Variable resistance hardness sensor

Abstract

To provide a variable resistance pressure sensitive thin film-based hardness sensor capable of measuring harness through pressing contact with a detection target by providing flexible cover layers above and below electrodes and a pressure sensing layer to sandwich the electrodes and the pressure sensing layer.SOLUTION: A variable resistance hardness sensor is provided, comprising an upper cover layer, a pair of electrodes, a pressure sensitive layer, and a lower cover layer arranged in the described order. The upper and lower cover layers are independently formed of one or more elastic materials, where an elastic material of the lower cover layer has a Young's modulus that is one-tenth or less of that of an elastic material of the upper cover layer.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a soft hardness sensor that determines hardness from a change in resistance when an indenter comes into contact with an object to be measured. [Background technology]

[0002] In recent years, there has been an urgent need to develop soft sensor devices to provide a sense of touch to soft robots capable of collaborating with humans. In particular, hardness sensors that can detect the hardness of an object are extremely important for providing a sense of touch to such robots, as they can detect the force sensation in real time when touching the surface of an object or when grasping an object. However, most hardness sensors to date have been based on inorganic MEMS (Micro Electro Mechanical Systems) technology, and improving flexibility and sensitivity for soft applications has been an implementation challenge.

[0003] Regarding soft hardness sensors, Non-Patent Document 1 discloses a stretchable wearable hardness sensor designed to simultaneously recognize applied force and deformation by incorporating a piezoresistive strain sensor and a piezoresistive pressure sensor. The strain sensor has a structure consisting of a silver nanowire (AgNW) / carbon nanotube (CNT) / polydimethylsiloxane (PDMS) composite (this structure is called an "AgCP composite"), and has a gauge factor of 1.45 and is durable. On the other hand, the pressure sensor is made by dropping a flake silver powder / carbon nanotube (CNT) / polydimethylsiloxane (PDMS) (amorphous calcium fluoride phosphate (AFCP) water-in-oil emulsion onto an AgCP electrode using the drop-cast method) and has a force sensing capability of 0.46 N. -1The device has a durability of over 15,000 load / unload cycles. Because the pressure sensor can detect pulse waves from the carotid artery, these two reliable sensors can be attached with polydimethylsiloxane (PDMS) to form a hardness sensor, which can easily measure the hardness of a substance when it comes into contact with it using Young's modulus (elastic coefficient). By placing this hardness sensor on the skin of patients with systemic sclerosis, an incurable disease, and healthy individuals and comparing and evaluating the measured hardness, the progression of skin hardening can be measured. Since the results are consistent with those obtained using a commercially available durometer, the researchers believe that this hardness sensor may be useful for diagnosing systemic sclerosis. [Prior art documents] [Non-patent literature]

[0004] [Non-Patent Document 1] Meng-Yang Liu et al., Nano Energy, 98, 107242 (2022) Summary of the Invention [Problem to be solved by the invention]

[0005] The present invention aims to provide a resistance change type hardness sensor that can measure hardness by pressing and contacting an object to be detected by sandwiching a flexible cover layer above and below a pair of electrodes and a pressure-sensitive layer. [Means for solving the problem]

[0006] The present invention comprises the following items. [1] A resistance change type hardness sensor having an upper cover layer, a pair of electrodes and a pressure-sensitive layer, and a lower cover layer in this order, wherein the upper cover layer and the lower cover layer are made of one or more elastic materials, and the Young's modulus of the elastic material constituting the lower cover layer is one-tenth or less of the Young's modulus of the elastic material constituting the upper cover layer.

[0007] [2] The resistance change type hardness sensor according to [1], wherein the one or more elastic materials have a Young's modulus of 0.1 to 2.0 MPa.

[0008] [3] The resistance change type hardness sensor according to [2], wherein the one or more elastic materials are silicone rubber. [4] The resistance change type hardness sensor according to [1], wherein the pressure-sensitive layer contains an elastic material and a conductive filler.

[0009] [5] The resistance change type hardness sensor according to [3], wherein the pressure-sensitive layer further contains a surfactant in addition to the elastic material and the conductive filler. [Effects of the Invention]

[0010] According to the present invention, by sandwiching an upper cover layer and a lower cover layer above and below a pair of electrodes and a pressure-sensitive layer, it is possible to provide a soft hardness sensor that can measure the hardness of a detection object from the resistance change that occurs when a probe (indenter) comes into pressure contact with the detection object. In such a resistance change type hardness sensor, by optimizing the hardness of the upper cover layer and the lower cover layer, it can become a sensor device that can detect the reaction force received from the detection object with high sensitivity and provide information on the softness and hardness. [Brief explanation of the drawings]

[0011] [Figure 1] FIG. 1 is a schematic diagram showing one embodiment of a resistance change type hardness sensor of the present invention. [Figure 2] FIG. 2 is a schematic diagram showing a pair of electrodes and a pressure-sensitive layer formed on a substrate film in the resistance change type hardness sensor of the present invention. [Figure 3] Figure 3 shows an image of the structural change in the pressure-sensitive layer when a resistance change type hardness sensor is placed on a detection target with a different Young's modulus and pressure is applied. [Figure 4]4 is a graph showing the change in resistance when pressure is applied in the following cases: when neither the upper nor lower cover layer is used (Comparative Example 1), when PDMS is used for both the upper and lower cover layers (Example 2), when PDMS is used for the upper cover layer and silicone rubber is used for the lower cover layer (Example 3), and when silicone rubber is used for both the upper and lower cover layers (Example 4). In the graph, the vertical axis represents the increment in resistance R (R / R0), which is proportional to the initial resistance value R0, and the horizontal axis represents the applied pressure (kPa). [Figure 5] Figure 5 is a graph plotting the rate of resistance change over time when a resistance change type hardness sensor attached to a finger is pressed into a metal piece and konjac with the same amount of pressure to detect changes in the electrical signal. Note that the upper cover layer of the resistance change type hardness sensor is the side that comes into contact with the finger, and the lower cover layer is the side that comes into contact with the metal piece or konjac. [Figure 6] 6 is a graph showing the change in resistance over time when PDMS was used for both the upper and lower cover layers (Example 1), when PDMS was used for the upper cover layer and a 2:1 mixture of PDMS and silicone rubber was used for the lower cover layer (Comparative Example 2), and when silicone rubber was used for both the upper and lower cover layers (Example 4). In the graph, the vertical axis represents the increment in resistance R (R / R0), which is proportional to the initial resistance value R0, and the horizontal axis represents time (seconds). [Figure 7] 7 is a graph showing the rate of resistance change with strain for the following cases: when PDMS was used for both the upper and lower cover layers (Example 1); when PDMS was used for the upper cover layer and a 2:1 mixture of PDMS and silicone rubber was used for the lower cover layer (Comparative Example 2); and when silicone rubber was used for both the upper and lower cover layers (Example 4). In the graph, the vertical axis represents the increment in resistance R (R / R0) proportional to the initial resistance value R0, and the horizontal axis represents Young's modulus (MPa). DETAILED DESCRIPTION OF THE INVENTION

[0012] Hereinafter, a resistance change type hardness sensor 10 (hereinafter simply referred to as "hardness sensor 10") of the present invention will be described in detail with reference to FIGS. Fig. 1 is a schematic diagram of a hardness sensor 10 of the present invention, showing a configuration in which a base film 5, a pair of electrodes 4, and a pressure-sensitive layer 3 are provided between an upper cover layer 1 and a lower cover layer 2. Fig. 2 is a schematic plan view of the hardness sensor 10 of Fig. 1 in a configuration in which a pair of electrodes 4 and a pressure-sensitive layer 3 are provided on the base film 5.

[0013] The upper cover layer 1 and the lower cover layer 2 are made of one or more elastic materials, such as rubber, a resin with sufficient elasticity, or a rubber composite material in which rubber and resin are mixed in a predetermined ratio.

[0014] Specifically, in addition to silicone rubbers such as polydimethylsiloxane (PDMS) and liquid silicone rubber (LSR), thermosetting elastomers such as fluororubber, and thermoplastic elastomers based on styrene, olefin, vinyl chloride, urethane, ester, and amide are used. Of these, silicone rubbers such as polydimethylsiloxane (PDMS) are preferably used in the present invention. These materials may be used alone or in combination of two or more. Different elastic materials may be used for the upper cover layer 1 and the lower cover layer 2.

[0015] The shapes of the upper cover layer 1 and the lower cover layer 2 may be various, such as circular, oval, rectangular, or horseshoe, depending on the shape of the substrate film 5 described below, but are usually rectangular.

[0016] The thickness of the upper cover layer 1 and the lower cover layer 2 is usually 1 to 1000 μm, preferably 100 to 500 μm, for example 250 μm. When the thickness of the upper cover layer 1 and the lower cover layer 2 is within the above range, the film has stability and durability, which is suitable.

[0017] The Young's modulus of the elastic material constituting the upper cover layer 1 and the lower cover layer 2 is preferably 0.1 to 2.0 MPa. The Young's modulus of the elastic material constituting the lower cover layer, which comes into contact with the detection target, is preferably at most about one-tenth of the elastic material of the upper cover layer, which presses the indenter. In other words, the Young's modulus of the lower cover layer 2 need only be one-tenth of that of the upper cover layer 1.

[0018] In addition to the elastic materials forming the upper cover layer 1 and the lower cover layer 2, functional polymer films such as polyesters such as polyethylene naphthalate (PEN), polycarbonate, polyimide, polyphenylene sulfide, polyethersulfone, polyethylene, polypropylene, fluororesins (PTFE, PFA, etc.), polymethyl methacrylate, and polyamide can be used for the base film 5. Of these, polyethylene naphthalate films are preferably used in the present invention.

[0019] The base film 5 may be made of a material that also serves as a spacer interposed between the upper cover layer 1, the pair of electrodes 4 and the pressure-sensitive layer 3, and the lower cover layer 2. Suitable materials for this purpose include transparent photocurable resins such as epoxy acrylate and urethane acrylate, and thermosetting resins such as silicone, polyester, and epoxy, which have flexibility, elasticity, and adhesiveness.

[0020] 1 and 2, a pair of electrodes 4 is disposed on a base film 5, and a pressure-sensitive layer 3 is further formed so as to cover at least a portion of the pair of electrodes 4. When the pressure-sensitive layer 3 is pressed by contact with an object to be detected, the resistance value changes. The hardness sensor of the present invention detects this change in resistance value.

[0021] The pair of electrodes 4 may be linear electrodes, or may be comb-shaped electrodes in which the electrodes are formed in a comb shape and the teeth of the combs interlock with each other. Forming the electrodes in this manner increases the electrode surface area in the pressure-sensitive layer 3, ensuring good electrical continuity in the pressure-sensitive layer 3 when the hardness sensor 10 is pressed. As a result, pressure sensitivity is improved.

[0022] The electrode 4 is typically a patterned electrode produced by printing or vapor deposition, such as flexographic printing, screen printing, inkjet printing, and gravure printing, or by vapor deposition, such as chemical vapor deposition (CVD) or physical vapor deposition (PVD).

[0023] Any material may be used for the electrode 4 as long as it is conductive and durable against expansion and contraction. Specific examples include metals such as gold, nickel, tantalum, aluminum, and chromium, or alloys thereof; resin composite silver paste materials such as conductive adhesives in which silver particles are uniformly dispersed in an organic binder such as epoxy resin; and conductive polymers such as PEDOT:PSS. Commercially available resin composite silver paste materials, such as silver paste (DOTITE XA-9521) (manufactured by Fujikura Kasei Co., Ltd.), may also be used.

[0024] The pressure-sensitive layer 3 is a layer whose resistance value changes depending on the magnitude of the pressure. The pressure-sensitive layer 3 is formed across a pair of electrodes 4 so that a conductive path is created by contact between conductive fillers and the like in the pressure-sensitive layer 3. When the pressure-sensitive layer 3 is pressed, the contact state of the conductive fillers and the like in the pressure-sensitive layer 3 changes, and as the strain of the pressure-sensitive layer 3 increases, the contact increases as shown in Figure 3, forming a percolation network and forming conductive paths 7, 7' that are optimal for exhibiting conductivity.

[0025] The pressure-sensitive layer 3 may be made of one or more metals, other conductive or semiconductive elements and oxides, or conductive or semiconductive organic or inorganic polymers. Specifically, the pressure-sensitive layer 3 preferably contains an elastic material and a conductive filler.

[0026] The elastic material used for the pressure-sensitive layer 3 may be the same as or different from the elastic material used for the upper cover layer 1 and the lower cover layer 2. A suitable elastic material for the pressure-sensitive layer 3 is silicone rubber such as polydimethylsiloxane (PDMS).

[0027] The conductive filler is a powder, particle, or fiber of a conductive material. Specific examples include, without limitation, carbon nanotubes (CNTs) and metal powders (e.g., titanium, tantalum, zirconium, vanadium, niobium, hafnium, aluminum, silicon, tin, chromium, molybdenum, tungsten, lead, manganese, beryllium, iron, cobalt, nickel, platinum, palladium, osmium, iridium, rhenium, rhodium, ruthenium, gold, silver, cadmium, copper, zinc, germanium, arsenic, antimony, bismuth, boron, scandium, and lanthanide and actinide series metals). Of these, carbon nanotubes (CNTs) are particularly suitable. Carbon nanotubes (CNTs) are carbon materials with diameters of 0.4 to 50 nm and have excellent electrical conductivity, thermal conductivity, and heat resistance. Carbon nanotubes (CNTs) include single-wall nanotubes (SWNTs), double-wall nanotubes (DWNTs), and multi-wall nanotubes (MWNTs). MWNTs are usually used, but any of them may be used.

[0028] The amount of conductive filler dispersed in the elastic material can be adjusted appropriately according to pressure sensitivity, but is usually 1.0 to 10.0 wt%, preferably 1.0 to 5.0 wt% from the perspective of improving characteristics, specifically 2.3 wt%.

[0029] In addition to the elastic material and conductive filler described above, a binder may be added to the pressure-sensitive layer 3 within a range that does not impair the effects of the present invention. The binder is preferably a resin whose glass transition point (Tg) is in a temperature range higher than room temperature, and examples of the binder include acrylic resin, vinyl chloride-vinyl acetate copolymer resin, polyester resin, and polyamide resin.

[0030] It is preferable that the pressure-sensitive layer 3 further contains a surfactant in addition to the elastic material and the conductive filler.

[0031] The surfactant is added to control the dispersibility of the conductive filler in the pressure-sensitive layer 3. In particular, when the elastic material constituting the pressure-sensitive layer 3 does not easily wet the surface of the conductive filler, making it difficult for the conductive filler to disperse in the pressure-sensitive layer 3, it is effective to add a surfactant to impart affinity to both the elastic material and the conductive filler.

[0032] The surfactant may be either an ionic surfactant or a nonionic surfactant. Ionic surfactants include anionic surfactants, cationic surfactants, and amphoteric surfactants. Nonionic surfactants include ester type, ether type, and ester-ether type. In the present invention, for example, ester-ether type polyoxyethylene-10-stearyl ether (PSE) (C 18 H 37 (OCH2CH2) n OH, n=1 to 100) is preferably used. From the viewpoint of improving properties, it is preferable to use polyoxyethylene-10-stearyl ether (PSE) with n=10 in the present invention.

[0033] FIG. 3 shows how a hardness sensor 10 is placed on detection targets 6a and 6b and an indenter is pressed against them from above. FIG. 3(a) shows the case where a hard detection target is used, and FIG. 3(b) shows the case where a soft detection target is used. As shown in FIG. 3, when an indenter is pressed vertically from above the upper cover layer 1, the detection target 6 is pressed and deformed, generating deformation resistance. This deformation resistance changes with pressure. This rate of change is related to the Young's modulus of the detection target 6. This Young's modulus can be calculated, for example, by performing two-wire resistance measurements using a digital multimeter and measuring the rate of change in the resistance of the indenter (R / R0) based on Ohm's law (R=V / I, R: sensor resistance value, I: fixed current (1.25 A), V: variable voltage).

[0034] When conductive paths 7, 7' are formed within the pressure-sensitive layer 3, resistance decreases, allowing current to flow more easily. The hardness sensor 10 detects the current at this time and calculates hardness from the increase in resistance (R / R0) relative to the change in pressure (kPa). Figure 3 is an illustration of the structural change in the pressure-sensitive layer 3 when objects with different Young's moduli are pressed against the hardness sensor 10.

[0035] Of the detection targets 6, the hard object 6a is made of, for example, stainless steel or iron, and the Young's modulus calculated from the rate of resistance change when an indenter is pressed against stainless steel at a pressure of 1 N for 30 seconds is approximately 200 GPa. On the other hand, the soft object 6b is made of, for example, silicone rubber or edible konjac, and the Young's modulus calculated from the rate of resistance change when an indenter is pressed against silicone rubber at a pressure of 1 N for 30 seconds is approximately 160 kPa.

[0036] In a resistance change type hardness sensor 10 having a length of 20 mm, a width of 40 mm, and a total thickness of 1.5 mm including the components in FIG. 1 (upper cover layer 1, lower cover layer 2, pressure-sensitive layer 3, base film 5, and detection object 6), when a load of 10 kPa is applied and the Young's modulus of the detection object 6 is 160 kPa to 200 GPa, the Young's modulus of the upper cover layer 1 is 160 to 2000 kPa and the Young's modulus of the lower cover layer 2 is 160 to 2000 kPa, and preferably the Young's modulus of the upper cover layer 1 is 2000 kPa and the Young's modulus of the lower cover layer 2 is 160 kPa.

[0037] The resistance change type hardness sensor 10 of the present invention can be manufactured by a known method. For example, an electrode pattern is formed by depositing poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS) on a polyethylene naphthalate (PEN) film. The electrode pattern is formed by printing or vapor deposition. When printing is performed, flexographic printing, screen printing (e.g., stencil printing), inkjet printing, gravure printing, etc. are used. When vapor deposition is performed, chemical vapor deposition (CVD) or physical vapor deposition (PVD) is used.

[0038] Next, an aqueous solution containing multi-walled carbon nanotubes (MWCNT), polydimethylsiloxane (PDMS), and polyoxyethylene-10-stearyl ether (PSE) is applied to the electrode pattern by, for example, stencil printing to form the pressure-sensitive layer 3. Note that, depending on the viscosity of the printing ink and the sensor shape, printing methods (for example, inkjet printing or gravure printing) or coating methods other than those described above may also be applied.

[0039] Furthermore, polydimethylsiloxane (PDMS) is attached as an upper cover layer 1 and a lower cover layer 2 to the upper and lower surfaces of the device consisting of the electrodes 4 and the pressure-sensitive layer 3, thereby obtaining a resistance change type hardness sensor 10.

[0040] The resistance change type hardness sensor 10 of the present invention is capable of detecting the force sense when grasping an object or touching the surface of an object, and is therefore expected to be applied to robot sensing technology and technology for palpating specific areas within an object, such as technology for palpating tumors in a living body. [Example]

[0041] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to the following examples. [Example 1] (1) Fabrication of the resistance change type hardness sensor 10 On a 50 μm-thick polyethylene naphthalate (PEN) film (Q65HA; manufactured by DuPont), poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonic acid) (PEDOT:PSS) (Clevios (registered trademark) S V4 STAB; manufactured by Heraeus) was formed as electrode 4 by stencil printing (film thickness: 5 μm), and annealed at 120° C. for 30 minutes. The distance between the electrodes was 1 mm. Next, multi-walled carbon nanotubes (MWCNTs), polydimethylsiloxane (PDMS), polyoxyethylene-10-stearyl ether (PSE) (C 18 H 37 (OCH2CH2) nA solution material (PDMS: 45.9 wt%, MWCNT: 2.3 wt%, PSE: 6.4 wt%, pure water: 45.4 wt%) was prepared using PEDOT:PSS (OH, n=10) and pure water. The solution material was deposited on a PEN substrate coated with PEDOT:PSS by stencil printing, and annealed at 50°C for 2 hours and then at 120°C for 1 hour to form a pressure-sensitive layer 3. PDMS (film thickness: 250 μm) was attached to the top and bottom of the device consisting of the obtained electrode 4 and pressure-sensitive layer 3 as the upper cover layer 1 and the lower cover layer 2 so that the film thickness ratio of the upper cover layer 1 and the lower cover layer 2 was 1:1, thereby obtaining a resistance change type hardness sensor 10.

[0042] (2) Performance evaluation of the resistance change type hardness sensor 10 A pressure of 5 kPa was applied to the resistance change type hardness sensor 10, and the resistance change value in the pressure sensitive layer 3 with respect to the applied pressure was measured. The resistance change type hardness sensor 10 showed clear sensitivity even to a weak pressure of 5 kPa. It also showed sensitivity to a pressure of approximately 120 kPa, making it clear that it has a relatively wide dynamic range. Next, the resistance change type hardness sensor 10 was mounted on a finger, and the change in resistance value was measured when objects of different hardness were grasped. Figure 5 is a graph plotting the rate of resistance change over time when the resistance change type hardness sensor 10 attached to the finger was pressed into a metal piece and konjac with the same amount of pressure to detect the change in electrical signal. When the resistance change type hardness sensor 10 was pressed into konjac, the resistance change rate (R / R0) was 0.07, but when pressed into a piece of metal, it was 0.025. This result shows that even when attached to a finger, it is possible to detect changes in electrical signals depending on the hardness of the object. The resistance change rate of the sensor increased by nearly three times when gripping a soft object compared to a hard object, which is presumably due to the difference in the change rate of the sensor housing for the same pressure.

[0043] [Example 2] (1) Fabrication of the resistance change type hardness sensor 10 A silver paste (DOTITE XA-9521) was formed as an electrode 4 on a PEN film (film thickness: 50 μm) by screen printing (film thickness: 30 μm), and annealed at 150° C. for 90 minutes. Next, a solution material (PDMS: 45.9 wt%, MWCNT: 2.3 wt%, PSE: 6.4 wt%, pure water: 45.4 wt%) was prepared from MWCNT, PDMS, PSE, and pure water. The solution material was formed into a film by stencil printing, heated at 50°C for 2 hours, and then annealed at 120°C for 1 hour to form a pressure-sensitive layer 3 (film thickness: 700 μm). PDMS (Young's modulus: approximately 2 MPa) with a thickness of 250 μm was attached to the top and bottom of the device consisting of the obtained electrode 4 and pressure-sensitive layer 3 as an upper cover layer 1 and a lower cover layer 2, thereby obtaining a resistance change type hardness sensor 10 (Figure 1). (2) Performance evaluation of the resistance change type hardness sensor 10 A weak pressure of 5 kPa was applied to the resistance change type hardness sensor 10, and the resistance change value in the pressure sensitive layer with respect to the applied pressure was measured.

[0044] [Example 3] A resistance change type hardness sensor 10 was fabricated in the same manner as in Example 2, except that the upper cover layer 1 was made of PDMS and the lower cover layer 2 was made of Ecoflex (registered trademark) (silicone rubber, Young's modulus: approximately 160 kPa). In the same manner as in Example 2, the performance of the resistance change type hardness sensor 10 was evaluated.

[0045] [Example 4] A resistance change type hardness sensor 10 was fabricated in the same manner as in Example 2, except that the upper cover layer 1 and the lower cover layer 2 were made of Ecoflex. In the resistance change type hardness sensor 10, when the upper cover layer and the lower cover layer had the same film thickness, the Young's modulus of the elastic material constituting the lower cover layer was one-tenth of the Young's modulus of the elastic material constituting the upper cover layer. In the same manner as in Example 2, the performance of the resistance change type hardness sensor 10 was evaluated.

[0046] [Comparative Example 1] A hardness sensor was produced in the same manner as in Example 2, except that neither the upper cover layer 1 nor the lower cover layer 2 was formed. In the same manner as in Example 2, the performance of the hardness sensor was evaluated.

[0047] Comparative Example 2 A hardness sensor was produced in the same manner as in Example 2, except that the upper cover layer 1 was made of PDMS and the lower cover layer 2 was made from a 2:1 (wt %) mixed solution of PDMS and Ecoflex. In the hardness sensor of Comparative Example 2, the Young's modulus of the lower cover substrate is 0.82 MPa (measured value), and when the upper and lower cover layers have the same film thickness, the Young's modulus of the elastic material constituting the lower cover layer is approximately one-fifth of the Young's modulus of the elastic material constituting the upper cover layer. In the same manner as in Example 2, the performance of the hardness sensor was evaluated.

[0048] FIG. 4 shows the resistance change rate versus applied pressure when the materials of the upper cover layer 1 and the lower cover layer 2 are changed as in Examples 2 to 4. Compared to Comparative Example 1, in which no cover layer was provided, the sensitivity of the sensor was improved in Examples 2 to 4 by attaching the upper cover layer 1 and the lower cover layer 2. Among these, Example 3, in which the upper cover layer 1 was made of PDMS and the lower cover layer 2 was made of Ecoflex (registered trademark), had the highest sensitivity. This is presumably because the softness of the contact surface between the indenter of the resistance change type hardness sensor 10 and the object increased, changing the housing change rate.

[0049] Hardness sensing was performed using the resistance change type hardness sensors 10 of Examples 1 and 4 and Comparative Example 2. The resistance change type hardness sensor 10 of Example 4, in which both the upper cover layer 1 and the lower cover layer 2 were Ecoflex and the Young's modulus of the lower cover layer 2 was 1 / 10 that of the upper cover layer 1, had a resistance change rate (R / R0) of 0.175, whereas the hardness sensor of Comparative Example 2, in which the Young's modulus of the lower cover layer 2 was approximately 1 / 5 that of the upper cover layer 1, had a resistance change rate (R / R0) of 0.131, a decrease of 25% (FIG. 6). This result indicates that the resistance change type hardness sensor 10 has the highest sensitivity when the Young's modulus of the lower cover layer 2 is 1 / 10 or less that of the upper cover layer 1.

[0050] The graph in FIG. 7 shows the relationship between sensitivity and an applied pressure of 1 N (62.5 kPa), with the Young's modulus (MPa) of the lower cover layer 2 on the horizontal axis and the rate of resistance change (R / R0) on the vertical axis. In the resistance change type hardness sensor 10 of Example 1, in which both the upper cover layer 1 and the lower cover layer 2 were made of PDMS and the Young's modulus of the lower cover layer 2 was the same as that of the upper cover layer 1, the rate of resistance change (R / R0) was 0.897 when the Young's modulus was 1.9 MPa. In the hardness sensor of Comparative Example 2, in which the Young's modulus of the lower cover layer 2 was approximately 1 / 5 that of the upper cover layer 1, the rate of resistance change (R / R0) was 0.87 when the Young's modulus was 0.8 MPa. In the resistance change type hardness sensor 10 of Example 4, in which the Young's modulus of the lower cover layer 2 was 1 / 10 that of the upper cover layer 1, the rate of resistance change (R / R0) was 0.824 when the Young's modulus was 0.16 MPa. These results show that the softer the lower cover layer 2, the higher the sensitivity. [Explanation of symbols]

[0051] 1 Top cover layer 2 Lower Cover Layer 3 Pressure-sensitive layer 4 electrodes 5. Base film 6 Detection target 6a Detection target (hard object) 6b Detection target (soft object) 7,7' Conductive path 10 Resistance change type hardness sensor

Claims

1. an upper cover layer, a pair of electrodes and a pressure-sensitive layer, and a lower cover layer in this order; the upper cover layer and the lower cover layer are each independently made of one or more elastic materials, A resistance change type hardness sensor, characterized in that the Young's modulus of the elastic material constituting the lower cover layer is not more than one-tenth of the Young's modulus of the elastic material constituting the upper cover layer.

2. 2. The resistance change type hardness sensor according to claim 1, wherein the one or more elastic materials have a Young's modulus of 0.1 to 2.0 MPa.

3. 3. The resistance change type hardness sensor according to claim 2, wherein the one or more elastic materials are silicone rubber.

4. 2. The resistance change type hardness sensor according to claim 1, wherein the pressure-sensitive layer contains an elastic material and a conductive filler.

5. 5. The resistance change type hardness sensor according to claim 4, wherein the pressure-sensitive layer further contains a surfactant in addition to the elastic material and the conductive filler.

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