Fish sensor

Researchers at Georgia Tech have engineered a motion detector that mimics the underwater flow measurements made by the blind cavefish.

Researchers at Georgia Tech have built a motion detector by copying the way that the blind cavefish senses the movement of water around it through the use of the gel-covered hairs that cover its body.

Vladimir Tsukruk, a professor at the Georgia Tech School of Materials Science and Engineering, and graduate students Michael McConney and Kyle Anderson conducted preliminary experiments with a simple artificial hair cell microsensor made of a common epoxy-based polymer called SU-8.

They found that, by itself, the sensor could not achieve the high-sensitivity or long-range detection of hydrodynamic disturbances created by moving or stationary bodies in a flow field. The hair cell needed a gel-like capsule – called a cupula – to overcome these challenges, similar to the one found on the fish itself.

‘After covering the hair cell with synthetic cupula, our bio-inspired microsensor had the ability to detect flow better than the blind fish. The fish can detect flow slower than 100 micrometres per second, but our system demonstrated flow detection of several micrometres per second,’ said Tsukruk. ‘Adding the cupula allowed us to detect a much smaller amount of flow and expand the dynamic range because it suppressed the background noise.’

In addition, the hydrogel encapsulation protects the sensors and increases their ability to withstand deformation as a result of impact. It also helps the hairs better withstand the marine environment by resisting corrosion and micro-organism growth.

Before starting to synthesise the gel-like material in the laboratory, the research team used optical microscopy and confocal fluorescence microscopy to determine the size, shape and properties of real cavefish cupula.

One type of cupula they found was cylindrical shaped with a height approximately five times larger than its diameter. The tallest part of the cupula was far enough away from the surface that it was exposed to free-flowing water and could bend with the hair to detect changes in flow.

To create the synthetic cupula in the laboratory, McConney dropped a solution of poly(ethylene glycol) tetraacrylate dissolved in methanol directly on the hair flow sensor. Once the droplet dried, he lowered another droplet until it made contact with the last drop and continued adding droplets until he constructed a tall hydrogel structure. Once the entire cupula structure dried, McConney exposed it to ultraviolet light to crosslink it, forming a 3D network.

To date, the researchers have fabricated an array of eight microsensors on a CMOS substrate and shown that the array is able to detect an oscillating object under the water.

They are currently looking for industrial partners to efficiently scale up their research so that they can fabricate arrays of thousands of the sensors and test them in real marine environments.

The front view of a hair sensor before and after being coated with the poly(ethylene glycol)-based hydrogel material