Nature+Publishing+Group
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Expanding Gas Storage Capacity of Nanoporous Materials
A KAIST research team led by Professor Jihan Kim of the Department of Chemical and Biomolecular Engineering has successfully proposed a rational defect engineering methodology that can greatly enhance the gas storage capacity of nanoporous materials. The team conducted a high-throughput computational screening of a large experimental metal-organic framework database to identify 13 candidate materials that could experience significant methane uptake enhancement with only a small proportion of linker vacancy defects.
This research was published online on November 16 in Nature Communications, with M.S. candidate Sanggyu Chong from KAIST as the first author and post-doctorate researcher Günther Thiele from the Department of Chemistry at UC Berkeley as a contributing author.
Metal-organic frameworks, hereinafter MOF, are crystalline nanoporous materials that are comprised of metal clusters and organic linkers continuously bound together by coordination bonds. Due to their ultrahigh surface areas and pore volumes, they have been widely studied for various energy and environment applications.
Similar to other crystalline materials, MOFs are never perfectly crystalline and are likely to contain several different types of defects within their crystalline structures. Among these defects, linker vacancy defects, or the random absence of linker vacancies in their designated bonding positions, are known to be controllable by practicing careful control over the synthesis conditions.
The research team combined the concepts of rational defect engineering over the linker vacancy defects and the potential presence of inaccessible pores within MOFs to propose a methodology where controlled the introduction of linker vacancy defects could lead to a dramatic enhancement in gas adsorption and storage capacities.
The study utilized a Graphic Processing Unit (GPU) code developed by Professor Kim in a high-throughput computational screening of 12,000 experimentally synthesized MOFs to identify the structures with significant amounts of pores that were inaccessible for methane. In determining the presence of inaccessible pores, a flood-fill algorithm was performed over the energy-low regions of the structure, which is the same algorithm used for filling an area with color in Microsoft Paint.
For the MOFs with significant amounts of inaccessible pores, as determined from the screening, the research team emulated linker vacancy defects in their crystalline structures so that the previously inaccessible pores would be newly merged into the main adsorption channel with the introduction of defects for additional surface area and pore volume available for adsorption. The research team successfully identified 13 structures that would experience up to a 55.56% increase in their methane uptake with less than 8.33% of the linker vacancy defects.
The research team believes that this rational defect engineering scheme can be further utilized for many other applications in areas such as selective adsorption of an adsorbate from a gas mixture and the semi-permanent capture of gas molecules.
This research was conducted with the support of the Mid-career Research Program of the National Research Foundation of Korea.
Figure1. A diagram for flood fill algorithm and example of identification of inaccessible regions within the MOFs, using the flood fill algorithm
Figure2. Methane energy contours before and after detect introduction
2017.12.04 View 7134 -
New Photocatalyst Converts Carbon Dioxide to 99% Pure Fuel
(Professor Song, Ph.D. candidates Kim, and Lim (from left))
A KAIST research team led by Professor Hyunjoon Song of the Department of Chemistry developed a metal oxide nanocatalyst that converts carbon dioxide to 99% pure methane. This technology directly uses sunlight to convert carbon dioxide into methane, which is more efficient in terms of energy storage capacity, compared to the conventional way of storing the electricity produced by solar cells in batteries. The research team used cheap catalytic materials to significantly enhance the reaction efficiency and selectivity of the chemical energy storage method.
This research was conducted as a joint research project with Professor Ki Min Nam at Mokpo National University with co-first authors Dr. Kyung-Lyul Bae and Ph.D. candidates Jinmo Kim and Chan Kyu Lim. The study was published in Nature Communications on November 7.
Although there is growing interest in sunlight as an energy resource, its usage has been limited to daytime and the power output varies with the weather. If sunlight could be directly converted to chemical energy, such as fuel, the limitations of energy storage and its usage could be overcome. In particular, the usage of sunlight to convert carbon dioxide, a main cause of the greenhouse effect in our atmosphere, is of great interest since both energy and environmental issues can be addressed. However, the stability of carbon dioxide made it difficult to convert it to other molecules. Thus, there was a need for a catalyst with enhanced efficiency and selectivity.
Professor Song’s team synthesized zinc oxide nanoparticles, often used in sun cream. The nanoparticles were then bound to copper oxide as single particles, forming a colloidal form of zinc oxide-copper oxide nanoparticles. Zinc oxides produce high energy electrons using light, and this energy is used to convert carbon dioxide into methane. Further, zinc oxide can also produce electrons with light and transfer the electrons to copper oxide. Similar to the principles of photosynthesis in leaves, the electron transfer reaction could be maintained for a long time. As a consequence, although the reaction was conducted in aqueous solution, methane of 99% purity could be obtained from carbon dioxide.
Conventional heterogeneous photocatalysts were in solid powder form with irregular structures and were not dispersed in water. Professor Song’s team used a nanochemical synthesis method to control the structure of the catalyst particles to be regular and maintained over a large surface area. This led to increasing carbon dioxide conversion activity by hundreds of fold in solution compared to existing catalysts.
Professor Song said, “A long time will be needed for the commercialization of the direct conversion reaction of carbon dioxide using sunlight. However, the precise control of catalyst structures at nanoscale would enhance the efficiency of photocatalyst reactions.” He continued, “Applying this method to various phtocatalysts will maximize the catalysts performance.”
(Figure 1. Scheme for carbon dioxide conversion reaction using nano photocatalyst in aqueous solution)
(Figure 2. Structure, photocatalytic CO2 conversion, and stability of ZnO-Cu2O nanocatalyst )
2017.11.13 View 7857 -
Mutant Gene Network in Colon Cancer Identified
The principles of the gene network for colon tumorigenesis have been identified by a KAIST research team. The principles will be used to find the molecular target for effective anti-cancer drugs in the future. Further, this research gained attention for using a systems biology approach, which is an integrated research area of IT and BT.
The KAIST research team led by Professor Kwang-Hyun Cho for the Department of Bio and Brain Engineering succeeded in the identification. Conducted by Dr. Dongkwan Shin and student researchers Jonghoon Lee and Jeong-Ryeol Gong, the research was published in Nature Communications online on November 2.
Human cancer is caused by genetic mutations. The frequency of the mutations differs by the type of cancer; for example, only around 10 mutations are found in leukemia and childhood cancer, but an average of 50 mutations are found in adult solid cancers and even hundreds of mutations are found in cancers due to external factors, such as with lung cancer.
Cancer researchers around the world are working to identify frequently found genetic mutations in patients, and in turn identify important cancer-inducing genes (called ‘driver genes’) to develop targets for anti-cancer drugs. However, gene mutations not only affect their own functions but also affect other genes through interactions. Therefore, there are limitations in current treatments targeting a few cancer-inducing genes without further knowledge on gene networks, hence current drugs are only effective in a few patients and often induce drug resistance.
Professor Cho’s team used large-scale genomic data from cancer patients to construct a mathematical model on the cooperative effects of multiple genetic mutations found in gene interaction networks. The basis of the model construction was The Cancer Genome Atlas (TCGA) presented at the International Cancer Genome Consortium. The team successfully quantified the effects of mutations in gene networks to group colon cancer patients by clinical characteristics.
Further, the critical transition phenomenon that occurs in tumorigenesis was identified using large-scale computer simulation analysis, which was the first hidden gene network principle to be identified. Critical transition is the phenomenon in which the state of matter is suddenly changed through phase transition. It was not possible to identify the presence of transition phenomenon in the past, as it was difficult to track the sequence of gene mutations during tumorigenesis.
The research team used a systems biology-based research method to find that colon cancer tumorigenesis shows a critical transition phenomenon if the known driver gene mutations follow sequentially. Using the developed mathematical model, it can be possible to develop a new anti-cancer targeting drug that most effectively inhibits the effects of many gene mutations found in cancer patients. In particular, not only driver genes, but also other passenger genes affected by the gene mutations, could be evaluated to find the most effective drug targets.
Professor Cho said, “Little was known about the contribution of many gene mutations during tumorigenesis.” He continued, “In this research, a systems biology approach identified the principle of gene networks for the first time to suggest the possibility of anti-cancer drug target identification from a new perspective.”
This research was funded by the Ministry of Science and ICT and the National Research Foundation of Korea.
Figure1. Formation of giant clusters via mutation propagation
Figure2. Critical transition phenomenon by cooperative effect of mutations in tumorigenesis
2017.11.10 View 7326 -
Highly Flexible Organic Flash Memory for Foldable and Disposable Electronics
A KAIST team reported ultra-flexible organic flash memory that is bendable down to a radius of 300μm. The memory exhibits a significantly-long projected retention rate with a programming voltage on par with the present industrial standards.
A joint research team led by Professor Seunghyup Yoo of the School of Electrical Engineering and Professor Sung Gap Im of the Department of Chemical and Biomolecular Engineering said that their memory technology can be applied to non-conventional substrates, such as plastics and papers, to demonstrate its feasibility over a wide range of applications.
With Dr. Seungwon Lee and Dr. Hanul Moon playing the role of leading authors, the research was published in Nature Communications on September 28.
Flash memory is a non-volatile, transistor-based data-storage device that has become essential in most electronic systems in daily life. With straightforward operation mechanisms and easy integration into NAND or NOR array architecture, flash memory has been established as the most successful and dominant non-volatile memory technology by far.
Despite promising demonstrations in the early stages of organic electronics, the overall progress in this field has been far slower than that of thin-film transistors (TFTs) or other devices based on flexible materials. It has been challenging, in particular, to develop flash memory that simultaneously exhibits a significant level of flexibility and performance. This is mainly due to the scarcity of flexible dielectric layers, which are responsible for the tunneling and blocking of charges.
The solution processing used for the preparation of most of the polymeric dielectric layers also makes it difficult to use them in flash memory due to the complexity involved in the formation of the bilayer dielectric structure, which is the key to flash memory operations.
The research team tried to overcome these hurdles and realize highly flexible flash memory by employing thin polymeric insulators grown with initiated chemical vapor deposition (iCVD), a vapor-phase growth technique for polymers that was previously shown to be promising for the fabrication of flexible TFTs. It was further shown that these iCVD-based polymeric insulators, when coupled with rational device design and material choice, can make a significant contribution to flash memory as well.
Memory using conventional polymer insulating films has often required a voltage as high as 100 V (volt) in order to attain long memory retention. If the device is made to operate at a low voltage, the short retention period of less than a month was problematic.
The KAIST team produced flash memory with programming voltages around 10 V and a projected data retention time of over 10 years, while maintaining its memory performance even at a mechanical strain of 2.8%. This is a significant improvement over the existing inorganic insulation layer-based flash memory that allowed only a 1% strain.
The team demonstrated the virtually foldable memory devices by fabricating the proposed flash memory on a 6-micrometer-thick ultrathin plastic film. In addition, it succeeded in producing them on printing paper, opening a way for disposable smart electronic products such as electronic paper and electronic business card.
Professor Yoo said, " This study well illustrates that even highly flexible flash memory can be made to have a practically viable level of performance, so that it contributes to full-fledged wearable electronic devices and smart electronic paper."
(Figure 1. Structure of flexible flash memory )
(Figure 2. Foldable flash memory)
2017.11.06 View 8555 -
High-Speed Motion Core Technology for Magnetic Memory
(Professor Kab-Jin Kim of the Department of Physics)
A joint research team led by Professor Kab-Jin Kim of the Department of Physics, KAIST and Professor Kyung-Jin Lee at Korea University developed technology to dramatically enhance the speed of next generation domain wall-based magnetic memory. This research was published online in Nature Materials on September 25.
Currently-used memory materials, D-RAM and S-RAM, are fast but volatile, leading to memory loss when the power is switched off. Flash memory is non-volatile but slow, while hard disk drives (HDD) have greater storage but are high in energy usage and weak in physical shock tolerance.
To overcome the limitations of existing memory materials, ‘domain wall-based, magnetic memory’ is being researched. The core mechanism of domain wall magnetic memory is the movement of a domain wall by the current. Non-volatility is secured by using magnetic nanowires and the lack of mechanical rotation reduced power usage. This is a new form of high density, low power next-generation memory.
However, previous studies showed the speed limit of domain wall memory to be hundreds m/s at maximum due to the ‘Walker breakdown phenomenon’, which refers to velocity breakdown from the angular precession of a domain wall. Therefore, there was a need to develop core technology to remove the Walker breakdown phenomenon and increase the speed for the commercialization of domain wall memory.
Most domain wall memory studies used ferromagnetic bodies, which cannot overcome the Walker breakdown phenomenon. The team discovered that the use of ‘ferrimagnetic‘ GdFeCo at certain conditions could overcome the Walker breakdown phenomenon and using this mechanism they could increase domain wall speed to over 2Km/s at room temperature.
Domain wall memory is high-density, low-power, and non-volatile memory. The memory could be the leading next-generation memory with the addition of the high speed property discovered in this research.
Professor Kim said, “This research is significant in discovering a new physical phenomenon at the point at which the angular momentum of a ferrimagnetic body is 0 and it is expected to advance the implementation of next-generation memory in the future.”
This research was funded by the National Research Foundation of Korea (NRF) grant funded by the Korea Government (MSIP) (No. 2017R1C1B2009686, NRF-2016R1A5A1008184) and by the DGIST R&D Program of the Ministry of Science, ICT and Future Planning (17-BT-02).
(Figure 1. Concept Map of Domain Wall Memory Material using Ferrimagnetic Body)
(Figure 2. Scheme and Experimental Results of Domain Wall Speed Measurements)
2017.10.30 View 7621 -
Solutal Marangoni Flows of Miscible Liquid Drive Transport without Surface Contamination
(Professor Hyoungsoo Kim, Department of Mechanical Engineering, KAIST)
A research team led by Hyoungsoo Kim, a professor of Mechanical Engineering at KAIST, succeeded in quantifying the phenomenon called, the Marangoni effect, which occurs at the interface between alcohol and water. It is expected that this finding will be a valuable resource used for effectively removing impurities from a surface fluid without any contamination, and developing materials that can replace surfactants.
This research, co-conducted with a research team led by Professor Howard A. Stone at Princeton University, was published online in Nature Physics on July 31.
The Marangoni effect, also known as tears of wine, is generated when two fluids having a different surface tension meet, causing finite mixing, spreading time and length scale. Typically, people believe that infinitely miscible liquids immediately mix together; however, it is not always true according to this paper.
The typical surface tension of alcohol is three times lower than that of water, and this different surface tension generates the Marangoni-driven convection flow at the interface of the two liquids. In addition, there is a certain amount of time required for them to mix.
This phenomenon has been discussed many times since it was discovered in early the 20th century, yet there was a limit to quantifying and explaining it.
Professor Kim, considering the mixing and spreading mechanism, used various flow visualization techniques and equipment for capturing high speed images in his experiment.
Through the flow visualization methods, the team succeeded in quantifying and explaining the complex, physicochemical phenomenon generated between water and alcohol. Moreover, they developed a theoretical model to predict the physicochemical hydrodynamic phenomena.
The theoretical model can predict the speed of Marangoni-driven convection flow, the area of a drop of alcohol and the time required to develop the flow field. Hence, this model can map out types of materials (e.g., alcohol) and the volume of a drop of liquid as applicable to target a specific situation.
Moreover, the research team believes that the interfacial flow enables the driving of bulk flows and that it can be a source of technology for effectively delivering drugs and removing impurities from a surface of substance without causing secondary contamination.
Above all, the results show a possibility for replacing surfactant with alcohol as a material used for delivering drugs. In the case of the drug delivery, some drugs are encapsulated with a surfactant in order to be effectively transported in vivo; however, the surfactant accumulates in the body, which can cause various side effects, such as heart disease. Therefore, using new materials like alcohol for drug delivery will contribute to preventing the side effects caused by the surfactant.
“The surfactant is used for delivering drugs, but it is difficult to be expelled from the body. This will cause various side effects, such as heart diseases in asthmatic patients,” said Professor Kim. “I hope that using new materials, like alcohol, will free people from these side effects.”
(Marangoni-driven convection flow generated at the interface between water and alcohol, and the flow visualization results)
- A drop of alcohol on a water surface
- Comparison of mixing structures on the surface
- Marangoni mixing flow under the free surface
2017.08.18 View 7415 -
Cooperative Tumor Cell Membrane-Targeted Phototherapy
A KAIST research team led by Professor Ji-Ho Park in the Bio and Brain Engineering Department at KAIST developed a technology for the effective treatment of cancer by delivering synthetic receptors throughout tumor tissue. The study, led by Ph.D. candidate Heegon Kim, was published online in Nature Communications on June 19.
Cancer targeted therapy generally refers to therapy targeting specific molecules that are involved in the growth and generation of cancer. The targeted delivery of therapeutics using targeting agents such as antibodies or nanomaterials has improved the precision and safety of cancer therapy.
However, the paucity and heterogeneity of identified molecular targets within tumors have resulted in poor and uneven distribution of targeted agents, thus compromising treatment outcomes.
To solve this problem, the team constructed a cooperative targeting system in which synthetic and biological nanocomponents participate together in the tumor cell membrane-selective localization of synthetic receptors to amplify the subsequent targeting of therapeutics. Here, synthetic and biological nanocomponents refer to liposomes and extracellular vesicles, respectively.
The synthetic receptors are first delivered selectively to tumor cell membranes in the perivascular region using liposomes. By hitchhiking with extracellular vesicles secreted by the cells, the synthetic receptors are transferred to neighboring cells and further spread throughout the tumor tissues where the molecular targets are limited.
Hitchhiking extracellular vesicles for delivery of synthetic receptors was possible since extracellular vesicles, such as exosomes, mediate intercellular communications by transferring various biological components such as lipids, cytosolic proteins, and RNA through a membrane fusion process. They also play a supportive role in promoting tumor progression in that tumor-derived extracellular vesicles deliver oncogenic signals to normal host cells.
The team showed that this tumor cell membrane-targeted delivery of synthetic receptors led to a uniform distribution of synthetic receptors throughout a tumor and subsequently led to enhanced phototherapeutic efficacy of the targeted photosensitizer.
Professor Park said, “The cooperative tumor targeting system is expected to be applied in treating various diseases that are hard to target.”
The research was funded by the Basic Science Research Program through the National Research Foundation funded by the Ministry of Science, ICT & Future Planning, and the National R&D Program for Cancer Control funded by the Ministry for Health and Welfare.
(Ph.D. candidates Hee Gon Kim (left) and Chanhee Oh)
Figure 1. A schematic of a cooperative tumor targeting system via delivery of synthetic receptors.
Figure 2. A confocal microscopic image of a tumor section after cooperative targeting by synthetic receptor delivery. Green and magenta represent vessels and therapeutic agents inside a tumor respectively.
2017.07.07 View 9555 -
Bio-based p-Xylene Oxidation into Terephthalic Acid by Engineered E.coli
KAIST researchers have established an efficient biocatalytic system to produce terephthalic acid (TPA) from p-xylene (pX). It will allow this industrially important bulk chemical to be made available in a more environmentally-friendly manner.
The research team developed metabolically engineered Escherichia coli (E.coli) to biologically transform pX into TPA, a chemical necessary in the manufacturing of polyethylene terephthalate (PET). This biocatalysis system represents a greener and more efficient alternative to the traditional chemical methods for TPA production. This research, headed by Distinguished Professor Sang Yup Lee, was published in Nature Communications on May 31.
The research team utilized a metabolic engineering and synthetic biology approach to develop a recombinant microorganism that can oxidize pX into TPA using microbial fermentation. TPA is a globally important chemical commodity for manufacturing PET. It can be applied to manufacture plastic bottles, clothing fibers, films, and many other products. Currently, TPA is produced from pX oxidation through an industrially well-known chemical process (with a typical TPA yield of over 95 mol%), which shows, however, such drawbacks as intensive energy requirements at high temperatures and pressure, usage of heavy metal catalysts, and the unavoidable byproduct formation of 4-carboxybenzaldehyde.
The research team designed and constructed a synthetic metabolic pathway by incorporating the upper xylene degradation pathway of Pseudomonas putida F1 and the lower p-toluene sulfonate pathway of Comamonas testosteroni T-2, which successfully produced TPA from pX in small-scale cultures, with the formation of p-toluate (pTA) as the major byproduct. The team further optimized the pathway gene expression levels by using a synthetic biology toolkit, which gave the final engineered E. coli strain showing increased TPA production and the complete elimination of the byproduct.
Using this best-performing strain, the team designed an elegant two-phase (aqueous/organic) fermentation system for TPA production on a larger scale, where pX was supplied in the organic phase. Through a number of optimization steps, the team ultimately achieved production of 13.3 g TPA from 8.8 g pX, which represented an extraordinary yield of 97 mol%.
The team has developed a microbial biotechnology application which is reportedly the first successful example of the bio-based production of TPA from pX by the microbial fermentation of engineered E. coli. This bio-based TPA technology presents several advantages such as ambient reaction temperature and pressure, no use of heavy metals or other toxic chemicals, the removable of byproduct formation, and it is 100% environmentally compatible.
Professor Lee said, “We presented promising biotechnology for producing large amounts of the commodity chemical TPA, which is used for PET manufacturing, through metabolically engineered gut bacterium. Our research is meaningful in that it demonstrates the feasibility of the biotechnological production of bulk chemicals, and if reproducible when up-scaled, it will represent a breakthrough in hydrocarbon bioconversions.”
Ph.D. candidate Zi Wei Luo is the first author of this research (DOI:10.1038/ncomms15689).The research was supported by the Intelligent Synthetic Biology Center through the Global Frontier Project (2011-0031963) of the Ministry of Science, ICT & Future Planning through the National Research Foundation of Korea.
Figure: Biotransformation of pX into TPA by engineered E. coli.
This schematic diagram shows the overall conceptualization of how metabolically engineered E. coli produced TPA from pX. The engineered E. coli was developed through reconstituting a synthetic metabolic pathway for pX conversion to TPA and optimized for increased TPA yield and byproduct elimination. Two-phase partitioning fermentation system was developed for demonstrating the feasibility of large-scale production of TPA from pX using the engineered E. coli strains, where pX was supplied in the organic phase and TPA was produced in the aqueous phase.
2017.06.05 View 10375 -
Observation of the Phase Transition of Liquid Crystal Defects
KAIST researchers observed the phase transition of topological defects formed by liquid crystal (LC) materials for the first time.
The phase transition of topological defects, which was also the theme of the Nobel Prize for Physics in 2016, can be difficult to understand for a layperson but it needs to be studied to understand the mysteries of the universe or the underlying physics of skyrmions, which have intrinsic topological defects.
If the galaxy is taken as an example in the universe, it is difficult to observe the topological defects because the system is too large to observe some changes over a limited period of time. In the case of defect structures formed by LC molecules, they are not only a suitable size to observe with an optical microscope, but also the time period in which the phase transition of a defect occurring can be directly observed over a few seconds, which can be extended to a few minutes. The defect structures formed by LC material have radial, circular, or spiral shapes centering on a singularity (defect core), like the singularity that was already introduced in the famous movie "Interstellar,” which is the center point of black hole.
In general, LC materials are mainly used in liquid crystal displays (LCDs) and optical sensors because it is easy to control their specific orientation and they have fast response characteristics and huge anisotropic optical properties. It is advantageous in terms of the performance of LCDs that the defects of the LC materials are minimized. The research team led by Professor Dong Ki Yoon in the Graduate School of Nanoscience and Technology did not simply minimize such defects but actively tried to use the LC defects as building blocks to make micro- and nanostructures for the patterning applications. During these efforts, they found the way to directly study the phase transition of topological defects under in-situ conditions.
Considering the LC material from the viewpoint of a device like a LCD, robustness is important. Therefore, the LC material is injected through the capillary phenomenon between a rigid two-glass plate and the orientation of the LCs can be followed by the surface anchoring condition of the glass substrate. However, in this conventional case, it is difficult to observe the phase transition of the LC defect due to this strong surface anchoring force induced by the solid substrate.
In order to solve this problem, the research team designed a platform, in which the movement of the LC molecules was not restricted, by forming a thin film of LC material on water, which is like oil floating on water. For this, a droplet of LC material was dripped onto water and spread to form a thin film. The topological defects formed under this circumstance could show the thermal phase transition when the temperature was changed. In addition, this approach can trace back the morphology of the original defect structure from the sequential changes during the temperature changes, which can give hints to the study of the formation of topological defects in the cosmos or skyrmions.
Prof. Yoon said, “The study of LC crystal defects itself has been extensively studied by physicists and mathematicians for about 100 years. However, this is the first time that we have observed the phase transition of LC defects directly.” He also added, "Korea is leading in the LCD industry, but our basic research on LCs is not at the world's research level."
The first author of this study is Dr. Min-Jun Gimand supported by a grant from the National Research Foundation (NRF) and funded by the Korean Government (MSIP). The research result was published on May 30, 2017 in Nature Communications.
Figure 1. The phase transition of the LC topological defect on cooling.
Figure 2. Polarizing optical microscopy images of topological defects depending on the strength of the director field. (a,b,e) Convergent director field arrangements of LC molecules and corresponding schematic images; (c,d,f) Divergent director field arrangements of LC molecules and corresponding schematic images.
2017.06.02 View 8673 -
Controlling 3D Behavior of Biological Cells Using Laser Holographic Techniques
A research team led by Professor YongKeun Park of the Physics Department at KAIST has developed an optical manipulation technique that can freely control the position, orientation, and shape of microscopic samples having complex shapes. The study has been published online in Nature Communications on May 22.
Conventional optical manipulation techniques called “optical tweezers,” have been used as an invaluable tool for exerting micro-scale force on microscopic particles and manipulating three-dimensional (3-D) positions of particles. Optical tweezers employ a tightly-focused laser whose beam diameter is smaller than one micrometer (1/100 of hair thickness), which can generate attractive force on neighboring microscopic particles moving toward the beam focus. Controlling the positions of the beam focus enabled researchers to hold the particles and move them freely to other locations so they coined the name “optical tweezers,” and have been widely used in various fields of physical and biological studies.
So far, most experiments using optical tweezers have been conducted for trapping spherical particles because physical principles can easily predict optical forces and the responding motion of microspheres. For trapping objects having complicated shapes, however, conventional optical tweezers induce unstable motion of such particles, and controllable orientation of such objects is limited, which hinder controlling the 3-D motion of microscopic objects having complex shapes such as living cells.
The research team has developed a new optical manipulation technique that can trap complex objects of arbitrary shapes. This technique first measures 3-D structures of an object in real time using a 3-D holographic microscope, which shares the same physical principle of X-Ray CT imaging. Based on the measured 3-D shape of the object, the researchers precisely calculates the shape of light that can stably control the object. When the shape of light is the same as the shape of the object, the energy of the object is minimized, which provides the stable trapping of the object having the complicated shape.
Moreover, by controlling the shape of light to have various positions, directions, and shapes of objects, it is possible to freely control the 3-D motion of the object and make the object have a desired shape. This process resembles the generation of a mold for casting a statue having desired shape so the researchers coined the name of the present technique “tomographic mold for optical trapping (TOMOTRAP).” The team succeeded in trapping individual human red blood cells stably, rotating them with desired orientations, folding them in an L-shape, and assembling two red blood cells together to form a new structure. In addition, colon cancer cells having a complex structure could be stably trapped and rotated at desired orientations. All of which have been difficult to be realized by the conventional optical techniques.
Professor Park said, “Our technique has the advantage of controlling the 3-D motion of complex shaped objects without knowing prior information about their shape and optical characteristics, and can be applied in various fields including physics, optics, nanotechnology, and medical science.”
Dr. Kyoohyun Kim, the lead author of this paper, noted that this technique can induce controlled deformation of biological cells with desired shapes. “This approach can be also applied to real-time monitoring of surgical prognosis of cellular-level surgeries for capturing and deforming cells as well as subcellular organelles,” added Kim.
Figure 1. Concept of optical manipulation techniques
Figure 2. Experimental setup
Figure 3. Research results
2017.05.25 View 7856 -
Processable High Internal Phase Pickering Emulsion Using Depletion Attraction
Professor Siyoung Choi’s research team from the KAIST Department of Chemical & Biomolecular Engineering used physical force to successfully produce a stable emulsion.
Emulsions, commonly known as cosmetic products, refer to stably dispersed structures of oil droplets in water (or water droplets in oil). Pickering emulsions refer to emulsions stabilized using solid particles, instead of detergent. Traditionally, it is said that water and oil do not mix. Until recently, detergent was added to mix oil and water for dispersion. Emulsions have traditionally been produced using this technique and are currently used for products such as mayonnaise, sun block, and lotion.
On the other hand, Pickering emulsions have been used after stabilization of chemical treatments on solid particle surfaces to enhance adsorption power. However, there were limitations in its application, since the treatment process is complex and its applicable range remains limited. Instead of chemical treatment on Pickering emulsion surfaces, the research team mixed small macromolecules a few nanometer in size with larger solid particles (tens of nanometers to a few micrometers). This induced depletion force was used to successfully stabilize the emulsion.
Depletion force refers to the force a large number of small particles induces to aggregate the bigger particles, in order to secure free space for themselves. In short, the force induces an attraction between larger particles. Until now, depletion force could only be applied to solids and solid particles. However, the research team used macromolecules and large particles such as solid particles and oil droplets to show the applicability of depletion force between solids and liquids. By introducing macromolecules that act as smaller particles, hydrophilic solid particles enhanced the adsorption of solid particles to the oil droplet surface, while preventing dissociation from the particle surface, resulting in the maintenance of a stable state.
The research team confirmed the possibility of the simple production of various porous macromolecular materials using stable Pickering emulsions. Such porous macromolecules are expected to be applicable in separation film, systems engineering, drug delivery, and sensors, given their large surface area.
Professor KyuHan Kim, the first author said, “Until now, depletion force has only been used between solid colloid particles. This research has scientific significance since it is the first example of using depletion force between solid particles and liquid droplets.”
Professor Choi said, “Beyond its academic significance, this technology could contribute to industries and national competitiveness.” He continued, “Since this technology uses physical force, not chemical, to produce stable emulsion, it can be used regardless of the type of solid particle and macromolecule. Further, it could be used in customized porous material production for special purposes.”
The research was published in Nature Communications online on February 1. In particular, this research is significant since an undergraduate student, Subeen Kim, participated in the project as a second author through the KAIST Undergraduate Research Program (URP). This research was funded by the National Research Foundation of Korea.
(Figure 1: Images of the inner structure of porous macromolecules produced using the new technology)
(Figure 2: Images showing the measurement of rheological properties of Pickering emulsions and system processability)
(Figure 3: Images showing a stable Pickering emulsion system)
2017.04.19 View 8185 -
Expanding the Genetic Code of Mus Musculus
Professor Hee-Sung Park of the Department of Chemistry, who garnered attention for his novel strategy of installing authentic post-translational modifications into recombinant proteins, expanded his research portfolio to another level. Professor Park’s team was the first to report the generation of a mouse strain with an expanded genetic code, allowing site-specific incorporation of unnatural amino acids.
Professor Park published the research on the new chemical biology method for achieving selective chemical modifications in proteins in Science last September. The research team, this time in collaboration with Professor Chan Bae Park of the Department of Physiology at the Ajou University School of Medicine, demonstrated temporal and spatial control of protein acetylation in various organs of the transgenic mouse using a recombinant green fluorescent protein as a model protein. This research was published in the online edition of Nature Communications on February 21.
This approach enables the rapid onset of position-specific acetylation of a target protein at any developmental stage, facilitating temporal and spatial control of protein acetylation in various organs of the transgenic mouse. Such temporal and spatial control of protein acetylation will be of prime importance for investigating many essential biological processes and human diseases at the tissue and organism level.
Almost all human proteins, the products of about 25,000 genes, are known to undergo various post-translational modifications during and after synthesis. Post-translation modifications regulate the function of cellular proteins, playing a key role in many essential processes such as delivering signals and body growth. However, the unusual protein modifications, aroused from genetic and/or environmental factors, trigger severe diseases including cancer, dementia, and diabetes.
The team inserted transgenes into the mouse genome to allocate the site-specific addition of unnatural amino acids. The researchers inserted a modified version of lysine into the house mice, which allowed for the control of the acetylation. They used recombinant green fluorescent proteins from transgenic house mice as models for control of the acetylation.
The team was also able to regulate the acetylation of specific temporal and spatial frames in the mice, restraining the abnormality in proteins to certain organs such as the liver and kidneys. The research team said the strategy will provide a powerful tool for systematic in vivo study of cellular proteins in the most commonly used mammalian model organisms for human physiology and disease. Professor Park said, “This method can be easily extended to generate a wide range of custom-made transgenic mouse strains for further investigating diverse proteins of interest.” He added, “This method can be further extended to generate a wide range of custom-made transgenic mouse strains, opening a new paradigm for investigating anti-cancer and cerebral disease treatments.
This work was supported by grants from KAIST Systems Healthcare and the Medicinal Bioconvergence Research Center and the Intelligent Synthetic Biology Center of the Global Frontier Project funded by the Ministry of Science, ICT & Future Planning and the Ministry of Food and Drug Safety.
(Figure:Temporal and spatial control of in vivo protein acetylation)
(a) Temporal expression of acetylated GFPuv in the AcK-GFPamber mouse. The expression of GFPuv in skeletal muscle, liver, and lung tissues was detected only in the AcK-injected mouse. Scale bar, 200 µm. (b) Western blotting of anti-FLAG-immunoprecipitated proteins from tissues of the AcK-GFPamber mouse. Acetylated GFPuv was produced after AcK injection. (c) Spatial expression of acetylated GFPuv in the AcK-GFPamber mouse. Acetylated GFPuv was observed only in skeletal muscle when AcK was directly delivered to the tissues. Sacle bar, 200 µm.
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