Vinylidene fluoride polymer flexible piezoelectric hydrogel with island structure and preparation method and application thereof

Abstract

The present application belongs to the field of piezoelectric gel materials, and particularly relates to a polyvinylidene fluoride polymer flexible piezoelectric hydrogel with island structure and a preparation method and application thereof. The flexible piezoelectric hydrogel is a three-dimensional continuous network formed by cross-linking of a hydrophilic polymer as a continuous phase, and polymer microspheres with uniform particle size formed by a hydrophobic polyvinylidene fluoride polymer as a uniformly dispersed dispersed phase. The island structure significantly improves the mechanical properties and piezoelectric properties of the gel. The piezoelectric gel material can be used for human sensing, detecting motion signals and pulse signals, and has high industrial production potential and biomedical application prospect.

Description

Technical Field

[0001] This invention belongs to the field of piezoelectric gel materials, specifically relating to a flexible piezoelectric gel of vinylidene fluoride polymer with an island structure, its preparation method, and its application. Background Technology

[0002] The piezoelectric effect refers to the phenomenon that certain materials can generate electric charge when subjected to external mechanical stress, or undergo mechanical deformation when an electric field is applied. These materials are called piezoelectric materials. Piezoelectric materials mainly fall into two categories: inorganic piezoelectric materials and organic piezoelectric materials. Representative inorganic piezoelectric materials include lead zirconate titanate, barium titanate, and potassium sodium niobate. Although they have high piezoelectric coefficients, their impact and tensile properties are poor, making them difficult to apply to flexible devices. Furthermore, high-performance piezoelectric ceramics are extremely sensitive to the purity of raw materials and sintering processes, resulting in high costs. Organic piezoelectric materials are mainly piezoelectric polymers, with representative materials being polyvinylidene fluoride (PVDF) and its copolymers (P(VDF-TrFE)). Although the piezoelectric coefficient of piezoelectric polymers is lower than that of piezoelectric ceramics, as flexible matrices, they are ideal materials for manufacturing flexible sensors and energy harvesters, and can be used in electronic skin and implantable medical devices.

[0003] In recent years, emerging composite piezoelectric materials have been continuously developed, aiming to combine the advantages of each component material to achieve customizable properties. For example, combining piezoelectric ceramics with piezoelectric polymers can achieve a combination of high piezoelectricity and flexibility. However, the interfacial bonding between inorganic and organic materials is a research challenge; a poor interface can lead to ineffective stress transfer, resulting in charge leakage. Moreover, the design and controllable fabrication of the microstructure in composite materials are also very complex. Among various composite materials, piezoelectric hydrogels have extremely broad application prospects. They combine the flexibility, stretchability, and biocompatibility of hydrogels (hydrophilic three-dimensional network polymers) with the mechanical-to-electrical energy conversion characteristics of piezoelectric materials, aiming to create novel smart materials that can sense pressure, deformation, and human physiological activities and generate corresponding electrical signals.

[0004] The applications of piezoelectric gel materials (mainly hydrogels) primarily include: biomedicine and health monitoring, flexible electronics and wearable devices, soft robotics, the Internet of Things, and smart sensing. Taking in-situ detection of physiological signals as an example, piezoelectric gels can be fabricated into ultra-thin, flexible patches that can be attached to the skin, muscles, or specific organs of the body to monitor weak physiological pressure signals in real time, such as intracranial pressure, intraocular pressure, muscle tension, and tendon activity. These signals are crucial for diagnosing glaucoma, muscle strain, and neurological diseases. However, to move piezoelectric gels from the laboratory to practical applications, a series of formidable challenges must be overcome. Strong piezoelectricity and high flexibility / stretchability are often mutually exclusive. Currently, inorganic piezoelectric nanoparticles or organic piezoelectric nanofibers are often incorporated into hydrogel networks. However, the difficulty lies in uniform dispersion and interfacial bonding. Fillers are prone to agglomeration, and the mismatch between the filler's modulus and that of the flexible polymer matrix can lead to stress concentration points and reduced mechanical properties. Currently, the piezoelectric coefficients of most piezoelectric hydrogels are much lower than those of traditional piezoelectric materials. Macroscopic piezoelectricity can be enhanced by oriented internal electric dipole moments or fillers through physical / chemical methods (such as stretching and electric field polarization). Secondly, the piezoelectric response can be amplified by optimizing its internal microstructure.

[0005] PVDF and its copolymers are a class of semi-crystalline polymer piezoelectric materials, whose piezoelectricity originates from their unique molecular and crystal structures. The β phase is the fundamental cause of the piezoelectric effect; it has an all-trans conformation, with fluorine and hydrogen atoms on either side of the carbon chain, forming a strong dipole moment. When these dipole moments align under an external electric field, the material exhibits macroscopic piezoelectricity. Combining PVDF or its copolymers with hydrogels holds promise for overcoming the mechanical mismatch between rigid electronic devices and soft tissues, as well as the conduction barriers between electronic signals. However, PVDF and its copolymers are hydrophobic, while hydrogels are hydrophilic. Achieving a stable and uniform combination of the two, while maintaining mechanical flexibility and high electromechanical sensitivity, remains a challenge.

[0006] CN120590679A discloses a cellulose-based polyvinylidene fluoride porous composite piezoelectric material. The preparation method includes the following steps: dissolving cellulose in a pre-cooled solvent system and performing gel treatment to obtain a cellulose hydrogel; performing solvent exchange on the cellulose hydrogel and then freeze-drying it once to obtain a cellulose aerogel; immersing the cellulose aerogel in a PVDF solution, using non-solvent-induced PVDF regeneration treatment, and performing a second freeze-drying to obtain the cellulose-based polyvinylidene fluoride porous composite piezoelectric material.

[0007] CN113831553A discloses a method for preparing PVDF-TrFE chitosan hydrogel, using PEDOT:PSS as a conductive filler, PVDF-TrFE as a piezoelectric material, and a biocompatible cross-linked chitosan quaternary ammonium salt solution as a flexible network framework, and obtaining PVDF-TrFE / cross-linked chitosan quaternary ammonium salt hydrogel by thermogel molding.

[0008] CN116531553A discloses a piezoelectric hydrogel for repairing diabetic wounds. By mass, its active ingredients include: 38-70 parts of a hydrophilic organic polymer, 20-60 parts of a hydrophobic piezoelectric polymer, and 1-5 parts of a reinforcing medium. However, the inherent hydrophobicity of PVDF and the hydrophilicity of PVA inevitably involve non-solvent-induced phase separation (NIPS). The spatial distribution of PVDF chains in the hydrophilic network is often uncontrolled. Therefore, the piezoelectric hydrogel obtained in this patent lacks sufficient mechanical strength and is unsuitable for use as a wearable biosensor material. Summary of the Invention

[0009] To address the technical deficiencies of existing polymer piezoelectric gels, the present invention aims to provide a polymer flexible piezoelectric gel with a controllable island structure and its preparation method, thereby overcoming the core bottleneck of the existing technology: In existing vinylidene fluoride polymer-based piezoelectric hydrogels, vinylidene fluoride polymer and hydrophilic polymer have natural thermodynamic incompatibility, making it impossible to achieve homogeneous co-solubility. There is a lack of effective means to control the microstructure of the dispersed phase of vinylidene fluoride polymer, making it impossible to form a stable and uniform island topology. This leads to the industry pain point that it is difficult to achieve both strong piezoelectricity and high flexibility / stretchability, and multi-component inability to achieve long-term stable compatibility. This invention utilizes a binary composite solvent system comprising fluorinated alcohols, coupled with a two-phase separation strategy of liquid-liquid phase separation (LLPS) morphology pre-regulation and NIPS structure fixation, to prepare a flexible piezoelectric gel with a controllable island structure of vinylidene fluoride polymer. The gel consists of monodisperse microspheres formed from hydrophobic vinylidene fluoride polymers as island phases, uniformly dispersed within a three-dimensional continuous marine structure formed by cross-linking of hydrophilic polymers. The vinylidene fluoride polymer island phases enable efficient stress transfer and dipole orientation, thereby simultaneously enhancing the material's mechanical stability and piezoelectric response. Hydrogel sensors prepared based on this gel exhibit stable and highly sensitive responses to subtle biomechanical signals, enabling the monitoring of physiological signals in multiple scenarios, including finger movements, facial micro-expressions, laryngeal movements, and radial artery pulse. Particularly for monitoring limb muscle group movements, by combining the motion signals collected by the sensor with a long short-term memory neural network, accurate identification of fitness movements and automatic detection of incorrect movement patterns can be achieved. The flexible piezoelectric gel provided by this invention has broad application prospects in the fields of intelligent wearable bioelectronics and personalized fitness monitoring.

[0010] To achieve the above-mentioned objectives, the technical solution adopted by this invention is as follows:

[0011] A flexible piezoelectric hydrogel with a sea-island structure is formed by a three-dimensional continuous network formed by cross-linking of hydrophilic polymers as the continuous phase and uniformly dispersed polymer microspheres formed by hydrophobic polyvinylidene fluoride polymers as the dispersion.

[0012] The dispersed phase (island phase) is non-agglomerated and non-fused, embedded in the three-dimensional network of the continuous phase (marine phase), forming a stable island topology. Compared with hydrogels lacking island structure, the flexible piezoelectric gel of vinylidene fluoride polymer with specific island structure of the present invention exhibits significantly enhanced mechanical properties (elongation at break, tensile strength, and toughness).

[0013] This invention employs a binary complex solvent composed of HFIP and a strongly polar aprotic solvent (such as dimethyl sulfoxide, DMSO). Leveraging the amphiphilic and compatibilizing properties of HFIP, it addresses the industry pain point of thermodynamic incompatibility between PVDF and hydrophilic polymers at the molecular level: on one hand, the strong fluorine-fluorine interaction between the fluoroalkyl structure of HFIP and PVDF disrupts the intermolecular forces of PVDF, achieving complete dissolution of hydrophobic PVDF; on the other hand, the terminal hydroxyl groups of HFIP form strong hydrogen bonds with the hydroxyl groups of hydrophilic polymers (such as PVA), achieving uniform dissolution of the hydrophilic polymer. This binary complex solvent simultaneously achieves complete and homogeneous co-solubility of PVDF and hydrophilic polymers, providing a uniform and stable precursor system for subsequent controllable phase separation. This fundamentally avoids the problems of premature random aggregation of PVDF and uncontrolled phase separation in existing single-solvent systems. Existing technologies, including gels formed from hydrophilic polymers and hydrophobic polymers such as vinylidene fluoride polymers, cannot achieve homogeneous co-solubility of the two phases, inevitably leading to uncontrolled phase separation and preventing the formation of the island structure specific to this invention.

[0014] This invention utilizes the thermodynamic incompatibility between PVDF and hydrophilic polymers, precisely controlling the interfacial tension between the two phases by adjusting the HFIP content in the compound solvent, triggering a controllable LLPS process. PVDF spontaneously aggregates in a homogeneous solution to form monodisperse polymer microdroplets with uniform particle size, which are then uniformly dispersed within the continuous phase of the hydrophilic polymer, forming a stable dispersion system of "island precursor-ocean precursor." This fundamentally avoids the problem of random aggregation and irregular phase domain formation of PVDF in existing technologies, laying a precise morphological foundation for the final island structure formation. Existing technologies fail to recognize the pre-regulatory role of the LLPS process on the PVDF phase morphology, and further fail to achieve precise control of the LLPS process through the solvent system, thus failing to achieve monodispersity and homogenization of the PVDF dispersed phase, and therefore cannot obtain island structures.

[0015] This invention employs a coupled process of first crosslinking and phase-locking followed by solvent exchange to permanently fix the pre-regulated morphology of LLPS: First, a freeze-thaw cycle crosslinking treatment is used to form a stable three-dimensional crosslinked network of the hydrophilic polymer continuous phase, completely locking the spatial positions of the PVDF microdroplets formed during the LLPS process and preventing them from fusing and agglomerating during subsequent solvent exchange. Then, solvent exchange is performed through non-solvent-induced phase separation (NIPS), replacing the organic solvent with non-solvent water. During this process, the spatially locked PVDF microdroplets complete crystallization and phase transformation, forming stable, monodisperse PVDF polymer microsphere islands, ultimately yielding a highly controllable morphology and a long-term stable island-structured piezoelectric gel. Existing technologies mostly use a single NIPS process, without pre-regulating the morphology and locking the spatial position of the PVDF phase through the LLPS process. During solvent exchange, PVDF is prone to agglomeration and fusing, resulting only in random porous structures or irregular aggregates, failing to form the uniform island structure of this invention.

[0016] Compared to hydrogels lacking an island structure, the flexible piezoelectric gel of vinylidene fluoride polymer (PVDF) of this invention, with a specific and controllable island structure, exhibits significantly enhanced mechanical properties, with substantial improvements in elongation at break, tensile strength, and toughness. Furthermore, through the aforementioned island structure design, the strong hydrogen bonding between PVDF and hydrophilic polymers (such as polyvinyl alcohol, PVA), combined with dehydrofluorination during solvent exchange, can efficiently induce PVDF to form a high content of electroactive β and γ crystalline phases. This allows the proportion of electroactive phases in the hydrogel to reach over 90%, and in preferred embodiments over 95%, significantly improving the piezoelectric coefficient and piezoelectric output performance of the PVDF-based piezoelectric gel. This makes it suitable as a flexible sensor material for highly sensitive detection of human muscle activity.

[0017] Furthermore, in the polyvinylidene fluoride polymer flexible piezoelectric gel, the size of the polyvinylidene fluoride polymer as the dispersed phase is 20-200 μm, preferably 50-100 μm.

[0018] This invention also provides a method for preparing the above-mentioned polyvinylidene fluoride piezoelectric gel with an island structure, comprising the following steps:

[0019] S1. Dissolve the hydrophilic polymer and the vinylidene fluoride polymer in a compounded organic solvent to form a pregel solution; the compounded organic solvent is a mixture of an aprotic polar organic solvent and a fluorinated alcohol;

[0020] S2. Place the pregel solution described in S1 into a gel mold, and after a freeze-thaw process, an organic gel is formed. Collect the organic gel.

[0021] S3. The organic gel described in S2 is placed in deionized water for solvent replacement, so that the organic gel is transformed into a hydrogel, and the polyvinylidene fluoride piezoelectric gel with island structure is collected.

[0022] Further, in step S1, the hydrophilic polymer is selected from polymers containing multiple hydroxyl groups, such as one or more of polyvinyl alcohol (PVA), polyethylene glycol (PEG), cellulose, sodium alginate, chitosan, hyaluronic acid, and gelatin.

[0023] Further, in step S1, the vinylidene fluoride polymer is a self-polymer of vinylidene fluoride, or a copolymer of vinylidene fluoride and other halogen-containing olefins, and the number of repeating units of vinylidene fluoride in the copolymer accounts for 50-80%, preferably 60-75%; specifically selected from one or more of polyvinylidene fluoride (PVDF), poly(vinylidene fluoride-trifluoroethylene), abbreviated as P(VDF-TrFE), poly(vinylidene fluoride-trifluorochloroethylene), abbreviated as P(VDF-CTFE), poly(vinylidene fluoride-hexafluoropropylene), abbreviated as P(VDF-HFP), poly(vinylidene fluoride-trifluoroethylene-trifluorochloroethylene), abbreviated as P(VDF-TrFE-CTFE), and poly(vinylidene fluoride-trifluoroethylene-trifluorochloroethylene), abbreviated as P(VDF-TrFE-CFE).

[0024] Furthermore, the molecular weight of the vinylidene fluoride polymer is 3 × 10⁻⁶. 5 ~1.5×10 6 g / mol (weight-average molecular weight), preferably 8 × 10⁻⁶ g / mol. 5 ~1.2×10 6 g / mol.

[0025] Further, in step S1, the mass ratio of the hydrophilic polymer to the vinylidene fluoride polymer is 10:1-3, preferably 10:1-2.

[0026] Further, in step S1, the aprotic polar organic solvent is selected from at least one of dimethyl sulfoxide (DMSO), N,N-dimethylformamide (DMF), and N,N-dimethylacetamide (DMAc); the fluorinated alcohol is selected from at least one of hexafluoroisopropanol (HFIP), 2,2,2-trifluoroethanol, perfluorotert-butanol, and pentafluoropropanol. Even further, in the compound solvent, the fluorinated alcohol accounts for 10-20% by volume.

[0027] The inventors discovered that the solvent in step S1 is crucial for obtaining a piezoelectric gel with a suitable island structure. The solvent in this invention contains a certain amount of fluorinated alcohol. When the hydrophobic polymer (e.g., PVDF) and hydrophilic polymer (e.g., PVA) in step S1 are mixed with an aprotic polar solvent (e.g., DMSO) and a fluorinated alcohol (e.g., HFIP), low-density polystyrene (LLPS) is initiated, forming a homogeneous blend solution. After freeze-thaw cycles, a PVDF / PVA organic gel is obtained. The final hydrogel is prepared by exchanging the solvent with distilled water. The presence of the fluorinated alcohol can precisely regulate the interfacial tension of the hydrophobic / hydrophilic polymer, inducing LLPS and driving the self-assembly of monodisperse PVDF droplets to minimize the interfacial energy of the system. Through precise control of the fluorinated alcohol, monodisperse PVDF microspheres without aggregation are achieved, fundamentally solving the compatibility problem between PVDF and the hydrophilic PVA matrix. Subsequently, the liquid, deformable island structure formed by LLPS is permanently frozen into a stable microstructure of hydrogel through the liquid-solid phase transition in steps S2 and S3. This process simultaneously completes the gelation and molding of the hydrogel and synergistically promotes the efficient transformation of the electroactive crystalline phase in the hydrogel.

[0028] Furthermore, in step S1, the amount of compound solvent used satisfies the requirement that the mass concentration of the hydrophilic polymer in the pregel solution is 7.5-15 wt%.

[0029] Furthermore, in step S2, during the freeze-thaw process, the freezing temperature is -80 to -60 ℃, the freezing time is 12-24 h, the thawing temperature is 20-25 ℃, and the thawing time is 6-12 h.

[0030] Furthermore, in step S3, the solvent replacement time is 48-72 hours, and the water content of the hydrogel obtained after solvent replacement is 70-80%.

[0031] The polyvinylidene fluoride piezoelectric gel with an island structure is composed of a hydrophilic polymer containing multiple hydroxyl groups and a hydrophobic fluorinated piezoelectric polymer with a piezoelectric effect. The island structure refers to the fact that the hydrophilic polymer in the hydrogel is a continuous phase, and the hydrophobic fluorinated polymer is uniformly dispersed in the continuous phase, just like islands dispersed in the sea, hence the name island structure.

[0032] The present invention also provides the use of the above-mentioned flexible piezoelectric gel with island structure as a human body sensor, including detecting human activity and pulse signal patterns.

[0033] This invention precisely controls interfacial tension using fluorinated alcohols, inducing the formation of monodisperse, non-agglomerated island structures. The dispersed phase, vinylidene fluoride polymer, is uniformly anchored in the hydrophilic continuous phase as microspheres, forming a spatially ordered array of piezoelectric response units. Simultaneously, this synergistically promotes the efficient transformation of the electroactive β / γ crystalline phase of the vinylidene fluoride polymer, fundamentally avoiding the defects of PVDF agglomeration, insufficient crystalline phase transformation, and disordered distribution of piezoelectric units found in ordinary physical blending systems. The piezoelectric coefficient d of the gel in this invention... 33 It can reach 20~45 pC / N, which is much higher than that of ordinary PVDF / PVA blended gels (10~18 pC / N). It maintains a linear piezoelectric response within a wide strain range of 0.1%~50%. It has no signal hysteresis or distortion for weak physiological / motor signals such as human pulse, micro-expression, vocal cord vibration, and joint micro-movements. It solves the core pain points of ordinary blended gels, such as weak piezoelectric performance, signal distortion, and inability to stably detect weak physiological signals of the human body.

[0034] The present invention has the following beneficial effects:

[0035] The island-structured polymer piezoelectric gel of this invention not only exhibits high elasticity, high toughness, and excellent fatigue resistance, capable of withstanding repeated large deformations while maintaining structural integrity, but its efficient piezoelectric response also ensures stable and sensitive electrical signal output under external mechanical stimulation. This efficient combination of mechanical and piezoelectric properties overcomes the limitations of traditional piezoelectric materials, such as poor flexibility and susceptibility to damage, as well as the weak mechanical strength and limited functionality of ordinary hydrogels, providing a key material foundation for achieving long-term, stable operation of devices in dynamic environments.

[0036] The polymer piezoelectric gel with an island structure of the present invention ingeniously solves the problem of poor interfacial compatibility after blending hydrophilic polymers and hydrophobic piezoelectric polymers in traditional methods. The special island structure not only improves the mechanical properties of the piezoelectric hydrogel, but also enhances its piezoelectric response performance.

[0037] The island-structured polymer piezoelectric hydrogel of this invention shows great application potential in the fields of flexible electronics and biomedicine. It can serve as a self-powered flexible sensor, directly attached to or implanted in the human body, enabling real-time monitoring of various physiological signals without an external power source, providing a new paradigm for health monitoring and human-computer interaction. Simultaneously, its excellent biocompatibility gives it a safety advantage over traditional inorganic piezoelectric materials in cutting-edge fields such as implantable devices and tissue engineering, laying a core technological foundation for the development of next-generation intelligent, biocompatible medical devices. Attached Figure Description

[0038] Figure 1 This is a photograph of the PVDF / PVA island-structured piezoelectric hydrogel prepared in Example 1.

[0039] Figure 2 The images show the SEM and EDS images of the PVDF / PVA island-structured piezoelectric hydrogel prepared in Example 1.

[0040] Figure 3 The image shows the XRD pattern of the PVDF / PVA island-structured piezoelectric hydrogel prepared in Example 1.

[0041] Figure 4 This is a quantitative analysis diagram of the electroactive phase of the PVDF / PVA island-structured piezoelectric hydrogel prepared in Example 1.

[0042] Figure 5 The tensile and compressive stress-strain curves of the PVDF / PVA island-structured piezoelectric hydrogel prepared in Example 1 are shown.

[0043] Figure 6 The diagram shows the open-circuit voltage generated by the PVDF / PVA island-structured piezoelectric hydrogel prepared in Example 1 under different mechanical stresses.

[0044] Figure 7 The d of the PVDF / PVA island-structured piezoelectric hydrogel prepared in Example 1 33 Piezoelectric coefficient diagram.

[0045] Figure 8 This is a signal image of human finger movement detected by the PVDF / PVA island-structured piezoelectric hydrogel prepared in Example 1.

[0046] Figure 9 The image shows the signal of the PVDF / PVA island-structured piezoelectric hydrogel obtained in Example 1 for detecting human pulse. Detailed Implementation

[0047] The invention will be further illustrated by the following embodiments. These embodiments are purely illustrative and are only used to specifically describe the invention, and should not be construed as limiting the invention. The invention will be further described below with reference to the accompanying drawings and embodiments.

[0048] The weight-average molecular weight of PVDF is 1.2 million, the weight-average molecular weight of PVA is 98,000, and the degree of alcoholysis is 99%.

[0049] Example 1

[0050] Take 1 g of PVA powder and 100 mg / 200 mg / 300 mg / 400 mg of PVDF powder, add 10 mL of a mixed solvent of DMSO and HFIP (DMSO to HFIP volume ratio 9:1), and magnetically stir at 90 °C for 1 h to obtain a homogeneous pregel solution. Then pour the pregel solution into a suitable mold and freeze at -80 °C for 24 h. After thawing for 6 h, obtain an organic gel. Place the organic gel in deionized water for solvent replacement for 60 h, changing the water every 12 h to obtain a PVDF / PVA island-structured piezoelectric hydrogel. The hydrogel morphology is as follows. Figure 1 As shown.

[0051] Take one piece of PVDF / PVA hydrogel with different PVDF contents, weigh and record the weight, then immerse it in liquid nitrogen for 30 seconds to freeze it completely. After taking it out, freeze dry it in a freeze dryer for 48 hours. Weigh the dried hydrogels and calculate the water content. The results are shown in Table 1. The water content of the PVDF / PVA hydrogels is greater than 70%.

[0052] Table 1. Water content of different PVDF / PVA hydrogels (mean ± SD, n = 4)

[0053]

[0054] Figure 2 The images show SEM and EDS images of the PVDF / PVA island-structured piezoelectric hydrogel obtained in Example 1. The SEM images show that as the PVDF content increases, the average size of the PVDF phase increases, concentrated in the range of 20-200 μm, preferably between 50-100 μm. When PVDF accounts for 40 wt% of the PVA mass, the PVDF phase aggregates and transforms into an irregular morphology. The EDS images also fully verify that these island-phase particles are composed solely of PVDF molecules (fluorine is distributed throughout these particles). Notably, the PVDF particles have a porous structure, and the PVDF content only affects their size, not their porous morphology.

[0055] The transformation of PVDF crystal phases during hydrogel formation was studied using XRD analysis, and the results are as follows: Figure 4 As shown, the initial PVDF is mainly an α-phase with characteristic peaks of 18.2°, 19.8°, and 26.6°, corresponding to the (020), (110), and (002) crystal planes, respectively. In the PVDF / PVA hydrogel, the characteristic peaks of the α-phase almost completely disappear, and the characteristic peak (20.6°) representing the β-phase appears, corresponding to the (110) and (200) crystal planes.

[0056] To further quantify the PVDF phase transformation within the PVDF / PVA hydrogel, we performed quantitative analysis of the electroactive phases (β and γ phases) in PVDF using ATR-IR. This was primarily achieved through a 510 cm⁻¹ spectral density measurement. -1 (β and γ phases), 614 cm -1 (α phase), 763 cm -1 (α phase), 840 cm -1 (β and γ phases), 1275 cm -1 (β phase) and 1234 cm -1 The characteristic peaks at the (γ phase) are used to distinguish the three crystal forms. The relative fraction of the electroactive phase (F) is calculated. EA The findings and results are shown below. Figure 5 The ratio of β and γ phases in PVDF / PVA hydrogels is more than twice that of pure PVDF, reaching up to 95.1%.

[0057] Example 2

[0058] Take 1 g of PVA powder and 200 mg of PVDF powder, add 10 mL of a mixed solvent of DMSO and HFIP (DMSO to HFIP volume ratio of 8:2), and magnetically stir at 90 °C for 1 h to obtain a homogeneous pregel solution. Then pour the pregel solution into a suitable mold and freeze at -80 °C for 24 h. After thawing for 6 h, obtain an organic gel. Place the organic gel in deionized water for solvent replacement for 60 h, changing the water every 12 h to obtain the PVDF / PVA piezoelectric hydrogel.

[0059] Example 3

[0060] The other conditions and operations are the same as in Example 2, except that the volume ratio of DMSO to HFIP is 7:3.

[0061] Example 4

[0062] Take 1 g of HPC powder and 200 mg of PVDF powder, add 10 mL of a mixed solvent of DMSO and HFIP (DMSO to HFIP volume ratio of 9:1), and magnetically stir at 90 °C for 1 h to obtain a homogeneous pregel solution. Then pour the pregel solution into a suitable mold and freeze at -80 °C for 24 h. After thawing for 6 h, obtain an organic gel. Place the organic gel in deionized water for solvent replacement for 60 h, changing the water every 12 h to obtain the PVDF / HPC piezoelectric hydrogel.

[0063] Comparative Example 1

[0064] Take 1 g of PVA powder and 200 mg of PVDF powder, add 10 mL of a mixed solvent of DMSO and water (DMSO to water volume ratio of 6:4), and magnetically stir at 90 ℃ for 1 h to obtain a homogeneous pregel solution. Then pour the pregel solution into a suitable mold and freeze at -80 ℃ for 24 h. After thawing for 6 h, obtain an organic gel. Place the organic gel in deionized water for solvent replacement for 60 h, changing the water every 12 h to obtain the PVDF / PVA hydrogel.

[0065] In this comparative example, due to the high water content in the solvent, the PVDF molecular chains could not be fully opened, and the crystalline regions could not be dissolved. This prevented the transformation of the crystalline phase from the α phase to the electroactive phase (β / γ phase) in subsequent operations. Furthermore, the mechanical properties were poor, with tensile strength and elongation at break far inferior to the island-structured hydrogel in Example 1. Therefore, the hydrogel prepared in the comparative example could not be used for piezoelectric sensing testing.

[0066] The mechanical properties of the PVDF / PVA island-structured piezoelectric hydrogel of Example 1 were tested using a universal testing machine. Tensile strength, elongation at break, toughness, Young's modulus, compressive strength, and compressive modulus of the hydrogel samples were determined by stress-strain curves. Specific tensile test conditions were as follows: the hydrogel sample was cut into a dumbbell shape with a thickness of 1 mm, a length of 12 mm, and a width of 2 mm. The tensile strength, elongation at break, toughness, and Young's modulus were recorded at a speed of 50 mm / min. Specific compressive test conditions were as follows: a cylindrical hydrogel sample was prepared with a diameter of 10 mm and a height of 5 mm. The compressive strength and compressive modulus of the sample were recorded at a compression speed of 1 mm / min. The results are as follows: Figure 3 As shown, the Young's modulus of PVDF / PVA hydrogels increases with increasing PVDF content, while tensile strength, elongation at break, and toughness initially increase and then decrease. The highest tensile strength, elongation at break, and toughness are observed when the PVDF content accounts for 20% of the PVA content. For PVDF1 / PVA10 and PVDF2 / PVA10, the PVDF island microspheres in these two hydrogels are relatively small and evenly distributed. These uniformly dispersed PVDF microspheres effectively dissipate energy, resulting in high elongation at break and toughness. However, in PVDF3 / PVA10 and PVDF4 / PVA10, the PVDF microspheres are larger and aggregate due to their higher content, leading to a relatively uneven distribution. The larger and less uniformly distributed PVDF island phases are more prone to cracking in the gel network during stretching, resulting in greater fracture and poorer toughness. Compared to pure PVA hydrogels, PVDF improves the compressive strength of the hydrogel. With increasing PVDF content, the compressive strength and compressive modulus gradually increase. The specific mechanical properties of different PVDF / PVA hydrogels are shown in Table 2.

[0067] Table 2. Numerical values ​​of specific mechanical properties and piezoelectric coefficients of different PVDF / PVA hydrogels (mean±SD, n=3)

[0068]

[0069] To verify the piezoelectric properties of the PVDF / PVA hydrogel, the hydrogel was cut to the required size and shape (30 mm in diameter, 1 mm in thickness), ensuring a smooth cut surface and edges. Surface moisture was blotted dry with filter paper, but not dehydrated. Copper electrodes were attached to both the top and bottom surfaces of the hydrogel, and thin leads were attached to each electrode using conductive silver paste. The hydrogel generator with the leads connected was placed on a sheet of encapsulation film, and then another sheet of encapsulation film was placed on top, ensuring the leads extended from one side and that the encapsulation layer was in close contact with the hydrogel and electrodes. The two leads of the generator were connected to an electrometer, and different pressure stimuli were applied. The generated voltage peaks and waveforms were observed and recorded. The results are as follows: Figure 6 As shown, Figure 6 The open-circuit voltage signals generated by PVDF2 / PVA10 piezoelectric hydrogel under different pressures show that the voltage signal gradually increases and stabilizes with increasing pressure, reaching a maximum of approximately 120 mV. This demonstrates that PVDF / PVA piezoelectric hydrogel can effectively convert external mechanical stimuli into electrical signals.

[0070] The optimal piezoelectric output of PVDF2 / PVA10 piezoelectric hydrogel stems from its moderate microsphere size and uniform dispersion, which allows it to form a continuous stress transfer network within the PVA matrix. This structure enables efficient stress transfer through the PVDF island phases under stress, maximizing dipole rearrangement and achieving optimal electromechanical conversion efficiency. When PVDF accounts for more than 30% of the PVA content, the "island" size increases significantly and agglomerates, leading to localized stress concentration and a weakened piezoelectric response. A uniform spherical island phase structure maintains a uniform stress distribution within the matrix, with each island phase functioning as an independent piezoelectric response unit, synergistically enhancing the overall output performance of the material. Conversely, in R-PVDF2 / PVA10 piezoelectric hydrogels with a non-uniform island structure, PVDF agglomeration areas easily become stress concentration points, disrupting matrix continuity, reducing charge separation efficiency, and ultimately resulting in the worst piezoelectric output.

[0071] In addition, such as Figure 7 As shown, the piezoelectric coefficient of the PVDF2 / PVA10 piezoelectric hydrogel is 42.7 pC N. -1 The piezoelectric coefficient of the piezoelectric hydrogel in Comparative Example 1 is 11.8 pC N. -1 This demonstrates that the island structure significantly improves the piezoelectric properties of the hydrogel.

[0072] Application examples

[0073] Human Sensing Performance Testing of PVDF / PVA Island-Structured Piezoelectric Hydrogel

[0074] The encapsulated piezoelectric hydrogel PVDF2 / PVA10 from Example 1 was used as a wearable sensor to monitor daily activities and physiological signals. When applied to the finger joints, it can detect finger bending movements, as shown in the results... Figure 8 As shown, Figure 8 The results showed the piezoelectric signals generated when the finger was bent at 30°, 45°, 60°, and 90°. Furthermore, pulse monitoring was performed by applying the hydrogel to the radial artery on the wrist, and the results were as follows... Figure 9 As shown, a stable pulse signal is displayed, and each pulse wave can be distinguished into three signal peaks: shock wave (P1), tidal wave (P2), and diastolic wave (P3). This signal indicates that the volunteer's cardiovascular condition is good.

Claims

1. A flexible piezoelectric hydrogel of vinylidene fluoride polymer with an island structure, characterized in that, The marine phase consists of a three-dimensional continuous network formed by cross-linking of hydrophilic polymers as the continuous phase, and uniformly sized polymer microspheres formed by hydrophobic vinylidene fluoride polymers as the dispersed phase.

2. The flexible piezoelectric hydrogel according to claim 1, characterized in that, The dispersed phase (island phase) is non-aggregated and non-fused, and is embedded in the three-dimensional network of the continuous phase (marine phase), forming a stable island topology.

3. The flexible piezoelectric hydrogel according to claim 1, characterized in that... The size of the polyvinylidene fluoride polymer as the dispersed phase is 20-200 μm, preferably 50-100 μm.

4. The method for preparing the flexible piezoelectric hydrogel with island-shaped vinylidene fluoride polymer according to any one of claims 1-3, characterized in that, Includes the following steps: S1. Dissolve the hydrophilic polymer and the vinylidene fluoride polymer in a compounded organic solvent to form a pregel solution; the compounded organic solvent is a mixture of an aprotic polar organic solvent and a fluorinated alcohol; S2. Place the pregel solution described in S1 into a gel mold, and after a freeze-thaw process, an organic gel is formed. Collect the organic gel. S3. The organic gel described in S2 is placed in deionized water for solvent replacement, so that the organic gel is transformed into a hydrogel, and the polyvinylidene fluoride piezoelectric gel with island structure is collected.

5. The preparation method according to claim 4, characterized in that, In step S1, the hydrophilic polymer is selected from polymers containing multiple hydroxyl groups, such as polyvinyl alcohol (PVA), hydroxypropyl cellulose (HPC), ethyl cellulose (EC), hydroxypropyl methyl cellulose (HPMC), polyethylene glycol (PEG), sodium alginate, chitosan, hyaluronic acid, and gelatin, or one or more of these.

6. The preparation method according to claim 4, characterized in that, In step S1, the vinylidene fluoride polymer is a self-polymer of vinylidene fluoride, or a copolymer of vinylidene fluoride and other halogen-containing olefins, wherein the number of repeating units of vinylidene fluoride in the copolymer accounts for 50-80%, preferably 60-75%; specifically selected from one or more of polyvinylidene fluoride (PVDF), poly(vinylidene fluoride-trifluoroethylene), abbreviated as P(VDF-TrFE), poly(vinylidene fluoride-trifluorochloroethylene), abbreviated as P(VDF-CTFE), poly(vinylidene fluoride-hexafluoropropylene), abbreviated as P(VDF-HFP), poly(vinylidene fluoride-trifluoroethylene-trifluorochloroethylene), abbreviated as P(VDF-TrFE-CTFE), and poly(vinylidene fluoride-trifluoroethylene-trifluorochloroethylene), abbreviated as P(VDF-TrFE-CFE); Furthermore, the weight-average molecular weight of the vinylidene fluoride polymer is 3 × 10⁻⁶. 5 -1.5×10 6 g / mol, preferably 8×10 5 -1.2×10 6 g / mol.

7. The preparation method according to claim 4, characterized in that, In step S1, the mass ratio of the hydrophilic polymer to the vinylidene fluoride polymer is 10:1-3, preferably 10:1-2.

8. The preparation method according to claim 4, characterized in that, In step S1, the aprotic polar organic solvent is selected from at least one of dimethyl sulfoxide (DMSO), N,N-dimethylformamide (DMF), and N,N-dimethylacetamide (DMAc); the fluorinated alcohol is selected from at least one of hexafluoroisopropanol (HFIP), 2,2,2-trifluoroethanol, perfluorotert-butanol, and pentafluoropropanol; furthermore, the volume percentage of the fluorinated alcohol in the compound solvent is 10-20%.

9. The preparation method according to claim 4, characterized in that, In step S1, the amount of compound solvent used is such that the mass concentration of the hydrophilic polymer in the pregel solution is 7.5-15 wt%. And / or, in step S2, during the freeze-thaw process, the freezing temperature is -80 to -60 ℃, the freezing time is 12-24 h, the thawing temperature is 20-25 ℃, and the thawing time is 6-12 h; And / or, in step S3, the solvent replacement time is 48-72 hours, and the water content of the hydrogel obtained after solvent replacement is 70-80%.

10. Use of the flexible piezoelectric gel with an island-shaped structure of vinylidene fluoride polymer as described in any one of claims 1-3 as a human body sensor, including detecting human activity and pulse signal patterns.

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