Final material prior to removal from mold after MICP process.
Context
MICP is a biological process where bacteria (Sporosarcina pasteurii) break down urea into ammonia and carbonic acid. The resulting rise in pH (alkaline environment) causes carbonate ions to be released from the carbonic acid that then bond with calcium ions to produce calcium carbonate.
Mold design
Using Rhino and a laser cutter to fabricate an acrylic mold, we created a form for the crystals to grow in. Screen printing mesh further contained the form while creating a semi-exposed surface to maximize the nutrient flow for the bacteria.
Diagram of acrylic mold with considerations for assembly and mold filling.
Experiment
We prepared 4 different compositions to experiment with how aggregate type and size affected the strength of the final composite.
After a day, the bacteria + sand-only mixtures had pH 8.5, indicating healthy bacteria.
The bacteria + grapeseed-sand mixtures had a pH of 6, indicating the bacteria was not active and couldn’t be used moving forward. We learned that grapeseed has antimicrobial properties which likely killed the bacteria.
Filling the mold, checking pH after 2 days of reaction to ensure bacteria were alive, scraping off exterior crystallization before removing from mold
Results and findings
The crystallization had formed tightly around the mesh and sand aggregates, causing the outer crust to be peeled off when the object was removed from the mold.
With the openings of the mesh blocked, nutrients and oxygen could not easily penetrate the rest of the aggregate which caused a “crust” to be formed on the outside and left the center more brittle due to lower crystal precipitation.
The openings of the mesh were 149µm. It’s possible that the fineness of the mesh increased the surface area for the calcium carbonate crystals to form on, which resulted in the openings of the mesh getting filled more rapidly or more completely.
Additionally, the “C” shape also made it susceptible to breaking while removing the mesh because it was easy to put torque onto the form.
Future steps
While the precipitation of calcium carbonate can be readily achieved (even in fairly loose laboratory conditions lacking full sterilization), the aggregate, mesh size, and geometry have meaningful impact on the integrity of the final object.
In a future iteration, I would use a slightly more open mesh and potentially modify the concentrations of bacteria or the nutrient bath to modulate the rate of calcium precipitation. The early formation of a crust provides visual confirmation that the reaction is occurring but ultimately can reduce the strength of the overall composite if the reaction cannot penetrate into the center.
