Sabbagh, B. ; Stolovicki, E. ; Park, S. ; Weitz, D. A. ; Yossifon, G. Tunable Nanochannels Connected in Series for Dynamic Control of Multiple Concentration-Polarization Layers and Preconcentrated Molecule Plugs. Nano Letters 2020, 20, 8524-8533. Publisher's VersionAbstract
Integration of ionic permselective medium (e.g., nanochannels, membranes) within microfluidic channels has been shown to enable on-chip desalination, sample purification, bioparticle sorting, and biomolecule concentration for enhanced detection sensitivity. However, the ion-permselective mediums are generally of fixed properties and cannot be dynamically tuned. Here we study a microfluidic device consisting of an array of individually addressable elastic membranes connected in series on top of a single microfluidic channel that can be deformed to locally reduce the channel cross-section into a nanochannel. Dynamic tunability of the ion-permselective medium, as well as controllability of its location and ionic permselectivity, introduces a new functionality to microfluidics-based lab-on-a-chip devices, for example, dynamic localization of preconcentrated biomolecule plugs at different sensing regions for multiplex detection. Moreover, the ability to simultaneously form a series of preconcentrated plugs at desired locations increases parallelization of the system and the trapping efficiency of target analytes.
Wu, Z. ; Werner, J. G. ; Weitz, D. A. Microfluidic fabrication of Phase-Inverted Microcapsules with Asymmetric Shell Membranes with Graded Porosity. ACS Macro Letters 2020, 10, 116-121. Publisher's VersionAbstract
Microcapsules with liquid cores and solid shells are attractive as dispersible protective micron-sized containers. Applications that rely on molecular mass transport often require a combination of size selectivity, high permeability, and mechanical stability. Capsule architectures that combine all these features represent a material property, design, and fabrication challenge. In this work, the design of an asymmetric microcapsule shell architecture is reported to achieve a good combination of the desired features. Poly(methyl methacrylate) phase-inverted microcapsules featuring an asymmetric graded macroporous shell covered with a dense skin separation layer are obtained from water-in-oil-in-water double emulsion drops that are phase-inverted in a water-based coagulation bath. The phase-inverted microcapsules exhibit good mechanical stability and allow for high permeability of its shell membrane with molecular size dependence.
Hwang, H. ; Weitz, D. A. ; Spaepen, F. Stiffness of the interface between a colloidal body-centered cubic crystal and its liquid. PNAS 2020, 117, 25225-25229. Publisher's VersionAbstract
Equilibrium interfaces were established between body-centered cubic (BCC) crystals and their liquid using charged colloidal particles in an electric bottle. By measuring a time series of interfacial positions and computing the average power spectrum, their interfacial stiffness was determined according to the capillary fluctuation method. For the (100) and the (114) interfaces, the stiffnesses were 0.15 and 0.18 kBT/σ2 (σ: particle diameter), respectively, and were isotropic in the plane of the interface. For comparison, similar charged colloids were used to create an interface between a face-centered cubic (FCC) crystal and its liquid. Its stiffness was significantly larger: 0.26 kBT/σ2. This result gives experimental support to the explanations offered for the preferential nucleation of BCC over FCC in metallic alloys.
Brenner, M. ; Sorensen, P. ; Weitz, D. Science and Cooking: Physics Meets Food, From Homemade to Haute Cuisine; 1st ed. W.W. Norton, 2020; pp. 320. Publisher's Version
Sun, Z. ; Wu, B. ; Ren, Y. ; Wang, Z. ; Zhao, C. ‐X. ; Hai, M. ; Weitz, D. A. ; Chen, D. Diverse Particle Carriers Prepared by Co-Precipitation and Phase Separation: Formation and Applications. ChemPlusChem 2020, 86, 49-58. Publisher's VersionAbstract
Nanoparticles with diverse structures and unique properties have attracted increasing attention for their widespread applications. Co‐precipitation under rapid mixing is an effective method to obtained biocompatible nanoparticles and diverse particle carriers are achieved by controlled phase separation via interfacial tensions. In this Minireview, we summarize the underlying mechanism of co‐precipitation and show that rapid mixing is important to ensure co‐precipitation. In the binary polymer system, the particles can form four different morphologies, including occluded particle, core‐shell capsule, dimer particle, and heteroaggregate, and we demonstrate that the final morphology could be controlled by surface tensions through surfactant, polymer composition, molecular weight, and temperature. The applications of occluded particles, core‐shell capsules and dimer particles prepared by co‐precipitation or microfluidics upon the regulation of interfacial tensions are discussed in detail, and show great potential in the areas of functional materials, colloidal surfactants, drug delivery, nanomedicine, bio‐imaging, displays, and cargo encapsulation.
Liu, Y. ; Yang, G. ; Jin, S. ; Zhang, R. ; Chen, P. ; Tengjisi, ; Wang, L. ; Chen, D. ; Weitz, D. A. ; Zhao, C. ‐X. J-Aggregate-Based FRET Monitoring of Drug Release from Polymer Nanoparticles with High Drug Loading. Angewandte Chemie 2020, 59, 20065-20074. Publisher's VersionAbstract
Understanding drug‐release kinetics is critical for the development of drug‐loaded nanoparticles. We developed a J‐aggregate‐based Förster‐resonance energy‐transfer (FRET) method to investigate the release of novel high‐drug‐loading (50 wt %) nanoparticles in comparison with low‐drug‐loading (0.5 wt %) nanoparticles. Single‐dye‐loaded nanoparticles form J‐aggregates because of the high dye‐loading (50 wt %), resulting in a large red‐shift (≈110 nm) in the fluorescence spectrum. Dual‐dye‐loaded nanoparticles with high dye‐loading using FRET pairs exhibited not only FRET but also a J‐aggregate red‐shift (116 nm). Using this J‐aggregate‐based FRET method, dye‐core–polymer‐shell nanoparticles showed two release processes intracellularly: the dissolution of the dye aggregates into dye molecules and the release of the dye molecules from the polymer shell. Also, the high‐dye‐loading nanoparticles (50 wt %) exhibited a slow release kinetics in serum and relatively quick release in cells, demonstrating their great potential in drug delivery.
Lin, S. ‐Z. ; Chen, P. ‐C. ; Guan, L. ‐Y. ; Shao, Y. ; Hao, Y. - K. ; Li, Q. ; Li, B. ; Weitz, D. A. ; Feng, X. ‐Q. Universal Statistical Laws for the Velocities of Collective Migrating Cells. Advanced Biosystems 2020, 4 2000065. Publisher's VersionAbstract
Migratory dynamics of collective cells is central to the morphogenesis of biological tissues. The statistical distribution of cell velocities in 2D confluent monolayers is measured through large‐scale and long‐term experiments of various cell types lying on different substrates. A linear relation is discovered between the variability and the mean of cell speeds during the jamming process of confluent cell monolayers, suggesting time‐invariant distribution profile of cell velocities. It is further found that the probability density function of cell velocities obeys the non‐canonical q‐Gaussian statistics, regardless of cell types and substrate stiffness. It is the Tsallis entropy, instead of the classical Boltzmann–Gibbs entropy, that dictates the universal statistical laws of collective cell migration. The universal statistical law stems from cell–cell interactions, as demonstrated by the wound healing experiments. This previously unappreciated finding provides a linkage between cell‐level heterogeneity and tissue‐level ensembles in embryonic development and tumor growth.
Gerber, G. ; Weitz, D. A. ; Coussot, P. Propagation and adsorption of nanoparticles in porous medium as traveling waves. Physical Review Research 2020, 2 033074. Publisher's VersionAbstract

Generally, one attempts to globally predict or interpret, from numerical simulations, the history of the effluents exiting porous media (i.e., the breakthrough curve), without a clear view of the detailed evolutions of deposition inside the medium. We developed a simple physical frame of description of the colloidal particle transport and adsorption, which allows to predict the main characteristics of transport and deposition in porous media from a set of directly measurable (macroscopic) physical parameters. More precisely, we show that the deposition distribution is basically a traveling wave propagating in the medium with a shape (frontal or extended) and velocity depending on the flow rate and the availability of particles with regards to the adsorption capacity. This in particular makes it possible to predict or interpret the breakthrough curve shape from a physical approach. We also show that additional effects may be included, such as a multiporosity leading to confinement effects (delayed deposition in less accessible regions). The validity of the model is checked from original direct visualizations by confocal microscopy of particle adsorption in time and space for nanoparticle suspensions flowing through a bead packing. This makes it possible to measure the evolution of the deposition profiles in time distinguishing the deposition in confined regions. The model appears to successfully predict the different trends: traveling wave, global deposition profile shape, profiles of deposition in confined regions.

    Zhang, L. ; Abbaspourrad, A. ; Parsa, S. ; Tang, J. ; Cassiola, F. ; Zhang, M. ; Tian, S. ; Dai, C. ; Xiao, L. ; Weitz, D. A. Core-Shell Nanohydrogels with Programmable Swelling for Conformance Control in Porous Media. ACS Applied Materials & Interfaces 2020, 12, 34217-34225. Publisher's VersionAbstract

    Conformance control during waterflooding in an oil reservoir is utilized to redistribute water and increase the sweep efficiency and hence oil production. Using preformed gel particles can effectively redirect the flow by blocking the high-permeability zones and forcing water into low-permeability zones where the oil is trapped. However, the size of such gel particles can limit their applications deeper within the reservoir and can result in shear-induced degradation near the well bore. Here, we fabricate core–shell nanohydrogels with delayed swelling behavior; their volume increases by a factor of 200 after about 30 days in brine under reservoir conditions. We study their effect on the flow behavior in a three-dimensional porous medium micromodel consisting of randomly packed glass beads. Using confocal microscopy, we directly visualize the spatial variations of flow in the micromodel before and after nanohydrogel injection and swelling. The swollen nanohydrogels block some pores reducing the permeability of the micromodel and diverting the water into low-permeability regions. A core flood experiment further confirms that the nanohydrogels can significantly reduce the permeability of a reservoir sample and divert the fluid flow. Our results demonstrate that these core–shell nanohydrogels might be useful for flow control in porous media and can be used as a conformance control agent.

    Wang, Y. ; Cao, T. ; Ko, J. ; Shen, Y. ; Zong, W. ; Sheng, K. ; Cao, W. ; Sun, S. ; Cai, L. ; Zhou, Y. ‐L. ; et al. Dissolvable Polyacrylamide Beads for High-Throughput Droplet DNA Barcoding. Advanced Science 2020, 7 1903463. Publisher's VersionAbstract
    Droplet‐based single cell sequencing technologies, such as inDrop, Drop‐seq, and 10X Genomics, are catalyzing a revolution in the understanding of biology. Barcoding beads are key components for these technologies. What is limiting today are barcoding beads that are easy to fabricate, can efficiently deliver primers into drops, and thus achieve high detection efficiency. Here, this work reports an approach to fabricate dissolvable polyacrylamide beads, by crosslinking acrylamide with disulfide bridges that can be cleaved with dithiothreitol. The beads can be rapidly dissolved in drops and release DNA barcode primers. The dissolvable beads are easy to synthesize, and the primer cost for the beads is significantly lower than that for the previous barcoding beads. Furthermore, the dissolvable beads can be loaded into drops with >95% loading efficiency of a single bead per drop and the dissolution of beads does not influence reverse transcription or the polymerase chain reaction (PCR) in drops. Based on this approach, the dissolvable beads are used for single cell RNA and protein analysis.
    Liu, Y. ; Yang, G. ; Baby, T. ; Tengjisi, ; Chen, D. ; Weitz, D. A. ; Zhao, C. ‐X. Stable Polymer Nanoparticles with Exceptionally High Drug Loading by Sequential Nanoprecipitation. Angewandte Chemie 2020, 59, 4720-4728. Publisher's VersionAbstract
    Poor solubility often leads to low drug efficacy. Encapsulation of water‐insoluble drugs in polymeric nanoparticles offers a solution. However, low drug loading remains a critical challenge. Now, a simple and robust sequential nanoprecipitation technology is used to produce stable drug‐core polymer‐shell nanoparticles with high drug loading (up to 58.5 %) from a wide range of polymers and drugs. This technology is based on tuning the precipitation time of drugs and polymers using a solvent system comprising multiple organic solvents, which allows the formation of drug nanoparticles first followed by immediate precipitation of one or two polymers. This technology offers a new strategy to manufacture polymeric nanoparticles with high drug loading having good long‐term stability and programmed release and opens a unique opportunity for drug delivery applications.
    Kong, T. ; Shum, H. C. ; Weitz, D. A. The Fourth Decade of Microfluidics. Small 2020, 16, 2000070. Publisher's Version kong2020.pdf
    Zhu, Z. ; Liu, J. - D. ; Liu, C. ; Wu, X. ; Li, Q. ; Chen, S. ; Zhao, X. ; Weitz, D. A. Microfluidics-Assisted Assembly of Injectable Photonic Hydrogels Toward Reflective Cooling. Small 2020, 16, 1903939. Publisher's VersionAbstract
    Development of fast curing and easy modeling of colloidal photonic crystals is highly desirable for various applications. Here, a novel type of injectable photonic hydrogel (IPH) is proposed to achieve self‐healable structural color by integrating microfluidics‐derived photonic supraballs with supramolecular hydrogels. The supramolecular hydrogel is engineered via incorporating β‐cyclodextrin/poly(2‐hydroxypropyl acrylate‐coN‐vinylimidazole) (CD/poly(HPA‐co‐VI)) with methacrylated gelatin (GelMA), and serves as a scaffold for colloidal crystal arrays. The photonic supraballs derived from the microfluidics techniques, exhibit excellent compatibility with the hydrogel scaffolds, leading to enhanced assembly efficiency. By virtue of hydrogen bonds and host–guest interactions, a series of self‐healable photonic hydrogels (linear, planar, and spiral assemblies) can be facilely assembled. It is demonstrated that the spherical symmetry of the photonic supraballs endows them with identical optical responses independent of viewing angles. In addition, by taking the advantage of angle independent spectrum characteristics, the IPH presents beneficial effects in reflective cooling, which can achieve up to 17.4 °C in passive solar reflective cooling. The strategy represents an easy‐to‐perform platform for the construction of IPH, providing novel insights into macroscopic self‐assembly toward thermal management applications.
    Parsa, S. ; Santanach-Carreras, E. ; Xiao, L. ; Weitz, D. A. Origin of anomalous polymer-induced fluid displacement in porous media. Physical Review Fluids 2020, 5 022001. Publisher's VersionAbstract

    Immiscible displacement of fluids with large viscosity mismatch is inherently unstable due to viscous fingering, even in porous media where capillary forces dominate. Adding polymer to the displacing fluid reduces the viscosity mismatch and suppresses the viscous fingering instability thereby increasing the fluid displacement leading to extensive use in applications such as oil recovery. Surprisingly, however, an increase in displacement occurs even for very large viscosity mismatches. Moreover, significant additional displacement is observed when the polymer solution is followed by additional water flow. Thus, the fundamental physics of this phenomenon remains unclear. To understand this behavior, we use confocal microscopy to visualize the displacement of oil in a three-dimensional micromodel of a porous medium and simultaneously measure the local flow velocities of the displacing fluid. We find that the increased displacement results from a counterintuitive effect: polymer retention in the medium and the resultant local changes in flow. Typically retention is avoided since it reduces the permeability of the medium; instead, we find that large and heterogeneous local changes in flow lead to sufficiently large enough viscous forces at the interface of the immiscible fluids resulting in increased displacement.

      Wheeler, M. A. ; Clark, I. C. ; Tjon, E. C. ; Li, Z. ; Zandee, S. E. J. ; Couturier, C. P. ; Watson, B. R. ; Scalisi, G. ; Alkwai, S. ; Rothhammer, V. ; et al. MAFG-driven astrocytes promote CNS inflammation. Nature 2020, 2020, 593-599. Publisher's VersionAbstract

      Multiple sclerosis is a chronic inflammatory disease of the CNS1. Astrocytes contribute to the pathogenesis of multiple sclerosis2, but little is known about the heterogeneity of astrocytes and its regulation. Here we report the analysis of astrocytes in multiple sclerosis and its preclinical model experimental autoimmune encephalomyelitis (EAE) by single-cell RNA sequencing in combination with cell-specific Ribotag RNA profiling, assay for transposase-accessible chromatin with sequencing (ATAC–seq), chromatin immunoprecipitation with sequencing (ChIP–seq), genome-wide analysis of DNA methylation and in vivo CRISPR–Cas9-based genetic perturbations. We identified astrocytes in EAE and multiple sclerosis that were characterized by decreased expression of NRF2 and increased expression of MAFG, which cooperates with MAT2α to promote DNA methylation and represses antioxidant and anti-inflammatory transcriptional programs. Granulocyte–macrophage colony-stimulating factor (GM-CSF) signalling in astrocytes drives the expression of MAFG and MAT2α and pro-inflammatory transcriptional modules, contributing to CNS pathology in EAE and, potentially, multiple sclerosis. Our results identify candidate therapeutic targets in multiple sclerosis.

      Selective cell encapsulation, lysis, pico-injection and size-controlled droplet generation using traveling surface acoustic waves in a microfluidic device
      Mutafopulos, K. ; Lu, P. J. ; Garry, R. ; Spink, P. ; Weitz, D. A. Selective cell encapsulation, lysis, pico-injection and size-controlled droplet generation using traveling surface acoustic waves in a microfluidic device. Lab on a Chip 2020, 20, 3914-3921. Publisher's VersionAbstract
      We generate droplets in a microfluidic device using a traveling surface acoustic wave (TSAW), and control droplet size by adjusting TSAW power and duration. We combine droplet production and fluorescence detection to selectively-encapsulate cells and beads; with this active method, the overwhelming majority of single particles or cells are encapsulated individually into droplets, contrasting the Poisson distribution of encapsulation number that governs traditional, passive microfluidic encapsulation. In addition, we lyse cells before selective encapsulation, and pico-inject new materials into existing droplets.
      Prodanović, R. ; Ung, W. L. ; Đurđić, K. I. ; Fisher, R. ; Weitz, D. A. ; Ostafe, R. A High-Throughput Screening System Based on Droplet Microfluidics for Glucose Oxidase Gene Libraries. molecules 2020, 25, 2418. Publisher's VersionAbstract

      Glucose oxidase (GOx) is an important industrial enzyme that can be optimized for specific applications by mutagenesis and activity-based screening. To increase the efficiency of this approach, we have developed a new ultrahigh-throughput screening platform based on a microfluidic lab-on-chip device that allows the sorting of GOx mutants from a saturation mutagenesis library expressed on the surface of yeast cells. GOx activity was measured by monitoring the fluorescence of water microdroplets dispersed in perfluorinated oil. The signal was generated via a series of coupled enzyme reactions leading to the formation of fluorescein. Using this new method, we were able to enrich the yeast cell population by more than 35-fold for GOx mutants with higher than wild-type activity after two rounds of sorting, almost double the efficiency of our previously described flow cytometry platform. We identified and characterized novel GOx mutants, the most promising of which (M6) contained a combination of six point mutations that increased the catalytic constant kcat by 2.1-fold compared to wild-type GOx and by 1.4-fold compared to a parental GOx variant. The new microfluidic platform for GOx was therefore more sensitive than flow cytometry and supports comprehensive screens of gene libraries containing multiple mutations per gene.

      Haney, B. ; Werner, J. G. ; Weitz, D. A. ; Ramakrishnan, S. Stimuli responsive Janus microgels with convertible hydrophilicity for controlled emulsion destabilization. Soft Matter 2020, 16, 3613-3620. Publisher's VersionAbstract

      Although the utilization of rigid particles can afford stable emulsions, some applications require eventual emulsion destabilization to release contents captured in the particle-covered droplet. This destabilizing effect is achieved when using stabilizers that respond to controlled changes in environment. Microgels can be synthesized as stimuli responsive polymeric gel networks that adsorb to oil/water interfaces and stabilize emulsions. These particles are commonly hydrogels that swell and collapse in water in response to environmental changes. However, amphiphilic functionality is desired to enhance the adsorption abilities of these hydrogels while maintaining their stimuli responsivity. Microfluidic techniques are used to synthesize Janus microgels with two opposing stimuli responsive hemispheres. The particles have a temperature responsive domain connected to a pH responsive network where each side changes its hydrophilicity in response to a change in temperature or pH, respectively. The Janus microgels are amphiphilic in acidic conditions at 19 °C and alkaline conditions at 40 °C, while the opposite conditions cause a reduction of the amphiphilicity. By stabilizing emulsions with these dual responsive microgels, “smart” droplets that respond to environmental cues are formed. Emulsion droplets remain stable with smaller diameters when aqueous solution conditions favor amphiphilic particles yet, coalesce to larger droplets upon changing pH or temperature. These responsive Janus microgels represent the advancing technology of responsive droplets and demonstrate the applicability of microgels as emulsion stabilizers.

      Yuan, Y. ; Brouchon, J. ; Calvo-Calle, J. M. ; Xia, J. ; Sun, L. ; Zhang, X. ; Clayton, K. L. ; Ye, F. ; Weitz, D. A. ; Heyman, J. Droplet encapsulation improves accuracy of immune cell cytokine capture assays. Lab on a Chip 2020, 20, 1513-1520. Publisher's VersionAbstract

      Quantification of cell-secreted molecules, e.g., cytokines, is fundamental to the characterization of immune responses. Cytokine capture assays that use engineered antibodies to anchor the secreted molecules to the secreting cells are widely used to characterize immune responses because they allow both sensitive identification and recovery of viable responding cells. However, if the cytokines diffuse away from the secreting cells, non-secreting cells will also be identified as responding cells. Here we encapsulate immune cells in microfluidic droplets and perform in-droplet cytokine capture assays to limit the diffusion of the secreted cytokines. We use microfluidic devices to rapidly encapsulate single natural killer NK-92 MI cells and their target K562 cells into microfluidic droplets. We perform in-droplet IFN-γ capture assays and demonstrate that NK-92 MI cells recognize target cells within droplets and become activated to secrete IFN-γ. Droplet encapsulation prevents diffusion of secreted products to neighboring cells and dramatically reduces both false positives and false negatives, relative to assays performed without droplets. In a sample containing 1% true positives, encapsulation reduces, from 94% to 2%, the number of true-positive cells appearing as negatives; in a sample containing 50% true positives, the number of non-stimulated cells appearing as positives is reduced from 98% to 1%. After cells are released from the droplets, secreted cytokine remains captured onto secreting immune cells, enabling FACS-isolation of populations highly enriched for activated effector immune cells. Droplet encapsulation can be used to reduce background and improve detection of any single-cell secretion assay.

      Liu, J. ; Chen, H. ; Shi, X. ; Nawar, S. ; Werner, J. ; Huang, G. S. ; Ye, M. ; Weitz, D. A. ; Solovev, A. A. ; Mei, Y. F. Hydrogel microcapsules with photocatalytic nanoparticles for removal of organic pollutants . Environmental Science Nano 2020, 7 656-664. Publisher's VersionAbstract

      Droplet-based microfluidics is used to fabricate thin shell hydrogel microcapsules for the removal of methylene blue (MB) from aqueous solutions. The microcapsules composed of a poly(methacrylic acid) hydrogel shell exhibit unique properties, including permeation, separation, purification, and reaction of molecular species. Photocatalytic TiO2 and ZnO nanoparticles encapsulated in the microcapsules, i.e. photocatalyst in capsule (PIC), are used to remove organic pollutants using an adsorption–oxidation mechanism. A prototype flow microreactor is assembled to demonstrate a controllable water purification approach in short time using photocatalysts. Our studies of aqueous and homogeneous hydrogel environments for the photocatalysts provide important insights into understanding the effectiveness of MB removal. Hydrogel capsules have MB removal rate comparable to homogeneous particles. Further reduction of both capsule and photocatalyst sizes can potentially aid in quicker water purification.