Researchers have discovered that sea urchin spines function not only as protective structures but also as natural sensors capable of detecting water movement. The finding has led to the development of a new bio-inspired metamaterial sensor that could support future technologies in underwater monitoring, aerospace systems and advanced sensing applications.
The research was led by Prof Wang Zuankai, Associate Vice President (Research and Innovation) and Dean of the Graduate School at The Hong Kong Polytechnic University (PolyU), in collaboration with scientists from City University of Hong Kong and Huazhong University of Science and Technology.
The team identified a mechanoelectrical sensing mechanism in the spines of the long-spined sea urchin (Diadema setosum). The phenomenon arises from the unique internal structure of the spine, which consists of a gradient porous skeletal framework known as stereom. This porous architecture allows the spine to generate electrical signals when exposed to water movement.
During experiments, researchers observed that when a droplet of seawater struck the tip of a spine, the spine rotated rapidly within a second. Electrical measurements showed that the impact produced a voltage of around 100 millivolts inside the spine. When the spine was submerged, flowing water generated electrical signals measuring several tens of millivolts. Notably, the response occurred even in spines from deceased specimens, confirming that the effect results from structural properties rather than biological cells.
The stereom structure inside the spine contains pores that vary in size and distribution along its length. Larger pores and lower solid density appear near the base, while smaller pores and higher density occur near the tip. As water moves through this gradient porous structure, interactions between the liquid and the solid surface generate shear forces that influence the electric double layer at the interface. This process causes charge separation and produces measurable voltage differences.
To replicate the natural structure, the researchers used vat photopolymerisation 3D printing to produce artificial samples made from polymer and ceramic materials. Testing showed that designs with gradient porous structures generated voltage outputs roughly three times higher than non-gradient structures under the same water flow conditions.
The team also created a bionic mechanoreceptor composed of a 3 × 3 array of gradient porous units. This device can detect and record underwater electrical signals in real time while identifying the direction of water flow without requiring an external power supply.
According to the researchers, the structure-based sensing principle opens opportunities for applications beyond underwater detection. Similar designs could be adapted to measure pressure, vibration or electromagnetic signals, potentially contributing to technologies such as brain-computer interfaces and advanced sensing systems. The study highlights how insights from biological materials can inspire new engineering solutions and expand the possibilities of next-generation sensing technologies.
