Principles And Biomedical Applications Of Light-Powered Micro/Nanomotors

In recent years, micro/nanomotors (MNMs) have become an increasingly important research field of nanotechnology. Due to the influence of viscous resistance and Brownian motion, inertia cannot maintain the motion of MNMs in the environment of low Reynolds number, so a constant force is needed to realize the continuous motion of the motors. Classification of MNMs can be made according to the different sources of energy supply. Common sources of energy are chemical fuels and external physical field. Chemically-powered MNMs are propelled by chemical reactions of chemical fuels in solution, which results in an asymmetric concentration gradient field or gas production. The main driving mechanisms of chemically-powered MNMs are self-electrophoresis, self-diffusiophoresis and bubble propulsion. The MNMs driven by physical field, however, convert the light field, magnetic field, electric field, ultrasonic field, temperature field or/and other external energy forms into mechanical energy to serve as the propulsion force.Chemically-powered MNMs have relatively strong driving force and are easy to be prepared. H2O2 is currently the most widely used kind of chemical fuel, but the application of them in biomedical field is limited due to the toxicity and side effects of H2O, in vivo. While there are other biocompatible chemically-powered MNMs, but they are still in need of improvement in terms of the catalytic efficiency and movement velocity when put into practical application. The fuel-free MNMs powered by external physical field have the advantages of remote control and high biocompatibility. For example, MNMs driven by magnetic field or ultrasonic field have the advantages of high energy penetration, strong propulsion force and relatively small requirements for surrounding media, but their manufacturing process are complicate, requiring expensive equipment and elaborately set equipment parameters.Among them, light-powered micro/nanomotors are attracting more and more attention due to their unique advantages of high controllability, good programmability, easy operability and potential applications in the field of environmental remediation and biomedicine. As an external control signal, light can be precisely tuned and adjusted by changing parameters such as light intensity, light frequency, propagation direction and reversible selection of the on/off mode of light source. Light-powered MNMs can easily realize remote wireless control with high spatial and temporal resolution.In this review, we aim to comprehensively summarize the current research status of light-powered MNMs, introducing their design principles, preparation process and motion mechanism. Particularly, we summarize the motion patterns of light-powered MNMs, and separately introduce the motion behavior modes of the individual motor (directional motion, rotation) and collective motors (aggregation, disaggregation and directional motion). Over the past decade, different components of light-powered MNMs have been prepared successfully, and different shapes of light-powered MNMs have been springing up continuously, which are able to carry out more complex and multifunctional tasks. Numerous progresses have also been achieved in the scope of directional motion phototaxis and collective behavior of light-powered MNMs. In order to provide a clearer insight into the whole concept of light-powered MNMs, representative examples of new achievements are enumerated in this review. In addition, as one of the most attractive but also the most challenging goal, biomedical applications of light-powered MNMs are summed up in details, complementing the latest advances on the basis of previous research results. Finally, we discuss the pivotal problems and challenges of light-powered MNMs confronted right now. We hope that this review can provide guidance to the development of light-powered MNMs and promote further improvement of the field in the near future.