A fluid-gated strain sensor based on non-uniform strain distribution in microfluidics

Abstract

A fluid-gated strain sensor based on microchannel non-uniform strain distribution comprises a microchannel as a stretching carrier and a functional material liquid conductor filled in the microchannel, wherein a discrete microcolumn array is distributed in the microchannel, and the gaps between the discrete microcolumn array form an intelligent gating driven by strain; the discrete microcolumn array will produce non-uniform strains that are unequal up and down under strain drive, wherein the connection with the microchannel substrate has the same strain as the microchannel, and the strain transmission is increasingly attenuated toward the middle, thereby causing adjacent microcolumns to approach or even contact each other, thereby achieving gate closure; the closed gating causes the cross-section of the conductive path constructed by the functional material liquid conductor to approach zero at the gate, thereby causing the resistance at both ends of the microchannel filled with the functional material liquid conductor to show an order of magnitude change, thereby converting the strain signal into an electrical signal, thereby achieving high-sensitivity, large-stretchability strain sensing; the present invention has the advantages of both a high gauge factor and a large stretch range.

Description

Technical Field

[0001] The invention belongs to the technical field of sensors, and in particular relates to a fluid-gated strain sensor based on microchannel non-uniform strain distribution. Background Art

[0002] With the advent of the intelligent era, there are more requirements for the development of sensors that are widely used in the fields of intelligent robots, human-computer interaction, health detection, and motion monitoring. Among many sensor categories, strain sensors, as sensing devices that can convert local deformation into electrical signals, have their unique functions and applications, such as detecting local strain of structural parts or sensing the joint movements of people / robots. Nowadays, the most commonly used strain gauges are limited in applications with special needs such as ultra-sensitive perception or large-scale motion perception due to their small strain range and low strain coefficient. Therefore, it is of great significance to develop strain sensors with a large tensile range and high strain coefficient.

[0003] The sensing principle of the most common strain gauge is mainly based on the dimensional change of conductive materials such as copper and gold: when the conductive material is stretched, its length changes, and the cross section also changes under the action of the Poisson effect, resulting in a change in resistance, and the strain can be sensed by detecting the change in resistance value. This type of strain sensor completely relies on the size effect caused by the Poisson effect, and its gauge factor is only related to the Poisson ratio of the material.

[0004] In recent years, in order to develop highly sensitive strain sensors, scholars at home and abroad have proposed a strategy for constructing strain sensors based on microcracks. They deposited hard conductive materials on flexible polymer substrates, and constructed conductive pathways that can quickly switch between on and off as strain changes through microcracks in the conductive materials caused by the mismatched mechanical properties between the soft substrate and the hard conductor (Gong, S., et al. Local Crack-Programmed Gold Nanowire Electronic Skin Tattoos for In-Plane Multisensor Integration. Advanced Materials 2019, 31, e1903789; Lee, J., et al. Ultrasensitive Strain Sensor Based on Separation of Overlapped Carbon Nanotubes. Small 2019, 15, e1805120); Although such a method can indeed bring about an order of magnitude increase in the gauge factor, at the same time, the potential interface failure problem caused by the mismatched mechanical properties will lead to the peeling and shedding of the conductive material on the polymer surface, making it difficult for such sensors to work stably and for a long time under large stretching. In order to solve the problem of large stretchability, some studies have constructed strain sensors by filling liquid conductors in microchannels (Xu, S., et al. Biocompatible Soft Fluidic Strain and Force Sensors for Wearable Devices. Advanced Functional Materials 2019, 29, 1807058; Zhou, L., et al. All-Printed Flexible and Stretchable Electronics with Pressing or Freezing Activatable Liquid-Metal-Silicone Inks. Advanced Functional Materials 2019, 30, 1906683). The working principle of this type of device is the same as that of the commonly used strain gauge. Although it has a very large stretch range of more than 100%, its gauge factor is still limited. Therefore, the development of a strain sensor that can have both a high gauge factor and a large stretch range is still an unresolved problem. Summary of the invention

[0005] In order to overcome the shortcomings of the above-mentioned existing sensor technology, the purpose of the present invention is to provide a fluid-gated strain sensor based on microchannel non-uniform strain distribution, which has the advantages of high gauge coefficient and large stretching range.

[0006] In order to achieve the above object, the technical solution adopted by the present invention is:

[0007] A fluid-gated strain sensor based on microchannel non-uniform strain distribution includes a microchannel 1 as a stretching carrier and a functional material liquid conductor 2 filled in the microchannel 1. A discrete microcolumn array 3 is distributed in the microchannel 1, and the gaps between the discrete microcolumn array 3 form an intelligent gating 4 driven by strain.

[0008] The discrete micro-pillar array 3 will produce non-uniform strains that vary from top to bottom under strain drive, wherein the connection with the microfluidic channel substrate has the same strain as the microfluidic channel, and the strain transmission becomes increasingly attenuated toward the middle, thereby causing adjacent micro-pillars to approach or even contact each other, thereby achieving gate closure 5; gate closure 5 causes the cross-section of the conductive path constructed by the functional material liquid conductor 2 at the gate to approach zero, thereby causing the resistance 6 at both ends of the microfluidic channel filled with the functional material liquid conductor 2 to show an order of magnitude change, thereby converting the strain signal into an electrical signal, thereby achieving high-sensitivity, large-stretchability strain sensing.

[0009] The materials of the microchannel 1 and the discrete microcolumn array 3 are various types of polymers with good stretchability and can be molded, such as polydimethylsiloxane, aliphatic aromatic random copolyesters, etc., which are selected according to the maximum stretchable range. The size of the microchannel does not affect the sensing performance.

[0010] The material of the functional liquid conductor 2 is various types of liquid materials with good conductivity, such as ionic liquid, room temperature liquid metal, carbon material dispersion, etc.; the functional liquid conductor 2 is freely extruded and refluxed in the microchannel 1, and there is no possibility of interface failure.

[0011] The smart gating 4 is related to the unit shape of the discrete micro-column array 3. Smart gating constructed with different unit shapes such as circle, diamond, square, etc. has different longitudinal contact lengths, resulting in different capacitance variation ranges. The size and shape are selected according to needs.

[0012] The critical strain size corresponding to the gate closure 5 is related to the unit size and gap size of the discrete micro-pillar array 3, and has a decisive influence on the sensitive interval and detection range of strain sensing. The size arrangement rule is selected according to the needs.

[0013] The absolute value of the resistor 6 at both ends of the microchannel is affected by the conductivity of the functional material liquid conductor 2. The conductor material is selected according to the test conditions. The material with higher conductivity has a larger test range. A method for preparing a fluid-gated strain sensor based on microchannel non-uniform strain distribution includes the following steps:

[0014] 1) Preparation of imprint mold: A microfluidic channel replica structure with a discrete micropillar array is etched on a silicon wafer through a patterned masking template;

[0015] 2) Preparation of microchannels: bonding the microchannel mold structure to the flat film through oxygen plasma activation to form a sealed channel;

[0016] 3) Preparation of strain sensor: A strain sensor filled with a liquid conductor of functional material is prepared by injection and capillary filling methods.

[0017] The step 1) is specifically as follows: preparing an imprinting mold by photolithography and dry etching processes, spin coating photoresist on the polished surface of a silicon wafer and patterning a microchannel with a discrete microcolumn array by photolithography development, and preparing a silicon mold with a microchannel replica structure by dry etching using the photoresist as a mask.

[0018] The step 2) is specifically as follows: the polymer material and the curing agent are mixed and stirred evenly, and then poured onto the silicon mold and the flat polyimide film respectively, and then flattened by spin coating, and then the bubbles are removed by vacuum equipment, and then placed in an oven and heated at 80°C for 60 minutes, and then taken out, cooled to room temperature, and demolded to obtain an unpackaged microfluidic structure; then the microfluidic structure and the polymer on the flat polyimide film are treated with oxygen plasma to achieve surface activation, and the microfluidic structure is buckled with the activated surface of the polymer film and maintained at a pressure of 1MPa for about 3 minutes to complete the bonding, thereby realizing the preparation of the microfluidic channel.

[0019] The step 3) is specifically as follows: pre-puncture a hole at one end of the encapsulated microchannel with a syringe, then absorb the functional substance liquid conductor material and inject it from the other end, and the functional substance liquid conductor gradually fills the gap in the microchannel under the action of capillary force and the thrust of the syringe; finally, copper wires are used to lead out electrodes at the positions where the syringes puncture holes at both ends of the microchannel, and the homogeneous material of the microchannel is dripped to complete the sealing to obtain the strain sensor.

[0020] Compared with the prior art, the present invention has the following beneficial effects:

[0021] 1. The sensor of the present invention introduces a discrete microcolumn array into a microfluidic channel. The gaps between the arrays serve as a liquid conductor path for a functional material to form a gate. Under the action of strain, the discrete microcolumn array generates uneven strain, which causes the gate to gradually narrow or even close, causing the on-off switching of the conductive path, thereby obtaining an order of magnitude change in resistance. Therefore, it has a strain factor of up to thousands and a stretching range far greater than 100%, which can achieve ultra-sensitive strain sensing and a wide range of strain detection.

[0022] 2. The resistance change of the sensor of the present invention comes from the flow of the functional material liquid conductor caused by the gate opening and closing, and the flow behavior of the functional material liquid conductor will not have potential interface failure problems, so it can have large stretching, long-term stability and reliability.

[0023] 3. The gated opening and closing in the sensor of the present invention will cause the lateral gap and the longitudinal gap to contract and relax in sequence, forming a squeeze-reflux cycle similar to the atria and ventricles in the heart. This liquid flow mode driven by volume difference has a direct driving force, thereby achieving low-hysteresis sensing. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 It is the overall structure diagram of the present invention.

[0025] Figure 2 It is a schematic cross-sectional view of the present invention when it is not stretched.

[0026] Figure 3 It is a cross-sectional schematic diagram of the present invention during stretching.

[0027] Figure 4 It is a top view schematic diagram of the present invention during stretching.

[0028] Figure 5 The morphology SEM images of the gate of Example 1 before and after stretching.

[0029] Figure 6 This is the resistance response characteristic of Example 1.

[0030] Figure 7 This is the stable response characteristic of Example 1 after 50,000 stretches. DETAILED DESCRIPTION

[0031] The technical solution of the present invention will be described in detail below with reference to the accompanying drawings and embodiments.

[0032] like Figure 1 , Figure 2As shown, Example 1, a fluid-gated strain sensor based on microchannel non-uniform strain distribution, includes a microchannel 1 as a stretching carrier and a functional material liquid conductor 2 filled in the microchannel 1, a discrete microcolumn array 3 is distributed in the microchannel 1, and the gaps between the discrete microcolumn array 3 form a strain-driven intelligent gate 4.

[0033] like Figure 3 As shown, the discrete micro-pillar array 3 will produce non-uniform strains that vary from top to bottom under strain drive, wherein the connection with the micro-channel substrate has the same strain as the micro-channel, and the strain transmission is increasingly attenuated toward the middle, thereby causing adjacent micro-pillars to approach or even contact each other, achieving gated closure 5;

[0034] like Figure 4 As shown, the gate closure 5 causes the cross-section of the conductive path constructed by the functional material liquid conductor 2 to approach zero at the gate, causing the resistance 6 at both ends of the microchannel filled with the functional material liquid conductor 2 to show an order of magnitude change, thereby converting the strain signal into an electrical signal to achieve high-sensitivity, large-stretchability strain sensing.

[0035] The method for preparing a fluid-gated strain sensor based on microchannel non-uniform strain distribution comprises the following steps:

[0036] 1) Preparation of the imprint mold: The imprint mold was prepared by photolithography and dry etching processes, and the photoresist AZ4620 was spin-coated on the polished surface of a 1 mm thick silicon wafer at a speed of 1000 rpm and pre-baked at a temperature of 95° C. for 5 min. A microchannel pattern with a discrete square array was obtained by exposure and development. The array unit had a side length of 150 μm, a gap of 50 μm, a microchannel width of 2 mm, and a length of 5 cm. Subsequently, a silicon mold with a depth of about 250 μm and a microchannel replica structure was prepared by dry etching using the photoresist as a mask;

[0037] 2) Preparation of microchannels: polydimethylsiloxane and curing agent were mixed in a mass ratio of 10:1 and stirred evenly, and then cast on a silicon mold and a flat polyimide film respectively. After being spin-coated at a rotation speed of 2000 rpm, the film was flattened using a vacuum device to remove bubbles and then placed in an oven to heat at 80°C for 60 minutes. After that, the film was taken out, cooled to room temperature, and demolded to obtain an unpackaged microchannel structure. Then, the microchannel structure and the polymer on the flat polyimide film were treated with oxygen plasma to achieve surface activation. After the microchannel structure and the activated surface of the polymer film were interlocked, the bonding was completed at a pressure of 1 MPa for about 3 minutes to achieve the preparation of the microchannel.

[0038] 3) Preparation of strain sensor: Pre-puncture a hole at one end of the encapsulated microchannel with a syringe, then absorb the functional material liquid conductor 2, i.e., ionic liquid 1-ethyl-3-methylimidazolium trifluoromethanesulfonate [EMIM]OTF and inject it from the other end. Under the action of capillary force and syringe thrust, the ionic liquid gradually fills the gap in the microchannel; finally, copper wires are used to lead out the electrodes at the puncture positions of the syringe at both ends of the microchannel, and polydimethylsiloxane is dripped on the puncture positions to complete the sealing and obtain the strain sensor.

[0039] The microchannel with discrete array obtained in Example 1 was filled with UV curing glue and stretched by about 30%. The UV glue was cured by UV irradiation and the microchannel was cut off. The gate closure of the microchannel section was observed using a scanning electron microscope. The observed gate change morphology is as follows: Figure 5 As shown, gating can achieve on-off switching under the action of non-uniform strain distribution.

[0040] The two ends of the fluid-gated strain sensor based on the non-uniform strain distribution of the microchannel obtained in Example 1 are fixed on a tensile testing machine, and a strain of 0-500% is applied to the sensor through the mobile end of the testing machine, and the resistance value of the resistor 6 at both ends of the microchannel corresponding to the strain is recorded. The test data of the resistance value and the strain are shown in FIG. Figure 6 As shown, the sensor has a much higher gauge factor than the strain gauge over the full range.

[0041] The two ends of the fluid-gated strain sensor based on the non-uniform strain distribution of the microfluidic channel obtained in Example 1 were fixed on a tensile testing machine, and a strain of 50% was applied to the sensor for 50,000 consecutive times through the mobile end of the testing machine, and the resistance response of the two ends of the microfluidic channel during the period was recorded. The test data of the resistance response is shown in FIG. Figure 7 As shown in the figure, the sensor still maintains good stability and reliability under long-term large stretching.

Claims

1. A fluid-gated strain sensor based on microchannel non-uniform strain distribution, characterized in that: The invention comprises a microfluidic channel (1) as a stretching carrier and a functional material liquid conductor (2) filled in the microfluidic channel (1), a discrete microcolumn array (3) is distributed in the microfluidic channel (1), and the gaps in the discrete microcolumn array (3) form a strain-driven intelligent gating (4); The discrete micro-pillar array (3) will generate non-uniform strains that vary from top to bottom under strain drive, wherein the connection with the microfluidic channel substrate has the same strain as the microfluidic channel, while the strain transmission becomes increasingly attenuated toward the middle, thereby causing adjacent micro-pillars to approach or even contact each other, thereby achieving gate closure (5); gate closure (5) causes the cross-section of the conductive path constructed by the functional material liquid conductor (2) at the gate to approach zero, thereby causing the resistance (6) at both ends of the microfluidic channel filled with the functional material liquid conductor (2) to show an order of magnitude change, thereby converting the strain signal into an electrical signal, thereby achieving high-sensitivity, large-stretchability strain sensing; The materials of the microchannel (1) and the discrete microcolumn array (3) are various types of polymers that are stretchable and moldable, including polydimethylsiloxane and aliphatic aromatic random copolyesters, and are selected according to the maximum stretchable range; The material of the functional substance liquid conductor (2) is various types of liquid materials with conductivity, including ionic liquid, room temperature liquid metal, and carbon material dispersion; the functional substance liquid conductor (2) is freely extruded and refluxed in the microchannel (1); The smart gating (4) is related to the unit morphology of the discrete micro-column array (3). Smart gating constructed with different unit morphologies has different longitudinal contact lengths, resulting in different capacitance variation ranges. The size and morphology are selected according to needs. The unit morphology includes round, diamond, and square.

2. The fluid-gated strain sensor based on microchannel non-uniform strain distribution according to claim 1, characterized in that: The critical strain size corresponding to the gate closure (5) is related to the unit size and gap size of the discrete micro-column array (3), and has a decisive influence on the sensitive interval and detection range of strain sensing. The size arrangement rule is selected according to needs.

3. The fluid-gated strain sensor based on microchannel non-uniform strain distribution according to claim 1, characterized in that: The absolute value of the resistance (6) at both ends of the microchannel is affected by the conductivity of the functional material liquid conductor (2). The conductor material is selected according to the test conditions. The material with higher conductivity has a larger test range.

4. The method for preparing a fluid-gated strain sensor based on microchannel non-uniform strain distribution according to claim 1, characterized in that: The following steps are involved: 1) Preparation of imprint mold: A microfluidic replica structure with a discrete micropillar array is etched on a silicon wafer through a patterned masking template; 2) Preparation of microchannels: The microchannel mold structure is bonded to the flat film through oxygen plasma activation to form a sealed channel; 3) Preparation of strain sensor: A strain sensor filled with a liquid conductor of functional material is prepared by injection and capillary filling methods.

5. The method according to claim 4, characterized in that The step 1) is specifically as follows: preparing an imprint mold by photolithography and dry etching, spin coating photoresist on the polished surface of a silicon wafer and patterning a microfluidic channel with a discrete microcolumn array by photolithography development, and preparing a silicon mold with a microfluidic channel replica structure by dry etching using the photoresist as a mask.

6. The method according to claim 4, characterized in that The step 2) is specifically as follows: the polymer material and the curing agent are mixed and stirred evenly, and then poured onto the silicon mold and the flat polyimide film respectively, and then flattened by spin coating, and then the bubbles are removed by vacuum equipment, and then placed in an oven and heated at 80°C for 60 minutes, and then taken out, cooled to room temperature, and demolded to obtain an unpackaged microfluidic structure; then the microfluidic structure and the polymer on the flat polyimide film are treated with oxygen plasma to achieve surface activation, and the microfluidic structure is buckled with the activated surface of the polymer film and maintained at a pressure of 1MPa for 3 minutes to complete the bonding, thereby realizing the preparation of the microfluidic channel.

7. The method according to claim 4, characterized in that The step 3) is specifically as follows: pre-puncture a hole at one end of the encapsulated microchannel with a syringe, then absorb the functional material liquid conductor material and inject it from the other end, and the functional material liquid conductor gradually fills the gap in the microchannel under the action of capillary force and the thrust of the syringe; finally, copper wires are used to lead out electrodes at the positions where the syringes puncture holes at both ends of the microchannel, and the homogeneous material of the microchannel is dripped to complete the sealing to obtain the strain sensor.

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