Imagine a soft material that changes its colour almost instantly when you press it. A gentle touch could make it shift from one colour to another, while releasing the pressure would quickly return it to its original state. Researchers have now developed a new photonic hydrogel that can do exactly this—with a response time of only about 0.1 milliseconds.
The material, developed by Youfeng Yue and his team, combines the flexibility of a hydrogel with a carefully engineered nanoscale structure that can control how light is reflected. The result is a soft material capable of full-colour switching across a wide visible range, while remaining mechanically stable even after more than 10,000 repeated cycles.
The technology could eventually be useful for advanced displays, highly sensitive stress sensors and other optical devices.
How Does a Photonic Material Produce Colour?
Unlike conventional coloured materials that rely on pigments or dyes, photonic materials can produce colour through their physical structure.
Photonic crystals contain repeating structures at extremely small scales. When light interacts with these structures, certain wavelengths are strongly reflected through a phenomenon known as Bragg diffraction. The reflected wavelength determines the colour that we see.
The important feature is that if the spacing of these structures changes, the reflected wavelength can also change.
This creates an opportunity to build materials whose colour responds to external forces.
Researchers have already explored soft photonic materials that can change their colour when exposed to electrical, magnetic, chemical, thermal or mechanical stimulation. Such materials could be particularly useful for sensors because their colour provides a visible indication of what is happening to them.
However, one major challenge has remained: speed.
Why Existing Colour-Changing Gels Are Too Slow
Many colour-changing photonic gels can produce a relatively broad range of colours, but their response times can range from several seconds to even hours.
That is far too slow for applications such as video-rate displays or sensors that need to monitor rapidly changing forces.
One reason is that many gels change their volume when they absorb or release solvent. This swelling and shrinking process depends on the movement of liquid through the material.
The larger the gel, the farther the solvent must travel, making the response increasingly slow.
Another approach is to physically deform the material. In principle, mechanical deformation should be much faster. But conventional soft photonic materials often behave in a viscoelastic way, meaning their internal polymer networks take time to rearrange and relax.
The researchers therefore needed a material that could deform and recover almost immediately.
The Secret: A Highly Elastic Hydrogel
Yue and his team focused on chemically crosslinked hydrogels containing large amounts of water.
In these materials, polymer chains are connected into a stable network. Because they do not depend heavily on slow, reversible physical bonds, their mechanical relaxation can be extremely rapid.
For example, the researchers explain that a chemically crosslinked hydrogel can have a theoretical viscoelastic relaxation time as short as about one microsecond under suitable conditions.
However, another process called poroelasticity can still slow the response. Poroelasticity involves the movement of liquid through the polymer network when the material is deformed.
For a typical millimetre-scale gel, this process can take much longer than the elastic response of the polymer network.
The researchers therefore designed their material so that, under sufficiently rapid deformation, the gel behaves almost like an incompressible elastic material.
This allows it to respond rapidly when compressed.
A Bio-Inspired Nanostructure
The biggest challenge was incorporating a photonic structure into the soft hydrogel without destroying its fast mechanical response.
The team solved this by developing a bio-inspired architecture.
Inside the hydrogel matrix are thousands of rigid lamellar bilayer domains arranged in a highly ordered direction. These tiny structures perform two important jobs.
First, they act as reflective elements. Their nanoscale spacing determines which wavelengths of visible light are reflected, producing different colours.
Second, they reinforce the surrounding soft hydrogel.
This combination gives the material an unusual balance: it remains soft and deformable, while also being strong enough to withstand repeated high-speed deformation.
When the hydrogel is compressed, the spacing between its internal reflective structures changes. That changes the wavelength of light being reflected—and therefore changes the colour visible to the observer.
When the pressure is removed, the elastic structure rapidly returns to its original configuration.
This is how the hydrogel achieves its ultrafast colour switching: mechanical compression changes the spacing of the internal photonic layers, instantly shifting the reflected wavelength.
Full-Colour Control
One of the most impressive features of the new material is its broad colour range.
The researchers demonstrated reversible colour tuning from approximately 340 to 640 nanometres, covering a large portion of the ultraviolet-to-visible transition and visible spectrum.
This means the material is not restricted to switching between just two colours.
Instead, its reflected colour can be continuously tuned across a broad range by changing the applied mechanical deformation.
This capability is particularly important for applications where a display or sensor needs to represent different levels of force using different colours.
Extremely Small Forces Can Trigger It
Another important feature is the small amount of pressure required to actuate the material.
The photonic hydrogel can respond to compressive pressures of less than 3 kilopascals.
That is significant because it means the material can detect relatively small mechanical stresses.
The researchers also demonstrated a spatial resolution of approximately 10 micrometres.
Together, these properties could make the material useful for mapping complicated stress and strain patterns with high spatial detail.
Built to Survive Repeated Use
Fast switching would not be very useful if the material quickly degraded.
Fortunately, the researchers found that the photonic hydrogel has strong mechanical stability.
It maintained reversible colour switching through more than 10,000 high-frequency cycles without significant degradation.
The rigid internal bilayer structures contribute to this durability by reinforcing the soft hydrogel network.
The material can also withstand high-speed vibrations, including those generated by ballistic impacts, demonstrating that its mechanical structure can remain stable under demanding conditions.
Potential Applications
The combination of speed, colour range, sensitivity and durability opens several possibilities.
Full-Colour Displays
Because the hydrogel can switch colours much faster than many previous photonic gels, it could potentially be used in mechanically driven optical displays.
Instead of using conventional electronic pixels, future devices might use controlled mechanical deformation to produce changing colours.
Stress and Strain Sensors
The material could also act as a visual stress sensor.
When pressure is applied, its colour changes. By observing the colour, researchers could potentially determine how mechanical stress is distributed across a surface.
This could be particularly useful where complicated stress patterns need to be observed directly.
Biological Applications
One particularly interesting possibility involves living cells.
Cells constantly generate mechanical forces as they move, grow and contract. The researchers suggest that the soft photonic hydrogel could potentially function as a scaffold that visually reveals these tiny forces.
Because the material can respond to pressures on the scale of kilopascals and offers high spatial resolution, it could provide a new way to study mechanical behaviour in biological systems.
Optical and Telecommunication Devices
The researchers also suggest that ultrafast, tunable photonic materials could eventually find applications in optical and telecommunication technologies, where controlling the behaviour of light rapidly is important.
A New Direction for Smart Soft Materials
The work by Yue and his team demonstrates that hydrogels do not have to be slow, passive materials.
By combining chemical crosslinking, nanoscale photonic structures and a carefully engineered mechanical architecture, the researchers created a soft material that can respond to mechanical forces almost instantly while producing a wide range of colours.
Its approximately 0.1-millisecond response time, broad 340–640 nm colour range, 10-micrometre spatial resolution, low actuation pressure and durability beyond 10,000 cycles make it particularly interesting for future optical technologies.
More broadly, the study shows how nanoscale structural design can give soft materials completely new capabilities. A simple mechanical force can be translated into a rapid and visible optical signal.
That could make these colour-changing hydrogels useful not only for futuristic displays, but also for sensing forces that are otherwise too small or too fast for the human eye to detect.
Reference: Yue, Y., Kurokawa, T., Haque, M. et al. Mechano-actuated ultrafast full-colour switching in layered photonic hydrogels. Nat Commun 5, 4659 (2014). https://doi.org/10.1038/ncomms5659
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