Why is the human brain spectacular? Filled with billions of neurons, it is impressive beyond the capability of any computer, fluidly taking in data, processing it, and acting—all with the assistance of your senses and your muscles, as you think and move. Consider all that goes on in your mind in a single day, and all that you do—powered by continual activity—and imagine the actual brainpower scientists must expend to come up with biocompatible material that can mimic tissue and human organs.
While surgical implants have been in use for nearly sixty years, they are often made of metal that can cause scar tissue and inflammation to accumulate. As the medical field evolves, scientists are continually looking for improved ways to make implants that are safe and effective. Now, a team at MIT has made implants that are not only soft enough for the human body, but conductive enough to interact with the human brain.
The researchers, led by Professor of Mechanical and Civil and Environmental Engineering Xuanhe Zhao, are 3D printing with soft, rubber-like and conductive polymers that can be used to monitor neural function as well as stimulate different areas of the brain for patients suffering from nervous system disorders like Parkinson’s, conditions like epilepsy, depression, and more.
Recent research by other scientists has also yielded 3D-printed GlioMesh hydrogels for the treatment of rare and aggressive brain cancer, implants for healing serious brain injuries, and even other items like 3D-printed mouse skulls for the purposes of Alzheimer’s research. Progressive studies like these are meant to offer a better quality of life for patients, and in some cases, they may even save lives.
In this study, the researchers have created neural probes, with all of the devices involved made from conductive polymers converted from a liquid to a material described as similar to a thick toothpaste. This allows for extrusion via a 3D printer and the fabrication of conductive patterns.
Key to the research was the conversion of the typically liquid conducting polymer into a more viscous material, capable of printing with a modified desktop system. This material is conventionally used as a coating for spray-on electrical devices, such as touch screens, but cannot be easily printed in 2D, let alone 3D. By mixing nanofibers of the conducting material—poly (3,4-ethylenedioxythiophene) polystyrene sulfonate, or PEDOT:PSS—with water and an organic solvent, the team was able to create a printable hydrogel concoction.
Among the parts printed was a small, confetti-sized electrode made up of a flexible, clear polymer onto which the conducting polymer was printed in lines just 10 microns thin. The device was then inserted into the brain of a mouse, with a single neuron monitored by the 3D-printed probe as the mouse moved about its surroundings. Gathering data with this type of testing allows scientists to customize medical treatments and create implants for a variety of neurological ailments.
Due to its soft nature, the electrode is more appropriate for implantation than typical electrodes made up of stiff metal wires and a needle probe. Moreover, because metal electrodes conduct electricity via electrons, a conversion process is needed to convert the ions produced by neurons into the proper electrical signal. In contrast, the water-based nature of the 3D-printed devices allows ions to pass through easily.
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.
The researchers also produced an array made up of multiple electrodes covered with a round plastic well to demonstrate the feasibility of fabricating neuron cultures for studying neural activity using the electrodes beneath.
“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.”
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