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I am automating a small-scale laboratory chemical process. One step is thermal equilibration of DNA in about 0.5 ml of aqueous buffer, which involves raising the liquid to 90°C and then immediately cooling it to 4°C, holding it at each temperature for about 10 minutes. The liquid must not turn to ice, but +/- 3-5 degrees is an acceptable range of temperature control. I'd like the temperature to move from 90C to 4C within about 1 minute.

Ideally, I'd like this to take place in a removable 1.5 ml test tube, or alternatively in a 3 mm OD/1mm ID hose, and be simple and inexpensive to install. It's OK if it requires some manual interventionl, as long as it can be set up in advance. This is going to be a device that is set up during the day and run overnight.

My impression is that a wirewound resistive heater would work best to heat to 90°C. For cooling, I've considered heat pipes and Peltier cooling plates, as well as having the test tube/hose immersed in a small water bath that is heated and cooled from below.

It seems like a Peltier plate would not transfer heat efficiently in and out of the tube, because it would be a flat surface in contact with a round surface, giving minimal contact area. But I haven't seen flexible heat pipes, and I don't know if they'd work to bring the tube to a specific temperature. A water bath seems like a fine solution, but a solution that directly controls the temperature of the tube without introducing a source of contamination would be better. What would you recommend?

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    \$\begingroup\$ hot and cold water sources, two electrically operated water valves and a timer \$\endgroup\$
    – jsotola
    Mar 5 at 18:05
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    \$\begingroup\$ Hi and welcome! To get the non-answer answer out of the way -- there are standard PCR machines already; how does your process differ from that (could it be ran on one anyway?), or could one be modified to fit your process? As for the question, how fast should heating/cooling be? How accurate the temperatures and times? \$\endgroup\$
    – Tim Williams
    Mar 5 at 18:17
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    \$\begingroup\$ There isn't actually any question to answer. I don't see a question in the text or the title. You should ask a question that is relevant to electrical or electronic engineering. Given that a Lab test set-up for DNA is likely to be many thousands of dollars, don't you think that buying a proper piece of calibrated equipment is likely to be worthwhile especially if your home-spun solution is called into question? \$\endgroup\$
    – Andy aka
    Mar 5 at 18:19
  • \$\begingroup\$ Hi Tim. The equipment I'm building is part of a larger apparatus that handles a bunch of tedious washing and incubation steps, which a PCR machine by itself doesn't do. Liquid handling machines also exist, but are expensive and offer a level of precision control that isn't necessary for this experiment. \$\endgroup\$
    – AllAmericanBreakfast
    Mar 5 at 18:23
  • \$\begingroup\$ Andy, good point. I updated so that this is actually a question. This apparatus is not for testing DNA. It would be used for a method called SELEX, which generates short pieces of synthetic DNA that bind a specific target. This process involves a huge amount of tedious manual labor, which I am hoping to automate. Sequencing and testing the output DNA, as you suggest, would not be left to a homebrew device like this. \$\endgroup\$
    – AllAmericanBreakfast
    Mar 5 at 18:31

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