New Paradigm of Personal Health Monitoring: Nanomaterial-based In-textile Sensors

doi:10.21203/rs.3.rs-2346614/v1

The use of nanomaterial-based in-textile sensors for personal health monitoring has recently emerged as a new paradigm for healthcare. This approach offers several advantages over traditional monitoring methods, such as increased comfort, convenience, and accuracy. In-textile sensors are integrated into clothing or wearable devices, allowing for continuous and unobtrusive monitoring of a range of physiological parameters, such as heart rate, body temperature, and blood pressure. Nanomaterials such as graphene and carbon nanotubes enable the sensors to be highly sensitive and selective, allowing for the detection of even small changes in the monitored parameters. However, there are also challenges and limitations to the widespread adoption of in-textile sensors for personal health monitoring. One major challenge is the integration of the sensors into wearable devices, as the sensors must be flexible, lightweight, and durable in order to be comfortable and practical for daily use. Additionally, there are concerns about the potential toxicity of some nanomaterials, and more research is needed to assess the safety of these materials for long-term use in personal health monitoring. In this review, we provide an overview of the current state of research on in-textile sensors for personal health monitoring, including the potential benefits and challenges of this technology. This review highlights the potential of in-textile sensors for personal health monitoring, as well as the challenges and limitations of this technology. Further research is needed to address these challenges and fully realize the potential of in-textile sensors for healthcare.

Health sciences/Health care

Physical sciences/Engineering

wearable

e-textile

in-textile sensors

nanomaterials

health monitoring

A rapidly expanding market for flexible and wearable sensors has emerged over the past few decades due to the exploding demand for real-time fitness tracking, health monitoring, and illness prognosis. Numerous well-known consumer and medical devices have adopted wearable sensor technology ¹, such as smartphones ², Fitbit® ³, and pulse oximeters. Due to their competitive hardware capacity, compact design, and reduced cost, wearable sensors are commonly employed in the medical field ^4–10. For example, by enabling rehabilitation outside of the hospital in an ambulatory setting, wearable technology lowers the expense of intensive therapy ¹¹.

Wearable sensors need to possess high stability, specificity, and sensitivity to be used as personal healthcare tools ¹². Over the past few years, researchers have been working to improve the capacity of wearable sensors to continually, promptly, and accurately monitor personal health parameters to increase the effectiveness and comfort of personal health monitoring ¹³. Some researchers have focused on raising the conductivity of the sensing components used in wearable sensors to enhance the sensors’ ability to convert diverse impulses into electrical signals ^14,15, which is essential for detecting minute variations in physiological parameters ¹⁶. An important sub-category of wearable sensors is in-textile sensors.

Fiber, as the most fundamental component of clothing, may be utilized in weaving and knitting to create a variety of designs and styles. So far, a variety of nanomaterials, such as primarily inorganic or metal oxide, have been successfully introduced into synthetic fibers as a result of the advent of nanotechnology to produce functionalized fibers with unique features^17,18. Those multi-functional fibers enable the production of smart clothing with the ability of energy collection^19,20, energy storage^21–24, and health monitoring^25,26.

The creation of in-textile sensors has made use of nanomaterials because of their high conductivity, quick electron transfer kinetics, and large aspect ratio, which can significantly improve the functionality of wearable sensors ²⁷. For example, metal nanoparticles (i.e., silver, copper, gold) based solutions or inks are widely used in textile printing and coating processes, which form highly conductive layers onto the fiber or fabric surfaces. However, the metal nanoparticle solutions/inks are expensive, unfriendly to the environment, and not biocompatible ^28–31. Carbon-based nano nanoparticles (i.e., graphene, carbon nanotube, carbon black) have demonstrated high potential for fabrication of in-textile sensors as their products have high conductivity and mechanical strength, and their suspensions can be cheaply obtained in large volumes as commodities [c-d]. Polymers are insulators. However, some composites based on a conjugated backbone (with sp² hybridized carbon atoms) are semi-conductive. They become promising candidates for in-textile sensors as they have many unique chemical and physical properties, such as controllable length, lightweight, as well as good thermal properties (Fig. 1).

In this review, an overview is provided on the current progress and innovations in wearable sensors with nanomaterial capabilities for monitoring human health. Narrative reviews are given on the fabrication of e-textile using nanomaterials, unique characteristics, and uses of nanomaterials in human healthcare (such as detecting chemicals and ions in bodily fluids, physiological signals, and motion states). Future potential prospects for the creation of wearable sensors powered by nanomaterials and their uses are also included.

2.1 Search Strategy

The following databases were used for the search: PubMed and Google Scholar. The following keywords were used for searching: wearable, E-textile, in-textile sensors, nanomaterials, and health monitoring.

2.2 Inclusion and Exclusion Criteria

Overall, the search covered 100 articles. 34 papers are fully reviewed and discussed. The inclusion criteria were (1) Studies focused on using nanomaterials to fabricate functional fibers, (2) Studies published in peer-reviewed English journals, (3) Studies describing the completed research, and (4) Studies published after 2012. The publication was chosen as they were in the period of rapid implementation of nanomaterials for in-textile sensors. The exclusion criteria were other publication types such as dissertations, editorials, and book or book chapters.

Among the selected articles, 12 articles were used to cover a wide range of carbon-based nanomaterials (single- and multi-carbon nanotubes, nanospheres, graphene, flakes, carbon black, etc.), 11 articles were related to metal-based nanomaterials (nanoparticles, nanowires, liquid metal nanoparticles, and nanocrystals), and 11 articles were related to polymer-based nanomaterials (nanoparticles, nanofibers, and nanowires) for in-textile sensors. These articles covered a variety of topics concerning the design, validation, and potential application of smart sensors in healthcare.

2.3 Study Selection

The screening process consisted of three major steps. The first author independently screened the titles. The first and second authors were then to screen the full text of the publication. The primary goal was to evaluate the technical content, such as solid data analysis and broad impact. Finally, the first and second authors also screened recent review papers about nanomaterial-based E-textile for more eligible studies.

3.1 Carbon-based Nano In-textile Sensors

Wang et al. ³² developed an electrochemical fabric using sensing carbon nanotube fibers with active coatings for sensing glucose, ions, pH, and other factors (Table 1). Chitosan was used to detect glucose levels, and ions were sensed using ionophore (ion-selective) and PEDOT:PSSS, a common material many researchers are using for wearable health monitoring. PANI was used for pH monitoring, while Ag/AgCl fibers were used as reference electrodes to obtain accurate readings. To provide stability for the material, a PVB layer was used with the Ag/AgCl fibers. This electrochemical fabric exhibited an electrical conductivity of 102–103 S cm^− 1 and tensile strength of 102–103 MPa with approximately 1000 bends. Glucose sensing was determined to be 0–200 × 10^− 6 M with a sensitivity of 2.15 nA µM^− 1, which covers the average sweat glucose concentration for humans. Regarding ion sensing, 10–160 × 10^− 3 M Na⁺ with a sensitivity of 45.8 mV dec^− 1, 2–32 × 10^− 3 M K⁺ with a sensitivity of 35.9 mV dec^− 1, and 0.5–2.53 × 10^− 3 M Ca²⁺ with a sensitivity of 52.3 mV dec^− 1 were observed. Wang et al. summarized that their work provides a highly flexible nanomaterial coating for health monitoring purposes that maintains structural integrity and measurement consistency even while under repeated deformations. However, they noted that glucose oxidase does not have a long lifetime when used in the active layer of sense, and there is a previously known issue with safety regarding Carbon Nanotubes (CNTs).

Aziz et al. ³³ proposed a smart-fabric sensor developed with single-walled carbon nanotubes filled with PVDF/PEDOT:PSS. The sensor response for temperature monitoring was found to be linear and stable with a sensitivity of 38 kΩ °C^− 1, and for bending angles, it was found to be 90 kΩ deg^− 1. Aziz et al. claimed that this sensor could be applied to wearable, foldable applications, stating that it is stable, highly robust, and quick to respond and recover and noting recovery times to be approximately 2 s. In the design of this device, damage from mechanical bending was prevented with the use of PVDF, which bound the SWCNT composite ink. Despite these noteworthy discoveries, only one stimulus can be measured at a time. In addition, Aziz et al. mentioned that the concentration ratios between SWCNT and polymer composition require further research.

Zhang et al. ³⁴ designed a multi-functional e-textile with a printable smart pattern using carbon nanotubes and silk fibroin. It was composed of a conductive core and a dielectric sheath. This smart textile can harvest 18 mW m^− 2 of power from human motion, with 26.42 mF cm^− 2 of capacitance and 0.42 mA cm^− 2 of current density. This development has the potential to be an essential facilitator in using sensors for in-textile health monitoring and other electrical sensors on smart clothes. Because it is 3D printed, it can be designed by the customer, providing great flexibility in its application while still allowing it to be incorporated into large-scale production. The product development is shown in Fig. 2(a).

Gao et al. ³⁵ used carbon nanotubes to create a flexible sensor that analyzes metabolites and electrolytes in sweat and measures skin temperature. It measures the concentration of two sweat metabolites, glucose and lactate, and two electrolytes, sodium and potassium ions. The glucose and lactate sensors were observed to have sensitivities of 2.35 nA µM^− 1 and 220 nA mM^− 1, respectively, and the Na⁺ and K⁺ sensors were observed to have sensitivities of 64.2 mC dec^− 1 and 61.3 mV dec^− 1, respectively. The temperature sensor was found to have a temperature sensitivity of 0.18% °C^− 1 and a range of 20–40°C, which is ideal for measuring the temperature of the skin. The sensor’s mechanical deformation was characterized by radii of curvature of 1.5 cm and 3 cm before and after bending, respectively, with minimal output changes in the device. Although this device was not developed with in-textile capabilities in mind, it has the potential to be incorporated into in-textile products. The product is shown in Fig. 2(b).

Ko et al. ³⁶ used single-walled carbon nanotubes to make a flexible strain sensor that can monitor human motion. It is composed of a brittle conductive silver coating layer and a stretchable conductive layer. The sensor demonstrated high sensitivity, with gauge factors of 570, 3550, 1968, and 1012- for the strain range of 0–1.5%, 1.5–5%, 5–10%, and 10–20%, respectively. It also showed promising stability and durability with a low lag response time. Because of the woven structure, the sensor displayed notable elasticity and was able to measure and detect the angle and motion of a finger. The advantages of this device are that it is easy to manufacture and integrate into clothes. The device is shown in Fig. 2(c).

Afroj et al. ³⁷ coated graphene flakes (a carbon-based nanomaterial) on textile yarn to create a flexible and washable textile sensor. Previously, manufacturing electroconductive textile yarn was unappealing due to its high expense and un-scalability for greater manufacturing; however, Afroj et al. explained the use of a high-speed yarn dyeing technique to overcome this challenge. The sensor was shown to produce high-temperature sensitivity, flexibility, and washability—the latter two characteristics being essential for in-textile applications. A low-power, self-powered RFID/Bluetooth was used to collect and send the data to a smartphone, which displayed the data the yarn collected. The resistance of the sensor decreased at a constant rate as the temperature increased from 20–60°C, which is similar to the results of previous studies on similarly reduced GO films. It also displayed repeatable consistency with temperature sensitivity in the range of 25–55°C. The device is shown in Fig. 2(d).

Table 1

Summary Of Recent Development of Carbon-Based Nanosensors
Research Group	Nanomaterial	Experimental Results	Applications	Pros and Cons
Wang et al. ³²	Carbon nanotube fibers	Electrical conductivity: 102–103 S cm^− 1 Tensile strength: 102–103 MPa; 1000 bends Glucose sensing: 0–200 × 10^− 6 M Sensitivity: 2.15 nA µM^− 1	Electrochemical fabric	Pros: Highly flexible, maintains structural integrity & detection ability. Cons: Glucose oxidase cannot be used for a long time. Safety issue with CNTs.
Aziz et al. ³³	Single-walled carbon nanotubes (SWCNTs)	@25–100°C: Linear & stable Impedance: 2.95–5.8 MΩ Sensitivity: 38 kΩ °C^− 1 Bending movement sensitivity: 90 kΩ deg^− 1	Smart-fabric sensor	Pros: Wearable, affordable, robust, stable, quick response/recovery. Cons: SWCNT and polymer ratios require further research. Can only measure one stimulus at a time.
Zhang et al. ³⁴	Carbon nanotubes	Power density: 18 mW m^− 2 Capacitance: 26.42 mF cm^− 2 Current density: 0.42 mA cm^− 2	Printable in-textile energy harvesting	Pros: Customer designed, 3D printed, harvests energy for health monitoring sensors.
Gao et al. ³⁵	Single-walled & multi-walled carbon nanotubes	Sensitivities: 2.35 nA µM^− 1 (glucose) 220 nA mM^− 1 (lactate) 64.2 mV dec^− 1 (Na⁺) 61.3 mV dec^− 1 (K⁺) 0.18% °C^− 1 for 20–40°C	Perspiration analyzing wearable sensor	Pros: Simultaneous and multiplexed screening of target biomarkers, mechanically flexible, wearable, fully integrated, measures sweat metabolites & electrolytes and skin temperature.
Ko et al. ³⁶	Single-walled carbon nanotubes (SWCNTs)	Gauge factors: 570, 3550, 1968, and 1012 for ranges of strain of 0–1.5%, 1.5–5%, 5–10%, and 10–20%, respectively	In-textile ultrasensitive strain sensor	Pros: Stable, durable, low lag response time, elastic. Able to measure and detect the angle and motion of a finger. Easy to manufacture and integrate into clothes.
Afroj et al. ³⁷	Graphene flakes	20–60°C – sensor resistance decreased 25–55°C – consistent temp. sensitivity	In-textile temperature sensor	Pros: High-temperature sensitivity, flexible, washable, wireless.
Abshirini et al. ³⁸	Multi-walled carbon nanotubes	Total bending & recovery time: 3.5 & 7 s, Piezo resistance change: 6–11% Gauge factor: 4.3	3D printable strain sensor with potential in-textile capabilities	Pro: High sensitivity, linear response under low strain rate. Appropriate for real-time load sensing in highly flexible structures and human motion recognition.
Liu et al. ³⁹	Carbon black nanospheres	Sensitivity: 81.6 kPa^− 1 Pressure range: 0–100 kPa Response/relaxation time: 6 ms/30 ms	Wearable pressure sensor for detecting sound and human physiological activity	Pros: Low cost, efficient recycling of carbon black, high sensitivity, large pressure range, fast response time, ability to detect repeated motion and vibrations.
Park et al. ⁴⁰	Wrinkled carbon nanotubes	Response time: <20 ms Wrinkles provide strain relief and improve pressure sensitivity by 12,800-fold	Flexible piezoresistive pressure sensor for human physiological signals	Pros: Low cost, highly sensitive, flexible, can monitor pulsatile blood flow and voice detection.
Shi et al. ⁴¹	Graphene and carbon nanotubes	CNT films: 90.2% transmittance w/ 550 nm to 87.1% after graphene hybridization Sheet resistance: ~4 kΩ sq^− 1; up to 20% resistance to buckling from the strain	Wearable strain sensor for human motion	Pros: Improves strength and load capabilities of CNT networks, can detect human motions, durable, highly sensitive, fast response.
Cao et al. ⁴²	Carbon nanotubes	Conductivity: 0.2 kΩ sq^− 1 Air permeability: 88.2 mm s^− 1	Washable self-powered in-textile gesture/touch sensor	Pros: Washable, self-powered, highly conductive, sensitive, air permeable, suitable for large-scale manufacturing, resilient to deformation.
Kang et al. ⁴³	Single-walled carbon nanotubes (SWNTs)	Pressure sensitivity: 0.335 kPa^–1 @10 kPa, 0.103 kPa^− 1 @50 kPa The pressure sensor changed 3.29% after 1000 loads (< 10 kPa)	Wearable sensor using hydrogel transfer printing of conductive nanonetworks	Pros: Versatile, measures finger motions, low-cost, demonstrates the ability to transfer print conductive nanonetworks for sensors on different surfaces.

3.2 Metal-based Nano In-textile Sensors

Zhang et al. ⁴⁴ presented a method to connect multiple health sensors using a metal nanoparticle ink that can be printed as a flexible printed circuit board (FPCB) (Table 2). This development shows promising results with the capability of continuous temperature, blood oxygen saturation, humidity, and electrocardiography (ECG) and electromyography (EMG) monitoring and reporting. Unfortunately, Zhang et al. noted that the device would begin to deteriorate at approximately 60°C as well as in water, which will undermine its future candidacy as an in-textile physiological health sensor. Despite this weakness, the Ag nanoparticles demonstrate stable conduction after soaking in water for 100 h. All data regarding the sensors appear to demonstrate accurate readings that predictably change with changing environmental conditions. The FPCB is shown in Fig. 3(a).

Thamizhanban et al. ⁴⁵ demonstrated a trimethylamine sensor fabric using Ag nanoparticles incorporated in glycolipids (Fig. 3(b)). It had a transient response for 1–500 ppm TMA, and a stable response for up to 15 days for 100 ppm TMA. This sensor, intended specifically for monitoring the health of workers in food-processing industries, has the potential to reduce the number of trimethylaminuria cases. An interesting aspect of this device is the use of a self-assembly process involving glucose-silver nanoparticles, which could be applicable to the development of sensors for healthcare, medicine, and other environmental science applications. Another attractive characteristic of this device is that it maintains its accuracy and sensitivity even when encapsulated in a porous thermoplastic film.

Arquilla et al. ⁴⁶ designed an in-textile ECG sensor that integrates ECG electrodes with silver nanoparticle coatings in clothing for daily use (Fig. 3(c)). This development provides a more comfortable and convenient method of measuring ECG signals. Arquilla et al. stated that it could also measure electrodermal activity (EDA), electroencephalography (EEG), and EMG, in addition

to ECG, which are undisturbed during physical manipulation, such as stretching, bending, or washing. Due to the consistency of the sensor, it is posed as a promising product to monitor ECG signals with no statistically significant difference from traditional gel electrodes (R-R interval: t = 1.43, p > 0.1; HR: t = − 0.70, p > 0.5; comfort: V = 15, p > 0.5).

Gong et al. ⁴⁷ developed a transparent gold nanowire mesh electrode using bottom-up self-assembly (Fig. 3(d)). This electrode has mesh pore sizes between 8–52 µm and a sheet resistance approximately 40 times smaller than what Gong et al. had previously reported for non-mesh Au nanowires, suggesting that the mesh concept is more effective. In addition, they discovered that the width and thickness of the mesh/bundle are controllable by changing the time and temperature of the aging process for the material. It was reported that these electrodes were washable and flexible and could be easily transferred onto other substrates. This electrode has great potential to be intermingled with in-textile applications for health monitoring, especially for measuring ECG signals. More research is needed to determine whether the use of gold nanowires or silver nanoparticles is more effective for this application.

Table 2

Summary Of Recent Development of Metal-Based Nanosensors
Research Group	Nanomaterial	Experimental Results	Applications	Pros and Cons
Zhang et al. ⁴⁴	Ag, Ni, Cu, Ni/Ag core-shell NPs	Numerical data are contained within the figures of the article	Printing metal NP ink on human skin for physiological signal monitoring	Pros: Measures temperature, blood oxygen saturation, ECG/EMG, and humidity. Detects signal progression of pandemics. Con: Decomposes at 60°C and in water.
Thamizhanban et al. ⁴⁵	Ag, Cu, and Zn NPs	Transient response: 1–500 ppm TMA Stable response: 100 ppm TMA (for 15 days)	Trimethylamine sensor fabric	Pros: TMA level sensing for health. Sensing performance is stable after encapsulation in thermoplastic films.
Arquilla et al. ⁴⁶	Ag NPs	No significant difference in R-R interval or heartrate between traditional and textile electrodes: R-R interval: t = 1.43, p > 0.1 HR: t = − 0.70, p > 0.5 Comfort: V = 15, p > 0.5	In-textile ECG electrodes for ECG monitoring	Pros: Results are promising, accurate, can detect EDA, EMG, and EEG, no resistance change during stretching, bending, or washing.
Gong et al. ⁴⁷	Au nanowires	Mesh pore size: 8–52 µm Sheet resistance: ~40 times smaller vs. previous	Transparent nanomesh electrode for potential in-textile applications	Pros: Washable, transparent, flexible, suitable for different substrates, can use pen or cutting machine lithography, bottom-up self-assembly, wearable electronics.
Lan et al. ⁴⁸	Au nano popcorn (popcorn shape)	Fabrication time: <5 min Sensitivity: 0.19 kPa^− 1 Response time: 93 ms	Gold nano popcorn for stretchable in-textile pressure sensor	Pros: AuNPC increases performance, can be woven in-textile, stretchable, has fast fabrication & response, is highly sensitive.
Li et al. ⁴⁹	Ag nanocrystals	Response time: 2 mm s^− 1 Recovery time: 4 s Strain change detection: 0.1%	In-textile human motion monitoring/analysis sensor	Pros: Silver is more convenient to synthesize, stable, and highly conductive. Textile-based, stretchable, can monitor human motion, good adaptable strain-electric response, large-scale fabrication.
Guo et al. ⁵⁰	Au NPs	Large strains: 100% Low detection limit: +/-0.09% Responsivity: <12 ms Reproducibility: 6000 + cycles	Optical strain sensor for physiological monitoring (pulse to motion)	Pros: Flexible, stretchable, low-cost, can measure large strain, low detection limit, responsive, durable, minimal hysteresis, MRI compatible.
Yang et al. ⁵¹	Galinstan liquid metal NPs	Open-circuit voltage: 268.06 V Short-circuit current: 12.06 µA Transferred charge: 103.59 nC Sensitivity: 2.52 V kPa^− 1 Stretchability: ~260%	LM nanoparticle-based electrodes for tactile sensing	Pros: Self-powered, stretchable, has great potential in future in-textile sensors and e-skins.
Chen et al. ⁵²	PdAu, Ag, Au, Pt, Pd NPs	Max. output voltage: 500 V Max output current density: 14 mA m^− 2 Max power density: 2.5 W m^− 2 with 20 MΩ resistance	Self-powered lactate detection	Pros: Bendable, lightweight, and portable, high selectivity and sensitivity, durable.
Kim et al. ⁵³	Ag NPs	Resistance: 0.9 Ω cm^− 1 Strain sensing range: 220% Sensitivity: ∼5.8 × 10⁴ High stability after 5000 cycles of stretching deformation	In-textile strain sensor for gesture recognition and posture improvement	Pros: Stretchable, low initial electrical resistance, high sensitivity, large range of strain sensing, minimal hysteresis, stable after 5000 cycles, can improve exercise posture through strain monitoring.
Li et al. ⁵⁴	Liquid metal NPs	Sensing area: 7.07 cm² Effective after 100 wear cycles over 5 days	Textile-integrated liquid metal electrodes (TILEs) for impedance measuring and health monitoring	Pros: Flexible, robust, easy to activate conductivity, reusable, improved wearability, spray-on, comparable to wet electrodes.

3.3 Polymer-based Nano In-textile Sensors

Liu et al. ⁵⁵ demonstrated the application of polyaniline nanofiber, also known as PANI, as an in-textile pressure sensor (Fig. 4(a)) (Table 3). This nanofiber material exhibited high surface roughness, which improved the performance of the sensor, as reflected by its high sensitivity of 46.48 kPa^− 1 and large linear range of < 4.5 kPa. It also demonstrated short response and relax times of 7 ms and 16 ms, respectively, with a 0.46 Pa detection limit, which exemplified the sensitivity of the sensor. Even after 250 cycles of loads, the low-cost pressure sensor developed by Liu et al. was still able to maintain its high sensitivity in detecting both carotid and wrist pulse. Liu et al. stated that this design shows merit in becoming a potential product in wearable health monitoring.

Kinnamon et al. ⁵⁶ developed an in-textile biosensor that detects influenza A virus using nanoporous polyamide (Fig. 4(b)). The study described the device to have a limit of detection of 10 ng mL^− 1 and a linear dynamic range of 10 ng mL^− 1 to 10 µg mL^− 1, enabling it to effectively detect the virus when present. It was suggested that this device be used as a technological method to prevent influenza outbreaks and further implemented in preventing pandemics or tracing the spread of pandemics. Although the results appear promising, it was noted that further research is needed to determine the biosensor’s effectiveness in different environmental conditions, investigate more realistic sampling methods, and establish a method for transferring the biosensor’s measurements to a display.

Gualandi et al. ⁵⁷ proposed a chemical sensor using a conductive polymer known as PEDOT:PSS incorporated as an in-textile OECT sensor (Fig. 4(c)). This sensor could measure active redox molecules from sweat, specifically adrenaline, dopamine, and ascorbic acid. The potentials for the use of this device were under 1 V, which led to the need for direct skin contact—a downfall of the design; however, it proved effective even after repeated washing cycles and mechanical deformations without any apparent degradation to its performance. Furthermore, the absorbed power was approximately 10^− 4 W, which was noted as being considerably low, making it suitable for portable applications. Overall, the performance of this device was comparable to a non-textile OECT sensor, which would greatly facilitate its transition to in-textile capabilities.

Jin et al. ⁵⁸ designed an e-textile physiological sensor that measures ECG, EMG, and physical motions using polyvinylidene fluoride nanofibers (Fig. 4(d)). It was noted that this sensor was highly conductive, up to four times more than the pristine material, due to the silver layer on top of the sensor. The stretchability of this material was 800%, and its cyclic degradation stayed around 0.56 even after 5000 cycles of 50% strain. The initial sheet resistance and conductivity were 0.047 Ω sq^− 1 and 9903 S cm^− 1, respectively, which speak to the effectiveness of this sensor. Electrocardiograms, electromyograms, and other motions can be monitored even when the subject is active without degradation of the readings. This sensor is also impressively stretchable, which may be a positive factor in its outlook.

Table 3

Summary Of Recent Development of Polymer-Based Nanosensors
Research Groups	Nanomaterial	Experimental Results	Applications	Pros and Cons
Liu et al. ⁵⁵	Polyaniline nanofiber	Sensitivity: 46.48 kPa^− 1 Linear range: <4.5 kPa Response time: 7 ms Relax time: 16 ms Detection limit: 0.46 Pa Stable: >250 loading cycles	In-textile pressure sensor	Pros: Flexible, in-textile electrode, highly conductive, high sensitivity, wide range, fast response and relax time, low detection limit, can detect wrist and carotid pulse, low cost.
Kinnamon et al. ⁵⁶	Nanoporous Polyamide	Linear dynamic range: 10 ng mL^− 1 to 10 µg mL^− 1 Limit of detection: 10 ng mL^− 1	Screen-printed in-textile biosensor to detect influenza A virus	Pros: Detects virus before symptoms, predicts potential influenza outbreaks, in-textile. Cons: Response of the sensor to realistic conditions and integration with IoT needs further research.
Gualandi et al. ⁵⁷	Conductive Polymer (PEDOT:PSSS)	Operating potential: <1 V Absorbed power: ~10^− 4 W Comparable to non-textile OECT performance	In-textile wearable chemical sensor	Pros: Low potential during operation and absorbed power, practical for portable/wearable applications, detects active redox molecules, can be deformed. Cons: Requires direct skin contact.
Jin et al. ⁵⁸	Polyvinylidene fluoride (PVDF) nanofibers	Stretchability: 800% Cyclic degradation: 0.56 after 5000 cycles of 50% strain Initial conductivity: 9903 S cm^–1 Sheet resistance: 0.047 Ω sq^–1	Physiological sensor for ECG, EMG, and physical motion	Pros: In-textile, durable, reduced crack growth of material, highly conductive, skintight.
Yang et al. ⁵⁹	Polyvinylidene fluoride nanofiber membrane	Sensitivity: 4.2 kPa^− 1 Response time: <26 ms Detection limit: 1.6 Pa Gurley value: 17.3 s/100 mL	Pressure sensor for physiological and spatial pressure monitoring	Pros: Excellent breathability, very sensitive, fast response, low detection limit, cost-effective, suitable for large-scale production, can monitor heart rate and respiration.
Ma et al. ⁶⁰	Polyaniline nanofiber (PANI)	Sensitivity: 0.04–0.10 kPa^− 1 Durable for 500 cycles Sensing range: 0–20 kPa Response/recovery time: 0.40 s Breaking strength: 25.00 MPa Elongation at break: 19.00%	Flexible mechanical sensor	Pros: Believed to effectively monitor the forces of extrusion, folding, and pressure. Suitable for various applications, like motion detection, human health tests, vocal cord vibration recognition, etc. An effective way to develop health monitoring sensors based on flexible fabrics.
Zhou et al. ⁶¹	Hollow polyaniline spheres (HPS), PANI	Tensile strength: 9.3 MPa Stretchability: >493% Toughness: 2.6 MJ m^− 3 Strain range: 0–300%, GF: 2.9; 300–450%, GF: 7.4 Response time: 0.22 s	Hydrogel pressure sensor for monitoring human motions, physiological activities, and deformations	Pros: Promising as a piezoresistive-type pressure sensor and visualization of pressure. High performance, flexible, applied in monitoring various human motions, physiological activities, and bending/vibration deformations.
Promphet et al. ⁶²	Chitosan (polymer nanoparticle)	In the pH range for human sweat (pH 4.0–7.0) Differentiates between low (< 10 mM) and high (≥ 12.5 mM) lactate levels	Lactate (colorimetric) & and sweat pH sensor	Pros: Able to differentiate the fitness and potential muscle fatigue. High potential for wearable monitoring of human health.
Wang et al. ⁶³	Polypyrrole nanowires	Glucose concentration: 1 nm to 1 µM, current increases; response time 0.5 s. Sensitivity: 0.773 NCR dec^− 1 Standard deviation: 7.1%	Glucose sensor	Pros: Excellent electrical performance, short response time, stability, high sensitivity, selectivity. Compatible with portable/wearable electronics.
Tessarolo et al. ⁶⁴	PEDOT: PSS	Presence of EG: enhances sensitivity up to 50 kPa applied pressure Pristine PEDOT:PSS ink: up to 100 kPa of pressure w/ low sensitivity	In-textile pressure sensor	Pros: In-textile, direct depositing onto fabric, pressure response range can be selective, comfortable, and low-cost.
Peng et al. ⁶⁵	PVDF nanofibers	Crystallinity: 49.57% Piezoelectric voltage constant: 0.4323 mV N^− 1	Piezoelectric PVDF nano yarns for future sensors	Pros: Good softness, the potential for wearable, and flexible sensors, robust, good piezoelectric properties.

Currently, the integration of polymer-, carbon-, and metal-based nanomaterials into in-textile health monitoring sensors is a popular research topic seeing numerous advancements. Many groups of researchers have been investigating the use of self-powered devices incorporating metal nanoparticles, as illustrated by Yang et al. and Chen et al. ⁵¹, ⁵². This idea is essential in the work of developing practical in-textile sensors for monitoring any kind of physiological or environmental factor, and the fact that self-powered electrodes with many of the requisite properties for in-textile sensors have already been developed reflects the strong potential for their future implementation. PVDF, PANI, and PEDOT:PSS are some of the most commonly used materials due to their flexibility. Carbon black nanospheres, proposed by Liu et al., ushered in the idea of using typical waste products such as diesel soot as a base material for developing pressure sensors and may pave the way for more creative ideas on recycling waste materials ³⁹.

Many of the reviewed articles have mentioned the difficulties of integrating electronics with in-textile applications without compromising the properties of clothing that consumers hold as a standard, including breathability and stretchability, to name a few. However, each article has since overcome these difficulties with each of their designs by incorporating a different version of nanomaterial, some being easy to manufacture via 3D printing ⁴⁴, flexible ³², or breathable ⁵⁹. These difficulties aside, other concerns may arise. Wang et al. stated that although the use of glucose oxidase might be extended with the application of non-enzymatic active material else, it will not last long. They also mentioned that the use of CNTs poses a safety risk unless sealing is carefully applied to the fibers ³². Supposing this to be true, this seal must not interfere with the sensor and must maintain the mechanical property standard of the clothing. Aziz et al. noted that further investigation is needed on the polymer composition and SWCNT concentration ratios for better optimization. They also noted that only one stimulus could be detected at a time with the proposed device ³³. Zhang et al. developed an e-textile sensor that can be printed on the skin and onto textiles; however, it was noted that the print would decompose in water at 60°C ⁴⁴. Although this may be viewed as a positive characteristic, it may pose difficulty for washing clothes in hot wash cycles for in-textile applications or hot showers for on-patient applications. Kinnamon et al. discussed the need for investigating a sensor’s responses to more realistic conditions before it can be considered for real-world applications. This would also include testing with IoT wireless reporting platforms ⁵⁶. Above all, one major limitation potentially found with all reporting but most explicitly mentioned by Gualandi et al. is the need for direct skin contact for optimal results ⁵⁷.

An e-textile is regarded as a textile structure permanently integrated, sewn, or attached with electrical and electronic capability^66–68. The e-textile system is complicated theoretically, requiring several inputs from the textile, wearable electronics, and computing to create the hybrid system. Nanoparticle-based e-textiles can be mainly produced with three different methods. First, the conductive and sensitive fibers could be produced by wet-spinning and electrospun ^69,70. Then, fibers could be integrated into textiles by traditional manufacturing methods (i.e., sewn, embroidered, woven or knitted).

Spinning methods are convenient for producing flexible and thin fibers, but it is a challenge to produce high-strength fibers. While the conventional processes, such as embroidery, sewing, weaving, or knitting, require the fiber to have a good mechanical strength to withstand the force during fabrication. Heng et al. demonstrated that twisted fiber with higher thickness and tensile strain could improve the mechanical strength of fibers ⁷¹. On the other hand, the fiber can sustain less strength by modified manufacturing methods, in which the functional fibers are fastened by the normal yarns ⁷².

The coating methods are another process for in-textile sensor fabrication, in which the nanoparticle suspensions are dip-coated, spin-coated, or spray-coated onto textiles to produce conductive sheets ^73–75. These methods are simple, low-cost, and effective. However, the fiber/fabric has a large amount of porous inside that is difficult to produce a uniform coating on its surface as compared to film substrates. With pressure from the roll-to-toll coating process, it could improve the uniformity of coating and generate stable coatings on the textile surface ^30,76.

Besides, the in-textile sensor can be produced by printing (i.e., screen printing, inkjet printing, direct write printing), in which the nanoparticle inks go through the nozzle or mesh and form the accurate geometry onto fabrics ^77–79. These are promising methods for mass production. It needs to study the adhesion, and mechanical properties of the in-textile sensors as the border between nanoparticles and the textile may be weak.

Future directions in research on the implementation of polymer-, carbon-, and metal-based nanomaterials in in-textile sensors for health monitoring may include the use of recycled materials such as carbon black nanospheres ³⁹. This idea may prove promising to not only clean exhaust waste from the atmosphere by repurposing it but also allow the development of highly sensitive fabrics for health monitoring applications.

Another future direction will possibly be a trend towards more use of AuNPC, noted for its popcorn shape, or other nanoparticles that allow fabrication processes to be further enhanced. The fabrication process using metallic MoS₂ nanosheets and grown Au nanostructures produces conductive fibers with the necessary mechanical and conduction properties in less than 5 minutes, which is ideal for the rapid manufacturing of in-textile health monitoring devices ⁴⁸.

The most promising future direction for in-textile health monitoring is eliminating the use of fused batteries by developing self-powered in-textile sensors, a concept proven by Chen et al. Chen et al. developed a self-powered electrochemical system using triboelectric nanogenerators for the detection of lactate concentration in sweat ⁵². With further developments in the use of recycled materials, rapid manufacturing, and self-powered devices in in-textile health monitoring systems, these systems may soon see widespread daily use for medical patients.

Three different nanomaterials have been used to fabricate in-textile sensors: polymer-, carbon-, and metal-based nanomaterials. PVDF, PANI, and PEDOT:PSS are some of the most frequently used polymer-based nanomaterials, proving to be effective in offering flexibility and enhancing the piezoelectric properties needed for electrodes to work sufficiently. SWCNTs are the most popular carbon-based nanomaterial due to their flexibility and the ease with which other materials can be incorporated with SWCNTs. Regarding the use of metal-based nanoparticles, silver nanoparticles are the most common due to their stability, conductivity, and convenience in synthetization. Future directions regarding the implementation of polymer-, carbon-, and metal-based nanomaterials in in-textile sensors for health monitoring may include the use of recycled materials such as carbon black nanospheres.

Competing interests

The author(s) declare no competing interests.

Data availability

All data generated or analyzed during this study are included in this published article (and its Supplementary Information files).

Author Contributions

A.S. and R.L. drafted the work, A.S. and R.L. searched and selected individual articles. L.X. reviewed the work. Z.T.H.T. reviewed the work and provided the funding.

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No competing interests reported.

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New Paradigm of Personal Health Monitoring: Nanomaterial-based In-textile Sensors

Status:

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Abstract

Figures

1. Introduction

2. Review Method

2.1 Search Strategy

2.2 Inclusion and Exclusion Criteria

2.3 Study Selection

3. Review Of Different Sensors

3.1 Carbon-based Nano In-textile Sensors

3.2 Metal-based Nano In-textile Sensors

3.3 Polymer-based Nano In-textile Sensors

4. Discussion

Conclusions

Declarations

References

Additional Declarations

Supplementary Files

Status:

Version 1