Fully Bioresorbable and Flexible Electrocorticography (ECoG) Array for Low Invasive Monitoring of Epileptic Seizures

Abstract

Implantable, bioresorbable, and flexible bioelectronics are leading the evolution towards non-invasive, high fidelity next-generation implanted devices, especially for recording acute/chronic neurophysiological signals from the brain. These devices promise to reduce foreign body response to implanted devices, saving neural tissue and improving recording fidelity and stability, extending implant life, and eliminating risks associated with surgical resection of implanted devices in acute recordings. Electrocorticography (ECoG) recordings are used in assessing the completeness of the treatment in patients with epilepsy. The latter is a central nervous system disorder in which brain activity becomes abnormal, causing seizures (a sudden, uncontrolled electrical disturbance in the brain) that lead to changes in behavior, movements, or feelings and sometimes loss of awareness. State-of-the-art devices for ECoG monitoring of seizures in patients with epilepsy are microelectrodes fabricated on rigid substrates with a high elastic modulus compared to the brain tissues. These implanted devices carry a high risk of failure and tissue damage due to mechanical mismatch with the soft brain tissue. Also, there is additional risk and cost to the patient when device resection is necessary. In this work, we designed and developed inkjet-printed microelectrode arrays on a flexible and biodegradable substrate for monitoring epilepsy. We fabricated the devices using a low-cost, additive inkjet printing process. The device has low-impedance gold microelectrodes on both polyimide (flexible substrate) and polycaprolactone (PCL, biodegradable substrate). We optimized the number of printed gold layers and parameters for a light-based sintering approach, achieving a minimum sheet resistance of 2.84 Ω/sq on PI at 3 layers and 2.54 Ω/sq at 9 layers on PCL. The mean impedance of 100 x 100 μm electrodes on PI and 300 x 400 μm electrodes on PCL measured 34.4 kΩ and 30.1 kΩ respectively. We recorded high fidelity epileptic seizure activity on multiple channels per device in an in vivo rat seizures model, where seizures were pharmacologically induced. Lastly, an accelerated degradation test was performed to assess the bioresorbable potential of our electrodes built on PCL. The work overall demonstrates the potential of inkjet printing for building low invasiveness, bioresorbable ECoG devices for applications in implanted neural interfaces. The scalability of the technology and approach used to build these devices also opens the door for potential near-future translation to clinical applications.