Facile synthesis of tough metallosupramolecular hydrogels by using phosphates as temporary ligands of ferric ions to avoid inhibition of polymerization

Abstract

Forming carboxyl-Fe 3+ coordination bonds as physical crosslinks is an effective strategy to develop tough hydrogels. Considering the inhibition of ferric ions on free-radical polymerization, these coordination bonds cannot be formed during the reaction, and a soaking process of preformed hydrogels is usually required for mechanical enhancement, resulting in uncontrollable gradient structure, long preparation time, and unnecessary waste of metallic ions. A facile strategy is reported here to prepare tough metallosupramolecular hydrogels by polymerization and in situ formation of coordination bonds with phosphates as the temporal ligands of Fe 3+ ions. The phosphate ligands in the precursor solution form coordination complexes with the Fe 3+ ions, which avoids the inhibition and ensures the polymerization. After swelling the resultant hydrogel in water, the ligands are substituted by carboxyl groups of the gel matrix due to the variation of local pH. The equilibrated hydrogel with carboxyl-Fe 3+ coordination bonds as the physical crosslinks possesses excellent mechanical properties that can be tuned over a wide range by adjusting the polymer compositions and the concentrations of phosphate ligands and Fe 3+ ions. This strategy should be applicable to other systems to enable synthesis of functional hydrogels with Fe 3+ ions as the additive toward specific applications in engineering and biomedical fields.

Tough and Robust Metallosupramolecular Hydrogels Enabled by Ti3C2Tx MXene Nanosheets

“…In recent years, there has been a growing interest in developing metallosupramolecular hydrogels with enhanced toughness and robustness to overcome the limitations of conventional hydrogels [7,[23][24][25]. One approach to achieve this is by incorporating nanosheets into the hydrogel matrix, which can provide additional mechanical reinforcement and improve the overall performance of the hydrogel.…”

Effect of Salt on Dynamic Mechanical Behaviors of Polyampholyte Hydrogels

“…Creating mechanically robust and tough hydrogels has flourished over the past two decades, − which has enabled the practical application of hydrogels in various fields requiring load-bearing, such as scaffolds for tissue engineering, − medical implants, wound dressings, , flexible and wearable devices, − etc. The basic principle for developing tough hydrogels is incorporating built-in energy dissipation mechanisms through covalent/noncovalent bonds as sacrificial bonds. , Recently, incorporating dynamic noncovalent bonds into hydrogels as reversible sacrificial bonds has attracted extensive attention since the dynamic bonds endow hydrogels with not only high toughness but also self-healing ability. , Hydrogels with ionic bonds, such as polyampholyte hydrogels (PA gels), − polyion-complex hydrogels (PIC gels), , and polyelectrolytes complexed with multivalent ionic groups, − are a typical representative of hydrogels with dynamic noncovalent bonds. Among them, the PA gels consisting of polymer networks bearing randomly dispersed cationic and anionic repeat groups exhibit tunable multiple mechanical properties over wide ranges.…”