In a discovery that could reshape underwater sensing technology, scientists have found that sea urchin spines do far more than defend the animal—they act as natural sensors capable of detecting water flow and generating electrical signals.

 

A research team led by WANG Zuankai, Associate Vice President (Research and Innovation) and Chair Professor of Mechanical Engineering at The Hong Kong Polytechnic University, working with researchers from City University of Hong Kong and Huazhong University of Science and Technology, has uncovered a previously unknown phenomenon: mechanoelectrical perception in sea urchin spines.

 

Their findings, published in the journal Nature, reveal that the spines’ unique porous internal structure allows them to convert water movement into measurable electrical signals.

 

The discovery centers on the long-spined sea urchin Diadema setosum. Researchers found that when a droplet of seawater hits the tip of one of its spines, the spine rotates rapidly—within a second. Electrical tests showed the impact produced a voltage of about 100 millivolts inside the spine. When the spine was submerged, water flow triggered electrical signals of several tens of millivolts.

 

Surprisingly, the same effect occurred even in dead spines, proving the phenomenon is not driven by living cells but by the spine’s structure itself.

 

The key lies in the spine’s stereom, a porous skeletal framework whose pores gradually change in size along the length of the spine. Larger pores with lower solid density sit at the base, while smaller pores with higher density appear at the tip, forming a gradient structure.

 

As water flows through this network, interactions between the liquid and solid surfaces generate electrical charge differences. The gradient structure intensifies this interaction, amplifying the voltage signal and enhancing the spine’s sensitivity to water movement.

 

Inspired by the natural design, researchers used advanced 3D printing to replicate the spine’s porous gradient structure using polymer and ceramic materials.

 

The artificial samples significantly outperformed conventional designs. Under water-flow stimulation, the spine-inspired structure produced voltage outputs about three times higher and signal amplitudes roughly eight times greater than non-gradient designs.

 

The team then built a 3 × 3 array bionic mechanoreceptor made from gradient porous units. The device can detect underwater electrical signals in real time and precisely locate where water flow strikes—all without requiring external power.

 

Researchers say the gradient porous structure enhances signal transmission, boosting the sensitivity and accuracy of the sensor. Replicating the design in different materials could expand its uses far beyond water-flow detection.

 

Potential applications include monitoring marine environments, managing underwater infrastructure, and enabling next-generation technologies such as brain-computer interfaces, vibration sensing, and even aerospace systems.

 

Prof Wang Zuankai said, "Compared to traditional mechanoreceptors, our design excels in manufacturability, structural design flexibility, material versatility, geometric and performance control, and real-time underwater self-sensing. Leveraging gradients of porous materials and 3D printing technologies, we aspire to produce more nature-inspired metamaterial sensors with a range of materials, pore sizes and surface features that support potential applications in many fields."

 

Prof Wang’s team has been at the forefront of biomimetic engineering, previously developing lotus leaf-inspired self-cleaning surfaces, Araucaria leaf-inspired liquid transport systems, and anti-icing materials based on fungal spore-shooting mechanisms.

 

He believes discoveries like the sea urchin spine sensor highlight how nature still holds untapped engineering solutions.

 

"For natural porous materials, mechanical properties such as strength may not be the primary function, but rather a by-product of complex biomineralisation. Uncovering previously unknown mechanisms that lie beyond a material's traditionally recognised function helps us to more comprehensively understand and fully utilise these natural resources. This is crucial for advancing biomimetic research," he added.