Tunable Crosslinking, Reversible Phase Transition, and 3D Printing of Hyaluronic Acid Hydrogels via Dynamic Coordination of Innate Carboxyl Groups and Metallic Ions

This article reports tunable crosslinking,
reversible phase transition,
and three-dimensional printing (3DP) of hyaluronic acid (HyA) hydrogels
via dynamic coordination of Fe3+ ions with their innate
carboxyl groups for the first time. The concentrations of Fe3+ and H+ ions and the reaction time determine the tunable
ratios of mono-, bi-, and tridentate coordination, leading to the
low-to-high crosslinking densities and reversible solid–liquid
phase transition of HyA hydrogels. At the monodentate-dominant coordination,
the liquid hydrogels have low crosslinking densities (HyA_L). At the
mixed coordination of mono-, bi-, and tridentate bonding, the solid
hydrogels have medium crosslinking densities (HyA_M). At the tridentate-dominant
coordination, the solid hydrogels have high crosslinking densities
(HyA_H). The reversible solid–liquid phase transitions among
HyA_L, HyA_M, and HyA_H were achieved via controlling the concentrations
of Fe3+ and H+ ions and reaction time. When
the crosslinking densities are between HyA_L and HyA_M, the hydrogels
become 3D printable (HyA_P). HyA_P hydrogels were 3D-printed successfully
using cold-stage or direct writing methods, and the 3D constructs
achieved better structural stability using the latter method. In the
direct exposure culture with bone marrow-derived mesenchymal stem
cells, the 3D-printed HyA_H (HyA_H_3D) and HyA_H hydrogels showed
higher average cell adhesion densities than the HyA_M, HyA_P, and
HyA_L hydrogel groups under both direct and indirect contact conditions.
For all hydrogel groups, cell adhesion densities under direct contact
conditions were statistically lower than the same groups under indirect
contact conditions. In this article, we elucidated the mechanisms
of dynamic coordination and the relationships among the key parameters
in controlling the tunable crosslinking, reversible phase transition,
and 3DP of HyA hydrogels without blending with other polymers or adding
functional groups. This approach can be potentially adapted to crosslink
and 3D print other polymeric hydrogels with carboxyl groups, which
is promising for a wide range of applications.