This paper aimed at describing a feasible technique for digitally customising 3D-printed face masks to individual face scans, which offers some important advantages. Firstly, such methodology customisable using free software thereby lowering the barrier to individual use of the design and offering a relatively cheap solution for public health sectors. Secondly, when placed filter-side down on the print-bed, it was possible to print the mask without supporting structures as each layer would have sufficient overlap from the previous layer to support it (Fig. 4). This feature results in savings of time when printing as well as avoiding material wastage.
Therefore, the main advantages of the proposed methodology are: 1) customisation for perfect fit resulting in avoidance of air leakage and extended-use comfort, 2) local fabrication eliminating issues with supply chain disruption during crisis, 3) reusable item of equipment lowering long term costs of PPE (It is also possible to scribe text on the model, for mask user identification), and 4) documented high levels of particle filtration by using a tried and tested filter system. The main limitations include requirement for technology application (Apple Iphone and computer able to run Blender, 3D printer with sufficient features to print more advanced filaments, human resources trained to use all of these items), an extensive printing time of around 5 hours per mask, supply of 3D printing filament and P3R filters and, high initial costs per mask.
In searching for a filter material solution, it was clear that a P3 standard of filtration was desirable considering the small dimension of the coronavirus and the need to protect healthcare workers to prevent failure of healthcare systems in the face of the pandemic.2 Therefore, the front of the mask was redesigned with a thread fitting 40 mm x 1/7” (EN 148-1:1999), to which the P3R filters of several manufacturers can be fitted. The P3 standard provides a high level of protection filtering more than 99.97% of airborne particles and protects against solid and liquid aerosols. P3 filters have an Occupational Exposure Limit (OEL) rating of 50x and an Assumed Protection Factor (APF) of 20x.
A P3R filter is reusable and may be used for several shifts. Some manufacturers suggest use of a P3R filter can be extended to several months. Filters can be disinfected with alcohol in between shifts, and in the selected system, a disposable prefilter eliminates most of the particle load, sparing the main filter. These features increase the system’s economy. In addition, the manufacturer instructions for use for the SR-510 P3R filter allow an operating temperature of 55⁰C for this filter. Meanwhile, data from a study looking at inactivation of a similar virus (SARS-Cov) revealed that in the absence of protein contamination, the virus could be inactivated by a temperature of 56⁰C.7 It is not unreasonable, within the headroom allowed by manufacturers, that disinfection in a warm-air oven at 56⁰C would inactivate the viral load and preserve the filter’s properties. Thermal sensitivity testing of the current pathogen is needed, and manufacturer testing of their filters at 60⁰C or higher is also needed, to permit more certainty in reprocessing of filters for reuse.
The material used herein was a thermoplastic copolyester, which is produced from 50% bio-based feedstock and is resistant to high temperatures due to long term thermo-oxidative stability as well as good UV resistance. Such material is highly elastic with a Shore D of 24 and passes ISO 10993-10 irritation/intracutaneous reactivity testing, ISO 10993-5 cytotoxicity testing and USP class VI for medical implants. Critically for this application, it has a high inter-layer adhesion due to its slower crystallization behaviour. The manufacturer suggests it for wearable items such as smartwatch bands as well as mouthguards. Our initial 3D printing tests of the mask design resulted in failed prints due to various problems, which were solved by adopting the final printing settings described above. Such settings are applicable to the specific model of 3D printer used, while others may require different settings.
Among the limitations of the present methodology are: the complex photogrammetry techniques used to capture the 3D image of the face and the use of a thin filter material in the filter holder at the front of the mask. To overcome the first limitation, various facial scanning methods were considered, including using intraoral dental scanners to scan the facial skin, using desktop model scanners to scan casts of the face and using various mobile phone methods. The choice of Bellus3D was based on the favourable ratio between the quality of the facial scan versus the technique’s simplicity and speed, accepting that it is limited to functioning only on recent Apple Iphone™ (Apple Inc, Cupertino, CA) mobile phone models. Since the method is quick, one unit of hardware can be used to scan several faces per hour, maintaining the concept of ease of accessibility for our concept. Furthermore, the technical report design of this article aimed only at showing the potential of an automated software tool tool to customise 3D-printed masks to face scans. Therefore, future prospective laboratory and clinical would be recommended to address qualitatetive and quantitative fit results, as well as the long-term stability and airborne particle protection levels. Finally, despite the automated use of the plug-in presented herein, basic CAD knowledge is still required in this method. On the other hand, software straightforward tools and plug-ins are currently being developed to simplify digital design procedures, especially considering the current focus on solutions to prevent COVID-19 transmission. Even if some level of supply of FFP3 masks is established, these need to be available in a variety of sizes and properly fit-tested for each healthcare worker.