**6. Mechanism by ultrasound-mediated microbubble destruction**

Ultrasound facilitates the delivery of drugs and genes. In the insonified field, the presence of microbubbles reduces the peak negative pressure which is necessary to enhance drug delivery. This happened because microbubbles acting as nuclei for cavitations decrease the threshold of ultrasound energy. Microbubble gets destroyed by ultrasound due to the gradual diffusion of gas at low acoustic power, formation of a shell defect with diffusion of gas, immediate expulsion of the microbubble shell at high acoustic power, and dispersion of microbubbles into several smaller bubbles.
