EXPERIMENTAL brain implants that have allowed paralysed people to control robotic arms by thought alone are dazzling in their potential, but today鈥檚 implants eventually lose their link to the brain. This now seems to be the result of an immune response which severs neural connections near electrodes. Finding drugs or coatings that prevent this process could enable permanent neural prosthetics.
鈥淭he promise is enormous,鈥� says , a bioengineer at the Georgia Institute of Technology in Atlanta. The feasibility of tapping brain signals has been demonstrated, he says, but the reliability to date is 鈥渢oo poor to be clinically useful鈥�.
Previously researchers assumed that the scar tissue that appears around brain implants causes them to fail. But Bellamkonda notes that scars tend to heal weeks before electrodes fail. The timing鈥檚 off, he says.
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When his team implanted electrodes into rats鈥� brains, they noticed that immune cells called macrophages collected around the electrodes four months after surgery. Control rats that merely received a scar-inducing operation 鈥� but no electrode 鈥� attracted far fewer of these cells. Furthermore, neurons adjacent to an electrode tended to lose their connections to surrounding neurons (Journal of Neural Engineering, ). Bellamkonda says that the immune cells severed these connections.
鈥淣eurons adjacent to an electrode tended to lose their connections to surrounding neurons鈥�
Coating electrodes with anti-inflammatories can reduce scarring, but these drugs run out quickly. Materials that somehow enable a more prolonged drug-release 鈥� or coating electrodes in proteins that help to keep neuron connections intact 鈥� might stop connections from breaking down, Bellamkonda says.
at the University of Pittsburgh in Pennsylvania, who develops neural prosthetics, adds that immune cells aren鈥檛 the only challenge. Bone and skin that regrow over implants can also affect their performance.