Product Survey: Dry Block Heaters

Heating Up
by Harald Zähringer, Labtimes 02/2017

Creative life science researchers may cover a lot of ground with nothing more than a simple block heater, a few Epp-cups and a micropipette. No wonder, then, that dry block heaters dominate the scenery on many lab benches.

Block heaters are often equipped with interchangeable modular aluminium blocks adapted to different vessels, containers, vials and tubes, typically used in life science labs. Dry blocks based on ­resistance heating cover temperatures from room temperature, or slightly above, to about 150 °C. Peltier driven systems may heat to similar temperatures but can also cool down probes to -10 °C or even -20 °C with additional insulation.

Peltier elements are built from two different types of Bismuth Telluride semiconductors, doped with foreign atoms: N-type semiconductors with electron excess and P-type semiconductors with holes or electron deficiency. If both ends of a N/P-type semiconductor pair are electrically joined, a thermocouple is formed.

Heat sink

Connecting a D.C. source to one junction of the thermocouple gives rise to the so called Peltier effect: electrons moving through the circuit absorb heat while passing the cold (P/N) junction (heat source) and transport it to the hot (N/P) junction (heat sink) on the opposite side of the system. However, a single thermocouple pumps only little heat quantities from the cold junction to the hot junction. Hence, Peltier systems are composed of numerous thermocouples working thermally in parallel (though they are connected electrically in series).

The temperature is usually adjusted in both types of dry blocks with so called proportional, integral, derivative (PID) control units that restrict deviations from the set values to less than 0.5 °C. Though block heaters with analogue temperature setting and turning knobs are still available, most models are digitally controlled via push buttons, touch pads or touch screens. Set and actual values for temperature are indicated on LCD displays and may be chosen in most dry blocks from a variety of stored temperature programmes.

Dry blocks are applied by life science researchers, e.g., for isothermal amplification of nucleic acids, which has sparked considerable interest in recent years as a convenient replacement for traditional PCR. Isothermal nucleic acid amplification has been established in many different forms, with Loop-Mediated Isothermal Amplification (LAMP) and Recombinase Polymerase Amplification (RPA) being the most popular techniques.

Though particular amplification methods differ in certain aspects, all have one thing in common: the temperature is kept constant during the whole amplification process – which can be achieved with a cheap dry block.

Constant temperature

But you don’t even need a dry block for the isothermal amplification reaction to occur. Rebecca Richards-Kortum’s team at the Rice University in Houston, Texas, has developed a low-cost RPA-based method for isothermal amplification of HIV-1 DNA that applies body heat to incubate the reaction (PLoS ONE 9: e112146).

RPA is perfectly suited for simple ­nucleic acid tests (NATs), which are increasingly used for rapid diagnosis of infectious diseases, especially in low resource settings: recombinase polymerase is pretty immune to impurities, it is available in lyophilised form, amplifies the DNA within a few minutes and, last but not least, the enzyme’s temperature optimum is 37 °C.

So, why not incubate RPA reactions with body heat?

The Texan group recruited ten volunteers and initially let them incubate RPA mock reactions (50 µl water in a 2 ml microcentrifuge tube) in four different body locations: “in a closed fist, placed in a rear trouser pocket, held in the axilla (outside of clothing), or taped to the abdomen (under clothing)”. It’s not really surprising that the measured average temperature of mock reactions incubated in the axilla (34.8 °C), was closest to the temperature optimum of the enzyme. Hence, the axilla was chosen for further RPA incubation experiments.

The axilla turned out to be the best location for body heat incubation of RPA reactions. Reaction tubes were placed in zipper locking plastic bags and secured in initial mock reactios under the arm with either a bandage, an elastic sweatband or a strip of African chitenje fabric. The group chose the chitenje fabric for further incubation experiments.

Body heat incubation

The tubes containing the RPA reactions were secured with a strip of cotton fabric under the axilla of the volunteers, which had to incubate the samples for twenty or thirty minutes in an office at room temperature or in a cold room at 10 °C. To compare the efficiency of body heat incubation with standard methods, reactions were also heated to 37 °C in a dry heating block under the same environmental conditions.

The results of the body-heated incubation experiments are crystal clear: ten out of ten reactions, containing ten or one hundred copies of HIV-1 DNA and conducted in the cold room at 10 °C, were positive and yielded detectable amounts of DNA. Almost the same holds true for incubation experiments performed at room temperature. The group had to classify only one reaction (out of ten) that contained one hundred copies of HIV-1-DNA as negative.

Chemical heater

If you have issues with incubating your RPA reactions under your arm, you may also consider chemical heaters as an alternative to electricity-driven dry heat blocks. Richards-Kortum teamed up with the Austrian born, in-vitro diagnostic specialist Bernhard Weigl, now at the Intellectual Ventures Laboratory in Bellevue, Washington, to design a chemical heater based on the exothermic reaction of solid magnesium with methanol (PLoS ONE 10: e0139449).

The setting of the chemical heater is pretty simple: a PCR tube is placed into an isolated five millilitre tube harbouring a small fuel pack, filled with mechanically alloyed magnesium-iron, sodium chloride and anhydrous copper (II) chloride (chloride is added to accelerate the redox reaction).

Injecting a few gram of methanol starts the exotherm reaction that brings the methanol to the boil at around 65 °C – which is the perfect temperature for loop-mediated isothermal amplification of nucleic acids.

Though the chemical heater is still in the proof-of-concept phase, it might be useful, especially in developing countries, to perform LAMP or other isothermal amplification reactions without electric dry heat blocks.

First published in Labtimes 02/2017. We give no guarantee and assume no liability for article and PDF-download.

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