Digital stiffness programmability is fulfilled with a heterogeneous mechanical metamaterial. The prototype consists of an elastomer matrix containing tessellations of diamond shaped cavities selectively confined with semi-rigid plastic beam inserts along their diagonals. Unit-cell perturbations by placing or removing each insert reshape the global constitutive relation whose lower and upper bounds corresponding to the configurations with all holes empty and all inserts in place, respectively, are significantly distant from each other thanks to a gap between the moduli of the elastomer and the inserts. Bidirectional operation is achieved by mixing insert orientations where longitudinal inserts enhance the macroscopic stiffness in compression and transverse ones tension. Arranged digital representations of such local insert states form the explicit encoding of global patterns so that systematic stiffness programming with minimal changes in mass is enabled both statically and in situ. These characteristics establish a new paradigm in actively tuning vibration isolation systems according to shifts in the resonance of base structures.
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