PAPR filter adapters for COVID-19

The early months of 2020 have been a uniquely hellish time. Between the rapidly increasing incidence of COVID cases, varying levels of social distancing, closing of schools, and economic contraction, we've all been trying to find ways to improve the situation for ourselves and others. For some, that has meant sewing masks, for others it has meant designing invasive ventilators (yikes).

As engineers and hackers, a common first instinct in response to a crisis or discovering someone in need is to try to engineer around the problem. In these cases, however, it's important to understand the true needs of the people you're looking to help, rather than your assumption about their needs.

Several years ago, my partner was peripherally involved in the process of Oregon State University starting a Center for Civic Engagement, a group with a charge that included coordinating community groups that needed resources with groups within OSU that may have them. During this process, we talked quite a bit at home about the dynamics of engineering groups designing exactly the wrong solution to a problem. Many of the efforts of people designing invasive ventilators are perfect examples. The person that used a windshield wiper motor to squeeze a bag comes to mind. Such a machine would almost certainly destroy the lungs of a patient in no time.

As engineers, it is imperative that we

• listen first,
• develop requirements from the user second,
• design a solution,
• then solicit feedback.
• Armed with the feedback from the user, go back to step two and start over.

Batteries

During the course of this project, I was contacted by a friend of mine, who runs the emergency department at a hospital here in Oregon. He had been tracking COVID for a while, and strategizing about how best to respond. His hospital was in possession of several devices called Powered Air Purifying Respirators, or PAPR, and he rightly concluded that they would be a good way to protect himself and his staff while they deal with the projected influx of COVID patients. The problem, however, was that the non rechargeable battery packs were in very short supply. He sent me an image of the battery and asked if I knew anything about the battery chemistry and whether it would be possible to charge them anyway.

Ultimately, they are not rechargeable. Furthermore, the 3M batteries last 12 hours, and he was only able to find 12 batteries to buy on the open market. This is, of course 144 hours of total run time, or 24 hours of runtime per PAPR (they have 6). This would provide some protection, but it's clearly inadequate for a longterm crisis. Cool, so I had my first concrete way I could help out, and feel some control over this crazy situation. Let's circle back on our process: I listened for the need, PAPR batteries; now I need the requirements. In this case the requirements are:

1. Readily available
2. Many hour runtime
3. Must provide at least as much airflow (CFM) as original
4. Must be convenient and rugged.

The first design that I prototyped was easily rejected. This design was simply a mechanical adapter that interfaced with the custom connector on the 3M PAPR battery and blower, and a cable that had a Deans connector, common in radio control models. The choice of Deans connector was to leverage the ubiquitous, powerful, and cheap batteries used in R/C models. The places this design fell down were in points 3 and 4. Firstly, the original 3M battery is 6 volts. This is an awkward voltage for LiPo batteries, as it falls between 1 and 2 batteries in series. Secondly, it's not convenient to put the battery in a pocket, then have a cable to another cable. Finally, the cables could easily be disconnected.

The second design is much better. I printed and assembled six of them. This design completely encloses the battery and electronics, and utilizes an off-the-shelf voltage regulator. I elected to use what's called a Battery Eliminator Circuit (or BEC) to regulate the voltage because it's completely contained, well made, readily available, and is less likely to fail than hand assembled circuits.

Where this design falls down, though, is in point 4, and slightly point 2. The battery case has a belt clip for mounting on the web straps of the PAPR blower, but it's a little soft, and the nurses have dropped a few. When the case is dropped it can break. With point 2, the BEC may be a simple linear regulator, and therefore wastes more energy in converting the battery voltage. Castle Creations doesn't provide a definitive answer on this, so it's an assumption. Finally, the lowest voltage you can set the BEC to is 4.8 volts, which is higher than required for our design, so it's providing more than the needed power, and this reduces runtime.

Further feedback has suggested a few more changes for the battery pack: One is to make the belt clip smaller, and stronger, to reduce the likelihood of a fall. A second is to add a battery monitor alarm. They over discharged one battery because they didn't pay attention to it. There are commodity buzzers that can be easily added to the batteries. Finally, the pins that connect to the PAPR blower can be pushed out if the cable isn't installed carefully. For that, I think the pins will have to be glued into place.

The design for the battery is available publicly in the A360 cloud. Also, the stl files are available for download individually from GrabCad.

Filters

Shortly after contacting me about the batteries, a need for filters also became apparent. To begin with, the PAPRs that the hospital had on hand were chemical filters, not necessarily HEPA. Second, it is impossible to buy new HEPA filters for the PAPRs in this current environment. I was asked, therefore, to come up with a solution to use vacuum filters with the PAPRs. Like the batteries, we can come up with a simple set of requirements:

1. Readily available
2. Instils confidence (no tape)
3. Rugged
4. Must be HEPA, and resistant to droplets and aerosols

The first things I started considering was whether, and how to, enclose the filter. Originally, I had thought to fully enclose the filter in plastic, and close the sides with essentially gaffer's tape. I mentioned this to the MD and he was concerned that it would appear rickety, and wouldn't instill confidence among the team. Ultimately I figured out that the filter media doesn't need to be enclosed.

Next, I had to find a filter that would be close to the right size and shape. All of this effort would be completely wasted if the filters it's designed around are hard to find and buy. Also, there's a lot of questionable marketing around HEPA. Unfortunately, HEPA is now a generic term, and it appears that no certification body is able to prevent non-tested nor certified products from using the term on their packaging. I did find a small Milwaukee filter that has a "Certified HEPA" logo. It's very difficult to trace the certification, but as the filter is first party, and name brand, it's among the most trustworthy filters I could find. The filter has a simple mounting scheme, and has only one penetration on the "clean" side of the filter, making it a perfect candidate. I already knew the screw thread size for the original filter canisters, so I developed a cap over the HEPA filter, and adapted it to the PAPR adapter. It all looked good in CAD...

Unfortunately, because I threw it together without having a PAPR to work with (I had ordered a military surplus one on eBay, but it hadn't arrived yet) it didn't fit at all. Once my PAPR came in the mail, I measured it and modeled it in CAD and designed another version. Because the diameter of the filter was much larger than that of the filter canisters that the PAPR is designed for, I had to make the adapter quite long to allow the filter to hang over the side of the case.

The second attempt fit perfectly, and I started printing them on my Prusa i3 clone. The problem I faced now, however, was that it was taking 26-or-so hours to print them. Not only do I not trust leaving my printer over night, but the room I use as my lab is where my cats are kept at night, and I trust them even less than the printer! To deal with some of these problems (remembering that I need 6-7 of these filter adapters, just for one hospital), I asked around for people that might have printer cycles that I could borrow. Fortunately, an engineer at the company I work for is an alum of HP Vancouver (Washington state), and was able to put me in touch with their 3D printing team!

HP is running a very cool program that connects designs and prototype services with medical facilities that need extra support. While I didn't get connected with this team exactly, I do want to give them a major shout out, and thank them for doing this. The team I was put in contact with was the printing R&D group that works on a powered plastic process based on PA-12 (a type of Nylon). Their printer was able to print 6 filter adapters at once, and a single batch takes about 18 hours, regardless of how many parts are contained within.

That's all very cool, but now we're changing the process used to make parts, so the assumptions I made while designing the second draft are no longer valid. In particular, FDM printers can enclose any volume with little added mass (this is a tunable parameter called infill). However, the process used in the HP printers is more like stereo lithography (I'm not sure about the details of their process; whether they sinter the substrate with a laser, or bind it with resin, but the effect is the same), wherein any enclosed volume will be filled with the substrate, and become rather heavy. So, an engineer at HP and I went through a few rounds of design review and came up with a cool looking design that met the required criteria above, while also minimizing the mass of the part.

The design we ultimately printed has a large adapter diameter to enclose the end of the filter, and mate with the bayonet features on the filter. Then a long tube with a 2-3 mm wall thickness connects the filter internal cavity to the PAPR blower. A cool wheel feature in the middle of the adapter is meant to provide support to the filter, and meet a matching feature on the PAPR body. The role of that feature is to try to prevent the part from breaking if the filter is hit with a wall, door, bed, or medical equipment.

Once the parts came in from HP (and it only took about a week), I couldn't wait to give the system a test. In the above image, I'm testing the full system with a CFM meter that's provided with the PAPR. In this test, the battery is a 2200mAh R/C airplane battery, in the battery case with the voltage regulator. The HP-printed filter adapter and a cloth "sock" that my wife sewed to provide some additional protection to liquid droplets is supporting the PAPR blower. On the output of the PAPR is a 3M CFM meter that they provide in the kit. It maxes out at 6CFM, and this system surpasses that flow rate easily.

At the hospital

The team at the hospital has been using the kit for a few weeks now, and their impression has been positive overall. Discounting the broken battery covers, there haven't been any complaints. In fact, the filter performance has been so good that they're able to get a few more hours of runtime out of the 3M batteries with an acceptable flow rate.

Thanks and acknowledgements

I want to thank my Doctor friend for giving me the opportunity to help out, and provide a sense of control in the completely out of control time. Also, I want to sincerely thank the folks at HP who were able to act so quickly (about a week from first email to parts in the mail!) in helping out. I want to thank Tabor Kelly for making the introductions. Finally, I want to thank you for reading!

Design files and disclaimer

I've provided a public link to the design files in Autodesk Fusion 360. You're free to download them, and use them as you see fit. However, I'm explicitly disclaiming any responsibility for any damages that may arise from the use of these files, and I make no warranty regarding their use or applicability for any purpose.