Abstract:
A nanofiber membrane is formed on a microfiber membrane. The nanofiber membrane may be electro sprayed directly onto the microfiber membrane and becomes integrated with the microfiber membrane to form a filter. The microfiber membrane provides structural integrity to for the nanofiber membrane, and an additional microfiber membrane may be added to sandwich the nanofiber membrane.

Description:
RELATED APPLICATION 
     This application is a continuation in part of and claims priority from U.S. patent application Ser. No. 10/951,254; filed on Sep. 27, 2004; which is incorporated herein by reference. 
     This application is related to U.S. patent application Ser. No. 10/394,757 (entitled Electrospray Emitter for Microfluidic Channel, filed May 21, 2003). 
    
    
     GOVERNMENT FUNDING 
     The invention described herein was made with U.S. Government support under Grant Number ECS-9876771 awarded by National Science Foundation (NSF)/Nanobiotechnology Center (NBTC). The United States Government has certain rights in the invention. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to nanofiber filters, and in particular to a nanofiber filter supported by microfibers. 
     BACKGROUND OF THE INVENTION 
     Due to their extremely small porous dimensions and high surface to volume ratio, nanofibers have been expected to be utilized as substrates for many applications such as high performance waste water filtration or biological contaminants filtration membranes. However, there has been little sign of potential products or usage of nanofibers for these applications because of their weak mechanical strength. The average dimension of nanofibers are less than 100 nm and sometimes as small as 20 nm. In this dimension, although they are layered and formed as thick membranes, the mechanical strength of the resulting structures is not sufficient to withstand macroscopic impacts for filtration applications such as normal liquid or air flows passing through them. 
     SUMMARY OF THE INVENTION 
     Nanofibers are formed on a microfiber membrane material to provide structural integrity. In one embodiment, the nanofibers are formed using electrospray deposition from a microfluidic source. In one embodiment, the source is an electrospray source, such as a microfabricated tip provides a solution from an electrostatically formed Taylor cone formed on the tip. Distances between source and surface, as well as solution viscosity may be varied to form nanofiber membranes with differing properties. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of an electrospinning system for creating polymer nanofibers according to an example embodiment of the invention. 
         FIG. 2  is an exploded block view example of a channel with integrated electrospray emitter according to an embodiment of the invention. 
         FIG. 3  is a representation of a Taylor cone established on a tip of an emitter in  FIG. 2  according to an example embodiment of the invention. 
         FIG. 4  is a schematic cross section diagram of a nanofiber membrane supported by a microfiber membrane according to an example embodiment of the invention. 
         FIG. 5  is a scanning electron microscope image of a nanofiber membrane according to an example embodiment of the invention. 
         FIG. 6  is a schematic cross section diagram of multiple nanofiber membranes supported by multiple microfiber membranes according to an example embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the following description, reference is made to the accompanying drawings that form a part hereof, and in which is shown by way of illustration specific embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilized and that structural, logical and electrical changes may be made without departing from the scope of the present invention. The following description is, therefore, not to be taken in a limited sense, and the scope of the present invention is defined by the appended claims. 
     An electrospray system is first described for creating nanofibers formed of various materials. A method for forming and supporting nanofiber membranes is then described, along with the resulting structure. The nanofiber membranes may be formed on microfiber membranes, and the resulting structure may be used as a filter in one embodiment. Various methods of forming the nanofiber membranes other than the electrospray system may also be used. 
     A microfluidic electrospray system is shown at  100  in  FIG. 1 . A microfluidic channel  110  is coupled at one end to a triangular tip  115 , acting as a source for formation of nanofibers. Both are supported by a substrate  120 . A reservoir  125  provides a polymer solution in one embodiment to the channel  110  and to the tip  115 . Another end of the microfluidic channel  110  is coupled to a reservoir  125  formed in the substrate  120 . The reservoir in one embodiment is coupled to a capillary tube  130 , or other plumbing to provide the polymer solution to the reservoir and channel. A conductor, such as a gold wire  135  is coupled to the reservoir for coupling the reservoir to a power supply  137 . The substrate is mounted on an x,y,z stage for moving the substrate laterally in a desired manner. 
     In one embodiment, the substrate  120  is positioned between approximately 5 mm to 12.5 mm from holder  145  on which a silicon substrate  150  with aluminum coating  155  is supported. The substrate and aluminum coating  155  are coupled to a ground via a conductor  160 , forming a counter electrode. By applying a potential via power supply  137  with respect to the grounded substrate  150 , a Taylor cone is established on tip  115 , resulting in a liquid jet  170  being formed at the tip and moving toward the substrate  150 . In one embodiment, the term Taylor cone is used to refer to any type of structure that result in a thin stream of liquid moving toward the substrate  150 . By moving the substrate  120  by use of the x,y,z stage  140 , the liquid jet moves across the substrate  150 , creating nanofibers on the substrate in desired positions. Z corresponds to the distance between the tip and the substrate. Stage  140  may be moved to create a membrane of substantially randomly oriented fibers. In further embodiments, no x,y stage need be used, and the substrate may be positioned proximate the tip  115  to produce nanofibers in a desired position on the substrate. 
     The term “nanofibers” is meant to cover fibers within the dimensions described herein, and smaller fibers. 
     The microfluidic coupling allows new possibilities for materials processing and nanostructure formation. The source allows for smaller source to substrate distances and permits operation at lower voltages than conventional sources. The shorter distance, referred to as a deposition distance, enables greater control of nanofiber morphology and more localized deposition of the fibers. In one embodiment, nanofibers are formed within a 5 mm diameter circle on the substrate  150 . 
     In one example, the electrospray device substrate  120  is attached on the x,y,z stage  140  and adjusted to form a deposition distance between the tip  115  and counter electrode/substrate of approximately 0.5 cm to 1.5 cm. A 300 nl/minute flow rate is created by coupling a syringe pump to the capillary tube  130 . A potential is applied to the wire  135  of approximate 2000V to 8500V. 
     Approximately 500 nm of aluminum is optionally sputter-deposited on the silicon wafer and used as the counter electrode for nanofiber deposition. In one embodiment, the counter electrode is attached to a rotating optical chopper, with rotation rate varied between 40 RPM to 800 RPM. In a further embodiment, nanofibers are directly deposited on the silicon wafer without the need for the Al layer. In this embodiment, the silicon wafer acts as the counter electrode. 
     Further detail of an electrospray device is shown at  200  in  FIG. 2 . In one embodiment, a top chip  210  has a microchannel  215  embossed therein. The device further comprises an emitter film  220 , having a triangular or trapezoidal shaped tip  230 . It should be noted that any type of source, such as commercially available electrospray sources may be used to provide an electrospray of desired materials in addition to the sources described herein. Electrospray techniques involve the use of an applied voltage to extract material from a surface. 
     In one embodiment, the emitter comprises a larger body portion that is rectangular, with the tip  230  extending from the rectangular portion. A bottom chip  240  is thermally bonded with the top chip  210 , sandwiching a portion of the emitter film to hold it firmly between the chips. In one embodiment, the film covers a portion of the length of the channel at one end of the bonded chips as indicated at  250 . The tip  230  extends laterally from the channel at end  250 . A reservoir  260  is coupled to the other end of the channel  215 . 
     The triangle tip  230  is approximately 3 um thick, and acts like a nozzle or wick that prevents liquid from spreading laterally at the exit of the fluidic channel. In one embodiment, the tip has an apex with an approximately 90 degree angle, and the angles adjacent the channel are approximately 45 degrees. The angle of the apex may be varied, such as between 40 and 120 degrees. At smaller apex angles, liquid may spread at the base of the triangle contacting the microchannel chip, as the wetting angle of solutions in the channel may be smaller than the angles the base of the triangle makes with the chip. Different apex angles may be optimal for solutions with different wetting angles. The base of the triangular tip is approximately 100 micrometers, and the height is approximately 50 micrometers. Thus, the base extends well beyond both sides of the channel when centered approximately at the center of the channel. 
     The shape of the tip  230  helps form and fix a position of a Taylor cone, as shown in  FIG. 3 . When a difference in potential is applied to the device, a liquid droplet with a critical curvature for establishing a Taylor cone is formed at the apex of the triangle. A liquid jet  320  is formed at the apex. Highly charged small liquid droplets are made extending toward the counter electrode. Excess electrostatic force extracts liquid from the apex of the Taylor cone to establish the liquid jet. Other shapes of emitter film may also be used, such as trapezoidal shaped emitter films. While an electrospray emitter is described as the source for nanofibers, other sources may also be used to create oriented nanofibers. 
     Polyethylene oxide was used as the nanofiber solution in one embodiment. It was prepared by dissolving PEO monomer (MW 100,000) at weight concentration of 6% to 10% in a mixture of 50% deionized water and 50% ethanol. Other concentrations may also be used. PEO polymeric solution is electrosprayed to the rotational counter electrode. The deposition distance is set at 2 cm and the position of the triangular tip was set at 2.0 cm laterally away from the center of the counter electrode. In addition to PEO, there are many organic such as polyaniline, poly lactic acid or inorganic solutions like silica that may be used. 
     For a spinning process, a flow rate of 300 nl/minute is maintained with the syringe pump. 7000V was applied to the gold wire at the fluid source with the metalized substrate at ground potential. A Taylor cone is maintained at the apex of the triangle tip with a stable total ion current of 15 nA. 
     In various embodiments, nanofiber size and morphology depend on process parameters, which may be varied significantly. Such parameters include the deposition distance, applied electric field strength, and rotational speed of the counter electrode. At smaller deposition distances, the polymer may arrive at the counter electrode as a solution, resulting in a structure resembling a membrane with holes, rather than fibers. In one embodiment, the deposition distance is set to 0.75 cm, and a Taylor cone is established with 3500V applied to the gold electrode. This resulted in approximately 14.8 nA of total ion current and columnar nanofibers with an average diameter of 200 nm. Nanofibers appear to have partially dried while traveling to the counter electrode. 
     With a distance of approximately 1.0 cm, a Taylor cone is established at about 4000V, and an ion current of about 14.5 nA. Thinner nanofibers are formed in this case, with an average diameter of approximately 100 nm. With a distance of 1.5 cm, the Taylor cone is also established at 4000V, resulting in columnar nanofibers with an average diameter of approximately 100 nm. 
     From the above examples, the nanofiber size decreased from 200 nm to 100 nm while the deposition distance was increased from 0.5 cm to 1.0 cm. Extension of the deposition distance to more than 1.0 cm may not influence the nanofiber diameter. Once the fibers form in transit, the nanofiber size appears to be fixed, and the fibers are deposited on the surface as a solid. 
     Applied electric field strength was varied from 4000 V/cm to 8500 V/cm at a distance of 1.0 cm in one example embodiment. At 4000 V/cm, cylindrical nanofibers are formed with an average diameter of 100 nm. At 5500 V/cm, the diameter is almost the same, but branched nanofibers with small diameter of 30 to 60 nm may be fabricated between the main nanofibers. 
     In one embodiment, various solutions of PEO may be used. Weight concentrations of 5, 10, 20 and 30% of PEO in a solvent of 50% deionized water and 50% ethanol may be utilized. Other concentrations may also be used, as well as entirely different solutions that are capable of forming wires. 
     Polyaniline (PANI) (48 mg, emeraldine base; Mw approximately 20,000, purchased from Aldrich, Wis., USA) may be dissolved in chloroform (1.5 ml) and doped with 10-camphorsulfonic acid (122 mg). PEO (48 mg, M w  approximately 900,000 purchased from Aldrich) may be added to the chloroform solution and stirred overnight. The concentration of PEO/PANI-HCSA may range from 0.5 to 2.0 wt. %. The amount of PEO mixed with PA may be varied from 10 to 80 wt. % in one embodiment. 
     In one embodiment, a Taylor cone is established with a potential of 4500 V applied to a 20 ul dropet and the counter electrode. Nanofibers may be generated for approximately 5 to 10 seconds. The length of the nanofiber is controlled by the volume of the droplet loaded on the tip. The length may also be controlled by controlling the potential. Removing the potential at desired times results in removing the Taylor cone, and hence stopping production of the nanofiber at a desired time and distance. Nanofibers may be deposited immediately after the polymeric solution is loaded to reduce effects of evaporation. In addition to the arrow shaped tip, triangle-shaped and straight metal wire tips may be employed. It may be more difficult to establish a Taylor cone with some tip shapes. 
     Diameters of nanofibers deposited from the various solutions may be in the 100 to 200 nm range for the 5% solution, 200-300 nm range for 10%, 300-500 nm for 20% and 500 to 1800 nm range for 30%. The polymer viscosity increases with concentration. The viscosity of a 30% solution is very high. Lower viscosity solutions appear to result in smaller diameter fibers. 
     Deposition distance may also be varied. In one embodiment, the distance is varied between 0.5 to 1.5 cm with a PEO solution of 10%. The counter electrode is not spun in this embodiment. Changes may be observed in the nanofiber morphology. In the case of a 0.5 cm deposition distance, deposited polymer resembles a membrane. This may be the result of the short transit distance, in which the polymer may arrive at the counter electrode as a wet polymer, allowing them to merge to form larger fibers, or bond together to make a fibrous web. At a distance of 0.75 cm, cylindrical nanofibers may be formed of diameter 200 to 850 nm range. In this case, the nanofibers appear to have partially dried while traveling to the counter electrode. At 1.0 cm distances, thinner nanofibers appear to be created, having average diameters of approximately 153 nm. A 5% solution resulted in nanofibers as small as 45 nm. 
     In one embodiment, the tips may be reused after surface cleaning. A wide range of polymeric material, such as highly viscous polymeric solutions can be electrospun from the tip. The short deposition distance as compared to syringe based electrospinning provides for easy control of the orientation of the nanofibers. The tips also provide the capability of electrospinning of colloidal suspensions mixed with a polymer solution to fabricate nanofibers composite materials. In addition to the formation of nanofibers, tips may be used to electrospray liquids, chemicals and for particulate deposition on a surface. 
     In still further embodiments, a solution of poly(methyl methacrylate) (PMMA) is used for fiber formation. 4 wt. % and 5.5 wt. % PMMA solutions may be prepared by dissolving 67.2 mg and 92.4 mg of PMMA (Mw 495,000) in 2 ml of anisole (phenyl methyl ether), respectively. A pipette or other type of applicator may be utilized to provide 30 ul of solution on the silicon tip. A voltage of 4000 to 7000 V may be applied between the tip and counter electrode to establish the Taylor cone and extract a liquid jet from its apex. 
     Target substrates may include many different materials, such as silicon, aluminum, thin film aluminum on silicon, and non-conducting substrates, such as silicon dioxide, silicon nitride, glass slides, cover slips and others. Such non-conductive substrates are mounted on the counter electrode in the path of the extracted liquid jet. 
     With highly volatile solvents in the solution used to form a Taylor cone may be stable only for several seconds prior to evaporation. A side effect of such volatile solvents appears to be the formation of more than one polymer liquid jet being extracted from a silicon tip per deposition cycle. This may lead to fibers of different sizes being deposited on the same substrate. When multiple polymer jets are extracted, the diameters of such jets may have very small diameters. Reducing the size of the microfabricated tip may also consistently create nanofibers with very small diameters. 
     In one embodiment, using the 4 wt. % solution of PMMA in anisole, fibers were produced having an average diameter of approximately 85.2 nm. Fibers deposited using 4 wt. % solution of PMMA range from 81.4 to 326.5 nm with an average of 190 nm. Fibers deposited using 5.5 wt. % solution of PMMA range from 88.5 to 346 nm with an average of 206 nm. 
     The smallest diameter fibers extracted from the solutions were deposited when more than one polymer jet was extracted from the silicon tip. The multiple jets produced fibers of various sizes, instead of a single jet producing fibers of approximately the same size. 
     In one embodiment, a microfiber membrane or filter  410  in  FIG. 4  may be supported on the counter electrode. The nanofibers are then formed directly onto the microfiber membrane  410  to form a nanofiber membrane  420 . In one embodiment, the nanofibers arrive at the microfiber membrane  410  at least partially wet. This state provides a tight bonding with the microfiber and also helps the nanofibers bond together to form a membrane with increased structural integrity. The microfiber membrane provides mechanical strength for the resulting microfiber supported nanofiber membrane  420 . 
     In one embodiment, an integration region  430  is formed where the nanofibers penetrate into the microfibers various distances. The distances are a function of the relative diameters of the fibers, and the force at which the nanofibers are projected towards the microfiber membrane  410 . Whether or not the nanofibers are not completely dried, the penetration also provides a bond between the resulting microfiber and nanofiber membranes. If the nanofibers are at least partially wet on arrival, a spun thermal bond may result, and provide good adhesion of the nanafibers to the microfibers. 
     In one embodiment, the microfiber membrane may be formed directly on the counterelectrode or a substrate coupled to the counterelectrode. The nanofibers may then be spun onto the microfiber membrane. In various embodiments, different materials may be interposed between the membranes, or the nanofiber membrane may be formed directly onto the microfiber membrane. In further embodiments, the nanofiber membrane may be produced independently of the microfiber membrane, and then placed onto it. The membranes may then be held together by suitable adhesive, or mechanical frame or other means of coupling the membranes. 
     A second microfiber membrane  440  may be placed over the nanofiber membrane to provide a filter type structure that has support for the nanofiber membrane from both sides. This second microfiber membrane  440  may be held in place may many different means as described above, or may be formed directly onto the nanofiber membrane using known microfiber deposition processes. If applied in a partially wet manner, the adhesion may be increased. 
     In one embodiment, the microfiber membranes may have diameters in the um range, or may be larger if desired. In further embodiments, other filter type substrates may be used to support nanofiber membranes, such as ceramic filters, nano porous membrane filter or ion exchange membrane filter. 
       FIG. 5  is a scanning electron microscope image of a nanofiber membrane  500  according to an example embodiment of the invention. A scale bar  510  indicates 5 um. A conventional membrane type filter typically consists of fibers of 20-50 micrometer in diameter. The mean pore size is approximately 50 um. On the other hand, the mean pore size of nanofiber membrane is much smaller as illustrated at  500 . The average pore size in one embodiment is less than 100 nm. (Pore size is usually described as the diameter of pore.) This is very suitable for capturing ultra fine particles or molecules, and also provides a significant difference of surface to volume ratio over microfiber membranes. 
     The weak point of nanofibers with average diameters of less than 100 nm was the mechanical resistibility for the air or liquid flow. Because of this weakness, it was difficult to commercialize the nanofiber based filtration product, although it has vast potential. By direct electrospinning of nanofibers onto the microfiber substrates as well as the construction of another layer of microfibers on the surface of nanofiber membrane a mechanically stable filtration membrane is created. Because of the high surface to volume ratio, the nanofiber membrane can significantly improve the filtration performance, such as the capture of nicotine molecules in tobacco smoke. 
     In one embodiment, the nanofibers are blown or formed to provide a membrane that is between approximately 20 nm to 1 um thick, with nanofiber diameters of approximately 100 to 200 um. The microfiber membrane may be approximately 10 um to 100 um or thicker in various embodiments, depending on the amount of structural support desired. In one embodiment, the diameter of the nanofibers and thickness of the nanofiber layers are selected as a function of molecule size to be filtered. For smaller molecule sizes, smaller diameter nanofibers may be used to decrease the resulting pore size in the membrane. The thickness of the membrane may also be increased. For larger molecule sizes, larger diameter nanofiber may be used in a thinner layer if desired. 
     One layer of nanofibers may be sufficient for many air filter applications. Liquid applications may require a microfiber layer on both sides of the nanofiber membrane. In still further embodiments, a second nanofiber membrane may be formed on top of the second microfiber membrane as illustrated in  FIG. 6  at  600 . Still further layers of nanofibers and microfibers may be added to form a stacked sandwich of microfiber and nanofiber membranes. Four microfiber membranes,  610 ,  615 ,  620  and  625  sandwich three nanofiber membranes  630 ,  635  and  640  in one embodiment. Still further layers may be added if desired. 
     As previously mentioned, the microfiber membranes may be formed in many different manners, such as by deposition. The nanofiber membranes may be formed using the above described electrospray device, or by other means, that may not include the use of a tip as described. The diameter of the fibers and thicknesses of the resulting membranes may be varied for different applications. Further, the number of layers of nanofiber membranes and microfiber membranes may also be varied.