An Inexpensive Hand Printed 3D Printed Centrifuge

Matthew Boeker and John Gianniniboeker1@stolaf.edu

St. Olaf College, 1500 St. Olaf Ave, Northfield MN 55057

IntroductionCentrifuges are very useful tools that rely on centripetal force for separating various

small organisms or parts of organisms from the media that they are in. This can be helpful for educational experiments such as separating DNA from lysed cell parts, or separating out the different parts of blood. Simpler uses also include pelleting small cells such as Tetrahymena for demonstrational or research purposes.

Centrifuges can be extremely expensive, ranging anywhere from $200 to upwards of $50,000. These prices normally put them out of the range for pre collegiate science programs, despite their many uses. In addition, most centrifuges require a source of electricity to function. In this experiment, we created a design for a 3D printed hand operated centrifuge. Our design pelleted Tetrahymena out of media in minimal time and appears to function as an inexpensive alternative to more costly machines.

MethodsWe used DesignSpark Mechanical 2.0 to create the design for the

centrifuge. The files for the design can be found HERE. We then modified the diameter of the centrifuge so that we had three models (7.5 cm, 10 cm and 15 cm). We printed these centrifuges using ABS plastic on a Lulzbot Taz® 3D printer (Figure 1).

In order to culture the Tetrahymena, we used the same procedure that was used in An Inexpensive Method for Sterilizing Tetrahymena Media Using a Microwave https://pages.stolaf.edu/opn-lab/experiments/.

We used about 3 feet of 1mm nylon string to feed through the two holes in the center of the centrifuge. We then tied the ends of the string together to make a loop. We added two dowel sticks to either end of the loop in order to make handles. Figure 2 shows the fully assembled 15 cm centrifuge.

In order to test the effectiveness of each centrifuge, we used two 1.5 ml micro centrifuge tubes filled with Tetrahymena culture on opposite sides of the centrifuge. Then, holding the handles, we spun the centrifuge so that there were coils in the string. From there, we pulled on the handles to force the centrifuge to spin. We continued this motion for the duration of the test. Figures 3 and 4 demonstrate this action.

We conducted three trials at 30 seconds, 1 minute and 2 minutes for each of the centrifuges tested. The timing started as soon as the centrifuge was brought up to full speed, which was indicated by a “whirring” sound. When the time was up, the centrifuge was allowed

to come to a stop slowly, without any jerky movements. We also compared our Tetrahymena to three controls of the solution with no centrifuging.

We looked for pelleting of the Tetrahymena and also took a small sample of the supernatant to look at under the microscope. We took a drop from the top of the supernatant and placed it on a microscope slide. We then used an Olympia Microscope at 100x magnification to look for the presence of Tetrahymena, and recorded the density present on the slide.

ResultsIn the three trials of control where no centrifuging was done, the Tetrahymena were too

numerous to count with no pelleting on the bottom of the micro centrifuge tube. We found that the best results came from the 10 cm centrifuge at 2 minutes of spinning, with an average of just under 3 Tetrahymena per slide. However, we did see similar results in the 1 minute trials as well, with an average of 4 Tetrahymena per slide. During the three 30 second trials, the number jumped up to about 15 Tetrahymena per slide. Figure 5 shows these results. Pelleting was observed during all of the trials with all centrifuges. Figure 6 shows an example of what the pellets looked like. Both the 7.5 cm and 15 cm centrifuges provided pelleting in all of their trials as well, but with higher average numbers of Tetrahymena per slide than the 10 cm centrifuge.

Figures

Figure 1. From left to right, the 10 cm, 7.5 cm and 15 cm diameter centrifuges

Figure 2. Fully assembled 15 cm centrifuge complete with handles and Tetrahymena media

Figure 3. After the initial coiling of the string, pull the string to its full extension. This should start the spinning of the centrifuge

Figure 4. After “stretching” the string, the coils shorten the length of the string. Pulling on the handles during this short phase will start the cycle over and keep the centrifuge spinning

Figure 5. Graph showing the average number of Tetrahymena in the supernatant after a certain amount of time of centrifuging in the 10 cm centrifuge

Figure 6. Tetrahymena pellet after 30 seconds of centrifuging

DiscussionFor our testing purposes, the 10 cm centrifuge worked the best. Depending on the

circumstances, however, any of the 3 centrifuges could be the best option. The speed of the centrifuge is extremely variable depending on who is operating it. The more force you put into pulling on the strings, the faster the speed of the centrifuge. This does get taxing, especially if you are spinning for 2 minutes. The larger the centrifuge, the harder it is to get it to an appropriate speed. This is why we think that the 10 cm centrifuge had better test numbers that the 15 cm centrifuge, especially during the 30 second trials.

This method of centrifugation is a very inexpensive and easy way to pellet out organisms or particles that are around the same size as Tetrahymena, which can range from 8000 microns 3 to 100,000 microns 3. Further testing needs to be done to see if these centrifuges have the power to pellet out even smaller particles. The only cost prohibitive part of this design is having access to a 3D printer. The cost of the filament plastic required should amount to less than $1 per centrifuge. An inexpensive method for growing the Tetrahymena is also provided in the paper An Inexpensive Method for Sterilizing Tetrahymena Media Using a Microwave https://pages.stolaf.edu/opn-lab/experiments/.

The results from this study show that this is a very effective way to centrifuge Tetrahymena, but the correct technique must be used at all times. It is extremely important to let the centrifuge come to rest on its own. Any jerky movements will send parts of the pellet back into the supernatant. Time is also quite important, as the motile Tetrahymena will swim out of the pellet and back into the supernatant very quickly. This would not be as important in a non-motile organism or particle. In this study however we think that this motility is the reason why we saw any Tetrahymena activity in the 1 and 2 minute trials.

The goal of this study was to prove that the design was functional and could be used to separate Tetrahymena from the media. This design did that, and is also perfect for classroom demonstrations, hands on science courses, or places where electrical centrifuges are not an option.