The modern cell culture incubator is much more than a box in which to keep cells warm. Talk of maintaining humidity, IR sensors, HEPA filters and decontamination cycles loom just as large in reviews and ads as mentions of temperature control. Yet how the box is heated (and perhaps cooled), how temperature uniformity is maintained and the speed at which the set point can be obtained and restored remain the heart of an incubator’s functionality.
After they’re set up, and if left undisturbed, most incubators can maintain a 37°C temperature to within about one-tenth of a degree. To do this, they rely on one of two technologies. The first surrounds the box with water, making use of water’s large thermal capacity to keep things inside at a constant temperature. The other option is to heat the air inside the box more directly. Each technology has distinct advantages and disadvantages related to humidity, decontamination and more.
With a much greater specific heat capacity than air, water traditionally has been used to regulate the temperature inside lab incubators. A jacket of water circulates around the outside of the incubator; the water exchanges heat with the inner chamber via natural convection and provides a fairly uniform interior temperature and thermal buffer against outside air.
Such a buffer can be especially useful in the event of a power outage, enabling a water-jacketed incubator to hold its heat about four to five times longer than units without a large surrounding thermal mass.
But water jackets are known to leak, laments Linda Reilly, Ph.D., director of the cell culture core facility at University of California, San Francisco (UCSF), who has used water-jacketed incubators for years.
They’re also very heavy and must be emptied before they can be moved. After they’re moved, they can be refilled and started up again—but it takes about 24 hours before they’re back to a stable operating temperature.
All that mass does have an advantage, though: It tends to dampen vibration that may have a negative effect on the culture of sensitive cells.
Water-jacketed incubators are certainly still being made, marketed, promoted and improved. Yet it seems most new models instead have a surrounding jacket of air (generally within some kind of insulation), with the inside air being warmed by one or more heaters. Because the interior chamber is heated directly, such models are able to set up faster and can recover temperature more quickly. (There is at least one incubator on the market that surrounds the inner chamber with a thin layer of high-thermal-capacity gel, enabling it to offer the advantages of both water- and air-jacketed models.)
Some air-jacketed models rely on natural convection to keep the heat evenly distributed inside the chamber, and others add mechanical assistance. Forced air, though, can lead to increased evaporation from the cultures, even with a humidity pan present, notes Scott Waniger, associate director of the National Cell Culture Center in Minneapolis.
Fans, in addition to creating vibration, can give contaminants such as fungi and bacteria a place to call home. At the very least, fans require the addition of another HEPA filter that will require periodic changing.
Another advantage of air-jacketed incubators is the possibility of incorporating heat decontamination, typically using either a 90°Cmoist-heat cycle or a 180°Cdry-heat cycle. This is not practical to do on a water-jacketed model. (A note of caution: Many incubators incorporate instrumentation that can be damaged by high heat. This includes IR sensors used to monitor CO2 concentration, which must be removed prior to decontamination.)
More to think about
Shakers and other instruments placed in an incubator can generate heat and could cause a well-insulated box to go above the set temperature. If this is a concern—or if the incubator is not housed in an air-conditioned space—a unit with refrigeration capacity may be in order.
Sometimes placement of an incubator can affect its performance. It’s always a good idea to avoid placing incubators in direct sun or next to an oven or autoclave. Reilly has even learned to be wary of sources of strong air currents, like ceiling vents.
Manufactures generally test their incubators for hot and cold spots and often publish temperature-uniformity statistics. Waniger periodically tests this for himself, mapping the temperature by placing leads and probes at different spots inside the incubator. Minor discrepancies can be compensated for by moving things around; larger ones may require avoiding sections of the incubator.
But perhaps the single best way to control an incubator’s temperature, Waniger says, “is training of the operator to keep [the] door closed.”
The image at the top of the page is from Sanyo Biomedical.