Wax-coated plastic morphs between soft and stiff

Future robots may sport a wax coating that allows them to transform from soft and squishy to stiff and strong.

Dipping foam or plastic in molten wax creates structures whose flexibility and strength can be tuned with temperature, researchers report June 30 in Macromolecular Materials and Engineering. The study introduces a novel class of inexpensive materials that can be both floppy and firm, says mechanical engineer Robert Shepherd of Cornell University.

Researchers could one day use the materials to build remote-controlled surgical or search-and-rescue robots that can squeeze through tight spaces when heated and firmly push obstacles aside when cooled.

Engineers have previously created shape-shifting materials, such as balloons filled with sand and liquids loaded with iron (SN: 8/10/13, p. 8). But the liquids aren’t that strong and the sand bags aren’t very adjustable, says Shepherd. So MIT mechanical engineer Nadia Cheng and colleagues looked for materials that they could control with heat.

“We melted down crayon wax, beeswax, candle wax, all sorts of things,” says Cheng, who is now at Empire Robotics in Boston. The researchers found that dipping packing foam into melted batik wax — used in fabric dyeing — led to the best combination of stiffness and strength once the wax hardened.

The team also dunked 3-D printed scaffolds of rubbery plastic into vats of wax, let the wax solidify and then measured the materials’ strength by squashing them with a contraption akin to a tiny car-crusher. Wax-drenched materials were up to 100 times as strong as uncoated ones, the researchers found. “It’s a dramatic difference,” says Shepherd.

To control the materials’ flexibility, the researchers wrapped heating wire around tubes of waxed foam. Heating the coils caused the foam to curve in specific spots, like a human spine bending at the neck or waist. As long as the temperature stayed below 71° Celsius, about as hot as it takes to cook an egg, the material bent only in the spots the hot wire touched.

Turning up the heat further let the materials heal themselves. When the team crushed coated plastic, the waxy skins began to crack and peel. Melting the skin back together restored much of the plastic’s strength.

The work is still in the early stages, but it’s a promising new idea, says robotics engineer Erik Steltz of iRobot in Bedford, Mass. He says the robotics community is getting closer to making shape-shifting robots a reality.