  Current strategies for designing tunable locally resonant metamaterials are
based on tuning the stiffness of the resonator; however, this approach presents
a major shortcoming as the effective mass density is constant at high
frequency. Here, this paper reports a type of tunable locally elastic
metamaterial-'called tunable fluid-solid composite'-inspired by the functions
of heart and vessels in animals and humans. The proposed metamaterial consists
of several liquid or gas inclusions in a solid matrix, controlled through a
pair of embedded pumps. Both the band gap and effective mass density at high
frequency can be tuned by controlling the liquid distribution in the unit cell,
as demonstrated through a combination of theoretical analysis, numerical
simulation, and experimental testing. Finally, we show that the tunable
fluid-solid metamaterial can be utilized to manipulate wave propagation over a
broad frequency range, providing new avenues for vibration isolation and wave
guiding.
