Coaxial electrospinning is an advanced fabrication technique for producing core-shell nanofibers with distinct inner and outer layers. Unlike conventional electrospinning, which uses a single nozzle to produce homogeneous fibers, coaxial electrospinning employs a dual-nozzle design to simultaneously eject two different materials, forming a composite fiber with a core-shell structure. This method enables precise control over the fiber's morphology, composition, and functionality, making it suitable for applications requiring encapsulation or controlled release, such as drug delivery systems.
The setup for coaxial electrospinning consists of two concentric needles or capillaries, where the inner needle delivers the core material and the outer needle supplies the shell material. Both materials are fed through separate syringes and controlled by independent pumps to regulate flow rates. A high-voltage power supply generates an electric field between the nozzle and a grounded collector, stretching the polymer solutions into ultrafine fibers through electrostatic forces. The key to successful coaxial electrospinning lies in optimizing parameters such as solution viscosity, conductivity, flow rates, and voltage to ensure stable Taylor cone formation and uniform fiber deposition. For instance, the shell solution typically has a higher viscosity to encapsulate the core effectively, while the core solution must maintain compatibility to prevent phase separation.
Material compatibility is critical in coaxial electrospinning, as the core and shell solutions must form a stable interface without mixing or precipitation. Common core materials include hydrophilic polymers like polyethylene oxide (PEO) or polyvinyl alcohol (PVA), which can encapsulate bioactive molecules such as proteins, growth factors, or drugs. The shell material often consists of hydrophobic or biodegradable polymers like polycaprolactone (PCL), polylactic acid (PLA), or poly(lactic-co-glycolic acid) (PLGA), providing structural integrity and controlled degradation. Solvent selection also plays a crucial role; miscible solvents prevent interfacial instability, while immiscible solvents can lead to phase separation. For example, using water as a core solvent and organic solvents like chloroform or dimethylformamide (DMF) for the shell can facilitate stable core-shell formation due to their immiscibility.
The versatility of coaxial electrospinning allows for tailoring nanofiber properties to specific applications. In drug delivery, the core-shell structure enables sustained or triggered release of therapeutic agents. The shell acts as a diffusion barrier, controlling the rate at which the encapsulated drug is released into the surrounding environment. By adjusting the shell thickness or material, release kinetics can be modulated from days to weeks. For instance, PLGA shells degrade hydrolytically, releasing the core payload gradually as the polymer erodes. Additionally, temperature-responsive or pH-sensitive polymers can be incorporated into the shell to achieve stimuli-responsive release, enhancing therapeutic precision.
Encapsulation efficiency is another advantage of coaxial electrospinning. Unlike surface adsorption or blending, where drugs may leach out prematurely, the core-shell configuration protects sensitive molecules from degradation or burst release. This is particularly beneficial for proteins or growth factors that require prolonged activity in vivo. Studies have demonstrated that coaxial electrospinning can achieve encapsulation efficiencies exceeding 90%, with minimal loss of bioactivity due to the mild processing conditions compared to harsher methods like spray drying or emulsion techniques.
Beyond drug delivery, coaxial electrospinning finds utility in encapsulating volatile compounds, enzymes, or probiotics for food and industrial applications. The shell provides a protective barrier against environmental factors like moisture, oxygen, or heat, extending the shelf life of unstable compounds. For example, essential oils encapsulated in zein or chitosan shells exhibit enhanced stability against evaporation and oxidation. Similarly, enzymes immobilized within nanofibers retain catalytic activity while being shielded from denaturation.
The scalability of coaxial electrospinning remains a challenge, as maintaining uniformity across large production volumes requires precise control over process parameters. However, advancements in multi-jet systems and automated collectors are improving throughput without compromising fiber quality. Future directions may explore hybrid techniques combining coaxial electrospinning with other nanofabrication methods, such as electrospraying or 3D printing, to create hierarchical structures with multifunctional capabilities.
In summary, coaxial electrospinning offers a robust platform for fabricating core-shell nanofibers with tailored properties for encapsulation and controlled release. Its dual-nozzle design, coupled with careful material selection, enables precise engineering of nanofibers for diverse applications beyond biomedical uses. By leveraging the unique advantages of core-shell architectures, this technique continues to expand possibilities in drug delivery, functional textiles, and industrial encapsulation.