阅读说明：本技术 一种正压力不敏感型叉指电容式应变传感器及其制备方法 (Positive pressure insensitive interdigital capacitive strain sensor and preparation method thereof ) 是由 张东光 王志民 张�杰 杨嘉怡 吴亚丽 于 2021-09-18 设计创作，主要内容包括：本发明涉及应变传感器,具体是一种正压力不敏感型叉指电容式应变传感器及其制备方法。本发明解决了现有电容式应变传感器无法区分拉力与正压力、无法应用于大拉伸应变的问题。一种正压力不敏感型叉指电容式应变传感器,包括柔性上基板、柔性下基板；柔性上基板的下表面开设有叉指型微流控通道,且叉指型微流控通道的两个接线端与柔性上基板的上表面之间各开设有一个上下贯通的填充孔；柔性上基板的下表面和柔性下基板的上表面粘合在一起；叉指型微流控通道内填充有液态金属叉指电极；两个填充孔的孔口均封堵有粘接剂。本发明适用于人机界面、软机器人、电子皮肤等领域。(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 the tension and the 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 communicated up and down 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 is bonded with the upper surface of the flexible lower substrate; liquid metal interdigital electrodes are filled in the interdigital microfluidic channel; the orifices of the two filling holes are plugged with adhesives. The invention is suitable for the fields of human-computer interfaces, soft robots, electronic skins and the like.)

1. A positive pressure insensitive interdigital capacitive strain sensor is characterized in that: 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 communicated up and down 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) is bonded with the upper surface of the flexible lower substrate (2); liquid metal interdigital electrodes (5) are filled in the interdigital micro-fluidic channel (3); the orifices of the two filling holes (4) are plugged with adhesives (6).

2. The positive pressure insensitive interdigital capacitive strain sensor of claim 1, wherein: the flexible upper substrate (1) and the flexible lower substrate (2) are both rectangular, the thickness of the upper substrate and the thickness of the lower substrate are both smaller than 1mm, and the upper substrate and the lower substrate are both made of PDMS; the diameters of the two filling holes (4) are both 1 mm; the liquid metal interdigital electrode (5) is 50 microns thick, the distance between two adjacent fingers is 200 microns, the length of each finger is 1cm, and the width of each finger is 100 microns; the adhesive (6) adopts Sil-Poxy silica gel adhesive; two wiring ends of the liquid metal interdigital electrode (5) are respectively connected with a conducting wire (13).

3. A method for manufacturing a positive pressure insensitive interdigital capacitive strain sensor, which is used for manufacturing a positive pressure insensitive interdigital capacitive strain sensor according to claim 2, wherein: the method 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 protrusions (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 curing 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) on the lower surface;

step S1.4: a filling hole (4) which is through up and down is drilled between two wiring ends of the interdigital micro-fluidic 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 rectangles;

step S5: placing a drop of liquid metal (12) at each of the two filling holes (4);

step S6: firstly, filling two drops of liquid metal (12) into the interdigital microfluidic channel (3) by adopting a vacuum filling method to form a liquid metal interdigital electrode (5), then respectively inserting a conducting wire (13) into two wiring ends of the liquid metal interdigital electrode (5), and then plugging orifices of two filling holes (4) by adopting an adhesive (6), thereby completing the preparation.

4. The method for preparing a positive pressure insensitive interdigital capacitive strain sensor according to claim 3, wherein: in step S1, the photolithography process sequentially includes: gluing, pre-baking, exposing, post-baking and developing;

when coating the photoresist, the coated photoresist is SU-83035 negative photoresist, the spin-coating speed is set to 500rpm and kept for 11s, and then the spin-coating speed is adjusted to 2000rpm and kept for 30 s;

pre-baking at 95 deg.C for 15 min;

during exposure, the exposure light source is ultraviolet light, the exposure time is 4s, and the exposure energy is 250mJ/cm²；

Baking at 65 deg.C for 1min and 95 deg.C for 5 min;

during development, the used developer is SU-8 developer.

5. The method for preparing a positive pressure insensitive interdigital capacitive strain sensor according to claim 3, wherein: in the steps S1 and S2, the curing is performed by using a heating plate, the heating temperature is 80 ℃, and the heating time is 4 hours.

6. The method for preparing a positive pressure insensitive interdigital capacitive strain sensor according to claim 3, wherein: in step S1, the filling hole (4) is drilled by a perforator.

7. The method for preparing a positive pressure insensitive interdigital capacitive strain sensor according to claim 3, wherein: in the step S1 and the step S2, the PDMS is formed by mixing an elastomer matrix and a curing agent according to a mass ratio of 10: 1.

8. The method for preparing a positive pressure insensitive interdigital capacitive strain sensor according to claim 3, wherein: in step S3, the lower surface of the flexible upper substrate (1) and the upper surface of the flexible lower substrate (2) are bonded together by using plasma.

9. The method for preparing a positive pressure insensitive interdigital capacitive strain sensor according to claim 3, wherein: in step S6, the vacuum filling method includes the following steps: placing the flexible upper substrate (1) and the flexible lower substrate (2) in a vacuum chamber for 20 min; after the vacuum is released, atmospheric pressure pushes two drops of liquid metal (12) to flow into the interdigital micro-fluidic channel (3) to form a liquid metal interdigital electrode (5).

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 have attracted great interest in the fields of human-machine interfaces, soft robots, electronic skins, etc., which can help to monitor the movements of soft robots or to convert human body movements into electrical signals.

The sensing mechanism of the strain sensor is mainly three types: capacitive sensing, piezoresistive sensing, piezoelectric sensing. Among these sensing mechanisms, capacitive 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 is made ofIt can be known that no matter tension or positive pressure can change capacitance, crosstalk problem exists between tension sensing and positive pressure sensing, so that the parallel polar plate capacitive strain sensor cannot distinguish tension from positive pressure.

For the interdigital capacitive strain sensor capable of decoupling the tension sensing and the positive pressure sensing, the capacitance of the interdigital capacitive strain sensor is generated between electrodes, and one unit capacitance C of the interdigital capacitance C_uIs composed of three parallel capacitors, two fringe field capacitors C_su1And C_su2And a parallel field capacitance C_mIn which the fringe field capacitance is composed ofCalculated parallel field capacitance ofCalculated as total capacitance of And (6) calculating.

The electrodes, which are key elements of the interdigital capacitive strain sensors, should have good electrical conductivity even under tensile conditions. However, the stretchable material film, such as silver nanowire or carbon nanotube network, currently selected as the electrode of the interdigital capacitive strain sensor, can maintain conductivity under moderate tensile strain, but cannot maintain conductivity under tensile strain exceeding 100%, thereby resulting in that the interdigital capacitive strain sensor cannot be applied to large tensile strain.

Ju Y H et al (Ju Y H, Han C J, Kim K S, et al, UV-Curable Adhesive Tape-Assisted Patterning of Metal Nanowires for ultra simple tensile Sensor [ J ]. Advanced Materials Technologies,2021:2100776.) designed an ultra simple tensile Sensor with UV-Curable Tape auxiliary Metal nanowire Patterning, which uses interdigitated electrode structure, silver Nanowires as electrode material, the Sensor capacitance drops regardless of whether tensile or compressive force is applied to the Sensor, cross talk between tensile and compressive sensing, the Sensor cannot distinguish tensile and compressive forces, and the Sensor' S electrodes can only maintain conductivity under moderate tensile strain, and cannot be applied to large tensile strain.

An interdigital transducer Based on silicon Foam is designed by Hesam Mahmoudlezhah et al (Hesam Mahmoudlezhah und Masoumeh, Anderson Iain, Rosset Samuel. intermediate Sensor Based on a silicon Foam for substrate magnetic management. J. Macromolecular porous communicating, 2020:), which adopts an interdigital electrode structure, forms interdigital electrodes on a flexible Printed Circuit Board (PCB) and is covered with compressible elastic Foam, can only be applied to detect pressure and not to detect tension, and can only detect pressure in the range of 50N, and the detection range is very small.

Patent No. cn202110131844.x (application date 2021/30/2021, published 6/15/2021) discloses a flexible interdigital capacitive sensor structure and a method for manufacturing the same. The sensor consists of a substrate layer, a middle interdigital electrode layer and an upper packaging layer, an interdigital electrode structure is also adopted, the electrode is formed by uniformly mixing liquid silicon rubber, rigid conductive fiber particles, a diluent and a synergist, the capacitance of the sensor can be reduced no matter tension or pressure is applied to the sensor, the problem of crosstalk 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% and is not suitable for large tensile strain.

Patent CN202010311401.4 (application date 2020, 4/20, and published date 2020, 8/7) discloses an interdigital counter electrode type flexible touch sensor based on the super capacitance sensing principle, in which an interdigital electrode of the sensor is prepared by printing conductive ink on a flexible substrate by 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 tension.

In summary, the current research and patents on the flexible interdigital sensor are lacking, and the current flexible interdigital sensor has the following limitations:

(1) most of the flexible interdigital sensors are pressure sensors, the preparation and performance research of the flexible interdigital sensors under tensile load is lacked, and although some flexible interdigital sensors can detect the tensile force, the problem of crosstalk exists between the tensile force sensing and the 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.

Researches show that the earthworms are composed of a plurality of segments which are arranged in parallel, when the earthworms move, the segments are alternately expanded and contracted, the tactile response is realized by means of the body surface muscle structure and the myoelectric reaction, and the nerve cables are adopted to transmit external stimulation signals, so that an excellent perception mechanism is formed, and an important bionics inspiration is provided for the invention. A positive pressure insensitive interdigital capacitive strain sensor is designed based on the earthworm electromyographic reaction principle, and the problems that the existing 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, aiming at solving the problems that the conventional 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 communicated up and down 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 is bonded with the upper surface of the flexible lower substrate; liquid metal interdigital electrodes are filled in the interdigital microfluidic channel; the orifices of the two filling holes are plugged with adhesives.

The flexible upper substrate and the flexible lower substrate are both rectangular, the thickness of the upper substrate and the thickness of the lower substrate are both smaller than 1mm, and the upper substrate and the lower substrate are both made of PDMS; the diameters of the two filling holes are both 1 mm; the liquid metal interdigital electrode is 50 microns thick, the distance between two adjacent fingers is 200 microns, the length of each finger is 1cm, and the width of each finger is 100 microns; the adhesive is Sil-Poxy silica gel adhesive; two wiring ends of the liquid metal interdigital electrode are respectively connected with a conducting wire.

A method for preparing a positive pressure insensitive interdigital capacitive strain sensor (the method is used for preparing the positive pressure insensitive interdigital capacitive strain sensor), which 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 an interdigital protrusion 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 interdigital protrusions are completely covered by the first PDMS layer, and then curing the first PDMS layer;

step S1.3: stripping the cured first PDMS layer to obtain a flexible upper substrate with an interdigital microfluidic channel on the lower surface;

step S1.4: drilling a filling hole which is communicated up and down between 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 curing the second PDMS layer;

step S2.3: stripping the cured second PDMS layer to obtain 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 rectangles;

step S5: placing a drop of liquid metal at each of the openings of the two fill holes;

step S6: firstly, filling two drops of liquid metal into the interdigital microfluidic channel by adopting a vacuum filling method to form a liquid metal interdigital electrode, then respectively inserting a conducting wire into two wiring ends of the liquid metal interdigital electrode, and then plugging the orifices of the two filling holes by adopting an adhesive, thereby completing the preparation.

In step S1, the photolithography process sequentially includes: gluing, pre-baking, exposing, post-baking and developing;

when coating the photoresist, the coated photoresist is SU-83035 negative photoresist, the spin-coating speed is set to 500rpm and kept for 11s, and then the spin-coating speed is adjusted to 2000rpm and kept for 30 s;

pre-baking at 95 deg.C for 15 min;

during exposure, the exposure light source is ultraviolet light, the exposure time is 4s, and the exposure energy is 250mJ/cm²；

Baking at 65 deg.C for 1min and 95 deg.C for 5 min;

during development, the used developer is SU-8 developer.

In the steps S1 and S2, the curing is performed by using a heating plate, the heating temperature is 80 ℃, and the heating time is 4 hours.

In step S1, the filling hole is drilled by using a punch.

In the step S1 and the step S2, the PDMS is formed by mixing an elastomer matrix and a curing agent according to a mass ratio of 10: 1.

In 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 step S6, the vacuum filling method includes the following steps: placing the flexible upper substrate and the flexible lower substrate in a vacuum chamber for 20 min; after the vacuum is released, atmospheric pressure pushes two drops of liquid metal to flow into the interdigital type microfluidic channel to form a liquid metal interdigital electrode.

In operation, as shown in fig. 15, the liquid metal interdigital electrode generates capacitance by edge effect and parallel field. The invention distinguishes the tensile force from the positive pressure according to the change of the distance between two adjacent fingers of the liquid metal interdigital electrode. The liquid metal interdigital electrode is positioned on the same plane, the thickness of the liquid metal interdigital 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 pulled. As shown in FIG. 16, when the liquid metal interdigital electrode is subjected to a tensile force, the distance d between two adjacent fingers of the liquid metal interdigital electrode₀Increase in electrode thickness t₀Reduced length of each finger l₀Each reduced, width w of each finger₀The capacitance is increased, and the capacitance is reduced along with the increase of the tensile force according to a calculation formula of the fringe capacitance, the parallel field capacitance and the total capacitance. As shown in fig. 17, when the present invention is subjected to positive pressure, the thickness t of the liquid metal interdigital electrode₀The change is negligible, and the space d between two adjacent fingers caused by the Poisson effect₀The change can be ignored, and the capacitance can be obtained to be only slightly changed according to a calculation formula of the fringe capacitance, the parallel field capacitance and the total capacitance. Therefore, the tensile force can be distinguished from the positive pressure by comparing the change of the distance between two adjacent fingers generated by the tensile force and the positive pressure of the invention and the change of the capacitance caused by the change. In other words, the invention hasThe positive pressure exerted thereon is insensitive, thereby enabling a distinction between tensile forces and positive pressure. Meanwhile, due to the conductivity and the fluidity of the liquid metal and the ductility of PDMS, the invention can detect the tensile strain of 100 percent, has a sensitivity coefficient of-0.3 and has good durability. In addition, the invention has low hysteresis<0.01). The present invention has good ductility and can be twisted, bent, and folded into a roll, as shown in fig. 18. As shown in fig. 19, the present invention has a high scalability (100%), a capacitance change almost linearly behaves in a tensile strain range of 0% to 100%, and a sensitivity coefficient of-0.3, and thus can be applied to a large tensile strain. As shown in fig. 20, the present invention can be applied to detect human finger joint movement, wrist joint movement, and elbow joint movement.

To verify the excellent performance of the present invention, the following tests were performed:

as shown in FIG. 21, the strain-capacitance change curve was obtained by stretching the present invention at two stretching rates of 5mm/min and 20mm/min to give a tensile strain of 0% to 50%. The curve shows that: the present invention exhibits low hysteresis in the 50% stretch strain range, with a 20mm/min stretch rate compared to the change in capacitance at a 5mm/min stretch rate, which is increased after a 20mm/min stretch time, but only in the 0.01 range.

As shown in fig. 22, the present invention was repeatedly stretched under a tensile strain of 30%, thereby obtaining a cyclic strain-capacitance change curve. 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 simultaneously 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 did not change during the addition and removal of the weight, indicating that the invention is not sensitive to positive pressure in the unstretched condition; at 20% tensile strain, the capacitance only changes by 0.004 during the addition and removal of the weight, indicating that the invention is not sensitive to positive pressure even under tensile conditions.

From the above results, it can be seen that: the invention has the characteristic of insensitivity to positive pressure, can distinguish the tensile force from the positive pressure, can be applied to large tensile strain, and simultaneously has good sensitivity, low hysteresis and durability.

The invention effectively solves the problems that the existing capacitive strain sensor can not distinguish the tension and the 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 structure diagram 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 first schematic diagram of the working principle of the invention.

Fig. 16 is a second schematic diagram of the working principle of the present invention.

Fig. 17 is a third schematic diagram of the working principle of the present invention.

FIG. 18 is a schematic representation of the invention with good ductility.

FIG. 19 is a graph showing the capacitance change under different tensile strain conditions according to the present invention.

FIG. 20 is a schematic diagram of the present invention applied to detecting human finger joint motion, wrist joint motion, and elbow joint motion.

FIG. 21 is a schematic representation of the hysteresis of the present invention under different draw rate conditions.

FIG. 22 is a schematic diagram of the capacitance change under a certain tensile strain and rapid stretching condition according to the present invention.

FIG. 23 is a schematic diagram of the change in capacitance of the present invention under different tensile strain conditions with a positive pressure applied.

In the figure: the manufacturing method comprises the following steps of 1-flexible upper substrate, 2-flexible lower substrate, 3-interdigital microfluidic channel, 4-filling hole, 5-liquid metal interdigital electrode, 6-adhesive, 7-first silicon chip, 8-interdigital protrusion, 9-first PDMS layer, 10-second silicon chip, 11-second PDMS layer, 12-liquid metal and 13-lead.

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 communicated up and down 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; liquid metal interdigital electrodes 5 are filled in the interdigital microfluidic channel 3; the openings of the two filling holes 4 are blocked by adhesive 6.

The flexible upper substrate 1 and the flexible lower substrate 2 are both rectangular, the thickness of the two is less than 1mm, and the two are both made of PDMS; the diameters of the two filling holes 4 are both 1 mm; the thickness of the liquid metal interdigital electrode 5 is 50 micrometers, the distance between two adjacent fingers is 200 micrometers, the length of each finger is 1cm, and the width of each finger is 100 micrometers; the adhesive 6 is Sil-Poxy silica gel adhesive; two terminals of the liquid metal interdigital electrode 5 are respectively connected with a conducting wire 13.

A method for preparing a positive pressure insensitive interdigital capacitive strain sensor (the method is used for preparing the positive pressure insensitive interdigital capacitive strain sensor), which 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 an interdigital protrusion 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 curing 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 on the lower surface;

step S1.4: a filling hole 4 which is through up and down is 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: peeling off 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 rectangles;

step S5: placing a drop of liquid metal 12 at each of the two fill holes 4;

step S6: firstly, filling two drops of liquid metal 12 into the interdigital microfluidic channel 3 by adopting a vacuum filling method to form a liquid metal interdigital electrode 5, then respectively inserting a conducting wire 13 into two wiring ends of the liquid metal interdigital electrode 5, and then plugging the orifices of the two filling holes 4 by adopting an adhesive 6, thereby completing the preparation.

In step S1, the photolithography process sequentially includes: gluing, pre-baking, exposing, post-baking and developing;

when coating the photoresist, the coated photoresist is SU-83035 negative photoresist, the spin-coating speed is set to 500rpm and kept for 11s, and then the spin-coating speed is adjusted to 2000rpm and kept for 30 s;

pre-baking at 95 deg.C for 15 min;

during exposure, the exposure light source is ultraviolet light, the exposure time is 4s, and the exposure energy is 250mJ/cm²；

Baking at 65 deg.C for 1min and 95 deg.C for 5 min;

during development, the used developer is SU-8 developer.

In the steps S1 and S2, the curing is performed by using a heating plate, the heating temperature is 80 ℃, and the heating time is 4 hours.

In step S1, the filling hole 4 is drilled by using a punch.

In the step S1 and the step S2, the PDMS is formed by mixing an elastomer matrix and a curing agent according to a mass ratio of 10: 1.

In 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 step S6, the vacuum filling method includes the following steps: placing the flexible upper substrate 1 and the flexible lower substrate 2 in a vacuum chamber for 20 min; after the vacuum is released, the atmospheric pressure pushes the two drops of liquid metal 12 to flow into the interdigital micro-fluidic 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 that 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 spirit and scope of the invention, and these changes and modifications are within the scope of the invention.