Ultra-sensitive wide linear range underwater tactile sensor and preparation method thereof

Abstract

The invention discloses an ultra-sensitive underwater tactile sensor with a wide linear range and a preparation method of the ultra-sensitive underwater tactile sensor, and belongs to the technical field of underwater tactile sensing. The ultra-sensitive wide-linear-range underwater tactile sensor adopts a sandwich structure and comprises a pair of flexible electrodes and a sensing layer clamped between the flexible electrodes, and the sensing layer is nano composite ionic gel with a micro-hemisphere array structure. The nano composite ionic gel is formed by compounding a polyvinylidene fluoride-co-hexafluoropropylene (PVDF-HFP) matrix, an ionic liquid and hydrophobic silicon dioxide nano particles. In order to solve the technical problems that the sensitivity of an underwater tactile sensor is reduced under high pressure, the linear range is narrow, signals are easy to saturate and the like in the prior art, high-linearity and high-sensitivity response in a wide pressure range is realized by starting from intrinsic modification of materials and constructing the nano-composite ionic gel sensing layer, and the underwater tactile sensor is particularly suitable for underwater application.

Description

Technical Field

[0001] This invention relates to the field of underwater tactile sensor technology, specifically to an ultrasensitive, wide linear range underwater tactile sensor and its fabrication method. Background Technology

[0002] Current underwater robots, lacking tactile perception capabilities, cannot perform the dexterity and complex tasks of human divers. Therefore, to improve the intelligence and autonomy of underwater robots, endowing them with tactile perception capabilities through tactile sensors, thus enabling human-like autonomous flexibility, is an essential technological approach. Tactile sensors are one of the core components for achieving dexterity and precision in underwater robots. However, the underwater environment, especially the extreme conditions of deep-sea environments such as high pressure, high salinity, and low temperatures, poses severe challenges to the performance of tactile sensors.

[0003] Traditional piezoresistive, capacitive, and piezoelectric tactile sensors, when used underwater, suffer from high rigidity in their packaging materials and are prone to nonlinear deformation in flexible substrates, leading to decreased sensitivity and narrow linear range. Furthermore, factors such as hydrostatic pressure, ion permeation, and dielectric instability can easily cause signal baseline drift and reduced reliability.

[0004] In recent years, iontronic tactile sensing technology has shown great potential due to its ability to achieve extremely high capacitance and sensitivity per unit area by utilizing the electric double layer (EDL) effect formed at the interface between ion conductors and electrodes. However, existing iontronic sensors still face problems such as ion leakage, limited linear response range under high pressure, and insufficient long-term stability underwater. To improve linearity, existing technologies often employ complex designs such as multilayer structures, porous structures, and multi-scale interlocking structures. However, these methods often come at the cost of sacrificing sensitivity, increasing manufacturing complexity, or increasing device thickness, making it difficult to meet the comprehensive requirements of underwater robots for sensors that are lightweight, thin, sensitive, have a wide measurement range, and withstand high pressure.

[0005] Therefore, there is an urgent need to develop a tactile sensor that is simple in structure, feasible in manufacturing process, and can combine ultra-high sensitivity and ultra-wide linear response range in underwater high-pressure environments. Summary of the Invention

[0006] The purpose of this invention is to provide an ultrasensitive underwater tactile sensor with a wide linear range and its preparation method, which solves the technical problems of underwater tactile sensors in the prior art, such as decreased sensitivity under high pressure, narrow linear range, and easy signal saturation. Starting from the intrinsic modification of materials, by constructing a nanocomposite ion gel sensing layer, high linearity and high sensitivity response over a wide pressure range are achieved, which is particularly suitable for underwater applications.

[0007] To achieve the above objectives, the technical solution of the present invention is as follows: An ultrasensitive, wide linear range underwater tactile sensor includes a sandwich structure, comprising a pair of flexible electrodes and a sensing layer sandwiched between the pair of flexible electrodes; the sensing layer is a nanocomposite ion gel with a micro-hemispherical array structure; the raw materials of the nanocomposite ion gel include polyvinylidene fluoride-co-hexafluoropropylene (PVDF-HFP), ionic liquid, and hydrophobic silica nanoparticles.

[0008] Furthermore, the hydrophobic silica nanoparticles have a diameter of 20 nm, and the weight ratio of the hydrophobic silica nanoparticles to polyvinylidene fluoride-co-hexafluoropropylene (PVDF-HFP) is 1:2 to 1:1.

[0009] Furthermore, the ionic liquid is 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide salt [EMIM][TFSI].

[0010] Furthermore, the flexible electrode is a composite electrode formed by sputtering gold onto a polyimide film.

[0011] Furthermore, it also includes an encapsulation layer for integrating the sensing layer and the flexible electrode into a waterproof enclosure, the encapsulation layer being made of polydimethylsiloxane.

[0012] A method for fabricating an ultrasensitive, wide linear range underwater tactile sensor includes the following steps: S1. Mold preparation and fluorination treatment: The membrane structure of the sensor was designed using SolidWorks, and the mold was prepared by photosensitive resin through photocuring 3D printing. The surface of the mold was then fluorinated. S2. Preparation of nanocomposite ion gel solution: Dissolve polyvinylidene fluoride-co-hexafluoropropylene (PVDF-HFP) in an organic solvent, add ionic liquid and hydrophobic silica nanoparticles, stir and mix evenly to obtain a homogeneous nanocomposite ion gel precursor solution. S3. Sensing layer molding: The precursor solution obtained in step S2 is poured into the fluorinated mold obtained in step S1. After removing the solvent and solidifying the gel, it is demolded to obtain a nanocomposite ion gel sensing layer with a micro-hemispherical array structure. S4. Device assembly and packaging: The sensing layer obtained in step S3 is placed between a pair of flexible electrodes and integrated into the substrate using a packaging material to obtain the underwater tactile sensor.

[0013] Furthermore, the fluorination treatment step in S1 specifically involves: after the mold is plasma cleaned, it is reacted with a fluorinating agent at 60-100°C for 2-6 hours under vacuum or inert atmosphere; the fluorinating agent is 1H,1H,2H,2H-perfluorodecyltrimethoxysilane.

[0014] Furthermore, the organic solvent in S2 is acetone; the weight ratio of polyvinylidene fluoride-co-hexafluoropropylene PVDF-HFP, ionic liquid and organic solvent is 1:3:9, the stirring temperature is 30-45℃, and the time is 2-4 hours.

[0015] Furthermore, the encapsulation material in S4 is a polydimethylsiloxane prepolymer, which is coated and cured by spin coating or casting.

[0016] By adopting the above technical solution, the present invention has the following advantages: 1. This invention provides an ultrasensitive underwater tactile sensor with a wide linear range and its fabrication method. By doping hydrophobic silica nanoparticles into a PVDF-HFP / ionic liquid gel network, the intrinsic mechanical properties of the gel are effectively controlled. The nanoparticles act as physical cross-linking points, enhancing the gel network strength and delaying the "hardening" of the microstructure and the saturation of contact area growth under high hydrostatic pressure. This achieves high linearity (R0) over a wide pressure range (e.g., a linear range of 500 kPa to 900 kPa). 2 >99.8% and high sensitivity (up to 56 kPa) -1 The unification of ) solves the contradiction of traditional flexible sensors that "high sensitivity results in a narrow measurement range, while a wide measurement range results in low sensitivity", and has excellent sensing performance.

[0017] 2. This invention provides an ultrasensitive, wide linear range underwater tactile sensor and its fabrication method. Specifically designed for underwater background pressure environments, the sensor can effectively distinguish weak changes in contact force even in the presence of hydrostatic pressure, and can be used for geometric contour recognition of target objects. Combined with waterproof encapsulation using materials such as PDMS, it exhibits strong long-term stability and resistance to environmental interference (such as turbulence and salinity), making it suitable for complex underwater scenarios such as deep-sea exploration and demonstrating excellent environmental adaptability.

[0018] 3. This invention provides an ultrasensitive underwater tactile sensor with a wide linear range and its fabrication method. The sensor features a simple structure and controllable fabrication process. It employs a classic sandwich structure, with the sensing layer being a single-structure nanocomposite ionogel, eliminating the need for complex multilayer, interlocking, or multi-scale structural designs. The microhemispherical array can be molded and replicated using mature processes such as photolithography and 3D printing. The fabrication method is simple, reproducible, and cost-controllable, facilitating large-scale production and integrated deployment on platforms such as underwater robots.

[0019] 4. This invention provides an ultrasensitive underwater tactile sensor with a wide linear range and its fabrication method. It utilizes an EDL (electric double layer) formed at the electrode-ion gel interface, outputting a capacitance signal by varying the contact area A with pressure. The combination of a nanocomposite ion gel sensing layer with a micro-hemispherical array structure and flexible electrodes, along with the capacitance output dominated by changes in the contact area and the capacitance per unit surface area of ​​the EDL, achieves ultrasensitive sensing functionality with a wide linear range. Attached Figure Description

[0020] Figure 1 This is a schematic diagram of the structure of the ultra-sensitive wide linear range underwater tactile sensor of the present invention; Figure 2 This is a process flow diagram for the fabrication of the ultrasensitive wide linear range underwater tactile sensor of the present invention; Figure 3 This is a schematic diagram illustrating the principle of the ionized tactile sensing of the present invention; Figure 4 This is a schematic diagram of the compressive deformation process of the micro-hemispherical structure of the present invention.

[0021] Reference numerals: 1. A pair of flexible electrodes; 2. Sensing layer; 3. Encapsulation layer. Detailed Implementation

[0022] The technical solution of the present invention will be specifically described below with reference to the accompanying drawings. It should be noted that, in this document, relational terms such as "first" and "second" are used only to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Moreover, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus.

[0023] A supersensitive, wide linear range underwater tactile sensor, specifically as follows: Figure 1As shown, the ultra-sensitive wide linear range underwater tactile sensor can endow an underwater robot with human-like tactile perception ability, and can still sense weak contact forces in an underwater operation scenario with background pressure, and on this basis, identify the geometric contour of the target object being touched. It includes a sandwich structure, and the sandwich structure includes a pair of flexible electrodes 1 and a sensing layer 2 sandwiched between the pair of flexible electrodes; the ultra-sensitive wide linear range underwater tactile sensor further includes a packaging layer 3 for integrally waterproof-packaging the sensing layer 2 and the flexible electrode 1. Among them, the sensing layer 2 is a nano-composite ionic gel with a micro-hemisphere array structure, and the raw materials of the nano-composite ionic gel include polyvinylidene fluoride-co-hexafluoropropylene PVDF-HFP, ionic liquid, and hydrophobic silica nanoparticles. The diameter of the hydrophobic silica nanoparticles is 20 nm, and the weight ratio of the hydrophobic silica nanoparticles to polyvinylidene fluoride-co-hexafluoropropylene PVDF-HFP is 1:2 to 1:1. The ionic liquid is 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [EMIM][TFSI]. The flexible electrode is a PI-Au composite electrode formed by sputtering gold on a polyimide film. The material of the packaging layer 3 is polydimethylsiloxane.

[0024] A preparation method of an ultra-sensitive wide linear range underwater tactile sensor is specifically as Figure 2 shown, and includes the following steps: S1. Mold preparation and fluorination treatment: Design the mold structure of the sensor through solidworks, prepare the mold by photo-curing 3D printing with photosensitive resin, and perform fluorination treatment on the surface of the mold; the specific steps of the fluorination treatment are: after the mold is cleaned by plasma, react with the fluorinating agent at 60-100 °C for 2-6 hours in a vacuum or inert atmosphere; the fluorinating agent is 1H,1H,2H,2H-perfluorodecyltrimethoxysilane.

[0025] S2. Preparation of nano-composite ionic gel solution: Dissolve polyvinylidene fluoride-co-hexafluoropropylene PVDF-HFP in an organic solvent, add an ionic liquid and hydrophobic silica nanoparticles, stir and mix evenly to obtain a homogeneous nano-composite ionic gel precursor solution; among them, the organic solvent is acetone; the weight ratio of polyvinylidene fluoride-co-hexafluoropropylene PVDF-HFP, ionic liquid to the organic solvent is 1:3:9, the temperature of stirring and mixing is 30-45 °C, and the time is 2-4 hours.

[0026] S3. Molding of the sensing layer: Pour the precursor solution obtained in step S2 into the fluorinated mold obtained in step S1, remove the solvent and cure the gel to form a mold, and then demold to obtain a nano-composite ionic gel sensing layer with a micro-hemisphere array structure; S4. Device assembly and packaging: Place the sensing layer obtained in step S3 between a pair of flexible electrodes, and perform integrated packaging through a packaging material to obtain an ultrasensitive wide linear range underwater tactile sensor. The packaging material is a polydimethylsiloxane prepolymer, which is coated by spin coating or pouring and then cured.

[0027] Example 1: An ultrasensitive wide linear range underwater tactile sensor for an environment with a water depth of about 100 meters

[0028] This example provides an ultrasensitive wide linear range underwater tactile sensor suitable for an operating scenario with a water depth of about 100 meters (background pressure of about 1 MPa). As Figure 1 shown, the ultrasensitive wide linear range underwater tactile sensor includes a pair of flexible electrodes 1 and a sensing layer 2 sandwiched between the pair of flexible electrodes; the ultrasensitive wide linear range underwater tactile sensor also includes a packaging layer 3 for integrally waterproof packaging the sensing layer 2 and the flexible electrodes 1. The sensing layer 2 is a nanocomposite ionic gel with a micro-hemisphere array structure, and the sensing layer 2 has a regularly arranged micro-hemisphere array structure. A pair of flexible electrodes 1 are made of a polyimide (PI) film with a thickness of 50 μm, and a 100 nm thick gold layer is plated on it by magnetron sputtering.

[0029] The preparation method of the ultrasensitive wide linear range underwater tactile sensor is as follows. Specifically, as Figure 2 shown: (1) Mold preparation and fluorination: Use SolidWorks software to design a three-dimensional model of a mold with a concave hemisphere array (hemisphere radius R = 100 μm, center spacing 200 μm), and use a photosensitive resin to make the mold by photolithography 3D printing technology. Place the mold in a plasma cleaner and process it for 5 minutes to activate the surface. Subsequently, add a few drops of 1H,1H,2H,2H-perfluorodecyltrimethoxysilane as a fluorinating agent to the activated mold surface, place it in a vacuum dryer, evacuate it, and then put it in an 80 °C oven and react for 4 hours. After completion, a mold with a hydrophobic fluorinated surface is obtained.

[0030] (2) Preparation of nanocomposite ionic gel solution: Weigh 1.0 g of PVDF-HFP, add 9.0 g of acetone, and stir until completely dissolved. Subsequently, add 3.0 g of ionic liquid [EMIM][TFSI] and 1.0 g of hydrophobic silica nanoparticles with an average particle size of 20 nm (Si with a weight ratio of 1: is added to the solution. Place the mixed system on a 38 °C constant temperature magnetic stirrer and stir at a speed of 500 rpm for 3 hours until a uniform, viscous pale yellow precursor solution is formed.

[0031] (3) Sensing layer formation: Carefully pour the prepared precursor solution into the fluorinated mold prepared in step (1), ensuring that the solution fills all the concave hemispherical structures. Place it in a fume hood and let it stand at room temperature for 12 hours to allow acetone to fully volatilize and the gel to solidify and form. Subsequently, gently remove the cured ion gel film from the mold to obtain the sensing layer 2 of the nano-composite ion gel with a regular micro-hemisphere array on its surface.

[0032] (4) Device assembly and encapsulation: Cut the obtained sensing layer 2 to a suitable size and place it between a pair of flexible electrodes 1, ensuring that the gold layer of the electrode is in surface contact with the micro-hemisphere array of the gel. After mixing the PDMS prepolymer and the curing agent in a weight ratio of 10:1 and degassing, pour and cover the sensing layer 2 and a pair of flexible electrodes 1, and place it in an oven at 70 °C for 2 hours to cure, forming an integrated waterproof encapsulation layer 33, and finally obtaining an ultrasensitive wide linear range underwater tactile sensor.

[0033] The performance of the ultrasensitive wide linear range underwater tactile sensor prepared in this embodiment was tested. In the pressure range of 0 - 500 kPa, the capacitance response of the sensor showed an excellent linear relationship (R² > 99.8%), and the sensitivity reached about 38 kPa. -1 In a test environment simulating a water depth of 100 m (about 1 MPa hydrostatic pressure plus contact force), the sensor could still clearly distinguish weak contact force changes in the order of 50 Pa and successfully reproduce the surface profile of the test object. After more than 10,000 compression - release cycle tests, the sensor signal output remained stable, showing good durability.

[0034] Example 2: An ultrasensitive wide linear range underwater tactile sensor for an environment with a water depth of about 50 m

[0035] The main difference between this embodiment and Example 1 lies in the preparation of the nano-composite ion gel solution, aiming to provide a sensor with higher sensitivity for a scenario with a water depth of about 50 m (lower background pressure).

[0036] When preparing the nano-composite ion gel solution, the dosages of each raw material are as follows: 1.0 g of PVDF-HFP, 9.0 g of acetone, 3.0 g of ionic liquid [EMIM][TFSI], and 0.5 g of hydrophobic Si nanoparticles (the weight ratio of Si to PVDF-HFP is 1:2). The remaining steps of mold preparation, fluorination, sensing layer formation, device assembly and encapsulation are exactly the same as those in Example 1.

[0037] Performance tests show that due to the reduced nanoparticle filling ratio, the gel modulus is relatively low, and the micro-hemispheroids are more prone to deformation under low pressure. The sensor in this embodiment still exhibits excellent linearity (R²>99.5%) within the 0-300 kPa pressure range, and its sensitivity is improved to approximately 56 kPa. -1 It is more suitable for delicate operations in shallow and medium water where the perception of weak forces is more critical.

[0038] Theoretical Analysis: The sensing mechanism of the ultra-sensitive, wide linear range underwater tactile sensor of this invention is based on the ionized double-layer capacitance effect. In this ionized tactile sensor design, the ion-electron double layer (EDL) formed at the interface between the ion gel and the flexible electrode is a key component for achieving high sensitivity and high resolution tactile sensing. For example... Figure 3 As shown, the ionogel contains a large number of low-molar-concentration positive and negative ion pairs. When a voltage is applied, electrons on the electrode and counterions in the ionogel aggregate at nanometer-scale intervals in the contact area, thereby increasing the capacitance. When the sensor is subjected to a contact force, the ionogel comes into contact with the flexible electrode. As the contact force increases, the microstructure of the ionogel is compressed and deformed, causing the contact area between it and the electrode to increase with increasing pressure, thus increasing the capacitance C. EDL Its value is defined by the Gouy-Chapman-Stern model:

[0039] Among them, C H Represents the Helmholtz layer, C D Indicates the diffusion layer. It is a comprehensive parameter; after simplified derivation, the double-layer capacitance value C EDL The capacitance C is determined by UAC (electric double-layer capacitance per unit surface area), which is a constant determined solely by the sensor's material system. In other words, the sensor's capacitance output value C... EDL It is determined by the contact area A between the ion gel and the electrode.

[0040] Hemispherical structures are robust, predictable, and structurally controllable during deformation, avoiding the instability of other structures (such as pyramids, micropillars, and random bumps). Therefore, this invention proposes a nanoparticle-filled hemispherical ionogel tactile unit. The pressure-sensing behavior of the ionized micro-hemispherical tactile unit originates from the coupling effect of mechanical deformation and electrical double-layer capacitance (EDL) modulation. Figure 4 As shown, when an external force is applied to a deformable hemispherical structure, the double-layer capacitance UAC per unit surface area and the contact area A between the ion gel and the electrode evolve with pressure. The analytical relationship between the capacitance value CEDL and the pressure P and the nanoparticle filling rate is as follows:

[0041] in, Capacitance per unit area when there is no voltage. It is a dimensionless ion enrichment efficiency factor. Let R be the thickness of the compressed shell, and R be the radius of the micro-hemispheric. An effective volume of EDL is defined near the electrode. It is a geometric constant determined by the number of elements and the effective area. The modulus of the unfilled ionogel matrix. It is Poisson's ratio.

[0042] Through the above theoretical analysis, the analytical relationship between the sensor's output capacitance value CEDL, pressure P, and nanoparticle filling rate can be quantified. This will guide the design of sensors with appropriate nanoparticle filling schemes for different water depth application scenarios, ensuring the achievement of functional requirements that combine ultra-sensitivity and wide linearity.

[0043] In summary, this invention successfully fabricates a tactile sensor suitable for underwater high-pressure environments, possessing both ultra-sensitivity and an ultra-wide linear range, through the combination of innovative nanocomposite ionogel materials and a simple microstructure design. It has promising application prospects and industrialization potential.

[0044] Finally, it should be noted that although the present invention has been described with reference to specific embodiments, those skilled in the art should recognize that the above embodiments are only used to illustrate the present invention and are not intended to limit the present invention. Various equivalent changes or substitutions can be made without departing from the concept of the present invention. Therefore, any changes or modifications to the above embodiments within the essential spirit of the present invention will fall within the scope of the claims of the present invention.

Claims

1. An ultra-sensitive, wide linear range underwater tactile sensor, characterized in that, The invention includes a sandwich structure comprising a pair of flexible electrodes and a sensing layer sandwiched between the pair of flexible electrodes; the sensing layer is a nanocomposite ion gel with a micro-hemispherical array structure; the raw materials of the nanocomposite ion gel include polyvinylidene fluoride-co-hexafluoropropylene (PVDF-HFP), ionic liquid, and hydrophobic silica nanoparticles.

2. The ultra-sensitive wide linear range underwater tactile sensor according to claim 1, characterized in that, The hydrophobic silica nanoparticles have a diameter of 20 nm, and the weight ratio of the hydrophobic silica nanoparticles to polyvinylidene fluoride-co-hexafluoropropylene (PVDF-HFP) is 1:2 to 1:

1.

3. The ultra-sensitive wide linear range underwater tactile sensor according to claim 1, characterized in that, The ionic liquid is 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide salt [EMIM][TFSI].

4. The ultra-sensitive wide linear range underwater tactile sensor according to claim 1, characterized in that, The flexible electrode is a composite electrode formed by sputtering gold onto a polyimide film.

5. The ultra-sensitive wide linear range underwater tactile sensor according to claim 1, characterized in that, It also includes an encapsulation layer for integrating the sensing layer and the flexible electrode into a waterproof encapsulation, the encapsulation layer being made of polydimethylsiloxane.

6. A method for fabricating an ultrasensitive, wide linear range underwater tactile sensor as described in any one of claims 1-5, characterized in that, Includes the following steps: S1. Mold preparation and fluorination treatment: The membrane structure of the sensor was designed using SolidWorks, and the mold was prepared by photosensitive resin through photocuring 3D printing. The surface of the mold was then fluorinated. S2. Preparation of nanocomposite ion gel solution: Dissolve polyvinylidene fluoride-co-hexafluoropropylene (PVDF-HFP) in an organic solvent, add ionic liquid and hydrophobic silica nanoparticles, stir and mix evenly to obtain a homogeneous nanocomposite ion gel precursor solution. S3. Sensing layer molding: The precursor solution obtained in step S2 is poured into the fluorinated mold obtained in step S1. After removing the solvent and solidifying the gel, it is demolded to obtain a nanocomposite ion gel sensing layer with a micro-hemispherical array structure. S4. Device assembly and packaging: The sensing layer obtained in step S3 is placed between a pair of flexible electrodes and integrated into the substrate using a packaging material to obtain the underwater tactile sensor.

7. The method for fabricating an ultrasensitive wide linear range underwater tactile sensor according to claim 6, characterized in that, The fluorination process in S1 specifically involves: after plasma cleaning, the mold is reacted with a fluorinating agent at 60-100°C for 2-6 hours under vacuum or inert atmosphere; the fluorinating agent is 1H,1H,2H,2H-perfluorodecyltrimethoxysilane.

8. The method for fabricating an ultrasensitive wide linear range underwater tactile sensor according to claim 6, characterized in that, The organic solvent in S2 is acetone; the weight ratio of polyvinylidene fluoride-co-hexafluoropropylene PVDF-HFP, ionic liquid and organic solvent is 1:3:9, the stirring temperature is 30-45℃ and the time is 2-4 hours.

9. The method for fabricating an ultrasensitive wide linear range underwater tactile sensor according to claim 6, characterized in that, The encapsulation material in S4 is a polydimethylsiloxane prepolymer, which is coated by spin coating or casting and then cured.

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