Stretchable flexible circuit based on metal-liquid metal slip structure

Abstract

The application provides a stretchable flexible circuit based on a metal-liquid metal slip structure, and belongs to the field of flexible electronics. The stretchable flexible circuit comprises a flexible substrate, at least one conductive layer, an electronic component layer and a flexible encapsulation layer, and each layer is arranged in a stacked manner. The conductive layer comprises a structure in which metal layers and liquid metal layers are alternately stacked. The metal layers comprise at least two groups, and each group of metal layers comprises a plurality of metal segments which are isolated from each other by notches. The projections of the metal segments of two adjacent groups of metal layers in a direction perpendicular to the stacking direction are arranged in a staggered manner, and constitute a slip structure when stretched. The liquid metal layer is filled in the gaps between the adjacent metal layers and the metal segments. The structure cooperatively dissipates stress through the cohesive flow of the liquid metal layer and the interface slip between the metal segments when stretched, so that the circuit can realize a stretchability of 80%-150% without needing a serpentine wiring, while maintaining the high electrical conductivity and high wiring density of the metal, and is suitable for the fields of flexible electronics and wearable sensing.

Description

Technical Field

[0001] This invention belongs to the field of flexible electronics technology, specifically relating to a stretchable flexible circuit based on a metal-liquid metal sliding structure. Background Technology

[0002] In recent years, flexible electronic devices, characterized by flexibility, stretchability, and biocompatibility, have become a research frontier, with applications expanding widely to fields such as health monitoring, human-computer interaction, robot sensing, and smart agriculture. In these applications, devices often need to be attached to dynamically changing biological surfaces or flexible carriers. This requires that their circuits not only be able to fit flexibly in the initial state but also have the ability to adapt to large and repeated stretching deformations during long-term use while maintaining stable electrical performance.

[0003] To achieve circuit stretchability, existing technologies mainly follow two approaches: First, designing stretchable structures, such as fabricating metal wires into serpentine or fractal geometries to absorb strain through in-plane buckling. While this method retains the high conductivity of the metal, it typically sacrifices wiring density and integration, and the complex structure presents challenges in manufacturing and reliability. Second, employing intrinsically stretchable conductive materials, such as conductive polymers or liquid metals. Among these, gallium-based liquid metals have attracted considerable attention due to their excellent fluidity and conductivity.

[0004] However, existing liquid metal circuit technologies, whether microchannel infusion or direct printing, suffer from problems such as complex processes, limited patterning precision (difficulty in achieving precise wiring with linewidths less than 100 micrometers), weak interface adhesion, and susceptibility to leakage or reliability degradation under repeated stretching. Specifically: (1) Soldering compatibility barriers with standard electronic components: A very thin layer of gallium oxide (Ga2O3) quickly forms on the surface of liquid metal in the air. This oxide film, combined with the inherent high surface tension of liquid metal, significantly hinders effective connection with conventional electronic components. In addition, commonly used lead-free solders often undergo chemical corrosion or diffusion reactions when in contact with liquid metal, further weakening the reliability of the connection. During the heating and soldering process, liquid metal circuits themselves are prone to deformation, displacement, or even breakage due to their high fluidity and poor mechanical stability, leading to circuit structure failure. These factors together make it difficult to directly apply traditional reflow soldering processes, and currently, alternative interconnection solutions such as anisotropic conductive adhesives, mechanical pressing, or surface modification must be relied upon. However, these methods still generally face problems such as high contact resistance, insufficient long-term reliability, complex processes, or poor dynamic stability, which seriously restrict the integration and practical application of liquid metal circuits with existing microelectronic component systems.

[0005] (2) Technical bottlenecks in high-density integration and precision manufacturing: The fluidity of liquid metal provides stretchability on the one hand, but on the other hand, it makes it extremely difficult to fabricate fine lines with clear edges, high aspect ratios, and high precision. Without physical barriers, liquid metal patterns are prone to migration and diffusion, leading to short circuits or performance degradation. Achieving high-density pads, microvia interconnects, and multilayer wiring structures similar to those on traditional printed circuit boards (PCBs) remains a huge challenge for existing liquid metal fabrication technologies. Most reported liquid metal circuits have relatively simple functions and low integration, making it difficult to support complex microsystems containing multiple sensors, signal processing, and wireless communication modules.

[0006] (3) Compatibility issues between manufacturing processes and the traditional microelectronics industry: Most current liquid metal patterning technologies, such as elastomer-based microfluidic technology and 3D printing direct writing technology, suffer from complex processes, limited patterning accuracy, difficulty in achieving precise wiring, weak interface adhesion, and susceptibility to leakage or reliability degradation under repeated stretching. Furthermore, these processing methods are non-standard and emerging manufacturing paradigms. They are often difficult to directly integrate with the existing large-scale, low-cost, high-precision process chains of the PCB industry. This disconnect in technological pathways sets extremely high technology transfer thresholds and cost barriers for their transition from laboratory demonstrations to large-scale production and commercial applications.

[0007] In summary, existing technologies struggle to simultaneously achieve high conductivity, high tensile strength, high wiring density, and ease of fabrication. Serpentine metal circuits sacrifice density; conductive elastomers lack sufficient conductivity; and existing liquid metal circuits face bottlenecks in precision machining and interface stability. Therefore, a novel circuit architecture and fabrication method is urgently needed to synergistically achieve conductivity, tensile strength, and high-density integration at both the material and structural levels, thereby advancing the practical application of next-generation high-performance flexible electronics. Summary of the Invention

[0008] To address the shortcomings of existing technologies, the present invention aims to provide a stretchable flexible circuit based on a metal-liquid metal sliding structure. This circuit, by introducing a unique sliding and interlacing structure in the conductive layer, can achieve significant stretching without the need for complex serpentine wiring, while maintaining the high conductivity and high wiring density of the metal conductors.

[0009] To achieve the above-mentioned objectives, the present invention provides a stretchable flexible circuit based on a metal-liquid metal sliding structure, comprising a flexible substrate, at least one conductive layer, an electronic component layer, and an encapsulation layer stacked sequentially, wherein the layers are arranged in a stacked manner. The conductive layer comprises a structure in which metal layers and liquid metal layers are alternately stacked; The metal layer comprises at least two groups, and each group of metal layers contains multiple metal segments. Adjacent metal segments are separated by laser-cut slits to form grooves and isolate them from each other. The metal segments of two adjacent metal layers are staggered in their projections perpendicular to the stacking direction; A liquid metal layer is disposed between at least two adjacent sets of metal layers and fills the gaps between the metal segments to form a continuous conductive path; slippage occurs during stretching, forming a metal-liquid metal slippage structure; An electronic component layer is disposed above or below the conductive layer; A flexible encapsulation layer covers the conductive layer and the electronic component layer.

[0010] This invention constructs a multilayered "metal-liquid metal" sliding structure on a flexible substrate. When the stretchable flexible circuit is stretched, the liquid metal layer in its conductive layer undergoes cohesive flow. Simultaneously, the interlaced metal segments in the upper and lower metal layers undergo relative and orderly interfacial sliding under the lubrication of the liquid metal. This flow-sliding synergistic mechanism efficiently dissipates the macroscopic tensile strain applied to the circuit and transforms it into microscopic interfacial movement and liquid metal deformation, thereby significantly reducing the mechanical stress borne by the metal segments themselves and effectively preventing plastic deformation or fracture.

[0011] The metal layer, as a rigid conductive substrate, can effectively encapsulate and fix the liquid metal layer in the middle, constraining the random flow and diffusion of the liquid metal and preventing leakage and displacement during processing and use. At the same time, the metal layer has excellent rigidity and formability, providing a stable processing substrate for laser engraving, greatly improving the precision and consistency of patterning, helping to achieve fine wiring at the microscale, and ensuring the high-density integration characteristics of the circuit.

[0012] Preferably, the maximum stretching ratio of the stretchable flexible circuit is not less than 50%, the resistance change rate of the conductive layer is not higher than 2%, and the minimum linewidth reaches 5 μm.

[0013] More preferably, the stretchable flexible circuit has a resistance change rate of no more than 1.6% for the conductive layer under the conditions of an elongation rate of no less than 80% and a liquid metal layer thickness of no less than 50 μm, and a minimum linewidth of 5 μm.

[0014] More preferably, the resistance change rate of the conductive layer in the stretchable flexible circuit is no higher than 1.1% under the conditions that the stretching rate is not less than 100% and the liquid metal layer thickness is not less than 150 μm.

[0015] More preferably, the conductivity of the stretchable flexible circuit is not less than 1.79 × 10⁻⁶. 7 S / m.

[0016] Preferably, the flexible substrate is made of an elastic polymer material.

[0017] More preferably, the flexible substrate is made of polydimethylsiloxane or an aliphatic aromatic copolyester.

[0018] Preferably, the electronic component layer includes at least one of a sensor, integrated circuit, resistor, capacitor, or light-emitting device.

[0019] Preferably, the metal layer is made of conductive metal or conductive metal alloy.

[0020] More preferably, the metal layer is made of copper foil, silver, or gold.

[0021] Copper foil, as a preferred conductive layer material, combines high conductivity, high structural strength, and excellent process adaptability. Copper foil can provide stable constraint and interface support for the liquid metal layer, effectively suppressing the diffusion, migration, and leakage of the liquid metal during processing and use, thus improving the overall stability of the circuit structure. Simultaneously, the smooth surface and moderate rigidity of copper foil significantly improve the precision and edge quality of laser engraving patterns, facilitating micron-level fine wiring. Furthermore, copper foil is fully compatible with traditional PCB soldering processes, allowing for reliable interconnection with electronic components using conventional solder without the need for complex surface modification of the liquid metal. This greatly reduces integration difficulty and manufacturing costs, enabling stretchable flexible circuits to be efficiently integrated with existing microelectronics manufacturing systems.

[0022] Compared to conventional pure liquid metal circuits, this structure, with the support of copper foil, retains the high strain adaptability of liquid metal while overcoming the shortcomings of pure liquid metal circuits, such as low processing precision, poor interface connectivity, and insufficient structural stability. It also combines the high conductivity of metal with the process compatibility of traditional PCBs, and has significant advantages in wiring density, processing convenience, device integration, and reliability.

[0023] Preferably, the thickness of the metal layer is no greater than 50 μm.

[0024] More preferably, the thickness of the metal layer is 6-35 μm.

[0025] Preferably, the liquid metal layer is made of gallium or a gallium-based alloy.

[0026] More preferably, the thickness of the liquid metal layer is no greater than 300 μm.

[0027] More preferably, the thickness of the liquid metal layer is 50-250 μm.

[0028] More preferably, the thickness of the liquid metal layer is 50-150 μm.

[0029] Preferably, different metal layer thicknesses correspond to different ranges of spacing between adjacent metal layer scribes, and the scribe extension direction is perpendicular to the stretching direction of the circuit.

[0030] The macroscopic tensile properties of the circuit are positively correlated with the thickness of the liquid metal layer and the design of the metal segments (such as the notch spacing). A thicker liquid metal layer provides stronger interfacial lubrication and stress dissipation during stretching, resulting in a higher achievable stretching rate. Larger notch spacing in the metal segments leads to longer slip strokes and stronger adaptability to large strains. However, the length of the metal segments must match the length of their natural fracture fragments under cyclic stretching to avoid uncontrollable overall fracture due to excessive length, ensuring only controllable local micro-fractures occur and maintaining the stability of the conductive path. During stretching, electrical connections are primarily ensured by the upper and lower metal segments. Under extreme strain, the continuous liquid metal layer serves as a backup conductive path, forming dual electrical integrity protection. This design ensures that the circuit experiences only controllable local damage during repeated deformation, significantly improving durability and cycle life.

[0031] More preferably, when the thickness of the metal layer is 6 μm, the spacing between adjacent scribe lines is 1-3 mm; When the thickness of the metal layer is 10 μm, the spacing between adjacent scribe lines is 2-4 mm; When the thickness of the metal layer is 20 μm, the spacing between adjacent scribe lines is 3-6 mm; When the thickness of the metal layer is 35 μm, the spacing between adjacent scribe lines is 4-10 mm.

[0032] To achieve the technical objective, this invention also provides a method for fabricating a stretchable flexible circuit based on a metal-liquid metal sliding structure, comprising the following steps: A flexible substrate is provided, and a structure in which metal layers and liquid metal layers are alternately stacked is formed on the flexible substrate to obtain a laminated blank; Each metal layer is patterned separately, while the liquid metal layer is patterned and its thickness is controlled using a mask, ultimately resulting in a conductive layer. A first encapsulation layer is covered on the conductive layer, and a window is made in the preset electronic component soldering area to fix the electronic component on the metal layer of the windowed area to form an electrical connection. A secondary encapsulation is performed on the area where electronic components are located to obtain a stretchable flexible circuit.

[0033] The process of building a laminated preform specifically includes: providing a flexible substrate, and constructing a layered structure on the flexible substrate by alternating layers of metal and liquid metal to form a laminated structure of at least three layers; the laminated structure has metal layers as the upper and lower surfaces, and liquid metal layers are set between adjacent metal layers as interlayers, that is, a three-layer structure has two metal layers sandwiching one liquid metal layer, a five-layer structure has three metal layers sandwiching two liquid metal layers, and so on to form a multi-layer composite structure.

[0034] Taking a three-layer structure as an example, the fabrication process of the conductive layer includes: patterning the upper and lower surface metal layers respectively, and patterning the liquid metal layer through a mask to finally obtain the conductive layer.

[0035] The post-conductive layer processing includes: covering the conductive layer with a first encapsulation layer, opening a window in the preset electronic component soldering area, fixing the electronic component to the metal layer in the windowed area to form an electrical connection; A secondary encapsulation is performed on the area where electronic components are located to obtain a stretchable flexible circuit.

[0036] Preferably, the patterning process is achieved by laser engraving, including: processing multiple grooves on each metal layer according to the metal layer groove distribution designed by the software to obtain mutually isolated metal segments, and controlling the groove pattern of each metal layer so that the metal segments are arranged alternately on the projection and form a sliding structure when stretched; liquid metal layer fills the gaps between adjacent metal layers and metal segments.

[0037] Preferably, the thickness of the photomask is no more than 300 μm.

[0038] More preferably, the thickness of the mask is 50-250 μm.

[0039] More preferably, the thickness of the mask is 50-150 μm.

[0040] To achieve the technical objectives, the present invention also provides the application of the aforementioned stretchable flexible circuit in wearable health monitoring devices, soft robot sensing skin, or bio-interface integrated sensing systems.

[0041] Preferably, in the application of a bio-interface integrated sensing system, the electronic component layer includes a temperature sensor and a processing circuit electrically connected to the temperature sensor; and the stretchable flexible circuit is conformally attached to the surface of the plant organ.

[0042] Compared with the prior art, the beneficial effects of the present invention include at least the following: (1) By using a sandwich and interleaved design of “metal-liquid metal”, the high conductivity of rigid conductors and the strain dissipation capability of liquid medium are combined at the microscale. This structure allows the circuit wires to adopt a simple straight layout, which can achieve high wiring density while withstanding tensile strain of up to 80%-150%, and the finest line width reaches 5 μm, breaking through the limitations of traditional serpentine wiring in terms of wiring density, line width accuracy and tensile strength.

[0043] (2) The conductive path is dominated by metal, and its resistance value changes very little during cyclic tensile testing. The resistance change rate of the conductive layer is no higher than 1.6% at 80% strain and no higher than 1.1% at a strain of not less than 100%, with a conductivity of 1.79 × 10⁻⁶. 7 With an S / m, its electrical stability and conductivity are far superior to intrinsically stretchable conductors based on conductive polymers or nanocomposites.

[0044] (3) The stretchable flexible circuit structure provided by this invention is highly compatible with mainstream flexible electronics manufacturing processes (such as laser patterning and surface mount technology). Electronic components can be stably connected to metal pads using standard soldering processes, without the need to develop special liquid metal interconnect technology. The interface is reliable and easy to integrate into the system.

[0045] (4) The flexible substrate, encapsulation material, metal foil and liquid metal used in the preparation process can all be commercially available materials, without the need for complex synthesis or pretreatment. The core patterning and structure definition are achieved through precision laser processing, which is flexible and highly accurate, and is conducive to large-scale preparation.

[0046] (5) The circuit has high tensile strength, high conductivity, high density and good biocompatibility. It is not only suitable for in-situ monitoring of plant physiological information (such as temperature and humidity), but also widely used in advanced fields such as wearable health monitoring devices, soft robot electronic skin, and human-computer interaction interfaces that require conformal attachment and sensing of dynamic curved surfaces. Attached Figure Description

[0047] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the accompanying drawings used in the description of the embodiments or the prior art will be briefly introduced below.

[0048] Figure 1 A schematic diagram of the stretchable flexible circuit based on a metal-liquid metal sliding structure provided by the present invention.

[0049] Figure 2 This is a schematic diagram of the fabrication process for a stretchable flexible circuit.

[0050] Figure 3This is an enlarged stretched view of the structure at point A in the schematic diagram of a stretchable flexible circuit.

[0051] Figure 4 The resistance change during the stretching of a thin liquid metal flexible circuit.

[0052] Figure 5 These are actual photographs of the circuit under different stretching rates.

[0053] Figure 6 This is an actual photograph of the circuit with the finest line width achieved through laser engraving.

[0054] Figure 7 The resistance change during the stretching of a relatively thick liquid metal flexible circuit.

[0055] Figure 8 This is a comparison of the temperature measured on the blade by the flexible circuit with the ambient temperature.

[0056] Figure 9 Comparison of the number of cyclic stretching cycles for different notch spacings.

[0057] Explanation of reference numerals in the attached figures: A - Conductive layer of metal-liquid metal slip structure; 1 - Substrate layer; 2-1 First metal layer; 2-2 Second metal layer; 3 - Liquid metal layer; 4 - Electronic component layer; 5 - Encapsulation layer. Detailed Implementation

[0058] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. The components of the embodiments of the present invention described and given in the accompanying drawings can generally be arranged and designed in various different configurations. Therefore, the following detailed description of the embodiments of the present invention provided in the accompanying drawings is not intended to limit the scope of the claimed invention, but merely to illustrate selected embodiments of the invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

[0059] The commercial dry release agent was purchased from Guangdong Wokesen New Materials Co., Ltd., model 717 epoxy resin high temperature release agent (WOCAX·717); the polydimethylsiloxane PDMS was purchased from Dow Corning Incorporated, USA, model Sylgard 184, which contains two components: prepolymer and curing agent; the liquid platinum silicone Ecoflex00-30 was purchased from BASF, USA, which contains two components: PART-A and PART-B.

[0060] This invention proposes a "metal-liquid metal" slip structure, taking a three-layer structure as an example, such as... Figure 1 As shown in Figure A, its core principle lies in the fact that when the circuit is stretched, the intermediate liquid metal layer 3 can efficiently dissipate stress through cohesive flow and interface sliding with the first metal layer 2-1 and the second metal layer 2-2, thereby protecting the brittle metal conductive layer from breakage. This allows the circuit to use linear high-density wiring while maintaining the high conductivity of the metal. This structure adapts to deformation through the flow and sliding of the liquid metal layer at 0%, 50%, and 100% stretching, maintaining electrical continuity. A detailed explanation follows with reference to the accompanying drawings and embodiments.

[0061] Example 1 like Figures 1-3 As shown in the figure, the fabrication of a stretchable flexible circuit based on a metal-liquid metal sliding structure provided in this embodiment specifically includes the following steps: Step 1: Clean a glass slide ultrasonically with ethanol and deionized water and dry it. Spray a layer of commercial dry release agent evenly on its surface and dry it in an oven at 80°C for 10 minutes to facilitate the subsequent peeling of the substrate.

[0062] Step 2: Mix the PDMS prepolymer and curing agent at a ratio of 10:1 and degas. Spin-coat the mixture onto the treated glass slide at 1000 rpm for 20 seconds to form a liquid film.

[0063] Step 3: Place the glass slide on a 95°C hot plate and heat it for 1 hour to form a smooth, flexible PDMS substrate layer 1 with a thickness of approximately 80 μm.

[0064] Step 4: The surface of the flexible PDMS substrate 1 is hydrophilized using an oxygen plasma treatment machine (18 W power, 60 s time) to enhance the adhesion of the subsequent metal layer.

[0065] Step 5: On the plasma-treated flexible PDMS substrate 1, a 6 μm double-sided lithium-ion battery copper foil is smoothly attached as the first metal layer 2-1. A 50 μm thick photomask is used to cover the copper foil, and liquid gallium metal is scraped into the pattern. 10 μL of 15% hydrochloric acid solution is added to remove the copper foil oxide layer, allowing the liquid metal to fully fill the photomask, resulting in liquid metal layer 3. Next, a second 6 μm thick copper foil is attached as the second metal layer 2-2, and a laminator is used to apply uniform pressure to ensure tight adhesion. After the liquid metal solidifies, the photomask is removed, forming a complete three-layer conductive structure of "copper foil-gallium-copper foil".

[0066] Step Six: Using a UV precision laser processing machine (wavelength 355 nm), the upper and lower copper foil layers are scanned and etched according to the design documents. The spacing between adjacent etch marks is 1 mm to remove excess material, forming a precise double-sided circuit pattern. Figure 1 The conductive layer A of the metal-liquid metal slip structure is shown.

[0067] Step 7: Mix the two components of Ecoflex-00-30 silicone rubber at a 1:1 mass ratio, degas, and then pour the mixture onto the patterned conductive layer surface. Cure at room temperature for 4 hours to form an encapsulation layer 5 with a thickness of approximately 200 μm. After curing, use an ultraviolet laser again to create windows at the preset component pad locations to precisely expose the underlying copper foil, i.e., the second metal layer 2-2.

[0068] Step 8: Apply no-clean solder paste to the copper foil pads at the window opening. Precisely place the electronic component layer 4 onto the pads. Reflow soldering is performed using a hot air gun at 180°C for 30 seconds, followed by cleaning with ethanol to remove any residue.

[0069] Step 9: Apply Ecoflex-00-30 adhesive to the area where the electronic component layer 4 is soldered, and cure at 60°C for 1 hour. Finally, cut the entire circuit into a fixed dumbbell shape to facilitate subsequent tensile characterization experiments. Gently peel it off from the glass slide to obtain an independent, stretchable, flexible circuit.

[0070] A tensile test was performed on the stretchable flexible circuit prepared in this embodiment. The two ends of the circuit were fixed to the stretching table using double-sided tape, and the top lead was connected to a multimeter. The resistance was measured using the four-wire method. The circuit resistance before stretching was recorded as R0, and the resistance after stretching was recorded as R. The rate of change of resistance was R / R0. The stretching rate and resistance change were recorded simultaneously. Figure 4 As shown, its overall maximum tensile strength is approximately 80%. Under 80% tensile strain, its conductor resistance change rate R / R0 is approximately 1.6%, exhibiting excellent electrical stability. Actual photographs under different tensile conditions are shown below. Figure 5 As shown, where Figure 5 Image a in the image was taken under the initial stretched state. Figure 5 Image b in the image was taken when the entire image was stretched to 60%.

[0071] The laser engraving parameters of the stretchable flexible circuit prepared in this embodiment were optimized to achieve a minimum linewidth of 5 μm. A real-world image is shown below. Figure 6 As shown, where Figure 6 Figure a in the middle and Figure 6 Image b in the image is an actual photograph taken with an electron microscope. Figure 6 Image c in the image is a scanning electron microscope image, demonstrating that a minimum linewidth of 5 μm can be achieved through condition optimization.

[0072] Example 2 This embodiment provides a stretchable flexible circuit based on a metal-liquid metal slip structure and its fabrication method, and applies it to the monitoring of micro-temperature changes in plant leaves.

[0073] This embodiment details the use of a low-modulus substrate and a relatively thick liquid metal layer to fabricate a flexible circuit with higher tensile strength that can conformally attach to plant leaves for temperature monitoring. The specific fabrication steps are as follows: Step 1: Clean a glass slide with ethanol and deionized water and dry it. Spray a layer of commercial dry release agent evenly and dry it in an oven at 80°C for 10 minutes.

[0074] Step 2: Ecoflex-00-30 silicone rubber. Mix the two components in a 1:1 mass ratio and degas. Spin-coat onto a glass slide at 2000 rpm for 40 seconds.

[0075] Step 3: Allow the glass slide to cure at room temperature (25°C) for 1 hour to form a flexible substrate layer 1. Ecoflex-30 has a lower modulus, allowing for better conformal bonding with the blade.

[0076] Step 4: The surface of the Ecoflex-30 substrate is hydrophilized using an oxygen plasma treatment machine (18 W power, 60 s time) to enhance the adhesion of subsequent metal layers.

[0077] Step 5: To obtain better tensile properties and a lower rate of change in resistance, this embodiment uses a 150 μm thick mask to control the thickness of the liquid metal layer 3.

[0078] On a plasma-treated Ecoflex-30 substrate, a 10 μm double-sided lithium-ion battery copper foil is smoothly attached as the first metal layer 2-1. A 150 μm thick mask is used to cover the copper foil, and liquid gallium metal is scraped into the pattern. 10 μL of 15% hydrochloric acid solution is added to remove the copper foil oxide layer, allowing the liquid metal to fully fill the mask, resulting in liquid metal layer 3. Next, a second 10 μm thick copper foil is attached as the second metal layer 2-2 and fixed using a laminator. After the liquid metal solidifies, the mask is removed, forming a complete three-layer conductive structure of "copper foil-gallium-copper foil".

[0079] Step Six: Using a UV precision laser processing machine (wavelength 355 nm), the upper and lower copper foil layers are scanned and etched according to the design documents. The spacing between adjacent etch marks is 2 mm. Excess parts are removed to form a precise double-sided circuit pattern. Figure 1 The conductive layer A of the metal-liquid metal slip structure is shown.

[0080] Step 7: Mix the two components of Ecoflex-00-30 silicone rubber at a 1:1 mass ratio, degas, and then pour the mixture onto the patterned conductive layer surface. Cure at room temperature for 4 hours to form an encapsulation layer 5 with a thickness of approximately 200 μm. After curing, use an ultraviolet laser again to create windows at the preset component pad locations to precisely expose the underlying copper foil, i.e., the second metal layer 2-2.

[0081] Step 8: Apply no-clean solder paste to the copper foil pads at the window opening. Precisely place the electronic component (layer 4) onto the pads. Reflow soldering is performed using a hot air gun at 180°C for 30 seconds, followed by cleaning with ethanol to remove any residue.

[0082] Step 9: Apply Ecoflex-00-30 adhesive to the areas where components are soldered, and cure at 60°C for 1 hour. Finally, cut the entire circuit into a dumbbell shape and test it using a cyclic tensile testing machine. The circuit ultimately achieved a lifespan of over 1000 cycles under 100% strain cyclic tensile stress, demonstrating high durability.

[0083] The circuit prepared in this embodiment was subjected to a tensile test. Both ends of the circuit were fixed to a tensile stage using double-sided tape, and the top lead was connected to a multimeter. The resistance was measured using the four-wire method. The circuit resistance before stretching was recorded as R0, and the resistance after stretching was recorded as R. The rate of change of resistance was R / R0. The stretching rate and resistance change were recorded simultaneously. Figure 7 As shown, the fabricated circuit as a whole can withstand a maximum tensile rate of approximately 150%. Under 100% tensile strain, its wire resistance change rate is approximately 1.1% (see [reference]). Figure 7 The electrical stability is better than that of Example 1, thanks to the better dissipation of forces by the thicker liquid metal slip layer.

[0084] The circuit and external shape of this embodiment were redesigned and processed, and the outer shape was laser-engraved into a leaf-like shape. Figure 2 Gently peel it off from the glass slide to obtain an independent, stretchable, flexible circuit.

[0085] The obtained flexible temperature sensing circuit was gently attached to the upper surface of healthy cotton seedling leaves. The ambient temperature recorded by a commercially available high-precision temperature sensor (placed in a cool, shaded area near the stem roots) was used as a control. Continuous monitoring for 24 hours yielded the following results: Figure 8 As shown. The leaf temperature measured by the circuit of this invention follows the same trend as the ambient temperature, but it can more sensitively capture the micro-temperature fluctuations caused by transpiration and light changes in the leaf itself, confirming its effectiveness and application potential in in-situ monitoring of plant physiology.

[0086] Example 3 The preparation process of Example 3 is the same as that of Example 2, except that the spacing between adjacent grooves in step six is ​​4 mm.

[0087] Example 4 The preparation process of Example 4 is the same as that of Example 2, except that the copper foil thickness in step five is 35 μm and the spacing between adjacent scribe lines in step six is ​​4 mm.

[0088] Example 5 The preparation process of Example 5 is the same as that of Example 2, except that the copper foil thickness in step 5 is 35 μm and the spacing between adjacent scribe lines in step 6 is 10 mm.

[0089] Comparative Example 1 The preparation process of Comparative Example 1 is the same as that of Example 2, except that the spacing between adjacent grooves in step six is ​​8 mm.

[0090] Comparative Example 2 The preparation process of Comparative Example 1 is the same as that of Example 2, except that the copper foil thickness in step five is 35 μm and the spacing between adjacent scribe lines in step six is ​​12 mm.

[0091] The stretchable flexible circuits prepared in Examples 2, 3, 4, and 5, and Comparative Examples 1 and 2, were tested using a cyclic stretching machine. The resistance change during the stretching process is shown in the figure. Figure 9 As shown, the stretchable flexible circuits prepared in Examples 2-5 have a lifespan exceeding 1000 cycles under 100% strain cyclic stretching, achieving high durability. In contrast, the stretchable flexible circuits prepared in Comparative Examples 1 and 2 only have a lifespan of 10 cycles under 100% strain cyclic stretching. This is because during cyclic stretching, the copper foil segments corresponding to different copper foil thicknesses are too long, causing wrinkles that affect the normal stretching of the circuit. Liquid metal accumulates at the wrinkles, leading to circuit breakage. This demonstrates that the circuits have poor lifespan and cannot reach the normal lifespan level of circuits.

[0092] In summary, the present invention demonstrates through Examples 1 and 2 that: Co-design and controllable performance: By adjusting the thickness of the liquid metal layer (50-150 μm) and the substrate modulus (PDMS to the softer Ecoflex), the overall tensile properties (80% to 150%) and resistance stability of the circuit can be effectively controlled to meet the monitoring needs of plants with different growth rates.

[0093] This invention demonstrates through examples and comparative examples that: By adjusting the grooves in the metal layer, the reusability of the stretchable circuit and the groove range corresponding to the thickness of the metal layer are verified, which has certain guiding significance for industrialization.

[0094] This patent optimizes the ratio between the notch spacing and the metal layer thickness, enabling the circuit to withstand 1000 cycles of strain tension, achieving high durability.

[0095] High density and high conductivity: The sliding structure allows the circuit to withstand large strain without serpentine traces, achieving linear high-density wiring with a line width / spacing of less than 100 μm and a minimum line width of 5 μm, while maintaining the high conductivity of copper.

[0096] The process is universal and biocompatible: the method is compatible with mainstream flexible electronics processes (spin coating, laser processing, surface mount welding), and all materials (PDMS, Ecoflex, gallium, copper) have good biocompatibility, making them suitable for long-term plant wear.

[0097] Precise monitoring capabilities: such as Figure 8 As shown, this circuit can stably and sensitively monitor micro-changes in plant leaf temperature, solving the problem of traditional methods being unable to obtain leaf microenvironment data in situ, non-destructively, and over a long period of time.

[0098] Finally, it should be noted that the above-described embodiments are merely specific implementations of the present invention, used to illustrate the technical solutions of the present invention, and not to limit it. The scope of protection of the present invention is not limited thereto. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that any person skilled in the art can still modify or easily conceive of changes to the technical solutions described in the foregoing embodiments within the technical scope disclosed in the present invention, or make equivalent substitutions for some of the technical features; and these modifications, changes, or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention, and should all be covered within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A stretchable flexible circuit based on a metal-liquid metal sliding structure, characterized in that, It includes a flexible substrate, at least one conductive layer, an electronic component layer, and a flexible packaging layer, with each layer arranged in a stacked manner; The conductive layer comprises a structure in which metal layers and liquid metal layers are alternately stacked; The metal layer comprises at least two groups, and each group of metal layers contains multiple metal segments. Adjacent metal segments are separated by laser-cut slits to form grooves and isolate them from each other. The metal segments of two adjacent metal layers are staggered in their projections perpendicular to the stacking direction; The liquid metal layer is disposed between at least two adjacent metal layers and fills the gap between the metal segments to form a continuous conductive path; it slips during stretching, forming a metal-liquid metal slip structure.

2. The stretchable flexible circuit according to claim 1, characterized in that, The maximum stretchability of the stretchable flexible circuit is not less than 50%, the resistance change rate of the conductive layer is not higher than 2%, and the minimum linewidth reaches 5 μm.

3. The stretchable flexible circuit according to claim 2, characterized in that, The conductivity of the stretchable flexible circuit is not less than 1.79 × 10⁻⁶. 7 S / m.

4. The stretchable flexible circuit according to claim 1, characterized in that, The flexible substrate is made of an elastic polymer material; The metal layer is made of conductive metal or conductive metal alloy; The liquid metal layer is made of gallium-based elemental material or gallium-based alloy.

5. The stretchable flexible circuit according to claim 4, characterized in that, The thickness of the metal layer is no greater than 50 μm; The thickness of the liquid metal layer is no more than 300 μm.

6. The stretchable flexible circuit according to claim 5, characterized in that, Different ranges of spacing between adjacent grooves in the metal layer are set according to different metal layer thicknesses, including: when the thickness of the metal layer is 6 μm, the spacing between adjacent grooves is 1-3 mm; When the thickness of the metal layer is 10 μm, the spacing between adjacent scribe lines is 2-4 mm; When the thickness of the metal layer is 20 μm, the spacing between adjacent scribe lines is 3-6 mm; When the thickness of the metal layer is 35 μm, the spacing between adjacent scribe lines is 4-10 mm; The direction of the groove extension is perpendicular to the stretching direction of the circuit.

7. The method for fabricating a stretchable flexible circuit according to any one of claims 1-6, characterized in that, Includes the following steps: A flexible substrate is provided, and a structure in which metal layers and liquid metal layers are alternately stacked is formed on the flexible substrate to obtain a laminated blank; Each metal layer is patterned separately, while the liquid metal layer is patterned and its thickness controlled using a mask, ultimately resulting in a conductive layer. A first encapsulation layer is covered on the conductive layer, and a window is made in the preset electronic component soldering area to fix the electronic component on the metal layer of the windowed area to form a conductive connection. A secondary encapsulation is performed on the area where electronic components are located to obtain a stretchable flexible circuit.

8. The preparation method according to claim 7, characterized in that, The patterning process is achieved by laser engraving, including: processing multiple grooves on each metal layer according to the metal layer groove distribution designed by the software to obtain mutually isolated metal segments, and controlling the groove pattern of each metal layer so that the metal segments are arranged alternately on the projection and form a sliding structure when stretched; liquid metal layer fills the gaps between adjacent metal layers and metal segments.

9. The application of the stretchable flexible circuit according to any one of claims 1-6 in wearable health monitoring devices, soft robot sensing skin, or bio-interface integrated sensing systems.

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