Junki Nakagawa

Thermal phonon transport in silicon nanowires and two-dimensional phononic crystal nanostructures

Thermal phonon transport in silicon nanowires (Si NWs) and two-dimensional phononic crystal (2D PnC) nanostructures was investigated by measuring thermal conductivity using a micrometer-scale time-domain thermoreflectance. The impact of nanopatterning on thermal conductivity strongly depends on the geometry, specularity parameter, and thermal phonon mean free path (MFP) distribution. Thermal conductivities for 2D PnC nanostructures were found to be much lower than that for NWs with similar characteristic length and surface-to-volume ratio due to stronger phonon back scattering. In single-crystalline Si, PnC patterning has a stronger impact at 4 K than at room temperature due to a higher specularity parameter and a longer thermal phonon MFP. Nanowire patterning has a stronger impact in polycrystalline Si, where thermal phonon MFP distribution is biased longer by grain boundary scattering.

Crystal structure dependent thermal conductivity in two-dimensional phononic crystal nanostructures

Thermal phonon transport in square- and triangular-lattice Si phononic crystal (PnC) nanostructures with a period of 300 nm was investigated by measuring the thermal conductivity using micrometer-scale time-domain thermoreflectance. The placement of circular nanoholes has a strong influence on thermal conductivity when the periodicity is within the range of the thermal phonon mean free path. A staggered hole structure, i.e., a triangular lattice, has lower thermal conductivity, where the difference in thermal conductivity depends on the porosity of the structure. The largest difference in conductivity of approximately 20% was observed at a porosity of around 30%. This crystal structure dependent thermal conductivity can be understood by considering the local heat flux disorder created by a staggered hole structure. Numerical simulation using the Monte Carlo technique was also employed and also showed the lower thermal conductivity for a triangular lattice structure. Besides gaining a deeper understanding ...

Evaluation of wearable gyroscope and accelerometer sensor (PocketIMU2) during walking and sit-to-stand motions

Recently healthcare of the elderly people has become a serious issue in medical and rehabilitation areas. In order to know their functional mobility and provide sufficient medical treatment, it is important to measure their body state precisely and objectively. Therefore we developed a wearable and wireless sensor of gyroscope and accelerometer (PocketIMU2) as an easy and precise measurement of human motions. In the sensor, we employed a small and high accurate LiNbO3 crystal to achieve joint angle computation with simple integration of angular velocity. In the current paper, we evaluate the accuracy of the sensor in two important basic motion, such as a walking and sit-to-stand motions. Computed joint angles of ankle, knee, and hip are compared to the reference data measured from a optical motion capture system in term of coefficients of correlation and root mean square error. As a result, coefficient of correlation showed very high value for all joint angles, and root mean square error was adequately small. This strongly supports the usage of our developed gyroscope and accelerometer sensor for monitoring human body movement for medical usage.

Thermal conduction in Si and SiGe phononic crystals explained by phonon mean free path spectrum

Thermal phonon transport in single-crystalline Si, amorphous SiGe, and poly-SiGe nanostructures was investigated experimentally at room temperature. The characteristic length dependence of thermal conductivity was compared across these three materials by changing the shortest distance between the circular holes of phononic crystals formed in the membranes. The dependences clearly differ for these materials, and these differences can be explained by the thermal phonon mean free path spectra of the three materials. Nanostructuring has a larger impact on thermal conductivity reduction when the characteristic length is comparable to that in the region where the thermal phonon mean free path spectrum is dense. The results suggest that thermal phonon mean free path spectra can be estimated qualitatively by thermal conductivity measurements with characteristic length sweeps.

Muscle Synergy Analysis of Human Standing-Up Motion with Different Chair Heights and Different Motion Speeds

Although standing-up motion is an important activity of daily living, it remains unclear how people perform the motion in different situations. As described in this paper, muscle synergy analysis is applied to standing-up motions performed at different circumstances, such as two different heights and at three different speeds. Results elucidated three invariant groups of synchronized muscle activations: The first synergy pulls the ankle and raises the hip. The second synergy extends the upper body. The third synergy stabilizes posture. Results also show that people controlled the activation coefficient of each synergy differently during all motions. The slower the standing-up motion is, the longer each synergy activates to adapt to the slower motion speed. Results of this study show that people use the same group of synchronized muscle activation and only control the activation coefficient to achieve adaptive standing-up motion.

Analysis of contribution of muscle synergies on sit-to-stand motion using musculoskeletal model

Recently, declining physical ability of elderly people has become an extremely important social issue. To improve their daily living activities, the standing-up motion is emphasized in this study as an important daily motion. Synergy analysis is applied to the standing-up motion to extract four important groups of muscle activations (synergies). Furthermore, the effect of synergies on body movement is calculated based on a musculoskeletal model of the human body. Results suggest that the first synergy works as preparation of the motion by pulling the ankle and flexing the hip. The second synergy controls the joint moment of the hip and knee joints to raise the hip and move the center of mass forward. The third synergy controls the ankle joint according to movement of the center of mass. The last synergy stabilizes the posture change from a seated to a standing position. Our findings imply that it is important to train those functional muscle activity to enhance the ability of standing-up motion.

Extraction and Evaluation of Proficiency in Bed Care Motion for Education Service of Nursing Skill

In the areas of care nursing, conventional physical skill education which has been conducted face-to-face prevents both experts and beginners from realizing the effects of their education and practice. In order to solve this problem, we aim to develop an effective skill education service of bed care motion with slide sheet which can prevent the outbreak of lumbago. In this study, we calculated the lumbar burden with measured body joint trajectory, foot reaction force, and muscle activities in the motion of experts and non-experts. Based on these analysis results, nursing skill was extracted and evaluated for skill education service.

Analysis of Joint Correlation between Arm and Lower Body in Dart Throwing Motion

As the population continues to age, the number of elderly people requiring healthcare is increasing. In order to improve their physical function, they need to get physical training. There are activities which require integrated arm movements and lower body movements. However there are no quantitative testing methods of the degree of recovery for the coordination between arm movements and lower body movements. In this study, we focus on dart throwing motion as arm movements in lower body movements and suggest the quantitative evaluation of the coordination between arm movements and lower body movements in dart throwing motion. Normalized correlation coefficient (NCC) between arm and lower body was computed at different throwing distances. In addition the standard deviation of the NCC was computed in order to investigate the stability of the joint correlation evaluation. This analysis shows that the correlation between elbow and ankle, or between elbow and knee, are increased at throwing long distance. We suggest that the NCC between elbow angle and right knee angle may be used for the evaluation of the joint correlation between arm movements and lower body movements in dart throwing motion.

Measurement of just noticeable difference of hip joint for implementation of self-efficacy: in active and passive sensation and in different speed

Self-efficacy, which is a belief to achieve a goal, is important for sufficient enhancement of physical ability of elderly people. For implementation of self-efficacy to assistive systems, it is necessary to subliminally change reference trajectories of the system within the range at which people cannot recognize the difference (just-noticeable difference: JND). This study clarified that elderly people have weaker position sense rather than young people when they are moved passively in relatively fast speed. New reference hip trajectories are computed to gradually extend hip flexion of human standing-up motion based on the measured JND. Graphical Abstract

Skill Evaluation and Education Services for Bed-Care Nursing with Sliding Sheet with Regression Analysis

Sliding sheet is a tool for changing positions and postures of bedridden patients. In the present study, in order to provide education service of sliding sheet, we examined skill points during the pulling movement of sliding sheet with regression analysis. Further, we proposed an automatic education system with Kinect camera basing on the results of the analysis. In the regression analysis, according to prior research, we included bending angle of waist, opening angle of legs, and angle of arm flex as three independent variables and moment of lumbar extension as a dependent variable. According to the results of regression analysis, we found that in order to minimize the load of lumbar during the pulling motion, participant should bend their body close to the bed, spread their legs widely to lower their center of gravity, and straighten their arms to the bed. Furthermore, basing on the results of our analysis, we proposed an evaluation service with Kinect camera, overlaying recommended posture on picture of learners’ body and giving evaluations on the load of their lumbar to the learners. We believe that our education service will be useful for the spread of sliding sheet.