Moisture-sensitive mechanical metamaterials with unusual and re-programmable hygroscopic deformation modes.

Mechanical metamaterials are of great interest due to their counterintuitive deformation under various physical fields. However, the research on metamaterials responding to moisture is still rare and controllable hygroscopic deformation is vital for sensoring, actuating, and stress elimination in a moisture environment. Inspired by the hygroscopic deformation of pinecones, this work studies 2D moisture-sensitive mechanical metamaterials exploiting bi-material curved strips as building blocks by simulations and experiments, which especially demonstrates repeatable programming ability to realize customized unusual hygroscopic deformations. Depending on the structural design of geometrical parameters and material configurations, the metamaterials exhibit a tunable coefficient of hygroscopic expansion from negative to positive, and unusual hygroscopic deformation modes including anisotropic, shearing, gradient, bending, and 3D deformation of 2D structures. Programmable metamaterials of arbitrary hygroscopic deformation are achieved by pixelated design and coding the building blocks. More importantly, the hygroscopic deformation is re-programmable by adopting erasable moisture-proof coatings on specific areas of metamaterials, i.e. , it can continuously provide different customized deformation modes in a sample.