## 5 Quality Assurance: Raw Materials and Final Printed Parts  
### Raw Materials  
The materials used for the printing activities carried out in context of the makervsvirus movement were selected according to the type of item to be printed and the preferences of the individual hubs in Karlsruhe and Ulm with which the KIT was cooperating.  
As an immediate reaction to the shortage of PPE, polylactic acid (PLA) became the material of choice for the makers due to its general popularity for the fused filament fabrication process [23]. PLA is a thermoplastic and a so-called biopolymer, because it is produced from renewable resources such as corn starch, tapioca roots, or sugar cane, which makes it environmentally friendlier than classic polymers. It is also biodegradable under certain aerobic conditions such as in industrial composting. It can be printed at relatively low temperatures (190-220 \({}^{\circ}\)C) and only requires the print bed to be moderately heated to 50-60 \({}^{\circ}\)C to ensure a secure attachment of the print to the platform. Post-processing is easy, and PLA is perfect for complicated geometries and has excellent coloring options. Therefore, it is favored for many prototyping applications and provides sufficient strength in combination with good aesthetic qualities for most applications. In turn, PLA has its drawbacks, such as its relatively low temperature resistance [24]: it begins to soften at around 50 \({}^{\circ}\)C. It is also not considered food safe [25], mainly because bacteria can build up during the printing process itself or in crevices due to defects in the printed parts, which is always a concern.  
As a consequence of PLA's drawbacks, polyethylene terephthalate glycol (PETG) was subsequently selected as the material of choice for printing most of the PPE equipment. PETG, like PLA, belongs to the polyester group of plastics and is a popular 3D printing material for applications requiring high impact resistance and ductility [26]. PETG is a more specialized filament, with a more sensitive process window compared to PLA. However, for most applications where physical properties of a component are stressed, a material such as PETG is preferred over PLA. Parts made of PETG have a much better temperature resistance than PLA, with some varieties showing stability up to 250 \({}^{\circ}\)C. In general, most PETG parts remain stable up to a temperature of 70 \({}^{\circ}\)C, facilitating its use in products intended for temperature-based sterilization processes. PETG is also able to better handle UV-light and other weather-based influences compared to PLA, favoring its outdoor usage. PETG is not biodegradable, but can be recycled into new filament or other products without loss in quality. Finally, it is considered to be food safe, but as every 3D printed part will contain numerous nooks, crannies, cracks, holes, etc. that bacteria can thrive in, a proper cleaning or sterilization before usage is crucial [27].  
The application of 3D printing for the production of PPE such as the face shields brings a number of requirements that have to be fulfilled:  
* being sturdy and mechanically sound enough to withstand continuous usage for long amounts of time;* being flexible enough to be assembled together with elastic components that will stress the part, especially because of the head bands;
* being light weight and not feeling like a burden when being worn;
* having the ability to be sterilized so they can be used under safe conditions in a medical environment.  
Considering these requirements, both PLA and PETG satisfy these criteria and can be used for the production of PPE. However, since PETG is more resistant to elevated temperatures (sterilization) and because of its higher impact resistance, it was the favored material for PPE in the makervsvirus hubs. PLA allows inexperienced makers to quickly produce good printing results due to its ease of use, but is not as mechanically strong and temperature resistant.