A highly sensitive and fast-response gradient hierarchical porous hydrogel, its preparation method, and its application in a piezoelectric sensor for blood pressure monitoring.

Abstract

This invention discloses a highly sensitive and fast-response gradient hierarchical porous hydrogel, its preparation method, and its application in a piezoelectric sensor for blood pressure monitoring. The method includes: S1. Precursor solution preparation: Weighing monomers, cationic crosslinking agents, and photoinitiators according to a specific ratio, and dissolving the monomers, cationic crosslinking agents, and photoinitiators in water to prepare a precursor solution. The monomers include acrylic acid and acrylamide. S2. Electric field induction treatment: Applying an electric field to the precursor solution to induce the cationic crosslinking agent to migrate and form a concentration gradient. S3. Photocrosslinking reaction: Placing the electric field-induced precursor solution under light irradiation to perform a crosslinking and curing reaction, thereby obtaining the gradient hierarchical porous hydrogel. The gradient hierarchical porous hydrogel exhibits ultra-high sensitivity and fast response in low-pressure regions. Integrating it into a wearable wristband system enables real-time, non-invasive monitoring of human arterial pulse signals.

Description

Technical Field

[0001] This invention relates to the field of piezoelectric sensor technology, specifically to a highly sensitive and fast-response gradient hierarchical porous hydrogel, its preparation method, and its application in a piezoelectric sensor for blood pressure monitoring. Background Technology

[0002] Hypertension has become a major cardiovascular disease worldwide and can lead to numerous health problems. Currently, clinical blood pressure (BP) measurement mainly relies on cuff-based devices, which are not suitable for wearable applications in daily life. With the rapid development of wearable electronic devices, wearable continuous healthcare monitoring technology has attracted increasing attention and provides a portable strategy for detecting abnormalities in the human body. Significant progress has been made in the materials and mechanisms of existing wearable sensors: for example, capacitive pressure sensors made of graphene / PDMS composite materials have improved sensitivity and linear response through the design of microstructured dielectric layers; MXene-based sensors (such as Ti3C2T) have also seen advancements. X Thin-film resistive pressure sensors achieve high sensitivity through high conductivity and interlayer slippage; stretchable conductive networks constructed using silver nanowires (AgNW) or liquid metal (EGaIn) are widely used in flexible electrocardiogram (ECG) electrodes and strain sensing units. However, these studies still face several limitations, such as poor biocompatibility with human skin and reliance on external or frequent power supply replacements, which restrict the long-term operational capability of the devices. The application of next-generation wearable electronic devices urgently requires self-powered flexible sensors with accurate sensing capabilities, high safety, and convenience. The design of self-powered sensors capable of directly harvesting ambient energy can avoid the use of bulky power devices, thereby greatly expanding the scope of practical applications.

[0003] Hydrogels, due to their high water content, excellent biocompatibility, and tunable mechanical and optical properties, have become important materials for wearable electronic devices. Their inherent softness and flexibility enable close and stable contact with dynamic biological surfaces. Furthermore, their ability to support ionic conductivity is advantageous for bioelectronic interfaces and physiological sensors. For example, Chinese patent application No. 201510870067.5 discloses a piezoelectric gel, its preparation method, and its applications. This piezoelectric gel has a porous structure, comprising an elastic gel substrate, a conductive polymer, and a neutral electrolyte solution completely permeated within the porous structure. The conductive polymer is loaded onto the elastic gel substrate. By permeating the conductive polymer into the elastic gel substrate, the physical conductivity of the piezoelectric gel is improved. When the piezoelectric gel is subjected to pressure, the conductive polymer undergoes p-type doping, causing its main chain to lose electrons. Simultaneously, negative ions are embedded, and the flow of electrons and negative ions generates an electric current, giving the piezoelectric gel its piezoelectric properties. However, the sensitivity and response speed of this piezoelectric gel to pressure are insufficient to clearly distinguish subtle characteristic peaks in pulse waves, making it unsuitable for clinical blood pressure monitoring.

[0004] In summary, there is an urgent need to develop a highly sensitive and fast-response hydrogel piezoelectric sensor that can clearly distinguish subtle characteristic peaks in pulse waves to achieve blood pressure monitoring. Summary of the Invention

[0005] The purpose of this invention is to provide a highly sensitive and fast-response gradient hierarchical porous hydrogel, its preparation method, and its application in a piezoelectric sensor for blood pressure monitoring.

[0006] In a first aspect, the present invention provides a method for preparing a highly sensitive and fast-responding gradient hierarchical porous hydrogel, comprising the following steps: S1. Preparation of precursor solution: Weigh the monomer, cationic crosslinking agent and photoinitiator according to the ratio, and dissolve the monomer, cationic crosslinking agent and photoinitiator in water to prepare a precursor solution, wherein the monomer includes acrylic acid and acrylamide; S2. Electric field induced treatment: An electric field is applied to the precursor solution to induce the cationic crosslinking agent to migrate and form a concentration gradient; S3. Photocrosslinking reaction: The precursor solution, which has been treated with an electric field, is placed under light to carry out a crosslinking and curing reaction to obtain the gradient hierarchical porous hydrogel.

[0007] Optionally, in step S1, the mass ratio of the monomer, cationic crosslinker, and photoinitiator is 80~120:0.8~1.2:1.6~2.4, and the mass ratio of acrylic acid and acrylamide in the monomer is 0.5~1.5:0.5~1.5.

[0008] Optionally, in step S1, the cationic crosslinking agent is 1-vinyl-3-butylimidazolium bromide, and the photoinitiator is ammonium persulfate.

[0009] Optionally, in step S2, the strength of the electric field is set to 2~4 V cm. –1 The induction treatment time is 5-15 min.

[0010] Optionally, in step S3, the illumination is ultraviolet light, with an intensity of 4~12 W and an illumination time of 10~60 min.

[0011] Secondly, the present invention provides a highly sensitive and fast-response gradient hierarchical porous hydrogel, which is prepared by the aforementioned method for preparing a highly sensitive and fast-response gradient hierarchical porous hydrogel.

[0012] Optionally, the gradient hierarchical porous hydrogel includes macroporous structures and microporous structures. The pore size of the macroporous structures is 10 μm to 30 μm, and the pore size of the microporous structures is 200 nm to 1 μm. The number of microporous structures increases in a gradient along the migration direction of the cationic crosslinking agent.

[0013] Thirdly, this invention provides an application of a highly sensitive and fast-response gradient hierarchical porous hydrogel in a piezoelectric sensor.

[0014] Optionally, the piezoelectric sensor based on the gradient hierarchical porous hydrogel can achieve a piezoelectric sensitivity of over 4000 mV / MPa at pressures below 120 kPa, with a piezoelectric sensitivity of S. P =|ΔV / ΔP|, where ΔV and ΔP correspond to the changes in output voltage and applied pressure, and the response time can reach less than 0.045 seconds.

[0015] Fourthly, the present invention provides an application of the aforementioned highly sensitive and fast-response gradient hierarchical porous hydrogel in a piezoelectric sensor for blood pressure monitoring.

[0016] In summary, the present invention has at least one of the following beneficial effects: This invention provides a highly sensitive and fast-response gradient hierarchical porous hydrogel, its preparation method, and its application in blood pressure monitoring. By utilizing the ion-specific effect induced by ammonium persulfate, the gel network is uniformly contracted to form a hierarchical porous structure. Combined with an electric field-induced cationic crosslinker migration strategy, a gradient distribution of crosslinking density is further introduced, thereby synergistically enhancing its piezoelectric properties. In subsequent embodiments, the gradient hierarchical porous hydrogel of this invention achieves an open-circuit voltage of 548.13 mV at a pressure of 277.78 kPa. The piezoelectric sensor based on this gradient hierarchical porous hydrogel achieves a piezoelectric sensitivity of over 4000 mV / MPa at pressures below 120 kPa, with a response time of less than 0.045 seconds, and exhibits good mechanical stability and cycle durability, significantly outperforming existing similar materials. Based on this hydrogel, this invention further developed a wearable wristband sensing system, successfully achieving high-fidelity acquisition of the radial artery pulse wave. Through an improved cuff calibration method, continuous and non-invasive blood pressure monitoring was achieved, clearly distinguishing characteristic peaks such as the main wave, tidal wave, and dicrotic wave. This invention provides new material design strategies and feasible technical solutions for next-generation flexible wearable health management devices that are high-performance, self-powered, and comfortable to wear. Attached Figure Description

[0017] Figure 1 The image shows a SEM image (left) of the gradient hierarchical porous hydrogel in Example 1 of this invention and a partial X-ray photoelectron spectroscopy elemental composition analysis image (right).

[0018] Figure 2 This is a SEM image of the homogeneous hierarchical porous hydrogel in Embodiment 2 of the present invention.

[0019] Figure 3 This is a schematic diagram of the apparatus for testing the piezoelectric properties of a hydrogel sensor according to Embodiment 1 of the present invention. Two gold-plated electrodes are attached tightly to the bottom of the gel, and a connection source is led out from the lower end of the electrodes. A cylindrical indenter with a diameter of 5 mm is fixed on the stretching machine.

[0020] Figure 4 These are stress-strain curves of the hydrogels in Examples 1 and 2a.

[0021] Figure 5 (a) is a piezoelectric performance diagram of the hydrogel of Example 1 pressed on the anode side and cathode side, and the hydrogel of Example 2a; (b) stress-strain curves of the hydrogel of Example 1 pressed on the anode side and cathode side repeatedly, with the filled area representing the measured range and the inner dashed line representing the average value.

[0022] Figure 6 This is the Young's modulus distribution of the gradient hierarchical porous hydrogel from the anode side to the cathode side in Example 1.

[0023] Figure 7This is a schematic diagram showing the results of compressive stress-strain simulation of the gradient hierarchical porous hydrogel in Example 1 on the cathode and anode sides.

[0024] Figure 8 This is a cyclic compressive stress-strain fatigue test of the hydrogel in Example 1.

[0025] Figure 9 The piezoelectric properties of the gradient hierarchical porous hydrogel of Example 1 are as follows: (a) output voltage under increasing cyclic pressure; (b) piezoelectric stability under low stress (1 kPa); (c) response time and recovery time test (under 60 kPa pressure); (d) piezoelectric cyclic stability under 30 kPa pressure: continuous loading for 700 s, with local magnification in the time ranges of 0–40 s and 660–700 s respectively.

[0026] Figure 10 These are fitting curves of the output voltage of the gradient multi-level porous hydrogel of Example 1 under increasing cyclic pressure when the anode side and cathode side are pressed, and the hydrogel of Example 2a.

[0027] Figure 11 The following illustrates the heart rate monitoring application of the gradient hierarchical porous hydrogel of Example 1: (a) The raw current signal of the sensor under pulse action. A partial magnified view shows two typical pulse cycles, where P, D, and T represent the main wave (P-wave), tidal wave (T-wave), and dicrotic wave (D-wave), respectively; (b) Extraction of the current signal from a single pulse cycle; (c) Overlap of raw signals (red area) and average results (black line), where the peak values ​​of P1 and P2 represent the sum of the forward traveling wave and the reflected waves from the hand and lower limb, minus the end-diastolic pressure; (d) Fast Fourier Transform results of the pulse signal; (e) A typical pulse response signal and its integrated signal, as well as the converted vascular pressure waveform, where the signal undergoes periodic knock waves (P) and dicrotic waves (D), with systolic blood pressure (SBP) and diastolic blood pressure (DBP) set to 60 mmHg and 110 mmHg, respectively; (f) The converted vascular pressure waveform, P... s P d and P m These represent the systolic blood pressure, diastolic blood pressure, and mean blood pressure within one cycle, respectively. Detailed Implementation

[0028] This invention provides a highly sensitive and fast-response gradient hierarchical porous hydrogel, its preparation method, and its application in a piezoelectric sensor for blood pressure monitoring. To make the objectives, technical solutions, and effects of this invention clearer and more explicit, the invention is further described in detail below. It should be understood that the specific embodiments described herein are only for explaining the invention and are not intended to limit the invention.

[0029] In recent years, continuous blood pressure monitoring technology based on pulse wave analysis has attracted widespread attention. Its core principle lies in extracting characteristic parameters (such as pulse wave transit time (PTT) and waveform characteristic K-value) from peripheral arterial pulse waves and estimating blood pressure values ​​through mathematical models. This method has the advantages of being non-invasive, continuous, and portable, but its accuracy is highly dependent on the performance of the pulse wave sensor. This invention innovatively proposes a highly sensitive and fast-response gradient hierarchical porous hydrogel and its preparation method. The piezoelectric sensor based on this gradient hierarchical porous hydrogel, due to its high sensitivity, can clearly distinguish subtle characteristic peaks in the pulse wave. Simultaneously, its rapid response enables the capture of rapid pulse rate changes and waveform details, perfectly meeting the performance requirements of blood pressure monitoring applications. Furthermore, its low power consumption and even self-powered capability meet the needs of portable devices for continuous blood pressure monitoring. This invention is based on this research.

[0030] In some embodiments of the present invention, a method for preparing a highly sensitive and fast-responding gradient hierarchical porous hydrogel is provided, comprising the following steps: S1. Preparation of precursor solution: Weigh the monomer, cationic crosslinking agent and photoinitiator according to the ratio, and dissolve the monomer, cationic crosslinking agent and photoinitiator in water to prepare a precursor solution, wherein the monomer includes acrylic acid and acrylamide; S2. Electric field induced treatment: An electric field is applied to the precursor solution to induce the cationic crosslinking agent to migrate and form a concentration gradient; S3. Photocrosslinking reaction: The precursor solution, which has been treated with an electric field, is placed under light to carry out a crosslinking and curing reaction to obtain the gradient hierarchical porous hydrogel.

[0031] In some embodiments of the present invention, in step S1, the mass ratio of the monomer, cationic crosslinker, and photoinitiator is 80~120:0.8~1.2:1.6~2.4, and the mass ratio of acrylic acid and acrylamide in the monomer is 0.5~1.5:0.5~1.5; preferably, the mass ratio of the monomer, cationic crosslinker, and photoinitiator is 90~110:0.9~1.1:1.8~2.2, and the mass ratio of acrylic acid and acrylamide in the monomer is 0.8~1.1:0.8~1.1.

[0032] In some embodiments of the present invention, in step S1, the cationic crosslinking agent is 1-vinyl-3-butylimidazolium bromide, and the photoinitiator is ammonium persulfate.

[0033] In some embodiments of the present invention, in step S2, the intensity of the electric field is set to 2~4 V cm. –1 The induction treatment time is 5-15 min.

[0034] In some embodiments of the present invention, in step S3, the illumination is ultraviolet light irradiation, the light intensity is 4~12 W, preferably 6~10 W, and the illumination time is 10~60 min, preferably 20~40 min.

[0035] In some embodiments of the present invention, a gradient hierarchical porous hydrogel is provided. The gradient hierarchical porous hydrogel includes macroporous and microporous structures. The pore size of the macroporous structures is 10 μm to 30 μm, and the pore size of the microporous structures is 200 nm to 1 μm. The number of microporous structures increases gradientally along the migration direction of the cationic crosslinking agent. The surface of the gradient hierarchical porous hydrogel with more microporous structures is defined as the cathode, and the surface with more macroporous structures is defined as the anode. The internal structure of the hydrogel is more porous on the anode side and more compact on the cathode side. The number of microporous structures increases gradientally from the anode side to the cathode side of the gradient hierarchical porous hydrogel. The N-type hydrogel uses imidazole cations. + Peak area quantification was used to determine the increase in cationic crosslinking agent from the anodic side to the cathode side of the gradient hierarchical porous hydrogel. Calculations showed that the Ni of the imidazole cation from the anodic side to the cathode side... + The integral area ratio of the N–C peak in acrylamide increased from 1:45.3 and 1:41.4 to 1:1.1, forming a distinct gradient structure.

[0036] In some embodiments of the present invention, the present invention provides an application of a highly sensitive and fast-response gradient hierarchical porous hydrogel in a piezoelectric sensor.

[0037] In some embodiments of the present invention, the piezoelectric sensor based on the gradient hierarchical porous hydrogel can achieve a piezoelectric sensitivity of over 4000 mV / MPa at pressures below 120 kPa, with a piezoelectric sensitivity of S. P =|ΔV / ΔP|, where ΔV and ΔP correspond to the changes in output voltage and applied pressure, respectively, and the response time can reach less than 0.045 seconds. The piezoelectric sensor includes the gradient hierarchical porous structure hydrogel and electrodes disposed on the surface of the gradient hierarchical porous structure hydrogel. The piezoelectric sensitivity is measured by applying test pressure to the anode side of the gradient hierarchical porous structure hydrogel.

[0038] In some embodiments of the present invention, the present invention provides an application of the aforementioned highly sensitive and fast-response gradient hierarchical porous hydrogel in a piezoelectric sensor for blood pressure monitoring. The blood pressure is based on the piezoelectric signal generated by the displacement of the tissue surface caused by pressure changes. A mathematical model is established to convert the piezoelectric pulse signal into a blood pressure wave. The piezoelectric response can be converted into vascular pressure (F) according to the following formula. BP ): .in Depending on the structure and material properties of the piezoelectric sensor, It is external resistance. This is the piezoelectric pulse signal from the device, and C is a correction coefficient related to the test subject's initial blood pressure (BP). That is, the vascular pressure F. BP It is equal to the product of the piezoelectric conversion coefficient 1 / αR and the definite integral of the piezoelectric pulse signal V(τ) over time t in the time interval [0,t], plus the system's initial zero-point bias constant C.

[0039] The present invention will be described in detail below with reference to embodiments and comparative examples. Unless otherwise specified, all raw materials used are commercially available. Example

[0040] This embodiment provides a highly sensitive and fast-response gradient hierarchical porous hydrogel, its preparation method, and its application in a blood pressure monitoring piezoelectric sensor. The preparation method of the gradient hierarchical porous hydrogel piezoelectric sensor includes the following steps: S1. Preparation of precursor solution: Weigh 3 g of acrylic acid (AA, CAS:79-10-7) and 2.85 mL of acrylamide (AAm, CAS:79-06-1) as monomers, and 0.06 g of 1-vinyl-3-butylimidazolium bromide (VBIMBr, CAS:1033461-45-8, accounting for 1% of the total mass of monomers) as a cationic crosslinking agent. Dissolve them together in deionized water and bring the volume to 20 mL. Stir ultrasonically for 10 min until completely dissolved to obtain a homogeneous and transparent stock solution, wherein the concentration of acrylic acid monomer is 15 wt% and the concentration of acrylamide monomer is 15 wt%. Take 5 mL of this stock solution and add 0.03 g of ammonium persulfate (APS, CAS:7727-54-0) as a photoinitiator. Shake until homogeneous to obtain the precursor solution.

[0041] S2. Electric Field Induction Treatment: The precursor solution is injected into a 10 mm × 10 mm × 10 mm cubic silicone mold. The mold has openings on both the left and right surfaces, and a transparent electrode (ITO glass) is attached to each. The two transparent electrodes are connected to the positive and negative terminals of an adjustable DC power supply via wires. A stable electric field is applied between the two ITO glass plates using the adjustable DC power supply, with the electric field strength set to 3.0 V cm⁻¹. –1 After being energized and left to stand for 10 minutes, the cationic crosslinking agent (VBIMBr) gradually forms a concentration gradient as it migrates to the negative electrode under the induction of the electric field.

[0042] S3. Photocrosslinking reaction: The precursor solution, which has been treated with an electric field, is irradiated under an 8 W ultraviolet lamp (wavelength 365 nm) for 30 min to carry out a crosslinking and curing reaction, thereby obtaining a hydrogel with a gradient hierarchical porous structure.

[0043] The hydrogel structure prepared in Example 1 was characterized using field emission scanning electron microscopy (SEM, JSM 7800F). Before SEM testing, the hydrogel needed to be freeze-dried to remove internal water while preserving its intact internal network structure. The test results of the hydrogel in Example 1 are as follows: Figure 1 As shown, the hydrogel possesses a gradient hierarchical porous structure, including micropores and macropores. The pore size of the micropores ranges from 200 nm to 1 μm, while the pore size of the macropores ranges from 10 to 30 μm. From the anode surface to the cathode surface, i.e., along the migration direction of the cationic crosslinking agent, the number of micropores within the hydrogel increases gradient. This verifies that the cationic crosslinking agent diffuses to the negative electrode to form a denser network. The macroscopic network framework exhibits a significant gradient distribution in pore size, with the hydrogel's internal structure being more porous on the anode side and denser on the cathode side. Furthermore, X-ray photoelectron spectroscopy was used to measure the relative nitrogen content at three locations (top, middle, and bottom) from the anode to the cathode side. The experiment deconvoluted the peaks at binding energies of 398.3, 399.6, and 401.4 eV, which originated from the nitrogen atom on the dicyandiamide cyano group, the N–C bond in dimethylacrylamide, and the N atom on the imidazole cation, respectively. + N-imidazolium cation + Peak area can be used to quantify the increase in cationic crosslinking agent from the anode to the cathode. Calculation results show that the N of imidazole cations from the anode side to the cathode side... + The integral area ratio of the N–C peak in AAm increased from 1:45.3 and 1:41.4 to 1:1.1, which means that the cationic crosslinking agent gradient on the gel is large, the gradient increases from the anode side to the cathode side, and the gradient structure is obvious.

[0044] Example 2 This embodiment provides a series of homogeneous hierarchical porous hydrogels with different hierarchical pore densities, achieved by controlling the amount of photoinitiator ammonium persulfate (APS), specifically including Examples 2a, 2b, and 2c. The preparation method differs from Example 1 in that the electric field induction treatment in step S2 is omitted, and different masses of APS are added in step S1. Details are as follows: Example 2a (0.03 g APS): S1. Preparation of precursor solution: Prepare the stock solution according to the method in S1 of Example 1 (3 g AA, 2.85 mL AAm, 0.06 g VBIMBr, deionized water to a final volume of 20 mL, ultrasonically stirred until completely dissolved). Take 5 mL of this stock solution, add 0.03 g ammonium persulfate (APS) as a photoinitiator, and shake until homogeneous to obtain the precursor solution.

[0045] S2'. Photocrosslinking reaction (without electric field induction): The precursor solution is injected into a 10 mm × 10 mm × 10 mm cubic silicone mold (no ITO glass is needed, and no electric field is applied), and then placed under an 8 W ultraviolet lamp (wavelength 365 nm) for 30 min to carry out the crosslinking and curing reaction, thereby obtaining a homogeneous hierarchical porous hydrogel.

[0046] Example 2b (0.01 g APS): The only difference from Example 2a is that the mass of APS added in step S1 is 0.01 g, and the rest of the preparation steps are exactly the same.

[0047] Example 2c (0.001 g APS): The only difference from Example 2a is that the mass of APS added in step S1 is 0.001 g, and the rest of the preparation steps are exactly the same.

[0048] The hydrogels prepared in Examples 2a, 2b, and 2c were characterized using the same testing methods as in Example 1. Figure 2 As shown, the SEM image of Example 2a (0.03 g APS) reveals a distinct hierarchical porous structure within the hydrogel. The macropores have a particle size of 15–20 μm, and their interiors and framework are densely packed with micropores ranging in size from 200 nm to 1 μm. The macropores and micropores are interconnected, forming a hierarchical "pore-within-a-pore" structure. The SEM image of Example 2b (0.01 g APS) also shows a distinct hierarchical porous structure, with numerous micropores distributed within the inner walls and framework of the macropores. Compared to Example 2b, the hydrogel of Example 2a exhibits a significantly increased number of micropores, resulting in a more compact overall network structure and marked local chain segment shrinkage. This verifies the strong dehydration and network shrinkage effect caused by the specific effect of high-concentration ammonium persulfate ions. SEM images of Example 2c (0.001 g APS) show that the internal pore structure of the hydrogel is relatively uniform, with the pore size mainly distributed in the range of 6 to 15 μm. No obvious hierarchical pore structure was observed, and the network skeleton is relatively complete but lacks secondary pore structure.

[0049] Example 3 This embodiment provides a series of homogeneous hierarchical porous hydrogels of different thicknesses, with thickness control achieved by changing the mold height. Specifically, it includes Example 3a (10 mm thickness) and Example 3b (5 mm thickness). The preparation method is basically the same as that of Example 2a (0.03 g APS, no electric field), only the mold size is changed. Details are as follows: Example 3a (thickness 10 mm) S1. Preparation of precursor solution: Prepare the stock solution according to the method in S1 of Example 1 (3 g AA, 2.85 mL AAm, 0.06 g VBIMBr, deionized water to a final volume of 20 mL, ultrasonically stirred until completely dissolved). Take 5 mL of this stock solution, add 0.03 g ammonium persulfate (APS) as a photoinitiator, and shake until homogeneous to obtain the precursor solution.

[0050] S2. Photocrosslinking reaction (without electric field induction): The precursor solution is injected into a 10 mm × 10 mm × 10 mm cubic silicone mold (no ITO glass is needed, and no electric field is applied), and placed under an 8 W ultraviolet lamp (wavelength 365 nm) for 30 min to carry out crosslinking and curing reaction, thereby obtaining a homogeneous hierarchical porous hydrogel with a thickness of 10 mm.

[0051] Example 3b (thickness 5 mm) The only difference from Example 3a is that the height of the silicone mold in step S2 is replaced with 5 mm (the length and width are still 10 mm × 10 mm), and the rest of the preparation steps are exactly the same, resulting in a homogeneous hierarchical porous hydrogel with a thickness of 5 mm.

[0052] The mechanical and piezoelectric properties of the hydrogels from Examples 1 and 2a were characterized using a universal tensile testing machine (AGS-X, China). Voltage signals were recorded using an electronic multimeter (Keithley 2450). The piezoelectric property test graphs are shown below. Figure 3 As shown, the bottom of the hydrogel to be tested is closely attached to two gold-plated electrodes, and the lower end of the electrodes leads out to the connection source. The universal tensile testing machine uses a cylindrical indenter to perform surface pressure test. The indenter diameter is 5mm, the compression speed is set to 20mm / min, and the indenter applies pressure on the anode side of the hydrogel.

[0053] The compressive stress-strain curves of the gradient hierarchical porous hydrogel of Example 1 and the homogeneous hierarchical porous hydrogel of Example 2a are shown below. Figure 4 As shown, the hydrogels exhibit drastically different behaviors during loading. The internal structure of the gradient hierarchical porous hydrogel in Example 1 is more porous on the anode side and more compact on the cathode side, allowing it to exhibit significant deformation even in low-pressure regions. In high-pressure regions, the pressure it can withstand increases exponentially, but the deformation at 5 kPa accounts for only 21.5% of the total deformation, indicating that the hydrogel of Example 1 has high pressure sensitivity. Conversely, the homogeneous hydrogel of Example 2a exhibits significantly lower deformation in low-pressure regions, with only 6.7% deformation at 5 kPa, showing a marked decrease in sensitivity.

[0054] The open-circuit voltage (point pressure, compression speed of 100 mm / min) of the hydrogel in Example 1 under pressure applied to the anode side and the cathode side, and the open-circuit voltage of the hydrogel in Example 2a under pressure were tested respectively. The test results are as follows: Figure 5 As shown in (a), when the hydrogel of Example 1 is placed with the anode side facing up and the cathode side facing down, the cylindrical indenter of the universal tensile testing machine acts on the anode surface (the large pore surface of the gradient multi-level pore structure), and the hydrogel piezoelectric sensor exhibits high sensitivity and high output voltage characteristics. However, when the hydrogel of Example 1 is placed in reverse, the cylindrical indenter of the universal tensile testing machine acts on the cathode surface (the small pore surface of the gradient multi-level pore structure), and the open circuit voltage performance of the hydrogel piezoelectric sensor is even worse than that of the homogeneous multi-level pore structure. The specific test results are shown in Table 1.

[0055] Table 1. Open-circuit voltages of hydrogel piezoelectric sensors in Examples 1 and 2a

[0056] The main reasons for this are twofold: firstly, the mechanical properties of the hydrogel on both sides of the gradient direction are different in Example 1. The difference in pressure gradient diffusion when applying pressure in different directions results in different deformations of the hydrogel, which in turn leads to different magnitudes of ion currents; secondly, the internal pore size of the hydrogel cathode surface in contact with the electrode in Example 1 is smaller, thus increasing the resistance to ion charge migration. Under pressure, the difference in diffusion between anions and cations is amplified, resulting in a larger open-circuit voltage due to pressure on the anode surface. Figure 5 (b) shows that in the point pressure test mode of the universal tensile testing machine, under the same point pressure, the deformation of the anode side is greater than that of the cathode side.

[0057] The Young's modulus distribution of the hydrogel of Example 1 from the anode side to the cathode side was tested using an electronic universal tensile testing machine with a micro-indenter (a fine needle indenter with a diameter of 1 mm and a compression rate of 10 mm / min). Figure 6 As shown, it can be seen that the Young's modulus increases significantly with a gradient, indicating that the gradient gel network of Example 1 has obvious mechanical gradient properties. The specific test results are shown in Table 2.

[0058] Table 2. Distribution of Young's modulus of the hydrogel in Example 1 from the anode side to the cathode side.

[0059] To understand the deformation of the gel under different directions of pressure gradient, a finite element method (FEM) simulation was performed using the solid mechanics module of COMSOL Multiphysics. The color depth of the gel represents the strain along the pressure direction. The FEM simulation of the gel was simplified using a 20-layer multilayer model. The overall modulus of the multilayer plate was determined based on... Figure 6 The obtained data were allocated, and then pressures of 1 kPa, 100 kPa, 200 kPa, and 300 kPa were applied to the hydrogel of Example 1, respectively. Figure 7As shown, under the same stress, pressing the less cross-linked hydrogel on the anode side of Example 1 results in greater gel deformation than pressing the cathode side, and this difference becomes more pronounced with increasing pressure. When the pressure reaches 300 kPa, the maximum gel strain on the cathode side is 39.2%, while the maximum gel strain on the anode side is 47.2%. These simulation results are consistent with... Figure 5 b together verified that the gradient gel network has obvious mechanical gradient properties.

[0060] In addition to the significant modulus gradient, the gradient hierarchical porous hydrogel of Example 1 also exhibits excellent resilience, maintaining its mechanical properties after undergoing 100 compression fatigue cycles in a short period of time. Figure 8 As shown in the figure, this characteristic helps ensure the durability of the gradient multi-level porous hydrogel as a flexible sensor device.

[0061] Figure 9 (a) shows the magnitude of the output voltage of the hydrogel of Example 1 (force applied on the anode side, point pressure, compression rate 100 mm / min) under increasing cyclic pressure. The test results are shown in Table 3.

[0062] Table 3. Output voltage of the hydrogel in Example 1 under increasing cyclic pressure.

[0063] As can be seen, the output voltage of the hydrogel piezoelectric sensor in Example 1 gradually increases with increasing pressure, and shows a trend of rapid increase within a small stress range and slow increase within a large stress range. The piezoelectric sensitivity of the hydrogel piezoelectric sensor is defined as S. P =|ΔV / ΔP|, where ΔV and ΔP are the changes in output voltage and applied pressure, respectively. When the pressure is below 120 kPa, the sensitivity of the gradient hierarchical porous hydrogel piezoelectric sensor is 4028.27 mV / MPa, while when the pressure is above 120 kPa, the pressure sensitivity drops to 399.71 mV / MPa. The higher sensitivity of the sensor in the low-pressure region stems from the larger pressure gradient diffusion on the side of the larger pores under low pressure. The decrease in pressure sensitivity under high pressure is likely due to the high modulus on the side of the smaller pores. This characteristic gives the hydrogel piezoelectric sensor of Example 1 excellent characteristics as a piezoelectric sensor: high sensitivity (within a small stress range) and a large range (able to withstand large stresses).

[0064] In addition, the piezoelectric stability of the sensor under low stress (1 kPa) was tested. Figure 9 (b) It can be seen that the hydrogel piezoelectric sensor of Example 1 meets the requirements for low-pressure sensing scenarios. More notably, the hydrogel piezoelectric sensor of Example 1 has an extremely fast response time ( Figure 9 c) Under a pressure of 60 kPa, the response time is only 0.043 s, and the discharge time is only 0.106 s, indicating that this gradient hierarchical porous structure also has the ability to respond and rebound quickly. Furthermore, this hydrogel piezoelectric sensor also exhibits excellent durability and superior piezoelectric cycling stability. Figure 9 As shown in Figure d, the piezoelectric sensor based on the gradient hierarchical porous hydrogel of Example 1 maintains a stable electrical response under nearly 100 cycles of repeated loading and unloading at a pressure of 30 kPa. In summary, the hydrogel piezoelectric sensor based on the gradient hierarchical porous structure of Example 1 exhibits excellent performance in terms of high output voltage, high sensitivity, fast response, and large range, making it highly promising for applications in portable wearable devices.

[0065] The piezoelectric properties of homogeneous hierarchical porous hydrogels prepared in Example 3a (10 mm thickness) and Example 3b (5 mm thickness) were tested using a 5 mm diameter cylindrical indenter in a point-pressure manner. The experimental results show that gel thickness significantly affects its sensitivity, range, and open-circuit voltage. For the thinner hydrogel (5 mm thickness), its sensitivity reaches approximately 90 mV / MPa in the low-pressure region (<50 kPa), significantly higher than the sensitivity of the 10 mm thick hydrogel in this pressure range (approximately 66 mV / MPa). This is because the thinner hydrogel, when subjected to point pressure, undergoes more concentrated deformation, resulting in more intense pressure gradient diffusion. This leads to significant differences in ion migration even at lower pressures, thus achieving higher low-pressure sensitivity. However, the range of the 5 mm thick hydrogel is limited; when the pressure exceeds approximately 150 kPa, its output voltage tends to saturate, making it difficult to distinguish larger pressure changes. In contrast, while the thicker hydrogel (10 mm thick) exhibits slightly lower low-pressure sensitivity, it withstands a wider range of compressive deformation, has a longer pressure transmission path, and demonstrates a more significant cumulative effect of cation and anion migration within the gel. Therefore, its measurement range can be extended to over 277.78 kPa, and it can still generate an open-circuit voltage as high as 548.13 mV at 277.78 kPa. Based on these results, the thicker hydrogel demonstrates significant advantages in terms of wide measurement range and high voltage output.

[0066] The principle of using the hydrogel piezoelectric sensor in Example 1 to monitor blood pressure via an extravascular sensor is based on measuring the piezoelectric signal generated by the displacement of the tissue surface due to pressure changes. A mathematical model is established to convert the piezoelectric pulse signal into a blood pressure wave. The piezoelectric response can be converted into vascular pressure (F) using the following formula. BP ): .in Depending on the structure and material properties of the piezoelectric sensor, It is external resistance. This is the piezoelectric pulse signal from the device, and C is a correction coefficient related to the test subject's initial blood pressure (BP). It should be emphasized that the coefficient only changes the amplitude, not the waveform characteristics. That is, the vascular pressure F... BP The piezoelectric conversion coefficient 1 / αR is equal to the product of the definite integral of the piezoelectric pulse signal V(τ) over the time interval [0,t] with respect to time t, and then superimposed with the system's initial zero-point bias constant C. Therefore, to calibrate these parameters, an improved cuff method is used to capture the piezoelectric response of the radial artery under different cuff pressures. This allows the extraction of systolic blood pressure (SBP) and diastolic blood pressure (DBP) to solve for these unknown parameters. Finally, the blood pressure value can be obtained from the converted piezoelectric pulse wave.

[0067] First, the hydrogel piezoelectric sensor of Example 1 (the hydrogel is directly attached to the wrist pulse point, with two electrodes attached between the bottom and the skin, connected to a source meter to measure the current signal) was worn on the radial artery of the left hand of a healthy adult volunteer. The source meter recorded the periodic output current signal of the sensor in different physiological states in real time. Figure 11 As shown in a, because it is proportional to the voltage response signal, the pulse piezodynamics formula can still be used. Extracting a single pulse cycle ( Figure 11 (b) Three characteristic peaks are clearly distinguishable: the main wave (P-wave), the tidal wave (T-wave), and the dicrotic wave (D-wave). These characteristic peaks correspond to the pressure wave reflection characteristics of the arterial system during cardiac systole and diastole, and are important indicators for assessing arterial stiffness and cardiovascular health. Furthermore, the original signals are superimposed to assess stability and eliminate fluctuations caused by the volunteer's arm swing. Figure 11 c). The annotated P1 and P2 peaks represent the sum of the forward traveling wave and the reflected waves from the hand and lower limb, minus the end-diastolic pressure. Calculate the corresponding Fast Fourier Transform results for the above multi-cycle pulse waves ( Figure 11d) The precise response frequencies of the arterial pulse and its higher harmonics verified the accuracy of the testing device. Finally, an improved cuff method was used to capture the piezoelectric response of the radial artery under different cuff pressures. Specifically, a cuff was placed on the arm to apply static pressure, and a flexible piezoelectric sensor (a gradient hierarchical porous hydrogel piezoelectric sensor) was attached to the skin near the radial artery at the wrist. The expected static pressure was applied, and the device then tested the pulse signal. Since pressure alters blood propagation, the corresponding arterial pulsation signal can be detected to reflect the vascular state. As the pressure increases, the arterial pulsation signal successively experiences two pressure values: systolic blood pressure (SBP) and diastolic blood pressure (DBP). Based on the piezoelectric signals corresponding to these two values, unknown parameters in the conversion formula can be calibrated. In the actual experiment, by gradually increasing the cuff pressure and recording the changes in the sensor output signal with external pressure, when the cuff pressure was lower than the diastolic blood pressure (60 mmHg), the sensor output waveform was complete, and the amplitude decreased slightly with increasing cuff pressure. When the cuff pressure was between the diastolic and systolic blood pressure (60-110 mmHg), the waveform showed characteristic changes, the dicrotic wave gradually disappeared, and the amplitude of the main wave decreased significantly. When the cuff pressure exceeded the systolic blood pressure (>110 mmHg), the pulse signal basically disappeared. Therefore, SBP was set to 110 mmHg and DBP to 60 mmHg. Figure 11 e shows the typical piezoelectric response of the volunteer and its integral signal, as well as the converted vascular pressure waveform, used to assess the state of blood propagation, where periodic percussion waves (P) and diabetic waves (D) are easily identifiable. Figure 11 f shows the converted vascular pressure waveform in detail, P s P d and P m These represent the systolic blood pressure, diastolic blood pressure, and mean blood pressure within one cycle, respectively. In summary, the gradient hierarchical porous gel sensor prepared in this application successfully achieved high-fidelity acquisition of the radial artery pulse wave and can realize continuous, non-invasive blood pressure monitoring through an improved cuff calibration method. Its high sensitivity, fast response, good stability, and self-powered potential provide new material choices and system solutions for the development of next-generation portable health management devices.

[0068] In summary, this application successfully developed a novel gradient hierarchical porous hydrogel by employing a one-step polymerization strategy, combining electric field-induced cationic crosslinker migration with the ion-specific effect of ammonium persulfate. The sulfate radicals generated under ammonium persulfate illumination act as structure-building ions, promoting uniform shrinkage of the gel network and forming a hierarchical porous structure within the original framework. This unique structure significantly enhances the piezoelectric ionic properties of the gel by increasing the internal charge migration resistance and built-in potential difference during deformation. Further introducing a gradient distribution of crosslinking density through electric field guidance, the resulting gradient hierarchical porous hydrogel exhibits an open-circuit voltage as high as 548.13 mV at 277.78 kPa, an ultra-high sensitivity of 4028.27 mV / MPa in the low-pressure region (<120 kPa), and a fast response time of 0.043 seconds. Furthermore, this hydrogel piezoelectric sensor exhibits excellent mechanical robustness, cyclic stability, and direction-dependent piezoelectric response characteristics, attributed to the synergistic effect of the hierarchical pores and gradient mechanical properties. Based on these superior properties, a gradient hierarchical porous hydrogel was successfully integrated into a wearable wristband system, enabling real-time, non-invasive monitoring of human arterial pulse signals. Through an improved cuff calibration method, the sensor can achieve continuous blood pressure estimation, clearly distinguish pulse wave characteristic peaks, and accurately track changes in systolic and diastolic blood pressure. This application not only verifies the effectiveness of gradient hierarchical porous structure engineering in enhancing piezoelectric ion performance but also provides a feasible material solution for next-generation self-powered, flexible health management devices.

[0069] It should be understood that the application of the present invention is not limited to the examples above. Those skilled in the art can make improvements or modifications based on the above description, and all such improvements and modifications should fall within the protection scope of the appended claims.

Claims

1. A method for preparing a highly sensitive and fast-response gradient hierarchical porous hydrogel, characterized in that, Includes the following steps: S1. Preparation of precursor solution: Weigh the monomer, cationic crosslinking agent and photoinitiator according to the ratio, and dissolve the monomer, cationic crosslinking agent and photoinitiator in water to prepare a precursor solution, wherein the monomer includes acrylic acid and acrylamide; S2. Electric field induced treatment: An electric field is applied to the precursor solution to induce the cationic crosslinking agent to migrate and form a concentration gradient; S3. Photocrosslinking reaction: The precursor solution, which has been treated with an electric field, is placed under light to carry out a crosslinking and curing reaction to obtain the gradient hierarchical porous hydrogel.

2. The method for preparing the highly sensitive and fast-response gradient hierarchical porous hydrogel according to claim 1, characterized in that, In step S1, the mass ratio of the monomer, cationic crosslinking agent and photoinitiator is 80~120:0.8~1.2:1.6~2.4, and the mass ratio of acrylic acid and acrylamide in the monomer is 0.5~1.5:0.5~1.

5.

3. The method for preparing the highly sensitive and fast-response gradient hierarchical porous hydrogel according to claim 1 or 2, characterized in that, In step S1, the cationic crosslinking agent is 1-vinyl-3-butylimidazolium bromide, and the photoinitiator is ammonium persulfate.

4. The method for preparing the highly sensitive and fast-response gradient hierarchical porous hydrogel according to claim 1 or 2, characterized in that, In step S2, the strength of the electric field is set to 2~4 V cm. –1 The induction treatment time is 5-15 minutes.

5. The method for preparing the highly sensitive and fast-response gradient hierarchical porous hydrogel according to claim 1 or 2, characterized in that, In step S3, the illumination is ultraviolet light, with an intensity of 4~12 W and an illumination time of 10~60 min.

6. A highly sensitive and fast-response gradient hierarchical porous hydrogel, prepared by the method for preparing the highly sensitive and fast-response gradient hierarchical porous hydrogel according to any one of claims 1 to 5.

7. The highly sensitive and fast-response gradient hierarchical porous hydrogel according to claim 6, characterized in that, The gradient hierarchical porous hydrogel includes macroporous structures and microporous structures. The pore size of the macroporous structures is 10 μm to 30 μm, and the pore size of the microporous structures is 200 nm to 1 μm. The number of microporous structures increases in a gradient along the migration direction of the cationic crosslinking agent.

8. The application of the highly sensitive and fast-response gradient hierarchical porous hydrogel as described in claim 6 or 7 in a piezoelectric sensor.

9. The application of the highly sensitive and fast-response gradient hierarchical porous hydrogel according to claim 8 in piezoelectric sensors, characterized in that, The piezoelectric sensor based on the aforementioned gradient hierarchical porous hydrogel achieves a piezoelectric sensitivity exceeding 4000 mV / MPa at pressures below 120 kPa, with a piezoelectric sensitivity of S. P =|ΔV / ΔP|, where ΔV and ΔP correspond to the changes in output voltage and applied pressure, and the response time can reach less than 0.045 seconds.

10. The application of the highly sensitive and fast-response gradient hierarchical porous hydrogel of claim 6 or 7 in a piezoelectric sensor for blood pressure monitoring.

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