One of our most vulnerable organs, the brain is soft, while brain implants are typically made from metal or other rigid materials that can eventually cause inflammation and scar tissue buildup. In response, Massachusetts Institute of Technology (MIT) engineers are developing soft, flexible neural implants that can conform to the brain’s contours and monitor activity without aggravating surrounding tissue. Such alternatives may also be useful in 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 civil and environmental engineering, the research team has developed a way to 3D print neural probes and other electronic devices that are as soft and flexible as rubber.
The team transformed the consistency of an electrically conductive polymer from a liquid-like substance into something closer to viscous toothpaste. That material can be fed through a conventional 3D printer to make stable, electrically conductive patterns.
The team modified 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 PEDOT:PSS nanofibers. Nanofibers provide conductivity when they come in contact, act as a sort of tunnel through which any electrical charge can flow.
To thicken the polymer while retaining the material’s inherent electrical conductivity, the team first freeze-dried the material leaving behind a dry matrix of nanofibers. Then they remixed the nanofibers with a solution of water and an organic solvent to form a hydrogel.
They made hydrogels with various concentrations of nanofibers and found that a range between 5% to 8% by weight of nanofibers produced a material that was both electrically conductive and suitable for feeding into a 3D printer.
As a proof of concept, they printed a small, rubbery electrode, about the size of a piece of confetti. It consisted of a layer of flexible, transparent polymer, over which they printed the conducting polymer, in thin, parallel lines that converge at a tip, measuring about 10µm wide. They implanted the electrode inside a mouse’s brain and found it could pick up electrical signals from a single neuron.
In principle, such soft, hydrogel-based electrodes might be more sensitive than conventional metal electrodes because most metal electrodes conduct electricity in the form of electrons, whereas neurons in the brain produce electrical signals in the form of ions.
“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,” says Baoyang Lu, one of the co-authors. “Because the electrode’s whole volume is active, its sensitivity is enhanced.”