Programming function into mechanical forms by directed assembly of silk bulk materials

Significance Engineering multiple functions in a single material format is a key design parameter to fabricate devices that can perform at the confluence between biology and technology. This can be achieved by designing materials with hierarchical structures across several scales or by embedding active molecules at the point of material formation. These approaches have been successfully pursued to engineer 2D materials formats. However, current technologies have limited the formation of 3D constructs with orthogonal functions. In this study, we demonstrate an entirely water-based sol–gel–solid process to generate 3D mechanical forms that embed biological (and other) functions. This approach is a step toward the development of multifunctional devices that may liaise between the biotic and abiotic worlds. We report simple, water-based fabrication methods based on protein self-assembly to generate 3D silk fibroin bulk materials that can be easily hybridized with water-soluble molecules to obtain multiple solid formats with predesigned functions. Controlling self-assembly leads to robust, machinable formats that exhibit thermoplastic behavior consenting material reshaping at the nanoscale, microscale, and macroscale. We illustrate the versatility of the approach by realizing demonstrator devices where large silk monoliths can be generated, polished, and reshaped into functional mechanical components that can be nanopatterned, embed optical function, heated on demand in response to infrared light, or can visualize mechanical failure through colorimetric chemistries embedded in the assembled (bulk) protein matrix. Finally, we show an enzyme-loaded solid mechanical part, illustrating the ability to incorporate biological function within the bulk material with possible utility for sustained release in robust, programmably shapeable mechanical formats.