# Interplay of the Mass Transport and Reaction Kinetics for Lateral Flow Immunoassay Integrated on Lab-on-Disc

## TL;DR

This paper introduces a new design parameter for improving the sensitivity of lateral flow immunoassays by integrating them with centrifugal microfluidic platforms.

## Contribution

The introduction of the Transport Reaction Constant (TRc), which incorporates film thickness to better predict surface-based assay performance.

## Key findings

- Integrating LFAs with a centrifugal Lab-on-Disc increases test sensitivity and lowers the limit of detection.
- The dwell time of the analyte at the test line is increased by retarding the liquid front using centrifugal force.
- The TRc parameter provides a scalable way to optimize diagnostic devices based on reaction kinetics.

## Abstract

What are the main findings?

Introduced a new Transport Reaction Constant (TRc) that extends
the traditional Damköhler number by incorporating film thickness, enabling
better prediction of surface-based assay performance.

Demonstrated that integrating lateral flow assays (LFAs) with a
centrifugal Lab-on-Disc (LFA-CD) improves sensitivity by both increasing
analyte dwell time at the test line and allowing for the increase of sample
volume.

What is the implication of the main finding?

TRc provides a generalizable design parameter for optimizing LFA
and other surface-reaction assays under flow, complementing existing
dimensionless numbers like Da.

The use of LFA-CD platforms facilitates the increase in test
sensitivity and lowering the limit of detection.

The presented approach provides a scalable, low-cost pathway to
prototype next-generation diagnostic devices whose microfluidic geometry and
fluidic controls can be optimized based on the analytes’ reaction kinetics.

Lateral Flow Assays (LFAs) are ubiquitous test platforms due to their affordability and simplicity but are often limited by low sensitivity and lack of flow control. The present work demonstrates the combination of LFAs with centrifugal microfluidic platforms that allows for enhancement of LFAs’ sensitivity via the increase in the dwell time of the analyte at the test line as well as by passing a larger sample volume through the LFA strip. The rate of advancement of the liquid front in the radially positioned NC strip is retarded by the centrifugal force generated on spinning disc; therefore, the dwell time of the liquid front above the test line of LFA is increased. Additionally, integrating a waste reservoir enables passive replenishment of additional sample volume increases total probed volume by approximately 20% (from 50 μL to 60 μL). Comprehensive analysis, including COMSOL multiphysics simulation, was performed to deduce the importance of parameters such as channel height (100–300 μm), disc spin rate (0–2000 rpm), and reaction kinetics (fast vs. slow binding kinetics). The analysis was validated by the experimental observation of the slower-reacting CD79b protein on the test strip. For slower-reacting targets like CD79b, fluorescence intensity increased by ~40% compared to the static LFA. A new merit number, TRc (Transport Reaction Constant), is introduced, which refines the traditional Damköhler number (Da) by including the thickness of the liquid layer (such as the height of the microchannel), which affects the final sensitivity of the assays and is designed to reflect the role channel height plays for surface-based assays (in contrast to the bulk assays).