Erika Eiser

High-throughput generation of hydrogel microbeads with varying elasticity for cell encapsulation

Elasticity of cellular microenvironments strongly influences cell motility, phagocytosis, growth and differentiation. Currently, the relationship between the cell behaviour and matrix stiffness is being studied for cells seeded on planar substrates, however in three-dimensional (3D) microenvironments cells may experience mechanical signalling that is distinct from that on a two-dimensional matrix. We report a microfluidic approach for high-throughput generation of 3D microenvironments with different elasticity for studies of cell fate. The generation of agarose microgels with different elastic moduli was achieved by (i) introducing into a microfluidic droplet generator two streams of agarose solutions, one with a high concentration of agarose and the other one with a low concentration of agarose, at varying relative volumetric flow rate ratios of the two streams, and (ii) on-chip gelation of the precursor droplets. At 37 degreesC, the method enabled a approximately 35-fold variation of the shear elastic modulus of the agarose gels. The application of the method was demonstrated by encapsulating two mouse embryonic stem cell lines within the agarose microgels. This work establishes a foundation for the high-throughput generation of combinatorial microenvironments with different mechanical properties for cell studies.

Complete wetting from polymer mixtures.

Coexisting polymer phases are characterized by very small interfacial energies, even well below their critical solution temperature. This situation should readily lead to the exclusion of one of the phases from any interface that favors the other. Such complete wetting behavior from a binary mixture of statistical olefinic copolymers is reported. By means of a self-regulating geometry, it is found that the thickness of a wetting layer of one of the phases at the polymer-air interface, growing from the other coexisting phase, attains macroscopic dimensions, increasing logarithmically with time. These results indicate that binary polymer mixtures could be attractive models for the study of wetting phenomena.

DNA-functionalized colloids: Physical properties and applications

The specificity and reversibility of the hydrogen-bonding between two complementary strands in DNA make this bio-molecule a unique binding agent. When DNA is grafted to nano-and micrometer sized colloids it can lead to specific binding between particles coated with complementary strands of single-stranded DNA. DNA-coated colloids hold great promise as the building blocks of a new generation of complex, self-assembling colloidal materials. This brief review sketches the recent developments and present status of the research on DNA-coated colloids with special emphasis on their role as potential building blocks in complex, self-assembling materials and as highly sensitive bio-sensors. Although the present review cannot be comprehensive, it hopefully highlights the promise of DNA-coated colloids as versatile and still largely unexplored form of soft matter.

Multistep kinetic self-assembly of DNA-coated colloids

Equilibrium self-assembly relies on the relaxation of disordered mixtures of building blocks towards an ordered ground state. The main drawback of this traditional approach lies in the kinetic traps that often interrupt the progression of the system towards equilibrium and lead to the formation of arrested phases. The latest techniques to control colloidal interactions open up the possibility of exploiting the tendency to dynamically arrest in order to construct amorphous materials with a specific morphology and local separation between multiple components. Here we propose strategies to direct the gelation of two-component colloidal mixtures by sequentially activating selective interactions. We investigate morphological changes in the structure of the arrested phases both by means of molecular dynamics simulations and experimentally by using DNA-coated colloids. Our approach can be exploited to assemble multicomponent mesoporous materials with possible applications in hybrid photovoltaics, photonics and drug delivery.

Arrested demixing opens route to bigels

Understanding and, ultimately, controlling the properties of amorphous materials is one of the key goals of material science. Among the different amorphous structures, a very important role is played by colloidal gels. It has been only recently understood that colloidal gels are the result of the interplay between phase separation and arrest. When short-ranged attractive colloids are quenched into the phase-separating region, density fluctuations are arrested and this results in ramified amorphous space-spanning structures that are capable of sustaining mechanical stress. We present a mechanism of aggregation through arrested demixing in binary colloidal mixtures, which leads to the formation of a yet unexplored class of materials––bigels. This material is obtained by tuning interspecies interactions. Using a computer model, we investigate the phase behavior and the structural properties of these bigels. We show the topological similarities and the geometrical differences between these binary, interpenetrating, arrested structures and their well-known monodisperse counterparts, colloidal gels. Our findings are supported by confocal microscopy experiments performed on mixtures of DNA-coated colloids. The mechanism of bigel formation is a generalization of arrested phase separation and is therefore universal.

Fluctuation-dissipation theorem in an aging colloidal glass

We provide a direct experimental test of the fluctuation-dissipation theorem (FDT) in an aging colloidal glass. The use of combined active and passive microrheology allows us to independently measure both the correlation and response functions in this nonequilibrium situation. Contrary to previous reports, we find no deviations from the FDT over several decades in frequency (1 Hz-10 kHz) and for all aging times. In addition, we find two distinct viscoelastic contributions in the aging glass, including a nearly elastic response at low frequencies that grows during aging.

Volume and porosity thermal regulation in lipid mesophases by coupling mobile ligands to soft membranes

Short DNA linkers are increasingly being exploited for driving-specific self-assembly of Brownian objects. DNA-functionalized colloids can assemble into ordered or amorphous materials with tailored morphology. Recently, the same approach has been applied to compliant units, including emulsion droplets and lipid vesicles. The liquid structure of these substrates introduces new degrees of freedom: the tethers can diffuse and rearrange, radically changing the physics of the interactions. Unlike droplets, vesicles are extremely deformable and DNA-mediated adhesion causes significant shape adjustments. We investigate experimentally the thermal response of pairs and networks of DNA-tethered liposomes and observe two intriguing and possibly useful collective properties: negative thermal expansion and tuneable porosity of the liposome networks. A model providing a thorough understanding of this unexpected phenomenon is developed, explaining the emergent properties out of the interplay between the temperature-dependent deformability of the vesicles and the DNA-mediated adhesive forces.

Hydrodynamic flow in the cytoplasm of plant cells

Plant cells show myosin‐driven organelle movement, called cytoplasmic streaming. Soluble molecules, such as metabolites do not move with motor proteins but by diffusion. However, is all of this streaming active motor‐driven organelle transport? Our recent simulation study ( Houtman et al., 2007 ) shows that active transport of organelles gives rise to a drag in the cytosol, setting up a hydrodynamic flow, which contributes to a fast distribution of proteins and nutrients in plant cells. Here, we show experimentally that actively transported organelles produce hydrodynamic flow that significantly contributes to the movement of the molecules in the cytosol. We have used fluorescence recovery after photobleaching and show that in tobacco Bright Yellow 2 (BY‐2) suspension cells constitutively expressing cytoplasmic green fluorescent protein (GFP), free GFP molecules move faster in cells with active transport of organelles than in cells where this transport has been inhibited with the general myosin inhibitor BDM (2,3‐butanedione monoxime). Furthermore, we show that the direction of the GFP movement in the cells with active transport is the same as that of the organelle movement and that the speed of the GFP in the cytosol is proportional to the speed of the organelle movement. In large BY‐2 cells with fast cytoplasmic streaming, a GFP molecule reaches the other side of the cell approximately in the similar time frame (about 16 s) as in small BY‐2 cells that have slow cytoplasmic streaming. With this, we suggest that hydrodynamic flow is important for efficient transport of cytosolic molecules in large cells. Hydrodynamic flow might also contribute to the movement of larger structures than molecules in the cytoplasm. We show that synthetic lipid (DOPG) vesicles and ‘stealth’ vesicles with PEG phospholipids moved in the cytoplasm.

Effect of Inert Tails on the Thermodynamics of DNA Hybridization

The selective hybridization of DNA is of key importance for many practical applications such as gene detection and DNA-mediated self-assembly. These applications require a quantitative prediction of the hybridization free energy. Existing methods ignore the effects of non-complementary ssDNA tails beyond the first unpaired base. We use experiments and simulations to show that the binding strength of complementary ssDNA oligomers is altered by these sequences of non-complementary nucleotides. Even a small number of non-binding bases are enough to raise the hybridization free energy by approximately 1 kcal/mol at physiological salt concentrations. We propose a simple analytical expression that accounts quantitatively for this variation as a function of tail length and salt concentration.

Hydrodynamic flow caused by active transport along cytoskeletal elements

We develop a simple lattice model to describe the hydrodynamic influence of active mass transport along bio-filaments on freely diffusing mass in the cell. To quantify the overall mass transport we include Brownian motion, excluded volume interactions, active transport along the filaments, and hydrodynamic interactions. The model shows that the hydrodynamic forces induced by molecular motors attached to the filaments give rise to a non-negligible flux close to the filament. This additional flux appears to have two effects. Depending on the degree of filament occupation it can exert a sufficiently large influence on unbound motors and cargo to modify their transport and also regulate the flux of motors bound to the filament. We expect such a mechanism is important in situations found in plant cells, where directional transport spans the entire cell. In particular, it can explain the cytoplasmic streaming observed in plant cells.