How 3D Printing Can Help Restore Hong Kong’s Corals
As the marine vessel of the Swire Institute of Marine Science (SWIMS) of the University of Hong Kong 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” appear to be a jarring sight. Two years ago, Typhoon Mangkhut swept away 80% of the local corals, leaving behind debris, broken coral skeletons and some surviving species in some of the Hong Kong waters. It may take up to decades for corals to regrow to its former state naturally. Yet a team of SWIMS researchers (Vriko Yu, David Baker and Phil Thompson) as well as architects (Christian J Lange, Lidia Ratoi, Dominic Co Lim, and Jason Hu) 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 steadily on the seafloors of Moon Island, Point One (near Jockey Club HSBC WWF Hong Kong Hoi Ha Marine Life Centre’s platform) and Coral Beach (in Cheung Chau), where most of the local corals grow. Weighing around 20kg and 65cm-long in diameters, each tile is composed of 3 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 coral bits 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) and Director, Fabrication and Material Technologies Lab & Robotic Fabrication Lab, admits that one of the greatest difficulties the team faces is the breaking of the clay units from time to time.
Yet they decide to settle on clay instead of plastic or concrete. The architects observe that there are similar marine projects such as those in France which also print 3D coral structures, though with a different material. “But there are a lot of conditions like water temperate or salinity to consider in the subtropical waters of Hong Kong,” Ratoi explains. “Some other marine species aside 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 at the centre of the room. Layers after layers, dark brown clay is squeezed out from this 0.6cm-wide mouthpiece like thick toothpaste according to the team’s design. The clay is then baked at 1125 degree Celsius. The product will then shrink by 11 per cent in size.
The team deployed the test tiles back in March this year. “We did some test 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, suggesting that they do not have factory equipment, “We had to stop for a while due to Covid-19, and I still teach [at the Faculty] 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 suggests that printing clay may not become a trend in the short run for it isn’t the easiest. “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, pointing to a clay model with an overhanging part.
Lange is positive about synthesising clay, however, and can see printing similar and more structure for coral restoration here in Hong Kong and beyond. “Also, 3D printing clay is cost effective,” Lange says.
Speaking of the new 3D printing technology, David M. Baker (Director of SIRMS and Associate Professor, School of Biological Sciences) also says "It’s key strength is permitting the ability to produce highly complex structures. Our reef tiles could not be produced using conventional mold casting technique, 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 observes from 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 at their basement. Perhaps they have another city in the pipeline.