Researchers led by Prof. TIAN Xingyou and Prof. ZHANG Xian from the Hefei Institutes of Physical Science (HFIPS) of the Chinese Academy of Sciences, together with Prof. YANG Yanyu from Zhengzhou University, have used gallium indium alloy (EGaIn) to initiate the polymerization and serve as flexible fillers to construct liquid metal/polyvinyl alcohol(PVA)/P(AAm-co-SMA) double network hydrogel.

"The resulting material was super-stretchable and self-healing," said LI Xiaofei, first author of the study, "it will promote the research and practical application of hydrogels and liquid metal in smart devices."

The study was published in Materials Horizons.

Most conductive hydrogels suffer from inferior mechanical properties and lack desirable self-recovery and self-healing capabilities, severely limiting the potential applications of hydrogels. Liquid metals such as EGaIn can toughen polymers by conforming to their changing shapes. In addition, gallium (Ga) in EGaIn can initiate the free-radical polymerization of vinyl monomer.

In this study, the researchers constructed a liquid metal/PVA/P(AAm-co-SMA) double network hydrogel (LM hydrogel) with EGaIn as both the polymerization initiator and the flexible fillers.

The PVA network used PVA microcrystals and coordination interaction of Ga³⁺ and PVA as cross-links, while the P(AAm-co-SMA) network used hydrophobic association and the EGaIn microspheres. The LM hydrogel was endowed with excellent super-stretchability (2,000%), toughness (3.00 MJ/m³), notch resistance, and self-healing property (> 99% at 25 °C after 24 hours) due to the multiple physical cross-links and the synergistic effect of the rigid PVA microcrystal network and the ductile P(AAm-co-SMA) hydrophobic network.

"The developed sensors can be used in health monitoring and motion identification through human-computer interaction," said LI Xiaofei, "thanks to the sensitive strain sensing capability of the LM hydrogel."

As a result of EGaIn's low infrared emissivity and remarkable photothermal properties, the LM hydrogel shows great promise in infrared camouflage.

This work was supported by the National Natural Science Foundation of China and the HFIPS Director's Fund.

Schematic structure and application of the liquid metal hydrogel. (Image by LI Xiaofei)