Process Parameters of High Voltage Power Supply for Coaxial Electrospinning Preparation of Hollow Fibers
Coaxial electrospinning extends conventional electrospinning technology to produce fibers with core sheath structures, enabling the fabrication of hollow fibers when the core material is subsequently removed. This technique employs concentric nozzles through which two different solutions flow simultaneously, with the outer solution forming the sheath and the inner solution forming the core. High voltage applied to the nozzle assembly creates the electric field that drives the jet formation and fiber solidification. The process parameters controlled by the high voltage power supply critically determine the fiber morphology and the success of hollow fiber formation.
The electrospinning process initiates when the electric field at the nozzle tip overcomes the surface tension of the solution, forming a Taylor cone from which a thin jet emerges. In coaxial electrospinning, the outer and inner solutions must both be drawn into the jet while maintaining the core sheath structure. The viscosity, conductivity, and surface tension of both solutions affect the jet stability and the resulting fiber morphology. The applied voltage determines the electric field strength and thus the electrostatic forces acting on the solutions.
Voltage magnitude affects the jet acceleration and the fiber diameter. Higher voltages create stronger electric fields that accelerate the jet more rapidly, typically producing finer fibers. However, excessive voltage can cause jet instability, leading to branching or beading defects. For coaxial electrospinning, the voltage must be appropriate for both the outer and inner solutions, which may have different electrical properties. The optimal voltage range depends on the solution properties, the nozzle geometry, and the working distance to the collector.
The solution flow rates for the outer and inner fluids must be coordinated to maintain the core sheath structure in the jet. The outer flow rate must be sufficient to encapsulate the inner fluid, but not so high that the core becomes too small relative to the sheath. The inner flow rate determines the core diameter, which becomes the hollow lumen diameter after core removal. The high voltage affects the flow rate requirements through its influence on the jet speed and the mass consumption rate.
Working distance, the gap between the nozzle tip and the collector, affects the flight time available for solvent evaporation and fiber solidification. Longer distances provide more time for drying, which is important for solutions with slower evaporation rates. The electric field strength for a given voltage decreases with distance, so longer distances may require higher voltages to maintain adequate jet acceleration. The working distance also affects the fiber deposition pattern on the collector.
Nozzle geometry for coaxial electrospinning includes the outer needle diameter, the inner needle diameter, and the relative positioning of the needle tips. The outer needle diameter affects the initial jet diameter and the fiber size. The inner needle diameter affects the core size. The needle tip alignment, whether the inner needle is recessed within, flush with, or protruding from the outer needle, influences the core sheath interface formation. These geometric parameters interact with the voltage and flow rates to determine the fiber structure.
Core removal to create hollow fibers can be accomplished through various methods depending on the core material. Dissolution in a selective solvent that removes the core without affecting the sheath is common when the core is a sacrificial polymer. Thermal decomposition can remove organic core materials at elevated temperatures. Extraction with supercritical fluids provides gentle removal without damaging the sheath structure. The core removal process must be complete to achieve true hollow fibers, but must not collapse or damage the sheath.
Fiber collection methods affect the fiber alignment and the resulting mat structure. Stationary collectors produce randomly oriented fiber mats. Rotating drum collectors align fibers in the direction of rotation. Patterned collectors with conductive and insulating regions can create ordered fiber arrays. The collector geometry and the electric field distribution between the nozzle and collector influence the fiber deposition pattern.
Process monitoring during electrospinning enables detection of instabilities and adjustment of parameters to maintain fiber quality. Optical observation of the Taylor cone and jet reveals the jet stability and the core sheath formation. Current measurement at the collector indicates the charge carried by the fibers, which relates to the jet characteristics. In situ imaging techniques can observe the fiber formation in real time, providing feedback for parameter optimization.
Scale up considerations for coaxial electrospinning include multiple nozzle arrangements and increased throughput. Multi nozzle systems can increase production rate but require careful design to avoid electric field interference between nozzles. Solution delivery systems must maintain consistent flow rates to all nozzles. The high voltage power supply must have adequate current capability to drive multiple jets. Process parameter optimization must account for the interactions in multi nozzle configurations.
