Stop-Gap N95 Masks in the Time of COVID-19

A University of Toronto and University Health Network APIL collaborative, with CIGITI-SickKids, Queen’s University, Lakehead University, NOSM, and USASK.

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Green NIOSH Mannequin, Blue “SFM” Silicone Inlay, Red Adapter, Yellow Intersurgical HME Filter
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“I3D” (Montana derivative) with DIY rubber seal on Left and “SFM” on Right, with filter caps. SLS printed.
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side-profile of NIH “SFM” mask body with UHN APIL-designed silicone inlay in blue

Design Lessons

Mask Body

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UHN APIL’s “SSR MB-ON” mask, harness and Intersurgical HME filter, derived from “Simple Silicone Respirator”
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Side-profile of “SSR MB-ON” silicone mask, 3DP harness, silicone straps, green Intersurgical HME filter.
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AccutFIT 9000, blue outside sample port, clear inside sample port, closed end syringe-cut, filter sealed to top.

Fluid Dynamics Testing

During initial particulate filtration testing, we discovered that the pressure drop across the filters used in our masks, with a surface area of <50 x 50 sq mm, resulted in significant flow acceleration due to excessive pressure drops across the filter. This increases the drag of particles across the spectrum (20 to 1000 nm) through the filter, and amplifies any imperfections in the seal around the wearer’s face. To address this, we will need to design and perform pressure-drop testing to determine the ideal surface area for each of our proposed filter materials, based on a flow rate of 30 to 80 L/min (passing NIOSH 95 testing standard) and a maximum of negative 5 cmH2O pressure drop (tailoring to human comfort). Breathing at negative 1 to 5 cmH2O represents a tolerable level of work of breathing, and not the maximal inspiratory force a healthy person can generate. For reference, during quiet breathing, we generate ~ negative 1 cmH2O through our lungs during inspiration, whilst expiration is passive.

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A sample of 5-ply Woodbridge INOAC filter, Vaughn, ON

Quantitative Fit-Testing — The Clinical Keystone

We see that apart from designing and production, one of our essential charges is testing and validation of solutions. That is what we understand by the term “clinically proven”. We have used quantitative fit-testing (QNFT) as outlined above to select the most workable solution of SSR MB-ON for health care institutions, which has tested 200 in overall for fit-factor of the QNFT rubric for different face types. But we have not yet been able to endorse any current 3DP alone stop-gap option, because of seal deficits, filtration surface area limits, resulting in underperforming overall QNFT.

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Next Steps

Design & Build

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Future Directions

Upon arriving at final design(s) that 1) provide an adequate and comfortable seal, with easy work of breathing, 2) are readily decontaminable and reusable, and 3) potentially reach a particulate filtration efficiency of 95% for 30nm particles (as required by NIOSH), we will recruit ~ fifty volunteers to perform quantitative fit testing to ensure the mask provides an adequate fit across a range of individuals. When this is complete, we will disseminate the final design(s) for widespread open-use. If necessitated by supply disruptions and continued demand, these design(s) will be ready for large-scale production by industry partners in Ontario and beyond.

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