CURRENTLY BEING UPDATED: This instructable should be back in perfect shape by the end of the week. Sorry for any inconvenience!
In this Instructable, we’re going to build a battery-powered Petrifilm™ incubator! The completed incubator can be used to perform visual, on-site bacterial analysis of water samples in the developing world. Creating your own device will only require modest DIY skills, and about $200. This project was developed by the Greater Austin Area chapter of Engineers without Borders.
Motivation
A recent study in Science Advances found that 4 billion people do not have enough clean water to meet their basic needs. Providing water to these populations will be one of the greatest challenges of the 21st century, and rapidly identifying water contamination is a critically important step. Field experience has shown that Petrifilm plates are a highly effective method for detecting harmful bacteria in water. The plates are both quantitative and visually striking, making it easy to communicate their results with local communities.
Petrifilm plates require a 37±1°C incubation for 48 hours before samples can be analyzed. Since commercial incubator systems are typically prohibitively expensive or require constant electrical power, their use in the field has been limited. This need has lead many field workers to incubate the samples using their own body heat. Body heat incubation which is uncomfortable, inconsistent, and greatly limits the number of samples that can be processed at one time. We felt a low cost device could be designed to affordably meet this need and greatly reduce the burden of those in the field.
The incubator described here is low-cost, portable, battery-powered, and reliable. In a 22-25°C environment, the charged device can incubate 40+ samples beyond the 48 hours required to complete cultivation. In colder environments, a larger battery pack may be required.
Construction of the incubator should only take about 2 hours
Materials (For online sources and prices see attached BillofMaterials.pdf)
The total cost is estimated at $150 to $200.
Tools:
In this step, use the templates printed in Step 2 to cut the foam insulation into the appropriate patterns.
Together, these foam patterns should fully insulate your cooler. If they do not, adjust the number of middle foam layers until the cooler is full. Note: For best performance, we recommend use of closed cell foam with a high R-value (capacity of a material to resist heat flow).
In this step, we will cut all of the cardboard components of the incubator using the templates provided.
Next, we will be creating the heating pad spacer. This will be utilized to suspend the heating pads in the bottom of the chamber.
In this step, a port is drilled for the system's thermometer.
Note: For this incubator, our foam bottom layer is 0.57" thick. If your foam has a different thickness, adjust the height of the thermometer port to account for this discrepancy. (With thicker foams, move the port up)
Next, we will continue this port into the inner chamber
In this step, the electrical heating system of the incubator is assembled.
The required components are:
Wiring the Incubator:
The heating pad and thermocouple will be mounted in the heating assembly in the next step.
The heating pad spacer is used to suspend the pads in the inner chamber while preventing direct contact with the Petrifilms.
In this step, we will be mounting the thermometer in our incubator.
IMPORTANT: After this step, the inner chamber will be permanently mounted in the incubator. Ensure the heating pads and thermocouple are properly inserted in the system.
A good seal is important for optimal insulation, and to ensure the incubator operates effectively.
In this step we will trim one of the foam middle layers to accommodate the electrical system.
Now that you have constructed the Petrifilm™ Incubator, you should test it in a controlled environment before relying upon it in the field.
We recommend that you obtain the E. Coli Petrifilms™ from 3M and and establish a control using sterile water. Additionally, test water from an actual creek likely to be contaminated with fecal matter. Any slow-running creek where people walk their dogs without cleaning up is a likely target.
Follow 3M's instructions for inoculation. We hope to make a separate instructable about this step and the use of the Petrifilm™ in general, but this is a summary.
This instructional video by 3M may clarify the above steps.
Check the thermometer a few times during the incubation period to see that it is very close to 37°C or 98°F. After 48 hours, remove the Petrifilms™. If you have colonies in the sterile water or if you have no colonies at all in the dirty creek water, you must investigate further until you understand why the incubator did not give both a negative and positive result. If however you understand the results and think the Petrifilm™ Incubator is ready for use, then congratulations! You now have a portable, battery powered incubator that can give you useful field information without readily available electrical power. It is important to ensure you have a full charge in the battery. When not in use, disconnect it from the charger and ensure it is in the off position. Despite these precautions, batteries lose energy continuously and it is important that you charge it before embarking on your journey and, if possible, before heading into the field. A fully charged battery left for a month will likely be discharged! If the ambient temperature is very low, keep the incubator in a warm place to ensure battery life lasts through an entire incubation period.
For more details on Petrifilm™ incubation, see the 3M Petrifilm™ data sheet.
This project was developed and is maintained by the Engineers Without Borders USA, Greater Austin Chapter, in the 2015-2016 school year. This is a combined professional and student chapter. Many people contributed to this project, but Evan Bartilson was a key contributor.
We are actively seeking users to test this Incubator and report your experience back to us. Please contact us via the emails listed on our contact page or by sending email to Robert L. Read [email protected].