6,912 research outputs found
The Glass Transition of Thin Polymer Films: Some Questions, and a Possible Answer
A simple and predictive model is put forward explaining the experimentally
observed substantial shift of the glass transition temperature, Tg, of
sufficiently thin polymer films. It focuses on the limit of small molecular
weight, where geometrical `finite size' effects on the chain conformation can
be ruled out. The model is based on the idea that the polymer freezes due to
memory effects in the viscoelastic eigenmodes of the film, which are affected
by the proximity of the boundaries. The elastic modulus of the polymer at the
glass transition turns out to be the only fitting parameter. Quantitative
agreement is obtained with our experimental results on short chain polystyrene
(Mw = 2 kg/mol), as well as with earlier results obtained with larger
molecules. Furthermore, the model naturally accounts for the weak dependence of
the shift of Tg upon the molecular weight. It furthermore explains why
supported films must be thinner than free standing ones to yield the same
shift, and why the latter depends upon the chemical properties of the
substrate. Generalizations for arbitrary experimental geometries are
straightforward.Comment: 7 pages, 4 figure
On-chip inverted emulsion method for fast giant vesicle production, handling, and analysis
Liposomes and giant unilamellar vesicles (GUVs) in particular are excellent compartments for constructing artificial cells. Traditionally, their use requires bench-top vesicle growth, followed by experimentation under a microscope. Such steps are time-consuming and can lead to loss of vesicles when they are transferred to an observation chamber. To overcome these issues, we present an integrated microfluidic chip which combines GUV formation, trapping, and multiple separate experiments in the same device. First, we optimized the buffer conditions to maximize both the yield and the subsequent trapping of the vesicles in micro-posts. Captured GUVs were monodisperse with specific size of 18 ± 4 µm in diameter. Next, we introduce a two-layer design with integrated valves which allows fast solution exchange in less than 20 s and on separate sub-populations of the trapped vesicles. We demonstrate that multiple experiments can be performed in a single chip with both membrane transport and permeabilization assays. In conclusion, we have developed a versatile all-in-one microfluidic chip with capabilities to produce and perform multiple experiments on a single batch of vesicles using low sample volumes. We expect this device will be highly advantageous for bottom-up synthetic biology where rapid encapsulation and visualization is required for enzymatic reactions
Fast Membranes Hemifusion via Dewetting between Lipid Bilayers
The behavior of lipid bilayer is important to understand the functionality of
cells like the trafficking of ions between cells. Standard procedures to
explore the properties of lipid bilayer and hemifused states typically use
either supported membranes or vesicles. Both techniques have several
shortcoming in terms of bio relevance or accessibility for measurements. In
this article the formation of individual free standing hemifused states between
model cell membranes is studied using an optimized microfluidic scheme which
allows for simultaneous optical and electrophysiological measurements. In a
first step, two model membranes are formed at a desired location within a
microfluidic device using a variation of the droplet interface bilayer (DiB)
technique. In a second step, the two model membranes are brought into contact
forming a single hemifused state. For all tested lipids, the hemifused state
between free standing membranes form within hundreds of milliseconds, i.e.
several orders of magnitude faster than reported in literature. The formation
of a hemifused state is observed as a two stage process, whereas the second
stage can be explained as a dewetting process in no-slip boundary condition.
The formed hemifusion states are long living and a single fusion event can be
observed when triggered by an applied electric field as demonstrated for
monoolein
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