A new versatile x–y–z electrospinning equipment for nanofiber synthesis in both far and near field

This work describes a versatile electrospinning equipment with rapid, independent, and precise x–y–z movements for large-area depositions of electrospun fibers, direct writing or assembly of fibers into sub-millimeter and micron-sized patterns, and printing of 3D micro- and nanostructures. Its versatility is demonstrated thought the preparation of multilayered functional nanofibers for wound healing, nanofiber mesh for particle filtration, high-aspect ratio printed lines, and freestanding aligned nanofibers.


Results and discussion
Design and set-up of the electrospinning equipment. A drawing of the electrospinning equipment is presented in Fig. 1. The equipment is designed for rapid and independent x-y-z movements without compromising accurate positioning and precision movements to allow its versatile use for depositing electrospun fibers over large area, writing and assembling fibers into precise sub-millimeter patterns with micron-size features, or precision printing of 3D microstructures. Besides precision movements, an accurate control over the electric field is a must to not only control fiber deposition but also its diameter and size. This combination is responsible for highly reproducible fiber production.
Large-area electrospinning. Electrospun fibers can be deposited over a large area to form a fiber mat, as shown in Fig. 2. The x-y movement of the collector base can be programmed to obtain different sample sizes and shapes. For example, a rotational plate motion in Fig. 2A defines a circular deposition area as shown in Fig. 2A inset. Similarly, programming a raster movement (Fig. 2B) deposits a rectangular fiber mat on the collector base, as shown in Fig. 2B inset. It ensured uniform deposition and thickness across the entire electrospun sample. Thus, it is possible to seamlessly electrospun multiple layers of functional fibers, as illustrated in Fig. 3. Figure 3B present an electrospun PVA (polyvinyl alcohol) fiber loaded with 10 wt.% amoxicillin and clavulanic acid (Fig. 3C) sandwiched between HPMC (hydroxypropyl methylcellulose) layers (Fig. 3A). The HPMC nanofibers are hypoallergenic and have excellent biocompatibility and skin adhesion 24,25 , while PVA is an excellent matrix for storing and releasing drug molecules 26,27 . The two HPMC layers are 5.2 μm thick and consist of    Fig. 4 and Supporting Information. HPMC has good solubility in an appropriate mixture of water and ethanol. Moreover, ethanol has a lower surface tension, faster evaporation rate, and is slightly (ca. 20%) more viscous than water. It, therefore, favors the preparation of thinner nanofibers. Indeed, adding more ethanol (i.e., H 2 O: EtOH from 2.0 to 1.5) sharply decreased the fiber diameter by roughly 150 nm from 410 ± 45 nm (Fig. 4A) to 240 ± 70 nm (Fig. 4B) and mean pore diameters of 210 ± 110 nm and 170 ± 60 nm, respectively. An HPMC dissolved in 1 H 2 O:1 EtOH solution produces thin nanofibers (Fig. 4C) with a mean diameter of 100 ± 30 nm and a mean pore diameter of 140 ± 50 nm. It has the smallest pores among the three samples and the best drug dosing characteristics. Therefore, it serves as the HPMC layers in the HPMC/PVA/HPMC multilayer sample presented in Fig. 3.   Fig. 5, where a single strand of 120 nm PEO nanofiber was precisely drawn into a 500 mm mesh (Fig. 5A) covering a 20 mm × 20 mm area of the carbon substrate as shown by the optical microscope picture in Fig. 5B. A closer inspection shows the strands are perfectly aligned and intercepting perpendicularly (i.e., 90 ± 2°) at the nodes to form a square opening, as shown in Fig. 5C. Individual strand does appear to twist a little bit, resulting in slight slack. However, the sides of the mesh opening deviate by less than 10% from 500 mm. The entire pattern is written in 100 s at a speed of 18 mm/s. Similarly, PEO nanofibers can be directly written on conducting ITO glass ( Fig. 5D), demonstrating its potential in microelectronic fabrication. Here, thicker PEO strands were electrospun to create a similar mesh pattern over a 10 mm × 10 mm area. Finally, these 150 nm PEO nanofibers were laid down in a regular grid pattern, as shown in Fig. 5E. A strong electric field and fast writing speed give nanofibers of narrow diameter. It is done by increasing applied voltage, lowering spinneret height, and increasing x-y movements. It is possible to detach and lift the electrospun mesh from the substrate as a sieve filter for particles. It would represent an ideal capture surface for investigating particulate filtration, especially of aerosols and droplets with sizes of 0.1 to 10 μm responsible for the airborne transmission of respiratory diseases. The computational calculations in Fig. 6 show that filtration efficiency will depend on the mesh opening size. The simulation done in Comsol Multiphysics was conducted for mesh openings of 50 and 5 microns, and for particle sizes ranging from 0.3 to 10 microns. The filtration efficiency increases with finer mesh openings and larger particulates. The results demonstrate that even coarse filter mesh openings of 5 μm (Fig. 6A) can reach MERV10 standards at higher flow and lower pressure drops (Fig. 6B). Particulate capture also relies on nanofiber's material and surface properties as it is governed by the molecular interactions between particles and surfaces, and the capture rate can be enhanced by increasing particle adhesion and minimizing rebound and resuspension.
Near-field printing of 3D micro-and nanostructures. Figure 7 displays examples of near-field printing of PEO nanofiber on a copper wire surface. The 90 µm diameter wire is 8 cm long and held in place as illustrated in Fig. 7A. Along the length of the wire, strands of PEO nanofibers were printed atop each other to create a narrow wall. The spinneret height (z-axis) was positioned 4 mm above the wire for printing the five strands of 8.6 ± 0.08 µm PEO fiber in Fig. 7B. The spinneret was programmed to move a 10-mm step upward every 10 s while the voltage was varied from 3.0 to 3.6 kV during printing to produce the 43 ± 0.1 µm high printed feature (Fig. 7B inset). The printing was done at a speed of 18 mm/s. A slight warping observed under the scanning electron microscope was an artifact caused by sample preparation. Fifty layers of 440 ± 16 nm PEO nanofibers shown in Fig. 7C were printed at a speed of 23 mm/s and a starting spinneret height of 3 mm with incremental upward z-movement of 10 µm per 15 s. The voltage was varied from 2.0 to 3.4 kV during printing. An incision made on the printed sample shows perfect alignment of the nanofibers along the entire thickness (Fig. 7C inset). www.nature.com/scientificreports/ Figure 7D shows a printed nanostructure consisting of 140 printed strands of 102 ± 10 nm PEO nanofiber on the copper wire. It was printed at a high speed of 300 mm/s with the z-axis adjusted upward by 10 µm every 20 s. Despite its narrow width, the thin nanofiber filaments are perfectly aligned, as shown in Fig. 7D inset. The freestanding feature is 102 nm thick and 14.3 ± 0.002 µm high giving it an aspect ratio of 142, while samples in Fig. 7B and C have aspect ratios of 5 and 50, respectively. This clearly demonstrates the new electrospinning equipment can print 3D micro-and nanostructures with control and precision. Figure 8 illustrate near-field electrospinning could be used to weave freestanding nanofibers between two copper wires acting as warp beams in a miniature "loom". Here, PVA nanofibers were strung between the wires at a rapid speed of 20 mm/s along the x-axis with the y-axis being offset to the left at 20 µm per minute to create the suspended aligned nanofibers in Fig. 8B. The scanning electron micrograph clearly shows each individual strand is separate and has a diameter of 130 ± 12 nm. It is robust and can be handled and prepared for microscopy examination. It is possible to program the near-field electrospinning process to weave more complex design to create functional textile mats from different electrospun nanofiber materials.

Conclusions
The versatile electrospinning equipment can seamlessly operate in both far-field and near-field conditions, made possible by the rapid but precise x-y-z movements with accurate positioning and electrical field control that enables large-area deposition of electrospun fibers, direct writing of nanofibers into sub-millimeter and micron-size patterns, and printing of 3D micro-and nanostructures. The equipment was successfully used to prepare multilayered functional nanofibers for biomedical application of wound healing. In addition, submillimeter square mesh made of nanofibers were printed over an extended area for studying dynamics of particulate capture and filtration. Finally, high aspect ratio nanofiber lines with narrow width were printed at high speed to demonstrate 3D nano-printing process.

Methods
Electrospinning equipment. The base collector was mounted on a motion platform with independently x and y movements controlled by two sets of timing belt, screw shaft, and stepper motor (SKU:81042, 42BYG) through a main board connected to Arduino (Megablock MegaPi) and a PC computer that enable rapid long-distance movements (100 mm/s), precise positioning (± 5 μm), and fine micron-step motions (1.8 ± 0.09°/PHASE and 400 steps/mm) for a seamless large-scale precision electrospinning. The movement can be programmed to deposit, write, or print different patterns of electrospun fibers. The z-movement controlled the distance of the spinneret from the collector and used a set of stepper motor and shaft to give precision movements as small as www.nature.com/scientificreports/ 5 mm. The interchangeable spinneret assembly was supported by a cantilevered 3D printed arm that also held a light source and a USB camera (LED Torch Lighting EVER, 8 LED USB camera microscope x1000 magnification) for monitoring the electrospinning. The base and the spinneret were connected to a programmable power supply (NanoNC PS355) and calibrated to deliver precise electric field for controlled electrospinning. The spinneret was designed and fabricated in the laboratory and connected to a syringe pump (kdScientific KDS200) that delivers 0.004µL/h to 3520mL/h of electrospinning solutions. The electrospinning equipment was housed in a transparent chamber equipped with a set of DC fan and humidifier/dehumidifier units to maintain the temperature and humidity during electrospinning. The chamber is connected to a local exhaust to vent solvents. Detailed description of the electrospinning equipment and operation is available in Supplementary Information.