Marine mussels secret catechol-containing adhesive proteins that enable these organisms to bind to various surfaces underwater. Synthetic mimics of these proteins have been created to function as adhesives and coatings for a wide range of applications. Here, we demonstrated the use of in situ electrical field stimulation to deactivate the adhesive property of catechol-containing adhesive that is in direct contact with a surface. Johnson–Kendall–Roberts (JKR) contact mechanics test was performed using a titanium (Ti) sphere in the presence of a pH 7.5 aqueous buffer. The Ti sphere also served as a conductive electrode for applying electricity to the adhesive, while a platinum (Pt) wire served as the counter electrode. Work of adhesion (W adh) decreased with increased levels of applied voltage and current, exposure time to the applied electricity, and salt concentration of the interfacial buffer. Application of 9 V for 1 min completely deactivated the adhesive. UV–vis diffuse reflectance spectra and tracking of catechol oxidation byproduct, hydrogen peroxide, confirmed that catechol was oxidized as a result of applied electricity. Contact mechanics testing further confirmed that the Young’s modulus of the adhesive increased by nearly 4 folds at the interface as a result of oxidative cross-linking, even though the modulus of the bulk of the adhesive was unaffected by applied electricity. The accumulation of hydroxyl ions near the cathode increased the local solution pH, which promoted oxidation-induced cross-linking of catechol and subsequently decreased its adhesive property. Tuning adhesive properties through in situ electrochemical oxidation provides on-demand control over the adhesive, which will potentially add another dimension in designing synthetic mimics of mussel adhesive proteins.
A hydrogel is a three-dimensional polymer network with high water content and has been attractive for many biomedical applications due to its excellent biocompatibility. However, classic hydrogels are mechanically weak and unsuitable for most physiological load-bearing situations. Thus, the development of tough hydrogels used in the biomedical field becomes critical. This work reviews various strategies to fabricate tough hydrogels with the introduction of non-covalent bonds and the construction of stretchable polymer networks and interpenetrated networks, such as the so-called double-network hydrogel. Additionally, the design of tough hydrogels for tissue adhesive, tissue engineering, and soft actuators is reviewed.
Reactive oxygen species (ROS) can degrade organic compounds and function as a broad-spectrum disinfectant. Here, dopamine methacrylamide (DMA) was used to prepare catechol-containing microgels, which can release ROS via metal-catechol interaction. A combination of the microgel and iron magnetic nanoparticle (FeMNP) significantly reduced the concentration of four organic dyes (Alizarin Red S, Rhodamine B, Crystal Violet, and Malachite Green) and an antibiotic drug, ciprofloxacin, dissolved in solution. Degradation of dye occurred across a wide range of pH levels (pH 3–9). This simple combination was also antimicrobial against both Escherichia coli and Staphylococcus aureus. Electron paramagnetic resonance spectroscopy (EPR) results indicate that singlet oxygen was generated during the reaction between catechol and FeMNP at both pH 3 and 7.4, which was responsible for the degradation of organic compounds and bactericidal features of the microgel. Unlike autoxidation that only occurs at a neutral to basic pH, FeMNP-induced catechol oxidation generated singlet oxygen over a wide range of pH level. Additionally, catechol chelates heavy metal ions, resulting in their removal from solution and repurposed these metal ions for dye degradation. This multifunctional microgel can potentially be used for environmental applications for the removal of organic pollutants and heavy metal ions from wastewater, as well as reducing bacterial infection in biomedical applications.
Many asteroids are likely to be have been shattered by collisions into fragments and reaccumulated as gravitationally‐bound rubble piles. These bodies may contain large porosities, although this picture may be complicated by compaction inside the asteroid body. Estimates of asteroid mass and volume imply a negative correlation between size and porosity for stony asteroids. Asteroids that are suspected to be metallic appear to contain larger porosities than stony asteroids of similar sizes. To understand these observations, we develop models for cold compaction of fragments of different materials. The initial boulder size distributions are assumed to be narrow. We focus on macro‐porosity between the boulders and do not consider micro‐porosity inside the boulders. In our model of silicate/chondritic boulders, compaction is assumed to occur through cataclastic fracturing, which creates small pieces that fill the pores between residual large boulders, leading to fractal‐like distributions. This fracturing occurs when compression leads to stresses exceeding the tensile strength of the boulders. Combining this model with data on meteorite strength, we suggest that the compaction of chondritic boulders can be significant at pressures of several megapascals. In our model of metal boulders, we consider cold welding and boulder deformation (through ductile yielding or brittle‐like fracturing, depending on the stress and intrinsic crack size). Given the properties of iron meteorites, we infer that compaction in metallic rubble piles, caused by ductile or brittle deformation, is small, and that cold welding may lead to large (≳50%) porosities if the boulders are of ∼1 m sizes.
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