A team of MIT engineers that is developing flexible neural implants has found a way to 3D print neural probes and electronic devices that are rubbery and soft in quality. The innovative devices, made from an electrically conductive polymer, could offer a safer alternative to conventional brain implants and metal-based electrodes, which can cause inflammation and scar tissue build up.
Protected by the hard skull, the human brain is a soft and fragile organ. Despite this, metals and other hard materials are still conventionally used for brain implants, which can lead to a number of problems. To overcome the problems associated with hard brain implants, a team led by Xuanhe Zhao, MIT professor of mechanical engineering and civil and environmental engineering, has been investigating the potential of soft and flexible neural implants, which can conform to the brain without inflaming or aggravating the tissue.
The team recently made a significant advancement in its research by developing a way to 3D print neural probes and other electronic devices using a soft polymer with conductive properties. The material, which originally had a liquid-like consistency, was made more viscous so it could be extruded using a conventional 3D printer to make stable and conductive patterns.
The printable polymer was adapted from liquid polymer solutions that are typically sprayed onto touchscreens and other devices as an antistatic coating. The material in question, PEDOT:PSS, had to be thickened so it could be formed in a 3D pattern. To achieve a more viscous conductive polymer, the team freeze-dried the liquid material, removing the liquid to result in a dry matrix of conductive nanofibers. These brittle nanofibers were then mixed with a hydrogel made of water and an organic solvent.
After testing several hydrogel and nanofiber concentrations ratios, the team found that a range between 5-8% by weight of nanofibers resulted in a toothpaste-like material that was both printable and electrically conductive. “Initially, it’s like soap water,” Zhao said. “We condense the nanofibers and make it viscous like toothpaste, so we can squeeze it out as a thick, printable liquid.”
To demonstrate the technique, the team 3D printed a number of soft electronic devices, including a rubbery electrode, which was implanted into the brain of a mouse. The neural probe was able to monitor activity from a single neuron as the mouse ran about. “Traditionally, electrodes are rigid metal wires, and once there are vibrations, these metal electrodes could damage tissue,” Zhao added. “We’ve shown now that you could insert a gel probe instead of a needle.”
The flexible neural implants could actually provide scientists with a higher-resolution picture of the brain’s activity, because it is made from a water-based material that ions (the type of electrical signal the brain produces) can pass through. With metal implants, by contrast, ions must be converted to electrons to be read, which can result in a loss of some signals.
The 3D printed neural implants could ultimately help in the development of customized therapies for patients with neurological disorders. The 3D printed devices also have the potential to be used as long-term brain implants.
“We hope by demonstrating this proof of concept, people can use this technology to make different devices, quickly,” explained Hyunwoo Yuk, a graduate student in Zhao’s group. “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.”