Terra-cotta ceramics have been used for thousands of years, but a particularly 21st-century application gives the material an entirely new purpose.
Marine scientists and architects from the Swire Institute of Marine Science (SWIMS) of the University of Hong Kong (HKU) and its Robotic Fabrication Lab of the faculty of architecture, respectively, worked together to print 3D terra-cotta tiles that will act as artificial reefs. The result is a mesmerizing, organic swirl of line and negative space that reads like a burnt orange topographic map—and mimics the natural patterns of the coral itself.
Coral presence has declined in Hong Kong over the past several years, according to Green Power, a Hong Kong-based environmental group. In fact, the coral population has declined 80% in one area, Double Island, Sai Kung, over the past decade. This is troubling since coral reefs are some of the most diverse and valuable ecosystems on earth; they “support more species per unit area than any other marine environment,” including millions of species yet to be discovered, according to the National Oceanic and Atmospheric Administration (NOAA).
Green Power attributes the decline to a combination of global warming, pollution, net fishing, and water sports. Researchers at HKU also cite bioerosion (gradual deterioration of coral habitat), coral bleaching, and partial mortality events (basically the rapid dying off of a species over a short time) due to red tide in 2015 and 2016, according to team member Vriko Yu. This is where man-made or artificial reefs come in—they help restore lost coral populations by reintroducing an environment amenable to regrowth—and they’ve been made of everything from purposefully submerged shipwrecks to cement sculptures.
The robotic 3D printing process offers unique advantages in the design and production of artificial reefs, according to team leader Christian Lange. It makes production easier and more efficient, by allowing the team to create large pieces in a short amount of time. It also enabled the team to make tiles with different designs and functions, something that wouldn’t be possible with a typical mold. To do this, the team 3D printed terra-cotta clay into the reef tile pattern and fired it to 1125 degrees Celcius to produce the 128 tiles they’ve made so far. Each tile is just shy of two feet in diameter. Why terra-cotta? It’s highly porous with “nice surface micro-texture” for marine organisms to latch on to, says team member Dave Baker, and an eco-friendly alternative to conventional materials such as cement or metal, the HKU team says.
“Though these tiles could be produced with other methods, such as making a double-sided mold, it would be quite complex to do since the design of the tile is very three-dimensional,” Lange says. “3D printing offers the advantage to produce objects and parts much more cost-effectively. But the most powerful advantage of it is that it could print each object with a different design without increasing the cost.” This way, they can design tiles that have different functions without incurring significant additional costs.
While all the coral tiles are identical in this pilot study, the team will use different designs in the next iteration to figure out how they affect the species. The designs can also be specific to the environment and underwater conditions where they are placed; for instance, the team designed these tiles to prevent sedimentation buildup, a major problem in Hong Kong waters.
And since the tiles interlock, they appear as though the ocean floor has been retiled and renovated. It has in some sense. The reef tile design provides a “structurally complex foundation,” according to an HKU statement, which gives coral fragments lots of nooks and crannies to anchor onto, and prevents sediment buildup, which is a major threat to corals, they say.
The team placed the reef tiles seeded with coral fragments over about a 430-square-foot area across three sites within Hoi Ha Wan Marine Park in Hong Kong this past July. Researchers will monitor coral growth on the tiles over the next two years. This is just the beginning of the project: The team plans to develop new tile designs and further expand seabed restoration in the area.
