Ultrasonically Propelled Micro- and Nanorobots

Ultrasound at sufficiently low amplitudes, specifically in the MHz frequency range, does little harm to the biological samples (such as cells and tissues) and provides an advantageous and well-controlled means to efficiently power microswimmers. In this review, a state-of-the-art overview of ultrasonically propelled micro- and nanorobots from the perspective of chemistry, physics, and materials science is given. First, the well-established theory of ultrasound propulsion for micro/nanorobots is introduced. Second, the setup designs for ultrasound propulsion of micro/nanorobots are classified. Following this, the presentative fabrication methods of ultrasonic micro/nanorobots are summarized in detail. After this, the mechanisms of ultrasound propulsion for micro/nanorobots are explored and discussed. The hybrid motion of magnetic-, light-, and catalytic-driven micro/nanorobots with ultrasonic actuation is then summarized and discussed. Subsequently, this review highlights and discusses representative potential applications of ultrasound-powered functional micro/nanorobots in biomedical, environmental, and other relevant fields. Lastly, this review presents a future outlook on the ultrasound-driven micro/nanorobots.