Design and Reprogrammability of Zero Modes in 2D Materials from a Single Element.

Mechanical extremal materials, a class of metamaterials that exist at the bounds of elastic theory, possess the extraordinary capability to engineer any desired elastic behavior by harnessing mechanical zero modes - deformation modes that demand minimal or, ideally, no elastic energy. However, the potential for arbitrary construction and reprogramming of metamaterials remains largely unrealized, primarily due to significant challenges in qualitatively transforming zero modes within the confines of existing metamaterial design frameworks. This work presents a method for explicitly defining and in situ reprogramming zero modes of 2D extremal materials by employing straight-line mechanisms (SLMs) and planar symmetry, which prescribe and coordinate the zero modes, respectively. The method is used to design, test, and reprogram centimeter-scale isotropic, orthotropic, and chiral extremal materials by reorienting the SLMs in place, enabling these materials to smoothly and reversibly interpolate between extremal modalities (e.g., unimode to bimode), material properties (e.g., negative to positive Poisson's ratios), and selectively enable chirality without changing the metamaterial's global structure. This methodology provides a straightforward and explicit strategy for the design and tuning of all varieties of 2D extremal materials, enabling dynamic mechanical metamaterial construction to completely cover the gamut of elastic properties.