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

Nadeen O. Chahine ^* , Faye H. Chen , Clark T. Hung  and Gerard A. Ateshian ^*

^* Musculoskeletal Biomechanics Laboratory, and Cellular Engineering Laboratory, Department of Biomedical Engineering, Columbia University, New York, New York; and Cartilage Biology and Orthopaedics Branch, National Institute of Arthritis and Musculoskeletal and Skin Diseases, National Institutes of Health, Bethesda, Maryland

Correspondence: Address reprint requests to Gerard A. Ateshian, Dept. of Mechanical Engineering, Columbia University, SW Mudd, 500 West 120th St., Mail Code 4703, New York, NY 10027. Tel.: 212-854-8602; Fax: 212-854-3304; E-mail: ateshian{at}columbia.edu.

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, 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 chondroitin sulfate concentration and decreasing NaCl ionic bath environment. Above 1 M 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 () of cartilage (), with the balance contributed by the intrinsic structural modulus of the solid matrix (i.e., ). A strong dependence of this intrinsic modulus on salt concentration was found; therefore, it appears that proteoglycans contribute structurally to the magnitude of in a manner independent of osmotic pressure.

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