Synovial fluid (SF) contains lubricant macromolecules, hyaluronan (HA), and proteoglycan 4 (PRG4). The synovium not only contributes lubricants to SF through secretion by synoviocyte lining cells, but also concentrates lubricants in SF due to its semi-permeable nature. A membrane that recapitulates these synovium functions may be useful in a bioreactor system for generating a bioengineered fluid (BF) similar to native SF. The objectives were to analyze expanded polytetrafluoroethylene membranes with pore sizes of 50 nm, 90 nm, 170 nm, and 3 mu m in terms of (1) HA and PRG4 secretion rates by adherent synoviocytes, and (2) the extent of HA and PRG4 retention with or without synoviocytes adherent on the membrane: Experiment I: Synoviocytes were cultured on tissue culture (TC) plastic or membranes +/- IL-1 beta TGF-beta 1 + TNF-alpha, a cytokine combination that stimulates lubricant synthesis. HA and PRG4 secretion rates were assessed by analysis of medium. Experiment 2: Bioreactors were fabricated to provide a BF compartment enclosed by membranes +/-adherent synoviocytes, and an external compartment of nutrient fluid (NF). A solution with HA (1 mg/mL, MW ranging from 30 to 4,000 kDa) or PRG4 (50 mu g/mL) was added to the BF compartment, and HA and PRG4 loss into the NF compartment after 2, 8, and 24 h was determined. Lubricant loss kinetics were analyzed to estimate membrane permeability. Experiment 1: Cytokine-regulated HA and PRG4 secretion rates on membranes were comparable to those on TC plastic. Experiment2: Transport of HA and PRG4 across membranes was lowest with 50 nm membranes and highest with 3 mu m membranes, and transport of high MW HA was decreased by adherent synoviocytes (for 50 and 90 nm membranes). The permeability to HA mixtures for 50 nm membranes was similar to 20 x 10(-8) cm/s (- cells) and similar to 5 x 10(-8) cm/s (+ cells), for 90 nm membranes was similar to 35 x 10(-8) cm/s (- cells) and similar to 19 x 10(-8) cm/s (+ cells), for 170 nm membranes was similar to 74 x 10(-8) cm/s (+/- cells), and for 3 mu m membranes was similar to 139 x 10(-8) cm/s (+/- cells). The permeability of 450 kDa HA was similar to 40x lower than that of 30 kDa HA for 50 nm membranes, but only similar to 2.5x lower for 3 mu m membranes. The permeability of 4,000 kDa HA was similar to 250x lower than that of 30 kDa HA for 50 nm membranes, but only x lower for 3 mu m membranes. The permeability for PRG4 was similar to 4 x 10(-8) cm/s for 50 nm membranes, similar to 48 x 10(-8) cm/s for 90 nm membranes, similar to 144 x 10(-8) cm/s for 170 nm membranes, and similar to 336 x 10(-8) cm/s for 3 mu m membranes. The associated loss across membranes after 24 h ranged from 3% to 92% for HA, and from 3% to 93% for PRG4. These results suggest that semi-permeable membranes may be used in a bioreactor system to modulate lubricant retention in a bioengineered SF, and that synoviocytes adherent on the membranes may serve as both a lubricant source and a barrier for lubricant transport. Biotechnol. Bioeng. 2010;106: 149-160. (C) 2009 Wiley Periodicals, Inc.