Droplet microarray: miniaturized platform for rapid formation and high-throughput screening of embryoid bodies

Electrode Droplet Microarray (eDMA): An Impedance Platform for Label-Free Parallel Monitoring of Cellular Drug Response in Nanoliter Droplets

Label-free real-time monitoring of cellular behavior using impedance spectroscopy is important for drug development and toxicological assessments. Parallelization and miniaturization of such experiments are essential for increasing throughput and enabling experiments with low abundant stem or primary cells. Traditional methods are not miniaturized and require large volumes of reagents and number of cells, limiting their suitability for cost effective high-throughput screening of cells of limited availability. Here, the fabrication, optimization, and application of a bioelectrical signaling monitoring system - electrode droplet microarray (eDMA) are demonstrated. The eDMA platform is based on preparation of a hydrophilic-superhydrophobic patterns covering an array of individually addressable microelectrodes, which confines cells to individual microelectrodes, allowing for parallel, real-time, and label-free detection of cellular responses to drug treatments in nanoliter droplets. The real-time monitoring of cytotoxic effect of an anticancer drug is demonstrated over 48 h with real-time calculation of the half-inhibitory concentration (IC50) values through impedance spectroscopy. This demonstrates eDMA's ability to dynamically assess responses to various drugs in parallel at any given time point, which is crucial for functional personalized oncology. Specifically, the platform can be employed for monitoring anticancer drug toxicity using limited patient samples, where the miniaturization provided by eDMA is essential.

ANDeS: An automated nanoliter droplet selection and collection device

The Droplet Microarray (DMA) has emerged as a tool for high-throughput biological and chemical applications by enabling miniaturization and parallelization of experimental processes. Due to its ability to hold hundreds of nanoliter droplets, the DMA enables simple screening and analysis of samples such as cells and biomolecules. However, handling of nanoliter volumes poses a challenge, as manual recovery of nanoliter volumes is not feasible, and traditional laboratory equipment is not suited to work with such low volumes, and small array formats. To tackle this challenge, we developed the Automated Nanoliter Droplet Selection device (ANDeS), a robotic system for automated collection and transfer of nanoliter samples from DMA. ANDeS can automatically collect volumes from 50 to 350 nL from the flat surface of DMA with a movement accuracy of ±30 µm using fused silica capillaries. The system can automatically collect and transfer the droplets from DMA chip into other platforms, such as microtiter plates, conical tubes or another DMA. In addition, to ensure high throughput and multiple droplet collection, the uptake of multiple droplets within a single capillary, separated by air gaps to avoid mixing of the samples within the capillary, was optimized and demonstrated. This study shows the potential of ANDeS in laboratory applications by using it for the collection and transfer of biological samples, contained in nanoliter droplets, for subsequent analysis. The experimental results demonstrate the ability of ANDeS to increase the versatility of the DMA platform by allowing for automated retrieval of nanoliter samples from DMA, which was not possible manually on the level of individual droplets. Therefore, it widens the variety of analytical techniques that can be used for the analysis of content of individual droplets and experiments performed using DMA. Thus, ANDeS opens up opportunities to expand the development of miniaturized assays in such fields as cell screening, omics analysis and combinatorial chemistry.

Palladium-Catalyzed Combinatorial Synthesis of Biphenyls on Droplet Microarrays at Nanoliter Scale

The rising costs of pharmaceutical research are currently limiting the productivity of drug discovery and development, but can potentially be diminished via miniaturization of the synthesis and screening of new compounds. As droplet microarrays already present themselves as a versatile tool for highly miniaturized biological screening of various targets, their use for chemical synthesis is still limited. In this study, the influential palladium-catalyzed Suzuki-Miyaura reaction is successfully implemented at the nanoliter scale on droplet microarrays for the synthesis of an 800-compound library of biphenyls. Each reaction is carried out in individual 150 nL droplets. Remarkably, the synthesis of these 800 compounds requires a minimal amount of reagents, totaling 80 µmol, and a solvent volume of 400 µL. Furthermore, the cleavage kinetics and purity of the obtained biphenylic compounds are investigated. Via the solid-phase synthesis approach, the compounds could be purified from excess reactants and catalyst prior to the analysis and a UV-cleavable linker allows for fast and additive-free cleavage of each compound into the individual 100 nL droplet. This novel approach expands the toolbox of the droplet microarray for miniaturized high-throughput chemical synthesis and paves the way for future synthesis and screening of chemical compounds in a single platform.

Droplet microarray platforms for high-throughput drug screening

High-throughput screening platforms are fundamental for the rapid and efficient processing of large amounts of experimental data. Parallelization and miniaturization of experiments are important for improving their cost-effectiveness. The development of miniaturized high-throughput screening platforms is essential in the fields of biotechnology, medicine, and pharmacology. Currently, most laboratories use 96- or 384-well microtiter plates for screening; however, they have disadvantages, such as high reagent and cell consumption, low throughput, and inability to avoid cross-contamination, which need to be further optimized. Droplet microarrays, as novel screening platforms, can effectively avoid these shortcomings. Here, the preparation method of the droplet microarray, method of adding compounds in parallel, and means to read the results are briefly described. Next, the latest research on droplet microarray platforms in biomedicine is presented, including their application in high-throughput culture, cell screening, high-throughput nucleic acid screening, drug development, and individualized medicine. Finally, the challenges and future trends in droplet microarray technology are summarized.

Design of a self-cleanable multilevel anticounterfeiting interface through covalent chemical modulation

Counterfeit products have posed a significant threat to consumers safety and the global economy. To address this issue, extensive studies have been exploring the use of coatings with unclonable, microscale features for authentication purposes. However, the ease of readout, and the stability of these features against water, deposited dust, and wear, which are required for practical use, remain challenging. Here we report a novel class of chemically functionalizable coatings with a combination of a physically unclonable porous topography and distinct physiochemical properties (e.g., fluorescence, water wettability, and water adhesion) obtained through orthogonal chemical modifications (i.e., 1,4-conjugate addition reaction and Schiff-base reaction at ambient conditions). Unprecedentedly, a self-cleanable and physically unclonable coating is introduced to develop a multilevel anticounterfeiting interface. We demonstrate that the authentication of the fluorescent porous topography can be verified using deep learning. More importantly, the spatially selective chemical modifications can be read with the naked eye via underwater exposure and UV light illumination. Overall, the results reported in this work provide a facile basis for designing functional surfaces capable of independent and multilevel decryption of authenticity.

One-Step Fabrication of Droplet Arrays Using a Biomimetic Structural Chip

In the field of one-step efficient preparation of dewetting droplet arrays, the process is hampered by the requirement for low chemical wettability of solid surfaces, which restricts the complete transition of wetting state and its broad prospects in biological applications. Inspired by the physical structure of the lotus leaf, enabling it to promote the change of the infiltration state of an aqueous solution on the surface, we developed a method of one-step fabrication of droplet arrays on the biomimetic structural chip designed in the present work. This greatly reduces the need for chemical modification techniques to achieve low wettability and reduces the reliance on complex and sophisticated surface preparation techniques, thus improving the fabrication efficiency of droplet arrays fully generated on a chip by one-step operation without the need for extra liquid phase or the control of harsh barometric pressure. We also studied the influence of dimensions of the biomimetic structure and the preparation process parameters such as number of smears and speed of smearing on the preparation rate and uniformity of the droplet arrays. The amplification of templating DNA molecules in the droplet arrays prepared in a one-step fabrication way is also performed to verify its application potential for DNA molecular diagnosis.

Leaf-Inspired Patterned Organohydrogel Surface for Ultrawide Time-Range Open Biosensing

Droplet arrays show great significance in biosensing and biodetection because of low sample consumption and easy operation. However, inevitable water evaporation in open environment severely limits their applications in time-consuming reactions. Herein, inspired by the unique water retention features of leaves, it is demonstrated that an open droplet array on patterned organohydrogel surface with water evaporating replenishment (POWER) for ultrawide time-range biosensing, which integrated hydrophilic hydrogel domains and hydrophobic organogel background. The hydrogel domains on the surface can supply water to the pinned droplets through capillary channels formed in the nether organohydrogel bulk. The organogel background can inhibit water evaporation like the wax coating of leaves. Such a unique bioinspired design enables ultrawide time-range biosensing in open environment from a few minutes to more than five hours involving a variety of analytes such as ions, small molecules, and macromolecules. The POWER provides a feasible and open biosensing platform for ultrawide time-range reactions.

Automated high-throughput image processing as part of the screening platform for personalized oncology

Cancer is a devastating disease and the second leading cause of death worldwide. However, the development of resistance to current therapies is making cancer treatment more difficult. Combining the multi-omics data of individual tumors with information on their in-vitro Drug Sensitivity and Resistance Test (DSRT) can help to determine the appropriate therapy for each patient. Miniaturized high-throughput technologies, such as the droplet microarray, enable personalized oncology. We are developing a platform that incorporates DSRT profiling workflows from minute amounts of cellular material and reagents. Experimental results often rely on image-based readout techniques, where images are often constructed in grid-like structures with heterogeneous image processing targets. However, manual image analysis is time-consuming, not reproducible, and impossible for high-throughput experiments due to the amount of data generated. Therefore, automated image processing solutions are an essential component of a screening platform for personalized oncology. We present our comprehensive concept that considers assisted image annotation, algorithms for image processing of grid-like high-throughput experiments, and enhanced learning processes. In addition, the concept includes the deployment of processing pipelines. Details of the computation and implementation are presented. In particular, we outline solutions for linking automated image processing for personalized oncology with high-performance computing. Finally, we demonstrate the advantages of our proposal, using image data from heterogeneous practical experiments and challenges.

Droplet Microarray Based Screening Identifies Proteins for Maintaining Pluripotency of hiPSCs

Human induced pluripotent stem cells (hiPSCs) are crucial for disease modeling, drug discovery, and personalized medicine. Animal-derived materials hinderapplications of hiPSCs in medical fields. Thus, novel and well-defined substrate coatings capable of maintaining hiPSC pluripotency are important for advancing biomedical applications of hiPSCs. Here a miniaturized droplet microarray (DMA) platform to investigate 11 well-defined proteins, their 55 binary and 165 ternary combinations for their ability to maintainpluripotency of hiPSCs when applied as a surface coating, is used. Using this screening approach, ten protein group coatings are identified, which promote significantly higher NANOG expression of hiPSCs in comparison with Matrigel coating. With two of the identified coatings, long-term pluripotency maintenance of hiPSCs and subsequent differentiation into three germ layers are achieved. Compared with conventional high-throughput screening (HTS) in 96-well plates, the DMA platform uses only 83 µL of protein solution (0.83 µg total protein) and only ≈2.8 × 105 cells, decreasing the amount of proteins and cells ≈860 and 25-fold, respectively. The identified proteins will be essential for research and applications using hiPSCs, while the DMA platform demonstrates great potential for miniaturized HTS of scarce cells or expensive materials such as recombinant proteins.

Role of chemistry in bio-inspired liquid wettability

Chemistry and topography are the two distinct available tools for customizing different bio-inspired liquid wettability including superhydrophobicity, superamphiphobicity, underwater superoleophobicity, underwater superoleophilicity, and liquid infused slippery property. In nature, various living species possessing super and special liquid wettability inherently comprises of distinctly patterned surface topography decorated with low/high surface energy. Inspired from the topographically diverse natural species, the variation in surface topography has been the dominant approach for constructing bio-inspired antiwetting interfaces. However, recently, the modulation of chemistry has emerged as a facile route for the controlled tailoring of a wide range of bio-inspired liquid wettability. This review article aims to summarize the various reports published over the years that has elaborated the distinctive importance of both chemistry and topography in imparting and modulating various bio-inspired wettability. Moreover, this article outlines some obvious advantages of chemical modulation approach over topographical variation. For example, the strategic use of the chemical approach has allowed the facile, simultaneous, and independent tailoring of both liquid wettability and other relevant physical properties. We have also discussed the design of different antiwetting patterned and stimuli-responsive interfaces following the strategic and precise alteration of chemistry for various prospective applications.

"Cells-to-cDNA on Chip": Phenotypic Assessment and Gene Expression Analysis from Live Cells in Nanoliter Volumes Using Droplet Microarrays

In vitro cell-based experiments are particularly important in fundamental biological research. Microscopy-based readouts to identify cellular changes in response to various stimuli are a popular choice, but gene expression analysis is essential to delineate the underlying molecular dynamics in cells. However, cell-based experiments often suffer from interexperimental variation, especially while using different readout methods. Therefore, establishment of platforms that allow for cell screening, along with parallel investigations of morphological features, as well as gene expression levels, is crucial. The droplet microarray (DMA) platform enables cell screening in hundreds of nanoliter droplets. In this study, a "Cells-to-cDNA on Chip" method is developed enabling on-chip mRNA isolation from live cells and conversion to cDNA in individual droplets of 200 nL. This novel method works efficiently to obtain cDNA from different cell numbers, down to single cell per droplet. This is the first established miniaturized on-chip strategy that enables the entire course of cell screening, phenotypic microscopy-based assessments along with mRNA isolation and its conversion to cDNA for gene expression analysis by real-time PCR on an open DMA platform. The principle demonstrated in this study sets a beginning for myriad of possible applications to obtain detailed information about the molecular dynamics in cultured cells.

Pluripotent stem cell assays: Modalities and applications for predictive developmental toxicity

This manuscript provides a review focused on embryonic stem cell-based models and their place within the landscape of alternative developmental toxicity assays. Against the background of the principles of developmental toxicology, the wide diversity of alternative methods using pluripotent stem cells developed in this area over the past half century is reviewed. In order to provide an overview of available models, a systematic scoping review was conducted following a published protocol with inclusion criteria, which were applied to select the assays. Critical aspects including biological domain, readout endpoint, availability of standardized protocols, chemical domain, reproducibility and predictive power of each assay are described in detail, in order to review the applicability and limitations of the platform in general and progress moving forward to implementation. The horizon of innovative routes of promoting regulatory implementation of alternative methods is scanned, and recommendations for further work are given.

Miniaturized droplet microarray platform enables maintenance of human induced pluripotent stem cell pluripotency

The capacity of human induced pluripotent stem cells (hiPSCs) for indefinite self-renewal warrants their application in disease modeling, drug discovery, toxicity assays and efficacy screening. However, their poor proliferation ability, inability to adhere to surfaces without Matrigel coating and tendency to spontaneously differentiate in vitro hinder the application of hiPSCs in these fields. Here we study the ability to culture hiPSCs inside 200 ​nL droplets on the droplet microarray (DMA) platform. We demonstrate that (1) hiPSCs can attach to the Matrigel (MG)-free surface of DMA and show good viability after 24 h culture; (2) hiPSC do not spontaneously differentiate when cultured on the MG-free surface of DMAs; (3) culturing of hiPSCs in 200 ​nL as compared to 2 ​mL culture leads to higher expression of the Nanog pluripotency marker. Overall, the results demonstrate the possibility to culture undifferentiated hiPSCs in 200 ​nL droplets on DMA, thereby opening the possibility for high-throughput screenings of hiPSCs with various factors without compromising the results through the involvement of animal-derived materials, such as Matrigel.

Droplet Microarray Based on Nanosensing Probe Patterns for Simultaneous Detection of Multiple HIV Retroviral Nucleic Acids

Multiplexed detection of viral nucleic acids is important for rapid screening of viral infection. In this study, we present a molybdenum disulfide (MoS2) nanosheet-modified dendrimer droplet microarray (DMA) for rapid and sensitive detection of retroviral nucleic acids of human immunodeficiency virus-1 (HIV-1) and human immunodeficiency virus-2 (HIV-2) simultaneously. The DMA platform was fabricated by omniphobic-omniphilic patterning on a surface-grafted dendrimer substrate. Functionalized MoS2 nanosheets modified with fluorescent dye-labeled oligomer probes were prepatterned on positively charged amino-modified omniphilic spots to form a fluorescence resonance energy transfer (FRET) sensing microarray. With the formation of separated microdroplets of sample on the hydrophobic-hydrophilic micropattern, prepatterned oligomer probes specifically hybridized with the target HIV genes and detached from the MoS2 nanosheet surface due to weakening of the adsorption force, leading to fluorescence signal recovery. As a proof of concept, we used this microarray with a small sample size (<150 nL) for simultaneous detection of HIV-1 and HIV-2 nucleic acids with a limit of detection (LOD) of 50 pM. The multiplex detection capability was further demonstrated for simultaneous detection of five viral genes (HIV-1, HIV-2, ORFlab, and N genes of SARS-COV-2 and M gene of Influenza A). This work demonstrated the potential of this novel MoS2-DMA FRET sensing platform for high-throughput multiplexed viral nucleic acid screening.

Miniaturized Drug Sensitivity and Resistance Test on Patient-Derived Cells Using Droplet-Microarray

Testing the sensitivity of patient-derived tumor cells ex vivo can potentially help determining the appropriate treatment for each patient and spot the development of resistance to a given therapy. The number of cells obtainable from a biopsy is, however, often insufficient for performing ex vivo tests in conventional microtiter plates. Here, we introduce a novel Droplet-Microarray platform based on a hydrophilic-superhydrophobic patterned surface that enables screenings using only 100 cells and 30 picomoles of a drug per individual nanoliter-sized droplet. We demonstrate that the dose-response of as few as 100 primary patient-derived chronic lymphocytic leukemia (CLL) cells to anticancer compounds on the Droplet-Microarray platform resembles the dose-response obtained in 384-well plates requiring 20,000 tumor cells per experiment. The extremely miniaturized Droplet-Microarray platform thus carries great potential for ex vivo drug sensitivity and resistance tests on patient-derived tumor cells and potentially for implementing such tests in medical practice of precision medicine.

Picoliter droplet array based on bioinspired microholes for in situ single-cell analysis

The division of aqueous samples into microdroplet arrays has many applications in biochemical and medical analysis. Inspired by biological features, we propose a method to produce picoliter droplet arrays for single-cell analysis based on physical structure and interface. A 0.9 pL droplet array with an RSD (relative standard deviation) less than 6.3% and a density of 49,000 droplets/cm² was successfully generated on a PDMS chip (polydimethylsiloxane) from a micromachined glass mold. The droplet generation principle of the wetting behavior in the microholes with splayed sidewalls on the PDMS chip by liquid smearing was exploited. The feasibility of the picoliter droplets for bacterial single-cell analysis was verified by the separation of mixed bacteria into single droplets and isolated in situ bacteria propagation.

Sessile Liquid Features as Molds for Silicone Elastomers

Liquid applied to a chemically patterned (wetting/nonwetting, lyophilic/lyophobic) substrate forms a 3-dimensional contoured surface, the shape of which depends on the volume of liquid applied and the shape of the three-phase contact lines of air (or other phase in contact), liquid, and the wetted pattern. The resulting binary contoured interface with air, which consists of flat unwetted regions of the substrate and the mean curvature liquid-vapor interfaces of the sessile structures, can be used as a mold for imprinting solid polymers by curing liquid resins in contact. The success, flexibility with regard to shape, and reproducibility of this molding process depend on numerous issues. These include the substrate surface chemistry, the liquid application method, properties of the liquid (vapor pressure, surface tension, viscosity, and permeability in the resin), the contact angles of the liquid on the patterned substrate, and the resin curing chemistry and conditions. We investigate the room temperature platinum(0)-catalyzed curing of the most commonly studied commercial silicone elastomer, Sylgard 184, using molds composed of sessile drops of liquids on circular wetting features (bare silicon wafer) patterned on covalently attached fluoroalkylsilyl monolayers. Liquids reported are water, glycerol, an ethylene glycol oligomer, and an ionic liquid. The vapor pressure of water and its permeability in dimethyl silicone were important (and problematic) issues that could be controlled by adjusting humidity. The ionic liquid N-ethyl-N'-methylimidazolium methanesulfonate poisoned/inhibited the curing chemistry and affected silicone cross-link density and the resulting feature shape, but its lack of vapor pressure was useful in studying flow coating as a scalable liquid application method. The ethylene glycol oligomer exhibited compatibility with (and diffusion into) the silicone. Glycerol proved to be the most well-behaved and controllable liquid studied and was used to demonstrate that condensation/evaporation can be used to adjust feature shape.

Direct-Write Bioprinting Approach to Construct Multilayer Cellular Tissues

As a cellular-assembly technique, bioprinting has been extensively used in tissue engineering and regenerative medicine to construct hydrogel-based three-dimensional (3D) tissue-like models with prescribed geometry. Here, we introduced a unique direct-write bioprinting strategy to fabricate a bilayer flat tissue in a hydrogel-free approach. A printed retina pigmented epithelium layer (RPE) was applied as living biopaper for positioning a fibroblast layer without using any hydrogel in bioink. We adjusted the number of cells in the inkjet droplets in order to obtain a uniform printed cell layer and demonstrated the formation of a bilayer construct through confocal imaging. Since our printing system introduced low levels of shear stress to the cells, it did not have a negative effect on cell survival, although cell viability was generally lower than that of control group over 1 week post-printing. In conclusion, our novel direct-write bioprinting approach to spatiotemporally position different cellular layers may represent an efficient tool to develop living constructs especially for regeneration of complex flat tissues.

Facile One Step Formation and Screening of Tumor Spheroids Using Droplet-Microarray Platform

Tumor spheroids or microtumors are important 3D in vitro tumor models that closely resemble a tumor's in vivo "microenvironment" compared to 2D cell culture. Microtumors are widely applied in the fields of fundamental cancer research, drug discovery, and precision medicine. In precision medicine tumor spheroids derived from patient tumor cells represent a promising system for drug sensitivity and resistance testing. Established and commonly used platforms for routine screenings of cell spheroids, based on microtiter plates of 96- and 384-well formats, require relatively large numbers of cells and compounds, and often lead to the formation of multiple spheroids per well. In this study, an application of the Droplet Microarray platform, based on hydrophilic-superhydrophobic patterning, in combination with the method of hanging droplet, is demonstrated for the formation of highly miniaturized single-spheroid-microarrays. Formation of spheroids from several commonly used cancer cell lines in 100 nL droplets starting with as few as 150 cells per spheroid within 24-48 h is demonstrated. Established methodology carries a potential to be adopted for routine workflows of high-throughput compound screening in 3D cancer spheroids or microtumors, which is crucial for the fields of fundamental cancer research, drug discovery, and precision medicine.