Stretchable Piezoelectric Sensor Mimics Dolphin Sonar for Soft Robots

A research team led by Professor Lee Nae-ung in the Department of Materials Science and Engineering at Sungkyunkwan University has developed a stretchable, electricity-generating nanocomposite material and used it to build an artificial ultrasonic sensory synapse device that mimics how dolphins perceive their surroundings underwater, as reported by 로봇신문 (Robot Newspaper). The work was published in the journal Advanced Materials.

Dolphins navigate murky, dark waters by emitting ultrasonic clicks and listening for the echoes that bounce off nearby objects and terrain — a process known as echolocation. As wearable and robotic-skin electronics advance, researchers have been looking for ways to replicate this kind of ultrasonic sensing and neural-style signal processing. But conventional inorganic piezoelectric materials, which generate electricity under mechanical stress, tend to be brittle, while flexible polymer alternatives often fail to spring back after stretching or lose much of their piezoelectric performance, making stretchable ultrasonic sensors difficult to realize.

To get around this, the team applied a cross-linking method that lightly binds the flexible polymer P(VDF-TrFE) using a soft chain molecule, PEG-diamine, and blended in nanometer-scale barium titanate (BaTiO3) particles known for strong electrical generation. By inducing robust hydrogen bonds between the polymer and the nanoparticles, the researchers were able to significantly expand the material's electrically active crystal structure without sacrificing its inherent stretchiness.

The resulting nanocomposite maintained stable piezoelectric performance even when stretched 50% beyond its original length, and it showed no performance degradation after more than 1,000 repeated pressing and stretching cycles, demonstrating notable durability.

The team then integrated this stretchable ultrasonic sensor with a flexible artificial neural device — a synaptic transistor. When ultrasound passes through water, the sensor detects it and generates an electrical signal, which is then relayed to the synaptic device so that it retains information about the ultrasonic signal much like a living nerve cell would. The researchers say this could serve as a starting point for developing artificial ultrasonic sensory systems that emulate the biological pathway through which sensory cells in a dolphin's ear pick up ultrasonic vibrations and relay them to the brain's neural network.

Professor Lee said, "This achievement is significant because it overcomes the fragility of conventional piezoelectric materials while achieving both elasticity and high performance simultaneously," adding that the technology is expected to play a key role in future soft underwater exploration robots as well as wearable medical devices that conform naturally to skin for health monitoring.

The research was supported by South Korea's Ministry of Science and ICT and the National Research Foundation of Korea through its nano and materials technology development program and basic research program. The paper, titled "Intrinsically Stretchable Piezoelectric Nanocomposites for Artificial Ultrasonic Sensory Synapse," is available via DOI at https://doi.org/10.1002/adma.74349.