Direct pressure measurements in a hyaluronan ultrafiltration concentration polarization layer

Background: The pressure distribution across a polarized layer forming during the ultrafiltration of macromolecular solutions depends on the physicochemical properties of the ultrafiltered molecule. Theoretical and experimental studies are in disagreement over the nature of this pressure profile: some predict a constant pressure distribution, while others predict a non-zero 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 ultrafiltration experiment. Methods: A steady-state polarized layer was created by pumping solvent at a constant flow rate through a hyaluronan matrix in a specially designed filtration cell. Polymer concentration and concentration gradient were measured using a previously described laser-based refractometric technique. For purposes of this study the filtration cell incorporated an in-dwelling miniature pressure transducer that traversed the polarized layer and measured pressures. Results: Polarized layer concentration profiles were similar to those previously reported. We measured a small, but non-zero, pressure drop of 7.9 mmHg across the polarized layer (95% confidence interval: 6.6–9.3 mmHg), which can be compared with the approximately 400 mmHg pressure drop across the ultrafiltration membrane. The measured change in polarized layer pressure was of the same order as that predicted theoretically. 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 polymer matrices should take such effects into account.