Easy Repair: Novel structural model heals with heat

The capacity of biological tissues to heal after being wounded is one of their most enviable traits. In recent years, materials scientists have been trying to emulate this capability by developing synthetic self-healing or easily mendable materials for products ranging from aerospace parts to athletic gear (SN: 2/17/01, p. 101: Scientists develop self-healing composites).

Now, Mila Boncheva and George Whitesides of Harvard University are tapping the vertebral spine for inspiration. Using millimeter-scale polymer beads for vertebrae and thin elastic threads for muscles and ligaments, the researchers have created spinelike structures that can deform drastically, even become damaged, yet still return to their original forms. The researchers describe two of these structures in the June 16 Angewandte Chemie International Edition.

“I’ve never seen anything like it,” comments Richard Syms of Imperial College in London.

To make one of the structures, Boncheva and Whitesides strung 10 hourglass-shape beads on an elastic thread, which they knotted tightly. The thread exerted compressive forces on the beads, which lined up perpendicular to one another, their waists snugly meshing. Each bead had a small patch of low-melting-point solder on each side of its waist, and these patches bonded the beads into a solid structure.

When part of the chain of beads was held on a surface, it supported around 250 grams–roughly the weight of two sticks of butter–applied at the other end. With more weight, one of the soldered joints gave out. Gentle shaking in a beaker of warm water realigned the chain. After the solder cooled and hardened, the researchers could repeat the breaking and reforming process.

To make their second structure, Boncheva and Whitesides strung the beads and knotted the ends of the string, then attached each end to a support 1 centimeter from the end beads. The resulting tensile forces on the beads mimicked those in a traction splint, which is sometimes used to hold fractured bones in place. When a solder joint broke, this system required only heating–no shaking–to re-form.

“It’s a clever system,” says Richard Wool of the University of Delaware in Newark. It could prove useful for designing vehicle-escape panels, car windshields, or even impact-resistant military-tank parts that could regain their original shape when heated, he speculates.

Although they’re not yet sure how to do it, the researchers aim to scale down their system using micro- or even nanoscale parts to replace the beads, solder, and elastic thread, says Boncheva.

With such miniaturization, researchers might use this strategy for installing healing properties into materials’ internal microstructures.

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