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Lee, S. M.

Publications and source records attributed to Lee, S. M..

16 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2014

Diffusion-driven and excitation-dependent recombination rate in blue InGaN/GaN quantum well structures

We report on diffusion-driven and excitation-dependent carrier recombination rate in multiple InGaN/GaN quantum wells by using photoluminescence, light-induced absorption, and diffraction techniques. We demonstrate gradually increasing with excitation carrier diffusivity and its correlation with the recombination rate. At low carrier densities, an increase in radiative emission and carrier lifetime was observed due to partial saturation of non-radiative recombination centers. However, at carrier densities above ∼5 × 10{sup 18} cm{sup −3}, a typical value of photoluminescence efficiency droop, a further increase of diffusivity forces the delocalized carriers to face higher number of fast non-radiative recombination centers leading to an increase of non-radiative losses.

36 MATERIALS SCIENCE↗
NASA NTRS2009

Splanchnic Compression Improves the Efficacy of Compression Stockings to Prevent Orthostatic Intolerance

Purpose: Post-spaceflight orthostatic intolerance (OI) is observed in 20-30% of astronauts. Previous data from our laboratory suggests that this is largely a result of decreased venous return. Currently, NASA astronauts wear an anti-gravity suit (AGS) which consists of inflatable air bladders over the calves, thighs and abdomen, typically pressurized from 26 to 78 mmHg. We recently determined that, thigh-high graded compression stockings (JOBST , 55 mmHg at ankle, 6 mmHg at top of thigh) were effective, though to a lesser degree than the AGS. The purpose of this study was to evaluate the addition of splanchnic compression to prevent orthostatic intolerance. Methods: Ten healthy volunteers (6M, 4F) participated in three 80 head-up tilts on separate days while (1) normovolemic (2) hypovolemic w/ breast-high compression stockings (BS)(JOBST(R), 55 mmHg at the ankle, 6 mmHg at top of thigh, 12 mmHg over abdomen) (3) hypovolemic w/o stockings. Hypovolemia was induced by IV infusion of furosemide (0.5 mg/kg) and 48 hrs of a low salt diet to simulate plasma volume loss following space flight. Hypovolemic testing occurred 24 and 48 hrs after furosemide. One-way repeated measures ANOVA, with Bonferroni corrections, was used to test for differences in blood pressure and heart rate responses to head-up tilt, stand times were compared using a Kaplan-Meyer survival analysis. Results: BS were effective in preventing OI and presyncope in hypovolemic test subjects ( p = 0.015). BS prevented the decrease in systolic blood pressure seen during tilt in normovolemia (p < 0.001) and hypovolemia w/o countermeasure (p = 0.005). BS also prevented the decrease in diastolic blood pressure seen during tilt in normovolemia (p = 0.006) and hypovolemia w/o countermeasure (p = 0.041). Hypovolemia w/o countermeasure showed a higher tilt-induced heart rate increase (p = 0.022) than seen in normovolemia; heart rate while wearing BS was not different than normovolemia (p = 0.353). Conclusion: BS may be an effective countermeasure to post-space flight OI. The addition of splanchnic compression is more effective than the previous thigh-high garments. These stockings are readily available, inexpensive, and can be worn for days following landing as astronauts re-adapt to Earth gravity.

Platts, Steven H.↗
NASA NTRS2009

Gender Differences in Baroreflex Sensitivity after Bed Rest

Two potential contributing factors to post-spaceflight orthostatic intolerance are decreases in baroreflex sensitivity (BRS) and sympathetic nervous system response. The purpose of this study was to examine the shape of the BRS curve and sympathetic response to a wide range of blood pressures (BP) before and during 6 head-down bed rest (BR). METHODS: Normal volunteers were tested one day before BR (20M, 1 0F) and near BR days 30 (20M, 10F), 60 (16M, 8F), and 90 (1 0M, 5F). BP was pharmacologically manipulated by 10-min infusions of phenylephrine (PE) and sodium nitroprusside (SNP) at 3 increasing concentrations with a 20-min rest between PE and SNP. Electrocardiogram and continuous finger blood pressure were recorded. A blood sample was drawn at the end of each infusion to measure plasma norepinephrine levels. The spontaneous baroreflex slope (SBS), a measure of BRS, was calculated as the slope of a sequence of 3 or more beats in which the systolic BP (SBP) and following R-R interval (RR) both increased or decreased. The data included saturated responses at the upper but not the lower end of the BP range. Mean response curves were constructed using second-order mixed model analysis. Results are based on term significance in the models. RESULTS RR: RR was lower during BR than pre BR (p<0.001). Pre BR males were modeled by a linear RR response to SBP (p=0.000) while females had a quadratic response which saturated at high SBP (p=0.019). By day 30, both genders were modeled by a linear response; compared to males, females had an attenuated (lower slope) RR response to changes in SBP (p=0.031). SBS: SBS vs SBP analysis showed a lower SBS during BR (p<0.001) when compared to pre BR. Females had a higher SBS than males pre BR (p=0.006). Females exhibited saturating SBS at higher SBP (p=0.016) on day 30, while males were modeled by a linear SBS response to SBP (p=0.035). NE: Females had different NE response to diastolic BP than males pre BR (p=0.035) and on day 30 (p=0.005). CONCLUSION: NE, RR and SBS responses to BP are affected by gender and BR. Not only do gender and BR baseline differences exist, but gender and BR also influence the slope and saturation of the BRS curves. Attenuated and saturating RR and SBS responses, as well as differences in baseline values, may contribute to the higher rates of orthostatic intolerance in women and after bed rest.

Arzeno, Natalia M.↗
NASA NTRS2001

Maximal exercise as a countermeasure to orthostatic intolerance after spaceflight

Previous investigators have suggested that maximal exercise performed 24 h before the end of bed rest, a spaceflight analog, restores prebed rest plasma volume, baroreflex responses, and orthostatic tolerance. PURPOSE: In this case report, we examined the effect of a similar exercise protocol 24 h before a Shuttle landing on the orthostatic responses of four crewmembers (EX) after spaceflights of 8-14 d. Four additional crewmembers (CON) served as controls and did not perform exercise during the final day of the flight. METHODS: Each crewmember performed a 10-min stand test approximately 10 d before launch (L-10) and within 1-2 h of landing (R+0). Cardiac stroke volume was measured (Doppler ultrasound) supine and during each min of standing for three EX and three CON subjects. RESULTS: Preflight, all crewmembers completed the stand test and each group had similar heart rate and blood pressure responses. Postflight, all subjects also completed the 10-min stand test. Each group had similarly elevated supine and standing heart rates, elevated diastolic and mean arterial blood pressures, and reduced pulse pressures compared to L-10. However, postflight cardiac output, mean +/- SEM, (EX: 4.5+/-0.6 L x min(-1); CON: 3.1+/-0.3 L x min(-1)) and stroke volume (EX: 43+/-7 mL x beat; CON: 30+/-6 mL x beat) were higher after 10 min standing in the EX subjects compared to CON subjects. CONCLUSIONS: For these four crewmembers, maximal exercise performed 24 h before landing may have helped maintain stroke volume but did not maintain heart rate and blood pressure responses during standing compared to preflight.

Flight Experiment↗
NASA NTRS1999

Inflight exercise affects stand test responses after space flight

PURPOSE: The purpose of this study was to determine whether exercise performed by Space Shuttle crew members during short-duration space flights (9-16 d) affects the heart rate (HR) and blood pressure (BP) responses to standing within 2-4 h of landing. METHODS: Thirty crew members performed self-selected inflight exercise and maintained exercise logs to monitor their exercise intensity and duration. Two subjects participated in this investigation during two different flights. A 10-min stand test, preceded by at least 6 min of quiet supine rest, was completed 10-15 d before launch (PRE) and within 4 h of landing (POST). Based upon their inflight exercise records, subjects were grouped as either high (HIex: > or = 3 times/week, HR > or = 70% HRmax, > or = 20 min/session, N = 11), medium (MEDex: > or = 3 times/week, HR < 70% HRmax, > or = 20 min/session, N = 10), or low (LOex: < or = 3 times/week, HR and duration variable, N = 11) exercisers. HR and BP responses to standing were compared between groups (ANOVA, P < or = 0.05). RESULTS: There were no PRE differences between the groups in supine or standing HR and BP. Although POST supine HR was similar to PRE, all groups had an increased standing HR compared with PRE. The increase in HR upon standing was significantly greater after flight in the LOex group (36 +/- 5 bpm) compared with HIex or MEDex groups (25 +/- 1 bpm; 22 +/- 2 bpm). Similarly, the decrease in pulse pressure (PP) from supine to standing was unchanged after space flight in the MEDex and HIex groups but was significantly greater in the LOex group (PRE: -9 +/- 3; POST: -19 +/- 4 mm Hg). CONCLUSIONS: Thus, moderate to high levels of inflight exercise attenuated HR and PP responses to standing after space flight.

manned↗
NASA NTRS1999

Space Physiology Studies

Calculations suggest that exercise in space to date has lacked sufficient loads to maintain musculoskeletal mass. Lower body negative pressure (LBNP) produces a force at the feet equal to the product of the LBNP and body cross-sectional area at the waist. Supine exercise within 50-60 mm Hg LBNP improves tolerance to LBNP and produces forces similar to those occurring during upright posture on Earth. Thus, exercise within LBNP may help prevent deconditioning of astronauts by stressing tissues of the lower body in a manner similar to gravity and also, may provide a safe and effective alternative to centrifugation in terms of cost, mass, volume, and power usage. We hypothesize that supine treadmill exercise during LBNP at one body weight (50-60 mm Hg LBNP) will provide cardiovascular and musculoskeletal loads similar to those experienced while upright in lg. Also, daily supine treadmill running in a LBNP chamber will maintain aerobic fitness, orthostatic tolerance, and musculoskeletal structure and function during bed rest (simulated microgravity).

Hargens, A. R.↗
NASA NTRS1997

Upright exercise or supine lower body negative pressure exercise maintains exercise responses after bed rest

Adaptation to bed rest or space flight is accompanied by an impaired ability to exercise in an upright position. We hypothesized that a daily, 30-min bout of intense, interval exercise in upright posture or supine against lower body negative pressure (LBNP) would maintain upright exercise heart rate and respiratory responses after bed rest. Twenty-four men (31 +/- 3 yr) underwent 5 d of 6 degree head-down tilt: eight performed no exercise (CON), eight performed upright treadmill exercise (UPex), and eight performed supine treadmill exercise against LBNP at -51.3 +/- 0.4 mm Hg (LBNPex). Submaximal treadmill exercise responses (56, 74, and 85% of VO2peak) were measured pre- and post-bed rest. In CON, submaximal heart rate, respiratory exchange ratio, and ventilation were significantly greater (P < or = 0.05) after bed rest. In UPex and LBNPex, submaximal exercise responses were similar pre- and post-bed rest. Our results indicate that a daily 30-min bout of intense, interval upright exercise training or supine exercise training against LBNP is sufficient to maintain upright exercise responses after 5 d of bed rest. These results may have important implications for the development of exercise countermeasures during space flight.

NASA Center ARC↗
NASA NTRS1997

Validity of a heart rate monitor during work in the laboratory and on the Space Shuttle

Accurate heart rate measurement during work is required for many industrial hygiene and ergonomics situations. The purpose of this investigation was to determine the validity of heart rate measurements obtained by a simple, lightweight, commercially available wrist-worn heart rate monitor (HRM) during work (cycle exercise) sessions conducted in the laboratory and also during the particularly challenging work environment of space flight. Three different comparisons were made. The first compared HRM data to simultaneous electrocardiogram (ECG) recordings of varying heart rates that were generated by an ECG simulator. The second compared HRM data to ECG recordings collected during work sessions of 14 subjects in the laboratory. Finally, ECG downlink and HRM data were compared in four astronauts who performed cycle exercise during space flight. The data were analyzed using regression techniques. The results were that the HRM recorded virtually identical heart rates compared with ECG recordings for the data set generated by an ECG simulator. The regression equation for the relationship between ECG versus HRM heart rate data during work in the laboratory was: ECG HR = 0.99 x (HRM) + 0.82 (r2 = 0.99). Finally, the agreement between ECG downlink data and HRM data during space flight was also very high, with the regression equation being: Downlink ECG HR = 1.05 x (HRM) -5.71 (r2 = 0.99). The results of this study indicate that the HRM provides accurate data and may be used to reliably obtain valid data regarding heart rate responses during work.

STS Shuttle Project↗
NASA NTRS1997

Resistance exercise training and the orthostatic response

Resistance exercise has been suggested to increase blood volume, increase the sensitivity of the carotid baroreceptor cardiac reflex response (BARO), and decrease leg compliance, all factors that are expected to improve orthostatic tolerance. To further test these hypotheses, cardiovascular responses to standing and to pre-syncopal limited lower body negative pressure (LBNP) were measured in two groups of sedentary men before and after a 12-week period of either exercise (n = 10) or no exercise (control, n = 9). Resistance exercise training consisted of nine isotonic exercises, four sets of each, 3 days per week, stressing all major muscle groups. After exercise training, leg muscle volumes increased (P < 0.05) by 4-14%, lean body mass increased (P = 0.00) by 2.0 (0.5) kg, leg compliance and BARO were not significantly altered, and the maximal LBNP tolerated without pre-syncope was not significantly different. Supine resting heart rate was reduced (P = 0.03) without attenuating the heart rate or blood pressure responses during the stand test or LBNP. Also, blood volume (125I and 51Cr) and red cell mass were increased (P < 0.02) by 2.8% and 3.9%, respectively. These findings indicate that intense resistance exercise increases blood volume but does not consistently improve orthostatic tolerance.

NASA Discipline Musculoskeletal↗
NASA NTRS1979

Lunar building materials: Some considerations on the use of inorganic polymers

The use of inorganic polymer systems synthesized from the available lunar chemical elements, viz., silicon, aluminum, and oxygen to make adhesives, binders, and sealants needed in the fabrication of lunar building materials and the assembly of structures is considered. Inorganic polymer systems, their background, status, and shortcomings, and the use of network polymers as a possible approach to synthesis are examined as well as glassy metals for unusual structural strength, and the use of cold-mold materials as well as foam-sintered lunar silicates for lightweight shielding and structural building materials.

Lee, S. M.↗
NASA NTRS1978

Numerical solutions and laser-Doppler measurements of spin-up

The spin-up flow in a cylinder of homogeneous fluid has been examined both experimentally and numerically. A series of laser-Doppler measurements was made of the zonal flow over a range of Ekman numbers and Rossby numbers at various locations in the interior of the flow. These measurements exceed previous ones in accuracy. The weak inertial modes excited by the impulsive start are detectable. The numerical simulations used the primitive equations in axisymmetric form and employed finite-difference techniques on both constant and variable grids. The number of grid points necessary to resolve the Ekman layers was determined. A thorough comparison of the simulations and the experimental measurements is made which includes the details of the amplitude and frequency of the inertial modes. Agreement to within the experimental tolerance is achieved. Analytical results for conditions identical to those in the experiments are not available but some similar linear and nonlinear theories are also compared with the experiments.

Warn-Varnas, A.↗
NASA NTRS1972

Polymeric coatings using electronic excitation

Process has been developed for glow discharge polymerization which is accomplished in inert atmosphere by using vacuum chamber. Polymeric coating, in this type of environment, produces high molecular weight coating polymers that have good stability and are resistant to abrasions and solvents.

Lee, S. M.↗