Shape memory in self-adapting colloidal crystals

Reconfigurable, mechanically responsive crystalline materials are central components in many sensing, soft robotic, and energy conversion and storage devices^1-4. Crystalline materials can readily deform under various stimuli, and the extent of recoverable deformation is highly dependent upon bond type^1,2,5-10. Indeed, for structures held together via simple electrostatic interactions, minimal deformations are tolerated. In contrast, structures held together by molecular bonds, in principle, can sustain much larger deformations and more easily recover their original configurations. In this report, we study the deformation properties of well-faceted colloidal crystals engineered with DNA. These crystals are large in size (> 100 µm) and have a body-centered cubic (BCC) structure with a high viscoelastic volume fraction (> 97%). Therefore, they can be compressed into irregular shapes with wrinkles and creases, and remarkably, these deformed crystals, upon rehydration, assume their initial well-formed crystalline morphology and internal nanoscale order within seconds. For most crystals, such compression and deformation would lead to permanent, irreversible damage. The significant structural changes to the colloidal crystals are accompanied by dramatic and reversible optical property changes. For example, while the original and structurally recovered crystals exhibit near-perfect (> 98%) broadband absorption in the UV-vis region, the deformed crystals exhibit significantly increased reflection (up to 50% of incident light at certain wavelengths), mainly due to increases in their refractive index and inhomogeneity.