Flow cells are common tools for studying and imaging biofilms on different surfaces. They consist of a flow chamber that allows for continuous medium feed and a glass coverslip viewing window for imaging the biofilm under a microscope. There are a few commercially available flow cells, but a microbial electrochemistry flow cell is not yet available and much more difficult to construct given the requirement for anaerobic conditions and the many electrical connections needed. We present below our own design, which we have used for about a year now (since 2013). This design has the option of using a flat metal anode or an ITO coverslip as your anode. You can either replicate this one, using the Solidworks file below, or get inspiration to build your own version!
The first thing you notice is many screws! The screws give rigidity to the flow cell and provide an equal pressure on the rubber gasket that prevents leakage. Notice the center screws are a bit skewed since we needed a side hole for the reference electrode right at the center of the flow cell (black arrow). The flow cell itself is made of 1.2 cm thick plexiglass and the front and back covers are aluminum plates of 0.2 cm thickness. The front cover has the viewing window that is 0.8 cm X 2.1 cm. It has a sloped surface to accommodate the size of the objective on the microscope. The bottom cover has threads for the scews. It also has a hole around the flow cell viewing window to avoid reflectance of the microscope beam. Also, it has a little hole for a sprig-loaded screw connection (more on this later). Let’s open it!
On the far side of the picture above, you can see the main rubber gasket used. It is ~1/16” thick silicone rubber gasket (McMaster Carr 5827T32). It is cut by hand to fit the flow cell and the screws/viewing window. In this picture, we have introduced the reference electrode, a Leakless Miniature Ag/AgCl Reference Electrode (Model ET072). This is introduced using a PEEK connector and the plexiglass has a thread for this connector. The same thread is used for the influent and effluent lines at each side of the flow cell and stainless steel connectors are used.
The first few times we used this flow cell, we realized there was a small leak at the viewing window, close to the glass coverslip. We added an extra thin gasket (~0.5 mm) around the coverslip to create a better seal.
The plexiglass has a small dip to accommodate a 0.2mm coverslip. The flow cell was designed for a 22 x 40 mm coverslip, as this is a standard size sold coated with ITO in the US. Notice the edges are rounded to allow for the coverslip to be pulled out. The flow cell chamber itself is 0.6 cm X 1.9 cm with a triangular shape along the inlet and outlet. The chamber is 0.8 mm deep, along with a 0.2 mm coverslip gives ~ 1 mm of imaging working distance if using the metal electrode. The electrode is placed close to the center of the chamber and is a stainless steel machined screw with a flat surface that has been sputter coated with gold. Notice the hole next to the electrode, which is the reference electrode connection. Also, on the side and outside of the chamber, there is a stainless steel spring-loaded plunger that can make a connection to an ITO coverslip. In future designs, we will move this plunger closer to the chamber. So…where is the cathode?!? The cathode is actually the effluent screw connection made of stainless steel. This way, the hydrogen produced at the cathode is not being produced inside the flow cell, but on the way out.
Both the gold-coated stainless steel anode and the spring-loaded plunger screw into the plexiglass and come out from the bottom of the flow cell, allowing an electrical connection to either one. This is advantageous for an inverted confocal microscope, like the one we have at SCEB.
Finally, we use a Long-distance 40x objective to image the biofilms on the gold-coated electrode. The one shown here has a working distance of up to 3.3 mm, which is larger than our 1 mm requirement. On the left is a picture of a Geobacter sulfurreducens biofilm grown on the flow cell.
Download .SLDASM file
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Download .SLDASM file
Download .easm file