt solution to obtain the hydrogel. This invention, by immersing the hydrogel precursor in a sodium salt solution of a certain concentration, produces a flexible sensor made from the hydrogel that exhibits both temperature and pH responsiveness. Furthermore, immersing the hydrogel precursor in a sodium salt solution of a certain concentration further improves the temperature sensitivity of the flexible sensor made from the hydrogel.

Description

Technical Field

[0001] This invention relates to the field of hydrogel sensor technology, and more specifically, to a hydrogel, its preparation method, and its application. Background Technology

[0002] Body temperature and sweat pH are important indicators of human health. On one hand, body temperature reflects the body's metabolic state; for every 1°C drop in body temperature, the basal metabolic rate decreases by 6%–7%. When room temperature drops, the body increases its basal metabolic rate to generate more heat to maintain a stable body temperature. Therefore, monitoring body temperature can assess whether the body's metabolic function is normal. On the other hand, changes in sweat pH (acidity / alkalinity) not only indicate whether the body is dehydrated but can also help diagnose skin-related symptoms. Flexible sensors, with their ability to acquire important biological information from the human body (such as body temperature or sweat pH), are currently widely used in the field of flexible wearable electronics.

[0003] To meet the demand for multifunctionality, temperature sensing and pH sensing functions need to be integrated into flexible wearable electronic devices. However, most resistive sensors currently only have a single temperature sensing function or a single pH sensing function. Integrating both resistive temperature and resistive pH sensors into flexible wearable electronic devices would complicate the design and manufacturing of the devices. Furthermore, sensors that simultaneously provide temperature and pH responses are currently rare.

[0004] A Chinese patent describes a thermosensitive hydrogel, its preparation method, and its application. The thermosensitive hydrogel is prepared by adhering poly(N-isopropylacrylamide) and polyvinyl alcohol (PVA) to the surface of graphene oxide (GO), and then applied to a resistive flexible temperature sensor. However, the temperature sensor made from this thermosensitive hydrogel lacks pH-responsive characteristics. Furthermore, the temperature sensitivity of the temperature sensor made from this thermosensitive hydrogel needs further improvement.

[0005] Therefore, it is necessary to develop a hydrogel that combines temperature and pH response characteristics to meet its application in sensors that simultaneously possess temperature and pH response properties. Summary of the Invention

[0006] The primary objective of this invention is to overcome the limitation of existing hydrogel-based resistive flexible sensors, which only exhibit single temperature or pH response characteristics, and to provide a method for preparing hydrogel materials. This invention involves immersing a hydrogel precursor in a sodium salt solution of a specific concentration, resulting in a hydrogel-based flexible sensor that possesses both temperature and pH response characteristics. Furthermore, immersing the hydrogel precursor in a sodium salt solution of a specific concentration further enhances the temperature sensitivity of the resulting hydrogel-based flexible sensor.

[0007] A further object of the present invention is to provide a hydrogel.

[0008] Another object of the present invention is to provide the application of the above-mentioned hydrogel in the fabrication of flexible sensors.

[0009] Another object of the present invention is to provide a flexible sensor.

[0010] The above-mentioned objective of this invention is achieved through the following technical solution:

[0011] A method for preparing a hydrogel includes the following steps:

[0012] S1. Mix polyvinyl alcohol, graphene oxide and solvent to obtain GO-PVA solution;

[0013] S2. The GO-PVA solution is mixed with a thermosensitive polymer solution and frozen to obtain a hydrogel precursor;

[0014] S3. The hydrogel precursor is soaked in a sodium salt solution to obtain the hydrogel;

[0015] In step S2, the mass ratio of polyvinyl alcohol in the GO-PVA solution, graphene oxide in the GO-PVA solution, and thermosensitive polymer in the thermosensitive polymer solution is 1:(0.015~0.040):(0.8~1.2);

[0016] The lower critical dissolution temperature of the thermosensitive polymer in the thermosensitive polymer solution is 30-50°C.

[0017] The polyvinyl alcohol is 1750±50;

[0018] The sodium ion concentration in the sodium salt solution is 0.5–1.5 mol / L.

[0019] In this invention, the selected thermosensitive polymer has a low critical dissolution temperature of 30-50°C. This low critical dissolution temperature range is close to the physiological temperature range of the human body. Hydrogels made from thermosensitive polymers with this low critical dissolution temperature range are suitable for the field of flexible wearable electronics.

[0020] In their previous research (CN117720747A), the inventors of this invention discovered that by adjusting the ratio of graphene oxide, PVA, and thermosensitive polymers, as well as by using specific combinations of PVA, the sensitivity of a thermosensitive hydrogel-based resistive flexible temperature sensor to temperature response can be improved.

[0021] The inventors of this invention further discovered that immersing the hydrogel precursor (i.e., the thermosensitive hydrogel of CN117720747A) in a sodium salt solution of a certain concentration resulted in a flexible sensor made from the hydrogel exhibiting excellent pH response characteristics. This may be because, with changes in pH, the -COOH functional groups in graphene oxide ionize (forming -COO₂). - The number of ) is different, while -COO - with Na + There is a strong attraction between them, and the probability of them colliding changes, thus causing Na... + Changes in motion speed lead to changes in the electrical resistance of the hydrogel. For example, as the pH increases, the -COOH functional groups in graphene oxide gradually ionize to form -COO groups. - The quantity increases, -COO - with Na + The probability of collision increases, Na + The decrease in motion speed leads to an increase in the resistance of the hydrogel, thus the hydrogel prepared by this invention has pH-responsive properties.

[0022] Furthermore, the inventors of this invention have discovered through research that immersing the hydrogel precursor in a sodium salt solution of a certain concentration further improves the temperature sensitivity of the flexible sensor made from the hydrogel. The principle is as follows: as the temperature changes, the presence of sodium ions causes a more significant change in the resistance of the hydrogel, thus improving the temperature sensitivity of the flexible sensor made from the hydrogel obtained by this invention.

[0023] The polyvinyl alcohol 1750±50 used in this invention is commercially available and can be purchased from companies such as Shanghai Yuanye Biotechnology Co., Ltd., Aladdin Reagent (Shanghai) Co., Ltd., and Shanghai Maclean Biochemical Technology Co., Ltd.

[0024] Preferably, in step S1, the degree of oxidation of the carboxyl groups in the graphene oxide is 0.05% to 0.25%. The degree of oxidation of the carboxyl groups in the graphene oxide can be determined with reference to the literature "Determination of Carboxyl Functional Group Content in Graphene Oxide by Infrared Spectroscopy - Gong Shuishui".

[0025] Preferably, in step S1, the mixing process is as follows: graphene oxide and solvent are mixed to obtain a dispersion, and then polyvinyl alcohol is added to the dispersion for dissolution.

[0026] More preferably, the dissolution process involves stirring at 90–120°C for 0.5–1 hour.

[0027] Preferably, in step S1, the mass-to-volume ratio of graphene oxide to solvent in the GO-PVA solution is (0.015~0.040)g:5mL.

[0028] Preferably, in step S1, the mass-to-volume ratio of polyvinyl alcohol and solvent in the GO-PVA solution is (0.9-1.2) g: 5 mL.

[0029] Preferably, in step S1, the solvent is water.

[0030] Preferably, in step S2, the number average molecular weight of the thermosensitive polymer is 200,000 to 400,000.

[0031] Preferably, in step S2, the thermosensitive polymer is one or both of poly(N-isopropylacrylamide) or poly(N-vinylcaprolactam).

[0032] More preferably, the thermosensitive polymer is poly(N-isopropylacrylamide).

[0033] Preferably, in step S2, the mass-to-volume ratio of the thermosensitive polymer to the solvent in the thermosensitive polymer solution is (0.9-1.2) g: 5 mL.

[0034] Preferably, in step S2, the preparation process of the thermosensitive polymer solution is as follows: the thermosensitive polymer and the solvent are mixed and placed at -5 to 2°C for 24 to 36 hours.

[0035] More preferably, the solvent is water.

[0036] Preferably, in step S2, the temperature for freeze-forming is -20 to -30°C.

[0037] Preferably, in step S2, the freezing time is 24 to 36 hours.

[0038] Preferably, in step S2, the lower critical dissolution temperature of the thermosensitive polymer in the thermosensitive polymer solution is 30-40°C.

[0039] Preferably, in step S3, the sodium ion concentration in the sodium salt solution is 0.8–1.5 mol / L. Within this sodium ion concentration range, the resulting flexible sensor exhibits better temperature sensitivity.

[0040] Preferably, in step S3, the sodium salt is an inorganic sodium salt.

[0041] More preferably, the inorganic sodium salt is at least one of sodium chloride, sodium sulfate, or sodium carbonate.

[0042] More preferably, the inorganic sodium salt is sodium chloride.

[0043] The soaking time described in step S3 can be controlled according to the actual situation, as long as the sodium ion content in the hydrogel no longer changes.

[0044] Preferably, in step S3, the soaking time is 12 to 36 hours.

[0045] Preferably, in step S3, the soaking is carried out at 15–30°C.

[0046] A hydrogel was prepared by the above-described preparation method.

[0047] This invention also protects the application of the above-mentioned hydrogel in the fabrication of flexible sensors.

[0048] Preferably, the flexible sensor is a flexible sensor with both temperature and pH response.

[0049] Preferably, the flexible sensor is a resistive flexible sensor.

[0050] A flexible sensor comprising the aforementioned hydrogel.

[0051] Preferably, the flexible sensor further includes a flexible substrate and a metal electrode electrically connected to the hydrogel.

[0052] The above-mentioned flexible sensor fabrication method includes the following steps: two metal electrodes are respectively disposed at both ends above a flexible substrate, and the above-mentioned hydrogel is placed above the two metal electrodes and in contact with the two metal electrodes to obtain the flexible sensor.

[0053] Preferably, the metal electrode is prepared by coating both ends of the flexible substrate with a conductive paste.

[0054] More preferably, the conductive paste is at least one of conductive gold paste, conductive silver paste, or conductive copper paste.

[0055] Preferably, the flexible substrate is made of one or both of polyethylene terephthalate or polyimide.

[0056] Compared with the prior art, the beneficial effects of the present invention are:

[0057] This invention describes a flexible sensor made from a hydrogel obtained by immersing a hydrogel precursor in a sodium salt solution of a certain concentration. The sensor exhibits both temperature and pH responsiveness. Furthermore, immersing the hydrogel precursor in a sodium salt solution of a certain concentration further improves the temperature sensitivity of the flexible sensor. Attached Figure Description

[0058] Figure 1 This is a schematic diagram of the flexible sensor structure.

[0059] Figure 2 The graph shows the relative resistance of the flexible sensors made from hydrogels in Examples 1-3 as a function of temperature.

[0060] Figure 3 The graph shows the relative resistance of the flexible sensor made of the comparative hydrogel as a function of temperature, which is the comparison example 1.

[0061] Figure 4 The graph shows the relative resistance of the flexible sensor prepared from the hydrogel of Example 1 and the comparative hydrogel of Comparative Example 1 as a function of pH value. Detailed Implementation

[0062] In this invention, the technical features described in an open-ended manner include both closed-ended technical solutions composed of the listed features and open-ended technical solutions that include the listed features. In this invention, numerical intervals (i.e., numerical ranges) are involved. Unless otherwise specified, the distribution of selectable numerical values ​​within the numerical interval is considered continuous, and includes the two endpoints of the numerical interval (i.e., the minimum and maximum values), as well as every numerical value between these two endpoints. Unless otherwise specified, when the numerical interval only refers to integers within the numerical interval, it includes the two endpoint integers of the numerical range, as well as every integer between the two endpoints, which is equivalent to directly listing every integer. When multiple numerical ranges are provided to describe features or characteristics, these numerical ranges can be merged. In other words, unless otherwise specified, the numerical ranges disclosed herein should be understood to include any and all subranges included therein. The "numerical value" in the numerical interval can be any quantitative value, such as a number, percentage, ratio, etc. The "numerical interval" can broadly include numerical interval types such as percentage intervals, ratio intervals, and proportion intervals.

[0063] The reagents used in the various embodiments and comparative examples of this invention are described below:

[0064] Polyvinyl alcohol 1750±50: Shanghai Yuanye Biotechnology Co., Ltd., S30196-500g;

[0065] Graphene oxide: Nanjing Nanotechnological Technology Co., Ltd., Model: XF002-2, Single-layer graphene oxide powder;

[0066] Poly(N-isopropylacrylamide): Shanghai Maclean Biochemical Technology Co., Ltd., number average molecular weight ~300,000.

[0067] Example 1

[0068] This embodiment provides a method for preparing a hydrogel, including the following steps:

[0069] 1) Mix 5 mL of deionized water and 36 mg of graphene oxide and sonicate for 0.5 h to obtain a dispersion; then add 1 g of polyvinyl alcohol to the dispersion and stir at 120 °C for 0.5 h to dissolve it to obtain a GO-PVA solution.

[0070] 2) Mix 5 mL of deionized water and 1 g of poly(N-isopropylacrylamide) and let stand at 0 °C for 24 h to obtain a poly(N-isopropylacrylamide) solution;

[0071] 3) Mix the GO-PVA solution from step 1) and the poly(N-isopropylacrylamide) solution from step 2) evenly at room temperature, pour into a culture dish, and freeze at -20℃ for 24 hours to form a hydrogel precursor.

[0072] 4) Immerse the hydrogel precursor from step 3) in a 1 mol / L NaCl solution, and remove it from the NaCl solution to obtain the hydrogel; wherein the immersion temperature is 25℃ and the immersion time is 24h.

[0073] Example 2

[0074] This embodiment provides a method for preparing a hydrogel. The difference from Embodiment 1 is that the concentration of the NaCl solution in step 4) is 0.5 mol / L.

[0075] Example 3

[0076] This embodiment provides a method for preparing a hydrogel. The difference from Embodiment 1 is that the concentration of the NaCl solution in step 4) is 1.5 mol / L.

[0077] Example 4

[0078] This embodiment provides a method for preparing a hydrogel. The difference from Embodiment 1 is that in step 4), the 1 mol / L NaCl solution is replaced with a 0.5 mol / L sodium sulfate solution.

[0079] Example 5

[0080] This embodiment provides a method for preparing a hydrogel. The difference from Embodiment 1 is that the amount of graphene oxide used in step 1) is 24 mg; and the amount of poly(N-isopropylacrylamide) used in step 2) is 0.8 g.

[0081] Comparative Example 1

[0082] This comparative example provides a method for preparing a control hydrogel, which differs from Example 1 in that step 4) is omitted. That is, the control hydrogel in this comparative example is the hydrogel precursor obtained in step 3) of Example 1.

[0083] Comparative Example 2

[0084] This comparative example provides a method for preparing a comparative hydrogel, which differs from Example 1 in that the concentration of the NaCl solution in step 4) is 2 mol / L.

[0085] Performance and Testing

[0086] Flexible sensors were fabricated using the hydrogels from each embodiment and the comparative hydrogel from Comparative Example 1. The fabrication method of the flexible sensors included the following steps:

[0087] Two silver electrodes are obtained by coating conductive silver paste at both ends on a flexible polyethylene terephthalate substrate. Then, a hydrogel is placed on top of and in contact with the two silver electrodes to form a flexible sensor. Figure 1 As shown in the figure, 1 is a flexible polyethylene terephthalate substrate, 2 and 3 are two silver electrodes, and 4 is a hydrogel.

[0088] 1. Temperature test

[0089] The two silver electrodes of the flexible sensor are connected to the positive and negative interfaces of a digital bridge. The hydrogel is then placed in contact with heated ceramic sheets at different temperatures. The resistance of the flexible sensor at different temperatures is measured, and the temperature sensitivity is calculated using the formula shown below:

[0090] S = (ΔR / R0) / ΔT;

[0091] Where S is the sensitivity, R0 is the initial resistance, ΔR is the relative change in resistance, and ΔT is the relative change in temperature.

[0092] 2. pH test

[0093] The two silver electrodes of the flexible sensor were connected to the positive and negative interfaces of the digital bridge. 100 μL of buffer solution with different pH values ​​was dropped onto the hydrogel to test the resistance of the flexible sensor at different pH values.

[0094] Figure 2 The graph shows the relative resistance of the flexible sensors made from hydrogels in Examples 1-3 as a function of temperature. Figure 3 The graph shows the relative resistance of the flexible sensor fabricated from the hydrogel in Comparative Example 1 as a function of temperature. Figure 2 It can be seen that the sensitivities of the flexible sensors in Examples 1-3 are 2.23%℃- 1 1.27%℃- 1 and 1.49%℃- 1 And from Figure 3 It can be seen that the sensitivity of the flexible sensor in Comparative Example 1 is 0.96%℃- 1The temperature sensitivity of the flexible sensors prepared by the hydrogels in Examples 4 and 5 is similar to that in Example 1. Therefore, it can be seen that the preparation method of the present invention, through soaking in a sodium salt solution, can improve the temperature response sensitivity of the flexible sensors prepared by the hydrogels.

[0095] Figure 4 This is a graph showing the relative resistance of the flexible sensors fabricated from the hydrogels of Example 1 and Comparative Example 1 as a function of pH. Figure 4 It can be seen that the resistance change rate of the flexible sensor prepared in Example 1 increases significantly with increasing pH value. In contrast, the resistance change rate of the flexible sensor prepared with the comparative hydrogel in Comparative Example 1 shows almost no significant change. The pH test results of the flexible sensors prepared in other examples are similar to those of Example 1, indicating that the flexible sensor prepared with the hydrogel of the present invention has pH-responsive characteristics.

[0096] In step 4) of Comparative Example 2, the sodium ion concentration in the sodium salt solution was too high. Since sodium ions have a strong hydration capacity, an excessively high sodium ion concentration would disrupt the interaction between poly(N-isopropylacrylamide) and polyvinyl alcohol and water molecules in the hydrogel, thereby damaging the internal structure of the hydrogel and preventing the hydrogel from improving its temperature response sensitivity.

[0097] Obviously, the above embodiments of the present invention are merely examples for clearly illustrating the present invention, and are not intended to limit the implementation of the present invention. Those skilled in the art can make other variations or modifications based on the above description. It is neither necessary nor possible to exhaustively describe all embodiments here. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the claims of the present invention.

Claims

1. A method for preparing a hydrogel, characterized by, The method comprises the following steps: S1. mixing polyvinyl alcohol, graphene oxide and solvent to obtain a GO-PVA solution; S2. mixing the GO-PVA solution with a temperature-sensitive polymer solution, freeze forming to obtain a hydrogel precursor; S3. soaking the hydrogel precursor in a sodium salt solution to obtain the hydrogel; In step S1, the carboxyl group in the graphene oxide corresponds to an oxidation degree of 0.05% to 0.25%; In step S2, the mass ratio of polyvinyl alcohol in the GO-PVA solution, graphene oxide in the GO-PVA solution and temperature-sensitive polymer in the temperature-sensitive polymer solution is 1:(0.015-0.040):(0.8-1.2); The low critical solution temperature of the temperature-sensitive polymer in the temperature-sensitive polymer solution is 30-50℃; The polyvinyl alcohol is polyvinyl alcohol 1750±50; The sodium ion concentration in the sodium salt solution is 0.5-1.5 moL / L.

2. The preparation method according to claim 1, characterized in that, In step S1, the mass-volume ratio of graphene oxide and solvent in the GO-PVA solution is (0.015-0.040) g:5 mL.

3. The preparation method according to claim 1, characterized in that, In step S2, the temperature-sensitive polymer is one or both of poly(N-isopropyl acrylamide) and poly N-vinyl caprolactam.

4. The preparation method according to claim 1, characterized in that, In step S3, the sodium salt is an inorganic sodium salt.

5. The method of claim 1, wherein the step of forming the first and second layers is performed by a method selected from the group consisting of: sputtering, evaporation, and chemical vapor deposition. In step S3, the soaking time is 12-36 h.

6. A hydrogel, characterized by, Prepared by the preparation method of any one of claims 1-5.

7. Use of the hydrogel of claim 6 in the preparation of a flexible sensor.

8. The use according to claim 7, characterized in that, The flexible sensor is a flexible sensor with temperature response and pH response.

9. A flexible sensor, characterized by The flexible sensor comprises the hydrogel of claim 6.

10. The flexible sensor of claim 9, wherein, The flexible sensor further comprises a flexible substrate and a metal electrode electrically connected to the hydrogel. The flexible sensor further comprises a flexible substrate and a metal electrode electrically connected to the hydrogel.

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