THz generation by optical rectification and competition with other nonlinear processes

We present a study of the competition between THz generation by optical rectification in ZnTe crystals with two-photon absorption and second harmonic generation. The incident pump field for optical rectification is shown to be depleted by two-photon absorption and the generated THz field is shown to be significantly absorbed upon tight focusing by free-carrier absorption

Rebreathing-Induced Hypoxia Improves Insulin Sensitivity in Adults with Type 2 Diabetes

Hypoxia stimulates glucose uptake through an insulin-independent pathway. PURPOSE: To examine the acute effect of rebreathing-induced hypoxia on plasma glucose and insulin levels in adults with type 2 diabetes. We hypothesized that rebreathing-induced hypoxia would attenuate the increase in glucose and insulin levels during an oral glucose tolerance test. METHODS: Nine individuals with type 2 diabetes (4 women, age: 53±10 years, body mass index: 35±7 kg/m2, HbA1c: 7.1±0.5%) visited the laboratory on two occasions. On both visits, a 2-hour, 75 g oral glucose tolerance test was conducted while simultaneously performing a rebreathing-induced hypoxia protocol (RIH) or breathing room air (Norm). Venous blood samples were collected 0, 30, 60, 90, and 120 min following ingestion of the high-glucose drink to measure plasma glucose and insulin levels. The rebreathing-induced hypoxia protocol consisted of two series of five 2-min rebreathing bouts in a low-volume, closed circuit system interspersed with two minutes of breathing room air. The first and second series of rebreathing bouts were performed within the first 30 min and 30-60 min after ingestion of the high-glucose drink, respectively. RESULTS: Rebreathing-induced hypoxia resulted in a nadir oxygen saturation of 88±4% and a nadir fraction of inspired oxygen of 12±5%. Plasma glucose responses to the oral glucose tolerance test were not different between conditions, however, insulin levels were lower during rebreathing-induced hypoxia than normoxia (RIH vs. Norm: 0: 17±13 vs. 21±14; 30: 37±28 vs. 42±26; 60: 57±32 vs. 77±52; 90: 88±67 vs. 116±81; and 120: 114±96 vs. 136 ±111 ulU/ml, p=0.03), suggesting an acutely improved insulin sensitivity. Accordingly, the insulin area under the curve was lower during rebreathing-induced hypoxia than normoxia (124±84 vs. 157±102, p=0.02). CONCLUSION: Exposure to short and intermittent rebreathing-induced hypoxia following ingestion of a high-glucose drink acutely improves insulin sensitivity in adults with type 2 diabetes. Rebreathing-induced hypoxia could therefore represent a novel and simple strategy to improve glycemic control in individuals living with type 2 diabetes

Feedback Control for Online Training of Neural Networks

International audienc

On the Elastic Stability of Folded Rings in Circular and Straight States

Single-loop elastic rings can be folded into multi-loop equilibrium configurations. In this paper, the stability of several such multi-loop states which are either circular or straight are investigated analytically and illustrated by experimental demonstrations. The analysis ascertains stability by exploring variations of the elastic energy of the rings for admissible deformations in the vicinity of the equilibrium state. The approach employed is the conventional stability analysis for elastic conservative systems which differs from most of the analyses that have been published on this class of problems, as will be illustrated by reproducing and elaborating on several problems in the literature. In addition to providing solutions to two basic problems, the paper analyses and demonstrates the stability of six-sided rings that fold into straight configurations

Impact of Intermittent Hypoxia on the Glycemic Response to an Oral Glucose Tolerance Test

Hypoxia triggers glucose uptake independently from the action of insulin. PURPOSE: To determine the acute effect of intermittent hypoxia, defined as alternating short bouts of breathing hypoxic and room air, on plasma glucose levels during an oral glucose tolerance test in healthy individuals. We hypothesized that exposure to intermittent hypoxia would attenuate the increase in glucose levels in response to an oral glucose tolerance test. METHODS: Nine individuals (5 men, age: 24 ± 4 years, height: 175 ± 9 cm, weight: 71.0 ± 13.5 kg, HbA1c: 5.4 ± 0.1%) participated in the study. Participants visited the laboratory on two occasions. On both visits, a 2-hour oral glucose tolerance test was performed, with venous blood samples collected 0, 30, 60, 90 and 120 minutes following the ingestion of a 75 g glucose drink. On visit 1, an intermittent hypoxia (IH) protocol, consisting of eight 4-minute hypoxic cycles at a targeted arterial oxygen saturation of 80% interspersed with breathing room air to resaturation, was performed following ingestion of the glucose drink. On visit 2, an intermittent normoxia protocol consisting of eight 4-minute normoxic cycles interspersed with breathing room air was performed following ingestion of the glucose drink. Visit order was randomized and participants were blinded to the condition. RESULTS: As expected, intermittent hypoxia resulted in a lower arterial oxygen saturation than intermittent normoxia (IH: 83 ± 3, IN: 98 ± 1%, p \u3c 0.01) which corresponded to lower levels of inspired oxygen (IH: 10.9 ± 0.7, IN: 20.9 ± 0.3%, p \u3c 0.01). Plasma glucose responses to the oral glucose tolerance test were not different between conditions (IH vs. IN: 0: 90 ± 7 vs. 89 ± 6; 30: 135 ± 21 vs. 137 ± 24; 60: 110 ± 28 vs. 108 ± 25; 90: 96 ± 18 vs. 88 ± 14; and 120: 101 ± 19 vs. 83 ± 14 mg/dl, p = 0.29). Intermittent hypoxia triggered an increase in cardiac output (6.1 ± 0.9 to 6.8 ± 1.3 L/min, p \u3c 0.01) caused by an increase in heart rate (67 ± 10 to 79 ± 12 bpm, p \u3c 0.01). CONCLUSION: Contrary to our hypothesis, intermittent exposure to hypoxia did not attenuate the increase in plasma glucose levels during an oral glucose tolerance test in individuals with normal glycemic control. It remains to be determined whether intermittent hypoxia can attenuate the increase in plasma glucose levels in response to an oral glucose tolerance test in individuals with impaired glucose tolerance

Impaired Erythropoietin Response to a Single Session of Intermittent Hypoxia in Patients with Type 2 Diabetes

Patients with type 2 diabetes (T2D) exhibit, on average, a 20% decline in maximal oxygen consumption when compared to healthy adults. Hemoglobin mass strongly correlates to maximal oxygen consumption. A reduced total blood volume has been observed in patients with T2D, suggesting that a reduced hemoglobin mass contributes to the decreased maximal oxygen consumption in this population. Hypoxia stimulates the release of erythropoietin (EPO), the hormone regulating red blood cell production. We previously showed that intermittent hypoxia, consisting of alternating short bouts of breathing hypoxic and normoxic air, increases EPO levels. PURPOSE: To determine the effect of a single session of intermittent hypoxia on serum EPO levels and hemoglobin mass in patients with T2D. We hypothesized that a single session of intermittent hypoxia would raise serum EPO levels and lead to an increase in hemoglobin mass in patients with T2D. METHODS: Ten patients with T2D (4 women, age: 53 ± 10 years, body mass index: 36.2 ± 8.5 kg/m2, HbA1c: 7.2 ± 1.2%) were exposed to an intermittent hypoxia protocol consisting of eight 4-min cycles at a targeted oxygen saturation of 80% interspersed with normoxic cycles to resaturation. Air was made hypoxic by titrating nitrogen into a breathing circuit. Pulmonary gas exchange, oxygen saturation, and hemodynamics were continuously measured throughout the protocol. EPO levels were measured before and 4.5 hours after the beginning of the protocol. Hemoglobin mass was assessed via carbon monoxide rebreathing before and seven days following intermittent hypoxia. RESULTS: Intermittent hypoxia lowered oxygen saturation (­­97 ±­ 2 to 81 ± 2%, p\u3c0.01), which resulted from a lower fraction of inspired oxygen (20.8 ±­ 0.1 to 11.1 ± 1.0%, p\u3c0.01). There was no significant change in EPO levels following exposure to intermittent hypoxia (11.9 ± 5.3­ to 12.1 ± 4.3 mU/ml, p=0.83). There was also no change in hemoglobin mass in response to intermittent hypoxia (864 ± 152­ to 850 ± 150 g, p=0.64). Intermittent hypoxia did not affect mean arterial pressure (94 ± 5 to 97 ± 7 mmHg, p=0.18) but increased cardiac output (9.1 ± 2.7 to 9.8 ± 2.8 L/min, p=0.03) due to an increase in heart rate (78 ± 9 to 84 ± 10 bpm, p\u3c0.01). CONCLUSION: A single session of intermittent hypoxia did not increase serum EPO levels or hemoglobin mass in patients with T2D. These findings suggest an impaired EPO response to decreased oxygen levels in patients with T2D, which may contribute to the reduced hemoglobin mass and total blood volume observed in this population

Understanding Arctic Surface Temperature Differences in Reanalyses

Reanalyses in the Arctic are widely used for model evaluation and for understanding contemporary climate change. Nevertheless, differences among reanalyses in fundamental meteorological variables including surface air temperature are large. A review of surface temperature differences is presented with a particular focus on differences in contemporary reanalyses. An important consideration is the significant differences in Arctic surfaces, including the central Arctic Ocean, the Greenland Ice Sheet, and non-glaciated land. While there is significant correlation among reanalyses in annual time series, there is substantial disagreement in mean values. For the period 1980-2013, the trend in annual temperature ranges from 0.3 to 0.7K per decade. Over the central Arctic Ocean, differences in mean values and trends are larger. Most of the uncertainty is associated with winter months. This is likely associated with the constraint imposed by melting processes (i.e. 0 deg. Celsius), rather than seasonal changes to the observing system

Multiple equilibrium states of a curved-sided hexagram:Part I-Stability of states

The stability of the multiple equilibrium states of a hexagram ring with six curved sides is investigated. Each of the six segments is a rod having the same length and uniform natural curvature. These rods are bent uniformly in the plane of the hexagram into equal arcs of 120deg or 240deg and joined at a cusp where their ends meet to form a 1-loop planar ring. The 1-loop rings formed from 120deg or 240deg arcs are inversions of one another and they, in turn, can be folded into a 3-loop straight line configuration or a 3-loop ring with each loop in an "8" shape. Each of these four equilibrium states has a uniform bending moment. Two additional intriguing planar shapes, 6-circle hexagrams, with equilibrium states that are also uniform bending, are identified and analyzed for stability. Stability is lost when the natural curvature falls outside the upper and lower limits in the form of a bifurcation mode involving coupled out-of-plane deflection and torsion of the rod segments. Conditions for stability, or lack thereof, depend on the geometry of the rod cross-section as well as its natural curvature. Rods with circular and rectangular cross-sections will be analyzed using a specialized form of Kirchhoff rod theory, and properties will be detailed such that all four of the states of interest are mutually stable. Experimental demonstrations of the various states and their stability are presented. Part II presents numerical simulations of transitions between states using both rod theory and a three-dimensional finite element formulation, includes confirmation of the stability limits established in Part I, and presents additional experimental demonstrations and verifications

Multiple equilibrium states of a curved-sided hexagram: Part II-Transitions between states

Curved-sided hexagrams with multiple equilibrium states have great potential in engineering applications such as foldable architectures, deployable aerospace structures, and shape-morphing soft robots. In Part I, the classical stability criterion based on energy variation was used to study the elastic stability of the curved-sided hexagram and identify the natural curvature range for stability of each state for circular and rectangular rod cross-sections. Here, we combine a multi-segment Kirchhoff rod model, finite element simulations, and experiments to investigate the transitions between four basic equilibrium states of the curved-sided hexagram. The four equilibrium states, namely the star hexagram, the daisy hexagram, the 3-loop line, and the 3-loop "8", carry uniform bending moments in their initial states, and the magnitudes of these moments depend on the natural curvatures and their initial curvatures. Transitions between these equilibrium states are triggered by applying bending loads at their corners or edges. It is found that transitions between the stable equilibrium states of the curved-sided hexagram are influenced by both the natural curvature and the loading position. Within a specific natural curvature range, the star hexagram, the daisy hexagram, and the 3-loop "8" can transform among one another by bending at different positions. Based on these findings, we identify the natural curvature range and loading conditions to achieve transition among these three equilibrium states plus a folded 3-loop line state for one specific ring having a rectangular cross-section. The results obtained in this part also validate the elastic stability range of the four equilibrium states of the curved-sided hexagram in Part I. We envision that the present work could provide a new perspective for the design of multi-functional deployable and foldable structures

Evaluation of the Surface Representation of the Greenland Ice Sheet in a General Circulation Model

Simulated surface conditions of the Goddard Earth Observing System model, version 5 (GEOS 5) atmospheric general circulation model (AGCM) are examined for the contemporary Greenland Ice Sheet (GrIS). A surface parameterization that explicitly models surface processes including snow compaction, meltwater percolation and refreezing, and surface albedo is found to remedy an erroneous deficit in the annual net surface energy flux and provide an adequate representation of surface mass balance (SMB) in an evaluation using simulations at two spatial resolutions. The simulated 1980-2008 GrIS SMB average is 24.7+/-4.5 cm yr(- 1) water-equivalent (w.e.) at.5 degree model grid spacing, and 18.2+/-3.3 cm yr(- 1) w.e. for 2 degree grid spacing. The spatial variability and seasonal cycle of the simulation compare favorably to recent studies using regional climate models, while results from 2 degree integrations reproduce the primary features of the SMB field. In comparison to historical glaciological observations, the coarser resolution model overestimates accumulation in the southern areas of the GrIS, while the overall SMB is underestimated. These changes relate to the sensitivity of accumulation and melt to the resolution of topography. The GEOS-5 SMB fields contrast with available corresponding atmospheric models simulations from the Coupled Model Intercomparison Project (CMIP5). It is found that only a few of the CMIP5 AGCMs examined provide significant summertime runoff, a dominant feature of the GrIS seasonal cycle. This is a condition that will need to be remedied if potential contributions to future eustatic change from polar ice sheets are to be examined with GCMs