Page 25 - Tailoring Electrospinning Techniques for Regenerative Medicine - Marc Simonet

solution viscosity resulting in a higher average fiber diameter.35,40,44,45 This may be attributed to the greater resistance of the solution, when stretched in the jet.76 Additionally the jet instability occurs at a longer distance away from the needle, reducing the travelling jet path length and its stretching time.77
To initiate the electrospinning jet the electrostatic repulsion force, resulting from the same charge on the solution, needs to exceed the solution’s surface tension. The surface tension leads to a decreasing surface area, resulting in Rayleigh instability in the jet. This may cause the formation of droplets or beaded fibers.38,39 A solution with a higher viscosity can suppress this e ect resulting in the production of bead-free fibers. Another way to obtain such bead-free fibers is to add solvents with a low surface tension like ethanol.78 Adding a surfactant, soluble or not, may also help to spin bead-free fibers and to reduce the voltage required to initiate the jet.79
A solution with a higher conductivity or dielectric constant has a higher capacity to carry charges. Higher charges in the jet increase the repulsion forces and the stretching rate, and reduce the stable jet length, resulting in a reduction of the fiber diameter. Additionally, repulsion forces counteract surface tension, suppressing the Rayleigh instability and hence reduce the formation of beaded fibers.40–44 The charge carrying capacity of the solution can
be increased by adding salt or selecting a di erent solvent.40,44,80,81
2.5.2 Process parameter 1 At a fixed solution concentration, the flow rate
defines the polymer deposition rate. For a given
voltage there is a specific flow rate range that
maintains a stable Taylor cone and hence stable spinning process.25,45 Increasing the flow rate while maintaining a stable process, will lead to larger fiber diameters.40,82 Above a certain flow rate the fibers will not dry anymore between spinneret and target and, due to the remaining solvent, fuse together on the target.83
The applied voltage di erence induces the needed charges to the solution and when the electrostatic force overcomes the surface tension the electrospinning process is initiated. Most electrospinning work with a single nozzle spinneret has been done with 0.5 to 2 kV/cm spinning distance. Changing the applied voltage influences the fiber morphology. A higher voltage creates higher columbic forces and a stronger electric field, generally leading to smaller fiber diameters.36,46,47 However, above a certain voltage limit, depending on the given set of electrospinning parameters, the process becomes unstable. The coronal discharge e ect (electrical discharge brought on by the ionization of the fluid) and an imbalanced flow rate vs. applied voltage will lead to a heterogeneous fiber morphology.25,84–86 The electrospinning process is driven by the potential di erence between spinneret and target. This enables the use of a positive or negative DC voltage
GENERAL INTRODUCTION
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