Liquid metal-based flexible hybrid electronics for monitoring human pulse rate and method of fabrication

Abstract

The application relates to the technical field of flexible electronics, and specifically discloses a liquid metal-based flexible hybrid electronic device for monitoring human pulse rate and a preparation method. The device comprises an intrinsically stretchable circuit formed by patterning liquid metal on a rubber substrate, and micro functional elements integrated in the form of a patch, which are collectively encapsulated by an elastomer. The preparation method comprises six steps of circuit printing, hot pressing, chemical etching, selective infiltration, stripping transfer and integrated encapsulation. The key is to utilize the selective infiltration of liquid metal to copper for patterning, and utilize the characteristics of semi-solid polymer for soft transfer. The device of the application benefits from its intrinsic stretchable characteristics, can conformally attach to human skin and synchronously deform, effectively suppresses motion artifacts, and thus realizes accurate, real-time and long-term monitoring of pulse rate in various states such as sitting, walking and running. The preparation method is simple in process and low in cost, and is suitable for large-scale production.

Description

Technical Field

[0001] This invention relates to the field of flexible electronics, and in particular to a liquid metal-based flexible hybrid electronic device for monitoring human pulse rate and its preparation method. Background Technology

[0002] Human pulse rate is a core physiological parameter reflecting the function of the cardiovascular system. It is closely related to various cardiovascular diseases and is also an important indicator for assessing human health and exercise status, playing a significant role in disease management. Currently, the mainstream sensors for monitoring human pulse rate use the photoplethysmography (PPG) principle. This involves illuminating the skin with highly penetrating red light, and a photodiode capturing the light signal reflected from the subcutaneous tissue and converting it into an electrical signal. When blood flows through the detection site, blood vessels dilate, shortening the light reflection path and increasing the photoelectric intensity, thus non-invasively reflecting the human pulse rate information.

[0003] With the increasing popularity of proactive health concepts, the flexibility of PPG sensors has garnered significant attention, aiming to achieve wearable monitoring of the human pulse rate. This will revolutionize the traditional rigid PPG sensor format, which is worn on the body via a clip or watch, providing a highly reliable detection interface and thus ensuring the accuracy of the acquired physiological information. Flexible hybrid electronics, due to its integration of the technological advantages of flexible printing and traditional electronics, has become the preferred solution for developing flexible PPG sensors.

[0004] However, existing flexible electronic devices still have shortcomings in terms of tensile strength and dynamic conformal adhesion to the skin, which limits their monitoring accuracy under complex conditions such as motion. Therefore, there is an urgent need to develop a flexible hybrid electronic device that is intrinsically stretchable and can deform synchronously with human skin in order to achieve high-fidelity, cross-timescale dynamic physiological signal monitoring. Summary of the Invention

[0005] To improve the monitoring accuracy of flexible hybrid electronic devices in various states such as sitting, walking, and running, this application provides a liquid metal-based flexible hybrid electronic device for monitoring human pulse rate and its preparation method.

[0006] Firstly, this application provides a liquid metal-based flexible hybrid electronic device for monitoring human pulse rate, employing the following technical solution: A liquid metal-based flexible hybrid electronic device for monitoring human pulse rate includes an intrinsically stretchable elastic circuit, a micro-functional element, and an elastic encapsulation layer. The intrinsically stretchable elastic circuit includes a rubber substrate and a patterned liquid metal disposed on the rubber substrate. The micro-functional element is integrated with the intrinsically stretchable elastic circuit. The elastic encapsulation layer covers and encapsulates the intrinsically stretchable elastic circuit and the functional element. The liquid metal is a gallium-based alloy with a melting point below room temperature.

[0007] In one specific implementation, the liquid metal is gallium indium or gallium indium tin.

[0008] In one specific implementation, the micro-functional components include a surface-mount red LED, a surface-mount photodiode, a surface-mount resistor, a surface-mount capacitor, and a microprocessor chip, all of which are mounted on an intrinsically stretchable elastic circuit.

[0009] In one specific implementation, the rubber precursor of the rubber substrate is uncured silicone rubber Ecoflex and / or uncured silicone rubber PDMS.

[0010] Secondly, this application provides a method for fabricating a liquid metal-based flexible hybrid electronic device for monitoring human pulse rate, which adopts the following technical solution: A method for fabricating a liquid metal-based flexible hybrid electronic device for monitoring human pulse rate includes the following steps: Circuit printing: Printing circuit patterns onto thermal transfer paper to obtain thermal transfer paper with circuit patterns; Hot-press patterning: The hot transfer paper with the circuit pattern is brought into contact with the copper-clad film, and the pattern is transferred to the copper layer surface of the copper-clad film by hot pressing, thus obtaining a copper-clad film with the circuit pattern. Chemical etching: The copper-clad film with circuit patterns is immersed in an etching solution to remove the copper layer that is not protected by the pattern, resulting in a patterned copper substrate. Selective wetting: Remove the pattern material from the patterned copper substrate surface to expose the copper pattern, and wet the copper pattern with liquid metal in a protective solution to obtain a copper-clad film with patterned liquid metal; Peel-off transfer: An uncured rubber precursor is cast onto a copper-clad film with patterned liquid metal, heated to semi-cured to form a rubber substrate, and the patterned liquid metal is transferred onto the rubber substrate by mechanical peeling to form an elastic circuit. Integrated packaging: Micro-functional components are attached to the elastic circuit for integration, a rubber precursor is cast again, and heated to complete curing to complete the encapsulation of the device, resulting in a liquid metal-based flexible hybrid electronic device for monitoring human pulse rate.

[0011] In one specific implementation, the hot pressing conditions in the hot pressing patterning step are: temperature 120°C and pressure 0.5 MPa.

[0012] In one specific implementation, in the chemical etching step, the etching solution is a 20wt% ferric chloride solution, the etching temperature is 90°C, and the etching time is 3-5 minutes.

[0013] In one specific implementation, in the selective wetting step, the protective solution is a 5% (v / v) acetic acid solution, and the wetting time is 5 minutes.

[0014] In one specific implementation, the heating to semi-curing step in the peel transfer step is performed as follows: when the rubber precursor is uncured silicone rubber Ecoflex, heating is performed at 60°C for 10 minutes; when the rubber precursor is uncured silicone rubber PDMS, heating is performed at 100°C for 45 minutes.

[0015] In one specific implementation, the rubber substrate has a thickness of 200 micrometers during the peeling transfer step.

[0016] In summary, this application has the following beneficial effects: 1. Intrinsic stretchability and conformal fit: Using liquid metal as a conductor, the circuit itself has high stretchability. Combined with a low-modulus silicone rubber substrate, the elastic modulus of the device matches the skin, enabling imperceptible, conformal wear and synchronous deformation with limb movement, greatly reducing motion artifacts.

[0017] 2. High-precision monitoring: The liquid metal-based flexible hybrid electronic device used to monitor human pulse rate can stably and accurately capture pulse rate signals when the human body is in different activity states such as sitting, walking, and jogging.

[0018] 3. Simple process and low cost: The preparation method of this application combines mature printed circuit board (PCB) micromachining technology with soft material transfer technology, which does not require complex photolithography or expensive equipment, and is conducive to large-scale preparation.

[0019] 4. Reliable integration: Functional components are integrated with the liquid metal circuit via surface mount technology and are fixed by an elastomer, eliminating the need for soldering and ensuring the reliability of the connection under deformation. Attached Figure Description

[0020] Figure 1 These are images illustrating the device wear, microscopic details, and mechanical stretching of Embodiment 1 of this application. Detailed Implementation

[0021] The present application will be further described in detail below with reference to the embodiments.

[0022] Example Example 1

[0023] This embodiment provides a liquid metal-based flexible hybrid electronic device for monitoring human pulse rate, including an intrinsically stretchable elastic circuit, a micro-functional element, and an elastic encapsulation layer. The micro-functional element is integrated with the intrinsically stretchable elastic circuit, and the elastic encapsulation layer covers and encapsulates the intrinsically stretchable elastic circuit and the functional element.

[0024] This application also provides a method for fabricating a liquid metal-based flexible hybrid electronic device for monitoring human pulse rate, comprising the following steps: Circuit printing: Design a circuit pattern based on the PPG principle on a computer, reserve positions for functional components in the pattern, and use an inkjet printer to print carbon ink onto thermal transfer paper to obtain thermal transfer paper with circuit pattern.

[0025] Hot-pressing pattern: The heat transfer paper with the circuit pattern is brought into contact with the copper-clad film. Under the conditions of 120℃ and 0.5MPa, the carbon ink pattern is heat-transferred to the surface of the copper-clad film using a hot press, resulting in heat transfer paper with the circuit pattern. The copper-clad film thickness is 100 micrometers, and the copper layer thickness is 18 micrometers.

[0026] Chemical etching: The copper-clad film with the circuit pattern is immersed in a 20wt% ferric chloride solution at 90℃ for 3-5 minutes to complete the etching process and remove the copper layer not protected by carbon ink. After etching, the carbon ink particles are removed by washing with ethanol to obtain the patterned copper substrate.

[0027] Selective wetting: The pattern material on the patterned copper substrate surface is removed, exposing the copper pattern. The copper pattern is then immersed in a 5% (v / v) acetic acid solution using a liquid metal of gallium indium tin alloy (Ga: 68.5%, In: 21.5%, Sn: 10%) for 5 minutes, ensuring the liquid metal adheres only to the surface of the copper pattern. This yields a copper-clad film with patterned liquid metal. The weakly acidic environment inhibits the oxidation of the liquid metal, allowing for better selective wetting of the copper.

[0028] Peel-off transfer: After removing the copper-clad film with patterned liquid metal on its surface from the acetic acid solution, it is baked at 80°C for 20 minutes. Subsequently, uncured silicone rubber Ecoflex0030 is poured onto the copper-clad film with patterned liquid metal to form a coating with a thickness of approximately 200 micrometers. It is then heated at 60°C for 10 minutes until semi-cured to form a rubber substrate. The patterned liquid metal is then transferred onto the rubber substrate by mechanical peeling to form an elastic circuit.

[0029] Integrated packaging: A surface-mount red LED (660nm wavelength), a surface-mount photodiode, a surface-mount resistor, a surface-mount capacitor, and a microprocessor chip are integrated onto a flexible circuit. Uncured Ecoflex 0030 silicone rubber is then poured on top and heated until fully cured, completing the device encapsulation and yielding the desired result. Figure 1 The image shows a liquid metal-based flexible hybrid electronic device for monitoring human pulse rate.

[0030] Example 2

[0031] The only difference between this embodiment and Embodiment 1 is that the method for fabricating the liquid metal-based flexible hybrid electronic device for monitoring human pulse rate includes the following steps: Circuit printing: Design a circuit pattern based on the PPG principle on a computer, reserve positions for functional components in the pattern, and use an inkjet printer to print carbon ink onto thermal transfer paper to obtain thermal transfer paper with circuit pattern.

[0032] Hot-pressing pattern: The heat transfer paper with the circuit pattern is brought into contact with the copper-clad film. Under the conditions of 120℃ and 0.5MPa, the carbon ink pattern is heat-transferred to the surface of the copper-clad film using a hot press, resulting in heat transfer paper with the circuit pattern. The copper-clad film thickness is 100 micrometers, and the copper layer thickness is 18 micrometers.

[0033] Chemical etching: The copper-clad film with the circuit pattern is immersed in hydrochloric acid containing 25 wt% hydrogen peroxide at room temperature for 10-15 minutes to complete the etching process and remove the copper layer not protected by carbon ink. After etching, the carbon ink particles are removed by washing with acetone to obtain the patterned copper substrate.

[0034] Selective wetting: The pattern material on the patterned copper substrate surface is removed, exposing the copper pattern. The copper pattern is then immersed in a 5% (v / v) acetic acid solution using a gallium-indium alloy liquid metal (Ga: 80%, In: 20%) for 5 minutes, ensuring the liquid metal adheres only to the surface of the copper pattern. This yields a copper-clad film with patterned liquid metal. The weakly acidic environment inhibits the oxidation of the liquid metal, allowing for better selective wetting of the copper.

[0035] Peel-off transfer: After removing the copper-clad film with patterned liquid metal on its surface from the acetic acid solution, it is baked at 80°C for 20 minutes. Subsequently, uncured silicone rubber PDMS (Dow Corning DC184) is cast onto the copper-clad film with patterned liquid metal to form a coating with a thickness of approximately 200 micrometers. Heating at 100°C for 45 minutes until semi-cured forms a rubber substrate. The patterned liquid metal is then mechanically peeled off and transferred onto the rubber substrate to form an elastic circuit.

[0036] Integrated packaging: The surface-mount red LED (wavelength 660nm), surface-mount photodiode, surface-mount resistor, surface-mount capacitor and microprocessor chip are attached to the flexible circuit for integration. Uncured silicone rubber Ecoflex0030 is poured again and heated until fully cured to complete the encapsulation of the device, resulting in a liquid metal-based flexible hybrid electronic device for monitoring human pulse rate.

[0037] Example 3

[0038] The only difference between this embodiment and Embodiment 1 is that the method for fabricating the liquid metal-based flexible hybrid electronic device for monitoring human pulse rate includes the following steps: Circuit printing: Design a circuit pattern based on the PPG principle on a computer, reserve positions for functional components in the pattern, and use an inkjet printer to print carbon ink onto thermal transfer paper to obtain thermal transfer paper with circuit pattern.

[0039] Hot-pressing pattern: The heat transfer paper with the circuit pattern is brought into contact with the copper-clad film. Under the conditions of 120℃ and 0.5MPa, the carbon ink pattern is heat-transferred to the surface of the copper-clad film using a hot press, resulting in heat transfer paper with the circuit pattern. The copper-clad film thickness is 100 micrometers, and the copper layer thickness is 18 micrometers.

[0040] Chemical etching: The copper-clad film with the circuit pattern is immersed in a 20wt% ferric chloride solution at 90℃ for 3-5 minutes to complete the etching process and remove the copper layer not protected by carbon ink. After etching, the carbon ink particles are removed by washing with ethanol to obtain the patterned copper substrate.

[0041] Selective wetting: The pattern material on the patterned copper substrate surface is removed, exposing the copper pattern. The copper pattern is then immersed in a 5% (v / v) acetic acid solution using a liquid metal of gallium indium tin alloy (Ga: 68.5%, In: 21.5%, Sn: 10%) for 5 minutes, ensuring the liquid metal adheres only to the surface of the copper pattern. This yields a copper-clad film with patterned liquid metal. The weakly acidic environment inhibits the oxidation of the liquid metal, allowing for better selective wetting of the copper.

[0042] Peel-off transfer: After removing the copper-clad film with patterned liquid metal on its surface from the acetic acid solution, it was left to stand under natural conditions for 24 hours. Subsequently, a mixture of uncured silicone rubber Ecoflex0030 and uncured silicone rubber PDMS (Dow Corning DC184) at a mass ratio of 0.8:1 was poured onto the copper-clad film with patterned liquid metal to form a coating with a thickness of approximately 200 micrometers. Heating at 80°C for 20 minutes until semi-cured formed a rubber substrate, the patterned liquid metal was then mechanically peeled off and transferred onto the rubber substrate to form an elastic circuit.

[0043] Integrated packaging: The surface-mount red LED (wavelength 660nm), surface-mount photodiode, surface-mount resistor, surface-mount capacitor and microprocessor chip are attached to the flexible circuit for integration. Uncured silicone rubber Ecoflex0030 is poured again and heated until fully cured to complete the encapsulation of the device, resulting in a liquid metal-based flexible hybrid electronic device for monitoring human pulse rate.

[0044] Performance testing For Examples 1-3, the following performance tests were performed: The liquid metal-based flexible hybrid electronic device obtained in Example 1 was attached to the brachial artery of the human arm. Upon power-up, the device's LED illuminated and it began to function. The connection between the functional elements and the liquid metal elastic circuit could be observed under an optical microscope. Pulse rate signals were collected from the human body in three states: sitting, walking, and jogging.

[0045] Following the above method, the liquid metal-based flexible hybrid electronic device obtained in Examples 2 and 3 was attached to the brachial artery of the human arm, and pulse rate signals of the human body were collected in three dimensions: sitting, walking, and jogging. The detection results are shown in Table 1.

[0046] The test results are shown in Table 1.

[0047] Table 1

[0048] As shown in Table 1, with the support of the intrinsic stretchability of the liquid metal-based flexible hybrid electronic device in Example 1, the pulse rate signals of the human body in the three dimensions of sitting, walking and jogging were successfully collected at 62.5 bpm, 85.7 bpm and 131.2 bpm, respectively.

[0049] This demonstrates that by using liquid metal as a conductor, the circuit itself possesses high tensile strength. Combined with a low-modulus silicone rubber substrate, the device's elastic modulus matches that of the skin. Functional components are integrated with the liquid metal circuit via patch technology and secured by an elastomer, eliminating the need for soldering and ensuring reliability at the connection points under deformation. This enables seamless, conformal wear and synchronized deformation with limb movement, significantly reducing motion artifacts. Furthermore, the liquid metal-based flexible hybrid electronic device can stably and accurately capture pulse rate signals in various activity states, such as sitting, walking, and jogging, and can be worn for extended periods, achieving pulse rate monitoring across time scales. The fabrication methods in Examples 1-3 combine mature printed circuit board (PCB) microfabrication technology with soft material transfer technology, eliminating the need for complex photolithography or expensive equipment, thus facilitating large-scale fabrication.

[0050] This is likely because traditional rigid or ordinary flexible sensors can slide or separate from the skin under dynamic conditions, producing "motion artifacts" and interfering with the signal. This application, however, uses liquid metal circuitry and a silicone rubber substrate to create a "biomimetic" substrate that matches the mechanical properties of the skin. Silicone rubber provides a soft matrix, and liquid metal acts as a conductor, giving the entire circuit network intrinsic stretchability similar to rubber. The entire device can conformally adhere to the skin and deform synchronously. This helps eliminate interface instability caused by mechanical mismatch, thereby maximally suppressing motion artifacts and ensuring signal clarity and readability under different motion states. A surface-mount red LED and a surface-mount photodiode constitute the PPG sensing unit. The light emitted by the LED penetrates the skin, is modulated by subcutaneous blood, and is received by the photodiode and converted into a weak current signal. Surface-mount resistors / capacitors and a microprocessor chip, together with the sensing unit, constitute a complete signal conditioning and processing circuit. Resistors and capacitors may be used to construct analog circuits such as filtering and amplification, while the chip is responsible for analog-to-digital conversion, preliminary calculations, or communication. Surface mount integration is key, helping to avoid fragile mechanical connections like solder joints. Physical fixation and electrical connections are achieved through subsequent rubber encapsulation, thereby maintaining the integrity of the circuit under tension.

[0051] In traditional fabrication methods, liquid metal surfaces have an oxide film and high surface tension, making it difficult to directly pattern precisely on a substrate. However, it exhibits good wettability to certain metals. The fabrication method of this application first obtains a fine copper pattern through printing and etching. In a weakly acidic environment, the liquid metal selectively wets and adheres to the copper surface without adhering to the surrounding insulating areas. The copper pattern acts as a template and anchor points, precisely defining the shape and position of the final liquid metal circuit. The fabrication method of this application employs a transfer strategy on semi-cured rubber. When the uncured silicone rubber precursor is cast onto a substrate with a liquid metal pattern and heated to a semi-cured state, the rubber possesses sufficient viscosity and strength. The liquid metal is encapsulated within the rubber matrix. Upon stretching, the rubber matrix deforms, and the internal liquid metal, being a fluid, flows accordingly, resulting in minimal resistance change. Furthermore, due to the strong interfacial bonding, it does not detach. The flexible encapsulation material, when the device is stretched, can disperse stress to the surrounding stretchable circuitry and substrate, protecting the connection between rigid components and flexible circuitry from stress concentration damage. Therefore, this application can improve the monitoring accuracy of flexible hybrid electronic devices in various states such as sitting, walking, and running.

[0052] This specific embodiment is merely an explanation of this application and is not intended to limit it. After reading this specification, those skilled in the art can make modifications to this embodiment without contributing any inventive step, but such modifications are protected by patent law as long as they fall within the scope of the claims of this application.

Claims

1. A liquid metal-based flexible hybrid electronic device for monitoring human pulse rate, characterized in that, The device includes an intrinsically stretchable elastic circuit, micro-functional components, and an elastic encapsulation layer. The intrinsically stretchable elastic circuit includes a rubber substrate and patterned liquid metal disposed on the rubber substrate. The micro-functional components are integrated with the intrinsically stretchable elastic circuit. The elastic encapsulation layer covers and encapsulates the intrinsically stretchable elastic circuit and the functional components. The liquid metal is a gallium-based alloy with a melting point below room temperature; the liquid metal is gallium indium or gallium indium tin. The micro-functional components include a surface-mount red LED, a surface-mount photodiode, a surface-mount resistor, a surface-mount capacitor, and a microprocessor chip. The surface-mount red LED, surface-mount photodiode, surface-mount resistor, surface-mount capacitor, and microprocessor chip are all attached to the intrinsically stretchable elastic circuit.

2. The liquid metal-based flexible hybrid electronic device for monitoring human pulse rate according to claim 1, characterized in that, The rubber precursor of the rubber substrate is uncured silicone rubber Ecoflex and / or uncured silicone rubber PDMS.

3. A method for fabricating a liquid metal-based flexible hybrid electronic device for monitoring human pulse rate as described in any one of claims 1-2, characterized in that, Includes the following steps: Circuit printing: Printing circuit patterns onto thermal transfer paper to obtain thermal transfer paper with circuit patterns; Hot-press patterning: The hot transfer paper with the circuit pattern is brought into contact with the copper-clad film, and the pattern is transferred to the copper layer surface of the copper-clad film by hot pressing, thus obtaining a copper-clad film with the circuit pattern. Chemical etching: The copper-clad film with circuit patterns is immersed in an etching solution to remove the copper layer that is not protected by the pattern, resulting in a patterned copper substrate. Selective wetting: Remove the pattern material from the surface of the patterned copper substrate to expose the copper pattern, and wet the copper pattern with liquid metal in a protective solution to obtain a copper-clad film with patterned liquid metal; Peel-off transfer: An uncured rubber precursor is cast onto a copper-clad film with patterned liquid metal, heated to semi-cured to form a rubber substrate, and the patterned liquid metal is transferred onto the rubber substrate by mechanical peeling to form an elastic circuit. Integrated packaging: Micro-functional components are attached to the elastic circuit for integration, a rubber precursor is cast again, and heated to complete curing to complete the encapsulation of the device, resulting in a liquid metal-based flexible hybrid electronic device for monitoring human pulse rate.

4. The method for fabricating a liquid metal-based flexible hybrid electronic device for monitoring human pulse rate according to claim 3, characterized in that, In the hot pressing patterning step, the hot pressing conditions are: temperature 120℃ and pressure 0.5MPa.

5. The method for fabricating a liquid metal-based flexible hybrid electronic device for monitoring human pulse rate according to claim 3, characterized in that, In the chemical etching step, the etching solution is a ferric chloride solution with a mass concentration of 20 wt%, the etching temperature is 90°C, and the etching time is 3-5 minutes.

6. The method for fabricating a liquid metal-based flexible hybrid electronic device for monitoring human pulse rate according to claim 3, characterized in that, In the selected immersion step, the protective solution is a 5% (v / v) acetic acid solution, and the immersion time is 5 minutes.

7. The method for fabricating a liquid metal-based flexible hybrid electronic device for monitoring human pulse rate according to claim 3, characterized in that, In the peeling and transfer step, the heating to semi-curing is performed as follows: when the rubber precursor is uncured silicone rubber Ecoflex, it is heated at 60°C for 10 minutes; when the rubber precursor is uncured silicone rubber PDMS, it is heated at 100°C for 45 minutes.

8. The method for fabricating a liquid metal-based flexible hybrid electronic device for monitoring human pulse rate according to claim 7, characterized in that, In the peeling and transfer step, the thickness of the rubber substrate is 200 micrometers.

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