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Please check out our DNA Thermal Cycler / PCR Instrument section to find manufacturers that sell these products
Thermal cyclers, also known as PCR machines or thermocyclers, are essential for any laboratory that relies on molecular biology. An indispensable tool for DNA amplification, the thermal cycler often becomes a nonstop workhorse that lab workers expect to function properly and efficiently. Due to the wide range of choices available from manufacturers, a good thermal cycler is not hard to find, as long as the lab first identifies what it needs in a thermal cycler, and then seeks the type that can best rise to the occasion.
Behind the technology
The basic idea of a thermal cycler is that it provides a thermally controlled environment for PCR samples. A thermal cycler usually contains a heating block with holes or depressions in it that receive sample tubes (though other types of sample vessels are now possible also; see below). For the PCR reactions to work properly, the block must change temperature at specific times, and spend specific durations of time at specific temperatures. The researcher programs the temperature cycling information into the thermal cycler either by computer or via a console on the instrument, or uses a preprogrammed routine built into the machine.
Applications of thermocyclers
Thermal cyclers are used in a range of disciplines. The majority of thermocyclers reside in research laboratories, where they are mainly used for amplifying DNA. In addition, researchers take advantage of thermal cyclers for cloning DNA and sequencing genes. Clinical facilities also house thermal cyclers for genetic tests that are used in diagnosing diseases.
Types of thermal cyclers
Thermal cyclers are available in a variety of sizes and models. You can choose from among the larger sizes of traditional models, smaller compact models designed to save lab space, and even “mini” or “personal” models that can be placed atop a desk. For the number-crunching capacity of much larger systems, some manufacturers make thermal cyclers that can be linked together to create a larger PCR system controlled by a central computer. Labs wanting to maximize their throughput capabilities may benefit from the linked PCR machine system.
Considerations for purchasing
There are several factors to bear in mind when considering the purchase of a thermal cycler.
1) Temperature change
Most thermal cyclers use a Peltier element to regulate the control of the temperature block. Ideally, the thermal cycler would change the temperature inside the reaction tubes instantaneously. In practice, it changes the temperature immediately surrounding the outside of the sample tube. For the reactions to work properly, the temperature needs to be changed at precise times and as quickly as possible. Ideal conditions would include having the fastest possible temperature change, but some types of work may not require the fastest of the fast—how fast depends on your particular experimental needs.
All thermal cycler manufacturers aim to make heating blocks that change temperature uniformly and as quickly as possible. If fast temperature changes are important to your work, look for thermal cyclers with a heating block made of silver. Silver heating blocks work best for fast temperature changes, and for encouraging temperature uniformity throughout the heating block, because silver conducts heat better than other materials used for heating blocks.
2) Capacity and vessel type
The thermal cycler block comes in a variety of sample capacities, with wells for sample tubes numbering up to 384, for example.
The issue of capacity is also related to the type of containers that you plan to use for the individual PCR reactions. The ubiquitous 0.2-mL PCR tube is still the most commonly used PCR reaction vessel. However, it is also possible to find thermal cycler blocks that can hold a variety of other containers, including 0.5-mL PCR tubes, yet more tubes that are distinct from the traditional PCR tubes (such as larger conical tubes), multiwell plates, and tube strips.
3) Heated lid
In times past, researchers added a layer of mineral oil to the top of each PCR reaction to prevent the reaction fluid from evaporating and condensing on the inside of the tube lids. Today, instead of dealing with messy mineral oil, many thermal cyclers are equipped with a heated lid that lies on top of the samples in the temperature block, preventing evaporation and the resulting condensation. This is not just a matter of convenience—evaporation and condensation can lead to data disaster, even more so in small-volume samples, because they change the concentration of reagents in the PCR tubes, which of course can result in skewed results or unwanted variation. A specialized, heated thermal cycler lid can help to prevent these problems, especially for sample tubes at the edges and corners of the heating block, which are more vulnerable to evaporation and condensation.
4) Flexibility, modularity, and adaptability
In earlier thermal cycler models, and indeed in some of today’s high-quality models, the heating block was one solid component. Today many manufacturers offer thermal cyclers with interchangeable heating blocks. This is useful if you need to use a different number of samples, or a different kind of reaction vessel, for example, changing from PCR tubes to multiple-well plates.
In addition, some manufacturers offer thermal cyclers with modular heating block systems. In these systems, the single large heating block can be replaced by two or more smaller heating blocks that together equal the same size of the original. Some thermal cyclers allow the smaller heating blocks to be controlled independently, so one researcher could have, e.g., three different experiments running at once. Alternatively, three different researchers could run their own PCR reactions simultaneously.
Other thermal cyclers use just one temperature block, but the researcher can control the temperature of different areas of the block separately (as if it were comprised of multiple smaller blocks). Known as a gradient function, the ability to program different temperatures into the same block is helpful for experimenting with primer annealing temperatures, for example, or for just generally optimizing PCR reaction conditions.
Latest developments in thermal cyclers
Table 1 – Purchasing considerations for thermal cyclers
The PCR machine is evolving at a rapid pace. Today, you can even pack a thermal cycler with you on your next trip. Biologists who do field work can now accomplish more in less time by analyzing samples on site. Even if your work is confined to the lab rather than the field, you can find greater convenience and efficiency in the newest thermal cyclers. Some now include WiFi capability, which means that you can connect to them wirelessly through a mobile device like a tablet or smart phone. The convenience of having your data streamed directly into your hand cannot be overstated, and the ability to control the thermal cycler remotely can be a huge convenience. You can make tweaks to the thermal cycler’s program from afar if necessary, without having to haul back into the lab for important adjustments.
The good news is that most PCR machines are extremely dependable, so in many ways, you really can’t go wrong. Considering the most important aspects of a thermal cycler will help you to winnow your choices down to the best one for your lab.
A summary of purchasing considerations for thermal cyclers is given in Table 1.
Caitlin Smith is a freelance science writer who has a Ph.D. in Neuroscience from Yale University and postdoctoral work in Electrophysiology and Synaptic Plasticity; e-mail: [email protected].
Please check out our DNA Thermal Cycler / PCR Instrument section to find manufacturers that sell these products