Flexible composite hardness sensor based on liquid metal integrated injection and method

Abstract

The invention belongs to the technical field of flexible electronic sensors, and particularly discloses a flexible composite hardness sensor based on liquid metal integrated injection and a method, and the flexible composite hardness sensor comprises a pressure sensing layer, a strain sensing layer and a film supporting layer which are stacked in sequence; a pressure sensing channel matched with the strain sensing layer is arranged on the bottom surface of the pressure sensing layer; a strain sensing channel matched with the film supporting layer is arranged on the bottom surface of the strain sensing layer; the composite structure combining the pressure sensing layer and the strain sensing layer is adopted, the contact pressure and the film surface strain are sensed at the same time in a single flexible device through the integrated liquid metal injection technology, the two kinds of signals do not interfere with each other and are output respectively, high-precision recognition and real-time feedback of the material hardness can be achieved, and the application range is wide. Compared with a traditional single sensing element, the sensor has a remarkable function integration advantage.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of flexible electronic devices and intelligent sensors, and particularly relates to a flexible composite hardness sensor based on liquid metal integrated injection and a method. BACKGROUND

[0002] With the rapid development of wearable flexible sensors and robot-assisted technology, tactile perception as a key information channel in human-computer interaction has attracted widespread attention. In particular, in high-precision application scenarios such as medical surgery, rehabilitation assistance, and tissue engineering, how to enable robots to obtain tactile feedback capabilities similar to human hands has become an important technical challenge to improve operation sensitivity and safety.

[0003] Traditional flexible tactile sensors are usually based on capacitive, resistive or piezoelectric principles to detect single physical quantities such as pressure and strain. However, in complex contact behaviors such as distinguishing the hardness of biological tissues, it is often difficult to achieve accurate identification relying solely on pressure or strain information. In particular, during medical surgery, doctors rely on intuitive judgments of tissue hardness to avoid damaging important organs or diseased areas, so it is particularly important to endow surgical robots with fine tactile recognition capabilities.

[0004] Currently, widely used tissue hardness detection methods such as nanoindentation, negative pressure suction, and shear wave elastography have high precision in laboratories or large equipment, but they are difficult to integrate into the end of a micro robot or wearable system due to their large system size, complex structure, and the need for handheld operation, and are not suitable for real-time in-situ measurement. In addition, some micro flexible sensors achieve time-sharing measurement of strain and pressure by constructing a multi-layer composite structure, but they have poor structural stability, complicated preparation process, and coupled interference of response signals.

[0005] In the field of microfluidics and liquid metal material applications, liquid metal has been gradually used to manufacture stretchable flexible sensing elements due to its good electrical conductivity, flexibility, and injection molding advantages. However, most current liquid metal sensors are still limited to single-function channel structures, making it difficult to simultaneously achieve high-sensitivity perception of multiple physical quantities in limited space, and the interlayer bonding method generally relies on multi-step injection or point-to-point connection, affecting device consistency and reliability. SUMMARY

[0006] To solve the above technical problems, the application provides a flexible composite hardness sensor based on liquid metal integrated injection, which can simultaneously achieve high-sensitivity pressure and strain decoupling perception in limited space.

[0007] To achieve the above purpose, the application uses the following technical solutions:

[0008] The application provides a flexible composite hardness sensor based on liquid metal integrated injection, which comprises a pressure sensing layer, a strain sensing layer and a film support layer which are sequentially stacked, the pressure sensing layer, the strain sensing layer and the film support layer sequentially form a complete structure through plasma bonding from top to bottom; the bottom surface of the pressure sensing layer is provided with a pressure sensing channel matched with the strain sensing layer; the bottom surface of the strain sensing layer is provided with a strain sensing channel matched with the film support layer; the strain sensing channel and the pressure sensing channel are vertically arranged in structure to form an orthogonal response system; the strain sensing channel is specially used for responding to in-plane tensile strain, and the pressure sensing channel is used for responding to vertical normal pressure, the functions of the two are decoupled and do not interfere with each other.

[0009] Preferably, the pressure sensing channel comprises a main liquid storage cavity arranged on the bottom surface of the pressure sensing layer, and branch microchannels are symmetrically arranged outside the main liquid storage cavity in a radial distribution; the pressure sensing channel adopts a micro-pump structure, the main liquid storage cavity is located at the center of the pressure sensing layer, and the branch microchannels are arranged and expanded to the four directions, so that the liquid metal is displaced in the pressure sensing channel under stress, the length and cross-sectional area of the channel are changed, and the resistance change is caused.

[0010] Preferably, the strain sensing channel comprises a ring-shaped flow channel group and a fan-shaped ring-shaped flow channel group, the ring-shaped flow channel group comprises annular serpentine microchannels symmetrically arranged on the bottom surface of the strain sensing layer; the fan-shaped ring-shaped flow channel group comprises serpentine microchannels symmetrically arranged on both sides of the ring-shaped flow channel group; and both ends of the serpentine microchannels are provided with branch grooves.

[0011] Preferably, the strain sensing layer is provided with injection channels respectively communicating with the branch grooves, the injection channels comprise two pairs of perfusion holes symmetrically arranged on both sides of the serpentine microchannels and respectively communicating with the corresponding branch grooves, the perfusion holes communicate with lead-out holes symmetrically arranged on both sides of the main liquid storage cavity of the pressure sensing layer, and the lead-out holes respectively communicate with the corresponding branch microchannels.

[0012] Preferably, the main liquid storage cavity is in a semispherical recessed shape, the diameter of the main liquid storage cavity is smaller than the inner diameter of the ring-shaped flow channel group; the lengths of the branch microchannels are equal; the lengths of the branch grooves at both ends of any serpentine microchannel are equal; the serpentine microchannel is a horizontal multi-grid structure for enhancing the resistance response of the surface deformation; and the serpentine microchannel is horizontally provided with parallel arranged slender grid bars, and the slender grid bars form a comb-shaped horizontal multi-grid structure.

[0013] Preferably, the strain sensing channel and the pressure sensing channel are filled with conductive liquid metal; electrodes are arranged in the lead-out holes, the bottom ends of one pair of electrodes are located in the perfusion holes, and the bottom ends of the other pair of electrodes are located in the lead-out holes; the liquid metal is Galinstan, which has good conductivity, low viscosity and high surface tension characteristics, and is suitable for microchannel flow; and the liquid metal is filled into the microchannels by integrated injection, one end is injected and the other end is exhausted, so as to ensure full injection and eliminate bubbles.

[0014] Preferably, the width and depth of the serpentine micro-channel cross-section are ≥100 microns, and the length is 4500 microns; the strain sensing channel is located at the bottom of the strain sensing layer, so that it is in close contact with the contact surface to improve the deformation detection sensitivity.

[0015] Preferably, the hardness recognition function of the sensor calculates the hardness index H based on the ratio of the pressure resistance change value ΔRp and the strain resistance change value ΔRs, and the specific relationship is:

[0016]

[0017] The hardness index H is used to distinguish different hardness materials such as glass, PDMS, and silicone.

[0018] A material hardness recognition method of a flexible composite hardness sensor based on liquid metal integrated injection, comprising the following steps:

[0019] S11, integrate the sensor into the end effector of the robot, apply pressure to the surface of the material to be detected through the end of the robot, and the sensor generates contact deformation.

[0020] S12, use the signal acquisition module to simultaneously acquire the resistance change signals of the pressure sensing channel and the strain sensing channel, obtain the pressure signal and the strain signal, and convert them into voltage output through the signal conversion module.

[0021] S13, calculate the ratio between the strain signal and the pressure signal to determine the hardness grade of the contact material, and the larger the ratio, the softer the material.

[0022] Preferably, the signal acquisition module includes an ADC acquisition module connected to the four electrode columns, a multi-stage amplification module connected to the ADC acquisition module, and an OLED display module connected to the multi-stage amplification module. The OLED is used for the display module to display the real-time values of the pressure signal and the strain signal and the hardness ratio; the ADC acquisition module, the multi-stage amplification module, and the OLED display module are all connected to the power module to realize stable power supply and signal processing; the ADC acquisition module is an analog-to-digital conversion acquisition module; and the OLED display module is an organic light-emitting diode display module.

[0023] A preparation method of a flexible composite hardness sensor based on liquid metal integrated injection, comprising the following steps:

[0024] S21, use a soft lithography method to prepare a mask and a mold for a microfluidic channel, and make a pressure sensing layer, a strain sensing layer, and a thin film support layer, each layer of which is prepared by soft lithography mold turning technology using PDMS.

[0025] S22, using PDMS (polydimethylsiloxane) and crosslinking agent is mixed in 10:1 ratio and cast molding, the pressure sensing layer containing pressure sensing channel, strain sensing layer containing strain sensing channel and film support layer are prepared.

[0026] S23, plasma surface treatment is carried out on each layer, and the composite structure is formed by interlayer alignment bonding; each layer is activated by using plasma surface treatment technology, and is bonded in turn to form an overall structure.

[0027] S24, liquid metal is injected by one-piece injection method, the liquid metal is injected into the micro flow channel, the two end injection mode is used, one end is injected, one end is exhausted, so that no air bubble is left in the flow channel; the liquid metal Galinstan is injected into the pressure sensing channel and the strain sensing channel from the perfusion hole and the lead-out hole by using a syringe, until the pressure sensing channel and the strain sensing channel are filled with liquid metal, and the electrode column is inserted into the lead-out hole to form a signal lead-out end.

[0028] S25, the electrode column is connected, and the sensor is integrally packaged, the perfusion hole and the lead-out hole are closed between the electrode column by using PDMS, and the sealing and insulation operation is completed.

[0029] S26, calibration is carried out by a microcontroller, and is applied to a haptic hardness identification scene.

[0030] The thickness of the pressure sensing layer, the strain sensing layer and the film support layer is controlled to be less than 7mm, and the total thickness of the sensor is controlled to be about 25mm, so that the flexible attachment and portable integration requirements are met.

[0031] Compared with the prior art, the beneficial effects of the present application are as follows:

[0032] 1, the composite structure of the pressure sensing layer and the strain sensing layer is adopted in the present application, the contact pressure and the membrane surface strain are simultaneously sensed in a single flexible device by the integrated liquid metal injection technology, the two types of signals are not interfered with each other and are output respectively, the high-precision identification and real-time feedback of material hardness can be realized, compared with the traditional single sensing element, the present application has significant functional integration advantages; the present application can simultaneously realize high-sensitivity pressure and strain decoupling sensing in limited space, and the structure is compact and easy to manufacture and integrate.

[0033] 2, the pressure sensing channel and the strain sensing channel are independently arranged in the present application, so that the functions of the two are not interfered with each other, and the pressure signal and the strain signal are respectively output by corresponding electrodes; by monitoring the resistance change rate of the liquid metal in the pressure sensing channel and the strain sensing channel, the ratio between the pressure signal and the strain signal is calculated, so that the hardness detection and identification of the touched object can be realized.

[0034] 3、The application sets up pressure sensing channels and strain sensing channels, which are used as the conductive paths of the strain and pressure sensing unit, and the resistance changes when the channels are deformed, so that the response detection of mechanical stimulation is realized; the strain sensing channels and the pressure sensing channels adopt serpentine and micropump composite structures respectively, and have a large resistance change rate in the deformation state, which can improve the sensitivity and dynamic response capability of the device, and is especially suitable for small load identification scenes; the strain sensing channel adopts a quarter grid structure and builds an equivalent bridge model, and through the optimization of the layout of the liquid metal channel, the error caused by temperature fluctuations or internal material tension changes is effectively eliminated, which can improve the stability and repeatability of strain detection; compared with traditional metal strain gauges or carbon-based composite material sensors, the application has higher decoupling capability and signal purity.

[0035] 4、The sensor structure of the application is a PDMS multilayer flexible film, which has good mechanical compliance, biocompatibility and attachability, can be easily integrated into various terminal execution parts such as robot fingers, flexible clamps, organ chips, realizes the expansion of tactile function without changing the structure, and is suitable for surgical robots, rehabilitation robots and wearable medical devices.

[0036] 5、The application can complete liquid metal perfusion through a single injection operation, which can significantly reduce the problems of bubble entrainment and interface delamination caused by traditional multiple injection; at the same time, multiple sensing channels are integrated into one, the preparation process is simplified and has strong repeatability, and the feasibility of batch and standardized production is provided.

[0037] 6、The matching test system provided by the application includes an ADC acquisition module, a multi-stage amplifier and an OLED display unit, which can convert resistance changes into voltage signals in real time, and complete display through a microcontroller, and is suitable for on-site data acquisition and analysis without relying on high-precision instruments.

[0038] 7、In experimental verification, the sensor of the application can stably identify the hardness difference between various common materials (such as glass, PDMS and silicone), the output strain-pressure ratio change trend is clear and has good repeatability, which confirms the application value and practicability of the application in flexible tactile recognition and hardness quantitative analysis.

[0039] 8、The application can realize accurate decoupling sensing of the pressure and strain information generated by the contacted object, realizes the identification and quantitative evaluation of the material hardness by simultaneously acquiring the pressure signal and the strain signal and performing fusion analysis on the two, and is suitable for medical robots, rehabilitation equipment and wearable electronic products.

[0040] 9、The application forms a close-fitting whole structure through plasma bonding, and the bottom layer is provided with a film support layer; the strain sensing channel is a closed loop structure connected at the head and tail, and is connected with an injection channel; the strain sensing channel is injected with conductive liquid metal; the application realizes the recognition of the hardness of the contact material by measuring the resistance changes of the pressure sensing channel and the strain sensing channel respectively and calculating the ratio of the two, and can realize the decoupling of the pressure and strain signals, compact structure, and high-stability hardness recognition device suitable for integrated in the end of the robot finger. BRIEF DESCRIPTION OF DRAWINGS

[0041] Figure 1 is the explosion schematic diagram of embodiment 1 of the application;

[0042] Figure 2 is the overall structure schematic diagram of embodiment 1 of the application;

[0043] Figure 3 is the structure schematic diagram of the pressure sensing layer in embodiment 1 of the application;

[0044] Figure 4 is the structure schematic diagram of the strain sensing layer in embodiment 1 of the application;

[0045] Figure 5 is Figure 4 is the local enlarged schematic diagram of the serpentine microchannel in the above-mentioned;

[0046] Figure 6 is the signal output and circuit connection module schematic diagram of embodiment 1 of the application;

[0047] Figure 7 is the contact test comparison result schematic diagram of different hardness materials in embodiment 2 of the application;

[0048] Figure 8 is the real object schematic diagram of the sensor of the application (after the liquid metal is injected);

[0049] Figure 9 is the real object schematic diagram of the electrical signal acquisition and processing platform of embodiment 2 of the application;

[0050] Figure 10 is the strain / pressure ratio and material hardness relationship curve schematic diagram of the sensor of the application.

[0051] In the figure: pressure sensing layer 1, strain sensing layer 2, film support layer 3, main liquid storage cavity 11, branch microchannel 12, serpentine microchannel 21, branch groove 22, injection channel 4, perfusion hole 41, lead-out hole 42. DETAILED DESCRIPTION

[0052] The application will be further described in detail through specific embodiments, but the scope of the application is not limited.

[0053] Example 1

[0054] A flexible composite hardness sensor based on integral injection of liquid metal, such as Figures 1-6 As shown in the picture, the actual product is as follows. Figure 8 As shown, it includes a pressure sensing layer 1, a strain sensing layer 2, and a thin film support layer 3 stacked in sequence; the bottom surface of the pressure sensing layer 1 is provided with a pressure sensing channel that cooperates with the strain sensing layer 2, and the strain sensing layer 2 is bonded to the back side of the pressure sensing layer 1; the bottom surface of the strain sensing layer 2 is provided with a strain sensing channel that cooperates with the thin film support layer 3, and the three layers are bonded together by plasma to form a complete structure, forming a complete flexible composite sensor body.

[0055] like Figures 2-3 As shown, the pressure sensing channel includes a main liquid storage cavity 11 disposed on the bottom surface of the pressure sensing layer 1. The main liquid storage cavity 11 is a hemispherical concave shape. Four radially distributed branch microchannels 12 are symmetrically arranged on the outside of the main liquid storage cavity 11. The branch microchannels 12 form a closed loop structure. The main liquid storage cavity 11 and the branch microchannels 12 together form the pressure sensing channel. When the upper surface is subjected to normal pressure, the liquid metal flows in the pressure sensing channel, causing a change in resistance, thereby outputting a pressure signal.

[0056] like Figures 4-5 As shown, the strain sensing channel includes an annular flow channel group and a fan-shaped annular flow channel group. The annular flow channel group includes two symmetrically arranged annular serpentine microchannels 21 on the bottom surface of the strain sensing layer 2. The fan-shaped annular flow channel group includes two symmetrically arranged serpentine microchannels 21 on both sides of the annular flow channel group. The serpentine microchannels 21 have a transverse multi-grid structure. Both ends of the serpentine microchannels 21 are provided with branch grooves 22. The serpentine microchannels 21 are connected end to end and arranged symmetrically to form a Wheatstone bridge structure, which is used to detect strain changes caused by tension. The cross-sectional width of the serpentine microchannels 21 is 100 μm, the depth is 100 μm, and the channel length is about 4500 μm. Every two serpentine microchannels 21 are located on the same axis.

[0057] like Figure 2 As shown, the strain sensing layer 2 is provided with injection channels 4 that are respectively connected to the branch grooves 22. The injection channels 4 are connected to the outside of the sensor through the outlet holes 42. The injection channels 4 include two pairs of infusion holes 41 symmetrically arranged on both sides of the serpentine microchannels 21 and respectively connected to the corresponding branch grooves 22. Each infusion hole 41 is connected to a serpentine microchannel 21. The infusion holes 41 are connected to the outlet holes 42 symmetrically arranged on both sides of the main liquid storage chamber 11 on the pressure sensing layer 1. The outlet holes 42 are respectively connected to the corresponding branch microchannels 12. The outlet holes 42 and the infusion holes 41 are arranged coaxially. The outlet holes 42 are used to insert electrode posts (copper needles) and are sealed by PDMS encapsulation.

[0058] The strain sensing channel and pressure sensing channel are filled with Galinstan liquid metal as a variable resistance material for sensor signal output; each lead-out hole 42 is equipped with an electrode post, with the bottom of one pair of electrode posts located in the infusion hole 41 and the bottom of the other pair of electrode posts located in the lead-out hole 42.

[0059] like Figure 6 As shown, the pressure sensing channel of pressure sensing layer 1 is a closed-loop microfluidic channel, and the output is a pressure signal channel. The strain sensing channel of strain sensing layer 2 is a bridge-type serpentine channel, and the output is a strain signal channel. The corresponding output electrodes of the two are connected to the external ADC acquisition module and amplifier module, respectively, to realize independent acquisition of dual channels. Due to the separation of the upper and lower structures and the independent arrangement of channels, the pressure signal and strain signal do not interfere with each other electrically, thus achieving efficient decoupling.

[0060] The sensor in this embodiment has an overall size of 25mm×25mm×2.5mm and is made by molding PDMS material. The ratio of PDMS prepolymer to crosslinking agent is 10:1, which has excellent flexibility and biocompatibility. The strain sensing channel and pressure sensing channel are prepared by SU-8 photoresist template. After molding, they are bonded by plasma treatment. Liquid metal is injected into the strain sensing channel and pressure sensing channel through the injection hole 41 through a syringe. The venting and liquid injection are completed simultaneously. After liquid injection, the electrode column is inserted as an electrode and then sealed with PDMS.

[0061] When in use, this sensor can be integrated into a flexible mechanical finger or a bio-attachment platform. It reads the pressure and strain responses through two voltage signals, and determines the hardness of the touched object by the ratio. Experiments on materials such as glass, PDMS, and silicone have verified that it has a clear and distinguishable signal discrimination ability, and has a sensitive response and good repeatability.

[0062] A method for material hardness identification based on a flexible composite hardness sensor with integrated liquid metal injection includes the following steps:

[0063] S11. Integrate the sensor into the end effector of the robot. Apply pressure to the surface of the material being tested by the robot end effector, and the sensor will deform upon contact. Integrate the sensor into a flexible finger sleeve structure to simulate the tactile scenario of the robot's fingers. Make the sensor surface contact the material to be tested, such as PDMS, silicone, glass, etc., to ensure that the sensor film surface is in full contact with the sample being tested.

[0064] S12, the signal acquisition module is used to simultaneously collect the resistance change signals of the pressure sensing channel and the strain sensing channel, to obtain the pressure signal and the strain signal, and to convert the signals into voltage output through the signal conversion module; the signal acquisition module comprises an ADC acquisition module connected with the four electrode columns, a multi-stage amplification module connected with the ADC acquisition module, and an OLED display module connected with the multi-stage amplification module, and the OLED is used to display the real-time values of the pressure signal and the strain signal and the hardness ratio; the ADC acquisition module, the multi-stage amplification module and the OLED display module are all connected to the power module.

[0065] As shown in Figure 8 , 9 , the four electrode columns of the sensor are connected with the ADC acquisition module respectively, the ADC acquisition module is connected with the two-stage amplification module, and the output of the amplification module is connected to the OLED display screen; the ADC acquisition module, the amplification module and the OLED screen are all connected to the power module to form a complete data acquisition system; the sensor is placed in a static state, and a standard loader is used to gradually apply vertical pressure to it; at the same time, the voltage signal from the upper pressure sensing channel is read through an oscilloscope, and the voltage signal from the strain sensing channel is output through the strain bridge structure; in the experiment, the pressure sensing channel is internally filled with a semispherical cavity structure of liquid metal, which deforms in volume under stress, causing the resistance of the channel to change; the strain sensing channel is a serpentine closed-loop microchannel structure, which is affected by the overall stretching of the film, and its length increases and the resistance rises, forming a strain voltage signal; when the pressure on the membrane surface is continuously loaded, the strain signal and the pressure signal output by the sensor are represented as Vpressure and Vstrain respectively, Vpressure is the voltage change from the closed-loop micro-pump structure, and Vstrain is the bridge output signal from the serpentine microchannel 21.

[0066] S13, the ratio between the strain signal and the pressure signal is calculated to determine the hardness grade of the contact material, and the larger the ratio, the softer the material.

[0067] Two groups of signals under the same loading conditions of different materials are recorded, and the data relationship as shown in Figure 10 is drawn: the ratio Vstrain / Vpressure is drawn into a function curve, which is used to reflect the hardness grade of different materials; the larger the ratio, the greater the material deformation and the softer the material; the smaller the ratio, the harder the material; by using this relationship, a quantitative calibration model between the voltage ratio output by the sensor and the material hardness can be established.

[0068] S14, repeated loading and releasing experiments are performed on the sensor to monitor the output stability and response time, and to verify the repeatability and sensitivity thereof; as shown in Figure 7As shown, the three types of materials, PDMS, silica and glass, produce different electrical signal change curves under the same loading volume, showing that the sensor of the application has good resolution capability and data repeatability in multi-material tactile recognition.

[0069] According to Figure 7 The electrical signal change curves in the application show that the sensor of the application can produce significantly different electrical signal responses for different materials under the same loading volume: the output curves of the three types of materials, PDMS, silica and glass, are significantly separated in the entire test interval, and monotonously increase with the pressure sensor output, without significant fluctuations or overlaps, showing good data stability and repeatability; among them, the strain output amplitude of silica is the largest, followed by PDMS, and glass is the smallest, which can accurately reflect the hardness and deformation difference of the materials. Therefore, the sensor of the application has excellent multi-material tactile recognition resolution capability and data repeatability, and can provide reliable support for rapid differentiation and accurate detection of different material characteristics.

[0070] A preparation method of a flexible composite hardness sensor based on liquid metal integrated injection, comprising the following steps:

[0071] S21, a soft lithography method is used to prepare a mask and a mold for the microfluidic channel.

[0072] Preparation of the mold for the pressure sensing layer 1 and the strain sensing layer 2: spin-coat SU-82050 photoresist on a silicon wafer with a thickness of 100 μm; perform 65℃ and 95℃ double-stage pre-baking treatment on the spin-coated silicon wafer to remove the solvent; contact the mask used for constructing the pressure sensing channel and the strain sensing channel with the silicon wafer coated with photoresist, and perform ultraviolet exposure; after exposure, perform post-baking treatment, and then develop using SU-8 developer, to obtain a microfluidic mold containing the pressure sensing layer 1 with the pressure sensing channel and the strain sensing layer 2 with the strain sensing channel.

[0073] S22, mix PDMS and crosslinking agent at a ratio of 10:1 and cast to form a pressure sensing layer 1 containing a pressure sensing channel, a strain sensing layer 2 containing a strain sensing channel, and a thin film support layer 3. Fill the microfluidic mold with PDMS (prepolymer and curing agent at a mass ratio of 10:1) respectively, and solidify at 75℃ for 2 hours to obtain the pressure sensing layer 1 and the strain sensing layer 2.

[0074] S23, perform plasma surface treatment on each layer, and form a composite structure through interlayer alignment bonding.

[0075] PDMS film for making sensor: the cured PDMS structure is peeled off from the mold to obtain a PDMS film with a microstructured pressure sensing layer 1 (provided with a hemispherical concave main liquid storage cavity 11 and a branched microchannel 12) and a strain sensing layer 2 (provided with a strain sensing channel in a serpentine closed loop structure); after plasma treatment of the surfaces of the two PDMS layers, they are aligned and laminated, so that the pressure sensing layer 1 and the strain sensing layer 2 form a closed microchannel structure; the pressure sensing channel in the structure is used to respond to the pressure signal, and the strain sensing channel is used to sense the tensile strain.

[0076] Packaging and punching of the laminated structure and the film support layer 3 of the flexible substrate: the pressure sensing layer 1, the strain sensing layer 2 and the film support layer 3 of the flexible substrate are packaged in multiple layers, with the pressure sensing layer 1 on the top, the main liquid storage cavity 11 facing down, the serpentine microchannel 21 in the strain sensing layer 2 in the middle layer, and the film support layer 3 at the bottom; before packaging, a punch is used to punch at four predetermined perfusion hole 41 positions to form liquid metal injection channels 4; after packaging, lead-out holes 42 are punched on the outside of the sensor.

[0077] S24, injecting liquid metal by one-piece injection method, injecting liquid metal into the microchannel, using two-end injection method, one end injection, one end exhaust, ensuring no air bubbles remaining in the channel.

[0078] Injecting liquid metal and inserting electrode column: using a syringe to inject liquid metal Galinstan (gallium-tin-indium alloy) from the perfusion hole 41 until the liquid metal completely fills the main liquid storage cavity 11 and the serpentine microchannel 21 along the branched microchannel 12 and the branched groove 22 respectively and there is no air bubble.

[0079] S25, connecting the electrode column and packaging the sensor as a whole.

[0080] Inserting four electrode columns into the injection channel 4, with the bottom ends of one pair of electrode columns located in the perfusion hole 41, and the bottom ends of the other pair of electrode columns located in the lead-out hole 42, the two pairs of electrode columns respectively measuring the pressure of the strain sensing channel and the pressure of the pressure sensing channel, while ensuring that the bottom ends of the electrode columns respectively contact the liquid metal in the strain sensing channel and the pressure sensing channel; the two pairs of electrode columns are respectively located at the two ends of the fan ring channel group; low-viscosity PDMS is used to seal the injection port and the electrode column to prevent leakage of the liquid metal, and is cured at 75°C for 1 hour.

[0081] S26, calibration by microcontroller and application to tactile hardness identification scene.

[0082] Example 2

[0083] A material hardness identification method of a flexible composite hardness sensor based on one-piece injection of liquid metal, comprising the following steps:

[0084] S11, integrate the sensor into the robot end effector, apply pressure to the surface of the detected material through the robot end, and the sensor generates contact deformation; integrate the sensor into the flexible finger sleeve structure to simulate the tactile scene of the robot finger; contact the sensor surface to the material to be measured, such as PDMS, silica gel, glass, etc., to ensure that the sensor film surface is in full contact with the measured sample.

[0085] S12, use the signal acquisition module to simultaneously collect the resistance change signals of the pressure sensing channel and the strain sensing channel, obtain the pressure signal and the strain signal, and convert them into voltage output through the signal conversion module; the signal acquisition module includes an ADC acquisition module connected with four electrode columns, a multi-stage amplification module connected with the ADC acquisition module, and an OLED display module connected with the multi-stage amplification module, and the OLED is used for the display module to display the real-time values of the pressure signal and the strain signal and the hardness ratio; the ADC acquisition module, the multi-stage amplification module, and the OLED display module are all connected to the power module.

[0086] As shown in Figure 8 , 9 , the four electrode columns of the sensor are respectively connected with the ADC acquisition module, the ADC acquisition module is connected with the two-stage amplification module, and the amplification module output is connected to the OLED display screen; the ADC acquisition module, the amplification module, and the OLED screen are all connected to the power module to form a complete data acquisition system; the sensor is placed in a static state, and a standard loader is used to gradually apply vertical pressure to it; at the same time, record: read the voltage signal from the upper pressure sensing channel through the oscilloscope; output the voltage signal from the strain sensing channel through the strain bridge structure; in the experiment, the pressure sensing channel is internally filled with a semispherical cavity structure of liquid metal, which deforms in volume under stress, causing the channel resistance to change; the strain sensing channel is a serpentine closed loop microchannel structure, which is affected by the overall stretching of the film, its length increases, and the resistance rises, forming a strain voltage signal; when the film surface is continuously loaded with pressure, the strain signal and the pressure signal output by the sensor are represented as Vpressure and Vstrain, respectively; Vpressure is the voltage change from the closed loop micro pump structure; Vstrain is the bridge output signal from the serpentine micro channel 21.

[0087] S13, calculate the ratio between the strain signal and the pressure signal to determine the hardness level of the contacted material, and the larger the ratio, the softer the material.

[0088] Record two groups of signals under different materials and the same loading conditions, and draw as Figure 10The data relationship shown is as follows: the ratio Vstrain / Vpressure is plotted as a function curve to reflect the hardness level of different materials. The larger the ratio, the greater the deformation and the softer the material; the smaller the ratio, the harder the material. Specifically, when the Vstrain / Vpressure ratio is larger, it indicates that the material has a larger strain signal and more obvious deformation under the same pressure, indicating that the material is soft, easily deformable, and has lower hardness. Conversely, when the ratio is smaller, the material has a smaller strain signal and smaller deformation under the same pressure, indicating that the material is more rigid, less easily deformable, and has higher hardness. Using this relationship, a quantitative calibration model between the sensor output voltage ratio and the material hardness can be established.

[0089] S14. Conduct repeated loading and releasing experiments on the sensor to monitor its output stability and response time, verifying its repeatability and sensitivity; such as Figure 7 As shown, PDMS, silicone, and glass produce different electrical signal change curves under the same loading volume, demonstrating the good discrimination capability and data repeatability of the sensor of the present invention in multi-material tactile recognition.

[0090] Example 3

[0091] A method for fabricating a flexible composite hardness sensor based on integral liquid metal injection includes the following steps:

[0092] S21. The mask and mold for microfluidic channels are prepared using soft photolithography.

[0093] The mold for fabricating pressure sensing layer 1 and strain sensing layer 2 was prepared as follows: SU-82050 photoresist was spin-coated onto a silicon wafer with a thickness controlled at 100 μm; the spin-coated silicon wafer was subjected to a two-stage pre-baking treatment at 65°C and 95°C to remove the solvent; the mask used to construct the pressure sensing channel and strain sensing channel was brought into contact with the photoresist-coated silicon wafer for ultraviolet exposure; after exposure, a post-baking treatment was performed, followed by development with SU-8 developer to obtain a microfluidic mold containing pressure sensing layer 1 with pressure sensing channel and strain sensing layer 2 with strain sensing channel.

[0094] S22. PDMS and crosslinking agent are mixed in a 10:1 ratio and molded to obtain a pressure sensing layer 1 containing pressure sensing channels, a strain sensing layer 2 containing strain sensing channels, and a thin film support layer 3. PDMS (prepolymer and curing agent in a 10:1 mass ratio) is poured into a microfluidic mold and cured at 75°C for 2 hours to obtain pressure sensing layer 1 and strain sensing layer 2.

[0095] S23. Perform plasma surface treatment on each layer and form a composite structure through interlayer alignment bonding.

[0096] PDMS film for making sensor: the cured PDMS structure is peeled off from the mold to obtain a PDMS film with a microstructured pressure sensing layer 1 (provided with a hemispherical concave main liquid storage cavity 11 and a branched microchannel 12) and a strain sensing layer 2 (provided with a strain sensing channel in a serpentine closed loop structure); after plasma treatment of the surfaces of the two PDMS layers, alignment lamination is performed to form a closed microchannel structure of the pressure sensing layer 1 and the strain sensing layer 2; the pressure sensing channel in the structure is used to respond to the pressure signal, and the strain sensing channel is used to sense the tensile strain.

[0097] Packaging and punching of the laminated structure and the film support layer 3 of the flexible substrate: multi-layer packaging of the pressure sensing layer 1, the strain sensing layer 2 and the film support layer 3 of the flexible substrate is performed, wherein the pressure sensing layer 1 is at the top, the main liquid storage cavity 11 faces down, the serpentine microchannel 21 is in the strain sensing layer 2 in the middle layer, and the film support layer 3 is at the bottom; before packaging, a punch is used to punch at four predetermined perfusion hole 41 positions to form a liquid metal injection channel 4; after packaging, a lead-out hole 42 is punched on the outside of the sensor.

[0098] S24, injecting liquid metal by one-piece injection method, injecting liquid metal into the microchannel, using two-end injection method, one end injection, one end exhaust, ensuring no air bubbles remaining in the channel.

[0099] Injecting liquid metal and inserting electrode column: using a syringe to inject liquid metal Galinstan (gallium-tin-indium alloy) from the perfusion hole 41 until the liquid metal completely fills the main liquid storage cavity 11 and the serpentine microchannel 21 along the branched microchannel 12 and the branched groove 22 respectively and there is no air bubble.

[0100] S25, connecting the electrode column and packaging the sensor as a whole.

[0101] Inserting four electrode columns into the injection channel 4, wherein the bottom ends of one pair of electrode columns are located in the perfusion hole 41, and the bottom ends of the other pair of electrode columns are located in the lead-out hole 42, two pairs of electrode columns respectively measure the pressure of the strain sensing channel and the pressure of the pressure sensing channel, while ensuring that the bottom ends of the electrode columns respectively contact the liquid metal in the strain sensing channel and the pressure sensing channel; two pairs of electrode columns are respectively located at the two ends of the fan ring channel group; low-viscosity PDMS is used to seal the injection port and the electrode column to prevent liquid metal leakage, and is cured at 75°C for 1 hour.

[0102] S26, calibration by microcontroller and application to haptic hardness recognition scene.

[0103] The above examples are specific embodiments of the present application, but the embodiments of the present application are not limited by the above examples, and any other combinations, changes, modifications, substitutions, simplifications that do not exceed the design ideas of the present application all fall within the protection scope of the present application.

Claims

1. A flexible composite hardness sensor based on integral injection of liquid metal, characterized in that, It includes a pressure sensing layer, a strain sensing layer, and a thin film support layer stacked in sequence; the bottom surface of the pressure sensing layer is provided with a pressure sensing channel that cooperates with the strain sensing layer; the bottom surface of the strain sensing layer is provided with a strain sensing channel that cooperates with the thin film support layer.

2. The flexible composite hardness sensor based on integral liquid metal injection according to claim 1, characterized in that, The pressure sensing channel includes a main liquid storage chamber disposed on the bottom surface of the pressure sensing layer, and branch microchannels symmetrically arranged radially on the outside of the main liquid storage chamber.

3. The flexible composite hardness sensor based on integral liquid metal injection according to claim 2, characterized in that, The strain sensing channel includes an annular flow channel group and a fan-shaped annular flow channel group. The annular flow channel group includes serpentine microchannels symmetrically arranged in annular shape on the bottom surface of the strain sensing layer. The fan-shaped annular flow channel group includes serpentine microchannels symmetrically arranged on both sides of the annular flow channel group. Both ends of the serpentine microchannels are provided with branch grooves.

4. The flexible composite hardness sensor based on integral liquid metal injection according to claim 3, characterized in that, The strain sensing layer is provided with injection channels that are connected to the branch slots. The injection channels include two pairs of symmetrically arranged perforations on both sides of the serpentine microchannel and connected to the corresponding branch slots. The perforations are connected to the outlet holes that are symmetrically arranged on both sides of the main liquid storage chamber on the pressure sensing layer. The outlet holes are connected to the corresponding branch microchannels.

5. The flexible composite hardness sensor based on integral liquid metal injection according to claim 4, characterized in that, The main liquid storage chamber is a hemispherical depression; the serpentine microchannel has a transverse multi-grid structure.

6. The flexible composite hardness sensor based on integral liquid metal injection according to claim 5, characterized in that, The strain sensing channel and pressure sensing channel are filled with liquid metal; each of the outlet holes is equipped with an electrode post, with the bottom end of one pair of electrode posts located inside the infusion hole and the bottom end of the other pair of electrode posts located inside the outlet hole.

7. The flexible composite hardness sensor based on integral liquid metal injection according to claim 6, characterized in that, The width and depth of the serpentine microchannel cross-section are ≥100 micrometers.

8. A method for material hardness identification based on a flexible composite hardness sensor with integrated liquid metal injection according to any one of claims 1-7, characterized in that, Includes the following steps: S11. Integrate the sensor into the end effector of the robot. Apply pressure to the surface of the material being detected by the end effector of the robot, and the sensor will undergo contact deformation. S12. The signal acquisition module simultaneously acquires the resistance change signals of the pressure sensing channel and the strain sensing channel to obtain the pressure signal and strain signal, and converts them into voltage output. S13. Calculate the ratio between the strain signal and the pressure signal to determine the hardness grade of the material in contact. The larger the ratio, the softer the material.

9. The material hardness identification method according to claim 8, characterized in that, The signal acquisition module includes an ADC acquisition module connected to four electrode posts, a multi-stage amplification module connected to the ADC acquisition module, and an OLED display module connected to the multi-stage amplification module. The OLED is used to display the real-time values ​​of the pressure signal and strain signal and the hardness ratio. The ADC acquisition module, the multi-stage amplification module, and the OLED display module are all connected to the power supply module.

10. A method for fabricating a flexible composite hardness sensor based on integral liquid metal injection according to any one of claims 1-7, characterized in that, Includes the following steps: S21. The mask and mold for microfluidic channels are prepared using soft photolithography. S22. PDMS and crosslinking agent are mixed in a 10:1 ratio and molded to obtain a pressure sensing layer containing pressure sensing channels, a strain sensing layer containing strain sensing channels, and a thin film support layer. S23. Perform plasma surface treatment on each layer and form a composite structure by interlayer alignment bonding; S24. Liquid metal is injected into the microchannel using an integrated injection method. The liquid metal is injected into the microchannel using a two-end injection method, with one end for injection and the other end for venting, to ensure that there are no air bubbles left in the channel. S25. Connect the electrode posts and encapsulate the sensor as a whole; S26. Calibrate using a microcontroller and apply it to tactile hardness recognition scenarios.

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