Super Tough Hydrogels with Self-adaptive Network Facilitated by Liquid Metal

Liquid metals (LM) refer to Ga-based alloys with low melting points such as Eutectic GaInSn alloys (EGaInSn), which have excellent fluidity, flexibility and low toxicity at room temperature. In this work, by incorporating EGaInSn into poly(acrylic acid) (PAA) hydrogels as dynamic crosslinkers and effective energy-dissipating units, tough and strong PAA-LM hydrogels are prepared through the interactions between the oxide layers on the surface of LM and carboxyl groups of PAA chains. Moreover, owing to the fluidity of LM, one LM droplet as a cross-linking point may change its shape or break into more and smaller droplets in the hydrogels upon stretching. Then hydrogels with self-adaptive networks can be fabricated by mechanical training through cyclic stretch at a certain elongation. Thus, the networks of the hydrogels are further homogenized, and the hydrogels are significantly toughened during the training due to the increased cross-linking density from the larger number of smaller LM droplets. After a 5-cycle training at the maximum strain of 500% and 24 h recovering, the tensile strength and initial modulus of the PAA-LM hydrogels can be increased from 355 kPa to 597 kPa and 71 kPa to 166 kPa, respectively, while the elongation at break only decreases from 1491% to 1034%.