The quick reaction of a compass needle pointing north is a familiar sight. That miniature magnet balanced on a nearly frictionless point is an elegant and simple device for navigating our world. Discovered by the Greeks or Chinese in the first century BC, the lodestone was as frightening as it was fascinating. When the material was cut into a long slender shape and placed into a pool of water it had the strange ability to automatically spin around to the same (north-south) direction every time. Over the centuries we have learned that the Earth itself is a giant magnet with field lines streaming out of the north pole and into the south. The lodestone, or ferromagnetic (ferro- meaning iron) material is also a magnet with north and south poles. In general, when a magnet is placed into a constant magnetic field it experiences a torque and spins around until it matches the field lines. A constant magnetic field is one that is of the same magnitude in all locations.
When the magnetic field is not constant, and changes magnitude throughout space (has a gradient), things get more interesting. If you dropped a small magnet into such a magnetic field gradient, the small magnet would feel a force and move toward the location where the field is strongest.
How do we make a magnetic field gradient? A good-sized permanent magnet will do the trick. Sometimes these are shaped as a big U, sometimes they are just a rectangular block of metal, and sometimes, for laboratory uses, they are shaped like a donut. The benefit of the donut shape is that it has a hole in the center where a test tube can slide in. If you can imagine the magnetic field inside of the donut hole it is easy to picture that the magnetic field is strongest along the outside of the tube where it touches the magnet, and is weakest at the center of the tube where there is no direct contact with the magnet. These donut-shaped magnets are an essential part of a tool called magnetic activated cell sorting.
In biological research it is very important to be able to isolate a particular cell population from a serum containing thousands of different cells. After the cells of interest are collected they can be grown in petri dishes where scientists can observe their growth patterns, gene expression, and reactions to applied molecules or drugs. To collect cells using magnetic activated sorting a scientist must first know what kind of cell he or she wants to isolate. The next step is to identify a handful of unique proteins on the outside of that cell. Importantly, these proteins have the job of binding to peptides outside of the cell to mediate information and nutrient exchange. The magnetic activated cell sorting process takes advantage of this binding ability.
After identifying the proteins outside the cells, the scientist creates or purchases magnetic beads made of an iron-oxide core with a coating of peptides that will specifically bind to those proteins. When the magnetic beads and the serum are mixed together inside of a test tube, the peptides on the magnetic beads specifically bind with the proteins on the cells to make a conjoined magnetic-bead-cell. Now, the tube is place inside of the magnet and the magnetic-bead-cells are forced against the walls of the tube where the magnetic field is the strongest. The remaining unwanted cells are not magnetic and remain in the center of the tube. They are rinsed out and replaced by clean media. Only then is the test tube removed from the magnet and the magnetic-bead-cells leave the sides of the tube and go into the media. The media and cells are put into a petri dish and that isolated population can be grown and studied further.
Magnetic activated cell sorting is a common tool, and many of the magnetic beads are produced commercially and are easily obtained within a day or two. Additionally, the work of many scientists has produced extensive documentation of the proteins that identify a variety of cell populations. Isolating specific cell types is quicker and easier than ever before. Magnets continue to serve us as we navigate new frontiers.