pH-Based Regulation of Hydrogel Mechanical Properties Through Mussel-Inspired Chemistry and Processing

Fe3+-induced bioinspired chitosan hydrogels for the sustained and controlled release of doxorubicin

A novel Fe³⁺-induced mussel-inspired CCS–NACCS hydrogel was developed for the sustained and controlled release of doxorubicin (DOX).

Regulatory parameters of self-healing alginate hydrogel networks prepared via mussel-inspired dynamic chemistry

Key parameters that influence the self-healing and water retention properties of hydrogels made of alginate–dopamine conjugates have been revealed.

Recent advances in clay mineral-containing nanocomposite hydrogels

Clay mineral-containing nanocomposite hydrogels have been proven to have exceptional composition, properties, and applications, and consequently have attracted a significant amount of research effort over the past few years. The objective of this paper is to summarize and evaluate scientific advances in clay mineral-containing nanocomposite hydrogels in terms of their specific preparation, formation mechanisms, properties, and applications, and to identify the prevailing challenges and future directions in the field. The state-of-the-art of existing technologies and insights into the exfoliation of layered clay minerals, in particular montmorillonite and LAPONITE®, are discussed first. The formation and structural characteristics of polymer/clay nanocomposite hydrogels made from in situ free radical polymerization, supramolecular assembly, and freezing-thawing cycles are then examined. Studies indicate that additional hydrogen bonding, electrostatic interactions, coordination bonds, hydrophobic interaction, and even covalent bonds could occur between the clay mineral nanoplatelets and polymer chains, thereby leading to the formation of unique three-dimensional networks. Accordingly, the hydrogels exhibit exceptional optical and mechanical properties, swelling-deswelling behavior, and stimuli-responsiveness, reflecting the remarkable effects of clay minerals. With the pivotal roles of clay minerals in clay mineral-containing nanocomposite hydrogels, the nanocomposite hydrogels possess great potential as superabsorbents, drug vehicles, tissue scaffolds, wound dressing, and biosensors. Future studies should lay emphasis on the formation mechanisms with in-depth insights into interfacial interactions, the tactical functionalization of clay minerals and polymers for desired properties, and expanding of their applications.

Stimuli-responsive supramolecular hydrogels with high extensibility and fast self-healing via precoordinated mussel-inspired chemistry

Supramolecular hydrogels have the advantages of stimuli responsiveness and self-healing compared to covalently crosslinked hydrogels. However, the existing supramolecular hydrogels are usually poor in mechanical properties especially in extensibility. In addition, these supramolecular hydrogels need a long self-healing time and have low self-healing efficiency. In this manuscript, we report a novel strategy to develop highly extensible and fast self-healing supramolecular hydrogels by using pre-coordinated mussel-inspired catechol-Fe³⁺ complexes as dynamic crosslinkers. The hydrogel can be fabricated and cast into various shapes by one-step photo-crosslinking. Thus fabricated hydrogels can be stretched beyond 10 times their original lengths, and the high extensibility can completely recover within a very short time (less than 20 minutes) even after the hydrogels are entirely cut apart. Utilizing the dynamic nature of supramolecular hydrogels, we can realize different mechanical behaviors including strength, extensibility and recoverability by varying the loading conditions. In addition, the hydrogels respond to multiple stimuli including mechanical force, temperature and certain chemicals because of the dynamic catechol-Fe³⁺ bond.

Rate-Dependent Stiffness and Recovery in Interpenetrating Network Hydrogels through Sacrificial Metal Coordination Bonds

Four-arm poly(ethylene glycol) (PEG) star polymers modified with 3-hydroxy-4-pyridinone (HOPO) end groups were shown to form transient, coordination networks upon addition of trivalent cations In3+, Fe3+, and Al3+. These coordination-based hydrogels exhibited high activation energies of viscoelasticity (34 kT) and characteristic bond lifetimes tunable over 2 orders of magnitude and could be incorporated into poly(hydroxyethylacrylamide)-based covalent scaffolds to create interpenetrating network hydrogels. Measurements carried out in compression and tension demonstrate that the secondary coordination network imparts toughness and stiffness to the overall material, and unlike traditional interpenetrating networks (IPNs), the extent of toughening is dependent on the rate at which the materials are deformed. The dynamic character of the coordination network also allows recovery after mechanical damage following high amplitude strains.

Biomedical and Clinical Importance of Mussel-Inspired Polymers and Materials

The substance secreted by mussels, also known as nature's glue, is a type of liquid protein that hardens rapidly into a solid water-resistant adhesive material. While in seawater or saline conditions, mussels can adhere to all types of surfaces, sustaining its bonds via mussel adhesive proteins (MAPs), a group of proteins containing 3,4-dihydroxyphenylalanine (DOPA) and catecholic amino acid. Several aspects of this adhesion process have inspired the development of various types of synthetic materials for biomedical applications. Further, there is an urgent need to utilize biologically inspired strategies to develop new biocompatible materials for medical applications. Consequently, many researchers have recently reported bio-inspired techniques and materials that show results similar to or better than those shown by MAPs for a range of medical applications. However, the susceptibility to oxidation of 3,4-dihydroxyphenylalanine poses major challenges with regard to the practical translation of mussel adhesion. In this review, various strategies are discussed to provide an option for DOPA/metal ion chelation and to compensate for the limitations imposed by facile 3,4-dihydroxyphenylalanine autoxidation. We discuss the anti-proliferative, anti-inflammatory, anti-microbial activity, and adhesive behaviors of mussel bio-products and mussel-inspired materials (MIMs) that make them attractive for synthetic adaptation. The development of biologically inspired adhesive interfaces, bioactive mussel products, MIMs, and arising areas of research leading to biomedical applications are considered in this review.

Self-healing metal-coordinated hydrogels using nucleotide ligands

A supramolecular gel formed by coordination of Zn(2+) with adenosine monophosphate (AMP) is reported. The adenine base, the monophosphate, and Zn(2+) are all important for gel formation. Mechanically disrupted gels can re-form upon centrifugation; applications of this gel for guest-molecule entrapment are explored.

Elastic sealants for surgical applications

Sealants have emerged as promising candidates for replacing sutures and staples to prevent air and liquid leakages during and after the surgeries. Their physical properties and adhesion strength to seal the wound area without limiting the tissue movement and function are key factors in their successful implementation in clinical practice. In this contribution, the advances in the development of elastic sealants formed from synthetic and natural materials are critically reviewed and their shortcomings are pointed out. In addition, we highlight the applications in which elasticity of the sealant is critical and outline the limitations of the currently available sealants. This review will provide insights for the development of novel bioadhesives with advanced functionality for surgical applications.

Mussel-Inspired Protein Nanoparticles Containing Iron(III)-DOPA Complexes for pH-Responsive Drug Delivery

A novel bioinspired strategy for protein nanoparticle (NP) synthesis to achieve pH-responsive drug release exploits the pH-dependent changes in the coordination stoichiometry of iron(III)-3,4-dihydroxyphenylalanine (DOPA) complexes, which play a major cross-linking role in mussel byssal threads. Doxorubicin-loaded polymeric NPs that are based on Fe(III)-DOPA complexation were thus synthesized with a DOPA-modified recombinant mussel adhesive protein through a co-electrospraying process. The release of doxorubicin was found to be predominantly governed by a change in the structure of the Fe(III)-DOPA complexes induced by an acidic pH value. It was also demonstrated that the fabricated NPs exhibited effective cytotoxicity towards cancer cells through efficient cellular uptake and cytosolic release. Therefore, it is anticipated that Fe(III)-DOPA complexation can be successfully utilized as a new design principle for pH-responsive NPs for diverse controlled drug-delivery applications.

High-performance TiO2 nanoparticle/DOPA-polymer composites

Many natural materials are complex composites whose mechanical properties are often outstanding considering the weak constituents from which they are assembled. Nacre, made of inorganic (CaCO3 ) and organic constituents, is a textbook example because of its strength and toughness, which are related to its hierarchical structure and its well-defined organic-inorganic interface. Emulating the construction principles of nacre using simple inorganic materials and polymers is essential for understanding how chemical composition and structure determine biomaterial functions. A hard multilayered nanocomposite is assembled based on alternating layers of TiO2 nanoparticles and a 3-hydroxy-tyramine (DOPA) substituted polymer (DOPA-polymer), strongly cemented together by chelation through infiltration of the polymer into the TiO2 mesocrystal. With a Young's modulus of 17.5 ± 2.5 GPa and a hardness of 1.1 ± 0.3 GPa the resulting material exhibits high resistance against elastic as well as plastic deformation. A key feature leading to the high strength is the strong adhesion of the DOPA-polymer to the TiO2 nanoparticles.

Protein-responsive assemblies from catechol-metal ion supramolecular coordination

Supramolecular self-assembly driven by catechol-metal ion coordination has gained great success in the fabrication of functional materials including adhesives, capsules, coatings and hydrogels. However, this route has encountered a great challenge in the construction of nanoarchitectures in the absence of removable templates, because of the uncontrollable crosslinking of catechol-metal ion coordination. Herein, we show that a supramolecular approach, combining both catechol-metal ion coordination and polymer self-assembly together, can organize polymers into hybrid nanoassemblies ranging from solid particles, homogeneous vesicles to Janus vesicles. Without the introduction of a specific binding ligand or complicated molecular design, these assemblies can totally disassemble in response to proteins. UV/vis absorption, fluorescence quenching and recovery investigations have confirmed that proteins can seize metal ions from the hybrid nanoassemblies, thus causing the degradation of catechol-metal ion coordination networks.

Light-responsive iron(III)-polysaccharide coordination hydrogels for controlled delivery

Visible-light responsive gels were prepared from two plant-origin polyuronic acids (PUAs), alginate and pectate, coordinated to Fe(III) ions. Comparative quantitative studies of the photochemistry of these systems revealed unexpected differences in the photoreactivity of the materials, depending on the polysaccharide and its composition. The roles that different functional groups play on the photochemistry of these biomolecules were also examined. Mannuronic-rich alginates were more photoreactive than guluronic acid-rich alginate and than pectate. The microstructure of alginates with different mannuronate-to-guluronate ratios changed with polysaccharide composition. This influenced the gel morphology and the photoreactivity. Coordination hydrogel beads were prepared from both Fe-alginate and Fe-pectate. The beads were stable carriers of molecules as diverse as the dye Congo Red, the vitamin folic acid, and the antibiotic chloramphenicol. The photoreactivity of the hydrogel beads mirrored the photoreactivity of the polysaccharides in solution, where beads prepared with alginate released their cargo faster than beads prepared with pectate. These results indicate important structure-function relationships in these systems and create guidelines for the design of biocompatible polysaccharide-based materials where photoreactivity and controlled release can be tuned on the basis of the type of polysaccharide used and the metal coordination environment.

Robust, self-healing hydrogels synthesised from catechol rich polymers

Catechol rich polymers yield robust, self-healing hydrogels.

Fe3+-induced oxidation and coordination cross-linking in catechol–chitosan hydrogels under acidic pH conditions

Fe³⁺-induced oxidation and coordination cross-linking in catechol–chitosan (CCS) hydrogels under acidic pH conditions; EDTA was used to disintegrate the coordination cross-linking system.

Zinc induced polyelectrolyte coacervate bioadhesive and its transition to a self-healing hydrogel

High performance Dopa-modified poly(acrylic acid) adhesives formed under the assistance of zinc ions can transform to a self-recovery polymer hydrogel with a pH trigger.

Air/water interfacial formation of freestanding, stimuli-responsive, self-healing catecholamine Janus-faced microfilms

A catecholamine freestanding film is discovered to be spontaneously formed at the air-water interface, and the film has unique properties of robust surface adhesiveness, self-healing, and stimuli-responsive properties. The interfacial film-producing procedure is a simple single step containing polyamines and catechol(amine)s. It is found that oxygen-rich regions existing at an air-water interface greatly accelerate the catecholamine crosslinking reaction.

Jack of all trades: versatile catechol crosslinking mechanisms

Catechols play an important role in many natural systems. They are known to readily interact with both organic (e.g., amino acids) and inorganic (e.g., metal ions, metal oxides) compounds, thereby providing a powerful system for protein curing. Catechol crosslinked protein networks, such as sclerotized cuticle and byssal threads of the mussel, have been shown to exhibit excellent mechanical properties. A lot of effort has been devoted to mimicking the natural proteins using synthetic catechol-functionalized polymers. Despite the success in developing catechol-functionalized materials, the crosslinking chemistry of catechols is still a subject of debate. To develop materials with controlled and superior properties, a clear understanding of the crosslinking mechanism of catechols is of vital importance. This review describes the crosslinking pathways of catechol and derivatives in both natural and synthetic systems. We discuss existing pathways of catechol crosslinking and parameters that affect the catechol chemistry in detail. This overview will point towards a rational direction for further investigation of the complicated catechol chemistry.

Self-assembly of stiff, adhesive and self-healing gels from common polyelectrolytes

Underwater adhesion has numerous potential medical, household, and industrial applications. It is typically achieved through covalent polymerization and cross-linking reactions and/or the use of highly specialized biological or biomimetic polymers. As a simpler alternative to these covalent and biomimetic strategies, this article shows that stiff, gel-like complexes that adhere to various substrates under water can also be prepared through the ionic cross-linking of common, commercial polyelectrolytes. The gels form spontaneously when synthetic polycations, such as poly(allylamine) (PAH), are mixed with strongly binding multivalent anions, pyrophosphate (PPi) and tripolyphosphate (TPP). The PAH/PPi and PAH/TPP gels exhibit very high storage moduli (G∞′ ≈ 400 kPa), self-heal when torn, and adhere to both hydrophilic and hydrophobic substrates under water (with short-term tensile adhesion strengths of 350–450 kPa). Furthermore, these gels can be dissolved on demand (if adhesion needs to be reversed) by changing the ambient pH, which controls the ionization state of the polyelectrolyte and ionic cross-linker. These properties suggest that synthetic polycations cross-linked with PPi and TPP ions could provide a simple, inexpensive, and scalable platform for underwater adhesion.

Novel hydrogel actuator inspired by reversible mussel adhesive protein chemistry

A novel hydrogel actuator that combines ionoprinting techniques with reversible catechol-metal ion coordination chemistry found in mussel adhesive proteins is developed. Deposited metal ions increase the local crosslinking density, which induces sharp bending of the hydrogel. Reversibly bound metal ions can be removed and reintroduced in a different pattern so that the hydrogel can be reprogrammed to transform into a different 3-dimentional shape.

Metal-coordination: Using one of nature's tricks to control soft material mechanics

Growing evidence supports a critical role of dynamic metal-coordination crosslinking in soft biological material properties such as self-healing and underwater adhesion1. Using bio-inspired metal-coordinating polymers, initial efforts to mimic these properties have shown promise2. Here we demonstrate how bio-inspired aqueous polymer network mechanics can be easily controlled via metal-coordination crosslink dynamics; metal ion-based crosslink stability control allows aqueous polymer network relaxation times to be finely tuned over several orders of magnitude. In addition to further biological material insights, our demonstration of this compositional scaling mechanism should provide inspiration for new polymer material property-control designs.

Mussel-mimetic protein-based adhesive hydrogel

Hydrogel systems based on cross-linked polymeric materials which could provide both adhesion and cohesion in wet environment have been considered as a promising formulation of tissue adhesives. Inspired by marine mussel adhesion, many researchers have tried to exploit the 3,4-dihydroxyphenylalanine (DOPA) molecule as a cross-linking mediator of synthetic polymer-based hydrogels which is known to be able to achieve cohesive hardening as well as adhesive bonding with diverse surfaces. Beside DOPA residue, composition of other amino acid residues and structure of mussel adhesive proteins (MAPs) have also been considered important elements for mussel adhesion. Herein, we represent a novel protein-based hydrogel system using DOPA-containing recombinant MAP. Gelation can be achieved using both oxdiation-induced DOPA quinone-mediated covalent and Fe(3+)-mediated coordinative noncovalent cross-linking. Fe(3+)-mediated hydrogels show deformable and self-healing viscoelastic behavior in rheological analysis, which is also well-reflected in bulk adhesion strength measurement. Quinone-mediated hydrogel has higher cohesive strength and can provide sufficient gelation time for easier handling. Collectively, our newly developed MAP hydrogel can potentially be used as tissue adhesive and sealant for future applications.

pH-dependent cross-linking of catechols through oxidation via Fe3+ and potential implications for mussel adhesion

The mussel byssus is a remarkable attachment structure that is formed by injection molding and rapid in-situ hardening of concentrated solutions of proteins enriched in the catecholic amino acid 3,4-dihydroxy-L-phenylalanine (DOPA). Fe³⁺, found in high concentrations in the byssus, has been speculated to participate in redox reactions with DOPA that lead to protein polymerization, however direct evidence to support this hypothesis has been lacking. Using small molecule catechols, DOPA-containing peptides, and native mussel foot proteins, we report the first direct observation of catechol oxidation and polymerization accompanied by reduction of Fe³⁺ to Fe²⁺. In the case of the small molecule catechol, we identified two dominant dimer species and characterized their connectivities by nuclear magnetic resonance (NMR), with the C6-C6 and C5-C6 linked species as the major and minor products, respectively. For the DOPA-containing peptide, we studied the pH dependence of the reaction and demonstrated that catechol polymerization occurs readily at low pH, but is increasingly diminished in favor of metal-catechol coordination interactions at higher pH. Finally, we demonstrate that Fe³⁺ can induce cross-links in native byssal mussel proteins mefp-1 and mcfp-1 at acidic pH. Based on these findings, we discuss the potential implications to the chemistry of mussel adhesion.

Smart hydrogels as functional biomimetic systems

This review discusses the principles underlying stimuli-responsive behavior of hydrogels and how these properties contribute to their biomimetic functions and applications.

Versatile tuning of supramolecular hydrogels through metal complexation of oxidation-resistant catechol-inspired ligands

The mussel byssal cuticle employs DOPA-Fe³⁺ complexation to provide strong, yet reversible crosslinking. Synthetic constructs employing this design motif based on catechol units are plagued by oxidation-driven degradation of the catechol units and the requirement for highly alkaline pH conditions leading to decreased performance and loss of supramolecular properties. Herein, a platform based on a 4-arm poly(ethylene glycol) hydrogel system is used to explore the utility of DOPA analogues such as the parent catechol and derivatives, 4-nitrocatechol (nCat) and 3-hydroxy-4-pyridinonone (HOPO), as structural crosslinking agents upon complexation with metal ions. HOPO moieties are found to hold particular promise, as robust gelation with Fe³⁺ occurs at physiological pH and is found to be largely resistant to oxidative degradation. Gelation is also shown to be triggered by other biorelevant metal ions such as Al³⁺, Ga³⁺ and Cu²⁺ which allows for tuning of the release and dissolution profiles with potential application as injectable delivery systems.

25th anniversary article: reversible and adaptive functional supramolecular materials: "noncovalent interaction" matters

Supramolecular materials held together by noncovalent interactions, such as hydrogen bonding, host-guest interactions, and electrostatic interactions, have great potential in material science. The unique reversibility and adaptivity of noncovalent intreractions have brought about fascinating new functions that are not available by their covalent counterparts and have greatly enriched the realm of functional materials. This review article aims to highlight the very recent and important progresses in the area of functional supramoleuclar materials, focusing on adaptive mechanical materials, smart sensors with enhanced selectivity, soft luminescent and electronic nanomaterials, and biomimetic and biomedical materials with tailored structures and functions. We cannot write a complete account of all the interesting work in this area in one article, but we hope that it can in a way reflect the current situation and future trends in this prosperously developing area of functional supramolecular materials.

Mussel-inspired histidine-based transient network metal coordination hydrogels

Transient network hydrogels cross-linked through histidine-divalent cation coordination bonds were studied by conventional rheologic methods using histidine-modified star poly(ethylene glycol) (PEG) polymers. These materials were inspired by the mussel, which is thought to use histidine-metal coordination bonds to impart self-healing properties in the mussel byssal thread. Hydrogel viscoelastic mechanical properties were studied as a function of metal, pH, concentration, and ionic strength. The equilibrium metal-binding constants were determined by dilute solution potentiometric titration of monofunctional histidine-modified methoxy-PEG and were found to be consistent with binding constants of small molecule analogs previously studied. pH-dependent speciation curves were then calculated using the equilibrium constants determined by potentiometric titration, providing insight into the pH dependence of histidine-metal ion coordination and guiding the design of metal coordination hydrogels. Gel relaxation dynamics were found to be uncorrelated with the equilibrium constants measured, but were correlated to the expected coordination bond dissociation rate constants.

Complexes of polydopamine-modified clay and ferric ions as the framework for pollutant-absorbing supramolecular hydrogels

Clay-based functional hydrogels were facilely prepared via a bioinspired approach. Montmorillonite (clay) was exfoliated into single layers in water and then coated with a thin layer of polydopamine (PDOPA) via in situ polymerization of dopamine under basic aqueous conditions. When a small amount of ferric salt was added into aqueous suspensions of the polydopamine-coated clay (D-clay), D-clay and Fe(3+) ions could rapidly self-assemble into three-dimensional networks through the formation of coordination bonds. Consequently, supramolecular hydrogels were formed at very low D-clay contents. Rheological measurements show that the D-clay/Fe(3+) hydrogels exhibit fairly elastic response in low stain range, and have self-healing capability upon removal of applied large stress. More importantly, the hydrogels can be used as adsorbents to effectively remove Rhodamine 6G (Rh6G), an organic pollutant, from water. UV-vis absorption spectra of the Rh6G-loaded hydrogels show bands related to π-π stacking interactions between the aromatic moieties of PDOPA and Rh6G, confirming the formation of PDOPA/Rh6G complex on the surface of D-clay.