Recent Studies on Hydrogels Based on H2O2-Responsive Moieties: Mechanism, Preparation and Application

H2O2 is essential for cellular processes and plays a vital role in the regulation of cell signaling pathways, which can be viewed as a warning signal for many kinds of disease including cancer, cardiovascular disease, reproductive abnormalities, diabetes, and renal failure. A H2O2-responsive hydrogel (H2O2-Gel) is a promising candidate for biomedical applications because of its good biocompatibility, similarity to soft biological tissues, ease of preparation, and its ability to respond to H2O2. In this study, the H2O2-responsive moieties used to fabricate H2O2-Gels were reviewed, including thioethers, disulfide bonds, selenides, diselenium bonds, diketones, boronic, and others. Next, the preparation method of H2O2-Gel was divided into two major categories according to their reaction mechanisms: either self-crosslinking or mechanisms entailing the addition of difunctional crosslinkers. Last, the applications of H2O2-Gels were emphasized, which have been viewed as desirable candidates in the fields of drug delivery, the detection of H2O2, glucose-responsive systems, ROS scavengers, tissue engineering, and cell-encapsulation.


Introduction
In recent years, relevant researchers have developed a variety of stimulation mechanisms by investigating the changes in the microenvironment of the human body [1][2][3]. Following this research, quite a number of stimuli-responsive materials have been developed with sensibility to acidity [4], alkali [5], temperature [6], mechanical force [7], magnetic fields [8], ultrasound [9], reactive oxygen species (ROS) [10], etc. ROS are emerging as critical signaling molecules which are abundant in the human microenvironment [11]. The term ROS encompasses a wide range of molecules, such as hydrogen peroxide (H 2 O 2 ), singlet oxygen ( 1 O 2 ), hydroxyl radical, superoxide, etc. [12]. The stable control of reactive oxygen species level is of great significance to maintain the normal physiological activities of human bodies [13]. The overexpression of reactive oxygen species can easily destroy the structure of many biological macromolecules, such as proteins and nucleic acids, and thus result in a variety of diseases [14]. Therefore, it is of great theoretical and practical significance to design and develop smart materials with ROS responsive behaviors. H 2 O 2 is a representative case of ROS that is particularly important in the physiological regulation in organisms [15]. It plays an important role in cell signal transduction, differentiation, and proliferation, and also plays an irreplaceable role in the diagnosis and treatment of many diseases [16]. Especially in diseases related to oxidative stress, a change in the H 2 O 2 level is a good indication of the location of the lesion site [17]. When oxidative stress and injury occur in the body, the concentration of H 2 O 2 is higher than the normal level [18]. Therefore, the over expression of H 2 O 2 in vivo is directly related to inflammation, bleeding, Alzheimer's disease, diabetes, renal function decline or tumors, and other Until now, different types of H2O2-responsive materials have been explored, includ ing MRI contrast agents, fluorescent sensors, transistors, drug delivery systems, mito chondria-directed tools, etc. [20,21]. Gu et al. published a review paper on the topic o H2O2-responsive materials, in which the biomedical applications were emphasized [17]. I is worth noting that hydrogels are widely used in the fields of drug delivery, tissue engi neering, surgical dressing, etc. [22][23][24][25]. As a result, hydrogel based on H2O2-responsiv moieties is a major kind of H2O2-responsive material [26,27] which can be defined as H2O2 responsive hydrogel (H2O2-Gel). There is a great deal of significance in performing a re view of the development of H2O2-Gel and in the explication of the guidelines for designing suitable formations, which is also our interest in this study. Hydrogels are three-dimen sional (3D) cross-linked polymer networks, which can absorb and retain large amounts o water [28]. Due to their flexible structure, stimuli-responsive hydrogels can perform work by converting an external stimulation into mechanical motions or chemical conversions which can be an effective framework for soft actuators. In this review, we focus on over viewing the responsive mechanism of H2O2-responsive moieties and the applications o H2O2-Gel, especially in biomedicine. The preparation methods are also summarized. W expect to provide guidance and designate the direction of further studies on H2O2-respon sive materials. Until now, different types of H 2 O 2 -responsive materials have been explored, including MRI contrast agents, fluorescent sensors, transistors, drug delivery systems, mitochondriadirected tools, etc. [20,21]. Gu et al. published a review paper on the topic of H 2 O 2responsive materials, in which the biomedical applications were emphasized [17]. It is worth noting that hydrogels are widely used in the fields of drug delivery, tissue engineering, surgical dressing, etc. [22][23][24][25]. As a result, hydrogel based on H 2 O 2 -responsive moieties is a major kind of H 2 O 2 -responsive material [26,27] which can be defined as H 2 O 2 -responsive hydrogel (H 2 O 2 -Gel). There is a great deal of significance in performing a review of the development of H 2 O 2 -Gel and in the explication of the guidelines for designing suitable formations, which is also our interest in this study. Hydrogels are three-dimensional (3D) cross-linked polymer networks, which can absorb and retain large amounts of water [28]. Due to their flexible structure, stimuli-responsive hydrogels can perform work by converting an external stimulation into mechanical motions or chemical conversions, which can be an effective framework for soft actuators. In this review, we focus on overviewing the responsive mechanism of H 2 O 2 -responsive moieties and the applications of H 2 O 2 -Gel, especially in biomedicine. The preparation methods are also summarized. We expect to provide guidance and designate the direction of further studies on H 2 O 2 -responsive materials.

Thioether (-S-)
Thioether can be regarded as a compound analogous to ether in which the oxygen is replaced by sulfur. Thioether is commonly synthesized from potassium or sodium sulfide and halohydrocarbon and is widely distributed in proteins [29]. Benefiting from the higher atomic number of sulfur compared to oxygen, thioether usually shows higher chemical activity than ether and can be transferred into sulfoxide and sulfone with the treatment of H 2 O 2 (Table 1) [30].

Thioether (-S-)
Thioether can be regarded as a compound analogous to ether in which the oxygen is replaced by sulfur. Thioether is commonly synthesized from potassium or sodium sulfide and halohydrocarbon and is widely distributed in proteins [29]. Benefiting from the higher atomic number of sulfur compared to oxygen, thioether usually shows higher chemical activity than ether and can be transferred into sulfoxide and sulfone with the treatment of H2O2 (Table 1) [30]. Napoli et al. investigated the mechanism for the oxidative destabilization of poly(propylene sulfide) [31]. Even at concentrations as low as 0.03-vol.% H2O2, the transformation from thioether to sulfoxide or sulfone can be achieved. The NMR shifts of the methyl group in the α and β position to sulfur were examined in detail to reveal the changes. After treating the material with H2O2, a doublet at ~1.53 ppm apparated corresponding to the methyl in β position to -SOx-moieties. Moreover, a high-frequency shift of the peaks corresponding to the protons in the α position to the -S-(2.65 and 2.93 ppm) [31,32] 2

Thioether (-S-)
Thioether can be regarded as a compound analogous to ether in which the oxygen is replaced by sulfur. Thioether is commonly synthesized from potassium or sodium sulfide and halohydrocarbon and is widely distributed in proteins [29]. Benefiting from the higher atomic number of sulfur compared to oxygen, thioether usually shows higher chemical activity than ether and can be transferred into sulfoxide and sulfone with the treatment of H2O2 (Table 1) [30]. Napoli et al. investigated the mechanism for the oxidative destabilization of poly(propylene sulfide) [31]. Even at concentrations as low as 0.03-vol.% H2O2, the transformation from thioether to sulfoxide or sulfone can be achieved. The NMR shifts of the methyl group in the α and β position to sulfur were examined in detail to reveal the changes. After treating the material with H2O2, a doublet at ~1.53 ppm apparated corresponding to the methyl in β position to -SOx-moieties. Moreover, a high-frequency shift of the peaks corresponding to the protons in the α position to the -S-(2.65 and 2.93 ppm) [33,34] 3

Thioether (-S-)
Thioether can be regarded as a compound analogous to ether in which the oxygen is replaced by sulfur. Thioether is commonly synthesized from potassium or sodium sulfide and halohydrocarbon and is widely distributed in proteins [29]. Benefiting from the higher atomic number of sulfur compared to oxygen, thioether usually shows higher chemical activity than ether and can be transferred into sulfoxide and sulfone with the treatment of H2O2 (Table 1) [30]. Napoli et al. investigated the mechanism for the oxidative destabilization of poly(propylene sulfide) [31]. Even at concentrations as low as 0.03-vol.% H2O2, the transformation from thioether to sulfoxide or sulfone can be achieved. The NMR shifts of the methyl group in the α and β position to sulfur were examined in detail to reveal the changes. After treating the material with H2O2, a doublet at ~1.53 ppm apparated corresponding to the methyl in β position to -SOx-moieties. Moreover, a high-frequency shift of the peaks corresponding to the protons in the α position to the -S-(2.65 and 2.93 ppm) [35] 4

Thioether (-S-)
Thioether can be regarded as a compound analogous to ether in which the oxygen is replaced by sulfur. Thioether is commonly synthesized from potassium or sodium sulfide and halohydrocarbon and is widely distributed in proteins [29]. Benefiting from the higher atomic number of sulfur compared to oxygen, thioether usually shows higher chemical activity than ether and can be transferred into sulfoxide and sulfone with the treatment of H2O2 (Table 1) [30]. Napoli et al. investigated the mechanism for the oxidative destabilization of poly(propylene sulfide) [31]. Even at concentrations as low as 0.03-vol.% H2O2, the transformation from thioether to sulfoxide or sulfone can be achieved. The NMR shifts of the methyl group in the α and β position to sulfur were examined in detail to reveal the changes. After treating the material with H2O2, a doublet at ~1.53 ppm apparated corresponding to the methyl in β position to -SOx-moieties. Moreover, a high-frequency shift of the peaks corresponding to the protons in the α position to the -S-(2.65 and 2.93 ppm) [36] 5

Thioether (-S-)
Thioether can be regarded as a compound analogous to ether in which the oxygen is replaced by sulfur. Thioether is commonly synthesized from potassium or sodium sulfide and halohydrocarbon and is widely distributed in proteins [29]. Benefiting from the higher atomic number of sulfur compared to oxygen, thioether usually shows higher chemical activity than ether and can be transferred into sulfoxide and sulfone with the treatment of H2O2 (Table 1) [30]. Napoli et al. investigated the mechanism for the oxidative destabilization of poly(propylene sulfide) [31]. Even at concentrations as low as 0.03-vol.% H2O2, the transformation from thioether to sulfoxide or sulfone can be achieved. The NMR shifts of the methyl group in the α and β position to sulfur were examined in detail to reveal the changes. After treating the material with H2O2, a doublet at ~1.53 ppm apparated corresponding to the methyl in β position to -SOx-moieties. Moreover, a high-frequency shift of the peaks corresponding to the protons in the α position to the -S-(2.65 and 2.93 ppm) [37] 6

Thioether (-S-)
Thioether can be regarded as a compound analogous to ether in which the oxygen is replaced by sulfur. Thioether is commonly synthesized from potassium or sodium sulfide and halohydrocarbon and is widely distributed in proteins [29]. Benefiting from the higher atomic number of sulfur compared to oxygen, thioether usually shows higher chemical activity than ether and can be transferred into sulfoxide and sulfone with the treatment of H2O2 (Table 1) [30]. Napoli et al. investigated the mechanism for the oxidative destabilization of poly(propylene sulfide) [31]. Even at concentrations as low as 0.03-vol.% H2O2, the transformation from thioether to sulfoxide or sulfone can be achieved. The NMR shifts of the methyl group in the α and β position to sulfur were examined in detail to reveal the changes. After treating the material with H2O2, a doublet at ~1.53 ppm apparated corresponding to the methyl in β position to -SOx-moieties. Moreover, a high-frequency shift of the peaks corresponding to the protons in the α position to the -S-(2.65 and 2.93 ppm) [38] 7

Thioether (-S-)
Thioether can be regarded as a compound analogous to ether in which the oxygen is replaced by sulfur. Thioether is commonly synthesized from potassium or sodium sulfide and halohydrocarbon and is widely distributed in proteins [29]. Benefiting from the higher atomic number of sulfur compared to oxygen, thioether usually shows higher chemical activity than ether and can be transferred into sulfoxide and sulfone with the treatment of H2O2 (Table 1) [30]. Napoli et al. investigated the mechanism for the oxidative destabilization of poly(propylene sulfide) [31]. Even at concentrations as low as 0.03-vol.% H2O2, the transformation from thioether to sulfoxide or sulfone can be achieved. The NMR shifts of the methyl group in the α and β position to sulfur were examined in detail to reveal the changes. After treating the material with H2O2, a doublet at ~1.53 ppm apparated corresponding to the methyl in β position to -SOx-moieties. Moreover, a high-frequency shift of the peaks corresponding to the protons in the α position to the -S-(2.65 and 2.93 ppm) [39,40] Napoli et al. investigated the mechanism for the oxidative destabilization of poly(propy lene sulfide) [31]. Even at concentrations as low as 0.03-vol.% H 2 O 2 , the transformation from thioether to sulfoxide or sulfone can be achieved. The NMR shifts of the methyl group in the α and β position to sulfur were examined in detail to reveal the changes. After treating the material with H 2 O 2 , a doublet at~1.53 ppm apparated corresponding to the methyl in β position to -SO x -moieties. Moreover, a high-frequency shift of the peaks corresponding to the protons in the α position to the -S-(2.65 and 2.93 ppm) was also observed. These phenomena indicate that both the sulfoxide and sulphone moieties are included in the backbone of H 2 O 2 -treated poly(propylene sulfide). Kramer and Deming further demonstrated the H 2 O 2-sensitive mechanism of thioether, in which the thioether moiety in a pyranose derivative can be oxidized into sulphone-containing product with a high yield [32].

Disulfide Bond (-S-S-)
Disulfide bonds are commonly included in a variety of proteins, which can be generated from the oxidation of thiol groups. An S anion from one sulfhydryl group can act as a nucleophile and attack another sulfhydryl moiety to create a disulfide bond. Particularly, H 2 O 2 can be used to break the S-S bond and yield corresponding thiolated compounds ( Table 1). Due to the high activity of sulfhydryl groups, the degraded thiolated compounds have the capacity of forming the original disulfide-bearing substance. Therefore, the disulfide-based H 2 O 2 -responsive behavior can be regarded as a reversible process [33].

Selenide (-Se-)
Selenium (Se) is a nonmetallic element in the oxygen family that mainly exists in several allotropic forms. It is widely spread in volcanic areas and sulfide ores, which can be used in photocells, solar cells, and in xerography. Selenium can be bound into proteins and serves as an indispensable microelement in humans. Its chemical properties are similar to sulfur, but with a higher reactivity. As a result, the H 2 O 2 -treated selenide derivatives are usually transferred into the selenone form (Table 1).
Zhang's group illustrated the structural change with the oxidation of a selenide block copolymer with H 2 O 2 at 0.1%, and the oxidized residue was analyzed by XPS. The binding energy of Se 3d5's shift from 55.9 eV to 59.5 eV was clearly observed, suggesting the higher valency of selenium which greatly matches the formation of selenone groups. The presence of O=Se=O groups can also be demonstrated based on the characteristic absorption bands at 904 and 880 cm −1 . 1 H-NMR and 77 Se-NMR further confirmed the transformation from -Se-to O=Se=O groups, which shows a H 2 O 2 -responsive behavior [35].

Diselenium Bond (-Se-Se-)
A diselenium bond can be regarded as a linkage containing two atoms of selenium combined with an element or radical. Due to the similar chemical properties of selenium and sulfur, a diselenium bond can also be broken with treatment from H 2 O 2 . The diselenide bond has an energy of around 176 kJ/mol, indicating its easy oxidization in the presence of H 2 O 2 while undergoing a phase transition from hydrophobic diselenide to hydrophilic selenic acid (Table 1) [41]. An attractive advantage of diselenide and disulfide bonds is their hydrolytic stability in physiological conditions [36]. Moreover, the diselenid-based H 2 O 2 -responsive behavior can also be regarded as a reversible process. The degraded hydrophilic selenic acid can rebuild the diselenium bond under ambient atmosphere.
Diselenium derivatives can be prepared by the oxidation of aryl selenols [42]. However, as aryl selenols are most conveniently prepared from the diselenides themselves, this method is not likely to have preparative value. Thus, diselenium derivatives are usually synthesized via other routes. For example, electron-deficient aryl halides can be used as substrates [43], and their reaction with the diselenide dianion gives rise to the desired molecules. Arylamines can also serve as potential precursors via the reaction of diazonium salts with sodium hydrogenselenide. Another typical route to diaryl diselenides is based on the reaction of arylaldehydes with sodium hydrogenselenide, which, after treatment with sodium borohydride and piperidine, leads to the target compounds.

Diketone
Benzil is a yellow crystalline containing a diketone moiety made by oxidizing benzoin [44]. Sawaki et al. have reported that benzil can transform into benzoic anhydride via a Baeyer-Villiger type reaction with H 2 O 2 in alkaline organic solvents [39]. Then the products are usually presented in benzoic form due to hydrolysis (Table 1). However, the reported conditions were far from biological. Nagano's group developed a simple strategy to adjust the reactivity between benzil and H 2 O 2 based on the modification of the benzene ring [40]. Following this method, 5-benzoylcarbonylfluorescein derivatives were synthesized for the detection of hydrogen peroxide at the cellular level together with the d-PeT mechanism to control fluorescence. Moreover, other peroxalate ester-containing derivatives have been reported to show similar H 2 O 2 -responsive properties [45][46][47].

Boronic
The reaction between H 2 O 2 and benzeneboronic acid in aqueous solutions can generate phenol and boric acid in high yields, which can be viewed as a homolytic orelectrophilic substitution in the benzene ring (Table 1) [48]. Generally, the rate of the reaction follows a dose-dependent manner. Moreover, in addition to benzeneboronic acid, quite a number of aryl-substituted boronic acids were developed, which can be exploited as H 2 O 2 -responsive tools with high accuracy, specificity, and sensitivity. Chang's group proposed a strategy for the optical detection of H 2 O 2 relying on the selective H 2 O 2 -triggered transformation of arylboronates to phenols [37]. With the H 2 O 2 -triggered hydrolytic deprotection of the boronates, our groups have developed a series of open, colored, and fluorescent products [49][50][51][52].
Interestingly, when the arylboronic acid ester is connected to a suitable linker, e.g., a typical -Ar-CH 2 -O-linkage, after the formation of a phenol derivative triggered by H 2 O 2 , a sequential self-immolative process can take place via a 1,4-/1,6 elimination reaction. This character of arylborates has enabled the development of H 2 O 2 -triggered degradable materials [38].

Others
To meet the increasing demand for H 2 O 2 -responsive materials and ROS scavengers, some other types of H 2 O 2 -sensitive linkages have been employed. Tellurium (Te)-containing organic compounds have gained attention because of their lower electronegativity, which is thought to be more easily oxidized by H 2 O 2 than selenium/sulfur-containing ones [53]. Moreover, tellurium-containing compounds were evidenced to exhibit less toxicity than selenium analogues. It has been reported that the tellurium atoms in Te-containing organic compounds can be completely oxidized with 100 µM H 2 O 2 .
Peptides and enzymes have also been used to fabricate H 2 O 2 -responsive materials. Sung et al. reported a peptide oligomer as a H 2 O 2 -responsive crosslinker for long-term tissue engineering applications. The degradation can take place by reacting with 5 mM H 2 O 2 [54,55].
Otherwise, some metallic oxides and metal salts can be used as H 2 O 2 -responsive moieties owing to their catalytic action or redox activity. For example, the catalytic reaction between the H 2 O 2 and MnO 2 contributes to the formation of H 2 O and O 2 [56]. Fe(II) can react with H 2 O 2 to generate hydroxyl radicals [57].

Preparation
Generally, hydrogels fall into two categories [58][59][60]: (i) Chemical gel: Hydrogels that are covalently cross-linked networks in which the equilibrium swelling state depends on the crosslink density and interaction parameters [61,62]; (ii) Physical gel: The networks are constructed by noncovalent linkages, e.g., hydrogen bonding, ionic bonding, hydrophobic interactions, molecular entanglements, etc. These interactions are reversible.
As for H 2 O 2 -responsisve hydrogels, the responsiveness is mainly based on the linkages summarized in the abovementioned section. Such responsiveness is quite different from a response to temperature, pH, or electric fields, and the latter of these is mainly presented in physical gels. As a result, H 2 O 2 -responsisve hydrogels are mainly presented as chemical gels in the literature, in which a chemical cross-linking approach is usually involved. The cross-linking approach can be classified into two generic groups.

Self-Crosslinking
For this case, the H 2 O 2 -Gels were usually prepared based on the self-crosslinking behavior of the polymeric chains without the addition of any other crosslinkers. The H 2 O 2 -sensitive moieties, e.g., sulfydryl, selenol, phenol, etc., are mainly incorporated in the polymeric chains as pendent or terminal groups which can be further involved in the condensation reactions to achieve the three-dimensional structures.
Wan's group synthesized a sulfydryl-bearing oligomer (o-DHLA) by treating dihydrolipoic acid with equimolar 2,2-dimethoxypropane under the catalysis of p-toluenesulfonic acid [63]. The obtained o-DHLA was achieved with sulfydryl units as terminal groups, the latter of which can react with each other to yield -S-S-containing microgels with the addition of polyvinyl alcohol. The formation of an -S-S-linkage makes the obtained microgels desirable H 2 O 2 -responsive drug delivery systems. Yang's group employed cysteine-containing keratin as the base material, in which the sulfydryl units can afford -S-S-linkages under the effect of oxygen [64].
Ma et al. [65] prepared a boronate-based copolymer comprising N-isopropylacrylamide, hydroxyethyl methacrylate, and (4-(hydroxymethyl)-phenylboronic acid. The presence of N-isopropylacrylamide endowed the obtained copolymer's temperature-responsive characters. As a result, the sol-gel transformation occurs when the temperature is higher than a certain value ranging from 18 to 37 • C. The lower critical solution temperature (LCST) and upper critical solution temperature (UCST) can be controlled by the proportion of N-isopropylacrylamide. The presence of boronate moieties makes the obtained hydrogel exhibit H 2 O 2 -responsive characters. A similar strategy can also be found in another patent [66].

Addition of Difunctional Crosslinkers
Zhao et al. [67] prepared H 2 O 2 -responsive hydrogels using cystamine dihydrochloride as the difunctional crosslinkers. In their strategy, catecholamine-modified chitosan was prepared and used to fabricate the hydrogels. The amino groups in cystamine can react with catecholamine moieties via the formation of benzoquinone. As a result, the disulfide bonds were introduced into the matrix of the obtained hydrogels, which endowed the hydrogels with H 2 O 2 -responsive degradation abilities.
Our group proposed an alternative approach to synthesizing H 2 O 2 -responsive hydrogels, in which the difunctional crosslinkers and polyhydric matrix are used. For example, the arylboronic acid unit on 1,4-phenylenediboronic acid can react with the vicinal diol groups of a polysaccharide to form a chemical hydrogel with high efficiency. The B−C bond in the obtained hydrogel can be broken with the addition of H 2 O 2 in a dose-dependent manner, which can give rise to the degradation behavior and endow the H 2 O 2 -responsive behavior to the hydrogel (Figure 1) [50]. The boronate-based difunctional crosslinkers have been also reported in other studies [68].

Drug Delivery
Drug-loaded hydrogels have generated increasing interest in recent years because o their extraordinary advantages in versatile drug administration, ease of preparation, high biocompatibility, and improved patient compliance [69]. Moreover, the stimuli-respon siveness can be achieved by including the responsive linkages into the hydrogel networks e.g., temperature-responsive, pH-responsive, photo-responsive, etc. H2O2 is usually in volved in a variety of pathological effects in organisms, in which the aberrant expression of H2O2 is directly linked with cancer, inflammation, phyma, etc. The exploration of H2O2 responsive drug delivery systems with high selectivity and specificity has great signifi cance for improving antitumor efficacy and preventing inflammation.
In Wan's study, a serious of thioketal-containing microgels have been developed tha show a dual responsive behavior to H2O2 and glutathione. Paclitaxel was loaded into the dual responsive thioketal microgels to achieve an antitumor preparation which is sensi tive enough to the tumor-site H2O2 and glutathione levels and able to release paclitaxe accordingly. Moreover, the obtained dual responsive thioketal microgels can be degrada ble into biocompatible and low toxic byproducts after performing their delivering tasks Following this method, the antitumor efficacy of chemotherapy drugs can be significantly improved by loading them into the dual responsive platform. Meanwhile, side effects can also be effectively reduced [63].
Duvall et al. [70] explored poly(propylene sulfide) nanoparticles and hydrogels a H2O2-responsive drug carriers. Generally, the conversion between sulfoxides/sulfone and hydrophobic sulfide can cause solubility changes. The hydrophobicity of poly(pro pylene sulfide) makes it a good candidate for loading hydrophobic drugs, similar to cur cumin. After delivery in vivo, the exposure of poly(propylene sulfide) to H2O2 enables a transition to its hydrophilic form, which triggers particle swelling and H2O2-responsive drug release.
Our group developed H2O2-Gels by using difunctional crosslinkers. A high level o H2O2 can destroy the B-C bond in the obtained hydrogels and thereby break the three

Drug Delivery
Drug-loaded hydrogels have generated increasing interest in recent years because of their extraordinary advantages in versatile drug administration, ease of preparation, high biocompatibility, and improved patient compliance [69]. Moreover, the stimuliresponsiveness can be achieved by including the responsive linkages into the hydrogel networks, e.g., temperature-responsive, pH-responsive, photo-responsive, etc. H 2 O 2 is usually involved in a variety of pathological effects in organisms, in which the aberrant expression of H 2 O 2 is directly linked with cancer, inflammation, phyma, etc. The exploration of H 2 O 2 -responsive drug delivery systems with high selectivity and specificity has great significance for improving antitumor efficacy and preventing inflammation.
In Wan's study, a serious of thioketal-containing microgels have been developed that show a dual responsive behavior to H 2 O 2 and glutathione. Paclitaxel was loaded into the dual responsive thioketal microgels to achieve an antitumor preparation which is sensitive enough to the tumor-site H 2 O 2 and glutathione levels and able to release paclitaxel accordingly. Moreover, the obtained dual responsive thioketal microgels can be degradable into biocompatible and low toxic byproducts after performing their delivering tasks. Following this method, the antitumor efficacy of chemotherapy drugs can be significantly improved by loading them into the dual responsive platform. Meanwhile, side effects can also be effectively reduced [63].
Duvall et al. [70] explored poly(propylene sulfide) nanoparticles and hydrogels as H 2 O 2 -responsive drug carriers. Generally, the conversion between sulfoxides/sulfones and hydrophobic sulfide can cause solubility changes. The hydrophobicity of poly(propylene sulfide) makes it a good candidate for loading hydrophobic drugs, similar to curcumin. After delivery in vivo, the exposure of poly(propylene sulfide) to H 2 O 2 enables a transition to its hydrophilic form, which triggers particle swelling and H 2 O 2 -responsive drug release.
Our group developed H 2 O 2 -Gels by using difunctional crosslinkers. A high level of H 2 O 2 can destroy the B-C bond in the obtained hydrogels and thereby break the threedimensional networks in H 2 O 2 -Gels. When the drug was loaded into the hydrogel, a typical H 2 O 2 -responsive behavior was easily achieved. It should be noted that hydrogels, which are diffusion-controlled drug delivery systems, usually exhibit an initial burst release. Since drug-release during the initial stage depends mostly on diffusional escape, a sudden release in a short period usually takes place. It can be regarded as an "initial burst release" [71,72]. A serious initial burst release may be toxic and give rise to suppressed bioavailability. In our study, the model drug was poured into halloysite nanotubes, a natural aluminosilicate clay mineral with a hollow tubular structure, before loading it into the matrix of H 2 O 2 -Gels (Figure 2, DLHCH-1: the therapeutic agent was pre-loaded into the halloysite nanotubes; DLHCH-2: the therapeutic agent was directly loaded into the matrix of hydrogels). Following this method, the "initial burst effect" was effectively suppressed due to the halloysite nanotubes. Our strategy provides an alternative way to design H 2 O 2 -Gels which may meet the requirements for the application of topical preparations to prevent inflammation ( Figure 2) [50]. dimensional networks in H2O2-Gels. When the drug was loaded into the hydrogel, a typical H2O2-responsive behavior was easily achieved. It should be noted that hydrogels, which are diffusion-controlled drug delivery systems, usually exhibit an initial burst release. Since drug-release during the initial stage depends mostly on diffusional escape, a sudden release in a short period usually takes place. It can be regarded as an "initial burst release" [71,72]. A serious initial burst release may be toxic and give rise to suppressed bioavailability. In our study, the model drug was poured into halloysite nanotubes, a natural aluminosilicate clay mineral with a hollow tubular structure, before loading it into the matrix of H2O2-Gels (Figure 2, DLHCH-1: the therapeutic agent was pre-loaded into the halloysite nanotubes; DLHCH-2: the therapeutic agent was directly loaded into the matrix of hydrogels). Following this method, the "initial burst effect" was effectively suppressed due to the halloysite nanotubes. Our strategy provides an alternative way to design H2O2-Gels which may meet the requirements for the application of topical preparations to prevent inflammation ( Figure 2) [50]. Zhen et al. focused on exploiting the boronate-based polymeric network as microneedles, which can be used as skin patches for acne vulgaris treatment. Due to the presence of boronate moieties, the antibiotic loaded composition possesses a H2O2-responsive release behavior [68].
There numerous reports focused on the development of H2O2-Gel as drug delivery systems [73,74]. However, there is still a lack of commercially available products. Further studies should pay attention to this point. Some other studies are still required. The potential side effects of the degraded products should also be investigated. Additionally, the specificity in non-enzyme systems should also be improved since the body fluid is a very complicated system.

Detection
H2O2 has been demonstrated to be a metabolite of many biochemical reactions. Qualitative H2O2 detection is an important tool which can be used to discover early lesions such as those present in cancer. As typical soft materials, hydrogels show a biophysical similarity to soft biological tissues and thereby possess a desirably good affinity with them. Moreover, the chemosensor moieties can be covalently linked or physically incor- Zhen et al. focused on exploiting the boronate-based polymeric network as microneedles, which can be used as skin patches for acne vulgaris treatment. Due to the presence of boronate moieties, the antibiotic loaded composition possesses a H 2 O 2 -responsive release behavior [68].
There numerous reports focused on the development of H 2 O 2 -Gel as drug delivery systems [73,74]. However, there is still a lack of commercially available products. Further studies should pay attention to this point. Some other studies are still required. The potential side effects of the degraded products should also be investigated. Additionally, the specificity in non-enzyme systems should also be improved since the body fluid is a very complicated system.

Detection
H 2 O 2 has been demonstrated to be a metabolite of many biochemical reactions. Qualitative H 2 O 2 detection is an important tool which can be used to discover early lesions such as those present in cancer. As typical soft materials, hydrogels show a biophysical similarity to soft biological tissues and thereby possess a desirably good affinity with them. Moreover, the chemosensor moieties can be covalently linked or physically incorporated into the matrix of hydrogels to endow the product with various diagnostic applications. In past decades, studies on utilizing H 2 O 2 -Gels as detection tools were reported [75,76].

Electrical
Tian's group developed a hydrogel-based biosensor system by incorporating Cytochrome c into the matrix of hydrogel consisting of Fmoc-L-lysine, Fmoc-L-phenylalanine, and sodium carbonate, in which the inherent bioactive activity of Cytochrome c was retained. After treatment with H 2 O 2 , the biochemical reaction between Cytochrome c and H 2 O 2 can result in a fast and sensitive change in the redox formal potential. Moreover, the cellular level H 2 O 2 can be quantitively determined. Such H 2 O 2 -responsive behavior was demonstrated to exhibit excellent specificity over commonly used ions, ascorbic acid, and other ROS. The good stability and reproducibility, as well as the high specificity and sensitivity, may pave a new path to further understanding the role of H 2 O 2 in pathological changes [18].
Li et al. prepared luminophore-incorporated polyaniline-polyacrylamide hydrogel that exhibited favorable biocompatibility and good conductivity. The obtained hydrogel showed a high detection sensitivity upon H 2 O 2 based on an electrochemiluminescence method. The limit of detection of H 2 O 2 was as low as 2.9 × 10 −9 M. The detection concentration ranged from 5 × 10 −5 to 1 × 10 −8 M. The results indicated that the prepared H 2 O 2 -Gel can be utilized to detect cellular H 2 O 2 [77].
The electrochemical detection of H 2 O 2 can also be achieved by constructing Hemin-G4/Au-containing composite hydrogels. In Hou's study [78], the excellent catalytic activity of Hemin-G4 and the high conductivity of gold nanoparticles were employed to endow the obtained hydrogels with a good electrochemical response towards H 2 O 2 . Moreover, A549 cells can adhere to the surface of the obtained hydrogels, which can be used to detect extracellular H 2 O 2 .
Electrical detection methods usually show low limit of detection (LOD) levels. The enzyme-incorporated ones exhibit good specificity, making them desirable detection tools in bioanalysis. Nevertheless, there is still a need for other detention methods that are low-cost and free of energy.

Fluorescent
Hydrogels with a fluorescent response to H 2 O 2 are fundamentally important in biology and pathophysiology which have practical value in early clinical diagnosis. In Yang's study [79], an intensive chemiluminescence hydrogel was proposed by incorporating luminol and hemin into the matrix of guanosine-derived hydrogel. The obtained composite hydrogel demonstrated enzyme-like activity towards the H 2 O 2 -mediated oxidation of luminol, in which the blue light can be visibly observed.
Our group synthesized a novel type of fluorescein-derived fluorescence probe by installing boronic acid groups at the 3 and 6 -positions of the xanthenone moiety in fluorescein. The obtained fluorescein derivative displayed almost no emission behavior (Φ f = ca.0.01). After treatment with H 2 O 2 , the fluorescein derivative can be degraded into fluorescein with strong fluorescence (Φ f = 0.94) [80]. The "turn-on" response in fluorescence resulted in a H 2 O 2 -detecting ability. Moreover, the two arylboronic acid groups in fluorescein derivative can react to diol units, which makes it a desirable crosslinker to fabricate the H 2 O 2 -responsive fluorescein change in the obtained hydrogels. Following this method, hydrogels with a "turn-on" fluorescence behavior upon H 2 O 2 administration were developed ( Figure 3A,B). The obtained hydrogel shows no emission behavior under the physiological H 2 O 2 concentration, while a fast non-fluorescent to fluorescent change can be achieved under pathological H 2 O 2 concentration [51].
Concerning the lack of an indicator to monitor the release behavior in real time for H 2 O 2 -Gels, the obtained hydrogels were also explored as drug carriers. As shown in Figure 3C, a very small amount of agent was released from DHNTs@PVA@PA under a low level of H 2 O 2 (0.02 µM), while the 4-fold increased initial-burst amount can be tracked for PTX@PVA@PA (DHNTs@PVA@PA: the therapeutic agent was pre-loaded into the halloysite nanotubes; PTX@PVA@PA: the therapeutic agent was directly loaded into the matrix of hydrogels). The significant increase of the release rates can be observed in 200 µM H 2 O 2 and compared to those in 0.02 µM H 2 O 2 , suggesting a typical H 2 O 2 -responsive release behavior. Figure 3D shows a remarkable increase of the fluorescence intensity over time from 5 to 40 min. The relationship between the release rate and fluorescence intensity was investigated and displayed in Figure 3E. The good relationship is quite beneficial to invisibly monitoring the drug release behavior. Concerning the lack of an indicator to monitor the release behavior in real time for H2O2-Gels, the obtained hydrogels were also explored as drug carriers. As shown in Figure 3C, a very small amount of agent was released from DHNTs@PVA@PA under a low level of H2O2 (0.02 μM), while the 4-fold increased initial-burst amount can be tracked for PTX@PVA@PA (DHNTs@PVA@PA: the therapeutic agent was pre-loaded into the halloysite nanotubes; PTX@PVA@PA: the therapeutic agent was directly loaded into the matrix of hydrogels). The significant increase of the release rates can be observed in 200 μM H2O2 and compared to those in 0.02 μM H2O2, suggesting a typical H2O2-responsive release behavior. Figure 3D shows a remarkable increase of the fluorescence intensity over time from 5 to 40 min. The relationship between the release rate and fluorescence intensity was investigated and displayed in Figure 3E. The good relationship is quite beneficial to invisibly monitoring the drug release behavior.

Colorimetric
As a typical pH indicator, phenolphthalein solution can present a red color under an alkalescence environment based on the transformation between the phenolic hydroxyl groups and the benzoquinone unit. In our study, this colorimetric behavior was employed for the visual detection of H 2 O 2 . Boronic acid pinacol ester groups were used to substitute the hydrogel groups in phenolphthalein. As a result, the transformation between the phenolic hydroxyl groups and benzoquinone unit was blocked. Then, the phenolphthalein derivative was used to crosslink polyvinyl alcohol chains in a weak basic solution to yield a H 2 O 2 -Gel based on the dynamic covalent bonds. After treatment with H 2 O 2 under a pathological concentration, the B-C linkage in the network can be rapidly degraded into Ar-OH groups and B-OH groups result in the formation of phenolphthalein. Since the hydrogel was prepared under an alkalescence environment, the obtained hydrogel showed a distinct colorimetric response to H 2 O 2 ( Figure 4) [52].
As a typical pH indicator, phenolphthalein solution can present a red color under an alkalescence environment based on the transformation between the phenolic hydroxyl groups and the benzoquinone unit. In our study, this colorimetric behavior was employed for the visual detection of H2O2. Boronic acid pinacol ester groups were used to substitute the hydrogel groups in phenolphthalein. As a result, the transformation between the phenolic hydroxyl groups and benzoquinone unit was blocked. Then, the phenolphthalein derivative was used to crosslink polyvinyl alcohol chains in a weak basic solution to yield a H2O2-Gel based on the dynamic covalent bonds. After treatment with H2O2 under a pathological concentration, the B-C linkage in the network can be rapidly degraded into Ar-OH groups and B-OH groups result in the formation of phenolphthalein. Since the hydrogel was prepared under an alkalescence environment, the obtained hydrogel showed a distinct colorimetric response to H2O2 (Figure 4) [52]. Yang et al. [81] reported a 2D photonic crystal-based horseradish peroxidase/bovine serum albumin (HRP/BSA) composite hydrogel. BSA and HRP was crosslinked by glutaraldehyde, in which BSA and HRP acts as a scaffold and a recognition unit, respectively. The obtained hydrogel can monitor H2O2 since HRP can selectively decompose H2O2 along with heme inactivation. The conformational change of HRP can decrease the crosslinking density and enlarge the particle spacing. As a result, the Debye ring of the 2-D photonic crystals also decreased, which provided a colorimetric response to H2O2 for the obtained hydrogel.
Compared to electrical and fluorescent methods, the colorimetric method shows a superior low cost, an easy operation, and operates free of energy. Moreover, the latter Yang et al. [81] reported a 2D photonic crystal-based horseradish peroxidase/bovine serum albumin (HRP/BSA) composite hydrogel. BSA and HRP was crosslinked by glutaraldehyde, in which BSA and HRP acts as a scaffold and a recognition unit, respectively. The obtained hydrogel can monitor H 2 O 2 since HRP can selectively decompose H 2 O 2 along with heme inactivation. The conformational change of HRP can decrease the crosslinking density and enlarge the particle spacing. As a result, the Debye ring of the 2-D photonic crystals also decreased, which provided a colorimetric response to H 2 O 2 for the obtained hydrogel.
Compared to electrical and fluorescent methods, the colorimetric method shows a superior low cost, an easy operation, and operates free of energy. Moreover, the latter method can be used without any sophisticated or expensive analytical instruments, which makes it a desirable detection tool for actual production and daily life.

Glucose-Responsive
Diabetes is regarded as a serious metabolic disorder, which affects more than onetenth of the population in the world [82,83]. Long-term high glucose levels can result in serious cardiovascular disease, renal failure, abnormal bone metabolism, and many other complications. The glucose-controlled insulin release system is usually required in the treatment of diabetes, especially for suppressing diabetes ulcers [84]. Among these systems, glucose oxidase is an important intermediary for the transformation of glucose signals into the generation of H 2 O 2 [85]: Therefore, with the addition of glucose oxidase, H 2 O 2 -Gel can also be used as glucoseresponsive systems. Kiritoshi et al. prepared H 2 O 2 -Gel via a typical radical copolymerization of 2-methacryloyloxyethyl phosphorylcholine and triethylene glycol dimethacrylate. The obtained polymer can be easily degraded in the presence of H 2 O 2 (974 mmol/L). In Kiritoshi's strategy, the glucose oxidase can be loaded into the matrix of the obtained hydrogel without a loss in catalytic activity. Following this method, the obtained H 2 O 2 -Gel is also available in glucose-responsive insulin delivery systems in a concentration-dependent manner [86].
In past decades, an increasing number of studies have focused on transforming H 2 O 2 -Gels into glucose-responsive systems with the addition of glucose oxidase [64,87]. For example, Yang et al. developed a hydrogel that formed in situ by loading glucose oxidase into keratin hydrogels, which showed promising applications for the treatment of diabetic wounds [64].

ROS Scavengers
An excessive ROS level can raise the oxidative damage to cellular macromolecules and thereby deteriorate many inflammation-related diseases. Moreover, the ROS accumulated in a wound can induce strong inflammation and serve as a strong barrier that inhibits tissue regeneration, which is a puzzling problem for diabetic ulcers [88]. Some H 2 O 2 -Gels are promising ROS scavengers in clinical applications [35,89], especially those that have the capacity to consume H 2 O 2 in a concentration-dependent manner [90].
Liu's group developed a type of ROS-scavenger by using a difunctional arylboronic derivative as crosslinker [91]. The obtained H 2 O 2 -Gels can act as an effective ROS-scavenger agent to promote wound-healing by reducing the ROS level around the wound, which is quite beneficial to inhibiting aseptic inflammation. Moreover, the H 2 O 2 -Gel was also explored as a drug carrier to allow the release of therapeutic agents, such as antibacterial agents and growth factors. As a result, the H 2 O 2 -Gel supported a systemic therapy strategy to promote wound-healing in complicated infections.
Gao et al. presented a novel type of thioether-containing injectable hydrogel with H 2 O 2 -responsive behaviors, which can be used as ROS-scavengers for myocardial infarction treatment [92]. The resulting animal model showed that the obtained H 2 O 2 -Gels can significantly consume excessive ROS, accelerate angiogenesis, and improve cardiac functions.

Tissue Engineering
Hydrogels have been demonstrated to be a desirable candidate for cartilage regenerative substrates owing to their attractive advantages including mild reaction conditions and tunable structures. The controllable gelatinizing process, especially for the gelation time, is the key issue when hydrogel is used for articular cartilage engineering. Rahbarghazi et al. developed a library of alginate-based hydrogels crosslinked via horseradish peroxidase. Generally, horseradish peroxidase can serve as an efficient enzyme catalysis in catalyzing the conversion from H 2 O 2 to H 2 O and O 2 [93]: As a result, the gelation time can be well-controlled based on the concentration of H 2 O 2 , which is a result of the deactivation effect of horseradish peroxidase by H 2 O 2 [94].
It is worth noting that the tissue engineering materials are usually composed of different parts to meet the demand in terms of therapeutics. Liu's group developed an injectable, post-trauma microenvironment-responsive, H 2 O 2 depleting, and drug-loaded hydrogel [95], which can be used to lower the H 2 O 2 levels in damaged brain tissue. The poly (propylene sulfide)-containing matrix endowed the obtained hydrogel with H 2 O 2responsive abilities. The released curcumin can further eliminate the ROS, promoting the regeneration and recovery of neurons. Following this method, a systematic and multi-channel promoting effect can be achieved, which is quite useful in tissue engineering materials.

Cell-Encapsulation
Hydrogel particles have been studied in the latest decade for encapsulating and delivering cells in humans, which meets many of the demands for treating diseases and producing antibodies for vaccine use. The main advantage of hydrogel-based cell-encapsulation technology is a mild reaction condition which does not threaten the lives of the enclosed cells.
Sakai et al. reported a facile approach for preparing mammalian cell-encapsulating hydrogel particles based on the horseradish peroxidase-crosslinked hydrogelation [96]. Gelatin bearing phenolic hydroxyl moieties were employed as the polymeric substrate and dissolved in a cell-suspending solution containing horseradish peroxidase. The mixed solution was then dropped into 1 mM H 2 O 2 to form hydrogels with a spherical shape. The α-H in phenolic hydroxyl moieties usually exhibits high activity with the presence of horseradish peroxidase and H 2 O 2 [19]. These phenomena suggested that the H 2 O 2 -responsive behavior can be used to establish a cell-encapsulation method in hydrogel particles.
In Sakai's proposed strategy [19], the individual cell can be encapsulated into a thin hydrogel sheath. This approach can be controlled by the addition of HRP and exhibits no significant adverse effects to the encapsulated cells. Apparently, the proposed approach based on the H 2 O 2 -responsive behavior is expected to be used to extend the applications of cell encapsulation technology.

Conclusions and Perspective
Recent studies of H 2 O 2 -Gels have been reviewed in this paper. The summarized H 2 O 2responsive linkages and preparation methods may serve as guides for researchers who are interested in H 2 O 2 -Gels. We expect to provide guidance and designate the direction of further studies on H 2 O 2 -Gels. In combination with drug delivery, the detection of H 2 O 2 , glucose-responsive systems, ROS scavengers, tissue engineering, and cell-encapsulation, H 2 O 2 -Gels could be advanced to have much more promising applications in biomedicine. Nevertheless, commercially available H 2 O 2 -Gels, as well as official standards or criteria, are also urgently needed. There are still some limitations for this issue. The level of H 2 O 2 usually differs among individuals, and the complexity of bodily fluids should also be considered. Therefore, it is far more difficult to establish in vitro and in vivo correlation (IVIVC), which is rarely studied but indeed remains a key issue that should be focused on. On the other hand, costly proteinases seriously limit large-scale production. Nonproteinase ones usually suffer from other problems such as stability, complexity in the synthesis approach, the potential toxicity of the degraded by-products, etc. Thus, longerduration studies are encouraged to conduct deeper investigations to make H 2 O 2 -Gels "over the counter".