1 Introduction
The magic of nature is that each animal has its own unique ability to survive. Mussels in the ocean have their own unique survival secrets. Wild mussels usually grow on the surface of rocks and reefs, showing strong cohesive forces
[1,2]. They can also adhere to the outer surface of the hull and affect the navigation of the ship. Studies have found that mussels can adhere to almost any material surface, and can form permanent adhesion on mineral, paraffin, polypropylene, metal, glass, teeth and bone surfaces. In the underwater environment, the mussel foot plate can adhere to the adhered solid surface regardless of whether the adhered solid surface is hydrophilic or hydrophobic. It can even adhere to the surface of the famous superhydrophobic material PTFE
[3]. On mica, a hard, inert material with a micro-smooth surface, mussels can still rely on foot plates to produce adhesion forces equivalent to three times their own weight
[4]. Ordinary adhesives are often unable to bond underwater, and they will lose their bonding ability if they are immersed underwater for a long time, while mussels can bond underwater and have a strong and lasting bonding force.
People have been studying the macroscopic and microscopic structure of mussel adhesion proteins, and gradually revealed what structure endows mussels with this extraordinary ability. Mussel foot proteins (Mfps) are secreted by Mussel foot glands to form Byssus and Plaque, by which mussels adhere to the substrate surface (as shown in Figure 1).
The filopodia and footplate are composed of a variety of adhesive proteins (Mfps), and almost every adhesive protein contains L-3,4-dihydroxyphenylalanine (DOPA)
[6,7]. By analyzing the composition and adhesion mechanism of mussel adhesion protein, researchers have prepared a variety of DOPA-modified biomimetic underwater adhesive materials by biomimetic means, which can realize underwater adhesion of substrates of different materials.It has replaced the traditional underwater mechanical connection in more and more fields, thus providing an effective way to simplify the process, save energy, reduce costs and improve economic efficiency for various industries
[8,9][10]. In addition, this kind of underwater adhesive material has good underwater adhesive performance, biocompatibility, degradability and other characteristics, and also has broad development space in medical fields such as surgery and tissue repair
[11].
2 Composition of Mussel Adhesion Protein
Since Waite et al began to study the adhesion of mussel in 1983, the composition and structure of mussel adhesion protein have been clarified, and great progress has been made in the study of its adhesion mechanism
[5,12⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓~50].
It has been found that mussels adhere to various solid surfaces by Byssus and Plaque. Mussels produce silk feet and foot plates through a physiological process similar to "injection molding production". Their strong feet will produce ditches. Mussels squeeze liquid protein into the trench, and in just a few seconds, they form a hair-like silk foot, each of which has a sticky plate at the end, which can firmly adhere to rocks and solid surfaces. The results showed that there were six kinds of adhesion proteins in filopodia and podocytes, which were Mfp-1, Mfp-2, Mfp-3, Mfp-4, Mfp-5 and Mfp-6, respectively
[5]. Their molecular weight, isoelectric point and DOPA content are shown in Table 1.
The molecular weight of Mfp-1 is about 110 kDa, and the content of DOPA is about 15 mol%. Mfp-1 is mainly distributed in the surface layer of filopodia and pedal disc, and its function is to wrap and protect other proteins in filopodia and pedal disc
[19,41⇓⇓⇓~45]. Mfp-3 is located at the contact interface between the foot plate and the substrate surface, and its function is to make the foot plate adhere to the substrate. The main components of Mfp-3 are glycine (25 mol% ~ 29 mol%) and asparagine (10 mol% ~ 18 mol%). Mfp-3 can not only be crosslinked, but also the hydroxyl groups on DOPA can form covalent bonds and hydrogen bonds with the substrate surface, which is the main source of adhesion between the foot plate and the solid surface. The adhesion energy of Mfp-3 could reach 0.3 mJ/m
2, which was equivalent to the adhesion force of 0.98 N provided by one foot plate. Mfp-3 can be divided into two subclasses, called fast Mfp-3 (Mfp-3F) and slow Mfp- 3 (Mfp- 3S). Among them, Mfp-3S is the first known adhesive protein capable of self-aggregation
[25,26]. The molecular weights of Mfp-2 and Mfp-4 are relatively large. The content of DOPA in Mfp-2 and Mfp-4 was relatively low, but these two proteins accounted for the highest proportion of all adhesive proteins, with a mass percentage of more than 60%. Mfp-2 and Mfp-4 constitute the main structure of the foot plate of the mussel. Mfp-2 and Mfp-4 are mainly distributed in the foot plate, which together constitute the main structure of the mussel filamentous foot and foot plate. Mfp-2 and Mfp-4 can increase the strength and toughness of the foot plate through the intermolecular crosslinking reaction initiated by Mfp 6
[47⇓~49]. Mfp-5 has a lower molecular weight (10 kDa) and the highest DOPA content (30 mol%). Mfp-5 is located at the contact interface between the foot plate and the substrate surface, and is also an important source of adhesive force between the foot plate and the solid surface (the adhesive energy is 14 mJ/m
2)
[27,33,35,46]. Mfp-6 has a large molecular weight of about 11.6 kDa and is mainly distributed in the bottom of the foot disc. The main function of Mfp-6 is to initiate and control the oxidative crosslinking process among Mfp-3, Mfp-5, Mfp-2 and Mfp- 4
[50].
3 Adhesion mechanism of mussel adhesive protein
The above studies showed that DOPA was a certain proportion of various adhesion proteins in the foot plate of mussel, which showed that DOPA played a key role in the curing adhesion process.
The catechol group in DOPA obtains cohesion through interactions with various groups in the system (such as hydrogen bonding, metal coordination, π-π interaction, cation-π interaction, and van der Waals force) (as shown in Figure 2a)
[26,28].
The benzene ring of the catechol group has a π bond, and the phenolic hydroxyl group has a lone pair of electrons and a hydrogen atom, so that it can form different chemical bonds with different substrates, and its adhesion mechanism is also different
[4,50,51]. The phenolic hydroxyl group of the catechol group not only has lone pair electrons and can accept hydrogen atoms of other hydrogen donors to form hydrogen bonds, but also can provide hydrogen atoms to form hydrogen bonds with other atoms with lone pair electrons, so that the phenolic hydroxyl group of the catechol group can generate hydrogen bonding with a polar and hydrophilic substrate surface, and has competition and replacement effects on water.
In addition, the phenolic hydroxyl group of catechol has weak acidity and reducibility, and can react with some metals, metal oxides and metal ions to form reversible organometallic complexes. The catechol group in the reduced state makes DOPA have excellent adhesion ability, but the phenolic hydroxyl group in the catechol group is very easy to oxidize to produce a benzoquinone structure.Subsequently, the quinone group can continue to undergo different reactions with other groups, such as a Schiff base reaction with a sulfhydryl group, a Michael addition reaction with an amino group or the formation of an imine structure, and a coupling reaction between the o-benzoquinone groups. The covalent bond greatly improves the adhesion strength between the mussel adhesion protein and the substrate surface
[51⇓⇓⇓~55]. In addition, when contacted with the substrate with aromatic ring derivatives, the benzene ring in DOPA structure can also produce π-π interaction with DOPA, which can enhance the adhesion performance of DOPA. These covalent and non-covalent interactions of DOPA make mussels have strong adhesion ability and can adhere to almost any substrate. Fig. 2b shows the adhesion mechanism of catechol groups to different substrate surfaces. Therefore, the introduction of catechol structure into macromolecular compounds has become a basic idea for the design of mussel-like adhesives. In addition, catechol groups were found to contribute to the formation of protein coacervates, which are ionized water-immiscible fluids that exhibit excellent wettability on almost all underwater substrates
[21].
Underwater bonding is a complex process. Recent studies have shown that not only catechol groups but also some organic cations play a key role in the underwater bonding process.
Maier et al. First synthesized a series of mussel adhesion protein analogues (as shown in Figure 3), and then tested their adhesion by advanced mechanical testing equipment, and found that compounds C and d did not have underwater adhesion ability, while compounds a and B had better underwater adhesion ability
[56]. The reason is that compounds C and d lack amino cation structure or catechol structure respectively, while compounds a and B have both amino cation structure and catechol structure. This study demonstrates that the coexistence of amino cations and catechol structure is a necessary condition for the underwater bonding ability of these compounds.
On the basis of the above experiments, Wilker and Waite further studied the experimental results and showed that when mussels adhered underwater, a layer of hydrated cations would be attached to the solid surface
[57~58]. The presence of the hydrated cation layer hinders the contact of the solid surface with the catechol groups and thus hinders the formation of chemical bonds between the catechol groups and the substrate surface. However, mussel adhesion proteins contain positively charged structures (such as lysine residues), and the hydrated cations are removed by the electrostatic repulsion of the positive charges, so that the catechol groups are in contact with the substrate surface. Under the action of various chemical bonds, mussel adhesion proteins can achieve strong adhesion with solid surfaces (as shown in Figure 4). It can be seen that catechol groups and organic cations must exist simultaneously if bionic adhesives want to have underwater adhesion ability.
4 Research progress of underwater biomimetic adhesives
Mussel adhesive proteins have broad application prospects due to their excellent underwater adhesive properties. Therefore, the preparation method of this kind of adhesive has been paid much attention. At present, there are three ways to obtain mussel adhesion proteins or their imitations.
1。 Bioextraction: Researchers have long attempted to isolate adhesive proteins from mussel foot plates and use them as adhesives. In 1990, BioPolymers successfully obtained mussel adhesive protein by directly extracting natural adhesive protein components from mussel foot glands, and developed a super adhesive (Cell-Tak). Although the adhesive proteins extracted from mussels have been commercialized and show excellent wet adhesive properties, the adhesive proteins extracted from mussels by bioextraction have poor water solubility, difficult operation and high cost, and only about 1 G of adhesive proteins can be extracted from 10 000 mussels, which is difficult to develop on a large scale
[59].
2。 Biological gene editing method: Researchers tried to extract mussel adhesion protein from recombinant Escherichia coli by gene editing. However, because these bacteria lack mussel tyrosinase, they can not convert tyrosine into DOPA. The method of obtaining mussel adhesion protein by biological gene editing and recombination technology not only faces the immaturity of technology and theory, but also has the problem of high cost
[60].
3。 Artificial synthesis by biomimetic means: Through the in-depth study of the adhesion ability of the foot plate of Mytilus edulis, the chemical composition and tissue structure of the foot plate of Mytilus edulis have been determined, and the adhesion mechanism of Mussel foot plate has been basically clarified. On this basis, in order to obtain adhesive materials similar to mussel adhesion proteins, researchers have prepared a variety of underwater adhesive materials similar to mussel adhesion proteins in structure and composition by biomimetic means
[12⇓⇓⇓⇓~17,59,61⇓⇓⇓⇓⇓⇓⇓~69]. Because the biomimetic underwater adhesive has the advantages of simple synthesis method, low preparation cost, easy to expand production and so on, the preparation of DOPA functionalized polymers by chemical methods to obtain high adhesion underwater adhesive materials has attracted more and more attention.
4.1 Coacervate layer type underwater adhesives
The adhesion of Mfp-3s to solid surface is essentially a process in which polyelectrolyte complexes of adhesive proteins self-aggregate to form coacervate through the interaction with water. Coacervates are colloidal systems composed of ionized macromolecular polymers or proteins that behave as dense and partially water-insoluble fluids. The coacervate may be injection molded and adhered to a wet solid surface. Complex coacervates are generally composed of polyanions and polycations. The formation of coacervates is usually the result of weak chemical bonds and non-specific interactions (such as electrostatic, hydrophobic, and hydrogen bonding), followed by underwater delamination, solidification, and adhesion through the interaction of polyions with water. These properties make complex coacervates ideal underwater adhesives. The coacervate of mussel adhesion protein plays an important role in the process of mussel wet adhesion. Mfp-3s, for example, is the only known natural protein complex capable of self-aggregation due to electrostatic attraction and hydrophobic interaction. The interfacial energy of Mfp-3S coacervate is 61 mJ/m
2, which is enough to make it adhere to most solid surfaces
[70].
Coacervate adhesives can be cured underwater from prepolymerization to curing, so they can be applied to related medical fields such as postoperative suture and tissue repair. The coacervate may seal the wound and keep the injured tissue from being exposed to the air, while also immobilizing the tissue and providing bioactive agents that improve patient comfort, accelerate tissue healing, and prevent infection.
Stewart et al. reported a polyphosphate adhesive with side chains containing phosphate, primary amine, and catechol in proportions similar to those of natural adhesives (as shown in Figure 5)
[71]. The polyelectrolyte aggregates to form a coacervate adhesive upon a change in temperature or pH. The results showed that when the pH value was 7. 4, the ratio of Mg
2+(Mg
2+ to amino group was 1 ∶ 0. 6), and the ratio of NaIO
4(NaIO
4 to catechol was 1 ∶ 2) as an oxidative crosslinking agent, the shear force reached 765 kPa after soaking in water at 37 ℃ for 24 H.
Waite et al. reported a polyelectrolyte underwater adhesive (QCS/PAAcat) initiated by ion exchange
[72]. The anion of the adhesive was composed of acrylic acid and catechol functionalized acrylic acid (7:3) random copolymer (as shown in fig. 6). The cation consists of the bis (trifluoromethane) sulfonimide (Tf
2N
-) quaternary ammonium salt of chitosan. Dissolving the two prepared polymers in DMSO, wherein the DMSO is an organic solvent with a low dielectric constant, so that the two dissolved polymers are not ionized. The DMSO solution was then injected onto a water-soaked glass sheet. The miscibility of DMSO with water allows solvent exchange to occur. Acrylic acid was deprotonated by water to form anions, and then the polyelectrolyte was complexed by the electrostatic attraction of anions and cations to form a coacervate, which gathered on the glass surface and cured within 25 s. The adhesive strength was tested by surface force analyzer (SFA), and the result was W
ad≥2 J/m
2.
Generally, polyelectrolyte complexes and coacervates are formed by mixing oppositely charged polymers or proteins in solution. Kim et al. reported a polyelectrolyte composed of two positively charged polymers (as shown in fig. 7)
[73]. The two positive charges used were Rmfp-1 (recombinant Mfp-1) and MADQUAT (poly (2- (trimethylammonium) ethyl methacrylate)). Different from the traditional positive and negative electrocoagulation, the polyelectrolyte forms a coacervate by overcoming the electrostatic repulsion through the strong short-distance cation-π interaction. This coacervate aqueous solution of the same charge has a low surface energy (<1 mJ/m
2). The W
ad=4.8 mJ/m
2 of bond strength was tested by SFA. The resulting coacervate consists of a thin and brittle polyelectrolyte framework with a regular circular pore structure, which implies a strong electrostatic correlation between the polyelectrolyte frameworks. This study provides a new mechanism for the preparation of polyelectrolyte complexes. It is well known that conventional coacervates generally have a low interface, and homo-charge coacervates have a very low interfacial tension due to their weak and sparse cross-linked network structure, which makes such positively charged coacervations useful for trapping, carrying, or encapsulating a wide range of anionic biomolecules and particles.
4.2 Hydrogel underwater adhesives
Hydrogel is an extremely hydrophilic three-dimensional network structure, which swells rapidly in water and can maintain a large volume of water without dissolution. As a three-dimensional network of highly hydrophilic expansion, hydrogels have a high degree of structural similarity with biological tissues. Hydrogels have been designed as platforms for various biomedical applications, such as drug delivery, tissue engineering, and implantable devices
[30,74]. In particular, hydrogels with biocompatibility and wet adhesion have attracted great interest as a new tissue adhesive, which is expected to replace surgical sutures and wound tapes
[75,76].
Although hydrogel is a good biocompatible material, the mechanical strength of hydrogel is weak, which leads to the fracture of hydrogel-like underwater adhesives under tension. Therefore, it is of great value to improve the bonding strength and internal stress of hydrogel adhesives
[77].
Polyethylene glycol (PEG) is a widely used polymer, and the hydrogels prepared from it generally have excellent hydrophilicity and biocompatibility. Yang et al. reported a terpolymer iCMBA obtained by copolymerization of PEG, citric acid, and dopamine (as shown in fig. 8)
[78]. The iCMBA was used as a precursor to prepare a series of iCMBAs hydrogels. This series of hydrogels have strong adhesion to wet tissue surface, which is 2.5 to 8.0 times higher than that of fibrin glue used in clinic. The adhesive strength of the iCMBA-P
200D
0.3 hydrogel prepared by adding 8% NaIO
4 to iCMBA was (8 515. 1 ± 1 167) kPa in dry state and (242. 4 ± 52. 3) kPa in aqueous state. The hydrogel has the characteristics of a tissue-like elastomer, has the capabilities of stopping bleeding and promoting wound healing, simultaneously has better biocompatibility and degradability, cannot cause obvious inflammatory reaction, and can be used as a surgical tissue adhesive, a sealant and a hemostatic agent.
Polysaccharides are also ideal materials for tissue regeneration and drug delivery due to their excellent biocompatibility and biodegradability. Cho et al. prepared a class of alginic acid hydrogels using a calcium-free crosslinking method (as shown in fig. 9)
[79]. The alginic acid-catechol hydrogel not only has adjustable physical and mechanical properties, but also has low immunogenicity and no cytotoxicity. By changing the content ratio of alginic acid/catechol, the maximum adhesive strength can reach 6 kPa. It can be seen that the crosslinking of alginic acid hydrogels by oxidation of catechol can overcome several disadvantages of the current crosslinking method using divalent cations (i.e., cytotoxicity, inflammatory response, limited control of physical and mechanical properties). The novel biomimetic alginate hydrogel can provide an adjustable, functional and biocompatible three-dimensional scaffold for tissue engineering and cell transplantation.
Cho et al. reported a kind of DOPA modified human gelatin synthesized by enzyme catalysis, and then cross-linked with Fe
3+ ions to obtain tissue adhesive (as shown in Figure 10)
[80]. They mainly obtained human gelatin through two steps, first extracting extracellular matrix from human adipose tissue, and then isolating gelatin from extracellular matrix. Under the action of tyrosinase, tyrosine residues in human gelatin are converted into DOPA through enzymatic reaction. Under the action of Fe
3+ ions, DOPA modified gelatin forms viscous DOPA-Fe
3+ gelatin hydrogel within a few seconds through the complexation of DOPA and Fe
3+ ions. The results showed that the DOPA-Fe
3+ gelatin hydrogel maintained good stability in water environment and exhibited good mechanical properties.The bond strength of the hydrogel was (10. 197 ± 0.237) kPa. They studied the hemostatic effect of DOPA-Fe
3+ gelatin hydrogel on hemorrhagic rats. Shortly after the injection of DOPA-Fe
3+ gelatin hydrogel, the bleeding in the liver was stopped. The overall results showed that DOPA-Fe
3+ gelatin hydrogel had good viscoelasticity and hemostatic properties, and could be used as a tissue adhesive in surgery.
Inspired by the adhesion process of mussels, Yao et al. reported a class of high-performance hydrogel adhesives (as shown in Figure 11)
[81]. They first prepared a copolymer PMD with positive charges and catechol groups on the side chain, and then mixed the copolymer with polyacrylic acid (PAA) to prepare PAA/PMD hydrogel. The PMD component simulates the mussel adhesion protein, and the PAA component simulates the rock surface. PMD and PAA chains can be non-covalently cross-linked by hydrogen bonds, charge interactions and chain entanglement. The catechol group provides the hydrogel with strong adhesion ability. The maximum tensile force and shear force of the hydrogel were 1. 1 MPa and 1. 6 MPa, respectively. The PAA/PMD hydrogel has good extensibility and repeatable adhesion.
4.3 Intelligent underwater adhesives
Although researchers have developed many different types of underwater adhesive materials by preparing polyelectrolyte complexes, hydrogels and macroscopic supramolecular assemblies, most of them are permanent and irreversible. Based on the demand of various smart devices for reversible adhesion, in recent years, people have tried to prepare reversible underwater adhesion materials by regulating reversible hydrogen bonding, π-π interaction, cation-π interaction through external stimuli such as temperature, pH value and light, and some progress has been made. Smart adhesives can respond to externally applied stimuli and can reversibly change between a bonding state and a non-bonding state. These smart adhesives have gained a lot of attention in various fields of materials science and engineering (underwater intelligent devices, intelligent climbing robots, reversible wound dressings, etc.)
[82,83].
Park et al. reported a new class of thermosensitive injectable tissue adhesion composite hydrogels HA/Pluronic F127 (shown in fig. 12)
[84]. They mixed catechol-modified HA with thiol-terminated Pluronic F-127 to prepare a lightly crosslinked HA/Pluronic composite gel structure. Rheological studies of HA/Pluronic show that the sol-gel transition is rapid and reversible depending on temperature. HA/Pluronic hydrogels can be injected into the body at room temperature in a sol state, but immediately become a gel at body temperature. The hydrogel formed in situ shows excellent tissue adhesion performance and good stability. The catechol unit endows HA/Pluronic with strong tissue bonding strength. Due to the temperature response of Pluronic, the adhesive strength of the hydrogel to soft tissue can be adjusted from (1. 47 ± 0.23) kPa to (7. 18 ± 0.93) kPa.
Narkar et al. reported a reversible underwater adhesive based on catechol-borate ester (as shown in Figure 13)
[85]. The adhesive contains a network structure combined by catechol and phenyl boric acid, when the pH value is 3, the catechol is in a reduction state (namely an adhesive state), the adhesive and a wet glass surface form a stronger interface bond, and the adhesive strength is W
ad=2000 mJ/m
2; At a pH of 9, the catechol is oxidized to quinone (i.e., the non-adhesive state), and the interfacial bonding strength is reduced by more than one order of magnitude, and the adhesive strength is W
ad=180 mJ/m
2, resulting in a reversible transition between the adhesive and non-adhesive States. At the same time, boric acid also acts as a protecting group to prevent the irreversible oxidation and crosslinking of catechol.
Zhou et al. First synthesized a thermosensitive copolymer p (DMA-co-MEA-co-NIPAAm), and then decorated it on a mushroom-like PDMS fiber array to prepare a new underwater thermoresponsive adhesive (TRGA)
[86]. It has high adhesion to the substrate above the critical temperature (LCST, 34 ℃), with an adhesion force of 8.1 kPa at 40 ℃. However, the adhesive force was low below the LCST, and the adhesive force was 1. 8 kPa at 20 ℃. Fe
3O
4 nanoparticles (Fe
3O
4NPs) were added to the TRGA to prepare the Fe
3O
4-TRGA. Under the stimulation of near-infrared (NIR) irradiation, strong surface plasmon resonance (SPR) is generated on the surface of Fe
3O
4 nanoparticles, which leads to a rapid increase in local temperature, resulting in the configuration conversion of PNIPAAm in water, thus adjusting the adhesion strength of Fe
3O
4-TRGA. Because the adhesive can quickly and reversibly control the adhesion, it can quickly capture and release underwater objects, as shown in Figure 14.
Zhou et al. reported that a novel thermoresponsive biomacromolecule underwater adhesive Chitosan-Catechol-pNIPAM was prepared by grafting Catechol groups and temperature-responsive pNIPAM onto the main chain of Chitosan (as shown in Figure 15). Chitosan-Catechol-pNIPAM has good biocompatibility
[87]. When the temperature is below or above the critical temperature (LCST, 35 ℃), Chitosan-Catechol-pNIPAM shows typical sol-gel transition characteristics, that is, it can switch between adhesion and non-adhesion. When the temperature changed from 10 ℃ to 40 ℃, the bond strength of the adhesive varied from 0. 63 kPa to 3. 18 kPa. The adhesive shows good thermal response underwater adhesion capability on various substrates such as glass, hydrogel, pigskin and the like, and particularly when the Fe
3O
4NPs is doped into the Chitosan-Catechol-pNIPAM in situ, the adhesive can realize dynamic adhesion on human skin under the irradiation of near-infrared laser. Chitosan-Catechol-pNIPAM can be coated on the outer surface of a syringe needle as a novel hemostatic needle to stop bleeding. The adhesive is expected to have a wide application prospect in the medical field.
4.4 Underwater biomimetic adhesive without catechol structures
Because natural mussel adhesion proteins contain DOPA, the bionic imitation of mussel adhesion proteins is often limited to the simple study of its chemical structure, but if we study the essential reasons for the adhesion of mussel adhesion protein, we will often produce new bionic ideas.
Wang et al. prepared a series of anth-PEI adhesives (as shown in fig. 16) by introducing anthracene rings, which are stimulus-responsive to light and heat, into the polyethyleneimine skeleton
[88]. Under the stimulation of light and heat, the cross-linked anthracene dimers in the network of the system have reversible dynamic formation and breakage characteristics, resulting in reversible adhesion with solid surfaces. The adhesive strength of 9-anth-PEI could be adjusted from (50.5 ± 7.4) kPa to (606.7 ± 30.3) kPa under the stimulation of light and heat, and the adhesive strength of 9-anth-PEI reached (606.7 ± 30.3) kPa after 1 min of UV irradiation. In addition, the adhesive has good fluidity and wide applicability to various base materials. The adhesive can spontaneously diffuse on the solid surface and absorb water at the interface to achieve full contact between the adhesive and the solid surface (as shown in fig. 16), thereby improving the bonding strength.
Li et al. reported a new class of supramolecular adhesive materials based on LMWMs (low molecular weight monomers) as shown in fig. 17
[89]. P1 has a closed-loop crown ether structure, which has good ductility and spatial dispersion, so that it can easily generate an easy and firm force with the substrate surface through supramolecular interactions (hydrogen bonding, van der Waals interaction, π-π stacking, hydrophobic interaction, metal complexation, etc.), so that P1 has a long-term stable underwater adhesion capability. In the underwater bonding test, the bonding strength is 3. 237 MPa when the iron sheet is used as the substrate, and the adhesive can be firmly bonded to the surface of various substrates.
Kamperman et al. reported a fully synthetic in situ curing adhesive based on complex coacervates
[90]. The adhesive consists of two polymers with opposite charges. Positive and negative charges were introduced to the main chain of poly (N-isopropylacrylamide) (PNIPAM) with thermal responsiveness, respectively (as shown in fig. 18). As a complex fluid coacervate, it can be injection-coated at room temperature. Physical crosslinking is caused by the densification of the PNIPAM chains due to their aggregation at elevated temperatures. Macroscopically, when the temperature is increased to the critical temperature (LCST, 23 ℃) of the polyelectrolyte complex PNIPAM, the polyelectrolyte is transformed into a non-flowing coacervate while maintaining the water content in the system. The bonding strength W
ad of the adhesive can be adjusted between 0.02~3.8 J/m
2 by temperature control. Because the adhesive has a hydrophobic area and still retains a large amount of water in the phase transition process, the adhesive shows good bonding strength to both hydrophilic and hydrophobic substrates in an underwater environment.
The above polymers without catechol structure can also produce underwater bonding ability through weak chemical bonds such as π-π interaction of aromatic rings and hydrogen bonding between oxygen atoms and bonded surfaces. It can be seen that biomimetic simulation does not necessarily have to simulate the structure of natural compounds, and simulating their electronic effects may be a more basic and effective perspective.
5 Application Prospect of Underwater Bionic Adhesive
Adhesive bonding, welding and mechanical connection are the three major connection technologies in the contemporary era. The application of adhesives has penetrated into various fields of the national economy. However, most of the adhesives can only have excellent bonding strength in the air, and can not be bonded underwater. Ordinary adhesives will gradually lose the bonding effect when they are immersed in water for a long time. The main reason is that water molecules enter the bonding interface to hydrate, swell and degrade the adhesive molecules, thus losing their bonding ability
[91⇓~93].
The biomimetic underwater adhesive not only can realize direct underwater curing bonding, but also has long-term and durable bonding force in the underwater environment. The bionic underwater adhesive also has the general characteristics of common adhesives, such as light weight, non-destructive bonding and low pollution, and can adapt to extreme marine environments such as high pressure, low temperature, corrosion and pollution through reasonable technological design
[94]. In addition, due to its good biocompatibility, underwater adhesives have also been widely concerned in the medical field
[95].
5.1 Anticorrosion and Rapid Repair of Ships
The ship is in the marine environment for a long time, because the seawater is a high salt system, the metal material parts of the ship in contact with the seawater for a long time will be corroded to varying degrees, which will cause corrosion and water seepage of the ship shell or parts, pipeline leakage, and even affect the safety of the ship. If the underwater adhesive is coated on the surface of a metal part, the protection time of the coating is greatly prolonged due to the characteristic of lasting adhesion, so that the underwater adhesive can play a long-term anti-corrosion effect. In the process of navigation, ships are very vulnerable to various structural damage, such as delamination, crack, notch, hole and fracture. These damages will significantly reduce the static and dynamic bearing capacity of composite materials, and even threaten the safety of ships. The underwater adhesive can quickly, conveniently and directly carry out underwater plugging and repairing, so that the ship can quickly recover its endurance. The underwater adhesive has the advantages of underwater coating, simple coating process, low production cost, high bonding strength and the like, and has wide application prospect in the fields of marine emergency repair, ship anticorrosion and the like
[96,97].
5.2 Surgical wound suturing and tissue repair
In daily life, all kinds of medical operations are very common. The traditional way is to suture the wound with needle and thread after the operation, and then remove the suture after the wound heals. Although biodegradable sutures are now available, the suture process can still cause pain to patients, and the incision is usually covered with drugs and gauze, which also brings inconvenience to patients'lives. However, bionic underwater adhesive can solve the above problems well
[98⇓~100].
Considering the particularity of human body environment, medical adhesives should meet the following conditions: a. High bonding strength under wet condition; b。 Ood biocompatibility, no toxicity to human body and no allergic reaction; c。 Body temperature curing,; d. No swelling effect; e。 Can be degraded in vivo; f。 The curing process is not exothermic. Due to the severity of medical requirements, there is no medical adhesive that can fully meet the above conditions. The bionic underwater adhesive has good wet bonding ability, biocompatibility, and can be degraded and absorbed by the human body, which fully meets the requirements of future medical adhesives. Medical bionic underwater adhesive will have great application value in the medical fields of wound administration, tissue bonding, fistula sealing, prosthesis fixation, ulcer coverage and hemostasis
[101].
5.3 Intelligent bionic equipment
Because the intelligent underwater adhesive can generate reversible adhesive force under the stimulation of light, heat, pH and the like, the intelligent underwater adhesive can be used in the field of intelligent underwater robots, and the reversible adhesive force can be used for functions such as underwater monitoring, underwater action, underwater transportation, wet climbing and the like. Smart underwater adhesives can also be used in wearable devices (for detecting electrocardiogram signals on wet skin), and can also be used in medical applications such as the preparation of medical tapes with reversible adhesion to wet skin. Because the intelligent underwater adhesive can respond quantitatively to external stimuli and can be adjusted according to the required shape changes, it is an ideal framework for the construction of flexible energy storage devices. It can be seen that intelligent underwater adhesives will have a wide range of applications in many fields
[83,87].
6 Conclusion and prospect
In the past few decades, inspired by the superior adhesion ability of mussel adhesion proteins, the development of underwater biomimetic adhesives has gradually become a research hotspot. In the development of underwater biomimetic adhesive, people have always been concerned about two issues: the structure of mussel adhesion protein and the mechanism of underwater adhesion? How to apply the existing research results to the development strategy of biomimetic underwater adhesives? In the early days, it was believed that the presence of DOPA was the key component determining the adhesion of underwater adhesives. Recently, other functional groups in proteins, such as amine cations, have also been identified as important for underwater adhesion. In this review, we first discussed the composition and adhesion mechanism of mussel adhesion proteins.Then the coacervate, hydrogel, smart and DOPA-free underwater adhesives designed based on electrostatic interaction, hydrogen bonding, π-π interaction, cation-π interaction, cation complexation, metal coordination, hydrophobic force, dynamic chemical bond and the combination of these chemical bonds are reviewed.
To understand the bonding mechanism of biomimetic underwater adhesives and further develop their applications, the following key issues need to be solved urgently:
1。 How to improve the bonding strength of underwater adhesives as much as possible so that they can really replace the traditional underwater connection methods?
2。 How to shorten the curing time of underwater adhesives so that they can play a role quickly underwater?
3。 How to improve the cohesion of underwater adhesives, that is, how to design the internal network structure of adhesives, so that adhesives have stronger cohesion;
4。 Can we design a new type of bionic underwater adhesive based on the known chemical bonds and micromechanical principles?
5. DOPA is the main structure of adhesion and solidification in mussel adhesion protein, but the optimal DOPA content to produce the maximum adhesion strength has not yet been determined.
6。 How do the two-dimensional and three-dimensional structures of natural adhesion proteins stack, and how do their stacking patterns affect their adhesion ability. The effect of the two-dimensional and three-dimensional packing structure of underwater adhesive molecules on the adhesive properties has not been reported.
7。 As a biomaterial used in clinic, safety is an important indicator of tissue adhesive, so it is necessary to study the biocompatibility, toxicity, degradability and scope of application of DOPA medical adhesives.
To sum up, although there are still many areas to be studied and improved, underwater adhesive technology has outstanding advantages such as wet bonding ability, strong adaptability, wide application range, light weight, low pollution, good biocompatibility, and can be degraded and absorbed by human body.It has been paid more and more attention and has become a new research hotspot in the field of material science. Underwater adhesive technology will have a broad development space in the field of ocean engineering and medicine in the future.
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