3 research outputs found
Direct Measurements of Effect of Counterion Concentration on Mechanical Properties of Cationic Vesicles
Theoretical analyses of charged membranes
in aqueous solutions
have long predicted that the electric double layer surrounding them
contributes significantly to their mechanical properties. Here we
report the first, direct experimental measurements of the effect of
counterion concentration on the bending and area expansion modulus
of cationic surfactant vesicles. Using the classical technique of
micropipet aspiration coupled with a modified experimental protocol
that is better suited for cationic vesicles, we successfully measure
the mechanical properties of a double-tailed cationic surfactant,
diethylesterdimethyl ammonium chloride (diC18:1 DEEDMAC) in CaCl<sub>2</sub> solutions. It is observed that the area expansion modulus
of the charged membrane exhibits no measurable dependence on the counterion
concentration, in accordance with existing models of bilayer elasticity.
The measured bending modulus, however, is found to vary nonmonotonically
and exhibits a minimum in its variation with counterion concentration.
The experimental results are interpreted based on theoretical calculations
of charged and bare membrane mechanics. It is determined that the
initial decrease in bending modulus with increasing counterion concentration
may be attributed to a decreasing double layer thickness, while the
subsequent increase is likely due to an increasing membrane thickness.
These mechanical moduli measurements qualitatively confirm, for the
first time, theoretical predictions of a nonmonotonic behavior and
the opposing effects of ionic strength on the bending rigidity of
charged bilayers
Origins of Microstructural Transformations in Charged Vesicle Suspensions: The Crowding Hypothesis
It is observed that
charged unilamellar vesicles in a suspension
can spontaneously deflate and subsequently transition to form bilamellar
vesicles, even in the absence of externally applied triggers such
as salt or temperature gradients. We provide strong evidence that
the driving force for this deflation-induced transition is the repulsive
electrostatic pressure between charged vesicles in concentrated suspensions,
above a critical effective volume fraction. We use volume fraction
measurements and cryogenic transmission electron microscopy imaging
to quantitatively follow both the macroscopic and microstructural
time-evolution of cationic diC18:1 DEEDMAC vesicle suspensions at
different surfactant and salt concentrations. A simple model is developed
to estimate the extent of deflation of unilamellar vesicles caused
by electrostatic interactions with neighboring vesicles. It is determined
that when the effective volume fraction of the suspension exceeds
a critical value, charged vesicles in a suspension can experience
ācrowdingā due to overlap of their electrical double
layers, which can result in deflation and subsequent microstructural
transformations to reduce the effective volume fraction of the suspension.
Ordinarily in polydisperse colloidal suspensions, particles interacting
via a repulsive potential transform into a glassy state above a critical
volume fraction. The behavior of charged vesicle suspensions reported
in this paper thus represents a new mechanism for the relaxation of
repulsive interactions in crowded situations
Microfluidic Generation of Composite Biopolymer Microgels with Tunable Compositions and Mechanical Properties
To develop an understanding of the
nature of complex, spatiotemporal
interactions between cells and the extracellular matrix (ECM), artificial
ECMs formed from hydrogels with a particular spectrum of properties
are being developed at a rapid pace. We report the microfluidic generation
of small, monodisperse composite agaroseāgelatin hydrogel modules
(microgel particles) that can be used for cell encapsulation and can
serve as instructive cellular microenvironments. The agarose component
of the microgels gelled under reduced temperature, while gelatin modified
with phenolic hydroxyl groups underwent peroxidase-catalyzed gelation.
Microgel composition, structure, morphology, and rigidity were tuned
in a high-throughput manner. The results of this work are important
for the generation of libraries of cell-laden polymer microgels for
single-cell analysis, tissue engineering, and fundamental studies
of the role of local microenvironments in cell fate