Stable jet and guiding electrode combination

Electrospun nanofibrous wall schematic. Adapted from Kim et al 2014

The combination of a stable jet coupled with guiding electrode has the potential to precisely and accurately control the deposition of electrospun nanofibers. Near field electrospinning has already shown that stable jet control is possible (more). Due to the charges carried by the electrospinning jet, it can be influenced by the external electric field. Researchers have used sharp edges and pins to guide incoming electrospinning jet (more). Preferential deposition of nanofibers on the struts of metal grids provides further evidence that electrospinning jet is sensitive to small differences to local electric field (more).

Several preliminary studies have demonstrated the result of stable jet electrospinning with guiding electrode. Using a sharp electrode tip with 50 um apex diameter placed at 2 mm from a solution droplet, the electrospun nanofiber was collected as a coiled, standing cylinder on top of the apex. These contrast with the randomly deposited fibers when the collector was replaced with a plate or a plate with a sharp electrode tip below it [Kim et al 2010]. Such difference in behavior is probably due to the degree of point guidance exerted by the collector to the charged jet. With this demonstration of electrospinning jet control, Kim et al [2014] was able to construct a standing wall made out of nanofibers layered on top of one another. A narrow conducting strip collector on a non-conducting substrate was used to isolate the electric field from the needle tip to the collector. The electrospinning jet was able to traverse along the length of the narrow strip to lay down the nanofiber on top of one another to build up a wall. Similar concept has been used to deposit stable melt electrospinning jet onto a patterned collector [Brown et al 2014]. The patterned collector may comprise of a flat substrate with patterned voids or a raised patterned platform (eg. pyramid).

A combination of near-field electrospinning and patterned substrate with micro-structures has shown that the accuracy and precision of nanofiber deposition can be brought down to within 5.15 µm as compared to 25 µm from a planar substrate at the same collector speed [Zheng et al 2010]. Bisht et al (2011) used low voltage, near field electrospinning to suspend nanofiber across carbon post with diameter of 30 µm and interpostal distance of 100 µm. Using a patterned silicon substrate with micro-pillars of diameter ranging from 1.6 µm to about 9 µm as collector, Zheng et al (2010) were able to successfully deposit a single strand of nanofiber across the micro-pillars thereby providing a new method for integrating nanofibers into micro/nano systems. To deposit a linear nanofiber strand across the pillar, the collector speed needs to at least match the jet spinning speed. Higher collector speed will result in a reduction in the fiber diameter due to stretching.

To determine the best method of improving electrospun fiber deposition accuracy, Xu et al (2014) compared three different methods using polyethylene oxide as the model polymer. The accuracy of fiber deposition is determined by the distance between the fibers deposited as the spinneret move back and forth. All three methods to be tested were based on near field electrospinning. The first method is near-field electrospinning without any modification. The second uses alternate polarity of the applied voltage at each deposition. The third concept is to use a guiding electrode below the collector plate. The gap between the fibers are 74 µm for normal near field electrospinning, 20 µm for alternating polarity and 7 µm with a guiding electrode. Their results showed that the use of a guiding electrode together with near field electrospinning is able to get the best accuracy in fiber deposition. Kim et al (2018) used inkjet printing with conductive Ag nanoparticles loaded ink to form patterns on a paper as a target for near field electrospun fibers. The conductive printed pattern served as a guiding electrode for the electrospinning jet. Poly(vinylidene fluoride) (PVDF) solution was electrospun from a height of 750 µm and a 150 µm offset from the edge of the pattern. The sensitivity of the electrospinning jet towards the electric field can be seen as the fibers are stacked on the edge of the conductive pattern where the relative electric field was much higher at the edge than at its center. When the pattern lines formed acute angle, right-angle or obtuse angle, the accuracy of the deposited fibers are influenced by slight changes in the relative electric field. From acute angle to right-angle, the electric field singularity increases from the edge to the intersection between the lines. In this case, the fibers were stacked directly on the edge of the line and to the middle of the intersection. For lines forming obtuse angle, the deposited fibers followed the edge of the lines by veered off the line at the intersection.

Analysis electric field singularity and electrospun fiber images stacked on the curved shape conductive patterns. a The distribution of relative electric field according to the variation of height of needle tip and the position of conductive pattern. b-d SEM and CCD camera images are fiber images stacked on acute, right and obtuse angled corners [Kim et al 2018]

Zheng et al (2021) carried out further investigations on using near field electrospinning and a sharp pin guiding electrode below the collector as a means of getting highly precise and accurate fiber deposition. The role of the guiding electrode is to focus the electrical field and with near field electrospinning. Using polyethylene oxide (PEO) as the model polymer, they were able to stack up to layers of fibers to create 3D complex structures with 10 to 80 fibers layers and height of 10 to 110 µm. In melt electrospinning, stacking of fiber layers has been limited to 50 layers [Brown et al 2014, Hochleitner et al 2015] due to the presence of residual charges which limits the accuracy of fiber placement. However, with solution electrospinning, residual solvents present in the deposited fibers may also facilitate migration of charges on the fiber to the ground which allow more layers of fiber to be stacked. A parameter that affects fiber placement is the collector motion velocity. A higher velocity would increase fiber placement accuracy as the polymer jet is stretched and reduces fiber diameter. A higher applied voltage also led to better fiber alignment due to greater focusing of the electric field between the nozzle and the guiding electrode. Although up to 80 fiber layers have been constructed, the effect of residual charges and the shielding of the electric field by the higher fiber layer cannot be prevented. Hence the accuracy and precision of the fiber deposition will be reduced as the layers build up. For a 60-layer fiber wall, Zheng et al (2021) reported a width of 94.3 µm and height of 102.2 µm although the fiber thickness is about 1.8 µm.

Guiding electrode setup for stacking layers of electrospun polyethylene oxide (PEO) fibers [Zheng et al 2021]

 

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