1 Introduction
Hydrogels consist of a chemically or physically cross-linked 3D network of hydrophilic polymer chains, possessing the ability to absorb and retain large amounts of water
[1]. Due to their intrinsic chemical and structural similarities to biological tissues, such as high water content, mechanical softness, hydrophilicity, and excellent biocompatibility, hydrogels have been widely used in tissue engineering
[2], bioelectronics
[3-4], and biomedicine
[5-6], among others. As an advanced material combining mechanical properties with liquid transport characteristics, hydrogel fibers synergistically integrate the advantages of hydrogel materials and fiber structures, capable of absorbing and retaining large amounts of water while maintaining their structural integrity
[7], and have become one of the most extensively studied materials in interdisciplinary research fields
[8-9], with widespread applications in wound dressings, wearable devices
[10-11], optoelectronics
[12-16], sensors
[17], and energy batteries
[18], among other fields. For instance, hydrogel optical fibers with high transparency, excellent mechanical flexibility, and stretchability have been developed for optogenetic modulation
[12-14], biosensors
[17], and as tissue waveguides for photothermal therapy
[13]. Currently, various manufacturing strategies such as wet spinning and microfluidic spinning
[2]have been employed for the continuous and scalable production of hydrogel fibers, yielding fibers with diameters ranging from 100 to 3000 µm and maximum lengths reaching kilometers
[19]. Furthermore, by leveraging the properties of functional polymers and hydrogel materials, hydrogel fibers are engineered to possess tailored attributes, such as unique optical transparency, electronic conductivity, and responsiveness to specific triggers (i.e., heat, magnetic fields, or ultrasound)
[20], among others.
PVA is a water-soluble synthetic polymer material, with its molecular chains rich in hydroxyl groups (—OH)
[21], possessing excellent fiber-forming ability, tunable crystallinity, and hydrogen bond network structure, thus becoming an ideal matrix material for preparing gel fibers, showing broad application prospects in flexible electronics, biomedicine, and smart textiles
[22]and other fields
[23]. As a high-performance functional material, PVA-based gel fibers have attracted widespread attention in fields such as flexible electronics
[24], biomedicine
[25], and human-computer interaction due to their unique physicochemical properties. This material exhibits excellent mechanical properties, which can be achieved by improving the microstructure
[26], regulating crosslinking density
[27], and introducing nano-reinforced phases
[28](such as silica nanofibers
[29], cellulose nanocrystals, or carbon nanotubes
[30], etc.) to achieve a wide range of mechanical property adjustments, while also possessing outstanding fatigue resistance
[31], making them suitable for long-term dynamic monitoring in wearable devices
[32]. Secondly, PVA-based gel fibers possess intelligent responsiveness, capable of producing reversible swelling-shrinking behavior in response to external stimuli (such as temperature, pH
[33], humidity, and electric fields, etc.)
[34], giving them significant application value in biomimetic actuators and adaptive sensors. By introducing conductive fillers (such as polyaniline, graphene, or poly(3,4-ethylenedioxythiophene)-poly(styrene sulfonate), etc.) and soaking in ionic liquids
[35]and other methods, highly sensitive flexible strain sensors can be prepared for real-time monitoring of human movement and physiological signals (such as breathing, pulse, etc.). Furthermore, in the field of biomedicine, the porous structure of PVA-based gel fibers can load drugs or growth factors to achieve controlled release, significantly improving disease treatment efficiency
[36-37].
Zhou et al.
[38]introduced preparation techniques for hydrogel fibers and analyzed and summarized the advantages and disadvantages of various preparation methods. They focused on elaborating the application progress of hydrogel fibers in biomedical fields such as wound dressings, drug delivery, surgical sutures, and tissue engineering, and pointed out that hydrogel fibers used in biomedicine still face several issues (limited selection range of raw materials, difficulty in constructing complex structural hydrogel fibers, poor mechanical properties of hydrogel fibers, etc.). Li et al.
[32]introduced significant progress and breakthrough innovations in global hydrogel fiber research over the past three decades, deeply explored internal structural changes, factors affecting mechanical strength, reinforcement methods, and other key factors influencing their moisture absorption characteristics, and comprehensively outlined the development trends and huge potential applications of hydrogel fibers. Chen et al.
[39]centering on the material composition, structural design, functional construction, and application fields of hydrogel fibers, systematically reviewed the development context of hydrogel fibers, while meticulously analyzing their limitations and challenges, providing guidelines for constructing functionalized or biomimetic hydrogel fibers and their practical applications. Although many literature reviews have summarized the preparation methods and application scenarios of hydrogel fibers, no literature has focused on analyzing the crosslinking principles, preparation methods, and application fields of hydrogel fibers from the perspective of a specific raw material.
This paper aims to analyze the crosslinking methods, preparation methods, and application fields of PVA-based gel fibers, and to prospect future research directions for PVA-based gel fibers, providing a reference for further research and application of PVA-based gel fibers.
2 Crosslinking methods for PVA-based gel fibers
PVA-based gel fibers are widely used in biomedical, textile, and environmental fields due to their excellent hydrophilicity, biocompatibility, and mechanical properties. Crosslinking is a key method for improving the performance of PVA-based gel fibers; it enhances the mechanical strength, stability, and functionality of the fibers. Based on differences in crosslinking mechanisms, crosslinking methods for PVA-based gel fibers are mainly divided into two categories: physical crosslinking and chemical crosslinking.
2.1 Physically cross-linked gel fibers
Physical crosslinking refers to the process by which PVA molecular chains are interconnected to form a three-dimensional network structure through non-covalent interactions, such as hydrogen bonds, van der Waals forces, ionic bonds, coordination bonds, and chain entanglement. These physical interactions are weak and reversible; under certain conditions (such as changes in temperature or solvent), the crosslinked structure can undergo decrosslinking. Physical crosslinking methods for PVA-based gel fibers include: freeze-thaw cycling, stretching-induced crystallization, ionic crosslinking, and radiation crosslinking (
Figure 1).
The freeze-thaw cycling method involves subjecting a PVA solution to repeated freezing and thawing treatments. During freezing, ice crystals form in the solution water, causing local increases in PVA molecular chain concentration and bringing them closer together. Upon thawing, cross-linked networks are formed between molecular chains through physical interactions such as hydrogen bonding and chain entanglement.
[40]. Wang et al.
[41]fabricated fibers via freeze-thaw cycling, featuring an inner layer containing bacterial cellulose (BC)/PVA/glycerol (Gly) and an outer layer of bioactive PVA/silk microfibre (SMF) gel. Their tensile strengths reached 7.34–9.26 MPa longitudinally and 4.46–7.34 MPa circumferentially. Dong et al.
[42]mixed PVA, gelatin (Gel), Gly, poly(3,4-ethylenedioxythiophene)-polystyrene sulfonate (PEDOT:PSS), and sodium chloride (NaCl), then fabricated gel fibers through a simple freeze-thaw cycle. The fibers achieved an elongation at break of 613% and a tensile strength of 1.1 MPa. Guo et al.
[43]prepared a high-strength gel fiber composed of PVA and reduced graphene oxide (rGO) for in vivo monitoring via freeze-thaw cycling. The hydrogel fibers exhibited high tensile strength (8.34 ± 0.66 MPa) and elastic modulus (1.15 ± 0.10 MPa) (
Figure 2).
The freeze-thaw cycling method is simple to operate, requires no chemical reagents, and is environmentally friendly; the prepared PVA-based gel fibers exhibit high flexibility and certain mechanical strength, and the gel network structure can be regulated by the number of freeze-thaw cycles. However, the crosslinking process is time-consuming, usually requiring multiple cycles to achieve ideal crosslinking effects; the resulting gel fibers have poor crosslinking uniformity, are prone to stress concentration internally, which affects the overall performance of the fibers.
The stretching-induced crystallization method involves externally stretching PVA gel fibers to align the disordered PVA molecular chains along the stretching direction. The inter-chain distance decreases, enhancing interactions and forming crystalline regions. These crystalline regions act as physical cross-linking points, connecting molecular chains to construct a stable three-dimensional network structure, thereby achieving cross-linking. Luo et al.
[44]utilized the stretching method to prepare a PVA-based hydrogel fiber containing tea polyphenols (TPs) with excellent mechanical and shape memory properties. The stretched PVA/TP hydrogel fibers can rapidly recover from various temporary shapes and exhibit superior mechanical properties (13.82 MPa) (
Figure 3). Gel fibers prepared via the stretching-induced crystallization method are environmentally friendly (requiring no chemical cross-linkers), have tunable properties (precise control of crystallinity and mechanical performance through stretching parameters), and possess excellent mechanical strength (highly oriented molecular chains enhance strength). However, this method demands high equipment precision and process control, increasing costs and technical difficulty; the production process involves time-consuming multi-stage stretching, resulting in low efficiency and hindering large-scale manufacturing.
Ionic crosslinking is a method for preparing gel fibers by forming ionic bonds between ionic groups on polymer chains and metal ions (such as Ca
2+) to construct a three-dimensional network structure. This process involves mild conditions, and the ionic bonds are reversible, making them susceptible to environmental factors such as pH and ion concentration. Lin et al.
[45]used calcium chloride (CaCl
2) to regulate the multiscale structure of PVA hydrogel fibers, thereby achieving an amorphous crosslinked network and obtaining high fracture strain. Furthermore, the mechanical properties of gel fibers crosslinked via coordination bonds can be modulated by the type of ligand and the type of metal ion. Using higher-valence metal ions or increasing the concentration can enhance mechanical properties.
Wu et al.
[46]utilized zirconium ions (Zr
4+) with polyacrylic acid (PAA) and PVA to prepare a biomimetic hydrogel fiber with high mechanical performance and environmental tolerance. The prepared biomimetic hydrogel fibers exhibited high toughness (162.25±21.99 MJ/m
3), and maintained excellent mechanical properties at -40 °C and 30% RH (stress >50 MPa, strain >200%) (
Figure 4). The ionic crosslinking reaction conditions are mild and the operation is simple; the degree of crosslinking can be flexibly controlled by ion concentration, type, etc.; it endows the fibers with pH/ion responsiveness. However, structural stability is greatly affected by the environment (such as salt concentration, pH); gel fibers prepared with some heavy metal ions pose a risk of biological toxicity.
The radiation cross-linking method utilizes high-energy rays (such as gamma rays and electron beams) to irradiate PVA gel fibers, generating free radicals on the PVA molecular chains. The interaction of these free radicals initiates cross-linking reactions between the molecular chains, forming a three-dimensional network structure. Idris et al.
[47]utilized gamma-ray radiation cross-linking to prepare a bioactive layer of polypyrrole (Py)/PVA composite matrix and performed cladding modification on optical fibers, thereby improving their biocompatibility and mechanical stability. The radiation cross-linking process requires no chemical cross-linking agents, can be conducted at ambient temperature and pressure with high efficiency, and induces cross-linking on both the surface and interior of the gel fibers, effectively enhancing the overall performance of the fibers. However, radiation equipment is expensive with high investment costs, and there are certain safety risks requiring strict protective measures to ensure operator safety.
The physical crosslinking process is simple and does not require the use of toxic chemical crosslinkers, aligning with green and environmentally friendly concepts; the resulting PVA gel fibers exhibit good biocompatibility and degradability. Meanwhile, the physical crosslinking process does not alter the chemical structure of PVA molecular chains, effectively preserving their original properties. However, the network structure formed by physical crosslinking has poor stability and low crosslinking strength. Under conditions of high temperature, high humidity, or prolonged external force, crosslinking points are prone to dissociation, leading to a decline in the mechanical properties of the fibers, which limits their application in fields requiring high strength and stability.
2.2 Chemically cross-linked gel fibers
Chemical crosslinking is a process that utilizes chemical reactions to enable active groups (such as hydroxyl groups) on PVA molecular chains to react with crosslinking agents, connecting the PVA chains via covalent bonds to form a stable three-dimensional network structure. The high bond energy of covalent bonds imparts high stability to the network structure formed by chemical crosslinking.
[48]. The chemical crosslinking methods for PVA-based gel fibers include: aldehyde crosslinking, carboxylic acid crosslinking, boric acid crosslinking, and photo-crosslinking (
Figure 5).
Aldehyde crosslinking utilizes aldehyde compounds (such as formaldehyde, glutaraldehyde, etc.) where the aldehyde groups react with hydroxyl groups on the PVA molecular chains via acetalization to form covalent bonds, achieving crosslinking between PVA molecular chains. Huang et al.
[49]introduced the inorganic binder tetraethyl orthosilicate (TEOS) and the chemical crosslinker glutaraldehyde (GA) to prepare a chemically crosslinked PVA-based gel fiber. This minimized polymer crystal scattering (crystal size approximately 3.5 nm) and increased the refractive index (RI) of the hydrogel (
Figure 6).
Kim et al.
[50]Dialdehyde cellulose (DAC) was introduced as a crosslinking agent into the coagulation bath of PVA/lignin spinning dope, followed by wet spinning to prepare PVA-based gel fibers. Adding DAC, a bio-based aldehyde cellulose, to the fibers is environmentally friendly and enhances their water stability. Aldehyde crosslinkers exhibit high reactivity and fast crosslinking rates, enabling high degrees of crosslinking in short periods; by controlling the type, concentration, and reaction conditions of the aldehyde crosslinker, the degree of crosslinking and the properties of the gel fibers can be precisely regulated; the resulting PVA gel fibers possess good chemical stability. However, aldehyde crosslinkers such as formaldehyde are toxic and have pungent odors, posing health risks to operators, while residual aldehydes can cause environmental pollution.
Carboxylic acid crosslinking utilizes polycarboxylic acids such as citric acid and succinic acid to undergo esterification reactions with the hydroxyl groups of PVA, forming a crosslinked network through heat treatment. This reaction typically requires high temperatures (150~180 ℃) and an acidic catalyst to promote it. Nataraj et al.
[51]used citric acid as a green crosslinking agent to fabricate water-stable electrospun nanofibers from highly hydrophilic PVA. The crosslinked nanofibers exhibited improved water resistance, thermal stability, and biocompatibility, expanding their biological applications (
Figure 7). Carboxylic acid crosslinking agents are non-toxic and biodegradable; the crosslinked PVA possesses excellent heat resistance and dimensional stability; the introduction of carboxyl groups can improve material hydrophilicity. However, the reaction requires relatively high temperatures, resulting in high energy consumption; the crosslinking speed is slow, leading to long process cycles; excessive crosslinking may cause a decrease in material flexibility.
The boric acid crosslinking method is based on the complexation reaction between boric acid and the hydroxyl groups on the PVA molecular chains under certain conditions, forming borate ester bonds to achieve crosslinking of the PVA molecular chains. This crosslinking reaction exhibits a certain degree of reversibility; under different pH and temperature conditions, the borate ester bonds can dissociate and reform. Hong et al.
[52]added boric acid as a crosslinking agent to the PVA spinning solution, and then, after sufficient crosslinking between boric acid and PVA, added rare non-metallic attapulgite clay (ATT) as a reinforcing material. Attapulgite clay-reinforced and boric acid-crosslinked PVA-based gel fibers (PVA/B/ATT fibers) were prepared via wet spinning. The strength of these fibers reached 14.2±0.3 cN/dtex. The boric acid crosslinking reaction proceeds under mild conditions without requiring high temperature or high pressure, imposing low demands on equipment; the crosslinking process is reversible; boric acid and its derivatives are relatively non-toxic and environmentally friendly. However, the stability of borate ester bonds is significantly affected by environmental factors (such as pH and temperature); the controllability of the crosslinking reaction is poor, making it difficult to precisely regulate the degree of crosslinking.
Photocrosslinking is a free radical polymerization reaction between PVA molecular chains initiated by ultraviolet or visible light irradiation in the presence of a photoinitiator. Song Xiaoyan et al.
[53]utilized ultraviolet light irradiation to prepare photocrosslinked composite nanofibers from electrospun PVA-styrylpyridinium salt (PVA-SBQ)/glycidyl methacrylate-modified hyaluronic acid (GMHA) composite nanofibers. The nanofibers did not dissolve instantly upon water immersion after UV radiation, and their morphology became more distinct with increased irradiation time. The photocrosslinking process is rapid and efficient, completing within seconds; it is suitable for continuous production and large-scale manufacturing.
The cross-linked network structure formed by chemical cross-linking is stable with high cross-linking strength, significantly enhancing the mechanical properties, chemical stability, and solvent resistance of PVA gel fibers. However, chemical cross-linking typically requires the use of chemical cross-linking agents; some of these agents are toxic and may remain in the fibers, posing potential hazards to the environment and human health. Although it is difficult to completely remove residual cross-linking agents, chemical cross-linking remains favored by researchers.
[54-55]. Furthermore, the chemical cross-linking process may alter the chemical structure of PVA molecular chains, affecting their biocompatibility and degradability.
3 Preparation method of polyvinyl alcohol-based gel fibers
Different crosslinking methods determine the selection of subsequent preparation processes. Preparing PVA-based gel fibers requires a comprehensive consideration of the characteristics of both PVA and gel fiber materials to coordinate their properties and produce gel fibers with excellent performance. Current preparation methods for PVA-based gel fibers include: mold method, dip coating, electrospinning, wet spinning, and microfluidic spinning (
Table 1).
From
Figure 8it can be seen that: (1) The breaking elongation of gel fibers prepared using the mold method and wet spinning is mainly concentrated in the range of 300%~700%. However, the tensile strength of wet-spun gel fibers is slightly higher than that of mold-method gel fibers, and wet spinning can produce fibers with higher tensile strength (>50 MPa). The main reason is that the preparation of hydrogel fibers by wet spinning in the cited literature involves post-treatments such as drawing and annealing, which increase the crystallinity of the gel fibers, thereby enhancing their mechanical properties.
[56].
(2) Gel fibers prepared by electrospinning exhibit high tensile strength but low elongation at break (<150%). Electrospinning utilizes the force of a high-voltage electrostatic field to overcome the surface tension of a polymer solution or melt, thereby stretching and refining the material to form nanofibers arranged in either disordered or ordered patterns. However, due to the extreme fineness of the nanofibers, the resulting samples exist in the form of nanofiber membranes. Within these membranes, an intricate three-dimensional network structure is formed, characterized by high porosity and interconnectivity. Consequently, gel fibers produced via electrospinning possess certain tensile strength but demonstrate relatively low elongation at break.
[57].
(3) Gel fibers prepared by microfluidic spinning exhibit low strength but a wide range of elongation at break (150%~800%). Microfluidic spinning utilizes microfluidic chips to fabricate gel fibers with specific dimensions and structures; the differences in microfluidic chips and the preparation process determine the mechanical properties of the gel fibers.
[58].
3.1 Mold method
The mold method involves injecting a PVA solution into molds of specific shapes (such as polytetrafluoroethylene tubes, glass capillaries, or polyvinyl chloride tubes), where the solution forms gel fibers within the mold through physical or chemical crosslinking (e.g., freeze-thaw cycles or chemical crosslinkers). Wang et al.
[59]injected a PVA/silver nanowire (AgNWs) solution into polytetrafluoroethylene tubes of different diameters and, utilizing a synergistic process of freeze-thaw, salting-out, and drying annealing, successfully prepared PVA hydrogel fibers with ultra-high conductivity (approximately 958 S/cm) and excellent mechanical properties (tensile strength of approximately 6.2 MPa, elongation >300%, toughness of approximately 10 MJ/m
3). (
Figure 9). Shi et al.
[60]prepared flexible hydrogel fibers with excellent resistance to extreme environments by adding glycerol to a mixture of PVA and PEDOT:PSS, injecting it via syringe into glass capillaries with an inner diameter of approximately 1000 μm, and subsequently subjecting it to freeze-thaw cycles.
Liu et al.
[61]uniformly mixed PVA with NaCl and injected it into a polyvinyl chloride tube to prepare a robust, highly tough, and highly cycle-stable ionically conductive PVA hydrogel fiber. The fiber exhibited excellent strength (20 MPa), toughness (67.2 MJ/m
3), good conductivity (0.38 S/m), a high gauge factor of 1.9 (at 100%~300% strain), and excellent sensing stability (1200 cycles at 50% strain) (
Figure 10). Currently, most PVA gel fibers are prepared using the molding method
[62], which is simple to operate and has low equipment requirements; it can precisely control fiber dimensions and shapes, making it suitable for small-batch customized production. However, production efficiency is low, making continuous production difficult to achieve; fiber surface quality depends on mold precision, prone to issues such as bubbles and uneven surfaces, limiting its large-scale application.
3.2 Dip coating method
Dip coating is a method in which a substrate is immersed in a PVA solution and then withdrawn at a predetermined rate, allowing the solution to adhere to the substrate surface. Subsequently, through processes such as freeze-thaw cycles and crosslinking, a PVA gel coating is formed on the substrate surface. Typically, the coating thickness (ranging from nm to µm) primarily depends on the viscosity of the PVA solution. Substrates with poor hydrophobicity may result in discontinuous hydrogel coatings; this can be addressed via surface modification techniques, such as plasma treatment or chemical oxidation/grafting, to enhance surface hydrophilicity.
Liu et al.
[20]Hydrogel fibers were prepared by immersing oxygen plasma-treated silica fibers into a PVA solution. The PVA solution gelled during air drying, a process in which PVA chains compacted and formed nanocrystalline domains, serving as physical cross-linking sites for the final PVA hydrogel coating (
Figure 11).
Wu et al.
[30]successfully prepared PVA/PAA hybrid hydrogel fibers with high mechanical properties (tensile strength of 3.8 MPa, toughness of 13.7 MJ/m
3), excellent anti-freezing performance, and high sensitivity by covering the hydrogel fibers with a layer of multi-walled carbon nanotubes (MWCNTs). (
Figure 12).
The dip-coating process is simple and has low equipment costs; it can prepare gel fibers on substrate surfaces of various shapes, offering strong substrate adaptability and enabling multifunctional composites. However, the coating thickness is difficult to control precisely, and uniformity is poor.
3.3 Electrospinning
Electrospinning is a method where a PVA solution forms a charged jet under a high-voltage electric field; the jet is stretched and refined by electrostatic forces, solidifies after solvent evaporation, and finally deposits on a collecting device to form nanoscale PVA gel fibers. Li et al.
[63]Using PVA as the substrate and curcumin and indigo as mixed indicators, nano-indicator fibers were prepared via electrospinning technology for freshness detection.
Zhao et al.
[64]prepared hydroxylated boron nitride nanosheet/waterborne polyurethane/PVA composite nanofibers with good thermal conductivity via electrospinning. They exhibit excellent thermal conductivity (thermal conductivity of 0.629 W/mK), outstanding hydrophilicity, and good mechanical properties (tensile strength and elastic modulus of 5.87 and 35.22 MPa, respectively) (
Figure 13). Valarmathi et al.
[57]prepared chitosan (CS)/PVA/Gel and copper-strontium substituted hydroxyapatite (Cu-Sr-HAP) composite nanofibers using the electrospinning method. Their tensile strength reached 65.45 ± 1.27 MPa, and the elongation at break reached 121.28% ± 1.76%.
Tavakoli et al.
[65]prepared core-shell nanofibers containing a PVA core and a gelatin (Gel) shell with advanced platelet-rich fibrin (A-PRF) using coaxial electrospinning. The tensile stress of the PVA/(Gel/A-PRF) core-shell nanofibers was 7.43 ± 0.38 MPa, and the elastic modulus was 102.05 ± 9.36 MPa (
Figure 14).
Electrospinning can prepare nanoscale ultrafine fibers with a large specific surface area; the fibers have high porosity and controllable structure, which can endow them with unique properties; fiber morphology and structure can be altered by adjusting parameters. However, production efficiency is low and output is small; there are high requirements for solution viscosity; high-voltage equipment is required, posing safety hazards, making large-scale industrial production difficult.
3.4 Wet spinning
Wet spinning involves preparing a PVA spinning solution, which is extruded through a spinneret into a coagulation bath (containing sodium citrate, sodium sulfate
[66-67]and ammonium sulfate
[68-69], etc.). In this process, the solvent in the solution undergoes diffusion and exchange with the coagulation bath, causing the PVA solution to coagulate and form gel fibers. Using wet spinning to prepare gel fibers offers significant advantages, enabling continuous production of fibers with diameters ranging from tens to hundreds of micrometers and maximum lengths reaching several kilometers. Nie et al.
[70]introduced the antifreeze salt CaCl
2into the PVA solution, which was then injected into a liquid nitrogen coagulation bath to obtain polymer chains with uniform structure but weak cross-linking. Subsequently, water evaporation induced densification of the polymer network, further stabilizing the gel fibers. These fibers exhibit excellent mechanical strength (5.99 MPa), elongation (600%), toughness (14.2 MJ/m
3), high transparency (84%), freeze resistance (-196 ℃), weavability, and long-term stability (
Figure 15). Wu et al.
[71]successfully prepared PEDOT:PSS/PVA/Ag composite conductive fibers using wet spinning technology. With the addition of Ag particles to the composite fibers, the surface resistance of the fibers decreased, while the tensile strength reached 40 MPa and the elongation at break achieved 44.4%.
Zhou et al.
[56]injected the PVA spinning solution into an ethanol coagulation bath, then transferred it to a sodium citrate solution for solvent exchange, followed by stretching and annealing treatment to obtain high-strength (141.67 MPa), tough (157.93 MJ/m
3), and high water content (40.6%) hydrogel fibers (
Figure 16).
Lou et al.
[72]Lou et al. [72] prepared gel fibers with a core layer of hydroxypropyl methylcellulose (HPMC) and PVA and a PVA shell layer using continuous coaxial wet spinning. The fibers were then immersed in a NaCl solution to produce PVA@PVA-HPMC hydrogel fibers, which exhibit significant tensile strength (6.7 MPa), elongation at break (450%), excellent conductivity (9.23 S/m), and outstanding freeze resistance (below -20 °C). Wet spinning offers high production efficiency and enables continuous manufacturing; it can produce large-diameter, high-strength fibers suitable for industrial applications, and fiber structure can be controlled by adjusting coagulation bath composition and spinning parameters. However, the preparation process requires large amounts of solvents and coagulation baths, resulting in high energy consumption for subsequent treatment; solvent recovery is difficult, posing risks of environmental pollution.
3.5 Microfluidic spinning
Microfluidic spinning is a method for preparing PVA gel fibers by utilizing microchannels generated in a microfluidic chip to interact PVA solution with other fluids (such as immiscible liquids), forming a uniform and stable liquid stream that is subsequently solidified.
[58,73]. By controlling the concentration and viscosity of the precursor solution, flow rate, coagulation strategy, and channel geometry of the microfluidic chip, fibers with controlled dimensions and structures can be produced. Wang et al.
[74]utilized microfluidic spinning technology to prepare TEMPO-oxidized cellulose nanofiber-graphene nanoplatelet/PVA-sodium alginate (SA)-tannic acid (TA) (abbreviated as TOCN-GN/PVA-SA-TA or TGG) composite gel fibers, achieving a tensile strength of 0.96 MPa, an elongation at break of 150%, and a conductivity of 2.66 S/m, while also demonstrating enhanced energy dissipation and fatigue resistance.
Zhao et al.
[75]prepared antibacterial PVA gel microfibers containing metal-organic framework particles with Ag
+(AMOF), Zn
2+(ZMOF), and cellulose nanocrystals (CNC) via microfluidic spinning, exhibiting a tensile strength of 4.78 MPa, excellent elongation at break of 768.3%, as well as good antibacterial and antioxidant properties (
Figure 17). Microfluidic spinning allows precise control over fiber size and structure, enabling the fabrication of uniform fibers; it facilitates the integration of multiple functions to produce fibers with special structures and properties. However, the equipment is sophisticated and costly; microchannels are prone to clogging, requiring high solution purity and strict operational conditions; production throughput is low, making it difficult to meet large-scale manufacturing demands.
3.6 3D Printing
3D printing is a method that precisely controls the position and shape of PVA gel material according to a computer-designed three-dimensional model through layer-by-layer deposition, ultimately forming PVA gel fiber products. Since Chuck Hull invented stereolithography (SLA) in 1984, 3D printing has undergone nearly 40 years of development to form a complete technical system. This technology breaks through the limitations of traditional subtractive manufacturing by directly constructing three-dimensional entities from digital models, bringing revolutionary changes to the manufacturing industry.
[76]. It is mainly applied in industrial manufacturing
[77], biomedical healthcare
[78-79], and the construction sector
[80-81]. Current bottlenecks include limited material performance, relatively slow printing speeds, and complex post-processing techniques
[82]. With the integrated development of technologies such as materials science and digital twins, it will play a greater value in fields like personalized customization and distributed manufacturing
[83].
Wang et al.
[84]utilized 3D printing technology to construct patterned electronic skins from PVA-based conductive hydrogel microfibers. The microfibers exhibited excellent mechanical properties (tensile strength of 85 kPa, elongation at break of 440%) and conductivity (electrical conductivity of 2.71 S/m). The resulting electronic skin demonstrated impressive stretchability and sensitivity (
Figure 18). Cui et al.
[85]combined electrospinning and 3D printing technologies to fabricate finer and more uniform nanofibers based on PVA and CS. These fibers showed good catalytic efficiency in immobilizing xylanase for corn straw degradation. 3D printing enables complex structural design, allowing for the preparation of personalized and customized fiber products; it requires no molds, has short production cycles, and can precisely control fiber spatial structure and performance distribution. However, it suffers from slow printing speeds and low production efficiency; it imposes high requirements on material flowability and curing properties; and equipment costs are high. Furthermore, printing precision is limited by both equipment and materials, posing obstacles to large-scale application.
4 Application of Polyvinyl Alcohol-Based Gel Fibers
PVA-based gel fibers prepared by different fabrication processes exhibit broad application prospects in fields such as biomedicine, flexible electronics, smart textiles, self-healing materials, and supercapacitors, owing to their excellent biocompatibility, high water absorption, and tunable mechanical properties.
4.1 Biomedicine
The applications of PVA-based gel fibers in biomedicine mainly include wound dressings, drug delivery, and tissue engineering.
Ideal wound dressings must combine passive and active protection to promote wound healing. Passive protection means it can be flexibly applied to the injured area, reducing contact between the wound and dust or bacteria in the air, and preventing secondary damage caused by external physical friction. In terms of tensile properties and elastic modulus, most PVA-based hydrogel fibers are suitable for skin
[86]. Active protection refers to being equipped with antibacterial and anti-inflammatory functions, capable of slowly releasing drugs to kill bacteria or promote wound healing
[87].
PVA-based gel fibers possess a porous three-dimensional network structure capable of efficiently absorbing wound exudate while maintaining an optimal moist environment to promote wound healing. By incorporating antimicrobial agents (such as silver nanoparticles, chitosan, or antibiotics), they can be endowed with anti-infective properties to reduce bacterial proliferation. Furthermore, the flexibility and breathability of PVA-based gel fibers allow them to conform to the skin, minimizing secondary injury during dressing changes.
[88]. Ju et al.
[89]replaced antibiotics with anti-Pseudomonas aeruginosa Nekophages (highly specific viruses capable of killing drug-resistant bacteria) and mixed them with PVA solution, encapsulating them into coaxial nanofibers via electrospinning. Phage-loaded electrospun nanofibers enable stable long-term storage, rapid release (within 10 min), and sustained release (over 2 h) of phages, demonstrating potential for developing wound dressings to treat antibiotic-resistant infections. Liu et al.
[90]developed a novel fiber-type medical dressing with excellent biocompatibility, pH monitoring capability, controlled drug release, and fluid absorption capacity using sodium alginate (SA), PVA, and curcumin (CUR) as raw materials, based on wet spinning and weft knitting technologies. CUR is a natural diketone compound extracted from turmeric; in addition to its antibacterial and anti-inflammatory effects, it is also a pH-sensitive substance that exhibits different colors under varying pH conditions. This gel fiber fabric demonstrates good swelling properties, drug retention function, breathability, moisture permeability, and biocompatibility with biological tissues.
Ding et al.
[91]Based on microfluidic spinning technology, a novel fibrous medical dressing with excellent breathability, antibacterial, and hemostatic properties was prepared using SA, microcrystalline cellulose (MCC), PVA, and Euphorbia. It exhibits good swelling performance (swelling mass ratio of 794% ± 7.3%), reasonable water vapor permeability (1311.30 ± 14.6 g/m
2·24 h) and porosity (>70%), which are beneficial for wound healing. In addition, the dressing shows good antibacterial activity against Escherichia coli and Staphylococcus aureus (
Figure 19).
The high specific surface area, good biocompatibility, tunable porous structure, and ease of encapsulating hydrophilic drugs make PVA-based gel fibers ideal drug carriers
[92]. Drug loading capacity and delivery rate are primarily related to the microstructure of hydrogel fibers
[93]. The specific microstructure enables hydrogel fibers to respond to changes in the wound microenvironment or the production of certain substances. Zhang et al.
[94]developed a rapidly dissolving drug delivery system using PVA nanofibers based on natural deep eutectic solvents (DES) embedded with honey and acetylsalicylic acid. These nanofibers exhibit good biocompatibility, rapid dissolution in artificial saliva (suitable for oral drug delivery requiring fast dissolution), and accelerated wound healing (increasing the skin healing rate in rats six days after surgery to 85.2%) (
Figure 20).
Afzal et al.
[95]utilized wet spinning technology to extrude a solution containing PVA/polycaprolactone (PCL) and a simulated drug into a coagulation bath for solidification, followed by washing and drying to prepare PVA/PCL fibers. The fibers exhibit excellent tensile properties (elongation at break ranging from 46.61% to 102.73%), sufficient liquid absorption capacity (approximately 15 g/g water absorption in distilled water for gel fibers with 20% PVA and 6% PCL concentrations), swelling ratio (approximately 108% swelling in distilled water for gel fibers with 20% PVA and 6% PCL concentrations), and degradation rate (approximately 15.4% degradation for gel fibers with 20% PVA and 6% PCL concentrations). Furthermore, they demonstrate good antibacterial activity (inhibiting Staphylococcus aureus), controllable drug release profiles for the model drug, and are suitable for drug delivery applications.
The structure of PVA-based gel fibers is similar to that of the natural extracellular matrix (ECM), providing an ideal microenvironment for cell adhesion, proliferation, and differentiation. By compounding with collagen
[87], nanofibers
[96], and hydroxyapatite
[97]and other materials, their bioactivity and mechanical properties can be enhanced, making them suitable for the regeneration of tissues such as skin, cartilage, and blood vessels
[98]. Abdelaziz et al.
[99]incorporated flaxseed extract rich in polyphenolic compounds into PVA nanofibers prepared by electrospinning to improve their application in bone tissue engineering, promote osteoblast function, and significantly upregulate gene expression in osteoblast-like cells, demonstrating their strong potential for bone tissue regeneration (
Figure 21).
Gohari et al.
[100]prepared PVA-based gel fibers loaded with human adipose-derived stem cells (ADSCs) via microfluidic spinning and enzymatic crosslinking. These fibers exhibit certain mechanical properties (tensile strength of 413 kPa, elongation at break of 410%), ideal cell viability (very few dead cells), and growth capacity (at day 14, the mitochondrial activity of cells embedded in the gel fibers was more than three times higher than that in non-crosslinked gel fibers). The encapsulated ADSCs were able to proliferate and grow within the gel fibers, forming filamentous cellular structures in both single and bundled configurations, which can be used for filamentous tissue engineering (
Figure 22).
4.2 Flexible Electronics
Flexible electronic devices designed based on PVA hydrogel fibers primarily rely on pressure, chemical, and temperature
[101]. Pure PVA gel fibers have low conductivity; to improve the conductivity of gel fibers, researchers introduce carbon nanoparticles (graphene), carbon nanotubes, and MXenes
[84], nano-metal particles
[102]and conductive components such as polypyrrole into the PVA gel fiber matrix, thereby improving their conductivity, sensitivity, and stability.
Sun et al.
[103]developed an all-polymer conductive ultrafine fiber (PU@PVA-PEDOT:PSS SI-CF) with ideal performance using microfluidic spinning technology, constructing a strain-insensitive conductive polymer hydrogel core with a semi-interpenetrating network into a helical structure. The prepared ultrafine fibers exhibit excellent stretchability (up to 500%), high conductivity (147 S/cm), ultra-low conductivity strain insensitivity (only 5% resistance change at 100% strain), and superior durability (2000 stretching-releasing cycles), showing great potential for applications in flexible wearable electronics as stretchable wires, self-powered sensors, and electrothermal heaters (
Figure 23). Xue et al.
[104]prepared a PVA-based gel fiber with high strength, high wear resistance, and degradability for various green energy applications via electrospinning. Through the interaction of PVA with Zr
4+, polyvinylpyrrolidone, and CS, the mechanical properties of the gel fiber were improved (tensile strength of 6.7 MPa), and it was applied to biodegradable triboelectric nanogenerators, self-driven humidity sensors, and passive daytime radiative cooling.
4.3 Smart textiles
Smart textiles have received widespread attention in recent years. PVA-based gel fibers possess excellent flexibility and conductivity (especially when compounded with carbon nanotubes, graphene, or conductive polymers), and are widely used in fields such as electronic skin and human-computer interaction, engaging in health monitoring, disease diagnosis, and treatment processes.
[105]. The resistance of conductive hydrogel fibers changes under pressure deformation, temperature, or environmental variations, which forms the basis for their application as fiber-based smart textiles.
[106-107]. For example, Cheng et al.
[108]prepared stretchable sensors by wet spinning PVA/waterborne polyurethane/MXene into a CaCl
2coagulation bath. They exhibit excellent mechanical and electrical properties (with a gel fiber sensitivity of 5.61 when the elongation at break exceeds 358%) and can be woven into various fabric forms according to actual requirements.
Furthermore, when salt ions are introduced into the PVA network, they can impart ionic conductivity to the hydrogel fibers
[69,109-110]. Many researchers have combined binary solvents with PVA gel fibers to overcome the limited use of ionically conductive hydrogel fibers under low-temperature conditions, preparing cold-resistant electronic skins and sensors with excellent performance
[111-112]. Hu et al.
[113]utilized strategies of wet spinning, solvent exchange, and multi-level hydrogen bond enhancement to construct continuous PVA co-gel fibers, achieving the continuous production of fibers with high strength (37.3 MPa), high strain (>700%), high conductivity (0.543 S/m), and high environmental stability (excellent fiber stability under extreme dryness and low temperatures of -60 °C), demonstrating potential in wearable strain sensors and physiological signal monitoring (
Figure 24). Zhu et al.
[114]prepared oxidized cellulose nanofiber-MWCNT/PVA-SA-TA hydrogel fibers via simple microfluidic spinning. The prepared fibers exhibited excellent mechanical properties (tensile strength of 8.06 MPa and breaking strain of 438%). They demonstrated significant conductivity (1.57 S/m) and anti-freezing properties at -20 °C. Sensors based on these gel fibers feature a wide detection range (0~250%), high sensitivity, fast response time (120 ms), and outstanding fatigue resistance (500 cycles), successfully detecting human motion and holding great potential for applications in the field of smart wearable textiles.
4.4 Self-healing materials
The dynamic hydrogen bonds and reversible cross-linked structures in PVA molecular chains endow them with self-healing capabilities. When the material suffers mechanical damage, its molecular chains can recombine to restore original performance.
Ideal self-healing materials should be able to restore their original properties relatively quickly
[115]. The self-healing ability of PVA-based hydrogel fibers mainly relies on the reversible breaking and reformation of dynamic covalent bonds within the molecular chains, as well as dynamic hydrogen bonds
[116]. When the material suffers mechanical damage, its molecular chains can recombine to restore original properties. To date, reversible chemical bonds (including disulfide bonds
[117-118], boronic ester bonds
[105], Diels-Alder reactions, and reversible free radical reactions) and non-covalent interactions (including hydrophobic interactions, host-guest interactions, hydrogen bonds, and various weak molecular interactions) have been used to fabricate self-healing hydrogel fibers.
Zhu et al.
[119]prepared a MOF-based PVA hydrogel fiber random laser capable of withstanding extreme damage. Due to the dynamic chemical bonding within the PVA hydrogel fibers, the MOF-based optical fiber random laser exhibits excellent self-healing capabilities (even complete fractures can recover their initial morphology and performance) (
Figure 25). Ma et al.
[120]used PVA fibers as raw materials to prepare highly water-absorbent gel fibers through surface hydrophilic modification. After surface modification of the PVA fibers, hydrophilic functional groups such as sodium carboxylate were successfully introduced onto the fiber surface, increasing both the surface roughness and the contact between these hydrophilic groups and the cement matrix. The water absorption performance of the fibers was improved (increased by 53.6 times), thereby promoting continuous hydration reactions of cement at crack locations. This enhanced the water absorption rate (74.1 times) and water retention rate (93%) of the modified fibers, achieving recovery of mechanical properties (increased compressive strength) and self-healing of internal cracks (with a recovery rate higher than that of ordinary cement).
4.5 Supercapacitor
PVA-based gel fibers possess high specific surface area and excellent ion transport capabilities, making them suitable as electrodes or solid-state electrolytes for supercapacitors. By incorporating conductive materials (such as polyaniline, MXene, or reduced graphene oxide), their electrochemical performance can be significantly enhanced, achieving high energy density and rapid charge-discharge rates, which are applicable to flexible energy storage devices.
Xu et al.
[121]prepared a hydrogel fiber-based flexible supercapacitor wrapped with a fluffy and soft sensitive film made of reduced graphene oxide modified cotton fiber (rGCF). It exhibits excellent swelling resistance, tensile properties (elongation at break up to 300%, tensile strength of 1 MPa), excellent specific capacitance (618 F/g), and high energy density (37.3 Wh/kg) (
Figure 26). Yu et al.
[122]prepared MXene/PVA fibers with good stretchability and electrochemical performance via wet spinning for fiber-based supercapacitors. As an efficient energy storage device, it possesses high gravimetric capacitance (119.3 F/g) and areal capacitance (130.9 mF/cm
2), demonstrating strong durability under various mechanical states and stable electrochemical behavior after stretching.
5 Conclusion and Outlook
As a multifunctional material, PVA-based gel fibers demonstrate broad application prospects. They combine the functionality and biocompatibility of hydrogel matrices with the advantages of high production efficiency and wide-ranging applications inherent to fibers.
This review first classifies the crosslinking methods of PVA-based gel fibers, primarily dividing them into physically crosslinked and chemically crosslinked gel fibers. The physical crosslinking process is simple and does not require toxic or harmful chemical crosslinkers; however, the resulting gel fiber network structure exhibits poor stability and low crosslinking strength, limiting its application in fields demanding high strength and stability. Chemically crosslinked gel fibers possess a stable network structure and high crosslinking strength, but they typically require chemical crosslinkers, some of which are toxic and pose potential hazards to the environment and human health. Different crosslinking methods determine the selection of subsequent preparation processes. This paper summarizes various methods for preparing PVA-based gel fibers: (1) The mold method is simple to operate but has low production efficiency, and the surface quality of the prepared gel fibers depends on mold precision, making large-scale application difficult; (2) The dip-coating method involves surface treatment on fiber substrates with a simple process, strong substrate adaptability, and the ability to achieve multifunctional composites, but coating thickness is difficult to control precisely and uniformity is poor; (3) Electrospinning can prepare nanoscale ultrafine fibers, altering fiber morphology and structure by adjusting parameters, but requirements for solutions and equipment limit its large-scale industrial production; (4) Wet spinning offers high production efficiency and continuous production capabilities, enabling the fabrication of large-diameter, high-strength fibers suitable for industrial applications, but it entails high energy consumption for post-processing; (5) Microfluidic spinning allows precise control over fiber size and structure, achieving multifunctional composites and preparing fibers with special structures and properties. However, it requires sophisticated equipment, microchannels are prone to clogging, production throughput is low, and it struggles to meet large-scale production demands; (6) 3D printing enables complex structural design and the fabrication of personalized fiber products, allowing precise control over fiber spatial structure and performance distribution. However, it suffers from slow printing speeds, low production efficiency, high requirements for material flowability and curing properties, high equipment costs, and obstacles to large-scale application. Finally, the applications of PVA-based gel fibers prepared by different processes in fields such as biomedicine, flexible electronics, smart textiles, and supercapacitors are discussed in detail.
Currently, although significant progress has been made in the synthesis methods and functional exploration of PVA-based gel fiber materials, their transition from the laboratory to practical applications still faces multiple challenges. Structurally, homogeneous and disordered monopolar network designs cannot simultaneously dissipate energy effectively, trapping them in the dilemma of the "strength-ductility" contradiction. In terms of performance, issues such as solvent evaporation, mechanical fatigue, and environmental sensitivity severely constrain their long-term stability. Regarding applications, laboratory methods like wet spinning, stretch forming, and electrospinning struggle to achieve continuous, high-speed, and stable production on a kilometer scale or beyond, hindering their industrialization. Looking ahead, breaking through these bottlenecks urgently requires a transformation from concept to technology. Future research should: (1) Focus on resolving the strength-ductility contradiction through bionic hierarchical structural design (e.g., constructing rigid-flexible systems reinforced with nanofibers) and dynamic cross-linking networks (e.g., introducing dynamic non-covalent bonds such as ionic coordination bonds, hydrogen bonds, and host-guest interactions into stable networks formed by covalent bonds). (2) Commit to achieving intelligent and stable performance of PVA-based gel fibers. For instance, interface regulation can be performed via encapsulation strategies and intrinsic anti-drying strategies to enhance the long-term stability of gel fibers. Simultaneously, efforts should be dedicated to making PVA-based gel fibers biodegradable and environmentally friendly by introducing hydrolyzable or enzymatically degradable units such as ester bonds and peptide bonds, enabling controlled degradation at the end of their service life to meet environmental requirements; furthermore, toxic organic solvents should be abandoned in favor of "green" processing systems using water, ionic liquids, or supercritical carbon dioxide as media. (3) Devote itself to interdisciplinary integration and innovation. This involves weaving highly conductive and highly stretchable PVA-based gel fibers into stretchable conductors or bio-electrodes, and interlacing or compounding gel fibers with traditional textile fibers (cotton, polyester) to develop smart fabrics with functions such as temperature regulation, humidity control, luminescence, or color change. Ultimately, through these "structure-performance" integrated innovations, there is confidence that the next generation of high-performance gel fibers can be precisely applied in fields such as smart wearable devices, human-computer interaction, and biomedical engineering, realizing a substantial leap from basic research to industrial value.
Key Projects in the Field of Social Development under the Key R&D Program of Ningxia Hui Autonomous Region(2022CMG02006)
Tianjin Key Research and Development Project(20YFZCSN00130)
Achievement Transfer and Commercialisation under the Heilongjiang Provincial Central Guidance Fund for Local Science and Technology Development(ZY23CG35)
2024 Jixi City Golden Rooster Action Talent Scheme(JJXDCXLJ2024H04)
Technology Project of Shaoputou District of Zhongwei City
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