Microfluidic Induced Controllable Microdroplets Assembly in Confined Channels

Advances in Nanoarchitectonics: A Review of "Static" and "Dynamic" Particle Assembly Methods

Particle assembly is a promising technique to create functional materials and devices from nanoscale building blocks. However, the control of particle arrangement and orientation is challenging and requires careful design of the assembly methods and conditions. In this study, the static and dynamic methods of particle assembly are reviewed, focusing on their applications in biomaterial sciences. Static methods rely on the equilibrium interactions between particles and substrates, such as electrostatic, magnetic, or capillary forces. Dynamic methods can be associated with the application of external stimuli, such as electric fields, magnetic fields, light, or sound, to manipulate the particles in a non-equilibrium state. This study discusses the advantages and limitations of such methods as well as nanoarchitectonic principles that guide the formation of desired structures and functions. It also highlights some examples of biomaterials and devices that have been fabricated by particle assembly, such as biosensors, drug delivery systems, tissue engineering scaffolds, and artificial organs. It concludes by outlining the future challenges and opportunities of particle assembly for biomaterial sciences. This review stands as a crucial guide for scholars and professionals in the field, fostering further investigation and innovation. It also highlights the necessity for continuous research to refine these methodologies and devise more efficient techniques for nanomaterial synthesis. The potential ramifications on healthcare and technology are substantial, with implications for drug delivery systems, diagnostic tools, disease treatments, energy storage, environmental science, and electronics.

Hierarchical Self-Assembly of Dipolar ZnO Nanoparticles and Microdroplets

In this work, we investigated the orientation and the polarization of ZnO nanoparticles, which serve as building blocks of highly monodisperse microspheres, using a droplet microfluidic-assisted synthesis method. We observe, for the first time, a square lattice organization of liquid microdroplets, in a steady state, at the oil/water interface. Such square organization reveals clearly a dipolar organization of ZnO nanoparticles at the surfaces of droplets at the early stage of ZnO nanocrystal aggregation and microsphere formation. We discuss different models of organization of ZnO nanoparticles and show that the well-known tip-streaming effect in droplets in microfluidics explains the reason for the obtained dipolar droplets. The square organization is illustrated and explained.

Microfluidic formation of crystal-like structures

Crystal-like structures find application in several fields ranging from biomedical engineering to material science. For instance, droplet crystals are critical for high throughput assays and material synthesis, while particle crystals are important for particles and cell encapsulation, Drop-seq technologies, and single-cell analysis. Formation of crystal-like structures relies entirely on the possibility of manipulating with great accuracy the micrometer-size objects forming the crystal. In this context, microfluidic devices offer versatile tools for the precise manipulation of droplets and particles, thus enabling fabrication of crystal-like structures that form due to hydrodynamic interactions among droplets or particles. In this review, we aim at providing an holistic representation of crystal-like structure formation mediated by hydrodynamic interactions in microfluidic devices. We also discuss the physical origin of these hydrodynamic interactions and their relation to parameters such as device geometry, fluid properties, and flow conditions.

Microfluidic Formation of Honeycomb-Patterned Droplets Bounded by Interface Bilayers via Bimodal Molecular Adsorption

Assembled water-in-oil droplets bounded by lipid bilayers are used in synthetic biology as minimal models of cell tissue. Microfluidic devices successfully generate monodispersed droplets and assemble them via droplet interface bilayesr (DIB) formation. However, a honeycomb pattern of DIB-bounded droplets, similar to epithelial tissues, remains unrealized because the rapid DIB formation between the droplets hinders their ability to form the honeycomb pattern. In this paper, we demonstrate the microfluidic formation of a honeycomb pattern of DIB-bounded droplets using two surfactants with different adsorption rates on the droplet surface. A non-DIB forming surfactant (sorbitan monooleate, Span 80) was mixed with a lipid (1,2-dioleoyl-sn-glycero-3-phosphocholine, PC), whose adsorption rate on the droplet surface and saturated interfacial tension were lower than those of Span 80. By changing the surfactant composition, we established the conditions under which the droplets initially form a honeycomb pattern and subsequently adhere to each other via DIB formation to minimize the interfacial energy. In addition, the reconstituted membrane protein nanopores at the DIBs were able to transport molecules. This new method, using the difference in the adsorption rates of two surfactants, allows the formation of a honeycomb pattern of DIB-bounded droplets in a single step, and thus facilitates research using DIB-bounded droplet assemblies.

Droplets as Carriers for Flexible Electronic Devices

Coupling soft bodies and dynamic motions with multifunctional flexible electronics is challenging, but is essential in satisfying the urgent and soaring demands of fully soft and comprehensive robotic systems that can perform tasks in spite of rigorous spatial constraints. Here, the mobility and adaptability of liquid droplets with the functionality of flexible electronics, and techniques to use droplets as carriers for flexible devices are combined. The resulting active droplets (ADs) with volumes ranging from 150 to 600 µL can conduct programmable functions, such as sensing, actuation, and energy harvesting defined by the carried flexible devices and move under the excitation of gravitational force or magnetic force. They work in both dry and wet environments, and adapt to the surrounding environment through reversible shape shifting. These ADs can achieve controllable motions at a maximum velocity of 226 cm min-1 on a dry surface and 32 cm min-1 in a liquid environment. The conceptual system may eventually lead to individually addressable ADs that offer sophisticated functions for high-throughput molecule analysis, drug assessment, chemical synthesis, and information collection.

Analyte capture in an array of functionalized droplets for a regenerable biosensor

We describe in this work an advanced microfluidic chip for the capture of bioanalyte on the surface of droplets arranged in a dense array. We show the procedure for generating, functionalizing, and arranging the droplets inside the device for capturing a specific bioanalyte. Then, we demonstrate the capacity of the array to capture analyte from a cross-flowing liquid, using a biotin/streptavidin model. The paper also proposes to use the droplets array, after integration with acoustic detection, as a regenerable detection interface for bioanalyte sensing. We model the arrangement of droplet in dense array and show that they present a larger effective capture surface and shorter capture distance than standard flat surface biosensor of the same footprint. As the droplets can be easily evacuated and replaced inside the device analysis chamber, the proposed biosensor would allow biointerface regeneration and chain measurement without dismounting the device.

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.

Self-assembly of droplets in three-dimensional microchannels

Self-assembly of droplets guided by microfluidic channels have potential applications ranging from high throughput assays to materials synthesis, but such demonstrations have been limited primarily to two-dimensional (2D) assembly of droplets in planar microfluidic devices. We demonstrated the use of three-dimensional (3D) microchannels to self-assemble droplets into ordered 2D and 3D arrays by designing microchannels with axial gradients in height and controlling the volume fraction of the droplets in the channel. In contrast to previous demonstrations, ordered 2D arrays of droplets were assembled at low volume fractions of the dispersed phase. Interestingly, we found that the self-assembly of droplets in microchannels was highly path dependent. The assembly of droplets was governed by transitions in the cross-sectional shapes of the microchannel, not the final geometry of the chamber for the assembly of droplet, which is a hitherto rarely explored phenomenon. The assembled droplets were used as templates for the fabrication of millimeter scale, anisotropic hydrogel fibers with ordered pore sizes (∼250 μm). These demonstrations suggested that 3D microchannels would be a viable platform for the manipulation of droplets, and applicable for the continuous synthesis of complex materials with 3D morphologies.

DROPLAY: laser writing of functional patterns within biological microdroplet displays

In this study, we introduce an optofluidic method for the rapid construction of large-area cell-sized droplet assemblies with user-defined, re-writable, two-dimensional patterns of functional droplets. Light responsive water-in-oil droplets capable of releasing fluorescent dye molecules upon exposure were generated and self-assembled into arrays inside a microfluidic device. This biological architecture was exploited by the scanning laser of a confocal microscope to 'write' user defined patterns of differentiated (fluorescent) droplets in a network of originally undifferentiated (non-fluorescent) droplets. As a result, long lasting images were produced on a droplet fabric with droplets acting as pixels of a biological monitor, which can be erased and re-written on-demand. Regio-specific light-induced droplet differentiation within a large population of droplets provides a new paradigm for the rapid construction of bio-synthetic systems with potential as tissue mimics and biological display materials.