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Author: Stephan Koehler (23)


Dec
2017

Colour is one of the most important visual attributes of food and is directly related to the perception of food quality. The interest in natural colourants, especially β-carotene that not only imparts colour but also has well-documented health benefits, has triggered the research and development of different protocols designed to entrap these hydrophobic natural molecules to improve their stability against oxidation. Here, we report a versatile microfluidic approach that uses single emulsion droplets as templates to prepare microparticles loaded with natural colourants.

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Dec
2017

Emulsions of two immiscible liquids can slowly coalesce over time when stabilized by surfactant molecules. Pickering emulsions stabilized by colloidal particles can be much more stable. Here, we fabricate biocompatible amphiphilic dimer particles using a hydrogel, a strongly hydrophilic material, and achieve large contrast in the wetting properties of the two bulbs, resulting in enhanced stabilization of emulsions.

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Mar
2017

Perforated single-layered graphene has demonstrated selectivity and flux that is orders of magnitude greater than state-of-the-art polymer membranes. However, only individual graphene sheets with sizes up to tens of micrometers have been successfully fabricated for pressurized permeation studies. Scaling-up and reinforcement of these atomic membranes with minimum cracks and pinholes remains a major hurdle for practical applications.

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May
2016

Block copolymers with a low hydrophilic-to-lipophilic balance form membranes that are highly permeable to hydrophilic molecules. Polymersomes with this type of membrane enable the controllable release of molecules without membrane rupture. However, these polymersomes are difficult to assemble because of their low hydrophobicity.

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May
2016

We develop an optical imaging technique for spatially and temporally tracking biofilm growth and the distribution of the main phenotypes of a Bacillus subtilis strain with a triple-fluorescent reporter for motility, matrix production, and sporulation. We develop a calibration procedure for determining the biofilm thickness from the transmission images, which is based on Beer-Lambert's law and involves cross-sectioning of biofilms. To obtain the phenotype distribution, we assume a linear relationship between the number of cells and their fluorescence and determine the best combination of calibration coefficients that matches the total number of cells for all three phenotypes and with the total number of cells from the transmission images.

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May
2016

Heterogeneous 3D cell microenvironment arrays are rapidly assembled by combining surface-wettability-guided assembly and microdroplet-array-based operations. This approach enables precise control over individual shapes, sizes, chemical concentrations, cell density, and 3D spatial distribution of multiple components. This technique provides a cost-effective solution to meet the increasing demand of stem cell research, tissue engineering, and drug screening.

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Mar
2016

Drop-based microfluidics have recently become a novel tool by providing a stable linkage between phenotype and genotype for high throughput screening. However, use of drop-based microfluidics for screening high-affinity peptide binders has not been demonstrated due to the lack of a sensitive functional assay that can detect single DNA molecules in drops. To address this sensitivity issue, we introduced in vitro two-hybrid system (IVT2H) into microfluidic drops and developed a streamlined mix-and-read drop-IVT2H method to screen a random DNA library.

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Feb
2016

We look at the drying process in a simple glass channel with dominant capillary effects as is the case in microfluidics. We find drying kinetics commonly observed for confined geometry, namely a constant period followed by a falling rate period. From visualization of the air/water interface with high resolution, we observe that the drying rate decreases without a drying front progression although this is the usually accepted mechanism for confined geometries.

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Oct
2015

A key viral property is infectivity, and its accurate measurement is crucial for the understanding of viral evolution, disease and treatment. Currently viral infectivity is measured using plaque assays, which involve prolonged culturing of host cells, and whose measurement is unable to differentiate between specific strains and is prone to low number fluctuation. We developed a rapid, targeted and culture-free infectivity assay using high-throughput drop-based microfluidics.

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Oct
2015

Recombination is an important driver in the evolution of viruses and thus is key to understanding viral epidemics and improving strategies to prevent future outbreaks. Characterization of rare recombinant subpopulations remains technically challenging because of artifacts such as artificial recombinants, known as chimeras, and amplification bias. To overcome this, we have developed a high-throughput microfluidic technique with a second verification step in order to amplify and sequence single recombinant viruses with high fidelity in picoliter drops.

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Aug
2015

Quantitative protein analysis of single cells is rarely achieved due to technical difficulties of detecting minute amounts of proteins present in one cell. We develop a mix-and-read assay for drop-based label-free protein analysis of single cells. This high-throughput method quantifies absolute, rather than relative, amounts of proteins and does not involve antibody labeling or mass spectrometry.

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Jul
2015

The development of ecofriendly sorbents for fast and efficient removal of heavy metals from aqueous media still remains a significant challenge. Here, we report that this task can be addressed by creating a porous naturally occurring polymer, as illustrated by functionalizing lignin with large numbers of mesopores and functional groups. We show that surface-functionalized porous lignin (SFPL), obtained by a two-step process, has a large surface area of 22.

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Jun
2015

Prevention of undesired leakage of encapsulated materials prior to triggered release presents a technological challenge for the practical application of microcapsule technologies in agriculture, drug delivery, and cosmetics. A microfluidic approach is reported to fabricate perfluoropolyether (PFPE)-based microcapsules with a high core-shell ratio that show enhanced retention of encapsulated actives. For the PFPE capsules, less than 2% leakage of encapsulated model compounds, including Allura Red and CaCl2 , over a four week trial period is observed.

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Feb
2015

Squamates classified as 'subarenaceous' possess the ability to move long distances within dry sand; body elongation among sand and soil burrowers has been hypothesized to enhance subsurface performance. Using X-ray imaging, we performed the first kinematic investigation of the subsurface locomotion of the long, slender shovel-nosed snake (Chionactis occipitalis) and compared its biomechanics with those of the shorter, limbed sandfish lizard (Scincus scincus). The sandfish was previously shown to maximize swimming speed and minimize the mechanical cost of transport during burial.

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Sep
2013

Drag on intruders in granular beds: a boundary layer approach.

Phys Rev E Stat Nonlin Soft Matter Phys 2013 Sep 30;88(3):030201. Epub 2013 Sep 30.
Jonathan Goldsmith, Hong Guo, Shelby N Hunt, Mingjiang Tao, Stephan Koehler
We performed a parametric study of the drag on vertical intruders with uniform cross sections of different sizes and shapes, from which we developed a semiempirical model. Baffling techniques were used to isolate the contributions of the intruder's different subsurfaces, and we observed size effects and force focusing on edges. We propose a boundary layer approach, whereby the drag is the surface integral of an effective stress over a monolayer of particles contacting the intruder.

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Dec
2013

In recent years, lithium has proved an effective augmentation strategy of antidepressants in both acute and treatment-resistant depression. Neuroprotective and procognitive effects of lithium have been evidenced. Brain-derived neurotrophic factor (BDNF) has been shown to play a key role in the pathophysiology of several neurological and psychiatric disorders.

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Aug
2012

Quantification of the physical properties of tissue biopsies and cell-remodeled hydrogels is critical for understanding tissue development and pathophysiological tissue remodeling. However, due to the low modulus, small size, irregular shape, and anisotropy of samples from these materials, accurate viscoelastic characterization using standard rheometric methods is problematic. The goal of this work is to utilize image analysis to extend rotational rheometry to these samples.

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Apr
2011

Rheology of evolving bidisperse granular media.

Phys Rev E Stat Nonlin Soft Matter Phys 2011 Apr 26;83(4 Pt 1):041305. Epub 2011 Apr 26.
Raenell Soller, Jessica Cook, Stephan A Koehler
We investigate the mixing of bidisperse distributions of spherical particles for slowly rotating vanes by pouring smaller beads, diameter d(1), onto a monodisperse bed composed of larger beads, diameter d(0), and monitoring the torque and lift forces. If the mixing beads are too small, d(1)/d(0)<0.05, the drag and lift are unaltered.

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Aug
2009

Rheology of draining steady-state foams.

Phys Rev E Stat Nonlin Soft Matter Phys 2009 Aug 20;80(2 Pt 1):021504. Epub 2009 Aug 20.
Raenell Soller, Stephan A Koehler
Aqueous foams continuously age due to fluid drainage and bubble coarsening, which makes it difficult to perform steady-state rheological measurements. Consequently we have developed the foam drainage rheology technique, where perfusion counteracts fluid drainage and bubble replenishment counteracts bubble coarsening during measurement of the shear stresses by a rheometer. We evaluate published power-law and Herschel-Bulkley models and find that parameters derived from emulsion experiments cannot describe our results.

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May
2008

Rheology of steady-state draining foams.

Phys Rev Lett 2008 May 19;100(20):208301. Epub 2008 May 19.
Raenell Soller, Stephan A Koehler
We developed the foam drainage rheology technique in order to perform rheological measurements of aqueous foams at a set liquid fraction epsilon and fixed bubble radius R without the usual difficulties associated with fluid drainage and bubble coarsening. The shear stress exhibits a power-law dependence on strain-rate, tau approximately gamma[over]n where n approximately 0.2.

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Aug
2006

Drag and lift on rotating vanes in granular beds.

Phys Rev E Stat Nonlin Soft Matter Phys 2006 Aug 22;74(2 Pt 1):021305. Epub 2006 Aug 22.
Raenell Soller, Stephan A Koehler
We have performed systematic experiments on vane intruders of different sizes and aspect ratios that are immersed and slowly rotated in beds of monodisperse glass beads of different diameters. We find that the torque and lift force on the vane increase with bead size. The measured torque on the rotating vanes follows a scaling behavior that depends on the effective immersion depth and the effective vane diameter.

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Feb
2006

Visceral hyperalgesia plays a pivotal role in manifestation of symptoms in patients with functional gastrointestinal disorders. In clinical studies combined treatment of peppermint- and caraway oil significantly reduced symptoms. Thus, the aim of this study was to characterize the effects of peppermint- and caraway oil, individually and in combination, on visceral nociception in a rat model of post-inflammatory visceral hyperalgesia.

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Oct
2002

Foam drainage varies with surfactant. We present direct measurements of the flow velocity profiles across single Plateau borders, which make up the interconnected channel-like network for liquid flow. For protein foams the interface is rigid, whereas small-surfactant foams show significant interfacial mobility.

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