  Auxetics refers to structures or materials with a negative Poisson's ratio,
thereby capable of exhibiting counter-intuitive behaviors. Herein, auxetic
structures are exploited to design mechanically tunable metamaterials in both
planar and hemispherical configurations operating at megahertz (MHz)
frequencies, optimized for their application to magnetic resonance imaging
(MRI). Specially, the reported tunable metamaterials are composed of arrays of
inter-jointed unit cells featuring metallic helices, enabling auxetic patterns
with a negative Poisson's ratio. The deployable deformation of the
metamaterials yields an added degree of freedom with respect to frequency
tunability through the resultant modification of the electromagnetic
interactions between unit cells. The metamaterials are fabricated using 3D
printing technology and a ~20 MHz frequency shift of the resonance mode is
enabled during deformation. Experimental validation is performed in a clinical
(3.0 Tesla) MRI, demonstrating that the metamaterials enable a marked boost in
radiofrequency (RF) field strength under resonance matched conditions,
ultimately yielding a dramatic increase in the signal-to-noise ratio (SNR) (~
4.5X) of MRI. The tunable metamaterials presented herein offer a novel pathway
towards the practical utilization of metamaterials in MRI, as well as a range
of other emerging applications.
