In-Depth Study of the Interaction Mechanism between the Lignin Nanofilms: Toward a Renewable and Organic Solvent-Free Binder

Lignin is an abundant biorenewable resource with an annual production of 50 million metric tons. Despite the abundance and high potential for applications, only ∼2% of the produced lignin was used for industrial applications. One of the main reasons for the low applicability is the lack of fundamental studies. In particular, the molecular binding mechanism of lignin is a key for the development and design of lignin into higher-value products. In this study, the interaction forces between homogeneous lignin nanofilms as thin as a phenylpropane unit monolayer (∼11 Å) are directly measured using a surface forces apparatus (SFA) at various concentrations of intervening electrolyte solution. The measured adhesion force decreases with increasing electrolyte concentration, the inverse of what would be expected according to the electric double layer theory. These findings, along with detailed analyses using Derjaguin–Landau–Verwey–Overbeek (DLVO) and hydrophobic theories, strongly indicate that hydrophobic interaction accounts for a large proportion of the interaction forces. Additional measurements between methyl-terminated self-assembled monolayer and lignin film confirm that hydrophobic interactions dominated the overall interaction potential of lignin films. Furthermore, lignin-supplemented activated carbon composites show enhanced compressive strength, which indicates the potential use of lignin as an ecofriendly reinforcing binder

Switch of Surface Adhesion to Cohesion by Dopa-Fe³⁺ Complexation, in Response to Microenvironment at the Mussel Plaque/Substrate Interface

Although Dopa-Fe³⁺ complexation is known to play an important role in mussel adhesion for providing mechanical properties, its function at the plaque/substrate interface, where actual surface adhesion occurs, remains unknown, with regard to interfacial mussel adhesive proteins (MAPs) type 3 fast variant (fp-3F) and type 5 (fp-5). Here, we confirmed Dopa-Fe³⁺ complexation of interfacial MAPs and investigated the effects of Dopa-Fe³⁺ complexation regarding both surface adhesion and cohesion. The force measurements using surface forces apparatus (SFA) analysis showed that intrinsic strong surface adhesion at low pH, which is similar to the local acidified environment present during the secretion of adhesive proteins, vanishes by Dopa-Fe³⁺ complexation and alternatively, strong cohesion is generated in higher pH conditions similar to seawater. A high Dopa content increased the capacity for both surface adhesion and cohesion, but not at the same time. In contrast, a lack of Dopa resulted in both weak surface adhesion and cohesion without significant effects of Fe³⁺ complexation. Our findings shed light on how mussels regulate Dopa functionality at the plaque/substrate interface, in response to the microenvironment, and might provide new insight for the design of mussel-inspired biomaterials

Essential Role of Thiols in Maintaining Stable Catecholato-Iron Complexes in Condensed Materials

The load-bearing proteins in mussel holdfasts rely on condensed tris-catecholato-Fe3+ coordination complexes for their toughness and shock-absorbing properties, and this feature has been successfully translated into synthetic materials with short-term high-performance properties. However, oxidation of catecholic DOPA (3,4-dihydroxyphenylalanine) remains a critical impediment to achieving materials with longer-lasting performance. Here, following the natural mussel pathway for protein processing, we explore how DOPA oxidation impacts coacervation of mussel foot protein-1 (mfp-1) and its capacity for phase-specific metal uptake in vitro. Without metal, DOPA oxidation changed the rheological properties (i.e., viscosity, loss, and storage moduli) of mfp-1 coacervate droplets. However, oxidation-dependent changes were recovered with dithiothreitol (DTT), completely restoring the behavior of mfp-1 coacervates prior to oxidation. With metal, mfp-1 coacervates exhibited gel-like behavior with high viscosity and cohesive forces by forming recognizable bis- and tris-catecholato-Fe complexes, linked to increased energy dissipation and toughness of byssus. These results indicate that Fe3+-mediated conversion of liquid–liquid phase-separated polymers into metal-coordinated networks is thorough and rapid, and DTT effectively maintains redox integrity. Our study provides much-needed improvements for processing catechol-functionalized polymers into high-performance materials

Peptidomimetic Wet-Adhesive PEGtides with Synergistic and Multimodal Hydrogen Bonding

The remarkable underwater adhesion of mussel foot proteins has long been an inspiration in the design of peptidomimetic materials. Although the synergistic wet adhesion of catechol and lysine has been recently highlighted, the critical role of the polymeric backbone has remained largely underexplored. Here, we present a peptidomimetic approach using poly­(ethylene glycol) (PEG) as a platform to evaluate the synergistic compositional relation between the key amino acid residues (i.e., DOPA and lysine), as well as the role of the polyether backbone in interfacial adhesive interactions. A series of PEG-based peptides (PEGtides) were synthesized using functional epoxide monomers corresponding to catechol and lysine via anionic ring-opening polymerization. Using a surface force apparatus, highly synergistic surface interactions among these PEGtides with respect to the relative compositional ratio were revealed. Furthermore, the critical role of the catechol–amine synergy and diverse hydrogen bonding within the PEGtides in the superior adhesive interactions was verified by molecular dynamics simulations. Our study sheds light on the design of peptidomimetic polymers with reduced complexity within the framework of a polyether backbone

Mussel-Inspired Anchoring of Polymer Loops That Provide Superior Surface Lubrication and Antifouling Properties

We describe robustly anchored triblock copolymers that adopt loop conformations on surfaces and endow them with unprecedented lubricating and antifouling properties. The triblocks have two end blocks with catechol-anchoring groups and a looping poly­(ethylene oxide) (PEO) midblock. The loops mediate strong steric repulsion between two mica surfaces. When sheared at constant speeds of ∼2.5 μm/s, the surfaces exhibit an extremely low friction coefficient of ∼0.002–0.004 without any signs of damage up to pressures of ∼2–3 MPa that are close to most biological bearing systems. Moreover, the polymer loops enhance inhibition of cell adhesion and proliferation compared to polymers in the random coil or brush conformations. These results demonstrate that strongly anchored polymer loops are effective for high lubrication and low cell adhesion and represent a promising candidate for the development of specialized high-performance biomedical coatings