Self-Strengthening of Force-Adaptive Hydrogels Cross-Linked with Pluronic Micelles through Mechanical Training

Hydrogels with excellent flexibility and stimulus responsiveness have important applications in flexible devices such as artificial muscles and soft robots. Various hydrogels can respond to thermal, electrical, and pH stimuli, while it remains challenging to fabricate force-adaptive hydrogels that can adapt to external mechanical stimuli to enhance their mechanical properties, mimicking the strengthening behavior of human muscles after physical training and exercising. Herein, a force-adaptive, self-strengthening poly(acrylic acid) (PAA) hydrogel was fabricated using block copolymer Pluronic F127 (PEO₉₉-PPO₆₅-PEO₉₉) diacrylate and zirconium ions (Zr⁴⁺) as cross-linkers. The Pluronic F127 diacrylate (PF127DA) phase-separated and self-assembled into nanosized micelles in the hydrogels, and zirconium ions (Zr⁴⁺) formed metal coordination with the carboxyl groups in PAA, which endowed the hydrogels with high tensile strength and high stretchability. Notably, the mechanical properties of the hydrogels were significantly improved after the adequate cyclic stretch training and sufficient rest time. The tensile strength of the hydrogel increased from 1633 to 2463 kPa, and the elongation at break increased from 829% to 1026% after training at 500% strain and resting for 24 h. Small-angle X-ray scattering (SAXS) showed that the sizes of Pluronic micelles became smaller after training, and low-field nuclear magnetic resonance (LF-NMR) demonstrated a more homogeneous hydrogel network with a higher cross-linking density. During cyclic stretching, the nanosized Pluronic micelles separated and reconstructed into smaller micelles, generating additional cross-linking domains and accounting for a higher cross-linking density. The homogeneity of the hydrogel network also increased through the reconstruction of Pluronic micelles and the break recombination of the carboxyl-Zr⁴⁺ coordination cross-links, thereby improving the mechanical properties of the hydrogel through training significantly. This research presents an effective approach for developing hydrogels with high tensile strength, high toughness, and force-adaptive capabilities and shows potential to create self-strengthening hydrogels like muscles for future soft robots through mechanical training.