4.1. Performance and Future Work
The open source nutating mixer is designed to perform the basic gentle 3-dimensional nutating rotation in both CW and CCW directions. The mixing provided by the nutating mixer is thorough yet gentle mixing that prevents foaming. This functionality is enough for most basic mixing needs, such as mixing blood samples or biological DNA samples. Both the speed and the tilt angle of the open source nutating mixer are fixed to provide appropriate agitation for mixing samples in small containers. However, some laboratories may want other tilt angles, as discussed above, or may need the ability to vary the speed of mixing. This open source system can be modified to a variable speed adjustable mixer in the future by adding a potentiometer to the circuit. In this case, a 45 or higher RPM motor can be used, and the potentiometer can adjust the speed from 0 RPM to 45 RPM according to the requirements of the experiments. This modification would have only a modest additional cost associated with the cost of the motor (<US$
20, which would substitute for the as-designed motor) and potentiometer (US$
2). To go further, it is possible to add an Arduino and LCD display to adjust the time and the speed of rotation, which would again increase costs further (e.g., US$
10 display and $
20 microcontroller or less if using the ATmega168 microcontroller with a custom PCB fabricated with an open source mill [45
]) and provide functionality well beyond what is currently commercially available. In addition, as the mixer as designed can operate with up to 3 kg of load. This is well above what would be expected of a fully loaded sample platform. Thus, the platform can be redesigned to hold multiple mats in a vertical stack simultaneously to mix more samples using the same instrument. The nutating mixer can already be used in cold rooms, but for use in incubators, a high-temperature 3-D printed polymer can be substituted for the PLA used here. As these potential future upgrades make clear one of the major advantages of open source 3-D printed scientific equipment, is that users can customize the equipment to meet their needs with minimal additional costs.
4.2. Cost Analysis
The overall cost of the 3-D printed and purchased equipment based on the BOM in shown in Table 1
and Table 2
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
]. 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
]; (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
] 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
]. 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 of disciplines). However, it would appear challenging for those with no experience in 3-D printing. For those with experience and access to a basic RepRap or similar the time investment for setting up a print again is trivial. For example, although only the two flexible components were printed out on a Lulzbot Mini here, all of the components could have been. The first five components can be printed on any such hard plastic FFF 3-D printer using default settings, or the settings recommended here. There are hundreds of thousands of these 3-D printers deployed globally, which are thus readily accessible to most labs. A tuned DIY RepRap or a commercial self-bed leveling 3-D printer can be left unattended after the file has been sent to print just as long as 2-D print jobs can be left to print without the printer being monitored by a user. Thus, although the actual 3-D print time is much longer, the time that labor is focused only on printing is less than half an hour. The two flexible 3-D prints must be made on an advanced (or upgraded) 3-D printer (e.g., the Lulzbot Mini with the Flexystruder) and may not be available even in every workshop with a 3-D printer. In these cases, the local FabLab [59
], makerspaces [61
], hackerspaces [62
], and even public libraries [63
] often have 3-D printing services available either for free, or for the cost of materials. Most universities now have at least basic 3-D printing capabilities somewhere on campus. For researchers wishing to fabricate the open source desktop nutating mixer with no access to 3-D printers locally, an on demand quasi-local 3-D print service can be used, such as MakeXYZ [64
]. The costs for a 3-D printing service will be more than the costs of the materials alone, but in general reasonable, because of the high degree of competition, quotes are available immediately for users in any given area.
Lastly, once all of the components are gathered they must be assembled. Having designed the system, the researchers in this study could build the device in about 10 min. To remain conservative, it is estimated that for a novice builder the build time would be under 30 min. (with the assembly instructions in this paper).
Thus, the overall cost in labor to source, print, and assemble a 3-D printable open source desktop nutating mixer is about 1 h. This indicates that it is profitable for an organization to use the open source version if their labor costs are under $250/h, even for the least expensive commercial equivalent (or under $330/h for the average commercial system). Finally, a point should be made about the life cycle cost advantages of the open source mixer. As all of the files are shared and the BOM is known, regardless of the failure mode of the device it is easily repaired from readily available components (e.g., reprint a broken plastic component or replace the DC motor). This ease of repair and upgrading is simply not available for all of the commercial systems, which would demand the purchasing of a replacement device. Thus, the value of the open source tool can be considered higher than the commercial functional equivalent, even though the open source tool costs less money to build upfront.