*2.1. Hemostatic Properties*

Hemostasis is the first stage of wound healing, starting with wound formation and ending with thrombosis. The physiological mechanism of human hemostasis is a complex dynamic process consisting of three processes: vasoconstriction, formation of platelet embolism, and coagulation (Figure 2). Vasoconstriction is a transient reflex contraction that is the first step in preventing blood loss during injury (Figure 2a). Thromboembolism is the second critical stage of hemostasis, involving platelet adhesion, activation, and aggregation into embolization. During the damage, platelets bind to von Willebrand factor (VWF) aggregates via their GP1b receptor and directly adhere to exposed subendothelial collagen via two receptors (glycoprotein VI (GP VI) and integrin α2β1) [27]. This adhesion triggers an activation process of platelets, which leads to a change in shape and the release of particles rich in substances such as serotonin, thromboxane A2 (TXA2), and adenosine diphosphate (ADP). Because they alter the conformation of GPIIb/IIIa, a receptor on the surface of platelets that enables platelets to bind to fibrinogen and induce platelets to aggregate into the walls of damaged blood arteries (Figure 2b). Primary hemostasis is a brief stage of hemostasis that finally results in the creation of a first embolism. The extrinsic and intrinsic pathways are two separate processes that take place during the second stage of hemostasis [28]. The extrinsic pathway, which starts when a blood artery is damaged, involves the interaction of tissue factor (TF) that is exposed to the blood and coagulation factor VII (FVII). When factor XII comes into contact with a foreign substance having a negatively charged surface, intrinsic pathways are activated. The activation of factor X, an enzyme that assists in the conversion of prothrombin to thrombin (thrombin transforms fibrinogen to fibrin monomers), is the result of these two processes (common pathway). Finally, fibrin monomers self-polymerize into fibrils, which laterally aggregate to create fibers and establish a hydrogel (Figure 2c). The interfiber and interfiber crosslinking caused by factor XIIIa stabilizes the fibrin gel and enhances clot stiffness.

According to cell-based models of hemostasis, the coagulation process has three overlapping stages: initiation, amplification, and propagation. When TF-bearing cells are exposed to flowing blood after vascular damage, the initiation phase begins, which results in the generation of activated factors IX and X as well as thrombin. Such thrombin serves several important roles during the amplification stage. Along with activating platelets, it also causes the cofactors V and VIII to become Va and VIIIa on the surface of the activated platelets. Thrombin activates factor XI on the surface of the platelet during this stage [8]. The proliferation that takes place on the surface of activated platelets during the last stage of this cellular model and eventually results in a burst of thrombin that is sufficient to create the fibrin mesh. Hydrogels can facilitate the hemostasis process on wet surfaces through three typical approaches: i) direct or indirect participation in the coagulation system to activate the physiological hemostatic process; ii) enrichment of the wound site with coagulation components (e.g., polymeric polysaccharides, inorganic zeolite clay, etc.) by physical and chemical means; iii) physical closure of blood vessels using the strong adhesion of the material to the tissue.

**Figure 2.** Schematic diagram of the hemostatic process: (**a**) vascular spasm. Immediately after injury, blood vessels are stimulated to constrict to reduce blood loss; (**b**) platelet plug formation. Once platelets are activated, the shape changes, releasing serotonin, ADP, and thromboxane A2, etc., causing more platelets to concentrate at the site of injury to form a thrombus; (**c**) coagulation cascade. Endogenous and exogenous coagulation pathways are activated, involving the interaction of factor (XII) and coagulation factor VII in the blood, with the two pathways eventually converging into a common pathway that activates factor X, which further converts prothrombinogen to thrombin. The generated thrombin activates Factor XIII, which promotes the conversion of fibrinogen into fibrin chains for hemostasis. Figures modified from [11] with permission. Copyright 2020. **Figure 2.** Schematic diagram of the hemostatic process: (**a**) vascular spasm. Immediately after injury, blood vessels are stimulated to constrict to reduce blood loss; (**b**) platelet plug formation. Once platelets are activated, the shape changes, releasing serotonin, ADP, and thromboxane A2, etc., causing more platelets to concentrate at the site of injury to form a thrombus; (**c**) coagulation cascade. Endogenous and exogenous coagulation pathways are activated, involving the interaction of factor (XII) and coagulation factor VII in the blood, with the two pathways eventually converging into a common pathway that activates factor X, which further converts prothrombinogen to thrombin. The generated thrombin activates Factor XIII, which promotes the conversion of fibrinogen into fibrin chains for hemostasis. Figures modified from [11] with permission. Copyright 2020.

### According to cell-based models of hemostasis, the coagulation process has three overlapping stages: initiation, amplification, and propagation. When TF-bearing cells are *2.2. Adhesion Properties*

### exposed to flowing blood after vascular damage, the initiation phase begins, which results 2.2.1. Four Theories of Bio-Adhesion

in the generation of activated factors IX and X as well as thrombin. Such thrombin serves several important roles during the amplification stage. Along with activating platelets, it also causes the cofactors V and VIII to become Va and VIIIa on the surface of the activated platelets. Thrombin activates factor XI on the surface of the platelet during this stage [8]. The proliferation that takes place on the surface of activated platelets during the last stage of this cellular model and eventually results in a burst of thrombin that is sufficient to create the fibrin mesh. Hydrogels can facilitate the hemostasis process on wet surfaces through three typical approaches: i) direct or indirect participation in the coagulation system to activate the physiological hemostatic process; ii) enrichment of the wound site with coagulation components (e.g., polymeric polysaccharides, inorganic zeolite clay, etc.) by physical and chemical means; iii) physical closure of blood vessels using the strong adhesion of the material to the tissue. *2.2. Adhesion Properties*  2.2.1. Four Theories of Bio-Adhesion There are four typical bio-adhesion theories, including wetting, mechanical, diffusion, and fracture theories [29] (Figure 3). Wetting is a phenomenon caused by liquid diffusion along a solid surface. Surface tension, capillary force, Van der Waals force, etc., are the basic causes of wet adhesion [30] (Figure 3a). Wet adhesion is an approach in which hydrogels can enhance adherence. According to the wetting hypothesis, capillary actions are brought on by curved water surfaces and tight contact between the interfaces, which There are four typical bio-adhesion theories, including wetting, mechanical, diffusion, and fracture theories [29] (Figure 3). Wetting is a phenomenon caused by liquid diffusion along a solid surface. Surface tension, capillary force, Van der Waals force, etc., are the basic causes of wet adhesion [30] (Figure 3a). Wet adhesion is an approach in which hydrogels can enhance adherence. According to the wetting hypothesis, capillary actions are brought on by curved water surfaces and tight contact between the interfaces, which results in shortdistance interactions [31]. The heterogeneity of the hydrogel is a key element in determining its adhesiveness. Hydrogel adhesions to hydrophobic surfaces are poor in the air, yet they may cling to hydrophilic surfaces quite effectively. This is because water will moisten the surface and create a water meniscus at the border of the contact region. As a consequence, in addition to molecular interactions, capillary adhesions also exist at the interface of the two adhesives [32]. The groups on the polymer can interact with the interface, however, when a hydrogel is submerged in water, pressure is usually needed to remove free water. The interface adhesion is related to the surface energy, therefore, appropriate surface energy is necessary to afford firm adhesion [33]. Inspired by the structures of tree frog toes, Nguyen et al. evaluated the wet stickiness of the contact interface under various circumstances [34]. The findings demonstrated that, in comparison to the surface of the non-patterned plate, the micro pattern cushion's surface boosted the interface adhesion in a wet environment. The effect of contact shape and substrate morphology on adhesion was investigated by Liu et al. [30]. The findings demonstrated that the interstitial liquid's contact geometry, and capillary density are strongly connected to the wet adhesion, and the ratio of capillary and adhesive forces is key to elucidating the mechanism of wet adhesion.

results in short-distance interactions [31]. The heterogeneity of the hydrogel is a key element in determining its adhesiveness. Hydrogel adhesions to hydrophobic surfaces are

mechanism of wet adhesion.

**Figure 3.** (**a**) Wetting theory. (**b**) Diffusion theory. (**c**) Fracture theory. Figures modified from [35] with permission. (**d**) Mechanical theory. (**e**) The principle of double network hydrogel enhances adhesion. The blue network represents polyacrylamide, the orange network represents alginate, and the red dots represent calcium ions. (**f**) Principle of topological binding. The chitosan chains are dissolved in a solution at pH = 5. The chitosan chains diffuse into the gel at pH = 7, using hydrogen bonds to form a new network. Figures modified from [36] with permission. **Figure 3.** (**a**) Wetting theory. (**b**) Diffusion theory. (**c**) Fracture theory. Figures modified from [35] with permission. (**d**) Mechanical theory. (**e**) The principle of double network hydrogel enhances adhesion. The blue network represents polyacrylamide, the orange network represents alginate, and the red dots represent calcium ions. (**f**) Principle of topological binding. The chitosan chains are dissolved in a solution at pH = 5. The chitosan chains diffuse into the gel at pH = 7, using hydrogen bonds to form a new network. Figures modified from [36] with permission.

poor in the air, yet they may cling to hydrophilic surfaces quite effectively. This is because water will moisten the surface and create a water meniscus at the border of the contact region. As a consequence, in addition to molecular interactions, capillary adhesions also exist at the interface of the two adhesives [32]. The groups on the polymer can interact with the interface, however, when a hydrogel is submerged in water, pressure is usually needed to remove free water. The interface adhesion is related to the surface energy, therefore, appropriate surface energy is necessary to afford firm adhesion [33]. Inspired by the structures of tree frog toes, Nguyen et al. evaluated the wet stickiness of the contact interface under various circumstances [34]. The findings demonstrated that, in comparison to the surface of the non-patterned plate, the micro pattern cushion's surface boosted the interface adhesion in a wet environment. The effect of contact shape and substrate morphology on adhesion was investigated by Liu et al. [30]. The findings demonstrated that the interstitial liquid's contact geometry, and capillary density are strongly connected to the wet adhesion, and the ratio of capillary and adhesive forces is key to elucidating the

On the other hand, a rough interface, in accordance with mechanical theory, increases the surface area that can make contact as well as the viscoelastic and plastic energy dissipation during joint breakage (Figure 3d) [35]. The first bonding hypothesis, known as me-On the other hand, a rough interface, in accordance with mechanical theory, increases the surface area that can make contact as well as the viscoelastic and plastic energy dissipation during joint breakage (Figure 3d) [35]. The first bonding hypothesis, known as mechanical interlocking (or mechanical interlocking), was proposed by MAC Bain and Hopkins in 1925 [37] and referred to the interlocking of the adhesive with the microscopic rough surface of the bonded object. Traditional alveolar bone filling methods achieve adhesion between the alveolar bone and the pretreated tooth surface, which is facilitated by mechanical interlocking [38]. Inspired by this phenomenon, bonding and adhesive can be achieved by filling the hydrogel in the pores of the substance. By utilizing the catechol-mediated synergized adhesion and interlocking in the pores of diatom silica, Lee et al. prepared a diatomaceous earth/polysaccharide elastic hydrogel, which showed good biocompatibility and can be strongly adhered to the skin [39]. In a separate study, sodium alginate and acrylamide were employed as raw ingredients by Yuan et al. to create a mechanically interlocked double-net hydrogel with enhanced adhesive capabilities and mechanical properties [40]. Such a hydrogel was competent in blocking the perforation and encouraging the healing of tissues.

In addition, the adhesion of the hydrogel to the mucosal layer can be explained by diffusion theory. For instance, certain bioadhesive polymers may dissolve in mucus mucin, and flexible polymer chains can physically entangle with mucin chains to provide an advantageous adhesion effect (Figure 3b) [35]. According to the theory of diffusion, adhesion is produced by the reciprocal diffusion of molecules between the substrate and the adhesive, therefore, macromolecular chains require sufficient solubility and flexibility for mutual diffusion. Due to the presence of an interpenetrating network structure, the hydrogel can create interdiffusion among polymers to increase its adhesion qualities. Feng et al. prepared silk fibroin/konjac glucomannan sponges with an interpenetrating network for wound dressing, which showed high biocompatibility for cell adhesion and proliferation [41]. In another study, Vorwald et al. created fibrin-alginate interpenetrating network hydrogels, which combined the excellent adhesion and stimulatory characteristics of fibrin with the adaptable mechanical properties of alginate for cellular orientation and distribution by combining [42]. The fracture theory was typically used to determine the maximal tensile stress. The toughness of the stiff polymer without flexible chains may also be calculated using this method (Figure 3c).

### 2.2.2. Influence Factors of Wet Surface Adhesion

In practice, the adhesion of hydrogels involves a complex interaction of chemical, physical and structural factors (Figure 4). Therefore, understanding these factors will help us to design the wet adhesion strategy for a given application. In this section, the chemical, physical, and structural factors that influent the wet surface adhesion of hydrogels are briefly summarized (Table 1). *Gels* **2023**, *8*, x FOR PEER REVIEW 7 of 23

**Figure 4.** The practical application of hydrogels in wet adhesion involves complex interactions that can be divided into three main categories: chemical, physical, and bionic structural factors. (**a**) Chemical factors, such as catechol chemistry, in situ activation, and reactions of intrinsic groups of the material. (**b**) Physical factors, such as hydrophobic aggregation, physical entanglement, and energy dissipation. (**c**) Biomimetic structural factors, such as micro-patterning and capillary forces. Figure modified from [43] with permission. **Figure 4.** The practical application of hydrogels in wet adhesion involves complex interactions that can be divided into three main categories: chemical, physical, and bionic structural factors. (**a**) Chemical factors, such as catechol chemistry, in situ activation, and reactions of intrinsic groups of the material. (**b**) Physical factors, such as hydrophobic aggregation, physical entanglement, and energy dissipation. (**c**) Biomimetic structural factors, such as micro-patterning and capillary forces. Figure modified from [43] with permission.

The adhesion capacity of hydrogels is usually related to the formation of ionic, cova-

primary way to enhance adhesion. Covalent bonds have higher underwater adhesion strength than intermolecular forces because they require sharing of electrons. By creating covalent bonds, hydrogels exhibit underwater adhesion and cohesion, making them attractive to substrates containing reactive functional groups [45]. When the hydrogel is used underwater, the presence of chemical or physical crosslinks prevents it from overswelling and impairs adhesion [46]. However, covalent-based underwater bonding usually involves surface modification or is dependent on the specific adhered surface [47]. Catechol chemistry is one of the most frequently used interactions among the many functional groups. Studies have uncovered facile methods to synthesize catechol polymers and molecules that can be grafted and coated multifunctionally into other materials [48]. Due to their inherent good biocompatibility, catechols are widely used for tissue adhesion in humid environments [49] (Figure 4a). Moreover, the groups in catechol hydrogels (containing ortho-quinone groups) can establish covalent bonds with the nucleophiles on the surface of the tissue and thus adhere to the membranes of biological tissues [50]. Zhou et al. spliced the catechol moiety of dopamine (DA) in the polymer chain of hyaluronic acid

(1) Typical Chemical Bond

### (1) Typical Chemical Bond

The adhesion capacity of hydrogels is usually related to the formation of ionic, covalent, and metal coordination bonds between the hydrogels and tissue [44]. Covalent bonding, in which functional groups chemically react with groups on the target surface, is the primary way to enhance adhesion. Covalent bonds have higher underwater adhesion strength than intermolecular forces because they require sharing of electrons. By creating covalent bonds, hydrogels exhibit underwater adhesion and cohesion, making them attractive to substrates containing reactive functional groups [45]. When the hydrogel is used underwater, the presence of chemical or physical crosslinks prevents it from over-swelling and impairs adhesion [46]. However, covalent-based underwater bonding usually involves surface modification or is dependent on the specific adhered surface [47]. Catechol chemistry is one of the most frequently used interactions among the many functional groups. Studies have uncovered facile methods to synthesize catechol polymers and molecules that can be grafted and coated multifunctionally into other materials [48]. Due to their inherent good biocompatibility, catechols are widely used for tissue adhesion in humid environments [49] (Figure 4a). Moreover, the groups in catechol hydrogels (containing ortho-quinone groups) can establish covalent bonds with the nucleophiles on the surface of the tissue and thus adhere to the membranes of biological tissues [50]. Zhou et al. spliced the catechol moiety of dopamine (DA) in the polymer chain of hyaluronic acid hydrogels, which established covalent bonds with nucleophiles (amines, thiols, and hydroxyl groups) and thus adhered to the wet biological tissue [51]. In another study, Ma et al. prepared in situ photo-responsive chitosan (CS) hydrogels based on the imine cross-linking method. After exposing the hydrogels to UV light, *o*-nitrophenyl was converted to the *o*-nitrobenzaldehyde group. This group was further cross-linked with the amino group on the tissue surface, thus forming a covalent bond that allows the wet tissue adhesion [52].

Covalent connections can be further classified into permanent and dynamic covalent bonds in physiological contexts, which have significant advantages in the application of hydrogel bio-adhesion. Permanent covalent bonds can be carbon–sulfur, carbon–carbon junctions, carbon–nitrogen, and silicon–oxygen bonds. Permanent covalent bonds are stable, hard, and irreversible. By irreversibly rupturing covalent or neighboring bonds, these bioadhesive hydrogels can be separated. Adhesives with permanent covalent typically adhered to the surface by reacting with other functional groups. Dynamic covalent bonds (DCBs) can reversibly break and form covalent bonds under specific conditions. DCBs can be formed by the linkage of disulfides, imides, hydrazides, phenylboronic acids, and Diels–Alder reactions. The use of dynamic covalent bonding, combined with the favorable properties of chemical and physical adhesion, allows the design of robust and reversible bioadhesive hydrogel dressings. During the adhesion process, DCBs can assemble a hydrogel network with self-healing capabilities between two adherent gel networks or interfaces. Such gels are widely used for wound closure and wound hemostasis. For example, Yang et al. developed an injectable mussel mucoadhesive self-healing hydrogel with good bioadhesive and hemostatic properties based on C-N single bonds and C-N double bonds [53]. Compared to conventional covalently linked hydrogels, hydrogels containing DCBs exhibited significant self-healing ability and good cytocompatibility. Using aminoglycoside, aldehyde-based hyaluronic acid, and adipic dihydrazide grafted hyaluronic acid as raw materials, Li et al. created the dynamic covalently cross-linked hydrogels. Such hydrogels were formed based on the imine and hydrazone cross-linking and showed strong and long-lasting antimicrobial properties, good biocompatibility, and self-healing ability [54].

(2) Hydrogen Bond

A hydrogen bond is created by the dipole–dipole attraction of two electronegative atoms. The hydrogen bond is an important driving force for hydrogel formation. For example, a novel pH-sensitive hydrogel was prepared by compositing konjac glucomannan, dopamine hydrochloride, L-cysteine hydrochloride, and epigallocatechin acid esters. Hydrogen bonds and catechol-mediated coordination were responsible for the hydrogel formation. This hydrogel possessed injectability and adhesion properties for potential use in drug delivery and release controllability [55]. In another study, Zhang et al. designed the catechol-modified polylysine/polyacrylamide hydrogel. The numerous hydrogen bonds rendered the hydrogel's strong adhesion, high compressive strength, and effective hemostasis, and can be used to seal bleeding sites [56]. Typically, there existed a counterpart between the hydrogel-wet surface hydrogen bonds and water molecules–induced hydrogen bonds. To enhance the wet surface adhesion of hydrogel, the water molecules-induced hydrogen bonds should be suppressed. A typical way to inhibit hydrogen bonding caused by water molecules is to repel or absorb water molecules from the surface (Figure 5a,b). The water film formed by these water molecules is considered to be the hydration layer. At the molecular level, the hydration layer at the hydrogel-attachment interface can be resisted by hydrophobic interactions. Therefore, hydrophobic solvents and hydrophobic monomers are usually used in hydrogels in order to improve wet adhesion [45,57]. Spraying the hydrophobic solvent on the surface of the hydrogel can also form a thin hydrophobic layer with a small contact angle with water [58]. When external pressure is applied to the hydrogel, the hydration layer can be broken and discharged from the adhered surface. Similarly, some hydrogels contain hydrophobic monomers (e.g., with long carbon chains or aromatic ring structures) that can repel water molecules and stick firmly thereto [59,60].

In contrast, the method of absorbing the hydration layer is simple than repelling the hydration layer. The absorption of the hydration layer is based on the fact that hydrogels contain highly hygroscopic materials [61]. For example, Wang et al. prepared an in situ photocuring hygroscopic hydrogel adhesive by using polyvinylpyrrolidone (PVP, as a hygroscopic polymer), acrylic acid, crosslinker, and photoinitiator [62]. Cong et al. designed an anthracene-based polyethyleneimine underwater adhesion hydrogel, and the high water absorption of polyethyleneimine can promote the absorption of interfacial water molecules and enable the hydrogel to adhere firmly to other substances [63].

(3) Van der Waals Force

Environmental stimuli can easily disrupt some of the adhesions generated based on chemical reactions. By contrast, less reactive physical factors can be used to durably strengthen wet adhesion. Van der Waals forces, electrostatic forces, hydrogen bonding, and hydrophobic interactions are representative examples of non-covalent interactions between interfaces, which affords physical adhesion. Van der Waals force is a weak force, especially in water, therefore, it is not the main driver of the adhesion of hydrogels and other substances. However, van der Waals forces can be aggregated into large volumes of material to provide strong adhesion. For example, geckos can stay and crawl normally on vertical walls due to the aggregated van der Waals forces generated by the contact of their feet with the walls [64]. Yi et al. created a bio-inspired wet hydrogel adhesive using polyethylene glycol hydrogel as a starting material by exploiting the water absorption properties, capillary forces, and van der Waals forces of hydrogels [65]. The mixture has excellent biological application potential and exhibited extraordinary reversible adherence to dry, wet, and submerged substrates. In another study, Sato et al. found that the Van der Waals forces are responsible for the adhesion of submicron silica particles to the surfaces of poly(acrylamide) and poly(dimethyl-acrylamide) hydrogels [66].

### (4) Electrostatic Interactions

Adding electrostatic interactions to hydrogels can improve biological wet adherence. Through electrostatic interactions, hydrogels can firmly adhere to the surface of different substances [67]. To create hydrogels with strong adhesion, Huang et al. prepared hybrid hydrogels with ionic and hydrophobic cross-linked networks, with strong adhesion and high toughness properties attributed to the synergistic effect of electrostatic interactions and hydrophobic junctions [68]. Tian et al. designed a strong and stable adhesive hydrogel with synergistic hydrophobic interactions and dynamic electrostatic forces using 2-acrylamido-2-methylpropanesulfonic acid, gelatin, CS, ethyl 2-methoxyacrylate, and acrylic acid as

raw materials. The hydrogel could adhere stably and firmly to various tissue surfaces [69]. Song et al. prepared a multifunctional physical hydrogel adhesive using catechol-modified CS and polyvinyl alcohol as raw materials. Dynamic hydrogen bonding and electrostatic interactions improved the persistence and reproducibility of this hydrogel adhesive [70].
