  Biological composites offer self-healing properties, biocompatibility, high
responsivity to external stimuli, and multifunctionality, due, in part, to
their complex, hierarchical microstructure. Such materials can be inexpensively
grown, and self-assembled from the bottom up, enabling democratized,
sustainable manufacturing routes for micro- and nano-devices. While biological
composites have been shown to incorporate rich photonic structures, their
phononic properties have hitherto remained unexplored. In this study, we
demonstrate that biological composites in the form of micron-thick
decellularized onion cell scaffolds behave as an organic phononic material,
with the presence of band gaps forbidding the propagation of elastic waves in
select frequency ranges. We show that the onion cells' phononic properties can
be phenotypically tuned, and anticipate these findings will yield new
biologically-derived, "green", and genetically tailorable phononic materials.
