## 4 Discussion
To support the efforts towards the fight against COVID-19, the National Institutes of Health (NIH) [30] in the U.S. has made the computer-aided design (CAD) models of PPE products freely available on its website, and has also divided them into two categories. Non-critical products include air exchangers, filter adaptors, in-line filter housing, and pneumotachometers. Products that have safety implications include ventilator splitters, Venturi valves, and flow restriction devices that pose a much greater risk if used or produced incorrectly. This digital library was created to support the manufacturing of PPE and other medical devices in short supply due to the COVID-19 outbreak. The NIH, Food and Drug Administration (FDA), Veterans Affairs (VA), America Makes, and contributors acknowledged that they cannot ensure the quality, safety, and efficacy of these designs when manufactured without proper quality controls and processes. As a result, ASTM International [30] has enabled public access to relevant ASTM standards used in the production and testing of PPE such as for face masks, medical gowns, gloves, and hand sanitizers. Other efforts, such as the U.K. Government Office for Product Safety and Standards, have also published guidelines to assist manufacturing companies planning to convert their operations to produce PPE. These include ways to have products approved for sale or to be donated to the government to be used by healthcare workers. It also provides guidance on what manufacturers have to do to fulfil those obligations, and contacts to U.K. Notified Bodies that can assess PPE for COVID-19 [31]. However, it must be noted that at the time of writing, there are still no specific standards available when using 3D printing for the production of PPE to ensure total compliance with healthcare protocols. In terms of the digital value chain when designing and producing parts for 3D printing, the ISO document, \"ISO 17296-4:2016 Additive manufacturing - General principles - Overview of data processing\" [32], suggests that data exchange methods could include the file type, data enclosed formatting of such data, and what this can be used for. Examples of suitable formats for data exchange include.STL,.STEP, and.AMF. Of these, the.STL file is still the most commonly used format, and this has been observed in the file formats available in the digital library for COVID-19 CAD files. One of the drawbacks is that.STL files do not specify units. Newer file formats have been made available, such as the.AMF file format that can contain metadata such as information of the CAD source, design and production specifications, supplier information, and production data. The.AMF file format is also able to support constellations that describe the build pack, stacking, nesting, or orientation of parts. The.AMF file format is also XML-based, and users with a text editor are able to quickly open, view, and edit the metadata content. Other useful formats that could provide a more accurate and efficient manufacturing workflow include the.3 MF file format. 3D printing in a bridge manufacturing mode has tremendously supported the fight against the COVID-19 pandemic [33]. Local or distributed manufacturing [34; 35], supply chain modifications, and enhanced education can help us achieve a more stable supply chain, as well as mitigate a number of shortcomings [28; 36; 37]. The labor requirements for pre- and post-production steps of 3D printing, in addition to the throughput of the 3D printer, limited range of raw materials, and multi-material process constraints, call for further technological advancements to improve the competitiveness of 3D printing, and to deal with raw material shortages [38]. This study was limited to open source parts because they are accessible to everyone, and therefore have the biggest impact. The work used few exemplar parts to highlight the potential of 3D printing, however there is a wide range of similar open source parts made available in open access repositories.