Mohammad Ali Zoljalali Archives - The MicroFluidic Circle

Microfluidic Heat Exchanger, a Unique Solution for Cooling of the Latest Electronic Devices

admin — Wed, 13 Mar 2019 13:20:20 +0000

When a lot of electro-mechanical systems have been miniaturized and integrated by compact design, thermal management in a small volume should be simultaneously considered. As the devices or systems become smaller, heat flux increases in general. Therefore, an effective cooling strategy for the micro-devices is required especially when the cooling target is made from microfabrication processes. The microfluidic heat exchanger is one of the most promising devices for cooling down the electronic systems because it can be also made by the microfabrication processes¹. This device which is also called microchannel heat sink has been considered as an effective heat removal tool and has caught much attention during the past decades, due to its advantages including high heat transfer performance, mild pressure loss and easy fabrication².

It is evident that the former electronic cooling systems majorly employ heat pipes, fins and a combination of both. The use of the fin and heat pipe based cooling system manages to occupy large volume and appeals a huge setback from the perspective of compactness of the device. For example, a normal heat pipe combined with a fan can dissipate (300W/m2K) energy and at lower magnitude with only fins (80W/m2K)³. But these cannot dissipate the latest range 2000W/cm2K, due to this, the above devices have limitations for recent electronic components. Increasing the surface area and using appropriate liquid coolant is one of the better options to move further. The concept of the microchannel heat sink was first coined by Tuckerman and Pease. His path-breaking work initiated many other researchers to compare their numerical and experimental work with other microfluidic heat exchanger shapes. The geometry of inlet and outlet manifolds which are responsible for distributing and gathering of fluid in channels is an important factor for designing microfluidic heat exchangers. Accordingly, the basic manifold designs are conservative and bifurcation type, as shown in Fig. 1. In a conservative design, there is a single manifold region directly connected to the channels distributing the flow. The dashed line in the manifold shows an alternative for improving flow uniformity and consequently temperature gradient by making the manifold non-uniform or triangular. In bifurcation type, the flow from the inlet is divided into two streams; each is then further subdivided into two more till the number of divisions matches the number of channels⁴.
Beside this, channels geometry, channels porosity, the fluid type, using ribs in channels and etc. are the parameters which have a significant effect on microfluidic heat exchangers efficiency.

Figure 1. Schematic for (a) conservative and (b) bifurcation type manifold structures⁴

A heat exchanger is used to transfer the thermal potential of one medium to another, with and without the direct interaction or mixing between them. The ability to transfer heat strongly depends on the contact area between the two medium, a larger area to volume ratio results in more heat transfer. The fluids are typically transferred by a piping network of relatively smaller size, upon entering the heat exchanger it has to be distributed within the heat exchanger so as to increase its contact surface area. Particular care is needed to be focused on this fluid distribution, as the flow rate through each fluid path of the heat exchanger has considerable effects on its performance with regards to pressure drop and heat transfer. The pressure drop in the heat exchanger manifold and channels can be measured experimentally, thus providing the information about the flow distribution characteristics. Such heat exchangers are distinguished by a very high ratio of surface area to volume, low thermal resistances, small volumes, lower total mass, and low inventory of working fluids.

However, it is pertinent to note that the importance of novel technologies in microchannels is not the only hour of demand for high-density electronic components but also to cool data centers, workstation computers, Nozzle cooling for 3D printers, supercapacitors thermal management and cooling of artificial organs. Therefore, the requirement of new sink design to enhance the existing cooling is persuasive by many factors and might provide new insight into many interdisciplinary fields.

Besides the above well-established benefits and application of the microchannel, it has also its own operational limitations as follows⁵:

Due to the increased surface area, friction factor increases in microchannels leading to higher pressure drop and it further intensifies while introducing high viscous fluids.

The use of nanofluids or two-phase fluid develops corrosion in the channels and causes a decrement in heat transfer due to fouling effects.

The non-uniformity of fluid flow distribution in microchannels leads to the development of hot spots on the electronic device and decreasing its lifespan.

Identifying an effective manufacturing process for microchannel that provides near zero surface roughness is difficult.

If point 4 is reality then the concern about early turbulence effects and higher pressure drop can be solved. But, most importantly the primarily responsible factor that influences heat transfer in the microchannel can be cornered.

Multiple interesting microchannel configurations have been proposed in the last decades to fulfill the demands on cooling of the latest electronic devices. Numerous studies are in process for a better understanding of fluid flow characteristic in microchannels and different ways for improvement of efficiency of microfluidic heat exchangers. In the coming years, this device seems to be the most reliable cooling technology due to its superior command overheat carrying capability and it will revolutionize the electronics industry.

^{1. E. S. Cho, J. W. Choi, J. S. Yoon and M. S. Kim, “Modeling and simulation on the mass flow distribution in microchannel heat sinks with non-uniform heat flux conditions,” International Journal of Heat and Mass Transfer, vol. 53, p. 1341–1348, 2010.}
^{2. Y.-T. Mu, L. Chen, Y.-L. He and W.-Q. Tao, “Numerical study on temperature uniformity in a novel mini-channel heat sink with different flow field configurations,” International Journal of Heat and Mass Transfer, vol. 85, pp. 147-157, 2015.}
^{3. V. R. B. Y. Tullius JF, “A review of cooling in microchannels,” Heat Transfer Eng, vol. 32, no. 8, pp. 527-541, 2011.}
^{4. O. K. Siddiqui and S. M. Zubair, “Efficient energy utilization through proper design of microchannel heat exchanger manifolds: A comprehensive review,” Renewable and Sustainable Energy Reviews, vol. 74, pp. 969-1002, 2017.}
^{5. G. Narendran, N. Gnanasekaran and D. A. Perumal, “A Review on Recent Advances in Microchannel Heat Sink Configurations,” Recent Patents on Mechanical Engineering, vol. 11, 2018.}

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Mohammad Ali Zoljalali

Mohammad Ali Zoljalali is a Master's degree student at the University of Mazandaran. He is working on microfluidics by the focus on microchannels heat sinks and Drug Delivery.

https://www.researchgate.net/profile/Mohammad_Ali_Zoljalali

The post Microfluidic Heat Exchanger, a Unique Solution for Cooling of the Latest Electronic Devices appeared first on The MicroFluidic Circle.

How is Microfluidics Used in Aging Research?

admin — Wed, 19 Dec 2018 14:30:16 +0000

Aging has been always a significant concern for mankind and they have been seeking a solution to overcome this challenge from the beginning. Biology as a scientific method presents important approaches which can revolutionize aging studies. Approaches that improve understanding of the underlying molecular mechanisms of aging, as well as their contributions to age-associated diseases. Studying the replicative aging phenomenon in the budding yeast has led to significant findings on how aging is regulated by evolutionarily conserved enzymes and molecular pathways. Identifying and characterizing the factors that modulate longevity is central to understanding the basic mechanisms of aging. Among model organisms used for research related to aging, the budding yeast has proven to be an important system for defining pathways that influence lifespan. Replicative lifespan is defined by the number of daughter cells a mother cell can produce before senescing. Over the past 10 years, replicative life span analysis has been performed on several thousand yeast strains, identifying several hundred genes that influence replicative longevity.¹

Based on the conservative method of aging studies, tracking of mother cells and observing molecular markers during the process of aging face different limitations. Fifty years after Mortimer and Johnston’s discovery, the technology used to analyze replicative aging remained essentially the same. To measure the number of daughter cells produced by each mother cell, Mortimer and Johnston grew yeast cells on an agar plate and used a micromanipulator (a microscope with a dissector) to remove daughter cells after each cell division². This is still the most widely used method for analyzing yeast lifespan. However, because the cells are grown on an agar plate, it is almost impossible to follow cell and organelle morphologies and track molecular markers throughout the lifespan of individual cells. Such high-resolution, single-cell analysis is critical for developing a mechanistic understanding of cellular aging and death. In addition, the traditional assay is laborious and time-consuming, which makes it very difficult to perform large-scale screening for mutants with lifespan phenotypes.³

Recently, the microfluidic system which is capable of retaining mother cells in the micro pads while removing daughter cells automatically, making it possible to observe fluorescent reporters in single cells throughout their lifespan. Figure 1 shows a microfluidic device for monitoring the aging process of budding yeast.

Figure 1. (a) Array of micropads. The chip contains 200 micropads arranged in an array format(b) Schematic illustration of microfluidic dissection. Mother yeast cells are held between a soft PDMS pad and thin cover glass.⁴ Copyright (2018) National Academy of Sciences, U.S.A.

The basic unit of the device consists of a microfluidic device with micro pads that can physically trap the mother cells while allowing the removal of daughter cells (Video 1). There are two significant mechanisms for the separation of mother and daughter cells from each other in microfluidics. The first one is separation based on the cells sizes which means that mother cells are trapped in micro pads duo to their bigger sizes and daughter cells are removed by fluid flow. However, as the mother cells get older their and their daughter sizes get bigger that this phenomenon can disrupt the procedure. Geometric confinement by itself alone is not good enough solution because it is sensitive to the height of the micro pad: if it is too high, the mother cells will not be stably trapped; if it is low enough to stably trap mother cells, there is a certain probability that daughter cells will be trapped and jam the device. Modifying the mother cell surface to create adhesion between micro pad walls and Mother cells surfaces is another way for separation. In this method, mother cells are trapped by a combination of geometric confinement (the height of the micro pad is comparable to the size of mother cells) and adhesion between biotin labeled mother cell surface and BSA-Avidin modified glass.

http://ufluidix.com/circle/wp-content/uploads/2018/12/sm05.mp4

Challenges:

However, many issues prevent the use of microfluidic devices in a high-throughput manner for lifespan screens.⁵

First, although the time required to monitor the entire lifespan of the yeast cell has been dramatically reduced, the throughput is limited to 1-4 channels per device.
Second, the basis of this dissection method is the size of cells but the point is that as the mother cells get older, they generate large daughter cells that also become trapped by the micro pads.
Third, the micro pad design often allows more than one cell to be trapped; multiple cells can be trapped underneath one micro pad, whereas no cells are trapped under others.
Finally, in some cases, cell-surface labeling and chemical modification of the device are required, which has proven to be technically challenging for fabrication and to introduce adverse effects on replicative lifespan.

Microfluidic dissection platform that allows us to track single yeast cells over their entire replicative lifespans with high-resolution imaging capability, is a possibility that the yeast-aging research community has long-awaited for it. Not only can this technology replace the tedious manual microdissection methods to determine lifespan data, but it can also be used for high-resolution in vivo fluorescence imaging of aging cells under exactly controlled environmental conditions⁶. This technology thus opens up unique possibilities for future aging research:Its capability for phenotypic tracing is essential (i) to explore the relevance of cell-to-cell heterogeneity in the aging process, and (ii) to address important questions, such as whether certain phenotypes in cellular youth affect old-age behaviour, or how damage is asymmetrically inherited by the mother cell and removed from the daughter cell, and how this changes with age. By elimination of mentioned disadvantages, there are promising signs that this technology will enable novel investigations in the quest for molecular mechanisms underlying the aging process and will permit large-scale screens into the aging phenotype for its capability to be easily multiplexed and to allow aging experiments in an unsupervised manner.

^{1. Replicative life span analysis in budding yeast. GL, Sutphin, JR, Delaney and M, Kaeberlein. 2014, Methods in molecular biology, Vol. 1205, pp. 341-357.}
^{2. Single Cell Analysis of Yeast Replicative Aging Using a New Generation of Microfluidic Device. Zhang, Yi , et al. 11, 2012, PLOS ONE, Vol. 7, pp. 1-10.}
^{3. Molecular phenotyping of aging in single yeast cells using a novel microfluidic device. Xie, Zhengwei, et al. 2012, Aging Cell, Vol. 11, pp. 599–606.}
^{4. Whole lifespan microscopic observation of budding yeast aging through a microfluidic dissection platform. Lee, Sung Sik , et al. 13, 2012, PNAS, Vol. 109, pp. 4916–4920.}
^{5. High-throughput analysis of yeast replicative aging using a microfluidic system. Jo, Myeong Chan , et al. 2015, PNAS, Vol. 112, pp. 9364–9369.}
^{6. A simple microfluidic platform to study age-dependent protein abundance and localization changes in Saccharomyces cerevisiae. Cabrera, Margarita, et al. 5, 2017, Microbial Cell, Vol. 4, pp. 169-174.}

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Mohammad Ali Zoljalali

Mohammad Ali Zoljalali is a Master's degree student at the University of Mazandaran. He is working on microfluidics by the focus on microchannels heat sinks and Drug Delivery.

https://www.researchgate.net/profile/Mohammad_Ali_Zoljalali

The post How is Microfluidics Used in Aging Research? appeared first on The MicroFluidic Circle.