The spores of Bacillus subtilis will be contained in a compartment which is impermeable to bacteria and liquid, but can still let through volatiles. Metabolites, water, and alanine should be separated from the spores by a breakable membrane. Mixing these compounds will cause germination.
The material we use for our sticker is TPX®, or polymethylpentene. This polymer is available as thin, transparent sheets. The advantage is that it is relatively cheap, strong, and capable of letting through volatiles. The radius of the pores in TPX® is between 1 an 10 nm, which is at least 50x larger than the average badmeat-volatile, but still small enough to keep liquid and bacteria in (see the figure below).
Comparison of the size of the TPX pores, volatiles and Bacillus subtilis.
Material
The material should not break too fast and should resist a human grip strength with a minimum of 40 pounds pinch and pressure.
The product should be made of a material that is light and easy to handle.
The material should be durable and not too expensive.
The volatiles and oxygen should penetrate through the material, while the spores, bacteria and liquid should stay inside.
The material should fit in a meat package.
The material should not be toxic or become toxic for the bacteria
The material should be able to cope with a temperature of at least 125 Celsius.
Measurements
Needs a visible feedback system for the human eye and should easy to understand for the customer.
The product should keep the visible feedback stored over time.
Appearance
Product should be made of attractive colors and shape.
Safety
The bacteria should not escape from the sticker nor harm the environment. Therefore the product should provide adequate support and be reliable.
Customer comfort
Easy to use the visible feedback device.
The product may provide different degrees of strength depending on the development of the customer strength.
For the development of the sticker, the PDCA (Plan Do Check Act) strategy was used to develop an ultimate device that keeps track of the needs of the customer and the environment in the project design.
The following questions where raised:
Who is your external customer?
The customer who is against food spoilage and customers that are willing to buy it.
What value do you deliver to that customer?
A replacement for the "use by date and sell by date" system: a new indicator for when meat is getting spoiled.
Who, in you iGEM group, delivers that value?
The design engineers and the biologists of iGEM Groningen 2012.
How do they deliver that value?
By doing research and creating a new indicator to predict when meat get spoiled.
Two different materials were carefully chosen for our final product. At first, the outer bag should be resistant enough to support the bacteria inside the sticker and give strength to the pressure that is applied by the customer. Its physical properties and characteristics should not be affected when too much pressure or deformation is applied on the sticker. It should be light and small for easy removing and placing in the meat package.
Second, the case should be made from a light material and the surface should be flat to provide grip and avoid sharp edges or other risks for the user.
Taking into account all these characteristics, we considered three materials for the sticker: polyetheen (sandwich bag), Polyvinyl chloride (cling film) and Polymethylpentene (PMP) als commonly known as TPX. We chose TPX as the material for the outer layer of the sticker, because this material fulfills all the requirements mentioned above.
In addition, we looked for another material needed for the inner compartment of the sticker, that separates the medium from the spores untill the customer wants to use it. At first, polyvinyl chloride turned out to be too weak for the outer layer of the sticker because it is easily broken. However, these properties are very suitable for the needed the breakable inner compartment of the sticker. Not a lot of pressure or strength is needed to break the inner compartment, which is handy for the customer when handling the sticker.
The prototype the inner compartment is made with cling film with side affix of 1mm thickness the inner pack was filled up with Luria broth and Food warden bacteria (link bacteria). In the outer layer from the sticker consist out of TPX with inside filled cling film package and spoors. The important part was that the surface of the TPX should have sufficient oxygen exchange to the medium; otherwise the bacterium does not growth optimal. We made experiments with flasks closed with 50ml medium and 84ml air to calculate the minimum oxygen exchange. In this experiment the bacterium growth in 37 Celsius degree. The 50 ml medium in the flasks has a diameter of average 6cm where 84ml of air average of 21% oxygen at a starting point is applied on the medium in the flask. That makes 17.64ml of total oxygen that is needed for the growth of 50ml medium. From out the TPX characteristic and mathematical calculation we made the following graph picture 2.
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