Abstract
At the beginning of the COVID-19 pandemic when traditional N95 respirators were in short supply in the United States, there was a need for alternative products that did not rely on traditional avenues of sourcing and manufacturing. The purpose of this research was to develop and test alternatives to N95 respirators that could be produced locally without specialized materials and processes. Through an interdisciplinary team of experts, new mask designs that use repurposed filtration media and commercially available components were developed and tested for filtration and fit against current N95 standards. Filtration efficiency test results showed that the filtration media can be used for high-quality facemasks and quantitative fit testing demonstrated that the new mask designs could be viable alternatives to traditional N95 facemasks when those masks are in short supply. Manufacturing viability was tested utilizing a workforce to create 6000 masks over 10 days. The ability to quickly produce masks at scale using a workforce without specialized skills demonstrated the feasibility of the mask designs and manufacturing approach to address shortages of critical healthcare equipment, mitigate risk for healthcare and essential workers, and minimize the transmission and spread of disease.
Introduction
N95 filtering facepiece respirators (FFRs) are the most used respiratory protective device in U.S. healthcare [1,2]. N95 FFRs filter at least 95% of the most penetrating airborne particle size (0.1–0.3 μm) [3] and are used in a variety of medical and nonmedical applications as a form of personal protective equipment (PPE). In the U.S., N95 respirators are regulated, tested, and certified by the National Institute of Occupational Safety and Health (NIOSH). To be approved as an N95 FFR, the design must be tested for filtration efficiency, airflow resistance, and material safety [4,5]. A surgical N95 FFR is a NIOSH-approved N95 respirator that has additional certification by the Food and Drug Administration.
In addition to N95 FFR certification, each wearer needs to be individually fitted the mask because of diverse face shapes and complexity of mask fit associated with a good mask seal to achieve optimal respiratory function. Healthcare organizations provide certified FFRs to their employees who work in specific environments and each wearer must go through medical approval and a yearly Occupational Safety and Health Administration (OSHA)-accepted fit test protocol as a component of health and safety regulations [6]. OSHA-accepted fit testing protocols must be conducted by qualified individuals and may be performed using a qualitative Fast Test-30 fit test or a quantitative test using a using a condensation nuclei counter (e.g., PortaCount, TSI Inc., Shoreview, MN) test instrument [6].
N95 FFRs are critical PPE for healthcare providers caring for COVID-19 patients. The early months of the COVID-19 pandemic in the United States resulted in severe shortages of critical personal protective equipment, such as N95 FFRs, that plagued the safety of healthcare and essential workers for many months [7–9]. As a result of PPE shortages, healthcare and essential workers were likely un-necessarily exposed to the virus. There is growing evidence of the need to outfit more healthcare and essential workers with N95-equivalent FFRs to mitigate future disease outbreaks [10].
The supply of N95 FFRs rebounded during the second half of 2021; however, the supply chain for masks in the United States remained vulnerable [11], and in many locations, the supply of N95 FFRs was inadequate as positive COVID-19 cases continued to increase. While manufacturers increased their production capacity of N95 respirators, the demand for N95 respirators far exceeded the supply, particularly in rural communities and nonhealthcare industries. The supply breakdown of N95 FFRs can be attributed to a decrease in U.S. based PPE manufacturing (approximately 90% of masks are imported), trends toward on-demand PPE manufacturing, government policy and planning, and a significant increase of N95 FFR use in healthcare settings during the COVID-19 pandemic [12–17].
The purpose of the research reported here was to develop and test alternative N95 respirators by creating mask designs that can be produced locally without N95 specific materials and processes.
Method
Design Process.
Protective mask alternatives were developed using interdisciplinary expertise from design, engineering, chemistry, industrial hygiene, and medicine. The design exploration was an iterative process that involved testing material filtration efficiency, component testing, experimenting with the overall shape of mask design, qualitative and quantitative fit testing, and manufacturing development. The decision makers and participants in the mask design process are listed in Table 1. They include leadership, connectors, industry professionals, a design and engineering team, a research and testing team, an industrial hygiene team, and a medical clinician team. Three masks (MNmask v1, MNmask Reusable, and MNmask Procedural; Minneapolis, MN) were developed to make use of flexible manufacturing alternatives and sourced components that are characterized by short lead times. MNmask v1 was refined to fit a larger population of wearers and is referred to as MNmask v2 in this paper.
Table 1.
Decision makers and participants in the mask development process
| 1. Leadership (L) | University and Medical Center Leadership including Department Heads, Deans, Provosts, VP University level and CEOs |
| 2. Connectors (C) | Institute for Engineering in Medicine and members of a team that provided a direct connection to the leadership team, medical clinicians, industry experts, office of general council, and technology/commercialization. |
| 3. Industry (I) | Filtration, PPE, and medical device industry professionals |
| 4. Design & engineering (D&E) | Design and engineering faculty and researchers |
| 5. Research & testing (R&T) | Filtration, chemistry, and engineering faculty and researchers |
| 6. Industrial hygienists (IH) | Industrial hygienists and environmental, health, and safety faculty |
| 7. Medical clinicians (MC) | Medical MDs and healthcare workers |
The design process for N95 mask alternatives began with a critical and urgent supply chain shortage, an explicit need communicated by leadership at the University of Minnesota Medical School. The University of Minnesota Institute for Engineering in Medicine then called on University of Minnesota leaders, industry, design, science, engineering, and medical faculty to together address the problem.
The development of a problem statement to direct the project began by evaluating current N95 FFRs on the market. We determined early in the evaluation process that nearly all commercial N95 designs required specialized manufacturing equipment such as molding, shaped heat sealing, and bonded components. Additionally, at the beginning of the pandemic, the supply chain for filtration media, elastic bands, and aluminum bendable nose wires was nearly depleted. The lack of access to manufacturing equipment and mask components forced the team to focus on design initiatives that circumvented traditional avenues of sourcing and manufacturing.
Initially, off-the-shelf filtration products such as the furnace, vacuum, and coffee filters were evaluated for their filtration efficiency [18]. Most of the products tested needed several layers to be effective, which makes breathing difficult because of airflow resistance, and adds complexity to the manufacturing process. Further, we determined that the structure and composition of the alternative media could cause skin irritation and might not be safe for breathing. This problem was solved through a collaborative relationship that was established with Cummins filtration (Nashville, TN), a company that designs, manufactures and sells air, fuel, hydraulic, and lubricant filtration products for diesel and gas-powered equipment around the world. This enabled access to nonendangered filtration media candidates for the alternative mask design.
Filtration Efficiency Tests.
The filtration efficiency of the media candidate was tested using samples provided by Cummins Filtration. The method of fractional efficiency measurement was applied for the media efficiency test [19] and a computer-controlled system was used for testing. The system is capable of testing size-dependent filter collection efficiency for particulates from 20 nm to 1000 nm under a variety of filtration velocities.
Design Evaluation and Fit Testing.
The MNmask v1 and MNmask v2 (a later revised version of MNmask v1) designs were evaluated through fit testing using healthcare professionals (n = 30) and nonhealthcare professionals (n = 10) volunteers.
A second study compared the fit of MNmask v1 and MNmask v2 to three commercial N95 FFRs and one KN95 respirator and was conducted with nonhealthcare professionals (n = 9, 5 F, 4 M.) Table 2 lists the properties of the masks that were tested. The participants fit tested each mask according to the OSHA standard 29 CFR 1910.134 using the quantitative fit testing method and a PortaCount™ Protest instrument (TSI Inc., Shoreview, MN) [20]. The test included normal breathing, deep breathing, head movement side to side, head movement up and down, talking (rainbow passage), and bending over.
Table 2.
Details of masks fit tested including company, model, and size range
All studies using human participants were approved by the University of Minnesota Institutional Review Board.
Pilot Manufacturing System.
Traditional N95 manufacturing factories and equipment are not viable options for producing alternative facemasks, as they require specialized supply chains and use specialized equipment. MNmask v1, MNmask v2, MNmask Reusable, and MNmask Procedural designs were developed so they could be built using simple manufacturing processes and with materials that are readily available when traditional supply chains are slowed or broken because of emergency, pandemic, or war.
A pilot manufacturing system was developed for MNmask v1. The assembly process included the preparation and assembly steps shown in Fig. 1. Production tools included a sheet cutter, an impulse heat sealer, an adhesive transfer tape dispenser, and a heavy-duty hand stapler. None of the steps require specialized skills to complete and the manufacturing system is flexible and scalable. A quality control process was developed and conducted by the assembler after each step with a final quality control check on each mask conducted by the assembly team leader.
Fig. 1.
Basic manufacturing steps for MNmask v1
Results
Through component testing on four participants, filter media testing, and fit testing, four mask designs were developed that use readily accessible materials and components are easy to manufacture using nonspecialized tools, and does not require skilled labor. Table 3 provides a timeline of events that occurred during the creation of the initial mask designs, the decision makers and participants for each step, and the design phase that took place over the course of 2.5 months. As an example of the process, MNmask v1 went through over 40 design iterations during the design development process by using feedback from pilot quantitative PortaCount™ Protest instrument (TSI Inc., Shoreview, MN) and qualitative (Fast Test-30) fit tests.
Table 3.
Timeline of the mask development process including date, occurrence, decision maker/participants, and design phase
| Date | Occurrence | Decision-makers and participants | Design phase | |
|---|---|---|---|---|
| Early March 2020 | Supply chain shortage declared by WHO; University and Medical Center leadership prioritize solution to N95 shortage. | L | Explicit need | |
| Mar. 18, 2020 | Institute for Engineering in Medicine (IEM) converges experts to address N95 mask shortage in response to a request from Vice Dean for Research at Med School. | L, C, I, D&E, R&T, MC | Research and problem statement development | |
| Mar. 18, 2020 | Problem statement developed in coordination with industry experts. | I, D&E | ||
| Mar. 19, 2020 | Filtration testing of alternative media begins at Center for Filtration Research (CFR) on university campus; Cummins Industries indicates they have media supply available to test and utilize. | L, C, I, R&T | ||
| Mar. 19, 2020 | N95 concept meetings: Two directions emerged—Reusable mask options utilizing three-dimensional printing and/or oxygen mask base and disposable mask options. | C, D&E, R&T, MC | ||
| Mar. 21, 2020 | The first disposable prototype (MNmask v1) was developed and presented to the team. | L, C, D&E | Filtration research and media testing | Prototyping and testing |
| Mar. 21–25, 2020 | MNmask v1 prototypes developed with nontraditional filtration recommended by CFR. | D&E | ||
| Mar. 26, 2020 | Filter Media from Cummins arrives on campus. | C, I | ||
| Filtration efficiency of Cummins media tested by CFR. | R&T | |||
| MNmask v1 prototyped in Cummins media and tested using TSI porta-count quantitative fit test (OSHA standard). | D&E, R&T | |||
| Filtration media and prototype fit tests pass NIOSH N95 standards. | R&T, D&E | |||
| Mar. 27, 2020 | Filtration research and MNmask v1 testing results presented to hospital CEO, leadership, and clinicians; Approval and seed funding to move forward with MNmask v1 development. | L, C, D&E, R&T, IH, MC | ||
| Mar. 29, 2020 | Leadership and medical clinicians organize evaluation of MNmask v1. FT-30 qualitative fit testing of mask occurs at the hospital for 20 clinicians. Four sizes of masks were tested. D&E team attends session remotely. Mediocre results across a range of face shapes. | L, C, D&E, R&T, IH, MC | ||
| Apr. 1, 2020 | Mask designs and manufacturing methods were discussed with the Office of General Council. A decision was agreed upon to license mask designs and production methods for free. | C, D&E, R&T | ||
| March to April 2020 | Media chemical composition and skin safety research are led by a chemist and filtration engineer to ensure the human safety of media. | R&T, IH | ||
| MNmask v1 design refinement continues through component and quantitative fit testing; Mask design refined to two sizes; Second round of hospital fit tests produces more promising results. | L, C, D&E, R&T, IH, MC | |||
| Apr. 2020 | MNmask Procedure design is presented to leadership as a procedural mask alternative and is a simplified modification of MNmask v1. MNmask Reusable design is presented as an alternative for those that do not fit into MNmask v1 | |||
| Mid-April 2020 | MNmask v1 design hardens. The manufacturing process developed with insights from industry contacts. Process tested with a small group of workers to ensure product quality and worker safety. | C, D&E, I, IH | Design hardening and manufacturing | |
| April to May 2020 | UVGI and heat decontamination protocols were developed and tested by CFR and hospital for N95 supply and MNmask v1. | R&T, IH | ||
| Mid-April to May 2020 | Full-scale manufacturing of MNmask v1 using a small workforce. Over 10 working days, 10–20 students worked 3–6 h shifts and produced 6000 masks. Phased-in manufacturing steps determined the most productive for small-scale production. | L, C, IH, D&E | ||
| May to June 2020 | Final quality control of 6000 MNmask v1 and 200 MNmask Re-usable supply ready for delivery to the hospital. 500 MNmask Procedural design kits assembled and distributed across campus. | L, C, D&E, IH | ||
Design Descriptions.
The features of the four facemask designs are highlighted in Table 4. Figure 2 illustrates the features of MNmask v1 and Table 5 describes MNmask v1 and MNmask v2 components. The masks have a large surface area, which increases the breathing area and sits away from the mouth, which enables clear communication during use.
Table 4.
Features and components of four mask designs
Fig. 2.
Features of MNmask v1
Table 5.
Final components of MNmask v1 and MNmask v2
| Materials | Purpose |
|---|---|
| Cummins filter media EX101 | Functional filter of nanoparticles |
| Closed-cell foam, ½ in. to ¾ in. width | Provides a soft structure and improved fit |
| Adhesive transfer tape | Provides a strong hold between media and foam/nose wire |
| Nose wire | Creates a contour around the nose bridge |
| Staples | Attaches band to mask |
| Stretchable band | Provides secure mask fit and seal |
| Duct tape | Creates a seal over staples and dart points |
The facemasks are made using commercially available tape, rubber bands, staples, adhesive-backed foam, and bendable nose bridge components. Design details, such as foam around the full perimeter of the mask, provide an improved fit across a diverse population, and a large mask surface area improves comfort lowers the resistance to breathing.
Filter Media Performance.
The masks use a filter media from Cummins Filtration that is typically used in industrial applications. The filtration performance of the Cummins Filtration media was tested by subjecting the media to particulates in the size range of 0.03–0.4 μm, which is representative of the size range for virus-containing aerosols and droplets (the diameter of the coronavirus itself is about 0.125 μm) [21]). From the data shown in Fig. 3, the Cummins Filtration media exhibits a nearly equivalent filtration performance as the filtration media found in a certified N95 mask, and the Cummins EX101 media filtered over 95% of the particles over the entire range tested. The filtration efficiency of the media is lower for particles larger than 0.4 μm because of impaction and interception, and higher for particles smaller than 0.03 μm because of Brownian diffusion.
Fig. 3.
Performance of Cummins filtration media EX101 and EX103, and 3 M N95 8210+ media
Although the Cummins media exhibited a pressure drop per area higher than that of the N95 media, this airflow resistance limitation is compensated for by using a larger filter area for the MNmask designs. This increased filter area also helps with better filtration efficiency (than reported in Fig. 3) by reducing the face velocity through filter material in actual use. Because the Cummins media is not electrostatically charged, it will likely have a longer shelf life and be more robust that charged materials during decontamination protocols such as ultraviolet germicidal irradiation (UVGI) or hydrogen peroxide vapor treatments [18].
Mask Fit.
The preliminary fit tests demonstrated that MNmask v1 might be suitable as an N95 alternative for some healthcare users, as about 40% of users passed the qualitative fit test.
The results of the quantitative fit tests comparing MNmask v1 and MNmask v2 to three commercial N95 FFRs and one KN95 facemask are presented in Fig. 4 and Table 6. An overall fit factor score of 100 is needed to pass N95 FFR according to the OSHA Respiratory Protection Standards.
Fig. 4.
Mean fit factor scores for each test activity and overall fit factor for six masks
Table 6.
Overall fit factor results for six masksa
| KN95 GB2626-2019 | MNmask v1 | MNmask v2 | N95 KCC 62355 | N95 3M Aura 9210+ | N95 3M 1860 | |
|---|---|---|---|---|---|---|
| Mean ± S.D. | Mean ± S.D. | Mean ± S.D. | Mean ± S.D. | Mean ± S.D. | Mean ± S.D. | |
| Range | Range | Range | Range | Range | Range | |
| Normal breathing 1 | 4.9 ± 2.7 | 126.6 ± 218.9 | 804.8 ± 1048.9 | 40.6 ± 36 | 470.2 ± 316.7 | 203.9 ± 194.5 |
| 2.4–9.1 | 6.2–691.0 | 61–2933 | 8.2–105 | 218–1202 | 2.9–725 | |
| Deep breathing | 5.3 ± 2.3 | 81.8 ± 119.2 | 556.2 ± 630.8 | 39.4 ± 33.6 | 306.6 ± 180.2 | 200.1 ± 295.2 |
| 2.5–9.0 | 5.0–385.0 | 40–1591 | 8.4–96 | 131–624 | 9.6–1027 | |
| Head side to side | 5.6 ± 4.0 | 97.1 ± 152.0 | 629.9 ± 656.2 | 33.1 ± 22.3 | 396.8 ± 254.5 | 146.3 ± 110.5 |
| 2.0–14.0 | 5.0–484.0 | 62–1857 | 3.1–79 | 232–1096 | 7.1–421 | |
| Head up and down | 4.6 ± 2.5 | 86.3 ± 121.3 | 376.7 ± 323.8 | 24.7 ± 18.7 | 401.4 ± 620.1 | 119.2 ± 93.1 |
| 2.2–8.1 | 6.0–375.0 | 72–883 | 2.5–57 | 39–2148 | 3–336 | |
| Talking | 11.4 ± 7.4 | 92.1 ± 133.1 | 250.2 ± 200.7 | 40.6 ± 22.3 | 119.9 ± 99.4 | 50.8 ± 48.3 |
| 1.8–27.0 | 8.9–423.0 | 39–596 | 10.0–78.0 | 30–324 | 19–186 | |
| Bending over | 5.1 ± 3.3 | 87.5 ± 122.3 | 413.0 ± 426.6 | 25.7 ± 26.3 | 303.3 ± 356.7 | 92.3 ± 61.7 |
| 2 .0–11.0 | 4.1–355.0 | 44–1279 | 3.2–74 | 21–1255 | 5.3–220 | |
| Normal breathing 2 | 3.6 ± 1.0 | 122.2 ± 187 | 762.1 ± 930.7 | 31.2 ± 23.4 | 405.7 ± 429.5 | 137.2 ± 77.5 |
| 2.2–5.1 | 3.5–537.0 | 49–2515 | 2.7–65 | 42–1332 | 3.8–268 | |
| Overall fit factora | 4.9 ± 2.2 | 93.3 ± 141.3 | 438.0 ± 436.1 | 29.0 ± 22.5 | 220.9 ± 169.2 | 89.6 ± 45.4 |
| 2.3–8.2 | 5.1–443.0 | 55–1132 | 4.3–69 | 52–671 | 4.9–168 | |
| % Pass rate | 0.00% | 22.22% | 77.78% | 0.00% | 88.89% | 55.56% |
N = 9.
Eight of nine participants had a fit factor score of over 100 when wearing the N95 3 M Aura 9210+ mask. Seven participants obtained a fit factor score of over 100 for MNmask v2. Five participants obtained a fit factor score of over 100 wearing the N95 3 M 1860. Two participants obtained a fit factor score of over 100 wearing the MNmask v1. No participants passed the quantitative fit test wearing the N95 KCC 62355 mask or the KN95 GB2626-2019 mask.
Manufacturing.
The pilot manufacturing process was implemented in a University of Minnesota academic building (conveniently empty during the pandemic) with students (n = 20) hired as hourly workers with a team leader trained and assigned for each shift. Assembly stations were spaced out over several rooms to maintain appropriate social distancing. A phased production schedule was the most efficient way of managing the socially distanced student workers, so only a select number of steps were running each day.
Over the course of 10 non-consecutive days, 6000 MNmask v1 masks were fabricated. The peak production rate was 258 masks/hour, which occurred during a shift with 19 student workers (i.e., about 13 masks/hour/worker).
In addition, 200 MNmask Reusable filter designs were manufactured, and 500 Procedural design facemasks kits were assembled, each with materials to make 100 masks (50,000 total masks).
Discussion
The pandemic-triggered rapid design and development process for these masks demonstrated that a large, multidisciplinary team can deliver a usable product even when operating remotely. Along with following a rigorous design process that involved repeated design-test iterations, four factors were essential to this project. First was the understanding that excellent facemask fit is required for a mask to meet N95 standards as well as an understanding that faces come in a wide range of shapes and sizes, which means that testing on a broad range of participants is required for mask evaluation.
The second was the availability of high-quality filtration media that was not part of the normal N95 supply chain. Media normally used in industrial applications proved to be suitable for high-quality mask fabrication.
Third, was access to state-of-the-art particle filtration testing resources. For example, an early test of the filtration media revealed that it had a higher pressure drop (i.e., airflow resistance) than N95 material, which drove the initial design of the masks to ensure they had sufficient filtration media area to enable easy breathing while wearing the mask. The ability to conduct quantitative fit testing with real-time results also facilitated the design process. For example, the designer could view the fit score while adjusting the position of the securing headbands to optimize the placement and tension of those bands.
Fourth was establishing a rigorous and Institutional Review Board-approved test process using a range of participants. Our fit test results reinforce that a high-quality facemask requires both high-quality material and a high-quality fit to the face. For example, while more participants (eight of nine) passed the fit test wearing the N95 3 M Aura 9210+ than passed wearing the MNmask v2 mask (seven of nine), the average fit factor score was highest for MNmask v2 (mean score = 438 compared to mean score = 221 for the Aura 9210+.) Both masks are comprised of filter media that passes the N95 standard for filtration efficiency, so these results are likely due to the adjustable band feature of MNmask v2, which led to a large variation in scores across participants as seen by the large standard deviation in Table 6 (436 compared to 169 for the Aura 9210+.) These results also confirm that no mask will effectively fit all participants.
Equally important as the design was the ability to manufacture the masks at low-volume using a workforce without specialized skills, and without automation or specialized equipment. Manufacturing was considered early and continuously throughout the design process. For example, design options that involved gluing seams were rejected as high-quality adhesive operations are better suited to an automated operation. An advantage of a simple manufacturing process is that it can be easily scaled. The process we developed has almost no fixed costs and the variable costs are dominated by the cost of nonspecialized labor. Assuming the availability of labor and the availability of generic assembly space, the process can scale up to any desired production rate. Further, the manufacturing method can, with no difficulties, be spread across multiple locations with appropriate quality control measures.
This approach to design and manufacturing is very well suited to emergency response situations as was the case with facemasks during the pandemic. It does not replace established methods for high-volume facemask manufacturing where the fixed cost of automation and specialized production machines that enable exceptionally high production rates will result in a lower cost and more consistent product than can be achieved by a process that is dominated by the variable cost of labor and the variations that are inherent to manual assembly.
Conclusion
The ability to quickly produce masks at scale using a novice workforce demonstrates the feasibility of the mask design and manufacturing approach to address shortages of critical healthcare equipment, mitigate exposure risk for healthcare and essential workers, and minimize the transmission and spread of disease. While these designs were originally developed for healthcare workers, they offer a practical solution to support safe working conditions during the pandemic across a variety of industries and are suitable for use by the general population.
The short-term impact of this mask project is that our method can protect military personnel, healthcare providers, and essential workers by providing alternative masks that can be used if the local supply is depleted. The long-term impact of the mask project is that it can provide design knowledge and an instruction repository that can be used for guidance in a future emergency or pandemic.
Further research is ongoing to fully characterize the filtration media, optimize the facemask design, expand performance and usability testing for healthcare workers, and test manufacturing reliability across different mask manufacturing methods.
Acknowledgment
The authors wish to thank Dr. John Bischof, director of the Institute for Engineering in Medicine, for his leadership in bringing our team together to work on this project.
Funding Data
Funding for this research was provided by the University of Minnesota Institute for Engineering in Medicine and the University of Minnesota Office of Discovery and Translation. This research was supported by the National Institutes of Health's National Center for Advancing Translational Sciences, Grant No. UL1TR002494. The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Institutes of Health's National Center for Advancing Translational Sciences.
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