CN113776699B - Positive pressure insensitive interdigital capacitive strain sensor and preparation method thereof - Google Patents
Positive pressure insensitive interdigital capacitive strain sensor and preparation method thereof Download PDFInfo
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- CN113776699B CN113776699B CN202111111958.4A CN202111111958A CN113776699B CN 113776699 B CN113776699 B CN 113776699B CN 202111111958 A CN202111111958 A CN 202111111958A CN 113776699 B CN113776699 B CN 113776699B
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Classifications
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
- G01B7/00—Measuring arrangements characterised by the use of electric or magnetic techniques
- G01B7/16—Measuring arrangements characterised by the use of electric or magnetic techniques for measuring the deformation in a solid, e.g. by resistance strain gauge
- G01B7/22—Measuring arrangements characterised by the use of electric or magnetic techniques for measuring the deformation in a solid, e.g. by resistance strain gauge using change in capacitance
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01L—MEASURING FORCE, STRESS, TORQUE, WORK, MECHANICAL POWER, MECHANICAL EFFICIENCY, OR FLUID PRESSURE
- G01L1/00—Measuring force or stress, in general
- G01L1/14—Measuring force or stress, in general by measuring variations in capacitance or inductance of electrical elements, e.g. by measuring variations of frequency of electrical oscillators
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01L—MEASURING FORCE, STRESS, TORQUE, WORK, MECHANICAL POWER, MECHANICAL EFFICIENCY, OR FLUID PRESSURE
- G01L1/00—Measuring force or stress, in general
- G01L1/14—Measuring force or stress, in general by measuring variations in capacitance or inductance of electrical elements, e.g. by measuring variations of frequency of electrical oscillators
- G01L1/142—Measuring force or stress, in general by measuring variations in capacitance or inductance of electrical elements, e.g. by measuring variations of frequency of electrical oscillators using capacitors
- G01L1/148—Measuring force or stress, in general by measuring variations in capacitance or inductance of electrical elements, e.g. by measuring variations of frequency of electrical oscillators using capacitors using semiconductive material, e.g. silicon
Abstract
The invention relates to a strain sensor, in particular to a positive pressure insensitive interdigital capacitive strain sensor and a preparation method thereof. The invention solves the problems that the existing capacitive strain sensor can not distinguish tension from positive pressure and can not be applied to large tensile strain. A positive pressure insensitive interdigital capacitive strain sensor comprises a flexible upper substrate and a flexible lower substrate; an interdigital microfluidic channel is formed on the lower surface of the flexible upper substrate, and a filling hole which is vertically communicated is formed between two wiring ends of the interdigital microfluidic channel and the upper surface of the flexible upper substrate; the lower surface of the flexible upper substrate and the upper surface of the flexible lower substrate are bonded together; the interdigital microfluidic channel is filled with liquid metal interdigital electrodes; the openings of the two filling holes are plugged with adhesive. The invention is suitable for the fields of human-machine interfaces, soft robots, electronic skins and the like.
Description
Technical Field
The invention relates to a strain sensor, in particular to a positive pressure insensitive interdigital capacitive strain sensor and a preparation method thereof.
Background
Strain sensors made of soft stretchable materials, which can help monitor the motion of soft robots or convert human body motion into electrical signals, are of great interest in the field of human-machine interfaces, soft robots, electronic skin, etc.
The sensing mechanisms of strain sensors are mainly three: capacitive sensing, piezoresistive sensing, piezoelectric sensing. In these sensing mechanismsCapacitive sensing is preferred over other sensing mechanisms due to its low temperature coefficient, low power consumption and low hysteresis behavior. But for parallel plate capacitance, the capacitor is composed ofIt can be known that the capacitance is changed no matter the tension or the positive pressure, and the crosstalk problem exists between the tension sensing and the positive pressure sensing, so that the parallel polar plate capacitance type strain sensor cannot distinguish the tension from the positive pressure.
For an interdigital capacitive strain sensor capable of decoupling tension sensing from positive pressure sensing, capacitance of the interdigital capacitive strain sensor is generated between electrodes, and one unit capacitance C of the interdigital capacitance C u Consists of three parallel capacitors, two fringe electric field capacitors C su1 And C su2 A parallel field capacitance C m Wherein the fringe field capacitance is defined byCalculated, the parallel field capacitance is defined by +.>Calculated, the total capacitance is defined by +.> And (5) calculating.
The electrodes, which are the key elements of the interdigital capacitive strain sensor, should have good electrical conductivity even under tensile conditions. However, the tensile material films currently selected as interdigital capacitive strain sensor electrodes, such as silver nanowires or carbon nanotube networks, while maintaining electrical conductivity at moderate tensile strains, cannot maintain electrical conductivity at tensile strains exceeding 100%, thus resulting in interdigital capacitive strain sensors not being applicable to large tensile strains.
Ju Y H et al design an ultra-simple and easy tensile pressure sensor of ultraviolet curing adhesive tape auxiliary metal nanowire patterning in (Ju Y H, han C J, kim K S, et al UV-Curable Adhesive Tape-Assisted Patterning of Metal Nanowires for Ultrasimple Fabrication of Stretchable Pressure Sensor [ J ]. Advanced Materials Technologies, 2021:2100776.), this kind of sensor adopts interdigital electrode structure, silver nanowire as electrode material, to this sensor both exert pulling force and pressure, this sensor electric capacity can all take place to descend, there is the crosstalk problem between pulling force sensing and the pressure sensing, the sensor can't distinguish pulling force and pressure, and this sensor ' S electrode can only keep electric conductivity under moderate tensile strain, can't be applied to big tensile strain.
Hesam Mahmoudinezhad Masoumeh et al (Hesam Mahmoudinezhad Masoumeh, anderson Iain, rosset Samul. Interdigitted Sensor Based on a Silicone Foam for Subtle Robotic Manipulation. [ J ]. Macromolecular rapid communications, 2020:) designed a silicon foam-based interdigital sensor that uses an interdigital electrode structure with interdigital electrodes formed on a flexible Printed Circuit Board (PCB) and covered with a compressible resilient foam that could only be used to detect pressure, could not be used to detect tension, and could only detect pressure in the range of 50N, with a small detection range.
Patent CN202110131844.X (application date 2021, 1, 30, publication date 2021, 6, 15) discloses a flexible interdigital capacitance sensor structure and a preparation method thereof. The sensor consists of a basal layer, a middle interdigital electrode layer and an upper packaging layer, an interdigital electrode structure is adopted, the electrode is formed by uniformly mixing liquid silicone rubber, rigid conductive fiber particles, a diluent and a synergist, the capacitance of the sensor can be reduced no matter whether tension or pressure is applied to the sensor, the crosstalk problem exists between tension sensing and pressure sensing, the sensor cannot distinguish tension and pressure, and the sensor only has a strain working range of 0% -45%, so that the sensor is not suitable for large tensile strain.
Patent CN202010311401.4 (application date of 4 months of 20 days of 2020, publication date of 8 months of 2020) discloses an interdigital counter electrode type flexible touch sensor based on super capacitor sensing principle, wherein interdigital electrodes of the sensor are prepared by printing conductive ink on a flexible substrate by adopting a screen printing process, and the sensor also adopts an interdigital electrode structure, but can only be used for detecting pressure and is not suitable for detecting tensile force.
In summary, the current research and lack of patents on flexible interdigital sensors have the following limitations regarding the current flexible interdigital sensors:
(1) Most of flexible interdigital sensors are pressure sensors, preparation and performance research of the flexible interdigital sensors under tensile load are lacking, and part of flexible interdigital sensors can detect tensile force, but crosstalk problems exist between the tensile force sensing and positive pressure sensing, so that the sensors cannot distinguish the tensile force from the pressure.
(2) Many flexible interdigital sensors have a small detection range and are not suitable for large tensile strains.
The research shows that the earthworms are composed of a plurality of segments which are arranged in parallel, when the earthworms move, the segments relax and contract alternately, the tactile response is realized by means of the body surface muscle structure and myoelectric reaction, and the nerve cable is adopted to transmit external stimulation signals, so that an excellent perception mechanism is formed, and important bionic teaching is provided for the earthworms. The positive pressure insensitive interdigital capacitive strain sensor is designed based on the earthworm myoelectric reaction principle, and the problems that the traditional capacitive strain sensor cannot distinguish tension from positive pressure and cannot be applied to large tensile strain can be solved.
Disclosure of Invention
The invention provides a positive pressure insensitive interdigital capacitive strain sensor and a preparation method thereof, which aim to solve the problems that the traditional capacitive strain sensor cannot distinguish tension from positive pressure and cannot be applied to large tensile strain.
The invention is realized by adopting the following technical scheme:
a positive pressure insensitive interdigital capacitive strain sensor comprises a flexible upper substrate and a flexible lower substrate; an interdigital microfluidic channel is formed on the lower surface of the flexible upper substrate, and a filling hole which is vertically communicated is formed between two wiring ends of the interdigital microfluidic channel and the upper surface of the flexible upper substrate; the lower surface of the flexible upper substrate and the upper surface of the flexible lower substrate are bonded together; the interdigital microfluidic channel is filled with liquid metal interdigital electrodes; the openings of the two filling holes are plugged with adhesive.
The flexible upper substrate and the flexible lower substrate are rectangular, the thickness of the flexible upper substrate and the flexible lower substrate is smaller than 1mm, and the flexible upper substrate and the flexible lower substrate are made of PDMS; the diameters of the two filling holes are 1mm; the thickness of the liquid metal interdigital electrode is 50 mu m, the distance between two adjacent finger parts is 200 mu m, the length of each finger part is 1cm, and the width of each finger part is 100 mu m; the adhesive adopts Sil-Poxy silica gel adhesive; two terminals of the liquid metal interdigital electrode are respectively connected with a wire.
The preparation method of the positive pressure insensitive interdigital capacitive strain sensor (the method is used for preparing the positive pressure insensitive interdigital capacitive strain sensor) is realized by adopting the following steps:
step S1: preparing a flexible upper substrate; the method comprises the following specific steps:
step S1.1: selecting a first silicon wafer, and forming interdigital bulges on the upper surface of the first silicon wafer by adopting a photoetching process;
step S1.2: spin-coating a first PDMS layer on the upper surface of the first silicon wafer, ensuring that the first PDMS layer fully covers the interdigital protrusions, and then solidifying the first PDMS layer;
step S1.3: stripping the cured first PDMS layer, thereby obtaining a flexible upper substrate with an interdigital microfluidic channel on the lower surface;
step S1.4: a filling hole which is vertically penetrated is respectively drilled between the two wiring ends of the interdigital microfluidic channel and the upper surface of the flexible upper substrate;
step S2: preparing a flexible lower substrate; the method comprises the following specific steps:
step S2.1: selecting a second silicon wafer;
step S2.2: spin-coating a second PDMS layer on the upper surface of the second silicon wafer, and then solidifying the second PDMS layer;
step S2.3: stripping the cured second PDMS layer, thereby obtaining a flexible lower substrate;
step S3: bonding the lower surface of the flexible upper substrate and the upper surface of the flexible lower substrate together;
step S4: cutting the flexible upper substrate and the flexible lower substrate into rectangular shapes;
step S5: placing a drop of liquid metal at each of the openings of the two filling holes;
step S6: firstly, filling two drops of liquid metal into an interdigital microfluidic channel by adopting a vacuum filling method to form a liquid metal interdigital electrode, then inserting a lead into two wiring ends of the liquid metal interdigital electrode respectively, and then plugging the orifices of the two filling holes by adopting an adhesive, thereby completing the preparation.
In the step S1, the steps of the photolithography process are sequentially as follows: gluing, pre-baking, exposing, post-baking and developing;
when the photoresist is coated, the coated photoresist is SU-8 3035 negative photoresist, the spin coating speed is firstly set to be 500rpm for 11 seconds, and then the spin coating speed is adjusted to be 2000rpm for 30 seconds;
in the pre-baking process, the baking temperature is 95 ℃, and the pre-baking time is 15min;
during exposure, the exposure light source is ultraviolet light, the exposure time is 4s, and the exposure energy is 250mJ/cm 2 ；
In post-baking, baking at 65deg.C for 1min, and baking at 95deg.C for 5min;
when developing, the developing solution is SU-8 developing solution.
In the step S1 and the step S2, the curing is performed by adopting a heating plate, the heating temperature is 80 ℃, and the heating time is 4 hours.
In the step S1, the filling hole is drilled by a perforator.
In the step S1 and the step S2, PDMS is formed by mixing an elastomer matrix and a curing agent according to a mass ratio of 10:1.
In the step S3, plasma is used to bond the lower surface of the flexible upper substrate and the upper surface of the flexible lower substrate together.
In the step S6, the vacuum filling method specifically includes the following steps: placing the flexible upper substrate and the flexible lower substrate in a vacuum chamber for 20min; after releasing the vacuum, the atmospheric pressure pushes two drops of liquid metal to flow into the interdigital microfluidic channel to form a liquid metal interdigital electrode.
In operation, as shown in fig. 15, the liquid metal interdigitated electrodes create capacitance through edge effects and parallel fields. The invention distinguishes the pulling force from the positive pressure according to the change of the distance between the adjacent two finger parts of the liquid metal interdigital electrode. The liquid metal interdigital electrode is positioned on the same plane, the thickness of the electrode is only 50 mu m, and the distance between two adjacent finger parts is easy to change when the liquid metal interdigital electrode is stressed by tensile force. As shown in FIG. 16, when the invention is under tension, the spacing d between two adjacent fingers of the liquid metal interdigital electrode 0 Increase the thickness t of the electrode 0 Reduced length l of each finger 0 Each of which has a reduced width w 0 All increase, and the capacitance can be reduced along with the increase of the tensile force according to the calculation formulas of the fringe capacitance, the parallel field capacitance and the total capacitance. As shown in FIG. 17, the thickness t of the liquid metal interdigitated electrode when the present invention is subjected to positive pressure 0 The distance d between two adjacent fingers caused by poisson effect is negligible 0 The change is negligible, and the capacitance can be obtained to only slightly change according to the calculation formulas of the fringe capacitance, the parallel field capacitance and the total capacitance. Therefore, the tension can be distinguished from the positive pressure by comparing the change of the distance between two adjacent fingers caused by the tension and the positive pressure and the change of capacitance caused by the change. In other words, the present invention is insensitive to the positive pressure applied thereto, thereby enabling the distinction of the tensile force from the positive pressure. Meanwhile, due to the conductivity and fluidity of the liquid metal and the ductility of PDMS, the invention can detect the tensile strain of up to 100%, has a sensitivity coefficient of-0.3 and has good durability. In addition, the invention has low hysteresis<0.01). As shown in FIG. 18, the present invention has good ductility and can be twisted, bent, and folded into rolls. As shown in FIG. 19, the present invention has high scalability (100%) of 0% toIn the 100% tensile strain range, the capacitance change is almost linear, and the sensitivity coefficient is-0.3, so that the method can be applied to large tensile strain. As shown in fig. 20, the present invention can be applied to detect finger joint movement, wrist joint movement, elbow joint movement of a human body.
To verify the excellent performance of the present invention, the following tests were performed:
as shown in FIG. 21, the present invention was stretched at two stretching rates of 5mm/min and 20mm/min to give a stretching strain of 0% to 50%, thereby obtaining a strain-capacitance change curve. The curve shows that: the invention exhibits low hysteresis in the 50% tensile strain range, with a 20mm/min draw rate that increases after a 20mm/min time lag compared to the change in capacitance at a 5mm/min draw rate, but with a change in capacitance in the range of only 0.01.
As shown in fig. 22, the cyclic strain-capacitance change curve was obtained by repeatedly stretching the present invention under a tensile strain condition of 30%. The curve shows that: the invention exhibits good stability and durability under continuous cyclic dynamic loading.
As shown in fig. 23, positive pressure-capacitance change curves were obtained by applying positive pressures of 0N, 10N, 15N, 17.5N under tensile strain conditions of 0% and 20%. The curve shows that: at 0% tensile strain, the capacitance was unchanged during the addition and removal of the weights, indicating that the invention is insensitive to positive pressure in the unstretched condition; at 20% tensile strain, the capacitance only changed by 0.004 during addition and removal of the weight, indicating that the invention is insensitive to positive pressure even under tensile conditions.
From the above results, it can be seen that: the invention has positive pressure insensitivity, can distinguish pulling force from positive pressure, can be applied to large tensile strain, and has good sensitivity, low hysteresis and durability.
The invention effectively solves the problems that the existing capacitive strain sensor can not distinguish tension from positive pressure and can not be applied to large tensile strain, and is suitable for the fields of human-computer interfaces, soft robots, electronic skins and the like.
Drawings
Fig. 1 is a schematic perspective view of the present invention.
Fig. 2 is a schematic cross-sectional view of the present invention.
Fig. 3 is a schematic cross-sectional view of the present invention.
Fig. 4 is a schematic diagram of step S1.1 in the present invention.
Fig. 5 is a schematic diagram of step S1.2 in the present invention.
Fig. 6 is a schematic diagram of step S1.3 in the present invention.
Fig. 7 is a schematic diagram of step S1.4 in the present invention.
Fig. 8 is a schematic diagram of step S2.1 in the present invention.
Fig. 9 is a schematic diagram of step S2.2 in the present invention.
Fig. 10 is a schematic diagram of step S2.3 in the present invention.
Fig. 11 is a schematic diagram of step S3 in the present invention.
Fig. 12 is a schematic diagram of step S4 in the present invention.
Fig. 13 is a schematic diagram of step S5 in the present invention.
Fig. 14 is a schematic diagram of step S6 in the present invention.
Fig. 15 is a schematic diagram of the working principle of the present invention.
Fig. 16 is a schematic diagram of the working principle of the present invention.
Fig. 17 is a schematic diagram of the working principle of the present invention.
Fig. 18 is a schematic view of the invention with good extensibility.
FIG. 19 is a graph showing the capacitance change under different tensile strain conditions according to the present invention.
Fig. 20 is a schematic view of the present invention applied to detect finger joint movement, wrist joint movement, elbow joint movement of a human body.
FIG. 21 is a schematic of hysteresis of the present invention under different draw rate conditions.
FIG. 22 is a schematic representation of the capacitance change under a certain tensile strain and rapid stretching condition of the present invention.
FIG. 23 is a graphical representation of the change in capacitance of the present invention with positive pressure applied under different tensile strain conditions.
In the figure: the liquid metal micro-fluidic chip comprises a 1-flexible upper substrate, a 2-flexible lower substrate, a 3-interdigital micro-fluidic channel, 4-filling holes, 5-liquid metal interdigital electrodes, 6-adhesives, 7-first silicon chips, 8-interdigital protrusions, 9-first PDMS layers, 10-second silicon chips, 11-second PDMS layers, 12-liquid metal and 13-wires.
Detailed Description
A positive pressure insensitive interdigital capacitive strain sensor comprises a flexible upper substrate 1 and a flexible lower substrate 2; an interdigital microfluidic channel 3 is formed on the lower surface of the flexible upper substrate 1, and a filling hole 4 which is vertically communicated is formed between two wiring ends of the interdigital microfluidic channel 3 and the upper surface of the flexible upper substrate 1; the lower surface of the flexible upper substrate 1 and the upper surface of the flexible lower substrate 2 are bonded together; the interdigital microfluidic channel 3 is filled with liquid metal interdigital electrodes 5; the openings of both filling holes 4 are plugged with adhesive 6.
The flexible upper substrate 1 and the flexible lower substrate 2 are rectangular, the thickness of the flexible upper substrate and the flexible lower substrate is smaller than 1mm, and the flexible upper substrate and the flexible lower substrate are made of PDMS; the diameters of the two filling holes 4 are 1mm; the thickness of the liquid metal interdigital electrode 5 is 50 mu m, the distance between two adjacent finger parts is 200 mu m, the length of each finger part is 1cm, and the width of each finger part is 100 mu m; the adhesive 6 is Sil-Poxy silica gel adhesive; one wire 13 is connected to each of the two terminals of the liquid metal interdigital electrode 5.
The preparation method of the positive pressure insensitive interdigital capacitive strain sensor (the method is used for preparing the positive pressure insensitive interdigital capacitive strain sensor) is realized by adopting the following steps:
step S1: preparing a flexible upper substrate 1; the method comprises the following specific steps:
step S1.1: selecting a first silicon wafer 7, and forming interdigital bulges 8 on the upper surface of the first silicon wafer 7 by adopting a photoetching process;
step S1.2: spin-coating a first PDMS layer 9 on the upper surface of the first silicon wafer 7, ensuring that the first PDMS layer 9 covers all the interdigital protrusions 8, and then solidifying the first PDMS layer 9;
step S1.3: stripping the cured first PDMS layer 9, thereby obtaining a flexible upper substrate 1 with the interdigital microfluidic channel 3 on the lower surface;
step S1.4: a filling hole 4 which is vertically penetrated is respectively drilled between the two wiring ends of the interdigital microfluidic channel 3 and the upper surface of the flexible upper substrate 1;
step S2: preparing a flexible lower substrate 2; the method comprises the following specific steps:
step S2.1: selecting a second silicon wafer 10;
step S2.2: spin-coating a second PDMS layer 11 on the upper surface of the second silicon wafer 10, and then curing the second PDMS layer 11;
step S2.3: stripping the cured second PDMS layer 11, thereby obtaining a flexible lower substrate 2;
step S3: bonding the lower surface of the flexible upper substrate 1 and the upper surface of the flexible lower substrate 2 together;
step S4: cutting the flexible upper substrate 1 and the flexible lower substrate 2 into rectangular shapes;
step S5: placing a drop of liquid metal 12 at each of the openings of the two filling holes 4;
step S6: firstly, two drops of liquid metal 12 are filled into the interdigital microfluidic channel 3 by adopting a vacuum filling method to form liquid metal interdigital electrodes 5, then two leads 13 are respectively inserted into two terminals of the liquid metal interdigital electrodes 5, and then the openings of the two filling holes 4 are plugged by adopting an adhesive 6, so that the preparation is completed.
In the step S1, the steps of the photolithography process are sequentially as follows: gluing, pre-baking, exposing, post-baking and developing;
when the photoresist is coated, the coated photoresist is SU-8 3035 negative photoresist, the spin coating speed is firstly set to be 500rpm for 11 seconds, and then the spin coating speed is adjusted to be 2000rpm for 30 seconds;
in the pre-baking process, the baking temperature is 95 ℃, and the pre-baking time is 15min;
during exposure, the exposure light source is ultraviolet light, the exposure time is 4s, and the exposure energy is 250mJ/cm 2 ；
In post-baking, baking at 65deg.C for 1min, and baking at 95deg.C for 5min;
when developing, the developing solution is SU-8 developing solution.
In the step S1 and the step S2, the curing is performed by adopting a heating plate, the heating temperature is 80 ℃, and the heating time is 4 hours.
In the step S1, the filling hole 4 is drilled by a perforator.
In the step S1 and the step S2, PDMS is formed by mixing an elastomer matrix and a curing agent according to a mass ratio of 10:1.
In the step S3, plasma is used to bond the lower surface of the flexible upper substrate 1 and the upper surface of the flexible lower substrate 2 together.
In the step S6, the vacuum filling method specifically includes the following steps: placing the flexible upper substrate 1 and the flexible lower substrate 2 in a vacuum chamber for 20min; after releasing the vacuum, the atmospheric pressure pushes two drops of liquid metal 12 to flow into the interdigital microfluidic channel 3 to form the liquid metal interdigital electrode 5.
While specific embodiments of the invention have been described above, it will be appreciated by those skilled in the art that these are by way of example only, and the scope of the invention is defined by the appended claims. Various changes and modifications to these embodiments may be made by those skilled in the art without departing from the principles and spirit of the invention, but such changes and modifications fall within the scope of the invention.
Claims (7)
1. A preparation method of a positive pressure insensitive interdigital capacitive strain sensor is characterized by comprising the following steps of: the method is used for preparing a positive pressure insensitive interdigital capacitive strain sensor, and the sensor comprises a flexible upper substrate (1) and a flexible lower substrate (2); an interdigital microfluidic channel (3) is formed on the lower surface of the flexible upper substrate (1), and a filling hole (4) which is vertically communicated is formed between two wiring ends of the interdigital microfluidic channel (3) and the upper surface of the flexible upper substrate (1); the lower surface of the flexible upper substrate (1) and the upper surface of the flexible lower substrate (2) are bonded together; the interdigital microfluidic channel (3) is filled with liquid metal interdigital electrodes (5); the openings of the two filling holes (4) are plugged with an adhesive (6);
the flexible upper substrate (1) and the flexible lower substrate (2) are rectangular, the thickness of the flexible upper substrate and the flexible lower substrate is smaller than 1mm, and the flexible upper substrate and the flexible lower substrate are made of PDMS; the diameters of the two filling holes (4) are 1mm; the thickness of the liquid metal interdigital electrode (5) is 50 mu m, the distance between two adjacent finger parts is 200 mu m, the length of each finger part is 1cm, and the width of each finger part is 100 mu m; the adhesive (6) adopts Sil-Poxy silica gel adhesive; two wiring ends of the liquid metal interdigital electrode (5) are respectively connected with a lead (13);
the method is realized by the following steps:
step S1: preparing a flexible upper substrate (1); the method comprises the following specific steps:
step S1.1: selecting a first silicon wafer (7), and forming interdigital bulges (8) on the upper surface of the first silicon wafer (7) by adopting a photoetching process;
step S1.2: spin-coating a first PDMS layer (9) on the upper surface of the first silicon wafer (7), ensuring that the first PDMS layer (9) covers all the interdigital protrusions (8), and then solidifying the first PDMS layer (9);
step S1.3: stripping the cured first PDMS layer (9) to obtain a flexible upper substrate (1) with an interdigital microfluidic channel (3) formed on the lower surface;
step S1.4: a filling hole (4) which is penetrated up and down is respectively drilled between two wiring ends of the interdigital microfluidic channel (3) and the upper surface of the flexible upper substrate (1);
step S2: preparing a flexible lower substrate (2); the method comprises the following specific steps:
step S2.1: selecting a second silicon wafer (10);
step S2.2: spin-coating a second PDMS layer (11) on the upper surface of the second silicon wafer (10), and then curing the second PDMS layer (11);
step S2.3: stripping the cured second PDMS layer (11) to obtain a flexible lower substrate (2);
step S3: bonding the lower surface of the flexible upper substrate (1) and the upper surface of the flexible lower substrate (2) together;
step S4: cutting the flexible upper substrate (1) and the flexible lower substrate (2) into rectangular shapes;
step S5: placing a drop of liquid metal (12) at each of the orifices of the two filling holes (4);
step S6: firstly, filling two drops of liquid metal (12) into an interdigital microfluidic channel (3) by adopting a vacuum filling method to form liquid metal interdigital electrodes (5), then inserting a lead (13) into two wiring ends of the liquid metal interdigital electrodes (5), and then plugging the orifices of the two filling holes (4) by adopting an adhesive (6), thereby completing the preparation.
2. The method for manufacturing the positive pressure insensitive interdigital capacitive strain sensor of claim 1, wherein the method comprises the following steps: in the step S1, the steps of the photolithography process are sequentially as follows: gluing, pre-baking, exposing, post-baking and developing;
when the photoresist is coated, the coated photoresist is SU-8 3035 negative photoresist, the spin coating speed is firstly set to be 500rpm for 11 seconds, and then the spin coating speed is adjusted to be 2000rpm for 30 seconds;
in the pre-baking process, the baking temperature is 95 ℃, and the pre-baking time is 15min;
during exposure, the exposure light source is ultraviolet light, the exposure time is 4s, and the exposure energy is 250mJ/cm 2 ；
In post-baking, baking at 65deg.C for 1min, and baking at 95deg.C for 5min;
when developing, the developing solution is SU-8 developing solution.
3. The method for manufacturing the positive pressure insensitive interdigital capacitive strain sensor of claim 1, wherein the method comprises the following steps: in the step S1 and the step S2, the curing is performed by adopting a heating plate, the heating temperature is 80 ℃, and the heating time is 4 hours.
4. The method for manufacturing the positive pressure insensitive interdigital capacitive strain sensor of claim 1, wherein the method comprises the following steps: in the step S1, the filling hole (4) is drilled by a perforator.
5. The method for manufacturing the positive pressure insensitive interdigital capacitive strain sensor of claim 1, wherein the method comprises the following steps: in the step S1 and the step S2, PDMS is formed by mixing an elastomer matrix and a curing agent according to a mass ratio of 10:1.
6. The method for manufacturing the positive pressure insensitive interdigital capacitive strain sensor of claim 1, wherein the method comprises the following steps: in the step S3, plasma is used to bond the lower surface of the flexible upper substrate (1) and the upper surface of the flexible lower substrate (2) together.
7. The method for manufacturing the positive pressure insensitive interdigital capacitive strain sensor of claim 1, wherein the method comprises the following steps: in the step S6, the vacuum filling method specifically includes the following steps: placing the flexible upper substrate (1) and the flexible lower substrate (2) in a vacuum chamber for 20min; after releasing the vacuum, the atmospheric pressure pushes two drops of liquid metal (12) to flow into the interdigital microfluidic channel (3) to form a liquid metal interdigital electrode (5).
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