Sputtered Iridium Oxide Microsupercapacitors Operating in Physiological Electrolytes

Abstract

Implantable devices (e.g. biosensors, neural prosthetics) are increasingly an option for the prevention, diagnosis, and treatment of human diseases and disorders, leading to patient rehabilitation and health improvement. Such devices are typically powered by an external source through wires or an internal battery. Unfortunately, these conventional power sources pose challenges and concerns; while wires are primary sources of infection and patient discomfort, implanted batteries pose safety concerns and suffer from the need for periodic replacements that involve a surgery which is an additional risk. Therefore, there is a need for an alternative power source that safely operates in human body with sustained durability. Supercapacitors are a class of electrochemical energy storage devices that may complement or even replace the implanted batteries. Besides the favorable traits of supercapacitors, including high power density, rapid charging and discharging, and long cycle life, the major advantage of supercapacitors particularly towards implantable applications lies in the possibility for directly utilizing physiological fluid as a source of electrolyte. This makes it unnecessary to house additional electrolyte in a package and thus precludes toxic electrochemical reactions associated with battery electrolytes. Such a package-free configuration also enables significant miniaturization of supercapacitors and may open up a possibility for implantation into previously unexplored body organs such as blood vessel, eyeball, or bladder. Here, we present sputtered iridium oxide microsupercapacitors operating in physiological electrolytes as progress toward human-body implantable energy storage devices. Supercapacitors having micropatterned interdigital electrodes coated with sputtered iridium oxide films are fabricated and characterized in two types of physiological electrolytes: a phosphate-buffered saline (PBS) solution and an inorganic model of interstitial fluid (model-ISF) that mimics the body fluid more closely. These devices exhibit a maximum volumetric capacitance density of 342 F cm-3, an energy density of 47.5 mWh cm-3, and a power density of 40.5 W cm-3 in PBS. It is observed that 55-74% of these values are still available in model-ISF. The energy and power density values obtained here are higher than those for other reported microsupercapacitors characterized in conventional aqueous or solid-state electrolytes. The devices maintain a proportional increase in current density from a scan rate of 20 to 200 mV s-1 and retain 40% of the initial voltage against a self-discharging for 6 hours. They also exhibit a capacitance retention of 100% under 10000 charge-discharge cycles at a scan rate of 200 mV s-1. The demonstrated energy storage performance and the durability in the physiological electrolytes warrant investigation of the proposed microsupercapacitors in biological fluids toward human-body energy storage applications.