Charge density-driven IPN formation and recovery in hyaluronic acid/ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si60.svg" display="inline" id="d1e890"> <mml:mi>κ</mml:mi> </mml:math> -carrageenan hydrogels: Novel insights from e

This study investigates how ion valency and coordination behavior modulate the formation, viscoelasticity, and recovery of hyaluronic acid (HA)/ κ -carrageenan ( κ CG) hydrogels. Hydrogels were prepared by combining HA and κ CG in the presence of monovalent (K + ), divalent (Ca 2＋ ), and trivalent (Al 3＋ ) ions at concentrations of 15–60 mM. Microrheology, Fourier-transform infrared (FTIR) spectroscopy, and bulk oscillatory rheology were employed to correlate molecular interactions with mechanical behavior. Frequency sweep measurements revealed that the moduli of the hyaluronic acid/ κ -carrageenan (HA– κ CG) blend increased by nearly two orders of magnitude relative to HA alone, confirming synergistic network reinforcement. Among ion-enriched systems, Ca 2＋ produced the highest G plateau ′ ( ∼ 20 kPa) and lowest modulus loss between 25 ° C and 37 ° C ( < 10 % ), while Al 3＋ induced intermediate moduli. Recovery tests demonstrated that Ca 2＋ -enriched hydrogels recovered ∼ 55% of their initial elasticity within 15 min, compared to ∼ 30% for K + and ∼ 40% for Al 3＋ systems. Microrheology confirmed corresponding trends in probe confinement, with subdiffusive exponents ( α ) ranging from 0.18 (K + ) to 0.32 (Ca 2＋ ). FTIR analysis revealed that while K + primarily stabilized κ CG helices, Ca 2＋ reinforced sulfate junction zones, and Al 3＋ introduced additional HA–carboxylate coordination. Collectively, these results demonstrate that both ion charge and concentration govern the balance between elasticity and deformability in HA– κ CG systems, defining whether true interpenetrating or semi-interpenetrating networks emerge. These insights establish design guidelines for improvement of mechanical performance in chemically unmodified HA-based hydrogels. • κ -carrageenan enhances structure in chemically unmodified hyaluronic acid hydrogels. • IPN formation in hyaluronic acid– κ -carrageenan gels is tunable via ion charge density. • Elasticity–recovery behavior shifts with ion charge and concentration