**5. Conclusion and prospect**

With the development of ultrasound contrast agents and the understanding of the biological effects of ultrasound, ultrasound-mediated gene delivery has been proven the great potential in the treatment of various diseases. Ultrasound contrast agents, including microbubbles, nanoparticles, and nanobubbles, can be used as gene vectors through intravenous or local injection into lesion site. With ultrasound irradiation at a certain level of acoustic intensity, the cavitation effect, sonoporation, and thermal effects occur, which can enhance the permeability of local tissue and promote the gene delivery into the pathological tissue.

Although ultrasound-mediated gene delivery has a broad application in animal study, there is still a long way for its application in human body. The main

problems, which need to be solved, may include the following aspects. First, many cationic materials are applied for the preparation of the ultrasound contrast agents. They have high gene-carrying capacity, but their biocompatibility is still doubtful. Second, ultrasound security is also an important concern. Unlike diagnostic ultrasound energy, the intensity applying in gene delivery is greater. Studies have shown that severe cavitation effects can lead to membrane rupture, DNA rupture, nuclear fragmentation, endothelial cell damage, microvascular leakage, hemolysis, myocardial injury, and even left ventricular function [19, 124]. More investigations need to be made to optimize the ultrasonic parameters so as to maximize the gene transfection efficiency and reduce the adverse side effects on the normal tissues and organs. In addition, the different types of ultrasound equipment used in various laboratories also bring some difficulties for the repeatability, which hinder the progress of ultrasound-mediated gene transfection technology to some degree. At the same time, it is believed that ultrasound will make more progress in gene delivery and bring about greater medical revolution in the future.
