NANOROBOTICS AND MICROROBOTICS (A FERREIRA, SECTION EDITOR) Engineering Ultrasound Fields to Power Medical Micro/Nanorobots Mariana Leal-Estrada 1 & Miguel Valdez-Garduño 1 & Fernando Soto 2 & Victor Garcia-Gradilla 1 Accepted: 21 December 2020 # The Author(s), under exclusive licence to Springer Nature Switzerland AG part of Springer Nature 2021 Abstract Purpose of Review Increasing demand for precision medicine devices has led to the development of self-propelled nano/microdevices capable of operating at small scale while performing biomedical tasks. This work intends to highlight the most recent advances on ultrasound propelled micro/nanorobots and provide an insight into the challenges and their future applications. Recent Findings Diverse types of acoustic fields have been used to power microrobots. Focused ultrasound (FU) is capable of concentrating acoustic energy in a single zone, optimal for applications where high penetration or localized actuation is required. In addition, standing wave ultrasound (SWU) robots work on nodal planes, have demonstrated multiple advanced biomedical tasks, suitable for lab-on-a-chip applications. Finally, traveling wave ultrasound (TWU) employs tunable resonant elements responding to specific frequencies, holding great promise for in-vivo applications. Summary Methods to propel microrobots with acoustic field are presented: FU, SWU, and TWU. Including their setup, design, propulsion mechanism, and applications. Keywords Ultrasound . Focused . Standing wave . Traveling wave . Micro/nanorobot . Nanomotor Introduction In recent years, advances in micro/nanorobotic systems have taken relevance given the need for devices with actuation ca- pabilities at small scales [1, 2]. This is especially important when it comes to engineering micro robotic designs that can be employed on biomedical applications, where they present a unique ability to access hard to reach regions of the body and perform useful tasks. For instance, microrobots have been applied in a wide range of practical applications, including targeted drug delivery [3–5], microsurgery [6, 7], detoxifica- tion [8], and diagnosis [9–12]. We define micro/nanorobots (MNRob) as devices, ranging from nano to microscale, that can convert the energy from different sources into actuation and locomotion toward performing a useful medical task [13]. Medical microrobots can perform diverse roles, including transporting therapeutic payloads [14–16], detecting and iso- lating biotargets [14, 17, 18], accelerating chemical reaction through enhanced fluid mixing [19–22], and applying me- chanical force toward tissue penetration [6, 23, 24]. Each specific task requires a motor design with a different shape, energy conversion, and material composition, enabling it to perform its entrusted task in optimal conditions while remaining biocompatible. The mechanism employed by the MNRob to harvest energy and transform it is one of the most important conditions that determine its design; therefore, a wide variety of approaches have been proposed. These tiny engines can convert local fuels or external power sources [25] to enable efficient small-scale locomotion [26, 27]. Despite the great progress in the use of micro robots in biomedical applications, there are many challenges to over- come. For instance, MNRob with chemical engines have shown great potential in drug delivery applications due to their This article belongs to the Topical Collection on Nanorobotics and Microrobotics * Victor Garcia-Gradilla vgg@cnyn.unam.mx Mariana Leal-Estrada g6_leal17@ens.cnyn.unam.mx Miguel Valdez-Garduño g6_vald17@ens.cnyn.unam.mx Fernando Soto fernandosotoalvarez@gmail.com 1 Center for Nanoscience and Nanotechnology, UNAM, Km 107 Carretera Tijuana- Ensenada, 22800 Ensenada, BC, Mexico 2 School of Medicine, Stanford University, Palo Alto, CA 94304-5427, USA https://doi.org/10.1007/s43154-020-00033-2 / Published online: 7 January 2021 Current Robotics Reports (2021) 2:21–32