CN109490380B

CN109490380B - Humidity sensor, application and equipment thereof, and preparation method of humidity sensor

Info

Publication number: CN109490380B
Application number: CN201811614121.XA
Authority: CN
Inventors: 吴进; 吴子轩
Original assignee: National Sun Yat Sen University
Current assignee: National Sun Yat Sen University
Priority date: 2018-12-27
Filing date: 2018-12-27
Publication date: 2020-08-25
Anticipated expiration: 2038-12-27
Also published as: CN109490380A

Abstract

The invention provides a humidity sensor, application thereof, equipment and a preparation method of the humidity sensor.

Description

Humidity sensor, application and equipment thereof, and preparation method of humidity sensor

Technical Field

The invention relates to the technical field of sensors, in particular to a humidity sensor, application and equipment thereof and a preparation method of the humidity sensor.

Background

The flexible stretchable sensor can be closely attached to a curved surface with a complex structure, such as the skin of a human body, a robot or a prosthetic limb, to acquire various information, and therefore, the flexible stretchable sensor has a wide application prospect in a plurality of fields such as electronic skin, wearable electronics and health care in recent years. It is desirable that a flexible sensor that is attached to the skin of a human body have the function of being stretchable and self-healing like the skin of a human. The stretchability allows the flexible sensor to move with the skin of the person so that the person does not feel uncomfortable. The self-repairing property enables the electronic device to self-repair the electrical, mechanical and other properties of the electronic device when the electronic device is cracked, scratched or exceeds the limit deformation range, thereby prolonging the service life of the device. The flexible stretchable humidity sensor has wide application requirements in the fields of health care, environmental monitoring and the like. For example, applying a flexible moisture sensor to the skin of a person can measure the moisture of the skin of the person to assess the physiological health characteristics of the person, metabolic conditions, or the effectiveness of skin care products. Information on wound healing can be obtained by measuring the moisture at the wound site. The humidity sensor can also be used for measuring the respiratory rate of a human body and detecting diseases such as sleep respiratory interruption and the like.

The traditional humidity sensitive material comprises graphene and MoS₂Carbon nano-Carbons (CNTs), porous ceramics, conductive polymers, etc., which are inferior in stretchability. The flexible/stretchable humidity sensors reported at present integrate the traditional humidity sensitive materials on the flexible/stretchable substrate through structural process regulation so as to realize the flexible/stretchable function. However, this method is complicated and does not allow or only allows very limited stretching rates. The maximum tensile strain of the currently reported humidity sensors is only 60%. Therefore, how to obtain a stretchable humidity sensor through a simple preparation method is a problem to be urgently solved.

Disclosure of Invention

Based on this, there is a need for a humidity sensor, its application, an apparatus and a method for manufacturing a humidity sensor.

The invention provides a humidity sensor, which comprises a gel, wherein the gel comprises a polymer network, a solvent combined with the polymer network to form the gel together, and an electrolyte salt dissolved in the solvent.

In one embodiment, the polymer network has functional groups for bonding with water molecules to form hydrogen bonds.

In one embodiment, the functional group is a hydrophilic group and comprises-NH₂、SO₃ ^-and-OH, preferably comprising-NH₂、SO₃ ^-and-OH.

In one embodiment, the solvent comprises water and optionally an organic solvent, and the organic solvent is preferably a small molecular polyol with 1-10 carbon atoms, and is more preferably at least one of glycerol and ethylene glycol.

In one embodiment, the gel is a double-network gel comprising a first polymer network and a second polymer network.

In one embodiment, the first polymer network is used for coating the solvent, the second polymer network is used for enhancing the mechanical strength of the gel, preferably, the mass ratio of the first polymer network to the second polymer network is 16: 1-1.3: 1, preferably, the first polymer network is polyacrylamide, and the second polymer network is carrageenan.

In one embodiment, the electrolyte salt includes at least one of potassium chloride, calcium chloride, and sodium chloride, preferably calcium chloride.

In one embodiment, the gel has transparency and the gel has a tensile strain greater than 1200%.

In one embodiment, the gel further comprises an electrode for measuring a parameter capable of reflecting the ion migration rate of the electrolyte salt in the gel, preferably, the parameter is the ion conductivity or ion resistivity of the electrolyte salt, or the resistance of the gel.

In one embodiment, the device further comprises a detection device of the parameter, electrically connected with the electrode and used for measuring the parameter through the electrode.

The invention also provides an application of the humidity sensor, wherein the humidity sensor is used for detecting relative humidity change in the environment through the change of the ion migration rate of the electrolyte salt in the gel.

In one embodiment, the water molecules are capable of hydrogen bonding with functional groups of the polymer network.

The invention also provides equipment which is a wearable humidity sensing device, a humidity sensing electronic skin, a human-computer interface, a flexible robot or medical equipment, and the equipment comprises the humidity sensor.

In one embodiment, the device further comprises a flexible substrate and/or a prompter, wherein the humidity sensor is arranged on the surface of the flexible substrate in a stacking mode, and the prompter gives out a prompt when the humidity sensor senses that the relative humidity exceeds or falls below a set value range.

The invention also provides a preparation method of the humidity sensor, which comprises the following steps:

uniformly mixing a monomer, a cross-linking agent, an initiator, a second polymer, an electrolyte salt and a solvent under a heating condition to obtain a mixed solution, wherein the monomer, the cross-linking agent and the initiator are used for forming a first polymer network, and the second polymer and the solvent can form a gel;

and carrying out cross-linking polymerization reaction on the monomer, the cross-linking agent and the initiator in the mixed solution to form the first polymer network, and then cooling to form the second polymer network to obtain the gel.

In one embodiment, when the solvent is water, the method further comprises soaking the gel in an organic solvent, wherein the organic solvent is preferably a small molecular polyol with 1-10 carbon atoms, and is more preferably at least one of glycerol and ethylene glycol.

According to the humidity sensor provided by the invention, by utilizing the characteristics of the gel, water molecules are adsorbed and dissolved in the solvent of the gel to influence the concentration of the polymer, and the change of the concentration of the polymer influences the migration of anions and cations of electrolyte salt in the gel, so that an electrical signal is generated, and the response of the gel to humidity is formed. The gel is used as the humidity sensor, so that the sensor has excellent stretchability and flexibility, and can be widely applied to wearable devices.

Drawings

FIG. 1a is a schematic diagram of a one-pot synthesis process of a double-network gel according to an embodiment of the present invention;

FIG. 1b is a schematic diagram of water molecule adsorption by functional groups in the double-network gel according to the embodiment of the present invention;

FIG. 2 is a schematic view of the combination of glycerol, ethylene glycol, and calcium chloride with water molecules in a double-network gel according to an embodiment of the present invention;

FIG. 3 is a schematic diagram of charge transfer of a dual network gel as a humidity sensor according to an embodiment of the present invention;

FIG. 4 is a graph showing the flexibility and tensile properties of a double-network gel according to an embodiment of the present invention;

FIG. 5 is a graph showing the anti-freezing effect of the dual-network gel after being stored at-18 ℃ in the embodiment of the invention;

FIG. 6 is a self-repairing characteristic test chart of the double-network gel according to the embodiment of the present invention;

FIG. 7 is a graph of optical transmittance test data for a dual network gel in accordance with embodiments of the present invention;

FIG. 8a is a graph showing the dynamic response of a dual network gel to humidity as a humidity sensor in accordance with an embodiment of the present invention;

FIG. 8b is a real-time humidity response curve of glycerol modified dual-network gel as a humidity sensor in accordance with an embodiment of the present invention;

FIG. 8c is a linear fit of a dual network gel as a humidity sensor to humidity response according to an embodiment of the present invention;

FIG. 8d is a diagram of a response test of glycerol modified double-network gel as a humidity sensor for detecting human breath humidity according to an embodiment of the present invention;

FIG. 8e is a graph showing response time and recovery time test curves for a one-cycle test of glycerol-modified double-network gel as a humidity sensor in accordance with an embodiment of the present invention;

FIG. 8f is a graph showing the response of the glycerol modified double-network gel to humidity detection in human breath for one month as a humidity sensor in accordance with an embodiment of the present invention;

FIG. 9a is a graph of mass loss versus time for a dual network gel according to an embodiment of the present invention at a relative humidity of 65% and a temperature of 25 ℃;

FIG. 9b is a graph showing the mass loss of a dual-network gel according to an embodiment of the present invention with time at a relative humidity of 55% and a temperature of 40 ℃;

FIG. 9c is a graph of mass loss versus time for a dual-network gel according to an embodiment of the present invention at a relative humidity of 37% and a temperature of 60 ℃;

FIG. 9d is a graph showing the change of relative resistance with time of a dual-network gel according to an embodiment of the present invention in an environment with a relative humidity of 65% and a temperature of 25 ℃;

FIG. 9e is a graph showing the change of relative resistance with time of a dual-network gel according to an embodiment of the present invention in an environment with a relative humidity of 55% and a temperature of 40 ℃;

FIG. 9f is a graph showing the change of relative resistance with time of a dual-network gel according to an embodiment of the present invention in an environment with a relative humidity of 37% and a temperature of 60 ℃.

Wherein the reference numbers are as follows: the preparation method comprises the following steps of 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 and 11.

Detailed Description

In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is described in further detail below by way of embodiments with reference to the accompanying drawings. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.

Embodiments provide a humidity sensor including a gel including a polymer network having functional groups for binding with water molecules, a solvent combined with the polymer network to form the gel, and an electrolyte salt dissolved in the solvent.

According to the humidity sensor provided by the embodiment of the invention, when the humidity in the environment is higher, water molecules can be adsorbed and dissolved in the solvent of the gel, the concentration of the electrolyte salt and the polymer in the solvent is diluted, and the concentration of the polymer is reduced, so that the blocking effect on the anion and cation migration of the electrolyte salt dissolved in the solvent is reduced, the ion migration speed is increased, and when the humidity in the environment is lower, the water molecules are desorbed, the concentration of the polymer is increased, the blocking effect on the ion migration is increased, the ion migration speed is reduced, and the change of the ion migration speed can be reflected by the conductance or the resistance of the gel, so that the response of the gel to the humidity is formed. The gel is used as the humidity sensor, so that the sensor has excellent stretchability and flexibility, and can be widely applied to wearable devices.

The polymer network may have functional groups for binding with water molecules, making it easier for water molecules to be adsorbed by the gel and enter the solvent. Further, the polymer network may act as a solid matrix to confine the electrolyte salt ions and the solvent, replacing the container required for ionic liquid or liquid metal conductors.

The functional group is a hydrophilic group, preferably the functional group comprises-NH₂，SO₃ ^-and-OH, more preferably-NH₂，SO₃ ^-and-OH, said-NH₂、SO₃ ^-and-OH can combine with water molecules to form hydrogen bonds. It will be appreciated that the sensitivity of the humidity sensor is related to whether water molecules are able to enter the solvent faster as the humidity increases in the external environment and to exit the solvent faster as the humidity decreases in the external environment. By using-OH, SO₃ ^-、-NH₂The water molecules can be combined with the water molecules through hydrogen bonds formed by the water molecules, the water molecules are promoted to be rapidly adsorbed and enter the solvent, the binding force provided by the hydrogen bonds is relatively weak, and the water molecules in the solvent can be rapidly desorbed when the relative humidity in the external environment is reduced, so that the humidity sensor has high sensitivityAnd (4) degree.

The solvent is used for dissolving the electrolyte salt to form anions and cations. Preferably, the solvent comprises water and the gel is a hydrogel. The solvent may or may not include an organic solvent, and in one embodiment, in order to solve the problem that moisture in the hydrogel tends to dry up due to easy evaporation, resulting in reduced stretchability, conductivity and stability of the humidity sensor, the solvent further includes an organic solvent, and the organic solvent is preferably a hygroscopic small molecule polyol, and more preferably at least one of glycerol and ethylene glycol. The organic solvent can select the alcohol solvent that can improve gel water retention, for example the hygroscopicity small molecule polyol that the C atomicity is 1 ~ 10, solves the problem that moisture easily evaporates in the gel, can improve the high temperature resistance of gel simultaneously, can also reduce the freezing point of solvent, improves the freezing resistance of gel, makes the sensor can normally work in the temperature range of broad. In one embodiment, the organic solvent is at least one of glycerol and glycol, and the freezing point of the mixed solution of the glycerol and/or glycol and water can reach a negative temperature, so that the freezing resistance of the gel can be improved. Furthermore, the micromolecular polyhydric alcohol solvents contain a plurality of-OH groups, and can form hydrogen bonds with water molecules, so that the adsorption of the water molecules is promoted, and the sensitivity of humidity detection is further improved. And the organic solvent does not reduce the mechanical deformability of the gel.

The gel may have an ultra-high tensile strain, which may be in excess of 1200%, and in one embodiment, 1225% greater than 20.4 times the currently reported stretchable moisture sensor (tensile strain 60%) with the best stretchability.

The gel may have transparency, and preferably, the gel has an optical transmittance of more than 60% in a visible light band. The humidity sensor with the transparent gel may be mounted on a display screen or window, for example, to form a transparent humidity sensing layer.

The gel may be porous and may be more conducive to the adsorption of water molecules. In one embodiment, a pore former may be added during the formation of the polymer network of the gel to form uniform micropores, increasing the porosity of the gel.

In order to enhance the mechanical strength of the gel, improve the scalability and service life of the gel, it is preferable that the gel is a double-network gel including a first polymer network and a second polymer network. The first polymer network and the second polymer network may be homogeneously mixed.

The first polymer network and the second polymer network have different physical properties. The first polymer network can be selected from polymers capable of forming gel with a solvent and used for coating the solvent to form gel, and the second polymer network is uniformly distributed in the gel and used for enhancing the mechanical strength of the gel. The second polymer network is preferably a flexible network of polymer molecules, which gives the gel a good flexibility and elasticity overall.

The second polymer network may be a network structure physically cross-linked by ionic bonds, for example may be selected from one or more of carrageenan, chondroitin sulphate, gelatin, alginate.

The first polymer network may be a high molecular polymer network structure chemically cross-linked by covalent bonds, and may be selected from one or more of polyacrylamide, polyaniline, and polyvinylamine. The first polymer network is obtained by polymerization and crosslinking reaction of monomers, a crosslinking agent and an initiator.

The polymer network enables efficient energy dissipation due to the sacrificial bonds provided by the unfolding of the polymer chains and the ionic crosslinking of the second polymer network, resulting in an increase in the elastic modulus of the gel.

The first polymer network and/or the second polymer network may have the functional group, and in a preferred embodiment, the first polymer network and the second polymer network each have a different functional group, for example each has-NH₂、SO₃ ^-and-OH. Due to the presence of the functional groups, the first polymer network and the second polymer network can be bonded together by the action of hydrogen bonds.

The electrolyte salt may include at least one of potassium chloride, calcium chloride, and sodium chloride, preferably calcium chloride. The calcium chloride can be combined with water molecules to form a hydrate, so that the adsorption of the water molecules is further promoted, the sensitivity of the humidity sensor can be improved, and the water-retaining property of the humidity sensor can also be improved.

Referring to fig. 1, in one implementation, as shown in fig. 1a, the first polymer network is polyacrylamide 5 and the second polymer network is carrageenan network 6. The electrolyte salt is calcium chloride 2. The solvent is water or a combination of water and ethylene glycol and/or glycerol. The polyacrylamide 5 is obtained by cross-linking and polymerizing monomer acrylamide 3 and cross-linking agent N, N' -methylene bisacrylamide 1 under the action of an initiator. Acrylamide 3, N' -methylene bisacrylamide 1, carrageenan 4, calcium chloride 2 and an initiator are uniformly heated and mixed in water, firstly, polyacrylamide 5 is formed through polymerization, then, the temperature is reduced to form a carrageenan network 6, and the carrageenan network 6, the solvent, the calcium chloride 2 dissolved in the solvent and the polyacrylamide 5 form double-network gel together. The mass ratio of the polyacrylamide 5 to the carrageenan 4 is preferably 16: 1-1.3: 1, and more preferably 5: 1-3: 1. In this example, the double-network gel contains a large amount of-NH at the same time₂、SO₃ ^-and-OH functional groups that can combine with water molecules 8 to form a number of hydrogen bonds 7, as shown in fig. 1b, the sensitivity to humidity response is very high.

Because the double-spiral structure of the carrageenan and the polymer molecular chain of the polyacrylamide, the effective energy dissipation can be realized by the expansion of the polymer chain and the sacrificial bond provided by the carrageenan double-spiral in the stretching process of the gel, so that the gel has better stretching strain property and can bear deformation such as stretching bending to a greater extent. Meanwhile, hydrogen bonds can be formed between the polyacrylamide and the carrageenan, so that a synergistic effect is formed between the first polymer network and the second polymer network, and in the gel stretching deformation process, the energy dissipation is promoted by the dynamic fracture and recombination of the hydrogen bonds, so that the stretching strain property of the gel is further enhanced.

In one embodiment, when the solvent is water, the hydrogel obtained may be further soaked in an organic solvent, such as ethylene glycol and/or glycerol, for a period of time, and there is a concentration difference between the water molecules and the ethylene glycol and/or glycerol, causing the molecules to diffuse, so that the ethylene glycol and/or glycerol replaces part of the water, and there are three forms of water, "free water", "intermediate water" and "strongly bound water" in the hydrogel. "free water" is not bound to the polymer network and is freely replaceable with ethylene glycol and/or glycerol; "intermediate water" loosely binds to the polymer network through hydrogen bonds and can be slowly displaced by ethylene glycol and/or glycerol; the strong binding water is not easy to be replaced. Referring to fig. 2, glycerol molecules 9 and/or glycol molecules 10, which replace the water solvent, have a large amount of-OH, and are easily combined with water molecules 8, thereby improving the water retention of the gel and allowing the sensor to have a wide operating temperature range.

Because a large number of hydrogen bonds are formed between the polyacrylamide and the carrageenan, the hydrogen bonds can be easily regenerated, and therefore the gel can be self-repaired after being broken. After the gel is fractured, a large number of hydrogen bonds can adhere the two ends of the gel again on the surface of the fractured surface, and self-repairing can be realized at room temperature. In addition to a large number of hydrogen bonds between polymer chains, the organic solvent ethylene glycol and glycerol molecules can also form a large number of hydrogen bonds with the polyacrylamide or the carrageenan, so that the self-repairing capability of the gel can be further improved by containing ethylene glycol and/or glycerol in the solvent. Furthermore, under the heating condition, when the sol-gel transition temperature is heated, the double-spiral structure of the carrageenan which is the first polymer network polymer is dissociated into single chains, and the single chains of the carrageenan at the interface are recombined into the double-spiral structure to repair the cracked surface after cooling, so the self-repairing effect of the gel can be further improved under the heating condition. Meanwhile, the self-repairing capability of the gel can be improved by adding electrolyte salt ions.

The humidity sensor senses humidity in the external environment through the ion migration rate of the electrolyte salt in the gel. In one embodiment, the humidity sensor further comprises electrodes for measuring a parameter reflecting the ion migration rate of the electrolyte salt in the gel, preferably measuring the ion conductivity or ion resistivity of the electrolyte salt in the gel, or the electrical resistance of the gel. The electrode can be a metal sheet, a metal film or conductive paste, such as a copper film, an aluminum film, a silver paste, an aluminum paste, a carbon paste, and the like. Two of the electrodes may be disposed at both ends of the gel, respectively.

The humidity sensor may further include a detection device electrically connected to the two electrodes, respectively, for measuring the ionic conductivity or ionic resistivity of the electrolyte salt in the gel through the electrodes.

The embodiment of the invention also provides an application of the humidity sensor, and the humidity sensor is used for detecting the humidity change in the environment through the change of the ionic conductivity of the electrolyte salt in the gel. The change in the concentration of the electrolyte salt in the solvent caused by the entry or exit of water molecules into or out of the solvent in the gel affects the rate of ion migration of the electrolyte salt, i.e., ion conductivity or ion resistivity. Referring to fig. 3, when the humidity of the environment increases, the anions and cations of the electrolyte salt move between the two electrodes 11, and the water molecules are adsorbed and dissolved in the hydrogel, so that the concentration of the high molecular polymer decreases, the inhibition effect on the ion migration decreases, the ion mobility increases, and the current increases. When the ambient humidity is reduced, water molecules are evaporated and separated from the hydrogel, so that the concentration of the polymer is increased, the inhibition effect on ion migration is increased, the ion mobility is reduced, and the current is reduced. Adsorption and desorption are reversible processes.

The resistance R of the gel in the humidity sensor is rho L/A, and R, rho and L, A are the resistance, resistivity, length and cross-sectional area of the gel respectively. The resistance of the gel is determined by the resistivity or the conductivity, and when the migration rate of the anions and cations of the electrolyte salt in the gel is changed, the resistance of the whole gel is changed. Thus, the humidity sensor can detect humidity changes in the environment by measuring changes in the gel resistance or conductance.

The embodiment of the invention also provides equipment applying the humidity sensor, such as a wearable humidity sensing device, humidity sensing electronic skin, a human-computer interface, a flexible robot or medical equipment.

The apparatus may also include a flexible substrate and the humidity sensor laminated on a surface of the flexible substrate. The device may further comprise a reminder for giving a reminder when the humidity sensor senses that the relative humidity in the environment exceeds or falls below the set value range.

The humidity sensor can be used for monitoring humidity in the environment to improve health and comfort of a living environment, and also can be used for monitoring various physiological information such as human respiration (respiration is an important physiological signal closely related to human health care and activities), human skin humidity (the humidity level of the skin contains a large amount of valuable physiological and metabolic information) and humidity around a wound (the healing condition of the wound can be evaluated by monitoring the humidity level around the wound) and the like to monitor the health condition of a human body.

The humidity sensor combines stretchability, transparency, and self-healing properties, widening the range of applications for flexible and wearable electronics. The stretchability of the electronic humidity sensing skin enables the sensor to be attached to the skin of a human body and move with the human body with minimal interference, for example, the electronic humidity sensing skin can be directly attached to the skin of the human body to monitor the health condition of the human body or the humidity change in the environment in real time for reminding. Its transparency makes it of great interest in consumer electronics, military, displays and new energy devices. The self-repairing property of the self-repairing adhesive enables the electronic device to be automatically repaired after the surface of the electronic device is scratched and mechanically damaged, and the service life of the electronic device is obviously prolonged.

The gel can be obtained by heating and mixing a polymer capable of forming the gel with a solvent dissolved with electrolyte salt, and then cooling. Other functional components, such as the first polymer network or the raw material for forming the first polymer network, may be added to the mixed system under heating, and the added components are uniformly compounded in the gel during the cooling to form the gel. After the gel is formed, other solvents can be further used for replacing the original solvent, and the solvent component in the gel can be changed. For example, by substitution with glycerol or ethylene glycol, a glycerol or ethylene glycol modified double-network gel is obtained.

For the double-network gel, the embodiment of the invention also provides a preparation method of the humidity sensor, which comprises the following steps:

s10, uniformly mixing a monomer, a cross-linking agent, an initiator, a second polymer, an electrolyte salt and a solvent under a heating condition to obtain a mixed solution, wherein the monomer, the cross-linking agent and the initiator are used for forming a first polymer network, and the second polymer and the solvent can form gel;

s20, enabling the monomer, the cross-linking agent and the initiator to perform cross-linking polymerization reaction in the mixed solution to form the first polymer network, then cooling to enable the second polymer to form a second polymer network, and forming the gel with the solvent.

In step S10, the mass ratio of the first polymer network-forming monomer to the second polymer network-forming polymer is preferably 16:1 to 1.3: 1. More preferably, the mass ratio of the first polymer network forming monomers to the second polymer network forming polymers is 5: 1.

In step S10, preferably, the monomer, the crosslinking agent, the second polymer, and the electrolyte salt are uniformly mixed at 75 to 100 ℃, the electrolyte salt is dissolved in the solvent, then the temperature is reduced to 60 to 75 ℃, the initiator is added, and the mixture is uniformly mixed to obtain the mixed solution.

In step S20, the monomer, the cross-linking agent, and the initiator may undergo cross-linking polymerization under light or heat to form the first polymer network according to the type of the initiator.

In one embodiment, the heating may be performed under sealed conditions, preferably at a temperature of 80 ℃ to 105 ℃. After the first polymer network is formed, the solution is cooled to 0-10 ℃ to form gel.

When the solvent is water, the method for preparing the humidity sensor may further include step S30, soaking the gel in the organic solvent, so that part of the water in the gel is replaced by the organic solvent. The soaking time is preferably 1 to 12 hours. The organic solvent is preferably a small molecular polyol having 1-10 carbon atoms, and more preferably at least one of glycerol and ethylene glycol.

Example 1

S10, placing 4g of acrylamide powder, 0.8g of carrageenan powder, 0.1g of calcium chloride powder and 0.003g of N, N' -methylene bisacrylamide powder in 50mL of deionized water, and magnetically stirring at 75 ℃ until the mixture is uniform; cooling to 60 ℃, adding 0.02g of ammonium persulfate, and uniformly stirring by magnetic force to obtain a mixed solution;

s20, placing the mixed solution in an oven to perform sealed reaction for 1-2 hours at 95 ℃ to form a polyacrylamide network, and then placing the mixed solution in a low-temperature environment at 4 ℃ to perform reaction for 1-2 hours to form a carrageenan network, so as to obtain the polyacrylamide/carrageenan double-network gel.

Example 2

s20, placing the mixed solution in an oven to perform sealed reaction for 1-2 hours at 95 ℃ to form a polyacrylamide network, and then placing the mixed solution in a low-temperature environment at 4 ℃ to perform reaction for 1-2 hours to form a carrageenan network, so as to obtain polyacrylamide/carrageenan double-network gel;

s30, soaking the polyacrylamide/carrageenan double-network gel in 100% of glycol for 1 hour to obtain the glycol modified double-network hydrogel.

Example 3

Substantially the same procedure as in example 2 was conducted, except that the 100% ethylene glycol solution in S30 was replaced with a 100% glycerin solution.

Example 4

Substantially the same procedure as in example 2 was conducted, except that the 100% ethylene glycol solution was replaced with a 20% ethylene glycol solution in S30.

Example 5

Substantially the same procedure as in example 2 was conducted, except that the 100% ethylene glycol solution in S30 was replaced with a 20% glycerin solution.

Examples of the experiments

1. Flexibility and stretchability of gels

Referring to fig. 4, when the double-network gel prepared in example 1 (DN in fig. 4a) and the double-network gel prepared in example 2 (EG-DN in fig. 4a, labeled with a colored dye) were placed in an environment at 40 ℃ and a relative humidity of 55% for 50 hours or more, it was observed that the double-network gel prepared in example 1 showed significant volume shrinkage, while the glycol-modified double-network gel prepared in example 2 remained intact after 50 hours (as in fig. 4a), and then the glycol-modified double-network gel placed for 50 hours was subjected to twisting, bending and stretching experiments, wherein the twisting still could reach 540 ° and the bending could reach 155 ° (as in fig. 4 b). The tensile strain of the double-network gel prepared in example 2 can reach 1225% (as shown in fig. 4 c), which shows that the double-network gel prepared in the invention has excellent flexibility and tensile strain, and the organic solvent ethylene glycol can improve the water retention property and tensile strain of the gel.

2. Effect of solvent on gel freezing resistance

After the double-network gels prepared in examples 1 to 5 were kept at-18 ℃ for 1 hour, as shown in fig. 5, it was found that the gel (1 # in fig. 5) using water as a solvent was completely frozen and did not have stretchability, but the gel (2 # to 5# in fig. 5) containing ethylene glycol or glycerol in the solvent was not frozen and was able to undergo 500% tensile deformation without breaking under the same conditions. This shows that the freezing point of the gel can be reduced to below 0 ℃ by the glycol or glycerol contained in the solvent, and the frost resistance of the gel can be greatly improved.

3. Self-repairing property

The self-healing properties of the gel can be evaluated by acting as a wire to light a small bulb in the circuit. Referring to fig. 6, the double-network gel prepared in example 2 was cut by a knife, and the experimental results show that the gel is conductive before being cut by the knife (as shown in fig. 6 a), becomes non-conductive after being cut into two sections (not shown in the figure), but the gel cut into two sections is butted and heated, so that the conductivity is recovered after the gel is self-repaired (as shown in fig. 6 b), and further the gel after the self-repair is stretched, the gel still has large tensile strain (as shown in fig. 6 c), but the resistance is increased due to the stretching, and the self-repair cannot reach 100%.

4. Transparency of

Referring to fig. 7, the optical transmittance of the visible light band of the double-network hydrogel prepared in examples 1 to 3 is high, and the background pattern of the word "zhongshan university" in the gel coverage area is still clearly visible.

5. Humidity sensitivity characteristics

The humidity sensor with the double-electrode chemical resistance structure is prepared by using the double-network gel prepared in the embodiment 1-5, a fixed voltage is applied to the humidity sensor by using a Gishley (Keithley) semiconductor test system, and the change delta G/G of the relative conductance is measured under different relative humidity conditions₀(where Δ G is relative to its initial conductance G₀Change in conductance) to evaluate the characteristics of the humidity sensor. Humidity sensing tests were conducted in the room with sensor exposure time and purge time of 300s, with dry air purging, and the response defined as the relative conductance change. Referring to FIG. 8, the dual network gels prepared in examples 1-3 were tested as humidity sensors for their response to a range of different relative humidities (from 4% to 90%). In the test process, the sensitivity, the response speed and the signal recovery speed of the sensor can be determined, the effective detection range of the sensor to the humidity can be found, and the change relation of the response size of the sensor along with the humidity can be obtained. And fitting the response-relative humidity curve to obtain a corresponding relation equation.

As shown in a dynamic response curve of fig. 8a, the double-network gels prepared in examples 1 to 3 all increase immediately with the increase of indoor relative humidity, and when the indoor relative humidity is 90%, conductance response ratios of the glycol-modified double-network gel prepared in example 2 and the glycerol-modified double-network gel prepared in example 3 as humidity sensors are respectively as high as 239% and 978%, which indicates that the solvent containing glycol and glycerol can promote adsorption of water molecules and improve the sensitivity of the humidity sensor;

as shown in fig. 8b, the glycerol-modified double-network gel prepared in example 3 as a humidity sensor can still detect the conductance response when the humidity is as low as 4%, and the wide detection range of the relative humidity from 4% to 90% has significant response, and the conductance is increased by over 543 times as the relative humidity is increased from 4% to 90%.

As shown in fig. 8c, the response of the dual-network gel humidity sensor prepared in the example of the present invention has a good linear relationship with the relative humidity, and the response sensitivities (slopes) of the dual-network gel prepared in example 1, the ethylene glycol modified dual-network gel prepared in example 2, and the glycerol modified dual-network gel prepared in example 3 as humidity sensors were calculated to be 0.24, 6.2, and 10, respectively, from the linearly fitted relative humidity response curve shown in fig. 8c, and the glycerol modified dual-network gel prepared in example 3 as humidity sensors had the highest sensitivity.

As shown in fig. 8d, the ethylene glycol modified double-network gel prepared in example 2 and the glycerol modified double-network gel prepared in example 3 have high sensitivity and are sufficient to detect the difference in humidity between the exhaled air and the environment due to respiration of a human body as humidity sensors, the conductance of the sensors is suddenly increased during exhalation, and then the signals are rapidly and completely restored during inhalation.

The response and recovery time is defined as the time to reach 90% signal change in the overall amplitude of the response factor. As shown in fig. 8e, the response time and recovery time of the glycerol modified dual-network gel humidity sensor prepared in example 3 for one breath are only 0.27s and 0.3s, which indicates that the sensor has the characteristics of fast response speed and fast recovery.

As shown in fig. 8f, the glycerol modified double-network gel prepared in example 3 is used as a humidity sensor to monitor the breath detection of a human body for one month, and it can be seen from the figure that the gel prepared in example 3 has good stability as a humidity sensor, and in the test, the response is basically stable at 49.3%, and the error is only 0.98%.

The double-network gel prepared by the embodiment of the invention can be used as a humidity sensor to detect the humidity change of the gas exhaled by a human body during breathing, and has the advantages of quick response, quick recovery and good repeatability, and the human body breathing experiment is preferably carried out outdoors. The humidity sensor is used for monitoring the respiration of a human body, so that important physiological signals related to the human body can be obtained, and the application has high practicability.

6. Water retention and stability of gels

The gel humidity sensors prepared in example 1, example 2 and example 3 were exposed to different environments for stability testing.

As shown in fig. 9a and 9d, the dual-network gel humidity sensor prepared in example 1 in the normal temperature and humidity environment (relative humidity 65%, 25 ℃) loses most of moisture for only 20 hours and loses the conductive ability for 10 hours; the quality and the resistance of the double-network gel humidity sensor prepared in the embodiment 2 are basically unchanged; the dual-network gel humidity sensor prepared in example 3 tends to be stable after 30% of mass loss, and the resistance becomes 400% and gradually tends to be stable.

As shown in fig. 9b and 9e, the dual-network gel humidity sensor prepared in example 1 loses most of moisture and loses conductivity only in 8 hours in an environment with a relative humidity of 55% and at 40 ℃; the mass loss of the dual-network gel humidity sensor prepared in the embodiment 2 is 45% and then the dual-network gel humidity sensor tends to be stable, and the resistance is 150% and tends to be stable; the dual-network gel humidity sensor prepared in example 3 tends to be stable after 30% of mass loss, but the resistance is greatly improved.

As shown in fig. 9c and 9f, the weight loss of the double-network gels prepared in examples 1 to 3 is increased at a relative humidity of 37% and in a high-temperature drying environment at 60 ℃, but after the high-temperature drying environment lasts 74 hours under the extremely severe conditions, the weight loss rate of the glycol or glycerol modified double-network gels prepared in examples 2 and 3 is still lower than that of the double-network gel prepared in example 1. The humidity sensor prepared in example 1 lost most of the moisture and lost the conductive ability for only 8 hours; the mass loss of the double-network gel humidity sensor prepared in the embodiment 2 is 60% and then the sensor tends to be stable, and the resistance is changed to 12000%; the dual-network gel humidity sensor prepared in example 3 tends to be stable after 55% of mass loss, but the resistance becomes 8000% of the original resistance and tends to be stable.

The above experiments show that the glycol or glycerol modified double-network gel prepared in examples 2 and 3 has better water retention, stronger resistance to high temperature drying environment and better gel stability than the non-modified double-network hydrogel prepared in example 1 in the same humidity and temperature environment, which indicates that the glycol or glycerol has hygroscopicity and can improve the water retention of the gel. Meanwhile, the weight loss of the glycol modified double-network gel is smaller than that of the glycerol modified double-network gel under mild conditions, but is larger than that of the glycerol modified double-network gel under extreme conditions, and under moderate conditions (relative humidity is 55%, and in an environment of 40 ℃), the mass loss is in a transition state along with the prolonging of the exposure time (as shown in figure 9b, about 40 hours). This shows that the glycol modified double-network gel has better water retention under milder conditions; under extremely harsh conditions, the double-network gel modified by the glycerol has better water retention. This is probably due to the lower vapour pressure, higher boiling point and higher viscosity of glycerol than ethylene glycol, which results from the maldistribution of glycerol in the gel at room temperature.

From a conductive point of view, the double network gel prepared in example 1 became almost non-conductive after 27 hours or more of exposure to the environment, mainly as a result of the gel losing a large portion of the water. In contrast, the ethylene glycol or glycerol modified double network gels prepared in examples 2 and 3 maintained conductivity under all experimental conditions, further demonstrating that partial replacement of water in the hydrogel with ethylene glycol and glycerol significantly improved the water retention and stability of the gel even under harsh environmental conditions.

The technical features of the embodiments described above may be arbitrarily combined, and for the sake of brevity, all possible combinations of the technical features in the embodiments described above are not described, but should be considered as being within the scope of the present specification as long as there is no contradiction between the combinations of the technical features.

The above-mentioned embodiments only express several embodiments of the present invention, and the description thereof is more specific and detailed, but not construed as limiting the scope of the present invention. It should be noted that, for a person skilled in the art, several variations and modifications can be made without departing from the inventive concept, which falls within the scope of the present invention. Therefore, the protection scope of the present patent shall be subject to the appended claims.

Claims

1. The humidity sensor is characterized by comprising a gel, wherein the gel comprises a polymer network, a solvent and an electrolyte salt, the solvent and the electrolyte salt are combined with the polymer network to form the gel together, the electrolyte salt is dissolved in the solvent, the polymer network has a functional group used for being combined with water molecules, the gel is a double-network gel and comprises a first polymer network and a second polymer network, the first polymer network is used for coating the solvent, the second polymer network is used for enhancing the mechanical strength of the gel, the mass ratio of the first polymer network to the second polymer network is 16: 1-1.3: 1, the first polymer network is polyacrylamide, and the second polymer network is carrageenan.

2. The humidity sensor of claim 1, wherein said functional group is a hydrophilic group, said functional group comprising-NH₂、SO₃ ^-and-OH.

3. The humidity sensor according to claim 1, wherein the solvent comprises water and an organic solvent, and the organic solvent is a small molecule polyol having 1-10 carbon atoms.

4. The humidity sensor according to claim 3, wherein the organic solvent is at least one of glycerol and ethylene glycol.

5. The wetness sensor of claim 1, wherein the gel has a transparency, and the gel has a tensile strain greater than 1200%.

6. The humidity sensor of claim 1, wherein said electrolyte salt comprises at least one of potassium chloride, calcium chloride, and sodium chloride.

7. The humidity sensor according to claim 1, further comprising an electrode for measuring a parameter of ion migration rate of the electrolyte salt in the gel, the parameter being ion conductivity or ion resistivity of the electrolyte salt, or resistance of the gel.

8. Use of a humidity sensor according to any of claims 1 to 7 in the detection of a change in relative humidity in an environment by a change in the rate of ion transport of the electrolyte salt in the gel.

9. An apparatus being a wearable humidity sensing device, a human-machine interface, a flexible robot or a medical apparatus, characterized in that the apparatus comprises a humidity sensor according to any of claims 1-7.

10. The apparatus of claim 9, wherein the wearable humidity sensing device is a humidity sensing electronic skin.

11. A method for manufacturing a humidity sensor according to any one of claims 1 to 6, comprising the steps of: