Towards All-Printed Lateral Flow Biosensors

Abstract

Lateral flow biosensors are analytical devices that detect biomaterials with physicochemical signals, such as optical signals. Unlike other biosensors, lateral flow biosensors are based on porous membranes, which use capillary force to transport biomaterials spontaneously. However, lateral flow biosensors are fabricated in batch mode, which means that membranes need to be cut from the rolls, pretreated, and assembled using a step-by-step process. Thus, there is a need for a more efficient manufacturing process. This thesis aims to accelerate the fabrication process by developing a method wherein the whole device is printed directly, including the printable substrates, as well as by developing a clog-free process for depositing expensive reagents. These novel printable porous media were developed using printing inks that contained various pigments and polymer binders. To this end, candidate formulations were screened from nine hundred inks formulations via wicking experiments. The results of these tests showed that the most promising formulations were based on calcium carbonates and latex polymers. This formulation was then used to develop printable porous media that can easily be printed into complex patterns, with changeable wicking speeds within each pattern. In addition, a bio- colorimetric assay of alkaline phosphates conducted on these porous media showed strong color signals that were comparable to the traditional membrane-based lateral flow strips. Clog-free printing processes were investigated by using a piezoelectric inkjet printer to print silica sols and six nanoparticle inks. The results of these tests showed that the vibration of the piezoelectric layer and the deposition of particles on the printhead surfaces induced clogging issues. Over time, the silica sols formed multilayer deposits on the print head surface, which subsequently detached due to the vibration of the piezoelectric layer. Consequently, these large sheets of silica clogged the nozzles during printing. This clogging issue was eliminated by adjusting the pH value of the silica sol inks to 3.1. The hydrophobic cationic polystyrene nanoparticles form a sub-monolayer on the printhead surface, which causes air entrainment and promotes air bubble adhesion into the interior of the print head surface when the piezoelectric layer deforms. Thus, alternate surface chemistries for the print head and ink particle surfaces may be required in order to print hydrophobic ink materials. Overall, this enhanced understanding of these clogging mechanisms helps to explain why printer performance varies when different particles are used.