Sterilizable porous filtration media containing nanofibers

Abstract

Provided herein are sterilizable porous filtration media, methods of making and methods of using the same.

Description

[0001] This application is a divisional application of the invention patent application with the Chinese patent application number 202180027590.5 and the invention title "Sterilizable Porous Filter Medium Containing Nanofibers", which entered the Chinese national phase on October 10, 2022, from the PCT international application PCT / US2021 / 017211 filed on February 9, 2021.

[0002] Cross - reference to related applications

[0003] This application claims the benefit of the priority of U.S. Provisional Patent Application No. 62 / 977,884, filed on February 18, 2020, the entire content of which is incorporated herein by reference in its entirety. Technical field Background art

[0004] Most filtration applications in the biopharmaceutical industry require a sterilization step performed by the drug manufacturer or a pre - sterilization step performed by the filter manufacturer. The sterilization step can include, for example, steam - in - place sterilization or autoclave sterilization. These sterilization methods apply high temperature or high energy to the membrane and filtration device. In addition, most devices sold for critical filtration applications pass a 100% integrity test, which means that each device sold will undergo at least one test. Most such integrity tests require wetting the filter medium inside the device, followed by testing and drying. The membrane structure needs to withstand these harsh conditions to be successfully used in these critical filtration applications. Unfortunately, for current nanofiber - based liquid filtration membranes, in both cases with heat (e.g., autoclaving) and without heat (e.g., integrity testing), the membrane structure stability and performance are negatively affected by wet / dry cycles. Summary of the invention

[0005] Methods and compositions related to nanofiber structures (e.g., nanofiber mats) are provided herein that exhibit high permeability and rejection after any wet - dry process (including moist heat sterilization (e.g., autoclaving, steam - in - place sterilization, and / or intermittent sterilization)).

[0006] In some aspects, methods for producing a porous non - woven liquid filtration medium containing polymer nanofibers that is compatible with moist heat sterilization are provided, the method comprising heating the porous non - woven nanofiber - containing liquid filtration medium to at least the glass transition temperature (T g ) of the nanofibers but not exceeding the melting temperature (T m) up to a time span required to achieve at least thermal equilibrium with the heating medium (excluding temperature rise and cooling).

[0007] In some aspects, provided herein is a liquid filtration medium comprising a porous polymer nanofiber mat that is compatible with moist heat sterilization, the liquid filtration medium having been heated at a temperature that is at least the glass transition temperature (T g ) of the polymer nanofibers but not exceeding the melting temperature (T m ) of the polymer nanofibers for up to at least the time required for thermal equilibrium (excluding temperature rise and cooling).

[0008] In certain aspects, provided herein is a method for sterilizing a porous nonwoven liquid filtration medium containing polymer nanofibers, the method comprising heating the liquid filtration medium to a temperature that is at least the glass transition temperature (T g ) of the polymer nanofibers but not exceeding the melting temperature (T m ) of the polymer nanofibers for at least 1 hour, and sterilizing the heat-treated liquid filtration medium using moist heat sterilization.

[0009] In further aspects, provided herein is a method for removing bacteria from a liquid sample, the method comprising heating a porous nonwoven liquid filtration medium containing polymer nanofibers to a temperature that is at least the glass transition temperature (T g ) of the polymer nanofibers but not exceeding the melting temperature (T m ) of the polymer nanofibers for at least 1 hour; sterilizing the liquid filtration medium using moist heat sterilization; and passing the liquid sample containing bacteria through the sterilized liquid filtration medium.

[0010] Certain aspects provided herein include a method for removing virus particles from a liquid sample, the method comprising heating a porous nonwoven liquid filtration medium containing polymer nanofibers to a temperature that is at least the glass transition temperature (T g ) of the polymer nanofibers but not exceeding the melting temperature (T m ) of the polymer nanofibers for up to a time span required to achieve thermal equilibrium; sterilizing the liquid filtration medium using moist heat sterilization; and passing the liquid sample containing virus particles through the sterilized liquid filtration medium.

[0011] Thus, in certain aspects, provided herein is a high-pressure sterilizable porous nonwoven nanofiber liquid filtration medium prepared according to the methods disclosed herein, and a critical filtration device comprising such a liquid filtration medium for critical filtration applications. In certain embodiments, such liquid filtration media are particularly useful for sterile filtration (aseptic) applications.

[0012] In one aspect, the present invention is a method for producing a porous nonwoven liquid filtration medium containing polymer nanofibers that is compatible with any wet or dry treatment (such as integrity testing or steam sterilization), the method comprising heating the porous nonwoven liquid filtration medium containing nanofibers to at least the glass transition temperature (T g ) of the nanofibers but not exceeding the melting temperature (T m ) of the nanofibers for at least 1 hour.

[0013] In another aspect of the present invention, the liquid filtration medium is prepared by electrospinning a polymer solution or melt to produce a porous nonwoven polymer nanofiber mat.

[0014] In another aspect of the present invention, relative to a corresponding filtration medium that has not been heated to at least the glass transition temperature (T g ) of the nanofibers but not exceeding the melting temperature (T m ) of the nanofibers for at least 1 hour prior to sterilization, the liquid filtration medium resists changes in liquid permeability after sterilization.

[0015] In another aspect of the present invention, the liquid filtration medium exhibits a bubble point pressure of 5 psi to 150 psi (psig).

[0016] In another aspect of the present invention, the liquid filtration medium exhibits a bubble point of 15 psi or greater.

[0017] In another aspect of the present invention, compared to the same liquid filtration medium that has not been heated to at least the T g of the polymer nanofibers but not exceeding the T m of the polymer nanofibers for at least 1 hour, the liquid filtration medium exhibits a smaller change in bubble point after sterilization.

[0018] In another aspect of the present invention, the bubble point is measured using as the wetting fluid.

[0019] In another aspect of the present invention, the liquid filtration medium exhibits a log reduction value (LRV) of at least 1 of Brevundimonas diminuta as measured according to ASTM F838-83.

[0020] In another aspect of the present invention, the liquid filtration medium exhibits a log reduction value (LRV) of at least 4 or at least 8 of Brevundimonas diminuta as measured according to ASTM F838-83.

[0021] In another aspect of the present invention, the liquid filtration medium has a porosity of about 80% to about 95%.

[0022] In another aspect of the present invention, the liquid filtration medium exhibits a liquid permeability greater than about 1000 LMH / psi.

[0023] In another aspect of the present invention, the liquid filtration medium exhibits a liquid permeability greater than about 100 LMH / psi.

[0024] In another aspect of the present invention, the liquid filtration medium exhibits a liquid permeability greater than about 10 LMH / psi.

[0025] In another aspect of the present invention, compared to the same filtration medium that has not been heated to at least the T of the polymer nanofibers g but not exceeding the T of the polymer nanofibers m for at least 1 hour, the liquid filtration medium exhibits a higher liquid permeability after sterilization.

[0026] In another aspect of the present invention, the liquid filtration medium exhibits a reduction in liquid permeability after sterilization of no more than 40%.

[0027] In another aspect of the present invention, the liquid filtration medium exhibits a reduction in liquid permeability after sterilization of no more than 30%.

[0028] In another aspect of the present invention, the liquid filtration medium exhibits a reduction in liquid permeability after sterilization of no more than 15%.

[0029] In another aspect of the present invention, the nanofibers have a fiber diameter of about 5 nm to about 1000 nm.

[0030] In another aspect of the present invention, the liquid filtration medium comprises 1) a symmetric nanofiber mat or 2) an asymmetric nanofiber mat that exhibits a fiber diameter that varies across the thickness of the entire nanofiber mat such that the average fiber diameter of one layer of the nanofiber mat is different from other layers of the nanofiber mat.

[0031] In another aspect of the present invention, the average fiber diameter varies continuously from one layer to another of the asymmetric nanofiber mat.

[0032] In another aspect of the present invention, the ratio of the average fiber diameter of one layer of the asymmetric nanofiber mat to another layer is at least 1.15.

[0033] In another aspect of the present invention, the average fiber diameter on at least one layer of the asymmetric nanofiber mat is from about 5 nm to about 1000 nm.

[0034] In another aspect of the present invention, the average fiber diameter on at least one layer of the asymmetric nanofiber mat is from about 5 nm to about 150 nm.

[0035] In another aspect of the present invention, the average fiber diameter on at least one layer of the nanofiber mat is about 100 nm.

[0036] In another aspect of the present invention, the average fiber diameter on at least one layer of the nanofiber mat is about 5 nm.

[0037] In another aspect of the present invention, the polymer is selected from: thermoplastic polymers, thermosetting polymers, nylon, polyimide, aliphatic polyamides, aromatic polyamides, polysulfone, cellulose acetate, polyethersulfone, polyurethane, poly(ureaurethane), polybenzimidazole, polyetherimide, polyacrylonitrile, poly(ethylene terephthalate), polypropylene, polyaniline, poly(ethylene oxide), poly(ethylene naphthalate), poly(butylene terephthalate), styrene-butadiene rubber, polystyrene, poly(vinyl chloride), poly(vinyl alcohol), poly(vinylidene fluoride), poly(vinyl butene), and their copolymers, derivative compounds, or blends.

[0038] In another aspect of the present invention, the polymer is an aliphatic polyamide.

[0039] In another aspect of the present invention, the polymer is selected from: nylon-6, nylon-6,6, nylon 6,6-6,10, nylon-6 copolymers, nylon-6,6 copolymers, nylon 6,6-6,10 copolymers, and any mixtures thereof.

[0040] In another aspect of the present invention, the polymer is nylon-6,6.

[0041] In another aspect of the present invention, the method includes heating the nanofiber mat to a temperature about 1 °C to about 80 °C lower than T m

[0042] In another aspect of the present invention, the method includes heating the nanofiber mat to a temperature about 56 °C lower than T m

[0043] In another aspect of the present invention, the method includes heating the nanofiber mat to a temperature about 75 °C lower than T m

[0044] In another aspect of the present invention, the method includes heating the nanofiber mat to a temperature about 100 °C to about 200 °C higher than T g

[0045] In another aspect of the present invention, the method includes heating the nanofiber mat to a temperature about 140 °C to about 158 °C higher than T g

[0046] In another aspect of the present invention, the method includes heating the nanofiber mat to about 190 °C to about 208 °C.

[0047] ​​​​​In another aspect of the present invention, the method includes heating the nanofiber mat to about 208 °C.

[0048] In another aspect of the present invention, the method includes heating the nanofiber mat in a non-oxidizing environment (such as an inert atmosphere oven).

[0049] In another aspect of the present invention, the method includes heating the nanofiber mat for at least about 1 hour to at least about 24 hours.

[0050] In another aspect of the present invention, the method includes heating the filter medium for at least about 12 hours.

[0051] In another aspect of the present invention, the method includes any wet-dry treatment, such as integrity testing, moist heat sterilization (including autoclaving, in-situ steam sterilization, and / or intermittent sterilization).

[0052] In another aspect of the present invention, the porous nonwoven nanofiber-containing liquid filter medium is electrospun onto the surface of a porous support or a non-porous support.

[0053] In another aspect of the present invention, the nanofiber mat is electrospun onto the surface of a porous support or a non-porous support, wherein the root mean square height of the surface of the porous nonwoven support is less than about 70 μm.

[0054] In another aspect of the present invention, the support comprises one or more layers produced by meltblowing, wet-laying, spin-bonding, calendering, electrospinning, electroblowing, or any combination thereof.

[0055] In another aspect of the present invention, the support comprises thermoplastic polymers, polyolefins, polypropylenes, polyesters, polyamides, their copolymers, polymer blends, cellulose-based, and combinations.

[0056] In another aspect of the present invention, the tightest pore diameter of the nanofiber layer is smaller than the tightest pore diameter of the porous nonwoven support.

[0057] In another aspect of the present invention, the porous support comprises one or more layers selected from the group consisting of spunbond nonwoven, meltblown nonwoven, needlepunched nonwoven, hydroentangled nonwoven, wet-laid nonwoven, resin-bonded nonwoven, electrospun nonwoven, electroblown nonwoven, woven fabric, knitted fabric, paper, and combinations thereof.

[0058] In another aspect, the present invention relates to a porous nonwoven nanofiber liquid filter medium that can be autoclaved, prepared by the method according to the present invention.

[0059] In one aspect, the present invention is a liquid filtration medium comprising a porous polymeric nanofiber mat that is compatible with any wet or dry process (such as integrity testing or moist heat sterilization), the liquid filtration medium having been heated at a temperature of at least the glass transition temperature (T g ) of the polymeric nanofibers but not exceeding the melting temperature (T m ) of the polymeric nanofibers for up to at least 1 hour.

[0060] In another aspect of the present invention, the liquid filtration medium is prepared by electrospinning a polymer solution or melt to produce a porous nonwoven polymeric nanofiber mat.

[0061] In another aspect of the present invention, relative to a corresponding filtration medium comprising a nanofiber mat that has not been heated to at least the T g of the polymeric nanofibers but not exceeding the T m of the polymeric nanofibers for up to at least 1 hour, the liquid filtration medium resists changes in liquid permeability after sterilization.

[0062] In another aspect of the present invention, the liquid filtration medium exhibits a bubble point pressure of 5 psi to 150 psi.

[0063] In another aspect of the present invention, the liquid filtration medium exhibits a bubble point of 15 psi or greater.

[0064] In another aspect of the present invention, compared to the same liquid filtration medium comprising a nanofiber mat that has not been heated to at least the T g of the polymeric nanofibers but not exceeding the T m of the polymeric nanofibers for up to at least 1 hour, the liquid filtration medium exhibits a smaller change in bubble point after sterilization.

[0065] In another aspect of the present invention, the bubble point is measured using as the wetting fluid.

[0066] In another aspect of the present invention, the liquid filtration medium exhibits a log reduction value (LRV) of at least 1 of Brevundimonas diminuta as measured according to ASTM F838-83.

[0067] In another aspect of the present invention, the liquid filtration medium exhibits a log reduction value (LRV) of at least 4 of Brevundimonas diminuta as measured according to ASTM F838-83.

[0068] In another aspect of the present invention, the liquid filtration medium exhibits a log reduction value (LRV) of at least 8 of Brevundimonas diminuta as measured according to ASTM F838-83.

[0069] In another aspect of the present invention, the liquid filtration medium has a porosity of from about 80% to about 95%.

[0070] In another aspect of the present invention, the liquid filtration medium exhibits a liquid permeability greater than about 1000 LMH / psi after autoclaving.

[0071] In another aspect of the present invention, the liquid filtration medium exhibits a liquid permeability greater than about 100 LMH / psi after autoclaving.

[0072] In another aspect of the present invention, the liquid filtration medium exhibits a liquid permeability greater than about 10 LMH / psi after autoclaving.

[0073] In another aspect of the present invention, relative to a corresponding nanofiber mat-containing liquid filtration medium that has not been heated to at least the T of the polymer nanofibers g but not exceeding the T of the polymer nanofibers m for at least 1 hour, the liquid filtration medium exhibits a relatively high liquid permeability after sterilization.

[0074] In another aspect of the present invention, the liquid filtration medium exhibits a reduction in liquid permeability after autoclaving of no more than 40%.

[0075] In another aspect of the present invention, the liquid filtration medium exhibits a reduction in liquid permeability after autoclaving of no more than 30%.

[0076] In another aspect of the present invention, the liquid filtration medium exhibits a reduction in liquid permeability after autoclaving of no more than 15%.

[0077] In another aspect of the present invention, the liquid filtration medium does not exhibit a significant change in liquid permeability after autoclaving.

[0078] In another aspect of the liquid filtration medium, the nanofiber mat of the liquid filtration medium has a fiber diameter of from about 5 nm to about 1000 nm.

[0079] In another aspect of the liquid filtration medium, the liquid filtration medium comprises 1) a symmetric nanofiber mat or 2) an asymmetric nanofiber mat that exhibits a fiber diameter that varies across the thickness of the nanofiber mat such that the average fiber diameter of one layer of the nanofiber mat is different from other layers of the nanofiber mat.

[0080] In another aspect of the liquid filtration medium, the average fiber diameter varies continuously from one layer of the nanofiber mat to another.

[0081] In another aspect of the liquid filtration medium, the ratio of the average fiber diameter of one layer of the nanofiber mat to another layer is at least 1.15.

[0082] On the other hand of the liquid filtration medium, the average fiber diameter on at least one layer of the nanofiber mat is from about 5 nm to about 1000 nm.

[0083] On the other hand of the liquid filtration medium, the average fiber diameter on at least one layer of the nanofiber mat is from about 5 nm to about 100 nm.

[0084] On the other hand of the liquid filtration medium, the average fiber diameter on at least one layer of the nanofiber mat is about 100 nm.

[0085] On the other hand of the liquid filtration medium, the average fiber diameter on at least one layer of the nanofiber mat is about 5 nm.

[0086] On the other hand of the liquid filtration medium, the polymer is selected from: thermoplastic polymers, thermosetting polymers, nylon, polyimide, aliphatic polyamides, aromatic polyamides, polysulfone, cellulose acetate, polyethersulfone, polyurethane, poly(ureaurethane), polybenzimidazole, polyetherimide, polyacrylonitrile, poly(ethylene terephthalate), polypropylene, polyaniline, poly(ethylene oxide), poly(ethylene naphthalate), poly(butylene terephthalate), styrene-butadiene rubber, polystyrene, poly(vinyl chloride), poly(vinyl alcohol), poly(vinylidene fluoride), poly(vinyl butene), and their copolymers, derivative compounds, or blends.

[0087] On the other hand of the liquid filtration medium, the polymer is an aliphatic polyamide.

[0088] On the other hand of the liquid filtration medium, the polymer is selected from: nylon-6, nylon-6,6, nylon 6,6-6,10, nylon-6 copolymers, nylon-6,6 copolymers, nylon 6,6-6,10 copolymers, and any mixtures thereof.

[0089] On the other hand of the liquid filtration medium, the polymer is nylon-6,6.

[0090] On the other hand, the nanofiber mat has been heated to about 1 °C to about 80 °C lower than T m

[0091] On the other hand of the liquid filtration medium of the present invention, the nanofiber mat has been heated to about 56 °C lower than T m

[0092] On the other hand of the liquid filtration medium of the present invention, the nanofiber mat has been heated to about 75 °C lower than T m

[0093] On the other hand of the liquid filtration medium of the present invention, the nanofiber mat has been heated to about 100 °C to about 200 °C higher than T g ​

[0094] In another aspect of the liquid filtration medium of the present invention, the nanofiber mat has been heated to about 140 °C to about 158 °C higher than T g High.

[0095] In another aspect of the liquid filtration medium of the present invention, the nanofiber mat has been heated to about 190 °C to about 208 °C.

[0096] In another aspect of the liquid filtration medium of the present invention, the nanofiber mat has been heated to about 208 °C.

[0097] In another aspect of the liquid filtration medium of the present invention, the nanofiber mat has been heated in a non-oxidizing environment (such as an inert atmosphere oven).

[0098] In another aspect of the liquid filtration medium of the present invention, the nanofiber mat has been heated for at least about 1 hour to at least about 24 hours.

[0099] In another aspect of the liquid filtration medium of the present invention, the nanofiber mat has been heated for at least about 12 hours.

[0100] In another aspect of the liquid filtration medium of the present invention, the wet-dry treatment (such as integrity testing, moist heat sterilization) includes autoclaving, in-situ steam sterilization, and / or intermittent sterilization.

[0101] In another aspect of the liquid filtration medium of the present invention, the porous nonwoven nanofiber-containing liquid filtration medium is electrospun onto the surface of a porous support or a non-porous support.

[0102] In another aspect of the liquid filtration medium of the present invention, the nanofiber mat is electrospun onto the surface of a porous support or a non-porous support, wherein the root mean square height of the surface of the porous nonwoven support is less than about 70 μm.

[0103] In another aspect of the liquid filtration medium of the present invention, the support comprises one or more layers produced by meltblowing, wet-laying, spunbonding, calendering, electrospinning, electroblowing, or any combination thereof.

[0104] In another aspect of the liquid filtration medium of the present invention, the support comprises thermoplastic polymers, polyolefins, polypropylenes, polyesters, polyamides, their copolymers, polymer blends, cellulose-based, and combinations.

[0105] In another aspect of the liquid filtration medium of the present invention, the smallest pore diameter of the nanofiber layer is smaller than the smallest pore diameter of the porous nonwoven support.

[0106] In another aspect of the liquid filtration medium of the present invention, the porous support comprises one or more layers selected from the group consisting of: spunbond nonwoven, meltblown nonwoven, needled nonwoven, hydroentangled nonwoven, wet-laid nonwoven, resin-bonded nonwoven, electrospun nonwoven, electroblown nonwoven, woven fabric, knitted fabric, paper, and combinations thereof.

[0107] In another aspect, the present invention includes a sterilizable porous nonwoven nanofiber liquid filtration medium prepared by the method according to the present invention.

[0108] In another aspect, the present invention includes a filtration device for critical filtration, the filtration device comprising one or more layers of the porous composite medium of the present invention.

[0109] In one aspect, the present invention includes a porous composite medium comprising: a porous asymmetric flat membrane prefilter having a tight layer and an open layer, and a pore size that gradually increases between the tight layer and the open layer; and a rejection layer comprising the liquid filtration medium according to any one of claims 48 to 97 disposed on the tight layer of the porous asymmetric flat membrane, wherein the pore size of the rejection layer is smaller than the pore size of the tight layer of the porous asymmetric flat membrane prefilter.

[0110] In another aspect, the porous composite medium has a bubble point as measured with a liquid, which is at least 20% greater than the bubble point of the porous flat membrane prefilter alone.

[0111] In another aspect, the porous composite medium has a mean flow pressure of isopropyl alcohol in the range of about 10 psi to about 130 psi.

[0112] In another aspect, the porous composite medium has a porous asymmetric polymer flat membrane comprising one or more layers produced by solution phase inversion, thermally induced phase separation, vapor induced phase separation, track etching, biaxial stretching, solvent etching, and combinations thereof.

[0113] In another aspect, the present invention relates to a filtration device for critical filtration, the filtration device comprising the porous composite medium of the present invention.

[0114] In another aspect, the filtration device has a composite filtration medium, the composite filtration medium being positioned in the device such that the porous asymmetric polymer flat membrane is upstream of the rejection filtration layer in the filtration direction, whereby the porous asymmetric flat membrane provides prefiltration of the sample and the rejection filtration layer provides further filtration of the sample.

[0115] In one aspect, the present invention relates to a method for sterilizing a porous nonwoven liquid filtration medium containing polymer nanofibers, the method comprising heating the liquid filtration medium to at least the glass transition temperature (T g ) of the polymer nanofibers but not exceeding the melting temperature (T m ) of the polymer nanofibers for at least 1 hour, and sterilizing the heat-treated liquid filtration medium using moist heat sterilization.

[0116] In one aspect, the present invention relates to a method for removing bacteria from a liquid sample, the method comprising heating a porous nonwoven liquid filtration medium containing polymer nanofibers to at least the glass transition temperature (T g ) of the polymer nanofibers but not exceeding the melting temperature (T m ) of the polymer nanofibers for at least 1 hour; sterilizing the liquid filtration medium using moist heat sterilization; and passing the liquid sample containing bacteria through the sterilized liquid filtration medium.

[0117] In one aspect, the present invention relates to a method for removing virus particles from a liquid sample, the method comprising heating a porous nonwoven liquid filtration medium containing polymer nanofibers to at least the glass transition temperature (T g ) of the polymer nanofibers but not exceeding the melting temperature (T m ) of the polymer nanofibers for at least 1 hour; sterilizing the liquid filtration medium using moist heat sterilization; and passing the liquid sample containing virus particles through the sterilized liquid filtration medium.

[0118] In another aspect of the present invention, the liquid filtration medium exhibits a virus log reduction value (LRV) greater than about 6.

[0119] In another aspect of the present invention, the liquid filtration medium exhibits a virus log reduction value (LRV) greater than about 3.

[0120] In another aspect of the present invention, the liquid filtration medium exhibits a virus log reduction value (LRV) greater than about 2. BRIEF DESCRIPTION OF THE DRAWINGS

[0121] Figure 1 Shows the difference in water permeability of selected nanofiber media before and after sterilization without pre-sterilization heat treatment (left) or with pre-sterilization heat treatment (right). The column labels with hatched lines show the percentage (%) decrease in water permeability after sterilization. The sterilization conditions were three autoclave cycles, each cycle at 135 °C for 60 minutes followed by a 15-minute drying time.

[0122] Figure 2A graph is a main effect plot depicting the percentage loss of water permeability. The points show the average percentage loss of each factor with (right) or without (left) pre-sterilization heat treatment.

[0123] Figure 3 Shows representative scanning electron microscope (SEM) images that display the morphology of a selected nanofiber medium before and after the sterilization process with (bottom) or without (top) pre-sterilization heat treatment.

[0124] Figure 4 Shows that the water permeability in a heat-treated asymmetric nylon-66 (N66) nanofiber mat remains unchanged after up to 12 autoclave cycles. Notably, the bubble point (BP) of the mat increases after the first 3 autoclave cycles (AC-3×) and remains consistent during additional autoclave cycles. Such mats were measured to have complete bacterial retention at a BP of 15 psi.

[0125] Figure 5 Presents representative scanning electron microscope (SEM) images that show that multiple autoclave cycles essentially do not change the morphology of the nanofibers in a heat-treated asymmetric nylon-66 medium (a) before sterilization, (b) after 3 autoclave cycles, (c) after 9 autoclave cycles, and (d) after 12 autoclave cycles. For all conditions, the fiber diameter is in the range of 95 - 105 nm. DETAILED DESCRIPTION

[0126] Overview

[0127] Typically, as part of the guidelines proposed by the International Council for Harmonization of Technical Requirements for Pharmaceuticals for Human Use (ICH), porous filtration media are sterilized prior to their aseptic use. Among all the methods available for the sterilization of porous filtration media for aseptic use, the most widely used is moist heat sterilization (in-situ steam sterilization) or autoclave sterilization in the form of saturated steam under pressure. Steam (moist heat) sterilization is non-toxic, inexpensive, rapidly microbicidal, sporicidal, and is the preferred sterilization method for aseptic use. However, such sterilization methods can have a negative impact on the filtration properties of the media that are critical for aseptic use. Liquid filtration membranes (e.g., pads) based on nanofibers that have not been subjected to steam sterilization can exhibit high liquid permeability and microbial retention. However, as demonstrated herein, the morphology of nanofiber-based liquid filtration membranes changes when subjected to wet-dry processing (including conventional steam sterilization methods), potentially resulting in a significant loss of water permeability. Therefore, it is necessary to design a porous filtration media that is robust and can withstand wet-dry processing at room temperature (as used in integrity testing), steam sterilization protocols, and dry sterilization protocols (e.g., gamma irradiation).

[0128] Without being bound by any particular theory, after a wetting agent (e.g., water or water and / or alcohol) invades the void space of a porous nanofiber pad by steam condensation during integrity testing (prior to the autoclave sterilization process) or during the autoclave process, collapse of the nanofiber pad occurs during the drying process. The autoclave sterilization cycle typically includes a post-sterilization drying step that occurs as the temperature cools. During such drying steps, the liquid level within the pores of the nanofiber pad rapidly drops, and the surface tension (Laplace force) of the dropping liquid level pulls the nanofibers together, resulting in a three-dimensional collapse mainly across the entire pad thickness (in the direction of liquid evaporation). The three-dimensional collapse compresses the pad and reduces the porosity, resulting in a loss of water permeability. In contrast to pads with larger pore sizes (where the nanofibers have larger diameters), nanofiber pads with smaller effective pore sizes (i.e., pads with finer nanofibers) experience a greater loss. Therefore, the impact of wet-dry processing (including moist heat sterilization) is particularly critical for filtration media containing finer nanofibers and for applications that require a higher retention guarantee.

[0129] In certain embodiments, the filter media and nanofiber mats disclosed herein resist structural collapse by influencing the mechanical modulus of the nanofibers, as defined by the polymer molecular weight, orientation, and crystallinity of the nanofibers. Such strategies for increasing the robustness of the nanofiber media and improving its ability to resist changes in water permeability after sterilization can include, for example, process modifications, material selection, and / or structural design. For example, in some embodiments, the effects after autoclaving can be mitigated by a combination of heat treatment, selection of an appropriate nanofiber polymer, and / or use of an asymmetric filter structure.

[0130] Accordingly, in certain aspects, the methods and filter media disclosed herein are particularly relevant in the biopharmaceutical industry. In certain aspects, the present disclosure provides methods that can be used with as-spun porous filter media to produce filter media that are robust enough to withstand wet-dry processing, including steam sterilization protocols (e.g., autoclaving), and in such cases reduce the processing time and cost of specific sterile filtration methods. In some aspects, the methods provided herein can be used to prepare nanofiber structures that resist collapse (e.g., maintain structural integrity) after moist heat sterilization.

[0131] Definition

[0132] For convenience, certain terms employed in the specification, examples, and appended claims are grouped together herein.

[0133] As used herein, unless the context clearly dictates otherwise, the singular forms “a,” “an,” and “the” are intended to include the plural forms as well.

[0134] The term “about” means within an acceptable error range of a particular value as determined by one of ordinary skill in the art. As used herein, “about” refers to an amount within 10% of a given value. In other words, these values include the recited value with a variation of 0 - 10% around that value (X ± 10%).

[0135] An "asymmetric" arrangement of nanofibers or an "asymmetric nanofiber mat" can mean that the average diameter of the nanofibers on one layer of the filter medium (e.g., a fiber mat or a support) is significantly different from the average diameter of the nanofibers on another layer of the filter medium; or is characterized by a fiber diameter that varies across the thickness of the entire nanofiber mat such that the average fiber diameter of one layer of the nanofiber mat is different from the other layers of the nanofiber mat. A gradient of fiber diameter across the thickness or cross-section of the entire nanofiber mat can also be described by a 'hourglass' structure or can also be characterized by having an asymmetry where the fiber diameter varies across the thickness of the entire nanofiber mat such that the average fiber diameter of one layer of the nanofiber mat is different from the other layers of the nanofiber mat. For example, the nanofiber mat can be in the form of a sheet having at least two layers (e.g., a composite filter medium such as a composite nanofiber mat), where one layer (i.e., the "top layer") is disposed on the top surface of the sheet, another layer (i.e., the "bottom layer") is disposed on the bottom surface of the sheet, and optionally one or more additional layers (i.e., the "intermediate layer") are disposed between the top layer and the bottom layer of the nanofiber mat, wherein the average fiber diameter of at least one of the layers containing nanofibers is different from the average fiber diameter of the nanofibers of another layer. An "asymmetric" filter medium (e.g., a nanofiber mat) also includes a structure where the average diameter of the nanofibers continuously increases from one surface of the medium to the other surface (sometimes referred to as a "tight" layer and an "open" layer, respectively). For example, a filter medium of the present invention including a nanofiber mat can be formed by simultaneously or sequentially forming a nonwoven structure of two or more different average fiber diameter nanofibers. By varying the relative rates at which different nanofibers are formed, an asymmetric structure can be prepared where the fiber diameter continuously varies from one surface to the opposite surface. The rate of change of the average fiber diameter across the thickness of the fiber support can be "slow" or relatively abrupt. It should be recognized that the term "layer" refers to a region of the medium where the average fiber diameter is relatively constant, but does not require a well-defined boundary. In an alternative embodiment, the asymmetric arrangement of nanofibers comprises nanofibers of the same average fiber diameter with different packing densities within the filter medium. For example, the layers of a composite filter medium (e.g., a composite nanofiber mat) can be prepared from nanofibers of substantially the same average fiber diameter, except that the percentage of the total volume of each layer occupied by the nanofibers can be different. See, for example, U.S. Patent No. 4,261,834 and U.S. Patent No. 4,629,563 (incorporated herein by reference in their entirety).

[0136] As used herein, the term "electrospinning" or "electrospun" refers to the electrospinning method of producing nanofibers from a polymer solution or suspension or melt by applying an electric potential to such a solution. Electrospinning methods (including suitable equipment for performing such electrospinning methods) for preparing electrospun nanofiber mats for filtration media are disclosed in International Publication Nos. WO2005 / 024101, WO 2006 / 131081, and WO 2008 / 106903 (each incorporated herein by reference in its entirety and each assigned to Elmarco of Liberec, Czech Republic). "Electroblowing" describes such an electrospinning method: wherein the polymer solution discharged from the spinning head is blown by blowing gas discharged from a gas injection nozzle to form a fibrous web of fibers.

[0137] As used interchangeably herein, the terms "filtrate" or "permeate" refer to a solution that has passed through a filter or membrane (e.g., the electrospun nanofiber composition used herein), as well as a solution that has passed through the filter or membrane.

[0138] The terms "including" and "includes" or variants thereof are inclusive in a manner similar to the term "comprising". The phrases "consisting essentially of" include embodiments containing the specified materials or steps, as well as those including materials and steps that do not substantially affect one or more of the basic and novel features of the embodiment.

[0139] As used herein, the term "log reduction value" or "LRV" refers to the common logarithm (base 10) of the ratio of the particle concentration in the feed to the particle concentration in the filtrate measured under standardized conditions.

[0140] The "bubble point test" provides a convenient way to measure the effective pore size. It is calculated by the following formula:

[0141]

[0142] where P is the bubble point pressure, γ is the surface tension of the probe fluid, r is the pore radius, and θ is the liquid-solid contact angle. The maximum pore size (or first bubble point) is recorded when gas flow through the sample is detected, and the average flow pore size corresponds to the pore size calculated at the pressure at the intersection of the wet curve and the semi-dry curve (which corresponds to the pore size that can account for 50% of the total gas flow).

[0143] Membrane manufacturers assign a nominal pore size rating to their membranes based on the rejection characteristics of commercially available membrane filters. When reported herein, the maximum pore size is determined by the bubble point test as set forth in ASTM Designation F316-03, “Standard Test Methods for Pore Size Characteristic of Membrane Filters by Bubble Point and Mean Flow Pore Test,” re-approved in 2011, and reported in nanometers (nm). Unless otherwise indicated, all BPs are measured using (Porous Materials Incorporated, Ithaca, NY) as the wetting fluid.

[0144] As used herein, the term “nanofiber” refers to a fiber having an average diameter or cross-section less than 1000 nm. In some embodiments, the number average cross-section of the nanofibers disclosed herein is less than 800 nm, less than 700 nm, less than 600 nm, less than 500 nm, less than 400 nm, or less than 200 nm. In certain embodiments, the number average diameter of the nanofibers disclosed herein is at least 5 nm, at least 20 nm, at least 30 nm, at least 40 nm, or at least 50 nm. In certain embodiments, the number average diameter of the nanofibers disclosed herein is between 5 nm and 500 nm, between 5 nm and 200 nm, between 5 nm and 100 nm, between 5 nm and 50 nm, between 50 nm and 500 nm, or between 50 nm and 200 nm. As used herein, the term “diameter” includes the largest cross-section of a non-circular shape.

[0145] As used herein, the term “nanofiber mat” refers to an assembly of a plurality of nanofibers such that the thickness of the mat is typically at least about 10 times greater than the diameter of an individual fiber in the mat. The nanofibers can be arranged randomly in the mat or aligned along one or more axes.

[0146] The term “nonwoven” means a web comprising a large number of randomly distributed fibers. The fibers can typically be bonded to each other or can be unbonded. The fibers can be staple fibers or continuous fibers. The fibers can include a single material or multiple materials, as a combination of different fibers or as a combination of similar fibers each composed of a different material.

[0147] As used herein, the term "permeability" refers to the rate at which a given volume of fluid passes through a given area of a filter medium across a given pressure drop. The common unit of permeability for each psi of pressure drop is liters per square meter per hour, abbreviated as LMH / psi. Such measurements are obtained by passing deionized water through a sample of the filter medium having a given area. Hydraulic (head pressure) or pneumatic (air pressure above the water) pressure is used to force the water through the sample.

[0148] The term "polymer (or polymeric)" refers to natural or synthetic organic compounds of relatively high molecular weight, the structure of which can be represented by repeating small units, monomers (e.g., nylon, polyethylene, rubber, cellulose). Natural biopolymers such as DNA and proteins are the basis of biological structure and function. Synthetic polymers are usually formed by addition polymerization or condensation polymerization of monomers. Nylon-6 is referred to herein as nylon-6 or N6. N66 can be interchangeably referred to as N6,6, N6 / 6 or N66.

[0149] The term "porosity" is used herein to represent the degree of empty space in a material and is the fraction of the volume of empty space relative to the total volume. The percentage porosity is calculated based on the following formula: % porosity = 100 × [1 - (basis weight / (pad thickness × polymer density))], where the basis weight is in g / m 2 , the polymer density is in g / m 3 , and the pad thickness is in m.

[0150] As used herein, the term "retentate" refers to the components or parts of a solution that are retained and do not pass through a filter or membrane (e.g., the electrospun nanofiber composition used herein), as well as the components or parts of a solution that have not passed through the filter or membrane. In the case of using a Stirred Cell, the liquid with solute retained on the upstream layer of the filter or membrane in the Stirred Cell is called the retentate. In the case of a TFF cassette or spiral device, the liquid flowing through the feed / retentate channel of the cassette or spiral device and returning from the device to the feed tank is called the retentate.

[0151] As used herein, a "semicrystalline" polymer refers to a polymer that, when present in the solid state, contains a multiphase structure (i.e., amorphous and crystalline parts with the same chemical composition but different physical properties). When heated, such polymers generally exhibit a glass transition temperature (T g ) in the amorphous phase and a melting temperature (T m ) in the crystalline phase; while an amorphous polymer only has T g and no T m .

[0152] The terms "variation" and "coefficient of variation" are used interchangeably herein and refer to a standardized measure of the dispersion of a probability distribution or a frequency distribution. It is typically expressed as a percentage and is defined as the ratio of the standard deviation to the mean.

[0153] Method for Producing a Sterilizable Nonwoven Nanofiber Structure

[0154] In some aspects, provided herein are methods for producing a liquid filtration medium comprising porous nonwoven polymer nanofibers that is compatible with moist heat sterilization. In certain embodiments, such methods include improving a "as-spun" nanofiber structure known in the art to a moist heat sterilizable form by using a unique combination of parameters including, for example, precision heat treatment, polymer selection, and / or specific nanofiber structure. Also, in some aspects, provided herein is a liquid filtration medium comprising a porous polymer nanofiber mat that is compatible with moist heat sterilization as disclosed herein (e.g., prepared by any of the methods disclosed herein).

[0155] In some embodiments, the liquid filtration medium comprising porous nonwoven polymer nanofibers (or at least the filtration medium comprising the nanofiber mat) is subjected to a heat treatment, wherein they are heated to at least the glass transition temperature (T g ) of the polymer nanofibers (e.g., nanofibers comprising one or more polymers) but not exceeding the melting temperature (T m ) of the polymer nanofibers. As described herein, such heat treatment is preferably carried out in a non-oxidizing atmosphere. For example, the heat treatment can be carried out in an anaerobic or inert atmosphere oven as known in the art. The transition temperatures and melting temperatures of certain exemplary polymers are provided in Table 1 and are known in the art. Where the glass transition temperature or melting temperature is provided as a range, for the purposes herein, the midpoint of the range should be considered the corresponding temperature.

[0156] Table 1: Glass transition temperatures and melting temperatures of exemplary polymers

[0157] Polymer <![CDATA[Transition temperature (T g )]]> <![CDATA[Melting temperature (T m )]]> Polyethylene (High Density) -125℃ 130° to 140°C Polyethylene (Low Density) -130℃ 85° to 125°C Polytetrafluoroethylene (PTFE) 120° to 130°C 320° to 330°C Polyethylene Terephthalate (PET) 70° to 80°C 245° to 265°C Nylon 6 40° to 60°C 210° to 220°C Nylon 6 / 6 50° to 60°C 240° to 265°C Nylon 6 / 10 45° to 55°C 215° to 220°C Polyphenylene Sulfide 85° to 95°C 275° to 290°C Polypropylene -20° to -5°C 165° to 175°C Polyvinylidene Fluoride -30° to -20°C 155° to 185°C

[0158] In certain embodiments, the liquid filtration medium (e.g., as-spun nanofiber mat) is heated to a temperature lower than the T m of the polymer nanofibers in the medium. In some embodiments, the liquid filtration medium is heated to a temperature lower than the T m of the polymer nanofibers by about 1 °C to about 80 °C. In certain embodiments, the liquid filtration medium is heated to a temperature lower than the T mA temperature that is about 1°C, about 5°C, about 10°C, about 15°C, about 20°C, about 25°C, about 30°C, about 35°C, about 40°C, about 45°C, about 50°C, about 55°C, about 60°C, about 65°C, about 70°C, about 75°C, or about 80°C lower. In some such embodiments, the liquid filtration medium can be heated to a temperature that is lower than the T of the polymer nanofibers by m about 5°C to about 15°C, about 10°C to about 20°C, about 15°C to about 25°C, about 20°C to about 30°C, about 25°C to about 35°C, about 30°C to about 40°C, about 35°C to about 45°C, about 40°C to about 50°C, about 45°C to about 55°C, about 50°C to about 60°C, about 55°C to about 65°C, about 60°C to about 70°C, about 65°C to about 75°C, or about 70°C to about 80°C. In some embodiments, the liquid filtration medium is heated to a temperature that is lower than the T of the polymer nanofibers by m about 56°C lower. In certain embodiments, the liquid filtration medium is heated to a temperature that is lower than the T of the polymer nanofibers by m about 75°C lower.

[0159] In some embodiments, the liquid filtration medium (or at least the filtration medium comprising the nanofiber mat) comprising polymer nanofibers disclosed herein is heated to a temperature higher than the T of the polymer nanofibers in the medium. g In certain embodiments, the liquid filtration medium is heated to a temperature that is about 100°C to about 200°C higher than the T of the polymer nanofibers. In some embodiments, the liquid filtration medium is heated to a temperature that is higher than the T of the polymer nanofibers by g about 100°C, about 105°C, about 110°C, about 115°C, about 120°C, about 125°C, about 130°C, about 135°C, about 140°C, about 145°C, about 150°C, about 155°C, about 160°C, about 165°C, about 170°C, about 175°C, about 180°C, about 185°C, about 190°C, about 195°C, or about 200°C. In some such embodiments, the liquid filtration medium is heated to a temperature that is higher than the T of the polymer nanofibers by g about 100°C, about 105°C, about 110°C, about 115°C, about 120°C, about 125°C, about 130°C, about 135°C, about 140°C, about 145°C, about 150°C, about 155°C, about 160°C, about 165°C, about 170°C, about 175°C, about 180°C, about 185°C, about 190°C, about 195°C, or about 200°C. In some such embodiments, the liquid filtration medium is heated to a temperature that is higher than the T of the polymer nanofibers by gA temperature of from about 100°C to about 110°C, from about 105°C to about 115°C, from about 110°C to about 120°C, from about 115°C to about 125°C, from about 120°C to about 130°C, from about 125°C to about 135°C, from about 130°C to about 140°C, from about 135°C to about 145°C, from about 140°C to about 150°C, from about 145°C to about 155°C, from about 150°C to about 160°C, from about 155°C to about 165°C, from about 160°C to about 170°C, from about 165°C to about 175°C, from about 170°C to about 180°C, from about 175°C to about 185°C, from about 180°C to about 190°C, from about 185°C to about 195°C, or from about 190°C to about 200°C. In some embodiments, the liquid filtration medium (e.g., polymeric nanofibers) is heated to a temperature higher than the T of the polymeric nanofibers g About 140°C to about 158°C higher.

[0160] In some embodiments, the liquid filtration medium comprises nylon 6 / 6 polymeric nanofibers and is heated to a temperature of from about 190°C to about 210°C (e.g., for at least 1 hour). In some embodiments, the liquid filtration medium comprising nylon 6 / 6 polymeric nanofibers is heated to a temperature of about 208°C (e.g., for at least 1 hour).

[0161] In certain embodiments, the nanofiber medium is heat treated for at least 1 hour, at least 2 hours, at least 3 hours, at least 4 hours, at least 5 hours, at least 6 hours, at least 7 hours, at least 8 hours, at least 9 hours, at least 10 hours, at least 11 hours, at least 12 hours, at least 13 hours, at least 14 hours, at least 15 hours, at least 16 hours, at least 17 hours, at least 18 hours, at least 19 hours, at least 20 hours, at least 21 hours, at least 22 hours, at least 23 hours, or at least 24 hours. In some embodiments, the nanofiber medium is heat treated for no more than 2 days, no more than 36 hours, no more than 24 hours, no more than 23 hours, no more than 22 hours, no more than 21 hours, no more than 20 hours, no more than 19 hours, no more than 18 hours, no more than 17 hours, no more than 16 hours, no more than 15 hours, no more than 14 hours, no more than 13 hours, no more than 12 hours, no more than 11 hours, no more than 10 hours, no more than 9 hours, no more than 8 hours, no more than 7 hours, no more than 6 hours, no more than 5 hours, or no more than 1 hour. In certain embodiments, the nanofiber medium is heat treated for about 1 hour, about 2 hours, about 3 hours, about 1 hour, about 5 hours, about 6 hours, about 7 hours, about 8 hours, about 9 hours, about 10 hours, about 11 hours, about 12 hours, about 13 hours, about 14 hours, about 15 hours, about 16 hours, about 17 hours, about 18 hours, about 19 hours, about 20 hours, about 21 hours, about 22 hours, about 23 hours, or about 24 hours. In certain embodiments, the nanofiber medium is heat treated for 1 hour to 21 hours, 1 hour to 18 hours, 1 hour to 12 hours, or 1 hour to 6 hours.

[0162] In some embodiments, the liquid filtration media disclosed herein are composed of an asymmetric nanofiber mat that exhibits fiber diameters that vary across the thickness of the nanofiber mat such that the average fiber diameter of one layer of the nanofiber mat is different from other layers of the nanofiber mat. Alternatively, the asymmetric nanofiber structure can be a composite multi-layer structure where the average fiber diameter of one or more layers of one layer is different from one or more layers of another layer. The layers on two “outer” surfaces have an average fiber diameter different from one or more “inner” layers. The “outer” surface is defined as the surface that contacts the spinning substrate or faces the electrode from which the nanofibers are generated. Everything between the two outer surfaces is defined as the inner layer. In certain embodiments, the ratio of the average fiber diameter of one layer of the asymmetric nanofiber mat to another layer is at least 1 to 2. In some embodiments, the ratio of the average fiber diameters is at least 1 to 1.75, 1 to 1.5, 1 to 1.25, or 1 to 1.15. Thus, in certain embodiments, the ratio of the average fiber diameter of one layer of the asymmetric nanofiber mat to another layer is at least 1.15.

[0163] The asymmetric nanofiber structure disclosed herein can have an average fiber diameter of about 5 nm to about 1000 nm on at least one layer. Thus, the asymmetric nanofiber structure can have a nanofiber diameter of about 1000 nm, 950 nm, 900 nm, 850 nm, 800 nm, 750 nm, 700 nm, 650 nm, 600 nm, 550 nm, 500 nm, 450 nm, 400 nm, 350 nm, 300 nm, 250 nm, 200 nm, 150 nm, 100 nm, 90 nm, 80 nm, 70 nm, 60 nm, 50 nm, 40 nm, 30 nm, or 20 nm on at least one layer. In some embodiments, the average nanofiber diameter on at least one layer is 5 nm to 20 nm, 15 nm to 25 nm, 20 nm to 30 nm, 25 nm to 35 nm, 30 nm to 40 nm, 35 nm to 45 nm, 40 nm to 50 nm, 45 nm to 55 nm, 50 nm to 60 nm, 55 nm to 65 nm, 60 nm to 70 nm, 65 nm to 75 nm, 70 nm to 80 nm, 75 nm to 85 nm, 80 nm to 90 nm, 85 nm to 95 nm, 90 nm to 100 nm, 95 nm to 105 nm, 100 nm to 110 nm, 105 nm to 115 nm, 110 nm to 120 nm, 115 nm to 125 nm, 120 nm to 130 nm, 125 nm to 135 nm, 130 nm to 140 nm, 135 nm to 145 nm, 140 nm to 150 nm, 145 nm to 155 nm, 150 nm to 160 nm, 155 nm to 165 nm, 160 nm to 170 nm, 165 nm to 175 nm, 170 nm to 180 nm, 175 nm to 185 nm, 180 nm to 190 nm, 185 nm to 195 nm, 190 nm to 200 nm, 195 nm to 205 nm, 200 nm to 210 nm, 205 nm to 215 nm, 210 nm to 220 nm, 215 nm to 225 nm, 220 nm to 230 nm, 225 nm to 235 nm, 230 nm to 240 nm, 235 nm to 245 nm, 240 nm to 250 nm, 245 nm to 255 nm, 250 nm to 260 nm, 255 nm to 265 nm, 260 nm to 270 nm, 265 nm to 275 nm, 270 nm to 280 nm, 275 nm to 285 nm, 280 nm to 290 nm, 285 nm to 295 nm, 290 nm to 300 nm, 295 nm to 305 nm, 300 nm to 310 nm, 305 nm to 315 nm, 310 nm to 320 nm, 315 nm to 325 nm, 320 nm to 330 nm, 325 nm to 335 nm, 330 nm to 340 nm,335 nm to 345 nm, 340 nm to 350 nm, 345 nm to 355 nm, 350 nm to 360 nm, 355 nm to 365 nm, 360 nm to 370 nm, 365 nm to 375 nm, 370 nm to 380 nm, 375 nm to 385 nm, 380 nm to 390 nm, 385 nm to 395 nm, 390 nm to 400 nm, 395 nm to 405 nm, 400 nm to 410 nm, 405 nm to 415 nm, 410 nm to 420 nm, 415 nm to 425 nm, 420 nm to 430 nm, 425 nm to 435 nm, 430 nm to 440 nm, 435 nm to 445 nm, 440 nm to 450 nm, 445 nm to 455 nm, 450 nm to 460 nm, 455 nm to 465 nm, 460 nm to 470 nm, 465 nm to 475 nm, 470 nm to 480 nm, 475 nm to 485 nm, 480 nm to 490 nm, 485 nm to 495 nm, 490 nm to 500 nm, 500 nm to 550 nm, 525 nm to 575 nm, 550 nm to 600 nm, 575 nm to 625 nm, 600 nm to 650 nm, 625 nm to 675 nm, 650 nm to 700 nm, 675 nm to 725 nm, 700 nm to 750 nm, 725 nm to 775 nm, 750 nm to 800 nm, 775 nm to 825 nm, 800 nm to 850 nm, 825 nm to 875 nm, 850 nm to 900 nm, 925 nm to 975 nm, or 950 nm to 1000 nm. In still further embodiments, the average fiber diameter on at least one layer of the asymmetric nanofiber structure is less than about 5 nm.

[0164] In some embodiments, the nanofibers and / or the solution (from which the nanofibers are spun (or blown)) comprise a polymer or polymer blend. For example, in some embodiments, the polymer or polymer blend is a semi-crystalline polymer. In some embodiments, the polymer or polymer blend is nylon-6, nylon-6,6, nylon 6,6-6,10, a nylon-6 copolymer, a nylon-6,6 copolymer, a nylon 6,6-6,10 copolymer, and any mixture thereof. In certain embodiments, the polymer is nylon 6 or nylon 6,6. In some embodiments, the polymer is nylon 6,6.

[0165] In some embodiments, the polymer or polymer blend is selected from thermoplastic polymers, thermosetting polymers, nylons, polyimides, aliphatic polyamides, aromatic polyamides, polysulfones, cellulose acetate, polyethersulfone, polyurethanes, poly(ureaurethane), polybenzimidazoles, polyetherimides, polyacrylonitrile, poly(ethylene terephthalate), polypropylene, polyaniline, poly(ethylene oxide), poly(ethylene naphthalate), poly(butylene terephthalate), styrene-butadiene rubber, polystyrene, poly(vinyl chloride), poly(vinyl alcohol), poly(vinylidene fluoride), poly(vinyl butene), and their copolymers, derivative compounds, or blends. As used herein, the term "nylon" may include nylon-6, nylon-6,6, nylon 6,6-6,10, and their copolymers, derivative compounds, blends, and combinations.

[0166] In some embodiments, the polymer or polymer blend is selected from nylon-6, nylon-4,6, nylon-6,6, nylon 6,6-6,10, polyaramide, polyurethane (PU), polybenzimidazole, polycarbonate, polyacrylonitrile, polyvinyl alcohol, polylactic acid (PLA), ethylene-vinyl acetate copolymer (PEVA), PEVA / PLA, polymethyl methacrylate (PMMA), PMMA / tetrahydroperfluorooctyl acrylate (TAN), poly(ethylene oxide) (PEO), collagen-PEO, polystyrene (PS), polyaniline (PANI) / PEO, PANI / PS, polyvinylcarbazole, poly(ethylene terephthalate) (PET), polyacrylic acid-poly(pyrenylmethanol) (PAA-PM), polyamide (PA), silk / PEO, polyvinylphenol (PVP), poly(vinyl chloride) (PVC), cellulose acetate (CA), PAA-PM / PU, polyvinyl alcohol (PVA) / silica, polyacrylamide (PAAm), lactic acid-glycolic acid copolymer (PLGA), polycaprolactone (PCL), poly(2-hydroxyethyl methacrylate) (HEMA), poly(vinylidene fluoride) (PVDF), PVDF / PMMA, polyetherimide (PEI), polyethylene glycol (PEG), poly(ferrocenedimethylsilane) (PFDMS), nylon 6 / montmorillonite (Mt), ethylene-vinyl alcohol copolymer, polyacrylonitrile (PAN) / TiO2, polycaprolactone (PCL) / metal, polyvinylpyrrolidone, poly(m-phenylene isophthalamide), polyethylene (PE), polypropylene (PP), nylon-12, poly(ethylene terephthalate) (PET), poly(ethylene naphthalate) (PEN), polyethersulfone (PES), polyvinyl butyral (PVB), PET / PEN, and their copolymers, derivative compounds, or blends.

[0167] The liquid filtration medium disclosed herein can be prepared by electrospinning or electroblowing a polymer solution or melt to produce a porous nonwoven polymer nanofiber structure (such as a nanofiber mat). Preferably, the liquid filtration medium is prepared by electrospinning, and the resulting porous nonwoven polymer nanofiber mat comprises one or more polymers. Electrospinning is a method of producing nanofibers from a mixture of polymers (e.g., a polymer solution, suspension, or polymer melt). The method involves applying an electric potential to such a polymer solution or polymer melt. Certain details of electrospinning methods for preparing electrospun nanofiber mats or membranes (including suitable equipment for performing the electrospinning method) are described in International Patent Application Publications WO2005 / 024101, WO2006 / 131081, and WO2008 / 106903, each of which is incorporated herein by reference.

[0168] During the electrospinning method, the fibers are produced by a spinning electrode by applying a high voltage to the electrode and the polymer solution, where the fibers are charged or spun towards a collecting electrode and collected as a highly porous nonwoven mat on a substrate between the electrodes. In the case where the polymer solution discharged from the spinning head is blown onto the substrate with a blowing gas discharged from a gas injection nozzle, it is called electroblowing.

[0169] The disclosed nanofiber mat may have a thickness of from about 1 μm to about 500 μm. In some embodiments, the nanofiber mat has a thickness of at least 5 μm, 10 μm, 15 μm, 20 μm, 25 μm, 30 μm, 35 μm, 40 μm, 45 μm, 50 μm, 55 μm, 60 μm, 65 μm, 70 μm, 75 μm, 80 μm, 85 μm, 90 μm, 95 μm, 100 μm, 105 μm, 110 μm, 115 μm, 120 μm, 125 μm, 130 μm, 135 μm, 140 μm, 145 μm, 150 μm, 155 μm, 160 μm, 165 μm, 170 μm, 175 μm, 180 μm, 185 μm, 190 μm, 195 μm, 200 μm, 205 μm, 210 μm, 215 μm, 220 μm, 225 μm, 230 μm, 235 μm, 240 μm, 245 μm, 250 μm, 255 μm, 260 μm, 265 μm, 270 μm, 275 μm, 280 μm, 285 μm, 290 μm, 295 μm, 300 μm, 305 μm, 310 μm, 315 μm, 320 μm, 325 μm, 330 μm, 335 μm, 340 μm, 345 μm, 350 μm, 355 μm, 360 μm, 365 μm, 370 μm, 375 μm, 380 μm, 385 μm, 390 μm, 395 μm, 400 μm, 405 μm, 410 μm, 415 μm, 420 μm, 425 μm, 430 μm, 435 μm, 440 μm, 445 μm, 450 μm, 455 μm, 460 μm, 465 μm, 470 μm, 475 μm, 480 μm, 485 μm, 490 μm, 495 μm, or 500 μm.

[0170] In some embodiments, the average fiber diameter of the nanofiber structure produced by the liquid filtration media disclosed herein (e.g., nanofiber mats) is from about 5 nm to about 1000 nm. The fiber diameter can have a wide distribution within the range of 16 - 36% CoV. In some such embodiments, the average nanofiber diameter does not exceed 1000 nm, 950 nm, 900 nm, 850 nm, 800 nm, 750 nm, 700 nm, 650 nm, 600 nm, 550 nm, 500 nm, 450 nm, 400 nm, 350 nm, 300 nm, 250 nm, 200 nm, 150 nm, 100 nm, 90 nm, 80 nm, 70 nm, 60 nm, 50 nm, 40 nm, 30 nm, or 20 nm. In some embodiments, the average nanofiber diameter is from 5 nm to 20 nm, 15 nm to 25 nm, 20 nm to 30 nm, 25 nm to 35 nm, 30 nm to 40 nm, 35 nm to 45 nm, 40 nm to 50 nm, 45 nm to 55 nm, 50 nm to 60 nm, 55 nm to 65 nm, 60 nm to 70 nm, 65 nm to 75 nm, 70 nm to 80 nm, 75 nm to 85 nm, 80 nm to 90 nm, 85 nm to 95 nm, 90 nm to 100 nm, 95 nm to 105 nm, 100 nm to 110 nm, 105 nm to 115 nm, 110 nm to 120 nm, 115 nm to 125 nm, 120 nm to 130 nm, 125 nm to 135 nm, 130 nm to 140 nm, 135 nm to 145 nm, 140 nm to 150 nm, 145 nm to 155 nm, 150 nm to 160 nm, 155 nm to 165 nm, 160 nm to 170 nm, 165 nm to 175 nm, 170 nm to 180 nm, 175 nm to 185 nm, 180 nm to 190 nm, 185 nm to 195 nm, 190 nm to 200 nm, 195 nm to 205 nm, 200 nm to 210 nm, 205 nm to 215 nm, 210 nm to 220 nm, 215 nm to 225 nm, 220 nm to 230 nm, 225 nm to 235 nm, 230 nm to 240 nm, 235 nm to 245 nm, 240 nm to 250 nm, 245 nm to 255 nm, 250 nm to 260 nm, 255 nm to 265 nm, 260 nm to 270 nm, 265 nm to 275 nm, 270 nm to 280 nm, 275 nm to 285 nm, 280 nm to 290 nm, 285 nm to 295 nm, 290 nm to 300 nm, 295 nm to 305 nm, 300 nm to 310 nm, 305 nm to 315 nm, 310 nm to 320 nm, 315 nm to 325 nm,from 320 nm to 330 nm, from 325 nm to 335 nm, from 330 nm to 340 nm, from 335 nm to 345 nm, from 340 nm to 350 nm, from 345 nm to 355 nm, from 350 nm to 360 nm, from 355 nm to 365 nm, from 360 nm to 370 nm, from 365 nm to 375 nm, from 370 nm to 380 nm, from 375 nm to 385 nm, from 380 nm to 390 nm, from 385 nm to 395 nm, from 390 nm to 400 nm, from 395 nm to 405 nm, from 400 nm to 410 nm, from 405 nm to 415 nm, from 410 nm to 420 nm, from 415 nm to 425 nm, from 420 nm to 430 nm, from 425 nm to 435 nm, from 430 nm to 440 nm, from 435 nm to 445 nm, from 440 nm to 450 nm, from 445 nm to 455 nm, from 450 nm to 460 nm, from 455 nm to 465 nm, from 460 nm to 470 nm, from 465 nm to 475 nm, from 470 nm to 480 nm, from 475 nm to 485 nm, from 480 nm to 490 nm, from 485 nm to 495 nm, from 490 nm to 500 nm, from 500 nm to 550 nm, from 525 nm to 575 nm, from 550 nm to 600 nm, from 575 nm to 625 nm, from 600 nm to 650 nm, from 625 nm to 675 nm, from 650 nm to 700 nm, from 675 nm to 725 nm, from 700 nm to 750 nm, from 725 nm to 775 nm, from 750 nm to 800 nm, from 775 nm to 825 nm, from 800 nm to 850 nm, from 825 nm to 875 nm, from 850 nm to 900 nm, from 925 nm to 975 nm, or from 950 nm to 1000 nm. In still further embodiments, the average fiber diameter is less than about 5 nm.

[0171] Two desirable characteristics of liquid filtration membranes are high permeability and reliable rejection. In certain embodiments, the electrospun nanofiber media disclosed herein are highly porous polymeric materials where the “pore” diameter scales linearly with the fiber diameter while the porosity is relatively independent of the fiber diameter. In certain embodiments, the porosity of the electrospun nanofiber media falls within the range of about 70% to 95% (e.g., about 75% to 95%, about 80% to 95%). In some embodiments, the permeability of the electrospun nanofiber media provided herein is significantly higher than that of an immersion cast membrane having a similar thickness and pore size rating.

[0172] In some embodiments, the liquid filtration media provided herein (e.g., nanofiber mats) have a bubble point (BP) obtained using a wetting fluid (i.e., determined by a bubble point test as set forth in ASTM designation F316-03, “Standard Test Method for Pore Size Characteristics of Membrane Filters by Bubble Point and Mean Flow Pore Test,” reapproved in 2011) of 5 psi to 150 psi (e.g., 10 psi, 11 psi, 12 psi, 13 psi, 14 psi, 15 psi, 16 psi, 17 psi, 18 psi, 19 psi, 20 psi, 21 psi, 22 psi, 23 psi, 24 psi, 25 psi, 50 psi, 75 psi, 100 psi, 125 psi, or 150 psi).

[0173] In some embodiments, the nanofiber structure (e.g., nanofiber mat) has a maximum pore size of no more than 500 nm, 450 nm, 400 nm, 350 nm, 300 nm, 250 nm, 200 nm, 150 nm, 100 nm, or 50 nm as determined by the bubble point test. In some embodiments, the produced nanofiber structure (e.g., nanofiber mat) has a pore size of 5 nm to 20 nm, 15 nm to 25 nm, 20 nm to 30 nm, 25 nm to 35 nm, 30 nm to 40 nm, 35 nm to 45 nm, 40 nm to 50 nm, 45 nm to 55 nm, 50 nm to 60 nm, 55 nm to 65 nm, 60 nm to 70 nm, 65 nm to 75 nm, 70 nm to 80 nm, 75 nm to 85 nm, 80 nm to 90 nm, 85 nm to 95 nm, 90 nm to 100 nm, 95 nm to 105 nm, 100 nm to 110 nm, 105 nm to 115 nm, 110 nm to 120 nm, 115 nm to 125 nm, 120 nm to 130 nm, 125 nm to 135 nm, 130 nm to 140 nm, 135 nm to 145 nm, 140 nm to 150 nm, 145 nm to 155 nm, 150 nm to 160 nm, 155 nm to 165 nm, 160 nm to 170 nm, 165 nm to 175 nm, 170 nm to 180 nm, 175 nm to 185 nm, 180 nm to 190 nm, 185 nm to 195 nm, 190 nm to 200 nm, 195 nm to 205 nm, 200 nm to 210 nm, 205 nm to 215 nm, 210 nm to 220 nm, 215 nm to 225 nm, 220 nm to 230 nm, 225 nm to 235 nm, 230 nm to 240 nm, 235 nm to 245 nm, 240 nm to 250 nm, 245 nm to 255 nm, 250 nm to 260 nm, 255 nm to 265 nm, 260 nm to 270 nm, 265 nm to 275 nm, 270 nm to 280 nm, 275 nm to 285 nm, 280 nm to 290 nm, 285 nm to 295 nm, 290 nm to 300 nm, 295 nm to 305 nm, 300 nm to 310 nm, 305 nm to 315 nm, 310 nm to 320 nm, 315 nm to 325 nm, 320 nm to 330 nm, 325 nm to 335 nm, 330 nm to 340 nm, 335 nm to 345 nm, 340 nm to 350 nm, 345 nm to 355 nm, 350 nm to 360 nm, 355 nm to 365 nm, 360 nm to 370 nm, 365 nm to 375 nm, 370 nm to 380 nm, 375 nm to 385 nm,The maximum pore size is from 380 nm to 390 nm, from 385 nm to 395 nm, from 390 nm to 400 nm, from 395 nm to 405 nm, from 400 nm to 410 nm, from 405 nm to 415 nm, from 410 nm to 420 nm, from 415 nm to 425 nm, from 420 nm to 430 nm, from 425 nm to 435 nm, from 430 nm to 440 nm, from 435 nm to 445 nm, from 440 nm to 450 nm, from 445 nm to 455 nm, from 450 nm to 460 nm, from 455 nm to 465 nm, from 460 nm to 470 nm, from 465 nm to 475 nm, from 470 nm to 480 nm, from 475 nm to 485 nm, from 480 nm to 490 nm, from 485 nm to 495 nm, or from 490 nm to 500 nm.

[0174] In some embodiments, the porosity of the liquid filtration medium (e.g., nanofiber mat) is at least 70%, at least 75%, at least 80%, at least 85%, at least 90%, or at least 95%. In some embodiments, the porosity is from 70% to 95%, from 75% to 95%, from 80% to 95%, from 85% to 95%, or from 90% to 95%.

[0175] In some embodiments, the variation in fiber diameter of the liquid filtration medium (e.g., nanofiber mat) is no more than 30%, no more than 29%, no more than 28%, no more than 27%, no more than 26%, no more than 25%, no more than 24%, no more than 23%, no more than 22%, no more than 21%, no more than 20%, no more than 19%, no more than 18%, no more than 17%.

[0176] In some embodiments, the liquid filtration medium (e.g., nanofiber mat) has a permeability of at least 10 LMH / psi to 20000 LMH / psi. In some embodiments, the liquid filtration medium (e.g., nanofiber mat) has a permeability of at least 10 LMH / psi to 20000 LMH / psi. In some such embodiments, the liquid filtration medium has a permeability of at least 6000 LMH / psi, at least 5750 LMH / psi, at least 5500 LMH / psi, at least 5250 LMH / psi, at least 5000 LMH / psi, at least 4750 LMH / psi, at least 4500 LMH / psi, at least 4250 LMH / psi, at least 4000 LMH / psi, at least 3750 LMH / psi, at least 3500 LMH / psi, at least 3250 LMH / psi, at least 3000 LMH / psi, at least 2750 LMH / psi, at least 2500 LMH / psi, at least 2250 LMH / psi, at least 2000 LMH / psi, at least 1750 LMH / psi, at least 1500 LMH / psi, at least 1250 LMH / psi, at least 1000 LMH / psi, at least 975 LMH / psi, at least 950 LMH / psi, at least 925 LMH / psi, at least 900 LMH / psi, at least 875 LMH / psi, at least 850 LMH / psi, at least 825 LMH / psi, at least 800 LMH / psi, at least 775 LMH / psi, at least 750 LMH / psi, at least 725 LMH / psi, at least 700 LMH / psi, at least 675 LMH / psi, at least 650 LMH / psi, at least 625 LMH / psi, at least 600 LMH / psi, at least 575 LMH / psi, at least 550 LMH / psi, at least 525 LMH / psi, at least 500 LMH / psi, at least 475 LMH / psi, at least 450 LMH / psi, at least 425 LMH / psi, at least 400 LMH / psi, at least 375 LMH / psi, at least 350 LMH / psi, at least 325 LMH / psi, or at least 300 LMH / psi or at least 100 LMH / psi, or at least 10 LMH / psi. In some embodiments, in some embodiments, the liquid filtration medium has a permeability of at least 300 LMH / psi to at least 400 LMH / psi, at least 450 LMH / psi to at least 550 LMH / psi, at least 400 LMH / psi to at least 500 LMH / psi, at least 550 LMH / psi to at least 650 LMH / psi, at least 700 LMH / psi to at least 800 LMH / psi,At least 750 LMH / psi to at least 850 LMH / psi, at least 900 LMH / psi to at least 1000 LMH / psi, at least 850 LMH / psi to at least 950 LMH / psi, at least 1000 LMH / psi to at least 1050 LMH / psi, at least 1100 LMH / psi to at least 1200 LMH / psi, at least 1150 LMH / psi to at least 1250 LMH / psi, at least 1200 LMH / psi to at least 1300 LMH / psi, at least 1250 LMH / psi to at least 1350 LMH / psi, at least 1300 LMH / psi to at least 1400 LMH / psi, at least 1350 LMH / psi to at least 1450 LMH / psi, at least 1400 LMH / psi to at least 1500 LMH / psi, at least 1450 LMH / psi to at least 1550 LMH / psi, at least 1500 LMH / psi to at least 1600 LMH / psi, at least 1550 LMH / psi to at least 1650 LMH / psi, at least 1600 LMH / psi to at least 1700 LMH / psi, at least 1650 LMH / psi to at least 1750 LMH / psi, at least 1700 LMH / psi to at least 1800 LMH / psi, at least 1750 LMH / psi to at least 1850 LMH / psi, at least 1900 LMH / psi to at least 2000 LMH / psi, at least 1950 LMH / psi to at least 2050 LMH / psi, at least 2100 LMH / psi to at least 2200 LMH / psi, at least 2150 LMH / psi to at least 2250 LMH / psi, at least 2200 LMH / psi to at least 2300 LMH / psi, at least 2250 LMH / psi to at least 2350 LMH / psi, at least 2300 LMH / psi to at least 2400 LMH / psi, at least 2350 LMH / psi to at least 2450 LMH / psi, at least 2400 LMH / psi to at least 2500 LMH / psi, at least 2450 LMH / psi to at least 2550 LMH / psi, at least 2500 LMH / psi to at least 2600 LMH / psi, at least 2550 LMH / psi to at least 2650 LMH / psi, at least 2600 LMH / psi to at least 2700 LMH / psi, at least 2650 LMH / psi to at least 2750 LMH / psi, at least 2700 LMH / psi to at least 2800 LMH / psi, at least 2750 LMH / psi to at least 2850 LMH / psi, at least 2900 LMH / psi to at least 3000 LMH / psi,from at least 2950 LMH / psi to at least 3050 LMH / psi, from at least 3100 LMH / psi to at least 3200 LMH / psi, from at least 3150 LMH / psi to at least 3250 LMH / psi, from at least 3200 LMH / psi to at least 3300 LMH / psi, from at least 3250 LMH / psi to at least 3350 LMH / psi, from at least 3300 LMH / psi to at least 3400 LMH / psi, from at least 3350 LMH / psi to at least 3450 LMH / psi, from at least 3400 LMH / psi to at least 3500 LMH / psi, from at least 3450 LMH / psi to at least 3550 LMH / psi, from at least 3500 LMH / psi to at least 3600 LMH / psi, from at least 3550 LMH / psi to at least 3650 LMH / psi, from at least 3600 LMH / psi to at least 3700 LMH / psi, from at least 3650 LMH / psi to at least 3750 LMH / psi, from at least 3700 LMH / psi to at least 3800 LMH / psi, from at least 3750 LMH / psi to at least 3850 LMH / psi, from at least 3900 LMH / psi to at least 4000 LMH / psi, from at least 3950 LMH / psi to at least 4050 LMH / psi, from at least 4100 LMH / psi to at least 4200 LMH / psi, from at least 4150 LMH / psi to at least 4250 LMH / psi, from at least 4200 LMH / psi to at least 4300 LMH / psi, from at least 4250 LMH / psi to at least 4350 LMH / psi, from at least 4300 LMH / psi to at least 4400 LMH / psi, from at least 4350 LMH / psi to at least 4450 LMH / psi, from at least 4400 LMH / psi to at least 4500 LMH / psi, from at least 4450 LMH / psi to at least 4550 LMH / psi, from at least 4500 LMH / psi to at least 4600 LMH / psi, from at least 4550 LMH / psi to at least 4650 LMH / psi, from at least 4600 LMH / psi to at least 4700 LMH / psi, from at least 4650 LMH / psi to at least 4750 LMH / psi, from at least 4700 LMH / psi to at least 4800 LMH / psi, from at least 4750 LMH / psi to at least 4850 LMH / psi, from at least 4900 LMH / psi to at least 5000 LMH / psi, from at least 4950 LMH / psi to at least 5050 LMH / psi, from at least 5100 LMH / psi to at least 5200 LMH / psi, from at least 5150 LMH / psi to at least 5250 LMH / psiFrom at least 5200 LMH / psi to at least 5300 LMH / psi, from at least 5250 LMH / psi to at least 5350 LMH / psi, from at least 5300 LMH / psi to at least 5400 LMH / psi, from at least 5350 LMH / psi to at least 5450 LMH / psi, from at least 5400 LMH / psi to at least 5500 LMH / psi, from at least 5450 LMH / psi to at least 5550 LMH / psi, from at least 5500 LMH / psi to at least 5600 LMH / psi, from at least 5550 LMH / psi to at least 5650 LMH / psi, from at least 5600 LMH / psi to at least 5700 LMH / psi, from at least 5650 LMH / psi to at least 5750 LMH / psi, from at least 5700 LMH / psi to at least 5800 LMH / psi, from at least 5750 LMH / psi to at least 5850 LMH / psi, from at least 5900 LMH / psi to at least 6000 LMH / psi, or at least 5950 LMH / psi, at least 6000 LMH / psi, at least 7000 LMH / psi, at least 8000 LMH / psi, at least 9000 LMH / psi, at least 10000 LMH / psi, at least 12000 LMH / psi, at least 14000 LMH / psi, at least 16000 LMH / psi, at least 18000 LMH / psi, at least 20000 LMH / psi.

[0177] The quantitative measure of microbial retention by a filter membrane is usually expressed as a log reduction value (sometimes referred to as the log retention value or LRV). The LRV is the logarithm of the ratio of the particle concentration in the challenge solution to the particle concentration in the filter effluent: LRV = Log{[CFU] 挑战 / [CFU] 流出物}.

[0178] In cases where the filter retains all microorganisms under the test conditions, the LRV reported is usually greater than the value obtained when a single microorganism passes through the filter. For example, at a challenge particle concentration of 4.77×10 7 CFU / cm 2 for a device with an effective filtration area of 13.8 cm 2 the maximum measurable LRV is 8.22. When no particles pass through the filter, the LRV is reported as greater than 8.22.

[0179] The pore size rating of a membrane is an indication that the membrane has successfully passed the relevant standardized bacterial challenge test. The most common pore size rating is 0.22 μm, which is designated for membranes that pass the standard test method for determining the bacterial retention of membrane filters utilized for liquid filtration (Standard Test Method for Determining Bacterial Retention of Membrane Filters Utilized For Liquid Filtration) (ASTM F838-83 test), and can be verified to produce a sterile effluent after a challenge with ≥10 7 CFU / cm 2 challenge with Brevundimonas diminuta. Brevundimonas diminuta (ATTC#19146) (formerly known as Pseudomonas diminuta) is an aerobic Gram-negative bacterium (bacillus). Due to its small size, Brevundimonas diminuta is the standard microorganism used to verify the sterilization of membrane filters, etc. Thus, the liquid filtration media disclosed herein has a log reduction value (LRV) of at least 1 of Brevundimonas diminuta as measured according to ASTM F838-83. Alternatively, the liquid filtration media disclosed herein has a log reduction value (LRV) of at least 8 of Brevundimonas diminuta as measured according to ASTM F838-83. Preferably, the liquid filtration media exhibits complete retention of microorganisms, e.g., Brevundimonas diminuta as measured according to ASTM F838-83. In certain embodiments, the liquid filtration media is challenged with virus particles and exhibits a virus log reduction value (LRV) of greater than about 6. In certain other embodiments, the liquid filtration media disclosed herein is capable of purifying biomaterials of interest, including virus-like particles, proteins, and conjugated polysaccharides such as those present in vaccines. Generally, the biomaterials have a molecular weight of about or greater than 500 kDa. Thus, the liquid filtration media disclosed herein can exhibit retention of standardized macromolecules and / or particles of a given size (such as dextran used in a standard method such as “dextran sieving”).

[0180] In some embodiments, a liquid filtration medium (e.g., a porous nonwoven nanofiber-containing liquid filtration medium) further comprises a porous nonwoven support. The nanofibers can be electrospun or electroblown onto the surface of the porous nonwoven support, wherein the root mean square height of the surface of the porous nonwoven support is less than about 70 μm. In some such embodiments, the support comprises one or more layers produced by meltblowing, wet-laying, spunbonding, calendering, electrospinning, electroblowing, or any combination thereof. The support can comprise a thermoplastic polymer, a polyolefin, polypropylene, polyester, polyamide, a copolymer thereof, a polymer blend, or any combination. Preferably, the smallest pore diameter of the nanofiber layer is less than the smallest pore diameter of the porous nonwoven support.

[0181] In some embodiments, the porous support comprises one or more layers selected from: spunbond nonwoven, meltblown nonwoven, needlepunched nonwoven, hydroentangled nonwoven, wet-laid nonwoven, resin-bonded nonwoven, electrospun, electroblown, woven fabric, knitted fabric, paper (including surface-modified paper), and any combination thereof.

[0182] In some embodiments, provided herein is a porous composite medium comprising a porous asymmetric polyethersulfone (PES) flat membrane prefilter and a rejection layer comprising a liquid filtration medium prepared by any method disclosed herein. In some embodiments, the porous asymmetric PES flat sheet membrane prefilter has a dense layer and a porous layer, and a pore size that gradually increases between the dense layer and the porous layer. The rejection layer comprising the liquid filtration medium disclosed herein can be disposed on the dense layer of the porous asymmetric PES flat membrane. In a preferred embodiment, the pore diameter of the rejection layer is less than the pore diameter of the dense layer of the porous asymmetric PES flat membrane prefilter. In some such embodiments, the porous composite medium has a bubble point, as measured with a liquid, that is at least 20% greater than the bubble point of the porous flat membrane prefilter alone. Preferably, the porous composite medium has an average flow bubble point of isopropyl alcohol in the range of about 10 psi to about 130 psi. In some embodiments, the porous asymmetric PES flat membrane disclosed herein comprises one or more layers produced by solution phase inversion, thermally induced phase separation, vapor induced phase separation, track etching, biaxial stretching, solvent etching, and combinations thereof. Accordingly, provided herein is a filtration device for critical filtration, the filtration device comprising such a porous composite medium as disclosed herein. In such a critical filtration device, the porous asymmetric PES flat membrane can provide prefiltration of a sample, while the rejection filtration layer provides further filtration of the sample. In some such embodiments, the porous composite filtration medium is preferably positioned such that the porous asymmetric PES flat membrane is upstream of the rejection filtration layer in the filtration direction.

[0183] It should be noted that the liquid filtration media disclosed herein (e.g., heat-treated porous nonwoven asymmetric liquid filtration media containing polymer nanofibers) are compatible with moist heat sterilization. Moist heat is typically used in combination with aseptic applications commonly used in, for example, the biopharmaceutical industry. Such impactful wet-dry processes (including moist heat sterilization methods known in the art) include, but are not limited to, saturated steam flowing under pressure (i.e., in-situ steam sterilization), autoclave sterilization, and tyndallization. Preferably, the liquid filtration media are compatible with autoclave sterilization such that they resist changes in the nanofiber structure (e.g., nanofiber morphology) and filtration parameters (e.g., permeability, porosity, LRV, and BP) described herein.

[0184] In some embodiments, the liquid filtration media provided herein resist changes in liquid permeability after sterilization relative to corresponding filtration media that do not contain semi-crystalline polymer nanofibers, do not contain an asymmetric nanofiber structure, have not been heat-treated, or any combination thereof. In some such embodiments, the sterilized liquid filtration media exhibit no more than a 15% decrease in liquid permeability after sterilization. For example, the autoclaved liquid filtration media of the present invention (e.g., autoclaved at least once or up to at least 12 times) do not exhibit a change in liquid permeability. Preferably, such sterilized liquid filtration media exhibit a liquid permeability decrease of no more than 1% to 15%, such as a 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, or 15% decrease in liquid permeability. More preferably, the disclosed sterilized liquid filtration media exhibit a liquid permeability decrease of no more than 11%. In the most preferred embodiments, the sterilized liquid filtration media (e.g., autoclaved at least once or up to at least 12 times) exhibit an increase in liquid permeability, such as an increase in liquid permeability of at least 1% to 6%. Preferably, the sterilized liquid filtration media exhibit an increase in liquid permeability of at least 6%.

[0185] Similarly, a sterilized liquid filtration medium (e.g., autoclaved at least once or up to at least 12 times) resists changes in the bubble point (BP) as disclosed herein. In some such embodiments, the sterilized liquid filtration medium does not exhibit a change in BP. For example but not limited to, the sterilized liquid filtration medium exhibits or maintains a BP of 5 psi to 150 psi. Preferably, the sterilized liquid filtration medium exhibits or maintains a bubble point of 20 psi or greater. Most preferably, the liquid filtration medium exhibits a smaller change in BP after sterilization compared to a corresponding filtration medium that does not contain semi-crystalline polymer nanofibers, does not contain an asymmetric nanofiber structure, has not been heat-treated, or any combination thereof. In some such embodiments, BP is measured with water. In a preferred embodiment, BP is measured with an alcohol (e.g., ethanol and / or isopropyl alcohol). In a more preferred embodiment, BP is measured with a solution containing water and an alcohol. Most preferably, BP is measured with a wettable fluid.

[0186] In some embodiments, a sterilized liquid filtration medium (e.g., autoclaved at least once or up to at least 12 times) resists changes in porosity. Preferably, such a sterilized liquid filtration medium exhibits or maintains a porosity of about 80% to about 95%.

[0187] In some embodiments, a sterilized liquid filtration medium (e.g., autoclaved at least once or up to at least 12 times) has or maintains its log reduction value (LRV). In a preferred embodiment, the sterilized liquid filtration medium has an LRV of at least 8 for Brevundimonas diminuta as measured according to ASTM F838-83. More preferably, the sterilized liquid filtration medium exhibits complete retention of microorganisms.

[0188] In other aspects of the present invention, a method for removing bacteria from a liquid sample is disclosed, the method comprising heating a porous nonwoven polymer nanofiber-containing liquid filtration medium as disclosed herein to at least the T of the polymer nanofibers as disclosed herein g but not exceeding the T of the polymer nanofibers m . For example, the heat treatment can be carried out for at least 1 hour and is preferably carried out in a non-oxidizing environment (such as an inert atmosphere oven). Then, the heat-treated liquid filtration medium is sterilized using moist heat sterilization (such as autoclave sterilization), and the liquid sample containing bacteria is passed through the sterilized liquid filtration medium. In a preferred embodiment, before and / or after autoclave sterilization, the liquid filtration medium has an LRV of at least 8 for Brevundimonas diminuta as measured according to ASTM F838-83.

[0189] Similarly, certain aspects of the present invention include a method for removing virus particles from a liquid sample, the method comprising heating the porous nonwoven liquid filtration medium containing polymer nanofibers disclosed herein to at least the T of the polymer nanofibers as disclosed herein g but not exceeding the T of the polymer nanofibers m . For example, such heat treatment can be carried out for at least 1 hour and is preferably carried out in a non-oxidizing environment (such as an inert atmosphere oven). Similarly, then, the heat-treated liquid filtration medium is sterilized using moist heat sterilization (such as autoclave sterilization), and the liquid sample containing virus particles is passed through the sterilized liquid filtration medium. In some such embodiments, the liquid filtration medium exhibits a virus log reduction value (LRV) greater than about 6 before and / or after autoclave sterilization.

[0190] Test Method

[0191] When reported herein, the basis weight is determined according to ASTM procedure D-3776 / D3776M-09a(2017), "Standard Test Methods for Mass Per Unit Area(Weight)of Fabric", and is reported in g / m 2 .

[0192] When reported herein, the porosity is calculated by dividing the basis weight of the sample (in g / m 2 units) by the polymer density (in g / cm 3 units), dividing by the sample thickness (in microns), multiplying by 100, and subtracting the resulting value from 100, i.e., % porosity = 100 - [basis weight / (density × thickness) × 100].

[0193] When reported herein, the fiber diameter is determined as follows: Scanning electron microscope (SEM) images of each layer of the nanofiber mat sample are taken (for example, at magnifications of 20000, 40000, or 60000 times). The diameters of at least ten (10) clearly resolvable nanofibers are measured from each SEM image and recorded. Irregular portions (i.e., agglomerates of nanofibers, polymer drops, intersections of nanofibers, etc.) are not included in the determination of the fiber diameter.

[0194] When reported herein, the nanofiber mat thickness was determined according to ASTM procedure 01777-96, "Standard Test Method for Thickness of Textile Materials," and reported in nanometers (nm) or micrometers (μm).

[0195] When reported herein, the maximum pore size was determined by the bubble point test as stated in ASTM designation F316-03, "Standard Test Method for Pore Size Characteristics of Membrane Filters by Bubble Point and Mean Flow Pore Test," re-approved in 2011, and reported in nanometers (nm).

[0196] When reported herein, the log reduction value (LRV) of bacteria was determined according to the standard test method of ASTM F838-83, "Standard Test Method for Determining the Bacterial Retention of Membrane Filters Used for Liquid Filtration."

[0197] Unless otherwise indicated, all BPs were measured using (Porous Materials Incorporated, Ithaca, NY) as the wetting fluid.

[0198] Examples

[0199] Example 1

[0200] Studies were conducted using a unique combination of properties selected from structure, material, and method to improve as-spun electrospun nanofiber mats (starting materials) into a type that can be autoclaved.

[0201] Four types of nanofiber mats were tested; each using a method that included electrospinning a polymer solution from a spinning electrode onto a nonwoven substrate (i.e., using a manufacturing-scale electrospinning device). The properties of the mats were reported immediately after production without any modification (referred to throughout this text as "as-spun" characterization).

[0202] Table 2 describes four nanofiber mats and their associated properties. All selected membranes had a fiber diameter of approximately 100 nm on at least one layer of the filter mat, such that the filter medium exhibited complete retention of Brevundimonas diminuta based on size-based separation as measured according to ASTM F838-83. In addition, all membranes exhibited a porosity of 85 - 95%, a bubble point of at least 15 psi (psig) (considered sufficient for complete retention), and a liquid permeability of greater than 3000 LMH / psi. The maximum pore size of the nanofiber mat was determined by the bubble point test as stated in ASTM designation F316-03 and using Determined as the wetting fluid.

[0203] Table 2: Four different nanofiber membranes were selected, which included two different materials (nylon 6 and nylon 66) and two different structures (symmetric and asymmetric).

[0204]

[0205] Classify the nanofiber mat types (as described in Table 2) as 'no heat treatment' (see Figure 1 , left picture) or 'heat treatment' (see Figure 1 , right picture). The heat treatment step includes heating the nanofiber mat roll in a non-oxidizing environment (e.g., in an anaerobic / inert atmosphere oven) at 208 °C for up to 12 hours.

[0206] Prior to autoclaving (or heat treatment and autoclaving), all nanofiber membranes described in Table 2 were subjected to an additional wetting-drying step, which included wetting in water or an isopropyl-water solution and drying at 80 °C for 12 hours. Typically, filtration devices (e.g., filtration devices applied in the industry) undergo wet-dry treatment during integrity testing before reaching the customer's site, where they are sterilized before application. Thus, the inclusion of such steps ensures that all possible effects of drying have been taken into account.

[0207] Autoclaving was performed under aggressive conditions, which included one or more cycles of heating to 135 °C for 60 minutes followed by a 15-minute drying time. Generally, such autoclaving parameters exceed the sterilization procedures typically implemented in the industry, which are carried out at 126 °C.

[0208] Results

[0209] The sterilization process caused a significant (and expected) decrease in the water permeability of all untreated mat types (see Figure 1 , left picture, comparing the white column and the column with cross-hatching). Notably, due to the use of strong solvents and the elevated temperature (80 °C for 6 hours) required for the electrospinning method, the polymer undergoes molecular breakage during the polymer dissolution step when used to produce nanofibers. This may be that the electrospinning method itself promotes molecular chain degradation through electrostatic forces. The intrinsic viscosity of the nanofiber polymer is significantly lower than that of the original polymer, indicating the harmful effects of the solvent and the electrospinning method on fiber strength. Further degradation occurs in the autoclave under high heat and humid conditions. This is similar to the phenomenon commonly referred to as hydrolytic instability (i.e., when exposed to high humidity and temperature, the cured polymer material lacks resistance to reverting to a semi-solid or liquid form).

[0210] In contrast, most nanofiber mats that were subjected to heat treatment prior to autoclaving showed significantly less reduction in water permeability; notable exceptions were the heat-treated nanofiber mats of asymmetric and symmetric nylon-66, which unexpectedly showed an improvement in water permeability (see Figure 1 , right-hand picture, especially "asymmetric-N66").

[0211] Solid-state polymerization can occur within a condensation polymer (such as nylon), where solid prepolymers (as well as dry monomers) follow a step-growth chemical process utilizing end-group functionality to produce higher molecular weights. Without wishing to be bound by any particular theory, heat treatment can increase the molecular weight using solid-state condensation. Solid prepolymer crystals react at a temperature below the melting point (T m ) and in an inert gas (i.e., a non-oxidizing environment), and transform monomer single crystals into highly oriented polycrystalline polymer aggregates. The increase in the molecular weight of the polymer via heat treatment is confirmed by zero-shear melt viscosity data, which is known to be associated with the average molecular weight of the polymer material. Additionally, the heat treatment step was found to improve the polymer crystallinity, as demonstrated by differential scanning calorimetry (DSC) thermograms. Both the molecular weight and the crystallinity affect the strength of the nanofiber mat. It should be noted that the "wettability" of the nanofiber mat remains unchanged before and after heat treatment.

[0212] Such data suggest that the change in the mechanical properties of the nanofiber mat is responsible for the observed robustness.

[0213] Example 2

[0214] To understand the importance of the combination of structure, polymer material, and method (e.g., asymmetric vs. symmetric, N66, and heat treatment) and their relative contributions in mitigating permeability loss, the following analysis was conducted. Table 3 describes the layout of the experimental design, sorted first by'method', then by'material', and then by'structure'. The effects of individual factors and combined factors on water permeability loss were then evaluated (as Figure 1 shown). Notably, simply by establishing this type of sorting order, the nanofiber mats are automatically arranged in ascending order of the observed water permeability loss, thus indicating the differences in the relative contributions of these parameters (see the 'output' column of Table 3).

[0215] Table 3: Experimental design for understanding key parameters

[0216]

[0217] Using the main effects plot (see Figure 2),The average response values of the method parameters are compared with the relative intensities of the effects of the various factors of the experiment. In short, when the line is horizontal (parallel to the x-axis), there is no main effect; the various levels of the factor affect the response in the same way, and the average % permeability loss of the response is the same across all factor levels. However, when the line is not horizontal, then the factor affects the result (i.e., permeability). The different levels of the factor affect the response differently. In addition, the steeper the slope of the line, the greater the importance of the main effect. Figure 2 It clearly shows that the following order is evident in terms of influence: method > material > structure. Additional analysis confirmed that all three effects have a statistically significant impact on permeability loss.

[0218] Example 3

[0219] The morphology of the asymmetric nanofiber mats was also studied by scanning electron microscopy (SEM), and the asymmetric mats were subjected to at least 3 rounds of autoclaving (AC 3×). Mats containing nylon-6 (N6) and not treated with heat treatment showed the most drastic structural changes. Figure 3 The SEM micrographs show that the fibers that were oriented in a straight array before sterilization appear to become wavy and contain more fusions at the fiber intersections (see upper picture). On the other hand, heat-treated nylon-66 (N66) nanofibers did not show significant morphological differences before and after sterilization (see lower picture), and are consistent with the observed mat robustness, as confirmed by the minimal loss of water permeability (if any).

[0220] Example 4

[0221] The heat-treated asymmetric N66 nanofiber mats that showed a significant improvement in water permeability were subjected to more than 3 autoclave sterilization cycles. With all three factors appropriate (heat treatment, nylon-66, and asymmetry), the nanofiber mats were robust enough to withstand up to 12 autoclave cycles and did not show any significant negative impact on water permeability (see Figure 4 ). In addition to the observed improvement in water permeability, the bubble point (or BP; using as the wetting fluid) (which is a measure of the retention properties of the mat) increased after the first 3 autoclave sterilization cycles and remained consistent in subsequent autoclave cycles. This increase in BP provides additional assurance of the mat's retention performance.

[0222] In addition, multiple autoclave cycles appear to have little effect on the nanofiber morphology in the best-performing mats (i.e., heat-treated asymmetric nylon-66 mats), even after 12 autoclave cycles (12×AC) (seeFigure 5 )。 Data indicate that heat-treated asymmetric N66 pads are robust against moist heat sterilization. After up to 12 autoclave cycles, such nanofiber pads show no decline to a modest increase in water permeability.

[0223] Incorporated by reference

[0224] All publications, patents, and patent applications mentioned herein are hereby incorporated by reference in their entirety, as if each individual publication, patent, or patent application was specifically and individually indicated to be incorporated by reference. In case of conflict, the present application (including any definitions herein) shall govern.

[0225] Equivalents

[0226] Those skilled in the art will recognize or be able to ascertain using only routine experimentation many equivalents to the specific embodiments described herein. Such equivalents are intended to be encompassed by the following claims.

Claims

1. A method for producing a porous nonwoven liquid filtration medium containing polymer nanofibers, the method comprising heating the porous nonwoven nanofiber-containing liquid filtration medium to at least the glass transition temperature (T g ) of the nanofibers but not exceeding the melting temperature (T m ) of the nanofibers for at least 1 hour, wherein the polymer is an aliphatic polyamide or polyethersulfone (PES), and wherein the liquid filtration medium comprises an asymmetric nanofiber mat or a symmetric nanofiber mat, the asymmetric nanofiber mat exhibiting fiber diameters that vary across the thickness of the entire nanofiber mat such that the average fiber diameter of one layer of the nanofiber mat is different from other layers of the nanofiber mat, and the symmetric nanofiber mat exhibiting a consistent fiber diameter across the thickness of the entire nanofiber mat.

2. The method according to claim 1, wherein the liquid filtration medium is prepared by electrospinning a polymer solution or melt to produce a porous nonwoven polymer nanofiber mat.

3. The method according to claim 1, wherein the corresponding filter medium that has not been heated to at least the glass transition temperature (T g ) of the nanofibers but not exceeding the melting temperature (T m ) of the nanofibers for at least 1 hour prior to sterilization, and the liquid filter medium resists changes in liquid permeability after sterilization.

4. The method according to claim 1, wherein the liquid filtration medium exhibits a bubble point pressure of 5 psi to 150 psi.

5. The method according to claim 1, wherein the liquid filtration medium exhibits a log reduction value (LRV) of at least 1 for Brevundimonas diminuta as measured according to ASTM F838-83.

6. The method according to claim 1, wherein the liquid filtration medium has a porosity of 80% to 95%.

7. The method according to claim 1, wherein the liquid filtration medium exhibits a liquid permeability of greater than 1000 LMH / psi.

8. The method according to claim 1, wherein the liquid filtration medium exhibits a post-sterilization liquid permeability reduction of no more than 40%.

9. The method according to claim 1, wherein the nanofibers have a fiber diameter of 5 nm to 1000 nm.

10. The method according to claim 1, wherein the average fiber diameter varies continuously from one layer to another of the asymmetric nanofiber mat.

11. The method according to claim 1, wherein the ratio of the average fiber diameter of one layer to another layer of the asymmetric nanofiber mat is at least 1.

15.

12. The method according to claim 1, wherein the average fiber diameter on at least one layer of the asymmetric nanofiber mat is 5 nm to 1000 nm.

13. The method according to claim 1, the method comprising heating the nanofiber mat to a temperature that is 1 °C to 80 °C lower than T m .

14. The method according to claim 13, the method comprising heating the nanofiber mat to be 56 °C lower than T m lower.

15. The method according to claim 13, the method comprising heating the nanofiber mat to 75 °C lower than T m lower.

16. The method according to claim 1, the method comprising heating the nanofiber mat to 100 °C to 200 °C higher than T g .

17. The method according to claim 1, the method comprising heating the nanofiber mat in a non-oxidizing environment.

18. The method according to claim 17, the method comprising heating the nanofiber mat in an inert atmosphere oven.

19. The method according to claim 1, the method comprising heating the nanofiber mat for at least 1 hour to at least 24 hours.

20. The method according to claim 1, wherein the porous nonwoven nanofiber-containing liquid filtration medium is electrospun onto the surface of a porous support or a non-porous support.

21. The method according to claim 1, wherein the aliphatic polyamide is nylon-6.

22. A liquid filtration medium prepared by the method according to claim 1.

23. A liquid filtration device, the liquid filtration device comprising the liquid filtration medium according to claim 22.

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