Mussel-Inspired Self-Healing Double-Cross-Linked Hydrogels by Controlled Combination of Metal Coordination and Covalent Cross-Linking

Mussel-inspired hydrogels held together by reversible catecholato–metal coordination bonds have recently drawn great attention owing to their attractive self-healing, viscoelastic and adhesive properties together with their pH-responsive nature. A major challenge in these systems is to orchestrate the degree of catechol oxidation that occurs under alkaline conditions in air and has a great impact on the aforementioned properties because it introduces irreversible covalent cross-links to the system, which stiffens the hydrogels but consume catechols needed for self-healing. Herein, we present a catechol-based hydrogel design that allows for the degree of oxidative covalent cross-linking to be controlled. Double cross-linked hydrogels with tunable stiffness are constructed by adding the oxidizable catechol analogue, tannic acid, to an oxidation-resistant hydrogel construct held together by coordination of the dihydroxy functionality of 1-(2′-carboxyethyl)-2-methyl-3-hydroxy-4-pyridinone to trivalent metal ions. By varying the amount of tannic acid, the hydrogel stiffness can be customized to a given application while retaining the self-healing capabilities of the hydrogel’s coordination chemical component

Mussel-Inspired Self-Healing Double-Cross-Linked Hydrogels by Controlled Combination of Metal Coordination and Covalent Cross-Linking

Mussel-inspired hydrogels held together by reversible catecholato–metal coordination bonds have recently drawn great attention owing to their attractive self-healing, viscoelastic and adhesive properties together with their pH-responsive nature. A major challenge in these systems is to orchestrate the degree of catechol oxidation that occurs under alkaline conditions in air and has a great impact on the aforementioned properties because it introduces irreversible covalent cross-links to the system, which stiffens the hydrogels but consume catechols needed for self-healing. Herein, we present a catechol-based hydrogel design that allows for the degree of oxidative covalent cross-linking to be controlled. Double cross-linked hydrogels with tunable stiffness are constructed by adding the oxidizable catechol analogue, tannic acid, to an oxidation-resistant hydrogel construct held together by coordination of the dihydroxy functionality of 1-(2′-carboxyethyl)-2-methyl-3-hydroxy-4-pyridinone to trivalent metal ions. By varying the amount of tannic acid, the hydrogel stiffness can be customized to a given application while retaining the self-healing capabilities of the hydrogel’s coordination chemical component

Self-Healing Mussel-Inspired Multi-pH-Responsive Hydrogels

Self-healing hydrogels can be made using either reversible covalent cross-links or coordination chemistry bonds. Here we present a multi-pH-responsive system inspired by the chemistry of blue mussel adhesive proteins. By attaching DOPA to an amine-functionalized polymer, a multiresponsive system is formed upon reaction with iron. The degree of polymer cross-linking is pH controlled through the pH-dependent DOPA/iron coordination chemistry. This leads to the formation of rapidly self-healing high-strength hydrogels when pH is raised from acidic toward basic values. Close to the p<i>K</i>_a value, or more precisely the pI value, of the polymer, the gel collapses due to reduced repulsion between polymer chains. Thereby a bistable gel-system is obtained. The present polymer system more closely resembles mussel adhesive proteins than those previously reported and thus also serves as a model system for mussel adhesive chemistry

Precipitation of Inorganic Phases through a Photoinduced pH Jump: From Vaterite Spheroids and Shells to ZnO Flakes and Hexagonal Plates

This report demonstrates a new way to precipitate inorganic phases through pH jumps driven by optical excitation of a photobase. The level of the pH jumps is manipulated by either the wavelength range or illumination time used. The method is demonstrated by precipitation of CaCO₃ and ZnO. Vaterite spheroids and shells for CaCO₃ as well as flakes and hexagonal plates for ZnO are formed under various controllable conditions. Notably, ZnO films could be formed and patterned directly on quartz substrates using a photomask. The film thickness was easily controlled by illumination time. This method is an important step toward self-organized crystallization with spatiotemporal control fully in the hands of the experimenter

Transparent Aggregates of Nanocrystalline Hydroxyapatite

Assemblies of nanoparticles into transparent aggregates have solicited strong research interest in the form of both crystalline or amorphous aggregates of nanoparticles. In the present work, we make short-range ordered several millimeter-sized transparent aggregates of citrate modified calcium phosphate nanoparticles and discuss the mechanism of their formation. Microparticles of hydroxyapatite (HAP) nanocrystals and amorphous calcium phosphate (ACP) were synthesized with citrate as a growth and assembly modifier. Millimeter-sized transparent aggregates of these microparticles were made with 0 to 7.5% citrate/Ca²⁺. The degree of crystallinity, i.e., the ratio between nanocrystalline HAP and ACP in the microparticles, was determined by Rietveld refinement of powder X-ray diffraction data with an internal standard. It was found to decrease with increasing citrate concentration. Citrate also reduced the nanocrystallite size at low citrate concentrations. Above ∼3% added citrate, the crystallite size did not reduce further. Transparent aggregates were obtained by drying a suspension of particles. The aggregates lacked long-range order and in many cases featured spiral fractures partially propagating through the aggregates. The assembly mechanisms were studied by in situ video imaging, polarized optical microscopy, transmission electron microscopy, and confocal microscopy. The transparent aggregates consisted of polydisperse microparticles. The transparent aggregates form due to evaporation, but sedimentation leads to vertical size segregation with larger microparticles preferentially located at the bottom of the sample

Calcium-phosphate-osteopontin particles for caries control

<p>Caries is caused by acid production in biofilms on dental surfaces. Preventing caries therefore involves control of microorganisms and/or the acid produced. Here, calcium-phosphate-osteopontin particles are presented as a new approach to caries control. The particles are made by co-precipitation and designed to bind to bacteria in biofilms, impede biofilm build-up without killing the microflora, and release phosphate ions to buffer bacterial acid production if the pH decreases below 6. Analysis of biofilm formation and pH in a five-species biofilm model for dental caries showed that treatment with particles or pure osteopontin led to less biofilm formation compared to untreated controls or biofilms treated with osteopontin-free particles. The anti-biofilm effect can thus be ascribed to osteopontin. The particles also led to a slower acidification of the biofilm after exposure to glucose, and the pH always remained above 5.5. Hence, calcium-phosphate-osteopontin particles show potential for applications in caries control.</p

Osteopontin Reduces Biofilm Formation in a Multi-Species Model of Dental Biofilm

<div><p>Background</p><p>Combating dental biofilm formation is the most effective means for the prevention of caries, one of the most widespread human diseases. Among the chemical supplements to mechanical tooth cleaning procedures, non-bactericidal adjuncts that target the mechanisms of bacterial biofilm formation have gained increasing interest in recent years. Milk proteins, such as lactoferrin, have been shown to interfere with bacterial colonization of saliva-coated surfaces. We here study the effect of bovine milk osteopontin (OPN), a highly phosphorylated whey glycoprotein, on a multispecies <i>in vitro</i> model of dental biofilm. While considerable research effort focuses on the interaction of OPN with mammalian cells, there are no data investigating the influence of OPN on bacterial biofilms.</p><p>Methodology/Principal Findings</p><p>Biofilms consisting of <i>Streptococcus oralis, Actinomyces naeslundii, Streptococcus mitis, Streptococcus downei</i> and <i>Streptococcus sanguinis</i> were grown in a flow cell system that permitted <i>in situ</i> microscopic analysis. Crystal violet staining showed significantly less biofilm formation in the presence of OPN, as compared to biofilms grown without OPN or biofilms grown in the presence of caseinoglycomacropeptide, another phosphorylated milk protein. Confocal microscopy revealed that OPN bound to the surface of bacterial cells and reduced mechanical stability of the biofilms without affecting cell viability. The bacterial composition of the biofilms, determined by fluorescence <i>in situ</i> hybridization, changed considerably in the presence of OPN. In particular, colonization of <i>S. mitis</i>, the best biofilm former in the model, was reduced dramatically.</p><p>Conclusions/Significance</p><p>OPN strongly reduces the amount of biofilm formed in a well-defined laboratory model of acidogenic dental biofilm. If a similar effect can be observed <i>in vivo</i>, OPN might serve as a valuable adjunct to mechanical tooth cleaning procedures.</p></div

Biofilms grown in the presence of OPN, hybridized with EUB338 and species-specific probes SMIT, SSAN, ANAES, SDOW or SORA2.

<p>EUB338 targets all organisms in the biofilms and was labelled with Atto633 (red). Species-specific probes were labelled with Cy3 (green). <b>A.</b> <i>S. mitis</i> SK24, the dominant organism in biofilms grown without OPN, accounted for 14% of the bacterial biovolume. <b>B–F.</b> The relative biovolumes of all other organisms increased in biofilms grown with OPN, as compared to biofilms grown without OPN. <i>S. sanguinis</i> SK150 (<b>B</b>) was the most abundant organism in the biofilms (48% of the biovolume). <i>A. naeslundii</i> AK6 was a prominent colonizer in basal layers of the biofilms (<b>C</b>, 22% of the biovolume in the basal layer), but was detected less frequently in upper layers of the biofilm (<b>D</b>, 9% of the total biovolume). <i>S. downei</i> HG594 (<b>E</b>, 11% of the biovolume) and <i>S. oralis</i> SK248 (<b>F</b>, 3% of the biovolume) represented smaller fractions of the bacterial biofilm.</p

Bacterial composition of biofilms grown in the presence and absence of OPN.

<p>In biofilms grown without OPN (−OPN), <i>S. mitis</i> SK24 was the predominant organism. When OPN was present in the medium (+OPN), the abundance of <i>S. mitis</i> was dramatically lower, and the relative abundance of all other organisms increased. <i>S. sanguinis</i> SK150 became the predominant organism.</p

Quantification of biofilm formation by crystal violet staining.

<p>Biofilms were grown in flow channels for 30 h on 1/10 diluted THB containing 26.5 µmol/L OPN, 26.5 µmol/L CGMP or none of the two proteins. <b>A.</b> Photograph showing biofilms grown with (right channel) and without OPN (left channel) after crystal violet staining. When OPN was present in the medium, less biofilm formed in the flow channels. <b>B.</b> Quantification of the biofilm biomass by spectrophotometry. OD₅₈₅ was significantly lower when biofilms were grown in the presence of OPN (+OPN), as compared to biofilms grown on THB only (−OPN). No such effect was observed when CGMP was present in the medium (CGMP). Error bars indicate standard deviations.</p