Working off the idea that we wanted a portable device quickly led us to a consumer 3D printer allowing us to do rapid prototyping. The printer we used is an Ultimaker 2+ provided to us by Ultimaker. We wanted a device that could show a fluorescent signal so we came we came up with our first designs. The fluorescent molecules produced by our system are excited by UV-LED’s with a specific wavelength. The emission spectrum is visible by the naked eye and this is how we planned to read out the signal. The membrane sealed sample vials containing our bacterial system are placed in the device from the top. Initially we started off with a device which fits one sample. However we soon realised that adding a positive and negative control makes out device more reliable. Once the button is pressed the UV-LED’s are turned on and the signal can be compared to the positive and negative control in order to determine the result. This approach was problematic since lighting conditions, background signal and observers are not a set variable. This prompted us rethink the way the device works and version 2 was on the way.

The first working prototype included space for 3 membrane sealed sample vials, a positive control, the sample and a negative control. In order to take out the variables of the lighting conditions, background signal and observers we wanted to use cheap and simple electronics in order to read off the signal. This led to the removal of the ‘windows’ in the front and a bulkier design to accommodate the electronics. The improved computational approach to read off the signal was based on Light Depended Resistors (LDR). This component changes its resistance according to the amount of light that it receives. This resistance can be measured via a simple circuit (See Figure) and processed by the arduino microprocessor. Once the button is pressed the device excites the sample vials with UV light and the emission spectrum is detected by the LDR’s. The signal of the sample is compared to the positive and negative control and the final result is displayed on an integrated OLED display on the front. Although this is a mayor improvement over trying to detect the signal by eye, this way of detecting the light is unreliable.

To improve the measurement capability of the system, various components were exchanged. First we chose to calibrate our device using fluorescein, as this circumvents the use of recombinant bacteria in this part of the prototyping faze. The UV-LED were exchanged for 475nm blue Cree LED’s, which will excite fluorescein provided in the InterLab study kit. This wavelength is much safer too use as it is not harmful to the retina. Second, circuit containing LDR’s was replaced. Whilst cheap, LDR’s are not very suitable of taking precise measurements. To improve on this we chose to implement photodiodes, a semiconductor which converters light into electrical current. However, this signal needs amplification, for which small transimpedance amplifier (TIA) circuit containing an operational amplifier was used (figure X). Furthermore, a small piece of orange filter paper was placed in between the photodiode and the sample. The filter paper blocks much of the blue, thereby reducing the amount of background signal. These parts were combined in a prototype setup as seen in figure X. Measurements were taking using a fluorescein dilution series, the results of which are shown in figure X..
The resulting graphs show a very nice calibration curve in the range of 0 to 2 mM of fluorescein. In order to check repeatability the whole setup was built again and the results are shown in Figure Xb. The results show the same very nice calibration curve and proves that this system is reliable and reproducible.