Programming Sequential Reagent Delivering Using a Pseudo-One Dimensional Paper Network

Abstract

Lateral flow tests (LFTs) have been identified as a diagnostic technology well suited for point-of-care use in low resource settings. In addition to being rapid, affordable, disposable, and easy to use, fluid transport of sample and reagents through the device occurs due to the capillary pressure of the strip material, rather than through the use of pumps. While strip based LFTs are generally limited to performing a single fluidic step, LFTs can be reconfigured into two-dimensional paper networks (2DPNs) that automatically carry out multi-step fluidic operations. Development of a functional 2DPN, however, is largely based on an "estimate and check" method - a process that may require multiple iterations before achieving an appropriate device design. Here, we use simple electrical circuit analogies of capillary pressure, fluidic resistance, and volumetric flow rate to model the fluidic behavior of 2DPN devices and predict designs with the desired functionality. We identify key design principles for a previously-developed 2DPN and introduce an alternative linear network (Psuedo-1DPN) that is similar in appearance to the classic strip LFT, but can perform the discrete sequential fluid delivery required of a multi-step assay. Tuning the capillary pressure and geometry of the device, we can predict the experimentally observable release trends of multiple assay reagents. Furthermore, this model allows the ability to easily modify design parameters, such as reagent volume, spacing between delivery steps, and total number of delivery steps, such that the desired system behavior is achieved. Additionally, we quantitatively characterize the impact fluid source pad materials have on the reagent release through a strip, a critical tool for both identifying appropriate fabrication materials and predicting overall system behavior. We find that materials of different porosity and structure exhibit unique fluid release profiles; both the delivery rate and total percentage of fluidic content released over time will dramatically vary given the type of material used in device. Finally, we demonstrate the utility of this Pseudo-1DPN using a multi-step, enzyme-linked immunosorbent (ELISA) based assay for human immunodeficiency virus (HIV) p24 antigen.