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

Bioresponsive and transformable coacervate actuated by intestinal peristalsis for targeted treatment of intestinal bleeding and inflammation

Developing an oral in situ-forming hydrogel that targets the inflamed intestine to suppress bleeding ulcers and alleviate intestinal inflammation is crucial for effectively treating ulcerative colitis (UC). Here, inspired by sandcastle worm adhesives, we proposed a water-immiscible coacervate (EMNs-gel) with a programmed coacervate-to-hydrogel transition at inflammatory sites composed of dopa-rich silk fibroin matrix containing embedded inflammation-responsive core-shell nanoparticles. Driven by intestinal peristalsis, the EMNs-gel can be actuated forward and immediately transform into a hydrogel once contacting with the inflamed intestine to yield strong tissue adhesion, resulting from matrix metalloproteinases (MMPs)-triggered release of Fe3+ from embedded nanoparticles and rearrangement of polymer network of EMNs-gel on inflamed intestine surfaces. Extensive in vitro experiments and in vivo UC models confirmed the preferential hydrogelation behavior of EMNs-gel to inflamed intestine surfaces, achieving highly effective hemostasis, and displaying an extended residence time ( > 48 h). This innovative EMNs-gel provides a non-invasive solution that accurately suppresses severe bleeding and improves intestinal homeostasis in UC, showcasing great potential for clinical applications.

Characterization of a novel antioxidant byssal protein from Mytilus coruscus foot

Mussel byssal proteins are of biomimetic importance for the development of novel underwater bio-adhesive agents. It is important to maintain a reduced state during the process of byssus adhesion. There are 19 mussel foot proteins (MFPs) have been reported in previous studies, among which only MFP-6 had been confirmed as an antioxidant protein in mussel byssus due to the function of cysteines, and playing an essential role in the redox balance of mussel byssus during adhesion process. Although the other four MFPs (MFP-16 ~ MFP-19) also have abundant cysteines, their function is still unknown. In this study, a novel mussel foot protein, named MFP-20, was identified from Mytilus coruscus foot. The sequential features, expression profile, and function of recombinant MFP-20 were verified. The results showed that MFP-20 has more abundant cysteines than other MFPs, the relative expression of mfp-20 was upregulated in Fe3+ stress and low pH seawater. In addition, different adhesive substrates induced significant changes of expression level of mfp-20. Furthermore, rMFP-20 showed strong antioxidant capacity in the DPPH assay, and the abundant cysteines in its sequence may play vital roles in the antioxidation activity. Our findings revealed the possible function of MFP-20 with a totally different sequence from the reported MFP-6 and provided new clues for exploring the redox balance of mussel byssus during the adhesion process.

Antibacterial polylysine-containing hydrogels for hemostatic and wound healing applications: preparation methods, current advances and future perspectives

The treatment of various types of wounds such as dermal wounds, multidrug resistant bacteria-infected wounds, and chronic diabetic wounds is one of the critical challenges facing healthcare systems. Delayed wound healing can impose a remarkable burden on patients and health care professionals. In this case, given their unique three-dimensional porous structure, biocompatibility, high hydrophilicity, capability to provide a moist environment while absorbing wound exudate, permeability to both gas and oxygen, and tunable mechanical properties, hydrogels with antibacterial function are one of the most promising candidates for wound healing applications. Polylysine is a cationic polymer with the advantages of inherent antibacterial properties, biodegradability, and biocompatibility. Therefore, its utilization to engineer antibacterial hydrogels for accelerating wound healing is of great interest. In this review, we initially discuss polylysine properties, and then focus on the most recent advances in polylysine-containing hydrogels (since 2016) prepared using various chemical and physical crosslinking methods for hemostasis and wound healing applications. Finally, the challenges and future directions in the engineering of these antibacterial hydrogels for wound healing are discussed.

Metal Ion-Containing Hydrogels: Synthesis, Properties, and Applications in Bone Tissue Engineering

Hydrogel, as a unique scaffold material, features a three-dimensional network system that provides conducive conditions for the growth of cells and tissues in bone tissue engineering (BTE). In recent years, it has been discovered that metal ion-containing hybridized hydrogels, synthesized with metal particles as the foundation, exhibit excellent physicochemical properties, osteoinductivity, and osteogenic potential. They offer a wide range of research prospects in the field of BTE. This review provides an overview of the current state and recent advancements in research concerning metal ion-containing hydrogels in the field of BTE. Within materials science, it covers topics such as the binding mechanisms of metal ions within hydrogel networks, the types and fabrication methods of various metal ion-containing hydrogels, and the influence of metal ions on the properties of hydrogels. In the context of BTE, the review delves into the osteogenic mechanisms of various metal ions, the applications of metal ion-containing hydrogels in BTE, and relevant experimental studies in vitro and in vivo. Furthermore, future improvements in bone repair can be anticipated through advancements in bone bionics, exploring interactions between metal ions and the development of a wider range of metal ions and hydrogel types.

Adaptive Adhesions of Barnacle-Inspired Adhesive Peptides

The strategy of robust adhesion employed by barnacles renders them fascinating biomimetic candidates for developing novel wet adhesives. Particularly, barnacle cement protein 19k (cp19k) has been speculated to be the key adhesive protein establishing the priming layer in the initial barnacle cement construction. In this work, we systematically studied the sequence design rationale of cp19k by designing adhesive peptides inspired by the low-complexity STGA-rich and the charged segments of cp19k. Combining structure analysis and the adhesion performance test, we found that cp19k-inspired adhesive peptides possess excellent disparate adhesion strategies for both hydrophilic mica and hydrophobic self-assembled monolayer surfaces. Specifically, the low-complexity STGA-rich segment offers great structure flexibility for surface adhesion, while the hydrophobic and charged residues can contribute to the adhesion of the peptides on hydrophobic and charged surfaces. The adaptive adhesion strategy identified in this work broadens our understanding of barnacle adhesion mechanisms and offers valuable insights for designing advanced wet adhesives with exceptional performance on various types of surfaces.

Microenvironment-responsive Cu-phenolic networks coated nanofibrous dressing with timely macrophage phenotype transition for chronic MRSA infected wound healing

Methicillin-resistant Staphylococcus aureus (MRSA) infection is a pressing clinical issue that impedes wound healing. Pro-inflammatory M1 macrophages is required to clear bacteria and recruit various cell types during the initial phase of wound healing, but timing of this process is crucial. Herein, a microenvironment-responsive nanofibrous dressing capable of timely macrophage phenotype transition in vivo is constructed by coating copper ions (Cu2+)-polydopamine (PDA) networks on poly (ε-caprolactone) fiber (PCL-fiber) membrane. During the initial post-implantation period, the nanofibrous dressing show pH-sensitive Cu2+ release in the acidic infection microenvironment. The release Cu2+ have a direct killing effect on MRSA, and promote the proinflammatory M1 phenotype of macrophages to enhance the antibacterial macrophage response. Later, PDA to become a reactive oxygen species (ROS) scavenger when in microenvironments with elevated ROS levels, which conferred the dressing with an immunomodulatory activity that convert M1 macrophages into M2 macrophages. In vivo examination in an MRSA infected full-thickness skin wounds of rat model demonstrates that this dressing significantly facilitated infection eradication and wound healing through modulating local inflammatory phenotype. Overall, this study offers a simple and effective approach for timely manipulation of inflammation progression to promote infected wound healing.

Coordination-driven robust antibacterial coatings using catechol-conjugated carboxymethyl chitosan

To prevent bacterial contamination on solid surfaces, a simple yet efficient antibacterial coating was developed in a substrate-independent manner by using the catechol-conjugated carboxymethyl chitosan (CMC-DOPA). The CMC-DOPA was firstly synthesized via an aza-Michael reaction with methyl acrylate and the subsequent acyl substitution with dopamine. The coating strategy consists of spin-coating-assisted deposition of CMC-DOPA on polydopamine-coated substrates and coordination-driven crosslinks between catechol groups and Fe3+ ions in sequence, producing the multilayered CMC-DOPA films. The film thickness was controllable depending on the concentration of CMC-DOPA. Compared to bare controls, the CMC-DOPA-coated substrates reduced the bacterial adhesion by up to 99.8 % and 96.2 % for E. coli and S. aureus, respectively. It is demonstrated that the CMC-DOPA coating can be a robust antibacterial coating across various pH environments, inhibiting bacterial adhesion by 78.7 %, 95.1 %, and 93.2 %, respectively, compared to the control, even after 7 days of acidic, physiological, and alkaline pH treatment. The current coating approach could be applied to various substrates including silicon dioxide, titanium dioxide, and polyurethane. Given its simple and versatile coating capability, we think that the coordination-driven CMC-DOPA coating could be useful for various medical devices and implants.

Sustainable polysaccharide and protein hydrogel-based packaging materials for food products: A review

Sustainable food packaging is a necessary element to ensure the success of a food system, the accomplishment of which is weighed in terms of quality retention and ensured products safety. Irrespective of the raised environmental concerns regarding petroleum-based packaging materials, a sustainable analysis and a lab to land assessment should be a priority to eliminate similar fates of new material. Functionalized bio-based hydrogels are one of the smartest packaging inventions that are expected to revolutionize the food packaging industry. Although in this review, the focus relies on recent developments in the sustainable bio-based hydrogel packaging materials, natural biopolymers such as proteins and polysaccharides from which hydrogels could be obtained, the challenges encountered in hydrogel-based packaging materials and the future prospects of hydrogel-based food packaging materials are also discussed. Moreover, the need for 'Life Cycle Assessment' (LCA), stress on certifications and a sustainable waste management system is also suggested which can bring both food and packaging into the same recycling bins.

Multifunctionality in Nature: Structure-Function Relationships in Biological Materials

Modern material design aims to achieve multifunctionality through integrating structures in a diverse range, resulting in simple materials with embedded functions. Biological materials and organisms are typical examples of this concept, where complex functionalities are achieved through a limited material base. This review highlights the multiscale structural and functional integration of representative natural organisms and materials, as well as biomimetic examples. The impact, wear, and crush resistance properties exhibited by mantis shrimp and ironclad beetle during predation or resistance offer valuable inspiration for the development of structural materials in the aerospace field. Investigating cyanobacteria that thrive in extreme environments can contribute to developing living materials that can serve in places like Mars. The exploration of shape memory and the self-repairing properties of spider silk and mussels, as well as the investigation of sensing-actuating and sensing-camouflage mechanisms in Banksias, chameleons, and moths, holds significant potential for the optimization of soft robot designs. Furthermore, a deeper understanding of mussel and gecko adhesion mechanisms can have a profound impact on medical fields, including tissue engineering and drug delivery. In conclusion, the integration of structure and function is crucial for driving innovations and breakthroughs in modern engineering materials and their applications. The gaps between current biomimetic designs and natural organisms are also discussed.

Modular Synthesis and Patterning of High-Stiffness Networks by Postpolymerization Functionalization with Iron–Catechol Complexes

Bioinspired iron-catechol cross-links have shown remarkable success in increasing the mechanical properties of polymer networks, in part due to clustering of Fe³⁺-catechol domains which act as secondary network reinforcing sites. We report a versatile synthetic procedure to prepare modular PEG-acrylate networks with independently tunable covalent bis(acrylate) and supramolecular Fe³⁺-catechol cross-linking. Initial control of network structure is achieved through radical polymerization and cross-linking, followed by postpolymerization incorporation of catechol units via quantitative active ester chemistry and subsequent complexation with iron salts. By tuning the ratio of each building block, dual cross-linked networks reinforced by clustered iron-catechol domains are prepared and exhibit a wide range of properties (Young's moduli up to ∼245 MPa), well beyond the values achieved through purely covalent cross-linking. This stepwise approach to mixed covalent and metal-ligand cross-linked networks also permits local patterning of PEG-based films through masking techniques forming distinct hard, soft, and gradient regions.

Bioinspired Oxidation-Resistant Catechol-like Sliding Ring Polyrotaxane Hydrogels

Adaptable hydrogels have been used in the biomedical field to address several pathologies, especially those regarding tissue defects. Here, we describe unprecedented catechol-like functionalized polyrotaxane (PR) polymers able to form hydrogels. PR were functionalized with the incorporation of hydroxypyridinone (HOPO) moieties into the polymer backbone, with a degree of substitution from 4 to 22%, depending on the PR type. The hydrogels form through the functionalized supramolecular systems when in contact with a Fe(III) solution. Despite the hydrogel formation being at physiological pH (7.4), the HOPO derivatives are extremely resistant to oxidation, unlike common catechols; consequently, they prevent the formation of quinones, which can lead to irreversible bounds within the matrix. The resulting hydrogels demonstrated properties lead to unique hydrogels with improved mechanical behavior obtained by metallic coordination crosslinking, due to the synergies of the sliding-ring PR and the non-covalent (reversible) catechol analogues. Following this strategy, we successfully developed innovative, cytocompatible, oxidative-resistant, and reversible crosslinked hydrogels, with the potential of being used as structural self-materials for a variety of applications, including in the biomedical field.

Different Curcumin-Loaded Delivery Systems for Wound Healing Applications: A Comprehensive Review

Curcumin or turmeric is the active constituent of Curcuma longa L. It has marvelous medicinal applications in many diseases. When the skin integrity is compromised due to either acute or chronic wounds, the body initiates several steps leading to tissue healing and skin barrier function restoration. Curcumin has very strong antibacterial and antifungal activities with powerful wound healing ability owing to its antioxidant activity. Nevertheless, its poor oral bioavailability, low water solubility and rapid metabolism limit its medical use. Tailoring suitable drug delivery systems for carrying curcumin improves its pharmaceutical and pharmacological effects. This review summarizes the most recent reported curcumin-loaded delivery systems for wound healing purposes, chiefly hydrogels, films, wafers, and sponges. In addition, curcumin nanoformulations such as nanohydrogels, nanoparticles and nanofibers are also presented, which offer better solubility, bioavailability, and sustained release to augment curcumin wound healing effects through stimulating the different healing phases by the aid of the small carrier.

pH-Universal Catechol-Amine Chemistry for Versatile Hyaluronic Acid Bioadhesives

Catechol, a major mussel-inspired underwater adhesive moiety, has been used to develop functional adhesive hydrogels for biomedical applications. However, oxidative catechol chemistry for interpolymer crosslinking and adhesion is exclusively effective under alkaline conditions, with limited applications in non-alkaline conditions. To overcome this limitation, pH-universal catechol-amine chemistry to recapitulate naturally occurring biochemical events induced by pH variation in the mussel foot is suggested. Aldehyde moieties are introduced to hyaluronic acid (HA) by partial oxidation, which enables dual-mode catechol tethering to the HA via both stable amide and reactive secondary amine bonds. Because of the presence of additional reactive amine groups, the resultant aldehyde-modified HA conjugated with catechol (AH-CA) is effectively crosslinked in acidic and neutral pH conditions. The AH-CA hydrogel exhibits not only fast gelation via active crosslinking regardless of pH conditions, but also strong adhesion and excellent biocompatibility. The hydrogel enables rapid and robust wound sealing and hemostasis in neutral and alkaline conditions. The hydrogel also mediates effective therapeutic stem cell and drug delivery even in dynamic and harsh environments, such as a motile heart and acidic stomach. Therefore, the AH-CA hydrogel can serve as a versatile biomaterial in a wide range of pH conditions in vivo.

Mussel-inspired biomaterials: From chemistry to clinic

After several billions of years, nature still makes decisions on its own to identify, develop, and direct the most effective material for phenomena/challenges faced. Likewise, and inspired by the nature, we learned how to take steps in developing new technologies and materials innovations. Wet and strong adhesion by Mytilidae mussels (among which Mytilus edulis-blue mussel and Mytilus californianus-California mussel are the most well-known species) has been an inspiration in developing advanced adhesives for the moist condition. The wet adhesion phenomenon is significant in designing tissue adhesives and surgical sealants. However, a deep understanding of engaged chemical moieties, microenvironmental conditions of secreted proteins, and other contributing mechanisms for outstanding wet adhesion mussels are essential for the optimal design of wet glues. In this review, all aspects of wet adhesion of Mytilidae mussels, as well as different strategies needed for designing and fabricating wet adhesives are discussed from a chemistry point of view. Developed muscle-inspired chemistry is a versatile technique when designing not only wet adhesive, but also, in several more applications, especially in the bioengineering area. The applications of muscle-inspired biomaterials in various medical applications are summarized for future developments in the field.

Identification of the fibroin of Stigmaeopsis nanjingensis by a nanocarrier-based transdermal dsRNA delivery system

Stigmaeopsis nanjingensis (Ma and Yuan) (Acari: Tetranychidae) is an important pest of bamboo-feeding behavior and silk production by the female adult mites is seriously harmful to bamboo leaves. Due to its small size, silking and cocooning, its management is difficult. This study discusses a fast and easy method for management of the pest by disturbing the spinning behavior. Stigmaeopsis nanjingensis is host specific and feeds only on bamboo leaves. Leaf margins of bamboo are highly hydrophobic, which makes dsRNA difficult to immerse. Hence, it is a challenge to apply the commonly used feeding method to inhibit gene expression in mites. In this study, we deliver dsRNA to interfere with the expression of fibroin by body wall permeation with a nanocarrier-based delivery system. The dsRNA/nanocarrier formulation droplets could enter the body cavity within 2 min after falling on the mite. The fibroin silencing efficiency was 75.4%, and the results of electron microscopy showed that dsRNA/nanocarrier damage the morphological structure of the silk thread. This study demonstrated the effectiveness of a nanocarrier-based percutaneous dsRNA delivery system in S. nanjingensis and its effect on the fibroin gene that influences the spinning behavior of S. nanjingensis. These findings may provide a new delivery system for RNAi-based control of spider mites that utilize protective webbing in the field.

Viscoelastic analysis of mussel threads reveals energy dissipative mechanisms

Mussels use byssal threads to secure themselves to rocks and as shock absorbers during cyclic loading from wave motion. Byssal threads combine high strength and toughness with extensibility of nearly 200%. Researchers attribute tensile properties of byssal threads to their elaborate multi-domain collagenous protein cores. Because the elastic properties have been previously scrutinized, we instead examined byssal thread viscoelastic behaviour, which is essential for withstanding cyclic loading. By targeting protein domains in the collagenous core via chemical treatments, stress relaxation experiments provided insights on domain contributions and were coupled with in situ small-angle X-ray scattering to investigate relaxation-specific molecular reorganizations. Results show that when silk-like domains in the core were disrupted, the stress relaxation of the threads decreased by nearly 50% and lateral molecular spacing also decreased, suggesting that these domains are essential for energy dissipation and assume a compressed molecular rearrangement when disrupted. A generalized Maxwell model was developed to describe the stress relaxation response. The model predicts that maximal damping (energy dissipation) occurs at around 0.1 Hz which closely resembles the wave frequency along the California coast and implies that these materials may be well adapted to the cyclic loading of the ambient conditions.

Effect of tannic acid on the mechanical and adhesive properties of catechol-modified hyaluronic acid hydrogels

Hyaluronic acid (HA) is applied in various fields, including pharmaceutical science, owing to its favorable biological properties such as moisture retention, non-toxicity, biodegradability, biocompatibility and biodegradability. In particular, many studies have aimed at its application in the form of a hydrogel. However, the applications of HA hydrogels are limited owing to their poor mechanical properties. In this study, an HA-catechol conjugate (HA-Cat) was synthesized by reacting the HA polymer with dopamine to improve its adhesion to various substrates. The HA-Cat hydrogel was prepared via oxidative crosslinking using a small amount of NaIO₄ as the oxidant, and the hydrogel formation was investigated by rheological and mechanical studies. Further, the effect of tannic acid (TA) on the adhesive strength and compressive strength of the HA-Cat/TA hydrogels was examined according to the amount of NaIO₄ used for crosslinking and the TA contents. Both the adhesive and compressive properties of the HA-Cat hydrogels were improved with the addition of TA. The HA-based hydrogels containing TA have great potential as cost-effective and biocompatible medical adhesives.

Structure-property relations in linear viscoelasticity of supramolecular hydrogels

Extraordinary mechanical properties of supramolecular gels (fracture toughness, fatigue resistance, injectability and self-healing ability) are strongly affected by their viscoelastic response driven by rearrangement (association and dissociation) of physical bonds. The kinetics of rearrangement is traditionally studied in small-amplitude shear oscillatory tests by analyzing the effect of the frequency of oscillations ω on the storage G' and loss G'' moduli. Conventional Maxwell-type models describe observations rather poorly when the gels reveal a pronounced flattening of the graphs G''(ω) at high frequencies. A simple model is derived in linear viscoelasticity of supramolecular gels. Its advantage is that the model reproduces experimental data correctly, on the one hand, and involves only four material constants, on the other. Based on the analysis of experimental data on gels cross-linked by coiled-coil complexes, covalent and ionic bonds, phenylboronic acid-diol complexes and metal-ligand coordination bonds, the model is applied to develop structure-property relations that describe the influence of chemical structure of supramolecular gels (concentration of polymer chains and type and molar fraction of temporary bonds) and environmental conditions (temperature, pH and ionic strength of buffer solutions) on their viscoelastic response.

Nanoparticle-Assembled Vacuolated Coacervates Control Macromolecule Spatiotemporal Distribution to Provide a Stable Segregated Cell Microenvironment

Membraneless coacervate compartments in the intracellular and pericellular space mediate critical cellular functions. Developing synthetic coacervates that emulate the morphological, physical, and functional complexity of these natural coacervates is challenging but highly desirable. Herein, a generalizable nanoparticle assembly (NPA) strategy is developed, which is applicable to interactive core-shell nanoparticles with different chemical makeups, to fabricate vacuolated coacervates. The obtained NPA coacervates contain stable internal vacuoles to provide segregated microcompartments, which can mediate the spatially heterogeneous distribution of diverse macromolecules via restricted diffusion. It is further shown that the vacuolated NPA coacervates can harbor and retain macromolecular medium supplements to regulate the functions of cells encapsulated in vacuoles. Furthermore, the restricted macromolecule diffusion can be abolished on demand via the triggered coacervate-hydrogel transition, thereby altering the exposure of encapsulated cells to environmental factors. It is believed that the NPA strategy provides new insights into the design principles of hierarchical coacervates that hold promising potential for a wide array of biomedical applications.

Mussel-inspired adhesive and polypeptide-based antibacterial thermo-sensitive hydroxybutyl chitosan hydrogel as BMSCs 3D culture matrix for wound healing

Hydrogels have gained great attentions as wound dressing. Binding to the tissue and preventing wound infection were the basic requirements for an "ideal dressing". We employed l-DOPA and ε-Poly-l-lysine to modify thermo-sensitive hydroxybutyl chitosan (HBC) to obtain (l-DOPA) - (ε-Poly-l-lysine)-HBC hydrogels (eLHBC). The eLHBC exhibited an almost 1.5 fold (P < 0.01) increase in wet adhesion strength compared to HBC. Upon the introduction of ε-Poly-l-lysine, eLHBC presented inherent antimicrobial property and prevented wound infection and inflammation response. Bone marrow mesenchymal stem cells (BMSCs) encapsulated in the eLHBC (BMSCs ⊂ eLHBC) could secret cytokins and growth factors via paracrine and promote the migration of fibroblast cells. BMSCs ⊂ eLHBC enhanced the complete skin-thickness wound healing via promoting collagen deposition and inhibiting infection and inflammation in vivo with wound closure rate being above 99 % after 15 days. The bioinspired, tissue-adhesive eLHBC could serve as advanced wound dressings for facilitating tissue repair and regeneration.

Fast-Curing Mussel-Inspired Adhesive Derived from Vegetable Oil

The development of functional materials based on renewable resources is of great significance in today's resource shortage. Here, we present an effective way to synthesize a mussel-inspired adhesive from acrylated epoxidized soybean oil (AESO), a renewable and commercially available small molecular material with a molecular weight around 1200 Da, by a one-step esterification reaction with the affordable 3,4-dihydroxybenzoic acid (DHA). By taking advantages of both the double bond and the catechol moiety presented in this small molecular adhesive, a short curing time was achieved with UV irradiation. An average bonding strength around 1.4 MPa at a curing time of only around 10 min on a glass substrate was observed, which reached 3.1 MPa (average 2.8 MPa) at a curing time of 2 h under ambient conditions. The curing time is much shorter, and the bonding strength is obviously stronger than the conditions where conventional oxidation agents such as IO₄^- or oxidation/coordination agents such as Fe³⁺ are used as the curing agent. Furthermore, the AESO-g-DHA can be used as an underwater adhesive, and an appreciable bonding strength up to 0.64 MPa was observed, which is superior than most of currently known commercialized glues. Given that the adhesive could be synthesized from low-cost renewable resources in one step, it might be a potential candidate for large-scale practical application.

Silk Hydrogels with Controllable Formation of Dityrosine, 3,4-Dihydroxyphenylalanine, and 3,4-Dihydroxyphenylalanine-Fe3+ Complexes through Chitosan Particle-Assisted Fenton Reactions

The oxidation of tyrosine residues of silk fibroin involves the generation of dityrosine and 3,4-dihydroxyphenylalanine (DOPA). However, it remains a challenge to selectively control the reaction pathway to produce dityrosine or DOPA in a selective fashion. Here, silk hydrogels with controllable formation of not only dityrosine and DOPA but also DOPA-Fe3+ complexes within the cross-linked networks were developed. The use of chitosan particles in the Fenton reaction allowed the interaction of Fe3+ ions with silk fibroin to be limited through the adsorption of Fe3+ ions onto chitosan particles by manipulating contact time between the reaction medium and chitosan particles. This led to significant suppression of the premature formation of β-sheet structures that cause steric hindrance to the collisions between tyrosyl radicals and thus enabled higher selectivity toward the formation of dityrosine than DOPA. Remarkably, the addition of ethylenediaminetetraacetic acid (EDTA) to the chitosan particle-assisted Fenton reactions resulted in hydrogels that significantly favored the formation of DOPA over dityrosine due to the increase in the hydroxylation of phenol in the presence of EDTA. Despite the existence of Fe3+-EDTA complexes, Raman spectra indicated the DOPA-Fe3+ complexation in the hydrogels. Mechanistically, the hydrogel networks with small-sized and uniformly distributed β-sheet structures as well as the abundance of DOPA appear to make non-EDTA-chelated Fe3+ ions more accessible to complexation with DOPA. These findings have important implications for understanding the oxidation of tyrosine residues of silk fibroin by metal-catalyzed oxidation systems with potential benefits for future studies on silk protein-based hydrogels capable of generating intrinsic adhesive features as well as for exploring dual-cross-linked silk hydrogels constructed by chemical cross-linking and metal-coordinate complexation.

In situ mechanical reinforcement of polymer hydrogels via metal-coordinated crosslink mineralization

Biological organic-inorganic materials remain a popular source of inspiration for bioinspired materials design and engineering. Inspired by the self-assembling metal-reinforced mussel holdfast threads, we tested if metal-coordinate polymer networks can be utilized as simple composite scaffolds for direct in situ crosslink mineralization. Starting with aqueous solutions of polymers end-functionalized with metal-coordinating ligands of catechol or histidine, here we show that inter-molecular metal-ion coordination complexes can serve as mineral nucleation sites, whereby significant mechanical reinforcement is achieved upon nanoscale particle growth directly at the metal-coordinate network crosslink sites.

Biological organic-inorganic materials, such as self-assembling metal-reinforced mussel holdfast threads, remain a popular source of inspiration for materials design and engineering. Here the authors show that metal-coordinate polymer networks can be utilized as simple composite scaffolds for direct in situ crosslink mineralization.

Fabrication and Characterization of Polysaccharide Metallohydrogel Obtained from Succinoglycan and Trivalent Chromium

In the present study, a polysaccharide metallohydrogel was successfully fabricated using succinoglycan and trivalent chromium and was verified via Fourier transform infrared spectroscopy, differential scanning calorimetry analysis, thermogravimetric analysis (TGA), field emission scanning electron microscopy, and rheological measurements. Thermal behavior analysis via TGA indicated that the final mass loss of pure succinoglycan was 87.8% although it was reduced to 65.8% by forming a hydrogel with trivalent chromium cations. Moreover, succinoglycan-based metallohydrogels exhibited improved mechanical properties based on the added concentration of Cr³⁺ and displayed a 10 times higher compressive stress and enhanced storage modulus (G′) of 230% at the same strain. In addition, the pore size of the obtained SCx could be adjusted by changing the concentration of Cr³⁺. Gelation can also be adjusted based on the initial pH of the metallohydrogel formulation. This was attributed to crosslinking between chromium trivalent ions and hydroxyl/carboxyl groups of succinoglycan, each of which exhibits a specific pH-dependent behavior in aqueous solutions. It could be used as a soft sensor to detect Cr³⁺ in certain biological systems, or as a soft matrix for bioseparation that allows control of pore size and mechanical strength by tuning the Cr³⁺ concentration.

Copper–CNT interfacing with Cu-doped polydopamine in CNT carpet: copper nucleation and resistance decrease upon soft annealing

The annealing of carpets of CNT coated with Cu-doped polydopamine lowers the carpets sheet resistance and induces the reduction of Cu ions into metallic Cu particles nucleating at the CNT surface.

Adhesive Catechol-Conjugated Hyaluronic Acid for Biomedical Applications: A Mini Review

Recently, catechol-containing polymers have been extensively developed as promising materials for surgical tissue adhesives, wound dressing, drug delivery depots, and tissue engineering scaffolds. Catechol conjugation to the polymer backbone provides adhesive properties to the tissue and does not significantly affect the intrinsic properties of the polymers. An example of a catecholic polymer is catechol-conjugated hyaluronic acid. In general, hyaluronic acid shows excellent biocompatibility and biodegradability; thus, it is used in various medical applications. However, hyaluronic acid alone has poor mechanical and tissue adhesion properties. Catechol modification considerably increases the mechanical and underwater adhesive properties of hyaluronic acid, while maintaining its biocompatibility and biodegradability and enabling its use in several biomedical applications. In this review, we briefly describe the synthesis and characteristics of catechol-modified hyaluronic acid, with a specific focus on catechol-involving reactions. Finally, we discuss the basic concepts and therapeutic effects of catechol-conjugated hyaluronic acid for biomedical applications.

Supramolecular and Physically Double-Cross-Linked Network Strategy toward Strong and Tough Elastic Fibers

The strength and toughness are two trade-off properties of a material, yet Nature can achieve strong and tough materials by introducing sacrificial bonds into a system. Here, we present a four-component multiblock copolymer (mBCP) approach toward strong and tough elastic fibers, by introducing terpyridine moieties into poly(ether ester) mBCP elastomers. After coordination with Fe(II), supramolecular cross-links are formed within the physically cross-linked thermoplastic elastomers. The toughening elastic fibers with a double-cross-linked network structure show high tensile strength (ca. 300 MPa) and toughness (ca. 100 MJ m^-3). In addition, they display excellent resilience with enhanced self-healing properties. Our strategy provides a promising way for the development of strong and tough elastomers by introducing metal-ligand sacrificial bonds into mBCPs elastomers.

Bioadhesive hydrogels demonstrating pH-independent and ultrafast gelation promote gastric ulcer healing in pigs

Hydrogels are soft materials used in an array of biomedical applications. However, the in situ formation of hydrogels at target sites, particularly in dynamic in vivo environments, usually requires a prolonged gelation time and results in poor adhesion. These limitations cause considerable loss of both hydrogel mass and encapsulated therapeutic cargoes, thereby compromising treatment outcomes. Here, we report the development of a hydrogel based on thiourea-catechol reaction to enhance the bioadhesion. Compared with classical bioadhesive hydrogels, our hydrogels show enhanced mechanical properties, exceedingly short curing time, and pH-independent gelation with a much lower oxidant concentration. We further report the robust adhesion of our hydrogels to acidic gastric tissues and easy delivery to the porcine stomach via endoscopy. The delivered hydrogels adhered to ulcer sites in vivo for at least 48 hours. Hydrogel treatment of gastric ulcers in rodent and porcine models accelerated ulcer healing by suppressing inflammation and promoting re-epithelization and angiogenesis. The improved retention of proregenerative growth factors and reduced exposure to external catabolic factors after hydrogel application may contribute to the observed therapeutic outcomes. Our findings reveal a promising biomaterial-based approach for treating gastrointestinal diseases.

Linear Coordination Polymer Synthesis from Bis-Catechol Functionalized RAFT Polymers

Catechol-Fe(III) complexes contain some of the strongest known metal-chelate coordination bonds. Despite this, they have until now not been utilized in (polymeric linker) linear coordination polymer (LCP) synthesis. With the view of generating catechol end-functional polymers, a new, symmetrical bis-catechol functionalized trithiocarbonate reversible addition fragmentation chain transfer (RAFT) agent is synthesized (CatDMAT). Acrylamide (AM) and dimethylacrylamide (DMA) polymerizations are conducted with CatDMAT using direct photoactivation RAFT polymerization to yield bis-catechol end-functionalized homo- and block-copolymers of molecular weight 10-15 kDa. Catechol-Fe(III) LCPs are successfully formed from the telechelic catechol polymers by bis-complexation to Fe(III). The tetrahedral bis-complex is detected by UV-vis spectroscopy (λ_max  = 570 nm), while increases in relative viscosity and M_n,GPC over their respective uncomplexed polymers confirm the occurrence of supramolecular polymerization. The catechol-LCPs are shown to undergo oxidation and crosslinking in aqueous solution after 24 h.

Synthesis and biological evaluation of a bioinspired, tissue-adhesive gellan gum-based hydrogel designed for minimally invasive delivery and retention of chondrogenic cells

A dopamine-modified, bioinspired gellan gum hydrogel (STM-148B) with improved physicochemical and biological characteristics, suitable for minimally invasive cell delivery and retention in the context of cartilage repair, is herein presented. STM-148B's putative game-changing design characteristics include a highly biocompatible, animal-free and chemically defined composition, reproducibility of manufacture and ease of formulation. STM-148B undergoes rapid ionic crossinking by physiologically relevant mono and divalent cations to form stable 3D hydrogels that possess excellent tissue adhesiveness, such that additional fixation aids are rendered superfluous. STM-148B hydrogels maintain viability of mammalian cells and further promote up-regulation of the expression of healthy chondrogenic extracellular matrix markers upon stimulation. STM-148B is currently undergoing pre-clinical safety and efficacy assessment as a medical device for cell delivery and retention focussing on regeneration of hyaline-like cartilage and may represent a valuable addition to the armamentarium of tissue-engineering therapies for treatment of focal cartilage lesions.

Tools for probing host-bacteria interactions in the gut microenvironment: From molecular to cellular levels

Healthy function of the gut microenvironment is dependent on complex interactions between the bacteria of the microbiome, epithelial and immune (host) cells, and the surrounding tissue. Misregulation of these interactions is implicated in disease. A range of tools have been developed to study these interactions, from mechanistic studies to therapeutic evaluation. In this Digest, we highlight select tools at the cellular and molecular level for probing specific cell-microenvironment interactions. Approaches are overviewed for controlling and probing cell-cell interactions, from transwell and microfluidic devices to engineered bacterial peptidoglycan fragments, and cell-matrix interactions, from three-dimensional scaffolds to chemical handles for in situ modifications.

Compartmentalized processing of catechols during mussel byssus fabrication determines the destiny of DOPA

Inspired largely by the role of the posttranslationally modified amino acid dopa (DOPA) in mussel adhesion, catechol functional groups have become commonplace in medical adhesives, tissue scaffolds, and advanced smart polymers. Yet, the complex redox chemistry of catechol groups complicates cross-link regulation, hampering fabrication and the long-term stability/performance of mussel-inspired polymers. Here, we investigated the various fates of DOPA residues in proteins comprising mussel byssus fibers before, during, and after protein secretion. Utilizing a combination of histological staining and confocal Raman spectroscopy on native tissues, as well as peptide-based cross-linking studies, we have identified at least two distinct DOPA-based cross-linking pathways during byssus fabrication, achieved by oxidative covalent cross-linking or formation of metal coordination interactions under reducing conditions, respectively. We suggest that these end states are spatiotemporally regulated by the microenvironments in which the proteins are stored prior to secretion, which are retained after formation-in particular, due to the presence of reducing moieties. These findings provide physicochemical pathways toward greater control over properties of synthetic catechol-based polymers and adhesives.

Recent progress in synthesis and application of mussel-inspired adhesives

The rapid and robust adhesion of marine mussels to diverse solid surfaces in wet environments is mediated by the secreted mussel adhesive proteins which are abundant in a catecholic amino acid, l-3,4-dihydroxyphenylalanine (Dopa). Over the last two decades, enormous efforts have been devoted to the development of synthetic mussel-inspired adhesives with water-resistant adhesion and cohesion properties by modifying polymer systems with Dopa and its analogues. In the present review, an overview of the unique features of various mussel foot proteins is provided in combination with an up-to-date understanding of catechol chemistry, which contributes to the strong interfacial binding via balancing a variety of covalent and noncovalent interactions including oxidative cross-linking, electrostatic interaction, metal-catechol coordination, hydrogen bonding, hydrophobic interactions and π-π/cation-π interactions. The recent developments of novel Dopa-containing adhesives with on-demand mechanical properties and other functionalities are then summarized under four broad categories: viscous coacervated adhesives, soft adhesive hydrogels, smart adhesives, and stiff adhesive polyesters, where their emerging applications in engineering, biological and biomedical fields are discussed. Limitations of the developed adhesives are identified and future research perspectives in this field are proposed.

Blue light-induced low mechanical stability of ruthenium-based coordination bonds: an AFM-based single-molecule force spectroscopy study

A HA–Ru^II complex was conjugated to a hyaluronan polymer through amide bonds. In AFM experiments using the “multi-fishhook” approach, the cantilever tip made contact with the polymeric molecule, resulting in stretching, indicated by sawtooth-like force-extension curves.

Deciphering How the Viscoelastic Properties of Mussel-Inspired Metal-Coordinate Transiently Cross-Linked Gels Dictate Their Tack Behavior

In recent years, researchers have incorporated mussel-inspired metal-coordinate cross-links into various types of gels to improve their mechanical properties, particularly toughness and self-healing. However, not much is understood about how the linear mechanical properties of these gels dictate their tack properties. In this study, we use shear rheology and tack tests to explore correlations between linear viscoelastic properties (i.e., plateau modulus, G_p, and characteristic relaxation time, τ_c) and tack behavior (i.e., peak stress, σ_max, and energy dissipation per volume, EDV) of transiently cross-linked hydrogels comprised of histidine-functionalized 4-arm PEG coordinated with Ni²⁺. By using the Ni²⁺-histidine ratio and polymer wt % of the transient networks to control their viscoelastic properties, we demonstrate a strong dependence of σ_max on G_p and τ_c. The observed correlation between network dynamics and mechanics under tensile load is in good quantitative agreement with a theoretical framework for σ_max, which includes the linear viscoelastic properties as parameters. EDV is also influenced by G_p and τ_c, and the EDV after reaching σ_max is additionally dependent on the polymer wt %. These findings are consistent with previously proposed molecular mechanics of reversible His_xNi²⁺ cross-links and provide us with new insights into the correlations between bulk mechanics and adhesive dynamics of gels with transient metal-coordinate cross-links.

Metal-crosslinked ɛ-poly-L-lysine tissue adhesives with high adhesive performance: Inspiration from mussel adhesive environment

Strong glue of mussels has long been considered as an ideal model to design synthetic bio-adhesives but the adhesive strength of metal-crosslinked mussel-inspired glues is not often satisfactory. Herein, inspired by the adhesive environment of mussels, we obtained metal-crosslinked ε-poly-L-lysine adhesives with high adhesive performance by introducing the elements of suitable adhesive environment (SAE) into the adhesives. The elements of SAE were clarified as weak alkaline conditions (pH ∼ 7.4) and low Fe³⁺ contents. The adhesive strength (∼105 kPa) of the metal-crosslinked adhesives endowed with the elements of SAE (PL-Cat/Fe-SAE) was about 8 times higher than that of fibrin glues. The high adhesive strength was found to originate from distinctive interfacial adhesion and cohesion strength of PL-Cat/Fe-SAE. PL-Cat/Fe-SAE showed strong interfacial adhesion capacity and nearly comparable cohesion strength to those PL-Cat/Fe adhesives with higher Fe³⁺ contents. The nearly comparable cohesion strength of PL-Cat/Fe-SAE was then found to be due to more amount of stable tris-complex existed in PL-Cat/Fe-SAE. In addition, PL-Cat/Fe-SAE was able to efficiently close the full thickness skin incisions. The study highlighted the importance of introducing SAE elements into the design of tissue adhesives and provided a facile and efficient strategy for constructing tissue adhesives with high adhesive performance.