Sometimes in research, it takes many twists, turns and time to uncover a simple answer.
Professor Geoff Spinks and his collaborators spent nearly two decades developing exotic materials for use as artificial muscles – with positive results – only to find that the best performing systems could be made from ordinary, everyday fishing line.
Think about it: developing a process on a small scale that will mimic one of the most powerful ‘motors’ in nature is a huge challenge. The idea of a material that expands and contracts like a human muscle is appealing to researchers who are looking for new ways to power robotics, wearable devices and tiny machines that can propel themselves through liquids (such as blood) for use in medicine.
“Muscles are amazing machines that produce much more mechanical power than any similar-sized motor or engine,” Professor Spinks says. “They are compact and silent, can generate large and fast movements and can operate for billions of cycles. Matching all these performances in a synthetic system has been beyond the reach of scientists to date.”
To arrive at a simple answer, scientists at the UOW-headquartered ARC Centre of Excellence for Electromaterials Science (ACES), working with researchers from Canada, Korea and the United States, first had to understand what kind of materials would respond to stimuli like a human muscle and why they responded that way.
The one that almost got away
Professor Spinks remembers well the email he received from his then PhD student Javad Foroughi with the subject line ‘black snake’. “We had sent Javad on an exchange to work with our partners at the University of Texas in Dallas (UTD). They are experts in making carbon nanotube fibres and when combined with our expertise in characterisation, we had developed a way to make artificial muscles from the carbon nanotubes.
“Javad was meant to be taking this work a step further using techniques developed by our Texas partners to create a yarn from both their carbon nanotubes and our conducting polymer materials. Our calculations predicted that we should get a better performance by combining the two materials.”
But as is the case sometimes in research, Foroughi was put to work on something else – the routine task of testing the properties of the new carbon nanotube yarn on its own. “I had the carbon nanotube yarn in a beaker of liquid and I put in an electrode, rather like rigging up a battery, and applied a charge,” Foroughi said.
The fibre started writhing around like a black snake. I had never seen this before and I thought this was very interesting, perhaps even significant.Javad Foroughi
He recorded a short video and sent it back to Professor Spinks in Australia. “I didn’t think it was all that significant,” Professor Spinks says. “But the emails from Javad kept coming. He sent another one with the subject line ‘kangaroo’ showing a piece of the carbon nanotube fibre lifting a small piece of blue tack, up and down, jumping if you will, like its namesake. To his credit, Javad’s persistence and the evidence he collated convinced me that he was indeed onto something worth investigating.”
Back in Australia, Professor Spinks set about trying to understand the source of the rotation and test the performance limits. After a while, he and Javad were able to generate very large and very fast rotations. The new artificial muscle, made from carbon nanotube threads ten times smaller than a human hair, produced a rotating action 1,000 times larger than previously known systems.
When immersed in a liquid electrolyte and with a voltage applied, the thread absorbed some of the surrounding liquid. As it swelled, the untethered end of the twisted yarn started to turn. The amount of rotation was about 2,500 degrees for each centimetre of thread length. On a per weight basis, the thread generated nearly as much power and torque as a conventional electric motor.
Meanwhile, research partners at the University of British Columbia in Vancouver, Canada, had discovered the carbon nanothreads also shortened in length when a voltage was applied. When everyone next met up they put two and two together and realised the shortening and the rotation were a property of the way the thread was structured. It was twisted.
Twisted line hooks catch of the day
The discovery that twisting the fibre resulted in a twisting muscle-like action led to a paper published in Science, but perhaps more significantly, it allowed the team to improve the performance of the artificial muscle and explore other materials.
The UTD team went on to find that similar a twisting action could be produced by filling the yarn with candle wax to make hybrid yarn. Heating the wax generated the torsional or twisting movement. They also observed that overtwisting these yarns generated coils, that when heated, contracted by up to 10 percent.
“We were very excited by this development,” Professor Spinks says. “It meant we had taken our muscle out of the beaker, which opened it up for more applications, particularly in our speciality area of wearable bionic devices.”
At this stage the team did not know why the coiling amplified the change in tension. Further research revealed more, and finally they discovered that similar effects from twisting into coils occurred in polymer fibres that have specific molecular structures.
“The pathway to this discovery was by no means obvious,” Professor Spinks says. “If we had not been developing the carbon nanotubes we would not have observed their very large torsional actuation. That work led us to investigate further the effect of twist and the discovery that overtwist-induced coiling.”
Professor Spinks demonstrated the ‘coiling’ by attaching one end of a piece of fishing line to an electric drill and hanging a weight off the other to apply some tension. The weight was fixed to prevent it from rotating as the drill twisted the fibre.
“At first the twisted fibre shortened but maintained a uniform shape. But at a critical point, a loop or coil formed in the fibre and further twisting produced more coils. Before too long the whole fibre was a spring-like coil. To set this shape we applied a little bit of heat using a hairdryer and let it cool. If we then hung a weight off the polymer coil and applied some more heat, the coil contracted.”
For more convenience and better temperature control, they wrapped a conductive material around the fibre and applied heat by passing a current.
Reeling it in
Ultimately, the international collaboration developed an artificial muscle from fishing line, which was capable of generating a contraction and force that compares favourably with our own muscle. Depending on how the fishing line muscles are made, the contractions can be as high as 50 percent with response times in a fraction of a second.
The fishing line muscles can also produce forces up to 100 times higher than similar-sized natural muscle. The discovery relied on developing a better understanding of how the material structure affected the performance of the artificial muscles. As the researchers introduced new generations of electromaterials, such as conducting polymers and carbon nanotubes, the performance of the artificial muscles improved.
Research continues to further improve the efficiency of the artificial muscle as well as explore stimuli, other than heat, to generate movement. Professor Spinks and his team have also begun to use the technology in wearable devices. They’ve landed a big one, and it will simply be a matter of time before we see this innovation in health devices, ‘intelligent clothing’ and industrial applications.
Which means they could be coming to a vein near you some time soon.