## 1 Introduction  
The COVID-19 pandemic has illustrated the vulnerability of conventional global supply chains. Over the past decade, other major disruptions have also exposed this vulnerability [1, 2]. For example, the eruption of an Icelandic volcano in 2010 affected time-critical air shipments [3]. The Japanese earthquake and tsunami in 2011 caused several months of disruption in the automotive industry [4]. Thailand's 2011 floods had a huge impact on the supply chains of computer manufacturers in need of hard disks drives [5]. After Hurricane Harvey in 2017, there was a mass disruption in the U.S. chemical production, which caused shortages of essential industrial building materials [6]. The avian influenza A (H5N1) affected trading and transportation systems in major cities, and caused a destructive effect on global food supply and business operation [7]. The coronavirus disease (COVID-19) pandemic has once again demonstrated the fragility of conventional global supply chains. Cost minimization and cost-cutting measures are the most significant drivers that lead to fragility and vulnerability of global supply chains [7; 8]. To date, a plethora of medical equipment businesses are relying on overseas production in developing countries to minimize costs. This has led to shortages in the supply of much-needed medical and non-medical products required to fight the COVID-19 pandemic. As a result, there is an unprecedented shortage of personal protective equipment (PPE) for healthcare workers, lack of ventilators and spare parts for patients, insufficient supply of sanitizer solutions, and scarcity of coronavirus test kits for the general public. Countries that are affected have imposed bans on the export of PPE and other products critical for health. These bans have subsequently caused a greater disruption of medical supplies, and illustrate the vulnerability of global supplies and logistics amid a large upheaval from the pandemic.  
Over the past decade, 3D printing (additive manufacturing) has been increasingly implemented in the medical industry. For example, medical and preoperative models [9], surgical implants [10; 11; 12; 13], and tools and templates for operations [14] have been fabricated using 3D printing. Major advantages of 3D printing are its ability to offer mass customization, to produce freeform parts on demand, and to cater to lot size indifference and other patient-specific designs [15; 16; 17]. In the current COVID-19 pandemic, 3D printing has been increasingly used to print spare parts for medical devices and protective gear, due to the unavailability of supplies. According to the search volume data of Google, the term \"3D printing\" was searched 2.2 times more in April 2020 than in the same period last year. Key benefits of 3D printing over conventional manufacturing are faster production, digital storage and traceability of part files, reduction in delivery time, and the ability to produce components regardless of the complexity of part geometry [18; 19].  
To date, there are only a few publications related to COVID-19 and 3D printing. More specifically, the following six topics have been investigated: open source ventilators [20], connectors for breathing devices [21], sourcing PPE [22], pressure-regulated ventilator splitting spare parts [23], respirators [24], and 3D printed ventilator parts [25]. Thus, there is reason to believe that the productivity of 3D printing with regard to the COVID-19 pandemic is not fully understood. The aim of this paper is to investigate the productivity of the most promising open source 3D printing solutions for the COVID-19 pandemic.