A highly efficient extraction protocol for magnetic particles on a digital microfluidic chip

• Introduction of a new particle enrichment protocol on a dloc-device.
• The protocol was realized by a spatially fixed magnet with a permanent magnetic field.
• Operating conditions were examined for a proper execution of this protocol.
• An immunoassay was executed on the dloc, involving this protocol.
• Reaction dynamics were optimized by the aid of an innovative stirring process.

We present a novel, simple and highly efficient protocol for the extraction of magnetic particles out of individual droplets on a digital lab-on-chip in the presence of a spatially fixed magnet with a permanent magnetic field. In this approach, the particles were extracted from the droplet by the interplay of capillary, magnetic and electrowetting forces, thereby avoiding the use of mechanical components that would be needed for removing the magnet when particle resuspension is required. This droplet manipulation allowed the execution of very efficient and fast washing protocols on the digital microfluidic (DMF) platform. To demonstrate the effectiveness of this particle extraction protocol, an IgG immunoassay was implemented on the DMF platform. Our improved protocol reduced the overall assay variability to 3% coefficient of variation (CV) while all incubation and washing steps were automatically performed on-chip. In addition, the suspended magnetic particles allowed the introduction of a very efficient mixing strategy by using the magnetic particles as magnetic stirrers, resulting in an improvement of 90% in detection limit compared to a passive mixing strategy, solely based on diffusion.