Cells are made of many different biomolecules. For example, the outer membrane is made of lipid molecules, the nucleus contains DNA, RNA, proteins, etc. Besides that, cells are surrounded by other molecules suspended in their media. Each biomolecule has different characteristics: electric charges (polarization), mass, etc. How could we separate them?
A centrifuge is a device that uses the centrifugal force to separate the different materials suspended in the liquid so at the end, you will have the heavier elements down at the bottom and subsequent lighter molecules staked up. Two factors are of main importance: a motor with torque enough to achieve the required G forces (calculated based in the number or revolutions per minute-rpm-) and the rotor radius. The torque is proportional to the radius of the rotor and so will be the force.
A brushless motor was selected due to the advantages they have: they are designed to last, they have a powerful torque and they can have a decent control of rpms. We ordered some of them and started to build the centrifuge design proposed by WAAG. However, we wanted to test our hack abilities and another design was made using a HDD motor. Old HDDs have brushless motors and because they break quite often, they are easy to find for free. So we made the two designs.
One of the students took this project on his own. When dismantling the HDD you could save some powerful neodymium magnets, the silica dishes and the brushless motor. Do not take the motor apart of the case because this case can work as the base of your device.
Brushless motors can be DC or AC motors. In the case of HDDs, they are AC. They do not have any positive and negative poles to connect. Instead, they have three (or more) different electromagnets (inductors on the stator) inside. Each electromagnet can be activated when passing a digital pulse through it. The idea is to activate each one of them in a sequential way: turn on inductor one and the rotor will move 120 degrees, then activate inductor two while turning off all others and the motor will move 120 degrees more, finally activate inductor number three to complete a full cycle and then repeat. The idea is to send three pulses sequentially and if you regulate the duty cycle you can control the spin velocity of the motor: the shorter the space between pulses the faster the motor will rotate. How can we find the electromagnets terminals? and how to activate them? The next image shows the connections of the HDD motor:
The idea is to measure the resistance between each terminal. There are three inductors sharing a common node. So, if we measure the resistance between each terminal while keeping one terminal as reference, there will be a combination that will give us the same value to each terminal. That will be the right combination and that means you have found the common node and the three terminals. Connect the common node to ground and all other nodes to arduino analog outputs. The electronic circuit and the code is available in our forum section.
Imported Brushless Motors
We bought some brushless motors from China and started building our centrifuge using the WAAG design. We made some improvements to the original design and added code to start, to increase the velocity and to stop the motor. The code and circuits are uploaded in our forum section.