  Engineered microbubbles can be acoustically driven to cavitate against a
substrate to produce localized erosion and fragmentation. This mechanical
action has therapeutic applications in the treatment of biomineralizations,
such as in urinary stone disease. However, current methods for quantifying the
mechanical action of cavitation on a substrate are slow or imprecise. In this
paper, we describe the design of a device that applies calibrated pressures to
microbubbles engineered to target a calcium-containing hydroxyapatite substrate
under physiological conditions and quantifies the result via an automated
submerged mass measurement with high precision and low drift. Measurements of
microbubble-mediated mass loss were observed to be linear with time, with
variance that was comparable to the resolution of the instrument. The rate of
mass loss with microbubbles present was 5.5-fold greater than in the absence of
microbubbles. This research instrument captures the essential mechanical and
physiological features of in vivo microbubble-mediated erosion and
fragmentation of urinary stones and has been used to optimize the parameters of
this treatment in a clinical trial for a promising new approach to the
treatment of nephrolithiasis. In addition to clinically relevant therapeutic
applications, this approach will contribute to broader understanding of
acoustic cavitation against a substrate.
