Shape-shifting robots

A team of researchers from Virginia Tech in the US has demonstrated how objects can have multiple functions by constructing them out of materials that are designed to change shape, which could have wider implications for manufacturing. 

This object can change shape from a surface vehicle to a quadcopter.

The team was led by Michael Bartlett, assistant professor in mechanical engineering and funded through a DARPA Young Faculty award and Director’s fellowship. Bartlett explained: “When we started the project, we wanted a material that could do three things: change shape, hold that shape, and then return to the original configuration, and to do this over many cycles.” He added: “One of the challenges was to create a material that was soft enough to dramatically change shape, yet rigid enough to create adaptable machines that can perform different functions.”

For the basic material they used an endoskeleton made of a low melting point alloy, or LMPA, which was embedded inside a rubber skin. Normally, when a metal is stretched too far, the metal becomes permanently bent, cracked, or stretched into a fixed, unusable shape but embedding it in rubber allowed the composite to hold a desired shape, enabling soft morphing materials to become instantly load bearing.

A key element is the way that the material is structured, and for this the team used the principles behind kirigami, the Japanese art of making shapes out of paper by cutting (not to be confused with origami, which uses folding). The material is structured with patterned kirigami incisions that allow the composite to morph quickly from one shape to another, and which can become fixed to load bearing structures. 

To return the structure back to its original shape, soft, tendril-like heaters were incorporated next to the LMPA mesh. The heaters cause the metal to be converted to a liquid at 60ºC, which is 10 percent of the melting temperature of aluminum. The elastomer skin keeps the melted metal contained and in place, and then pulls the material back into the original shape, reversing the stretching, giving the composite what the researchers call ‘reversible plasticity’. After the metal cools, it again contributes to holding the structure’s shape.

Dohgyu Hwang, a graduate student who co-authored the report, explained: “These composites have a metal endoskeleton embedded into a rubber with soft heaters, where the kirigami-inspired cuts define an array of metal beams. These cuts combined with the unique properties of the materials were really important to morph, fix into shape rapidly, then return to the original shape.”

This kirigami-inspired composite design can be used to create complex shapes, from cylinders to balls to the bumpy shape of the bottom of a pepper. The shape could also be changed quickly, in one test it took less than 1/10 of a second following impact with a ball to change the shape and fix it into place. Also, if the material breaks, it can be healed multiple times by melting and reforming the metal endoskeleton.

There’s a wide range of potential applications for this technology. The research team used it to create a functional drone that autonomously morphs from a ground to air vehicle as well as a small submarine that could retrieve objects from the bottom of an aquarium. Other uses could include so-called soft robotics, whereby machines can carry out various functions and to self-heal if damaged. The technology could also be applied to wearable devices.

You can find further information, including a video showing how this material works, from vt.edu.


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