Team:Grenoble-Alpes/pipetting module
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PIPETTING MODULE
HOW TO CONTROL THE PIPETTE ?
In our system, we need to pipette automatically different volumes to execute our biological process.
The solution we chose is to use an electronic pipette Biohit e10. The main advantage is that after removing the cap of the pipette, we can solder wires to the electronic card to control the different buttons of the pipette and it will work by itself!
The button functions are described in the following figure:
To make the pipette work, we need a power supply of 5 Volts and 1.5 Ampers at least. Wires are connected from an Arduino card (microcontroller) to the buttons to control them later.
HOW TO MOVE THE PIPETTE ?
We needed a pipetting system moving vertically above the samples on the circular plate. For that, we used a linear guide with a lead screw and a motor allowing to move the pipette vertically with a good precision. All the pieces, like the guiding bars, the coupler and the lead screw have been bought on a professional website [2] to guarantee us a fluid and precise movement. It provides the precision required to pipette at the wanted depth in the Eppendorf tubes. The precision reached is about one-tenth of mm.
This linear guide system has been fixed on a wooden plate between two aluminum bars. The plate has been first engraved with a laser-cutting machine to mark the positions to drill the holes for the screws at the right position.
The vertical movement of the pipette is realized by a motor controlled with an Arduino microcontroller.
HOW TO FIX THE PIPETTE ON A LINEAR GUIDE ?
We printed a piece in 3D to hold the pipette and to fix it to the linear guide.
A piece designed on a Computer-Aided Design (CAD) software and printed 3D is fixed on the linear guide and holds the pipette. The dimensions have been measured carefully to fit perfectly with the pipette. Some foam rubber is added to get a better fixation of the pipette with higher pressure forces.
One issue of this system was to take a pipette tip on the circular plate with the pipette. The pipette had to go down, insert in the tip and push a bit to attach. However, the force required to push was too strong and instead of inserting in the tip, the pipette was detaching from the 3D-printed holding piece. This problem was solved by drilling a hole at the back of the 3D-printed piece. With a screw passing through this hole and nuts, the pipette is now perfectly fixed.
LEARN MORE
About the pipette choice
In the first stage of the project development, we thought of 3 main ideas to realize the functions of the pipetting module.
A simple syringe, whose piston is connected to a motor.
The use of a simple syringe with a motor pulling or pushing the piston has the advantage of being an affordable solution. However, we cannot control how much we really pull or push the piston with our motor: if our stepper motor misses a step, it would be very difficult for us to notice it. Hence, there is no repeatability of the biological process possible.
A regular pipette with an “artificial thumb” that would control it Another system would have been to use a regular pipette and create a mechanical system able to push and turn the button to pipette and change the volume. The challenge here is to get enough precision on the system to pipette the right volume. We realized that it would have needed too much time to create such a system with at least one stepper motor, and many mechanics. Moreover, there was still with the uncertainty that we could reach the needed precision. Hence, we had to find another way to make our pipetting module.
An electronic pipette connected to our Arduino microcontroller Finally, we decided to use an electronic pipette and simply connect the physical buttons to our microcontroller (Arduino). By sending an electronic signal, we can trigger the pipette buttons and let the internal system of the electronic pipette handle the actual pipetting. This method allows us to have a guaranteed accuracy on our pipetting (the same as if we were manually triggering the different buttons of the pipette). The primary goal of this module is to be precise, and an electronic pipette connected to our Arduino is the best way to obtain this accuracy. Below, one can find the specifications of the pipettes with the systematic and random errors in function of the volume.
For us, the big advantage of some electronic pipette models is that even the tip ejection is electronically controlled and saves us from adding another mechanical system. One drawback of electronic pipettes is their price: count at least 600€ for a new one. However, those devices work on battery that tends to stop working before the rest. As a result, a lot of functioning second-hand electronic pipette without a battery can be found on the internet. We were lucky to have partners who gave us electronic pipettes for free. We first worked on the pipette Biohit e10 that can pipette up to 10μL but to execute the biological process entirely with only one pipette, the ideal volume range is from 10μL to 300μL. We bought a first Biohit e300 pipette from the US that somehow stopped working from one day to another. Then we stopped looking because this model was sold out everywhere. We asked a partner company if they had any pipette, and they had the same model, but also non-functional. And here is the nice story, we saw that one electronic component was burnt and we managed to replace it with the component of the other pipette. To sum up, from to broken pipettes, we built one working pipette.
About the pipette choice
This is how an optocoupler works: We send a current through the diode that will emits some light. On the other side, a phototransistor receives the light and converts it into a current that will finally close the circuit from the 5V to the GND of the button like a switch and like that trigger the button.