HYBRID ELECTROSPINNER FOR CORE-SHELL FIBER FABRICATION

Abstract

Electrospinning (ES) provides the technical community with a readily available method to produce polymer fibers ranging from nanoscale to microscale. Here, we present a novel “hybrid electrospirming apparatus,” whereby, modifications to a melt electrospinner have allowed fabrication of core-sheath fibers with polymer sheaths and solution-based cores. These modifications include a split polymer melt heating block, coaxial block spinneret equipped with heaters and multiple feed ports for core and sheath material, and a wiring system for heat which requires multiple switches for safety and on-demand heat activation. Successful demonstration of coaxial fiber fabrication is demonstrated using polycaprolactone-polyethylene oxide blend shell and fluorescent gelatin core materials.

Description

STATEMENT REGARDING FEDERALLY SPONSORED

RESEARCH OR DEVELOPMENT

Research was sponsored by the Army Research Laboratory and was accomplished under Cooperative Agreement Number W911NF-15-2-0020. The views and conclusions contained in this document are those of the authors and should not be interpreted as representing the official policies, either expressed or implied, of the Army Research Laboratory or the U.S. Government.

THE NAMES OF THE PARTIES TO A JOINT RESEARCH AGREEMENT

Not applicable.

REFERENCE TO SEQUENCE LISTING, A TABLE, OR A COMPUTER PROGRAM LISTING COMPACT DISC APPENDIX

Not applicable.

BACKGROUND OF THE INVENTION

Electrospinning (ES) provides the technical community with a readily available method to produce polymer fibers ranging from nanoscale to microscale. ES produces fibers with small cross-sections and high surface area, making them ideal for a multitude of applications. Structures produced using ES methods exhibit a high surface-to-volume ratio, tunable porosity, and controllable composition. ES is of interest to the technical community in areas involving novel ES methods and materials including enhanced filtration [D. Aussawasathien, et al., Journal of Membrane Science, 2008, R. Gopal et al., Journal of Membrane Science, 2007. K. M. Yun, et al., Chemical Engineering Science, 2007. X. H. Qin, et al., Journal of Applied Polymer Science, 2006] augmented biomedical tissue regeneration [D. Liang, et al., Advanced Drug Reviews, 2007, Kim et al., Biomaterials, 2003], and advanced fabrication of liquid crystal polarizers [Y. YF, et al., Advanced Materials, 2007]. Although ES is the common term today, it was initially described by Formhals in a series of patents as an experimental setup for the production of polymer filaments using electrostatic force. The first patent filed by Formhals in 1934 on ES was issued for the production of textile yarns, with a process consisting of a movable thread collecting device that gathered threads in a stretched condition. He was granted related patents in 1938, 1939, and 1940[K. J. Pawlowski, et al., Materials Research Society Symposia, 2004]. ES was first observed in 1897 by Rayleigh, with related electrospraying studied in detail in 1914 and a patent issued to Antonin Formhals in 1934 [J. Zeleny, Physical Reviews, 1914, A. Formhals, Patent US1975504A, 1934]. In 1969, the published work of Taylor set the foundation for ES [G. Taylor, Proceedings “Electrically driven jets,” Proceedings of the Royal Society of London A: Mathematical, Physical, and Engineering Sciences, 1969].

ES involves the delivery of a liquid polymer to a spinneret (sometimes referred to as a capillary or needle) [I. S. Chronakis, Journal of Materials Processing Technology, 2005, Z. M. Huang, et al. Journal of Composites Science, 2003, J. Doshi et al., Journal of Electrostatics, 1995] that is held at a high voltage relative to a collection plate [J. L. Skinner et al., Proceedings of SPIE—The International Society for Optical Engineering, 2015]. Polymer is pumped to the tip of the spinneret, and electric charge is initiated in the collection plate. The initiated voltage creates an electrostatic force that pulls polymer from the spinneret to an electrode deposition surface. An initial short region (microns to millimeters) where the fiber is essentially straight is called the stable region. At the point where lateral perturbations cause transverse fiber velocities, the instability region starts. The instability region consists of polymer fiber moving in a whipping motion from the stable region toward the collection plate, while solvent evaporates off the polymer jet. Polymer fibers are then deposited onto the collection surface. Fiber size depends largely on solution flow rate, supplied electric current, and fluid surface tension [S. V. Fridrikh, et al., Physical Reviews Letters, V. Beachley et al., Materials Science Engineering C, 2009, A. Koski, et al., Materials Letters, 2004]. Given the time scales associated with fiber deposition by ES, charges on the metallic collection plate move instantaneously. Motion of charge in the polymer (much slower than motion of charge in metals) is dictated by ionic mobility in the polymer [D. H. Reneker, et al., Journal of Applied Physics]. Any effort to control the electric field within ES must take into account the high-frequency cutoff enforced by polymer limitations. The low-frequency cut-off for dynamic field control relates to the spatial fiber deposition rate and time constants associated with the instability region.

In solution ES, polymers which are pre-dissolved in solvent are used. Although solution ES is more common, melt ES can also be performed, in which, solid polymers are used. Melt-ES does not require solvent evaporation, instead creating a liquid polymer from a solid, in which phase transition of the polymer is associated with increased temperature [P.D. Dalton, et al., Biomacromolecules, 2006, J.S. Kim et al., Polymer Journal, 2000, L. Larrondo et al., Journal of Polymer Science and Polymer Physics, 1981, S. Lee et al., Journal of Applied Polymer Science, 2006, J. Lyons, et al., Polymer, 2004]. Lack of solvent in melt ES is beneficial for two reasons. First, the lack of harsh solvent requires less precaution during polymer preparation, and second, the instability region caused by solvent evaporation is a non-issue. Melt-ES was first patented by Norton in 1936 [C. L. Norton, US Patent 2048651,1936] but it was not until 1981 that a three-paper series describing electrostatics and polymer melts was published by Larrondo and Manley [L. Larrondo, et al., Journal of Polymer Science Part B, 1981].

BRIEF SUMMARY OF THE INVENTION

An apparatus designed for hybrid ES, whereby, materials which consist of core-sheath fibers can be fabricated is disclosed herein. The hybrid electrospinner designed allows a polymer melt to encase a solution-based (solution or solution-based) core, forming coaxial or core-sheath structured fibers. ES in the dual feed system designed will force polymer shell and core solution into a high voltage electric field generated between the spinneret and the collection plate. The electric field described is initiated by an externally applied voltage. The electric field used during ES creates a force on polymer shell and core solution, which results in deformation of the polymer/solution stream to lower surface area. Polymer shell and core solution are then pulled by the electrostatic force from spinneret to collection plate, forming core-shell fibers with a solid shell and liquid core.

For core-shell or coaxial structures to be fabricated on melt ES, a novel ES spinneret had to be designed. Because melt ES involves dry polymers, the spinneret had to be equipped not only with concentric spinneret configuration, but also heaters to melt the dry polymer used for the shell, prior to ES. The novel ES spinneret designed is comprised of an outer and inner annulus contained within a block spinneret. The block spinneret also contains two channels for cartridge heaters and a channel that incorporates a feed control mechanism for the outer annulus region.

The hybrid electrospinner heating design will include cartridge heaters and a Proportional-Integral-Derivative (PID) controller. In the heating design, two safety switches were added to require the main power input controller and also a secondary control switch on the heaters. The secondary control switch enables powering of the PM controller, but not activating the heaters until desired.

The fiber sheath created during ES with the hybrid electrospinner is delivered to the outer annular region of the coaxial block spinneret from a split block used to melt the polymer prior to ES. The split block design allows polymer to be molded to the correct shape for the hybrid electrospinner, and also for easy cleaning. The split block melt chamber contains a threaded fitting to attach to the sheathing feed of the spinneret, and an isolated heater cartridge holder to maintain the heat input but separate fed polymer from the cartridge heater.

The core (solution) feed for the hybrid electrospinner was designed to provide control over flow rate and provide consistent flow through the coaxial block spinneret within the applied electric field without damaging any electronics. This was accomplished by feeding the core solution through a syringe regulated by a commonly used syringe pump.

The hybrid electrospinner was designed and fabricated to electrospin monoaxial polymer fibers, or coaxial (core-sheath/shell) fibers with a solution-based core and polymer shell.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a schematic of the design used for the hybrid ES apparatus. The set-up shown is designed to enable production of materials which contain fibers composed of a polymer melt sheath and a solution- based core, although monoaxial polymer fibers can be electrospun as well.

FIG. 2 is a schematic plan view of a coaxial (core-sheath) spinneret design incorporating two heaters to melt polymer prior to ES.

FIG. 3 is a perspective view of a coaxial (core-sheath) spinneret design incorporating two heaters to melt polymer prior to ES.

FIG. 4: is a photograph of a coaxial block spinneret prototype. Image shows two ports for heater cartridges as well as two feed ports. Side port allows polymer feed for fiber sheath, while top port is used for solution, core feed.

FIG. 5 is a wiring schematic for a PID heater controller. The control loop shown has two main power controls, one for overall power, and a second for heater power. The separated control design allows the PID controller to be activated and set without activating the heaters until desired.

FIG. 6 is a photograph of a PID controller and power switch control panel.

FIG. 7 is a perspective transparent view of the polymer melting chamber design with split block.

FIG. 8 is a perspective view of the split block design of the melting chamber, which allows for polymer melt to be molded for delivery into the outer annulus of the coaxial block spinneret for sheath material delivery, as well as for easy cleaning.

FIG. 9 is a photograph of the split block melt chamber attached to the side of the coaxial block spinneret. Here, heater cartridges have been inserted into the melt chamber as well as the spinneret to provide heat at multiple points along the polymer path prior to extrusion from the spinneret and ES.

FIG. 10 is a photograph of a syringe pump used to deliver solution core to the coaxial block spinneret. The syringe pump allows for control over the core solution delivery to the spinneret using a stepper motor for calibrated flow rate set by the user.

FIG. 11 is a far field micrograph of monoaxial electrospun fibers produced from polypropylene melt in the hybrid electrospinner. During ES, polypropylene melt was subjected to an electric field strength of 367 kV/m. The resulting fibers had an average diameter of 20 μm and presented a smooth, consistent surface.

FIG. 12 is another far field micrograph of monoaxial electrospun fibers produced from polypropylene melt in the hybrid electrospinner. During ES, polypropylene melt was subjected to an electric field strength of 367 kV/m. The resulting fibers had an average diameter of 20 μm and presented a smooth, consistent surface.

FIG. 13 is a photograph taken during ES. Photo shows polymer/solution jet deforming into a cone shape. This is typical during ES, where deformation incurs as a mechanism to reduce surface area during electric field exposure. Solution core can be seen pulling into the polymer stream, while polymer sheath material maintains the overall structure of the stream, which will result in coaxial fiber formation.

FIG. 14 is an epifluorescent micrograph showing coaxial electrospun fiber containing a polycaprolactone/polyethylene oxide blend polymer shell and fluorescent gelatin core. The fiber produced was fabricated in an electric field strength of 400 kV/m, and a diameter of approximately 20 μm with a core diameter of 9 to 15 μm.

DETAILED DESCRIPTION OF THE INVENTION

A schematic of a hybrid electrospinner is shown in FIG. 1. Said hybrid electrospinner is comprised of: a heating block 30, which is further comprised of a polymer melt chamber 1, and at least one heater cartridge 2; a hybrid spinneret 3; a syringe pump 4; a thermocouple 5; a temperature controller 6; a high voltage power supply 7; and a collection plate 9. Whereby said heating block 30 is further comprised of a communication 10 between said polymer melt chamber 1 and said hybrid spinneret 3 for delivery of polymer melt 11. Said at least one heater cartridge 2 is designed to be inserted and removable from said heating block 30. Said syringe pump 4 delivers core solution 13 to the hybrid spinneret 3. The design shown allows a polymer melt 11 to encase a core 14, where said core can be solution-based or solution, forming coaxial fibers or core-sheath structured fibers 15. ES in this dual feed system will force polymer shell 17 and core solution 13 into a high voltage electric field 18 generated between the hybrid spinneret 3 and the collection plate 9. The electric field 18 described is initiated by an externally applied high voltage power supply 7. The electric field 18 used during ES creates a force on polymer shell 17 and core solution 13, which results in deformation of the polymer/solution stream to lower surface area. Polymer shell and core solution are then pulled by the electrostatic force from the hybrid spinneret 3 to the collection plate 9, forming core-sheath structured fibers 15 with a solid shell and liquid core as shown in FIGS. 13 and 14.

For coaxial or core-sheath structured fibers 15 to be fabricated by melt ES, a novel ES hybrid spinneret 3 had to be designed. Solution ES of coaxial structures involves a simple change of the spinneret to contain one instead of two concentric spinnerets, which are each fed a separate polymer or solution. Because melt ES involves dry polymers, the hybrid spinneret 3 had to be equipped not only with concentric spinneret configuration, but also heater cartridges 2 to melt the dry polymer used for the shell, prior to ES. The novel ES hybrid spinneret 3 designed contains an outer annular wall 19 and inner annular wall 20 contained within the hybrid spinneret. Melted polymer enters the hybrid spinneret 3 through a communication 10 comprising a polymer entry port 21, allowing polymer flow and fiber sheath formation within the space created between the outer annular wall 19 and inner annular wall 20. Core solution-based material entry 22 occurs via a core solution feed 31, which allows core solution to flow into the space enclosed by the inner annular wall 20. The outer annular wall 19 and inner annular wall 20 terminate at an extrusion port 23 where the polymer and solution-based material are subjected to the electric field 18 for ES of core-sheath structured fibers 15. The hybrid spinneret 3 also contains two channels 24 for cartridge heaters 2 to be removably inserted, and a channel that incorporates a feed control mechanism for the outer annular region.

A schematic model of the coaxial melt ES hybrid spinneret 3 is shown in FIGS. 2 and 3, while a photograph of the hybrid spinneret 3 is shown in FIG. 4.

The hybrid electrospinner heating design includes heater cartridges 2 and a temperature controller 6 comprising a Proportional-Integral-Derivative (PID) controller 25. The wiring diagram for the heating mechanism designed is shown in FIG. 5. In FIG. 5, a completed heater control loop is shown, excluding the thermocouple 5 that the PID controller 25 collects data from. FIG. 5 schematic contains two 144 Ohm heater cartridges 2, a PID controller 25, a heater power switch 26, a main power switch 27 and a high voltage power supply 7 comprising a 120-volt power source. The heater cartridges 2 are comprised of HDC0031 heaters that are matched up to an Omega PID controller 25 for temperature control. Also seen in the wiring schematic are the two switches added as a safety precaution, requiring not only the main power input controller, but also a secondary control switch on the heaters. The secondary control switch enables powering of the PID controller, but not activating the heaters until desired. Controller switches 26 and 27 on the panel are shown in FIG. 6.

The feed mechanism for the fiber sheath creating during ES is shown in FIGS. 7 and 8. The design shown is comprised of a heating block 30 with a polymer melt chamber 1, which provides a feed 28 to the outer annulus 19 region of the hybrid spinneret 3. Said feed 28 can be threaded to accommodate a threaded fitting 33, for attachment to the polymer entry port 21 of the hybrid spinneret 3. The heating block 30 design comprises a split block shown in FIG. 8 so that melt polymer mold can be prepared in the melt chamber 1 mold, and also for ease of cleaning. The heating block 30 is also comprised of an isolated melt chamber heater cartridge channel 29 to accept a melt chamber heater cartridge, which maintains heat input, but is separate from the fed polymer. In FIG. 9, a photograph of the heating block 30 is shown attached to the side of the hybrid spinneret 3. Active control or gravity feed are both possible with the feed mechanism designed.

The core solution feed 31 for the hybrid electrospinner was designed to provide control over flow rate and provide consistent flow through the hybrid spinneret 3 within the applied electric field 18 without damaging any electronics. This was accomplished by feeding the core solution through a syringe 32 regulated by a commonly used syringe pump 4 shown in FIG. 10.

The hybrid electrospinner was designed and fabricated to electrospin monoaxial polymer fibers, or coaxial (core-sheath/shell) fibers with a solution core and polymer shell. In FIGS. 11 and 12, an example of monoaxial fibers electrospun with the hybrid electrospinner are shown. In FIGS. 13 and 14, an example of coaxial fibers electrospun with the hybrid electrospinner are shown.

It is understood that the foregoing examples are merely illustrative of the present invention. Certain modifications of the articles and/or methods may be made and still achieve the objectives of the invention. Such modifications are contemplated as within the scope of the claimed invention.

Claims

1. A Hybrid ES apparatus comprising: a. a heating block, comprised of a polymer melt chamber further comprised of a feed at a bottom end of said polymer melt chamber, and at least one heating block heater cartridge channel isolated from said polymer melt chamber to accept at least one removable heating block heater cartridge;b. a hybrid spinneret comprised of a concentric spinneret further comprising an outer annular wall and an inner annular wall, where a space is created between said outer annular wall and inner annular wall to accept a melted polymer from said polymer melt chamber feed through a polymer entry port of said hybrid spinneret where said melted polymer enters said space between said outer annular wall and inner annular wall, anda space is created inside said inner annular wall isolated from said space between said outer annular wall and inner annular wall, which accepts a core solution from a core solution feed located at a first end of said hybrid spinneret;where said outer annular wall and inner annular wall terminate at a polymer extrusion port located at a second end of said hybrid spinneret; andwhereby said hybrid spinneret is further comprised of at least one removable spinneret heater cartridge channel isolated from said outer annular wall, which accepts at least one removable spinneret heater cartridge.

2. The Hybrid ES apparatus of claim 1 where said feed and said polymer entry port are threaded to accommodate a threaded fitting for attachment of said heating block to said hybrid spinneret.

3. The Hybrid ES apparatus of claim 1 where a syringe and a syringe pump delivers core solution to the space created by said inner annular wall.

4. The Hybrid ES apparatus of claim 1 where said heating block comprises a split block to facilitate cleaning.

5. The Hybrid ES apparatus of claim 1 further comprising a temperature controller comprised of a Proportional-Integral-Derivative (PID) controller, which controls the temperature of said heating block and said hybrid spinneret.

6. The Hybrid ES apparatus of claim 5 further comprising a main power input controller, and a secondary control switch for said heater cartridges to enable powering of said PID controller without activating said heater cartridges until desired.