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Proof of concept
In the whole project, we designed and generated three kinds of E-pili, GsPilA, GmPilA, and PaPilA. Then we used SEM (scanning electron microscope) to view the micro-structure of the pilis we have produced so that we can prove that the pilis may be the E-pili. After the proof, we started to examine the functions of the E-pili.
After cultivated E. coli, we can see some glistening particles on the plate, which we believed that they are the sign that pili were generated. We then use a blender to separate the E-pili from the E. coli and use a stirring filtration to collect E-pili from the mixture. These two devices allow us to have E-pili mass production, meaning that people can produce it in factories. What is more, compare to using centrifuges, using stirring filtration cost less with mass production.
From a drop of the mixture, we can see the glistening particles. To further prove our assumption, we used a scanning electron microscope to print the structure. We observed a uniform porous structure, which was the same as what the paper said. Therefore, we concluded that our E. coli generated the pili we want.
To further prove the function of E-pili, we used our first design. In the design, we chose glass slides as our bases and chose carbon nano tubes as the bottom and top electrodes, because it is relatively cheap and has relatively high electrical conductivity. We produced three kinds of batteries with three kinds of pili, GsPilA, PaPilA, and GmPilA. To prevent an open circuit during the examination, we added three tiers after drying the previous tier for each kind of pili.
In our examination, we chose to contact pili directly instead of electrode. Results showed that PaPilA achieved 0.005 mA and 0.3 V, which are the highest current and voltage we can achieve. On average, PaPilA has a median voltage, highest current but the lowest output. In contrast, GsPilA has the lowest current and highest voltage. What is more, it has maximum production. For GmPilA, it has the median voltage, current, and output. This examination not only directly demonstrates that these pili can function appropriately, but also they have different properties.
In order to increase the voltage of the battery, we designed the series circuits. We first draw carbon nano tubes on tapes with the same length and width and cut the tapes down, named them from 1 to 4. Then, stick the tapes on a glass slide parallelly. To form unit 1, we uncovered the left end of NO.2 and added pili drop between 1 and 2. When the pili are relatively dry, we stuck the end of NO.2 tape back. Unit 1 was formed. To form unit 2, we repeated the process between the right end NO.2 and NO.3 tapes, and for unit 3, we did the same thing between the left end on NO.3 and NO.4.
After making the battery, we first tested the voltage of each unit. Then, to determine whether our battery follows Ohm's Law, we tested the following combination: unit 1+2, unit 2+3, and unit 1+2+3. Through the results of each combination, we discovered that our battery follows Ohm's Law because the voltage of each combination is the total voltage of each component, while the current does not change a lot. However, our current and voltage are not high enough because of the contact resistance.
To decrease the contact resistance of the battery, we discussed with Cong Xu, who gave us some advice about designing the battery. Then, we used a 3D printer to print out our latticed design battery. In the design, carbon nano tubes are added first to the containers. After the material dried, we added three tiers of GsPilA (since it has the maximum production).
The new battery is designed to be formed series circuits in the way shown in figure 4. (B). Also, compare to our first generation, it has the potential of being standardized. An essential character is that it has more significant contact areas with the air, meaning that the battery is sensitive to humidity. Therefore, we experimented with testing the relationship between humidity and voltage. According to the result, under 50% to 60% humidity, the battery can generate the highest voltage, while under other humidity that is higher or lower than 50% and 60%, the battery generates a lower voltage. The whole figure is in the shape of a parabola that has the highest point.
After visiting Seeed, our team members were inspired by their enterprise spirit about open-sourcing hardware and one of their sensors. We then designed the new structure of the battery, in the shape of AA and AAA batteries. In this way, we hope that more people can use our product. To make the new battery, we first added carbon nano tubes on the 3D printed containers. After the tubes were dried, we added some GsPilA to the top of the electrode. By connecting each cell, we can form AA or AAA batteries. To test the design's feasibility, we examined the voltage, current, and resistance of a signal cell. The result shows that those properties were not affected. Therefore, our new design was successful.
However, contact resistance still bothered us. To solve the problem, we met with Professor Qin. Professor Qin came up with the idea of using a PCB plate as the base of the battery instead of the glass slide. In this way, our battery can decrease contact resistance and open-sourcing since the process of producing the battery is easier. After learning from him, we immediately designed the PCB plate under the instruction of Cong Xu and sent it to a company to produce. The plate consists of 4 parts: copper wire, bottom, top, and PCB body. To make the battery on the PCB plate, we added GsPilA on section B and folded section A.
After the battery generated relatively steady voltage and current, we believed that it is ready to solve problems in real life. We want to apply the battery in the power bank in human life, especially for those adventurers who enjoy traveling through forests, mountains, or deserts. Unlike traditional power banks that cannot recharge without electricity, our battery can recharge only depending on the air's moisture difference, which ensures that travelers have enough electricity to use.
What is more, since the natural world takes a large portion of our Earth, we are eager to use the battery to solve nature problems.
Our first goal is to protect migratory birds. Since our battery is for each piece, it is light and thin. Compared to the current GPS's battery used on migratory birds, it will not affect them a lot, meaning that small migratory birds can use GPS. In the trip to the Dapeng Leucadendron Floridum Wetland Park, we learned that our battery could also power those sensors which require electricity wires to power. Since it is hard to install electric wires, the sensors' location is currently limited by wires. With our battery, those sensors can monitor more areas.
Finally, we found an excellent application for powering the Marine Litter Detective, designed by Seeed and MakerBay. Their detectors are to record the current in the ocean so that they can trace the pattern of litter in the sea and find out the source of the litter. However, their detectors will not be recycled, indicating that they will be left in the ocean, causing pollution. For their product, pollution only comes from two places: shells and batteries. MakerBay is working on finding new materials to replace the plastic shells, but have no idea about the batteries until we appeared. Our project gives them the possibility to develop a hundred-percent degradable detector. In the future, we will further collaborate with them.