Abstract
This work presents a heterogeneous immunoassay using the integrated functionalities of a channel and droplets in a digital microfluidic (DMF) platform. Droplet functionality in DMF allows for the programmable manipulation of discrete sample and reagent droplets in the range of nanoliters. Pressure-driven channels become advantageous over droplets when sample must be washed, as the supernatant can be thoroughly removed in a convenient and rapid manner while the sample is immobilized. Herein, we demonstrate a magnetic bead-based, enzyme-linked immunosorbent assay (ELISA) using ~60 nL of human interleukin-6 (IL-6) sample. The wash buffer was introduced in the form of a wall-less virtual electrowetting channel by a syringe pump at the flow rate of 10 μL/min with ~100% bead retention rate. Critical parameters such as sample wash flow rate and bead retention rate were optimized for reliable assay results. A colorimetric readout was analyzed in the International Commission on Illumination (CIE) color space without the need for costly equipment. The concepts presented in this work are potentially applicable in rapid neonatal disease screening using a finger prick blood sample in a DMF platform.
Highlights
Immunoassay is a standard approach in biomedical research and clinical diagnostics for measuring target analytes that relies on antibody–antigen binding
The diagnostics of cardiac diseases [1,2], sepsis [3], malaria [4] and traumatic brain injury [5] rely on immunoassays to detect and quantify the disease markers
This corresponds to a ~15X reduction in sample volume compared with a typical enzyme-linked immunosorbent assay (ELISA) in digital microfluidic (DMF)
Summary
Immunoassay is a standard approach in biomedical research and clinical diagnostics for measuring target analytes that relies on antibody–antigen binding. Supernatant separation followed by droplet-based sample washing has been proven to be highly efficient in reducing the background signal on the DMF chip, and could be used in various immunoassay applications [22]. These DMF-based ELISA rely on photomultiplier tubes (PMTs)—a costly piece of equipment that may not be accessible in resource-limited regions—for chemiluminescent detection due to their higher sensitivity. To reduce the need for blood transfusions for newborns, this assay shows that only ~60 nL of sample can be enough for a single ELISA This corresponds to a ~15X reduction in sample volume compared with a typical ELISA in DMF. The developed ELISA protocol can be adapted to a wide range of heterogeneous immunoassays in DMF, and shows the potential of using DMF devices for rapid diagnostics in newborns [18] using nL quantities of sample
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