Wearable Printed Temperature Sensors: Short Review on Latest Advances for Biomedical Applications

152 IEEE REVIEWS IN BIOMEDICAL ENGINEERING, VOL. 16, 2023

Wearable Printed Temperature Sensors: Short Review on Latest Advances for

Biomedical Applications Saleem Khan , Shaukat Ali , Arshad Khan

, and Amine Bermak , Senior Member, IEEE (Methodological Review)

Abstract—The rapid growth in wearable biosensing de- vices is driven by the strong desire to monitor the human health data and to predict the symptoms of chronic dis- eases at an early stage. Different sensors are developed for continuous monitoring of various biomarkers through wearable and implantable sensing patches. Temperature sensor has proved to be an important physiological pa- rameter amongst the various wearable biosensing patches.

This paper highlights the recent progresses made in print- ing of functional nanomaterials for developing wearable temperature sensors on polymeric substrates. A special focus is given to the advanced functional nanomaterials as well as their deposition through printing technologies.

The geometric resolutions, shape, physical and electrical characteristics as well as sensing properties using different materials are compared and summarized. Wearability is the main concern of these newly developed sensors, which is summarized by discussing representative examples. Fi- nally, the challenges concerning the stability, repeatability, reliability, sensitivity, linearity, ageing, and large-scale man- ufacturing are discussed with future outlook of the wear- able systems.

Index Terms—Biodegradable, nanomaterials, printing, temperature sensors, wearable Electronics.

I. INTRODUCTION W EARABLE sensors and transducers on polymeric foils have attracted significant attention and are growing rapidly with the fast advent in miniaturized thin film electron- ics and the need for medical practitioner to obtain real time data from their patients [1]–[5]. The unique properties such as mechanical flexibility, lightweight and in some cases biocom- patibility and biodegradability make them distinguished from silicon-based devices [6]–[9]. The conformal integration of thin polymeric substrates to nonplanar surfaces without significant

Manuscript received 5 April 2021; revised 12 September 2021; ac- cepted 16 October 2021. Date of publication 20 October 2021; date of current version 6 January 2023. This work was supported by NPRP from Qatar National Research Fund (a member of Qatar Foundation) under Grants PRP10-0201-170315 and NPRP11S-0110-180246, and by Qatar National Library. (Corresponding author: Saleem Khan.)

The authors are with the College of Science and Engineering, Hamad

Bin Khalifa University, Qatar Foundation, Doha 5825, Qatar (e-mail: sakhan3@hbku.edu.qa; shaali@hbku.edu.qa; arkhan4@hbku.edu.qa; abermak@hbku.edu.qa).

Digital Object Identifier 10.1109/RBME.2021.3121480 degradation in the sensor signals make them ideal for wearable biosensing related applications [4], [10], [11]. The lower cost of polymeric substrates as well as the functional nanomateri- als and their manufacturing through printing technologies en- hance the attractiveness for cost-effective portable and wearable sensing applications [12]–[16]. Printing is preferred on foils due to their incompatibility with higher temperatures in clean room processes. Printing enables position-specific deposition of functional materials that significantly reduces the materi- als wastage[14]. A number of interesting sensing devices and systems have been reported and the number still rising with a particular focus on wearable biosensing devices [9], [10], [17]–[19]. Among the list of various biosensors developed, temperature sensors are widely reported as standalone or part of a sensing patch that can be placed on human skin [19]–[24].

Temperature is a main vital physiological sign that is used to determine the heat exchanges occurring between the epidermal tissues and external environment. For human body, the tempera- ture monitoring on human skin is broadly explored as compared to the deep body temperature sensing [23], [25]–[27]. Human body suffers from the thermal stresses upon experiencing to a range of diseases, for instance normal fever to more chronic diseases such as cardiovascular, diabetic, pulmonological diag- nostics, cancer, and other syndromes etc. [28], [29]. The rise in skin as well as deep body temperatures are considered as preliminary indicator for a seriously developing disease, and continuous monitoring becomes essential in most of the cases [27], [28], [30], [31]. Therefore, wearable biosensing patches developed on biocompatible polymeric substrates are crucial for real-time monitoring of biomarkers that can send data wirelessly to the hospital facilities for expert observation and opinion of the physicians remotely [32]–[36]. Fig. 1 shows a broader overview of the wearable biosensing scenarios and potential biomarkers used for real time health monitoring.

Selection of the substrates is critical to the wearable tempera- ture sensors besides the materials properties, as the deployment onnonplanarsurfacessuchashumanskinrequiresgoodsensitiv- ity and accuracy at different orientations such as setting, moving, stretching, and bending etc. [37]–[39]. This can be achieved through various strategies by placing the sensors on places which are not subjected to physiological movements or make a good selection of all the materials for compliant integration to human

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KHAN et al.: WEARABLE PRINTED TEMPERATURE SENSORS: SHORT REVIEW ON LATEST ADVANCES

153 Fig. 1.

Overview of wearable electronics enablers and perpsective application areas of temperature sensing. Proof of concept devices are attached to human skin. [20], [13], [17], [69], [140], [145] [reproduced with permission]. skin, that can easily mitigate the unwanted noise signals [40]– [44]. Besides the good sensitivity and accuracy, the temperature sensorsaredesiredtohavegoodrepeatability,sensitivetominute changes as the temperature changing window is very small (i.e.,

25–40 °C), and lastly the robustness with enhanced stability against varying climatic conditions [45]–[48]. The stretchable interconnects play significant role and are therefore considered as the main complementary component for developing wearable biosensing patches, as the deformable interconnects can absorb the stresses with negligible effects on the central sensing devices [49]–[51]. Therefore, a good combination of soft lightweight substrates, advanced functional nanomaterials making soft, thin sensing films and stretchable interconnects [52], and most im- portantly a robust fabrication technique for processing these low Tg (glass transition temperature) materials at ambient conditions, are desired for development of reliable wearable sensing patches [9], [16], [53]–[55]. This paper highlights the latest developments made in geometric designs of the sensory cells, engineered nanomaterials (both inorganic and organic) and the fabrication methods particularly the solution-based printing technologies.

II. WEARABLE TEMPERATURE SENSORS: DESIGNS AND STATE OF THE ART

Temperature sensing is one of the central physiological parameters used to determine the human body temperature with particular emphasis on patients suffering from prolonged chronic diseases, normal fewer, unconscious, and injured pa- tients undergoing a surgical treatment and for the health status of the medical staff [56]–[58]. Wearable temperature sensing finds its attractions not only in the medical related field, but in general it is also useful for monitoring and tracing the body tem- perature of healthy people doing extensive outdoor activity [59], [60]. Besides the heavy workout for fitness activities of athletes and sportsmen, wearable temperature sensor is very useful for laborers working in very harsh environmental conditions. The rising levels of climatic conditions particularly the temperature and humidity, lead to dehydrating the individuals and causes fatigue and many other serious implications towards their health.

Therefore, developing wearable temperature sensors not only to monitor the human health, but observing the local ambient environment is equally important. Fig. 2 shows schematic of the wearable sensing models employing temperature sensor accompanied with the wireless data transmission models.

A. Temperature Sensors Conventionallytemperaturesensorsaredevelopedindifferent geometric structures depending on the application, materials processability as well as the manufacturability in the desired shape [23], [61]–[63]. These various structures are distinguished by the temperature sensing mechanism through certain changes and upon physical interaction with the hot surfaces. The two broad categories of temperature sensors are based on contact andnon-contact-basedsensingmechanism.Contacttypesensors are used to monitor a wide range of surfaces including solids, liquids and gaseous phases. However non-contact-based sensor can sense remotely the thermal irradiations emitted from the hot surfaces. The prominent temperature sensors include ther- mostats, thermistors, resistive temperature detectors (RTDs), thermocouples, negative temperature co-efficient thermistors (NTC) and silicon-based sensors etc. Details of the working principle of these sensors are provided in [64], [65]. Among these, the bulk resistance changes of a RTD (resistance tem- perature detector), thermally sensitive resistors (thermistors), the commonly used mercury-based thermometers, optical and handheld infrared monitoring sensors etc. are few of the com- monly used types of temperature sensors. Each type of the sensor proposes advantages over other, however certain limitations for the wearable related applications restrict the use of specific type of sensors to be adopted. RTDs and thermistors are usually adopted in many applications in general due to their reliable and fast response, structural stability, good accuracy and lastly their ease in manufacturing make them ideal for batch produc- tion at much depreciated costs [66]–[68]. The thermal IR and optical sensors on the other hand are commonly used in the indoor medical facilities, but these are developed on wafer-based substrates and the signal conditioning circuits too embedded on rigid PCBs (printed circuit boards) making it challenging for lightweight, portable, and wearable related applications. The conformal attachment of the sensors to human skin directly needs all the contributing materials to be flexible or stretchable enough to absorb the stresses caused by the deformations due to human physical activity [23], [67], [69]. For this particular case, thelargeareacoverageofthesensorsplayssignificantrolewhich is enabled specifically by the RTD and thermistor geometric approaches. Fig. 3. shows designs of the proposed geometric structures ideal for such situations. Key performance factors de- sired for the wearable sensors are sensitivity, accuracy, detection at lower temperature ranges, reliability, and repeatability upon

154 IEEE REVIEWS IN BIOMEDICAL ENGINEERING, VOL. 16, 2023

Fig. 2.

Big picture of a wireless monitoring system and wireless data transmission model. Biosensors embedded in a wearable gadget with installed signal processing board and wireless communication. Model also shows proposed schematic for real-time monitoring, where the recorded data is transmissted to the control-station via bluetooth and data sent to the cloud, which is accesible remotly to the medical experts.

Fig. 3.

Schematic of teh printed temperature sensors, (a) Schematic of a RTD, (b) Inkjet printed RTD placed on a human skin, (c) Schematic of a thermistor, (d) Printed thermistor with Ag IDEs and temperature sensing layer of carbon nanocomposite. wearing these sensors on non-planar surfaces. A linear response in the sensitivity leads to increased accuracy, which is highly demanding especially for human body temperature sensing. The sensing ranges of the temperature is a key parameter, and selec- tion of suitable material which is very sensitive to the minute changes occurring in the human body [61], [70]. Response time of the sensor is another key parameter, which plays significant role in detection of the thermal variations. Early detection of the symptoms helps in timely diagnosis and treatment. Real-time monitoring is dependent on various important parameters such as response time, stability, and hysteresis etc. of the sensor.

Resolution refers to the measurement of the smallest amount of changes measured by the sensor and therefore, becomes an important parameter, when it comes to wearable tempera- ture sensors. Lastly, for wearability, the sensors require to be developed on biocompatible substrates, having lightweight and beconformableontonon-planar surfaces without significant loss in the sensor response [61], [70], [71].

III. TEMPERATURE SENSING MATERIALS Materials’ selection is the crucial and main enabling factor in developing wearable biosensing applications. The materials used for developing wearable biosensors are desired to have matching properties both with the underlying substrate as well as other constituent materials making the sensor structure. Gen- erally, the materials are required to be biocompatible, portable, lightweight, soft, flexible/stretchable as well as conforming to the non-planar surface without significant degradation both physically as well as in the sensing response [28], [34], [72]. As the wearable sensors are attached directly on the human skin, the sensors are desired not to pose any health or medical risk.

In this scenario, biocompatibility is highly desired for wearable and implantable sensing devices. This includes biocompatibility of the substrate as well the deployed materials for construction of the sensing device. Various biosensors such as multi-function brain sensor, implantable pressure-strain sensor etc. have been reported which are developed completely on biocompatible sub- strates as well as utilized the similar sensing materials [73], [74].

These fabrication methods however have distinguished benefits as well as critical limitations. Among these, the temperature sensors developed through printing technologies require certain properties which are distinctive for each fabrication type. Con- ventionally the materials used for printing are in colloidal or nanocomposite form, where the rheological properties of the solutionistunedspecifictotheprintingtechnology.Forinstance, solution viscosity, surface tension, work of adhesion, spreading co-efficient and nanoparticles concentration in the solution are some of the main characteristics considered, when choosing a printing technique. Printed wearable temperature sensors are

KHAN et al.: WEARABLE PRINTED TEMPERATURE SENSORS: SHORT REVIEW ON LATEST ADVANCES

155 developed either in the form of RTD or thermistors, therefore intrinsic conducting materials as well temperature sensing con- ductor/semiconductor are used in construction of the sensor structure. A wide variety of materials are developed with the desired solution properties for different printing technologies.

This section highlights few of the broad categories of printing materials used specifically for developing wearable temperature sensors utilizing printing technologies.

A. Metallic Conductors The change in electrical resistance of metallic conductors with the corresponding temperature variations is linked to the intrinsic thermal co-efficient resistance (TCR) of the material [46], [66]. These resistance change caused by the phononic interactions are central to the utilization of these materials for temperature sensors. Higher TCR values (either positive or negative) are the key selection criterion for using these metal- lic conductors for a specific range of temperature variations.

Pure metals such as Gold (Au), Copper (Cu), Platinum (Pt),

Nickel (Ni), Aluminum (Al) and silver (Ag) are commonly used for the temperature sensing applications [61], [62], [75].

The structures are developed in the form of wires, thin solid films as well as in the liquid form contained in a closed flow channel. The advanced manufacturing techniques especially the thin film deposition at high resolution patterning of few of these metallic conductors have attracted significant interest. The colloidal solutions made from the nanoparticles suspensions and processed through controlled deposition techniques such as additive manufacturing play a major role in reducing the material wastage and thus contribute a lot in minimizing the overall manufacturing cost [76], [77]. The colloids are made by mixing metallic nanoparticles in suitable solvents exhibiting properties in between a solution and suspension. The mixture formed is a mostly heterogenous having dispersive properties at tunable scattering phases depending on the nanoparticles’ sizes [77], [78]. The average nanoparticle sizes as well as the aspect ratios are kept in close ranges to facilitate uniform dispersion.

Further additives in the form of surfactants are added for tuning the rheological properties such as viscosity and surface tension etc. as well as enhancing the solution stability and prolonged shelf-life [79]–[81]. These metallic conductors are patterned as independent transducer layers such as RTD as well as constituent layersofthinfilmdevices.Nickel(Ni)basedtemperaturesensors have widely been reported due to its attractive features such as higher sensitivity, linearity, high reference point of resistance, wide sensing range (0-100 °C) and above all its availability in the market at lower prices compared to other metallic conductors [28], [68], [82]–[84]. Some of these metallic conductors are prone to oxidation particularly Cu and Ag, therefore multilayer coatingintheformofananocompositeisfabricated.Highlyther- mal sensitive material i.e., Ni is coated with a more stable layer of Pt as protective covering [82]–[85]. Most of these depositions however occur at very controlled conditions by using clean room processes. To simplify the manufacturing process and accessible for exploration in ambient conditions, deposition of colloids of these metallic nanoparticles through wet processing technology is more attractive. Au nanoparticles solution has been explored extensively and stabilized inks are developed at commercial- ized grade by several groups. Au is more stable, having linear response, resilient to environmental impacts, and above all it is biocompatible [28], [68], [86]. Currently, Au based patterning is practiced on thermally stabilized polyimide substrates due to its relatively higher sintering temperatures i.e., 250 °C [62], [86]. This challenge needs to be addressed and ideal sintering temperature of within 150 °C would enable the Au patterning on a wide range of polymeric substrates.

B. Inorganic/Organic Conducting Materials Organic conductors are the polymeric materials that produce electrical conducting properties similar to metallic or inorganic semiconductors [13], [87]–[89]. The chemical structure of these intrinsic conducting polymers can be tuned in controlled clean room environments to get the desired electrical, chemical, and mechanical properties [89]. Initial work on the p-doped or- ganic conductor was reported which led to the development of many similar types afterwards. Few of the commonly used polymeric conductors include but not limited to, are polyacety- lene, polypyrrol, polyphenylene, poly (p-phenylene vinylene), polythiophene polyaniline, polyaniline doped with camphor sul- fonic acid and PEDOT:PSS (3, 4-polyethylenedioxythiopene- polystyrene sulfonic acid), graphene etc. [90]–[92]. Among the list, PEDOT-PSS and graphene have attracted significant interest, due to their abundant availability, easy processing, good electrical, chemical, and mechanical properties [87], [93], [94].

The easy manufacturing and processing techniques especially the wet fabrication routes have contributed significantly to the extensive research on these materials. Many exciting appli- cations have been explored and are particularly explored for temperature related applications. The microstructure of poly- meric materials especially the PEDOT-PSS is responsible for the thermal sensing properties [95]. The core-shell structure is formed by the EPDOT-PSS nanocrystal, where the PEDOT remains at the core of the grain and PSS surrounds the core.

The bulk resistivity of PEDOT-PSS layer is mainly influenced by the insulating PSS part of the material. Effective bound- ary size reduces as a result of smaller number of particles boundaries at higher temperatures, this influences the overall electrical resistance. the electrons do not have sufficient thermal energy to surpass these boundaries at lower temperatures, and therefore the electrical resistance increases [96]. The strong mechanical properties of PEDOT-PSS also make it an ideal candidate for wearable related applications, as it retains the strong adhesion with the substrates upon bending at very low deflection angles. Similarly, the carbon-based nanomaterials and solutions are also explored extensively for temperature related sensing applications. Carbon nanotubes in its pristine form as well in a nanocomposite have been deployed as thermistors on polymeric substrates. Graphene in the carbonaceous family has outperformed many metals and CNTs in terms of electri- cal and thermal properties and is therefore adopted for many thermal management and energy storage related applications.

The monolayers of graphene sheets and very strong physical

156 IEEE REVIEWS IN BIOMEDICAL ENGINEERING, VOL. 16, 2023 structure at nanoscale produce excellent thermal properties. The strong linear relationship between electrical conductance and temperature makes graphene a promising competitor compared to the commonly used metallic conductors as RTDs. Deposi- tion of monolayer graphene sheets are mostly done in clean room processes through CVD (chemical vapor deposition) on electronic grade silicon wafers. To deploy on large areas of polymeric substrates and make the deposition process simple and cost-effective, recued graphene oxide in solution form has been reported widely.

C. Organic-inorganic Hybrid and Nanocomposite Materials

Hybrid organic-inorganic materials is an interesting filed of engineered materials and the exciting properties of both the material complement each other to produce extra-ordinary prop- erties. For instance, the electrical conductivity offered by inor- ganic metallic conductors, when mixed by organic elastomers produce advanced stretchable and flexible composites that are ideal for wearable sensing applications. Beyond the electrical properties, these hybrid materials also exhibit enhanced physical characteristics including good optical, luminescence, chemical inertness, selectivity in chemical as well as biochemical sensing environments. A wide variety of sensing applications have been developed utilizing these hybrid materials covering almost all the fields of physical, chemical, electrical and electrochemical sensors. A wide variety of organic-inorganic hybrid materials, their synthesis and applications are reviewed extensively [97], [98]. The hybrid organic-inorganic materials are broadly divided into two categories depending on the bonding strengths. Weak

Vander Waals, electrostatic or hydrogel bonding occur in one type, whereas in the second type, covalent or ionic bonds are formed between the molecules. A number of synthesis tech- niques are adopted, among which sol-gel, hydrothermal and solvothermal processes are commonly used. Hybrid nanocom- posites are tailored materials synthesized with tuned physical, electrical and mechanical properties ideally required for a spe- cific application [87], [89], [99], [100]. Metallic nanoparticles either in the powdered form or mixed in a solvent are dispersed in polymeric matrix, that develops into a uniform and stable mix- ture. Nanoparticles within the matrix form conducting networks within the bulk of nanocomposite. For the metallic conductors, materials are deposited as nanocomposite thin layer successively to enhance adhesion, prevent the underlying materials from oxidation and other environmental impacts [101], [102]. To make nanocomposites deployable on larger areas using additive manufacturingtechnologies,nanoparticlesofthesemetalliccon- ductors are mixed in polymeric matrix materials in various ratios targeting the desired application [13], [21], [103]. A uniform dis- persion of conducting fillers is highly desired to achieve repeat- able processing and testing results. Different mixing techniques such as mechanical stirring and ball milling are used for uniform dispersion followed by functionalization of the nanofillers in some cases [104], [105]. The mixing ratios of fillers and matrix materials are determined based on the percolation threshold, where a certain level of electrical conduction is achieved by reaching the optimal combination percentages. These conduct- ing nanocomposites are key to the development of a wide range of sensing devices, for instance pressure, temperature, humidity, and proximity sensors etc. [105] Nanocomposite materials are made by mixing both metallic and organic conducting fillers.

Temperature sensing is recorded mostly in the form of elec- trical resistance variation in the bulk of nanocomposite layer as against the thermal gradient. Theoretically, the conducting fillers forming a network of connected threads within the poly- meric matrix are responsible for resistivity variations making it sensitive to the temperature changes. Multiple conducting nanofillers are explored for the nanocomposite synthesis that are used for temperature sensing and monitoring [106]–[109].

For instance, a nanocomposite of graphite mixed in PDMS is applied for making a large area sensing patch containing 64 sensors, each with an area of 4 × 4 cm2 [106]. Cu based inter- digital electrodes developed on a PI substrate are also applied to monitor the conductivity changes in a nanocompite based layer. A detail comparative study is conducted on the thermal sensing performance of Carbon and Ag mixed in PDMS sepa- rately at a temperature range of 25–150 °C [110]. Carbon based nanocomposite exhibited clearer response and dependance on temperature upto 150 °C, whereas Ag based nanocomposite produced best response at 120 °C. The different configurations and geometric structures could easily be explored with using nanocomposites as the thermal sensing layer particularly in the shape of a discrete resistor as well in a wheatstone bridge design [111].

IV. SUBSTRATES The interesting features such as flexibility, bendability, lightweight and wearability etc. of the polymeric substrates that provides a supporting ground for the low-cost fabrication of large area devices [12], [112], [113]. The bendability at small angular deflections makes them ideal for fast speed pro- duction through Roll-to-roll manufacturing [114]. The flexible substrates are desired to have stable properties comparable to the standard planar rigid substrates [115]. The three different types of flexible substrates developed largely so far are thin glass [116], [117], thin metallic foils [118], [119] and polymer basedplasticssheets[120].Amongtheseplasticbasedsubstrates are more suitable for low-cost wearable related applications, as the thin brittle glass is expensive as well as prone to breaking.

Similarly, thin metal foil-based substrates are relatively heavy and require extensive surface treatments to make it suitable for thin film deposition of functional nanomaterials. In this scenario, plastic based polymeric substrates offer reasonable tradeoffs concerning the physical, mechanical, and chemical inertness etc.

One main hindrance in using plastic based substrates is about its low glass transition temperatures (Tg), usually within the range of 250 °C. This however covers the range for biocompatible substrates required for most of the wearable and implantable devices [120], [121]. Biocompatibility of the substrate material is central to the development of wearable biosensors. Biocom- patibility is termed an important factor which is responsible for the safe interface between living cells/tissues and the sensing

KHAN et al.: WEARABLE PRINTED TEMPERATURE SENSORS: SHORT REVIEW ON LATEST ADVANCES

157 Fig. 4.

Key enablers for realizing a flexible and wearable sensing device. Electronic functional inks in the form of colloids and nanocomposites having organic-inorganic hybrids combined with suitable cost-effective additive manufacturing i.e., printing technologies. Large area flexible substartes are used as supporting layers and enable the wearability an dfoldability of the developed structures. module without causing any harm to the deployed surface. As the substrates are in intimate contact with human skin or with the tissues, air permeability and biocompatibility is strictly required to avoid any medical risk or complications [122], [123]. Bio- compatibility need to be assessed both for in-vitro and in-vivo applications particularly focusing on the toxicity to the cells, cell attachment, leachates and cell culturing. Mismatch properties or non-biocompatibility can lead to cell damage as well as inflammatory issues and immune system disorder [122]. Various substrates have been developed considering the biocompatibility aspects, that include SU-8, air permeable membrane, carbonized silk fibers, cotton fabric, cellulose based and most importantly polymer based biomaterials [32]. Among the list of potential polymeric substrates, PDMS is ideal for wearable electronics.

The viscoelastic properties and biocompatibility make it more suitable for deployment on nonplanar surfaces. The recently developed cellulose based substrates are also used, however the porosity and liquid absorption capacity of the substrate make it challenging for human body worn biosensing applications [124], [125]. Polyurethane is commonly reported substrate used to ex- ploit the biocompatibility [126]. The semipermeable membrane allows a certain amount of humidity and oxygen desired for the normal functioning of living cells and tissues. One major challenge with most of the biocompatible substrates is its limited upper processing temperature. As the metallic conductors, for instance Au require higher temperatures (i.e., ∼250 °C), which is beyond the Tg of these substrates. Therefore, to IR or flashlight sintering techniques are used to address this issue. In flashlight sintering, only the top surface of the thin film gets heated with tuned light intensity and power without damaging the underlying substrate. Flashlight sintering too has certain limitations i.e., transparency of the substrates as well as deformation of the upper contacting surface with the sintered pattern. Therefore, further research in developing biocompatible substrates and reliable sintering technique offering low temperature operation and producing higher instantaneous temperatures are highly desired.

V. KEY ENABLING WEARABLE SENSING TECHNOLOGIES Development of electronic devices and sensors that are suit- able for deployment on nonplanar surface, require all the con- stituent materials to be in perfectly matching in terms of their processability, mechanical and electromechanical properties etc. [2], [3], [32] Key enablers for such devices are the synthesizing colloidal solutions of functional nanomaterials with suitable rheological properties for the specific printing technology and its compliant integration on a polymeric based flexible substrate [12], [62]. Fig. 4 depicts the key enablers desired for making flexible and wearable sensing devices.

A. Printing Technologies Printing is an important key enabler for patterning func- tional materials on diverse substrates at ambient conditions.

The cost-effective manufacturing through printing and covering processing areas larger than wafer scale are the main attractions.

Printing technologies can be used both for high resolution pat- terning as well as coating larger areas in a much simpler way as compared to the conventional clean room processing [12], [27]. Depositing materials on demand at specified location in a controlled fashion make printing technologies distinguished from other manufacturing techniques due to the reduced material wastage. The materials wastage is lowered as printing is done in a single step as against the many steps involved in the clean room processes that use several subtractive steps for patterning structures [114], [128]. Various printing technologies have been adopted as well as modified rendering the expertise from con- ventional text printing techniques. Few of the prominent printing technologies used include but not limited to, are gravure, offset, flexography, inkjet, screen, aerosol, transfer and micro-contact printing etc. All the techniques have specific set of requirements and produce results based on their capabilities. Rheological properties of the solutions and nanocomposites come at first place when choosing a printing technique. For instance, inkjet,

158 IEEE REVIEWS IN BIOMEDICAL ENGINEERING, VOL. 16, 2023 slot-dies, and aerosol jet printing requires materials in the vis- cosity range of 5-12 cPS, however higher viscosities are desired for screen as well as gravure and offset printing techniques.

Similarly,thepatternresolutionandfilmthicknessesachievedon the substrate depends on the selection of the printing technique.

Screen and offset printing produce thick films (in the range of

0.2-0.8 µm), while inkjet and aerosol are used to print thin films (in the range of (.01-.2 µm). Film thickness is tuned by multiple printing cycles depending on the desired thicknesses. Printing speed is another important parameter considered for selection of a specific printing technique. Gravure, offset and flexography are famous for their high-speed production, as they are easy to be installed on a roll-to-roll printing system. Speeds as high as ∼

150 m/min can be achieved with these systems at repeatable and reproducible film formation. Whereas inkjet, screen and aerosol jet printing are low speed processes and are mostly adopted for producing prototypes or installed at research lab facilities for evaluation.

Besides printing technologies, selection of suitable mate- rials also plays significant role in determining the key fac- tors of a temperature sensor. Fabrication processes to deposit organic/inorganic materials in a patterned or in the form of thin films is determined by the rheological properties of the nanoparticle solutions. For wearable electronics and devices’ development on flexible substrates, the materials and processing techniques need to respect all the limitations. In standard manu- facturing processes, the different combination steps of photore- sist coating, exposure to sensitive light, etching, target material depositionfollowedbylift-offetc.makethemanufacturingcom- plicated and very expansive as significant amount of material is wasted. This also adds to the budget of electronic and material pollution directly or indirectly. On the other hand, the compli- cated and sophisticated processes restrict the manufacturing to certaingroupswhocanacquireandmaintaintheprocessingcosts of such manufacturing facilities. Thus, solution based additive manufacturing offers significant advantages when it comes to low-cost manufacturing at less capital investments. Therefore, besides the development of other electronic devices, temper- ature sensors are also developed using printing technologies [30], [82], [129].

Printing has evolved as an attractive approach for rapid manu- facturing of thin film electronics. Chemical solutions or colloids of the functional nanomaterials are stabilized with the help of additives and surfactants are used for printing [130]–[132].

Properties and material content are tuned according to the set requirements of a specific printing process and target function- ality to be achieved. The controlled deposition and dispensing parameters are adjusted collectively by the materials properties as well as the actuation mechanism parameters [133]. The most important solution properties influencing the actuation mech- anism are viscosity, surface tension, particle content, average nanoparticle sizes and solvents’ vaporization point etc. [130], [134]. Substrate surface conditions also play major role and contribute significantly to the final shape of the printed structures and resolution. Surface properties such as the hydrophilicity, surface energy, lower contact angles, higher work of adhesion etc. are required to be at moderate levels in achieving good

Fig. 5. (a). Schematic of the multi-layer thermistor, (b) Chemcial com- position of PEDOT-PSS and its linkages with GOPS, (c). Schematic of step-by-step manufacturing of tehrmistor, (d) Patterned IDEs and sensing material using inkjet printing technolgy. [20] [reproduced with permission] . printingresults[135],[136].Thetwotypesoftemperaturesensor considered in this review are RTDs and thermistor, which em- ploy printing technologies capable of patterning high resolution conducting structures as well as deposition a uniform thermal sensing layer.

B. Printed Temperature Sensors Printed RTDs work on the principle of changing resistance as against the temperature rise and therefore, the thermal co- efficient of resistance (TCR) is used to determine the corre- sponding temperature variations [137]–[139]. The all-printing method of fabrication in the shapes of meander, spiral or circular make the manufacturing simple and cost-effective. For RTDs, the sensor is completed in a single step by printing the desired structure using conducting nanoparticle ink only. The same type of ink is used for the interconnects as well as for the contacting pads. Therefore, the process of making RTD is straight forward and making it through printing technology in a single step is much advantageous. On the other hand, thermistor is made of two different materials and therefore, two printing cycles are executed to complete the manufacturing process. Interdigital electrodes (IDEs) are the first layer of the device, which is printed using metallic ink [13]. Inter digital spacing between the consecutive electrodes, play significant role in sensing response.

Therefore, various studies have been performed to optimize the spacingwiththecorrespondingthermallysensitivematerial. The second printing cycle uses thermally sensing material deposited in the active area covered by IDEs, either using printing or a thin film coating technology. Significant progress has been made in exploring both the approach of RTDS and thermistors in recent years by investigating different materials and manufacturing processes. Mixture of conducting polymer composites made by combination of reduced graphene oxide (rGO) and poly- hydroxybutyrate (PHB) for making both RTD and thermistor structures. Ag based electrodes are used and direct printing technique as well as drop casting is used to pattern and deposit ink respectively [13]. Fig. 5 shows schematics and printed model of a new type of thermistor, where the temperature sensing material is patterned rather than complete filling of the effective area surrounded by the IDEs. The type of sensor is claimed to be

KHAN et al.: WEARABLE PRINTED TEMPERATURE SENSORS: SHORT REVIEW ON LATEST ADVANCES

159 much effective to the conventional thermistors where the whole sensing layer is deposited as a thin film.

High resolution Ag patterning in meander is done with inkjet printingtechnologyonacellulosebasedbiocompatiblesubstrate [30], [140]. An all-printing approach for patterned deposition of PEDOT-PSS and carbon ink is used to design a whetstone bridge [20], [141]. Similarly, inkjet printed sensors fabricated from mixture of carbon and PEDOT:PSS to have TCR value of around 0.25% /°C. Inkjet printing of graphene/PEDOT:PSS ink is developed on top on skin mountable polyurethane plas- ter (adhesive bandage). A new addition in the RTD design is implemented, where Ag based conducting stripes are added in the pathway inkjet printed graphene/PEDOT:PSS [67]. A small wearable patch comprising multiple sensory cells such as acceleration, ECG and temperature sensor are developed mainly by using an inkjet printing technology. For temperature sensor, PEDOT:PSS and CNTs were mixed to obtain an ink compatible with inkjet printing technology [142]. An efficient skin mountable temperature sensor is reported as part of a multisensory patch used for wearable health monitoring ap- plication. An inkjet printable ink was synthesized by mixing

CNT ink and PEDOT:PSS in a 3:1wt% ratio for development of skin based temperature sensor [69]. A similar type of wearable temperature sensor is reported made in the shape of a band for human wrist. Inkjet printing of PEDOT:PSS is used to developthistemperaturesensor[143],[144].Screenprintingofa nanocomposite of PEDOT:PSS, Ag nanoparticles and graphene ink is used to impregnate a stretchable fabric for development of wearable self-powered temperature sensor. This type of sensor is claimed to offer very attractive features such as ultra-thinness, light weight, high flexibility, and stretchability etc. enabling the sensor to conformally contact with the human skin [35]. A highly stretchable but strain-insensitive sensor is developed by using different mixtures of SWCNTs, MWCNTs and AgNps. The synthesized ink is mixed in certain ratios which is compatible with using Microplotter ink printing system [145].

Aerosol jet printing has recently been produced exciting re- sults leading the field of printed electronics for high resolution patterning as much less efforts. In this scenario, a Cu-CuNi tem- perature sensor is reported on Kapton substrate. The nanopar- ticles are deposited using aerosol jet printing followed by laser sintering at lower powers respecting the thermal properties of the

Kapton substrate [82]. Besides the solidified sensing structures of the temperature sensors, liquid based metallic conductor are also used in a 3D printed microchannels as reported in.

Biocompatible polylactic acid material is used as substrate for the microfluidic channel made by 3D printer and filled with liquid metal (Galinstan, Rotometal). The sensor is deployed on human ear and is used detect core body temperatures [146].

Transfer printing is used to develop ultra-flexible and biocom- patible temperature sensor following a meander structure made of Au/Cr. The temperature sensitive material is integrated in a polyurethane substrate using transfer printing approach. Ini- tially the Au based microstructures are developed using pho- tolithography techniques on a PI (polyimide) coated Si wafer [33]. Table I summarizes few of the representative designs, the sensing materials, substrates, the fabrication method as well as the encapsulation materials used to cover the sensing layers by providing further mechanical strength and protection from the varying climatic conditions.

VI. TEMPERATURE SENSING PERFORMANCE BASED ON DIFFERENT MATERIALS AND STRUCTURES

Printed temperature sensors work on the principle of re- sistance change against corresponding temperature rise, and therefore the TCR is mostly considered as the measurement parameter for sensitivity [57], [129], [147]. For wearable tem- perature sensors, the body temperature varies in an effort to transfer heat between living cells and the external environ- ment [148], [149]. This heat is dissipated through the skin or respiratory system. Real-time monitoring is enabled by the wearable temperature sensors to determine the instantaneous changes occurring in the body under various conditions [150].

Accurate measurement is important in recording the real time data regardless of the motion or motion-less state of the body.

Although the conformal interface and deployment of flexible sensors offer the possibility to detect these minute changes in real-time [151]–[153]. They still require a good combination of sensitivity, rapid response, hysteresis-free, robustness against varying conditions and stability during normal physiological movement [154], [155]. As the sensors are used to monitor the human health condition and the data would be used to make a decision on emergency basis, therefore a reliable sensing data is of prime importance [156], [157]. To improve the sensitivity and reliability, a wide range of functional nanomaterials in pristine form as well nanocomposites are used to achieve better performance [61], [72], [158]–[160].

A. Performance of Conducting Materials-Based Sensors

Various interesting researches have been conducted recently withafocusonaddressingthenumerousaspectsofperformance, reliability, wearability, and biocompatibility etc. In this scenario, an ultrathin temperature sensor produced by embedding the Au meander structure in a semipermeable polyurethane membrane through transfer printing [33]. The sensor has been tested 24/7 and also calibrated during water bath. The sensor has presented a TCR value of 0.002778 /°C by placing at multiple body locations such as underarm, forearm and comparable perfor- mance to mercury-based sensors are claimed to be achieved.

The development of sensor on biocompatible, breathable, and stretchable substrate is interesting, as it permeates the water and air without causing any significant degradation to the sensor performance [33]. Ag is the commonly used printed materials in the form of RTD, where the bulk resistance variation of the printed patterns is corelated with the change in temperature [31], [66], [161], [162].

A high-resolution printed Ag meander structure is developed on a bacterial nanocellulose substrate, which is biocompatible and biodegradable. The sensor exhibited a positive co-efficient resistance (PTC) by producing sensitivity of 0.06 /°C [140].

Inkjet printing of a large area Ag printed meander using inkjet technology exhibited a TCR value of .0029/ °C in the tem- perature range of 20–60 °C [30]. PEDOT:PSS is reported by

160 IEEE REVIEWS IN BIOMEDICAL ENGINEERING, VOL. 16, 2023

TABLE I SUMMARY OF DESIGN, MATERIALS AND FABRICATION METHODS OF SOME LATEST REPRESENTATIVE RESEARCH most of the research for developing temperature sensors, be- sides the metallic conductor [95], [163], [164]. Being a poly- mer and highly conductive, the intrinsic flexibility makes it an attractive candidate especially when it comes to electrically conducting structures on polymeric substrate [58], [93], [126].

A PEDOT:PSS based temperature sensor has been reported on Kapton and cotton fabric substrates, which are suitable for wearable related applications [30]. The sensor is claimed to be highly stable and sensitive towards small changes in the temperature and can detect a variation down to 0.1 °C [144].

Cross-linked PEDOT-PSS is used for developing thermistor- based temperature sensor where, IDEs were inkjet printing using

Ag nanoparticles-based ink and the sensing area is covered with PEDOT-PSS in a meander patterned configuration. Fig. [5] shows the schematic and printed sensor using this architecture.

The sensor showed NTC and a higher sensitive of 0.77% /°C is reported. Sensitivity of the PEDOT:PSS is enhanced by mixing with GOPS [20].

B. Performance of Nanocomposites-Based Sensors Metallic and polymeric-based nanocomposites are also ex- tensively explored for a wide range of biosensing applications [94], [165], [166]. For temperature sensor, rGO [137], [166] and its nanocomposite especially with PEDOT:PSS and PHB are exploited both for the RTD and thermistor configurations [56], [87], [167]. Ag based IDEs are used for the thermistor,

KHAN et al.: WEARABLE PRINTED TEMPERATURE SENSORS: SHORT REVIEW ON LATEST ADVANCES

161 Fig. 6.

Sensitivity and temperature responses of (i), Thermistor using rGO/PEDOT-PSS, testing under different temperature and humidity conditions, (ii) Sensor develoepd based on Seebeck co-efficient by using nanocomposites of MWCNTs/AgNWs, SWCNTs/AGNWs and MWC- NTS/SWCNTs. (iii) Temperature response of nanocomposite of PEDOT:PSS/AgNPs thermocouple. [20], [145], [35] [reproduced with permission]. and a printing mechanism is adopted for the tuned nanocom- posite solutions. The sensors are reported to have NTC both for the rGO and rGO/PHB nanocomposites. TCR values in the range of 0.018-0.03 is reported minimum for the rGO and maximum for the mixing ratio of 12 wt.% [13], [144]. A skin mountable polyurethane adhesive bandage is used to develop temperature sensor. An inkjet printable nanocomposite solution is prepared by mixing graphene/PEDOT:PSS and patterned as meander shape through inkjet printing technology. Sensor re- sponse is enhanced by printing Ag stripes in the pathways of graphene/PEDOT-PSS patterns to mitigate the chances of signal degradation. The sensor is responsive and tested in the temper- ature ranges suitable for human health monitoring i.e., 35–45

°C. The graphene/PEDOT:PSS behave as NTC while recording a TCR value of 0.006/°C in the operating range of temperatures [67]. A temperature sensor in a straight wider line is reported to have higher sensitivity i.e., 1.3%/°C. The wide sensing pattern is developed by printing a nanocomposite solution of PEDOT:PSS and CNTs, connected with Ag based interconnects [69], [144].

Fig. 6 gives some representative sensitivities responses at dif- ferent humidity and temperature conditions. The graph shows data for two different devices i.e., thermistor and thermocouple, where the sensor response is measured as resistance for the thermistor and Seebeck co-efficient is used to determine the varying temperature values.

C. Sensors’ Performance Based on Un-Conventional Architectures

Further developments in the field of wearable sensing ap- plication are proposed by exploiting some un-conventional ar- chitectures and materials. For instance, a wheatstone bridge configuration is designed and produced using an all-printed approach. The sensors are made of carbon nanoparticles ink and a mixture of PEDOT:PSS with DMSO (dimethyl sulfoxide) and evaluated the TCR as positive and negative co-efficient respectively. The TCR is evaluated also a function of mixing ratios of PEDOT:PSS and DMSO at 0.3 wt.% and 3 wt.% which resulted in the range of 0.009 /°C to 0.0025/°C respectively.

Carbon based ink produced a positive TCR i.e., 0.0022/°C [141].

Another interesting alternative proposed for monitoring of core body temperature by monitoring inside the human ear rather than placing the sensor on top of skin. The sensor is developed through 3D printing and is able to be worn on a human ear to record the core body temperature from the tympanic membrane.

An infrared sensor is developed through 3D printing of liquid

162 IEEE REVIEWS IN BIOMEDICAL ENGINEERING, VOL. 16, 2023

TABLE II SUMMARY OF THE PRINTED SENSORS RESPONSES metal in a wearable module and is connected with wireless communication system for real-time monitoring [146]. Textile based temperature sensors are very attractive, as the intrinsic wearability and stretchability features comply with most of the sensor requirements. Thermoelectric approach is used to determine the temperature response by developing the sensor in a scalable way using nanocomposites of Ag, PEDOT:PSS and graphene [168]. Mixtures of this ultrasensitive materials is printed through stencil coating. An output voltage of 1.1mV is generated for a temperature difference of 100 K, with a high durability up to 800 cycles of 20% strain. The sensor is also explored for the dependency on stretching direction and has exhibited corresponding temperature-sensing properties [35].

Another interesting development is proposed about exploit- ing the Seebeck effect to mitigate the strain-related noise that might occur in the temperature sensing data. Three different nanomaterials i.e., SWCNTs, MWCNTs and AgNWs are used in different composition and printed using high resolution Mi- croplotter technology. Main idea behind this development is that the difference the Seebeck coefficients generate a voltage which is proportional to the difference of cold and hot points of the junction. A large array of sensor is developed and de- ployed on a human hand. The Seebeck coefficient from the mixing nanocomposite of SWCNTs/AgNWs could reach up to 37mV/°C. Similarly, the MWCNTs/SWCNTs produced See- beck coefficient of 23mV/°C, lower than the SWCNTs/AgNWs combination. Both the combination resulted in good linearity and reproducibility of the sensors’ outcome [64], [101]. An- other similar approach for exploiting the Seebeck response of

CU and CuNi films deposited by using an aerosol jet print- ing. A higher Seebeck coefficient is achieved i.e., 40 µV/°C, claimed to be the highest sensitivity reported so far for sensor developed on polymeric substrates. The stability tests were performed after bending the sensors for 200 cycles at different deflection angles, reported a negligible variation within 2.5% of the Seebeck coefficient [82]. Table II summarizes the re- sponses of representative devices by including the designs, TCR, temperature ranges, sensitivity and wearability spots on the human body.

The fluorinated polymer CYTOP (CTX-809A) was chosen as the passivation layer, due to its low water vapor permeability and good adhesion on the substrate

KHAN et al.: WEARABLE PRINTED TEMPERATURE SENSORS: SHORT REVIEW ON LATEST ADVANCES

163 Fig. 7.

Sensor responses with applying encapsulations. Sensor is encapsulated with water repellant layer. (a) Cross sectional view of the covering film. (b) Comparison and effect on the sensitivity with and without encapsulation. [33], [20] [reproduced with permission] .

VII. ENCAPSULATION MATERIALS Covering the printed structures is important to protect the sensing layer as well the complementary signal readout in- terconnections from the environmental impacts. Encapsulation is important not only for the organic based materials, which are more prone to climatic condition, but metallic based RTDs especially made of Cu and Ag are prone to oxidation. Physical, electrical, and mechanical properties of the encapsulant layer need to be in matching with the underlying sensing and metallic layers. A very thin layer of insulation, which is sufficient enough to block any penetrating air or humidity. Localized deposition of the encapsulant layer is usually deposited especially at the active sensing area. Whole area of the substrate does not need to be covered for the wearable sensing patches. The substrates are usu- ally biocompatible and sometime with microporous substrates are selected to maintain the normal environmental conditions on the human skin. Therefore, most of the wearable temperature sensors reported so far always have applied a suitable encapsu- lant at the effective sensing area and selecting a biocompatible substrate at the same time.

A semipermeable membrane is used as encapsulant layer for developing a breathable and stretchable temperature sensor. The membrane covers multiple underlying films used to develop the sensor that provides a strong base simultaneously to maintain the planarity and neutrality of the base sensing layer [33]. A

UV-epoxy based encapsulant is applied on a spray coated CNT layer and evaluated against non-encapsulated sensors [169].

The encapsulated sensors produced much linear response with minimal hysteresis as compared to the unencapsulated sensors.

A fluoropolymer (CYTOP) CTX-809A is applied as passivation layer on a printed PEDOT:PSS sensing layer [20]. A drop casting technique is applied to produce a thin layer with thickness of about 10µm. Sufficient passivation against humidity as well as strengthenstheunderlyingsensinglayerisreported.Fig.7shows schematic of a sensor with and without encapsulant layer and their sensivity responses. Stability in an all-printed sensor is observed by comparing the performance of an encapsulated and nonencapsulated sensing film. Inkjet printed PEDOT:PSS and

DMSO are used as NTC (negative thermal coefficient) tempera- ture sensors, and a barrier foil is used as top encapsulant. A damp heat accelerated lifetime test was performed for 400 hours at

65 °C and 85% RH. Encapsulant materials not only influences physical properties but improved the electrical characteristics of the film as well. Electrical resistance of encapsulated sensor increased by 10% relative to the base resistance. However, a 13% increaseinthebaseresistancewasobservedwithunencapsulated sensor, showing more stability in the sensor performance [141].

In another test, a nanocomposite of graphene/PEDOT:PSS is evaluated with and without encapsulation [67]. An electronic grade coating (EGC) material was drop casted on the printed area of the sensor in argon environment and dried at room tem- perature. The resistance response after repeated cyclic and accel- erated tests confirm the stable resistance response of the covered sensor, however the uncoated sensor diverged significantly from the initial base resistance. The resistance vs. temperature slope reversed from negative (NTC) to positive (PTC) while testing in ambient condition accompanied with prominent hysteresis.

The environmental variation specially humidity caused serious implications to the performance of the sensor when not coated with proper encapsulant [67]. Applying a low-moisture perme- able membrane is another approach used to make the sensor waterproof. These materials come in solution form and can be printed deterministically at the desired sensing area [69]. Lami- nation of a dry photoresist film on thermally sensitive material is arapidalternativeforapplyingencapsulationlayers[170],[171].

An aluminum encapsulation layer is deposited on a PEDOT:PSS layer using atomic layer deposition (ALD) technique [172] as a protection against humidity for temperature sensors. PDMS is a commonly used and readily available biocompatible encap- sulant, which is inert to chemical etchants and environmental variations etc. The easy processing and deposition/coating make

164 IEEE REVIEWS IN BIOMEDICAL ENGINEERING, VOL. 16, 2023

Fig. 8.

Representaive examples of the wearable sensing patches deployed at different spots on a human body, (a-b) Fully printed PEDOT:PSS temperature sensor with flexible PCB circuit, (c) Stretchable sensor fabricate dform polymer nanocomposite and mounted on foreahd, (d-e) multisensor patch made of conventional rigid sensor and embedded in a flexible substrate, (f) ECG and tempearture sensor mounted on human skin with gelss-less approach, (g) high resolution RTD tempearture sensor mounted on human finger, (h) Highly stretchable and ultrasensitive tempartrue sensor, simulated an dtested on human skin. [20], [13], [17], [69], [140], [145] [reproduced with permission]. it attractive to be used in any experimental investigations [13], [145]. To make the sensor suitable for implantable applications, thickness of the encapsulant needs to be minimized. Therefore, for ultrflexible sensors, parylene C coating (∼1µm thick) layer is commonly practiced [61].

VIII. WEARABILITY DESIGNS AND PLACEMENT PREFERENCES ON HUMAN BODY

The sensor patches need to be deployed in a conformal way on the human skin, such that the noise generated by the physio- logical movements and or by the chemical analytes excreted by the human body are avoided. Therefore, selection of the most feasible location in the human body to detect the signal easily is of prime importance. For continuous monitoring, the system needs to be light-weight and not to interrupt the daily routine activities of the individual. With the fast advancements in miniaturized microelectronics, it has become possible to deploy these sensor patches on human body seamlessly. Fig. 8 shows some representative examples, where wearable sensing patches are attached to human body at different body spots to determine various biomarkers. To develop a full system, a hybrid approach towards utilizing both printed sensors and Si based signal conditioning circuits. Different designs are pur- sued depending on the type of sensors and target biological or physiological analytes. Here we focus mainly on the de- signs specific to temperature sensing. For the sensor developed on ultra-flexible substrate, a biocompatible adhesive based on hypoallergenic polyvinylethylether is used. This adhesive is biocompatible and can stay for longer period as 7 days without causing any irritation to human skin. This is complemented by the breathable substrate, which contains semipermeable mem- brane that allows to maintain the normal climatic conditions on the human skin [33]. These types of sensors are developed without firm integration of the electronic circuitry and attaching the senor patch to human skin does not qualify completely for the wearable systems. A biopatch is implemented by using

NFC (near field communication) technique. The biopatch can detect, save, and transmit the recorded temperature to a nearby receiver wirelessly. Besides the temperature sensor, the biopatch comprises a microcontroller (RF430FRL152H) for running the

NFC communication standard as well as a non-volatile memory for saving the recorded temperature values [173].

A surfaces mount BLE module for wireless communication has presented very promising results. The overall thin module is capable of integration to the human skin as well as tried on

KHAN et al.: WEARABLE PRINTED TEMPERATURE SENSORS: SHORT REVIEW ON LATEST ADVANCES

165 Fig. 9.

Wearable temperature sensor for monitoing of deep body temperature using an IR detector, (a) design of the earable smart device with communication model and tools, (b) Image of wearing the smart device and circuitry within the device (c) Detection mechanism of temperature in the IR sensor and output voltage of the thermistor vs. ambient, (d) Output voltage of IR sensor as a function of time by increasing temperature and calibrated output voltage as a function of the temperature difference. [146] [reproduced with permission]. several other nonplanar surfaces [20]. Multilayer integration on a reusable kirigami structured substrate is proposed by plac- ing ECG, acceleration, and temperature sensors. The kirigami structure in base substrate allows the slight deformations caused by the physiological movements of the human body parts. The sensor is attached to the human chest using a bi-adhesive tape.

This is a major discrepancy and using bi-adhesive tape could be detrimental to human skin for longer use [144], [174]. A large area sensor patch employing both ECG and temperature sensors are reported by mounting on a human chest without using a gel usually required for the ECG electrodes. A new gel-less approach is developed made by mixing biocompatible

PDMS and PEIE, which upon curing are become sticky and provide a firm conformal contact with human skin [69]. The sensor response is in acceptable range both for the ECG and temperature, however serious issue with this design is the data readout through wired connections. A more compact wearable sensor is presented for measuring ECG, PPG, and body temper- ature. The sensor patch is developed using a central board for data acquisition and processing, powering board and batteries, all attached to the human chest [17]. A relatively easy approach is the integration temperature sensors in a wearable band. The miniaturized and compact electronic components are easy to be packaged in band and transmit data through BLE module in a nearby connected device [143].

Other than the skin temperature, wearable devices for mea- suring core-body temperature are also investigated as shown in Fig. 9. A smart 3D earable package is developed used to determine the core body temperature. The sensor is based on infrared sensor. All the data processing circuits, and wireless modules are embedded in a single package and mounted on a

3D printed designed gadget to be worn on human ear [146].

Another similar approach by making a foam-based Y-shaped sensor is developed, equipped with temperature sensors and electronics, and focuses on ergonomic aspect. This type of sensor is ideal for continuous monitoring of core-body tem- perature especially for the mobile patients [25]. Monitoring the temperature of a diabetic patient especially in the scenario of diabetic foot ulcers (DFU) is an interesting application. A personalized temperature sensor equipped shoe is developed to monitor the foot temperature of a diabetic patient. The tem- perature rise time at the wearable plantar surface could be a potential indicative biomarker for differences in soft tissue biomechanics and vascularization during diabetes onset and progression.

IX. CHALLENGES AND FUTURE OUTLOOK Despite the fact that significant progress has been made to- wards realizing wearable sensors in general and particularly the temperature sensors. There are many technological challenges before it is widely accepted by the masses at large. Biocompat- ibility: Most of the devices and design reported here are based on placing the polymeric substrate directly on the human skin or using a bi-adhesive tape as a temporary solution for deploying as wearable module. This however is faced with challenges of biocompatibility, covering large area of the skin may cause irri- tation and toxicity of the some of the metallic conductors etc. [1], [123], [175], [176]. Other than skin related incompatibilities, the analytical procedure, reliability of the sensor data, real-time ac- quisition and decision-making protocols, security and powering the devices using batteries need a significant amount of research.

Putting hazardous and somehow carcinogenic materials on or near the human skin for long term need extensive investigation and finding alternative powering devices that are compatible with human skin, need to be explored. Signal readout: Making interconnections with the printed devices and circuits is another challenging task. Conducting epoxies have been in practice

166 IEEE REVIEWS IN BIOMEDICAL ENGINEERING, VOL. 16, 2023 commonly for making connections to printed pads, however due to the lower glass transition temperatures of biocompatible substrates, these epoxies cannot be applied. Making a normal physical contact would be feasible for laboratory level tests, however for real time applications, reliable interconnections need to be explored. The contact resistance needs to be in negligible, otherwise the minor variations due to inappropriate contacts with the pads could lead to incorrect data especially in the case of human body temperature generated data. Further, interconnect lines and electrical connections between contacting pads and the signal processing circuits make the system less reliable due to the noise factor occurring during physiological movements as well as by varying contact resistances. This will require tremendous work on the electronic circuit side by incorporating onboard filters to mitigate the noise and unwanted errors adding to the temperature detection signal. The human body temperature varies in a very close range, and therefore, a minor deviation from the actual values will result in a misleading situation.

Majority of the reported sensors are deployed and directly interfaced with human forearm/wrist, forehead or on the chest.

As, these parts of the body are always exposed to external environmental conditions, and a proper encapsulation with ac- ceptable working conditions of the sensors are highly desired.

The biocompatible materials as well as breathable substrates are more challenging especially for the wearable sensing ap- plication. The interface between human skin or tissue and the mounting sensing materials is difficult to predict as several biological reactions are occurring in the human body and the materials can interact differently. The use of natural biomaterials would lead to address the issues, of biocompatibility, however reproducing the similar sensing performance obtained with conventional materials would be much challenging. To reduce the materials waste, development of bioresorbable sensors is another interesting approach, but that too needs the whole set of constituent materials to be biodegradable. Surface mount integration of thin films maintain the lightweight, miniaturized electronic circuitry and wireless communication modules are at the center of further developments in the field of wearable temperature sensors. Spots to place sensors on the human body are to be properly selected and the areas that produce significant thermal signals are to be selected for sensor mounting. The supporting substrates as well as the complementary materials making the sensor geometry are preferred to be sufficiently stretchable for accommodating the deformations occurring due to the physiological movements. Incorporating such materials that allow stretchability also produce relatively low sensitivities as well as nonlinearity in the sensor response. Therefore, a good combination of compliant materials and substrates is a major challenge for producing sensors with reliable responses, good sensitivity range, hysteresis-free and lower time of detections.

Majority of the reported flexible and wearable sensors are fab- ricated using the newly emerging additive manufacturing tech- nologies by utilizing solution based functional nanomaterials.

Inkjet printing is the most popular manufacturing technique reported so far and has commonly applied for deposition of the temperature sensing layer. These manufacturing techniques are claimed to be cost-efficient due to the ease in manufacturing by localized deposition of materials as well as less materials wastage. The accessibility of such manufacturing tool in com- mon research labs and easy processing steps make it ideal for low-cost fabrication. However, the high throughput and batch manufacturing by roll-to-roll fabrication is the ultimate solution towards cost-effective systems. Currently, the standalone system such as inkjet printing are mostly reported, which are ideal for proof-of-concept devices and making prototypes through discrete deposition steps. Making an integrated manufacturing process flow for patterned deposition of metal lines, sensing layers, interconnects as well as encapsulants could possibly provide a solution towards large scale fabrication at lowers costs.

Similarly, few important features of the printed devices such as stability, repeatability, mismatch, sensitivity accuracy, long term reliability and lastly the large-scale fabrication at depre- ciated costs are considered to be the most challenging aspects for successful development wearable electronic systems. Being developed from solution-based nanomaterials, the temperature limitations of biocompatible substrates restrict the higher ther- mal annealing of the printed patterns. This is very crucial for the stability of the devices as incomplete sintering may lead to inconsistent sensor responses as well as it may cause the delamination of the printed structures at lower bending angles.

The printed materials are mostly in their amorphous form, that makes it less attractive when it comes to accurate responses at insignificant instantaneous changes especially when triggered by the human body temperatures. The layers could possibly crack with physiological movements or in some cases localized delamination occurs when subjected to higher stresses, which changes the base resistance and have significant bearing on the repeatability and reliability of the sensor. This has been observed in the reported literature that sensors performance is degraded after certain bending cyclic tests. Therefore, investigating such materials which are compliant to the polymeric substrates and produce signals repeatedly in the same operating window with minimal deviations even after indefinite bending cycles, are critically needed. Ageing effect of the organic materials is another serious challenge that need to be considered critically.

Encapsulationispreferredinsuchscenariostoprotecttheprinted sensing layers which are prone to varying climatic conditions, however materials and sufficiently thin coating is required that does not influence the performance of the temperature sensing.

Repeatability of the sensors’ response in close ranges with insignificant variations need to be guaranteed. Also, the sensors need to be tested in diverse climatic conditions and calibrated according to the set environment. The different type of sen- sor based on their deployability on the human skin such as body-wornsensorpatches,hand-bands,orimplantablescenarios need to be properly evaluated and compared based on their performance, acceptability from the users, data reliability and reproducibility and after all the ease in using and manipulating the whole wearable system. A realistic approach towards, a clinically viable, robust, user-friendly and acceptable design of the wearable sensory system is needed.

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167 X. CONCLUSION Wearable electronics are foreseen to revolutionize the health- care system by providing an easy alternative to clinical diag- nostics and early detection of various chronic diseases through continuous monitoring of human body fluids and physiological parameters. Temperature sensing is one of the prime physiologi- cal biomarkers, which is used to monitor and determine the heat exchanges occurring between the inner body tissues and external environment. The fast developments in thin film manufacturing technology are expected to provide solutions for replacing bulky and rigid electronics, that could easily and fully integrate onto nonplanar surfaces, particularly on the human body. The large area manufacturing enabled by the high througput printing tech- nologies is ideal for development of wearable sensing modules, as covering wider surface of the skin is desired for getting reliable data regardless of the large physiological deformations occurring due to movements. This review highlights the latest developments made in the advancements of wearable tempera- ture sensors. Remarkable efforts have been made towards realiz- ing flexible and biocompatible temperature sensors concerning the new geometric designs, utilizing functional nanomaterials in their pristine form as well as exploring strategies to make suitable nanocomposites that are more suitable for temperature sensing applications in real time. Flexible temperature sensor is reported either in the RTD or thermistor geometries, which are easily achievable to fabricate through additive manufacturing approaches. Metallic conductors are mostly adopted for the

RTD structure, however a more focus on the PEDOT:PSS and carbonaceous materials is observed to be reported in the latest developments. Printing is the broadly adopted manufacturing technique especially the inkjet printing technology that used these functional materials in their solution form with respec- tive rheological properties. A comparative study based on the processability through printing, suitable materials, biocompat- ibility, stability, sensitivity, and reproducibility of results is performed. It is observed that certain nanocomposites partic- ularly mixtures of graphene and PEDOT:PSS could achieve higher sensitivities. Despite the fact that all printed devices have been realized on flexible substrates, it is still challenging to maintain the reproducible sensing response in the suitable range of temperature sensing suitable for wearable applications. Few of the challenges are highlighted faced by the development of wearable sensors related to the material, geometry, interfacing with the signal conditioning circuits, attachment of the sensors to human body using biocompatible interfaces or adhesive, compact wireless communication modules and a user-friendly design of the wearable gadget.

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