Product Survey: Benchtop centrifuges
by Harald Zähringer, Labtimes 02/2015
Since almost every laboratory protocol requires at least one centrifugation step, benchtop centrifuges belong to the most basic and essential laboratory devices.
Benchtop centrifuges are rather simple-structured machines, composed of a brushless electric motor that directly drives a rotor without additional gearing or a belt drive. The spinning rotor generates a relative centrifugal force (RCF or g-force), which depends on the rotational frequency, given in revolutions-per-minute (rpm), and the distance from the rotor-axis.
Centrifugation tubes, cups or bottles filled with probe suspensions are inserted into cavities or buckets symmetrically arranged along the radius of the rotor. The suspended particles are squeezed by the g-force to move away from the rotor-centre in a size and density-dependent fashion, and settle at the bottom of the centrifugation cup.
Similar to swirling water that deposits material at the riverbank, microvortices may be used to trap and collect cells.
According to their sizes, benchtop centrifuges are usually categorised as microcentrifuges, general purpose and small benchtop centrifuges. Lifting a general purpose centrifuge to place it on a workbench takes the strong back and arms of a weightlifter or a chain-block. Weighing between 50 and well above 100 kilograms, easy transportability is not really a plus point of general purpose centrifuges. As the name implies, their biggest advantage is versatility. They may be equipped with various rotors, such as fixed-angle, horizontal and swinging-bucket rotors that can handle almost any popular tube, cup, bottle, microtiter plate, PCR strip or even cell culture flask.
General purpose centrifuges may reach maximal g-forces of approximately 30,000 g with fixed-angle rotors and around 5,000 g with swinging bucket rotors. Most models are available as uncooled or cooled versions, with capacities of up to several litres.
Fixed-angle rotors of general purpose centrifuges made of aluminium are pretty massive and having to regularly lift them, considerably increases the daily workload. Lightweight carbon fibre rotors, weighing only half of similarly-sized aluminium rotors are a clever alternative, offering more than just improved ergonomics: they are corrosion and fatigue-resistant (in contrast to aluminium rotors), extremely durable and, last but not least, repairable after damages. Carbon fibre rotors are slightly more expensive than conventional aluminium rotors, since the manufacturing process involves some hand crafting. The higher costs are compensated by energy savings during each centrifuge run, due to a lower mechanical load and faster ramp-up times.
Metal or plastic is also the question when it comes to rotor lids. Traditional, stainless steel lids are almost unbreakable and safely protect the lab personnel from toxic aerosols, microorganisms or other biohazards, if they are provided with airtight rubber seals. But you can’t see through a stainless steel lid after opening the centrifuge cover, to determine whether a centrifuge tube is broken or has been leaking during a centrifuge run. This problem is avoided with transparent plastic lids, which are also highly resistant to corrosion. Most manufacturers rely on high-impact resistant, polyphenylsulfone lids that easily withstand the radial forces occurring during centrifugation as well as the fast temperature changes upon cooling.
Small benchtop centrifuges are the little cousins of general purpose centrifuges with smaller footprints, lower weights and capacities, usually well below one litre. They are still very flexible, despite their lesser size, and may be equipped with different types of fixed-angle and swing-out rotors, delivering similar g-forces to the larger rotors, spinning in their bigger cousins.
Scaling down small benchtop centrifuges even further leads to microcentrifuges, the classical workhorses of molecular biology or biochemistry labs, predominantly working with 1.5 or 2.0 ml Eppendorf cups or PCR tubes and strips. Hence, the typical microcentrifuge comes with a fixed-angle, standard rotor, running 24 tubes in 1.5 or 2.0 ml sizes, which may be replaced by additional rotors, holding 0.5 ml tubes or PCR strips.
Larger microcentrifuges may attain remarkable accelerations of roughly 30,000 g and are available as refrigerated versions. On the other side of the microcentrifuge spectrum are pocket-sized mini microcentrifuges with small footprints and tiny plastic rotors achieving centrifugal forces of only 3,000 g.
But even the smallest microcentrifuge is a giant compared to the centrifuge-on-a-chip developed by the Dino Di Carlo group at the University of California in Los Angeles, USA (Mach et al., Lab Chip 2011, 11, 2827-34). The Di Carlo centrifuge-on-a-chip approach does not need any moving parts to separate cells by size and density, similar to a benchtop centrifuge. It is rather based on a microfluidic chip that creates microscale fluid vortices to “trap and release particles and cells in suspension” as the group describes in their paper.
The idea behind the centrifuge chip is pretty simple: the particle solution is injected through a narrow inlet channel into the middle of an orthogonal, right-angled vortex chamber that opens up at the end of the inlet. The outlet channel of the chamber is exactly placed at the opposite side of the inlet. Similar to the water of a swirling river, the microfluidic stream creates a vortex in both chamber areas that traps larger particles, while smaller particles escape the vortex traps and follow the main stream flowing straight from the inlet to the outlet channel.
The Di Carlo team has already applied the centrifuge chip to isolate circulating cancer cells from blood. Since tumour cells are larger than blood cells, such as leukocytes, erythrocytes, lymphocytes or platelets, they are trapped in the microvortices and can be isolated and further analysed (Che et al., PLoS ONE 8(10): e78194). Meanwhile, the Californian start-up company Vortex Biosciences is commercialising Di Carlo’s idea and trying to integrate the centrifuge chip into an automated setup, to routinely collect circulating tumour cells in cancer patients.
First published in Labtimes 02/2015. We give no guarantee and assume no liability for article and PDF-download.
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