Team:SIS Korea/Hardware
Hardware
To replicate an environment in which Escherichia coli (E. coli) can be cultured and one in which the degradation of plastics is optimized, our hardware system incorporates various sensors and motors, which allow for the control of both the pH and temperature. The materials included: two acrylic containers, three different filters, four 12V DC motors, four Peltier modules, four two-channel relay modules, two current-passing channels, two waterproof temperature sensors, a pH meter, and two Arduino Unos. The two acrylic containers were designed first, in order to provide a suitable environment for the degradation of plastics and to optimize the overexpression of the plastic-degrading enzyme in E. coli, and its secretion. The height, length, and width of the cases were each set to 15cm, which allows for each of them to contain two liters of the solution with enough remaining space. Holes were made on one side and the top of the cases, serving as spaces for tubes; these tubes are used to transfer the solution between the two cases and from another reservoir containing a new solution.
The container in which degradation occurs is divided into four sections, which were divided using three filters that were created with varying sizes of iron mesh. Since the plastic waste is added from the top of the container, the first filter - which consists of square openings with side length 10mm - was installed nearest to the top of the container. Here, the microplastics must be degraded until they can fit through the holes to move onto the next section. This means that: assuming the microplastics are shaped as either circles or ellipses, the diameter or minor axis must measure at most 10√(2) or about 14.14mm to fit through the openings. Below that, there is a second filter with an opening size of 5mm, which similarly means that the microplastics must be at most 5√(2) or about 7.07mm to fit through. The same process is repeated for the last filter, which has openings of size 2mm. Thus, the most degraded solution remains on the bottom section, which is the section that is removed first every 24 hours, as the removing pump is attached to this section.
Four 12V DC motors connected with water pumps are used in this system. The first pump is used to transfer the new culture medium to the container in which the overexpression and secretion of the enzyme are performed. The second pump transfers this enzyme to the container in which the degradation of the plastics occurs, and the third pump removes the solution from the degradation container when the enzyme is inactivated and the rate of degradation decreases. The last motor-pump set transfers the NaOH solution into the degradation container to maintain a constant pH value, in order to promote the degradation process.
The replacement of the media needed to occur due to the growth curve of E. coli in the container for production and secretion of the enzyme. The growth curve is divided into four phases: the lag phase, where the bacteria are first introduced to the solution and the population is stable, the exponential phase, where the population grows exponentially, the stationary phase, when the population has reached its maximum and does not increase nor decrease, and finally the death phase, when the population decreases rapidly. The bacteria went into the stationary phase at around 24 hours, when there was a similar number of deaths as the number of bacteria produced. As we need high efficiency for the overexpression of plastic-degrading enzymes, the solution had to be replaced every 24 hours, as efficiency drops rapidly in the death phase.
The four Peltier modules (TEC1-12706) attached to plastic heat sinks maintain the temperature for each of the two containers, which is possible as one of the ceramic plates of a Peltier module absorbs heat and the other dissipates heat. Two modules were attached to the exterior sides of each container, and aluminum plates were attached in between the module and container for higher efficiency in heat conduction. The connection for one of the modules is reversed to allow both heating and cooling of the solution, and fans were attached to the heat sinks to maintain a consistent temperature.
To regulate the actuation of each pump and Peltier modules, four two-channel relay modules were used. Each relay module consists of two current-passing channels connected to an Arduino Uno, and one Arduino Uno was connected to two relay modules. For our system, one Arduino controls the water pumps and the other controls the Peltier modules, and two separate codes had to be written for them.
Two waterproof temperature sensors (model SEN050007) are also connected to the Arduino regulating the Peltier modules. And an analog pH meter, connected to the Arduino controlling the water pumps, is also used to measure the pH value of the solution in which the degradation of the plastic occurs. A 12V power supply was used to provide the power needed for the motors and Peltier modules, and an additional two pipes were inserted into the containers for aeration and circulation of the solutions, needed for culturing and regulating the pH value.
