Over the past few months, a group of committed volunteers from the Philadelphia Makers Meetup have come together to address the shortage of lifesaving face masks. Our objective was to design a practical, DIY device that could sanitize reusable PPE in accord with the following 6 criteria:
- Quick, cheap and simple to build – less than 2 days, using around $200 in parts (readily available from Amazon)
- No programming or advanced electrical skills required
- Reliable, safe and easy to use with low maintenance
- Quick operating cycle – 30 minutes at 75C (158F), once the cabinet has warmed to the operating temperature.
- High throughput -100+ masks per day with minimal risk of mask damage
- Compact and portable – easy to move to where it’s needed
Our team, which includes a PhD with deep domain expertise in microbiology and a PhD in Electrical Engineering, researched the many recommended methods for sanitizing facemasks to deactivate the Covid-19 virus. These methods include: UV light exposure, vaporized hydrogen peroxide, ethylene oxide, ozone, isopropyl alcohol spray, moist-heat and dry-heat. For a variety of reasons and after much consideration, we settled on the dry-heat method as the most appropriate DIY build approach. And while exposure at 70C (158F) for 30 minutes is an optimal method for pasteurization of pathogens in food and vaccines (and to inactivate common human viruses), some recently updated studies suggest that 75C (167F) is better when it comes to the Covid-19 virus. See, for example (PDF download)Of course, the guidance may change as more research is conducted.
So, we are recommending a 30 minute cycle, to commence only once the temperature controller has reached a steady reading of 75C (157F). Be sure to allow the cabinet to thoroughly pre-heat (which should take less than 15 minutes depending on your build). Or, for simplicity, set-and-forget for a full hour on the timer (erring on the side of caution).
Early in the design process we settled on the use of incandescent light bulbs as the heat source (cheap, long-lasting and easy to replace). Think the original Easy-Bake Oven, or a soft-pretzel warmer; but even this decision led to complex tradeoffs and many iterations. Initially, we tested with a single, very high-wattage bulb. But to minimize the risk of mask ignition (and burning your hands) we consequently had to enclose the bulb in a bulky protective cage; moreover, high wattage bulbs are more expensive and less durable. So we settled on 4 low wattage bulbs.
Of course, to make the cabinet simple and inexpensive, we decided to use wood. But our testing showed that it took a long time to reach the requisite temperature because most of the energy went to heating the bulk of the wood (and surrounding atmosphere). So we realized the need to incorporate some kind of insulation. However, the most readily available – Styrofoam board – was found to outgas at 75C (creating its own form of contamination). Needing something more heat-resistant, we ultimately turned to the more high-tech (but still Amazon-available, K-Flex foam. All as detailed in our Materials List.
At this juncture, we thought we were finally ready to go – and proceeded to lab testing. We worked with three types of samples – masks that had been worn extensively, unused masks that we ‘contaminated’ with an organism that is bio-similar to the COVID pathogen (in its vulnerability to heat) and pristine (untreated) masks out of the box. Ran the cycle (for a full hour) and then waited to see what would grow in the petri dish from swabs of the masks. Well, weren’t we surprised to find that swabs from all of the masks (even those unused and presumably sterile) resulted in still-viable organisms growing in the medium. Was there a problem with the scientific guidance or the multiple probes deployed to monitor the heat and duration during these tests? Well, it turned out, none of the above. Rather, we had ’discovered’ that there are organisms – including certain bacteria and spores – which can’t be deactivated at 75C. The relief came from the discovery that there was no trace of the coronavirus biosimilar microorganism, and that these relatively common (and, also relatively non-pathogenic, though heat resistant) organisms were present in the previously worn masks having been introduced through the exhalation of the human subjects. What’s more, because we use a fan to insure even heat distribution, we ended up cross-contaminating the sterile masks as well.
What to do? One option would have been to dramatically increase the heat (to 100 or even 130c) – but that would likely damage the masks while posing other dangers and complexities. And so we settled on recommending that each mask be self-contained and labeled by user (even a paper lunch bag will work). This helps to insure that users get back a mask decontaminated from the virus that causes Covid-19, along with only those heat-resistant organisms that were already present in their personal exhalations.
I could go on. We ultimately went through many builds, all while adhering to the criteria above enumerated (which dramatically constrained our design options).
For the results – including detailed, and of course free, DIY instructions – please visit Lightboxdiy.com and start building. Of course, we make no warranties that the LightBox will actually deactivate the Covid-19 virus. This isn’t a medical device. And if you make any substantial changes/improvements you make could change the operating results. So make sure to carefully test that the requisite temperature is achieved (and maintained) with the heat uniformly distributed throughout the cabinet. The ramp time should not exceed 15 minutes, assuming a sufficiently robust heat source and ample insulation.
I’d like to thank my volunteer teammates, Karen Hogan, Edward Polkowski, Brad Denenberg, Andrej Patoski and Evan Gurgui. And, of course, where would we be without Dale Dougherty who encouraged us to keep at this work (and who has committed himself to sharing this and other life-saving innovations with the worldwide Maker community).
Please let us know how you are doing at the community page on our website, including details and pictures of your own builds (and suggested improvements which you’d like to share). You can also email me directly, firstname.lastname@example.org