Instrumentation & Measurement Magazine 25-9 - 6

(Fig. 2) [3], [8]. Especially soft and lightweight materials like
silicone rubbers and transparent electrodes were exploited
to create artificial leaves which can be installed on the plants
to create the effective plant leaf-silicone material pair that
creates especially high charges in the plant tissue. The soft
structures and the transparency of the devices do not harm
the tissue and allow photosynthesis. It was even possible to
coat plant leaves directly with a thin layer of silicone rubber
to enhance triboelectric charging of the leaf surface and enable
plant-based wind energy harvesting [5]. Such biohybrid
energy harvesters have the advantage that they use only minimal
quantities of artificial materials to realize a device which
can produce electricity in power outputs comparable to artificial
triboelectric and piezoelectric energy harvesters using
similar mechanical forces [1], [5]. Yet, it is essential to analyze
their performance in a manner adapted to the organism, and
this requires modifying and revising methods applied to artificial
systems.
Setups to Measure Triboelectric
Phenomena on Plant Leaves
The setup to measure triboelectric charging of biological and
living tissue is similar to those used to analyze triboelectricity
of artificial materials, but it requires special modifications and
considerations to be applied to living organisms like plants.
A general parameter that is of interest is analyzing which material
pair creates particularly high voltages and currents.
Therefore, the potential differences generated in a specific
plant species after contact with different materials would be
tested or, inversely, different plant species would be analyzed
using a certain material to contact them to compare how the
signal varies among species. For such analysis, it is important
to keep parameters like impact force, frequency, contact area,
separation between leaf and material constant to focus the
analysis on the parameter (e.g., species or contact material).
Especially during longer tests, plant growth conditions like
sufficient lighting, watering, correct levels of humidity and
temperature also need to be considered.
A typical setup is illustrated in Fig. 3a. It requires: a stage
which can be varied in x,y,z direction comprising a sample
holder onto which the plant leaf can be fixed (e.g., glued or mechanically)
freshly picked from the plant or still attached to it; a
load cell under the leaf to control and track the impact force; an
actuator which actuates the sample that will be contacted with
the leaf, where the actuator should be controllable in terms of
frequency, amplitude, and force; devices controlling and at
least tracking humidity and temperature in the environment;
and a Faraday cage to reduce noise during signal acquisition.
Specific examples of which components could be used to realize
such a setup are given in [1] and [3].
An ideally inert metal electrode (e.g., a gold or platinum
electrode) is then inserted into the plant tissue at the stem (Fig.
1b) where the metal/electrolyte interface enables recording
the electric charges in the plants. The reactions that occur at
such interfaces need to be considered when measuring most
bioelectrical signals. The electrode is then connected to the
data acquisition devices.
The measurement circuit used to analyze the voltage, transferred
charges, or short circuit current in the tissue is displayed
in Fig. 3b. The instrumentation to record such signals are typically
high-impedance electrometers to measure currents in the
nA to μA range and oscilloscopes that enable a high frequency
analysis of the currents. However, especially for energy harvesting,
information on how much of the generated charges can
be stored in an energy storage device like a capacitor can also
provide useful information. A typical capacitor charging circuit
is shown in Fig. 3c in which a diode bridge is used to rectify the
alternating current produced by the plant-hybrid generator.
Performing the Analysis
In a characteristic experiment, the sample fixed on the actuator
is repeatedly actuated towards the plant leaf, transiently
Linear actuator
V
I
Faraday
cage
Electrode
Defined material
Detached leaf or plant
Force sensor
µ-positioning
stage x, y, z
(a)
(b)
DAQ
Tissue
electrode
+
-
+
-
Electrode
@ contact
material
(c)
Fig. 3. (a) Overview of the whole testing setup dedicated to measure potential differences in plants caused by leaf triboelectrification. (b) Details of the
acquisition circuit. Voltage and current are measured in two separated experiments. In the voltage configuration the input stage is a unity gain buffer followed by
an amplifier. In the current setup the input stage is a transpimpedance amplifier followed again by an amplification stage. These measurements are performed by
a high impedance electrometer (Keithley 6517B). (c) Circuit for energy harvesting: rectifying circuit typically used for charging a capacitor tracking the voltage over
the capacitor to analyze the charging dynamics.
6
IEEE Instrumentation & Measurement Magazine
December 2022
Tissue
electrode
Electrode @
contact material
Low-leakage
diode bridge
Capacitor

Instrumentation & Measurement Magazine 25-9

Table of Contents for the Digital Edition of Instrumentation & Measurement Magazine 25-9