Conventional magnetic particle-based assays involve the step-by-step transfer of reagents and washing buffers between sample vials, offering high capture efficiencies and minimal particle loss. However, they are batch processes and suffer from having multiple manual steps that are both laborious and time-consuming. Researchers in Turkey, the UK and the Netherlands have collaborated to develop a magnetic particle-based assay platform in which functionalised magnetic particles are transferred sequentially through laminated volumes of reagents and washing buffers, significantly reducing assay times.
Magnetic microparticles can be used for assays in place of conventional enzymes (ELISA), radioisotopes (RIA) or fluorescent moieties (fluorescent immunoassays). Typically consisting of a core of iron oxide nanoparticles in a polymer or silica matrix, the microparticles exhibit superparamagnetic properties, meaning that they are only magnetic in the presence of a magnetic field. When the field is removed, they lose their magnetic properties and can redisperse in a solution. Their small size yields high surface-to-volume ratios making them suitable for miniaturisation of assays.
Phaseguides are lines of microstructures that are incorporated into the floor of a microfluidic channel. Around one-quarter the height of the microchannel, they direct the advancing air-liquid interface as a solution is introduced into the device, with the meniscus being pinned on the phaseguide. The advancing liquid is unable to cross between boundaries until the section of chip bordered by the phaseguide is full. This allows phaseguides to be designed to precisely control the route the liquid takes through the microfluidic device.
Researchers at Cankaya University in Turkey, the University of Hull in the UK, and Mimetas and the Leiden Academic Centre for Drug Discovery in the Netherlands, have used phaseguide technology to produce a miniaturized magnetic microparticle assay and tested it in two standard clinical diagnostic assays.
The design of the microfluidic device featured a rectangular chamber 4 mm long x 10 mm wide that was split into five lanes. Each lane was fed by a single inlet channel, with air vent channels to allow air to be expelled during the filling of the system, and featured phaseguides that were designed to direct the advancing air-liquid through the chip. All solutions for the experiments were prepared using high purity water (18.2 MΩ cm at 25 °C) from an ELGA® LabWater purification system.
The microfluidic device was filled with liquids one lane at a time by pipetting aqueous solutions into the inlet holes. Microparticles were moved through a series of reagents and washing steps in the different channels using a large magnet. Images of the particles were captured for fluorescence analysis before and after the magnet was moved.
A streptavidin-biotin binding assay and a C-reactive protein (CRP) immunoassay were performed as proof of concept of the chip’s functionality. Both are standard assays in clinical diagnostics. For the streptavidin-biotin binding assay, streptavidin-functionalised magnetic particles were moved through fluorescently-labelled biotin and a series of wash steps were performed. To perform a sandwich CRP assay, primary antibody-functionalised magnetic particles were moved through a solution of CRP, fluorescently-labelled secondary antibody and washing buffer.
The phaseguide-assisted liquid lamination platform was shown to successfully perform magnetic particle-based assays in both one-step (streptavidin–biotin) and two-step (CRP) formats. An increase in fluorescence intensity was seen once the beads had migrated through the biotin reagent, indicating successful binding of the biotin to the streptavidin-functionalised particles. Increased fluorescence at the end of the assay was seen with increased concentrations of CRP, demonstrating potential for the quantitative detection of CRP. Condensing multiple reaction and washing processes into a single step allowed both assays to be performed in less than 8 seconds.
The next steps in assay development would be investigation into other clinically relevant biomarkers and looking at real clinical samples, rather than standard solutions. The technique could also be extended to solid-phase extraction and nucleic acid assays, enabling DNA and RNA analysis from a sample matrix. Due to the speed of the process, the platform offers opportunities to perform clinical assays more efficiently. The platform shows potential for point-of-care analysis in particular as it does not require the use of a pumping mechanism, instead needing only manual pipetting of the reagents, making it promising as a portable system.
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