Abstract
NMR is used to measure sodium flow driven by a 1D concentration gradient inside poly-acrylamid (pAA) hydrogel. A sodium concentration jump from 0.5 M NaCl to 0 M NaCl is applied at the bottom of a cylindrical pAA sample. The sodium level and hydrogen level are measured as a function of time and position inside the sample for 5 days. Then a reversed step is applied, and ion flow is measured for another 5 days. During the measurement, the cylindrical sample is radially confined and allowed to swell in the axial direction. At the same time, sodium and moisture in the sample are measured on a 1D spatial grid in the axial direction. A quadriphasic mixture model (Huyghe and Janssen in Int J Eng Sci 35:793, 1997) is used to simulate the results and estimate the diffusion coefficient of sodium and chloride. The best fit results were obtained for D_{Na^+} = 1.15times 10^{-5} cm2/s and D_{Cl^-} = 2.15times 10^{-5} cm2/s, at 25 degrees centigrade. Different time constants were observed for swelling and deswelling.
Highlights
Biological tissues and cells are in continuous renewal and need constant supply of nutrients and removal of waste products
The material consists of a hydrophilic copolymer gel, which has been synthesized by means of polymerization of acrylic acid (AA) and acrylamide (AAm) monomers
Before the cylindrical pAA sample is placed in the sampleholder, it is blotted and weighed to determine the swelling degree at t 1⁄4 0, when the sample is in equilibrium with a 0.5 M NaCl external bath concentration
Summary
Biological tissues and cells are in continuous renewal and need constant supply of nutrients and removal of waste products. Because of its high affinity to water, it tends not to bind to the negatively charged macromolecules, but rather build double layers around it. Sodium plays a vital role in the water binding capacity of the extracellular space. Sodium concentration is a measure of the fixed charge density (cfc) of the tissue which in turn is a measure of disc degeneration.[1,2,5,13,18] The ocular lens relies on internally directed ion and water flow for its circulation.[22]. A thorough understanding of sodium diffusion in presence of negatively charged macromolecules is essential in the context of the physiology of the disc, the eye as well as in tissueengineering applications
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