When Bottom-Up Meets Top-Down

In nature, materials are built in a “bottom-up” manner, relying on the self-assembly of fundamental building blocks, and have evolved over thousands of years of natural selection to achieve impressive performance. Materials in nature are sustainably designed in a hierarchical and multifunctional way to be lightweight while also providing toughness and resilience, and to possess biotic and abiotic resistance. Since the industrial revolution and accompanying advancement of modern material science, most of our industrial materials are synthetic polymers and metals, which, unlike natural materials, are processed into products using a “top-down” approach. Many synthetic polymers are nondegradable, and their production processes are associated with a significant negative environmental impact. Marelli et al. (1) were the first to apply the top-down approach usually reserved for man-made materials to natural silk embedded with functional materials such as enzymes and light- and strain-responsive nanoparticles. This work was mainly possible due to the thermoplastic properties of silk, which enable compression molding, and its stiffness, which enables machining. Significant attention has been accorded in recent years to biopolymers, mainly those with β-sheet–forming ability, such as silk, elastin, and fibrin. Silk fibroin is one of several structural proteins that can be used to generate biopolymer-based materials with controlled functionalities (2). The bottom-up process in silk fibroins is characterized by the formation of a hierarchical …