Method for manufacturing a wireless passive biaxial strain sensor based on a nested double loop antenna

Abstract

The present application relates to the technical field of flexible materials, and particularly relates to a preparation method of a wireless passive biaxial strain sensor based on a nested double-loop antenna. The strain sensor designed and prepared by the method is composed of a nested double-loop antenna, the capacitance and inductance characteristics of the inner and outer loop structures under the tensile condition are regulated, two decoupled resonance frequencies are generated, and the x-axis and y-axis strain specificity sensitivities are respectively realized. Through laser scanning, an ink composed of liquid metal and styrene-ethylene / butylene-styrene copolymer (SEBS) two self-healing materials is activated, and the preparation of the stretchable biaxial strain sensor is realized. The circuit-free design realizes the bidirectional strain monitoring without crosstalk, and provides a new scheme for biomechanical sensors.

Description

Technical Field

[0001] This invention relates to the field of flexible materials technology, and in particular to a method for fabricating a wireless passive biaxial strain sensor based on a nested double-loop antenna. Background Technology

[0002] The rapid increase in chronic diseases and population aging has exacerbated the demand for real-time, precise physiological monitoring technologies. Strain sensors have become a key tool for obtaining critical health data by tracking biomechanical signals such as joint movement, respiration, pulse, and tumor growth, playing an important role in personalized healthcare.

[0003] Wireless strain sensors have garnered significant attention for eliminating power constraints, particularly crucial in implantable scenarios where battery replacement is impractical. Existing approaches integrate resistive or capacitive sensing units with wireless transmission modules, where mechanical strain modulates the resistance and capacitance, and an RF antenna converts the resistance signal into a wireless output. These systems still face fundamental limitations: battery-dependent designs impact lifespan and miniaturization, making them difficult to adapt to implantable environments. Wireless charging systems, on the other hand, require additional rectifier diodes. Recent advances focus on antenna-sensor co-design, where antenna resonant frequency shift is directly correlated with strain, eliminating signal processing circuitry and improving miniaturization and flexibility. This represents a key trend in integrated sensing and communication architectures, fundamentally unifying sensing and wireless communication capabilities.

[0004] Human tissues, such as the heart wall and joints, undergo complex anisotropic deformations. Single-axis sensors inevitably miss these multidimensional features, thus limiting diagnostic accuracy. Current biaxial strain sensing strategies, such as orthogonally arranged multiple sensors or layered silicon sensor grids, rely on physical cabling for multipath transmission and do not fundamentally solve the problem of strain signal crosstalk. They suffer from low signal fidelity and spatial resolution and depend on complex and power-intensive compensation algorithms. Therefore, decoupling multiaxial strain remains an ongoing design challenge. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to provide a method for fabricating a wireless passive biaxial strain sensor based on a nested double-ring antenna. The method uses liquid metal and SEBS, two self-healing materials, to form a composite material, and proposes a laser scanning in-situ activation technology for the liquid metal composite material. This not only overcomes the shortcomings of the prior art, but also meets the requirements of flexible electronic devices for stretchable wires.

[0006] To achieve the aforementioned objective, the technical solution of the present invention is implemented as follows: a method for fabricating a wireless passive biaxial strain sensor based on a nested dual-loop antenna; the fabrication method includes the following steps:

[0007] S1. Design of nested dual-loop antenna:

[0008] The antenna was designed and simulated using HFSS software, and the antenna pattern was saved.

[0009] Preparation of S2 and LM inks:

[0010] LM ink is obtained by mixing LM particles obtained by ultrasonication of surfactant with SEBS solution and then stirring magnetically.

[0011] Patterning of S3 and LM inks:

[0012] S31. Apply LM ink to a polytetrafluoroethylene substrate by scraping. Place the applied LM ink in a fume hood and transfer the ink and substrate onto a glass slide after the solvent has completely evaporated.

[0013] S32. Using an ultraviolet laser and based on the preserved antenna pattern, cut out the required coil pattern from the ink and substrate transferred to the glass slide, and remove the excess ink and substrate outside the coil pattern;

[0014] Activation of S4 and LM inks:

[0015] The coil pattern formed by LM ink is activated by laser scanning, and the coil pattern changes from an insulating state to a conductive state;

[0016] S5. Coil transfer and packaging:

[0017] S51. Imprint the coil pattern onto the SEBS base film, and remove the polytetrafluoroethylene film after the solvent has completely evaporated.

[0018] S52. A layer of SEBS is spin-coated onto the coil pattern after the polytetrafluoroethylene film has been removed as an encapsulation layer. After the solvent has completely evaporated, a wireless passive biaxial strain sensor is obtained.

[0019] Preferably, in step S1, the nested dual-loop antenna is configured as a nested dual-loop structure, including an outer loop antenna and an inner loop antenna;

[0020] The outer loop antenna is configured to use a resonant frequency f1 of 0.91 GHz, and the outer loop antenna has a strain sensitivity of 3.73 MHz / % in the x-axis direction and a strain sensitivity of 1 MHz / % in the y-axis direction.

[0021] The inner loop antenna is configured to use a resonant frequency f2 of 2.23 GHz, and the inner loop antenna has a sensitivity of 8.36 MHz / % to strain in the y-axis direction and a sensitivity of 1.46 MHz / % to strain in the x-axis direction.

[0022] Preferably, the length of the outer ring antenna along the x-direction (42.2 mm) is greater than the length along the y-direction (32 mm), and the ratio of the length in the y-direction to the length in the x-direction is 0.758, which is much greater than Poisson's ratio (approximately equal to 0.5). When the outer ring antenna is stretched along the x-axis, the inductance increases significantly by 17.3% (30% strain), and when the outer ring antenna is stretched along the y-axis, the inductance increases slightly by 2.6% (30% strain).

[0023] Furthermore, the outer ring antenna incorporates a cross-linked capacitance. When the outer ring antenna is stretched along the x-axis, the cross-linked capacitance area S increases, the gap d decreases, and C=εS / d. The overall capacitance increases significantly by 1.04% (30% strain).

[0024] When the outer ring antenna is stretched in the y-axis direction, the change in capacitance interdigitation is small, and the capacitance decreases slightly by 0.086% (30% strain).

[0025] In summary, when the outer ring antenna is stretched along the x-axis, both capacitance and inductance increase significantly, and the frequency drops; when stretched along the y-axis, the inductance increases slightly and the capacitance decreases slightly, forming self-compensation, and the frequency remains unchanged.

[0026] Preferably, the length of the inner ring antenna along the x-direction is 9.1 mm, which is less than the length along the y-direction of 21 mm, and the ratio of the length of the inner ring antenna along the x-direction to the length of the inner ring antenna along the y-direction is 0.433, which is close to Poisson's ratio (approximately equal to 0.5).

[0027] The inner loop antenna's inductance increases by 20.7% (30% strain) when stretched along the y-direction, while its inductance remains essentially unchanged (approximately 0.44 * 10⁻⁶). -7 H); the capacitance of the inner loop antenna is relatively small (approximately 1.15 * 10). -13 F), and the change in capacitance and inductance is small when the inner loop antenna is stretched bidirectionally, and this frequency change depends on the change in inductance.

[0028] Preferably, in step S2, the LM particles (liquid metal) are gallium-indium alloys, and the mass ratio of gallium to indium is 75.5:24.5; the surfactant is ethyl 3-mercaptopropionate, and during ultrasonic operation, the surfactant concentration is 0.1 mM, the time is 30 min, the power is 30%, the working time is 3 s, and the interval is 2 s; the elastomer polymer is styrene-ethylene / butene-styrene copolymer, i.e., SEBS solution, type H1221, and the mass ratio of SEBS particles to organic solvent in the SEBS solution is SEBS:n-hexane:toluene = 1:1:4; the mass ratio of the LM particles to the SEBS in the SEBS solution is 6:1-8:1; the magnetic stirring time is 30-60 min, and the rotation speed is 300-600 rpm.

[0029] Preferably, in step S31, the coating is performed using an adjustable scraper with a scraper height of 50 μm; the volume of the LM ink is 3-5 mL; the glass slide is 20 cm long, 10 cm wide, and 5 mm thick; and the solvent has a complete evaporation time of more than 12 h.

[0030] Preferably, in step S32, the laser cutting parameters are: speed 100-500 mm / s, power 50-80%, frequency 30-50 kHz, and number of revolutions: 20-100.

[0031] Preferably, in step S4, the laser is an ultraviolet laser; the laser scanning range should completely encompass the wire pattern; the laser activation parameters are a speed of 500 mm / s, a power of 40-60%, a frequency of 40-50 kHz, and a rotation number of 1.

[0032] Preferably, in step S5, the SEBS base film and SEBS encapsulation are prepared by spin coating of SEBS solution, the mass ratio of each component in the solution is SEBS:n-hexane:toluene = 1:1:2 or 1:1:3, the spin coating speed is 300-500 rpm, and the time is 10 s; the solvent evaporation time is more than 12 h.

[0033] The beneficial effects of this invention are reflected in:

[0034] This invention provides a method that, for the first time, uses a single integrated antenna structure to simultaneously and independently measure strain in two orthogonal directions (x-axis and y-axis) without any batteries, chips, or complex external circuitry. Its innovative nested dual-loop antenna design utilizes geometric anisotropy, allowing the two resonant frequencies to specifically respond to strain in different directions, thus physically avoiding signal crosstalk (<5%). This overcomes the technical bottleneck of existing wireless multi-axis sensors that rely on complex resistor networks, physical wiring, or external decoding circuits. This provides a completely new solution for the accurate monitoring of complex vector deformations in flexible, confined spaces. Attached Figure Description

[0035] Figure 1 This is a dimension diagram of the nested double-loop antenna designed in this invention;

[0036] Figure 2 The image shows the nested double-loop antenna prepared in Example 1 attached to a human wrist.

[0037] Figure 3 The S11 curves are shown for different x-axis strain levels in Example 2.

[0038] Figure 4The linear fitting curves of strain degree and frequency shift under x-axis strain conditions in Example 2 are as follows: f1 shifts significantly to the left (sensitivity 3.73 MHz / %), while f2 remains stable (sensitivity 1 MHz / %).

[0039] Figure 5 The S11 curves are shown for different y-axis strain levels in Example 3.

[0040] Figure 6 The linear fitting curves of strain degree and frequency shift under y-axis strain conditions in Example 3 are as follows: f2 shifts significantly to the left (sensitivity 8.36 MHz / %), and f1 shifts slightly to the left (sensitivity 1.46 MHz / %). Detailed Implementation

[0041] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of them. Unless otherwise specified, the embodiments and features in the embodiments of this application can be combined with each other. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0042] like Figure 1-6 As shown:

[0043] This invention provides a method for fabricating a wireless passive biaxial strain sensor based on a nested double-loop antenna. The principle of the method is as follows: The designed nested double-loop antenna system consists of inner and outer loops, which can generate two decoupled resonant frequencies. According to the formula f ∝ 1 / √LC, the capacitance and inductance characteristics are controlled through structural design (the length of the outer loop antenna in the x-direction (42.2 mm) is slightly longer than its length in the y-direction (32 mm), and the ratio of the length in the y-direction to the length in the x-direction is 0.758, which is much greater than Poisson's ratio (approximately 0.5). When the outer loop is stretched in the x-axis direction, the inductance increases significantly, while when stretched in the y-axis direction, the inductance increases slightly. Simultaneously, the outer loop antenna introduces a cross-linked capacitor. When stretched in the x-axis direction, the area s of the cross-linked capacitor increases, the gap d decreases, C=εS / d, and the overall capacitance increases significantly. When stretched in the y-axis direction, the change in the cross-linked capacitor gap is small, and the capacitance decreases slightly.

[0044] In summary, when the outer loop antenna is stretched along the x-axis, both capacitance and inductance increase significantly, and the frequency drops. When stretched along the y-axis, the slight increase in inductance and slight decrease in capacitance create self-compensation, and the frequency remains unchanged. The inner loop antenna has a length of 9.1 mm in the x-axis and 21 mm in the y-axis, with x / y = 0.433, close to Poisson's ratio (approximately 0.5). Stretching in the y-axis results in a significant increase in inductance, while stretching in the x-axis remains essentially unchanged. The inner loop capacitance is small, and its change is not significant during bidirectional stretching; this frequency change depends on the change in inductance. This circuitless design achieves a specific response of the outer loop antenna to x-axis strain and the inner loop antenna to y-axis strain, enabling crosstalk-free bidirectional strain monitoring.

[0045] In practical applications, such as Figure 1 The diagram shows the dimensions of the nested double-loop antenna. L1 represents the vertical length of the outer loop antenna, which is 42.2 mm. W1 represents the horizontal length of the outer loop antenna, which is 32 mm. L2 represents the vertical length of the inner loop antenna, which is 9.1 mm. W2 represents the horizontal length of the inner loop antenna, which is 21 mm.

[0046] Example 1

[0047] First, 0.5 mL of LM was injected into 15 mL of 0.1 mM ethyl 3-mercaptopropionate ethanol solution, and the mixture was sonicated for 30 min at 30% power (20 kHz) using an ultrasonic cell disruptor to form a uniform suspension. Then, the suspension was washed three times by centrifugation with anhydrous ethanol to obtain solid LM particle precipitate.

[0048] Next, SEBS solutions (SEBS:n-hexane:toluene = 1:1:2 and 1:1:4) were prepared. 5g of SEBS elastomer particles were dissolved in mixed solvent 1 (composed of 5g n-hexane and 10g toluene) and mixed solvent 2 (composed of 5g n-hexane and 20g toluene), respectively. The solutions were then treated in an ultrasonic cleaner at 100% power (50 kHz) for 6 hours until complete dissolution, yielding transparent and homogeneous SEBS solutions (1:1:2 and 1:1:4).

[0049] Then, the LM particles obtained above are mixed with the SEBS solution (1:1:4) at a mass ratio of 8:1 and magnetically stirred at 500 rpm for 40 minutes to ensure that the LM particles are uniformly dispersed in the SEBS matrix, forming the final composite ink.

[0050] Patterning and activation followed. A PTFE film was fixed onto PET, and composite ink was coated onto a 50 μm thick film using a doctor blade. After the solvent evaporated for 12 hours, a solid film was formed. The film was then laser-cut using a 355 nm UV laser at a scanning speed of 500 mm / s, a frequency of 30 kHz, and 80% power. After 20 repeated scans, a nested double-loop antenna pattern was formed. After removing excess material, the pattern was fully scanned and activated using a 40% power UV laser to transform it from an insulating state to a conductive state.

[0051] Finally, the process involves transfer and encapsulation. A SEBS solution (1:1:2) is spin-coated onto a silicon wafer, and a PTFE film with an antenna pattern is pressed onto this film, side down. After curing for 12 hours, the PTFE is peeled off, completing the pattern transfer. Another layer of SEBS solution (1:1:2) is then spin-coated and cured for 12 hours to achieve double-sided SEBS encapsulation, resulting in the final biaxial strain sensor.

[0052] Example 2

[0053] The vector strain sensor prepared in Example 1 was attached to the stretching platform of the tensile testing instrument using PI tape, ensuring that the stretching direction was strictly aligned with the x-direction of the antenna structure (error <1°). An x-axis strain was applied. The readout antenna was connected to a vector network analyzer (SVA1075X, scanning band 0–3 GHz), and its S11 parameters were monitored in real time via near-field coupling between the loop antenna and the sensing antenna. The tensile testing instrument was controlled using Kinese software (strain 0%–30%, step size 5%), with each strain step held for 10 s to eliminate hysteresis. The S11 parameters of the sensing antenna were recorded. Figure 3 As shown: (Under x-axis stretching conditions, f1 shifts from 1.017 GHz to 0.905 GHz with a sensitivity of 3.73 MHz / %, and f2 shifts from 2.472 GHz to 2.442 GHz with a sensitivity of only 1 MHz / %.)

[0054] Example 3

[0055] The vector strain sensor prepared in Example 1 was attached to the stretching platform of the tensile tester using PI tape, ensuring that the stretching direction was strictly aligned with the y-direction of the antenna structure (error <1°). A y-axis strain was applied. The readout antenna was connected to a vector network analyzer (SVA1075X, scanning band 0–3 GHz), and its S11 parameters were monitored in real time via near-field coupling between the loop antenna and the sensing antenna. The tensile tester was controlled using Kinese software (strain 0%–30%, step size 5%), with each strain step held for 10 s to eliminate hysteresis. The S11 parameters of the sensing antenna were recorded. Figure 5As shown: (Under y-axis stretching conditions, f2 has a sensitivity of 8.36 MHz / % from 2.416 GHz to 2.165 GHz, and f1 has a sensitivity of only 1.46 MHz / % from 1.007 GHz to 0.963 GHz).

[0056] As can be seen from the above method and the preparation and testing processes of Examples 1, 2, and 3 applying this method, the wireless passive biaxial strain sensor obtained by the method provided by this invention achieves completely wireless sensing signal reading without the need for a built-in battery or physical wires. It can simultaneously decouple and distinguish strain in two orthogonal directions (x-axis and y-axis), eliminating the measurement limitations of single-axis sensors. It ensures that the sensor as a whole has good stretchability to adapt to the dynamic deformation of complex curved surfaces such as human skin, joints, or soft robots, and can solve the signal crosstalk problem commonly found in multi-axis sensors, improving measurement accuracy and reliability.

[0057] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for fabricating a wireless passive biaxial strain sensor based on a nested dual-loop antenna, characterized in that, The preparation method includes the following steps: S1. Design of nested dual-loop antenna: The antenna was designed and simulated using HFSS software, and the antenna pattern was saved. Preparation of S2 and LM inks: LM ink is obtained by mixing LM particles obtained by ultrasonication of surfactant with SEBS solution and then stirring magnetically. Patterning of S3 and LM inks: S31. Apply LM ink to a polytetrafluoroethylene substrate by scraping. Place the applied LM ink in a fume hood and transfer the ink and substrate onto a glass slide after the solvent has completely evaporated. S32. Using an ultraviolet laser and based on the preserved antenna pattern, cut out the required coil pattern from the ink and substrate transferred to the glass slide, and remove the excess ink and substrate outside the coil pattern; Activation of S4 and LM inks: The coil pattern formed by LM ink is activated by laser scanning, and the coil pattern changes from an insulating state to a conductive state; S5. Coil transfer and packaging: S51. Imprint the coil pattern onto the SEBS base film, and remove the polytetrafluoroethylene film after the solvent has completely evaporated. S52. A layer of SEBS is spin-coated onto the coil pattern after the polytetrafluoroethylene film has been removed as an encapsulation layer. After the solvent has completely evaporated, a wireless passive biaxial strain sensor is obtained. In step S1, the nested dual-loop antenna is configured as a nested dual-loop structure, including an outer loop antenna and an inner loop antenna; The outer loop antenna is configured to use a resonant frequency f1 of 0.91 GHz, and the outer loop antenna has a strain sensitivity of 3.73 MHz / % in the x-axis direction and a strain sensitivity of 1 MHz / % in the y-axis direction. The inner loop antenna is configured to use a resonant frequency f2 of 2.23 GHz, and the inner loop antenna has a sensitivity of 8.36 MHz / % to strain in the y-axis direction and a sensitivity of 1.46 MHz / % to strain in the x-axis direction.

2. The method for fabricating a wireless passive biaxial strain sensor based on a nested double-loop antenna according to claim 1, characterized in that, The length of the outer ring antenna along the x-direction is greater than the length along the y-direction, and the ratio of the length in the y-direction to the length in the x-direction is 0.

758. When the outer ring antenna is stretched along the x-axis, the inductance increases by more than 17.3%, and when the outer ring antenna is stretched along the y-axis, the inductance increases by less than 2.6%.

3. The method for fabricating a wireless passive biaxial strain sensor based on a nested double-loop antenna according to claim 2, characterized in that, The length of the inner loop antenna along the x-direction is less than its length along the y-direction, and the ratio of the length of the inner loop antenna along the x-direction to its length along the y-direction is 0.433; the tensile inductance of the inner loop antenna along the x-direction is 0.44 * 10⁻⁶. -7 H.

4. The method for fabricating a wireless passive biaxial strain sensor based on a nested double-loop antenna according to claim 2, characterized in that, The outer ring antenna incorporates a cross-linked capacitor. When the outer ring antenna is stretched along the x-axis, the area S of the cross-linked capacitor increases, the gap d decreases, C=εS / d, and the capacitance increases by more than 1.04%. When the outer ring antenna is stretched along the y-axis, the capacitance decreases by less than 0.086%. The capacitance of the inner ring antenna is 1.15*10. -13 F.

5. The method for fabricating a wireless passive biaxial strain sensor based on a nested double-loop antenna according to claim 2, characterized in that, In step S2, the LM particles are gallium-indium alloys with a gallium to indium mass ratio of 75.5:24.5; the surfactant is ethyl 3-mercaptopropionate, and during ultrasonic operation, the surfactant concentration is 0.1 mM, the ultrasonic time is 30 min, the power is 30%, the working time is 3 s, and the interval is 2 s; the elastomer polymer is styrene-ethylene / butene-styrene copolymer, i.e., SEBS solution, type H1221; the mass ratio of SEBS particles to organic solvent in the SEBS solution is SEBS:n-hexane:toluene = 1:1:4; the mass ratio of the LM particles to the SEBS in the SEBS solution is 6:1-8:1; the magnetic stirring time is 30-60 min, and the rotation speed is 300-600 rpm.

6. The method for fabricating a wireless passive biaxial strain sensor based on a nested double-loop antenna according to claim 1, characterized in that, In step S31, the coating is performed using an adjustable scraper with a blade height of 50 μm; the volume of the LM ink is 3-5 mL; the glass slide is 20 cm long, 10 cm wide, and 5 mm thick; and the solvent has a complete evaporation time of more than 12 hours.

7. The method for fabricating a wireless passive biaxial strain sensor based on a nested double-loop antenna according to claim 1, characterized in that, In step S32, the laser cutting parameters are: speed 100-500 mm / s, power 50-80%, frequency 30-50kHz, and number of revolutions: 20-100.

8. The method for fabricating a wireless passive biaxial strain sensor based on a nested double-loop antenna according to claim 1, characterized in that, In step S4, the laser is an ultraviolet laser; the laser scanning range should completely include the wire pattern; the laser activation parameters are a speed of 500 mm / s, a power of 40-60%, a frequency of 40-50 kHz, and a rotation number of 1.

9. The method for fabricating a wireless passive biaxial strain sensor based on a nested double-loop antenna according to claim 1, characterized in that, In step S5, the SEBS base film and SEBS encapsulation are prepared by spin coating of SEBS solution. The mass ratio of each component in the solution is SEBS:n-hexane:toluene = 1:1:2 or 1:1:

3. The spin coating speed is 300-500 rpm and the time is 10 s. The solvent evaporation time is more than 12 h.

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