Software workflow

Bill Of Materials: You could see a complete BoM here (upload the bill).

Amount of time: 2 hours.

Total costs: 0 €.

1. You only need two applications to run your laser engraver:

• Inkscape (for creating the design and and the.gcode file). A .gcode file stores the required machining orders for the laser to cut the designed geometry.

• Universal Gcode Sender (send the .gcode file to the Arduino).

2. In order to create the shape of your microfluidic chip, you need to have a feasible design. There are many open software applications, as:

• DraftSight (2D): although you could design directly with Inkscape, an specific software for design may result handy at some point of despair.

• Freecad (3D).

CAUTION: Alternative versions of a .dxf file will not be imported successfully.

3. If the design is made in a software different to Inkscape, you should export the file as a .dxf file (R2013). Inkscape can import this sort of files.

4. Once the design is created and imported into Inkscape, you should place it within the canvas with the required proportions. For NEJE laser engraver, the dimensions of the bed is 38 x 38 mm. You should find how Inkscape work area relates to the effective area of the bed.

5. Once the dimensions have been corrected and fixed, you should convert the lines to a “path” format, clicking in the “path” tab and selecting “stroke to path”. We have tried to omit this step, and it seems it does not matter skip this step.

6. Click on “extensions” tab and select “generate laser gcode”, and set the cutting parameters. These parameters should be set experimentally. Speed, laser power, passes vary depending on the specific tape selected, or the surface where you stick the tape. We encourage you to set your desired parameters, based on your specific scenario.

7. Once the .gcode file has been successfully created, open the Universal Gcode Sender and and set the ports and baud rate (115200 bps).

8. Now you need to set your “Zero machine coordinates”. From “Machine control” tab, move the X- and the Y- to the coordinates that you want to set as (0,0). And click on “Reset Zero”

9. Import the file. (File mode tab -> Browse). And finally send it to the laser engraver.

Calibration

Bill Of Materials: You could see a complete BoM here (upload the bill).

Amount of time: 2 hours.

Total costs: 10 €.

There are three factors that you should consider for calibration purposes:

1. Choosing the tape: Depending on which depth of channel suits your needs, you should choose one tape or another. Cutting parameters should be established by each user, according to their particular needs.

ADVICE: We are using brown packing tape. The deep of the tape is 50 microns and the parameters that have fit to us are the following: Travel Speed 200, Laser Speed 50, Laser Power 12000, Power-On Delay 0, Passes 2, and Pass Depth 1.

2. Calibrating the dimensions of the Inkscape canvas: we highly recommend you to know the correction factors to apply to your design. A good practice should involve (for instance) designing a 10 x 10 mm square in Inkscape and send it to NEJE laser engraver, and measure it afterwards to know exactly how dimensions translate between software and manufacturing hardware.

3. Getting to know the effective work area of bed of the NEJE laser engraver: if you set the laser over the Zero-machine coordinate, and the, in the “command tab”, you insert the command s01, and M3, you should see how the laser turns on. If you click on X+ or Y+, you should be able to navigate over the edges of the bed, and thus, determine (and engrave) them onto the bed or any other surface that you might consider.

Disclaimer: The programs that we have used in these step are not developed by our group and are hosted in our Github just to keep the version that have worked for us.

Cutting

Bill Of Materials: You could see a complete BoM here (upload the bill).

Amount of time: Depending on the project

Total costs: less than 5 €.

Although cutting is a pretty straight forward process, please, take this advises into consideration:

1. Mind your eyes: Please, do not hesitate to use the safety glasses devoted to look to the laser.

2. Use your final polymerization surface to support your tape: we have used an acetate sheet that fits in our final polymerization chamber. Ensure your surface are horizontal and without bubbles between the tape and itself. Lifting off the table from the surface in which it has been cuttered may cause the deformation of the original pattern.

3. Chlorine content in many tapes: As many tapes contain a remarkable amount of chlorine, we advise you to cut in a ventilated place and to avoid inhaling this.

4. Enjoy not always you get the results that you are planning. Take the time to set your arrangement and enjoy this process!

Making a vacuum pump and vacuum chamber

Bill Of Materials: You could see a complete BoM here (upload the bill).

Amount of time: 3 hour.

Total costs: 20 €.

1. First you need to modify the compressor into a vacuum pump. To achieve it, we recommend consulting the following video

ADVICE: The time spended in the degassing process depend on the strength of your vacuum. For fast degassing (and also for making larger vacuum chambers) consider to spend a little more in a better vacuum pump. The ones from air conditioner systems works well for almost any purpose and fits well for the next plasma treatment steps.

2. Once you have converted the compressor into a functional vacuum pump, you can create a vacuum chamber. Please, take into account that the vacuum chamber should be able to accommodate the recipient where the PDMS, the curing agent and the army of air bubbles are.

3. You might want to provide your vacuum chamber with a venting valve for pressurizing the chamber alternatively, as we explain in the following point.

4. You should seal every possible leakage with PTFE tape. Air leaks are not desirable, and your duty is to avoid them.

Degassing

Bill Of Materials: You could see a complete BoM here (upload the bill).

Amount of time: 30 minutes

Total costs: 0 €.

1. Degassing involves extracting the air embedded in the mix of PDMS and the curing agent. First you need to place the recipient which holds the mixture of PDMS and bubbles inside the vacuum chamber.

2. Turn on the pump. The vacuum pressure provided by the pump is more than enough for our purposes. Leave it for 20 seconds and you will see the bubbles migrating towards the surface.

3. Once a significant amount of bubbles has moved towards the surface, let the airflow inside the vacuum chamber. Many bubbles will burst, and thus the air will be successfully extracted.

4. Repeat the process enough times to remove all the trapped bubbles.

5. Once the PDMS is homogeneous, you can pour the mixture into the polymerization chamber, where you are planning the PDMS to cure.

CAUTION: If you do not want to trap air bubbles when pouring the mixture , do not rise the recipient. Pour it as close as possible from the bottom of the polymerization chamber.

Polymerization process: curing time and heating

Bill Of Materials: You could see a complete BoM here (upload the bill).

Amount of time: 1 hour.

Total costs: 0 €.

1. This is the final step of the whole process. Once the PDMS is ready to cure, you may want to have the material ready in an hour, or maybe you can leave it curing overnight.

2. If you place the mixture inside a drying oven (65ºC), your PDMS will be cured in just half and an hour.

ADVICE: The relation between time and temperature varies, but it seems to be linear. Although this relation seems to vary from one commercial brand to another. 24 hours of polymerization should be enough at room temperature, but sometimes it is not enough. Test it experimentally with a small PDMS quantity.

3. Elaborate a chart with the parameters that suit your needs better.

CAUTION: When heating, you should take into account the materials of the polymerization chamber. Mind the temperature, as an excess of heat may melt the materials of the polymerization chamber.

Calibrating the microwave

Bill Of Materials: You could see a complete BoM here (upload the bill).

Amount of time: 2 hours.

Total costs: 1 €.

1. CBefore even thinking about treating the PDMS, make sure that the plasma is generated successfully. You just need to test the whole display without PDMS inside the bottle. Calibrating the microwave means knowing how the it works experimentally, so take a pen and a paper.

2. You should make a chart with some parameters as:

1. Power.

2. Pressure.

3. Time until spark.

4. Amount of aluminum foil.

5. Temperature of the bottle.

6. Amount of water inside the glass.

3. Sparks will help to generate the plasma inside the bottle. In this regard, you need to place some aluminum foil inside the bottle. Measuring the required amount of aluminum foil is key for the experiment to succeed. We cut pieces of 20 x 20 mm of aluminum foil and use them as our arbitrary building blocks of aluminum foil.

4. First, you need to get some sparks out of the aluminum foil. If you get no sparks, add more aluminum foil, or increase the power. Take notes about how it behaves.

5. Once you get some sparks it is time to generate plasma. Vacuum pressure is key. 1 Pa was recommended. We were limited by the resolution of our manometer. We could only read 755 mmHg as (negative) relative pressure. We have no certainty about the actual value of the pressure.

ADVICE: Notice that plasma color gives us a lot of relevant information about what is going on inside the bottle. A blue plasma means that oxygen content is significant. There are many sources online where you can see what colour you should expect.

6. You can use a glass of water to absorb any excess of energy. Note how water affects the experimental arrangement.

7. Once the treatment is done, the borosilicate bottle should not exceed 150 ºC. Test it with an IR thermometer gun.

8. Once you are happy with the experimental outcome, you should proceed to the next step and bake the PDMS.

Baking the PDMS

Bill Of Materials: You could see a complete BoM here (upload the bill).

Amount of time: 2 hours.

Total costs: 0 €.

1. This protocol repeats the protocol before, with the exception that now we use the PDMS and the glass and analyze the new results.

2. Although plasma will fill the bottle, you need something to separate the PDMS chip and the aluminum foil. You do not want the PDMS to burn, as it will notoriously change its properties. Glass will prevent the PDMS to burn.

3. Make sure that PDMS and aluminum foil are separated by a glass plate, or any other glass surfaces that you might have at hand.

4. Follow what you have learned in the previous step and bake it.

5. Once the treatment is done, test the hydrophobicity of the PDMS, placing a drop of water on its surface. A treated PDMS chip should be hydrophilic. And a burnt chip will be absolutely hydrophobic

6. You should elaborate another chart with the relevant information about the process, studying the surface tension of the PDMS with respect to a drop of water.

7. Once the surfaces have been treated, they should be joined together immediately, and leave it a few hours to get the desired effect.

8. Two surfaces have joined when the bonding is strong enough to be broken before the surfaces are split up.