An Overview on the Adhesion Mechanisms of Typical Aquatic Organisms and the Applications of Biomimetic Adhesives in Aquatic Environments

Water molecules pose a significant obstacle to conventional adhesive materials. Nevertheless, some marine organisms can secrete bioadhesives with remarkable adhesion properties. For instance, mussels resist sea waves using byssal threads, sandcastle worms secrete sandcastle glue to construct shelters, and barnacles adhere to various surfaces using their barnacle cement. This work initially elucidates the process of underwater adhesion and the microstructure of bioadhesives in these three exemplary marine organisms. The formation of bioadhesive microstructures is intimately related to the aquatic environment. Subsequently, the adhesion mechanisms employed by mussel byssal threads, sandcastle glue, and barnacle cement are demonstrated at the molecular level. The comprehension of adhesion mechanisms has promoted various biomimetic adhesive systems: DOPA-based biomimetic adhesives inspired by the chemical composition of mussel byssal proteins; polyelectrolyte hydrogels enlightened by sandcastle glue and phase transitions; and novel biomimetic adhesives derived from the multiple interactions and nanofiber-like structures within barnacle cement. Underwater biomimetic adhesion continues to encounter multifaceted challenges despite notable advancements. Hence, this work examines the current challenges confronting underwater biomimetic adhesion in the last part, which provides novel perspectives and directions for future research.

Development of a multifunctional injectable temperature-sensitive gelatin-based adhesive double-network hydrogel

Gelatin-based bioadhesives are suitable for the treatment of wounds due to their inherent biocompatibility, lack of immunogenicity, and potential for modification. However, common limitations with such adhesives include their adhesive strength and versatility. In the present study, a multifunctional injectable temperature-sensitive gelatin-based adhesive double-network hydrogel (DNGel) was engineered using facile dual-syringe methodology. An integrative crosslinking strategy utilized the complexation of catechol-Fe³⁺ and NIPAAm-methacryloyl. As anticipated, the DNGel exhibited multifunctional therapeutic properties, namely temperature-sensitivity, mechanical flexibility, good adhesive strength, injectability, self-healing capability, antibacterial activity, and the capability to enable hemostasis and wound healing. The bioinspired dynamic double-network was stabilized by a number of molecular interactions between components in the DNGel, providing multifunctional therapeutic performance. In addition, comprehensive in vitro and in vivo testing confirmed that the adhesive hydrogel exhibited effective antihemorrhagic properties and accelerated wound healing by the promotion of revascularization, representing considerable potential as a next-generation multifunctional smart adhesive patch.

Water- and Acid-Sensitive Cu2O@Cu-MOF Nano Sustained-Release Capsules with Superior Antifouling Behaviors

Marine biofouling is one of the technical bottlenecks restricting the development of the global marine economy. Among the commercial self-polishing antifouling coatings, cuprous oxide is an irreplaceable component because of its efficiency and broad-spectrum antibacterial activity. However, one of the biggest obstacles to achieving long-term antifouling is the "initial burst and final decay" of cuprous oxide in the coating. Here, we lock the copper ions by establishing an antifouling unit composed of Cu₂O (core) and Cu-based metal-organic framework (Cu-MOF, shell). Cu-MOF is densely grown in situ on the periphery of Cu₂O by acid proton etching. The shell structure of Cu-MOF can effectively improve the stability of the internal Cu₂O and thus achieve the stable and slow release of copper ions. Furthermore, Cu₂O@Cu-MOF nanocapsules can also achieve active defense by rapid and complete dissolution of Cu₂O@Cu-MOF at local acidic microenvironment (pH ≤ 5) where the adhesion of fouling organisms occurs. Super-resolution fluorescence microscopy is used to explain the sterilization mechanism. Relying on the water- and acid-sensitive properties of Cu-MOF shell, the stable, controlled and efficient release of copper ions has been achieved for the Cu₂O@Cu-MOF nanocapsules in the self-polishing antifouling coatings. Thus, these controlled-release nanocapsules make long-term antifouling promising.

T1-Positive Mn2+-Doped Multi-Stimuli Responsive poly(L-DOPA) Nanoparticles for Photothermal and Photodynamic Combination Cancer Therapy

In this study, we designed near-infrared (NIR)-responsive Mn2+-doped melanin-like poly(L-DOPA) nanoparticles (MNPs), which act as multifunctional nano-platforms for cancer therapy. MNPs, exhibited favorable π-π stacking, drug loading, dual stimuli (NIR and glutathione) responsive drug release, photothermal and photodynamic therapeutic activities, and T1-positive contrast for magnetic resonance imaging (MRI). First, MNPs were fabricated via KMnO4 oxidation, where the embedded Mn2+ acted as a T1-weighted contrast agent. MNPs were then modified using a photosensitizer, Pheophorbide A, via a reducible disulfide linker for glutathione-responsive intracellular release, and then loaded with doxorubicin through π-π stacking and hydrogen bonding. The therapeutic potential of MNPs was further explored via targeted design. MNPs were conjugated with folic acid (FA) and loaded with SN38, thereby demonstrating their ability to bind to different anti-cancer drugs and their potential as a versatile platform, integrating targeted cancer therapy and MRI-guided photothermal and chemotherapeutic therapy. The multimodal therapeutic functions of MNPs were investigated in terms of T1-MR contrast phantom study, photothermal and photodynamic activity, stimuli-responsive drug release, enhanced cellular uptake, and in vivo tumor ablation studies.

pH- and temperature-responsive radially porous silica nanoparticles with high-capacity drug loading for controlled drug delivery

The design of smart and functional nanocarriers for drug delivery systems that use a variety of organic and inorganic materials has led to the development of nanomedicines with improved therapeutic efficiency and reduced side effects. In this study, a pH- and temperature-responsive, controlled-release system with a high capacity for drug loading was developed based on radially porous silica nanoparticles composed of functionalized ligands and polymer encapsulation. This drug delivery system uses radially oriented mesoporous silica nanoparticles as the drug carrier, and control of the surface chemistry of those nanocarriers allows high-capacity loading efficiency of target drugs and stimuli-responsive release kinetics governed by pH and temperature. The delivery of ibuprofen was chosen to test this system, and a maximum loading efficiency of ca. 270 wt% was established, which was 3 times greater than that in previous studies for silica nanoparticles such as SBA-15, MCA-41, and MCM-48. In addition, the pH- and temperature-responsive release of ibuprofen was achieved when the surface of the nanocarriers was treated by pH-responsive amine functionalization and a temperature-responsive surface coating of agarose gel. Finally, cytotoxicity testing using the fibroblast cells showed that the developed silica nanocarriers have no toxicity on the cells, which should allow these nanocarriers to be applied as a nanomedicine in drug delivery systems.

Expanding the DOPA Universe with Genetically Encoded, Mussel-Inspired Bioadhesives for Material Sciences and Medicine

Catechols are a biologically relevant group of aromatic diols that have attracted much attention as mediators of adhesion of "bio-glue" proteins in mussels of the genus Mytilus. These organisms use catechols in the form of the noncanonical amino acid l-3,4-dihydroxyphenylalanine (DOPA) as a building block for adhesion proteins. The DOPA is generated post-translationally from tyrosine. Herein, we review the properties, natural occurrence, and reactivity of catechols in the design of bioinspired materials. We also provide a basic description of the mussel's attachment apparatus, the interplay between its different molecules that play a crucial role in adhesion, and the role of post-translational modifications (PTMs) of these proteins. Our focus is on the microbial production of mussel foot proteins with the aid of orthogonal translation systems (OTSs) and the use of genetic code engineering to solve some fundamental problems in the bioproduction of these bioadhesives and to expand their chemical space. The major limitation of bacterial expression systems is their intrinsic inability to introduce PTMs. OTSs have the potential to overcome these challenges by replacing canonical amino acids with noncanonical ones. In this way, PTM steps are circumvented while the genetically programmed precision of protein sequences is preserved. In addition, OTSs should enable spatiotemporal control over the complex adhesion process, because the catechol function can be masked by suitable chemical protection. Such caged residues can then be noninvasively unmasked by, for example, UV irradiation or thermal treatment. All of these features make OTSs based on genetic code engineering in reprogrammed microbial strains new and promising tools in bioinspired materials science.

Recent Trends in Mussel-Inspired Catechol-Containing Polymers (A Review)

Syntheses and applications of mussel-inspired polymeric materials have gained a foothold in research in recent years. Mussel-inspired chemistry coupled to Michael addition and Schiff’s base reactions was the key success for this intensive research. Unequivocally, The basic building brick of these materials is catechol-containing moiety, namely, 3,4-dihydroxyphenyl-L-alanine (L-DOPA or DOPA) and dopamine (DA). These catechol-based units within the chemical structure of the material ensure chiefly its adhesive characteristic to adherends of different natures. The newly-made catechol-bearing polymeric materials exhibit unique features, implying their importance in several uses and applications. Technology advent is being advantaged with these holdfast mussel protein-like materials. This review sheds light into the recent advances of such mussel-inspired materials for their adhesion capacity to several substrata of different natures, and for their applications mainly in antifouling coatings and nanoparticles technology.

Enhanced Cell Adhesion on a Nano-Embossed, Sticky Surface Prepared by the Printing of a DOPA-Bolaamphiphile Assembly Ink

Inspired by adhesive mussel proteins, nanospherical self-assemblies were prepared from bolaamphiphiles containing 3,4-dihydroxyphenylalanine (DOPA) moieties, and a suspension of the bolaamphiphile assemblies was used for the preparation of a patterned surface that enhanced cell adhesion and viability. The abundant surface-exposed catechol groups on the robust bolaamphiphile self-assemblies were responsible for their outstanding adhesivity to various surfaces and showed purely elastic mechanical behaviour in response to tensile stress. Compared to other polydopamine coatings, the spherical DOPA-bolaamphiphile assemblies were coated uniformly and densely on the surface, yielding a nano-embossed surface. Cell culture tests on the surface modified by DOPA-bolaamphiphiles also showed enhanced cellular adhesivity and increased viability compared to surfaces decorated with other catecholic compounds. Furthermore, the guided growth of a cell line was demonstrated on the patterned surface, which was prepared by inkjet printing using a suspension of the self-assembled particles as an ink. The self-assembly of DOPA-bolaamphiphiles shows that they are a promising adhesive, biocompatible material with the potential to modify various substances.

Rapid and sensitive detection of microRNA via the capture of fluorescent dyes-loaded albumin nanoparticles around functionalized magnetic beads

MicroRNAs (miRNAs) play important roles in gene regulation and cancer development. Nowadays, it is still a challenge to detect low-abundance miRNAs. Here, we present a magnetic fluorescent miRNA sensing system for the rapid and sensitive detection of miRNAs from cell lysates and serum samples. In this system, albumin nanoparticles (Alb NPs) were prepared from inherent biocompatible bovine serum albumin (BSA). A large number of fluorescent dyes were loaded into Alb NPs to make Alb NPs serve as signal molecular nanocarriers for signal amplification. Benefited from the reactive functional groups-carboxyl groups of Alb NPs, p19 protein, a viral protein that can bind and sequester short RNA duplex effectively and selectively, was modified successfully to the surface of the fluorescent dyes-loaded Alb NPs, thus enabling the probe:target miRNA duplex recognition and binding. Followed by the introduction of gold nanoparticles coated magnetic microbeads (Au NPs-MBs), which were prepared through a novel and simple method, the system combined the merits of the rapid and efficient collection given by MBs with the good affinities to attach probe molecules endowed by the coated gold layer. A broad linear detection range of 10fM-10nM and a low detection limit of 9fM were obtained within 100min by detecting a model target miRNA-21. The feasibility of this method for rapid and sensitive quantification might advance the use of miRNAs as biomarkers in clinical praxis significantly.

Recent approaches in designing bioadhesive materials inspired by mussel adhesive protein

Marine mussels secret protein-based adhesives, which enable them to anchor to various surfaces in a saline, intertidal zone. Mussel foot proteins (Mfps) contain a large abundance of a unique, catecholic amino acid, Dopa, in their protein sequences. Catechol offers robust and durable adhesion to various substrate surfaces and contributes to the curing of the adhesive plaques. In this article, we review the unique features and the key functionalities of Mfps, catechol chemistry, and strategies for preparing catechol-functionalized polymers. Specifically, we reviewed recent findings on the contributions of various features of Mfps on interfacial binding, which include coacervate formation, surface drying properties, control of the oxidation state of catechol, among other features. We also summarized recent developments in designing advanced biomimetic materials including coacervate-forming adhesives, mechanically improved nano- and micro-composite adhesive hydrogels, as well as smart and self-healing materials. Finally, we review the applications of catechol-functionalized materials for the use as biomedical adhesives, therapeutic applications, and antifouling coatings. © 2016 The Authors. Journal of Polymer Science Part A: Polymer Chemistry Published by Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017, 55, 9-33.