What Is a Centrifuge?

A centrifuge is a common piece of laboratory equipment specializing in separating components that differ in size via centrifugal force. Centrifugal force is the inherent force caused by rotation that sends particles away from the center of rotation. In a centrifuge, this force is paired with very high speeds to facilitate the separation of components in a liquid or liquids in a solution.

This separating force has immense potential when utilized at high speeds, offering a valuable separation tool that is standard in many protocols. Centrifuges have many industrial and scientific applications, such as cell culture harvesting and blood separation. They are also found as spinning components within more complex scientific instruments.

What Is Centrifugation?

Centrifugation separates substances based on their size and density. Due to the natural laws of sedimentation, larger or more dense particles settle more quickly than smaller, less dense particles. This principle drives centrifugation’s many applications and informs the various types of centrifuge and centrifuge systems available.

There is a centrifuge for every clinical and research need, making it a highly variable tool. The proper centrifuge is integral to proper laboratory workflow, functioning, and successful downstream testing.

Parts of the Centrifuge

Centrifuges can be modified in size and functionality to fit their intended purpose best; however, the basic construction and format of the centrifuge are standard and linked to centrifugal principles. Within the centrifuge is the motor responsible for the high-speed spin and centrifugal g-forces achieved during centrifugation. This powerful motor ranges in top speeds across the different types of centrifuges, maxing out at an astonishing speed of 150,000 rpm.

Connected to this strong motor are centrifuge arms that attach the rotor adaptors and tubes to the motor. The centrifuge arms facilitate energy transfer from the central motor to the separation samples. The motor, arms, and rotor adaptors combine into one cohesive piece called the rotor. The rotor is the internal mechanism responsible for the centrifugation.

The separation tube that holds the sample to be centrifuged sits within rotor adapters attached to the centrifuge arms. These adaptors can be oriented at an angle to facilitate easy pellet formation and the most efficient separation. The angle at which the samples are oriented is linked to the type of centrifuge, with 45-degree angles being most common among fixed-angle centrifuges and 90-degree angle or swing-bucket centrifuges having more variability across centrifuge models and uses.

A sample’s angle is achieved either by loading the sample tube onto the rotor directly at the intended angle or by loading it into rotor adaptors that adjust the angle during centrifugation, like those found in swing-bucket centrifuges. The spinning motion of the centrifuge is slowed and stopped by a series of brakes to decelerate the rotor quickly and smoothly, thus minimizing the disturbance of the sample layers formed in the spin.

Safety is a primary concern in the development and use of centrifuge machines because of the immense velocity achieved during operation and the potential of rotors or tubes to fly off and cause injury. Some centrifuges have two lids to protect the rotor, samples, and user during centrifugation. The first lid is screwed into the rotor until it is tightly fastened close and the second lid encloses the vessel into the machine.

Modern centrifuges also come with lid interlocks that prevent the centrifuge from initiating centrifugation if the lid is not properly closed. Additional safety features commonly found in centrifuges include automatic rotor recognition and imbalance detectors. This ensures the proper rotors are used—and in proper balance—to protect the user from potential accidents.

Centrifuges have a main control panel, generally a display screen or series of buttons that allow the user to modify the speed in relative centrifugal force (RCF), rotations per minute (RPM), and time in minutes and hours of spin steps. While RCF values are consistent between all centrifuges, its relationship with RPMs is dependent on the physical properties of the instrument and can vary between different models. Some centrifuges also come with temperature control to maintain the samples at a specific temperature.

Centrifuge Tubes and Sample Types

Centrifuges can accommodate a large range of different sample tubes and containers depending on the size and design of the rotor or rotor adaptors. The tubes used in centrifugation hold the samples for optimal separation, and they must sit flush and secure within the rotor. Some centrifuges provide adaptors that allow the use of many different tubes in the same centrifuge to ensure a snug fit every time.

Types of Centrifuges

Centrifuges employ two formats of separation:

• Sedimentation centrifugation relies on the principle that particles suspended in a solution settle according to gravity and their unique densities. Centrifuges utilize the sedimentation principle to separate particles based on their density. Sedimentation does not involve the presence of a filter; instead, the particles separate into distinct layers and pellets based on their weight. These layers are referred to as a density gradient.
• With filtration centrifugation, centrifugal force pulls the samples through a filter. Filtration separates solid particles from their liquid supernatant. This separation style relies heavily on the particle size and the pore size of the filter in use. Filtration predominantly utilizes separation tubes called columns, which contain filters of specific sizes designed to capture or remove particles of interest.

Fixed-Angle and Swing-Bucket Centrifuges

Angles play an important role in centrifugation, allowing for succinct pelleting and clear layers. Centrifuges employ two main methods to maintain a specific angle during the spin, with the most common angle being 45 degrees. The larger the spin angle, the tighter the pellet will be, and the smaller the angle, the more spread and thin the pellet will form.

Most simple centrifuges use what is considered a fixed-angle system, meaning the rotor is set up with permanent slots for tubes to be directly loaded onto the instrument at the intended angle. Fixed-angle centrifuges have many advantages, including having fewer moving parts and metal contact within the instrument, which increases the machine’s lifetime. The stationary tube holder also allows for a higher maximum spin speed and maximum g-force than a swing bucket alternative.

Swing buckets allow the rotor and sample to move with centrifugal force to an angled position during the spin step. Once the top speed is reached, the rotor spins at a fixed ratio from the center of rotation, achieving the critical 45-degree angle required for pellet formation. Because of the rotor motion during the high-velocity spin, swing buckets have limited g-force capacity. However, they are useful for various tube formats, including 96-well plates, and have a high sample capacity.

Centrifuge Sizes

Centrifuges can have varying functionalities depending on their size and inner rotational power. Typically, centrifuges are classified by size and function: floor-standing and benchtop centrifuges, including multipurpose and microcentrifuges.

• Floor-standing centrifuges offer the tradeoff of bench space for floor space and are capable of high-speed and high-capacity protocols. One important type of floor-standing centrifuge is an ultracentrifuge, which is an incredibly fast and refrigerated centrifuge meant for specialized applications.
• Alternatively, benchtop centrifuges are optimized for laboratory countertops instead of the floor. There are two major types of benchtop centrifuges: microcentrifuges and multipurpose centrifuges.
  • Microcentrifuges are small in size and mainly used for quick spin-down steps instead of sedimentation-based separations. They take up very little space, are highly portable, and can plug directly into most biohazard hood spaces.
  • Multipurpose centrifuges have a larger chamber and higher top speed than microcentrifuges. They offer significant versatility in rotor compatibility and functionality across laboratory spaces.

Using Centrifuges With Microbubbles

Akadeum’s microbubble technology works hand-in-hand with centrifugation by utilizing microbubbles’ natural buoyancy. As centrifugation sorts particles by density sedimentation, it facilitates the floatation of the microbubbles attached to target cells, meaning simple cell sorting can be achieved in one centrifugation step.

Explore how microbubbles use centrifugation to gently sort cells using antibodies. Contact our team with any questions!