Capacitive storage in mycelium substrate

Abstract

The emerging field of living technologies aims to create new functional hybrid materials in which living systems interface with artificial ones. Combining research into living technologies with emerging developments in computing architecture has enabled the generation of organic electronics from plants and slime mould. Here, we expand on this work by studying capacitive properties of a substrate colonised by mycelium of grey oyster fungi, Pleurotus ostreatus. Capacitors play a fundamental role in traditional analogue and digital electronic systems and have a range of uses including sensing, energy storage and filter circuits. Mycelium has the potential to be used as an organic replacement for traditional capacitor technology. Here, were show that the capacitance of mycelium is in the order of hundreds of pico-Farads. We also demonstrate that the charge density of the mycelium `dielectric' decays rapidly with increasing distance from the source probes. This is important as it indicates that small cells of mycelium could be used as a charge carrier or storage medium, when employed as part of an array with reasonable density.

Full text

Capacitive storage in mycelium substrate
Alexander E. Beasley∗1, Anna L. Powell1, and Andrew Adamatzky1
1Unconventional Computing Laboratory, UWE, Bristol, UK
March 18, 2020
Abstract
The emerging field of living technologies aims to create new functional hybrid materials in which living
systems interface with artificial ones. Combining research into living technologies with emerging devel-
opments in computing architecture has enabled the generation of organic electronics from plants and
slime mould. Here, we expand on this work by studying capacitive properties of a substrate colonised
by mycelium of grey oyster fungi, Pleurotus ostreatus. Capacitors play a fundamental role in traditional
analogue and digital electronic systems and have a range of uses including sensing, energy storage and
filter circuits. Mycelium has the potential to be used as an organic replacement for traditional capacitor
technology. Here, were show that the capacitance of mycelium is in the order of hundreds of pico-Farads.
We also demonstrate that the charge density of the mycelium ‘dielectric’ decays rapidly with increasing
distance from the source probes. This is important as it indicates that small cells of mycelium could be
used as a charge carrier or storage medium, when employed as part of an array with reasonable density.
Keywords: fungi, capacitance, mycelium, storage, biocomputing
1 Introduction
The study of novel substrates for sensing, storing and processing information draws on work from the fields of
unconventional computing, living technology and organic electronics. The field of unconventional computing
aims to define the principles of information processing in living, physical and chemical systems and applies
this knowledge to the development of future computing devices and architectures [1]. Research into living
technologies is focused on the co-functional integration of animate and non-organic systems [9]. Finally, or-
ganic electronics [54, 31] looks to use naturally occurring materials as analogues to traditional semi-conductor
circuits [57], which often requires functionalisation using polymers or metallic compounds to exploit ionic
movement [34]. The development of organic electronics promises a technology that is low-cost and has low
production temperature requirements [37]. However, difficulties such as relatively low gain of organic tran-
sistors (approx. 5), and the behavioural variability inherently means that there is a large amount of research
effort being placed into organic thin film transistors [66, 59, 24, 58], organic LEDs [52, 40], and organic
capacitors [35, 51, 41]. The capacitive properties of a device allows it to either store energy or react to
AC/DC signals differently. There are a number of applications in which this property may be utilised, such
as energy harvesting [7], memory [60], or filter circuits [16]. Hybrid electronic circuits are a concept that
looks to combine traditional silicon, semi-conductor devices with elements found in nature [36, 8].
The capacitive properties of living tissues [38] have a wide range of potential applications, e.g. the estima-
tion of a plan root system size [18, 45], quantifying the DNA content of eukaryotic cells [56], analysing water
transport pathways in plants [13], measuring heat injury in plants [65], measuring contents of minerals in
bones [62], gauging firmness of apples [11], sugar contents of citrus fruits [12], maturity of avocados [64], esti-
mating depth of epidermal barriers [15], studies of endo- and exocytosis of single cells [48], and approximating
mass and morphology of microbial colonies [25, 42, 53].
∗Corresponding author: Alexander Beasley, alex.beasley@uwe.ac.uk
1

In the present study we focus on the capacitive properties of the mycelium of the grey oyster fungi
Pleurotus ostreatus for several reasons.
Firstly, research into the capacitive properties of fungi is lacking, despite their huge potential for bioelec-
tronic applications. Fungi are the largest, most widely distributed and oldest group of living organisms on
the planet [17]. The smallest fungi are microscopic single cells, the largest, Armillaria bulbosa, occupies 15
hectares and weighs 10 tons [55]. Fungi sense light, chemicals, gases, gravity and electric fields [5] as well
as demonstrating mechanosensing behaviour [32]. Thus, their electrical properties can be tuned via various
inputs.
Secondly, fungi have the potential to be used as distributed living computing devices, i.e. large-scale
networks of mycelium, which collect and analyse information about environment and execute some decision
making processes [2].
Finally, there is a growing interest in developing buildings from pre-fabricated blocks of substrates
colonised by fungi [50, 4, 20]. A recent initiative aims to grow monolithic constructions in which living
mycelium coexists with dried mycelium, functionalised with nanoparticles and polymers [3]. In such a case,
fungi could be act as optical, tactile and chemical sensors, fuse and process information and perform decision
making computations [2].
Providing local charge to areas of mycelium allows the storage of information inside the substrate. Iden-
tifying the area around which the induced charge can be detected allows the construction of an array in the
substrate where each cell can contain individual bits of information. Determining the capacitive properties
of fungi takes a step towards the realisation of fungal analogue circuits — circuits that use fungi to replace
traditional semiconductors.
The rest of this paper is organised as follows. Section 2 describes the experimental methods used for the
analysis of the substrate. Section 3 presents the results with discussions. Finally, conclusions are drawn in
Sect. 4.
2 Experimental method
Mycelium of the grey oyster fungi Pleurotus ostreatus (Ann Miller’s Speciality Mushrooms Ltd, UK) was
cultivated on damp wood shavings (Fig. 1(a)). Control samples of the growth medium, woodshavings, were
not colonised by mycelium. Iridium-coated stainless steel sub-dermal needles with twisted cables (Spes
Medica SRL, Italy) were inserted in the colonised substrate. An LCR meter (BK Precision, model number)
was used to provide a nominal reading of the capacitance of the sample with probes at 10 mm, 20 mm, 40 mm
and 50 mm separation.
The samples were charged using a bench top DC power supply (BK precision 9206) to 50 V. The power
supply output was de-activated and the discharge curve was measured using a bench top digital multi-meter
(DMM) Fluke 8846A (Fig. 1(b)). To fully characterise the capacitance of the samples, both the charge and
discharge curves were monitored [23] with a number of probe separations (e.g. 10 mm, 20 mm, 40 mm, and
50 mm). Measurements from the DMM were automated through a serial terminal from a host PC. All bench-
top equipment was high impedance to limit power lost through leakage in the test equipment. All plots were
generated using MATLAB. The sample interval was approximately 0.33 s.
3 Results
Capacitance measurement
Samples of the growth medium and mycelium were measured for their capacitance using a standard bench
top LCR meter (Tab. 1). The measured capacitance value of the mycelium substrate was two to four fold
greater than that of the growth medium alone. The values of capacitance were also effected by the moisture
content such that, if the capacitance of the mycelium was measured straight after it is sprayed with water,
the capacitance typically increased compared to that of dry substrate.
2

(a)
Mycelium Sample +
-V
~10mm
Iridium-coated stainless
steel sub-dermal needles
(b)
Figure 1: Experimental setup. (a) A sample of the mycelium under test. (b) Voltage discharge measuring
set up for mycelium samples
3

Table 1: Capacitance of mycelium and growth mediums.
Sample Electrode spacing Capacitance (pF)
Dry wood shavings 10 mm 37.6
20 mm 37.4
Damp wood shavings 10 mm 57
20 mm 58.6
Mycelium (drying)
10 mm 184
20 mm 144
30 mm 146
40 mm 120
50 mm 118
Mycelium (freshly watered)
10 mm 193
20 mm 186
30 mm 134
40 mm 125
50 mm 139
0 5 10 15 20 25 30 35
Seconds [s]
0
10
20
30
40
50
Voltage [V]
(a)
0 5 10 15 20 25 30 35
Seconds [s]
0
10
20
30
40
50
Voltage [V]
(b)
Figure 2: Discharge of a substrate after being charged to 50 V with probes separation of 10 mm. (a) Dry
wood shavings. (b) Damp wood shavings — shavings are immersed in water for half and hour after which
excess water is drained. Data are discrete. Line is for eye guidance only.
0 5 10 15 20 25 30 35
Seconds [s]
0
10
20
30
40
50
Voltage [V]
10mm
20mm
40mm
50mm
Figure 3: Mycelium sample is charged to 50 V then allowed to discharge. Electrode spacing is varied (10 mm,
20 mm, 40mm and 50 mm). Data are discrete. Line is for eye guidance only.
4

Table 2: Discharge curve fitness approximation coefficients (with 95% confidence bounds)
Sample a b
Dry wood shavings 104.9 (102.6, 107.2) -0.4079 (-0.4151, -0.4007)
Damp wood shavings 6205 (5164, 7245) -0.6261 (-0.6464, -0.6058)
Mycelium w/10 mm probe separation 2276 (2198, 2354) -0.4446 (-0.4481, -0.441)
Mycelium w/20 mm probe separation 2688 (2544, 2833) -0.4639 (-0.4695, -0.4583)
Mycelium w/40 mm probe separation 1569 (1458, 1680) -0.3948 (-0.4021, -0.3875)
Mycelium w/50 mm probe separation 1413 (1311, 1516) -0.3817 (-0.3891, -0.3743)
0 200 400 600 800 1000 1200 1400
Seconds [s]
-1
-0.5
0
0.5
1
1.5
2
2.5
Voltage [V]
Positive
Negative
(a)
0 200 400 600 800 1000 1200 1400
Seconds [s]
0
5
10
15
Voltage [V]
(b)
Figure 4: Charging mycelium to 50V with source electrodes 10 mm apart. Substance was charged from
approx. 10 minutes, readings are taken for approx. 22 minuets in total. (a) Sense electrodes were placed
in series, with the source terminals (10 mm away from either positive or negative electrodes). (b) Sense
electrodes were in parallel from the source electrodes (10 mm clearance). Data are discrete. Line is for eye
guidance only.
Discharge characteristics
Discharge characteristics of the growth medium and mycelium samples are shown in Figs. 2–3. Discharge
curves were produced by setting up the DC power supply and DMM in parallel with each other. The
substrate being tested is then charged to 50 V and the power supply output is disabled. The DMM continued
to periodically measure the remaining charge in the substrate for a period of time. The sample interval was
approximately 0.33 s.
The discharge curves for both the growth medium and the mycelium are very steep - approximated by
an exponential (1) .
f(x) = a·eb·x(1)
Where the parameters for 95% fitness for different mediums can be found in Table 2.
The discharge time is governed by equation τ=RC, where τis the time constant, Ris a resistance, and
Cis a capacitance.
With capacitance in the order of pico-Farads, and input impedance of the source and measurement
equipment in the order of mega-Ohms, it is expected that the discharge will be in the order of seconds.
Comparing the discharge curves of the growth medium to that of the mycelium samples, it is evident that
the discharge was not as steep in the mycelium due to the increase in capacitance over the growth medium.
However, it was still in the pico-Farad range and, therefore, the majority of charge was lost after just over
5 s. Increasing the separation distance of the probes (Fig. 3) had only a minimal effect on the capacitance of
the substrate (shown in Tab. 1), and therefore minimal effect on the discharge curve.
Observing the charging and discharging behaviour of the sample around the source electrodes helps to
build a better picture of the current density of the mycelium substrate. Figure 4 shows how we placed the
measurement equipment electrodes 10 mm away from the source electrodes in the mycelium, in three different
5

+
+
+
+
Increasing distance
Source
Measure
Measure
Measure
(a)
(b)
(c)
Figure 5: Measurement probes are arranged around the source probes to examine the charge field in the
substrate.
locations shown in Fig. 5. The sample was then charged for approximately 10mins before the supply was
turned off. The electrodes placed in ‘series’ (locations (a) and (b) on Fig. 5) with the charge electrodes
(Fig. 4(a)) show minimum voltage detected beyond the supply electrodes in the horizontal plane, when the
supply is active. Placing the measurement probes in ‘parallel’ (location (c) on Fig. 5) with the supply probes
(Fig. 4(b)) demonstrates the fact that considerably more current is conducted between the supply probes
in the vertical plane. When the supply is de-activated, the voltage around the supplies collapses almost
instantly.
Charge characteristics
The charge curves in the specimens around the supply electrodes provide an insight into the ability of the
substrate to conduct current. The sense electrodes were distanced from the source electrodes by varying
amounts. Initially, the growth medium was studied on its own (Fig. 6). The dry wood shavings (Fig. 6(a))
showed very low voltage across the sense electrodes placed 10 mm away from the source electrodes (parallel).
The dry shavings essentially acted as an open circuit and the measurement electrodes picked up noise. Damp
wood shavings form a more contiguous mass and the introduction of the water helped to conduct current
(Fig. 6(b)). From a 50 V source a maximum voltage of approx. 15 V was reached across the measurement
electrodes. Although a different charge curve was generated for the two repeated runs with electrodes at
different distances, we were primarily interested in the maximum observed voltage over the period Fig. 6(c)
superimposes the charge curves from both dry and damp growth medium on to the same axis and Fig. 8
shows all charge curves for growth medium and mycelium.
Preforming similar experiments with the mycelium substrate (Fig. 7), it was observed that moving the
measurement electrodes further from the supply reduced the measured Vmax over the ˜
22 min measurement
window. Figure 9 shows that the maximum measured voltage dropped rapidly as the distance from the
source electrodes increased. At 10 mm away from the source, less than 1/5th of the supply was measured over
22 mins, decreasing to less than 1/10th at 15mm separation. Figure 10 shows that, for the shortest paths
between the two source electrodes, there was a higher current density, indicated by the field lines being closer
together. As we move further away from the centre of the two probes, the current density decreased (arrows
are shown further apart). Beyond the two probes in the ‘y–direction’, we would expect to experience very
little current flow, however there are still fringing field effects which give rise to the small voltages shown in
Fig. 4(a).
Additionally, the moisture content of the mycelium had an impact on its ability to conduct current.
Figure. 11 shows that, if the sample of mycelium is continually charged over a period where it is also drying
out, the conducted charge can decrease rapidly as the water content vanishes. The measurement electrodes
for this sample are 5 mm away from the source, however we see a decrease in measured voltage of almost
15 V over the measurement window.
6

0 20 40 60 80 100 120 140
Seconds [s]
-2
0
2
4
6
8
Voltage [V]
10-3
(a)
0 200 400 600 800 1000 1200 1400
Seconds [s]
0
5
10
15
20
Voltage [V]
10mm
20mm
(b)
0 20 40 60 80 100 120 140
Seconds [s]
0
5
10
15
20
Voltage [V]
Dry shavings 10mm probes
Damp shavings 10mm probes
Damp shavings 20mm probes
(c)
Figure 6: Charge dynamics of growth substrate with measurement equipment set in parallel with charge
electrodes. Electrode pairs were 10 mm apart. (a) Dry wood shavings. (b) Damp wood shavings. (c) Dry
and damp wood shavings. Data are discrete. Line is for eye guidance only.
0 200 400 600 800 1000 1200 1400
Seconds [s]
0
10
20
30
40
Voltage [V]
5mm
10mm
15mm
20mm
50mm
Figure 7: Charging of mycelium sample with measurement equipment set to measure at different distances
from supply (5 mm, 10mm, 15 mm, 20 mm, and 50 mm). Supply electrodes and measurement electrodes were
arranged in parallel with each other.
7

0 20 40 60 80 100 120 140
Seconds [s]
-5
0
5
10
15
20
25
30
Voltage [V]
Dry shavings 10mm probes
Damp shavings 10mm probes
Damp shavings 20mm probes
Mycelium 5mm
Mycelium 10mm
Mycelium 15mm
Mycelium 20mm
Mycelium 50mm
Figure 8: Charge characteristics of wet and dry growth medium and and mycelium with measurement probes
at different distances from source probes.
5 10 15 20 25 30 35 40 45 50
Distance [mm]
0
10
20
30
40
Voltage [V]
Figure 9: Maximum measured voltage at increasing distance from supply probes in mycelium. Data are
discrete. Line is for eye guidance only.
8

+
Fringing fields
Figure 10: Current density between the two source electrodes. Current flow is shown in the physical direction
rather than convention.
0 200 400 600 800 1000 1200 1400
0
5
10
15
20
25
Voltage [V]
Figure 11: Mycelium charge measured 5 mm away from 50V source while mycelium sample starts to dry out.
Data are discrete. Line is for eye guidance only.
9

4 Conclusions
Mycelium exhibits traditional capacitive characteristics. The capacitance of the substance is in the order of
100’s of pico-Farads. Whilst this does not represent vast amounts of storage, it is up to four fold more than
that of the growth medium alone. Most noticeably, the charge carrying capability of the substance drops off
rapidly when measurements are taken away from the source electrodes. This shows great potential for the
use of mycelium networks to conduct or store charge in local ‘hot spots’ that are isolated from other areas
in the immediate vicinity. However, it is crucial that the moisture content of the mycelium is kept constant
since the ability to carry charge is strongly influenced by moisture content.
Any potential analogue circuits implemented with live mycelium will be vulnerable to environmental
conditions, especially humidity, availability of nutrients and removal of metabolites. Ideally, the mycelium
networks should be stabilized so they continue functioning whilst drying. This stabilization can be achieved
either by coating or priming the mycelium with polyaniline (PANI) or poly(3,4-ethylenedioxythiophene) and
polystyrene sulfonate (PEDOT-PSS). This approach has been proven to be successful in experiments with
slime mould P. polycephaum [6, 6, 19], and thus it is likely that a similar technique may be applied to
fungi. Moreover, PABI and PEDOT-PSS incorporated in, or interfaced with, mycelium can bring additional
functionality in terms of conductive pathways [63], memory switches [30, 21] and synaptic-like learning [10, 33].
An optional route toward the functional fixation of mycelium would be doping the networks with substances
that affect the electrical properties of mycelium, such as carbon nanotubes, graphene oxide, aluminium oxide,
calcium phosphate. Similar studies conducted in our laboratory using slime mould and plants have shown
that such an approach is feasible [28, 27]. Moreover using a combination of PANI and carbon nanotubes
in the mycelium network afford it supercapacitive properties [22, 26]. Another potential direction of future
studies would be to increase the capacity of the mycelium as a result of modifying the network geometry
by varying nutritional conditions and temperature [14, 29, 46, 47], concentration of nutrients [49] or with
chemical and physical stimuli [5]. With regards to the impact of our finding for the field of unconventional
computing, we believe further research on experimental laboratory implementation of capacitive threshold
logic [43, 39], adiabatic capacitive logic [44] and capacitive neuromorphic architectures [61] will yield fruitful
insights.
Acknowledgements
This project has received funding from the European Union’s Horizon 2020 research and innovation pro-
gramme FET OPEN “Challenging current thinking” under grant agreement No 858132.
Author contributions
A.B. conceived the idea of experiments. A.A. and A.P. prepared the substrate colonised by mycelium. A.B.
performed experiments, collected data and produced all plots in the manuscript. A.B. and A.A. prepared
manuscript (wrote and reviewed all contents). A.P. reviewed manuscript.
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