Dilutions in Paper Networks
Precise sample dilution is a particular type of mixing, and one often required to perform chemical reactions and binding-based assays. In conventional microfluidic systems continuous dilution requires combining two fluid streams in a channel using expensive pumps and providing some means to mix the two fluids. We created a paper dilution circuit that mixes two fluids and allows control over the dilution factor by simply changing the shape of the paper.
A sample can be diluted accurately by controlling the relative addition of sample and diluent. By modifying the relative resistance of two inlet channels (in this case, by the length of each channel arm), their relative contributions can be controlled, allowing the creation of a dilution circuit.
The image shows dilution of a blue fluid (top leg) with a colorless buffer (right leg). The dilution factor is set by the relative flow rates of the two fluids. The flowrates are not set by pumps, but rather by the relative fluidic “resistances” of the two inlet legs according to Darcy’s Law. For a given paper material, the resistance is simply proportional to the length of the leg and the viscosity of the fluid. As the length of thedilution arm increases, the volumetric flow rate of the diluent decreases, leading to a reduced dilution factor in the common channel downstream. Serial dilutions are also possible by adding multiple dilution arms, allowing a wide range of dilutions without a single pump or pipetting step.
For more information see:
“Microfluidics without pumps: reinventing the T-sensor and H-filter in paper networks,” Lab on a Chip (2011).
Jennifer L. Osborn, Barry Lutz, Elain Fu, Peter Kauffman, Dean Y. Stevens, and Paul Yager.
Department of Bioengineering, University of Washington, Seattle, WA, USA