Engineers 3D print soft, rubbery brain implants

March 30, 2020

The brain is one of our most vulnerable organs, as soft as the softest tofu. Brain implants, on the other hand, are typically made from metal and other rigid materials that over time can cause inflammation and the buildup of scar tissue.

MIT engineers are working on developing soft, flexible neural implants that can gently conform to the brain's contours and monitor activity over longer periods, without aggravating surrounding tissue. Such flexible electronics could be softer alternatives to existing metal-based electrodes designed to monitor brain activity, and may also be useful in brain implants that stimulate neural regions to ease symptoms of epilepsy, Parkinson's disease, and severe depression.

Led by Xuanhe Zhao, a professor of mechanical engineering and of civil and environmental engineering, the research team has now developed a way to 3D print neural probes and other electronic devices that are as soft and flexible as rubber.

The devices are made from a type of polymer, or soft plastic, that is electrically conductive. The team transformed this normally liquid-like conducting polymer solution into a substance more like viscous toothpaste -- which they could then feed through a conventional 3D printer to make stable, electrically conductive patterns.

The team printed several soft electronic devices, including a small, rubbery electrode, which they implanted in the brain of a mouse. As the mouse moved freely in a controlled environment, the neural probe was able to pick up on the activity from a single neuron. Monitoring this activity can give scientists a higher-resolution picture of the brain's activity, and can help in tailoring therapies and long-term brain implants for a variety of neurological disorders.

"We hope by demonstrating this proof of concept, people can use this technology to make different devices, quickly," says Hyunwoo Yuk, a graduate student in Zhao's group at MIT. "They can change the design, run the printing code, and generate a new design in 30 minutes. Hopefully this will streamline the development of neural interfaces, fully made of soft materials."

Yuk and Zhao have published their results in the journal Nature Communications. Their co-authors include Baoyang Lu and Jingkun Xu of the Jiangxi Science and Technology Normal University, along with Shen Lin and Jianhong Luo of Zheijiang University's School of Medicine.

From soap water to toothpaste

Conducting polymers are a class of materials that scientists have eagerly explored in recent years for their unique combination of plastic-like flexibility and metal-like electrical conductivity. Conducting polymers are used commercially as antistatic coatings, as they can effectively carry away any electrostatic charges that build up on electronics and other static-prone surfaces.

"These polymer solutions are easy to spray on electrical devices like touchscreens," Yuk says. "But the liquid form is mostly for homogenous coatings, and it's difficult to use this for any two-dimensional, high-resolution patterning. In 3D, it's impossible."

Yuk and his colleagues reasoned that if they could develop a printable conducting polymer, they could then use the material to print a host of soft, intricately patterned electronic devices, such as flexible circuits, and single-neuron electrodes.

In their new study, the team report modifying poly (3,4-ethylenedioxythiophene) polystyrene sulfonate, or PEDOT:PSS, a conducting polymer typically supplied in the form of an inky, dark-blue liquid. The liquid is a mixture of water and nanofibers of PEDOT:PSS. The liquid gets its conductivity from these nanofibers, which, when they come in contact, act as a sort of tunnel through which any electrical charge can flow.

If the researchers were to feed this polymer into a 3D printer in its liquid form, it would simply bleed across the underlying surface. So the team looked for a way to thicken the polymer while retaining the material's inherent electrical conductivity.

They first freeze-dried the material, removing the liquid and leaving behind a dry matrix, or sponge, of nanofibers. Left alone, these nanofibers would become brittle and crack. So the researchers then remixed the nanofibers with a solution of water and an organic solvent, which they had previously developed, to form a hydrogel -- a water-based, rubbery material embedded with nanofibers.

They made hydrogels with various concentrations of nanofibers, and found that a range between 5 to 8 percent by weight of nanofibers produced a toothpaste-like material that was both electrically conductive and suitable for feeding into a 3D printer.

"Initially, it's like soap water," Zhao says. "We condense the nanofibers and make it viscous like toothpaste, so we can squeeze it out as a thick, printable liquid."

Implants on demand

The researchers fed the new conducting polymer into a conventional 3D printer and found they could produce intricate patterns that remained stable and electrically conductive.

As a proof of concept, they printed a small, rubbery electrode, about the size of a piece of confetti. The electrode consists of a layer of flexible, transparent polymer, over which they then printed the conducting polymer, in thin, parallel lines that converged at a tip, measuring about 10 microns wide -- small enough to pick up electrical signals from a single neuron.

The team implanted the electrode in the brain of a mouse and found it could pick up electrical signals from a single neuron.

"Traditionally, electrodes are rigid metal wires, and once there are vibrations, these metal electrodes could damage tissue," Zhao says. "We've shown now that you could insert a gel probe instead of a needle."

In principle, such soft, hydrogel-based electrodes might even be more sensitive than conventional metal electrodes. That's because most metal electrodes conduct electricity in the form of electrons, whereas neurons in the brain produce electrical signals in the form of ions. Any ionic current produced by the brain needs to be converted into an electrical signal that a metal electrode can register -- a conversion that can result in some part of the signal getting lost in translation. What's more, ions can only interact with a metal electrode at its surface, which can limit the concentration of ions that the electrode can detect at any given time.

In contrast, the team's soft electrode is made from electron-conducting nanofibers, embedded in a hydrogel -- a water-based material that ions can freely pass through.

"The beauty of a conducting polymer hydrogel is, on top of its soft mechanical properties, it is made of hydrogel, which is ionically conductive, and also a porous sponge of nanofibers, which the ions can flow in and out of," Lu says. "Because the electrode's whole volume is active, its sensitivity is enhanced."

In addition to the neural probe, the team also fabricated a multielectrode array -- a small, Post-it-sized square of plastic, printed with very thin electrodes, over which the researchers also printed a round plastic well. Neuroscientists typically fill the wells of such arrays with cultured neurons, and can study their activity through the signals that are detected by the device's underlying electrodes.

For this demonstration, the group showed they could replicate the complex designs of such arrays using 3D printing, versus traditional lithography techniques, which involve carefully etching metals, such as gold, into prescribed patterns, or masks -- a process that can take days to complete a single device.

"We make the same geometry and resolution of this device using 3D printing, in less than an hour," Yuk says. "This process may replace or supplement lithography techniques, as a simpler and cheaper way to make a variety of neurological devices, on demand."
-end-


Massachusetts Institute of Technology

Related Brain Articles from Brightsurf:

Glioblastoma nanomedicine crosses into brain in mice, eradicates recurring brain cancer
A new synthetic protein nanoparticle capable of slipping past the nearly impermeable blood-brain barrier in mice could deliver cancer-killing drugs directly to malignant brain tumors, new research from the University of Michigan shows.

Children with asymptomatic brain bleeds as newborns show normal brain development at age 2
A study by UNC researchers finds that neurodevelopmental scores and gray matter volumes at age two years did not differ between children who had MRI-confirmed asymptomatic subdural hemorrhages when they were neonates, compared to children with no history of subdural hemorrhage.

New model of human brain 'conversations' could inform research on brain disease, cognition
A team of Indiana University neuroscientists has built a new model of human brain networks that sheds light on how the brain functions.

Human brain size gene triggers bigger brain in monkeys
Dresden and Japanese researchers show that a human-specific gene causes a larger neocortex in the common marmoset, a non-human primate.

Unique insight into development of the human brain: Model of the early embryonic brain
Stem cell researchers from the University of Copenhagen have designed a model of an early embryonic brain.

An optical brain-to-brain interface supports information exchange for locomotion control
Chinese researchers established an optical BtBI that supports rapid information transmission for precise locomotion control, thus providing a proof-of-principle demonstration of fast BtBI for real-time behavioral control.

Transplanting human nerve cells into a mouse brain reveals how they wire into brain circuits
A team of researchers led by Pierre Vanderhaeghen and Vincent Bonin (VIB-KU Leuven, Université libre de Bruxelles and NERF) showed how human nerve cells can develop at their own pace, and form highly precise connections with the surrounding mouse brain cells.

Brain scans reveal how the human brain compensates when one hemisphere is removed
Researchers studying six adults who had one of their brain hemispheres removed during childhood to reduce epileptic seizures found that the remaining half of the brain formed unusually strong connections between different functional brain networks, which potentially help the body to function as if the brain were intact.

Alcohol byproduct contributes to brain chemistry changes in specific brain regions
Study of mouse models provides clear implications for new targets to treat alcohol use disorder and fetal alcohol syndrome.

Scientists predict the areas of the brain to stimulate transitions between different brain states
Using a computer model of the brain, Gustavo Deco, director of the Center for Brain and Cognition, and Josephine Cruzat, a member of his team, together with a group of international collaborators, have developed an innovative method published in Proceedings of the National Academy of Sciences on Sept.

Read More: Brain News and Brain Current Events
Brightsurf.com is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to Amazon.com.