Coated magnetic beads are capable of interacting with and binding to a corresponding target within a sample. Binding specific biomarkers to the surface functional group present on the bead (e.g., streptavidin) ensures that the interaction is limited to specific cells. Recovery of material for further studies is greatly simplified when beads are concentrated from suspension, by means of an external magnet.
Properties of Magnetic Beads
Microbeads can access the minute scale-lengths of biological cells necessary for cell separation. The biomagnetic beads are functionalized with antibodies or proteins, which endow them with specificity. It is possible for a number of beads to attach to a single cell. Each cell is then confronted by the net impact of all beads binding to it. For micron size cells, the number of attached beads is likely to be in the range of tens.
During a biomagnetic separation process, a space varying field is generated from a magnetic source. Magnetizable objects - in this case, superparamagnetic beads - will move in the direction of the magnetic field gradient. Cells experience the net influence of the magnetic force over all beads attached to them and are drawn to the magnet. For flow-through applications, for instance, separation is possible by situating the magnetic source at an appropriate angle to the net hydrodynamic flow of the solution and the cells within it.
Utilizing beads of uniformly consistent size and surface adhesion is essential to reduce particle variability and improve the level of reproducibility within and between processes.
The magnetic force will determine the separation speed of the magnetic beads. In all cases, the magnetic force is a consequence of field gradients acting on magnetic moments. A magnetic force capable of separating cells without overwhelming the sample or damaging cells during the process is necessary. As such, a homogenous magnetic gradient ensures that particles further from the magnet in the solution are subject to sufficient magnetic force to draw them to the magnet, and particles closer to the magnet do not suffer from undue stress due to prolonged exposure to a high magnetic field, which can damage the cells.
For optimal performance, it is necessary to saturate the magnetic beads, ensuring a constant, fixed magnetic moment. Saturated beads will experience a dipolar interaction and will form chains or clusters. These clusters will be less affected by drag forces and will have a higher magnetic moment, allowing them to move faster than solitary beads.
This article explains the process of magnetic activated cell sorting. If you are interested in knowing more about this process, download our free basic guide to magnetic bead cell sorting:
Two primary systems can be applied for implementing magnetic bead cell separation: direct and indirect.
In a direct system, biomagnetic beads activated with an appropriate surface ligand are circulated directly into the sample containing the cell mixture. Beads bind with targeted cells during incubation, and are subsequently magnetically retrieved.
An indirect system is somewhat more complicated. Indirect processes proceed in several steps. Sensitized target cells are incubated with an affinity ligand. Following a wash step, the cells are then incubated with beads activated with a secondary surface ligand.
Selection of a particular system is reliant upon the purpose and ultimate goals of the procedure. Sufficient procedural variety exists to ensure an appropriate methodology can be applied in all cases.
Overall, the success of a magnetic bead cell separation process relies on (1) the selection of suitable biomagnetic beads and (2) the application of an appropriate magnetic force. Choosing the right beads will ensure specificity and reproducibility, while applying the correct magnetic force will increase bead capture, decrease process time, and minimize cell damage. Taken together, these two factors will play a key role in determining a protocol’s success, resulting in a separation process that is specific to the needs of a given application.
- The Character and Quality of Magnetic Bead Cell Separation Processes
- Microbeads/Nanoparticles for Cell Sorting
- Methods of Magnetic Cell-Sorting