While most reconfigurable materials can toggle between two distinct states, a new material’s shape can be finely tuned, adjusting its physical properties as desired. The material has potential applications in next-generation energy storage and bio-implantable microdevices.
Most materials that are designed to change shape require a persistent external stimulus to change 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, enabling it to be finely controlled to attain any in-between states, remain in these configurations even upon the removal of the stimulus, and be easily reversed. If a low current is applied, a resulting chemical reaction changes the shape by a controlled, small degree. If a high current is applied, the shape changes substantially. Remove the electrical control and the configuration is retained.
Defects and imperfections exist in all materials and can often determine a material’s properties. In this case, the team built in defects to imbue the material with the properties they wanted.
The researchers designed a silicon-coated lattice with microscale straight beams that bend into curves under electrochemical stimulation, taking on unique mechanical and vibrational properties. The materials were created using an ultra-high-resolution 3D printing process called two-photon lithography. Using this novel fabrication method, they were able to build in defects in the architected material system, based on a pre-arranged design. In a test of the system, a sheet of the material was fabricated that, under electrical control, revealed a Caltech icon.
A material with such a finely controllable ability to change shape has potential in future energy storage systems because it provides a pathway to create adaptive energy storage systems that would enable batteries to be significantly lighter, safer, and to have substantially longer lives. Some battery materials expand when storing energy, creating a mechanical degradation due to stress from the repeated expanding and contracting. Architected materials can be designed to handle such structural transformations.