As you will already know, many engineering materials, such as metals, show Hookean elasticity in which the tensile stress applied to a sample is directly proportional to the resultant strain. Within the range of Hookean elasticity, the stress-strain curves on loading and unloading are identical. Such linear elasticity is the usual assumption in engineering design. However, the elasticity of most materials in living systems is much more complicated.
In this TLP, you will learn that some biomaterials exhibit non-linear stress-strain curves. Mammalian skin, for instance, exhibits a J-shaped stress-strain curve, as do healthy arterial walls. Materials with J-shaped curves are usually tough, and have other advantages. For example, materials with different elastic behaviour such as S-shaped stress-strain curves are prone to elastic instabilities such as aneurysms when used for tubes under pressure.
This TLP also discusses viscoelasticity. Many biomaterials show time-dependent stress-strain curves. Associated with this, the loading and unloading curves do not superimpose on each other. Although deformation is elastic (i.e. recoverable), energy is absorbed during the deformation. This is particularly important in spider silk: when a fly hits the web its energy should be absorbed by the deformation of the web. If spider silk showed elasticity without energy absorption, then the web instead would act as a trampoline!