Assembly of Chitin Nanofibers into Porous Biomimetic Structures via Freeze Drying.

The intricate hierarchical architectures in natural creatures are usually derived from assembly of molecular building blocks into nanoscale structures that then organize into micro- and macroscopic sizes. An example is the complex structure in arthropods (crustaceans, insects) constructed primarily of chitin. Because of chitin's inherent insolubility in common solvents, processes for mimicking the fascinating natural chitin-based nanostructures are still at an early stage of development. Here, we present a facile freeze-drying approach to assemble chitin nanofibers (20 nm diameter) into a variety of structures whose size and morphology are tunable by adjusting freezing temperature and heat transfer characteristics. We show that reducing the freezing rate allows controllable formation of structures ranging from oriented sheets to three-dimensional aperiodic nanofiber networks that mimic the size and interconnectivity of the white beetle cuticle. The formation of nanofibrous structures is not predicted by the widely used particle encapsulation model of freeze-drying. We reason that this structure occurs due to a combination of attractive interactions of the nanofibers and a slow freezing rate that encapsulates and preserves the network structure. The method outlined here is likely applicable to creating fine nanofibrous structures with other polymers and materials classes with size ranges useful in diverse applications such as tissue engineering, filtration, and energy storage.