Intermediate filaments are filaments 10 nm in diameter that make up an important 
component of the cytoskeleton in most metazoan taxa. They are most familiar for 
their role as the fibrous component of α-keratins such as skin, hair, nail, and 
horn but are also abundant within living cells. Although they are almost 
exclusively intracellular in their distribution, in the case of the defensive 
slime produced by hagfishes, they are secreted. This article surveys the 
impressive diversity of biomaterials that animals construct from intermediate 
filaments and will focus on the mechanisms by which the mechanical properties of 
these materials are achieved. Hagfish slime is a dilute network of hydrated 
mucus and compliant intermediate filament bundles with ultrasoft material 
properties. Within the cytoplasm of living cells, networks of intermediate 
filaments form soft gels whose elasticity arises via entropic mechanisms. Single 
intermediate filaments or bundles are also elastic, but substantially stiffer, 
exhibiting modulus values similar to that of rubber. Hard α-keratins like wool 
are stiffer still, an effect that is likely achieved via dehydration of the 
intermediate filaments in these epidermal appendages. The diverse mechanisms 
described here have been employed by animals to generate materials with 
stiffness values that span an impressive eleven orders of magnitude.