## 4 Discussion
### Cost Analysis  
The overall cost of the 3-D printed and purchased equipment based on the BOM in shown in Tables 1 and 2 is US$37, which can be compared to other nutating mixers commercially available on the market, as summarized in Table 3. The closest available commercial products on both instrument suppliers' websites and Amazon cost on average US$373, which is 10 times the cost of the open source nutating mixer, thus saving 90% of the commercial cost. These cost savings are in agreement with the savings observed in other scientific tools in past studies [6; 11; 12; 31]. Finally, a point should be made about the range in costs of the commercial systems shown in Table 3. The most expensive system also included variable speed, which are not included in the lowest cost models. As noted above, this functionality could be added to the open source system, while maintaining the same cost advantage in percent for the materials costs. These cost savings, however, assume that there is no labor cost for the purchasing of components, 3-D printing, and assembly. Zero labor costs are relevant in the following situations: (1) where the 3-D printing and the assembly of the device is used as a learning tool in order to provide students with experience in the construction of scientific equipment or open hardware [57; 58]; (2) where the labor is provided by unpaid interns or volunteers (e.g., undergraduates volunteering for research to gain experience and improve their resume/CVs); or, (3) where there is no opportunity cost to using existing salaried employee (e.g., the use of a lab manager or RA, TA, or other position that is paid a fixed cost, and for which there is no opportunity cost for them working on the fabrication of the device. In general, in academic institutions these conditions can be readily met in most labs. In the labs, however, where this is not the case (e.g., industrial labs) it is instructive to look at the potential cost of labor for the fabrication of the open source desktop nutating mixer. The labor involved is represented by three tasks: (1) purchasing the six primary components, as outlined in Table 2; (2) 3-D printing the seven 3-D printed parts listed in Table 1; and, (3) assembling the device when all of the components have been gathered. The labor for each of these tasks will be analyzed separately. First, purchasing the components is a low-skill task, particularly when the Amazon hyperlinks in references [36; 37; 38; 39; 40] are still active and purchasing is occurring in the United States (U.S.). The total time for this task would be about 5 min for anyone with an existing Amazon account and shipping is free for those with Amazon prime. In the future, if these components are no longer available on these hyperlinks than they will need to be sourced from other websites, which will have an additional time cost. It should also be pointed out that it may be possible to decrease the cost of the device by careful comparison shopping for the components, and many of the components are probably already available for no cost at institutional Fablabs [59; 60]. Regardless, this subtask can be undertaken by the lowest-cost worker in an organization (e.g., a receptionist at a company) and represents a trivial or non-existent cost. Next, the 3-D printing of the components listed in Table 1 can be considered a moderate skilled task (although FFF 3-D printing is a rapidly expanding skill set seen in young workers in a wide array