Making crystalline materials flexible requires a trade-off in the number of interactions between molecular components; more of these generally maintain crystallinity, whereas fewer allow flexibility. This puts limits on the degree of flexibility or crystal-lattice expansion for molecular crystals. Akif Tezcan and colleagues take advantage of the porosity of ferritin protein crystals to infuse the crystals with a polymer network. Upon the addition of water, the protein–polymer composite is able to expand isotropically to twice its size while maintaining periodicity. This expansion is reversible when salt is added. The non-covalent interactions between the ferritin lattice and the polymer network make the material tougher than either component alone, and provide self-healing behaviour when cut. This new class of composite materials combines the structural order of ferritin crystals with the chemical tunability of polymer hydrogels.
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