Unit-I WEARABLE SENSORS

Introduction
Wearable technology is any kind of electronic device designed to be worn on the user's body.
Such devices can take many different forms, including jewellery, accessories, medical devices,
and clothing or elements of clothing. The term wearable computing implies processing or
communication capabilities, but in reality, the sophistication among wearable’s can vary.

The most sophisticated examples of wearable technology include artificial intelligence (AI)
hearing aids, Google Glass and Microsoft'sHoloLens, and a holographic computer in the form of
a virtual reality(VR) headset. An example of a less complex form of wearable technology is a
disposable skin patch with sensors that transmit patient data wirelessly to a control device in a
healthcare facility.

Need for wearables:

Fundamentally, wearables can perform the following basic functions or unit operations
 Sense
 Process(Analyze)

 Store
 Transmit
 Apply(Utilize)

Of course, the specifics of each function will depend on the application domain and the wearer,
and all the processing may occur actually on the individual or at a remote location (e.g.,
command and control center for first responders, fans watching the race, or viewers enjoying the
mountaineer’s view from the Mount Everest base camp).

Figure 2 is a schematic representation of the unit operations associated with obtaining and
processing situational data using wearables. For example, if dangerous gases are detected by a
wearable on a first responder, the data can be processed in the wearable and an alert issued.
Simultaneously, it may be transmitted to a remote location for confirmatory testing and the
results– along with any appropriate response (i.e., put on a gas mask) – can be communicated to
the user in real-time to potentially save a life. This same philosophy can also be used by an avid
gamer who might change his strategy depending on what “weapons” are available to him and
how his opponents are performing Each of these scenarios requires personalized mobile
information processing, which can transform the sensory data into information and then to
knowledge that will be of value to the individual responding to the situation. While wearables
are being used in many fields, as discussed, this chapter will focus primarily on wearables in the
healthcare domain.
Wearables provide an unobtrusive way to longitudinally monitor an individual – not just during
the day but, over the individual’s life-time. Such an expansive view of the individual will be
valuable in detecting changes over time and help in early detection of problems and diseases
leading to preemptive care and hence, a better quality of life. Inferring the potential of wearables
in other application domains should be straightforward and can be accomplished by instantiating
the fundamental principles and concepts presented here.

ATTRIBUTES OF WEARABLES
A sensor is defined as “a device used to detect, locate, or quantify energy or matter, giving a
signal for the detection of a physical or chemical property to which the device responds”. Not all
sensors are necessarily wearable, but all must have sensing capabilities. The key attributes
required of an ideal wearable are shown in Figure 4.
From a physical standpoint, the wearable must be lightweight and the form factor should be
variable to suit the wearer. For instance, if the form factor of the wearable to monitor the vital
signs of an infant prone to sudden infant death syndrome prevents the infant from physically)
lying down properly, it could have significant negative implications. The same would apply to
an avid gamer _ if the form factor interferes with her ability to play “naturally,” the less likely
that she would be to adopt or use the technology. Aesthetics also plays a key role in the
acceptance and use of any device or technology. This is especially important when the device is
also seen by others (the essence of fashion).

Therefore, if the wearable on a user is likely to be visible to others, it should be aesthetically

pleasing and, optionally, even make a fashion statement while meeting its functionality. In fact,
with wearables increasingly becoming an integral part of everyday lives, the sociological facets
of the acceptance of wearables opens up exciting avenues for research. Ideally, a wearable
should become such an integral part of the wearer’s clothing or accessories that it becomes a
“natural” extension of the individual and “disappears” for all intents and purposes. It must have
the flexibility to be shape-conformable to suit the desired end use; in short, it should behave like
the human skin. The wearable must also have multi-functional capability and be easily
configurable for the desired end-use application.

Wearables with single functionality (e.g., measuring just the heart rate) are useful, but in
practical applications, more than one parameter is typically monitored; and, having multiple
wearables _ one for each function or data stream would make the individual look like a cyborg
and deter their use even if the multiple data streams could be effectively managed. The
wearable’s responsiveness is critical, especially when used for real-time data acquisition and
control (e.g., monitoring a first responder in a smoke-filled scene). Therefore, it must be “always
on.” Finally, it must have sufficient data bandwidth to enable the degree of interactivity, which
is key to its successful use. Thus, the design of wearables must be driven by these attributes

Taxonomy for Wearables

Figure 5 shows the proposed taxonomy for wearables. To begin with, they can be classified
as single function or multi-functional. They can also be classified as invasive or non invasive.
Invasive wearables (sensors) can be further classified as minimally invasive, those that penetrate
the skin (subcutaneous) to obtain the signals, or as an implantable, such as a pacemaker.
Implantable sensors require a hospital procedure to be put into place inside the body. Non-
invasive wearables may or may not be in physical contact with the body; the ones not in contact
could either be monitoring the individual or the ambient environment (e.g., a camera for
capturing the scene around the wearer or a gas sensor for detecting harmful gases in the area).
Non-invasive sensors are typically used in systems for continuous monitoring because their use
does not require extensive intervention from a healthcare professional.
Wearables can also be classified as active or passive depending upon whether or not they need
power to operate; pulse oximetry sensors fall into the former, while a temperature probe is an
example of a passive wearable that does not require its own power to operate. Yet another view
of wearables is the mode in which the signals are transmitted for processing _ wired or wireless.
In the former, the signals are transmitted over a physical data bus to a processor; in the wireless
class of wearables, the communications capability is built into it, which transmits the signals
wirelessly to a monitoring unit. Sensors can be for one-time use or they can be reusable. Finally,
wearables can be classified based on their field of application, which can range from health and
wellness monitoring to position tracking as shown in the figure. “Information processing” is
listed as one of the application areas because many of these traditional functions such as
processing e-mail can now be done on a wearable in the form of a wristwatch. It is important to
note that not all the classes are mutually exclusive. For instance, a wearable can be multi-
functional, active, non-invasive, and be reusable for health monitoring. The proposed taxonomy
serves two key functions: first, it helps in classifying the currently available wearables so that
the appropriate ones can be selected depending upon the operating constraints; second, it helps
in identifying opportunities for the design and development of newer wearables with
performance attributes for specific areas that need to be addressed

Components of wearable Systems.

The main components of wearable devices areas follows

Control:
Wearable-specific microcontrollers are small, so as to be comfortable and discrete. On the other
hand, the distinctive shapes and colors can function as a decorative element. Several of the
boards available are hand- washable (minus the power source).
Input/Output:
In place of pins, these boards have metal eyelets which you can loop conductive thread through
to sew soft circuit connections. Some boards also have snaps— or eyelets large enough to solder
on snaps — for easy removal.
ConductiveTextiles:
A material containing metals such as silver or stainless steel, through which an electrical current can
flow is said to be conductive. Wearable systems can make use of these materials in a variety of
ways, such as:
1.Threadformakingcircuits
2.Fabricforcapacitivetouchsensors
3.Hook-and-loopforswitches
Sensors:
Sensors gather information about the environment, the user, or both. Examples of the former
include light, temperature, motion (ACC), and location (GPS). Examples of the latter include
heart rate (ECG), brain waves (EEG), and muscle tension (EMG). A few wearable
microcontrollershavebasicsensorsonboard.Othermanufacturersoffera range of external sensor
modules that connect to the main board.
Power:
When scoping out a wearable design one of the first things to consider is the power requirement.
Do you just want to illuminate a few LEDs, or do you want to run a servomotor? Boards with an
integrated holder for a lithium coin battery are nice for low-power projects that need to be self-
contained. However, boards with a standard JST connector (with or without a circuit to charge
LiPo batteries) are more versatile.
Actuators
One generic way to describe a wearable system is: In response to X, where X is the input from a
sensor, Y happens. Actuators such as LEDs, buzzers or speakers, and servomotors are what
make things happen.

Networking
To communicate with smart devices, the internet, or other wearable systems, you need wireless
connectivity. In addition to Wi-Fi and Bluetooth, wearable-friendly options include
  BLE,which has lower power consumption than classic bluetooth ,a range of 50m, and a
data transmission rate up to 1 Mbps
 NFC, a radio frequency field with a range of approximately 20cm and data transmission
rate up to about 400 Kbps

Types of Wearable Systems

Smart watches:
These days, the watches are tech-enabled. They double up as a fitness tracker, and sleep monitor
in addition to being the classic time-keeping device. Smart watches provide us with many other
features including enabling us to make & attend phone calls and check messages. Some watches
have the feature of playing FM radio or audio&videofiles with a Bluetooth headset. They
generally connect to the smart phone via an app and act as a supporting device. They are often
referred to as a ‘Wearable Computer’ on your wrist because of the bundle of features that can
use through the touch screen.

FitnessTrackers:

Fitness Trackers are among the wearable technology devices wearable on the wrist. Fitness
trackers were primarily launched to perform the function of pedometer, i.e. counting the number
of steps but they have evolved to become an overall health monitor since then. They perform
various functions including tracking your heartbeat, monitoring your sleep, calories burned, and
other metrics. They share the data to the app on the smartphone. Intoto,they make a perfect
health tracker. Some devices are enabled to regularly share the information on the metrics of the
wearer to their physicians to keep them informed and help early detection of any issue.

SmartJewelry:

Jewelry no more acts like pieces of ornaments on your neck or hand, they have become smart.
Smart Jewelry are those wearables like necklaces, wristbands, bracelets, earrings that are tech-
enabled to help you track your steps, track monitor your heartbeat &sleep, and some even notify
you of incoming calls.
Game Simulators:

The rise of VR in gaming has given rise to many wearable devices that simulate an environment
and make the experience more realistic, engrossing, and adventurous. The devices include VR
Headsets (also called Head-Mounted Displays or HMDs) that create a visual simulation and
bands that come with built-in sensors to detect your movements. These bands enable you to
control your movements through hand gestures.
 SmartClothing:

The advancement of technology with IoT has fostered many inventions including Smart Clothes.
Smart clothes are also popularly known as E- Textile as they come integrated with electronic
devices that measure the health metrics of the wearer. Smart clothes help measure health-related
aspects like heart rate, respiration rate, sleep, the body temperature, and provide you with that
information. Smart clothing also includes smart shoes that examine your health, steps, fatigue,
and collect other metrics to help you improve health and prevent injury.

Smart Glasses:

Ranging from simple smart glasses that are equipped with Bluetooth wireless music and hands-
free calling to the glasses that can live stream videos to take photos, to advanced smart glasses
that are AR-enabled to give you an immersive experience, these smart glasses are the of
eyewear. Smart glasses can enable the user to read text messages and reply to them hands-free.
Smart glasses by some companies are equipped with features like internet access and browsing
through voice commands

Heartbeat Trackers & BloodPressure Monitors:

There are fitness trackers for a specific use case like monitoring the heartbeat or regularly
measuring the blood pressure. These devices help track the metrics among the people who suffer
from related diseases. The fitness trackers record and provide the measurements to the wearer
regularly. Some devices are enabled to share the data with the physician.

SmartEarbuds:

New to enter the wearable technology market are earbuds. Though Bluetooth earbuds are
existing for a while now, they aren’t considered among wearable technology because they do
not collect and send data. But some companies are making earbuds smart. Smart earbuds have a
built-in gyroscope, GPS, and compass. The sensors in the earbuds relay the information to the
smartphone, which enables it to know your direction and movement. Hence, the smart earbuds
are equipped to provide directions in real-time.

SmartContactLens:

Smart Contact Lens is among the recent inventions made possible with
IoT.Thesmartcontactlensescurrentlyavailableinthemarketarehelpful for medical reasons. It helps
monitor eyes for various diseases like Diabetes, Glaucoma, and cataracts. It helps in the
treatment of farsightedness. A part from medical reasons, some companies are working on smart
contact lenses that are AR-enabled work on solar power, and capture and store images and ideos.
Smart lenses are among the implantable devices.

Applications of wearables

Currently other applications within healthcare are being explored, such as:
 Applications for monitoring of glucose, alcohol, and lactate or blood oxygen, breath
monitoring, heart beat, heart rate and its variability, electromyography (EMG),
electrocardiogram (ECG) and electroencephalogram(EEG),body temperature, pressure
(e.g. in shoes), sweat rate or sweat loss, levels of uric acid and ions – e.g. for preventing
fatigue or injuries or for optimizing training patterns, including via "human-integrated
electronics"
 Forecasting changes in mood, stress, and health
 Measuring blood alcohol content
 Measuring athletic performance
 Monitoring how sick the user is
 Detecting early signs of infection
 Long-term monitoring of patients with heart and circulatory problems that records an
electrocardiogram and is self-moistening
 Health Risk Assessment applications, including measures of frailty and risks of
age-dependent diseases
 Automatic documentation of care activities
 Days-long continuous imaging of diverse organs via a wearable bio
adhesive stretchable high-resolution ultrasound imaging patch or e.g. a wearable
continuous heart ultrasound imager (potential novel diagnostic and monitoring tools)
 Sleep tracking
 Cortisol monitoring for measuring stress
 Measuring relaxation or alertness e.g. to adjust their modulation or to measure efficacy of
modulation techniques
 Epidermal skin technology. According to Science Daily, the Terasaki Institute for
Biomedical Innovation invented wearable electronic skin for monitoring health. A next-
generation of wearables, this ultra-thin e-skin patch can be attached to the wearer's chest
area along with a small wireless transmitter by using water spray and can be worn for up
to a week. It is sensitive enough to pick up and record electro signals, such as heartbeats
and muscle movements, which can be sent to health care providers via the cloud so they
can monitor the user's vitals remotely. This powerful wearable is a stepping stone for
monitoring chronic illnesses such as heart failure and diabetes.
 Health monitoring. People use wearable technology to track and receive notifications
for their heart rate and blood pressure, watch their calorie in take or manage their training
regimens. The COVID- 19 pandemic boosted the use of wearable technology, as
consumers gained a broader awareness of personal hygiene and taking precautions to
prevent the spread of infections. Apple, for instance, updated its Cardiogram app by
 introducing a new sleeping beats- per-minute feature that monitors heart rate fluctuations
for COVID- 19 patients.
 Entertainment and gaming. The gaming and entertainment industries were the first to
adopt VR headsets, smart glasses and controllers. Popular VR head-mounted displays,
such as Oculus Quest, Meta Quest and Sony PlayStation VR, are used for all types of
entertainment purposes, including gaming, watching movies and virtual travelling
 Fashion and smart clothing. Clothing known as smart clothing, or intelligent fashion,
has been gaining wide popularity over the past few years. Smart jackets, such as Levi's
jacket made with Google's Project Jacquard technology whose threads are composed of
electrical fibers, enable the wearer to answer calls, play music or taking photos.
Smartwatches,wristbands, smart shoes and smart jewelry are also popular examples of
wearable technology.
 Military. These wearables include technology that tracks soldiers' vitals, VR-based
simulation exercises and sustainability technology, such as boot inserts that estimate how
well the soldiers are holding their equipment weight and how terrain factors can affect
their performance.
 Sports and fitness. Sports use wearable athletic devices that are either built into the
fabric of the sports apparel or are incorporated into sports equipment, such as bats and
balls. The GPS and Bluetooth-linked devices relay real-time data to coaches for analysis
through connected electronic devices such as laptops. Besides wearable athletic devices,
familiar wearable technology such as Fitbit, AppleWatch, Garmin, Samsung Galaxy
Watch and Polar are usedextensively totrackvariousareasofthe player'shealthand
performance metrics.

Advantages of WearableTechnology

 Rapid data results can help drive improvements

 Detailed data can supplement loss analysis and loss trends.
 Can help build a business case for senior management
 Data from wearable sensors offers promising job risk analysis and evaluation
opportunities for safety and ergonomics practitioners
 Enhance employee wellness programs

Disadvantages of WearableTechnology
 Requires a time commitment to review and analyze data
 Requires financial commitments and planning
 Devices could lead to distraction.
 Data security and privacy could be compromised with legal, financial, and personal
consequences
 Devices could lead to over-trust or under-trust

Sensors for Wearable Systems

 Introduction
When designing wearable systems to be used for physiological and biomechanical parameters
monitoring, it is important to integrate sensors easy to use, comfortable to wear, and minimally
obtrusive. Wearable systems include sensors for detecting physiological signs placed on-body
without discomfort, and possibly with capability of real-time and continuous recording. The
system should also be equipped with wireless communication to transmit signals, although
sometimes it is opportune to extract locally relevant variables, which are transmitted when
needed. Most sensors embedded into wearable systems need to be placed at specific body
locations, e.g. motion sensors used to track the movements
ofbodysegments,oftenindirectcontactwiththeskin,e.g.physiological sensors such as pulse meters
or oximeters. However, it is reasonable to embed sensors within pieces of clothing to make the
wearable system as less obtrusive as possible.
. A key technology for wearable systems is the possibility of implementing robust, cheap
microsystemsenablingthecombinationofalltheabovefunctionalitiesin a single device. This
technology combines so-called micro-electro- mechanical systems (MEMS) with advanced
electronic packaging technologies. The former allows complex electronic systems and
mechanical structures (including sensors and even simple motors) to be jointly manufactured in
a single semiconductor chip. A generic wearable system can be structured as a stack of different
layers. The lowest layer is represented by the body, where the skin is the first interface with the
sensor layer. This latter is comprised of three sub-layers: garment and sensors, conditioning and
filtering of the signals and local processing. The processing layer collects the different sensor
signals, extracts specific features and classifies the signals to provide high-level outcomes for
the application layer. The application layer can provide the feedback to the user and/or to the
professional, according to the specific applications and to the user needs

Sensors for Wearable Systems

Biomechanical Sensors
Biomechanical sensors are thought to be used to record kinematic parameters of body segments.
Knowledge of body movement and gesture can be a means to detect movement disturbances
related to a specific pathology or helpful to contextualize physiological information within
specific physical activities. An increasing of heartrate, for example, could be either due to an
altered cardiac behaviour or simply because the subject is running.

Inertial Movement Sensors

Monitoring of parameters related to human movement has a wide range of applications. In the
medical field, motion analysis tools are widely used both in rehabilitation and in diagnostics. In
the multimedia field, motion tracking is used for the implementation of life like videogame
interfaces and for computer animation. Standard techniques enabling motion analysis are based
on stereo-photogrammetric, magnetic and electromechanical systems. These devices are very
accurate but they operate in a restricted area and/or they require the application of obtrusive
parts on the subject body. On the other hand, the recent advances in technology have led to the
design and development of new tools in the field of motion detection which are comfortable for
the user, portable and easily usable in non-structured environments. Current prototypes realized
by these emergent technologies utilize micro-transducers applied to the subject body or textile-
based strain sensors.
The first category, instead, includes devices based on inertial sensors (mainly accelerometers
and gyroscopes) that are directly applied on the body segment to be monitored. These sensors
can be realized on a single chip (MEMS technology) with low cost and outstanding
miniaturization. Accelerometers are widely used for the automatic discrimination of physical
activity and the estimation of body segment
inclinationwithrespecttotheabsolutevertical.Accelerometersaloneare not indicated for the
estimation of the full orientation of body segments. The body segment orientation by using the
combination of different sensors through data fusion techniques (Inertial Measurement Units,
IMU). Usually, tri-axial accelerometers (inclination), tri-axial gyroscopes (angular velocity),
magnetometers (heading angle) and temperature sensors (thermal drift compensation) are used
together. Main advantages of using accelerometers in motion analysis are the very low
encumbranceandthelowcost.Disadvantagesarerelatedtothepossibility of obtaining only the
inclination information in quasi-static situations (the effect of the system acceleration is a noise
and the double integration of acceleration to estimate the segment absolute position is
unreliable).
Accelerometers are widely used in the field of wearable monitoring systems, generally used in
the monitoring of daily life activities (ADL). Physical activity detection can be exploited for
several fields of application, e.g. energy expenditure estimation, tremor or functional use of a
body segment, assessment of motor control, load estimation using inverse dynamics techniques
or artificial sensory feedback for control of electrical neuromuscular stimulation Usually, three-
axial accelerometers are used.
They can be assembled by mounting three single-axis accelerometers in a box with their
sensitive axes in orthogonal directions or using a sensor based on one mass .An accelerometer
measures the acceleration and the local gravity that it experiences. Considering a calibrated tri-
axial accelerometer (the accelerometer signal (y) contains two factors: one is due to the gravity
vector (g) and the other depends on the system inertial acceleration (a), both of them expressed
in the accelerometer reference frame :The inclination vector (z) is defined as the vertical unit
vector, expressed in the accelerometer coordinate frame .In static conditions, only the factor due
to gravity is present and the inclination of the accelerometer with respect to the vertical is
known. In dynamic conditions, the raw accelerometer signal does not provide a reliable
estimation of the inclination, since the inertial acceleration is added to the gravity factor. This
estimation error grows as the subject movements become faster(e.g. running, jumping).
 Many algorithms have been developed and tested to
performareliableestimationofthesubjectbodyinclination:mostofthem use low pass filters with
very low cut-off frequency in order to extract z ,others implement more complex techniques
which use a model-based approach mainly based on Kalman filter techniques. An example of
integration of these sensors in a garment was developed in the frame of the Proetex project
(FP6-2004- IST-4-026987), which aimed at using textile and fibre based integrated smart
wearables for emergency disaster intervention personnel. The ProeTEX motion sensing platform
is used to detect long periods of user immobility and user falls to the ground and it is realized by
means of two tri-axial accelerometer modules. One accelerometer is placed in the higher part o
the trunk (collar level) in order to detect inactivity and falls to the ground. The second sensor is
placed in the wrist region and its aim is to achieve more accuracy in inactivity detection, since
an operator can move his arms while his trunk is not moving. The core of the motion sensor is
the processing algorithm described in, which allows to perform a reliable estimation of the body
inclination even in the case of intense physical activity such as running or jumping. This
algorithm allows a good estimation of subject activities and generated fall alarms with very high
sensitivity and extremely low level of false positives.
Respiration Activity sensor
The most challenging vital sign to accurately record during continuous monitoring is the
respiratory activity due to the fact that the signals are affected by movement artifacts and
filtering or feature recognition algorithms are not very effective. Monitoring of respiratory
activity involves the collection of data on the amount and the rate at which air passes into and
out of the lungs over a given period of time. In literature, there are several methods to do this,
both directly, by measuring the amount of air exchanged during the respiration activity, and
indirectly, by measuring parameters physically correlated to breathing, such as changes in thorax
circumference or cross section, or trans-thoracic impedance.

Direct methods are based on as pyrometer that measures directly the airflow in the lung
exchanged during inspiration and expiration, but of course it cannot be integrated into a
wearable system because it employs a mouthpiece, which could interfere with the freedom of
movements, disrupting the normal breathing pattern during measurement, thus causing
discomfort for the user.
Indirect method exploits displacements of the lung that are transmitted to the thorax wall and
vice versa, and therefore measurements of chest-abdominal surface movements can be used to
estimate lung volume variation. In literature, a number of devices have been used to measure rib
cage and abdominal motion including mercury in rubber strain gauges, linear differential
transducers, magnetometers, and optical techniques, but almost all cannot be comfortably
integrated into a wearable system. For reference only, it is worth while citing a more
sophisticated technique, called stereo photogrammetry, which makes it possible to estimate the
three-dimensional coordinates of points of the thorax, estimating therefore volume variations.
Nevertheless, this system presents a considerable drawback in that it is cumbersome, extremely
expensive, and can only be used in research environments or in laboratory applications. Indirect
techniques that can be implemented in wearable systems are respiratory inductive
plethysmography, impedance plethysmography, piezo resistive and/or piezoelectric
pneumography. These systems are minimally invasive and do not interfere with physical
activity.

Inductive Plethysmography

The inductive plethysmography method for breathing monitoring consists of two elastic
conductive wires placed around the thorax and the abdomen to detect the cross sectional area
changes of the ribcage and the abdomen region during the respiratory cycles. The conductive
wires are insulated and generally sewn in a zig-zag fashion onto each separate cloth band. They
can be considered as a coil and are used to modulate the output
frequencyofasinewavecurrentproducedbyanelectricoscillatorcircuit. As a matter of fact, the sine
wave current generates a magnetic field, and the cross-sectional area changes due to the
respiratory movements of the rib cage and of the abdomen determine a variation of the magnetic
field flow through the coils. This change in flow causes a variation of the self- inductance of
each coil that modulates the output frequency of the sinusoidal oscillator. This relationship
allows for monitoring the respiratory activity by detecting the frequency change in the oscillator
output signal .For accurate volumetric measurements using RIP ,it is Assumed that the cross-
sectional area within the ribcage and the abdomen coil, respectively, ref lects al l the changes
occurring within the respective lung compartment, and further that the lung volume change is the
sum of the volume changes of the two compartments. Under optimal situations, lung volume can
be approximated with an error lessthan10%.
Impedance Plethysmography
This technique consists of injecting a high frequency and low amplitude current through a pair
of electrodes placed on the thorax and measuring the trans-thoracic electrical impedance
changes. As a matter of fact, there is a relationship between the flow of air through the lungs and
the impedance change of the thorax. The measurements can be carried out by using either two or
four electrode configurations. Electrodes can be made of fabric and integrated into a garment or,
even, embedded into an undershirt. It is worthwhile noting that by measuring the trans-thoracic
electrical impedance it is possible to non-invasively monitor, in addition to breathing rate also
tidal volume, functional residual capacity, lung water and cardiac output.

Pneumography Based on Piezoresistive Sensor

Piezoresistive pneumography is carried out by means of piezoresistive sensors that monitor the
cross-sectional variations of the rib cage. The piezoresistive sensor changes its electrical
resistance if stretched or shortened and is sensitive to the thoracic circumference variations that
occur during respiration. Piezoresistive sensors can be easily realized as simple elastic wires or
by means of an innovative sensorized textile technology. It consists of a conductive mixture
directly spread over the fabric. The lightness and the adherence of the fabric make the
sensorized garments truly unobtrusive and cumbersome, and hence comfortable for the subject
wearing them. This mixture does not change the mechanical properties of the fabric and
maintains the wearability of the garment.

Plethysmography Based on Piezoelectric Sensor

This method is based on a piezoelectric cable or strip which can be simply fastened around the
thorax thus monitoring the thorax circumference variations during the respiratory activity. A
possible implementation can be a coaxial cable whose dielectric is a piezoelectric polymer
(p(VDF- TrFE)), which can be easily sewn in a textile belt and placed around the chest. The
sensor is sensitive to the thorax movements and produces a signal directly proportional to the
thorax expansion in terms of charge variation, which was converted in an output voltage
proportional to the charge by means of a charge amplifier. A suitable local processor can enable
implementation of the Fast Fourier Transform in real time and extraction of the breathing rate.

Wearable ground force sensor

A wearable ground reaction force (GRF) sensor is a device designed to measure the forces exerted on
the body while interacting with the ground during movement, such as walking, running, or jumping.
These forces include both vertical and horizontal components and can provide valuable information
about an individual’s biomechanics, posture, and gait. GRF sensors are commonly used in fields like
sports science, rehabilitation, biomechanics research, and even in wearable technology for personal
fitness tracking.

These sensors typically work by embedding pressure-sensitive materials or force transducers into a
wearable format, such as shoes, insoles, or pads. The sensors measure the force applied at different
points of contact with the ground and can then be analyzed to assess the quality of movement, identify
abnormalities in gait, or monitor fatigue levels.

Applications for wearable GRF sensors include:

1. Sports Performance Analysis: Understanding how athletes interact with the ground during
training or competition, providing insights into stride efficiency, force production, and injury
prevention.
2. Rehabilitation: Monitoring patients during recovery from injury, such as a sprained ankle or
knee surgery, to ensure they are putting weight on the right areas or maintaining proper
alignment while walking or running.
 3. Balance and Posture: In gait analysis or for older adults, to track and improve balance and
posture to prevent falls.
4. Prosthetics and Orthotics: Analyzing the force distribution on artificial limbs to improve their
design and function, ensuring a more natural gait.

The quantitative analysis of gait variability using kinematics and kinetic characterisations can be
helpful to medical doctors in monitoring patients’ recovery status in clinical applications.
Moreover, these quantitative results may help to strengthen their confidence in the rehabilitation.
Walking speed, stride length, the centre of mass (CoM) and the centre of pressure (CoP) have
been considered as factors in the evaluation of walking gait . According to one study on slip type
falls, friction force was used to draw up important safety criteria for detecting safe gait, so the
transverse components of ground reaction force (GRF) may provide important information for
quantifying gait variability.