solder, and then wrap the exposed areas with electrical
tape as described above.
9. After completing the circuit, use electrical tape to hold the
wires to the sides and bottom of the plastic bowl to keep
them out of the path of the tube holder when the centrifuge
spins (Header Image; Figure 2).
10. Finally, use scissors or wire cutters to remove the front and
back “flaps” on the lid of the plastic bowl, since they likely will
not snap into place given the location of the switch and jack.
Alternatively, readers can skip this step and simply leave the
front and back flaps up when using the OPN Minifuge.
The completed centrifuge should now spin when hooked up to
the AC/DC adapter and plugged into a wall socket (Figure 2A). If
not, unplug the adapter from both the socket and the centrifuge,
and check the contacts on all of the electrical connections in the
OPN Minifuge. If the motor still does not spin, disconnect the
adapter from the both the socket and centrifuge again, and try
switching the positive and negative connections on the DC jack to
ensure the current is flowing in the proper direction for the motor.
Also, instead of using an AC/DC adapter, readers can power the
OPN Minifuge using batteries that are connected to a similar adapter
plug, which further makes the device portable.
Most hazards for this project are self-evident, such as the significant dangers of working with power tools and electrical wiring.
For example, readers should obviously exercise great care when
working with hand or shop tools, and be sure to wear the proper
eye and ear protection as well. Readers should also make sure that
the OPN Minifuge is never plugged in or connected to the AC/DC
adapter when working on the electrical wiring. This last point is
particularly important because an AC/DC adapter can hold its
charge for some time even after it has been unplugged. In addition, to reduce the risk of overloading, which could cause an electrical fire, the current and voltage rating of the AC/DC adapter
should never exceed the limits of the fan motor (e.g., we used a
12-volt, 230-mA adapter to power a 12-volt, 250-mA fan). As a
result, those who lack experience in these areas should work with
a trained craftsperson to avoid injury. Such a technician should
also be able to provide guidance on assembling the OPN Minifuge
and help with troubleshooting the set-up in the event that the
centrifuge does not spin when first plugged in.
Calibration and Testing
We used a stroboscope to determine that the OPN Minifuge spins
at approximately 1,200 rpm when holding two microfuge tubes
containing 1 mL of water each. Given the 80 mm radial distance
to the middle of each tube, we calculated that the OPN Minifuge
generates a force of roughly 129 g. Next, as a durability test, we
ran the centrifuge for over 8 continuous hours with 1 mL of water
in each micro-centrifuge tube, and the device operated without
incident or mechanical failure. Finally, we tested the centrifuge by
using it to pellet Tetrahymena thermophila, a small ciliated proto-
zoan often used in our research lab. As a control, we used a Gilson
GmC Lab mini-centrifuge, which spins at 6,000 rpm and generates
a force of up to 2,910 g according to its published specifications
In these tests, we used Tetrahymena cultures that were prepared
by adding 500 µL of stock cells to separate 125-mL flasks contain-
ing 25 mL of modified Neff media (Cassidy-Hanley et al., 1997).
Although these flasks of media had previously been autoclaved to
ensure a sterile environment (Bozzone, 2000), they had reached
room temperature by the time that the stock cells were added.
The cultures were then placed on a shaker table and incubator (at
70 rpm and 30°C) for 24 to 48 hours before testing to generate a
range of initial cell densities (Stewart & Giannini, 2016).
From these cultures, we spun down 1-mL samples of Tetrahy-
mena in micro-centrifuge tubes for 0, 10, 20, 40, and 80 seconds.
From each sample, we pipetted 20 µL from the middle of the
supernatant column onto a microscope slide, and added 4 µL of
5% glutaraldehyde to fix the cells. We next covered the sample
with a coverslip and examined each slide under a light micro-
scope at 40× magnification, counting ten fields of view per slide
for Tetrahymena and one slide for each time interval. We repeated
the experiment a total of eight times for each device, averaged the
tallies for each time period in each of the eight tests, calculated the
corresponding grand means from those results, and then charted
the respective relative frequencies as a proportion of the initial
(0-second) average cell count (Figure 6).
Although the Gilson mini-centrifuge slightly out-performed our
design, the samples showed considerable reduction in cell count after
just 10 seconds of spinning in the OPN Minifuge, and nearly complete
removal of Tetrahymena from the supernatant by 80 seconds. Never-
theless, readers should conduct their own preliminary tests after
assembling their centrifuge, especially if using different parts than
those described above or using the device for a different application.
Given its functionality and affordability, we hope that the OPN
Minifuge will help to expand the use of this important piece of
Figure 6. The relative frequency of Tetrahymena thermophila
cells in the middle of the supernatant as a function of
centrifugation time (0, 10, 20, 40, or 80 seconds) in a Gilson
GmC Lab mini-centrifuge or the OPN Minifuge.