new methods to electrospin nanofibers · journal of engineered fibers and fabrics 34 volume 6,...

Journal of Engineered Fibers and Fabrics 32 http://www.jeffjournal.org Volume 6, Issue 3 - 2011

New Methods to Electrospin Nanofibers

George G. Chase, Jackapon Sunthorn Varabhas, Darrell H Reneker

University of Akron, Akron, OH UNITED STATES

Correspondence to: George G. Chase email: [email protected]

ABSTRACT Electrospinning from a single jet is commonly used to produce very fine polymeric fibers. The mass production rate of the electrospinning from a single jet is relatively low. Alternative methods to launch multiple jets and increase production rates are described here. Few variations on the electrospinning process are reported in literature. In this paper we discuss three novel methods to launch polymer jets via electric fields. Multiple pendant drop electrospinning jets from porous tubes produced fibers with average fiber diameter smaller than 400 nanometers. Bubble launched electrospinning and blown-film methods also result in multiple jets but of larger fiber diameters. These processes have not been optimized to produce small fibers. The three methods here have production rates on the order of 0.03 to 9.00 g/hr per jet. These methods should be scalable.

INTRODUCTION Submicron sized fibers have a number of potential applications such as wound healing, tissue growth, and filtration. Electrospinning is one method for producing such fibers. In electrospinning literature these fibers are referred to as nanofibers. Due to the low mass production rates of electrospun fibers from traditional single jets [1-5], alternative methods with potential for higher production rates are desired.

The single jet electrospinning process is commonly used by many research groups to create submicron size fibers. A polymer solution is pumped by a syringe pump through a small needle to form a small pendant droplet at the needle tip. The needle is charged to a high voltage, typically about 20kV, to charge the polymer solution in the droplet. The electrical forces overcome the surface tension and the charged liquid jet launches from the apex of the pendant droplet hanging from the needle. When the jet has traveled a few centimeters from the droplet, the interaction between electrical, surface, and molecular forces becomes unstable and the jet bends or loops into an expanding coil [6]. The electrical charges cause the jet to stretch and the diameter to

reduce while the solvent evaporates and the jet dries into a solid fiber.

A few methods for producing electrospun nanofibers at higher mass production rate are available commercially by ELMARCO (Liberec, Czech Republic) and Nanostatics (Columbus, Ohio). In the ELMARCO design liquid jets launch from the surface of a rotating cylindrical drum or from wires [7,8]. The Nanostatics design uses an array of packed needles to launch multiple simultaneous jets. Other designs to increase the production rate are reported but to our knowledge not yet commercialized [9-15]. The mass production rates of these designs are limited by the spacing between jet or nozzle and electric field interference that occurs when the jets are too close together. Electric field interference can and delay the onset of the bending instability and can cause larger fibers to form.

Yarin et.al [15] used a magnetic fluid placed underneath the immiscible polymer solution to create multiple jets on the surface of a polymer solution by applying magnetic fields to the liquid bath. The electric field was applied to the solution once multiple liquid jets were created by the magnetic field. Multiple liquid jets were report to have density of 26 jets per square centimeter. This design claims to solve design and clogging problems of multiple nozzles in a close packed arrangement.

The successes of the methods described above are encouraging. Many other electrospinning designs are possible and yet to be explored. Three methods are described below that are simple yet effective to implement.

MATERIALS AND METHODS Materials Similar electrospinning solutions were used to explore the electrospinning designs. The solutions, at different concentrations, were made by mixing Polyvinylpyrrolidone (PVP) (MW 360,000, Sigma-Aldrich, St. Louis, Missouri) in ethanol, by a magnetic stir bar for 4 to 6 hours.


Methods Three methods are described here to launch multiple electrospun liquid jets. The methods include jets from an array of pendular drops on a porous tube, jets launched from air bubbles in the polymer solution, and jets launched from curved thin films of polymer solution. The three methods should be scalable.

Multiple Pendant Droplet Electrospinning From A Porous Tube One way to launch multiple jets is by electrically charging multiple pendant drops. These multiple pendant droplets are formed at orifices formed in an array on the surface of a plastic tube. Two types of cylindrical tubes to create multiple pendant droplets were investigated: a porous walled cylindrical tube and a solid cylindrical tube with machined holes.

The wall material of the porous walled tube is made of polypropylene foam that has pores with an average diameter of about 40 microns. Machined holes were drilled halfway through the wall from outer surface, as shown in Figure 1, to create points of lesser flow resistance. Polymer solution, when pumped into the interior of the tube, preferentially flows through the pores and through the machined holes to form drops on the outside surface of the tube at the location of the drilled holes.

FIGURE 1. Cutaway view drawing showing the cylindrical porous wall tube with its axis oriented horizontally. Small diameter holes are drilled half way through the walls. The left and right ends of the tube are plugged with rubber stoppers. The tube is filled with polymer solution through one of the openings in the top.

The solid cylindrical tube design is similarly constructed with an array of holes but the holes are drilled completely through the tube wall, as shown in Figure 2. Polymer solution is loaded into the tube and is forced through the holes by air pressure of a few kilopascals applied at the top of the manifold. Drops of polymer solution form at each of the drilled holes on the surface of the tube. Figure 3 shows the conical shaped drops hanging on the surface of the tube after applying the voltage. The holes diameters are 0.2 - 0.5 mm in diameter and are space 1 cm

apart. Theron et.al.[1] reported a reasonable distance between each jet to be 1 cm in order to achieve uniformity of electrospun fibers and stable electrospun jets. The spacing between jets was tested and we found the optimum to be in the range of 0.7 – 1.0 cm.

A wire mesh electrode is inserted inside the tube to distribute the electric charge to the polymer in the tube. The electrode, inside of a transparent Plexiglas tube, is shown in Figure 4. An experimental setup with a tube, reservoir, and collector surface is shown in Figure 5. Mass production rates of submicron size fibers were roughly proportional to the number of drilled holes and are consistent with values reported in literature [6,16,17].

Typical production rates and experimental parameters for the porous wall tube and the solid wall tube are compared in Table I. As with single jet electrospinning, the fiber diameter and production rates depend on a number of parameters including applied voltage, polymer solution concentration, and distance from the tube to the grounded collector. For the tube design the fiber size and production rates also depend upon the hole spacing. Figure 6 shows the charged jets repel each other which affect the path of the jets which in turn affects the fiber diameter. Figure 7 shows SEM images of fibers created by the porous and solid wall tubes. The solid walled tube produced slightly larger diameter fibers (0.3 to 3 microns) compared to the porous wall tube (0.2 to 1 microns). The solid wall tube produced fibers at about 10 times the rate of the porous wall tube.

FIGURE 2. Solid cylindrical tube used as a manifold to create multiple pendant drops of polymer at the bottom surface. One opening at the top of the manifold is used to charge the tube with polymer solution and the other openings are used as vents.


TABLE I. Average diameter and mass production rare of PVP at specified operating conditions for the porous wall tube and solid wall tube.

Parameters & Responses Porous wall tube

Solid wall tube

Fiber diameter range (micron) Production rate (g/hr/1 jet)

0.2 - 1

0.03 – 0.04

0.3 -3

0.45 – 0.50

Polymer concentration (%wt) Gap distance (cm) Applied voltage (kV) Orifice diameter (mm) Orifice spacing (cm) Applied pressure (kPa)

15.0 15.2 50.0 0.5 1.0

1.0 – 2.0

10.0 20.0 40.0 0.2 2.0

0.25 – 0.50

FIGURE 3. Close-up photo of conical drops formed during electrospinning at the locations of the drilled holes on the porous wall tube manifold.

FIGURE 4. Image of wire mesh electrode inside of a transparent Plexiglas tube.

FIGURE 5. Experimental set up to create sheets submicron fibers on a conveyor belt. Polymer solution is delivered to the manifold from a polymer reservoir by low air pressure.

Bubble Launched Electrospinning In this design, bubbles are used to create a curved surface similar to a pendant drop commonly used in electrospinning. Wilson and Taylor [18] studied the conditions to launch liquid jets from the apex of a soap bubble and determined that the voltage required to launch liquid jet is a function of bubble radius and surface tension of liquid. As the radius of the droplet or soap bubble increase, voltage required to launch liquid jet increases as well.

Experiments were conducted with air bubbles in polymer solutions [19]. Figure 8 shows a plot of experimental data relating the electric field strength required to launch a liquid jet, from the apex of polymeric bubble, as a function of bubble diameter and polymer concentration. The plot shows for each bubble diameter the relation between concentration and electric field strength (ie. the applied voltage divided by the distance from the bubble to the grounded collector) goes through a minimum at a concentration of about 12%. As the bubble diameter increases, the electric field strength required to launch a jet decreases. This is the opposite to what Taylor [20] reported for drops, where the voltage required to launch a liquid jet increases as radius of droplet increases. The effect of bubble film thicknesses requires further investigation to explain the relation between electric field required to launch and the bubble diameter.


FIGURE 6. Photographs of the path of the electrospinning jets from drops on the solid wall tube. (A) Three jets are illuminated with a steady light and a long exposure time during which many coils of the jet pass downward through the image. (B) The same jets in (A) but illuminated with a single flash that almost, but not completely, stopped the motion of the electrospinning jet segments.

FIGURE 7. SEM images of fibers created from (A) the porous wall and (B) the solid wall tubes at the experimental conditions listed in Table 1.

FIGURE 8. 3-D plot between electric field, bubble diameter, and PVP solution concentration.

By using a high frame rate camera, the photos in Figure 9 show the deformation of an air bubble in PVP solution in an electric field. The bubble deformation is similar to that of a pendant drop in an electric field. Theoretical calculations show a conducting fluid can launch from a conical shaped drops within an electric field when the semi-vertical angle is 49.3 degrees [1,21]. The location of the greatest charge density is at the apex of the conical shape drop and this location is where the electrical forces can exceed the interfacial forces to launch liquid jets [22]. Yong [23] reports producing electrospun fibers of diameters in the range of 50 – 800 nm from a solution of 2% wt of PEO in alcohol and distilled water.

A prototype design to implement bubble launched jets is shown in Figure 10. In this design, an electrified ring is used to aid in directing the fibers towards the collector and to increase the distance between the bubble and the collector. Forced air was blown past the inside of the electrified ring to minimize fiber collection on the ring. However, as shown in Figure 10B, the air flow can cause the fibers to twist together to form a yarn. Electrified rings have been used to steer electrospinning jets and to control fiber deposition elsewhere [24,25].

To form the bubbles, pressurized air was slowly forced through a small tube at the bottom of a small vertical cylindrical reservoir filled with a 12 wt% PVP solution. The bubbles were about 5mm in diameter and produced at a rate of about 1 per second. The bubbles floated to the top of the reservoir by buoyance force. The electrified ring was positioned 10 cm above bubble and charged at 25.0 kV, slightly above the voltage required to launch the liquid jet form the apex of the polymeric bubble.

FIGURE 9. Series of images from a high frame rate camera show a liquid jet launching from the apex of a PVP solution bubble. Time zero was taken to be the frame that electrical potential was applied to the bubble. These images were taken at speed of 500 frames per second. Glints on the bubble surface are reflections from nearby objects in the experimental set up.


FIGURE 10. (A) Experimental setup to electrospin fibers from air bubbles in polymer solutions. Air bubbles emerge at the bottom of a cylindrical reservoir of polymer solution and rise to the top of the solution. When charged, a jet launches from the bubble towards the wire mesh collector. The electrified ring is used to guide the jet towards the collector. Air is blown through the ring to minimize fibers collecting on the ring. (B) An image of fibers collected on the wire mesh collector. When conditions are right, the air blowing through the ring causes the electrospinning fibers to twist into a yarn.

A grounded wire mesh collector was placed 25 cm above electrified ring to allow enough distance for the liquid jet to stretch and dry. The electrospun fibers had diameters in the range of 1.0 – 5.0 micros (Figure 11). The mass production rate was in the range of 8.0 – 10.0 g/hr per jet. Smaller fibers can be produced by adjusting the solution recipe, but this was not explored in the current work.

FIGURE 11. SEM images of PVP fiber fabricated by the bubble launch electrospinning jet process.

Stable jets launched from the bubbles were observed, as long as the bubbles were on the surface to support the liquid jet. As a bubble burst on the surface, another bubble rises to take its place. If the bubble production rate is increased then multiple bubbles may occupied the free surface of the polymer. The

jets from multiple simultaneous bubbles tended to have a shorter life due to competition to launch and electrical charge repulsions between the jets. Shorter jet lives resulted in reduced production rates per jet.

Blown Polymer Film Electrospinning In this design, electrospun jets are launched from a polymer film stretched across openings in a wire mesh. Air is blown against the underside of the film to deform the film into a hemispherical shape not unlike that of the bubbles previously described. The film is electrically charged to launch the jets. Also, when the air blows against the film with sufficient force, the film may burst and cause a spray of drops that may stretch to form fibers.

Similar to a liquid jet launching from the apex of a polymeric bubble, the liquid jet launches from the apex of a curved polymeric film. The experimental set up to implement this idea is shown in Figure 12. A metal hexagonal mesh drum with 8.0 millimeter openings rotates on its axis. As it rotates, part of the drum submerges in a polymer reservoir. A polymer film stretched across the mesh is lifted upward by the rotating drum. An air nozzle positioned under the upper part of the drum blows air against the underside of the polymer film to deform it into a hemispherical shape at the highest point in the drum rotation. This produced electrospun PVP fibers with diameters in the range of 2 to 5 microns (Figure 13). Further study is required to optimize and reduce average fiber size.

FIGURE 12. Blown polymer film electrospinning jet design apparatus. (A) A wire drum rotates on its axis; it submerges in a polymer solution and emerges carrying a film across the orifices. (B) Pressurized air blown under the film causes the film to spray. (C) The jet collects on a grounded wire mesh above the rotating wire drum.


FIGURE 13. Microscopic image of fibers made by the polymer blown film electrospinning jet method.

CONCLUSIONS Three alternative methods to produce electrospun fibers are presented in this paper. Liquid jets can be launched from a liquid drops on the surface of porous and solid wall tubes. Jets can also be launched from the curved surface of an air bubble in a polymer solution and from the curved surface of a polymer film. The fibers produced by these methods were larger in diameter than the fibers typically produced by single-jet electrospinning. All of these methods launched multiple jets and had production rates greater than a single jet. In future work the processes should be studied to determine optimum conditions and to reduce fiber size.

ACKNOWLEDGEMENT This research was funded by the Coalescence Filtration Nanofibers Consortium at the University of Akron. REFERENCES [1] Theron, S.A.; Yarin, A.L.; Zussman E and

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AUTHORS’ ADDRESSES George G. Chase Jackapon Sunthorn Varabhas Darrell H Reneker University of Akron Chemical Engineering Whitby Hall 411A Akron, OH 44325-3906 UNITED STATES

new methods to electrospin nanofibers · journal of engineered fibers and fabrics 34 volume 6,...

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