PVDF porous membrane for virus removal and preparation method and filter element thereof

Abstract

The application provides a PVDF porous membrane for removing viruses, a preparation method and a filter element, the porous membrane is made of PVDF material, comprises a main body, one side of the main body is a first porous surface, the other side of the main body is a second porous surface, the SEM measurement average pore size gradient of the main body is 2-12 nm / μm; the pore size gradient of the porous membrane is small; in the wet state of the porous membrane main body, the part of capturing 20 nm colloidal gold is D20, D20 is the area from the first porous surface and located at 20%-100% of the film thickness, part of the 20 nm colloidal gold is located at the second porous surface; the thickness of D20 is 15-35 μm; the porous membrane has good interception effect on 20 nm colloidal gold; meanwhile, the pore size of the small pore area is large, so that the membrane flux is high and the flux attenuation is slow, the mechanical strength is high, and the porous membrane is particularly suitable for filtering high-concentration protein fluid; the LRV value of the porous membrane for 20 nm colloidal gold is greater than 2 and less than 4, the double-layer stacking is used in actual use, and the risk of virus leakage is avoided; the preparation method is simple, fast, effective, green and environment-friendly.

Description

Technical Field

[0001] This invention relates to the field of membrane materials technology, and more specifically to a PVDF porous membrane for virus removal, its preparation method, and filter element. Background Technology

[0002] Membrane separation technology (including enrichment, concentration, and filtration purification) is a modern, highly efficient separation technology. Compared with traditional distillation and rectification techniques, it has advantages such as high separation efficiency, safe operation at room temperature, no pollution during processing (no impact on product quality), and high product yield. The core of membrane separation technology is the separation membrane, and the most common separation membrane currently is the polymer porous membrane. It is a type of separation membrane made from organic polymers as raw materials according to a certain process. Depending on the type of polymer, polymer porous membranes can be further subdivided into cellulose polymer porous membranes, polyamide polymer porous membranes, sulfone polymer porous membranes, fluorine-containing polymer porous membranes, etc. In addition, they can also be classified according to the pore size into microfiltration porous membranes, ultrafiltration porous membranes, nanofiltration porous membranes, and reverse osmosis membranes. Currently, polymer porous membranes are mainly used in biopharmaceuticals, medical applications, seawater desalination, and industrial semiconductor manufacturing.

[0003] In recent years, in addition to plasma fractionation preparations derived from human blood, there is also a need to improve the safety of biopharmaceuticals against viruses. Therefore, pharmaceutical manufacturers have researched incorporating virus removal / inactivation processes into their manufacturing processes. They have discovered that by using virus-removing membranes to filter the corresponding fluids, viruses can be efficiently removed without denaturing proteins, thus meeting the requirements for medical safety.

[0004] For example, US Patent 20200238221A1 (Sartorius Corporation application) discloses a porous monolayer polymer membrane, wherein at least one major surface of the polymer membrane has a surface porosity of at least 40%, and the total porosity of the polymer membrane is 0.8 to 1.4 times that of the at least 40% surface porosity; and the polymer membrane has an asymmetry factor of 1.5 to 10; this polymer membrane is a monolayer porous membrane with good flux and long service life, and is mainly used for filtering viruses, proteins or macromolecules; however, the average pore size of this polymer membrane is relatively large, and it can only retain large particles with a particle size of several hundred nanometers, and cannot retain small viruses with a particle size of about 20 nm (currently, the typical parvovirus is mouse parvovirus, whose particle size is about 20 nm).

[0005] To further improve the interception efficiency of parvovirus, Chinese patent CN1759924B (applied by EMD Millipore) discloses a multilayer composite ultraporous membrane. This composite ultraporous membrane includes at least one first porous membrane layer having a first surface and an equivalent second surface, and at least one second porous membrane layer having an equivalent first surface and a second surface. The first and second layers are superimposed and have a porosity transition region from the equivalent first surface of the second layer to the equivalent second surface of the first layer. At least one of the layers is an asymmetric ultraporous membrane. This composite structure... The resulting membrane structure has a strong retention effect on parvoviruses, meeting the needs of practical applications; however, it also has the following problems. First, since this porous membrane is a composite membrane, there is a risk of delamination / separation during pleating. In addition, the membrane-forming material used to prepare this composite membrane is mainly polyethersulfone. The sulfone group of polyethersulfone has benzene rings on both sides, which makes its membrane hydrophilicity poor. Even if hydrophilic additives are added during the preparation process or hydrophilic modification is performed after membrane preparation, the hydrophilicity is still poor. This results in the final membrane having a certain adsorption effect on proteins, which leads to a general protein yield and reduces economic benefits.

[0006] To further improve protein yield, Chinese patent CN105980037B (application by Asahi Kasei Corporation) also discloses a virus-removing membrane comprising a hydrophilized synthetic polymer (preferably polyvinylidene fluoride). The membrane is integrally formed without composites. In the cross-section of this virus-removing membrane, the thickness of the portion capturing colloidal gold with a diameter of 20 nm or more but less than 30 nm is 10 μm or more but less than 30 μm in a wetted state. Simultaneously, the portion capturing colloidal gold with a diameter of 15 nm is located at 60% or more but less than 100% of the membrane thickness from the first side. This membrane structure ensures sufficient and efficient retention of small viruses of 20 nm and above. Furthermore, the PVDF membrane, after hydrophilic treatment (either before or after membrane preparation), exhibits high hydrophilicity, resulting in protein adsorption far lower than that of commercial PES membranes, thereby improving protein yield.

[0007] However, this porous membrane also has certain drawbacks. First, to ensure retention efficiency, the pore size of the membrane's micropore region is extremely small (even excessively small). It has a good retention rate for 15nm colloidal gold (the area capturing 15nm colloidal gold is located at more than 60% and less than 100% of the membrane thickness from the first side). This easily leads to a significant increase in the amount of protein retained by the membrane (especially the protein retention rate in large-size, high-concentration protein fluids is too high), thus the protein yield still cannot meet the needs of practical applications (proteins are very expensive, and it is desirable to maximize protein yield). Second, because the pore size of its micropore region is very small, although the membrane initially has a good flux, small viruses and / or proteins will quickly block the pores, causing the membrane flux to decay too quickly and the filtration speed to be too slow in the middle and later stages (especially when filtering high-concentration protein fluids, the flux decay is very obvious).

[0008] Furthermore, the virus-removing membrane produced in this patent is a hollow fiber membrane, which has low pressure resistance (the actual operating pressure does not exceed 0.1 MPa) and is relatively easy to damage. This makes the manufacturing process of the virus-removing membrane component and its filter relatively complex. Moreover, since the membrane is a hollow fiber membrane, it can only be used as a single layer in actual use, which easily leads to the risk of virus leakage (because it is difficult to precisely control every process node during the membrane manufacturing process, which can easily lead to some small defects in the main structure of the membrane. These defects cannot be detected during integrity and other related tests; however, these defects will be amplified during long-term filtration, eventually leading to virus leakage).

[0009] In conclusion, the aforementioned problems have, to some extent, limited the development of antiviral membranes. Summary of the Invention

[0010] To address the shortcomings of existing technologies, the present invention aims to provide a PVDF porous membrane for virus removal, its preparation method, and filter element. This PVDF porous membrane not only has a good retention effect on small viruses such as 20nm, but also has a large flux, a slow flux decay, and high mechanical strength.

[0011] To achieve the above objectives, the present invention provides the following technical solution: a PVDF porous membrane for virus removal, comprising a body having non-directional tortuous pathways within the body, one side surface of the body being a first porous surface, and the other side surface of the body being a second porous surface, wherein the average pore size measured by SEM of the first porous surface is greater than the average pore size measured by SEM of the second porous surface.

[0012] The average pore size variation gradient of the porous membrane, as measured by SEM, is 2-12 nm / μm.

[0013] In the porous membrane substrate under wet conditions, the region that captures colloidal gold with a diameter of 20 nm is designated as D20. D20 is the region extending from the first porous surface and covering 20%-100% of the membrane thickness, with some of the 20 nm colloidal gold located at the second porous surface. The thickness of D20 is 15-35 μm.

[0014] The porous membrane has an LRV value greater than 2 and less than 4 for 20 nm colloidal gold.

[0015] The porous membrane of this invention is made of PVDF material, which is widely available and cost-effective. Compared to PES: PES membranes generally have low hydrophilicity, and even after hydrophilic modification, their hydrophilicity remains relatively low. This results in PES membranes having a certain adsorption effect on proteins, especially in the small pore areas (virus-retention areas), where protein adsorption is stronger, making it unsuitable for processing some high-concentration protein fluids (such as blood products, dual antibodies). In contrast, the hydrophilic-modified PVDF membrane has high hydrophilicity, far exceeding that of commercially available PES membranes, thus significantly reducing protein adsorption. Even when filtering high concentrations... When used with protein fluids, the adsorption of proteins remains low, which meets practical needs. Compared to cellulose membranes, cellulose membranes have lower mechanical strength and are relatively brittle (low elastic modulus). Furthermore, cellulose membranes can only be transported under humid conditions, making storage and transportation cumbersome and costly. During transportation, some areas of the membrane are prone to drying out, leading to membrane shrinkage. Even after wetting the shrunken areas and restoring them to their initial state, filtration performance may still be affected. Therefore, PVDF porous membranes are particularly suitable for use as virus removal membranes, offering unique advantages.

[0016] In the membrane body structure of the porous membrane provided by this invention, it can be clearly seen that the pore sizes on the two outer surfaces of the porous membrane are different, with a certain gap; the pore size on one outer surface is larger, while the pore size on the other outer surface is relatively smaller; the outer surface with the relatively larger pore size is referred to as the first porous surface (both the macropore surface and the liquid inlet surface) in this invention, while the outer surface with the relatively smaller pore size is referred to as the second porous surface (both the micropore surface and the liquid outlet surface) in this invention; the body has non-directional tortuous pathways, which refer to randomly oriented grooves. The membrane has a porous structure with a discrete distribution of pores, and the non-directional tortuous pathways are interconnected. The fibers forming the porous membrane structure are continuous. It can be understood that "continuous" means that all the fibers are interconnected as a whole, as if formed in one piece, without the use of additional adhesives or the like to connect them. Unless torn by external force, the network of fibers cannot be separated from each other. At the same time, the continuous network of fibers is also interconnected with the first porous surface and the second porous surface. Such a membrane structure ensures the filtration accuracy of the porous membrane and is beneficial to improving the retention of small viruses by the porous membrane.

[0017] As is well known, the typical parvovirus is mouse parvovirus, with a particle size of approximately 20 nm. This invention reflects the retention capacity of a porous membrane for various parvoviruses by measuring the location and retention status of 20 nm colloidal gold within the membrane. After the porous membrane retains colloidal gold of a certain particle size, the distribution of the colloidal gold within the membrane can be determined according to Chinese Patent CN105980038B – Test Method for Virus-Removing Membranes: A section is cut from the virus-removing porous membrane after filtering the colloidal gold solution. The brightness distribution of multiple sites in the cross-section stained with colloidal gold is measured using an optical microscope. Since colloidal gold absorbs light, the brightness shift depends on the amount of colloidal gold captured. It should be noted that background noise can be removed by analyzing the brightness distribution as needed. Then, a displacement graph with film thickness on the horizontal axis and brightness on the vertical axis is generated; thereby obtaining the region where colloidal particles of a certain size are trapped in the film thickness direction; (in this invention, the first porous surface is at 0% of the film thickness, and the second porous surface is at 100% of the film thickness); in addition, gold element determination can be performed on the film cross-section by EDS to understand the distribution of colloidal gold of the corresponding particle size in the film cross-section; of course, those skilled in the art can also obtain the distribution of colloidal gold in the film cross-section by other measurement methods, and the above measurement methods are for reference only.

[0018] After testing, it was found that the region D20, where 20nm colloidal gold is retained, is located in the area from the first porous surface and within 20%-100% of the main body thickness (i.e., D20 can be either 40%-100% or 20%-100% of the main body thickness). Simultaneously, some 20nm colloidal gold is retained at the second porous surface, and the thickness of the D20 region is 15-35μm. The retention of 20nm colloidal gold by the porous membrane indicates that, on the one hand, the thickness of the D20 region is relatively... The thickness, combined with the tortuous pathways, ensures that the porous membrane has a high retention rate for various small viruses (particle size of 20nm and above). Tests show that the porous membrane has an LRV value greater than 2 and less than 4 for 20nm colloidal gold. This means that the porous membrane needs to be stacked in two layers in actual use to ensure that the overall LRV is greater than 4 (because current regulations require an LRV greater than 4). Even if small defects occur in the porous membrane during the membrane fabrication process, it can still maintain a good virus retention effect.

[0019] On the other hand, compared to patent CN201580007740.0 (application by Asahi Kasei Corporation), the pores in the small-pore region (virus-retaining region) of this invention are relatively large. The 20nm colloidal gold is already retained at the second porous surface, making it impossible to retain 15nm colloidal gold further. As is well known, the pore size has a significant impact on membrane flux. The relatively large pore size in the small-pore region of this invention increases the flux of the PVDF membrane. It is also less prone to clogging, and flux decay is greatly reduced. At the same time, the relatively large pore size in the small-pore region prevents the porous membrane from retaining proteins, especially some large proteins, reducing the adsorption capacity for proteins (because the smaller the membrane pores, the larger the specific surface area, and the stronger the adsorption capacity). This greatly improves the protein yield (and proteins in virus-removing solutions are very expensive, so people want to maximize protein yield). Furthermore, due to the reduction in protein adsorption, flux decay is also further reduced, ensuring that the porous membrane can filter various high-concentration protein fluids rapidly for a long time.

[0020] Furthermore, the porous membrane of this invention is integrally formed, and the pore size of the porous membrane gradually changes with the overall thickness of the membrane (within a certain region, the pore size may remain essentially unchanged with the membrane thickness), without abrupt changes. Therefore, the rate of change of membrane pore size with thickness is reflected by measuring the average pore size change gradient using SEM. The average pore size change gradient is calculated as: (Average pore size measured by SEM on the first porous surface - Average pore size measured by SEM on the second porous surface) / thickness. A larger value indicates a faster pore size change, and a smaller value indicates a smaller pore size change. In this invention, the average pore size change gradient of the porous membrane is 2-12 nm / μm, which has a suitable pore size change gradient value, indicating that this invention... The membrane pore size changes with thickness, but the change is neither too rapid nor does it contain excessively large pores (excessively large pores lead to low overall mechanical strength and poor pressure resistance, making it easily damaged under pressure). Therefore, the porous membrane of this invention exhibits good overall mechanical strength and pressure resistance, making it less prone to damage under high pressure. Simultaneously, the pore size gradient is not too small, which would result in low retention efficiency or low flux, failing to meet the needs of practical applications. Under the combined effect of this pore size gradient and the D20 retention position and thickness, the porous membrane not only ensures high virus retention efficiency but also possesses a large flux with slow flux decay and high mechanical strength!

[0021] The porous membrane of this invention has a single-layer asymmetric membrane structure, meaning it is integrally formed without undergoing any "composite" or similar processes. The entire membrane is made of PVDF material, and the material is uniform throughout, with no material variation. The only change in the membrane's structure is its overall structure. In contrast, composite membranes have multiple layers, and the pore size changes abruptly as one layer transitions to another. Therefore, this porous membrane is less prone to delamination.

[0022] It should be noted that although there is a certain amount of colloidal gold in some regions along the film thickness direction, it is very low. Therefore, these regions are not considered to be areas where colloidal gold is trapped. These regions only have some residual colloidal gold (rather than trapped). Therefore, in the virus-removing membrane, it is preferable to form a continuous area along the film thickness direction that captures colloidal gold with a diameter of 20 nm, which is the region where colloidal gold of the corresponding particle size is truly trapped.

[0023] Here, in the membrane thickness direction, a first distance a is measured from the first porous surface of the virus-removing porous membrane to the portion of the colloidal gold capture site closest to the first porous surface;

[0024] In addition, in the membrane thickness direction, a second distance b is measured from the first porous surface of the virus-removing porous membrane to the portion of the colloidal gold capture site closest to the second porous surface; then, the value A (expressed as a percentage of a / c) of the first distance a divided by the membrane thickness c of the virus-removing membrane is calculated at multiple sites, and the average value of the value A at multiple sites is used as the first reach.

[0025] Furthermore, the second distance b divided by the membrane thickness c of the virus-free membrane is calculated at multiple sites, and the value B is expressed as a percentage (expressed as a percentage of b / c). The average value of the values ​​B at multiple sites is used as the second reachability. The area where colloidal gold with a diameter of 20 nm is retained is A20-B20 (i.e., the D20 region described in this invention, so the thickness of the D20 region = (B20-A20) * membrane thickness c); the area where colloidal gold with a diameter of 30 nm is retained is A30-B30 (i.e., the D30 region described in this invention).

[0026] As a further improvement of the present invention, in the porous membrane body in a wet state, the region that captures colloidal gold with a diameter of 30 nm is D30. D30 is a region located from the first porous surface and at 15%-97% of the membrane thickness, and the ratio of the thickness of D30 to the thickness of D20 is 0.6-0.95:1.

[0027] To better understand the structure of the porous membrane, in addition to retention experiments using 20nm colloidal gold, retention tests were also conducted using colloidal gold with a particle size of 30nm. By observing the retention locations of colloidal gold with different particle sizes in the porous membrane, a better understanding of the membrane structure was achieved. The tests revealed that D30 is located in the region extending from the first porous surface and spanning 15%-97% of the membrane thickness (i.e., D30 can be either 15%-90% or 20%-97% of the main thickness). However, 30nm colloidal gold could not be retained at the second porous surface, thus demonstrating the superiority of this invention. The pores of the porous membrane are not too large, thus avoiding impact on the retention efficiency of 20nm colloidal gold. Simultaneously, the ratio of D30 thickness to D20 thickness is 0.6-0.95:1, with the D30 region thickness being smaller than the D20 thickness, and a suitable ratio existing between them. This further demonstrates that the pore size of the membrane's micropore region changes essentially with the thickness gradient, retaining colloidal gold of different particle sizes at different locations. This ensures both the overall flux and retention efficiency of the membrane, while also giving it high mechanical strength. Furthermore, the pore size in the micropore region is generally large, minimizing protein retention and ensuring a high protein yield for the porous membrane.

[0028] As a further improvement of the present invention, in the porous membrane body in the wet state, the thickness of the part that captures colloidal gold with a diameter of 20-40 nm is 20-48 μm; when the diameter of the captured colloidal gold is 20-40 nm, the colloidal gold capture gradient K1 of the porous membrane is 1.1-4 nm / μm.

[0029] Colloidal gold capture gradient = Change in the diameter of the colloidal gold being captured / Thickness of the main region of the colloidal gold being captured for the corresponding diameter.

[0030] Currently, parvoviruses are generally considered to be various viruses with a particle size of 20-40nm; that is, the virus removal membrane mainly removes various viruses with a particle size of 20-40nm; therefore, a 40nm colloidal gold retention test was also conducted. The part D20 that captures colloidal gold with a diameter of 20nm is A20-B20; the part D40 that captures colloidal gold with a diameter of 40nm is A40-B40; then the part that captures colloidal gold with a diameter of 20-40nm is A40-B20 from the surface of the first porous membrane at the main thickness, and the thickness of this part is (B20-A40)*c, where c is the overall thickness of the membrane. Note that if the thicknesses of the two membranes are different, c is the average of the thicknesses of the two membranes.

[0031] Tests revealed that the thickness of the region capturing colloidal gold with a diameter of 20-40nm was 20-48μm. This thickness, combined with the synergistic effect of the PVDF material itself and the tortuous pathways, ensured efficient and sufficient capture of 20-40nm colloidal gold, while also enabling the porous membrane to have a large flux.

[0032] Further observation of the membrane structure revealed that the average pore size of the main body in this invention continuously decreases in a gradient from the region near the first porous surface to the region near the second porous surface. The average pore size changes continuously in the membrane thickness direction, meaning that colloidal gold particles of different sizes are trapped in different main body regions. The location where the colloidal gold is trapped within the main body can effectively reflect the pore size. The colloidal gold trapping gradient is obtained by the ratio between the change in colloidal gold particle size and the thickness of the main body where the corresponding colloidal gold particle size is trapped. The magnitude of this value can also reflect, to some extent, the change in membrane pore size with thickness across the membrane cross-section. The larger the value, the faster the membrane pore size changes with thickness within a certain region of the membrane cross-section.

[0033] When the diameter of the captured colloidal gold is 20-40 nm, the area capturing 40 nm colloidal gold is A40-B40; the area capturing 20 nm colloidal gold is A20-B20. When calculating the average gradient of the captured colloidal gold diameter, this invention uses the average value of the corresponding colloidal gold capture region to represent the entire region of colloidal gold capture. That is, C40 represents the entire region of 40 nm colloidal gold captured in the main film structure, and C40 = (A40 + B40) / 2; C20 represents the entire region of 20 nm colloidal gold captured in the main film structure, and C20 = (A20 + B20) / 2. Therefore, the change in colloidal gold diameter is 40 nm - 20 nm = 20 nm.

[0034] The thickness of the host body for capturing the corresponding colloidal gold diameter is: C20-40=(C20-C40)*c; when the diameter of the captured colloidal gold is 20-40nm, the average gradient of the captured colloidal gold diameter along the thickness direction is K1=20nm / C20-40μm;

[0035] Calculations showed that K1 = 1.1-4 nm / μm. This small value indicates that in the region near the second porous surface of the porous membrane (i.e., the relatively small pore region in the membrane body), the pore size changes little with the membrane thickness. This region is the key area for virus retention. The small change in pore size with membrane thickness in this region ensures that the entire porous membrane has high retention efficiency for various viruses (especially small 20 nm viruses), meeting the needs of practical applications. At the same time, the pore size change in the small pore region is not too small, further ensuring that the porous membrane has a high flux.

[0036] As a further improvement of the present invention, in the porous membrane body in a wet state, the capture peak value of colloidal gold with a diameter of 20 nm is L, wherein the L region is located in the area from the first porous surface and at 60-80% of the thickness of the body, and the distance between the L region and the second porous surface is 5-15 μm.

[0037] Tests revealed that the area where 20nm colloidal gold is retained is mainly located near and including the second porous surface. The capture peak L indicates the area where the membrane retains the most 20nm colloidal gold. If the capture peak L is too close to (or even directly on) the second porous surface, there is a risk of virus leakage, especially during filtration interruptions. Even with the double-layer stacked porous membrane of this invention, this risk cannot be completely avoided. Conversely, if the capture peak is too far from the second porous surface, it indicates that the 20nm colloidal gold is retained prematurely within the membrane, leaving a large unused area and resulting in a relatively small overall pore size. This would greatly affect the flux of the porous membrane, resulting in excessively low flux and loading capacity. Furthermore, the capture peak value is related to both the membrane material and the pore structure, meaning that different virus-removing membranes require different capture peak values. Since the pore size of the small pore region in this invention is relatively far, the capture peak value L is a certain distance from the second porous surface. It is located in the region from the first porous surface and at 60%-80% of the main body thickness, and the distance between the peak value L and the second porous surface is 5-15 μm. This structure ensures that the porous membrane can efficiently trap various tiny viruses without the risk of virus leakage, while also ensuring that the porous membrane has high flux and loading capacity.

[0038] After cutting a section from the virus-free porous membrane after filtering a 20nm colloidal gold solution, the section was measured using an optical microscope. The darkest part of the section was found to be the capture peak. Alternatively, after testing with an EDS spectrometer, the location of the peak was found to be the capture peak.

[0039] Furthermore, to achieve higher protein yields, whether in laboratory validation or actual production processes in biopharmaceutical companies, a top-wash operation (often at 30 psi) is generally required after virus filtration using a membrane filter (or porous membrane). This is because some protein is still retained in the porous membrane during virus filtration, and since proteins are very expensive, a top-wash operation is necessary to remove them from the porous membrane to further improve protein yield. Taking laboratory validation as an example (although the actual production process may differ, the mechanism is the same and can be characterized through laboratory validation), the top-wash operation typically occurs during continuous filtration. When the flow rate of the membrane filter (or porous membrane) decreases to 75% or only a small amount of feed remains, the external pressure is removed and the process is paused for a period of time (e.g., 5 to 15 minutes) to allow the protein to be released from the pore structure. Subsequently, a liquid such as a buffer solution is added to elute the released protein from the porous membrane, thereby improving the protein yield.

[0040] However, while top washing can indeed improve protein yield, during the removal of external pressure, viruses with stronger motility are more likely to be released from the porous structure and move to the effluent surface of the porous membrane. During top washing, these released and moved viruses are easily eluted, resulting in a virus concentration in the top wash solution that is often much higher than that in the filtrate obtained during retention filtration. Therefore, the top wash solution usually needs to be processed separately. However, if the virus concentration in the top wash solution is too high, various purification processes are required, which are cumbersome, time-consuming, and labor-intensive. Furthermore, it is even considered that the integrity of the porous membrane may be compromised.

[0041] In this invention, the distance between the capture peak L region and the second porous surface is 5-15 μm, providing sufficient thickness. Simultaneously, the average pore size gradient of the membrane cross-section (especially the micropore region) is relatively small, ensuring a very low virus concentration in the wash buffer (i.e., still efficiently retaining viruses as small as 20 nm). This meets the needs of practical applications and further improves protein yield, resulting in good economic benefits!

[0042] As a further improvement of the present invention, in the porous membrane body in a wet state, the region on both sides of the L portion where the number of 20nm colloidal gold captured is not less than 0.8L is the capture peak region; the thickness of the capture peak region is 5-20μm, and the ratio of the thickness of the capture peak region to the membrane thickness is 15-40%.

[0043] In the cross-section of the porous membrane, the position where the most 20nm colloidal gold is retained is called the capture peak L. A significant amount of colloidal gold is also retained in the regions on either side of the capture peak L. In this invention, the region on either side of the capture peak L where at least 0.8L of 20nm colloidal gold is captured is defined as the capture peak region. The existence of the capture peak region ensures that the porous membrane can adequately retain 20nm colloidal gold. However, if the thickness of the capture peak region is too small, there is a risk of leakage of the retained virus, especially during high-pressure, high-capacity filtration or when filtration is interrupted and restarted (including top washing). Conversely, if the thickness of the capture peak region is too large, it can easily affect the flux and slow down the filtration speed. In this invention, the thickness of the capture peak region is 5-20μm, and the ratio of the capture peak region thickness to the membrane thickness is 15-40%. This thickness ensures that the porous membrane adequately retains 20nm colloidal gold, eliminating the risk of virus leakage during special situations such as top washing, while also ensuring a large overall flux of the porous membrane.

[0044] Furthermore, the presence of a certain thickness of the capture peak region further reflects that the pore size of the porous membrane's micropore region changes with the thickness gradient, further ensuring the membrane's mechanical strength. At the same time, the porous membrane has a slower flux decay, enabling it to filter various protein fluids efficiently over a long period of time.

[0045] During EDS testing, the location of the captured peak area can be considered as an area where the gold content at the corresponding location is not less than 80% of the gold content at the peak location; during optical microscopy testing, the location of the captured peak area can be considered as the difference in brightness between that location and the location without colloidal gold not less than 80% of the difference in brightness between the captured peak location and the location without colloidal gold; of course, those skilled in the art can also obtain the above parameters through other measurement methods, and the above measurement methods are for reference only.

[0046] As a further improvement of the present invention, when the diameter of the captured colloidal gold is 20-100 nm, the colloidal gold capture gradient K2 of the porous membrane is 1.5-8 nm / μm;

[0047] Colloidal gold capture gradient = Change in the diameter of the colloidal gold being captured / Thickness of the main region of the colloidal gold being captured for the corresponding diameter.

[0048] When the diameter of the captured colloidal gold is 20-100nm, the region capturing 100nm colloidal gold is A100-B100; the region capturing 20nm colloidal gold is A20-B20. In calculating the colloidal gold capture gradient, this invention uses the average value of the corresponding colloidal gold capture region to represent the entire region of colloidal gold capture. That is, C100 represents the entire region of 100nm colloidal gold captured in the main film structure, and C100 = (A100 + B100) / 2; C20 represents the entire region of 20nm colloidal gold captured in the main film structure, and C20 = (A20 + B20) / 2. Therefore, the change in the diameter of the colloidal gold is 100nm - 20nm = 80nm.

[0049] The thickness of the host body for capturing colloidal gold of the corresponding diameter is: C20-100=(C20-C100)*c; when the diameter of the captured colloidal gold is 20-100nm, the average diameter variation gradient of the captured colloidal gold along the thickness direction is K1=100nm / C20-100μm;

[0050] In this invention, the main structure of the porous membrane can be divided into two regions. The region with relatively large pore sizes mainly serves as a pre-filtration area, while the region with relatively small pore sizes mainly serves to trap small viruses. 100nm colloidal gold can be considered as large particulate impurities, while 20nm colloidal gold is considered as small particulate impurities (or small viruses). Therefore, the value of K2 can be considered as the change in pore size with thickness across the entire membrane cross-section. A larger K2 indicates a faster change in membrane pore size with thickness; however, K2 cannot be too large, as excessively rapid changes will lead to… Excessively large pores in the porous membrane can lead to low overall mechanical strength, making it susceptible to damage under pressure. Conversely, a slow change in K2 can result in low retention efficiency or low flux, failing to meet the demands of practical applications. In this invention, the colloidal gold capture gradient K2 = 1.5-8 nm / μm provides a suitable capture gradient, enabling the porous membrane to efficiently retain various small viruses (especially mouse parvovirus with a particle size of approximately 20 nm). This minimizes the risk of virus leakage and achieves both high protein yield and high retention efficiency.

[0051] As a preferred option, K2:K1 = 1.5-3.5. This indicates that the overall pore gradient of the membrane is greater than the pore size variation in the micropore region, but without excessive variation. This ensures sufficient and efficient retention of small viruses, increases membrane flux, and provides high mechanical strength, allowing for stable filtration for extended periods under high pressure (e.g., 45 psi).

[0052] As a further improvement of the present invention, the thickness of the porous membrane is 25-55 μm; and the ratio of the thickness of the D20 region to the membrane thickness is 51-85%; the specific surface area of ​​the porous membrane is 6-12 m². 2 / g.

[0053] The membrane thickness can be determined by characterizing the membrane structure using a scanning electron microscope, followed by calculation using computer software (such as Matlab, NIS-Elements, etc.) or manual measurement. Of course, those skilled in the art can also obtain the above parameters using other measurement methods (such as freeze-drying followed by measurement with a measuring tool). These measurement methods are for reference only. When the membrane thickness is too small, its mechanical strength will be low; simultaneously, due to the short filtration time, effective filtration will be impossible. When the membrane thickness is too large, the filtration time will be too long, resulting in excessive time costs. Furthermore, based on the inherent characteristics of PVDF, the thickness of the porous membrane in this invention is 25-55 μm, ensuring that the PVDF porous membrane not only has high mechanical strength but also enables effective filtration with high filtration efficiency, short filtration time, and low time costs. Additionally, since the PVDF porous membrane of this invention is used in a double-layer stacked configuration in actual use, it further ensures the practicality of machining.

[0054] Meanwhile, measurements showed that the ratio of the thickness of the D20 region to the membrane thickness was 51-85%, indicating that most areas in the membrane body are regions with relatively small pore sizes (i.e., what we call the separation layer, where the pore size is relatively small compared to the pore size of other areas in the membrane body). The existence of these areas ensures sufficient retention of small viruses. In order to make the porous membrane have a high flux, while the thickness of the D20 (small pore region) is relatively large, the overall membrane thickness (which can also be considered as the thickness of the large pore region) should be appropriately reduced.

[0055] Based on this, it is necessary to ensure that the pre-filtration layer effectively retains large particles to reduce the possibility of clogging of the porous membrane. The pores on the side of the D20 region closer to the first porous surface also have a larger diameter, which can also effectively retain and accommodate larger particles of impurities. That is to say, the specific small pore regions with relatively large pore sizes in this application (the pore size is relatively large compared to the pore sizes of other virus-removing membranes) also have a certain pre-filtration effect, which can supplement and assist the pre-filtration layer. Therefore, even if the large pore regions of the porous membrane in this application are relatively thin, with the synergistic effect of the specific large thickness and relatively large pore size small pore regions, it can still effectively retain larger particles without causing rapid clogging of the porous membrane.

[0056] As is well known, the larger the specific surface area of ​​a membrane, the stronger its adsorption capacity for proteins, resulting in a lower protein yield. The specific surface area of ​​the porous membrane of this invention, as measured by the BET specific surface area test method, is 6-12 m². 2 The large surface area of ​​the porous membrane further demonstrates that the adsorption of proteins by the porous membrane of the present invention is very low. Even when filtering some high-concentration protein fluids, a high protein yield can still be obtained, ensuring economic benefits.

[0057] As a further improvement of the present invention, the average pore size of the first porous surface measured by SEM is 200-500 nm; the pore area ratio of the first porous surface is 10-25%.

[0058] A certain number of pores with a certain diameter exist on the first porous surface of the membrane. As is well known, the size, number, and shape of the pores in the membrane have a significant impact on the filtration accuracy (retention efficiency) and flux of the membrane. In this invention, the pores on the first porous surface are basically circular, some are circular, and some are elliptical. The average pore size measured by SEM on the first porous surface is 200-500 nm (preferably 250-450 nm). This pore size is used to remove some large particulate impurities, improve the overall dirt holding capacity of the membrane, and at the same time enable the porous membrane to have a high flux. Meanwhile, the pore area ratio on the first porous surface is 10-25% (the ratio of the sum of the pore areas to the membrane area). The first porous surface has a suitable average pore size measured by SEM and a corresponding pore area ratio, which ensures that the porous membrane has a large flux, facilitates the rapid passage of fluid through the porous membrane, shortens the filtration time, and also has a large tensile strength to meet the needs of practical applications.

[0059] The average pore size of a membrane surface can be measured by characterizing the membrane structure using a scanning electron microscope, followed by measurement and calculation using computer software (such as Matlab, NIS-Elements, etc.) or manually. During membrane fabrication, in the direction perpendicular to the membrane thickness (if the membrane is a flat sheet, this direction is planar; if the membrane is a hollow fiber membrane, this direction is perpendicular to the radius), its characteristics, such as pore size distribution, are roughly uniform and consistent. Therefore, the average pore size of a portion of the corresponding plane can be used to reflect the overall average pore size of that plane. In actual measurement, the membrane surface can be characterized using an electron microscope to obtain the corresponding SEM image. Since the pores on the membrane surface are roughly uniform, a certain area can be selected, such as 1 μm² (1 μm x 1 μm) or 25 μm² (5 μm x 5 μm). The specific area size depends on the actual situation. Then, the pore diameter of all pores on this area can be measured using appropriate computer software or manually, and then calculated to obtain the average pore diameter of the surface. Of course, those skilled in the art can also obtain the above parameters through other measurement methods. The above measurement methods are for reference only.

[0060] The method for testing the porosity can involve characterizing the membrane structure using a scanning electron microscope, followed by measurement and calculation using computer software (such as Matlab, NIS-Elements, etc.) or manually; a specific area is selected, for example, 1 μm. 2 (1μm x 1μm) or 25μm 2(5μm multiplied by 5μm), the specific area size depends on the actual situation. Then, the area of ​​all holes on this area is measured by appropriate computer software or manually, and then calculated and summed, and then divided by the corresponding area to obtain the porosity of the surface. Of course, those skilled in the art can also obtain the above parameters by other measurement methods. The above measurement methods are for reference only. In addition, the average diameter of the first fiber in the subsequent features can also be obtained by a similar method (i.e., the average diameter of several first fibers on a certain area).

[0061] As a further improvement of the present invention, the first porous surface includes a plurality of continuous first fibers, and adjacent continuous first fibers surround each other to form a circular hole; the average diameter of the first fiber is 40-120 nm; the ratio of the average diameter of the first fiber to the average diameter of the first hole is 0.15-0.45.

[0062] In the membrane structure of the porous membrane provided by this invention, it can be clearly seen that there are several first fibers on the first porous surface of the membrane. The average diameter of the first fibers is 40-120 nm. The first fibers of this thickness ensure the stability of the first pores, which facilitates a certain pre-filtration of the fluid and traps impurity particles of a certain size. At the same time, it ensures the mechanical strength of the membrane. In addition, the ratio of the average diameter of the first fiber to the average diameter of the first pore (the average diameter of the first pore is equivalent to the average diameter of the first porous surface) is 0.15-0.45. This ratio, together with the PVDF material, further ensures the stability of the first pores, making them less prone to collapse or shrinkage (when filtering fluid, the first porous surface comes into contact with the fluid first, and the first pores are most likely to collapse or shrink). This allows for long-term filtration under high pressure, with a filtration pressure reaching 45 psi. It also results in a large membrane flux.

[0063] As a further improvement of the present invention, the average pore size of the second porous surface measured by SEM is 60-160 nm; the pore area ratio of the second porous surface is 4-15%; and the ratio of the average pore size of the liquid inlet surface to the average pore size of the liquid outlet surface is 1.5-5.

[0064] Existing technology suggests that to trap viruses as small as 20nm, membrane pores should be small, with the smallest pore diameter not exceeding 40nm. However, our measurements revealed a surprising difference: the average pore diameter of the second porous surface, measured by SEM, is greater than 40nm, typically ranging from 60-160nm. This indicates that the average pore diameter of the second porous surface of our membrane is larger (in this invention, the average pore diameter of the second porous surface can be considered the average pore diameter of the smallest pore in the membrane). This significantly differs from existing technology and can be seen as overcoming technical bias. Compared to existing technology, the larger average pore diameter at the liquid outlet allows for higher protein yields, exceeding 98% (because smaller pores result in stronger protein adsorption). Simultaneously, small pores are easily clogged, leading to rapid flux decay and short lifespans in some existing virus removal membranes, despite high initial flux. Larger pores, on the other hand, are less prone to clogging, resulting in slower initial flux decay. This allows for long-term, high-efficiency filtration. As for why such "large" pores can still retain 20nm colloidal gold, this is primarily related to the PVDF material. Secondly, the main body of this invention features a tortuous pathway, with multiple layers of relatively large pore structures continuously stacked in the membrane thickness. The non-overlapping portions of the stacked pore structures are much smaller than the average pore size measured by SEM (the solid portion of the lower layer blocks the pore structure of the upper layer, preventing the passage of the feed liquid and various particulate matter within it). In other words, the channel through which the feed liquid passes formed by the stacked pore structures is much smaller than the average pore size measured by SEM. Therefore, by using an appropriate thickness, the path of the stacked pore structures is further improved, thereby achieving efficient retention of 20nm colloidal gold. Furthermore, the pore area ratio on the second porous surface is 4-15%, which, together with the appropriate average pore size of the second porous surface, further ensures the sufficient retention of 20nm colloidal gold by the porous membrane, while also exhibiting good filtration speed.

[0065] Preferably, in this invention, the pore area ratio of the first porous surface is at least 3% larger than that of the second porous surface. That is, the pore area ratio of the first porous surface (macropore surface) is larger, and the pore area ratio of the second porous surface (micropore surface) is smaller. This structural arrangement allows the fluid to pass quickly through the first porous surface (macropore surface), which acts as a pre-filter for the fluid, ensuring that the membrane as a whole has good flux. Meanwhile, the second porous surface (micropore surface) is relatively dense and can efficiently trap various small viruses (especially mouse parvovirus with a particle size of about 20 nm), making it less likely for small viruses to leak, thus meeting the needs of practical applications.

[0066] In this invention, the pore size of the porous membrane gradually changes with the overall membrane thickness (within a certain region, the pore size may remain essentially unchanged with membrane thickness), without abrupt changes. Besides the pore size gradient reflecting the change in membrane pore size with thickness, the ratio of the average pore sizes of the two outer surfaces further better reflects this change. This ratio can be called the asymmetry factor; the smaller the value (closer to 1), the stronger the symmetry of the two outer surfaces of the porous membrane; the larger the value, the greater the asymmetry. Measurements show that the ratio of the average pore size of the first porous surface to that of the second outer surface is 1.5-5, preferably 2-4. This indicates that the porous membrane of this invention exhibits a small asymmetry between its two outer surfaces. This asymmetry not only ensures a large flux and slow flux decay of the porous membrane, but also, in conjunction with a suitable thickness, ensures high virus retention efficiency, meeting practical requirements.

[0067] It's important to note that for hydrophilic PVDF porous membranes, which inherently possess a certain protein adsorption rate, the adsorption of proteins by the porous membrane is inevitable. Therefore, the closer the protein yield gets to 100%, the greater the difficulty becomes, and this difficulty does not increase linearly. For example, increasing the protein yield from 80% to 90% is relatively easy, while increasing it from 90% to 95% is drastically more difficult, and increasing it from 95% to 98% is even more challenging. Once the protein yield has reached over 98%, further increases become exponentially more difficult.

[0068] As a further improvement of the present invention, the porous membrane has a tensile strength of 8-20 MPa and an elongation at break of 50-200%; the flux of the porous membrane is greater than 50 L*h. -1 *m -2 @30psi; the protein yield of the porous membrane is not less than 98%.

[0069] The tensile strength and elongation at break are important indicators for evaluating the mechanical strength of porous membranes. Under certain conditions, the greater the tensile strength of a porous membrane, the better its mechanical strength. Tensile strength refers to the membrane's ability to withstand parallel tensile forces. During testing under specific conditions, the membrane sample is subjected to a tensile load until it fails. Based on the maximum tensile load at failure and the change in the membrane sample's dimensions (length), the tensile strength and elongation at break can be calculated. Both tensile strength and elongation at break can be measured using a universal tensile testing machine. The testing method for tensile strength is well-known in the field, for example, according to ASTM standards. D790 or ISO178 explain the tensile strength test procedure in detail. Measurements show that the tensile strength of the PVDF porous membrane of this invention is 8-20 MPa, and the elongation at break is 50-200%. This indicates that the porous membrane of this invention has high tensile strength and elongation at break, good mechanical properties, high industrial practical value, and fully meets market demands. Furthermore, a pressure resistance test was conducted, and the porous membrane's pressure resistance is greater than 45 psi, meaning that under a pressure of 45 psi, the membrane pores do not collapse or shrink, ensuring efficient interception of small viruses from various protein fluids. The operating pressure during actual filtration can also be 45 psi, further accelerating the filtration speed and shortening the filtration time. In addition, the elastic modulus is greater than 450 MPa, further demonstrating that the porous membrane has high elasticity. During filtration, the membrane can deform appropriately, ensuring long-term efficient filtration and preventing virus leakage due to membrane brittleness.

[0070] Permeation flux, also known as permeation rate or simply flux, refers to the amount of substance that passes through a unit area of ​​a porous membrane per unit time under a certain operating pressure during the separation process. The magnitude of the flux reflects the filtration speed; a higher flux indicates a faster filtration speed. In this invention, the flux of the porous membrane is greater than 50 L / h. -1 *m -2 @30psi indicates a relatively high flux, suggesting that the porous membrane has a fast filtration speed. While ensuring retention efficiency, the fluid can pass through the porous membrane quickly, resulting in lower time costs and higher economic benefits.

[0071] IVIG is an injectable immunoglobulin and serves as a protein model for antibodies. In this invention, IVIG acetate buffer at a concentration of 30 g / L was used as the test solution (a high-concentration liquid). Filtration was performed under a pressure of 30 psi. After 60 minutes, the flux of the porous membrane was no less than 50% of the initial flux. This demonstrates that the PVDF porous membrane exhibits low protein adsorption, slow flux decay, and the ability to stably and efficiently filter various fluids over a long period. It is particularly suitable for filtering high-concentration, unstable protein liquids (such as some blood products and dual antibodies).

[0072] Furthermore, the protein yield of the porous membrane is not less than 98%, indicating that the effective substance protein in the fluid is not easily adsorbed onto the membrane. On the one hand, it will not clog the membrane pores, ensuring that the porous membrane still has a long service life. On the other hand, it ensures that the content of the effective substance protein in the fluid changes very little, and the protein is basically not lost, thus ensuring economic benefits. The test method for protein yield can refer to patents CN105980037B-Virus Removal Membrane, CN101816898B-Ultraporous Membrane and its Preparation Method, CN1759924B-Ultraporous Membrane and its Preparation Method, etc.

[0073] On the other hand, the present invention also provides a method for preparing a PVDF porous membrane for virus removal, comprising the following steps:

[0074] S1. Prepare a casting solution and cast it onto a carrier to form a liquid film; the casting solution comprises the following components by weight: 18-30 parts of PVDF resin, 5-25 parts of hydrophilic additives, 10-20 parts of small molecule additives, and 45-75 parts of organic solvents; the mass percentage of PVDF resin in the casting solution is greater than 20%.

[0075] S2. A gas flow with a relative humidity of 60-90% is blown onto the surface of the liquid film to process it, thereby obtaining a raw film; wherein the relative velocity between the gas flow and the liquid film is 0.1-6 m / s, and the duration is not less than 10 s; and the carrier temperature is at least 5°C lower than the liquid film temperature;

[0076] S3. Immerse the raw film in the extraction bath for further extraction and solidification, then wash and dry to obtain the film.

[0077] S4. The film is hydrophilically treated to obtain a porous membrane.

[0078] As a further improvement of the present invention, the number average molecular weight of the PVDF resin is 300,000 to 1,200,000; the hydrophilic additive is at least one of polyethylene glycol, polyvinylpyrrolidone, and polyvinyl alcohol; and the small molecule additive is at least one of LiCl, NH4Cl, nano-silica, acetone, butanone, and tetrahydrofuran.

[0079] As a further improvement of the present invention, in step S2 the liquid film temperature is 15-50℃ and the carrier temperature is 0-40℃; in step S3, the processing time is not less than 3 minutes and the extraction bath includes at least one of water and small molecule alcohol.

[0080] As a further improvement of the present invention, the small molecule alcohol is ethanol or isopropanol; the organic solvent is at least one selected from N-methylpyrrolidone, dimethylformamide, dimethylacetamide, trimethyl phosphate, triethyl phosphate, and γ-butyrolactone.

[0081] As a further improvement of the present invention, 1-5 parts by weight of a surfactant are added to the casting solution, wherein the surfactant is a fluorinated surfactant.

[0082] By adopting the above technical solution, in preparing the PVDF virus-removing membrane of the present invention, a casting solution is first prepared. The casting solution mainly includes PVDF resin, hydrophilic additives, small molecule additives, and organic solvents. Among them, PVDF resin is a film-forming polymer with good film-forming processing performance. The final film has good mechanical properties and is also resistant to pollution, making it suitable for use in the field of virus removal. In addition, the solid content of PVDF resin in the casting solution of this application needs to be strictly controlled and its solid content needs to be greater than 20%. This is because the inventors of this application have found that if the solid content of PVDF resin in the casting solution is too low, it is difficult to obtain a virus-removing membrane with an ideal membrane structure. The organic solvent is at least one of N-methylpyrrolidone, dimethylformamide, dimethylacetamide, trimethyl phosphate, triethyl phosphate, and γ-butyrolactone (it can be one of these solvents or a mixed solvent obtained by mixing multiple solvents). The organic solvent is used to fully dissolve the PVDF resin to form a uniform, stable, and clear casting solution, which is then used to form a porous membrane with an ideal pore size through subsequent phase separation and curing processes.

[0083] Preferably, the PVDF resin has a number-average molecular weight of 300,000 to 1,200,000. This molecular weight of PVDF resin is beneficial for forming a uniform, stable casting solution with high solid content, and for obtaining a film with high mechanical properties. At the same time, small molecule additives are added to the casting solution. The small molecule additives are at least one of LiCl, NH4Cl, nano-silica, acetone, butanone, and tetrahydrofuran. The small molecule additives not only help to make the final porous membrane have an ideal pore size, obtain a larger flux and maintain efficient virus retention, but also improve the uniformity of the pore structure of the porous membrane, thereby improving the tensile strength and other mechanical properties of the porous membrane. In addition, a hydrophilic additive is added to the casting solution. The hydrophilic additive is at least one of polyethylene glycol, polyvinylpyrrolidone, and polyvinyl alcohol. The addition of the hydrophilic additive can improve the hydrophilicity of the porous membrane and promote phase separation of the casting solution, thereby increasing the porosity of the membrane pores. The hydrophilic additive in the casting solution works synergistically with the extraction solution to promote more reasonable phase separation and extraction solidification of the casting solution, which is conducive to obtaining an ideal membrane pore structure, less prone to defects and macropores, and with relatively uniform pore size. Preferably, a small amount of surfactant—fluorocarbon surfactant, such as Fluorad FC-170C—can also be added to the casting solution. The addition of surfactant is more conducive to obtaining an ideal gradient membrane pore structure. It should be noted that the membrane formation process of different porous membranes varies greatly. The casting solution system in this application is only suitable for PVDF membrane formation and is not suitable for polyethersulfone and cellulose porous membranes. The inventors speculate that this is related to the properties of the membrane forming materials themselves.

[0084] A well-formulated casting solution significantly impacts the structure and properties of the final porous membrane, affecting factors such as pore size distribution, thickness, and flow rate (flux). A suitable casting solution formulation ensures the final porous membrane has appropriate thickness and achieves ideal pore size. Furthermore, the high-solids-content casting solution in this application has a reasonable viscosity, is easy to handle, and can be manually cast (e.g., by hand-pouring, casting, or spreading on a casting surface) or automatically cast (e.g., by pouring or additional casting on a moving bed). Various devices known in the art can be used for casting. Casting equipment includes, for example, mechanical coaters, which include blades, doctor blades, or spray / pressurization systems. Various casting speeds are suitable, as known in the art, such as casting speeds of approximately 2-6 feet per minute (fpm), depending on the specific circumstances.

[0085] After the casting solution is cast into a liquid film, the liquid film undergoes phase separation treatment. A high-humidity airflow is blown onto the surface of the liquid film to promote phase separation and form a growing film. It is well known that the faster the phase separation rate, the smaller the pores in the resulting film. In this invention, by coordinating a certain airflow humidity (60-90%), a suitable casting solution formula, and controlling the temperature difference between the carrier and the liquid film (at least 5°C), the liquid film undergoes relatively rapid phase separation on the air side. This results in smaller pores on the air side (but due to the limited water vapor content in the airflow, the pores will not be too small, and the film will not have pores with a diameter of around 20 nm), while simultaneously forming a film structure with a relatively small pore size gradient. Furthermore, the relative velocity between the airflow and the liquid film is 0.1-6 m / s, and the duration is not less than 5 seconds. This phase separation treatment aims to obtain the ideal pore size and thickness of the porous membrane and facilitates the formation of an ideal membrane structure, i.e., ideal fibers. In addition, the liquid film temperature can be understood as the casting solution temperature; a certain temperature difference between the carrier and the liquid film can adjust the phase separation rate.

[0086] The biofilm is then immersed in an extraction bath for further extraction and solidification. The extraction bath is at least one of water and a small molecule alcohol (C1-C4 small molecule alcohol), preferably at least one of ethanol and isopropanol. Under the action of the extraction bath, PVDF will be precipitated more completely, and the processing time is not less than 3 minutes to ensure the formation of a biofilm with an ideal structure.

[0087] Next, to improve the hydrophilicity of the film, a grafting method was used to hydrophilize it. The hydrophilic treatment solution contained 8% hydroxypropyl acrylate, 23% 3-butanol, and the balance water by volume percentage. The prepared hydrophilic treatment solution was bubbled and stirred to remove oxygen for 20 minutes, maintaining the temperature of the hydrophilic treatment solution at 45°C during treatment. Subsequently, the film was placed in a nitrogen atmosphere and cooled to below -60°C, then irradiated with at least 25 kGy of gamma rays using Co60 as the radiation source. After irradiation, the film was placed under a low pressure of approximately 13.4 Pa and allowed to stand for 15 minutes. The film was then contacted with the aforementioned stirred and deoxygenated hydrophilic treatment solution and allowed to react for 1 hour. After the reaction, the film was dried with 2-propanol under vacuum at 60°C to obtain the porous membrane.

[0088] The resulting PVDF porous membrane has an ideal membrane structure, high flux, slow flux decay, high mechanical strength, and good tensile strength. It is particularly suitable for processing some very difficult-to-filter materials (high concentration, poor stability) (such as blood products, dual antibiotics), has good versatility, and a wide range of applications.

[0089] In addition, this application also provides a filter cartridge for virus removal, which includes the porous membrane according to any one of claims 1-11, and the filter cartridge includes 2-3 stacked porous membranes.

[0090] Preferably, the membrane filter element can be a capsule filter element or a needle filter element; in order to achieve a logarithmic removal rate greater than 4, 2-3 filter membranes can be stacked together, thereby greatly reducing the risk of virus leakage.

[0091] The beneficial effects of this invention are as follows: A PVDF porous membrane for virus removal comprises a main body having non-directional tortuous pathways. One side surface of the main body is a first porous surface, and the other side surface is a second porous surface. The average pore size measured by SEM of the first porous surface is larger than that measured by SEM of the second porous surface. The average pore size variation gradient of the porous membrane measured by SEM is 2-12 nm / μm, exhibiting a relatively small pore size variation gradient. In the wetted porous membrane body, the region that captures colloidal gold with a diameter of 20 nm is designated as D20. D20 is a region extending from the first porous surface and located at 20%-100% of the membrane thickness. A 20nm colloidal gold layer is located on the second porous surface; the thickness of D20 is 15-35μm; thus ensuring that the porous membrane has a strong retention effect on small viruses with a particle size of 20nm and above; at the same time, the pore size of the small pore region is relatively large, thus achieving a high protein yield and high flux, with slow flux decay, and also possessing high mechanical strength, making it particularly suitable for high-concentration protein fluids; the LRV value of the porous membrane for 20nm colloidal gold is greater than 2 and less than 4, and in actual use, it is used in double-layer stacking to ensure that there is no risk of virus leakage; in addition, this invention also provides a method for preparing this porous membrane, which is convenient, fast, effective, simple to operate, green and environmentally friendly, and suitable for large-scale promotion. Attached Figure Description

[0092] Figure 1 The image shown is a scanning electron microscope (SEM) image of the first porous surface in the porous membrane prepared in Example 1, with a magnification of 10K×.

[0093] Figure 2 The image shown is a scanning electron microscope (SEM) image of the second porous surface in the porous membrane prepared in Example 1, with a magnification of 10K×.

[0094] Figure 3 The image shows a scanning electron microscope (SEM) image of the longitudinal section of the porous membrane prepared in Example 1, with a magnification of 10K×.

[0095] Figure 4 The image shown is a scanning electron microscope (SEM) image of the first porous surface in the porous membrane prepared in Example 2, with a magnification of 20K×.

[0096] Figure 5 The image shown is a scanning electron microscope (SEM) image of the second porous surface in the porous membrane prepared in Example 2, with a magnification of 20K×.

[0097] Figure 6 The image shows a scanning electron microscope (SEM) image of the longitudinal section of the porous membrane prepared in Example 3, with a magnification of 2K×.

[0098] Figure 7 The image shows a scanning electron microscope (SEM) image of the longitudinal section of the porous membrane prepared in Example 3, near the first porous surface, with a magnification of 20K×.

[0099] Figure 8 The image shows a scanning electron microscope (SEM) image of the longitudinal section of the porous membrane prepared in Example 3, near the second porous surface, with a magnification of 20K×.

[0100] Figure 9 The image shows a scanning electron microscope (SEM) image of the longitudinal section of the porous membrane prepared in Example 4 near the second porous surface after being trapped by 20 nm colloidal gold, with a magnification of 20K×.

[0101] Figure 10 The image shows the cross-sectional distribution of colloidal gold content in the porous membrane prepared in Example 4; (the bottom part is the first porous surface, and the top part is the second porous surface).

[0102] Figure 11 This is a schematic diagram of the porous membrane flux testing device of the present invention;

[0103] Figure 12 This is a schematic diagram of the testing apparatus used for testing the retention efficiency of colloidal gold in porous membranes according to the present invention. Detailed Implementation

[0104] To more clearly illustrate the overall concept of this application, a detailed description is provided below using examples. Unless otherwise specified, in the following examples, the raw materials and equipment used to prepare the porous membrane are commercially available; wherein, a Hitachi S-5500 scanning electron microscope is used to characterize the structural morphology of the virus-removing membrane.

[0105] Example 1

[0106] A method for preparing a PVDF porous membrane for virus removal includes the following steps:

[0107] S1. Prepare a casting solution and cast it onto a carrier to form a liquid film; the casting solution comprises the following components by weight: 24 parts PVDF resin, 15 parts hydrophilic additive, 40 parts small molecule additive, and 30 parts organic solvent; wherein the number average molecular weight of the PVDF resin is 800,000; the hydrophilic additive is polyvinylpyrrolidone; the small molecule additive is LiCl; and the organic solvent is N-methylpyrrolidone.

[0108] S2. An airflow with a relative humidity of 75% is blown onto the surface of the liquid film for treatment to obtain a raw film; wherein the relative velocity between the airflow and the liquid film is 2.5 m / s and the duration is 25 s; the liquid film temperature is 25°C and the carrier temperature is 10°C.

[0109] S3. Immerse the raw film in an extraction bath for further extraction and solidification for 6 minutes. The extraction bath is water. Then wash and dry to obtain the film.

[0110] S4. The film is subjected to hydrophilic treatment, specifically by grafting. The hydrophilic treatment solution contains 8% hydroxypropyl acrylate, 23% 3-butanol, and the balance water by volume percentage. The prepared hydrophilic treatment solution is bubbled and stirred to remove oxygen for 20 minutes, maintaining the temperature of the hydrophilic treatment solution at 45°C during treatment. Subsequently, the film is placed in a nitrogen atmosphere and cooled to approximately -65°C, then irradiated with approximately 27 kGy of gamma rays using cobalt-60 as the radiation source. After irradiation, the film is placed under a low pressure below approximately 13.4 Pa and allowed to stand for 15 minutes. The film is then contacted with the aforementioned hydrophilic treatment solution after stirring and removing oxygen, and allowed to stand for 1 hour. After the reaction, the film is dried with 2-propanol under vacuum at 60°C to obtain the porous membrane.

[0111] The main difference between Examples 2-6 and Example 1 lies in the different casting solution formulations and process parameters for each step in the preparation of porous membranes, as detailed in the table below:

[0112] Examples 2-6 Casting solution formulation

[0113]

[0114] Preparation process of Examples 2-6

[0115]

[0116] Example 7

[0117] A method for preparing a PVDF porous membrane for virus removal includes the following steps:

[0118] S1. Prepare a casting solution and cast it onto a carrier to form a liquid film; the casting solution comprises the following components by weight: 18 parts PVDF resin, 8 parts hydrophilic additive, 32 parts small molecule additive, 22 parts organic solvent, and 3 parts surfactant; wherein the number average molecular weight of the PVDF resin is 1.2 million; the hydrophilic additive is polyvinyl alcohol; the small molecule additive is LiCl; the organic solvent is dimethylacetamide; and the surfactant is a fluorinated surfactant.

[0119] S2. An airflow with a relative humidity of 80% is blown onto the surface of the liquid film for treatment to obtain a raw film; wherein the relative velocity between the airflow and the liquid film is 3.5 m / s, the duration is 20 s; the liquid film temperature is 25°C, and the carrier temperature is 5°C;

[0120] S3. Immerse the raw film in an extraction bath for further extraction and solidification for 5 minutes. The extraction bath is water. Then wash and dry to obtain the film.

[0121] S4. The film is subjected to hydrophilic treatment, specifically by grafting. The hydrophilic treatment solution contains 8% hydroxypropyl acrylate, 23% 3-butanol, and the balance water by volume percentage. The prepared hydrophilic treatment solution is bubbled and stirred to remove oxygen for 20 minutes, maintaining the temperature of the hydrophilic treatment solution at 45°C during treatment. Subsequently, the film is placed in a nitrogen atmosphere and cooled to approximately -65°C, then irradiated with approximately 27 kGy of gamma rays using cobalt-60 as the radiation source. After irradiation, the film is placed under a low pressure below approximately 13.4 Pa and allowed to stand for 15 minutes. The film is then contacted with the aforementioned hydrophilic treatment solution after stirring and removing oxygen, and allowed to stand for 1 hour. After the reaction, the film is dried with 2-propanol under vacuum at 60°C to obtain the porous membrane.

[0122] Comparative Example 1

[0123] In Example 1, the relative humidity of the airflow in S2 was set to 30%, and the carrier temperature was controlled at 25°C, the same as the liquid film temperature, while all other conditions remained unchanged.

[0124] Tests revealed that the average pore size of the second porous surface (liquid outlet surface) of the porous membrane was 200 nm. This average pore size was too large, and its LRV (Lead Value Value) was too small (less than 1) when conducting the 200 nm colloidal gold retention test, making it virtually impossible to retain 20 nm colloidal gold and failing to meet the requirements of actual experiments. Therefore, subsequent tests on 30 nm and 40 nm colloidal gold were not conducted.

[0125] Comparative Example 2

[0126] The weight parts of PVDF resin in the S1 casting solution of Example 1 were set to 12 parts, and the carrier temperature was controlled at 25°C, the same as the liquid film temperature, while all other conditions remained unchanged.

[0127] Because the solid content of PVDF resin in the casting solution is too low and the carrier temperature is not controlled, the overall gradient of the membrane is too large. At the same time, the average pore size of the second porous surface is too small, only 30 nm. This results in low flux of the porous membrane, rapid flux decay, and short service life, which cannot meet the needs of actual use.

[0128] 1. Structural Characterization

[0129] The morphology of the nanoscale polymer porous membranes obtained in each embodiment was characterized using scanning electron microscopy, and the required data were then obtained; the specific results are shown in the table below:

[0130] Table 1:

[0131]

[0132] Table 2

[0133]

[0134] Table 3

[0135]

[0136] Table 4

[0137]

[0138] Note: As shown in Tables 1-4, the porous membranes prepared in Examples 1-7 of this invention all possess ideal membrane structures. These porous membranes are integrally formed without any composite process, making the preparation process simple. The membrane pore size varies with the thickness gradient, thus achieving efficient retention of small viruses up to 20 nm in size. They also exhibit high flux, slow flux decay, long service life, and low protein adsorption, making them suitable for applications in virus removal, particularly for filtering high-concentration protein fluids.

[0139] Performance characteristics

[0140] Membrane flux is calculated as follows:

[0141] The formula for calculating membrane flux (J) is: J = V / (T × A) where:

[0142] J -- Membrane flux unit: L*h -1 *m -2

[0143] V -- Sampling volume (L); T -- Sampling time (h); A -- Effective membrane area (m2)

[0144] The operating conditions used for determining the porous membrane separation performance in this invention are as follows: the feed solution is deionized water, the operating pressure is 30 psi, the operating temperature is 25°C, and the solution pH is 7; the flux testing device is... Figure 11 ;

[0145] 20nm colloidal gold retention test Figure 12

[0146]

[0147]

[0148] After testing, it was found that the porous membranes prepared in Examples 1-7 had a high LRV for 20nm colloidal gold, and the LRV remained high after top washing, indicating that the porous membrane of the present invention has a sufficient retention effect on viruses of 20nm and above. In addition, the porous membrane has good flux and fast filtration speed, and also has high mechanical properties. According to regulations, the LRV value of the entire component must be greater than 4, so it can be used in double-layer stacking in actual use, which can meet the needs of practical applications.

[0149] Furthermore, according to the test method used in paragraph 114 of CN201010154974.7-Ultraporous membrane and its preparation method: protein yield test of the sample was carried out, and it was found that the protein yield of the porous membrane of the present invention is not less than 98%, and can even reach 99%, with basically no protein retention or adsorption, ensuring economic benefits; while the protein yield of Comparative Example 2 is less than 90%, which cannot meet the needs of practical applications.

[0150] In addition, the pressure resistance test was conducted on all samples of Examples 1-7. The pressure resistance was greater than 50 psi. They were able to filter the corresponding fluid stably and quickly under a pressure of 45 psi. At the same time, during the integrity test, they were subjected to a pressure of 60 psi and the membrane pores remained intact.

[0151] The above description is merely a preferred embodiment of the present invention. The scope of protection of the present invention is not limited to the above embodiments. All technical solutions falling within the scope of the present invention's concept are within the scope of protection of the present invention. It should be noted that for those skilled in the art, any improvements and modifications made without departing from the principles of the present invention should also be considered within the scope of protection of the present invention.

Claims

1. A PVDF porous membrane for virus removal, comprising a body having non-directional tortuous pathways within the body, one side surface of the body being a first porous surface, and the other side surface of the body being a second porous surface, characterized in that: The average pore diameter measured by SEM of the first porous surface is greater than that measured by SEM of the second porous surface. The average pore size variation gradient of the porous membrane, as measured by SEM, is 2-12 nm / μm. In a porous membrane substrate in a wetted state, the region that captures colloidal gold with a diameter of 20 nm is designated as D20. D20 is a region extending from the first porous surface and covering 20%-100% of the membrane thickness, with a portion of the 20 nm colloidal gold located at the second porous surface. The thickness of D20 is 15-35 μm, and the ratio of the thickness of the D20 region to the membrane thickness is 51-85%. The porous membrane has an LRV value greater than 2 and less than 4 for 20 nm colloidal gold; The average pore size of the second porous surface, as measured by SEM, is 60-160 nm.

2. The PVDF porous membrane for virus removal according to claim 1, characterized in that: In the porous membrane substrate under wet conditions, the region that captures colloidal gold with a diameter of 30 nm is designated as D30. D30 is the region extending from the first porous surface and located at 15%-97% of the membrane thickness. The ratio of the thickness of D30 to the thickness of D20 is 0.6-0.95:

1.

3. The PVDF porous membrane for virus removal according to claim 1, characterized in that: In the porous membrane substrate under wet conditions, the thickness of the portion that captures colloidal gold with a diameter of 20-40 nm is 20-48 μm; when the diameter of the captured colloidal gold is 20-40 nm, the colloidal gold capture gradient of the porous membrane is K1 = 1.1-4 nm / μm. Colloidal gold capture gradient = Change in the diameter of the colloidal gold being captured / Thickness of the main region of the colloidal gold being captured for the corresponding diameter.

4. The PVDF porous membrane for virus removal according to claim 1, characterized in that: In a porous membrane substrate in a wet state, the capture peak value for colloidal gold with a diameter of 20 nm is L. The L region is located in the area from the first porous surface and at 60-80% of the substrate thickness, and the distance between the L region and the second porous surface is 5-15 μm.

5. The PVDF porous membrane for virus removal according to claim 4, characterized in that: In a porous membrane substrate in a humid state, the region on both sides of the L portion where the number of colloidal gold particles at 20 nm is not less than 0.8 L is the capture peak region; the thickness of the capture peak region is 5-20 μm, and the ratio of the thickness of the capture peak region to the membrane thickness is 15-40%.

6. The PVDF porous membrane for virus removal according to claim 1, characterized in that: When the diameter of the captured colloidal gold is 20-100 nm, the colloidal gold capture gradient K2 of the porous membrane is 1.5-8 nm / μm; Colloidal gold capture gradient = Change in the diameter of the colloidal gold being captured / Thickness of the main region of the colloidal gold being captured for the corresponding diameter.

7. The PVDF porous membrane for virus removal according to claim 1, characterized in that: The thickness of the porous membrane is 25-55 μm; the specific surface area of ​​the porous membrane is 6-12 m². 2 / g.

8. The PVDF porous membrane for virus removal according to claim 1, characterized in that: The average pore size of the first porous surface, as measured by SEM, is 200-500 nm; the pore area ratio on the first porous surface is 10-25%.

9. The PVDF porous membrane for virus removal according to claim 1, characterized in that: The first porous surface includes a plurality of continuous first fibers, with adjacent continuous first fibers forming a circular first hole; the average diameter of the first fiber is 40-120 nm; the ratio of the average diameter of the first fiber to the average diameter of the first hole is 0.15-0.

45.

10. The PVDF porous membrane for virus removal according to claim 1, characterized in that: The pore area ratio on the second porous surface is 4-15%; the ratio of the average pore diameter of the first porous surface to the average pore diameter of the second porous surface is 1.5-5.

11. The PVDF porous membrane for virus removal according to claim 1, characterized in that: The porous membrane has a tensile strength of 8-20 MPa and an elongation at break of 50-200%. The flux of the porous membrane is greater than 50 L*h -1 *m -2 @30psi; The protein yield of the porous membrane is not less than 98%.

12. A method for preparing a PVDF porous membrane for virus removal according to any one of claims 1-11, characterized in that: Includes the following steps: S1. Prepare a casting solution and cast it onto a carrier to form a liquid film; the casting solution comprises the following components by weight: 18-30 parts PVDF resin, 5-25 parts hydrophilic additive, 30-50 parts small molecule additive, and 20-40 parts organic solvent; the mass percentage of PVDF resin in the casting solution is greater than 20%; S2. An airflow with a relative humidity of 60-90% is blown onto the surface of the liquid film for treatment to obtain a raw film; wherein... The relative velocity between the airflow and the liquid film is 0.1-6 m / s, and the duration is not less than 10 s; and the carrier temperature is at least 5°C lower than the liquid film temperature. S3. Immerse the raw film in the extraction bath for further extraction and solidification, then wash and dry to obtain the film. S4. The film is hydrophilically treated to obtain a porous membrane.

13. The method for preparing a PVDF porous membrane for virus removal according to claim 12, characterized in that, The number-average molecular weight of the PVDF resin is 300,000 to 1,200,000. The hydrophilic additive is at least one of polyethylene glycol, polyvinylpyrrolidone and polyvinyl alcohol; The small molecule additive is at least one of LiCl, NH4Cl, nano-silica, acetone, butanone, and tetrahydrofuran.

14. The method for preparing a PVDF porous membrane for virus removal according to claim 12, characterized in that, In step S2, the liquid film temperature is 15-50℃, and the carrier temperature is 0-40℃. In step S3, the processing time is not less than 3 minutes, and the extraction bath includes at least one of water and small molecule alcohol.

15. The method for preparing a PVDF porous membrane for virus removal according to claim 14, characterized in that, The small molecule alcohol is ethanol or isopropanol; The organic solvent is at least one of N-methylpyrrolidone, dimethylformamide, dimethylacetamide, trimethyl phosphate, triethyl phosphate, and γ-butyrolactone.

16. The method for preparing a PVDF porous membrane for virus removal according to claim 12, characterized in that, The casting solution also contains 1-5 parts by weight of a surfactant, which is a fluorinated surfactant.

17. A filter cartridge for virus removal, characterized in that: The filter element includes the porous membrane according to any one of claims 1-11, and the filter element includes 2-3 stacked porous membranes.

Images