Electrotherapeutic devices require an electrode for coupling with the body. The most common electrodes are made of conducting corrosion resistance materials (e.g., TiN, Ir-IrO2, Pt) plus a coupling layer (e.g., electrolyte). The electrode is the location where redox reaction take place between the device and the tissue. As such, it must conduct both electrons and ions. The reactions can be capacitive, involving the charging and discharging of the electrode-electrolyte double layer, or faradaic. Capacitive charge-injection is more desirable than faradic charge-injection because no chemical species are created or consumed during a stimulation pulse. Most noble metal based electrodes are faradic or pseudo-capacitive, which can lead to performance changes over time. In addition, under the high rate of charge injection and high current density conditions of a neuromuscular stimulation pulse, access to all the accessible charges is limited by the interfacial resistance and low surface area at the electrode [1]. A particularly critical point is the passage of current between the surface of the skin and the electrical contact connected by wire to the device, which requires a low stable resistance that does not vary with time, humidity [2].
We have developed new hybrid mixed-ionic-electronic conductors (MIECs) that have the potential to overcome these deficiencies. The MIECs are an interconnected network of electrical and ionic conductors in an elastomeric matrix that provide: (1) high surface area for efficient capacitive charge-discharge; (2) high ionic conductivity for low interfacial resistance; (3) low ohmic resistance; and (4) excellent flexibility and toughness. Carbon nanotubes (CNTs) are the electrical conductors in the MIEC and hyaluronic acid (HA), along with moisture and ions, is the ionic conductor. Unlike the current state-of-the-art, conducting noble metals, this system exhibits good mechanical properties, high conductivity (up to 3000 mS/cm), high moisture retention (up to 100wt%) and high ion mobility, leading to facile electrode kinetics. This simple yet efficient system is promising for the fabrication of a variety of high performance flexible electrodes.
Molecular Design Strategies toward Improvement of Charge Injection and Ionic Conduction in Organic Mixed Ionic–Electronic Conductors for Organic Electrochemical Transistors