Advances in Near Field Electrospinning

Near field electrospinning is one of the leading techniques in accurate placement of nanofibers. This technique has evolved from using AFM tips to supply a limited volume of solution for electrospinning to nozzle-based system where a continuous stream of solution can be extruded. The precision and accuracy of the fiber deposition has also vastly improved from simple formation of oriented fibers on a collector to precise deposition of the fiber at a specific point. Despite the progress, several intrinsic limitation or characteristic of near-field electrospinning still needs to be noted when employing this technique.

Wet gelatin fiber from near-field electrospinning. [Xue et al 2014. PLoS ONE 9(4): e93590. doi:10.1371/journal.pone.0093590. This work is licensed under a Creative Commons Attribution 4.0 International.]

Spinneret

To spin nanofibers at such close distance, the initial radius of the jet needs to be small since stretching of the solution will be limited. By using an atomic force microscope tip with a small drop of solution at the tip, a small initial spinning radius can be achieved [Kameoka et al 2003]. This method, known as near-field electrospinning, has been shown to be capable of spinning nanofiber over trenches and also to create nanofiber patterns [Sun et al 2006]. However, having a tip with a drop of solution limits the length of fibers that can be produced before having to go for a refill. Using a spinneret with a reservoir of solution generally produces fibers with diameter of a few micrometers [Gupta et al 2007; Xue et al 2014] as there is a limit to which the capillary size can be reduced while allowing the solution to flow through it. The fibers may also be wet at deposition thus giving the fibers a semi-circular cross-section [Xue et al 2014]. Nevertheless, this did not stop researchers from using extremely fine needle to spin nanofibers [Chang et al 2008, Camillo et al 2013]. Chang et al (2008) used a 100 µm diameter needle tip to electrospin polyethylene oxide while Camillo et al (2013) a µm-diameter tip Tungsten spinneret in a 26 gauge needle to electrospin conjugated polymer, poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) blended with polyethylene oxide. King et al (2019) showed in their near field electrospinning of polydioxanone solution, when the needle diameter reduces, there is a significant reduction in the resultant fiber diameter. An 18 Gauge needle (0.838 mm inner diameter) gave a fiber diameter of about 25 µm and a 23 Gauge needle (0.337 mm inner diameter) gave a fiber diameter of 6 µm.

Low voltage

Further development in the near field electrospinning spinning process have attempted to improve fiber deposition precision and reducing fiber diameter. Camillo et al (2013) was able to fabricate 100 nm diameter fiber at an applied voltage of 1.5 kV and a tip to collector distance of 500 µm using a modified fine tip spinneret. Separate reports by Chang et al (2008) and Bisht et al (2011) have shown that higher voltage leads to a significant increase in the fiber diameter (in the micrometer range) and loss of jet stability. The remedy is to significantly reduce the voltage used in the electrospinning process to about 200 to 600 V with tip to collector distance at about 0.5 to 1 mm. However, the charges on the solution drop at the tip of the needle were insufficient to break free from the surface tension to initiate electrospinning without assistance. Chang et al (2008) used a tungsten probe tip and Bisht et al (2011) used a glass microprobe tip (1 to 3 µm tip diameter) to mechanically draw the solution at the tip of the needle to initiate electrospinning. In the study by Chang et al (2008), reduction of electrospinning voltage from 1.5 kV (at tip to collector distance of 500 µm) to 600 V reduces the fiber diameter from 3 µm to 50 nm. Using a lower voltage of 200 V with tip to collector distance of 1 mm, Bisht et al (2011) was able to pattern nanofibers (polyethylene oxide) with diameter below 20 nm. Similar to electrospinning with longer tip to collector distance, it is likely that there is an optimum voltage which the fiber diameter obtained will be at its finest. Voltage higher or lower than this value will see an increase in the fiber diameter. Song et al (2015) showed that when the voltage for electrospinning polystyrene was increased from 400 to 500V, at a tip to collector distance of 20 µm, the fiber diameter reduced from close to 160 nm to about 60 nm. Such fiber diameter response to voltage is due to a balance of stretching of the jet and the speed at which it hit the collector. While increasing voltage causes greater stretching which reduces the fiber diameter, this also causes greater jet acceleration where the stretching terminates when the jet hit the collector.

To use a low working voltage in near field electrospinning while eliminating the need to use a physical object to initiate electrospinning, an alternative is to use a higher voltage for initiation of electrospinning and switch to a lower voltage once the jet has erupted from the nozzle. Huang et al (2014) used this concept with a movable stage collector to produce ordered patterns with interfiber pitch of 50 µm. By controlling the height between the nozzle tip and the collector stage and the speed of stage, fibers with different orientation and cross-sectional shape can be obtained. Generally, closer distance between nozzle tip and collector (ranging from 0.5 mm to 2 mm) results in flat fibers due to impaction of the electrospinning jet. To maintain a high precision of fiber deposition for low voltage near field electrospinning, a smaller distance between collector and nozzle tip is recommended. In the experiments by George et al (2020) using polyacrylonitrile (PAN) solution, when the distance increases to 20 mm, coiled wires are formed instead. As the distance increases, the electrospinning jet has a longer "runway" to pick up speed. When the velocity of the electrospinning jet exceeds that of the collector movement, the wire will start to buckle and form coils on the collector. King et al (2019) described the concept of initiating the electrospinning process with a higher voltage followed by reduction to the desired voltage. Using polydioxanone solution, an initial voltage of -1.8 kV was applied for tip to collector distance of less than 2 mm and -2.0 kV for distances greater or equal to 2 mm for 10 s. The voltage was then reduced to -1.1 kV as the working voltage. A limitation of the setup is that the landing point at electrospinning jet initiation cannot be determined although subsequent adjustment can be made after the jet has landed or the structure can be built up based on the displacement relative to the landing point. To control the landing point of the electrospinning jet, a target point may be set. George et al (2020) used patterned carbon pillars with diameter of about 20 µm and spacing of less than 40µm on a silicon substrate to direct the deposition of electrospun fibers. The electrospun fibers were found to preferentially deposit on the edge of the pillars and an array of fibers were suspended across all the pillars with predetermined spacing. The pillars were also found to be able to guide the fiber to deposit on it even if the electrospinning jet was slightly offset by a small angle.

Patterned deposition of wires on a carbon pillar array [George et al 2020].

Low voltage, near field electrospinning has shown characteristics that differ from conventional near field electrospinning using higher voltage. Fiber diameter has already been shown to be smaller using this technique. With low voltage, near field electrospinning, the fiber diameter was found to be sensitive to the collector stage movement due to mechanical stretching; low velocity giving rise to larger fiber diameter and vice versa [Bisht et al 2011]. King et al (2019) also showed that as the velocity of the translational collector stage increases, the fiber diameter reduces. The electrospun polydioxanone fibers diameter collected at stage velocity of 10 mm/s and 100 mm/s were 10 µm and 3.7 µm respectively. Instead of very fine spinneret tip, Bisht et al (2011) showed that it is possible to spin fibers with diameter less than 100 nm using a 27 gauge needle (approx. 200 µm inner diameter).

Aligned nanofibers using near-field electrospinning of poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene]/polyethylene oxide solution [Camillo D D et al Nanoscale 2013; 5: 11637. doi:10.1039/C3NR03094F. This work is licensed under a Creative Commons Attribution 3.0 Unported License.]

Table: Precision of fiber deposition using Near Field Electrospinning

Description	Pitch between fibers	Fiber diameter	Material	Reference
Near-Field electrospinning	250 µm	15 - 50 µm	Polyhedral oligomeric silsesquioxane-poly(carbonate-urea)urethane, Polyhedral oligomeric silsesquioxane-polycaprolactone-poly(carbonate-urea)urethane	Gupta et al 2007
Near-Field electrospinning	100 µm	1 µm	Polyvinylidene fluoride	Chang et al 2012
Near-field electrospinning, distance 0.5 cm	50 µm	150 nm	Polyethylene oxide	Chang et al 2008
Near-field electrospinning, distance 0.5 cm, speed 20 cm/s	20 µm	700 nm	Chitosan / Polyethylene oxide	Fuh et al 2013
Near-field electrospinning, distance 0.5 mm, speed 5 cm/s	10 µm	1 µm	Polyethylene oxide	Hellmann et al 2009
Near-field electrospinning, distance 20 µm, speed 2 mm/s	~5 µm	~75nm	Polystyrene	Song et al 2015
Near-field electrospinning, distance 0.5 mm, speed 50 cm/s	100 µm	100 nm	Poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene]/Polyethylene oxide	Camillo et al 2013
Near-Field electrospinning distance. 1.8 mm.	Stacking	10 µm	polydioxanone	King et al (2019)

Electric field

Electrospinning jet can be very sensitive to variation in electric field. Thus a target with electric field profile that attracts the jet may be used to guide the electrospinning jet towards the desired landing point. Bisht et al (2011) demonstrated the precision and accuracy of low voltage, near field electrospinning by suspending fiber across carbon post with diameter of 30 µm and interpostal distance of 100 µm. Min et al (2013) used near field electrospinning to deposit semiconducting poly(3-hexylthiophene) (P3HT):PEO-blend organic nanowire over multiple field-effect transistors on a flexible polyarylate substrate at a speed of 13.3 cm/s with regular spacing of 50 µm and fiber diameter of 289 nm. They have also demonstrated the ability to spin highly aligned nanowires from other materials such as poly(9-vinyl carbazole) (PVK) and poly{[N,N'-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5'-(2,2'-bithiophene)}.

The combination of near-field electrospinning and a guiding electrode has the potential to obtain precise and accurate fiber deposition (Read Stable jet and guiding electrode combination, stable-guided-jet.html). Xu et al (2014) used a guiding electrode behind the collector to create a direct line from the nozzle tip to it. This significantly dampens the deviation of the electrospinning jet from its original path as a result of electrostatic repulsion from the preceding deposited nanofiber. Without the guiding electrode, the near-field electrospun fibers have a spread of 74 µm. With the guiding electrode, the spread was reduced to just 7 µm. This raises the possibility of building up 3D structures using electrospinning. 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].

Charges

The charges retained on the deposited fiber have also been shown to negatively impact the ability to construct ordered structures in near-field electrospinning. Even with a conductive collector, build-up of residual charges due to stationary deposition of fibers has been shown to cause oscillation in the otherwise stable jet [Zheng et al 2010]. The effect of residual charges can also be seen when helical fibers start to form when the collector speed is unable to match the fiber ejection speed [Zheng et al 2010]. The impact of residual charge on controlled fiber deposition is magnified when a non-conducting material is used as the collector. Zheng et al (2014) showed that using a DC high voltage power supply for near field electrospinning on a polyethylene terephthalate (PET) substrate generated only randomly oriented nanofibers. The charges retained on the deposited fiber were sufficiently high to deflect the initiating jet on the tip of the nozzle. To reduce the charges on the deposited fibers, an AC high voltage power supply was used instead for near field electrospinning. This allows a vertical electrospinning jet to be maintained and collection of closely spaced fibers (about 20 µm inter-fiber spacing).

Collector

In near field electrospinning, one of the risks is electrical shorting due to the proximity of the charged needle tip to the grounded collector. Any electrical shorting will disrupt the electrospinning process and result in discontinuous fiber. While using a lower voltage may reduce this risk, an alternative is to use a less conductive collector. Liu et al (2014) used a rotating glass tube with a copper foil lining at the inner surface of the tube for the collection of electrospun oriented polyvinylidene fluoride (PVDF) fiber. While initial fiber alignment was excellent, the alignment starts to deteriorate after prolonged fiber deposition which can be attributed to the presence of residual charges.

The influence of residual charges on the precision of fiber deposited is more pronounced when an insulating surface and a conductive surface were used as collectors. Choi et al (2017) used a hydrophobic and insulating acrylic substrate as collector. To increase conductivity of selected region of the collector, plasma treatment was carried out to render those region hydrophilic. The collector was placed in a high humidity environment such that the hydrophilic region will be slightly conductive due to the presence of water molecule attached to it. Near field electrospinning of polyurethane showed that on the insulating hydrophobic surface, the fibers were twisted and curved due to weak electric field profile between the emitter and the collector surface and the inability of charges to escape. In contrast, on the hydrophilic region, the fiber were placed in accordance to the movement of the emitter relative to the collector. This shows that electrical charges on the electrospinning jet needs to escape for precise deposition and even an insulating substrate with slight conductivity is crucial for ordered fiber arrangement.

Concentration

In electrospinning, it is generally agreed that with higher concentration, the diameter of the fibers increased due to greater viscosity which resist stretching. In near field electrospinning, similar observation has been reported where concentration increases, fiber diameter increased [Chang et al 2008; Zheng et al 2012]. However, in separate studies by Pan et al (2014) using poly(γ-benzyl α, l-glutamate) and Pan et al (2015) using polyvinylidene fluoride (PVDF) reported reduction in fiber diameter with increasing concentration. Pan et al (2015) attributed this to a higher charge accumulation in higher concentration PVDF solution. However, more studies need to be carried out to verify this hypothesis.

Melt 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.

Box structured scaffolds printed upon a microscope slide, demonstrating the uniformity of the filament patterns. (A) Due to the high deposition accuracy on a glass slide there are optical effects when light is held in the background. (B) and (C) SEM images of such scaffolds tilted at 30° to show the regularity of the scaffold. [Hochleitner et al 2015. Biofabrication 2015; 7: 035002. This work is licensed under a Creative Commons Attribution 3.0 Unported License.]

Multi-nozzles

For near-field electrospinning to be used at an industrial level, a multi-nozzles system will certainly increase the output if the patterns are repeating. Therefore, it is important to understand the factors that influence precision and accurate deposition of nanofibers when multiple nozzles are used. Wang et al (2015) investigated the parameters for multiple nozzles near field electrospinning. The parameters investigated include electrode-to-collector distance ranging from 2 to 6 mm and nozzle spacing between 2.1 to 3.5 mm. From mass production of conventional electrospinning, it is known that the electric field interference between the electrospinning jets will have an impact on the process. Similarly, when there are multiple nozzles, the distance between the deposition points are larger than the distance between the nozzles. With more nozzles, the deposition distance increases. In their setup, two to six nozzles are placed in two rows. Further research is needed to determine the impact of the fiber deposition when more nozzles in other configurations are used. The accuracy and precision of the fiber deposition from each nozzle will also need to be investigated when used as a group.

(a) Deposition distribution changes with different electrode-to-collector distance. (b) Deposition distribution changes with different nozzle spacing. (c) Deposition distribution changes with different applied voltage. [Wang et al 2015. This work is licensed under a Creative Commons Attribution 3.0 Unported License.]

Using dual nozzles setup, Wang et al (2017) did a detailed study on the parameters influencing the deposition gap between them. Parameters such as needle length, needle spacing, applied voltage and electrode to collector distance were examined. The top three dominant parameters were found to be needle spacing, electrode to collector distance and needle length. It is easy to understand the influence of needle spacing and electrode to collector distance on the gap between deposited fibers. As electrospinning jets tend to repel one another, increasing the distance between the electrode and collector allows greater divergence upon deposition. With increasing needle length, the gap between deposited fibers increases. This has been attributed to its influence on the electric field profile. With a longer needle, the vertical component of the electric field gets reduced as the charges were spread over a longer length of the needle. In an investigation by Wang et al (2018) using multiple nozzles arranged linearly, they found that the deposition distance between fibers were nearer to the inter-nozzle distance when the number of nozzles increases. This trend is the same regardless of voltage and tip to collector distance. This is contrary to an earlier investigation by another group [Wang et al 2015] which reported increasing fiber deposition distance with increasing nozzle numbers. However, it is important to note that in the earlier investigation, the nozzles were arranged in two rows while this setup is in a single row. In the study by Wang et al (2018), although the average deposition distance between the fibers were reduced with more nozzles, the coefficient of variation on deposition distance increases. That means there are greater spread in the fiber distribution. Reduction in deposition distance between fibers when the number of nozzles increases may be attributed to electric field shielding from neighbouring nozzles. Deposition distance between fibers from marginal nozzles were greater than intermediate nozzles since the intermediate nozzles experience electric field shielding. For the same voltage, there is also less volume charge density per nozzle leading to a weaker electric field strength per nozzle. Therefore, a smaller interfiber distance was recorded for more nozzles. Zhang et al (2019) showed that the nozzle arrangement has a significant impact on near field electrospinning of polyethylene oxide (PEO) due to electric field interference between the nozzles. Between a linear nozzle arrangement and a 2-rows alternating nozzle arrangement, the latter has a greater electric field interference. As the electric field interference increases mutual repulsion and reduces the electric field strength near the nozzle tip, the average diameter of the resultant fiber from the 2-rows alternating nozzle arrangement is thicker than the linear arrangement. The same interference also causes greater uneven fiber deposition and variance in fiber diameter. Increasing the number of nozzles also increases electric field interference regardless of whether they are arranged linearly or 2-rows alternating. This also causes a deterioration in the quality of the fiber output. In near field electrospinning, there is a limit at which the voltage can be increased without causing electric arcing. Therefore, parameters that influence electric field interference needs to be addressed for multi-nozzles near-field electrospinning setup.

One method that has been used successfully in conventional multi-nozzles electrospinning to create a more uniform electric field strength across linearly arranged nozzles is to have the nozzle tips aligned in an arc vertically [Jiang et al 2019]. That is, the center nozzles are arranged at a height nearest to the collector and the height progressively increases towards the ends. This creates a more uniform electric field since the shielded center nozzles are now nearer to the collector. By varying the depth of the arc, the electric field strength across the nozzles can be adjusted. Comparing the electrospinning of polyethylene oxide (PEO) solution from 9 nozzles arranged linearly, between nozzle tips arranged in the same plane and in an arc, the former had solution dripping from the center nozzles while the latter showed continuous electrospinning across all 9 nozzles at optimal arc arrangement.

Core-shell fibers

Near-field electrospinning has evolved to enable the production of core-shell fibers. Pan et al (2015) used a co-axial nozzle for electrospinning of poly(vinylidene fluoride) (PVDF) hollow fiber tubes. To create the hollow tubes, compressed air was ejected from the core of the nozzle during electrospinning. The core nozzle has an inner diameter of 0.63 mm while the outer nozzle has an inner diameter of 1.07 mm. The resultant fibers have a diameter of about 11 µm with inner lumen diameter of about 3 µm.

Assisted Airflow

Assisted airflow electrospinning or electroblowing uses a curtain of blowing air around the electrospinning nozzle tip to facilitate jet initiation and stretching of the jet. This reduces the voltage required for electrospinning and also helps to direct the electrospinning jet. When used with near field electrospinning, this increases the precision and accuracy of the fiber deposition. However, unlike most near field electrospinning setup, the distance between the nozzle tip and collector cannot be too close or there will be air turbulence from the blowing air bouncing off the solid substrate. Jiang et al (2018) showed that the optimum distance for assisted airflow near field electrospinning was 2 mm for polyethylene oxide (PEO) solution. Both precision and accuracy were improved with assisted airflow. When combined with optimised collector velocity, positioning errors of less than 5% has been printed for complex patterns.

Other Variations

Some researchers have used other unique materials and devices to create near field electrospinning. Coppola et al (2020) constructed an alternative form of near-field electrospinning, named, pyro-electrohydrodynamic tethered electrospinning. In their setup, a pyroelectric material, lithium niobate (LN) crystal plate was used to generate the electric field. This LN crystal plate was placed behind a glass microslide collector. A small drop of poly(lactic-co-glycolic acid) (PLGA) solution was placed at a distance of less than 1 mm from the crystal plate. Heating of the lithium niobate (LN) crystal plate generates charges and at sufficiently high voltage, a single stable electrospinning jet would erupt from the PLGA solution droplet. The target collector would move as the fiber deposited on it to form the desired structure. The resultant fiber has a diameter between 10 µm < d < 30?µm. The precision of the fiber deposition is such that the multiple layers of fibers can be stacked on top of one another. A wall made of 10 fiber layers was constructed with good superimposition and homogenous stacking, no spaces or defects.

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