In paper networks, the passive wicking action of the membranes moves fluids and analytes through the network to perform the chemistry of the assay. For some applications however, it may be beneficial to actively control the flow of the fluid to ensure the chemistry or process occurs as intended. A recently published communication in Lab on a Chip, “Paper on a disc: balancing the capillary-driven flow with a centrifugal force” by Hwang et al, demonstrates an interesting method for controlling the flow of fluid through a piece of paper on a disc.
The authors establish a model that predicts the how far a fluid will flow into a paper that is spinning by balancing the force of capillary action intrinsic to the paper with the centrifugal force due to the rotation of the paper on a disc (Figure 1). With this model, they are able to predict the final position of a fluid front for a paper spinning at a certain RPM. As one would expect, the faster a paper spins, the lower on the strip the equilibrium position is due to the increased centrifugal force (Figure 2).
Figure 1: (A) The authors' schematic for the balance of forces that influence in a spinning paper. (B) The plastic disc that holds the paper membranes for spinning in these experiments.
Figure 2: (A) The progression of the fluid front in a paper spun at contant RPM. (B) A plot of the progression of the fluid front for different RPM values. (C) A fit of the results of the experiments to the predicted model in the paper.
By using a computer controlled centrifuge with programmable routines, the authors were able to generate constant flow rates in the paper by varying the spin speed according the position of the fluid front. In addition, by a repeated spinning and wicking, the fluid can be run through the paper and drained out again. Processes such as these could prove useful for future disc devices that utilize immunochromatographic processes on paper strips. Overall, this paper provides an excellent groundwork for controlling flow in lab-on-a-disc devices, but it adds the need for an external centrifuge and controller. This controller removes the benefits of having a non-instrumented assay for point-of-care diagnostics, as it adds cost and complexity to the system.