Propulsion mechanisms and applications of multiphysics- driven micro- and nanomotors

Abstract

Multiphysics-driven synthetic micro/nanomotors can convert chemical or other types of energies, such as magnetic, electrical, photochemical, thermal, acoustic energies into mechanical energies in terms of motion and velocity. Significant efforts have been dedicated to preparing micro/nanomotors as well as the applications of micro/nanomotors. This paper provides a comprehensive review on the research for a large variety of micro/nanomotors recently investigated in China. Three types of propulsion mechanisms are proposed for multiphysics-driven synthetic micro/nanomotors, i.e., chemical propulsion, external-physical-fields propulsion and hybrid propulsion. It is shown that chemical propelled micro/ nanomotors can convert the chemical energy caused by the redox reactions into mechanical energy, such as concentration- gradient propelled micro/nanomotors, self-electrophoresis micro/nanomotors and bubble-propelled micro/nanomotors. Among the diverse chemical propelled micro/nanomotors, the micro/nanomotors prepared by rolled-up or eletrodeposition technique show a higher velocity and a more controllable motion. External-physical-fields propelled micro/nanomotors can transfer light, magnetic energy, ultrasonic energy into mechanical energy. There is no need to require any additional chemical fuel which is harmful to human body such as hydrogen peroxide, strong acid, alkaline, Br2, or I2 solutions, for the micro/nanomotors propelled by external-physical-fields. Therefore, it has better biocompatibility and broader applications than the chemical propelled micro/nanomotors. Hybrid propelled micro/nanomotors can combine the chemical propulsion mechanisms with external-physical-fields propulsion mechanisms simultaneously. The hybrid propelled micro/nanomotors not only can move in the chemical fuel, but also can be propelled by light, magnetism or ultrasonic energy. It has more powerful applicability and a broader application than that propelled by the other two propulsion mechanisms. Although the existed researches of micro/nanomotors are still of limitations, such as ambiguous propulsion mechanisms, low velocity, imprecise motion control and poor biocompatibility, the Chinese researchers have made significant progress and breakthroughs in recent years. The micro/nanomotors, which move autonomously in fluids, are studied for a growing number of applications that include targeted drug delivery, separation, biosensor, and fabrication of active biomimetic systems, environmental remediation and other emerging applications. The micro/ nanomotors can be used as adsorbent of microscale particle to achieve reversible swarming and separation of micro/ nanomotors, eliminate the pollutants in environment and the toxin in human body. In recent years, the micro/nanomotors can also be used as microscale sensor to measure micro-fluid viscosity, search and repair the microcracks. In addition, the micro/nanomotors can be applied for drug carrier to pick up and deliver the drug particles to targeted locations and can effectively control-release drug molecules to avoid drug accumulation. Nowadays, the micro/nanomotors also can be applied to control and assemble the microparticles and this technique is expected to be applied for biological single-cell analysis system. Although the micro/nanomotors can be applied in different fields, the real application of micro/ nanomotors is still far away to reach, especially to be used in human body. In conclusion, various propulsion mechanisms have been demonstrated in this review. Furthermore, typical applications in various fields are also presented. The prospects, challenges and future trends of the investigation on micro-and nanomotors are discussed. It is helpful for researches who are working in micro/nano motors research area as well as in engineering applications.