Flexible photonic skins emit light when stretched, enabling real-time underwater signaling and leak detection. The 3D printed design conforms to curved surfaces and works autonomously.
(Nanowerk Spotlight) Communication under water is difficult in ways that challenge even the most advanced technologies. Light signals scatter. Radio waves disappear quickly in seawater. Sound travels farther but at the cost of distortion, delay, and interference from both natural and artificial sources. For divers, soft robots, or marine equipment, even the simplest task—such as signaling that a valve is leaking or that a human is in distress—can become a significant engineering challenge.
Most existing systems depend on fragile electronics, rigid parts, or external power sources that are not compatible with flexible, dynamic environments. As more activity moves into the oceans, including autonomous exploration, search operations, and industrial inspection, the technical limitations of current tools are becoming increasingly clear.
At the center of this problem is a need for visibility without wiring, signaling without electronic components, and sensing without any power supply. The objective is not just to transmit information, but to do so under pressure, in saltwater, and across irregular surfaces such as fingers, pipes, or moving robotic limbs.
One approach that has gained attention is the use of mechanoluminescent materials. These are substances that produce light when they are physically deformed, either through stretching, compression, or vibration. Since they do not require electricity, batteries, or switches, they can potentially serve as self powered indicators in environments where conventional lighting fails.
The concept is promising, but existing materials come with significant tradeoffs. Mechanoluminescent composites made from zinc sulfide embedded in silicone are chemically stable and stretchable, but their designs are often restricted to flat films that do not adapt well to curved or flexible surfaces. These materials may be either mechanically robust or bright enough to see, but rarely both. Stress tends to concentrate in specific areas, leading to uneven light output and early material failure. These limitations have kept the technology from being deployed in practical underwater systems.
The team reports the development of a 3D printed, self powered light emitting device that combines a stretchable mechanoluminescent composite with a type of geometry known as auxetic. Auxetic structures expand laterally when stretched, allowing them to conform naturally to curved and moving surfaces. The result is a material that behaves like a soft photonic skin, capable of producing light under mechanical stress while remaining flexible, durable, and waterproof.
Schematic representation of the fabrication process for the self-powered mechanoluminescent photonic skin. The photonic skin exhibits exceptional conformability and produces uniform, bright mechanoluminescent signals when deformed. (Image: Reprinte from DOI:10.1002/adma.202502743, CC BY)
The researchers created this photonic skin by printing a composite ink made from zinc sulfide phosphor particles suspended in a silicone matrix. When the material is stretched or compressed, friction occurs between the rigid phosphor particles and the soft silicone. This interaction causes an imbalance of electric charge, which then generates a brief local electric field. That field excites electrons in the zinc sulfide, which fall back to a lower energy state by emitting visible light. This phenomenon is called triboluminescence.
The researchers carefully tuned the formulation of the ink to ensure that it could be printed in precise shapes and remain mechanically stable after curing. A high phosphor concentration, around 80 percent by weight, was needed to achieve sufficient brightness. However, this level of loading also made the material more brittle.
To overcome this, the printed structures were encapsulated in a thin outer layer of transparent silicone. This shell acted as a protective barrier, improved stretchability, and helped redistribute mechanical stress throughout the material.
This design significantly improved both mechanical and optical performance. The photonic skin could stretch by up to 50 percent without cracking or delaminating. It produced uniform and visible light across its surface and retained more than 70 percent of its initial brightness after 10,000 cycles of repeated stretching and relaxing. In controlled testing, its brightness reached over 7 candela per square meter at moderate strain levels, a level easily visible to the human eye even in dim conditions.
Finite element simulations confirmed that the auxetic structures adhered better to curved surfaces than conventional film designs. The researchers evaluated various patterns and found that auxetic geometries consistently maintained higher contact area when placed on cylindrical, spherical, or saddle shaped surfaces. This led to more even stress distribution and more consistent light emission.
By comparison, standard flat or positively shaped structures tended to wrinkle or lift away, reducing their usefulness on non-flat surfaces.
The material’s performance was not only theoretical. The team demonstrated several practical applications. In one example, they fabricated a glove with strips of photonic skin attached to each finger. Bending one or two fingers triggered brief pulses of light, which could be used to transmit Morse code. A single finger bend signaled a short pulse, while two fingers indicated a long one. Using this method, the glove wearer was able to signal messages such as “SOS” or “UP” without any electronic components, and the light remained visible under water, even in cold or high salinity conditions.
In another demonstration, the photonic skin was applied to a simulated gas tank with a small puncture. When compressed air was introduced, the escaping gas caused local deformation of the skin. This movement triggered a flash of light at the leak site, allowing for immediate visual detection. The device responded more quickly and with greater brightness than traditional mechanoluminescent films. Its ability to conform closely to curved surfaces made it particularly well suited for real world equipment monitoring, where pipes and vessels often have complex shapes.
The researchers also mounted the skin on a soft robotic fish and placed it in water tanks at different temperatures. The device continued to emit light in response to mechanical motion at room temperature, in heated water, and in ice water. After 28 days of immersion in a saltwater solution, the material showed no loss in luminescent intensity, confirming its environmental durability.
There are still limitations to consider. The material responds only to physical deformation, so it cannot detect chemical signals or respond to non-mechanical inputs. Light output is still dependent on the correct balance of phosphor loading and silicone elasticity, which may require further optimization for different use cases. The color of light is determined by the phosphor used and is not tunable during operation.
While the material performs well in laboratory conditions, large scale deployment would require further testing under real oceanic conditions and over longer durations.
Even so, the photonic skin described in this work represents a clear step forward. It combines structure and function in a single, integrated material system that emits light without any external power. Its fabrication is based on 3D printing, which allows rapid prototyping and easy adaptation to different sizes and shapes. By relying only on mechanical inputs, the material sidesteps many of the failure points associated with electronics, such as corrosion, short circuits, or battery depletion.
The research by Sun and colleagues offers a useful platform for expanding the role of light based communication and sensing in extreme environments. Future work could explore multi color signaling, integration with robotic systems, or adaptation for airborne or space applications.
The underlying concept—using mechanical movement as both a power source and a signal—opens new possibilities for designing soft, autonomous systems that remain functional even when power and electronics are unavailable.
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