Mille-feuille Filter : A Non-woven Nano-cellulose Based Virus Removal Filter for Bioprocessing

Virus removal filters, produced from synthetic surface-modified polymers or regenerated cellulose by phase inversion, are vital to the production of therapeutic proteins such as monoclonal antibodies and plasma proteins. Use of these filters is also one of the most expensive purification steps in the downstream processing of proteins due to high sales price and being limited to a single use. In this thesis, a virus removal filter produced from Cladophora sp. algal nanocellulose has been characterized. The mille-feuille (‘‘a thousand leaves’’) filter paper is the first non-woven, wet-laid filter paper composed of 100% native nanocellulose that is capable of removing the ‘‘worst-case’’ model viruses, the non-enveloped parvoviruses, i.e., minute virus of mice (MVM; 18–20 nm), from water with a log10 reduction value (LRV) ≥5.78 (≥99.9998%). The mille-feuille filter features a unique internal stratified architecture that is the result of nanofiber self-assembly into 2D nanosheets during manufacturing. Such an internal structure has several benefits for achieving highly selective virus removal with high flux. The pore size distribution can be tailored to sizes from 10 to 25 nm by altering drying conditions, i.e. temperature and drying rate; therefore, the filter can be customized to target the size cut-off of the smallest virus particles known. The mille-feuille filter has achieved up to 200 L m-2 h-1 (LMH) bar-1 in flux. Furthermore, protein recovery rates of 99% were measured during bovine serum albumin (BSA) filtration. Protein recovery was determined to be dependent on the protein size and charge. Filtration of cell culture media was also investigated, and no fouling was observed with fluxes of 400 LMH for an 11 µm filter and 140 LMH for a 33 µm filter at 3 bar. An LRV of >4.8 was measured for the 33 µm filter at 3 bar, but only 2.2 was measured for the 11 µm filter at 3 bar using the small-size ФX174 bacteriophage as a model virus. Furthermore, the virus reduction was discovered to be pressure dependent, with the LRV increasing with trans membrane pressure (TMP). The tendency to virus breakthrough was partly mitigated at low TMPs by filter cross-linking. In summary, the mille-feuille filter paper has the characteristics to be a promising virus removal filter for both upstream and downstream applications. Further studies shall focus on the area of protein filtration to gain a better understanding of how buffer conditions and the physical characteristics of proteins contribute to filter fouling