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Agitating Samples the Right Way

Laboratory stirring and mixing are carried out by either magnetic stirrers (including hot-plate models), table-type agitators and shakers, shaker-incubators, or overhead stirrers. All have their niches.

Angelo DePalma, PhD

Angelo DePalma is a freelance writer living in Newton, New Jersey. You can reach him at

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Laboratory stirring and mixing are carried out by either magnetic stirrers (including hot-plate models), table-type agitators and shakers, shaker-incubators, or overhead stirrers. All have their niches.

“Stirrers are laboratory staples that work with numerous other instruments, including homogenizers or pH meters, or by themselves to dissolve solutes or mix reactions,” notes Marta LaForest, sales and marketing manager for laboratory products at Caframo (Wiarton, ON). A recent survey by Lab Manager Magazineshows the greatest uptake for stirrers in life sciences, medicine, pharmaceutical, chemical, and food and beverage industries.

Choice of stirrer type depends on the sample’s size, composition, and physical properties. Magnetic stirrers are generally used for nonviscous liquids of up to a few liters in volume. Caframo specializes in overhead stirrers operating from bench scale (two-liter capacity) to hightorque models for mixing up to 80 liters of viscous product.

Viscosity’s the thing

Thick, viscous materials or very large volumes do not lend themselves to shaking or magnetic stirring. Magnetic coupling may not be strong enough to overcome viscosity, and stir-bar vortices may not reach high enough into the fluid to provide uniform mixing. These samples almost always require overhead stirrers, which supply more energy to the sample.

Samples for which viscosity changes— either to more or less viscous— also benefit from overhead stirring. Joe Novotny, chief engineer at Eberbach (Ann Arbor, MI) cites starch electrophoresis, where starch is added to a buffer. “Once it gets hot and the starch starts dissolving, then polymerizing, the solution becomes quite viscous.” Similarly, materials that change phase, say, from gel to liquid or vice versa, are good candidates for overhead stirring.

During the last 15 years, overhead stirrer manufacturers have shifted from using AC motors controlled by a rheostat to DC brush motors that provide better control. Today, many models employ small brushless DC motors. As electronics became smaller, more compact, and available for many types of instrumentation, overhead stirrers have benefited from the ability to program stirring “methods” directly into the instrument. These involve mostly stirring patterns, for example, slow to fast.

The new brushless DC motors incorporate Hall effect sensors that enable the stirrer to “see” how fast they’re spinning. “Stirrers can change speed, unattended, in response to what is occurring in the sample,” Mr. Novotny says. “On older models the operator needed to be present to increase motor power in such situations.”

According to Ms. LaForest, the principal benefit of brushless DC motors is their efficiency. “All the power from this type of motor goes into mixing the product instead of toward generating heat or extraneous motion.”

Computerization has been a significant trend in most lab instruments, including stirrers and shakers. However, beyond regulated industries where processes must be validated, little call exists for connecting overhead stirrers to computers. “I’ve seen shakers hooked up to computers for recording stirring time and speed,” Mr. Novotny says, “but by and large this is unnecessary because the control features are already built into the instrument.”

Not everyone agrees with that statement. Caframo’s overhead stirrers lack the ability for computer control, “but that feature is on its way,” says Ms. LaForest. Networking to a computer helps in logging results and quantifying viscosity and speed values. “The main benefits here are repeatability and consistency,” she says. “Operators want to set up a run and do it exactly the same way next time.”

All shook up

A significant number of mixing applications do not require the robustness of an overhead stirrer. Many chemists go through their entire careers, for example, using only magnetic stir-bars or hotplate stirrers. Another industry that tends to avoid overhead stirrers is biotechnology, particularly for cell culture. While large-scale biomanufacturing does indeed employ impeller- type stirring at a small scale, one is more likely to see a shaker or tabletop rocking-type mixer.

Even at a large scale, biotech is moving away from stainless steel impellers, due to the shear stress they place on cells. Some single-use bioreactor bags still employ older stirring technologies, but these are giving way to magnetic stirrers, superconductively levitated stir-wheels, external actuators that compress and expand the bag, and gas-actuated paddle wheels.

At a small scale, however, biological mixers are the way to go. These come in several designs ranging from vortex mixers for single samples that require short-term mixing to sophisticated temperature-controlled “table” models for multiple samples and microtiter plates.

Temperature control is a critical feature for cell and microbiological cultures undergoing agitation. One common design is a shaking water bath, which shakes samples at up to 400 strokes per minute. Joseph Costello, North American sales and marketing manager for Grant Instruments (Hillsborough, NJ), says, “Water baths work well, within very tight temperature tolerances and good heat transfer to the sample.” But he also admits that they can be messy, given the fact that the bottles are sitting in water.

On the other hand, microtiter plate shakers often run at faster frequencies, since the volumes are minuscule and samples tend to sit in place inside the wells.

Some somatic cells and most stem cells are so sensitive to shear that the only type of agitation they survive is gentle rocking on tabletop mixers. Until fairly recently, cultures grown in shake flasks were agitated in this manner. Wave Biotech, now a unit of GE Healthcare, built its entire business around tabletop rocking of plastic bioreactor bags ranging in size from a few milliliters up to 1000 liters.

One consideration when selecting a mixer or stirrer is what will occur downstream of your particular operation. If scale-up is not on the horizon, almost any suitable stirring or mixing solution will do. On the other hand, agitation does not always scale smoothly from plates to vials to large reactors. “But that is why companies have process development groups,” Mr. Costello says.