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Polyvinyl alcohol (PVA) is a synthetic polymer containing CH2CH(OH) repeating units. The vinyl monomers that make up the polymer are not prepared directly from vinyl alcohol due to their instability, leading to the tautomerization mechanism of acetaldehyde. It is obtained by polymerization of homopolymers consisting of protected monomer units such as vinyl acetate, vinyl esters or vinyl ethers. The first patent for the preparation of polyvinyl alcohol dates back to 1924 and was invented by W.O. Herrmann et al., who obtained a polyvinyl alcohol solution by saponifying polyvinyl alcohol esters with caustic soda solution. Since then, the production of this polymer has steadily increased to become a plastic material used in many applications, with an annual growth rate of more than 4% until 2030. PVA can be obtained by various synthetic routes, of which the method developed by Hermann and Haehnel is still the most commonly used. The process is based on the free radical polymerization of vinyl acetate followed by hydrolysis of the ester group in the strong base methanol. The physicochemical properties of the obtained product depend on the degree of polymerization and the hydrolysis kinetics, followed by precipitation, washing and drying. Vinyl ester monomers can also be used as PVA precursors in the same way as vinyl acetate. The homopolymer is obtained by cationic polymerization of vinyl ethers (CH2=CHOR) with Lewis acid catalysts and hydrolyzed under acidic conditions to give PVA. This process preferably leads to isotactic PVA.
The main point of this paper
Basic properties of PVA:
PVA is a synthetic and semi-crystalline polymer with multiple hydroxyl groups, known for its oxygen barrier, dyeing properties and mechanical strength.
The physicochemical properties of PVA are closely related to the preparation method, especially the hydrolysis state, which can be fully hydrolyzed or partially hydrolyzed.
Nomenclature and characteristics of PVA:
The nomenclature of PVA reflects its apparent viscosity value and degree of hydrolysis (DH) of a 4 wt.% solution at 20°C, such as PVA-10-98, which means a viscosity of 10 mPa and 98% hydrolysis.
The degree of hydrolysis affects the crystallinity, melting temperature, mechanical strength, solubility in water and compatibility with other excipients of PVA.
Application areas of PVA:
PVA is widely used in biopharmaceutical products (such as surgical sutures, contact lenses, wound dressings) and internal biomedical applications (such as artificial kidney membranes, articular cartilage) due to its unique properties.
In the textile industry, PVA is used as a thickener, surfactant, sizing agent, microcapsule membrane material, coating and water-soluble synthetic fiber.
Characteristics of PVA fiber:
PVA fiber is known for its moisture absorption and wear resistance, and also has potential biomedical applications, such as improving textile comfort, designing conductive structures, and as highly absorbent textile fabrics.
Electrospinning technology:
Electrospinning is a method for making nano- to micron-scale fibers by adjusting operating, process and formulation parameters.
The method is based on the electrostatic force to stretch the polymer solution to form continuous fibers, involving equipment such as high-voltage power supply, injection pump and collector.
Factors affecting electrospinning:
Process parameters such as collector type, distance, voltage, nozzle inner diameter and spinning solution flow rate will affect the physical properties of the fiber.
The physicochemical properties (viscosity, surface tension, conductivity) and chemical properties (molecular weight, concentration, solubility) of the solution also have an important influence on the electrospinning process.
Advances in electrospinning technology:
The latest advances in PVA electrospinning technology are reviewed, including process parameters (voltage, distance, flow rate, and collector), solution parameters (molecular weight and concentration), and environmental parameters (humidity and temperature) that affect the structure, mechanical, and chemical properties of PVA-based electrospinning matrices.
Applications of PVA electrospinning:
The importance of PVA electrospinning in biomedical applications, including wound dressings, drug delivery, tissue engineering, and biosensors, is emphasized.
PVA electrosprayed particle formation:
The new field of particle formation by PVA electrospraying is highlighted as a promising research direction.
Limitations and advantages of PVA matrices:
The limitations (e.g., reduced mechanical strength due to water solubility, limited biodegradability) and advantages (e.g., good biocompatibility, biodegradability, good mechanical properties, and compatibility with other materials) of different PVA matrices are reviewed.
Future research suggestions:
Suggestions for future research are proposed, including the exploration of new processes, systematic investigation of process and formulation parameters, polymer rheology studies, and control of interactions between compounds during the process.
Factors affecting the electrospinning process:
The increase in temperature will affect the electrospinning process, including accelerating the movement of the jet molecular chain, increasing the solution conductivity, reducing the solution viscosity and surface tension, thereby affecting the diameter and morphology of the nanofibers.
This article focuses on electrospun structures from polyvinyl alcohol (PVA) with potential applications in the biomedical field. PVA is a versatile polymer used as a basis for a wide range of electrospun structures. Recent papers have highlighted the process modifications required to achieve controlled morphology and encapsulation of active ingredients. Control of morphology depends on process parameters such as flow rate, applied voltage, needle size, distance between needle and collector, as well as formulation parameters such as molecular weight of polymer and drug, solution viscosity/volatility, surface tension and viscosity, taking into account the extent of hydrolysis of PVA, which plays an important role in the physicochemical properties of the solution.
Due to its hydrophilicity, biocompatibility, non-toxicity and mechanical strength, this polymer has been widely used in filtration, gas sensors, wound dressings, tissue engineering and scaffolds, drug delivery, and to a lesser extent cancer therapy. Its mechanical and structural properties can also be tailored to specific applications by blending with other polymers such as chitosan or introducing nano-objects.
In recent years, the versatility of the electrospinning process has enabled the modification of the resulting structural morphology to be consistent with the desired properties, such as promoting cell proliferation on the surface or controlling the release of active substances. The development of new processes associated with new morphologies or new surface states represents the most innovative research opportunity. However, the development of new processes based on electrospinning requires a systematic investigation of process and formulation parameters, including polymer rheology studies in polymer blends or polymer-active ingredient blend solutions to control the interactions between the compounds during the process.
