Instrumentation & Measurement Magazine 25-9 - 8

them, for example the impact force or the contact area. As triboelectric
generators in general, and also plants, have high
internal impedances, it is required to analyze at which matching
impedance the power output is highest. This can be done
by measuring the current generated by the plant hybrid generator
through a series of known resistors.
The application of plant-hybrid energy harvesters involves
the conversion of wind energy into electricity. This requires
additional analysis. Here, effects of wind speed, wind direction,
orientation of the leaves to the wind, and resulting leaf
oscillations play a role. Such parameters can be best analyzed
in an instrumentation capable to simulate natural wind conditions.
Ideal are phytochambers with wind generators (as
partially shown in Fig. 2b) that not only allow to control wind
speed and direction but also parameters like lighting, humidity,
temperature, which affect plant growth to best control the
experimental setup [3].
Other Ways Plants Convert Energy
Next to triboelectricity, living plant tissue enables further
phenomena for converting an externally applied energy into
potential differences. As described above, the entire plant's
cellular tissue and the apoplast act as an ionic conductor, and
plants hence resemble branched electrodes with a large surface
area. This leads to the fact that plants can act as antennas
to receive (and also transmit) radio frequency (RF) radiation.
Resembling water-based or ionic liquid antennas, plants receive
RF radiation from a broad frequency range including
typical 50 Hz noise, low and medium frequency radio communication,
ultra-high frequency FM broadcast radio stations,
Global System for Mobile Communications (GSM) and RF
from broadband cellular network [5]. This can be measured by
spectrum analyzers connected to the tissue using shielded antenna
cables to reduce effects from circuit components. The RF
energy harvesting can be combined with wind energy harvesting
to enhance the power output which was realized, using
only slight variations of the circuit [5].
Improving
Performance and
Sustainability
One factor to enhance
the performance is the
impact force, and this
parameter can be easily
experimentally varied to
better analyze the behavior
by adjusting the actuation
amplitude of the test setup.
In an outdoor scenario in
which wind will be used,
the impact force depends
on the wind speed, leaf and
branch motion, and the
overall mechanical system.
In general, higher impact
8
force typically leads to an increase in the voltage amplitude
but does not change its polarity. Too high impact forces, however,
can damage the cuticle and the cells. Especially damage
of the cuticle, which is leading to an electrical contact between
the upper cuticle surface and the inner cellular tissue or even
wetting of the leaf surface, will significantly reduce the triboelectric
charging [1]. Here again, soft materials like elastomers,
for example silicones, have advantages over stiffer materials
as their elastic, soft deformation reduces stress on the cuticle.
Moreover, soft materials that are capable to conformably adapt
to the plant's leaf microstructure can increase the effective surface
area that is in contact and hence generates charges.
Indeed, another parameter that has a strong influence on
the signal amplitude is the contact area. This is a parameter
that can be easily controlled on the macroscale, e.g., by choosing
a sample of a certain size like 10 by 10 mm. However, it is
very difficult to control this parameter on the micro- or even
nanoscale. Leaves are rough surfaces that have a hierarchical
surface structure that often ranges from nanoscale (surface
waxes) to millimeter scale features (trichomes, hairs, etc.).
If such a rough surface contacts a rigid surface, the contact
area is determined by the structure and could be less than the
sample area. On the other hand, using materials like elastomers
that adapt to the leaf's structural features may increase
this effective contact area and the voltages generated. Hence,
the analysis of contact electrification between two materials
should always be supported by an analysis of the surface structure
with profilometers and microscopy.
As the triboelectric effect is strongly material dependent,
further engineering of the contact material by surface treatments
and chemical functionalization will likely increase the
power output. It is important that impact forces and stiffness
of materials do not damage the leaves, even though they can
repair or self-heal to some extent. Moreover, transparent materials
are better suited to permit photosynthesis when installed
on top of the leaf. Thus, balancing these parameters will allow
power output to be maximized while keeping the plant alive.
Fig. 5. Roadmap for future developments for the next five to ten years for using plant-hybrid systems as self-powered
sensing and energy harvesting platforms.
IEEE Instrumentation & Measurement Magazine
December 2022

Instrumentation & Measurement Magazine 25-9

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