12,13 Positive pressure capillary microfluidic devices are capable of complex operations with minimal user intervention, 14 operate with a small device footprint, and are compatible with robotic pipetting equipment. The structurally simplest form of capillary microfluidics use positive pressure generated by droplets deposited at different inlets and outlets to control the speed and direction of flow, with smaller droplets generating higher positive pressures and self-flowing via a conduit into larger droplets. There are many different approaches for using capillary effects for flow control in microchannel-based microfluidics. 6–11 Here, we review microchannel-based, deterministic capillary microfluidics. Porous capillary microfluidics rely primarily on stochastic capillary flow within a network of pores and are well reviewed elsewhere. Paper-based capillary microfluidics have potential for global health applications and have been widely adopted thanks to low cost and low entry barriers as new devices can rapidly be prototyped using cutters, scissors, or common printers. New descriptors such as self-powered, autonomous, advanced, integrated, and circuit have been associated with capillary microfluidics to reflect their evolving capabilities and inherent advantages related to minimally-instrumented operation and point-of-care testing.Īlthough the word capillary was initially proposed in the context of microfabricated Si microfluidics, 5 over the last decade, the term capillary microfluidics has become strongly associated with the use of porous materials such as paper-based (and to a lesser extent thread-based) devices. 4 Capillary microfluidics are often referred to as ‘passive’ because they typically afford no real-time control over flow, as opposed to ‘active’ devices with external peripheral control. Capillary effects are governed by the interplay between surface tension of a liquid and the geometry and surface chemistry of its solid support. 3 Capillary microfluidic devices use capillary effects (also called capillary action or capillary force) to manipulate liquids. 1,2 Although microfluidic chips are small, their operation often requires complex peripheral equipment so that instead of living up to their promise as lab-on-chip devices, the reality resembles ‘lab-around-a-chip’ limiting the application of microfluidics in point-of-care settings. 1 Introduction Microfluidic devices are miniaturized liquid handling systems with potential for biomedical applications because of their small sample and reagent volume requirements, potential for efficient mass transport to functionalized surfaces, ease of automation, low-cost, and disposability.
The combination of better analytical models and lower entry barriers (thanks to advances in rapid manufacturing) make CCs both a fertile research area and an increasingly capable technology for user-friendly and high-performance laboratory and diagnostic tests. Finally, we highlight recent developments in rapid prototyping of CCs and the benefits offered including speed, low cost, and greater degrees of freedom in CC design. We discuss applications of CCs starting with the most common usage in automating liquid delivery steps for immunoassays, and highlight emerging applications such as DNA analysis. We discuss the basic physical laws governing capillary flow, deconstruct CCs into basic circuit elements including capillary pumps, stop valves, trigger valves, retention valves, and so on, and describe their operating principle and limitations. We identify three distinct waves of development driven by microfabrication technologies starting with early implementations in industry using machining and lamination, followed by development in the context of micro total analysis systems (μTAS) and lab-on-a-chip devices using cleanroom microfabrication, and finally a third wave that arose with advances in rapid prototyping technologies. To both reflect recent experimental and conceptual progress, and distinguish from paper-based capillary microfluidics, we adopt the more recent terminology of capillaric circuits (CCs). Here, we review the history and progress of microchannel-based capillary microfluidics spanning over three decades. Capillary microfluidics can deliver liquids in a pre-programmed manner without peripheral equipment by exploiting surface tension effects encoded by the geometry and surface chemistry of a microchannel. Microfluidics offer economy of reagents, rapid liquid delivery, and potential for automation of many reactions, but often require peripheral equipment for flow control.
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