Instrumentation & Measurement Magazine 25-9 - 5

the cells and in the apoplast (the " space " between the cells),
rendering the tissue an ionic conductor. Thus, it can act as an
electrode. It is well known that plants indeed use electrophysiological
signals to communicate between cells, resembling
to some extent those signals produced by our nerve cells.
These electrophysiological signals are different from the electrical
charges which we treat here, produced by triboelectric
phenomena on the surface of leaves, as they often include active
cellular processes like controlled release of ions through
the cell membrane. Yet, this type of electrophysiological signals
are difficult to use for energy harvesting but the fact that
the plants also exploit inter-cellular electrical communication
shows that the propagation of charges through the tissue
is not only possible but definitely a common and essential
phenomenon occurring in living plants. Further reading on
electrophysiological signals can be found for example in [6].
Instead, more interesting for energy harvesting is the following:
the essential structure for triboelectrically-generated
potential differences in the tissue due to static charges on the
leaf is the above-described double layer that consists of a dielectric
layer (cuticle) on top of an ion-conductive electrode
(cellular tissue). When the leaf cuticle is touched by another
material, charges are created. In doing so, it is not necessary
that a shear contact occurs (like in the example of a balloon
rubbed against hair). A vertical contact and separation motion
is indeed sufficient to create triboelectric charges. These
charges on the cuticle are then electrostatically induced into
the electrode (the tissue) beneath, and for this a mechanical
separation between the layers is necessary, which is typically
better in vertical contact and release motions. Simply connecting
an electrode to the plant tissue allows the charges to be
analyzed and harvested (Fig. 1b).
Factors that Influence Triboelectric
Charging
Triboelectrification is a phenomenon that is strongly dependent
on the pair of materials which comes into contact. How
factors like electron, ion, and material transfer upon contact
contribute to charge generation is yet not fully understood,
even on less complex, artificial surfaces. Nevertheless, some
processes have become clear also on leaves. The so-called triboelectric
series [7] qualitatively orders artificial materials in
terms of their capability to develop positive or negative surface
charges. Contacting typical artificial materials from the
triboelectric series with plant leaves lead to the following observations:
First, the lipids and waxes of the cuticles of plant
leaves tend to charge positively [4], especially after contact
with materials like silicone rubbers, natural rubber, or fluorinated
polymers like polytetrafluoroethylene (PTFE) which
themselves, in turn, tend to charge mainly negatively [1]. Most
other polymers, metals, and natural materials like wood, as
well as the contact between two leaves, lead to marginal charging
[1]. Second, further crucial factors that affect the charge
generation are the impact force, the contact area, the frequency
in which a repeated contact occurs and environmental parameters
like humidity and temperature [1], [5]. Also, the
micro-nano surface structure can further enhance the charge
generation. However, hitting leaf surfaces too strongly may
also harm the leaves. A third feature that needs to be considered
is thus that materials should not damage the leaf during
impact, which renders soft materials more suitable. Moreover,
materials should ideally be transparent to not hamper photosynthesis
of the leaf. Significant damage of the cuticle and the
cells underneath will lead to a different behavior, but plants
can to some extend also self-heal and renew tissue to adapt
to mechanical stress. Thus, when measuring triboelectricallygenerated
signals, especially those on living organisms, the
above-mentioned factors require special consideration which
must be reflected in the setups used for data acquisition and
for energy harvesting approaches.
Plant-hybrid Triboelectric Energy
Harvesting
The combination of living plants and artificial materials was
used to construct wind energy harvesters, which make use of
the leaf motion and oscillations in the wind to create electricity
Fig. 2. (a) Design concept of plant-hybrid wind energy harvesters in which an artificial leaf is installed on a plant leaf. The artificial leaf is made of a material
which enhances leaf triboelectrification and wind drives the contact and release motion. (b) Characterizing plant-hybrid wind energy harvesting in a phytochamber
with wind machine (adapted from [3], used with permission under Creative Commons CC BY license).
December 2022
IEEE Instrumentation & Measurement Magazine
5

Instrumentation & Measurement Magazine 25-9

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