Academic literature on the topic 'Magnetic nanoswimmers'

Flagella can be used to make magnetically-controlled microfluidic and nanoscale devices for biomedical applications in both vitro and vivo environments. They are capable of operating with high precision on the cellular and subcellular level. So far, scientists and engineers have successfully used monolithic inorganic materials or photoactive polymers [1] to mimic the helical bacterial flagella whose rotary-propulsion mechanism effectively overcomes the dominant viscous forces that prevail in a low Reynolds-number environment. Here, we focus on bacterial flagella and their rotary motion. The bacterial flagellum is an ideal biomaterial for constructing self-propelling nanoswimmers because it can reversibly change its geometry in response to different environmental stimuli such as pH, the local concentration of certain organic solvents, and mechanical force on the flagella. The bacterial flagellum is very easy to manipulate because it is composed of flagellin which can be mechanically isolated through vortexing and centrifugation, which enables flagella to be used as nanoscale sensors and mechanical transducers. Our project focuses on fabricating a bacterial flagella forest which consists of an ordered array of flagella on a glass substrate. Flagella are attached to magnetic nanobeads via biotin-avidin bonding for actuation by oscillating magnetic field.

: Nanoshuttles are unique structures which resemble double-headed arrows or a nanorod with sharp tips for better penetration into the tumor cells, reduction of toxicity and minimization of off-targeting effect. These biologically-inspired multimetallic or bimetallic nanoswimmers are capable of transporting cargoes from one end to another via self-propulsion in efficient manner. Encapsulation with pH- and heat-sensitive polymers allows nanoshuttles to release cargos at the targeted site in a controlled fashion. This review article focuses on the methods of preparation and characterization of nanoshuttles with applications in the field of antineoplastic, antibacterial, erectile dysfunction, electrochemical biosensing, anticounterfeiting, on-demand and targeted delivery system for imaging as well as cell ablation therapy. Magnetic nanoshuttles exhibit modified optical properties for utilization in diagnostic imaging for sensitive and early diagnosis of diseases. Smart drug delivery is achieved when nanoshuttles are combined with nanomotors to exhibit distinctive, rapid and unidirectional movement in the bloodstream. Cost-effective synthesis of nanoshuttles will extend their applications in the commercial sectors by overcoming the limitations like scale-up and regulatory approval. In the near future, nanoshuttles will diversify in the fields of energy conversion, energy storage, 3D printing, stem cell fabrication and theranostics.

From flocking birds to migrating cells, ‘active matter’ is ubiquitous in the natural world. Almost all known life forms are based on self-propelled entities working collectively to create large-scale structures, networks, and movements. Artificially designed self-propelled objects can allow the study of active matter phenomena with a level of control that is not possible in natural, biological systems. With this motivation, we develop micro/nanoscale swimmers whose swimming mechanism is inspired by microscopic, flagellated bacteria. Among different ways of powering swimmers, the magnetic field deserves special mention due to its inherent biocompatibility, minimal dependence on the properties of the surrounding medium, and remote powering mechanism. Along with providing an insight into the non-equilibrium phenomena of active matter, the helical swimmers can also impact future biomedical practices with intelligent, multifunctional entities swarming toward a diseased site and delivering therapeutics with high accuracy. When an oscillating magnetic field is applied to helical structures, motility is induced in the form of back-and-forth motion, but the directionality is unspecified and thereby represents a zero force, zero torque, active colloid system. These are called reciprocal swimmers, and their degree of randomness in the reciprocal sequence plays an important role in determining their effective motility. We show the results at high activity levels where the degree of randomness is further affected by the presence of the surface, which in turn results in a non-monotonic increase of motility as a function of the magnetic drive. The magnetic swimmers show enhanced diffusivity compared to their passive counterparts, and their motility can be tuned externally. However, to achieve a self-propelled velocity, we use the ratchet principle to break reciprocal symmetry in time. The thermal ratchets can extract useful work from random fluctuations and are common on the molecular scale, such as motor protein. We use the ratchet principle to induce net motility in an externally powered magnetic colloid, which otherwise shows reciprocal (back and forth) motion. The swimmers show net motility with enhanced diffusivity, in agreement with numerical calculations. We further discuss the preliminary experimental results and modelling pertaining to collective dynamics of the helical magnetic nanoswimmers. Additionally, we have studied non-magnetic tracer beads suspended in a medium containing many swimmers and found the diffusivity of the beads to increase under magnetic actuation, akin to measurements performed in dense bacterial suspensions. Crucial aspects of studying the active swimmers pertain to their behaviour under different physical conditions. We demonstrate controlled manipulation of magnetic helices within two types of optical confinement: an optical bowl and a flat potential, both formed by manipulating an optical tweezer. The interaction of helical swimmers with optical confinement is modelled and further confirmed by experiments. Combining optical and magnetic forces in a single nanostructure can allow multiple investigations pertaining to colloidal physics, including micro-rheology, hydrodynamics and confinement effects. In summary, we envision that developing helical magnetic swimmers will provide a new model system to investigate fundamental non-equilibrium phenomena and play a vital role in developing intelligent theragnostic probes for biomedical applications.

There has been a surge of interest in recent years to design and fabricate various types of motile micro/nanoparticles that can be maneuvered using chemical, optical, thermal, electrical or magnetic energy sources. A collection of such motile particles can be used as a model system to study various active matter phenomena, which can answer fundamental questions related to non-equilibrium statistical physics. This motile micro/Nano system can also be important in various biomedical applications like targeted drug delivery, microsurgery, biochemical sensing and disease diagnosis. Among several actuation schemes, magnetic actuation deserves a special mention owing to its non-invasive and non-chemical mode. In this thesis, micron sized helical structures have been fabricated, which are rendered magnetic by a thin coating of magnetic materials and actuated by rotating _led. Using this system, a study on generalized dynamics of elongated structures has been done, which is also supported by analytical theory and numerical calculations. Both experiments and numerical simulations show the existence of multiple cut-off frequencies related to the stability of different dynamical modes of an elongated structure, whose analytical expressions have been derived in the present work. Despite the observation of rich dynamical configurations, the field dependent directionality of this mode of actuation by rotating field fails to qualify the experimental system as active matter, thus hinders the possibility of using it as a model system to study a wide variety of phenomena like pattern formation in swarm motion, synchronization, etc. In this thesis, a different actuation technique has been demonstrated, which decouples the directionality from the applied external field, thus enabling the helical micro swimmers to propel in any direction. The system presented here is the rest experimental demonstration of magnetically actuated active matter system, where the helical structures show back and forth motion in a random direction, thus resembling a reciprocal swimmer and shows enhanced diffusivity in accordance to earlier theoretical predictions. We further extended this idea to report how the same actuating field used for reciprocal motion can be tuned to break the temporal symmetry to design a non-reciprocal swimmer. The actuation principle is based on the idea of `Brownian Motor', where the reciprocity is broken using asymmetric _led pattern and incorporating thermal actuations into the system. A detailed numerical study of the dynamics of the system is reported here which sets the criteria to build a system with optimal performance where tuning from reciprocal to non-reciprocal actuation can be achieved in a simple manner. In a related project, we report an actuation scheme to manoeuvre geometrically identical nanostructures in deferent directions, and subsequently position them at arbitrary locations with respect to each other. In comparison to the other techniques where controlling the directionality and actuation are powered by separate energy sources, the experiment reported here shows how these two factors can be controlled only by magnetic _ends. The technique shown here requires proximity of the nanostructures to a solid surface and is applicable for independent positioning of any number of micro/nanobots; thus, can be useful in applications that require remote and independent control over individual components in microfluidic environments. Finally, we report couple of experimental techniques to study the hydrodynamic interactions between helical swimmers. In one of the techniques, we investigate possibility of synchronization between two rotating helical swimmers at low Reynolds number conditions. In the other technique, we discuss different ways to study motion of a collection of magnetically actuated helical swimmers. The methods presented here show different fabrication schemes that are useful to avoid magnetic agglomeration. The preliminary experiments reported here can be useful to study the behaviour of a collection of magnetic swimmers coupled via hydrodynamic interactions.

Micron scale robots going inside our body and curing various ailments is a technolog¬ical dream that easily captures our imagination. However, with the advent of novel nanofabrication and nanocharacterization tools there has been a surge in the research in this field over the last decade. In order to achieve locomotion (swim) at these small length scales, special strategies need to be adopted, that is able to overcome the large viscous damping that these microbots have to face while moving in the various bod¬ily fluids. Thus researchers have looked into the swimming strategies found in nature like that of bacteria like E.coli found in our gut or spermatozoa in the reproductive mucus. Biomimetic swimmers that replicate the motion of these small microorganisms hold tremendous promise in a host of biomedical applications like targeted drug delivery, microsurgery, biochemical sensing and disease diagnosis. In one such method of swimming at very low Reynolds numbers, a micron scale helix has been fabricated and rendered magnetic by putting a magnetic material on it. Small rotating magnetic fields could be used then to rotate the helix, which translated as a result of the intrinsic translation rotation coupling in a helix. The present work focussed on the development of such a system of nanopropellers, a few microns in length, the characterization of its dynamics and velocity fluctuations originating from thermal noise. The work has also showed a possible application of the nanopropellers in microrheology where it could be used as a new tool to measure the rheological characteristics of a complex heterogeneous environment with very high spatial and temporal resolutions. A generalized study of the dynamics of these propellers under a rotating field, has showed the existence of a variety of different dynamical configurations. Rigid body dynamics simulations have been carried out to understand the behaviour. Significant amount of insight has been gained by solving the equations of motion of the object analytically and it has helped to obtain a complete understanding, along with providing closed form expressions of the various characteristics frequencies and parameters that has defined the motion. A study of the velocity fluctuations of these chiral nanopropellers has been carried out, where the nearby wall of the microfluidic cell was found to have a dominant effect on the fluctuations. The wall has been found to enhance the average level of fluctuations apart from bringing in significant non Gaussian effects. The experimentally obtained fluctuations has been corroborated by a simulation in which a time evolution study of the governing 3D Langevin dynamics equations has been done. A closer look at the various sources of velocity fluctuations and a causality study thereof has brought out a minimum length scale below which helical propulsion has become impractical to achieve because of the increased effect of the orientational fluctuations of the propeller at those small length scales. An interesting bistable dynamics of the propeller has been observed under certain experimental conditions, in which the propeller randomly switched between the different dynamical states. This defied common sense because of the inherent deterministic nature of the governing Stokes equation. Rigid body dynamics simulations and stability analysis has shown the existence of time scales in which two different dynamical states of the propeller have become stable. Thus the intrinsic dynamics of the system has been found to be the reason behind the bistable behaviour, randomness being brought about by the thermal fluctuations present in the system. Finally, in a novel application of the propellers, they have been demonstrated as a tool for microrheological mapping in a complex fluidic environment. The studies done in this work have helped to develop this method of active microrheology in which the measurement times are orders of magnitude smaller than its existing counterparts.

Micron scale robots going inside our body and curing various ailments is a technolog¬ical dream that easily captures our imagination. However, with the advent of novel nanofabrication and nanocharacterization tools there has been a surge in the research in this field over the last decade. In order to achieve locomotion (swim) at these small length scales, special strategies need to be adopted, that is able to overcome the large viscous damping that these microbots have to face while moving in the various bod¬ily fluids. Thus researchers have looked into the swimming strategies found in nature like that of bacteria like E.coli found in our gut or spermatozoa in the reproductive mucus. Biomimetic swimmers that replicate the motion of these small microorganisms hold tremendous promise in a host of biomedical applications like targeted drug delivery, microsurgery, biochemical sensing and disease diagnosis. In one such method of swimming at very low Reynolds numbers, a micron scale helix has been fabricated and rendered magnetic by putting a magnetic material on it. Small rotating magnetic fields could be used then to rotate the helix, which translated as a result of the intrinsic translation rotation coupling in a helix. The present work focussed on the development of such a system of nanopropellers, a few microns in length, the characterization of its dynamics and velocity fluctuations originating from thermal noise. The work has also showed a possible application of the nanopropellers in microrheology where it could be used as a new tool to measure the rheological characteristics of a complex heterogeneous environment with very high spatial and temporal resolutions. A generalized study of the dynamics of these propellers under a rotating field, has showed the existence of a variety of different dynamical configurations. Rigid body dynamics simulations have been carried out to understand the behaviour. Significant amount of insight has been gained by solving the equations of motion of the object analytically and it has helped to obtain a complete understanding, along with providing closed form expressions of the various characteristics frequencies and parameters that has defined the motion. A study of the velocity fluctuations of these chiral nanopropellers has been carried out, where the nearby wall of the microfluidic cell was found to have a dominant effect on the fluctuations. The wall has been found to enhance the average level of fluctuations apart from bringing in significant non Gaussian effects. The experimentally obtained fluctuations has been corroborated by a simulation in which a time evolution study of the governing 3D Langevin dynamics equations has been done. A closer look at the various sources of velocity fluctuations and a causality study thereof has brought out a minimum length scale below which helical propulsion has become impractical to achieve because of the increased effect of the orientational fluctuations of the propeller at those small length scales. An interesting bistable dynamics of the propeller has been observed under certain experimental conditions, in which the propeller randomly switched between the different dynamical states. This defied common sense because of the inherent deterministic nature of the governing Stokes equation. Rigid body dynamics simulations and stability analysis has shown the existence of time scales in which two different dynamical states of the propeller have become stable. Thus the intrinsic dynamics of the system has been found to be the reason behind the bistable behaviour, randomness being brought about by the thermal fluctuations present in the system. Finally, in a novel application of the propellers, they have been demonstrated as a tool for microrheological mapping in a complex fluidic environment. The studies done in this work have helped to develop this method of active microrheology in which the measurement times are orders of magnitude smaller than its existing counterparts.