Magnetophoretic long jump of magnetic microparticles in an engineered magnetic stray field landscape for highly localized and large throughput on-chip fractionation

Polydisperse batches of magnetic microparticles are an obstacle to the realization of efficient and reliable diagnostic Lab-on-a-chip systems. We have developed a method to spatially sort magnetic particles according to their magnetophoretic properties using a fast, single-shot procedure. 

The demand for sensitive and rapid point-of-care testing is rising worldwide due to the experience with the recent COVID pandemic and an increased risk for future outbreaks. Magnetic particle-based sensing devices (often referred to as Lab-on-a-chip systems) could be a possible solution in this regard. Magnetic particles offer the advantage that they can be surface-functionalized with almost any chemical compound, allowing the capture of disease-specific bioentities from a patient sample. Secondly, their on-chip motion can be very precisely controlled remotely by using a combination of engineered micrometer-scaled magnetic stray field landscapes and homogenous macroscopic fields. Research has been conducted in this field for around 20 years, but still, market-ready devices are merely on the horizon.

An intricate challenge is the question of how to deal with the inherent polydispersity of commonly employed superparamagnetic beads, that is, particles with sizes, magnetic properties, and surface characteristics varying around a mean value. These variations typically decrease the reliability of a biodetection analysis. An initial sorting event, where particles of equal properties are separated into individually adressable fractions, is therefore a desirable preprocessing step.

Magnetic stripe domains of gradually increasing width and in-plane magnetization (upper schematic) are employed to sort spherical magnetic microparticles according to their specific characteristics. These could be particle size, magnetic content distribution, or particle surface properties. Experiments for differently-sized superparamagnetic particles immersed in water show a clear spatial separation above the magnetic stripe pattern (see lower microscope image). The separated particles can be used for further analysis in Lab-on-a-chip applications.

We have worked out a way to realize this on-chip fractionation of superparamagnetic beads based on a specifically designed magnetic stray field landscape and recently published our results in the journal Lab on a Chip. The field landscape is realized by utilizing ion bombardment induced magnetic patterning (IBMP) to fabricate in-plane magnetized stripe domains of increasing stripe width and an alternating head-to-head/tail-to-tail magnetization configuration (see upper panel in the image). Capturing commercially available Dynabeads inside the field landscape, we were successful in demonstrating a fast separation of differently-sized beads at varying locations on the magnetically patterned chip substrate (see lower panel in the image) by applying a short sequence (≈ 3 seconds) of external magnetic field pulses. The physical mechanism of this finding was analyzed in more detail using high-speed microscope imaging and field landscape simulations.

For the future, we aim to refine the field landscape design and investigate magnetic particles of equal size but different surface functionalizations to leap towards a robust sorting procedure that potentially could work by itself as a Lab-on-a-chip-enabling biodetection technique.

R. Huhnstock et al, Lab on a chip 26, 494-506 (2026)
DOI: 10.1039/D5LC01000D

Dr. Rico Huhnstock, AGE - Functional thin films