(Invited) Development of Flexible Conductive Polymer Electrodes for Recording and Stimulation of Neural Tissue
Authors: Merrill Daniel , Thiessen Alexander , Smith Christopher , McDonnall Daniel 
Published: February 2020

We describe the development of thin, flexible conductive polymer electrodes for recording and stimulation of excitable tissue including brain, spinal cord, peripheral nerve, and muscle. The electrode material characteristics are intended to mitigate the foreign body response. Manufacturing is based on 3-D robotic deposition of alternating layers of insulating polymer, and polymer doped with electrically conductive particles forming the electrodes and interconnections. The robot can produce multilayer structures and supports synthesis of the electrode array with integrated electronics at low cost. We have performed in vivo and in vitro testing, including flexural tests which demonstrate electrical and mechanical reliability for over 100 million flexions. The capacitive nature of the electrodes provides reversible charge storage capacity that is comparable with other high charge capacity materials.

Manufacturing Process: The robotic system first deposits an insulating substrate using a silicone co-polymer which is well established as a chronic implant material. After the substrate cures, a layer of conductive ink based on polymer doped with electrically conductive particles is deposited to form the electrodes and interconnections. Next, an overcoat of insulating polymer is deposited. This process may be repeated to form multilayer structures, similar to the process of printed circuit board manufacturing. Because the ink is mostly polymer by volume fraction, an entire structure has mechanical properties of a homogeneous polymer. This mechanical matching of insulating and conducting phases provides high flexibility and flexural durability. Composite structures with features as small as 100 µm trace width, 200 µm pitch and 30 µm thin insulating layers are achieved with high reproducibility including optional connection directly onto printed circuit boards.

Electrode Testing
: Mechanical and electrical performance of the devices has been verified through mechanical compression, stretch, and bend tests, and accelerated lifetime soak testing. These stress tests have verified the conductivity of the traces and electrodes and the insulation of the polymer for over 100 million bend cycles (tested per EN 45502-2) and lifetimes equivalent to over 400 days in vivo. The electrodes have also been successfully validated in chronic in vivo studies in rat, cat, rabbit and pig models.

Electrical Characterization
: DC resistances of the ECoG array vary with trace length and thickness, and are generally on the order of several hundred ohms over several inches of trace. Electrical impedance spectroscopy (EIS) was performed after soaking the arrays in saline solution for at least 12 h, and yielded trace-to-saline impedances below 10 kΩ for frequencies in the range of 0.2 Hz to 1 MHz. Cyclic voltammetry was performed with a Pt counter electrode and a fresh Ag/AgCl reference electrode with a scan rate of 20 mV/s. The reversible charge storage capacity (RCSC) increased with cycling as is common with other materials used for stimulation. After several cycles, the RCSC is on the order of 1 – 1.5 mC/cm2, comparable with other high charge storage capacity materials. This is consistent with the capacitive nature of the platinum embedded within dielectric polymer.

Demonstration of Chronic Wireless ECoG Recording
: We have developed a wireless recording system used in conjunction with the flexible electrode array. A four-electrode array was implanted subdurally over sensory cortex in swine for chronic ECoG monitoring. Leads from the array passed to a subcutaneous transmitter placed between the scapulae. The transmitter sent data to an external transceiver via infrared transmission. We demonstrated wireless transmission of evoked ECoG recordings over 63 days as the paw was electrically stimulated.