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Hyperexpandable macromolecular crystals exhibit self-healing capabilities

By Ahmad R. Kirmani June 14, 2018
self-healing macromolecular crystals
A ferritin crystal-hydrogel hybrid (above) undergoing an expansion-contraction cycle as probed by x-ray scattering (below) over time. Credit: Nature Publishing Group

Single crystals are repeating units of atomic planes exhibiting long-range periodic order. This inherent symmetry leads to a slew of enticing chemical and physical properties. Semiconducting single crystals, silicon for example, are the epitome of near-flawless transport of charge through them. A fundamental limitation that this long-range order levies on crystals is their intrinsic brittleness. Flexing them at first leads to defect formation on the atomic scale, and finally, to a loss of order and physical breakdown at the macroscopic level. This constraint often limits the use of single crystals to laboratory-based model studies, making them incompatible with real-life applications that require flexibility and stretchability.

A recent breakthrough from a research group at the University of California, San Diego may overcome this limitation. The group demonstrated hyperexpandable macromolecular crystals with self-healing capabilities. As reported in a study published recently in Nature, the researchers fabricated a hybrid of ferritin crystals—a protein that stores iron—and hydrogel polymers. These hybrids exhibit properties of a new form of material that breaks down the compromise between structural order and flexibility.

An important criterion to achieve the goal was to look for mesoporous lattices that would allow effective penetration of a polymer network, targeting extensive interaction between the two phases so as to maintain integrity of the hybrid material. This led the research team to synthesize flexible hybrid materials that integrated copolymer hydrogels into the mesoporous lattices of ferritin crystals. “To fabricate this material, we exploited the porous nature of the crystals of ferritin to form an extensive network of hydrogels within the crystal lattices. This process essentially ‘bonds’ the protein molecules that form the crystals with the ‘snake-like’ chains of polyacrylate/acrylamide polymers,” says Akif Tezcan, who led the research team.

The hybrid materials display a wide array of extraordinary properties. The research team was able to induce expansion of the ferritin-hydrogel hybrids by placing them in water. Addition of a salt solution (sodium chloride or calcium chloride) led to contraction of the expanded crystals. The expansion-contraction cycles were observed in real-time using x-ray scattering and optical microscopy. The crystals were found to expand to 570% of their original volume and contract back without loss of structural order, exhibiting excellent self-healing behavior.

The biggest surprise, however, was the observed enhanced order at the atomic level after an expansion-contraction cycle. This resulted in higher-resolution x-ray diffraction data of the ferritin crystals reported to date. “The concept of integrating protein crystals with polymer networks provides a potential route for improving protein crystallography, in general. X-ray crystallography is the premier method for determining the atomic structure of protein molecules and the demonstration that we can actually improve the resolution of the protein crystal structures via polymer bonding could benefit the entire field,” says Tezcan regarding one of the many possible applications of the discovery.

Importantly, the ability of these materials to expand and contract in response to external stimuli can be utilized for the storage and controlled release of biomolecules such as nucleic acids as well as large biological agents for therapeutic and diagnostic purposes.

The advance is garnering wide-spread attention in the research community. Monica Olvera de la Cruz, a professor of chemistry at Northwestern University working on self-assembly of heterogeneous molecules, is excited about the self-healing properties of these hybrids and says there is plenty of room for applications in biotechnology. “When placed in water the structure swells and when monovalent and divalent salts are added the structure contracts without breaking the crystalline structure. This work provides the guidelines to interface proteins and synthetic materials that can undergo mechanical changes for multiple biomimetic functions and biotechnology applications including filtration, separation, and sensing.”

Read the abstract in Nature.