TPU dimples give robots the human touch

West Lafayette, Indiana – A team at Purdue University has used 3D printed TPU to show that a patterned sheet of domes can form an energy-storing skin. They claim it is strong enough to perform mechanical tasks, and could even store and process data like a mechanical computer.

When the dimpled circles are pressed, they change from being convex and standing proud from the surface to being concave hollows. Structures like this with two stable states demonstrate the concept of bi-stability.

‘Bi-stability is an important concept found in nature,’ said Andres Arrieta, assistant professor of mechanical engineering at Purdue. ‘Earwigs, for example, have bistable, foldable wings that snap to an open state with very little energy. We are working to make programmable structures inspired from this bi-stability.’

The groups started with a flat, one-inch square sheet with a pop-up dome that was 3D-printed with TPU. When pressed with a finger it would snap from convex to concave. But when they printed a 3x3 grid of the domes, new behaviours appeared.

‘When you invert two domes that are close to another, they start interacting,’ Arrieta said. ‘And when you start making patterns of these domes on a sheet, the sheet itself begins to curve globally. Depending on which domes are inverted, you get different shapes.’

They moved on to larger grids and more complex patterns, and by pressing various domes in or out, the sheet could be made to form a cylinder, a star or a saddle shape. “These individual bi-stable domes are combining to form a new metamaterial, which itself has multiple stable states,’ Arrieta said. ‘We call it hierarchical multistability.”

When the domes are concave, the gripper arms remain open

To demonstrate how it might be used, they built a robotic gripper using two lines of domes. When concave, the gripper arms remained open. If a small amount of air pressure was applied to make the domes convex, the arms closed sufficiently tightly to grasp and hold a small weight.

“Grasping something is easy, but maintaining that grasp requires constantly expending energy,” Arrieta said. “This is true for both humans and machines. But what's interesting about this gripper is that when we invert the domes that make the gripper close, we are actually storing energy in the skin. 

The gripper arms are using that energy to maintain its grasp, rather than requiring some external energy source. In essence, we are using the structure itself as a mechanical battery.”

In future, they hope they will be able to apply this technology to flexible robotics, such as a robotic hand whose grip resembles a human one. Arrieta also thinks mechanical computing might be possible.

“When you think about it, these up-and-down domes are a lot like the 1s and 0s of computer data,” Arrieta said. ‘We can imagine “programming” a sheet like this by pressing the domes in certain locations in a certain order, and then “reading” that data mechanically based on the shape of the sheet. 

“This can be done without power or a central processor of any kind. In this way, future machines will function much more like animals, which use mechanical sensing and processing to react much more quickly.”

The work has been published in the journal Advanced Science.