When we think about a gecko, we try to think of something besides car insurance. Not just out of preserving endangered species from the threat of industrial demolition, but also because there is something more to a gecko than surface-level personality and bright, entrancing colors.
Sticky gecko feet are moving research forward on the inner-workings of human cartilage, according to a blog post on the University of Delaware's website.
This is significant because cartilage — typically a mysterious substance — has kept scientists awake night after night for nearly a century.
Sticky gecko feet might unveil secrets of cartilage
Recently the sticky toes of this charismatic reptile have inspired the creation of a vast swathe of inventions — from the next generation of Velcro to Super-Man-like suits that help U.S. military personnel scale walls.
"There's nothing else in the world like it," said Associate Professor of mechanical engineering David Burris, of the University of Delaware and principal investigator in UD's Materials Tribology Laboratory. "It's an extraordinary material. We've been studying it since the 1930s, and we still don't know how it does what it does."
However, Burris and his colleagues are bringing us a step closer to closing the case in cartilage — which also means we're taking a step toward engineering a class of superior synthetic joints that approach the functionality of the real thing.
Necessity of cartilage
Cartilage is a strong yet flexible fluid-ridden tissue that cushions the space between, and keeps bones from grinding against each other. Reducing friction is crucial for joint health and avoiding chronic conditions like osteoarthritis — a common affliction affecting 60 million Americans' mobility, according to the journal Science Direct.
When cartilage disappears, "you're not walking," said Burris. "You're in tremendous pain."
The research community has long known the critical lubricating nature of cartilage in joints. But the question of how the material achieves lubrication has lived on as a specter of mystery in the scientific world.
One school of thought thinks friction is low because a thin layer of synovial fluid between cartilage surfaces in the joint, keeping surfaces from touching.
However, this doesn't solve one of cartilage's most intriguing characteristics: it's really sticky. But it slides into joints with ease to allow us a full range of motion.
"Unusual properties don't develop in nature for no reason," said Burris. "The chance that an exceptionally lubricious material like cartilage also developed unusual adhesion by coincidence looks negligible to me. It has to be a critical part of the lubrication story that no one has looked into."
With a 2019 grant from the National Science Foundation, Burris and colleagues have been researching this part of the story — seeking the answers to the stickiness of cartilage by constructing customized devices to take unique measurements.
So far they've found that cartilage sticks to surfaces to the same degree that gecko feet stick to other surfaces — nature's gold standard for uncommonly high surface adhesion.
Gecko toes adhere like cartilage
Gecko toes have millions of microscopic hairs — each acting like individual springs that conform and stick to the wall of a house (or a tree, or a car). Coarse pieces of cartilage have bumps that work the same way — they cohere to an opposing cartilage surface one by one in the joint.
This makes a seal between two surfaces, allowing various types of pressure to build. Negative pressure keeps the joints together, while positive pressure supports body weight and keeps water — the lubrication itself — from leaking.
Casting doubt on conventional wisdom
It's counter-intuitive to think of intimate contact between body parts as inherently harmful, and a source of grinding and wearing out of connecting surfaces. But cartilage needs close quarters to work.
"In a car engine, you have to keep surfaces separated in order to lubricate," said Burris. "And you do so by running constantly at a couple thousand RPMs. We're not constantly running; we're not constantly moving our joints. We're sitting around the vast majority of the time, so nature has come up with a completely different solution for lubrication."
The tight sucking action between two pieces of cartilage, Burris discovered, is also responsible for a different mystery of cartilage: cracking. When we crack our knuckles or other joints in the body, there's a rupture of the interface when two very sticky cartilage surfaces are suddenly pulled apart.
As Burris and colleagues continue their research on cartilage — and all the mysteries it holds — one thing's for certain: this newest breakthrough in a series of cartilage-related quandaries is the result of 10 years' work, and the common sense around the material may have to catch up to the times.