Publications by Year: 2018

Kim, Q. H. ; Shin, D. ; Park, J. ; Weitz, D. A. ; Jhe, W. Initial growth dynamics of 10 nm nanobubbles in the graphene liquid cell. Appl. Nanosci. 2018. Publisher's VersionAbstract

The unexpected long lifetime of nanobubble against the large Laplace pressure is one of the important issues in nanobubble research and a few models have been proposed to explain it. Most studies, however, have been focused on the observation of relatively large nanobubbles over 100 nm and are limited to the equilibrium state phenomena. The study on the sub-100 nm sized nanobubble is still lacking due to the limitation of imaging methods which overcomes the optical resolution limit. Here, we demonstrate the observation of growth dynamics of 10 nm nanobubbles confined in the graphene liquid cell using transmission electron microscopy (TEM). We modified the classical diffusion theory by considering the finite size of the confined system of graphene liquid cell (GLC), successfully describing the temporal growth of nanobubble. Our study shows that the growth of nanobubble is determined by the gas oversaturation, which is affected by the size of GLC.

Li, Q. ; Zhang, Y. - W. ; Wang, C. - F. ; Weitz, D. A. ; Chen, S. Macroscopic Self-Assembly: Versatile Hydrogel Ensembles with Macroscopic Multidimensions (Adv. Mater. 52/2018). Adv. Mater. 2018, 30, 1870400. Publisher's VersionAbstract

A new microfluidic‐assisted self‐healing‐driven assembly strategy enabling continuous and controllable construction of programmed ordered assemblies is developed by Su Chen and co‐workers in article number 1803475. This allows self‐assembly to be carried out on the macroscopic scale toward tissue materials and light‐emitting diode devices.

Zhu, Z. ; Zhang, J. ; Tong, Y. -long; Peng, G. ; Cui, T. ; Wang, C. - F. ; Chen, S. ; Weitz, D. A. Reduced Graphene Oxide Membrane Induced Robust Structural Colors toward Personal Thermal Management. ACS Photonics 2018, 6 116-122. Publisher's VersionAbstract
Angle-independent structural colors are prepared by membrane separation-assisted assembly (MSAA) method with modified reduced graphene oxide (rGO) as the substrate membrane. We show that the wrinkled and crumpled rGO laminates not only ensure uneven morphology of colloidal film but improve color saturation by decreasing coherent scattering. In addition, we study the influence of stopband position on thermal insulation property of the colloidal film for the first time. High absolute temperature difference of 6.9 °C is achieved comparing with control sample. And films with longer stopband positions indicate better thermal insulation performance because of inherent slow photon effect in photonic structure. This general principle of thermal insulation by colloidal films opens the way to a new generation of thermal management materials.
Montessori, A. ; Lauricella, M. ; Stolovicki, E. ; Weitz, D. ; Succi, S. Topological aspects of jetting to dripping transition in step emulsifiers. Physics of Fluids 2018, 31, 021703. Publisher's VersionAbstract
Fully three-dimensional, time-dependent, direct simulations of the non-ideal Navier-Stokes equations for a two-component fluid shed light into the mechanism which inhibits droplet breakup in step emulsifiers below a critical threshold of the width-to-height (w/h) ratio of the microfluidic nozzle. Below w/h ∼ 2.6, the simulations provide evidence of a smooth topological transition of the fluid from the confined rectangular channel geometry to an isotropic (spherical) expansion of the fluid downstream the nozzle step. Above such threshold, the transition from the inner to the outer space involves a series of dynamical rearrangements which keep the free surface in mechanical balance. Such rearrangements also induce a backflow of the ambient fluid which, in turn, leads to jet pinching and ultimately to its rupture, namely, droplet formation. The simulations show remarkable agreement with the experimental value of the threshold, which is found around w/h ∼ 2.56.
Li, Q. ; Zhang, Y. - W. ; Wang, C. - F. ; Weitz, D. A. ; Chen, S. Versatile Hydrogel Ensembles with Macroscopic Multidimensions. Adv. Mater. 2018, 30, 1803475. Publisher's VersionAbstract
Methods allowing construction of macroscopic programmed materials in a flexible and efficient fashion are highly desirable. However, the existing approaches are far removed from such materials. A new self‐healing‐driven assembly (SHDA) strategy to fabricate various programmed materials by using uniform gel beads (microsize of 212 µm or millimeter size of 4 mm) as building blocks is described here. In virtue of hydrogen bonds and host–guest interactions between gel beads, a series of linear, planar, and 3D beaded assemblies are fabricated via SHDA in microfluidic channels in a continuous and controlled manner. From the perspective of practical applications, the use of gel assemblies is exploited for tissue engineering with controlled cells coculture, as well as light conversion materials toward white‐light‐emitting diodes (WLEDs). The SHDA strategy developed in this study gives a new insight into the facile and rapid fabrication of various programmed materials toward biological tissue and optoelectronic device.
Wang, H. - F. ; Ran, R. ; Liu, Y. ; Hui, Y. ; Zeng, B. ; Chen, D. ; Weitz, D. A. ; Zhao, C. - X. Tumor-Vasculature-on-a-Chip for Investigating Nanoparticle Extravasation and Tumor Accumulation. ACS Nano 2018, 12, 11600–11609. Publisher's VersionAbstract
Nanoparticle tumor accumulation relies on a key mechanism, the enhanced permeability and retention (EPR) effect, but it remains challenging to decipher the exact impact of the EPR effect. Animal models in combination with imaging modalities are useful, but it is impossible to delineate the roles of multiple biological barriers involved in nanoparticle tumor accumulation. Here we report a microfluidic tumor-vasculature-on-a-chip (TVOC) mimicking two key biological barriers, namely, tumor leaky vasculature and 3D tumor tissue with dense extracellular matrix (ECM), to study nanoparticle extravasation through leaky vasculature and the following accumulation in tumor tissues. Intact 3D tumor vasculature was developed with selective permeability of small molecules (20 kDa) but not large ones (70 kDa). The permeability was further tuned by cytokine stimulation, demonstrating the independent control of the leaky tumor vasculature. Combined with tumor spheroids in dense ECM, our TVOC model is capable of predicting nanoparticles’ in vivo tumor accumulation, thus providing a powerful platform for nanoparticle evaluation.
Ziblat, R. ; Weaver, J. ; Arriaga, L. R. ; Chong, S. ; Weitz, D. A. Determining the Lipid Specificity of Insoluble Protein Transmembrane Domains. Lab Chip 2018, 18, 3561-3569. Publisher's VersionAbstract
While the specificity of protein–lipid interactions is a key feature in the function of biological membranes, studying the specifics of these interactions is challenging because most membrane proteins are insoluble in water due to the hydrophobic nature of their transmembrane domains (TMDs). Here, we introduce a method that overcomes this solubility limitation and identifies the affinity profile of protein TMDs to specific lipid formulations. Using 5 human TMDs as a sample group, our results demonstrate that TMDs are highly selective and that these specific lipid–TMD interactions can involve either a single lipid, or the combination of multiple lipid species.
Shi, W. ; Didier, J. E. ; Ingber, D. E. ; Weitz, D. A. Collective Shape Actuation of Polymer Double Emulsions by Solvent Evaporation. ACS Applied Materials & Interfaces 2018, 10, 31865–31869. Publisher's VersionAbstract
We demonstrate that the shape actuation of waterin-oil-in-water double emulsion droplets can be achieved by controlling solvent evaporation in a model system, where the oil phase consists of hydrophobic homopolymer/amphiphilic block copolymer/solvent. A gradient of interfacial tension is created in the polymer shell, which drives significant deformation of the droplets in constant volume. The deformed droplets recover to their initial shape spontaneously, and shape actuation of droplets can be further tuned by osmotic pressure. Our model system provides a new prototype for developing shape-responsive droplets in a solvent environment.
Eggersdorfer, M. L. ; Seybold, H. ; Ofner, A. ; Weitz, D. A. ; Studart, A. R. Wetting controls of droplet formation in step emulsification. Proceedings of the National Academy of Sciences 2018, 115, 9479-9484. Publisher's VersionAbstract
The formation of droplets is ubiquitous in many natural and industrial processes and has reached an unprecedented level of control with the emergence of milli- and microfluidics. Although important insight into the mechanisms of droplet formation has been gained over the past decades, a sound understanding of the physics underlying this phenomenon and the effect of the fluid’s flow and wetting properties on the droplet size and production rate is still missing, especially for the widely applied method of step emulsification. In this work, we elucidate the physical controls of microdroplet formation in step emulsification by using the wetting of fluidic channels as a tunable parameter to explore a broad set of emulsification conditions. With the help of high-speed measurements, we unequivocally show that the final droplet pinch-off is triggered by a Rayleigh–Plateau-type instability. The droplet size, however, is not determined by the Rayleigh–Plateau breakup, but by the initial wetting regime, where the fluid’s contact angle plays a crucial role. We develop a physical theory for the wetting process, which closely describes our experimental measurements without invoking any free fit parameter. Our theory predicts the initiation of the Rayleigh–Plateau breakup and the transition from dripping to jetting as a function of the fluid’s contact angle. Additionally, the theory solves the conundrum why there is a minimal contact angle of α = 2π/3 = 120° for which droplets can form.
Rotem, A. ; Serohijos, A. W. R. ; Chang, C. B. ; Wolfe, J. T. ; Fischer, A. E. ; Mehoke, T. S. ; Zhang, H. ; Tao, Y. ; Ung, W. L. ; Choi, J. - M. ; et al. Evolution on the biophysical fitness landscape of an RNA virus. Mol. Biol. Evol. 2018, 35, 2390–2400. Publisher's VersionAbstract

Viral evolutionary pathways are determined by the fitness landscape, which maps viral genotype to fitness. However, a quantitative description of the landscape and the evolutionary forces on it remain elusive. Here, we apply a biophysical fitness model based on capsid folding stability and antibody binding affinity to predict the evolutionary pathway of norovirus escaping a neutralizing antibody. The model is validated by experimental evolution in bulk culture and in a drop-based microfluidics that propagates millions of independent small viral subpopulations. We demonstrate that along the axis of binding affinity, selection for escape variants and drift due to random mutations have the same direction, an atypical case in evolution. However, along folding stability, selection and drift are opposing forces whose balance is tuned by viral population size. Our results demonstrate that predictable epistatic tradeoffs between molecular traits of viral proteins shape viral evolution.

Qi, L. - Y. ; Shang, L. ; Chen, X. ; Ye, L. ; Zhang, W. ; Feng, P. ; Zou, W. ; Cao, N. ; Zhou, H. - H. ; Weitz, D. A. ; et al. A Versatile Strategy to Fabricate 3D Conductive Frameworks for Lithium Metal Anodes. Adv. Mater. Interfaces 2018, 5 1800807. Publisher's VersionAbstract
The suppression of lithium dendrite is critical to the realization of lithium metal batteries. 3D conductive framework, among different approaches, has shown very promising results in dendrite suppression. A novel cost‐effective and versatile dip‐coating method is presented here to make 3D conductive framework. Various substrates with different geometries are coated successfully with copper, including electrically insulating glass fiber (GF) or rice paper and conducting Ni foam. In particular, the as‐prepared copper coated GF shows promising results to serve as the lithium metal substrate by the electrochemical battery tests. The method significantly broadens the candidate materials database for 3D conductive framework to include all kinds of intrinsically insulating 3D substrates.
Werner, J. G. ; Deveney, B. T. ; Nawar, S. ; Weitz, D. A. Dynamic Microcapsules with Rapid and Reversible Permeability Switching. Adv. Funct. Mater. 2018, 28, 1803385. Publisher's VersionAbstract
Dynamic microcapsules are reported that exhibit shell membranes with fast and reversible changes in permeability in response to external stimuli. A hydrophobic anhydride monomer is employed in the thiol–ene polymerization as a disguised precursor for the acid‐containing shells; this enables the direct encapsulation of aqueous cargo in the liquid core using microfluidic fabrication of water‐in‐oil‐in‐water double emulsion drops. The poly(anhydride) shells hydrolyze in their aqueous environment without further chemical treatment, yielding cross‐linked poly(acid) microcapsules that exhibit trigger‐responsive and reversible property changes. The microcapsule shell can actively be switched numerous times between impermeable and permeable due to the exceptional mechanical properties of the thiol–ene network that prevent rupture or failure of the membrane, allowing it to withstand the mechanical stresses imposed on the capsule during the dynamic property changes. The permeability and molecular weight cutoff of the microcapsules can dynamically be controlled with triggers such as pH and ionic environment. The reversibly triggered changes in permeability of the shell exhibit a response time of seconds, enabling actively adjustable release profiles, as well as on‐demand capture, trapping, and release of cargo molecules with molecular selectivity and fast on‐off rates.
Montessori, A. ; Lauricella, M. ; Succi, S. ; Stolovicki, E. ; Weitz, D. Elucidating the mechanism of step emulsification. Phys. Rev. Fluids 2018, 3 072202. Publisher's VersionAbstract

Three-dimensional, time-dependent direct simulations of step emulsification microdevices highlight two essential mechanisms for droplet formation: first, the onset of an adverse pressure gradient driving a backflow of the continuous phase from the external reservoir to the microchannel, and second, the striction of the flowing jet which leads to its subsequent rupture. It is also shown that such a rupture is delayed and eventually suppressed by increasing the flow speed of the dispersed phase within the channel, due to the stabilizing effect of dynamic pressure. This suggests a new criterion for dripping-jetting transition, based on local values of the capillary and Weber numbers.

Werner, J. ; Nawar, S. ; Solovev, A. A. ; Weitz, D. A. Hydrogel Microcapsules with Dynamic pH-Responsive Properties from Methacrylic Anhydride. Macromolecules 2018, 51, 5798-5805. Publisher's VersionAbstract
Dynamic microcapsules are a highly sought-after class of encapsulant for advanced delivery applications with dynamically tunable release profiles, as actively manipulatable microreactors, or as selective microtraps for molecular separation and purification. Such dynamic microcapsules can only be realized with a nondestructive trigger-response mechanism that changes the permeability of the shell membrane reversibly, as found in hydrogels. However, the direct synthesis of a trigger-responsive hydrogel membrane around a water drop without the use of sacrificial templates remains elusive due to the incompatibility of the synthesis chemistry with aqueous emulsion processing. Here, we report on a facile approach to fabricate reversibly responsive hydrogel microcapsules utilizing reactive anhydride chemistry. Cross-linked and hydrophobic poly(methacrylic anhydride) microcapsules are obtained from microfluidic double emulsion drop templating that enables direct encapsulation of hydrophilic, water-suspended cargo within the aqueous core. Hydrolysis in aqueous environment yields microcapsules with a poly(acid) hydrogel shell that exhibit high mechanical and chemical stability for repeated cycling between its swollen and nonswollen states without rupture or fatigue. The permeability of the microcapsules is strongly dependent on the degree of swelling and hence can be actively and dynamically modified, enabling repeated capture, trap, and release of aqueous cargo over numerous cycles.
Li, W. ; Zhang, L. ; Ge, X. ; Xu, B. ; Zhang, W. ; Qu, L. ; Choi, C. - H. ; Xu, J. ; Zhang, A. ; Lee, H. ; et al. Microfluidic fabrication of microparticles for biomedical applications. Chem. Soc. Rev. 2018, 15, 5646-5683. Publisher's VersionAbstract
Droplet microfluidics offers exquisite control over the flows of multiple fluids in microscale, enabling fabrication of advanced microparticles with precisely tunable structures and compositions in a high throughput manner. The combination of these remarkable features with proper materials and fabrication methods has enabled high efficiency, direct encapsulation of actives in microparticles whose features and functionalities can be well controlled. These microparticles have great potential in a wide range of bio-related applications including drug delivery, cell-laden matrices, biosensors and even as artificial cells. In this review, we briefly summarize the materials, fabrication methods, and microparticle structures produced with droplet microfluidics. We also provide a comprehensive overview of their recent uses in biomedical applications. Finally, we discuss the existing challenges and perspectives to promote the future development of these engineered microparticles.
Zhang, H. ; Cui, N. ; Cai, Y. ; Lei, F. ; Weitz, D. A. Single-cell sequencing leads a new era of profiling transcriptomic landscape. Journal of Bio-X Research 2018, 1 2-6. Publisher's VersionAbstract
Understanding the complexity of biological systems requires a comprehensive analysis of their cell populations. Ideally, this should be done at the single cell level, because bulk analysis of the full population obscured many critical details due to artifacts introduced by averaging. However, this has been technically challenging due to the cumbersome procedure, low throughput, and high costs of performing analysis on a single-cell basis. Excitingly, technical improvements in single-cell RNA sequencing are making it economically practical to profile the transcriptomics of large populations of cells at the single-cell level, and have yielded numerous results that address important biological and medical questions. Further development of the technology and data analysis will significantly benefit the biomedical field by unraveling the function of individual cells in their microenvironments and modeling their transcriptional dynamics.
Liu, E. Y. ; Jung, S. ; Weitz, D. A. ; Yi, H. ; Choi, C. - H. High-throughput double emulsion-based microfluidic production of hydrogel microspheres with tunable chemical functionalities toward biomolecular conjugation. Lab Chip 2018, 18, 323–334. Publisher's VersionAbstract
Chemically functional hydrogel microspheres hold significant potential in a range of applications including biosensing, drug delivery, and tissue engineering due to their high degree of flexibility in imparting a range of functions. In this work, we present a simple, efficient, and high-throughput capillary microfluidic approach for controlled fabrication of monodisperse and chemically functional hydrogel microspheres via formation of double emulsion drops with an ultra-thin oil shell as a sacrificial template. This method utilizes spontaneous dewetting of the oil phase upon polymerization and transfer into aqueous solution, resulting in poly(ethylene glycol) (PEG)-based microspheres containing primary amines (chitosan, CS) or carboxylates (acrylic acid, AA) for chemical functionality. Simple fluorescent labelling of the as-prepared microspheres shows the presence of abundant, uniformly distributed and readily tunable functional groups throughout the microspheres. Furthermore, we show the utility of chitosan's primary amine as an efficient conjugation handle at physiological pH due to its low pKa by direct comparison with other primary amines. We also report the utility of these microspheres in biomolecular conjugation using model fluorescent proteins, R-phycoerythrin (R-PE) and green fluorescent protein (GFPuv), via tetrazine–trans-cyclooctene (Tz–TCO) ligation for CS-PEG microspheres and carbodiimide chemistry for AA-PEG microspheres, respectively. The results show rapid coupling of R-PE with the microspheres' functional groups with minimal non-specific adsorption. In-depth protein conjugation kinetics studies with our microspheres highlight the differences in reaction and diffusion of R-PE with CS-PEG and AA-PEG microspheres. Finally, we demonstrate orthogonal one-pot protein conjugation of R-PE and GFPuv with CS-PEG and AA-PEG microspheres via simple size-based encoding. Combined, these results represent a significant advancement in the rapid and reliable fabrication of monodisperse and chemically functional hydrogel microspheres with tunable properties.
Klepac, D. ; Kostkova, H. ; Petrova, S. ; Chytil, P. ; Etrych, T. ; Kereiche, S. ; Raska, I. ; Weitz, D. A. ; Filippov, S. K. Interaction of spin-labeled HPMA-based nanoparticles with human blood plasma proteins – the introduction of protein-corona-free polymer nanomedicine. Nanoscale 2018, 10, 6194–6204. Publisher's VersionAbstract
In this paper, we revised the current understanding of the protein corona that is created on the surface of nanoparticles in blood plasma after an intravenous injection. We have focused on nanoparticles that have a proven therapeutic outcome. These nanoparticles are based on two types of biocompatible amphiphilic copolymers based on N-(2-hydroxypropyl)methacrylamide (HPMA): a block copolymer, poly(ε-caprolactone) (PCL)-b-poly(HPMA), and a statistical HPMA copolymer bearing cholesterol moieties, which have been tested both in vitro and in vivo. We studied the interaction of nanoparticles with blood plasma and selected blood plasma proteins by electron paramagnetic resonance (EPR), isothermal titration calorimetry, dynamic light scattering, and cryo-transmission electron microscopy. The copolymers were labeled with TEMPO radicals at the end of hydrophobic PCL or along the hydrophilic HPMA chains to monitor changes in polymer chain dynamics caused by protein adsorption. By EPR and other methods, we were able to probe specific interactions between nanoparticles and blood proteins, specifically low- and high-density lipoproteins, immunoglobulin G, human serum albumin (HSA), and human plasma. It was found that individual proteins and plasma have very low binding affinity to nanoparticles. We observed no hard corona around HPMA-based nanoparticles; with the exception of HSA the proteins showed no detectable binding to the nanoparticles. Our study confirms that a classical “hard corona–soft corona” paradigm is not valid for all types of nanoparticles and each system has a unique protein corona that is determined by the nature of the NP material.
Anbari, A. ; Chien, H. - T. ; Datta, S. S. ; Deng, W. ; Weitz, D. A. ; Fan, J. Microfluidic Model Porous Media: Fabrication and Applications. Small 2018, 14, 1703575. Publisher's VersionAbstract
Complex fluid flow in porous media is ubiquitous in many natural and industrial processes. Direct visualization of the fluid structure and flow dynamics is critical for understanding and eventually manipulating these processes. However, the opacity of realistic porous media makes such visualization very challenging. Micromodels, microfluidic model porous media systems, have been developed to address this challenge. They provide a transparent interconnected porous network that enables the optical visualization of the complex fluid flow occurring inside at the pore scale. In this Review, the materials and fabrication methods to make micromodels, the main research activities that are conducted with micromodels and their applications in petroleum, geologic, and environmental engineering, as well as in the food and wood industries, are discussed. The potential applications of micromodels in other areas are also discussed and the key issues that should be addressed in the near future are proposed.
Choi, C. - H. ; Lee, H. ; Weitz, D. A. Rapid Patterning of PDMS Microfluidic Device Wettability Using Syringe-Vacuum-Induced Segmented Flow in Nonplanar Geometry. ACS Appl. Mater. Interfaces 2018, 10, 3170–3174. Publisher's VersionAbstract
We present a simple and rapid method to spatially pattern the surface wetting properties of PDMS microfluidic devices by layer-by-layer (LbL) deposition of polyelectrolytes using syringe-vacuum-induced segmented flow in nonplanar geometry. Our technique offers selective surface modification in microfluidic chips with multiple flow-focusing junctions, enabling production of monodisperse double- and triple-emulsion drops.