Dependency theory | EPFL Graph Search

Dependency theory is the idea that resources flow from a "periphery" of poor and underdeveloped states to a "core" of wealthy states, enriching the latter at the expense of the former. A central contention of dependency theory is that poor states are impoverished and rich ones enriched by the way poor states are integrated into the "world system". This theory was officially developed in the late 1960s following World War II, as scholars searched for the root issue in the lack of development in Latin America. The theory arose as a reaction to modernization theory, an earlier theory of development which held that all societies progress through similar stages of development, that today's underdeveloped areas are thus in a similar situation to that of today's developed areas at some time in the past, and that, therefore, the task of helping the underdeveloped areas out of poverty is to accelerate them along this supposed common path of development, by various means such as investment, tec

Deformation behavior of nanoporous polycrystalline silver layers sintered by low temperature bonding: experiment and simulation

Saba Zabihzadeh

Integrated circuit packaging technology has become a prime design consideration for the develop-ment of electronic system concepts. One key issue is the bonding layer between chip and substrate. Currently, high-lead solder materials are being used, which apart from their environmental problem, limit the reliable operation temperature of devices to temperatures lower than 175ºC. Low-temperature bonding by sintering nanograin sized silver is emerging as a promising replacement in power electronics modules working at high temperatures. Although silver has been studied as bonding material since three decades, it has not been widely used in the packaging industry which might be ascribed to the limited knowledge on its mechanical behavior. In this work, the microstructure of thin layers of nanoporous silver sintered under different conditions are studied and linked to the mechanical behavior (at room temperature) using X-ray diffraction in-situ deformation tests and finite element simulations. First the microstructure of several samples such as grain size distribution, defect structures, porosity, pore morphology and ligament size distribution are investigated and are related to the corresponding sintering conditions. Electron microscopy, high resolution X-ray imaging, and X-ray diffraction techniques are the employed characterization methods. We found a strong dependency of the microstructure on the sintering conditions. Afterward, series of X-ray diffraction in-situ continuous, load-unload and stress relaxation tensile tests are performed to study the deformation mechanisms at room temperature. All samples exhibit small elongation and a large hysteresis in the stress-strain curve. From the change in the peak position upon unloading the residual elastic strain inside the sample is calculated while the evolution of the peak broadening provides information on the recovery mechanisms and induced dislocations. A significant recovery in the peak position and peak broadening after unloading and during relaxation is observed for all specimens. This is attributed to the presence of the pores. We found that the density of generated dislocations during tensile deformation depends on the porous structure. The results suggest that there exist two contributions to deformation: one related to the pores and other to intra-granular dislocations. The competition between both mechanisms is discussed in terms of porosity and grain size. Finally, 3D FE microstructure-based simulations are performed to allow a better correlation between the porous morphology and mechanical behavior. The 3D reconstructed images of nanoporous silver obtained from X-ray ptychography are used as an input to create finite element meshes of the actual porous morphology. For homogenization purpose, a representative volume element approach is chosen. The apparent elastoplastic response of the bulk silver is extracted from the porous samples using inverse homogenization method. The simulations allow distinguishing between the interplay and role of the ligament size, pore morphology and porosity, and provide a better comprehension on the experimental observations. We show that the proposed model has a predictive character for general nanoporous silver microstructures.

EPFL2016

Engineering Dynamic Hydrogels For In Vitro Organoid Culture

Delphine Lorraine Perle Blondel

Over the past decade, methods to culture stem cells in three dimensions have opened up a plethora of new opportunities for basic and translational research in the life sciences. In particular, the use of natural extracellular matrix (ECM) surrogates, such as mouse tumour-derived Matrigel, unleashed the possibility to recapitulate complex multicellular behaviours in vitro, culminating in the successful derivation of miniaturized organs, termed organoids. While organoids hold tremendous potential as model systems in basic biology, the translational potential of these systems is hampered by their strict dependency on animal-derived matrices that suffer from batch-to-batch variability, potential immunogenicity, and ethical concerns. Recent efforts in engineering covalently crosslinked synthetic hydrogels have attempted to substitute these ill-defined organoid culture systems. However, although these bio-artificial matrices have shown significant potential for 3D organoid culture, their stability and their inherent elastic nature hamper organoid development. Indeed, the native ECM, just like Matrigel, is a highly viscoelastic and dynamic 3D milieu that can relax in response to tissue-induced stress by breaking and subsequently rearranging its network, thus permitting cellular remodelling without compromising the macroscopic stability of the material over time. Therefore, there is a need to develop the next generation of synthetic organoid culture matrices, exhibiting in vivo-like stress-relaxation properties. This thesis provides a perspective on how chemically defined, bio-inspired hydrogels can be designed as potential replacements for native ECM-based matrices for 3D organoid culture. In contrast to traditional synthetic hydrogels that cannot accommodate the dynamic mechanical changes occurring during organoid development, four types of synthetic matrices were developed that are crosslinked, either fully or partially, through dynamic physical bonds. The introduction of such network dynamics is shown to facilitate key morphogenetic processes, such as tissue budding, that are normally absent in purely elastic networks. A unifying hypothesis emerging from these results is that stress relaxation is a crucial requirement for 3D organoid culture. The mechanisms by which stress relaxation influence ISC fate and organoid development remain to be further studied. Preliminary experiments indicated that the stem cells may sense and respond to these characteristics through differential activation of mechanosensing pathways including the transcriptional co-activator YAP. The dynamic hydrogels developed in this thesis hold great promise as tunable environments for stem cell-based self-organization, enabling morphogenetic processes that may be suppressed in conventional synthetic hydrogels systems predominantly used in 3D cell culture applications.

EPFL2019

Feed-forwards meet recurrent networks in vehicle trajectory prediction

Alexandre Massoud Alahi, Mohammadhossein Bahari

For an autonomous car, getting surprised is the worst thing that can happen. To prevent that, plenty of studies are trying to forecast traffic participants’ actions especially in an urban scene using recurrent networks. Recurrent networks are used for temporal tasks in many application domains like Natural Language processing and computer vision. Despite the tendency in the literature to use recurrent neural networks for trajectory prediction, we argue that because of small dependency in trajectory sequences of a vehicle, a feed-forward neural network can be used, instead. In this paper, we will compare these two methods in vehicle trajectory prediction while considering the vanilla models or taking the scene into account. In order to have more variations in trajectories, roundabouts are used as a case study. Our results show that the proposed feed-forward network has competitive results with a recurrent network with 6 times faster processing time.