Bead mill homogenizers utilize beads inside a closed system which are rapidly agitated to grind and homogenize the sample. The actual causes of homogenization are collisions between the sample and the beads (which reduce particle size on a macro-scale), collisions between the beads or between the beads and the tube (which reduce particle size on a micro-scale), and to a lesser extent, shear forces caused by the rapidly moving beads.
Bead mills use different types of motion to agitate the beads, although most involve shaking. Below is a video which captures the mechanism of action for the Precellys 24, which uses an extremely rapid 3D shaking motion.
Because bead mill homogenizers utilize disposable consumables, there is an extremely low chance of cross-contamination. Additionally, since homogenization occurs within a closed tube, there is a very low chance of creating aerosols or other user exposure to the sample. This is especially important if the sample contains toxic, infectious, or otherwise hazardous substances. Certain bead mill homogenizers are designed to provide a higher level of protection to sample exposure than others. See our application page on BSL 3 homogenization for more information on hazardous applications and suitable homogenizer configurations.
Because there is no motor which needs to power a probe, bead mill homogenizers can be readily designed for high-throughput applications, and most allow for the homogenization of multiple samples simultaneously. Many bead mill homogenizers are designed for up to 24 samples, and high-throughput bead mills are usually significantly less expensive than high-throughput setups using other technologies. Furthermore, the use of various types and sizes beads makes bead mills suitable for varied applications. For instance, bead mills can be used to dissociate tissue and recover live cells, or to recover intact nuclei from cells. Bead mills can also be used without any liquid, making them good for milling or dry grinding of solid samples.
The primary drawback to bead mill homogenizers are the maximum sample size limitations. Due to the nature of the mechanism of action, it is impractical to design bead mills homogenizers for large volumes, and they generally only accommodate samples up to a few grams or milliliters. (Larger milling instruments utilizing beads in a rotary drum do exist, but these are dissimilar to the laboratory-scale bead mills.)
Since they use collisions between beads to homogenize the sample, bead mill homogenizers may also leave small amounts of microscopic particulate matter in the sample. If your application could be affected by small amounts of the bead material, then a bead mill may not be appropriate for your application. You may, however, be able to select a different bead material which would not interfere with your downstream analytical method.
Almost all bead mill homogenizers have a number of different beads are available for use, and various applications call for different beads. Larger beads are more appropriate for breaking down large or dense structures while small beads are better for micromilling already-small particles into an even smaller size. Bead shape and density matters as well. Denser beads will be more effective at breaking down tougher, harder materials. Irregularly shaped beads can help break down tough or fibrous materials as well, as their edges will deliver the force of the moving beads to a smaller area for greater impact.
Sample throughput and sample size are the two key things to look for. Ensure that your sample size range is appropriate for the instrument and its capacity fits your throughput requirements. If you have particularly high throughput requirements, you may also want to inquire about the expected run time for your sample, since the power of the bead mill can be the difference between a 30 second run time and a 3 minute run time.
Due to the many different mechanisms by which they agitate the samples, there is no single indicator of the power of a bead mill homogenizer. If you believe your sample may be difficult to homogenize, ensure that you ask if the instrument will be up to the task.