Patent Publication Number: US-7592277-B2

Title: Nanofiber mats and production methods thereof

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is related to U.S. Patent Publication No. 2005/0224998, filed as U.S. application Ser. No. 10/819,942, on Apr. 8, 2004, entitled “Electrospray/Electrospinning Apparatus and Method,” the entire contents of which are incorporated herein by reference. This application is related to U.S. Patent Publication No. 2005/02249999, filed as U.S. application Ser. No. 10/819,945, on Apr. 8, 2004, entitled “Electrospinning in a Controlled Gaseous Environment,” the entire contents of which are incorporated herein by reference. This application is related to U.S. Patent Publication No. 2006/0228435, filed as U.S. application Ser. No. 10/819,916, on Apr. 8, 2004, entitled “Electrospinning of Fibers Using a Rotating Spray Head,” the entire contents of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to the field of fiber mats including multicomponent fiber mats and processes of forming such mats. 
     2. Description of the Related Art 
     Fibers and nanofibers are finding new applications in the pharmaceutical, filter, catalysts, clothing, and medical industries. Techniques such as electrospinning have been used to form fibers and nanofibers. For example, electrospinning techniques have been used to form fibers as small as a few nanometers in a principal direction. The phenomenon of electrospinning involves the formation of a droplet of polymer at an end of a needle, the electric charging of that droplet in an applied electric field, and an extraction of the polymer material from the droplet into the environment about the tip such as to draw a fiber of the polymer material from the tip. 
     Glass fibers have been manufactured in a sub-micron range for some time. Small micron diameter fibers have been manufactured and used commercially for air filtration applications for more than twenty years. Polymeric melt blown fibers have recently been produced with diameters less than a micron. Several value-added nonwoven applications, including filtration, barrier fabrics, wipes, personal care, medical and pharmaceutical applications may benefit from the interesting technical properties of nanofibers and nanofiber webs. Electrospun nanofibers have a dimension less than 1 μm in one direction and preferably a dimension less than 100 nm in this direction. Nanofiber webs have typically been applied onto various substrates selected to provide appropriate mechanical properties and to provide complementary functionality to the nanofiber web. In the case of nanofiber filter media, substrates have been selected for pleating, filter fabrication, durability in use, and filter cleaning considerations, as described in U.S. Pat. No. 6,673,136, the entire contents of which are incorporated herein by reference. 
     Conventional techniques for electrospinning produce mats of fibers or nanofibers having a uniform chemical composition throughout the mat. Even if the electrospin medium (i.e., the liquid or dissolved polymer) is a mix of various polymers, the fibers produced would have a uniform composition at any given location in the resultant fiber mat, i.e., the composition at any point being determined by the polymer constituency at the time of electrospinning. In addition, the conventional electrospinnning techniques produce fibers of a uniform fiber thickness at any point in the resultant fiber mat, as factors preset on the electrospinning device such as for example the electric field strength and the drying rate determine the fiber thickness produced. 
     Recently, Smith et al in U.S. Pat. No. 6,753,454, the entire contents of which are incorporated herein by reference, describe a technique for electrospinning fibers simultaneously or sequentially from multiple polymer-containing reservoirs. In this technique, the reservoirs for electrospinning were connected via a switch to a common power supply generating the requisite electric field by which the fibers are electrospun. As such, the fibers electrospun from the separate reservoirs collect onto a common ground electrode. Smith et al describe one utility of an alloyed fiber mat in the field of medical dressings where one side of the fiber composite is predominantly a set of hydrophilic fibers and the other side is predominantly a set of hydrophobic fibers. Smith et al also describe a polymer membrane forming the medical dressing that is generally formulated from a plurality of fibers electrospun from a substantially homogeneous mixture of any of a variety of hydrophilic and at least weakly hydrophobic polymers, that can be optionally blended with any of a number of medically important wound treatments, including analgesics and other pharmaceutical or therapeutical additives. For example, Smith et al describe polymeric materials suitable for electrospinning into fibers that may include absorbable and/or biodegradable polymeric substances that react with selected organic or aqueous solvents, or that dry quickly. Smith et al also describe that essentially any organic or aqueous soluble polymer or any dispersions of such polymer with a soluble or insoluble additive suitable for topical therapeutic treatment of a wound may be employed. 
     A schematic representation of the apparatus of Smith et al is shown in  FIG. 1 .  FIG. 1  depicts an electrospinning apparatus  10  for the production of a fiber mat. The term “fiber mat” is used to define a plurality of fibers formed by forming fiber after fiber on each other. Respective fibers in the fiber mat can intermingle or be separate from other fibers in the fiber mat. Conventionally, the electrospinning apparatus  10  produces fibers that weakly adhere to each other. 
     The electrospinning apparatus shown in  FIG. 1  is capable of producing fiber mats from separate electrospinning devices. The electrospinning apparatus  10  has two electrospinning devices  10   a  and  10   b  that each produces a same electric field  12  that extracts a polymer melt or solution  14  extruded from a tip  16  of an extrusion element  18  to a collection electrode  20 . An enclosure/syringe  22  stores the polymer solutions  14  in each of the electrospinning devices  10   a  and  10   b . A voltage power source  24  is electrically connected with one electrode through a wire  26  to each of the electrospinning devices  10   a  and  10   b , and the other electrode of the power source  24  is electrically connected to ground. A switch  25  connects either of the electrospinning devices  10   a  and  10   b  to the power supply  24 . The electric field  12  created between the tip  16  and the collection electrode  20  causes the polymer solution  14  to overcome cohesive forces that hold the polymer solution together. A jet of the substance  14  is drawn from the tip  16  toward the collection electrode  20  by the electric field  12  (i.e., electric field extracted), and dries during flight from the extrusion element  18  to the collection electrode  20  in a fiber extraction region  27  to form polymeric fibers, which can be collected downstream on the collection electrode  20 . 
     However, fibers produced from the apparatus in  FIG. 1  can suffer from poor adherence among the fibers that constitute the fiber mat due to the electrospun substances having the same electric polarities which in turn results in the collected fibers being repelled from each other as the fibers coalesce together on the collection electrode  20 . 
     SUMMARY OF THE INVENTION 
     One object of the present invention is to provide apparatuses and methods for producing fiber mats. 
     Another object of the present invention is to provide fiber mats having an intermixed region of first and second fibers. 
     Another object of the present invention is to provide a fiber mat having first fibers with a first diameter and second fibers with a second diameter different than the first diameter. 
     Another object of the present invention is to provide a fiber mat having first fibers made of a first material and second fibers made of a second material. 
     According to one aspect of the present invention, there is provided a novel apparatus that includes a first electrospinning device configured to electrospin first fibers of a first substance, a second electrospinning device configured to electrospin second fibers of a second substance, and a biasing device configured to bias the first electrospinning device with a first electric polarity and to bias the second electrospinning device with a second electric polarity of opposite polarity to the first electric polarity to promote attraction and coalescence between the first and second fibers such that first and second fibers combine in a mat formation region. 
     According to a second aspect of the present invention, there is provided a novel method for producing the fiber mat, the method includes electrospinning under the first electric polarity fibers from the first substance, electrospinning under the second electric polarity fibers from the second substance, and coalescing the first and second fibers to form the fiber mat. 
     According to a third aspect of the present invention, there is provided a novel mat of fibers, the mat having a plurality of first and second fibers intermixed therein; having a cross section fiber density of at least (2.5×10 13 )/d 2  fibers/cm 2 , where a value of d is given in nm, less than 500 nm, and represents an average diameter d along a length of one fiber of the plurality of first and second fibers. 
     According to a fourth aspect of the present invention, there is provided a novel composite fiber mat that includes at least one of first and second fibers, and particles directly attached to a surface of the at least one of the first and second fibers along a longitudinal direction of the fibers, the particles being attached by a fiber material of the at least one of the first and second fibers. 
     It is to be understood that both the foregoing general description of the invention and the following detailed description are exemplary, but are not restrictive of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete appreciation of the present invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein: 
         FIG. 1  is a schematic illustration of a conventional electrospinning apparatus; 
         FIG. 2  is a schematic illustration of a dual electrospinning apparatus having horizontal extrusion elements according to one embodiment of the present invention; 
         FIG. 3  is a schematic illustration of a fiber distribution according to one embodiment of the present invention; 
         FIG. 4  is a schematic illustration of a dual electrospinning apparatus of one embodiment of the present invention having extrusion elements forming a predetermined angle from vertical direction; 
         FIG. 5A  is a schematic illustration of a mat of multicomponent fibers according to one embodiment of the present invention; 
         FIG. 5B  is a SEM micrograph of the fibers in a mat region produced according to the present invention; 
         FIGS. 5C-5E  are schematic illustrations of fiber distributions in regions corresponding to a first end, a central portion, and a second end of a fiber mat of the present invention; 
         FIG. 6A  is a schematic illustration of an electrospinning apparatus having a plurality of extrusion elements used in another embodiment of the present invention; 
         FIG. 6B  is a schematic illustration of an electrospinning apparatus having a particle delivery device according to another embodiment of the present invention; 
         FIG. 7A  is a schematic illustration of an electrospinning apparatus having an opposed particle delivery device according to another embodiment of the present invention; 
         FIG. 7B  is a SEM micrograph of a particle/fibers of the present invention; and 
         FIG. 8  is a flowchart depicting a method of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views, and more particularly to  FIG. 1 , the inventors of the present invention have determined that one effect of the poor adherence between fibers formed in the apparatus of  FIG. 1  is that the fiber web tends to break into smaller parts. One factor contributing to the poor adherence derives from the use of a common potential supply provided by a power supply  24 . The inventors of the present invention have discovered that the above deficiencies can be overcome if fibers of the fiber web are collected in a state where the fibers have opposite electrical charges on respective fibers in the web. Thus, in one embodiment of the present invention, two electrospinning devices (i.e., a first electrospinning device and a second electrospinning device) are operated at opposite electrical polarities. As a result, the respective electrospun fibers have opposite charge and electrostatically attract to each other in a mat formation region. 
     Thus, in one embodiment of the present invention, the apparatus  11  shown in  FIG. 2  includes at least two electrospinning devices  11   a  and  11   b . The apparatus  11  is a plural electrospinning apparatus and is configured to produce a fiber mat formed of fibers with different components. The electrospinning devices  11   a  and  11   b  can be any known electrospinning device having the requisite opposite biases applied. The electrospinning devices  11   a  and  11   b  are disposed in one embodiment of the present invention opposite to each other with an optional collection electrode  20  provided between the electrospinning devices  11   a  and  11   b . In addition, the electrospinning device  11   a  can be connected to a first high voltage power source  24   a  through a wire  26   a  with the power source  24   a  grounded. Similarly, the electrospinning device  11   b  can be connected to a second high voltage power source  24   b  through a wire  26   b  with the power source  24   b  grounded. The substance electrospun from the electrospinning devices  11   a  and  11   b  becomes fibers in corresponding fiber formation regions  18   a  and  18   b  and those fibers coalesce in a mat formation region that could be defined by the collection electrode  20 , if present. If an impermeable collection electrode is not present, the fibers attract to each other and collect into a mat in a region where the resultant electric potential is zero. The collection electrode can have any orientation that is suitable to collect the fibers and has a shape selected to match a desired shape of the fiber mat. Exemplary shapes of the collection electrode  20  include but are not limited to a hook, a ring, a web, and/or a net. 
     The formation of the fiber mat is described in an illustrative example with reference to the apparatus in  FIG. 2 , which is not intended to limit the present invention. Both electrospinning devices  11   a  and  11   b  of  FIG. 2  simultaneously extrude respective electrospin mediums  14 . The electrospin mediums  14  used in each of the devices  1   a  and  11   b  are different for the purpose of the present example. After the electrospin mediums  14  are extruded from the extrusion elements  18   a  and  18   b , the electrospun substances travel towards each other and electrostatically attract to each other due to the opposite electrical charges of the fibers. Upon contact, the fibers remain attached and collected by the collection electrode, if present. By grounding the collection electrode  20 , the charged fibers would be not only electrostatically attracted to each other but also attracted to the collection electrode  20 . 
     The two power sources  24   a  and  24   b  could be identical or different. The power sources independently control an electric potential of each of the electrospinning devices  11   a  and  11   b . The power sources  24   a  and  24   b  are configured to provide opposite polarities to the devices  11   a  and  11   b . The power sources are configured with the apparatus geometry to supply an electric field strength of 10,000 to 500,000 V/m 
     In such a configuration, the fibers produced by the electrospinning device  11   a  are extruded towards the fibers produced by the electrospinning device  11   b . When the fibers from the two devices are attracted to and collide with each other, for example due to the opposite electric charges on the respective fibers, the fibers form a fiber mat having fibers, according to one aspect of the invention, with a high fiber-to-fiber adherence as well as a high degree of interpenetration. 
     In one embodiment of the present invention the fibers extruded from the first and second electrospinning devices can have an average diameter of less than 500 nm, preferably less than 100 nm. Larger diameter fibers such as fibers less than 5 μm can also be electrospun in the present invention. An average separation of adjacent fibers in the fiber mat can be less than an average diameter of the fibers, preferably less than half of an average diameter of the fibers. Further, a cross sectional density of the fibers per cm 2  is calculated as a function of various parameters. For example, the cross sectional density is calculated with reference to  FIG. 3 , by dividing a length “a” of a side of a cube (which represents a region of the mat) by a sum of (i) an average diameter “d” of the fibers in the fiber mat, and (ii) an average separation of adjacent fibers “s” (i.e., the distance between two adjacent outer fiber surfaces, as shown in  FIG. 3 ). Further, the quantity obtained is squared to obtain the cross sectional density over a side surface of the cube. 
       FIG. 3  depicts various individual fibers not yet coalesced into a fiber mat. Using conventional electrospinning, as described previously by Smith et al, the fibers retain common, like charge and tend to be repulsive, thus not densely coalescing. As such, the fibers tend to contact infrequently at points along lengths of the fiber. By contrast, according to the present invention, the fibers have opposite charge and thus attract. Hence, the separation “s” between fibers in the mat of the present invention is smaller, yielding a denser network of coalesced fibers. For example, if the length a of the side of the cube is considered to be 1 cm, and the average separation s is considered to be equal to or approximate to the average diameter d of the fibers, then the cross section density will vary with the average diameter d of the fibers in a cross section of the mat, and will have a value equal to at least (2.5×10 13 )/d 2  fibers/cm 2 , where a value of d is given in nm. Moreover, the inventors have found that the mat produced can have an average separation smaller than the average diameter of the fibers, and thus the cross section density above calculated represents only one value in a range of cross section density that could be achieved with the present invention. The inventors of the present invention have also found that the average separation distance s between adjacent fibers can be as small as 10 nm. Observed fiber mats regions showing the compactness of the fibers (due to the electrostatic attraction) are shown and discussed later with regard to  FIG. 5B . 
     Indeed, while the criterion of (2.5×10 13 )/d 2  fibers/cm 2  is realized in one embodiment of the present invention, utilizing the electrospinning devices  11   a  and  11   b  of the present invention, the present invention is not limited to only this density criterion. For example, the density criterion of (2.5×10 13 )/d 2  fibers/cm 2  will scale with the average separation distance s obtained by electrospinning the materials of opposite polarity, which in the present invention depending on various factors such as the fiber materials, fiber diameters, applied bias, etc. can range from a separation distance of 10×d to a value of 1/10×d, and can include all values in between. 
     In another embodiment of the present invention, the fibers coalesce in a region where the first and second electrospun substances include a solvent content. The region includes a mat formation region where the solvent content of the electrospun substances is less than 10 weight % and/or a mat formation region where the solvent content is greater than 20 weight % depending on the polymer and other conditions under which electrospinning is being carried out. If the solvent content is less than 10 weight %, then minimal or no consolidation appears among the fibers that coalesce. On the contrary, if the solvent content is greater than 20 weight %, the fibers coalesce and consolidate together. Preferably, the regions have the solvent content less than 2 weight % to prevent consolidation and a solvent content greater of 30 weight % to promote consolidation. 
     In another embodiment of the present invention, the fibers of opposite polarities can collide with each other in a fiber formation region where evaporation of a solvent and consolidation of the electrospun substance into fibers is not complete, thus providing a mechanism for consolidation of the fibers at or along junctions between the opposite polarity fibers. 
     In one embodiment of the present invention, the collection electrode is disposed below the electrospinning devices  11   a  and  11   b . In another embodiment, a chamber or enclosure  28  is provided around the region in which the various fibers collide with each other to control a gaseous environment as disclosed in U.S. application Ser. No. 10/819,945. 
     According to the present invention, any arrangement of at least two electrospinning devices that (i) produce fibers charged with electric charges having an opposite polarity and (ii) electrospin the fibers such that the electrospun fibers are capable of electrostatically attracting each other to produce the fiber mat of the present invention. Indeed,  FIG. 4  shows another embodiment of the present invention having at least two electrospinning devices  11   a  and  11   b  that produce fiber mats having the properties described above.  FIG. 4  shows that the substances electrospun by the extrusion elements  18   a  and  18   b  are directed to each other under a predetermined angle Φ from a horizontal direction such that the drying fibers electrostatically attract to each other to form the fiber mat. As previously discussed, the collection electrode  20  can optionally be provided to collect the fiber mat. 
     A distance from each extrusion element of the electrospinning devices  11   a  and  11   b  to the collection electrode  20  is preferably in a range between 5 and 50 cm, but the distance depends on a temperature of the ambient, on the properties of the polymer substance extruded, and the drying rate of the extruded substance, as would be known by those skilled in the art. 
     The composition of the fibers electrospun from the electrospinning devices  11   a  and  11   b  could be identical or different. If different materials are used for the substance of each device, the fiber mat can have a chemical composition that varies along a length of the fiber mat. Further, the average diameter of the fibers electrospun from the electrospinning devices  11   a  and  11   b  could be identical or different. 
     The fibers and nanofibers produced by the present invention include, but are not limited to, acrylonitrile/butadiene copolymer, cellulose, cellulose acetate, chitosan, collagen, DNA, fibrinogen, fibronectin, nylon, poly(acrylic acid), poly(chloro styrene), poly(dimethyl siloxane), poly(ether imide), poly(ether sulfone), poly(ethyl acrylate), poly(ethyl vinyl acetate), poly(ethyl-co-vinyl acetate), poly(ethylene oxide), poly(ethylene terephthalate), poly(lactic acid-co-glycolic acid), poly(methacrylic acid) salt, poly(methyl methacrylate), poly(methyl styrene), poly(styrene sulfonic acid) salt, poly(styrene sulfonyl fluoride), poly(styrene-co-acrylonitrile), poly(styrene-co-butadiene), poly(styrene-co-divinyl benzene), poly(vinyl acetate), poly(vinyl alcohol), poly(vinyl chloride), poly(vinylidene fluoride), polyacrylamide, polyacrylonitrile, polyamide, polyaniline, polybenzimidazole, polycaprolactone, polycarbonate, polydimethylsiloxane-co-polyethyleneoxide, polyetheretherketone, polyethylene, polyethyleneimine, polyimide, polyisoprene, polylactide, polypropylene, polystyrene, polysulfone, polyurethane, polyvinylpyrrolidone, proteins, SEBS copolymer, silk, and styrene/isoprene copolymer. 
     Additionally, polymer blends can also be produced as long as the two or more polymers are soluble in a common solvent. A few examples would be: poly(vinylidene fluoride)-blend-poly(methyl methacrylate), polystyrene-blend-poly(vinylmethylether), poly(methyl methacrylate)-blend-poly(ethyleneoxide), poly(hydroxypropyl methacrylate)-blend poly(vinylpyrrolidone), poly(hydroxybutyrate)-blend-poly(ethylene oxide), protein blend-polyethyleneoxide, polylactide-blend-polyvinylpyrrolidone, polystyrene-blend-polyester, polyester-blend-poly(hyroxyethyl methacrylate), poly(ethylene oxide)-blend poly(methyl methacrylate), poly(hydroxystyrene)-blend-poly(ethylene oxide). 
     Examples of suitable hydrophilic polymers include, but are not limited to, linear poly(ethylenimine), cellulose acetate and other grafted cellulosics, poly (hydroxyethylmethacrylate), poly (ethyleneoxide), and polyvinylpyrrolidone. Examples of suitable polymers that are at least weakly hydrophobic include acrylics and polyester such as, poly(caprolactone), poly (L-lactic acid), poly (glycolic acid), similar co-polymers of theses acids. As described in Smith et al, polymer solutions may optionally be applied in a sterile condition. 
     As suggested hereinabove, other additives, either soluble or insoluble, may also be included in the liquid(s) to be electrospun into the fibers. Preferably, these additives are medically important topical additives provided in at least therapeutic effective amounts for the treatment of the patient. Such amounts depend greatly on the type of additive and the physical characteristics of the wound as well as the patient. Generally, however, such additives can be incorporated in the fibers in amounts ranging from trace amounts (less than 0.1 parts by weight per 100 parts polymer) to 500 parts by weight per 100 parts polymer, or more. Examples of such therapeutic additives include, but are not limited to, antimicrobial additives such as silver-containing antimicrobial agents and antimicrobial polypeptides, analgesics such as lidocaine, soluble or insoluble antibiotics such as neomycin, thrombogenic compounds, nitric oxide releasing compounds such as sydnonimines and NO-complexes that promote wound healing, other antibiotic compounds, bacteriocidal compounds, fungicidal compounds, bacteriostatic compounds, analgesic compounds, other pharmaceutical compounds, adhesives, fragrances, odor absorbing compounds, and nucleic acids, including deoxyribonucleic acid, ribonucleic acid, and nucleotide analogs. 
     Once the various fibers intermingle with each other, a seed of the fiber mat is formed. The core of the fiber mat  41  is shown in core region  42  in  FIG. 5A . Region  42  of the fiber mat  41  includes various fibers electrospun by a corresponding electrospinning device. However, after the core region  42  is formed, due to the opposite arrangement of the electrospinning devices and the disposition of the collection electrode there between, fibers from each respective electrospinning device penetrate less into the core region  42  and the newly electrospun fibers start to accumulate on each side of the core region  42 , in regions  40  and  44  respectively. Thus, each region  40  and  44  includes mainly the fibers produced from the substance held by the electrospinning device closest to that side of the core region  42 . If the electrospinning devices are continuing to electrospin fibers, few newly electrospun fibers can penetrate the regions  40 ,  42 , and  44 , and new regions  38  and  46  form on the regions  40  and  44 , respectively. The newly formed regions  38  and  46  include almost exclusively the fibers electrospun from each of the respective electrospinning devices. 
       FIG. 5B  shows a SEM micrograph of the fibers formed in the core region  42  of the mat. The thick fibers in  FIG. 5B  have been obtained by using 22.5% of polystyrene in dimethylformamide and the thin fibers have been obtained by using 20% of polycaprolactone in dimethylformamide/methylene chloride (20/80). The SEM micrograph shown in  FIG. 5B  represents a plan view of fibers in the mat. 
       FIGS. 5C-5E  schematically illustrate a change in the distribution of the fibers in the plan view of the mat when the plan view of the mat is (i) close to one side of the mat (see  FIG. 5C ), (ii) substantially at equal distances from the sides of the mat (see  FIG. 5D ), and (iii) close to the other side of the mat (see  FIG. 5E ). The sides of the mat are those exposed surfaces of the mat after formation, defined by the last fibers formed during the electrospinning process performed by the device shown in  FIG. 2 .  FIG. 5C  shows that the concentration of first fibers is higher than the concentration of the second fibers and  FIG. 5E  showing a reverse of those concentrations. The first and second fibers are illustrated in  FIGS. 5C-5E  as having different thicknesses. However, the thickness of the fibers in the figures is intended to distinguish the two fibers and not to limit the fibers of the mat to fibers having different thicknesses. In other words, the two fibers shown in  FIGS. 5C-5E  could be fibers having the same thickness and different chemical compositions. 
     Referring back to  FIG. 5A , in the regions  38 ,  40 ,  44 , and  46 , the fibers electrospun from the opposed electrospinning devices do not intermingle as strong as in the region  42 , and these regions can be reduced or suppressed. For example, using the device shown in  FIG. 2 , a fiber mat can be produced to have only a region such as region  42  as the fibers coming from the respective electrospinning devices interact and intermingle with each other without having to penetrate the fiber mat. 
     In another embodiment of the present invention, a metal frame, used to collect the nanofibers, can be rotated either continuously or intermittently by design, to obtain highly-interpenetrated or interwoven fiber mats and/or to produce mats with a uniform distribution of the first and second fibers. In other words, the changing in fiber concentration in a plan view of the mat described above could be reduced if the metal frame rotates such to expose parts of the metal frame preferentially to the first electrospinning device and then to the second electrospinning device. Thus, the layers of the mat do not merely lie on top of one another, but in one embodiment of the present invention interpenetrate at the layer boundaries. 
     For example, in this embodiment, the collector  20  shown in  FIG. 2  can be rotated, thus functioning as a rotational collector. More specifically, the collector  20  can be rotated around the shown vertical axis to expose gradually one side of the collector  20  to fibers from the electrospinning device  11   a  and then to expose the same side to fibers from the electrospinning device  11   b.    
     Alternatively, the collector  20  in  FIG. 4  could be rotated about the shown vertical axis to expose sequentially one quadrant of the upper collector to fibers from the electrospinning device  22   a  and then to expose the same quadrant to fibers from the electrospinning device  22   b.    
     As disclosed in U.S. application Ser. No. 10/819,945, control of the gaseous environment about the extrusion element  18  improves the quality of the fiber electrospun with regard to the distribution of nanofiber diameter and with regard to producing smaller diameter nanofibers. For example, by modifying the electrical properties of the gaseous environment about the extrusion element  18 , the voltage applied to the extrusion element can be increased and a pulling of the liquid jet from the extrusion element  18  can be improved. In particular, injection of gases in an enclosure around the electrospinning devices appears to reduce the onset of a corona discharge (which would disrupt the electrospinning process) around the extrusion element tip, thus permitting operation at higher voltages enhancing the electrostatic force. Further, injection of electronegative gases reduces the probability of bleeding-off charge in a Rayleigh instability region of the fiber, thereby enhancing the stretching and drawing of the fiber under the processing conditions. However, controlling the gaseous environment about the extrusion elements  18  is performed to enhance the electrostatic force and the drawing of the fibers. 
     As shown in  FIG. 2 , by maintaining a liquid pool  30  at the bottom of the chamber  28 , the amount of solvent vapor present in the ambient about the electrospinning environment can be controlled by altering a temperature of the chamber  28  and/or the solvent pool  30 , thus controlling the partial pressure of solvent in the gaseous ambient in the electrospinning environment. Optionally, a flow controller  34  can be used to control a flow rate of gaseous species to the fiber extraction fiber from a gas supply  32 . 
     Further, an atmosphere in the enclosure is controlled such that at least one of an evaporation rate of a solvent from the first and second electrospun substances and an electrical resistance of the atmosphere is varied. The liquid of the liquid pool  30  includes, for example, at least one of dimethylformamide, formamide, dimethylacetamide, methylene chloride, chlorobenzene, chloroform, carbon tetrachloride, chlorobenzene, chloroacetonitrile, carbon disulfide, dimethylsulfoxide, toluene, benzene, styrene, acetonitrile, tetrahydrofuran, acetone, methylethylketone, dioxanone, cyclohexanone, cyclohexane, dioxane, 1-nitropropane, tributylphosphate, ethyl acetate, phosphorus trichloride, methanol, ethanol, propanol, butanol, glycol, phenol, diethylene glycol, polyethylene glycol, 1,4-butanediol, water, other acid, other alcohol, other ester alcohol, other ketone, other ester, other aromatic, other amide, and other chlorinated hydrocarbon, and the flow controller  34  controls a supply of, for example, at least one of electronegative gases, ions, and energetic particles. A gas supply includes a supply of at least one of CO 2 , CO, SF 6 , CF 4 , N 2 O, CCl 4 , CCl 3 F, and CCl 2 F 2 . 
       FIG. 6A  shows in more detail an electrode spin device  51  of an electrospinning device, similar to the spin head disclosed in U.S. application Ser. No. 10/819,942. The electrospinning device  51  shown in  FIG. 6A  produces an electric field  12  that extrudes the electrospin medium  14 . The electric field  12  is directed by an electrode  36  through one or a plurality of extrusion elements  18  formed in a wall of the enclosure  22 , in which the solution  14  is enclosed. Details of the enclosure  22  and the extrusion elements  18  are given in U.S. Ser. No. 10/819,425, previously incorporated by reference. The enclosure  22  is made of an insulating material or an electrical permeable material. The extrusion elements  18  are provided in the wall of the enclosure  22  opposite to the electrode  36 , to define between the extrusion elements  18  and the electrode  36  a space  38 . The enclosure  22  communicates through a passage  40  with a source  42  of the electrospin medium  14 . Various possible arrangements of the electrodes  20  and  36 , distances between these electrodes, various constructions of the extrusion elements and their materials, the dimensions of the extrusion elements, and the voltage applied to the extrusion elements are disclosed in U.S. patent application Ser. No. 10/819,942. In one embodiment of the present invention, electrospinning devices  11   a  and  11   b  are configured as electrospinning device  51 . 
     As illustrative of the process of the present invention, the following non-limiting examples are given to illustrate selection of the polymer and solvent for the fibers, the tip diameter of the extrusion elements, the collector material, the solvent pump rate, the electric field, and the polarity of the fibers: 
     EXAMPLE I 
     a poly(ethylenimine) solution of a molecular weight of 1050 kg/mol for the first fibers and a poly(caprolactone) solution of a molecular weight of 100 kg/mol for the second fibers, 
     a solvent of dimethylformamide (DMF) for both the first and second fibers, 
     extrusion elements tip diameter of 1000 μm for both fibers, 
     an Al ring collector, 
     0.5 to 1.0 ml/hr pump rate providing the polymer solution to the extrusion elements, 
     a gas flow rate in the range of 0.5 to 50 lpm, 
     an electric field strength of 2 kV/cm for electrospinning the first and second fibers, 
     positive polarity for the first fibers and negative polarity for the second fibers, and 
     a gap distance between the tip of the extrusion elements and the collector of 17.5 cm. 
     Using the above substances for electrospinning and the above conditions, a mat having the first fibers made of a material different than the second fibers is obtained. The resultant fiber diameter depends on several variables and for a given set of variables, will vary from polymer to polymer. This example further represents a mat of hydrophilic and hydrophobic fibers. 
     EXAMPLE II 
     a polystyrene solution of a molecular weight of 1050 kg/mol for the first fibers and a polystyrene solution of a molecular weight of 2000 kg/mol for the second fibers, 
     a solvent of dimethylformamide DMF for both the first and second fibers, 
     extrusion elements tip diameter of 1000 μm for both fibers, 
     an Al ring collector, 
     0.5 to 1.0 ml/hr pump rate providing the polymer solution to the extrusion elements, 
     a gas flow rate in the range of 0.5 to 50 lpm 
     an electric field strength of 2 kV/cm for the first fibers, 
     an electric field strength of 5 kV/cm for the second fibers, 
     positive polarity for the first fibers and negative polarity for the second fibers, and 
     a gap distance between the tip of the extrusion elements and the collector of 17.5 cm. 
     The resultant fiber mat includes first fibers with a first average diameter and second fibers with a second average diameter, different than the first average diameter. In this illustration, the molecular weight characteristics of the electrospin medium and the electric field influence the resultant fiber diameter size, with the electric field applied to the extrusion elements extruding the first fibers at 2 kV/cm and the electric field applied to the extrusion elements extruding the second fibers at 5 kV/cm. 
     Additionally, in one embodiment, particles can be injected into a fiber extraction region of the electrospinning devices to produce fibers with partially embedded particles. The particles can be injected under similar conditions to those described above for the fiber electrospinning conditions. For instance,  FIGS. 2 ,  6 B, and  7 A show a particle delivery device  50  that delivers particles to a fiber forming region such that the delivered particles collide and combine with at least one of the first and second electrospun substances to form fibers having attached particles. For instance,  FIG. 2  shows a particle delivery device  50  that delivers particles to a fiber forming region such that the delivered particles collide and combine with at least one of the first and second electrospun substances to form fibers including the particles. The particle delivery device  50  can include a particle guide device  52  that guides the particles into a part of fiber forming region. The particle delivery device  50  can include at least one of a nebulizer and an atomizer. The particle delivery device  50  may have a collimator  56  configured to collimate the particles. The particle delivery device  50  can also have a particle source  58 , a gaseous carrier source  60  in communication with particles output by the particle source  58 , and a flow regulator  62  configured to regulate a gas flow from the gaseous carrier source. The speed of the particles admitted into the chamber  28  thus depends on the gas flow from the regulator  62 . In one embodiment not shown in  FIG. 2 , the particle delivery device  50  can be replaced entirely by an electrospray device similar to the electrospinning devices  11   a  and  11   b . The electrospray device replacing the particle delivery device  50  can supply the materials discussed above for the particle delivery device  50 . As such, a gaseous medium can be used (see  FIG. 6A , flow controller  34  and gas supply  32 ) in a vicinity of the electrospray device to affect the electrosprayed particles. The particle delivery device  50  can operate in parallel to or in the absence of the electrospray device. 
     The particle delivery device  50  can supply at least one of a metallic material, an organic compound, an oxide material, a semiconductor material, an electroluminescent material, a phosphorescent material, a medical compound, and a biological material. 
     The particle delivery device  50  in one embodiment of the present invention can be a Collision nebulizer that provides suspended nanosized particles into a first carrier (e.g., a carrier gas) to form an aerosol. The Collision nebulizer can be connected to a diffusion dryer to evaporate traces of water (or other vapors) from the aerosol before injecting the aerosol of particles into a region about where the substance to be extruded is electrospun, i.e., where the fibers are produced. Commercially available Collision nebulizers such as for example available from BGI, Waltham, Mass., are suitable for the present invention. The nebulizer of the present invention can provide electrically charged airborne particles to a region of where the substance  14  to be extruded is electrospun. For example, nanosized silicon particles suspended in carbon tetrachloride and then nebulized in the Collision nebulizer can provide an aerosol of silicon particles for injection into a region where the substance  14  to be extruded is electrospun. Suspension of the particles in a carrier fluid can be obtained not only by nebulization but also by atomization, condensation, dried dispersion, electrospray, or other techniques known in the art. 
     The present inventors have discovered that charging the particles provided by the particle delivery device  50  with an electric charge opposite to the electric charge with which the electrospin medium  14  is charged, not only promotes the attraction of the particles to the fibers but also tends to prevent the particles from coalescing with each other during deposition on the fibers. In other words, because the particles have the same electric charge, the particles tend to repel each other, and stay separate from each other on the fibers. In addition, by having the particles charged with a charge opposite to the charge of the fibers, more particles can interact with the fibers due to the electric attraction between the particles and the fibers. Therefore, the process of charging the particles oppositely to the charge of the fibers can achieve a high rate of collision between the particles and the fibers. 
     The inventors of the present invention have discovered that, if the particles provided collide with the electrospun material before the electrospun material is completely dried, the particles can attach to the fibers. However, some particles may interact with the electrospun material after the material has dried but can nevertheless be entrapped in the fiber mats of the present invention. 
     The particles included into the fiber mats of the present invention can be composed of a variety of materials including but not limited to pharmaceuticals, polymers, biological matter, ceramics, and metals. Even particles that do not mix with the polymer solution can be included in the fiber mats of the present invention. The particles delivered in the present invention have a diameter ranging preferably from 5 nanometers to 100 nanometers, and can have diameters as large as a few microns (e.g., 1-5 μm). 
     In one embodiment of the present invention, the particles can be provided from an electrospray device. By electrospraying, an electrospray material is charged to a high electric potential and then expelled by the high electric field at the tip of the electrospray device. Due to the high electric charges on the particles of the material, the expelled electrosprayed particles form a mist of electrically charged particles. 
     The electrospray device constituting the particle delivery device  50 , in this embodiment, is placed to a side of the extrusion element  18  of the electrospinning device  11   a  to provide particles directed toward a horizontal path as shown in  FIG. 6B , although other directions may also be used. The electrospinning device  11   a  is configured to provide the fibers directed toward a vertical path, although other directions may also be used, such that the path of the fibers intersects the path of the particles, as shown in  FIG. 6B . Optionally, a chamber  28  could be placed around the extrusion element  18 . 
     In another embodiment, the particle delivery device  50  and the electrospinning device  11   a  can be disposed in a horizontal arrangement as shown in  FIG. 7A . Thus, both the fibers and the particles are expelled horizontally into the chamber  28 , with the fibers and the particles being collected by the collection electrode  20 , which can be placed vertically, as shown, or horizontally if the particle delivery device  50  and the electrospinning device  11   a  are directed to the horizontal direction. 
       FIG. 7B  is a micrograph of a particle/fiber composite made by the present invention. In preparing the particle/fiber composite shown in  FIG. 7B , an electrospray nozzle and an electrospinning head, maintained at (˜20 kV but at different polarities) were set up facing each other separated by a distance of 15-30 cm in a cross-shaped glass chamber. In other experiments, the electrospinning was done in a vertical direction (as described above) and the electrospray was carried at right angles to the vertical direction, at a distance of 9-15 cm from the tip of the electrospinning needle. 
     The distance between the spinhead needle and the sprayhead needle was controlled. If the distance is too close the fibers tend to be attracted and deposited on the sprayhead. If the distance is too far apart the sprayed particles will not adequately be attached to the nanofibers. The ranges given above have been found to be appropriate, but the present invention is not so limited and other distances are suitable for the present invention. 
     The particles in  FIG. 7B  are PCL (polycaprolactone) produced by electrospraying a 1% (w/w) solution of the polymer in methylene chloride in an atmosphere of carbon dioxide. The solution of the polymer was pumped into a stainless steel hypodermic syringe needle (guage 25) at a flow rate of 0.5 ml per hour. The needle was connected to the negative terminal of a 20 kV power supply. 
     The fibers in  FIG. 7B  are polystyrene electrospun from a 25% (w/w) solution in DMF using a similar 25 gauge stainless steel needle. The flow rate of the polymer into the needle was controlled at 0.5 ml per hour. The needle was connected to a positive terminal of a 20 kV power supply. 
     A ground plate was used at the bottom of the chamber and served to collect the nanofiber with attached particles product formed. 
     Other electrospinning devices could be used along with electrospinning device  11   a  in  FIGS. 6B and 7A  such as for example the electrospinning devices  11   a  and  11   b  in  FIG. 2  to produce multicomponent fiber mats (as described above) that include attached particles. 
       FIG. 8  is a flowchart depicting one method of the present invention. In step  810 , first fibers are electrospun under a first electric polarity from a first substance. In step  820 , second fibers are electrospun under a second electric polarity of opposite polarity to the first electric polarity from a second substance. In step  830 , the electrospun first and second fibers are coalesced to form a fiber mat. However,  FIG. 8  does not imply that steps  810  and  820  are only sequential. In fact, the steps  810  and  820  according to the present invention can be performed simultaneously or sequential function of the desired characteristics of the mat to be formed. 
     The method optionally includes providing the first and second substances with different chemical compositions. The method can as well provide first and second substances of the same chemical composition or material. The method can combine fibers of the same average diameter or different average diameters. Hence, the method can produce in the fiber mat first and second fibers of the same or different chemical composition or material. Additionally, the method can produce a fiber mat having fibers of the same or different average diameters included therein. 
     Furthermore, by electrospinning for example identical or different fibers from the two electrospinning devices  11   a  and  11   b , a particle/fiber mat composite having a cross sectional density (as before) of (2.5×10 13 )/d 2  fibers/cm 2  can be achieved that includes attached particles. 
     In step  830 , coalescing optionally includes electrostatically attracting the fibers of the first and second electrospun substances due to opposite electric charges on the first and second electrospun fibers, and combining the first and second electrospun fibers in a region where the first and second electrospun fibers include a solvent content. Coalescing the first and second fibers includes combining the first and second fibers in a region where the solvent content of the first and second electrospun fibers is low enough to prevent fibers adhering to each other or combining the first and second fibers in a region where the solvent content of the first and second electrospun fibers is high enough to obtain adhesion and to produce partial blending of the first and second fibers, the solution content being variable for each polymer-solvent combination, and preferably between 20 and 80 weight %. 
     The method optionally controls an atmosphere in a vicinity of the electrospun first and second fibers so as to adjust at least one of an evaporation rate of a solvent from the first and second fibers and an electrical resistance of the atmosphere. The controlling of the atmosphere can be achieved by providing a vapor pressure of a liquid to the atmosphere and/or controlling a temperature of a vapor pool container containing the liquid. The vapor includes, for example, at least one of dimethylformamide, formamide, dimethylacetamide, methylene chloride, chlorobenzene, chloroform, carbon tetrachloride, chlorobenzene, chloroacetonitrile, carbon disulfide, dimethylsulfoxide, toluene, benzene, styrene, acetonitrile, tetrahydrofuran, acetone, methylethylketone, dioxanone, cyclohexanone, cyclohexane, dioxane, 1-nitropropane, tributylphosphate, ethyl acetate, phosphorus trichloride, methanol, ethanol, propanol, butanol, glycol, phenol, diethylene glycol, polyethylene glycol, 1,4butanediol, water, other acid, other alcohol, other ester alcohol, other ketone, other ester, other aromatic, other amide, and other chlorinated hydrocarbon. The controlling of the atmosphere can include providing a gas supply of at least one of electronegative gases, non-electronegative gases, ions, and energetic particles and the supply can include supplying at least one of CO 2 , CO, SF 6 , CF 4 , N 2 O, CCl 4 , CCl 3 F, and CCl 2 F 2 . 
     The method can include collecting the first and second fibers on a collection electrode and the collection electrode optionally includes at least one of a loop, a net, a hook, and a web. The collection electrode can be a grounded electrode. 
     The electrospinning under a first electric polarity and the electrospinning under a second electric polarity can include extracting the first and second fibers in opposing directions towards each other and the method can include storing at least one of the first and second substances in a compartment having extrusion elements mounted in a wall of the compartment. If the compartment is present, then the method can include radiating an electric field from the compartment by an electrode disposed inside the compartment. 
     The method can provide the first and second substances in a solvent and also can provide at least one of the first and second substances with a polymeric substance included in the solvent. The providing at least one of the first and second substances with a polymeric substance can include providing in the first and second substances different polymeric substances dissolved by the solvent. 
     By controlling one or more of an electric field, a solvent composition, a polymer type, flow rate, and a gas environment, the present embodiment can create fibers of different diameters. Such information on setting such parameters is known in the art of electrospinning, see for example U.S. Pat. No. 6,110,590 and the patent references disclosed in that patent, the entire contents of which are incorporated by reference herein. Electrospinning of the present invention can electrospin for example from the two electrospinning devices shown in  FIG. 2  fibers of different average diameters provided all other variables including for example the polymer type and the solvent are the same if different applied electric fields are used. For example, by applying an electric field strength of 10,000 to 100,000 V/m in a vicinity of one of the electrospinning devices, nanofibers can be produced having an average diameter less than 1 μm. And for example, by applying an electric field strength of 50,000 to 200,000 V/m in a vicinity of one of the electrospinning devices, nanofibers can be produced having an average diameter less than 500 nm. By applying an electric field strength of 150,000 to 400,000 V/m in a vicinity of one of the electrospinning devices, nanofibers can be produced having an average diameter less than 100 nm. 
     The method, during electrospinning, can deliver particles in a vicinity of the electrospun first and second fibers such that the particles combine with at least one of the electrospun first and second fibers. Combining the particles with the electrospun fibers would preferably occur for electrospun fibers having a solvent content, as described above. 
     The particles can be delivered by at least one of a nebulizer, an atomizer, and an electrospray device. A collimator can be used to collimate the particles. Particles from a particle source can be mixed and transported with a gaseous carrier, such as for example entraining the particles in a regulated flow of the gaseous carrier. As understood in the art, the speed of the particles depends on the gas flow rate. As illustrated here, the particles can be delivered by an electrospray device. 
     The particles can be at least one of a metallic material, an organic material, an oxide material, a semiconductor material, an electroluminescent material, a phosphorescent material, a medical compound, and a biological material. The particles can be nanoparticles having an average diameter less than 500 nm. 
     The coalescing can combine the first and second fibers to produce a region in the fiber mat in which adjacent fibers have a separation less than an average diameter d of one fiber of the first and second fibers, the average diameter being determined along a length of the one fiber. As such, a region in the fiber mat can have a cross section fiber density of at least (2.5×10 13 )/d 2  fibers/cm 2 , where d is an average diameter of one fiber of the first and second fibers and a value of d is given in nm. 
     Applications 
     As noted a fiber mat can be formed by the present invention in which one set of fibers has a first average diameter and a second set of fibers has a second average diameter such that the first set serves as a mechanical support for the second set. In one embodiment, the second set of fibers includes nanofibers having a diameter not limited to but preferable less than 500 nm. 
     Another application of the fiber mat of the present invention is for a medical product that substitutes the functions of the human or animal skin in medical cases (e.g., burns) in which the skin has been destroyed. It is know that a large percentage of the people suffering burns die because the functions performed by the skin cannot be substituted by any device. The main functions of the skin are (i) to prevent foreign objects to penetrate from outside the organism into the organism, (ii) to remove exudates away from a wound surface, and (iii) to allow certain fluids (water) to leave the organism. A plurality of fibers having a same chemical composition cannot achieve these two opposing functions. However, a mat of fibers composed of fibers with different chemical compositions can perform the functions of the skin when one of the fibers has function (i) and the other fiber has function (iii). Thus, the two fibers that simulate the human skin could be for example hydrophobic and hydrophilic fibers. The hydrophobic fibers include at least one of poly(alkyl acrylate), polybutadiene, polyethylene, polylactones, polystyrene, polyacrylonitrile, polyethylene terephthalate), polysulfone, polycarbonate, and poly(vinyl chloride), and the hydrophilic fibers include at least one of poly(acrylic acid), poly(ethylene glycol), poly(vinyl alcohol), poly (vinyl acetate), cellulose, poly(acrylamide), proteins, poly (vinyl pyrrolidone), and poly(styrene sulfonate). 
     The present inventors have found that the integrity of a mat having two fiber types displaying different functions is better when these fibers are formed as a mat where one surface of the mat includes mainly of the first type of fiber and the other surface of the second type of fiber with a gradient mix of the two fibers within the thickness of the fiber mat. The composition of the mat therefore changes from fiber type one to fiber type two across the thickness of the mat. The integrity of two separately spun layers of nanofiber mats made of the first fiber and of the second fiber sandwiched together, by comparison to the mat of the present invention, is considerably lower. 
     Another application of the mat of fibers is in the filtration field. Various filters commercially available include nanofibers to filter nanosized particles. However, the commercially available filters lack good adherence of the nanofibers to a substrate on which the nanofibers are formed. This problem causes the nanofibers to easily break away from the filter and to contaminate the medium. The fiber mat of the present invention solves that problem because the two different fibers have a high adherence and because one of the fibers could be formed with a high thickness to offer the required mechanical strength and the other fibers are nanofibers to offer the nanosized filtration function. Alternatively, the first fibers have a first elastic modulus and the second fibers have a second elastic modulus several times the elastic modulus of the first fibers, in a range of two to twenty, preferably in a range of two to five. Accordingly, the mat of fibers of the present invention has a good adherence and filtration function. 
     Numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein.