How 3D Printing Can Restore Corals
As the marine vessel of the Swire Institute of Marine Science (SWIMS) of the University of Hong Kong (HKU) edges closer to Moon Island in Sai Kung’s Hoi Ha Wan, a cluster of reddish hexagons looms under the sea. But these aren’t corals. Neither colourful nor teeming with much life, these “beehives” are a jarring sight. Two years ago, Typhoon Mangkhut swept away 80 percent of the local corals, leaving behind debris and broken coral skeletons in Hong Kong waters.
It may take up to decades for the coral to regrow to its former state naturally. But a team of SWIMS researchers and architects at HKU are on an ambitious rescue mission.
The Reformative Coral Habitats project, conducted by Hong Kong’s first robotic fabrication laboratory at HKU’s Faculty of Architecture, has been printing out the world’s first terracotta tiles. The peculiar-looking structures, flatter than most coral species, sit on the sea floor in a few places around Hong Kong, including Coral Beach, on the territory's island of Cheung Chau. Weighing around 20kg and 65cm-long in diameter, each tile is composed of three parts: the legs, nine-grid layers and six coral-like layers. Together, the tile units act as an anchoring bed for corals to attach and grow. Assistant lecturer Lidia Ratoi explains, “The tiles aren’t conventional tiles. Hong Kong’s subtropical climate entails much underwater sedimentation. That’s why our tiles have a lot of perforations so that sediment doesn’t deposit on the surface and suffocate the corals.”
The coral-biomimicry layers are designed according to the geometry of a brain coral, so that it does not interfere with actual corals’ growth patterns. There are eight pockets on each tile to host corals. “The pockets are big enough to allow the corals to grow both horizontally, to become sturdy, and vertically, so that they can compete with other underwater species such as algae, which may suffocate them,” Ratoi explains. “Corals grow only during the warm season. It’s vital that they are forced to grow in a vertical direction.”
These pieces of coral planted in the pockets are parts broken off from the original corals before the typhoon. “So they’re genetically the same [as the corals before],” says Lange, who hopes that their project can speed up the restoration of the local coral population. Clay, similar to the calcium found in corals, may not be the strongest material when it comes to engineering a “city” that can withstand the strong ocean currents. Christian Lange, associate professor (teaching), admits that one of the greatest difficulties the team faces is the breaking of the clay units from time to time.
Yet they decided to settle on clay instead of plastic or concrete. There are similar marine projects, such as those in France, that print 3D coral structures with a different material. “But there are a lot of conditions like water temperature or salinity to consider in the subtropical waters of Hong Kong,” Ratoi explains. “Some other marine species aside from corals may not be able to survive the changes in water, but clay doesn’t damage the underwater conditions.”
Surprisingly, the material they’re using doesn’t contain any special ingredients at all. “It’s just clay you can find on any shelf from the UK,” Lange says, pulling out a blob of soft, dark brown clay from the printing machine. “Hong Kong doesn’t have its own clay, but I would like to try working with local materials in the future,” he says.
The architecture team works in the locked basement behind an inconspicuous door in HKU’s Knowles Building car park. Here, instead of cars, an ABB Foundry Plus 2 robot is parked in the centre of the room. Layer after layer, dark brown clay is squeezed out from this 0.6cm-wide mouthpiece like thick toothpaste. The clay is then baked at 1,125 degree celsius.
The team deployed the test tiles back in March this year. “We did some tests in a simulated environment, and we observed that the corals were growing [on the clay units],” Ratoi reveals. Upon the initial success, the team went on to produce a total of 128 pieces over the next two months. “We can’t really do it on a manufacturing scale,” Lange says. “We had to stop for a while due to Covid-19, and I still teach during the day.”
So when the first full set of tiles were deployed on June 13, the team was overjoyed. It was a breakthrough in technology. “What’s important is the collaboration between two departments in working towards coral restoration,” the architect says. He says that printing clay may not become a trend in the short run as it isn’t the easiest material to work with. “When compared to printing other materials, you have to print two structures for clay. Apart from the main product, you may also have to print another structure to support it so that it doesn’t fall down,” he explains.
Speaking of the new 3D printing technology, David M Baker, director of SIRMS and associate professor, School of Biological Sciences, also says "Its key strength is permitting the ability to produce highly complex structures. Our reef tiles could not be produced using conventional mould casting techniques, and the complexity is a key driver of the quality of the habitat we are providing for our corals and the other wildlife within the site. As we install the tiles, we notice that immediately many species of fish quickly occupy them."
The marine team deployed the last set of tiles last week. In the next two years, they will monitor the corals’ re-growth. The marine team estimates that the effort will generate a total area of restored coral habitat of about 40 square metres. "This is quite small compared to the total area of coral communities in Hong Kong," says Baker. "However, even small patches of corals can enhance local biodiversity by creating a home for other species, and small patches can be very important in the long-term by generating propagules—baby corals that can settle on nearby areas thus spreading corals naturally in our area."
Although it’s still too early to perceive significant results, Ratoi expects that the coral parts will join to form bigger coral structures. “We’ve placed an Autonomous Reef Monitoring System to detect signs of life,” says Lange, as he studies a picture sent over by the marine team that shows green and black marine organisms attached to their tiles. Until then, the team will keep printing away in their basement.
See also: The Woman And The Sea: Female Free Divers Take Conservation To New Depths