Method for manufacturing anisotropic recoverable flexible circuit based on balsa wood liquid metal

Abstract

The invention provides a method for manufacturing an anisotropic recoverable flexible circuit based on balsa wood liquid metal, which comprises the following steps of: S1, preparing a semi-lignin-removed balsa wood chip or an all-lignin-removed balsa wood chip as a balsa wood substrate; s2, the liquid metal is subjected to stirring pretreatment, and an oxidation film is generated on the surface of the liquid metal; s3, printing a conductive circuit pattern on the surface of the cork wood substrate by the pretreated liquid metal through a transfer printing method; and the hollow channel in the cork base is filled with the pretreated liquid metal in a vacuum-assisted mode. The flexible circuit prepared by adopting the technical scheme of the invention does not cause LM leakage and has the characteristics of anisotropy and high conductivity, the cork wood substrate can be naturally degraded, and LM can be recycled by a DES solution, so that the flexible circuit is more environment-friendly.

Description

Technical Field

[0001] The present invention relates to the field of flexible electronic technology, and in particular to a method for manufacturing anisotropic recyclable flexible circuits based on balsa wood liquid metal. Background Art

[0002] Flexible electronics typically consist of flexible packaging materials and deformable conductive circuits. The choice of flexible packaging material directly influences the anisotropy and mechanical properties of the flexible electronics themselves. Wood, particularly balsa, has a unique layered, porous structure and nearly straight tubular shape that not only naturally encapsulates conductive materials and constructs conductive circuits, but also exhibits excellent flexibility and a large internal surface area after removing lignin, making it highly attractive for flexible electronics applications.

[0003] Initial research involved infusing molten Sn-Bi alloy into balsa wood using a vacuum-pressure method. The Sn-Bi alloy solidified at room temperature, mechanically strengthening the material and imparting anisotropic conductivity. However, the preparation process was complex, requiring high-temperature vacuum treatment at 250°C and consuming significant energy. Furthermore, the inherent lignin and hemicellulose structures within the balsa wood were not removed, and the presence of the solid alloy reduced the material's potential for flexible applications such as bending and stretching. To further enhance its application in flexible electronics, researchers delignified balsa wood films, infused epoxy resin into the hollow channels for polymerization, and deposited silver nanowires on the film's surface. This resulted in the fabrication of high-definition, transparent, flexible conductive films for flexible electronic screens. Unfortunately, this deposition method is prone to defects or cracks at the interface between the balsa wood cell walls and the polymer during flexible bending or other deformations due to shrinkage during polymerization and polymer aging. This significantly degrades the anisotropy and mechanical properties of the composite. Furthermore, the conductivity is achieved by depositing silver nanowires on the surface, making it impossible to construct circuits within the film or interconnect the silver nanowires on the surface. To create conductive circuits within the internal channels of balsa wood, researchers chemically delignified the wood, leaving behind a large number of hydroxyl groups on the inner walls of the channels, which are highly absorbent. Multi-walled carbon nanotubes (MWCNTs) were then embedded within the delignified wood structure. However, because balsa wood lacks the corresponding hydroxyl groups on its surface, designable circuit patterns cannot be formed on the wood surface, limiting the application of this method in flexible electronics.

[0004] Gallium-based liquid metal is widely used in conductive circuit applications in flexible electronics, intelligent sensing, biomedicine, and other fields due to its excellent electrical conductivity, thermal conductivity, and fluidity at room temperature, as well as its low melting point, low viscosity, low toxicity, and low saturated vapor pressure. The vacuum method is a relatively mature, convenient, and low-cost method for filling hollow, multi-channel, straight-walled balsa wood structures with gallium-based liquid metal as conductive circuits. However, the low wettability of gallium-based liquid metal itself and the high fluidity after filling the channels, which leads to the easy leakage of liquid metal when the balsa wood material deforms, remain difficult to solve. In addition, the balsa wood-conductive circuit composite materials used in the prior art contain epoxy resin, rubber, and polyimide matrix materials, which are difficult to degrade and recycle. Summary of the Invention

[0005] In response to the above technical problems, the present invention discloses a method for manufacturing anisotropic recyclable flexible circuits based on balsa wood liquid metal. This method is conducive to filling the hollow channels inside delignified balsa wood with vacuum assistance to construct internal anisotropic conductive circuits, eliminating the leakage of gallium-based liquid metal due to high fluidity; and the material is degradable and recyclable.

[0006] To this end, the technical solution adopted in the present invention is:

[0007] A method for manufacturing anisotropic recyclable flexible circuits based on balsa wood liquid metal comprises the following steps:

[0008] Step S1, preparing semi-delignified balsa wood chips or fully delignified balsa wood chips as balsa wood substrate;

[0009] Step S2, stirring the liquid metal to pre-treat it so as to form an oxide film on its surface;

[0010] Step S3: Print the conductive circuit pattern of the pretreated liquid metal onto the surface of the balsa wood substrate by transfer printing; and fill the hollow channel inside the balsa wood substrate with the pretreated liquid metal by vacuum assistance.

[0011] Furthermore, the liquid metal is gallium-based liquid metal (gallium-indium eutectic EGaIn, LM).

[0012] This technical solution uses a stirring pretreatment of the liquid metal to produce an excess oxide film with good wettability on its surface, making it easier for the liquid metal to fill the hollow channels inside the delignified balsa wood with the aid of a vacuum to construct internal conductive circuits. During the filling process, there is no leakage due to high fluidity. Through transfer printing, the surface pattern can be constructed on the substrate surface to achieve anisotropic and highly conductive properties. The preparation process is simple, the balsa wood substrate can be naturally degraded, and the LM can be recycled by the DES solution. The flexible electronic components obtained by this method are more environmentally friendly.

[0013] As a further improvement of the present invention, in step S1, the semi-delignified balsa wood chips are prepared by completely immersing the balsa wood chips in a NaOH and Na2SO3 solution, vacuum treating them for 0.5-1.5 hours, washing them to remove residual chemicals, and then freeze-drying them for at least 40 hours to obtain the semi-delignified balsa wood chips. Furthermore, in the NaOH and Na2SO3 solution, the concentration of NaOH is 2.0-3.0 M, and the concentration of Na2SO3 is 0.3-0.5 M. More preferably, in the NaOH and Na2SO3 solution, the concentration of NaOH is 2.5 M, and the concentration of Na2SO3 is 0.4 M.

[0014] The fully delignified balsa wood chips are prepared by placing balsa wood slices in an acetate buffer solution (pH 4.6) containing NaClO2 at 80-90°C for 2-6 hours, washing, and freeze-drying for at least 40 hours to obtain fully delignified balsa wood chips. Furthermore, the concentration of the NaClO2 acetate buffer solution is 0.5-1.5 wt%, and further, the concentration of the NaClO2 acetate buffer solution is 1.0 wt%.

[0015] As a further improvement of the present invention, in step S1, semi-delignified balsa wood is used as the balsa wood substrate.

[0016] As a further improvement of the present invention, in step S2, the pretreatment is stirring, the stirring speed is 500-700 rpm / min, and the time is more than 3 hours.

[0017] As a further improvement of the present invention, in step S2, the pretreatment is stirring, the stirring speed is 600 rpm / min, and the time is 4 hours.

[0018] As a further improvement of the present invention, in step S3, the conductive circuit pattern includes at least three independent conductive circuit patterns.

[0019] As a further improvement of the present invention, the conductive circuit pattern includes a first conductive circuit, a second conductive circuit, a third conductive circuit, a fourth conductive circuit, and a fifth conductive circuit. The second conductive circuit, the third conductive circuit, the fourth conductive circuit, and the fifth conductive circuit are connected in parallel and in series with the first conductive circuit.

[0020] The present invention also discloses an anisotropic recyclable flexible circuit based on balsa wood liquid metal, which is prepared by the method for manufacturing an anisotropic recyclable flexible circuit based on balsa wood liquid metal as described above.

[0021] The present invention also discloses a flexible electronic component, which includes the anisotropic recyclable flexible circuit based on balsa wood liquid metal as claimed in claim 8.

[0022] Compared with the prior art, the present invention has the following beneficial effects:

[0023] First, the technical solution of the present invention pre-treats gallium-based liquid metal (galium-indium eutectic EGaIn, LM) to produce an excess oxide film with good wettability on the surface of the liquid metal. This allows the LM to facilitate vacuum-assisted filling of the hollow channels within delignified balsa wood to construct internal conductive circuits, eliminating the risk of LM leakage due to high fluidity. Using a transfer method, a stamp with the designed circuit shape is imprinted on the LM with the excess oxide film and then transferred to the balsa wood surface, completing the surface patterning to achieve anisotropic and highly conductive properties, with a conductivity of up to 9.8×10 2 S·m -1 (σ z )(parallel to the LM direction, i.e. parallel to the direction of the hollow channel inside the balsa wood), 8.9×10 -6 S·m -1 (σ x )(perpendicular to the LM direction, that is, perpendicular to the direction of the hollow channel inside the balsa wood); the balsa wood-LM composite material is prepared and used for flexible applications of multi-channel mechanical sensing.

[0024] Second, the balsa wood substrate in the balsa-LM composite material of the present invention can be naturally degraded, and the LM can be recovered using a DES solution. The DES solution contains choline chloride and oxalic acid in a 1:2 molar ratio, mixed in an 80°C water bath until the solution is transparent. When the balsa-LM composite material comes into contact with the DES solution, the LM detaches from the balsa wood substrate and condenses into spheres due to high surface tension, with a mass recovery rate of LM exceeding 96%. This demonstrates that the present invention is recyclable, more environmentally friendly, and has significant scientific and commercial significance. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 1 is a flow chart of a method for manufacturing anisotropic recyclable flexible circuits based on balsa wood liquid metal according to an embodiment of the present invention; wherein (a) is a flow chart of step S1, and (b) is a flow chart of step S2.

[0026] Figure 2 These are light microscopic and electron microscopic images of balsa wood, semi-delignified balsa wood, and fully delignified balsa wood according to the embodiments of the present invention; wherein, a)-c) are light microscopic images, electron microscopic images, and electron microscopic partial enlarged images of balsa wood; d)-f) are light microscopic images, electron microscopic images, and electron microscopic partial enlarged images of semi-delignified balsa wood; h)-j) are light microscopic images, electron microscopic images, and electron microscopic partial enlarged images of fully delignified balsa wood.

[0027] Figure 3 These are the LMWA mechanical properties, electrical conductivity stability and flexibility results of the three balsa wood substrates of the embodiments of the present invention; among them, a) is the comparison result of the bending recovery after wrapping cylinders of different radii with semi-delignified balsa wood and fully delignified balsa wood substrates; b) is the flexible display of the semi-delignified balsa wood substrate - knotted state; c) is the flexible display of the semi-delignified balsa wood substrate - curled state; d) is the flexible display of the semi-delignified balsa wood substrate - twisted state; e) is the flexible display of the semi-delignified balsa wood substrate - bent state; f)-h) are the flexible displays of the LMWA conductor of the semi-delignified balsa wood substrate.

[0028] Figure 4 : These are performance diagrams of the LM filled into balsa wood material and performance diagrams of the prepared stress sensor in an embodiment of the present invention; wherein, a) is a schematic diagram of the hollow straight channel inside the balsa wood material filled with LM that has been pretreated by mechanical stirring with vacuum assistance; b)-c) are schematic diagrams and SEM results of the cross-section of the PDF-wood substrate after pre-treatment with LM; d)-e) are schematic diagrams and SEM results of the longitudinal section of the PDF-wood substrate after pre-treatment with LM; f) is a diagram of the conductivity anisotropy results in the X and Z directions after the PDF-wood substrate is pre-treated with LM; g) is a comparison diagram of the conductivity anisotropy of different materials; h) is a design diagram of the surface circuit pattern of the LMWA conductor on the cross-sectional surface of a semi-delignified balsa wood substrate; i) is a physical diagram of the surface circuit pattern of a stress sensor prepared with an LMWA conductor on a semi-delignified balsa wood substrate; j) is a simplified schematic diagram of the surface and internal circuit of a stress sensor prepared with an LMWA conductor on a semi-delignified balsa wood substrate; k) is a diagram of the voltage measured over four channels of a stress sensor prepared with an LMWA conductor on a semi-delignified balsa wood substrate as a function of stress.

[0029] Figure 5 Schematic diagram of recycling of recyclable flexible circuits according to an embodiment of the present invention; wherein, a) is a schematic diagram of the recycling process of LMWA circuits in DES solution, b) is a schematic diagram of the recycling process of PDF-wood-based LMWA circuits in DES solution, and c) is a schematic diagram of the recycling process of CDF-wood-based LMWA circuits in DES solution. DETAILED DESCRIPTION

[0030] The preferred embodiments of the present invention are described in further detail below.

[0031] A method for manufacturing anisotropic recyclable flexible circuits based on balsa wood liquid metal, such as Figure 1 As shown, the following steps are included:

[0032] In step S1, balsa wood is cut to the desired size and chemically treated to remove lignin, resulting in partially delignified flexible wood or completely delignified flexible wood, which serves as the substrate. Gallium-based liquid metal (galium-indium eutectic EGaIn, LM) is mechanically stirred at 600 rpm for 4 hours. This creates a large, highly wettable oxide film on the surface, allowing the LM to vacuum-assisted fill the hollow channels within the delignified balsa wood, creating internal conductive circuits. This eliminates the risk of LM leakage due to its high fluidity.

[0033] Step S2, a highly controllable and selective LM conductive circuit is constructed on the substrate surface by transfer printing, and the hollow straight channels inside different balsa wood substrates can be filled by vacuum assistance, such as Figure 2 The resulting balsa wood-liquid metal / wood anisotropic (LMWA) composite flexible electronics are shown in Figure a). After service life, over 96% of the LM by weight can be recovered through cleaning with an environmentally friendly DES solution. The balsa wood substrate is also naturally biodegradable, making the material system highly environmentally friendly. Balsa wood without lignin removal was used as a control sample, with all other steps being identical. A total of three balsa wood-based flexible electronic components were obtained.

[0034] Specifically, the preparation method of the semi-delignified balsa wood is as follows: the semi-delignified balsa wood is prepared from balsa wood through a simple chemical delignification treatment process, and the balsa wood chips are completely immersed in a NaOH and Na2SO3 solution, in which the concentration of NaOH is 2.5M and the concentration of Na2SO3 is 0.4M. Then the solution is vacuum treated for 1 hour to ensure that the mixed solution fully penetrates into the channels inside the balsa wood. The obtained substrate is washed with deionized water to remove residual chemicals, and then freeze-dried for 48 hours to obtain semi-delignified balsa wood slices. The preparation method of the fully delignified balsa wood is as follows: the original balsa wood slices are placed in an acetate buffer solution (pH 4.6) containing 1wt% NaClO2, and treated at 85°C for 4h to obtain a white wood chip base; the obtained base is rinsed with deionized water multiple times, and freeze-dried for 48h to obtain fully delignified balsa wood slices.

[0035] After delignification, the lignin content of semi-delignified balsa wood decreased from 24.5 wt% to 14.3 wt% of the total weight of the original wood, a decrease of 41.6%, while the lignin content of fully delignified balsa wood decreased to 2.0 wt%, a decrease of 91.8%.

[0036] Light microscopy and electron microscopy were performed on balsa wood, semi-delignified balsa wood and fully delignified balsa wood, such as Figure 2 As shown, during the delignification process, the mass percentages of hemicellulose and cellulose removed from semi-delignified balsa wood are 29.6% and 9.5%, respectively, while the mass percentages of hemicellulose and cellulose removed from fully delignified balsa wood are 51.4% and 8.5%, respectively.

[0037] The mechanical properties, conductivity stability and flexibility of LMWA of LM conductive circuits based on three balsa wood substrates were compared. Figure 3 As shown in the figure, the LM transfer process fills the pores of DF-wood and transforms DF-wood into a dense material with higher mechanical strength than semi-delignified balsa wood and fully delignified balsa wood. When the real-time resistance change during 1000 120° bending is used to reflect the shape recovery rate of the LMWA conductor, it is observed that the shape recovery rate is about 85%, as shown in the figure. Figure 3 The limit of the wrapping radius of semi-delignified balsa wood is 0.75 mm, and that of fully delignified balsa wood is 3.5 mm. Flexible electronic components based on semi-delignified balsa wood have excellent flexibility because they can withstand various types of deformation, such as bundling, rolling, twisting and bending, without any obvious cracks or damage, such as Figure 3 As shown in b)-3e), the flexible electronic components based on semi-delignified balsa wood have higher flexibility than the flexible electronic components based on fully delignified balsa wood.

[0038] The size of the product in this embodiment is 60×60×1mm 3 After transferring the patterned LM, the LM can not only wet the surface but also penetrate into the conduits of the semi-delignified balsa wood to form LMWA conductors without sacrificing the flexibility of the semi-delignified balsa wood. Figure 3 f-3h. Fully delignified balsa wood is less flexible than semi-delignified balsa wood, but it can also be used as a substrate for LM transfer. The following experiments focus on flexible electronic components using semi-delignified balsa wood as a substrate.

[0039] Using vacuum assistance, the mechanically stirred pre-treated LM can be filled into the hollow straight channel structure in the balsa wood material. Figure 4 As shown in a), after filling the hollow structure of the semi-delignified balsa wood substrate, its cross section is as follows Figure 4 b) and 4c); longitudinal section Figure 4 d) and 4e) show the results of LM completely filling the internal structure.

[0040] In order to confirm the anisotropic electrical properties of the LMWA conductor after filling with LM, the anisotropic conductivity of the conductor parallel to the LM direction (σ z ) and perpendicular to (σ x) LM direction conductivity. Along the LM distribution direction, LMWA has high conductivity, conductivity (σ z ) is 9.8×10 2 S·m -1 ,like Figure 4 f). However, in the direction perpendicular to the LM distribution (σ x ) has an electrical conductivity of only 8.9×10 -6 S·m -1 The natural structure of parallel hollow spaces inside balsa wood provides an anisotropic template to make the LMs in different channels parallel and unconnected: the combination of this intrinsic balsa wood internal structure and the ordered orientation of the LMs results in an excellent anisotropic conductivity ratio (σ z / σ x )≈1.1×10 8 ,like Figure 4 Compared with the conductivity of other anisotropic materials, the conductivity of LMWA is much higher than that of composite materials such as carbon nanotubes, and slightly higher than that of metal microcoils with anisotropic orientation in a polymer matrix.

[0041] In the transfer printing method, the cross-sectional surface of semi-delignified balsa wood substrate was constructed. Figure 4 h) shows the five independent LM pattern structures. These five independent patterns can be regarded as separate resistors, and channel 0 can be regarded as a series resistor with the other four branch parallel resistors. The simplified circuit diagram is shown in Figure 4 j) When channels 1 to 4 are squeezed with different stresses, the stress deforms the internal channel structure of the semi-delignified balsa wood substrate, causing the internal liquid LM structure and resistance to change, thereby changing the voltage loaded on the 1MΩ resistor connected in series with the four channels. By changing the voltage of different channels, the resistance of the four branch paths can be obtained, thus realizing stress sensing, such as Figure 4 k).

[0042] Furthermore, this embodiment also conducted a constant temperature recycling experiment of electronic products based on LMWA conductors, such as Figure 5 As shown. The oxide film (Ga2O3) produced on the surface of pretreated LM has good wettability with the surface of delignified wood. The goal of LMWA recycling is achieved through the dissolution reaction of the oxide film of DES and Ga-based LM. In this experiment, LEDs are embedded in the LMWA electronic component circuit to detect whether the circuit is powered. After it is gradually immersed in the DES solution, the three LED lights go out one by one, proving that the LM circuit is dissolved and the conductivity disappears. Figure 5 It is worth noting that a key challenge for smart sensing devices and electronic systems is to terminate functions on demand. Figure 5Figures 5b and 5c show the overall process of dissolving the LM circuit and recovering the LM, LED, and semi-delignified or fully delignified balsa wood. Before the device was immersed in DES, the LED light was on. After immersion in DES, the LM circuit began to gradually separate from the different balsa wood substrates. After separation, the separated LM droplets settled at the bottom of the beaker containing DES due to their high density. Due to the high interfacial tension of LM in DES, the LM droplets coalesced and transformed into spheres, reducing the interfacial area and lowering the surface energy. After immersion in the DES solution for 24 hours at room temperature, the majority of the LM was separated and recovered, yielding clean balsa wood substrate materials, Ga-based LM, and LED. In experiments, this method recovered over 96% of the LM printed on semi-delignified and fully delignified balsa wood. This green, simple, and feasible strategy achieved recovery rates superior to previously reported LM paper-based electronics (91%) and LM PVA-based electronics (96%).

[0043] The above is a further detailed description of the present invention in conjunction with specific preferred embodiments, and the specific implementation of the present invention should not be considered to be limited to these descriptions. For those skilled in the art of the present invention, without departing from the concept of the present invention, several simple deductions or substitutions can be made, which should be considered to fall within the scope of protection of the present invention.

Claims

1. A method for manufacturing anisotropic recyclable flexible circuits based on balsa wood liquid metal, characterized in that: The steps include: Step S1, preparing semi-delignified balsa wood chips or fully delignified balsa wood chips as balsa wood substrate; Step S2, stirring the liquid metal to pre-treat it so as to form an oxide film on its surface; Step S3, printing a conductive circuit pattern on the surface of the balsa wood substrate using the pretreated liquid metal by transfer printing; The pre-treated liquid metal is filled into the hollow channel inside the balsa wood substrate in a vacuum-assisted manner.

2. The method for manufacturing anisotropic recyclable flexible circuits based on balsa wood liquid metal according to claim 1, characterized in that: In step S1, the semi-delignified balsa wood chips are prepared by the following steps: completely immersing the balsa wood chips in a NaOH and Na2SO3 solution, vacuum treating for 0.5-1.5 hours, washing to remove residual chemicals, and then freeze-drying for more than 40 hours to obtain semi-delignified balsa wood chips; The fully delignified balsa wood chips are prepared by the following steps: placing balsa wood slices in an acetate buffer solution containing 0.5-1.5wt% NaClO2, treating at 80-90°C for 2-6h, washing and freeze-drying for more than 40 hours to obtain fully delignified balsa wood chips.

3. The method for manufacturing anisotropic recyclable flexible circuits based on balsa wood liquid metal according to claim 2, characterized in that: In step S1, semi-delignified balsa wood is used as the balsa wood substrate; in the NaOH and Na2SO3 solutions, the concentration of NaOH is 2.0-3.0M, and the concentration of Na2SO3 is 0.3-0.5M.

4. The method for manufacturing anisotropic recyclable flexible circuits based on balsa wood liquid metal according to claim 1, characterized in that: In step S2, the pretreatment is stirring, the stirring speed is 500-700 rpm / min, and the time is more than 3 hours.

5. The method for manufacturing anisotropic recyclable flexible circuits based on balsa wood liquid metal according to claim 4, characterized in that: In step S2, the pretreatment is stirring, the stirring speed is 600 rpm / min, and the time is 4 hours.

6. The method for manufacturing anisotropic recyclable flexible circuits based on balsa wood liquid metal according to claim 1, characterized in that: In step S3, the conductive circuit pattern includes at least three independent conductive circuit patterns.

7. The method for manufacturing anisotropic recyclable flexible circuits based on balsa wood liquid metal according to claim 6, characterized in that: The conductive circuit pattern includes a first conductive circuit, a second conductive circuit, a third conductive circuit, a fourth conductive circuit, and a fifth conductive circuit. The second conductive circuit, the third conductive circuit, the fourth conductive circuit, and the fifth conductive circuit are connected in parallel and in series with the first conductive circuit.

8. Anisotropic recyclable flexible circuit based on balsa wood liquid metal, characterized by: The anisotropic recyclable flexible circuit is prepared by the method for manufacturing an anisotropic recyclable flexible circuit based on balsa wood liquid metal as described in any one of claims 1 to 7.

9. A flexible electronic component, characterized in that: It comprises the anisotropic recyclable flexible circuit based on balsa liquid metal as claimed in claim 8.

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