Thermal Cycling Airflow Reactor I (Part 7)

Design challenges

In previous projects, I have found that low-cost 60mm PTC car heaters can be adapted for efficient heating of microreactors via forced air convection. The 150W 12V heaters have an integrated fan and provide relatively fast heating times, 10x faster than PTC block heaters coupled to fan-driven heatsinks.

An overall summary can be found at: https://www.hackster.io/jim-haseloff/airflow-microreactor-projects-327330 - with links to Hackster project pages with more technical information. The efficiency of the 60mm car heater has opened up the prospect of building low-cost temperature cycling incubators, using switched fans and airflows.

There are a couple of major challenges - one is to engineer dual airflows that are respionsible for heating and cooling respectively, simply switched by alternate powering of fan systems. Second is to switch to a temperature resistant polymer for the 3D printed reactor vessels. For this, I have been trialling Nylon, ONE PET and GreenTec Pro filaments, which offer HDT values over 100ºC, but are generally more difficult materials to work with.

Heat resistant 3D printing polymers

In order to move to thermal cycling applications, I have been experimenting with different heat-resistant filaments, including Nylon (polyamide), ONE PET and a new organic polymer, GreenTEC Pro. I used Nylon filament to print prototype vessels that were tailored to hold the fan-heater assembly and twin exhaust and inlet blower fans. These first prototypes were built with relatively thin walls (3 mm) and lid that fitted over the top of the base. Heating profiles are shown above - the vessels heated quickly, reaching 95ºC setpoints within a few minutes. Very simple, untuned heating control software was used in these experiments, so there were noticeable temperature oscillations around the set point. Successfully printed Nylon vessels performed well at higher temperatures, reaching over 100ºC, with no sign od the softening and sagging that could be seen with PLA prints at higher temperatures (>80ºC). Attempts to build larger vessels with long print times (around 24h in draft mode) were beset by a variety of technical problems - stringing and nozzle blockages with nylon, and brittle prints with ONE PET.

GreenTEC Pro is a relatively new material from Extrudr in Austria. It appears to be derived from lignin (wood) starting materials. The GreenTEC Pro filament printed like PLA, reliably and without problems - albeit at higher temperature 230ºC and with hotter glass support plate (90ºC). I have switched to the new biopolymer and have been able to able to test its advertised heat tolerance. It tolerates up to 115ºC without deformation. The material has a smooth matt surface, and is easy to work after printing, allowing drilling filing, etc. It is also available in a wider range of colours than Nylon.

Switched airflows

The availability of higher power fan heaters, and temperature resistant polymers for 3D printing - allows the prospect of building thermal cycling devices capable of driving PCR reactions as well as isothermal diagnostics and DNA assembly reactions. PCR reactions require high temperatures (~95ºC) and rapid transition between set points. The images below show the GreenTEC Pro 3D printed components used to build prototypes to test this. The reactor was based on a 3-part stacked design - with base, containing heaters and fans, sample holder and lid. The main walls of the reactor were 9mm thick, with 30% 3D cubic infill, and used a flange-socket arrangement to join the base and lid.

The fans were arranged to drive two cycles of air flow - one for recycled flow of heated air, the other for blower fan driven intake and exhaust of air for the cooling part of the cycle. The different directions of air flow are shown below. 50mm blower fans were used for these, each mounted in custom designed holders.

3D printed components

The custom prototype reactor was designed in Autodesk Fusion 360. The base, fan holders, sample holder and lid parts are designed to slot together at defined physical interfaces, and can be independently redesigned - potentially useful to accommodate detection circuitry or altered air flow.

The parts were printed using GreenTEC Pro filament and assembled with the electrical components for testing.

The wired prototype was hooked up to a custom programmable controller (https://www.hackster.io/jim-haseloff/programmable-test-rig-d7df62) - images of the touchscreen interface and XOD sketch are shown below. The controller allows setting of different set points, and provides a PID controller for regulating temperature shifts.

Controller

When powered up, the prototype was tested by heating to set points, including the the upper limit of its temperature range, at 95ºC, and held there. Infra red images were taken to estimate surface temperatures. The 9mm thick walls of the GreenTEC Pro prototype reduced external surface temperatures, and the profile was consistent with teh expected patterns of airflow.

Temperature cycling

Further a program for cyclic temperature changes was implemented to test rates of change. In first tests, the controller was set up to cycle between 95ºC, 55ºC and 72ºC - common conditions for polymerase chain reactions. The speed of heating and cooling allowed a cycle time of around 350 secs per iteration.

While the latest designs give feasible rates of temperature change and control - there is still a question of even heating. In this design, the sample tubes are suspended over the fan heater inlet and return air channel. The reasoning was that rapid air flow in the small enclosed volume would help equalise temperatures across the vessel. However, IR images and temperature sensors show that a temperature differential can build up across the sample tube rack (see below).

To deal with this - the vessel was modified to:

(i) reverse the direction of airflow, so that the fab heater was directly blowing onto the sample tubes. (The existing design deliberatelt relied on baffling and indirect airflow to maximise air mixing).

(ii) This required the development of a manifold to evenly direct the flow of heated air across the samples.

The redesign could potentially increase the speed and efficiency of heating/cooling, and allows more precise control of airflow and thermal exchange through 3D design of manifold systems - taking advantage of the heat-resistant properties of the GreenTEC Pro material.

Project continued at: