1-MHz ultrasound enhances internal diffusivity in agarose gels

Ultrasound sonification stimulates the release of pharmaceutical compounds from hydrogels. At the surface of hydrogels, cavitation, cavities formed in liquid, activates to stimulate that release under low-frequency ultrasound. Under high-frequency ultrasound, although cavitation activities are highly suppressed, the compounds are still released. Although it remains elusive how high-frequency ultrasound stimulates this release, one hypothesis is that the internal diffusivity is enhanced. In this study, internal diffusivities in agarose gels were estimated with fluorescent recovery after photobleaching (FRAP) analysis. Under 1-MHz ultrasound sonification, internal diffusivity in agarose gels was enhanced. The enhancement of internal diffusivity was larger than that with temperature elevation alone, although temperature elevation was also observed along with the ultrasound sonification. Thus, we found that high-frequency ultrasound sonification enhances internal diffusivity in agarose gels. This enhancement was, at least in part, independent of temperature elevation.