Magnetic beads and separators have grown in popularity as a biotechnological tool over the past few decades. As we gained experience with the technology, we learned that all magnetic separators are not created equal. It turns out that it isn’t just the magnitude of the magnetic force that improves separation time, it is the way it spreads out over the working area that matters the most. A homogeneous magnetic separator is designed to ensure that every point within the working volume experiences the same magnetic force. This is especially important when scaling up a biomagnetic separation process from milliliters to liters.
Effect of a magnetic field on a paramagnetic nanoparticle
One of the most important magnetic properties of nanoparticles is their superparamagnetism. This means that they have no residual magnetism when they are in a zero magnetic field. Once they are placed into a magnetic field their magnetic dipoles align with the field lines and they become magnetized. The transition between magnetized and demagnetized is close to instantaneous due to the small size of the nanoparticles.
The strength of a magnetic field rapidly decreases as distance from the magnet increases. This means that every permanent magnet will set up a magnetic field gradient.
The classic magnetic separator used in biological applications is simply a bar magnet that a test tube is placed next to. The magnetic particles are in solution within the test tube. The particles closest to the magnet will rapidly move down the gradient toward the magnet, while those farther away, on the other side of the test tube, will move more slowly, or sometimes may not be magnetized at all.
One way to fix this is to use a stronger magnet to collect those particles farthest away, but this means that those closest to the magnet will experience a very high magnetic force. In the case of cell separation this is dangerous because the high force could cause the cell to burst. Additionally, a strong magnetic force could overcome the electrostatic forces keeping individual particles from clumping together and could cause irreversible agglomeration of the particles. This is also undesirable because large clumps of particles will have unexpected behavior during the separation process, and if large enough, may simply sink to the bottom of the tube.
A homogeneous magnetic separator is engineered to ensure that every point within the working volume experiences the same magnetic force. Therefore, all particles will become magnetized and move down the magnetic field gradient at a similar velocity, and the cells attached to the particles will experience equal forces. This speeds up separation time and increases cell viability.
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