Microswimmer

Other common terms may be more descriptive, including information about the object shape, e.g., microtube or microhelix, its components, e.g., biohybrid, spermbot,[2] bacteriabot,[3] or micro-bio-robot,[4] or behavior, e.g., microrocket, microbullet, microtool or microroller.

[15] This paper continues to inspire ongoing scientific discussion; for example, recent work by the Fischer group from the Max Planck Institute for Intelligent Systems experimentally confirmed that the scallop principle is only valid for Newtonian fluids.

For example, one of the first bioinspired microswimmers consisted of human red blood cells modified with a flagellum-like artificial component made of filaments of magnetic particles bonded via biotin–streptavidin interactions.

[36] More recently, biomimetic swimming inspired by worm-like travelling wave features,[37] shrimp locomotion,[38] and bacterial run-and-tumble motion,[39] was demonstrated by using shaped light.

Motile systems have developed in the natural world over time and length scales spanning several orders of magnitude, and have evolved anatomically and physiologically to attain optimal strategies for self-propulsion and overcome the implications of high viscosity forces and Brownian motion, as shown in the diagram on the right.

For instance, lipid vesicles integrated with enzymes such as transmembrane adenosine 5’-triphosphatase, membrane-bound acid phosphatase, or urease exhibit enhanced mobility correlating with the enzymatic turnover rate.

[96] One of the current engineering challenges is to create miniaturized functional vehicles that can carry out complex tasks at a small scale that would be otherwise impractical, inefficient, or outright impossible by conventional means.

Rather than being electronic devices on a chip, micromotors are able to move freely through a liquid medium while being steered or directed externally or by intrinsic design, which can be achieved by various mechanisms, most importantly catalytic reactions,[98][99][100][101] magnetic fields,[102][103] or ultrasonic waves.

[111] With biomimetic approaches, researchers were able to imitate the flagella-based motion strategy of sperm and Escherichia coli bacteria by reproducing their respective flagellum shape and actuating it with magnetic fields.

Hydrophobic surfaces produce a large contact angle with the liquid which has the effect of exerting less frictional drag torque on the microswimmer body resulting in a lower step-out frequency required for movement.

[131][132] Using the concept of flexible oar, Dreyfus et al reported a micro swimmer that exploit elastic property of a slender filament made up of paramagnetic beads.

Under this test, the propulsion method that performed the fastest was the microswimmer with the tubular body and flexible planar tail due to taking advantage of the helical and corkscrew motion generated at an angle of 30-degree misalignment from the external magnetic field.

Taking inspiration from Whitesides, who used the decomposition of hydrogen peroxide (H2O2) to propel cm/mm-scale objects on a water surface,[135] Sen et al. (2004) fabricated catalytic motors in the micrometer range.

[99] Further analysis of the Pt/Au rods showed that they were capable of performing chemotaxis towards higher hydrogen peroxide concentrations,[100] transport cargo,[101] and exhibited steerable motion in an external magnetic field when inner Ni segments were added.

[101] Interest has been shown in using high-frequency sound waves for microswimmer navigation due to being cleared as safe for clinical studies by the U.S. Food and Drug Administration which would allow them to be used in biomedical applications.

Bioinspired by microvelia beetles which are capable of gliding on the water at high speeds, microswimmers are proposed to take advantage of the Marangoni effect which is the mass transfer across a gradient of the surface tension for a fluid.

Choi et al. demonstrated that photopatterned microswimmers without any mechanical actuation system or external force are capable of traversing a fluid through a polyvinyl alcohol (PVA) fuel source which causes the surface tension of the water once it is dissolved.

[144] Focusing on adaptation, existing approaches at the colloidal scale mostly rely on external feedback, either to regulate motility via the spatiotemporal modulation of the propulsion velocity and direction [145][140][146][147] or to induce shape changes via the same magnetic or electric fields,[148][149][150] which are also driving the particles.

On the contrary, endowing artificial microswimmers with an internal feedback mechanism, which regulates motility in response to stimuli that are decoupled from the source of propulsion, remains an elusive task.

For example, through simple shape design, platinum micromotors can execute a variety of motion trajectories, from highly linear to orbital, when exposed to hydrogen peroxide.

[151] Efficient switching between different propulsion states can, for instance, be reached by the spontaneous aggregation of symmetry-breaking active clusters of varying geometry,[152][153][154][155] albeit this process does not have the desired deterministic control.

Shape-shifting colloidal clusters reconfiguring along a predefined pathway in response to local stimuli [164] would combine both characteristics, with high potential toward the vision of realising adaptive artificial microswimmers.

[167][168] Initially microorganisms were used as the motor units for artificial devices, but in recent years this role has been extended and modified toward other functionalities that take advantage of the biological capabilities of these organisms considering their means of interacting with other cells and living matter, specifically for applications inside the human body like drug delivery or fertilisation.

In that sense, self-sufficient microorganisms naturally function very similar to what we envision for artificially created microrobots: They harvest chemical energy from their surroundings to power molecular motor proteins that serve as actuators, they employ ion channels and microtubular networks to act as intracellular wiring, they rely on RNA or DNA as memory for control algorithms, and they feature an array of various membrane proteins to sense and evaluate their surroundings.

In principle, these abilities also qualify them as biological microrobots for novel operations like theranostics, the combination of diagnosis and therapy, if we are able to impose such functions artificially, for example, by functionalisation with therapeutics.

[174][175] The quest on how to navigate or steer to optimally reach a target is important, e.g., for airplanes to save fuel while facing complex wind patterns on their way to a remote destination, or for the coordination of the motion of the parts of a space-agent to safely land on the moon.

[1] Only a few decades later, microswimmers aiming to become true microscale surgeons evolved from an intriguing science-fiction concept to a reality explored in many research laboratories around the world, as already highlighted by Metin Sitti in 2009.

[198][1] These active agents that can self-propel in a low Reynolds number environment might play a key role in the future of nanomedicine, as popularised in 2016 by Yuval Noah Harari in Homo Deus: A Brief History of Tomorrow.

[208] A recent study by Mirkhani N, et al. demonstrated drug delivery using rotating magnetic fields (RMF) controlled magnetostatic bacteria (MTB) on a mouse tumor model.

Experimental validation using the mouse tumor model has confirmed the efficacy of the RMF control in enhancing the translational velocity and the penetration of MTB into deep tissues.