Mycelium Hub — IoT Protocol Stack and Electrode Design for Fungal Networks

Living mycorrhizal mycelium carries electrical signals in response to chemical, mechanical and humidity stimuli — a biological channel that can be instrumented. We present the electrode design for stable coupling to the mycelial mass, a lightweight protocol stack for bidirectional pulse transmission, and the environmental constraints that bound reliability.

Abstract

Mycorrhizal fungi form continuous mycelial networks that extend over hectares in temperate and tropical forests. These networks are not passive — they transport water, nutrients and chemical signals between plants, and in response to mechanical, chemical and humidity stimuli they generate measurable electrical pulses. The literature on fungal electrical signalling [1,2] now covers a decade of controlled laboratory observations: the question is no longer whether mycelium conducts and modulates electrical signals, but how reliably those signals can be used as a communication medium. This article summarises our ongoing work on three fronts: an electrode design that couples stably to the mycelial mass without destroying it, a lightweight protocol stack for bidirectional pulse communication over short mycelial channels, and the environmental envelope within which the medium remains usable. We close with the open problems that must be solved before mycelium-based biological IoT moves out of the laboratory.

Keywords: mycelium hub, mycelial network, fungal electrical signalling, biological IoT, mycorrhizal network, carbon nanotube electrode, bioelectronic communication.

1. Why Mycelium

Three properties make mycelial networks interesting as a biological communication substrate:

• Spatial reach. A single mycorrhizal network in a mature forest connects hundreds to thousands of plants over hectares, with no artificial infrastructure. The medium already exists; the engineering problem is coupling to it.
• Intrinsic error correction. Mycelium is highly redundant — a single hypha can be severed without disconnecting the network. The medium carries signals in parallel through multiple paths, which is desirable for low-power sensing in environments where cable damage is likely.
• Low energy footprint. The medium does not need external power. The energy for signal propagation comes from the biology itself; the instrumentation only reads and modulates.

The limitations are equally clear: bandwidth is far below any electronic alternative, propagation speed is slow (centimetres to metres per second rather than light-speed), and reliability depends on environmental conditions that are hard to control in a forest. For niche applications — distributed soil moisture sensing, chemical presence detection, root-zone biosecurity — these limitations are acceptable in exchange for the operational advantages.

2. Electrode Design

The physical coupling between instrumentation and mycelium is the first engineering challenge. Our electrode design has been through four iterations; the current version meets four requirements.

2.1 Material Selection

Metal electrodes release ions over time, damaging the biology and causing drift in the measurement. We use composite electrodes with a carbon nanotube (CNT) core and a biocompatible polymer binder, which are inert in the mycelial environment, mechanically compliant, and have sufficient electrical conductivity for the signal levels involved. The CNT geometry also provides a high effective surface area, which lowers the contact resistance to the mycelium.

2.2 Geometry and Insertion Depth

The electrode is a cylindrical probe, 1–2 mm diameter, 10–30 mm long. Insertion is gentle — a slow push into the substrate following a pre-drilled pilot hole, with the mycelium growing around and into the electrode surface over the following 48–72 hours. This growth phase is critical: freshly inserted electrodes have contact resistance of 100 kΩ or more; after mycelial growth, this drops to 1–10 kΩ, which is the operational range.

Electrode spacing of 20–100 mm between a transmitting and a receiving pair gives usable signal-to-noise at bench scale. Larger spacings require higher transmitter power, which approaches the biological damage threshold.

2.3 Substrate Humidity Window

Mycelial conductivity is strongly dependent on substrate moisture. Below about 40 % water content (by weight) in a lignocellulosic substrate, the signal falls to noise level; above about 80 %, the medium saturates and the signal becomes blurred by free water conduction. The operational window is 50–70 % moisture, which maps to typical forest soil conditions in temperate climates but is narrower than one might wish.

For deployed sensors, this means the medium is seasonally bounded — reliable in moist conditions, unreliable in drought.

2.4 Long-term Stability

Our longest continuous bench recording is 90 days with a single electrode pair in a Pleurotus ostreatus substrate. Baseline potentials shifted by 15 mV over this period; stimulus responses remained detectable throughout. Electrode replacement is required every 3–6 months in substrates with high humidity, due to biofouling of the CNT surface.

3. Signal Characterisation

Three types of signals have been documented in our recordings and are consistent with the published literature:

• Baseline potentials. The mycelium exhibits a resting potential between electrode pairs that drifts slowly (mV per hour) and is stable enough to serve as a reference for impulse detection.
• Spontaneous spikes. The mycelium generates spontaneous electrical pulses at a rate of a few per hour, with amplitudes of 1–10 mV and durations of 10–100 seconds. These are not noise; their rate correlates with substrate conditions and stimulus history.
• Evoked responses. Deliberate stimuli — mechanical touch, controlled humidity pulse, chemical addition (sugar solutions, aromatic compounds) — produce reliable, repeatable response waveforms, with latencies of seconds to minutes and amplitudes of 1–20 mV.

The evoked responses are the basis for the protocol stack.

4. Protocol Stack

Communication over a mycelial medium requires a protocol stack adapted to the medium’s unusual properties: slow propagation, variable latency, unreliable individual pulses.

4.1 Physical Layer

The physical layer transmits individual pulses with:

• amplitude 1–5 V applied, 5–20 mV measured at receiver,
• pulse width 2–10 seconds,
• inter-pulse interval 10–30 seconds (necessary for the medium to return to baseline).

Pulse shaping is a square wave with soft edges; sharper edges couple to free-water conduction and are less selectively detected by the mycelium.

4.2 Data Link

The data link layer frames pulses into 5-bit symbols with a start marker and parity, giving an effective payload rate of 3–5 bits per minute. This is bytes per hour, not kilobytes per second — the use cases must be adapted accordingly.

Error correction uses repeat-until-acknowledged (ARQ) for important frames and blind triplication for small telemetry. More sophisticated codes (Reed-Solomon, Hamming) are being evaluated but add latency that is already costly.

4.3 Network Layer Considerations

The mycelial network is physically not planar — hyphae branch and reconnect in three dimensions. Signals from a transmitter reach a receiver by multiple paths with slightly different latencies; the receiver sees a smeared, lower-amplitude version of the transmitted pulse. Multi-hop routing is in principle possible by chaining electrode pairs, but at bench scale the losses from each hop reduce the usable range to about two or three hops.

For current deployment-focused experiments, we treat the network as single-hop between a transmitting electrode pair and a receiving pair, and use the native biological connectivity as the medium between them.

5. Measured Transfer Characteristics

Across approximately 300 hours of controlled transmission experiments, we observe:

• Bit rate: 3–5 bits per minute reliably; 10 bits per minute at the edge of reliability.
• Bit error rate: 1–5 % at the operational humidity window, rising rapidly outside it.
• End-to-end latency: 15–60 seconds for a 10-bit payload, depending on distance and humidity.
• Range: 20–100 mm at bench scale with current electrode geometries; longer distances achievable with multi-hop at proportionally higher error rates.

These numbers put the medium firmly in the category of ultra-low-rate, high-latency biological sensing, not in competition with conventional radio or wired IoT.

6. Open Problems

Four open problems stand between bench reliability and field deployment:

• Scaling beyond laboratory substrates. Forest soil is a much more variable medium than a lignocellulosic substrate in a controlled grow chamber. Field reliability under seasonal moisture, temperature and biological competition is unproven.
• Robustness in outdoor conditions. Temperature cycles, UV at the electrode surface near the soil, and freeze-thaw stress all degrade the electrode and the surrounding mycelium.
• Interface with conventional IoT. The protocol stack above terminates at a wire; to be useful in a sensor network, it must then couple into a LoRa, 802.15.4 or similar radio stack with appropriate protocol translation.
• Long-term electrode drift. Baseline shifts of 10–20 mV over 90 days are tolerable; shifts of 100 mV or more, which we see in some substrates, require recalibration that disrupts the measurement.

Each of these is a research problem, not an engineering problem — the answers are not currently in the literature.

7. Research Outlook

The near-term use cases for mycelium-based biological IoT are:

• Distributed soil-moisture sensing at ecosystem scale, where many low-rate sensors complement a few high-rate electronic sensors.
• Chemical presence detection (specific nutrients, allelopathic compounds, pollutants) by instrumenting the native chemotactic response of the mycelium.
• Root-zone biosecurity — detecting the arrival of pathogen biomass or invasive species through changes in the baseline response pattern of the local mycelial network.

Longer-term, the integration of mycelial sensing with conventional IoT infrastructure opens the possibility of ecosystem-scale, low-power monitoring networks that combine biological and electronic components. Whether this reaches operational maturity depends on the next decade of work on the open problems above.

8. Conclusion

Living mycorrhizal mycelium is a working biological communication medium at bench scale. Signals are detectable, repeatable, and codable into a protocol stack sufficient for low-rate telemetry. The medium is slow, humidity-dependent and bandwidth-limited; it is not a replacement for conventional IoT but a complement in environments where the biological medium already exists and conventional infrastructure is costly or impossible to deploy.

The engineering is now downstream of the biology. The next three to five years of work will determine whether mycelium-based sensing enters operational use in environmental monitoring and agriculture, or remains a laboratory curiosity.

IOAS operates a long-running research project on biological communication substrates including mycelial networks. For enquiries and collaboration: director@ioas.pro.

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