Stability of thin-film wireless recording and stimulation devices for epilepsy monitoring

The stability of thin-film multielectrode devices fabricated on flexible polyimide substrates was evaluated for wireless subdural recording and stimulation in intracranial electrocorticogram (ECoG) monitoring prior to epilepsy surgery. Devices were fabricated with either 48 or 64 sputtered iridium oxide (SIROF) electrodes. Power and bidirectional data transfer were provided via 121 kHz and 13.56 MHz RF links, respectively. A 64-channel application specific integrated circuit (ASIC) was developed for the devices and provided both the recording and stimulation capability. The ASIC, supporting discrete electronic components, and data/power coils were incorporated on a polyimide substrate and protected from the saline environment by a combination of amorphous silicon carbide (a-SiC) and silicone encapsulants. Soak testing at 37°C in saline under continuous power, and unpowered soak testing at 87°C, were conducted for 29 days and 12 days, respectively. Normal function of the ASIC and device, as measured by the consistency in RMS signal strength and stimulation driving voltage, was observed over the test periods. The charge capacity measured by cyclic voltammetry and the impedance of the SIROF electrodes remained stable over 20 days at 87°C. The results suggest that a-SiC and silicone encapsulants can protect active electronics for implantation periods up to 29 days, the maximum length anticipated for ECoG monitoring in epilepsy patients.