Introduction:

A critical feature of personal protective respirator a conformal fit to the wearer’s face.  If inspired air flows preferentially around the edges of the mask, then airborne particulates can be inspired by the user.  Similarly, if expired airflow exits around the mask, then the wearer may not adequately protect others from airborne droplets containing infectious material. 

When commercial N95 respirators are in short supply, innovators must utilize alternative materials to create airway filtration. For example, availability of replaceable N95 filters of various configurations designed for reusable industrial respirator facemasks may be available, even though the reusable facemasks are not.  In other cases, materials not intended for airway filtration may be useful if the risk/benefit calculus of airborne droplet vs novel device material exposure to warrants this substitution.

Finally, the facemask should be reusable, able to be rapidly manufactured and low cost in order to have the greatest impact on a large population in need of personal airway protection.

These three specifications were used to design and develop a fabrication method for a reusable scaffold that is customized to the face of individual wearer.  This scaffold, may be able to be cleaned with soap and water and hold filter materials from a variety of sources, ranging from OSHA compliant N95 filters to cotton fabric to polypropylene surgical wrap. Our preliminary testing indicated that the majority of users pass qualitative and quantitative fit testing (OSHA 1910.134 App A), while some users must utilize additional mitigations such as a CPAP nasal gel pad to complete fit and improve comfort.

Comprehensive testing, such as for biocompatibility (ISO 10993) and for fluid protection has not been completed.  Nor has this device been cleared by FDA (21 CFR 878.4040,  ProCODE MSH).  Various improvements in design and fabrication methodology are ongoing.

Outline of fabrication methods:

1. Obtain an optical scan of user’s face as STL using technology with 1 mm resolution such as commercial structured light printer.
2. Merge assembly in CAD of filter build-out and user’s face STL together to yield one STL as a Boolean combination of the respirator/mask scaffold.
3. Trim STL of merged file to encompass only the area around respirator/mask scaffold with a 2.5 cm border
4. 3D print combined STL of face respirator/scaffold.  PLA and a FFD consumer grade printer work well.
5. Vacuum thermoform a 2 mm PTEG sheet over 3D print made in #4.
6. Cut thermoformed sheet around scaffold, leaving at least a 1 cm wide flange that fits flat against the contours of the user’s face.  This flange, extending laterally from the mask, is critical because it forms the airtight seal against the face that enables passage of the OSHA fit test.
7. Cut out sections of the thermoformed mask for selected filter fitting adapters
8. 3D print filter fitting adapters from STLs using PLA on an FFD printer (STLs for fittings are provided open source)
9. Pinch cut O-rings from EPDM or similar elastic sheet
10. Fabricate filter housings using 3D printer using geometry of available N95 or other airway filter material.
11. Screw filter fitting adapters into thermoformed mask, sealing them hermetically with O-ring and silicone flexible cement
12. Insert filter housing into adapter, sealing with silicone flexible cement
13. Insert filters into filter housings and snap top and bottom pieces together
14. Staple elastic straps
15. Test fit using OSHA guidance
16. CPAP gel cushion material inserted along the bridge of  the nose may improve comfort and fit, especially in individuals with a broad face. Alternatively a 3/8” x 1/16” closed cell foam strip placed along the mask at the bridge of the nose creates a good seal in wearers with a small sized face.
17. Position and tension of the elastic straps may also need adjustment to avoid deformation of the thermoformed mask.

Example: Thermoformed mask/respirator scaffold

Other files provided:

1. Sample STL of scan of end-user’s face
2. Sample STL of end-user’s face combined with CAD of mask designed for one type of industrial N95 filters
3. Sample filter fitting CAD files, kindly provided by Ethan Reggia (ereggia@umd.edu)