Theoretical and experimental evaluation of microstreaming created by a single microbubble: Application to sonoporation

It is hypothesized that microstreaming might play a role in the sonoporation process, inducing shear stresses which create tension and stretching over the cell membrane and thus lead to its transient permeabilization. In this study, the results of microscopic particle-image velocimetry (PIV) for large bubbles are presented and compared to those obtained using a numerical model. Air bubbles were created in a water solution and then attached to a wall. For each bubble, the microstreaming was measured over a plane located at a distance of 50 μm from the bubble wall. Bubbles were excited using a single element transducer at 28 kHz and 7 kPa. The acoustic microstreaming generated by the air bubble was calculated for the same excitation signal by using a theoretical model based on an analytical solution of the time-averaged Navier-Stokes equation. This approach was also applied to estimate the flow around an encapsulated contrast microbubble using an excitation signal centered at 2.87 MHz with a pressure amplitude of 50 kPa. PIV measurements show that a maximal average velocity of 0.25 mm/s occurs near the bubble resonant size (244 μm diameter bubble). For this bubble, the shear stress is also maximal. Theoretical data are in good qualitative agreement with PIV measurements. Using the contrast agent microbubble model, a maximal flow velocity and shear stress of 4 mm/s and 19 Pa respectively are obtained for a 2.5 μm diameter bubble. The shear stresses are much higher than those produced by normal blood flow (0.5 - 2 Pa). These results suggest that bubbles are capable of exerting significant shear stresses on the cell membrane, affecting likely the sonoporation process.