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to its normal activity. Proteins
seem to drive the piezoelectric
effect inside a human body.
The basic building blocks
of proteins are amino acids,
which have electric dipoles
derived from their polar side
groups. For example, g-glycine
and D -alanine are amino acid
materials that have a strong
piezoelectric response ( 7 ). The
piezoelectric effect in amino
acids is obtained by a change
in the molecule electric dipole
amplitude in response to either
an applied mechanical force
or an electric field. Artificially
made biocompatible piezoelec-
tric materials also exist, such as
polyvinylidene fluoride (PVDF)
and its copolymers ( 8 ).
Yang et al. make an important
contribution by developing a
new method for scaling flexible
piezoelectric glycine thin films.
These films are self-assembled
by evaporating a solvent from
a glycine-polyvinyl alcohol solu-
tion. The films are also biocom-
patible and degradable.
Biocompatible piezoelectric materials
can be fabricated and designed for mul-
tiple purposes inside a human body. These
include monitoring local dynamic pressure
changes ( 9 ), such as heartbeats, breathing,
blood flow, and intraocular and cranial
pressure, and forming physical movements
inside the human body, such as muscle
activities ( 10 ). Another purpose is promot-
ing the healing of injuries by induced local
electric fields, such as in the case of local
growth of neurons and accelerated repair of
injured bones ( 11 ).
Flexible polycrystalline piezoelectric thin
films are more suitable than bulk size crys-
tals for applications inside a human body
because of the dynamic nature and flexibility
of human organs. The thin films have a pro-
nounced piezoelectric response when grown
with a preferred polar crystallographic ori-
entation in-vertical to the film plane.
Using the piezoelectric effect to replace
human muscle functionality by applying
a mechanical force requires a source of
electric energy. The optimal energy source
would be the human body itself, which
provides a mechanical energy that can
be converted into an electrical energy by
the piezoelectric materials. These devices
are called piezoelectric energy harvesters,
which can be attached as flexible thin films
to local sites inside the human body.
The piezoelectric harvesters can gener-
ate enough electric energy to operate the
device even from tiny mechanical move-
ments at extremely low frequencies ( 12 ),
such as heartbeats, blood flow, contraction
and expansion of lungs, walking, and eye
blinking. The piezoelectric energy harvest-
ers can be extremely flexible, lightweight,
and positioned close to the piezoelectric
actuator within the human body. The elec-
tric energy supplied by the piezoelectric
harvester should be accumulated in a bio-
compatible electric capacitor that enables
a controlled release of electric energy on
demand to the piezoelectric actuator ( 13 ).
Biocompatible capacitors can be made of
piezoelectric thin films by using their di-
Implanted microchips, developed for
controlled drug release at local sites in-
side a human body ( 14 ), can, in principle,
be used in a piezoelectric device. The im-
planted microchip controls the entire op-
eration of the device, which includes the
sensor, actuator, electric capacitor, and en-
ergy harvester (see the figure). The opera-
tions order of the microchip begins with a
signal received from the piezoelectric sen-
sor that indicates the misfunctioning of a
certain muscle. Then, an electric pulse is
sent through the electric capacitor to the
piezoelectric actuator to apply a mechani-
cal force to fix the problem. Finally, a feed-
back electric signal is received from the
sensor that communicates the resulting ef-
fect. These operations should continue un-
til the problem is fixed. In this
way, the device operates au-
tonomously where immediate
intervention is needed without
any external interference or ex-
ternal power supply.
An external human body
communication to the im-
planted microchip can be
achieved by using a wireless
communication ( 15 ). The pur-
pose of such a communication
is to receive real-time data on
the implanted piezoelectric
device activity and transmit
operational commands to the
microchip. A wireless commu-
nication to a human body from
an external device requires a
substantial power source. It
suffers from poor transmis-
sion through biological tis-
sues. It also needs a relatively
large antenna, which limits the
minimal size of the implant-
able microchip and prevents
implementation in organs such
as brain, heart, and spinal cord
that could be damaged by the
Extensive research activities are cur-
rently done on every aspect of biocompat-
ible piezoelectric sensing, actuating, and
energy harvesting. A major task would
be to integrate them into an autonomous
biocompatible device implanted on a local
site inside a human body that optimally
functions without any external interven-
tion, replacing the functionality of a local
muscle for the normal operation of a hu-
man organ. j
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Testing the implant
Piezoelectric thin films
implanted on mouse
muscles allow for testing
for biocompatibility and
targeted for use in humans.
Piezoelectric films that actuate, sense, and harvest energy combined with a
biocompatible microchip and capacitor create a self-contained system that works
automatically without an external interface.
Piezoelectric implants require a sensor to identify a problem in a muscle, apply
corrections with actuation, monitor the change, and signal back to a microchip that
controls this process. Ideally, the microchip is also implanted, and the energy for the
processing is harvested from the local physical movements as well as vibrations.
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