MCC micro-capsule
To allow WB800 properly function in the concrete to fill the crack, we have to ensure the bacteria remain as spore – a stable and dormant state in which it will remain uninterrupted by other factors until the cracks appearing. We need to insulate them from outside environment and revive them when cracks appear. Under this consideration, we design a kind of micro-capsule composed of MCC material sheltering the spore from outside. The MCC, short for microcrystalline cellulose, as a stable and non-toxic composite material, can certainly ensure the insolation between spores and outer environments. In the meantime, the use of micro-capsule can gather substantial spores together, thus promoting the spores’ concentration each unit volume and enhance the repairing effect. So we design the micro-capsule to properly embed the bacteria as spores in the concrete.
Team:SZU-China/Design
Overview
As has been described in the background, we have to construct a self-repairing device in the concrete, which is achieved by genetic modified Bacillus subtilis embedded inside by micro-capsule.
To construct the bacteria properly functioning in the concrete, we should first improve the alkali resistance, germination rate and mineralization capacity of Bacillus subtilis significantly. Then, we use the special technology to make the spore of Bacillus subtilis into microcapsules, while adding the necessary nutrients for germination together. Finally we will incorporate the microcapsule into concrete in a certain proportion.
When the microcracks inside the building appears, the microcapsules are torn and the water is infiltrated to dissolve the nutrient, which stimulates the spore into the germination state and finally returns to normal activity. Carbonic anhydrase of Bacillus subtilis promotes CO2 hydration to produce CO32-, which binds to free Ca + in the environment to form calcium carbonate deposits, thereby enabling microcracks to self-repair and isolate the external environment from further corrosion of the internal reinforcement.
Here are the brainstorms of how we design our self-repairing system.
Which chassis to choose?
WB800
The chassis we chose this year is Bacillus subtilis because of its ability to form spore, which enable the bacteria to lie dormant for extended periods and protect itself from the harsh environment, like extreme pH, high temperature, and desiccation. In all the strains of Bacillus Subtilis, B.S168 is the most common one. However, since this strain secrete the digestive enzyme which will certainly interrupt the exogenous gene expression, we choose its modified strain – WB800, which has no digestive enzyme.
How to embed the bacteria into the concrete?
What function will the bacteria perform?
To activate the spore for germination
As mentioned above, the embedded spore remains dormant in the micro-capsule. In order to wake them up for function, we transferred the gene gerA into our bacteria and overexpress it. This gene encodes the nutrient receptor on the inner membrane which can detect the outer nutrient substance for example, L-alanine, and activates the germination and initiate our system through complex signaling pathways.
To resist certain alkaline environment
Considering the working circumstance of our system is concrete which has rather high pH, We need to enhance the alkali resistance of our bacteria. Thus we transferred two alkali resistant genes.
1. The expression of gene tupA is a kind of enzyme which can shift glucuronic acid and L-glutamic acid into polyglucuronic acid and poly-γ-L-glutamic acid. These polymers will form a layer of protection barrier on the cell wall and neutralize the extracellular hydroxyl.
2. In the meantime we transferred the second alkali resistant gene, nhaC. It is the coding gene of the proton pump on the cell membrane. And this specific proton pump play a key role in regulating cytoplasmic pH value by coupling net hydrogen ions in and sodions out simultaneously, thus can balance the endocellular pH.
To produce CaCO3 for crack filling
The key point of self-repairing system is to allow our bacteria producing calcium carbonate for crack filling. In order to realize this, we transferred the gene of carbonic anhydrase. It is a highly efficient enzymes catalyzes the reversible hydration of CO2 and forming bicarbonate and protons rapidly. The bicarbonate radicals will diffuse out of the cell along with the concentration gradient and combine with the hydroxyl and calcium iron in the concrete environment, which will become calcium carbonate sediment. Now the calcium carbonate can go and fill the micro cracks.
Here is a video for the specific mechanism described above.
Reference
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