Page 26 - Tailoring Electrospinning Techniques for Regenerative Medicine - Marc Simonet

CHAPTER 1
source to influence which polymer groups are more prone to be on the fiber surface and thereby change for example hydrophobicity or bioactivity of the electrospun mesh.48–50 Using an AC voltage source limits the residual charges on the jet, resulting in fibers with larger diameter. Additionally, having less charge build up on the target, allows to spin for prolonged time periods and hence overall thicker sca olds can be produced.87
Most research equipment uses a single nozzle as a spinneret. To combine polymer properties, to create fibers with specific drugs or bioactive cues, to create hollow fibers or to electrospin polymers or materials which cannot be electrospun on their own, coaxial51–53 or side-by-side54,55 spinnerets can be used. Compared to the other spinnerets, a single nozzle o ers a lot of flexibility and is simple to use. The polymer flow can be tightly adjusted and controlled by static fluid pressure, a pressurized gas system or by a syringe pump. This in contrast to a needleless system, where the flow is mainly set by an interplay of solution properties, equipment design and applied voltage.56,57 This makes the process more di icult to control, though needleless systems have in general a much higher production rate than nozzle based systems; few g/h for a single nozzle vs. up to kg/h for needless devices.58,59
The electrospun fibers can be collected on a solid target, in liquid or in the air. Selecting the design of
k Discussion at conferences and meetings and my own experiences 24
the target depends on the intended application and desired mesh morphology. Setups to electrospin in liquids or air are o en used for fiber bundles or yarn production,30,31,60–62 whereas solid targets enable a direct fabrication of 3D sca olds such as for example a blood vessel or a heart valve.24,63
Varying the distance between spinneret and target changes the electric field strength and the jet flight time, which both directly influence the fiber diameter. In general an increase in the spinning distance decreases the fiber diameter.64,65 Since the fibers dry during flight, decreasing the distance can lead to fused fibers. This may give additional mechanical strength to, but also reduces the porosity of the electrospun mesh.47,83,88
2.5.3 Ambient parameters
The relative humidity and temperature surrounding the electrospinning process influence the fiber morphology and the process stability. Their influence on the process stability and hence reproducibility was, at least partially, held responsible for discussions that electrospinning was an art rather than a science.9 It was well known in the electrospinning community that for instance spinning in the morning or winter can result in a di erent mesh and fiber morphology compared to the a ernoon respectively summer; if spinning was at all possiblek. During the last few years, these reproducibility issues have