Mille-feuille (''a thousand leaves'') paper is the first non-woven, wet-laid filter paper, composed of 100% native cellulose, which is capable of removal of the ''worst-case'' model virus, the non-enveloped parvoviruses, i.e. minute virus of mice (MVM; 18-20 nm), from water with a log 10 reduction value (LRV) >5 (>99.999%). We further illustrate how the flow rate across the mille-feuille paper can be increased exponentially so that flux rates in the order of 350 L m À2 h À1 bar À1 can be potentially achieved.
Pore-size distribution (PSD) is the most critical parameter for size-exclusion virus removal filters. Yet, different dry- and wet-state porometry methods yield different pore-size values. The goal of this work is to conduct comparative analysis of nitrogen gas sorption (NGSP), liquid-liquid and cryoporometry with differential scanning calorimetry (CP-DSC) methods with respect to characterization of regular and cross-linked virus removal filter paper based on cellulose nanofibers, i.e. the mille-feuille filter. The filters were further characterized with atomic force and scanning electron microscopy. Finally, the removal of the worst-case model virus, i.e. minute virus of mice (MVM; 20 nm, nonenveloped parvovirus) was evaluated. The results revealed that there is no difference of the obtained PSDs between the wet methods, i.e. DSC and liquid-liquid porometry (LLP), as well as no difference between the regular and cross-linked filters regardless of method. MVM filtration at different trans membrane pressure (TMP) revealed strong dependence of the virus removal capability on applied pressure. It was further observed that cross-linking filters showed enhanced virus removal, especially at lower TMP. In all, the results of this study highlight the complex nature of virus capture in size-exclusion filters.
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