Strain sensor and method for manufacturing same

Abstract

Provided is a strain sensor having high sensitivity to bending and tension. This strain sensor comprises: a flexible resin base material; conductive regions disposed on the resin base material and containing a laser-induced graphene; and a pair of electrodes electrically connected to the conductive regions. At least some of the conductive regions further contain a light-absorbing nanomaterial.

Description

Strain sensor and manufacturing method thereof

[0001] The present invention relates to a strain sensor and a method for manufacturing the same.

[0002] Strain sensors have been commercialized using many materials and device structures. However, their performance (e.g., sensitivity, selectivity, stability, and manufacturing process cost) is still limited, and most sensors are based on MEMS (Micro Electro Mechanical Systems) technology using silicon semiconductors. In recent years, there has been a growing demand for flexible sensors that can be attached to various surfaces, replacing the rigid, inflexible sensors based on MEMS technology.

[0003] For example, bending sensors (e.g., Non-Patent Document 1) have been reported for attaching to bending parts of a human body, animal, machine, etc. to measure bending radius, curvature, etc., and tensile sensors (e.g., Non-Patent Document 2) have been reported for attaching to stretchable parts of a human body, animal, machine, etc. to measure elongation. These bending sensors and tensile sensors (collectively referred to as "strain sensors" in this specification) have a flexible resin substrate, a conductive pattern disposed on the resin substrate, and electrodes. The bending or tensile state is identified by a change in the resistance value of the conductive pattern that occurs when the sensor is bent or pulled. Here, the strain sensors in Non-Patent Document 1 and Non-Patent Document 2 use laser-induced graphene (LIG), which is obtained by carbonizing a resin material with a laser, for the conductive pattern and electrodes.

[0004] Altynay Kaidarova, et al., “Wearable multifunctional printed graphene sensors”, npj Flexible Electronics, 2019, 3:15Ashok Chhetry, et al., “MoS2-Decorated Laser-Induced Graphene for a Highly Sensitive, Hysteresis-free, and Reliable Piezoresistive Strain Sensor”, ACS Appl. Mater. Interfaces 2019, 11, 22531-22542

[0005] In order to put the strain sensor into practical use, it is necessary for the resistance value to change significantly in response to tension or bending. However, the strain sensors described in Non-Patent Documents 1 and 2 do not have sufficient resistance change.

[0006] SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a strain sensor having high sensitivity to bending and tension, and a method for manufacturing the same.

[0007] The present invention provides a strain sensor having a flexible resin substrate, a conductive region including laser-induced graphene arranged on the resin substrate, and a pair of electrodes electrically connected to the conductive region, wherein at least a portion of the conductive region further includes a nanomaterial having light absorption properties.

[0008] The present invention provides a method for manufacturing a strain sensor having a flexible resin substrate and a conductive region including laser-induced graphene arranged on the resin substrate, the method including the steps of: preparing the resin substrate; forming a nanomaterial layer including a nanomaterial having light absorption properties on the resin substrate; and irradiating the nanomaterial layer with a laser to form laser-induced graphene on the surface of the resin substrate.

[0009] The present invention provides a method for manufacturing a strain sensor having a flexible resin substrate and a conductive region including laser-induced graphene arranged on the resin substrate, the method including the steps of: preparing a resin graphene formation substrate; forming a nanomaterial layer including a light-absorbing nanomaterial on the graphene formation substrate; irradiating the nanomaterial layer with a laser to form laser-induced graphene on the surface of the graphene formation substrate; applying a material of the resin substrate onto the graphene formation substrate and solidifying or curing it to form the resin substrate; and peeling off the resin substrate and the laser-induced graphene from the graphene formation substrate.

[0010] According to the present invention, a strain sensor having high sensitivity to bending and tension and a method for manufacturing the same are provided.

[0011] 1A to 1C are diagrams schematically illustrating a method for manufacturing an LIG strain sensor of the present invention. FIG. 2A is a plan view of a bending sensor according to one embodiment of the present invention, FIG. 2B is an enlarged cross-sectional view taken along line A-A in FIG. 2A, and FIG. 2C is an enlarged view of region α in FIG. 2B. FIG. 3A is a plan view of a tensile sensor according to one embodiment of the present invention, FIG. 3B is a cross-sectional view taken along line A-A in FIG. 3A, and FIG. 3C is an enlarged view of region β in FIG. 3B. FIGS. 4A to 4F are process diagrams illustrating a method for manufacturing a tensile sensor according to one embodiment of the present invention. FIG. 5A is a photograph of the conductive region of a first bending sensor produced in an example, observed with an SEM, and FIG. 5B is a photograph of the conductive region of a second bending sensor produced in an example, observed with an SEM. FIG. 6 is a graph showing the relationship between bending strain and the rate of change in resistance for the first bending sensor and the second bending sensor in an example. FIG. 7 is a graph showing the change in current when the bending strain of the first bending sensor in the example is changed every 30 seconds. FIG. 8 is a graph showing the change in the resistance change rate over time when the first bending sensor is bent and restored every 5 seconds so that the bending strain is 0.32%. FIG. 9 is a diagram comparing the gauge factors of the first bending sensor and a known bending sensor. FIG. 10A is a photograph of the conductive region of the first tensile sensor fabricated in the example, observed with an SEM, and FIG. 10B is a photograph of the conductive region of the third tensile sensor (comparative example) fabricated in the example, observed with an SEM. FIG. 11 is a Raman spectrum of the LIG in the conductive region of the first tensile sensor and the third tensile sensor fabricated in the example, and of the LIG remaining on the LIG formation substrate side. FIG. 12 is a graph showing the change in tensile strain and resistance value of the first tensile sensor, the second tensile sensor, and the third tensile sensor fabricated in the example. Fig. 13 is a graph showing the change in the rate of change in resistance value over time when the first tensile sensor produced in the example is repeatedly stretched and contracted. Fig. 14 is a graph showing the change in the rate of change in resistance value over time when the first tensile sensor produced in the example is repeatedly stretched and contracted. Fig. 15 is a graph showing the change in current when the bending strain amount of the first tensile sensor produced in the example is changed every 30 seconds.Fig. 16 is a graph showing the change in current when the tensile strain of the first tensile sensor fabricated in the example is increased by 2% every 100 seconds up to 10% while repeatedly stretching and contracting. Fig. 17 is a graph comparing the gauge factors of the first tensile sensor, the second tensile sensor, and a known tensile sensor.

[0012] In this specification, a numerical range expressed using "to" means a range that includes the numerical values ​​before and after "to" as the lower and upper limits. In numerical ranges described in stages in this specification, the upper or lower limit described in a certain numerical range may be replaced with the upper or lower limit of another numerical range described in stages.

[0013] Conventionally, laser-induced graphene (hereinafter also referred to as "LIG") used in strain sensors and the like has generally been formed by directly irradiating a resin material with a laser and carbonizing the resin material. However, the inventors have conducted extensive research and found that the resistance value of LIG obtained by this method is difficult to change even when stretched or bent, and that the sensitivity is low when used in a strain sensor. In contrast, the inventors have found that by using a light-absorbing nanomaterial during LIG formation, LIG ​​can be obtained whose resistance value changes significantly when stretched or bent.

[0014] A method for forming an LIG using a nanomaterial will be described using the schematic diagrams (cross-sectional views) of FIGS. 1A to 1C. In this method, first, as shown in FIG. 1A, a nanomaterial layer 13 containing a light-absorbing nanomaterial is formed on a resin material 11, which will be the material for the LIG. Then, as shown in FIG. 1B, a laser beam L is irradiated onto the nanomaterial layer 13 in the region where the LIG will be formed. In the region irradiated with the laser beam L, the nanomaterial absorbs the light energy from the laser beam L and converts it into heat. As a result, as shown in FIG. 1C, the resin material 11 in that region is efficiently carbonized, generating LIG 12. Note that, as shown in FIG. 1C, the nanomaterial in the nanomaterial layer 13 is thought to be pushed out of the LIG as the LIG 12 is generated. The reason why the change in resistance value of LIG12 formed by such a method increases with stretching or bending is speculated to be due to, for example, the difference in the microstructure of the formed LIG12 from that of conventional LIG or the interaction between LIG and nanomaterials, but is not certain.

[0015] Here, the strain sensor of the present invention has a conductive region including a LIG formed by the above-described method. When the LIG is formed by the above-described method, a nanomaterial adheres to the periphery of the LIG. Therefore, whether the LIG in the conductive region is formed by the above-described method can be determined by, for example, elemental analysis, by confirming whether the nanomaterial adheres to at least a portion of the conductive region (e.g., the side surface of the LIG).

[0016] Hereinafter, an embodiment in which the strain sensor of the present invention is a bending sensor for identifying bending strain of an object to be inspected, and an embodiment in which the strain sensor is a tensile sensor for identifying elongation strain of an object to be inspected will be described as examples, although the strain sensor of the present invention is not limited to these applications.

[0017] 1. Bending Sensor A bending sensor according to one embodiment of the present invention will be described with reference to the drawings. Fig. 2A shows a plan view of bending sensor 20 of this embodiment, Fig. 2B shows an enlarged cross-sectional view taken along line A-A in Fig. 2A, and Fig. 2C shows an enlarged cross-sectional view of region α in Fig. 2B.

[0018] The bending sensor 20 of this embodiment includes a flexible resin substrate 21, a conductive region B on the resin substrate 21 in which an LIG 22 is disposed, and a pair of electrodes 25.

[0019] The resin substrate 21 of the bending sensor 20 may be a resin substrate that can bend to follow the bending of an object to be inspected by the bending sensor 20 (for example, a human body, an animal, a machine, etc.) and that can form the LIG 22 (conductive region B) by the above-described method. However, it is more preferable that the resin substrate 21 has durability that makes it less likely to break or crack even when repeatedly bent.

[0020] The resin substrate 21 may be composed of one layer, or may be composed of two or more layers. The resin substrate 21 may be composed of only resin, or may partially contain an inorganic material such as a filler, as long as the purpose and effect of this embodiment are not impaired. The type of resin contained in the resin substrate 21 is appropriately selected depending on the application of the bending sensor 20, the desired flexibility, strength, etc. Examples of such resins include polyimide, silicone resin, polyurethane, polyethylene, etc. The resin substrate 21 may contain only one of these, or two or more types. Among various resins, polyimide is preferred from the viewpoints of strength and ease of forming LIG.

[0021] The shape and thickness of the resin substrate 21 are appropriately selected depending on the type of resin, the application of the bending sensor 20, the strength and flexibility of the resin substrate 21, and the like.

[0022] In this specification, the conductive region B refers to a region including LIG 22 and capable of conducting electricity when a voltage is applied between the pair of electrodes 25. In the present embodiment, the conductive region B includes not only LIG 22 but also a nanomaterial.

[0023] The formation pattern of the LIG 22 in the conductive region B of this embodiment is not particularly limited. For example, the LIG 22 may be formed randomly or regularly. The LIG 22 may be formed on a line having a straight, zigzag, wavy, or other shape. The extension direction of the line is not particularly limited, and may be parallel, perpendicular, or oblique to the line connecting the pair of electrodes 25, or may be any other direction. In this case, the LIG 22 may be formed continuously on the line or may be formed intermittently, such as in a dashed or dotted line. Note that, as shown in FIG. 2A , in this embodiment, the LIG 22 is continuously formed on a straight line perpendicular to the line connecting the pair of electrodes (two electrodes) 25. Furthermore, a plurality of linear LIGs 22 are arranged parallel to each other between the pair of electrodes 25. In this specification, the term "parallel" refers not only to a case where the LIGs 22 are completely parallel to each other, but also to a case where there is a deviation to the extent that does not impair the purpose and effects of the invention. For example, when lines parallel to both are superimposed, the angle between them is within 0±5°. The same applies to perpendicular, and the angle between two lines is also included in the term "perpendicular." Here, when the LIGs 22 are formed on parallel lines as in this embodiment, there may be a gap between the lines that does not interfere with electrical conduction.

[0024] 2C, the LIG 22 has a protruding portion 22a protruding from the surface of the resin substrate 21. The height of the protruding portion 22a from the surface of the resin substrate 21 is not particularly limited.

[0025] In this embodiment, a nanomaterial is typically attached to the outer wall of the protruding portion 22a of the LIG 22, or a nanomaterial layer 23 containing the nanomaterial is present in the gap between two adjacent LIGs 22. The nanomaterial may be any material capable of absorbing laser light and converting it to heat. The shape of the nanomaterial is not particularly limited, and may be particulate, tubular, or fibrous. The size of the nanomaterial may be on the nanometer order, with smaller sizes being preferable from the perspective of facilitating the conversion of light energy to thermal energy. For example, when the nanomaterial is particulate, the average particle diameter may be approximately 1 nm to 1000 nm, for example, 20 nm to 500 nm. On the other hand, when the nanomaterial is tubular or fibrous, the diameter may be on the nanometer order, for example, the average diameter may be 1 to 1000 nm, or 20 to 500 nm. On the other hand, the length of the tubular or fibrous nanomaterial is not particularly limited as long as it does not impair the objectives and effects of this embodiment, and may be on the nanometer order or micron order. For example, the length may be 100 nm to 100 μm, or may be about 1 μm to 60 μm.

[0026] The size of the nanomaterial can be obtained by, for example, measuring the size of about 200 particles using a transmission electron microscope or a scanning electron microscope and calculating the arithmetic average. In addition, in the case of a particulate nanomaterial, the BET-equivalent particle diameter may be used as the size of the nanomaterial. In this specification, unless otherwise specified, the particle diameter of a particulate nanomaterial means the BET-equivalent particle diameter. The BET-equivalent particle diameter (nm) is calculated by multiplying the BET specific surface area (m ２ / g) is converted using the following formula (1): BET converted particle diameter (nm) = 6000 / (BET specific surface area (m ２ / g) × ρ (g / cm ３ (In the above formula (1), ρ represents the density of the nanomaterial. For example, the density ρ of zinc oxide is 5.61 g / cm ３ The BET specific surface area refers to a specific surface area measured by the BET method using a known device (for example, a fully automatic specific surface area measuring device, trade name: Macsorb HM Model-1201, manufactured by Mountec Co., Ltd.).

[0027] Examples of nanomaterials include zinc oxide (ZnO) nanoparticles, carbon nanotubes (CNTs), gold (Au) nanoparticles, and silver (Ag) nanoparticles, which can be used alone or in combination. Among these, zinc oxide (ZnO) nanoparticles are particularly preferred, and zinc oxide (ZnO) nanoparticles, either alone or in combination with other nanomaterials, are particularly preferred. In the bending sensor 20 of this embodiment, nanomaterials may be attached to areas other than the conductive region B, or a nanomaterial layer 23 may be formed.

[0028] The shape of the conductive region B is appropriately selected depending on the application of the bending sensor 20. In this embodiment, the conductive region B is rectangular, but is not limited thereto. The area of ​​the conductive region B is also not particularly limited, and in this embodiment, the conductive region B is an area of ​​2 mm (perpendicular to the linear LIG 22) × 10 mm (parallel to the linear LIG 22), but may be larger or smaller.

[0029] The structure and position of the pair of electrodes 25 of the bending sensor 20 are not particularly limited as long as they can pass electricity through the conductive region B. In this embodiment, the two opposing electrodes 25 are arranged to sandwich the conductive region B. Note that each electrode 25 only needs to be made of a conductive material, and may be made of LIG or a material other than LIG, such as silver, copper, a carbon material, a conductive polymer, or an alloy thereof.

[0030] The bending sensor 20 may include components other than the resin substrate 21, the conductive region B, and the electrodes 25. For example, the bending sensor 20 may further include a battery (not shown) for supplying electricity to the electrodes 25, a means for measuring current (not shown), a means for storing measured values, a monitor (not shown) for displaying measured values, a communication means for transmitting measured values ​​to an external device, etc. The bending sensor 20 may also further include a fixing means (not shown) for fixing the bending sensor 20 to an object to be inspected.

[0031] (Method of Using the Bending Sensor) A method of using the bending sensor 20 of this embodiment will be described. When using the bending sensor 20, the above-mentioned electrodes 25 are connected to an external power supply, ammeter, etc. (neither of which are shown). Note that if the bending sensor 20 is equipped with a power supply, ammeter, etc., this step does not need to be performed.

[0032] The resin substrate 21 is fixed to an object to be inspected (not shown) by a known method so that the surface on which the conductive region B is disposed faces outward, for example. A constant voltage is then applied to the conductive region B via the electrode 25, and the resulting current (resistance) is monitored. For example, the degree to which the bending sensor 20 is bent (bending radius or bending curvature) can be determined by calculating the change in the current (resistance) from the current (resistance) when the bending sensor 20 is not bent. Furthermore, more sensitive measurements can be achieved by bending the sensor 20 in a direction that brings the opposing electrodes closer to each other. When using a bending sensor, combining multiple bending sensors can more accurately identify the bending direction.

[0033] (Modification) In the above, the conductive region B and the electrodes 25 are disposed on only one surface of the resin substrate 21. However, the conductive region B and the electrodes 25 may be disposed on both surfaces of the resin substrate 21. In this case, one bending sensor 20 can measure bending strain amounts (bending radius and bending curvature) in various directions.

[0034] Furthermore, the bending sensor 20 has a feature that the resistance value changes not only depending on the amount of bending strain but also depending on the amount of received ultraviolet light. Therefore, the bending sensor 20 can also be used to identify the amount of received ultraviolet light.

[0035] (Manufacturing Method) An example of a method for manufacturing the bending sensor 20 described above will be described below, but the method is not limited to this method.

[0036] In this method, first, the resin substrate 21 is prepared (substrate preparation step). At this time, the resin substrate 21 may be prepared on which the electrodes 25 and other components are previously arranged. In addition, a masking process or the like may be performed to prevent the nanomaterial from adhering to regions other than the region where the LIG will be formed in the subsequent nanomaterial layer formation step.

[0037] Next, a nanomaterial layer 23 containing the nanomaterial is formed in the conductive region B (the region where the LIG 22 is to be formed) of the resin substrate 21 (nanomaterial layer formation process). The nanomaterial layer 23 may be formed over the entire surface of the resin substrate 21, or may be formed only in the region where the LIG 22 is to be formed. The method for forming the nanomaterial layer 23 is not particularly limited. For example, a dispersion liquid in which the nanomaterial is dispersed in an appropriate solvent may be prepared, and the dispersion liquid may be applied and solidified by a known method. The type of solvent is not particularly limited as long as it does not corrode the resin substrate 21 and can uniformly disperse the nanomaterial. The method for applying the dispersion liquid is also not particularly limited. Examples include spin coating, inkjet coating, and application using a dispenser. After application of the dispersion liquid, the coating film may be solidified by natural drying or by heating.

[0038] In this case, the thickness of the nanomaterial layer 23 to be formed is not particularly limited as long as a sufficient amount of LIG can be formed by laser irradiation, which will be described later.

[0039] Next, the region where the nanomaterial layer 23 has been formed is irradiated with a laser in a desired pattern (laser irradiation step). The laser to be irradiated may be a continuous wave laser or a pulsed laser, as long as it can carbonize the surface of the resin substrate 21 and form the LIG 22 in the desired region. The wavelength of the laser to be irradiated is not particularly limited as long as it can form the LIG, and examples thereof include a CO laser (wavelength 5 μm) and a CO ２ Infrared lasers such as lasers (wavelength 9.3 to 10.6 μm) are preferred, and CO lasers are particularly preferred because the nanomaterials can easily absorb energy. ２ However, as long as the LIG can be formed, the laser is not limited to an infrared laser, and a laser in the visible or ultraviolet range can also be used. Examples of such lasers include F ２Examples of laser sources include an excimer laser (wavelength 157 nm), an ArF excimer laser (wavelength 193 nm), a KrF excimer laser (wavelength 248 nm), and a XeCl excimer laser light source (wavelength 308 nm). The laser irradiation pattern is appropriately selected according to the desired pattern of the LIG 22. At this time, the region where the electrode 25 is to be formed may also be irradiated with the laser to form the electrode 25 made of LIG. Alternatively, a separate step of forming the electrode 25 may be performed.

[0040] After the LIG 22 is formed in the conductive region B, the resin base material 21 may be rolled, if necessary, so as to generate cracks in the LIG 22. By generating cracks in the LIG 22, the sensitivity of the bending sensor 20 can be further increased.

[0041] 2. Tensile Sensor A tensile sensor according to one embodiment of the present invention will be described with reference to the drawings. Fig. 3A shows a plan view of the tensile sensor 30 of this embodiment, Fig. 3B shows an enlarged cross-sectional view taken along line A-A in Fig. 3A, and Fig. 3C shows an enlarged cross-sectional view of region β in Fig. 3B.

[0042] The tensile sensor 30 of this embodiment includes a flexible resin substrate 31, a conductive region C on the resin substrate 31 where an LIG 32 is disposed, and a pair of electrodes 35.

[0043] The resin substrate 31 of the tensile sensor 30 may be a flexible resin substrate that can stretch in accordance with the stretch of the object (e.g., a human body, an animal, a machine, etc.) being inspected by the tensile sensor 30. The resin substrate 31 preferably has durability that makes it difficult for breakage or cracking to occur even when repeatedly stretched.

[0044] The resin substrate 31 may be composed of one layer or two or more layers. The resin substrate 31 may be composed of only resin, or may partially contain an inorganic material such as a filler, as long as the object and effect of this embodiment are not impaired. Examples of resins contained in the resin substrate 31 include silicone-based resins such as dimethylpolysiloxane (PDMS), aromatic polyester resins such as Ecoflex (manufactured by BASF), and urethane-based resins. Among these, PDMS is preferred from the viewpoint of its good extensibility and ease of handling.

[0045] The shape and thickness of the resin substrate 31 are appropriately selected depending on the type of resin, the application of the tensile sensor 30, the strength and extensibility of the resin substrate 31, and the like.

[0046] In this specification, the conductive region C refers to a region including the LIG 32, which is capable of conducting electricity between a pair of electrodes 35 when a voltage is applied between the electrodes 35. In this embodiment, the conductive region C includes not only the LIG 32 but also a nanomaterial.

[0047] In the conductive region C of this embodiment, the LIGs 32 are preferably arranged without gaps so as to connect one electrode 35 to the other electrode 35. Such LIGs 32 may be, for example, a collection of straight, zigzag, wavy, or other lines. The extension direction of the lines is not particularly limited and may be parallel to, perpendicular to, or in another direction relative to the line connecting the pair of electrodes (two electrodes) 35. However, a parallel direction is preferable from the viewpoint of improving the sensitivity of the tensile sensor 30. Note that, as shown in FIG. 3A , in this embodiment, the LIGs 32 are continuously arranged on a straight line parallel to the line connecting the pair of electrodes (two electrodes) 35. Furthermore, a plurality of linear LIGs 32 are arranged parallel to each other between the pair of electrodes 35.

[0048] In the conductive region C of this embodiment, as shown in FIG. 3C , LIG ​​32 is embedded in the resin substrate 31. In this embodiment, the surface of the resin substrate 31 and the top surface of LIG 32 are located at approximately the same position. However, the top surface of LIG 32 may be located higher or lower than the surface of the resin substrate 31. The top surface of LIG 32 may be smooth or rough. In this embodiment, a nanomaterial (nanomaterial layer 23) is present between the LIG 32 and the resin substrate 31 in the region (embedded portion 32a) where LIG 32 is embedded in the resin substrate 31. In this embodiment, the nanomaterial is present in this region because it is manufactured by the method described below. However, the nanomaterial may be attached to or the nanomaterial layer 23 may be disposed in a region other than the embedded portion 32a. Here, the type and shape of the nanomaterial present in the embedded portion 32a are the same as the type and shape of the nanomaterial contained in the conductive region B of the bending sensor 20 described above.

[0049] The shape of the conductive region C is appropriately selected depending on the application of the tensile sensor 30. In this embodiment, the conductive region C has a rectangular shape, but is not limited thereto. The area of ​​the conductive region C is also not particularly limited, and in this embodiment, the conductive region C is an area of ​​2 mm (perpendicular to the linear LIG 32) × 10 mm (parallel to the linear LIG 32), but may be larger or smaller.

[0050] The structure and position of the pair of electrodes 35 of the tensile sensor 30 are not particularly limited as long as they can pass electricity through the conductive region C. In this embodiment, the pair of electrodes 35 are arranged on both ends of the linear LIG 32 so as to sandwich the conductive region C. With such an arrangement, the resistance change when the tensile sensor 30 is pulled parallel to the linear LIG 32 tends to be large, improving the sensitivity of the tensile sensor 30. Note that each electrode 35 may be made of a conductive material, and may be made of LIG or a material other than LIG, such as silver, copper, a carbon material, a conductive polymer, or an alloy thereof.

[0051] The tensile sensor 30 may include components other than the resin substrate 31, the conductive region C, and the electrodes 35. For example, the tensile sensor 30 may further include a battery (not shown) for supplying electricity to the electrodes 35, a means for measuring current (not shown), a means for storing measured values, a monitor (not shown) for displaying measured values, a communication means for transmitting measured values ​​to an external device, etc. The tensile sensor 30 may also further include a fixing means (not shown) for fixing the tensile sensor 30 to the test object.

[0052] (Method of Using the Tension Sensor) A method of using the tension sensor 30 of this embodiment will be described. First, the above-mentioned electrode 35 is connected to an external power supply, ammeter, etc. (neither of which are shown). Note that if the tension sensor 30 is equipped with a power supply, ammeter, etc., this step does not need to be performed.

[0053] The tensile sensor 30 is then fixed to the test object (not shown) so that it is pulled in a direction that increases the distance between the pair of electrodes 35. A constant voltage is then applied to the conductive region C via the electrodes 35, and the amount of current (resistance value) is monitored. For example, by calculating the amount of change in the amount of current (resistance value) from the amount of current (resistance value) when the tensile sensor 30 is not stretched, it is possible to determine the amount of tensile strain occurring in the tensile sensor 30, i.e., the extent to which the tensile sensor 30 is stretched. Furthermore, using multiple tensile sensors 30 and arranging them in different directions allows the condition of the test object to be determined in more detail.

[0054] (Manufacturing Method) An example of a manufacturing method for the above-described tensile sensor 30 will be described below with reference to FIGS. 4A to 4F. However, the present invention is not limited to this method. In this method, as shown in FIG. 4A, a LIG formation substrate (graphene formation substrate) 39 is prepared (hereinafter also referred to as the "LIG formation substrate preparation step"). Then, as shown in FIG. 4B, a nanomaterial layer 33 containing a nanomaterial is formed on the LIG formation substrate 39 (hereinafter also referred to as the "nanomaterial layer formation step"). Furthermore, as shown in FIG. 4C, a laser is irradiated onto the nanomaterial layer 33 to carbonize the LIG formation substrate 39, thereby forming LIG 32 (hereinafter also referred to as the "laser irradiation step"). Then, as shown in FIG. 4D, a material for a resin substrate 31 is applied to the LIG formation substrate 39 and solidified or cured to form the resin substrate 31 (hereinafter also referred to as the "resin substrate formation step"). 4E, the resin base material 31 and the LIG 32 are peeled off from the LIG formation substrate 39 (hereinafter also referred to as a "peeling step"). Thereafter, a step of forming an electrode 35 (FIG. 4F) and the like are performed as needed. Note that the manufacturing method may further include other steps.

[0055] In the LIG formation substrate preparation step, a LIG formation substrate 39, which is a material for the LIG 32, is prepared. The LIG formation substrate 39 may be flexible or rigid. The type of the LIG formation substrate 39 is not particularly limited, and may be a known resin substrate for forming LIG. Examples of the LIG formation substrate 39 include substrates made of polyimide, silicone resin, etc. Among these, a substrate made of polyimide is preferred from the viewpoint of ease of forming LIG.

[0056] In the nanomaterial layer forming step, the nanomaterial layer 33 is formed on the LIG formation substrate 39. The method for forming the nanomaterial layer 33 is the same as the nanomaterial layer forming step in the method for manufacturing the bending sensor 20 described above.

[0057] In the laser irradiation step, a laser is irradiated in a desired pattern onto the region where the nanomaterial layer 33 is formed. The type of laser to be irradiated and the irradiation method are the same as those in the laser irradiation step in the manufacturing method of the bending sensor 20 described above.

[0058] In the resin substrate formation process, a material for the resin substrate 31 (e.g., a resin varnish or a solution containing a resin precursor) is applied to a desired thickness on the LIG formation substrate 39 on which the LIG 32 has been formed. The material is then solidified or cured to embed the LIG 32 in the resin substrate 31. The application and solidification (curing) of the material for the resin substrate 31 may be performed only once or may be repeated until the desired film thickness is achieved. The LIG 32 described above is porous. Therefore, when the resin substrate 31 is formed by this method, the material for the resin substrate 31 penetrates into the LIG 32, firmly bonding the formed resin substrate 31 and the LIG 32. Furthermore, nanomaterials are typically attached to the periphery of the LIG 32 formed in the laser irradiation process. Therefore, when the resin substrate 31 is formed by this method, the nanomaterial penetrates between the resin substrate 31 and the LIG 32.

[0059] In the peeling step, the resin base 31 is peeled off from the LIG formation substrate 39. At this time, since the resin base 31 and the LIG 32 are firmly bonded to each other as described above, the LIG 32 is cleaved between the resin base 31 and the LIG formation substrate 39, and the LIG 32 is peeled off together with the resin base 31, as shown in FIG. 4E . Therefore, the LIG 32 can be disposed in the conductive region C of the resin base 31.

[0060] Thereafter, the electrodes 35 are formed by a known method (for example, by applying a metal paste), although the electrodes 35 may also be formed in the above-mentioned laser irradiation step.

[0061] The present invention will be described in more detail below with reference to examples. However, the scope of the present invention is not limited by these examples, and modifications of the embodiments are possible without departing from the spirit of the present invention.

[0062] 1. Bending Sensor (1) Preparation of First Bending Sensor A flexible resin substrate (thickness 125 μm) made of polyimide was prepared. Separately, a slurry (manufactured by Sumitomo Osaka Cement Co., Ltd.) was prepared by dispersing ZnO nanoparticles with an average particle diameter (BET equivalent particle diameter) of approximately 25 nm in isopropyl alcohol (IPA). The BET equivalent particle diameter was calculated based on the BET specific surface area (m ２ / g) was calculated using the general formula (1) above. The BET specific surface area was determined using a fully automatic specific surface area measuring device, Macsorb HM Model-1201, manufactured by Mountech Co., Ltd. The BET specific surface area of ​​ZnO nanoparticles is approximately 40 m ２ The ZnO nanoparticle concentration in the slurry was 21.5% by mass. The slurry was applied to the resin substrate by spin coating (500 rpm). After that, the IPA was removed, and a ZnO layer with a thickness of approximately 500 nm was formed on the resin substrate. ２ Using a laser system (VLS2.30, Universal Laser Systems), a 4.0 W CO laser was applied to the ZnO layer by computer-controlled one-step scanning. ２ The laser was irradiated to form LIG. More specifically, a 2 mm wide, 10 mm long region of the ZnO layer was irradiated with the laser multiple times in a 100 μm pitch, in a linear pattern of 10 mm in length. This resulted in a conductive region in which 10 mm long LIGs were arranged in parallel. Then, a 7.5 W CO ２ A laser was irradiated to form a pair of electrodes (LIG) outside the conductive region, parallel to the LIG in the conductive region. Each electrode was 10 mm long and 10 mm wide. To increase the sensitivity of the bending sensor, the bending sensor was rolled to a diameter of 3 mm, and cracks were introduced into the LIG in the conductive region. A photograph of the conductive region of the bending sensor observed with a scanning electron microscope (SEM) is shown in Figure 5A.

[0063] (2) Fabrication of a Second Bending Sensor A bending sensor was fabricated in the same manner as described above, except that the laser irradiation pitch when forming LIG in the conductive region was changed to 200 μm. FIG. 5B shows a photograph of the conductive region of the bending sensor observed with an SEM.

[0064] (3) Evaluation The first bending sensor and the second bending sensor were each evaluated as follows.

[0065] Change in resistance value with bending strain The first bending sensor and the second bending sensor were each fixed to an actuator having a single-axis stepping motor. Furthermore, the sensors were connected to a data logger. Figure 6 shows the rate of change in resistance value with bending strain when each sensor was bent so that the surface having the LIG (conductive region) was facing outward. The rate of change in resistance value is calculated by {(resistance value when strain is applied - initial resistance value) / initial resistance value}. As shown in Figure 6, in both the first bending sensor and the second bending sensor, the rate of change in resistance value increased with increasing bending strain.

[0066] Relationship between bending and current As described above, the first bending sensor was fixed to the actuator, and the amount of bending strain was increased every 30 seconds, and then decreased every 30 seconds. The change in current when the amount of bending strain was increased is shown in Figure 7. As shown in Figure 7, if the amount of bending strain was the same, the current when bending and when returning was approximately the same, demonstrating its usefulness as a bending sensor.

[0067] Change in resistivity with number of repeated bending cycles As described above, the first bending sensor was fixed to the actuator, and the sample was bent and stretched every 5 seconds so that the bending strain was 0.32%. The change in the rate of resistance change over time is shown in Fig. 8. As shown in Fig. 8, the rate of resistance change did not change significantly even when the sample was bent and stretched repeatedly over a long period of time.

[0068] Comparison with known bending sensors The results of comparing the gauge factor of the second bending sensor with that of known bending sensors are shown in Figure 9. The known bending sensors used were, for example, those having the configuration shown in Table 1 below. Table 1 also lists the gauge factor of the second bending sensor.

[0069] As shown in FIG. 9, the bending sensor of the present invention exhibited a higher gauge factor than the known bending sensor at low strain amounts.

[0070] 2. Tensile Sensor (1) Preparation of First Tensile Sensor A flexible LIG substrate (125 μm thick) made of polyimide was prepared. Separately, a slurry (manufactured by Sumitomo Osaka Cement Co., Ltd.) was prepared in which ZnO nanoparticles with an average particle diameter (BET equivalent particle diameter determined by the above method) of approximately 25 nm were dispersed in isopropyl alcohol (IPA). The concentration of ZnO nanoparticles in the slurry was 21.5 mass%. The slurry was applied to the resin substrate by spin coating (500 rpm). Thereafter, the IPA was removed, and a ZnO layer with a thickness of approximately 500 nm was formed on the resin substrate. Then, CO ２ A CO laser was applied to the ZnO layer by computer-controlled one-step scanning using a laser system (VLS2.30, Universal Laser Systems). ２ The laser was irradiated. More specifically, a 2 mm wide, 10 mm long region of the ZnO layer was irradiated with the laser multiple times at a pitch of 100 μm in a 10 mm long linear pattern. As a result, a conductive region in which 10 mm long LIGs were arranged in parallel was formed on the LIG formation substrate.

[0071] Next, a polydimethylsiloxane (PDMS) precursor and a crosslinker (Sylgard 184, manufactured by Dow) were mixed at a mass ratio of 10:1. After degassing, the mixture was spin-coated onto the LIG-forming substrate at 700 rpm and cured at 90°C for 10 minutes. Further spin-coating was performed at 700 rpm and cured at 90°C for 2 hours. After 2 hours, the resin substrate (PDMS) and LIG were peeled off from the LIG-forming substrate. Then, a pair of electrodes was formed using silver paste on both ends (perpendicular to the LIG) of the linear LIG transferred to the resin substrate (PDMS), forming a tensile sensor. A photograph of the conductive region of the tensile sensor observed with a scanning electron microscope (SEM) is shown in FIG. 10A.

[0072] (2) Fabrication of Second Tensile Sensor A tensile sensor was fabricated in the same manner as the first tensile sensor, except that the pair of electrodes was arranged parallel to the LIG in the conductive region.

[0073] (3) Preparation of a third tensile sensor (comparative example) A tensile sensor was prepared in the same manner as the first tensile sensor, except that a conductive region was formed on a polyimide LIG substrate without forming a ZnO layer, and then transferred to a resin substrate made of PDMS. A photograph of the conductive region of the tensile sensor observed with a scanning electron microscope (SEM) is shown in FIG. 10B.

[0074] (4) Evaluation The first tensile sensor, the second tensile sensor, and the third tensile sensor were each evaluated as follows.

[0075] - Confirmation of LIG by Raman Spectroscopy Figure 11 shows the Raman spectra of the LIG after transfer to PDMS and the LIG remaining on the LIG formation substrate (polyimide) side of the first and third tensile sensors. As shown in Figure 11, graphene was confirmed on both the PDMS and polyimide sides of the first tensile sensor. On the other hand, no graphene remained on the polyimide side of the third tensile sensor (comparative example). Furthermore, no 2D band was observed in the LIG after transfer to PDMS.

[0076] Change in resistance value with tensile strain Figure 12 shows the rate of change in resistance value when the first tensile sensor, second tensile sensor, and third tensile sensor were each fixed to the actuator in the same manner as above and pulled so that the distance between the electrodes increased. The rate of change in resistance value is calculated by {(resistance value when strain is applied - initial resistance value) / initial resistance value}. As shown in Figure 12, the third tensile sensor (comparative example) showed no change in resistance value. In contrast, the first tensile sensor and the second tensile sensor each showed a change in resistance value in response to tensile strain. In particular, the rate of change in resistance value of the first tensile sensor was large.

[0077] Change in resistivity with number of repeated tensile tests Figure 13 shows the change in the rate of resistance change over time when the first tensile sensor was stretched and contracted every 5 seconds so that the tensile strain was 4%. As shown in Figure 13, the rate of resistance change did not change significantly even when the first tensile sensor was stretched and contracted repeatedly over a long period of time. Similarly, Figure 14 shows the change in the rate of resistance change over time when the first tensile sensor was stretched and contracted every 5 seconds so that the tensile strain was 8%. In this case, too, as shown in Figure 14, the rate of resistance change did not change significantly even when the first tensile sensor was stretched and contracted repeatedly over a long period of time.

[0078] Relationship between tensile strain and current (1) For the first tensile sensor, the change in current when the tensile strain amount was increased by 2% every 10 seconds up to 10%, and then decreased by 2% every 10 seconds is shown in Figure 15. As shown in Figure 15, if the tensile strain amount was the same, the current when pulled and when released was approximately the same, demonstrating its usefulness as a tensile sensor.

[0079] Relationship between tensile strain and current (2) For the first tensile sensor, the change in current when the tensile strain amount was increased by 2% every 100 seconds up to 10% while repeatedly stretching was shown in Figure 16. As shown in Figure 16, if the tensile strain amount was the same, the current showed approximately the same value, demonstrating its usefulness as a tensile sensor.

[0080] Comparison with known tensile sensors The results of comparing the gauge factors of the first and second tensile sensors with the gauge factors of known tensile sensors are shown in Figure 17. The known tensile sensors used had the configuration shown in Table 2 below. Table 2 also shows the results for the second tensile sensor.

[0081] As shown in FIG. 17, the tensile sensors of the present invention (the first tensile sensor and the second tensile sensor) exhibited a much higher gauge factor than the known tensile sensor at a low strain amount.

[0082] This application claims priority based on Japanese Patent Application No. 2024-087976, filed May 30, 2024. The contents of the specification and drawings of that application are incorporated herein by reference in their entirety.

[0083] The strain sensor of the present invention has high sensitivity to bending and tension, and is therefore very useful for measurements in a variety of technical fields.

[0084] 11 Resin material 12, 22, 32 LIG 13, 23, 33 Nanomaterial layer 20 Bending sensor 21, 31 Resin substrate 22a Protrusion 25, 35 Electrode 30 Tension sensor 39 Graphene formation substrate (LIG formation substrate) B, C Conductive region

Claims

1. A strain sensor comprising: a flexible resin substrate; a conductive region including laser-induced graphene disposed on the resin substrate; and a pair of electrodes electrically connected to the conductive region, wherein at least a portion of the conductive region further comprises a nanomaterial having light absorption properties.

2. The strain sensor according to claim 1, wherein, when a cross section perpendicular to the surface of the resin substrate is observed, the laser-induced graphene includes protrusions protruding from the surface of the resin substrate, and the nanomaterial is attached to the outer walls of the protrusions.

3. A strain sensor as described in claim 1, wherein, when a cross section perpendicular to the surface of the resin substrate is observed, the laser-induced graphene has an embedded portion embedded in the resin substrate, and the nanomaterial is present between the resin substrate and the laser-induced graphene in the embedded portion.

4. The strain sensor according to claim 1, wherein the laser-induced graphene is arranged in a plurality of lines parallel to each other in the conductive region.

5. The strain sensor of claim 1, wherein the nanomaterial comprises zinc oxide nanoparticles.

6. The strain sensor according to claim 1, wherein the nanomaterial includes at least one material selected from the group consisting of carbon nanotubes, gold nanoparticles, and silver nanoparticles.

7. The strain sensor according to claim 2, wherein the resin substrate contains polyimide, and the strain sensor is used as a bending sensor.

8. The strain sensor according to claim 3, wherein the resin substrate contains a silicone-based resin or a urethane-based resin, and the strain sensor is used as a tensile sensor.

9. The strain sensor according to claim 8, wherein in the conductive region, the laser-induced graphene is arranged in a plurality of lines parallel to each other, and the pair of electrodes is arranged at both ends of the lines.

10. A method for manufacturing a strain sensor having a flexible resin substrate and a conductive region including laser-induced graphene arranged on the resin substrate, the method comprising the steps of: preparing the resin substrate; forming a nanomaterial layer including a nanomaterial having light absorption properties on the resin substrate; and irradiating the nanomaterial layer with a laser to form laser-induced graphene on the surface of the resin substrate.

11. A method for manufacturing a strain sensor having a flexible resin substrate and a conductive region including laser-induced graphene arranged on the resin substrate, the method comprising: a step of preparing a resin substrate for graphene formation; a step of forming a nanomaterial layer including a nanomaterial having light absorption properties on the graphene formation substrate; a step of irradiating the nanomaterial layer with a laser to form laser-induced graphene on a surface of the graphene formation substrate; a step of applying a material of the resin substrate onto the graphene formation substrate and solidifying or curing it to form the resin substrate; and a step of peeling off the resin substrate and the laser-induced graphene from the graphene formation substrate.

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