Transparent materials with stiff and tough hierarchical structures

Materials that are transparent in the visible spectrum are useful in applications that range broadly from displays in portable devices to large-area panels and windows used in the construction industry. The high strength and hardness of silica-based glasses make them most suitable choice in many of these applications. However, such properties come at the cost of the low fracture resistance and low damage tolerance of glasses, which result in brittle and unpredictable failure with potentially dangerous and harmful outcomes. Strong and fracture resistant transparent materials are therefore in high demand in several structural applications. Inspired by the hierarchical structure of biological composites, researchers have been able to fabricate synthetic materials that combine high strength and toughness. Nevertheless, reconciling these mechanical properties with optical transparency does not constitute a trivial task. In this review article, we describe and discuss some of the most promising biologically templated and biologically inspired composite materials that have been proposed to combine optical transparency, strength and fracture toughness. The replication of some of the hierarchical features of biological materials within their structure allows these composites to take advantage of synergistic toughening mechanisms that act at different length scales and provide high resistance against fracture. Understanding how they are fabricated and which mechanisms contribute to their toughness is key to draw guidelines for the synthesis of future transparent, strong and tough composites that are safer and more reliable than state-of-the-art silica-based glasses.