SAN FRANCISCO, March 10 (Xinhua) -- A research team at Stanford University has taken a type of brittle plastic and modified it chemically to make it as bendable as a rubber band, while slightly enhancing its electrical conductivity.
The result, as reported in Science Advances, is a flexible electrode compatible with human's supple and sensitive nerves.
"This flexible electrode opens up many new, exciting possibilities down the road for brain interfaces and other implantable electronics," said Zhenan Bao, a professor of chemical engineering with the U.S. university in Northern California. "Here, we have a new material with uncompromised electrical performance and high stretchability."
The brain is soft and electronics are stiff, which can make combining the two challenging, such as when neuroscientists implant electrodes to measure brain activity and perhaps deliver tiny jolts of electricity for pain relief or other purposes.
For more than a decade, Bao's lab has been working to make electronics soft and flexible so that they feel and operate almost like a second skin.
Along the way, the team has started to focus on making brittle plastics that can conduct electricity more elastic. The material they have now is still a laboratory prototype, but the team hopes to develop it as part of their long-term focus on creating flexible materials that interface with the human body.
Electrodes are fundamental to electronics. Conducting electricity, these wires carry back and forth signals that allow different components in a device to work together.
In human brains, special thread-like fibers called axons play a similar role, transmitting electric impulses between neurons. The current generation of electronic implants can't stretch and contract with the brain and make it complicated to maintain a good connection.
"One thing about the human brain that a lot of people don't know is that it changes volume throughout the day. It swells and deswells," postdoctoral research fellow Yue Wang, the first author on the paper, was quoted as saying in a news release. "If we have an electrode with a similar softness as the brain, it will form a better interface."
Designed to make a more seamless connection between the stiff world of electronics and the flexible organic electrodes in human bodies, Bao's stretchable plastic started with two essential qualities: high conductivity and biocompatibility, meaning that it could be safely brought into contact with the human body.
However, it was very brittle, and stretching it even 5 percent would break it.
The plastic was made up of two different polymers that were tightly wound together. One was the electrical conductor. The other polymer was essential to the process of making the plastic.
When these two polymers combined, they created a plastic that was like a string of brittle, sphere-like structures. It was conductive, but not flexible. The researchers hypothesized that if they could find the right molecular additive to separate the two polymers, they could prevent this crystallization and give the plastic more stretch.
After testing more than 20 different molecular additives, they found one that did the trick. It was a molecule similar to the sort of additives used in soap. This additive transformed the plastic's chunky and brittle molecular structure into a fishnet pattern with holes in the strands to allow the material to stretch and deform.
When its elasticity was tested, the new material became slightly more conductive when stretched to twice its original length, and it remained conductive even when stretched 800 percent its original length.
"We thought that if we add insulating material, we would get really poor conductivity, especially when we added so much," said Bao, referring to a common understanding that adding material to a conductor usually weakens its ability to transmit electrical signals.
But thanks to their precise understanding of how to tune the molecular assembly, the researchers got the best of both worlds: the highest possible conductivity for the plastic while at the same transforming it into a very robust and stretchy substance.
"By understanding the interaction at the molecular level, we can develop electronics that are soft and stretchy like skin, while remaining conductive," said Wang.