Non-homogeneous electrospun fiber distribution by collector modification

Electrospinning is commonly known to produce a nonwoven fibrous membrane with relatively uniform top surface topography. However, with clever modification of the collector, it is possible to construct a fibrous structure with non-uniform fiber organization. Such non-uniform fiber organization helps to segment the membrane into small areas with specific requirement such as in tissue engineering or sensors. Electrospun fibers that are more aligned or thinner have been shown to exhibit greater transparency then randomly organized fibers and this may create patterned windows.

There are a few methods which the collector can be modified to influence fiber deposition on specific region. The most common method is to modify the electric field distribution. The electrospinning jet is sensitive to small difference in the local electric field. Electrospinning jet using polycaprolactone solution was able to discern conductive lines with widths of 350 µm and spacing of 1.7 mm [Wu et al 2010]. The actual resolution is better than this as an imprint of conductive letters of size 1 mm by 1 mm on an insulating plastic has been made using electrospun fibers [Wu et al 2010]. Conductive collecting substrate with textured surface such as wire mesh or grids has been used to form patterned/textured nanofibrous membrane due to its electric field profile. Ortega et al (2013) used a raised metal ring to create a fibrous structure with non-uniform fiber organization. In this setup, given that the electrical potential of the ring and the ground is uniform, the electrospinning jet will be attracted to the surfaces that are closer to it. In this case, it will get attracted to the raised area of the ring and the ground location which is at a slight distance away from the corner where the ring touches the ground. This forms a "parallel electrodes collector" configuration where the corner of the ring which touches the ground acts like the gap in the "parallel electrodes collector" configuration. This causes the fibers to lean in parallel from the ground against the wall of the ring forming region of lower fiber density [Ortega et al 2013]. As the height of the ring increases, the fibers get deposited further from the corner. This creates a single membrane with distinct regions of high fiber density and lower fiber density [Ortega et al 2013].

Uneven distribution of electrospun fibers on a single "cell" template.

Kim et al (2016) constructed an electrospun tube with areas of high transparency and low transparency. Higher transparency regions are made out of aligned fibers while randomly oriented fibers form the low transparency regions. Sectors comprising of highly aligned nanofibers were created by having non-conductive cellophane tape on the rotating drum surface. The non-conductive cellophane tape creates a "gap" within a more conductive surface and this aligned fibers formed across the width of the cellophane tape similar in principle to the parallel electrodes collector for collecting aligned fibers. The sectors comprising of oriented fibers have much greater transparency than the other sectors which is made out of randomly oriented fibers. By deliberately specifying areas of higher transparency, they were able to roll the mat into a tube with sections that has greater transparency. This may facilitate nerve reconstructive surgery by having both edges of the tube more transparent.

(a,b) The set up for fabrication of designed mat with aligned and random nanofibers (c) Results of electric field distribution simulation (Top view): surface and variation of electric field. [Kim et al Scientific Reports 2016; 6: 23761. This work is licensed under a Creative Commons Attribution 4.0 International.]

Another method is to vary the electrospinning tip to collector distance in a rotating setup. By varying the diameter of a rotating cylindrical collector, the circumferential rotation speed will also be varied with the same rpm (revolution per minute). With a smaller diameter the surface rotation speed will be slower than part of the cylinder with a larger diameter. Differences in fiber morphology and distribution along a rotating cone along its longitudinal axis has been demonstrated by Zhou et al (2017). The differences were more pronounced when the needle tip to the cone axis distance was low. For example, fiber diameter was smaller at the larger end of the cone collector due to its higher surface velocity. Porosity of the membrane was also higher at the larger diameter section of the cone due to larger deposition area.