Team:WashU StLouis/Hardware
In order to detect airborne fungal spores, they first need to be captured. 3 in. diameter circular thin scratched polyethylene disks will be hung above crops in a farmer’s field, Approximately one an acre will need to be tested each day. These disks are taken down at the end of the day, inserted into a tube with rehydrated cells, which contact the disk and any spores germinating on it. First, a blank solution that only has the cells in it is poured into the observation tray and inserted into the device. The switch is turned on to take a blank measurement, and a light will flash green, the switch turned off, and the tray removed by the user. Then, the liquid from the tube with the disk is poured into the removable observation tray and inserted into the device. The operation switch is turned back on, and now when the reading is complete, a light flashes green and each window on the front of the device will turn green or red depending on which fungi are detected. The tray can then be removed and cleaned with ethanol.
Designed with extensive feedback from farmers, NGOs, government officials, and other iGEM teams, the housing best represents the nature of the project. The exterior of the device is 3D printed from a cheap but durable ABS plastic to improve the speed we could prototype. The progression of housings shown below reflect the constant adaptation to different feedback and improvements in functionality. The windows on the front of the housing represent different crops shown in pictures so that farmers who can’t read are able to use it, and it spans language barriers more easily. The housing was built so that these windows can be easily modified by simply inserting a different sheet of plexiglass with different plants included. The code for the arduino microcontroller can be easily programmed to produce different light signals behind the windows. Farmers preferred a green color if that crop was immune to whatever was found on the collection disk and red if it was susceptible. The housing contains mounting platforms for the LED indicator lights, a dock for the detection chamber, and is easily held closed with 4 bolts around the outside.
The detection chamber houses the actual fluorescence detection. Once the tray has been inserted into the detection chamber with the blank or the liquid that has contacted the disk, the switch is turned on. An excitation LED inside the detection chamber is turned on which is filtered to specific wavelengths, and excites any fluorescent proteins made by the engineered cells. These fluorescent proteins emit light at wavelengths that pass through another filter which removes the excitation light before being absorbed into a photodiode which turns the light’s energy into an electrical signal.
The device is run using an arduino uno microprocessor which is easily coded and modified. The electrical output from the detection chamber is amplified and filtered by two concurrent operational amplifiers before being read by an input into the arduino. Depending on the input voltage, and how it compares to a blank reading, the arduino will indicate which lights should turn red or green in the windows, indicating the presence or absence of a virulent fungi. The circuitry is fairly simple and easily adjusted to meet the needs of different iGEM teams, simplify repairs or replacement of parts by the user, and expand to greater detection capabilities.
We created a small solar array to charge the 9V battery powering the hardware. This was requested as a more reliable way for farmers to have access to the power needed to operate our hardware. The solar array trickle charges the 9V battery over 8-10 hrs. by producing a maximum voltage of 12V and 13.38 mA. The voltage needs to be larger than 9V to charge the battery and in direct sunlight the solar array easily reaches 11V through a window. The amperage is large enough to charge the battery at between 13-14mA and smaller than 1/10 the battery capacity of 175mAh so not to damage the battery.
