Conductive hydrogel strain sensor for measuring body movement

Abstract

The invention provides a hydrogel strain sensor with double conductive mechanisms. The hydrogel strain sensor comprises an ion conduction mechanism and an electron conduction mechanism. Wherein the ion conduction mechanism is realized by the electrolyte, and the electron conduction mechanism is realized by the conductive filler. The hydrogel strain sensor comprises a double-net-shaped hydrogel layer, the first layer is a polymer net generated by hydrogel subjected to physical crosslinking, and the second layer is a polymer net generated by hydrogel subjected to chemical crosslinking. The polymer networks generated by the first layer of hydrogel and the second layer of hydrogel penetrate through each other through physical crosslinking. Furthermore, the double-mesh hydrogel can adsorb an aqueous solution, the adsorption amount is about 50-75% by weight of the hydrogel, the aqueous solution comprises an ion-conducting salt, and the amount of the ion-conducting salt is about 5-25% by weight of the hydrogel; the conductive filler comprises two or more of graphene, a carbon nano tube and MXene. In addition, the sensor further comprises a stretchable conductive electrode which is located outside the hydrogel layer and can be selected from a conductive elastomer, a stretchable metal net or a stretchable conductive fabric. The sensor has the characteristics of high sensitivity, high stability and high reliability, and can be widely applied to the fields of online diagnosis and treatment, man-machine interaction, intelligent wearable equipment and the like.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to U.S. Patent Application 18 / 655,362, filed May 6, 2024, and the disclosure of which is incorporated herein by reference in its entirety. TECHNICAL FIELD

[0002] The present invention relates to an electrically conductive hydrogel strain sensor, and more particularly, to a hydrogel strain sensor that can be attached to the skin to measure body motion. BACKGROUND

[0003] Conventional strain sensors fix a metal strip on a test sample and measure the strain of the test sample by changes in the resistance of the metal strip with elongation. This type of strain sensor is often used to measure the strain of objects that undergo small deformations, such as the length of structural steel or wing components, but cannot be applied to record the motion of body parts or internal organs. For example, when recording the motion of an appendage in the body part, such as an arm, leg, or finger, the skin can undergo stretching deformations from small to large, and conventional strain sensors cannot accurately measure such deformations. Therefore, there is a need to develop new types of strain sensors to meet the high-precision measurement requirements of skin deformations.

[0004] Therefore, hydrogel strain sensors have emerged. Compared with the metal strips used in conventional strain sensors, hydrogels have greater elasticity and can stretch and contract with the skin motion of the arm or leg to which they are attached. In this context, the term “hydrogel” refers to a three-dimensional hydrophilic polymer network composed of hydrophilic polymers that can absorb water equivalent to at least 10% of the total volume of the hydrogel. Since hydrogels can be made extremely elastic and flexible, and stretch and contract in a manner similar to human skin, they are very suitable for attaching to the human body.

[0005] In Chinese Patent Applications CN115607106A, CN115558229A, and CN113787800A, related hydrogel strain sensors are disclosed, however, these strain sensor materials are either not sensitive enough to measure subtle resistance changes or lack sufficient flexibility in a practical and wearable strain sensor.

[0006] Therefore, there is still a lack of improved hydrogel strain sensors, i.e., wearable sensors that can be used to measure body motion. SUMMARY

[0007] A dual conductive mechanism hydrogel strain sensor with ion conductive mechanism and electron conductive mechanism is provided. The hydrogel strain sensor includes a double network hydrogel, wherein the double network hydrogel includes a first layer of chemically crosslinked hydrogel-forming polymer network and a second layer of physically crosslinked hydrogel-forming polymer network, wherein the first layer and the second layer of hydrogel-forming polymer network are interpenetrated by physical crosslinking. Further, the first and second layer of crosslinked hydrogel-forming polymer network can absorb aqueous solution, and the absorption amount is about 50-75% of the weight percentage of the hydrogel. The aqueous solution includes ion conductive salt, and the content of the ion conductive salt is about 5-25% of the weight percentage of the hydrogel. The conductive filler includes two or more of graphene, carbon nanotube and MXene. The sensor further includes a stretchable conductive electrode, which is located outside the hydrogel layer and can be selected from a conductive elastomer, a stretchable metal mesh or a stretchable conductive fabric.

[0008] In another aspect, the first layer of chemically crosslinked crosslinked hydrogel-forming polymer network includes a polyvinyl alcohol-based polymer; and the second layer of physically crosslinked hydrogel-forming polymer network includes an acrylamide-based polymer or a urethane-based polymer.

[0009] In another aspect, the ion conductive salt is selected from sodium chloride, calcium chloride, lithium chloride or potassium chloride.

[0010] In another aspect, an encapsulation layer, such as a protective silicone or polyurethane layer, can be formed on the hydrogel.

[0011] The dual conductive mechanism hydrogel strain sensor can be further connected to a wearable flexible strain measurement device.

[0012] A method for preparing a hydrogel strain sensor with dual conductive mechanism is also provided. The method includes mixing a water-soluble synthetic polymer for forming a first layer of chemically crosslinked hydrogel-forming polymer network, a polymerizable monomer for forming a second layer of physically crosslinked hydrogel-forming polymer network, and an ion conductive salt solution to form a first mixture; adding an electron conductive filler to the mixture, and then adding a crosslinking agent to crosslink the polymerizable monomer. The resulting hydrogel is a double network hydrogel with the first layer of chemically crosslinked hydrogel-forming polymer network and the second layer of physically crosslinked hydrogel-forming polymer network interpenetrated. Next, a sensing material is cut from the resulting double network hydrogel, and an electrode is connected to the sensing material.

[0013] In another aspect, an initiator is added to form the second layer of physically crosslinked hydrogel-forming polymer network.

[0014] In another aspect, the mixture includes: 0.5-15% by weight of a water soluble synthetic polymer for forming a first layer of chemically cross-linked hydrogel-forming polymer network; 5-20% by weight of a polymerizable monomer; 5-25% by weight of a salt to facilitate ionic conduction; 0.005-3% by weight of an electronically conductive filler; 50-75% by weight of deionized water; a cross-linking agent; an initiator for a sol-gel process; and an accelerator for a sol-gel process.

[0015] wherein the cross-linking agent can be N,N'-methylenebisacrylamide, the initiator can be ammonium persulfate, and the accelerator can be N,N,N',N'-tetramethylethylenediamine. BRIEF DESCRIPTION OF DRAWINGS

[0016] Figure 1 shows a schematic of a conductive hydrogel in an embodiment;

[0017] Figure 2 shows a strain sensor employing a conductive hydrogel; Figure 1

[0018] Figure 3 shows a strain sensor connected to a strain sensor measurement device and a transmission circuit; and

[0019] Figure 4 shows a graph of strain sensor resistance versus time. DETAILED DESCRIPTION

[0020] Figure 1 ​A hydrogel 100 is shown schematically that not only has mechanical properties that mimic human skin, but also has sufficient electrical conductivity to be used in a strain sensor. By mimicking the elasticity of human skin, a strain sensor made of the hydrogel 100 can accurately reflect the movement of various human appendages. To make a hydrogel with elasticity, the present application uses two polymer networks 30 and 40, each of which can be a cross-linked polymer network. Notably, the two polymer networks 30 and 40 are interpenetrated by physical bonding to form a double network hydrogel. As shown in the hydrogel 100, the first polymer network 30 and the second polymer network 40 are interpenetrated by hydrogen bonding. Due to the introduction of hydrogen bonds, the hydrogel network can easily cope with its own stretching by breaking hydrogen bonds and restoring its original shape by reforming hydrogen bonds. By means of reversible physical bonds, the hydrogel material can be deformed sensitively by both small and large strain rates to more closely mimic the movement of human skin. Because in a body movement, human skin can stretch in multiple directions, but can quickly recover to the non-stretched state when at rest.

[0021] The first and second polymer networks 30 and 40 can be formed of one or more synthetic polymers, such as polyacrylate, polyvinyl alcohol, polyethylene glycol, polymers of methacrylate (e.g., hydroxyl methacrylate), polyvinylpyrrolidone, acrylamide polymers, polyurethane. In addition, the first and second polymer networks 30 and 40 can also be formed of one or more natural polymers, such as chitosan, polysaccharides, alginate, gum, pectin, and collagen. In the following examples, polyvinyl alcohol and polyacrylamide will be chosen to prepare the polymer networks 30 and 40. However, it is noted that other polymers (such as the polymers described above) can also be chosen to prepare the hydrogel 100.

[0022] To enable the hydrogel to mimic the properties of human skin, the water content of the hydrogel can be further selected. Typically, the water content will be selected to be 50-75% by weight (human skin has a water content of about 70%). In addition, the high water content of the hydrogel also helps to improve its sensitive electrical properties. The aqueous solution contains an ionic conductive salt, which is present in an amount of about 5-25% by weight of the hydrogel.

[0023] To use the hydrogel 100 as a strain sensor, an electrically conductive additive is further added to the hydrogel matrix. The hydrogel 100 of the present application is an electrically conductive hydrogel with dual conductive mechanisms of ionic and electronic conduction, which includes ions 10 and conductive particles 20. By adding ions and conductive particles to the hydrogel, the electrical resistance of the hydrogel changes even in a small body / muscle movement. In addition, the strain sensor made of the hydrogel 100 has a fast recovery time, which is very important for measuring dynamic body movements.

[0024] During the formation of the hydrogel, one or more salts are added to introduce ions 10 into the hydrogel 100 matrix, where the salts include sodium chloride, potassium chloride, lithium chloride, calcium chloride. The conductive particles 20 can be selected from carbon-based materials (e.g., graphite, graphene, carbon nanotubes, Ketjen black, super P), MXenes (i.e., transition metal carbides, nitrides, or carbonitrides in the form of atomically thin layers), metal nanoparticles (gold nanoparticles, silver nanoparticles, copper nanoparticles), conductive polymers (e.g., one or more of polyacetylene, polyaniline, polypyrrole, polythiophene, poly-para-phenylene, polyphenylene, polyfuran, poly(3,4-ethylenedioxythiophene): poly(styrenesulfonic acid (PEDOT:PSS)). In this way, the resistivity of the hydrogel can be selectively adjusted to be between a low resistivity of a few hundred ohms to a high resistivity of the order of mega-ohms.

[0025] The formation of the hydrogel 100 represents that the polymer networks 30 and 40 have the correct conformation and the conductive particles are sufficiently and uniformly dispersed throughout the polymer networks, so that a uniform conductive performance is achieved in the hydrogel. For example, this can be achieved by dissolving the water-soluble polymer used to form the first layer of polymer network, and adding cross-linkable monomers, ion-conducting salts and conductive particles. In this way, the conductive particles can be more easily uniformly dispersed in the mixture. Only after confirming that the mixture has been mixed uniformly, the cross-linking of the monomers is carried out to obtain a structure as shown in Figure 1

[0026] Figure 2 A strain sensor 200 is shown, which includes a hydrogel 100 as shown in Figure 1 It is worth noting that the strain sensor 200 also contains electrodes 210. When the sensor is fixed to a human body part and measures human motion, the electrodes 210 must be closely attached to the hydrogel, and therefore, in order to ensure that they are sufficiently flexible, the electrodes 210 can be made of conductive mesh or conductive elastomers and the like. Examples include elastomers (e.g., rubber containing conductive particles), elastomer fabrics with metal mesh (e.g., elastomer fabric embedded with silver or copper mesh), conductive elastomer yarns woven into a stretchable structure, and the like. In general, the electrodes should be flexible enough to adhere to the surface of the hydrogel sensing material without causing delamination, and have certain mechanical properties. In this way, when the human body moves, the entire sensor patch can deform together, which is the ideal case for the electrodes.

[0027] ​The strain sensor 200 can optionally further comprise one or more encapsulation layers 220 for protecting the hydrogel 100. The encapsulation layer 220 can be an elastomer such as polyurethane, silicone, polydimethylsiloxane, thermoplastic polyurethane elastomer or hydrogenated styrene-butadiene block copolymer. Leads 230 can extend from the electrodes 210 for connection to a readout circuit of the strain sensor.

[0028] Figure 3 A systematic overview of the signal conditioning, regulation, processing and wireless transmission pathways is provided to facilitate multi-channel body measurements. As Figure 3 shown, the signal processing system 300 comprises a voltage divider 310, a low pass filter 320, a microcontroller 330 with an analog-to-digital converter and a wireless transceiver 340 such as a Bluetooth transceiver.

[0029] The signal conditioning pathway of each sensor is implemented through analog circuitry and is closely related to the respective converted signal. With this configuration, it is ensured that the final analog output of each pathway is finely resolved and kept within the input voltage range of the analog-to-digital converter. Furthermore, through the computational and serial communication capabilities of the microcontroller, it is possible to effectively calibrate, compensate and transmit the conditioned signals to the wireless transceiver on the main board. The transceiver facilitates the wireless data transmission to a Bluetooth-enabled mobile device. The mobile device is equipped with a custom-developed application that contains a user-friendly interface for sharing or uploading data to a cloud server.

[0030] The wireless receiver can receive the signals, wherein the wireless receiver is part of a signal processor such as a computer or a mobile device. The raw data of the strain gauge resistance versus time received by the computer is shown in Figure 4 where the mechanical signal from the movement is converted into an electrical signal because the distance between the conductive particles changes when the movement of the body part causes the deformation of the strain sensor. As Figure 4 shown, the resistance of the hydrogel changes as the distance between the conductive particles changes.

[0031] The present invention also relates to a method of preparing a conductive hydrogel with dual conductive mechanisms.

[0032] The method of preparing the hydrogel comprises the following procedure: (S1) A water-soluble synthetic polymer is added to deionized water at a high temperature of 60-90 °C (e.g. 85 °C) and stirred magnetically for four hours to allow it to dissolve; (S2) Further monomers are dissolved therein using magnetic stirring for subsequent copolymerization; (S3) Ionic conductive salt is dissolved therein using magnetic stirring; (S4) Use a planetary mixer to mix the electronically conductive filler into the hydrogel solution prepared above; (S5) Dissolve the crosslinking agent in it by magnetic stirring; (S6) Dissolve the initiator in it by magnetic stirring; (S7) Dissolve the accelerator in it by magnetic stirring; (S8) The prepared hydrogel solution is degassed by vacuuming; (S9) Pour the degassed hydrogel solution into a glass substrate with 1 mm thick silicon spacers to perform sol-gel treatment. - Gel formation; (S10) Use a laser to cut the hydrogel sample into the required size and shape; (S11) Connect the flexible silver-plated webbing electrode to the hydrogel; (S12) Encapsulate the hydrogel with electrodes attached; and (S13) Connect the hydrogel strain sensor to the flexible circuit board.

[0033] In some embodiments, the prepared hydrogel has the following components and contents: 0.5-15% by weight of water-soluble synthetic polymers are used to form the first layer of chemically cross-linked hydrogel to generate a polymer network; 5-20% by weight of polymerizable monomers; A 5-25% salt content by weight facilitates ion conduction; 0.005-3% by weight of electronically conductive filler; Crosslinking agent; Initiators for the sol-gel process; and Accelerators used in the sol-gel process.

[0034] Example 1

[0035] Preparation of hydrogels with dual conductive mechanisms and carbon nanotube fillers

[0036] At 85 °C, 1.2 g of polyvinyl alcohol was added to 20 ml of deionized water and stirred magnetically for several hours until dissolved; then, at room temperature, 4.69 g of acrylamide was further added to the solution, and after being dissolved by magnetic stirring for two hours, 3.45 g of sodium chloride was added and also stirred magnetically for one hour until it was completely dissolved, obtaining a hydrogel solution. Then, 10 ml of carbon nanotube solution was added to the hydrogel solution prepared above and uniformly dispersed by a planetary mixer at 900 rpm for 5 minutes; then, 0.007 g of crosslinking agent (N,N'-methylene bisacrylamide) was dissolved into the mixture solution by magnetic stirring at room temperature, and 0.02 g of sol-gel initiator (ammonium persulfate) was added, while 8 μL of accelerator (N,N,N',N'-tetramethyl ethylenediamine) was added dropwise into the solution, forming a hydrogel precursor solution, which was stirred magnetically at room temperature until the initiator and accelerator were uniformly mixed; finally, the hydrogel precursor solution was poured onto a glass substrate with a 1 mm silicon spacer, and a glass sheet was covered on the hydrogel precursor solution to carry out the sol-gel process and control the thickness of the hydrogel sheet. The prepared sensor is extremely sensitive to the unnoticeable force and stretch on the human body, and can detect the signals of whole body movement even in the strain range of 0-75%; that is, the sensitivity of the sensor is GF≥10, and the recovery speed is less than 50 milliseconds.

[0037] Conductive filler

[0038] The conductive filler includes graphene, MXene and carbon nanotubes. Among them, the aqueous dispersion of single-layer graphene with different concentrations is purchased from XFNANO; the aqueous dispersion of MXene (Ti3C2) with different concentrations is provided by Beijing Beikexin Material Technology Co., Ltd.; and the aqueous dispersion of carbon nanotubes used in this embodiment is multi-walled carbon nanotubes with different concentrations purchased from XFNANO.

[0039] Flexible silver-plated fabric electrode

[0040] The silver-plated fabric is made of 100% silver fiber, which has a bacteriostatic rate of >99.9%, a surface resistance of <1 Ω / S, a shielding efficiency of >55 DB, and can be cut into the required width by laser cutting.

[0041] Packaging layer

[0042] Two encapsulation methods were used, including direct encapsulation with commercially available polyurethane (PU) film and encapsulation with two layers of silicone rubber on the top and bottom supplemented with silicone rubber adhesive. The commercially available polyurethane tape used was provided by Shanghai Hongsheng Medical Technology, the silicone rubber was the Ecoflex series product of the American Smooth On company, and the silicone rubber adhesive (also known as Sil Poxy) was also purchased from the American Smooth On company, which can form a firm but still flexible connection between the silicone parts, ensuring the stretchability of the hydrogel strain sensor after encapsulation.

[0043] Due to the interaction between the hydrogel and the ions, the prepared sensor has high sensitivity of GF≥10 and fast recovery speed of less than 50 ms. This interaction ensures good ion conduction and high sensitivity. In addition, the addition of carbon nanotube / graphene / MXene conductive agent is beneficial to electron conduction, thereby quickly recovering the resistance, i.e., fast signal response. The double cross-linked polymer network can produce high stretchability and toughness, thereby providing durability. The chemical cross-linking network also maintains the structure of the hydrogel. The physical cross-linking network can maintain the water content and introduce hydrogen bonds that can produce reversible rupture during stretching, making the hydrogel have good stretchability.

[0044] The silver-plated fabric electrode has excellent flexibility, high conductivity, and does not produce oxidation when in contact with the hydrogel, so it can be well fitted with the hydrogel as an electrode.

[0045] The polyurethane (PU) film or silicone rubber has good biocompatibility and can be in direct contact with the skin, so it can provide an ultra-thin encapsulation layer for the hydrogel to prevent the hydrogel from dehydrating and prolong its service life.

[0046] During the stretching process, the changes in resistivity and conductivity are derived from the changes in the arrangement of the structure within the hydrogel and the movement of ions between the conductive fillers, which are helpful for detecting the force on the skin.

[0047] As used herein and unless otherwise defined, the terms such as "substantially," "essentially," "approximately," and "about," are used to describe and account for minor variations in the value. When recited together with an event or circumstance, these terms can include examples in which the event or circumstance occurs exactly, as well as examples in which the event or circumstance occurs nearly. By way of illustration, when used with a numerical value, these terms include a range of variation of less than or equal to ±10% of the numerical value, such as less than or equal to ±5%, less than or equal to ±4%, less than or equal to ±3%, less than or equal to ±2%, less than or equal to ±1%, less than or equal to ±0.5%, less than or equal to ±0.1%, or less than or equal to ±0.05%.

[0048] The preceding description is provided to enable any person skilled in the art to practice the present application as claimed. The context of the specification makes it clear that any refer ence to a method using terms such as "comprise", "comprising", "comprised of", "comprised between" or "comprising between" is used herein to describe only a procedure, process, or technique. Any reference to a method using terms such as "comprising", "comprise", "comprised of", "comprised between" or "comprising between" is used herein to describe only a procedure, process, or technique. The description is not intended to limit the application to the exact embodiments disclosed and, therefore, many modifications and variations will be apparent to those skilled in the art. These are intended to be included herein. The above described embodiments are presented for purposes of best mode of practicing the present application and to enable others skilled in the art to best utilize the application.

Claims

1. A hydrogel strain sensor having dual conductive mechanisms of ionically and electronically conducting, characterized in that, comprising: a double network hydrogel comprising: a first layer of chemically crosslinked hydrogel-forming polymer network; a second layer of physically crosslinked hydrogel-forming polymer network, wherein the first and second layers of hydrogel-forming polymer network are interpenetrated by physical crosslinking; an aqueous solution, wherein the first and second layers of crosslinked hydrogel-forming polymer network can absorb the aqueous solution by 50-75% weight percentage of the hydrogel, wherein the aqueous solution comprises an ionically conductive salt at a content of 5-25% weight percentage of the hydrogel; an electronically conductive filler selected from two or more of graphene, carbon nanotube, and MXene; and a stretchable electronically conductive electrode located outside of the hydrogel, wherein the stretchable electronically conductive electrode is selected from an electronically conductive elastomer, a stretchable metal mesh, or a stretchable electronically conductive fabric.

2. The hydrogel strain sensor with dual conductive mechanisms of claim 1, wherein the first layer of physically crosslinked hydrogel-forming polymer network comprises a polyvinyl alcohol-based polymer; and the second layer of chemically crosslinked hydrogel-forming polymer network comprises an acrylamide-based polymer or a urethane-based polymer.

3. The hydrogel strain sensor with dual conductive mechanisms of claim 1, wherein the ionically conductive salt is selected from sodium chloride, calcium chloride, lithium chloride, or potassium chloride.

4. The hydrogel strain sensor with dual conductive mechanisms of claim 1, wherein the outside of the hydrogel further comprises an encapsulation layer.

5. The hydrogel strain sensor with dual conductive mechanisms of claim 1, wherein the encapsulation layer is selected from silicone or polyurethane.

6. A strain measurement device, characterized by comprising the hydrogel strain sensor with dual conductive mechanisms of claim 1, and a wearable flexible strain measurement device connected to the hydrogel strain sensor with dual conductive mechanisms.

7. A method for preparing a hydrogel strain sensor with dual conductive mechanisms according to claim 1, characterized in that, comprising: mixing a water-soluble synthetic polymer for forming the first layer of chemically crosslinked hydrogel-forming polymer network, a polymerizable monomer for forming the second layer of physically crosslinked hydrogel-forming polymer network, and an ionically conductive salt solution to form a first mixture; adding the electronically conductive filler to the first mixture; adding a crosslinking agent to the first mixture to crosslink the polymerizable monomer and obtain the double network hydrogel having the first layer of chemically crosslinked hydrogel-forming polymer network and the second layer of physically crosslinked hydrogel-forming polymer network interpenetrated by each other; cutting a sensing material from the double network hydrogel; and connecting an electrode to the sensing material.

8. The method of claim 7 for preparing the hydrogel strain sensor with dual conductive mechanisms, further comprising adding an initiator to form the second layer of physically crosslinked hydrogel-forming polymer network.

9. The method of claim 8 for preparing the hydrogel strain sensor with dual conductive mechanisms, wherein the first layer mixture comprises: 0.5-15% weight percentage of the water-soluble synthetic polymer for forming the first layer of chemically crosslinked hydrogel-forming polymer network; ​ 5-20% by weight of the polymerizable monomer; 5-25% by weight of the salt, which helps to enhance ionic conduction; 0.005-3% by weight of the electronically conductive filler; 50-75% by weight of deionized water; the crosslinking agent; the initiator for sol-gel process; and the accelerator for sol-gel process.

10. The method of claim 9, wherein the crosslinking agent is N,N'-methylenebisacrylamide.

11. The method of claim 9, wherein the initiator is ammonium persulfate.

12. The method of claim 9, wherein the accelerator is N,N,N',N'-tetramethylethylenediamine.

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