Measurements of Magnetic Fiber Dynamics in Magnetic Fields using Optical and Electrical Techniques

<p>The fabrication of piezoelectric ceramics (Piezoceramics) currently relies on a costly dice and fill process to create an array of aligned pillars. These pillars act as waveguides, improving the performance of the piezoceramic wafers over the bulk piezoceramic alone. It is theorised the creation of aligned pores in the piezoceramic may exhibit the same waveguiding effect, removing the need for the dice and fill process. A technique for creating these pores is in development at Callaghan Innovation, New Zealand, where nickel coated carbon fibers are added to the ceramic slurry, aligned with a magnetic field, and attracted to the bottom of a mold. The number of fibers and degree of alignment dictate the waveguiding effectiveness and hence the performance of the piezoceramic. Additionally the time taken for fibers to form an array in the bottom of the mold dictate the piezoceramics fabrication time. Thus it is crucial to be able to measure the alignment and magnetically assisted sedimentation of these fibers in-situ. However the ceramic slurry is opaque, hence the optical methods traditionally can not be implemented. This thesis describes the development and implementation of an electrical technique using the anisotropic conductance of fibers, for measuring fiber dynamics during the fabrication of piezoceramics. The results of this electrical technique are compared to both optical monitoring results in a transparent solution, and models for the motion of rigid cylinders in a fluid suspension. The change in conductance corresponding to fiber rotation was found to have a time constant corresponding to fiber rotation which is a scalar multiple of that of transmission microscopy and the mathematical modeling. This is a product of the geometry of the electrode configurations used to measure conductance. Furthermore, for fiber rotation, the fiber concentration in the solution changes the effective fluid viscosity due to hydrodynamic turbulence created by the rotating fibers. The conductance change corresponding to the magnetically assisted fiber settling is in good accordance with both the optical observations and mathematical modeling for 50 mPas solutions, however for 30 mPas solutions the modeling underestimates the settling time by 20%. The maximum fiber concentration to create a single layer of aligned fibers in the bottom of the mold was found to be 12 fibers=mm³. Exceeding this limit results in a secondary and tertiary layer of fibers forming directly below the fiber suspension injection location.</p>

Measurements of Magnetic Fiber Dynamics in Magnetic Fields using Optical and Electrical Techniques

<p>The fabrication of piezoelectric ceramics (Piezoceramics) currently relies on a costly dice and fill process to create an array of aligned pillars. These pillars act as waveguides, improving the performance of the piezoceramic wafers over the bulk piezoceramic alone. It is theorised the creation of aligned pores in the piezoceramic may exhibit the same waveguiding effect, removing the need for the dice and fill process. A technique for creating these pores is in development at Callaghan Innovation, New Zealand, where nickel coated carbon fibers are added to the ceramic slurry, aligned with a magnetic field, and attracted to the bottom of a mold. The number of fibers and degree of alignment dictate the waveguiding effectiveness and hence the performance of the piezoceramic. Additionally the time taken for fibers to form an array in the bottom of the mold dictate the piezoceramics fabrication time. Thus it is crucial to be able to measure the alignment and magnetically assisted sedimentation of these fibers in-situ. However the ceramic slurry is opaque, hence the optical methods traditionally can not be implemented. This thesis describes the development and implementation of an electrical technique using the anisotropic conductance of fibers, for measuring fiber dynamics during the fabrication of piezoceramics. The results of this electrical technique are compared to both optical monitoring results in a transparent solution, and models for the motion of rigid cylinders in a fluid suspension. The change in conductance corresponding to fiber rotation was found to have a time constant corresponding to fiber rotation which is a scalar multiple of that of transmission microscopy and the mathematical modeling. This is a product of the geometry of the electrode configurations used to measure conductance. Furthermore, for fiber rotation, the fiber concentration in the solution changes the effective fluid viscosity due to hydrodynamic turbulence created by the rotating fibers. The conductance change corresponding to the magnetically assisted fiber settling is in good accordance with both the optical observations and mathematical modeling for 50 mPas solutions, however for 30 mPas solutions the modeling underestimates the settling time by 20%. The maximum fiber concentration to create a single layer of aligned fibers in the bottom of the mold was found to be 12 fibers=mm³. Exceeding this limit results in a secondary and tertiary layer of fibers forming directly below the fiber suspension injection location.</p>

Crosstalk between myosin II and formin functions in the regulation of force generation and actomyosin dynamics in stress fibers

REF52 fibroblasts have a well-developed contractile machinery, the most prominent elements of which are actomyosin stress fibers with highly ordered organization of actin and myosin IIA filaments. The relationship between contractile activity and turnover dynamics of stress fibers is not sufficiently understood. Here, we simultaneously measured the forces exerted by stress fibers (using traction force microscopy or micropillar array sensors) and the dynamics of actin and myosin (using photoconversion-based monitoring of actin incorporation and high-resolution fluorescence microscopy of myosin II light chain). Our data revealed new features of the crosstalk between myosin II-driven contractility and stress fiber dynamics. During normal stress fiber turnover, actin incorporated all along the stress fibers and not only at focal adhesions. Incorporation of actin into stress fibers/focal adhesions, as well as actin and myosin II filaments flow along stress fibers, strongly depends on myosin II activity. Myosin II-dependent generation of traction forces does not depend on incorporation of actin into stress fibers per se, but still requires formin activity. This previously overlooked function of formins in maintenance of the actin cytoskeleton connectivity could be the main mechanism of formin involvement in traction force generation.

Computational Analysis of Fiber Dynamics in Human Nasal Cavity

Elongated particles, such as asbestos and mineral fibers, are considered severe inhalation hazards due to their ability to penetrate into the deep lung. Frequently the dynamic behavior of the fibrous particles is attributed to their unique needle-like geometry. Therefore, understanding the interactions of the inhaled elongated particles with the airflow environment is of great significance. In this study, the transport and deposition of elongated micro-fibers in a realistic human nasal cavity is investigated numerically. The motion of the micro-fiber is resolved by solving the system of equations governing its coupled translational and rotational motions. The governing equations included the drag, the hydrodynamic torques that were evaluated using the Jeffrey model. The influence of the shear lift force was also included in these simulations. The no-slip wall boundary condition for airflow in the airways was used. Since the surface of airways is covered with mucus, when a fiber touches the surface, it was assumed to be deposited with no rebound. The study allows a close look at the non-spherical particle-flow dynamics with respect to the translation, rotation, coupling, and how the rotation affects the particle’s macroscopic transport and deposition properties. A series of simulations for different microfiber diameters and aspect ratios were performed. The simulation results are compared with the existing experimental data, and earlier computational model predictions and good agreements were obtained. The present study also seeks to provide additional insight into the transport processes of microfibers in the upper respiratory tract.

Actin stress fiber dynamics in laterally confined cells

Multiple cellular processes are affected by spatial constraints from the extracellular matrix and neighboring cells. In vitro experiments using defined micro-patterning allow for in-depth analysis and a better understanding of how these constraints impact cellular behavior and functioning. Herein we focused on the analysis of actin cytoskeleton dynamics as a major determinant of mechanotransduction mechanisms in cells. We seeded primary human umbilical vein endothelial cells onto stripe-like cell-adhesive micro-patterns with varying widths and then monitored and quantified the dynamic reorganization of actin stress fibers, including fiber velocities, orientation and density, within these live cells using the cell permeable F-actin marker SiR-actin. Although characteristic parameters describing the overall stress fiber architecture (average orientation and density) were nearly constant throughout the observation time interval of 60 min, we observed permanent transport and turnover of individual actin stress fibers. Stress fibers were more strongly oriented along stripe direction with decreasing stripe width, (5° on 20 μm patterns and 10° on 40 μm patterns), together with an overall narrowing of the distribution of fiber orientation. Fiber dynamics was characterized by a directed movement from the cell edges towards the cell center, where fiber dissolution frequently took place. By kymograph analysis, we found median fiber velocities in the range of 0.2 μm/min with a weak dependence on pattern width. Taken together, these data suggest that cell geometry determines actin fiber orientation, while it also affects actin fiber transport and turnover.

Centrosomes control kinetochore-fiber plus-end dynamics via HURP to ensure symmetric divisions

SUMMARYDuring mitosis centrosomes can affect the length of kinetochore-fibers (k-fibers) and the stability of kinetochore-microtubule attachments, implying that they regulate k-fiber dynamics. The exact cellular and molecular mechanisms by which centrosomes regulate k-fibers remain, however, unknown. Here, we created human non-cancerous cells with only one centrosome to investigate these mechanisms. Such cells formed highly asymmetric bipolar spindles that resulted in asymmetric cell divisions. K-fibers in acentrosomal spindles were shorter, more stable, had a reduced poleward microtubule flux at minus-ends, and more frequent pausing events at their plus-ends. This indicates that centrosomes regulate k-fiber dynamics both locally at minus-ends and far away at plus-ends. At the molecular level we find that the microtubule-stabilizing protein HURP is enriched on the k-fiber plus-ends in the acentrosomal spindles of cells with only one centrosome. HURP depletion rebalance k-fiber stability and dynamics in such cells, and improved spindle and cell division symmetry. Our data further indicate that HURP accumulates on k-fibers inversely proportionally to half-spindle length. We propose that centrosomes regulate k-fiber plus-ends indirectly via length-dependent accumulation of HURP. Thus by ensuring equal k-fiber length, centrosomes promote HURP symmetry, reinforcing the symmetry of the mitotic spindle and of cell division.