Multi-Ciliated Microswimmers -- Metachronal Coordination and Helical Swimming

The dynamics and motion of multi-ciliated microswimmers with a spherical body and a small number N (with 5 < N < 60) of cilia with length comparable to the body radius, is investigated by mesoscale hydrodynamics simulations. A metachronal wave is imposed for the cilia beat, for which the wave vector has both a longitudinal and a latitudinal component. The dynamics and motion is characterized by the swimming velocity, its variation over the beat cycle, the spinning velocity around the main body axis, as well as the parameters of the helical trajectory. Our simulation results show that the microswimmer motion strongly depends on the latitudinal wave number and the longitudinal phase lag. The microswimmers are found to swim smoothly and usually spin around their own axis. Chirality of the metachronal beat pattern generically generates helical trajectories. In most cases, the helices are thin and stretched, i.e. the helix radius is about an order of magnitude smaller than the pitch. The rotational diffusion of the microswimmer is significantly smaller than the passive rotational diffusion of the body alone, which indicates that the extended cilia contribute strongly to the hydrodynamic radius. The swimming velocity vswim is found to increase with the cilia number N with a slightly sublinear power law, consistent with the behavior expected from the dependence of the transport velocity of planar cilia arrays on the cilia separation.Comment: 15 pages, 14 figure

The price of populism: financial market outcomes of populist electoral success

Following financial research on the importance of public policy for asset prices, we hypothesize that the success of populist movements impacts risk assessments in financial markets. Building a novel dataset, findings show for a sample of Western democracies that the success of populist parties has a direct impact on volatility in major domestic market indexes, measured from option prices spanning national elections. Despite its anti-capitalist rhetoric, the political insecurity generated by populist movements on the far left only partially translates into financial insecurity in the context of institutionalized democracies. In turn, we find the electoral success of right-wing populists to reduce risk assessments, which could be driven by its frequent association with rent-seeking and big business

Flagellated and Ciliated Microswimmers

The propulsion mechanism and the swimming dynamics of various ciliated microorganisms are investigated. Ciliated microswimmers, ranging from a single flagellated sperm cell to multiciliated microswimmers, propel themselves by cilia attached to their cell membrane. The underlying complex biomachinery of a cilium, the axoneme, employs an evolutionary developed mechanism, which is tailored to generate an optimal beating pattern to propel the swimmer through the environment it encounters. In this work mesoscale hydrodynamics simulations are used to simulate the whip-like motion of the cilium at low Reynolds numbers. The particle-based approach of multi-particle collision dynamics enables simulations of self-propelled microswimmers in complex confinements where steric and hydrodynamic interactions strongly influence the swimming dynamics. Details of cilia arrangement and beat shape are critical in understanding propulsion and surface attraction. The axonemal beating of cilia and flagella is modeled by a semi-flexible polymer with periodically changing intrinsic curvature. In the spirit of a minimalistic modeling approach, the axoneme is only bend along one degree of freedom, creating a defined beat plane. The first part discusses surface attraction and guidance of sperm cells swimming in confinement. In particular, the motion of sperm in geometrically structured (zigzag) microchannels provides an interesting geometry for the manipulation and sorting of sperm cells. Sperm swim along the channel walls, but are deflected from the sidewall at sharp bends. The simulation results are in qualitative agreement with recent microfluidic experiments and provide a better insight into the mechanisms of sperm navigation under strong confinement. The effective adhesion of a sperm cell to a flat surface depends both on the envelope of its planar beat shape and on the orientation of its beat plane. A proposed self-propelled steric model explains the average deflection around corners. Further investigation of various beat patterns with increasing wavelength results in complex surface attraction dynamics of the sperm cell. The insight from the steric model helps to understand the surface attraction in terms of the beat-shape envelope. It is found that when the beat pattern exceeds a critical wavelength, the flagellum buckles and beats in a complex three-dimensional shape, which strongly increases surface attraction. Indeed, the analysis of three-dimensional experimental holographic data of freely swimming human sperm cells shows that on average the beat pattern is relatively planar but exhibits regular nonplanar components twice per beat. By comparing this high-resolution experimental data with simulation results, a possible explanation for the nonplanar beating is obtained. Simulated sperm with imposed planar bends and two orders of magnitude smaller twist than bending rigidity undergo a twist instability and exhibit a three-dimensional beat pattern. Simulations allow to map the phase space of the twist instability, which shows no dependence on the bending rigidity, but a sharp transition from planar to three-dimensional beating below a critical twist rigidity. A localized twist wave goes through the cilium, which twists the cilium at a very narrow segment close to the point of minimal in-plane bending. This creates essentially two beat planes, separating the cilium in two segments of planar beating before and after the twisting region. In the second part, propulsion and synchronization of multi-ciliated spherical swimmers with different cilia densities and arrangements are studied. Instead of pre-imposing the intrinsic curvature, a ratchet-like mechanism drives the ciliary beat pattern. Therefore, the beat period can be influenced by the flow generated from the motion of the other cilia. The propulsion velocity of ciliated spherical swimmers increases sub-linearly with increasing cilia density. Large differences in propulsion speed for equal numbers of cilia with different arrangements on the sphere are found. For symmetric ciliated swimmers, the emergence of a stable synchronization state is found to depend on the initial condition. In some symmetric 9-cilia swimmers, long stable phases of synchronization emerge. Swimmers whose phase difference increases due to phase slips have a slower propulsion velocity than swimmers which develop a constant phase-lag between cilia. Turning to an oscillator model for cilia synchronization, the emergence of metachronal coordination in different topologies above a surface is studied. The oscillators are modeled as hydrodynamically interacting spheres propelled along a circular trajectory. Non-dimensionalization of the model provides the radial confinement strength as the only control parameter. Boundary effects influence the synchronization as well as the confinement strength. In open chains of oscillators as well as in circular arrangements, stable large-scale patterns of synchronization emerge until a critical confinement strength. No long-term coordination emerges above a critical confinement strength in any of the studies topologies. Finally, the cilium model is used to simulate a tuft of cilia, modeled to describe the placement of cilia in brain ventricles of mice. It is found that the particle flux towards the surface is located in hot-spots where the flux is significantly enhanced compared to purely diffusive transport. This shows the important role of ciliary beating in molecular transport towards primary cilia on the surface of the ventricles

Towards dynamical network biomarkers in neuromodulation of episodic migraine

Computational methods have complemented experimental and clinical neursciences and led to improvements in our understanding of the nervous systems in health and disease. In parallel, neuromodulation in form of electric and magnetic stimulation is gaining increasing acceptance in chronic and intractable diseases. In this paper, we firstly explore the relevant state of the art in fusion of both developments towards translational computational neuroscience. Then, we propose a strategy to employ the new theoretical concept of dynamical network biomarkers (DNB) in episodic manifestations of chronic disorders. In particular, as a first example, we introduce the use of computational models in migraine and illustrate on the basis of this example the potential of DNB as early-warning signals for neuromodulation in episodic migraine.Comment: 13 pages, 5 figure

Digitalization challenging physical culture and education – Current issues in sport pedagogical research

This Special Issue focuses on technology and how people are making sense of this related to body, movement, exercise and health through a sport pedagogical lens

Frequency-modulated atomic force microscopy operation by imaging at the frequency shift minimum: The dip-df mode

Rode S, Schreiber M, Kühnle A, Rahe P. Frequency-modulated atomic force microscopy operation by imaging at the frequency shift minimum: The dip-df mode. Review of Scientific Instruments. 2014;85(4):43707.In frequency modulated non-contact atomic force microscopy, the change of the cantilever frequency (Delta f) is used as the input signal for the topography feedback loop. Around the Delta f(z) minimum, however, stable feedback operation is challenging using a standard proportional-integral-derivative (PID) feedback design due to the change of sign in the slope. When operated under liquid conditions, it is furthermore difficult to address the attractive interaction regime due to its often moderate peakedness. Additionally, the Delta f signal level changes severely with time in this environment due to drift of the cantilever frequency f(0) and, thus, requires constant adjustment. Here, we present an approach overcoming these obstacles by using the derivative of Delta f with respect to z as the input signal for the topography feedback loop. Rather than regulating the absolute value to a preset setpoint, the slope of the Delta f with respect to z is regulated to zero. This new measurement mode not only makes the minimum of the Delta f(z) curve directly accessible, but it also benefits from greatly increased operation stability due to its immunity against f(0) drift. We present isosurfaces of the Delta f minimum acquired on the calcite CaCO3(1014) surface in liquid environment, demonstrating the capability of our method to image in the attractive tip-sample interaction regime. (C) 2014 AIP Publishing LLC

True Atomic-Resolution Imaging of (1014) Calcite in Aqueous Solution by Frequency Modulation Atomic Force Microscopy

Rode S, Oyabu N, Kobayashi K, Yamada H, Kühnle A. True Atomic-Resolution Imaging of (1014) Calcite in Aqueous Solution by Frequency Modulation Atomic Force Microscopy. Langmuir. 2009;25(5):2850-2853.Calcite (CaCO(3)) is one of the most abundant minerals on earth and plays an important role in a wide range of different fields including, for example, biomineralization and environmental geochemistry. Consequently, surface processes and reactions such as dissolution and growth as well as (macro)molecule adsorption are of greatest interest for both applied as well as fundamental research. An in-depth understanding of these processes requires knowledge about the detailed surface structure in its natural state which is quite often a liquid environment. We have studied the most stable cleavage plane of calcite under liquid conditions using frequency modulation atomic force microscopy. Using this technique, we achieved true atomic-resolution imaging, demonstrating the high-resolution capability of frequency modulation atomic force microscopy in liquids. We could reproduce contrast features reported before using contact mode atomic force microscopy, originating from the protruding oxygen atom of the carbonate groups. Besides this contrast, however, our results, indeed, indicate that we obtain more detailed structural information, revealing the calcium sublattice of the (10 (1) over bar4) cleavage plane

TPMS-based membrane lung with locally-modified permeabilities for optimal flow distribution

Membrane lungs consist of thousands of hollow fiber membranes packed together as a bundle. The devices often suffer from complications because of non-uniform flow through the membrane bundle, including regions of both excessively high flow and stagnant flow. Here, we present a proof-of-concept design for a membrane lung containing a membrane module based on triply periodic minimal surfaces (TPMS). By warping the original TPMS geometries, the local permeability within any region of the module could be raised or lowered, allowing for the tailoring of the blood flow distribution through the device. By creating an iterative optimization scheme for determining the distribution of streamwise permeability inside a computational porous domain, the desired form of a lattice of TPMS elements was determined via simulation. This desired form was translated into a computer-aided design (CAD) model for a prototype device. The device was then produced via additive manufacturing in order to test the novel design against an industry-standard predicate device. Flow distribution was verifiably homogenized and residence time reduced, promising a more efficient performance and increased resistance to thrombosis. This work shows the promising extent to which TPMS can serve as a new building block for exchange processes in medical devices