Anesthesia Gas Sensor

Cite As

Nano-Micro Lett.

And Humidity Sensors

Yibing Luo1, Jianye Li1, Qiongling Ding1, Hao Wang1, Chuan Liu1, Jin Wu1 *

Highlights

• A systematic summary of the research progress of hydrogel-based gas and humidity sensors is presented. • The sensing mechanism of hydrogel-based gas and humidity sensors is elaborated. • The potential of hydrogel-based vapor sensors in different fields of application is demonstrated.

ABSTRACT  Breathing is an inherent human activity; however, the composi- tion of the air we inhale and gas exhale remains unknown to us. To address this, wearable vapor sensors can help people monitor air composition in real time to avoid underlying risks, and for the early detection and treatment of diseases for home healthcare. Hydrogels with three-dimensional polymer networks and large amounts of water molecules are naturally flexible and stretchable. Functionalized hydrogels are intrinsically conductive, self-healing, self-adhesive, biocompatible, and room-temperature sensitive. Compared with traditional rigid vapor sensors, hydrogel-based gas and humidity sensors can directly fit human skin or clothing, and are more suitable for real-time monitoring of personal health and safety. In this review, current studies on hydrogel-based vapor sensors are investigated. The required properties and optimization methods of wearable hydrogel-based sen- sors are introduced. Subsequently, existing reports on the response mechanisms of hydrogel-based gas and humidity sensors are summarized. Related works on hydrogel-based vapor sensors for their application in personal health and safety monitoring are presented. Moreover, the potential of hydrogels in the field of vapor sensing is elucidated. Finally, the current research status, challenges, and future trends of hydrogel gas/ humidity sensing are discussed.

KEYWORDS  Health and safety monitoring; Gas and humidity sensor; Functionalized hydrogel; Wearable sensor; Flexible and

Stretchable Sensor

1 State Key Laboratory of Optoelectronic Materials and Technologies and the Guangdong Province Key Laboratory of Display Material

1  Introduction

With the rapid growth of industrialization and urbaniza- tion, the quality of life has taken a notable leap; however, this consequently increased environmental problems.

Polluting gases, such as ­NO2, ­NH3, CO, and ­H2S, and volatile organic compounds, including benzene, ethanol, and formaldehyde, are emitted into the environment by vehicle exhaust and factories. When their concentration exceeds a certain threshold, which is as low as 1 ppm, this causes indelible harm to human health and the environ-

Ment . Moreover, Flammable And Explosive Gases,

such as ­H2 and ­CH4, pose a direct threat to the safety of

Human Life And Property . To Avoid Underlying

risks, we require devices that can accurately detect the presence and concentration of these toxic, flammable, and explosive gases in real time; thereby, prompting appropri- ate protective measures.

Vapor-sensing devices are often found in the medical

And Health Care Industries . As Human Exhalation

contains a large number of gas and water molecules, moni- toring and analyzing information related to it, such as res- piratory rate and composition of exhalation, can achieve the non-invasive and comfortable early prevention and treatment of certain diseases [21, 22]. For example, the acetone concentration in the breath can be used as a bio- marker for diabetes, and the detection of breath humidity

Enables Respiratory Monitoring . However, Expen-

sive and bulky testing equipment and complex operational requirements, which require testing at a specific location guided by professionals only, limit the accessibility of the technology to ordinary people. Therefore, portable and wearable gas/humidity sensors are better alternatives for the easy and real-time monitoring of health and safety, compared to bulky and expensive large instruments, such as gas chromatographs. In addition, by setting certain alarm thresholds, people can detect toxic, flammable, and explosive gases in their environment to avoid unnecessary danger or detect certain diseases at the early stage to avoid their progression.

For portable and wearable sensors, flexible and wear- able electronic systems have received copious atten- tion because of their huge potential applications in human–machine interfaces, health monitoring, and smart

Skins For Robots . Flexible And Wearable Vapor

sensors can fit directly on human skin or clothing, and

Which Greatly Broaden Their Application . Moreo-

ver, they can achieve real time monitoring of target data, thereby allowing people to determine their surrounding environment and physiological health status anytime and anywhere. In contrast, traditional gas/humidity sensors, such as semiconductor, electrochemical, and optical gas sensors, have limited mechanical deformability, which makes them inherently unsuitable for flexible and wear- able devices. For flexible applications, sensitive materials are commonly integrated on a flexible substrate .

However, devices prepared using this approach have lim- ited strain tolerance (typically ≤ 50%) and poor adapta- tion of materials with different mechanical properties .

Moreover, based on the application of wearable devices, sensors are expected to operate at room temperature, which reduces additional heating power consumption and decreases thermal risks, such as burns. For example, widely studied metal–oxide semiconductor vapor sensors require high temperatures to ensure the sensing operation of their sensitive material, which is contrary to the opera- tion of wearable devices . Therefore, the combined requirements for wearable gas/humidity sensors, such as stretchability and room-temperature operation, prompt the need for a novel sensitive material that can achieve high strength, stretchability, and room-temperature sensibility.

Hydrogel is a gel composed of polymer network chains and a large amount of water molecules with a three-dimen-

Sional Network Structure . It Is Extremely Hydro-

philic and rapidly swells in water to retain a large volume of water without dissolving [51, 52]. The hydrophilic func- tional groups on the polymer backbone endows the water absorption of hydrogels, whereas the crosslinking between the polymer network chains resist dissolution . Thus, hydrogels are permeable to chemical and biological mol- ecules and transparent to light and sound waves, which are liquid-like properties that are attributed to their high-water content. Meanwhile, the crosslinked structure of the poly- mer network chains endows the hydrogels with flexibility and stretchability, similar to that of elastic solids .

Since 1954, when Wichterle and Lim first synthesized hydrogel , it has been widely used in various fields, including food preparation, bioengineering, agricul-

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application requirements, hydrogels can be functional- ized to achieve specific properties and structures . Thus, functionalized hydrogels are more suited to the complex requirements for flexible and wearable vapor sensors. In particular, it allows gas and humidity sensing at room temperature, thereby reducing the risk of explo- sion, while reducing the power consumption. In addition, its deformability allows the sensor to fit more closely to complex curved surfaces, such as human skin, and follow various body movements without mechanical damage. The intrinsic conductivity allows hydrogel-based vapor sen- sors to directly respond to electrical parameters for easier collection and processing. Self-healing ability permits the sensor to heal without changing its response after mechanical damage, thereby extending its lifespan. High transparency and biocompatibility enhance the aesthetic wearability and potential of hydrogel-based vapor sensors for medical applications. Recent studies have reported the use of hydrogels as the sensitive material for gas-sensing applications, such as ­NO2, ­NH3, ­CO2, and ­O2 . In addition, hydrogels have a unique swelling property that makes them naturally sensitive to humidity for humidity sensing. Several studies reported the direct use of hydro- gel-based vapor sensors for practical applications, such as smart masks, electronic skin, respiratory analysis, and wireless monitoring, demonstrating their great prospects for applications in environmental monitoring, medical health, and other fields . In contrast to flexible sensing schemes that integrate sensitive materials in an elastic substrate, the intrinsically sensitive, conductive, and stretchable properties of hydrogels have increased the efficiency and convenience for the preparation of sensors.

By attaching hydrogel vapor sensors to the human skin or integrating them into clothing and accessories, personal health and safety monitoring can be easily achieved with- out additional equipment.

Breathing Is An Inherent Human Activity; However,

the composition of the air we inhale and gas we exhale remains unknown. Using hydrogel as the sensitive mate- rial, flexible stretchable and wearable vapor sensors can acquire the information of the gas content and concentra- tion in the inhaled air and exhaled gas through the con- tinuous testing of ambient air or human exhaled gas at room temperature, thereby realizing the real-time moni- toring of personal health and safety. Currently, reviews have focused on hydrogel research in vapor sensing when introducing recent work on flexible wearable sensors, or

Describing Research On Hydrogels . However, Only

general information on hydrogel-based vapor sensors is provided, and a systematic overview of hydrogel research in gas/humidity sensing and their applications for personal health and safety monitoring is not available. The research on hydrogel-based gas/humidity sensors is increasing, and concerns about health and safety are growing. Therefore, a review focused on the research of hydrogel-based gas/ humidity sensors and their role on personal health and safety is needed. This paper presents a well-rounded dis- cussion of hydrogel-based vapor sensors in terms of their properties and optimization, existing mechanisms and applications for personal health and safety monitoring (Fig. 1). The current research status and problems to be solved for hydrogel-based vapor sensors are summarized.

Finally, the conceivable future research trends for hydrogel gas/humidity sensing are discussed.

2  Design And Optimization Of Hydrogels

Hydrogels are classified based on their properties. Accord- ing to the source, hydrogels can be divided into natural

And Synthetic Hydrogels . Natural Hydrogels, Such

as collagen, chitosan (CS), hyaluronic acid, alginate, gela- tin, elastin, chondroitin sulfate, and heparin, have excellent Fig. 1   Schematic of the functionalized hydrogel-based vapor sensors

© The Authors

biocompatibility, and are used in a wide range of applica- tions, such as bioengineering, medical health, and food

Processing . Nonetheless, Their Unsatisfactory

mechanical deformability, uncontrollable structure, and deg- radation limit their broader spectrum of applications [48, 49]. Meanwhile, synthetic hydrogels with enhanced mechan- ical deformability and controlled structure and degrada- tion, such as polyethylene glycol (PEG), polyvinyl alcohol (PVA), poly(2-hydroxyethyl methacrylate) (PHEMA) and polyacrylamide (PAM), can be used in sensors, actuators,

Soft Robots, And Wastewater Treatment . However,

synthetic hydrogels have lower biocompatibility than that of natural hydrogels, making them unsuited for health-related applications [48, 49]. According to the different methods of crosslinking polymer chains, they can be divided into physi- cally and chemically crosslinked hydrogels . Physi- cally crosslinked hydrogels are formed by various strong/ weak intermolecular interactions between polymer chains and usually have poor mechanical deformability, which limit their applications . In contrast, chemically crosslinked hydrogels are crosslinked by covalent bonding between polymer chains, resulting in superior mechani-

Cal Deformability . However, The Degradation Of

biocompatibility owing to the residual chemical crosslink- ers, organic solvents, and photoinitiators in chemically crosslinked hydrogels limit their usage in biomedical-related applications.

Based on the practical application of wearable vapor sen- sors, large deformability, long-term stability, conductivity, self-healing, high responsiveness, and biocompatibility are desired. As different hydrogels have their own advantages and disadvantages, simple hydrogels cannot meet these complex requirements for stretchable and wearable vapor sensors. Hence, simple hydrogels should be optimized and modified to meet practical application demands. In the recent decades, important advances have focused on the preparation of hydrogels with the modulation of their properties, such as porosity, biocompatibility, electrical con- ductivity, and mechanical deformability, by different design and optimization approaches to meet different application requirements. In this section, we will briefly introduce some inherent and functionalized properties of hydrogels, and the corresponding design and optimization approaches from the application requirements of flexible wearable vapor sensors to provide possible theoretical support for the design and preparation of gas/humidity sensors for personal health and safety monitoring.

2.1  Mechanical Deformability

Flexibility and stretchability are important requirements for hydrogel-based wearable vapor sensors. These properties enable the sensor to tightly fit to irregular surfaces, such as human skin, thereby ensuring the user’s comfort while wear- ing the sensor. In addition, the sensor can stretch without being damaged with body movement. During usage, vari- ous degrees of mechanical deformation or even mechanical damage are generated in the actual operation. This requires the flexibility and robust mechanical properties of the sen- sor, while maintaining its high sensitivity. However, conven- tional hydrogels have low mechanical deformability with a small fracture energy, low elastic modulus, and brittleness, which does not meet the large mechanical deformability demand for flexible and wearable vapor sensors .

Thus, various technical approaches need to be introduced to enhance the toughness and stretchability of hydrogels for practical applications.

The direct crosslinking of polymer chains or free polym- erization of vinyl monomers with crosslinkers is the con- ventional approach to prepare hydrogels. Nonetheless, the reflective activity differences between the species leads to the fracture of the hydrogel owing to the local stress concen- tration under external forces. The uneven crosslinking den- sity between polymer chains, varying chain lengths between crosslinking points, and presence of dangling chains and rings are the factors that contribute to the localized stress

Concentration . Therefore, Increasing The Homoge-

neity of hydrogel and avoiding local stress concentration can improve the mechanical deformability of hydrogels. Sakai et al. designed a homogeneous tetra-polyethylene glycol

Hydrogel . They Synthesized Symmetric Tetrahedral

macromolecular monomer phases with the same PEG arm length, as shown in Fig. 2a. The two monomers were ter- minal tetraamine polyethylene glycol (TAPEG) and termi- nal tetra-NHS polyethylene glycol (TNPEG). By adjusting the stoichiometric ratio of the two monomers, a series of hydrogels is prepared. The hydrogels prepared with a stoi- chiometric ratio of 1:1 had a maximum compressive strength of 2.5 MPa, which was five times higher than that of con- ventional agarose or acrylamide gels. In their subsequent

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studies, the importance of the homogeneity of polymer crosslinking in enhancing the mechanical deformability of hydrogels was demonstrated [116, 117].

The mechanical deformability of hydrogels can also be enhanced by introducing mechanical dissipation mecha- nisms. Several microcracks and nanocracks are inevita- bly introduced during the preparation and processing of hydrogels. These microcracks and nanocracks can eas- ily be propagated under stress application, resulting in fracture of the hydrogel at the macroscopic level. With the introduction of a mechanical dissipation mechanism, the cracks in hydrogels are inhibited. A double-network (DN) structure is a common approach for introducing a mechanical energy dissipation mechanism. The dual net- work is composed of polymer network chains with oppo- site properties [104, 118–122]. One polymer network is composed of a soft, ductile, and loosely crosslinked polymer with high molecular weight, whereas the other polymer network is composed of a rigid, brittle, and densely crosslinked polymer, as illustrated in Fig. 2b.

During the stretching of DN hydrogels, the mechanical energy is dissipated by sacrificing the breakage of chemi- cal bonds in the densely crosslinked polymer network, or by the perturbation of non-covalent interactions, thereby improving the mechanical deformability of the hydro-

Gel . Wu Et Al. Used A One-Pot Method

to prepare PAM/Carrageenan (Carr) DN hydrogels for ­NO2 and ­NH3 sensing. The DN hydrogel has an elastic modulus of 299 kPa, which is four times higher than that of a single-network Carr hydrogel and 33 times higher than that of a single-network PAM hydrogel. This implies that the deformability of DN PAN/Carr hydrogels is sig- nificantly improved and can be adapted to the needs of flexible wearable vapor sensor applications.

Fig. 2   a Schematic of the synthesis of tetra-PEG hydrogel. Reproduced with permission . Copyright 2008, American Chemical Society. b Schematic of the synthesis of DN hydrogels. c Schematic diagram of the synthesis of clay NPs/PNIPAMNC. Reproduced with permission .

Combining The Two Toughening Mechanisms Men-

tioned above is a good approach to improve the mechani- cal deformability of hydrogels. Haraguchi et al. pro-

Posed Nanocomposite (Nc) Hydrogels , As Shown In

Fig. 2c. They prepared clay nanoparticles (NPs)/poly(N- isopropyl acrylamide) NC (PNIPAMNC) hydrogels using a NIPAMNC monomer and well-dispersed clay NPs. Mul- tiple polymer chains are linked to a single clay NP under various non-covalent interactions, such as electrostatic and coordination interactions. The homogeneous disper- sion of the clay NPs in the flexible network alleviated the problem of localized stress concentration, whereas the non-covalent interactions between the clay NPs and polymer chains interfere with the dissipation of mechani- cal energy. By the superposition of the two toughening effects, the hydrogel could reach a tensile strength of 109 kPa and elongation of 1.424%.

2.2  Conductivity

Conductivity is an important characteristic of hydrogel- based vapor sensors. The change in electrical parameters as the response signal makes the subsequent signal collec- tion and processing easier and more convenient. Thus, the conductivity of the hydrogel has notable effects on its gas/ humidity response performance. In particular, modulating the conductivity of the hydrogel according to the actual usage requirements can effectively improve the response characteristics of the vapor sensor. In addition, hydrogels with intrinsic conductivity have high flexibility, stretchabil- ity, and biocompatibility, which allow them to meet certain special application scenarios. The inherent stretchability eliminates the need for hydrogels to integrate additional substrates for the fabrication of sensors, thereby avoiding a series of complex mechanical engineering and laminat- ing processes, and unsuitable fit after lamination. However, traditional hydrogels are not inherently conductive. Thus, different special hydrogel treatments are required to modu- late this property.

As shown in Fig. 3, conductive hydrogels can be divided into electron- and ion-conductive hydrogels, according to their conductive modes. Direct crosslinking of conduc- tive materials is used to synthesize electrically conductive hydrogels, as shown in Fig. 3a. Common conductive mate- rials such as graphene, carbon nanotubes, and conductive polymers like polyaniline (PANI), polypyrrole (PPY), and poly(3,4-ethylenedioxythiophene) (PEDOT) are commer- cially available . They can be directly crosslinked by doping molecules to form conductive hydrogels. Reduced graphene oxide hydrogel (RGOH) is synthesized from gra- phene oxide using a one-pot hydrothermal self-assembly method by Wu et al. . This hydrogel is electronically conductive, and can be used for ­NO2 and ­NH3 sensing.

Fig. 3   Main approaches for producing conductive hydrogels. Reproduced with permission . Copyright 2020, Royal Society of Chemistry

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Another common method is polymerization and gelation of the hydrogel precursor solution with conductive fillers, such as metal NPs, carbon nanotubes, and conductive polymers, as depicted in Fig. 3b . In addition, electron-con- ductive hydrogels can be prepared using a non-conductive hydrogel as the substrate carrying a conductive polymer precursor, as depicted in Fig. 3c, in which the conductive polymer polymerizes in situ to form an interpenetrating net- work conductive hydrogel with uniform density .

In existing vapor sensor research, ion-conductive hydrogels are more commonly used as sensing materials. This type of hydrogels achieves their conductive properties with the addi- tion of salt solution with high concentration, thereby intro- ducing a large number of free ions (Fig. 3d) [127, 146, 147].

The ionic conductivity of hydrogels is commonly enhanced by preparing polyelectrolyte networks or constructing ion channels in hydrogel networks [148, 149]. Wu et al.

used the salt infiltration method to obtain ion-conductive hydrogels by adding calcium chloride ­(CaCl2) to PAM/Carr DN hydrogels. The addition of ­CaCl2 enhanced the conduc- tivity, inhibited the dissolution of ­NH3 and other gases, and improved the ­NO2 selectivity of the hydrogel. In addition, it promoted the redox ­NO2 reaction in the hydrogel and elec- trode, which enhanced the response characteristics of the hydrogel.

2.3  Anti‑Freezing And Water Retention Abilities

The operation of wearable vapor sensors follows the work and living environment of the wearer; thus, they are likely to be subjected to complex environments, such as sub-zero temperatures, or harsh or dry environments. However, traditional hydrogel materials do not have this capability.

At low temperatures, the water molecules in the hydrogel tend to condense into ice, which eventually leads to freez- ing, stiffness, brittleness, and loss of working ability of the hydrogel. In low-humidity or high-temperature environ- ments, the water molecules in the hydrogel easily volatilize and evaporate, resulting in the excessive water loss in the hydrogel, making them easy to crumple, dry, harden, and lose its working ability. Therefore, to achieve hydrogel-based vapor sensor with proper operation for different complicated environments, its resistance to freezing and dehydration is an important parameter.

The encapsulation of hydrogel sensors with elastomeric materials is an effective approach to prevent the evaporation of water from hydrogels [151, 153, 154]. Yuk et al.

have prepared a hydrogel–elastomer hybrid with an extremely strong interfacial and functional microstructure, inspired by the structure and function of mammalian skin, as shown in Fig. 4a.

The hybrid retains stretchability, while enhancing the water- retention capacity of the hydrogel, thereby enabling it to work in dry environments. Despite the improvement of the water- retention capacity by isolating the hydrogel from the external environment with elastomer encapsulation, this method has minimal effects in lowering the freezing point of the hydrogel to improve its freezing resistance. Moreover, the additional elastomer encapsulation limits the application of hydrogels for humidity and gas sensing. Another common method to enhance the freezing resistance and water storage capacity of hydrogels is the addition of hygroscopic salts, such as sodium

Chloride And ­Cacl2 . Wu Et Al. Added Lithium

bromide (LiBr) to PAM/Carr hydrogels using the immersion method, which significantly inhibited dehydration and freez- ing (Fig. 4b). The PAM/Carr hydrogels without and with LiBr were simultaneously placed at a temperature of 25 °C and relative humidity (RH) of 43%. As illustrated in Fig. 4b2, the volume of the hydrogel without LiBr decreased significantly and became fragile after 48 h, whereas the hydrogel with LiBr has minimal variations with enhanced water storage capac- ity. In addition, the freezing point of the hydrogel decreased from − 13.7 to − 43.7 °C after the addition of LiBr, as shown in Fig. 4b3, which indicates the improved freezing resistance.

In addition, the organohydrogel can be obtained by replacing a portion of the aqueous solution in the hydrogel with an organic solution to improve the defects of conventional hydrogels, such

As Cold And Dryness Resistance . Meanwhile, This

method can effectively improve the mechanical stretchability and multi-stimuli sensitivity of hydrogels. Ding et al. used a facile one-pot polymerization method to prepare PVA/cel- lulose nanofibril (CNF)–glycerol (Gly) DN organohydrogel with significantly improved water storage and freezing resist- ance (Fig. 4c), which could carry out normally after placed at − 20 °C for a long time.

© The Authors

Fig. 4   a Encapulation: a1 Schematic diagram of elastomer encapsulated hydrogel; a2 Variation of the weight ratio of hydrogel and hydrogel- elastomer hybrid with time. Reproduced with permission . Copyright 2016, Springer Nature. b Adding hygroscopic salts: b1 Schematic structure of LiBr-PAM/Carr DN hydrogel; b2 Trends in weight of hydrogels with different concentrations of LiBr content over time under room temperature drying environment; b3 DSC spectra of these hydrogels. Reproduced with permission . Copyright 2022, Springer Nature. c Organohydrogel: c1-c2 Schematic diagram of PVA/CNF-Gly preparation method; c3 DSC spectra of the organohydrogels with different CNF contents; c4 Weight trends over time for organic hydrogels with different CNF contents under high temperature and dry conditions. Reproduced

2.4  Self‑Healing Ability

Self-healing capability refers to the ability of a material to repair and restore its original properties after being dam- aged. Although we can improve the mechanical deform- ability of hydrogel through various toughening methods for wearable vapor sensors, the devices still exhibit mechanical damage owing to various factors. If a sensor has the self- healing ability to recover and restore its original operation even after being mechanically damaged, it can extend its lifespan, and improve its reliability and durability.

The self-healing properties of hydrogels can be classified into physical and chemical self-healing, according to dif- ferent mechanisms. Physical self-healing is achieved by the polymer chain reconfiguration under reversible non-covalent interactions, such as hydrogen bonding, hydrophobic inter- actions, metal–ligand coordination, host–guest interactions, and combinations of multiple intermolecular interactions

. In Contrast, Chemical Self-Healing Is Achieved

by the dynamic covalent chemistry in hydrogels, including native boron ester complexation, Schiff bases, acylhydrazone bonds, disulfide bonds, and other dynamic chemical bonds and reactions . The general reversible interactions in physical and chemical self-healing are shown in Fig. 5.

The self-healing properties of hydrogels can be achieved by designing specific molecular structures. For example, introducing more hydrogen bonding donor/acceptor groups, such as –OH, –NH2, and –COOH, can increase the self- healing ability of hydrogels by reversibly forming hydrogen bonds when the hydrogel is mechanically damaged. Zhang et al. used the freezing/thawing method to prepare a PVA hydrogels with self-healing ability against mechanical damage at room-temperature. This self-healing process can occur without additional stimuli or healing agents. Instead, self-healing occurred when the cutting surface possesses a sufficient amount of free hydroxyl groups and a sufficiently high rate of chain migration to the cutting surface. Hence, adding more hydrogen bonding donor/acceptor groups, such as hydroxyl groups, can promote the self-healing of hydro- gels. In addition, self-healing can be achieved by the hydro- phobic part of hydrogels by the autonomous loading between hydrophobic structural domains/conjugation, metal–ligand coordination by the reversible chelation between ligands and metal atoms, and host–guest by combining interac- tions with different binding affinities . Therefore, these relevant functional groups can be introduced into the hydrogel to realize their physical self-healing. Furthermore, these interactions can be combined to form multiple inter-

Molecular Interactions . Li Et Al. Synthesized

a self-healing ABA triblock copolymer hydrogel. The recon- struction of the catechol-mediated hydrogen bonding and aromatic interactions allow the hydrogel to withstand high strain, repeatability, and rapid nondestructive self-healing.

Deng et al. prepared a covalent dynamic gel using bis-acylhydrazine functionalized poly(ethylene oxide) ­(A2) and tris[(4-formylphenoxy)methyl]ethane ­(B3) as raw materials. This hydrogel formed amide bonds through the dynamic chemical reactions between aldehyde and hydrazine groups to achieve self-healing under microacidic conditions, resulting in chemical self-healing, of which the acylhydra- zone bond is a common reversible interaction. However, the acylhydrazone bond can only undergo dynamic reversible reactions under microacidic conditions; thus, its applicable pH range can be broadened by introducing other dynamic covalent bonds. For example, self-healing hydrogels can be achieved by introducing corresponding functional groups, such as phenylboronic acid and diol, which can undergo a dynamic chemical reaction to generate a phenylboronic ester complexation; thiol/disulfide bond exchange reaction that form a dynamic covalent bond disulfide bond; and the amine bond Schiff base formed by the nucleophilic attack of the amine on the aldehyde group as a dynamic covalent bond

. By Utilizing The Dynamic Reaction Of Aminated

gelatin, adipic acid dihydrazine, and oxidized dextrose, Chen et al. synthesized a self-healing hydrogel. The dam- aged hydrogel can recover its original properties under the dynamic action of imine and acylhydrazone bonds. Moreo- ver, dynamic reversible reactions, such as the Diels–Alder

© The Authors

(DA) and reversible radical reactions, can achieve chemical self-healing .

2.5  Other Properties

In addition to the properties mentioned in Sects. 2.1–2.4, other properties, such as biocompatibility, biodegradation, self-adhesiveness, and transparency, broaden the applica- tion of wearable hydrogel-based vapor sensors. Hydrogels with biocompatibility are suitable for biological and medical applications because they are non-toxic and non-damaging to human tissues. During the use of the sensors, the health and safety of the human body are ensured to accomplish special requirements. Zhou et al. synthesized a bio- compatible hydrogel using extracellular matrix-derived biopolymer gelatin and chondroitin sulfate. The hydrogel also has self-healing and adhesive properties that can be used to seal or reattach ruptured tissues in surgical proce- dures, such as by injection. Liang et al. used PAM and CS to prepare a PAM–CS DN hydrogel-based ­O2 sensor.

This hydrogel-based ­O2 sensor has promising application on wounds for real-time monitoring of ­O2 concentration and promoted wound healing owing to the excellent biocompat- ibility, biodegradability, and antimicrobial properties of CS, and hydrophilic and bioinert nature of PAM.

Biodegradability is a desirable property for the specific applications of sensors, in response to the trend of green development. Hydrogels with degradability can degrade spontaneously or on demand after completing a response task. Gao et al. demonstrated the successful synthe- sis of supramolecular polymer-reinforced hydrogels with biodegradable properties using poly(N-acryloyl 2-glycine) (PACG) and gelatin methacrylate (Gelma) as raw materi- als by photoinitiated polymerization. The hydrogel has a high tensile strength (1.1 MPa), high compressive strength (12.4 MPa), large Young’s modulus (320 kPa), and high compressive modulus (837 kPa). The biodegradability of the hydrogels can be regulated by adjusting the PACG/ Gelma ratio. Using black phosphorus (BP) nanosheets and thermosensitive hydrogel [poly(D,L-lactide)-poly(ethylene glycol)-poly(D,L-lactide) (PDLLA–PEG–PDLLA:PLEL)] as raw materials, Shao et al. prepared a novel photother- mal therapy (PTT) system for the postoperative treatment of ex-vivo biocompatibility, and can gelate rapidly into a film under near-infrared irradiation, which suitable for the PTT removal of tumor tissues after tumor resection.

Self-adhesion is another important property for wearable hydrogel vapor sensors. This enhances the adhesion of the sensor to target substrates, such as clothing and skin with- out using additional adhesives. Strong adhesion allows the sensor to fit more closely to substrates, such as skin, to bet- ter collect target stimuli. The strong adhesion of hydrogels requires the interaction of chemistry, connection topology, and dissipative mechanics. Liang et al. synthesized a series of hydrogels with good adhesion properties. They achieved self-healing as well as on-demand solubility of the hydrogels using biodynamic bonding cross-linking of ferric iron, protocatechaldehyde containing catechol and aldehyde groups, and quaternized CS (QCS). In addition, the hydro- gel is injectable and biocompatible with antibacterial and hemostatic characteristics, which enable its application in treating skin incisions and wounds. Inspired by plants, Gan et al. developed a plant-derived adhesive hydrogel.

The hydrogel was constructed with Ag–lignin NPs, pectin, and acrylic acid as the raw materials for a dynamic catechol redox system based on the dynamic redox reaction of cat- echol triggered by lignin NPs. The hydrogels demonstrated long-term reproducible adhesion owing to the constant gen- eration of catechol moieties in the redox system, high tough- ness, and antimicrobial properties, which highlight their applicability in surgical or other biomedical applications.

High transparency allows the invisibility of hydrogel devices for its use in optoelectronic devices, commercial electronics, and military equipment. In addition, transparent sensors are more comfortable and aesthetically pleasing for people to wear on a daily basis. Therefore, transparency is another important characteristic of wearable hydrogel-based vapor sensors. Han et al. synthesized transparent, stretchable, adhesive, and conductive hydrogels by the in- situ polymerization of polydopamine-doped PPy nanofibers in a PAM network. The hydrogel can achieve a transmittance of more than 70% of the visible spectrum even at a thickness of more than 1 mm, and can meet the requirements of vari- ous bioelectronic devices, especially human–machine inter- faces. With acrylamide and PVA as the primary materials, and using N,N-methylenebisacrylamide (MBAA) crosslink- ing, Ge et al. prepared a DN conductive hydrogel with high stretchability (> 500%), high transparency (> 90%), good adhesion, and biocompatibility. The high sensitiv- ity of pressure of 0.05 ­kPa−1 under 0–3.27 kPa enables its

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successful application in the detection of speech signals, airflow, and limb movements.

3  Hydrogel‑Based Gas Sensors

In daily work and life, people are constantly exposed to unknown gases and can even step into dangerous environ- ments with irreversible consequences if they are not careful.

To address this, portable and wearable gas sensors are used to act as a second sense to accurately identify dangerous gases that cannot be easily distinguished, detect risks, and avoid dangerous areas, thereby avoiding damage to human health and safety. In addition, wearable gas sensor can be used in the medical field to achieve non-contact diagno- sis and treatment of various diseases, such as diabetes and asthma.

Currently, gas sensing, including ­NH3, ­NO2, ­O2, and ­CO2, has been achieved using hydrogels as gas-sensitive materi- als. For this application, hydrogels exhibit flexibility and stretchability to ensure the comfort of the users, which real- izing real-time and reliable monitoring of the concentration of target gases at room temperature, thereby offering long- term protection for health and safety. In addition, the unique self-healing ability of the hydrogel allows the sensor able to repair itself under mechanical damage during use and extend its applicable life. Thus, hydrogel-based gas sensors have been used in various fields from environmental moni- toring, health care, personal safety protection, to smart life, which demonstrate their promising applications. This sec- tion reviews the recent research progress of hydrogel-based gas sensors with a brief explanation of their gas-sensitive mechanisms based on existing studies. At the end of this section, the performance parameters of hydrogel-based sen- sors are compared with those using other materials with ­NO2 and ­NH3 as target gas examples (Table 1) to highlight the advantages and potential of hydrogel as a sensitive material for flexible and wearable gas sensors.

3.1  Gas‑Sensing Mechanism

Compared with traditional semiconductor- and electrochem- ical-based gas sensors, conductive hydrogel-based gas sen- sors are still in its infancy, and their mechanisms are still unclear. Nonetheless, various studies are trying to unveil the mechanism behind hydrogel gas sensing, which have made a significant contribution to the clarification of their sensing mechanisms.

Hydrogel-based gas sensors can function as essential elec- trochemical sensors. As shown in the Fig. 6a, a simple gas sensor is formed by adding one electrode at the ends of a hydrogel. In this case, the hydrogel serves as an electrolyte that carry different conductive particles, whereas the elec- trodes at each end are the sensitive (working electrode) and counter electrodes, respectively. A constant DC voltage is applied at the two electrodes, and the current and other elec- trical parameters between them are monitored in real time.

As the sensor is exposed to a certain concentration of the tar- get gas, a redox reaction occurs between the gas molecules and electrode at the interface of the sensitive electrode and hydrogel. Depending on the target gas, electrons are released to or captured by the electrode, resulting in a change in the current or other electrical parameters between the electrodes, thereby generating a response signal. The amplitude of the response signal further changes with the concentration of the target gas.

Recent studies have validated the sensing process of hydrogel gas sensors [67, 69, 150, 206]. Take hydrogel- based ­NO2 gas sensor as an example, Wu et al. elabo- rated its sensing mechanism and further verified it through experiments. As shown in Fig. 6b, the response reaction of ­NO2 occurs at the cathode-hydrogel junction, where ­NO2 is reduced and ­OH− ions are consequently produced in the hydrogel with the same number of consumed elec- trons, thereby increasing the current. In addition, the Ag electrode is oxidized (anode) to produce AgCl, which allow more electrons and ions to participate in the conduction and increase in current. To verify this conjecture, they designed an electrode-masking experiment, as shown in Fig. 6d. The cathode or anode electrodes were separately covered, and their respective response change curves were collected (Fig. 6e). If the anode is covered, the sensor continuously responds, whereas if the cathode is covered, the sensor does not produce sensing signals for ­NO2. Further, after the response reaction, the Ag electrode was observed under a scanning electron microscope (SEM). Figure 6c shows an additional layer of AgCl added to the anode electrode, which reflects the reliability of the previous deduction from the side. In addition, they investigated the effect of the number of Ag electrode turns on the ­NO2 response. Under the same temperature and humidity, the ­NO2 response of the sensor with five turns of the Ag electrode was significantly greater

© The Authors

than that with one turn, as depicted in Fig. 6f. This result confirms the significant influence of the electrodes on the sensing reaction of hydrogel-based gas sensors, as an elec- trochemical reaction.

To further investigate the effect of the electrodes on the sensing performance of hydrogel-based gas sensors, Wu et al. tested the ­NO2 response of different electrode materials on the same hydrogel substrate in another study.

With Ag as the cathode material and Ag or CuSn as the anode material, they tested the change of the sensor response to ­NO2 under different anodes, respectively. Figure 6g illus- trates the increased sensitivity from 31.18 to 60.02% ­ppm−1 when anodic Ag was replaced with CuSn alloy owing to the stronger oxidation tendency of Cu and Sn; thus, the electrode material has a strong influence on the gas-sensing performance. In addition to the increased sensitivity, the CuSn anode displays better corrosion resistance than that with the Ag anode, leading to a prolonged lifespan.

Ambient humidity has a great impact on the response characteristics of hydrogel-based gas sensors. In low-humid- ity environments, the conductive channels in the hydrogel are not connected, and the sensing loop is equivalent to that of a broken circuit, which prevents further response reaction with the target gas. As the ambient humidity is gradually increased, the conductive pathway in the hydrogel is con- nected, and the target gas can further react with the hydrogel and electrode. Zhang et al. explored the role of RH in the sensing properties of hydrogel-based gas sensors. Taking ­NH3 as an example, they used poly-L-aspartic acid (PAA)/L- glutamic acid (GA) composite hydrogels as the research Table 1   Sensing performance of hydrogel-based and material based other ­NO2/NH3 sensors C: Concentration; Res./Sens.: response or sensitivity; ­tres/trec: response time/recovery time; D: deformability; Temp.: operating temperature; RT: room-temperature; ­Za: the impedance modulus in the air; ­Zg: the impedance modulus in the target gas; ­Ra: sensor resistance in air; ­Rg: sensor

Temp

Ref.

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Fig. 6   a Schematic of a simple hydrogel-based gas sensor and its sensing process. Reproduced with permission . Copyright 2023, Wiley‐VCH. b Schematic of the sensing mechanism of hydrogel for ­NO2 sensing. c SEM image of the Ag anode after prolonged ­NO2 sensing, indicating the generation of an AgCl layer. d Schematic of the electrode-masking experimental setup. e Real-time current curves of the hydrogel sensor for different ­NO2 concentrations when the anode or cath- ode is encapsulated. f Dynamic response curves of the hydrogel sensors with different numbers of Ag electrode turns to different ­NO2 concentrations. Reproduced with permission . Copyright 2021, Wiley‐VCH. g Effect of anode material change on the sensitivity of hydrogel-based ­NO2 sensors. Reproduced with permis-

© The Authors

object. The hydrogels were placed in environments with dif- ferent humidity and ­NH3 concentrations, and their complex impedance maps were tested from 20 Hz to 20 MHz at 1 V.

Their corresponding equivalent circuits were provided. At low-humidity values (10–50% RH; Fig. 7g), no proton-con- ducting channels were noted in the hydrogel-sensitive film, and its impedance did not change significantly even with the introduction of ­NH3 (Fig. 7a–b). At moderate humidity val- ues (Fig. 7h), hydrophilic groups, such as carboxyl, amino, and amide groups, absorb water molecules to form a hydro- gen bonding network and proton conduction pathways. The introduced ­NH3 can combine with water molecules through hydrogen bonding, which are adsorbed on the sensitive membrane, thereby affecting the impedance of the device (Fig. 7c). At high-humidity values (Fig. 7i), the hydrophilic hydrogel-sensitive membrane adsorbs more water molecules and is activated. The water molecules are self-ionized into ­H+ and ­OH−, with some of the ­H+ ions combining with other water molecules to form ­H3O+ under the grotto mechanism (Eq. 1). In addition, the sensitive membrane exhibits ionic conductivity. When ­NH3 is introduced, the hydrogel-sensi- tive membrane takes up a large amount of ­NH3 by acid–base Fig. 7   a-d Complex impedance plots of the PAA/GA sensor at different humidity values with or without 50 ppm ­NH3. e Complex imped- ance plot of the PAA/GA sensor with different ­NH3 concentrations at 80% RH. f Impedance modulus of the PAA/GA sensors with and without 50 ppm ­NH3 at different humidity levels. ­NH3 sensing mechanism of PAA/GA: g at low RH values, h at moderate RH values, and i at high RH values. Reproduced with permission . Copyright 2021, Elsevier

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adsorption. On the one hand, ­H2O reacts with the dissolved

­Nh3 To Form ­Nh4

+ under a proton transfer reaction (Eq. 2). Furthermore, the carboxyl group in GA reacts with ­NH3 to

+ And ­Rcoo− In The

liquid phase (Eq. 3). Under the combined effect of multiple components, the resistance of the hydrogel-sensitive mem- brane decreases, and its conductivity increases, producing a strong response signal (Fig. 7d). Therefore, humidity is a key factor for the ­NH3 adsorption and response of hydrogels.

In particular, as the humidity increases, the ­NH3 adsorption,

+ And ­Oh− Ions Increases, Thereby

achieving more pronounced changes in the conductance of the hydrogel. The research on the response mechanism of hydrogel- based gas sensors is still in its initial stage. Thus, the effects of the electrode materials, properties of the hydrogel, and external environment on the gas response of the hydrogel need to be further investigated by designing reliable experi- ments. We are anticipating the increased study on this aspect with more researchers joining the exploration and investiga- tion of the hydrogel gas-sensing mechanisms in the future.

3.2  No2 Gas Sensors

NO2 is one of the most studied gases in current hydrogel- based gas sensors. It is a reddish-brown gas with an irritat- ing odor and is often produced by burning fossil fuels. It is extremely toxic, as little as 1 ppm is sufficient to cause head- aches, breathing problems or to cause eye, nose or throat infections. When the concentration of ­NO2 exceeds 20 ppm, it can even be directly harmful to human health . There- fore, it is significant to develop high-performance and low- cost wearable ­NO2 sensors to protect people’s daily health and safety. RGOH is an electronically conductive hydrogel material with a three-dimensional porous structure, and its large specific surface area enables it to adsorb more gases, which is very favorable for gas sensing. With the aim of real time monitoring of ­NO2 and ­NH3, Wu et al. developed

4 + Rcoo−

a hydrogel-based gas sensor with detection limit as low as 4.1 ppb ­NO2 and 1.48 ppm ­NH3, respectively. They use reduced graphene oxide hydrogels modified with ­NaHSO3 to form a sulfide one (S-RGOH). Compared with the unmodi- fied device, the response of S-RGOH to ­NO2 and ­NH3 was improved by 118.6 and 58.9 times, respectively. Neverthe- less, considering the applications of portable and wearable gas sensors, the material composition of the sensors should be green, non-toxic and biocompatible. In order to guarantee that the sensor itself should not cause harm to the human body during usage, green and non-polluting substances that are harmless to human beings could be chosen to modify the hydrogel, so that the sensing performance can be improved while protecting human health and safety. Wu and his team

Improved Their Previous Study By Choosing To Use

green, non-polluting vitamin C (VC) as a modifier for the reduced graphene oxide hydrogel (V-RGOH). The sensor maintains good performance while being green and harm- less. The response of V-RGOH devices was 52.5 and 36.3% for 100 ppm ­NH3 and 10 ppm ­NO2 with detection limits as low as 0.42 and 0.07 ppm, respectively. The strong interac- tion between VC and the measured gas, and the combination of multiple active sites due to the three-dimensional porous structure, improve the sensing performance of RGOH for ­NH3 and ­NO2. Whereas the multiple sensitivities to ­NO2 and ­NH3 still affect the selectivity of the hydrogel for each of the two gases. Further enhancements are needed to improve the selectivity of hydrogel-based gas sensors for ­NO2 and ­NH3, respectively. The potential of metal oxide semiconduc- tor (MOS) materials for gas sensing is well documented, and although most MOS-based sensors need to operate at relatively high temperatures, the excellent sensitivity and selectivity they exhibit for a wide range of gases is very promising. Therefore, combining metal oxides with RGOH solves the problem of high-temperature operation while improving the overall response and selectivity of the sensor.

To further increase the selectivity of hydrogel-based gas sen- sors for ­NO2, Wu et al. prepared a three-dimensional graphene hydrogel modified with ­SnO2 ­(SnO2/RGOH). The ­SnO2/RGOH ­NO2 sensor has a very low theoretical detec- tion limit of 2.8 ppm and a high sensitivity of 4.3 ­ppm−1 at room temperature. Moreover, as illustrated in Fig. 8a, the selectivity of the sensor for ­NO2 is greatly enhanced by the increased sensitivity of the device to ­NO2 and the improved conductivity of the material due to the presence of the p–n junction between RGOH and ­SnO2.

© The Authors

Fig. 8   a Schematic diagram of the selectivity of ­SnO2/RGOH for ­NO2. b Diagram of microheater. Reproduced with permission . Copyright 2020, American Chemical Society. c Comparison of the response ability of the original hydrogel and the self-healed hydrogel to ­NO2. Reproduced with permission . Copyright 2021, Wiley‐VCH. d Schematic of wireless sensing and alarm for ­NO2. d1-d4 Sensor alarm process for 2 ppm ­NO2: when the current exceeds the threshold, the red light comes on and the buzzer sounds. d5-d7 Wireless reception of ­NO2 sensing signals on a smartphone. Reproduced with permission . Copyright 2022, Royal Society of Chemistry. e Real‐time responses of the PVA–CNF DN organohydrogel towards 0.4–2 ppm ­NO2 at 25, − 10 and − 20 °C. Reproduced with permission . Copyright 2022, Wiley‐VCH. f Structure of self-powered ­NO2 sensor/power source. g Schematic diagram of self-powered ­NO2 sensing system. Reproduced

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However, in order to keep these sensors undisturbed by humidity and temperature changes at room temperature, the authors in all three studies mentioned above integrated a micro-heating device outside the hydrogel, as depicted in Fig. 8b. Although this device can improve the stability of the hydrogel-based gas sensor to some extent, it brings higher preparation cost and difficulty, as well as increases the power consumption and safety risk during practical use. A more convenient and safe approach is to introduce hygroscopic salts into the hydrogel to enhance its gas-sensitive response properties while improving its water-retaining ability and stability. Wu et al. developed a salt infiltrated hydrogel that can realize real-time monitoring of ­NO2 at room temper- ature without any other treatments. They used a simple solu- tion infiltration method to incorporate ­CaCl2 into the PAM/ Carr DN hydrogel. In the presence of ­CaCl2, this hydrogel ­NO2 sensor exhibited a high sensitivity of 119.9%, short response and recovery time (29.8 and 41.0 s, respectively), good linearity, low theoretical limit of detection (LOD) of 86 ppt, high selectivity, stability, and ionic conductivity. The sensor is self-healing as it recovers after being truncated and does not affect the conductive and sensitive proper- ties, which means it is more tolerant, as shown in Fig. 8c.

Moreover, the sensitive characteristics of the sensor are not affected by tensile deformation, and it can maintain the sens- ing of ­NO2 even at 100% strain with shortened response and recovery time. Wei et al. also prepared a ­NO2 gas sensor operating at room temperature using PAM/CArr DN hydro- gel with active metal electrodes. ­CaCl2 and Gly were intro- duced to improve its conductivity and mechanical deform- ability as well. The sensor can not only withstand 1,400% tensile strain due to its unique DN structure, but also exhibits a high sensitivity of 60.02% ­ppm−1 and the ultralow LOD of 6.8 ppb ­NO2. The sensor also features excellent selectiv- ity for ­NO2 and still works well at 50% tensile strain. Even further, the authors implemented wireless sensing of ­NO2 by collecting and processing data from real-time monitor- ing. When the concentration of ­NO2 exceeds the threshold, both the smartphone and the alarm system will give an early warning, as shown in Fig. 8d.

Nonetheless, the operation of hydrogel gas sensors in extreme environments still presents challenges. For exam- ple, in environments where the temperature is too high or too low, there is a risk of evaporation or freezing of the water in the hydrogel that prevents it from working properly for detection. The introduction of organic polyol into hydrogel can solve the problem to some extent. For example, Gly has a lower freezing point than water, and the hydroxyl groups in it combine with surrounding free water molecules through hydrogen bonding, which effectively inhibits the formation of hydrogen bonds between water molecules at sub-zero temperatures, and directly preventing ice crystal formation and lowering the freezing point . Ding et al.

developed a ­NO2 hydrogel sensor capable of operating at room temperature or even at − 20 °C, as illustrated in Fig. 8e. They used a PVA–CNF DN organohydrogel with a water/ Gly binary solvent to realize high-efficiency sensing of ­NO2.

The sensor has ultrahigh sensitivity (372% ­ppm−1), low LOD (2.23 ppb), fast response and recovery time (41/144 s for 250 ppb ­NO2), and high selectivity to ­NO2. After being cut or remolded, it can still heal, recover and retain its original conductive and sensitive properties perfectly, which implies better endurance and more application possibilities, demon- strating the potential of the sensor for actual use.

Nevertheless, all of the above ­NO2 sensors require addi- tional power supply devices to provide energy for their oper- ation, and the rigidity and bulkiness of traditional external power supplies limit the overall nature of the flexible vapor sensor, sacrificing advantages such as convenience and com- fort. Thus, the development of self-powered vapor sensor is of great practical importance. Since the hydrogel-based gas sensor is based on electrochemical sensing, an electrochemi- cal cell can be coupled to the hydrogel-based gas sensor to achieve self-powered sensing. Wu et al. constructed a novel self-powered flexible ­NO2 sensor using the struc- ture of Zn-zinc trifluoromethanesulfonate (Zn(OTf)2)/PAM hydrogel-Carbon, as demonstrated in Fig. 8f. The sensor has ultra-high sensitivity (1.92% ­ppb−1), very low theoreti- cal LOD (0.1 ppb) and excellent ­NO2 selectivity. It works even in complex environments such as stretching, bending, sub-zero, and high humidity environment. The authors also assembled it with miniaturized circuit modules, as shown in the Fig. 8g, and successfully built a self-powered integrated wireless ­NO2 monitoring system for real-time and remote gas detection.

3.3  Nh3 Gas Sensors

Ammonia is another toxic and harmful gas from human excretion and industrial production. Although it has a pun- gent odor, it is easily detectable only when its concentration

© The Authors

is higher than 50 ppm . It can scorch the skin, eyes, and mucous membranes of respiratory organs, and if people inhale too much, it can cause lung swelling and even death.

Only more than 35 ppm of ­NH3 is sufficient to potentially destroy the human sensory and respiratory systems [216, 217]. Ammonia is widely used in the electronics industry, food processing, chemical and scientific research fields.

Thus, real-time monitoring of the presence and concentra- tion of ­NH3 is necessary to ensure human safety. In addition, ammonia is one of the gases in the body’s exhaled breath that can be used to mark the presence of a certain disease . Generally, the concentration of ­NH3 in human exha- lation is roughly between 0.5 to 2 ppm. When the concen- tration increases, it indicates possible kidney failure, liver dysfunction, peptic ulcer and other diseases. While when the concentration decreases, it indicates possible asthma . In general, whether in protecting the human body or health monitoring, we require ­NH3 sensors with excellent performance to protect our wholesome life.

Recently, it has been shown that oxygen-containing

−And − ­Nh2 Play A

non-negligible role in promoting the adsorption of ­NO2 and ­NH3 molecules, so functionalized hydrogels such as PVA and PAM rich in oxygen-containing groups are also commonly used in the preparation of gas sensors [221, 222]. Zhi et al. prepared ­NH3 and ­NO2 hydrogel gas sensors based on PVA and CA double networks with the response of 31.60%, response/recovery times of 46/9 s for 40 ppm ­NH3 and low LODs of 140.85 ppb while being stretchable. Further, in order to make the hydrogel have stronger mechanical deformability as well as better water storage capacity for complex practical application envi- ronments, Wu and his team introduced the Gly into chemically derived ion-conductive PAM/CArr DN hydro- gels to prepare a high-performance stretchable room-tem- perature ­NH3 and ­NO2 sensor. It can not only withstand various stringent mechanical deformations, including up to 1.200% strain, wide ranges of bending and torsion, but also has high sensitivity (78.5 ­ppm−1 to ­NO2 and 1.3 ­ppm−1 to ­NH3) and low LOD (1.2 ppb for ­NO2 and 220 ppb for ­NH3). And the gas-sensitive performance of the sensor will not be impaired by the strong mechanical deforma- tion, which means that the sensor will not fail to work properly in practical wearable applications due to human movement or other possible stretching scenarios. After- wards, they introduced ethylene glycol (Eg) modification on the basis of PAM/CArr DN hydrogel to prepare ­NH3 and ­NO2 hydrogel gas sensors that can operate at room temperature . The sensor has high sensitivity to ­NH3 (1.4 ­ppm−1), low LOD (92 ppb), and retains sensitive characteristics under various mechanical deformations (Fig. 9a). In addition, it has extremely high resistance to drying and freezing (avoid drying within a year and its freezing point is below − 130 °C), high transparency and self-healing ability, enabling adaptation to a wider range of practical environments and application situations.

However, the above gas sensors are both ­NH3 and ­NO2 sensitive, it is difficult to exclude the interference of ­NO2 gas on the response signal in practical applications, and the research and development of gas sensors with high ­NH3 selectivity are warranted. ­NH3 has a relatively high solubil- ity in water, so increasing the ambient humidity and con- sequently the content of water molecules in the hydrogel can effectively improve the adsorption of ­NH3 molecules and enhance the response of the sensor to ­NH3. Liu et al.

developed an environmentally friendly and non-toxic biomass hydrogel-based ­NH3 sensor employing PAA and GA as sensing materials based on a humidity-activated mechanism at room temperature. The sensor has a response of 9.2 to 50 ppm ­NH3 at room temperature with 80% RH.

The PAA/GA hydrogel has better sensing characteristics for ­NH3 in high humidity environments, especially its selectiv- ity (Fig. 9b). The detailed mechanism has been mentioned above. In addition, they made a test on human exhalation, the sensor was able to accurately identify the presence of 5 ppm ­NH3 in the exhaled gas, which means that the sensor has the potential to be used in healthcare applications.

Similarly, Liu et al. prepared a highly sensitive, non- toxic, and environmentally friendly electrical ­NH3 hydrogel sensor under humid environment at 25 °C using biomass hydrogel GA and poly-L-glutamate (PGA) as sensitive composite materials. The sensor showed a response of up to 8.4 to 50 ppm ­NH3 at 80% RH with a LOD as low as 0.5 ppm, and good ­NH3 selectivity. And the high response performance of PGA/GA mainly comes from the synergistic effect unique to PGA and GA. As shown in Fig. 9c-d, firstly, PGA and GA have good chemical compatibility because of their similar chemical structures. Secondly, the appropriate amount of PGA in PGA/GA enables the hydrogel to maintain a certain adsorption capacity for water and ­NH3. As men- tioned above, at low humidity, the adsorbed water will form a hydrogen bonding network with some groups available for

1 3

proton leap (grotto mechanism), when the device is in weak proton conduction. At high humidity, the sensitive mate- rial adsorbs more water to form a condensed water layer, and the proton conductivity makes ­H2O and ­NH3 start to move. The adsorbed ­NH3 easily forms ammonium salts with

+ And ­H3O+ Ions On

the surface of the material and the water layer enhances the mobility of these ions in water, which greatly increases the conductivity of the sensor. In addition, during sensing, ­NH3 is physically adsorbed and dissolved in the ligand water,

+ Ions. And Carboxylic Acid Groups Are Rich

in GA and PGA, which can also react with ­NH3 to generate

­Nh4

+. Therefore, the PGA/GA hydrogel ­NH3 sensor has bet- ter response characteristics in high humidity environments. Fig. 9   a Response of PAM/CArr DN hydrogel to ­NH3 in different tensile strains. Reproduced with permission . Copyright 2020, American Chemical Society. b Response (purple column) of PAA/GA sensor to ­NH3 and other interfering gases. Reproduced with permission . Copy- right 2021, Elsevier. c-d ­NH3 sensing mechanism of PGA/GA sensors: c at low RH values and b at high RH values. Reproduced with permis- sion . Copyright 2020, Elsevierc. e Diagram of the breath testing process for ­NH3. f Response of hydrogels to exhalation with different ­NH3 contents. Reproduced with permission . Copyright 2021, Elsevier

Zhang And Her Team Used Thiol-Ene Photochemistry

to encapsulate citric acid (CA) into crosslinked polyeth- ylene glycol diacrylate (PEGDA) hydrogels to prepare a highly sensitive and selective CA/PEGDA hydrogel-based ­NH3 gas sensor working at high humidity (80% RH). The sensor exhibited a high response (6.20) to 20 ppm ­NH3 at room temperature, as well as a low LOD (50 ppb). This is due to the equilibrium water content of the hydrogel com- posite and the moderate acid dissociation constants of the acid groups. This sensor exhibits good chemical stability, while the stable hydrogel structure facilitates the detection of ­NH3 in humid environments. Specifically, CA has an extremely strong hydrophilicity and ­NH3 adsorption ability.

At 80% RH, the large amount of adsorbed water enhances the activity of the carboxyl group, which causes the car- boxyl group to react with a large amount of adsorbed ­NH3 to form ammonium salts. The ammonium salt has stronger hydrophilicity than the carboxyl group, and the adsorption of water increases, leading to CA membrane deliquescence.

Unlike CA, PEGDA exhibits relatively poor hydrophilicity, and the amount of ­NH3 adsorbed is relatively small even at higher moderate environments, blocking the formation

Of Carboxylates. Compounding Ca With Pegda Allows

the hydrogel to maintain a good ­NH3 adsorption capacity while still possessing an appropriate hydrophilicity. As a result, CA/PEGDA has both high ­NH3 response and good reproducibility. In addition, the carboxyl group in the CA structure can provide more adsorption sites for ­NH3, thus, the CA/PEGDA composite has good response properties to ­NH3 at 80% RH. They also tested for the presence of ­NH3 in human exhalation, the sensor can accurately identify differ- ent concentrations of ­NH3 in the exhaled breath, as shown in Fig. 9e-f, demonstrating the potential for clinical application of the sensor.

However, the hydrogel ­NH3 sensors described above, especially the last three examples, typically have a higher response in high humidity environments. Although this fea- ture is more friendly for breath detection applications, there is a problem of low response for ­NH3 monitoring in other situations. Subsequent development of hydrogel-based gas sensors with high sensitivity and selectivity for ­NH3 at room temperature and normal RH or even low RH should be con- tinued to adapt to more complex application scenarios.

3.4  O2 Gas Sensors

Oxygen is an essential gas that is indispensable for the sur- vival of living things. It is involved in the aerobic decompo- sition of carbohydrates that provide energy for life activities . When the concentration of oxygen is below 17%, it causes adverse physiological reactions such as difficulty in breathing and weakness. When the oxygen concentration is below 12%, there is even a risk of death by asphyxiation [224, 225]. Nevertheless, too high oxygen concentration is not a good thing. When people are exposed to high partial pressures of oxygen ­(PO2), the central nervous system and the pulmonary system will suffer damage, some of which

Is Even Irreversible . However, Sometimes A High

oxygen concentration environment is still desirable, such as anesthesia machines and hyperbaric chambers in hospitals [224, 229]. In order to avoid the danger of oxygen poison- ing or hypoxia, a monitoring of the oxygen concentration is required, and it is necessary to develop sensor devices that can monitor oxygen concentration over a wide range.

Liang et al. prepared a room-temperature ­O2 sensor (in an oxygen-free environment) using a PAM-CS DN struc- tured organohydrogel. It is capable of monitoring the full ­O2 concentration range with a theoretical minimum detection limit as low as 5.7 ppm, sensitivity as high as 0.2% ­ppm−1, and good resistance to temperature and humidity. In addi- tion, the PAM-CS organohydrogel sensor has stretchability, which allows sensing even under 100% stretching without the loss of sensing performance (Fig. 10a1); self-healing capability, which can self-heal and compound even after being cut and restore the original response characteristics (Fig. 10a2); and self-adhesive capability, which allows the adhesion on the skin surface without the use of additional binders (Fig. 10a3). Since the oxygen concentration in exhaled breath is lower than that in air, human respiration can be identified by changes in oxygen concentration. As shown in Fig. 10a4–a5. The sensor is sensitive to human exhalation, indicating the possibility of its application for respiratory monitoring.

In parallel to respiratory monitoring, oxygen sensors could also be used in other medical applications such as cell culture experiments or monitoring of transcutaneous oxygen pressure ­(tcPO2). When the temperature of the

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Fig. 10   a1 Response to 1% ­O2 under different tensile strains. a2 Response of original and self-healed organohydrogels to 1% ­O2. a3 Photo- graphs showing the organohydrogel (marked by the yellow circle) adhered firmly to various materials. a4 Real-time response curve of the sensor to human exhalation in the air. a5 Diagram of the breath testing process for ­O2. Reproduced with permission . Copyright 2022, Springer Nature. SEM of b1 PI film and b2 hydrogel on artificial skin. b3 Schematic diagram of the ­tcPO2 sensing device and sensing process. b4 Illustra- tion of placing the sensor on the foot to measure ­tcPO2. b5-b6 Test results of ­tcPO2 sensing when the foot is in different states. Reproduced with permission . Copyright 2021, American Association for the Advancement of Science

© The Authors

skin surface rises, oxygen diffuses out from the capillar- ies into subcutaneous tissues and the skin surface. At this time, monitoring the oxygen partial pressure of the skin can reflect the actual oxygen supply to the tissue cells, thus directly reflecting the microcirculation of the skin and indi- rectly reflecting the macrovascular condition [231, 232].

Liebisch et al. prepared a Clark-type oxygen micro- sensors that can operate in an aqueous environment. They used a PHEMA hydrogel layer containing a buffer solu- tion as the electrolyte, platinum as the electrode material for the sensor, and polydimethylsiloxane (PDMS) as the gas permeable membrane. They also monitored oxygen in monolayer cultures with T-47D breast cancer cells, which further demonstrates the applicability of the sensor in cell culture oxygen concentration detection. In terms of oxygen sensors for ­tcPO2 monitoring, it is also possible to monitor physiological changes in the human body, such as blood flow rate and the amount of local oxygen supplied to the tissues. Kim and his team propose a skin device inter- face with tissue-like characteristics that is formed by using an ultrathin type of functionalized hydrogel (PAM). Its soft- ness and mechanical deformability allow it to adhere well to human skin as shown in Fig. 10a-b, while the combination of a highly porous three-dimensional (3D) network and the super-low thickness of the interfacial hydrogel allows for rapid diffusion and transport of the target bioanalyte. Then the authors fabricated an electrochemical device with three electrodes (Pt as working and counter electrode and Ag/ AgCl as reference electrode) for the detection of ­tcPO2. To prevent the oxygen from circulating inside and outside the device, they further encapsulated the sensor with a multi- layer oxygen barrier film made of PI/Al/PI (polyimide). As depicted in Fig. 10c, measurements are performed by first applying gentle heating to the skin across a hydrogel inter- face through which oxygen molecules are extracted from the blood vessels under the skin to reach the device. The diffused oxygen is then electrochemically reduced at the working electrode, and the change in reduction current is measured to obtain information about ­tcPO2. By connect- ing this sensor to the human foot (Fig. 10d), it is capable of detecting the reduction current and ­tcPO2 at different foot positions, with a trend between them roughly as illustrated in Fig. 10e-f. However, in order to obtain more accurate data, more detailed research is needed in the future, such as improving electrode fabrication and measurement protocols.

3.5  Co2 Gas Sensors

CO2 is ubiquitous and can be produced whenever carbona- ceous materials are burned, and as a by-product of respira- tion, ­CO2 will inevitably be produced as long as organisms are still breathing. The concentration of ­CO2 should not be higher than 1,000 ppm in human working and living envi- ronments . Exceeding ­CO2 levels may not only have adverse effects on human health, such as headaches, fatigue, eye diseases, nasal, or respiratory diseases, but also bring about the greenhouse effect and global warming [235, 236].

However, ­CO2 has neither color nor odor, so it is difficult for human-being to detect its presence through the human sensory system, let alone know its concentration. Hence, portable and wearable gas sensors are needed to help people detect the presence and concentration of ­CO2 in their lives and work, and prevent unknown risks.

One way to measure ­CO2 is to first inject a gas containing ­CO2 into an aqueous solution and reflect the amount of ­CO2 by measuring the concentration of dissolved ­CO2. Wang et al.

prepared hydrogel membranes from branched polyeth- yleneimine (BPEI) and partially oxidized dextran (PO-DEX) by in situ Schiff base reaction to achieve rapid, real-time, and continuous monitoring of dissolved ­CO2. The BPEI/PO-DEX films swell in water, and the swelling will be greater when ­CO2 is introduced to react with the amino group in BPEI. The movement of the Fabry–Perot stripe on the reflectance spec- trum of the BPEI/PO-DEX film is used as a sensing signal to allow sensitive detection of dissolved ­CO2. In the similar vein, Li et al. developed a gas sensor for detecting dis- solved ­CO2 concentration. They prepared a hydrogel sensing grating by replicating a photoluminescent surface grating on a thin film of azo molecular glass on the surface of a hydrogel using soft lithography to achieve highly sensitive detection of low-concentration ­CO2. The sensing reaction is performed in water, and ­CO2 gas needs to be bubbled in the water of the cuvette to react with water to produce carbonic acid. By reacting with the tertiary amine group on the PDMAPMA (dimethylaminopropyl methacrylamide) polymer chain, the positively charged polymer chain generates electrostatic repulsion, which causes the hydrogel grating to swell, result- ing in a photometric change and ­CO2 sensing. Subsequently, repeated purging of the hydrogel with an inert gas such as argon or nitrogen can remove the adsorbed ­CO2 from the surface, making the hydrogel grating reusable.

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