Acoustic actuation techniques offer a promising tool for contactless manipulation of both synthetic and biological micro/nano agents that encompass different length scales. The traditional usage of sound waves has steadily progressed from mid-air manipulation of salt grains to sophisticated techniques that employ nanoparticle flow in microfluidic networks. State-of-the-art in microfabrication and instrumentation have further expanded the outreach of these actuation techniques to autonomous propulsion of micro-agents. In this review article, we provide a universal perspective of the known acoustic micromanipulation technologies in terms of their applications and governing physics. Hereby, we survey these technologies and classify them with regards to passive and active manipulation of agents. These manipulation methods account for both intelligent devices adept at dexterous non-contact handling of micro-agents, and acoustically induced mechanisms for self-propulsion of micro-robots. Moreover, owing to the clinical compliance of ultrasound, we provide future considerations of acoustic manipulation techniques to be fruitfully employed in biological applications that range from label-free drug testing to minimally invasive clinical interventions.
Technical design features for improving the way a passive elastic filament produces propulsive thrust can be understood by analyzing the deformation of sperm-templated microrobots with segmented magnetization. Magnetic nanoparticles are electrostatically self-assembled on bovine sperm cells with nonuniform surface charge, producing different categories of sperm-templated microrobots. Depending on the amount and location of the nanoparticles on each cellular segment, magnetoelastic and viscous forces determine the wave pattern of each category during flagellar motion. Passively propagating waves are induced along the length of these microrobots using external rotating magnetic fields and the resultant wave patterns are measured. The response of the microrobots to the external field reveals distinct flow fields, propulsive thrust, and frequency responses during flagellar propulsion. This work allows predictions for optimizing the design and propulsion of flexible magnetic microrobots with segmented magnetization.
bacterium, [7] alga [8] ). Since most of the above-described organisms are aquatic, their anatomical features precipitate biomimetic approaches to design new microrobots that imitate efficient swimming motion of such organisms.Within such aquatic creatures, a unique swimming mechanism is demonstrated by organisms (e.g., octopus, squid, cuttlefish) that belong to the Cephalopoda family. The cephalopods exhibit a jet-propulsion phenomenon whereby they sequentially inflate and deflate bodies to pump fluid which imparts the necessary thrust to move forward. [9][10][11][12][13][14] This sequential inflation and deflation in cephalopods can be attributed to their elastic bodies which function like a mass-spring system. [14][15][16] These naturally occurring mass-spring resonators have been a motivation to design artificial robotic systems that closely imitate the cephalopod-inspired motion. [14] Previously, different fabrication methods (e.g., mold casting, [9] 3-D printing, [10] shape memory alloys, [11] dielectric elastomers, [13] elastic membranes [14] ) have produced microrobotic designs that mimic members of the Cephalopoda family. Although the aforementioned fabrication methods closely imitate the anatomy of cephalopods up to centimeter scale, their implementation at micro-and nano-scale can be challenging owing to fabrication constraints. Such precise replication of anatomical features at micro-to nano-scale requires multi-step fabrication processes. [5,[17][18][19][20] Specifically, the synthesis of micro-scale movable components that enable Aquatic organisms within the Cephalopoda family (e.g., octopuses, squids, cuttlefish) exist that draw the surrounding fluid inside their bodies and expel it in a single jet thrust to swim forward. Like cephalopods, several acoustically powered microsystems share a similar process of fluid expulsion which makes them useful as microfluidic pumps in lab-on-a-chip devices. Herein, an array of acoustically resonant bubbles are employed to mimic this pumping phenomenon inside an untethered microrobot called CeFlowBot. CeFlowBot contains an array of vibrating bubbles that pump fluid through its inner body thereby boosting its propulsion. CeFlowBots are later functionalized with magnetic layers and steered under combined influence of magnetic and acoustic fields. Moreover, acoustic power modulation of CeFlowBots is used to grasp nearby objects and release it in the surrounding workspace. The ability of CeFlowBots to navigate remote environments under magnetoacoustic fields and perform targeted manipulation makes such microrobots useful for clinical applications such as targeted drug delivery. Lastly, an ultrasound imaging system is employed to visualize the motion of CeFlowBots which provides means to deploy such microrobots in hard-to-reach environments inaccessible to optical cameras.The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/smll.202105829.
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