A flexible skin structure fusing inside-out sensing and a method of making the same

Abstract

The application discloses a flexible skin structure integrating surface and internal sensing and a preparation method thereof, and the flexible skin structure integrating surface and internal sensing comprises an elastic base, a plurality of channel structures are arranged in the elastic base, liquid metal is injected into the channel structures, and the liquid metal forms a liquid metal network in the channel structures of the elastic base; the liquid metal network serves as a sensing system and comprises an SSPP unit and a piezoresistive channel structure arranged in parallel on the periphery of the SSPP unit; and the cable comprises a flexible flat cable connected with the piezoresistive channel structure and a coaxial cable connected with the SSPP unit. The application discloses the flexible skin structure integrating surface and internal sensing and the preparation method thereof, and has the advantages of compact structure, high function integration degree and simple preparation process. Through the multi-modal flexible electronic skin, surface layer and deep layer information can be cooperatively sensed and processed; surface layer pressure sensing and deep layer medium sensing can be realized, and the flexible skin structure has multi-modal sensing functions.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of flexible electronic devices, in particular to a flexible multi-modal sensing electronic skin based on liquid metal micro-channel, which integrates surface tactile perception and inner penetrating medium perception, and a preparation method thereof. BACKGROUND

[0002] With the rapid development of flexible electronic devices, wearable sensors and human-computer interaction systems, flexible electronic skin as a key carrier shows broad application prospects in intelligent medical monitoring, artificial intelligence, robot perception and other fields. At present, the common flexible electronic skin mostly adopts piezoresistive, piezoelectric, capacitive and other mechanisms to realize the collection of surface physical signals such as tactile, strain and temperature. However, the existing technology focuses on surface signal perception, and it is difficult to realize "penetrating" information detection of the target internal or deep tissue at the same time, which limits the functional expansion and information dimension in complex application scenarios.

[0003] In order to realize higher level human-computer interaction and environmental perception ability, the existing technology begins to try to integrate multi-modal sensing into the same flexible device. On the one hand, liquid metal, due to its excellent electrical conductivity, flexibility and stretchability, becomes an ideal material for realizing flexible conductive paths and channel structures, and is widely used in flexible piezoresistive sensors. On the other hand, as an artificial surface plasmon polariton (Spoof Surface Plasmon Polaritons, SSPPs), it has the characteristics of electromagnetic wave localization enhancement, tunable frequency response and medium sensitivity, which provides a new path for non-contact deep medium perception.

[0004] Although the existing technology has made certain progress in flexible pressure perception and electromagnetic wave penetration detection, there is still a lack of an ultra-thin electronic skin that can efficiently integrate liquid metal micro-channels and SSPP structures, which can not only meet the flexible and high-sensitivity tactile perception, but also has the non-invasive detection ability of internal medium changes of the target. SUMMARY

[0005] The present application overcomes the shortcomings of the prior art and provides a flexible skin structure with compact structure, high functional integration and simple preparation process, which integrates surface and inner perception, and a preparation method thereof. Through the multi-modal flexible electronic skin, the surface and deep information can be cooperatively perceived and processed. The flexible skin structure can realize surface pressure perception and deep medium perception, and has multi-modal sensing function.

[0006] In order to achieve the above object, the technical scheme adopted by the present application is: a flexible skin structure fusing surface and internal perception, comprising: an elastic substrate, a plurality of channel structures are arranged in the elastic substrate, a liquid metal is injected into the channel structures, the liquid metal forms a liquid metal network in the channel structures of the elastic substrate, the liquid metal network serves as a perception system, comprising: an SSPP unit, and a piezoresistive channel structure arranged in parallel on the periphery of the SSPP unit; a cable comprising a flexible flat cable connected with the piezoresistive channel structure, and a coaxial cable connected with the SSPP unit.

[0007] In a preferred embodiment of the present application, the piezoresistive channel structure is arranged in parallel on both sides of the SSPP unit, forming a double-channel perception system.

[0008] In a preferred embodiment of the present application, the elastic substrate comprises a substrate layer, a flexible film with through holes, and a flexible film with channels arranged in sequence; the flexible film with channels and the flexible film with through holes are matched to form a channel structure in the middle, and a through slot is further arranged on the flexible film with channels, which is in communication with the through holes on the flexible film with through holes.

[0009] In a preferred embodiment of the present application, the flexible flat cable and the coaxial cable are embedded between the substrate layer and the flexible film with through holes at one end connected with the liquid metal network, and the one end connected with the liquid metal network corresponds to the through holes on the flexible film with through holes.

[0010] In a preferred embodiment of the present application, the through holes of the flexible film with through holes and the through slot on the flexible film with channels are respectively plugged by the substrate layer and the packaging film, so as to confine the liquid metal in the channel structure of the elastic substrate.

[0011] In a preferred embodiment of the present application, an electromagnetic shielding layer is further arranged on the elastic substrate; the elastic substrate is a PDMS material elastic substrate, and the PDMS is polydimethylsiloxane; the liquid metal is an indium-tin alloy with a melting point of 6℃, a density of 6.03 g / cm, and a purity of 99%;

[0012] The liquid metal is a conductive medium.

[0013] In a preferred embodiment of the present application, a preparation method of a flexible skin structure fusing surface and internal perception is used to prepare the flexible skin structure fusing surface and internal perception, comprising the following steps:

[0014] Obtaining a substrate layer, a flexible film with through holes, a flexible film with channels, a liquid metal, a flexible flat cable, a coaxial cable, a packaging film, and an electromagnetic shielding layer.

[0015] The side of the flexible film with channel structure is matched with the flexible film with through hole, and the channel structure is formed between the flexible film with channel and the flexible film with through hole; the other side of the flexible film with through hole is matched with the base layer, and the coaxial line and one end of the flexible flat cable are respectively embedded between the flexible film with through hole and the base layer; the contact end of the coaxial line corresponds to the SSPP unit in the channel structure and contacts the hole position on the flexible film with through hole to transmit the radio frequency signal; the contact end of the flexible flat cable corresponds to the piezoresistive channel structure in the channel structure and contacts to conduct the piezoresistive signal; the through hole on the flexible film with through hole is injected with liquid metal, and the liquid metal enters the channel structure to form a liquid metal network;

[0016] The encapsulation film is covered on the through hole of the flexible film with channel injected with liquid metal, and the liquid metal is limited in the channel structure of the elastic base;

[0017] The electromagnetic shielding layer is arranged on the outside of the base layer or the flexible film with channel (it can be covered on the outside of the base layer or the flexible film with channel, but only one can be covered).

[0018] In a preferred embodiment of the present application, the preparation method of the base layer, the flexible film with through hole and the encapsulation film comprises:

[0019] The polydimethylsiloxane prepolymer and the curing agent are weighed and mixed in a mass ratio of 10:1 to obtain a PDMS solution; the PDMS solution is degassed in a vacuum degassing box; the PDMS solution is spin-coated on a culture dish to form a PDMS liquid film; and the PDMS liquid film is degassed and then stepwise cured and cross-linked in a constant-temperature oven to obtain a flexible substrate film.

[0020] In a preferred embodiment of the present application, the preparation method of the flexible film with channel comprises:

[0021] In step S1, the glass substrate is surface treated by using a cleaning liquid combined with an ultrasonic cleaning process; high-purity nitrogen is used for blowing and removing residual liquid on the surface of the substrate; and a clean and dry glass substrate surface is obtained;

[0022] In step S2, a photomask plate is designed based on a preset channel pattern, and a spiral topological configuration is adopted to obtain a photomask plate with a channel pattern;

[0023] In step S3, based on the surface of the pretreated glass substrate, a photosensitive dry film is uniformly covered on the substrate surface by using a precise film pasting process; a dry film layer is formed by using a hot-pressing composite device to realize the close adhesion between the dry film and the substrate under the set temperature and pressure parameters; and a coated substrate with a uniform photosensitive layer is obtained;

[0024] In step S4, a contact photolithography method is used to perform patterning treatment on the coated substrate, so that the channel structure pattern on the mask plate is completely transferred to the photosensitive dry film layer;

[0025] Step S5, using sodium carbonate solution to wash off the unexposed part of the dry film glass substrate, to obtain a dry film glass substrate containing channel structure;

[0026] Step S6, before implementing the PDMS pouring process, the dry film glass substrate with channel structure needs to be treated with surface modification, to obtain a silanized dry film glass substrate;

[0027] Step S7, using prepolymer pouring forming process to prepare flexible structure film on the modified substrate surface, to obtain a flexible film with channel;

[0028] In one preferred embodiment of the present application, the side of the flexible film with channel provided with channel is matched with the flexible film with through hole, and the step of forming channel structure between the flexible film with channel and the flexible film with through hole comprises:

[0029] Firstly, low-adhesion clean tape is used to remove the particulate contaminants on the surface of the flexible film with through hole and the flexible film with channel; then the flexible film with channel and the flexible film with through hole to be bonded are placed in a plasma treatment device, and surface activation treatment is performed under the condition of 10±1W radio frequency power and 8-10 NI / h oxygen flow rate for 120±5s, so that the contact surface forms hydroxyl active group; the flexible film with channel and the flexible film with through hole after treatment are aligned and matched in the opposite way of the activated surface, a pressing force of 0.01-0.05MPa is applied and is left for 10-15min to promote preliminary combination between molecules; finally, the assembly is transferred to a constant temperature heating platform at 65±5℃ for stepwise bonding and curing, the interface molecules are allowed to diffuse sufficiently by keeping the temperature at 50±2℃ for the first 30min, and then the temperature is raised to 70±2℃ for 90min to complete the covalent bond;

[0030] The through hole of the flexible film with through hole is provided with a flow guide structure coaxially aligned with the through groove on the flexible film with channel for injecting liquid metal.

[0031] The step of covering the encapsulation film on the through hole of the flexible film with through hole for injecting liquid metal, and limiting the liquid metal in the channel structure of the elastic substrate comprises:

[0032] The film is processed into an encapsulation film with a covering area exceeding the edge of the liquid metal injection port by 1.0±0.2mm by cutting; Sil-Poxy silicone adhesive is selected to be coated on the periphery of the injection port with a uniform thickness of 25±5μm, and after optical alignment, a matching pressure of 0.05±0.01MPa is applied; in order to eliminate thermal interface stress, the encapsulation film is completed in a constant temperature environment of 23±2℃ during the packaging process, and the adhesive realizes molecular level penetration without thermal resistance layer by keeping the standing time of 30±2min during the curing stage.

[0033] Specifically, the step of fixing the flexible flat cable and the coaxial cable on the piezoresistive network and the SSPP unit includes: selecting an 8 pin FPC flat cable, and matching the silver copper core points of the flat cable with the through holes on the flexible film with the through holes, and forming a closed loop with the liquid alloy in the piezoresistive network. The connection end of the coaxial cable and the flexible film adopts a bifurcated welding tail, the signal line is connected with the SSPP unit in the liquid metal network through the polygonal through holes on both sides of the through hole film, and the ground wire is clamped between the through hole flexible film and the flexible substrate.

[0034] The packaging process is completed by coating Sil-Poxy silica adhesive between the through hole flexible film and the flexible substrate at a uniform thickness of 25±5 mu m, applying a bonding pressure of 0.05±0.01 MPa, and completing the packaging process in a constant temperature environment of 23±2 DEG C.

[0035] Specifically, the packaging film adopts a flexible PDMS packaging film with a thickness of 500±50 mu m.

[0036] The present application solves the defects in the technical background, and has the beneficial technical effects of:

[0037] The application discloses a flexible skin structure integrating surface and internal perception and a preparation method thereof, and realizes collaborative perception and processing of surface layer and deep layer information through a multi-modal flexible electronic skin.

[0038] The application is a flexible multi-modal sensing electronic skin integrating surface tactile perception and internal penetrating medium perception based on a liquid metal microchannel, and a preparation method thereof. BRIEF DESCRIPTION OF DRAWINGS

[0039] The application will be further described below in combination with the drawings and examples.

[0040] Figure 1 A schematic diagram of a flexible multi-modal sensing electronic skin structure of a preferred embodiment of the application Figure 1 ;

[0041] Figure 2 A schematic diagram of a flexible multi-modal sensing electronic skin structure of a preferred embodiment of the application Figure 2 ;

[0042] Figure 3 A structure schematic diagram of a liquid alloy sensing network of a preferred embodiment of the application

[0043] Figure 4Preparation flowchart of the flexible film with channel for the preferred embodiment of the present application;

[0044] Figure 5 Structure schematic diagram of the SSPP unit for the preferred embodiment of the present application;

[0045] Figure 6 Pressure sensing signal acquisition and processing circuit schematic diagram for the preferred embodiment of the present application;

[0046] Figure 7 HFSS-based dispersion relation simulation of the SSPP unit of the present application Figure 1 ;

[0047] Figure 8 S11 parameter simulation diagram of the SSPP structure of the present application by high-frequency electromagnetic simulation software;

[0048] Figure 9 Electric field intensity nephogram distribution of the SSPP unit of the present application;

[0049] Figure 10 Electric field vector simulation result distribution diagram of the SSPP unit of the present application in the x-y plane;

[0050] Figure 11 Electric field vector simulation result distribution diagram of the SSPP unit of the present application in the x-z plane;

[0051] Figure 12 Magnetic field vector distribution diagram of the SSPP unit of the present application in the x-y plane;

[0052] Wherein, 1, electromagnetic shielding layer; 2, encapsulation film; 3, flexible film with channel; 4, liquid metal network; 51, flexible film with through hole; 52, base layer; 6, flexible flat cable; 7, coaxial cable; 8, piezoresistive channel structure; 9, SSPP unit, 91, transition section; 92, surface waveguide section. DETAILED DESCRIPTION

[0053] The present application will now be further described in detail by reference to the drawings and embodiments, which are all simplified schematic diagrams and only schematically illustrate the basic structure of the present application, and thus only show the components related to the present application.

[0054] It should be noted that if directional indicators (such as up, down, bottom, top, etc.) are involved in the embodiments of the present invention, these directional indicators are only used to explain the relative positional relationship and movement of the components in a specific posture. If the specific posture changes, the directional indicators will also change accordingly. The terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features. Therefore, features defined with "first" and "second" may explicitly or implicitly include one or more of that feature. Unless otherwise explicitly specified and limited, the terms "set," "connected," and "linked" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a direct connection or an indirect connection through an intermediate medium; they can refer to the internal connection of two components. For those skilled in the art, the specific meaning of the above terms in the present invention can be understood according to the specific circumstances.

[0055] Example 1, as Figures 1-6 As shown, a flexible skin structure integrating surface and internal sensing includes: an elastic substrate with several layers of channel structures, in which liquid metal is injected, forming a liquid metal network 4 within the channel structures of the elastic substrate, and the liquid metal network 4 making electrical contact with cables. An electromagnetic shielding layer 1 is also disposed on the elastic substrate; in this embodiment, the elastic substrate is made of PDMS (polydimethylsiloxane) material; the liquid metal is an indium tin alloy with a melting point of 6°C, a density of 6.03 g / cm³, and a purity of 99%; the liquid metal is a conductive medium. The electromagnetic shielding layer 1 is made of copper foil.

[0056] Specifically, the elastic substrate includes a base layer 52, a flexible membrane 51 with through holes, and a flexible membrane 3 with channels stacked sequentially. After the flexible membrane 3 with channels and the flexible membrane 51 with through holes are joined together, a channel structure is formed in the middle. The flexible membrane 3 with channels is also provided with through grooves, and the through grooves are connected to the through holes on the flexible membrane 51 with through holes.

[0057] Specifically, the liquid metal network 4, as a sensing system, includes: an SSPP unit 9, and a piezoresistive channel structure 8 arranged in parallel around the SSPP unit 9; the cables include a flexible flat cable 6 connected to the piezoresistive channel structure 8, and a coaxial cable 7 connected to the SSPP unit 9. Furthermore, in this embodiment, the piezoresistive channel structure 8 is arranged in parallel on both sides of the SSPP unit 9, forming a dual-channel sensing system.

[0058] Specifically, the end of the flexible flat cable 6 and the coaxial cable 7 connected with the liquid metal network 4 is embedded between the base layer 52 and the flexible film 51 with holes, and the end of the flexible flat cable 6 and the coaxial cable 7 connected with the liquid metal network 4 corresponds to the hole on the flexible film 51 with holes.

[0059] Specifically, the hole of the flexible film 51 with holes and the groove of the flexible film 3 with channels are respectively plugged by the base layer 52 and the encapsulation film 2, so as to limit the liquid metal in the channel structure of the elastic base.

[0060] Embodiment two, the flexible skin structure of the fusion of inside and outside perception realizes multi-modal signal processing, including the following steps:

[0061] 1. Piezoresistive acquisition: the dual optimization of sensing performance is realized through the spiral topology structure design, which is specifically manifested as:

[0062] According to the resistance law R = pi / S , the electrode channel length l is maximized in the limited area of 20mm², so that the initial resistance R 0 is increased to the order of 50-100Ω (preferably 75±5Ω), which significantly improves the detection signal-to-noise ratio of the resistance change (8-10 times higher) compared with the straight channel design (usually 5-10Ω); the spiral configuration enables a single pressure point to simultaneously induce the cooperative deformation of 3-5 adjacent channels.

[0063] According to the sensitivity formula of Δ R / R 0=2Δ S / S 0, the effective sensing area change rate Δ S / S 0 is amplified to 3.2-4.8 times of the straight line.

[0064] The general circuit for acquiring piezoresistive signals acquires piezoresistive signals by reading the voltage change in the circuit through a resistance with a suitable resistance value. The pressure sensing signal acquisition and processing circuit is shown in Figure 6 , which also uses an LM324 operational amplifier with an amplification factor of 21. It is powered by the 5V power supply interface of the development board and is grounded with the development board. The sensor is internally powered by the 3.3V power supply interface of the development board, and the voltage dividing resistor is selected to be 1kΩ.

[0065] SSPP signal analysis: the SSPP structure channel of the SSPP unit 9 in the flexible electronic skin is connected with the SDR module (radio module), and the 1MHz signal f0 is transmitted through the 5.9GHz carrier f DThe transmission is connected to the SSPP module in the flexible electronic skin through the coaxial line 7, the output end normalized gain is 0.5; the RX port receives the signal, filters out the high-frequency noise through the low-pass filter (cut-off frequency 2MHz, bandwidth 0.5MHz), and then resamples, stores 100 data per second; then, the amplitude of the signal is calculated in the Python module. The above Python module is built by Gnuradio. When the medium concentration or property in the environment changes, the dielectric constant of the surface material of the SSPP unit 9 and the piezoresistive channel structure 8 outside the SSPP unit 9 changes, thereby affecting the resonance characteristics and transmission efficiency (S parameter) of the microwave signal on the SSPP unit 9. The amplitude of the received signal is calculated to judge the medium outside the flexible electronic skin.

[0066] In example three, on the basis of example one or example two, a signal processing system realized by a flexible skin structure integrating surface and internal perception, comprising:

[0067] A deep medium recognition and surface (piezoresistive contact recognition) detection integrated platform is constructed to realize synchronous perception and wireless remote transmission of multi-modal signals. The system integrates a flexible skin structure (flexible SSPP sensor) and an embedded SDR module (radio module) for real-time acquisition and spectral analysis of electromagnetic signals. In the deep substance recognition function, a flexible skin structure (flexible SSPP sensor) is used, and an electromagnetic shielding layer 1 (copper foil) and an impedance matching circuit (transition section 91) are introduced into the flexible skin structure (flexible SSPP sensor), which significantly improves the anti-interference ability and spectral response sensitivity of the system.

[0068] In example four, a preparation method of a flexible skin structure integrating surface and internal perception is used to prepare a flexible skin structure integrating surface and internal perception, comprising the following steps:

[0069] The base layer 52, the flexible film 51 with through holes, the flexible film 3 with channels, the liquid metal, the flexible flat cable 6, the coaxial line 7, the packaging film 2, and the electromagnetic shielding layer 1 are obtained.

[0070] One side of the flexible film 3 with channels is matched with the flexible film 51 with through holes, and a channel structure is formed between the flexible film 3 with channels and the flexible film 51 with through holes; the other side of the flexible film 3 with channels is matched with the base layer 52, and one end of the coaxial line 7 and the flexible flat cable 6 is respectively embedded between the flexible film 3 with channels and the base layer 52; the contact end of the coaxial line 7 corresponds to the SSPP unit 9 in the channel structure and contacts through the hole position on the flexible film 51 with through holes to transmit radio frequency signals; the contact end of the flexible flat cable 6 corresponds to the piezoresistive channel structure 8 in the channel structure to conduct piezoresistive signals; the through hole on the flexible film 51 with through holes is injected with liquid metal, and the liquid metal enters the channel structure to form a liquid metal network 4;

[0071] covering the encapsulation film 2 on the through hole of the flexible film 51 injected with liquid metal, limiting the liquid metal in the channel structure of the elastic substrate;

[0072] setting the electromagnetic shielding layer 1 on the outside of the substrate layer 52 and / or the flexible film 3 with channels.

[0073] Embodiment five, a preparation method of a flexible skin structure integrating surface and internal sensing, for preparing a flexible skin structure integrating surface and internal sensing, comprising the following steps:

[0074] Step 1, obtaining a substrate layer 52, a flexible film 51 with through holes, a flexible film 3 with channels, a liquid metal, a flexible flat cable 6, a coaxial cable 7, an encapsulation film 2, and an electromagnetic shielding layer 1.

[0075] In this embodiment, the preparation method of the substrate layer 52, the flexible film 51 with through holes, and the encapsulation film 2 comprises:

[0076] The polydimethylsiloxane prepolymer and the curing agent are mixed in a mass ratio of 10:1 to obtain a PDMS solution; the PDMS solution is degassed in a vacuum degassing box; the PDMS solution is spin-coated on a culture dish to form a PDMS liquid film; after degassing treatment, the PDMS liquid film is placed in a constant temperature oven for stepwise curing and crosslinking curing to obtain a flexible substrate film; and several pieces of the flexible substrate film are used as the substrate layer 52, the flexible film 51 with through holes, and the encapsulation film 2. More specifically, the polydimethylsiloxane prepolymer and the curing agent are accurately weighed and mixed in a mass ratio of 10:1, and are stirred at a low speed of 300-500 rpm for 5 min to form a homogeneous mixture; the mixture is degassed in a vacuum degassing box for 30 min; 2.5 g of the mixture is placed in a plasma-treated flat-bottomed culture dish, and a programmed spin coating process is used to spin coat the mixture at a speed of 550 r / min for 18 s to complete the initial flattening, and then spin coat the mixture at a speed of 900 r / min for 9 s to realize thickness homogenization; the spin-coated culture dish is placed in a vacuum environment of -0.1 MPa for degassing treatment for 25 min; and finally, the culture dish is placed in a constant temperature oven at 55°C for stepwise curing treatment for 110 min, wherein the temperature is maintained at 48°C for the first 30 min to promote molecular diffusion, and the temperature is increased to 68°C for the last 90 min to complete crosslinking curing, thereby obtaining a functional flexible substrate film with a thickness uniformity deviation of ≤5%.

[0077] In this embodiment, the preparation method of the flexible film 3 with channels comprises:

[0078] Step S1, the surface of the glass substrate is treated by using a cleaning solution combined with an ultrasonic cleaning process; high-purity nitrogen is used to blow and remove the residual liquid on the surface of the substrate; a clean and dry glass substrate surface is obtained; specifically, the substrate is placed in a heating platform or a vacuum heating cavity at 115°C for heat treatment, and the treatment time is 15 min, so that the surface adsorbed water molecules are completely removed, and a clean and dry glass substrate surface is obtained.

[0079] Step S2, a photomask plate is designed based on a preset microchannel pattern, and a spiral topological configuration is adopted to obtain a photomask plate with a microchannel pattern; specifically, the microchannel line width is set to 50 μm. Under the design constraints of meeting the minimization of the area of a single sensing unit (the target value is about 20 mm²), the maximum length of the microchannel is realized by spiral path optimization. At the same time, under the condition of ensuring that there is no signal crosstalk between adjacent channels and meeting the requirements of subsequent bonding process, the pitch of the spiral pattern is compressed and set. The size of the SSPP unit 9 is d = 6 mm, a = 3 mm, w = 0.7 mm, and the thickness t = 0.6 mm, h = 0.7 mm; the cutoff frequency is about 6.5 GHz, and the SSPP unit 9 is also provided with a transition section 91 and a surface waveguide section 92 to reduce energy reflection and realize radio frequency energy input and output, as shown in Figure 4 The dispersion coefficient of the SSPP unit 9 can be obtained by the following formula: wherein, k 0 is the free space wave vector, ε d is the dielectric constant of the dielectric, ε m is the equivalent dielectric constant of the metal structure; the drawn microchannel image is transferred to a PET polyester film to obtain a photomask plate.

[0080] Step S3, as shown in Figure 4 , based on the pretreated glass substrate surface, a photosensitive dry film is uniformly covered on the substrate surface by a precision film pasting process, and a heat pressing composite device is used to realize the close adhesion between the dry film and the substrate under the set temperature and pressure parameters, to form a photosensitive dry film layer and obtain a film-coated substrate with a uniform photosensitive layer; specifically, the thickness of the photosensitive dry film layer is 35 μm, and the thickness tolerance of the dry film layer is controlled within ± 5%, and finally a film-coated substrate with a uniform photosensitive layer is obtained.

[0081] Step S4, the film substrate is patterned by using a contact photolithography method, so that the microchannel pattern on the mask plate is completely transferred to the dry film layer; the contact photolithography method includes: the glass substrate coated with Dupont series dry film is tightly attached to the pre-prepared photolithography mask plate in a film surface-mask ink layer opposite manner, a uniform clamping pressure of 0.1 MPa is applied through a special clamp to ensure gapless contact between the interfaces, and the pattern transfer is performed under a 365 nm wavelength UV light source for an accurate exposure time of 5.5 s, wherein the exposure process is carried out under an illumination of 15 mW / cm², so that the microchannel pattern on the mask plate is completely transferred to the dry film layer.

[0082] Step S5, the unexposed part of the Dupont dry film on the dry film glass substrate is washed away by using a sodium carbonate solution, and a dry film glass substrate containing a channel structure is obtained; specifically, a negative photoetching process is used to develop the exposed substrate, a Dupont series negative photoresist is selected as a photosensitive medium, a cross-linking reaction occurs in the ultraviolet exposed area to form a channel structure, and the unexposed area is selectively dissolved by a 1.4% mass fraction sodium carbonate solution; the specific implementation process includes: placing the exposed substrate in a clean culture dish, injecting a developing solution at 21°C, and then oscillating at a frequency of 55 times per minute for 110 seconds to complete the pattern development, then using 18.2 MΩ·cm deionized water for pulse washing (5 seconds / time, repeated 3 times), and then placing it in a 0.4 MPa high-purity nitrogen gas blowing for 28 minutes in a 49°C precision oven for curing, to obtain a dry film glass substrate containing a channel structure.

[0083] Step S6, before the PDMS casting process is implemented, the dry film glass substrate with a channel structure needs to be surface modified to obtain a silanized dry film glass substrate; specifically, a gas phase silane reagent is used to modify the substrate surface in a sealed reaction cavity to form a monolayer release interface, which specifically includes: placing the patterned substrate in a vacuum drying box, introducing 1H, 1H, 2H, 2H-perfluorooctyltriethoxysilane vapor, and reacting at 75°C for 28 minutes to form a 1 nm thick hydrophobic fluorosilane molecular layer, thereby obtaining a silanized dry film glass substrate.

[0084] Step S7, a flexible microstructure film is prepared on the modified substrate surface by using a prepolymer casting process to obtain a flexible film 3 with channels; specifically, polydimethylsiloxane (PDMS) prepolymer and curing agent are mixed in a mass ratio of 10:1, vacuum degassing treatment is performed (-0.095 MPa, 28 min), and then the mixture is spin-coated on the surface of the silanized substrate at a speed of 1500 rpm to form a liquid film layer with a thickness of 450 μm, followed by curing at 78°C for 115 min, to finally obtain an elastic PDMS film with an accurately replicated channel structure, i.e., a flexible microchannel film. A punch is used to punch holes at the injection ports reserved on both sides of each group of electrode channels.

[0085] Step 2, the side of the flexible film 3 provided with micro-channels is matched with the flexible film 51 provided with through holes, and a channel structure is formed between the flexible film 3 provided with micro-channels and the flexible film 51 provided with through holes.

[0086] In this embodiment, the side of the flexible film 3 provided with micro-channels is matched with the flexible film 51 provided with through holes, and a channel structure is formed between the flexible film 3 provided with micro-channels and the flexible film 51 provided with through holes. The steps include: first, using a low-adhesion clean tape to remove the particulate contaminants on the surface of the flexible film 51 provided with through holes and the flexible film 3 provided with micro-channels; then, placing the flexible film 3 provided with micro-channels and the flexible film 51 provided with through holes to be bonded in a plasma processing device, and performing a surface activation treatment under the condition of 9W radio frequency power and 8NI / h oxygen flow rate for 115s, so that the contact surface forms a hydroxyl active group; the flexible film 3 provided with micro-channels and the flexible film 51 provided with through holes after treatment are aligned and matched in a manner that the activation surfaces face each other, a pressing force of 0.01MPa is applied and left for 10min to promote the initial combination between molecules; finally, the assembly is transferred to a constant temperature heating platform of 60℃ for stepwise bonding and curing, the interface molecules are allowed to diffuse sufficiently by keeping 48℃ for the first 30min, and then the temperature is raised to 68℃ for 90min to complete the covalent bond; the through hole of the flexible film 51 provided with through holes is provided with a flow guide structure coaxially aligned with the through groove on the flexible film 3 provided with micro-channels for injecting liquid metal. Specifically, the positional accuracy deviation is ≤50 μ m, to ensure the reliable electrical connection between the subsequent flexible flat cable 6 and the micro-channel electrode.

[0087] The other side of the flexible film 3 provided with micro-channels is matched with the substrate layer 52 to obtain an elastic substrate with micro-channels, and the coaxial line 7 and one end of the flexible flat cable 6 are respectively embedded between the flexible film 3 provided with micro-channels and the substrate layer 52; the elastic substrate with micro-channels is connected with the pre-made flexible flat cable 6, the flexible flat cable 6 is an FPC flexible flat cable, the surface wave guide section 92 at both ends of the SSPP unit 9 is connected with the coaxial line 7, and a thin film structure with regular appearance is obtained. Liquid metal is injected into the micro-channel of the thin film structure through the injection port of the through hole on the flexible film 51 provided with through holes, and the liquid metal enters the channel structure including the piezoresistive channel structure 8 and the SSPP unit 9 to form a liquid metal network 4. The contact end of the coaxial line 7 corresponds to the SSPP unit 9 in the channel structure and contacts through the hole on the flexible film 51 provided with through holes to transmit radio frequency signals; the contact end of the flexible flat cable 6 corresponds to the piezoresistive channel structure 8 in the channel structure and contacts to conduct piezoresistive signals.

[0088] Step 3, the encapsulation film 2 is covered on the through hole of the flexible film 51 injected with liquid metal, and the liquid metal is limited in the channel structure of the elastic substrate. The specific steps include: the film is processed into the encapsulation film 2 by cutting, the encapsulation film 2 covers the area beyond the edge of the liquid metal injection port by 0.8 mm; the Sil-Poxy silicone adhesive is selected to be coated on the periphery of the injection port with a uniform thickness of 20 μm, and after optical alignment, a lamination pressure of 0.04 MPa is applied; in order to eliminate the thermal interface stress, the encapsulation film 2 is completed in a constant temperature environment of 21 ℃ during the packaging process, and the adhesive realizes molecular level penetration without thermal resistance layer by keeping a standing time of 28 min in the curing stage. The encapsulation film 2 adopts a flexible PDMS encapsulation film with a thickness of 450 μm.

[0089] Step 4, the electromagnetic shielding layer 1 is arranged outside the substrate layer 52 and / or the flexible film 3 with channels. The electromagnetic shielding layer 1 adopts a copper foil; the copper foil is cut into a suitable rectangular film according to the size of the SSPP unit 9, and is attached to the flexible film 3 with channels and the encapsulation film 2 through the adhesive.

[0090] Example 6, a preparation method of a flexible skin structure integrating surface and internal sensing, for preparing a flexible skin structure integrating surface and internal sensing, comprising the following steps:

[0091] Step 1, obtaining a substrate layer 52, a flexible film 51 with through holes, a flexible film 3 with channels, liquid metal, a flexible flat cable 6, a coaxial cable 7, an encapsulation film 2, and an electromagnetic shielding layer 1.

[0092] In the embodiment, the preparation method of the substrate layer 52, the flexible film 51 with through holes, and the encapsulation film 2 comprises:

[0093] The polydimethylsiloxane prepolymer and the curing agent are mixed in a mass ratio of 10:1 to obtain a PDMS solution; the PDMS solution is degassed in a vacuum degassing tank; the PDMS liquid film is formed by spin coating the PDMS solution on a culture dish; the flexible substrate film is obtained by stepwise curing and cross-linking curing of the PDMS liquid film in a constant temperature oven; and several pieces of the flexible substrate film are used as the base layer 52, the flexible film with holes 51, and the packaging film 2. Specifically, the polydimethylsiloxane prepolymer and the curing agent are accurately weighed and mixed in a mass ratio of 10:1, and a homogeneous mixture is formed by stirring at a low speed of 400 rpm for 7 min; the mixture is degassed in a vacuum degassing tank for 30 min; 3.0 g of the mixture is placed in a flat-bottomed culture dish treated by plasma, and a programmed spin coating process is used to spin coat the mixture at a speed of 600 r / min for 20±2 s to complete the initial flattening, and then at a speed of 1000 r / min for 10 s to realize thickness homogenization; the culture dish after spin coating is placed in a vacuum environment of -0.1 MPa for degassing treatment for 30 min; finally, the flexible substrate film with a thickness uniformity deviation of ≤5% is prepared by stepwise curing treatment in a constant temperature oven at 60℃ for 120 min, in which the temperature is maintained at 50℃ for the first 30 min to promote molecular diffusion, and then the temperature is increased to 70℃ for the last 90 min to complete the cross-linking curing.

[0094] In this embodiment, the preparation method of the flexible film with channels 3 includes:

[0095] In step S1, the surface of the glass substrate is treated by using a cleaning liquid combined with an ultrasonic cleaning process; high-purity nitrogen gas is used for blowing and removing residual liquid on the surface of the substrate; and a clean and dry glass substrate surface is obtained. Specifically, the substrate is placed in a heating platform or a vacuum heating cavity at 120℃ for heat treatment for 20 min to completely remove the surface-adsorbed water molecules and obtain a clean and dry glass substrate surface.

[0096] In step S2, a photomask plate is designed based on a preset microchannel pattern, and a spiral topological configuration is used to obtain a photomask plate with a microchannel pattern. Specifically, the microchannel line width is set to 50 μm. Under the design constraints of minimizing the area of a single sensing unit (the target value is about 20 mm²), the length of the microchannel is maximized by optimizing the spiral path. At the same time, the pitch of the spiral pattern is compressed under the condition of ensuring no signal crosstalk between adjacent channels and meeting the requirements of subsequent bonding processes. The size of the SSPP is d = 6 mm, a = 3 mm, w = 0.7 mm, and the thickness t = 0.6 mm, h = 0.7 mm; the cutoff frequency is about 6.5 GHz, and the two ends contain a transition section 91 and an end waveguide section 92 to reduce energy reflection and realize the input and output of radio frequency energy, and the SSPP unit 9 is as shown in Figure 4As shown, the dispersion coefficient can be derived from the following equation: wherein k 0 is the free space wave vector, ε d is the dielectric constant of the dielectric, ε m is the equivalent dielectric constant of the metal structure; the drawn microchannel image is transferred to a PET polyester film to obtain a photolithographic mask.

[0097] Step S3, based on the pre-processed glass substrate surface, the photosensitive dry film is uniformly covered on the substrate surface by precise film pasting process, and the dry film is tightly attached to the substrate under the set temperature and pressure parameters by using a hot pressing composite device, forming a photosensitive dry film layer, and obtaining a film-coated substrate with a uniform photosensitive layer; Specifically, the thickness of the photosensitive dry film layer is 37 pm, and the thickness tolerance of the dry film layer is controlled within ± 5%, and finally a film-coated substrate with a uniform photosensitive layer is obtained.

[0098] Step S4, the film-coated substrate is patterned by using a contact photolithography method, so that the microchannel pattern on the mask is completely transferred to the photosensitive dry film layer; The contact photolithography method includes: the glass substrate coated with Dupont series photosensitive dry film is tightly attached to the pre-prepared photolithographic mask in a film surface-mask ink layer opposite manner, a uniform clamping pressure of 0.1-0.3 MPa is applied through a special clamp to ensure gapless contact at the interface, and the pattern transfer is carried out under a 365 nm wavelength UV light source with an accurate exposure time of 6 s, wherein the exposure process is carried out under the condition of illumination 17.5 mW / cm², so that the microchannel pattern on the mask is completely transferred to the photosensitive dry film layer.

[0099] Step S5, the unexposed part of the Dupont photosensitive dry film on the dry film glass substrate is washed away by using a sodium carbonate solution, and a dry film glass substrate containing a channel structure is obtained; Specifically, a negative photoetching process is used to develop the exposed substrate, and Dupont series negative photoresist is selected as the photosensitive medium, and a cross-linking reaction occurs in the ultraviolet exposed area to form a channel structure, and the unexposed area is selectively dissolved by a 1.5% mass fraction of sodium carbonate solution, and the specific implementation process includes: placing the exposed substrate in a clean culture dish, injecting a 23℃ developing solution, and oscillating at a frequency of 60 times / minute for 120 seconds to complete the pattern development, then using 18.2 MΩ·cm deionized water for pulse washing (5 seconds / time, repeated 3 times), and then placing it in a 0.4 MPa high-purity nitrogen gas blowing after blowing, and then placing it in a 50℃ precision oven for 30 minutes to obtain a dry film glass substrate containing a channel structure.

[0100] Step S6, before the PDMS casting process is implemented, the dry film glass substrate with channel structure needs to be subjected to surface modification treatment to obtain a silanized dry film glass substrate; specifically, a gas phase silane reagent is used to perform molecular self-assembly modification on the surface of the substrate in a closed reaction cavity to form a monolayer release interface, which specifically includes: placing the patterned substrate in a vacuum drying box, introducing 1H, 1H, 2H, 2H-perfluorooctyltriethoxysilane vapor, and reacting at 80°C for 30 minutes to form a hydrophobic fluorosilane molecular layer with a thickness of 1.5 nm, thereby obtaining a silanized dry film glass substrate.

[0101] Step S7, a flexible microstructure film is prepared on the surface of the modified substrate by using a prepolymer casting process to obtain a flexible film 3 with channels; specifically, after polydimethylsiloxane (PDMS) prepolymer and a curing agent are mixed in a mass ratio of 10:1 and subjected to vacuum degassing treatment (-0.097 MPa, 30 min), the mixture is spin-coated on the surface of the silanized substrate at a speed of 2000 rpm to form a liquid film layer with a thickness of 500 μm, and then the liquid film layer is cured at 80°C for 120 min, thereby obtaining an elastic PDMS film with a precisely replicated channel structure, i.e., a flexible microchannel film. A puncher is used to punch holes at the injection ports reserved on both sides of each electrode channel.

[0102] Step 2, the side of the flexible film 3 with microchannels is aligned with the flexible film 51 with through holes, and a channel structure is formed between the flexible film 3 with channels and the flexible film 51 with through holes.

[0103] In this embodiment, the side of the flexible film 3 with microchannels is aligned with the flexible film 51 with through holes, and a channel structure is formed between the flexible film 3 with channels and the flexible film 51 with through holes, which includes the following steps: first, low-adhesion clean tape is used to remove particulate contaminants on the surfaces of the flexible film 51 with through holes and the flexible film 3 with channels; then the flexible film 3 with channels and the flexible film 51 with through holes to be bonded are placed in a plasma treatment device, and surface activation treatment is performed under the condition of 10 W radio frequency power and 9 NI / h oxygen flow rate for 120 s to form hydroxyl active groups on the contact surfaces; the flexible film 3 with channels and the flexible film 51 with through holes after treatment are aligned and attached in a manner that the activated surfaces face each other, 0.03 MPa of pressing force is applied and the assembly is left to stand for 12.5 min to promote preliminary intermolecular bonding; finally, the assembly is transferred to a constant-temperature heating platform at 65±5°C for stepwise bonding and curing, the interface molecules are allowed to diffuse sufficiently by keeping the temperature at 50°C for the first 30 min, and then the temperature is raised to 70°C for 90 min to complete covalent bonding; the through holes of the flexible film 51 with through holes are provided with a flow guide structure coaxially aligned with the through grooves on the flexible film 3 with channels for injecting liquid metal. Specifically, the positional accuracy deviation of the flow guide structure is ≤50 μ μm to ensure reliable electrical connection between the subsequent flexible flat cable 6 and the microchannel electrode.

[0104] The other side of the flexible membrane 3 with channels is mated with the substrate layer 52 to obtain an elastic substrate with microchannels. One end of the coaxial line 7 and the flexible flat cable 6 are respectively embedded between the flexible membrane 3 with channels and the substrate layer 52. The elastic substrate with microchannels is connected to the pre-fabricated flexible flat cable 6, which is an FPC flexible flat cable. The surface waveguide sections 92 at both ends of the SSPP unit 9 are connected to the coaxial line 7 to obtain a well-formed thin film structure. Liquid metal is injected into the microchannels of the thin film structure through the injection port of the through hole on the flexible membrane 51. The liquid metal enters the channel structure, including the piezoresistive channel structure 8 and the SSPP unit 9, to form a liquid metal network 4. The contact end of the coaxial line 7 corresponds to the SSPP unit 9 in the channel structure and contacts the hole on the flexible membrane 51 with through holes to transmit radio frequency signals. The contact end of the flexible flat cable 6 contacts the piezoresistive channel structure 8 in the channel structure to conduct piezoresistive signals.

[0105] Step 3: The encapsulation film 2 is placed over the through-hole of the flexible film 51 with vias into which the liquid metal is injected, confining the liquid metal within the channel structure of the elastic substrate. Specific steps include: processing the film into an encapsulation film 2 with a coverage area extending 1.0 mm beyond the edge of the liquid metal injection port by cutting; applying a uniform thickness of 25 μm of Sil-Poxy silicone adhesive to the periphery of the injection port, followed by optical alignment and application of a bonding pressure of 0.05 MPa; to eliminate thermally induced interfacial stress, the encapsulation film 2 is encapsulated in a constant temperature environment of 23°C, and a 30-minute resting time is maintained during the curing stage to allow the adhesive to penetrate at the molecular level without forming a thermally resistive layer. The encapsulation film 2 is a 500 μm thick flexible PDMS encapsulation film.

[0106] Step 4: The electromagnetic shielding layer 1 is disposed on the outside of the substrate layer 52 and / or the flexible membrane 3 with channels. The electromagnetic shielding layer 1 is made of copper foil; the copper foil is cut into a suitable rectangular film according to the size of the SSPP unit 9, and then attached to the flexible membrane 3 with channels and the encapsulation film 2 with adhesive.

[0107] Example 7: A method for preparing a flexible skin structure with integrated surface and interior sensing, comprising the following steps;

[0108] Step 1: Obtain a substrate layer 52, a flexible membrane 51 with through holes, a flexible membrane 3 with channels, liquid metal, a flexible flat cable 6, a coaxial cable 7, an encapsulation film 2, and an electromagnetic shielding layer 1.

[0109] In this embodiment, the preparation method of the substrate layer 52, the flexible film 51 with through holes, and the encapsulation film 2 includes:

[0110] A PDMS solution was obtained by weighing and mixing polydimethylsiloxane prepolymer and curing agent at a mass ratio of 10:1; the solution was degassed in a vacuum degasing chamber; the PDMS solution was spin-coated onto a petri dish to form a PDMS liquid film; the PDMS liquid film was degassed and then cured in a constant temperature oven in a stepwise manner and cross-linked to obtain a flexible substrate film; several flexible substrate films were used as the base layer 52, the flexible film 51 with through holes, and the encapsulation film 2, respectively. Specifically, polydimethylsiloxane prepolymer and curing agent were precisely weighed and mixed at a mass ratio of 10:1, and stirred at low speed of 500 rpm for 10 min to form a homogeneous mixture. The mixture was then degassed in a vacuum degaussing chamber for 30 min. 3.5 g of the mixture was placed in a plasma-treated flat-bottomed petri dish, and a programmed spin-coating process was used to spin-coat the dish sequentially at a speed of 650 rpm for 22 s to achieve initial flattening, and then at a speed of 1100 rpm for 11 s to achieve thickness homogenization. The spin-coated petri dish was then placed in a vacuum environment of -0.1 MPa for 35 min to undergo degaussing treatment. Finally, the dish was subjected to a step-curing treatment in a constant temperature oven at 65℃ for 130 min, with the temperature maintained at 52℃ for the first 30 min to promote molecular diffusion, and then increased to 72℃ for the last 90 min to complete cross-linking and curing, resulting in a functional flexible substrate film with a thickness uniformity deviation of ≤5%.

[0111] In this embodiment, the method for preparing the flexible membrane 3 with channels includes:

[0112] Step S1: The glass substrate is surface treated using a cleaning solution combined with ultrasonic cleaning process; high-purity nitrogen is used to purge and remove residual liquid from the substrate surface; a clean and dry glass substrate surface is obtained; specifically, the glass substrate is placed in a heating platform at 125°C or in a vacuum heating chamber for heat treatment for 30 minutes to completely remove adsorbed water molecules from the surface and obtain a clean and dry glass substrate surface.

[0113] Step S2 involves designing a photomask based on a preset microchannel pattern, employing a spiral topology to obtain a photomask with a microchannel pattern; specifically, the microchannel linewidth is set to 50 μm. Under the design constraint of minimizing the area of ​​a single sensing unit (target value approximately 20 mm²), the microchannel length is maximized through spiral path optimization. Simultaneously, while ensuring no signal crosstalk between adjacent channels and meeting the requirements of subsequent bonding processes, the spacing of the spiral pattern is compressed. Figure 5 As shown, the dimensions of SSPP unit 9 are d =6mm, a =3mm, w =0.7mm, thickness t= 0.6mm, h = 0.7mm; the cutoff frequency is about 6.5GHz, and the SSPP unit 9 is further provided with a transition section 91 and a surface waveguide section 92 at both ends, so as to reduce energy reflection and realize radio frequency energy input and output, and the SSPP unit 9 is as shown in Figure 4 The dispersion coefficient of the SSPP unit 9 can be obtained by the following formula: Wherein k 0 is the free space wave vector, ε d is the dielectric constant of the dielectric, ε m is the equivalent dielectric constant of the metal structure; the drawn microchannel image is transferred to a PET polyester film to obtain a photoetching mask.

[0114] Step S3, based on the pre-processed glass substrate surface, the photosensitive dry film is uniformly covered on the substrate surface by precise film pasting process, and the dry film is tightly attached to the substrate under the set temperature and pressure parameters by using a hot pressing composite device, to form a photosensitive dry film layer, and obtain a film-coated substrate with a uniform photosensitive layer; specifically, the thickness of the photosensitive dry film layer is 39µm, and the thickness tolerance of the dry film layer is controlled within ±5%, and finally a film-coated substrate with a uniform photosensitive layer is obtained.

[0115] Step S4, the film-coated substrate is subjected to patterned processing by using a contact photoetching method, so that the microchannel pattern on the mask is completely transferred to the photosensitive dry film layer; the contact photoetching method comprises: tightly attaching the glass substrate coated with Dupont series photosensitive dry film to the pre-prepared photoetching mask in a film surface-mask ink layer opposite manner, applying a uniform clamping pressure of 0.3MPa by using a special clamp to ensure gapless contact at the interface, and performing pattern transfer under a 365nm wavelength UV light source with an accurate exposure time of 6.5s, wherein the exposure process is carried out under an illumination of 20mW / cm², so that the microchannel pattern on the mask is completely transferred to the photosensitive dry film layer.

[0116] Step S5, the unexposed part of the Dupont photosensitive dry film on the dry film glass substrate is washed away by using a sodium carbonate solution, to obtain a dry film glass substrate containing a channel structure; specifically, a negative photoetching process is used to develop the exposed substrate, and Dupont series negative photoresist is selected as the photosensitive medium, the cross-linking reaction occurs in the ultraviolet exposed area to form a channel structure, and the unexposed area is selectively dissolved by using a 1.6% mass fraction sodium carbonate solution, and the specific implementation process comprises: placing the exposed substrate in a clean culture dish, injecting a developing solution at 25℃, and then oscillating at a frequency of 65 times / minute for 130 seconds to complete pattern development, then using 18.2MΩ·cm deionized water for pulse washing (5 seconds / time, repeated 3 times), blowing with 0.4MPa high-purity nitrogen gas, and then placing in a 51℃ precision oven for curing for 32 minutes, to obtain a dry film glass substrate containing a channel structure.

[0117] Step S6, before the PDMS casting process is implemented, the dry film glass substrate with channel structure needs to be subjected to surface modification treatment to obtain a silanized dry film glass substrate; specifically, a gas phase silane reagent is used to modify the substrate surface in a closed reaction cavity to form a monolayer release interface, specifically including: placing the patterned substrate in a vacuum drying box, introducing 1H, 1H, 2H, 2H-perfluorooctyltriethoxysilane vapor, reacting at 85°C for 32 minutes to form a 2nm-thick hydrophobic fluorosilane molecular layer, thereby obtaining a silanized dry film glass substrate.

[0118] Step S7, a flexible microstructure film is prepared on the modified substrate surface by using a prepolymer casting process to obtain a flexible film 3 with channels; specifically, polydimethylsiloxane (PDMS) prepolymer and a curing agent are mixed at a mass ratio of 10:1, vacuum degassing treatment is performed (-0.1 MPa, 32 min), and the mixture is spin-coated on the surface of the silanized substrate at a speed of 2500 rpm to form a 550μm-thick liquid film layer, which is then cured at 82°C for 125 min to finally obtain an elastic PDMS film with a precisely replicated channel structure, i.e., a flexible microchannel film. A puncher is used to punch holes at the injection ports reserved on both sides of each electrode channel.

[0119] Step 2, the side of the flexible film 3 with microchannels is aligned with the flexible film 51 with through holes, and a channel structure is formed between the flexible film 3 with channels and the flexible film 51 with through holes.

[0120] In this embodiment, the side of the flexible film 3 with microchannels is aligned with the flexible film 51 with through holes, and a channel structure is formed between the flexible film 3 with channels and the flexible film 51 with through holes, which includes the following steps: first, low-adhesion clean tape is used to remove particulate contaminants on the surfaces of the flexible film 51 with through holes and the flexible film 3 with channels; then the flexible film 3 with channels and the flexible film 51 with through holes to be bonded are placed in a plasma treatment device and subjected to surface activation treatment under the conditions of 11W radio frequency power and 10 NI / h oxygen flow rate for 125s to form hydroxyl active groups on the contact surfaces; the treated flexible film 3 with channels and the flexible film 51 with through holes are aligned and attached with the activated surfaces opposite to each other, a pressing force of 0.05 MPa is applied and left to stand for 15 min to promote preliminary molecular bonding; finally, the assembly is transferred to a constant temperature heating platform at 70°C for stepwise bonding and curing, the interface molecules are allowed to diffuse sufficiently by keeping the temperature at 52°C for the first 30 min, and then the temperature is raised to 72°C for 90 min to complete the covalent bond; the through holes of the flexible film 51 with through holes are provided with a flow guide structure coaxially aligned with the through grooves on the flexible film 3 with channels for injecting liquid metal. Specifically, the positional accuracy deviation is ≤50 μm to ensure reliable electrical connection of the subsequent flexible flat cable 6 with the microchannel electrode.

[0121] The other side of the flexible film 3 with channels is matched with the base layer 52 to obtain an elastic base with microchannels, and the coaxial line 7 and one end of the flexible flat cable 6 are respectively embedded between the flexible film 3 with channels and the base layer 52; the elastic base with microchannels is connected with the pre-prepared flexible flat cable 6, the flexible flat cable 6 is an FPC flexible flat cable, the surface wave guide section 92 at both ends of the SSPP unit 9 is connected with the coaxial line 7, and a thin film structure with regular appearance is obtained. Liquid metal is injected into the microchannels of the thin film structure through the injection port of the through hole on the flexible film 51 with holes, and the liquid metal enters the channel structure including the piezoresistive channel structure 8 and the SSPP unit 9 to form a liquid metal network 4. The contact end of the coaxial line 7 corresponds to the SSPP unit 9 in the channel structure and contacts through the hole on the flexible film 51 with holes to transmit radio frequency signals; the contact end of the flexible flat cable 6 corresponds to the piezoresistive channel structure 8 in the channel structure and contacts to conduct piezoresistive signals.

[0122] Step 3: The encapsulation film 2 is covered on the through hole of the flexible film 51 with holes for injecting liquid metal to limit the liquid metal in the channel structure of the elastic base. The specific steps include: the film is processed into an encapsulation film 2 with a covering area exceeding the edge of the liquid metal injection port by 1.2 mm through cutting; Sil-Poxy silicone adhesive is selected to be coated on the periphery of the injection port with a uniform thickness of 30 μm, and after optical alignment, a lamination pressure of 0.06 MPa is applied; in order to eliminate thermal interface stress, the encapsulation film 2 is completed in a constant temperature environment of 25℃ during encapsulation, and a standing time of 32 min is maintained in the curing stage to enable the adhesive to realize molecular level penetration without generating thermal resistance layer. The encapsulation film 2 adopts a flexible PDMS encapsulation film with a thickness of 550 μm.

[0123] Step 4: The electromagnetic shielding layer 1 is arranged on the outer side of the base layer 52 and / or the flexible film 3 with channels. The electromagnetic shielding layer 1 adopts a copper foil; the copper foil is cut into a suitable rectangular film according to the size of the SSPP unit 9, and is attached to the flexible film 3 with channels and the encapsulation film 2 through adhesive.

[0124] Example Eight, on the basis of Example Five, the SSPP unit 9 and the piezoresistive channel structure 8 surrounding the SSPP unit 9 (SSPP is artificial surface plasmon) in the application are subjected to systematic electromagnetic characteristic simulation analysis through high frequency structure simulation software (HFSS), and the simulation result is as shown in Figures 7-12 Figure 7 ​The dispersion relation curve of the SSPP unit 9 is shown, and simulation data show that when the working frequency is lower than 5GHz, the structure does not exhibit obvious wavelength compression characteristics, and when the frequency rises to more than 6GHz, the dispersion curve presents good smooth characteristics, based on the frequency characteristic analysis, the application preferably selects 5-6GHz as the best working frequency band.

[0125] The application performs systematic simulation analysis on the S11 parameters of the SSPP unit 9 and the piezoresistive channel structure 8 outside the SSPP unit 9 through high-frequency electromagnetic simulation software. Figure 8 The simulation curve shows that in the frequency range of 1-10GHz, the S11 parameter presents obvious frequency selection characteristics, wherein two significant resonance valley points appear near 2.3GHz and 3.5GHz, and the S11 parameter reaches the extreme values of-40dB and-35dB respectively, indicating that the two frequency points have excellent energy coupling efficiency; in the working frequency band of 5-6GHz, the S11 parameter is stably maintained in the interval of-20dB to-25dB, showing good signal transmission performance; when the frequency exceeds 7GHz, the S11 parameter rapidly deteriorates to more than-10dB, indicating that the structure has a natural suppression effect on high-frequency signals. It is particularly worth noting that there is a transition region near 4.2GHz, and the S11 parameter rapidly rises from-30dB to-15dB, which provides a clear demarcation basis for frequency band selection. By analyzing the change trend of the S11 parameter curve, it can be confirmed that the SSPP structure has good signal transmission efficiency (S11<-20dB) and moderate quality factor in the frequency band of 5-6GHz, which makes it particularly suitable as the core sensitive unit of the microwave sensor, providing an optimal frequency band selection scheme for realizing high-sensitivity material detection, and the multi-resonance point characteristics of the S11 parameter curve also provide potential possibilities for realizing multi-band composite sensing.

[0126] As Figure 9As shown, the electric field intensity cloud distribution of the SSPP unit 9 is provided, and the electric field distribution characteristics of the SSPP unit 9 in the application are analyzed in detail by high-frequency electromagnetic simulation software. The electric field cloud map clearly shows the electromagnetic field distribution characteristics of the SSPP unit 9 at a specific operating frequency. The electric field intensity shows a clear local enhancement effect at the edge of the groove of the periodic structure of the SSPP unit 9. This enhancement effect rapidly decays with the increase of the distance from the structure surface, which conforms to the typical field distribution law of surface plasmons. It can be observed from the cloud map that the electric field energy is mainly concentrated in the metal-dielectric interface of the SSPP unit 9, and shows an exponential decay distribution in the cross section perpendicular to the propagation direction, while in the longitudinal section along the propagation direction, it shows a periodic fluctuation characteristic. It is particularly worth noting that obvious electric field hot spot areas are formed at the key structural dimensions of the SSPP unit 9 (such as groove depth and period length), and the electric field intensity in these areas reaches the maximum value, verifying the high binding ability of the structure to the electromagnetic field. At the same time, the simulation results show that the electric field distribution has excellent stability in the operating frequency band of 5-6 GHz, and there is no obvious energy leakage phenomenon, which is crucial for realizing high-sensitivity material detection. Through quantitative analysis of the intensity distribution of the electric field cloud map, the structural parameters of the SSPP unit 9 can be accurately optimized to ensure that it can achieve the best electromagnetic field and interaction effect with the measured substance in the sensing application, providing a reliable simulation basis for subsequent sensor performance improvement.

[0127] Figure 10 and Figure 11 The electric field vector simulation results of the SSPP unit 9 in the x-y plane and the x-z plane are shown respectively. The electric field vector distribution of the SSPP unit 9 obtained by high-frequency electromagnetic simulation clearly shows the vector characteristics of the electromagnetic field in the 5 GHz operating frequency band. The electric field vector mainly shows strong orientation along the propagation direction (z-axis) of the SSPP structure, and forms an obvious vector aggregation effect at the edge of the metal groove, with the maximum field strength amplitude. It can be seen in the x-y cross section that the electric field vector propagates in the form of a surface wave at the dielectric-metal interface, and the vector direction shows a periodic oscillation characteristic, and presents an exponential decay law with the increase of the distance from the surface. It is particularly worth noting that at the groove depth mutation of the SSPP unit 9, the electric field vector is significantly deflected and forms a local vortex structure, which effectively enhances the interaction strength of the electromagnetic field and the measured substance. In the x-z section, the electric field vector forms an alternating distribution pattern along the propagation direction, and the phase difference of the vector of adjacent period units is π / 2, which conforms to the typical propagation characteristics of surface plasmons. Through quantitative analysis of the distribution law of the vector diagram, it can be confirmed that the SSPP unit 9 has excellent field binding ability in the 5 GHz frequency band.

[0128] Figure 12Further, the magnetic field vector distribution diagram of the x-y plane is given, and the simulation results clearly show that the magnetic field vector is mainly oriented along the Y direction. The above series of simulation results not only verify the excellent electromagnetic field binding ability and signal transmission characteristics of the SSPP unit 9 in the 5-6 GHz frequency band, but also provide an important theoretical basis for the structural optimization of the sensor. By setting the geometric parameters of the SSPP unit 9, selective response to specific frequency band microwave signals is successfully achieved, which lays a solid foundation for subsequent sensor performance testing and application implementation.

[0129] In the ninth embodiment, on the basis of the eighth embodiment, as shown in Figure 6 , the piezoresistive signal of the piezoresistive channel structure 8 outside the SSPP unit 9 is collected by the piezoresistive signal collection circuit. The piezoresistive signal collection circuit uses a high-precision analog switch chip ADG333ABRSZ to realize intelligent switching control of 8-way piezoresistive sensors (i.e., the piezoresistive channel structure 8). When the ADG333 switch is switched to the Vref (2.5V) reference voltage terminal, the in-phase input terminal and the anti-phase input terminal of the operational amplifier LMV321IDBVR have equal potentials, forming a virtual short state. At this time, no current passes through the piezoresistive channel, and the system is in a measured state. When the switch is switched to the ground potential, a constant potential difference of 2.5V is formed across the piezoresistive channel, and the feedback resistor R f and the operational amplifier LMV321IDBVR together constitute a non-inverting amplifier circuit, and the resistance value of the piezoresistive channel can be accurately calculated by measuring the voltage change at the output terminal. The output voltage of the spiral piezoresistive network follows the formula:

[0130] ;

[0131] where R f is the feedback resistor between the inverting input terminal and the output terminal of the amplifier (1kΩ±1%), and R i is the dynamic resistance of the spiral piezoresistive network. The liquid alloy spiral piezoresistive channel (i.e., the piezoresistive channel structure 8) presents a baseline resistance of 10Ω±5% in the unpressurized state, and when subjected to external pressure, the change in the cross-sectional area of the internal microchannel causes the resistance value to linearly increase to about 20Ω. This change is digitized and collected by a 24-bit sigma-delta ADC. The reference voltage is provided by a low-dropout linear regulator AMS1117-2.5 chip, which converts a 5V input voltage to a precise reference voltage of 2.5V. The P1-P8 output terminals of the circuit use push-pull driving design and can output 2.5V high level and ground level. The digital control terminals D2-D9 control the switching state of the 8 groups of single-pole double-throw switches of the ADG333 chip through the timing signals generated by the FPGA. Through time division multiplexing, the 8-way piezoresistive signal is detected in a loop, meeting the application requirements of tactile sensing. By controlling the switch of the ADG333 chip, the voltage values of the 8 piezoresistive spirals (i.e., the piezoresistive channel structure 8) are scanned and measured. In this embodiment, R f has a resistance of 10Ω, and Ri The range of Ω is 10-20Ω; therefore, the range of the measured voltage is 2.5-5V, but it is not limited to this. In other embodiments, the specific parameter settings can be adjusted according to actual usage requirements.

[0132] Working principle:

[0133] This invention relates to a flexible multimodal sensing electronic skin based on liquid metal microchannels, integrating surface tactile sensing and deep penetrating medium sensing, and its fabrication method. The multimodal flexible electronic skin enables collaborative sensing and processing of surface and deep information, achieving both surface pressure sensing and deep medium sensing, thus possessing multimodal sensing capabilities. This invention integrates a piezoresistive sensing module and an artificial surface plasmon sensing module, maintaining the overall structural flexibility, thinness, and miniaturization while achieving simultaneous sensing of multimodal physical signals.

[0134] This invention provides a flexible multimodal sensing electronic skin based on liquid metal microchannels, integrating surface tactile sensing and inner penetrating medium sensing, and its fabrication method. The method includes: fabricating a flexible film 3 with channels using soft photolithography according to preset microchannel dimensions and functional requirements, and integrating periodically grooved SSPP units 9 and piezoresistive channel structures 8 within it; fabricating a flexible substrate layer 52 using polydimethylsiloxane (PDMS) casting, and bonding it to the microchannel film through plasma surface treatment to obtain an elastic composite substrate with microchannels; connecting the obtained flexible substrate layer 52 to one side of a flexible printed circuit board (FPC) with flexible flat cables 6, and simultaneously laminating a flexible film 51 with through holes on the other side of the FPC to form a complete piezoresistive sensing unit structure; connecting the microchannel portion embedded with artificial surface plasmon resonance structures to an RF signal interface via an IPEX connector to construct an SSPP detection module with electromagnetic wave response capability. Liquid metal (such as eutectic gallium indium or Galinstan) is injected into the channel structure through a reserved injection port to form a liquid metal electrode network with excellent conductivity. Finally, the injection port is sealed with a flexible encapsulation film 2 to obtain an integrated, ultra-thin multimodal electronic skin device. This invention integrates the SSPP unit 9 with the piezoresistive channel structure 8 of liquid metal, achieving coupling and synergy between surface tactile sensing and deep medium response functions while maintaining the device's flexibility and miniaturization characteristics. This provides an innovative technical path for multimodal, high-sensitivity, and non-invasive human signal detection.

[0135] The application adopts soft lithography micro-nano processing technology to construct a multilayer channel structure on an elastic substrate, and combines the excellent flexibility of PDMS (polydimethylsiloxane) material to prepare a micro-channel array with good mechanical compliance and light transmission. After the channel is formed, liquid metal is injected as a conductive medium to realize a high-conductivity and high-stretchability sensing network. Meanwhile, the micro-channel film is firmly integrated with the upper and lower substrates by using plasma bonding technology to form a micro-structure platform with high stability and high sealing performance. The manufacturing process does not require high temperature or complex equipment, and has good low-cost batch production capacity.

[0136] The above specific embodiments are specific supports for the scheme idea of the application, and cannot limit the protection scope of the application. Any equivalent changes or equivalent modifications made on the basis of the technical scheme of the application according to the technical idea of the application still belong to the protection scope of the technical scheme of the application.

Claims

1. A flexible skin structure integrating surface and internal perception, characterized in that, include: An elastic substrate has several layers of channel structures, in which liquid metal is injected. The liquid metal forms a liquid metal network in the channel structures of the elastic substrate. The liquid metal network serves as a sensing system, including: SSPP units and piezoresistive channel structures arranged in parallel around the SSPP units; the cable includes a flexible flat cable connected to the piezoresistive channel structures and a coaxial cable connected to the SSPP units. The elastic substrate includes a base layer, a flexible membrane with through holes, and a flexible membrane with channels stacked sequentially; the flexible membrane with channels and the flexible membrane with through holes are joined together to form a channel structure in the middle, and the flexible membrane with channels is also provided with through grooves, which are correspondingly connected to the through holes on the flexible membrane with through holes. One end of the flexible flat cable and coaxial cable connected to the liquid metal network is embedded between the substrate layer and the flexible membrane with through holes, and the end of the flexible flat cable and coaxial cable connected to the liquid metal network corresponds to the through holes on the flexible membrane with through holes. The through holes of the flexible membrane and the through grooves on the flexible membrane with channels are respectively sealed by the substrate layer and the encapsulation film, confining the liquid metal in the channel structure of the elastic substrate; an electromagnetic shielding layer is also provided on the elastic substrate. Furthermore, the contact end of the coaxial cable corresponds to the SSPP unit in the channel structure and makes contact through the hole on the flexible film with through holes to transmit radio frequency signals; the contact end of the flexible flat cable makes contact with the piezoresistive channel structure in the channel structure to conduct piezoresistive signals.

2. The flexible skin structure integrating surface and interior perception according to claim 1, characterized in that: include: The piezoresistive channel structures are arranged in parallel on both sides of the SSPP unit to form a dual-channel sensing system.

3. A flexible skin structure integrating surface and interior perception according to claim 2, characterized in that: The elastic substrate is made of PDMS material, where PDMS is polydimethylsiloxane. The liquid metal used is an indium-tin alloy with a melting point of 6°C, a density of 6.03 g / cm³, and a purity of 99%. The liquid metal is a conductive medium.

4. A method for preparing a flexible skin structure integrating surface and internal perception, characterized in that: The method for preparing a flexible skin structure with fused surface and interior sensing as described in any one of claims 1 to 3 includes the following steps; Obtain a substrate layer, a flexible membrane with through holes, a flexible membrane with channels, liquid metal, a flexible flat cable, a coaxial cable, an encapsulation film, and an electromagnetic shielding layer; One side of the flexible membrane with microchannels is aligned with a flexible membrane with through holes, forming a channel structure between the two membranes. The other side of the flexible membrane with through holes is aligned with a substrate layer, and one end of a coaxial cable and a flexible flat cable are respectively embedded between the flexible membrane with through holes and the substrate layer. Liquid metal is injected into the through holes of the flexible membrane with channels, and the liquid metal enters the channel structure to form a liquid metal network. A thin film is placed over the through-hole of a flexible membrane with channels to inject liquid metal, thus confining the liquid metal within the channel structure of the elastic substrate. The electromagnetic shielding layer is placed on the outside of the substrate or a flexible membrane with channels.

5. The method for preparing a flexible skin structure integrating surface and internal perception according to claim 4, characterized in that: Methods for preparing a substrate layer, a flexible membrane with through-holes, and an encapsulation film include: A PDMS solution was obtained by weighing and mixing polydimethylsiloxane prepolymer and curing agent at a mass ratio of 10:1; the solution was degassed in a vacuum degasing oven; the PDMS solution was spin-coated onto a petri dish to form a PDMS liquid film; the PDMS liquid film was degassed and then cured in a constant temperature oven in a stepwise manner and cross-linked to obtain a flexible substrate film.

6. The method for preparing a flexible skin structure integrating surface and internal perception according to claim 5, characterized in that: A method for preparing a flexible membrane with channels includes: Step S1: The glass substrate is surface treated using a cleaning solution combined with ultrasonic cleaning process; and residual liquid on the substrate surface is removed; a clean and dry glass substrate surface is obtained. Step S2: Based on the preset microchannel pattern, a photomask is fabricated using a spiral topology to obtain a photomask with a pattern of channels. Step S3: Based on the pretreated glass substrate surface, a photosensitive dry film is uniformly covered on the substrate surface through a precision film lamination process. A hot-pressing composite device is used to achieve tight bonding between the dry film and the substrate under set temperature and pressure parameters to form a photosensitive dry film layer and obtain a coated substrate with a uniform photosensitive layer. Step S4: The coated substrate is patterned using a contact photolithography method so that the microchannel pattern on the mask is completely transferred to the photosensitive dry film layer. Step S5: Use sodium carbonate solution to wash away the unexposed portion of the DuPont photosensitive dry film on the dry film glass substrate to obtain a dry film glass substrate containing a channel structure. Step S6: Before implementing the PDMS casting process, the surface of the dry film glass substrate with the channel structure needs to be modified to obtain the silanized dry film glass substrate. Step S7: A flexible microstructure film is prepared on the surface of the modified substrate using a prepolymer casting molding process to obtain a flexible film with channels.

7. The method for preparing a flexible skin structure integrating surface and internal perception according to claim 4, characterized in that: The steps of forming a channel structure between the flexible membrane with channels and the flexible membrane with through holes include: First, low-adhesion cleaning tape is used to remove particulate contaminants from the surfaces of the flexible membranes with through holes and through holes. Then, the flexible membranes with through holes and through holes to be bonded are placed in a plasma treatment device and subjected to surface activation treatment for 120±5s under RF power of 10±1W and oxygen flow rate of 8-10 NI / h, so that hydroxyl active groups are formed on the contact surface. The treated flexible membranes with through holes and through holes are aligned and bonded with the activated surfaces facing each other, and a pressure of 0.01-0.05MPa is applied and left to stand for 10-15min to promote initial intermolecular bonding. Finally, the components are transferred to a constant temperature heating platform at 65±5℃ for step-by-step bonding and curing. The temperature is maintained at 50±2℃ for the first 30min to allow the interfacial molecules to diffuse fully, and then the temperature is raised to 70±2℃ and held for 90min to complete the covalent bonding. The through holes of the flexible membrane are provided with a flow guiding structure that is coaxially aligned with the channel grooves on the flexible membrane for injecting liquid metal. The steps of covering the through-holes of a flexible membrane with micropores with an encapsulation film over the liquid metal to confine the liquid metal within the channel structure of an elastic substrate include: The film is processed by cutting to form an encapsulation film whose coverage area extends 1.0±0.2mm beyond the edge of the liquid metal injection port; Sil-Poxy silicone adhesive is applied to the periphery of the injection port with a uniform thickness of 25±5μm, and after optical alignment, a bonding pressure of 0.05±0.01MPa is applied; in order to eliminate thermally induced interfacial stress, the encapsulation process is completed in a constant temperature environment of 23±2℃, and the curing stage is kept for 30±2min to allow the adhesive to penetrate at the molecular level without forming a thermally resistive layer; The encapsulation film is a flexible PDMS encapsulation film with a thickness of 500±50μm.

Images