Improving Uniformity of Electrospun Membrane

A close examination of electrospun membrane cross section typically reveals non-uniform membrane thickness with the thickest part at the center and gets thinner closer to the periphery. This challenge is even more pronounced in multiple nozzle electrospinning where repulsive forces between the electrospinning jets forces the fibers to be deposited in distinct and separate locations. Such non-uniform distribution of fibers as a membrane may lead to poor application performance. Tensile testing on the membrane also frequently show failure by delamination between fibers sheets instead of a clean breakage. To understand the reason for the non-uniform distribution of the fibers, it is important to understand the electrospinning jet behavior.

Distinct and separated fiber deposition locations from each nozzle.

During electrospinning, the jet typically accelerates along the electrical potential gradient and it follows the path where the potential difference gradient is the steepest. For a point source, the steepest potential gradient is the shortest distance from the source to the collector. As the fiber accumulates, there is a drop in the potential difference between the source and the area where fibers have deposited due to an accumulation of residual charges. This diverts the jet to the periphery of the deposited fibers and the deposition expands outwards. This results in greater fiber deposits at the point closest to the source and taper off away from its center. Depending on the material, the rate of electrical charges lost from the fiber will influence the degree of spread or area covered by fibers.

There are a few ways which the electrospun membrane uniformity can be improved. This is usually achieved either mechanically or by changing the electric field profile or response. Mechanical movement may be applied either to the spinneret or the collector where one part will move relative to the other part. Such movement forces the fibers to be spread out over the deposition area. However the limitation is the fiber deposition will correspond to the direction of movement instead of a random spread. Aligned fibers may be fabricated instead of a uniform layer of randomly oriented fibers. More complex collector movement has been used to improve electrospun fibers mat uniformity. Pathalamuthu et al (2019) describes a spirograph-based collector movement for collection of electrospun polyacrylonitrile (PAN) fibres. The spiropath path comprises multiple circular movements passing through a point centre. Movement speed of the collector needs to be kept below 300 rpm or the air flow generated by the collector movement would disrupt the depositing jet. On a static collector, the coefficient of variation was 12.5%. However, the coefficient of variation with the spirograph-based collector movement was 2.4%. This demonstrated a significant improvement in electrospun mat thickness uniformity.

To reduce the effect of potential field gradient on the electrospinning jet deposition, a base plate electrode that is almost at the same level as the nozzle tip may be used. Unlike having a single nozzle which creates a point electrical source to a collector, the base electrode set up a uniform electric field from the source to the collector. This reduces the bias in the electric potential gradient and potentially improves fiber deposition uniformity. In a demonstration of the importance of electric field uniformity on the fiber distribution, Wu et al (2016) used auxiliary electrodes to create a uniform electric field from a solution reservoir source to a flat collector in a free surface electrospinning setup. Without the auxiliary electrodes, the edges of the reservoir containment is a source of high charge concentration and electric field strength. Comparing the fiber distribution across the collector with and without the auxiliary electrodes showed better fiber uniformity in the former where the electric field is more uniform.

Since the presence of residual charges on deposited fibers would divert or repel further incoming fibers, a less conducting collector may favour a more uniform fiber distribution. For some solutions, the deposited fibers are known to creep up towards the spinneret or form 3D assembly when the collector is a conductive plate. When the collector is switched to a non-conducting plate, fiber deposition becomes well spread out over the collector [Sun et al 2012]. A non-conducting plate prevents the charges on the deposited fiber to dissipate immediately. The residual charges on the fibers would divert incoming electrospinning jet to other region with greater potential difference. Therefore, a more uniform fiber distribution can be expected. However, this potentially reduces the thickness of the membrane that can be collected since the reduction in potential difference between the collected membrane and the spinning source may divert the electrospinning jet to other parts of the setup.

Most electrospinning collectors comprises of a flat and smooth surface. This provides a uniform electric field across the surface and the electrospinning jets will self organize and deposit fibers on points on the collector with the greatest potential difference. An alternative method is to guide the jets to deposit in a more uniform manner on the collector. This has been achieved using an uneven surface with high voltage applied to the collector and grounding the nozzle tip [Koenig et al 2019]. When the collector features numerous sharp points and given a high voltage, it encourages the electrospinning jet to travel to discrete points on the collector. When the discrete points are optimally spaced, the jets will attempt to cover all the points on the collector resulting in a more uniform fiber coverage.