Flexible strain sensor array for human multi-directional motion monitoring and preparation and decoupling method

Abstract

The present application relates to the field of flexible electronics and wearable sensing technology, and provides a flexible strain sensor array for monitoring multi-directional movement of human body and a preparation and decoupling method. The flexible strain sensor array is formed by stacking three flexible strain sensors in a vertical direction, and the included angle between every two layers is 120°. Each flexible strain sensor comprises, from top to bottom, a substrate layer, a strain sensing layer and a packaging layer. The strain sensing layer is a conductive composite layer doped with liquid metal droplets. An electrode connecting part is arranged in an electrode area of the strain sensing layer and is installed with a lead wire. The packaging layer covers the strain sensing layer. The present application can form a stable conductive path without additional sintering, and has simple manufacturing process and good tensile performance. Through a decoupling formula, the present application can realize accurate monitoring of the three-dimensional spatial movement state of human joints.

Description

Technical Field

[0001] This application relates to the fields of flexible electronics and wearable sensing technology, specifically to a flexible strain sensor array for monitoring multi-directional human motion and its fabrication and decoupling methods. Background Technology

[0002] With the rapid development of flexible electronics technology and smart wearable devices, the monitoring of multi-directional human motion has significant application value in fields such as healthcare, sports rehabilitation, and smart wearables, and has therefore attracted widespread attention. Flexible strain sensor arrays, capable of conforming to human skin and operating stably under complex multi-directional deformation scenarios such as joint flexion and extension, and limb swinging, have become core components for achieving accurate human motion monitoring. Among them, liquid metal, due to its high conductivity, excellent ductility, and good deformation adaptability, shows significant application potential in the field of flexible electronics, especially in the monitoring of multi-directional human motion signals. It can effectively capture strain signals of different directions and amplitudes of the human body, providing support for multi-directional motion monitoring.

[0003] However, current flexible strain sensors based on liquid metal composite materials typically require additional mechanical sintering or post-processing steps to form the conductive network. This not only results in complex manufacturing processes and low production efficiency but also potentially affects the flexibility and deformation adaptability of the sensor array, hindering large-scale fabrication and failing to meet the mass application requirements of sensor arrays for multi-directional human motion monitoring. Furthermore, current sensor arrays generally suffer from limitations in decoupling multi-directional motion signals due to reliance on complex sensor structure design, algorithm optimization, and high precision requirements in sensor fabrication. Therefore, providing a simple, stable, and readily adaptable method for fabricating flexible strain sensor arrays that can form arrayed conductive pathways without additional sintering, and also offering a relatively easy method for decoupling multi-directional motion signals, has become a pressing technical challenge in this field. Summary of the Invention

[0004] The purpose of this application is to provide a flexible strain sensor array for monitoring multi-directional human motion and a method for its fabrication and decoupling, in order to solve the problems of complex manufacturing processes and the need for additional sintering steps in the fabrication of sensors using liquid metal composite materials in the prior art, as well as the signal decoupling problem of the sensor array.

[0005] To achieve the above objectives, this application adopts the following technical solution:

[0006] A flexible strain sensor array for monitoring multi-directional human motion, wherein the flexible strain sensor array consists of three flexible strain sensors stacked alternately in the vertical direction, and each flexible strain sensor includes a base layer, a strain sensing layer and an encapsulation layer arranged sequentially from top to bottom;

[0007] The strain sensing layer is a conductive composite material layer used to generate a change in resistance when subjected to tensile deformation in order to achieve strain detection, and it has electrode connection parts at both ends.

[0008] The encapsulation layer covers one side of the strain sensing layer and encapsulates the electrode connection portion.

[0009] Further optimization involves ensuring that the angle between any two of the three flexible strain sensors in the horizontal direction is 120°.

[0010] Further optimization involves using a thermoplastic polyurethane composite gallium-based liquid metal conductive composite material for both the strain sensing layer and the electrode connection portion; and using thermoplastic polyurethane for both the substrate layer and the encapsulation layer.

[0011] Further optimization resulted in the strain sensing layer being shaped like a paper cutout.

[0012] Further optimization involves connecting the electrode connection portion to a copper wire.

[0013] The method for fabricating a flexible strain sensor array for multi-directional human motion monitoring includes the following steps:

[0014] Step 1: Print the strain sensing layer and the substrate layer sequentially on the printing platform;

[0015] Step 2: Peel off and flip the composite structure of strain sensing layer and substrate layer obtained in Step 1 from the printing platform, so that the strain sensing layer is facing up and the substrate layer is facing down on the printing platform.

[0016] Step 3: Arrange electrode connection parts at both ends of the upper surface of the strain sensing layer after it is reversed, connect the copper wire to the electrode connection parts, and then print the encapsulation layer for encapsulation to obtain a flexible strain sensor that can be used for multi-directional human motion monitoring.

[0017] Step 4: Repeat steps 1-3 until three flexible strain sensors that can be used for multi-directional human motion monitoring are prepared. Stack them in an alternating manner in the vertical direction, with the angle between each pair of strain sensors in the horizontal direction being 120°. Heat and solidify to form a flexible strain sensor array.

[0018] Further optimization involves defining the three strain sensors as a bottom-layer flexible sensor, a middle-layer flexible sensor, and an upper-layer flexible sensor.

[0019] A decoupling algorithm for a flexible strain sensor array used for multi-directional human motion monitoring includes the following steps:

[0020] Step 1: Install the sensor array at the joint of the human body. Define the plane where the sensor array is located when the joint is not rotating as the horizontal plane. The bottom layer of sensors is arranged along the X-axis of the horizontal plane, and its center is located at the origin of the coordinate system. The plane that is perpendicular to the horizontal plane and includes the Z-axis is the vertical plane.

[0021] Step 2: Obtain the relative resistance changes of the bottom layer flexible sensor, the middle layer flexible sensor, and the top layer flexible sensor in the sensor array;

[0022] Step 3: Based on the relative resistance change, calculate the horizontal rotation angle θ and vertical rotation angle φ of the human joint using the following formula:

[0023]

[0024]

[0025] Where ε1, ε2 and ε3 are the strains of the bottom flexible sensor, the middle flexible sensor and the top flexible sensor, respectively, Vx is the strain difference between the middle flexible sensor and the top flexible sensor, k is the strain level coupling coefficient and k' is the strain-angle conversion coefficient.

[0026] Further optimization involves determining the directions of the horizontal rotation angle θ and the vertical rotation angle φ using the following direction functions:

[0027] Horizontal direction function

[0028]

[0029] ,

[0030] Vertical direction function

[0031] ,

[0032] Where R1, R2, and R3 represent the relative resistance changes of the bottom layer sensor (S1), the middle layer sensor (S2), and the top layer sensor (S3), respectively. Num The calculation formula is obtained, where ∆R Num This represents the change in resistance relative to the initial resistance R0.

[0033] Further optimization involves setting the horizontal rotation angle θ to a range of -35° to 35° and the vertical rotation angle φ to a range of -45° to 75°.

[0034] The technical solution of the present invention achieves the following beneficial technical effects:

[0035] 1. By first printing the strain sensing layer and then forming the substrate layer, liquid metal droplets are naturally deposited to the bottom of the strain sensing layer during the solidification process, thereby forming a stable conductive path without the need for additional mechanical sintering.

[0036] 2. Under the influence of gravity, metal ions in liquid metal droplets naturally settle and form a deposition layer with a high concentration of metal ions at the bottom of the strain sensing layer. This deposition layer has excellent conductivity, which is the key to achieving high-sensitivity strain detection. By flipping the strain sensing layer to face upwards, it is easy to directly install conductive connectors and copper wires on it to achieve efficient electrical signal transmission.

[0037] 3. Three flexible strain sensors are stacked vertically, with each pair of sensors forming a 120° angle on the horizontal plane. After heating and curing, a flexible sensor array is formed. This design not only enables it to sensitively respond to strain signals from any direction within the horizontal plane, overcoming the deficiency of single-directional sensors in responding to multi-directional motion, but also makes the vertically stacked three-layer structure thin and flexible, allowing it to conform to joints with large curvatures, such as the wrist and neck. Based on the spatial configuration of this array, combined with signal decoupling methods, the resistance change signals output by the three sensors can be separated into horizontal and vertical rotation angles, thereby quantitatively monitoring the complex motion posture of human joints in three-dimensional space (e.g., left-upward and right-downward deflection of the wrist). Experiments show that the measurement error of the human joint rotation angle can be controlled within 5%, achieving high-precision multi-directional motion monitoring. Attached Figure Description

[0038] Figure 1 This is an exploded view of the flexible strain sensor and printing platform in Embodiment 1 of this application;

[0039] Figure 2 This is a schematic diagram of the structure of the strain sensing layer of Embodiment 1 provided in this application, including the electrode connection portion and the copper wire thereon;

[0040] Figure 3 This is an isometric view of the finished flexible strain sensor based on liquid metal composite material in Embodiment 1 provided in this application;

[0041] Figure 4 This is a side view of the finished flexible strain sensor based on liquid metal composite material in Embodiment 1 provided in this application;

[0042] Figure 5 This is the strain-relative resistance curve of the thermoplastic polyurethane elastomer and liquid metal elastic composite material of Example 1 provided in this application;

[0043] Figure 6This is a graph showing the relationship between the number of cycles and the change in relative resistance of the thermoplastic polyurethane elastomer and liquid metal elastic composite material of Example 1 provided in this application;

[0044] Figure 7 , Figure 8 This is a schematic diagram of the fabrication method of the flexible sensor array in Embodiment 2 provided in this application;

[0045] Figure 9 This is a schematic diagram of the three-layer stacked flexible sensor array in Embodiment 2 provided in this application;

[0046] Figure 10 This is an exploded view of the flexible sensor array in Embodiment 2 provided in this application;

[0047] Figure 11 This is an isometric view of the flexible sensor array in Embodiment 2 provided in this application;

[0048] Figure 12 This is a three-dimensional spatial angle diagram provided in this application;

[0049] Figure 13 These are curves showing the relationship between the rotation angle of a human joint and the relative resistance changes of each sensor in the sensor array in Embodiment 3 provided in this application. Among them, 13a is the curve showing the relationship between the horizontal rotation angle θ and the relative resistance changes of the three flexible strain sensors, and 13b is the curve showing the relationship between the vertical rotation angle φ and the relative resistance changes of the three flexible strain sensors.

[0050] Figure 14 This is a comparison chart of the actual and calculated values ​​of the rotation angle described in Embodiment 3 of this application, where 14a is a scatter plot comparing the actual and calculated values ​​of the horizontal rotation angle θ, and 14b is a scatter plot comparing the actual and calculated values ​​of the vertical rotation angle φ.

[0051] Figure 15 This is a microstructure morphology diagram of the strain sensing layer in the flexible sensor provided in Embodiment 1 of this application;

[0052] Reference numerals: 1-Printing platform, 2-Base layer, 3-Strain sensing layer, 4-Electrode connection, 5-Copper wire, 6-Encapsulation layer, S1-Bottom layer flexible sensor, S2-Middle layer flexible sensor, S3-Upper layer flexible sensor. Detailed Implementation

[0053] Example 1

[0054] This embodiment provides a strain sensor for monitoring multi-directional human motion, such as... Figure 1As shown, the flexible strain sensor includes a substrate layer 2, a strain sensing layer 3, an electrode connection portion 4, and an encapsulation layer 6. The substrate layer 2, made of thermoplastic polyurethane (TPU), serves as the bottom surface of the flexible strain sensor, possessing high tensile strength and adaptability to complex curved surfaces, allowing it to conform to areas such as the elbow and knee joints of the human body. The strain sensing layer 3, disposed on one side of the substrate layer 2, is made of a conductive composite material of thermoplastic polyurethane and gallium-based liquid metal (TPU / LM), formed on the printing platform 1 using a microelectronic direct-write printing process. Figure 2 As shown, the sensing structure of the strain sensing layer 3 is paper-cut in shape, resembling a scissor fork, allowing it to undergo recoverable deformation under tension. This deformation generates a change in resistance during tensile deformation, enabling strain detection. Two electrode connection portions 4 are also provided at both ends of the strain sensing layer 3 for connection to external circuits. Copper wires 5 are mounted on the electrode connection portions 4. The encapsulation layer 6, made of thermoplastic polyurethane (TPU), covers the strain sensing layer 3, encapsulating the electrode connection portions 4, the conductive paths of the strain sensing layer 3, and the copper wires 5. This prevents liquid metal leakage and ensures safety during use.

[0055] like Figure 15 a to Figure 15 e. The conductive pathways and electrode microstructures of this embodiment were characterized using electron microscopy. For example... Figure 15 As shown in Figure a, the front of the conductive path exhibits a paper-cut-like structure, with clearly visible V-shaped cuts arranged periodically along the stretching direction. Figure 15 b and Figure 15 As shown in Figure c, the electrode connection 4 has a flat surface before stretching, and the gallium-based liquid metal is uniformly distributed (the bright area in the figure represents the gallium-based liquid metal). After stretching, the electrode area deforms along with the substrate, but the liquid metal droplets remain connected without significant breakage, demonstrating that the composite material has excellent ductility. Figure 15 d and Figure 15 The electron microscope image of e shows gallium-based liquid metal droplets in the bright area and thermoplastic polyurethane matrix in the dark area. The gallium-based liquid metal droplets form an interconnected conductive network within the polymer. The droplets are spherical or ellipsoidal in shape with clear boundaries, and some droplets are connected by necks. This microstructure confirms that the liquid metal undergoes phase separation and forms discrete droplets during solidification, and the contact points between the droplets constitute strain-sensitive conductive pathways.

[0056] In this embodiment, the thickness of the substrate layer 2 is 100μm, the thickness of the strain sensing layer 3 is 150μm, the thickness of the encapsulation layer 6 is 200μm, and the overall thickness of the sensor is controlled at 450μm, so that it can maintain mechanical reliability even under strain exceeding 500%.

[0057] like Figure 4As shown in the figure, the strain-relative resistance change curve of the flexible strain sensor in this embodiment shows that its sensitivity (GF) can reach a maximum of 0.81, which is several times higher than that of a pure liquid metal sensor, enabling precise monitoring of motion signals such as human joint bending. Figure 5 As shown, after more than 2,000 cycles of tensile testing, the minimum and maximum resistance values ​​of the flexible strain sensor did not change significantly, demonstrating good cyclic stability.

[0058] Example 2

[0059] This embodiment provides a flexible strain sensor array for monitoring multi-directional human motion, which is formed by stacking three independent flexible strain sensors sequentially in the vertical direction. See [link / reference needed]. Figure 6 , Figure 7 , Figure 8 and Figure 9 .

[0060] The specific preparation steps are as follows:

[0061] S100, Preparation of Flexible Materials and Liquid Metal-Based Composites

[0062] Thermoplastic polyurethane particles and dimethylformamide solution were mixed at a mass ratio of 1:2 and magnetically stirred at 80°C for 30 min to obtain a uniform thermoplastic polyurethane solution. After cooling, the solution was mixed with gallium-based liquid metal at a mass ratio of 1:3 and stirred continuously at room temperature for about 1 h. At this time, no gallium-based liquid metal particles were visible to the naked eye, and a uniform thermoplastic polyurethane / gallium-based liquid metal fluid was obtained.

[0063] S200, Print the strain sensing layer on the printing platform.

[0064] A thermoplastic polyurethane / gallium-based liquid metal fluid was printed on printing platform 1 using a microelectronic printer. The printing thickness was 150μm and the size was 20×30mm. After printing, the printing platform was heated to 80℃ and heated for about 10 minutes to quickly evaporate the dimethylformamide solvent and solidify the strain sensing layer 3.

[0065] S300, Forming a substrate layer on the strain sensing layer.

[0066] Take a thermoplastic polyurethane solution and print a 100μm thick thermoplastic polyurethane film on the cured strain sensing layer 3 as a base layer. After printing, heat the printing platform to 80℃ and continue heating for about 10 minutes to cure the base layer 2. At this time, the strain sensing layer 3 is located between the base layer 2 and the printing platform 1.

[0067] S400, peel and flip

[0068] The semi-finished flexible strain sensor after curing in step S3 is peeled off from the printing platform 1, then flipped over and placed back on the printing platform 1. The flipped base layer 2 is located between the strain sensing layer 3 and the printing platform 1.

[0069] S500, Install wires and form an encapsulation layer

[0070] Electrode connection 4 and copper wire 5 are both installed on the upward-facing side of strain sensing layer 3. Then, thermoplastic polyurethane solution is used to print a 200μm thick encapsulation layer 6 on top of strain sensing layer 3, covering electrode connection 4 and copper wire 5. After printing, the printing platform is heated to 80°C and heated for about 10 minutes to cure encapsulation layer 6. Thus, strain sensing layer 3, electrode connection 4 and copper wire 5 are encapsulated between base layer 2 and encapsulation layer 6, resulting in a strain sensor for multi-directional human motion monitoring.

[0071] S600, Fabrication of sensor array

[0072] Repeat steps S1 to S5 three times to fabricate three independent strain sensors for multi-directional human motion monitoring. Then, stack the three flexible strain sensors vertically, with each pair of sensors forming a 120° angle on the horizontal plane. After stacking, heat the flexible strain sensor array on printing platform 1 at 80°C for 5 minutes to tightly bond the base layer 2 and encapsulation layer 6 of each sensor, forming a strain sensor for multi-directional human motion monitoring (e.g., ...). Figure 10 and Figure 11 ).

[0073] This array structure can sensitively respond to strain signals from any direction in the horizontal plane, overcoming the shortcomings of single-direction sensors in responding to multi-directional motion.

[0074] It should be noted that, as Figure 9 As shown, the 120° angle between every two sensors in the horizontal direction refers to the independent layout orientation of the three sensors before stacking. After stacking, viewed from a top angle, the directional angle between adjacent sensors is 60°, but this is the geometric projection result of the 120° angle and does not affect the sensor array's sensitive coverage of strain in the 360° direction within the horizontal plane.

[0075] Example 3

[0076] This embodiment provides a decoupling method for strain sensors used for monitoring multi-directional human motion, which is used to monitor the rotational posture of human joints (such as wrists and necks) in three-dimensional space.

[0077] S1. Coordinate System Definition and Sensor Installation

[0078] A sensor array is installed at the joint of the human body. When the joint is not rotating, the plane containing the sensor array is defined as the horizontal plane (XOY plane), and the center of joint rotation is defined as the origin of the coordinate system. The bottom sensor S1 is arranged along the X-axis of the horizontal plane, with its center located at the origin of the coordinate system. The plane perpendicular to the horizontal plane and containing the Z-axis is defined as the vertical plane. The horizontal rotation angle θ is defined as the angle by which the joint deviates from the X-axis in the horizontal plane (positive value indicates rightward deflection, negative value indicates leftward deflection); the vertical rotation angle φ is defined as the angle by which the joint deviates from the horizontal plane in the vertical plane (positive value indicates downward deflection, negative value indicates upward deflection) (e.g., ...). Figure 12 ).

[0079] S2. Acquire the resistance change signal of the sensor array.

[0080] The relative resistance changes of the bottom layer sensor S1, the middle layer sensor S2, and the top layer sensor S3 are obtained by an external digital source meter and denoted as R1, R2, and R3, respectively, where R = ΔR / R0, ΔR is the resistance change, and R0 is the initial resistance.

[0081] S3. Determine the direction of joint rotation.

[0082] Compare the relative resistance changes of the three sensors: the direction of the sensor with the largest change is the direction of joint rotation. For example, if the change in S2 is the largest, the joint rotates in the direction of S2.

[0083] Furthermore, the state of tension or compression can be further determined based on the sign of the change in relative resistance: a positive change in relative resistance indicates that the sensor is stretched and the joint rotates in the direction of the sensor; a negative change indicates that the sensor is compressed and the joint rotates in the opposite direction.

[0084] S4. k-value calibration and fitting

[0085] Since the relationship between the relative resistance change and strain of a flexible strain sensor is nonlinear within the strain range of less than 60%, and only approaches linearity after the strain exceeds 60%, in order to reduce errors and better match the nonlinear strain response of thermoplastic polyurethane elastomers and liquid metal composite elastomers under small deformations and the near-linear strain response under large deformations, a composite model of exponential decay model and linear model is used to fit the k value.

[0086]

[0087] Among them, the index term In the initial stage of fitting nonlinearity, the linear term Supplement the subsequent linear trend, constant term The overall offset is balanced, which can adapt to both low strain (high gradient) and high strain (near linear) regions.

[0088] By calibrating the relative resistance change-angle relationship and strain-angle relationship of the three-layer sensing layer under pure horizontal rotation in the range of -35° to 35° and pure vertical rotation in the range of -45° to 75°, the values ​​α = 0.3, β = 4.8, and γ = 0.2 were obtained. =0.2, k′=0.472.

[0089] S5. Calculate the horizontal and vertical rotation angles.

[0090] Based on the strain gauge measurement principle and geometric analysis, the horizontal rotation angle θ and vertical rotation angle φ of human joints are calculated using the following formula:

[0091]

[0092]

[0093] Where ε1, ε2, and ε3 represent the strains of the bottom flexible sensor S1, the middle flexible sensor S2, and the top flexible sensor S3, respectively; Vx represents the strain difference between the S2 and S3 flexible sensors; k is the strain horizontal coupling coefficient; and k′ is the strain-angle conversion coefficient. The directions of the horizontal and vertical rotation angles are determined by the functions sgn(x) and sgn(y), respectively.

[0094] Based on the relationship between the relative resistance changes of the three sensors and the strain, and the characteristic that the sensors have the same amount of expansion and contraction but opposite relative resistance changes under the same deflection angle and opposite deflection directions, the deflection direction of the human joint can be obtained. The direction function is as follows:

[0095]

[0096]

[0097]

[0098] Where R1, R2, and R3 represent the relative resistance changes of the bottom flexible sensor S1, the middle flexible sensor S2, and the top flexible sensor S3, respectively. Num The calculation formula is obtained, where ∆R Num This represents the change in resistance relative to the initial resistance R0.

[0099] The relationship between strain and relative resistance change was determined through calibration experiments, such as... Figure 5 and Figure 13 As shown, both clearly demonstrate that when the strain is less than 60%, the relative resistance change increases rapidly and nonlinearly with increasing strain; when the strain is greater than 60%, the relative resistance change tends to increase linearly. Figure 5 This reflects the intrinsic resistance-strain response characteristics of the material. Figure 13This verifies the reproducibility of this characteristic in real joint motion scenarios. The two verifications mutually confirm each other, eliminating the possibility of randomness caused by testing methods or installation errors, and proving that the nonlinear-to-linear transition is an inherent law of thermoplastic polyurethane / gallium-based metal solution composite materials. Based on this, this embodiment uses a composite model combining an exponential decay model and a linear model for k-value fitting to simultaneously accommodate the nonlinear response in the low-strain region and the near-linear response in the high-strain region.

[0100] S6. Verification Results

[0101] like Figure 14 As shown, the above method was used to measure and calculate a total of 8 sets of data for four wrist joint movements (upper left, upper right, lower left, and lower right). The calculated values ​​of the horizontal rotation angle θ and the vertical rotation angle φ were in high agreement with the actual values, with the error controlled within 5%, which verified the accuracy and practicality of this method.

[0102] The specific judgment logic is as follows: Based on the signs of the calculated horizontal rotation angle θ and vertical rotation angle φ, the combined rotation direction of the joint is determined:

[0103] When θ < 0 and φ > 0, it is determined to be a deflection to the lower right;

[0104] When θ < 0 and φ < 0, it is determined to be a deflection to the upper right;

[0105] When θ > 0 and φ > 0, it is determined to be a deflection to the lower left;

[0106] When θ > 0 and φ < 0, it is determined to be a deflection to the upper left.

[0107] The results of the four sets of actions measured in the experiment are shown in Table 1 below:

[0108] Table 1

[0109]

[0110] Experimental results show that the rotation direction obtained by judging the sign of θ and φ is completely consistent with the actual movement, and the calculated values ​​of the horizontal rotation angle and the vertical rotation angle are highly consistent with the actual measured values, with the error controlled within 5%. This verifies that the decoupling method can accurately identify the composite rotation direction of the joint in space and realize the effective monitoring of multi-directional human movement.

[0111] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. It will be apparent to those skilled in the art that the present invention is not limited to the details of the above exemplary embodiments, and can be implemented in other specific forms without departing from the spirit or basic characteristics of the present invention.

Claims

1. A flexible strain sensor array for monitoring multi-directional human motion, characterized in that, The flexible strain sensor array consists of three flexible strain sensors stacked alternately in the vertical direction. Each flexible strain sensor includes a base layer (2), a strain sensing layer (3), and an encapsulation layer (6) arranged sequentially from top to bottom. The strain sensing layer (3) is a conductive composite material layer, which is used to generate a change in resistance when subjected to tensile deformation in order to achieve strain detection. Electrode connection parts (4) are provided at both ends. The encapsulation layer (6) covers the strain sensing layer (3) and encapsulates the electrode connection portion (4) inside.

2. The flexible strain sensor array for monitoring multi-directional human motion according to claim 1, characterized in that, In the three flexible strain sensors, the angle between any two sensors in the horizontal direction is 120°.

3. The flexible strain sensor array for monitoring multi-directional human motion according to claim 1, characterized in that, The strain sensing layer (3) and the electrode connection part (4) are both made of a conductive composite material formed by thermoplastic polyurethane and gallium-based liquid metal; the base layer (2) and the encapsulation layer (6) are both made of thermoplastic polyurethane.

4. The flexible strain sensor array for monitoring multi-directional human motion according to claim 1, characterized in that, The strain sensing layer (3) is in the shape of paper cutout.

5. The flexible strain sensor array for monitoring multi-directional human motion according to claim 1, characterized in that, The electrode connection part (4) is connected to a copper wire (5).

6. A method for fabricating a flexible strain sensor array for monitoring multi-directional human motion, characterized in that, Includes the following steps: Step 1: Print the strain sensing layer (3) and the substrate layer (2) sequentially on the printing platform (1). Step 2: Peel off and flip the composite structure of strain sensing layer (3) and base layer (2) obtained in step 1 from the printing platform (1) so that the strain sensing layer (3) faces up and the base layer (2) faces down on the printing platform (1); Step 3: Electrode connection parts (4) are arranged at both ends of the upper surface of the strain sensing layer (3) after inversion, and copper wires (5) are connected to the electrode connection parts (4). Then, an encapsulation layer (6) is printed for encapsulation to obtain a flexible strain sensor that can be used for multi-directional human motion monitoring. Step 4: Repeat steps 1-3 until three flexible strain sensors that can be used for multi-directional human motion monitoring are prepared. Stack them in an alternating manner in the vertical direction, with the angle between each pair of strain sensors in the horizontal direction being 120°. Heat and solidify to form a flexible strain sensor array.

7. The preparation method according to claim 6, characterized in that, The three strain sensors are the bottom layer sensor (S1), the middle layer sensor (S2), and the top layer sensor (S3).

8. A decoupling algorithm for a flexible strain sensor array for monitoring multi-directional human motion, characterized in that, The flexible strain sensor array is a flexible strain sensor array prepared by the preparation method according to any one of claims 6 or 7, comprising the following steps: Step 1: Install the sensor array at the joint of the human body. Define the plane where the sensor array is located when the joint is not rotating as the horizontal plane. The bottom sensor (S1) is arranged along the X-axis of the horizontal plane, and its center is located at the origin of the coordinate system. The plane that is perpendicular to the horizontal plane and includes the Z-axis is the vertical plane. Step 2: Obtain the relative resistance changes of the bottom flexible sensor (S1), the middle flexible sensor (S2), and the top flexible sensor (S3) in the sensor array; Step 3: Based on the relative resistance change, calculate the horizontal rotation angle θ and vertical rotation angle φ of the human joint using the following formula: Wherein, ε1, ε2, and ε3 are the strains of the bottom flexible sensor (S1), the middle flexible sensor (S2), and the top flexible sensor (S3), respectively, and V x For the strain difference between the middle layer flexible sensor (S2) and the upper layer flexible sensor (S3), k is the strain horizontal coupling coefficient and k′ is the strain-angle conversion coefficient.

9. The decoupling method according to claim 8, characterized in that, The directions of the horizontal rotation angle θ and the vertical rotation angle φ are determined by the following direction functions: Horizontal direction function ， Vertical direction function ， Where R1, R2, and R3 represent the relative resistance changes of the bottom layer sensor (S1), the middle layer sensor (S2), and the top layer sensor (S3), respectively. Num The calculation formula is obtained, where ∆R Num This represents the change in resistance relative to the initial resistance R0.

10. The decoupling method according to claim 8, characterized in that, The horizontal rotation angle θ ranges from -35° to 35°, and the vertical rotation angle φ ranges from -45° to 75°.

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