Bead Formation in Electrospinning

Beads need not be always spherical or spindle like collapsed shapes and prune-like beads (Shawon and Sung 2004) have been described. When a particularly volatile solvent such as THF is used to electrospin PS from 13 wt% solution, cup-shaped beads, likely resulting from the collapse of thin-walled spherical beads, were observed. Even more complex bead morphologies, such as porous cups with PMMA/acrylonitrile (8wt%) (Liu and Kumar 2005) or raisin-like morphologies with polycarbonate/(THF/CHCl3 1 1) (14-15 wt%) (Krishnappa et al. 2003), have been reported. Several complex shapes of beads encountered in electrospinning are illustrated in Fig. 3.8. 

Reneker and colleagues, however, suggested factors such as surface tension, viscosity, and charge density of the jet to be the primary factors 

Several approaches are available to control bead formation in electrospinning of polymer nanofibers  

Increasing the applied voltage increases the surface charge of the jet and helps to reduce the frequency of occurrence of beads. However, this will generally suppress bead formation only at adequate feed rates 

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Figure 4.4 Example of bead formation during electrospinning SEM micrographs of poly (propyl carbonate) (PPC) beads prepared by electrospinning a PPC solution in dichloro-methane. (Reprinted with permission from Biomaterials, Electrospun aliphatic polycarbonates as tailores tissue scaffold materials by A. Welle, M. Kroger et al., 28, 2211-2219. Copyright...

Other studies revealed that the formation of bead defects in electrospun fibers has been initiated by varying solution surface tension and solution charge density, and by charge neutralization [42]. As attractive characteristics of electrospinning, these structures may also exhibit wide variations in their shapes and surface morphologies [50-51]. 

PEO is frequently used to faeilitate eleetrospinning of diffieult to proeess polymers, since it has good electro-spinnability, bioeompatibility and solubility in aqueous solutions. Duan et al. [168] reported that ultrafine fibers eould not be eleetrospim from chitosan solution in aqueous acetic acid, but formed when a eertain amoimt of PEO was added. Phase separation might have occurred which resulted in chitosan and PEO rich fibers of different diameters. Lou et al. [169] foimd that the chitosan/PEO ratio is an important factor in fiber formation, and a ratio of 60 40 resulted in the optimum fiber mat morphology with fiber diameters below 100 nm. They also confirmed the non-toxicity and the enhanced cell proliferation of the ehitosan/PEO fiber mats. By heating the polymer solution with hot air before it was ejected, Desai et al. [166] was able to electrospin blend solution with PEO concentration as low as 5% without bead formation. 

Despite the fact that the electrospinning technique is relatively easy to use, there are a number of process parameters that can greatly affect fiber formation and structure. Listed in order of relative impact to the electrospinning process, the most important parameters are applied voltage, polymer flow rate, and capillary-collector distance. All three parameters can influence the formation of nanofibers with bead-like defects. 

In the electrospinning technique, the ejected charged jet was affected by electrical forces, so it is needed to have high electrical properties, i.e., a good dielectric constant, to enhance the density of charges at the surface of the jet for better stretching and uniform formation of fibers with bead-free morphology, ... 

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