How can we specify a Biomagnetic Separation Process?

When a new CLIA-IVD kit is transferred from R&D to production, all the manufacturing protocols should be adapted to the new throughput and volume. Biomarker specifications, buffers and coating protocols would benefit from the cumulated experience in non-magnetic kits. Coupling an antibody to magnetic beads is quite similar to doing it in colloidal gold or latex particles. But the washing protocols using Biomagnetic Separation are something new. The use of classical (and dirty) centrifugation method makes not so much sense when we can use the magnetic properties of the beads. Similar reasoning applies to the use lateral flow filtration or other complex and time-consuming non-magnetic separation techniques.

But, even if using Biomagnetic Separation seems to be the obvious choice, they are some problems in practice. The first one is how to specify the process itself. When we discuss with IVD-kits manufacturers about their needs on magnetic bead separation, they usually refer to:

• Separation time: how long it takes to separate the solid phase from the buffer.

• Magnetic beads losses: what is the maximum amount of beads (and the coupled biomarkers) they accept to be lost during the process.

• Batch volume: what is the required size of the lot and, sometimes, how to make compatible the different scales required during the process (development, validation lots, full production).

• The need of avoiding irreversible aggregation: Having irreversible aggregation of the beads during the separation process force to expend a lot of resources to re-suspend them, and to check that the re-suspension is done properly. As each single kit (ml) should have the same characteristics, failure to do it correctly implies an increase in the variability.

However, all these parameters are ‘functional’: they describe the consequences of the biomagnetic separation, but does not describe the process itself. What it is missed here is asking ourselves about the key parameter that defines a biomagnetic separation process. What defines the behavior of the magnetic beads during the process?

The answer is quite simple, but usually overlooked. The key parameter in Biomagnetic Separation processes is the magnetic force. The magnetic beads move at a definite speed, determined by the net force resultant of the competition between magnetic force and the drag force. This late is caused by the buffer’s viscosity.

It is important to notice that we are talking about the Magnetic Force, not the magnetic field. We should a basic concept: Applying a perfectly spatially-uniform Magnetic Field over a magnetic bead does not generate Magnetic Force. It will just generate a torque that would align the bead’s magnetic moment with the applied field. If we wish to generate a Magnetic Force (and consequently a movement of the magnetic beads), that we need to apply is a Magnetic Field that is not uniform in space.

When we use a simple permanent magnet for separating magnetic beads in a vial, we can do it because the applied magnetic field is not uniform. Then it make no sense to try to define the process by the value of the magnetic field (usually expressed in Tesla or Gauss), as what is key is how it changes with distance.