Intracortical microelectrodes currently have the greatest potential for achieving a functional neural prosthesis in patients with neurodegenerative diseases or spinal cord injury. Device efficacy is lacking in long-term performance as seen in both chronological histology and biopotential recording studies.
Some researchers have shown that small single polymer fibers (less than 7-μm diameter) do not induce an encapsulation layer in the rat subcutis so we have extended this concept to neural probe design. In this experiment we investigated the brain-tissue response of polymer probes with 4-μm feature sizes that are capable of withstanding insertion forces when penetrating the rat neocortex. This polymer probe has both a stiff penetrating shank (70-μm by 42-μm) and fine polymer structures (4-μm by 5-μm) that extend laterally from the shank. Our testing verifies that despite a flexible substrate and small dimensions, these devices are mechanically robust and practical as neural probes. We developed a microfabrication process using SU-8 and parylene to create the relatively thick probe shank and thin lateral arms.
In vivo testing was conducted on seven Sprague-Dawley rats. These parylene devices were chronically implanted in the motor cortex for 4-weeks and then imaged using fluorescence microscopy. Cellular encapsulation and neuronal loss were assessed using a Hoechst counterstain and the immunomarker NeuN (neuronal nuclei).
The tissue reactivity immediately around the fine-feature structures is greatly reduced, showing mild cell encapsulation (90±68% increase) relative to the probe shank (460±320% increase). Neuronal loss was only (21±25%) out to 25-μm relative to significant loss around the probe shank (47±19%). Additionally, laminin+, fibronectin+, and Ox42+ tissue often showed greater intensity and thickness at the shank, indicating that the dense scar formation typical of cortical implants was mitigated around the fine lateral structure.
These results suggest that using MEMS-based microfabrication to create sub-cellular structures will significantly reduce encapsulation, which should extend the longevity of neural probes. We also believe this concept could be beneficial to any implantable sensor capable of scaled geometries.