I’ve been asked to design a respirator cartridge that fits to anesthesia masks (possibly used and sterilized), and some CPAP masks (either sharing the same standard 22mm fitting or with adapters) as an emergency N95 alternative. I’m neutral on filter media, but researchers at UF identified using both layers of Halyard H600 surgical wrap as an effective medium when there’s enough surface area. However, their design doesn’t 3d print well, and is rather large and heavy.
I have a design that provides greater surface area of filter media for breathability, doesn’t depend on rubber bands, and is much lighter (47g printed in PETG, which is the heaviest of the common 3d printing thermoplastics). I’m doing some fine-tuning on that should go out for proper N95 testing in a few days once I’ve dialed in the parameters. I made an adapter that allows me to test with a CPAP mask I was given for the purpose (it has a standard 22mm fitting on one end, and the other end engages the CPAP mask taper) and it’s quite breathable.
@HalfNormal I know you’ve been concerned about amateur efforts here. Obviously this doesn’t replace the real thing. But it was driven by a physician in NJ asking for help because of physicians and residents intubating COVID-19-postiive patients without any shield or respirator at all, and my current plan is that assuming good results from an N95 test it goes next to NIH. Since this is your field, do you have thoughts in particular on availability of used, sterilized (or, if supply is not a problem, new) anesthesia masks? It’s the good face fit that I’m most concerned about; that’s the part I don’t really influence because it’s a cartridge not a mask that I’m designing.
Here’s the work in progress repository:
Here are pictures of it attached to a CPAP mask, with tape over the holes in the CPAP mask for a test seal.
The frames are meant to hold mechanically, but can and probably should also be taped together, front to back. Belt and suspenders.
Here’s a rendering of the design, showing the frames that hold the filter medium in place both as they are printed (next to the base unit) and also how they are installed on the base unit.
First of all thanks for your efforts. It is really appreciated!
I am not too sure of your question. Is the mask for the health care worker or patient? I am unaware of a cartridge mask for health care workers but that does not mean it does not exist. A CPAP mask alone would be overly bulky and I am not sure of the fit while moving or talking which is tested with the N95 mask fit.
It’s a cartridge to add to an anesthesia mask to make a well-fitting N95 emergency stand-in for providers. For CPAP masks there are different designs, and one possibility is a simpler frame not intended for sleeping that re-purposes only the inserts that include the silicone face fitting for seal but not the bulk of the CPAP strap system. Haven’t gotten there yet; focused first on the request for repurposing the anesthesia masks. I don’t actually have them yet. I have a donated CPAP system that has a standard (I think) 22mm fitting that is supposed to be the same fitting as the anesthesia masks and which I measured to design the cartridge.
The availability question is about how many disposable but sterilizable anesthesia masks you see actually disposed of that could instead be autoclaved and repurposed, and/or what the supply chain currently looks like for new anesthesia masks. I’ve heard normal demand is down due to rescheduled elective surgery so there might be a glut of supply unless the manufacturers have pivoted? This started with an attempt to reuse masks that would otherwise be disposed of, though.
It’s hard to have a good sense of where things stand overall; things change so rapidly. Definitely a chance that by the time I’m done this will have turned out to have been useless, but better to have tried than shrugged.
Yes, anesthesia masks are not affected by the PPE shortage. Since surgeries are down they should be plentiful. The real issue would be how to secure the mask to a person’s face. They are not made to be secured in a vertical position. The patient is laying supine. They are single use and made of plastic so I doubt they would hold up to being autoclaved.
I hope this helps.
Good to hear that it makes sense to you that if this works it makes sense that it wouldn’t cause immediate downstream shortages. That’s one of my concerns.
It was anesthesiologists apparently who came up with the idea of sterilizing them; I assumed autoclave but maybe a different protocol, I didn’t ask. I don’t actually know how they are thinking of attaching them, either! But this was from folks whose alternative is making respirators out of to-go containers and such. Maybe attachment fittings will be the next request I get!
It’s clear that there are areas of NYC and NJ that don’t have lots of resources and have to get scrappy. What’s particularly sad is that the NYC area really does have a lot of resources overall and is having trouble providing for health care workers; Detroit is in a similarly bad place but has neither the resources nor the media presence and so rarely makes it above the fold. (This is why the bulk of the face shields I’ve shipped non-locally have gone to Detroit.)
We are are reusing masks that have been sterilized using a hydrogen peroxide mist technique. They are also laundering tyvek jumpsuits. We have also repurposed expired N95 masks for droplet precaution only. The hospital is actually very fortunate in that we are trying to conserve and are not running out. My heart goes out to those hard hit areas and of course the healthcare professionals trying to make the best of a bad situation.
Once again I cannot thank you enough for all you are doing.
I have the current iteration of the frame down to 40g of PLA and printable on a 200mm x 200mm bed:
I have a (not tested yet) adapter for various straps (20-22mm CPAP, ½" elastic, ¼" elastic) that can be adapted by simple parameter changes for other strap widths:
It fits around the inlet where it connects to the cartridge so you can just print the right holder for the straps you have. This way you don’t have to have masks with strap attachments on them, or strap attachments that fit your head for this use; so if the straps for anesthesiology masks don’t hold well when you are walking around instead of laying supine, you aren’t constrained by those straps. At least, that’s the theory; needs tested in practice.
@HalfNormal do the anesthesiology masks, at least the ones you work with, have a 22mm outlet fitting, or inlet fitting? I’ve been working from other masks so far, waiting on arrival of an order of anesthesiology masks for testing, and hoping to get test article ready for N95 fitment tests at a hospital about an hour away this week…
Thanks again! Is that dark ring for attaching straps fixed to it, or is it a removable part that I can replace with something optimized for different straps?
Aha, more people have tested the concept, minus my cartridges!
My concern would be fitting the stack-up they show in that video underneath a face shield. My cartridges add less than a centimeter of depth relative to the mask outlet.
I tried a CPR mask. It is conceptually similar, but it has a 22mm inlet instead of an 22mm outlet, and the pillow seal is not adjustable for fit. It didn’t work well with my glasses. Sounds like the anesthesiology masks will fit better and be cheaper. Better, faster, cheaper; pick three!
Eight days after my first pictures above, they don’t really look any different, but after adjusting lots of things by fractions of millimeters I have a set that doesn’t fall apart even when I hold it in my hand and shake it violently, and we’re now arranging for an N95 fitment test hopefully to include both one particular CPAP mask (quite comfortable to wear) and an anesthesiology mask. I have an adapter for a CPR mask but the CPR mask just isn’t as comfortable to wear, at least the one I got, so I’m thinking of that as a last resort.
Now printing the second of three sets (one each in PETG, PLA+, and PLA) planning for Bitter Fit tests next week.
For sterilizing PETG, ABS, ASA, or any of the high temperature thermoplastics, the 60⁰C for 30-45 minute protocol will be fine and could be used for an assembled cartridge, but that won’t work for PLA or PLA+.
When throwing away the filter medium and sterilizing the parts, UV-A or hydrogen peroxide mist should be appropriate protocols for any plastic. The lifetime of these parts is unlikely to reach the point where UV damage to the plastic is a consideration.
The bitter fit tests didn’t happen yet (I’m appropriately in line behind actual fit tests for staff) but I’m told that today a group of physicians and engineers who had been thinking about designing something similar will be reviewing the design.
Maybe attachment fittings will be the next request I get!
