Formation of Bubbles in a Multisection Flow-Focusing Junction

The formation of bubbles in a flow-focusing (FF) junction comprising multiple rectangular sections is described. The simplest junctions comprise two sections (throat and orifice). Systematic investigation of the influence on the formation of bubbles of the flow of liquid and the geometry of the junction identifies regimes that generate monodisperse, bidisperse, and tridisperse trains of bubbles. The mechanisms by which these junctions form monodisperse and bidisperse bubbles are inferred from the shapes of the gas thread during breakup: these mechanisms differ primarily by the process in which the gas thread collapses in the throat and/or orifice. The dynamic self-assembly of bidisperse bubbles leads to unexpected groupings of bubbles during their flow along the outlet channel.Chemistry and Chemical Biolog

Bubbles navigating through networks of microchannels

This paper describes the behavior of bubbles suspended in a carrier liquid and moving within microfluidic networks of different connectivities. A single-phase continuum fluid, when flowing in a network of channels, partitions itself among all possible paths connecting the inlet and outlet. The flow rates along different paths are determined by the interaction between the fluid and the global structure of the network. That is, the distribution of flows depends on the fluidic resistances of all channels of the network. The movement of bubbles of gas, or droplets of liquid, suspended in a liquid can be quite different from the movement of a single-phase liquid, especially when they have sizes slightly larger than the channels, so that the bubbles (or droplets) contribute to the fluidic resistance of a channel when they are transiting it. This paper examines bubbles in this size range; in the size range examined, the bubbles are discrete and do not divide at junctions. As a consequence, a single bubble traverses only one of the possible paths through the network, and makes a sequence of binary choices (“left” or “right”) at each branching intersection it encounters. We designed networks so that, at each junction, a bubble enters the channel into which the volumetric flow rate of the carrier liquid is highest. When there is only a single bubble inside a network at a time, the path taken by the bubble is, counter-intuitively, not necessarily the shortest or the fastest connecting the inlet and outlet. When a small number of bubbles move simultaneously through a network, they interact with one another by modifying fluidic resistances and flows in a time dependent manner; such groups of bubbles show very complex behaviors. When a large number of bubbles (sufficiently large that the volume of the bubbles occupies a significant fraction of the volume of the network) flow simultaneously through a network, however, the collective behavior of bubbles—the fluxes of bubbles through different paths of the network—can resemble the distribution of flows of a single-phase fluid.Chemistry and Chemical Biolog

Stretchable Microfluidic Radiofrequency Antennas

Highly stretchable and robust antennas are fabricated by injecting liquid metal into a microfluidic channel that consists of two types of silicone rubber with different stiffness. The resulting antennas exhibit high mechanical stability under strain, while retaining high stretchability; these antennas can be stretched by up to a tensile strain of 120% with little degradation in radiation efficiency.Chemistry and Chemical BiologyEngineering and Applied Science

Cofabrication: A Strategy for Building Multicomponent Microsystems

This Account describes a strategy for fabricating multicomponent microsystems in which the structures of essentially all of the components are formed in a single step of micromolding. This strategy, which we call “cofabrication”, is an alternative to multilayer microfabrication, in which multiple layers of components are sequentially aligned (“registered”) and deposited on a substrate by photolithography. Cofabrication has several characteristics that make it an especially useful approach for building multicomponent microsystems. It rapidly and inexpensively generates correctly aligned components (for example, wires, heaters, magnetic field generators, optical waveguides, and microfluidic channels) over very large surface areas. By avoiding registration, the technique does not impose on substrates the size limitations of common registrations tools, such as steppers and contact aligners. We have demonstrated multicomponent microsystems with surface areas exceeding 100 $cm^2$, but in principle, device size is only limited by the requirements of generating the original master. In addition, cofabrication can serve as a low-cost strategy for building microsystems. The technique is amenable to a variety of laboratory settings and uses fabrication tools that are less expensive than those used for multistep microfabrication. Moreover, the process requires only small amounts of solvent and photoresist, a costly chemical required for photolithography; in cofabrication, photoresist is applied and developed only once to produce a master, which is then used to produce multiple copies of molds containing the microfluidic channels. From a broad perspective, cofabrication represents a new processing paradigm in which the exterior (or shell) of the desired structures are produced before the interior (or core). This approach, generating the insulation or packaging structure first and injecting materials that provide function in channels in liquid phase, makes it possible to design and build microsystems with component materials that cannot be easily manipulated conventionally (such as solid materials with low melting points, liquid metals, liquid crystals, fused salts, foams, emulsions, gases, polymers, biomaterials, and fragile organics). Moreover, materials can be altered, removed, or replaced after the manufacturing stage. For example, cofabrication allows one to build devices in which a liquid flows through the device during use, or is replaced after use. Metal wires can be melted and reset by heating (in principle, repairing a break). This method leads to certain kinds of structures, such as integrated metallic wires with large cross-sectional areas or optical waveguides aligned in the same plane as microfluidic channels, that would be difficult or impossible to make with techniques such as sputter deposition or evaporation. This Account outlines the strategy of cofabrication and describes several applications. Specifically, we highlight cofabricated systems that combine microfluidics with (i) electrical wires for microheaters, electromagnets, and organic electrodes, (ii) fluidic optical components, such as optical waveguides, lenses, and light sources, (iii) gels for biological cell cultures, and (iv) droplets for compartmentalized chemical reactions, such as protein crystallization.Chemistry and Chemical Biolog

Dual Sacrificial Molding: Fabricating 3D Microchannels with Overhang and Helical Features

Fused deposition modeling (FDM) has become an indispensable tool for 3D printing of molds used for sacrificial molding to fabricate microfluidic devices. The freedom of design of a mold is, however, restricted to the capabilities of the 3D printer and associated materials. Although FDM has been used to create a sacrificial mold made with polyvinyl alcohol (PVA) to produce 3D microchannels, microchannels with free-hanging geometries are still difficult to achieve. Herein, dual sacrificial molding was devised to fabricate microchannels with overhang or helical features in PDMS using two complementary materials. The method uses an FDM 3D printer equipped with two extruders and filaments made of high- impact polystyrene (HIPS) and PVA. HIPS was initially removed in limonene to reveal the PVA mold harboring the design of microchannels. The PVA mold was embedded in PDMS and subsequently removed in water to create microchannels with 3D geometries such as dual helices and multilayer pyramidal networks. The complementary pairing of the HIPS and PVA filaments during printing facilitated the support of suspended features of the PVA mold. The PVA mold was robust and retained the original design after the exposure to limonene. The resilience of the technique demonstrated here allows us to create microchannels with geometries not attainable with sacrificial molding with a mold printed with a single material

Evaluation of Lateral and Vertical Dimensions of Micromolds Fabricated by a PolyJet™ Printer

PolyJet™ 3D printers have been widely used for the fabrication of microfluidic molds to replicate castable resins due to the ease to create microstructures with smooth surfaces. However, the microstructures fabricated by PolyJet printers do not accurately match with those defined by the computer-aided design (CAD) drawing. While the reflow and spreading of the resin before photopolymerization are known to increase the lateral dimension (width) of the printed structures, the influence of resin spreading on the vertical dimension (height) has not been fully investigated. In this work, we characterized the deviations in both lateral and vertical dimensions of the microstructures printed by PolyJet printers. The width of the printed structures was always larger than the designed width due to the spreading of resin. Importantly, the microstructures designed with narrow widths failed to reproduce the intended heights of the structures. Our study revealed that there existed a threshold width (wd′) required to achieve the designed height, and the layer thickness (a parameter set by the printer) influenced the threshold width. The thresholds width to achieve the designed height was found to be 300, 300, and 500 μm for the print layer thicknesses of 16, 28, and 36 μm, respectively. We further developed two general mathematical models for the regions above and below this threshold width. Our models represented the experimental data with an accuracy of more than 96% for the two different regions. We validated our models against the experimental data and the maximum deviation was found to be <4.5%. Our experimental findings and model framework should be useful for the design and fabrication of microstructures using PolyJet printers, which can be replicated to form microfluidic devices

Fabrication of Complex 3D Fluidic Networks via Modularized Stereolithography

Stereolithography (SL) 3D printing has been widely applied for the fabrication of microchannels with photocurable resins and hydrogels, albeit with limitations in complexity and dimensions of attainable microchannels due to inadvertent polymerization of trapped photoresin within the channel voids and difficulty in evacuating trapped photoresin from channels after printing. Herein, a novel approach to circumvent these limitations by modularizing the fluidic network into printable subunits and assembling the printed subunits to reconstruct the fluidic network is proposed. This approach is validated by fabricating 2D and 3D hierarchical branching networks, lattice fluidic networks, helical channels, and serpentine channels, all of which are difficult to fabricate by a single attempt of 3D printing. The proposed approach offered 1) improves channel dimensions (channel w = 75 μm and h = 90 μm) and 2) increases complexity of fluidic network (up to 36 branching points). The principle of this approach is applicable to any SL printer and photocurable material for the fabrication of 3D microchannels. This approach should find applications in engineering tissue constructs recapitulating the complex 3D architecture of their vasculatures.</p

Fabrication of Complex 3D Fluidic Networks via Modularized Stereolithography

Stereolithography (SL) 3D printing has been widely applied for the fabrication of microchannels with photocurable resins and hydrogels, albeit with limitations in complexity and dimensions of attainable microchannels due to inadvertent polymerization of trapped photoresin within the channel voids and difficulty in evacuating trapped photoresin from channels after printing. Herein, a novel approach to circumvent these limitations by modularizing the fluidic network into printable subunits and assembling the printed subunits to reconstruct the fluidic network is proposed. This approach is validated by fabricating 2D and 3D hierarchical branching networks, lattice fluidic networks, helical channels, and serpentine channels, all of which are difficult to fabricate by a single attempt of 3D printing. The proposed approach offered 1) improves channel dimensions (channel w = 75 μm and h = 90 μm) and 2) increases complexity of fluidic network (up to 36 branching points). The principle of this approach is applicable to any SL printer and photocurable material for the fabrication of 3D microchannels. This approach should find applications in engineering tissue constructs recapitulating the complex 3D architecture of their vasculatures.</p

3D-printed peristaltic pump kit

Commercially available pumps designed for microfluidics are usually bulky, expensive, and not customizable. Herein, we developed a cost-effective micro-peristaltic pump consisting of 3D-printed and off-the-shelf components. With the developed components, pumps with different sizes (as small as 50 mm by 20 mm by 28 mm), and varying operating flowrates (0.21 μL/min to 56.6 μL/min) were readily assembled. We also demonstrated the operation of this pump inside an incubator for 7 days, envisaging the use in the continuous flow culture. This low-cost miniaturized peristaltic pumps shall find the use in lab-on-a-chip, organs-on-a-chip, and point-of-care microfluidic devices.</p

Fabrication of Freestanding Multicellular Discs using Thermo-responsive Hydrogels

Biological tissues are multicellular constructs that possess anisotropic physical and chemical properties. To miniaturize them and resemble the phenomena at the cellular interfaces, multicellular discs with a tunable interface are useful analytical tools. To realize such a construct and observe its maturation, a fabrication method needs to be developed where the cell-laden graft can be cultured statically for weeks to months before it can be harvested for further analysis. In this work, we report a method to fabricate freestanding multicellular discs with a diameter of 500 µm. Such discs offer multicellular properties of biological tissue with a tunable interface. The fabricated discs enabled the total analysis of the phenomena at the interface from the observation of the cell maturation to the complete separation of cell populations in downstream analysis, which shall be potentially useful for fundamental biological studies with controlled chemical and physical environments.</p