An important growth requirement for aerobic microorganisms, which include all yeast, algae, and some bacteria, is an abundance of oxygen (or CO₂ in the case of algae). Therefore it is common in bioreactors to see sterile air, or even pure oxygen, pumped into the liquid using a bubbler or sparger. With a bit of extra work, you can add a bubbler to your Pioreactor. Let's see how!

We'll be taking advantage of the PWM (pulse width modulation) channels on the Pioreactor. These channels supply up to 5V, and can be tuned to lower voltages using PWM. For example, setting PWM to 50% causes the output voltage to be 2.5V. This is useful for precise control, like for exact stirring RPMs or exact dosing amounts. We can also use the PWM channels for controlling other peripherals, like an air pump.

We purchased a simple air pump, like this one from ABRA or this one from adafruit. We connected the power to our Pioreactor's PWM 1, and connected the air outlet to our vial's luer lock using a short piece of 3mm inner-diameter tubing (also available at the above retailers). For sterile air, you can use a 0.2µm air filter in-between, too. Below is a photo of our setup. Note for demonstration purposes, we are using our custom "cut-out" version of our Pioreactor and a red LED instead of the usual infrared LED.

We also installed the latest version of the pioreactor-bubbler plugin. After following the installation instructions on the plugin's homepage, we were ready to go. The neat thing about this plugin is that it will automatically shut-off bubbling when the optical density measurement is taking place. This is pretty important, as air bubbles will cause major distortions in the path of light between the LED and sensors. Below, we can see this on-off cycle in (slow-mo) action:

### Future work

There are a few improvements one can make to this system. The most important parameter in dissolving oxygen into a liquid is the amount of surface area between air bubbles and the liquid. In the current setup above, are air bubbles are pretty large, which means that the surface area is minimized. If we made the air bubbles smaller, by using an air stone, or any other way to diffuse the air, we could achieve even better oxygen uptake in the liquid.