Electric field and charges

It is generally a challenge to gather a clump of fluffy and loosely arranged nanofibers as the electrospinning process tends to compact them. However, some studies have shown that it is possible to get loosely arranged nanofibers by modifying the electric field to remove the factors that causes fiber compaction.

Impacting of the nanofiber on the collector as it accelerates towards it is one of the causes of nanofiber compaction. A setup that is able to allow the electrospinning jet to slow down sufficiently prior to deposition will eliminate fiber compaction. Miyamoto et al diverted the jet path and also neutralizes the charges on the jet to achieve this. This causes the fiber to lay down gently on top of one another to give a fluffly and open nanofibrous clump.

Another cause of nanofiber compaction in a standard setup is that the electrospinning jet is depositing fibers on the collector is like hitting a wall (collector). Removing the "wall" such that the jet is not hitting against a "solid" surface will give rise to a loosely arranged nanofibers. There are a few setups that have been constructed to achieve this with varying level of effectiveness. Having a flat collector with sparsely arranged needles pointing towards the spinneret creates a collection setup consists mainly of empty space. Nanofibers will preferentially rest on the tip of the needles but its weight and the collision of the incoming fibers cause the resting fiber to collapse into the space between the needles. As a result, a loosely packed nanofibrous structure is formed [Phipps et al 2011]. In a different setup, an array of needles in a radial configuration and a space behind it (as shown in the diagram below) was used to fabricate 3-dimensional nanofibers [Blakeney et al 2011]. The presence of the needle array encourages the electrospinning jet to spin towards an almost empty space to be gathered. Fluffy and uncompressed clump of nanofibers has been constructed using this method.

Using a similar concept, Joseph et al (2015) used a collector comprises of sharp spokes evenly distributed around an axis and radiating towards the electrospinning spinnerets. Two spinnerets of opposing polarity were used for electrospinning fibers from the edge of the collector. To create a fluffy ball of nanofibers, electrospinning jet of opposing polarity is needed and the collector needs to be rotated. The opposing polarity is needed to cause collision of the electrospinning jet in mid-air. The rotation creates a pressure drop inside collector and the resultant inward airflow sucks the electrospun fibers to the center of the collector to form a fluffy clump of fibers.

In a simplification of the use of pointed tip for the construction of 3D nanofibrous structure, just a single needle tip has been shown to be capable of collecting 3D nanofibrous structure. However, it is necessary to move the spinning nozzle in the lateral direction to disperse the fiber deposition around the standing collector needle. Without lateral nozzle movement, fibers gathered on the pointed edge of the collector needle may repel further incoming fibers. Movement of the nozzle away from the collector needle exposes its side to the electrospinning jet and thus allowing fibers to be deposited along its side instead of just at the tip. Leong et al (2014) successfully demonstrated the feasibility of this concept using polycaprolactone (PCL) solution. However, observation of the fibers under scanning electron microscope (SEM) showed wide distribution in the fiber diameter. This may be due to the constantly changing electric field as a result of the nozzle movements.

Instead of using a solid needle tip, Leong et al (2016) showed that using a hypodermic needle with an insulated base as the collector was also able to produce 3D scaffold. This is despite having a slight variation in the needle tip profile as the hypodermic needle has a slanted tip and a hollow center. The electrospinning nozzle was moved laterally as per the earlier electrospinning setup [Leong et al 2014] and the electrospun PCL fibers showed the same wide fiber diameter distribution. Comparing the pore size between the 3D scaffold and 2D membrane using conventional electrospinning setup, the former has a pore size of about 42µm while the latter has a pore size of about 10µm.

The sensitivity of electrospinning jets in responding to slight variation in the electric field profile may be used to create an assembly of fibrous layers. Lubasova et al (2020) constructed a ridges or corrugated profile collector made of non-conductive cardboard substrate and metal plates were placed behind the cardboard substrate to direct fiber deposition. Responding to the presence of the electrically grounded conductive metal plate behind the cardboard substrate, the fibers are deposited on the surface with the back touching the metal plate and span the walls of the ridges at the plane of the edge of the metal plates. By progressively stacking metal plates behind the cardboard substrate, a new layer of electrospun fibers are formed on the collector.

Photographic image showing two nanofiber bridges between the two sides of the V-shaped substrate [Lubasova et al 2020].

Schematic of 3-D nanofibrous layer producing setups for variations A-D (a-d) [Lubasova et al 2020].

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