Anyone following me on twitter may have noticed I posted a few pictures of my progress during my build of a simple HEPA filter based laminar flow box (not sure if it’s actually considered a flow hood). The idea of this build being to greatly improve the reliability of the aseptic protocols in our lab.
To come up with the design I did a lot of research and ended up finding some great resources on various psychedelic mushroom growing forums, a surprisingly good resource. The design that is popular and that I ended up going with (at least for mk I of the flow hood) is essentially just a box with a fan blowing in one side and the HEPA filter mounted on another side. While the construction side of things is therefore rather simple, the crucial aspect is actually achieving a laminar airflow by pairing a suitable fan with your chosen filter.
I found that this guide by EvilMushroom666 explained the maths of figuring out what fan to pair with what filter really well. In summary, you need to source a fan that is capable of blowing enough air for the size of your filter at the given amount of pressure drop (back pressure) that your filter creates.
So step 1 was to find a filter that was reasonably priced and would give a decently sized working area. Unfortunately for us Brits, it seems there are far more options in the US than in the UK for reasonably priced HEPA filters, however Jasun Filtration had the Ultima MPW78 18×24 for £100. Right. Filter sorted.
Step 2 was where the inimitable EvilMushroom666’s guide lead the way for me and helped me to arrive at the necessary specification of the fan I would have to find. It seems that most people have to resort to squirrel cage blower fans to achieve the kinds of airflow necessary despite the high pressure drop of HEPA filters. However, after several hours of looking through fans on mouser.co.uk I managed to a find a beastly 12V DC fan that could handle the pressure drop and still put out enough air for our filter at around a quarter of the price of the squirrel cage blowers.
Now came the assembly, which was essentially just the building of a box. And though being located in a hackspace certainly helps with getting this done, this could all be done with very minimal tools should you not have access to a fully kitted out workshop. One thing to note was that my attempt to make the box airtight using the spare drain sealant lying around the hackspace didn’t actually work, however this was easily remedied with a bit of tape to seal all the box’s joints properly.
Finally, I attached a prefilter and we connected the fan to a power supply and overvolted it a tad to reach the required airflow. We tested to see if the air coming out of the filter was indeed laminar using a bit of liquid nitrogen and some water to produce a thick cloud of easily visible moist air.
The final test was to carry out a dummy plating procedure where the LB agar plates were not inoculated with anything but then incubated for a few days. Our first attempt was somewhat disheartening as the plates that were prepared in the “sterile” zone did infact become contaminated (though less than the controls). For attempt 2 we created a raised shelf in front of the filter on which to work, and this lead to success.
In this improved configuration, our controls, where the plating was done outside the clean airflow, were covered in a colourful mosaic of colonies. And, apart from the two plates that were furthest away from the filter at the corners of the clean work area, the rest of the plates were completely sterile.
Obviously, though this is an incredibly encouraging result, it has not quite proven that the “sterile” work area is indeed sterile. However since we would rather not waste time and petri dishes by doing further dedicated experiments to establish the effectiveness of the DIY flowhood, we have marked the 8 spaces in which petri dishes can be placed in the sterile area. Every time a plating protocol is carried out we shall record the what space it was carried out in and if/how many contaminant colonies grow on the plates. Over time we should build up a nice picture of the statistical likelihood of contamination based on the spatial position of within the sterile area.
This is only the beginning however. I have as yet not found any examples of people constructing a vertical laminar flow cabinet, and as you can see from the wiki page for this project, that is the eventual aim of the DIY flow hood mk II).