Hyunmin Choi - Academia.edu (original) (raw)

Papers by Hyunmin Choi

2009 IEEE 9th International Conference on the Properties and Applications of Dielectric Materials, 2009

In the devices structure of ITO/N,N'-diph enyl-N,N'bis(3-methylphenyl)-1,1'-biphenyl-... more In the devices structure of ITO/N,N'-diph enyl-N,N'bis(3-methylphenyl)-1,1'-biphenyl-4,4'-diam ine(TPD)/tris(8-hydroxyquinoline)aluminum (Alq3) electron-transport-layer(ETL) (2,9-Dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP))/Al, we have studied the efficiency improvement of organic light-emitting diodes depending on the thickness variation of BCP using electron transport layer. The thickness of TPD and Alq3 was manufactured 40nm, 60nm under a base pressure of 5x10−6Torr using at thermal evaporation, respectively. The TPD and Alq3 layer were evaporated to be deposition rate of 2.5Å/s. And the BCP was evaporated to be at a deposition of 1.0Å/s. As the experimental results, we found that the luminous efficiency and the external quantum efficiency of the device is superior to others when thickness of BCP is 5 nm. Also, operating voltage is lowest. Compared to the ones from the devices without BCP layer, the luminous efficiency and the external quantum efficiency were improved by a factor of four hundred ninety and five hundred, respectively. And operating voltage is reduced to about 2 V.

Materials, 2021

The aim of this study was to investigate the behavior of dental-derived human mesenchymal stem ce... more The aim of this study was to investigate the behavior of dental-derived human mesenchymal stem cells (d-hMSCs) in response to differently surface-treated implants and to evaluate the effect of d-hMSCs on local osteogenesis around an implant in vivo. d-hMSCs derived from alveolar bone were established and cultured on machined, sandblasted and acid-etched (SLA)-treated titanium discs with and without osteogenic induction medium. Their morphological and osteogenic potential was assessed by scanning electron microscopy (SEM) and real-time polymerase chain reaction (RT-PCR) via mixing of 5 × 106 of d-hMSCs with 1 mL of Metrigel and 20 μL of gel-cell mixture, which was dispensed into the defect followed by the placement of customized mini-implants (machined, SLA-treated implants) in New Zealand white rabbits. Following healing periods of 2 weeks and 12 weeks, the obtained samples in each group were analyzed radiographically, histomorphometrically and immunohistochemically. The quantitativ...

Sleep, 2018

relationships of the powers of different EEG frequencies in a single understandable index. The sc... more relationships of the powers of different EEG frequencies in a single understandable index. The scale ranges from 0-2.5 with values of zero only occurring during sleep and a value of 2.5 only occurring during wakefulness. A value of 1.25 predicts an equal probability of being awake or asleep. The current investigation is undertaken to obtain normal values of non-REM ORP in good sleepers in their home environments and to establish the repeatability of this measure from night to night. Methods: Good sleepers (GS) were recruited and defined as nonshift workers sleeping 7-9 hours per night who considered their sleep of good quality and didn't have daytime symptoms attributed to their sleep. Acceptable participants couldn't be habitual nappers, have chronic health or sleep diagnoses, use psychotropic or recreational drugs, or have reached the menopause. ESS, ISI and FOSQ 10 had to be in the normal range. OSA was excluded based on an HSAT measuring EEG (Prodigy). A second EEG study was performed one week later. Results: Thirty-three people completed the study (9 (20-35) females, 8 (20-35) males, 8 (35-50) males and 8 (35-50) females. The mean ORP for non-REM sleep during both nights of the study was 0.54 ± 0.17 (66 nights). Excluding three outlier nights, results in a tighter distribution of 0.51 ± 0.13. Distribution fitting suggests a normal distribution (Kolmogorov-Smirnov test, p=0.254). No difference was noted in non-REM ORP between first and second nights (0.55 ± 0.17 vs 0.53 ± 0.16). The study was powered to find a difference of 0.08 units between first and second nights with an α error (0.05) and β error (0.2). Conclusion: GS normal values for non-REM ORP is ≤0.76 units. Repeatability of ORP suggests no systematic second night effect. There is considerable variability in non-REM ORP between nights. Support (If Any): Financial and in-kind support was provided by Cerebra Health Inc.

Journal of the Korean Association for Research in Science Education, 2011

The purpose of this study was to investigate the elementary school teachers' perception of sc... more The purpose of this study was to investigate the elementary school teachers' perception of science writing. In this study, 10 elementary school teachers who have taught in the 3rd or 4th grade science lesson in 2010 were selected. Researchers constructed interview guide in three parts including the teachers' understanding of science writing, the status of science writing teaching and the difficulties of science writing in their classes. For the investigation, semi-structured in-depth interviews with 10 elementary school teachers were conducted individually. The results showed that the elementary school teachers were unfamiliar with the word ‘science writing’ and considered science writing as a writing using science learning contents. Also, they think that teaching science writing in their science lessons was not needed and didn't assess and provide detailed feedback with the students' written works. Most teachers needed teaching materials and assessment tools for sci...

Previous studies showed that nattokinase supplementation lowered blood pressure in prehypertensiv... more Previous studies showed that nattokinase supplementation lowered blood pressure in prehypertensives at rest. However, the effects of nattokinase on cardiac output (CO), blood pressure, and systemic vascular resistance in response to dynamic exercise are not clear in prehypertensives. Therefore, the purpose of this study was to determine the effect of nattokinase on cardiovascular responses during mild and moderate dynamic exercise. Ten prehypertensive subjects were recruited in this study. To demonstrate the effects of nattokinase, it was designed to randomized, double blind way for 8 wk nattokinase or placebo supplementation. Initially, all subjects were continuously instrumented to measure stroke volume, heart rate, cardiac output, total vascular conductance, and blood pressure with increasing workloads from rest through mild and moderate exercise corresponding to 40% and 60% VO2peak, respectively. The results showed that CO, systolic blood pressure, mean arterial pressure were si...

Journal of the Korean Association for Research in Science Education, 2011

The purpose of this study was to analyze abductive reasoning on the inquiry of scientists and ele... more The purpose of this study was to analyze abductive reasoning on the inquiry of scientists and elementary school gifted children in science. Subjects for this study were eight scientists and eight elementary school gifted children in science studying in the Academy of Gifted Child Education in Science affiliated with Seoul National University of Education. As a result, abductive reasoning on the scientific inquiry of scientists and gifted children showed the three stages of generating hypotheses, designing the experiments, and interpreting the results. The abductive reasoning in each stage characterized the five types as complex abduction, analogical abduction, observation-based abduction, logic-based abduction, selective abduction. The sub-reasoning process of the abductive reasoning of gifted children in science differed in some ways from that of scientists. First, for most scientists, representing a method or representing a casual explican appeared after searching for the characte...

The FASEB Journal, 2016

We found previously that exercise may evoke an exaggerated blood pressure (BP) response in prehyp... more We found previously that exercise may evoke an exaggerated blood pressure (BP) response in prehypertensive individuals due to an increase in peripheral vasoconstriction induced by endothelial dysfu...

The Korean Society of Sports Science, 2016

The renal vasoconstriction serves to shunt blood flow from the kidney toward active skeletal musc... more The renal vasoconstriction serves to shunt blood flow from the kidney toward active skeletal muscles during exercise. However, it is not well known that the stimulation of group Ⅲ muscle afferents reflexly increases sympathoexci-tation in the renal arteries. Therefore, we recorded single renal nerve filament response to tendon stretch before and after injection of gadolinium into the popliteal artery in decerebrate cats. In control experiment, the impulse activity of single renal nerve efferent was significantly increased during tendon stretch, while gadolinium significantly reduced the renal sympathetic impulse activity. The blood pressure responses induced by the mechanoreflex activation were significantly increased in both conditions, but the pressor response was substantially attenuated after injection of gadolinium. We conclude that the mechanical component of the exercise pressor reflex may contribute to evoke the renal vasoconstriction during exercise.

It is reported that beetroot juice supplementation reduces blood pressure in healthy subjects at ... more It is reported that beetroot juice supplementation reduces blood pressure in healthy subjects at rest. It is not known that beetroot juice supplementation alters the pressor response mediated during exercise and by the muscle metaboreflex activation in normotensive. Thus, the purpose of this study was to investigate the effects of dietary nitrate derived from beetroot on muscle metaboreflex in normotensive compared with placebo. Fifteen normotensive (22.7±0.5 yrs) were recruited in this study. All subjects were assigned in a randomized, double-blind, crossover design to receive beetroot (70 mL/day, ~5.6 mmol of NO3 ) and placebo (70 mL/day, no NO3 ) for 15 days. The subjects completed 2 min static handgrip exercise at 50% maximal voluntary contraction followed immediately by post exercise muscle ischemia(PEMI). On day 15, the same protocol was repeated after beetroot or placebo supplementation. All subjects were continuously instrumented to measure heart rate (HR), stroke volume (SV...

It is reported that beetroot juice supplementation reduces blood pressure in healthy subjects at ... more It is reported that beetroot juice supplementation reduces blood pressure in healthy subjects at rest. However, It is not known that beetroot juice supplementation alters the pressor response mediated during exercise and by the muscle metaboreflex activation in prehypertension. Thus, the purpose of this study was to investigate the effects of dietary nitrate derived from beetroot (BR) on muscle metaboreflex and endothelial function in prehypertension compared with placebo (PR). Eleven prehypertensives (24±2 yrs) were recruited in this study. All subjects were assigned in a randomized, double-blind, crossover design to receive BR (70 mL/ day, ~5.6 mmol of NO3 and PL (70 mL/day, no NO3 ) for 15 days. The subjects completed 2 min static handgrip exercise at 50% maximal voluntary contraction followed immediately by post exercise muscle ischemia(PEMI). On day 15, the same protocol was repeated after BR or PL supplementation. All subjects were continuously instrumented to measure heart ra...

Journal of the Korean Association for Research in Science Education, 2011

The purpose of this study was to analyze science-gifted students' knowledge-generation proces... more The purpose of this study was to analyze science-gifted students' knowledge-generation processes by analyzing students' inquiry journal. As a result, first, science-gifted students showed various knowledge-generation processes, but they were limited to inductive thinking and abductive thinking, and their thinking processes were very simple. Second, most of the knowledge-generation processes of science gifted were simple, repetitive and diagrammatic processes because of observation and empirical situation of a limited scope. And a simple and repetitive diagram was generated by a simple variable selection and design, observation in limited scope, unbiased intervention by subjective thinking, and absence of exploration or finding errors. And they showed often a logical leap of reasoning.

Journal of Sport and Leisure Studies, 2012

Journal of The Korean Association For Research In Science Education, 2013

The purpose of this study is to analyze observation characteristics through eye movement accordin... more The purpose of this study is to analyze observation characteristics through eye movement according to cognitive styles. To do this, we developed observation tasks that show the differences between wholistic cognitive style group and analytic cognitive style group, measured eye movement of university students with different cognitive styles after being given an observation task. The difference between two cognitive style groups is confirmed by analysing gathered statistics and visualization data. The findings of this study are as follows: First, to compare fixation time and frequency, we compared the average value of total time used in the observation task by the wholistic cognitive style group and analytic cognitive style group. The numbers of Fixation (total) and number of Fixations (30s), is based on the fact that the wholistic cognitive style group has more numbers of fixation (Total) and number of fixations (30s) means the wholistic cognitive style group can observe more points or overall features than the analytic cognitive style group, in contrast, the analytic cognitive style group tend to focus on a particular detail, and observe less numbers of points. Second, to compare observation object and area by cognitive style, the outcome of analysing visualization data shows that wholistic cognitive style group observes the surrounding environment of spider and web on a wider area, on the other hand, the analytic cognitive style group observes by focusing on the spider itself. Through the result of this study, there are differences in observation time, frequency, object, area, and ratio from the two cognitive styles. It also shows the reason why each student has varied outcome, from the difference of information following their cognitive styles, and the result of this study helps to figure out and give direction as to what observation fulfillment is more suitable for each student.

2009 IEEE 9th International Conference on the Properties and Applications of Dielectric Materials, 2009

In the devices structure of ITO/N,N'-diph enyl-N,N'bis(3-methylphenyl)-1,1'-biphenyl-... more In the devices structure of ITO/N,N'-diph enyl-N,N'bis(3-methylphenyl)-1,1'-biphenyl-4,4'-diam ine(TPD)/tris(8-hydroxyquinoline)aluminum (Alq3) electron-transport-layer(ETL) (2,9-Dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP))/Al, we have studied the efficiency improvement of organic light-emitting diodes depending on the thickness variation of BCP using electron transport layer. The thickness of TPD and Alq3 was manufactured 40nm, 60nm under a base pressure of 5x10−6Torr using at thermal evaporation, respectively. The TPD and Alq3 layer were evaporated to be deposition rate of 2.5Å/s. And the BCP was evaporated to be at a deposition of 1.0Å/s. As the experimental results, we found that the luminous efficiency and the external quantum efficiency of the device is superior to others when thickness of BCP is 5 nm. Also, operating voltage is lowest. Compared to the ones from the devices without BCP layer, the luminous efficiency and the external quantum efficiency were improved by a factor of four hundred ninety and five hundred, respectively. And operating voltage is reduced to about 2 V.

Materials, 2021

The aim of this study was to investigate the behavior of dental-derived human mesenchymal stem ce... more The aim of this study was to investigate the behavior of dental-derived human mesenchymal stem cells (d-hMSCs) in response to differently surface-treated implants and to evaluate the effect of d-hMSCs on local osteogenesis around an implant in vivo. d-hMSCs derived from alveolar bone were established and cultured on machined, sandblasted and acid-etched (SLA)-treated titanium discs with and without osteogenic induction medium. Their morphological and osteogenic potential was assessed by scanning electron microscopy (SEM) and real-time polymerase chain reaction (RT-PCR) via mixing of 5 × 106 of d-hMSCs with 1 mL of Metrigel and 20 μL of gel-cell mixture, which was dispensed into the defect followed by the placement of customized mini-implants (machined, SLA-treated implants) in New Zealand white rabbits. Following healing periods of 2 weeks and 12 weeks, the obtained samples in each group were analyzed radiographically, histomorphometrically and immunohistochemically. The quantitativ...

Sleep, 2018

relationships of the powers of different EEG frequencies in a single understandable index. The sc... more relationships of the powers of different EEG frequencies in a single understandable index. The scale ranges from 0-2.5 with values of zero only occurring during sleep and a value of 2.5 only occurring during wakefulness. A value of 1.25 predicts an equal probability of being awake or asleep. The current investigation is undertaken to obtain normal values of non-REM ORP in good sleepers in their home environments and to establish the repeatability of this measure from night to night. Methods: Good sleepers (GS) were recruited and defined as nonshift workers sleeping 7-9 hours per night who considered their sleep of good quality and didn't have daytime symptoms attributed to their sleep. Acceptable participants couldn't be habitual nappers, have chronic health or sleep diagnoses, use psychotropic or recreational drugs, or have reached the menopause. ESS, ISI and FOSQ 10 had to be in the normal range. OSA was excluded based on an HSAT measuring EEG (Prodigy). A second EEG study was performed one week later. Results: Thirty-three people completed the study (9 (20-35) females, 8 (20-35) males, 8 (35-50) males and 8 (35-50) females. The mean ORP for non-REM sleep during both nights of the study was 0.54 ± 0.17 (66 nights). Excluding three outlier nights, results in a tighter distribution of 0.51 ± 0.13. Distribution fitting suggests a normal distribution (Kolmogorov-Smirnov test, p=0.254). No difference was noted in non-REM ORP between first and second nights (0.55 ± 0.17 vs 0.53 ± 0.16). The study was powered to find a difference of 0.08 units between first and second nights with an α error (0.05) and β error (0.2). Conclusion: GS normal values for non-REM ORP is ≤0.76 units. Repeatability of ORP suggests no systematic second night effect. There is considerable variability in non-REM ORP between nights. Support (If Any): Financial and in-kind support was provided by Cerebra Health Inc.

Journal of the Korean Association for Research in Science Education, 2011

The purpose of this study was to investigate the elementary school teachers' perception of sc... more The purpose of this study was to investigate the elementary school teachers' perception of science writing. In this study, 10 elementary school teachers who have taught in the 3rd or 4th grade science lesson in 2010 were selected. Researchers constructed interview guide in three parts including the teachers' understanding of science writing, the status of science writing teaching and the difficulties of science writing in their classes. For the investigation, semi-structured in-depth interviews with 10 elementary school teachers were conducted individually. The results showed that the elementary school teachers were unfamiliar with the word ‘science writing’ and considered science writing as a writing using science learning contents. Also, they think that teaching science writing in their science lessons was not needed and didn't assess and provide detailed feedback with the students' written works. Most teachers needed teaching materials and assessment tools for sci...

Previous studies showed that nattokinase supplementation lowered blood pressure in prehypertensiv... more Previous studies showed that nattokinase supplementation lowered blood pressure in prehypertensives at rest. However, the effects of nattokinase on cardiac output (CO), blood pressure, and systemic vascular resistance in response to dynamic exercise are not clear in prehypertensives. Therefore, the purpose of this study was to determine the effect of nattokinase on cardiovascular responses during mild and moderate dynamic exercise. Ten prehypertensive subjects were recruited in this study. To demonstrate the effects of nattokinase, it was designed to randomized, double blind way for 8 wk nattokinase or placebo supplementation. Initially, all subjects were continuously instrumented to measure stroke volume, heart rate, cardiac output, total vascular conductance, and blood pressure with increasing workloads from rest through mild and moderate exercise corresponding to 40% and 60% VO2peak, respectively. The results showed that CO, systolic blood pressure, mean arterial pressure were si...

Journal of the Korean Association for Research in Science Education, 2011

The purpose of this study was to analyze abductive reasoning on the inquiry of scientists and ele... more The purpose of this study was to analyze abductive reasoning on the inquiry of scientists and elementary school gifted children in science. Subjects for this study were eight scientists and eight elementary school gifted children in science studying in the Academy of Gifted Child Education in Science affiliated with Seoul National University of Education. As a result, abductive reasoning on the scientific inquiry of scientists and gifted children showed the three stages of generating hypotheses, designing the experiments, and interpreting the results. The abductive reasoning in each stage characterized the five types as complex abduction, analogical abduction, observation-based abduction, logic-based abduction, selective abduction. The sub-reasoning process of the abductive reasoning of gifted children in science differed in some ways from that of scientists. First, for most scientists, representing a method or representing a casual explican appeared after searching for the characte...

The FASEB Journal, 2016

We found previously that exercise may evoke an exaggerated blood pressure (BP) response in prehyp... more We found previously that exercise may evoke an exaggerated blood pressure (BP) response in prehypertensive individuals due to an increase in peripheral vasoconstriction induced by endothelial dysfu...

The Korean Society of Sports Science, 2016

The renal vasoconstriction serves to shunt blood flow from the kidney toward active skeletal musc... more The renal vasoconstriction serves to shunt blood flow from the kidney toward active skeletal muscles during exercise. However, it is not well known that the stimulation of group Ⅲ muscle afferents reflexly increases sympathoexci-tation in the renal arteries. Therefore, we recorded single renal nerve filament response to tendon stretch before and after injection of gadolinium into the popliteal artery in decerebrate cats. In control experiment, the impulse activity of single renal nerve efferent was significantly increased during tendon stretch, while gadolinium significantly reduced the renal sympathetic impulse activity. The blood pressure responses induced by the mechanoreflex activation were significantly increased in both conditions, but the pressor response was substantially attenuated after injection of gadolinium. We conclude that the mechanical component of the exercise pressor reflex may contribute to evoke the renal vasoconstriction during exercise.

It is reported that beetroot juice supplementation reduces blood pressure in healthy subjects at ... more It is reported that beetroot juice supplementation reduces blood pressure in healthy subjects at rest. It is not known that beetroot juice supplementation alters the pressor response mediated during exercise and by the muscle metaboreflex activation in normotensive. Thus, the purpose of this study was to investigate the effects of dietary nitrate derived from beetroot on muscle metaboreflex in normotensive compared with placebo. Fifteen normotensive (22.7±0.5 yrs) were recruited in this study. All subjects were assigned in a randomized, double-blind, crossover design to receive beetroot (70 mL/day, ~5.6 mmol of NO3 ) and placebo (70 mL/day, no NO3 ) for 15 days. The subjects completed 2 min static handgrip exercise at 50% maximal voluntary contraction followed immediately by post exercise muscle ischemia(PEMI). On day 15, the same protocol was repeated after beetroot or placebo supplementation. All subjects were continuously instrumented to measure heart rate (HR), stroke volume (SV...

It is reported that beetroot juice supplementation reduces blood pressure in healthy subjects at ... more It is reported that beetroot juice supplementation reduces blood pressure in healthy subjects at rest. However, It is not known that beetroot juice supplementation alters the pressor response mediated during exercise and by the muscle metaboreflex activation in prehypertension. Thus, the purpose of this study was to investigate the effects of dietary nitrate derived from beetroot (BR) on muscle metaboreflex and endothelial function in prehypertension compared with placebo (PR). Eleven prehypertensives (24±2 yrs) were recruited in this study. All subjects were assigned in a randomized, double-blind, crossover design to receive BR (70 mL/ day, ~5.6 mmol of NO3 and PL (70 mL/day, no NO3 ) for 15 days. The subjects completed 2 min static handgrip exercise at 50% maximal voluntary contraction followed immediately by post exercise muscle ischemia(PEMI). On day 15, the same protocol was repeated after BR or PL supplementation. All subjects were continuously instrumented to measure heart ra...

Journal of the Korean Association for Research in Science Education, 2011

The purpose of this study was to analyze science-gifted students' knowledge-generation proces... more The purpose of this study was to analyze science-gifted students' knowledge-generation processes by analyzing students' inquiry journal. As a result, first, science-gifted students showed various knowledge-generation processes, but they were limited to inductive thinking and abductive thinking, and their thinking processes were very simple. Second, most of the knowledge-generation processes of science gifted were simple, repetitive and diagrammatic processes because of observation and empirical situation of a limited scope. And a simple and repetitive diagram was generated by a simple variable selection and design, observation in limited scope, unbiased intervention by subjective thinking, and absence of exploration or finding errors. And they showed often a logical leap of reasoning.

Journal of Sport and Leisure Studies, 2012

Journal of The Korean Association For Research In Science Education, 2013

The purpose of this study is to analyze observation characteristics through eye movement accordin... more The purpose of this study is to analyze observation characteristics through eye movement according to cognitive styles. To do this, we developed observation tasks that show the differences between wholistic cognitive style group and analytic cognitive style group, measured eye movement of university students with different cognitive styles after being given an observation task. The difference between two cognitive style groups is confirmed by analysing gathered statistics and visualization data. The findings of this study are as follows: First, to compare fixation time and frequency, we compared the average value of total time used in the observation task by the wholistic cognitive style group and analytic cognitive style group. The numbers of Fixation (total) and number of Fixations (30s), is based on the fact that the wholistic cognitive style group has more numbers of fixation (Total) and number of fixations (30s) means the wholistic cognitive style group can observe more points or overall features than the analytic cognitive style group, in contrast, the analytic cognitive style group tend to focus on a particular detail, and observe less numbers of points. Second, to compare observation object and area by cognitive style, the outcome of analysing visualization data shows that wholistic cognitive style group observes the surrounding environment of spider and web on a wider area, on the other hand, the analytic cognitive style group observes by focusing on the spider itself. Through the result of this study, there are differences in observation time, frequency, object, area, and ratio from the two cognitive styles. It also shows the reason why each student has varied outcome, from the difference of information following their cognitive styles, and the result of this study helps to figure out and give direction as to what observation fulfillment is more suitable for each student.