Kiyoshi Takano

A method for the design of 3D scaffolds for high-density cell attachment and determination of optimum perfusion culture conditions.

The application of in vitro cultured cells in tissue engineering or drug screening, aimed at complex soft tissues such as liver, requires in vivo physiological function of the cultured cells. For this purpose, the scaffold in which cells are cultured should provide a microenvironment similar to an in vivo one with a three-dimensional extracellular matrix, a high supply capacity of O(2) and nutrients, and high cell density. In this paper, we propose a method to design (1) the geometry of the scaffold, with a surface/volume ratio optimized to allow high-density (5 x 10(7)cells/mL) cell culture and (2) culture conditions that will supply optimal quantities of oxygen and nutrients. CFD modeling of mass transport was used to determine the shear stress as well as O(2) and glucose metabolism in the scaffold (20 mm width-35 mm length) for various flow rates. Validation of the model was done through comparison with flow resistance and micro-PIV experiments. CFD analysis showed the maximum metabolic rate densities for this scaffold are 6.04 x 10(-3)mol/s/m(3) for O(2) at 0.71 mL/min and 1.91 x 10(-2)mol/s/m(3) for glucose at 0.35 mL/min.

Low O2 metabolism of HepG2 cells cultured at high density in a 3D microstructured scaffold.

Among the features of in vivo liver cells that are rarely mimicked in vitro, especially in microchips, is the very high cell density. In this study, we have cultured HepG2 in a plate-type PDMS scaffold with a three-dimensional ordered microstructure optimally designed to allow cells to attach at a density of 108 cells/mL. After the first step of static open culture, the scaffold was sealed to simulate the in vivo oxygen supply, which is supplied only through the perfusion of medium. The oxygen consumption rate at various flow rates was measured. An average maximal cellular oxygen consumption rate of 3.4 × 10−17 mol/s/cell was found, which is much lower than previously reported values for hepatocytes. Nevertheless, the oxygen concentration in the bulk stream was not the limiting factor. It has been further confirmed by the reported numerical model that the mass transport resistance on the surface of a cell that limits the oxygen supply to the cell. These results further emphasize that access to a sufficient quantity of oxygen, especially through the diffusion-limited layer on the surface of a cell, is very important for the metabolism of hepatocytes at such a high density.

Active enhancement of evaporation of a liquid drop on a hot solid surface using a static electric field

Abstract This study aims at further improvement of evaporation or boiling heat transfer using an electric field. Active enhancement using an electric field would be extremely effective in film boiling, but favorable effects would be expected on nucleate and transition boiling, and on the maximum and minimum heat fluxes. In this paper the results of our preceding studies are first summarized. We have found that the application of an electric field enhances, to a great extent, the evaporation of a liquid drop on a hot solid surface. The time needed for a single drop to evaporate completely reduces to one-twentieth of that for evaporation without an electric field, when 300 V is applied to a drop of ethanol. The results of an additional experiment are presented in this paper. The heat transfer coefficient during evaporation of a liquid drop on a platinum surface is measured while the volume of the drop suspended at the tip of a thin needle nozzle is kept constant. The voltage applied between the drop and the heat transfer surface is varied from 0 V to 2000 V (for R113) or to 250 V (for ethanol). The maximum enhancement ratios of the evaporative heat transfer coefficients at the highest voltages are 7.6 for ethanol and 2.8 for R113.

Extending Network Coverage by Using Static and Mobile Relays during Natural Disasters

During natural disasters, such as earthquakes, apart of the Internet access infrastructure can be damaged, leaving many users disconnected. At the same time, many peopleneed to communicate to find their relatives and receive officialnotifications about the current situation. This paper presents anevaluation of different techniques for extending network coveragein such scenarios. We use real-world data to model the poweroutage probability of cellular base stations in Tokyo area andcombine it with information about batteries/power generatorsto create accurate maps of network coverage for different timeperiods after an earthquake. In our simulation, we use a realmap of evacuation sites, provided by Japanese government. Wefirst considered mobile nodes, moving between evacuation sites, and investigated their impact on network coverage. Then, wedeveloped an algorithm to determine the optimal locations forstatic relays ensuring different levels of network coverage. Ourresults show that even a small number of fixed relays, carefullyplaced between the evacuation sites, can outperform a muchhigher number of mobile nodes in terms of network coverage.

Big Data Based Service Area Estimation for Mobile Communications during Natural Disasters

Big Data analysis is considered as a powerful tool to help the disaster management in many fields. In this study, we investigated and applied the Big Data techniques for the disaster estimation of mobile communication systems. Firstly, we have developed an data analysis model by analyzing the large amount of data generated and collected from the previous earthquakes. Then, based on the model, we integrated all the necessary data and potential earthquakes estimations that are likely to strike Tokyo area in the near future, to calculate the mobile service area with probabilistic availability. Moreover, we could also predict the spatial and temporal changes of the service availability areas by building them into the geographic information system (GIS). Our developed model and simulation platform could help many decision-making bodies such as governments or network operators to assess the vulnerability of communication infrastructures and make better preparedness and fast response for large-scale disasters.

Capacity-aware cost-efficient network reconstruction for post-disaster scenario

Natural disasters can result in severe damage to communication infrastructure, which leads to further chaos to the damaged area. After the disaster strikes, most of the victims would gather at the evacuation sites for food supplies and other necessities. Having a good communication network is very important to help the victims. In this paper, we aim at recovering the network from the still-alive mobile base stations to the out-of-service evacuation sites by using multi-hop relaying technique. We propose to reconstruct the post-disaster network in a capacity-aware way based on prize collecting Steiner tree. The purpose of the proposed scheme is to achieve high capacity connectivity ratio in a cost efficient way. To provide more accurate evaluation results, we evaluate the proposed scheme by using the real evacuation site and base station data in Tokyo area, and utilizing the big data analysis based post-disaster service availability model.

Study on enhancement of condensation heat transfer using an electric field

Enhancement of condensation heat transfer using a nonuniform electric field was experimentally investigated for horizontal smooth and low-finned tubes. In the experiments, a wire electrode parallel to the tube was placed beneath the tube. The experimental parameters were the distance and voltage between the wire electrode and the tube, and the condensation heat flux. Results of the present experiment for the low-finned tube indicate that, as the applied voltage increases, the enhancement ratio increases steeply at a certain voltage and it reaches 2.4. It was observed that the condensate flow pattern falling down from the bottom of the tube changed from a flat film to circular columns at a critical voltage.

Recrystallization and Water Absorption Properties of Vitrified Trehalose Near Room Temperature

PurposeTo provide the physicochemical properties of vitrified trehalose for predicting its recrystallization.MethodsThin films of vitrified trehalose solutions were prepared at room temperature and exposed to various humid and temperature atmospheres. The in-situ amount of retained water in the vacuum-dried trehalose thin film during exposure was determined using its FTIR spectrum by quantifying the extremely infinitesimal amount of retained water in the trehalose solution. Recrystallization of the sample was also assessed by the FTIR spectrum of trehalose dihydrate.ResultsThe effective water absorption coefficient, hmeff, exponentially increased to the water activity of the trehalose sample, Aw, at 25°C and 40°C at which the increasing rates are comparable. The surface energy of trehalose dihydrate, γ, was found to be lower than the value calculated from the reported equation, neglecting the effects of the activity of the solute and solvent water.ConclusionsThe retained water in trehalose considerably increases its affinity for water vapor, and the change in this affinity with regard to the water activity is nearly independent of temperature. The dihydrate nucleation rate of trehalose-water system is maximal when trehalose weight ratio is ~0.8 at 25°C and is slightly higher (~0.85) at 40°C.

Population-Aware Relay Placement for Wireless Multi-Hop Based Network Disaster Recovery

Network disaster recovery is one of the greatest concerns for Mobile Network Operators (MNOs) and first responders during large-scale natural disasters such as earth- quakes. In many recent studies, wireless multi-hop networking has been demonstrated as an effective technique to quickly and efficiently extend the network coverage during disasters. In this paper, we specifically address the network deployment problem by proposing the Population-Aware Relay Placement (PARP) solution, which seeks the efficient deployment of a limited number of relays such that population coverage is maximized in the scenario of network disaster recovery. We provide a graph-based modeling and prove its NP-hardness accordingly. In order to efficiently solve this problem, we propose a heuristic solution, which is constructed in two steps. We first design a simple algorithm based on a disk graph to determine the Steiner locations, which is the biggest challenge in this problem. Then, we formulate the problem as an integer programming problem, which is inspired by the formulation of Prize-Collecting Steiner Tree (PCST). Thus, the integer problem is solved by exploring the similarity of the existing algorithm for PCST. To evaluate the proposed solution extensively, we present numerical results on both real-world and random scenarios, which validate the effectiveness of the proposed solution and show substantial improvement by comparing to the previous one.

Spatio-temporal data-driven analysis of mobile network availability during natural disasters

The accurate assessment of mobile network availability during large-scale natural disasters is essential for ensuring effective preparation and fast response. However, traditional network availability assessment models are ideal and cannot effectively take into account the spatio-temporal dynamics of mobile network failures in a disaster scenario. Therefore, their evaluation results are generally inaccurate and of coarse granularity, thus not meeting the strict requirements for disaster preparation and response. In this paper, we propose a data-driven analysis framework for the accurate assessment of mobile network availability by integrating essential geographical features from various sources, e.g., seismic intensity data, buildings and land usage data, base station location data, and many other data in related studies. Furthermore, we explore the spatio-temporal inter-correlations and dynamics of several key factors of network failures and their impacts on network availability by associating them with corresponding geographical features in a disaster scenario. We demonstrate our analysis framework with a synthetic earthquake scenario in the Tokyo area and validate our analysis by comparing to existing studies.