Team:Exeter/Implementation
Project Implementation
Coral Reef Conservation
When thinking about the applications of our calcium carbonate production technique, we were initially attracted to marine conservation, as well as clinical and biomedical fields. We started looking into calcite-based bone regeneration material for usage in implant dentistry. However, given the wealth of resources in marine biology that we had access to at Exeter University, we ultimately took an environmental route, finding ourselves drawn to the idea of the production of coral backbones to help promote coral regrowth.
Fish are essential for coral ecosystems as they graze on seaweed which would otherwise smother corals if left to grow. Natural coral reefs have crevices to attract fish for this exact purpose, therefore we would design our structures to imitate the natural reef by integrating hollow parts into our hypothetical coral design.
Current artificial coral backbones are made from a range of materials, including plastics, steel and concrete. There are many drawbacks associated with these materials; plastics often leach toxic chemicals into the ocean and have surfaces that are too smooth for the attachment of corals[1].Steel and concrete structures, despite being the materials of choice due to a high attachment rate, have large carbon footprints[2]. Their production leads to the release of carbon dioxide into the atmosphere, which is paradoxical given the fact that backbones are built to protect corals from rising ocean acidity which is a direct consequence of increasing carbon dioxide levels[3]. An additional disadvantage to these materials is that they are often implemented without proper planning and long-term management, and consequently end up accumulating on reefs and contributing to the pollution and degradation of the environment[4].
A 3D Printer for Carbon-Mitigated Coral Regeneration
Printing Coral Backbones
The ultimate aim of our project is the 3D-printing of calcium carbonate into custom shapes that would recreate the native backbone structures of coral reefs. This process involves genetically modifying bacteria with the ureolytic pathway, allowing them to precipitate calcium carbonate. Once these bacteria have been combined with the hydrogel in the printer nozzle, the resulting mixture is ready to be moulded into our pre-designed custom shapes. The precision allowed by a bioprinter would enable us to produce porous shapes with rough surfaces that would be more readily colonised by coral and should therefore be highly functional as coral backbones. As this production technique does not involve the mining of calcium carbonate, an extremely carbon intensive process, this technique would have the added benefit of being carbon-mitigated, making our project a truly environmentally friendly solution to the crisis affecting coral reefs[5]. An additional advantage to our 3D-printed custom shapes is that they are not restricted to the shapes achieved by evolution; they could potentially be designed to have even larger surface areas than natural corals and therefore a higher coral attachment rate.
Hydrogel Printer Ink
Central to our production technique is the use of a hydrogel, a material made of long chain polymers and water, which constitutes around 90% of its chemical makeup. The hydrogel encapsulates the calcium carbonate-precipitating bacteria and effectively acts as an 'ink' for our bioprinter. The non-biodegradability of our hydrogel means that it will not be degraded by a UV light like a pseudo-plastic and therefore will not infiltrate the food chain. Placing our artificial backbone into the ocean and integrating it into the ecosystem also means that non-toxicity is a highly important property of the hydrogel. For these reasons we chose to initiate our hydrogel experiments with agarose, a natural polymer that has the properties required for safe implementation in the ocean. The other material required for our coral structures is of course the calcium carbonate component. The calcium ions required by the bacteria for the precipitation of calcium carbonate would come from the calcium chloride -supplemented culture medium. Our CO2 would ideally be sourced from carbon capture facilities owned by power stations and the cement industry.
Our Progress Towards Implementing Our Project
The COVID-19 pandemic significantly reduced the time we were able to spend in the laboratory. We knew from the start that it would be unlikely that we could successfully precipitate calcium carbonate in a hydrogel and print a prototype for our coral backbones in the time we had. We therefore decided to focus our efforts on designing the printer hardware. We modelled the mechanics of fluid flow through various designs in order to determine which would lead to optimal fluid flow, in addition to the homogenous mixing of the bacteria and hydrogel within the nozzle. This is crucial to ensure the structural integrity of our coral backbones, as calcium carbonate that is unevenly distributed within the hydrogel may lead to structural weakness. Our initial model was designed in a way such that the bacteria and hydrogel-containing tubes would intersect in the nozzle chamber at relative angles which would generate a vortex, ensuring the homogeneity of the mixture. The final design, an image of which can be seen below, is the result of a collaboration with the Ashesi Ghana iGEM team. Having talked them through our fluid flow models and given them a rough design of the printer nozzle, the engineers within their team modified our design and were able to simulate the internal conditions of the nozzle in accordance with our model. Their modifications, which include a larger surface area, filleting, and a larger nozzle chamber volume, would render the nozzle more resistant to fractures caused by high pressure fluids.
The bioprinter imposes a size limit on the objects it can produce, therefore, were we to reach the stage of having a working prototype, we would print small structures in such a way that they could be pieced together like 'Lego' or sickle bricks to form the larger backbone structures required for effective coral restoration.
Despite not being able to print a prototype, this has not stopped us from consulting coral experts such as Steve Simpson, a professor of marine biology at the University of Exeter (see our Human Practices page for more details about this collaboration). Professor Simpson gave us invaluable information about coral restoration and advised us about the various ways in which we might go about implementing our structures in a marine environment, were we to have working prototypes. One of these methods is the breeding of corals in captivity, in which case we would go out on a coral spawning night and collect coral larvae. These would then be added to our structures and monitored to determine their attachment rate. Given a high attachment rate, these corals could then be placed in the sea.
Other Applications
Whereas this is our main application focus, the 3D printed calcium carbonate need not be restricted to marine conservation. Calcium carbonate has a wide range of applications and would be suitable for use in pharmaceutical and clinical fields, for example in tablets and capsules used for drug delivery.
References
1.https://www.nature.com/articles/s41598-018-33683-6
2.https://www.annualreviews.org/doi/pdf/10.1146/annurev-environ-041112-110510
