Effect of Hydration Time in Saline on the Swelling and Uniaxial Tensile Response of Annulus Fibrosus of the Intervertebral Disc.

The intervertebral disc (IVD) is a biphasic tissue in which the extracellular matrix (ECM) acts as a structural scaffold and regulates hydration and solute transport. The influence of hydration on the swelling and mechanical properties of the IVD, particularly the annulus fibrosus (AF), is not fully described in the literature. Hydration is assumed to affect inter- and intramolecular hydrogen bonding and hydrophilic interactions, thereby modulating tissue mechanics. This study aimed to assess the effect of hydration time on free swelling of AF and its impact on mechanical performance. AF specimens were divided into five groups, hydrated for 0, 10, 20, 30, or 40 min and subjected to uniaxial tensile testing until failure. Swelling-related geometric changes were correlated with tensile properties. Results demonstrated that hydration duration significantly influenced AF's structural and mechanical characteristics in anterior and posterior IVD regions. Hydration increases rapidly within 10-20 min, causing cross-sections to swell, stress capacity to decrease, and stiffness to remain unchanged. However, after 40 min, the tissue becomes swollen beyond physiological balance. These findings identify hydration duration as a critical factor regulating AF function and provide important insights for experimental standardization, numerical modeling, and hydrogels designed for intervertebral disc regeneration.