## 4 Design for 3D Printing in a Pandemic Situation  
A major advantage for 3D printing is the fast way iterations of specific designs can be brought into a physical form for testing and evaluation. While this is beneficial in the development phase of a product and shortens the time to market, in the case of a pandemic situation, this also can lead to problems.  
With the outbreak of the COVID-19, different makers and companies immediately sprung to action and released CAD designs for different medical appliances to overcome shortages in different areas. These included surgical face shields [16; 17], respiratory masks, and ventilator valves [18] as some examples. As all of these are crucial products for personal protection, the number of different CAD designs released posed some challenges for makers willing to help.  
First of all, most of the CAD designs, particularly those for respiratory masks, were not tested and proven beforehand. This resulted in minor issues such as masks not being comfortable to wear, or more severe issues with the masks not functioning properly or even harming the user (patient) by not providing sufficient air flow or lacking viral protection. Some designs also required a large amount of post-production assembly of additional parts, which in itself undermines the use of 3D printing for its ability to create complex parts without the need for additional manufacturing steps. There are some more intricate designs available for respiratory masks [19], but these often have highly individualized parts, making use of 3D scanning techniques to adjust them to the user.  
Face shields and surgical mask straps are the most commonly produced items in the current pandemic situation. Even for those rather simple products, a variety of different designs were published with the original face shield design (Figure 3a) by [PERSON] [20] being the most commonly used. To satisfy the high demand, print-time optimized models of the face shields were quickly generated and published as well as stacked versions of this model, making use of the whole z-axis of higher volume printers (Figure 3b).  
Similar efforts were being made for surgical mask straps. A wide variety of CAD designs is available at the moment, ranging from simple, easy, and fast to print types to very intricate ones with a quick release feature (Figure 4 a-c).  
Figure 4: Surgical mask straps; simple, (**a**) fast printing model, (**b**) complex quick-release model top, (**c**) in use.  
Figure 3: Headband designs for protective face shields; (**a**) simplified design for faster printing, (**b**) Prusa design, (**c**) final face shield in use.  
As most of these PPE designs are being published by private persons on various 3D printing community websites such as thingiverse [21] or myminifactory [22], they did not undergo any of the official functional testing, approval, or certification processes usually required for PPE products and even more importantly for medical devices. There are a limited number of certified products available, as the testing and certification process is time consuming. Despite the time constraints, Prusa Research had their design and face shield assembly certified by the Czech Ministry of Health. Often, the makers had their printed parts tested by hospitals or doctors individually for their applicability and sterilization properties. To avoid legal issues, the shields are delivered with a notice attached that these are not certified medical products and have to be tested for their intended use beforehand.  
As a consequence, makers have to make an educated decision on which designs to put into production in times of supply shortages. The designs have to fulfil a number of key properties: functionality as intended for the specific purpose, reliability/durability in use, printability, and short printing time are some of the points to consider when choosing a model.