The control the fluid flow in paper channels is critical in realizing integrated assays where multi-step chemical reactions are performed. Thus far, this has often involved the use of dissolvable solutes, absorbable materials, or magnetic particles with paper substrates to regulate or switch fluid flow. There is scope to simplify existing techniques in various aspects: the fabrication and assembly of actuation, the reduction in response time, and with lower costs. 

In this article published in Lab on Chip, authors presented an actuator device made entirely out of chromatography paper and incorporated with folds. By selectively wetting the fold with a small volume of water (four microliters), they were able to raise or lower the actuator’s tip and thus engage or break the fluidic contact between paper fluidic channels. This principle was then used to create two configurations of switches: single-pole single-throw (SPST) and single-pole double-throw (SPDT). The SPST and SPDT terminology is commonly used in the classification of commercial switches. The ‘pole’ refers to the number of inter-channel fluid connections being controlled by the actuator at a given time, while the ‘throw’ refers to the extreme (i.e. ON) position of the actuator. As a proof-of- concept, an
integrated colorimetric assay was demonstrated to detect the presence of three analytes – glucose, protein, and nitrite – in artificial saliva. 

Figure 1  Simply add water to an all-paper actuator to establish physical connection between two paper channels (Image credit/copyright: Lab on a Chip, RSC publishing) 

The actuator is very easy to fabricate merely by paper cutting and folding. The actuator’s response time is only a few seconds and there is scope to choose multiple switch layouts depending on the user’s needs. Most importantly, since everything is constructed in paper without any extra components, there are the benefits of easy storage, usage, and integration with other paper microfluidic devices. The authors are looking into ways to incorporate these switches with practical paper microfluidic assays.

For more information and full text paper, please find from this link on Lab Chip.  For technical details, please contact to Prof.  Santosh Pandey from Department of Electrical and Computer Engineering at Iowa State University, Ames, USA.