History of electrospinning. The distance between the needle and the collector is very important because the electric field increases as the distance decreases. It also shortens the pulling force and the time to pass through the fibrin-created area, thereby sufficiently evaporating the solvent from the fiber. As a result, if the distance between the needle and the collector is reduced, the fibers can grow and experience a bubble-like structural deformation. If the high voltage is chosen to be 15 kV and the distance between the needle tip and the collector is between 9 cm and 21 cm, the fiber morphology can be evaluated from the SEM images in Figure 10. At the shortest distance (9 cm), the fiber converges at the junctions before reaching the collector due to incomplete evaporation of the solvent. For the other four distances used, the fibers looked similar and the average fiber diameter increased slightly with distance from the collector. The tip-to-collector distance directly affects the transit time and the intensity of the electric field. In order to form fibers, the electrospinneret must have enough time to evaporate most of the solvent. At the same time, the intensity of the electric field increases and this increases the acceleration of the beam towards the collector. As a result, solvents may not have enough time to evaporate when they reach the collector.
History of electrospinning
Another important treatment parameter is the flow rate of the polymer solution in the syringe. A low flow rate is preferred to allow sufficient time for the polymer solution to polarize. If the flow rate is too high, thick diameter beaded fibers will be formed instead of fine diameter smooth fibers because of the short drying time before reaching the collector and also because of the low drag forces. As the feed rate is increased, there is a corresponding increase in fiber diameter or bubble size due to the larger volume of solution exiting the needle tip. As shown in Figure 11, Shamimz et al.[50] found that the current is kept constant as it decreases with other parameters. There is a decrease in bubble size and an increase in the diameter of the nanofibers. The implication here is that as the current decreases, the spot size can decrease until a bead-free structure is obtained.
The formation of nanofibers can be divided into woven and non-woven nanofibers. The type of collector used plays an important role in distinguishing between types or orientations of nanofibers. The use of a two-mass static collector [2] together with a directional collector represents the methods used in the development of aligned woven nanofibers. A rotating drum collector is used to collect nanofibers from aligned arrays, while a rotating disk is used to collect uniaxially aligned nanofibers. The aligned fibers obtained from the rotating drum correspond to the rotational speed applied to the drum [2]. This type of electrospinning process is more complex because the spin speed needs to be controlled evenly to produce nanofibers with such good alignment. A rotating disc collector is also used to collect continuous nanofibers as they can absorb the large electric field applied to the edge of the disc [2].
As shown in Figure 12, SEM images of various collectors were generated for many aspect ratios, including the wire mesh used by Wang X et al. will investigate. [52], Sandarai B. et al., [53], Li D. et al., [54], parallel or lattice bars and torsion or wheel bars studied by Xu CY et al. [55] and Ki CS et al [56], Kim et al [57] showed that different types of compounds used in the collector affect the structure of poly(L-lactide) (PLLA) and poly(lactide-coglycolide).
Fiber diameter and morphology can also be affected by environmental parameters such as humidity and temperature. For example, Mituppatham et al. [58], as shown in Figure 13, used the smaller fiber diameter with polyamide 6 fibers for the inverse relationship between solution viscosity and temperature. As for humidity, low humidity can completely dry the solvent and increase the speed. Contrary to solvent evaporation, high humidity leads to thicker fiber diameter because the charges on the nozzle can be neutralized and the drag forces are weakened.