Abbaspourrad, A. ; Datta, S. S. ; Weitz, D. A. Controlling release from pH-responsive microcapsules. Langmuir 2013, 29, 12697-12702.Abstract

We report a microfluidic approach to produce monodisperse pH-responsive microcapsules with precisely controlled release behavior. The solid microcapsule shells are composed of a biocompatible pH-responsive polymer and robustly encapsulate an active material; however, when exposed to a trigger pH, the shells degrade and ultimately release the microcapsule contents. We control the trigger pH by using polymers that dissolve at different pH values. We independently control the time at which the microcapsule contents are released by carefully controlling the shell thickness. Moreover, we independently control the rate at which the encapsulated contents are released by making hybrid shells composed of a mixture of a pH-responsive polymer and varying proportions of another, solid, pH-unresponsive polymer. This enables us to achieve monodisperse microcapsules that robustly encapsulate an active material, only releasing it when exposed to a desired pH, after a prescribed time delay, and at a prescribed rate.

Abate, A. R. ; Hung, T. ; Sperling, R. A. ; Mary, P. ; Rotem, A. ; Agresti, J. J. ; Weiner, M. A. ; Weitz, D. A. DNA sequence analysis with droplet-based microfluidics. Lab on a Chip 2013, 13, 4864-4869.Abstract

Droplet-based microfluidic techniques can form and process micrometer scale droplets at thousands per second. Each droplet can house an individual biochemical reaction, allowing millions of reactions to be performed in minutes with small amounts of total reagent. This versatile approach has been used for engineering enzymes, quantifying concentrations of DNA in solution, and screening protein crystallization conditions. Here, we use it to read the sequences of DNA molecules with a FRET-based assay. Using probes of different sequences, we interrogate a target DNA molecule for polymorphisms. With a larger probe set, additional polymorphisms can be interrogated as well as targets of arbitrary sequence.

Kim, S. - H. ; Kim, J. W. ; Kim, D. - H. ; Han, S. - H. ; Weitz, D. A. Enhanced-throughput production of polymersomes using a parallelized capillary microfluidic device. Microfluidics and Nanofluidics 2013, 14, 509-514.Abstract

We report a parallelized capillary microfluidic device for enhanced production rate of monodisperse polymersomes. This device consists of four independent capillary microfluidic devices, operated in parallel; each device produces monodisperse water-in-oil-in-water (W/O/W) double-emulsion drops through a single-step emulsification. During generation of the double-emulsion drops, the innermost water drop is formed first and it triggers a breakup of the middle oil phase over wide range of flow rates; this enables robust and stable formation of the double-emulsion drops in all drop makers of the parallelized device. Double-emulsion drops are transformed to polymersomes through a dewetting of the amphiphile-laden middle oil phase on the surface of the innermost water drop, followed by the subsequent separation of the oil drop. Therefore, we can make polymersomes with a production rate enhanced by a factor given by the number of drop makers in the parallelized device.

Rodriguez-Lopez, J. ; Shum, H. C. ; Elvira, L. ; Montero de Espinosa, F. ; Weitz, D. A. Fabrication and manipulation of polymeric magnetic particles with magnetorheological fluid. Journal of Magnetism and Magnetic Materials 2013, 326, 220-224.Abstract

Polymeric magnetic microparticles have been created using a microfluidic device via ultraviolet (UV) polymerization of double emulsions, resulting in cores of magnetorheological (MR) fluids surrounded by polymeric shells. We demonstrate that the resultant particles can be manipulated magnetically to achieve triggered rupture of the capsules. This illustrates the great potential of our capsules for triggered release of active ingredients encapsulated in the polymeric magnetic microparticles. (C) 2012 Elsevier BY. All rights reserved.

Lee, J. Y. ; Hwang, J. W. ; Jung, H. W. ; Kim, S. H. ; Lee, S. J. ; Yoon, K. ; Weitz, D. A. Fast dynamics and relaxation of colloidal drops during the drying process using multispeckle diffusing wave spectroscopy. Langmuir 2013, 29, 861-866.Abstract

The fast dynamics generated by the Brownian motion of particles in colloidal drops, and the related relaxation during drying, which play key roles in suspension systems, were investigated incorporating multispeckle diffusing wave spectroscopy (MSDWS). MSDWS equipment was implemented to analyze the relaxation properties of suspensions under a nonergodic and nonstationary drying process, which cannot be elucidated by conventional light scattering methods, such as dynamic light scattering and diffusing wave spectroscopy. Rapid particle movement can be identified by the characteristic relaxation time, which is closely related to the Brownian motion due to thermal fluctuations of the particles. In the compacting stage of the drying process, the characteristic relaxation time increased gradually with the drying time because the particles in the colloidal drop were constrained by themselves. Moreover, variations of the initial concentration and particle size considerably affected the complete drying time and characteristic relaxation time, producing a shorter relaxation time for a low concentrated suspension with small particles.

Kim, S. - H. ; Nam, J. ; Kim, J. W. ; Kim, D. - H. ; Han, S. - H. ; Weitz, D. A. Formation of polymersomes with double bilayers templated by quadruple emulsions. Lab on a Chip 2013, 13, 1351-1356.Abstract

Polymersomes, vesicles composed of bilayer membranes of amphiphilic block-copolymers, are promising delivery vehicles for long-term storage and controlled release of bioactives; enhanced stability of the membrane makes polymersomes potentially useful in a wide range of biological delivery applications by comparison with liposomes. However, unilamellar structure is intrinsically fragile when subjected to external stress. Here, we report a microfluidic approach to produce polymersomes with double bilayers, providing higher stability and lower permeability than unilamellar polymersomes. To achieve this, we developed a new design of a capillary microfluidic device to produce quadruple-emulsion drops which serve as a template for the polymersomes-in-polymersomes. When two bilayers are attracted by depletion in polymersomes-in-polymersomes, the inner polymersomes protrude and bud, forming double bilayers. We confirm these structures are indeed double bilayers using microaspiration and selective doping of the leaflets with nanoparticles. The resultant polymersomes have great potential as highly stable and biocompatible microcarriers for robust encapsulation and storage of bioactives such as drugs, cosmetics and nutrients.

Chen, H. ; Li, J. ; Wan, J. ; Weitz, D. A. ; Stone, H. A. Gas-core triple emulsions for ultrasound triggered release. Soft Matter 2013, 9 38-42.Abstract

Gas-in-oil-in-water-in-oil triple emulsions are fabricated with a microfluidic method. The encapsulating layers can be triggered for release by ultrasound, owing to the gas core. Due to the stability in the atmosphere, the emulsions are polymerized by using UV light outside the device to fabricate compound particles with a gas-in-liquid-in-solid structure.

Wang, W. ; Zhang, M. - J. ; Xie, R. ; Ju, X. - J. ; Yang, C. ; Mou, C. - L. ; Weitz, D. A. ; Chu, L. - Y. Hole-shell microparticles from controllably evolved double emulsions. Angewandte Chemie-International Edition 2013, 52, 8084-8087. [PDF]
Wilking, J. N. ; Zaburdaev, V. ; De Volder, M. ; Losick, R. ; Brenner, M. P. ; Weitz, D. A. Liquid transport facilitated by channels in Bacillus subtilis biofilms. Proceedings of the National Academy of Sciences of the United States of America 2013, 110, 848-852.Abstract

Many bacteria on earth exist in surface-attached communities known as biofilms. These films are responsible for manifold problems, including hospital-acquired infections and biofouling, but they can also be beneficial. Biofilm growth depends on the transport of nutrients and waste, for which diffusion is thought to be the main source of transport. However, diffusion is ineffective for transport over large distances and thus should limit growth. Nevertheless, biofilms can grow to be very large. Here we report the presence of a remarkable network of well-defined channels that form in wildtype Bacillus subtilis biofilms and provide a system for enhanced transport. We observe that these channels have high permeability to liquid flow and facilitate the transport of liquid through the biofilm. In addition, we find that spatial variations in evaporative flux from the surface of these biofilms provide a driving force for the flow of liquid in the channels. These channels offer a remarkably simple system for liquid transport, and their discovery provides insight into the physiology and growth of biofilms.

Abbaspourrad, A. ; Duncanson, W. J. ; Lebedeva, N. ; Kim, S. - H. ; Zhushma, A. P. ; Datta, S. S. ; Dayton, P. A. ; Sheiko, S. S. ; Rubinstein, M. ; Weitz, D. A. Microfluidic fabrication of stable gas-filled microcapsules for acoustic contrast enhancement. Langmuir 2013, 29, 12352-12357.Abstract

We introduce a facile approach for the production of gas-filled microcapsules designed to withstand high pressures. We exploit microfluidics to fabricate water-filled microcapsules that are then externally triggered to become gas-filled, thus making them more echogenic. In addition, the gas-filled microcapsules have a solid polymer shell making them resistant to pressure-induced buckling, which makes them more mechanically robust than traditional prestabilized microbubbles; this should increase the potential of their utility for acoustic imaging of porous media with high hydrostatic pressures such as oil reservoirs.

Liu, D. ; Herranz-Blanco, B. ; Makila, E. ; Arriaga, L. R. ; Mirza, S. ; Weitz, D. A. ; Sandler, N. ; Salonen, J. ; Hirvonen, J. ; Santos, H. A. Microfluidic templated mesoporous silicon-solid lipid microcomposites for sustained drug delivery. ACS Applied Materials & Interfaces 2013, 5 12127-12134.Abstract

A major challenge for a drug-delivery system is to engineer stable drug carriers with excellent biocompatibility, monodisperse size, and controllable release profiles. In this study, we used a microfluidic technique to encapsulate thermally hydrocarbonized porous silicon (THCPSi) microparticles within solid lipid microparticles (SLMs) to overcome the drawbacks accompanied by THCPSi microparticles. Formulation and process factors, such as lipid matrixes, organic solvents, emulsifiers, and methods to evaporate the organic solvents, were all evaluated and optimized to prepare monodisperse stable SLMs. FTIR analysis together with confocal images showed the clear deposition of THCPSi microparticles inside the monodisperse SLM matrix. The formation of monodisperse THCPSi solid lipid microcomposites (THCPSi SLMCs) not only altered the surface hydrophobicity and morphology of THCPSi microparticles but also remarkably enhanced their cytocompatibility with intestinal (Caco-2 and HT-29) cancer cells. Regardless of the solubility of the loaded therapeutics (aqueous insoluble, fenofibrate and furosemide; aqueous soluble, methotrexate and ranitidine) and the pH values of the release media (1.2, 5.0, and 7.4), the time for the release of 50% of the payloads from THCPSi SLMC was at least 1.3 times longer than that from the THCPSi microparticles. The sustained release of both water-soluble and -insoluble drugs together with a reduced burst-release effect from monodisperse THCPSi SLMC was achieved, indicating the successful encapsulation of THCPSi microparticles into the SLM matrix. The fabricated THCPSi SLMCs exhibited monodisperse spherical morphology, enhanced cytocompatibility, and prolonged both water-soluble and -insoluble drug release, which makes it an attractive controllable drug-delivery platform.

Koziej, D. ; Floryan, C. ; Sperling, R. A. ; Ehrlicher, A. J. ; Issadore, D. ; Westervelt, R. ; Weitz, D. A. Microwave dielectric heating of non-aqueous droplets in a microfluidic device for nanoparticle synthesis. Nanoscale 2013, 5 5468-5475.Abstract

We describe a microfluidic device with an integrated microwave heater specifically designed to dielectrically heat non-aqueous droplets using time-varying electrical fields with the frequency range between 700 and 900 MHz. The precise control of frequency, power, temperature and duration of the applied field opens up new vistas for experiments not attainable by conventional microwave heating. We use a non-contact temperature measurement system based on fluorescence to directly determine the temperature inside a single droplet. The maximum temperature achieved of the droplets is 50 degrees C in 15 ms which represents an increase of about 25 degrees C above the base temperature of the continuous phase. In addition we use an infrared camera to monitor the thermal characteristics of the device allowing us to ensure that heating is exclusively due to the dielectric heating and not due to other effects like non-dielectric losses due to electrode or contact imperfection. This is crucial for illustrating the potential of dielectric heating of benzyl alcohol droplets for the synthesis of metal oxides. We demonstrate the utility of this technology for metal oxide nanoparticle synthesis, achieving crystallization of tungsten oxide nanoparticles and remarkable microstructure, with a reaction time of 64 ms, a substantial improvement over conventional heating methods.

Rowat, A. C. ; Jaalouk, D. E. ; Zwerger, M. ; Ung, L. W. ; Eydelnant, I. A. ; Olins, D. E. ; Olins, A. L. ; Herrmann, H. ; Weitz, D. A. ; Lammerding, J. Nuclear envelope composition determines the ability of neutrophil-type cells to passage through micron-scale constrictions. Journal of Biological Chemistry 2013, 288, 8610-8618.Abstract

Neutrophils are characterized by their distinct nuclear shape, which is thought to facilitate the transit of these cells through pore spaces less than one-fifth of their diameter. We used human promyelocytic leukemia (HL-60) cells as a model system to investigate the effect of nuclear shape in whole cell deformability. We probed neutrophil-differentiated HL-60 cells lacking expression of lamin B receptor, which fail to develop lobulated nuclei during granulopoiesis and present an in vitro model for Pelger-Huet anomaly; despite the circular morphology of their nuclei, the cells passed through micron-scale constrictions on similar timescales as scrambled controls. We then investigated the unique nuclear envelope composition of neutrophil-differentiated HL-60 cells, which may also impact their deformability; although lamin A is typically down-regulated during granulopoiesis, we genetically modified HL-60 cells to generate a subpopulation of cells with well defined levels of ectopic lamin A. The lamin A-overexpressing neutrophil-type cells showed similar functional characteristics as the mock controls, but they had an impaired ability to pass through micron-scale constrictions. Our results suggest that levels of lamin A have a marked effect on the ability of neutrophils to passage through micron-scale constrictions, whereas the unusual multilobed shape of the neutrophil nucleus is less essential.

Choi, C. - H. ; Weitz, D. A. ; Lee, C. - S. One step formation of controllable complex emulsions: From functional particles to simultaneous encapsulation of hydrophilic and hydrophobic agents into desired position. Advanced Materials 2013, 25, 2536-2541. [PDF]
Kim, S. - H. ; Kim, J. W. ; Kim, D. - H. ; Han, S. - H. ; Weitz, D. A. Polymersomes containing a hydrogel network for high stability and controlled release. Small 2013, 9 124-131.Abstract

Capillary microfluidic devices are used to prepare monodisperse polymersomes consisting of a hydrogel core and a bilayer membrane of amphiphilic diblock-copolymers. To make polymersomes, water-in-oil-in-water double-emulsion drops are prepared as templates through single-step emulsification in a capillary microfluidic device. The amphiphile-laden middle oil phase of the double-emulsion drop dewets from the surface of the innermost water drop, which contains hydrogel prepolymers; this dewetting leads to the formation of a bilayer membrane. Subsequently, the oil phase completely separates from the innermost water drop, leaving a polymersome. Upon UV illumination of the polymersome, the prepolymers encapsulated within the interior are crosslinked, forming a hydrogel core. The hydrogel network within the polymersomes facilitates sustained release of the encapsulated materials and increases the stability of the polymersomes through the formation of a scaffold to support the bilayer. In addition, this approach provides a facile method to make monodisperse hydrogel particles directly dispersed in water.

Di Benedetto, F. ; Fasano, V. ; Persano, L. ; Maruccio, C. ; Mele, E. ; Potente, G. ; Weitz, D. A. ; De Lorenzis, L. ; Pisignano, D. Rolling particle lithography by soft polymer microparticles. Soft Matter 2013, 9 2206-2211.Abstract

Elastomeric polymeric microspheres are employed as a direct-writing tool for the continuous delivery of molecular materials. The mechanical properties enabling patterning are investigated and modelled. The proposed approach provides a low cost and versatile lithographic method for transferring features with real-time dynamic control.

Mazutis, L. ; Gilbert, J. ; Ung, L. W. ; Weitz, D. A. ; Griffiths, A. D. ; Heyman, J. A. Single-cell analysis and sorting using droplet-based microfluidics. Nature Protocols 2013, 8 870-891.Abstract

We present a droplet-based microfluidics protocol for high-throughput analysis and sorting of single cells. Compartmentalization of single cells in droplets enables the analysis of proteins released from or secreted by cells, thereby overcoming one of the major limitations of traditional flow cytometry and fluorescence-activated cell sorting. As an example of this approach, we detail a binding assay for detecting antibodies secreted from single mouse hybridoma cells. Secreted antibodies are detected after only 15 min by co-compartmentalizing single mouse hybridoma cells, a fluorescent probe and single beads coated with anti-mouse IgG antibodies in 50-pl droplets. The beads capture the secreted antibodies and, when the captured antibodies bind to the probe, the fluorescence becomes localized on the beads, generating a clearly distinguishable fluorescence signal that enables droplet sorting at similar to 200 Hz as well as cell enrichment. The microfluidic system described is easily adapted for screening other intracellular, cell-surface or secreted proteins and for quantifying catalytic or regulatory activities. In order to screen similar to 1 million cells, the microfluidic operations require 2-6 h; the entire process, including preparation of microfluidic devices and mammalian cells, requires 5-7 d.

Datta, S. S. ; Chiang, H. ; Ramakrishnan, T. S. ; Weitz, D. A. Spatial fluctuations of fluid velocities in flow through a three-dimensional porous medium. Physical Review Letters 2013, 111.Abstract

We use confocal microscopy to directly visualize the spatial fluctuations in fluid flow through a three-dimensional porous medium. We find that the velocity magnitudes and the velocity components both along and transverse to the imposed flow direction are exponentially distributed, even with residual trapping of a second immiscible fluid. Moreover, we find pore-scale correlations in the flow that are determined by the geometry of the medium. Our results suggest that despite the considerable complexity of the pore space, fluid flow through it is not completely random.

DiLauro, A. M. ; Abbaspourrad, A. ; Weitz, D. A. ; Phillips, S. T. Stimuli-responsive core-shell microcapsules with tunable rates of release by using a depolymerizable poly(phthalaldehyde) membrane. Macromolecules 2013, 46, 3309-3313.Abstract

Flow-focusing microfluidic techniques were used to provide access to core shell microcapsules in which the shell is composed of end tapped poly(phthalaldehydes) that depolymerize completely from head-to-tail in response to fluoride. Microcapsules made from these depolymerizable polymers provide an amplified response to the applied chemical signal, where the rate of the response can be tuned both by varying the length of the polymer and the thickness of the shell wall.

Munster, S. ; Jawerth, L. M. ; Leslie, B. A. ; Weitz, J. I. ; Fabry, B. ; Weitz, D. A. Strain history dependence of the nonlinear stress response of fibrin and collagen networks. Proceedings of the National Academy of Sciences of the United States of America 2013, 110, 12197-12202.Abstract

We show that the nonlinear mechanical response of networks formed from un-cross-linked fibrin or collagen type I continually changes in response to repeated large-strain loading. We demonstrate that this dynamic evolution of the mechanical response arises from a shift of a characteristic nonlinear stress-strain relationship to higher strains. Therefore, the imposed loading does not weaken the underlying matrices but instead delays the occurrence of the strain stiffening. Using confocal microscopy, we present direct evidence that this behavior results from persistent lengthening of individual fibers caused by an interplay between fiber stretching and fiber buckling when the networks are repeatedly strained. Moreover, we show that covalent cross-linking of fibrin or collagen inhibits the shift of the nonlinear material response, suggesting that the molecular origin of individual fiber lengthening may be slip of monomers within the fibers. Thus, a fibrous architecture in combination with constituents that exhibit internal plasticity creates a material whose mechanical response adapts to external loading conditions. This design principle may be useful to engineer novel materials with this capability.