Brain implants are vital for the future development of treatments for diseases from Parkinson’s and epilepsy to severe depression. They are also fundamental for helping people with paralysis and locked-in syndrome regain movement and communication.
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However, current implants are constructed from rigid materials that contrast to the soft tissue of the brain, which can cause inflammation and scar tissue to build over time.
Now, a team of engineers at MIT have established a soft, flexible neural implant that is able to flex and mold to the brain’s contours without damaging the tissue. The soft electrodes could be used to monitor brain activity and stimulate brain cells just as the existing electrode arrays do that are made from metal.
3D printing soft electrodes
In a paper published this week in the journal Nature Communications, the team at MIT has outlined how neural probes and devices can be 3D printed, resulting in a structure that is soft and flexible, like rubber.
The team used an electrically conductive soft plastic to construct the electrodes. Usually, the polymer is in the form of a liquid, however, the team devised a way to make the material more similar to the viscosity of toothpaste, which is able to be used in conventional 3D printers.
The paper sees the engineers crafting numerous soft electronic devices from the material. One such device was a rubbery electrode that was implanted into a mouse brain, where it was able to record and monitor the activity of a single neuron, thus demonstrating its potential use in devising methods to treat neurological disorders in humans.
Creating a polymer for use in 3D printers
Scientists have been intensely investigating conducting polymers for a number of years due to their unique properties such as their high degree of flexibility combined with excellent electrical conductivity. Currently, conductive polymers have found uses in various applications, including antistatic coatings, sensors, artificial muscles, and even solar energy conversion.
While polymer solutions are easily used in the form of a spray, making it easily used as a homogenous coating, until now it has proven difficult to use in 2D or 3D structures.
The team saw how the development of a printable conducting polymer would open the door to creating a number of electronic devices, not just single-neuron electrodes and brain electrodes.
To do this, the team modified poly (3,4-ethylenedioxythiophene) polystyrene sulfonate, known as PEDOT:PSS, to create a conducting polymer that can be 3D printed. Usually, the material is in the form of a liquid as it is a mixture of nanofibers and water. It is the nanofibers which give the substance its conductivity.
3D printers cannot work with liquid substrates, so the team devised a way to thicken the material without reducing its conductivity. To thicken the substrate the team freeze-dried the material, having the effect of removing the liquid and leaving behind a nanofiber sponge. Next, the team mixed the nanofibers with water and an organic solvent solution, creating a water-based hydrogel, a rubbery substance with embedded nanofibers.
Various concentrations of nanofibers were used to create a number of hydrogels. The analysis revealed that a 5 to 8% concentration of nanofibers was optimal, creating a substrate with the consistency of toothpaste that could be used in a 3D printer and retained excellent electrical conductivity.
3D printing neurological devices
The team at MIT has established a way to create a wide range of neurological devices from a substrate that is not only highly electrically conductive but also flexible like rubber. The material will overcome the limitations of previous rigid devices, reducing the detrimental impact on surrounding brain tissue.
Also, in utilizing 3D printing techniques, the devices will be able to be created on-demand using a simple and cost-effective method.
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