Characterizing Sparse Graphs by Map Decompositions

A map is a graph that admits an orientation of its edges so that each vertex has out-degree exactly 1. We characterize graphs which admit a decomposition into k edge-disjoint maps after: (1) the addition of any ℓ edges; (2) the addition of some ℓ edges. These graphs are identified with classes of sparse graphs; the results are also given in matroidal terms

Information Structures to Secure Control of Rigid Formations with Leader-Follower Structure

This paper is concerned with rigid formations of mobile autonomous agents using a leader-follower structure. A formation is a group of agents moving in real 2- or 3- dimensional space. A formation is called rigid if the distance between each pair of agents does not change over time under ideal conditions. Sensing/communication links between agents are used to maintain a rigid formation. Two agents connected by a sensing/communication link are called neighbors. There are two types of neighbor relations in rigid formations. In the first type, the neighbor relation is symmetric. In the second type, the neighbor relation is asymmetric. Rigid formations with a leader-follower structure have the asymmetric neighbor relation. A framework to analyze rigid formations with symmetric neighbor relations is given in our previous work. This paper suggests an approach to analyze rigid formations that have a leader-follower structure

Designing stem cell niches for differentiation and self-renewal

Mesenchymal stem cells, characterized by their ability to differentiate into skeletal tissues and self-renew, hold great promise for both regenerative medicine and novel therapeutic discovery. However, their regenerative capacity is retained only when in contact with their specialized microenvironment, termed the stem cell niche. Niches provide structural and functional cues that are both biochemical and biophysical, stem cells integrate this complex array of signals with intrinsic regulatory networks to meet physiological demands. Although, some of these regulatory mechanisms remain poorly understood or difficult to harness with traditional culture systems. Biomaterial strategies are being developed that aim to recapitulate stem cell niches, by engineering microenvironments with physiological-like niche properties that aim to elucidate stem cell-regulatory mechanisms, and to harness their regenerative capacity in vitro. In the future, engineered niches will prove important tools for both regenerative medicine and therapeutic discoveries

Sparse Hypergraphs and Pebble Game Algorithms

A hypergraph G=(V,E) is (k,ℓ)-sparse if no subset V′⊂V spans more than k|V′|−ℓ hyperedges. We characterize (k,ℓ)-sparse hypergraphs in terms of graph theoretic, matroidal and algorithmic properties. We extend several well-known theorems of Haas, Lovász, Nash-Williams, Tutte, and White and Whiteley, linking arboricity of graphs to certain counts on the number of edges. We also address the problem of finding lower-dimensional representations of sparse hypergraphs, and identify a critical behavior in terms of the sparsity parameters k and ℓ. Our constructions extend the pebble games of Lee and Streinu [A. Lee, I. Streinu, Pebble game algorithms and sparse graphs, Discrete Math. 308 (8) (2008) 1425–1437] from graphs to hypergraphs

Automating the multimodal analysis of musculoskeletal imaging in the presence of hip implants

In patients treated with hip arthroplasty, the muscular condition and presence of inflammatory reactions are assessed using magnetic resonance imaging (MRI). As MRI lacks contrast for bony structures, computed tomography (CT) is preferred for clinical evaluation of bone tissue and orthopaedic surgical planning. Combining the complementary information of MRI and CT could improve current clinical practice for diagnosis, monitoring and treatment planning. In particular, the different contrast of these modalities could help better quantify the presence of fatty infiltration to characterise muscular condition after hip replacement. In this thesis, I developed automated processing tools for the joint analysis of CT and MR images of patients with hip implants. In order to combine the multimodal information, a novel nonlinear registration algorithm was introduced, which imposes rigidity constraints on bony structures to ensure realistic deformation. I implemented and thoroughly validated a fully automated framework for the multimodal segmentation of healthy and pathological musculoskeletal structures, as well as implants. This framework combines the proposed registration algorithm with tailored image quality enhancement techniques and a multi-atlas-based segmentation approach, providing robustness against the large population anatomical variability and the presence of noise and artefacts in the images. The automation of muscle segmentation enabled the derivation of a measure of fatty infiltration, the Intramuscular Fat Fraction, useful to characterise the presence of muscle atrophy. The proposed imaging biomarker was shown to strongly correlate with the atrophy radiological score currently used in clinical practice. Finally, a preliminary work on multimodal metal artefact reduction, using an unsupervised deep learning strategy, showed promise for improving the postprocessing of CT and MR images heavily corrupted by metal artefact. This work represents a step forward towards the automation of image analysis in hip arthroplasty, supporting and quantitatively informing the decision-making process about patient’s management

ENVIRONMENTALLY FRIENDLY TECHNOLOGY: THE BEHAVIOUR OF NATURAL AND SYNTHETIC BINDER SYSTEMS WITHIN PAPER COATINGS

Coating shrinkage upon drying is a phenomenon well known to the paper coating industry, where it often causes changes in the final structure of the coating layer leading to poor results in terms of gloss, light scattering, surface strength, coverage, uniformity and printability. Such shrinkage has in previous studies been wrongly associated with shrinkage of the polymeric binders used in the coating formulation, by making erroneous comparison with solvent-based paint systems. Natural binders, as starch or proteins, which come from renewable resources and are therefore environmentally friendly, suffer more from this shrinkage phenomenon than synthetic binders. The aim of this research project was to improve the understanding of the processes involved in the drying of a coating layer and to create a model able to describe them. Shrinkage while the coating layer dries has been successfully measured by observing the deflection of coated strips of a synthetic elastically-deformable substrate. Ground calcium carbonate was used as the coating pigment, together with latex binders of both low and high glass transition temperature, Tg, respectively, and also with starch which is a natural film-forming water soluble binder. The final dry coatings were studied with mercury porosimetry and by scanning electron microscopy in order to characterise their porous structure. The flow and rheological properties of the coating colour formulations were measured in order to probe the particle-particle interaction between the different species in the wet coating colour. The void space of the dry coating layers was modelled using Pore-Cor, a software which generates simulated porous networks. A new algorithm was developed to model, within the simulated void space, the effective particles or "skeletal elements" representative of the solid phase of the dried porous system. The water-filled porous structures at the beginning of the shrinkage process (first critical concentration, FCC) were subsequently modelled by creating Pore-Cor structures with the same solid skeletal elements distribution as at the second critical concentration (at which the particles lock their positions), but with higher given porosity to account for the water present The capillary forces acting on the surface of the simulated coating were calculated, and found to be several orders of magnitude larger than the measured shrinkage forces. The shrinkage process was thus described as resulting from the effect of capillary forces in the plane of the coating layer resisted by a stick-slip process, where the capillary forces yield shrinkage only if a resistance force within the drying coating layer holds the structure in place and allows the menisci to form. The stick-slip theory was strongly supported by quantitative comparisons between the experimental forces required to intrude mercury, and the capillary forces within the simulated void structure.Omya AG, Oftringen, Switzerlan