Compressive and shear properties of alginate gel: Effects of sodium ions and alginate concentration

The equilibrium and viscoelastic properties of alginate gel crosslinked with Ca²⁺ were determined as a function of alginate concentration and duration of exposure to physiologic concentrations of NaCl. Compressive and shear stress relaxation tests and oscillatory shear tests were performed to measure the material properties at two time periods after storage in NaCl compared to no NaCl exposure. The effect of concentration was determined by testing 1-3% alginate gel in a bath of physiological NaCl and CaCl₂. After 15 h of exposure to NaCl, the compressive, equilibrium shear, and dynamic shear moduli decreased by 63, 84, and 90% of control values, respectively. The material properties exhibited no further changes after 7 days of exposure to NaCl. The loss angle and amplitude of the relaxation function in the shear also decreased, indicating less viscous behaviors in both dynamic and transient configurations. All moduli, but not the loss angle, significantly increased with increasing alginate concentration. The observed decrease in compressive and shear stiffness for alginate gel after exposure to Na⁺ was significant and indicated that physiological conditions will soften the gel over a time period of up to 7 days after gelation. The alginate gel retains significant solidlike behaviors, however, as measured by a loss angle of approximately 3°. This study provides the first available data for material properties of alginate gel tested in physiological saline.