Droplet Array Research Articles

Fabrication of porous polymer coating layers with selective wettability on filter papers via the breath figure method and their applications in oil/water separation.

A comb-like amphiphilic polymer (PBTF), composed of hydrophobic backbones and hydrophilic side chains, was employed to grow honeycomb coating layers in situ on a filter paper via directly casting a polymer solution and by the subsequent dynamic breath figure (BF) method. Through regulating the hydrophilic polymer side chain density and the solution concentration, a continuous honeycomb coating layer contouring to the filter paper surface profile, in addition to possessing a water contact angle (WCA) as high as 146°, was successfully fabricated. The present study also finds that increasing the hydrophilic side chain density will turn PBTF into a surfactant-like polymer, and thus, endow the PBTF solution with the capacity of numerous micro–nano-sized water droplets, rather than simply stabilizing the ordered water droplet arrays on the surface of the solution. With vast nano-sized water droplets in it, the once transparent PBTF solution changed into a translucent nano-emulsion, which demonstrates a strong Tyndall effect. While casting such nano-emulsion on a filter paper and then subjecting to the BF process, the polymeric solute takes both nano-emulsion intrinsic nano-sized water droplets and solvent evaporation-induced water droplets as templates and self-assembles into a bird-nest-like three-dimensional porous microstructure, which possesses micro–nano-sized communicating pores. By regulating the water content in the nano-emulsion, the bird-nest-like structure can be uniformly formed on the surface of the filter paper, which revealed a WCA of 152°. The coated filter papers possess selective wettability, and meanwhile, maintain the inherent permeability of the substrates, which therefore can be directly utilized as oil/water separation materials.

Rapid generation of hybrid biochemical/mechanical cues in heterogeneous droplets for high-throughput screening of cellular responses.

Cells in their native microenvironment are subjected to varying combinations of biochemical cues and mechanical cues in a wide range. Although many signaling pathways have been found to be responsive for extracellular cues, little is known about how biochemical cues crosstalk with mechanical cues in a complex microenvironment. Here, we introduced heterogeneous droplets on a microchip, which were rapidly assembled by combining wettability-patterned microchip and programmed droplet manipulations, for a high-throughput cell screening of the varying combinations of biochemical cues and mechanical cues. This platform constructed a heterogeneous droplet/microgel array with orthogonal gradual chemicals and materials, which was further applied to analyze the cellular Wnt/β-catenin signaling in response to varying combinations of Wnt ligands and substrate stiffness. Thus, this device provides a powerful multiplexed bioassay platform for drug development, tissue engineering, and stem cell screening.

Wafer-level fabrication of alkali vapor cells using in-situ atomic deposition

We demonstrate a new technique for filling mm-scale microfabricated silicon and glass cavities with alkali vapors at the wafer-scale. A single etched silicon wafer contains an array of cavities containing alkali precursor materials offset laterally from the cell array. The wafer is heated to create an array of alkali droplets on an upper glass wafer, which is then translated laterally under vacuum and bonded to create the cells. This technique can be implemented in a commercially available bonding tool, allows the fabrication of cells with arbitrary buffer gas contents and pressures and can potentially produce cells with dimensions below 100 µm.

Fast and environmentally friendly fabrication of superhydrophilic-superhydrophobic patterned aluminum surfaces

The superwettability patterned aluminum surface has been shown considerable attention for emerging applications. However, the available techniques are time-consuming and require hazardous chemicals, especially toxic fluoride. In this work, the superhydrophilic-superhydrophobic patterned surface of aluminum was successfully fabricated by using nanosecond laser (λ=1065 nm) and fluorine-free N-Octyltriethoxysilane (OcTES). The time required to make superhydrophilic surfaces is only a few minutes and switch it into superhydrophobic is only 30 min. The patterned surface had both superhydrophilicity (apparent water contact angle (WCA) closed to 0°) and superhydrophobicity (WCA of 156° and roll-off angle of 2.9°), which achieved the droplet arrays and the fluid channels with special patterns. The results indicated that the synergy between the rough surface and the dominant presence of polar Al-OH groups induced by laser ablation was critical for superhydrophilicity. The OcTES molecules containing non-polar C-C(H) groups were assembled on the rough surface by the formation of Si-O-Al bonds, which transformed the laser-treated surface from superhydrophilic into superhydrophobic. This work is of significance for the eco-friendly fabrication of superwettability patterned aluminum surface and shows potential applications in fluidic systems and liquid controls.

A flexible and cost-effective manual droplet operation platform for miniaturized cell assays and single cell analysis

Herein, we developed a flexible and cost-effective manual droplet operation system (MDOS) for performing miniaturized cell assays as well as single cell analysis. The MDOS consists of a manual x-y-z translation stage for liquid transferring and switching, a high-precision syringe pump for liquid driving and metering, a tapered capillary probe for droplet manipulation, a droplet array chip for droplet loading and reaction, sample/reagent reservoirs for storage, and a microscope for droplet observation, with a total expense of only $4,000. By using the flexible combination of three elementary operations of the x-y-z stage's moving and the pump's aspirating and depositing, the MDOS can manually achieve multiple droplet handling operations in the nanoliter to picoliter range, including droplet generation, assembling, fusion, diluting, and splitting. On this basis, multiple cell-related operations could be performed, such as nanoliter-scale in-droplet cell culture, cell coculture, drug stimulation, cell washing, and cell staining, as well as formation of picoliter single-cell droplets. The feasibility and flexibility of the MDOS was demonstrated in multi-mode miniaturized cell assays, including cell-based drug test, first-pass effect assay, and single-cell enzyme assay. The MDOS with the features of low cost, easy to build and flexible to use, could provide a promising alternative for performing miniaturized assays in routine laboratories, in addition to conventional microfluidic chip-based systems and automated robot systems.

Genetics, clinical features and outcomes of non-syndromic pituitary gigantism: experience of a single center from Sao Paulo, Brazil.

Non-syndromic pituitary gigantism (PG) is a very rare disease. Aryl hydrocarbon receptor-interacting protein (AIP) and G protein-coupled receptor 101 (GPR101) genetic abnormalities represent important etiologic causes of PG and may account for up to 40% of these cases. Here, we aimed to characterize the clinical and molecular findings and long-term outcomes in 18 patients (15 males, three females) with PG followed at a single tertiary center in Sao Paulo, Brazil. Genetic testing for AIP and GPR101 were performed by DNA sequencing, droplet digital PCR and array comparative genomic hybridization (aCGH). Pathogenic variants in the AIP gene were detected in 25% of patients, including a novel variant in splicing regulatory sequences which was present in a sporadic male case. X-LAG due to GPR101 microduplication was diagnosed in two female patients (12.5%). Of interest, these patients had symptoms onset by age 5 and 9years old and diagnosis at 5 and 15years, respectively. X-LAG, but not AIP, patients had a significantly lower age of symptoms onset and diagnosis and a higher height Z-score when compared to non-X-LAG. No other differences in clinical features and/or treatment outcomes were observed among PG based on their genetic background. We characterize the clinical and molecular findings and long-term outcome of the largest single-center PG cohort described so far.

Method for Passive Droplet Sorting after Photo-Tagging.

We present a method to photo-tag individual microfluidic droplets for latter selection by passive sorting. The use of a specific surfactant leads to the interfacial tension to be very sensitive to droplet pH. The photoexcitation of droplets containing a photoacid, pyranine, leads to a decrease in droplet pH. The concurrent increase in droplet interfacial tension enables the passive selection of irradiated droplets. The technique is used to select individual droplets within a droplet array as illuminated droplets remain in the wells while other droplets are eluted by the flow of the external oil. This method was used to select droplets in an array containing cells at a specific stage of apoptosis. The technique is also adaptable to continuous-flow sorting. By passing confined droplets over a microfabricated trench positioned diagonally in relation to the direction of flow, photo-tagged droplets were directed toward a different chip exit based on their lateral movement. The technique can be performed on a conventional fluorescence microscope and uncouples the observation and selection of droplets, thus enabling the selection on a large variety of signals, or based on qualitative user-defined features.

Space-based microgravity experiments on flame spread over randomly distributed n-decane-droplet clouds: Anomalous behavior in flame spread

The first combustion experiments aboard the Japanese Experiment Module “Kibo” on the International Space Station, titled “Elucidation of Flame Spread and Group Combustion Excitation Mechanism of Randomly Distributed Droplet Clouds (Group Combustion)” were conducted in 2017 in order to bridge the gap between droplet combustion and spray combustion through flame spread over droplet arrays, droplet-cloud elements and randomly distributed droplet clouds. n-Decane droplets were distributed randomly on a SiC-fiber lattice, and the flame spread and group-combustion excitation were observed. During the flame spread near the group-combustion-excitation limit, two types of anomalous behavior appeared: large-scale ignition and re-burning by a slow flame propagation in a burned area. We successfully simulated similar phenomena by a flame-spread experiment using droplet-cluster arrays. The large-scale ignition phenomenon is caused by the ignition of a flammable mixture generated by heating multiple droplets existing outside the local flame-spread limit. The re-burning phenomenon by the slow flame propagation in a burned area is caused by the ignition of a flammable mixture diluted by the combustion products after the radiative extinction of the group flame. There is the possibility of a cool-flame appearance in both phenomena.

Cytogenetically visible inversions are formed by multiple molecular mechanisms

Cytogenetically detected inversions are generally assumed to be copy number and phenotypically neutral events. While nonallelic homologous recombination is thought to play a major role, recent data suggest the involvement of other molecular mechanisms in inversion formation. Using a combination of short‐read whole‐genome sequencing (WGS), 10X Genomics Chromium WGS, droplet digital polymerase chain reaction and array comparative genomic hybridization we investigated the genomic structure of 18 large unique cytogenetically detected chromosomal inversions and achieved nucleotide resolution of at least one chromosomal inversion junction for 13/18 (72%). Surprisingly, we observed that seemingly copy number neutral inversions can be accompanied by a copy‐number gain of up to 350 kb and local genomic complexities (3/18, 17%). In the resolved inversions, the mutational signatures are consistent with nonhomologous end‐joining (8/13, 62%) or microhomology‐mediated break‐induced replication (5/13, 38%). Our study indicates that short‐read 30x coverage WGS can detect a substantial fraction of chromosomal inversions. Moreover, replication‐based mechanisms are responsible for approximately 38% of those events leading to a significant proportion of inversions that are actually accompanied by additional copy‐number variation potentially contributing to the overall phenotypic presentation of those patients.

Inkjet-printed CMUT humidity sensors with high sensitivity and low hysteresis

Capacitive micromachined ultrasonic transducer (CMUT) is a proven MEMS based gravimetric sensor featuring high mass sensitivity. It can be highly sensitive for relative humidity (RH) measurement when drop-casted with moisture-sensitive graphene oxide (GO) film. However, high hysteresis appears to be a critical problem for the sensor partially due to the non-uniformity of the sensing film. Here, we present miniaturized CMUTs that are functionalized by the inkjet-printed GO film and exhibit high sensitivity and meanwhile low hysteresis. Hexagon CMUTs with a side length of 500 μm were designed and fabricated by the nitride-to-oxide wafer bonding process. GO inks were prepared using a mixture of DI water and ethanol as the solvent. A square pattern formed by a 2D array of ink droplets was inkjet printed on the CMUT membranes. GO films with different thicknesses were deposited on three CMUTs. The humidity sensing performance of the sensors was evaluated by measuring the resonance frequency shifts induced by the added mass of adsorbed water molecules. The sensors delivered high RH sensitivity of up to 3.35 kHz/%RH, short response/recovery time down to 8 s/2 s, and hysteresis as low as 2.6 %RH. This work demonstrates that the inkjet printing technique is an excellent tool to functionalize CMUT with the advantages of minimized material consumption, batch-production capability, and exceptional control over the film thickness and quality. The CMUT printed with GO film has also shown to be a promising candidate as a miniaturized humidity sensing platform.

Static array of droplets and on-demand recovery for biological assays.

Microfluidics has revolutionized several research areas by providing compact yet powerful microanalytical devices that in many cases outperform conventional systems. Among different microfluidics technologies, droplet microfluidics has emerged as a powerful platform to enable analyses of biological samples and phenomena because of its simplicity and versatility. Droplet microfluidics enables high-throughput encapsulation, manipulation, and analysis of single cells while drastically reducing the cost and time required by conventional technologies. For many of these microanalysis systems, manipulation of individual droplets is extremely important as it enables multiplexed high dimensional phenotyping of the targets, going beyond surface phenotyping. One of the key manipulation steps that needs to be implemented with high precision is enabling long-term observation of droplets and recovery of a subset of these droplets for further analysis. This Perspective highlights the recent advances and provides an outlook on future developments that will enable highly complex analyses of biological samples.

In-plane growth of germanium nanowires on nanostructured Si(001)/SiO2 substrates

Germanium (Ge) nanowires (NWs) were grown in-plane on nano-structured Si(001)/SiO2 substrates by molecular beam epitaxy using gold (Au) as the solvent. The site-selective NW growth was enabled by a rectangular array of gold droplets on silicon (Si) tips with an Au nuclei density below 0.25 µm−2 on the surrounding silicon oxide (SiO2). The initial growth of Ge NWs starting from Si–Au droplets with SixGe1−x nucleation from ternary alloy is discussed from a thermodynamic point of view. The in-plane NW elongation occurred within 〈110〉 directions on the substrate and NWs were mainly bounded by two 55° inclined 111 facets and a less pronounced planar (001) top facet. Fully relaxed crystal lattices of Ge NWs were observed from two-dimensional reciprocal space maps of x-ray diffraction measurements.

Combining Mass Spectrometry of Picoliter Samples with a Multicompartment Electrodynamic Trap for Probing the Chemistry of Droplet Arrays.

Single droplet levitation provides contactless access to the microphysical and chemical properties of micrometer-sized samples. Most applications of droplet levitation to chemical and biological systems use nondestructive optical techniques to probe droplet properties. To provide improved chemical specificity, we coupled a multicompartment quadrupole electrodynamic trap (QET) with single droplet mass spectrometry. Our QET continuously traps a monodisperse droplet population (tens to hundreds of droplets) and allows for the simultaneous sizing of a single droplet using its Mie scattering pattern. Single droplets are subsequently ejected into the ionization region of an ambient pressure inlet mass spectrometer. We optimized two complementary soft ionization techniques for picoliter aqueous droplets: (1) paper spray (PS) ionization and (2) thermal desorption glow discharge (TDGD) ionization. Both techniques detect oxygenated organic acids in single droplets, with signal-to-noise ratios >100 and detection limits on the order of 10 pg. Sensitivity and reproducibility across single droplets are driven by the droplet deposition location and spray stability in PS-MS and the ionization region humidity and analyte evaporation rate in TDGD-MS. Importantly, the analyte evaporation rate can control the TDGD-MS quantitative capability because high evaporation rates result in significant ion suppression. This effect is mitigated by optimizing the vaporization temperature, droplet size range, and analyte volatility. We demonstrate quantitative and reproducible measurements with a droplet internal standard (<10% RSD) and compare the sensitivity of PS-MS and TDGD-MS. Finally, we demonstrate the application of QET-MS to the study of heterogeneous chemical kinetics with the reaction of gas phase O3 and aqueous maleic acid droplets.

Self-propulsion of active droplets without liquid-crystalline order

The swimming of cells, far from any boundary, can arise in the absence of long-range liquid-crystalline order within the cytoplasm, but simple models of this effect are lacking. Here we present a two-dimensional model of droplet self-propulsion involving two scalar fields, representing the cytoplasm and a contractile cortex. An active stress results from coupling between these fields; self-propulsion results when rotational symmetry is spontaneously broken. The swimming speed is predicted, and shown numerically, to vary linearly with the activity parameter and with the droplet area fraction. The model exhibits a Crowley-like instability for an array of active droplets.

Microstructural transition in monotectic alloys: A phase-field study

In the present study, we employ a multiphase-field model based on the grand chemical potential formulation to simulate the microstructural transition in a binary FeSn monotectic alloy. In a directional solidification environment, we systematically investigate the formation of monotectic microstructures. At equilibrium monotectic composition, we observe that depending on the imposed temperature gradient and the solidification velocity, the liquid L2 phase transforms into an array of droplets embedded in the solid phase matrix. Initiated at the solidification front, the minor liquid phase undergoes detachment and spherodization to form L2 droplets as a result of post-solidification ripening behind the growth front. We show that the wavelength of the oscillating phase is critical to observe a lamellar to droplet transition. A microstructural selection map at various growth conditions is delineated to illustrate the different monotectic microstructures. The present phase-field simulations, while providing significant insights into the formation of microstructures closes the gap with the in-situ observations reported earlier. In addition, the influence of lamellar spacing is investigated in detail through the Jackson and Hunt analytical relationship for the present material system.

Role of Vitamin D Antiviral Plants and Precaution in COVID-19

Energy concentration is essential to create compact, efficient microfluidic optical devices. Polyethylene glycol (PEG) is a suitable fluid for this purpose with its bipolarity (molecular dissolvability), nonvolatility, and high index of refraction (waveguiding). Its distinguished feature is a bistability in the phase transition process; i.e., a mixture of two PEG types (molecular weight: 300 and 2000), for example, takes both the liquid and solid phases in the temperature range of 2�??38 °C. One can use this phase transition to pause a sample flow at a specific position in microchannels. The bistability also realizes a rewritable signboard with a PEG droplet array, since both the clear (liquid) and milk-white (solid) states are stable at room temperature. The strong scattering in the solid phase is useful to confine a light beam (photonic localization). Mirrorless lasers, which have been studied extensively with microdroplets, can be constructed with dye-dispersed PEG, since the confined fluorescence induces a stimulated emission. A bistable laser emission has been demonstrated in the phase transition process of the microfluid. The light confinement is also useful to enhance the absorbance of inorganic materials, which hardly absorb excitation or probe light particularly in a microchannel. Chemists have recently developed organic-inorganic hybrid phosphors with an enhanced excitation efficiency. Spectroscopic analysis of these hybrids needs time-resolved measurements, since both rapid and slow processes occur in organic ligands and metal ions. The time-space conversion spectral measurement is suitable for microfluidic devices, since a nanosecond resolution is attainable with no need for a large pulsed laser.

A Femtoliter Droplet Array for Massively Parallel Protein Synthesis from Single DNA Molecules.

Advances in spatial resolution and detection sensitivity of scientific instrumentation make it possible to apply small reactors for biological and chemical research. To meet the demand for high-performance microreactors, we developed a femtoliter droplet array (FemDA) device and exemplified its application in massively parallel cell-free protein synthesis (CFPS) reactions. Over one million uniform droplets were readily generated within a finger-sized area using a two-step oil-sealing protocol. Every droplet was anchored in a femtoliter microchamber composed of a hydrophilic bottom and a hydrophobic sidewall. The hybrid hydrophilic-in-hydrophobic structure and the dedicated sealing oils and surfactants are crucial for stably retaining the femtoliter aqueous solution in the femtoliter space without evaporation loss. The femtoliter configuration and the simple structure of the FemDA device allowed minimal reagent consumption. The uniform dimension of the droplet reactors made large-scale quantitative and time-course measurements convincing and reliable. The FemDA technology correlated the protein yield of the CFPS reaction with the number of DNA molecules in each droplet. We streamlined the procedures about the microfabrication of the device, the formation of the femtoliter droplets, and the acquisition and analysis of the microscopic image data. The detailed protocol with the optimized low running cost makes the FemDA technology accessible to everyone who has standard cleanroom facilities and a conventional fluorescence microscope in their own place.

A Femtoliter Droplet Array for Massively Parallel Protein Synthesis from Single DNA Molecules

A Femtoliter Droplet Array for Massively Parallel Protein Synthesis from Single DNA Molecules

Droplet array for open-channel high-throughput SERS biosensing

Open-channel and high throughput are two important aspects of clinical diagnosis, correlation biochemical analysis, cell culture techniques and food safety. Here, we propose the mini-pillar based array for open-channel and high-throughput SERS detection of miRNA. The polydimethylsiloxane (PDMS) mini-pillars are used as a high-throughput platform, which have good anchoring and aggregation effects on microdroplets, greatly reducing the amount of analytical solution and facilitate the homogeneous sample distribution after evaporation. The deposited gold nanorods (Au NRs) on the pillars with optimized diameter served as SERS-active substrate, can greatly improve the sensitivity of SERS signal compared to other planar substrates. On the open-channel biological chip, sensitive, simultaneous, and specific detection of breast cancer marker miRNA-1246 can be performed. In this mini-pillar array SERS system, the limit of detection (LOD) is 10−12 M. The mini-pillar array shows enormous potential for open channel, high-throughput biomolecular detection, providing an opportunity for biomedical point-of-care testing (POCT) and drug screening.

Reconfigurable Integrated Optofluidic Droplet Laser Arrays.

Optofluidic lasers are an emerging technology for the development of miniaturized light sources and biological and chemical sensors. However, most optofluidic lasers demonstrated to date are operated at the single optical cavity level, which limits their applications in high-throughput biochemical sensing, high-speed wavelength switching, and on-chip spectroscopic analysis. Here, we demonstrated an optofluidic droplet laser array on a silicon chip with integrated microfluidics, in which four individual droplet optical cavities are generated and controlled by a 2 × 2 nozzle array. Arrays of droplets with a diameter ranging from 115 to 475 μm can be generated, removed, and regenerated on demand. The lasing threshold of the droplet laser array is in the range of 0.63-2.02 μJ/mm2. An image-based lasing threshold analysis method is developed, which enables simultaneous lasing threshold measurement for all laser units within the laser array using a low-cost camera. Compared to the conventional spectrum-based threshold analysis method, the lasing threshold obtained from the image-based method showed consistent results. Our droplet laser array is a promising technology in the development of cost-effective and integrated coherent light source on a chip for point-of-care applications.