'We offer a step by step description of how to make a battery powered 9-24 volt DC motor driven centrifuge for separation of cell in fluid suspension for under 20 dollars. This centrifuge may be used in the field (when no other technology is available), and is set in a cookie tin or coffee can to prevent accidental release of content.
Bullet shells act as the "buckets" for the centrifuge.
Paperclips are bent to hold the shell casings, which spin out when the motor begins to turn.
A DC motor speed controller allows control over acceleration and deceleration.
A motor spinning at 2000-2500 RPM with a radius of 2 inches will develop the
600 g of centrifugal force required to separate cells in fluid suspension.
We used 1 milliliter pipettes which fit into the bullet casings to spin-down a "cell pellet"
in a pleural fluid sample however a "virtual tube" of cellophane will also work .
This project was supported in part by the Center for Parabiotics Research.
Project team members: R Siderits, J Jaworski, W Lecorchick, O Ouattara
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Step 1: Finished Prototype
Here is an image of the completed centrifuge. We chose to place the motor, arms, and buckets on the outside of this cookie tin for demonstration purposes. You may choose a larger coffee tin or better yet, a 12" diameter, 2-3 inch deep cookie tin to place it inside.
Use the lid of the cookie tin to completely cover the centrifuge for safety.
The knob on the front of this image is the DC motor control. We used a two part plastic and a bottle cap as a mold for both the rotor head (center) and the speed controller knob.
Step 2: Parts
Here is a list of the parts that we used.
Start with a DC 12-24 Volt flat mount motor
Find a DC motor speed controller (or circuit)
Get a large coffee-can or cookie-tin (12" diameter)
Two 9 Volt batteries
Two paper clips
Two or more bullet casings to serve as "buckets"
Hammer, wood, nail, pointed "needle nose" pliers
Total project time is about 30 minutes
This will run on one 9 volt battery; however, two batteries in series or a 12 volt DC supply will really spin things up to full speed. The paper clips need to be sturdy. The bends and relative looseness help to absorb and distribute imbalances and thereby serve to dampen vibration or precession.
We have included sources for these parts as well as a circuit diagram for the DC motor controller.
Step 3: A Look Inside
These two images look into the cookie tin. The lower half holds the motor speed controller. We drilled a hole in the side of the cookie tin to let the shaft of the controller through and then put the nut on the outside to hold it in place.
The knob is formed from two part plastic (or epoxy, or JB weld) in a bottle cap.
The under surface of the lid shows the wires from the controller going up to the motor.
The motor is set in a hole in a piece of wood and has four small screws holding it to the lid.
Step 4: Computer Model of Prototype
This computer model was done in trueSpace 6.6 by Caligari corporation.
This model with the "Physics" simulation helped us develop the curves at the end of the paperclips and the placement of the hole in the bullet casing.
Step 5: The Rotor Head
These images show a close-up of the rotor head and the paper clips. A line drawn on a piece of paper that corresponds to the shape of the bends in the paper clips will assure that both are identical. We used needle-nosed pliers to bend the paper clips into this shape. The paper clips are set into the holes in the rotor head, not glued. As a matter of fact, there is no glue in this project.
The motor is pressure fit into the hole in the piece of wood. If it's a little loose, then a layer of tape around the motor will snug it up.
The hole in the rotor head is also a pressure fit.
Notice the bend in the "bucket end" of the paper clips. This bend allows the bullet casing buckets to swing out with centripetal force.
The green mark on the top of the rotor head marks the site where reflective tape was placed to measure revolutions per minute (RPM) by using a tachometer.
Step 6: Making the Bucket
These images show how we made the holes in the sides of the bullet casings for the paper clip rotor arms. We made a small wooden jig to hole the base of the casing while we put a small nail into the open end and tapped it with a hammer.
This will put a small hole in the end without distorting the casing. Notice in the first image what happens if you put the hole too close to the rim; it splits the casing.
Step 7: Pipette in Bucket on Rotor Arm Ready to Spin-out
Here is the paperclip rotor arm with the bullet casing "bucket" holding a bulb-down, 1 ml pipette.
When the rotor spins up, the bucket spins out and the fluid in the pipette is subjected to the centrifugal force.
Step 8: Take a Look at These Cell Pellets!
These two pipettes were spun for 5 minutes at about 2500 RPM with a rotor radius (paperclip) of 80 millimeter. This gives a Relative Centrifugal Force (g) of about 500. Not bad for one half dead 9 volt battery. This particular motor would spin up to 5000 RPM and give more than 800 g with a 2 inch radius. This is more than enough force to separate many types of cells in suspension.
The whitish cell pellet at the bottom represents the cells that have been separated from the fluid.
In this case we used a pleural fluid sample that was to be discarded. The separation was surprising.
Step 9: Figuring Out Centrifugal Force
Use of this "nomogram" allows you to determine the relative centrifugal force (RCF) in g's, by aligning the radius of rotation in mm (in this case, paper clip length for the rotor arm measured from the center of the motor spindle to the base of the bucket).
The Revolutions Per Minute (RPM) can be verified with a hand-held tachometer. We used an infra- red tachometer from Harbor Freight.
Step 10: Tips and Learning Points
Use plug in DC power supply instead of battery
Use solar panel (Harbor Freight) to recharge battery
Place a weight on top cover to dampen vibration
Use two part plastic or epoxy resin to make rotor head
Make a virtual tube using cellophane in the bucket
Consider using a tachometer to get a speed check
Make your own speed controller (see references)
Always use all appropriate safety gear!
Step 11: Long Arm Paperclip (14 Cm) With Tachometer
This image shows the "long" paperclip rotor arm (see "arch" to left, beyond lid of tin) which measured up to 14 cm from the center spindle of the DC motor. You can see the tachometer at the top of the image. With two 9 volt batteries in series or a 12-24 volt DC power supply this will "Tach" up to 2000 RPM for about 600 g's. Spinning it up that fast requires that you at least Duct Tape the unit down to a surface (and wear Goggles).
Step 12: This Small Solar Panel Can Recharge the 9 Volt Batteries.
This small solar panel can be used to recharge a rechargeable 9 volt battery. We purchased this type of charger from the Harbor Freight store for 11 dollars.
Step 13: References and Sources
Step 14: Disclaimer
but not limited to:
1) Full body armor.
2) Using a centrifuge that spins really fast.
3) Powertools that you never read the instructions for.
4) All tools that you never learned to use properly.
5) Anything that heats up, pinches, cuts, squeezes, flies off, carries electric current or causes other traumatic, caustic, or thermal injury.
We are sharing our experience, not telling you to do it.
If you choose to try this then - it is at your own risk!
- No really, we're not kidding about this.
Step 15: What Can This Project Be Used For?
Aside from a hobby or "field" solution for separation of cells in fluid suspension, this
prototype device could be used (in the absence of any other technological solution) for
initial sample preparation of cytology specimens.
There may be a demonstrable need for this capability in supporting evolving health care infrastructures in countries with restricted economy.
For example, in "developed" nations the PAP smear for cervical-vaginal cytology has decreased the incidence of cervical carcinoma by approximately 70%. However, in a country with
restricted economy this is not the case. A woman may need to transport a cervical
cytology specimen herself, sometimes hundreds of miles, and then wait months for the
sample to be prepared and interpreted.
Providing a basic functional centrifuge for cytology slide preparation may help facilitate
aspects of this difficult process.
Step 16: Watch It Spin!
This video segment shows the movement of the buckets as the motor speed is gradually increased.
Step 17: Good Luck!
We hope that you've enjoyed this How-To and that it may be useful to you.
The potential for this prototype to be used in remote areas in countries with restricted economy for use in supporting cervical vaginal cytology field preparation of sample may help to provide critically needed services for these populations.
Good luck and be safe (remember to build yours INSIDE of a cookie tine, not on top).