The design and investigation of hydrogel-based metamaterials with ultra large negative hygroscopic expansion ratio

A design strategy for a mechanical metamaterial with large negative hygroscopic expansion(NHE)was proposed in this paper.Different from the reported structures,the present metamaterial is designed by constructing repeated lattice microstructure consisting of curved ligaments incorporating hydrogel active layers and polymer support layers and straight polymer bars.When immersed in the solution environment,the swelling of hydrogel layer of such composite structure induces the reversed bending of the ligament,leading to the overall ultra-large shrink(negative expansion)deformation of the metamaterial.Through the new structural design,large NHE effects can be achieved.The theoretical investigation and finite element analysis(FEA)were conducted to demonstrate the large negative expansion effects of such metamaterial.The results showed that the effective NHE ratio of the metamaterial is dependent of the curvature of the curved ligament and the size of both the ligament and the connecting rod.The ultra-large NHE ratios about−80%for the 2D structure and−90%for the 3D version can be obtained by adopting the structural parameters.The newly designed metamaterials have potential applications in medical and other fields.