Full-Combinatorial Concentration Gradient Array with 3D Micro/Nanofluidics for Antibiotic Susceptibility Testing.

Recent advancements in micro/nanofluidics have facilitated on-chip microscopy of cellular responses in a high-throughput and controlled microenvironment with the desired physicochemical properties. Despite its potential benefits to combination drug discovery, generating a complete combinatorial set of concentration gradients for multiple reagents in an array format remains challenging. The main reason is limited layouts of conventional micro/nanofluidic systems based on two-dimensional channel networks. In this paper, we present a device with three-dimensional (3D) interconnection of micro/nanochannels capable of generating a complete combinatorial set of concentration gradients for two reagents. The device was readily fabricated by laminating a pair of multilayered monolithic films containing a Christmas tree-like mixer, a cell culture chamber array, and through-holes, all within each single film. We assessed the reliable generation of a full-combinatorial concentration gradient array and validated it by using numerical analysis. We applied the proposed device to test the antibiotic susceptibility of bacterial cells in a convenient one-step manner. Furthermore, we explored the potential of the device to accommodate the arrayed complete combinatorial set for two or more drugs, while extending the capabilities of our laminated object manufacturing method for realizing 3D micro/nanofluidic systems.

Programmable and Pixelated Solute Concentration Fields Controlled by Three-Dimensionally Networked Microfluidic Source/Sink Arrays.

Membrane-integrated microfluidic platforms have played a pivotal role in understanding natural phenomena coupled with solute concentration gradients at the micro- and nanoscale, enabling on-chip microscopy in well-defined planar concentration fields. However, the standardized two-dimensional fabrication schemes in microfluidics have impeded the realization of more complex and diverse chemical environmental conditions due to the limited possible arrangements of source/sink conditions in a fluidic domain. In this study, we present a microfluidic platform with a three-dimensional microchannel network design, where discretized membranes can be integrated and individually controlled in a two-dimensional array format at any location within the entire quasi-two-dimensional solute concentration field. We elucidate the principles of the device to implement operations of the pixel-like sources/sinks and dynamically programmable control of various long-lasting solute concentration fields. Furthermore, we demonstrate the application of the generated solute concentration fields in manipulating the transport of micrometer or submicrometer particles with a high degree of freedom, surpassing conventionally available solute concentration fields. This work provides an experimental tool for investigating complex systems under high-order chemical environmental conditions, thereby facilitating the extensive development of higher-performance micro- and nanotechnologies.

Fabricating and Laminating Films with Through-Holes and Engraved/Protruding Structures for 3D Micro/Nanofluidic Platforms.

Micro/nanofluidic devices have become popular for delicately processing biological, material, and chemical samples. However, their reliance on 2D fabrication schemes has hindered further innovation. Here, a 3D manufacturing method is proposed through the innovation of laminated object manufacturing (LOM), which involves the selection of building materials as well as the development of molding and lamination techniques. Fabrication of interlayer films is demonstrated with both multi-layered micro-/nanostructures and through-holes, using an injection molding approach and establishing strategic principles of film design. Utilization of the multi-layered through-hole films in LOM allows reducing the number of alignments and laminations by at least two times compared to conventional LOM. Using a dual-curing resin for film fabrication, a surface-treatment-free and collapse-free lamination technique is shown for constructing 3D multiscale micro/nanofluidic devices with ultralow aspect ratio nanochannels. The 3D manufacturing method enables the development of a nanochannel-based attoliter droplet generator capable of 3D parallelization for mass production, which implies the remarkable potential to extend numerous existing 2D micro/nanofluidic platforms into a 3D framework.

Performance characterization of transparent and conductive grids one-step-printed on curved substrates using template-guided foaming.

Next-generation electronic devices require electrically conductive, mechanically flexible, and optically transparent conducting electrodes (CEs) that can endure large deformations. However, patterning conditions of such CEs have been mainly limited to flat substrates because of the nature of conventional fabrication techniques; thus, comprehensive studies are needed to be conducted on this topic. Herein, we characterize the material and structural properties of CEs, curvature of substrates, and their operational performance. We use a single-step printing method, termed template-guided foaming (TGF), to fabricate flexible transparent conducting electrodes (FTCEs) on various substrates with initial curvatures. We adopted silver nanowires (AgNWs) and a conductive polymer (PEDOT:PSS) to characterize and compare the effect of initial substrate curvatures on the sheet resistance during inward and outward bending. The AgNW-based grids exhibited a considerably low sheet resistance, which was linearly proportional to the working curvature of the substrate, whereas PEDOT:PSS-based grids exhibited a relatively higher sheet resistance, which increased regardless of the initial and working curvatures of the substrate. Although both CE grids exhibited a high flexibility and transmittance during 10 000 cyclic tests, the initial curvature of the substrate affected the sheet resistance; hence, operational conditions of FTCEs must be considered to improve the repeatability and durability of such FTCE-integrated devices. Finally, we believe that our study introduces a novel methodology for the design, fabrication, and operation strategy of flexible electronic devices and wearable devices with high performances.

Full-Fiber Auxetic-Interlaced Yarn Sensor for Sign-Language Translation Glove Assisted by Artificial Neural Network.

Yarn sensors have shown promising application prospects in wearable electronics owing to their shape adaptability, good flexibility, and weavability. However, it is still a critical challenge to develop simultaneously structure stable, fast response, body conformal, mechanical robust yarn sensor using full microfibers in an industrial-scalable manner. Herein, a full-fiber auxetic-interlaced yarn sensor (AIYS) with negative Poisson's ratio is designed and fabricated using a continuous, mass-producible, structure-programmable, and low-cost spinning technology. Based on the unique microfiber interlaced architecture, AIYS simultaneously achieves a Poisson's ratio of-1.5, a robust mechanical property (0.6 cN/dtex), and a fast train-resistance responsiveness (0.025 s), which enhances conformality with the human body and quickly transduce human joint bending and/or stretching into electrical signals. Moreover, AIYS shows good flexibility, washability, weavability, and high repeatability. Furtherly, with the AIYS array, an ultrafast full-letter sign-language translation glove is developed using artificial neural network. The sign-language translation glove achieves an accuracy of 99.8% for all letters of the English alphabet within a short time of 0.25 s. Furthermore, owing to excellent full letter-recognition ability, real-time translation of daily dialogues and complex sentences is also demonstrated. The smart glove exhibits a remarkable potential in eliminating the communication barriers between signers and non-signers.

Pervaporation-assisted in situ formation of nanoporous microchannels with various material and structural properties.

Nanoporous structures are crucial for developing mixed-scale micro-/nanofluidic devices because they facilitate the manipulation of molecule transport along the microfluidic channel networks. Particularly, self-assembled particles have been used for fabricating various nanoporous membranes. However, previous self-assembly mechanisms relied on the material and structural homogeneities of the nanopores. Here, we present a pervaporation-assisted in situ fabrication method that integrates nanoporous membrane structures into microfluidic devices. The microfluidic devices contain a control-channel layer at the top, which induces local and addressable pervaporation, and the main-channel layer, which is present at the bottom with pre-designated locations for nanoporous microchannels; the layers are separated using a gas-permeable film. The target particle suspensions are loaded into the main channels, and their pervaporation is controlled through the gas-permeable film, which successfully assembles the particles at the pre-designated locations. This method yields nanoporous microchannels with various material and structural properties by fabricating heterogeneous nanopore arrays/junctions in series and other diverse structures along the microchannels. We validate the basic working principle of microfluidic devices containing nanoporous microchannels. Furthermore, we theoretically analyze the fundamental experimental results, which suggest the remarkable potential of our strategy to fabricate nanopore networks without using conventional nanofabrication methods.

Direct Single-Step Printing of Conductive Grids on Curved Surfaces Using Template-Guided Foaming.

Advanced transparent conductors have been studied intensively in the aspects of materials, structures, and printing methods. The material and structural advancements have been successfully accomplished with various conductive nanomaterials and spring-like structures for better electrical conductivity and high mechanical flexibility of the transparent conductors. However, the capability to print submicrometer conductive patterns directly and conformally on curved surfaces with low processing cost and high throughput remains a technological challenge to achieve, primarily because of the original two-dimensional (2D) nature of conventional lithography processes. In our study, we exploit a liquid-mediated patterning approach in the development of flexible templates, enabling printing of curvilinear silver grids in a single-step and strain-free manner at a submicrometer resolution within several minutes with minimum loss of noble metals. The template can guide arrays of receding liquid-air interfaces on curved substrates during liquid evaporation, thereby generating ordered 2D foam structures that can confine and assemble silver nanoparticles in grid patterns. The printed silver grids exhibit suitable optical, electrical, and Joule-heating performances, enabling their application in transparent heaters. Our technique has the potential to extend the existing 2D micro/nanofluidic liquid-mediated patterning approach to three-dimensional (3D) control of liquid-air interfaces for low-cost all-liquid-processed functional 3D optoelectronics in the future.

Double-Sided Microwells with a Stepped Through-Hole Membrane for High-Throughput Microbial Assays.

To improve the throughput of microwell arrays for identifying immense cellular diversities even at a single-bacteria level, further miniaturization or densification of the microwells has been an obvious breakthrough. However, controlling millions of nanoliter samples or more at the microscale remains technologically difficult and has been spatially restricted to a single open side of the microwells. Here we employed a stepped through-hole membrane to utilize the bottom as well as top side of a high-density nanoliter microwell array, thus improving spatial efficiency. The stepped structure shows additional effectiveness for handling several millions of nanoliter bacterial samples in the overall perspectives of controllability, throughput, simplicity, versatility, and automation by using novel methods for three representative procedures in bacterial assays: partitioning cells, manipulating the chemical environment, and extracting selected cells. As a potential application, we show proof-of-concept isolation of rare cells in a mixed ratio of 1 to around 106 using a single chip. Our device can be further applied to various biological studies pertaining to synthetic biology, drug screening, mutagenesis, and single-cell heterogeneity.

Multimodal and Covert-Overt Convertible Structural Coloration Transformed by Mechanical Stress.

Most materials and devices with structurally switchable color features responsive to external stimuli can actively and flexibly display various colors. However, realizing covert-overt transformation behavior, especially switching between transparent and colored states, is more challenging. A composite laminate of soft poly(dimethylsiloxane) (PDMS) with a rigid SiO2 -nanoparticle (NP) structure pattern is developed as a multidimensional structural color platform. Owing to the similarity in the optical properties of PDMS and SiO2 NPs, this device is fully transparent in the normal state. However, as their mechanical strengths differ considerably, upon compressive loading, a buckling-type instability arises on the surface of the laminate, leading to the generation of 1D or 2D wrinkled patterns in the form of gratings. Finally, an application of the device in which quick response codes are displayed or hidden as covert-overt convertible colored patterns for optical encryption/decryption, showing their remarkable potential for anticounterfeiting applications, is demonstrated.

Controlled open-cell two-dimensional liquid foam generation for micro- and nanoscale patterning of materials.

Liquid foam consists of liquid film networks. The films can be thinned to the nanoscale via evaporation and have potential in bottom-up material structuring applications. However, their use has been limited due to their dynamic fluidity, complex topological changes, and physical characteristics of the closed system. Here, we present a simple and versatile microfluidic approach for controlling two-dimensional liquid foam, designing not only evaporative microholes for directed drainage to generate desired film networks without topological changes for the first time, but also microposts to pin the generated films at set positions. Patterning materials in liquid is achievable using the thin films as nanoscale molds, which has additional potential through repeatable patterning on a substrate and combination with a lithographic technique. By enabling direct-writable multi-integrated patterning of various heterogeneous materials in two-dimensional or three-dimensional networked nanostructures, this technique provides novel means of nanofabrication superior to both lithographic and bottom-up state-of-the-art techniques.

Micro-/Nanofluidics for Liquid-Mediated Patterning of Hybrid-Scale Material Structures.

Various materials are fabricated to form specific structures/patterns at the micro-/nanoscale, which exhibit additional functions and performance. Recent liquid-mediated fabrication methods utilizing bottom-up approaches benefit from micro-/nanofluidic technologies that provide a high controllability for manipulating fluids containing various solutes, suspensions, and building blocks at the microscale and/or nanoscale. Here, the state-of-the-art micro-/nanofluidic approaches are discussed, which facilitate the liquid-mediated patterning of various hybrid-scale material structures, thereby showing many additional advantages in cost, labor, resolution, and throughput. Such systems are categorized here according to three representative forms defined by the degree of the free-fluid-fluid interface: free, semiconfined, and fully confined forms. The micro-/nanofluidic methods for each form are discussed, followed by recent examples of their applications. To close, the remaining issues and potential applications are summarized.

Long-Term and Programmable Bacterial Subculture in Completely Automated Microchemostats.

A controllable microchemostat can provide an ideal, powerful means to study the growth behavior of microorganisms by improving conventional macroscale chemostat. However, a challenge remains for implementing both continuous growth and active population control of microorganisms at the same time because they keep communicating with nearby culture environments by regulating their metabolism. Here, we present a novel microchemostat that enables reversible bacterial isolation, continuous chemical refreshment, and dynamic physicochemical stimulation. The microchemostat not only controls bacterial growth and subculture conditions in a completely automated and programmed manner but it also makes it possible to manipulate bacterial populations from a single bacterium to an ultrahigh density for long-term subculture periods with ultralow reagent consumption. Moreover, the microchemostat enables in situ measurement and feedback control of bacterial growth and population through various subculture programming modes that are sequentially performed using a single microchemostat over 720 h; to the best of our knowledge, this is the longest microchemostat culture of bacterial cells reported to date. Hence, we ensure that the microchemostat can be further applied to a wide range of microbial studies on a single chip, such as nutrient optimization, genetic induction, environmental selection, high-throughput screening, and evolutionary adaptation.