Preliminary ideas
In our project, The degree of dental caries was detected by an E.coli detectors which could monitor the presence of Streptococcus mutans. In the preliminary idea, we wanted to build a small device to realize the detection of Streptococcus mutans. This device needs four functions. The first is that the equipment can be heated to above 60 °C for microbial culture and pasteurization which can kill Streptococcus mutans and E.coli detectors to prevent them from leaking into the environment. Second, the device can automatically control the working time and temperature. The Third is that the instrument is small enough to be used at community hospitals, small clinics, and even home for point-of-care testing (POCT). Thirdly, a hand-held ultraviolet detector was used to monitor the fluorescence signal induced by Streptococcus mutans. After the idea emerged, we drew the sketch and picture of this device (Fig.1 and 2).
Fig.1. Sketch of the preliminary ideas
Fig.2. Design drawings of the device
For the preliminary device, we refer to the water-jacketed constant temperature incubator, and want to make it into a double-layer structure. The water layer is heated by the outer heating plate to achieve insulation. A thermostat is also embedded in the device to control the outer layer. The heating plate maintains the temperature inside the device constant at 37 °C. To facilitate the observation, we designed the top to be insulated glass. Finally, we also considered biosafety. The results showed that 60 °C for 1 hour can effectively kill the S.mutans and E.coli detector (Table.1). We designed the heater to ensure that the medium in the device can be heated to 60 degrees. We used a simple device to verify that our heating and temperature control system can work (Fig.3).
Table.1 Thermal sterilization experiment
Fig.3. Verification of heating and temperature control system
Improvement
In the process of testing, we found many defects in initial ideas. At first, tinfoil was chosen as water layer isolation material, which has good thermal conductivity. But the tinfoil is easy to break, which will lead to water contact with the wire, causing safety hazards. We use stainless steel instead of tin foil for water isolation, however, the heating time was significantly increased, which affected the efficiency. In order to overcome the shortcomings of the preliminary device, we have designed the air bath detection device after careful consideration and full discussion (Fig. 4). We improved the process and replaced the hydrothermal device in the preliminary device with an air-heating module. This not only reduced the difficulty of the production process, but also enhanced the heating effect. The detection protein we used is sfGFP, which needs to be irradiated with ultraviolet light to excite fluorescence, so we embedded a ultraviolet lamp inside the device to solve this problem. Because our testing scheme was adjusted under the advice of experts, this design only stayed on the design drawings, without product processing.
Fig.4. Design of the air bath detection device
In the later stage, we further optimized the gene loop design of E.coli detector and replaced the fluorescence gene with lacZ gene. The β-galactosidase expressed by lacZ gene can transform X-gal into blue substrate to realize naked eye visual detection (Fig.5, The product appears green in the medium system). In order to obtain professional guidance, we consulted Deng Rongkang, a medical equipment R & D Engineer. He introduced in detail the key points of the research and development of instruments, and guided us to design a more professional detection device (Fig. 6).
Fig.5. Color change of E.coli detector culture induced by CSP
Fig.6. Design of the metal bath detection device
In the new device, we changed the air bath scheme to metal bath device (Fig.7), and carefully designed the observation window, illumination, anaerobic culture, heating module, temperature control system, space sealing and other technologies. Based on the design drawing, we processed the shell of the equipment, heating module and temperature control system (Fig.8). How to keep the E.coli detector from reproducing during storage and transportation is a key issue. Based on the suggestions of experts and team members, we keep the broth in the detection tube, and put the dry powder of E.coli detector into the lid of the detection tube. We have a special design for the lid. When using, press the button on the lid to press the dry powder into the broth in the tube (Fig.9).
Fig.7. Design and testing of metal bath module
Fig.8. The construction of simple metal bath detection device
Fig.9. Detection tube
Further, we will use touch screen intelligent control system to improve man-machine interactive technology. We also hope to control the detection device through mobile phones (Fig.10).
Fig.10. The detection procedure of Streptococcus mutans