Flexible noninvasive blood glucose sensing antenna integrated with electromagnetic band gap and double-split resonant ring

Abstract

The invention relates to a flexible noninvasive blood glucose sensing antenna integrated with an electromagnetic band gap and a double-split resonant ring, which adopts a flexible substrate and a coplanar waveguide feed network, and integrates an electromagnetic band gap array structure formed by periodic metal through holes on a ground plane; two symmetrically-distributed double-split resonant rings and parasitic branches for impedance matching are constructed at the top of the sensing antenna.

Description

Technical Field

[0001] This invention belongs to the field of biomedical sensing technology, specifically relating to a wearable flexible microwave sensing antenna sensor and its structure for non-invasive and continuous monitoring of human physiological information (especially blood glucose concentration). Background Technology

[0002] Non-invasive blood glucose monitoring (NIGM) is an urgent clinical need for millions of diabetic patients worldwide, aiming to replace traditional invasive blood collection methods that are painful and inconvenient. Microwave sensing technology, with its high sensitivity to changes in the dielectric properties of biological tissues, has become one of the key technological pathways for achieving non-invasive monitoring. Its basic principle is that physiological fluctuations in blood glucose concentration cause corresponding changes in the dielectric constant and conductivity of tissues. This change can be captured by a flexible microwave sensing antenna closely attached to the skin surface, manifesting as a measurable shift in its S11 reflection parameters, amplitude, phase, and other resonant characteristics.

[0003] However, most existing microwave sensing devices use FR4 rigid substrates, which presents two major challenges in practical applications. Firstly, there is poor conformal fit. The human body surface (such as the wrist) has complex physiological curvatures, making it difficult for rigid sensors to achieve a tight, gapless fit. This easily leads to air gaps and motion artifacts, severely affecting the stability and signal-to-noise ratio of the sensing signal. Secondly, there are issues with wearing comfort and biosafety. Rigid structures are unsuitable for prolonged and dynamic wear, and energy distribution is difficult to control, potentially posing a risk of electromagnetic exposure to non-target tissues.

[0004] In addition, fluctuations in ambient temperature are another key bottleneck affecting monitoring accuracy. Temperature changes nonlinearly alter the dielectric constant of tissues by changing factors such as skin moisture content and blood flow velocity. The resulting signal drift may even mask weak signal characteristics caused by changes in blood glucose levels, posing a serious challenge to the accuracy and reliability of monitoring.

[0005] Therefore, there is an urgent need in this field for a new type of sensor that not only has excellent flexibility to achieve conformal fit with the skin, but also needs to improve the sensitivity and anti-interference ability of signal acquisition through ingenious structural design, so as to provide a solid sensing front-end foundation for achieving high-precision non-invasive blood glucose monitoring. Summary of the Invention

[0006] This invention provides a flexible, non-invasive blood glucose sensing antenna that integrates an electromagnetic bandgap array structure and a double-split resonant ring, aiming to improve the sensor's signal sensitivity, anti-interference capability, and reliability for wearable applications.

[0007] The technical solution of the present invention is as follows: A flexible, non-invasive blood glucose sensing antenna integrating an electromagnetic bandgap and a double-split resonant ring is provided. The antenna uses a flexible substrate and is characterized by employing a coplanar waveguide feeding network to integrate an electromagnetic bandgap array structure composed of periodic metal vias on the ground plane. Two symmetrically distributed double-split resonant rings and parasitic stubs for impedance matching are constructed on the top of the sensing antenna.

[0008] Furthermore, a rectangular feed pad 10 is provided in the middle of the base area of ​​the flexible non-invasive blood glucose sensing antenna. Two independent, uniformly sized circular units are symmetrically arranged on both sides of the rectangular feed pad 10 to form an electromagnetic bandgap array 6 structure. A trapezoidal transition region 5 is formed by smoothly contracting upward from the rectangular feed pad 10. A T-shaped section 9 is symmetrically arranged at the upper part of the trapezoidal transition region 5. Two T-shaped beams 4 are symmetrically arranged upward at both ends of the T-shaped section 9. The two T-shaped beams 4 are respectively connected to two identical split resonant rings located at the top through two symmetrically arranged parallel vertical short arms 8. Each split resonant ring consists of two concentric slotted rings with opposite opening directions, one slotted ring being the inner ring 7 and the other slotted ring being the outer ring 2.

[0009] Furthermore, matching short branches 3 are provided on the upper edges of both T-shaped crossbeams 4.

[0010] Furthermore, each split resonant ring is provided with a resonant ring protrusion 1 at its top.

[0011] Furthermore, the design dimensions, in mm, are as follows: the height of the electromagnetic bandgap array is 2, the radius of the circular unit constituting the electromagnetic bandgap array is 0.8, and the center-to-center distance between two circular units is 1. The width of the rectangular feed pad is 4, and the thickness of the rectangular feed pad is 0.5; the length of the main transmission line, from the bottom of the trapezoidal transition zone to the bottom of the T-section, is 24.6, and the length of the main transmission line to the top of the T-section, from the bottom of the trapezoidal transition zone to the top of the T-section, is 27.3. The distance between the two split resonant rings at the top and the two symmetrically distributed T-shaped beams is 3.7, and the width of the two symmetrically distributed parallel vertical short arms is 0.37; the width of the connection point of the T-section is 3.52. For each split resonant ring, the outer radius of the outer ring is 2, the inner radius of the outer ring is 1, the outer radius of the inner ring is 1.76, and the inner radius of the inner ring is 0.74.

[0012] Furthermore, the matching short branch thickness of the T-shaped beam is 0.53.

[0013] Furthermore, the flexible substrate is a polyimide film; the sensing antenna is statically bent, with a minimum bending radius of 1.1 mm. The design of the bending area, namely the main transmission line and the resonant ring sensing head, follows the flexible circuit board (FPC) bending specifications: 1) the bending axis is parallel to the long axis, and the traces are parallel to the bending axis; 2) no rigid components or electroplated through holes are placed in the bending area, and the resonant ring sensing head adopts a line pattern.

[0014] Furthermore, the overall size of the sensing antenna is 26 × 36 mm. 2 The overall thickness is 0.11 mm. Attached Figure Description

[0015] Figure 1 A schematic diagram of the geometric structure of a sensing antenna sensor.

[0016] Figure 2 A schematic diagram of the bending performance of the sensor antenna: (a) is a front view of the sensor bending, (b) is a side view of the sensor bending, and (c) is a rear view of the sensor bending.

[0017] Figure 3 A schematic diagram of the sensor's wearing position and its simulation performance curve.

[0018] Figure 4 Simulation diagram of electromagnetic field distribution of sensor at resonant frequency, (a) electric field, (b) current, (c) magnetic field. Figure 5 The performance evaluation comparison chart of the sensor in the in-situ test is shown in (a) as the root mean square error (RMSE) and (b) as the mean absolute relative error (MARD).

[0019] Figure 6 DTS error raster analysis plot of in vivo test results.

[0020] Figure 7 Specific Absorption Rate (SAR) simulation analysis of sensors in wearable applications.

[0021] The annotations in the attached figures are explained as follows: 1: Resonant ring protrusion; 2: Outer ring of the split resonant ring; 3: Matching short branch of the T-shaped beam; 4: T-shaped beam; 5: Trapezoidal transition region; 6: Electromagnetic bandgap array; 7: Inner ring of the split resonant ring; 8: Two symmetrically distributed parallel vertical short arms; 9: T-shaped section; 10: Rectangular feed pad Detailed Implementation

[0022] The present invention will be further described below with reference to the accompanying drawings and specific embodiments.

[0023] The core of the sensing antenna of this invention is a standard rectangular 26 × 36 mm piece. 2 On a flexible polyimide (PI) substrate, a single-piece copper conductive pattern, mirror-symmetrical along the central longitudinal axis, is fabricated using precision photolithography. Polyimide not only enables sensors to conform to the complex curves of the human body due to its excellent flexibility, but also provides an ideal platform for carrying high-frequency sensing circuits and ensuring wearing safety due to its extremely low dielectric loss, stable chemical properties, and excellent biocompatibility in the microwave frequency band.

[0024] The pattern starts at the bottom edge of the substrate, which is the base area for signal access and initial conditioning: at the bottom is a horizontal rectangular feed pad 10 ground plane, whose width forms a good electrical connection and impedance matching initial transition with external measuring equipment (such as the coaxial cable of a vector network analyzer), serving as the only port for microwave signal injection.

[0025] On either side of the feed pad 10, a symmetrical and closely arranged array of independent, uniformly sized circular units forms the surface feature of the electromagnetic bandgap array 6 structure on the plane. This periodic array is a calculated "band-stop filter" for suppressing electromagnetic surface waves in a specific frequency band. It introduces periodic discontinuities into the conventional ground plane, making it equivalent to a high-impedance surface in the target frequency band, thereby strongly hindering unnecessary leakage and crosstalk of electromagnetic energy along the sensor-skin contact surface. This deployment aims to more effectively constrain and guide signal energy to the upward main transmission path from the source, providing a clean, high signal-to-noise ratio signal environment for subsequent accurate sensing.

[0026] As the signal travels upward from the center, the copper foil pattern first undergoes a smooth, contracting trapezoidal transition region 5. This trapezoidal structure acts as an impedance transformer in the microwave circuit, enabling the signal to smoothly transition from a wider feed port to a narrower main transmission line. This greatly reduces signal reflections that may be caused by abrupt changes in size, ensuring the continuity and efficiency of energy transmission.

[0027] After the trapezoidal transition region 5, as this energy channel extends to the sensing core area near the top of the substrate, the geometry of the pattern becomes complex, forming two symmetrically distributed split resonant rings 2 for energy distribution and field enhancement: the end point of the main transmission line is a transverse rectangular strip, forming a standard T-shaped section 9, which is a passive power distribution network whose function is to accurately divide the input single microwave signal into two signals with equal amplitude and the same phase.

[0028] At the left and right ends of the T-section 9, two shorter, independent, symmetrically distributed T-shaped beams 4 extend horizontally outwards. Each T-shaped beam 4 has a matching short branch 3 designed at its upper edge to achieve precise resonance. In microwave theory, the resonator itself exhibits reactive characteristics. These two short branches, by introducing specific susceptance, precisely cancel the reactive component of the resonant head, thus ensuring that the entire resonant ring sensing head (the part of the sensing antenna above the T-section is called the resonant ring sensing head) presents a pure 50-ohm real impedance to the feed line at the operating frequency, achieving perfect matching and eliminating energy reflection. After power distribution and impedance matching, the two signals are guided by two parallel vertical short arms 8 that symmetrically rise from the upper edges of the two T-shaped beams 4, and finally injected into the two identical double-split resonant rings that form the core of the sensing function.

[0029] Each split resonant ring consists of two concentric slotted rings with opposite opening directions. The inner ring 7 is smaller, with its opening on the right, while the outer ring 2 is larger, with its opening on the left, and is directly connected to the parallel vertical short arm 8 supporting it. Each split resonant ring has a resonant ring protrusion 1 at its top. This special geometric arrangement allows for a very strong capacitance effect at the gap between the openings of the two rings during resonance, highly confining and dramatically amplifying the surrounding electric field energy, forming a highly localized "electromagnetic hotspot."

[0030] This localized electric field, enhanced by its geometric structure, penetrates the skin surface perpendicularly, resembling a miniature dipole probe. It is extremely sensitive to changes in the dielectric properties of subcutaneous tissue fluid. This sensitive probe deeply couples with the adjacent biological tissue. Any perturbation to the tissue's dielectric properties caused by changes in blood glucose concentration significantly alters the equivalent capacitance of the gap, directly leading to a measurable drift in the resonant frequency of the entire resonator. This drift ultimately manifests as a lateral shift of the S11 reflection parameter resonance valley point on a vector network analyzer. This enables a precise sensing process that transforms invisible physiological and chemical changes into accurately captured and quantified electromagnetic signals.

[0031] The design process is as follows: A preliminary flexible sensing antenna sensor structure was constructed, using commercially available polyimide (PI) film with excellent dielectric properties as the substrate. After optimization, the overall sensor size was determined to be 26 × 36 mm. 2 The overall thickness is 0.11 mm.

[0032] Design a coplanar waveguide (CPW) feed network and construct two symmetrically distributed improved circular split resonant rings and parasitic stubs for impedance matching on top of the sensing antenna. Simultaneously, integrate an electromagnetic bandgap array structure composed of periodic metal vias on the ground plane. For example... Figure 1 As shown.

[0033] To achieve optimal sensing performance, the precise geometric dimensions of the sensing antenna of this invention were ultimately determined through repeated iterations and parameter optimization using electromagnetic simulation software. The specific process is as follows: a complete three-dimensional model of the sensing antenna is established in the simulation software, accurately reproducing the geometric details, including the base, feed line, resonator, and EBG array.

[0034] In the model, the substrate material is set to polyamide (PI), and it is assigned a corresponding dielectric constant and loss tangent value; the conductive layer is set to copper, and it is assigned a corresponding conductivity. Secondly, to simulate the actual working environment, a waveport excitation is set at the feed port of the model, and open boundary conditions are set to simulate free-space radiation.

[0035] The core objective of the optimization is to enable the sensing antenna to generate a deep, sharp resonance near 2.4 GHz under no-load conditions, while ensuring miniaturization and flexibility, and to possess good impedance matching characteristics (i.e., the S11 reflection parameter is much less than -10 dB). To this end, extensive parametric scanning analysis was conducted on key geometric parameters affecting the sensing antenna performance, such as the radius and slot width of the split resonator, the width of the feed line, the length and position of the matching stubs, and the size and spacing of the electromagnetic bandgap array.

[0036] By systematically changing the values ​​of one or more parameters and observing their effects on resonant frequency, resonant depth, and input impedance, an optimal solution was ultimately found in the multidimensional parameter space using the software's built-in optimizer algorithm. This ensured that the sensing antenna maintained excellent performance both under no-load conditions (resonating at approximately 2.4 GHz) and after loading a human arm model (resonance shifted to approximately 1.9 GHz). The final dimensions described in this specification represent the final results of the electromagnetic simulation and optimization process, determining the key geometric dimensions of the sensing antenna. The detailed dimensions (unit: mm) of each part of the sensing antenna were ultimately determined as follows: the total length of the overall sensor profile is 36 mm, and the total width is 26 mm. In the base region, the height of the electromagnetic bandgap array is 2 mm, and the radius of the circular element constituting the electromagnetic bandgap array is 0.8 mm. In the feeding structure, the rectangular feeding pad has a width of 4 and a thickness of 0.5. The main transmission line length (from the bottom of the trapezoidal transition zone to the bottom of the T-section) is 24.6, and the length from the main transmission line to the top of the T-section (from the bottom of the trapezoidal transition zone to the top of the T-section) is 27.3. The distance between the two improved circular split resonant rings at the top and the two symmetrically distributed T-shaped crossbeams is 3.7. The matching short branches of the T-shaped crossbeams have a thickness of 0.53, and the width of the two symmetrically distributed parallel vertical short arms is 0.37. The width of the connection point of the T-section is 3.52. As the core sensing unit, the double-split resonant ring has an outer ring outer radius of 2 and an inner ring inner radius of 1, an outer ring outer radius of 1.76, and an inner ring inner radius of 0.74.

[0037] To ensure that the sensor described in this invention can achieve a close conformal fit with the human body surface (such as the wrist), the sensing antenna of this invention is 26 × 36 mm. 2 The FPC structure has undergone rigorous bending performance design. The total thickness of the FPC is approximately 0.11 mm, and its application scenario is "static bending," meaning a single bend to fit the wearing position, rather than dynamic repeated bending. According to the IPC-2223 industry standard, the minimum bending radius for static bending should be greater than 10 times the total thickness of the FPC. Therefore, the minimum allowable bending radius of the FPC structure of the sensing antenna of this invention is 1.1 mm. To achieve this flexibility standard, the sensor's design in the bending area (mainly the main transmission line and resonant ring sensing head) strictly follows the FPC bending specifications: 1) The bending axis is parallel to the 36 mm long axis (i.e., the direction of the main transmission line), and all critical traces are parallel to the bending axis (i.e., perpendicular to the bending direction) to minimize copper foil stress; 2) No rigid components or electroplated vias are placed in the bending area, and the sensing area uses linear patterns instead of solid copper plating to ensure sufficient substrate flexibility. These designs ensure that the sensor maintains excellent electrical performance and structural reliability even with a bending radius of 1.1 mm or greater.

[0038] Simulations were performed on the sensing antenna at a resonant frequency of 2.4 GHz. The results show that the electric field strength at the SRR slot is significantly enhanced, reaching a peak value exceeding 7000 V / m, verifying its highly localized energy sensing mechanism. Figure 4 As shown.

[0039] The sensing antenna was loaded onto a simplified three-layer (skin-fat-muscle) human arm model. Simulation results show that the sensing antenna has a resonance depth of -31 dB near 2.4 GHz under no-load conditions; after loading the human model, the main resonant frequency shifts to approximately 1.9 GHz, and the resonance depth becomes -16 dB, while still maintaining good impedance matching, proving its suitability for surface sensing applications, such as... Figure 3 As shown.

[0040] The sensing antenna of this invention was worn above the radial artery in the wrist of 10 healthy subjects and connected to a vector network analyzer (VNA) via a high-frequency cable. The VNA was set to a frequency of 1 GHz-10 GHz, and a commercial blood glucose meter (ACCU-CHEK® Guide) was used as a reference. An oral glucose tolerance test (OGTT) was performed and data was collected synchronously. During the complete dynamic response monitoring cycle, a full set of synchronous data was collected every 3 minutes, including the S11 spectrum and fingertip reference blood glucose value.

[0041] The process of building the prediction model is as follows: S1, Data Processing: First, the raw S11 spectral data was smoothed and denoised using a Savitzky-Golay filter. Then, to capture comprehensive blood glucose response information, S11 features were extracted from the main resonant frequency band (1-2 GHz) and an auxiliary sensitive frequency band (5-8 GHz). Finally, principal component analysis (PCA) was applied to reduce the dimensionality of the extracted high-dimensional S11 features, generating a microwave feature vector composed of 10 principal components (PCs), which retained over 95% of the original data variance. The collected skin temperature (… ) and ambient temperature ( ) are directly used as features, and the difference between the two is calculated ( Together, they form a temperature feature vector with three dimensions.

[0042] S2, Forming the Interactive Feature Vector: The 10-dimensional microwave feature vector obtained in the previous step is multiplied element-wise with the 3-dimensional temperature feature vector. This operation generates a new interactive feature vector containing 30 (10 × 3) interaction terms. Each interaction term represents a nonlinear interaction between a specific principal component of the microwave signal and a specific temperature feature, enabling subsequent models to learn how temperature is "modulated" rather than simply "superimposed" on the microwave signal.

[0043] S3, Forming a Comprehensive Feature Vector: The three sets of feature vectors generated in the previous steps are concatenated to form a final comprehensive feature vector with a total dimension of 43 for model input. This vector consists of: 10 S11 principal component features, 3 basic temperature features, and 30 interactive features.

[0044] S4 employs a Gaussian process regression (GPR) nonparametric model for blood glucose prediction. A constructed 43-dimensional comprehensive feature vector is used as the model input, and synchronously collected reference blood glucose values ​​are used as the output to train the GPR model. A squared exponential kernel function with automatic correlation determination (ARD) is used, and its hyperparameters are optimized by maximizing the log-marginal likelihood function.

[0045] S5. After the model training is completed, in actual monitoring, the real-time collected S11 and temperature data are processed through the above S1-S3 steps to generate a 43-dimensional feature vector, which is then input into the trained GPR model. The model can then output a high-precision predicted blood glucose value and simultaneously provide a quantification of the uncertainty of the prediction result.

[0046] Through the above steps, the method proposed in this invention can effectively decouple the interference of temperature fluctuations on blood glucose monitoring signals. Results show that on the independent test set, the root mean square error (RMSE) of the system prediction is as low as 8.15 mg / dL, the mean absolute relative error (MARD) is 5.15%, and the coefficient of determination (R²) of the data is [missing information]. 2 The value is 0.918, such as Figure 5 As shown. In the DTS error raster analysis, 98.8% of the data points fell within the clinically accurate A region, as shown. Figure 6 As shown.

[0047] Simulation analysis shows that the peak average specific absorptivity (SAR) of the sensing antenna of this invention under normal operating conditions is 8.66 × 10⁻⁶ g of tissue. -3 W / kg, far below the internationally recognized safety limit, such as Figure 7As shown above, the results demonstrate that this flexible sensing antenna structure integrating an electromagnetic bandgap and a double-split resonant ring is highly feasible and effective for achieving high-precision, high-reliability, and safe non-invasive blood glucose monitoring.

[0048] The above description of the disclosed embodiments enables those skilled in the art to make or use this application. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of this application. Therefore, this application is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

[0049] The above are merely embodiments of this application and are not intended to limit the scope of this application. Various modifications and variations can be made to this application by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the scope of the claims of this application.

Claims

1. A flexible, non-invasive blood glucose sensing antenna integrating an electromagnetic bandgap and a double-split resonant ring, employing a flexible substrate, characterized in that, A coplanar waveguide feeding network is used to integrate an electromagnetic bandgap array structure composed of periodic metal vias on the ground plane; two symmetrically distributed double-split resonant rings and parasitic stubs for impedance matching are constructed on the top of the sensing antenna.

2. The flexible non-invasive blood glucose sensing antenna according to claim 1, characterized in that, The base area of ​​the flexible non-invasive blood glucose sensing antenna is provided with a rectangular feed pad (10) in the middle. Two independent circular units of uniform size are symmetrically arranged on both sides of the rectangular feed pad (10) to form an electromagnetic bandgap array (6) structure. The rectangular feed pad (10) is smoothly contracted upward to form a trapezoidal transition area (5). T-shaped sections (9) are symmetrically arranged on the upper part of the trapezoidal transition area (5). Two T-shaped beams (4) are symmetrically arranged at both ends of the T-shaped sections (9). The two T-shaped beams (4) are connected to the same two split resonant rings at the top through two parallel vertical short arms (8) arranged symmetrically. Each split resonant ring is composed of two concentric slotted rings with opposite opening directions. One slotted ring is the inner ring (7) and the other slotted ring is the outer ring (2).

3. The flexible non-invasive blood glucose sensing antenna according to claim 2, characterized in that, Both T-shaped crossbeams (4) have matching short branches (3) on their upper edges.

4. The flexible non-invasive blood glucose sensing antenna according to claim 2, characterized in that, Each split resonant ring has a resonant ring protrusion (1) at its top.

5. The flexible non-invasive blood glucose sensing antenna according to claim 2, characterized in that, The design dimensions are as follows (in mm): The height of the electromagnetic bandgap array is 2; the radius of the circular unit constituting the electromagnetic bandgap array is 0.8; the center-to-center distance between two circular units is 1; the width of the rectangular feed pad is 4; the thickness of the rectangular feed pad is 0.5; the length of the main transmission line, from the bottom of the trapezoidal transition zone to the bottom of the T-section, is 24.6; the length of the main transmission line to the top of the T-section, from the bottom of the trapezoidal transition zone to the top of the T-section, is 27.3; the distance between the two split resonant rings at the top and the two symmetrically distributed T-shaped beams is 3.7; the width of the two symmetrically distributed parallel vertical short arms is 0.37; the width of the connection point of the T-section is 3.52; for each split resonant ring, the outer radius of the outer ring is 2, the inner radius of the outer ring is 1, the outer radius of the inner ring is 1.76, and the inner radius of the inner ring is 0.

74.

6. The flexible non-invasive blood glucose sensing antenna according to claim 3, characterized in that, The matching short branch thickness of the T-shaped beam is 0.

53.

7. The flexible non-invasive blood glucose sensing antenna according to any one of claims 1 to 6, characterized in that, The flexible substrate is a polyimide film; the sensing antenna is statically bent, with a minimum allowable bending radius of 1.1 mm. The design of the bending area, namely the main transmission line and the resonant ring sensing head, follows the flexible circuit board (FPC) bending specifications: 1) The bending axis is parallel to the long axis, and the traces are parallel to the bending axis; 2) No rigid components or electroplated through holes are placed in the bending area, and the resonant ring sensing head adopts a line pattern.

8. The flexible non-invasive blood glucose sensing antenna according to any one of claims 1 to 6, characterized in that, The overall size of the sensing antenna is 26 × 36 mm. 2 The overall thickness is 0.11 mm.

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