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.