Light-Driven Micro/Nanomotor for Promising Biomedical Tools: Principle, Challenge, and Prospect

A micro/nanomotor (MNM), as miniaturized machinery, can potentially bridge the application gap between the traditional macroscale motor and the molecular motor to manipulate materials at the cellular scale. The fascinating biomedical potential application for these tiny robots has been long envisioned by science fiction, such as "Fantastic Voyage", where complicated surgery can be performed at single cell precision without any surgical incision. However, to enter the highly conservative biomedical and healthcare industry in practice, the MNM must provide unique advantages over existing technology without introducing additional health risk, which has not been fully materialized. As an emerging approach, light-driven micro/nanomotors (LMNMs) have demonstrated several unique advantages over other MNMs, which will be addressed in this Account. As a control signal, light promises additional degrees of freedom to manipulate MNMs by modulating the light intensity, frequency, polarization, and propagation direction with spatial and temporal precision, which enables excellent controllability and programmability of LMNMs. Additionally, the fruitful knowledge and catalysts from the well-studied photocatalysis can be readily transferred to LMNMs for photoelectrochemical reactions, which provides a rich materials inventory for the development of advanced LMNM systems. A model LMNM in general can be regarded as a miniaturized solar cell combined with electrokinetic propulsion parts, where electric current is provided by the photovoltaic effect and then converted to propulsion thrust through a variety of electrokinetic mechanisms. It can be envisioned that the electric current may be further regulated with the onboard electronic circuit for advanced logic-controlled nanorobots. Finally, because incident photons instead of active chemicals provide the energy for LMNM propulsion, the highly active but toxic chemical fuels can be avoided, which suggested their better biocompatibility. It is essential to emphasize that all of these promises rely on the in-depth understanding of the photoelectrochemical reaction as well as the physics of electrokinetic phenomena, which requires further investigations. As a persistent endeavor, the biomedical application is the most attractive but challenging target for MNMs. Currently, most of the MNMs are demonstrated with in vitro conditions largely deviating from the biological environment, and nontrivial in vivo studies and cytotoxicity experiments are rarely reported. As merits of MNMs, the efficiency, biocompatibility, ion tolerance, and controllability critically determine the future success of MNMs. In this Account, existing and prospective solutions in these aspects are systemically discussed for light-propelled MNMs. We believe that, with a better understanding of the fundamental photoelectrochemical and electrokinetic processes, the development of motor design strategies, and improved fabrication methods, the promised practical biomedical application, such as early disease diagnosis, interventional therapy, targeted therapy, and microsurgery, could be realized in the near future.