  In the paper, we succeeded to freeze-dry monodisperse microbubbles without
degrading their size and acoustic properties. We used microfluidic technology
to generate highly monodisperse (coefficient of variation, CV<5%) microbubbles
and optimized their formulation along with a cryoprotectant. By using a
specific technique of retrieval of the bubble, we showed that freeze-drying the
microbubbles does not alter their size distribution. To compare the fundamental
resonance properties of the bubbles, we performed backscattered acoustic
characterization measurements. Our experimental results revealed that the
freeze-drying process conserved the acoustic properties of the bubbles. The
maximum backscattering power amplitude of fresh and freeze-dried monodisperse
PVA bubbles was around ten eight times higher than that of SonoVue at a similar
concentration in vitro. By solving the question of storage and transportation
of monodisperse bubbles, our work facilitates their penetration in the domain
of UCAs, for performing new tasks and developing novel non-invasive
measurements, such as pressure, unaccessible to the existing commercialized
bubbles.
