Hydrogels reinforced with rings on strings

Hydrogels have promise as tissue engineering scaffolds and in other biomedical applications. But these materials, which are made mostly of water held together by hydrophilic, cross-linked polymers, tend to be weak. To toughen up hydrogels, scientists have introduced sacrificial bonds: stretch the material, and these bonds break, dissipating energy so that rupturing the hydrogel is more difficult. These bonds can be slow to re-form, however, so the boost in strength doesn’t last.

Researchers at the University of Tokyo have come up with an alternative toughening strategy. A team led by Koichi Mayumi and Kohzo Ito created a hydrogel that contains polyethylene glycol chains threaded through pairs of covalently linked cyclodextrin rings to make a netlike structure (Science 2021, DOI: 10.1126/science.aaz6694).

When the hydrogel is stretched, the polymer chains slide through the rings, which maintain the cross-links between polymer strands. Stretch the polymer enough, and the polyethylene glycol crystallizes at points, increasing toughness. Studies show this slide-ring hydrogel is 10 times as tough as a hydrogel made from polyethylene glycol with conventional cross-links. The researchers say the slide-ring reinforcement strategy could be used with other gels made of semicrystalline polymers.