Study on high throughput nanomanufacturing of photopatternable nanofibers using tube nozzle electrospinning with multi-tubes and multi-nozzles
© The Author(s) 2017
Received: 1 December 2016
Accepted: 13 January 2017
Published: 27 January 2017
High throughput nanomanufacturing of photopatternable nanofibers and subsequent photopatterning is reported. For the production of high density nanofibers, the tube nozzle electrospinning (TNE) process has been used, where an array of micronozzles on the sidewall of a plastic tube are used as spinnerets. By increasing the density of nozzles, the electric fields of adjacent nozzles confine the cone of electrospinning and give a higher density of nanofibers. With TNE, higher density nozzles are easily achievable compared to metallic nozzles, e.g. an inter-nozzle distance as small as 0.5 cm and an average semi-vertical repulsion angle of 12.28° for 8-nozzles were achieved. Nanofiber diameter distribution, mass throughput rate, and growth rate of nanofiber stacks in different operating conditions and with different numbers of nozzles, such as 2, 4 and 8 nozzles, and scalability with single and double tube configurations are discussed. Nanofibers made of SU-8, photopatternable epoxy, have been collected to a thickness of over 80 μm in 240 s of electrospinning and the production rate of 0.75 g/h is achieved using the 2 tube 8 nozzle systems, followed by photolithographic micropatterning. TNE is scalable to a large number of nozzles, and offers high throughput production, plug and play capability with standard electrospinning equipment, and little waste of polymer.
In the last decade, electrospinning has seen its growth in multiple fields such as biomedical engineering, energy storage, and electronics [1, 2]. Applications using electrospun nanofibers include gas sensors, nerve guidance scaffolds, air filters, nano sensors, energy storage capacitors and solar cells [3–8]. A typical production rate of the electrospun nanofibers using a single spinneret is 0.01–0.1 g/h, which may be appropriate for lab usage or small scale experiments, but not for large scale usage and production . Techniques to produce high throughput nanofibers have been studied by scaling up the spinneret count [10, 11]. Multiple spinnerets can be implemented by using an array of metallic needles either in a linear  or a circular  array. Although the multiple metallic needle approach gives a production rate of 0.024 g/h per nozzle which is comparable to the single needle production rate , the metallic spinnerets are bulky, difficult to assemble, and expensive. To implement non-metallic jetting sources, Dosunmu et al.  have used a highly porous reservoir and a conductive wire to charge the polymer. While the technique gives a high production rate of 5 g/h, it shows a wide distribution of nanofiber diameters. By drilling holes half-way through the porous walls, Varabhas et al.  have improved the diameter uniformity but lowered the production rate to 0.3 g/h and increased the operating voltages to 60 kV. Using microfluidic channels, Srinivasta et al.  have demonstrated intricate janus architecture nanofibers, but the open channel approach has lowered throughput to 0.1 g/h with 8 nozzles. Also, drilled holes in plastic films  and drilled holes into syringe filters  have been reported. But these approaches suffer from non-uniformity in nanofiber diameter or not quite scalable.
Electrospinning of photopatternable polymers allows for direct patterning of the nanofibers using UV lithography of the nanofibers. Norbornene based co-polymer PN3TMA6 with photocrosslinkable units (methacrylate)  and Polyethylene oxide (PEO) with photo initiator (Irgacure) in hyaluronic acid  have been reported. Recently, multiple groups including the author group have reported on electrospinning and subsequent photopatterning using commercially available SU-8 [7, 21–24]. Patterned nanofibers have been subsequently carbonized after thermal treatment, resulting in good electrodes for high density energy storage devices such as supercapacitors [7, 21, 22]. While the process has large commercial implications in context of nanomanufacturing, so far only a single spinneret approach has been exercised. Recently, the authors have presented the tube nozzle electrospinning (TNE) process . In this paper, the TNE process is described in detail as a high throughput nanofiber fabrication process using a commercially available photopatternable epoxy SU-8 and the photolithographical patterning of the SU-8 nanofibers is detailed. In TNE, low density polyethylene (LDPE) tubes are adopted and a linear nozzle array has been formed using a computer numerical control (CNC) printed circuit board (PCB) milling machine. The semi vertical angle due to electrostatic repulsion between nozzles, the nanofiber diameter, the nanofiber production throughput and the growth rate as a function of the nozzle count, and photopatterning the electrospun nanofiber stacks are detailed.
Electrospinning setup: SNE and TNE
SU-8 2025 (Microchem Inc.) has an intrinsic viscosity of 4500 cSt with a solid content percentage of 68.45%. SU-8 2025 is optimized for the electrospinning process by diluting it in cylcopentanone (Sigma Aldrich Inc.) to reduce the solid content percentage down to 60.87%. The solution is stirred overnight using a magnetic stirring bar and stored in dark amber bottles.
The multiple intermittent electrospinning technique [10, 21] is used in this work to reduce charge accumulation and nanofiber repulsion effects. The electric charge can accumulate inside the collected nanofiber without sufficient resting time, which can limit the nanofiber stack height by repelling the incoming charged nanofiber during electrospinning. In this work, each cycle has 30 s of electrospinning and 30 s of resting to maximize the nanofiber stack height.
Results and discussion
Taylor cone formation and electric repulsion
Semi-vertical-angles measured for 2, 4 and 8 nozzle architectures showing compression of electrospinning cones due to high density nozzles
Comparison of porosity and average pore size for the 1 tube 2 nozzle TNE architecture with different nozzle diameters and TCDs at constant voltage = 12.5 kV
Nozzle diameter = 0.2 mm
Pore area (μm2)
Nozzle diameter = 0.5 mm
Pore area (μm2)
Production and growth rates of TNE
Comparison of mass throughput for SU-8 based TNE with 2, 4 and 8 nozzles
Nanofiber mass (mg)
Time of collection (min)
Mass throughput (g/h)
1 × 2
1 × 4
1 × 8
2 × 2
2 × 4
2 × 8
Mean and standard deviation of the measured nanofiber stack height was calculated for the membrane over a distance of 1 cm. The TNE technique increases the throughput rate as expected from the reduced SVAs with the higher density of nozzles. The decrease in SVAs not only increases the throughput rate of the process, but also increase the directionality of the electrospun nanofibers. Due to the electrostatic repulsion of adjacent cones, electrospun nanofibers tend to be confined in a narrow space much like the electric field shaping due to an array of charged metallic rings .
Using the 4 and 8 nozzles in the single tube architecture, a growth rate of 0.16 and 0.21 μm/s are obtained, respectively, using the multiple intermittent approach as shown in Table 3. The growth rate is observed to linearly increase until it starts to saturate at thicker collections at 480 s for both the 4 nozzle and 8 nozzle systems. Therefore, the growth rate is calculated in the linear growth region (<240 s) and given by the slope of the linear fitted curve. The saturation in height of the nanofibers is attributed to the electrostatic repulsion from the residual charge in the thicker stacked nanofibers.
Photolithographically patterned nanofibers
Structures with diameters of 20 and 40 μm are patterned with SU-8 nanofibers as shown in the inset of Fig. 8e, f, respectively. Patterning resolution is limited by the light scattering effect due to the refractive index mismatch between SU-8 (n = 1.68) and air (n = 1) where the nominal nanofiber diameter (100–500 nm) and the wavelength of the UV source (i-line, λ = 365 nm) are in the similar range. With the uneven surface of SU-8 nanofibers, the large proximity gap between the mask and the photosensitive nanofibers results in pattern enlargement or deformation due to UV light refraction and diffraction. The previously reported oil-lithography patterning technique  can suppress this artifact to achieve high-aspect-ratio and high resolution microstructures.
Tube nozzle electrospinning (TNE) with multiple nozzles formed on an LDPE tube has been successfully demonstrated, which offers a superior production capability to the conventional SNE approach. The non-metallic nozzles lower the electrostatic repulsion force between nanofibers which contributes to enhancing the growth rate of nanofibers. TNE with 8 nozzles on a plastic tube allows for the improved directionality of electrospun nanofibers, resulting in a low SVA of 12.28°. TNE with multiple nozzles shows the similar relationship of the nanofiber diameter to its operating parameters such as the nozzle diameter, the voltage and the TCD as the SNE system. The multiple intermittent growth technique is used to mitigate the effect of the accumulated charge and enhance the nanofiber growth rate. TNE demonstrates a high nanofiber collection rate of 0.46 and 0.75 g/h in the 1 × 8 and 2 × 8 nozzle architectures, respectively. An average deposition rate of 0.34 µm/s is obtained in TNE with the 2 × 8 nozzle architecture. Multi nozzle TNE shows potential of significant electrospinning time reduction which can ultimately contribute to reducing the manufacturing cost of nanofiber based devices. Lithographic patterning of thick SU-8 nanofibers without damaging the nanofibrous morphology is demonstrated. The result shows that TNE is an excellent nanofiber manufacturing approach with low nanofiber repulsion, high throughput production, high stack growth rate, and scalability.
SPF carried out the simulation, participated in its design, performed the experiments, and prepared the written material. PJ conceived of the study and design, and helped to draft the manuscript. DES participated in the experimental procedure concept and helped analyzing data. KTK participated in experimental setup and statistical analysis. YKY supervised the whole process including the design, experiment, analysis, and preparation of this manuscript. All authors read and approved the final manuscript.
The authors would like to thank Jessica Meloy and Dr. David Arnold for milling machine fabrication tips and training in the Interdisciplinary Microsystems Group (IMG Group), and David Hays and Al Ogden in Nanoscale Research Facilities at the University of Florida for assistance on the SEM measurements.
The authors declare that they have no competing interests.
This work is supported in part by NSF ECCS 1132413.
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