Physics:Open microfluidics

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Microfluidics refers to the flow of fluid in channels or networks with at least one dimension on the micron scale.[1][2] In open microfluidics, also referred to as open surface microfluidics or open-space microfluidics, at least one boundary confining the fluid flow of a system is removed, exposing the fluid to air or another interface such as a second fluid.[1][3][4]

Types of open microfluidics

Open microfluidics can be categorized into various subsets. Some examples of these subsets include open-channel microfluidics, paper-based, and thread-based microfluidics.[1][5][6]

Open-channel microfluidics

In open-channel microfluidics, a surface tension-driven capillary flow occurs and is referred to as spontaneous capillary flow (SCF).[1][7] SCF occurs when the pressure at the advancing meniscus is negative.[1] The geometry of the channel and contact angle of fluids has been shown to produce SCF if the following equation is true.

[math]{pf \over pw}\lt  cos(\theta)[/math]

Where pf is the free perimeter of the channel (i.e., the interface not in contact with the channel wall), and pw is the wetted perimeter[8] (i.e., the walls in contact with the fluid), and θ is the contact angle of the fluid on the material of the device.[1][5]

Paper-based microfluidics

Paper-based microfluidics utilizes the wicking ability of paper for functional readouts.[9][10] Paper-based microfluidics is an attractive method because paper is cheap, easily accessible, and has a low environmental impact. Paper is also versatile because it is available in various thicknesses and pore sizes.[9] Coatings such as wax have been used to guide flow in paper microfluidics.[11] In some cases, dissolvable barriers have been used to create boundaries on the paper and control the fluid flow.[12] The application of paper as a diagnostic tool has shown to be powerful because it has successfully been used to detect glucose levels,[13] bacteria,[14] viruses,[15] and other components in whole blood.[16] Cell culture methods within paper have also been developed.[17][18] Lateral flow immunoassays, such as those used in pregnancy tests, are one example of the application of paper for point of care or home-based diagnostics.[19] Disadvantages include difficulty of fluid retention and high limits of detection.

Thread-based microfluidics

Thread-based microfluidics, an offshoot from paper-based microfluidics, utilizes the same capillary based wicking capabilities.[20] Common thread materials include nitrocellulose, rayon, nylon, hemp, wool, polyester, and silk.[21] Threads are versatile because they can be woven to form specific patterns.[22] Additionally, two or more threads can converge together in a knot bringing two separate ‘streams’ of fluid together as a reagent mixing method.[23] Threads are also relatively strong and difficult to break from handling which makes them stable over time and easy to transport.[21] Thread-based microfluidics has been applied to 3D tissue engineering and analyte analysis.[24][25]


One of the main advantages of open microfluidics is ease of accessibility which enables intervention (i.e., for adding or removing reagents) to the flowing liquid in the system.[26] Open microfluidics also allows simplicity of fabrication thus eliminating the need to bond surfaces. When one of the boundaries of a system is removed, a larger liquid-gas interface results, which enables liquid-gas reactions.[1][27] Open microfluidic devices enable better optical transparency because at least one side of the system is not covered by the material which can reduce autofluorescence during imaging.[28] Further, open systems minimize and sometimes eliminate bubble formation, a common problem in closed systems.[1]

In closed system microfluidics, the flow in the channels is driven by pressure via pumps (syringe pumps), valves (trigger valves), or electrical field.[29] An example of one of these methods for achieving low flow rates using temperature-controlled evaporation has been described for an open microfluidics system, allowing for long incubation hours for biological applications and requiring small sample volumes.[30] Open system microfluidics enable surface-tension driven flow in channels thereby eliminating the need for external pumping methods.[26][31] For example, some open microfluidic devices consist of a reservoir port and pumping port that can be filled with fluid using a pipette.[1][5][26] Eliminating external pumping requirements lowers cost and enables device use in all laboratories with pipettes.[27]


Some drawbacks of open microfluidics include evaporation,[32] contamination,[33] and limited flow rate.[4] Open systems are susceptible to evaporation which can greatly affect readouts when fluid volumes are on the microscale.[32] Additionally, due to the nature of open systems, they are more susceptible to contamination than closed systems.[33] Cell culture and other methods where contamination or small particulates are a concern must be carefully performed to prevent contamination. Lastly, open systems have a limited flow rate because induced pressures cannot be used to drive flow.[4]


Like many microfluidic technologies, open system microfluidics has been applied to nanotechnology, biotechnology, fuel cells, and point of care (POC) testing.[1][4][34] For cell-based studies, open-channel microfluidic devices enable access to cells for single cell probing within the channel.[35] Other applications include capillary gel electrophoresis, water-in-oil emulsions, and biosensors for POC systems.[3][36][37] Suspended microfluidic devices, open microfluidic devices where the floor of the device is removed, have been used to study cellular diffusion and migration of cancer cells.[5] Suspended and rail-based microfluidics have been used for micropatterning and studying cell communication.[1]


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