Instrumentation & Measurement Magazine 26-1 - 37

model useful for the soil-plant configuration described below,
the chemo-electrical transduction properties have been studied
to demonstrate the conversion principle attributable to
microorganism processes. In this context, the effects of X-ray
irradiation were compared with the procedure based on heating
to evaluate the contribution of bacterial and enzymatic
activities in the voltage generation [28].
Focusing on a single plant and generation of electrical
signals, various studies have been addressed the context of
electrophysiology, health and applied stress in plants [29],
[30]. In particular, considering plants with implanted electrodes,
various principles can be highlighted that are mainly
correlated with the localization of the contacts. This approach
considers biological chemo-electrical transduction which will
include stimulation of plant tissue to modify the activities of
the plasma membrane, which in turn, affects the membrane
voltage [31]. In particular, the electrical responses of cells in the
immediate environment of a stimulus, including local periodic
electrical oscillations and propagating electric signals, are often
used for the study of the electrical activity in plants.
Although several conversion mechanisms which involve
biological processes of plants and the generation of electrical
signals are not entirely clear [30], two main potential
activities can be considered as transduction methods: variable
potentials, suitable for local damaging stimulus; and action
potentials, mainly induced by non-damaging quantities, with
propagation through living tissues [31], [32]. These principles
are mainly based on a passive influx of calcium, an inhibition
of the plasma membrane H+ activity and an anion efflux
which results in depolarization of the cells.
In order to describe the behavior of the whole system,
the soil-plant system will be now considered. This solution
addresses the possibility of using the sensing properties of
plants to make self-generating devices based on modifying
the metabolic processes of
the natural organisms present
in the living system as
a function of the specific
measurand. The electrodes
considered are inside the
soil. In particular, considering
the mechanism in [33],
its effect regards the change
of soil redox potential, thus
resulting in a measurable
electrical signal between
anode and cathode inside
the soil.
The consequence is
an output voltage correlated
with the change in
the measurand when other
parameters such as moisture,
soil mineralogy, and
organisms are kept invariant
or naturally filtered by
February 2023
the living organism considered. As an example, a soil-plant
system based on a Dimorphotheca ecklonis is used to measure
ultraviolet radiation, in which wavelength in the UV-A region
(315-400 nm) will be considered. The principle of operation is
based on the natural property of the plant to detect UV-A radiation
with photoreceptors that mediate plant responses.
The increased intensity of UV radiation implies an alteration
of the plant's metabolism, with a change in the fluids emitted
through the roots and organic acids [33]. This process involves
the production of root exudates which induce a modification
in terms of output voltage as a consequence of a change in soil
redox potential.
It is worth noting that from a general point of view, the
main advantage of this solution and more generally of living
sensors regards their ability to naturally filter out and reject
other physical quantities if they are different with respect to
the measurand. In this context the choice of soil and plant is
strongly related to the ability of the devices to be insensitive
to interfering or influencing quantities (e.g., temperature, humidity,
amount of water, natural plant growth). For all these
reasons, an exploration phase is particularly important and
fundamental during the conception phase.
As illustrated in the left side of Fig. 3, this approach must
include the characteristics, such as influences, electrode characteristics,
robustness, etc. The transduction mechanism and
the modeling are correlated with the soil-plant systems. It can
be described as a function of the bacterial activity, biological
wire system, telegrapher's equations applied in plants, equivalent
circuits, etc. A specific measurand defines the physical
quantity of interest to be measured through the plant.
In the next section, a case study and a synthesis of the
activity of a living sensor for measurement of UV-A is presented.
This system is of interest for different applications, in
particular for microclimate monitoring in cultural heritage
Fig. 3. A view of process used to develop living sensors.
IEEE Instrumentation & Measurement Magazine
37
Instrumentation & Measurement Magazine 26-1

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