Scientists created crystals that can stretch and then shrink back to their original state. They built the new form of material by chemically integrating protein crystals with polymer hydrogels. The new material doesn’t sacrifice its high structural order to gain flexibility or vice versa. Dynamic bonds that form between the hydrogel and proteins promote self-healing. Defects, where the crystal structure is out of alignment, that emerge when the crystal expands and contracts, self-heal. This self-healing is a property not expected in crystalline materials.
Often found in batteries or sensors, crystalline materials possess greatly desired properties such as highly efficient charge transport. However, these materials quickly break down when forced to bend or stretch. This limits their use in filtration, separation, energy conversion, and sensors. These new materials can expand and return to their original shape repeatedly without losing order and can self-heal defects when they do arise. Their design offers a blueprint for versatile, resilient crystalline materials.
The formation of solid matter typically involves a tradeoff between structural order of crystalline materials and the flexibility of amorphous materials. Flexible organic or inorganic molecular crystals cannot expand without fracturing, and crystalline materials rarely display self-healing behavior. Hydrogel polymers do not have the structural order of molecular crystals. Their highly elastic mechanical properties allow them to expand considerably and self-heal when equipped with dynamic bonding capabilities.
Scientists have now developed a new form of materials that seamlessly combine the structural order and arrangement of components typical of crystals, the adaptiveness and tunable mechanical properties of polymeric networks, and the chemical versatility of protein building blocks. To create the material, the team infused hydrogel precursors within the porous crystals of ferritin, a sphere-like iron-storage protein, and then polymerized it to create an elastic, faceted crystal. When placed in ion-free water, the integrated crystal-polymer materials uniformity expanded in all directions to 180% of their original dimensions and more than 500% of their original volume while retaining crystallinity and overall shape. Specific molecular contacts between the highly expanded crystals were reformed when the crystal contracted, leading to fully regained atomic-level ordering. Extensive dynamic bonding interactions between the hydrogel network and the ferritin molecules minimized the build-up of local strain. This same dynamic bonding enables the hybrid to heal lattice defects that emerge. Rapid expansion or contraction frequently led to fracturing, but cracks that propagated throughout the structure, some as wide as 20 μm, were spontaneously sealed to restore the crystal-polymer hybrid to nearly its original shape. The team observed the expansion-contraction behavior, ability to resist deformation, and self-healing in more complex hybrid crystals that contained well-defined, spatially differentiated regions.
F. Akif Tezcan
University of California, San Diego
This work was primarily supported by the Department of Energy (DOE) Office of Science, Office of Basic Energy Sciences, Materials Sciences and Engineering Division. Partial support for small-angle X-ray scattering (SAXS) studies was provided by the National Science Foundation. Crystallographic data were collected at the Stanford Synchrotron Radiation Light Source (SSRL) and the Crystallography Facility of the University of California, San Diego. SAXS data were collected at SSRL and the Advanced Photon Source (APS). Both facilities are supported by the DOE, Office of Science, Office of Basic Energy Sciences.
L. Zhang, J.B. Bailey, R.H. Subramanian, A. Groisman, and F. Akif Tezcan, “Hyperexpandable, self-healing macromolecular crystals with integrated polymer networks.” Nature 557, 86 (2018). [DOI: 10.1038/s41586-018-0057-7]
MRS Bulletin article: Hyperexpandable macromolecular crystals exhibit self-healing capabilities
Nature News and Views article: Perfect union of protein and gel creates hyperexpandable crystals