Angewandte Chemie International Edition
© WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
For full article and contact information, see Angew. Chem. Int. Ed. 2003, 42 (23), 2644 - 2647
Mechanical structures made of tiny beads
spontaneously heal after breaking
One of the many advantages that living beings have over man-made structures is their amazing ability to heal themselves. How practical it would be if broken objects could also repair themselves. Some steps toward self-healing materials have already been taken with the development of several plastics and ceramics with self-healing properties. George M. Whitesides and Mila Boncheva at Harvard University in Cambridge, MA (USA), have now found a new way to introduce self-healing powers into inanimate matter. They used millimeter-sized beads to build up structures that spontaneously return to their original linear arrangement after a break or dislocation, much like the spine in vertebral animals.
A spinal column consists of rigid structural elements, the vertebrae, which are separated by elastic discs and are held together by muscles and ligaments. This construction principle, carefully worked-out by Nature, renders the spine both strong and flexible, cushioning blows and protecting the sensitive spinal cord. The Harvard researchers have copied this successful blueprint. The role of the vertebra is taken on by hour-glass-shaped, millimeter-sized, plastic beads. By means of holes in their "waists", these are strung onto an elastic thread that is secured with knots at either end. The thread is thus under tension and acts like the muscles and ligaments. The tension puts pressure on the beads, which forces them into a linear formation in which each is perpendicular to the next. The beads are equipped with little copper plates that carry a little patch of solder. If the strand of beads is heated above the melting point of the solder and is then cooled down again, the solder binds the beads into a solid, compact rod. The solder thus acts like the vertebral discs. These micro-spines can carry up to 250 g before breaking into two pieces, which are still held together by the thread. Simply heating them up in hot water and lightly shaking the container are enough to repair the break. A second spinal column variation, in which the knotted ends of the threads are additionally fastened into a clamping mechanism, is even more stable. This construction was modeled after a traction device used to relieve strain on the spinal column after an injury. The traction forces cause the strand of beads to spontaneously return to the linear rod structure after an overload, simple heating then heals the break.