Paper-Based Microfluidics: DNA Detection via Wicking Distances of Microsphere Aggregates and the Effects of Laser-Etching on Wicking Speeds in Paper Channels

The development of patterning high-resolution microfluidic circuits onto cellulose paper in 2007 initiated widespread research into the use of the paper as a low-cost, easy-to-use alternative substrate over the glass and plastics of traditional microfluidics. Paper, as a porous hydrophilic material, naturally wicks fluid through itself, without the need for external pumps or power sources. The simplest paper-based devices are lateral flow devices, where liquid wicks in one direction along the paper strip. These are suitable for simple detection chemistries; however, for more complex reactions, devices that are more complicated are required, and this frequently entails the sequential delivery of different liquids or reagents to reaction zones. To achieve sequential delivery, one can manually deposit reagents at different times, or deposit them simultaneously and have them arrive at times dictated by their travel paths. Modifying channel lengths and widths is the easiest method, but is constrained by the device's footprint and available sample volume, as longer and wider channels will require larger volumes. Alternatively, one can attempt to modify the speed at which liquid wicks through the paper itself. This is predominantly dictated by the pore size of the paper and is typically modified by adding material to block the pores, slowing flow. Further, one can have the liquid flow outside the paper altogether in some kind of external capillary. This work, on the other hand, investigated the use of a CO2 laser to uniformly etch the surface of the paper. Depending on the degree of etching, this was able to both increase and decrease the wicking speed of water through paper channels. The simplest lateral flow devices are typically colorimetric, giving qualitative results. However, getting quantitative data can be quite a bit more difficult. Distance-based devices provide a user-friendly means of obtaining quantitative data without the need for any additional equipment, si