Because they can reversibly change their size and shape under different conditions, hydrogels are attractive for a range of applications, including artificial muscles, drug delivery, and sensors. The materials tend to be brittle, however, breaking easily when stretched, and this has prevented their commercialization. Now, researchers at Nagoya University and the University of Tokyo have designed hydrogels with temperature and pH sensitivities that are extremely stretchable as well as mechanically strong, reports Phys.org, based on a study published in Nature Communications.

The new hydrogel structure was inspired by recent research on a "slide-ring gel," in which molecules can slide through the holes in a figure-eight-shaped junction of cross-linked polymers, writes Lisa Zyga on Phys.org. By minimizing stress on the polymer network, the so-called pulley effect strengthens the hydrogel.

The resulting hydrogels can be stressed, compressed, coiled, and knotted without breaking. The strong hydrogels also cannot be easily cut with a sharp knife. In addition, they can absorb large amounts of water, becoming 620 times heavier and gaining a much larger volume when placed in water.

"In my opinion, the greatest significance of our work must be that not only chemists but also many researchers in other fields such as physics, biology, and engineering can easily obtain the extremely stretchable hydrogels if they have the polyrotaxane cross-linkers that we made," co-author Yukikazu Takeoka at Nagoya University told Phys.org.