Design of a Fully-Resorbable Soft Electronic Device for Neural Stimulation and Monitoring

Currently, clinical electrocorticography (EcoG) electrode matrices are embedded in inelastic silicone rubber. For subchronic studies, neural implants need to be removed during a second surgery in order to limit immune system reactions or infectious risks. However, any surgery involves risks for the patients and can lead to complications. The development of transient electronic devices opens the way to new medical possibilities for mid-term post-surgical follow-up, such as electrophysiological monitoring, on-demand and controlled drug delivery, or electrical stimulation in tissue regeneration. Implants able to degrade harmlessly inside the body eliminate the need of removal surgery with associated risks. Compared to non-degradable devices, they potentially reduce inflammation responses and chronic adverse conditions that could decrease device functionality.<br/>A bioelectronic device consisting of poly(lactic-co-glycolic acid) (PLGA) films as insulating substrates, sputtered molybdenum (Mo) as conductive tracks, and conductive polymer electrodes for tissue interface, has been fabricated using microfabrication techniques. With their mixed ionic/electronic conductivity and tunable mechanical, electrical, electrochemical, and biological properties, conductive polymers are materials of choice for the design of the conductive electrodes of such transient devices. Herein, resorbable materials based on sulfated hyaluronan modified with phenylboronic moieties (HAS-PBA) as dopants of poly(3,4-ethylenedioxy)thiophene (PEDOT) were specifically developed for this application.<br/>The thickness of the entire device is 40 µm thick with Mo tracks of only 500 nm, whereas the length and width are respectively 9 mm and 4 mm. Electrical impedance spectroscopy showed that the PEDOT:HAS-PBA layer reduced the electrode impedance (580 Ω at 1 kHz), providing a better interface than Mo metal alone (3400 Ω at 1 kHz). The charge storage capacity of electrode material showed an increase from 5 mC/cm² to 26 mC/cm² for Mo and PEDOT:HAS-PBA, respectively. The device was developed for a working period between 1 and 2 months, and a complete resorption around 3 months. In vitro characterization of device stability for electrophysiological monitoring followed by device degradation was carried out over a 3-month period. The next step will be to implant the device subdurally in contact with rat brain cortex for electrical stimulation and recording, and to explore its bioresorbability properties <i>in vivo</i>.<br/>In summary, we describe the development of a thin and flexible device that is suitable for different applications such as pre-surgical localization of epileptogenic foci or post-surgical monitoring of brain activity. The lifetime of our device is perfectly adapted to these applications.