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Protein nanowire films made of the pili of certain kinds of bacteria has a unique structure that allows it to utilize internal humidity gradients to generate voltage; In other words, the material has the ability to generate electricity from ambient moisture. We aim to increase the power output and increase their reliability as a power sources through arranging them into series circuits[*] and using voltage regulators[*].
Protein nanowire solution[*]
Carbon nanotube solution (CNT)[*]
Gold leaves and wires
Electrical wires (copper)
Repurposed water bath
The Bottom Electrodes
Before we can move on to any further testings regarding electricity, we need to make sure that we have a proper base for the film to sit on; it needs to have a low resistance and must not react with acids (in this case HCl). There are only few materials known to have these properties, such as gold, vitreous carbon[*], and carbon nanotubes. We eventually used carbon nanotube for preliminary prototyping due to its flexibility and its relative inexpensiveness. Meanwhile, we also tested gold electrodes because of its superior conductivity.
To produce our most basic prototype (illustration below), we directly dropcast the CNT into a mold made of duct tape sitting on a glass slide. After the nanotube has cured on top of a repurposed water bath at 80 degrees Celsius, we peel the mold off and leave behind the bottom electrode. After which, we dropcast the protein nanowire solution onto the electrode, and heat it again at 80 degrees Celsius to produce the final film.
The Top Electrodes
Since the film is very soft and delicate, we have to be careful when placing the top electrode onto the film; otherwise, the film might be penetrated and produce a short circuit. We tested multiple material candidates for the top electrodes, including cured CNT, yarns soaked with CNT, coated copper wire coils gold wires, gold leaves, etc.
Iterations of Modular Designs
In order to increase the device's current and voltage, we tested modular designs and series connections. The entire development process could be categorized into 3 generations.
The first generation utilizes readily available materials such as glass slides, plastic lids, copper wires, etc. However, they are not standardized and have relatively large errors, and tend to short circuit or break connection. They also have short lifespans due to the same reason. They are more experimental than functional, and paved way for later generations.
Design Based on PCR Tube Lids:
It utilizes the flexibility of PCR tube lids, which are made of a soft plastic. Since the lids are in series already, they are naturally a good basis for series circuits. We dropcasted the CNT and nanowire into each slot in order. Then, we use copper a wire to puncture the bottom of a lid subunit, and place the same wire on the top of the nanowire film of the next subunit, thus forming a series. However, there are some short circuit issues because the wires might penetrate too deep, and the copper wires are very prone to corrosion.
With Elongated CNT Strips:
This design is very simple because it utilizes lateral moisture gradient, which means the humidity of one edge of the film is higher than that of the opposite side, despite the power is much weaker (the common practice is utilizing vertical moisture gradient, which means there is a moisture different between the top and the bottom). In this case, one strip of CNT electrode is both the top electrode of one film and the bottom electrode of another film, forming a series circuit, while the strip also acts as the wire between two subunits. However, due to the weak adhesion between films and the glass slide, the films are very likely to become loose and lead to open circuit.
Design Utilizing Another Kind of Lid:
In previous iterations there are either short circuit or open circuit issues, mostly because the top and bottom electrodes are two separate parts. In this design we try to make the top and bottom electrodes adhere to one piece of lid to eliminate this problem. However, short circuit issues still exist.
The second generation is characterized by the extensive usage of 3D printing and modular sub-units. Therefore, they are theoretically more precise and have better longevity compared to the first generation. We also test gold electrodes in this generation because we suspect the inferiorty of CNT. However, because all of the electrodes and pili films are manually positioned, the second generation still have low fitting accuracy. As a result, some problems in the first generation, such as short circuit and open circuit, still exist.
The net-like structure is the extension of the container, on which CNT covers. Using such a structure can ensure the overall strength of the device, and sufficient contact between the top electrodes and film, without compromising the moisture entrance. However, the films shrink after they become dry, leading to a headspace between the top electrode and the film, thus forming an open circuit.
2nd and 3rd Design:
They are basically repetitions of the PCR tube lid design, but replacing the previous plastic lids with 3D printed capsules. One capsule act as the basis of one subunit and the roof of another one. However, because of the imprecise nature of manual constructions, there are some open circuit issues between the bottom of one subunit and the top of another one. At the same time, the top electrode is also very likely to penetrate the soft film and contact the bottom electrode, creating a short circuit. Therefore, we can only create subunits but cannot connect them.
The third generation utilizes Printed Circuit Board, which prints the desired circuit, in our case both the top and bottom electrode, onto a single polyimide sheet in a precise and compact manner. Highly standardized, it greatly improves the precision over previous generations because of its automatic production. As a result, it solves the short circuits and open circuits occur in the first and second generation; at the same time, it keeps certain advantages of them, such as modular one-piece construction and easy maintenance.
It also eliminates manual series connections because all of the sub-units are already printed onto a single polyimide sheet; in previous generations, we need to manually connect two sub-units, during which open circuit is likely to occur. It, thus, theoretically can help us to achieve series circuit.
The film is different from a normal battery in that it has very large inner resistance, therefore the device itself would consume a large amount of energy, leading to low power efficiency.
Equation between resistance and electromotive force[*]
I: current in the circuit (Ampere; A)
E: total electromotive force (Volt; V)
v: voltage applied on the load (Volt; V)
R: inner resistance of the film (Ohm; Ω)
r: the resistance of the load (Ohm; Ω)