Centrifuges do only one thing, but they do it very well, and their basic operating principle is as old as the universe itself. Centrifuges represent a practical application of Isaac Newton's First Law, or the Law of Inertia, which states, “In the absence of a net force, a body either is at rest or moves in a straight line with constant speed.” It is this law that explains the force generated when an otherwise straight-line motion is constrained into a rotational movement around a fixed axis. The resultant force, the result of centripetal acceleration, (often termed “centrifugal force,” a fictitious force peculiar to circular motion) is always perpendicular to the axis of rotation and can be as much as a million times the force of gravity. Centrifuges create this high force to quickly separate heavier particles from lighter ones, often in a liquid suspension so the particles can move easily. As a result, centrifuges have found uses in many diverse industries, from dairy (separating cream from whole milk) to nuclear (uranium purification and enrichment). Even the common automatic washer uses this principle of physics to express water from clothes during the spindry portion of the operating cycle.
In medical facilities, centrifuges are found primarily in clinical laboratory and blood-banking operations, with specialized (microhematocrit) centrifuges sometimes used in or near nursing areas. Medical centrifuges can be divided into three types—low-speed, high-speed, and ultracentrifuges. Another classification method is by size—floor-standing (about the size of an automatic washing machine), tabletop (about the size of a large microwave oven), or handheld, which is rarely found nowadays. Both low-speed and high-speed types are available with or without refrigeration and may be either floor-standing or tabletop size. Low-speed centrifuges generally operate up to 10,000 revolutions per minute (rpm) and are primarily used to separate red blood cells. High-speed units operate between 10,000 and 30,000 rpm and are the most versatile of the three types. Ultracentrifuges generally operate from 40,000 to 150,000 rpm and are always refrigerated to prevent sample deterioration caused by the heat created by air friction.
The basic components of a centrifuge, no matter the type, are the electric motor, the rotor or head, and the motordrive electronics. A fourth component, where applicable, is the cooling system used in refrigerated centrifuges. The electric motor and motor drive electronics are contained within the base of the centrifuge, while the rotor head is housed in a chamber, called “the bowl,” above the base. The bowl also contains a heavy safety shield that surrounds the rotor and protects the operator in the event the head breaks and propels fragments outwards.
Traditionally designed centrifuges use a motor with brushes that convey power to the armature. These wear out relatively quickly and are the component with the highest failure rate. To remediate this problem, early designs used longer-wearing brushes, but many modern models use induction motors instead. Induction motors, also known as “brushless motors,” use a magnetic field to induce the armature current. Since there is no physical contact with the armature, the failure rate and associated remedial maintenance are substantially reduced. Likewise, the use of permanently lubricated bearings, instead of older oil cup-and-fiber washer designs, also reduces maintenance requirements.
The rotor head, or simply the rotor, is fastened securely to the shaft of the motor and is made of either aluminum alloy (for low-speed and moderate-speed applications) or titanium (for high-speed applications). The two most common head varieties are fixed angle and swinging bucket. Fixed-angle rotors are precisely machined from a solid block of metal and contain between 4 and 40 holes bored at between 20 and 45 degrees to the vertical axis. Each hole contains a metal holder with a rubber cushion at the bottom. The diameter and depth are matched to the test tube being spun so that the tube just snugly fits into it. The cushion helps prevent tube breakage during operation and its absence usually guarantees a broken tube during centrifugation. The swinging bucket rotor normally includes between two and six swinging buckets (or tube holders), placed symmetrically around the shaft for balance, that hang vertically at rest, but swing out to 90 degrees to the vertical axis during centrifugation. Each bucket contains up to 62 compartments for holding the tubes, microscope slides, bottles, or cups being centrifuged. Most swinging bucket centrifuges are floor standing, although a few tabletop designs exist, due to the size of the bowl required to allow the bucket to swing on its axis.
A third rotor variety can only be described as a “special purpose” design since it is specific to a desired task, like cell washing. Cell-washing centrifuges, for example, automatically wash, decant, mix, and rewash red blood cells in a single rotor. Cell-washing rotor designs are proprietary and unique to the centrifuge or rotor manufacturer as are other special purpose rotor designs.
Motor-drive control is also considered proprietary since there is no single universal schematic common among the various manufacturers. Some legacy centrifuges have little more than a mechanical timer and a constant-speed motor. Most commonly, however, motordrive control is accomplished electronically on a single printed circuit board. Often this board includes the controls for duration and speed of centrifugation, the circuits for dynamic braking and the lid interlock, and a speed display. Some units include control of the acceleration and braking rates, and even a brush wear indicator. The duration of centrifugation is controlled by a simple timer. Speed is controlled by limiting the power going to the motor, while dynamic braking is performed by reversing the polarity of the power applied to the motor. The lid interlock performs two functions: It prevents the unit from starting when the lid is open and locks the lid closed when the rotor is moving. The speed display on modern units is a digital tachometer, usually utilizing a magnetand- coil sensor, but biomeds may run across legacy units that use a flexible shaft and mechanical tachometer. The brush wear indicator can be of several designs. One incorporates an electronic total run-time counter that indicates when the brushes should be checked based upon average wear specifications, while another design uses sensors on the brush caps to detect when they wear down to the lowest usable length. The designer may include other features on this board, such as a switch allowing continuous centrifugation or a bowl thermometer, but the aforementioned features are common to most modern centrifuges.
How to Manage the Device
Although centrifuges are not complex, critical, highly technical medical devices, various certifying bodies virtually require scheduled maintenance and the keeping of a detailed maintenance history for each centrifuge. As will be seen, there are many organizations involved in laboratory operations, and each has rules about centrifuge maintenance. Generally, however, all organizations require regular, documented preventive maintenance, following the manufacturer's procedure, as well as documentation of remedial maintenance performed both inhouse and under contract. Although it's not mandatory to maintain service records for the life of the item, it's not a bad idea. There's no telling what regulators might want or what a lawyer might seek.
While not regulatory bodies, the College of American Pathologists (CAP), the American Association of Blood Banks (AABB), and the Joint Commission (JC) do certify laboratories, blood banks, and healthcare facilities. As such, if a facility wishes to participate in one of their programs, such as laboratory certification or blood banking, the facility must abide by their rules and meet their standards. That also applies to their rules governing maintenance of centrifuges and other medical equipment. Rather than address each organization's policies in this article, I recommend that the supporting biomed have a meaningful discussion with the laboratory manager regarding the requirements of the particular organization or organizations with which the laboratory aligns itself. Alternatively, biomeds may obtain their own copy of the applicable accreditation or certification manual and thoroughly absorb the maintenance information.
Risk Management Issues
Centrifuges present a relatively large number of risks for their small size and cost. Older centrifuges lacking lid interlocks pose the risk of amputation of fingers. Newer models have a mechanism that prevents start-up while the lid is open and premature lid opening while the rotor is still moving. This is an obvious risk management issue and replacement of older models with safer ones must be a facility decision.
A less obvious risk issue revolves around the accuracy of the centrifuge's speed and timer. A centrifuge running too fast (that is, one rotating at a higher speed than indicated on the tachometer) can lyse red blood cells, causing the unwanted release of cell proteins into the plasma portion of the sample and potentially cause inaccurate test results. This can lead to misdiagnosis and inappropriate treatment. Over-centrifugation can cause the same effect, but there is a bit more leeway in timing than in speed. Under-centrifugation, either because of lower speed or less time, reduces sample separation, which can reduce plasma concentration and increase the number of red cells in the plasma. Both of these conditions can cause inaccurate test results and a resultant misdiagnosis and inappropriate treatment.
A third area of potential risk results from the centrifuges brushes. As the brushes wear, carbon dust is deposited inside the bottom around the motor area. A buildup of this dust can create a high-resistance path from the brushes to ground, increasing leakage current and shock potential. The biomed, by performing periodic electrical leakage current measurements, plays a critical role in mitigating the risk of electrical shock to the operator.
A major risk factor that can only be mitigated by the operator is exposure to blood-borne pathogens. As previously mentioned, failure to use cushions at the bottom of the tube holder can cause the sample tubes to shatter. Additionally, old and imbalanced rotors, old or defective tubes, or simply improper operation can cause sample tube breakage. Should this occur during operation, the sample (blood or other bodily fluids) will be sprayed throughout the interior of the bowl. Additionally, shards of glass will be strewn about both the bottom and sides of the bowl, with some of the smallest and most dangerous clinging to the sides with the sample residue acting as the glue. Therefore, universal precautions should always be taken when performing centrifugation, and biomeds should always be careful when opening and working on centrifuges.
No one knows exactly when the first centrifuge was invented, but the underlying principle has been around forever. The first dairy centrifuge to separate cream from milk was invented in 1864 by Antonin Prandtl of Munich. Likewise, the first continuous centrifugal separator was demonstrated in 1879. Improvement in centrifuges focused on those used by dairy facilities for milk separation since this was their primary application. Centrifuges did not appear in any quantity in medical facilities until the 1960s when interest in blood and its components rose dramatically. Today the centrifuge has come into its own as the primary means to separate patient samples of whole blood into its components for testing.
During its evolution, the basic design of the centrifuge has remained unchanged: It is still an electric motor moving a rotating head or rotor. Although centrifuges come in many different sizes for different applications, the bulk of the changes have been in the design of the rotor for specialized applications and its material to allow rotation at even higher speeds without destructing.
Today's centrifuges are relatively maintenance and failure free. Their primary weak area has always been their brushes and bearings. Even though some centrifuges use long-wearing brushes, the centrifuge's high speed causes rapid brush wear and, in the process of wear, they build up a good deal of conductive carbon powder in and around the bottom of the motor that can contribute to increased leakage current. This buildup must be removed during preventive maintenance checks and services, as well as when the brushes are replaced. The newest designs employ induction motors that are brushless. When bearings fail, they fail with the usual symptoms of excessive heat and noise.
American Association of Blood Banks, “Guidelines for the Use of Blood Warming Devices”
College of American Pathologists, “Laboratory Accreditation Manual”
ECRI Healthcare Product Comparison System for Centrifuges
The remainder of the common maintenance problems generally stem from the control-printed circuit board. A number of symptoms can be traced back to this board, the most common being the inability to energize the motor, loss of dynamic braking, and lid interlock problems.
Training and Equipment
Basic electrical-mechanical training coupled with midlevel electronics training is sufficient to maintain centrifuges. A good service manual is worth its weight in gold, especially when troubleshooting the printed circuit board. Regular hand tools are normally sufficient for servicing centrifuges since special tools are rarely, if ever, required. A small vacuum cleaner (of the type used in maintaining personal computers) with a high-efficiency particulate air (HEPA) filter is recommended for removing brush dust and residue from inside the centrifuge base.
The primary test equipment required for calibration is a tachometer, and a phototachometer is strongly recommended. Most centrifuges have a hole in the lid that facilitates the use of the stick-on, black-and-white label that many phototachometers use to measure centrifuge speed. A stopwatch is required to calibrate the timer. A word about calibration: Not all centrifuges can be calibrated in the traditional sense because they lack adjustments. Calibration in this case either certifies that the stated (for fixed, single-speed devices) speed is within the manufacturer's performance criteria or that the indicated speed (for devices with an integral tachometer) is actually attained. When calibrating centrifuges with only a speed control marked with arbitrary numbers, it is recommended that the biomed prepare a chart correlating the numbers on the speed control with the actual speed. Likewise, since most timers lack any adjustment, timer calibration normally certifies the timer is operating within its stated tolerance.
Future Development of Centrifuges
In addition to the brushless motors and permanently lubricated bearings found in newer centrifuges, increased use of microprocessor controls is anticipated. One area of increasing microprocessor application allows the operator to set and store various operational parameters (speed, time, etc.) for a particular application and recall with the push of a single button. This eliminates the need to set speed and time every time the operation is performed. Another application of microprocessor technology enables centrifuges to identify a particular rotor (via a barcode on the rotor) and automatically select preprogrammed operating parameters. These units are already found in the marketplace, but automated applications such as these are expected to increase to both reduce operator error and increase sample throughput. The core technology of centrifuges, a basic principle of physics, is considered fully mature, but incremental improvements in controls and features are expected to continue.
Robert Dondelinger, CBET-E, MS, is the senior medical logistician at the U.S. Military Entrance Processing Command in North Chicago, IL. E-mail: firstname.lastname@example.org