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Scientists Discover Way to Send Information into Black Holes Without Using Energy

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Scientists Built a Material That Lets Robots “See” What They Feel

Imagine a material that can literally show where and how hard it is being touched simply by changing its color. Scientists have developed a new type of hydrogel that can do exactly that. The material changes its optical appearance when stretched or mechanically deformed, creating visible patterns that reveal information about force and stress.

Researchers led by Liu have developed a fatigue-resistant mechanoresponsive color-changing hydrogel, or FMCH. Unlike many existing materials that may weaken, lose their optical response, or become unreliable after repeated deformation, this hydrogel is designed to withstand thousands of loading cycles while continuing to produce predictable color changes.

The technology could eventually help create more sensitive tactile sensors for robots, wearable devices, smart materials, and advanced optical systems.

A Material That Turns Mechanical Force Into Color

Mechanoresponsive materials are designed to respond to physical forces such as stretching, compression, bending, or pressure. Their ability to visually reveal mechanical deformation makes them attractive for sensors and other technologies.

However, developing a material that can repeatedly change color without suffering serious damage has been challenging.

The new FMCH addresses this problem by combining unusual optical behavior with strong mechanical properties.

When the hydrogel is in its original, undeformed condition, it appears dark under a circular polariscope. But when it is stretched, its color changes gradually.

As the material is stretched to as much as six times its original length, it can transition through several visible stages:

Black → Gray → Yellow → Purple

This color transformation is not simply a decorative effect. It contains information about the mechanical stress experienced by the material.

That makes the hydrogel potentially useful as a visual stress sensor.

Why Does the Color Change?

The color-changing behavior is connected to a phenomenon called photoelasticity.

When certain transparent or translucent materials experience mechanical stress, their internal optical properties can change. Under polarized light, these changes can produce visible patterns and colors.

In FMCH, the researchers carefully designed the material at the molecular level to control both its mechanical behavior and its optical response.

The hydrogel contains long polymer chains that become highly entangled with one another. These molecular entanglements allow the chains to move and slide relative to each other when the material is stretched.

Instead of individual polymer chains easily breaking under stress, the interconnected molecular structure helps distribute the applied force across the material.

This contributes to the hydrogel's remarkable toughness and ability to survive repeated deformation.

Built to Survive Thousands of Cycles

One of the most impressive characteristics of FMCH is its fatigue resistance.

Many materials can perform well during a single stretch but gradually deteriorate after being repeatedly deformed. For practical sensors and robotic systems, however, a material may need to respond to thousands or even millions of mechanical events.

The researchers found that FMCH could maintain its color-changing performance for up to 10,000 cycles under repeated mechanical loading.

The material also showed consistent behavior across different strain rates. This is important because real-world mechanical forces are rarely applied at exactly the same speed.

A robotic hand, for example, may gently touch an object in one situation and quickly grasp it in another.

A sensor must ideally respond reliably in both cases.

The experiments demonstrated that FMCH combines rapid short-term responsiveness with long-term stability, making it particularly interesting for dynamic sensing applications.

Exceptional Mechanical Strength

The hydrogel is not only optically responsive; it is also mechanically robust.

Researchers measured a fracture toughness of approximately 3,000 J m⁻², while its stretchability reached about six times its original length.

Its fatigue threshold was reported to be above 400 J m⁻².

These properties are significant because they show that the material can tolerate substantial deformation before serious damage develops.

The researchers also observed low hysteresis toughening during mechanical testing, suggesting that the hydrogel can dissipate mechanical energy while maintaining its overall performance.

Together, these properties make FMCH different from many conventional mechanoresponsive materials, where mechanical durability and optical sensitivity can be difficult to achieve simultaneously.

The Role of Hygroscopic Salts

Another important part of the material's design is the use of hygroscopic salts.

Hygroscopic substances have a strong ability to interact with and retain water. In this hydrogel, the amount of water influences its optical properties.

The researchers were therefore able to adjust the hydrogel's photoelastic response by controlling its water content through the amount of hygroscopic salt.

This creates an interesting molecular design strategy.

The researchers essentially control two important characteristics separately:

  • Molecular entanglement helps determine mechanical strength and toughness.

  • Water content and hygroscopic salts help tune optical behavior.

By balancing these factors, the team created a material that is both mechanically durable and optically responsive.

A New Kind of Robotic Touch

Perhaps the most exciting application is the use of FMCH as a vision-based tactile sensor for robots.

Traditional robots often depend on electronic sensors to determine whether they are touching an object and how much force they are applying.

The FMCH approach takes a different route.

When the material comes into contact with an object, mechanical stress creates optical patterns across its surface. A camera can capture these patterns, and the resulting image can provide information about what the robot is touching.

In experiments, the sensor could help distinguish several important properties, including:

Object shape: The stress distribution can reveal the geometry of the contacted object.

Spatial position: The location of the optical pattern can indicate where contact occurs.

Applied pressure: Different levels of force produce different stress distributions.

Material stiffness: How an object deforms and distributes force can provide clues about whether it is soft, flexible, or relatively rigid.

This effectively turns mechanical information into an image that a computer can analyze.

Why This Could Matter for Soft Robots

Soft robots are designed to interact safely and naturally with people and delicate objects. But giving these robots a reliable sense of touch remains a major challenge.

A conventional electronic sensing system can require multiple components, wiring, and complicated integration.

FMCH offers a potentially simpler approach. A camera can observe the hydrogel's changing optical patterns, while software interprets those images.

This could allow robots to gain information about their surroundings without requiring a large number of conventional force sensors.

The researchers describe their tactile sensor as low-cost and long-lasting, characteristics that could be valuable for future perceptive soft robots.

From Laboratory Material to Future Technology

Although FMCH is still a research-stage technology, its combination of mechanical durability and optical responsiveness makes it particularly promising.

A material that can repeatedly translate mechanical stress into visible information could have applications beyond robotics. Similar concepts could potentially be explored in wearable sensors, structural monitoring, interactive devices, medical technologies, and advanced optical systems.

The major achievement is not simply that the hydrogel changes color.

It is that the researchers have developed a material capable of doing so reversibly, predictably, and repeatedly while remaining mechanically tough.

The ability to survive up to 10,000 cycles while retaining its sensing performance demonstrates the potential for long-term dynamic applications.

A Colorful Future for Robot Sensing

FMCH represents an interesting step toward materials that can directly convert physical forces into visual information.

Instead of relying entirely on conventional electronic sensors, future robots could potentially use smart materials that visually reveal what they are feeling.

By carefully controlling molecular entanglements and hygroscopic salts, Liu and the research team have combined strength, stretchability, fatigue resistance, and optical responsiveness in one hydrogel.

The result is a material that does more than simply survive mechanical stress—it communicates that stress through color.

If the technology can be further developed and integrated with cameras and artificial intelligence, it could help robots better understand the objects they touch and interact with.

In the future, a robot may not just see the world around it.

It could see what it feels.

Reference: J. Liu, W. Li, Y. She, S. Blanchard, and S. Lin, “ Fatigue-Resistant Mechanoresponsive Color-Changing Hydrogels for Vision-Based Tactile Robots.” Adv. Mater. 37, no. 49 (2025): 2407925. https://doi.org/10.1002/adma.202407925

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