Publications by Year: 2011

Microgel Suspensions: Fundamentals and Applications; Fernandez‐Nieves, A. ; Wyss, H. W. ; Mattsson, J. ; Weitz, D. A., Ed. 1st ed. Wiley-VCH, 2011; pp. 484. Publisher's VersionAbstract
Providing a vital link between chemistry and physics on the nanoscale, this book offers concise coverage of the entire topic in five major sections, beginning with synthesis of microgel particles and continuing with their physical properties. The phase behavior and dynamics of resulting microgel suspensions feature in the third section, followed by their mechanical properties. It concludes with detailed accounts of numerous industrial, commercial and medical applications.
Zhao, Y. ; Shum, H. C. ; Adams, L. L. A. ; Sun, B. J. ; Holtze, C. ; Gu, Z. ; Weitz, D. A. Enhanced encapsulation of actives in self-sealing microcapsules by precipitation in capsule shells. Langmuir 2011, 27, 13988-13991. Publisher's VersionAbstract
Microcapsules with core-shell structures are excellent vehicles for the encapsulation of active ingredients; however, the actives often leak out of these structures over time, without observable damage to them. We present a novel approach to enhancing the encapsulation of active ingredients inside microcapsules. We use two components that can form solid precipitates upon mixing and add one each to the microcapsule core and to the continuous phase. The components diffuse through the shell in the same manner as the actives, but upon meeting, they precipitate to form solid particles within the shell; this significantly reduces leakage through the shell of the microcapsules. We show that the reduction in the leakage of actives is due to the blockage of channels or pores that exist in the shell of the capsules by the solid precipitates.
Zhao, Y. ; Shum, H. C. ; Chen, H. ; Adams, L. L. A. ; Gu, Z. ; Weitz, D. A. Microfluidic generation of multifunctional quantum dot barcode particles. Journal of the American Chemical Society 2011, 133, 8790-8793. Publisher's VersionAbstract
We develop a new strategy to prepare quantum dot (QD) barcode particles by polymerizing double-emulsion droplets prepared in capillary microfluidic devices. The resultant barcode particles are composed of stable QD-tagged core particles surrounded by hydrogel shells. These particles exhibit uniform spectral characteristics and excellent coding capability, as confirmed by photoluminescence analyses. By using double-emulsion droplets with two inner droplets of distinct phases as templates, we have also fabricated anisotropic magnetic barcode particles with two separate cores or with a Janus core. These particles enable optical encoding and magnetic separation, thus making them excellent functional barcode particles in biomedical applications.
Wyss, H. M. ; Henderson, J. M. ; Byfield, F. J. ; Bruggeman, L. A. ; Ding, Y. ; Huang, C. ; Suh, J. H. ; Franke, T. ; Mele, E. ; Pollak, M. R. ; et al. Biophysical properties of normal and diseased renal glomeruli. American Journal of Physiology-Cell Physiology 2011, 300, C397-C405. Publisher's VersionAbstract

The mechanical properties of tissues and cells including renal glomeruli are important determinants of their differentiated state, function, and responses to injury but are not well characterized or understood. Understanding glomerular mechanics is important for understanding renal diseases attributable to abnormal expression or assembly of structural proteins and abnormal hemodynamics. We use atomic force microscopy (AFM) and a new technique, capillary micromechanics, to measure the elastic properties of rat glomeruli. The Young's modulus of glomeruli was 2,500 Pa, and it was reduced to 1,100 Pa by cytochalasin and latunculin, and to 1,400 Pa by blebbistatin. Cytochalasin or latrunculin reduced the F/G actin ratios of glomeruli but did not disrupt their architecture. To assess glomerular biomechanics in disease, we measured the Young's moduli of glomeruli from two mouse models of primary glomerular disease, Col4a3(-/-) mice (Alport model) and Tg26(HIV/nl) mice (HIV-associated nephropathy model), at stages where glomerular injury was minimal by histopathology. Col4a3(-/-) mice express abnormal glomerular basement membrane proteins, and Tg26(HIV/nl) mouse podocytes have multiple abnormalities in morphology, adhesion, and cytoskeletal structure. In both models, the Young's modulus of the glomeruli was reduced by 30%. We find that glomeruli have specific and quantifiable biomechanical properties that are dependent on the state of the actin cytoskeleton and nonmuscle myosins. These properties may be altered early in disease and represent an important early component of disease. This increased deformability of glomeruli could directly contribute to disease by permitting increased distension with hemodynamic force or represent a mechanically inhospitable environment for glomerular cells.

Yao, N. Y. ; Becker, D. J. ; Broedersz, C. P. ; Depken, M. ; MacKintosh, F. C. ; Pollak, M. R. ; Weitz, D. A. Nonlinear viscoelasticity of actin transiently cross-linked with mutant alpha-actinin-4. Journal of Molecular Biology 2011, 411, 1062-1071. Publisher's VersionAbstract
Filamentous actin and associated actin binding proteins play an essential role in governing the mechanical properties of eukaryotic cells. They can also play a critical role in disease; for example, mutations in alpha-actinin-4 (Actn4), a dynamic actin cross-linking protein, cause proteinuric disease in humans and mice. Amino acid substitutions strongly affect the binding affinity and protein structure of Actn4. To study the physical impact of such substitutions on the underlying cytoskeletal network, we examine the bulk mechanical behavior of in vitro actin networks cross-linked with wild-type and mutant Actn4. These networks exhibit a complex viscoelastic response and are characterized by fluid-like behavior at the longest timescales, a feature that can be quantitatively accounted for through a model governed by dynamic cross-linking. The elastic behavior of the network is highly nonlinear, becoming much stiffer with applied stress. This nonlinear elastic response is also highly sensitive to the mutations of Actn4. In particular, we observe that actin networks cross-linked with Actn4 bearing the disease-causing K255E mutation are more brittle, with a lower breaking stress in comparison to networks cross-linked with wild-type Actn4. Furthermore, a mutation that ablates the first actin binding site (ABS1) in Actn4 abrogates the network's ability to stress-stiffen is standard nomenclature. These changes in the mechanical properties of actin networks cross-linked with mutant Actn4 may represent physical determinants of the underlying disease mechanism in inherited focal segmental glomerulosclerosis. (C) 2011 Elsevier Ltd. All rights reserved.
Wilking, J. N. ; Angelini, T. E. ; Seminara, A. ; Brenner, M. P. ; Weitz, D. A. Biofilms as complex fluids. Mrs Bulletin 2011, 36, 385-391. Publisher's VersionAbstract
Bacterial biofilms are interface-associated colonies of bacteria embedded in an extracellular matrix that is composed primarily of polymers and proteins. They can be viewed in the context of soft matter physics: the rigid bacteria are analogous to colloids, and the extracellular matrix is a cross-linked polymer gel. This perspective is beneficial for understanding the structure, mechanics, and dynamics of the biofilm. Bacteria regulate the water content of the biofilm by controlling the composition of the extracellular matrix, and thereby controlling the mechanical properties. The mechanics of well-defined soft materials can provide insight into the mechanics of biofilms and, in particular, the viscoelasticity. Furthermore, spatial heterogeneities in gene expression create heterogeneities in polymer and surfactant production. The resulting concentration gradients generate forces within the biofilm that are relevant for biofilm spreading and survival.
Windbergs, M. ; Weitz, D. A. Drug dissolution chip (DDC): A microfluidic approach for drug release. Small 2011, 7 3011-3015. Publisher's VersionAbstract

The first pharmaceutical dissolution testing device for multiparticulate drug delivery systems on a microfluidic chip is presented. This versatile platform technology allows for simultaneous analysis of the drug concentration in the dissolution medium and of the solid parts of the sample immobilized in the device with different analytical techniques. By mimicking the fate of drug release in the human body, this can contribute to the optimization of therapeutic effectiveness during the development of novel drug release systems.

Weitz, D. A. Colloidal glasses. In Glasses and Grains; 2011; Vol. 61, pp. 25-39. Publisher's VersionAbstract
Classes and granular materials share many features in common: Both can flow under some conditions but form disordered solids under other conditions. The similarity is captured within the jamming phase diagram, which considers how the solid-like state is fluidized with decreasing density, increasing shear stress, and increasing agitation, due to temperature in the case of molecular glasses and to shaking or some other form of agitation in the case of granular materials. Colloidal particles also undergo both jamming and glass transitions. They have the advantage that they are thermalized by temperature and that the particles themselves are large enough to be directly visualized. Thus, the study of the glass transition in colloids can provide an interesting comparison between molecular glasses and granular materials. This paper reviews the properties of colloidal suspensions near the colloidal glass transition, and explores both the glass-like properties and the jamming properties of these materials.
Wang, W. ; Xie, R. ; Ju, X. - J. ; Luo, T. ; Liu, L. ; Weitz, D. A. ; Chu, L. - Y. Controllable microfluidic production of multicomponent multiple emulsions. Lab on a Chip 2011, 11, 1587-1592. Publisher's VersionAbstract
A hierarchical and scalable microfluidic device constructed from a combination of three building blocks enables highly controlled generation of multicomponent multiple emulsions. The number, ratio and size of droplets, each with distinct contents being independently co-encapsulated in the same level, can be precisely controlled. The building blocks are a drop maker, a connector and a liquid extractor; combinations of these enable the scale-up of the device to create higher-order multicomponent multiple emulsions with exceptionally diverse structures. These multicomponent multiple emulsions offer a versatile and promising platform for precise encapsulation of incompatible actives or chemicals, for synergistic delivery and biochemical and chemical reactions, and for engineering multicompartment materials with controlled internal phases.
Vemula, P. K. ; Boilard, E. ; Syed, A. ; Campbell, N. R. ; Muluneh, M. ; Weitz, D. A. ; Lee, D. M. ; Karp, J. M. On-demand drug delivery from self-assembled nanofibrous gels: A new approach for treatment of proteolytic disease. Journal of Biomedical Materials Research Part A 2011, 97A, 103-110. Publisher's VersionAbstract
Local delivery of drugs offers the potential for high local drug concentration while minimizing systemic toxicity, which is often observed with oral dosing. However, local depots are typically administered less frequently and include an initial burst followed by a continuous release. To maximize efficiency of therapy, it is critical to ensure that drug is only released when needed. One of the hallmarks of rheumatoid arthritis, for example, is its variable disease activity consisting of exacerbations of inflammation punctuated by periods of remission. This presents significant challenges for matching localized drug delivery with disease activity. An optimal system would be nontoxic and only release drugs during the period of exacerbation, self-titrating in response to the level of inflammation. We report the development of an injectable self-assembled nanofibrous hydrogel, from a generally recognized as safe material, which is capable of encapsulation and release of agents in response to specific enzymes that are significantly upregulated in a diseased state including matrix metalloproteinases (MMP-2 and MMP-9) and esterases. We show that these self-assembled nanofibrous gels can withstand shear forces that may be experienced in dynamic environments such as joints, can remain stable following injection into healthy joints of mice, and can disassemble in vitro to release encapsulated agents in response to synovial fluid from arthritic patients. This novel approach represents a next-generation therapeutic strategy for localized treatment of proteolytic diseases. (C) 2011 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 97A: 103-110, 2011.
Tambe, D. T. ; Hardin, C. C. ; Angelini, T. E. ; Rajendran, K. ; Park, C. Y. ; Serra-Picamal, X. ; Zhou, E. H. ; Zaman, M. H. ; Butler, J. P. ; Weitz, D. A. ; et al. Collective cell guidance by cooperative intercellular forces. Nature Materials 2011, 10, 469-475. Publisher's VersionAbstract
Cells comprising a tissue migrate as part of a collective. How collective processes are coordinated over large multi-cellular assemblies has remained unclear, however, because mechanical stresses exerted at cell-cell junctions have not been accessible experimentally. We report here maps of these stresses within and between cells comprising a monolayer. Within the cell sheet there arise unanticipated fluctuations of mechanical stress that are severe, emerge spontaneously, and ripple across the monolayer. Within that stress landscape, local cellular migrations follow local orientations of maximal principal stress. Migrations of both endothelial and epithelial monolayers conform to this behaviour, as do breast cancer cell lines before but not after the epithelial-mesenchymal transition. Collective migration in these diverse systems is seen to be governed by a simple but unifying physiological principle: neighbouring cells join forces to transmit appreciable normal stress across the cell-cell junction, but migrate along orientations of minimal intercellular shear stress.
Thiele, J. ; Windbergs, M. ; Abate, A. R. ; Trebbin, M. ; Shum, H. C. ; Foerster, S. ; Weitz, D. A. Early development drug formulation on a chip: Fabrication of nanoparticles using a microfluidic spray dryer. Lab on a Chip 2011, 11, 2362-2368. Publisher's VersionAbstract
Early development drug formulation is exacerbated by increasingly poor bioavailability of potential candidates. Prevention of attrition due to formulation problems necessitates physicochemical analysis and formulation studies at a very early stage during development, where the availability of a new substance is limited to small quantities, thus impeding extensive experiments. Miniaturization of common formulation processes is a strategy to overcome those limitations. We present a versatile technique for fabricating drug nanoformulations using a microfluidic spray dryer. Nanoparticles are formed by evaporative precipitation of the drug-loaded spray in air at room temperature. Using danazol as a model drug, amorphous nanoparticles of 20-60 nm in diameter are prepared with a narrow size distribution. We design the device with a geometry that allows the injection of two separate solvent streams, thus enabling co-spray drying of two substances for the production of drug co-precipitates with tailor-made composition for optimization of therapeutic efficiency.
Sinha, N. N. ; Weitz, D. A. Cocktail physics. Physics World 2011, 24, 25-28. Publisher's VersionAbstract

Many top chefs are well known for taking a more scientific approach to cooking in recent years, and some “mixologists” are now extending the same philosophy to cocktails. Naveen N. Sinha and David A. Weitz explain the theory and techniques behind these increasingly exotic mixed drinks.

Studart, A. R. ; Studer, J. ; Xu, L. ; Yoon, K. ; Shum, H. C. ; Weitz, D. A. Hierarchical porous materials made by drying complex suspensions. Langmuir 2011, 27, 955-964. Publisher's VersionAbstract
Porous structures containing pores at different length scales are often encountered in nature and are important in many applications. While several processing routes have been demonstrated to create such hierarchical porous materials, most methods either require chemical gelation reactions or do not allow for the desired control of pore sizes over multiple length scales. We describe a versatile and simple approach to produce tailor-made hierarchical porous materials that relies solely on the process of drying. Our results show that simple drying of a complex suspension can lead to the self-assembly of droplets, colloidal particles and molecular species into unique 3D hierarchical porous structures. Using a microfluidic device to produce monodisperse templating droplets of tunable size, we prepared materials with up to three levels of hierarchy exhibiting monodisperse pores ranging from 10 nm to 800 mu m. While the size of macropores obtained after drying is determined by the size of initial droplets, the interconnectivity between macropores is strongly affected by the type of droplet stabilizer (surfactants or particles). This simple route can be used to prepare porous materials of many chemical compositions and has great potential for creating artificial porous structures that capture some of the exquisite hierarchical features of porous biological materials.
Steinhilber, D. ; Seiffert, S. ; Heyman, J. A. ; Paulus, F. ; Weitz, D. A. ; Haag, R. Hyperbranched polyglycerols on the nanometer and micrometer scale. Biomaterials 2011, 32, 1311-1316. Publisher's VersionAbstract
We report the preparation of polyglycerol particles on different length scales by extending the size of hyperbranched polyglycerols (3 nm) to nanogels (32 nm) and microgels (140 and 220 mu m). We use miniemulsion templating for the preparation of nanogels and microfluidic templating for the preparation of microgels, which we obtain through a free-radical polymerization of hyperbranched polyglycerol decaacrylate and polyethylene glycol-diacrylate. The use of mild polymerization conditions allows yeast cells to be encapsulated into the resultant microgels with cell viabilities of approximately 30%. (C) 2010 Elsevier Ltd. All rights reserved.
Sprakel, J. ; Lindstroem, S. B. ; Kodger, T. E. ; Weitz, D. A. Stress enhancement in the delayed yielding of colloidal gels. Physical Review Letters 2011, 106, 248303. Publisher's VersionAbstract
Networks of aggregated colloidal particles are solidlike and can sustain an applied shear stress while exhibiting little or no creep; however, ultimately they will catastrophically fail. We show that the time delay for this yielding decreases in two distinct exponential regimes with applied stress. This behavior is universal and found for a variety of colloidal gel systems. We present a bond-rupture model that quantitatively describes this behavior and highlights the role of mesoscopic structures. Our result gives new insight into the nature of yielding in these soft solid materials.
Shum, H. C. ; Santanach-Carreras, E. ; Kim, J. - W. ; Ehrlicher, A. ; Bibette, J. ; Weitz, D. A. Dewetting-induced membrane formation by adhesion of amphiphile-laden interfaces. Journal of the American Chemical Society 2011, 133, 4420-4426. Publisher's VersionAbstract
We introduce an approach for forming bilayer polymer membranes by adhesion of amphiphile-laden interfaces. This adhesion is induced by a reduction of solvent quality for the amphiphilic diblock copolymers through selective evaporation of good solvent in the solvent mixture. By combining this membrane formation mechanism with a double-emulsion-templated approach for vesicle formation, we fabricate monodisperse polymersomes that exhibit excellent membrane uniformity, and structural stability, using a method that has high encapsulation efficiency. Moreover, we also show that the technique is versatile and can be applied to different block copolymers. The ability to direct the assembly of amphiphiles into a membrane creates new opportunities to engineer the structures of vesicles on the level of the individual bilayer leaflets.
Shum, H. C. ; Zhao, Y. -jin; Kim, S. - H. ; Weitz, D. A. Multicompartment polymersomes from double emulsions. Angewandte Chemie-International Edition 2011, 50, 1648-1651. Publisher's VersionAbstract

Versatile chambers: A microfluidic technique is used to generate polymersomes with multiple compartments using double emulsions with different morphology as templates (see scheme). The polymersomes could be utilized for encapsulation of multiple substances and as reaction vessels that enable triggered reaction with controlled stoichometry of the reactants.

Seiffert, S. ; Dubbert, J. ; Richtering, W. ; Weitz, D. A. Reduced UV light scattering in PDMS microfluidic devices. Lab on a Chip 2011, 11, 966-968. Publisher's VersionAbstract
Microfluidic devices which consist of polydimethylsiloxane ( PDMS) are used extensively for the production of polymer microparticles through the use of droplet templating and on-chip photopolymerization. However, in existing methods, spatial confinement of the photochemical droplet solidification is impaired by UV light scattering inside the PDMS elastomer. We present a technique to load PDMS microfluidic devices with a fluorescent dye that absorbs the scattered UV light and shifts it to longer wavelengths. By this means, the stray light is no longer harmful, and UV exposure can be limited to a desired region on the microfluidic chip.
Muluneh, M. ; Sprakel, J. ; Wyss, H. M. ; Mattsson, J. ; Weitz, D. A. Direct visualization of pH-dependent evolution of structure and dynamics in microgel suspensions. Journal of Physics-Condensed Matter 2011, 23, 505101. Publisher's VersionAbstract
We use 3D confocal microscopy combined with image analysis and particle tracking techniques to study the structure and dynamics of aqueous suspensions of fluorescently labelled p(NIPAm-co-AAc) microgel particles. By adjusting the pH we can tune the interactions between the microgel particles from purely repulsive near neutral pH, to weakly attractive at low pH. This change in the interaction potential has a pronounced effect on the manner in which the suspensions solidify. We directly follow the evolution of the system after a quench from the liquid state to obtain detailed information on the route to kinetic arrest. At low pH and low concentration, dynamic arrest results mainly from crystallization driven by the attraction between particles; crystal nucleation occurs homogeneously throughout the sample and does not appear to be localized to geometric boundaries. Moreover, the growth of crystals is characterized by nucleation-limited kinetics where a rapid growth of crystal domains takes place after a long concentration-dependent lag time. At low pH and high concentration, relaxation of the suspension is constrained and it evolves only slightly, resulting in a disordered solid. At neutral pH, the dynamics are a function of the particle number concentration only; a high concentration leads to the formation of a disordered soft glassy solid.