Buckling upon Inflation in Soft Porous Nonlinear Geometric (SPoNGe) Metamaterials

Cellular metamaterials represent unique platforms to manipulate structure property relationships and enhance mechanical responses. While their unconventional behaviors have traditionally been obtained via pattern-transformations under compressive loading or deflation, we theoretically investigate and experimentally realize a new class of soft porous metamaterials that undergo buckling instability upon inflation, unlocking superior programming and sequencing capabilities for soft intelligent machines. Our soft porous nonlinear geometric (SPoNGe) metamaterial reimagines the traditional rubber slab with periodic holes by incorporating a single internal pressure cavity. This novel structure, actuated via a single internal positive pressure, can be engineered to exhibit global short-wavelength buckling modes with controllable circumferential lobe count of the cylindrical pores. First, we experimentally demonstrate the programmable post-buckling behavior by tuning the geometric parameters. Then, with a combination of analytical and numerical methods, we accurately predict the critical buckling pressure and pattern reconfiguration of the cellular metamaterial. By enabling different pattern rearrangements of the collapsing pores, we achieve a new actuation mechanism to suddenly reconfigure the global structure, selectively grasp slender objects, and operate multiple fluid channels with a single input.