Taking inspiration from a family of African fish called Mormyrids, which are mildly electric and feature organs that can locate nearby prey, scientists from City University of Hong Kong have developed an artificial sensor system that promises to enable robots and smart prosthetics to locate items without assistance from machine vision.

“We developed a new strategy for 3D motion positioning by electronic skin, bio-inspired by ‘electric fish',” explained the scientists.

The e-skin can be worn on the arm, thanks to its small control circuit [top right] and sensor [bottom left]. Source: Jingkun Zhou, Jian Li et al.

According to its developers, the sensor relies on capacitance to detect an object regardless of its conductivity, with one layer of the sensor functioning as a transmitter that produces an electrical field that travels beyond the surface of the device while another layer behaves as a receiver and is capable of detecting both the direction and the distance to an object. Thanks to this architecture, the sensor system can locate an object in three-dimensional space.

The sensor's electrode layers are composed of a biogel, printed on both sides of a dielectric substrate made of polydimethylsiloxane (PDMS), which is a silicon-based polymer widely used in biomedical applications. The biogel's ability to transmit and receive electrical signals is due to a pattern of microchannels embedded on its surface. The resulting sensor is thin, flexible, soft, stretchable and transparent, making it ideal for assorted applications requiring an object-sensing system to adapt to irregular surfaces, such as the human body.

The team added that the capacitive field surrounding the sensor is disturbed once an object comes within range, which can subsequently be detected by the receiver. Because the magnitude in the change of signal can indicate the distance to the target, using multiple sensors in an array enables the system to determine the position of the target in three dimensions, the researchers added. During trials of their system, the researchers explained that their system detected objects up to 10 cm away when used in the air and as far as 1 m away when used underwater.

For functionality, the sensors require a separate controller component connected via silver or copper wires. The controller performs several tasks including generating the driving signal to activate the transmitting layers, collecting signals from the receiving layers using 16-bit analog-to-digital converters and processing the data through a microcontroller unit. The microcontroller calculates the target object's position and transmits that information to a smartphone or other device via a Bluetooth Low Energy (BLE) transmitter, thereby avoiding the need to send raw data for computation and thus conserving energy.

Power is supplied by an integrated lithium-ion battery, which can be wirelessly recharged through a copper coil. The system is designed to consume minimal electrical power. While the controller is less flexible and transparent than the sensors, it is encapsulated in PDMS, making it both waterproof and biocompatible.

The team concluded that the system best detects objects about 8 mm in diameter while objects smaller than 4 mm might not be detected accurately, thereby limiting practical use cases for the system to things like tracking finger movements for human-machine interfaces. Additionally, the system can reportedly detect objects concealed behind a cloth or paper barrier.

The team envisions that this nature-inspired sensor could potentially lead to the development of new wearable sensors, such as devices for human-machine interfaces and thin and flexible e-skin.

An article detailing the new sensor, “Mormyroidea-inspired electronic skin for active non-contact three-dimensional tracking and sensing,” appears in the journal Nature Communications.