Procurement and Supply of Cell Storage Systems for the Biotechnology Laboratory

Specifying a cell storage system is an important decision for any life sciences laboratory. A poor choice can put valuable research at risk and add costs. A good choice can reduce aggravation and lower expenditures. Planners need to consider a variety of factors when selecting a cell storage system, including temperature needs, power demands, sample contamination, potential facility requirements, safety, and cryogen supply. This article compares the two types of storage systems—liquid nitrogen (LIN) freezers and mechanical freezers—to help the reader make more informed purchases.

Temperature

The first question that a planner should ask when selecting a storage system is “How cold is cold?” For many applications, cold enough means achieving the glass transition temperature of water (–130 °C). At this temperature, movement within a cell ceases, putting it into a state of suspended animation. This enables the sample to survive for a virtually indefinite period of time.

There are two primary options for storing samples at this temperature: 1) LIN freezers and 2) mechanical freezers. LIN units offer the coldest temperatures. They store samples in a pool of LIN at –196 °C, or, in the case of a vapor phase unit, in cold nitrogen vapor at temperatures of –170 to –190 °C. Mechanical freezers store samples at somewhat warmer temperatures. The coldest mechanical units achieve temperatures of –130 °C to –150 °C, although the vast majority of mechanical freezers are designed to cool to –80 °C.

In general, mechanical units are the more convenient option. The user simply plugs in the unit and sets the temperature to get started. However, to achieve temperatures at or below –130 °C, mechanical units must operate at the very edge of their ranges, making samples somewhat vulnerable to fluctuations in temperature. Consequently, these units are still somewhat rare. In contrast, LIN units are the more common and more reliable option for storage below the glass transition temperature and for long-term sample preservation.

Sample integrity

Figure 1 - LABS 40K freezer.

In recent years, the potential for contaminants such as viruses, bacteria, and fungi to migrate from cell to cell when immersed in LIN has become an issue. Laboratories are under pressure to protect samples from contamination. One way for them to do this is to provide additional protective containers around the vials that contain the samples. Another way is to store samples in a vapor phase unit, which keeps the samples below the glass transition temperature, without direct contact with the LIN.

The argument has been made that LIN vapor phase units have large temperature gradients and therefore provide inconsistent freezing. This may be true for some older units that have large lids. However, newer designs, equipped with improved control systems, maintain extremely consistent temperatures. For example, in a temperature profile test, the LABS 40K™ unit, equipped with a KRYOS® control system (Taylor-Wharton Cryogenics, Theodore, AL) (Figure 1), demonstrated the ability to maintain a consistent temperature of –180 to –187 °C across the top of the racks. In fact, when the lid was left open for 3 hr, the temperature still held within a range of –171 to –181 °C (see Figure 2). It is unlikely that a mechanical unit would perform as well with its door open for this extended period of time.

Figure 2 - Temperature of vapor phase unit.

Storage above the glass transition temperature

Conventional wisdom says that because mechanical freezers are convenient, they are the natural choice for storage above the glass transition temperature. However, the choice is not so simple. Users need to consider a host of factors, some of which may make an LIN freezer the practical choice above the glass transition temperature also. In reality, most facilities use a combination of mechanical and LIN units to meet their needs. Following are some factors to consider when making decisions for specific applications.

Facility planning issues

Facility issues such as power demands, space, location, cleanroom, and secure access requirements all play important roles. Users should consider questions such as, “How much electricity will I need to operate the freezers?” and “How do I supply units on upper floors?” A discussion of these issues follows.

Power

Power consumption is a primary consideration. All storage systems require some electricity to operate. However, mechanical freezers consume much more power to run compressors and refrigeration systems that achieve low temperatures. The electricity to run a mechanical freezer costs approx. $1100–$2200 per year, depending on the local price of electricity. Most facilities have 20–30 freezers; thus the annual power consumption can easily cost $44,000–$66,000. Conversely, an LIN unit consumes less power than a light bulb to run its automated controls. It achieves its low temperatures through cryogens.

Added to this higher power demand is air conditioning the room with mechanical freezers. The compressors on a mechanical unit emit about 3200 BTU of heat per hour. A room could quickly go from 70 to 100 °F, putting an increased load on the air conditioning system. For one –80 °C unit, this will add $300 per year, depending on local power costs. For the entire facility, this could mean an additional $6000–$9000 power cost. In some older facilities, sufficient air conditioning may not be available, requiring upgrades that may make mechanical freezers cost prohibitive.

Also, the possibility of a power disruption should be considered. During a disaster, or even a more common brownout or blackout, the lack of power can put samples at risk, especially if the proper backup systems are not in place. Mechanical freezers operate much in the same way as household freezers—without power, the unit does not generate cold temperatures. The typical –80 °C mechanical unit therefore uses high-pressure CO2 cylinders that keep the unit cold for approx. 5–7 hr if the power goes down. One should keep in mind that the gas coming from the cylinder is –50 °C, which is warmer than the intended temperature of a –80 °C freezer. For colder temperatures, an LIN backup cylinder is required. Conversely, a typical LIN freezer deprived of electricity can keep samples below the glass transition temperature for up to 10 days, provided the lid is kept closed. Backup systems will be discussed in detail below in the section on managing the supply chain.

Space

Because space is at a premium in most biotechnology laboratories, planners need to carefully consider their short- and long-term space needs when selecting units. A typical –80 °C freezer, with a capacity of 48,000 vials, takes up a little over 26 ft2, including the space to open the door and a 6-in. space around the unit for airflow. The unit may require a 9-in.-diam CO2 cylinder for backup. In contrast, a LABS 40K unit takes about 17 ft2 to hold 41,500 vials (see Figure 3). It must be kept in mind that the LIN unit uses a 26-in.-diam LIN cylinder. If a laboratory chooses to keep the LIN cylinder right next to the unit, the space requirement is about 30 ft2. However, planners may want to consider implementing alternative supply modes such as an external bulk or microbulk tanks located in less spaceconstrained areas to conserve space in the freezer room.

Figure 3 - Unit dimensions.

Another consideration is that LIN freezers are a good option in places in which headspace is limited. LIN freezers are built chest style—the average LIN unit is a little less than 4.5 ft tall, significantly shorter than a 6.5-ft mechanical freezer. An additional 1.5 ft of headspace should be allowed for the lid to open so that personnel can retrieve samples when necessary.

Weight and facility limitations

When selecting units for upper floors, weight limitations can restrict the size of the individual storage unit. The evaluation can become very detailed and specific; therefore it is a good idea to review the building codes prior to selecting a unit. Additionally, the physical dimensions of elevators, doorways, and corridors may limit the choice of units for upper floors. For example, a larger unit may be ideal for a first-floor installation but may not work on a second floor if there is not an elevator of suitable size. Likewise, larger units may not fit through narrow doors or corridors.

Validation

Validation is a process that has to be undertaken for both mechanical and LIN units. Validation includes several processes, including installation qualification (IQ), operational qualification (OQ), and performance qualification (PQ)—costing approx. $1000. This cost can increase depending on individual customer specifications and needs to be factored into the cost for each unit. In general, the fewer the units, the lower the validation cost. Thus, where possible, one should consider selecting the largest unit in a series to minimize this cost. For example, a LABS 80K™ unit provides about the same storage capacity of two LABS 40K units at half the validation cost.

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