WO2017083187A1

WO2017083187A1 - Electrospinning

Info

Publication number: WO2017083187A1
Application number: PCT/US2016/060494
Authority: WO
Inventors: Chase COFFMAN
Original assignee: Elektrofi, Inc
Priority date: 2015-11-12
Filing date: 2016-11-04
Publication date: 2017-05-18
Also published as: US20170137971A1

Abstract

Among other things, fibers are electrospun using an electrospinning structure comprising a base and at least one emitting element on the base, a first electrode arranged at a distance from the free end of the at least one emitting element, and optionally a collection element between the at least one emitting element and the first electrode, the collection element being configured to collect the fibers. The at least one emitting element has a projecting free end. At least a portion of the base, the at least one emitting element, or both include a porous material. The first electrode is configured to cause fibers to be produced from the free end of the at least one emitting element.

Description

Electrospinning

CROSS REFERENCE TO RELATED APPLICATION

Pursuant to 35 U.S.C. § 119(e), this application claims priority to U.S. Provisional Application Serial No. 62/254,492, filed November 12, 2015, the contents of which are hereby incorporated by reference.

TECHNICAL FIELD

This disclosure relates to electrospinning.

BACKGROUND

Electrospinning is a process that is useful, for example, for creating small scale fibers, including nano-scale fibers (sometimes called nanofibers), from liquid precursors. When an electric field is applied to a fluid interface, electrical charges near the surface rearrange themselves in a way that creates an outward electrostatic force. If the field is sufficiently high, the surface can be reconstituted in the form of jet that carries with it a flow of liquid. Many electrospinning systems utilize fluids containing dissolved polymers, which are known to possess good molecular cohesion. During the jetting process, this can help to suppress the breakup of the jet and instead promote the propagation of thin, continuous filaments. These can be collected downstream, possibly by a flat plate or another device, and used for various purposes.

Certain basic electrospinning systems initiate jetting from fluid interfaces that, in some way, are substantially unconstrained (i.e., so-called "free surface electrospinning"), as well as systems that feed liquid to the tip of a rigid emitting element (e.g. a solid needle or a capillary tube), where the morphology of the rigid apex at least partially influences the fluid interface. With regard to clarifying articles: Lukas, et al., "Self- organization of jets in electrospinning from free liquid surface: A generalized approach," Journal of Applied Physics 103, 084309 (2008) is instructive in the field of free surface electrospinning while Ponce de Leon, et al., "Parallel nanomanufacturing via

electrohydrodynamic jetting from microfabricated externally-fed emitter arrays," Nanotechnology 26 (2015) 225301 introduces an alternative approach embodying emitting elements.

Regardless of the system architecture, the effective production rate of fibers, including nano-scale fibers, has been lower than what would be suitable for commercial feasibility. For example, in many electrospinning systems embodying emitting elements, use of a single element is common. Attainable throughputs from such a system are, however, relatively limited in comparison to the needs of foreseeable commercial applications. Owing to the possibility of concurrent, spatially-discrete jetting sites, free surface technologies alleviate this issue to some extent.

Not being bound by certain free surface constraints, collections of emitting elements offer possibly greater recourse. Recognizing this, multiplexed systems of emitting elements have been proposed. Andrady, et al., in patent application publication U.S. 2005/0224998 Al, and Robertson, et al., in patent U.S. 7629030 B2, disclose pressurized manifolds comprising a fluid supply inlet on one side, typically capillary elements on another side, and a fully enclosed liquid volume in between. Although it does not embody emitting elements, the device described in Chase, et al., "New Methods to Electrospin Nanofibers," Journal of Engineered Fibers and Fabrics, Vol. 6, Issue 3 (2011) performs electrospinning from a hollow porous cylinder. A series of blind recesses are machined onto the outermost face and designed to function as discrete electrospinning sites. The underlying porous medium ballasts these sites against a flow of electrospinning fluid that is actively pumped into the inner part of the cylinder. In U.S. patent application publication 2014/0353860 Al, Velasquez-Garcia, et al., disclose an array of solid-body needles incorporating periodic external microstructures that promote passive wetting by an electrospinning fluid.

Electrospray is a technique different from electrospinning. "Electrospray" is a technique for extracting charged atoms, molecules, or droplets from electrospraying fluids (e.g., liquid metals and so-called "ionic liquids"), where the atoms, molecules, or droplets are not physically connected to one another; while "electrospinning" is a technique for producing continuous filaments (sometimes called fibers), or semi- continuous filaments, from electrospinning fluids (e.g., polymeric solutions). The continuous or semi-continuous filament is characterized by an aspect ratio (i.e., the ratio between the length and diameter of the filament) of greater than 2-to-l .

In patent U.S. 8791411 B2, Lozano, et al., describe a device designed to electrospray a subset of fluids known as "ionic liquids" using a passive hydraulic architecture (i.e., sans external pumping). The device is operated using a discontinuous electrode that is disposed a short distance from the free ends of the emitting elements. Individual holes in the electrode are aligned to the corresponding free ends of individual emitting elements such that the electrosprayed beam may propagate through the electrode. Legge, et al., "Electrospray Propulsion Based on Emitters Microfabricated in Porous Metals," Journal of Propulsion and Power, Vol. 27, No. 2, 2011; Courtney, et al., "Emission measurements from planar arrays of porous ionic liquid ion sources," J. Phys. D: Appl. Phys., 45 (2012) 485203; and Coffman, et al., "On the Manufacturing and Emission Characteristics of a Novel Borosilicate Electrospray Source," 49^th

AIAA/ASME/SAE/ASEE Joint Propulsion Conference, San Jose, CA, 14-17 July 2013, AIAA 2013-4035 offer further exposition on the element arrays and their intended use in an electrospray system for micropropulsion.

SUMMARY

In an aspect, fibers are produced using an electrospinning structure having a base and at least one emitting element on the base, a first electrode arranged at a distance from a free end of the at least one emitting element, and optionally, a collection element between the at least one emitting element and the first electrode. The at least one emitting element has a projecting free end. At least a portion of the base, the at least one emitting element, or both include a porous material. The first electrode is configured to cause fibers to be produced from the free end of the at least one emitting element. The collection element being configured to collect the fibers

In an aspect, fibers are formed by actions that include applying a voltage across at least a portion of an electrospinning structure having a base and at least one emitting element on the base such that a liquid passes along at least a part of a path from a source of the liquid to the at least one emitting element and to be emitted from the at least emitting element to form fibers.

In an aspect, at least one sol gel precursor, at least one magnetic material, and at least one solvent are mixed; a magnetic field is applied to the mixture to form protrusions on a surface of the mixture; and the solvent is removed from the mixture to form a porous electrospinning structure.

In an aspect, there are a set of elongated elements and a vessel to contain a volume of a liquid. Each of the elements projects from a base to a free end. Each of the elements includes a material that is porous to a liquid from which fibers are to be formed by electrospinning from the free end of each of the elements. The elements are configured so that at least portions of the bases are in contact with the liquid. In some embodiments, the base can be broader than the free end.

In an aspect, each of a set of elongated elements projects from a base to a free end. An electrode plate is arranged at a distance from the free ends of the elements for electrospinning from the free ends of the elements to the electrode plate. Each of the elements contains a material that is porous to a liquid from which fibers are to be formed by electrospinning from the free end of the element. The electrode plate is continuous at locations that correspond to locations of the free ends of the elongated elements. In some embodiments, the base can be broader than the free end.

In an aspect, each of a set of elongated elements projects from a base to a free end. Each of the elements includes a material that is porous to a liquid from which fibers are to be formed by electrospinning from the free end of the element. Surfaces of the porous material that are to be in contact with the liquid are hydrophilic. In some embodiments, the base can be broader than the free end.

In an aspect, there is a set of elongated elements, and a vessel to contain a volume of a liquid. Each of the elements projects from a base to a free end. Each of the elements contains a material that is porous to a liquid from which fibers are to be formed by electrospinning from the free end of the element. The vessel is unsealed. In some embodiments, the base can be broader than the free end. In an aspect, a liquid from which fibers are to be formed by electrospinning through an integrated porous body are conducted by capillarity from base ends of elements of the integrated porous body to free ends of the elements, and the fibers are electrospun from the free ends of the elements.

Embodiments can include one or more of the following features.

In some embodiments, at least a portion of the electrospinning structure is between the first electrode and a second electrode.

In some embodiments, the electrospinning structure includes a second electrode.

In some embodiments, the porous material includes a polymer, a metal, a ceramic, or a combination of any two or more of them. Examples of polymeric porous material can include a polyurethane, a polytetrafluoroethylene, a polyethylene, a polypropylene, a polyvinylidene fluoride, or an ethyl vinyl acetate or combinations of two or more of them. In some embodiments, the porous material is substantially hydrophilic.

In some embodiments, the porous material has an average pore size from about 0.1 microns to about 100 microns.

In some embodiments, at least a portion of the extrusion element has a porosity that allows a liquid to pass along at least a portion of a path that extends from a source of the liquid to the free end of the emitting element.

In some embodiments, the electrospinning structure includes a vessel for containing a liquid from which the fibers are to be formed. In some embodiments, the second electrode can be disposed in the vessel.

In some embodiments, a vessel is to contain a liquid from which the fibers are to be formed, and the second electrode and at least a portion of the extrusion element are disposed in the vessel.

In some embodiments, the first electrode is configured to collect the fibers. For example, the first electrode can include a plate. In some embodiments, the plate can be continuous at a location that corresponds to the location of the free end of the at least one emitting element. In some embodiments, the first electrode includes a screen.

In some embodiments, the apparatus includes the collection element. In some embodiments, a power source is electrically connected to the first and second electrodes. In some embodiments, the power source is configured to generate a voltage of from about 100 V to about 100,000 V.

In some embodiments, the at least one emitting element has an opening and the opening has a diameter of from about 10 μπι to about 250 μπι.

In some embodiments, the at least one magnetic material comprises magnetic nanoparticles (e.g., iron-containing nanoparticles). In some embodiments, the magnetic nanoparticles can be coated with a surfactant.

In some embodiments, the at least one sol gel precursor comprises a metal alkoxide, a metal chloride, or a metal nitrate, or a combination of any two or more of them. For example, the least one sol gel precursor can include tetramethyl orthosilicate, tetraethyl orthosilicate, or aluminum chloride, or a combination of any two or more of them.

In some embodiments, the catalyst can include an acid or a base.

These and other aspects and embodiments, and combinations of them can be expressed as methods, apparatus, systems, components, compositions, and in other ways.

Other features, objectives, and advantages of the subject matter disclosed herein will be apparent from the description, drawings, and the claims.

DESCRIPTION OF DRAWINGS

FIG. 1 is a side sectional schematic view of an electrospinning system including emitting elements and a base and being partially immersed in a bath of an electrified electrospinning fluid, such that electrospinning is seen to occur.

FIG. 2 is a side sectional schematic view of an electrospinning system in which a recess in a common base holds an electrified electrospinning fluid.

FIG. 3 is a side sectional view of an electrospinning system, where a collection element is disposed between emitting elements and a counter-electrode to intercept and collect electrospun fibers.

FIG. 4 is side schematic views of flow paths in and around three different emitting elements. FIG. 5 is a schematic side view of a "needle-like" emitting element.

FIG. 6 is a schematic side view of a "ribbon-like" emitting element.

FIG. 7 is a schematic side view of a "triangular" emitting element.

FIG. 8 is a schematic side view of a "tube-like" emitting element.

FIG. 9 is detailed schematic side, end, and top views of a "cylindrical" emitting element.

FIG. 10 is detailed schematic side, end, and top views of a "tube-like" emitting element including a blind recess and a base.

FIG. 11 is detailed schematic side, end, and top views of an emitting element.

FIG. 12 is detailed schematic side, end, and top views of a "tube-like" emitting element including a capped void and a base.

FIG. 13 is a perspective view of an electrospinning system, where a moving collection surface is disposed in the region between the emitting elements and the counter-electrode to intercept and collect the electrospun fibers and transport them away from the electrospinning zone.

FIG. 14 is a sectional schematic side view of two groups of emitting elements formed on two bases, each base being disposed in its own bath of an electrospinning fluid, and each base being biased to its own voltage with respect to a common counter- electrode.

FIG. 15 is a side sectional schematic view of an electrospinning system in which a container is available for intermittently or continuously replenishing a bath of an electrospinning fluid in which a base and elements are disposed.

DETAILED DESCRIPTION

This disclosure generally relates to electrospinning that uses capillarity that provides good hydraulic capacity; and emitting elements that do not require dedicated pumping components and are not prone to either spillover or clogging. At least in part, this owes to the electrospinning fluid being both internal and possibly external to a surface of the emitting elements. The term "emitting element" is used here broadly to include any element that includes a free end (e.g., a tip) from which fibers are to be electro-spun and an end opposite the free end that is located on a base. The emitting element projects from the base and ends in the free end, which is away from the base. Some (e.g., all) of the emitting elements can be located on a common base. In some embodiments, each emitting element can be located on an individual base, which can then be coupled together to form an electrospinning structure. In some embodiments, a combination of the above arrangements can be used. In some implementations, at least a portion of the emitting elements or base or bases, or both, are manufactured from porous materials which can include ensembles of liquid channels, and can confer good hydraulic properties (e.g., allowing a free flow of liquid) while substantially mitigating clogging of the channels. In some implementations, the porous materials can be hydrophilic to an electrospinning fluid, so as to permit the fluid to passively impregnate the porous material by capillary action.

The advantages of various embodiments of the electrospinning described here include: (1) reduction or obviation of an active hydraulic pumping requirement; (2) reduction of clogging or spillover or both; and (3) amenability to fluid flowing over the outer surface of the elements, as in the case of solid needles, or through the internal medium, as in the case of capillary tubes (see FIG. 4), or both. The latter is believed to help promote robust electrospinning throughputs.

This disclosure relates to electrospinning, including apparatuses and methods of making and using these apparatuses. The apparatuses described here (also referred to as the "electrospinning systems") can include an electrospinning structure containing one or more emitting elements formed on one or more bases. In some embodiments, some (e.g., all) of the emitting elements can be formed on a common base. In some embodiments, each emitting element can be located on an individual base, which can then be coupled together to form an electrospinning structure. In some embodiments, a combination of the above arrangements can be used. In some embodiments, the electrospinning system can include a vessel containing an electrospinning fluid, and at least a portion of the electrospinning structure is disposed in the electrospinning fluid. In some embodiments, the electrospinning structure itself forms a vessel for containing the electrospinning fluid. The electrospinning apparatuses described here typically includes two electrodes that are connected to a power supply and are used to generate an electric field that is appropriate for supporting electrospinning of the fluid from the emitting elements to produce fibers.

In general, an emitting element (also referred to as "element") includes two ends, which are interconnected by a material (see, e.g., FIGs. 5 thru 12). An electrospinning fluid can pass through the element, on the surface of the element, or both to reach the free end, from which the fibers are to be produced. In some embodiments, one end of an emitting element is formed onto a base and the other end is a free end. The free end can have a diameter larger than, equal to, or less than the end formed on the base. In some embodiments, one end of an emitting element and a base are formed as an integral article. In some embodiments, the electrospinning systems described here can include

morphologically similar but spatially distinguishable emitting elements, e.g., in an array. In some embodiments, each of the elements includes its own base and those bases can be joined to form a combined base for the elements. In some embodiments, a group of emitting elements can be formed on one common base. A combination of the above arrangement can also be used.

The emitting elements can exhibit needle-like form factors, in which case they can resemble sharpened pencil tips; ribbon-like form factors, in which case they can resemble the working side of a sharpened knife; triangular form factors, in which case they can resemble a thin extrusion of the projection of any needle-like form onto a two- dimensional plane; or tube-like form factors, in which case they can resemble capillaries; among possibly many other form factors.

The electrospinning systems can include one or more bases. We use the terms "base" and "bases" broadly to include, for example, any structure or structures that support(s) one or more emitting elements to form an array or other pattern. The base can have any external form factor so long as it offers appropriate mechanical rigidity to the elements that it supports. We use the term "array" broadly to include, for example, any regular or irregular arrangement of the emitting elements. This includes linear

arrangements, e.g., in a single row; planar arrangements, e.g., elements arranged in multiple rows protruding from a common face of the base, or other patterns. In some embodiments, both the emitting elements and the base or bases comprise porous materials containing ensembles of liquid flow paths, interconnected or otherwise. We use the term "porous materials" broadly to refer to any material having a porosity greater than zero. Typically, the emitting elements and the base (or the entire

electrospinning structure) can have a porosity that allows a liquid (e.g., an

electrospinning fluid) to pass through the base and an emitting element, and exit the free end of the emitting element to produce fibers. The term "porosity" is widely understood as the ratio of void volume to total volume of a three-dimensional porous body, where the total volume is determined by the macroscopic outer dimensions of the body. A typical way of measuring the porosity can include the submersion of a material in a fluid medium against which it is perfectly impermeable. The material has a volume of solid Ss and occupies a measured volume So in the impermeable medium. The corresponding porosity is calculated as (So-Ss)/So. For clarity, the porous materials referred to in this disclosure include materials in which the voids may or may not be interconnected to any degree, and materials that may elsewhere be referred to as either open-cell foams or sponges. Examples of suitable porous materials include porous metals; porous ceramics (including porous glasses); porous semiconductors (including porous silicon); porous polymers (or porous plastics); and combinations of any two or more of them. Examples of porous polymers include a polyurethane, a polytetrafluoroethylene, a polyethylene, a polypropylene, a polyvinylidene fluoride, or an ethyl vinyl acetate or combinations of any two or more of them. Regardless of the material, the pore size of the emitting elements or the base or both can be at least about 0.1 microns (e.g., at least about 0.5 microns, at least about 1 micron, at least about 5 microns, or at least about 10 microns) and/or at most about 100 microns (e.g., at most about 75 microns, at most about 50 microns, at most about 25 microns, or at most about 10 microns).

In some embodiments, each of the emitting elements can have an opening at its free end to allow an electrospinning fluid to exit the emitting elements to produce fibers. In some embodiments, the opening can have a diameter of at least about 10 microns (e.g., at least about 20 microns, at least about 50 microns, or at least about 100 microns) and/or at most about 250 microns (e.g., at most about 200 microns, at most about 150 microns, at most about 100 microns, or at most about 50 microns).

In some embodiments, the emitting elements and the one or more bases can be comprised of the same porous material. In some embodiments, the emitting elements and the one or more bases can be comprised of different porous materials.

In some implementations, the material composing the emitting elements or the material composing the one or more bases can be substantially hydrophilic. We use the term "hydrophilic" in the context of the so-called "wetting angle" that is widely understood. Perfect wetting is taken to mean a wetting angle of zero degrees, while perfect non-wetting is taken to mean a wetting angle of 180 degrees. In other words, "wetting angle" refers to the angle that forms between a flat surface and a line that is tangent to the interface of a droplet, local to its contact point, disposed on the same surface and as measured inside the liquid. For example, substantial hydrophilicity can be satisfied when a drop of an electrospinning fluid, placed on a solid, flat, and continuous surface (i.e., non-porous) that has a chemical composition identical to that of the porous material used here, makes a wetting angle of less than 90 degrees. Among several possible methods, this angle can typically be measured through the use of a goniometer. Without wishing to be bound by theory, it is believed that the condition of substantial hydrophilicity tends to ensure that electrospinning fluids can freely migrate through the porous base or bases and the emitting elements by capillary action.

In some embodiments, the electrospinning systems described here can include first and second electrodes that are connected to a power supply and are used to generate an electric field to perform electrospinning from the emitting elements. In some embodiments, the first electrode (also sometimes referred to as a counter electrode) is arranged downstream and at a distance from the free end of the at least one emitting element. The space between the free ends of the emitting elements and the first electrode can be used to receive the fibers to be formed. The distance from the free ends of the emitting elements and the first electrode is typically in the range from about 100 microns to 10 centimeters (e.g., from 200 microns to 5 centimeters, from 500 microns to 1 centimeter, or from 1 millimeter to 5 millimeters). In some embodiments, the electrospinning structure is arranged between the first and second electrodes such that, during use, the first and second electrodes establish an electric field (e.g., across at least a portion of the electrospinning structure) to cause electrospinning to occur from the emitting elements to produce electrospun fibers. In such embodiments, the second electrode can be in direct contact with or embedded within the electrospinning structure or can be arranged at a distance (e.g., less than about 10 cm) from the electrospinning structure (e.g., on the other side of the electrospinning structure from the first electrode). In some embodiments, the electrospinning structure itself can either include a second electrode or serve as a second electrode. In some embodiments, during use, the second electrode and at least a portion of the electrospinning structure (e.g., at least a portion of the base or bases in the electrospinning structure, or a portion of the emitting elements) can be disposed in an electrospinning fluid in a vessel to allow the fluid to pass through the electrospinning structure and be electrospinning from the free ends of the emitting elements to produce fibers.

In some embodiments, the first electrode can be configured to collect fibers. For example, the first electrode can be a plate having a continuous surface to collect fibers exiting the free ends of the emitting elements. In some embodiments, the first electrode can be continuous (e.g., without any holes, pores, or voids) at locations that correspond to the locations of the free and of the emitting elements. In some embodiments, the first electrode can be a screen. In some embodiments, the first electrode can have pores small enough to collect fibers. In some embodiments, the second electrode can be a continuous plate.

In some embodiments, the electrospinning systems described here can include a collection element between the electrospinning structure and the first electrode, and the collection element is configured to collect the fibers. The collection element can be in any suitable form, such as a plate or a screen. In general, the collection element should not nullify the electric field between the first and second electrodes.

In some embodiments, the electrospinning systems described here can further include a power source electrically connected to the first and second electrodes such that a difference in electric potential can be established. This potential produces an electric field in the region between the two electrodes, which includes a sub-region between the free ends of the emitting elements and the counter-electrode. This electric field is believed to concentrate near or at the free ends of the emitting elements and cause electrospinning fiber formation when the associated voltage is high enough. Modulation of the voltage from the power supply can be used to adjust the electric field in the region between at least one emitting element and the first electrode. When the voltage is high enough, the free ends of the emitting elements can begin to support the jetting of continuous liquid filaments, i.e., electrospinning. These filaments can propagate downstream in the general direction of the first electrode and can be collected by the first electrode or the collection element described above.

In some embodiments, the power source can be configured to generate a voltage of at least about 100 V (e.g., at least about 200 V, at least about 500 V, at least about 1,000 V, at least about 5,000 V, or at least about 10,000 V) to at most about 100,000 V (e.g., at most about 50,000V, at most about 10,000V, at most about 5,000 V, or at most about 1,000 V).

In some embodiments, the electrospinning fluid can be a solution including at least one solvent (e.g., an organic solvent or water or both) and at least one polymer dissolved in the solvent. The polymers that can be used in the electrospinning fluid depend on the fibers to be produced and can include polyolefins, polyesters, polyamides, polyethers, polyacrylates, vinyl polymers, and mixtures and co-polymers of any two or more of them. In some embodiments, the electrospinning fluid can include a polymer melt without any solvent.

A selection of micro-manufacturing techniques that are compatible with the disclosed materials can be used to pattern elements. In some embodiments, the manufacturing techniques can be "subtractive" techniques, in which a material is selectively removed from a monolithic structure to create emitting elements on a base. In some embodiments, the manufacturing techniques can be "integrated" techniques, in which the material composing the elements and the elements themselves are formed concurrently. Among the former techniques, plasma etching, laser etching, and electrochemical etching are examples. Among the latter, several forms of molding are typical examples. One exemplary integrated technique is powder sintering, in which micro- or nano-sized pieces of matter are thermally bonded in a mold that defines the form factor of the desired emitting elements or the one or more bases, or both. Another exemplary integrated technique is a sol-gel process for forming and freeze casting, in which, it is believed that generally, liquid precursors of porous materials are exposed to molds as they are processed to form rigid porous materials that exhibit the desired form.

In some embodiments, a magnetic molding process can be employed with either a sol-gel or a freeze cast. In the case of a sol -gel, the method can include (1) mixing at least one sol gel precursor, at least one magnetic material, and at least one solvent to form a mixture; applying a magnetic field to the mixture to form protrusions on a surface of the mixture; and removing the solvent from the mixture to form a porous electrospinning structure. The method can include adding a catalyst (e.g., an acid or a base) after forming protrusions on a surface of the mixture. The protrusions thus formed can then be converted into rigid porous emitting elements on a porous base by heating the mixture such that a sol-gel process occurs to convert the sol gel precursor into a porous polymer and that the solvent is removed to form a rigid electrospinning structure. In some embodiments, the magnetic material that can be used in the above sol gel process can include magnetic nanoparticles, such as iron containing nanoparticles (e.g., iron nanoparticles, iron oxide nanoparticles, magnetite nanoparticles, or hematite

nanoparticles, or combinations of two or more of them). In some embodiments, the sol gel precursor can include a metal alkoxide (e.g., tetramethyl orthosilicate or tetraethyl orthosilicate), a metal chloride (e.g., aluminum chloride), or a metal nitrate (e.g., aluminum nitrate), or combinations of two or more of them. In some embodiments, the magnetic nanoparticles can be coated with a surfactant by adding a surfactant into the mixture. It is believed that the surfactant can facilitate formation of the suspension of the nanoparticles in the solvent. Other additives, such as polymers or proton scavengers, can be added to the mixture described above.

As an example, the sol gel process described above can include preparation of a dispersion that includes a paramagnetic solvent, an acidic aluminum salt, a polymer, and a proton scavenger. The paramagnetic solvent is a so-called "carrier fluid" in which one of magnetite nanoparticles, hematite nanoparticles, or some other iron-containing nanoparticles can be disposed in a base solvent along with a surfactant (e.g., an oleic acid, or a soy lecithin). Such a fluid is responsive to magnetic fields and is also known as a ferrofluid. In advance of substantial gelation within the sol-gel, the dispersion can be exposed to a magnetic field such that a component of the field is orthogonal to the surface of the solution. When the field is strong enough, a pattern of so-called Rosensweig spikes can emerge on the surface of the dispersion, after which the process of gelation can be allowed to occur in the presence of the same field. The sol-gel can then be dried after substantial gelation to form either a xerogel or an aerogel green body in which the Rosensweig spikes, which can serve as the emitting elements described herein, have been frozen into place. Thereafter, the green body may or may not be thermally treated (i.e. sintered) to accrue additional mechanical rigidity.

In some modes of operation of the electrospinning system, the elements and one or more bases, or portions of them, are in fluid communication with a vessel containing an appropriate electrospinning liquid. In some embodiments, this vessel contains an open bath of the fluid, that is, one that is exposed to the ambient, in which case the bases may be at least partially submerged while other portions of the emitting elements or the free ends of the emitting elements remain above the nominal fluid level. In some

embodiments, the base or bases are fully immersed in the bath while the emitting elements themselves are at least partially immersed. In some embodiments, the vessel can be an open recess on the back of the one or more bases, in which case the base or bases themselves form the bottom wall of the vessel. In some embodiments, the fluid channels in the elements and the base or bases can ensure that the electrospinning fluid substantially impregnates both via capillary action, sans any recourse to hydrostatic forcing, even when the porous materials are mismatched.

In some embodiments, the electrospinning system described here can include a separate container containing an electrospinning fluid. The container can be in fluid communication with the vessel in which the emitting elements and one or more bases are disposed and intermittently or continuously replenish the electrospinning fluid contained in the vessel. During the course of operation, it is possible that strong gradients in hydraulic pressure may form between the free ends of the emitting elements (where the filaments will emanate) and the upstream bath of the electrospinning fluid. In some instances this will be acceptable, while in others it may be useful to manage the thermal excitation of the liquid (e.g., subject it to controlled heating) to reduce viscosity. Such action can mitigate gradients. In still other instances, similar heating may be useful for enabling electrospinning with solutions of high polymer concentration, or concentrations of other viscous substances that would otherwise be unworkable, such that the effective mass or volumetric rate of fiber deposition is enhanced. Given that the chain entanglement of dissolved fluid constituents, which is believed to play a role in discouraging jet disintegration (i.e., promoting continuous filaments in lieu of, for example, droplets), is possibly adversely affected by thermal management, the mentioned methods can be acceptable insofar as they preserve rheological properties that are appropriate to electrospinning. Reference to electrospinning fluids, therefore, is understood to encompass so-called "melts" or any other such fluid with rheological properties that are appropriate to electrospinning, and should be interpreted broadly.

During the course of operation, it is possible that certain emitting elements and possibly the underlying base or bases can be depleted of their fluid charges, where "fluid charge" is defined broadly as a volume of liquid stored by an emitting element or one or more bases, or a combination of both, when the electrospinning structure is not generating fibers.

For example, in a simplified situation where the voltage Ve on the power supply demarcates the threshold at which electrospinning occurs from the free ends of the emitting elements, the voltage Ve+ will engender fiber formation while the voltage Ve- will not. The "fluid charge" is then interpreted to mean the volume of liquid stored by an element or one or more bases, for any voltage less than or equal to Ve-, including the quiescent state involving zero voltage.

It is believed that depletion, including partial depletion, of the fluid charge could lead to intermittency in the electrospinning process, or in some cases its preclusion. In the event of the latter, spraying of droplets (i.e., electrospraying) could instead prevail. As a countermeasure, in some embodiments, the emitting elements could be temporarily submerged in a bath of the electrospinning fluid, up to and including the free ends, to restore the fluid charge. In other embodiments, the voltage on the power supply could be periodically modulated to prevent problematic depletion. For example, the voltage Ve+ could be enforced for a first time Tl (where Tl is shorter than the characteristic time for depletion), the voltage Ve- could be enforced immediately thereafter for a second time T2 (where T2 is longer than the characteristic time for restoration of the fluid charge), and then the voltage Ve+ could be reestablished.

In some embodiments, a first base or group of bases (e.g., each containing one or more emitting elements) may be in contact with a first reservoir of electrospinning fluid while other bases or groups of bases are in contact with separate reservoirs of electrospinning fluids. In some embodiments, a common counter-electrode can be disposed at a distance from the emitting elements attached to the first base or bases. This distance, however, need not be identical to the distance of the emitting elements attached to the other base or bases in contact with different fluid reservoirs. Furthermore, a single electrode can be disposed in all of the fluid reservoirs or separate electrodes can be disposed in each individual reservoir or groups of reservoirs. In the event of the latter, each electrode in an individual reservoir among different reservoirs can be connected to a common counter-electrode. Such an electrical configuration allows for separate reservoirs to be biased to different voltages. Depending upon several factors, this configuration could permit, for example, concurrent electrospinning of disparate fluids to produce fibers having different compositions.

Some implementations of the electrospinning systems are described below in more detail based on the illustrations in FIGs. 1-14.

FIG. 1 illustrates embodiments of an electrospinning system that includes an electrospinning structure having porous elements (1) formed on a common porous base (2) that is partially immersed in a bath of an appropriate electrospinning fluid (3) contained in a vessel (4). Depending upon the degree of immersion, the affixed ends of the elements (1) themselves may or may not reside below the nominal level of the electrospinning fluid (3). However, a flow (5) of the electrospinning fluid (3) that is enabled by capillary action permits the fluid to substantially impregnate the array of elements (1) and base (2) even when the fluid is not electrically stressed. A first electrode (6), the counter-electrode, is situated at a first distance from the free ends of the elements, while a second electrode (7) is disposed in the bath of electrospinning fluid at a second distance from the base (2). A power supply (8) electrically connects the electrodes (6) and (7) such that a difference in electrical potential can be established. This potential can be held predominantly in either polarity or it can periodically vary from one polarity to the other. For example, a sinusoidal wave centered about the electrical reference potential, e.g. "ground", may be utilized. In either case, the magnitude of the potential can also be periodically modulated so as to prevent problematic depletion of the "fluid charges" that occupy the elements and their bases.

During use of the system shown in FIG. 1, a difference in electrical potential can establish an electric field (9) in the region (101) between the two electrodes (6) and (7), including a sub-region (102) between the free ends of the elements (1) and the counter- electrode (6) and a sub-region (103) between the free ends of the elements (1) and the electrode (7). The quotient of the modulus of the difference in potential and the distance between the two electrodes is a first approximation to the magnitude of this field.

However, a measure of spatial non-uniformity typically occurs such that the field is specifically amplified local to the free ends of the elements (1). In general, this amplification depends upon factors that include, but are not be limited to, the geometry of the free ends, the material composing the elements, the material composing the bases, the electrical properties of the electrospinning fluid, and the relative spacings (101), (102), and (103) of the electrodes, or combinations of them. When the strength of the electric field in the vicinity of the free ends of the elements (1) is sufficiently large,

electrohydrodynamic jetting can occur. So long as the rheological properties of the electrospinning fluid (3) are appropriate, this jetting will result in the production of continuous filaments or fibers (10) that propagate away from the elements and are eventually intercepted and collected by the counter-electrode (6).

FIG. 2 depicts embodiments of an electrospinning system in which a face (201) of the base (2) is in contact with walls (11). The assembly of the walls (11) and the base forms a "recess" (202) that acts as a container for the electrospinning liquid (3) but keeps it partially exposed to the ambient atmosphere. The porous elements (1) and base (2) permit formation of a flow (5) of the electrospinning fluid (3) by capillary action to allow the fluid to substantially impregnate the elements (1) and base (2). During use, fibers (10) can be produced by generating an electric field (9) in electrodes (6) and (7) in a manner similar to the system shown in FIG. 1. Such a configuration can be useful, for example, in situations where it is desirable to electrospin fibers in a direction that is nominally aligned with the gravitational vector (203) prevailing in the local environment. Note that the walls (11) can be made from any suitable material (such as a porous material or a hydrophilic material).

FIG. 3 depicts embodiments of an electrospinning system in which a collection element (12) is disposed at a third distance (301) from the free ends of the elements (1), where the third distance is in general less than the distance (302) between the free ends of the elements (1) and the nearest plane of the counter-electrode (6). Appropriate collection elements can include, but are not limited to, semi-transparent screens, fixed objects, and moving objects upon which fibers can be continuously and substantially evenly deposited (i.e. a conveyor belt configuration), or combinations of two or more of these. Materials for such collection elements are not restricted to electrically conductive options and can include both conductors and dielectrics (insulators). When a dielectric is used as a collection element, it can be useful to modify the surface properties of the dielectric so as to prevent problematic accumulation of charge in the region of electric field between the elements (1) and electrode (6).

In some embodiments, the electrode (7) can be in direct contact with the base (2). In other embodiments, the base (2) itself or the base (2) and elements (1) together can serve as the electrode (7). The latter is particularly possible, for example, when the base (2) and elements (1) are both composed of porous metals. Such electrical architectures can be useful in helping to structure the electric field surrounding the free ends of the elements, where it is believed that strong amplification could play a role in augmenting the electrospinning throughput. FIG. 4 depicts the flows of an electrospinning fluid in and around three emitting elements. An electric field (9) acts on each of a capillary-type emitting element (13), a solid needle emitting element (14), and a porous emitting element (15) to produce nanofibers (10). In the case of the capillary-type element, the flow of liquid (16) is substantially inside the element. In the case of the solid needle element, the flow of liquid (17) is on the surface of the element. In the case of the porous element, the flow of liquid can be both inside the element (16) and on its surface (17).

FIG. 5 is a schematic illustrating a porous needle-like emitting element. A side- view (18) includes a dashed line (19) demarcating the location of the slice for a cross- sectional view (20). Atop-down view (21) of the element is also shown.

FIG. 6 is a schematic illustrating a porous ribbon-like emitting element. A side- view (22) includes a dashed line (23) demarcating the location of the slice for a cross- sectional view (24). A top-down view (25) of the element indicates that the element forms a ridge (601), similar to the working edge of a knife.

FIG. 7 is a schematic illustrating a porous triangular emitting element. A side- view (26) includes a dashed line (27) demarcating the location of the slice for a cross- sectional view (28). A top-down view (29) of the element shows that the element is a thin extrusion of its side profile (26).

FIG. 8 is a schematic illustrating a porous tube-like emitting element. A side- view (30) includes a dashed line (31) demarcating the location of the slice for a cross- sectional view (32). A top-down view (33) of the element shows that the element contains a void (49) within its center.

FIG. 9 is a more detailed schematic illustrating embodiments of a cylindrical emitting element (34) showing its porosity and its attachment to a base (2). The location of the slice for the cross-section (36) is the dashed line (35). A top-down view (37) of the element is also provided.

FIG. 10 is a more detailed schematic illustrating embodiments of a tube-like emitting element (38) showing its porosity and its attachment to a base (2). The location of the slice for the cross-section (40) is the dashed line (39). The cross-section (40) delineates a blind recess (41) near the center of the element. In some cases, this recess can extend through a portion of the height of the element, and in other cases, it can extend through the height of the element and into a portion of the base (2). A top-down view (37) of the element is also provided.

FIG. 11 is a more detailed schematic illustrating embodiments of a tube-like emitting element (43) showing its porosity and its attachment to a base (2). The location of the slice for the cross-section (45) is the dashed line (44). The cross-section (45) delineates a thru-hole (46) that extends through the body of the element and its underlying base. A top-down view (47) of the element is also shown.

FIG. 12 is a more detailed schematic illustrating embodiments of a tube-like emitting element (48) showing its porosity and its attachment to a base (2). The location of the slice for the cross-section (50) is the dashed line (49). The cross-section (50) delineates a recess (51) that extends through the base and partially into the element. The recess (51) is capped by a layer of a porous material (52). A top-down view (53) of the element is also provided.

FIG. 13 is a perspective view of embodiments of the electrospinning system in which a plurality of emitting elements (1) are formed on a common base (2), forming a planar array. The base (2) is partially immersed in a bath of an electrospinning fluid (3) that is contained in a vessel (4). A counter-electrode (6) is disposed at a distance (74) from the array and connected to a power supply (8) using an electrical lead (55). Another electrical lead (54) connects the same power supply (8) directly to the base (2), which in this case serves as the complementary electrode. A conveyor belt (12) is disposed in the region between the free ends of the elements (1) and the counter-electrode (6) to collect electrospun fibers. Its ends are attached to a mechanism (56) that is capable of moving it in relation to the array. When the voltage on the power supply (8) is appropriately modulated, such that a sufficient electrical field (9) prevails in the region between the electrodes, electrospinning begins to occur from the elements (1) to produce fibers (10), which propagate toward the conveyor belt (12).

FIG. 14 illustrates embodiments of the electrospinning system in which a first group of emitting elements (57) are attached to a base (58) that is partially immersed in a bath of a first electrospinning fluid (59) in a vessel (60). An electrode (61) is disposed in this vessel (60) and connected to a common counter-electrode (69) using a power supply (62). A second group of emitting elements (63) are attached to another base (64) that is partially immersed in a bath of a second electrospinning fluid (65) in a vessel (66). The second electrospinning fluid (65) can be the same as or different from the electrospinning fluid (59). An electrode (67) is disposed in the vessel (66) and connected to the common counter-electrode (69) using its own power supply (68). The power supply (62) biases the electrode (61) to a voltage VI, while the power supply (68) biases the electrode (67) to a voltage V2. VI and V2 generate the electric fields (70) and (71) in the spaces between the electrode (61) and the counter-electrode (69), and the electrode (67) and the counter-electrode (69), respectively. In general, the voltages VI and V2 can be the same or can be different. Electrospinning of fibers (72) can take place when the electric field (70) is sufficiently large, and electrospinning of fibers (73) can take place when the electric field (71) is sufficiently large. An advantage of this embodiment is that it can produce two different sets of fibers concurrently.

FIG. 15 illustrates embodiments of the electrospinning system described here in which an external container (75) containing an electrospinning fluid (3) to replenish the vessel (4) in which the base (2) and elements (1) are disposed. The container (75) includes a hydraulic line (76) equipped with a valve (77) such that additional electrospinning fluid can be added to the vessel (4) as needed. Such a configuration could help to prevent issues associated with a dwindling fluid bath during

electrospinning.

Other embodiments are within the scope of the following claims.

Claims

WHAT IS CLAIMED IS:

1. An apparatus for producing fibers, comprising:

an electrospinning structure comprising a base and at least one emitting element on the base, the at least one emitting element having a projecting free end, at least a portion of the base or the at least one emitting element or both comprising a porous material;

a first electrode arranged at a distance from the free end of the at least one emitting element and configured to cause fibers to be produced from the free end of the at least one emitting element; and

optionally, a collection element between the at least one emitting element and the first electrode, the collection element being configured to collect the fibers.

2. The apparatus of claim 1, further comprising a second electrode, wherein at least a portion of the electrospinning structure is between the first and second electrodes.

3. The apparatus of claim 1, wherein the electrospinning structure comprises a second electrode.

4. The apparatus of claim 1, wherein the porous material comprises a polymer, a metal, a ceramic, or a combination of any two of more of them.

5. The apparatus of claim 1, wherein the porous material comprises a polyurethane, a polytetrafluoroethylene, a polyethylene, a polypropylene, a

polyvinylidene fluoride, or an ethyl vinyl acetate, or a combination of any two or more of them.

6. The apparatus of claim 1, wherein the porous material has an average pore size from about 0.1 microns to about 100 microns.

7. The apparatus of claim 1, wherein the porous material is substantially hydrophilic.

8. The apparatus of claim 1, wherein at least a portion of the electrospinning structure has a porosity that allows a liquid to pass along at least a portion of a path that extends from a source of the liquid to the free end of the emitting element.

9. The apparatus of claim 1, wherein the electrospinning structure comprises a vessel for containing a liquid from which the fibers are to be formed.

10. The apparatus of claim 9, wherein a second electrode is disposed in the vessel.

11. The apparatus of claim 1, wherein the apparatus comprises a vessel for containing a liquid from which the fibers are to be formed, a second electrode and at least a portion of the electrospinning structure being disposed in the vessel.

12. The apparatus of claim 1, wherein the first electrode is configured to collect the fibers.

13. The apparatus of claim 12, wherein the first electrode comprises a plate.

14. The apparatus of claim 13, wherein the plate is continuous at a location that corresponds to the location of the free end of the at least one emitting element.

15. The apparatus of claim 11, wherein the first electrode comprises a screen.

16. The apparatus of claim 1, wherein the apparatus comprises the collection element.

17. The apparatus of claim 1, wherein the apparatus comprises a power source electrically connected to the first electrode and a second electrode.

18. The apparatus of claim 17, wherein the power source is configured to generate a voltage of from about 100 V to about 100,000 V.

19. The apparatus of claim 1, wherein the at least one emitting element has an opening and the opening has a diameter of from about 10 μπι to about 250 μπι.

20. A method of forming fibers, comprising:

applying a voltage across at least a portion of an electrospinning structure comprising a base and at least one emitting element on the base such that a liquid passes along at least a part of a path from a source of the liquid to the at least one emitting element and to be emitted from the at least one emitting element to form fibers.

21. A method, compri sing :

mixing at least one sol gel precursor, at least one magnetic material, and at least one solvent to form a mixture;

applying a magnetic field to the mixture to form protrusions on a surface of the mixture; and

removing the solvent from the mixture to form a porous electrospinning structure.

22. The method of claim 21, wherein the at least one magnetic material comprises magnetic nanoparticles.

23. The method of claim 22, wherein the magnetic nanoparticles comprise iron-containing nanoparticles.

24. The method of claim 22, wherein the magnetic nanoparticles are coated with a surfactant.

25. The method of claim 21, wherein the at least one sol gel precursor comprises a metal alkoxide, a metal chloride, or a metal nitrate, or a combination of two or more of them.

26. The method of claim 25, wherein the least one sol gel precursor comprises tetramethyl orthosilicate, tetraethyl orthosilicate, or aluminum chloride, or a combination of two or more of them.

27. The method of claim 25, further comprising adding a catalyst after forming protrusions on a surface of the mixture.

28. The method of claim 27, wherein the catalyst is an acid or a base.

29. An apparatus, comprising

a set of elongated elements, each of the elements projecting from a base to a free end, each of the elements comprising a material that is porous to a liquid from which fibers are to be formed by electrospinning from the free end of the element, and

a vessel to contain a volume of the liquid,

wherein the bases of the elements are configured so that at least portions of the bases are in contact with the liquid .

30. An apparatus, comprising

an electrode plate arranged at a distance from the free ends of the elements for electrospinning from the free ends of the elements to the electrode plate, the electrode plate being continuous at locations that correspond to locations of the free ends of the elongated elements.

31. An apparatus, comprising

a set of elongated elements, each of the elements projecting from a base to a free end, each of the elements comprising a material that is porous to a liquid from which fibers are to be formed by electrospinning from the free end of the element, surfaces of the porous material that are to be in contact with the liquid being hydrophilic.

32. An apparatus, comprising

a vessel to contain a volume of the liquid, the vessel being unsealed.

33. A method, compri sing

by capillarity, conducting a liquid from which fibers are to be formed by electrospinning through an integrated porous body, the liquid being conducted from base ends of elements of the integrated porous body to free ends of the elements, and

electrospinning the fibers from the free ends of the elements.