Ice-Templated Fabrication of Porous Materials with Bioinspired Architecture and Functionality

ConspectusBiological porous materials, such as polar bear hair, wood, bamboo, and cuttlebone, exhibit outstanding thermal and mechanical properties due to their hierarchical architectures. Specifically, polar bears can retain thermal homeostasis in the extremely cold Arctic due to outstanding thermal insulation property of their porous hairs; wood and bamboo exhibit excellent mechanical strength, highly efficient water and nutrient transport capacity, and low density due to their hierarchically aligned porous architecture; cuttlebone can resist large hydrostatic pressure in the deep-sea environment due to its mechanically efficient porous structure with lamellar septa connected by asymmetrically S-shaped walls. Obviously, the hierarchical architecture is crucial for the excellent performance of biological porous materials. Inspired by these natural design motifs, bioinspired materials with hierarchical architectures have been extensively explored for various applications where excellent mechanical, thermal, and electric properties are highly demanded. As a controllable, versatile, scalable, and environmental-friendly process, ice templating (or freeze casting) represents an effective approach to mimicking the sophisticated architecture of biological materials in synthetic counterparts, which has attracted wide attention from research groups across the world.In recent years, we have conducted comprehensive studies on the mechanism of the ice-templating approach and provided a rich toolbox based on this technique to meet various manufacturing demands. We systematically studied the material assembly process and the heat and mass transfer mechanism of the ice-templating technology from the aspects of cold surface, temperature field design, and intrinsic properties of the building blocks. First, bidirectional freezing guided by dual temperature gradients was developed to generate a long-range nacre–mimetic lamellar architecture. Next, complex hierarchical lamellar architectures were constructed through freezing on engineered cold surfaces with either wettability gradient or grooved pattern. Additionally, the "freeze-spinning" technique was developed to realize continuous and large-scale fabrication of fibers with aligned porous architecture mimicking polar bear hair. Finally, the applicability of the unidirectional ice-templating technique was broadened from soluble to insoluble polymeric materials through the freezing of emulsion droplets. These techniques have been widely used to fabricate a series of porous materials with bioinspired architectures, which hold great potential in thermal regulation, liquid transport, and mechanical functions for wide applications in various fields. Finally, a concise summary of this Account, including latest development, future challenges and opportunities in the ice-templated fabrication of bioinspired porous materials will be provided. This Account provides guidance for the design and ice-templated fabrication of bioinspired porous materials with multiscale architecture and functionality, which are crucial for various engineering applications.