Design, Build, Test, Learn Cycle

In the design process of our biophotovoltaic cells, several prototypes were created to work towards maximizing the amount of voltage we were able to measure easily.

First Prototype:

Our first attempt at creating a functional biophotovoltaic cell was a very inspiring experience. This prototype was created using the components we had spent time testing and optimizing including our conductive ink, biological ink, and hydrogels soaked in 1% salt BG-11 media, as well as materials we already had in the lab including copper wire, tape, a multimeter, paper, and a pipette tip box to help maintain sterility. The protocol we followed was created by us and based on the design found in the published Sawa et al journal. Here, three layers of the conductive ink were printed using the screen printing technique while three layers of the biological ink were pipetted on top of the anode. This was because the concentration of CMC in our biological ink was too low, and the ink was not viscous enough to also be screen printed without leaking outside of the pattern. Each layer of ink was given time to dry before the next was printed to achieve a layering effect. Once the final layer of biological ink was dry, squares of hydrogels were placed on top of each anode and cathode pairing to facilitate a hydrogen gradient in our biophotovoltaic cells. Here, the hydrogels were sized so that they would be touching each other. Finally, two small lengths of copper wire were cut, stripped, and one was taped to the first anode while the second was taped to the last cathode in order to read the voltage across five of our cells. Our GW Instek GDM-8245 multimeter was used to probe the wires to show the voltage drop across our cells. Because the wires were connected to the conductive ink using a small piece of tape, only a small portion of the copper wire was making contact with the ink. Along with this, the connection was not very strong and the wires would often shift, especially when the probe would make contact with the wires, leading to very unstable readings. With these connection considerations in mind, the output of this first prototype was reading between 0.12-0.25 mV. To ensure the voltage being read was because of the cyanobacteria cells, we measured across a control setup that consisted of just the conductive ink without any cyanobacteria cells. When we measured this voltage it came back as zero volts, showing that the voltage we were measuring in our prototype was directly caused by the cyanobacteria. Another note is that we were not seeing an increase in voltage drop across biophotovoltaic cells in series. One reason for this could have been due to the cyanobacteria spreading from the anode to the cathode, creating one large anode across the paper and removing the flow of electrons that would be seen in a concentration cell. This spreading of the cyanobacteria was caused by the biological not having a high enough concentration of CMC, and not being viscous enough to remain strictly on the anode. A future improvement for subsequent prototypes is creating a more viscous biological ink with a CMC concentration of 8%.

Second Prototype

After learning about the shortcomings of measurement of the first design, more supplies were purchased because it was clear that we needed to better the connections of the wires to the electrodes to obtain more consistent readings. The first change in materials included hydrogels that were cut down smaller to only cover the area of the anode and cathode of one biophotovoltaic cell at a time. Our first prototype was fluctuating between both positive and negative voltages, showing that the electrons were not strictly flowing from the anode to the cathode which we believed was caused by all of the hydrogels being in contact with one another. Along with this the hydrogels were soaked in a higher salt concentration of BG-11 media at 2% salt to encourage a stronger hydrogen gradient. Copper tape was used in place of the copper wires due to its high conductivity, flexibility, and increased contact surface area with the electrodes. By increasing the contact surface area a more stable reading was able to be measured. A new 87V Max multimeter was donated to our lab from Fluke and used in place of our old multimeter. With these changes in our new prototype, the multimeter was reading a consistent 17 mV across one biophotovoltaic cell, but also gave the same reading of 17 mV across four biophotovoltaic cells connected in series. In this prototype, we still were not able to measure an increase in voltage drop across multiple biophotovoltaic cells in series.

Image 3: Setup of Prototype 2

Third Prototype

For the third prototype, the goal was to be able to measure an additive voltage drop in voltaic cells in series. To do this, one consideration we had was printing the adjacent anodes and cathodes of different voltaic cells closer together so the oxygen diffusion from the anode to the cathode is across a shorter distance. Having a shorter distance is both faster for oxygen diffusion because the molecules do not have to travel as far, and more favorable for the oxygen molecules compared to having to diffuse across a longer distance so they would diffuse at a greater rate. When the oxygen molecules reach the cathode, they participate in the reduction half-reaction that contributes to the formation of an electrical current, so allowing more oxygen molecules to travel to the cathode results in a greater contribution to the electrical current formation. Another observation we had in our second prototype was that when we measured the voltage across the cells using the multimeter, the voltage would begin to drop. To address this, we attached a 10 microfarad capacitor across four of the voltaic cells in series. Capacitors act to temporarily store built up voltage in a circuit, and release this energy when needed. With this addition we were able to measure 454 millivolts across the four cells, compared to 174.5 millivolts across the four cells without a capacitor. Measuring across one set of electrodes with the capacitor resulted in 92 millivolts, therefore we concluded that the voltage drop from the four cells in series was additive, and our improvements worked.

Figure 23: Graph of the average voltage measured from each prototype

Future Directions

For subsequent prototypes, one goal is to be able to light up a diode. Having proven that we were able to build up a voltage across multiple biophotovoltaic cells in our third prototype, the next step would be to print more biophotovoltaic cells in series and measure if their voltage drop would continue to be additive. One consideration for this setup is that the more biophotovoltaic cells that are printed, the larger the value of the resistance will be between the first and last cells. Because of this, naturally, some voltage output will be lost due to the resistance. To combat this another design we could test is connecting multiple series of biophotovoltaic cells in parallel. There is no voltage drop over cells in parallel, but the distance between them will decrease which will decrease the resistance experienced by the current. Another way we would be able to build up more voltage would be by adding more capacitors to the individual biophotovoltaic cells connected in series. This would not only allow the biophotovoltaic cells to charge up their voltage before releasing it but would also give a more stable reading to the multimeter when measuring a voltage across the circuit.

Another future direction would be testing these biophotovoltaic cell prototypes using transformed cyanobacteria. Currently, we have only tested these prototypes using wild-type cyanobacteria, Synechocystis sp. PCC 6803. A part of our wet lab team’s efforts was put into genetically modifying cyanobacteria with psbE and psbF. The alpha and beta subunits respectively of cytochrome b559 play a role in the generation of electrons from water on photosystem II ¹. Transforming cyanobacteria with these has been proven by the wet lab team to work, so the next step would be growing enough transformed cyanobacteria to test our biophotovoltaic cells. The assumption would be that these transformed cells would be able to produce more voltage than the wild-type strain.

A third future direction would be looking into incorporating ethanol distillation into our prototypes. The ethanol production by our cyanobacteria is one of our modules in the wet lab aspect of our project. With this, we have found that ethanol can be distilled from the hydrogels sitting on top of the cyanobacteria cells in our biophotovoltaic cell design. This distillation process creates a cleaner way to produce ethanol and would be a strong addition to an upscaled version of our biophotovoltaic cell systems.