Complex sentence: Mechanical metamaterials are artificial structures with mechanical properties defined by their structure rather than their composition and can be seen as a counterpart to the rather well-known family of optical metamaterials and include acoustic metamaterials as a special case of vanishing shear.
Split into simpler sentences: Mechanical metamaterials are artificial structures with mechanical properties defined by their structure rather than their composition. They can be seen as a counterpart to the rather well-known family of optical metamaterials and include acoustic metamaterials as a special case of vanishing shear.