Highly Flexible Thread-Based Eutectogel Organic Electrochemical Transistors

Electronic devices made from organic materials and polymer systems have attracted much attention across bioelectronic applications that interface closely with the human body, ranging from health monitoring devices to consumer wearables. Versus conventional silicon, organic transistors and electronics realized on flexible substrates can more closely resemble the mechanical properties of the underlying human skin, for improved comfort and accuracy when recording biosignals. Particularly, organic electrochemical transistors (OECTs) have emerged as promising bioelectronic active components, due to their biofriendly composition, facile processing, and high transconductance. Often aqueous electrolytes are used for gating through formation of electrolytic double layers. However, such transistors are volatile and prone to degradation from dehydration over time.<br/>In this work, we utilize a two-prong approach to address this challenge. Firstly, the electrolyte used is a gelatin-supported deep eutectic solvent gel electrolyte or “eutectogel,” rather than an archetypical aqueous electrolyte solution. The eutectogel, comprised of choline chloride and ethylene glycol supported by 20 wt.% gelatin, provides a nonvolatile electrolyte for long term device usage without further encapsulation, and the gel support scaffold provides mechanical stability (ie can act as the substrate itself) and leakproof operation of the device. The resulting OECT devices show excellent device performance and stability of transistor operation in ambient conditions without further coatings over several days or weeks. Further, experiments suggest excellent water vapor permeability, which would allow the gel to sit on-body and still allow for the underlying skin to breathe. The second factor in our design is an active channel fiber, which contains PEDOT:PSS between source and drain electrodes deposited via a novel cleanroom-free electroless deposition approach. The fiber adds additional geometric freedom, such that the active channel can be placed anywhere within the eutectogel for transistor action, for more conformal electronics.<br/>The presented transistors operate at low voltage (within ~1 V), exhibit a maximum transconductance and ON/OFF ratio of 16 mS and 10³, respectively, for the champion device, and an average transconductance efficiency of 4.7±1.2 V^-1 at the applied biasing point of 0 V. A hydrogel equivalent OECT was fabricated and compared with the eutectogel OECT. The hydrogel OECT lost 75% volume over 72 hours and the max transconductance exhibited a 94% change after 24 hours. Conversely, the eutectogel OECT observed no measurable change in volume and only a 12% change in max transconductance over 7 days, likely due to the instability of PEDOT:PSS in ambient conditions itself. This novel device design and construction of 3D electronic circuits could lead to future implementations of smart wearables that better resemble the body it interfaces with.