Fish-Inspired Sensor “Touches” Using Electric Fields

The ability to detect a nearby presence without seeing or touching it may sound fantastical—but it’s a real ability that some creatures have. A family of African fish known as Mormyrids are weakly electric, and have special organs that can locate a nearby prey, whether it’s in murky water or even hiding in the mud. Now scientists have created an artificial sensor system inspired by nature’s original design. The development could find use one day in robotics and smart prosthetics to locate items without relying on machine vision.

“We developed a new strategy for 3D motion positioning by electronic skin, bio-inspired by ‘electric fish,’” says Xinge Yu, an associate professor in the Department of Biomedical Engineering at the City University of Hong Kong. The team described their sensor, which relies on capacitance to detect an object regardless of its conductivity, in a paper published on 14 November in Nature.

One layer of the sensor acts as a transmitter, generating an electrical field that extends beyond the surface of the device. Another layer acts as a receiver, able to detect both the direction and the distance to an object. This allows the sensor system to locate the object in three-dimensional space.

The e-skin sensor includes several layers, including a receiver and a transmitter.Jingkun Zhou, Jian Li et al.

The sensor electrode layers are made from a biogel that is printed on both sides of a dielectric substrate made of polydimethylsiloxane (PDMS), a silicon-based polymer that is commonly used in biomedical applications. The biogel layers receive their ability to transmit and receive electrical signals from a pattern of microchannels on their surface. The end result is a sensor that is thin, flexible, soft, stretchable, and transparent. These features make it suitable for a wide range of applications where an object-sensing system needs to conform to an irregular surface, like the human body.

The capacitive field around the sensor is disrupted when an object comes within range, which in turn can be detected by the receiver. The magnitude in the change of signal indicates the distance to the target. By using multiple sensors in an array, the system can determine the position of the target in three dimensions. The system created in this study is able to detect objects up to 10 centimeters away when used in air. The range increases when used underwater, to as far as 1 meter.

Jingkun Zhou, Jian Li et al.

To be functional, the sensors also require a separate controller component that is connected via silver or copper wires. The controller provides several functions. It creates the driving signal used to activate the transmitting layers. It also uses 16-bit analog-to-digital converters to collect the signals from the receiving layers. This data is then processed by a microcontroller unit attached to the sensor array, where it computes the position of the target object and sends that information via a Bluetooth Low Energy transmitter to a smartphone or other device. (Rather than send the raw data to the end device for computation, which would require more energy).

Power is provided by an integrated lithium-ion battery that is recharged wirelessly via a coil of copper wire. The system is designed to consume minimal amounts of electrical power. The controller is less flexible and transparent than the sensors, but by being encapsulated in PDMS, it is both waterproof and biocompatible.

The system works best when detecting objects about 8 millimeters in diameter. Objects smaller than 4 mm might not be detected accurately, and the response time for sensing objects larger than 8 mm can increase significantly. This could currently limit practical uses for the system to things like tracking finger movements for human-machine interfaces. Future development would be needed to detect larger targets.

The system can detect objects behind a cloth or paper barrier, but other environmental factors can degrade performance. Changes in air humidity and electromagnetic interference from people or other devices within 40 cm of the sensor can degrade accuracy.

The researchers hope that this sensor could one day open up a new range of wearable sensors, including devices for human-machine interfaces and thin and flexible e-skin.