Suspended microchannel resonators for ultralow volume universal detection

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2008.Includes bibliographical references (leaves 32-33).Universal detectors that maintain high sensitivity as the detection volume is reduced to the sub-nanoliter scale can enhance the utility of miniaturized total analysis systems ([mu]-TAS). Here the unique scaling properties of the suspended microchannel resonator (SMR) are exploited to show universal detection in a 10 pL analysis volume with a density detection limit of ~1 ([mu]g/cm³ (10 Hz bandwidth) and a linear dynamic range of six decades. Analytes with low UV extinction coefficients such as polyethylene glycol (PEG) 8 KDa, glucose, and glycine are measured with molar detection limits of 0.66 ([mu]M, 13.5 ([mu]M, and 31.6 ([mu]M, respectively. To demonstrate the potential for real-time monitoring, gel filtration chromatography was used to separate different molecular weights of PEG as the SMR acquired a chromatogram by measuring the eluate density. This work suggests that the SMR could offer a simple and sensitive universal detector for various separation systems from liquid chromatography to capillary electrophoresis. Moreover, since the SMR is itself a microfluidic channel, it can be directly integrated into ([mu]-TAS without compromising overall performance.by Sungmin Son.S.M

Carbonization Study of Cellulose Nanocrystals and Super Engineering Plastic Based Nano Composite Fibers

Department of Materials Science and EngineeringCellulose Nanocrystals (CNC) have been regarded as a versatile precursor for carbon nanomaterials. CNC can be converted into carbon materials by hydrothermal treatment and subsequent carbonization process. Due to high crystallinity and structural regularity of CNC, carbonized CNC would give well-ordered graphitic structure compared to other cellulose-based carbon materials. In chapter 2, carbonization study of CNC covers the effect of heat treatment conditions on the structural development mechanism of CNC over the range of carbonization temperature from 1000 to 2500 oC. We have conducted experiments to study the effect of oxidative stabilization process on the structural development of CNC-based graphite. Compared to the carbonization mechanism of pristine CNC, stabilized CNC was prepared by heat treatment at 250 oC for 1hr. In addition, the resultant graphitic structure of carbonized cellulose nanocrystals was systemically analyzed by transmission electron microscopy, x-ray photoelectron spectroscopy, and Raman spectroscopy. TEM data clarified that carbonized CNC prepared from stabilized samples (S-cCNC) gave rise to more highly ordered graphitic structure with little distortion site and defect points compare to D-cCNC over the whole temperature range of carbonization. Structural development mechanisms of both C- and S-cCNCs were systematically traced by Raman spectroscopy. Peak fitting results of Raman spectra evidenced structural conversion from disordered carbon to the graphitic structure. In chapter 3, PES/CNC composite fibers were prepared by dry-jet wet spinning and their morphology and tensile properties were characterized. CNC with high Young???s modulus, crystallinity and aspect ratio can be regarded as a nano-size reinforcing agent. Dispersion of CNC was investigated by Dynamic Light Scattering (DLS) and Scanning Electron Microscopy (SEM). Upon using bath-type sonication with a power of 20 J/s, 48 hr sonication time was required to obtain well-dispersed CNC phase in N,N-Dimethylacetamide (DMAc). Experimental results showed that the tensile modulus of PES/CNC1 composite fibers were 4.7 GPa about 17% higher than control PES fibers.clos

Precise single cell monitoring reveals principles of cell growth

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2013.Cataloged from PDF version of thesis.Includes bibliographical references (p. 104-107).Accumulation of mass is a fundamental cellular process that is associated with metabolism, signaling and regulation. Despite the wealth of knowledge in molecular processes the principles of growth in mammalian cells are poorly understood since growth has never been monitored with high precision. Development of the Suspended Microchannel Resonator (SMR), a microfluidic mass measurement system, enables femtogram cell mass resolution. With this, we developed a method to simultaneously measure molecular signals and single cell mass with high precision over multiple generations. First we investigated how cells control their size. It is known that proliferating cells govern the rate at which they build their biomass and divide, but the mechanism that is used to maintain size homeostasis remains unclear. We obtained over 1,000 hours of growth data from mouse lymphoblast and pro-B-cell lymphoid cell lines. Cell lineage analysis revealed a decrease in the growth rate variability at the G1/S phase transition, which suggests the presence of a growth rate threshold for maintaining size homeostasis. We could also identify unexpected aspects of the growth trajectory such as continuation of growth during M phase, large and switch-like drop in growth rate upon cytokinesis. We next studied the metabolic and energetic requirements necessary for cell growth by monitoring immediate single cell growth response to nutrient depletion. To this end, we developed a method to gently exchange the fluid surrounding a cell while constantly monitoring cell growth. We observed that cells immediately change the growth rate upon depletion of key nutrients such as glucose or glutamine. The growth rate change was surprisingly large but restored upon repletion of nutrients. This implies that immediate growth response integrates both loss of nutrient uptake and signaling associated with metabolism of the particular nutrient. We developed two platforms to measure single cell growth in high throughput. These advancements will broaden the application of the SMR to the study of primary cells or cancer cells.by Sungmin Son.Ph.D

Exploring the relationship between the spatial distribution of roads and universal pattern of travel-route efficiency in urban road networks

Urban road networks are well known to have universal characteristics and scale-invariant patterns, despite the different geographical and historical environments of cities. Previous studies on universal characteristics of the urban road networks mostly have paid attention to their network properties but often ignored the spatial networked structures. To fill the research gap, we explore the underlying spatial patterns of road networks. In doing so, we inspect the travel-route efficiency in a given road network across 70 global cities which provides information on the usage pattern and functionality of the road structure. The efficiency is quantified by the detour patterns of the travel routes, estimated by the detour index (DI). The DI is a long-standing popular measure, but its spatiality has been barely considered so far. In this study, we probe the behavior of DI with respect to spatial variables by scanning the network radially from a city center. Through empirical analysis, we first discover universal properties in DI throughout most cities, which are summarized as a constant behavior of DI regardless of the radial position from a city center and clear collapse into a single curve for DIs for various radii with respect to the angular distance. Especially, the latter enables us to know the scaling factor in the length scale. We also reveal that the core-periphery spatial structure of the roads induces the universal pattern, which is supported by an artificial road network model. Furthermore, we visualize the spatial DI pattern on the city map to figure out the city-specific characteristics. The most and least efficient connections of several representative cities show the potential for practical implications in analyzing individual cities.Comment: 11 pages, 6 figure

A microfluidic “baby machine” for cell synchronization

Common techniques used to synchronize eukaryotic cells in the cell cycle often impose metabolic stress on the cells or physically select for size rather than age. To address these deficiencies, a minimally perturbing method known as the “baby machine” was developed previously. In the technique, suspension cells are attached to a membrane, and as the cells divide, the newborn cells are eluted to produce a synchronous population of cells in the G1 phase of the cell cycle. However, the existing “baby machine” is only suitable for cells which can be chemically attached to a surface. Here, we present a microfluidic “baby machine” in which cells are held onto a surface by pressure differences rather than chemical attachment. As a result, our method can in principle be used to synchronize a variety of cell types, including cells which may have weak or unknown surface attachment chemistries. We validate our microfluidic “baby machine” by using it to produce a synchronous population of newborn L1210 mouse lymphocytic leukemia cells in G1 phase.National Cancer Institute (U.S.). Physical Sciences-Oncology Center (U54CA143874)National Institute of General Medical Sciences (U.S.) (EUREKA R01GM085457

Charge-spin correlation in van der Waals antiferromagenet NiPS3

Strong charge-spin coupling is found in a layered transition-metal trichalcogenide NiPS3, a van derWaals antiferromagnet, from our study of the electronic structure using several experimental and theoretical tools: spectroscopic ellipsometry, x-ray absorption and photoemission spectroscopy, and density-functional calculations. NiPS3 displays an anomalous shift in the optical spectral weight at the magnetic ordering temperature, reflecting a strong coupling between the electronic and magnetic structures. X-ray absorption, photoemission and optical spectra support a self-doped ground state in NiPS3. Our work demonstrates that layered transition-metal trichalcogenide magnets are a useful candidate for the study of correlated-electron physics in two-dimensional magnetic material.Comment: 6 pages, 3 figur

Strongly adhesive dry transfer technique for van der Waals heterostructure

That one can stack van der Waals materials with atomically sharp interfaces has provided a new material platform of constructing heterostructures. The technical challenge of mechanical stacking is picking up the exfoliated atomically thin materials after mechanical exfoliation without chemical and mechanical degradation. Chemically inert hexagonal boron nitride (hBN) has been widely used for encapsulating and picking up vdW materials. However, due to the relatively weak adhesion of hBN, assembling vdW heterostructures based on hBN has been limited. We report a new dry transfer technique. We used two vdW semiconductors (ZnPS3 and CrPS4) to pick up and encapsulate layers for vdW heterostructures, which otherwise are known to be hard to fabricate. By combining with optimized polycaprolactone (PCL) providing strong adhesion, we demonstrated various vertical heterostructure devices, including quasi-2D superconducting NbSe2 Josephson junctions with atomically clean interface. The versatility of the PCL-based vdW stacking method provides a new route for assembling complex 2D vdW materials without interfacial degradation.Comment: Accepted for publication in 2D Material

Direct observation of mammalian cell growth and size regulation

We introduce a microfluidic system for simultaneously measuring single cell mass and cell cycle progression over multiple generations. We use this system to obtain over 1,000 hours of growth data from mouse lymphoblast and pro-B-cell lymphoid cell lines. Cell lineage analysis revealed a decrease in the growth rate variability at the G1/S phase transition, which suggests the presence of a growth rate threshold for maintaining size homeostasis

Intracellular Water Exchange for Measuring the Dry Mass, Water Mass and Changes in Chemical Composition of Living Cells

We present a method for direct non-optical quantification of dry mass, dry density and water mass of single living cells in suspension. Dry mass and dry density are obtained simultaneously by measuring a cell’s buoyant mass sequentially in an H[subscript 2]O-based fluid and a D[subscript 2]O-based fluid. Rapid exchange of intracellular H[subscript 2]O for D[subscript 2]O renders the cell’s water content neutrally buoyant in both measurements, and thus the paired measurements yield the mass and density of the cell’s dry material alone. Utilizing this same property of rapid water exchange, we also demonstrate the quantification of intracellular water mass. In a population of E. coli, we paired these measurements to estimate the percent dry weight by mass and volume. We then focused on cellular dry density – the average density of all cellular biomolecules, weighted by their relative abundances. Given that densities vary across biomolecule types (RNA, DNA, protein), we investigated whether we could detect changes in biomolecular composition in bacteria, fungi, and mammalian cells. In E. coli, and S. cerevisiae, dry density increases from stationary to exponential phase, consistent with previously known increases in the RNA/protein ratio from up-regulated ribosome production. For mammalian cells, changes in growth conditions cause substantial shifts in dry density, suggesting concurrent changes in the protein, nucleic acid and lipid content of the cell.National Cancer Institute (U.S.). Physical Sciences-Oncology Center (U54CA143874)National Institutes of Health (U.S.) (Center for Cell Division Process Grant P50GM6876)National Institutes of Health (U.S.) (Contract R01CA170592)United States. Army Research Office (Institute for Collaborate Biotechnologies Contract W911NF-09-D-0001