Acoustic properties of surfactant microbubbles in relation to their lifetime in vitro as determined by diffusion

In this study, to make microbubbles less than 10 μm and to extend their lifetime, sodium laurate is chosen as the material for the coating of microbubbles. In studies of ultrasound contrast agents, the authors are the first to report on the use of sodium laurate as the material for manufacturing the microbubble contrast agent. In addition, the method of producing microbubbles using a syringe is proposed. Using sodium laurate and the syringe method, microbubbles with diameter less than 10 μm are produced. From the result of simulation, it is found that the dissolved gas concentration in the solution surrounding microbubbles plays an important role in determining microbubble lifetime. Microbubbles without coating are calculated to have a lifetime of less than 1 min when the dissolved concentration of gas is about 90%. On the other hand, the lifetime of microbubbles made from sodium laurate is about 5 min longer than the calculated lifetime. Therefore, it is hypothesized that when the dissolved gas in the solution is above a certain level, the mechanism governing annihilation of microbubbles is the dissolution of the surfactant coating layer into the solution, instead of the diffusion of gas from the microbubbles. Since the dissolution time of surfactants is longer than the dissolution time of gas, the lifetime becomes longer.