Quantitative laser Doppler anemometry measurements of the shear-stresses exerted on ultrasonic microbubbles attached to surfaces under physiological flow conditions

Ultrasonic contrast agents are currently being developed to tat-get and attach to specific sites of interest within the body. A microbubble, echogenic at intravascular frequencies and capable of targeting areas of atherosclerotic plaque has been developed in-house. The microbubble can be attached to agar and imaged under different flow rates. The shear stresses experienced by the attached microbubbles are unknown and difficult to measure in vivo. To this end we have developed a novel flow chamber and have used laser Doppler anemometry (LDA) to estimate the shear stresses experienced by the attached microbubbles

The biotinylated microbubbles were attached to streptavidin coated agar samples which were placed in wells in the flow chamber. The surface of the agar was flush with the base of the flow channel. Water containing seeding particles was passed over the surface of the agar at a constant flow volume. For LDA a front lens with focal point of 160 mm was used to focus the laser, giving a probe volume of 45 x 45 x 200 micron. Velocity data was collected from the surface of the agar through the flow volume to the top of the flow channel. The shear rate from the sample wall was then calculated allowing the shear stress at the wall to be calculated. The shear stresses on attached microbubbles were calculated for different flow volumes.

The velocity profile from the base of the chamber to the top was found to be parabolic. The peak velocities were 19.20 cm s(-1), 21.71 cm s(-1) and 22.97 cm s(-1) for flow rates of 80, 90 and 100 ml min(-1) respectively. The shear stresses at the surface of the agar, calculated from these profiles were 2.86 dynes cm(-2), 3.47 dynes cm(-2) and 4.50 dynes cm(2).