RESUMEN
The material quality of III-nitrides is severely limited by the lack of cost-effective substrates with suitable lattice and thermal expansion coefficients. A suspended ultrathin silicon membrane substrate ([Formula: see text]16 nm), fabricated by an easy process on SOI substrates, is thus designed for nitride epitaxial growth, which can effectively release the strain in the epi-layers, and has demonstrated large-area (Al)GaN growth with a smooth surface and greatly reduced defect density. This research provides a promising CMOS-compatible method for growing cost-effectively high-quality III-nitrides that can be used for the development of high-performance devices.
RESUMEN
In this review paper, several new approaches about the 3C-SiC growth are been presented. In fact, despite the long research activity on 3C-SiC, no devices with good electrical characteristics have been obtained due to the high defect density and high level of stress. To overcome these problems, two different approaches have been used in the last years. From one side, several compliance substrates have been used to try to reduce both the defects and stress, while from another side, the first bulk growth has been performed to try to improve the quality of this material with respect to the heteroepitaxial one. From all these studies, a new understanding of the material defects has been obtained, as well as regarding all the interactions between defects and several growth parameters. This new knowledge will be the basis to solve the main issue of the 3C-SiC growth and reach the goal to obtain a material with low defects and low stress that would allow for realizing devices with extremely interesting characteristics.
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Interspecies differences in locomotor efficiency have been extensively researched, but within-species variation in the metabolic cost of walking and its underlying causes have received much less attention. This is somewhat surprising given the importance of walking energetics to natural selection, and the fact that the mechanical efficiency of striding bipedalism in modern humans is thought to be related in some part to the unique morphology of the human foot. Previous studies of human running have linked specific anatomical traits in the foot to variations in locomotor energetics to provide insight into form-function relationships in human evolution. However, such studies are relatively rare, particularly for walking. In this study, relationships between a range of functional musculoskeletal traits in the human lower limb and the energetics of walking over compliant and noncompliant substrates are examined, with particular focus on the lower limb and foot. Twenty-nine young, healthy individuals walked across three surfaces-a noncompliant laboratory floor, and compliant 6 cm and 13 cm thick foams-at self-selected speeds while oxygen consumption was measured, from which the metabolic cost of transport was calculated. Lower limb lengths, calcaneus lengths, foot shape indices, and maximum isometric plantarflexion torques were also measured and subsequently tested for relationships with metabolic cost over these surfaces using linear regression. It was found that metabolic cost varied considerably between individuals within and across substrate types, but this variation was not statistically related to or explained by variations in musculoskeletal parameters considered to be adaptively important to efficient bipedal locomotion. This therefore provides no supportive evidence that variations in these gross anatomical parameters confer significant advantages to the efficiency of walking, and therefore suggest caution in the use of similar metrics to infer differences in walking energetics in closely related fossil species.
Asunto(s)
Metabolismo Energético , Pie/anatomía & histología , Caminata , Evolución Biológica , Fenómenos Biomecánicos , Marcha , Humanos , Locomoción , CarreraRESUMEN
Single-crystalline semiconductor nanomembranes (NMs) bonded to compliant substrates are increasingly used for biomedical research and in health care. Nevertheless, there is a limited understanding of how individual cells sense the unique mechanical properties of these substrates and adjust their behavior in response to them. In this work, we performed proliferation assays, cytoskeleton analysis, and focal adhesion (FA) studies for NIH-3T3 fibroblasts on 220 and 20 nm single-crystalline Si on polydimethylsiloxane (PDMS) substrates with an elastic modulus of â¼31 kPa. We also characterized cell response on bulk Si as a reference. Our in vitro studies show that varying the thickness of the NM between 20 and 220 nm affects the proliferation rate of the cells, their cytoskeleton, fiber organization, spread area, and degree of FA. For example, cultured cells on 220 nm Si/PMDS exhibit the same response as on bulk Si, that is, they are well-spread with a pentagonal (or dendritic) shape and show a good organization of stress fibers and FAs. On the other hand, the cells on 20 nm Si/PDMS are spherical, with fiber organization and FAs in undetectable levels. We explained the results of our in vitro studies through a shear-lag mechanical model. The calculated FA-substrate contact stiffnesses for fibroblasts on bulk Si and 220 nm Si/PDMS closely match, and they are significantly higher than the stiffness of the integrin clutches and the plaque. Conversely, focal contacts with 20 nm Si/PDMS have comparable lateral compliance to adhesion-mediating intracellular organisms. In conclusion, our work relies on recent advances in NM technology to fill a critical knowledge gap about how individual cells sense and react to the mechanical properties of NM-based substrates. Our findings will have a major impact on the design of flexible electronic materials for applications in biomedical science and health care.