  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.
