Review—Energy and Power Requirements for Wearable Sensors

Wearable sensing technology has quickly transformed from a science-fiction vision to a real-life technology in various fields such as defense, medical sciences, aerospace technology, food tech, etc. Wearable devices are drawing attention in the medical field as they provide relevant information about people’s health in real-time. These sensors are flexible, cost-effective, and highly sensitive, which makes them a favorable candidate for future sensing technology. Despite being relatively small, they frequently sense, collect, and upload a variety of physiological data to enhance quality of life. This could lead to a major change in the daily life of people, but for this change to happen, sustainable energy technology that can power flexible wearable devices is needed. Wearable sensors come in a variety of shapes and sizes and require energy for their proper functioning. As a result, it is critical to develop and choose dependable energy supply systems. This review paper discusses different energy sources that are used to power wearable devices along with various challenges that are in the realm of this technology. The future holds great possibilities for wearable sensing technology, which can be explored only if the power sourcing to these devices is more sustainable, eco-friendly, and efficient.

require in any place via renewable, accessible, and free methods. 11lexible solar cells (SCs) based on advanced nanomaterials with better mechanical robustness and transparency were revealed to be perfect candidates to address this pressing issue.Perovskite solar cells, dye-sensitized solar cells, and organic solar cells are the three types of innovative platforms that have been established.Flexiblewearable photovoltaic systems may be easily modified with any sensor device or substrate to provide energy to a wide range of electronic devices. 12Photovoltaic platforms, on the other hand, may be incorporated into hybrid platforms and employed in a variety of applications.
Dye-sensitized SCs are one of the most cost-effective photovoltaic devices, attracting attention because of their easy manufacturing process, low-cost raw materials, low-cost equipment, and use of environmentally acceptable components in their preparation.They can also become very flexible, making them perfect candidates for the creation of flexible-wearable SCs. 13 Fiber-shaped dye-sensitized SCs have piqued interest in recent years as potential energy production systems for the creation of the next generation of smart devices in this area.To realize the wearable notion, long and flexible thin-film SCs capable of adapting to curved surfaces, such as the human body, are required.Casadio et al. 14 investigated various critical features of long and flexible fiber-shaped dye-sensitized SCs with photoanodes made of affordable TiO 2 and a completely organic [5,4-d]thiazole-based sensitizer (TTZ5).In 3.5 cm-long devices, the effect of photoanode thickness on photovoltaic performance was investigated, and an ideal thickness of 10 μm was determined, yielding a maximum PCE of 1.57%.The SCs were then made long (10 cm) utilizing thin layer photoanodes of around 5 μm, which were proven to be the best choice for such devices.The as-obtained TTZ5 devices have dimensions that are desirable for future applications, as well as a phenomenal PCE of 1.23%, opening the door for further optimization of thin-film flexible long SCs.In addition, solidstate fibre solar cells that are stable and washable are being developed for future wearable electronics. 15lexible photovoltaic systems with a photoactive perovskite layer have also emerged as promising alternatives for powering wearable gadgets.These photovoltaic systems can become flexible and transparent, meeting the demand for green energy sources that are portable, wearable, and efficient.Self-powered greenhouses, solar curtains, wearable electronics, and building-integrated photovoltaics are all possible applications.Due to its solid-state nature and lowtemperature fabrication technique, the perovskite photovoltaic platform offers a promising approach for the fabrication of flexible devices. 16Because of their flexibility, semi-transparency, lightweight, and low processing temperature, organic SCs have been regarded as a viable substrate for the development of flexible and wearable Solar Systems.Most high-efficiency organic-based photovoltaic systems use a hetero-junction that consists of a conjugated polymeric substance or molecule as an electron donor and fullerenederived nanomaterials as electron acceptors. 17Photovoltaic devices that use fullerene as an electron acceptor layer, on the other hand, face several challenges, including low light absorption rates, fixed energy levels, and chemical compositions, all of which can negatively impact the open-circuit voltage and short-circuit current density of established photovoltaic devices.Furthermore, as compared to other solar devices, these devices exhibit weak flexibility, a brittle crystalline structure, and limited stretchability.Recently, significant efforts have been made to improve the efficiency of organic SCs, with the use of a solution-processed non-fullerene electron acceptor improving the PCE of organic solar cells to around 12%, with tuneable electronic and chemical properties and significantly improved device stability compared to fullerene-based organic photovoltaic devices. 18When a PM6: CH1007: PCBM ternary blend was placed on top of the perovskite layer to facilitate prolonged absorption, a high-performance hybrid SC was reported that outperformed both single-component perovskite and organic analogues.It results in a champion PCE of 23.80% for rigid devices and 21.73% for flexible devices, respectively.In addition, the hybrid construction aids in long-term stability for more than 1000 h.The flexible devices, on the other hand, have high mechanical fatigue endurance, with 95% of the initial PCE remaining after 1000 cycles of successive bendings.By combining flexible gadgets with a wearable temperature sensor, a self-powered temperature monitoring system was successfully demonstrated. 19onformable energy storage devices that can offer energy output whenever needed are projected to power next-generation wearable electronics.In this regard, the new integrated energy harvesting and storage system in a flexible assembly has presented a viable option.A flexible perovskite SC-driven photo-rechargeable lithium-ion capacitor for self-powering wearable strain sensors was described by Li et al. 20 With a discharge current density of 0.1 A g −1 , this flexible module achieves an overall efficiency of 8.41% and a high output voltage of 3 V.Even at a high current density of 1 A g −1 , it could still achieve a remarkable overall efficiency of over 6%, surpassing state-of-the-art photo-charging power sources.As a result, the self-powered strain sensor can easily capture precise and continuous data on physiological signals without the need for external power, accomplishing the synergy of energy harvesting, storage, and use inside one smart system.This multi-field coupled integrated platform is expected to provide considerable advantages in the development of self-powered wearable electronics.
Integration of wearable sensors with nanogenerators.-Selfpoweredsensors may capture and convert ambient energy to electricity, allowing them to operate in the absence of an external power source.Nanogenerators (NGs) can capture energy from various sources such as human mobility 21 environment 22 gas sensors 23 and biological nerve impulses 24 etc.Self-powered wearable sensors based on NGs can look at objects in a new context.As a result, low-cost self-powered sensors may be widely deployed and are a strong choice for data sources for big data, artificial intelligence (AI), and the Internet of Things (IoT).NGs could be utilized as pressure sensors as well as power sources.Triboelectric nanogenerators (TENGs) were utilized.Water droplets 25 wind flow 26 and even human pulse waveforms 27 have all been detected using piezoelectric nanogenerators (PENGs)-based sensors.Traffic monitoring and road and bridge monitoring 28 can benefit from NGbased self-powered sensors.The triboelectrification/contact electrification (CE) method is the basis for TENG functioning. 29The singleelectrode mode, the common vertical contact-separation mode, the freestanding triboelectric-layer mode, and the contact-sliding mode are the four working modes of TENGs. 30TENGs may be manufactured with a variety of inexpensive, low-maintenance, and environmentally friendly materials.The PENG utilizes external stimuli to generate an electrical signal.Sensors based on PENG/TENG are flexible and precise.AI-based on the back end may be utilized to instantly determine the enormous quantity of data gathered to recreate muscle action and properly determine workout routines.AI classification algorithms, for instance, may recognize and identify athletes' motions in real time. 31he data analysis technologies can track athletes' real-time training progress and make training recommendations. 32,33uman motion recognition, medical diagnosis and rehabilitation, sports training, human 3D motion modeling, and respiratory monitoring can benefit from self-powered wearable sensors that capture human motion data.These data can be utilized to identify human health or computer interaction in real time.Wen et al. 21reated a transparent and stretchable wrinkling TENG (WP-TENG) electrode based on a poly(3,4ethylenedioxythiophene): poly(4styrene sulfonate) (PEDOT: PSS) electrode and implanted the WP-TENG-based self-powered motion sensor at various places of a human arm.As illustrated in Fig. 2a, the WP-TENG was inserted on the skin above the arm muscles.Once the arm is bent, the muscles expand the sensor to a wider contact area, causing the sensor to produce a voltage fluctuation.A voltage of roughly 23 V is created as the output voltage.The voltage falls to zero upon release.Figure 2b shows how the peak voltage changes with the bending angle of the elbow and the rate of joint motion.Figure 2c shows the variation of the peak voltage with the frequency of joint motion.The self-powered motion sensor may use the peak voltage output to determine the bending angle of the elbow joint and count the peaks to monitor the motion frequency in real time.Self-powered motion sensors can also be utilized to recognize gestures.For those with language impairments, a combination of a self-powered motion sensor and a back-end data processing system based on machine learning (ML) can enable sign language recognition.Zhou et al. 34 created a stretchy sensor to recognize sign language.The flexible material is used to attach the stretchy sensor to a glove.Each finger creates an electrical signal as it moves.These signals are identified using machine learning methods, and the text is then transformed into vocal output.With a recognition accuracy of 98.63 percent and a recognition time of less than 1 s, the technique of ML processing employs a principal component analysis (PCA) algorithm for feature extraction and a support vector machine (SVM) algorithm for gesture recognition.A more sophisticated self-powered pressure/ touch sensor based on PENGs/TENGs might replace the front-end sensor, allowing impaired persons to live and interact normally.Other physiological processes of the human body, such as heartbeat, breathing, and vocal cords, may also be collected using self-powered motion sensors.The back-end data processing technology may provide real-time detection and early warning of human health without the need for external power.Voice, for example, may be identified by the vibrations of the vocal cords, which can be recorded using a biosensor applied to the throat's skin.Back-end data processing technologies can achieve voiceprint recognition and speech recognition.
Integration of wearable sensors with thermoelectric generators.-Thermoelectricgenerators (TEGs) are yet another sort of ECS Sensors Plus, 2024 3 022601 energy source that may be utilized to power wearable devices.TEGs have a long life span, emit no noise, and create the energy required to directly power wearable devices by converting heat into electrical energy.TEGs are also environmentally friendly.TEGs have been demonstrated to be useful in powering wearable sensors making use of heat generated by the human body.The human body is a fantastic portable source of energy, producing up to 58.2 W m −2 in waste heat while at rest 35 making it an excellent portable source of energy.Any amount of this waste heat may be used to power most low-energy wearable devices, eliminating the requirement for batteries as a backup energy source.TEG technology has progressed to the point where it can convert electricity more efficiently.TEGs may also be directly incorporated into wearable textiles 36 which is another advantage.In addition to monitoring glucose levels, heat-powered sensors have been employed in hearing aids and accelerometerbased rehabilitation devices and the results are shown in Fig. 3. 37 Thermoelectric energy provides certain benefits over other wearable sensor power sources, including the ability to generate heat.
TEGs provide continuous power as long as there is a temperature differential between the skin and the surrounding environment, which is typical for most practical settings. 38TEGs convert heat into electrical energy by using the Seebeck effect. 39When two distinct materials (for example, n-type and p-type semiconductors) form junctions at different temperatures, the carriers of electrons and holes will migrate to the cool ends.This phenomenon causes electric fields to be generated in both materials that are proportionate to the temperature differential. 40If there is a circuit connection, the current may flow.TEGs are created by inserting a heat sink between the n- and p-type semiconductors and the heat source.Individuals create heat as a consequence of their metabolic operations, which are used to keep their core body temperature stable.Heat escaping from the skin is transmitted to the surrounding environment by convection and radiation at a rate of 1 ∼ 10 mW cm −2 .The pace at which heat is transmitted varies depending on whatever portion of the body is being studied.Among other things, muscles operate as insulators whereas arteries are the most efficient heat transport organs in the body.When clothing is worn, heat transmission may be hindered, resulting in an average body heat transfer rate of around 5 mW cm −2 . 41As a consequence of poor conversion of body heat to electricity, electric generators that utilize body heat as an energy source may have various drawbacks.Thin TEGs with low energy consumption are required for efficient conversion.Additionally, flexible wristbands with TEG modules that record adequate accelerometer data from a user are required in certain applications. 41tegration of wearable sensors with batteries.-Batteriesare the devices used for storing and releasing electric energy.For storing energy, batteries use chemistry, in the form of chemical potential.In batteries, electricity is converted into a chemical potential form before it can be readily stored.Batteries contain two electrical terminals (cathode and anode), separated by an electrolyte.For storing and releasing energy, a battery is connected to an external circuit.Ions flow through the electrolyte, while electrons travel through the circuit.Electrons and ions can travel in either direction through the circuit and electrolyte in a rechargeable battery.When the electrons travel from the cathode to the anode, they enhance the chemical potential energy, thus charging the battery; when they move in another direction, they convert this chemical potential energy to electrical energy in the circuit, which results in discharging the battery.During charge or discharge, oppositely charged ions move through the electrolyte inside the battery, balancing the charge of the electrons moving through the external circuit and resulting in a rechargeable, long-lasting device.The battery can be unplugged from the circuit once it has been charged in order to store the chemical potential energy for later use as electricity.Wearable devices like various other portable electric technologies need batteries for their proper working.The batteries used for wearable devices must be small, thin, and exhibit a long life cycle.Some of the more common types of batteries used in wearable are: • Lithium-Ion (Li-Ion) and Lithium-Ion Polymer (LiPo, LIP, Li-poly) Alkaline batteries are not only safe to use but are easily replaceable.The AA and AAA come under the category of alkaline batteries.Button cells (sometimes known as AKA: coin cells) are another type of alkaline battery.The standard voltage of these batteries is 1.5 V, and their dimensions are 11.6 mm in diameter by 5.4 mm in height. 42When compared to lithium and silver oxide batteries, alkaline batteries have the lowest energy capacity and the least stable voltage as their voltage decreases gradually with use rather than delivering a constant and stable voltage before abruptly dropping at the end of life.A Nickel-metal-hybrid (Ni-MH) battery is a type of rechargeable battery.These batteries have nearly twice the capacity of a nickel-cadmium battery (Ni-Cd) of equal size, and their energy density approaches that of a lithium-ion battery.Nowadays, the most popular batteries for wearable are lithium-ion (Li-ion) and lithium-ion polymer (LiPo, Li-Po, LIP, Li-poly).Small Li-Polymer or Li-Coin Rechargeable cells are commonly utilized in wearable. 43,44Murat A. Yokus et al. 45 reported a wearable system with a flexible sensor array for non-invasive and continuous scrutinizing of human biochemistry as shown in the Fig. 4.
For continuous measurement of lactate, glucose, pH, and temperature, the system incorporates signal conditioning, processing, and transmission units.The system is powered by a 3.7 V, 150 mAh battery and consumes around 15 mA when actively measuring analyte concentrations and transmitting data, giving a battery life of 9.9 h for continuous measurement and wireless data transmission.anterior arm anterior (biceps brachii). 37.
ECS Sensors Plus, 2024 3 022601 Wei Gao et al. 46 presented a wearable flexible integrated sensing array (FISA) for simultaneous and selective screening of a panel of biomarkers in sweat.A single rechargeable lithium-ion polymer battery with a nominal voltage of 3.7 V with desired capacity is used to power the FISA.
Similarly, a commercially accessible thin film Li-ion battery, integrated on a flexible Kapton substrate is used to power a complete wearable sensor system for scrutinizing the concentration of lactate in sweat. 47Despite of the type of battery used, different designs have been explored to make sure that the batteries fit user requirements for keeping wearable devices safely and continuously powered. 48owever, one of the main limitations of a battery is the high toxicity of its electrolytes.Moreover, wearable sensors exhibit size constraints as they must be very compact in size.Thus, for small-size gadgets, rigid battery packs are not appropriate.Consequently, optimization of the performance of a battery inside a wearable sensor is still a topic for future work.
Integration of wearable sensors with biofuel cells.-Anew kind of energy recovery technology for wearable sensors is the usage of Biofuel Cells (BFCs).Fuel cells are electrochemical cells in which current is created by reactions happening between chemical species flowing into the cell at its anodic site and the oxidant at its cathodic site. 49Fuel cells are used to generate electricity.Because they can create continuous energy as long as the reactants are available, fuel cells vary from normal batteries in that they can store energy.Even though there are a variety of different fuel cell configurations, the proton-exchange membrane fuel cell is the most often utilized. 50uel and oxidant are kept apart by a membrane in this form of fuel cell, which allows only protons created at one end of the cell to pass it while keeping the quantity of oxidant present at the other end of the cell to a bare minimum.Electrons created at the anode are unable to flow through the membrane to reach the cathode; as a consequence, they must choose another route, which results in the production of current.The use of fuel cells to power wearable sensors provides a number of benefits over other types of power sources.The existence of reactants within the fuel cell eliminates the need to replace the batteries, which is the most significant benefit 51 as previously stated.Additionally, elderly or bedridden patients may benefit from the use of wearable sensors driven by BFCs that employ reactants that are naturally occurring in the human body, such as glucose or lactic acid.When BFCs are used to power wearable sensors, the power supply and bio-sensing may be merged to make the design more straightforward. 52Epidermal BFCs have been shown to be effective in oxidizing lactic acid in perspiration to create energy.The energy created by perspiration aids in the production of biofuels, which yield ten times more energy per unit area than any other biofuel now utilized in wearable sensors. 53Jia et al. 54 developed a BFC with bridge and island structures that were flexible, elastic, and compatible with wearable sensors, and they published their findings in science.This BFC was composed of two dotted rows that were joined together by spring-shaped components.The island and bridge constructions were created utilizing lithography 55 to create them out of gold.Efforts are also being made to increase the adaptability of biofuel cells by researchers.According to Shitanda and colleagues 56 a lactate paper-based biofuel cell is being developed that has the potential to be highly useful in wearable applications.It has the ability to produce 3.4 V open circuit voltage (OCV) with six cells in series and 4.3 mW output power with a 6 × 6 cell array, depending on the configuration.He and his team exhibited a BFC (bendable fitness bracelet) that collects sweat and uses lactate to power wearable gadgets.When used with a 20 mM lactate solution, it can yield a maximum output power of 74 watts and an output voltage of 0.39 volts. 57The biofuel cell array has shown to be quite effective in powering devices while maintaining low power consumption.In general, BFCs are extremely biocompatible and have cheap manufacturing costs, making them a good choice for medical applications.When it comes to supplying electricity for wearable sensors, they rely on biological compounds.More BFCs with great flexibility and tiny size have recently been explored, and the results are promising.BFCs, on the other hand, need more research in order to improve their PCE.Their output power, on the other hand, is highly dependent on the concentration of the analyte.
Integration of wearable sensors with hybrid energy system.-Hybridization of energy resources is the manual or automatic synchronization of two or more energy generators to provide a higher power supply and the system thus formed is called a Hybrid energy system. 58The hybridization of the energy resources increases the power conversion efficiency (PCE) of the system. 59Typically, a hybrid energy system consists of an energy harvester, a sensor module, and an energy storage system.Hybrid energy systems find application in wearable sensors as they can provide a high power supply and possess flexibility.Moreover, these energy systems help in achieving sustainable energy goals.The hybrid energy systems that have been developed so far are thermoelectric and solar energy sources, TENGs and solar energy sources, electromagnetic and thermal energy sources, and TENGs-PENGs sources.
In solar cells, the photons outside the band gap are converted into waste heat.Thus, combining solar cells with thermoelectric energy sources (TEGs) will help in the efficient conversion of these wasted photons and increase the total output power. 60Theoretically, such a hybrid system can attain an efficiency of 23% (higher than both PV and TEGs), due to additional energy supplied by TEGs.Recently, a PV-TEG-based bracelet was developed that can harness solar energy and heat from the human body. 61nother hybrid energy system used in wearable sensor applications is solar cells and TENGs.The system can efficiently harness both solar and mechanical energy.The solar energy is converted via dye-sensitized solar cells (fibre) and the body motion is converted into electrical signals using fibre-TENGs.In such systems, the electrical signals can further be converted into chemical energy using supercapacitors.
The hybrid energy system based on electromagnetic energy sources (ESs) and thermoelectric energy generators (TEGs) are also used for power supply in wearable sensors.In such systems, the electromagnetic energy as well as the heat produced is used to provide power supply to the wearable sensors.
The TENG-PENG hybrid system can harvest energy from human motion or the ambient environment.It consists of a TENG to harness energy from the environment (i.e., solar energy, wind energy, etc) and a PENG to convert mechanical energy from body motion into electrical signals.Such hybrid systems have been used to supply power to digital watches and mobile phones. 62,63In general, hybrid generators have fewer applications than single-source generators because they are generally less flexible; however, they are able to offset the limitations caused by harvesting energy from a single source.To realize high compatibility with various applications, a good hybrid energy harvester should be of high flexibility.

Comparison of Various Energy Sources
Table I analyzes the many types of energy sources that may be used to power wearable sensors along with their advantages and disadvantages.It is possible to employ these energy sources alone or in combination; their operation and performance make them appropriate for a wide range of applications; and they may be used to power a wide range of sensing devices.It is dependent on several elements, including the ambient environment, the continuous sensing and sensing frequency requirements, the target analyte being detected, the cost, and human behavior, among others, to determine the appropriateness of a certain energy source for a given application.There is a good possibility that a technology to power a specialized wearable sensor to meet the performance requirements of any particular application already exists.
A wearable hybrid self-charging power textile with sustainable energy collecting and storage was developed by Sheng et al. 64 The power textile is composed of two components: a novel coaxial fibershaped supercapacitor (fiber-SC) made by functionalized loading of a wet-spinning graphene oxide fibre as an energy storage unit, and a coaxial fiber-shaped polylactic acid/reduced graphene oxide/polypyrrole (PLA-rGO-PPy) triboelectric nanogenerator (fiber-TENG) that can harvest low-frequency and irregular energy during human motion as a power generation unit.The fiber-TENG is wearable, flexible, knittable, and versatile enough to be integrated with a range of portable devices.The coaxial fiber-SC has strong cycle stability and a high volumetric energy density.A completely stretchy TENG made of an inherently stretchable MXene/silicone elastomer and a silver nanowires (Ag NWs)-graphene foam nanocomposite was described by Yang et al. 65 The intrinsically stretchable TENG demonstrates steady electrical output across a range of severe deformation circumstances, long-term dependability, and high output performance (73.6 V, 7.75 μA, and 2.76 W m −2 ).Besides being used on human skin and clothing to monitor human motion and identify strength training postures, the naturally extensible TENG can also capture the sporadic mechanical energy from human bodies to power different energy storage devices, like commercial capacitors that power wearable electronics.Jiang et al. 66 reported triboelectric nanogenerator which is self-cleaning, ultraviolet (UV)protective, self-powered and antibacterial, for self-powered sensing and mechanical energy harvesting.The fabrication of nanogenerator is done with Ag nanowires through a facile method of electrospinning.An extensive literature review was provided by Sharma et al. 67 on solar cell efficiency, DC-DC power converters, microcontrollers, energy storage (battery/supercapacitor), and different design costs for wireless sensor networks that harvest solar energy.The use of forearm-mounted, commercially available flexible photovoltaic panels was the subject of research by Kim et al. 68 in their study of wearable sleeve devices and power converter systems.An examination of the effects of curvature reveals that, while a forearm-curving panel does lower output power, the angle with respect to the light source significantly impacts voltage characteristics and output power.The many classes of fibre batteries, such as zinc, lithium, and other forms of fibre batteries, have been investigated by Xiao et al. 69 Challenges.-An ideal wearable sensor or device should require little or no user intervention.1][72] However, a digital or analog wearable sensor's power consumption should be minimum, to achieve sustainable development goals.The wearable sensor driven by electromagnetic energy is cheap to produce, possesses high power generation capacity, and generates a large output current.However, the system possesses many limitations, i.e., the device can only work up to a specific range from the antenna, and the range depends upon the height of the antenna (10 times the height of the antenna), to attain operation at more considerable distances the system requires significant power consumption which can be attained by using additional supply from batteries and solar power.Moreover, the flexibility and miniaturization of electromagnetic energy-driven wearable sensors are very difficult.Thus, wearable sensors driven by electromagnetic energy are confined to applications requiring high power consumption and large sizes such as backpacks and bags.To overcome this challenge, wireless power supply can be explored using supercapacitors and fuel cells [73][74][75] this would connect many circuits, but again the connectivity in harsh environment conditions is an issue, and such devices would cease to function in low connectivity areas.Moreover, the fuel cells require continuous fuel supply which ultimately reduces the life cycle of the wearable sensor.In addition to this, the start-up time for a fuel cell is ECS Sensors Plus, 2024 3 022601 ECS Sensors Plus, 2024 3 022601 very large which brings challenges in collecting the real-time data from the wearable device.Also, the development of supercapacitors is in its early stages, and the research area has its challenges and problems.
The wearable sensors based on solar cells depend upon solar energy for their power supply.One of the critical challenges for such sensors is to perform on a rainy day or in bad light conditions.The device fails to work continuously in such conditions, which reduces its reliability and stability.Hence, such sensors are not ideal for military wearable applications.Furthermore, the piezoelectric wearable sensors still require an external power supply for performing their applications.These challenges need to be addressed so that the full potential applications of wearable sensors can be achieved in the future.New materials with high efficiency, flexibility, and stability need to be explored.The research idea would be the development of a self-energy harvesting wearable device that can operate in harsh conditions without compromising the stability and reliability of the device.In spite of all the above-mentioned discussions, some of the significant challenges that need to be addressed are as follows: Limited power supply.-Wearablesensors are often limited in terms of available power supply due to their small form factor and the need for portability.This limitation makes it challenging to provide a continuous and reliable power source for these devices.
Power efficiency.-Wearable sensors should be designed to operate efficiently to maximize battery life.Power optimization techniques such as low-power electronics, energy harvesting, and intelligent power management algorithms need to be implemented to minimize power consumption and extend the device's operational time.
Energy harvesting.-To overcome the limited power supply challenge, wearable sensors can utilize energy harvesting techniques.Energy can be harvested from various sources such as solar power, thermal gradients, kinetic energy, or ambient radiofrequency signals.However, efficiently converting and storing harvested energy remains a challenge.
Small form factor.-Wearable sensors are typically small in size and have limited physical space to accommodate bulky batteries or power storage devices.This constraint necessitates the use of miniaturized and lightweight power sources with high energy density, such as thin-film batteries or flexible supercapacitors.
Sensor integration.-Wearablesensors often comprise multiple sensors, each with different power requirements.Integrating these sensors into a compact form factor while managing their power needs can be complex.Power management circuits and intelligent algorithms are necessary to handle the varying power demands of different sensors effectively.
User comfort.-Wearablesensors should be comfortable to wear for prolonged periods, and adding bulky or heavy power sources can hinder user experience.Balancing power requirements with device weight and size is crucial to ensure user comfort and acceptance.
Data processing and transmission.-Wearablesensors generate a significant amount of data that needs to be processed and transmitted wirelessly.These operations consume power and can drain the battery quickly.Efficient data compression, onboard data processing, and optimized wireless communication protocols are essential to minimize power consumption during data handling.
Real-time monitoring.-Somewearable sensors require real-time monitoring capabilities, such as continuous heart rate monitoring or motion tracking.Achieving real-time monitoring while maintaining power efficiency is a significant challenge, as it often requires continuous sensor operation and data processing.
Addressing these challenges requires a multidisciplinary approach, involving advancements in battery technology, power management techniques, energy harvesting methods, miniaturization, and system-level optimization.Continued research and development in these areas will contribute to the successful development of energy-efficient and reliable wearable sensor systems.
The development of wearable sensors encompasses combined research efforts on various aspects such as materials, fabrication processes, and electrochemical analyses.During the last 50 years, extensive progress has been made in the field of wearable sensor development to enhance the capabilities and improve the reliability of measuring analytes in the body.For instance, as we witness the continuously growing number of diabetic patients, the demand rises for an efficient system of glucose monitoring as a part of personalized treatment and health management. 81Since diabetes is a chronic disease with severe to fatal complications but no direct cure so far, a closely monitored glucose level in the body in real-time is essential for diabetic patients.Although the development of personal blood glucometers has brought significant changes in healthcare for diabetes, there remain several limitations.As a point-of-care method, it is difficult to monitor glucose levels continuously, and abrupt but significant changes in blood glucose levels are often neglected. 82Above all, patients usually do not comply well with conventional treatments because of the pain and discomfort associated with the invasive monitoring process.New materials, concepts, and engineering techniques evolve toward the development of non-invasive and patient-friendly monitoring systems of blood glucose. 83While the latest improvements of invasive monitoring systems feature a minimal amount of blood specimen for a personal glucometer and continuous and accurate monitoring via an implantable-type device, invasive methods are still not free from physical pain and inconvenience associated with the protocols.As a new generation of glucose sensing, the non-invasive system can satisfy both patient-friendly paradigms and the reliability of glucose detection.The new studies on improving the novel non-invasive monitoring systems are likely to ensue with growing popularity.As, a number of studies confirmed the potential application of many alternative biofluids such as tears, saliva, (interstitial fluid) ISF, and sweat as viable analytes for glucose monitoring, the incorporation of painless monitoring devices and patient-friendly protocols will strengthen the feasibility of non-invasive systems readily adopted in commercial and medical applications.Indeed, the painless methods are still less accurate compared to direct blood glucose monitoring and are yet to establish a more reliable protocol against physiological variance among individuals.Therefore, the sensors based on non-invasive methods should be developed in such a way that their results will correlate with the standard results of the invasive methods. 84Particularly, the development of non-invasive sensors still has room for improvement in terms of immobilizing enzymes, increasing the sensitivity, and reinforcing the long-term stability of the sensors.Along with the technological aspects in the individual device level pending further research, the convenient design of the non-invasive glucose monitoring system and protocol associated with the acquisition of biofluid specimens may also need to be refined to promote patient-friendliness. Consequently, continued efforts and novel approaches to establish a reinforced biofluid-based wearable sensor whose reliability is comparable to that of a blood-based system hold great promise for convenient blood glucose monitoring in daily routine and represent a novel paradigm for diabetic healthcare in the future.Table II shows the performance comparison table among different energy and power harvesters.

Future Scope
The pace at which the field of wearable devices is budding is incredible.Wearable devices cover a wide range of commercial applications, and the future holds various promising opportunities in the form of start-ups.According to an estimate (figure 5), the field of ECS Sensors Plus, 2024 3 022601 wearable devices will be dominated by electrical and optical sensors by 2020 and beyond. 76Wearable devices and sensors have various fields of application, such as medical sciences, military defense, consumer electronics, industries, and infrastructure damage control. 59In the medical field, wearable sensors can be used for medical diagnosis and testing, including retina treatment and care, manufacturing hearing aids, monitoring the human body using waistbands and smartwatches, etc.One of the main areas of development and profit for wearable sensors is consumer electronics.This includes smart bands, smart watches, charging devices, smartphones, backpacks, jackets, etc.In addition, researchers are developing wearable textiles that can harvest solar energy and charge mobile phones and accessories.In defense, the military can be equipped with self-charging wearable devices and clothing that can significantly benefit operations in harsh environmental conditions.Industries use RFID tags and wireless wearable sensors to detect damages and defects in infrastructures like bridges, buildings, railways, etc. 59 The RFID chips are also being considered for monitoring purposes to counter national and international security breaches.However, the critical challenge is the continuous supply of energy to the wearable sensor, limiting its efficiency and life cycle.Scientists consider energy harvesting as a perfect solution to counter the problem.The energy can be harvested from the human body in the form of heat and motion or from the environment. 77Companies are already working on making wearable devices and sensors selfreliable in energy consumption.Moreover, the inclusion of machine learning and low-energy processors helps innovate and add pace to the development of wearable sensors.Recently, wearable sensors experienced a tremendous boom in the sports and fashion industry.The athletes monitor their progress while wearing such wearable sensors and improve accordingly.The fitness band has gained exceptional market value, and nowadays, these are commonly found in every household.The work is already in progress making these fitness bands more accurate and reliable.The optimization of sensor design can prove very helpful in extending the lifetime of wearable devices.3D and 4D printing can be used while designing wearable devices, as the technologies can print complex structures in one go and offer flexibility and stability to the structure.However, there are still challenges to overcome to make energy harvesting a mature solution for wearable devices, and researchers are investigating and finding novel solutions to power wearable devices and make them self-sustainable.
5 G and internet of things.-Theera of big data-driven product design has grown exponentially because of the emergence of 5 G and the Internet of Things (IoT).In addition, recent advancements in computer power and software frameworks have accelerated the emergence and evolution of artificial intelligence (AI). 78In this context, the digital twin, which integrates the physical and virtual worlds and analyses different sensor data with the aid of AI algorithms, has emerged as a cutting-edge technology, where the various sensors are highly desired for collecting environmental data.However, despite the fact that current sensor technologies, such as wearable sensors, microphones, inertial measurement units, etc, are often utilized as sensing components for a variety of applications, ECS Sensors Plus, 2024 3 022601 excessive power consumption and battery replacement remain issues.As self-powered sensors, triboelectric nanogenerators (TENGs) offer a workable framework for developing self-sustaining and low-power devices.This section briefly discusses the application of artificial intelligence to the design of intelligent sensor systems for the 5 G and IoT future. 79Intelligent systems are developing rapidly as a result of recent advances in computer power, artificial intelligence technologies, and software design.This makes it feasible to more effectively integrate virtual and physical items, and it will also make it easier to apply digital twins to different manufacturing and industrial processes.Different sensors are becoming increasingly and more crucial in digital twin applications.The development of low-power, self-sustaining systems is now possible thanks to the TENG-based intelligent system, which will work better with the digital twin.Additionally, sensor fusion with multimodality data is essential for the functional development of intelligent systems.More advanced sensors might be developed for intelligent systems with the further advancement of AI technologies, ranging from wearable sensors like gloves, socks, touchpads, exoskeletons, and electronic skin to the broad applications of healthcare, robot perception, smart homes, etc. Future healthcare applications will need to be very dynamic and time-sensitive, which presents a challenge for today's communication systems.In order to address the varied communication requirements of medical treatment applications related to the Internet of Things (IoT), 5 G networks are being developed and evolved.IoT devices that give rise to 5 G-aided smart healthcare networks need better cellular coverage and network performance.Smart healthcare and IoT applications would greatly benefit from 5 G networks.From an operational and financial perspective, smart healthcare and IoT applications are essential when it comes to the 5 G network. 80

Conclusions
Wearable sensing technology has quickly transformed from a science-fiction vision to a real-life technology in the past few decades.It is no longer associated with garments only.Wearable devices may be set up for customized mobile information processing for specialized uses such as immersive gaming, fitness tracking, public safety alerts, or entertainment and healthcare.There are several applications for wearable devices with green and sustainable power sources, including healthcare, soft robotics, artificial intelligence, and the Internet of Things.Wearable chemical and biochemical sensors have the potential to give new sources of non-invasive physiological information from a range of bodily fluids, such as sweat, tears, saliva, and interstitial fluids.This information could be used to improve the health of a person by allowing for early disease detection, which could lead to the deployment of early interventions.Figure 3 shows the available data and forecast of wearable sensors market size from 2023 to 2033 (USD Million).The key challenge to this technology is the power sourcing of the wearable device.Batteries, solar cells, nanogenerators, biofuel cells, and thermoelectric generators are the most commonly used power-sourcing options for wearable devices.However, the technology faces various challenges in terms of making power-sourcing options more sustainable and effective.In addition, the choice of material, device fabrication technique, electrode preparation, etc also add to the problems associated with wearable sensor technology.However, the future holds great opportunities for the technology as the IOT and 5 G are taking the world by storm and wearable sensor technology will greatly benefit from them.It will help in the quick transfer of data, enhancing the accuracy and efficiency of devices, and reducing their cost and size.The use of energy-harvesting power sources in conjunction with wearable systems should result in the best possible situation for an autonomous wearable system.The idea of a world without wearable technology is, indeed, unimaginable.A transdisciplinary approach will propel us ahead at breakneck speed on this exhilarating adventure towards the Holy Grail of wearables, while also allowing us to "do well by doing well."ORCID Sandeep Arya https://orcid.org/0000-0001-5059-0609Ajit Khosla https://orcid.org/0000-0002-2803-8532Vinay Gupta https://orcid.org/0000-0003-3223-7267

Figure 1 .
Figure 1.(a) The present state of the mobile biosignal monitoring and (b) its projected future state.(Reproduced from 9 with permission.Copyright 2014, Elsevier).

Figure 2 .
Figure 2. (a) WP-TENG was inserted on the skin above the arm muscles, (b) peak voltage variation with the bending angle, (c) variation of the peak voltage with the frequency of joint motion.(Reproduced from 33 under Creative Commons Attribution CC BY License.Copyright 2021, Beilstein-Institut Zur Forderung der Chemischen Wissenschaften).

Figure 4 .
Figure 4. (a) Polyimide (bottom), metallization (middle), and encapsulation (top) layers are visible in an exploded view of the flexible sensor array, (b) awearable multiplexing system that can be worn on the wrist inside a watch enclosure that was 3D printed and (c) Image of the flexible sensor array.45 .

Table I .
Comparison in wearable sensor power supply characteristics.

Table II .
Performance comparison among different Wearable energy harvesters.