Progressive Membrane Buildup

Electrospinning typically produces a two dimensional sheet of nonwoven nanofibrous mesh. Building a thick membrane beyond tens of microns is a challenge even if the spinning process is allowed to continue for several hours. Build-up of static charges on the deposited fibers may repel incoming fibers and this may limit the thickness of the resultant membrane [Tong et al 2013].

With some modifications of conventional electrospinning setup, it is still possible to construct a relatively thick scaffold. Eap et al (2015) used a mask to concentrate the electrospun fibers on a small area. The mask comprised of 2.5 mm thick poly(methyl methacrylate) with a hole (diameter of 25 mm) placed over the conductive collector. This raises the electrical potential in all location except the exposed collector under the hole. With this mask, the electrospinning jet deposition was localized in a small area thereby allowing a faster build up of nanofibers layer to give a 3-D scaffold. Tong et al uses positive and negatively charged electrospinning jet to deposit fibers on rotating drums [Tong et al 2013]. To prevent the collision of the opposing charged jets, they erected a barrier between the two collection drums. The drums were rotated such that alternate positive and negative charged nanofibers were collected. With this, they were able to buildup membrane with thickness of over 300 um compared to less than 200 um from conventional single jet electrospinning setup. Using the same concept, Gorji et al (2012) used positively and negatively charged electrospinning jets at opposite sides of a rotating drum. They were able to collect polyurethane nanofibers using this setup for 12 hours and the thickness of the membrane increases linearly over the 12 hours collection duration. At the end of 12 hours, the thickness of the mesh reaches about 275 µm. The weight of the fibers was also found to increase linearly throughout the 12 hours collection. This shows that using opposing charges in fiber collection is effective in eliminating charge build up which hinders the build-up of scaffold thickness. However, to construct a scaffold with height of a few centimetres, this method is clearly not practical.

Vong et al (2021) showed that with a ring guiding electrode, they were able to get a relatively fast buildup of electrospun fibers. The guiding electrode is either placed on the collector or below the collector to steer the electrospinning jet towards it. With a ring guiding electrode on the collector, the electrospinning jet will be depositing fibers along the edges of the ring. This will lead to a faster accumulation of fibers on the edges and the inner ring which facilitates fiber buildup. With this setup and an optimal distance between nozzle tip and collector of 5 cm, Vong et al (2021) was able to get polystyrene (PS) and polyacrylonitrile (PAN) fibrous scaffolds of 3 cm thickness after 10 min of deposition with the electrospinning nozzle circling over the ring. However with polyvinylpyrrolidone (PVP), the build profile is less ideal with a 1 cm buildup of fibers at the center of the ring. Interestingly, Vong et al (2021) showed that it is possible to build a higher structure by gradually increasing the distance between the nozzle tip and the collector during electrospinning. With a vertical speed of 0.5 cm/min they were able to increase the height of PS structure to 5 cm (60% increase) after 10 min of deposition. A starting distance between the nozzle tip and collector at 5 cm was optimal as a larger working distance was found to increase the spread of the deposited fibers and reduce the vertical buildup. A gradual vertical movement of the nozzle tip away from the collector is able to maintain an optimal distance as the fibrous structure grows taller.

3D electrospun structures of PS, PAN and PVP. (a.1) Top-view, (a.2) Side view, (a.3) SEM pictures and (a.4) High FPS camera picture during electrospinning of 3D PS. (b.1), (b.2), (b.3) and (b.4) are for 3D PAN. (c.1), (c.2), (c.3) and (c.0.4) are for 3D PVP [Vong et al 2021].

With progress in stable jet electrospinning, where the stability of the electrospinning jet can be maintained for at least several centimeters, there is a possibility of building a tall, ordered fibrous structure in a relatively short time. Stable jet for building ordered structures have been demonstrated in melt electrospinning and also through controlling solution parameters. Hochleitner et al (2014) was able to build a 2 mm thick structure out of ordered poly(2-ethyl-2-oxazoline) fibers. 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. Jin et al (2020) introduced a "lag effect" in near field melt electrospinning where the electrospinning jet is being dragged as it adheres to the moving collector. This happens when the collector moves faster than the ejection of the polymer melt. This tensile drag force resists chaotic bending from the electrical charges and improves accuracy of the deposited fibers. With this, they were able to create highly controlled patterns although the buildup of residual charges above 100 layers will interfere with the deposition if the adjacent fiber is too close. Therefore, they used a 80 µm gap between fibers for their fiber pattern. However, it may also be argued that in this case, it may be "tensile drawing" of the jet from the spinneret instead of "electrospinning" that pulls the fiber into the desired diameter since the "lag" due to the tension is greater than the effect of the electrostatic charges on the spinning jet. Unfortunately, the study did not verify whether the fiber diameter is due to tensile stretch or electrostatic stretch. 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.

In solution electrospinning, Yuan et al (2012) used ultrahigh molecular weight poly(ethylene oxide) (UHMWPEO) (Mw > 5000kDa) for blending with other polymers to form long and stable electrospinning jet. The presence of UHMWPEO raises the solution viscoelasticity and this contributes to the jet stability. Doping poly(L-lactide), polycaprolactone and chitosan solution with UHMWPEO has been shown to generate a stable electrospinning jet across a tip to collector distance of 15 cm. The resultant fiber diameters were about 1.5 µm or more. With the stable electrospinning jet, they were able to construct highly ordered structures. Unlike near-field electrospinning, given the substantial tip to collector distance, they were able to use this method to construct patterned three-dimensional fibrous scaffold [Yuan et al 2015]. With the ability to control electrospinning jet deposition location, the pore size of the resultant 3D structure can be controlled .

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