Direct Measurement of Osmotic Pressure of Glycosaminoglycan Solutions By Membrane Osmometry at Room Temperature

¹ Columbia University
² National Institutes of Health /NIAMS

^* To whom correspondence should be addressed. E-mail: ateshian{at}columbia.edu.

Submitted on November 30, 2004
Revised on January 13, 2005
Accepted on 6 June 2005

	Abstract

Articular cartilage is a hydrated soft tissue composed of negatively charged proteoglycans fixed within a collagen matrix. This charge gradient causes the tissue to imbibe water and swell, creating a net osmotic pressure that enhances the tissue's ability to bear load. In this study we designed and utilized an apparatus for directly measuring the osmotic pressure of chondroitin sulfate (CS), the primary glycosaminoglycan found in articular cartilage, in solution with varying bathing ionic strength (0.015 M, 0.15 M, 0.5 M, 1 M and 2 M NaCl) at room temperature. The osmotic pressure () was found to increase nonlinearly with increasing CS concentration and decreasing NaCl ionic bath environment. Above 1M NaCl, changes negligibly with further increases in salt concentration, suggesting that Donnan osmotic pressure is negligible above this threshold, and the resulting pressure is attributed to configurational entropy. Results of the current study were also used to estimate the contribution of osmotic pressure to the stiffness of cartilage based on theoretical and experimental considerations. Our findings indicate that the osmotic pressure resulting from configurational entropy is much smaller in cartilage (based on an earlier study on bovine articular cartilage) than in free solution. The rate of change of osmotic pressure with compressive strain is found to contribute approximately one-third of the compressive modulus (H_A^eff) of cartilage ( ~ H_A^eff/3), with the balance contributed by the intrinsic structural modulus (H_A) of the solid matrix (i.e. H_A ~ 2*H_A^eff/3 ). A strong dependence of this intrinsic modulus on salt concentration was found; therefore it appears that proteoglycans contribute structurally to the magnitude of H_A, in a manner independent of osmotic pressure .

Key Words: Articular Cartilage, Compressive Stiffness, Glycosaminoglycan, Osmotic Pressure

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