How would you detect a heart attack? There are some symptoms that might tell you that you are very likely having a heart attack. Although you might feel pain in the chest, shortness of breath or other known physical symptoms, that doesn’t mean you in are actually having one. Conversely, you may not experience these symptoms but an attack is well on its way. In addition to painful symptoms, an electrocardiogram can be used to further indicate if you’re having a heart attack, but it also isn’t always accurate. But what if you could detect a heart attack by monitoring cardiac specific biomarkers in the blood or saliva? Those attempts are well underway.
Detecting anything in bodily fluids is always a good time for our friend microfluidics to enter the scene. Even after the correct biomarker is established, it must be quantified in a quick and demonstrative way. While biomarker levels vary in statistically significant ways during disease, they may still occur at low levels, making detection very hard. John T McDevitt et al. fulfill this need in “A disposable bio-nano-chip using agarose beads for high performance immunoassays.” I actually worked in Dr. McDevitt’s lab while I was at Rice University, so I’m pretty familiar with his work, although I didn’t work on this project.
Agarose Beads
In this paper, Dr. McDevitt develops a system of agarose beads for immunoassays that can be implemented in a microfluidic system and tests it on C-reactive protein (CRP). CRP is a biomarker that is elevated during inflammation or heart attacks, which could make it useful when detecting or confirming a heart attack when used in conjunction with other biomarkers. This process uses a sandwich immunoassay, which we’ve seen before when detecting ovarian cancer. However, in this case, 280 µm agarose beads are used to house the immunoassay process. These beads are very porous, providing a large surface area to volume ratio, which affords many antibody-capturing locations to be stationed on and within the bead. Other beads made of polystyrene would only allow capture antibodies on the surface, and would not be able to detect as much of the biomarker in the same volume.
Protein-sensing Microarray
While the porosity of the bead allows the detection to be enhanced, the shape of the bead is also important in this immunoassay. This system is comprised of an array of microscale PDMS cups, each containing an agarose bead. The cups are shaped like inverted pyramids and feature a small hole at the bottom that allows for fluid to drain. This setup drives fluid through the beads by convection, as it is the only exit point for the fluid. In its simplest form, the system contains three stacked layers: the middle layer contains the microarray of inverted pyramids, which is surrounded by two PDMS layers containing the microfluidic injection and drainage channels. The middle layer is made of an epoxy, and actually originates from a lithography-produced silicon mold. The silicon mold looks exactly the same as the epoxy, but due to the cost and process to produce the silicon, it doesn’t make sense to use it in each system. Instead, it is used as a mold for PDMS, which itself is used to mold epoxy, giving us a copy of the original silicon. The resulting epoxy layer can be incorporated by irreversible covalent bonding to the PDMS layers.
CRP Detection with Agarose Beads
The agarose beads are first incubated with CRP specific capture antibodies, capturing the CRP as it is forced through the agarose beads. Detection antibodies are fluorescently tagged and are pumped through beads, specifically attaching to the anchored CRP. The fluorescence of the beads will correspond to CRP concentration in the sample. In order to calibrate the system properly, control beads are also included in the array which contain antibodies for another antigen. This system is able to produce a precise dose-response curve. Its limit of detection is about 1 ng/ml, which is far below the physiological range of CRP, but may be used while processing saliva, which must be diluted several times since it’s highly viscous. You can also see that the fluorescent green signal infiltrates the bead and is not limited to the surface. This demonstrates the porous agarose bead’s ability to capture more CRP and deliver a stronger signal.
Modelling the Microarray with Computational Fluid Dynamics
This system was also simulated using the computational fluid dynamics program COMSOL, and the results were similar to those found in the real world. Interestingly, there are some special considerations to think about when measuring the intensity of the beads. Not all flow through the array is equal. First, there is a bit of a pressure gradient across the beads such that the beads closest to the source experience the greatest pressure and those further away experience slightly lower pressure. The increased pressure causes more CRP and tagged antibodies through the beads resulting in a higher signal. Second, the size of the drain in each pyramid may vary, which would again affect the pressure experienced by each bead and its resulting intensity. Finally, any deviations in the bead shape will alter how it sits in the inverted pyramid. Once again, smaller beads sitting lower in the holes will experience higher pressures and signals.
Final Thoughts
This paper presents the final component in an immunoassay intended for microfluidic chips. While there are many other processes which must be carried out before the biomarker level can be measured, this method can also be used for a range of biomarkers and is only limited by the nature of the fluid and the concentration of the biomarker.
References
Du, N., Chou, J., Kulla, E., Floriano, P., Christodoulides, N., & McDevitt, J. (2011). A disposable bio-nano-chip using agarose beads for high performance immunoassays Biosensors and Bioelectronics, 28 (1), 251-256 DOI: 10.1016/j.bios.2011.07.027