May 3, 2016

Cornell Researchers Create New Material Capable of Shifting States

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For all the discussion surrounding artificial intelligence and robots recently, the stiff, dull metal exterior of robots has only recently begun to evolve. While human-like robots, with silicon skin, can simulate emotions but robots with the shape-shifting ability of the Transformers have yet to hit the market. However, Prof. Robert Shepherd, mechanical and aerospace engineering, is developing a material that could soon bring that to reality.

Shepherd and his team at  Organic Robotics Lab is working on a metal-rubber composite by harnessing the strength of a metallic alloy and the flexibility of a soft silicone foam. The material can withstanding heavy loads or deform under them upon command. The only requirement for switching between these properties is a change in temperature.

The material is capable o both withstanding heavy loads or deforming under them as needed.

Image courtesy of Ilsa van Meerbeek

The material is capable o both withstanding heavy loads or deforming under them as needed.

A metal salt and uncured silicone — a rubber like substance with adhesive properties — are used to synthesize the material. The two ingredients are poured into a mold, placed in a water-bath and then a vacuum chamber. The final step ensures that the air in the silicone foam’s two millimetre pores is replaced with the metal, forming the composite.

The team published their findings in the April 13th issue of Advanced Materials.

“We’ve basically made a rubber sponge and infiltrated that sponge with molten metal, which when cooled gives you a metal foam intertwined with a rubber foam,” Ilsa van Meerbeek grad said.

Choosing the right material is crucial, Meerbeek explained. Known as Field’s metal, it was chosen specifically for its low melting point, 62°C, and low toxicity, making it more practical for use in human environments.

Below 62°C, the metal is solid, ensuring that the entire composite remains rigid. At higher temperatures, the metal is molten and the silicone foam’s properties dominate, allowing it to deform. At these temperatures, the molten metal also “fills” in any cracks that may have formed, such that, the composite “self-heals.”

Meerbeek is optimistic about the material’s future and he did not rule out the possibility of mass-producing such materials in the near future.

“The only limiting factor is how big a mould and vacuum chamber you can make,” Meerbeek said.

Only a handful of other such materials have been developed.

“[Similar materials] typically have channels that are filled with the metal, so you get a two dimensional structure and what we have is three dimensional, widening the possible applications,” Meerbeek said.

The cracks that develop in this three dimensional structure does not propagate too far. However, in simple two dimensional structures, an entire channel is compromised by a tiny crack, weakening the overall structure.

“The reason we work on this is that traditional robots tend to be either very complicated in design and have high functionality or very simple and have limited functionality,” Meerbeek said. “We’re trying to bridge that gap by making soft material devices.”

The molten metal can fill any cracks that are formed and so can "self heal."

Image courtesy of Ilsa van Meerbeek

The molten metal can fill any cracks that are formed and so can “self heal.”

Besides its sheer science fiction like appeal, Meerbeek asserts that the material has multiple applications. The U.S Air Force, which partially funded this project through its Young Investigator Research Program, is interested in the possibility of creating wings that can alter their structure mid-flight using such materials.

“Morphing the wing would require changing its shape really quickly,” Meerbeek said.

Other devices would use the flexibility that it affords to squeeze through tight spaces and then become rigid as necessary.

“Soft material devices would be able to traverse rubble filled areas, often a result of earthquakes, but then become rigid to perform various tasks, perhaps supporting crumbling structures,” Meerbeek said.

Although the possibilities are enticing, Meerbeek insists that there are still hurdles to overcome.

“It withstands a decent load but the metal part breaks at a relatively low force compared to other metals,” Meerbeek said.

In fact, the team hopes to explore other manufacturing techniques, 3-D printing among them, to strengthen the internal structure of the composite.

“There’s also the issue of how to quickly heat the material while it’s in a device,” Meerbeek said. “But I don’t think these are insurmountable problems.”

Though still in its nascent stages, Meerbeek underlined the importance of integrating the material into existing systems. Indeed, the team aims to continue improving it, hoping that the material “could one day be whatever you wanted it to be.”