The self built replicate of the open source syringe pump.
Team:ETH Zurich/SyringePump
Syringe
Pump.
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AROMA is equipped with syringe pumps that perform the medium transfer tasks in our biosensor. One task of the pumps is to fill and empty the bubbling tank. Before each measurement we need to replace the medium into which the air is bubbled. This is why we need two big pumps. One adds fresh medium while the other one removes the old medium. After the bubbling step we bring the molecule of interest in contact with our engineered E.coli. For this task, a third pump sucks the medium into the microfluidic chip. This pump needs a precise low volume control and the capability to achieve a stop in the flow as quickly as possible. Otherwise the response cannot be read out during this time because E.coli do not rotate if the flow is too high . Syringe pumps are commercial available, but they are very large and very expensive. This is why we looked into building our own customized one.
Design
Initial idea
We initially planned to use an open source design available online. The following one seemed very promising:
instruction.
Unfortunately these initial prints did not fit our setup and were also poorly designed. Especially the mounting of the syringe itself is very complicated and one needs to disassemble half of the parts to just swap the pump. Additionally the measurements were too tight. For 3D printed parts one always needs to add a small factor to the dimensions of the parts, especially when leaving cutouts for screws and rods. At least this was the case for our 3D printer, an Ultimaker 2+ Extended. So essentially we ended up redesigning the whole syringe pump from scratch.
Our Concept
Mechanical Illustration of our Syringe pump design
Similarly to the open source design we used stepper motors which have a very high accuracy and therefore allow us to turn the syringe very slowly but still precisely. More information about the motor can be found here.. We kept the basic layout similar to the original design. We use three main parts connected via two 6mm rods.
- Part: The first part is attached directly to the motor via four screws. The rotor - the rotating part of the motor fits through the big hole connecting to the middle shaft. On the sides the two rods are clamped in tightly with the two top screws.
- Part: Similar to the previous part the two rods are clamped in. Additionally we guide the middle shaft with bearings. On top the syringe pump is being clicked in to the mount fitting perfectly. Compared to the original open source model our syringe pump is mounted significantly closer to the middle shaft in order to have less play. This significantly improves the accuracy of the syringe and makes it easier to replace.
- Part: The middle part is the only one out of the three actually moving. It is connected to the center shaft via a nut resulting in a back and forth movement when turning the shaft of the motor. Two linear bearings on both sides enable smooth movement along the two 6mm rods. The end of the syringe pump is connected to this part extruding and intruding it.
Implementation
The two different Versions
As described above, we need two different kinds of syringes. A small one sucking medium through the microfluidic chip and two big ones replacing the medium in the bubbling chamber. We used the same design for the base and simply changed the top mount of the syringes, leaving the mechanism the same. Both syringes are plastic one- use syringes, available at affordable prices. The small syringe has a volume of 10ml and the large one 60ml. This shows how easy it is to customize the syringe for different requirements.
The Assembly
Assembling the syringe pumps is quite straightforward: screwing everything together and placing the bearigns in the designed spots. We provide a part list and downloads of our 3D designs to replicate our setup on a special external page:
Optimization
In order to make the syringe actually usable we also need to provide the right software powering it. The software is integrated in ROS and part of our modular AROMA package. In the software one only has to enter the speed in ul per minute and the total ul one wants to pump. In order to achieve that we calibrate the pump by measuring the volume extruded after a certain number of steps of the motor. We thus obtained a calibration parameter representing the volume per step extracted by the motor. This parameter can be easily configured from the init file of the program, enabling simple calibration of new designs. For the small syringe we still have quite significant flow in the microfluidic chip right after the motor stops due to small air bubbles in the pipe which are mostly generated when the pump is connected to the chip. Those tiny air bubbles lead to a oscillating water column. This can be reduced significantly by making the syringe move in the opposite direction for a few seconds. We recorded a number of measurement data and calibrated the corrective movement accordingly.
Calibration Data to reduce flow after stopping the Syringe
| Input | Stopping movement | Time until complete rest | ||
|---|---|---|---|---|
| Amount in ul | Speed in ul/min | Amount in ul | Speed in ul/min | |
| 2 | 5 | 0.5 | 5 | 43 |
| 2 | 5 | 1 | 5 | 25 |
| 2 | 5 | 1 | 8 | 27 |
| 2 | 5 | 1.5 | 8 | 41 |
| 5 | 5 | 1 | 5 | 40 |
| 5 | 5 | 1 | 8 | 24 |
| 5 | 5 | 1.5 | 8 | 45 |
| 5 | 5 | 1 | 8 | 33 |
| 5 | 5 | 1 | 6 | 39 |
This are just a few of the measurements we did. We observed that the total input amount does not influence the ideal stopping movement significantly. Therefore we decided to use the 1ul and 8ul per second as an optional break command. Note that even with commercial syringe pumps one does not get a faster time until complete rest.
Conclusion and User feedback
Syringe Pump on the Robot