By inkjet-printing an array of antibodies onto a glass slide with a non-stick polymer coating, the D4 assay diagnostic tool from Duke University is a self-contained test that detects low levels of antigens - the protein markers of a disease - from a single drop of blood.
By creating a sensitive, easy-to-use "lab on a chip," the researchers plan to bring rapid diagnostic testing to areas that lack access to standard lab-based diagnostic technologies. The platform is described in the Proceedings of the National Academy of Sciences.
The current gold standard for quantitative diagnostic tests is the enzyme-linked immunosorbent assay (ELISA), which identifies how many specific antigens are present in a biological sample.
The D4 assay reportedly allows clinicians to avoid problems associated with ELISA – such as ease of use, time taken for diagnosis, and bulky lab instruments - without sacrificing sensitivity or accuracy.
The new assay can quickly identify a disease biomarker, and results can be read using a tabletop scanner or 3D printed smartphone attachment that uses the phone's camera to read the results.
Like ELISA, the D4 assay uses a matched pair of antibodies to detect and capture a target protein in a blood sample. The array contains two types of antibodies - immobilised capture antibodies and soluble detection antibodies - which are tagged with a fluorescent marker to allow the researchers to identify how much of the antigen is present. When a drop of blood is placed on the slide, the detection antibodies dissolve, separate from the array and bind to the target proteins in the blood. These fluorescing antibody-protein pairs then attach to the capture antibodies that are still on the slide.
Unlike other diagnostic tests, D4 assay's antibody array is printed on a novel polymer brush coating. When a sample is placed on the slide, the coating prevents the non-target proteins from attaching to the surface of the slide. By preventing unwanted proteins from binding to the assay, the polymer brush makes it easier to detect low levels of target proteins by getting rid of 'background noise' on the chip.
"The polymer brush allowed us to store all of the tools we need on the chip while maintaining a simple design," said Ashutosh Chilkoti, chair of the Department of Biomedical Engineering (BME) at Duke and senior author on the paper.
Using the D4 assay, researchers don't need to follow a complicated workflow to clear non-target proteins from their slide, as with the ELISA test. Instead, they simply need to wash the slide in a buffer solution to remove any extraneous particles.
"What's cool is that our assay can achieve comparable sensitivity to the ELISA within 15 minutes, and if further sensitivity is needed, longer incubation times can be used," says Daniel Joh, an MD-PhD student in the Chilkoti lab and co-author of the paper.
The team at Duke estimate that the D4 chips will cost less than $1 and the mobile phone attachment developed at the University of California, Los Angeles will be less than $30 when produced in bulk.