Rapid Prototyping of 2D Transistors via Gel and Skin

Characterization of 2D materials directly on their growth substrates is crucial for advancing their applications in bioelectronics and flexible electronics as well as fundamentally understanding the as-grown material properties. Traditional methods often require transferring these materials to different substrates, a process that can introduce defects, alter their properties, and limit their practical use. Characterizing 2D materials without transferring them preserves their intrinsic qualities and provides a more accurate understanding of their behavior in real-world conditions. This approach is particularly important for developing wearable technologies, such as graphene tattoos, where the materials must maintain their integrity and performance when integrated with flexible and soft substrates like human skin.<br/>Here, we present a unique system to study 2D material properties in a mechanically soft, flexible, and realistic environment suitable for future wearable applications. The system comprises commercially available medical-grade gel electrodes, with the embedded Ag/AgCl acting as the reference electrode similar to those used in electrolytic field-effect transistors. The hydrogel serves as the electrolyte, with the 2D material, such as graphene, simply placed on top of the gel. These gel electrolytes are extremely robust under mechanical strain, flexible, exhibit insignificant gate leakage currents, and maintain high stability across a broad temperature range (-50C to +110C). The gels are reusable, with samples stored in ambient conditions for over three years without degradation.<br/>Moreover, we show that utilizing human skin as an amplifier for transistor signals is feasible. By using the hypodermis as an electrolyte, we demonstrate that graphene transistor tattoos biased through the body exhibit remarkable charge carrier mobility up to 6500 cm2/V/s. Before testing on the skin, we validated our approach with low-cost, commercially available Ag/AgCl gel pads, which proved to be reliable test beds for transistor characterization. Using these gels, we introduced a novel approach for rapidly probing electronic properties. Similar to graphene, we demonstrated impressive performance with PtSe2-based transistors and MoS2-based transistors characterized via skin.