There is a wealth of clever engineering solutions in nature, too many to list. One of the richest sources of inspiration for mechanical engineers for years has been the snake.
The snake's mesmerising motion provides an obvious cue for robotics engineers. Robot arms that recreate a snake's articulation, such as OC Robotics or Comau's Racer3, are available now for remote handling and working in confined spaces.
However, what's not so widely known about snakes is their use of friction to move across a variety of terrain. If you've ever felt a snake's skin, you will know that the scales all lie in the same direction and are articulated to aid forward motion. Ever seen a snake placed on a smooth surface? They slither wildly, but do not move forward.
Dr Christian Greiner, from KIT university in Germany, has been working in the field of biomimetics for 10 years and investigating the way snakes move for the last 18 months. His latest research has looked at the way the snake's scales reduce friction to help them move.
He then used a standard solid state laser with a one micron base length to etch markings on to the steel pins. The patterns exactly resemble the scales.
When rubbed togetherm the results stunned Greiner and his engineering colleagues. Compared to an unlubricated smooth counterpart, rubbing the steel pins together resulted in 40% less friction.
“If we'd managed just a 1% reduction, our engineering colleagues would have been really delighted. 40% really is a leap forward,” he says.
First applications could be on the micro and nano-scale, such as sensors in car anti-lock braking systems and accelerometers or computer hard disk drives. However, there is no reason why the technique could not be scaled up for larger mechanical devices and materials.
“Scaling up would use the same technique. We might need some different tools – a crankshaft for example would need a laser that can rotate. But laser etching is already used to produce dimples, like on a golf ball, for components in automotive engines and in aerospace applications.”
Lubricants have been around for thousands of years, and are unlikely to be enitrely replaced any time soon, but Greiner says that “there are lots of applications where you can't have a lubricant”.
Harsh environments, such as freezing temperatures or vacuums, or where there is a lot of dust and dirt are ripe for improvement. Reliability is the challenge in such environments. moving parts are sealed, so that lubrication is not contaminated, or a solid state lubricant is used.
Space is the pre-eminent example of a harsh environment – lubricating the International Space Station is a big problem. The snake skin etching technique could significantly increase reliability and simplify designs in these applications.
However, an as of yet fully unresolved question is wear. The research revealed only very minor amounts of wear on the pins. However, it is an area that requires more investigation, says Greiner, for an extended period of time with more measurement.
Another unknown is how the etched surfaces behaviour with lubricants. When lubricated, the etched pins created three times more friction, says Greiner, because of the way the fluid flows in and around the textured surface. This was not unexpected.
“The species we mimicked – the royal python and a lizard called a sandfish skink—live in very dry environments and don’t secrete oils or other liquids onto their skin,” he says. “The biological system is optimised for a non-lubricated environment. More fluid analysis is required.”
A snake's skin resists backwards motion and the next step for the research is to develop polymers that reduce friction in only one direction. It is, Greiner says, early days and this later work is not yet scheduled for publication.
Overall he believes there are still massive opportunities and improvements in looking at nature for tribological purposes and the development of materials and surface coatings. It looks as though the snake will continue to provide rich pickings for engineers eager to innovate.