Sessile droplets for chemical and biological assays

Sliding of drops on mesoporous thin films

There is great interest in developing surfaces with enhanced properties for the sliding of liquid droplets. Here we show that both water and oil droplets placed on mesoporous thin film surfaces slide at relatively small tilt angles with respect to non-porous surfaces of the same material. The effect arises from a particular soft pinning at the contact line, which is a consequence of the fact that sessile droplets are partially "floating" onto a locally self-imbibed mesoporous film. Therefore, droplets present a reduced sliding angle and an enhanced sliding velocity in comparison to droplets on non-porous surfaces of the same material. The formed droplet-substrate interface is different to those observed on superhydrophobic or oil-infused surfaces, and involves a particular sliding dynamic. These findings would help to improve technical developments that require the precise handling of droplet mobility, whose interest span from chemical and biological assays in open microfluidic platforms to applications in energy and the environment.

Control of solutal Marangoni-driven vortical flows and enhancement of mixing efficiency

HYPOTHESIS

In droplet microfluidics applications, flow control and mixing in a small volume without any active external devices is a challenge. Vapor-mediated solutal Marangoni flows can be effectively generated by applying the vapor of a volatile liquid, which can be possibly controlled, and can eventually be used in a mixing enhancement device.

EXPERIMENTS

We investigated and controlled vapor-mediated solutal Marangoni flows by varying the local surface tension. We systematically tested the effects of different volatile liquids and their vapor concentration on the flow pattern. Furthermore, by varying the number of vapor sources, we generated and controlled multiple vortices, and analyzed them by particle image velocimetry (PIV). The proposed method was applied to a mixing enhancement application.

FINDINGS

We show that in addition to the surface tension of the volatile liquid, the vapor concentration also influenced the local surface tension along the interface, which in turn changed the internal flow velocity. To predict the flow velocity and oscillatory frequency of the solutal Marangoni flow, we developed a theoretical model based on scaling analysis that showed a good agreement with the experimental results. We believe that the current study will motivate low-cost and portable sample flow control and mixing systems in the near future.

Size-Controlled Patterning of Single-Crystalline Perovskite Arrays toward a Tunable High-Performance Microlaser

Single-crystalline halide perovskites with regular morphology are of great significance for laser applications because they can be used to fabricate a natural whispering-gallery-mode resonator. Although enormous efforts have been put to synthesize single-crystalline perovskites, controlling the lateral size and thickness of the crystal, particularly at the nanoscale, is still challenging. Here, we report a facile and high-throughput strategy to selectively one-step create micro/nanoscale size-controlled all-inorganic perovskite single-crystal arrays by surface-tension-confined evaporative assembly. Our method can be used to easily tune the single crystal size and selectively position the single crystal, with versatility in fabricating perovskite single-crystal arrays in a wafer scale. When the patterned size increases from 2 to 25 μm, the width of the CsPbClBr₂ perovskite microplates increased from 150 nm to 4.2 μm. Fixing the width of the microplates at 1.6 μm, with the increase of the sliding speed from 50 to 250 mm/min, we could significantly control the thicknesses from 270 to 430 nm. Additionally, our present study provides a characterization of lasers based on different three-dimensional structures, confirming their width-dependent lasing mode and thickness-dependent lasing threshold characteristic, which is beneficial for the tunability of a high-performance microlaser.

Hydrophobic/hydrophilic patterned surfaces for directed evaporative preconcentration

We present a microfluidic platform that rapidly deposits many samples and preconcentrates them, making it suitable for a wide range of high-throughput detection schemes.

Microbioreactors for Process Development and Cell-Based Screening Studies

Microbioreactors (MBRs) have emerged as potent cultivation devices enabling automated small-scale experiments in parallel while enhancing their cost efficiency. The widespread use of MBRs has contributed to recent advances in industrial and pharmaceutical biotechnology, and they have proved to be indispensable tools in the development of many modern bioprocesses. Being predominantly applied in early stage process development, they open up new fields of research and enhance the efficacy of biotechnological product development. Their reduced reaction volume is associated with numerous inherent advantages - particularly the possibility for enabling parallel screening operations that facilitate high-throughput cultivations with reduced sample consumption (or the use of rare and expensive educts). As a result, multiple variables can be examined in a shorter time and with a lower expense. This leads to a simultaneous acceleration of research and process development along with decreased costs.MBRs range from simple miniaturized cultivations vessels (i.e., in the milliliter scale with limited possibilities for process control) to highly complex and automated small-scale microreactors with integrated sensors that allow for comprehensive screenings in very short time or a precise reflection of large-scale cultivation conditions. Progressive developments and improvements in manufacturing and automation techniques are already helping researchers to make use of the advantages that MBRs offer. This overview of current MBR systems surveys the diverse application for microbial and mammalian cell cultivations that have been developed in recent years.

Hexagonal deposits of colloidal particles

Colloidal particles are essential materials for modern inkjet printing and coating. Here we demonstrate a versatile method to achieve hexagonal deposits of colloidal particles through droplet evaporation on hexagonal micropillar arrays. We identify how colloidal fluids turn into hexagonal deposits during evaporation with x-ray tomography. Interestingly, evaporation-driven hexagonal deposits are quite crack-free uniform. We attribute hexagonal deposit shape control to local contact line pinning by colloidal particles and geometric constraints by micropillar arrays. This deposition strategy offers a feasibility for high-quality evaporation-driven crack-free uniform polygonal deposits of colloidal particles for diverse applications.

Antifouling digital microfluidics using lubricant infused porous film

Electrowetting-driven digital (droplet-based) microfluidics has a tremendous impact on lab-on-a-chip applications. However, the biofouling problem impedes the real applications of such digital microfluidics. Here we report antifouling digital microfluidics by introducing lubricant infused porous film to electrowetting (more exactly, electrowetting on dielectric or EWOD). Such film minimizes direct contact between droplets and the solid surface but provides liquid-liquid contact between droplets and the lubricant liquid, which thus prevents unspecific adsorption of biomolecules to the solid surface. We demonstrate the compatibility of the lubricant infused film with EWOD to transport bio droplets. This configuration shows robust and high performance even for long cyclic operations without fouling in a wide range of concentrations of protein solutions. In addition, a variety of conductive droplets, including deionized (DI) water, saline, protein solution, DNA solution, sheep blood, milk, ionic liquid and honey, are examined, similarly showing high performance in cyclic transportations. In addition, using the same electrode patterns used in EWOD, transportations of dielectric (non-conductive) droplets including light crude oil, propylene carbonate and alcohol are also achieved. Such capability of droplet handling without fouling will certainly benefit the practical applications of digital microfluidics in droplet handling, sampling, reaction, diagnosis in clinic medicine, biotechnology and chemistry fields.

Microfabricated needle for hydrogen peroxide detection

A microfabricated needle-like probe has been designed and applied for hydrogen peroxide (H2O2) sampling and detection using a commercial, single-step fluorescent H2O2 assay. In this work, droplets of the assay reagent are generated and sent to the needle tip using a mineral-oil carrier fluid. At the needle tip, the sample is drawn into the device through 100 μm long hydrophilic capillaries by negative pressure. The sampled fluid is immediately merged with the assay droplet and carried away to mix and react, producing a sequence of droplets representing the H2O2 concentration as a function of time. We have characterized the assay fluorescence for small variations in the sample volume. With the calibration, we can calculate the concentration of H2O2 in the sampled liquid from the size and intensity of each merged droplet. This is a microfluidic data-logger system for on-site continuous sampling, controlled reaction, signal storage and on-line quantitative detection. It is a useful tool for monitoring dynamic chemical reactions in analytical chemistry and biological applications.

Controlling self-assembly and buckling in nano fluid droplets through vapour mediated interaction of adjacent droplets

HYPOTHESIS

Sessile droplets of contrasting volatilities can communicate via long range (∼O (1) mm) vapour-mediated interactions which allow the remote control of the flow driven self-assembly of nanoparticles in the drop of lower volatility. This allows morphological control of the buckling instability observed in evaporating nanofluid droplets.

EXPERIMENTS

A nanofluid droplet is dispensed adjacent to an ethanol droplet. Asymmetrical adsorption induced Marangoni flow (∼O (1) mm/s) internally segregates the particle population. Particle aggregation occurs preferentially on one side of the droplet leaving the other side to develop a relatively weaker shell which buckles under the effect of evaporation driven capillary pressure.

FINDINGS

The inter-droplet distance is varied to demonstrate the effect on the precipitate shape (flatter to dome shaped) and the location of the buckling (top to side). In addition to being a simple template for hierarchical self-assembly, the presented exposition also promises to enhance mixing rates (∼1000 times) in droplet-based bioassays with minimal contamination.

Organic-free, versatile sessile droplet microfluidic device for chemical separation using an aqueous two-phase system

This work presents a novel portable, versatile sessile droplet microfluidic (SDMF) device to perform liquid manipulation operations such as confining, splitting and colorimetric detection. Furthermore, chemical isolations based on an aqueous two-phase system (ATPS) for separating an analyte of choice from a complicated sample matrix can be carried out. ATPS extractions can replace conventional liquid-liquid extractions and take away the need for harmful organic solvents. Superhydrophobic (SH) surfaces were fabricated from a commercially available material, Ultra-Ever Dry® (UED®). On these SH surfaces, surface energy traps (SETs) were produced either by air plasma treatment (simultaneously) or laser micromachining (sequentially) to dock/pin an ATPS containing droplet onto the surface. Splitting of droplets or removing a precise volume of the top phase from a pinned extraction system was achieved with a sandwich-chip approach. For this, an additional SET patterned substrate was placed on top of the droplet and subsequently lifted. This multipurpose platform was used to isolate Cd from a mixture of several other metal ions (i.e. Mn, Ni, Cu, Pb, Fe) for its subsequent interference-free detection. An ATPS consisting of sodium sulfate and polyethylene glycol (PEG) as phase forming components and potassium iodine as extractant allowed separation of cadmium with an extraction efficiency of q(Cd2+) = 98.5%. Using a portable, cost-effective, smartphone-based UV/vis spectrometer, Cd was detected with a LoD of 3.4 ppm. Alternatively, the multipurpose platform can also be used as sampling platform for a benchtop UV/vis spectrometer, where a LoD of 0.53 ppm was obtained. Potential applications of the presented platform include sample preparation and separation that can be achieved by aqueous two-phase extractions, such as proteins, antibodies, DNA, cells, organic molecules and metal ions.

Bioinspired superwettable micropatterns for biosensing

The bioinspired micropatterns exhibit outstanding capacity in controlling and patterning microdroplets, which have offered new functionalities and possibilities towards a wide variety of emerging biological and biomedical applications.

Superwettable nanodendritic gold substrates for direct miRNA SERS detection

By combining a superwettable interface with a nanodendritic gold structure, we have fabricated a superwettable nanodendritic gold substrate for direct SERS detection of multiple concentrations of miRNAs. The nanodendritic gold substrate provides numerous hotspots for Raman signal enhancement, and the superwettable interface ensures the immobilization of droplets in superhydrophilic microwells, which hold great potentials for applications in disease diagnostics.

Drop mobility on superhydrophobic microstructured surfaces with wettability contrasts

Manipulation of drop motion has attracted considerable attention recently as it is pertinent to industrial/biological applications such as microfluidics. Wettability gradients/contrasts applied to microtextured, superhydrophobic surfaces are probable candidates for engineering drop motion by virtue of their wettability controllability and low contact angle hysteresis. In the present work, we present a systematic study of drop mobility induced via wettability contrasts. A millimetre-sized water drop, placed on the boundary between two surfaces with distinct, uniform arrays of pillars, immediately moved toward the surface more densely populated with asperities, which was relatively more hydrophilic. The velocity of the motion was found to increase proportionally with the difference in pillar densities on each surface, in circumstances where the rear side surface had sufficiently small contact angle hysteresis. To elucidate the underlying mechanism of drop motion, we implemented a surface energy analysis for each motion event. Motion was initiated by the excess surface free energy due to drop deformation and directed in favour of energy minimisation. Lastly, we propose a theory to predict the direction of the drop which at the same time acts as the criterion for the motion to ensue.

Static droplet array for culturing single live adherent cells in an isolated chemical microenvironment

We present here a new method to easily and reliably generate an array of hundreds of dispersed nanoliter-volume semi-droplets for single-cells culture and analysis. The liquid segmentation step occurs directly in indexed traps by a tweezer-like mechanism and is stabilized by spatial confinement. Unlike common droplet-based techniques, the semi-droplet wets its surrounding trap walls thus supporting the culturing of both adherent and non-adherent cells. To eliminate cross-droplet cell migration and chemical cross-talk each semi-droplet is separated from a nearby trap by an ∼80 pL air plug. The overall setup and injection procedure takes less than 10 minutes, is insensitive to fabrication defects and supports cell recovery for downstream analysis. The method offers a new approach to easily capture, image and culture single cells in a chemically isolated microenvironment as a preliminary step towards high-throughput single-cell assays.

Micro/nano acoustofluidics: materials, phenomena, design, devices, and applications

Acoustic actuation of fluids at small scales may finally enable a comprehensive lab-on-a-chip revolution in microfluidics, overcoming long-standing difficulties in fluid and particle manipulation on-chip. In this comprehensive review, we examine the fundamentals of piezoelectricity, piezoelectric materials, and transducers; revisit the basics of acoustofluidics; and give the reader a detailed look at recent technological advances and current scientific discussions in the discipline. Recent achievements are placed in the context of classic reports for the actuation of fluid and particles via acoustic waves, both within sessile drops and closed channels. Other aspects of micro/nano acoustofluidics are examined: atomization, translation, mixing, jetting, and particle manipulation in the context of sessile drops and fluid mixing and pumping, particle manipulation, and formation of droplets in the context of closed channels, plus the most recent results at the nanoscale. These achievements will enable applications across the disciplines of chemistry, biology, medicine, energy, manufacturing, and we suspect a number of others yet unimagined. Basic design concepts and illustrative applications are highlighted in each section, with an emphasis on lab-on-a-chip applications.

Microfluidic chip with movable layers for the manipulation of biochemicals

A simple and effective platform that can conglomerate various microfluidic functions in a single chip is essential for many bioassays, especially for point-of-care testing applications. Here, a chip that exploits surface tension in solutions with movable top and bottom layers is presented, for use in fluid transport, mixing, maintaining metered volumes, and biomolecule capture and release. The chip has open chambers in vertically mobile top layers and rotationally mobile bottom layers to exploit surface tension in biochemical solutions, and implements control over fluid motion. To manipulate biomolecules, a vertically mobile tip with a permanent magnet at the top layer performs collection, transport, release, and dispersion of magnetic beads. Thus, the chip orchestrates various fluidic control functions without using on-chip valves and pumps that increase operational and structural complexity. To demonstrate its utility, the chip performs automated DNA extraction by obtaining genomic DNA from a sample containing cells. Our approach provides a useful and effective alternative to numerous platforms that use active and passive on-chip components for bioassays.

Microfluidic Devices for Drug Assays

In this review, we give an overview of the current state of microfluidic-based high-throughput drug assays. In this highly interdisciplinary research field, various approaches have been applied to high-throughput drug screening, including microtiter plate, droplets microfluidics as well as continuous flow, diffusion and concentration gradients-based microfluidic drug assays. Therefore, we reviewed over 100 recent publications in the field and sorted them according to their microfluidic approach. As a result, we are showcasing, comparing and discussing broadly applied approaches as well as singular promising ones that might contribute to shaping the future of this field.

Redefining Molecular Amphipathicity in Reversing the "Coffee-Ring Effect": Implications for Single Base Mutation Detection

The "coffee ring effect" is a natural phenomenon wherein sessile drops leave ring-shaped structures on the solid surfaces upon drying. It drives a nonuniform deposition of suspended compounds on the substrates, which adversely affects many processes, including surface-assisted biosensing and molecular self-assembly. In this study, we describe how the coffee ring effect can be eliminated by controlling the amphipathicity of the suspended compounds, for example, DNA modified with hydrophobic dye. Specifically, nuclease digestion of the hydrophilic DNA end converts the dye-labeled molecule into an amphipathic molecule (one with comparably weighted hydrophobic and hydrophilic ends) and reverses the coffee ring effect and results in a uniform disk-shaped feature deposition of the dye. The amphipathic product decreases the surface tension of the sessile drops and induces the Marangoni flow, which drives the uniform distribution of the amphipathic dye-labeled product in the drops. As a proof of concept, this strategy was used in a novel enzymatic amplification method for biosensing to eliminate the coffee ring effect on a nitrocellulose membrane and increase assay reliability and sensitivity. Importantly, the reported strategy for eliminating the coffee ring effect can be extended to other sessile drop systems for potentially improving assay reliability and sensitivity.

Label-Free Sensing in Microdroplet-Based Microfluidic Systems

Droplet microfluidic systems have evolved as fluidic platforms that use much less sample volume and provide high throughput for biochemical analysis compared to conventional microfluidic devices. The variety of droplet fluidic applications triggered several detection techniques to be applied for analysis of droplets. In this review, we focus on label-free droplet detection techniques that were adapted to various droplet microfluidic platforms. We provide a classification of most commonly used droplet platform technologies. Then we discuss the examples of various label-free droplet detection schemes implemented for these platforms. While providing the research landscape for label-free droplet detection methods, we aim to highlight the strengths and shortcomings of each droplet platform so that a more targeted approach can be taken by researchers when selecting a droplet platform and a detection scheme for any given application.

Micro to Nanoscale Engineering of Surface Precipitates Using Reconfigurable Contact Lines

Nanoscale engineering has traditionally adopted the chemical route of synthesis or optochemical techniques such as lithography requiring large process times, expensive equipment, and an inert environment. Directed self-assembly using evaporation of nanocolloidal droplet can be a potential low-cost alternative across various industries ranging from semiconductors to biomedical systems. It is relatively simple to scale and reorient the evaporation-driven internal flow field in an evaporating droplet which can direct dispersed matter into functional agglomerates. The resulting functional precipitates not only exhibit macroscopically discernible changes but also nanoscopic variations in the particulate assembly. Thus, the evaporating droplet forms an autonomous system for nanoscale engineering without the need for external resources. In this article, an indigenous technique of interfacial re-engineering, which is both simple and inexpensive to implement, is developed. Such re-engineering widens the horizon for surface patterning previously limited by the fixed nature of the droplet interface. It involves handprinting hydrophobic lines on a hydrophilic substrate to form a confinement of any selected geometry using a simple document stamp. Droplets cast into such confinements get modulated into a variety of shapes. The droplet shapes control the contact line behavior, evaporation dynamics, and complex internal flow pattern. By exploiting the dynamic interplay among these variables, we could control the deposit's macro- as well as nanoscale assembly not possible with simple circular droplets. We provide a detailed mechanism of the coupling at various length scales enabling a predictive capability in custom engineering, particularly useful in nanoscale applications such as photonic crystals.

Superwettable Electrochemical Biosensor toward Detection of Cancer Biomarkers

Bioinspired superwettable micropatterns that combine two extreme states of superhydrophobicity and superhydrophilicity with the ability to enrich and absorb microdroplets are suitable for versatile and robust sensing applications. Here we introduce a superwettable microchip that integrates superhydrophobic-superhydrophilic micropatterns and a nanodendritic electrochemical biosensor toward the detection of prostate cancer biomarkers. On the superwettable microchip, the superhydrophobic area could confine the microdroplets in superhydrophilic microwells; such behavior is extremely helpful for reducing the amount of analytical solution. In contrast, superhydrophilic microwells exhibit a high adhesive force toward microdroplets, and the nanodendritic structures can improve probe-binding capacity and response signals, thus greatly enhancing the sensitivity. Sensitive and selective detection of prostate cancer biomarkers including miRNA-375, miRNA-141, and prostate-specific antigen on a single microchip is also achieved. Such a superwettable microchip with high sensitivity, low sample volume, and upside-down detection capability in a single microdroplet shows great potential to fabricate portable devices toward complex biosensing applications.

Simple scaling laws for the evaporation of droplets pinned on pillars: Transfer-rate- and diffusion-limited regimes

The evaporation of droplets can give rise to a wide range of interesting phenomena in which the dynamics of the evaporation are crucial. In this work, we find simple scaling laws for the evaporation dynamics of axisymmetric droplets pinned on millimeter-sized pillars. Different laws are found depending on whether evaporation is limited by the diffusion of vapor molecules or by the transfer rate across the liquid-vapor interface. For the diffusion-limited regime, we find that a mass-loss rate equal to 3/7 of that of a free-standing evaporating droplet brings a good balance between simplicity and physical correctness. We also find a scaling law for the evaporation of multicomponent solutions. The scaling laws found are validated against experiments of the evaporation of droplets of (1) water, (2) blood plasma, and (3) a mixture of water and polyethylene glycol, pinned on acrylic pillars of different diameters. These results shed light on the macroscopic dynamics of evaporation on pillars as a first step towards the understanding of other complex phenomena that may be taking place during the evaporation process, such as particle transport and chemical reactions.

Triggering molecular assembly at the mesoscale for advanced Raman detection of proteins in liquid

An advanced optofluidic system for protein detection based on Raman signal amplification via dewetting and molecular gathering within temporary mesoscale assemblies is presented. The evaporation of a microliter volume of protein solution deposited in a circular microwell precisely follows an outward-receding geometry. Herein the combination of liquid withdrawal with intermolecular interactions induces the formation of self-assembled molecular domains at the solid-liquid interface. Through proper control of the evaporation rate, amplitude of the assemblies and time for spectral collection at the liquid edge are extensively raised, resulting in a local enhancement and refinement of the Raman response, respectively. Further signal amplification is obtained by taking advantage of the intense local electromagnetic fields generated upon adding a plasmonic coating to the microwell. Major advantages of this optofluidic method lie in the obtainment of high-quality, high-sensitivity Raman spectra with detection limit down to sub-micromolar values. Peculiarly, the assembled proteins in the liquid edge region maintain their native-like state without displaying spectral changes usually occurring when dried drop deposits are considered.

Surface-tension-confined assembly of a metal–organic framework in femtoliter droplet arrays

The key factors determining HKUST-1 single-crystal growth are the internal flows in an evaporating droplet and consequently aggregation induced by the combination of metallic Cu(ii) and BTC ions.