Imagine a material that is incredibly lightweight, can be designed into almost any three-dimensional shape, and can stretch to more than four times its original length without permanently losing its shape. Researchers Jiang and Wang have developed a new class of mechanical metamaterials that brings these unusual properties together in a single material.
The new metamaterials combine highly complex 3D structures with an extraordinary level of reversible stretchability. The researchers achieved strains of more than 414%, roughly four times greater than previous materials with similarly complex 3D architectures.
This combination could eventually lead to lighter and more flexible robots, advanced wearable electronics, rehabilitation devices, highly efficient shock absorbers and other technologies that need materials capable of handling extreme deformation.
Why These Materials Matter
Traditional engineering materials usually involve a compromise between strength, weight, flexibility and structural complexity. Creating a material that is both highly stretchable and capable of forming complicated three-dimensional architectures has been particularly difficult.
Soft materials are already used in applications such as impact absorption, soft robotics, rehabilitation devices, tissue scaffolds, drug delivery systems and stretchable electronics. However, existing manufacturing techniques often limit either the material's stretchability or the complexity of its structure.
For example, some approaches create random foam-like structures, while others produce relatively simple layered or beam-based architectures. These methods can make useful soft materials, but they do not provide complete freedom to design complicated 3D structures.
Previous elastomer lattices with nanoscale architectures demonstrated stretchability of around 225%. However, their structures were restricted by the manufacturing process.
The new approach overcomes this limitation by allowing researchers to create much more freely designed 3D architectures while achieving reversible stretching beyond 400%.
The Secret Is in the Architecture
The material itself is made from highly stretchable elastomers, a class of polymers that can undergo large deformation and return toward their original shape.
But the researchers did something more interesting than simply choosing a stretchy material. They engineered the material at the structural level.
Instead of making a solid block of elastomer, they created a complex lattice, consisting of interconnected microscopic structures with carefully designed empty spaces.
This architecture dramatically reduces the material's overall density while allowing it to deform in controlled ways.
The resulting structures can have a density as low as 60 kilograms per cubic metre, which is only about 6% of the density of the bulk elastomer.
In other words, most of the volume is empty space.
That makes the material extremely lightweight while preserving the ability to withstand very large deformation.
A New Manufacturing Strategy
Creating such delicate elastomer structures is not easy.
Conventional additive manufacturing methods can struggle with elastomers and hydrogels because their liquid precursors may have high viscosity, require long curing times or cannot adequately support their own weight during fabrication.
Jiang and Wang addressed these problems using a strategy based on projection microstereolithography.
The researchers first create a temporary hollow scaffold using a material that can later be dissolved. This scaffold acts as a supporting framework during manufacturing.
The elastomer can then be formed around the carefully designed architecture. After processing, the temporary scaffold is removed, leaving behind the desired highly complex elastomer lattice.
This approach allows researchers to create structures at micro- to millimetre scales with much greater freedom than many previous techniques.
Most importantly, it makes it possible to combine complex 3D geometry with extreme stretchability.
More Than 414% Stretchability
One of the most striking characteristics of the new metamaterials is their reversible stretchability.
The researchers demonstrated strains of up to 414%.
A strain of 414% means the material can extend to more than five times its original length during the deformation process, depending on how strain is defined and reported. The important point is that the material can undergo an exceptionally large deformation and recover its structure when the load is removed.
This is far beyond what is normally possible for complex 3D architected materials.
The researchers report that the stretchability is around four times greater than existing counterparts with similar 3D structural complexity.
Such extreme deformation could be especially valuable where conventional rigid materials would crack, break or permanently deform.
Unexpected Mechanical Behaviour
The researchers also discovered that these metamaterials behave differently from conventional engineered materials when subjected to large forces.
During large-strain tension, the material's modulus, which describes its resistance to deformation, shows an approximately linear relationship with its density.
This is interesting because conventional architected materials often follow a power-law relationship in which mechanical properties depend strongly on the type of architecture.
Here, the researchers found that different structural designs could display a more unified relationship between density and stiffness under large tensile deformation.
This finding provides researchers with a useful design principle: instead of simply choosing a particular architecture, they can potentially tune mechanical behaviour by controlling the material's density and structural geometry.
Negative Stiffness Could Improve Energy Absorption
The metamaterials show another unusual behaviour under compression: programmable negative stiffness.
Normally, when you compress a material, it pushes back against the applied force. Negative stiffness represents a different mechanical response in which the structure can become easier to deform over part of its deformation path.
Although this sounds unstable, carefully designed negative-stiffness structures can be extremely useful for absorbing mechanical energy.
The researchers found that their metamaterials can exploit this behaviour to achieve very high energy-absorption efficiency.
This could make them attractive for applications involving impacts, vibrations and sudden mechanical loads.
Instead of simply resisting an impact, a carefully engineered structure can deform in a controlled manner and dissipate a large amount of energy.
Potential Applications
The combination of low density, complex 3D architecture and extreme stretchability opens the door to several applications.
Lightweight Connectors
The material can be designed into flexible connectors that join different 3D-printed components.
Because the connectors can stretch dramatically, they could accommodate movement or misalignment while keeping the overall structure lightweight.
Ultraefficient Dampers
Their unusual compression behaviour could make these metamaterials useful as dampers for absorbing vibrations and mechanical shocks.
Compared with conventional elastomer foams, the researchers demonstrated the potential for significantly improved energy-absorption performance.
Rehabilitation Devices
Another promising application is rehabilitation technology.
The metamaterials can be fabricated into customized 3D structures that conform to body shapes while providing specifically designed levels of support.
This could eventually help create rehabilitation devices that are lightweight, flexible and tailored to individual anatomical requirements.
Stretchable Electronics
The structures could also be combined with conductive materials to create lightweight electronics capable of stretching.
Because the 3D architecture can be deliberately designed, researchers can potentially control how electrical pathways are distributed through the structure.
This could enable 3D anisotropic conductivity, where electrical properties differ depending on the direction.
Such materials could become useful in wearable devices, flexible sensors and other unconventional electronic systems.
A Step Toward Bioinspired Materials
The researchers believe these metamaterials could also provide new opportunities for designing materials inspired by nature.
Biological materials often achieve remarkable combinations of light weight, flexibility and mechanical strength through hierarchical structures rather than simply relying on the properties of their basic ingredients.
The new metamaterials offer engineers a way to explore a similar design philosophy.
Instead of asking only, "What material should we use?", engineers can also ask, "What structure should we build from that material?"
This distinction is at the heart of metamaterial engineering.
What Could Come Next?
The technology could eventually move beyond passive structures.
If these elastomer lattices are combined with materials that respond to heat, light, chemicals, electricity or other external stimuli, their shapes could potentially be actively controlled.
That raises the possibility of advanced 4D-printed structures that change their three-dimensional form when exposed to a particular stimulus.
Future versions could therefore do more than simply stretch and recover. They might change shape, stiffness or functionality on demand.
The researchers' work demonstrates a new combination of properties: highly complex 3D architecture, extremely low density and reversible stretchability above 414%.
That combination was difficult to achieve using earlier manufacturing approaches. By overcoming the manufacturing challenge, Jiang and Wang have created a platform that could allow engineers to explore mechanical properties and applications that were previously difficult or impossible to achieve.
From soft robots and wearable electronics to rehabilitation systems and impact-absorbing structures, these super-stretchable metamaterials could offer a new way to build machines and devices that are lighter, softer, more adaptable and capable of surviving extreme deformation.
Reference: Jiang Y, Wang Q. Highly-stretchable 3D-architected Mechanical Metamaterials. Sci Rep. 2016 Sep 26;6:34147. doi: 10.1038/srep34147. PMID: 27667638; PMCID: PMC5035992.
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