Dynamic Assembly of Magnetic Colloidal Vortices.

Magnetic colloids in external time-dependent fields are subject to complex induced many-body interactions governing their self-assembly into a variety of equilibrium and out-of-equilibrium structures such as chains, networks, suspended membranes, and colloidal foams. Here, we report experiments, simulations, and theory probing the dynamic assembly of superparamagnetic colloids in precessing external magnetic fields. Within a range of field frequencies, we observe dynamic large-scale structures such as ordered phases composed of precessing chains, ribbons, and rotating fluidic vortices. We show that the structure formation is inherently coupled to the buildup of torque, which originates from internal relaxation of induced dipoles and from transient correlations among the particles as a result of short-lived chain formation. We discuss in detail the physical properties of the vortex phase and demonstrate its potential in particle-coating applications.

Magnetically Self-Assembled Colloidal Three-Dimensional Structures as Cell Growth Scaffold.

Understanding the chemical and physical conditions for cell growth is important from biological and medical aspects. Many tissues and cell types (e.g., epithelial cells and neurons) naturally grow on surfaces that span in three-dimensions and offer structural or mechanical support. The scaffold surface has to promote adhesion and cell proliferation as well as support their weight and retain its structural integrity. Here, we present a flexible method that uses self-assembly of micrometer superparamagnetic particles to produce appropriate scaffold surfaces with controllable general appearance in three dimensions, such as oriented membranes, branched structure, or void network. As a proof of principle, the Chinese hamster ovary epithelial cell line was successfully grown for several days on inclined membranes. Robustness of the oriented membrane architecture was probed with optical tweezers. We measured the magnetic force holding one particle in a self-assembled upright hexagonal sheet and modeled it as a sum of pair interaction forces between spatially arrested static dipoles.

Pattern formation and coarse-graining in two-dimensional colloids driven by multiaxial magnetic fields.

We study the pattern formation in a two-dimensional system of superparamagnetic colloids interacting via spatially coherent induced interactions driven by an external precessing magnetic field. On the pair level, upon changing the opening angle of the external field, the interactions smoothly vary from purely repulsive (opening angle equal to zero) to purely attractive (time-averaged pair interactions at an opening angle of 90°). In the experiments, we observed ordered hexagonal crystals at the repulsive end and coarsening frothlike structures for purely attractive interactions. In both of these limiting cases, the dense colloidal systems can be sufficiently accurately described by assuming pairwise additivity of the interaction potentials. However, for a range of intermediate angles, pronounced many-body depolarization effects compete with the direct induced interactions, resulting in inherently anisotropic effective interactions. Under such conditions, we observed the decay of hexagonal order with the concomitant formation of short chains and percolated networks of chains coexisting with free colloids. In order to describe and investigate these systems theoretically, we developed a coarse-grained model of a binary mixture of patchy and nonpatchy particles with the ratio of patchy and nonpatchy colloids as the order parameter. Combining genetic algorithms with Monte Carlo simulations, we optimized the model parameters and quantitatively reproduced the experimentally observed sequence of colloidal structures. The results offer new insight into the anisotropy induced by the many-body effects. At the same time, they allow for a very efficient description of the system by means of a pairwise-additive Hamiltonian, whereupon the original, one-component system features a two-component mixture of isotropic and patchy colloids.

Magnetic field directed assembly of magnetic non-spherical microparticles.

This study reports on the fabrication and assembly of anisotropic microparticles as versatile building blocks for directed magnetic assemblies. Although spherical microparticles have received extensive attention, the assembly of non-spherical magnetic microparticles remains underexplored. Herein, we present a fabrication approach that utilizes photolithography and soft lithography to create prism-shaped magnetic microparticles. In order to investigate their assembly, a switching rotating magnetic field was employed. To support our experimental findings, a numerical model which takes into account the magnetic dipole moments induced by the field of other particles was developed. This model helps in understanding the forces and torques governing particle behavior during assembly. Simulations were conducted using the numerical model to complement our experimental findings. In the two particle experiments, attractive magnetic interactions led to various configurations depending on initial positions. For three particles, a tip-to-tip configuration suggested closed or stable ring-like structures. Our work highlights the feasibility of producing highly responsive, non-spherical magnetic microparticles and their potential for assemblies. The versatile fabrication method, coupled with the added degree of freedom conferred by prismatic shapes, opens promising avenues for applications in biology and material science.

Self-assembled artificial cilia.

Due to their small dimensions, microfluidic devices operate in the low Reynolds number regime. In this case, the hydrodynamics is governed by the viscosity rather than inertia and special elements have to be introduced into the system for mixing and pumping of fluids. Here we report on the realization of an effective pumping device that mimics a ciliated surface and imitates its motion to generate fluid flow. The artificial biomimetic cilia are constructed as long chains of spherical superparamagnetic particles, which self-assemble in an external magnetic field. Magnetic field is also used to actuate the cilia in a simple nonreciprocal manner, resulting in a fluid flow. We prove the concept by measuring the velocity of a cilia-pumped fluid as a function of height above the ciliated surface and investigate the influence of the beating asymmetry on the pumping performance. A numerical simulation was carried out that successfully reproduced the experimentally obtained data.

Technical strategy in a patient with symptomatic thoracic aneurysm near the origin of the left subclavian artery and left internal thoracic artery coronary graft.

Thoracic endovascular aortic repair (TEVAR) is a safe and reliable technique utilized in the treatment for aortic aneurysms. However, in up to 40% of patients, devices are typically placed over the left subclavian artery (LSA) origin. In this report, we present a case of a successful TEVAR procedure following the transposition of the LSA with protective carotico-axillary/carotid bypass in a patient with a patent left internal thoracic artery (LITA)-left anterior descending (LAD) coronary artery bypass graft and right internal carotid artery (ICA) occlusion.

Distribution of risk factors in patients with premature coronary, supra-aortic branches and peripheral atherosclerotic disease.

OBJECTIVE: The aim of this study was to determine the distribution of major risk factors among patients with atherosclerotic disease aged ≤50 years. SUBJECTS AND METHODS: The study population comprised 944 patients aged ≤50 years with clinically significant manifestations of atherosclerotic disease compared with 350 consecutive (control) older patients (age >50 years). RESULTS: The most significant risk factors for atherosclerotic disease of the supra-aortic branches were: smoking (87.5%, p < 0.001), family history of atherosclerosis (52.3%, p < 0.001), diabetes mellitus (28.5%) and being male (56.9%) (p < 0.05 for both). In patients with coronary artery disease, there was considerable disparity in the prevalence of smoking (85.4%), hyperlipoproteinemia (67.4%), and family history of atherosclerosis (68%) (p < 0.001). For peripheral artery disease, the most significant risk factors were smoking (97%, p < 0.001), hyperlipidemia (p < 0.01), and family history of atherosclerotic disease (p < 0.01). When compared to controls, patients with premature atherosclerosis smoked more frequently, had hyperlipidemia, had a family history of atherosclerosis, and were more frequently of male sex. CONCLUSION: Premature atherosclerosis was most frequently associated with smoking, hyperlipidemia, family history of atherosclerotic disease, and male sex.

Transposition of Subclavian Artery - Is It the Appropriate Choice?

INTRODUCTION: To present patients with symptomatic subclavian artery disease and treatment possibility. CASE REPORT: A 53-year-old female was admitted with vertigo and left arm claudication. Duplex-scan and MSCT arteriography verified subclavian artery occlusion. After the endovascular treatment failure, the patient was switched to surgical treatment - subclavian artery transposition. After the successful surgical treatment, the patient was discharged on the second postoperative day. If the patient is a candidate for surgery, the literature review shows good initial and long-term results after the subclavian artery transposition, and emphasizes this technique as superior. CONCLUSION: Subclavian carotid transposition is a safe and effective method of treatment in patients after endovascular treatment failure or other indication. Also, the surgical treatment is technically demanding because of the difficult access to the vessel origin, and it requires experienced surgeons.

Hydrodynamic synchronization of autonomously oscillating optically trapped particles.

Ellipsoidal micron-sized colloidal particles can oscillate spontaneously when trapped in a focused laser beam. If two oscillating particles are held in proximity their oscillations synchronize through hydrodynamic interactions. The degree of synchronization depends on the distance between the oscillators and on their orientation. Due to the anisotropic nature of hydrodynamic coupling the synchronization is strongest when particles are arranged along the direction of oscillations. Similar behavior is observed for many oscillating particles arranged in a row. Experimental observations are well reproduced with a model that uses a phenomenological description of the optical force and hydrodynamic interactions. Our results show that oscillating ellipsoidal particles can serve as a model system for studying hydrodynamic synchronization between biological cilia.

Analysis of fluid flow around a beating artificial cilium.

Biological cilia are found on surfaces of some microorganisms and on surfaces of many eukaryotic cells where they interact with the surrounding fluid. The periodic beating of the cilia is asymmetric, resulting in directed swimming of unicellular organisms or in generation of a fluid flow above a ciliated surface in multicellular ones. Following the biological example, externally driven artificial cilia have recently been successfully implemented as micropumps and mixers. However, biomimetic systems are useful not only in microfluidic applications, but can also serve as model systems for the study of fundamental hydrodynamic phenomena in biological samples. To gain insight into the basic principles governing propulsion and fluid pumping on a micron level, we investigated hydrodynamics around one beating artificial cilium. The cilium was composed of superparamagnetic particles and driven along a tilted cone by a varying external magnetic field. Nonmagnetic tracer particles were used for monitoring the fluid flow generated by the cilium. The average flow velocity in the pumping direction was obtained as a function of different parameters, such as the rotation frequency, the asymmetry of the beat pattern, and the cilium length. We also calculated the velocity field around the beating cilium by using the analytical far-field expansion. The measured average flow velocity and the theoretical prediction show an excellent agreement.

Measurement of fluid flow generated by artificial cilia.

We observed and measured the fluid flow that was generated by an artificial cilium. The cilium was composed of superparamagnetic microspheres, in which magnetic dipole moments were induced by an external magnetic field. The interaction between the dipole moments resulted in formation of long chains-cilia, and the same external magnetic field was also used to drive the cilia in a periodic manner. Asymmetric periodic motion of the cilium resulted in generation of fluid flow and net pumping of the surrounding fluid. The flow and pumping performance were closely monitored by introducing small fluorescent tracer particles into the system. By detecting their motion, the fluid flow around an individual cilium was mapped and the flow velocities measured. We confirm that symmetric periodic beating of one cilium results in vortical motion only, whereas asymmetry is required for additional translational motion. We determine the effect of asymmetry on the pumping performance of a cilium, verify the theoretically predicted optimal pumping conditions, and determine the fluid behaviour around a linear array of three neighbouring cilia. In this case, the contributions of neighbouring cilia enhance the maximal flow velocity compared with a single cilium and contribute to a more uniform translational flow above the surface.

Confinement effect on interparticle potential in nematic colloids.

We studied the confinement effect on the interaction force in nematic liquid crystal colloids with spherical particles inducing planar anchoring. Using magneto-optical tweezers, we measured the spatial dependence of the quadrupolar structural interparticle force over 4 orders of magnitude. For small separations, the interparticle potential follows the power law, whereas for separations larger than the sample thickness, it decreases exponentially with the decay length proportional to the sample thickness. Experimental results are reproduced by using the Landau-de Gennes free-energy minimization approach.

Factors associated with hyperhomocysteinemia after renal transplantation.

Recent studies show that clinically stable renal transplant recipients have an increased prevalence of hyperhomocysteinemia (hyperHcy), but the mechanism of this disorder has not yet been elucidated. The aim of the present study was to evaluate the factors associated with hyperHcy after a successful renal transplantation. In 106 stable renal transplant recipients, total serum Hcy level (tHcy), folate, total protein, serum creatinine concentration, creatinine clearance, lipid status, body weight (BW), body mass index (BMI), and body fat (BF) were determined. The mean doses of cyclosporine, prednisolone, and azathioprine (mg/kg/day) were recorded. The mean serum tHcy level was significantly higher in renal transplant patients than in healthy controls (22.02 +/- 8.02 versus 13.0 +/- 3.3 micromol/ L; p < 0.001), and the incidence of patients with hyperHcy was 82%. Comparison of the group of 20 patients with tHcy level <15 micromol/L and the group of 86 patients with tHcy level >15 micromol/L revealed that the latter was significantly older, heavier, had been longer on dialysis before renal transplantation, and had older donors and poorer renal graft function. Significant correlation was found between tHcy level and recipient age, dialysis duration, BW, creatinine clearance, serum creatinine, and folate concentration. However, multivariate analysis indicated that creatinine clearance (p = 0.025) and BW (p = 0.03) were the only determinants of elevated total Hcy level in renal transplant recipients. HyperHcy persists after successful kidney transplantation in the majority of renal transplant recipients, and its appearance is primarily associated with creatinine clearance and body weight.

Interparticle potential and drag coefficient in nematic colloids.

Magneto-optic tweezers were used for measurements of liquid-crystal-mediated forces between spherical beads with tangential anchoring in thin nematic samples. Repulsive force, which results from the quadrupolar symmetry of defects around the immersed beads, decreases proportionally to 1/x6, with x being the bead separation. The velocity with which the particles are pushed apart also follows the same separation dependence. We thus find the effective drag coefficient gamma(eff) independent of x for surface-to-surface distances as small as 10% of the bead diameter.

Noise-enhanced performance of ratchet cellular automata.

We present the first experimental realization of a ratchet cellular automaton (RCA) which has recently been suggested as an alternative approach for performing logical operations with interacting (quasi)particles. Our study was performed with interacting colloidal particles which serve as a model system for other dissipative systems, i.e., magnetic vortices on a superconductor or ions in dissipative optical arrays. We demonstrate that noise can enhance the efficiency of information transport in RCA and consequently enables their optimal operation at finite temperatures.