2008
Lee, D. ; Weitz, D. A. Double emulsion-templated nanoparticle colloidosomes with selective permeability.
Advanced Materials 2008,
20, 3498-3503.
Publisher's VersionAbstractNanoparticle colloidosomes, shown in the SEM image, are generated by using water-in-oil-in-water double emulsions as templates. Hydrophobic silica nanoparticles that are dispersed in the oil phase stabilize the double emulsions, and subsequently become the shell of the colloidosomes upon removal of the organic solvent as shown in the figure.
lee2008.pdf Koester, S. ; Angile, F. E. ; Duan, H. ; Agresti, J. J. ; Wintner, A. ; Schmitz, C. ; Rowat, A. C. ; Merten, C. A. ; Pisignano, D. ; Griffiths, A. D. ; et al. Drop-based microfluidic devices for encapsulation of single cells.
Lab on a Chip 2008,
8 1110-1115.
Publisher's VersionAbstractWe use microfluidic devices to encapsulate, incubate, and manipulate individual cells in picoliter aqueous drops in a carrier fluid at rates of up to several hundred Hz. We use a modular approach with individual devices for each function, thereby significantly increasing the robustness of our system and making it highly flexible and adaptable to a variety of cell-based assays. The small volumes of the drops enables the concentrations of secreted molecules to rapidly attain detectable levels. We show that single hybridoma cells in 33 pL drops secrete detectable concentrations of antibodies in only 6 h and remain fully viable. These devices hold the promise of developing microfluidic cell cytometers and cell sorters with much greater functionality, allowing assays to be performed on individual cells in their own microenvironment prior to analysis and sorting.
koester2008.pdf Kim, J. - W. ; Lee, D. ; Shum, H. C. ; Weitz, D. A. Colloid surfactants for emulsion stabilization.
Advanced Materials 2008,
20, 3239-3243.
Publisher's VersionAbstractColloid surfactants are fabricated with precisely controlled geometry and used for emulsion stabilization. These amphiphilic dimer particles (left) combine the benefits of emulsion stabilization of particles and the amphiphilicity of molecular surfactants to afford better emulsion stabilization. Remarkably, these colloidal surfactants stabilize not only spherical emulsion droplets but also nonspherical ones (right).
kim2008.pdf Holtze, C. ; Rowat, A. C. ; Agresti, J. J. ; Hutchison, J. B. ; Angile, F. E. ; Schmitz, C. H. J. ; Koster, S. ; Duan, H. ; Humphry, K. J. ; Scanga, R. A. ; et al. Biocompatible surfactants for water-in-fluorocarbon emulsions.
Lab on a Chip 2008,
8 1632-1639.
Publisher's VersionAbstractDrops of water-in-fluorocarbon emulsions have great potential for compartmentalizing both in vitro and in vivo biological systems; however, surfactants to stabilize such emulsions are scarce. Here we present a novel class of fluorosurfactants that we synthesize by coupling oligomeric perfluorinated polyethers (PFPE) with polyethyleneglycol (PEG). We demonstrate that these block copolymer surfactants stabilize water-in-fluorocarbon oil emulsions during all necessary steps of a drop-based experiment including drop formation, incubation, and reinjection into a second microfluidic device. Furthermore, we show that aqueous drops stabilized with these surfactants can be used for in vitro translation (IVT), as well as encapsulation and incubation of single cells. The compatability of this emulsion system with both biological systems and polydimethylsiloxane (PDMS) microfluidic devices makes these surfactants ideal for a broad range of high-throughput, drop-based applications.
holtze2008.pdf Gardel, M. L. ; Kasza, K. E. ; Brangwynne, C. P. ; Liu, J. ; Weitz, D. A. ; Correia, J. J. ; Detrich, H. W. Mechanical response of cytoskeletal networks. In
Biophysical Tools for Biologists, Vol 2: In Vivo Techniques; Elsevier, 2008; Vol. 89, pp. 487-519.
Publisher's VersionAbstractThe cellular cytoskeleton is a dynamic network of filamentous proteins, consisting of filamentous actin (F-actin), microtubules, and intermediate filaments. However, these networks are not simple linear, elastic solids; they can exhibit highly nonlinear elasticity and athermal dynamics driven by ATP-dependent processes. To build quantitative mechanical models describing complex cellular behaviors, it is necessary to understand the underlying physical principles that regulate force transmission and dynamics within these networks. In this chapter, we review our current understanding of the physics of networks of cytoskeletal proteins formed in vitro. We introduce rheology, the technique used to measure mechanical response. We discuss our current understanding of the mechanical response of F-actin networks, and how the biophysical properties of F-actin and actin cross-linking proteins can dramatically impact the network mechanical response. We discuss how incorporating dynamic and rigid microtubules into F-actin networks can affect the contours of growing microtubules and composite network rigidity. Finally, we discuss the mechanical behaviors of intermediate filaments.
gardel2008.pdf Edd, J. F. ; Di Carlo, D. ; Humphry, K. J. ; Koster, S. ; Irimia, D. ; Weitz, D. A. ; Toner, M. Controlled encapsulation of single-cells into monodisperse picolitre drops.
Lab on a Chip 2008,
8 1262-1264.
Publisher's VersionAbstractEncapsulation of cells within picolitre-size monodisperse drops provides new means to perform quantitative biological studies on a single-cell basis for large cell populations. Variability in the number of cells per drop due to stochastic cell loading is a major barrier to these techniques. We overcome this limitation by evenly spacing cells as they travel within a high aspect-ratio microchannel; cells enter the drop generator with the frequency of drop formation.
edd2008.pdf Dickey, M. D. ; Chiechi, R. C. ; Larsen, R. J. ; Weiss, E. A. ; Weitz, D. A. ; Whitesides, G. M. Eutectic gallium-indium (EGaIn): A liquid metal alloy for the formation of stable structures in microchannels at room temperature.
Advanced Functional Materials 2008,
18, 1097-1104.
Publisher's VersionAbstractThis paper describes the rheological behavior of the liquid metal eutectic gallium-indium (EGaIn) as it is injected into microfluidic channels to form stable microstructures of liquid metal. EGaIn is well-suited for this application because of its theological properties at room temperature: it behaves like an elastic material until it experiences a critical surface stress, at which point it yields and flows readily. These properties allow EGaIn to fill microchannels rapidly when sufficient pressure is applied to the inlet of the channels, yet maintain structural stability within the channels once ambient pressure is restored. Experiments conducted in microfluidic channels, and in a parallel-plate rheometer, suggest that EGaIn's behavior is dictated by the properties of its surface (Predominantly gallium oxide, as determined by Auger measurements); these two experiments both yield approximately the same number for the critical surface stress required to induce EGaIn to flow (similar to 0.5 N/m). This analysis-which shows that the pressure that must be exceeded for EGaIn to flow through a microchannel is inversely proportional to the critical (i.e., smallest) dimension of the channel-is useful to guide future fabrication of microfluidic channels to mold EGaIn into functional microstructures.
dickey2008.pdf Clausell-Tormos, J. ; Lieber, D. ; Baret, J. - C. ; El-Harrak, A. ; Miller, O. J. ; Frenz, L. ; Blouwolff, J. ; Humphry, K. J. ; Koster, S. ; Duan, H. ; et al. Droplet-based microfluidic platforms for the encapsulation and screening of mammalian cells and multicellular organisms.
Chemistry & Biology 2008,
15, 427-437.
Publisher's VersionAbstractHigh-throughput, cell-based assays require small sample volumes to reduce assay costs and to allow for rapid sample manipulation. However, further miniaturization of conventional microtiter plate technology is problematic due to evaporation and capillary action. To overcome these limitations, we describe droplet-based microfluidic platforms in which cells are grown in aqueous microcompartments separated by an inert perfluorocarbon carrier oil. Synthesis of biocompatible surfactants and identification of gas-permeable storage systems allowed human cells, and even a Multicellular organism (C. elegans), to survive and proliferate within the microcompartments for several days. Microcompartments containing single cells could be reinjected into a microfluidic device after incubation to measure expression of a reporter gene. This should open the way for high-throughput, cell-based screening that can use >1000-fold smaller assay volumes and has similar to 500x higher throughput than conventional microtiter plate assays.
clausell-tormos2008.pdf Cho, E. C. ; Kim, J. - W. ; Fernandez-Nieves, A. ; Weitz, D. A. Highly responsive hydrogel scaffolds formed by three-dimensional organization of microgel nanoparticles.
Nano Letters 2008,
8 168-172.
Publisher's VersionAbstractWe present a robust and straightforward method for fabricating remarkably responsive hydrogel scaffolds consisting of submicron-sized microgel particles. We demonstrate that the microgel particles assemble either through bridging or depletion interactions to yield a structure that swells or deswells at a macroscopic level in much shorter times as compared to a bulk polymer gel of similar characteristics. This approach offers a new way of fabricating functional hydrogel scaffolds with controllable responsiveness to applied stimuli and excellent loading capability for a wide variety of materials, irrespective of chemistry, size, and shape.
cho2008.pdf Carroll, N. J. ; Rathod, S. B. ; Derbins, E. ; Mendez, S. ; Weitz, D. A. ; Petsev, D. N. Droplet-based microfluidics for emulsion and solvent evaporation synthesis of monodisperse mesoporous silica microspheres.
Langmuir 2008,
24, 658-661.
Publisher's VersionAbstractA novel method for the fabrication of monodisperse mesoporous silica particles is suggested. It is based on the formation of well-defined equally sized emulsion droplets using a microfluidic approach. The droplets contain the silica precursor/surfactant solution and are suspended in hexadecane as the continuous oil phase. The solvent is then expelled from the droplets, leading to concentration and micellization of the surfactant. At the same time, the silica solidifies around the surfactant structures, forming equally sized mesoporous particles. The procedure can be tuned to produce well-separated particles or alternatively particles that are linked together. The latter allows us to create 2D or 3D structures with hierarchical porosity.
carroll2008.pdf Brangwynne, C. P. ; Koenderink, G. H. ; MacKintosh, F. C. ; Weitz, D. A. Cytoplasmic diffusion: Molecular motors mix it up.
Journal of Cell Biology 2008,
183, 583-587.
Publisher's VersionAbstractRandom motion within the cytoplasm gives rise to molecular diffusion; this motion is essential to many biological processes. However, in addition to thermal Brownian motion, the cytoplasm also undergoes constant agitation caused by the activity of molecular motors and other nonequilibrium cellular processes. Here, we discuss recent work that suggests this activity can give rise to cytoplasmic motion that has the appearance of diffusion but is significantly enhanced in its magnitude and which can play an important biological role, particularly in cytoskeletal assembly.
brangwynne2008.pdf Brangwynne, C. P. ; Koenderink, G. H. ; MacKintosh, F. C. ; Weitz, D. A. Nonequilibrium microtubule fluctuations in a model cytoskeleton.
Physical Review Letters 2008,
100, 118104.
Publisher's VersionAbstractBiological activity gives rise to nonequilibrium fluctuations in the cytoplasm of cells; however, there are few methods to directly measure these fluctuations. Using a reconstituted actin cytoskeleton, we show that the bending dynamics of embedded microtubules can be used to probe local stress fluctuations. We add myosin motors that drive the network out of equilibrium, resulting in an increased amplitude and modified time dependence of microtubule bending fluctuations. We show that this behavior results from steplike forces on the order of 10 pN driven by collective motor dynamics.
brangwynne2008.pdf Bendix, P. M. ; Koenderink, G. H. ; Cuvelier, D. ; Dogic, Z. ; Koeleman, B. N. ; Brieher, W. M. ; Field, C. M. ; Mahadevan, L. ; Weitz, D. A. A quantitative analysis of contractility in active cytoskeletal protein networks.
Biophysical Journal 2008,
94, 3126-3136.
Publisher's VersionAbstractCells actively produce contractile forces for a variety of processes including cytokinesis and motility. Contractility is known to rely on myosin II motors which convert chemical energy from ATP hydrolysis into forces on actin filaments. However, the basic physical principles of cell contractility remain poorly understood. We reconstitute contractility in a simplified model system of purified F-actin, muscle myosin II motors, and a-actinin cross-linkers. We show that contractility occurs above a threshold motor concentration and within a window of cross-linker concentrations. We also quantify the pore size of the bundled networks and find contractility to occur at a critical distance between the bundles. We propose a simple mechanism of contraction based on myosin filaments pulling neighboring bundles together into an aggregated structure. Observations of this reconstituted system in both bulk and low-dimensional geometries show that the contracting gels pull on and deform their surface with a contractile force of similar to 1 mu N, or similar to 100 pN per F-actin bundle. Cytoplasmic extracts contracting in identical environments show a similar behavior and dependence on myosin as the reconstituted system. Our results suggest that cellular contractility can be sensitively regulated by tuning the (local) activity of molecular motors and the cross-linker density and binding affinity.
bendix2008.pdf Abate, A. R. ; Weitz, D. A. Single-layer membrane valves for elastomeric microfluidic devices.
Applied Physics Letters 2008,
92, 243509.
Publisher's VersionAbstractWe characterize single-layer membrane valves for elastomeric microfluidic devices. The devices are simple to fabricate using standard single-layer softlithography; moreover, they afford continuous control over flow rate. This combines the simplicity of stamped microfluidic devices with the precision control of membrane valves, which we demonstrate by steering objects in the flow using a simple device. (c) 2008 American Institute of Physics.
abate2008.pdf Abate, A. R. ; Lee, D. ; Do, T. ; Holtze, C. ; Weitz, D. A. Glass coating for PDMS microfluidic channels by sol-gel methods.
Lab on a Chip 2008,
8 516-518.
Publisher's VersionAbstractSoft lithography using polydimethylsiloxane (PDMS) allows one to fabricate complex microfluidic devices easily and at low cost. However, PDMS swells in the presence of many organic solvents significantly degrading the performance of the device. We present a method to coat PDMS channels with a glass-like layer using sol-gel chemistry. This coating greatly increases chemical resistance of the channels; moreover, it can be functionalized with a wide range of chemicals to precisely control interfacial properties. This method combines the ease of fabrication afforded by soft-lithography with the precision control and chemical robustness afforded by glass.
abate2008.pdf Abate, A. R. ; Krummel, A. T. ; Lee, D. ; Marquez, M. ; Holtze, C. ; Weitz, D. A. Photoreactive coating for high-contrast spatial patterning of microfluidic device wettability.
Lab on a Chip 2008,
8 2157-2160.
Publisher's VersionAbstractFor many applications in microfluidics, the wettability of the devices must be spatially controlled. We introduce a photoreactive sol-gel coating that enables high-contrast spatial patterning of microfluidic device wettability.
abate2008.pdf 2007
Kong, H. J. ; Kim, C. J. ; Huebsch, N. ; Weitz, D. A. ; Mooney, D. J. Noninvasive Probing of the Spatial Organization of Polymer Chains in Hydrogels Using Fluorescence Resonance Energy Transfer (FRET).
Journal of the American Chemical Society 2007,
129, 4518-4519.
Publisher's VersionAbstract
Spatial organization of polymer chains upon ionic or covalent cross-linking to form a hydrogel is examined on the molecular scale using a fluorescent resonance energy transfer (FRET) technique. Minimal changes in the conformation in the polymer chains but significant changes of the intermolecular association of polymer chains are found when polymer chains are cross-linked with a different number of cross-links. This study provides a previously undescribed technique to analyze hydrogel structure in a noninvasive manner.
kong2007.pdf Wyss, H. M. ; Miyazaki, K. ; Mattsson, J. ; Hu, Z. ; Reichman, D. R. ; Weitz, D. A. Strain-rate frequency superposition: A rheological probe of structural relaxation in soft materials.
Physical Review Letters 2007,
98, 238303.
Publisher's VersionAbstractThe rheological properties of soft materials often exhibit surprisingly universal linear and nonlinear features. Here we show that these properties can be unified by considering the effect of the strain-rate amplitude on the structural relaxation of the material. We present a new form of oscillatory rheology, strain-rate frequency superposition (SRFS), where the strain-rate amplitude is fixed as the frequency is varied. We show that SRFS can isolate the response due to structural relaxation, even when it occurs at frequencies too low to be accessible with standard techniques.
wyss2007.pdf Weeks, E. R. ; Crocker, J. C. ; Weitz, D. A. Short- and long-range correlated motion observed in colloidal glasses and liquids.
Journal of Physics-Condensed Matter 2007,
19, 205131.
Publisher's VersionAbstractWe use a confocal microscope to examine the motion of individual particles in a dense colloidal suspension. Close to the glass transition, particle motion is strongly spatially correlated. The correlations decay exponentially with particle separation, yielding a dynamic length scale of O(2-3 sigma) (in terms of particle diameter sigma). This length scale grows modestly as the glass transition is approached. Further, the correlated motion exhibits a strong spatial dependence on the pair correlation function g(r). Motion within glassy samples is weakly correlated, but with a larger spatial scale for this correlation.
weeks2007.pdf Utada, A. S. ; Chu, L. Y. ; Fernandez-Nieves, A. ; Link, D. R. ; Holtze, C. ; Weitz, D. A. Dripping, jetting, drops, and wetting: The magic of microfluidics.
Mrs Bulletin 2007,
32, 702-708.
Publisher's VersionAbstractThe following article is based on the Symposium X presentation given by David A. Weitz (Harvard University) on April 11, 2007, at the Materials Research Society Spring Meeting in San Francisco. The article describes how simple microfluidic devices can be used to control fluid flow and produce a variety of new materials. Based on the concepts of coaxial flow and hydrodynamically focused flow, used alone or in various combinations, the devices can produce precisely controlled double emulsions (droplets within droplets) and even triple emulsions (double emulsions suspended in a third droplet). These structures, which can be created in a single microfluidic device, have various applications such as encapsulants for drugs, cosmetics, or food additives.
utada2007.pdf