Combined Funnel, Concentrator, and Particle Valve Functional Element for Magnetophoretic Bead Transport Based on Engineered Magnetic Domain Patterns

We investigated the directed motion of superparamagnetic microparticles suspended in water above a magnetic stripe domain pattern in which the length of the stripes varies continuously and periodically. The recorded motion behavior of the particles opens up new possibilities for the precise diagnostic detection of pathogens in chip-based rapid tests.

By applying an external alternating magnetic field, we were able to move the particles laterally back and forth. Using an optical microscope with an attached high-resolution camera, we were able to observe a fascinating transformation of the particle arrangement: Starting from longer stripe domain lengths where the particles were forming a row, they were pushed together as they approached smaller stripe lengths, forming zigzag-shaped particle groups (clusters). This funnel-like behavior meant that only a limited number of particles could be moved within a group at the narrowest position with minimum stripe length. However, as the length of the stripe domains increased again after the narrowest position, the clusters could be dissolved back into particle rows. This interesting effect can potentially be used for disease detection: Assuming a pathogen is present in the liquid, which causes the particles to "stick" to each other (molecular bridges are formed) when the particles are in very close contact with each other, the clusters would not rearrange back into a row after leaving the narrowest stripe domain site. The change in the arrangement behavior would thus prove the presence of the pathogen. Another exciting observation was that at high frequencies for the applied alternating magnetic fields, the narrowest position is blocked, i.e. the incoming particles cannot get through and cause a sort of "particle jam". This behavior is similar to that of a valve, allowing us to demonstrate several important functionalities for particle transport-based rapid detection of pathogens on one and the same chip substrate.