Liquid metal composite conductive ink for flexible circuit and preparation method thereof

Abstract

The invention provides liquid metal composite conductive ink for a flexible circuit and a preparation method of the liquid metal composite conductive ink. The preparation method comprises the following steps: S1, mixing polyvinyl alcohol, deionized water and liquid metal, and performing ultrasonic treatment to obtain polyvinyl alcohol modified liquid metal nanoparticle dispersion liquid; s2, citric acid and silver nitrate are added into the polyvinyl alcohol modified liquid metal nanoparticle dispersion liquid for mixing reaction, and the liquid metal composite conductive ink for the flexible circuit is obtained. The flexible circuit prepared from the liquid metal composite material has initial conductivity, high tensile property and high conductivity.

Description

Technical Field

[0001] The present invention relates to the technical field of flexible conductive materials, and particularly relates to a liquid metal composite conductive ink for flexible circuits and a preparation method thereof. Background Art

[0002] Gallium-based liquid metal, with its characteristics of both fluid properties and high conductivity, has become an ideal material for the preparation of flexible functional circuits and is widely used in flexible electronics. The flexible circuits obtained by liquid metal printing have good electrical conductivity and dynamic adaptability, and hardly introduce stress during large deformations. However, due to the different material phase states of liquid metal and traditional printing materials, various current patterning methods for printed circuits are difficult to be directly applied to the preparation of liquid metal flexible circuits.

[0003] Currently, the preparation methods of gallium-based liquid metal flexible circuits mainly include solution-based synthesis, chemical vapor deposition, and physical vapor deposition. Solution-based synthesis can be printed over a large area, but it is difficult to achieve stretchability and requires heat treatment at high temperatures. Vapor deposition can form a condensation oxide film with excellent chemical purity, but it will generate harmful chemical precursors and requires vacuum treatment. In addition, bulk gallium-indium alloys have problems such as large surface tension and easy agglomeration during the preparation process. Therefore, ultrasonic treatment is often used to convert liquid metal into nanoparticles to obtain higher stability. However, the circuits made of nano metal particles perform poorly in terms of deformability.

[0004] Currently, the commonly used method to improve the deformability of liquid metal circuits is to modify and encapsulate them with polymers. Currently, there are many types of polymers used to modify liquid metal, such as polyvinylpyrrolidone, polyvinyl alcohol, cellulose, polydopamine, polyethylene, etc.

[0005] Polyvinyl alcohol is a water-soluble polymer. There are a large number of secondary hydroxyl groups on the main chain macromolecule, and it has many similarities with cellulose in terms of chemical properties. Its aqueous solution has good adhesiveness and film-forming properties, and has chemical properties such as long-chain polyol esterification, etherification, and acetalization. In related technologies, polyvinyl alcohol is modified on the surface of liquid metal nanoparticles. Most of them tightly attach polyvinyl alcohol to the surface of the droplets to prevent crack formation. However, the circuits formed after patterning do not have initial electrical conductivity. This is because there is an oxide layer on the surface of liquid metal particles, and the particles are insulated from each other due to the insulating oxide layer, resulting in non-conductivity of the whole. Before use, external force stretching and pressing are required to activate the conductivity of the circuit. During the activation process, the oxide layer shell breaks, releasing the liquid metal core, and the liquid metal particles are connected to form a conductive path. However, this method has a low film-breaking efficiency, and the additional external force will cause liquid metal leakage, damage the circuit structure, and affect the electrical conductivity, stretchability, and scratch resistance.

[0006] Based on this, it is necessary to provide a liquid metal composite conductive ink for flexible circuits, so that the circuit has initial conductivity and can improve the deformability of the circuit. Summary of the Invention

[0007] To solve the problems existing in the background technology, the present invention provides a liquid metal composite conductive ink for flexible circuits and a preparation method thereof. The flexible circuit prepared from the liquid metal composite material has initial conductivity, high stretchability and high conductivity. The technical solution of the present invention to solve the above technical problems is as follows: In the first aspect, the present invention provides a preparation method of a liquid metal composite conductive ink for flexible circuits, including the following steps: S1. Mix polyvinyl alcohol, deionized water and liquid metal, and ultrasonically obtain a dispersion of liquid metal nanoparticles modified with polyvinyl alcohol; S2. Add citric acid and silver nitrate to the dispersion of liquid metal nanoparticles modified with polyvinyl alcohol, mix and react to obtain the liquid metal composite conductive ink for flexible circuits.

[0008] In the method of the present invention, by mixing polyvinyl alcohol and liquid metal in water and ultrasonically treating, the liquid metal is dispersed into nanoparticles, and polyvinyl alcohol crosslinks into a network on the surface of the liquid metal nanoparticles to modify them. After adding citric acid and silver nitrate, silver ions are reduced to silver nanoparticles and adhere to the surface of the liquid metal nanoparticles. The finally obtained liquid metal composite conductive ink is a dispersion of liquid metal nanoparticles modified with citric acid and polyvinyl alcohol with silver particles attached to the surface.

[0009] The above liquid metal composite conductive ink is used for the preparation of the conductive layer of the flexible circuit. It is sprayed on the circuit pattern and then dried to remove the solvent to form a film to obtain the conductive layer of the flexible circuit. During the film formation process, water evaporates. Due to the presence of citric acid, the pH of the system decreases, which can effectively remove the oxide layer on the surface of the liquid metal nanoparticles, activate the liquid metal, and at the same time the capillary force increases, causing the liquid metal nanoparticles modified with polyvinyl alcohol with silver particles attached to the surface to aggregate and form a continuous conductive network, endowing the flexible circuit with initial conductivity; at the same time, during the film formation process, citric acid crosslinks with polyvinyl alcohol, further improving the tensile properties of the conductive material.

[0010] Preferably, the degree of polymerization of the polyvinyl alcohol is 1700-2200.

[0011] According to the above scheme, the liquid metal is a gallium-indium alloy, and the content of gallium in the gallium-indium alloy is 75.5 wt%, and the content of indium is 24.5 wt%.

[0012] Since polyvinyl alcohol is soluble in water at temperatures above 95°C, polyvinyl alcohol is first dissolved in a portion of water to prepare an aqueous polyvinyl alcohol solution, which is then mixed with the remaining deionized water and liquid metal.

[0013] In step S2, citric acid and silver nitrate can be added directly or prepared as an aqueous solution for addition. The amounts of citric acid and silver nitrate added are relatively small. To ensure the accuracy of their addition amounts and better dissolution, preferably, they are added in the form of an aqueous solution.

[0014] According to the above scheme, the mass ratio of polyvinyl alcohol to liquid metal is 1:15 - 30, the mass ratio of citric acid to liquid metal is 1:10 - 50, and the mass ratio of silver nitrate to liquid metal is 1:1800 - 9000.

[0015] The ratio of polyvinyl alcohol to liquid metal will affect the conductivity and stretchability of the prepared circuit. If the content of polyvinyl alcohol is too high, the conductivity of the prepared circuit will deteriorate; if the content is too low, the stretching performance will be poor.

[0016] In the reaction system of step S1, the content of liquid metal in the reaction system is 37.5 - 42.8 wt%, and in the reaction system of step S2, the content of liquid metal in the entire reaction system is 32.9 - 41.6 wt%.

[0017] According to the above scheme, the ultrasonic wave in step S1 is an inserted pulse ultrasonic wave, with an ultrasonic power of 500 - 750 W, a frequency of 20 - 40 kHz, an amplitude of 20 - 40%, a single pulse time of 5 s, a time interval between adjacent pulses of 5 s, a temperature of 5 - 15°C, and an ultrasonic time of 5 - 30 min.

[0018] According to the above scheme, in step S2, the stirring reaction is carried out for 3 - 10 min, and the reaction temperature is 18 - 25°C.

[0019] In a second aspect, the present invention provides a liquid metal composite conductive ink for flexible circuits prepared by the above preparation method.

[0020] In a third aspect, the present invention provides a flexible circuit conductive layer, which is prepared by the following method: using the above liquid metal composite conductive ink to form a circuit pattern on a substrate, and drying to remove the solvent to obtain the flexible circuit conductive layer.

[0021] According to the above scheme, the method for forming the circuit pattern is a mask spraying method, and the drying temperature is 40 - 80°C, and the time is 20 - 60 min.

[0022] According to the above scheme, the initial conductivity of the formed flexible circuit conductive layer is 4×10 6 S / m.

[0023] Fourthly, the present invention also provides an application of the above liquid metal composite conductive ink in the preparation of flexible electronic devices.

[0024] According to the above solution, the flexible electronic device includes a wearable device, a flexible sensor or a flexible electrode.

[0025] The beneficial effects of the present invention are as follows: 1) By modifying liquid metal nanoparticles with polyvinyl alcohol, a tough and friction-resistant film is formed after drying, improving the tensile performance and anti-friction performance of the prepared flexible circuit. At the same time, citric acid is added to the liquid metal composite material. During the film-forming process, as water evaporates, citric acid removes the oxide layer on the surface of liquid metal nanoparticles, activating the liquid metal particles to endow the circuit with initial conductivity. There is no need for the step of removing the oxide layer by external force stretching and pressing to activate the initial conductivity, avoiding the leakage of liquid metal and the damage of the flexible circuit, and thus avoiding the influence on its conductivity. At the same time, during the film-forming process, citric acid cross-links with polyvinyl alcohol, further enhancing the tensile performance of the flexible circuit and improving its tensile resistance change rate. 2) Silver nitrate is added during the preparation of the liquid metal composite conductive ink of the present invention. Silver ions are reduced to silver particles and adhere to the surface of liquid metal nanoparticles modified with polyvinyl alcohol. After being fabricated into a circuit, on the one hand, the conductivity of the circuit is improved. On the other hand, since silver particles can improve the anchoring between liquid metal nanoparticles, the stretchability of the circuit is further enhanced, and its tensile resistance change rate is improved. 3) The initial conductivity of the flexible circuit prepared from the liquid metal composite material of the present invention is 4×10 6 S / m. When the elongation rate reaches 700%, the tensile resistance change rate is 100%, and the electrical properties remain stable under the condition of cyclic large stretching. 4) The preparation method of the present invention is simple and convenient, capable of large-scale production. The prepared flexible circuit has broad application prospects in the fields of flexible and stretchable electronic devices, etc. 5) The main components of the liquid metal composite conductive ink of the present invention are non-toxic and harmless to the human body, can be used for human adhesion without causing adverse reactions, and can be used in wearable devices or health monitoring devices. Description of the Drawings

[0026] Figure 1 It is a schematic flow chart of the preparation method of the liquid metal composite conductive ink of the present invention; Figure 2 It is a schematic diagram of the reaction of liquid metal nanoparticles modified with polyvinyl alcohol with citric acid and silver nitrate solution of the present invention; Figure 3 It is a schematic diagram of the principle of the liquid metal composite conductive ink drying into a film of the present invention; Figure 4Test result graph of the tensile resistance change rate of the flexible circuit conductive layer prepared from the liquid metal composite conductive ink of Example 1, Comparative Example 1 and Comparative Example 2 of the present invention; Figure 5 Test result graph of the tensile resistance change rate of the flexible circuit conductive layer prepared from the liquid metal composite conductive ink in Example 1 of the present invention during multiple cyclic stretching; Figure 6 Test result graph of the tensile resistance change rate of the flexible circuit conductive layer prepared from the liquid metal composite conductive ink in Comparative Example 1 of the present invention during multiple cyclic stretching; Figure 7 Schematic diagram of the manufacturing process of the blood oxygen saturation detection circuit in Application Example 2 of the present invention; Figure 8 For the blood oxygen saturation detection circuit in Application Example 2 of the present invention, the detection results of PPG signal, PR and S P O 2 are shown in the graph, where Figure 8 A is the PR and SpO 2 , Figure 8 B is the near-infrared light PPG signal at wavelengths of 880 nm and 660 nm; Among them, 1. Flexible substrate, 2 and 5. Rigid substrates, 3. Mask, 4. Conductive layer, 6. Electronic components, 7. Encapsulation layer, 8. Mold, 9. Adhesive functional layer. Detailed implementation manners

[0027] The principles and features of the present invention will be described below in conjunction with the accompanying drawings and specific embodiments. The examples given are only used to explain the present invention and are not intended to limit the scope of the present invention.

[0028] In order to enable a flexible circuit based on liquid metal to obtain initial conductivity, and at the same time have good tensile properties and reduce the change rate of tensile resistance, the present invention provides a liquid metal composite conductive ink for flexible circuits. Polyvinyl alcohol (PVA) is used to modify liquid metal nanoparticles, and citric acid and silver nitrate are added to the dispersion of PVA-modified liquid metal nanoparticles for reaction. Silver ions are reduced to silver particles and attached to the surface of the liquid metal nanoparticles to obtain the liquid metal composite conductive ink. During the process of patterning it to form a flexible circuit conductive layer, citric acid is used to remove the oxide layer on the surface of the liquid metal particles, so that the prepared circuit has initial conductivity. At the same time, the cross-linking of citric acid and polyvinyl alcohol improves the tensile properties of the flexible circuit conductive layer.

[0029] The flow schematic diagram of the preparation method of the liquid metal composite conductive ink is as Figure 1 shown, and includes the following steps: S1. Mix polyvinyl alcohol, deionized water and liquid metal, and ultrasonically obtain a dispersion of PVA-modified liquid metal nanoparticles; S2. Add citric acid and silver nitrate into the dispersion liquid of liquid metal nanoparticles modified by polyvinyl alcohol, and mix and react to obtain the liquid metal composite conductive ink for flexible circuits.

[0030] In the method of the present invention, by mixing and ultrasonically treating polyvinyl alcohol and liquid metal in an aqueous solution, the liquid metal is dispersed into nanoparticles, and polyvinyl alcohol crosslinks into a network on the surface of the liquid metal nanoparticles to modify them. After adding citric acid and silver nitrate, silver ions are reduced to nano-silver particles and adhere to the surface of the liquid metal nanoparticles. The schematic structural diagram is as Figure 2 shown. The finally obtained liquid metal composite conductive ink is a dispersion liquid of liquid metal nanoparticles modified by citric acid and polyvinyl alcohol with silver particles attached to the surface.

[0031] The above-mentioned liquid metal composite conductive ink is used for the preparation of the conductive layer of a flexible circuit. It is sprayed on the circuit pattern, and then the solvent is removed by drying to form a film to obtain the conductive layer of the flexible circuit. The schematic diagram of the principle during the film-forming process is as Figure 3 shown. As water evaporates, due to the presence of citric acid, the pH of the system decreases, which can effectively remove the oxide layer on the surface of the liquid metal nanoparticles, activate the liquid metal, and at the same time, the capillary force increases, causing the liquid metal nanoparticles modified by polyvinyl alcohol with silver particles attached to the surface to aggregate, forming a connected conductive path and endowing the flexible circuit with initial conductivity. At the same time, during the film-forming process, citric acid crosslinks with polyvinyl alcohol, further improving the tensile properties of the material.

[0032] In some specific embodiments, the liquid metal is a gallium-indium alloy, and the content of gallium in the gallium-indium alloy is 75.5 wt%, and the content of indium is 24.5 wt%.

[0033] Since polyvinyl alcohol is soluble in water above 95 °C, polyvinyl alcohol is first dissolved in part of the water to prepare an aqueous solution of polyvinyl alcohol, and then mixed with the remaining deionized water and liquid metal.

[0034] In step S2, citric acid and silver nitrate can be directly added or added in the form of an aqueous solution. The addition amounts of citric acid and silver nitrate are relatively small. To ensure the accuracy of their addition amounts and better dissolution, preferably, they are added in the form of an aqueous solution.

[0035] In some specific embodiments, in step S1, the mass ratio of polyvinyl alcohol to liquid metal is 1:15 - 30, the mass ratio of citric acid to liquid metal is 1:10 - 50, and the mass ratio of silver nitrate to liquid metal is 1:1800 - 9000.

[0036] The ratio of polyvinyl alcohol to liquid metal will affect the conductivity and stretchability of the prepared circuit. If the content of polyvinyl alcohol is too high, the conductivity of the prepared circuit will become poor; if the content is too low, the stretchability will be poor.

[0037] In some specific embodiments, the polyvinyl alcohol solution, the citric acid solution, and the silver nitrate solution are all aqueous solutions. In the reaction system of step S1, the content of the liquid metal in the reaction system is 37.5 - 42.8 wt%, and in the reaction system of step S2, the content of the liquid metal in the entire reaction system is 32.9 - 41.6 wt%.

[0038] In some specific embodiments, the prepared liquid metal nanoparticles have a particle size of 250 ± 20 nm.

[0039] In some specific embodiments, the ultrasonic wave is an inserted pulse ultrasonic wave, with an ultrasonic power of 500 - 750 W, a frequency of 20 - 40 kHz, an amplitude of 20 - 40%, a single pulse time of 5 s, a time interval between two adjacent pulses of 5 s, a temperature of 5 - 15 °C, and an ultrasonic time of 5 - 30 min.

[0040] In some specific embodiments, in step S2, the stirring reaction is carried out for 3 - 10 min, and the reaction temperature is 18 - 25 °C.

[0041] The present invention provides a liquid metal composite conductive ink for flexible circuits prepared by the above preparation method.

[0042] The present invention further provides a flexible circuit conductive layer, which is prepared by the following method: using the above liquid metal composite conductive ink to form a circuit pattern on a substrate, and drying to remove the solvent to obtain the flexible circuit conductive layer.

[0043] In some specific embodiments, the method for forming the circuit pattern is the mask printing method, and the drying temperature is 40 - 80 °C and the time is 20 - 60 min.

[0044] In some specific embodiments, the initial conductivity of the formed flexible circuit conductive layer is 4 × 10 6 S / m.

[0045] Fourthly, the present invention further provides the application of the above liquid metal composite conductive ink in the preparation of flexible electronic devices.

[0046] According to the above scheme, the flexible electronic device includes a wearable device, a flexible sensor, or a flexible electrode.

[0047] The following are specific embodiments. In the following embodiments, the used polyvinyl alcohol has a degree of polymerization of 1799 and a degree of alcoholysis of 99%.

[0048] Example 1 In this example, a liquid metal composite conductive ink for flexible circuits was prepared. The specific process is as follows: (1) Add 0.6 g of polyvinyl alcohol (PVA) solution, 1 g of deionized water, and 1.5 g of liquid metal (LM) into a centrifuge tube, and perform water bath ultrasonic treatment for 20 min to obtain liquid metal nanoparticles modified with polyvinyl alcohol. The PVA solution is an aqueous PVA solution with a mass concentration of 10%. Insertion pulse ultrasonic treatment is adopted, with an ultrasonic power of 500 - 750 W, a frequency of 20 - 40 kHz, a temperature of 5 - 15 °C, an insertion pulse ultrasonic amplitude of 20 - 40%, a single pulse time of 5 s, and a time interval of 5 s between adjacent pulses; (2) Add 0.1 g of citric acid (CA) solution and 0.05 g of silver nitrate (AgNO 3 ) solution into the above liquid metal nanoparticles modified with polyvinyl alcohol, and stir and mix for 5 min to obtain liquid metal composite conductive ink. The CA solution is an aqueous CA solution with a mass concentration of 30%, and the silver nitrate solution is an aqueous silver nitrate solution with a concentration of 0.1005 mol / L.

[0049] Example 2 In this example, a liquid metal composite conductive ink for flexible circuits was prepared. The specific process is as follows: (1) Add 1 g of polyvinyl alcohol solution, 1 g of deionized water, and 1.5 g of liquid metal into a centrifuge tube, and perform water bath ultrasonic treatment for 20 min to obtain liquid metal nanoparticles modified with polyvinyl alcohol. The PVA solution is an aqueous PVA solution with a mass concentration of 10%, and the ultrasonic conditions are the same as those in Example 1; (2) Add 0.1 g of citric acid solution and 0.05 g of silver nitrate solution into the above liquid metal nanoparticles modified with polyvinyl alcohol, and stir and mix for 5 min to obtain liquid metal composite conductive ink. The CA solution is an aqueous CA solution with a mass concentration of 30%, and the silver nitrate solution is an aqueous silver nitrate solution with a concentration of 0.1005 mol / L.

[0050] Example 3 In this example, a liquid metal composite conductive ink for flexible circuits was prepared. The specific process is as follows: (1) Add 0.6 g of polyvinyl alcohol solution, 1 g of deionized water, and 1.5 g of liquid metal into a centrifuge tube, and perform water bath ultrasonic treatment for 20 min to obtain liquid metal nanoparticles modified with polyvinyl alcohol. The PVA solution is an aqueous PVA solution with a mass concentration of 10%; the ultrasonic conditions are the same as those in Example 1.

[0051] (2) Add 0.5 g of citric acid solution and 0.05 g of silver nitrate solution into the above liquid metal nanoparticles modified with polyvinyl alcohol, and stir and mix for 5 min to obtain liquid metal composite conductive ink. The CA solution is an aqueous CA solution with a mass concentration of 30%, and the silver nitrate solution is an aqueous silver nitrate solution with a concentration of 0.1005 mol / L.

[0052] Comparative Example 1 A liquid metal composite conductive ink for flexible circuits was prepared in this comparative example. The specific process was as follows: 0.6 g of polyvinyl alcohol solution, 1 g of deionized water, and 1.5 g of liquid metal were added to a centrifuge tube and sonicated in a water bath for 20 min to obtain polyvinyl alcohol-modified liquid metal nanoparticles. The polyvinyl alcohol solution was an aqueous solution of polyvinyl alcohol with a mass concentration of 10%, and the sonication conditions were the same as those in Example 1.

[0053] Comparative Example 2 A liquid metal composite conductive ink for flexible circuits was prepared in this comparative example. The specific process was as follows: (1) 0.6 g of polyvinyl alcohol solution, 1 g of deionized water, and 1.5 g of liquid metal were added to a centrifuge tube and sonicated in a water bath for 20 min to obtain polyvinyl alcohol-modified liquid metal nanoparticles. The polyvinyl alcohol solution was an aqueous solution of polyvinyl alcohol with a mass concentration of 10%, and the sonication conditions were the same as those in Example 1.

[0054] (2) 0.1 g of citric acid solution was added to the above polyvinyl alcohol-modified liquid metal nanoparticles and mixed to obtain a liquid metal composite conductive ink. The citric acid solution was an aqueous solution of citric acid with a mass concentration of 30%. Application Example 1 The liquid metal composite conductive inks prepared in Example 1, Comparative Example 1, and Comparative Example 2 above were used to form a conductive layer of a flexible circuit. Specifically, the liquid metal composite conductive ink was patterned to form a circuit pattern. For example, a circuit pattern could be formed on a flexible substrate layer by using a mask spraying method. Specifically, a mask engraved with the circuit pattern was placed on the flexible substrate, and the liquid metal composite conductive ink was evenly applied on the flexible substrate layer by spraying to form a conductive layer. Then, electronic components were bonded to the conductive layer and placed in an oven at 40 - 80 °C and dried for 20 - 60 min to cure and form a good electrical connection.

[0055] The performance of the flexible circuits prepared from the liquid metals in Examples 1 - 3, Comparative Example 1, and Comparative Example 2 was tested as follows.

[0056] 1. Test the initial conductivity Operation method: The initial resistance of the flexible circuits prepared from the liquid metals in Examples 1 - 3 and Comparative Example 1 was measured with a bench multimeter, and their initial conductivity was calculated.

[0057] Test results: For the flexible circuits prepared from the liquid metals in Examples 1 - 3, since the oxide layer on the surface of the liquid metal nanoparticles was removed by citric acid and the capillary force increased due to the evaporation of the solution moisture, the polyvinyl alcohol-modified liquid metal nanoparticles with silver particles attached to their surfaces would aggregate to form a continuous conductive network. Therefore, the circuits initially appeared silver, and the conductivity was 4×10 6S / m. In Comparative Example 1, the flexible circuit prepared from liquid metal was initially grayish-black and in an insulating state due to the presence of an oxide layer on the surface of the liquid metal particles, and the particles were insulated from each other by the insulating oxide layer. After external force stretching of the circuit, it became conductive, and the conductivity was 3.9×10 6 S / m.

[0058] The results show that the flexible circuits prepared in Examples 1-3 have better initial conductivity compared to Comparative Example 1.

[0059] 2. Test the change rate of tensile resistance Operation method: Stretch the flexible circuits prepared from liquid metal in Examples 1, Comparative Example 1, and Comparative Example 2 on a biaxial tensile testing machine, and measure the resistance change rate by using a bench multimeter and then calculating it.

[0060] The test results are as Figure 4 shown. When the tensile rate reaches 700%, the change rate of the tensile resistance in Comparative Example 1 has exceeded 6000%, the change rate of the tensile resistance in Comparative Example 2 remains within 500%, while the change rate of the tensile resistance in Example 1 can still remain within 100%. The flexible circuit prepared in Example 1 has a smaller change rate of tensile resistance compared to Comparative Example 1 and Comparative Example 2, and its tensile performance is higher than that of Comparative Example 1 and Comparative Example 2, while the tensile performance of Comparative Example 2 is higher than that of Comparative Example 1. In Examples 1 and 2, the cross-linking of citric acid and polyvinyl alcohol improved the stretchability of the material, so the change rate of the tensile resistance of the prepared circuit is linearly related to the tensile rate, and the curve of the change rate of the tensile resistance of the circuit prepared in Comparative Example 1 is approximately an exponential function. In addition, due to the addition of silver nitrate in Example 1, after silver ions are reduced to silver particles, they adhere to the surface of the liquid metal nanoparticles modified by polyvinyl alcohol, improving the anchoring between the liquid metal nanoparticles, and thus enhancing the stretchability of the circuit.

[0061] 3. Test the electrochemical performance under cyclic large tensile conditions Operation method: Stretch the flexible circuits prepared from liquid metal in Example 1 and Comparative Example 1 on a biaxial tensile testing machine for multiple cyclic stretches with a tensile rate of 300%, and measure the resistance change rate by using a bench multimeter and then calculating it.

[0062] The results of Example 1 are as Figure 5 shown. Due to the cross-linking effect of citric acid and polyvinyl alcohol and the improvement of particle anchoring by silver ions, the flexible circuit prepared in Example 1 can maintain a stable and consistent change rate of tensile resistance during multiple cyclic stretches, and the change rate of resistance is within 60%, and it can recover to the initial resistance, having stable electrochemical performance under cyclic large tensile conditions. The results of Comparative Example 1 are as Figure 6As shown, the stretching resistance change rate gradually increases with the increase in the number of cyclic stretching. At the beginning, the stretching resistance change rate is 1000%. After stretching 200 times, the resistance change rate approaches 10000% and cannot return to the initial resistance value, and it does not have stable electrochemical performance under the condition of cyclic large stretching. The results show that the flexible circuit prepared in Example 1 has better stretching performance.

[0063] Application Example 2 The above liquid metal composite conductive ink can be used to prepare a blood oxygen saturation detection circuit.

[0064] The structure of the flexible blood oxygen saturation detection circuit from bottom to top is a flexible substrate layer, a conductive layer, a packaging layer, an adhesion layer, and electronic components arranged on the conductive layer. The preparation materials of the flexible substrate layer, the packaging layer, and the adhesion layer are all PDMS, but their cross-linking agent mass fractions are different. The conductive layer is patterned from the liquid metal composite conductive material prepared in Example 1. The specific manufacturing process is as Figure 7 shown.

[0065] Put Sylgard 184 polydimethylsiloxane (PDMS) with a mass ratio of prepolymer to curing agent of 11:1 in a vacuum box to evacuate, let it stand for 40 - 60 minutes. After all the bubbles in the PDMS are eliminated, spin-coat the standing PDMS evenly on a common hard substrate. After the common hard substrate 2 spin-coated with PDMS stands at room temperature for 1 - 2 minutes, put it on the hot plate, adjust the temperature to 60 degrees, and the curing time is 40 minutes, then the flexible substrate layer 1 can be obtained on the common hard substrate 2. Then, place the mask 3 engraved with the circuit pattern on the flexible substrate layer 1, and use the spraying method to evenly apply the liquid metal composite conductive ink on the flexible substrate layer 1 to make the conductive layer 4. Peel the flexible substrate layer 1 coated with the conductive layer 4 as a whole from the hard substrate 2 and transfer it to the surface of a hard substrate 5 (such as sandpaper) that does not adhere to PDMS to obtain the overall independent flexible substrate layer 1 and conductive layer 4. Next, adhere the electronic components 6 to the conductive layer 4, and after drying and curing in an oven, a good electrical connection is formed. Then, place a square mold 8 on the flexible substrate layer 1, slowly inject PDMS into it until it evenly covers the entire circuit, and then apply a layer of viscous functional material on the upper surface of the PDMS, let it stand and react for 25 minutes, and remove the mold 8 to make the packaging layer 7 and the viscous functional layer 9. Thus, the flexible blood oxygen saturation detection circuit can be obtained.

[0066] Attach the flexible blood oxygen saturation detection circuit to the finger part of a healthy male to detect PPG signals, PR, and S P O 2 The detection results are as Figure 8 shown, where Figure 8 A is the PR and SpO of a normal person 2 ,Figure 8 B is the near-infrared light PPG signals at wavelengths of 880 nm and 660 nm. It can be seen that the AC of the near-infrared light PPG signal at a wavelength of 880 nm and the infrared light PPG signal at 660 nm collected both contain the photoplethysmogram, and the characteristics of the main wave amplitude, dicrotic wave amplitude, and dicrotic notch amplitude in the pulse wave are clearly visible. The collected PPG signals are processed to obtain PR and S P O 2 data. It can be seen that S P O 2 data is maintained at about 98%, meeting the level standard of healthy adult males.

[0067] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A method for preparing liquid metal composite conductive ink for flexible circuits, characterized in that: The steps include: S1. Mixing polyvinyl alcohol, deionized water and liquid metal, and obtaining a polyvinyl alcohol-modified liquid metal nanoparticle dispersion by ultrasonic treatment; S2. Add citric acid and silver nitrate to the liquid metal nanoparticle dispersion modified with polyvinyl alcohol, mix and react, and obtain liquid metal composite conductive ink for flexible circuits.

2. The method for preparing liquid metal composite conductive ink for flexible circuit according to claim 1, characterized in that: The liquid metal is a gallium-indium alloy, in which the gallium content is 75.5wt% and the indium content is 24.5wt%.

3. The method for preparing liquid metal composite conductive ink for flexible circuit according to claim 1, characterized in that: The mass ratio of polyvinyl alcohol to liquid metal is 1:15~30, the mass ratio of citric acid to liquid metal is 1:10~50, and the mass ratio of silver nitrate to liquid metal is 1:1800~9000.

4. The method for preparing liquid metal composite conductive ink for flexible circuit according to claim 3, characterized in that: The polyvinyl alcohol is added in the form of an aqueous solution. In the reaction system of step S1, the content of liquid metal in the reaction system is 37.5-42.8wt%. In the reaction system of step S2, the content of liquid metal in the entire reaction system is 32.9-41.6wt%.

5. The method for preparing the liquid metal composite conductive ink for flexible circuit according to any one of claims 1 to 4, characterized in that: In step S1, the ultrasound is an inserted pulse ultrasound with an ultrasound power of 500-750 W, a frequency of 20-40 kHz, an amplitude of 20-40%, a single pulse time of 5 s, a time interval of 5 s between two adjacent pulses, a temperature of 5-15° C., and a time of 5-30 min.

6. The method for preparing liquid metal composite conductive ink for flexible circuit according to claim 1, characterized in that: In step S2, the reaction is stirred for 3 to 10 minutes at a reaction temperature of 18 to 25°C.

7. Liquid metal composite conductive ink for flexible circuits prepared according to the preparation method according to any one of claims 1 to 6.

8. A flexible circuit conductive layer, characterized in that: The flexible circuit conductive layer is prepared by the following method: using the liquid metal composite conductive ink as claimed in claim 7 to form a circuit pattern on a substrate, drying and removing the solvent to obtain the flexible circuit conductive layer.

9. The flexible circuit conductive layer according to claim 8, characterized in that: The method for forming the circuit pattern is a mask printing method, the drying temperature is 40~80℃, and the time is 20~60min.

10. Use of the liquid metal composite conductive ink as claimed in claim 7 in the preparation of flexible electronic devices.

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