Flexible force sensor for generating interface capacitance and triboelectric effect through microfluid deformation

Abstract

Real-time monitoring of human body surface physiological information is a core function of wearable medical equipment. A micro-fluidic technology is paid attention to the field of biochemical physiological information sensing, but the potential of the micro-fluidic technology in the aspect of mechanical force physiological information sensing is not fully excavated, so that the realization of a multi-parameter integrated detection platform is hindered. The invention provides a dual-mode flexible physiological force sensor based on microfluid deformation. According to the sensor, capacitance and triboelectric effects are creatively fused, and the limitation of a single sensing mechanism is broken through. The liquid drop not only can be used as a deformable electrode to form a capacitance structure with the bottom electrode to realize static force detection, but also can be contacted with the PDMS dielectric layer for electrification to capture a dynamic force signal. By accurately regulating and controlling geometric parameters of the sensor, collaborative optimization of sensitivity and detection range is realized, and ultrafast dynamic response of 21ms is achieved. Experiments prove that stable underwater pulse monitoring can be carried out for 168 hours, and the biomedical potential of the underwater pulse sensor in the aspects of mechanical and biochemical signal fusion and overall physiological analysis is proved.

Description

Technical Field

[0001] The present invention relates to the field of flexible pressure sensors, and specifically to a flexible force sensor that generates interface capacitance and triboelectric effect through microfluid deformation, a sensor that detects dynamic and static pressure excitations using an active capacitance mechanism, and a sensor that detects static pressure excitations using a passive triboelectric mechanism. By regulating the geometric parameters of the sensor, the coordinated optimization of sensitivity and range is achieved. Background Art

[0002] Physiological information, including chemical and mechanical information, is an important clinical indicator for predicting human disease. In recent years, wearable medical devices based on novel mechanisms, materials, structures, and processes have undoubtedly provided real-time physiological monitoring solutions for addressing key strategic challenges in life and health. Among numerous physiological monitoring platforms, microfluidics technology, through precise manipulation of fluids at the microscale, allows for better control of molecular concentrations and interactions. It integrates basic functional units onto microchips, automating the entire analytical process and attracting widespread attention in the field of bio- and chemical-physiological signal sensing. Sweat, as a signal source for physiological indicators, offers the advantage of non-invasive monitoring. Liu et al. proposed a flexible microfluidic electrochemical sensor that, through optimized microfluidic structure, enables real-time, quantitative analysis of sweat sodium ion concentration. Zahed et al. developed a microfluidic integrated multimodal wearable hybrid patch that integrates biochemical (glucose, pH, temperature) and electrophysiological (ECG) sensing, combined with wireless transmission technology, to achieve real-time, continuous, and precise monitoring of sweat metabolites and cardiac activity. However, its potential in physiological force signal sensing remains underexplored, which hinders the integration of multi-parameter physiological information on the same platform.

[0003] The detection of physiological force information is mainly based on piezoresistance, piezoresistance, piezoelectricity and triboelectricity principles, each of which has its own unique advantages and limitations. As a unique example, Nie et al. used the electric double layer (EDL) capacitance formed at the interface between electrodes and electrolyte droplets to achieve pressure detection, successfully detecting tiny changes in blood pressure on the skin surface, and have demonstrated the feasibility of microfluidics in the field of mechanical force physiological information sensing. However, the response time of this device is limited and cannot meet the needs of high-frequency dynamic monitoring (such as high-speed blood flow analysis). At the same time, the physiological force detection process based on a single electric double layer capacitance mechanism still requires external energy supply. Therefore, the development of microfluidic-based physiological force information sensing devices is expected to expand the application of traditional microfluidics technology in the biomedical field, unify the collection of mechanical force and biochemical signals on a single platform, promote the coordinated analysis of physiological status, and realize multi-information fusion sensing, which is of great value. Summary of the Invention

[0004] The purpose of this invention is to provide a dual-mode flexible physiological force sensor based on the liquid-solid interface capacitance and triboelectric effect generated by microfluidic deformation. This sensor can simultaneously capture static and dynamic forces through both capacitive and triboelectric mechanisms, overcoming the limitations of traditional single sensing mechanisms. By adjusting microstructural parameters, two key parameters, sensitivity and range, can be dynamically adjusted, aiming to expand the sensor's application scenarios.

[0005] The present invention discloses a flexible force sensor that generates interface capacitance and triboelectric effect by deformation of microfluid. The flexible force sensor that generates interface capacitance and triboelectric effect by deformation of microfluid comprises a high-precision part and a large-range part. The high-precision part and the large-range part share the same PET substrate (6) and are distributed on both sides of the PET substrate (6).

[0006] The high-precision part includes a first PDMS structural layer (1), a first water droplet (2), a first PDMS friction layer (3), a first flow channel ITO electrode (4) and a first sensing cavity ITO electrode (5);

[0007] The large-range part includes a second flow channel ITO electrode (7), a second sensing cavity ITO electrode (8), a second PDMS friction layer (9), a second water droplet (10) and a second PDMS structural layer (11);

[0008] The first PDMS structural layer (1) comprises a sensing cavity, a water injection port and a sensing channel; the sensing cavity is a cylindrical cavity arranged at the center of the first PDMS structural layer (1) for accommodating a first water droplet (2); the water injection port is arranged at the same height as the sensing cavity; the sensing channel is arranged outside the sensing cavity and connected to the sensing cavity; the first water droplet (2) injects the water droplet into the sensing cavity through the water injection port; a circular hole is provided at the center of the first PDMS friction layer (3), the circular hole is connected to the sensing cavity, and is used to conduct the charge of the first water droplet (2) in the sensing cavity to the first sensing cavity ITO electrode (5); the first flow channel ITO electrode (4) and the first sensing cavity ITO electrode (5) are arranged between the first PDMS friction layer (3) and the PET substrate (6);

[0009] The large-range part and the high-precision part are symmetrically arranged with the PET substrate (6) as the center.

[0010] Furthermore, it is characterized in that the sensing chamber is filled with water through the water inlet, and the end of the microfluidic channel is closed. In the absence of external force, the air pressure at the end is balanced with the water pressure in the sensing chamber; when pressure is applied, the water droplets in the sensing chamber will be pressed into the sensing channel to varying degrees, forming a capacitor with the flow channel ITO electrode corresponding to the sensing channel, generating a capacitance signal; at the same time, the water droplets generate a triboelectric effect during the contact process with the friction layer, generating a voltage signal, thereby reflecting the magnitude of the applied pressure through the output electrical signal.

[0011] Furthermore, the sensing cavity of the first PDMS structural layer (1) has a diameter of 10 mm, a height of 0.3 mm, and three support pillars with a diameter of 1 mm are evenly distributed;

[0012] The sensing cavity of the second PDMS structural layer (11) has a diameter of 10 mm and a height of 1.1 mm, and is evenly distributed with four support pillars with a diameter of 1 mm;

[0013] The support column is used to prevent the sensing cavity from being unable to rebound due to excessive load, and can also change the Young's modulus of the sensing cavity to change the range and sensitivity of the sensor.

[0014] Furthermore, the shape of the sensing channel of the first PDMS structural layer (1) is the same as the shape of the first flow channel ITO electrode (4);

[0015] The shape of the sensing channel of the second PDMS structural layer (11) is the same as the shape of the second flow channel ITO electrode (7).

[0016] Furthermore, the first flow channel ITO electrode (4) is provided with an extraction electrode at the center of the outermost circle and on the same straight line as the first sensing cavity ITO electrode (5), for extracting the electrical signal from the sensor; the first sensing cavity ITO electrode (5) is a straight line structure, extending from the center to the edge of the device, and the extraction electrode is provided at the edge.

[0017] Furthermore, the method for preparing the flexible force sensor that generates interfacial capacitance and triboelectric effect by microfluid deformation includes:

[0018] A substrate is prepared; the substrate comprises a PET backing (6), a friction layer and an ITO electrode; the friction layer comprises a first PDMS friction layer (3) and a second PDMS friction layer (9); the ITO electrode comprises a first flow channel ITO electrode (4), a first sensing cavity ITO electrode (5), a second flow channel ITO electrode (7) and a second sensing cavity ITO electrode (8);

[0019] The microfluidic structure layer is prepared, and the preparation of the microfluidic structure layer further includes the following steps:

[0020] Step 1: preparing a positive mold of the first PDMS structure layer (1) and the second PDMS structure layer (11);

[0021] Step 2: preparing a first PDMS structure layer (1) and a second PDMS structure layer (11) based on the positive mold;

[0022] Step 3: Integrate the substrate and microfluidic structure layer.

[0023] Furthermore, the PET substrate (6) is a flexible polyethylene terephthalate substrate; a pre-designed mask is attached to the substrate and then an ITO electrode is sputtered to ensure that the metal electrode is deposited only in the designated flow channel area, thereby minimizing the parasitic capacitance effect; PDMS is spin-coated on the PET substrate (6) with the ITO electrode at speeds of 2000 rpm and 500 rpm respectively by a spin coater to form a friction layer (3) and a friction layer (9); a circular through-hole mask with a diameter of 2 mm is pre-attached to the center of the PET substrate (6), which is removed after spin coating to make a circular through-hole with a diameter of 2 mm; a heating box is set to 80°C and continuously heated for 30 minutes to uniformly solidify the friction layer.

[0024] Furthermore, the positive mold process for preparing the first PDMS structure layer (1) and the second PDMS structure layer (11) is the same;

[0025] Fix the pretreated silicon wafer in the center of the spin coater tray, add an appropriate amount of SU-8 photoresist to the center of the silicon wafer, set the spin coating parameters to obtain the desired photoresist thickness, and soft bake at 95°C for 5 minutes;

[0026] Design a reverse polyester film mask and 2 Expose to UV light for 35 seconds;

[0027] After exposure, bake on a 95°C hot plate for 5 minutes, develop the silicon wafer, and finally hard bake in a 140°C oven for 5 minutes;

[0028] After the silicon wafer cools to room temperature, the substrate is immersed in propylene glycol methyl ether acetate and isopropyl alcohol solutions for 15 seconds each. The residual SU-8 photoresist solvent is removed by cyclic development, thus completing the preparation of the PDMS positive mold.

[0029] Furthermore, in step 2, the liquid PDMS elastomer and the cross-linking agent are stirred and mixed in a ratio of 10:1, the mixture is placed in a vacuum box to extract the air in the mixture, and then the mixture without air is poured onto the positive mold of the first PDMS structural layer (1) and the second PDMS structural layer (11), and cured at 80°C for at least three hours to form a top PDMS structural layer with a thickness of 2.1 mm. Finally, the cured first PDMS structural layer (1) and the second PDMS structural layer (11) are removed from the positive mold by tweezers.

[0030] Furthermore, in step three, the first PDMS structure layer (1) and the second PDMS structure layer (11) peeled off and trimmed from the positive mold are aligned with the first flow channel ITO electrode (4) of the first PDMS friction layer (3) and the second flow channel ITO electrode (7) of the second PDMS friction layer (9) pre-prepared and spin-coated on the front and back sides of the flexible PET substrate respectively;

[0031] A vacuum plasma treatment apparatus is used to perform oxygen plasma treatment on the first PDMS structural layer (1), the second PDMS structural layer (11) and the flexible PET substrate (6) to achieve permanent bonding.

[0032] The beneficial effects achieved by the present invention are:

[0033] Dual-mode sensing mechanism improves sensing capability and versatility: The dual-mode flexible physiological force sensor of the present invention combines the capacitive mechanism and the triboelectric mechanism, and can sense static and dynamic forces at the same time, breaking through the single sensing mechanism of traditional sensors. The capacitive mechanism has high sensitivity and is suitable for the detection of dynamic and static forces, while the triboelectric mechanism can effectively capture dynamic force signals. This dual-mode sensing strategy enables the sensor to simultaneously detect the static and dynamic components in the physiological force signal, improving the versatility and applicability of the sensor. In addition, the introduction of the triboelectric effect enables the sensor to be self-powered without the need for an external power supply, thereby improving the independence and energy efficiency of the sensor.

[0034] Microstructure adjusts sensitivity and range to meet diverse application needs: This invention achieves fine control of sensitivity and range by precisely adjusting the microstructure parameters of the sensor (such as sensing cavity height, number of pillars, droplet volume, etc.). In the low pressure range (0–3.2kPa), the sensor can provide high sensitivity (1.862kPa -1 ), suitable for high-precision physiological signal monitoring; and within the high-pressure range (3.2–9.6 kPa), the sensor can still operate stably, ensuring adaptability to a wide range of pressures. Through the adjustability of the structural design, the sensor of the present invention can meet the different requirements for sensitivity and range in different application scenarios, and is widely used in wearable devices, medical monitoring, health management and other fields, providing accurate and reliable multi-parameter physiological monitoring data.

[0035] Miniaturization and ease of integration, optimized design, and complementary performance: This invention utilizes a simple, high-precision manufacturing process and boasts strong integration. By adjusting microstructural parameters, precise control of sensitivity and range is achieved, allowing sensors with different performance characteristics to share a common substrate, resulting in a single, integrated sensor with both high precision and a wide range. This optimized design not only makes the sensor suitable for diverse application needs but also facilitates integration with other physiological information monitoring systems while maintaining high performance, enhancing its applicability and practical value in wearable devices. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 This is the appearance diagram of a dual-mode flexible physiological force sensor based on liquid-solid interface capacitance and triboelectric effect generated by microfluidic deformation;

[0037] Figure 2 This is a schematic diagram of the force and microfluidic deformation of the flexible physiological force sensor;

[0038] Figure 3 This is a schematic diagram of the capacitance-pressure sensing mechanism of the flexible physiological force sensor;

[0039] Figure 4 This is the dynamic response curve of the sensor capacitance over time under a periodic pressure of 0.95kPa;

[0040] Figure 5 Schematic diagram of the triboelectric-pressure sensing mechanism of the flexible physiological force sensor;

[0041] Figure 6 This is the dynamic response curve of the sensor voltage over time under a periodic pressure of 0.95kPa;

[0042] Figure 7 The sensitivity curve of the sensor without air cavity is shown. The sensor has a height of 1.1mm, a height of 0.1mm, four supporting pillars, and a spin coating speed of 1000rpm.

[0043] Figure 8 It monitors the radial artery periodic pulse signal in real time and identifies the three characteristic waves in the detected pulse wave: percussion wave (P), tidal wave (T) and recoil wave (D). The sensitivity curve of the microfluidic flexible dual-mode pressure sensor shows two linear states under different sensitivities. DETAILED DESCRIPTION

[0044] The present invention will be further described below with reference to specific embodiments, and the advantages and features of the present invention will become clearer as the description proceeds. However, these embodiments are merely exemplary and do not constitute any limitation to the scope of the present invention. It should be understood by those skilled in the art that the details and forms of the technical solutions of the present invention may be modified or replaced without departing from the spirit and scope of the present invention, and such modifications and replacements fall within the scope of protection of the present invention.

[0045] As attached Figure 1As shown, a flexible force sensor that generates interfacial capacitance and triboelectric effects due to microfluidic deformation is described. The pressure sensor comprises a high-precision portion and a large-range portion. The pressure sensor includes a first PDMS structural layer 1, a first water droplet 2, a first PDMS friction layer 3, a first flow channel ITO electrode 4, a first sensing cavity ITO electrode 5, a PET substrate 6, a second flow channel ITO electrode 7, a second sensing cavity ITO electrode 8, a second PDMS friction layer 9, a second water droplet 10, and a second PDMS structural layer 11. The first PDMS structural layer 1, the first PDMS friction layer 3, the PET substrate 6, the second PDMS friction layer 9, and the second PDMS structural layer 11 are all circular and concentrically arranged. The first PDMS structural layer 1, the first water droplet 2, the first PDMS friction layer 3, the first flow channel ITO electrode 4, and the first sensing cavity ITO electrode 5 constitute the high-precision portion, while the second flow channel ITO electrode 7, the second sensing cavity ITO electrode 8, the second PDMS friction layer 9, the second water droplet 10, and the second PDMS structural layer 11 constitute the large-range portion.

[0046] The first PDMS structure layer 1 and the second PDMS structure layer 11 are both composed of a sensing cavity and a sensing channel. The sensing cavity is a cylindrical groove structure for accommodating water droplets. There are raised support columns in the groove. The purpose is to prevent the sensing cavity from being unable to rebound due to excessive load. At the same time, it can also change the Young's modulus of the sensing cavity to change the range and sensitivity of the sensor. The sensing channel in the first PDMS structure layer 1 and the second PDMS structure layer 11 is a maze-shaped surrounding groove structure connected to the sensing cavity. The above structures are all manufactured using soft lithography technology. The difference between the first PDMS structure layer 1 and the second PDMS structure layer 11 is that the number of support columns in the sensing cavity of the first PDMS structure layer 1 is 3, and the shape of the maze structure sensing channel is the same as the first flow channel ITO electrode 4; the number of support columns in the sensing cavity of the second PDMS structure layer 11 is 4, and the shape of the maze structure sensing channel is the same as the second flow channel ITO electrode 7.

[0047] The first PDMS friction layer 3 and the second PDMS friction layer 9 are thin film structures with circular holes in the middle for conducting the charges in the water droplets to the bottom electrode.

[0048] The first channel ITO electrode 4 and the second channel ITO electrode 7 are respectively arranged directly below the sensing channels of the first PDMS structural layer 1 and the second PDMS structural layer 11, with the same shape as the sensing channel, separated by the first PDMS friction layer 3 and the second PDMS friction layer 9. Their function is to form a flat plate capacitor with the first water droplet 2 and the second water droplet 10 respectively discharged into the sensing channel after the sensing cavity is subjected to force, thereby generating a capacitive signal. At the same time, the first water droplet 2 and the second water droplet 10 are respectively electrified by solid-liquid friction with the first PDMS friction layer 3 and the second PDMS friction layer 9 to generate a voltage signal. The outer circles of the first channel ITO electrode 4 and the second channel ITO electrode 7 have lead-out electrodes for external wires.

[0049] The first sensing cavity ITO electrode 5 and the second sensing cavity ITO electrode 8 are respectively arranged below the sensing cavity of the first PDMS structural layer 1 and the second PDMS structural layer 11. Their ends overlap with the through-holes of the first PDMS friction layer 3 and the second PDMS friction layer 9, respectively, to extract the charge from the water. The electrodes are not connected to external lead wires. The two electrodes are arranged on the PET substrate 6 by spray deposition. A mask must be attached to the PET substrate 6 before spraying. The first sensing cavity ITO electrode 5 and the second sensing cavity ITO electrode 8 are respectively in the same plane as the first flow channel ITO electrode 4 and the second flow channel ITO electrode 7. The first PDMS friction layer 3 and the second PDMS friction layer 9 are spin-coated on the front and back surfaces of the PET substrate to which the first flow channel ITO electrode 4 and the second flow channel ITO electrode 7 are attached.

[0050] The first PDMS structure layer 1 and the second PDMS structure layer 11 are permanently bonded to the first PDMS friction layer 3 and the second PDMS friction layer 9 respectively through plasma cleaning technology.

[0051] The high-precision part and the large-range part have similar structures. The two parts share a PET substrate 6, and the two parts are concentrically distributed on both sides of the PET substrate 6.

[0052] The first PDMS structure layer 1: The first PDMS structure layer 1 includes a sensing cavity, a water injection port, and a sensing channel. The sensing cavity is a cylindrical cavity with a diameter of 10 mm and a height of 0.3 mm. Three support columns with a diameter of 1 mm are evenly distributed. The water injection port is at the same height as the sensing cavity. The outside of the sensing cavity is connected to the sensing channel, which is 0.05 mm high and 1 mm wide.

[0053] First water drop 2: Inject the water drop into the sensing cavity of the first PDMS structure layer 1 through the water injection port, and the injection volume just reaches the volume of the airspace in the sensing cavity of the first PDMS structure layer 1.

[0054] First PDMS friction layer 3: The first PDMS friction layer 3 is spin-coated on the flexible PET substrate 6 at a speed of 2000 revolutions per minute using a spin coating technique. In the center of the spin-coated friction layer, there is an area with a diameter of approximately 2 mm without a spin coating layer. Before spin coating, it is covered with a mask to connect the first water droplet 2 in the sensing cavity of the first PDMS structural layer 1 to the corresponding first sensing cavity ITO electrode 5 at the bottom.

[0055] First flow channel ITO electrode 4: The first flow channel ITO electrode 4 has exactly the same shape as the sensing channel in the first PDMS structure layer 1. An extraction electrode is set at the center of the outermost circle to extract the electrical signal from the sensor. The extraction electrode is on the same straight line as the first sensing cavity ITO electrode 5.

[0056] First sensing cavity ITO electrode 5: used to transfer charge in the water in the sensing cavity, on the same plane as the first flow channel ITO electrode 4, with a width of about 2 mm, extending from the center to the edge of the device;

[0057] PET substrate 6: a flexible PET substrate material with a diameter of 40 mm. After a mask is set at the center of the substrate, the first PDMS friction layer 3 and the second PDMS friction layer 9 are spin-coated.

[0058] Second flow channel ITO electrode 7: The second flow channel ITO electrode 7 has exactly the same shape as the sensing channel in the second PDMS structure layer 11, but an extraction electrode is set at the center of the outermost circle to lead the electrical signal out of the sensor. The extraction electrode is on the same straight line as the second sensing cavity ITO electrode 8.

[0059] Second sensing cavity ITO electrode 8: used to transfer charge in the water in the sensing cavity, on the same plane as the second flow channel ITO electrode 7, with a width of about 2 mm, extending from the center to the edge of the device;

[0060] Second PDMS friction layer 9: The second PDMS friction layer 9 is spin-coated on the flexible PET substrate 6 at a speed of 500 revolutions per minute using a spin coating technique. In the center of the friction layer after spin coating, there is an area with a diameter of approximately 2 mm without a spin coating layer. Before spin coating, it is covered with a mask to connect the second water droplet 10 in the sensing cavity of the second PDMS structural layer 11 to the second sensing cavity ITO electrode 8 corresponding to the bottom.

[0061] Second water drop 10: The water drop is injected into the sensing cavity of the second PDMS structure layer 11 through the water injection port, and the injection volume just reaches the volume of the airspace in the sensing cavity of the second PDMS structure layer 11.

[0062] The second PDMS structural layer 11 includes a sensing cavity, a water inlet, and a sensing channel. The sensing cavity is a cylindrical cavity with a diameter of 10 mm and a height of 1.1 mm. Four support columns with a diameter of 1 mm are evenly distributed, and the water inlet is at the same height as the sensing cavity. The outside of the sensing cavity is connected to the sensing channel, which is 0.1 mm high and 1 mm wide.

[0063] The parts are concentrically aligned in the drawings.

[0064] The first and second structures are different, but the basic principles are the same. Taking the first as an example, the basic principle of the flexible physiological force sensor is as follows: Figure 2 As shown, the cross-sectional view is combined with the top view to intuitively show the structural deformation of the sensing cavity of the first PDMS structural layer 1 when under pressure and the deformation process of the first water droplet 2. Thanks to the high transparency of PDMS and ITO, even in the absence of back-end electrical signal output, the applied force can be estimated by observing the change in the position of the gas-liquid interface. The three figures show the flow of microfluids in the sensor and the deformation of the device under no force, slight force, and full load force conditions. When there is no external force, the microfluid is mainly stored in the sensing cavity. When it is slightly subjected to external force, the sensing cavity deforms, causing the fluid in the cavity to be discharged into the sensing channel. When it is fully loaded, the fluid in the cavity is discharged into the sensing channel in large quantities due to deformation, generating a capacitive effect and a triboelectric effect with the first flow channel ITO electrode 4 and the first PDMS friction layer 3.

[0065] The first and second have the same capacitance mechanism and friction mechanism, such as Figure 3 、 4 As shown in Figures 5 and 6, it is assumed that the vertical displacement of the contact point is proportional to the applied contact force, and the entire sensing cavity is deformed. Combined with the geometric differences between the sensing cavity and the sensing channel and the liquid volume conservation effect, the strain on the elastomer shell is amplified, resulting in a small mechanical compression in the sensing chamber to produce a significant fluid displacement, driving the droplets into the sensing channel and interacting with the flow channel ITO electrode ( Figure 3 ), the figure shows three situations: no external force, pressure, and pressure release, corresponding to three different capacitance mechanism diagrams. The fluid discharged from the sensing cavity acts as a variable electrode, and forms a flat-plate capacitor with the flow channel ITO electrode that changes with pressure.

[0066] Under the action of dynamic external force, the deflection of the flexible shell and the compression of the sensing cavity jointly change the position of the air-water interface, and its displacement is positively correlated with the change in capacitance ( Figure 4 ). The three marker sites in the figure correspond to Figure 3When the pressure increases to 0.95kPa, the output capacitance reaches 17.5pF in different states. Under the same pressure conditions, the output capacitance is repeatable. Notably, the applied force and the output electrical signal are highly aligned with minimal hysteresis, and there is virtually no electrical signal-pressure hysteresis loop. This force-electrical fit provides a solid foundation for the device's application as a reliable force sensor. Once the external pressure is released, the elastomer quickly recovers, generating a vacuum force that draws the fluid back into the sensing chamber and away from the coplanar electrodes, enabling the detection of dynamic and static force changes.

[0067] Triboelectric charging and electrostatic induction occur between the water and the underlying PDMS friction layer. To prevent capacitance between the water and the force applied in the initial sensing chamber from interfering with the experiment, the water is grounded. When the water first contacts and separates from the PDMS channel, the PDMS attracts electrons from the water. Consequently, due to the different electron affiliations, the end of the PDMS layer becomes negatively charged, while the end of the water becomes positively charged. When the external force is removed, the vacuum force causes the water to return to the sensing chamber, where the negatively charged PDMS and the positively charged ITO electrode balance the potential. When pressure is applied again to the water chamber, the sensing chamber is compressed, allowing water to flow into the channel. Once the water front (the interface between water and air) passes the ITO electrode, the positive charge in the water balances the negative charge on the PDMS layer, forcing electrons to flow from the ground to the ITO. When the pressure is released and the water flows back into the chamber, the negative charge on the PDMS layer forces the electrons to flow to the ground. The device uses water and the ITO at the bottom of the sensing channel as the lead-out electrodes. Therefore, for this mode, when the sensing chamber is subjected to dynamic force, there is a peak when the water front flows forward through the ITO electrode, and a trough when the water front leaves the ITO electrode backward, but the measured voltage values ​​are all above the base axis. The triboelectric mechanism requires liquid to flow through the electrode to generate an output signal. The four positions marked in the figure correspond to Figure 5 When the pressure increases to 0.95kPa, the output voltage is about 0.44V.

[0068] When subjected to external force, the water discharged from the sensing cavity into the sensing channel generates triboelectric effect and electrostatic induction with the PDMS friction layer at the bottom of the sensing channel. When water first contacts the PDMS friction layer, the PDMS becomes negatively charged due to electron transfer, while the water becomes positively charged (the water body is grounded to suppress parasitic capacitance). After the pressure is released, the vacuum force causes the water to flow back, and the negative charge of the PDMS is balanced with the potential of the ITO electrode. When pressure is applied again, the water front (the interface between water and air) passes through the ITO electrode, and the positive charge in the water balances the negative charge on the PDMS layer, thereby forcing electrons to flow from the ground to the ITO, triggering charge compensation. When the pressure is released, the flow reverses, forming a unipolar voltage pulse (a peak-to-valley waveform above the baseline). Figure 6 ).

[0069] Figure 7As shown in Figure 2, the sensitivity curve of the microfluidic physiological force sensor can be divided into two regions: low pressure (0–3.2 kPa) and high pressure (3.2–9.6 kPa). In the low pressure range, the sensor sensitivity can reach 1.862 kPa. -1 , and the output capacitance increases approximately linearly with the applied force; in the high pressure range, the sensitivity is 0.127kPa -1 Despite a uniform increase in applied pressure, the rate of increase in output capacitance slows significantly and rapidly approaches saturation. This phenomenon is attributed to the fact that the capacitance change depends on the final position of the water within the sensing channel: in the low-pressure range, the sensing chamber is filled with water, which is more compressible, and the water front can quickly move in response to external forces. As the applied pressure increases, water gradually drains from the sensing chamber, causing the air pressure at the end of the sensing channel to rise sharply, creating significant resistance, slowing the movement of the water front and ultimately saturating it.

[0070] Figure 8 The image shows a real-time recording of multiple periodic radial artery pulse signals, along with a high-resolution image of a segment of the pulse wave. Thanks to the sensor's high sensitivity, the captured pulse waveform is distinct and contains three identifiable peaks: the impact wave (P), the tidal wave (T), and the recoil wave (D). A complete pulse waveform consists of the forward wave P ejected from the left ventricle and the reverse waves T and D, generated by the resistance to blood flow in the distal cardiovascular system. Based on the information between these three peaks, the pulse wave stiffness index can be assessed, providing a basis for the non-invasive diagnosis of atherosclerosis.

[0071] The present invention also provides an embodiment of a method for preparing a physiological force sensor.

[0072] Substrate and electrode preparation

[0073] A flexible polyethylene terephthalate (PET) substrate (purchased from Taobao) with a diameter of 40 mm and a thickness of 250 μm was used as the substrate. A pre-designed mask was applied to the substrate before ITO sputtering to ensure that the metal electrodes were deposited only in the designated flow channel areas, thereby minimizing parasitic capacitance effects. A spin coater (IC8000S, Jiangsu Leibo Scientific Instrument Co., Ltd.) was used to spin-coat PDMS onto the PET substrate with ITO electrodes at speeds of 2000 rpm and 500 rpm, respectively, to form friction layers 3 and 9. A heating oven was set to 80°C for 30 minutes to ensure uniform curing of the friction layers.

[0074] Microfluidic structure layer preparation

[0075] Step 1: Preparation of positive mold. The manufacturing process of the first PDMS structure layer 1 and the second PDMS structure layer 11 is the same. Both need to use photolithography technology to pre-make the positive molds of the first PDMS structure layer 1 and the second PDMS structure layer 11. The structural layer with a special structure can be obtained by performing inversion on the prepared mold. The specific steps are as follows: fix the pretreated silicon wafer in the center of the tray of the glue machine, add an appropriate amount of SU-8 photoresist to the center of the silicon wafer, and set the spin coating parameters to obtain the required photoresist thickness, and soft bake at 95°C for 5 minutes. Use the commercial software Auto CAD to design the reverse polyester film mask (Mask) manufactured by SMIC Qiheng (Suzhou) at 22.5mW / cm 2 Expose to ultraviolet light for 35 seconds. This mask is used to block specific areas in the photolithography process to ensure precise transfer of the design pattern to the photoresist. After exposure, bake on a 95°C hotplate for 5 minutes, develop the silicon wafer, and finally hard bake in a 140°C oven for 5 minutes. After the silicon wafer cools to room temperature, the substrate is immersed in propylene glycol methyl ether acetate and isopropyl alcohol solutions for 15 seconds each. Residual SU-8 photoresist solvent is removed through cyclic development, completing the preparation of the PDMS positive mold. Due to the height difference between the sensing channel, the injection port, and the sensing chamber in this experiment, two rounds of photolithography were performed on the same silicon wafer backing plate.

[0076] Step 2: Preparation of structural layers. The first PDMS structural layer 1 and the second PDMS structural layer 11 need to be obtained on the positive mold prepared in step 1 by the casting technique. Specifically: the liquid PDMS elastomer (Sylgard 184, Dow Corning) and the cross-linking agent are stirred and mixed in a ratio of 10:1, the mixture is placed in a vacuum box to extract the air mixed in the mixture due to stirring, and then the mixture without air is poured onto the positive mold of the first PDMS structural layer 1 and the second PDMS structural layer 11 prepared by the photolithography technology in step 1, and cured at 80°C for at least three hours to form a top PDMS structural layer with a thickness of 2.1 mm, and finally the cured first PDMS structural layer 1 and the second PDMS structural layer 11 are removed from the positive mold by tweezers.

[0077] Step 3: Integration of substrate and structural layer. The first PDMS structural layer 1 and the second PDMS structural layer 11, which have been peeled and trimmed from the positive mold, are aligned with the first flow channel ITO electrode 4 of the first PDMS friction layer 3 and the second flow channel ITO electrode 7 of the second PDMS friction layer 9, which have been pre-prepared and spin-coated on the front and back sides of the flexible PET substrate. The first PDMS structural layer 1 and the second PDMS structural layer 11 are treated with oxygen plasma using a vacuum plasma treatment apparatus (VR-RS6, Guangzhou SunJune Technolog Co., Ltd.) to achieve permanent bonding. Finally, BD 301 The 2G needle is inserted into the injection port of the elastomer housing, and a glass syringe equipped with an injection pump is used to inject liquid into the microfluidic network at a calibrated flow rate. Water is chosen as the working fluid due to its advantages of non-toxicity, low density, and high dielectric constant.

[0078] The above is only the specific steps of the present application, and does not constitute any limitation on the protection scope of the present application; any technical solutions formed by equivalent transformation or equivalent replacement fall within the protection scope of the present application; and the parts not described in detail in the present application belong to the commonly known technology of those skilled in the art.

Claims

1. A flexible force sensor that generates interfacial capacitance and triboelectric effect due to microfluid deformation, characterized in that: The flexible force sensor for generating interface capacitance and triboelectric effect by deformation of microfluid comprises a high-precision part and a large-range part, the high-precision part and the large-range part share the same PET substrate (6) and are distributed on both sides of the PET substrate (6); The high-precision part includes a first PDMS structural layer (1), a first water droplet (2), a first PDMS friction layer (3), a first flow channel ITO electrode (4) and a first sensing cavity ITO electrode (5); The large-range part comprises a second flow channel ITO electrode (7), a second sensing cavity ITO electrode (8), a second PDMS friction layer (9), a second water droplet (10) and a second PDMS structural layer (11); The first PDMS structural layer (1) comprises a sensing cavity, a water injection port and a sensing channel; the sensing cavity is a cylindrical cavity arranged at the center of the first PDMS structural layer (1) for accommodating a first water droplet (2); the water injection port is arranged at the same height as the sensing cavity; the sensing channel is arranged outside the sensing cavity and connected to the sensing cavity; the first water droplet (2) injects the water droplet into the sensing cavity through the water injection port; a circular hole is provided at the center of the first PDMS friction layer (3), the circular hole is connected to the sensing cavity, and is used to conduct the charge of the first water droplet (2) in the sensing cavity to the first sensing cavity ITO electrode (5); the first flow channel ITO electrode (4) and the first sensing cavity ITO electrode (5) are arranged between the first PDMS friction layer (3) and the PET substrate (6); The large-range part and the high-precision part are symmetrically arranged with the PET substrate (6) as the center.

2. The flexible force sensor according to claim 1, wherein the microfluidic deformation generates interfacial capacitance and triboelectric effect, The sensing chamber is filled with water through the water inlet, and the end of the microfluidic channel is closed. In the absence of external force, the air pressure at the end is balanced with the water pressure in the sensing chamber. When pressure is applied, the water droplets in the sensing cavity will be pressed into the sensing channel to varying degrees, forming a capacitor with the ITO electrode of the flow channel corresponding to the sensing channel, generating a capacitance signal; at the same time, the water droplets produce a triboelectric effect during the contact process with the friction layer, generating a voltage signal, thereby reflecting the magnitude of the applied pressure through the output electrical signal.

3. The flexible force sensor according to claim 1, wherein the microfluidic deformation generates interfacial capacitance and triboelectric effect, The sensing cavity of the first PDMS structural layer (1) has a diameter of 10 mm, a height of 0.3 mm, and three support pillars with a diameter of 1 mm are evenly distributed; The sensing cavity of the second PDMS structural layer (11) has a diameter of 10 mm and a height of 1.1 mm, and is evenly distributed with four support pillars with a diameter of 1 mm; The support column is used to prevent the sensing cavity from being unable to rebound due to excessive load, and can also change the Young's modulus of the sensing cavity to change the range and sensitivity of the sensor.

4. The flexible force sensor according to claim 1, wherein the microfluidic deformation generates interfacial capacitance and triboelectric effect, The shape of the sensing channel of the first PDMS structural layer (1) is the same as the shape of the first flow channel ITO electrode (4); The shape of the sensing channel of the second PDMS structural layer (11) is the same as the shape of the second flow channel ITO electrode (7).

5. The flexible force sensor according to claim 1, wherein the microfluid deformation generates interfacial capacitance and triboelectric effect, The first flow channel ITO electrode (4) is provided with an extraction electrode at the center of the outermost circle and on the same straight line as the first sensing cavity ITO electrode (5), for extracting an electrical signal from the sensor; the first sensing cavity ITO electrode (5) is a straight line structure, extending from the center to the edge of the device, and the extraction electrode is provided at the edge.

6. The flexible force sensor according to claim 1, wherein the microfluidic deformation generates interfacial capacitance and triboelectric effect, The method for preparing the flexible force sensor that generates interface capacitance and triboelectric effect due to microfluid deformation includes: A substrate is prepared; the substrate comprises a PET backing (6), a friction layer and an ITO electrode; the friction layer comprises a first PDMS friction layer (3) and a second PDMS friction layer (9); the ITO electrode comprises a first flow channel ITO electrode (4), a first sensing cavity ITO electrode (5), a second flow channel ITO electrode (7) and a second sensing cavity ITO electrode (8); The microfluidic structure layer is prepared, and the preparation of the microfluidic structure layer further includes the following steps: Step 1: preparing a positive mold of the first PDMS structure layer (1) and the second PDMS structure layer (11); Step 2: preparing a first PDMS structure layer (1) and a second PDMS structure layer (11) based on the positive mold; Step 3: Integrate the substrate and microfluidic structure layer.

7. The flexible force sensor according to claim 6, wherein the microfluid deformation generates interfacial capacitance and triboelectric effect, The PET substrate (6) is a flexible polyethylene terephthalate substrate; a pre-designed mask is attached to the substrate and then an ITO electrode is sputtered to ensure that the metal electrode is deposited only in the designated flow channel area, thereby minimizing the parasitic capacitance effect; PDMS is spin-coated on the PET substrate (6) with the ITO electrode at speeds of 2000 rpm and 500 rpm respectively by a spin coater to form a friction layer (3) and a friction layer (9); a circular through-hole mask with a diameter of 2 mm is pre-attached to the center of the PET substrate (6), which is removed after spin coating to form a circular through-hole with a diameter of 2 mm; a heating box is set to 80° C. and continuously heated for 30 minutes to uniformly solidify the friction layer.

8. The flexible force sensor according to claim 6, wherein the microfluid deformation generates interfacial capacitance and triboelectric effect, The positive mold process for preparing the first PDMS structure layer (1) and the second PDMS structure layer (11) is the same; Fix the pretreated silicon wafer in the center of the spin coater tray, add an appropriate amount of SU-8 photoresist to the center of the silicon wafer, and set the spin coating parameters to obtain the desired photoresist thickness. Soft bake at 95°C for 5 minutes; Design a reverse polyester film mask and 2 Expose to UV light for 35 seconds; After exposure, bake on a 95°C hot plate for 5 minutes, develop the silicon wafer, and finally hard bake in a 140°C oven for 5 minutes; After the silicon wafer cools to room temperature, the substrate is immersed in propylene glycol methyl ether acetate and isopropyl alcohol solutions for 15 seconds each. The residual SU-8 photoresist solvent is removed by cyclic development, thus completing the preparation of the PDMS positive mold.

9. The flexible force sensor according to claim 6, wherein the microfluid deformation generates interfacial capacitance and triboelectric effect, In step 2, the liquid PDMS elastomer and the cross-linking agent are stirred and mixed in a ratio of 10:1, the mixture is placed in a vacuum box to extract the air in the mixture, and then the mixture without air is poured onto the positive mold of the first PDMS structural layer (1) and the second PDMS structural layer (11), and cured at 80° C. for at least three hours to form a top PDMS structural layer with a thickness of 2.1 mm. Finally, the cured first PDMS structural layer (1) and the second PDMS structural layer (11) are removed from the positive mold by tweezers.

10. The flexible force sensor according to claim 6, wherein the microfluid deformation generates interfacial capacitance and triboelectric effect, In step three, the first PDMS structure layer (1) and the second PDMS structure layer (11) peeled and trimmed from the positive mold are aligned with the first flow channel ITO electrode (4) of the first PDMS friction layer (3) and the second flow channel ITO electrode (7) of the second PDMS friction layer (9) pre-prepared and spin-coated on the front and back sides of the flexible PET substrate respectively; A vacuum plasma treatment apparatus is used to perform oxygen plasma treatment on the first PDMS structural layer (1), the second PDMS structural layer (11) and the flexible PET substrate (6) to achieve permanent bonding.

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