This shows the nanoarchitected metamaterial deforming to create the Caltech icon. Image: Julia Greer/Caltech.A newly developed type of architected metamaterial has the ability to change shape in a tunable fashion.
While most reconfigurable materials can toggle between two distinct states, the way a switch toggles on or off, the new material's shape can be finely tuned, adjusting its physical properties as desired. The material, which has potential applications in next-generation energy storage and bio-implantable micro-devices, was developed by a team of researchers from the California Institute of Technology (Caltech), the Georgia Institute of Technology and ETH Zurich in Switzerland.
The team was led by Julia Greer, professor of materials science, mechanics and medical engineering in Caltech's Division of Engineering and Applied Science. She creates materials out of micro- and nanoscale building blocks arranged into sophisticated architectures that can be periodic, like a lattice, or non-periodic in a tailor-made fashion, giving them unusual physical properties.
Most materials that are designed to change shape require a persistent external stimulus to change them from one shape to another and stay that way: for example, they may be one shape when wet and a different shape when dry – like a sponge that swells as it absorbs water.
By contrast, the new nanomaterial deforms through an electrochemically driven silicon-lithium alloying reaction. This means that it can be finely controlled to attain any ‘in-between’ states, remain in these configurations even upon the removal of the stimulus and be easily reversed.
Apply a little current, and the resulting silicon-lithium alloying reaction changes the shape of the material by a controlled, small degree. Apply a lot of current, and the shape changes substantially. Remove the electrical control, and the configuration is retained – just like tying off a balloon. The researchers report this new type of material in a paper in Nature.
Defects and imperfections exist in all materials, and can often determine a material's properties. In this case, the team chose to take advantage of that fact and build in defects to imbue the material with the properties they wanted.
"The most intriguing part of this work to me is the critical role of defects in such dynamically responsive architected materials," says Xiaoxing Xia, a graduate student at Caltech and lead author of the Nature paper.
For the paper, the team designed a silicon-coated lattice with microscale straight beams that bend into curves under electrochemical stimulation, taking on unique mechanical and vibrational properties. Greer's team created these materials using an ultra-high-resolution 3D printing process called two-photon lithography. Using this novel fabrication method, they were able to build defects into the architected material system, based on a pre-arranged design. In a test of the system, the team fabricated a sheet of the material that, under electrical control, reveals a Caltech icon.
"This just further shows that materials are just like people, it's the imperfections that make them interesting. I have always had a particular liking for defects, and this time Xiaoxing managed to first uncover the effect of different types of defects on these metamaterials and then use them to program a particular pattern that would emerge in response to electrochemical stimulus," says Greer.
A material with such a finely controllable ability to change shape has potential in future energy storage systems because it provides a way to create adaptive energy storage systems that would allow batteries, for example, to be significantly lighter and safer, and to have substantially longer lives, Greer says. Some battery materials expand when storing energy, creating a mechanical degradation due to stress from the repeated expanding and contracting. Architected materials like this one can be designed to handle such structural transformations.
"Electrochemically active metamaterials provide a novel pathway for development of next generation smart batteries with both increased capacity and novel functionalities. At Georgia Tech, we are developing the computational tools to predict this complex coupled electro-chemo-mechanical behavior," says Claudio Di Leo, assistant professor of aerospace engineering at the Georgia Institute of Technology.
This story is adapted from material from Caltech, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.