The fouling of hollow fiber filter membranes used in perfusion cell cultures is one of many hurdles in the transition toward continuous manufacturing. Fouling clogs membranes, increase the transmembrane pressure that contributes to unstable processes and even process failures.

Recently, Dominique WuDunn, associate scientist, AstraZeneca, and first author of a recent paper, and her colleagues directly compared three manufacturers’ filters in terms of fouling. They assessed the effects of pore size, inner diameter, and filter length on product retention in the bioreactor (sieving). They also considered the increase in transmembrane pressure in tangential flow filtration (TFF) perfusion cell culture.

The team produced a proprietary recombinant monoclonal antibody (mAb) using a cell line derived from Chinese hamster ovaries and tested a total of 10 hollow fiber filters from three manufacturers. Surface areas ranged from 155 to 1,000 cm2.  Pore sizes were either 0.2 or 0.65 µm. Internal diameters (ID) ranged from 1 to 2.6 mm, and filter length from 19.8 to 41.5 cm. A constant shear rate of 600 s−1 was used for all but one filter (because of technical considerations).

Internal diameter vs. length

As the researchers reported, “Filters with larger internal diameters maintain higher product sieving and lower hydraulic membrane resistance.” However, “the shortest hollow fibers also maintained the highest product sieving, but not necessarily the lowest hydraulic membrane resistance.”

For product sieving, Company A’s filter with 0.2 μm pore size and 2.6 mm internal diameter (ID) “maintained sieving above 98% for the entire duration of the cell culture, ending with yields of 93.4% and 93%,” they wrote. That filter had an 800 cm2 surface area and 19.8 cm length. Shear rate was 300 s−1.

Two other filters from the same company, with the same pore size and length, but with a 1.4 mm internal diameter and surface areas of 200 and 900 cm2 produced final yields of 57.9% and 42%, respectively.

The other filters performed comparably until day four. After that, the two 0.2 µm, 2.0 mm ID filters from Company B “maintained higher product sieving, ending with yields of 48.4% and 46.3%.”

Regarding other parameters, Company C’s 0.2 μm, 2.0 mm ID filters maintained the lowest level of hydraulic membrane resistance. The highest resistance occurred when the internal diameter shrank to 1.0 mm. The study also confirmed that larger pore sizes reduce both sieving and hydraulic membrane resistance.

The authors say this study shows that “…a shorter length hollow fiber, larger lumen ID, and lower shear rate can result in a significant reduction in fouling, given the same starting cell culture material.”

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