ELECTROSPINNING PROCESS FOR MANUFACTURE OF MULTI-LAYERED STRUCTURES

Abstract

Devices and methods for high-throughput manufacture of concentrically layered nanoscale and microscale fibers by electrospinning are disclosed. The devices include a hollow tube having a lengthwise slit through which a core material can flow, and can be configured to permit introduction of sheath material at multiple sites of Taylor cone formation formation.

Description

FIELD OF THE INVENTION

The present invention relates to systems and methods for the manufacturing of microscale or nanoscale concentrically-layered fibers and other structures by electrospinning.

BACKGROUND

Macro-scale structures formed from concentrically-layered nanoscale or microscale fibers (“core-sheath fibers”) are useful in a wide range of applications including drug delivery, tissue engineering, nanoscale sensors, self-healing coatings, and filters. On a commercial scale, the most commonly used techniques for manufacturing core-sheath fibers are extrusion, fiber spinning, melt blowing, and thermal drawing. None of these methods, however, are ideally suited to producing drug-loaded core-sheath fibers, as they all utilize high temperatures which may be incompatible with thermally labile materials such as drugs or polypeptides. Additionally, fiber spinning, extrusion and melt-blowing are most useful in the production of fibers with diameters greater than ten microns.

Core-sheath fibers with diameters less than 20 microns can also be produced by electrospinning, in which an electrostatic force is applied to a polymer solution to form very fine fibers. Conventional electrospinning methods utilize a needle to supply a polymer solution, which, upon activation of an electric field, is then ejected into a continuous stream toward a grounded collector. As the jet stream travels in the air, solvent evaporation occurs resulting in a single long polymer fiber. Core-sheath fibers have been produced using emulsion-based electrospinning methods, which exploit surface energy to produce core-sheath fibers, but which are limited by the relatively small number of polymer mixtures that will emulsify, stratify, and electrospin. Core-sheath fibers have also been produced using coaxial electrospinning, in which concentric needles are used to eject different polymer solutions: the innermost needle ejects a solution of the core polymer, while the outer needle ejects a solution of the sheath polymer.

Coaxial electrospinning has been used in the fabrication of core-sheath fibers for drug delivery in which the drug-containing layer (the “core”) is confined to the center of the fiber and is surrounded by a drug-free layer (the “sheath”). The sheath then serves as a diffusion barrier to a therapeutic agent in the core. Thus, release rates of the drug can be tightly controlled by varying the thickness, composition, and degradation profile of the sheath material as well as composition and concentration of the drug in the core Additionally, core-sheath fibers can be used for tissue engineering (e.g., incorporation of therapeutics to affect cell growth), filtration (e.g., incorporation of self-cleaning compounds such as titania), sensors (e.g., creation of hollow fibers to allow measurement of small analyte volumes), and as self-healing materials (e.g., spontaneous repair of surfaces with release of core contents). Core-sheath fibers can also be used as a way to create fibers from materials that would be otherwise unable to be electrospun (e.g., polymer pre-cursors such as poly(glycerol sebacic acid) or insulating materials such as Teflon). To do so, the material incompatible with electrospinning is confined in the center of the fiber and is surrounded by a material optimized for electrospinning; upon completion of the process the surrounding sheath material is removed (e.g., dissolved or melted away).

However, the creation of core-sheath fibers using a single needle has limited throughput. To increase throughput, coaxial nozzle arrays have been utilized, but such arrays pose their own challenges, as separate nozzles may require separate pumps, the multiple nozzles may clog, and interactions between nozzles may lead to heterogeneity among the fibers collected. Another means of increasing throughput, which utilizes a spinning drum immersed in a bath of polymer solution, has been developed by the University of Liberec and commercialized by Elmarco, S.R.O. under the mark Nanospider®. The Nanospider® improves throughput relative to other electrospinning methods, but to date core-sheath fibers have not been fabricated using the Nanospider®. There is, accordingly, a need for a mechanically simple, high-throughput means of manufacturing core-sheath fibers.

SUMMARY OF THE INVENTION

The present invention addresses the need described above by providing systems and methods for high-throughput production of core-sheath fibers by co-localizing multiple materials to multiple sites of Taylor cone formation, promoting the formation of multiple electrospinning jets and electrospun fibers incorporating a plurality of materials.

In one aspect, the present invention relates to a device for high-throughput production of core-sheath fibers by electrospinning. The device comprises a hollow vessel having a slit therethrough (the “core slit”), through which a solution of the core polymer can be introduced; the device also includes one or more features for the introduction of a sheath polymer into, above, beneath, or alongside the core slit. In some embodiments, the device comprises an additional slit or slits abutting the core slit on one or both slides through which solutions of sheath polymer can be introduced. In some embodiments, the sheath solution is contained within a bath or other vessel in which the hollow vessel containing the core solution is immersed. In some embodiments, the vessel includes structural features such as channels or regions of texture or smoothness through which the sheath polymer solution can run.

In another aspect, the present invention relates to a device for collection of electrospun fibers in yarn form. The device comprises a grounded or oppositely charged collector for electrospun yarns, the collector being configured to rotate so that fibers are twisted into yarns as they are collected from an electrospinning apparatus.

In yet another aspect, the present invention relates to methods of making core-sheath fibers and electrospun yarns using the devices of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, like reference characters generally refer to the same parts throughout the different views. Drawings are not necessarily to scale, as emphasis is placed on illustration of the principles of the invention

FIG. 1A-FIG. 1D is a schematic illustration of a fiber generated by the present invention.

FIG. 2 is a schematic illustration of a portion of an electrospinning apparatus according to an embodiment of the invention.

FIG. 3A-FIG. 3B is a schematic illustration of a portion of an electrospinning apparatus according to an embodiment of the invention.

FIG. 4A-FIG. 4B is a schematic illustration of a portion of an electrospinning apparatus according to another embodiment of the invention.

FIG. 5A-FIG. 5B is a schematic illustration of a portion of an electrospinning apparatus according to yet another embodiment of the invention.

FIG. 6 is a schematic illustration of a yarn-making apparatus according to an embodiment of the invention.

FIG. 7A-FIG. 7B includes photographs of an example of the present invention.

FIG. 8A-FIG. 8B is a photograph of another example of the present invention.

FIG. 9 is a schematic illustration of a portion of an electrospinning apparatus according to an embodiment of the invention.

FIG. 10A-FIG. 10C includes photographs of portion of an electrospinning apparatus according to certain embodiments of the invention.

FIG. 11A-FIG. 11C includes photographs of electrospinning apparatus of the invention muse.

FIG. 12 is a close up photograph of a Taylor cone from an operating electrospinning apparatus of the invention.

FIG. 13A-FIG. 13E includes scanning electron micrographs of electrospun core-sheath and homogeneous fibers formed on apparatuses of the invention.

FIG. 14A-FIG. 14C includes photographs and schematic illustrations of apparatuses utilizing pneumatic fluid supplies according to certain embodiments of the invention.

FIG. 15A-FIG. 15D includes schematic illustrations and photographs of apparatuses utilizing pneumatic fluid supplies according to certain embodiments of the invention.

FIG. 16A-FIG. 16E includes schematic illustrations of hydraulically-drive and mechanically-driven fluid supplies according to certain embodiments of the invention.

FIG. 17A-FIG. 17D includes photographs and schematic illustrations of gravity-driven fluid supplies according to certain embodiments of the invention.

FIG. 18A-FIG. 18D includes photographs of apparatuses in accordance with the invention having varying geometries (linear and round) and varying slit arrangements (single slits, many holes, few holes).

FIG. 19A-FIG. 19D includes photographs of diffusers in accordance with the invention.

FIG. 20A-FIG. 20D includes photographs of even polymer solution flows achieved with a change of the direction of flow in accordance with certain embodiments of the invention.

FIG. 21A-FIG. 21E includes photographs and schematic drawings of an electrospinning apparatus of the invention having a circular slit.

FIG. 22 includes cumulative dexamethasone release data from core-sheath fibers formed under varying flows of sheath polymer solution.

FIG. 23A-FIG. 23C includes schematic depictions of apparatuses according to embodiments of the invention.

FIG. 24A-FIG. 24B includes schematic depictions of apparatuses according to embodiments of the invention.

FIG. 25A-FIG. 25C includes schematic depictions of apparatuses according to embodiments of the invention.

FIG. 26 includes a schematic depiction of an angle in a wedge-shaped vessel according to certain embodiments of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention relates to electrospun fibers, including drug-containing electrospun fibers and yarns described in co-pending U.S. patent application Ser. No. 12/620,334 (United States Publication No. 2010/0291182), the entire disclosure of which is incorporated herein by reference for all purposes.

An example of a fiber produced by the devices and methods of the present invention is shown schematically in FIGS. 1a and 1b. Fiber 100 is generally tubular in shape, and is characterized by a length 110 and a diameter 111. Fibers generated by the devices and methods of the present invention are generally small enough to be useful for implantation to address a wide range of medical applications. As such, the fiber 100 has a diameter that is preferably up to about 20 microns. The length 110 of fiber 100 will vary depending on its intended use, and may range widely from micrometers to centimeters or greater. In a preferred embodiment, fiber 100 includes an inner radial portion 120 and an outer radial portion 130, as shown in FIGS. 1c and 1d. In this preferred embodiment, the total diameter 111 of the fiber is no more than about 20 microns, and the diameter of the outer radial portion is about 1-7 microns larger than the inner radial portion.

Examples of biodegradable polymers that can be used with this invention include: polyesters, such as poly(ε-caprolactonc), polyglycolic acid, poly(L-lactic acid), poly(DL-lactic acid); copolymers thereof such as poly(lactide-co-ε-caprolactone), poly(glycolido-co-ε-caprolactone), poly(lactide-co-glycolide), copolymers with polyethylene glycol (PEG); branched polyesters, such as poly(glycerol sebacate); poly(propylene fumarate); poly(ether esters) such as polydioxanone; poly(ortho esters); polyanhydrides such as poly(sebacic anhydride); polycarbonates such as poly(trimethylcarbonate) and related copolymers; polyhydroxyalkanoates such as 3-hydroxybutyrate, 3-hydroxyvalerate and related copolymers that may or may not be biologically derived; polyphosphazenes; poly(amino acids) such as poly (L-lysine), poly (glutamic acid) and related copolymers.

Examples, of biologically derived restorable polymers include: polypeptides such as collagen, elastin, albumin and gelatin; glycosaminoglycans such as hyaluronic acid, chondroitin sulfate, dermatan sulfate, keratan sulfate, heparan sulfate and heparin; chitosan and chitin; agarose; wheat gluten; polysaccharides such as starch, cellulose, pectin, dextran and dextran sulfate; and modified polysaccharides such as carboxymethylcellulose and cellulose acetate.

Examples of other dissolvable or resorbable polymers include polyethylene glycol and poly(ethylene glycol-propylene glycol) copolymers that are known as pluronics and reverse pluronics.

Examples of non-biodegradable polymers include: nylon 4, 6; nylon 6; nylon 6,6; nylon 12; polyacrylic acid; polyacrylonitrile; poly(benzimidazole) (PBI); poly(etherimide) (PEI); poly(ethylenimine); poly(ethylene terephthalate); polystyrene; poly(styrene-block-isobutylene-block-styrene); polysulfone; polyurethane; polyurethane urea; polyvinyl alcohol; poly(N-vinylcarbazole); polyvinyl chloride; poly (vinyl pyrrolidone); poly(vinylidene fluoride); poly(tetrafluoromethylene) (PTFE); polysiloxanes; and poly (methyl methacrylate).

Electrospun core-sheath fibers and other structures produced by the systems and methods of the invention may include any suitable drug, compound, adjuvant, etc. and may be used for any indication that may occur to one skilled in the art. In preferred embodiments, the drug or other material chosen is insoluble in the polymers and solvents comprising the core polymer solution, or the concentration of drug or material used exceeds the solubility limit of the drug or material in the polymers or solvents. Without limiting the foregoing, general categories of drugs that are useful include, but are not limited to: opioids; ACE inhibitors; adenohypophoseal hormones; adrenergic neuron blocking agents; adrenocortical steroids; inhibitors of the biosynthesis of adrenocortical steroids; alpha-adrenergic agonists; alpha-adrenergic antagonists; selective alpha-two-adrenergic agonists; androgens; anti-addictive agents; antiandrogens; antiinfectives, such as antibiotics, antimicrobials, and antiviral agents; analgesics and analgesic combinations; anorexics; antihelminthics; antiarthritics; antiasthmatic agents; anticonvulsants; antidepressants; antidiabetic agents; antidiarrheals; antiemetic and prokinetic agents; antiepileptic agents; antiestrogens; antifiungal agents; antihistamines; antiinflammatory agents; antimigraine preparations; antimuscarinic agents; antinauseants; antineoplastics; antiparasitic agents; antiparkinsonism drugs; antiplatelet agents; antiprogestins; antipruritics; antipsychotics; antipyretics; antispasmodics; anticholinergics; antithyroid agents; antitussives; azaspirodecanediones; sympathomimetics; xanthine derivatives; cardiovascular preparations, including potassium and calcium channel blockers, alpha blockers, beta blockers, and antiarrhythmics; antihypertensives; diuretics and antidiuretics; vasodilators, including general coronary, peripheral, and cerebral; central nervous system stimulants; vasoconstrictors; hormones, such as estradiol and other steroids, including corticosteroids; hypnotics; immunosuppressives; muscle relaxants; parasympatholytics; psychostimulants; sedatives; tranquilizers; nicotine and acid addition salts thereof; benzodiazepines; barbiturates; benzothiadiazides; beta-adrenergic agonists; beta-adrenergic antagonists; selective beta-one-adrenergic antagonists; selective beta-two-adrenergic antagonists; bile salts; agents affecting volume and composition of body fluids; butyrophenones; agents affecting calcification; catecholamines; cholinergic agonists; cholinesterase reactivators; dermatological agents; diphenylbutylpiperidines; ergot alkaloids; ganglionic blocking agents; hydantoins; agents for control of gastric acidity and treatment of peptic ulcers; hematopoietic agents; histamines; 5-hydroxytryptamine antagonists; drugs for the treatment of hyperlipiproteinemia; laxatives; methylxanthines; monoamine oxidase inhibitors; neuromuscular blocking agents; organic nitrates; pancreatic enzymes; phenothiazines; prostaglandins; retinoids; agents for spasticity and acute muscle spasms; succinimides; thioxanthines; thrombolytic agents; thyroid agents; inhibitors of tubular transport of organic compounds; drugs affecting uterine motility; anti-vasculogenesis and angiogenesis; vitamins; and the like; or a combination thereof.

FIG. 2 illustrates one embodiment of the present invention. Apparatus 200 comprises a hollow cylindrical tube 210 having a longitudinal slit 220 along a portion of or its entire length. Alternatively, multiple, disconnected slits can be spaced along the length. A core polymer solution 230 can be introduced into the lumen of tube 210 in a volume and/or at a flow rate sufficient for the surface of the solution to emerge through slit 220. In one example, tube 210 is 0.5-100 cm in diameter with a wall thickness of 50-5,000 microns. The cylindrical tube 210 is, in some embodiments made of a conducting material such as stainless steel, copper, bronze, brass, gold, silver, platinum, and other metals and alloys. Metals used to form portions of apparatuses of the invention may be polished, brushed, cast, etched (by acid or other chemical or mechanically) or unfinished. The metal finish may be chosen to affect an aspect of the performance of the apparatus 200; for example, the inventors have found that using polished brass improves the flow of polymer solution. Alternatively, non-metal materials or insulating materials may be used to form all or a part of the apparatus 200. Slit 220 preferably has a width sufficient to permit formation of Taylor cones 240 from the surface of the core polymer solution 230, the width of slit 220 being generally between 0.01 and 20 millimeters, and preferably between 0.1 to 5 millimeters. The length of tube 210 is preferably between 5 centimeters and 50 meters, and more preferably between 10 centimeters and 2 meters.

In certain alternate embodiments, multiple apparatuses 200 may be placed in rows comprising up to 50 units, either in parallel or end-to-end, with a preference for 10 or fewer units per row. An advantage of using multiple units versus one long unit for increased throughput is better control over the flow of the polymer solutions. Alternatively, multiple apparatuses may be placed in rows and operated via a central power supply and/or central polymer delivery system that distributes an electric voltage and polymer solution to multiple individual apparatuses.

The core polymer solution 230 preferably has a viscosity of between 1 and 100,000 centipoise, and is more preferably between 200 and 5,000 centipoise. Core polymer solution 230 is preferably pumped through the lumen of tube 210 and slit 220 at rates of between 0.01 and 1000 milliliters per hour per centimeter, more preferably between 5 and 200 milliliters per hour per centimeter. A voltage, preferably between 1 and 250 kV, more preferably between 20-100 kV, is applied. The positive electrode of the power supply is preferably connected to the conducting slit-cylinder directly or via a wire, such that a potential difference exists between the slit cylinder and a grounded collector 250. Grounded collector 250 is preferably placed at a distance between 1 and 100 centimeters from slit 220 and parallel to the axial dimension of tube 210. Grounded collector 250 consists of various geometries (e.g. rectangular, circular, triangular, etc.), rotating drum/rod, wire mesh, air gaps, or other 3D collectors including spheres, pyramids, etc. In alternate embodiments the collector is oppositely charged relative to the polymer solution(s). In some embodiments, the collector 250 includes one or more grounded or oppositely charged points (for example, two grounded points separated by a space), and fibers collect around the one or more points and/or between them. Upon application of a sufficient voltage, Taylor cones 240 and electrospinning jets 241 will form at the exposed surface of core polymer solution 230, and the jets will attract toward collector 250, forming homogeneous fibers.

The invention includes means for co-localizing sheath and core polymer solutions at multiple sites of Taylor cone formation so that core-sheath fibers are produced. In certain embodiments, devices of the invention comprise a hollow vessel having a lengthwise slit therethrough, through which a solution of the core polymer can be introduced. The devices additionally comprise two slits abutting the core slit on both slides through which solutions of the sheath polymer are supplied. Flow of both core and sheath polymer solutions is initiated and an electric field is introduced. These steps are performed in any suitable order: for example, in some embodiments, flow of the core polymer solution is initiated, a field is introduced and Taylor cones and electrospinning jets comprising core polymer solution are formed; then sheath polymer flow is initiated such that the sheath polymer is incorporated into Taylor cones and electrospinning jets. In other embodiments, the sheath polymer flow is initiated first, then the field is introduced and, after formation of Taylor cones and electrospinning jets, the core polymer flow is initiated. In still other embodiments, both polymer solutions are provided simultaneously, then the field is introduced, etc.

Application of an electric field of sufficient strength to apparatuses of the invention leads to formation of Taylor cones and electrospinning jets in the polymer solution or solutions. In some embodiments, Taylor cones and electrospinning jets are formed in the core polymer solution 230, then the sheath polymer solution 260 is added alongside or above the core polymer solution 230 so that the sheath polymer solution 260 is drawn up into Taylor cones 240 and electrospinning jets 241. In preferred embodiments, Taylor cones and jets are formed in the sheath polymer solution 260 and the core polymer solution 230 is added, preferably beneath the sheath polymer solution 260, so that it is incorporated or pulled into electrospinning jets. As illustrated in FIG. 9, this can be achieved, in preferred embodiments, by using nested wedge-shaped vessels 210, 270. A first slit 220 is located at one apex of the inner wedge shaped vessel; 210, and a second, larger wedge-shaped vessel 270 is arranged so that a second slit 271 is aligned with the first slit 220 and a gap exists between the inner wedge-shaped vessel 210 and the outer wedge-shaped vessel 270, permitting a solution of sheath polymer solution 260 to flow around the inner wedge shaped vessel 210. The wedge-shaped vessels 210, 270 may be oriented so that the slit is aligned with a vertical plumb line, or it may be angled with respect to a vertical plumb line so that extra core polymer solution 230 or extra sheath polymer solution 260 can run-off, preventing formation of inhomogeneities such as globs in the resulting fibers or other structures. The wedge shaped vessels, in preferred embodiments, include side walls that are angled 30° from the vertical, as shown in FIG. 26.

In alternate embodiments of the present invention, three parallel troughs are utilized, as illustrated in FIG. 5. Apparatus 300 comprises an inner trough 310 and two outer troughs 320, 330. The walls 311, 312 of inner trough 310 are optionally tapered, so that their thickness decreases to zero at the top of inner trough 310. Inner trough 310 is filled with a solution of core polymer solution 220, which is pumped through inner trough 310 from the bottom up at rates of between 0.01 and 1000 milliliters per hour per centimeter, more preferably between 10 and 50 milliliters per hour per centimeter. Alternatively, the solution can be fed in from the sides or a combination of the bottom and sides. Inner trough 310 has a height ranging preferably from 5-10 centimeters and a sufficient width to permit formation of Taylor cones and jets 240, 241, which emerge from the surface of core polymer solution 220, the width of inner trough 310 being generally between 0.01 and 20 millimeters, and preferably between 0.1 to 5 millimeters. Outer troughs 320, 330 are filled with sheath polymer solutions 260 to heights sufficient for the sheath polymer solution to be drawn into the sites of Taylor cone and jet initiation 240, 241. As shown in FIG. 5b, walls 311, 312 of inner trough 310 may incorporate a reciprocal periodic wave structure, forming regions of higher and lower width within inner trough 310, which structure biases the formation of Taylor cones and jets 240, 241 to regions in which the width of inner trough is locally maximized. The voltage is applied by attaching the positive electrode of the power supply to the inner walls of the trough, which is composed of a metallic conducting material such as stainless steel, copper, bronze, gold, silver, platinum and other alloys. The inner and/or outer troughs 310, 320, 330 are optionally angled with respect to a vertical plumb line so that extra core polymer solution 220 or extra sheath polymer solution 260 can run-off.

In certain alternate embodiments, such as that illustrated in FIG. 3, hollow cylindrical tube 210 will be arranged so that slit 220 points downward, and a sheath polymer solution 260 will be applied to the upward-facing external surface of tube 210 so that sheath polymer solution 260 runs down the sides of tube 210 and co-localizes with the core-sheath polymer at sites of Taylor cone and jet initiation 240, 241. Once the sheath polymer solution 260 is co-localized with the Taylor cone, it will be incorporated into the jet. The sheath polymer solution 260 is drawn toward and over the core fibers by varying the flow rate and viscosity of the sheath polymer solution 260, or by incorporating structural features 211 such as grooves, channels, coatings, and textured or smooth surfaces on the outer surface of hollow tube 210.

In certain alternate embodiments, as illustrated in FIG. 4, hollow tube 210 will be partially submerged in a bath 270 containing the sheath polymer solution 260. The volume of the sheath polymer solution 260 within bath 270 will be set at a level so that the top surface of the sheath polymer solution is at or near the sites of Taylor cone and jet initiation 240, 241. The degree to which sheath polymer solution 260 is co-localized with the core solution can be controlled by varying the viscosity of sheath polymer solution 260, or by incorporating structural features 211 on the outer surface of hollow tube 210 such as rings, teeth, grooves, channels, coatings, wires, wire meshes and textured or smooth surfaces. These structural features can be used to control the site of co-localization of the solutions mechanically (e.g., a channel), chemically (e.g., a hydrophilic coating is used to control the location of flow), or electrically (e.g., a structure such as metal teeth provides a site of charge concentration).

While the bath is depicted in FIG. 4 as being open, other arrangements of the hollow tube 210 and the bath 270 are preferred, such as the arrangement shown schematically in FIGS. 9-10: each of the hollow tube 210 and the bath 270 are generally wedge-shaped, and the slit 220 is located at one apex of the wedge shape, as is a corresponding slit 271 in the bath 270: the arrangement of the slit 271 of the bath 270 to the slit 220 of the hollow tube 210 is illustrated in FIG. 10. FIG. 11 shows multiple core-sheath Taylor cones 240 and electrospinning jets 241 emanating from the slit 270 when the apparatus is in use. A close-up image of a core-sheath Taylor cone is shown in FIG. 12.

In other embodiments, such as the one described in Example 2, infra, the sheath polymer solution 260 can be introduced directly to the sites of Taylor cone and jet initiation 240, 241, by using a syringe pump and needle. This method is superior to previously used coaxial nozzle arrays, as single bore needles are used, reducing the likelihood of clogging.

In an alternate embodiment, the invention comprises a collector plate configured as a drum 400, which can be placed into a yarn-spinning apparatus as shown in FIG. 6. At any point during collection of fibers (prior to initiation, during collection, or after collection initiation), the drum is engaged with a belt that is in turn engaged with a mandrel that can spin in one direction, and free ends of the collected fibers are attached to another drum engaged with another belt that is engaged with a different mandrel which spins in a direction opposite from that of the first mandrel. The resulting yarns can be post-processed into higher-order structures such as ropes by attaching opposite ends of multiple yarns to opposing drums, and spinning them in opposite directions as described above.

The structural uniformity of core-sheath fibers produced by the apparatuses and methods of the invention depends in part upon the supply of core polymer solution 230 and sheath polymer solution 260 to the interior and exterior of the hollow tube 210. Without wishing to be bound to any theory, it is believed that supplying fluid evenly over time and across the width of the slit permits the fluid surface exposed to the electrical field to be kept relatively even and flat and to prevent variations in electrical field strength across the long axis of the slit over time (except for electrical field variations originating from electrospinning jet formation). In certain embodiments of the invention, the evenness of fluid flow is reflected, among other ways, in the evenness of the meniscus within the slit or other elongate area in which Taylor cones or electrospinning jets 240, 241 form.

In preferred embodiments, core and/or sheath polymer solutions 230, 260 are provided to the interior and exterior of the hollow tube 210 at the slit 220 in a steady, laminar fashion such that fluid velocity and pressure of the core and/or sheath polymers 230, 260 are constant across the width of the slit 230 over time. Such steady, laminar flow can be achieved by a variety of methods, which may be used alone or combined, and the inventors have found that driving polymer flow pneumatically, hydraulically, mechanically (piston-driven) or by gravity can result in a suitably consistent supply of the required fluids; this aim can also be met by employing flow directing structures such as diffusers in flow paths for the core and sheath polymers 230, 260

With respect to pneumatic driving of fluids, FIG. 14 shows apparatuses of the invention utilizing reservoirs 231, 261 for core polymer solution 230 and sheath polymer solution 260, respectively. Each of the reservoirs includes one or more gas inputs 280, each of which preferably located opposite a conduit 232, 262 for the core and sheath polymer solutions 230, 260, respectively. For example, in the embodiments of FIG. 14, gas is provided via inputs 280 at the top of the reservoirs 231, 261, and polymer solutions exit via conduits 232, 262 at the bottom of the reservoirs. The conduits of the apparatus 200 preferably have a width that is roughly the same as a width of the slit 220, thus minimizing the formation of spreading flows and eddies that may result in variances of fluid velocity or pressure across the width of the slit 220. In some embodiments, turbulent and/or uneven flows are minimized by removing sharp angles or curves from the flow paths from the reservoirs 231, 261 through the conduits 232, 262 to the slit 220; the flow paths may be, in some embodiments, substantially linear. It will be appreciated that solutions can also be injected through the inputs 280 leading to reservoirs 231, 261 and 280 to permit continuous electrospinning.

Any suitable gas may be used to drive the flow of core and/or sheath fluids 230, 260, including air, but in preferred embodiments a non-reactive or inert gas is used such as Nitrogen, Helium, Argon, Krypton, Xenon, Carbon dioxide, Helium, Nitrous Oxide, Oxygen combinations thereof and the like. The gas used to drive flows is optionally insoluble in the solvents used in the core or sheath polymer solutions 230, 260 to prevent the formation of gas bubbles during electrospinning. Additional steps may be taken to prevent bubble formation during electrospinning, including de-gassing the core and sheath polymer solutions 230, 260 prior to use and separating the gas used to drive fluid flows from the polymer solutions 230, 260 through the use of an impermeable membrane or piston. In some embodiments, an inflatable balloon is used to displace polymer solutions 230, 260 from the reservoirs 231, 261. The reservoirs 231, 261 and the gas inputs 280 are preferably sufficiently airtight to prevent leakage at the gas pressures used.

As shown in FIG. 15, pneumatic driving mechanisms may include pressure regulators (FIG. 15A) to ensure that gas is provided at a constant pressure, which in turn will advantageously permit the maintenance of even fluid flows during electrospinning. In some embodiments, pneumatic pressure is generated through the use of a piston 285 to compress a fixed volume of gas in an airtight vessel such as a polymer solution reservoir. Finally as shown in FIGS. 15C-D, in some embodiments, multiple air inlets 280 are used to ensure pneumatic pressure is applied evenly across the width of the reservoir 231/261 and, in turn, that the fluid velocity and pressure is kept even across the width of the slit 220.

With respect to hydraulic driving of fluids, as shown in FIG. 16 A-B, in preferred embodiments a fluid 281 such as water will be used to displace a piston 285 which then displaces a polymer solution such as the core polymer solution 230 toward the slit 220. As discussed above, the piston 285 preferably moves through a reservoir or a conduit having a width approximately equal to a width of the slit 220, and the piston 285 itself preferably has a width substantially equal to the width of the slit 220. Also as discussed above, an inlet for the fluid 281 and the piston 285 can be disposed within a reservoir opposite a conduit, or in any other suitable arrangement.

In some embodiments, the piston includes one or more sealing features 286 such as gaskets or O-rings to prevent the driving fluid from mingling with the polymer solution. This aim may also be achieved in some embodiments by tailoring the surfaces of the piston 285 and/or the reservoir to repel the fluid 281 used to drive the piston 285—for example, in embodiments where water is used to drive the piston 285, the piston and the wall of the reservoir may include hydrophobic surfaces to prevent the migration of water past the piston.

With respect to piston-driven fluids, piston 285 may be made of any suitable material, including plastics, metals and combinations thereof. In some embodiments, the piston 285 is made of a material that is the same as or similar to a material included in the hollow tube 210; in other embodiments, the piston is non-conductive and/or includes a dielectric material. The piston preferably includes a material that is non-reactive with the polymer solutions 230, 260. The piston and/or the reservoir may include a coating or surface to render it non-reactive and/or to prevent a gas or liquid used to drive the piston from mingling with the polymer solution. The piston and/or the reservoir may also include a coating to minimize friction between the piston and the walls of the reservoir to prevent binding between the piston to the walls and variation in fluid velocities and pressures delivered to the slit 220.

Pistons may be driven pneumatically, hydraulically (as discussed above) or by mechanical actuators such as screw actuators or linear actuators. Multiple pistons may be used to drive core polymer solution 230 and sheath polymer solution 260. As shown in FIG. 16E, in some embodiments, sheath polymer solution is driven by multiple pistons 285A which are coupled to one-another to ensure the supply of sheath polymer solution is consistent on either side of the slit 220.

Pressure diffusers can be used to even out flow across a vessel and/or a slit for electrospinning. Pressure diffusers, as the term is used herein, refers to structures that obstruct at least a portion of a flow path to re-direct a relatively narrow stream of fluid over a larger area. A pressure diffuser may include holes, slits, or other apertures to permit fluid to flow through the diffuser. A diffuser may also include angled, curved, or beveled surfaces to force fluid contacting such surfaces to flow in desired directions around the diffuser. One or more diffusers can be arranged, in parallel or in series, across a flow path to more fully diffuse the flow of a solution. The diffuser can include surfaces parallel to, perpendicular to, or otherwise angled to a desired direction of flow. A selection of diffusers compatible with the invention are illustrated in FIG. 19 and are described in Example 5, below.

With respect to gravity-driven fluid flows, in such embodiments, a reservoir such as a core polymer solution reservoir 231 will be positioned above the hollow tube 210 and the slit 220, such that the polymer solution 230/260 will flow downward by gravity from the reservoir toward the slit. The apparatus 200 includes a vent or valve through which air can enter the reservoir 231/261 to occupy space vacated by polymer solution 230/260 as it flows toward the slit 220.

In some embodiments, the polymers used in the present invention include additives such as drug particles, metallic or ceramic particles to yield fibers having a composite structure.

Although the disclosure herein has focused on linear vessels having linear slits, any suitable geometry may be used, including round designs as shown in FIG. 21 and as described in Example 8. The methods and apparatuses described above can be adapted and/or combined to form core-sheath fibers using a round vessel having a round slit. Core polymers and sheath polymers can be provided to the slit in a round vessel using nested annular flow paths, as is illustrated in FIG. 21E; these annular flow paths are compatible with piston-driven, hydraulically-driven, or pneumatically driven polymer systems described above.

In addition, although the disclosure focuses on systems and methods utilizing a single lengthwise slit, any suitable aperture geometry may be used, including without limitation multiple short slits, holes, curved slits, slits and holes together, etc. Similarly, the invention includes systems and methods utilizing complex three-dimensional arrangements, such as that shown in FIG. 22, utilizing multiple disks 350, each disk containing three troughs in a manner similar to that shown in FIG. 5—a central trough 310 for the core polymer solution 220 flanked by troughs 320, 330 for the sheath polymer solution 260. In the system of FIG. 22, the polymer solutions 220, 260 are supplied by a central line 360 connected to each disk. Upon application of an electrical field, Taylor cone formation and formation of electrospinning jets occurs in a radially outward direction, and the resulting fibers are collected on a grounded collector 370 disposed circumferentially about and at a suitable distance from the disks 350.

Preferred embodiments of the invention utilize elongate areas including slits for electrospinning. Using elongate areas rather than, say, radially symmetrical or square areas advantageously permits multiple solutions or materials to be continuously and evenly supplied to sites of Taylor cone and electrospinning jet formation such that they are closely opposed, yet remain separate. In non-elongate areas such as squares, Taylor cones and electrospinning jets that form in the center of the area tend to deplete the supply of materials or polymer solutions in the center of the area, which materials cannot be replaced as efficiently and evenly while remaining in an unmixed fashion as is possible in narrower, more elongate areas. In addition, the use of elongate areas provides a straightforward path to scaling-up fiber production: as the long dimension of the elongate area increases, it is possible to form more Taylor cones and electrospinning jets within the area, yet by keeping a short dimension relatively constant, materials and polymer solution can be rapidly supplied from alongside or underneath the area to prevent depletion. Suitable dimensions for slits in apparatuses of the invention are disclosed in Examples 7 and 8, below.

The systems and methods described herein can be adapted to form structures other than core-sheath fibers. For example, core-sheath particles may be formed using core and/or sheath polymer solutions with low viscosity. Upon introduction on an electric field, Taylor cones and structures similar to electrospinning jets (which are referred to as “spray jets” herein) will form. Due to the low viscosity of the solutions, the spray jets will break-up midstream leading to particle formation. Optionally, vibration can be used to disrupt the flow of the core and/or sheath solutions to further encourage the formation of spray jets and/or particles.

The invention also includes combinations of the systems and methods described above. For example, structures incorporating multiple sheath polymers can be formed using a vessel/bath setup as described above in combination with a syringe pump to provide a second sheath polymer solution to sites of Taylor cone formation.

In some embodiments, one or more of the core polymer solution and the sheath polymer solution is delivered in a pulsatile manner to create fibers with gradients of core densities and/or sheath thicknesses.

The invention includes systems and methods in which limited or no structure is used to separate core and sheath polymer solutions 220, 260. As shown in FIG. 24C, multiple polymer solutions may mix poorly such that little or no structural separation between core and sheath polymer solutions 220, 260 is necessary to form structures with distinct cores and sheaths. In the embodiment depicted in FIGS. 24A-B, core polymer solution 220 is provided at discrete points within an electrospinning vessel; the remainder of the vessel is filled with sheath polymer solution, and a field is then applied to initiate electrospinning.

The devices and methods of the present invention may be further understood according to the following non-limiting examples:

Example 1

Formation of Homogeneous Fibers

To illustrate the principle by which multiple Taylor cones and electrospinning jets are generated by the systems and methods of the invention, homogeneous fibers made of poly(lactic co-glycolic acid) (L-PLGA) were manufactured in accordance with the present invention. A solution containing 4.5 wt % of 85/15 L-PLGA in hexafluoroisopropanol was pumped into one end of a 10 cm long hollow tube (1 cm diameter) having a 0.4 cm slit of the present invention at a rate of 8 milliliters per hour. A grounded, flat, rectangular collecting plate was placed approximately 15 centimeters from the slit of the cylinder, and a voltage of 25-35 kV was applied, and the resultant fibers were collected on the collecting plate and examined under scanning electron microscopy as illustrated in FIG. 7b.

Example 2

Formation of Core-Sheath Fibers

Core-sheath fibers were manufactured in accordance with the present invention, as shown in FIG. 8a. A rhodamine-containing core solution containing 15 wt % polycaprolactone in a 3:1 (by volume) chloroform:acetone solution was pumped through a hollow cylindrical tube having a slit therethrough at a rate of 10 ml/hour. Jets were formed by applying a voltage of 25 kV. Once the Taylor cones were stable, a syringe pump and needle filled with a fluorescein-containing sheath solution containing 15 wt % polycaprolactone in a 6:1 (by volume) chloroform:methanol solution was placed so that the needle was adjacent to one of the Taylor cones, and the sheath solution was pumped at a rate of 6 ml/hour. To verify the core-sheath structure of the resulting fibers, fluorescence micrographs were obtained which demonstrated that the rhodamine-containing core component was indeed surrounded by the fluorescein-containing sheath component, as shown in FIG. 8b.

Example 3

Electrospinning Conditions for Various Slit/Hole Geometries

Slit-surfaces of various geometries were fabricated and the formation of electrospinning jets from these surfaces was demonstrated. FIG. 18 shows slit-surfaces that are (A) continuously linear, (B) continuously circular, (C) continuously linear with holes, and (D) non-continuous holes. The respective dimensions of slits or holes and the electrospinning conditions used therefore are presented in Table 1, below:

TABLE 1
GEOMETRIES AND ELECTROSPINNING CONDITIONS
FOR APPARATUSES SHOWN IN FIG. 18:
Slit	Apparatus	Polymer	Slit	Flow	Flow	Electric
Geometry	Geometry	solution	dimensions	rate	Source	field
Continuously	Wedge	6 wt % PLGA	0.5 mm × 35 mm	60 ml/hr	Underneath	40 kV
linear		75/25 in TFE
Continuously	Annular or	2 wt % PLGA	1 mm × 80 mm	120 ml/hr	Underneath	40 kV
circular	Showerhead	85/15 in
		Chloroform/Methanol
		(6:1)
Continously	Tube	2.5 wt % PLGA	8 cm long	30 ml/hr	Ends	40 kV
linear		85/15 in
with holes		Chloroform/Methanol
		(6:1)
Non-	Tube	2.5 wt % PLGA	5 cm long	20 ml/hr	Ends	40 kV
continuous		85/15 in
holes		Chloroform/Methanol
		(6:1)

Example 4

Achieving Even Flow of Polymer Solutions Using Mechanical Piston

Even flow of polymer solution to a slit was achieved by the use of a mechanical piston. FIG. 25A-B depicts the apparatus used. The wedge-shaped slit fixture is attached to a chamber connected to a piston that is mechanically driven using a syringe pump. As the piston moves forward, it pushes solution uniformly towards the slit. Using a flow rate of 50 ml/h and a voltage of 50 kV, multiple electrospinning jets emerged along the entire length of the slit as shown in 25 C.

Example 5

Achieving Even Flow of Polymer Solutions Using Pressure Diffusers

Even flow of polymer solution to the slit was achieved by incorporating pressure diffusers to divert momentum of fluid flow across the slit. Shown in FIG. 19 are examples of such diffusers. In FIG. 19A, the diffuser is a triangular fixture that contains holes across its length to allow polymer solution to flow through. To demonstrate its ability to divert fluid flow, the diffuser was press-fit inside a container such that flow of solution is forced through its holes rather than around. As shown in FIG. 19B, a dyed solution of PLGA in chloroform:methanol that was pumped into the container from one inlet source encounters the diffuser, spreads across the length of the chamber, and then flows through the holes of the diffuser. The result is a more even distribution of fluid flow across the length of the chamber. Similarly, FIG. 19C shows a circular shaped pressure diffuser that contains holes across its surface. As shown in FIG. 19D series of these diffusers were press fit into a tube and filled with non-dyed polymer solution of PLGA in chloroform:methanol. A dyed solution of the same solution was then pumped into the tube from one inlet source at the bottom. Similar as before, the solution encounters the diffusers, spreads across the area of the tube, and then passes through the holes of the diffuse. The result is a more even distribution of fluid flow across the tube. Pressure diffusers can be incorporated into the apparatus of the invention to achieve even flow of polymer to the slit surface.

Example 6

Achieving Even Flow of Polymer Solutions Using Polymer Solution Re-Direction

Another method for even flow can be achieved by redirecting polymer solution to flow in the opposite direction of initial direction. Shown in FIG. 20 is an experiment in which a 2 wt % PEO solution in 60:40 (by vol) ethanol:water is pumped through a tube that faces down inside a container. The tube is placed 10 mm away from the bottom of the container and fluid flow is set at 50 ml/h. The solution contains a blue dye to visualize the fluid flow pattern. As demonstrated, solution initially travels in the downward direction and upon encountering the wall of the container, proceeds to spread across the bottom and rise up uniformly. This diversion of momentum of fluid flow concept can be incorporated into the apparatus of the invention to achieve even flow of polymer to the slit surface.

Example 7

Electrospinning of Core-Sheath Fibers Using Direct Feed of Polymer Solutions

Core-sheath fibers were manufactured using an apparatus according to the embodiment of FIGS. 9 and 10. The apparatus consists of an inner trough with a slit width of 0.5 mm, while the width of the outer trough is 2 mm. The length of the entire slit is 7 cm. These wedge-shaped slits were affixed to a base fixture that allowed polymer solution to be directly delivered from inlet ports originating from the underside of the fixture.

A sheath solution 260 of 2.8 wt % 85/15 PLGA in 6:1 (by vol) chloroform/methanol and a core solution 230 of 2.8 wt % 85/15 PLGA in 6:1 (by vol) chloroform/methanol containing 30% wt % dexamethasone drug with respect to PLGA was used. The sheath flow rate was set at 100 ml/h while the core flow rate was set at 50 ml/h. A voltage of 50 kV was applied.

Example 8

Electrospinning of Core-Sheath Fibers Using Pneumatic Feed of Polymer Solutions

Core-sheath fibers were manufactured using an apparatus according to the embodiment of FIGS. 9-10 and 14. The apparatus consists of an inner trough capable of holding 50 mls of polymer solution and outer troughs capable of holding 100 mls of sheath polymer solution. The slit width of the inner trough is 0.5 mm, while the width of the outer trough is 2 mm. The length of the slit is 3.5 cm. Polymer solution was delivered to the respective slits via pneumatic actuation using a syringe pump and empty syringe. A sheath solution of 6 wt % PLGA in hexafluoroisopropanol (HFIP) was delivered at 60 mL/min and a core solution 230 of 15 wt % PCL in 6:1 (by vol) chloroform/methanol containing 30% wt % dexamethasone drug with respect to PCL was delivered at a rate of 10 mL/min. A voltage of 50-60 kV was applied and numerous core-sheath jets were emitted from the slit-surface of the apparatus and fibers were collected. FIG. 11 shows multiple core-sheath Taylor cones 240 and electrospinning jets 241 emanating from the slit 270 when the apparatus is in use. The core-sheath structure of the resulting fibers was confirmed by scanning electron microscopy, as shown in FIGS. 13A-D, which includes multiple scanning electron micrographs of fibers 100 having distinct cores 120 comprising dexamethasone particles and sheaths 130. FIG. 13E shows a control fiber made from a single PLGA/PCL/dexamethasone blend which does not exhibit the core-sheath structure.

Example 9

Electrospinning of Core-Sheath Fibers Using Pneumatic Feed of Polymer Solutions

Fibers with various core-sheath structures were fabricated using an apparatus according to the embodiment of FIGS. 9-10 and 14. Core-sheath structure was varied by varying the outer sheath flow rate while keeping the core flow rate constant. The sheath solution 260 consisted of 6 wt % PLGA in hexafluoroisopropanol (HFIP) while the core solution 230 consisted of 15 wt % PCL in 6:1 (by vol) chloroform/methanol containing 30% wt % dexamethasone drug with respect to PCL. The core flow rate was kept constant at 20 ml/h while the sheath flow rate was adjusted to either 40 or 100 ml/h. A control fiber made from a PLGA/PCL/dexamethasone blend was also fabricated. To evaluate the different core-sheath structures, elution of the dexamethasone drug from fibers was evaluated. As shown in FIG. 22, varying the sheath flow rate had the effect of varying the release kinetics of dexamethasone. Without wishing to be bound to any any theory, the inventors hypothesize that greater sheath flow rates led to thicker sheaths, which restricted diffusion of drug from fiber cores more completely than in fibers formed in conditions of lower sheath flow.

Example 10

Electrospinning from Circular Fixture

An apparatus incorporating a round slit rather than a linear one has been used. A showerhead fixture was modified, replacing a center piece with a plug to form a circumferential slit. When a 1 wt % PLGA solution was provided to the slit, multiple Taylor cones and electrospinning jets were observed, as shown in FIGS. 21 A and D.

The term “and/or” is used throughout this application to mean a non-exclusive disjunction. For the sake of clarity, the term A and/or B encompasses the alternatives of A alone, B alone, and A and B together. The aspects and embodiments of the invention disclosed above are not mutually exclusive, unless specified otherwise, and can be combined in any way that one skilled in the art might find useful or necessary.

The term “elongate” is used throughout this application to refer to structures having at least two dimensions, one dimension being longer, and preferably substantially longer, than the other(s). For the sake of clarity, the term “elongate” encompasses structures that are linear, cylindrical, cuboidal, curved, curvilinear, toroidal, annular, angled, rectangular, etc. and any structure that could be formed by bending or curving one of the elongate structures listed above.

While several embodiments of the present invention have been described and illustrated herein, those of ordinary skill in the art will readily envision a variety of other means and/or structures for performing the functions and/or obtaining the results and/or one or more of the advantages described herein, and each of such variations and/or modifications is deemed to be within the scope of the present invention. More generally, those skilled in the art will readily appreciate that all parameters, dimensions, materials, and configurations described herein are meant to be exemplary and that the actual parameters, dimensions, materials, and/or configurations will depend upon the specific application or applications for which the teachings of the present invention is/are used. Those skilled in the art will recognize, or be able to ascertain using no more than routine experimentation, many equivalents to the specific embodiments of the invention described herein. It is, therefore, to be understood that the foregoing embodiments are presented by way of example only and that, within the scope of the appended claims and equivalents thereto, the invention may be practiced otherwise than as specifically described and claimed. The present invention is directed to each individual feature, system, article, material, kit, and/or method described herein. In addition, any combination of two or more such features, systems, articles, materials, kits, and/or methods, if such features, systems, articles, materials, kits, and/or methods are not mutually inconsistent, is included within the scope of the present invention.

The breadth and scope of the invention is intended to cover all modifications and variations that come within the scope of the following claims and their equivalents:

Claims

1. (canceled)

2. A method of forming an electrospun structure, the method comprising the steps of: providing first and second unmixed polymer solutions in an elongated area; andapplying an electric field to at least a portion of the elongated area, thereby forming a plurality of electrospinning jets, wherein the first polymer solution is incorporated in an inner radial portion of each jet and the second polymer solution is incorporated in an outer radial portion arranged about the inner radial portion of each jet.

3. The method of claim 2, wherein the electrospun structure is an elongate fiber having a diameter of less than 20 microns.

4. The method of claim 2, wherein the plurality of electrospinning jets includes at least 10 jets.

5. The method of claim 2, wherein the plurality of electrospinning jets includes at least 100 jets.

6. The method of claim 2, wherein the elongated area is a linear, curvilinear or annular slit.

7. The method of claim 6, wherein the slit is formed within an apex of a wedge-shaped vessel.

8. The method of claim 6, wherein the slit is formed within a tubular vessel.

9. The method of claim 2, wherein the elongated area is defined by at least two parallel plates.

10. A method of electrospinning a polymer fiber, the fiber having a core including a first material and a sheath including a second material disposed about the core, the method comprising the steps of: flowing a first polymer solution comprising the first material within an elongate area;applying an electric field to at least part of the elongate area, thereby forming a plurality of electrospinning jets originating from a plurality of Taylor cones; andproviding a second polymer solution to at least one of the plurality of Taylor cones, thereby incorporating the first and second polymer solutions into at least one of the electrospinning jets.

11. The method of claim 10, wherein the elongated area is a linear, curvilinear or annular slit.

12. The method of claim 11, wherein the slit is formed within an apex of a wedge-shaped vessel.

13. The method of claim 11, wherein the slit is formed within a tubular vessel.

14. The method of claim 11, wherein the step of providing the second polymer solution to the at least one Taylor cone comprises flowing the second polymer solution around the slit.

15. The method of claim 11 wherein the elongated area is defined by at least two parallel plates.

16. The method of claim 15, wherein the step of providing the second polymer solution to the at least one Taylor cone comprises flowing the second polymer solution around the at least two parallel plates.

17. A method of forming an electrospun structure using an apparatus comprising a first wedge-shaped vessel having a first slit at an apex thereof, and a second wedge-shaped vessel including a second sit at an apex thereof, the first wedge-shaped vessel disposed inside of the second vessel such that the first and second slits are aligned, the method comprising the steps of: flowing a first polymer solution comprising the first material through the first wedge-shaped vessel and the first slit;flowing a second polymer solution comprising the second material through the second wedge-shaped vessel and the second slit; andapplying an electrical field to at least part of at least one of the first and second slits, thereby forming a plurality of electrospinning jets, each jet incorporating the first and second polymer solutions.

18. The method of claim 17, wherein the apparatus includes a collecting area having at least one electrically grounded point thereon, the method further comprising the step of collecting the electrospun structure within the collecting area.