Nine-grid combination and re-entrant hybrid NPR structures. (a) Nine-grid combination multi-cell NPR structures, (b) the predicted deformation state of nine-grid combination structures based on FEA, (c)–(e) X-type, H-type and Sd-type x-y displacement relationships, (f) force-displacement curves and displacement relationships of X-type, H-type, and Sd-type structures [37]. (g) Geometric parameters of REHH structures, (h) y-axis and x-axis stretch, (i) displacement analysis results under tensile loading, (j) Poisson's ratio and elastic modulus vs. θ₁ for various internal angles θ₂ [53].