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Piezoelectric Energy Harvesting for Wearable and Implantable Devices

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Piezoelectric Energy Harvesting for Wearable and
Implantable Devices
M. Sami Zitouni
College of Engineering & IT
University of Dubai
Dubai, United Arab Emirates
mzitouni@ud.ac.ae
Maisam Wahbah
College of Engineering & IT
University of Dubai
Dubai, United Arab Emirates
mwahbah@ud.ac.ae
Abstract—Energy harvesting devices aim to harness the wasted
energy from various energy sources available in their envi-
ronment including thermal energy, kinetic energy, electrostatic
energy, and solar energy. The relevance of such devices in-
creased with the emergence of low power and micro/nano scale
electronics. By utilizing the human body motion, piezoelectric
energy can be generated. The human body motion can be
either external or internal including movements, breathing, and
blood pressure. This plays a critical role in the development
of self-powered wearable and implantable biomedical devices,
that can be used for personalized medical treatment and health
monitoring through processing and analysis of continuously
collected physiological signals. Thus, this article presents a study
of the state of the art energy harvesting from human body
methods that utilize the piezoelectric effect. Mainly, the focus is on
harvesting human body motion using wearable and implantable
devices. Further, issues and challenges in the current technologies
are discussed.
Index Terms—Piezoelectric, energy harvesting, wearable de-
vices, implantable devices
I. INTRODUCTION
Because of the decrease in power consumption of electronic
devices, implantable and wearable electronic devices have
become more popular. There is a strong push towards having
self powered devices which instead of constant charging, they
use energy harvesting [1]. One of the most promising ways
of energy harvesting is utilizing the properties of piezoelectric
materials. Ever since the piezoelectric effect was discovered
by the Curie brothers in the 1880, it has been a subject of
interest for human body energy harvesting [2]. Piezoelectric
materials act as a mechanical-to-electrical converter which
produce charges when stress or strain is applied as shown in
Figure 1 [3]. The amount of charge or polarization produced
was found to be proportional to the applied force which causes
mechanical deformation.
In 1990, Massachusetts Institute of Technology (MIT) was
able to produce the first energy harvesting device based on
human motion [4]. A decade after, the first wearable device
was fabricated as a shoe heel which was able to produce
around 1 W of power [5]. A small movement inside the
human body such as the fluctuation of blood pressure, was
successfully harvested to produce 2.3µW .
Other methods utilizing energy harvesting from kinetic move-
ment include electrostatic and electromagnetic energy harvest-
Fig. 1. Vibration energy harvesting process.
ing. However, piezoelectric materials have the highest energy
density, high efficiency, do not need a voltage source, and
require the least maintenance [2]. They are also convenient
because the human body is rich in vibrations from moving, to
blood pressure, to as simple as breathing. Challenges arise be-
cause piezoelectric materials are highly frequency dependent.
Since the smaller the size of an object, the higher the resonant
frequency [6], it is difficult to design energy harvesters to work
on the human body. They could also be bulky in size which
is difficult to integrate in small systems [4].
This article focuses on energy harvesting from the human
body to self power wearable devices that are usually inte-
grated in everyday personal objects or attached to body parts.
Another aspect of focus for this article is energy harvesting
inside the body through implantable devices usually through
surgical insertion. Section II describes the piezoelectric effect
in physical terms, followed by a discussion and comparison
of the piezoelectric materials for human energy harvesting in
section III. Finally, section IV will discuss both wearable and
implantable devices for energy harvesting.
II. PIEZOELECTRIC EFF EC T
The direct piezoelectric effect states that the formation of
a dipole moment in a piezoelectric material by changing its
atomic structure with stress, will result in a voltage difference
TABLE I
AMO UNT O F EN ERG Y PRO DUC ED B Y DIFF ER ENT B ODY PART S DU RIN G
DIFFERENT ACTIVITIES
Cycling Walking Jogging
Arm N/A N/A 210µW
Wrist N/A 33µW 100µW
Ankle 24µW 67µW 234µW
across the material. It was then discovered that electrical po-
larization deforms the piezoelectric material, which is known
as the converse piezoelectric effect [7].
In some piezoelectric materials, displacements between the
atoms occur under a specific temperature called The Curie
Temperature, which result in having permanent dipole mo-
ments within these materials. Such crystal structures are
known as pyroelectric. On the other hand, in ferroelectric
materials, the spontaneous electric polarization is reversible
by applying an external electric field.
Recently, many companies are manufacturing synthetic piezo-
electric materials for actuators and sensors in specific applica-
tions with improved piezoelectric response. The crystals in
these piezoelectric materials such as ferroelectric ceramics,
are randomly oriented below the Curie Temperature, where
each crystal has s different dipole moment orientation. When
stress is applied to the material, an increase in the dipole
moments will occur to some domains, while a decrease will
occur to others which results in no overall change. Thus,
these materials are treated with the poling process, in which a
constant electric field is applied to impose all dipoles to align
in the same orientation. Therefore, when force is applied to
the treated material, the voltage will be generated along this
unique polarization direction.
Applying compression on a poled piezoelectric ceramic will
decrease the dipole moment, which will cause the development
of a voltage with the same polarity as the poling voltage.
If tension is applied, the generated voltage will be opposite
to the poling voltage. Thus, the continuous application of
periodic compression and tension can be done to generate an
AC voltage [8]. In case of the converse piezoelectric effect,
the material will lengthen when a voltage in the same polarity
as the poling direction is applied, while it will shorten when
the applied voltage polarity is reversed [9].
III. PIEZOELECTRIC MATER IA LS
The type of Piezoelectric material has a major impact on
the performance of the energy harvesting device. Currently,
the main focus of researchers is on Lead zirconate titanate
(PZT), Polyvinylidene fluoride (PVDF), Piezoelectric
Composite Fabrics, and Relaxor Ferroelectrics. A vastly used
piezoelectric ceramic material in energy harvesting devices
is PZT, as it has stable temperature and good performance.
The downside of this material is its fragility, where applying
periodic high frequency vibration can cause it to break. Thus,
research is focused on developing a PZT material with higher
flexibility. PVDF is piezoelectric polymer material that is
commonly used in energy harvesting devices. PVDF features
Fig. 2. An overview of the piezoelectric energy harvesting systems for
biomedical applications by Panda et al. [12].
decent flexibility resulting in good fatigue resistance, and it
is able to provide relatively more energy in its lifespan. For
higher flexibility, Piezoelectric Composite Fabrics are used
for energy harvesting.
The favorability of using relaxor ferroelectric unimorphs
such as PMN-PT in energy harvesting devices is due to its
high piezoelectric coefficient and electromechanical coupling
coefficient, adding its high strain level [10].
Although inorganic piezoelectric materials are associated with
relatively high piezoelectric coefficients, they are fragile and
cannot sustain significant deformations from the soft and
highly flexible tissues of the human body. Thus, the recent
research focus is to develop stretchable piezoelectric energy
harvesters and sensors with large mechanical compliance while
maintaining high piezoelectricity. Currently, two strategies
have been investigated for developing stretchable piezoelectric
devices, either through intrinsically stretchable piezoelectric
materials with advanced material synthesis, or by applying
stretchable structural designs to provide stretchable properties
in conventional brittle materials [11].
IV. POEZOELECTRIC ENERGY HARV ES TI NG
APPLICATIONS
Figure 2 displays an overview of the piezoelectric energy
harvesting systems for biomedical applications presented by
Panda et al. [12]. This study focuses on wearable and im-
plantable piezoelectric energy harvesting applications.
A. Wearable Harvesters
Power can be harvested from outside the human body to
be incorporated with wearable devices such as shoes, and
watches. Power can be generated from body heat, walking,
small movements, and blood pressure fluctuations. Table I
shows the approximate amount of energy produced from
different types of activities [13]. The small amount of power
generated from the human body movement can be sufficient
for low power electronics.
Wearable harvesters usually consists sensors, signal condition-
ing circuits and wireless transmission technology. Applications
for such devices include on body monitoring, prosthetics and
orthotics, and portable electronic systems. On-body monitor-
ing and body sensing is one of the applications of importance
to the biomedical industry. It has been a popular topic in
healthcare and a lot of research interest has been going towards
the design of wearable devices with long-term efficiency.
A piezoelectric pulse generator for self power body sensing
nodes that utilizes piezoelectric impulse excitation mechanism
was developed by Jiang et al. [14]. This work proved to be
suitable for excitation by the low frequency random body
motion. An oscillator circuit as a proof of concept was created
that operates at a resonant frequency of 310MHz [14].
A simple wearable sensing system that utilizes harvesting
human body energy during activity was addressed in [15].
Piezoelectric fiber composite materials were used to harvest
energy from the elbow and knee joint. This is used to power
up sensors that transmit information about the well being of
the body itself such as ECG, and blood pressure sensors.
A curved piezoelectric generator using PVDF that harvests
electrical energy from low-frequency body movement was
developed by Jung et al. [16]. The generator on its own was
able to light up 476 commercial LED bulbs and generate
around 45Vand operates to frequencies as low as 1Hz. The
generator was tested as a power source for wearable devices
and was integrated with both a watch and a shoe-insole.
The insole generator was able to produce 14Vof average
power and the watch 22V. This power is used to prolong the
operating time of wearable electronics.
Kymissis et al. [5] used a unimorph strip made from a
piezoelectric composite material and a multi-layer laminate of
PVDF foil. These generators were built into a shoe to harvest
electrical energy. At a walking frequency of 1Hz, the PVDF
produced peaks of 20mW while the unimorph had a larger
response of 80mW . These methods provide the least invasive
and efficient solutions to be applied directly to the sole of
shoes. They are efficient to power low power and low duty
cycle devices.
Mhetre et al. [17] discussed the generation of voltage from
human exhalation to power up electronic devices. It was found
that as the age, weight, and height of a person increases, the
more the exhalation force and the more the output. The output
of the average person with normal breathing into a pipe with a
PVDF film varies between 0.2to 3mV . If more than one piezo
film is connected in series, the voltage generated is more.
A device was developed by Zhang et al. [18] that uses, a
stiffness spring with piezoelectric generator to harvest the
mechanical energy of the foot. The harvester was placed in
a shoe near the heel, where the presented results show that a
60kg person could harvest 235.2mJ per step.
A wearable insole based on PVDF array-based piezoelectric
harvesters was developed by Gao et al. [19], which was able
to generate 8.6mW power output during running. For non-
invasive human health monitoring, Li et al. [20] developed a
piezoelectric nano-generator based self-powered sweat sensing
device, which is attached on finger joints, and equipped with
wireless signal transmission to a user interface.
Beyaz et al. [21] presented a piezoelectric harvester belt
capable of generating 1.51.8Vduring normal and fast
respiration, respectively. Liu et al. [22] developed a wearable
sensor based on flexible piezoelectric harvester that generates
electric energy from respiration, shouting, and muscle move-
ment, e.g. wrist bending, which can be used for wearables.
Hybrid piezoelectric-triboelectric nanogenerator was utilized
by Liu et al. [23] for limb sensing and information interaction
through morse code detection, and by Zou et al. [24] as
biosensor.
B. Implantable Harvesters
Implantable devices such as cardiac monitors, cardiac pace-
maker and cardioverter defibrillator offer therapy and diag-
nostics for various health issues. Most of these devices are
powered by internal battery and cutting edge research achieved
great reduction in size with increased lifetime. However, the
batteries still suffer from limited operational lifetime which
force the need for surgery to replace it. This can be very
inconvenient to the patient as it expose him to the risk of
surgical procedures including post side effects and possible
mortality. Thus, considerable research work was motivated to
develop energy harvesting techniques that are able to harness
the wasted energy of human body in order to supply the
implantable devices as an alternative to the replaceable battery.
Karami and Inman [25] investigated powering a implantable
pacemaker in the chest with piezoelectric energy harvesting
device. It was proposed to use low frequency zig-zag structures
along with non linear magnetic configurations. This device
aimed to achieve a size of around four cubic centimeters. For
powering intra-cardiac active medical devices, Martin Deterre
et. al [26] presented an energy harvesting technique that is
based on a packaged implant which is subjected to stress be-
cause of cyclic blood pressure variations in the cardiac cavities.
The stresses were transmitted on a spiral shaped piezoelectric
transducer converting these vibrations into electrical energy to
power the device. The shapes of the harvester components
design have a major influence in the amount of harvested
mechanical energy. This work reported to achieve a power
of 4.15W/cm3at 1.5Hz.
A Combination of in vitro and in vivo experiments were
conducted by Zhang et al. [27] to attempt to utilize the
pulsation of ascending aorta in harvesting energy. The choice
of the ascending aorta was due to its high level of deformation
compared to other blood vessels making its power output is
the largest. Through the in vitro simulated experiment, the
proposed piezoelectric generator provided high output power
and flexibility. In vivo study was performed by implanting the
harvester to wrap around the ascending aorta, and the output
powered generated was lower than in vitro.
A method to power an implantable nano-sensor using ul-
trasounds was proposed in [28]. The nano-sensor was en-
capsulated in an artificial cell and attached to the human
tissue, where an external source produces ultrasounds to create
vibrations in piezoelectric wires of the nano-sensor to harvest
the energy required to power the device. Lewandowski et al.
[29] proposed the use of an implantable piezoelectric energy
generator that is able to harness the energy from stimulated
muscle in order to power electronic medical devices that are
implanted in human body. In this method, a stack piezoelectric
generator was connected with a muscle tendon in series, where
muscle contraction force was caused by activating the motor
nerve electrically to apply strain on the piezoelectric stack
in order to generate charge, which then was store in a load
capacitor. The generated energy was used to both stimulate
muscle and power an implanted medical device. Here it was
assumed that the power generated from stimulated muscle
contractions is larger than that need for the stimulations.
In [30], the authors provided results to verify the feasibility of
using a piezoelectric generator harvester that harvests energy
from stimulated muscle in extending the life of implanted
devices batteries or making them independent from external
power sources. In the conducted simulations, a force pulses
of 250Nwere applied on 8cm long implanted piezoelectric
harvester. The achieved power generation was 690µW which
is greater than the estimated input power of 46µW .
To monitor and diagnose patients that undergone total knee
replacement surgery, et al. [31] suggested the use of PZT
material for energy harvesting to provide low power sensors
and microprocessors with energy within the implants. This can
overcome the major limitations of embedded implant sensors,
allowing them to have self-replenishing power and to operate
for long term. An adequate electrical energy can be harvested
from the mechanical energy provided by ordinary physical
activities using an orthopedic implants encapsulated with PZT
ceramics. The presented results showed that three 1.2cm3PZT
ceramics encapsulated within a total knee replacement implant
was able to provide 4.5mW , where a conditioning circuit was
utilized to transform the Piezoelectric output to power a low
power microprocessor. The efficiency of transforming mechan-
ical energy into raw electrical energy said to be approximately
19%.
A tibial implant embedded with piezoelectric ceramics was
presented by Almouahed et al. [32] for the detection of post-
operative complications of total knee replacement surgery. The
change in pressure center was measured for different levels
of compression in order to detect issues such as soft-tissue
and imbalance complications tibiofemoral misalignment. The
harvested power by the piezoelectric ceramics was used to
power a wireless transmitter allowing it to send in vivo data
regarding the knee implant to outside the body for further
processing.
Beker et al. [33] proposed the use of a MEMS piezoelectric
energy harvester to stimulate auditory nerve inside cochlea by
implanting it on eardrum or ossicles. The proposed harvester
consisted of multiple cantilevers, each had different resonance
frequency to cover the hearing band. The piezoelectric en-
ergy harvesting device produced signals from each cantilever
that were used to stimulate the corresponding section of
the auditory nerve. This method eliminates the need of the
conventional cochlear implants components such as battery,
microphone, transmitter, and sound processor. A prototype
of this MEMS piezoelectric energy harvester with a single
cantilever was fabricated, and the results displayed it capability
to generate signals at a typical eardrum vibration, that can
stimulate the auditory nerve. Thus, this method can be a
potential self powered alternative to the conventional cochlear
implants.
For mechanical activity monitoring in self powered biome-
chanical implants, Piezoelectricity-driven hot-electron injec-
tors (p-HEI) can be used. Zhou et al. [34] self-limiting
capability of an energy harvester was exploited to operate
a p-HEI device. The proposed p-HEI device was said to be
capable of self activation with an input power of less than
5nW . A 0.5µm bulk CMOS process was used to fabricate
a prototype in order to verify the device functionality. The
results showed that the dynamics of the device was quasi-
linear with respect to time, given a fixed input power. Zhou
et al. [35] presented a method for photodynamic therapy by
providing in situ localized light using an implantable light
source powered by ultrasonic waves. The proposed implants
dimensions were 2×4×2mm3and 2×2×2mm3, and
they included a piezoelectric harvester attached to two red
LEDs. The dimensions of the introduced device allowed its
implantation inside a tumor to be done using a biopsy needle.
In vitro experiments were conducted, and the results implied
that the in situ light offered by the device was adequate for
photodynamic therapy with tolerable increase in the treatment
time.
A pacemaker powered by bio-compatible piezoelectric har-
vester was investigated by Azimi et al. [36]. It generates
pacing pulses by harvesting bio-mechanical energy from the
left ventricle, based on the potential limitations. Li et al. [37]
used a implantable piezoelectric energy generator to power
a cardiac pacemaker that pace porcine heart in vivo through
heartbeats kinetic energy extraction. The output current was
15µA, which powered a contemporary and functional cardiac
pacemaker.
V. CONCLUSION
The design of wearable and implantable energy harvesting
devices has drawn a lot of research attention in the past years.
Piezoelectric materials provides the perfect candidate for such
systems due to its long-term stability. Piezoelectricity allows
us to harvest energy from naturally available energy sources
such as vibrations, body movement, stress and strain. This
allows us to utilize it for the increasing demand for self-
powered electronics. This article provides a study of what
researchers have carried out to harness energy from the body
through implantable or wearable devices. Implantable devices
are required to self-power and hence increase their life and
reliability while wearable devices are targeted towards power-
ing electronics or monitoring the body vitals. This paves the
way towards a sustainable and personalized health-care system
driven by the advancement in biomedical signal processing
and artificial intelligence technologies implemented with em-
bedded self-powered wearable and implantable devices and
biosensors.
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... Over the last few years, piezoelectric elements have emerged as potential and reliable options for harnessing ambient mechanical vibrations to generate usable electrical energy. They have been used to operate sensor networks and wearable devices for personal health monitoring by converting the mechanical vibrations caused by human motion into electrical power [8][9][10][11]. As a typical example, by attaching a piezoelectric element to the sole of a subject's shoe, a voltage can be generated by walking and supplied to a wearable device without putting a burden on the subject [12,13]. In [14], a practical application of a piezoelectric shoe-based RFID device was demonstrated using various piezoelectric materials, such as polyvinylidene fluoride, and piezoelectric unimorph devices. ...
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Motion recognition and information interaction sensors with flexibility and stretchability are crucial functional module as interactive media between the mechanical motions and electric signal in intelligent robot and rehabilitation training system. Nanogenerator has a good application in the field of intelligent interaction with advantages of self-powered sensing ability, easy fabrication, considerable sensitivity and reliability. However, the singularity of the sensing mode limits its application scenarios. Hence, in this paper we develop a flexible and stretchable dual modes nanogenerator (FSDM-NG) based on the integration of piezoelectric and triboelectric principles for human motion sensing and information interaction. In piezoelectric mode,the FSDM-NG can effectively monitor the bending angle of joints(finger, wrist and elbow)from 30° to 90°. In triboelectric mode, the text and logic information transfer are encoded based on the Morse code and logic gate, respectively. Besides,the device has good adhesion, biosafety and robustly which make it works normally even under water environments. Combining these two sensing mechanisms, multiple modes of sensing from touch and stretch based on the FSDM-NG can be achieved for information interaction in real time. The proposed sensor has potential to adapt to more complex sensing, which may provide a new path for intelligent interaction of robots and rehabilitation training field.