[0001]    This application claims benefit from U.S. Provisional Application No. 61/176,115, filed May 7, 2009. 
     
    
     BACKGROUND 
       [0002]    Nanofibers ranging from 10 nm to 1000 nm have been used in filtration medium to capture submicron particles below 1000 nm. The ability of nanofibers to capture particles is believed to be due to combination of interception of submicron particles by the fibers as well as the Brownian motion or “random walk” of submicron particles, both of which facilitate the particles to be captured by the large surface/mass ratio of the nanofibers. 
         [0003]    Conventional filtration media may have a layer of nanofibers with a size distribution in a range of 100 to 300 nm laid on a substrate layer of the medium, as depicted in  FIG. 1A . Further increase of capture capability may be obtained by increasing the nanofiber surface area such as by reducing the fiber diameter and/or by increasing the packing density of the nanofibers, as measured in terms of grams of nanofibers per square meter. However, since the nanofibers do not possess structural rigidity, when more fibers are laid, the additional nanofibers often compress to form a dense (i.e. low porosity or void space) and thick layer that substantially increase the pressure drop across the medium. This lowers the permeability of the filtration medium to airflow, and resulted in undesirable breathability to the filtration medium. 
         [0004]    Consequently, it is desirable to develop an improved nanofiber filtration medium that has a high filtration efficiency but low pressure drop. It is also desirable to develop a method of making the nanofiber filtration medium having these improved properties. 
       SUMMARY 
       [0005]    According to one aspect, a filtration medium may include at least one substrate layer, and at least two nanofiber layers. The substrate layer is sandwiched between the nanofiber layers in a series to form an alternating laminate unit. 
         [0006]    According to another aspect, a method of making a filtration medium may include coating nanofibers onto a substrate layer either on one surface or on both surfaces to form a laminate unit, and stacking at least two of the units in a series to form an alternating laminate unit. 
         [0007]    According to a further aspect, a method of making a filtration medium may include coating nanofibers onto a substrate layer either on one surface or on both surfaces to form a laminate unit, and folding the laminate unit in a serpentine arrangement. 
         [0008]    As a best mode at the present time, a method of making a filtration medium which includes multiple nanofiber layers supported by substrate medium wherein all the nanofiber layers have open pores exceeding 98% by volume. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1A  depicts a prior art filtration medium with a nanofiber layer on a substrate layer having 0.12 grams per square meter of nanofibers. 
           [0010]      FIG. 1B  depicts the filtration medium of the present invention having the substrate layer sandwiched between the nanofiber layers having 0.12 grams per square meter of nanofibers on each side. 
           [0011]      FIG. 2A  depicts a prior art filtration medium of  FIG. 1A  having 0.24 grams per square meter of nanofibers. 
           [0012]      FIG. 2B  depicts the filtration medium of the present invention having two filtration media as depicted in  FIG. 1A  stacked up to form a laminate unit having 0.24 grams per square meter of nanofibers. 
           [0013]      FIG. 2C  depicts the filtration medium of the present invention having two filtration media as depicted in  FIG. 1B  stacked up to form a laminate unit having 0.48 grams per square meter of nanofibers. 
           [0014]      FIG. 3A  depicts a prior art filtration medium of  FIG. 1A  having 0.36 grams per square meter of nanofibers. 
           [0015]      FIG. 3B  depicts the filtration medium of the present invention having three filtration medium as depicted in  FIG. 1A  stacked up to form a laminate unit having 0.36 grams per square meter of nanofibers. 
           [0016]      FIG. 3C  depicts the filtration medium of the present invention having three filtration medium as depicted in  FIG. 1B  stacked up to form a laminate unit having 0.72 grams per square meter of nanofibers. 
           [0017]      FIG. 4A  depicts a prior art filtration medium of  FIG. 1  having 0.48 grams per square meter of nanofibers. 
           [0018]      FIG. 4B  depicts the filtration medium of the present invention having four filtration medium as depicted in  FIG. 1A  stacked up to form a laminate unit having 0.48 grams per square meter of nanofibers. 
           [0019]      FIG. 4C  depicts the filtration medium of the present invention having four filtration medium as depicted in  FIG. 1B  stacked up to form a laminate unit having 0.96 grams per square meter of nanofibers. 
           [0020]      FIG. 5A  depicts the thickness of nanofiber layer in filtration medium as measured against the prior art. 
           [0021]      FIG. 5B  depicts the thickness of filtration medium as measured against the prior art. 
           [0022]      FIG. 6  depicts the solid volume fraction of nanofibers of the filtration medium as measured against the prior art. 
           [0023]      FIG. 7  depicts the porosity of the filtration medium as measured against the prior art. 
           [0024]      FIG. 8A  depicts the capture efficiency and pressure drop for single layer and mutli-layer filtration media. 
           [0025]      FIG. 8B  depicts the capture efficiency and pressure drop for single layer and mutli-layer facemask filtration media. 
           [0026]      FIG. 8C  depicts the capture efficiency and pressure drop for a mutlilayer filtration medium with a minimum of 95% capture of particles of 0.3 micron in size. 
           [0027]      FIG. 9A  depicts a single nanofiber layer coated on a support porous layer. 
           [0028]      FIG. 9B  depicts a filtration medium with 5 layers of nanofibers, as folded from the single nanofiber layer of  FIG. 9A . 
           [0029]      FIG. 9C  depicts a single nanofiber layer coated on both sides of a support porous layer. 
           [0030]      FIG. 9D  depicts a filtration medium with 10 layers of nanofibers, as folded from the single nanofiber layer of  FIG. 9C . 
           [0031]      FIG. 10  depicts the capture efficiency and pressure drop for mutli-layer filtration media at various packing density subjected to particles at 50 nm. 
           [0032]      FIG. 11  depicts the capture efficiency and pressure drop for mutli-layer filtration media at various packing density subjected to particles at 100 nm. 
           [0033]      FIG. 12  depicts the capture efficiency and pressure drop for mutli-layer filtration media at various packing density subjected to particles at 200 nm. 
           [0034]      FIG. 13  depicts the capture efficiency and pressure drop for mutli-layer filtration media at various packing density subjected to particles at 300 nm. 
           [0035]      FIG. 14  depicts the capture efficiency and pressure drop for mutli-layer filtration media at various packing density subjected to particles at 400 nm. 
           [0036]      FIG. 15  depicts the relative quality factor and capture efficiency for multi-layer nanofiber filtration medium at 0.233 grams per square meter of nanofibers. 
           [0037]      FIG. 16  depicts the relative quality factor and capture efficiency for multi-layer nanofiber filtration medium at 0.350 grams per square meter of nanofibers. 
           [0038]      FIG. 17  depicts the relative quality factor and capture efficiency for multi-layer nanofiber filtration medium at 0.467 grams per square meter of nanofibers. 
       
    
    
     DETAILED DESCRIPTION 
       [0039]    Reference will now be made in detail to a particular embodiment of the invention, examples of which are also provided in the following description. Exemplary embodiments of the invention are described in detail, although it will be apparent to those skilled in the relevant art that some features that are not particularly important to an understanding of the invention may not be shown for the sake of clarity. 
         [0040]    Furthermore, it should be understood that the invention is not limited to the precise embodiments described below, and that various changes and modifications thereof may be effected by one skilled in the art without departing from the spirit or scope of the invention. For example, elements and/or features of different illustrative embodiments may be combined with each other and/or substituted for each other within the scope of this disclosure and appended claims. In addition, improvements and modifications which may become apparent to persons of ordinary skill in the art after reading this disclosure, the drawings, and the appended claims are deemed within the spirit and scope of the present invention 
         [0041]    A filtration medium may include one substrate layer and at least one nanofiber layers coated on at least one side of said substrate layer to form a bi-layer laminate unit, as depicted in  FIG. 1A . In another arrangement, a filtration medium may also include at least one substrate layer, and at least two nanofiber layers. The substrate layer may be sandwiched between the nanofiber layers in a series to form an alternating tri-layer laminate unit, as depicted in  FIG. 1B . The nanofiber layers and the substrate layer may adhere to one another to form a well-bonded laminate structure. The term “well-bonded laminate structure” means a multilayer filtration medium in which none of the layers are substantially corrugated. Using layers with small thickness may facilitate the formation of a well-bonded laminate structure. 
         [0042]    To increase capture efficiency while reduce pressure drop, the filtration medium may have multiple layers, as depicted in  FIGS. 2B ,  2 C,  3 B,  3 C,  4 B and  4 C. The arrows indicate the direction of airflow. The multilayer filtration medium may include more than one bi-layer or one tri-layer unit arranged in a series of parallel and repeating bi-layer or tri-layer laminate units, such that the filtration medium alternates between nanofiber layers and substrate layers. The number of bi-layers and tri-layers in the filtration medium may be determined by factors including the desired overall polymer packing density to reach the desired particle capture efficiency, the desired overall medium thickness, the thickness of each layer, and the allowed maximum pressure drop. The filtration medium may thus have higher filtration efficiency by virtue of increased packing density, but without incurring high pressure drop across the filtration medium. 
         [0043]    The filtration medium of the instant configuration may offer many advantages: the nanofibers may maintain a low solid volume fraction (or equivalently a higher porosity) in each nanofiber layer, the total thickness of the nanofiber layers in the filter may well exceed the single nanofiber layer having the same total polymer packing density (i.e. same grams per square meter or “gsm”), a high particle capture efficiency may be attained with submicron particles, a lower pressure drop may be achieved when compared to the single layer with the same packing density (i.e. same gsm), the substrate layer may act as a support providing mechanical stress (tensile) for the filtration medium, and the substrate layer may serve as a filter medium. 
         [0044]    One example of the multilayer filtration medium may include two nanofiber layers and two substrate layers stack together in an alternating configuration, as depicted in  FIG. 2B , which stands in contrast to the prior art that includes only a single nanofiber layer bonded to a single substrate layer, as depicted in  FIG. 2A . In this example, both the filtration media of  FIGS. 2A and 2B  contain 0.24 g/m 2  of nanofibers, while  FIG. 2C  contains 0.48 g/m 2 . 
         [0045]    Another example of the multilayer filtration medium may include three nanofiber layers and three substrate layers stack together in an alternating configuration, as depicted in  FIG. 3B , which stands in contrast to the prior art that includes only a single nanofiber layer bonded to a single substrate layer, as depicted in  FIG. 3A . In this example, both the filtration media of  FIGS. 3A and 3B  contain 0.36 g/m 2  of nanofibers while the three tri-layer stackup in  FIG. 3C  contains 0.72 g/m 2 . 
         [0046]    A further example of the multilayer filtration medium may include four nanofiber layers and four substrate layers stack together in an alternating configuration, as depicted in  FIG. 4B , which stands in contrast to the prior art that includes only a single nanofiber layer bonded to a single substrate layer, as depicted in  FIG. 4A . In this example, both the filtration media of  FIGS. 4A and 4B  contain 0.48 g/m 2  of nanofibers while the four-layer medium in  FIG. 4C  contains 0.96 g/m 2 . 
         [0047]    The nanofibers in the filtration medium may be obtained in a variety of ways. For example, nanofibers may be produced by electrospinning a polymer solution. In another example, nanofibers may be obtained by melt-blown polymers. Examples of applicable polymers may include polyolefin, polyacetal, polyamide, polyester, cellulose ether and ester, polyalkylene sulfide, polyarylene oxide, polysulfone, modified polysulfone polymers, nylon, polystyrene, polyacrylonitrile, polycarbonate and mixtures thereof. In one example, the nanofibers may be formed by electrospinning Nylon 6 polymer from a 98% formic acid solution. In another example, the nanofibers may be formed by electrospinning polystyrene or polyvinyl chloride from solutions in tetrahydrofuran (THF). In another example, the nanofibers may be formed by electrospinning polyethylene oxide (PEO) dissolved in water. In yet another example, polyethylene-terephthalate (PET) and polyethylene-naphthalate (PEN) may be electrospun or spin-melted into nanofibers directly from their polymer melts. So long as the resulting solution or polymer melt has a viscosity similar to that of honey, with viscosity of at least 2000-3000 cP at 25° C., the solution, melt or suitable candidate materials can thus be spun into nanofibers. Nanofibers may also be formed using other processes known to one skilled in the art. 
         [0048]    The nanofibers may have an average diameter of about 5 to about 900 nanometers, preferably from about 100 to about 300 nanometers, and more preferably from about 150 to about 250 nanometers. For example, electrospun Nylon 6 nanofibers may have an average fiber diameter from about 147 to about 249 nanometers, when the weight percentage of Nylon 6 in a polymer solution ranges from about 18% to about 24%, and under the electrospinning condition of 25 kV electrode voltage and 14 cm tip-to-collector distance. The average fiber diameter may be characterized by taking an SEM (scanning electron microscope) image and randomly selecting and measuring the diameters of 30 nanofibers from the image. By reducing the distance between the tip-to-collector, e.g. 5 centimeters, the fiber diameter can increase to 600-800 nanometers. One possible explanation for this reduction in diameter is that the fibers do not have time to get thinner (i.e. smaller in diameter) by stretching through charge repulsion from like charges deposited on the fiber surface. 
         [0049]    The substrate layer may include any porous and non-woven materials that may provide mechanical strength as support for the filtration medium. For example, the substrate layer may include microfibers. Examples of microfibers may include polyethylene, polyethylene, glass, cellulose acetate, activated carbon fiber or combinations thereof. The microfibers may have an average diameter of about 1 to about 30 microns, which may include finer microfibers having an average diameter of about 1 to about 20 microns and coarser microfibers having an average diameter of about 10 to 30 microns, such as activated carbon fiber. The content of the microfibers in the filtration medium may vary from about 10 to about 600 grams per square meter of filter area. 
         [0050]    In one example, the nanofibers may be directly electrospun onto the surface of a sheet of non-woven microfibers. In another example, the microfibers may be placed in a liquid, and nanofibers may be electrospun onto them. The liquid suspension may then be air-circulated, and the liquid may be removed under vacuum. Subsequently, the microfibers and nanofibers may be compressed mechanically together with a small amount of compatible adhesive to form a rigid structure. 
         [0051]    The substrate layer may include one or more additives, such as in a particulate, fiber, whisker, or powder form. Examples of additives may include anti-microbial substrates. The term “anti-microbial substrates” means any chemicals or particles that may be used to kill or make unviable microbes, viruses or bacteria. Examples of anti-microbial substrates may include nano-particles made of magnesium oxide (MgO), silver (Ag) compounds including silver nitrate, titanium oxide nanoparticles, Poly(N-benzyl-4-vinylpyridinium chloride), or combinations thereof. 
         [0052]    Examples of additives may also include adsorption particles. The term “adsorption particles” means nano-sized adsorbents, with molecule sizes from about 0.5 to about 100 nanometers, that may physically attract and adsorb particles and volatile organic compounds (VOCs) from a fluid stream to the surface of the adsorption particle. This attraction may involve electrostatic or chemical interaction. Examples of adsorption particles may include activated carbon, silica gel, activated alumina, zeolites, porous clay minerals, molecular sieves, or combinations thereof. Nano-sized absorbents made of zinc oxide, calcium oxide, cupric oxide, magnesium oxide, manganese dioxide, manganese oxide, aluminum oxide, and zeolite may also be used to filter specific molecules such as hydrogen sulphide. 
         [0053]    The additives may further include a plurality of desorption substances. The term “desorption substances” mean particles or vapor that may diffuse away from the surfaces or pores of the substrate layer. For example, desorption substances may include medication or fragrance particles or vapor. The desorption substances may be diffused gradually over time, rather than being released in a single dose or in multiple dose pulses. 
         [0054]    Desorption substances for treating asthma and respiratory diseases may be used in medical applications. Examples of desorption substances may include steroids for chronic obstructive pulmonary disease; albuterol powder for the treatment of asthma; respirable antisense oligonucleotides (RASONs) for attenuating specific disease-associated mRNAs; Spiriva HandiHaler® (tiotropium bromide, available from Boehringer Ingelheim) for the treatment of bronchospasm associated with chronic obstructive pulmonary disease; Qvar® (beclomethasone dipropionate, available from Ivax) for the treatment of asthma; Xopenex® (available from Sepracor) as inhaled solution for treatment of reversible obstructive airway disease; DuoNeb® (albuterol sulfate and ipratropium bromide, available from Dey Laboratories) for the treatment of bronchospasm associated with COPD; Foradil Aerolizer® (formoterol fumarate inhalation powder, available from Novartis) as bronchodilator for COPD, asthma and bronchospasm; Ventolin HFA® (albuterol sulfate inhalation aerosol, available from GlaxoSmithKline) for the treatment or prevention of bronchospasm; Tri-Nasal Spray® (triamcinolone acetonide spray, available from Muro Pharmaceutical) for treatment of nasal symptoms of allergic rhinitis in adults and children age 12 or older; Proventil HFA Inhalation Aerosol® (available from 3M Pharmaceuticals) for treatment or prevention of bronchospasm; Rhinocort Aqua Nasal Spray® (available from AstraZeneca) for nasal spray containing budesonide; or combinations thereof. 
         [0055]    The desorption substances may also be used in household, cosmetic or industrial applications to modulate the immediate surrounding environment. In one example, Symbicort® made by Astrazeneca and Serevent® made by GSK (GlaxoSmithKline) may be used for both adsorption and release of particles. These may be in powder form or liquid aerosol form that may be adsorbed onto the substrate layers. 
         [0056]    The filtration medium may also include one or more cover layers bonded to the laminate. The cover layer may include, for example, a non-woven material. The filtration medium may further include a hydrophobic layer bonded to one of the cover layers. The hydrophobic layer may be configured to allow free gas exchange to occur across the filtration medium, while preventing water and other aqueous liquids from entering. Thus the hydrophobic layer can prevent virus bearing water droplet from wetting and penetrating the cover. The hydrophobic layer may be non-polar. Examples of non-polar polymers include PTFE, glass composites and nylon. Polyethersulfone (PES) and acrylic copolymers may also be used to render the filtration medium hydrophobic, which may cause membranes to become non-wettable by most low-surface tension liquids. Biodegradable polymers may also be used, which may include aliphatic polyesters such as poly(lactic acid), poly(glycolic acid), polycaprolactone, and their copolymers. 
         [0057]    Geometries of the Multilayer Filtration Medium 
         [0058]    In one embodiment, the multilayer filtration medium may include a nanofiber layer coated on a porous substrate layer, as depicted in  FIG. 9A , which may be folded into a multilayer of nanofibers in a serpentine arrangement. For example, a multilayer filtration medium with five nanofiber layers is depicted in  FIG. 9B . In another embodiment, the multilayer filtration medium may include two nanofiber layers coated on a porous substrate layer, as depicted in  FIG. 9C , which may be folded into a multilayer of nanofibers in a serpentine arrangement. For example, a multilayer filtration medium with ten nanofiber layers is depicted in  FIG. 9D . 
         [0059]    Method of Making 
         [0060]    A method of making the multi-layer filtration medium may include coating nanofibers onto a substrate medium to form a laminate unit, as shown in  FIG. 1A , and stacking at least two of the units in a series of parallel and repeating bi-layer laminate units, such that the filtration medium alternates between nanofiber layers and substrate layers. 
         [0061]    Another method of making the multi-layer filtration medium may include coating nanofibers on both sides of a substrate to form a laminate unit, as shown in  FIG. 1B , and stacking at least two of the units in a series of parallel and repeating bi-layer laminate units, such that the substrates will be separated by two layers of nanofibers. 
         [0062]    Quality Factor 
         [0063]    Quality factor (QF) is defined as QF=ln(1−η)/ΔP, where is the collection efficiency provided by a filter in capturing particles of specific size, and ΔP is the pressure drop across the filter. A filter having a higher η and/or a lower ΔP, thus yielding a higher QF, may be said to have a better performance. 
         [0064]    The relative quality factor (RQF) is defined as RQF=QF 2 /QF 1 , where QF 1  is the quality factor of a filter as a performance baseline, and QF 2  is the quality factor of another filter to be compared with. 
         [0065]    Reference will now be made in detail to the preferred embodiments of the invention, examples of which are also provided in the following description. Exemplary embodiments of the invention are described in detail, although it will be apparent to those skilled in the relevant art that some features that are not particularly important to an understanding of the invention may not be shown for the sake of clarity. 
         [0066]    Furthermore, it should be understood that the filtration medium is not limited to the precise embodiments described below and that various changes and modifications thereof may be effected by one skilled in the art without departing from the spirit or scope of the invention. For example, elements and/or features of different illustrative embodiments may be combined with each other and/or substituted for each other within the scope of this disclosure and appended claims. 
         [0067]    The filtration medium is further illustrated by the following examples, which are not to be construed in any way as imposing limitations upon the scope thereof. On the contrary, it is to be clearly understood that resort may be had thereof which, after reading the description herein, may suggest themselves to those skilled in the art without departing from the spirit of the specification and/or the scope of the appended claims. 
       EXAMPLES 
     Example 1 
     Thickness Comparison of Filtration Media 
       [0068]    The determined thickness of nanofiber layer from electrospinning typical polymer/solvent combination, for example Polyethylene Oxide (PEO) in propanol, Nylon 6 (N6) in formic acid, Polystyrene (PS) in tetrahydrofuran (THF) and Polyvinyl Chloride (PVC) also in THF, is depicted in  FIG. 5A . The results have indicated that for filtration medium formed by coating a layer of nanofibers on a substrate medium (e.g. filtration medium as depicted in  FIGS. 1A ,  2 A,  3 A and  4 A), the thickness of nanofiber layer increased moderately, from 9.694 microns at 0.1167 gsm to only 16.92 microns at 0.7 gsm. In contrast, for filtration medium in the form of a laminate unit, the thickness of nanofiber layer increased significantly from 9.694 microns at 0.1167 gsm to 58.16 microns at 0.7 gsm. By including the thickness of substrate medium (approximately 1×10 −4  m),  FIG. 5B  have indicated that the sandwich laminate unit adopting filtration medium as depicted in  FIG. 1B  as the building block is thinner than the alternating unit formed by stacking of filtration medium as depicted in  FIG. 1A , under the same amount of nanofibers. 
         [0069]    As will be explained later, filtration media with a single layer of nanofibers coating have much lower air permeability than multi-layer arrangement (both alternating and sandwich laminate units) under the same amount of nanofibers. Results shown in  FIG. 5B  have suggested that the sandwich arrangement is superior to the alternating arrangement in terms of saving substrate medium, thus reducing overall thickness and limiting production cost. 
       Example 2 
     Solid Volume Fraction Comparison of Filtration Media 
       [0070]    The solid volume fractions of filtration media consist of single layer and multiple layers of nanofibers were compared, as depicted in  FIG. 6 . The results have indicated that when nanofibers were concentrated in a single layer, the solid volume fraction increased with the amount of nanofibers. To fabricate a filtration medium with nanofibers distributed sparsely along its thickness, the filtration medium was stacked against relatively low solid volume fraction until the target amount of nanofibers was met. For example, in the case of single layer of nanofibers, the solid volume fraction was at about 0.01065 at 0.1167 gsm of nanofibers, but increased substantially to about 0.0366 at 0.7 gsm. In contrast, a filtration medium formed by stacking 6 layers of filtration medium containing 0.7 gsm of nanofibers has a solid volume fraction maintained at 0.01065. 
         [0071]    The results in  FIG. 6  shows that nanofibers that were distributed sparsely along filtration medium thickness by multi-layer arrangement lead to desirable properties, such as increased porosity (or lower solid volume fraction) and reduced pressure drop. 
       Example 3 
     Porosity Comparison of Filtration Media 
       [0072]    The porosities of filtration media consist of single layer and multiple layers of nanofibers were compared, as depicted in  FIG. 7 . Porosity and solid volume fraction are related by: 
         [0000]      Porosity=1−Solid Volume Fraction.
 
         [0073]    Similar to the results as show in  FIG. 6 , nanofibers that were distributed sparsely along filtration medium thickness by multi-layer arrangement lead to desirable properties, such as increased porosity (or lower solid volume fraction) and reduced pressure drop. 
       Example 4 
     Capture Efficiency Versus Pressure Drop Comparison of Filtration Media 
       [0074]    The capture efficiency versus pressure drop of filtration media consist of single layer and multiple layers of nanofibers were compared, as depicted in  FIG. 8A . Each nanofiber layer in the multi-layer arrangement had 0.12 gsm of nanofibers coated on a substrate medium. 
         [0075]    The results have indicated that under a given amount of nanofibers (as labeled parameter on the curves), the capture efficiency was comparable between the single layer and multi-layer arrangement. However, the pressure drop has showed a marked difference. For example, while the substrate medium coated with 0.72 gsm of nanofibers (represented by hollows in  FIG. 8A ) yielded a pressure drop of 162.6 Pa, the filtration medium formed by stacking of 6 layers of substrate medium coated with 0.12 gsm of nanofibers (represented by solids in  FIG. 8A ) only yielded a pressure drop of 87.49 Pa. The results have shown multi-layer arrangement in the nanofiber filtration medium can improve air permeability (i.e. lower pressure drop), with small reduction in capture efficiency as a tolerable trade-off. 
       Example 5 
     Capture Efficiency Versus Pressure Drop Comparison for Facemasks 
       [0076]    To demonstrate the potential as a substitute to existing filtration products, the nanofiber filtration medium in multi-layer arrangement was compared against disposable conventional surgical facemasks, as depicted in  FIG. 8B . The dotted line in  FIG. 8B  represents stacking of substrate medium coated with 0.12 gsm of nanofibers, while the solid line represents stacking of conventional face masks. 
         [0077]    Model A4 was a filtration medium having 0.12 gsm of nanofibers coated on a conventional facemask. Model A4 had a capture efficiency of 44% and a pressure drop of 32.16 Pa. This capture efficiency was equivalent to having three face masks aligned in series. However, the pressure drop across three facemasks aligned in series was 46 Pa, i.e. 30% higher than that of the model A4 filtration medium. 
         [0078]    Model B8 was a filtration medium formed by stacking two layers of substrate medium coated with 0.12 gsm of nanofibers. Model B8 had a capture efficiency of 54% and a pressure drop of 29.16 Pa. This capture efficiency was equivalent to having four face masks aligned in series. However, the pressure drop of having four face masks aligned in series was 70 Pa, i.e. 240% higher than that of the model B8 filtration medium 
         [0079]    The results have shown that the filtration medium has outperformed conventional facemasks by having higher capture efficiency and/or lower pressure drop. 
       Example 6 
     Capture Efficiency Versus Pressures Drop Comparison with N95 Respirators 
       [0080]    The capture efficiency and pressure drop of various filtration media formed by stacking of substrate medium coated with nanofibers in various nanofiber packing densities (0.0584, 0.0875 and 0.1167 gsm) were compared against conventional N95 respirators, as depicted in  FIG. 8C . The results indicated that the pressure drop of conventional N95 respirators were about 140 to 147 Pa. Comparatively, the pressure drop of the filtration medium formed by stacking 15 layers of substrate medium coated with 0.0584 gsm of nanofibers was only 90 Pa (37% reduction in pressure drop). However, the filtration medium had a capture efficiency of 95% against 0.3-micron particles, which matched the capture efficiency of the N95 respirators. Nanofibers can also be coated on both sides of the substrate medium. By coating preferably 0.1167 gsm of nanofibers on each of the two sides of the substrate medium, the filtration medium depicted in  FIG. 1B  can be obtained. It has a capture efficiency of 54.89% against 0.3-micron particles, and a pressure drop of 29.16 Pa, which are identical to the filtration medium formed by stacking 2 layers of substrate medium coated with 0.1167 gsm of nanofibers on one side only (point P in  FIG. 8C ). The arrangement of  FIG. 1B  is advantageous since it saves one substrate medium.  FIG. 2C  shows the filtration medium formed by stacking 2 filtration medium of  FIG. 1B . Its capture efficiency and pressure drop are the same as the filtration medium formed by stacking 4 layers of substrate medium coated with 0.1167 gsm of nanofibers on one side only (point Q in  FIG. 8C ), and it saves two substrate medium. Similarly, the arrangement depicted in  FIG. 3C  can save three substrate medium (point R in  FIG. 8C ), and the arrangement shown in  FIG. 4C  can save four substrate medium (point S in  FIG. 8C ). 
       Example 7 
     Degree of Pressure Drop Reduction Depends on Target Capture Efficiency 
       [0081]    The capture efficiency and pressure drop of filtration media formed by stacking substrate medium coated with various quantities of nanofibers were compared against each other, as depicted in  FIGS. 10 to 14 . Capture efficiencies against 50, 100, 200, 300 and 400 nm particles were depicted in  FIGS. 10 ,  11 ,  12 ,  13  and  14 , respectively. 
         [0082]      FIG. 10  depicts the results of filtration media formed by substrate medium coated with a layer of nanofibers (data points marked with “1” next to them) versus filtration media formed by stacking of substrate medium coated with nanofibers in different quantities (data points marked with numbers other than “1”, indicating there is more than one nanofiber layer). For example, a substrate medium coated with 0.7 gsm of nanofibers attained 92% capture efficiency against 50 nm particles and a pressure drop of 164 Pa. In contrast, a filtration medium formed by stacking 6 layers of substrate medium, each coated with 0.1167 gsm of nanofibers attained nearly 90% of efficiency and a pressure drop of only 88 Pa (i.e., a 46% reduction). Furthermore, a filtration medium formed by stacking 12 layers of substrate medium, each coated with 0.0584 gsm of nanofibers also attained 90% efficiency and a lower pressure drop at 70 Pa (57% reduction). This phenomenon can be observed for other particle sizes, as depicted in  FIGS. 11 to 14 . 
         [0083]    In summary, the results in  FIGS. 10 to 14  have indicated that for the same total amount of nanofibers, the filtration medium with more nanofiber layers had a lower pressure drop. These results be viewed as artificially re-distributing the nanofibers sparsely throughout the filter medium thickness, thus increased the porosity of nanofiber layer and resulted in a more permeable medium. The decrease in pressure drop was found to be more significant in filter media with more nanofibers, e.g. 0.7 gsm of nanofibers. For filter media that contained lesser amount of nanofibers, e.g. 0.0875 gsm, the advantages obtained from pressure drop reduction may not be able to cover the increased cost due to extra consumption and processing of substrate medium. 
       Example 8 
     Relative Quality Factor and Capture Efficiency of Multi-Layer Nanofiber Filtration Media 
       [0084]    RQF and capture efficiency of multilayer filtration media having various nanofiber packing densities were measured, as depicted in  FIGS. 15 to 17 . The multilayer filtration medium having a packing density of 0.233 g/m 2  was depicted in  FIG. 15 . The multilayer filtration medium having a packing density of 0.35 g/m 2  was depicted in  FIG. 16 . The multilayer filtration medium having a packing density of 0.467 g/m 2  was depicted in  FIG. 17 . 
         [0085]    The results in  FIGS. 15 to 17  have indicated that for the same total amount of nanofibers in terms of grams per square meter, increasing the number of nanofiber layers in the multilayer filtration medium significantly reduced the pressure drop. 
         [0086]    While the examples of the filtration medium have been described, it should be understood that the filtration medium are not so limited and modifications may be made. The scope of the filtration medium is defined by the appended claims, and all devices that come within the meaning of the claims, either literally or by equivalence, are intended to be embraced therein.
A method of making a multilayer filter and the multilayer filter made by the method. The method includes generally two steps. The first step is to coat a layer of nanofibers on a single side or both sides of a substrate medium to obtain a composite filter medium, and the second step is to fold the composite filter medium in a serpentine fashion to form a multilayer filter. The second step may alternatively be accomplished by stacking up a number of sheets of the composite filter medium to form a multilayer filter having a structure of two layers of nanofibers being sandwiched between two layers of said substrate medium. The resulting multilayer filter produced by either method contains at least one structural unit which has two layers of nanofibers being sandwiched between two layers of the substrate medium.