Sensors & Actuators: A. Physical 366 (2024) 114993
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Sensors and Actuators: A. Physical
journal homepage: www.journals.elsevier.com/sensors-and-actuators-a-physical
A review on flexible wearables – Recent developments in non-invasive
continuous health monitoring
Nikolay L. Kazanskiy a, b, Svetlana N. Khonina a, b, Muhammad A. Butt a, *
a
b
Samara National Research University, 443086 Samara, Russia
IPSI RAS-Branch of the FSRC “Crystallography and Photonics” RAS, 443001 Samara, Russia
A R T I C L E I N F O
A B S T R A C T
Keywords:
Wearables
Smart watches
Smart contact lenses
Non-invasive glucose monitoring
Heart rate monitoring
Body temperature monitoring
Wearable sensors hold profound significance in our increasingly interconnected world, revolutionizing the way
we monitor and manage our health, daily activities, and environments. These compact, unobtrusive tools have
transcended the realm of mere gadgets to become indispensable tools for individuals and healthcare professionals
alike. By continuously collecting data on vital signs, physical activity, sleep patterns, and more, wearables
empower users to gain valuable insights into their well-being, facilitating proactive health management. Beyond
preventive medicine, wearables find applications in diverse fields, from sports performance optimization to
industrial safety monitoring, making them catalysts for transformative change in the way we live, work, and
thrive. As technology continues to advance, the significance of wearables in enhancing our lives and pushing the
boundaries of knowledge will only continue to grow. In this review, we focused on the recent advances in the
field of wearables for continuous non-invasive monitoring of cardiovascular, body temperature and blood
glucose. These sensors are commonly incorporated into garments or stylish accessories such as watches, allowing
individuals to monitor different facets of their health, including metabolic processes and vital signs. In premise,
the worldwide wearables market is undergoing substantial growth propelled by technological advancements,
expanding applications, and the need for inventive preventive healthcare solutions. Despite existing challenges,
the industry is well-positioned for ongoing expansion as it tackles these issues and pushes the frontiers of
wearable technology.
1. Introduction
Contemporary wearables can deliver precise measurements on par
with those achieved by standardized medical instruments. Conse­
quently, the line linking consumer-oriented wearables and medicalgrade apparatuses has become increasingly indistinct [1,2]. The initial
wave of wearables, including wristwatches, footwear, and headsets,
primarily focused on tracing an individual’s bodily activity, heart rate
(HR), or body temperature through biophysical monitoring [3–6]. As the
first-generation wearables gained widespread acceptance and proved
successful, attention has gradually shifted towards non-invasive or
minimally invasive biochemical and multi-modal monitoring, repre­
senting the next evolution in personalized preventive medicine [7].
These second-generation wearable sensing devices embody various form
factors such as on-skin patches, tattoos, tooth-mounted films, contact
lenses, textiles, and even more intrusive options like microneedles and
injectable tools [8–11]. A defining feature of second-generation wear­
able sensing devices is their utilization of biofluids, where bio­
recognition elements are employed to transform the presence of
identifiable analytes into measurable signals [2,12].
Wearable sensing devices play a pivotal role within the preventive
medicine ecosystem by allowing continuous, real-time examining of
diverse physiological parameters and health-related information [13,
14]. Typically, these devices are worn on the body or seamlessly inte­
grated into clothing and accessories, allowing them to capture HR, ac­
tivity levels, sleep patterns, temperature, and more data [15]. These
wearable sensing tools empower preventive medicine professionals to
remotely oversee patients, thus facilitating early recognition of health
Abbreviations: CGM, Continuous glucose monitoring; BP, Blood pressure; HR, Heart rate; BG, Blood glucose; CV, Cardiovascular; CVD, Cardiovascular disease;
IOP, Intraocular pressure; BTM, Body temperature monitoring; SCL, Smart contact lens; ECG, Electrocardiogram; GF, Gauge factor; e-skin, Electronic skin; IBI, Interbeat interval; PPG, Photoplethysmography; CNT, Carbon nanotube.
* Corresponding author.
E-mail addresses: kazanskiy@ipsiras.ru (N.L. Kazanskiy), khonina@ipsiras.ru (S.N. Khonina), butt.m@ssau.ru (M.A. Butt).
https://doi.org/10.1016/j.sna.2023.114993
Received 13 October 2023; Received in revised form 7 December 2023; Accepted 28 December 2023
Available online 2 January 2024
0924-4247/© 2023 Elsevier B.V. All rights reserved.
N.L. Kazanskiy et al.
Sensors and Actuators: A. Physical 366 (2024) 114993
issues and diminishing the necessity for frequent in-person medical
appointments. This capability is especially valuable for patients grap­
pling with chronic conditions like diabetes, heart disease, and hyper­
tension. Additionally, wearables possess the capacity to discern subtle
shifts in vital signs and activity levels, serving as potential indicators of
underlying health concerns [16]. For instance, irregular HR patterns
may signify arrhythmias, while alterations in sleep patterns might sug­
gest the presence of sleep disorders. Furthermore, wearable sensing
devices yield a wealth of personalized health data that can be harnessed
to tailor treatment plans and medication regimens to the unique needs of
each patient. This individualized approach contributes to more effective
and efficient preventive medicine delivery [17].
Some wearable devices also offer features that assist patients in
adhering to their medication schedules by sending timely reminders and
tracking their medication intake. This feature proves particularly valu­
able for individuals managing complex medication regimens. Moreover,
wearables are widely embraced for monitoring physical activity, calorie
expenditure, and sleep quality [18]. This data empowers individuals to
make healthier lifestyle choices and holds significant potential for pre­
ventive medicine. Moreover, smart contact lenses (SCLs) have surfaced
as an encouraging wearable medical gadget within the realm of pre­
ventive medicine [19–22]. Given its close association with human ac­
tivities and circadian rhythms, long-term continuous tracking greatly
enhances the monitoring of intraocular pressure (IOP) [23–25].
Certain wearable sensing devices, equipped with accelerometers and
gyroscope sensors, are adept at detecting falls, especially among elderly
populations [26]. The rapid recognition of a fall can trigger automatic
alerts to caregivers or emergency services, enhancing safety. In addition,
select wearables incorporate sensors designed to measure stress levels,
breathing patterns, and other physiological markers associated with
mental health. This data serves as a foundation for developing strategies
to reduce stress and enhance mental well-being.
Furthermore, the data obtained from wearable sensing devices can
be seamlessly integrated with electronic health records (EHRs) and
other preventive medicine systems. This integration facilitates a more
comprehensive view of a patient’s medical history, empowering pre­
ventive medicine providers to make more informed decisions. Wearable
sensing devices are increasingly leveraged in clinical trials to gather
objective, real-time data regarding participants’ health status and ac­
tivities. This usage enhances the quality and accuracy of research out­
comes. Lastly, by enabling early intervention, remote monitoring, and
preventive care, wearables have the potential to mitigate healthcare
costs associated with hospitalizations and emergency room visits.
Wearable sensing devices find extensive utility in both fitness
tracking and everyday life. They serve the dual purpose of continuously
monitoring individuals for the emergence of deteriorating symptoms.
When it comes to identifying patients at risk of respiratory decline, these
wearables can serve as a bridge between home isolation and standard
hospital care, potentially reducing the need for intensive care unit (ICU)
admissions [27,28]. Furthermore, wearable sensing devices possess the
capability to monitor the health status of individuals who may have
been exposed to health risks [29]. In this context, the monitoring of body
temperature and HR assumes paramount importance in detecting early
warning signs. An elevated body temperature often indicates the pres­
ence of a viral infection, a response triggered by the activation of the
immune system. As such, body temperature monitoring (BTM) has
gained widespread acceptance as a means of detecting early traces of
Covid-19 infection [30]. Numerous wearable tools are now accessible
for examining individuals who are at risk due to potential exposure.
Additionally, during a viral infection, a rise in HR and alterations in
pulse waveforms can signify physiological stress. Various studies have
demonstrated the feasibility of using wearable apparatuses for nonstop
HR supervising and the recognition of cardiac events [31].
For achieving highly precise temperature measurements, it is
essential to have direct interaction with the human body. However,
conventional rigid temperature sensing devices face limitations in
achieving a conformal fit on irregular surfaces. This challenge plays a
pivotal part in the advancement of flexile and wearable temperature
sensing devices. To meet the demands of wearability, it is imperative to
obtain temperature sensors that are elastic, flexible, biocompatible,
lightweight, robust, and non-irritating. Additionally, ensuring user
comfort during prolonged use or wear is of paramount importance.
Textiles, with their extensive variety of fibres, yarns, and fabrics that
extend beyond their traditional protective and aesthetic functions, pre­
sent an exceptional and flexible platform for integrating sensing capa­
bilities while also delivering comfort to the wearer [32]. Further details
on flexible temperature sensing devices can be found in [33].
The paper’s structure is as follows: Section 2 provides an overview of
the wearable sensing devices market from 2023 to 2032. In Section 3, we
delve into the operational principles of wearables. Section 4 explores the
predominant materials employed in wearable technology. Section 5
briefly covers the applications of wearables, encompassing cardiovas­
cular, temperature, and glucose monitoring. Furthermore, Section 6
elucidates the challenges associated with the widespread adoption of
wearables. Finally, Section 7 offers concluding remarks to complete the
paper.
2. Wearables market
Individuals can monitor their health and fitness status using wear­
ables, which are advanced technological devices designed to be worn.
These sensors are typically integrated into clothing or fashion acces­
sories like watches, enabling users to keep tabs on various aspects of
their well-being, including metabolic processes and vital signs such as
HR, blood pressure (BP), and blood oxygen saturation [34–36]. The
adoption of wearable technology is on the rise, driven by increased
awareness of health and safety, higher consumer spending on electronic
gadgets, urbanization trends, and improved lifestyles among the
growing population. These sensors are versatile and can be worn in
various locations according to one’s preference. While it may initially
sound like an exaggeration, thanks to sensors integrated into clothing
and various other forms, not just limited to wrist-worn devices, this
assertion holds true [37].
The growth of wearable technology is also facilitated by factors such
as affordability and ergonomic advancements in miniaturized elec­
tronics. Moreover, the widespread use of smartphones and connected
devices, along with the surging demand for compact, lightweight sen­
sors with enhanced performance capabilities, further contributes to the
expansion of wearable technology. Prominent examples of wearable
technology in this category include Fitbit, earpieces, and smartwatches.
In 2022, the worldwide market for wearables was valued at approxi­
mately USD 978.86 million, and it is anticipated to exceed USD 4336.7
million by 2032 as disclosed in Fig. 1. This growth is anticipated to occur
at a compound annual growth rate (CAGR) of 16.1% from 2023 to 2032
[38].
Wearable technology, including wristwear, bodywear, and eyewear,
is finding increasing applications in both preventive medicine and
consumer information infotainment [39]. These wearable products have
evolved significantly, with continuous advancements in technology,
resulting in smaller and more sophisticated devices. The expansion of
the wearables market is driven by several key factors. Firstly, ongoing
developments in sensor technology, wireless communication, and power
management have contributed to the steady expansion of this market.
Additionally, the utilization of wearables for newborns and the
increasing adoption of home and remote patient monitoring have
further fueled its growth. These sensors can be seamlessly connected to
other smart consumer electronics like tablets and smartphones, enabling
efficient monitoring and data collection.
Furthermore, the market has witnessed the introduction of innova­
tive tools such as smart shirts, smart rings, and smart glasses, thanks to
technological advancements. Fig. 2 illustrates the increasing prevalence
of wearables and offers a glimpse into the future of wearable technology.
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Sensors and Actuators: A. Physical 366 (2024) 114993
Fig. 1. Wearables market size.
Fig. 2. Wearables can be worn on any part of the body for examining daily activities and preventive medicine.
The rising demand for the Internet of Things (IoT) has also played a
significant role in driving adoption across various industries, including
consumer electronics, preventive medicine, and fitness. Despite these
positive trends, there are some challenges [40]. The relatively high cost
of wearables and concerns related to privacy and data security have
somewhat hindered market growth. However, the implementation of
stringent data storage regulations is expected to alleviate these concerns
and boost the popularity of wearables. The miniaturization of sensors,
especially through microelectronics-based technologies, has been
instrumental in the development of wearable technology. Smaller sensor
sizes have been a critical factor in encouraging the adoption of sensing
technologies, especially in the realm of wearable electronics. Major
industry players like STMicroelectronics [41], NXP Semiconductors
[42], and Broadcom [43] have invested significantly in research and
development to drive innovation in this space. The use of sensors
employing technologies like MEMS (Micro-Electro-Mechanical Sys­
tems), NEMS (Nano-Electro-Mechanical Systems), and CMOS (Comple­
mentary Metal-Oxide-Semiconductor) has also contributed to this trend.
In assumption, the global wearables market is experiencing signifi­
cant growth driven by technological advancements, increasing appli­
cations, and the demand for innovative preventive medicine solutions.
While challenges exist, the industry is poised for continued expansion as
it addresses these issues and pushes the boundaries of wearable tech­
nology. For instance, at the Consumer Electronics Show (CES) 2022,
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N.L. Kazanskiy et al.
Sensors and Actuators: A. Physical 366 (2024) 114993
Relief band Technologies LLC displayed the Relief band Sport smart­
watch, which offers treatment for nausea and vomiting associated with
various conditions, underscoring the potential of wearables in
enhancing preventive medicine and well-being.
signals. These signals encompass variations in current, resistance, and
voltage, underpinned by triboelectric, piezo-resistive, and piezo-electric
mechanisms, respectively (as elucidated in Fig. 3(a, b)).
Within this context, wearable pressure sensors can be systematically
classified into four distinct categories: piezo-resistive, piezo-electric,
triboelectric, and capacitive sensors [44]. Piezo-resistive sensors are
deployed for the monitoring of human movement signals, detecting al­
terations in resistivity [45]. They are distinguished by their extended
operational lifespans and swift response times. Conversely, wearable
piezoelectric sensors function by harnessing electrostatic induction and
the piezo-electric phenomenon to observe signals. Wearable capacitive
sensors, conversely, are applied for motion detection, relying on varia­
tions in capacitance as their fundamental operating principle. Typically,
wearable triboelectric sensors comprise a triboelectric layer situated
between two parallel electrodes. These sensors produce either current or
voltage signals when exposed to pressure, primarily via the triboelectric
effect and electro-static induction [46].
Wearable strain sensors, exemplified by resistive and capacitive
types, primarily operate on the principle of dimension change when
subjected to stretching (as depicted in Fig. 3(c)). These sensors have
garnered significant attention for their capacity to seamlessly interface
with the human body, making them invaluable for monitoring human
motion and healthcare applications [47]. Their mechanical elasticity
and flexibility enhance skin compatibility while also rendering them
resistant to sweat. Simultaneously, wearable biosensors, commonly
3. Working principle of wearables
Wearables and biosensors represent invaluable analytical in­
struments with the capacity to convert bio-signals into electrical or
optical signals. These bio-signals encompass diverse physiological pa­
rameters, such as body temperature, HR, and motion, which can be
continuously examined and calculated in real-time. Wearable devices
are taxonomically classified based on their monitoring modalities,
which include pressure, strain, electrochemical, temperature, and
optoelectrical sensors (as visually depicted in Fig. 3). Within this clas­
sification, wearable pressure sensors assume a fundamental role in the
surveillance of human health situations and the facilitation of humanmachine interactions. Importantly, the emergence of pressure sensors
incorporating two-dimensional (2D) materials has introduced innova­
tive avenues for the detection and surveillance of essential human
physiological signals. As an illustrative example, a pressure sensor ex­
hibits notable proficiency in tracking cardiovascular (CV) health by
discerning irregularities in BP and HRs. These flexible pressure sensors
exhibit exceptional portability and adaptability to the contours of
human skin, persistently transducing subtle bio-signals into electrical
Fig. 3. Illustration of mechano-transduction mechanisms employed in wearables, showcasing various sensor types, (a) piezo-resistive pressure sensor [51], (b) piezo
or triboelectric pressure sensors [51], (c) stress sensor [51], (d) electro-chemical biosensor [51], (e) temperature sensing device [51], (f) optoelectronic sensor [51].
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known as electrochemical biosensors, provide non-invasive surveillance
capabilities and signify a swiftly progressing frontier in telehealth
technology [48]. Electrochemical biosensors identify shifts in biological
binding interactions by quantifying modifications in current, resistance,
and capacitance at the sensor’s surface (as depicted in Fig. 3(d)). They
demonstrate exceptional proficiency in the surveillance of diverse
bodily fluids, encompassing saliva, interstitial fluid, tears, urine, and
sweat. These sensors are emerging as user-friendly and portable sub­
stitutes for conventional analytical tools within the healthcare domain.
Crucially, these bodily fluids contain a wealth of essential bio­
markers, encompassing proteins (such as hormones and lactate dehy­
drogenase), small molecules (like uric acid, glucose, amino acids, and
cortisol), and ions (including sodium, potassium, and calcium) [49,50].
These biomarkers furnish clinically pertinent information for the diag­
nosis and management of various metabolic disorders, incorporating
cancer, diabetes, and periodontal diseases. Perhaps, sweat biomarkers
provide invaluable insights into metabolic conditions, such as alcohol
and glucose levels. Furthermore, temperature serves as a decisive
marker of human well-being and illness, playing a pivotal role in both
daily life and preventive medicine contexts. Most temperature-sensing
devices operate by detecting temperature changes through alterations
in the electrical properties of materials, such as variations in current or
resistance (as depicted in Fig. 3(e)). Skin-integrated wearable temper­
ature sensing devices find application in measuring the baseline tem­
perature of newborns and infants. Furthermore, real-time, and
continuous BTM can offer valuable insights for the early detection of
fever, potentially aiding in the anticipation of illness in humans.
On another front, wearable optoelectronic sensors present a highly
promising option for preventive medicine applications because of their
ability to observe vital health indicators precisely and continuously.
These sensors utilize a sensing mechanism that captures specific wave­
lengths of light penetrating the skin and then optically detects changes
in blood vessel volume driven by the cardiac cycle (illustrated in Fig. 3
(f)). This category of wearables facilitates the tracing of various critical
signals, including arterial oxygen saturation, HR, and respiratory rate.
Fig. 4. Materials for wearables.
with conductive substances, and chemical stability [62]. Resistive sen­
sors and active electrodes are crafted by incorporating conductive ma­
terials into a pliable polymer matrix [63]. Rycewicz and colleagues
demonstrated the production of boron-doped diamond nanosheets
(BDDNS) on Kapton, a type of polymer substrate. The resulting
BDDNS/Kapton sensor exhibited the capacity to measure strains of up to
0.55%, suggesting its promising potential for utilization in low-strain
sensor applications [64].
4. Materials for flexible wearables
Materials for wearables play a pivotal role in shaping the function­
ality and comfort of these innovative tools. These sensors require ma­
terials that are flexible, lightweight, and biocompatible to ensure
seamless integration with the human body. Commonly used materials
include stretchable conductive polymers and elastomers, which allow
sensors to conform to the body’s contours while maintaining electrical
conductivity [52–55]. Additionally, advanced textiles with embedded
conductive threads or fibres offer a breathable and comfortable option
for wearable integration. For biometric and physiological monitoring,
biocompatible materials like silicone and medical-grade adhesives
ensure skin-friendliness and long-term wearability. Moreover, materials
that can withstand environmental factors such as moisture and tem­
perature variations are essential for robust performance [56]. The
continuous development and selection of appropriate materials continue
to drive innovation in wearable technology, enabling a wide range of
applications in healthcare, fitness, and beyond [57,58]. The materials
used in the construction of flexible sensors are generally categorized into
three main groups as presented in Fig. 4. We will describe each of these
three categories below.
4.2. Metal-based materials
Metal is the most employed conductor in force sensors. Flexible force
sensors are crafted using a range of metals, including copper, gold, ti­
tanium, nickel, magnesium, silvine, chrome, molybdenum, zinc, and
others [65]. These metals are employed in various forms, such as
metallic films, metallic nanomaterials, liquid solutions, metallic oxides,
and MXenes. Among the metallic films, you can find a diverse array of
materials like gold, copper, aluminium, silver, and zinc. In a study by
Hogas et al., a manufacturing approach was proposed, which combined
thin film deposition with electrospinning to create a strain sensor [66].
While this sensor demonstrated remarkable sensitivity, it suffered from a
low signal-to-noise ratio. Consequently, the electrode layers of flexible
force sensors often incorporate metallic films.
Liquid metals, which refer to amorphous metals in a liquid state at
room temperature, are ideal for ink printing applications. Among these
liquid metals, Mercury, and Eutectic Gallium Indium (EGaIn) are the
most frequently utilized. However, Mercury’s toxicity and health haz­
ards render it unsuitable for wearables. In contrast, EGaIn is a popular
choice in flexible electronics due to its biocompatibility [67]. Ali et al.
confirmed the linearity of the average capacitance change in electrode
channels filled with EGaIn liquid metal, demonstrating a capacitive
sensor with 0.11% sensitivity and a gauge factor (GF) of 0.9975 [68].
Liquid metal is gaining momentum for widespread adoption in the
biomedical and wearable electronics industries, owing to its flexibility,
resistivity, and biocompatibility [69].
Metal oxides have attracted substantial attention in the development
of flexible sensors due to their adjustable band gap, cost-effectiveness,
4.1. Polymers
Polymers find widespread application as foundational materials in
sensor technology. Noteworthy among these polymers, due to their
flexible attributes, are polydimethylsiloxane, polyimide, polyurethane,
polyethylene terephthalate, polyvinylidene fluoride, parylene, poly­
ethylene naphthalate, and more [59–61]. These materials are renowned
for their robustness, effective thermal conductivity, ease of unification
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large specific surface area, and ease of fabrication [70]. Furthermore,
they offer outstanding biocompatibility, rapid response times, and
durability across various operational conditions. Lee et al. engineered a
flexible strain sensor using zinc oxide nanowires (ZnO NWs) on a textile
substrate [71]. This flexible sensor demonstrated a gauge factor of
approximately 0.96 and featured channels with high conductivity and
exceptional resilience to mechanical deformation. Notably, when the
nanowires were allowed to grow unrestrictedly in reduced graphene
oxide for 3 h, the gauge factor increased to 7.64, highlighting zinc ox­
ide’s capacity to enhance sensitivity. This development opens avenues
for the utilization of metal oxides in the textile industry to create sensory
systems capable of detecting bending strains in the human body.
MXenes, a class of inorganic compounds composed of carbonitride
and metal carbides, can be selectively obtained by etching 3D layered
compounds in strong acidic or basic solutions. MXenes are denoted as
Mn+ 1Xn, where M represents a transition metal and X can be either
nitrogen or carbon. MXenes have recently garnered attention for pro­
ducing flexible force sensors. Sobolciak et al. fabricated a piezoresistive
sensor with electrospinning pads using modified MXenes, achieving a GF
of 4.5 [72]. MXenes exhibit excellent flexibility, oxidation resistance,
and high conductivity. However, expanding the scope of sensors made
with MXenes is an ongoing area of research [73].
exercise intensity, calorie expenditure, and sleep patterns. These sensors
also find applications in sports performance analysis, helping athletes
optimize their training regimens and prevent injuries. Beyond health­
care and fitness, wearables are employed in augmented reality and
virtual reality tools for precise motion tracking and immersive experi­
ences. In industrial settings, they enhance worker safety by monitoring
environmental conditions and detecting exposure to hazardous sub­
stances. Moreover, wearables have a role in improving the quality of life
for individuals with disabilities through assistive technologies like
prosthetic limbs controlled by neural signals [83]. The widely
researched applications of wearables are listed in Table 1. However, in
this section, we mainly examined the three most vital applications of
wearables as shown in Fig. 5.
5.1. Wearables for CV health monitoring
In recent years, while there has been a gradual improvement in living
standards, there has been a noticeable trend towards irregular living
habits and dietary choices. This shift is associated with an increased
strain on the heart and a heightened risk of heart disease. According to
the "Report on CV Health and Diseases in China 2019: An Updated
Summary," the prevalence of cardiovascular diseases (CVDs) in China
has been consistently rising year by year [84]. Currently, it is estimated
that there are approximately 330 million individuals suffering from CV
conditions, with CVD-related mortality topping the list of all diseases.
Furthermore, the treatment of CVDs comes with a substantial financial
burden. The World Heart Federation has projected that by the year
2030, the global cost of treating CVD will surge from around USD 863
billion in 2010 to an astonishing USD 1044 billion [85]. Consequently,
the inhibition of CVDs shows a pivotal role in not only lowering mor­
tality rates but also alleviating the economic strain associated with their
treatment [86].
CV conditions can be deduced from various signals originating dur­
ing the rhythmic beating of the heart. One such signal is the Electro­
cardiogram (ECG), which is generated due to the heart’s unique
electrical signal conduction system. This conduction system is made of
specialized cardiomyocytes [87]. Initiated by the sinoatrial node, these
cardiac muscle cells undergo stimulation, and the resulting electrical
impulse subsequently follows a specific pathway and time sequence,
ultimately resulting in the generation of the ECG signal on the body’s
surface. Furthermore, the opening and closing of each heart valve during
a heartbeat generate signals of heart sounds. Concurrently, as the heart
contracts rhythmically, the pulse waves created by the arteries amal­
gamate various pieces of info regarding the heart’s pumping activity and
the pressure waves throughout the arterial network. Within the process
of blood circulation, a complex yet subtle force is generated, leading to
the formation of seismocardiogram/ballistocardiogram (SCG/BCG)
signals, which are based on both the applied force and its corresponding
reaction force [88]. 73]. Similarly, during each heartbeat, the apex of
the heart impacts the chest wall, giving rise to an apexcardiogram (ACG)
signal [89]. Collectively, these indicators provide partial insights into
one’s CV condition.
In contemporary times, some commercially available monitoring
equipment, be it BP meters, ECG devices (such as Holter monitors), or
mattress systems designed to monitor BCG/SCG signals, often suffer
from being bulky and inconvenient to carry. Moreover, these traditional
monitoring methods are prone to issues like electrode detachment,
which can disrupt people’s normal daily activities and work. Anticipated
to address these challenges is an innovative flexible electronic technol­
ogy. Flexible electronics possess attributes such as lightweight con­
struction and high sensitivity, making them exceptionally well-suited for
seamless integration with the human skin. Recent advancements in
flexible materials and processing techniques have paved the way for
prolonged monitoring of the human body using flexible wearables and
devices [17]. This development has been further catalyzed by the pro­
liferation of 5th Generation Mobile Communication Technology (5 G),
4.3. Carbon-based materials
Carbon-based materials are extensively employed in the fabrication
of flexile force sensors, largely due to their remarkable conductivity,
lightweight characteristics, stability, and flexibility. This category of
materials typically encompasses carbon black, carbon nanotubes
(CNTs), graphene, and various graphene derivatives [74]. Frequently,
these materials are blended with polymers to create composites with
remarkably extraordinary levels of conductivity. For instance, carbon
black, possessing an amorphous structure akin to graphite, serves as a
cost-effective conductive nanoparticle that enhances both the strength
and electrical conductivity of materials when integrated into the flexible
matrix of composites [75]. When a silicone elastomer Eco-flex substrate
is layered with carbon black, the resulting sensor demonstrates an
impressive GF of up to 3.7, a stretchability of 500%, exceptional
repeatability (up to 10,000 cycles), and reduced hysteresis. In another
study, a porous structure formed by incorporating carbon black and
NaCl into Thermoplastic Polyurethane (TPU) resulted in a sensor with
high sensitivity, measuring at 5.5 kPa− 1, and an extensive range for
measuring pressure [76]. Carbon black stands out as an attractive choice
for wearables, thanks to its affordability and excellent conductivity.
Graphene is renowned for its excellent conductivity and high surface
area per unit mass. Nevertheless, its fabrication presents challenges. An
offshoot of graphene, graphene oxide, contains oxygen-containing
functional groups [77,78]. When in a water solution, it exhibits visco­
elastic properties, high viscosity, and printability. A resistive strain
sensor featuring a 3D conductive network structure is demonstrated
which exhibited remarkable sensitivity, repeatability, and response
rates [79]. Furthermore, Wang et al. suggested that fine-tuning the
sensor’s microstructure could effectively modulate its GF and subse­
quently designed a graphene strain sensor with a porous structure [80].
This adjustability makes it well-suited for wearable health monitoring
tools, as the sensor’s microstructure can adapt to the varying stress
conditions of different body parts.
5. Applications of wearables
Wearables have revolutionized various industries with their versa­
tility and ability to collect real-time data from the human body. In
healthcare, wearables are used for monitoring vital signs such as HR, BP,
and oxygen levels, enabling continuous patient monitoring and early
detection of medical conditions [51,81,82]. Fitness enthusiasts benefit
from wearables that track physical activity, providing insights into
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Table 1
Several types of wearables utilized in different applications.
Sensor type
Material
Analytes/sensing mechanism
Application
Reference
Wrist wearable
Wearable electronic
device
E-skin
Wearable on body
Smart contact lens
Smart contact lens
(self-powered)
Smart contact lens
(self-powered)
Skin-worn device
MEMS microphone sensor
Poly (3,4-ethylene-dioxythiophene (PEDOT) on the surface of thermoplastic
polyurethane (TPU) fiber
Polyimide
Highly conductive hydrogels based on Fe3+-lignin nanoparticles
Hyaluronate-modified Au@Pt bimetallic electrodes
Copper hexacyanoferrate cathode combined with glucose oxidase enzyme
Acoustic sensing
Relative resistive change
Cardiac monitoring
BTM
[126]
[127]
Sweat
Conductivity of hydrogel
Tear
Tear
BTM
Strain sensing
CGM
CGM
[101]
[128]
[22]
[129]
Carbon nanotube cathodes
Tear
CGM
[130]
Anode (agarose pilocarpine) and cathode (agarose PBS)
Interstitial fluid and sweat
biofluids
[94]
Smartwatch
Wearable on body
Wearable
multisensory
system
Wearable on body
Nafion-coated flexible electrochemical sensor patch
Two electrodes
Two EM-based sensors: a multiband slot antenna and a multiband-rejection
filter
Interstitial fluid
Galvanic skin resistance
Serum, animal tissues and
animal models of diabetes
Metabolic and
haemodynamic
parameters
CGM
CGM
CGM
Multi-walled carbon nanotube
Thermistor
[100]
Wearable on body
Acrylate copolymer (AC) and carbon black (CB)
Wearable glasses
Mirrors, laser, lens, camera
Smart lens
polydimethylsiloxane (PDMS), polystyrene-b-poly(ethylene-ranbutadiene)-b-polystyrene (SEBS) embedded with silver (Ag) flakes
(AgSEBS), Silbione liquid silicone rubber
Graphene-based textile
Ni2P/G ink-based electrodes
Hydrophilic polyvinylpyrrolidone film
Polydimethylsiloxane (PDMS) and silver nanowires (AgNWs)
Hardware sensing and wireless communication system
Gyroscope and accelerometer
Change in resistance of AC and
CB
Observing the radius of
curvature of the grid pattern of
the cornea
Observing the radius of
curvature of an eye
Body temperature
monitoring
Body temperature
monitoring
IOP monitoring
IOP monitoring
[25]
Heartrate monitoring
CGM
Humidity monitoring
IOP monitoring
Gait analysis
Motion tracking system
[133]
[134]
[135]
[136]
[137]
[138]
Ear wearable
Wearable on body
Wearable on body
Smart contact lens
Wearable on body
Wearable on body
ECG
sweat
Capacitive response
Piezoresistive strain
Inertial measurement unit
Inertial measurement units
[110]
[111]
[131]
[62]
[132]
Fig. 5. Wearables for CV health monitoring, BTM and CGM are discussed in this section.
complemented by edge computing and the Internet of Things (IoT), all of
which collectively enhance the capabilities of flexible electronics in
monitoring applications. Looking ahead, the convergence of these
emerging technologies promises to usher in a future of highly person­
alized and precise health management, revolutionizing how individuals
monitor and optimize their well-being [46].
A novel framework leveraging machine learning techniques to esti­
mate Inter-Beat Interval (IBI) from wrist Photoplethysmography (PPG)
signals is proposed [90]. The main goal is to enhance the precision of IBI
estimates while ensuring their compatibility with wearable devices,
considering typical computational power and memory constraints. This
was achieved through a dual approach. The initial method focuses on
optimizing a set of parameters associated with the interpolation of sys­
tolic peaks. During the training phase, the weights applied to neigh­
boring samples of systolic peaks was fine-tuned, minimizing the error
between the optimized peak positions and their corresponding ground
truth. The second method adopts a supervised machine learning
approach, treating R-peak detection as a classification task. This enables
accurate identification of systolic peaks in PPG signals by classifying
candidate peaks as either true or false systolic peaks.
To assess the effectiveness of these methods, the author conducted
experiments on a meticulously curated dataset comprising PPG and ECG
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signals from 46 volunteers, including those with normal sinus rhythm,
atrial fibrillation, and other unspecified cardiac arrhythmias. Compar­
ative analysis with state-of-the-art methods revealed excellent perfor­
mance, showcasing lower Mean Absolute Error (MAE) and Root Mean
Square Error (RMSE) in IBI estimation. This approach also demonstrated
heightened Precision and Recall in the detection of systolic peaks [90].
Introducing an innovative solution known as the Thin Soft Minia­
turized System (TSMS), a conformal piezoelectric sensor array, an active
pressure adaptation unit, a signal processing module, and an advanced
machine learning approach are successfully combined [91]. This
groundbreaking integration enables seamless, continuous wireless
monitoring of ambulatory artery BP in a wearable format. Through
meticulous material selection, precise control and sampling strategies,
and seamless system integration, the TSMS has demonstrated
remarkable improvements in interfacial performance while consistently
delivering Grade A-level measurement accuracy. In an initial study
involving 87 volunteers and ongoing clinical monitoring of two in­
dividuals with hypertension, the TSMS has proven itself as a dependable
BP measurement solution. This technology showcases immense poten­
tial for enhancing precision in BP control and facilitating the develop­
ment of personalized diagnosis schemes, marking a substantial
advancement in the field of wearable health monitoring. In Fig. 6(a), a
schematic representation of the TSMS design is presented as a wearable
wristband, featuring compact dimensions measuring 15 cm in length,
35 mm in width, and 4 mm in thickness. This TSMS comprises three
essential subsystems: a pulse sensing system based on piezoelectric
technology, an active pressure adaptation system, and a data sam­
pling/transmitting system (as depicted in Fig. 6(b)). The pulse sensing
Fig. 6. (a) Diagram illustrating the signal conversion process from piezo response to continuous BP, presented within a mobile graphic user interface (GUI) [91], (b)
Exploded view of the wireless wristband, featuring three integral subsystems [91], (c) illustration of the BP monitoring system which comprises key components,
including an optical fiber sensor, signal acquisition unit, data processing module, BP calculation component, smartwatch terminal, and mobile phone integration
[92], (d) a tailor-made ring designed to perfectly fit the user’s finger size [93], (e) finite element model of the human finger served as a guiding blueprint for the
design of the ring [93], (f) a cross-section of the bioimpedance sensing system featuring four electrodes [93], (g) image depicting sensor placement and enzymatic
chemical sensors for interstitial fluid (ISF) and sweat [94].
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system is equipped with a pair of sensors spaced 15 mm apart, utilizing
piezoelectric thin layers composed of lead zirconate titanate (PZT 5 H).
These sensors measure 3 mm × 3 mm in size and have a thickness of
200 µm, as illustrated in Fig. 6(b) [91]. The BP monitoring process is
depicted in Fig. 6(c).
Continuous and automatic monitoring of BP plays a crucial role in
the prevention of CVDs, particularly hypertension. This ongoing moni­
toring has also proven invaluable in assessing the effectiveness of
medications and diagnosing clinical hypertension. However, existing
wearable BP monitors often grapple with limitations related to porta­
bility, electrical safety, accuracy, and precise positioning. To address
these challenges, an innovative solution: the Optical Fiber SensorAssisted Smartwatch, designed for precise and continuous BP moni­
toring is proposed [92]. This smartwatch incorporates a fibre adapter
and a liquid capsule to enhance its BP-sensing capabilities. The fibre
adapter detects pulse wave signals, while the liquid capsule expands the
sensing area and ensures a comfortable fit against the body. Remark­
ably, the sensor boasts impressive specifications, including a sensitivity
of − 213 µW/kPa, a rapid response time of 5 ms, and exceptional
reproducibility over 70,000 cycles.
By leveraging pulse wave signal analysis and a machine learning
algorithm, this smartwatch achieves continuous and highly accurate BP
monitoring. Additionally, a dedicated wearable smartwatch equipped
with a signal processing chip, a Bluetooth transmission module, and a
specially crafted smartphone app for proactive health management is
presented. Comparison with commercial sphygmomanometer reference
measurements demonstrates the smartwatch’s reliability, with systolic
pressure and diastolic pressure errors falling within acceptable ranges:
− 0.35 ± 4.68 mmHg and − 2.54 ± 4.07 mmHg, respectively, accord­
ing to the criteria set by the British Hypertension Society (BHS) and the
Association for the Advancement of Medical Instrumentation (AAMI).
The integration of optical fibre technology into this smartwatch marks a
significant step forward in the realm of digital health, promising a
practical and effective paradigm for health monitoring and
management.
Smart rings offer a distinctive opportunity for continuous physio­
logical monitoring, presenting numerous advantages over other wear­
able devices. They are convenient to wear, impose minimal burden on
the user, particularly when compared to bulkier alternatives, are wellsuited for nocturnal use, and can be custom-sized to ensure consistent
sensor-to-skin contact. Continuous monitoring of BP plays a pivotal role
in diagnosing and prognosticating CV health. Traditional ambulatory BP
measurement tools rely on cumbersome inflatable cuffs, making them
impractical for frequent or continuous use.
In response to these challenges, ring-shaped bioimpedance sensors
harnessing the deep tissue sensing capabilities of bioimpedance without
being influenced by varying skin tones, as is the case with optical
methods are introduced [93]. This approach incorporates a compre­
hensive human finger finite element model, complemented by extensive
participant data, to derive optimal design parameters for electrode
placement and dimensions as shown in Fig. 6(d-f). This optimization
ensures the highest sensitivity to arterial volumetric changes. To esti­
mate BP, advanced machine learning algorithms are employed. The
results demonstrate the efficacy of our bioimpedance ring sensors, with
peak correlations reaching 0.81 and minimal errors (systolic BP: 0.11
± 5.27 mmHg, diastolic BP: 0.11 ± 3.87 mmHg) across a dataset of over
2000 data points and a wide range of BP values (systolic: 89–213 mmHg
and diastolic: 42–122 mmHg). These findings underscore the significant
potential of bioimpedance rings for precise and continuous BP estima­
tion, offering a promising avenue for better CV health monitoring [93].
To monitor the impact of daily activities on the body’s physiological
responses, it is essential to employ wearable devices capable of simul­
taneously tracking metabolic and hemodynamic parameters. In [94], a
non-invasive skin-worn device designed for concurrent monitoring of BP
and HR using ultrasonic transducers, as well as the measurement of
multiple biomarkers through electrochemical sensors is proposed (Fig. 6
(g)). The optimization efforts have resulted in an integrated device that
offers both mechanical resilience and flexibility, allowing it to conform
seamlessly to curved skin surfaces. Furthermore, the device’s ability to
reliably sense glucose levels in interstitial fluid, as well as lactate,
caffeine, and alcohol in sweat, all without any interference or crosstalk
between individual sensors was ensured.
In clinical trials involving human volunteers, the device effectively
recorded the physiological responses to a range of activities. This
included monitoring post-meal glucose production, tracking glucose
utilization via glycolysis, and observing corresponding increases in BP
and HR during oxygen depletion and lactate generation during exercise.
The combined acoustic and electrochemical sensing capabilities of this
integrated wearable device hold the potential to advance our compre­
hension of the body’s reactions to everyday actions. Moreover, this
technology may facilitate the early identification and anticipation of
irregular physiological shifts [94].
5.2. Wearables for BTM
The proliferation of digital health initiatives has spurred the
advancement of multifunctional flexible electronic devices. Among
these innovations, wearable systems play a pivotal role within the realm
of flexible electronic equipment. These systems proactively monitor
individuals’ vital signs, including parameters such as BP and humidity,
in real-time, thus simplifying the process of data acquisition and anal­
ysis. Consequently, personal health management has become more
advanced and convenient.
Among the various vital signs in human physiology, temperature
stands out as one of the most critical indicators that can effectively
convey essential information about metabolism and pathological con­
ditions [95]. This has led to a growing interest among researchers in the
continuous real-time monitoring of physiological temperature using
flexile sensors affixed to the skin [33]. Normally, the human body
maintains a temperature of around 37 ◦ C during the summer months.
However, when an individual develops a fever, their body temperature
can rise to 39 ◦ C or even 40 ◦ C. Beyond 42 ◦ C, immediate cooling
measures become imperative.
Conversely, the perils associated with a body temperature dropping
below 33 ◦ C are often underestimated. Hypothermia, characterized by a
sudden decline in core temperature, can induce severe shivering and, in
extreme cases, lead to fatality. Thus, the development of a flexile
wearable temperature sensing device capable of continuous real-time
monitoring of human body temperature and providing feedback on
abnormal temperature variations assumes paramount importance in the
establishment of a personalized medical system. A versatile acrylate
copolymer (AC) exhibiting an adjustable temperature response within
the range of 33–40 ◦ C was successfully synthesized [62]. Subsequently,
this AC material was utilized in the development of an exceptionally
sensitive wearable body temperature sensing (BTS), complemented by
the inclusion of carbon black (CB). The substantial expansion in volume
of the AC, coupled with the formation of a porous conductive network
through CB integration, facilitates a remarkable three-order change in
resistance in response to temperature fluctuations.
Moreover, this BTS exhibits a remarkable sensitivity (expressed as R/
R0), achieving an impressive 12.5% variation in response to temperature
shifts of as little as 0.5 ◦ C around the typical body temperature.
Importantly, this sensitivity remains consistent even after undergoing
200 cycles of heating and cooling. To enhance user-friendliness and
practicality, the BTS is ingeniously connected to light-emitting diodes
(LEDs) to visually depict this dynamic temperature-monitoring process.
Consequently, this meticulously crafted high-sensitivity BTS holds great
promise for a wide array of applications within the realm of intelligent
wearable electronics.
Wearables have garnered significant attention due to their seamless
interaction with the human body. In addition to serving protective and
aesthetic purposes, textiles are favored for wearable sensing devices due
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to their inherent softness, exceptional flexibility, and permeability [96,
97]. Consequently, they can conform to the contours of human skin,
enabling it to breathe naturally. The importance of wearable sensing
devices for monitoring changes in body temperature cannot be over­
stated, especially when continuous and extended monitoring is neces­
sary [98]. In such scenarios, the weight and comfort of these sensors
become pivotal factors. It is precisely at this juncture that textiles
emerge as an ideal sensor platform, adept at meeting these re­
quirements, thanks to their extensive array of fibres, yarns, and fabrics.
Epidermal electronic systems designed for detecting electrophysio­
logical signals, sensing, therapy, and drug delivery represent the cutting
edge of man–machine interfacing in healthcare. However, a significant
challenge persists in creating multifunctional bioapplications that boast
minimal invasiveness, biocompatibility, and stable electrical perfor­
mance while accommodating various mechanical deformations of bio­
logical tissues. A novel approach is proposed where a natural silk protein
combined with carbon nanotubes (CNTs) is harnessed to realize an
epidermal electronic tattoo (E-tattoo) system with multifunctional ca­
pabilities, effectively addressing these challenging issues [99]. The
method involves dispersing highly conductive CNTs onto biocompatible
silk nanofibrous networks with a porous nature, resulting in the con­
struction of ultrathin electronic patches that adhere seamlessly to the
skin. The individual components of this innovative system include
electrically and optically active heaters, a temperature sensor with a
temperature coefficient of resistance of 5.2 × 10− 3 ◦ C− 1, a stimulator
capable of achieving a penetration depth of over 500 µm in the skin for
drug delivery, and real-time electrophysiological signal detectors. This
strategic integration of E-tattoos onto human skin paves the way for a
new generation of electronic platforms, offering a breakthrough in
wearable and epidermal bioapplications [99].
The production and characterization of flexible temperature sensors
employing multi-walled carbon nanotubes (MWCNT) are presented in
[100] to achieve precise temperature measurements in various sensing
applications. The construction of these temperature sensors involved the
utilization of inkjet printing technology to deposit CNT ink onto soft
taffeta fabric. To enable this process, an aqueous conductive ink, based
on CNTs, was specially formulated. To shield the sensors from envi­
ronmental factors during usage and testing, a translucent polyurethane
(PU) welding tape was applied as an encapsulation layer. These sensors
operate as thermistors, demonstrating a linear increase in conductivity
with rising temperatures. The authors of this work conducted a
comparative evaluation of three differently patterned temperature sen­
sors, and the highest temperature coefficient of resistance (TCR) ach­
ieved was − 1.04% per degree Celsius, accompanied by a thermal index
Fig. 7. a) manufacturing procedure for e-skin featuring the eyes-pattern design along with the BTM sensor [101], (b) an optical snapshot of the wearable BTM sensor
[101], (c) An enlarged microscopic view of the temperature sensor within the developed e-skin [101], (d) schematic representation of the wearable device [102], (e)
depiction of the system-level block diagram (on the left) and the control panel interface for electronic adjustments and real-time signal visualization via a dedicated
smartphone application (on the right) [102], (f) Explanation of the operational principle behind the lactate biosensor [102], (g) Photograph captured during an
on-body test, showcasing the wearable device positioned on the thigh of a cyclist [102].
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of 1135 K [100].
The key technology of the human-machine interface has been
recognized for a wide range of intelligent applications due to its capacity
for seamless communication between humans and machines. Significant
attention has been garnered by wearable electronic skins (e-skins),
boasting a pliable and adaptable design, owing to their remarkable
ability to conform to uneven and textured surfaces, including human
skin and internal organs. Nevertheless, certain drawbacks have been
observed in most of the e-skins developed thus far, including mechanical
instability and the accumulation of residue at the interface between the
device and human skin. In [101], a mechanically robust e-skin featuring
a novel "eye" pattern design is proposed. The ingeniously devised eye
pattern dissipates mechanical stress and strain more efficiently than
previously employed patterns. It has been empirically confirmed that
the e-skin’s ability to allow the permeation of by-products through sweat
removal tests demonstrates superior performance compared to con­
ventional e-skins. Lastly, the real-time monitoring of body temperature
using a resistive-type thermometer integrated into the e-skin was
demonstrated. In Fig. 7(a), the production procedure, along with a
three-dimensional representation of the e-skin featuring the eye pattern
design, is presented. Moving to Fig. 7(b), an optical depiction of the
completed e-skin adorned with the eye pattern is shown. This e-skin
includes a BTM sensor, which is electrically connected via copper wires
using silver paste. As demonstrated in Fig. 7(c), a chromium-based
resistive temperature sensor is successfully manufactured on the e-skin
with the eye pattern [101].
The utilization of wearable sensing interfaces for the chemical
analysis of sweat presents an appealing alternative to conventional
blood-based procedures within the realm of sports. Despite the assertion
that sweat lactate serves as a pertinent biomarker in sports, the devel­
opment of a thoroughly validated wearable system to substantiate this
claim has remained elusive. In [102], a fully integrated sweat lactate
sensing system was designed for on-the-spot perspiration analysis. This
compact device can be comfortably affixed to the skin, enabling
real-time monitoring of sweat lactate levels during sporting activities
such as cycling and kayaking. The innovation within our system unfolds
on three fronts: a sophisticated microfluidic design for sweat collection
and analysis, a rigorously validated lactate biosensor, characterized by a
thoughtfully designed outer diffusion-limiting membrane, and an inte­
grated circuit for signal processing complemented by a custom smart­
phone application.
The sensor spans the expected range of lactate concentrations in
sweat (1–20 mM) while exhibiting an appropriate sensitivity (− 12.5
± 0.53 nA mM–1) and a commendable response time (<90 s). Further­
more, the sensor is minimally affected by changes in pH, temperature,
and flow rate. It also demonstrates exceptional attributes in terms of
reversibility, durability, and reproducibility. To validate the perfor­
mance of this sensing device, Xuan et al. conducted a substantial number
of on-body tests involving elite athletes engaged in controlled cycling
and kayaking sessions [102]. Additionally, the correlation findings be­
tween sweat lactate levels and other physiological indicators typically
measured in sports laboratories are discussed, including blood lactate,
perceived exhaustion, HR, blood glucose (BG), and respiratory quotient.
These results shed light on the potential of continuous sweat lactate
monitoring for enhancing sports performance assessment. The system,
as illustrated in Fig. 7(d), comprises four essential components: (i) An
electrochemical biosensor utilizing enzyme-based technology, which is
interconnected with an outer diffusion-limiting membrane, enabling the
measurement of sweat lactate levels; (ii) A sweat sampling cell and
pressure controller, both fabricated through 3D printing, featuring a
cuboid design with dimensions of 30 × 16 × 7 mm; (iii) A reusable
electronic board responsible for recording electrical currents and facil­
itating wireless data transmission; (iv) A custom-designed mobile
application tailored to the specific requirements of the system [102].
Fig. 7(e) displays the system-level block diagram of the device, which
highlights several components. The lactate biosensor, resembling
electronics like a potentiostat (depicted in green), generates a current
signal. This raw current signal is then processed (represented in purple),
converted from analog to digital (shown in grey) by an analogue-todigital converter, and transmitted wirelessly (highlighted in blue) to
the user interface on a mobile phone via an electronic board. The bio­
sensing element follows a "first-generation" approach and consists of
three essential parts: the redox mediator, the enzyme, and an external
diffusion-limiting layer. The operational principle is explained in Fig. 7
(f). In summary, the external diffusion-limiting membrane controls the
transfer of lactate from the sample to the enzyme layer, ensuring con­
stant concentration. Lactate is then enzymatically transformed by
lactate oxidase, producing hydrogen peroxide and pyruvate. Prussian
blue is present to reduce hydrogen peroxide, enabling real-time moni­
toring of lactate concentration by tracking the corresponding reduction
in current, which is directly proportional to the lactate content in the
sample.
The device can be easily attached to any part of the body using
medical tape and straps, ensuring that it doesn’t cause discomfort to the
wearer during physical activities. Fig. 7(g) shows that the device is
securely fastened to the skin, as demonstrated here underneath a cy­
clist’s sweatpants. Once it’s in position, the device records the current
signal, which is then transmitted via Bluetooth to a dedicated mobile
application. When the person’s sweat reaches a specific level, it is
directed through a microfluidic channel, passing over the detection area.
At this point, the measured current accurately reflects a meaningful
concentration of lactate in the sweat [102].
5.3. Wearables for CGM
Diabetes, an infamous chronic ailment, primarily arises from im­
mune system dysfunction or metabolic disorders, leading to the exces­
sive accumulation of glucose in the bloodstream. This prolonged
abnormal glucose level can trigger severe CV complications, including
hypertension, dyslipidemia, diabetic cardiomyopathy, CV autonomic
neuropathy, and myocardial infarction [103]. These complications
stand as the leading causes of morbidity and mortality among diabetic
patients [104]. Given the dire consequences and the alarming increase
in diabetes prevalence, closely monitoring, and regulating BG levels
becomes of paramount importance to preempt its spread and avert the
long-term CV risks. Traditionally, assessing BG via invasive blood sam­
pling is commonplace, but it proves uncomfortable and offers no
continuous monitoring for patients. Consequently, numerous alternative
non-invasive or minimally invasive BG detection approaches have
emerged, encompassing electrochemical, optical, microwave methods,
and more [20,105–107]. Among these, electrochemical analysis of
glucose concentration in biofluids like sweat, saliva, tears, and inter­
stitial fluid (ISF) stands out as the most promising method for achieving
continuous surveillance [108]. Notably, it boasts a relatively straight­
forward operating principle and a wearable detection setup, garnering
significant attention in ongoing research endeavors.
CGM has revolutionized the management of diabetes, offering two
main approaches: invasive and non-invasive methods. Invasive CGM
involves the insertion of a small sensor under the skin, typically in the
abdomen or arm, which continuously measures glucose levels in the
interstitial fluid (See Fig. 8(a)). This method provides highly accurate
and real-time data, enabling individuals with diabetes to make imme­
diate and precise adjustments to their insulin doses and lifestyle [109].
On the other hand, non-invasive CGM, often in the form of wearable
devices or patches, measures glucose levels through alternative means,
such as sweat, tears, or even breath (See Fig. 8(b)) [110,111]. While
non-invasive CGM offers the advantage of greater comfort and reduced
pain compared to invasive methods, it may be less accurate and have a
slight time delay. The choice between these two approaches depends on
an individual’s preferences, needs, and the specific characteristics of
their diabetes management, with both options contributing to improved
glycemic control and quality of life for those living with diabetes.
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Fig. 8. CGM methods (a) invasive CGM method [109], (b) non-invasive [110].
The rapid proliferation of smartphones and other connected tools has
fueled the demand for smaller, low-power, and more efficient sensors
with improved performance. Advances in nanotechnology and micro­
fabrication have accelerated the miniaturization process, making smart
sensors more affordable and facilitating the development of smart fab­
rics. Let’s say, Abbott Laboratories unveiled a continuous glucose
monitoring (CGM) device in January 2022, capable of providing
continuous monitoring for up to 14 days, reflecting the increasing con­
sumer demand for wearable preventive medicine tools [112].
Sweat serves as a compelling biofluid for health monitoring due to its
abundance in biochemical markers and its accessibility through
epidermal sensors. Within sweat, one can find small molecules like
glucose, and their concentrations are often correlated with those found
in the blood. However, achieving precise measurements of glucose
concentration in sweat proves challenging due to various interfering
factors. These factors encompass skin temperature, pH levels, contami­
nants from the surrounding environment (including glucose and other
biomarkers), the relatively low concentration of glucose in sweat, the
limited sample volumes available, and the natural evaporation of sweat.
To address these complexities and drawbacks, there is a growing de­
mand for integrated systems featuring multiple monitoring functions,
glucose sensors with high sensitivity and selectivity, as well as suitable
methods for collecting and transporting sweat samples. Capitalizing on
recent advancements in flexible materials, fabrication techniques, and
sensing methodologies, several promising platforms have emerged to
enable continuous monitoring of glucose levels in sweat.
A superior indicator of disorders and diseases compared to blood has
been identified in the form of saliva. Specifically, the level of salivary
glucose is regarded as a reliable marker for diabetes [106]. A strong
correlation between BG levels and salivary glucose levels has been
confirmed in preliminary studies, rendering continuous monitoring of
saliva glucose levels a highly promising method for tracking BG [113,
114]. Several notable advantages are presented by saliva as a diagnostic
fluid when compared to other bodily fluids such as blood, tears, sweat,
or urine. Firstly, it can be easily tested by individuals with minimal
training. Secondly, it is non-invasive, eliminating the risk of infections or
cross-contamination associated with frequent finger pricks. Thirdly, it is
particularly convenient for individuals who face challenges in extracting
blood samples, including infants, the elderly, and those with haemo­
philia. Lastly, saliva contains a wealth of disease-related biomarkers,
including those typically found in blood.
In contrast to various other biological fluids, such as saliva and
sweat, interstitial fluid (ISF) and tears exhibit remarkable stability in
both their volume and glucose concentration. This inherent stability has
prompted the development of indirect methods for monitoring BG
levels, involving the analysis of tear glucose concentration and its cor­
relation with BG levels. One approach involves the utilization of
enzyme-based amperometric and coulometric glucose sensors within
systems designed to analyze tear glucose concentration. These systems
have been demonstrated to establish a strong correlation between tears
and BG levels [107,115]. To achieve this, researchers have collected tear
fluid samples from both rabbits and humans using glass capillary tubes.
In [116], a smartphone-controlled, microneedle (MN)-based wear­
able CGM system is developed, revolutionizing long-term glucose
monitoring. This CGM system, utilizing a sandwich-type enzyme
immobilization strategy, meets the critical clinical need for monitoring
ISF glucose levels over a remarkable 14-day period. With a mean ab­
solute relative difference of 10.2% and a cost of under $15, our system
demonstrates efficiency comparable to leading commercial glucometers
and the FDA-approved CGM system FreeStyle Libre (Abbott Inc., Illinois,
USA). This self-developed CGM system not only accurately monitors
glucose fluctuations but also furnishes a wealth of clinical information.
This technology stands out in its ability to pinpoint the causes of indi­
vidual BG changes, offering valuable insights for precise glucose control.
In essence, our intelligently wearable CGM system emerges as a prom­
ising alternative for home-care diabetes management, providing a so­
phisticated and cost-effective solution for individuals seeking enhanced
control and understanding of their glucose levels.
In [117], a touch-actuated biosensor designed for monitoring glucose
levels in ISF is proposed. The biosensor comprises three integral com­
ponents: 1) a solid microneedle array (MA) facilitating painless skin
penetration; 2) an RI unit enabling ISF extraction through micro­
channels created by the MA; and 3) a sensing unit dedicated to glucose
monitoring. The biosensor’s innovative sensing strategy follows a
sequence of "skin penetration-RI extraction-electrochemical detection."
Compared to RI extraction alone, the introduced skin penetration-RI
extraction sampling strategy significantly enhances glucose extraction
flux by approximately 1.6 times, demonstrated both in vitro and in vivo.
Furthermore, this touch-actuated biosensor was integrated into a
wearable glucose monitoring system, complete with a wireless electro­
chemical detector and a smartphone application. In vivo, experiments
involving healthy and diabetic rats showcase a robust correlation be­
tween the results obtained from this wearable system and those from a
commercially available blood glucometer. This sampling strategy,
combining skin penetration and RI extraction, not only sets the stage for
the development of wearable platforms for glucose monitoring but also
holds promise for monitoring various ISF biomarkers, eliminating the
need for painful finger-stick blood sampling [117].
Concurrently, advancements in technology have led to the creation
of smart contact lenses, equipped with integrated glucose sensors and
analysis circuits. These innovative lenses enable real-time monitoring of
tear glucose levels while being worn [21,118,119]. Regarding optical
analysis techniques, Elsherif et al. introduced a wearable contact lens
optical sensor and conducted a comparative study across six groups,
examining glucose concentrations spanning from 0 to 50 mM, with in­
tervals of 10 mM [120]. However, their demonstrated sensitivity was
deemed impractical, given that the typical glucose concentration in tears
falls within the range of 0.05–5 mM. In contrast, a more recent optical
glucose sensor, developed by a team from MIT in 2020, offers increased
sensitivity. Nevertheless, it necessitates the use of a high-resolution
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optical spectrometer to discern Raman signatures [121]. This
non-invasive glucose measurement method employs an 830 nm-diode
laser to gauge glucose concentration based on the Raman shift. Conse­
quently, there is a need for optical glucose sensors to be both more
sensitive and straightforward in their operation.
An innovative system featuring body-matching, vasculature-inspired
quasi-antenna arrays designed as electromagnetic (EM) sensors is pro­
posed for immediate, continuous, and wireless detection of glucose
fluctuations in the bloodstream [122]. Individual users are catered to by
these sensors, with the utilization of EM waves and seamless integration
with a specialized machine learning-driven signal processing module.
Importantly, they are highly adaptable and discreetly incorporated into
wearable garments like socks, ensuring a conforming fit to curved skin
surfaces while maintaining resilience to movement. Rigorous calibration
for factors such as temperature, humidity, and motion is undergone by
the complete wearable system, resulting in exceptional accuracy in
tracking glucose variations. A remarkable 100% diagnostic accuracy
across a wide range of glucose fluctuations has been demonstrated in
in-vivo experiments involving diabetic rats and pigs. In human trials
conducted with diabetes patients and healthy individuals, an impressive
clinical accuracy of 99.01% was reached by CGM among the
twenty-eight subjects who underwent Oral Glucose Tolerance Tests. As a
result, this approach guarantees the faithful and continuous monitoring
of glucose variations across hypo-to-hyperglycemic levels with unpar­
alleled precision [122].
To improve the accuracy of glucose detection, Mao et al. concen­
trated on customizing the electrode material of the sensor [123]. Their
research led to the careful design of bimetallic organic frameworks
(bi-MOFs) made up of manganese and nickel ions, referred to as
NiMn-MOF, which consist of extremely thin nanosheets, as depicted in
Fig. 9(a). These thin nanosheets, combined with the incorporation of
different metal ions in the structure, optimize the electronic character­
istics, thus improving the electrical conductivity of the MOFs. The
preparation method has yielded impressive outcomes, demonstrating
the exceptional electrocatalytic performance of NiMn-MOF in glucose
detection. Specifically, NiMn-MOF shows an outstanding sensitivity of
1576 μA mM− 1cm− 2 in the linear range of 0 to 0.205 mM, with
additional linear ranges observed at 0.255–2.655 mM and
3.655–5.655 mM. Moreover, the NiMn-MOF sensor exhibits exceptional
repeatability, reproducibility, long-term stability, and an extremely low
limit of detection (LOD) of 0.28 μM (S/N = 3), providing a solid foun­
dation for practical applications of the sensor in the field of wearable
CGM, especially during activities such as dancing.
Notably, NiMn-MOF sensor, as designed, offers accurate glucose
measurement in sweat, underscoring its significant potential for
advancing the field of wearable CGM during physical exertion, such as
dancing. As shown in Fig. 9(b), the BIOSYS advanced biosensing system
recorded the signals corresponding to glucose levels in the dancer’s
sweat on their hands. In contrast, Fig. 9(c) displays the response current
curve of the NiMn-MOF monitor concerning sweat glucose levels during
the exercise. As the duration of the exercise extended, the biosensor’s
response current to sweat-borne glucose exhibited a progressive rise,
climbing from 64 to 183 nA [123].
In the study by Guo et al., they successfully developed multifunc­
tional SCLs incorporating MoS2 transistors [124]. This innovative lens,
placed on a PDMS substrate, featured multiple functions aimed at
enhancing personal health care. It included a glucose sensor utilizing
MoS2 nanosheets for the direct measurement of glucose levels in tears, a
photodetector for receiving optical data, and a temperature sensor based
on gold (Au) to monitor potential corneal diseases. The lens’s unique
serpentine mesh structure allowed it to seamlessly interact with tears,
being directly mounted on the contact lens. This design not only
increased sensing sensitivity but also ensured that blinking and vision
remained unaffected. Furthermore, comprehensive tests confirmed the
exceptional biocompatibility of this SCL, highlighting its immense po­
tential as the next-generation point-of-care wearable soft device for
managing personal health.
Recent advancements in research have focused on point-of-care tear
glucose sensors, expanding their scope beyond diabetes diagnosis to
include therapeutic applications. Keum et al. introduced a smart lens
device, seamlessly integrated with a biocompatible polymer [20]. This
point-of-care device featured ultrathin soft circuits and a microcon­
troller designed for detecting tear glucose levels, drug administration,
data transmission, and wireless power supply. Crucially, the study
Fig. 9. (a) prototype of working electrode [79], (b) conceptual image of a dancer wearing a diabetes metre [123], (c) The volunteer’s reaction signal in (b) [123], (d)
a schematic representation of the SCL for the treatment and diagnosis of diabetes. As a universal platform for diverse diagnostic and treatments, the SCL is implanted
with a biosensor, an f-DDS, a wireless power transmission scheme from a transmitter coil to a receiver coil, an ASIC chip, and a remote communication system [20].
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Sensors and Actuators: A. Physical 366 (2024) 114993
validated the accuracy of tear glucose measurements against BG levels
and demonstrated the lens’s capability to trigger drug delivery for dia­
betic retinopathy treatment. This groundbreaking work represents the
first step towards developing contact lenses that combine biometric
analysis and drug delivery, promising a future where personal health
care and medical devices seamlessly merge diagnostic and therapeutic
functions [125].
The versatile SCL comprises five key components: a live electrochemical biosensor, an adaptable drug delivery system for on-demand
use (f-DDS), a resonant inductive wireless energy transmission system,
an integrated circuit-based microcontroller chip paired with a power
management unit (PMU), and a remote radio frequency communication
system, as illustrated in Fig. 9(d). The real-time amperometric biosensor
is specially designed to detect glucose levels in tears, eliminating the
necessity for invasive blood tests. Additionally, the self-regulated pul­
satile f-DDS enables controlled drug release, which can be initiated
remotely. Wireless power transfer, facilitated by resonant inductive
coupling with a copper receiver coil, allows the lens to draw power from
an external source featuring a transmitter coil. The device establishes
communication with an external controller through RF communication.
In our evaluation, we investigate the potential use of this SCL for both
diagnosing diabetes and managing diabetic retinopathy therapy [20].
the development of energy-efficient sensors is imperative to reduce the
frequency of recharging or battery replacement [141].
In addition to these challenges, ensuring the seamless transmission of
data from sensors to receivers or smartphones, especially for sensors
integrated into clothing, requires the establishment of robust wireless
connectivity and data transfer protocols for real-time monitoring [142].
Calibration remains an ongoing concern to sustain accuracy, with the
added complexity of accounting for sensor data drift over time in sub­
sequent data analysis. The integration of wearable sensing devices with
other devices or systems can be intricate, necessitating the assurance of
compatibility with existing infrastructure and data processing tools
[143].
Given that wearable sensing devices gather sensitive health and
personal data, safeguarding data privacy and security is of paramount
concern. The implementation of robust encryption and data protection
measures is indispensable [144]. The absence of industry standards for
flexible sensors can obstruct interoperability and hinder their broad
adoption. Active standardization initiatives across various domains are
underway to address this issue.
Moreover, the heightened manufacturing costs associated with
wearable sensing devices pose a significant barrier to accessibility,
prompting concerted efforts to streamline production processes and
improve affordability. Overcoming this financial obstacle is pivotal to
ensuring that these technological advancements are widely available
and not limited to a select demographic. Furthermore, convincing users
to adopt wearable sensing devices can prove to be a formidable task,
given potential reservations related to privacy concerns, discomfort, or
skepticism about the necessity of continuous monitoring. Addressing
these reservations demands a comprehensive, multidisciplinary
approach, bringing together expertise from fields such as materials sci­
ence, electronics, mechanical engineering, data science, and user expe­
rience design. It is imperative to foster ongoing research and
development initiatives that strategically target these challenges, ulti­
mately paving the way for the full realization of the extensive potential
of wearable flexible sensors across a myriad of applications.
6. Challenges associated with the wide availability of wearables
In recent years, wearable flexible sensors have garnered significant
consideration due to their versatility in various purposes, such as
healthcare, fitness surveillance, and human-machine interfaces. None­
theless, they present a range of obstacles that must be surmounted to
foster their widespread adoption and effective deployment. The effec­
tiveness of wearable sensors is intricately tied to the diverse range of
human body types and activities, presenting notable challenges. Body
type variations, such as differences in skin thickness, texture, and elas­
ticity, can affect the contact and adhesion of sensors to the skin, influ­
encing the accuracy of data collection. Individuals engaged in different
physical activities may experience variations in motion, perspiration,
and body temperature, adding complexity to sensor calibration and data
interpretation. For instance, a wearable sensor designed for monitoring
heart rate during a sedentary task may encounter difficulties in accu­
rately capturing data when the user is engaged in vigorous exercise.
Moreover, diverse lifestyles and daily routines can impact the wear­
ability of these devices, as user comfort and compliance play crucial
roles in the long-term success of wearable sensor applications.
Addressing these challenges necessitates continuous innovation in
sensor design, incorporating adaptability to various body types and
activities, to enhance the reliability and usability of wearable technol­
ogy across a broad spectrum of users.
First and foremost, flexible sensors are routinely subjected to me­
chanical stresses, including bending and stretching [75]. The challenge
lies in ensuring their enduring functionality and resilience under such
conditions. This necessitates meticulous selection of materials and
manufacturing processes capable of withstanding repeated de­
formations. Furthermore, wearables must prioritize user comfort during
prolonged wear. This entails the utilization of soft, pliable, and
breathable materials. Attachment methods like straps, adhesives, or
other fastening mechanisms should be designed to prevent skin irrita­
tion or discomfort.
Additionally, maintaining sensor precision and reliability, particu­
larly after extended periods of usage, poses a formidable task. The
implementation of calibration procedures and mechanisms to correct for
drift becomes essential to guarantee the consistency of data over time.
For applications in the medical and healthcare realms, ensuring the
biocompatibility of the materials used in wearables is of paramount
importance to avert allergic reactions or tissue irritation in users [139,
140]. Many wearable sensing devices rely on power sources, making
effective power management a vital concern. Enhancing battery life or
7. Concluding remarks
Wearables hold immense importance in today’s world, revolution­
izing how we monitor and manage our health and lifestyle. These
compact and versatile devices provide real-time data on vital signs,
physical activity, and environmental factors, empowering individuals to
take proactive control of their well-being. From tracking HRs during
workouts to monitoring sleep patterns and even aiding in the manage­
ment of chronic illnesses, wearables offer a wide range of information
that can lead to healthier, more informed choices. Furthermore, their
potential extends beyond personal use, playing a pivotal role in
advancing medical research, telehealth services, and the overall
enhancement of healthcare delivery. With their ability to bridge the gap
between data and action, wearables are instrumental in fostering a
future where preventative healthcare and personalized well-being take
center stage.
Wearables for CV monitoring play a crucial role in maintaining heart
health and preventing CVDs. These devices provide continuous tracking
of vital metrics like HR, BP, and ECG data, offering individuals valuable
insights into their CV health. By detecting irregularities or anomalies in
real-time, wearables can assist in identifying potential issues like ar­
rhythmias or hypertension, allowing for early intervention and medical
consultation. Moreover, they empower individuals to make informed
lifestyle choices, such as adjusting exercise routines or managing stress,
to mitigate CV risks. Wearables also enable remote monitoring by
healthcare professionals, enhancing patient care and reducing the
burden on healthcare systems. In essence, wearables for CV monitoring
are a powerful tool in promoting heart health, preventing CVDs, and
improving the overall well-being of individuals.
Moreover, wearables for body temperature monitoring have
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assumed paramount importance, particularly in recent times. These
devices present non-intrusive and continuous methods of tracking a
person’s temperature, providing valuable insights into health status and
potential infections. They have played a key role in an international
response to the COVID-19 pandemic, as elevated body temperature is
often an early symptom of illness. Beyond pandemic management,
wearable temperature sensors hold significance in various healthcare
applications, such as monitoring fever in children, tracking temperature
trends in individuals with chronic conditions, and ensuring early
recognition of infections in high-risk populations. Moreover, they
empower individuals to proactively manage their health by enabling
prompt intervention when abnormal temperature patterns are detected.
In essence, wearable temperature sensors contribute to early diagnosis,
preventive healthcare, and overall well-being, making them a vital tool
in modern healthcare technology.
Finally, wearables for CGM have revolutionized the lives of in­
dividuals with diabetes. These devices offer a continuous and noninvasive means of tracking BG levels, reducing the need for frequent
finger pricks and enhancing the overall quality of life for those managing
this chronic condition. The importance of wearable glucose sensors ex­
tends beyond convenience; they provide critical data that enables better
glycemic control and helps prevent dangerous fluctuations in blood
sugar. By offering real-time insights, individuals with diabetes can make
more informed decisions about their diet, medication, and lifestyle
choices, reducing the risk of complications associated with poorly
managed blood sugar levels. Furthermore, these sensors enable health­
care professionals to remotely monitor patients, providing timely in­
terventions and personalized treatment adjustments. In sum, wearable
glucose sensors empower individuals with diabetes to take proactive
control of their health, improve their well-being, and reduce the longterm health risks associated with the disease.
The realization of wearables, while promising, comes with its share
of challenges. Miniaturization and power efficiency are constant hur­
dles, as sensors need to be compact and energy-efficient to be comfort­
ably worn for extended periods. Ensuring data accuracy and reliability
in dynamic environments remains a challenge, particularly for motionrelated metrics like step counts or HR during intense physical activ­
ities. Privacy and data security concerns arise as these devices collect
sensitive health information, necessitating robust safeguards against
unauthorized access. Interoperability and standardization issues can
hinder the seamless integration of wearable data into healthcare sys­
tems. Moreover, affordability and accessibility remain concerns, as not
everyone can access or afford these devices, potentially exacerbating
health disparities. Ethical considerations, such as consent for data usage
and the responsible handling of user information, add further
complexity. Overcoming these challenges is essential to fully harness the
potential of wearables for improving health and well-being while
addressing the ethical and logistical dimensions of this transformative
technology.
Data Availability
No data was used for the research described in the article.
Acknowledgement
This work was performed with funding from the Samara National
Research University Development Program for 2021–2030 within the
framework of the “Priority-2030″ program (in materials for wearables)
and the State Assignment of FSRC “Crystallography and Photonics” RAS
(in the wearable sensing applications).
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CRediT authorship contribution statement
Butt Muhammad A.: Formal analysis, Software, Writing – original
draft, Writing – review & editing. Kazanskiy Nikolay L.: Conceptuali­
zation, Data curation, Formal analysis, Resources, Software. Khonina
Svetlana N.: Investigation, Project administration, Resources, Software,
Supervision.
Declaration of Competing Interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
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Nikolay L. Kazanskiy graduated with honors (1981) from Kuibyshev Aviation Institute
(presently, Samara National Research University), majoring in Applied Mathematics. He
received his Candidate in Physics & Maths (1988) and Doctor in Physics & Mathematics
(1996) degrees from Samara National Research University. He is the director of Image
Processing Systems Institute of the RAS - Branch of the Federal Scientific-Research Centre
"Crystallography and Photonics" of the Russian Academy of Sciences, also holding a parttime position of a professor at Technical Cybernetics department of Samara National
Research University. He is a member of OSA, SPIE and IAPR. He co-authored 400 scientific
papers, 14 monographs, 57 inventions and patents. His current research interests include
diffractive optics, computer vision, optical sensors, mathematical modeling, lighting de­
vices design, and nanophotonics.
Svetlana N. Khonina is the main researcher of the Laser Measurements laboratory at
Image Processing Systems Institute of the Russian Academy of Sciences and a professor of
Computer Science department at Samara National Research University. She graduated
from Kuibyshev Aviation Institute (1989), received her Candidate’s and Doctor’s Degrees
in Physics & Mathematics from Samara State Aerospace University (1995) and (2001). She
is a co-author of 500 scientific papers, 7 books and 5 inventions. Her current research
interests include diffractive optics, singular optics, mode and polarization transformations,
nanophotonics, optical and digital image processing.
Muhammad Ali Butt received his PhD degree in Material Sciences from Universitat
Rovirai Virgili, Spain in year 2015. In 2018, he worked at Nicolaus Copernicus University,
Poland as a Research Assistant Professor. In 2013, he made a research stay at Optoelec­
tronic research Centre (ORC), University of Southampton, England. He works as a Senior
Scientist at Samara National Research University, Russia. He is a co-author of 161 research
papers and 8 book chapters. Research interests are optical waveguides, plasmonic sensors,
diffractive optics, and optical filters.
18