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Build your own Powered Air-Purifying Respirator

In response to the COVID-19 pandemic, Tetra Bio Distributed has developed a Powered Air-Purifying Respirator (Tetra PAPRa). Darryl Hwang, Ph.D., an Assistant Professor of Research at University of Southern California (USC) and a founder of Tetra Bio Distributed (TBD), initially started developing the Tetra PAPRa in 2020 as a follow up to the emergency 3D printed mask initiative in the early days of Personal Protective Equipment (PPE) shortages following the COVID pandemic. 


Galvanized by the COVID-19 pandemic, in the summer of 2020, Tetra Bio Distributed (TBD), a nonprofit organization dedicated to creating open-source solutions to cost-prohibitive medical technology, formed and began working towards effective and better fitting PPE. Some of the projects that TBD has created include: The Tetra Ventilator Splitter, which extends a single ventilator’s capacity to treat up to four patients, the Tetra Fitra – a fitment device, dramatically improving the fit of common flat fold N95 masks as well as the Tetra PAPRa – a powered air-purifying respirator.


In the earlier days of the pandemic, Daniel Stemen, manager of respiratory care for University of Southern California (USC), noted significant issues with N95 respirator masks in his field.  As the manager of respiratory care, Stemen was tasked with overseeing respiratory therapists treating COVID-19 patients, as well as testing the different pieces of Personal Protective Equipment (PPE) that his staff use to ensure that PPE are effective.


Stemen observed that a great many of the emergency use filtering face mask respirators (FFRs) the hospital had been given fit poorly. The lack of proper fit meant that his staff were at risk for COVID exposure, despite wearing PPE.


With the dilemma of ill-fitting PPE at hand, Stemen enlisted the aid of Tetra to design a fitment device that could go over a respirator. The result of this engineering effort was Fitra. This device was ultimately published in the Journal of Occupational and Environmental Medicine (, and it has been shown to improve the mask fit of flat fold N95 masks. The statistics gathered from Stemen’s study showed that community transmission among healthcare workers at USC was 0.27%, while other hospitals had community transmission rates of 5.5-7.7%, a dramatic decrease attributed largely to properly fitting PPE.


Despite the reduced risk of exposure and transmission amongst health-care workers, Dr. Hwang observed that after four hours or so of wearing properly fitting respirators, users began to develop headaches as they were re-breathing their exhaled carbon dioxide. Although property fitting respirators are much less likely to cause respiratory issues for those wearing them for shorter durations, the long hours and therefore longer exposure windows for health-care workers would mean a better solution was needed. With this in mind, Dr. Hwang created a working PAPRa in case of emergency need at hospitals.


At its core, the Tetra PAPRa utilizes a fairly straightforward idea of putting a fan behind a filter in an airtight box. The fan then pulls air through the filter and pushes that air through the hose. The hose in turn leads to a mask onto the wearer’s face. As the wearer breathes out, that air travels out through the filter in the front of the mask.


The first prototype device used a vacuum cleaner HEPA filter and a fan to pull air through to the wearer’s face, as well as a 12-volt power tool battery to provide power.  TBD is now working on the eighth iteration of that prototype device, with a goal to create a device that, when powered off, provides the same level of protection as a properly fit passive N95 and N99 respirator. And when powered on, provides enough airflow to the user to be comfortable to wear for hours at a time.


Despite Dr. Hwang’s initial goal being to create a working PAPRa in case of emergency need at hospitals, Tetra sees that the PAPRa has expanded its usefulness to a wider range of scenarios than just protecting health-care professionals. The organization has noted that the TETRA PAPRa can be a useful tool for immuno-compromised individuals, possibly allowing them to move more freely through society. It has also shown itself to be a useful breathing device for individuals who find they have to navigate fire seasons happening throughout various regions of the world and the associated smoke and air toxicity. 


As a nonprofit organization dedicated to creating open-source  medical technology, Tetra has made the STLs (stereolithography or Standard Tessellation Language) available for anyone to make the TETRA PAPRa if they want one. Those STLs can be found on their website under the heading, “Build a PAPRa”.

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