Move toward vehicles that morph

MIT engineers report they may have found a way for structures "“ and materials "“ to move like a bird, adjusting its wings to catch every current of air, essentially morphing from one shape into another.

The discovery could lead to an airplane that morphs on demand from the shape that is most energy efficient to another better suited to agility, or in the case of a boat could have a hull that changes shape to allow more efficient movement in choppy, calm or shallow waters.

This has been unobtainable with such conventional devices as hydraulics, which aren't practical for a variety of reasons "“ from cost to weight to ease of movement.

The work involves a new application of a familiar device: the rechargeable battery.

Batteries expand and contract as they are charged and recharged.

"This has generally been thought to be something detrimental to batteries. But I thought we could use this behaviour to another end: the actuation, or movement, of large-scale structures," said Yet-Ming Chiang, the Kyocera Professor in the Department of Materials Science and Engineering (MSE).

Several types of "active" materials are already used to move devices ranging from miniature motors to micropositioners. None, however, "can enable the large-scale structural morphing we've been working toward," Professor Steven R. Hall of the Department of Aeronautics and Astronautics said.

While searching for materials that would allow such morphing, engineers have recently focused on nature's approach to the problem. A plant the bends toward the light, quickly furls its leaves when touched, or pushes a concrete sidewalk aloft with its roots is essentially moving fluids between cells.

Chiang realised that the solid compounds used to store electrical energy in lithium rechargeable batteries could be made to work in a similar way. The movement of ions to and from these materials during charging and recharging, he thought, was analogous to the moving fluids in plants.

Chiang and Hall began testing commercially available rechargeable batteries of prismatic form, and designed their own devices composed of graphite posts surrounded by a lithium source.

They found that the batteries continued to expand and contract under tremendous stresses, a must for devices that will be changing the shape of, say, a stiff helicopter rotor that's also exposed to aerodynamic forces.

Other key advantages of the approach: The electrically activated batteries can operate at low voltages (less than 5 V) and the materials that make up the batteries are also inherently light.

"For things that fly, weight is important," Hall said.

The researchers have already demonstrated basic battery-based actuators that can pull and push with large force. Later this year, they hope to demonstrate the shape-morphing of a helicopter rotor blade. The morphing capability should allow for a more efficient design, ultimately making it possible for a vehicle to carry heavier loads.

The researchers emphasise that much work remains to be done, such as refining the design of the battery for optimal operation in a morphing vehicle. Chiang notes, however, that "we've been able to demonstrate the potential of this approach even using these very unoptimised off-the-shelf batteries."