PAPRa: An Open-source Powered Respirator in Response to the COVID-19 Pandemic
The COVID-19 pandemic of 2020 created an unprecedented need for respiratory personal protection equipment (PPE). Due to the world-wide spread of SARS-CoV-2 virus, global supply lines and manufacturing disruptions led to severe shortages, especially in the realm of face masks in general, and N95 rated protection specifically. This crisis led to rationing of masks of all filtration levels and the unavailability of medical grade filters used in treating respiratory patients. In addition, mask mandates were enforced in some regions of the US, reducing the already low supply of PPE for healthcare.
In addition to the issues with supply, the PPE currently available were not suitable for general public use. One inherent shortcoming of N95 rated passive masks is the reduced air flow which results in increased effort exerted by the wearer. Traditional disposable N95 masks are not rated to be worn more than 4 hours at a time, as a proper seal around the face will tend to have the wearer inhale their own exhaled carbon dioxide, a situation that leads to a condition called hypercapnia. Hypercapnia typically causes headaches, dizziness, fatigue, and inability to think clearly, meaning that those wearing properly fitted PPE had to work in difficult conditions, while those with improperly sealed PPE were exposed to the virus. Additionally, N95 masks are supposed to be single-use disposable respirators. During the shortages, healthcare workers were forced to reuse their masks which increased the risks of cross-contamination as well as possible reduction of protection. Hospitals with more financial resources returning towards powered air purifying respirator (PAPR) units.
During the Ebola epidemics previous to 2020, many healthcare providers purchase small numbers of PAPRs specifically to protect healthcare workers dealing with Ebola patients. There were two major differences between the protection needs of the Ebola epidemic and the COVID-19 pandemic. The first was the scale of healthcare workers who needed protection. For Ebola, the presumption was that you needed to protect only the healthcare workers administering aid to a very small number of possible Ebola patients. For COVID-19, the expectation was that all healthcare workers needed to be protected from all patients. The second difference was the nature of the protection. In previous epidemics, the assumption of PPE use was to protect the healthcare provider from the illness of the patient. With COVID-19, a further constraint was placed in protecting patients from possibly infected healthcare providers. Medically specified PAPRs vent the exhalation of the wearer into open space. The system works perfectly fine in isolated rooms where everybody is wearing the correct PPE, but poses a problem when used in common areas as with the use cases of the COVID-19 pandemic.
To address the lack of life saving PPE, TBD set out to design an open-source 3D printable PAPR with filtered exhalation using freely available components, which we named the PAPRa. The design starting point was to source a filter which was not a protected medical commodity. Looking for n95 equivalent performance, the rating of HEPA filters used in air purifiers provided the filtration efficiency necessary. The PAPRa was designed around the form factor of the Germ Guardian Type A HEPA filter. Using an existing and available filter allowed for access to replacement HEPA certified consumables without the need for custom manufactured filters. Multiple manufacturers make generic versions of this form factor, so the design would not be locked into a single provider of filter material
As the design progressed, we enumerated several design criteria beyond the use of HEPA filters, including ensuring a minimum of N95 levels of protection as determined by a quantitative fit test, 115 LPM of flow to the face of the wearer, the ability to modulate the amount of airflow to the face of the wearer, at least three hours of battery life at full power, a combined weight of less than 2 pounds for the entire device, and the ability to be mounted on MOLLE accessories, such as belts and backpacks.
Careful selection of the fan component was based on both power consumption and flow necessary to produce minimum safe conditions of 115 LPM while not being too loud or consuming too much power. After testing multiple models and manufacturers, the squirrel cage blower type gave the best performance for power supplied. To power the blower, the choice was made to use a 12 volt power tool battery. This design decision provided two crucial benefits. The first is the easy availability of 12 volt batteries which have their own established charging ecosystems. Second is convenient packaging which allows for secure connections while maintaining speedy battery exchange.
In order to take advantage of an existing battery design, a custom electronic board was engineered to serve as the power delivery system to the fan unit, the fan speed controller, and the battery level indicator. A 3D printable housing was modeled to contain both the board and serve as the retaining receptacle for the power tool battery. Currently, the designs for the PAPRa are focused towards using the Milwaukee M12 line of lithium ion batteries. Future plans are to create different boards and battery housing models to accommodate other brands. The intention has always been to provide options in terms of power source.
In recent years, the proliferation of consumer grade additive manufacturing platforms has dramatically increased in America and around the world. The ability to turn digital files into physical objects has captured the imaginations of many makers. The designs created for the PAPRa have been prototyped on popular consumer focused 3D printers such as the Prusa MK3S and the Creality Ender 3. Our tests have shown that devices printed on such machines are capable of producing the desired airtight seal and physical durability necessary for a useful device.
NIOSH does not certify designs, but rather certifies manufactured devices. As such, the current goal of TBD is to manufacture devices that can meet NIOSH certifications, and then distribute those devices to address what we see as a need in the marketplace for active N95 half facepiece respirators.
The PAPRa project, like all projects at Tetra Bio, is maintained as an open-source hardware project using a docs-as-code approach, with tools such as Asciidoc, and Hugo. The project repo can be found on GitHub (https://tetrabiodistributed.github.io/papra). We encourage anyone interested to check out our designs and provide suggestions on improvements. Tetra Bio is offering the designs freely and licensed under open source. The hope is to continue to create open sourced medical-related devices to combat future shortages.
