Using a dynamic fabrication process, hybrid, photoactivated microswimmers made from two different semiconductors, titanium dioxide (TiO2) and cuprous oxide (Cu2O) are developed, where each material ...occupies a distinct portion of the multiconstituent particles. Structured light‐activated microswimmers made from only TiO2 or Cu2O are observed to be driven in hydrogen peroxide and water most vigorously under UV or blue light, respectively, whereas hybrid structures made from both of these materials exhibit wavelength‐dependent modes of motion due to the disparate responses of each photocatalyst. It is also found that the hybrid particles are activated in water alone, a behavior which is not observed in those made from a single semiconductor, and thus, the system may open up a new class of fuel‐free photoactive colloids that take advantage of semiconductor heterojunctions. The TiO2/Cu2O hybrid microswimmer presented here is but an example of a broader method for inducing different modes of motion in a single light‐activated particle, which is not limited to the specific geometries and materials presented in this study.
All‐semiconductor, light‐activated microswimmers move with different modes of motion depending upon the light's wavelength. These “hybrid microswimmers” propel without toxic chemicals or precious metals, and may open a new class of fuel‐free active colloids. This system is expected to establish unique research directions including wavelength‐dependent self‐assembly, the performance of complex tasks at the microscale, and fuel‐free actuation via semiconductor heterojunctions.
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
Natural CO2 conversion can be triggered with high spatial control by shining light on precision‐tailored pristine leaves, as reported by Alexander Kuhn et al. in their Research Article (e202205298). ...The perfectly site‐selective evolution of oxygen under irradiation, symbolized by the bubbles released at the extremity of a leaf positioned at the air/water interface of a mountain lake, is used to generate significant motion of such all‐natural macro‐ and microswimmers.
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Biodegradable microswimmers offer great potential for minimally invasive targeted therapy due to their tiny scale, multifunctionality, and versatility. However, most of the reported systems focused ...on the proof‐of‐concept on the in vitro level. Here, the successful fabrication of facile hydrogen‐powered microswimmers (HPMs) for precise and active therapy of acute ischemic stroke is demonstrated. The hydrogen (H2) generated locally from the designed magnesium (Mg) microswimmer functions not only as a propellant for motion, but also as an active ingredient for reactive oxygen species (ROS) and inflammation scavenging. Due to the continuous detachment of the produced H2, the motion of the microswimmers results in active H2 delivery that allows for enhanced extracellular and intracellular reducibility. With the help of a stereotaxic apparatus device, HPMs were injected precisely into the lateral ventricle of middle cerebral artery occlusion (MCAO) rats. By scavenging ROS and inflammation via active H2, MCAO rats exhibit significant decrease in infarct volume, improved spatial learning and memory capability with minimal adverse effects, demonstrating efficient efficacy on anti‐ischemic stroke. The as‐developed HPMs with excellent biocompatibility and ROS scavenging capability holds great promise for the treatment of acute ischemic stroke or other oxidative stress induced diseases in clinic in the near future.
Facile hydrogen‐powered microswimmers (HPMs) for precise and active hydrogen therapy of acute ischemic stroke are demonstrated for the first time. The hydrogen generated locally from HPMs by consuming water functions not only as a propellant for the motion, but also as an active ingredient for reactive oxygen species and inflammation scavenging, which holds great efficacy for the treatment of acute ischemic stroke.
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Microscopic active droplets are able to swim autonomously in viscous flows. This puzzling feature stems from solute exchanges with the surrounding fluid via surface reactions or their spontaneous ...solubilization and from the interfacial flows resulting from these solutes' gradients. Contrary to asymmetric active colloids, these isotropic droplets swim spontaneously by exploiting the nonlinear coupling of solute transport with self-generated Marangoni flows; such coupling is also responsible for secondary transitions to more complex individual and collective dynamics. Thanks to their simple design and their sensitivity to physico-chemical signals, these droplets are fascinating to physicists, chemists, biologists, and fluid dynamicists alike in analyzing viscous self-propulsion and collective dynamics in active-matter systems, developing synthetic cellular models, or performing targeted biomedical or engineering applications. I review here the most recent and significant developments of this rapidly growing field, focusing on the mathematical and physical modeling of these intriguing droplets, together with their experimental design and characterization.
The Front Cover illustrates how magnesium microswimmers – represented by fish – locate sites of iron corrosion, for example the anchor in sea water in the picture. The microswimmers are guided by the ...pH gradient to the corrosion site, where they form a local protection layer. More information can be found in the Article by Alexander Kuhn and co‐workers.
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Locomotion and transport of microorganisms in fluids is an essential aspect of life. Search for food, orientation toward light, spreading of off-spring, and the formation of colonies are only ...possible due to locomotion. Swimming at the microscale occurs at low Reynolds numbers, where fluid friction and viscosity dominates over inertia. Here, evolution achieved propulsion mechanisms, which overcome and even exploit drag. Prominent propulsion mechanisms are rotating helical flagella, exploited by many bacteria, and snake-like or whip-like motion of eukaryotic flagella, utilized by sperm and algae. For artificial microswimmers, alternative concepts to convert chemical energy or heat into directed motion can be employed, which are potentially more efficient. The dynamics of microswimmers comprises many facets, which are all required to achieve locomotion. In this article, we review the physics of locomotion of biological and synthetic microswimmers, and the collective behavior of their assemblies. Starting from individual microswimmers, we describe the various propulsion mechanism of biological and synthetic systems and address the hydrodynamic aspects of swimming. This comprises synchronization and the concerted beating of flagella and cilia. In addition, the swimming behavior next to surfaces is examined. Finally, collective and cooperate phenomena of various types of isotropic and anisotropic swimmers with and without hydrodynamic interactions are discussed.
Interactions in active colloids Liebchen, Benno; Mukhopadhyay, Aritra K
Journal of physics. Condensed matter,
02/2022, Volume:
34, Issue:
8
Journal Article
Peer reviewed
Open access
The past two decades have seen a remarkable progress in the development of synthetic colloidal agents which are capable of creating directed motion in an unbiased environment at the microscale. These ...self-propelling particles are often praised for their enormous potential to self-organize into dynamic nonequilibrium structures such as living clusters, synchronized super-rotor structures or self-propelling molecules featuring a complexity which is rarely found outside of the living world. However, the precise mechanisms underlying the formation and dynamics of many of these structures are still barely understood, which is likely to hinge on the gaps in our understanding of how active colloids interact. In particular, besides showing comparatively short-ranged interactions which are well known from passive colloids (Van der Waals, electrostatic etc), active colloids show novel hydrodynamic interactions as well as phoretic and substrate-mediated 'osmotic' cross-interactions which hinge on the action of the phoretic field gradients which are induced by the colloids on other colloids in the system. The present article discusses the complexity and the intriguing properties of these interactions which in general are long-ranged, non-instantaneous, non-pairwise and non-reciprocal and which may serve as key ingredients for the design of future nonequilibrium colloidal materials. Besides providing a brief overview on the state of the art of our understanding of these interactions a key aim of this review is to emphasize open key questions and corresponding open challenges.
Cell delivery using magnetic microswimmers is a promising tool for targeted therapy. However, it remains challenging to rapidly and uniformly manufacture cell-loaded microswimmers that can be ...assembled into cell-supporting structures at diseased sites. Here, rapid and uniform manufacturable 2D magnetic achiral microswimmers with pores were fabricated to deliver bone marrow mesenchymal stem cells (BMSCs) to regenerate articular-damaged cartilage. Under actuation with magnetic fields, the BMSC-loaded microswimmers take advantage of the achiral structure to exhibit rolling or swimming motions to travel on smooth and rough surfaces, up inclined planes, or in the bulk fluid. Cell viability, proliferation, and differentiation tests performed days after cell seeding verified the microswimmers’ biocompatibility. Long-distance targeting and in situ assemblies into 3D cell-supporting structures with BMSC-loaded microswimmers were demonstrated using a knee model and U-shaped wells. Overall, combining the advantages of preparing an achiral 2D structured microswimmer with magnetically driven motility results in a platform for cell transport and constructing 3D cell cultures that can improve cell delivery at lesion sites for biomedical applications.
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9.
Chemotactic self-caging in active emulsions Hokmabad, Babak Vajdi; Agudo-Canalejo, Jaime; Saha, Suropriya ...
Proceedings of the National Academy of Sciences - PNAS,
06/2022, Volume:
119, Issue:
24
Journal Article
Peer reviewed
Open access
A common feature of biological self-organization is how active agents communicate with each other or their environment via chemical signaling. Such communications, mediated by self-generated chemical ...gradients, have consequences for both individual motility strategies and collective migration patterns. Here, in a purely physicochemical system, we use self-propelling droplets as a model for chemically active particles that modify their environment by leaving chemical footprints, which act as chemorepulsive signals to other droplets. We analyze this communication mechanism quantitatively both on the scale of individual agent-trail collisions as well as on the collective scale where droplets actively remodel their environment while adapting their dynamics to that evolving chemical landscape. We show in experiment and simulation how these interactions cause a transient dynamical arrest in active emulsions where swimmers are caged between each other's trails of secreted chemicals. Our findings provide insight into the collective dynamics of chemically active particles and yield principles for predicting how negative autochemotaxis shapes their navigation strategy.
We study, by means of an exact analytical solution, the motion of a spheroidal, axisymmetric squirmer in an unbounded fluid, as well as the low Reynolds number hydrodynamic flow associated to it. In ...contrast to the case of a spherical squirmer-for which, e.g. the velocity of the squirmer and the magnitude of the stresslet associated with the flow induced by the squirmer are respectively determined by the amplitudes of the first two slip ('squirming') modes-for the spheroidal squirmer each squirming mode either contributes to the velocity, or contributes to the stresslet. The results are straightforwardly extended to the self-phoresis of axisymmetric, spheroidal, chemically active particles in the case when the phoretic slip approximation holds.
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