Digital magnetic tagging for multiplexed suspension-based biochemical assays

Microarrays and suspension (or bead)-based technologies have attracted significant interest for their broad applications in high throughput molecular biology. However, the throughput of microarrays will always be limited by the array density and the slow diffusion of molecules to their binding sites. Suspension-based technologies, in which all the reactions take place directly on the surface of microcarriers functionalized with molecular probes, could offer true multiplexing due to the possibility of extending their detection capability by a straightforward expansion of the size of the chemical library of probes. To fully exploit their potential, the microcarriers must be tagged, but the number of distinct codes available from spectrometric/graphical/physical encoding methods is currently fairly limited. A digital magnetic tagging method based on magnetic microtags, which have been anisotropy engineered to provide stable magnetization directions which correspond to digital codes, is reported. The tags can be suspended in solution and functionalized with a variety of biological molecular probes. Magnetic tagging offers several benefits compared to the traditional optical encoding techniques currently employed. It offers minimal background signals, potential for a large number of distinct codes, miniaturization of devices, and the ability to write a code in situ. Experimental data showing the reading of individual magnetic microbars from samples comprising 50×20 μ m2 Ni elements, as well as micromagnetic simulations that show the feasibility of stray field detection, are presented. The stray fields of the magnetic microbars spanning a range of 60 mOe were detected by a microfabricated fluxgate sensor scanned in a raster fashion over the sample that was placed about 70 μm away. Free floating tags have also been fabricated for use in microfluidic systems. A magnetic lab-on-a-chip device could be used for tagging biomolecular probes for applications in genome sequencing, immunoassays, clinical diagnostics, drug discovery, and general pathogen detection and screening.