Surface-mounted strain sensor for soft robot and manufacturing method thereof

Abstract

The invention discloses a patch type strain sensor for a soft robot and a manufacturing method of the patch type strain sensor, and belongs to the field of sensors. The patch type strain sensor comprises a silica gel substrate, hydrogel, a first wire, a second wire, a silica gel adhesive and thin silica gel, the lower surface of the hydrogel adheres to the upper surface of the silica gel substrate through a silica gel adhesive, only the periphery of the contact face of the hydrogel and the silica gel substrate is coated with the silica gel adhesive, the two sides of the upper surface of the hydrogel are provided with a first wire and a second wire respectively, and the wires and the silica gel substrate are adhered through a conductive adhesive tape. The thin silica gel is located on the upper surface of the hydrogel and covers the contact points of the first wire and the hydrogel and the contact points of the second wire and the hydrogel. The material has the advantages of high conductivity, good tensile property and high resistance sensitivity.

Description

Technical Field

[0001] The present invention relates to the field of robots, and particularly to a patch-type strain sensor for soft robots. Background Art

[0002] Hydrogel robots, due to their use of flexible materials, exhibit significant advantages in terms of adaptability to complex environments, safety in human-machine interaction, optimization of motion energy efficiency, and functional integration. However, hydrogel robots are extremely susceptible to the influence of temperature and humidity in the environment. High temperature and low humidity can cause the hydrogel to lose water, and the ionic conductivity and resistance sensitivity will also be significantly affected. Therefore, it is extremely important to improve the performance stability of hydrogel-based strain sensors. Existing techniques for shielding hydrogels from the influence of temperature and humidity include adding heat stabilizers or cross-linking agents and introducing hydrophobic groups for hydrophobic modification. However, these methods often come at the cost of sacrificing the flexibility, biological activity, or function of the hydrogel, and require complex synthesis or expensive materials, making it difficult to industrialize. Summary of the Invention

[0003] Based on this, the present invention proposes a patch-type strain sensor for soft robots, aiming to stably identify the motion posture of the robot and be able to be used for a long time.

[0004] A patch-type strain sensor for a soft robot, the patch-type strain sensor comprising: a silicone substrate, a hydrogel, a first wire, a second wire, a silicone adhesive, and a thin silicone; the lower surface of the hydrogel is adhered to the upper surface of the silicone substrate by the silicone adhesive, and the silicone adhesive is only coated around the contact surface of the hydrogel and the silicone substrate. The first wire and the second wire are respectively arranged on both sides of the upper surface of the hydrogel, and conductive tapes are arranged at the positions on the silicone substrate corresponding to the contact points of the first wire and the second wire with the hydrogel; the thin silicone is located on the upper surface of the hydrogel and covers the contact points of the first wire and the second wire with the hydrogel.

[0005] Further, the material of the substrate is silicone.

[0006] Further, the thickness of the silicone substrate is 2 mm to 5 mm, the thickness of the hydrogel is 0.5 to 2 mm, the diameters of the first wire and the second wire are 0.4 to 0.6 mm, and the thickness of the thin silicone is 0.2 to 0.4 mm.

[0007] Further, the shape of the patch-type strain sensor is square or dumbbell-shaped.

[0008] A method for manufacturing a patch-type strain sensor for a soft robot, the method comprising:

[0009] Step 1: Bond the hydrogel to the silicone substrate using a silicone adhesive;

[0010] Step 2: Use conductive tape to paste the first wire and the second wire on the upper surface of the hydrogel;

[0011] Step 3: Use silicone adhesive to paste the thin silicone on the upper surface of the hydrogel;

[0012] The production method of the hydrogel is as follows:

[0013] Step (1): Mix chitosan CHI, acrylic acid AA, and deionized water to form an AA@CHI solution;

[0014] Step (2): After mixing polyvinyl alcohol PVA and deionized water, obtain a PVA solution;

[0015] Step (3): Dissolve aluminum nitrate crystals Al(NO3)3 in water to form an Al(NO3)3 solution;

[0016] Step (4): First mix the AA@CHI solution with the PVA solution to obtain an AA@CHI@PVA solution;

[0017] Step (5): Mix the AA@CHI@PVA solution with the Al(NO3)3 solution to obtain an AA@CHI@PVA-Al 3+ solution;

[0018] Step (6): Add a photoinitiator and N,N'-methylenebisacrylamide MBA to the AA@CHI@PVA-Al 3+ solution to obtain a hydrogel precursor solution;

[0019] Step (7): Transfer the solution obtained in step (6) into a mold and polymerize it under ultraviolet light irradiation to obtain a PAA@CHI@PVA-Al 3+ hydrogel, named PCPA hydrogel;

[0020] Step (8): Perform freeze-thaw cycle treatment on the PCPA hydrogel to obtain a Fr-PCPA hydrogel.

[0021] Furthermore, in step (1), chitosan CHI and acrylic acid AA are first mixed, and then deionized water is added; the mass ratio of chitosan, acrylic acid, and deionized water is 0.1:(1-2):(3-8);

[0022] In step (2), the mass ratio of polyvinyl alcohol to deionized water is 1:(5.6-29);

[0023] In step (3), the mass ratio of aluminum nitrate to deionized water is (0.1-0.7):2.

[0024] Further, in the step (6), the photoinitiator is any one of 2-hydroxy-4'-(2-hydroxyethoxy)-2-methylpropiophenone, 2-hydroxy-2-methyl-1-phenyl-1-propanone, and 2,4,6-trimethylbenzoyl-diphenylphosphine oxide; the mass ratio of the photoinitiator to the mass of acrylic acid is 1:(18.87 - 50).

[0025] Further, in the step (6), the mass fraction of N,N'-methylenebisacrylamide to the acrylic acid in the step (1) is 1:(166.67 - 1000).

[0026] Further, in the step (8), for the freeze-thaw cycle treatment step, it is frozen at -20°C for 8 - 24 hours, thawed at 25°C for 2 - 4 hours, and then the freeze-thaw cycle is repeated 3 - 6 times.

[0027] The PAA@CHI@PVA-Al 3+ hydrogel provided by the present invention uses Al 3+ as the main cross-linking agent and MBA as the auxiliary cross-linking agent to reduce the cytotoxicity of the gel. At the same time, the softness of the gel is ensured. Al 3+ can also serve as the conductive center to improve the conductivity of the overall hydrogel. At the same time, Al 3+ can also cross-link the hydroxyl groups of PVA. The presence of PVA greatly improves the mechanical properties of the hydrogel. The present invention has the following advantages:

[0028] (1) High conductivity:

[0029] The conductivity of the PCPA hydrogel can reach up to 19.61 mS / mm at most, and the average value of the conductivity of the Fr-PCPA hydrogel is slightly lower than that of the PCPA hydrogel. The high conductivity of the PCPA and Fr-PCPA hydrogels determines their good sensing performance.

[0030] (2) Good tensile properties:

[0031] The engineering strain of the Fr-PCPA hydrogel can reach up to 1238.46% at most, which can adapt to the large strain of the soft robot. At the same time, the stress is up to 281 kPa at most. It shows that a small stress can achieve a large strain of the hydrogel.

[0032] (3) High resistance sensitivity:

[0033] The Fr-PCPA hydrogel sensor has high sensitivity, the resistance change rate is as high as 2601.45%, and the strain sensitivity factor (GF value) is about 9.81 at most. The high resistance sensitivity is convenient for the extraction and detection of resistance signals. Description of the Drawings

[0034] Figure 1It is the flowchart of the curing and freeze-thaw of the Fr-PCPA hydrogel.

[0035] Figure 2 It is the schematic diagram of the assembly of the Fr-PCPA hydrogel and the silicone soft robot.

[0036] Figure 3 It is the comparison chart of the conductivity of the PCPA and Fr-PCPA hydrogels.

[0037] Figure 4 It is the comparison chart of the stress-strain of the PCPA and Fr-PCPA hydrogels.

[0038] Figure 5 It is the change diagram of the resistance change rate of the strain sensor of Fr-PCPA with time and strain.

[0039] Figure 6 It is the strain sensitivity coefficient diagram of the strain sensor of Fr-PCPA. Specific implementation method

[0041] In the first aspect, the present invention provides a method for fabricating and assembling the hydrogel strain sensor. The first step is to prepare a highly stretchable and highly resistive hydrogel, including the following steps:

[0042] (1) Dissolve chitosan (CHI) in acrylic acid (AA) to form an AA@CHI solution.

[0043] (2) Mix polyvinyl alcohol (PVA) with water to obtain a PVA solution.

[0044] (3) Dissolve aluminum nitrate crystals (Al(NO3)3) in water to form an Al(NO3)3 solution.

[0045] (4) First, mix the AA@CHI solution with the PVA solution to obtain an AA@CHI@PVA solution.

[0046] (5) Mix the AA@CHI@PVA solution with the Al(NO3)3 solution to obtain an AA@CHI@PVA-Al 3+ solution.

[0047] (6) Add a photoinitiator and a trace amount of N,N'-methylenebisacrylamide (MBA) to the AA@CHI@PVA-Al 3+ solution to obtain a hydrogel precursor solution.

[0048] (7) Transfer the solution into a mold and polymerize it under ultraviolet light irradiation to obtain a PAA@CHI@PVA-Al 3+ hydrogel, simply named PCPA hydrogel.

[0049] (8) Subject the PAA@CHI@PVA-Al 3+ hydrogel to freeze-thaw cycling to obtain the Fr-PCPA hydrogel.

[0050] Second, assemble the hydrogel sensor and the soft robot, the steps are as follows:

[0051] (9) Attach one side of the freeze-thawed hydrogel to the silicone soft robot, and stick silver wires on both ends of the other side of the hydrogel with conductive tape.

[0052] (10) Use silicone adhesive to stick the silicone soft robot substrate, the hydrogel, the wire, and the thin silicone together.

[0053] (11) Connect the wire to a digital source meter to record the change in gel resistance during the movement of the robot. The output voltage of the digital source meter is set to apply a 0.5 V positive electric field every two seconds, and the rest of the time is a blank electric field.

[0054] Furthermore, the mass ratio of chitosan, acrylic acid, and deionized water is 0.1 g : (1 - 2 g) : (3 - 8 g), preferably 0.1 g : (1.5 - 2 g) : (3 - 6 g), more preferably 0.1 g : (1.5 - 1.7 g) : (3 - 6 g), and most preferably, the mass fraction of CHI in the AA@CHI solution is 0.9 - 2.4%, and the mass fraction of AA is 10.9 - 24.3%. The addition order is chitosan, acrylic acid, deionized water.

[0055] Furthermore, the degree of alcoholysis of the polyvinyl alcohol is 97.5 - 99 mol%, and most preferably 98 - 99 mol%. The mass ratio of polyvinyl alcohol to deionized water is 1 g : (5.6 - 29 g), more preferably 1 g : (9 - 29 g), and most preferably 1 g : (19 - 29 g). Most preferably, the mass fraction of PVA in the PVA solution is 3.3 - 5%. The addition order is polyvinyl alcohol first and then deionized water.

[0056] Furthermore, the aluminum nitrate cannot be replaced by other aluminum salts, and the mass ratio of aluminum nitrate to deionized water is (0.1 - 0.7 g) : 2 g, and most preferably (0.3 - 0.5 g) : 2 g.

[0057] Furthermore, the photoinitiator is selected from any one of 2-hydroxy-4'-(2-hydroxyethoxy)-2-methylpropiophenone (2959), 2-hydroxy-2-methyl-1-phenyl-1-propanone (HMPP), and 2,4,6-trimethylbenzoyl-diphenylphosphine oxide (TPO). More preferably, photoinitiator 2959 and photoinitiator TPO. The mass fraction of the photoinitiator in acrylic acid is 2 - 5.3%, more preferably 2 - 3.3%. Most preferably 2.5 - 3.3%.

[0058] Further, the mass fraction of N,N'-methylenebisacrylamide in acrylic acid is 0.1-0.6%, and most preferably 0.1-0.2%.

[0059] Further, the molds used include a polytetrafluoroethylene mold and a glass-silicone combined mold. Most preferably, it is a glass-silicone combined mold with double-sided light transmission, consisting of two glass slides sandwiching a silicone sheet, fixed with a long-tail clip, the size of the glass slides is international standard, and the silicone is hollowed out in the middle for filling with the hydrogel precursor solution. The hollowed-out shapes include a square and a dumbbell shape. The thickness of the silicone is 0.8-5 mm. If the hydrogel is used in conductivity testing, the most preferred silicone thickness is 2-5 mm and the optimal shape is a square. If the hydrogel is used in tensile testing, the most preferred silicone thickness is 2-3 mm and the optimal shape is a dumbbell shape.

[0060] Further, the wavelength used for ultraviolet curing is 365 nm, and the curing time is preferably 20-40 min, and most preferably 25-35 min.

[0061] Further, in the freeze-thaw cycle treatment step, it is frozen at -20°C for 8-24 hours, thawed at 25°C for 2-4 hours, and the freeze-thaw cycle is 3-6 times. Most preferably, it is frozen at -20°C for 24 hours, thawed at 25°C for 4 hours, and the freeze-thaw cycle is 3-4 times.

[0062] Further, the assembly process includes a silicone substrate (1) of the silicone soft robot, PAA@CHI@PVA-Al 3+ hydrogel (2), a first wire (3), a second wire (4), a silicone adhesive (5), and a thin silicone (6). The silicone substrate (1) of the silicone soft robot is attached to PAA@CHI@PVA-Al 3+ hydrogel (2), the first wire (3) and the second wire (4) are respectively attached to both ends of the silicone bottom surface of PAA@CHI@PVA-Al 3+ hydrogel (2), and the thin silicone (6), the first wire (3), the second wire (4), PAA@CHI@PVA-Al 3+ hydrogel (2), and the silicone substrate (1) of the silicone soft robot are adhered with the silicone adhesive (5).

[0063] Further, the thickness of the thin silicone (6) is 0.1 mm.

[0064] In the second aspect, the present invention provides a novel gel polymer and a strategy for improving its stability. The main components of the hydrogel include: acrylic acid, chitosan, polyvinyl alcohol, aluminum nitrate, a photoinitiator, and a crosslinking agent. The hydrogel is encapsulated in a silicone sandwich to isolate the influence of temperature and humidity in the environment and improve the environmental stability of the hydrogel.

[0065] Thirdly, the present invention provides a general method for endowing a soft robotic skin with sensing ability. The specific steps of this method are as follows: Use a silicone adhesive to stick a thin silicone, silver wire, hydrogel, and the soft robotic shell together. Connect the other end of the un-stuck conductive part to a digital source meter, analyze the hydrogel resistance signal during the process, and the motion posture of the robot can be monitored in real time.

[0066] The following is an example to illustrate the preparation and assembly method of a patch-type strain sensor for a soft robot provided by the present invention.

[0067] I. Preparation of the hydrogel precursor solution:

[0068] (1) Dissolve chitosan (CHI) in acrylic acid (AA) to form an AA@CHI solution.

[0069] (2) Mix polyvinyl alcohol (PVA) with water to obtain a PVA solution.

[0070] (3) Dissolve aluminum nitrate crystals (Al(NO3)3) in water to form an Al(NO3)3 solution.

[0071] (4) First, mix the AA@CHI solution with the PVA solution to obtain an AA@CHI@PVA solution.

[0072] (5) Mix the AA@CHI@PVA solution with the Al(NO3)3 solution to obtain an AA@CHI@PVA-Al 3+ solution.

[0073] (6) Add a photoinitiator and a trace amount of N,N'-methylenebisacrylamide (MBA) to the AA@CHI@PVA-Al 3+ solution to obtain the hydrogel precursor solution.

[0074] In the above steps (1), (3), (4), (5), and (6), after mixing the materials in each step, they need to be stirred for 30 - 60 minutes, and the stirring speed is between 250 - 300 rpm. Stirring at too high a speed will generate a large number of bubbles, prolonging the subsequent ultrasonic treatment time. Stirring at too low a speed will increase the dissolution time, prolonging the total duration of the experiment and reducing the experimental efficiency.

[0075] In the above steps (1), (3), (4), (5) and (6), after the materials are mixed and stirred, they all need to be subjected to ultrasonic dispersion treatment for 30 - 60 min. Among them, the ultrasonic dispersion treatment time for steps (1), (3), (4) and (6) is 30 - 40 min. The ultrasonic dispersion time for step (5) depends on the concentration of Al(NO₃)₃. If the dissolution amount of Al(NO₃)₃ in step (4) is 0.1 - 0.3 g, the ultrasonic dispersion time in step (5) is 30 - 40 min. If the dissolution amount of Al(NO₃)₃ in step (4) is 0.4 - 0.7 g, the ultrasonic dispersion time in step (5) is 40 - 60 min.

[0076] In the above step (2), after mixing polyvinyl alcohol (PVA) and water, cover the bottle mouth with plastic wrap and let it stand at room temperature, waiting for the PVA to swell for 2 - 8 h, and then place it in an oil bath at 95 °C and stir for 4 - 9 h. If preparing a 5% mass fraction PVA solution, first swell at room temperature for 2 - 3 h, and then stir in an oil bath at 95 °C for 4 - 5 h; if preparing a 10% mass fraction PVA solution, first swell at room temperature for 3 - 4 h, and then stir in an oil bath at 95 °C for 5 - 6 h; if preparing a 15% mass fraction PVA solution, first swell at room temperature for 5 - 8 h, and then stir in an oil bath at 95 °C for 7 - 9 h; PVA solutions with a mass fraction above 15% affect the conductivity performance of the hydrogel in the present invention, so they are not included in the preparation method of the present invention.

[0077] In the above step (3), aluminum nitrate cannot be replaced by other aluminum salts. The mass ratio of aluminum nitrate to deionized water is (0.1 - 0.7 g) : 2 g, and the most preferred is (0.3 - 0.5 g) : 2 g.

[0078] In the above step (6), the photoinitiator is selected from 2 - hydroxy - 4' - (2 - hydroxyethoxy) - 2 - methylpropiophenone (2959), 2 - hydroxy - 2 - methyl - 1 - phenyl - 1 - propanone (HMPP), and 2,4,6 - trimethylbenzoyl - diphenylphosphine oxide (TPO). More preferably, the photoinitiators are 2959 and TPO. The mass fraction of the photoinitiator in acrylic acid is 2 - 5.3%, more preferably 2 - 3.3%. The most preferred is 2.5 - 3.3%.

[0079] In the above step (6), the mass fraction of N,N' - methylenebisacrylamide in acrylic acid is 0.1 - 0.6%, and the most preferred is 0.1 - 0.2%.

[0080] II. Curing and freeze - thaw cycling of the hydrogel:

[0081] (1) Transfer the solution into a mold and polymerize it under ultraviolet light irradiation to obtain PAA@CHI@PVA - Al3+ A hydrogel, simply named PCPA hydrogel.

[0082] (2) Subject the PAA@CHI@PVA-Al 3+ hydrogel to freeze-thaw cycles to obtain the Fr-PCPA hydrogel. Finally, the appearance of the hydrogel is as Figure 1 shown.

[0083] In the above curing step (1), the molds used are polytetrafluoroethylene molds and glass-silicone composite molds. Most preferably, it is a glass-silicone composite mold with double-sided light transmission, consisting of two glass slides sandwiching a silicone sheet, fixed by a long-tail clip clamping the glass and the silicone. The size of the glass slide is international standard, and the silicone is hollowed out in the middle for filling the hydrogel precursor solution. The hollowed-out shapes are square and dumbbell-shaped. The thickness of the silicone is 0.8 - 5 mm. If the hydrogel is used in conductivity tests, the most preferred silicone thickness is 2 - 5 mm, and the optimal shape is square. If the hydrogel is used in tensile tests, the most preferred silicone thickness is 2 - 3 mm, and the optimal shape is dumbbell-shaped.

[0084] In the above curing step (1), the wavelength used for ultraviolet curing is 365 nm, and the curing time is preferably 20 - 40 min, most preferably 25 - 35 min.

[0085] In the above freeze-thaw cycle step (2), freeze at -20 °C for 8 - 24 hours, thaw at 25 °C for 2 - 4 hours, and perform freeze-thaw cycles 3 - 6 times. Most preferably, freeze at -20 °C for 24 hours, thaw at 25 °C for 4 hours, and perform freeze-thaw cycles 3 - 4 times.

[0086] III. Assembly of the hydrogel and the soft robot:

[0087] (1) Attach one side of the freeze-thawed hydrogel to the silicone soft robot, and stick silver wires on both ends of the other side of the hydrogel with conductive tape.

[0088] (2) Use silicone adhesive to stick the silicone substrate of the silicone soft robot, the hydrogel, the wire, and the thin silicone together.

[0089] (3) Connect the wire to a digital source meter to record the change in gel resistance during the movement of the robot. The output voltage of the digital source meter is set to apply a 0.5 V positive electric field every two seconds, and the rest of the time is a blank electric field.

[0090] During the above assembly process, it includes a silicone soft robot substrate (1), Fr-PCPA hydrogel (2), a first wire (3), a second wire (4), silicone adhesive (5), and thin silicone (6). The silicone soft robot substrate (1) is attached to the Fr-PCPA hydrogel (2), and the first wire (3) and the second wire (4) are respectively attached to both ends of the silicone base surface of the Fr-PCPA hydrogel (2). The silicone soft robot substrate (1), Fr-PCPA hydrogel (2), first wire (3), second wire (4), and thin silicone (6) are adhered together with silicone adhesive (5). The specific structural schematic diagram is as shown in Figure 2 shown.

[0091] During the above assembly process, the thickness of the thin silicone (6) is 0.1 mm.

[0092] The following will be further elaborated in detail through specific embodiments. Additionally, the following embodiments are used for further illustration of the present invention and cannot be construed as limiting the protection scope of the present invention. Non-essential improvements and adjustments made by those skilled in the art based on the above content of the present invention fall within the protection scope of the present invention. The specific numerical values described in the following examples are merely examples within a suitable range, which means that those skilled in the art can make selections within a suitable range according to the description herein without being limited to the specific numerical values in the following examples. Embodiment

[0093] (1) Weigh 0.1 g of chitosan (CHI) and place it in a sample bottle. Then weigh 1.5 g of acrylic acid (AA) and drop it into the same sample bottle. Add 3 ml of deionized water to this sample bottle and stir it on a magnetic stirrer at 250 rpm for 30 min, and then perform ultrasonic treatment for 30 min to obtain an AA@CHI solution.

[0094] (2) Mix 1 g of polyvinyl alcohol (PVA) with 19 g of deionized water, let it stand at room temperature for 3 h, and then stir it in an oil bath at 95 °C for 4 h to obtain a 5% PVA solution by mass fraction.

[0095] (3) Mix 0.1 g of aluminum nitrate crystal (Al(NO3)3) with 2 ml of deionized water, stir it on a magnetic stirrer at 250 rpm for 30 min, and then perform ultrasonic treatment for 30 min to obtain an Al(NO3)3 solution.

[0096] (4) First, mix the AA@CHI solution with 1 ml of PVA solution, stir it on a magnetic stirrer at 250 rpm for 30 min, and then perform ultrasonic treatment for 30 min to obtain an AA@CHI@PVA solution.

[0097] (5) Mix the AA@CHI@PVA solution with 2 ml of Al(NO3)3 solution, stir on a magnetic stirrer at 250 rpm for 30 min, and then sonicate for 30 min to obtain the AA@CHI@PVA-Al 3+ solution.

[0098] (6) Add 55 mg of photoinitiator 2959 and 3 mg of N,N'-methylenebisacrylamide (MBA) to the AA@CHI@PVA-Al 3+ solution to obtain the hydrogel precursor solution.

[0099] (7) Transfer the solution into a glass-silicone mold and polymerize it under ultraviolet light with a wavelength of 365 nm for 35 min to obtain the PAA@CHI@PVA-Al 3+ hydrogel, simply named PCPA 0.1 .

[0100] (8) Transfer the PAA@CHI@PVA-Al 3+ hydrogel into a refrigerator at -20 °C and freeze it for 24 h, then thaw it at room temperature of 25 °C for 4 h. Repeat the freeze-thaw cycle three times, and simply name it Fr-PCPA 0.1 .

[0101] In the second step, assemble the hydrogel sensor and soft robot as follows:

[0102] (9) Attach one side of the hydrogel to the silicone soft robot, and stick silver wires to both ends of the other side of the hydrogel with conductive tape.

[0103] (10) Use silicone adhesive to stick the silicone soft robot substrate, hydrogel, wire, and thin silicone together. The thin silicone completely covers the hydrogel.

[0104] (11) Connect the wire to a digital source meter to record the change in gel resistance during the movement of the robot. The output voltage of the digital source meter is set to apply a 0.5 V positive electric field every two seconds, and the rest of the time is a blank electric field.

[0105] Figure 3 is the conductivity comparison chart of PCPA and Fr-PCPA hydrogels, where Fr-PCPA 0.1 and PCPA 0.1 indicate that the 6 ml hydrogel precursor solution contains 0.1 g of Al(NO3)3; Fr-PCPA 0.7 and PCPA 0.7 indicate that the 6 ml hydrogel precursor solution contains 0.7 g of Al(NO3)3. It can be seen from the figure that PCPA 0.1 , Fr-PCPA 0.1 , PCPA0.7 、Fr-PCPA 0.7 have conductivities of 4.32 mS / cm, 4.08 mS / cm, 19.61 mS / cm, and 18.10 mS / cm respectively. The conductivity of PCPA 0.7 is higher than that of PCPA 0.1 , and the conductivity of Fr-PCPA 0.7 is higher than that of Fr-PCPA 0.1 . The overall average conductivity of the Fr-PCPA hydrogel is lower than that of PCPA. This is because the greater the concentration of Al(NO3)3, the greater the concentration of conductive ions and the higher the conductivity; freeze-thaw cycles reduce the pore size of the hydrogel, hindering ion mobility and reducing conductivity.

[0106] Figure 4 is a stress-strain comparison diagram of PCPA and Fr-PCPA hydrogels. It can be seen from the figure that freeze-thaw cycles make the mechanical properties of the Fr-PCPA hydrogel superior to those of the PCPA hydrogel. The maximum fracture strain of the PCPA hydrogel is 741.96%, and the fracture strength is about 75 kPa; the maximum fracture strain of the Fr-PCPA hydrogel is 1238.46%, and the fracture strength is about 281 kPa.

[0107] Figure 5 is a graph showing the change in the resistance change rate of the strain sensor of Fr-PCPA with time and strain. It can be seen from the figure that when the strain is 100%, the resistance change rate is 291.99%; when the strain is 200%, the resistance change rate is 840.86%; when the strain is 300%, the resistance change rate is 1644.22%; when the strain is 400%, the resistance change rate is 2601.45%.

[0108] Figure 6 is a graph of the strain sensitivity coefficient (GF) of the strain sensor of Fr-PCPA. It can be seen from the figure that when the strain is 0 - 100%, GF is 2.86; when the strain is 100% - 200%, GF is 5.48; when the strain is 200 - 300%, GF is 8.02; when the strain is 300% - 400%, GF is 9.81.

Claims

1. A patch-type strain sensor for a soft robot, the patch-type strain sensor comprising: Silicone substrate, hydrogel, first wire, second wire, silicone adhesive, thin silicone; the lower surface of the hydrogel is adhered to the upper surface of the silicone substrate by the silicone adhesive, and the silicone adhesive is only coated around the contact surface of the hydrogel and the silicone substrate. The first wire and the second wire are respectively arranged on both sides of the upper surface of the hydrogel, and conductive tapes are arranged at the positions of the silicone substrate corresponding to the contact points of the first wire and the second wire with the hydrogel; The thin silicone is located on the upper surface of the hydrogel and covers the contact points of the first wire and the second wire with the hydrogel.

2. The patch-type strain sensor for a soft robot according to claim 1, wherein, The material of the substrate is silicone.

3. The patch-type strain sensor for a soft robot according to claim 1, wherein, The thickness of the silicone substrate is 2 mm to 5 mm, the thickness of the hydrogel is 0.5 to 2 mm, the diameters of the first wire and the second wire are 0.4 to 0.6 mm, and the thickness of the thin silicone is 0.2 to 0.4 mm.

4. The patch-type strain sensor for a soft robot according to claim 1, wherein The patch-type strain sensor is square or dumbbell-shaped.

5. A manufacturing method of a patch-type strain sensor for a soft robot as described in claim 1, the method comprising: Step 1: Bond the hydrogel to the silicone substrate using a silicone adhesive; Step 2: Paste the first wire and the second wire on the upper surface of the hydrogel using a conductive tape; Step 3: Paste the thin silicone on the upper surface of the hydrogel using a silicone adhesive; The manufacturing method of the hydrogel is: Step (1): Mix chitosan CHI, acrylic acid AA, and deionized water to form an AA@CHI solution; Step (2): Mix polyvinyl alcohol PVA and deionized water to obtain a PVA solution; Step (3): Dissolve aluminum nitrate crystal Al(NO3)3 in water to form an Al(NO3)3 solution; Step (4): First mix the AA@CHI solution with the PVA solution to obtain an AA@CHI@PVA solution; Step (5): Mix the AA@CHI@PVA solution with the Al(NO3)3 solution to obtain the AA@CHI@PVA-Al 3+ solution; Step (6): Add a photoinitiator and N,N'-methylenebisacrylamide MBA to the AA@CHI@PVA-Al 3+ solution to obtain a hydrogel precursor solution; Step (7): Transfer the solution obtained in step (6) into a mold and polymerize it under the irradiation of ultraviolet light to obtain a PAA@CHI@PVA-Al 3+ hydrogel, named PCPA hydrogel; Step (8): Perform freeze-thaw cycle treatment on the PCPA hydrogel to obtain a Fr-PCPA hydrogel.

6. The manufacturing method of the patch-type strain sensor of the soft robot according to claim 5, wherein, In the said step (1), chitosan CHI and acrylic acid AA are first mixed, and then deionized water is added; the mass ratio of chitosan, acrylic acid, and deionized water is 0.1:(1 - 2):(3 - 8); In the said step (2), the mass ratio of polyvinyl alcohol to deionized water is 1:(5.6 - 29).

7. The manufacturing method of the patch-type strain sensor of the soft robot according to claim 5, characterized in that, In the said step (6), the photoinitiator is any one of 2-hydroxy-4'-(2-hydroxyethoxy)-2-methylpropiophenone, 2-hydroxy-2-methyl-1-phenyl-1-propanone, and 2,4,6-trimethylbenzoyl-diphenylphosphine oxide; the mass ratio of the photoinitiator to the mass of acrylic acid is 1:(18.87 - 50).

8. The manufacturing method of the patch-type strain sensor of the soft robot according to claim 5, characterized in that, In the said step (6), the mass ratio of N,N'-methylenebisacrylamide to the mass fraction of acrylic acid in step (1) is 1:(166.67 - 1000).

9. The manufacturing method of the patch-type strain sensor of the soft robot according to claim 5, characterized in that, In the said step (8), for the freeze-thaw cycle treatment step, freeze at -20°C for 8 - 24 hours, thaw at 25°C for 2 - 4 hours, and then perform the freeze-thaw cycle 3 - 6 times.

10. The manufacturing method of the patch-type strain sensor of the soft robot according to claim 5, characterized in that In the said step (3), the mass ratio of aluminum nitrate to deionized water is (0.1 - 0.7):2.

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