Bubble nuclei in polyacrylamide hydrogels

Abstract

Bubble nuclei have been studied extensively in water for over 50 years with nuclei categorized as homogeneous or heterogeneous. However, it is unclear how those nuclei identified in water translate to viscoelastic hydrogels or tissues. In this study, bubble nuclei were evaluated in 17.5%, 20%, and 22.5% v/v% polyacrylamide (PA) hydrogels (n = 3 each). A 1.5 MHz focused ultrasound transducer was used to induce cavitation using 10-ms pulses with pressures ranging up to p+ = 8 = 89 MPa, and p− = 2 = 26 MPa and -6 dB focal dimensions of 9.4 × 1.15 mm2 (p−). Cavitation size and location were monitored with high-speed photography. When the concentration of PA increased from 17.5% to 20%, the area occupied by bubbles at 0.07 ms decreased from 0.14 mm2 to 0.06 mm2. The location of acoustic cavitation for replicate exposures in the 17.5% gel at 0.07 ms became more consistent as the acoustic pressure increased with no bubble overlap at p− = 2 = 21MPa and 18% bubble overlap at p− = 2 = 26MPa. These results suggest acoustic cavitation in PA hydrogels is dependent on the availability of bubble nuclei at each driving pressure. Future work includes investigating the distribution of bubble nuclei in tissues. [Work supported by NSF CAREER 1943937 and PSU Riess Fellowship].