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E-mail address: yingyingzhang tsinghua. Use the link below to share a full-text version of this article with your friends and colleagues. Learn more. Flexible and wearable electronics are attracting wide attention due to their potential applications in wearable human health monitoring and care systems. Carbon materials have combined superiorities such as good electrical conductivity, intrinsic and structural flexibility, light weight, high chemical and thermal stability, ease of chemical functionalization, as well as potential mass production, enabling them to be promising candidate materials for flexible and wearable electronics.
Herein, the latest advances in the rational design and controlled fabrication of carbon materials toward applications in flexible and wearable electronics are reviewed. Furthermore, the integration of multiple devices toward multifunctional wearable systems is briefly reviewed. Finally, the existing challenges and future opportunities in this field are summarized. Recently rapid development of flexible and wearable functional electronics has been changing conventional medical diagnosis mode by endowing it with combined features of wearability, comfortability, remote operation, and timely feedback, which promotes the emerging of wearable human activity monitoring and personal health management.
Wearable human activity and health monitoring systems can be utilized for continuous, noninvasive, real time, and comfortable monitoring of vital biometric signs, which provides important clinically related information for disease diagnosis, preventive healthcare and rehabilitation care.
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Various flexible materials, such as advanced carbon materials, metal nanomaterials, and conductive polymers, have been utilized for wearable electronics. Compared to other candidates, advanced carbon materials possess unique superiorities such as good electrical conductivity, high chemical and thermal stability, low toxicity, as well as ease to be functionalized, endowing them with great potentiality for applications in wearable electronics.
Carbon materials in macroscopic forms of fiber, film, and foam are reviewed in particularly considering their applications in flexible electronics.
Finally, the existing challenges and future perspectives in the field of advanced carbon materials for applications in wearable healthcare systems are discussed and proposed.
Higher GF means higher sensitivity. Besides, elastomers with low modulus, such as PDMS and Ecoflex, have been developed as the elastomeric dielectric. In the following, wearable strain and pressure sensors based on carbon materials CNTs, graphene, and other carbon materials in various macroscopic forms are discussed in sequence.
CNTs are promising materials for highly stretchable strain sensors because of their excellent electrical conductivity, extremely large aspect ratio, and outstanding flexibility. Besides, CNT fibers, which can be directly dry spun from superaligned CNT arrays, 79 had also been reported for fabrication of strain sensors.
Graphene, which possesses excellent flexibility and good electrical conductivity, is another promising material for flexible strain sensors. Two categories of graphene materials, including graphene grown by chemical vapor deposition CVD 47 , 53 , 76 , 80 , 81 and graphene derived from exfoliated graphite, 48 , 50 , 51 , 75 , 82 , 83 have been explored for wearable strain sensors.
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Compared to graphene grown by CVD, graphene exfoliated from natural graphite has advantages of mass production and low cost, benefiting for practical applications. In addition, graphene materials derived from GO solutions can be produced into various formats such as fibers, ribbons, films, and sponges, or easily combined with other materials in considering the requirements of the specific applications. From the above, flexible strain sensors based on CNTs and graphene generally show large sensing range but low sensitivity, or high sensitivity but small sensing range.
Combining 2D graphene with 1D conductive nanomaterials such as CNTs and Ag nanowires is an effective approach to improve the performance of the strain sensors based on mono nanocarbon materials. For example, graphite can be deposited on flexible substrates through drawing using a pencil, which can be further used for fabrication of flexible strain sensors.
Besides, graphite powder and carbon black can be combined with polymers to obtain conductive inks that are then used to fabricate flexible strain sensors through printing technique. Carbon materials can also be made from various natural biomaterials, such as silk, cotton, corncobs, 89 and mushrooms.
There also exist some limitations in using natural biomaterials to prepare carbon materials, such as the low carbon yield and the relatively low quality more defects and lower graphitization degree , which usually leads to inferior mechanical property and electrical conductivity.
The hierarchical structures the yarns are actually composed of paralleled or twisted microfibers of the commercial fabrics such as woven fabrics and knitted fabrics play important roles for achieving high performance of strain sensors. Besides, sensing performance of the wearable strain sensors can be tuned by selecting different woven structures, which is desirable for practical applications.
External pressure stimuli would enhance the electron conduction between conductive fillers, which consequently enable the detection of pressure.
Because of the giant tunneling piezoresistance between the interlocked microdomes, the obtained flexible pressure sensors showed a high sensitivity of Except for the utilization of carbon materials as conductive fillers in the polymer matrix, carbon materials with intrinsic flexibility can be combined with hierarchically structured polymer substrates to obtain flexible pressure sensors.
Similarly, graphene films have also been combined with polymers for flexible pressure sensors. Besides, the combination of hierarchically structured carbon with microstructured elastic substrates will lead to further improved performance of the fabricated pressure sensors. Very recently, based on flexible and conductive carbonized silk nanofiber membranes, our group have developed a transparent and flexible pressure sensor that showed a high sensitivity of As illustrated in Figure 5 g, a flexible pressure sensor attached on the fingertip can be utilized to detect varying pressure during the process of picking grapes, suggesting potentiality of flexible pressure sensors for artificial skins.
Body temperature as well as moisture of human skin and exhaled breath are other main vital signs which can provide important information of the personal health condition for medical diagnosis, putting in a request for wearable temperature and humidity sensors for monitoring body temperature and moisture. Here we focus on thermistors, and interested readers can refer to a recent review for the sensing mechanism of other kinds of temperature sensors.
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For example, by utilizing CNT—PEDOT:PSS nanocomposite ink for printing of a thermal response film, flexible temperature sensors have been reported, which show good flexibility and tunable sensitivity with range from 0. Due to the good electrical property of SWCNTs and the thermal motion of the segments of polymers, the soft thermal sensor exhibited good and repeatable thermal sensitivity.
Owing to its outstanding mechanical adaptability, the soft thermal sensor can be attached to a prosthetic hand for applications in soft intelligent robots Figure 6 c. Such flexible sensor can be conformably attached to human body to monitor the skin temperature. As shown in Figure 6 i, when the temperature sensor is attached onto a human neck, the subtle temperature change of the esophagus induced by drinking of hot water can be successfully monitored Figure 6 j.
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Such temperature sensor showed satisfactory performance with high sensitivity of 0. Moisture of human skin and exhaled breath plays an important role in the diagnosis of underlying dermatosis and pulmonary diseases, which makes a request for flexible and wearable humidity sensors.
CNTs have been generally functionalized with oxygenic functional groups , or other active components to achieve good sensing performance of humidity sensors. Assembly of rGO nanosheets is also promising for flexible electronics. An effective and reproducible strategy based on surface chemistry has been reported to assemble rGO ultrathin films for flexible noncontact humidity sensing. When the device was exposed to moist air, the rGO surface would adsorb water molecules through hydroxyl groups.
When the rGO ultrathin film was under the electrostatic field, absorbed water molecules on rGO surface would generate hydronium ions as charge carriers, and correspondingly increase the electrical conductivity of the rGO film, which made the rGO ultrathin film promising for humidity sensing.
Owing to the high sensitivity of the rGO ultrathin film for humidity sensing, flexible patterned humidity sensing matrix based on rGO ultrathin film can be used to detect contactless humidity with fast response and high spatial resolution Figure 7 c. The respiration frequency of a person after different exercises can be monitored by the designed GO film without external power, as demonstrated by the different numbers of voltage output pulses Figure 7 f.
Due to the fact that the humidity sensors are working based on the interaction water molecule and oxygenic functional groups on the carbon materials, the sensors generally exhibit relatively slow response and recovery.
In the future, combing functionalized carbon materials with other active components or exploring new working mechanisms could be feasible strategies to improve sensing performance. Electrochemical sensors are generally composed of three electrodes, namely, a working electrode, a reference electrode, and a counter electrode, among which the working electrode determines the selectivity and sensitivity of electrochemical sensors.
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The working electrode should be electrically conductive and contains active components such as specific chemicals or biological receptors to achieve selectivity and sensitivity. For example, when the working electrode is functionalized with catalysts e.
Figure 8 a. Working electrodes are generally modified with specific ionophore to achieve the sensitivity and selectivity.
More importantly, such potentiometric tattoo sensor can accomplish rapid and nearly instantaneous response to pH variation even under stretched states Figure 8 e , implying its potentiality for wearable applications.
Multiple trace heavy metals such as copper and zinc which can be found in body fluids such as blood, urine and sweat are also related to individual health conditions. Such mouthguard salivary biosensing platform showed prospect for the wearable and noninvasive monitoring and assessing of personal health conditions.
Biocompatibility, stability and interference rejection of contact lens sensors as well as the integration with wireless readout and communication circuits should be further improved, although contact lens sensors exhibit high sensitivity, fast response, and good repeatability.
In addition to flexibility, stretchability of wearable devices is important for the seamless integration with biological tissues because of the soft and stretchable nature of biological tissues. Researchers have recently developed stretchable electrochemical devices via screen printing technique by combining intrinsically stretchable ink or stretchable structure design with stretchable substrates e. As shown in Figure 9 b, the first degree stretching is attributed to the serpentine patterns, and the second degree stretching is ascribed to the intrinsic stretchability of the printed CNT lines.
The synergistic effect of the stretchable CNT lines and the wavy structure endows the printed electrochemical device with high tolerance to extreme multidimensional strains, as demonstrated by the structural integrity of the stretchable device under various mechanical deformations such as linear stretching and torsional twisting Figure 9 c. Except for stretchable polymers as substrates, intrinsically stretchable textile can also be used as substrates for stretchable electrochemical sensors.
Two recent review papers have introduced the strategies and nanomaterials for fabrication of stretchable conductors in detail. Most human body activities, such as heart beating, muscle movement, brain activity, and nerve function, are driven by low levels of electrical current. For example, ECG records the electrical signals induced by heart beats, providing important information about the physical condition of heart and being crucial in the clinical study of cardiovascular diseases.
To eliminate the influence of dehydration and possible inflammation of gel electrolytes, various dry electrodes that can be directly attached onto human skin without gel electrolytes have been developed. Electrical conductivity and mechanical flexibility of the dry electrodes are two vital parameters to determine the quality of measured signals.
Owing to their high electrical conductivity, good mechanical robustness, biocompatibility and high stability, carbon materials have been developed for applications in flexible conductors, , which show great potential as dry electrophysiological electrodes. For example, CNTs are usually dispersed in soft polymer matrix to combine the conductivity of carbon nanotubes and the flexibility of polymer to fabricate flexible and dry electrodes. Considering the uncomfortableness and unreliability caused by the relatively rigid wires, the direct use of conventional electrical wires in wearable electronics is not practical.
Recently, various microscale electrical conducting wires with high conductivity and excellent flexibility have been reported, such as CNT yarns, , graphene fibers, - as well as carbon or metal nanomaterial coated yarns. CNT fibers, which can be fabricated through wet spun and dry spun strategies, possess combined features of high electrical conductivity, high mechanical strength, and lightweight.
The developed CNT and graphene fibers can be used as promising substitutes of traditional heavy and rigid wires for wearable electronic systems. Flexible and wearable power systems are of great significance for the development of wearable electronics for the fact that power supply is indispensable in electronics. Considerable research efforts have been made to develop flexible or even stretchable energy devices including supercapacitors, electrochemical batteries e.
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Supercapacitors, as one kind of the most efficient power devices, have attracted significant attention during the past decades because of their high power density, long cycle life, rapid charging—discharging rate and good safety.
Carbon materials, including CNTs, 27 , , graphene, , - and other carbon materials e. In contrast, the energy storage of pseudocapacitance supercapacitors is accomplished through reversible chemical redox reactions at the surface of active materials.
The developed active materials for pseudocapacitance supercapacitors includes transitional metal oxides e. On the basis of the above, hybrid supercapacitors showing superior performance can be fabricated by combining carbon materials and pseudocapacitive materials, which is ascribed to the synergistic effect of the EDL and pseudocapacitance.
Except for flexibility, stretchability is more important for wearable applications of supercapacitors for the fact that human motions usually induce strain stimuli. The stretchability of 1D supercapacitors can be further improved by structural designing.
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For example, a highly compressible CNT sponge synthesized by CVD has been reported as an electrically conductive substrate to adsorb and in situ polymerize polypyrrole PPy monomers to achieve a core—shell structured CNT PPy sponge that can be utilized for flexible and compressible supercapacitors. In addition, the combination of CNTs with pseudocapacitive materials would lead to flexible supercapactiors with further improved energy density, which is of significance for practical applications.
Figure 15 d shows a stretchable supercapacitor which was fabricated by stacking two conductive textile electrodes into plane configuration with gel electrolytes sandwiched between the electrodes. Besides, the conductive textile can be functionalized with other active materials such as pseudocapacitive conductive polymers to further improve the energy density of the supercapacitor.
Compared to the Na Li —air batteries and Al Mg —air batteries which need nonaqueous electrolytes and strict humidity control during manufacture process and can hardly be electrically recharged, Zn—air batteries are more appealing for practical wearable applications because of their environmental and human benignity, rechargeability, relative low cost and abundant resource.
A typical air cathode is composed of electrocatalytic active components and a conductive substrate which serves as the pathway for oxygen diffusion and electron transfer as well as physical support for electrocatalytic active components. Rational design of air cathode is of great importance for achieving flexibility and superior performance of flexible Zn—air batteries. For typical electrocatalysts in powder, the fabrication of flexible air cathodes generally require electrochemical inactive and nonconductive polymeric binders to immobilize electrocatalysts on the flexible conductive supports.
Carbon materials, based on their high electrical conductivity, good thermal and chemical stability, large surface area, plentiful morphologies and good electrocatalytic activities, have been widely used for applications in the air cathodes of flexible Zn—air batteries.