4D printed chiral metamaterials with negative swelling behavior

Soft adaptable materials and structures that change their shapes, volumes and properties in response to surrounding environment are important and ideal for scientific and engineering applications. Existing soft gels exhibit a dramatic volumetric swelling deformation when immersed in solution. Developing an innovative material system that can undergo large and actively negative swelling deformation driven by solution is an interesting challenge. In this work, a water-driven mechanical metamaterial with negative swelling is designed and fabricated based on the coupled responses of the swelling of hydrogels and deformation characteristics of chiral metamaterials. The constructed metamaterial utilizes deformation characteristics of 4D printed composite structure to convert the swelling deformation of the hydrogel into a bending deformation of flexible ligaments, resulting in a volume reduction of the entire metamaterial. Activated by surrounding solution, the metamaterial can achieve large and adjustable effective negative swelling behaviors with the desired isotropic characteristics. Based on experimental data and finite element simulation results, the customized metamaterials can be implemented to produce the desired negative swelling deformation by adjusting the lattice geometry and microstructural parameters. These design concepts exploit the capabilities of existing soft materials and have great potential in a variety of applications.