Publications by Year: 2009

2009
Vader, D. ; Kabla, A. ; Weitz, D. A. ; Mahadevan, L. Strain-Induced Alignment in Collagen Gels. PLOS One 2009, 4 e5902. Publisher's VersionAbstract

Collagen is the most abundant extracellular-network-forming protein in animal biology and is important in both natural and artificial tissues, where it serves as a material of great mechanical versatility. This versatility arises from its almost unique ability to remodel under applied loads into anisotropic and inhomogeneous structures. To explore the origins of this property, we develop a set of analysis tools and a novel experimental setup that probes the mechanical response of fibrous networks in a geometry that mimics a typical deformation profile imposed by cells in vivo. We observe strong fiber alignment and densification as a function of applied strain for both uncrosslinked and crosslinked collagenous networks. This alignment is found to be irreversibly imprinted in uncrosslinked collagen networks, suggesting a simple mechanism for tissue organization at the microscale. However, crosslinked networks display similar fiber alignment and the same geometrical properties as uncrosslinked gels, but with full reversibility. Plasticity is therefore not required to align fibers. On the contrary, our data show that this effect is part of the fundamental non-linear properties of fibrous biological networks.

vader2009.pdf
Kasza, K. E. ; Vader, D. ; Koester, S. ; Wang, N. ; Weitz, D. A. Imaging Techniques for Measuring the Materials Properties of Cells. In Live Cell Imaging: A Laboratory Manual; CSHL Press, 2009. Publisher's VersionAbstract

The “materials properties” of a biological material include its composition and microscopic structure and the relationship between its structure and its mechanical properties. For living cells, the motor-driven internal motion also significantly impacts the properties, even independently of any remodeling of the cell structure that can occur. These materials properties dictate the passive mechanical response of the material to an applied force. The mechanical properties of cells and tissues are essential for their function and health and affect how cells actively respond to mechanical force in important biological processes, ranging from motility to differentiation and morphogenesis. The mechanical properties of bulk tissues can be determined by traditional rheological techniques that measure the force required to stretch, compress, or shear macroscopic tissue. However, individual cells are too small to be measured by such methods and have highly heterogeneous structures; thus techniques are required that can probe soft materials at the micrometer scale. A variety of microrheological techniques, developed to determine the materials properties of cells, reveal that living cells have materials properties that are quite unusual compared with common inert materials. Cells are active, nonequilibrium materials with a highly nonlinear elasticity. This article presents a subset of microrheological techniques that involve optical imaging of micrometer-sized probes on or within individual cells, describes how to analyze probe motions, and discusses limitations of the techniques.

Abate, A. R. ; Lee, D. ; Holtze, C. ; Krummel, A. ; Do, T. ; Weitz, D. A. Functionalized glass coating for PDMS microfluidic devices. In Lab-on-a-Chip Technology (Vol. 1): Fabrication and Microfluidics; Caister Academic Press, 2009; Vol. 1. Publisher's VersionAbstract

Microfluidic devices can perform multiple laboratory functions on a single, compact, and fully integrated chip. However, fabrication of microfluidic devices is difficult, and current methods, such as glass-etching or soft-lithography in PDMS, are either expensive or yield devices with poor chemical robustness. We introduce a simple method that combines the simple fabrication of PDMS with superior robustness and control of glass. We coat PDMS channels with a functionalized glass layer. The glass coating greatly increases the chemical robustness of the PDMS devices. As a demonstration, we produce emulsions in coated channels using organic solvents. The glass coating also enables surface properties to be spatially controlled. As a demonstration of this control, we spatially pattern the wettability of coated PDMS channels and use the devices to produce double emulsions with fluorocarbon oil.

Gaudreault, R. ; Di Cesare, N. ; Weitz, D. A. ; van de Ven, T. G. M. Flocculation kinetics of precipitated calcium carbonate. Colloids and Surfaces 2009, 340, 56-65. Publisher's VersionAbstract

When the percentage of filler in paper is increased, the optical properties are improved and the production cost lowered. However, fillers weaken paper strength by decreasing the fibre–fibre bonded area. Little is known about the optimum filler floc size or filler floc properties to allow developing optimum paper characteristics. Consequently, the kinetics of aggregation of scalenohedral precipitated calcium carbonate (PCC) filler was studied using various polymers (flocculants, coagulants and dry strength agents). The sodium salt of partially hydrolysed polyvinyl formamide copolymerized with acrylic acid (PVFA/NaAA) or C-starch lead to floc sizes, less sensitive to dosage within a certain range. Results from stability ratios correlate with PCC particle size. The change in particle size measured by photometric dispersion analysis (PDA) correlates well with the change in PCC particle size measured by light scattering/diffraction. Kinetic calculations show the orthokinetic aggregation times to be consistent with the experimental PDA results. The main uncertainty in the orthokinetic times is estimating the effective shear rate. It is proposed that the bridging surface area of PCC particles, the area which can form bonds between PCC particles or aggregates, should be used to study the kinetics of PCC aggregation, and not the total or projected surface area. In polymer induced aggregation, the PCC particle size increases to a plateau value with increasing polymer dosage. Two regions are most pronounced for C-PAM, PVFA/NaAA and A-starch. Region I corresponds to bridging flocculation. Region II is where the particle size reaches a plateau, and not the expected maximum predicted by classical polymer bridging theory or charge neutralisation theory, likely because of a competition between particle aggregation and polymer adsorption.

gaudreault2009.pdf
Carroll, N. ; Mendez, S. ; Edwards, J. ; Weitz, D. A. ; Petsev, Dimiter, N. Droplet-Based Microfluidics for Emulsion and Solvent Evaporation Synthesis of Monodisperse Mesoporous Silica Microspheres. In Structure and Functional Properties of Colloidal Systems; Taylor & Francis Group, 2009. Publisher's VersionAbstract

 

The miniaturization of chemical ow and analysis systems has opened up exciting avenues of scien ti c and engineering possibilities. Channels with widths in the tens of micrometer range are referred to as micro uidic devices. Fluidic behavior at the microscale may differ from that at larger scales in that interfacial tension, viscous effects, and energy dissipation can dominate the system. Micro uidics has received much attention in the scienti c community and many excellent reviews have been published [1,2]. A key advantage of micro uidics is the ability to perform experiments and bioassays using miniscule quantities of solution. This provides an economic bene t and is important for certain biosensing applications, experiments requiring single-molecule interrogation (e.g., deoxyribonucleic acid (DNA) sequencing [3,4]), or diffusion-limited regimes. Another benefit is that rapid measurements of these minute quantities can be performed with miniaturized analytical systems [5-7]. In some applications slow or minimal mixing is required, and the laminar ows obtained in microchannels become highly desirable. Water-in-oil emulsions can be formed in micro uidic devices to form a steady stream of monodisperse aqueous droplets with volumes as small as picoliters [8]. The drops can be loaded with reactants to perform chemical reactions of interest [9].

 

Zhou, E. H. ; Trepat, X. ; Park, C. Y. ; Lenormand, G. ; Oliver, M. N. ; Mijailovich, S. M. ; Hardin, C. ; Weitz, D. A. ; Butler, J. P. ; Fredberg, J. J. Universal behavior of the osmotically compressed cell and its analogy to the colloidal glass transition. PNAS 2009, 106, 10632-10637. Publisher's VersionAbstract
Mechanical robustness of the cell under different modes of stress and deformation is essential to its survival and function. Under tension, mechanical rigidity is provided by the cytoskeletal network; with increasing stress, this network stiffens, providing increased resistance to deformation. However, a cell must also resist compression, which will inevitably occur whenever cell volume is decreased during such biologically important processes as anhydrobiosis and apoptosis. Under compression, individual filaments can buckle, thereby reducing the stiffness and weakening the cytoskeletal network. However, the intracellular space is crowded with macromolecules and organelles that can resist compression. A simple picture describing their behavior is that of colloidal particles; colloids exhibit a sharp increase in viscosity with increasing volume fraction, ultimately undergoing a glass transition and becoming a solid. We investigate the consequences of these 2 competing effects and show that as a cell is compressed by hyperosmotic stress it becomes progressively more rigid. Although this stiffening behavior depends somewhat on cell type, starting conditions, molecular motors, and cytoskeletal contributions, its dependence on solid volume fraction is exponential in every instance. This universal behavior suggests that compression-induced weakening of the network is overwhelmed by crowding-induced stiffening of the cytoplasm. We also show that compression dramatically slows intracellular relaxation processes. The increase in stiffness, combined with the slowing of relaxation processes, is reminiscent of a glass transition of colloidal suspensions, but only when comprised of deformable particles. Our work provides a means to probe the physical nature of the cytoplasm under compression, and leads to results that are universal across cell type.
zhou2009.pdf
Weitz, D. A. Unjamming a Polymer Glass. Science 2009, 323, 214-215. Publisher's VersionAbstract

Small probes reveal that glass can melt in different ways.

weitz2009.pdf
Wang, B. ; Shum, H. C. ; Weitz, D. A. Fabrication of monodisperse toroidal particles by polymer solidification in microfluidics. Chemphyschem 2009, 10, 641-645. Publisher's VersionAbstract

Microdoughnuts: Polymer toroidal particles such as the one shown in the left picture have been prepared by a capillary microfluidic technique. Droplets of polymer solution undergo non‐uniform solidification to form the anisotropic polymer particles. By incorporating functional materials inside the polymer network, functional toroidal particles (center and right images) can be tailor‐made for specific applications such as magnetic actuation.

wang2009.pdf
Wahrmund, J. ; Kim, J. - W. ; Chu, L. - Y. ; Wang, C. ; Li, Y. ; Fernandez-Nieves, A. ; Weitz, D. A. ; Krokhin, A. ; Hu, Z. Swelling kinetics of a microgel shell. Macromolecules 2009, 42, 9357-9365. Publisher's VersionAbstract
Tanaka's approach to swelling kinetics of a solid gel sphere is extended to a spherical microgel shell. The boundary condition at the inner surface is obtained from the minimization of shear elastic energy. Temporal evolution of a shell is represented in a form of expansion over eigenfunctions of the corresponding diffusion equation. The swelling of Tanaka's solid spherical gel is recovered as a special case of our general Solution if the inner radius approaches zero. In another limiting case of it thin (balloon-like) shell, the set of eigenfunctions is reduced to a single exponential term. In the general case, a solid sphere swells slightly faster than the same sphere with in internal cavity. To test Our theoretical model, we prepared monodisperse poly-N-isopropylacrylamide (PNIPAM) hydrogel shells using a microfluidic device. The temporal dependence of the inner and outer radii of the shell were measured, and the data were fitted to our theoretical model. As a result, we obtained the collective diffusion constants for shrinking and for swelling processes. The obtained values for microgel shells are in excellent agreement with the previous results obtained for submillimeter PNIPAM solid spheres in the same temperature interval. Our model shows that the characteristic swelling time of a gel shell should be proportional to the square of the outer radius not to the thickness of the shell, agreeing with experimental observation.
wahrmund2009.pdf
Trepat, X. ; Wasserman, M. R. ; Angelini, T. E. ; Millet, E. ; Weitz, D. A. ; Butler, J. P. ; Fredberg, J. J. Physical forces during collective cell migration. Nature Physics 2009, 5 426-430. Publisher's VersionAbstract
Fundamental biological processes including morphogenesis, tissue repair and tumour metastasis require collective cell motions(1-3), and to drive these motions cells exert traction forces on their surroundings(4). Current understanding emphasizes that these traction forces arise mainly in 'leader cells' at the front edge of the advancing cell sheet(5-9). Our data are contrary to that assumption and show for the first time by direct measurement that traction forces driving collective cell migration arise predominately many cell rows behind the leading front edge and extend across enormous distances. Traction fluctuations are anomalous, moreover, exhibiting broad non-Gaussian distributions characterized by exponential tails(10-12). Taken together, these unexpected findings demonstrate that although the leader cell may have a pivotal role in local cell guidance, physical forces that it generates are but a small part of a global tug-of-war involving cells well back from the leading edge.
trepat2009.pdf
Tang, S. K. Y. ; Li, Z. ; Abate, A. R. ; Agresti, J. J. ; Weitz, D. A. ; Psaltis, D. ; Whitesides, G. M. A multi-color fast-switching microfluidic droplet dye laser. Lab on a Chip 2009, 9 2767-2771. Publisher's VersionAbstract
We describe a multi-color microfluidic dye laser operating in whispering gallery mode based on a train of alternating droplets containing solutions of different dyes; this laser is capable of switching the wavelength of its emission between 580 nm and 680 nm at frequencies up to 3.6 kHz-the fastest among all dye lasers reported; it has potential applications in on-chip spectroscopy and flow cytometry.
tang2009.pdf
Studart, A. R. ; Shum, H. C. ; Weitz, D. A. Arrested coalescence of particle-coated droplets into nonspherical supracolloidal structures. Journal of Physical Chemistry B 2009, 113, 3914-3919. Publisher's VersionAbstract
Colloidal and supracolloidal structures with anisotropic shape and surface chemistry are potential building blocks for the fabrication of novel materials. Droplets or bubbles are often used as templates for the assembly of particles into supracolloidal structures of spherical shape. Particle-coated droplets or bubbles have recently been shown to also retain nonspherical geometries after deformation, suggesting that the templating approach can also be used to produce supracolloidal structures with anisotropic shape. We show that partially coated droplets generated in a microcapillary device can undergo spontaneous coalescence into stable nonspherical structures. By positioning the droplets into regular arrays before coalescence, we produce anisotropic geometries with well-defined bonding angles between adjacent merged droplets. This approach allows for the fabrication of novel anisotropic supracolloidal structures with deliberately designed shapes.
studart2009.pdf
Shum, H. C. ; Bandyopadhyay, A. ; Bose, S. ; Weitz, D. A. Double emulsion droplets as microreactors for synthesis of mesoporous hydroxyapatite. Chemistry of Materials 2009, 21, 5548-5555. Publisher's VersionAbstract
We introduce a novel approach for synthesizing mesoporous hydroxyapatite (HAp, (Ca)(10), (PO(4))(6)(OH)(2)) using double emulsion droplets as microreactors. By using capillary microfluidic techniques, the size and the geometry of the droplet microreactors can be tuned easily. Double emulsion droplets offer the combined advantages of both shielding the reactants and on-demand addition of reactants; this makes them highly versatile microreactors. Such droplet microreactors also enable simple visualization of the HAp formation process as well as control over the porosity in the HAp that is synthesized. Powder formed with our technique demonstrates a remarkable microstructure, as well as significantly enhanced BET specific average surface area and nanoscale porosity. Our results present a novel synthesis approach for controlling the nanoscale porosity and the morphology of inorganic particles using double emulsion droplets.
shum2009.pdf
Shah, R. K. ; Kim, J. - W. ; Weitz, D. A. Janus supraparticles by induced phase separation of nanoparticles in droplets. Advanced Materials 2009, 21, 1949-1953. Publisher's VersionAbstract
Biphasic Janus particles with a precisely tunable internal morphology are fabricated using a novel, versatile, and robust technique. This technique can be used in conjunction with microfluidics to produce monodisperse particles, or can be combined with bulk emulsification techniques to produce large quantities of particles.
shah2009.pdf
Schmitz, C. H. J. ; Rowat, A. C. ; Koester, S. ; Weitz, D. A. Dropspots: A picoliter array in a microfluidic device. Lab on a Chip 2009, 9 44-49. Publisher's VersionAbstract
We present a simple microfluidic device that uses an array of well-defined chambers to immobilize thousands of femtoliter-to picoliter-scale aqueous drops suspended in inert carrier oil. This device enables timelapse studies of large numbers of individual drops, while simultaneously enabling subsequent drop recovery.
schmitz2009.pdf
Rowat, A. C. ; Bird, J. C. ; Agresti, J. J. ; Rando, O. J. ; Weitz, D. A. Tracking lineages of single cells in lines using a microfluidic device. Proceedings of the National Academy of Sciences of the United States of America 2009, 106, 18149-18154. Publisher's VersionAbstract
Cells within a genetically identical population exhibit phenotypic variation that in some cases can persist across multiple generations. However, information about the temporal variation and familial dependence of protein levels remains hidden when studying the population as an ensemble. To correlate phenotypes with the age and genealogy of single cells over time, we developed a microfluidic device that enables us to track multiple lineages in parallel by trapping single cells and constraining them to grow in lines for as many as 8 divisions. To illustrate the utility of this method, we investigate lineages of cells expressing one of 3 naturally regulated proteins, each with a different representative expression behavior. Within lineages deriving from single cells, we observe genealogically related clusters of cells with similar phenotype; cluster sizes vary markedly among the 3 proteins, suggesting that the time scale of phenotypic persistence is protein-specific. Growing lines of cells also allows us to dynamically track temporal fluctuations in protein levels at the same time as pedigree relationships among the cells as they divide in the chambers. We observe bursts in expression levels of the heat shock protein Hsp12-GFP that occur simultaneously in mother and daughter cells. In contrast, the ribosomal protein Rps8b-GFP shows relatively constant levels of expression over time. This method is an essential step toward understanding the time scales of phenotypic variation and correlations in phenotype among single cells within a population.
rowat2009.pdf
Rowat, A. C. ; Weitz, D. A. Understanding epigenetic regulation: Tracking protein levels across multiple generations of cells. European Physical Journal-Special Topics 2009, 178, 71-80. Publisher's VersionAbstract
Cells and organisms are remarkably robust: they alter the variety and levels of expressed genes and proteins in response to environmental stimuli, including temperature, chemicals, and the stiffness of their surroundings. Ultimately changes in gene and protein expression can result in a distinct phenotypic state, which in some cases is maintained over multiple generations; the ability to pass on a particular phenotypic state to progeny cells is critical for differentiation. Moreover, epigenetic regulation of phenotype is also thought to provide an evolutionary advantage for a population of cells adapting to a fluctuating environment on faster timescales than the occurrence of genetic mutations. However, simple methods to study patterns of gene and protein expression on multi-generational timescales are sparse. Here we describe a technique to study lineages of single cells over multiple generations using a microfluidic device; this reveals patterns of expression where protein levels are correlated across multiple generations. Such quantitative information of protein expression in the context of pedigree remains hidden when studying the population as an ensemble.
rowat2009.pdf
Ramsteiner, I. B. ; Jensen, K. E. ; Weitz, D. A. ; Spaepen, F. Experimental observation of the crystallization of hard-sphere colloidal particles by sedimentation onto flat and patterned surfaces. Physical Review E 2009, 79, 011403. Publisher's VersionAbstract
We present a confocal microscopy study of 1.55 mu m monodisperse silica hard spheres as they sediment and crystallize at the bottom wall of a container. If the particles sediment onto a feature less flat wall, the two bottom layers crystallize simultaneously and layerwise growth follows. If the wall is replaced by a hexagonal template, only layerwise growth occurs. Our results complement earlier numerical simulations and experiments on other colloidal systems.
ramsteiner2009.pdf
Peng, Y. ; Chen, W. ; Fischer, T. M. ; Weitz, D. A. ; Tong, P. Short-time self-diffusion of nearly hard spheres at an oil-water interface. Journal of Fluid Mechanics 2009, 618, 243-261. Publisher's VersionAbstract
Optical microscopy and multi-particle tracking are used to study hydrodynamic interactions of monodisperse polymethylmethacrylate (PMMA) spheres at a decalin-water interface. The short-time self-diffusion coefficient measured at low surface coverage has the form D(S)(S)(n) = alpha D(0)(1 - beta n), where n is the area fraction Occupied by the particles, and Do is the Stokes-Einstein diffusion coefficient in the bulk suspension of PMMA spheres in decalin. The measured values of a are found to be in good agreement with the numerical calculation for the drag coefficient of interfacial particles. The measured values of P differ from that obtained for bulk suspensions, indicating that hydrodynamic interactions between the particles have interesting new features at the interface.
peng2009.pdf
Nakamura, F. ; Heikkinen, O. ; Pentikaeinen, O. T. ; Osborn, T. M. ; Kasza, K. E. ; Weitz, D. A. ; Kupiainen, O. ; Permi, P. ; Kilpelaeinen, I. ; Ylaenne, J. ; et al. Molecular basis of filamin A-FilGAP interaction and its impairment in congenital disorders associated with filamin A mutations. Plos One 2009, 4. Publisher's VersionAbstract
Background: Mutations in filamin A (FLNa), an essential cytoskeletal protein with multiple binding partners, cause developmental anomalies in humans. Methodology/Principal Findings: We determined the structure of the 23(rd) Ig repeat of FLNa (IgFLNa23) that interacts with FilGAP, a Rac-specific GTPase-activating protein and regulator of cell polarity and movement, and the effect of the three disease-related mutations on this interaction. A combination of NMR structural analysis and in silico modeling revealed the structural interface details between the C and D beta-strands of the IgFLNa23 and the C-terminal 32 residues of FilGAP. Mutagenesis of the predicted key interface residues confirmed the binding constraints between the two proteins. Specific loss-of-function FLNa constructs were generated and used to analyze the importance of the FLNa-FilGAP interaction in vivo. Point mutagenesis revealed that disruption of the FLNa-FilGAP interface perturbs cell spreading. FilGAP does not bind FLNa homologs FLNb or FLNc establishing the importance of this interaction to the human FLNa mutations. Tight complex formation requires dimerization of both partners and the correct alignment of the binding surfaces, which is promoted by a flexible hinge domain between repeats 23 and 24 of FLNa. FLNa mutations associated with human developmental anomalies disrupt the binding interaction and weaken the elasticity of FLNa/F-actin network under high mechanical stress. Conclusions/Significance: Mutational analysis informed by structure can generate reagents for probing specific cellular interactions of FLNa. Disease-related FLNa mutations have demonstrable effects on FLNa function.
nakamura2009.pdf

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