Capillary microfluidics in microchannels: from microfluidic networks to capillaric circuits

Abstract

Microfluidics offer economy of reagents, rapid liquid delivery, and potential for automation of many reactions, but often require peripheral equipment for flow control. 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. Here, we review the history and progress of microchannel-based capillary microfluidics spanning over three decades. 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). 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. 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 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. 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. 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.

Advanced Wearable Microfluidic Sensors for Healthcare Monitoring

“…Passive microfluidic networks pump liquids via capillary forces generated by µ‐channels or osmotic pressure generated by hydrogel materials . The absence of control systems or mechanical moving parts leads to a highly minimalized system, and the analysis chamber can be located close to the inlets of liquids, combining the advantages of simplicity in manufacturing and low power consumption …”

“…CBVs can be molded with the same manufacturing material as that of the microfluidic networks and can be easily adjusted in terms of their geometry . Furthermore, utilizing a series of CBVs, a microfluidic system can manipulate liquids in a preprogrammed manner and realize time‐dependent sampling, which has great potential in capturing chronographic physiological information …”

Inner Surface Design of Functional Microchannels for Microscale Flow Control

“…Plasma technique offers a simple and effective way of direct implementation of microchannel functionalization . Figure E shows the schematic illustration of oxygen, UV/ozone (UVO), and fluorocarbon plasma treatment of Si/SiO 2 /polymer substrates.…”

“…Recently, they realized gas–water separation by simply constructing obstacle microstructures in the microchannels . Construction of hydrophobic obstacle microstructure in the microchannel increased the energy barrier for the flow of water, which could be explained by the Young–Laplace equation and the Gibbs inequality effect . Because of the large critical Laplace force in the outlet channel with microstructure, the water was guided into outlet channel without microstructure under a driving pressure of critical pressure, and gas flowed into another outlet channel (Figure B).…”

“…Plasma technique offers a simple and effective way of direct implementation of microchannel functionalization. [46] Figure 2E shows the schematic illustration of oxygen, UV/ozone (UVO), and fluorocarbon plasma treatment of Si/SiO 2 /polymer substrates. When the microchannel surfaces are exposed to oxygen/UVO plasma, high energy oxygen radicals are generated and bombarding the substrate surface to oxidize it and render it hydrophilic, which provides the microchannel with capillary filling capability.…”