Brain implants are typically made from metal and other rigid materials that over time can cause inflammation and the buildup of scar tissue. Engineers are 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.
The team also 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 that they could then feed through a conventional 3D printer to make stable, electrically conductive patterns. The team can change the design, run the printing code, and generate a new design in 30 minutes.
Conducting polymers are a class of materials with a 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. The liquid form of these polymers is difficult to use for two-dimensional, high-resolution patterning and is impossible to use for 3D.
The team modified poly (3,4-ethylene-dioxythiophene) 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 that, when they come in contact, act as a sort of tunnel through which any electrical charge can flow. If the polymer was fed 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. The team 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.
The 3D-printed rubbery electrode, about the size of a piece of confetti, consists of a layer of flexible, transparent polymer over which the conducting polymer was printed 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.
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.