Our article in PNAS: Crack engineering as a new route for the construction of arbitrary hierarchical

First-ever method to mass-produce 3D hierarchical and biomimetic structures in desired materials

Three-dimensional (3D) hierarchical structures are the fundamental elements of numerous biological surfaces that exhibit various fascinating functionalities useful for liquid and mass transport, energy conversion, signal transformation, and topological interaction, etc. In recent years, extensive efforts have been directed towards mimicking these natural inspirations for practical applications (Nature 424, 810, 2003; Science 346, 1096, 2014; Nat. Rev. Mater. 2, 17036, 2017). While breakthroughs in bio-inspired engineering have created a wide range of structures that enable various fascinating functionalities, these achievments remain conceptual because of the daunting challenge in creating desired hierarchical structures at affordable cost, in large scale, and/or with desired material. Gernally, pre-designed hierarchical structures can only be prototyped using expensive and sophisticated technologies (that is, the standard MEMS process and/or two-photon 3D printing technologies). These methods are limited by very low throughput and very high cost, and are applicable to only a few materials, e.g., Si and photoresist. As a result, the created rigid structures inevitably suffer from limited structural durability, flexibility, and optical transparency, not to mention the unaffordable cost, thereby preventing their broad real-life applications. Thus, it is imperative to develop an affordable and scalable strategy to facilely fabricate hierarchical structures and increase the materials diversity for smart interfacial materials.

For the first time we report an approach that enables mass-production of arbitrary hierarchical structures through a faithful replication process with broad material compatibility. In striking contrast to conventional wisdom, we demontrate that the previously strictly avoided detrimental crack phenomenon can be translated into a powerful tool to achieve structures and functions that are otherwise impossible to realize even using state-of-art facilities. Our innovation lies in that elastomer materials are associated with a high level of configurability by virtue of their elastic deformation characteristics; effetively controlling the cracking in elastic materials and subsequently configuration of the elastic crack can pave a road for 3D replication. We demonstated that this approach can realize faithful mass-production of various functioanl hierarchical structures reported before that could only be made at very small scale in silicon or photoresist only. We name this new approach configurable, elastic crack engineering (CECE)-assisted molding (CAM), which is a breakthrough in microfabrication technology, as it greatly extends the range of 3D micro-processible materials to broadly used structural plastics and other shapable materials, as well as tremdendously reduced the production cost (by the order of 10^4) and increased the production scope and throughput. Our work paves the way for the cost-effective, large-scale production of a variety of flexible, inexpensive, and transparent 3D hierarchical and biomimetic materials.