In a recently published study in scientific advancesresearchers proposed a novel approach to developing flexible and biodegradable electronics called MycelioTronics that could replace electronic substrate material.
In addition, researchers reported a method for the efficient and scalable growth and harvesting of this material based on a fungal mycelial “skin” derived from a naturally growing saprophytic fungus. Ganoderma lucidium.
Electronic devices, including wearables (e.g. cell phones) and untethered devices, are becoming irreversibly integrated into human life. Due to their limited lifetime, they generate enormous amounts of e-waste and hinder the realization of a green electronic future. The bleak situation points to challenges in manufacturing electronic devices with sustainable materials.
Biodegradable printed circuit boards (PCBs) are not available, and most graphene and carbon-based biomaterials still contain unsustainable substrates. Traditional integrated circuits (ICs), which make up the bulk of the PCBs used in mobile applications, use metals, ceramics, and polymers. There is an urgent need for biodegradable ICs based on plant-based materials that yield fully transient electronics, including biodegradable circuit elements. To date, advances in using mushroom mycelium with electronics and sensor platforms have only resulted in inconveniently bulky electronics that have limited sensor performance.
About the study
In the present study, researchers fabricated lightweight and shape-adaptive sensor patches based on G. lucidium Mycelial substrate and highlighted general processing techniques of mycelial skin for electronics. For example, they constructed conductive paths by metallizing mycelial surfaces by physical vapor deposition (PVD) of thin metal layers and subsequent laser ablation.
The development of the mycelial skin on the surface had three distinct phases, each giving a more mature skin. The young skin surface was a bright white color that took up increasingly dense layers on the separating grid. The skin became thicker and denser, and brown spots (or a rough crust) appeared on its surface, called the middle skin. In the third phase, the skin’s surface becomes completely covered with a brown crust called mature skin.
These skins, composed of living mycelium, were saturated with water and, after additional compression and drying, yielded final skins. Further optimization of the growth conditions could significantly accelerate and stabilize this process. However, the team achieved a maximum of five consecutive harvests from a growing medium over six weeks with sufficient good quality mycelium yield. The thermogravimetric analysis (TGA) of all three skin types showed their stability up to over 250 °C (high temperature). It ensured that this substrate could hold electrical components on its top surface using standard electronic processing techniques such as soldering.
The young mycelial skin had electrical properties comparable to paper-based substrates; thus electronic circuits made using this approach could withstand high current densities of up to 333 Amm−2. It also had good breakdown strength, relative permittivity, and conductivity. In addition, the researchers permanently constrained mycelial skins into numerous geometries by exploiting the soakability of its foam-like hyphen network. It was soaked in 2-propanol, then molded into the desired shape using a mold, and air drying this deformed skin in an environment yielded a fully functional MycelioTronic device.
Finally, the researchers illustrated the conformability of mycelial skins. To do this, they formed a conductor strip including an SMD LED (Surface Mounted Device – Light Emitting Diode) into a spiral structure without visibly reducing the luminosity of the LED. They also showed how MycelioTronic devices are encapsulated with a biodegradable shellac-ethanol varnish to ensure electrical insulation and its applications in wearable technology.
Researchers achieved untethered operation of a standalone circuit that directly contained a mycelial battery, a capacitive sensor, and other necessary communication modules. For biodegradable and sustainable batteries, the mycelial skin, combined with a highly ion-conducting electrolyte solution, imbibed large amounts of liquid, resulting in a flexible membrane.
The medium mycelial skin type showed the lowest resistivity, which was as low as 54.3 ± 19.8 ohms-cm with this electrolyte solution, making it a viable battery separator material. It also achieved MacMullin numbers as low as 6.7, making it comparable to commercial lithium-ion battery separators. Commercial Li-ion batteries typically use polyolefin polymer separators because they have excellent mechanical properties, are chemically stable, and can be manufactured with small enough pore sizes to integrate safety mechanisms. However, all of these are non-renewable petroleum products that are both expensive and unfavorable in terms of environmental impact. On the contrary, mycelial skin separators can be grown naturally and use fewer resources than paper-based materials.
In addition, the team demonstrated a untethered mycelial sensor board with a surface-mount datacom module powered by an integrated mycelial battery and an embedded impedance sensor. They incorporated this sensor structure and two 15 mm x 15 mm electrodes for the mycelial battery directly into our circuit by laser ablating a copper and gold-metallized mycelial skin. They also studied its performance as a humidity sensor in a controlled environment using a climate chamber. They gradually increased the relative humidity (rH) by 10% to 20% and 70% rH and performed impedance spectra from one Hertz (Hz) to 10 MHz under stable climatic conditions.
The battery delivers a high operating current of approximately two milliamps (mA) during normal operation and ~13.5 mA when transferring data to the circuit. When an object like a finger approached the sensor, its charge changed because the finger acted as a parasitic capacitance, resulting in significant changes in sensor capacitance. In addition to proximity detection, they also demonstrated the sensor’s aspiration detection capabilities. A momentary increase in humidity caused a detectable change in capacitance. After they stopped direct aspiration, the signal initially decreased until they observed an area of slower decrease caused by residual moisture adhering to the mycelial surface. Thus, with this eco-friendly MycelioTronic design, they could perform a completely untethered proximity and humidity measurement with an integrated sustainable power supply.
The MycelioTronic approach paves the way for sustainable electronics with high functionality and variability. At the end of the life of these electronics, reusable surface mount components could be easily disassembled from the board using simple tools such as a heat gun or soldering iron, leaving only the biodegradable substrate as a waste product. Likewise, the mycelial skin-based PCB would easily decompose into composting soil after removing the conventional ICs. It would lose 93.4% of its dry matter within 11 days, after which the remains of the sample would no longer be distinguishable from the soil. Unprocessed mycelial skins similarly disintegrate to 9.3% of their original mass after 11 days.
The fully biodegradable mycelial skin made possible the replacement of fossil and heavily processed electronic components. Coupled with traditional non-degradable circuit components, it achieves the high functionality of all traditional electronic devices without sacrificing sustainability. This mushroom material also showed high thermal stability, which due to its conformability makes it easy to manufacture electronic sensor boards in various shapes.
Overall, the study demonstrated the versatility of mushroom spawn skins as sustainable electronics, giving way to a more sustainable electronic device architecture.
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