Self-assembling flexible 3D MEAs for cortical implants

Introduction Flexible implants are known to reduce the mechanical mismatch between the soft brain tissue and the electrode arrays, allowing accurate signal recordings and neural stimulation and reducing inflammatory responses. Closer proximity to neurons in deeper layers allows selective stimulation of cells of different depths. In addition, a high density of penetrating electrode-shafts is necessary to ensure high resolution. The self-assembling of the shafts reduces the cost and workload, however, the sufficient stability of the shafts is a challenge. Methods In first trials gold disc electrodes (10 to 20 mm) are patterned between two polyimide layers (PI2611 HD Microsystems) forming the flexible substrate. Different structuring processes for the polyimide using wet development and dry etching are used to expose shafts with one to two electrodes each. Pits are patterned on one side of the shafts (5-600 mm), which are filled with polyimide that contracts due to thermoplastic behavior during polycondensation at 400°C in nitrogen athmosphere. This causes the shafts to stand up from the x-y plane into the z-direction. Results It was observed that shafts lifted it self up automatically in different angles up to 42°. Statistical data analysis has shown that there is a correlation between the direction and strength of the nitrogen flow of the oven and the lifting mechanism. In addition, it was identified that the polyimide also shows a positive influence on the size of the set-up angle. Conclusion The proposed method of fabricating self-assembling three-dimensional MEAs with PI is very promising and can be used to process MEAs with a high amount of penetrating electrode shafts. Shaft length up to 1.5 mm could be lifted up into the z-direction. In further investigations it will be tested to increase the lifting angle up to 90° to assure an easy implantation of the electrodes.