## 2 Materials and Methods  
The process for the fabrication of the Mask Fitter requires a face scan, followed by the design and 3D-printing of the Mask Fitter. For a face scan process, several options are available including photogrammetry, stereophotogrammetry, laser-beam scanning and structured light scanning [21]. An easy-to-use alternative is the \"face ID\" system implemented in the more recent generations of smartphones and tablets. The smartphones that at present can be used are any iPhoneTM with a TrueDepthTM camera (iPhone X onwards), or an iPad ProTM (all generations). For this survey, an iPad Pro 1 st generation was used. The necessary software, Bellus3D Dental Pro, can be downloaded from the Apple store. Bellus3D Dental Pro is available as a free download for basic features offering in-app purchase for advanced functionalities. This face scan system has been reported to allow trueness and precision in a clinically acceptable range [21; 22]. The design of the Mask Fitter has different options. The basic version of the Mask Fitter for a surgical mask can be generated and exported free of charge, whilst the generation of the Mask Fitter with holders' name or the FFP2 (\"tall\") version require payment of a USD 1.99 (USA currency) fee. After installing the software, the face scan acquisition procedure is straightforward. It consists of a voice guided procedure requiring a simple left, right, up, and down sequence of head movements (Figure 1). Figure 1: Face scan acquisition procedure. Of the three available scanning options (face, face + neck, full head) the second one is recommended by the authors of the present paper for the Mask Fitter purposes. Option one (face only) takes an insufficiently detailed scan of the chin area, while option three (full head) is redundant as the back of the head is not involved in the Mask Fitter design. At the end of an approximately 30 s scan process, followed by another approximately 20 s for data computing, the 3D-representation of the face is generated. After selection of the function \"mask fitting\" and subsequently the selection of the desired design (\"standard\" for surgical masks, \"tall\" for FFP2), a Mask Fitter consisting of an anatomic frame is automatically designed by the software with a 10 s processing time (Figure 2a). The generated Mask Fitter design can then be exported as a.stl file (Figure 2b). The free version of the software does not allow the face-scanning to be saved, but it does allow the sending of the.stl file of the generated design of the Mask Fitter via email. Once the file is received by email it can be 3D-printed. Partner services are suggested from the Company itself, but any 3D-printing services, available online at different, but reasonable prices (e.g., GBP 10-15 UK currency), can be used. Alternatively, and more conveniently, the Mask Fitter can be printed by any Office, Department, University, Company, Dental Laboratory or even personal 3D-printer (Figures 3-5) as there are no special requirements for the printer and the material. A basic Stereolithography (SLA--Resin) 3D-printer or a basic Fused Deposition Modeling (FDM--filament) 3D-printer can efficiently 3D-print the Mask Fitter. Figure 3: A GCode is generated by a 3D-printing slicing software (Ultimaker Cura). Figure 2: (**a,b**): The Mask Fitter design generated by the software. For the present study, a total of 20 dental staff were recruited for this service evaluation selected following \"convenience sample\" selection criteria, and they voluntarily agreed to try the Mask Fitter. A written consent was obtained from each of the participants, by which they agreed to participate in the study and to have their data used as part of the research. They were face-scanned and then their Mask Fitters were 3D-printed as previously described.