Liquid metal ink, preparation method and application thereof

Abstract

This invention provides a liquid metallic ink, its preparation method, and its applications. The liquid metallic ink of this invention comprises liquid metallic ink particles, a co-solvent, and water. The liquid metallic ink particles include EGaIn droplets, and the surface of the EGaIn droplets is coated with a polymer, which is a conjugated conductive polymer formed from poly(3,4-ethylenedioxythiophene) and poly(styrene sulfonate). Compared with traditional conductive inks, the liquid metallic ink of this invention offers higher stability, ease of processing, and better mechanical robustness. The ink printed on this inkjet printer maintains its linear shape without breaking even when the PVA substrate is deformed, showing promise for revolutionizing the printed electronics field. This invention also provides a preparation method and applications for the liquid metallic ink.

Description

Technical Field

[0001] This invention belongs to the field of 3D printing technology, specifically relating to a liquid metal ink, its preparation method, and its application. Background Technology

[0002] Gallium-based liquid metal (LM) particles, particularly gallium-indium eutectic alloys (EGaIn) composed of 75% gallium and 25% indium, have gained widespread attention in various applications, including stretchable electronics, microfluidics, soft robotics, microelectrodes, circuits, and catalysis, due to their unique fluid properties, low toxicity, and metallic conductivity. The inherent fluidity of EGaIn provides a significant advantage for the fabrication of flexible circuits. However, the high surface tension and density of EGaIn result in poor wettability with substrates, making the fabrication of EGaIn-based conductive circuits challenging. To address these drawbacks, various strategies have been reported, such as using surfactants to reduce surface tension, alloying with other metals, and utilizing advanced printing techniques to improve accuracy and stability. Current strategies for fabricating LM conductive circuits in flexible electronics involve a range of techniques, including injection molding, screen printing, soft lithography, selective wetting, reduction patterning, roller pen filling of metal, 3D printing, and laser patterning. The specific manufacturing process chosen depends on factors such as mold design, specialized equipment, production scale, material compatibility, and environmental conditions. While these LM molding methods are promising, they typically require numerous manual steps, making them both time-consuming and labor-intensive. Furthermore, their high cost, complexity, and need for specialized equipment and molds hinder large-scale production.

[0003] Inkjet printing technology offers a more efficient method for manufacturing conductive circuits, directly creating patterns without the need for molds or large amounts of equipment, significantly reducing production costs and time. The versatility and precision of inkjet printing allow for the efficient production of high-resolution, complex designs on a variety of substrates, making it ideal for the rapidly evolving field of flexible electronics. Inkjet printing technology is categorized into continuous inkjet (CIJ) and on-demand inkjet (DOD) systems based on the droplet ejection method. DOD inkjet printing is particularly attractive due to its environmental friendliness, ejecting droplets only when needed by generating pressure pulses. DOD inkjet printers are further classified into thermal, piezoelectric, electrostatic, and acoustic types. Thermal inkjet printers are a widely used type of DOD inkjet printer, generating droplets within microseconds by heating resistive elements in the ink chamber to 300-400°C, causing the ink to evaporate and form bubbles, which then expel the ink from the nozzle. These printers offer excellent value for money, producing small droplets that facilitate multi-layer deposition of various materials at different resolutions, and ensuring non-contact material handling with minimal waste.

[0004] Currently, inkjet printing with EGaIn-based inks faces several challenges, including unstable suspended matter and rapid oxidation in aqueous solutions, which hinders the printing process and reduces conductivity. Despite these challenges, water-based inks hold great potential compared to organic solvent-based liquid metals (LMs) due to their environmental and biocompatibility. In related technologies, some researchers have utilized alginate to achieve LM nano-sizing and develop biocompatible water-based inks, while others have used elemental sulfur to uniformly disperse liquid metals in bulk polymers. However, processing micro / nano LMs in organic systems often results in liquid metal insulation due to the non-conductive organic materials at the interface. Therefore, for most LM composites, post-processing methods such as mechanical sintering, laser sintering, or chemical treatment are required to restore conductivity. Furthermore, particle aggregation poses a challenge, leading to inconsistent printed patterns and potentially clogging printhead nozzles. Controlling the viscosity of EGaIn inks is also crucial; they are either too viscous, causing nozzle clogging, or too thin, resulting in poor resolution. To fully leverage the advantages of EGaIn in flexible electronics, the development of a liquid metal ink is urgently needed. Summary of the Invention

[0005] The present invention aims to at least solve one of the aforementioned technical problems existing in the prior art. To this end, the present invention provides a liquid metal ink that exhibits stable performance and good conductivity, making it suitable for use in flexible electronic products.

[0006] The present invention also provides a method for preparing liquid metal ink.

[0007] The present invention also provides a printed circuit.

[0008] The present invention also provides a sensor.

[0009] A first aspect of the present invention provides a liquid metal ink comprising liquid metal ink particles, a co-solvent, and water, wherein the liquid metal ink particles comprise EGaIn droplets, and the surface of the EGaIn droplets is coated with a polymer, wherein the polymer is a conjugated conductive polymer formed of poly(3,4-ethylenedioxythiophene) and poly(styrene sulfonate).

[0010] One of the technical solutions of the present invention concerning liquid metal ink has at least the following beneficial effects:

[0011] Typically, the high surface tension and density of EGaIn result in poor wettability with substrates. This invention addresses this by encapsulating EGaIn droplets with a conjugated conductive polymer formed from poly(3,4-ethylenedioxythiophene) and poly(styrene sulfonate), thereby reducing and altering the surface tension and density of EGaIn without affecting the contact angle and viscosity of the substrate. Furthermore, this invention mixes EGaIn droplets coated with the conjugated conductive polymer in different proportions of water-based co-solvents. Conductivity is achieved through simple mechanical extrusion without the need for additional laser or thermal activators, resulting in low cost and easy operation. The liquid metallic ink can be printed using common household inkjet printers without molds or specialized equipment, and can be printed on polymer substrates (such as polyamide, PVP, etc.) and even pulp paper at room temperature.

[0012] Compared to traditional conductive inks, the liquid metal ink of this invention offers greater stability, ease of processing, and better mechanical robustness. The inkjet-printed ink can maintain its linear shape without breaking even when the PVA substrate is deformed, and has the potential to revolutionize the field of printed electronics.

[0013] The liquid metal ink of this invention exhibits stable performance and good conductivity. When poly(3,4-ethylenedioxythiophene) (PEDOT) and poly(styrene sulfonate) (PSS) in the ink are coupled with EGaIn, a protective layer (i.e., the polymer coating the surface of the EGaIn droplets) is formed around the EGaIn particles, preventing the oxidation of surface Ga. The strong interaction between poly(3,4-ethylenedioxythiophene) and poly(styrene sulfonate) and EGaIn effectively shields the gallium core, preserving the Ga... + This state prevents further oxidation. The polymer not only stabilizes the ink but also improves its conductivity by maintaining the state of the gallium metal. Ink printed on substrates such as PVA has a resistance of 65kΩ.

[0014] Unlike existing insulating liquid metal inks, circuits printed with the liquid metal ink of this invention can conduct electricity through simple mechanical extrusion, without the need for thermal or laser sintering. Simply sliding the pattern several times (e.g., three times, depending on the film thickness and the length and width of the pattern) breaks down the surface oxide layer during the sliding process, causing the liquid metal core to coalesce and form a continuous conductive path. The cured ink then produces a shiny metallic color and conductivity.

[0015] The liquid metallic ink of this invention can achieve the desired performance by mixing EGaIn and PEDOT:PSS in different proportions of water (H2O) and a co-solvent. By adjusting the water ratio, the spreadability and stability of the ink on the substrate can be balanced, thereby optimizing the printing quality.

[0016] The liquid metal ink of this invention can print smooth, stable, and conductive patterns to produce printed LM lines with a resistance of 65kΩ on flexible PVA substrates, making them suitable as metal interconnects on hydrophilic substrates. This method greatly simplifies the creation of LM-based circuits, eliminates the need for thin-film metal deposition, and paves the way for more readily available and scalable manufacturing of flexible electronic devices.

[0017] According to some embodiments of the present invention, the cosolvent includes ethylene glycol.

[0018] The role of a cosolvent is to adjust viscosity, thereby regulating the printing process and adhesion to the substrate.

[0019] According to some embodiments of the present invention, the volume ratio of the co-solvent to water is 1:1.5 to 4.

[0020] If the co-solvent ratio is too high, the ink viscosity may be excessively high, which can obstruct ink flow within the printhead, affecting the stability and accuracy of inkjet printing. The ink may not effectively wet the substrate surface during printing, resulting in uneven ink distribution and fixation, impacting print quality and conductivity. Conversely, excessive water content significantly reduces ink viscosity, potentially causing the ink to flow excessively during printing, becoming difficult to control, or even leaking or seeping into the substrate, affecting print quality and accuracy. Furthermore, high water content may dilute the conductive components in the ink, resulting in discontinuous or unstable conductive paths in the printed circuit. Therefore, a co-solvent to water ratio of 1:1.5–4 is an optimized value that maintains the stability, conductivity, and print quality of liquid metal inks. Adjusting the ratio within this range balances ink flowability, adhesion, and conductivity, ensuring clear and stable printed circuit lines, suitable for applications such as flexible electronic devices.

[0021] According to some embodiments of the present invention, the average particle size D50 of the liquid metal ink particles is 100nm to 200nm.

[0022] Ink particles that are too small may cause the ink to flow excessively, making it difficult to precisely control the printing process and potentially resulting in blurry or unclear printed images. Small particles may also prevent the formation of sharp lines and patterns during printing, affecting print resolution and detail. Conversely, large particles increase ink viscosity, which can lead to poor ink flow within the printhead, impacting the stability and accuracy of inkjet printing. Large ink particles may also struggle to adhere effectively to the substrate, resulting in insufficient print adhesion and affecting print quality and durability.

[0023] Therefore, liquid metal ink particles with an average particle size in the range of 100nm to 200nm can maintain good flowability and viscosity control while ensuring printing resolution and pattern clarity. This particle size range also helps ensure the continuity and stability of the printed conductive path, thereby improving the reliability and performance of the printed circuit.

[0024] According to some embodiments of the present invention, the content of the conjugated conductive polymer in the liquid metal ink is 1 wt% to 20 wt%.

[0025] According to some embodiments of the present invention, the content of the conjugated conductive polymer in the liquid metal ink is 1 wt% to 10 wt%.

[0026] According to some embodiments of the present invention, the content of the conjugated conductive polymer in the liquid metal ink is 1 wt% to 5 wt%.

[0027] Insufficient conjugated conductive polymer content may lead to inadequate ink conductivity, resulting in unstable conductive paths or excessively high resistance values ​​in the printed ink. It may also weaken the ink's ability to encapsulate liquid metal ink particles, potentially affecting the ink's uniformity and stability during the printing process. Conversely, excessively high conjugated conductive polymer content will significantly increase ink viscosity, which may reduce flowability during inkjet printing and even affect printhead cleaning and maintenance.

[0028] According to some embodiments of the present invention, when the content of the conjugated conductive polymer in the liquid metal ink is 1 wt% to 20 wt%, the content of the liquid metal ink particles in the liquid metal ink is 50 μL to 500 μL.

[0029] The concentration of liquid metal ink particles in liquid metal inks ranges from 1 wt% to 20 wt% to achieve optimal performance and stability during the printing process. Specifically, too low a concentration of liquid metal particles may result in discontinuous or unstable conductive paths in the printed ink, affecting the circuit's conductivity. Too low a concentration may also make the liquid metal ink unstable during storage and printing, making it difficult to ensure print quality and durability. Conversely, too high a concentration of liquid metal particles increases ink viscosity, which may hinder ink flow within the printhead, affecting the stability and accuracy of inkjet printing. Liquid metals are relatively expensive, and excessively high concentrations significantly increase ink costs, hindering large-scale applications and market competitiveness.

[0030] According to some embodiments of the present invention, when the content of the conjugated conductive polymer in the liquid metal ink is 1wt% to 5wt%, the content of liquid metal ink particles in the liquid metal ink is 100 μL to 300 μL.

[0031] A second aspect of the present invention provides a method for preparing the liquid metal ink described in the first aspect of the present invention, the method comprising adding EGaIn, poly(3,4-ethylenedioxythiophene) and poly(styrene sulfonate) to water containing the co-solvent, ultrasonicating and then centrifuging.

[0032] One technical solution of the present invention relating to the preparation method of liquid metal ink has at least the following beneficial effects:

[0033] The preparation method of this invention requires no expensive equipment or complex process control, the reaction conditions are not harsh, the raw materials are readily available, the production cost is low, and it is easy to industrialize. It eliminates the need for expensive or toxic solvents, meeting environmental and health safety requirements. Ultrasonic treatment and centrifugation are relatively simple process steps, requiring no complex operating techniques. This increases the controllability and repeatability of production, improving production efficiency. Furthermore, raw materials such as EGaIn, poly(3,4-ethylenedioxythiophene), and poly(styrene sulfonate) are readily available and relatively inexpensive.

[0034] According to some embodiments of the present invention, the ultrasonic treatment is performed using a tip ultrasonic device.

[0035] Ultrasonic treatment is performed using a high-precision ultrasonic device, with the aim of breaking large EGaIn droplets into smaller particles through ultrasonic stimulation.

[0036] According to some embodiments of the present invention, the amplitude of the ultrasonic treatment is 10% to 30%.

[0037] According to some embodiments of the present invention, the ultrasonic treatment time is 60 min to 120 min.

[0038] After ultrasonic treatment, centrifugation was performed to collect the concentrated EGaIn ink for later use.

[0039] A third aspect of the present invention provides a printed circuit prepared from the liquid metal ink described in the first aspect of the present invention.

[0040] One of the technical solutions of the present invention concerning printed circuits has at least the following beneficial effects:

[0041] The printed circuit of the present invention, by using the liquid metal ink described in the first aspect of the present invention, possesses all the beneficial effects of that ink. Specifically:

[0042] It exhibits excellent conductivity. The ink formed by encapsulated EGaIn droplets can be printed on conventional home inkjet printers. After printing, a continuous conductive path is formed through simple mechanical extrusion, without the need for complex thermal or laser sintering processes. Because the conjugated conductive polymer effectively protects the EGaIn droplets from oxidation and maintains their conductivity, the printed circuit has a stable resistance value (65kΩ).

[0043] It has wide applicability. It can be printed on a variety of substrates such as flexible PVA substrates, making it suitable for the manufacture of various flexible electronic devices. The printed LM lines can serve as metal interconnects on hydrophilic substrates, ensuring stable performance even in humid environments.

[0044] Simplified manufacturing process. Eliminating traditional metal deposition and complex heat treatment processes greatly simplifies the manufacturing process of LM circuits. Conductivity can be achieved with just a few simple sliding patterns, reducing manufacturing costs and operational complexity.

[0045] Environmentally friendly and safe. Using water and ethylene glycol as solvents is more environmentally friendly and safer than traditional organic solvents. It eliminates the need for expensive or toxic chemicals, meeting environmental and health safety standards.

[0046] Print quality optimization. By adjusting the ratio of polymer and EGaIn, as well as the water content, the spreadability and stability of the ink on the substrate can be balanced, thereby optimizing print quality. The printed patterns are smooth and stable, maintaining their conductivity for a long time, making them suitable for electronic devices used for extended periods.

[0047] A fourth aspect of the present invention provides a sensor prepared from the liquid metal ink described in the first aspect of the present invention. Attached Figure Description

[0048] Figure 1 This is a schematic diagram of the synthesis and thermal printing process of LM@PEDOT:PSS.

[0049] Figure 2 This refers to the particle size distribution of EGaIn-PEDOT:PSS ink.

[0050] Figure 3 This is a scanning electron microscope image of liquid metal ink.

[0051] Figure 4 This is a transmission electron microscope (TEM) image of liquid metallic ink.

[0052] Figure 5 These are the X-ray photoelectron spectroscopy results for liquid metallic ink.

[0053] Figure 6 These are the contact angle test results of liquid metallic ink on different substrates.

[0054] Figure 7 These are schematic diagrams of the activation process of liquid metal ink and scanning electron microscope images of the printed circuit before and after activation.

[0055] Figure 8 This is a schematic diagram of a complex image created using liquid metal ink.

[0056] Figure 9 This is a graph showing the change in resistance over time when liquid metal ink is printed on a PVA film.

[0057] Figure 10 This is a schematic diagram of liquid metal ink printed on a PVA film, connected to an LED, and emitting light.

[0058] Figure 11 This is a graph showing the change in ΔR / Ro when the electronic skin monitors finger movements.

[0059] Figure 12 This is a graph showing the change in ΔR / Ro when the electronic skin monitors wrist movement.

[0060] Figure 13 The diagram shows EGaIn / PEDOT:PSS lines printed on paper, which allows the paper box to fold, exhibiting high stability and good conductivity, and activating the LED lights. Detailed Implementation

[0061] The following are specific embodiments of the present invention, and the technical solutions of the present invention will be further described in conjunction with the embodiments, but the present invention is not limited to these embodiments.

[0062] In a first aspect, some embodiments of the present invention provide a liquid metal ink comprising liquid metal ink particles, a co-solvent, and water, wherein the liquid metal ink particles comprise EGaIn droplets, and the surface of the EGaIn droplets is coated with a polymer, wherein the polymer is a conjugated conductive polymer formed of poly(3,4-ethylenedioxythiophene) and poly(styrene sulfonate).

[0063] It should be noted that the high surface tension and density of EGaIn typically result in poor wettability with the substrate. This invention addresses this by encapsulating EGaIn droplets with a conjugated conductive polymer formed from poly(3,4-ethylenedioxythiophene) and poly(styrene sulfonate), thereby reducing and altering the surface tension and density of EGaIn without affecting the contact angle and viscosity of the substrate. Furthermore, this invention mixes EGaIn droplets coated with the conjugated conductive polymer in different proportions of water-based co-solvents. Conductivity is achieved through simple mechanical extrusion without the need for additional laser or thermal activators, resulting in low cost and easy operation. The liquid metallic ink can be printed using common home inkjet printers without molds or specialized equipment, and can be printed on polymer substrates (such as polyamide, PVP, etc.) and even pulp paper at room temperature.

[0064] It should also be noted that, compared with traditional conductive inks, the liquid metal ink of the present invention offers higher stability, is easier to process, and has better mechanical robustness. The inkjet-printed ink can maintain its line shape without breaking even when the PVA substrate is deformed, and has the prospect of revolutionizing the field of printed electronics.

[0065] The liquid metal ink of this invention exhibits stable performance and good conductivity. When poly(3,4-ethylenedioxythiophene) (PEDOT) and poly(styrene sulfonate) (PSS) in the ink are coupled with EGaIn, a protective layer (i.e., the polymer coating the surface of the EGaIn droplets) is formed around the EGaIn particles, preventing the oxidation of surface Ga. The strong interaction between poly(3,4-ethylenedioxythiophene) and poly(styrene sulfonate) and EGaIn effectively shields the gallium core, preserving the Ga... + This state prevents further oxidation. The polymer not only stabilizes the ink but also improves its conductivity by maintaining the state of the gallium metal. Ink printed on substrates such as PVA has a resistance of 65kΩ.

[0066] It is understood that, unlike existing insulating liquid metal inks, circuits printed with the liquid metal ink of this invention can conduct electricity through simple mechanical extrusion, without the need for thermal or laser sintering. Simply sliding the pattern several times (e.g., three times, depending on the film thickness and the length and width of the pattern) breaks down the surface oxide layer during the sliding process, causing the liquid metal core to coalesce and form a continuous conductive path. The cured ink then produces a shiny metallic color and conductivity.

[0067] Furthermore, the liquid metallic ink of the present invention can achieve the desired performance by mixing EGaIn and PEDOT:PSS in different proportions of water (H2O) and co-solvent. By adjusting the water ratio, the spreadability and stability of the ink on the substrate can be balanced, thereby optimizing the printing quality.

[0068] The liquid metal ink of this invention can print smooth, stable, and conductive patterns to produce printed LM lines with a resistance of 65kΩ on flexible PVA substrates, making them suitable as metal interconnects on hydrophilic substrates. This method greatly simplifies the creation of LM-based circuits, eliminates the need for thin-film metal deposition, and paves the way for more readily available and scalable manufacturing of flexible electronic devices.

[0069] In conjunction with the first aspect, in some embodiments of the present invention, the co-solvent includes ethylene glycol.

[0070] The role of a cosolvent is to adjust viscosity, thereby regulating the printing process and adhesion to the substrate.

[0071] In conjunction with the first aspect, in some embodiments of the present invention, the volume ratio of the co-solvent to water is 1:1.5 to 4.

[0072] If the co-solvent ratio is too low, the ink viscosity may be too high, which can obstruct ink flow within the printhead, affecting the stability and accuracy of inkjet printing. The ink may not effectively wet the substrate surface during printing, resulting in uneven ink distribution and fixation, impacting print quality and conductivity. Conversely, excessive water content significantly reduces ink viscosity, potentially causing the ink to flow excessively during printing, becoming difficult to control, or even leaking or seeping into the substrate, affecting print quality and accuracy. Furthermore, high water content may dilute the conductive components in the ink, resulting in discontinuous or unstable conductive paths in the printed circuit. Therefore, a co-solvent to water ratio of 1:1.5–4 is an optimized value that maintains the stability, conductivity, and print quality of liquid metal inks. Adjusting the ratio within this range balances ink flowability, adhesion, and conductivity, ensuring clear and stable printed circuit lines, suitable for applications such as flexible electronic devices.

[0073] In conjunction with the first aspect, in some embodiments of the present invention, the average particle size D50 of the liquid metal ink particles is 100 nm to 200 nm.

[0074] Ink particles that are too small may cause the ink to flow excessively, making it difficult to precisely control the printing process and potentially resulting in blurry or unclear printed images. Small particles may also prevent the formation of sharp lines and patterns during printing, affecting print resolution and detail. Conversely, large particles increase ink viscosity, which can lead to poor ink flow within the printhead, impacting the stability and accuracy of inkjet printing. Large ink particles may also struggle to adhere effectively to the substrate, resulting in insufficient print adhesion and affecting print quality and durability.

[0075] Therefore, liquid metal ink particles with an average particle size in the range of 100nm to 200nm can maintain good flowability and viscosity control while ensuring printing resolution and pattern clarity. This particle size range also helps ensure the continuity and stability of the printed conductive path, thereby improving the reliability and performance of the printed circuit.

[0076] In conjunction with the first aspect, in some embodiments of the present invention, the content of the conjugated conductive polymer in the liquid metal ink is 1 wt% to 20 wt%.

[0077] In conjunction with the first aspect, in some embodiments of the present invention, the content of the conjugated conductive polymer in the liquid metal ink is 1 wt% to 10 wt%.

[0078] In conjunction with the first aspect, in some embodiments of the present invention, the content of the conjugated conductive polymer in the liquid metal ink is 1 wt% to 5 wt%.

[0079] Insufficient conjugated conductive polymer content may lead to inadequate ink conductivity, resulting in unstable conductive paths or excessively high resistance values ​​in the printed ink. It may also weaken the ink's ability to encapsulate liquid metal ink particles, potentially affecting the ink's uniformity and stability during the printing process. Conversely, excessively high conjugated conductive polymer content will significantly increase ink viscosity, which may reduce flowability during inkjet printing and even affect printhead cleaning and maintenance.

[0080] In conjunction with the first aspect, in some embodiments of the present invention, when the content of the conjugated conductive polymer in the liquid metal ink is 1wt% to 20wt%, the content of the liquid metal ink particles in the liquid metal ink is 50µL to 500µL.

[0081] The concentration of liquid metal ink particles in liquid metal inks ranges from 1 wt% to 20 wt% to achieve optimal performance and stability during the printing process. Specifically, too low a concentration of liquid metal particles may result in discontinuous or unstable conductive paths in the printed ink, affecting the circuit's conductivity. Too low a concentration may also make the liquid metal ink unstable during storage and printing, making it difficult to ensure print quality and durability. Conversely, too high a concentration of liquid metal particles increases ink viscosity, which may hinder ink flow within the printhead, affecting the stability and accuracy of inkjet printing. Liquid metals are relatively expensive, and excessively high concentrations significantly increase ink costs, hindering large-scale applications and market competitiveness.

[0082] In conjunction with the first aspect, in some embodiments of the present invention, when the content of the conjugated conductive polymer in the liquid metal ink is 1wt% to 5wt%, the content of liquid metal ink particles in the liquid metal ink is 100 μL to 300 μL.

[0083] In conjunction with the first aspect, in some embodiments of the present invention, the conjugated conductive polymers can be commercially available poly(3,4-ethylenedioxythiophene) and polystyrene sulfonate (PEDOT:PSS, which can be dispersed in water at a concentration of 1.5 wt.%).

[0084] In a second aspect, some embodiments of the present invention provide a method for preparing the liquid metal ink described in the first aspect of the present invention, the method comprising adding EGaIn, poly(3,4-ethylenedioxythiophene) and poly(styrene sulfonate) to water containing the co-solvent, ultrasonicating and then centrifuging.

[0085] It is understood that the preparation method of this invention does not require expensive equipment or complex process control, the reaction conditions are not harsh, the raw materials are readily available, the production cost is low, and it is easy to industrialize. It does not require the use of expensive or toxic solvents, meeting environmental protection and health and safety requirements. Ultrasonic treatment and centrifugation are relatively simple process steps, requiring no complex operating techniques. This increases the controllability and repeatability of production, improving production efficiency. Furthermore, raw materials such as EGaIn, poly(3,4-ethylenedioxythiophene), and poly(styrene sulfonate) are readily available and relatively inexpensive.

[0086] In conjunction with the second aspect, in some embodiments of the present invention, ultrasonic processing is performed using a tip ultrasonic device.

[0087] Ultrasonic treatment is performed using a high-precision ultrasonic device, with the aim of breaking large EGaIn droplets into smaller particles through ultrasonic stimulation.

[0088] In conjunction with the second aspect, in some embodiments of the present invention, the amplitude of the ultrasonic treatment is 10% to 30%.

[0089] In conjunction with the second aspect, in some embodiments of the present invention, the ultrasonic treatment time is 60 min to 120 min.

[0090] After ultrasonic treatment, centrifugation was performed to collect the concentrated EGaIn ink for later use.

[0091] In a third aspect, some embodiments of the present invention provide a printed circuit prepared from the liquid metal ink described in the first aspect of the present invention.

[0092] It is understood that the printed circuit of the present invention, by using the liquid metal ink described in the first aspect of the present invention, possesses all the beneficial effects of that ink. Specifically:

[0093] It exhibits excellent conductivity. The ink formed by encapsulated EGaIn droplets can be printed on conventional home inkjet printers. After printing, a continuous conductive path is formed through simple mechanical extrusion, without the need for complex thermal or laser sintering processes. Because the conjugated conductive polymer effectively protects the EGaIn droplets from oxidation and maintains their conductivity, the printed circuit has a stable resistance value (65kΩ).

[0094] It has wide applicability. It can be printed on a variety of substrates such as flexible PVA substrates, making it suitable for the manufacture of various flexible electronic devices. The printed LM lines can serve as metal interconnects on hydrophilic substrates, ensuring stable performance even in humid environments.

[0095] Simplified manufacturing process. Eliminating traditional metal deposition and complex heat treatment processes greatly simplifies the manufacturing process of LM circuits. Conductivity can be achieved with just a few simple sliding patterns, reducing manufacturing costs and operational complexity.

[0096] Environmentally friendly and safe. Using water and ethylene glycol as solvents is more environmentally friendly and safer than traditional organic solvents. It eliminates the need for expensive or toxic chemicals, meeting environmental and health safety standards.

[0097] Print quality optimization. By adjusting the ratio of polymer and EGaIn, as well as the water content, the spreadability and stability of the ink on the substrate can be balanced, thereby optimizing print quality. The printed patterns are smooth and stable, maintaining their conductivity for a long time, making them suitable for electronic devices used for extended periods.

[0098] In a fourth aspect, some embodiments of the present invention provide a sensor prepared from the liquid metal ink described in the first aspect of the present invention.

[0099] In conjunction with the fourth aspect, in some embodiments of the present invention, the sensor includes patches for monitoring various movements of the human body. By attaching these patches to different parts of the body, such as fingers, wrists, and the head, motion sensors with high responsivity can be developed. When the electronic tape is applied to the skin, it forms a conductive path that can detect changes in resistance (ΔR / Ro) during body movement. As the tape area moves, the tension on the conductive ink changes, thereby altering its resistance. These changes in resistance are then measured and analyzed to monitor and quantify specific movements of body parts.

[0100] The following will describe the concept and technical effects of the present invention clearly and completely with reference to embodiments, so as to fully understand the purpose, features and effects of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are all within the scope of protection of the present invention.

[0101] In the description of this invention, the terms "one embodiment," "some embodiments," "illustrative embodiment," "example," "specific example," or "some examples," etc., refer to specific features, structures, materials, or characteristics described in connection with that embodiment or example, which are included in at least one embodiment or example of the invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples.

[0102] Unless otherwise specified, "room temperature" in this invention means 25℃±5℃.

[0103] Unless otherwise specified, "about" in this invention means that the allowable error is within ±2%.

[0104] Unless otherwise specified in the examples, the procedures should be performed under standard conditions or conditions recommended by the manufacturer. Reagents or instruments whose manufacturers are not specified are all commercially available products.

[0105] The abbreviation PP represents a conjugated conductive polymer. PEDOT:PSS represents poly(3,4-ethylenedioxythiophene) and poly(styrene sulfonate). LM@PEDOT:PSS represents liquid metal ink.

[0106] In this embodiment, the conjugated conductive polymers are commercially available poly(3,4-ethylenedioxythiophene) and polystyrene sulfonate (PEDOT:PSS), supplied by Macklin (Shanghai) Chemical Technology Co., Ltd.

[0107] Example

[0108] In this embodiment, a liquid metal ink was first prepared, and then a polyvinyl alcohol polymer film was prepared by ourselves. The prepared ink was then used to print on the film, and a series of characterizations were performed.

[0109] The components of liquid metal ink are liquid metal ink particles, a co-solvent, and water. The liquid metal ink particles consist of EGaIn droplets, the surface of which is coated with a polymer, which is a conjugated conductive polymer formed by poly(3,4-ethylenedioxythiophene) and poly(styrene sulfonate).

[0110] in:

[0111] The cosolvent is ethylene glycol, with a content of 70:30 (water:EG, volume ratio).

[0112] The content of EGaIn droplets is 5 wt%.

[0113] The content of the conjugated conductive polymer is 5 wt%.

[0114] The remainder is water.

[0115] The preparation method of liquid metal ink is as follows:

[0116] Liquid metallic ink was prepared by adding 5 wt.% EGaIn and 5 wt.% PEDOT:PSS (i.e., conjugated conductive polymer) to an H2O:EG solution according to the specified ratio. The ink was ultrasonicated for 90 min at 20% amplitude in a cold water bath using a 1 / 16-inch micro-tip probe. After ultrasonication, the ink was centrifuged at 2000 rpm for 5 min to remove larger nanoparticles.

[0117] Preparation of polyvinyl alcohol (PVA) polymer films

[0118] PVA was prepared by mixing in a solution (5 wt.% PVA dissolved in water) at 80°C for 2 hours and then rotating at a high speed of 400 rpm. After the PVA was completely dissolved, the solution was poured into a mold and dried in an oven at 40°C for 6 hours. Then, to ensure uniform layer distribution, it was kept at room temperature until completely dry.

[0119] Printing process

[0120] After ink preparation, it was loaded into a 10mL ink cartridge (XL-805) with a print width of 0.57 inches (14.48mm). The liquid metallic ink on the PVA film was directly inkjet printed using an HP1200 printer with a thermal printhead. Optimized printing conditions were: droplet weights of 7 and 3 ng, printhead resolution of 1200 × 600 dpi, and substrate at room temperature (25°C). The PVA film was dried at 70°C for 10 minutes; this is the optimal temperature for solvent evaporation.

[0121] This flexible patch, based on EGaIn-PEDOT:PSS ink on a PVA substrate, is designed for testing electro-motion sensing devices.

[0122] PVA films were fabricated, each printed with EGaIn-PEDOT:PSS ink to form a patterned line 80 mm long and 5 mm wide. These PVA films were securely attached to the fingers and wrist joints of the experimenters to ensure that the films stretched and deformed synchronously with the joint movements. The ends of the wires were clamped with alligator clips and connected to a digital multimeter. The real-time changes in the wire resistance were measured using the digital multimeter throughout the process of repeatedly bending and straightening the fingers or wrists.

[0123] Characterization methods

[0124] It should be noted that all ultrasounds were performed using an ultrasound machine (Branson SFX550 Digital Ultrasound Cell Disruptor).

[0125] SEM images were captured using a SU8220 (Hitachi, Japan) measuring instrument. 10 μL of LM nanoparticle ink suspension diluted twice was dropped onto a silicon wafer and dried in the environment.

[0126] Transmission electron microscope (TEM) (JEOL JSM-7500FA) uses EGaIn / PEDOT:PSS ink, which has a dark color and a high concentration of LM nanoparticles. Therefore, the suspension was diluted with EtOH, then dropped onto the TEM copper grid and dried under ambient conditions.

[0127] Dynamic light scattering (DLS) measurements were performed using the Malvern Zetasizer nano-series at 25 °C. The ink was diluted to 1 μL in EtOH prior to each measurement.

[0128] Electromechanical coupling measurement; conductive traces (length: 40mm; width: 20mm) are used for electromechanical testing, following the manufacturing process.

[0129] After being treated with LM nanoparticle ink, the PVA with conductive traces was cut into dog bone shapes (length: 60mm) using a laser cutter (VLS 3.50, Universal Laser Systems, Inc.).

[0130] With the printed circuitry precisely positioned in the center of the dog-bone shape, these conductive-trace dog bones are subjected to uniaxial tensile loads within a general-purpose bread frame (Instron). Simultaneously, changes in sample resistance are measured using a multimeter. The tensile specimen is used to determine the maximum electromechanical strength at a displacement rate of 10 mm / min.

[0131] All resistance measurements were performed using a digital multimeter (Kitthley DMM6500). X-ray photoelectron spectroscopy (XPS) analysis was performed using a PHI 5000 VersaProbe III (ULVAC PHI, Japan).

[0132] The contact angle of the LM nanoparticles was measured using a DSA25 contact angle meter (Kruss, Germany).

[0133] Optical microscope SOPTOP (SZN71TR, China).

[0134] To measure the viscosity, a Pinkevich glass capillary viscometer was used.

[0135] Figure 1 The synthesis of LM@PEDOT:PSS and the subsequent thermal printing process are demonstrated. Figure 1 Part a shows the ink preparation process, part b shows the process of loading ink into the ink cartridge and printing using a thermal inkjet printer, and part c shows a schematic diagram of ink being printed on a PVA film.

[0136] The process of obtaining the properties of conductive LM@PEDOT:PSS printing inks involves four steps:

[0137] (i) Preparation of EGaIn ink;

[0138] (ii) Print using specific parameters;

[0139] (iii) Dry the film at about 70°C for about 10 min to evaporate the solvent, ensuring mechanical stability and adhesion to the substrate, although the film is still non-conductive at this stage;

[0140] (iv) Further drying at room temperature for 24 hours gives the films a shiny metallic color and conductivity, depending on their thickness.

[0141] The solution containing EGaIn droplets was sonicated with a probe for 1.5 hours. During this process, the ultrasonic stimulation broke down large EGaIn droplets into smaller particles. Simultaneously, these particles were stabilized by coating their surfaces with the conjugated polymer PEDOT:PSS. The surfaces of these particles became stable due to the coating with the conjugated polymer PEDOT:PSS. After centrifugation, the concentrated liquid metal ink was collected and loaded into an ink cartridge (XL-805) as a 0.57-inch (14.48 mm) printing tape.

[0142] Liquid metallic ink on PVA films was printed directly using an HP 1200 thermal printhead. Optimized printing conditions were: droplet weights of 7 and 3 ng, printhead resolution of 1200 × 600 dpi, and substrate at room temperature (25°C). Figure 1 In this study, the liquid metal ink on the PVA film containing circuits and lines is very stable and uniform, and does not affect its function as a conductive line after stretching and twisting.

[0143] The stability, dispersibility, and size of nanoparticles (NPs) are critical to liquid metal ink systems, as larger particles or aggregated nanoparticles can clog printer nozzles. Furthermore, NPs should have a narrow size distribution and be uniformly dispersed throughout the ink carrier to ensure optimal jetting performance.

[0144] The particle size distribution of EGaIn-PEDOT:PSS ink was characterized by dynamic light scattering (DLS), such as... Figure 2 As shown, the average particle size of the prepared LM ink is 164 nm.

[0145] Figure 3 The image shows a scanning electron microscope (SEM) image of liquid metallic ink in which EGaIn NPs with a smooth core-shell spherical structure are uniformly stacked together.

[0146] Figure 4 Transmission electron microscopy (TEM) images of liquid metal ink show that EGaIn NPs formed under continuous ultrasound are spherically isolated with clear boundaries and rounded shapes.

[0147] Furthermore, viscosity and contact angle are critical to ink performance. For example, thermal inkjet printing technology uses high temperatures to generate bubbles that eject ink from nozzles; therefore, the ink must contain an aqueous solution and be able to withstand these temperatures. Aqueous solvents often face challenges of poor dispersibility and adhesion in most conductive compounds, thus requiring the use of binders or surfactants to achieve high-quality ink uniformity. Imbalanced ink composition leads to a series of problems during printing, such as pattern distortion, surface cracking, and the coffee ring effect, becoming a serious obstacle to inkjet printing. To achieve optimal printing results, EG is used as a high-boiling-point co-solvent to reduce the solvent evaporation rate and provide suitable viscosity and contact angle values ​​for the printing process.

[0148] X-ray photoelectron spectroscopy (XPS) analysis of the ink revealed the chemical composition of its core and shell. For example... Figure 5 As shown, the measured XPS spectra indicate that the main elements are Ga 2p, Ga 3d, and carbon, corresponding to LM@PEDOT:PSS. The XPS results are referenced to the C1s peak with a binding energy of 284.6 eV.

[0149] Furthermore, such as Figure 6 As shown, contact angle tests revealed different values, with sample H-3 exhibiting 110.9°C on paper and 86.8°C on PVA substrates. Notably, the low contact angle (less than 90°C) of EGaIn ink droplets on the PVA film indicates excellent wettability. The contact angles of H-1 and H-3 were compared with the baseline H-2 ink. With the contact angle remaining essentially unchanged on paper, the contact angle of sample H-3 on the PVA film increased to 112.8°C. The high contact angle on the PVA film can be attributed to the higher EG content. Higher EG content results in higher ink viscosity, lower surface tension, less diffusion, and a larger contact angle. However, inks with lower EG content (H-1 and H-2) maintain a balance, allowing for better diffusion and a lower contact angle.

[0150] For the polymer binder of LM NPs, the examples selected the conductive polymer PEDOT:PSS, a conjugated polymer that is easily processed with aqueous solvents. This ensures its suitability for liquid metal ink applications without adversely affecting the ink's viscosity or contact angle properties. The content of PEDOT:PSS and EGaIn has a significant impact on the stability and performance of the liquid metal ink.

[0151] This invention first evaluated different dosages of EGaIn NPs to determine the optimal concentration for printing. 200 μL was determined to be the maximum suitable amount for LM, as higher concentrations resulted in difficulty in maintaining uniform and stable nanoparticles in solution, increasing the likelihood of nozzle clogging. Lower concentrations of low-metal nanoparticles, below 200 μL, exhibited poor conductivity. Tests were conducted using 200 μL of EGaIn NPs, with PEDOT:PSS concentrations varying to 1, 3, 5, and 10 wt.%.

[0152] The results showed that inks containing 1 and 3 wt.% PEDOT:PSS were unstable and had poor conductivity. Furthermore, higher concentrations of PEDOT:PSS (e.g., 10 wt.%) resulted in blurry images due to poor printability. Finally, adjusting the concentrations of PEDOT:PSS and EGaIn NPs to prepare inks achieved a good trade-off between stability and conductivity. In addition, this invention also attempted to use other surfactant polymers in the water-based ink system, such as polystyrene pyrrolidone (PVP), PVA, and polystyrene sulfonate (PSS). However, after loading these inks into cartridges, the printers failed to print, indicating a problem with the inks.

[0153] Furthermore, the wetting properties of EGaIn-PEDOT:PSS ink on PVA films were observed. The ink exhibited relatively hydrophilic properties, allowing it to spread and maximize contact due to its specific affinity for water. This caused the printed droplets to shrink into island shapes on the hydrophobic surface.

[0154] Figure 7 The activation process of conductive LM nanoparticle ink is shown. By sliding the pattern 3-4 times, the cured ink develops a shiny metallic color and conductivity, depending on the film thickness and the length and width of the pattern. During the sliding process, the mechanical action disrupts the surface oxide layer, causing the liquid metal core to coalesce and form a continuous conductive path. Repeated sliding ensures that the oxide layer is sufficiently disrupted, promoting the formation of a uniform conductive metal film. This activation process highlights the importance of mechanical activation in achieving the desired conductivity of LM nanoparticle inks, facilitating the production of high-quality conductive films that are easily activated. Figure 7 In the SEM images, before activation, the pattern shows that after 3-5 prints, the ink shrinks into island-like shapes, densely packed together. After activation, the pattern exhibits a different effect; the ink appears continuous and linear, demonstrating conductivity. The interconnected hollow spaces indicate good conduction in both the lateral and vertical directions.

[0155] Different circuits and complex images were printed on PVA film to demonstrate that the ink can create complex images, such as... Figure 8 As shown.

[0156] The stability of LM@PEDOT:PSS ink was assessed by printing square patterns on a PVA film using the same ink cartridge for several consecutive days, and measuring the change in its conductivity after curing under optimal conditions. Figure 9 As shown, the conductivity of LM@PEDOT:PSS patterns printed with the same cartridge remained constant for 7 consecutive days, then decreased. During this period, frequent nozzle clogging was observed, requiring multiple cleaning steps to maintain function. This behavior is likely due to the initiation of nanoparticle deposition or aggregation. After 14 days, the cartridge was no longer ejectible. This study suggests that the stability of LM@PEDOT:PSS inks can be improved by exploring alternative surfactants or binders.

[0157] exist Figure 10 In this study, a printed circuit was used to verify the conductivity of EGaIn-PEDOT:PSS ink on a PVA film. Connecting an LED to the circuit resulted in it emitting light with an amplitude of 3.05V, confirming the good conductivity of the pattern.

[0158] Furthermore, the liquid metallic ink LM@PEDOT:PSS of this invention can be used to manufacture electronic patches for monitoring various human body movements. By attaching these patches to different parts of the body, such as fingers and wrists, motion sensors with high responsiveness can be developed. When the electronic tape is applied to the skin, it forms a conductive path that can detect changes in resistance (ΔR / Ro) during body movement. As the tape area moves, the tension on the liquid metallic ink changes, thereby altering its resistance. These changes in resistance are then measured and analyzed to monitor and quantify specific movements of body parts.

[0159] Figure 11 and Figure 12 The changes in ΔR / Ro of the electronic skin are shown when monitoring finger and wrist movements, respectively. For example, when a finger bends, the tape stretches, causing a change in resistance detected by the sensor. Similarly, wrist and head movements cause corresponding changes in the resistance of the electronic patch, providing real-time monitoring of these movements. The application of LM@PEDOT:PSS ink in the electronic patch demonstrates its potential to create flexible wearable sensors that can accurately monitor and respond to human movement, paving the way for advancements in wearable technology and health monitoring systems.

[0160] Figure 13 The origami cube is made by printing EGaIn / PEDOT:PSS along the lines on the paper. It shows the process of energizing the LED light source by the printed EGaIn / PEDOT:PSS lines, demonstrating high stability and good conductivity.

[0161] The present invention has been described in detail above with reference to the embodiments. However, the present invention is not limited to the above embodiments. Within the scope of knowledge possessed by those skilled in the art, various changes can be made without departing from the spirit of the present invention.

Claims

1. A liquid metal ink, characterized in that, The components are liquid metal ink particles, a co-solvent, and water. The liquid metal ink particles include EGaIn droplets, the surface of which is coated with a polymer. The polymer is a conjugated conductive polymer formed by poly(3,4-ethylenedioxythiophene) and poly(styrene sulfonate). The co-solvent is ethylene glycol. The volume ratio of the co-solvent to water is 1:1.5 to 4. The average particle size D50 of the liquid metal ink particles is 100 nm to 200 nm. The liquid metal ink is prepared by adding EGaIn, poly(3,4-ethylenedioxythiophene) and poly(styrene sulfonate) to water containing the co-solvent, followed by ultrasonic treatment and centrifugation.

2. The liquid metal ink according to claim 1, characterized in that, The content of the conjugated conductive polymer in the liquid metal ink is 1wt% to 20wt%.

3. The liquid metal ink according to claim 2, characterized in that, The content of the liquid metal ink particles in the liquid metal ink is from 50 microliters to 500 microliters.

4. The liquid metal ink according to claim 1, characterized in that, The ultrasonic treatment is performed using a tip ultrasonic device; and / or the amplitude of the ultrasonic treatment is 10% to 30%; and / or the duration of the ultrasonic treatment is 60 min to 120 min.

5. A printed circuit, characterized in that, It is prepared from any one of the liquid metal inks according to claims 1 to 4.

6. A sensor, characterized in that, It is prepared from any one of the liquid metal inks according to claims 1 to 4.

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