Centrifuges are essential tools for all sorts of labs, with the needs of different types of labs dictating different core competencies. A molecular biology lab may want to spin a lot of small tubes, for example, a fermentation lab may need to spin large volumes, while a virology lab might need the relative centrifugal force (RCF) of an ultracentrifuge. Here we examine the centrifugation needs of the lab specializing in cell culture.
Spinning Cells
Let’s be honest: in a cell culture lab, the main function of a centrifuge is to spin down cells. To that end, the cell culture facility that Alison Killilea supervises at the University of California, Berkeley, uses “very basic, old centrifuges – they’re not even digital.” Most people who are just doing this for culturing and passaging cells “spin the cells at room temperature, and they don’t need to go very fast – somewhere around 3000 RPM,” she says.
Most commonly the cells will be spun in 15 or 50 ml conical tubes. The centrifuge should be equipped with a swing-out (or swinging-bucket) rotor so that the tubes will rotate 90 degrees, “to create a pellet of cells at the bottom of the tube,” points out Sergey Pryshchep, Cell and Molecular Biology Research Core director at Rensselaer Polytechnic Institute. “You don’t want to spread them out on the wall.”
Researchers may sometimes want to spin their cells in microcentrifuge tubes, or even in cell culture plates – so a centrifuge with interchangeable rotors, allowing for those options, is certainly a plus.
Labs working with human samples, especially, are going to want a biocontainment lid in case of cracked tubes or spills “so that if anything should occur within one bucket it doesn’t enter the entire chamber or compromise any of the other samples in other buckets,” says Richard Sicard, Thermo Fisher Scientific’s Commercial Manager for the Americas for benchtop centrifuges.
Several vendors offer tissue culture packages as a single part number that come with either a refrigerated or ambient temperature benchtop centrifuge, a swing-out rotor, buckets, and adapters for 15 and 50 ml conical tubes. Some, like those from Thermo Fisher Scientific, include a biocontainment lid as well. “We also allow you to build your own package – you can pick a different rotor, different buckets, etc.,” points out Elise Ambrose, product manager at Midwest Scientific (MIDSCI).
General Purpose
Centrifuges can be divided up in different ways, for example whether they sit on a bench or stand on the floor.
And among the benchtop units: whether they are refrigerated (many models offer the choice); how big they are (footprint); what their capacity is (how many tubes, and of what size); whether they are capable of running at low speed only, or can also hit higher, or even “ultra”-high speeds; whether they have interchangeable rotors or are dedicated to one application (like spinning plates or microfuge tubes). To some extent these categories can overlap in different ways as well. For example, a Corning® LSE™ compact centrifuge can accommodate fixed-angle rotors for 1.5 ml, 15 ml, or 50 ml tubes, or a swinging-bucket rotor for 5 ml tubes, but “is not designed” to allow for the larger tubes to swing out, points out Peter Will, Corning Life Science’s global product line manager for centrifugation.
Centrifuges often called “general purpose” or “universal” tend to do what the name implies – they’ll spin a variety of rotors, that accommodate a variety of plasticware, at a range of speeds (usually not including “ultra”). And while a strictly cell culture lab may not need such versatility, it might behoove them to invest in those extra capabilities if they can afford it – especially if there is only a single instrument dedicated to that space. “You never know what application might come up for which you need a centrifuge,” says Sicard.
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That goes for refrigeration as well. Mammalian cells are typically incubated at 37 degrees, and then after spinning re-suspended in pre-warmed medium – common sense says that if anything, you might want a heated centrifuge (which is available). But – especially for longer spins – “it’s nice for a centrifuge to be able to keep 4 degrees. For some cultures, you don’t want to activate the cells,” says Pryshchep. And Sicard points out that refrigeration can maintain ambient temperature if that’s required as well, controlling heat that might build up in the chamber.
General purpose centrifuges tend to have capacities ranging up from four 50 ml conical tubes (standardly termed 4 x 50 ml) – like the diminutive Eppendorf 5702 – to 4 x 750 ml, a size more often associated with a floor model. If space and finances allow, it might make sense to purchase a higher capacity instrument, if for no other reason than future-proofing. But for many academic institutions like the Fox Chase Cancer Center’s Cell Culture Facility “space is limited,” says manager Pamela Nakajima, noting that the footprint of the core’s 5702 is “very compact.”
Refrigeration itself takes up room as well. “Some manufacturers or models have the refrigeration unit on the side versus on the back,” notes Will. “Ideally you don’t want to take up that much space from left to right – there’s usually more dead space from front to back on a lab bench.”
Gravy
Most cell culture labs don’t think about acceleration and deceleration. They ramp up and down as quickly as the instrument will go, secure in the knowledge that the pellet will remain intact. But for some applications, such as when spinning down blood in a high-density liquid (to create a gradient), for example, “it’s nice to have the option not to use a high brake to stop the centrifuge,” says Pryshchep. Many instruments offer choices of acceleration and deceleration rates.
Some centrifuges offer other programming options as well – for example, to set a user-defined, password-protected programs “so that they don’t have to spend time to re-set the temperature and time and speed [between users], allowing them to work more efficiently,” explains Will.
One of the main “cons” or “tips” cited in user reviews is the need to properly balance samples.
Nakajima, for example, recommends that volumes on opposite sides of the rotor be within 1 ml of each other. Sicard points out that all of his company’s rotors have been tested to withstand a particular threshold of imbalance, up to which “it might vibrate but it’s not going to affect it.” But, “it’s not something we’re going to promote … We promote the best practice of always balancing your rotor.” Yet at least one line of rotors (Beckman Coulter’s ARIES™ Smart Balance) will automatically correct for an imbalance of up to 50 grams of opposing load – about which Pryshchep exclaims “if that’s possible, it’s a nice feature, I can tell you.”
Many other features, bells, and whistles are available for benchtop centrifuges as well, ranging from end-of-run indicator lights, beeps, and automatic door opening (Sicard recommends turning off the latter when using refrigeration), to easy cleaning, tool-less rotor changing, and automatic rotor recognition (so the instrument knows the maximum speed). Of course they’re not as crucial as having the capacity to accommodate the tubes you need to spin at the speed you need to spin them in a centrifuge within your budget that fits in the space allocated – but they may be nice to have nonetheless.
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