Imagine a material whose mechanical behavior can be changed simply by shining a blue light on it. Instead of replacing the material or physically rebuilding its structure, you could use light as a kind of control knob to switch its properties from one behavior to another.
That idea is becoming possible with a new generation of mechanical metamaterials—artificial materials whose unusual properties come mainly from their carefully designed internal structures rather than the substance they are made from.
A team led by Alexander Münchinger has demonstrated a particularly striking example. They created microscopic, three-dimensional metamaterials from liquid-crystal elastomers and used blue LED light to dramatically change how the structures respond to mechanical forces. In two different designs, light was able to flip the sign of an effective mechanical property, turning one type of behavior into its opposite.
What Makes Metamaterials Different?
Ordinary materials get most of their properties from their chemical composition. Steel is strong because of its structure at the material level, rubber is flexible, and glass is relatively rigid and brittle.
Metamaterials work differently.
Their properties can come primarily from the geometry and arrangement of their microscopic structures. By designing tiny beams, hinges, patterns, and repeating units, researchers can make a material behave in ways that are difficult or impossible to obtain from conventional materials.
This means that if scientists can make those tiny structures change shape when exposed to an external stimulus, they can potentially make the entire metamaterial change its effective properties.
The stimulus could be temperature, pressure, electricity, magnetic fields, solvents—or, in this case, light.
The new work takes this concept a major step further by creating responsive structures in three dimensions at the micrometer scale.
A Material That Responds to Light
The researchers used liquid-crystal elastomers, a special class of soft materials that combine the elasticity of polymers with the organized molecular structure of liquid crystals.
One important feature of these materials is that their mechanical behavior depends on the orientation of their internal molecular structure, known as the director field.
The researchers used advanced 3D laser printing to carefully arrange this director field into complex three-dimensional patterns.
This gave them control over how different parts of the microscopic structure would respond when illuminated.
The result is essentially a tiny mechanical machine made from a single responsive material.
When blue light from an LED is applied at intensities of roughly 10–30 W/cm², different parts of the liquid-crystal elastomer respond differently. That difference causes individual elements to bend or change shape.
Those small movements can then produce a much larger change in the overall mechanical behavior of the metamaterial.
First Demonstration: Flipping Poisson’s Ratio
The first design demonstrates something particularly unusual: the ability to switch the sign of the material's Poisson's ratio.
Poisson's ratio describes what happens to a material in one direction when it is stretched or compressed in another direction.
For example, take a normal rubber band and stretch it lengthwise. It becomes thinner from the sides. This corresponds to a positive Poisson's ratio.
But some specially designed materials behave in the opposite way. When stretched, they can also expand sideways rather than becoming narrower. These materials are called auxetic materials, and they have a negative Poisson's ratio.
The researchers designed their metamaterial using structures resembling a bow tie.
Under one condition, the structure behaves like a conventional material with a positive Poisson's ratio. After exposure to light, its microscopic elements change shape, causing the entire architecture to become auxetic.
In other words, light changes the material from non-auxetic to auxetic behavior.
The important point is that this isn't simply a small adjustment. The researchers demonstrate a reversal in the sign of the effective property.
It is similar to having a mechanical switch that can change the direction of a response.
Second Demonstration: Reversing Twist
The second metamaterial is even more unusual.
Instead of focusing on conventional elasticity, the researchers created a chiral metamaterial. Chiral structures have a handedness, meaning their geometry can resemble a right-handed or left-handed form.
Think of your left and right hands. They are mirror images, but you cannot rotate one into the other to make them identical.
The researchers designed microscopic chiral elements connected within a three-dimensional metamaterial structure.
When exposed to light, these elements change their handedness. This causes the relationship between applied strain and twisting to reverse.
The researchers describe this as a flip in the sign of twist per strain.
This behavior goes beyond conventional classical elasticity and enters the more specialized field of micropolar elasticity, where additional microscopic rotational effects can influence the mechanical response of a material.
So once again, light acts like a control mechanism that can make the structure switch between opposite forms of mechanical behavior.
Why 3D Printing Is Important
One of the biggest advances behind the research is the ability to manufacture complicated structures at the micrometer scale.
Previous stimulus-responsive metamaterials had important limitations. Many were demonstrated using relatively large, macroscopic models. Others were effectively two-dimensional rather than fully three-dimensional.
Some could change their properties when exposed to a stimulus, but the amount of change was relatively limited.
The new approach addresses these problems by combining microscale 3D printing with carefully controlled liquid-crystal orientation.
Instead of simply printing a shape, researchers can control how the material's internal molecular orientation is arranged throughout a three-dimensional structure.
That provides another level of design freedom.
Light Becomes a Mechanical Control Knob
The concept could eventually lead to materials whose properties can be continuously adjusted—or potentially switched between different states like a digital control.
This is one reason stimulus-responsive metamaterials are attracting attention.
Imagine a future material that could automatically change its mechanical behavior depending on sunlight. A building component might adjust its response to environmental conditions. A microscopic structure could change how it interacts with sound or vibrations when illuminated.
The researchers also suggest possible applications involving acoustic activity, where the chiral structures could potentially be used to optically control the sign of how sound behaves within a material.
Other possible applications include adjustable structures for biological research. For example, light-responsive metamaterials could potentially provide controllable mechanical environments for studying cells.
More broadly, the ability to control mechanical properties locally using light could open possibilities for advanced optomechanical systems, where light controls mechanical behavior.
From Fixed Materials to Programmable Materials
Traditional engineering materials are generally designed with fixed properties. Once manufactured, their stiffness, geometry, and mechanical response are mostly determined.
Responsive metamaterials offer a different philosophy.
Instead of asking, “What material should we build?”, scientists can ask, “What behavior should the material have, and how can we program its structure to produce it?”
The work by Münchinger and his team demonstrates an important step toward that idea.
Their microscopic structures don't merely become slightly softer or harder when exposed to light. In carefully designed architectures, light can make the material cross from one mechanical regime into another and even reverse the sign of an effective property.
That is a much stronger form of responsiveness.
The Future of Light-Controlled Metamaterials
The experiments are still a proof of concept, and the researchers point out that improvements in sample quality and the number of repeating structural units will be important for future development.
But the underlying idea is powerful.
With increasingly sophisticated 3D printing techniques, scientists could potentially create much more complicated responsive architectures, combining different geometries and molecular orientations within the same microscopic object.
Eventually, materials could be designed to respond differently to different light conditions, allowing their mechanical properties to be programmed spatially and dynamically.
The bigger vision is a new class of three-dimensional optomechanical metamaterials—materials whose behavior is not permanently fixed but can be actively controlled using light.
What once sounded like science fiction—a material that changes its mechanical personality when illuminated—is becoming a laboratory reality. And rather than simply making materials stronger or lighter, this technology could allow engineers to build materials whose behavior itself can be programmed and switched on demand.
Reference: Alexander Münchinger, Li-Yun Hsu, Franziska Fürniß, Eva Blasco, Martin Wegener, 3D optomechanical metamaterials, Materials Today, Volume 59, 2022, Pages 9-17, ISSN 1369-7021, https://doi.org/10.1016/j.mattod.2022.08.020.(https://www.sciencedirect.com/science/article/pii/S1369702122002292)
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