Hsiu-Hung Chen

Particle shape effect on heat transfer performance in an oscillating heat pipe

The effect of alumina nanoparticles on the heat transfer performance of an oscillating heat pipe (OHP) was investigated experimentally. A binary mixture of ethylene glycol (EG) and deionized water (50/50 by volume) was used as the base fluid for the OHP. Four types of nanoparticles with shapes of platelet, blade, cylinder, and brick were studied, respectively. Experimental results show that the alumina nanoparticles added in the OHP significantly affect the heat transfer performance and it depends on the particle shape and volume fraction. When the OHP was charged with EG and cylinder-like alumina nanoparticles, the OHP can achieve the best heat transfer performance among four types of particles investigated herein. In addition, even though previous research found that these alumina nanofluids were not beneficial in laminar or turbulent flow mode, they can enhance the heat transfer performance of an OHP.

Development of a reliable low-cost controlled cooling rate instrument for the cryopreservation of hematopoietic stem cells

BACKGROUND AIMS An optimal cooling rate is one of the critical factors influencing the survival of cells during cryopreservation. We describe a novel device, called the box-in-box, that has been developed for optimal cryopreservation of human hematopoietic stem cells (HSC). METHODS This work presents the design of the device, a mathematical formulation describing the expected temperature histories of samples during the freezing process, along with actual experimental results of thermal profile tests. In experiments, when the box-in-box device was transferred from room temperature to a -80 degrees C freezer, a cooling rate of -1 to -3.5 degrees C/min, which has been widely used for the cryopreservation of HSC, was achieved. In order to evaluate this device further, HSC cryopreservation was compared between the box-in-box device and a commercially available controlled-rate freezer (CryoMed). RESULTS The experimental data, including total cell population and CD34(+) hematopoietic progenitor cell recovery rates, viability and cell culture colony assays, showed that the box-in-box worked as well as the CryoMed instrument. There was no significant difference in either survival rate or the culture/colony outcome between the two devices. CONCLUSIONS The box-in-box device can work as a cheap, durable, reliable and maintenance-free instrument for the cryopreservation of HSC. This concept of a box-in-box may also be adapted to other cooling rates to support cryopreservation of a wide variety of tissues and cells.

Hydrophobic Surface Effect on Heat Transfer Performance in an Oscillating Heat Pipe

An experimental investigation of an oscillating heat pipe (OHP) with a superhydrophobic inner surface coated with a superhydrophobic self-assembled monolayer (SAM) of n-octadecyl mercaptan was conducted. The experimental results show that the oscillating motion in an OHP with a superhydrophobic surface can be generated and the OHP can function well. This is very different from the conventional wicked heat pipe, which cannot function if the inner surface is hydrophobic. The functionality of a superhydrophobic OHP is not sensitive to the wetting condition of the inner surface of the OHP. The investigation results in a better understating of heat transfer mechanism occurring in an OHP.

Ultra-high cooling rate utilizing thin film evaporation.

This research introduces a cell cryopreservation method, which utilizes thin film evaporation and provides an ultra-high cooling rate. The microstructured surface forming the thin film evaporation was fabricated from copper microparticles with an average diameter of 50 μm. Experimental results showed that a cooling rate of approximately 5[Formula: see text]10(4) °C/min was achieved in a temperature range from 10 °C to -187 °C. The current investigation will give birth to a cell cryopreservation method through vitrification with relatively low concentrations of cryoprotectants.

Particle enrichment employing grooved microfluidic channels

The well-studied chaotic micromixer has found its application on the enrichment of microparticles. Here, we report the use of such patterning flows produced by a grooved surface integrated into a microfluidic device to continuously concentrate the flowing particles of comparable sizes to the depth of the channel. The particle distributions after passing through the grooves illustrate the enrichment profiles and the size-dependent patterns. We expect that the alignment of the concentrated particles can facilitate the visualization of sizing and counting in cell-based devices.

Microfabricated thermal conductivity sensor: a high resolution tool for quantitative thermal property measurement of biomaterials and solutions

Obtaining accurate thermal properties of biomaterials plays an important role in the field of cryobiology. Currently, thermal needle, which is constructed by enclosing a manually winded thin metal wire with an insulation coating in a metallic sheath, is the only available device that is capable of measuring thermal conductivity of biomaterials. Major drawbacks, such as macroscale sensor size, lack of versatile format to accommodate samples with various shapes and sizes, neglected effects of heat transfer inside the probe and thermal contact resistance between the sensing element and the probe body, difficult to mass produce, poor data repeatability and reliability and labor-intense sensor calibration, have significantly reduced their potential to be an essential measurement tool to provide key thermal property information of biological specimens. In this study, we describe the development of an approach to measure thermal conductivity of liquids and soft bio-tissues using a proof-of-concept MEMS based thermal probe. By employing a microfabricated closely-packed gold wire to function as the heater and the thermistor, the presented thermal sensor can be used to measure thermal conductivities of fluids and natural soft biomaterials (particularly, the sensor may be directly inserted into soft tissues in living animal/plant bodies or into tissues isolated from the animal/plant bodies), where other more standard approaches cannot be used. Thermal standard materials have been used to calibrate two randomly selected thermal probes at room temperature. Variation between the obtained system calibration constants is less than 10%. By incorporating the previously obtained system calibration constant, three randomly selected thermal probes have been successfully utilized to measure the thermal conductivities of various solutions and tissue samples under different temperatures. Overall, the measurements are in agreement with the recommended values (percentage error less than 5%). The microfabricated thermal conductivity sensor offers superior characteristics compared to those traditional macroscopic thermal sensors, such as, (a) reduced thermal mass and thermal resistivity, (b) improved thermal contact between sensor and sample, (c) easy to manufacture with mass production capability, (d) flexibility to reconfigure sensor geometries for measuring samples with various sizes and shapes, and (e) reduced calibration workload for all sensors microfabricated from the same batch. The MEMS based thermal conductivity sensor is a promising approach to overcome the inherent limitations of existing macroscopic devices and capable of delivering accurate thermal conductivity measurement of biomaterials with various shapes and sizes.

Electrical Conductivity Measurements for the Ternary Systems of Glycerol/Sodium Chloride/Water and Ethylene Glycol/Sodium Chloride/Water and Their Applications in Cryopreservation

Electrical conductivity of a solution is a property that can be easily determined through the measurement of a conductivity probe. The present study demonstrates the measurements of electrical conductivity for two ternary solutions: glycerol/sodium chloride/water and ethylene glycol/sodium chloride/water. When the concentration of sodium chloride to water ratio (R) is fixed, the existence of either glycerol or ethylene glycol, both cryoprotective agents (CPAs), can be quantitatively determined by their depressive influence on electrical conductivity of the solution. The measurements were performed on solutions with a set of 10 different concentrations of CPAs, ranging from 3.2% to 50% (v/v), along with five ratios of NaCl/water solutions. Equations to fit the experimental measurements were devised to characterize the relations among electrical conductivity, CPAs concentration, and R. A conductivity meter used in this study required <5 s to read the solutions electrical conductivity, which is faster than the measurement using osmometry method. The charts of ternary solutions associated with their electrical conductivity and concentrations make it especially useful for monitoring the cryopreservation processes, including addition and removal of CPAs, to prevent osmotic damages to biological samples.

Electrowetting-on-dielectric assisted bubble detachment in a liquid film

Drawing inspiration from electrowetting-controlled droplets, the potential advantages of electrowetting for bubble dynamics are investigated experimentally. In this study, we present and characterize an open electrowetting-on-dielectric (EWOD) system for studying the bubble behavior. Both detachment and non-detachment processes of a small single bubble in a thick liquid film under EWOD were experimentally observed. The measurement of contact angle changes of the small air bubble shows relatively good agreement with Young-Lippmanns equation within the majority of the test voltage range, except for the saturation region. Meanwhile, we have experimentally demonstrated both the characteristics of single- and double-bubble detachment within a thin liquid film. Direct bubble detachment may occur when it touches the gas-liquid interface during the process of contact angle change, while indirect bubble detachment is highly possible due to the dramatic oscillation resulting from the detachment of adjacent bubbles...

Assessment of Cryoprotectant Concentration by Electrical Conductivity Measurement and Its Applications in Cryopreservation

This chapter presents an important application of the electrical conductivity measurement in cryopreservation. Long-term cryopreservation of cells and tissues is essential in both clinical treatments and fundamental researches. In order to reduce the cryo-injury to the cells during cryopreservation, cryoprotective agents (CPAs) should be added before freezing, but also removed after thawing duo to the cytotoxicity. In these steps, severe osmotic stresses may result in injuries to the cells too. Therefore, monitoring the addition and removal of CPAs to the cell samples is critical in order to prevent the osmotic injury. In this chapter, the electrical conductivity measurement was applied to assess the CPA concentration in cryopreservation. Firstly, the standard correlations between the CPA concentration and the electrical conductivity of the solutions (including CPA-NaCl-water ternary solutions and CPA-albumin-NaCl-water quaternary solutions) were experimentally obtained for a few mostly used CPAs. Then a novel “dilution-filtration” system with hollow fiber dialyzer was designed and applied to remove the CPA from the solutions effectively. Measurement of electrical conductivity was validated as a safer and easier way to on-line and real-time monitoring of CPA concentration in cell suspensions. This work demonstrated a very important application of electrical conductivity in the biomedical engineering field.

Wetting dynamics of multiscaled structures

Priming dynamics is one of the critical parameters in designing a capillary-driven thermal management system. We report both an experimental and simulation study of hierarchical structures with silicon pillars and silicon nanowires on the pillar surface. Liquid front velocity covered and uncovered was characterized using capillary wetting experiments and validated by numerical simulation and theoretical prediction. The water under cover moves one order of magnitude faster than the water in the uncovered case. The experimental results and the prediction are in good agreement for flow regimes in both the covered and the uncovered regions.