The centrifuge is an important piece of equipment for any biology lab, necessary for a wide variety of experiments. Cost, however, often prevents the use of centrifuges in many teaching labs, greatly limiting educational experiences for students. As a result, we describe how to construct a “homemade” single-speed mini-centrifuge using inexpensive parts easily acquired online or at hardware or other stores (e.g., a 4.7-quart plastic bowl and lid, standard computer cooling fan, AC/DC adapter, electrical wire, etc.). We further used this design and a commercial mini-centrifuge to pellet 1-mL samples of Tetrahymena thermophila, and we found that the two performed comparably. Finally, because our plans and the materials needed to build this device are all open source, we call our design the OPN Minifuge, and we hope that it will help to expand the scope of experiments that students can run in introductory and upper-level biology teaching labs.

## Introduction

Used to spin laboratory samples at high speeds to separate components, the centrifuge is a critical piece of equipment in any biology lab, necessary for everything from concentrating cells to harvesting organelles to purifying DNA. This makes the centrifuge essential for many introductory and advanced laboratory-teaching exercises. However, a low-end micro-centrifuge can cost $1,500 or more, and even mini-centrifuges run between$150 and $600 new. Because these prices can limit centrifuge access for many teaching labs and other facilities, several authors have begun proposing their own low-cost designs (Bhamla et al., 2017; WareJoncas et al., 2016, listing select references). Along these lines, we describe how to construct a single-speed mini-centrifuge based on a design for a computer fan stir plate (Whitesides, 2007). Our model can be assembled using materials easily purchased online, at thrift stores, surplus stores, or hardware stores for a total cost between$15 and \$45, depending on the items already on hand. We further call our design the OPN Minifuge in that its plans and materials are open source and accessible for all to use or modify as needed. (We chose the term “OPN” instead of “open” to avoid any confusion with other “open” equipment, software, or laboratories already in existence.)

## Materials and Methods

Although other items will suffice, we built the OPN Minifuge using the following materials (Figure 1):

• Hardware Store

• 8 × 8 × ¾-inch (or larger) plywood board

• 6.5-inch length cut from 1-inch wide × 1/16-inch thick flat aluminum bar

• 1-inch diameter wooden disk that is 1/4 to 3/8 inch thick

• 1 to 2 feet of 22-gauge (or similar) braided electrical wire

• Two to four #6 (or similar) screws that are each 1 inch long

• Quick-drying (5-minute) epoxy

• Electrical tape

• Online or Thrift/Hardware Store

• 90 × 90-mm, 12-V, 0.25-A DC CPU cooling fan

• 12-V, 0.23-A AC/DC adapter with a 2.1-mm connector plug

• One DC connection jack that fits a 2.1-mm plug

• One push-button or toggle switch

• Online or Supermarket/Hardware Store

• Sterilite® Ultraseal 4.7-quart bowl

Figure 1.

The parts and materials used to make the OPN Minifuge. (Epoxy labels have been blurred to obscure the brand, since any commercial quick-drying epoxy product can be used.)

Figure 1.

The parts and materials used to make the OPN Minifuge. (Epoxy labels have been blurred to obscure the brand, since any commercial quick-drying epoxy product can be used.)

After testing several different containers, we chose the Sterilite® Ultra-Seal 4.7-quart bowl because it had a flat bottom and was wide enough to allow the assembled motor and tube holder to spin freely, and shallow enough to allow easy access to the microfuge tubes. Of course, other containers with a flat bottom may fit these needs as well, especially if using a larger or smaller fan motor or aluminum bar. We also recommend using plywood as the base since it is less likely to warp or shrink over time compared to many other types of inexpensive woods.

To build the OPN Minifuge (Figure 2A), readers should follow the steps below, and we recommend reading the entire set of instructions first to become familiar with the process before trying to make the device.

Figure 2.

The completed OPN Minifuge. (A) Overhead view. (B) Close-up of the motor assembly.

Figure 2.

The completed OPN Minifuge. (A) Overhead view. (B) Close-up of the motor assembly.

1. Start by making the tube holder (Figure 2B) out of the aluminum bar and wooden disk.

• Use a hack saw to cut a 6.5-inch length from a 1-inch-wide flat aluminum bar that is 1/16th of an inch thick (tracing out the cut line in advance can lead to a straighter edge). Then, use a metal file or sandpaper to smooth the edge of the bar as needed. For schools that have a metal shop, a band saw, chop saw, or metal shear could be used to cut the aluminum bar.

• Alternatively, if readers have difficulty obtaining a 1-inch-wide × 1/16-inch-thick aluminum bar, other soft metal bars (e.g., brass) will suffice. Readers could also use a hack saw, band saw, or metal shear to cut a 6.5 × 1-inch rectangle from a 12 × 12-inch aluminum plate that is 1/16 inches thick.

• Next, use a drill press or a hand-held power drill to drill two 7/16-inch-wide holes at the opposite ends of the aluminum bar, since this diameter should hold many common microfuge tubes. However, make sure to measure the tubes in advance in case larger or smaller holes are needed. Also, each hole should be centered 5/8 inch away from the end of the bar along the mid-line (marking these locations in advance should result in better placed holes). Importantly, when drilling these holes, make sure to secure the aluminum bar in place (e.g., by using a vice that is bolted to the drill platform or tabletop, or by placing the aluminum bar on a wooden board and then using at least two clamps to hold the bar and board to the drill platform or tabletop, so that they will not move).

• After drilling these holes, bend the aluminum bar to a 30˚ angle, 1 inch from each end using a vice and a protractor to measure the angle. Also, make sure to bend each end of the bar in the same direction, so that both point the same way (Figure 2B). Alternatively, in schools that have a metal shop, the bar can be bent in a “brake” (i.e., a machine that bends sheet metal).

• Once finished with the aluminum bar, make the wooden disk by cutting a 3/8-inch long piece off of a 1-inch diameter wooden dowel. Since this piece should be flat and level, consider using a chop saw for this step. Alternatively, some hardware stores or lumber yards may cut this piece for free when purchasing the dowel. However, if cutting a wooden dowel proves too problematic, readers can instead use a circular flat-head plug with a 1-inch diameter as their wooden disk. Although these plugs are usually ¼ inch thick, they should provide enough clearance for the aluminum part to spin above the motor casing.

• Next, use sandpaper to rough up the bottom of the aluminum bar near its center and the top side of the wooden disk. Then, epoxy these two pieces together, making sure that the aluminum bar is centered directly on top of the wooden disk and that the two ends of the aluminum bar are pointing upward (Figure 2B). Also, press firmly on the pieces for at least 30 seconds to ensure a tight initial fit and a level attachment.

• Then, let the epoxy cure overnight for a strong bond. Place some weight (e.g., a small jar filled metal bolts) on top of the aluminum bar right above the wooden disk to press the two pieces together as the epoxy dries.

2. After the tube holder has dried, use scissors and wire cutters to remove the blades from the fan (Figure 3), which should increase the speed of the finished centrifuge. Specifically, cut off the fan blades as close to the fan head as possible using scissors (Figure 3A and 3B), and then remove any jagged edges by making a series of finer cuts with the wire cutters (Figure 3C and 3D). To simplify the construction process, however, readers can skip this step and leave the fan blades intact.

3. Next, use the wire cutters to remove the top corners of the fan casing and two of the bottom corners that are diagonally opposite from each other. This will leave two screw holes that are kitty-corner from each other on the bottom of the casing, which should provide a solid anchor for the assembly (Figure 2B). Alternatively, readers can leave all four of the bottom corners intact and use four 1-inch-long #6 screws to secure the fan to the plywood board. Readers can also skip this step as well, leaving the fan casing alone and using longer (e.g., 1.5-inch-long) screws to hold the assembly to the plywood board. Also, depending on the fan motor used, other screws (with a larger or smaller diameter or a different length) may be necessary.

4. Then, following the same process that is described above, epoxy the assembled tube holder onto the top of the plastic fan head, which is the side that spins (Figure 2B), and let the epoxy cure overnight for a solid bond.

5. Once the motor assembly has dried, prepare the plastic bowl for the push-button switch and DC jack that will later be inserted into the front and back faces of the centrifuge. Although there are many ways to make the holes for these components, we suggest the following approach, which in our experience tends to reduce the chances of cracking the plastic (Figure 4). Also, because our switch and jack both had a diameter of roughly ½ inch, we used that dimension for all of our measurements below. Of course, for larger or smaller components, readers would need to adjust their dimensions accordingly.

• First, clamp a 3 × 12-inch (or longer) rectangular board that is ½ to ¾ inches thick to a table top, so that one end of the board extends over the edge of the table by a few inches. Then, “hang” the plastic bowl on the corner of the board so that its front face is looking upwards, and clamp the bowl in place (Figure 4A). This set-up should enable readers to drill through the plastic bowl and into (or through) the wooden board without damaging the table.

• Next, drill a small hole (e.g., 1/8-inch) into the flat portion of the front face along the midline, and roughly 3/8 inch below the bottom of the raised ridge that extends around the bowl (Figure 4A and 4B). One way to place this initial hole is to press the final (i.e., largest) drill bit up to the face of the bowl where the plug will be located (the front face), and then mark the scratch that the tip makes on the plastic. Readers should also make sure to leave some space between the edge of the final hole and the raised ridge extending around the bowl to account for the washer and nut that will hold the switch in place.

• Then, enlarge the hole using a slightly bigger (e.g., 5/32-inch) drill bit, and repeat this process with increasingly larger drill bits (Figure 4C) until the hole is big enough to hold the switch (Figure 4D). For example, given the size of our switch, we ended with a ½-inch-wide hole. Although time consuming, we suggest this approach because simply drilling into the face of the bowl with a large bit can crack the plastic.

• After finishing the hole in the front face, repeat this process in the same location of the back face, so that the DC jack can later be inserted there.

• If the holes are placed in these locations, the switch and jack should be out of the way of the tube holder as it spins. Of course, depending upon the exact switch or jack used, readers may need to place these holes in different locations (e.g., just under the raised ridge that extends around the bowl) or use larger or smaller holes. For these reasons, readers should measure both the length and diameter of these components in advance to make sure that the holes are both the correct size and placed in the proper locations.

• Alternatively, readers can wait to drill the holes for these components until after securing the finished motor assembly to the plastic bowl and plywood board (Steps 6A–6C below). This approach may enable some readers to better place the holes for the switch and jack, so that they can stay out of the path of the spinning tube holder (especially if using a different plastic bowl).

6. Next, screw the completed motor assembly into the plywood board through the bottom of the plastic bowl (Figure 2B).

• To do so, first center the completed motor assembly on the bottom of the bowl, and then center the bowl on top of the plywood board.

• Next, predrill the holes for the screws through the bottom of the plastic bowl and into the board, using the holes in the corners of the fan motor casing to line up the drill bit. It is important to predrill these holes into the bottom of the bowl because the plastic may crack if the screws are sunk directly into the bowl without predrilling. Also, make sure to use a drill bit that is smaller than the diameter of the screw (e.g., for #6 screws, we used a 3/32-inch drill bit). Otherwise, the screws may not grip the plywood and hold the motor assembly in place.

• Once the holes have been predrilled, screw the completed fan motor and tube holder, plastic bowl, and plywood board together (Figure 2B).

7. Next, push the switch and the jack into their respective holes in the front and back of the bowl, and secure the components with the included washers and nuts (Figure 2A).

8. Then, with all of the pieces attached, use the additional lengths of braided wire to connect the positive (red) wire from the fan motor to the closest terminal of the push-button switch, the other terminal of the switch to the positive terminal of the DC jack, and the negative terminal of the jack to the negative (black) wire from the fan motor (Header Image; Figures 2A and 5).

• If the positive and negative terminals of the DC jack are not clearly marked on the component, readers should consult the spec sheet for the part. Alternatively, experienced readers could use a multimeter to identify which terminal is which, and other readers could ask an experienced technician for help with this process. We recommend identifying and labeling (if possible) the positive and negative terminals of the jack in advance because the centrifuge will not spin if the wrong connections are made, and the circuit would then need to be re-wired.

• When wiring the different circuit components together, make sure to use enough additional wire so that each length can later be taped down to the bottom or side of the plastic bowl and, thus, remain out of the way of the aluminum bar and microfuge tubes when the centrifuge spins (Figure 2). Also, if too much wire is used, readers can simply cut off the excess length before making their final connections.

• To splice two pieces of wire together, simply strip off roughly 1 inch of plastic coating (insulation) from the end of each wire, twist the exposed leads together, and then wrap the bare wire with electrical tape (starting/stopping about 1 inch above/below the exposed wire to ensure an insulated connection).

• To connect a piece of wire to one of the terminals of the push-button switch or DC jack, follow a similar approach, except twist the bare end of the wire tightly, thread it through the hole in the terminal, twist the wire back around itself, and wrap it with electrical tape as previously described.

• Also, for permanent connections, readers can solder the wires together using a soldering iron and electrical 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.

Figure 3.

Cutting the blades off of the computer fan using scissors (A, B) and wire cutters (C, D).

Figure 3.

Cutting the blades off of the computer fan using scissors (A, B) and wire cutters (C, D).

Figure 4.

Drilling holes in the front and back faces of the plastic bowl to hold the push-button switch and DC connection jack.

Figure 4.

Drilling holes in the front and back faces of the plastic bowl to hold the push-button switch and DC connection jack.

Figure 5.

A circuit diagram showing the wiring for the OPN Minifuge.

Figure 5.

A circuit diagram showing the wiring for 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.

## Hazards

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 protozoan 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 (Gilson, 2006).

In these tests, we used Tetrahymena cultures that were prepared by adding 500 µL of stock cells to separate 125-mL flasks containing 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 Tetrahymena 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 microscope 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).

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.

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.

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. Nevertheless, 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.

## Discussion

Given its functionality and affordability, we hope that the OPN Minifuge will help to expand the use of this important piece of equipment in biology teaching labs. With an affordable single-speed centrifuge, teachers can broaden the scope of laboratory experiments they conduct with their students and demonstrate many techniques used in modern research settings.

The OPN Minifuge also easily lends itself to modification and customization through the use of different fan motors, power sources, or other materials. For example, by using an AC/DC adapter with a lower current or voltage rating, readers can reduce the speed of the centrifuge. Similarly, by shortening the length of the aluminum bar, readers can reduce the force generated by the centrifuge. Alternatively, readers can increase the speed and/or force of the centrifuge by using a fan motor that spins at a higher rpm level or by extending the length of the aluminum bar. Readers can further change the capacity of the centrifuge by making a four-arm model from an aluminum plate or simply drilling more holes in the aluminum bar and bending it closer to the center to hold more microfuge tubes (although this last technique might require using a different container as well). As such, readers can build equipment that fits the exact requirements of their particular project or application, and we hope that the OPN Minifuge will serve as a useful tool for introducing students to advanced experiments and techniques in their biology teaching labs.

## Disclosures

The views set forth in this article are those of the authors and do not necessarily reflect the position or belief of any entity, institution, organization, group or other individual. We further declare that we have no conflicts of interest related to any product, brand, company, or website discussed in this article. Nor do we endorse any product, brand, website, or other item mentioned in this article or that could be used to make the OPN Minifuge. In fact, we encourage readers to safely and responsibly experiment with different items and materials to improve upon this design.

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