A preparation method and application based on a directly written printed flexible concave microstructure surface

Abstract

The present invention provides a preparation method for a surface of a flexible recessed microstructure based on direct writing printing, which relates to the technical field of direct writing printing and includes: coating a polymer on a support material and pre-curing to form a viscoelastic substrate; using a sacrificial ink to perform direct writing printing on the viscoelastic substrate to obtain a flexible material with recessed microstructures on the surface. The present invention uses a pre-cured viscoelastic material as the printing substrate, and a solution that is immiscible with the viscoelastic substrate, has shear thinning characteristics, and meets the conditions of direct writing printing as the direct writing printing ink, and cooperates with the direct writing printing technology to prepare a patterned microstructure, improve the printing accuracy, ensure the quality of the microstructure finished product, obtain a patterned surface with controllable microstructures, has the characteristics of being simple and convenient, fast, efficient, low-cost, green and environmentally friendly, can also avoid the pollution generated during processes such as mask and exposure etching, and can control the size of the extruded ink by adjusting the inner diameter of the nozzle and the extrusion pressure, with simple and flexible operation and strong controllability.

Description

Technical Field

[0001] The present invention relates to the technical field of direct writing printing, and particularly to a preparation method for the surface of a flexible concave micro-structure based on direct writing printing. Background Art

[0002] Disclosing the information of this background art section is only intended to enhance the overall understanding of the present invention, and it is not necessarily regarded as an admission or any form of implication that this information constitutes the prior art already known to those of ordinary skill in the art.

[0003] Currently, the manufacturing technology of concave micro-structures is developing. However, due to the disadvantages of low controllability, high cost, poor environmental friendliness, etc. in the current methods for preparing concave micro-structures, there is a lack of an effective method for manufacturing graphic concave micro-structures inside soft materials, and the drawbacks of the hindered development speed of concave micro-structures are becoming more and more obvious.

[0004] Existing methods for preparing micro-structures include etching, masking, printing, etc., which usually require complex equipment or operation methods to achieve. For example, in the masking method, a photolithography plate needs to be remade every time the program is modified, and different programs cannot be produced simultaneously, resulting in a long cycle; the etching steps are complex and cannot guarantee uniformity, etc. Moreover, the processes such as masking and exposure etching in these micro-structure preparation methods are costly and extremely prone to pollution. Due to the limitations of complex processes, low precision, poor controllability, etc. in these traditional printing methods, they gradually cannot meet the requirements of modern development.

[0005] Therefore, it is of great research significance to realize surface concave micro-structures with controllable morphology and high resolution in a low-cost, high-efficiency and flexible manner. Summary of the Invention

[0006] The object of the present invention is to address the defects existing in the prior art and propose a preparation method for the surface of a flexible concave micro-structure based on direct writing printing. By using a viscoelastic substrate and a sacrificial ink that is easy to remove, direct writing printing is carried out based on dynamic confinement to achieve the preparation of patterned micro-structures.

[0007] On the one hand, a preparation method for the surface of a flexible concave micro-structure based on direct writing printing includes the following steps:

[0008] S1: Prepare a viscoelastic substrate: Coat a polymer on a support material and pre-cure it to form a viscoelastic substrate;

[0009] S2: Prepare a substrate with a concave micro-structure on the surface: Use a sacrificial ink to perform direct writing printing on the viscoelastic substrate to obtain a flexible material with a concave micro-structure on the surface;

[0010] The polymer has viscoelasticity;

[0011] The sacrificial ink is a solution that is immiscible with the viscoelastic substrate, has shear-thinning properties, and meets the conditions for direct-write printing.

[0012] On the other hand, the application of the flexible concave microstructured surface obtained by the above-mentioned method for preparing a flexible concave microstructured surface based on direct-write printing in the fields of microchannels, microfluidics, capillary force adhesion, drug release, anti-fouling surfaces, flexible sensors, and electroluminescence;

[0013] Preferably, the application in the fields of flexible sensors and electroluminescence is as follows: the flexible concave microstructured surface is used as a template to prepare a surface with convex microstructures and applied in the fields of flexible sensors and electroluminescence;

[0014] Preferably, the application in the fields of capillary force viscosity and drug release is as follows: the capillary force of the flexible concave microstructured surface acts on the fluid and is applied in the fields of capillary force adhesion and drug release; more preferably, the fluid includes air and liquid.

[0015] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0016] In the present invention, a pre-cured viscoelastic material is used as the printing substrate, and a patterned concave micro-structure is prepared by directly writing and printing a sacrificial ink that is immiscible with the substrate on the surface of the viscoelastic substrate. By controlling the pre-curing degree of the viscoelastic material, changing the concentration of the sacrificial ink, and controlling the direct-write printing conditions, the printing accuracy is improved, the quality of the micro-structure finished product is guaranteed, and a patterned surface with controllable concave micro-structures is obtained. In the present invention, the ink is directly transferred onto the substrate by direct-write printing, and the ink infiltrates and deposits on the substrate to form the required concave micro-structures. It has the characteristics of simplicity, convenience, high speed, high efficiency, low cost, and environmental friendliness. It can also avoid the pollution generated during processes such as mask and exposure etching. Moreover, the size of the extruded ink can be controlled by simply adjusting the inner diameter of the nozzle and the extrusion pressure, and the operation is simple and flexible, with strong controllability.

[0017] The present invention uses a solution that is immiscible with the viscoelastic substrate, has shear thinning characteristics, and meets the conditions of direct writing printing as the direct writing printing ink. In combination with the viscoelastic substrate, on the one hand, the shear thinning performance of the ink is effectively utilized. The ink exhibits high viscoelasticity before extrusion, has good shear thinning characteristics when extruded through the nozzle, is easy to extrude, and the extruded ink segment restores high viscoelasticity, and can maintain good shape fidelity when deposited on the viscoelastic substrate, thereby enhancing the printing fidelity; that is, the present invention solves the defects of uneven ink marks and poor printing fidelity caused by the difficult viscosity control of conventional sacrificial inks, and improves the direct writing printing accuracy. On the other hand, the ink of the present invention uses an appropriate concentration to ensure that it has appropriate viscosity and surface tension. When the ink is deposited on the substrate, it exhibits high viscosity, and the behavior of the ink is mainly dominated by the surface tension. When the ink concentration is fixed, it is deposited on the substrate at a fixed three-phase contact angle, and the ink will not be completely embedded when it contacts the substrate, showing higher stability; at the same time, due to the viscoelasticity of the substrate inhibiting the spreading of the ink, and the parts where the ink contacts the substrate surface are symmetrically distributed. After the ink is deposited, as the solvent in the ink continuously volatilizes, its size gradually decreases. In combination with the inhibitory effect of the substrate on it, the size of the ink is further reduced (that is, the size of the recessed microstructure), making the size of the recessed microstructure much lower than the nozzle diameter, and improving the direct writing printing accuracy. In the third aspect, the pre-curing degree of the substrate affects the ink deposition depth. When the substrate reaches a certain pre-curing degree, the ink will be deposited to the corresponding position within a certain time. Once it reaches the fixed position, the ink will no longer sink. After the viscoelastic substrate is completely cured, removing the ink will obtain a flexible substrate with patterned recessed microstructures.

[0018] Polydimethylsiloxane (PDMS) is a low-cost and non-toxic material with good tensile properties, strong chemical stability, high optical transmittance, and biocompatibility, and is an ideal material as the matrix. The present invention uses pre-cured PDMS as the viscoelastic substrate and controls the pre-curing degree of the mixture. When the ink is extruded onto the substrate, the viscoelastic substrate can inhibit the spreading of the ink, keeping the ink in its original shape and avoiding the problem that the substrate in the conventional technology cannot restrict the spreading of the ink marks; and the viscoelastic substrate has good viscoelasticity, enabling the ink to be deposited to a fixed depth within a certain time, effectively controlling the printing depth of the microstructure, and avoiding the problem of insufficient ink deposition depth in the conventional technology, improving the quality of the finished recessed microstructure. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] The specification drawings constituting a part of the present invention are used to provide a further understanding of the present invention. The schematic embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute an improper limitation of the present invention.

[0020] Figure 1Schematic flow chart of the preparation method for the surface of the flexible concave microstructure based on direct writing printing in Embodiment 1 of the present invention;

[0021] Figure 2 Optical microscope images of the microstructures obtained by setting different printing spacings in Embodiment 1 of the present invention and three-dimensional topography maps obtained by stylus profilometry scanning, where a, b, and c are optical microscope images, and d, e, and f are the corresponding three-dimensional topography maps;

[0022] Figure 3 Deposit behavior diagrams of inks with different pre-curing times on the substrate in Embodiment 3 of the present invention;

[0023] Figure 4 Microstructure size diagrams with different pre-curing times in Embodiment 3 of the present invention;

[0024] Figure 5 Schematic diagram of the capillary action of the concave microstructure in the PDMS film of the patterned concave microstructure obtained in Embodiment 1 of the present invention;

[0025] Figure 6 is Figure 5 Optical microscope image of the concave microstructure filled with methylene blue aqueous solution in Detailed implementation manners

[0026] It should be noted that the following detailed descriptions are all exemplary and are intended to provide further explanations of the present invention. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by those of ordinary skill in the technical field to which the present invention belongs.

[0027] It should be noted that the terms used herein are only for describing specific implementation manners and are not intended to limit the exemplary embodiments according to the present invention. As used herein, unless otherwise clearly specified in the context, the singular form is also intended to include the plural form. In addition, it should be understood that when the terms "comprise" and / or "include" are used in this specification, they indicate the presence of features, steps, operations, devices, components, and / or combinations thereof.

[0028] Term explanation:

[0029] Precursor: A precursor is a form of existence before obtaining the target product. Most of them exist as organic-inorganic complexes or solid mixtures, and some exist in the form of sols. The PDMS material is an optically transparent elastomer material formed by the coupling reaction of the terminal active groups of macromolecular polydimethylsiloxane with a curing agent to complete the curing process. The precursor described in the present invention is the above-mentioned terminal active groups of macromolecular polydimethylsiloxane, which is a form of existence before obtaining the target product.

[0030] As introduced in the background art, there is currently no effective method for manufacturing graphic microstructures inside soft materials, and existing microstructural preparation methods have problems such as long production cycles, high costs, easy generation of pollution, low precision, and poor operability. The present invention provides a preparation method for a surface of a flexible recessed microstructure based on direct writing printing.

[0031] A typical embodiment of the present invention provides a preparation method for a surface of a flexible recessed microstructure based on direct writing printing, which includes the following steps:

[0032] S1: Prepare a viscoelastic substrate: Coat a polymer on a support material and pre-cure it to form a viscoelastic substrate;

[0033] S2: Prepare a substrate with a recessed microstructure on the surface: Use a sacrificial ink to perform direct writing printing on the viscoelastic substrate to obtain a flexible material with a recessed microstructure on the surface;

[0034] The polymer has viscoelasticity;

[0035] The sacrificial ink is a solution that is immiscible with the viscoelastic substrate, has shear thinning characteristics, and meets the conditions for direct writing printing.

[0036] The present invention uses a pre-cured viscoelastic material as a printing substrate, and prepares a patterned recessed microstructure by directly writing and printing a sacrificial ink on the surface of the viscoelastic substrate. By controlling the degree of pre-curing of the viscoelastic material, changing the concentration of the sacrificial ink, and controlling the direct writing printing conditions, the printing precision is improved, the quality of the microstructural finished product is ensured, and a patterned surface with controllable microstructures is obtained. The direct writing printing (DIW) technology has the advantages of simplicity, convenience, low cost, and environmental friendliness. In the present invention, the sacrificial ink is directly extruded onto the substrate by direct writing printing, and the ink infiltrates and deposits on the substrate to form the required recessed microstructure. This process can avoid pollution generated during processes such as masking and exposure etching.

[0037] In some embodiments of this embodiment, the support material is selected from metal plates, plastic plates, composite material plates, and polymer films.

[0038] Preferably, the metal plate includes copper plates, zinc plates, aluminum plates, and microcrystalline zinc plates.

[0039] Preferably, the plastic plate includes PVC plastic plates, PP plastic plates, ABS resin plates, and PE plastic plates.

[0040] Preferably, the composite material plate includes aluminized films and laminated papers.

[0041] Preferably, the polymer film includes polyethylene terephthalate (PET) films, biaxially oriented polypropylene (BOPP) films, and polyvinyl chloride (PVC) films.

[0042] In some embodiments of this embodiment, the thickness of the polymer is greater than the diameter of the nozzle during direct writing printing, so as to prevent the sacrificial ink from being completely embedded in or penetrating the viscoelastic substrate.

[0043] In some embodiments of this embodiment, the curing temperature for pre-curing is 70°C - 80°C, and the curing time is 5 - 15 minutes, preferably 70°C for 8 - 10 minutes.

[0044] The degree of pre-curing of the viscoelastic substrate affects the size of the concave microstructure. If the pre-curing time of the substrate is short, the surface of the substrate has a certain fluidity, and the substrate does not provide sufficient support for the ink extruded by direct writing printing, easily causing the ink to be completely embedded in the viscoelastic matrix, resulting in extremely small concave microstructure sizes. As the pre-curing time gradually increases, the surface of the substrate changes from a liquid state to a semi-liquid state, with weakened fluidity. The ink changes from being completely embedded in or even penetrating the substrate to being partially embedded, and the embedding depth becomes shallower and shallower, and the concave microstructure gradually appears. When the pre-curing time of the substrate further increases until the surface of the substrate is in a viscoelastic state, the ink presents a semi-embedded state on the surface of the substrate, and at this time, a viscoelastic substrate with a relatively obvious concave microstructure can be obtained. If the pre-curing time of the substrate is long, the surface of the substrate is completely cured, and the ink cannot be embedded in the substrate, and a concave microstructure cannot be formed. The pre-curing conditions adopted in the present invention can completely ensure that the surface of the substrate is in a viscoelastic state, and thus a flexible substrate with obvious structure and concave microstructure can be obtained.

[0045] In some embodiments of this embodiment, S2 specifically includes the following steps: using sacrificial ink to perform direct writing printing on the surface of the viscoelastic substrate. After the ink no longer deposits on the surface of the viscoelastic substrate, completely cure the viscoelastic substrate, and then remove the sacrificial ink to obtain a flexible substrate with a concave microstructure morphology on the surface.

[0046] In some embodiments of this embodiment, during direct writing printing, the nozzle diameter is 100 - 200 μm, and the adjusted air pressure range is 8 - 10 psi. The extrusion speed of the sacrificial ink is controlled by air pressure to ensure the size of the microstructure.

[0047] In some embodiments of this embodiment, the polymer includes polyethylene, polyisobutylene, polystyrene, plasticized polyvinyl chloride, polymethyl methacrylate, phenolic resin, natural rubber, or polydimethylsiloxane.

[0048] Among them, when the polymer is polydimethylsiloxane, it is specifically a mixture composed of a polydimethylsiloxane precursor and a curing agent, and the mass ratio of the polydimethylsiloxane precursor to the curing agent is 5 - 15:1, preferably 10:1;

[0049] The sacrificial ink is a solution that is immiscible with the viscoelastic polydimethylsiloxane substrate, has shear-thinning properties, and meets the conditions for direct ink writing printing; further preferably, it is one of an aqueous solution of polyvinyl alcohol (PVA) and an ethanol solution of polyacrylic acid (PAA).

[0050] A remarkable feature of the ink for direct ink writing printing (DIW) is that it can quickly self-heal after extrusion and has shear-thinning properties, that is, the viscosity of the ink decreases with the increase of shear rate or shear stress. When the ink is at a low flow rate or in a static state, due to the entanglement between molecules, the viscosity is relatively large, but when the flow rate becomes larger, under the action of shear stress, the interconnections decrease, and it rolls and rotates and shrinks into a mass, showing the phenomenon of shear thinning. The ink exhibits high viscoelasticity before extrusion, has a good shear-thinning effect when extruded through the nozzle, and recovers high viscoelasticity after deposition, and can maintain good shape fidelity.

[0051] At the same time, as the sacrificial ink to be removed after printing, the removal performance is also one of the key indicators. PVA is soluble in water, and the higher the water temperature, the greater the solubility. It is non-toxic and has no side effects on the human body, and has good biocompatibility and biodegradability; PAA is easily soluble in water, ethanol, isopropanol, etc., is immiscible with the substrate, and is easily removed. Therefore, PVA ink and PAA ink are the preferred sacrificial inks when combined with the viscoelastic polydimethylsiloxane substrate.

[0052] Preferably, the degree of polymerization of polyethylene is 1750 ± 50, and the mass ratio of polyvinyl alcohol to water in the aqueous solution of polyvinyl alcohol is 1:10 - 15, preferably 1:13.

[0053] Preferably, the mass ratio of polyacrylic acid to ethanol in the ethanol solution of polyacrylic acid is 1:1 - 3, preferably 1:2.

[0054] The surface tension of the ink affects its deposition effect on the viscoelastic substrate. If the surface tension is too large, the ink is likely to aggregate together, affecting the shape of the concave microstructures. If the surface tension is small, the ink is likely to spread and cannot maintain its shape, resulting in poor stability and affecting the shape of the concave microstructures. The concentrations of the aqueous PVA solution and the ethanol solution of PAA in the present invention can ensure that during the direct ink writing printing process, the ink maintains its shape on the viscoelastic substrate with high stability, ensuring the shape and dimensional accuracy of the concave microstructures. When printing the concave microstructures, the sacrificial ink with the above concentrations has appropriate concentrations, surface tension and viscosity, and is easy to form micro-pit structures, micro-groove structures and cross microstructures.

[0055] In some embodiments of this embodiment, the concave microstructures include micro-pit structures, micro-groove structures and cross microstructures;

[0056] Preferably, the depth of the micro-pit structure is 40 μm and the width is 60 μm.

[0057] Another typical embodiment of the present invention provides an application of the flexible concave microstructured surface obtained by the above preparation method of the directly written printing flexible concave microstructured surface in the fields of microchannels, microfluidics, capillary adhesion, drug release, anti-fouling surfaces, flexible sensors, and electroluminescence.

[0058] Preferably, the applications in the fields of flexible sensors and electroluminescence are as follows: The flexible concave microstructured surface is used as a template to prepare a surface with convex microstructures, and it is applied to the fields of flexible sensors and electroluminescence.

[0059] Preferably, the applications in the fields of capillary viscosity and drug release are as follows: The capillary force of the flexible concave microstructured surface acts on the fluid, and it is applied to the fields of capillary adhesion and drug release; Further preferably, the fluid includes air and liquid.

[0060] The following combines specific embodiments to further elaborate on the present invention. It should be noted that the specific embodiments are interpretations rather than limitations of the present invention.

[0061] Example 1

[0062] A preparation method of a directly written printing flexible concave microstructured surface includes the following steps:

[0063] S1: Prepare a viscoelastic substrate: Cut the polyethylene terephthalate (PET) film into uniformly sized block films (2.5 cm × 2.5 cm), ultrasonically clean the cut PET film with a mixed solution of deionized water and ethanol, and place it in an oven at 60 °C for drying after cleaning. Weigh the polydimethylsiloxane PDMS precursor and the curing agent and pour them into a beaker (the mass ratio of the precursor to the curing agent is 10:1), stir evenly with a glass rod, and then evacuate in a vacuum drying oven to remove the bubbles generated during stirring. Spin-coat on the PET block film using a spin coater, and the spin-coating conditions are 400 rpm for 10 s at low speed and 1000 rpm for 15 s at high speed. Place the spin-coated PET block film in a vacuum drying oven at 70 °C for pre-curing for 9 min to obtain the prepared viscoelastic substrate.

[0064] S2: Prepare a substrate with concave microstructures on the surface: Weigh a certain mass of polyvinyl alcohol (PVA) with an average degree of polymerization of 1750 ± 50 and mix it with deionized water. The mass ratio of PVA to deionized water is 1:13, and heat it to 85 °C in a water bath to completely dissolve it to obtain the required sacrificial ink.

[0065] Using direct writing printing technology, a nozzle with a diameter of 100 μm, adjusting the air pressure to 9 psi, respectively setting the programs required for preparing micro-pit structures, micro-groove structures and cross micro-structures. After setting the programs, the sacrificial ink is extruded and deposited on the prepared viscoelastic substrate. After the ink no longer deposits, the adhesive substrate is completely cured, and then the finished product is washed with deionized water to remove the PVA solute on the surface. After drying, a PDMS film with a patterned concave micro-structure is obtained.

[0066] Figure 2 Optical microscope images (upper) of the concave micro-structures with different printing spacings (the printing spacings are 400 μm, 300 μm, and 200 μm from left to right in sequence) and three-dimensional topography images (lower) of the micro-pit structures obtained by step profiler scanning for Example 1 are shown. It can be seen that the depth of the micro-pit structures prepared under the pre-curing conditions of Example 1 is about 40 μm, and the width is about 60 μm.

[0067] Example 2

[0068] A preparation method for a surface of a flexible concave micro-structure based on direct writing printing, comprising the following steps:

[0069] S1: Prepare a viscoelastic substrate: Cut a polyethylene terephthalate (PET) film into uniformly sized block films (2.5 cm × 2.5 cm), ultrasonically clean the cut PET film with a mixed solution of deionized water and ethanol, and place it in an oven at 60 °C for drying after cleaning. Weigh the polydimethylsiloxane (PDMS) precursor and the curing agent and pour them into a beaker (the mass ratio of the precursor to the curing agent is 10:1), stir evenly with a glass rod, and then evacuate in a vacuum drying oven to remove the bubbles generated during stirring. Spin-coat on the PET block film using a spin coater, and the spin-coating conditions are low speed 400 rpm, 10 s, high speed 1000 rpm, 15 s. Put the spin-coated PET block film into a vacuum drying oven at 70 °C for pre-curing for 9 min to obtain the prepared viscoelastic substrate.

[0070] S2: Prepare a substrate with a concave micro-structure on the surface: Weigh a certain mass of polyacrylic acid (PAA) and mix it with absolute ethanol, and the mass ratio of polyacrylic acid to absolute ethanol is 1:2 to obtain the required sacrificial ink.

[0071] Using direct writing printing technology, a nozzle with a diameter of 100 μm, adjusting the air pressure to 9 psi, respectively setting the programs required for preparing micro-pit structures, micro-groove structures and cross micro-structures. After setting the programs, the sacrificial ink is extruded and deposited on the prepared viscoelastic substrate. After the ink no longer deposits, the adhesive substrate is completely cured, and then the finished product is washed with deionized water to remove the PAA solute on the surface. After drying, a PDMS film with a patterned micro-structure is obtained.

[0072] Example 3

[0073] Effect of the degree of pre-curing of the substrate on the printed concave microstructure

[0074] PDMS films with patterned concave microstructures were prepared using different pre-curing times to study the effect of the degree of pre-curing of the substrate on the printed concave microstructure. The pre-curing times were increased in gradients between 5 - 20 min. The rest was the same as in Example 1.

[0075] As Figure 3 shown, as the pre-curing time of the PDMS substrate increases, the ink exhibits different deposition behaviors on the substrate. As Figure 4 shown, as the pre-curing time of the PDMS substrate increases, the size of the concave microstructures formed by the deposition of the ink on the substrate changes. Among them, Figure 3 the upper three figures and Figure 4 the leftmost figure are the deposition behavior of the ink on the substrate and the microstructure size diagram when the curing time is less than 8 min. It can be seen that the ink is completely embedded in the substrate and the microstructure size is small; Figure 3 the middle three figures and Figure 4 the second left figure are the deposition behavior of the ink on the substrate and the concave microstructure size diagram when the curing time is 8 - 10 min. It can be seen that the concave microstructures are obvious; Figure 3 the lower three figures and Figure 4 the rightmost and the second right figures are the deposition behavior of the ink on the substrate and the concave microstructure size diagram when the pre-curing time is more than 10 min. It can be seen that when the degree of pre-curing is too high, the ink cannot be embedded in the substrate and no concave microstructures can be formed.

[0076] When the pre-curing time of the substrate is short, it can be observed that the surface of the substrate still has a certain fluidity. At this time, the printed ink does not have enough support, and the ink is almost completely embedded in the PDMS surface, and only relatively small concave microstructures can be observed; when the pre-curing time of the substrate gradually increases, the surface of the substrate shows a semi-liquid property and the fluidity weakens. The ink is in a partially embedded state, and the embedding depth becomes shallower and shallower, and the concave microstructures gradually appear; when the pre-curing time of the substrate further increases to the surface of the substrate showing a viscoelastic state, the ink is in a semi-embedded state on the surface of the substrate. At this time, more obvious concave microstructures can be observed on the surface of the substrate; when the pre-curing time of the substrate is long, the surface of the substrate is completely cured, and the ink cannot be embedded in the substrate and no concave microstructures can be formed.

[0077] The behavior of the ink is mainly governed by surface tension. When the ink concentration is fixed, it will deposit on the substrate with a fixed three-phase contact angle. The ink will not be completely embedded when it collides with the substrate, showing better stability. During the printing process, since the sacrificial ink has a high viscosity and the printing speed is controllable, it will not cause ink splashing. Due to the viscoelasticity of the substrate inhibiting the spreading of the ink, when the substrate reaches a certain degree of pre-curing, the ink will deposit at the corresponding position within a certain time. Once it reaches the fixed position, the ink will not sink. Different degrees of pre-curing of the substrate result in different depths and volumes of the droplet embedded in the substrate.

[0078] Example 4

[0079] Patterned microstructured surfaces can be widely applied in fields such as microfluidics and drug release. Here, the application of the PDMS film with a patterned concave microstructure obtained by direct writing printing in Example 1 is demonstrated.

[0080] As Figure 5 shown, first, the PDMS film with a concave microstructure is subjected to plasma treatment, and then an aqueous solution of methylene blue is dropped on the treated PDMS film. Due to the capillary action of the concave microstructure, the aqueous solution of methylene blue will enter the concave microstructure, as Figure 6 shown. Since PDMS has good biocompatibility, the concave microstructure prepared on the PDMS film is expected to be applied in related fields such as capillary force adhesion and drug release.

[0081] The above are only the preferred embodiments of the present invention and are not used to limit the present invention. For those skilled in the art, the present invention can have various changes and modifications. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A preparation method for the surface of a flexible recessed microstructure based on direct writing printing, characterized in that, it includes the following steps: S1: Prepare a viscoelastic substrate: coat a polymer on a support material and pre-cure to form a viscoelastic substrate; S2: Prepare a substrate with a recessed microstructure on the surface: use a sacrificial ink to perform direct writing printing on the viscoelastic substrate to obtain a flexible material with a recessed microstructure on the surface; the polymer has viscoelasticity; the support material is selected from a metal plate, a plastic plate, a composite material plate, and a polymer film; the curing temperature of the pre-curing is 70°C - 80°C, and the curing time is 5 - 15 min; in the direct writing printing, the nozzle diameter is 100 - 200 μm, and the adjusted air pressure range is 8 - 10 psi; the polymer includes polyethylene, polyisobutylene, polystyrene, plasticized polyvinyl chloride, polymethyl methacrylate, phenolic resin, natural rubber, or polydimethylsiloxane; the sacrificial ink is a solution that is immiscible with the polydimethylsiloxane viscoelastic substrate, has shear thinning properties, and meets the conditions of direct writing printing.

2. The preparation method for the surface of a flexible recessed microstructure based on direct writing printing according to claim 1, characterized in that, the thickness of the polymer is greater than the diameter of the nozzle during direct writing printing.

3. The preparation method for the surface of a flexible recessed microstructure based on direct writing printing according to claim 1, characterized in that, the curing temperature of the pre-curing is 70°C, and the curing time is 8 - 10 min.

4. The preparation method for the surface of a flexible recessed microstructure based on direct writing printing according to claim 1, characterized in that, S2 specifically includes the following steps: use a sacrificial ink to perform direct writing printing on the surface of the viscoelastic substrate. After the ink no longer deposits on the surface of the viscoelastic substrate, completely cure the viscoelastic substrate, and then remove the sacrificial ink to obtain a flexible substrate with a recessed microstructure morphology on the surface.

5. The preparation method for the surface of a flexible recessed microstructure based on direct writing printing according to claim 1, characterized in that, in the direct writing printing, the nozzle diameter is 100 μm.

6. The preparation method for the surface of a flexible recessed microstructure based on direct writing printing according to claim 1, characterized in that, when the polymer is polydimethylsiloxane, it is specifically a mixture composed of a polydimethylsiloxane precursor and a curing agent, and the mass ratio of the polydimethylsiloxane precursor to the curing agent is 5 - 15:

1.

7. The preparation method for the surface of a flexible recessed microstructure based on direct writing printing according to claim 6, characterized in that, the mass ratio of the polydimethylsiloxane precursor to the curing agent is 10:

1.

8. The preparation method for the surface of a flexible recessed microstructure based on direct writing printing according to claim 1, characterized in that, the sacrificial ink is one of an aqueous solution of polyvinyl alcohol and an ethanol solution of polyacrylic acid.

9. The preparation method for the surface of a flexible recessed microstructure based on direct writing printing according to claim 8, characterized in that, the polymerization degree of the polyvinyl alcohol is 1750 ± 50, and the mass ratio of polyvinyl alcohol to water in the aqueous solution of polyvinyl alcohol is 1:10 - 15; or, the mass ratio of polyacrylic acid to ethanol in the ethanol solution of polyacrylic acid is 1:1 - 3; Alternatively, the concave microstructure includes a micropit structure, a microgroove structure, and an intersecting microstructure.

10. The preparation method of the flexible concave microstructure surface based on direct writing printing according to claim 9, characterized in that the mass ratio of polyvinyl alcohol to water in the polyvinyl alcohol aqueous solution is 1:

13.

11. The preparation method of the flexible concave microstructure surface based on direct writing printing according to claim 9, characterized in that the mass ratio of polyacrylic acid to ethanol in the polyacrylic acid ethanol solution is 1:

2.

12. The preparation method of the flexible concave microstructure surface based on direct writing printing according to claim 9, characterized in that the depth of the micropit structure is 40 μm and the width is 60 μm.

13. Application of the flexible concave microstructure surface obtained by the preparation method of the flexible concave microstructure surface based on direct writing printing according to any one of claims 1-12 in the fields of microchannels, microfluidics, capillary adhesion, anti-fouling surfaces, flexible sensors, and electroluminescence.

14. The application according to claim 13, characterized in that the application in the fields of flexible sensors and electroluminescence is: using the flexible concave microstructure surface as a template to prepare a surface with a convex microstructure and applying it to the fields of flexible sensors and electroluminescence.

15. The application according to claim 13, characterized in that the application in the field of capillary viscosity is: using the capillary force of the flexible concave microstructure surface to act on the fluid and applying it to the field of capillary adhesion.

16. The application according to claim 15, characterized in that the fluid includes air and liquid.

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