The secret to making something break less is to make it break more—at least at a microscopic level. When something brittle such as glass shatters, the only molecules involved in the breakage are the ones along the surface of the shards; inside individual fragments, the material is virtually unaffected. To reduce brittleness, researchers design materials that distribute stress below the surface, which prevents cracks from propagating and keeps the object from breaking up in the first place.
Zhigang Suo of Harvard University and his collaborators have now applied this principle to a class of materials called hydrogels, which are made up of water and networks of long molecules known as polymers, which act as scaffolding. Suo's hydrogels, which have the consistency of rubber, can stretch to 20 times their original size without breaking. By comparison, a typical rubber band snaps if it is stretched more than sixfold, Suo says. The new material also has remarkable toughness, which, in the technical sense, is the ability to absorb pressure, tension or an impact without breaking. The energy it takes to break this hydrogel is 10 times greater than for similar materials.
Previous hydrogels lacked toughness and often fell apart like tofu. The secret of Suo's hydrogel is that it contains not one polymer scaffolding but two. The first is made of long carbohydrate chains derived from algae. The chains, held together by positively charged calcium ions, pair up like the two sides of a zipper.
A secondary scaffolding consists of a synthetic polymer whose long chains link together in strong bonds. When an impact hits the material, the algae-derived chains unzip and the calcium ions disperse in the water. The secondary network distributes the stress deeper below the cracking surface, so the energy dissipates into a larger volume of the material. Once the stress is removed, the material self-heals because calcium ions, attracted to negatively charged segments in the algae chain, zip the primary network back together.
The new material, though not ready for prime time, shows that hydrogels may be strong enough for applications such as tissue engineering and prosthetics. “Today if your cartilage is damaged, it is very difficult to replace,” Suo says. And any artificial replacement would need to be at least as tough as the natural stuff. Suo and his collaborators published their work in the September 6 issue of Nature.
The energy it takes to break the new hydrogel is “really impressive,” says Hokkaido University's Jian Ping Gong, who in 2003 led the team that first pioneered double-network hydrogels. Gong, however, points out that the self-healing in the new material is not complete and is somewhat slow, taking several hours. To be useful in applications, the material will have to reach 100 percent healing, she says, and do so in a shorter time.