Engineers develop a plastic electrode that stretches like rubber but carries electricity like wires

March 13, 2017 by Shara Tonn
A printed electrode pattern of the new polymer being stretched to several times of its original length (top), and a transparent, highly stretchy “electronic skin” patch forming an intimate interface with the human skin to potentially measure various biomarkers (bottom). Credit: Bao Lab

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.

Chemical engineer Zhenan Bao is trying to change that. For more than a decade, her 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.

Now in Science Advances, Bao's team describes how they took one such brittle and modified it chemically to make it as bendable as a rubber band, while slightly enhancing its electrical conductivity. The result is a soft, flexible electrode that is compatible with our supple and sensitive nerves.

"This flexible electrode opens up many new, exciting possibilities down the road for brain interfaces and other implantable electronics," said Bao, a professor of chemical engineering. "Here, we have a new material with uncompromised electrical performance and high stretchability."

The material 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 .

Flexible interface

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 our brains, special thread-like fibers called axons play a similar role, transmitting electric impulses between neurons. Bao's stretchable plastic is designed to make a more seamless connection between the stiff world of electronics and the flexible organic electrodes in our bodies.

"One thing about the human brain that a lot of people don't know is that it changes volume throughout the day," says postdoctoral research fellow Yue Wang, the first author on the paper. "It swells and deswells." The current generation of electronic implants can't stretch and contract with the brain and make it complicated to maintain a good connection.

A robotic test instrument stretches over a curved surface a nearly transparent, flexible electrode based on a special plastic developed in the lab of Stanford chemical engineer Zhenan Bao. Credit: Stanford University

"If we have an electrode with a similar softness as the brain, it will form a better interface," said Wang.

To create this flexible electrode, the researchers began with a plastic that had two essential qualities: high conductivity and biocompatibility, meaning that it could be safely brought into contact with the human body. But this plastic had a shortcoming: It was very brittle. Stretching it even 5 percent would break it.

Tightly wound and brittle

As Bao and her team sought to preserve conductivity while adding flexibility, they worked with scientists at the SLAC National Accelerator Laboratory to use a special type of X-ray to study this material at the . All plastics are polymers; that is, chains of molecules strung together like beads. The plastic in this experiment was actually 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 these two tightly wound polymers, they could prevent this crystallization and give the plastic more stretch. But they had to be careful – adding material to a conductor usually weakens its ability to transmit electrical signals.

After testing more than 20 different molecular additives, they finally found one that did the trick. It was a molecule similar to the sort of additives used to thicken soups in industrial kitchens. 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 they tested their new material's elasticity, they were delighted to find that it became slightly more conductive when stretched to twice its original length. The plastic remained very 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. 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," Wang says.

Explore further: New research improves conductive plastic for health, energy, other technologies

More information: Yue Wang et al. A highly stretchable, transparent, and conductive polymer, Science Advances (2017). DOI: 10.1126/sciadv.1602076

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6 comments

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Sonhouse
not rated yet Mar 13, 2017
So what kind of conductivity are they talking about, in terms of ohms/square cm or some such measure. 0.01 ohms? 0.1 ohms? 1000 ohms? If it were low enough there would be appications outside neuron engineering.
derphys
not rated yet Mar 13, 2017
Problem : theses products are likely like bispnenyl endocrine disruptors used to make soft plastics ????
Sonhouse
not rated yet Mar 13, 2017
Here is the full text:

http://advances.s...076.full

The conductivity is rated at 4100 Seimens/cm which is about 2 milliohms. A good number. They didn't say how much current it would carry though. It would clearly be more sensitive to higher temperatures.
rustolio
not rated yet Mar 13, 2017
A resistance of 2 milliohms is futile. Temperature could definitely change it, but at body temperature probably not so much.
antialias_physorg
3 / 5 (1) Mar 13, 2017
The conductivity is rated at 4100 Seimens/cm which is about 2 milliohms

The currents you need to activate a neuron are very small (you need about 20mV of trans-membrane potential). You also don't need this continuously. Just breaching the threshold once for a very short time is enough for an activation that lasts for a couple of milliseconds.
Unless we're talking massive amounts of parallel activations (which make no sense from a functional point of view) this will not cause any heat issues.

A massive activation could only be sensible as an emergency measure to stop a circular activation pattern during an epileptic fit. But even there the activation time is very low.
Note: 'Brain defibrillators' already exist with a much cruder (i.e. higher power) approach and they do not damage brain tissue.
SiaoX
not rated yet Mar 13, 2017
theses products are likely like bispnenyl endocrine disruptors used to make soft plastics

Nope, some ionic liquids (organic salts) were used (not quite edible though). The bisphenol plasticizers aren't polar enough, as they're supposed to dissolve in PVC.
A resistance of 2 milliohms is futile.
At this video you can see green LED powered with this polymer and it glows quite brightly (i.e. current in range of few miliampers). Definitely enough for wearable electronics.

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