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Ionic osmotic effects increase fluid flow during permeation tests

Farrell, Mark and Riches, Philip (2011) Ionic osmotic effects increase fluid flow during permeation tests. Journal of Mechanics in Medicine and Biology.

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Abstract

Fluid flow is essential for the transport of metabolites to, from and within the intervertebral disc. By applying quadriphasic mixture theory experimentally, this study relates fluid flow within the bovine nucleus pulposus (NP) to the applied fluid pressure gradients and fixed charge and mobile ion concentration gradients. 24 plugs of NP tissue (diameter 10mm, height 1020 ± 122 µm (mean ± S.D)), orientated in the axial direction, were harvested from bovine tail discs. The plugs were permeated with either; 0M NaCl, 0.15M NaCl or 3M NaCl solutions by subjecting them to 30, 45 and 60kPa fluid pressure gradients applied in a random sequence. The hypertonic solution was assumed to render all non-fluid pressure gradients negligible, whilst the hypotonic solution was assumed to render the mobile ion concentration gradient negligible. The effects of these gradients on fluid flow were expressed as a percentage of the applied fluid pressure. Fluid velocity was significantly increased through the tissue in the isotonic case compared to the hypertonic case by up to 55% of the applied fluid pressure. The fixed charges accounted for between 26-43% and the mobile ion gradient responsible for 12-26% of this increased fluid flow. These results highlight the importance of using a constitutive equation for permeability that includes mobile ions and fixed charges as separate phases when modelling cartilaginous tissue in order to better describe fluid flow, and thus convective transfer of metabolites, within the tissue.