Direct pressure measurements in a hyaluronan ultrafiltration concentrationpolarization layer

Background : The pressure distribution across a polarized layer forming dur ing the ultrafiltration of macromolecular solutions depends on the physicoc hemical properties of the ultrafiltered molecule. Theoretical and experimen tal studies are in disagreement over the nature of this pressure profile: s ome predict a constant pressure distribution, while others predict a non-ze ro pressure gradient. Our objective was to gain insight into this issue by directly measuring both the pressure distribution and concentration profile in a hyaluronan concentration polarization layer during a model dead-end u ltrafiltration experiment. Methods: A steady-state polarized layer was crea ted by pumping solvent at a constant how rate through a hyaluronan matrix i n a specially designed filtration cell. Polymer concentration and concentra tion gradient were measured using a previously described laser-based refrac tometric technique. For purposes of this study the filtration cell incorpor ated an in-dwelling miniature pressure transducer that traversed the polari zed layer and measured pressures. Results: Polarized layer concentration pr ofiles were similar to those previously reported. We measured a small, but non-zero, pressure drop of 7.9 mmHg across the polarized layer (95% confide nce interval: 6.6-9.3 mmHg), which can be compared with the approximately 4 00 mmHg pressure drop across the ultrafiltration membrane. The measured cha nge in polarized layer pressure was of the same order as that predicted the oretically. Conclusion: The pressure profile across a hyaluronan polarized layer is non-constant, likely due to solute-solute interactions within the polarized layer. Such effects may be most pronounced for long-chain random coiling macromolecules. Further modeling of transport in such long-chain po lymer matrices should take such effects into account. (C) 2001 Elsevier Sci ence B.V. All rights reserved.