“EL ALIENTO DE VIDA”

Table 2.9: Electrospinning parameters.

Surface Tension

For the electrospinning process to occur, the charge of the polymer solution must be high enough to overcome the surface tension of the solution. If the polymer solution has a high concentration of free solvent molecules, there is a possibility that the molecules will group together and create a spherical bead shape due to the surface tension. To overcome this problem an increase in viscosity will help to create a greater interaction between the solvent and polymer molecules. When the solution is stretched during electrical charging the solvent and polymer molecules are less likely to congregate into a spherical shape. Instead the solvent molecules will spread over the entangled polymer molecules (267). Fig. 2.11 illustrates this occurrence.

distributed over the entangled polymer at a relatively high viscosity and (B) shows how the solvent molecules tend to congregate under the action of surface tension at a relatively low

viscosity (267).

Conductivity

The electrospinning process can only occur when a solution gains sufficient charge so that the repulsive forces within the solution can overcome the surface tension. The ability for a solution to incur stretching and drawing also depends on how well the solution can carry charges (267). A number of studies have reported that a high solution conductivity yielded fibres free from beads (271-273). One study reported that the addition of salts to the electospinning solution resulted in a higher charge density on the electrospinning jet surface, which in turn increases the charges carried by the jet, causing higher elongation and fibres with fewer beads (268).

Voltage

A high voltage is an essential element in the electrospinning process. A high voltage will induce the required charges on the polymer solution and together with the external electrical field an elctrospinning jet will emerge from the needle tip or rotating electrode when the electrostatic charge is high enough to overcome the surface tension of the solution (267). During the electrospinning process the viscoelastic solution will be stretched by the columbic repulsive 2

.8.3.2 Processing Condition Parameters

force in the jet. A higher voltage will increase the acceleration of the jet and stretch the polymer solution due to the increase in electrical charges (268). This may result is fibres with a smaller mean diameter, due to the stretching of the polymer jet (274, 275). However, if the acceleration is too fast the solvent will have less time to evaporate and will result in wet polymer fibres composed of mainly beads landing upon the collector (276).

Distance

It can be assumed that by increasing the distance the polymer solution has to travel, the stretching of the solution will be increased, resulting in a smaller fibre diameter. Increasing the distance also allows time for the solvent to evaporate before it reaches the collector (267). When the distance is too low, it may cause the fibres to merge, forming junctions (275), as shown in Fig. 2.12.

Flow Rate

Flow rate refers to the amount of solution released from the needle tip. It can be assumed that if a greater volume is drawn away from the needle tip at a given voltage, the fibre diameter may be greater than a lower flow rate at the same voltage (268). However, a greater volume of solution will take longer for the solvent to evaporate and so the distance must be considered when finding optimum spinning conditions (277).

The temperature, humidity, atmosphere and pressure may also have an influential effect on the polymer solution during electrospinning. The humidity is of particular importance as water drops may condense on the surface of the fibres, disrupting the fibre morphology (267). A study on the effect of relative humidity (RH) on the electrospinning of poly (acrylonitrile) (PAN) and polysulfone (PSU) showed great variation in the diameter and mechanical strength of the fibres with varying RH between 0% to 60%. For example the average fibre diameter of PAN increased

distance of 0.5 cm from the collector (241).

2

.8.3.3 Ambient Condition Parameters

from 150 nm at 0% RH to 630 nm at 60% RH while the average diameter of PSU fibres increased from 1.15 µm at 0% RH to 3.58 µm at 50% RH, all other parameter remained constant. PAN fibres were also shown to shift from a smooth surface to a rougher one with increasing RH. PSU exhibited varying changes in the surface morphology with increasing RH, from smooth to elongated pores and creases in the fibres (278).

A change in temperature is also known to affect the fibre morphology during electrospinning, due to the change in viscosity of the electrospinning solution and solvent evaporation (267). Polyurethane was shown to produce more uniform fibres, when electrospun at a higher temperature of approximately 70ºC, when compared with those spun at ambient room temperature. At the higher temperature several jets were produced from the tip of the needle, with a large angle between each jet. It was also observed that the deposition rate of the fibres was increased at a higher temperature, resulting in a thicker electrospun web (279).

Electrospun materials as medical dressings are still in their infancy, with many research institutions exploring new possibilities. These materials meet the majority of the requirements outlined for wound healing polymer devices, as their microfibrous and nanofibrous structures provide the nonwoven textile with desirable properties (280). The fibres produced from electrospinning are exceptionally small, with typical fibre diameters ranging from 5 nm to 500 nm (57). This offers many advantages including, a high surface area between 5 m2 g-1 to 100 m2 g-1 (281) and a high aspect ratio, where the length of the fibres can be as long as several kilometres due to the continuous process (260). The size of the fibres also provides an environment which closely mimics the extracellular matrix (ECM) required in healing (260). They also have a high porosity but small pore size, with reports of 93.6% porosity (280) and pore throat diameters ranging between 0.1 µm to 0.8 µm (282). These unique properties of electrospun webs provide the following benefits for materials in wound care and can be seen below in section 2.8.4.1.