Tuning negative stiffness mechanical metamaterial's snap-through behavior with a series-connected spring

Negative stiffness mechanical metamaterials have shown great promise for various applications and are progressing toward programmability. The key challenge in designing programmable metamaterials lies in achieving efficient tunability. However, due to the limitations of current tuning strategies, many existing programmable metamaterials fall short of expectations. Here, we introduce a strategy that facilitates efficient response transformation and performance tuning across scales for negative stiffness mechanical metamaterials. The implementation of this novel tuning approach requires only the adjustment of a serial spring's stiffness which is much easier and more practical than controlling the non-linear contributions of the buckling parts. A systematic theoretical model has been developed to illustrate this innovative approach, and the results have been validated through simulations and experiments. This research can serve as a reference for designing programmable negative stiffness mechanical metamaterials (NSMMs).