Preprocessing materials for external self-assembly Some materials are formed within cells, notably the hair shaft , or in a controlled extracellular space, such as spider silk glands. However, other biological materials form without requiring an organ for their production. Self-assembly can occur upon secretion in external environments, which generally simplifies biomimicry approaches , without the need for selfmicrofabrication. Yet some materials still depend on an organ for internal preprocessing prior to their secretion. For instance, the cuticle of insects is initially soft but hardens rapidly in contact with air . This chitin and protein biomolecular composite has a preformed microstructure that hardens into a tough exoskeleton through sclerotization . Catecholamines, such as dopamine derivatives, oxidize with air into quinones and crosslink cuticular proteins but the microstructure is performed by the epidermis . In other cases, materials rely on environmental conditions such as temperature changes, humidity absorption, or dehydration. Some aquatic species notably rely on their saline aqueous environment for material processing, such as barnacle adhesive , sandcastle worm cement , coral skeleton , and mussel byssus .