UNIT II

SIGNAL PROCESSING AND ENERGY HARVESTING FOR

WEARABLE DEVICES
 Introduction

• Use of mobile sensor-based platforms for human action recognition

is an ever-growing area of research.
• Recent advances in this field allow patients to wear several small
sensors with embedded processors and radios.
• Collectively, these sensors form a body sensor network
• Although BSNs have the potential to enable many useful
applications, limited processing power, storage and energy make
efficient use of these systems crucial.
• Moreover, user comfort is a major issue, which can cause patients to
become frustrated and stop wearing the sensor nodes.
• The interaction between the human body and these wearable nodes
here is defined as wearability.
 Wearability Issues
• Wearable devices and electronics present a unique interface of
technology and humanity, thus producing unique challenges that
need to account both for technological and human aspects of the
problem.
• Human behavior may affect the operation of wearable as much as
the technology advance.
• The following aspects may be considered as some of the
challenges facing the field of wearables:
Break-through applications of wearable electronics.
• From the dawn of history, the evolution of wearables is driven by
the practicality, utility, and convenience they provide.
• The challenge of modern wearable electronics in discovering
ubiquitous applications, as its future growth is contingent on
emerging applications in health, wellness, and other personal
needs.
• Novel applications of wearables may need to be supported by
extensions of sensing and data analytics capabilities, thus
presenting a compelling use case
Minimization of user burden and integration with everyday
wear items. The illustrations of wearable devices frequently
include pictures of individuals instrumented at every possible
location on the body, such as arms, legs, torso, etc.
Practically, such a scenario represents an unrealistically high user
burden and is unfeasible.
A related challenge is the seamless integration of wearable
electronics in everyday wear items, such as textiles clothing,
footwear and accessories.
Efficient and informative interpretation of data generated by
wearable devices.
• Wearable devices may generate an abundance of data, for e.g.,
health-related sensor signals.
• The challenge lies in the interpretation of such data streams and
connection with health outcomes, using sensor data to guide
behavioral interventions and health education.
• Emerging methods of artificial intelligence carry a promise of
solution to the problem of data analysis and interpretation
Ultra-low power operation.
• A wearable device should ideally sustain a lifetime operation without
or minimal user interference.
• In terms of power, this implies operation on a battery, energy
harvested from the body, or a combination thereof.
• This requires low-power operation both for analog and digital
electronics of a wearable.
• Wireless power delivery may be explored to seamlessly charge many
devices without need to connect each individual device to a charging
circuit (for e.g., charging all socks in a drawer), biofuel cells and
super capacitors may need to be utilized in the power subsystems.
Flexible and stretchable electronics
• Epidermal and body compliant electronic devices may be
considered a subset of wearables with additional requirements of
allowing shape changes in response to body movement, making
such devices especially sensitive to motion artifacts, demanding
high biocompatibility and adaptability to variation in human body
shapes, sizes, and characteristics.
Biocompatible communications:
• Communications from the body (to the outside world) and on the
body (between multiple wearables) demand new solutions, as
traditional radio methods experience challenges due to absorption
by body tissue.
• The related challenges include development of efficient methods
for communicating through or on the body, including the
organization of wearables in body sensor networks and their
integration into the Internet of Things Biodegradable Electronics.
• If wearable electronics are to become the true mainstream, the
challenge of sustainable, ecologically viable manufacturing, and
disposal needs to be addressed.
Privacy and Security:
• By definition, a wearable is an electronic device that resides on
or close to a person and is present in a variety of life situations.
• The challenges include protection of personal information,
preventing the unauthorized use of wearables for biometric
identification, and ownership of the data produced by wearables
• Functionality criteria constrains node placement to regions where
relevant data can be sensed.
• The number of nodes required to capture all relevant data can
vary based on the quality of information sensed at individual
locations.
• Convenience criteria include: (1) physical interference with
movement, (2) difficulty in removing and placing nodes, (3)
social and fashion concerns, (4) frequency and difficulty of
maintenance (charging and cleaning)
• This makes energy usage a primary constraint in designing BSNs,
limiting everything from data sensing rates and link bandwidth, to
node size and weight.
• Thus, one of the important goals in designing BSNs is to
minimize energy consumption while preserving an acceptable
quality of service.
• Energy consumption can be decreased by lower sampling
frequency, decreasing processing power, and simplifying signal
processing.
physical shape and placement of sensor
• The sensor such as 3-axis accelerometer can be used as body position
sensor.
• This sensor provides information of patient's position i.e. standing,
sitting, supine, inclined, left and right.
• Such accelerometer based sensor can easily be interfaced with any
microcontroller boards such as arduino uno, arduino mega etc.
• Design for wearable BSNs focuses on specific and important issues for
developing wearable computing systems that take into account the
physical shape of the sensors and their active relationship with the
human form.
• Design for wearability requires unobtrusive sensor node placement on
the human body based on application-specific criteria.
• Criteria for placement can vary with the needs of functionality and
• Criteria for placement can vary with the needs of functionality
and convenience.
• Functionality criteria constrains node placement to regions where
relevant data can be sensed.
• The number of nodes required to capture all relevant data can
vary based on the quality of information sensed at individual
locations.
• For example, in continuous healthcare monitoring, patients will
be expected to charge the sensors or replace the batteries on a
regular basis, as they do with cell phones and other electronics.
• However, the frequent need to charge and the bulk of the battery
can frustrate the users, causing them to no longer wear the
sensors.
• Furthermore, batteries are the heaviest component in the system.
• By decreasing power usage, the size and weight of each sensor
node can decrease, thus increasing patient comfort and device
wearability.
Technical challenges - sensor design and signal Acquisition

• As engineers seek to develop new and innovative wearable

diagnostics, many are focusing on smaller, more energy-efficient
devices.
• In the process, respondents say they are fighting the kind of typical
constraints that factor into many of today’s product development
efforts, citing top among them the areas of cost (38%), durability
(37%) and power management (35%).
• There are other unique challenges in designing diagnostic wearables to
be used by a patient, caregiver, or consumer in a non-medical setting.
• There are other unique challenges in designing diagnostic
wearables to be used by a patient, caregiver, or consumer in a
non-medical setting.
• High user expectations around ease of use, the need for intuitive
user interfaces and complete documentation, as well as the need
to account for the vagaries of uncontrolled home care settings top
the list of challenges cited by engineers.
• High user expectations around ease of use, the need for intuitive user
interfaces and complete documentation, as well as the need to account
for the vagaries of uncontrolled home care settings top the list of
challenges cited by engineers.
• Data collection and connectivity represent another area for concern.
• Nearly one third (30%) of respondents pointed to connectivity as a
challenge.
• Over two thirds (82%) agree that there isn’t a lot of clarify about how to
effectively capture and use the data or doing something medically
effective with it once collected.
• Nearly all (94%) cited a need to for ownership of data security and
privacy.
 Signal Acquisition
• Wearable sensors have demonstrated wide applications from
medical treatment, health monitoring to real-time tracking, human-
machine interface, smart home, and motion capture because of the
capability of in situ and online monitoring.
• Data acquisition is extremely important for wearable sensors,
including modules of probes, signal conditioning, and analog-to-
digital conversion.
• However, signal conditioning, analog-to-digital conversion, and
data transmission have received less attention than probes,
especially flexible sensing materials, in research on wearable
sensors.
• Wearable sensors are able to monitor various human body signals,
ranging from biophysical signals (including human motions,
respiration rates, bioelectricity, etc.) to biochemical signals (such as
body fluids, blood components, glucose, etc.).
• The target biophysical/biochemical signals are converted to
electrical signals by probes, the first component of the data
acquisition (DAQ) module.
• Then, the raw electrical signals are processed by the remaining two
components of the DAQ module, i.e., signal conditioning and
analog-to-digital conversion in sequence.
• The processed signals are eventually transmitted to terminals by the
data transmission (DT) module.
Wearable sensor working processes
 Including amplifiers pursuing high linearity and low power consumption;
filters suitable for signal and noise characteristics; high-performance ADCs
with both noise rejection and energy efficiency.
 Challenges
• Stability deals with the degree to which sensor performance and hence
response remain constant over time.
• Stability is a major issue faced by wearable chemical sensors, and by
many mechanical sensors that stretch or deform.
• For chemical sensors, continuous exposure to biofluids may lead to
biofouling, chemical changes, or irreversible non-specific adsorption on
the transducer surface.
• For mechanical sensors, they can reach strain limits or experience to
many actuation cycles, either resulting in mechanical material
degradation or failure.
• Optical and electrical sensors are often inherently robust, especially if
they rely on proven metal and semiconductor materials.
 Sampling frequency for reduced energy consumption

• The sampling frequency or sampling rate, fs, is the number of

samples divided by the interval length over in which occur,
thus fs = 1/T, with the unit sample per second, sometimes
referred to as hertz, for example e.g. 48 kHz is 48,000 samples
per second.
• According to the Nyquist-Shannon theorem, the sample rate
should be at least two times the highest frequency you intend to
capture to represent an audio signal accurately.
• Long battery runtime is one of the most wanted properties of
wearable sensor systems.
• The sampling rate has an high impact on the power consumption.
• However, defining a sufficient sampling rate, especially for
cutting edge mobile sensors is difficult.
• Often, a high sampling rate, up to four times higher than
necessary, is chosen as a precaution.
• Especially for biomedical sensor applications many contradictory
recommendations exist, how to select the appropriate sample rate.
• The demand for wearable technology steadily increased over all
fields of application in the last years.
• Requirements for wearable sensors differ from general purpose
computing because of its low-power system design.
• The choice of the appropriate sampling rate is one of the most
discussed topics in biomedical signal processing.
• On one hand there are interests to keep the sampling rate as high
as possible on the other hand there are interest to the lowest
possible sampling rate.
• This question must be answered in a early development stage
because many design decisions for hardware and software are
made based on the sampling rate.
• One argument for keeping the sampling rate high is to make sure
that all frequencies that might be relevant are available in the
digital domain. It depends on the application which sampling rate
has to be selected, e.g to derive only the heart rate or more
complex information from an electrocardiogram (ECG), e.g. heart
rate variability or arrhythmia
 Power Requirements- Solar cell, Vibration based, Thermal
based,
• The term "solar-powered wearable devices" refers to a new class
of technology that uses solar cells to produce and store electrical
energy.
• These solar cells, which are frequently created from photovoltaic
materials, may convert sunlight into power, allowing wearables to
operate autonomously without needing regular battery
replacements or wall-mounted charging.
• Energy harvesters such as solar cells (which obtain power from
the sunlight) or Thermoelectric Generators (TEGs) (which obtain
power from temperature differences) have very low efficiencies
and the energy they harvest is not always available (e.g., solar
cells in the night).
• Hence, they are used as backup sources that work alongside
rechargeable batteries.
• Using two or more energy sources in a hybrid energy harvesting
system is also a good option, since one power source can back up
the others when they are not available.
• Even though solar energy is not always available, it has become
the most used energy source for energy harvesting applications in
wearables.
• Solar cell performance depends on many factors, such as the
operating conditions of the load (sensor measurements and
connectivity), time, location and position of the solar cells, and it
is difficult to theoretically estimate the generated power when
they are included in a wearable device.
• Using energy harvesting not only increases the battery life of
portable devices with the energy available in the environment, but
it also reduces the ecological footprint compared to fossil fuels,
the main power source used worldwide.
• Solar energy is the most common energy source used in energy
harvesting systems since it is possible to obtain tens or even
hundreds of mW with relatively small-sized solar cells
Schematic of a typical solar energy harvesting circuit
 Benefits of Integrating Solar Power into Wearables
• Sustainability and Environmentally Friendly -The sustainability
of solar-powered wearables is one of its main features. These
gadgets help conserve natural resources and fight technological
waste by lowering reliance on throwaway batteries and
conventional charging techniques. Accepting solar technology in
wearables becomes a huge step in the correct way as the globe
moves towards a greener future.
• Extended Battery Life-Wearable technology's internal battery is
supplemented by solar power, thereby increasing the time between
charges. Outdoor enthusiasts, travelers, and hikers who frequently
find themselves away from power outlets may benefit the most
from this perk. Wearables that employ solar charging may
continually draw power from the sun, ensuring convenience and
uninterrupted usage.

• Freedom from Power Outlets-When the wearable's battery gets

low, you no longer need to carry charging wires around or look for
outlets. we may confidently go to isolated regions or participate in
outdoor activities with solar-powered wearables without worrying
about a dead gadget. The dependable power source, the sun is
• Energy-Efficient Technology-Technology for solar cells is
advancing and becoming more energy-efficient. This implies that
wearable solar devices may efficiently capture solar energy,
offering a workable and sustainable substitute for traditional
power sources. Users may utilise technology guilt-free knowing
they are helping to protect the environment by consuming less
energy.
 Solar Textiles: Integrating Solar Cells into Fabric
• The revolutionary development of solar textiles, sometimes
referred to as solar fabrics or photovoltaic textiles, mixes solar
cells with fabric to make it possible to include renewable energy
harvesting into regular apparel and textiles.
• From clothing and outdoor gear to architecture and transportation,
this cutting-edge technology has the potential to revolutionise
many different industries.
• The functionality of solar textiles extends beyond traditional
solar panels. They offer several advantages:
• Versatility
• Portability
• Energy Independence
• Aesthetic Appeal
Challenges and Advancements in Textile-based Solar Technology

• Efficiency: One of the primary challenges is improving the efficiency

of solar textiles. Flexible solar cells generally have lower efficiency
compared to rigid panels, so optimizing the energy conversion rate
without compromising flexibility is a crucial goal.

• Durability: Textiles are subjected to wear and tear, exposure to the

elements, and frequent washing, which can pose durability challenges
for integrated solar cells. Researchers are developing more robust
materials and coatings to enhance the lifespan of solar textiles.
• Flexibility and Comfort: Solar textiles must maintain their
flexibility and comfort to be practical for everyday use. Ensuring
that the solar components do not hinder the fabric's feel and drape
is essential for user acceptance.

• Scalability and Cost: Mass-producing solar textiles at an

affordable price is crucial for widespread adoption. Finding cost-
effective manufacturing methods and using sustainable materials
can help drive down production costs.
Energy Harvesting Techniques for Wearable Devices
• From smartwatches and fitness trackers to medical monitoring
gadgets, wearable technology has ingrained itself into our daily
lives.
• Assuring a consistent and stable power source to maintain the
operation of wearables is one of the major hurdles in the
development and acceptance of these devices.
• Wearables may now collect and store energy from their
surroundings to power themselves thanks to energy harvesting
techniques, which have emerged as a potential alternative.
 Applications of Solar-Powered Wearable Technology
Health Monitoring
• Solar-Powered Fitness Trackers
• Solar-Powered Smartwatches
Bright Clothing
• Solar-Powered LED Jackets
• Solar-Powered Backpacks
Outdoor Activities
• Solar-Powered GPS Watches:
• Solar-powered headlamps
• Solar-Powered Water Purifiers
Vibration based power Requirements of wearable devices
• With the vast amount of wearable technology available, the
demand for compact devices with smaller batteries, or no batteries
at all, and longer charge duration has presented a challenge.
• Consumers of wearable technology want the convenience of a
portable device without the need for frequent charging or bulky
and expensive batteries.
• Producers of wearable technology are then tasked with creating
devices that meet this demand.
• The use of piezoelectric components in wearable technology is a
solution for this issue.
• The use of piezoelectricity stands to reduce, or even eliminate, the
need for frequent charging of devices and batteries.
• Consumers will no longer be burdened with having to be near an
electrical outlet, which will in turn conserve electricity.
• As a result, wearable devices and more efficient batteries will
have longer usable lives.
• This will also reduce the environmental hazards presented by the
frequent disposal of batteries and electrical components into
landfills.
• Harvesting energy using piezoelectric ceramic involves the
conversion of energy from vibrations that occur during walking,
breathing, and moving on many parts of the body.
Placing sensors
• When stressed, piezoelectric components create an electrical
current that can be immediately used or stored. The amount of
energy produced is still relatively small and required body
movements aren’t often regular and predictable. Also, a large
surface area is often necessary to harvest a sufficient amount of
energy. This presents a challenge when thinking of the small size
needed in wearable devices.
 Direct and inverse piezoelectric effects
• Two promising factors in surmounting these obstacles are the
versatility of piezoelectric components and the fact that the
efficiency of piezoelectric energy harvesting has increased, while
the power requirements for current wearable devices have been
reduced.
• There are four different types of materials that can be used for
piezoelectric energy harvesting: ceramics, single crystals,
polymers, and composites.
• Of these, ceramic is the preferred material for this type of energy
harvesting because of its low cost, effective piezoelectric
properties and easy incorporation into energy harvesting devices.
• Piezoelectric vibration energy harvesting is the preferred method for use
with wearable devices since it is the most capable of producing the power
level needed for small-scale devices.
• There are two kinds of mechanical energy that can be scavenged from the
human body.
• The first is related to continuous activity, such as breathing and heart
beating; while the other is related to discontinuous movements, such as
walking and joint movements.
• The process of walking produces the largest amount of power compared
with other body motions.
• It has been recorded that a 68kg man is able to generate 67W when
walking at a speed of two steps per second.
• The easiest way to harvest this energy is through piezoelectric shoe
• Body joints are also attractive locations for harvesting energy due
to their high motion amplitude, fast angular velocity, large
impulse force, and high frequency of use in daily human
activities.
• For example, the knee joint produces high biomechanical energy
since it generates a larger torque in comparison to other human
joints.
• Knee joint motions are often related to gait motion, where
walking and running frequencies are normally in the range of 0.5-
5 Hz.
• Even for relatively minor activities such as eye blinking,
piezoelectric transducers have effectively been used to convert
• For example, a self-powered sensor was developed for both
energy harvesting and health rehabilitation monitoring, which
was based on polymeric piezoelectric nano/microfibers.
• Furthermore, continuous energy can be harvested from the
process of human breathing.
• There are two kinds of energy that can be collected in this case.
• The first relies on scavenging energy due to the intake and release
of air, which can produce approximately 1 W of power.
• The other relies on chest expansion, which requires a tight band
fixed around the chest of the user to generate around 0.83 W
when breathing normally.
 Wearable Piezoelectric Applications
• Piezoelectric components can be used for wearable technologies
and other new technologies. Their use presents vast possibilities
across many industries.
• Human comfort, convenience, health and safety have the potential
to be greatly improved with the availability and use of products
containing piezoelectric components.
• Many of these capabilities and products are already emerging in
today’s society.
• These include:
 A piezoelectric pacemaker that is powered by the rhythm of a beating
heart. This eliminates the need for invasive and dangerous surgery for
battery replacement
Footpath lighting powered by footsteps striking energy-absorbing tiles
The ability to power monitoring and sensor devices in remote and
dangerous places (bridges, pipelines, etc.). This eliminates the risk to
humans that arises when batteries need charging or replacing
A vehicle driver’s seat that uses piezoelectric sensors to monitor and
sense driver’s heart rate and respiration. It uses vibration sensors to
allow ventilation and massage features to be automatically activated in
the seat when driver stress is detected
 Wearable devices that can be charged by walking, running or other
physical activity
 Design Challenges in Wearables with Piezoelectric
Technology
• Material Choice- Textiles that have a greater elasticity perform at
a greater efficiency when harvesting piezoelectric energy. The
greater elasticity of the material increases the stresses occurring in
the garment and, consequently, increases the elongation of
piezoelectric elements. In addition, the garment must be form
fitting in order to increase the clothing pressure and increase the
piezoelectricity efficiency by increasing the strain exerted on the
harvester on the garment. However, with this increased tightness
of the garment on the user, this subsequently restricts the user’s
movements and their ability to harvest energy.
• Durability: Energy harvesters are required to have high
environmental durability and operational reliability. However, in
the case of piezoelectric energy harvesters, the material properties
may change during the manufacturing process, even if the
piezoelectric effect is caused by intrinsic physical properties such
as the crystal structure of the material. When a strain is repeatedly
applied to a material, macroscopic cracks may occur resulting in a
drop in the amount of power generated. Clarifying the mechanism
behind the deterioration of materials that occurs during the
conversion of kinetic energy into electric energy and taking
countermeasures are challenges for piezoelectric technology.
• Operating Frequency: It is a well-known issue with piezoelectric
energy harvesters that they do not harvest energy efficiency at
varying frequencies. These devices operate at a high frequency
whereas humans have an ultra-low frequency of around 1Hz. As
the operating bandwidth of piezoelectric energy harvesters is
quite high, this significantly limits their utility within real world
applications in wearable devices.
Human body as a heat source for power generation
Introduction
• Wearable devices are widely used in different areas, such as
personal health monitoring systems, electronic sports sensors,
smartwatches, and clothing.
• The main concern in evolving IoT technology embedded in smart
wearable devices is to find a sustainable power supply.
• Three different solutions have been proposed so far to overcome
this obstacle.
• The first is to achieve an enhanced design with a higher battery
capacity.
• The second is replacing batteries with a permanent power supply,
and the third is optimizing wearable devices so that they need less
input power.
• Batteries cannot be a suitable power source for new generations of
wearable gadgets due to their many practical limitations .
• In addition to being bulky, batteries have a limited capacity and often
either must be charged periodically or replaced after a short period of
operation
• Limitations on the use of batteries are especially more evident in
health care monitoring systems and implanted medical devices
• In these cases, the batteries must be able to provide the conditions of
continuous and uninterrupted operation of the mentioned systems.
• Because the power consumption of wearable devices is often in the
range of microwatt to milliwatt, the use of heat dissipated from the
human body as a source of electrical power required by wearable
devices has attracted the researchers’ interest
• It is anticipated that replacing bulky, low-capacity conventional
batteries with body-heat-driven micro-generators could pave the
way for further development of smart wearable technologies.
• In recent years, various approaches have been introduced to extract
energy from the human body.
• Some of these methods exploit physical body motions, and some of
them benefit from body heat dissipation.
• Available body energy from daily activities can be converted to
electricity utilizing piezoelectric devices, electrostatic-based
harvesters, electromagnetic generators, and triboelectric generators.
• The skin temperature is generally higher than the ambient
temperature.
 Human body as a heat source for power generation
• Body heat applied to a thermoelectric generator plus energy
harvesting to produce power for a wearable device achieves both
minimization of form factor and power consumption.
• Other consideration in powering wearable devices is the necessity
to impose weight and size constraints, particularly if you initially
choose a battery as the source of power.
• To limit size and weight you should use energy harvesting instead
of the battery.
• harvest energy from several environmental sources:
1. Light, using photovoltaics
2. Movement of the wearer
3. Radio frequency energy (RF)
4. Temperature differences using a thermoelectric generator (TEG)
An evaluation of these environmental sources reveals that
photovoltaic or RF harvesters limit the application of zero-power
wearables to environments where sufficient ambient light or RF
emissions is provided to satisfy the energy budget.
• Movement-based harvesting systems require an active wearer and
usually have unstable power generation characteristics.
• In contrast, the human body is a constant heat source and
typically a temperature difference exists between body core and
the environment.
• Even in a scenario where the wearer is stationary and situated in a
dark room (e.g., during sleep), energy can be produced. Lower
ambient temperatures, the presence of air convection, or increased
activity of the wearer can drastically increase the amount of
accumulated energy.
• Because the voltages produced by thermal harvesting are
typically too low to power wearable electronics, you must include
a high-efficiency dc-dc converter into a wearable system.
• Thermoelectric energy conversion of human body heat represents
a promising alternative as it is largely independent of external
factors.
• The average power harvested per square centimeter is higher
using the thermal harvester than an equally sized solar cell.
• However, the produced voltage is used to directly charge a super
capacitor as an energy buffer and the device is only operational if
the ambient temperature is lower than 25°C to 27°C.
• Wearable devices have been used to monitor a variety of health
and environmental measures and are now becoming increasingly
popular.
• The performance and efficiency of flexible devices, however, pale
in comparison to rigid devices, which have been superior in their
ability to convert body heat into usable energy.
• Thermoelectric generators are semiconductor devices that have
no moving parts and convert heat directly into electricity.
• When combined with thermal storage they can provide electricity
round the clock at as low as $0.06 per kilowatt-hour and could
achieve 16% efficiency.
• Energy Harvesting from Temperature Gradient at the Human
Body. The human body continuously radiates heat.
• Devices with direct contact with the human body can harvest this
wasted energy by means of thermoelectric generators (TEGs).
• Depending on the amount of physical activity, about 60 to 180 W
of heat generated by metabolism in the body is exchanged with
the surrounding environment through heat convection and
radiation.
• The waste heat recovery from the human body can be a reliable
way to produce electric power for supplying wearable devices.
• This can be done through various methods such as the conversion
of infrared emissions from the human body to electricity and
methods based on four scientific phenomena, i.e., the
electrokinetic effect, pyroelectricity , thermoelectricity and ionic
thermoelectricity
• The effect of changes in skin temperature and the body’s
thermophysiological responses to environmental temperature
stimuli has been investigated on the performance of wearable
thermoelectric generators
• Optimizing the performance of wearable thermoelectric
generators (WTEGs), it is necessary to consider the effect of the
human body's thermoregulatory responses, such as skin
temperature and sweating rate.
• The change in skin temperature directly affects the performance
of WTEGs, and the change in sweating rate can also affect the
thermal resistance between the skin and the hot plate and
overshadow the matching of thermal resistances during operation.
 Methods of human body energy harvesting
• Body energy harvesters are often divided into active and passive
methods depending on their work principles.
• Active strategies usually generate electricity from harnessed
body motions during daily activities such as running, walking,
and even chest movement due to breathing.
• In contrast, passive methods use the human body’s heat loss to
generate the power required by wearable devices.
 Active human body energy harvesters
• Active energy harvesters are defined as systems that require
external mechanical stimuli to generate electricity.
• Walking, running, bending, and stretching are examples of
intentional external mechanical stimuli in the body that can be used
to generate electricity.
• Even involuntary movements of the body, such as movements
caused by breathing or blood flow , can be used to stimulate active
systems and generate electricity.
• The active human body energy harvesters include generators based
on piezoelectric, electrostatic, electromagnetic, and triboelectric
effects.
• In addition to generating potential difference and high electric
power, piezoelectric generators do not need an initial electric
potential difference to start up;
• The structure’s scalability and simplicity, combining these
generators with other types is difficult.
• Also, the production of low electric current and spontaneous
electric discharge in low-frequency external stimulations are
among the disadvantages of these types of generators.
• Generators based on the electrostatic effect can produce
significant potential difference and electric power density using
low frequency external excitations, but they need initial electric
potential difference to start working.
• The electric current produced by these tools is also relatively low.
Also, the dielectric breakdown phenomenon is one of the other
disadvantages of these generators.
• Electromagnetic generators do not need an initial electric potential
difference to start working and can produce a high electric current.
However, on the other hand, when they receive low-frequency
external stimuli, they produce a slight electric potential difference.
These generators have a complex structure and combining them
with other generators is difficult
• By using low-frequency external excitations, triboelectric
generators produce significant electrical power density, and at the
same time, they have high energy conversion efficiency and
flexibility;
• But, in their nature, there are problems such as low electric current
generation, low durability, and the difficulty of integrating them with
other generators.
• In addition to the structural disadvantages mentioned for the above
generators, the irregular and random movement of body parts as the
external driving force of these wearable generators harms their output
stability.
• In addition, the low frequency of body movements causes a decrease
in the performance of piezoelectric and electromagnetic generators.
• On the other hand, if the user is unable to move for any reason,
including old age, disability, or illness, these generators will not be able
to produce electricity.
• Also, the output power of generators that use involuntary body
movements such as breathing is insignificant.
 Passive human body energy harvesters
• The body core temperature is usually kept constant in a healthy
person by thermoregulatory mechanisms. For this purpose, the
excess heat produced in the body is transferred through the skin
in both forms of sensible and latent heat (sweat evaporation) to
the surrounding ambient. Passive energy harvesting methods are
commonly used to harness the sensible thermal energy lost from
the skin. Passive systems can be divided into two categories:
contact and non-contact systems.
 The non-contact system
• In this category, physical contact between the scavenger and the
human body is not required.
• heat loss from the human body through radiation, as the main
heat dissipation mechanism from the body at room temperature,
can be harvested by photovoltaic structures.
• The human body is an almost ideal infrared emitter source for
powering wearable and implanted devices at a skin emissivity of
0.98 for infrared wavelength ranging from 1 to 14 μm
 Contact systems
• Harvesters in this category require direct contact with human
skin since they exploit the dissipated heat from the skin as a heat
source by means of a conduction mechanism to generate
electricity.