A gradient hydrogel and a method for preparing the same by density difference-driven bidirectional self-growth

Abstract

The present invention belongs to the technical field of functional materials, and specifically discloses a gradient hydrogel and a method for preparing the same by density difference-driven bidirectional self-growth, which comprises the following steps: using a mixed solution of an alkali and a salt as the bottom layer solution; spreading a mixed solution of a polymer monomer, polyphenol, ammonium persulfate and N,N'-methylenebisacrylamide above the bottom layer solution, and standing and reacting at 15-35 °C for a set time to obtain the hydrogel. The prepared hydrogel exhibits high sensitivity, a wide sensing range, good stability and a simple / rapid preparation process, and is thus suitable for monitoring micro-deformation and high-stress motion behaviors.

Description

Technical Field

[0001] The present invention belongs to the technical field of functional materials, and particularly relates to a gradient hydrogel and a method for preparing the same by density difference-driven bidirectional self-growth. Background Art

[0002] The statements herein only provide background art related to the present invention and do not necessarily constitute prior art.

[0003] The unique gradient structure of the human skin endows it with remarkable functionality. Extending from the epidermis to the dermis and subcutaneous tissue, it has excellent adaptability and flexibility and can effectively perform protection, sensing, and regulation functions. By mimicking the structure and functional properties of human skin, researchers have developed a variety of highly sensitive and wide-detection-range devices and applied them to different application scenarios (such as medical engineering, intelligent prosthetics, bionic robots, etc.). As a bionic skin material, hydrogel shows great potential in bionic sensor applications due to its excellent flexibility, biocompatibility, good electrical conductivity, and tunability. Endowing the hydrogel with a gradient microstructure is an effective strategy to broaden its application fields in bionic sensors, but it is still a challenge to achieve skin-like sensitivity and timely response to complex mechanical stimuli.

[0004] To simulate the gradient structure of the skin, various gradient hydrogels have been studied to explore their potential applications in more complex scenarios. Bionic gradient hydrogels usually have a gradient structure in terms of crosslinking density and component distribution. Researchers have explored various strategies to prepare gradient hydrogels, including gravity induction, electric / magnetic field stimulation, 3D printing technology, etc. By applying additional electrical or magnetic stimulation, the distribution of charged components can be reconfigured, thereby affecting the pore channels and the crosslinking network structure of the hydrogel. Using electrostatic interactions, hydrogen bonds, or interpenetrating aggregate networks to combine single-layer hydrogels with different properties to prepare bilayer or multilayer gradient hydrogels is also a desirable method. However, the above methods still have disadvantages such as complex preparation processes, generation of harmful chemicals, low sensitivity, and poor interfacial interactions. Therefore, there is an urgent need to develop a simple and rapid method for preparing gradient hydrogels. Summary of the Invention

[0005] Aiming at the deficiencies of the existing technology, the purpose of the present invention is to provide a gradient hydrogel and a method for preparing the same by density difference-driven bidirectional self-growth.

[0006] To achieve the above purpose, the present invention is realized by the following technical solutions:

[0007] In a first aspect, the present invention provides a method for preparing a gradient hydrogel by density difference-driven bidirectional self-growth, comprising the following steps:

[0008] Use a mixed solution of alkali and salt as the bottom layer solution;

[0009] Lay a mixed solution of polymer monomer, polyphenol, ammonium persulfate and N,N'-methylenebisacrylamide above the bottom layer solution, and let it stand and react for a set time at 15 - 35 °C to obtain the product.

[0010] A high-concentration / high-density salt solution can form a layer separation with the upper low-density solution to form an initial phase separation solution, which is beneficial to the subsequent self-growth reaction.

[0011] The stable complex formed by salts such as lithium chloride and the catechol part in polyphenols under alkaline conditions helps to achieve the redox balance between catechol and quinone, and further promotes the free radical polymerization reaction.

[0012] After polymerization, the polymer monomer forms a polymer chain, which serves as the backbone of the hydrogel. The polyphenol participates in the redox reaction to generate free radicals, promoting the polymerization of the monomer. Ammonium persulfate is the initiator, and N,N'-methylenebisacrylamide is the cross-linking agent.

[0013] In some embodiments, the mixed solution of polymer monomer, polyphenol, ammonium persulfate and N,N'-methylenebisacrylamide is slowly injected along the container wall, and spreads above the bottom layer solution under the action of the density difference.

[0014] The slow speed only needs to enable the mixed solution of polymer monomer, polyphenol, ammonium persulfate and N,N'-methylenebisacrylamide to spread above the bottom layer solution, avoiding the mixing of the two solutions.

[0015] In some embodiments, the alkali is potassium hydroxide or sodium hydroxide.

[0016] In some embodiments, the salt is lithium chloride, sodium chloride or zinc chloride.

[0017] In some embodiments, in the bottom layer solution, the concentration of the alkali is 0.03 - 0.05 g / ml, and the concentration of the salt is 0.08 - 0.5 g / ml.

[0018] In some embodiments, the polymer monomer is acrylamide or acrylic acid.

[0019] In some embodiments, the polyphenol is lignin, tannic acid, catechol, dopamine or vanillin.

[0020] In some embodiments, in the mixed solution of polymer monomer, polyphenol, ammonium persulfate and N,N'-methylenebisacrylamide, the concentration of the polymer monomer is 0.5 - 1.5 g / ml, the concentration of the polyphenol is 4 - 10 mg / ml, the concentration of ammonium persulfate is 4 - 10 mg / ml, and the concentration of N,N'-methylenebisacrylamide is 0.1 - 0.8 mg / ml.

[0021] In some embodiments, the time of the static reaction is 0.1 - 1 h.

[0022] In a second aspect, the present invention provides a gradient hydrogel prepared by the preparation method.

[0023] In a third aspect, the present invention provides the application of the gradient hydrogel in the preparation of a pressure sensor.

[0024] The beneficial effects obtained by one or more of the above embodiments of the present invention are as follows:

[0025] In the present invention, a precursor solution containing polymer monomers (such as polymer monomers like acrylamide and acrylic acid) is slowly added to a high - concentration salt solution (such as lithium chloride, sodium chloride, etc.). Due to the density difference between the two solutions, a barrier layer is formed by redox - induced free - radical polymerization. Due to the generation of free radicals and subsequent heat release, the interface is immediately ignited by a strong redox reaction. At the same time, free radicals undergo polymerization / gelation reactions from the interface to both sides, making the hydrogel show a two - way self - growth trend.

[0026] The polymerization / gelation difference is caused by the difference in precursor concentration between the upper layer and the bottom layer. From top to bottom, the cross - link density of the prepared hydrogel decreases. The bottom layer has a lower cross - link degree and a softer structure, but has a higher sensitivity as a strain sensor and is called the soft end. In contrast, the top layer with a high cross - link degree (high polymer network density) has a dense structure and excellent mechanical properties and is called the hard end. This strategy represents a significant advance in the development of biomimetic gradient hydrogels and provides a novel functional material design scheme.

[0027] When subjected to a small pressure, the soft end can exhibit a large deformation, making the hydrogel highly sensitive. When subjected to a greater pressure, due to the dense network structure of the hard end, the hydrogel structure will not be damaged, endowing the hydrogel sensor with a wide sensing range and good stability. The hydrogel simultaneously exhibits high sensitivity, a wide sensing range, good stability, and a simple / rapid preparation process, so it is suitable for monitoring micro - deformation and high - stress motion behaviors. This work not only opens up a new field for the preparation of gradient hydrogels but also significantly expands their applications in the field of soft electronics.

[0028] The preparation method of the present invention is simple, highly practical, and easy to promote. Without other stimuli at room temperature and without generating other harmful substances, a gradient hydrogel can be obtained by self - growth in just a few minutes.

[0029] The present invention has excellent universality. The prepared gradient hydrogel can adjust its mechanical properties, conductivity, and shape by changing the mold salt concentration, shape, salt composition, polyphenol, and monomer types, etc., to meet different application requirements. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] The accompanying drawings forming a part of this invention are used to provide a further understanding of the invention. The schematic embodiments of the invention and their descriptions are used to explain the invention and do not constitute an improper limitation to the invention.

[0031] Figure 1 It is a scanning electron microscope (SEM) image of the gradient hydrogel prepared in Example 1 of the present invention.

[0032] Figure 2 It is a compressive stress test graph of the hydrogels prepared in Examples 1-5 of the present invention; among them, a is a comparison graph of the mechanical property tests of the gradient hydrogels in Examples 1-5; b is a graph of the electrical conductivity tests of the gradient hydrogels in Examples 1-5.

[0033] Figure 3 It is a real-scene graph of different morphologies of the gradient hydrogel prepared in Example 4 of the present invention;

[0034] Figure 4 It is a graph of the sensing performance test of the gradient hydrogel pressure sensor prepared in Example 4 of the present invention. Among them, a is a comparison graph of the resistance of the gradient hydrogel under different pressures, b is a graph of the characterization of the pressure sensing sensitivity of the gradient hydrogel, and c is a graph of the characterization of the response time and recovery time of the gradient hydrogel. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0035] It should be noted that the following detailed description is illustrative and is intended to provide further explanation of the present invention. Unless otherwise specified, all technical and scientific terms used in the present invention have the same meaning as commonly understood by those of ordinary skill in the technical field to which the present invention belongs.

[0036] The present invention will be further described in detail below in conjunction with specific embodiments. It should be noted that the specific embodiments are explanations of the present invention rather than limitations.

[0037] Example 1

[0038] A method for preparing a gradient hydrogel by density difference-driven bidirectional self-growth, comprising the following steps:

[0039] Prepare a gradient hydrogel. First, add 4 g of acrylamide, 30 mg of sodium lignosulfonate, 30 mg of ammonium persulfate, and 2 mg of N,N'-methylenebisacrylamide to 5 mL of deionized water, and stir at room temperature to prepare a uniform solution as the upper layer.

[0040] Subsequently, 0.168 g of potassium hydroxide and 0.424 g of lithium chloride were added to 5 ml of deionized water, and stirred at room temperature to prepare a homogeneous solution as the bottom layer. Then, the bottom layer solution was first poured into a container, and then the upper layer solution was slowly poured along the wall of the container to prevent the two solutions from mixing due to shaking. Then, the above solution was placed at room temperature to obtain a gradient hydrogel.

[0041] The gradient hydrogel was connected to a texture analyzer and then to a digital source meter to monitor the change in resistance during repeated compression of the hydrogel.

[0042] Example 2

[0043] A method for preparing a gradient hydrogel by density difference-driven bidirectional self-growth, comprising the following steps:

[0044] Prepare a gradient hydrogel. First, 4 g of acrylamide, 30 mg of sodium lignosulfonate, 30 mg of ammonium persulfate, and 2 mg of N,N'-methylenebisacrylamide were added to 5 mL of deionized water, and stirred at room temperature to prepare a homogeneous solution as the upper layer.

[0045] Subsequently, 0.168 g of potassium hydroxide and 0.848 g of lithium chloride were added to 5 ml of deionized water, and stirred at room temperature to prepare a homogeneous solution as the bottom layer. Then, the bottom layer solution was first poured into a container, and then the upper layer solution was slowly poured along the wall of the container to prevent the two solutions from mixing due to shaking. Then, the above solution was placed at room temperature to obtain a gradient hydrogel.

[0046] The gradient hydrogel was connected to a texture analyzer and then to a digital source meter to monitor the change in resistance during repeated compression of the hydrogel.

[0047] Example 3

[0048] A method for preparing a gradient hydrogel by density difference-driven bidirectional self-growth, comprising the following steps:

[0049] Prepare a gradient hydrogel. First, 4 g of acrylamide, 30 mg of sodium lignosulfonate, 30 mg of ammonium persulfate, and 2 mg of N,N'-methylenebisacrylamide were added to 5 mL of deionized water, and stirred at room temperature to prepare a homogeneous solution as the upper layer.

[0050] Subsequently, 0.168 g of potassium hydroxide and 1.272 g of lithium chloride were added to 5 ml of deionized water, and stirred at room temperature to prepare a homogeneous solution as the bottom layer. Then, the bottom layer solution was first poured into a container, and then the upper layer solution was slowly poured along the wall of the container to prevent the two solutions from mixing due to shaking. Then, the above solution was placed at room temperature to obtain a gradient hydrogel.

[0051] Connect the gradient hydrogel to a texture analyzer and then to a digital source meter to monitor the change in resistance during repeated compression of the hydrogel.

[0052] Example 4

[0053] A method for preparing a gradient hydrogel by density difference-driven bidirectional self-growth, comprising the following steps:

[0054] Prepare the gradient hydrogel. First, add 4 g of acrylamide, 30 mg of sodium lignosulfonate, 30 mg of ammonium persulfate, and 2 mg of N,N'-methylenebisacrylamide to 5 mL of deionized water, and stir at room temperature to prepare a homogeneous solution as the upper layer.

[0055] Subsequently, add 0.168 g of potassium hydroxide and 1.696 g of lithium chloride to 5 ml of deionized water, and stir at room temperature to prepare a homogeneous solution as the lower layer. Then, first pour the lower layer solution into a container (the container shapes are pyramid, cylinder, flower shape, and cube structures), and then slowly pour the upper layer solution along the container wall to prevent the two solutions from mixing due to shaking. Then place the above solution at room temperature to obtain the gradient hydrogel.

[0056] Connect the gradient hydrogel to a texture analyzer and then to a digital source meter to monitor the change in resistance during repeated compression of the hydrogel.

[0057] Example 5

[0058] A method for preparing a gradient hydrogel by density difference-driven bidirectional self-growth, comprising the following steps:

[0059] Prepare the gradient hydrogel. First, add 4 g of acrylamide, 30 mg of sodium lignosulfonate, 30 mg of ammonium persulfate, and 2 mg of N,N'-methylenebisacrylamide to 5 mL of deionized water, and stir at room temperature to prepare a homogeneous solution as the upper layer.

[0060] Subsequently, add 0.168 g of potassium hydroxide and 2.119 g of lithium chloride to 5 ml of deionized water, and stir at room temperature to prepare a homogeneous solution as the lower layer. Then, first pour the lower layer solution into a container, and then slowly pour the upper layer solution along the container wall to prevent the two solutions from mixing due to shaking. Then place the above solution at room temperature to obtain the gradient hydrogel.

[0061] Connect the gradient hydrogel to a texture analyzer and then to a digital source meter to monitor the change in resistance during repeated compression of the hydrogel.

[0062] Example 6

[0063] A method for preparing a gradient hydrogel by density difference-driven bidirectional self-growth, comprising the following steps:

[0064] To prepare a gradient hydrogel, first add 4 g of acrylic acid, 30 mg of sodium lignosulfonate, 30 mg of ammonium persulfate, and 2 mg of N,N'-methylenebisacrylamide to 5 mL of deionized water, and stir at room temperature to prepare a homogeneous solution as the upper layer.

[0065] Subsequently, add 0.168 g of potassium hydroxide and 2.119 g of lithium chloride to 5 ml of deionized water, and stir at room temperature to prepare a homogeneous solution as the lower layer. Then, first pour the lower layer solution into a container, and then slowly pour the upper layer solution along the container wall to prevent the two solutions from mixing due to shaking. Then place the above solution at room temperature to obtain a gradient hydrogel.

[0066] Connect the gradient hydrogel to a texture analyzer, and then connect it to a digital source meter to monitor the change in resistance during the repeated compression of the hydrogel.

[0067] Example 7

[0068] A method for preparing a gradient hydrogel by density difference-driven bidirectional self-growth, comprising the following steps:

[0069] To prepare a gradient hydrogel, first add 4 g of acrylamide, 30 mg of alkali lignin, 30 mg of ammonium persulfate, and 2 mg of N,N'-methylenebisacrylamide to 5 mL of deionized water, and stir at room temperature to prepare a homogeneous solution as the upper layer.

[0070] Subsequently, add 0.168 g of potassium hydroxide and 2.119 g of lithium chloride to 5 ml of deionized water, and stir at room temperature to prepare a homogeneous solution as the lower layer. Then, first pour the lower layer solution into a container, and then slowly pour the upper layer solution along the container wall to prevent the two solutions from mixing due to shaking. Then place the above solution at room temperature to obtain a gradient hydrogel.

[0071] Connect the gradient hydrogel to a texture analyzer, and then connect it to a digital source meter to monitor the change in resistance during the repeated compression of the hydrogel.

[0072] Example 8

[0073] A method for preparing a gradient hydrogel by density difference-driven bidirectional self-growth, comprising the following steps:

[0074] To prepare a gradient hydrogel, first add 4 g of acrylamide, 30 mg of sodium lignosulfonate, 30 mg of ammonium persulfate, and 2 mg of N,N'-methylenebisacrylamide to 5 mL of deionized water, and stir at room temperature to prepare a homogeneous solution as the upper layer.

[0075] Subsequently, 0.168 g of potassium hydroxide and 1.8 g of sodium chloride were added to 5 ml of deionized water, and a homogeneous solution was prepared by stirring at room temperature as the bottom layer. Then, the bottom layer solution was first poured into a container, and then the upper layer solution was slowly poured along the wall of the container to prevent the two solutions from mixing due to shaking. Then, the above solution was placed at room temperature to obtain a gradient hydrogel.

[0076] The gradient hydrogel was connected to a texture analyzer and then to a digital source meter to monitor the change in resistance during repeated compression of the hydrogel.

[0077] Example 9

[0078] A method for preparing a gradient hydrogel by density difference-driven bidirectional self-growth, comprising the following steps:

[0079] Prepare a gradient hydrogel. First, 4 g of acrylamide, 30 mg of sodium lignosulfonate, 30 mg of ammonium persulfate, and 2 mg of N,N'-methylenebisacrylamide were added to 5 mL of deionized water, and a homogeneous solution was prepared by stirring at room temperature as the upper layer.

[0080] Subsequently, 0.168 g of sodium hydroxide and 2.119 g of lithium chloride were added to 5 ml of deionized water, and a homogeneous solution was prepared by stirring at room temperature as the bottom layer. Then, the bottom layer solution was first poured into a container, and then the upper layer solution was slowly poured along the wall of the container to prevent the two solutions from mixing due to shaking. Then, the above solution was placed at room temperature to obtain a gradient hydrogel.

[0081] The gradient hydrogel was connected to a texture analyzer and then to a digital source meter to monitor the change in resistance during repeated compression of the hydrogel.

[0082] Example 10

[0083] A method for preparing a gradient hydrogel by density difference-driven bidirectional self-growth, comprising the following steps:

[0084] Prepare a gradient hydrogel. First, 4 g of acrylamide, 30 mg of sodium lignosulfonate, 30 mg of ammonium persulfate, and 2 mg of N,N'-methylenebisacrylamide were added to 5 mL of deionized water, and a homogeneous solution was prepared by stirring at room temperature as the upper layer.

[0085] Subsequently, 4 g of zinc chloride were added to 5 ml of deionized water, and a homogeneous solution was prepared by stirring at room temperature as the bottom layer. Then, the bottom layer solution was first poured into a container, and then the upper layer solution was slowly poured along the wall of the container to prevent the two solutions from mixing due to shaking. Then, the above solution was placed at room temperature to obtain a gradient hydrogel.

[0086] The gradient hydrogel was connected to a texture analyzer and then to a digital source meter to monitor the change in resistance during repeated compression of the hydrogel.

[0087] The density difference-driven bidirectional self-growing gradient hydrogel described above was characterized and its performance was tested, and the results are as Figures 1 to 4 shown.

[0088] Figure 1 It is the scanning electron micrograph of the gradient hydrogel prepared in Example 1. It can be seen that the gradient hydrogel has a certain cross-linking gradient. The top layer structure is dense and has excellent mechanical properties, while the bottom layer has a lower cross-linking degree and is softer in texture.

[0089] Figure 2 It is the compressive stress test chart of the gradient hydrogels prepared in Examples 1-5 of the present invention. Among them, 2M is the gradient hydrogel prepared in Example 1, 4M is the gradient hydrogel prepared in Example 2, 6M is the gradient hydrogel prepared in Example 3, 8M is the gradient hydrogel prepared in Example 4, and 10M is the gradient hydrogel prepared in Example 5. The above concentrations are all the concentrations of lithium chloride in the bottom layer solution. As can be seen from Figure 2 a in, as the concentration of lithium chloride in the bottom layer solution increases, the compressive stress and compressive modulus of the prepared gradient hydrogel decrease in turn. As can be seen from Figure 2 b in, as the ion concentration increases from 2 M to 8 M, the conductivity of the gradient hydrogel shows an increasing trend. When the ion concentration in the bottom layer solution reaches 8 M, the conductivity reaches the maximum value. In addition, due to the penetration and diffusion of ions; the ion concentration gradually decreases along the gradient hydrogel from the bottom layer to the upper layer, resulting in a gradient distribution of ion content from the soft end to the hard end. To sum up, by adjusting the ion concentration in the bottom layer, the mechanical properties and conductivity of the gradient hydrogel can be adjusted to meet different application requirements.

[0090] Figure 3 It is the actual scene diagram of different morphologies of the gradient hydrogel prepared in Example 4 of the present invention. It can be seen that gradient hydrogels with different shapes can be prepared by using containers with different shapes to meet different application requirements.

[0091] Figure 4 It is the sensing performance test chart of the gradient hydrogel pressure sensor prepared in Example 4 of the present invention. According to Figure 4 a in, the gradient hydrogel sensor produces a clear resistance change under different pressures; according to Figure 4 b in, the sensitivity of the gradient hydrogel sensor is 44 Pa in the range of 0-5 kPa -1 ; in the range of 0-1 kPa, the sensitivity can reach 51.6 Pa -1 ; according to Figure 4 c in, the gradient pressure response time is 321 ms and the recovery time is 345 ms.

[0092] Table 1 is a comparison table of the strain of the gradient hydrogels prepared in Example 4 and Examples 6-10 of the present invention with the change of pressure. It can be seen that the gradient hydrogels prepared with different components all have excellent deformation ability under different pressure conditions, indicating that the preparation method of the gradient hydrogel has good universality.

[0093] Table 1 is a comparison table of the strain of the gradient hydrogels in Examples 4, 6-10 with the change of pressure

[0094]

[0095] It can be seen therefrom that the density-difference-driven bidirectional self-growing gradient hydrogel prepared by the present invention has an obvious gradient structure. By adjusting the mold shape, salt concentration, shape, salt composition, polyphenol and monomer types, etc., its stress-strain, sensitivity, conductivity and morphology can be adjusted, which well solves the disadvantages of the existing density-difference-driven bidirectional self-growing gradient hydrogel, such as complex mechanical preparation process, generation of harmful chemical substances, low sensitivity, and poor interfacial interaction. The constructed pressure sensor has stable output performance, high sensitivity and wide detection range, creating favorable conditions for the development of flexible wearable electronic devices.

[0096] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. For those skilled in the art, the present invention can have various changes and modifications. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A method for preparing a gradient hydrogel by density difference driven bidirectional self-growth, characterized in that: The steps include: A mixed solution of alkali and salt is used as a bottom solution, wherein the concentration of alkali is 0.03-0.05 g / ml and the concentration of salt is 0.08-0.5 g / ml; Spreading a mixed solution of polymer monomers, polyphenols, ammonium persulfate and N,N'-methylenebisacrylamide on the top of the bottom solution, and standing at 15-35°C for a set reaction time to obtain; In the mixed solution of the polymer monomer, polyphenol, ammonium persulfate and N,N'-methylenebisacrylamide, the concentration of the polymer monomer is 0.5-1.5 g / ml, the concentration of the polyphenol is 4-10 mg / ml, the concentration of the ammonium persulfate is 4-10 mg / ml, and the concentration of the N,N'-methylenebisacrylamide is 0.1-0.8 mg / ml; The polymer monomer is acrylamide or acrylic acid; The polyphenol is alkali lignin or sodium lignin sulfonate.

2. The density difference driven bidirectional self-growth preparation method of gradient hydrogel according to claim 1, characterized in that: A mixed solution of polymer monomers, polyphenols, ammonium persulfate and N,N'-methylenebisacrylamide is injected along the container wall at a low speed and spread on the bottom solution due to the density difference.

3. The density difference driven bidirectional self-growth preparation method of gradient hydrogel according to claim 1, characterized in that: The alkali is potassium hydroxide or sodium hydroxide; the salt is lithium chloride, sodium chloride or zinc chloride.

4. The density difference driven bidirectional self-growth preparation method of gradient hydrogel according to claim 1, characterized in that: The standing reaction time is 0.1-1h.

5. A gradient hydrogel, characterized in that: Prepared by the preparation method described in any one of claims 1 to 4.

6. Use of the gradient hydrogel according to claim 5 in preparing a pressure sensor.

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