Spider-style sensor detects vibrations

Some spiders court by scratching leaves and 'listening' to the vibrations with organs in their legs

By copying the design of an organ found in spiders' legs, engineers in South Korea have built a sensor that can detect miniscule vibrations.

It works because the vibrations open and close cracks in a very thin layer of platinum, changing its conductivity.

A similar slit-based system is found inside the joints of some spiders.

The team reports in the journal Nature that when they stick their sensor to the neck or wrist, it can read out what someone says - or their pulse.

Speaking to BBC News, Prof Mansoo Choi said the project began two years ago, when one of his colleagues at Seoul National University read a paper in the same journal.

It described how a particular species of wandering spider communicates with potential mates, metres away on the same plant, by scratching the leaves and "hearing" the vibrations.

The organ in the spiders' legs that detects these incredibly faint vibrations is made up of a series of slits. It is called the "lyriform organ" because the slits vary in length, like the strings of a lyre.

"We tried to mimic the cracked shape of the organ," Prof Choi said.

To do so, they placed a very thin layer of the metal platinum on top of a flexible polymer, and bent it.

They had to use just the right thickness of platinum, and bend it in just the right way, to get a pattern of parallel cracks that would act as a vibration sensor.

"Initially we failed many times," said Prof Choi.

"It took several months... but finally we worked out how ultra-high sensitivity could be derived from a controlled crack formation."

The sensor detected the simulated flutter of a ladybird's wings

When they applied electricity to the platinum layer, they found they could read out the frequency and the size of vibrations they delivered to the sensor, because the cracks were changing the metal's resistance as they opened and closed.

Crucially, the edges of the cracks in the platinum were rather rough, forming jagged lines when viewed through a microscope. If they were perfectly straight, all the current would stop at once and the sensor would simply be "on" or "off".

But with their slightly jagged edges, the cracks became very sensitive, allowing varying levels of current to pass through as the polymer underneath bent with the vibrations.

The team's spider-inspired design could record notes from a violin - or a person's speech and pulse

To test it further, the researchers simulated the tiny vibrations of a ladybird's wings - a gentle wiggle of just 14 micrometres (0.014mm). Their sensor was up to the task.

When they placed it on a violin, the sensor could distinguish between the different notes being played. It could also read out a subject's pulse if strapped to their wrist, and even allowed the team to tell the difference between spoken words when it was placed on someone's neck.

"When you speak, if you touch your neck - it's vibrating. Our sensor is capable of detecting vibrations like that, very precisely," Prof Choi explained.

The team has patented their design and is working on improving it further - in particular, making it more durable and using cheaper metals than platinum.

The lead author of the Nature paper that first caught the Korean team's attention was Prof Peter Fratzl, from the Max Planck Institute of Colloids and Interfaces in Potsdam, Germany.

He has written a commentary on the new paper, saying that the device's practical applications are impressive, but noting that it is still nowhere near as precise as the spidey-sense that inspired it.

Prof Fratzl told the Nature Podcast: "The vibration sensor of the spider combines a number of exquisite properties. It's extremely sensitive but it's also extremely discriminatory.

"There's a lot of room to improve on what [they] are doing technically, before we can even compare it to what the spider sensor is actually capable of."

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