A schematic of a “3-D” flexible electrode array. Note the Z-shaped part of the electrode array located between the cranial bone and the brain surface, and the tips (10 micrometers) of the protrusions at the bottom, which serve as recording sites. (credit: Johan Agorelius et al./Front. Neurosci.)
Researchers at Lund University have developed implantable multichannel electrodes that can capture signals from single neurons in the brain over a long period of time — without causing brain tissue damage, making it possible to better understand brain function in both healthy and diseased individuals.
Current flexible electrodes can’t maintain their shape when implanted, which is why they have to be attached to a solid chip. That limits their flexibility and irritates brain tissue, eventually killing surrounding nerve cells and making signals unreliable, says professor Jens Schouenborg.
He explains that recording neuronal signals from the brain requires an electrode that is bio-friendly (doesn’t cause any significant damage to brain tissue) and that is flexible in relation to the brain tissue (the brain floats in fluid inside the skull and moves around whenever a person breathes or turns their head).
“The electrode and the implantation technology that we have now developed have these properties,” he says. Described in an open-access paper in the journal Frontiers in Neuroscience, the new “3-D electrodes” are unique in that they are extremely soft (they even deflect against a water surface) and flexible in all three dimensions, enabling stable recordings from neurons over a long period of time.
Lund University | Breakthrough for electrode implants in the brain
How to implant soft electrodes
But the challenge was how to implant these electrodes in the brain. Visualize pushing spaghetti into a slab of meat. The solution: encapsulating the electrodes in a hard but dissolvable gelatin material, one that is also very gentle on the brain.
“This technology retains the electrodes in their original form inside the brain and can monitor what happens inside virtually undisturbed and normally functioning brain tissue,” said Johan Agorelius, a doctoral student in the project.
This allows for better understanding of what happens inside the brain and for developing more effective treatments for diseases such as Parkinson’s disease and chronic pain conditions, says Schouenborg.
Abstract of An array of highly flexible electrodes with a tailored configuration locked by gelatin during implantation—initial evaluation in cortex cerebri of awake rats
Background: A major challenge in the field of neural interfaces is to overcome the problem of poor stability of neuronal recordings, which impedes long-term studies of individual neurons in the brain. Conceivably, unstable recordings reflect relative movements between electrode and tissue. To address this challenge, we have developed a new ultra-flexible electrode array and evaluated its performance in awake non-restrained animals.
Methods:An array of eight separated gold leads (4 × 10 μm), individually flexible in 3D, were cut from a gold sheet using laser milling and insulated with Parylene C. To provide structural support during implantation into rat cortex, the electrode array was embedded in a hard gelatin based material, which dissolves after implantation. Recordings were made during 3 weeks. At termination, the animals were perfused with fixative and frozen to prevent dislocation of the implanted electrodes. A thick slice of brain tissue, with the electrode array still in situ, was made transparent using methyl salicylate to evaluate the conformation of the implanted electrode array.
Results: Median noise levels and signal/noise remained relatively stable during the 3 week observation period; 4.3–5.9 μV and 2.8–4.2, respectively. The spike amplitudes were often quite stable within recording sessions and for 15% of recordings where single-units were identified, the highest-SNR unit had an amplitude higher than 150 μV. In addition, high correlations (>0.96) between unit waveforms recorded at different time points were obtained for 58% of the electrode sites. The structure of the electrode array was well preserved 3 weeks after implantation.
Conclusions: A new implantable multichannel neural interface, comprising electrodes individually flexible in 3D that retain its architecture and functionality after implantation has been developed. Since the new neural interface design is adaptable, it offers a versatile tool to explore the function of various brain structures.