Flow network controlled shape transformation of a thin membrane through differential fluid storage and surface expansion

The mechanical properties of a thin, planar material, perfused by an embedded flow network, can be changed locally and globally by the fluid transport and storage, resulting in small or large-scale deformation, such as out-of-plane buckling. Fluid absorption and storage eventually cause the material to locally swell. Different parts can hydrate and swell unevenly, prompting a differential expansion of the surface. In order to computationally study the hydraulically induced differential swelling and buckling of such a membrane, we develop a network model that describes both the membrane shape and fluid movement, coupling mechanics with hydrodynamics. We simulate the time-dependent fluid distribution in the flow network based on a spatially explicit resistor network model with local fluid-storage capacitance. The shape of the surface is modeled by a spring network produced by a tethered mesh discretization, in which local bond rest lengths are adjusted instantaneously according to associated local fluid content in the capacitors in a quasi-static way. We investigate the effects of various designs of the flow network, including overall hydraulic traits (resistance and capacitance) and hierarchical architecture (arrangement of major and minor veins), on the specific dynamics of membrane shape transformation. To quantify these effects, we explore the correlation between local Gaussian curvature and relative stored fluid content in each hierarchy by using linear regression, which reveals that stronger correlations could be induced by less densely connected major veins. This flow-controlled mechanism of shape transformation was inspired by the blooming of flowers through the unfolding of petals. It can potentially offer insights for other reversible motions observed in plants induced by differential turgor and water transport through the xylem vessels, as well as engineering applications

Automatic 2-D/3-D Vessel Enhancement in Multiple Modality Images Using a Weighted Symmetry Filter

Automated detection of vascular structures is of great importance in understanding the mechanism, diagnosis and treatment of many vascular pathologies. However, automatic vascular detection continues to be an open issue because of difficulties posed by multiple factors such as poor contrast, inhomogeneous backgrounds, anatomical variations, and the presence of noise during image acquisition. In this paper, we propose a novel 2D/3D symmetry filter to tackle these challenging issues for enhancing vessels from different imaging modalities. The proposed filter not only considers local phase features by using a quadrature filter to distinguish between lines and edges, but also uses the weighted geometric mean of the blurred and shifted responses of the quadrature filter, which allows more tolerance of vessels with irregular appearance. As a result, this filter shows a strong response to the vascular features under typical imaging conditions. Results based on 8 publicly available datasets (six 2D datasets, one 3D dataset and one 3D synthetic dataset) demonstrate its superior performance to other state-ofthe- art methods

Theoretical and Computational Studies of the Lateral Phases on a Multicomponent Lipid-Bilayer Surface

Thesis (Ph.D.)--University of Washington, 2019The lipid-bilayer membrane is the fundamental structure of a cell plasma membrane and has been largely studied by biophysical experiments, theoretical modeling and computer simulations. Two major aspects of lipid bilayer morphologies are studied, namely lateral phase separation and pattern formation (spatial organization of lipid molecules), and shape change or deformation of the membrane (undulation of the surface). In this dissertation, I apply theoretical modeling, analytical and numerical computation as well as molecular simulation to study the lateral phases on the surface of a multicomponent lipid bilayer, which is able to stay in a homogeneous state, phase separate, or transform into heterogeneous states including modulated phases and microemulsions. The focal point is a lipid-bilayer vesicle of finite size and spherical topology. I calculate phase diagrams which reveal the effects of intrinsic finite sizes of vesicles on their surface pattern and lateral phase generation. I also calculate the structure factor of vesicles, which is measured by scattering signals, and theoretically develop an approximate model-independent interconversion between the three-dimensional signal and the two-dimensional structure factor of planar membranes, and I also stress the finite size effect in this relation. It is also observed that the lipid domain formation is coupled with membrane curvature distribution on the surface. I apply the coupling through domain bending properties, and create phase diagrams in which changing membrane mechanical properties can vary the lateral phases on a vesicle surface. In addition to the aforementioned continuum modeling, I also utilize coarse-grained molecular dynamics simulation to explore the phase behavior of asymmetric lipid bilayers consisting of a phase-separating leaflet and a homogeneous leaflet, in which the former would induce a tendency of phase separation in the latter. The molecular simulation, which keeps a record of each lipid molecule, could be a more biologically relevant model of plasma membranes