Solve / 2024
Making scalability part of the mechanism.
THE CENTRAL QUESTION
Magnetic separation is widely used because a magnet can collect labelled cells, particles or contaminants without direct contact. The difficulty appears when the flow channel becomes larger or the magnet moves farther away: the field gradient that pulls a particle drops rapidly. Conventional systems therefore gain throughput by repeating small channels and magnets, which makes the equipment bulky. This work asks whether scale can be built into the transport mechanism instead.
A static magnet pulls most strongly near its surface. As the workspace grows, particles farther away experience much less force, so increasing channel size can reduce capture speed and reliability. Simply using a larger magnet does not remove this steep spatial decay.
The experiment places a microfluidic chip near a rotating cylindrical permanent magnet. Inside the chip, patterned nickel micromagnets create local landscapes that move as the broad field rotates. Magnetic particles gather into swarms and translate across the surface, including against a perpendicular liquid flow.
Many-particle simulations and experiments reveal two regimes. At moderate driving rates, swarms remain synchronized with the rotating landscape; at higher rates they slip and spread. Resolving that transition helps define the useful frequency range instead of reporting only the fastest isolated demonstration.
Across the modeled and experimental comparison range, the reported transport speed is roughly two orders of magnitude above gradient-based separation. A separate experiment in porcine blood shows that the mechanism still operates in a viscous, crowded fluid, although the motion is slower than in simpler liquids.
The experiments use one chip. The proposed next step is to place many chips around the same large rotating field, allowing parallel channels to share the expensive actuation volume. The illustrated system near 100 millilitres per minute is a roadmap, not a completed machine; field uniformity, shear, coagulation, coatings and long-duration blood compatibility remain open questions.