“Shark-like” sensor detects electrical signals in water

The exotic properties of a quantum material could lead to shark-inspired devices for studying the marine environment

Taking inspiration from an organ that helps sharks hunt for their prey, materials scientists at Purdue University in Indiana have developed a sensor that can detect minute electrical signals travelling through salt water, and is robust enough to survive the harsh conditions of frigid seas.

The sensor, which is described in a paper in Nature, could help study marine ecosystems and organisms, or could detect the movements of shipping for commercial or military maritime applications, its developers claim.

Hammerhead sharks are particularly sensitive to electrical signals because of a high density of electroreceptors in their oddly-shaped heads

Sharks, in common with some other marine species, have the ability to detect electrical signals, which gives away the location of their prey. They do this thanks to an organ located near their mouth, called the ampullae of Lorenzini. This contains a jelly that can conduct ions in the seawater towards a membrane at the base of the organ which contains cells that can detect them. This inspired the Purdue team, led by materials engineer Shriram Ramanathan, to look for a method of replicating this ability.

The device they have developed gets its properties from a material that acts in bizarre ways, owing to the way its electrons behave. Samarium nickelate is a class of materials known as perovskites, which are also the subject of intensive research because of their potential in low-cost photovoltaic cells. They are also quantum materials, meaning that their properties result from quantum mechanical interactions.

The Purdue team, led by Shriram Ramanathan (centre in red) worked with many other institutions

Samarium nickelate has the unusual ability to conduct protons, which can pass from the water into the material and out again, and also undergoes a dramatic phase change from conductor to insulator. This phase change also causes it to become more transparent, and as it absorbs protons, it swells slightly. This potentially gives technologists a variety of ways to study the changes caused by electric signals in the water; moreover, the ability to exchange protons reversibly with the water protects it from corrosion.

Aluminium, by contrast, forms a protective oxide coating in water, but this prevents further interaction with the environment.

“Here, we start with the oxide material and we are able to maintain its functionality, which is very rare,” Ramanathan said. “If the material transmits light differently, then you can use light as a probe to study the property of the material and that is very powerful. Now you have multiple ways to study a material, electrically and optically.”

Ramanathan’s team found that the sensitivity of the detector was remarkably similar to a shark’s electrical sense. “We show that these sensors can detect electrical potentials well below one volt, on the order of millivolts, which is comparable to electric potentials emanated by marine organisms. The material is very sensitive. We calculated the detection distance of our device and find a similar length scale to what has been reported for electroreceptors in sharks.”

The research also involved collaborators fro, the Argonne National Laboratory, Rutgers University, the US National Institute of Standards and Technology, The Canadian Light Source at the University of Saskatchewan, Columbia University and the University of Massachusetts.