Flexible resistive sensor based on gallium-based liquid metal and application thereof

Abstract

The invention discloses a flexible resistive sensor based on gallium-based liquid metal and application thereof, the flexible resistive sensor comprises an upper PDMS substrate and a lower PDMS substrate, and the upper PDMS substrate and the lower PDMS substrate are provided with micro-channels; the micro-channel is filled with gallium-based liquid metal, and the gallium-based liquid metal is gallium-indium-tin alloy; when the sensor is used as a touch sensor, the sensor is provided with four fingerprint-shaped micro-channels which are symmetrically distributed on long and short axes of an ellipse, and the design of a Wheatstone bridge circuit is combined, so that pressure and temperature signals can be effectively decoupled, and high-precision pressure detection and independent temperature sensing are realized; when being used as a strain sensor, the strain sensor is provided with a snakelike micro-channel. The sensor has excellent flexibility, ductility and wear resistance, high-precision detection of pressure and temperature can be achieved, the bending angle of the finger joint can be monitored synchronously, the working performance is stable, and the dynamic range is wide; the method has obvious effects in object texture identification and finger motion state detection, and has important application value in flexible robots, intelligent prostheses and wearable intelligent devices.

Description

Technical Field

[0001] The present invention belongs to the technical field of flexible tactile sensors, and in particular relates to a flexible resistive sensor based on gallium-based liquid metal and applications thereof. Background Art

[0002] Information perception, as one of the core ways for robots to exchange information with the external environment, is driving the continuous development of robotics technology. Currently, the perception sensors widely used in the field of intelligent robots are mainly divided into two categories. The first category is non-contact visual sensors, which can quickly and accurately perceive information about target objects in environments with good lighting conditions. However, in environments with insufficient lighting or confined spaces, the data acquisition quality of visual sensors will be greatly reduced, making it difficult to characterize the detailed features of objects, and their application is limited in application scenarios requiring high precision. The second category is contact tactile sensors, which collect data through direct contact with objects and are less affected by environmental factors. Even in environments with insufficient lighting or confined spaces, they can still accurately obtain multi-dimensional information about objects, such as stress distribution, object shape, and surface morphology. In tasks that require delicate operations and adaptability to complex environments, by integrating tactile sensors, intelligent robots can better understand and respond to the external environment, thereby improving the accuracy of their operations.

[0003] However, traditional rigid tactile sensors struggle to adapt to irregular, dynamically changing surfaces, limiting their application in flexible electronics, soft robotics, and bio-integrated devices. Traditional flexible tactile sensors also suffer from issues such as material stability, poor sensitivity, and low biosafety. Gallium-based liquid metal, with its high conductivity, low viscosity, nearly unlimited stretchability, self-healing ability, low toxicity, and good biocompatibility, offers a promising solution to these challenges. Summary of the Invention

[0004] In view of the deficiencies in the prior art, the technical problem to be solved by the present invention is to provide a flexible resistive sensor based on gallium-based liquid metal and its application.

[0005] To achieve the above object, the present invention adopts the following technical solutions:

[0006] A flexible resistive sensor based on gallium-based liquid metal comprises upper and lower PDMS substrates, each provided with a microchannel; the microchannel is filled with gallium-based liquid metal, which is a eutectic alloy of gallium and indium, with a mass fraction of gallium of 75.5% and a mass fraction of indium of 24.5%.

[0007] Furthermore, when the flexible resistive sensor is used as a tactile sensor, there are four microchannels distributed on the same ellipse, the major axis and minor axis of the ellipse are two symmetrically distributed microchannels, and each microchannel is fingerprint-shaped; when the flexible resistive sensor is used as a strain sensor, there is only one microchannel, and the microchannel is snake-shaped.

[0008] Furthermore, when the flexible resistive sensor is used as a strain sensor, the sensitivity of the strain sensor is:

[0009]

[0010] Where ΔR is the resistance change of the strain sensor, R0′ is the initial resistance value of the strain sensor considering the copper wire, ε is the strain, A is the aspect ratio of the microchannel, R0 is the initial total resistance of the liquid metal of the strain sensor, and R w is the resistance of the copper wire.

[0011] An application of a flexible resistive sensor based on gallium-based liquid metal, using the flexible resistive sensor as a strain sensor and tactile sensor for grasp detection and object texture recognition.

[0012] Compared with the prior art, the present invention has the following beneficial effects:

[0013] This invention uses gallium-based liquid metal as the raw material. By fabricating two layers of PDMS substrate containing microchannels, bonding them together, and then injecting a gallium-indium alloy via a syringe, the resulting flexible resistive sensor based on gallium-based liquid metal exhibits excellent flexibility, ductility, and wear resistance. The sensor can achieve high-precision pressure and temperature detection, and simultaneously monitor finger joint flexion angle. With stable performance and a wide dynamic range, it has important applications in flexible robots, intelligent prosthetics, and wearable smart devices. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 Schematic diagram of the microchannel structure of the tactile sensor of the present invention;

[0015] Figure 2 is a structural diagram of a Wheatstone bridge circuit of the present invention;

[0016] Figure 3 is the resistance change rate of the tactile sensor of the present invention at different force application frequencies;

[0017] Figure 4 This is a schematic diagram of grabbing a water cup while wearing a tactile sensor;

[0018] Figure 5 is the resistance change rate of the tactile sensor during the process of grasping the water cup;

[0019] Figure 6Schematic diagram of the structure of the strain sensor of the present invention;

[0020] Figure 7 It is the resistance change rate of the strain sensor under different bending states of the finger. DETAILED DESCRIPTION

[0021] Specific embodiments are given below in conjunction with the accompanying drawings. The specific embodiments are only used to introduce the technical solutions of the present invention in detail and are not intended to limit the scope of protection of the present application.

[0022] The present invention provides a flexible resistive sensor based on gallium-based liquid metal, comprising upper and lower PDMS substrates, each of which is provided with microchannels. Gallium-based liquid metal is filled in the microchannels to form sensitive elements for sensing temperature and stretching changes.

[0023] The microchannel can be fingerprint-shaped, S-shaped, sawtooth-shaped, spiral coil-shaped, reverse snake-shaped or snake-shaped; when the flexible resistive sensor is used as a strain sensor, the microchannel is snake-shaped; when the flexible resistive sensor is used as a tactile sensor, there are four microchannels distributed on the same ellipse, and two microchannels are symmetrically distributed on the major axis and minor axis of the ellipse respectively. Each microchannel is fingerprint-shaped, making the tactile sensor's signal perception most sensitive.

[0024] The gallium-based liquid metal uses gallium, gallium-indium alloy or gallium-indium-tin alloy; when gallium is used, the microchannel needs to be surface modified; when gallium-indium-tin alloy is used, the microchannel needs to be treated to improve adhesion; gallium-indium alloy has the best electrical and thermal conductivity, and is particularly suitable for high-density integrated flexible circuits. At the same time, due to the moderate surface tension and controllable oxide layer of gallium-indium alloy, it is easier to compound with a flexible substrate through spraying, printing, etc., thereby improving the efficiency of sensor preparation. Therefore, the performance of the sensor is optimal when gallium-indium alloy is used.

[0025] To facilitate parameter measurement of the sensor, a copper wire is connected to each end of the sensor microchannel, and the copper wire is in contact with the gallium-based liquid metal. However, the contact between the copper wire and the gallium-based liquid metal produces an oxide layer, which seriously weakens the conductivity between the copper wire and the gallium-based liquid metal. To solve this problem, a layer of conductive CuGa2 is electroplated on the surface of the copper wire. CuGa2 has good wettability with the gallium-based liquid metal, which is used to prevent the formation of an oxide layer while maintaining good electrical contact between the copper wire and the gallium-based liquid metal.

[0026] When used as a tactile sensor, this flexible resistive sensor is subject to the combined effects of pressure and temperature during actual use. Filling four fingerprint-shaped microchannels with gallium-based liquid metal creates four resistors: two resistors symmetrically located at the ends of the minor axis of the ellipse, and two resistors symmetrically located at the ends of the major axis. When the tactile sensor is subjected to external force, the cross-sectional area of ​​the two microchannels on the minor axis of the ellipse decreases significantly, causing a change in their resistance, which can be used to sense stress. The cross-sectional area of ​​the two microchannels on the major axis of the ellipse does not change much. When the temperature of an object changes, the contact resistance between the tactile sensor and the object also changes. Because the four microchannels have the same geometry, the temperature-induced resistance change of the four resistors is the same. Therefore, the two resistors on the minor axis of the ellipse are sensitive to both force and temperature signals, while the two resistors on the major axis are more susceptible to temperature changes. Connecting a resistor located on both the minor and major axes of the ellipse to a Wheatstone bridge circuit eliminates the effects of temperature, allowing accurate force signals to be obtained by measuring the output voltage, while temperature sensing is achieved by measuring the resistance change of the two resistors on the major axis of the ellipse.

[0027] The Wheatstone bridge circuit consists of four bridge arms. The first bridge arm consists of a resistor R located on the minor axis of the ellipse. FT1 The second bridge arm consists of a resistor R located on the major axis of the ellipse. T1 The third and fourth bridge arms are composed of a fixed reference resistor R0 respectively. The input voltage is applied to two diagonal points of the bridge, and the output voltage is measured from the other two diagonal points.

[0028] When the tactile sensor is affected by temperature, since the material properties of the four resistors are the same, their resistance responds to temperature in the same way and their resistance increases in the same proportion. Assuming that all increase by ΔR T , then R FT1 and R T1 Both increase ΔR T , due to R FT1 and R T1 If the absolute value of the increase is the same (assuming the initial resistance is equal), the resistance change of the first and second bridge arms is the same. According to the Wheatstone bridge balance principle (R FT1 / R T1 = R0 / R0 initial balance), when R FT1 and R T1 When the same amount is increased or decreased, the output voltage Vout of the circuit remains unchanged at 0 (or an initial offset voltage close to 0), and the temperature signal is offset in the differential output of the circuit.

[0029] When the tactile sensor is subjected to external force, the resistance of the two resistors located on the major axis of the ellipse increases significantly. Assuming that both increase ΔR F, the resistance of the two resistors located on the minor axis of the ellipse remains almost unchanged, ΔR T ≈0, which destroys the bridge balance (R FT1 Significantly increased R T1 constant), will produce a force with the same magnitude as the external force (ΔR F ) is proportional to the output voltage Vout.

[0030] When the tactile sensor is affected by force and temperature at the same time, the temperature change causes the resistance of all resistors to change in the same proportion, for example, R FT1 and R T1 Both increase ΔR T ; External force changes cause R FT1 Additional ΔR F , R T1 In the bridge, R FT1 The total change is ΔR T +ΔR F , R T1 The total change is ΔR T +0≈ΔR T ; The bridge output voltage Vout mainly depends on (ΔR T +ΔR F )-ΔR T =ΔR F , the common change caused by temperature (ΔR T ) is offset by the difference again, and the output Vout mainly reflects the external force (ΔR F The output (Vout) of the Wheatstone bridge only responds to force (ΔR F ), and the influence of temperature is basically eliminated. T2 Acquire temperature signal, extremely insensitive to external force (ΔR F ≈0), and is very sensitive to temperature changes (ΔR T Therefore, direct measurement of R T2 Resistance change (ΔR T2 ), can directly and independently reflect the temperature change (ΔT), the resistance R T2 Acts as a pure temperature sensor.

[0031] To further verify the dynamic repeatability of the sensor, 9 times of 9N pressure with different frequencies and the same magnitude were applied to the sensor. The time for applying pressure 1-3 times was 12s; the time for applying pressure 4-6 times was 8s; the time for applying pressure 7-9 times was 4s, and the interval between applying pressure was 10s. Figure 3As shown in the experimental results, when the same pressure is applied to the sensor at different frequencies, the sensor's output also changes with the frequency of the applied pressure; and the fluctuation of the sensor's output is very small each time the pressure is applied. These experimental results show that the prepared sensor can well measure the pressure applied cyclically at different frequencies. This performance provides durability for real-time Braille recognition in blind-aiding devices.

[0032] In order to verify the real-time sensing performance of the sensor in dynamic operation, the designed fingerprint-type gallium-based liquid metal sensor and spiral coil-type sensor were closely attached to the finger surface ( Figure 4 ), a grasping experiment was conducted on a 500mL water cup (total weight about 650g), and the sensor output signal was monitored in real time ( Figure 5 When the thumb grasps the cup wall, the sensor output signal rises sharply, clearly capturing the initial contact force of the grasping action. After entering the holding phase, the signal remains stable, demonstrating the sensor's excellent anti-interference ability and low noise characteristics under continuous load. When the cup is released, the signal drops sharply to the baseline within 80ms. The experiment successfully distinguished the three phases of grasping, holding, and releasing, demonstrating that the sensor designed in this study can not only detect instantaneous changes in finger contact force with high precision, but also provide force feedback for curved surface contact.

[0033] In order to help visually impaired users understand more complex tactile information and provide richer environmental cognition, the sensor should not only have the ability to detect pressure, but also the ability to detect the bending of finger joints. By combining the two functions, it can help beginners to get real-time feedback on whether the recognized Braille is correct, and can judge the fingering and sliding speed through the joint bending data to provide corrective guidance. Based on this, the flexible resistive sensor is used as a strain sensor (such as Figure 6 As shown in Figure 2, it is used to test the bending of the finger joints. The mathematical modeling process of the strain sensor is as follows:

[0034] The strain sensor microchannel is divided into a part parallel to the strain direction and a part perpendicular to the strain direction, and the subscripts p and v are used to distinguish them. p is the length of the microchannel parallel to the strain direction, L v is the length of the microchannel perpendicular to the strain direction, then the total length of the microchannel L0=L p +L v ;The initial total resistance R0 of the strain sensor liquid metal is:

[0035]

[0036] Where R p0 is the initial resistance of the liquid metal in the microchannel parallel to the strain direction, R v0 is the initial resistance of the liquid metal in the microchannel perpendicular to the strain direction, L p0is the initial length of the microchannel parallel to the strain direction, L v0 is the initial length of the microchannel perpendicular to the strain direction, S0 is the area of ​​the strain sensor microchannel, and ρ is the resistivity of the gallium-based liquid metal;

[0037] After strain is applied to the strain sensor, the total resistance R1 of the liquid metal of the strain sensor is:

[0038]

[0039] Where R p1 is the resistance of the liquid metal in the microchannel parallel to the strain direction after the strain is applied, R v1 is the resistance of the liquid metal in the microchannel perpendicular to the strain direction after the strain is applied, and ε is the strain;

[0040] From formula (2), we can know that R p0 With R v0 The proportional relationship affects the resistance R1 of the sensor after strain is applied; the proportional relationship between the initial length of the microchannel perpendicular to the strain direction and the initial length of the microchannel parallel to the strain direction, that is, the horizontal and vertical microchannel length relationship is defined as the aspect ratio A:

[0041]

[0042] The aspect ratio A is related to the microchannel structure of the strain sensor and is dimensionless. The initial total resistance R0 of the strain sensor liquid metal and the total resistance R1 of the liquid metal after strain is applied can be expressed as:

[0043] R0=(A+1)R P0 (4)

[0044]

[0045] The ratio of the total resistance R1 of the sensor liquid metal after the strain is applied to the total resistance R0 of the sensor liquid metal before the strain is applied is:

[0046]

[0047] Equation (6) shows the change in resistance of the liquid metal in the strain sensor microchannel after strain is applied. In the actual liquid metal strain sensor, the liquid metal encapsulated in the microchannel needs to be led out using two metal wires to connect to the external circuit, so the resistance of the copper wire should also be considered.

[0048] R0′=R0+R w (7)

[0049] Where R0′ is the initial resistance of the strain sensor considering the copper wire, R w is the resistance of the copper wire;

[0050] From formula (6), the resistance R1 of the liquid metal in the microchannel of the strain sensor after the strain is applied can be obtained:

[0051]

[0052] Considering the strain sensor with lead wires, the resistance R1′ after the strain is applied is:

[0053]

[0054] Considering the resistance change rate of the strain sensor after the strain is applied to the lead wire, it is:

[0055]

[0056] Formula (10) is the mathematical model of the gallium-based liquid metal strain sensor considering the sensor microchannel structure. Under the action of input strain, the strain sensor outputs the resistance change rate, which is related to the aspect ratio A, the initial resistance of the liquid metal R0, and the copper wire resistance R w It is related to R0 and R w Under the same conditions, the output resistance change rate of the strain sensor is inversely proportional to the aspect ratio A at the same ΔR.

[0057] The sensitivity of the strain sensor can be derived from formula (10):

[0058]

[0059] Sensitivity (GF) is an important performance indicator of the sensor, which is the ratio of the sensor output change to the input change. For gallium-based liquid metal flexible strain sensors, when the input strain is the same, the greater the output resistance change rate, the greater the sensor sensitivity. From formula (11), it can be seen that the sensor sensitivity is inversely proportional to the aspect ratio A and is inversely proportional to R0 / (R0+R w ) is proportional to.

[0060] The strain sensor sample is fixed on the index finger of the subject, and the subject's index finger is randomly extended to 0°, bent to 50°, and 90°. The index finger is held for a period of time after each bend to facilitate observation of the sensor response. For example, within 0 to 20 seconds, the subject's index finger is extended to 0°, bent to 50°, extended to 0°, bent to 90°, and extended to 0°, completing the secondary index finger movement detection. The output of the strain sensor is shown in Figure 2. Figure 7 ,The experimental results show that the strain sensor can clearly distinguish the ,state of the index finger of the subject. The flexible strain sensor ,invented has great application potential in the field of finger ,bending detection and gesture recognition.

[0061] The two sensors are further applied to simultaneously detect joint motion and contact force during hand grasping. For example, when grasping an object, the movement of the fingers and the contact force between the fingers and the object's surface are two aspects that need to be detected. A pressure sensor and a strain sensor are fixed to the subject's index finger. The subject is asked to grasp a cup filled with water, hold it for a period of time, and finally release the cup. The pressure sensor measures the contact force applied by the thumb on the sensor surface when grasping the cup, and the strain sensor measures the degree of bending of the index finger when grasping the cup. During the grasping process, since the pressure on the sensor's force-bearing area is approximately evenly distributed, the output of the force-bearing area is almost equal. During the grasping process, the output of the strain sensor can accurately distinguish between the grasping and releasing of the cup. In summary, the gallium-based liquid metal flexible strain sensor of the present invention can be used simultaneously with a pressure sensor. In wearable experiments, it can effectively detect the movement of the subject's fingers when grasping a water cup and the contact force between the fingers and the surface of the object. At the same time, the symmetrical microchannel design can distinguish the pressure distribution. Detecting the motion state of the human hand when grasping an object demonstrates its good application prospects in the wearable field.

[0062] Combined with the above experimental studies, the gallium-based liquid metal pressure sensor and strain sensor of the present invention are feasible for wearable applications for object texture recognition. The synergistic integration of a highly sensitive fingerprint-type pressure unit and a stretch-type joint sensing unit enables multifunctional, all-in-one applications. By capturing real-time fingertip pressure distribution and joint bending data, they provide dynamic feedback on character recognition accuracy and correct fingering issues such as sliding direction and pressure uniformity. They can also provide quantitative pathological indicator analysis for diseases such as Parkinson's disease by continuously monitoring finger tremor frequency, joint stiffness, and bradykinesia. Their stability and ability to recognize complex textures further demonstrate their potential for wearable applications such as medical tactile feedback and precision operation training.

[0063] Any matters not described in the present invention are applicable to the prior art.

Claims

1. A flexible resistive sensor based on gallium-based liquid metal, comprising upper and lower PDMS substrates, each of which is provided with a microchannel; characterized in that: The microchannel is filled with gallium-based liquid metal, which is one of gallium, gallium-indium alloy and gallium-indium-tin alloy.

2. The flexible resistive sensor based on gallium-based liquid metal according to claim 1, characterized in that: When the flexible resistive sensor is used as a strain sensor, it contains only one microchannel and the microchannel is snake-shaped; when the flexible resistive sensor is used as a tactile sensor, there are four microchannels and they are distributed on the same ellipse, with two microchannels symmetrically distributed on the major axis and minor axis of the ellipse, and each microchannel is fingerprint-shaped.

3. The gallium-based liquid metal-based flexible resistive sensor according to claim 1, characterized in that: When the flexible resistive sensor is used as a strain sensor, the sensitivity of the strain sensor is: Where ΔR is the resistance change of the strain sensor, R0′ is the initial resistance value of the strain sensor considering the copper wire, ε is the strain, A is the aspect ratio of the microchannel, R0 is the initial total resistance of the liquid metal of the strain sensor, and R w is the resistance of the copper wire.

4. An application of a flexible resistive sensor based on gallium-based liquid metal according to any one of claims 1 to 3, characterized in that: Flexible resistive sensors are used as strain sensors and tactile sensors for grasp detection and object texture recognition.

Images