Melt Electrospun Structures

Electrospun polypropylene melt. [Fang et al. Journal of Nanomaterials, vol. 2012, Article ID 382639, 9 pages, 2012. doi:10.1155/2012/382639. This work is licensed under a Creative Commons Attribution 3.0 Unported License.]

Melt electrospinning is similar to conventional electrospinning in the range of structures that can be fabricated. Nonwoven melt electrospun fibrous mat can be constructed using conventional setup but with a heated nozzle. In contrast to conventional electrospinning where the electrospinning jet typically enters the bending instability phase, high viscosity of the melt makes its electrospinning jet more stable. This makes it easier to fabricate patterned fibrous structures using melt electrospinning.

Electrospinning melt polymers at close distance to the collector (30 mm or less) have been carried out to construct highly ordered structure. A translation stage is necessary to match the spinning speed so as to lay the fibers along a straight line. Electrospinning of polycaprolactone with an applied voltage of 10 kV, a translational collector speed of 2.5 x 10^-2 m/s was able to achieve this. The resultant structure has controlled pore size of 46 µm and was demonstrated to support good infiltration of cells [Farrugia et al 2013]. Increasing the tip to collector distance to allow more stretching of the electrospinning jet will cause a corresponding loss in the precision of fiber deposition as the jet enters the bending instability stage [Hellmann et al 2009].

SEM images of 3D melt printed PCL deposited in 120 degree turns taken at A) low magnification and B) higher magnification. C) SEM image of 3D melt printed PCL deposited in 90 degree turns. Images courtesy of Prof Paul Dalton.

3D scaffold has also been constructed using electrospinning melt and with ordered fiber deposition. Hochleitner et al (2014) was able to build a 2 mm thick structure out of ordered poly(2-ethyl-2-oxazoline) fibers. Hochleitner et al (2015) was able to use melt electrospinning at close distance between the tip and the collector (1 to 10 mm) to produce polycaprolactone fibers with diameter of about 800 nm and stack them on top of one another to form an array of box-structures with periodicity of about 100 µm and height of 80 µm. This creates new applications and opportunity for nanofiber structures where greater order brings benefits or better performance. In the work by Brown et al (2014), stacking of the electrospun polycaprolactone fibers was limited to 50 layers. Beyond this, the accurate fiber placement is lost and this was attributed to build-up of internal residual charge within the fiber. Incidentally, Hochleitner et al (2015) also limit the polycaprolactone fiber layer to 50. It was hypothesized that the preference of the fibers to stack on top of one another is due to shorter distance between the nozzle tip and the top of the fiber as compared to the ground and that residual charges are trapped inside the fiber instead of at the surface. Thus the electrospinning jet preferentially deposits on top of existing fiber layer. The impact of accumulated residual charges is significant for precision electrospinning as it restricts the number of fibers that can be stacked up continuously and it probably will affect the distance tolerance between fibers.

Box structured scaffolds printed upon a microscope slide, demonstrating the uniformity of the filament patterns. (A) Due to the high deposition accuracy on a glass slide there are optical effects when light is held in the background. (B) and (C) SEM images of such scaffolds tilted at 30° to show the regularity of the scaffold. [Hochleitner et al 2015. Biofabrication 2015; 7: 035002. This work is licensed under a Creative Commons Attribution 3.0 Unported License.]

Construction of melt electrospun tube follows the same concept as conventional electrospinning with the use of a rotating rod. However, the greater stability of the melt electrospinning jet makes controlling the helical alignment of the fibers on the rod easier. Ko (2014) was able to successfully construct polycaprolactone tube using melt electrospinning although the orientation of the fibers at close-up was random. However, there are sufficient fibers that are bundled in a helical orientation around the tube to be visible under the random fiber coverage. If it is shown to be possible to precisely control the orientation of melt electrospun fibers in the construction of a tubular structure, this may potentially be used in polymer stent application.

Other uniquely shaped fibrous structures have also been constructed using melt electrospinning. Brown et al (2014) used melt electrospinning on a dome shaped patterned collector. Long stable jet from melt electrospinning of polycaprolactone lay predominantly on the wire grids and formed a replica of the underlying patterned collector except that it is made out of raised fibers. The end result is a bowl-shaped fibrous structure with regular pores with average pore size of 250 µm. It is easy to envision that such porous structure has the potential for use in bone replacement graft.

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