Control Strategies in Guanine Biocrystallization

Highly reflective assemblies of guanine crystals generate a diverse array of optical phenomena in animals, from camouflage reflectors to image‐forming mirrors. The optical functions of guanine crystals and their related animal behaviors have been studied for over a century. However, only recently have we begun to unravel how organisms precisely control the structure and morphologies of these crystals which underly their photonic properties. In this minireview, we outline strategies employed by animals to control the formation and properties of guanine crystals, integrating both in vitro and in vivo studies. We frame these advances using analogies to ‘design’ principles in inorganic biomineralization—from the use of dopants and macromolecular templates in crystal morphology control to non‐classical crystallization pathways. We show that, despite their distinct biology, organic and inorganic biocrystallization share strikingly similar crystal design strategies. These strategies are a rich source of inspiration for bio‐inspired materials, especially in the development of sustainable optical materials. Finally, we outline key challenges for this exciting new field of organic biomineralization.