The gel doing an impersonation of a trampoline in the video above is a new synthetic material from Harvard engineers, a substance that stretches to more than 20 times its length and can withstand more force than human cartilage, the resilient tissue that cushions our joints.
This gel starts out as a powder of two different substances, whose molecules link among themselves when mixed with water. Its astounding ability to stretch without tearing comes from the two components’ different cross-linking styles. One component, seaweed extract alginate, forms cross-links using ionic bonds, in which atoms of opposite charges attract. The other component, contact lens ingredient polyacrylamide, forms cross-links using covalent bonds, in which atoms connect by sharing electrons. Ionic bonds are very flexible, while covalent bonds are very strong, and both have their roles to play: When the ball hits the gel, the alginate molecules separate, like two magnets forced apart, absorbing stress. But the gel holds together throughout the bounce thanks to the stronger covalent bonds of the polyacrylamide.
Previous exceptionally stretchy gels were one-time-use materials: they also involved two substances, but both cross-linked using covalent bonds, but one set always broke whenever the stuff was stretched. In contrast, this new gel can recover most of its strength after a period of rest, because the ionic bonds of the alginate reform, like magnets you’ve pulled apart that snap back together when you let them go.