Soft Thin-Film Battery Using Mixed-Conducting Particulate Composites for Bioelectronics

Recently, there is a great interest in developing biocompatible and flexible power-source in vivo neuro recordings because tethered and externally powered devices that interface with neural tissues can restrict natural motions and prevent social interactions in animal models. Most of the neuro recording devices generally rely on a widespread commercial electrochemical power source, such as Li-ion battery or supercapacitor, or similar rigid and bulky system for a wireless transmitter, such as RF power transfer, ultrasonic transduction, energy harvesting device, and magnetic resonant coupling device. However, these rigid power sources are not comfortable enough to be implanted in the neural tissue and could cause irritation, infections, and unnatural motion by reducing the motion freedom, particularly in small animals. Thereby, size, weight, and biocompatible materials are the key limiting features. <br/> In this work, we demonstrated a novel structure of highly conformable and biocompatible rechargeable thin-film battery using conducting polymer poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) as an anode, chitosan scaffolding in cellulose matrix as an electrolyte with separator, and mix-conducting Zn particulate composite as a cathode. Zinc is a trace element in animals that has dietary value and chitosan (CS) is a polycationic biopolymer, naturally found in the shells of shrimp. For the fabrication of cathode, Zinc microparticles (&lt; 45μm) were dissolved in CS-based polymers to create an ion-conducting scaffolding polymer matrix. The purpose of CS-based polymers is to enhance the conductivity (water uptake), contact resistance, and adhesiveness to the separator and current collector. The mixture composite provided reliable, flexible mechanical bonding between conformable substrates due to the bio-adhesive properties of CS. Note that the Zn concentration (w/v) determines the capacity of the battery. Each Soft thin-film battery layer was integrated through a blade coating onto a polyimide with Au current collector. The battery showed about 1.2V at an open-circuit voltage which was higher than theoretical the work function difference (0.74 ~ 0.8 eV) between PEDOT:PSS and Zn. The soft thin-film battery with an Au current collector for both the anode and the cathode with a low sheet resistance (~ 5 Ohm/sq) functions shows lightweight (~ 0.04 g/cm²), low solid electrolyte interphase impedance (~ 20 Ohm), and excellent flexibility. After packaging, the rechargeable soft thin-film battery, despite being thin (~200 μm), exhibits high conformability with high reliability and a high energy density (~ 10 mWh/g). In addition, we successfully integrated the battery to power the organic electrochemical transistor circuit to the amplified signal. This result shows the feasibility of realizing a fully implantable medical device along with soft thin-film batteries for signal processing or continuous neural recording in the future.