Hardware
So you decided to work with a photosynthetic organism in your iGEM project and are wondering if there is a way to grow it faster and more efficiently. So did we, which is why we built our own photobioreactor and want to help you find the correct system for you!
In the following paragraphs you will find examples of PBRs and some criteria which may help you to decide on the type of reactor that fits your project the most.
What types of PBRs are there?
PBRs are somewhat broadly divided into two types.
The first type are open systems, defined by a lack of barrier between the culturing medium and the surroundings. One example for an open system is the open Pond which is flat basin or pool filled with the culturing medium and the organism.
The other type are closed systems, where the culturing medium is located in a closed environment to prevent any exchange between it and the surroundings. There is a large selection of these, e.g. tube reactors which are an arrangement of long tubes oriented horizontally or vertically. There are more Tube systems which are just a different way of organizing the tubes, for example the tree arrangement with the tubes being oriented in a horizontal spiral.
Another setup for a closed system is the Foil system consisting of big plastic bags with a sealed top.
How to decide on what type of PBR you want to choose.
To decide what type of reactor would suit your project best depends on a handful of factors.
These vary from the best growth conditions of your organism, if you are working with a GMO to the materials, equipment and funds that you have access to.
The growth conditions are generally more easily monitored and controlled in a closed system. For example if your organism needs a specific temperature it is easier to reduce the heat evaporation or even heat up the system.
With a closed system it is also easier to ensure that a GMO cannot escape into the environment. This is rather hard to pull off with an open reactor and will probably necessitate the use of a kill switch (see our concept of a future implementation for more infos).
The amount of control you have with a closed system comes with the downside that these reactors are not only more expensive, but also more complicated to build. That is because you need secure ways to replace or recycle the culturing medium, methods for gas exchange and ways to monitor parameters like gas levels, temperature, etc.
An open system is thus more preferable if your organism is forgiving with its growth conditions, you're not working with a GMO or need a reactor to prove that the PBR concept you created is working.
In both industry and laboratories most work that requires PBRs is done with either algae or cyanobacteria. Most of these reactors can also be used with mosses, but that will require some tweaking.
Creating a model of the PBR.
The modeling of a 3D version of the PBR is a step that might not be necessary, though it might help you to visualize your reactor better. The model can also be used to create a simulation of the water flow and other variables and how they change if you tweak the reactor, without the need to rebuild parts or the whole reactor.
For creating a model of the reactor we worked with FreeCAD or Autodesk for the basic structural model of the system.
If you want to use this model for a computer simulation, you will need to convert it into a mesh. This can be done with another software, for example Meshmixer or Meshlab.
These meshes can then be imported into ANSYS or similar programs. ANSYS is used for simulating the flow of fluids. For this you will have to set a number of parameters and other things like the initial conditions.
Materials
If you are building a reactor for proving that your concept of a reactor works, it is advisable to do so with materials that are inexpensive. This is due to the fact that you might need to redo parts or the whole reactor.
Generally every PBR needs some kind of pump. Depending on how much water and with what pressure it needs to be pumped through the system you can decide on what pump would work better. For a reactor that needs small amounts of or a slow flow of water you can work with an aquarium pump. For a larger system with more water, etc. a pump used for a garden pond or something similar, might be the right choice.
If you need to disperse the water over a larger area you can connect the pump to a sprinkler. Though a standard garden hose that is perforated in the correct spots can be another simple solution.
The frame can most efficiently be built from plastic or wood. Plastic has the distinct advantage for being both inexpensive and waterproof. If you want to build with wood you can use some kind of plastic tarp to make it waterproof.
How to build a turf scrubber
For a basic Turf scrubber you need only a few parts.
You need a basin or similar in which the water can be collected after it flowed through the reactor. You then need a pump and a garden hose to get it back into the system. For the dispersal of the water over the length of the turf scrubber you can use either method described in the part above.
For the PBR itself you will need a plane surface as a basis. This can be an old tabletop, a piece of wood or a plastic sheet. If you are working with wood remember to waterproof it in some way.
On top of this surface you will need to install a grid on which your organism is able to grow. The mesh size for this grid will need to be chosen, based on the amount of water flowing through the system and the organism you’re working with. For example if only a small amount of water is flowing through the reactor you can work with a bigger size of mesh because the organism does not need to withstand too much force. Alos a moss might need smaller meshes in general to more effectively be grown since it needs more surface to hold on to.
The flat surface should be positioned in a way where the water flows right back into the basin. For that it might help to have it at a slight angle so that the end where the basin is is lower than the other side.
To minimize the loss of water you can also attach some railings to the sides. If you plan on replacing the mesh from time to time you can also use these railings for a way to hang the mesh into the system.
For fixating all the parts in place you can use nails and waterproof tape as well as zip ties.
When everything is in place you can test if your turf scrubber works as you intended. Leaks can be easily fixed with tape.
