Science fiction has conditioned us to imagine futuristic cities as chromed-out metropolises—gleaming monoliths twisting and towering over LED-lit autoways and radiant biospheres. But what if cities of the future look less like something out of Blade Runner and more like the apocalyptic hellscapes of Mad Max?
Most of today's urban centers are made from two of the most environmentally damaging building materials currently in use: steel and concrete. Together, their production accounts for a whopping 10 percent of the world's total greenhouse gas emissions. If you need a little bit of perspective, that's only slightly less than the annual carbon footprint of the entire transportation industry.
Yet, with the global population count expected to surpass 8 billion by 2025, megacities such as Tokyo, London, and New York will be increasingly relied upon to not only house growing numbers of people, but also to scale up in a way that's sustainable.
This construction conundrum was what inspired biomimetic engineer Michelle Oyen to tackle her newest—and sci-fi as hell—project: building cities out of bone.
Today, "bone cities" are more likely to conjure visions of Europe's haunting, skeleton-filled ossuaries than anything remotely futuristic. But they might just be a feasible solution to building taller, stronger, and more carbon-friendly structures.
"I fly back and forth a lot between the UK and the US, and I'd been harbouring a lot of guilt about the effect that had on my carbon footprint. I'd always assumed, as many of us do, that air travel is a huge contributor to carbon emissions," Oyen, who also leads the University of Cambridge's Department of Engineering, said in a statement. "But the truth is, while the emissions caused by air travel are significant, far more are caused by the production of concrete and steel, which of course is what most cities are built from."
Oyen's job is to look to nature for inspiration, so when faced with the problem of designing a building medium as strong as it is eco-friendly, that's exactly what she did. Instead of throwing money and resources at "greening" unsustainable materials, Oyen opted to create an entirely new one. According to her research, which received funding from the the US Army Corps of Engineers, synthetic bone and eggshell perfectly fit that bill.
Ounce for ounce, bone is actually stronger than steel, and a cubic inch of it can bear four times the load than its concrete equivalent. Bone's strength is derived from a composite of protein and insoluble salt or mineral called "hydroxyapatite," which gives it the benefit of both stiffness and resistance. The ratio of this composite in a human femur, for example, is about 50:50, while something like an eggshell contains approximately five percent protein and 95 percent mineral content.
Using these ratios as a starting point, Oyen and her team were able to successfully produce samples of synthetic bone and eggshell by "templating" hydroxyapatite directly onto a substrate of natural collagen.
"One of the interesting things is that the minerals that make up bone deposit along the collagen, and eggshell deposits outwards from the collagen, perpendicular to it," said Oyen. "So it might even be the case that these two composites could be combined to make a lattice-type structure, which would be even stronger—there's some interesting science there that we'd like to look into."
The group of engineers said their technique requires little energy and can be easily scaled, but because it currently requires the use of animal collagen as a protein source, they plan to replicate their results using more humane properties, such as synthetic polymers. It's also unclear how much the production of artificial bone would cost compared to concrete and steel.
There are still a lot of questions about how we could build cities of bone, but the most daunting is whether the construction industry would ditch the unsustainable for the avant-garde. Even wood, which has long been considered one of the most environmentally friendly building materials available, is only just starting to gain traction with urban city planners.
"All of our existing building standards have been designed with concrete and steel in mind. Constructing buildings out of entirely new materials would mean completely rethinking the whole industry," Oyen added.
"But if you want to do something really transformative to bring down carbon emissions, then I think that's what we have to do. If we're going to make a real change, a major rethink is what has to happen."