Stable electrospinning jet

For precision electrospinning, one approach is to have a stable and predictable jet path. An electrospinning jet generally comprises of three segments, the Taylor cone, the stable segment and the bending instability segment. Without any modifications, the stable segment of the electrospinning jet is about 10 mm from the jet initiation before bending instability sets in. In the bending instability segment, it is not possible to predict precisely the location electrospinning jet in space. Therefore, the key is to use stable region where the deposition location of the fiber is predictable thus forming pattern on a collector is like drawing using the electrospinning jet. Stable jet and guiding electrode combination is able to direct the electrospinning jet. However, given the speed of the electrospinning jet, the movement of the collector or the spinneret must be sufficiently fast to manage the fiber deposition speed. When the speed of the collector relative to the spinneret is too slow, the fibers would start to deposit on and on the side of the deposition path. Jeong and Lee (2016) used a slower collector speed to form microfiber patterns with ivy shoot-like geometries. Their studies also showed that the feed-rate will affect the formation and clarity of the ivy-shoot pattern as this parameter affects the rate of evaporation of the deposited fibers and its diameter.

Influence of the scan speed of the collector on the shape morphology of the fractal geometric microfibrous patterns: SEM images of the fabricated microfibrous patterns at scan speeds of (a) 10; (b) 25; (c) 50; (d) 75; (e) 150; and (f) 450 mm/s. The scan path distance of fabricated microfibrous patterns was (a-e) 500 µm and (f) 50 µm [Jeong and Lee. Materials 2016; 9: 266].

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.]

For electrospinning within the stable region of the jet, some researchers bring the collector to within millimetres of the nozzle tip.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.

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.

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. Sugimoto et al (2019) was able to use near field low voltage electrospinning to create ordered nanofibers made of M13 bacteriophage (phage). The distance and voltage used for their electrospinning was 5 mm and 600-700 V respectively. The resulting electrospun phage fibers showed high crystallinity with phage fibrils oriented in the direction of the fiber axis. In contrast, wet spun phage fibers have liquid crystalline phage structure which were loosely oriented through the long axis. With sufficient collector speed, they were able to construct highly aligned fibers. A 1000 cm/min wiring velocity was needed to get relatively accurate placement of the fibers. However, the variance was large at 10 µm either side of 20 µm fiber deposition interval. 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]. 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).

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. 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.

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. Such combination of near-field electrospinning and a guiding electrode has been shown to be able to obtain precise and accurate fiber deposition (Read Stable jet and guiding electrode combination).

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

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).

Left. Deflected electrospinning jet in near field electrospinning due to charge retention of deposited fibers on insulating substrate when DC high voltage power supply was used. Right. Vertical electrospinning jet in the same setup when an AC high voltage was used due to reduced charge on deposited fibers. [Zheng et al 2014. Journal of Nanomaterials, vol. 2014, Article ID 708186, 7 pages, 2014. doi:10.1155/2014/708186. This work is licensed under a Creative Commons Attribution 3.0 Unported License.]

The main contribution of bending instability in an electrospinning jet is the charges on the surface of the jet whose repulsion overcomes the inertia and the damping viscoelastic force of the jet. Moon et al (2021) showed that by applying the high voltage to the collector plate instead of the electrospinning nozzle, they are able to create a condition that effectively suppresses the bending instability of the electrospinning jet. Moon et al (2021) hypothesised 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 are 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)

Coppola et al (2020) constructed an pyro-electrohydrodynamic tethered near-field electrospinning where a pyroelectric material, lithium niobate (LN) crystal plate was used to generate the electric field behind the 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 10µm < d < 30µm. 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.

Electrospinning melt polymers at close distance to the collector (30 mm) has also been carried out to construct highly ordered structure. The translational speed of the stage needs to match the spinning speed to lay the fibers along a straight line. Electrospinning of polycaprolactone with a applied voltage of 10 kV, a translational collector speed of 2.5 x 10^-2 m/s was able to achieve this. The resultant structure has controlled pore size of 46 µm and was demonstrated to support good infiltration of cells [Farrugia et al 2013]. Increasing the tip to collector distance to allow more stretching of the electrospinning jet will cause a corresponding loss in the precision of fiber deposition as the jet enters the bending instability stage [Hellmann et al 2009]. In another demonstration of the stability and accuracy of melt electrospinning, Brown et al (2014) was able to stack electrospun polycaprolactone fibers lengthwise on top of one another until 50 layers were formed. Beyond that, the accurate fiber placement is lost which was attributed to build-up of internal residual charge within the fiber. It was hypothesized that the preference of the fibers to stack on top of one another is due to shorter distance between the nozzle tip and the top of the fiber as compared to the ground and that residual charges are trapped inside the fiber instead of at the surface. Thus the electrospinning jet preferentially deposits on top of existing fiber layer. The impact of accumulated residual charges is significant for precision electrospinning as it restricts the number of fibers that can be stacked up continuously and it probably will affect the distance tolerance between fibers.

SEM images of 3D melt printed PCL deposited in 120 degree turns taken at A) low magnification and B) higher magnification. C) SEM image of 3D melt printed PCL deposited in 90 degree turns. Images courtesy of Prof Paul Dalton.

To maintain a stable jet in melt electrospinning, Hochleitner et al (2016) investigated some parameters that may result in fiber pulsing which is the undesirable sectional oscillation of fiber diameter. Presence of fiber pulsing will affect both the quality of the fiber and its placement. Once the deposition speed and the collector speed have been optimized to collect a straight fiber, it was found that the flow rate and the applied voltage will influence the rate of pulsing. The flow rate will determine the amount of materials available for stretching and the voltage will apply the necessary force to stretch it. When the flow rate is excessive and the applied voltage is insufficient to stretch the material, pulsing will set in. However, there is a limit which the voltage can be increased beyond which arcing and breakdown of the electric field will occur. Therefore, an optimum value of applied voltage corresponding to the flow rate can be determined for stable fiber deposition. In near field melt electrospinning writing, the high viscosity and proximity between the spinneret tip and the collector meant that there is a certain stiffness in the electrospinning jet. This stiffness resist buckling from the electrical charges but it also generates a tensile drag force on the jet as the collector moves faster than the ejection of the polymer melt. When this translational motion is sufficiently high, it helps to straighten the deposited fibers on the collector although this also introduces a lag effect as the jet gets pulled away from the spinneret tip [Jin et al 2020]. Jin et al (2020) used this "lag effect" to introduce a higher tensile drag force which will resist chaotic bending from the electrical charges and improve accuracy of the deposited fibers. With this, they were able to create highly controlled patterns although the buildup of residual charges above 100 layers will interfere with the deposition if the adjacent fiber is too close. Therefore, they used a 80 µm gap between fibers for their fiber pattern. However, it may also be argued that in this case, it may be "tensile drawing" of the jet from the spinneret instead of "electrospinning" that pulls the fiber into the desired diameter since the "lag" due to the tension is greater than the effect of electrostatic charges on the spinning jet. Unfortunately, the study did not verify whether the fiber diameter is due to tensile stretch or electrostatic stretch.

Given the limitation of near field electrospinning, an alternative method is to try to lengthen the stable segment of the electrospinning jet such that the distance where the jet remains stable is large enough for stretching of the solution to the nanometer scale. Stable jet length of more than 5 cm can be expected through optimizing the spinning parameters. Table 2 is a summary of parameters that encourages greater jet stability.

Electrospinning relies on the presence of charges to create the stretching force required to extend the solution jet to form nanofibers. Since force from the charges applies in all directions, this contributes to bending instability in the electrospinning jet. Thus, reducing the charges of the electrospinning jet to the minimum required to extend the solution will reduce bending instability. This can be achieved by optimizing the voltage applied [Qini et al 2010] and selection of the solvent based on its conductivity and/or dielectric property [Sun et al 2012]. Shin et al (2001) showed that with aqueous PEO, the jet is stable below 1 kV/cm. Between 1 kV/cm to 1.5 kV/cm, bending instability sets in. However, when the jet is stable, it is not dry at deposition and results in coalescence of fluid instead of fiber formation. Thus, a more volatile solvent will be appropriate to compensate for short flight duration between initiation and deposition. Given the influence of charge density on the electrospinning jet on its stability, increasing salt additive to the solution led to a reduction in stable length [Yalcinkaya et al 2013]. Another way is to reduce the surface charge density of the jet by encouraging gas ionization. Korkut et al (2008) found that in a higher humidity environment, gas ionization increases and this led to greater stable jet length. Charge density on the electrospinning jet may also be reduced by using less conductive solvents or solvents with lower dielectric constant. Wu et al (2018) showed that when electrospinning solution with lower dielectric constant, jet stability increases. For polystyrene (PS), using N, N-dimethyl formamide (DMF) as the solvent gave a stable jet of only 3.5 mm. DMF is known to have a high dielectric constant. Using chloroform which has a low dielectric constant, the stable jet length increases to 21 mm. In the same study, tetrahydrofuran (THF) which has a higher dielectric constant and slightly higher boiling point than chloroform showed a stable jet length of 7.5 mm. However, more polymer-solvent combination will need to be tested to substantiate this hypothesis since chloroform has a lower boiling point than DMF and THF and this may have contributed to longer stable jet length.

In an electrospinning set up with a base-plate configuration, increasing the protrusion of the needle beyond the base-plate was found to increase the stable jet length [Korkut et al 2008]. This small adjustment in the needle protrusion may bring about other beneficial changes in the electric field to bring about greater stable jet length. Increment in stable jet length due to needle protrusion has been attributed to increasing tangential electric field on the electrospinning jet which creates a straight electrical passage from the needle tip to the collector [Reneker et al 2000, Yang et al 2008]. Hohman et al (2001) described these as fringe fields where the local electric field near the nozzle is higher than the average field between the two plates. There is also a limit to the needle protrusion beyond the base plate in exerting a tangential force to maintain a stable jet. When the needle tip is flushed to the base plate, no stable jet length was observed in the electrospinning of polyurethane and polyethylene oxide. With a 2 mm protrusion of the needle, a stable jet length was observed for each solution. However, with further increase in the protrusion, the stable jet length progressively shortened. This reduction in jet length was attributed to deformation of the electric field around the charged needle tip [Yalcinkaya et al 2013].

Increasing the electrospinning jet "stiffness" or resistance to bending may also facilitate in maintaining its stability. A higher surface tension may reduce the escalation of jet perturbation and improves stability. Solution with greater viscosity will also contribute to maintaining jet "stiffness". Zhou et al (2013) showed using poly(L-lactide acid) (PLLA) that increasing concentration and polymer molecular weight increases the stable electrospinning jet length. Higher viscosity will also increase the electrospinning jet radius at initiation and a larger diameter will also increase its resistance to bending [Yalcinkaya et al 2013]. Increasing the flow rate [Ou et al 2011] and larger nozzle diameter [Thompson et al 2007] will also increase the initial jet radius. Factors such as using more volatile solvent or increase the ambient spinning temperature will favor faster solvent evaporation and accelerating the "stiffening" of the electrospinning jet. Using solvents of different volatility, Zhou et al (2013) demonstrated increasing stable jet length from PLLA solution of increasing volatility. However, since different solvents were used to prepare the solution, it is also possible that there may be other factors in play although the result is in agreement with this concept. Yuan et al (2012) used ultrahigh molecular weight poly(ethylene oxide) (UHMWPEO) (Mw > 5000kDa) for blending with other polymers to form long and stable electrospinning jet. The presence of UHMWPEO raises the solution viscoelasticity and this contributes to the jet stability. Doping poly(L-lactide), polycaprolactone and chitosan solution with UHMWPEO has been shown to generate a stable electrospinning jet across a tip to collector distance of 15 cm. The resultant fiber diameters were about 1.5 µm or more. With the stable electrospinning jet, they were able to construct highly ordered structures. Unlike near-field electrospinning, given the substantial tip to collector distance, they were able to use this method to construct patterned three-dimensional fibrous scaffold [Yuan et al 2015]. Christ et al (2022) was able to obtain a long stable electrospinning jet with poly(methyl methacrylate) (PMMA) using 2-butanone as the solvent and a solution concentration of 35 wt%. Solvents of higher volatility such as tetrahydrofuran (THF), trichloromethane and dichloromethane were used but due to rapid evaporation of the solvent, residues of the polymer formed at the nozzle tip after a while and this led to clogging of the tip. 2-butanone with a higher boiling point was able to give consistent electrospinning. At an initial test concentration of 25 wt%, the stable length of the electrospinning jet using 2-butanone as the solvent is much shorter. This contrasts with the long stable jet when THF was used as the solvent at 25 wt%. In the case of THF, more rapid vaporization and stiffening of the jet probably allows the jet to maintain a stable jet with a lower concentration. However, the same property that contributes to clogging of the tip makes THF a poor choice. With 2-butanone, increasing the concentration of PMMA to 35 wt% was able to achieve the right balance of solvent vaporization, jet stiffness and uninterrupted electrospinning. To further facilitate the elongation of the electrospinning jet, a negative high voltage was applied to the rotating drum collector. Highly aligned PMMA fibers with angle of distribution within 80 +5° were successfully collected.

Table 2: Parameters affecting length of stable jet

Factors	Effect	Source
Static charges on jet	Increase bending instability therefore reduces stable jet length	Voltage applied to the spinning solution [Qin et al 2010] Conductivity and dielectric property of the solvent [Sun et al 2012]
Electric field (Tangential) [Reneker et al 2000, Yang et al 2008]	Stabilizes electrospinning jet. Increasing voltage increases stable jet length [Angammana 2011, Wu et al 2012]	Electric field from the spinning nozzle
Electric field (Base electrode with a short length of needle extending beyond base electrode) [Yang et al 2008]	Creates a uniform electric field and reduces tangential electric field effect. Reduces stable jet.	Electric field from base electrode
Electric field (Base electrode with a sufficient length of needle extending beyond base electrode) [Kiselev P et al 2012]	Creates a uniform electric field and while maintaining tangential electric field effect. Increases stable jet length.	Electric field from base electrode
Gas ionization [Korkut et al 2008]	Reduces surface charge density of the jet. Increases stable jet length.	Gas ionization from electrospinning needle; high electric field density
Electric field (focusing and guiding electrode)	Creates a invisible barrier that constrain the spinning jet. [Deitzel et al 2001] Improves stable jet length	Electric field profile generated by the auxiliary electrodes
Surface tension	Reduce escalation of perturbation thereby stabilizes the electrospinning jet [Reneker et al 2000].	Solution property
Viscosity	Increase solution viscosity stabilizes the jet as it reduces the escalation of perturbation	Solution concentration [Ou et al 2011] Molecular weight [Feng et al 2013] Melt polymer [Dalton et al 2008]
Spinning jet radius	A larger spinning jet radius is better able to resist bending and thus improves jet stability	Flow rate [Ou et al 2011] - Higher flow rate, larger jet radius [Milleret et al 2011, Wang et al 2009] Viscosity Nozzle diameter - Larger nozzle diameter, larger jet radius [Thompson et al 2007]
Flow rate	Modelling showed that larger flow rate leads to longer stable jet [He et al 2005]. This has been demonstrated experimentally by Angammana[Angammana 2011]
Solvent volatility	Faster vaporization and corresponding stiffening of electrospinning jet leading to longer stable jet length [Zhou et al 2013]	Solvent property

Deposited nanofibers within a narrow width

In a demonstration on the application of precise and accurate deposition of electrospun fibers, 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)}. 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.]

From the production quality perspective, there are several outputs that need to be measured in precision electrospinning such as fiber diameter consistency and variance in the pitch between fibers. One of the most important measurements on the process is probably the variance of the pitch between fibers and the pitch of the moving collector. For example, if the collector stage moved by 200 µm, the pitch between the fibers should be about 200 µm. The distance between the collector stage and a base line and the distance between the deposited fiber and the same baseline must also be measured to ensure that the process is under control. Yan et al (2013) did a pilot study on the variance of the pitch between fibers and the pitch of the moving collector using near field electrospinning of polyvinyl alchohol (PVA)/Chitosan. The distance between the spinneret tip and collector was 3 mm and the voltage applied was 2 kV. At a stage moving pitch of 400 µm and 200 µm, the measured spacing between the fibers were 387 µm and 203 µm respectively. This works out to an error of less than 3.25%. More rigorous studies are required to determine the ability of the near field electrospinning to control fiber spacing and deposition.

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