This review presents insights into the fundamental challenges of wet adhesion, and the applications of catechol-functionalized hydrogels in diverse areas.

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

Microgels that can generate antipathogenic levels of hydrogen peroxide (H2O2) through simple rehydration in solutions with physiological pH are described herein. H2O2 is a widely used disinfectant but the oxidant is hazardous to store and transport. Catechol, an adhesive moiety found in mussel adhesive proteins, was incorporated into microgels, which generated 1–5 mM of H2O2 for up to four days as catechol autoxidized. The sustained release of low concentrations of H2O2 was antimicrobial against both gram-positive (Staphylococcus epidermidis) and gram-negative (Escherichia coli) bacteria and antiviral against both non-enveloped porcine parvovirus (PPV) and enveloped bovine viral diarrhea virus (BVDV). The amount of released H2O2 is several orders of magnitude lower than H2O2 concentration previously reported for antipathogenic activity. Most notably, these microgels reduced the invectively of the more biocide resistant non-envelope virus by 3 log reduction value (99.9% reduction in infectivity). By controlling the oxidation state of catechol, microgels can be repeatedly activated and deactivated for H2O2 generation. These microgels do not contain a reservoir for storing the reactive H2O2 and can potentially function as a lightweight and portable dried powder source for the disinfectant for a wide range of applications.

A simple two-step, shaking-assisted polydopamine (PDA) coating technique was used to impart polypropylene (PP) mesh with antimicrobial properties. In this modified method, a relatively large concentration of dopamine (20 mg ml−1) was first used to create a stable PDA primer layer, while the second step utilized a significantly lower concentration of dopamine (2 mg ml−1) to promote the formation and deposition of large aggregates of PDA nanoparticles. Gentle shaking (70 rpm) was employed to increase the deposition of PDA nanoparticle aggregates and the formation of a thicker PDA coating with nano-scaled surface roughness (RMS = 110 nm and Ra = 82 nm). Cyclic voltammetry experiment confirmed that the PDA coating remained redox active, despite extensive oxidative cross-linking. When the PDA-coated mesh was hydrated in phosphate saline buffer (pH 7.4), it was activated to generate 200 μM hydrogen peroxide (H2O2) for over 48 h. The sustained release of low doses of H2O2 was antibacterial against both gram-positive (Staphylococcus epidermidis) and gram-negative (Escherichia coli) bacteria. PDA coating achieved 100% reduction (LRV ~3.15) when incubated against E. coli and 98.9% reduction (LRV ~1.97) against S. epi in 24 h.

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Hydroxyl radical (•OH) is a potent reactive oxygen species with the ability to degrade hazardous organic compounds, kill bacteria, and inactivate viruses. However, an off-the-shelf, portable, and easily activated biomaterial for generating •OH does not exist. Here, microgels were functionalized with catechol, an adhesive moiety found in mussel adhesive proteins, and hematin (HEM), a hydroxylated Fe3+ ion-containing porphyrin derivative. When the microgel was hydrated in an aqueous solution with physiological pH, molecular oxygen in the solution oxidized catechol to generate H2O2, which was further converted to •OH by HEM. The generated •OH was able to degrade organic dyes, including orange II and malachite green. Additionally, the generated •OH was antimicrobial against both Gram-negative (Escherichia coli) and Gram-positive (Staphylococcus epidermidis) bacteria with the initial concentration of 106 to 107 cfu/mL. These microgels also reduced the infectivity of a nonenveloped porcine parvovirus and an enveloped bovine viral diarrhea virus by 3.5 and 4.5 log reduction values, respectively (99.97–99.997% reduction in infectivity). These microgels were also functionalized with positively charged [2-(methacryloyloxy)­ethyl] trimethylammonium chloride, which significantly enhanced the antibacterial and antiviral activities through electrostatic interaction between the negatively charged pathogens and the microgel. These microgels can potentially serve as a lightweight and portable source of disinfectant for an on-demand generation of •OH with a wide range of applications.

Catechol-based bioadhesives generate hydrogen peroxide (H 2 O 2 ) as a byproduct during the curing process. H 2 O 2 can have both beneficial and deleterious effects on biological systems depending on its concentration. To control the amount of H 2 O 2 released from catechol-containing polyethylene glycol-based adhesive (PEG-DA), an adhesive was formulated with silica nanoparticles (SiNPs) prepared with increased porosity and acid treatment to increase Si−OH surface content. These SiNPs demonstrated increased surface area, which promoted interaction with catechol and resulted in an increased cure rate, bulk mechanical properties, and adhesive properties of PEG-DA. Most importantly, the SiNPs demonstrated a 50% reduction in the released H 2 O 2 while improving the cell viability and proliferation of three primary cell types, including rat dermal fibroblasts, human epidermal keratinocytes, and human tenocytes. Additionally, the SiNPs degraded into soluble Si, which also contributed to increased cell proliferation. Incorporation of porous and acid-treated SiNPs can be a useful approach to simultaneously modulate the concentration of H 2 O 2 while increasing the adhesive performance of catechol-based adhesives.

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