Alexander A. Solovev

Catalytic Microtubular Jet Engines Self‐Propelled by Accumulated Gas Bubbles

Strain-engineered microtubes with an inner catalytic surface serve as self-propelled microjet engines with speeds of up to approximately 2 mm s(-1) (approximately 50 body lengths per second). The motion of the microjets is caused by gas bubbles ejecting from one opening of the tube, and the velocity can be well approximated by the product of the bubble radius and the bubble ejection frequency. Trajectories of various different geometries are well visualized by long microbubble tails. If a magnetic layer is integrated into the wall of the microjet engine, we can control and localize the trajectories by applying external rotating magnetic fields. Fluid (i.e., fuel) pumping through the microtubes is revealed and directly clarifies the working principle of the catalytic microjet engines.

Self-Propelled Nanotools

We describe nanoscale tools in the form of autonomous and remotely guided catalytically self-propelled InGaAs/GaAs/(Cr)Pt tubes. These rolled-up tubes with diameters in the range of 280-600 nm move in hydrogen peroxide solutions with speeds as high as 180 μm s(-1). The effective transfer of chemical energy to translational motion has allowed these tubes to perform useful tasks such as transport of cargo. Furthermore, we observed that, while cylindrically rolled-up tubes move in a straight line, asymmetrically rolled-up tubes move in a corkscrew-like trajectory, allowing these tubes to drill and embed themselves into biomaterials. Our observations suggest that shape and asymmetry can be utilized to direct the motion of catalytic nanotubes and enable mechanized functions at the nanoscale.

Dynamics of Biocatalytic Microengines Mediated by Variable Friction Control

We describe the motion of self-propelled hybrid microengines containing catalase enzyme covalently bound to the cavity of rolled-up microtubes. The high efficiency of these hybrid microengines allows them to move at a very low concentration of peroxide fuel. The dynamics of the catalytic engines is mediated by the generation of front-side bubbles, which increase the drag force and make them turn. The specific modification of the inner layer of microtubes with biomolecules can lead to other configurations to generate motion from different chemical fuels.

Microbots Swimming in the Flowing Streams of Microfluidic Channels

We describe the motion of self-propelled catalytic Ti/Fe/Pt rolled-up microtubes (microbots) in the microchannels of a microfluidics system. Their motion is precisely controlled by a small magnetic field, and the transport of multiple spherical microparticles into desired locations is achieved. The microbots are powerful enough to propel themselves against flowing streams. The integration of “smart and powerful” microbots into microchip systems can lead to multiple lab-on-a-chip functions such as separation of cells and biosensing.

Light-Controlled Propulsion of Catalytic Microengines†

Control over the autonomous motion of artificial nano/ micromachines is essential for real biomedical and nanotechnological applications. Consequently, a complete nanomachine should be able to be turned on and off at will. Developments over the last few years on synthetic catalytic nano/microengines and motors have enabled the harvesting of chemical energy from local molecules and transforming it into an effective autonomous motion. Several impressive applications have recently reported the use of artificial micromachines for the detection of biomolecules with roving nanomotors, transport of animal cells in a fluid, and other microcargo delivery. Recently, the use of a light source has been implemented to propel microparticle-based motors generated by a selfdiffusiophoretic mechanism. Despite this interesting approach, the motion of the particles is limited by the dissolution of the materials and to the ultraviolet (UV) spectrum. Moreover, a reversible method to start and stop the propulsion of micromotors by a visible-light source remains a challenge. Here we report the tuning of the propulsion power of Ti/ Cr/Pt catalytic microengines (m-engines) through illumination of a solution by a white-light source. We show that light suppresses the generation of microbubbles, stopping the engines if they are fixed-to or self-propelled above a platinum-patterned surface. The m-engines are reactivated by dimming the light source that illuminates the fuel solution. The illumination of the solution with visible light in the presence of Pt diminishes the concentration of hydrogen peroxide fuel and degrades the surfactant, consequently reducing the motility of the microjets. Electrochemical measurements and analysis of the surface tension support our findings. We also study the influence of different wavelengths over the visible spectrum (500–750 nm) on the formation of microbubbles. Rolled-up Ti/Cr/Pt catalytic m-engines with diameters of 5–10 mm and a length of 50 mm were prepared as described previously elsewhere and in the Experimental Section. Microengines were immersed into solutions of aqueous H2O2 (2.5% v/v) as fuel and benzalkonium chloride (ADBAC) (0.5% v/v), as the surfactant, to determine the influence of white light on the mobility of the m-engines. At lower concentrations of both chemicals, the generation of microbubbles is significantly reduced. Thus, the motility of the catalytic m-engines is controlled by a small change in the fuel (H2O2 and/or surfactant) concentration. These conditions allow us to investigate a concentration range close to the metastable state, that is, where the probability of stopping the m-engines is high. Figure 1A

The smallest man-made jet engine

The design of catalytic engines powered by chemical fuels is an exciting and emerging field in multidisciplinary scientific communities. Recent progress in nanotechnology has enabled scientists to shrink the size of macroengines down to microscopic, but yet powerful, engines. Since a couple of years ago, we have reported our progress towards the control and application of catalytic microtubular engines powered by the breakdown of hydrogen peroxide fuel which produces a thrust of oxygen bubbles. Efforts were undertaken in our group to prove whether the fabrication of nanoscale jets is possible. Indeed, the smallest jet engine (600 nm in diameter and 1 picogram of weight) was synthesized based on heteroepitaxially grown layers. These nanojets are able to self-propel in hydrogen peroxide solutions and are promising for the realisation of multiple tasks.

Catalytic microstrider at the air-liquid interface.

Rolled-up microtubes work as catalytic striders at the air–liquid interface of hydrogen peroxide solution (sketched in the left image). Such micromachines, buoyed by oxygen bubbles, self-propel at the fuel surface by the bubble recoiling mechanism and dynamically self-assemble into patterns (right image) due to the meniscus-climbing effect.

Light Confinement by a Cylindric Metallic Waveguide in Dense Buffer Gas Environment

We report on the implementation of metallic microtubes in a system of rubidium vapor at 230 bar of argon buffer gas. The high-buffer gas pressure leads to a widely pressure broadened linewidth of several nanometers, interpolating between the sharp atomic physics spectra and the band structure of solid state systems. Tube-like metallic waveguide structures have been inserted in the high-pressure buffer gas system, allowing for guiding light in an optical dense gas over a length in the tube of up to 1 mm. The system holds promise for nonlinear optics experiments and the study of atom-light polariton condensation.

Geometry Design, Principles and Assembly of Micromotors

Discovery of bio-inspired, self-propelled and externally-powered nano-/micro-motors, rotors and engines (micromachines) is considered a potentially revolutionary paradigm in nanoscience. Nature knows how to combine different elements together in a fluidic state for intelligent design of nano-/micro-machines, which operate by pumping, stirring, and diffusion of their internal components. Taking inspirations from nature, scientists endeavor to develop the best materials, geometries, and conditions for self-propelled motion, and to better understand their mechanisms of motion and interactions. Today, microfluidic technology offers considerable advantages for the next generation of biomimetic particles, droplets and capsules. This review summarizes recent achievements in the field of nano-/micromotors, and methods of their external control and collective behaviors, which may stimulate new ideas for a broad range of applications.

Dynamic curvature control of rolled-up metal nanomembranes activated by magnesium

Dynamic curvature control of rolled-up metal nanomembranes using an active magnesium layer design is implemented in bio-oriented conditions to realize shape transformation of expansion, shrinking, un-rolling and re-rolling. The tube integrated with a catalytic Pt layer is proposed for a new type of smart drug delivery microsystem.