Colourful mantis shrimp on the sea floor

US Army funds robot that packs a punch like a mantis shrimp

Image credit: Dreamstime

Researchers from Harvard University and Duke University, in a US Army-funded project, have developed a small robot which mimics the mechanics of the strongest punch in the animal kingdom: that of the bizarre mantis shrimp.

The research sheds light on the biology of the mantis shrimp, which is famed for its superhuman vision, incredible strength and psychedelic colouring. Mantis shrimps are armed with club-like appendages which accelerate faster than a bullet from a rifle, breaking through shell and cartilage.

“The idea of a loaded spring released by a latch is a staple in mechanical design, but the research team cleverly observed that engineers have yet to achieve the same performance out of a 'Latch-Mediated Spring Actuator' that we find in nature,” said Dr Dean Culver, of the Army Research Laboratory.

“By closely mimicking the geometry of a mantis shrimp's physiology, the team was able to exceed accelerations produced by limbs in other robotic devices by more than tenfold.”

How mantis shrimps produce these extreme movements has fascinated biologists; recent advances in high-speed imaging have made it possible to watch these strikes, although some mechanics remain poorly understood.

Many small animals – and even some plants – produce ultra-fast movements by storing elastic energy and rapidly releasing it through a latching mechanism, like a mouse trap. In the mantis shrimp, there are two small structures - sclerites - in the muscle tendons, which act as the latch. While for a typical spring-loaded mechanism removing the latch causes the spring to immediately shut, when the sclerites unlatch there is a short delay before the elastic energy turns to kinetic energy.

“When you look at the striking process on an ultra-high-speed camera, there is a time delay between when the sclerites release and the appendage fires,” said Dr Nak-seung Hyun, a Harvard engineer. “It is as if a mouse triggered a mouse trap, but instead of it snapping right away, there was a noticeable delay before it snapped. There is obviously another mechanism holding the appendage in place, but no one has been able to analytically understand how the other mechanism works.”

Biologists have suggested that while the sclerites initiate the unlatching, the geometry of the hammer-like appendage acts as a secondary latch. This could allow the shrimp to control the movement of the arm while it continues to store energy. This theory was put to the test by studying the mechanics of the system and then building a robot with the same linkages, from which a mathematical model of the movement can be produced. Their robotic device is faster than any similar devices at the same scale.

The researchers mapped four distinct phases of the mantis strike, starting with the latched sclerites and ending with the strike itself. They found that, indeed, after the sclerites unlatch, the geometry of the mechanism takes over, holding the appendage in place until it reaches an over-cantering point. Only then is the latch released.

“This process controls the release of stored elastic energy and actually enhances the mechanical output of the system,” said Emma Steinhardt, a Harvard graduate student. “The geometric latching process reveals how organisms generate extremely high acceleration in these short duration movements, like punches.”

According to Culver, actuator architecture like this offers impressive capabilities to smaller mechanisms for delivering forces, with military applications.

“I think there’s a broader takeaway here,” he added. “Something the engineering community and defence research can keep in mind. We’re not done learning about mechanical performance from nature and biological systems. Things we take for granted, like a simple sprung actuator, are still ripe for further investigation at many scales.”

In 2017, researchers from the University of Illinois published a study detailing the development of an ultra-sensitive camera inspired by the vision system of the mantis shrimp. 

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