Characterization of acoustic droplet vaporization and inertial cavitation thresholds in acoustically-responsive tissue scaffolds

Hydrogel scaffolds are commonly used in tissue engineering as a substrate for cells or to deliver regenerative growth factors. We have previously demonstrated that acoustically responsive scaffolds can be developed by incorporating sonosensitive emulsions into hydrogel scaffolds. Growth factors, contained within the sonosensitive emulsions, can be released using ultrasound. Since ultrasound can be applied with both spatial and temporal control, this enables spatio-temporal release of growth factors within the ARS, which is unattainable with conventional hydrogel scaffolds. In this study, we expand this previous work by studying the thresholds associated with acoustic droplet vaporization (ADV) and inertial cavitation (IC) - two processes relevant to growth factor release. Acoustically-responsive scaffolds (ARS) were cast in OptiCells by doping fibrin hydrogels with perfluorocarbon droplets. ADV was generated in the ARS using a 2.5 MHz single element transducer. B-mode ultrasound was used to detect the onset of ADV whereas IC was detected with a broadband hydrophone. Results showed that ADV and IC thresholds are dependent on the number of acoustic cycles, the concentration of the ARS, and the structure of the droplets.