The most widely used technique to design paper microfluidic devices consists in using wax to hydrophobize paper.
Fabrication method:
It is possible to define hydrophobic barriers in paper by selectively depositing wax using a printing device. The latter melts the wax and prints the wanted pattern. As the wax touches the paper, it solidifies and a hot-plate is then used to melt the wax again to make it flow vertically into the paper until it reaches its other side. This way, the solvents are fully confined. Solid-ink printers, using an hydrophobic wax instead of regular ink, used to be commercialized by Tektronics and Xerox and were ideal for this application but the brand has discontinued the production in 2017. Instead, it is possible to use a microdispenser with a heatable printhead (Figure 1). A particularity of this technique is the possibility to create fully enclosed channels as shown in Figure 2
Figure 1 : A microdispensing device [2]. Figure 2 : The different channel types that can be obtained by wax printing [3].
Upsides/Downsides:
This method is the most used in the literature on paper-based microfluidics, but most of the research was done with a solid-ink printer which is no longer commercialized. Using a microdispenser, it remains a simple and fast method, well-suited for both prototyping and production since no mask is required. It is also environmentally friendly as it only requires paper and wax. Yet, this method requires an extra heating step in order for the wax to penetrate into the paper and the wax spreads laterally as it penetrates diminishing the resolution.
References:
[1] A. W. Martinez, S. T. Phillips, G. M. Whitesides, and E. Carrilho, “Diagnostics for the Developing World: Microfluidic Paper-Based Analytical Devices,” Analytical Chemistry, vol. 82, no. 1, pp. 3–10, Jan. 2010.
[3] D. M. Cate, J. A. Adkins, J. Mettakoonpitak, and C. S. Henry, “Recent Developments in Paper-Based Microfluidic Devices,” Analytical Chemistry, vol. 87, no. 1, pp. 19–41, Nov. 2014.
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