In a recent study published in Science Advances, researchers proposed a novel approach to develop flexible and biodegradable electronics called MycelioTronics, which could substitute for electronic substrate material.
Additionally, the researchers reported a method for the efficient and scalable growth and harvest of this material based on a fungal mycelium “skin” derived from a naturally growing saprophytic fungus, Ganoderma lucidum.
Electronic devices, including wearable (e.g., mobiles) and untethered devices, are integrated into human lives irrevocably. Owing to their limited lifespan, they generate enormous amounts of electronic waste, hindering the realization of a green electronic future. The grim state of affairs points to challenges in fabricating electronic devices with sustainable materials.
Biodegradable printed circuit boards (PCBs) are unavailable, and most graphene and carbon-based biomaterials still incorporate unsustainable substrates. Conventional integrated circuits (ICs) that take up the highest proportion of the mass of PCBs used in mobiles use metals, ceramics, and polymers. There is an urgent need for biodegradable ICs based on plant-based materials, yielding entirely transient electronics, including biodegradable circuit elements. So far, advancements in using fungus mycelium with electronics and sensing platforms have only yielded unfavorably bulky electronics exhibiting limited sensing performance.
About the study
In the present study, researchers fabricated lightweight and shape-adaptive sensor patches based on G. lucidum mycelium substrate and highlighted general processing techniques of mycelium skin for electronics. For instance, they constructed conductor paths by metalizing mycelium surfaces via physical vapor deposition (PVD) of thin metal layers and subsequent laser ablation.
Mycelium skin development on the surface exhibited three distinct phases, with each yielding more mature skin. The young skin surface had a bright white color that occupied increasingly dense layers on the separation grid. The skin grew thicker and denser, and brown patches (or a rough crust) appeared on its surface, referred to as the medium skin. In the third phase, the skin surface got completely masked with a brown crust, referred to as mature skin.
These skins, composed of living mycelium, were saturated with water and yielded final skins after additional compressing and drying. Further optimizing the growth conditions could accelerate and stabilize this process substantially. Nonetheless, the team achieved a maximum of five subsequent harvests from one growing medium over six weeks with sufficient mycelium skin yield of good quality. Thermogravimetric analysis (TGA) of all three skin types demonstrated their stability up to more than 250°C (high temperature). It ensured that this substrate could hold electrical components on its top using standard electronic processing techniques like soldering.
The young mycelium skin had electrical properties comparable to paper-based substrates; thus, electronic circuitry fabricated using this approach could sustain high current densities up to 333 A mm−2. It also had good breakdown strength, relative permittivity, and conductivity. Moreover, the researchers demonstrated forcing mycelium skins permanently into numerous geometries by exploiting the soakability of its foam-like hyphen network. It soaked 2-propanol, subsequently reshaping into the desired form using a mold, and air drying of this deformed skin in an ambient environment yielded a fully functional MycelioTronic device.
Finally, the researchers illustrated the shape-adaptiveness of mycelium skins. To this end, they reshaped a conductor strip, including a surface-mounted device–light-emitting diode (SMD-LED), into a helical structure, without visibly diminishing the LED’s luminosity. They also showed how to encapsulate MycelioTronic devices using a biodegradable shellac-ethanol varnish to ensure electrical insulation and its applications in wearable technology.
The researchers achieved the untethered operation of a standalone circuit directly incorporating a mycelium battery, a capacitive sensor, and other necessary communication modules. For biodegradable and sustainable batteries, mycelium skin soaked up large amounts of liquid in combination with a highly ion-conducting electrolyte solution, yielding a flexible membrane.
The type medium mycelium skin exhibited the lowest specific resistance, being as low as 54.3 ± 19.8 ohm-cm with this electrolyte solution, rendering it a viable battery separator material. Also, it attained MacMullin numbers as low as 6.7, making them comparable to commercial lithium-ion battery separators. Commercial Li-ion batteries typically use polyolefin polymer separators as they have excellent mechanical properties, are chemically stable, and can be produced with small enough pore sizes to incorporate safety mechanisms. However, all these are non-renewable petroleum products, both expensive and unfavorable in terms of environmental impact. On the contrary, mycelium skin separators can be grown naturally and consume fewer resources than paper-based materials.
Furthermore, the team demonstrated an untethered mycelium sensor board with a surface-mounted data communication module powered by an integrated mycelium battery and an embedded impedance sensor. They directly incorporated this sensor structure and two 15 mm by 15 mm electrodes for the mycelium battery in our circuit by laser ablation from copper-gold metalized mycelium skin. Further, they investigated its performance as a humidity sensor within a controlled environment using a climate chamber. They gradually incremented relative humidity (r.H.) by 10% to 20% and 70% r.H. performing impedance spectra from one hertz (Hz) to 10 MHz during stable climate conditions.
The battery supplies a high operating current of approximately two milli-ampere (mA) under standard operation and ~13.5 mA during data transmission to the circuit. When an object like a finger approached the sensor, its charging altered as the finger acted as a parasitic capacitance, resulting in distinct changes in sensor capacitance. In addition to proximity sensing, they also demonstrated the sensor’s aspiration sensing capabilities. A short-term rise in humidity caused a detectable change in capacity. After they terminated direct aspiration, the signal first decreased until they observed a region of slower decrease caused by residual moisture adhering to the mycelium surface. Thus, they could conduct entirely untethered proximity and humidity sensing with an integrated sustainable power supply using this environment-friendly MycelioTronic design.
The MycelioTronic approach makes way for sustainable electronics with high functionality and variability. After the end of the life of these electronics, reusable surface-mounted components could be easily dissembled from the board using simple tools like a heat gun or solder iron, leaving only the biodegradable substrate as a waste product. Likewise, the mycelium skin-based PCB would disintegrate readily in composting soil after the removal of the conventional ICs. It would lose 93.4% of its dry mass within 11 days, after which sample remnants would also be indistinguishable from the soil. Unprocessed mycelium skins disintegrate similarly down to 9.3% of their initial mass after 11 days.
Mycelium skin being entirely biodegradable rendered the replacement of fossil-based and heavily processed electronic components feasible. When coupled with conventional non-degradable circuit components, it achieved the high functionality of all conventional electronic devices without sacrificing sustainability. This fungal material also demonstrated high thermal stability facilitating the fabrication of electronic sensor boards in varied shapes because of their shape-adaptiveness.
Overall, the study demonstrated the versatility of fungal mycelium skins as sustainable electronics making way for a more sustainable architecture of electronic devices.