Soft, rubbery brain implants out of the 3D printer
In the future, such biocompatible flexible electronics could be used beyond brain activity monitoring, to actively stimulate neural regions and ease symptoms of epilepsy, Parkinson’s disease, and severe depression.
designed a soft and flexible neural electrode.
Images: courtesy of the researchers.
Led by Xuanhe Zhao, a professor of mechanical engineering and of civil and environmental engineering, the research team has developed a 3D-printer compatible conducting polymer hydrogel that they can use to 3D print neural probes and other electronic devices that are as soft and flexible as rubber.
In a paper published in the Nature Communications journal, the researchers report the printing of 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.”
Key ingredient to the 3D-printable toothpaste looking hydrogel are modified PEDOT:PSS nanofibers, providing the ink’s conductivity. The researchers made hydrogels with various concentrations of nanofibers until they got the proper consistence for use in a 3D printer. Even intricate patterns produced with a conventional 3D printer proved to remain stable and electrically conductive. One of the rubbery electrodes they produced was about the size of a piece of confetti, consisting of a layer of flexible, transparent polymer over which was printed the conducting polymer, in thin, parallel lines that converged at a tip, measuring about 10 microns wide. The tip was designed small enough to pick up electrical signals from a single neuron as they proved through in-vivo experiments.
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, explain the researchers. Any ionic current produced by the brain needs to be converted into an electrical signal that a metal electrode can register. 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 soft electrode is made from electron-conducting nanofibers, embedded in a hydrogel through which ions can pass freely. This makes the whole electrode volume active, increasing its sensitivity.
of individual neurons. Images: courtesy of the researchers.
The MIT researchers also fabricated a multi-electrode array with very thin printed electrodes, surrounded by a round plastic well. Such contraptions would typically be used by neuroscientists to study the activity of cultured neurons through the signals that are detected by the device’s underlying electrodes. With device, the researchers showed they could replicate within an hour the complex designs of electrode arrays traditionally designed with costly and time-consuming lithography techniques, often involving mask design or metal etching.
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