  Equilibrium bifurcation in natural systems can sometimes be explained as a
route to stress shielding for preventing failure. Although compressive buckling
has been known for a long time, its less-intuitive tensile counterpart was only
recently discovered and yet never identified in living structures or organisms.
Through the analysis of an unprecedented all-in-one paradigm of elastic
instability, it is theoretically and experimentally shown that coexistence of
two curvatures in human finger joints is the result of an optimal design by
nature that exploits both compressive and tensile buckling for inducing
luxation in case of traumas, so realizing a unique mechanism for protecting
tissues and preventing more severe damage under extreme loads. Our findings
might pave the way to conceive complex architectured and bio-inspired
materials, as well as next generation artificial joint prostheses and robotic
arms for bio-engineering and healthcare applications.
