Triphasic Theory for Swelling Properties of Hydrated Charged Soft Biological Tissues

Many hydrated soft tissues, such as articular cartilage, meniscus and intervertebral disc, change their dimensions, volume and weight when the ion concentration in the external bathing solution is changed (Parsons and Black 1979; Maroudas 1975, 1979; Maroudas and Bannon 1981; Myers et al 1984; Mow and Schoonbeck 1984; Eisenberg and Grodzinsky 1985, 1987). For example, for an unloaded specimen of cartilage soaked in NaCl solutions at constant temperature, the tissue dimensions decrease nonlinearly in an exponential manner with increasing NaCl concentration; this decrease approaches an asymptote at a high concentration, e.g., 2.5M NaCl (Mow and Schoonbeck 1984). Therefore, at the physiological state of 0.15M NaCl, cartilage is in a swollen state, with a swelling pressure resisted by the elastic stress developed in the collagen-proteoglycan solid matrix. This is often referred to as the collagen pre-stress or elastic stress in the solid matrix (Maroudas 1975, 1976, 1979; Maroudas and Bannon 1981; Grodzinsky et al 1981). For a normal cartilage bathed in 0.15M NaCl solution, the swelling pressure has been measured to be around 0.17 MPa (Maroudas 1975,1979; Maroudas and Bannon 1981).