Applications of Near Field Electrospinning

In precision electrospinning, near field electrospinning is the most widely investigated technique. Current progress including its hybrids is able to consistently deposit fibers less than 1 µm with pitch of less than 100 µm. This has enable further investigations into potential applications of this technique in terms of the type of structures that can be constructed and its usage.

Scaffolds constructed by oriented electrospun fibers have been shown to facilitate contact guidance of cells. These scaffolds are generally constructed using standard electrospinning setup but with a rotating collector. The oriented fibers are distributed over a wide area with no control on the pitch. With near field electrospinning, more in-depth studies on the influence of aligned fibers on cell behaviour can be investigated. Parameters such as fiber spacing and organization of the fibers on cell response will give further insight into cell behaviour. In a study by Fuh et al (2013), the behavior of embryonic kidney cells were tested on fiber (diameter about 700 nm) grids formed on polypyrrole substrates as shown in the figure below using near field electrospinning. The study showed distinct cell alignment on the parallel grids with fiber-fiber spacing of 20 µm demonstrating better cell orientations than 100 µm spacing. No cell orientation was recorded for other substrates. Investigation of cell spreading showed that the spreading is the same for parallel grid and square grid of the same spacing. However, cell spreading is reduced when the spacing between the fibers are reduced for both square and parallel grids. In separate study by Park et al (2011) using dermal fibroblasts on randomly oriented and aligned polycaprolactone (PCL) fibers, they found that cell alignment is better with higher density of aligned fibers below the cells. Their study showed that cell guidance is more effective when the fiber spacing is similar or smaller than the cell size.

Fiber grid patterns. Fiber grid with 20 µm spacing shows better influence on cells than 100 µm spacing [Fuh et al 2013].

The relative ease of forming a continuous line from point to point creates the possibility of using near field electrospinning to generate nanowires across nano-connection points. 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.

Changes in the electrical conductivity of materials are often used for gas sensing. In near field electrospinning, it is possible to deposit a single strand of fiber across conducting electrodes to function as ultra-small sensor. Chen et al (2011) investigated the gas sensing potential of a single polyaniline nanofiber field effect transistor. The sensor showed high sensitivity towards NH₃ with a 7% decrease in current when exposed to 1 ppm of NH₃ with a gate voltage of -10 V. Near-field electrospinning is also able to construct highly aligned nanofibers on conducting electrodes for the purpose of sound detection. Wang et al (2020) developed a non-resonating acoustic sensor based on the ability of sparsely distributed freely suspended electrospun piezoelectric fibers to pick up acoustic signals. A nanofiber mesh made of poly(vinylidene fluoride-co-trifluoroethylene) (P(VDF-TrFE)) with an average fiber diameter of 307 nm was produced by a dynamic near field electrospinning method across a parallel electrodes collector setup. This nanofiber mesh was found to be sensitive to acoustic waves from 200 Hz to 500 Hz which covers the most common sound frequencies encountered daily hence the acoustic sensors made from the electrospun (P(VDF-TrFE)) fibers were able to detect changes in the source frequencies. As the sound wave travels perpendicular to the suspended nanofibers, it causes them to vibrate and generate a voltage output. Further tests showed that the nanofiber mats were able to differentiate between different sound frequencies. The optimum length of the suspended nanofibers were found to be 3 mm. Such a short distance is well within the limits for parallel electrodes electrospinning.

Significant progress has also been made in melt electrospinning and its combination with near-field electrospinning. Hochleitner et al (2015) was able to use melt electrospinning at close distance between the tip and the collector (1 to 10 mm) to produce polycaprolactone fibers with diameter of about 800 nm and stack them on top of one another to form an array of box-structures with periodicity of about 100 µm and height of 80 µm.

Agarwal et al (2018) used precision melt electrospinning to align polycaprolactone (PCL) beside one another. Plates made out of these aligned PCL fibers were stacked on top of one another in a hierarchical helicoidal design to mimic the basic building blocks found in shells of mantis shrimp dactyl club. Such arrangement of fibers gave the structure superior impact toughness. To enhance inter-plate and inter-fibre bonding, the electrospun PCL fibers plates were first functionalized to introduce carboxyl and amino group before hot pressing. The tensile strength of hot-pressed unidirectionally oriented fibers between plates were compared with aligned fibers plates stacked in a 15° rotations between each layer to form a 3D helicoidal structure. Their result showed that the toughness of helicoidal PCL sample was 5 times and 1.5 times greater than bulk PCL block and unidirectionally oriented PCL fibers sample respectively.

Methodology, design, and SEM images of one layer of 3D Helicoidal Structure. (a) Schematics of near-field melt electrospinning (NFES)-the process used to first lay down patterns of electrospun fibres on top a collector; (b) illustration of a top view of the hierarchical helicoidal design of the basic building blocks found in shells of mantis shrimp dactyl club, as reported in the literature; (c) scanning electron microscope (SEM) image of a layer of highly aligned, dense fibre-structured samples of melt electrospinning fibres with diameters around 10?µm at 65x magnification [Agarwal et al 2018].

For normal solution near field electrospinning, it is still a challenge to reduce instability of the electrospinning jet such that the fibers can be stacked on top of one another to create a 3D pattern. In an attempt to improve the accuracy of near field electrospinning fiber deposition, 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. However, this is still insufficient to enable stacking of the fibers.

The influence of the guiding electrode on the electrospinning jet may be diminished due to the presence of the collector. Lee and Kim (2014) showed that it is possible to stack polyethylene oxide (PEO) on top of one another using a narrow collector. In their experiment, they used a platinum line on glass as the collector target. While they are able to stack the fibers above one another to form a wall, the shape of the 3D fibrous structure is restricted by the underlying pattern.

In a development of 3D printing capability using electrospinning, Moon et al (2021) showed that by having the high voltage applied to the collector instead of the nozzle tip, they are able to get a longer stable electrospinning jet. With this setup the distance between the tip and the collector was about 5 to 10 mm. They hypothesized that when a positive high voltage is applied on the collector, there is a migration of cations towards the induced negative polarity needle wall. However, as the movement of cation is slow, the presence of both cation and electrons in the electrospinning jet reduces the net interfacial charge on it. The reduced Coulombic repulsive forces on the electrospinning that causes bending instability is less than the stabilizing inertia and viscoelastic force hence there is greater jet stability. The diameter of the fibers were about 10 µm which is large for electrospun fibers. Such larger diameter also helped in the damping of the electrospinning jet. This setup for maintaining jet stability has been tested on several polymer solutions such as TPU, PLGA, polycaprolactone (PCL) and others. Stacks of up to 90 layers of electrospun microfibers made of different materials have been demonstrated.

Mechanism of a straight jet. Digital images of Taylor cones for (A) a single-phasic microfiber and (B) a biphasic microfiber. PLGA was used for the single-phasic microfiber; PLGA and TPU were used for the biphasic microfiber. Schematic diagrams of the forces applied to the polymeric jet for (C) the conventional jetting system and (E) the CREW system. SEM images of grid patterns that are produced by (D) the conventional jetting system and (F) the CREW system. Scale bars, 300 µm. (Moon et al 2021)

With the capability of maintaining a precise and accurate deposition of the electrospun fibers, they were able to construct ordered 3D structures using electrospinning. Although a diameter of 10 µm is large for electrospun fibers, this diameter is much smaller than filament-based 3D printing and may be used together with it to achieve more complex architecture.

Tandem scaffold.(A) Schematic diagram of the design of tandem scaffolds using PMMA and PLGA, where the PLGA was precisely positioned onto a PMMA pattern; (B) schematic diagram of the mechanism of bending motion of a PLGA/PMMA bilayered pattern at elevated temperature (here, 80°C); schematic diagram and digital photo of before and after heat treatment depending on its PLGA pattern, (C) perpendicular pattern, (D) horizontal pattern, (E) combined pattern, and (F) PMMA only [Moon et al 2021].

In power generation using piezo-electric fibers, near field electrospinning is able to produce single fibers to determine its power generation ability or to construct highly oriented fibers which allow the control of current flow. Pan et al (2014, 2015) tested the power output from aligned nanofibrous membranes made from different piezoelectric materials. Through their test on single nanofibers from near field electrospinning, they found that the maximum power from poly(γ-benzyl α, l-glutamate) was 138 pW while polyvinylidene fluoride (PVDF) fiber was able to generate 266 pW.

Precise deposition using near field electrospinning means that the fibers may be used in the construction of more complicated devices. Su et al (2021) combined techniques of melt near field electrospinning, micromolding, and skiving process to mass produce tadpole-like magnetic polycaprolactone/Fe₃O₄ (PCL/Fe₃O₄) microrobot. Melt electrospun PCL fibers were used as a sacrificial template to form PDMS channels. These PDMS channels were formed by layers of melt electrospun fibers stacked on top of one another. By varying the speed of the collector, near field melt electrospinning was able to produce fibers of different diameters. When the collector speed was 100 mm/min, the channel width after removing the fiber was 159 µm. When the collector speed was increased to 1500 mm/min, the channel width after removing the fiber was reduced to 25 µm. Injection molding was used to fill the PDMS channels with PCL/Fe₃O₄ solution. After solidification of the PCL/Fe₃O₄, the billets were removed and embedded in a frozen, water-soluble polymer for skiving into microslides. The tadpole-like microrobots were then released from the polymer by washing.

Characterization of the polydimethylsiloxane (PDMS) channels and magnetic polycaprolactone (PCL)/Fe3O4 asymmetric billet. (A) Scanning electron microscopy (SEM) images of the cross-section of PDMS channels fabricated with different printing speeds using 0.9 mm needles. (B) The width and depth of the channels against the printing speeds. (C) SEM images of the cross-section of asymmetric PDMS channels with designed shapes. (D) Optical microscope image of channel 2. (E) SEM image of magnetic PCL/Fe3O4 asymmetric billet from channel 2. The values of speed were marked on each image and the unit was mm min^-1 [Su et al 2021].

▼ Reference

Agarwal K,Zhou Y, Ali H P A, Radchenko I, Baji A, Budiman A S. Additive Manufacturing Enabled by Electrospinning for Tougher Bio-Inspired Materials. Advances in Materials Science and Engineering 2018; 2018: 8460751. Open Access
Bisht G S, Canton G, Mirsepassi A, Kulinsky L, Oh S, Dunn-Rankin D, Madou M J. Controlled Continuous Patterning of Polymeric Nanofibers on Three-Dimensional Substrates Using Low-Voltage Near-Field Electrospinning. Nano Lett. 2011; 11: 1831.
Chen D, Lei S, Chen Y. A Single Polyaniline Nanofiber Field Effect Transistor and Its Gas Sensing Mechanisms. Sensors 2011; 11: 6509. Open Access
Fuh Y K, Chen S Z, He Z Y. Direct-write, highly aligned chitosan-poly(ethylene oxide) nanofiber patterns for cell morphology and spreading control. Nanoscale Research Letters 2013; 8: 97. Open Access
Hochleitner G, Jungst T, Brown T D, Hahn K, Moseke C, Jakob F, Dalton P D, Groll J. Additive manufacturing of scaffolds with sub-micron filaments via melt electrospinning writing. Biofabrication 2015; 7: 035002. Open Access
Lee M, Kim H Y. Toward Nanoscale Three-Dimensional Printing: Nanowalls Built of Electrospun Nanofibers. Langmuir 2014; 30: 1210.
Moon S, Jones M S, Seo E, Lee J, Lahann L, Jordahl J H, Lee K J, Lahann J. 3D jet writing of mechanically actuated tandem scaffolds. Science Advances 2021; 7 Open Access
Pan C T, Yen C K, Wang S Y, Lai Y C, Lin L, Huang J C, Kuo S W. Near-field electrospinning enhances the energy harvesting of hollow PVDF piezoelectric fibers. RSC Adv 2015; 5: 85073.
Pan C T, Yen C K, Liu Z H, Li H W, Kuo S W, Lu Y S, Lai Y C. Poly(γ-benzyl α, l-glutamate) in Cylindrical Near-Field Electrospinning Fabrication and Analysis of Piezoelectric Fibers. Sensors and Materials 2014; 26: 63.
Park S H, Hong J W, Shin J H, Yang D Y. Quantitatively Controlled Fabrication of Uniaxially Aligned Nanofibrous Scaffold for Cell Adhesion. Journal of Nanomaterials 2011; 2011: 201969. Open Access
Su Y, Qiu T, Song W, Han X, Sun M, Wang Z, Xie H, Dong M, Chen M. Melt Electrospinning Writing of Magnetic Microrobots. Adv. Sci. 2021; 8: 2003177 Open Access
Wang W, Stipp P N, Ouaras K, Fathi S, Huang Y Y S. Broad Bandwidth, Self-Powered Acoustic Sensor Created by Dynamic Near-Field Electrospinning of Suspended, Transparent Piezoelectric Nanofiber Mesh. Small 2020, 16, 2000581 Open Access
Xu J, Abecassis M, Zhang Z, Guo P, Huang J, Ehmann K, Cao J. Accuracy Improvement of Nano-fiber Deposition by Near-Field Electrospinning. IWMF 2014, 9th International Workshop on Microfactories. Oct 5-8, 2014, Honolulu, USA. Open Access
Zheng G, Li W, Wang X, Wu D, Sun D, Lin L. Precision deposition of a nanofibre by near-field electrospinning. J Phys D: Appl Phys 2010; 43: 415501.

▲ Close list

▼ Credit and Acknowledgement