A Corn-Based Conductive Glue as Multi-Functional Tool for Edible Electronics: Interconnecting Devices and Food Monitoring

Edible electronics is a research field focused on developing disposable functional devices wherein all constituent components, including powering devices, electronic circuits and actuators among others, are designed to be safely ingested and processed by the gastrointestinal track.^[1] Numerous high-impact applications would be facilitated by the development of complex functional electronic devices that can be handled similarly to food, from a safety and recyclability perspective. These applications include food quality monitoring systems to prevent food spoilage and smart capsules for diagnosing and treating gastrointestinal disorders.^[1,2] The swift evolution of the field has already led to the development of distinct edible electronic devices as edible sensors,^[3] antennas,^[4] batteries^[5] and printed transistors.^[6] However, finding additive strategies for the assembly of these separate components is a fundamental and as-yet-largely unexplored aspect of manufacturing complex edible electronics. A main challenge is finding alternatives to soldering and wiring widely used in traditional electronics, which should provide efficient and stable mechanical and electrical interconnections between all components while being compliant with edibility constraints, including toxicity, size, and shape factors.
In this work, we have developed an edible electrically conductive adhesive made from edible ingredients and with a small footprint representing a unique enabler for interconnecting edible components. Our adhesive is made out of zein (a prolamine protein found in corn^[7]) as adhesive matrix and activated carbon (food additive E 153) as conductive filler. In particular, the formulated acrylic-like viscous ink is compatible with several deposition techniques, such as ink brushing and blade-coating. Our results show that incorporating activated carbon in zein does not only confer electrical conductivity, with a resistivity of 3x10³ Ω cm, but also it increases the overall mechanical robustness and adhesiveness with a twelve-fold enhancement in adhesive strength, reaching up to approximately 2 MPa. As a proof-of-concept, the conductive adhesive was validated in different applications relevant to edible electronics, such as surface mounting of devices on top of innovative edible substrates, interconnecting state-of-the-art edible batteries and conforming highly adhesive electrodes for fruit monitoring. Notably, the obtained circuits show good and long time functioning indicating a lack of resistive load and large time stability of the glue. On the other hand, the adhesive electrodes on fruit constitute long-term stable labels permitting food monitoring over ripening via bio-impedance spectroscopy measurements of fruits, proving a large multi-functionality of our material. Our findings open thus the door to development of fully edible circuitry and biodegradable edible sensors for agriculture.
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