What kinda material?

Alexandra Parmentier the material i used is protein solution.

Stable composite materials exist, which comprise negative-stiffness inclusions embedded in a positive-stiffness matrix, but we're talking essentially metals.

Don' t know whether biological heterogeneous materials may behave this way.

In general, negative stiffness can manifest itself when energy is stored in the deforming object, which could be the case of a protein.

Thank You Alexandra Parmentier