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Technology hidden in paper: How to use capillary action to drive liquid flow?
With the advancement of technology, we are always looking for easier, lower-cost and more portable diagnostic tools. In recent years, the development of paper-based microfluidic technology is one of the answers to this need. This technology builds on traditional lateral flow testing and revolutionizes medical diagnostics by utilizing capillary action to allow liquids to flow along designed channels in a porous paper substrate.
Paper-based microfluidic devices consist of a series of hydrophilic fibers through which fluids are transported and thus controlled in a passive manner.
Structure of paper microfluidics
The architecture of a paper microfluidic device mainly includes inlet, channel, flow amplifier, flow resistance, obstacles and outlet. These elements work together to allow liquid to flow efficiently inside the device. The initial inlet is a substrate (usually cellulose), and the channels are composed of a hydrophilic submillimeter network that directs the flow of liquid.
Flow principle and control
The movement of fluids in porous media such as paper is affected by permeability, geometry, and evaporative effects. Capillary action plays a key role in this process. The liquid flows automatically through capillary action without external pressure driving. In effect, paper becomes the medium for transporting liquids, and with precise design, a stable flow of liquids can be achieved.
Capillary flow during the wetting period can be approximated by Washburn's equation, and subsequent flow is laminar.
Exploration of manufacturing methods
There are various methods for manufacturing paper-based microfluidic devices, including wax printing, inkjet printing, photolithography, etc. Each of these techniques has advantages and disadvantages, ranging from the low-cost and rapid production of wax prints to the high-precision design of photolithography, all of which strive to design suitable channels for liquid flow. Wax printing, for example, uses hot-melt wax to create channels on paper, which can be done quickly and cost-effectively, albeit at a lower resolution.
Expansion of application fields
The application of paper microfluidic technology is not limited to medical diagnosis, but also extends to areas such as environmental testing and food safety. Due to their compact and lightweight nature, these devices are particularly suitable for use in resource-limited environments. What's more, these paper-based devices typically cost less than traditional microfluidic technologies, making them much more accessible worldwide.
However, with so many advantages, how to further improve the accuracy and reliability of this technology in various applications remains an unsolved problem.
Changes in Diagnostics
In diagnostic applications, paper-based microfluidic devices are designed to produce low-cost and user-friendly point-to-point devices that can be operated without the assistance of specialized personnel. This not only meets the demand standards proposed by the World Health Organization, but also helps improve diagnostic efficiency in resource-poor or remote areas. However, this device still faces two major challenges, one is user acceptance, and the other is how to simplify the operation process.
Conclusion
Although the potential of paper microfluidic technology is undoubtedly huge, its application and popularization still depend on continuous technological innovation and improvement of user experience. How will future developments change the way medical diagnosis is performed in our daily lives?
