We report a method to form multifunctional polymer coatings through simple dip-coating of objects in an aqueous solution of dopamine. Inspired by the composition of adhesive proteins in mussels, we used dopamine self-polymerization to form thin, surface-adherent polydopamine films onto a wide range of inorganic and organic materials, including noble metals, oxides, polymers, semiconductors, and ceramics. Secondary reactions can be used to create a variety of ad-layers, including self-assembled monolayers through deposition of long-chain molecular building blocks, metal films by electroless metallization, and bioinert and bioactive surfaces via grafting of macromolecules.Methods for chemical modification of bulk material surfaces play central roles in modern chemical, biological, and materials sciences, and in applied science, engineering, and technology (1-4). The existing toolbox for the functional modification of material surfaces includes methods such as self-assembled monolayer (SAM) formation, functionalized silanes, Langmuir-Blodgett deposition, layer-by-layer assembly, and genetically engineered surfacebinding peptides (5-9). Although widely implemented in research, many available methods have limitations for widespread practical use; specific examples include the requirement for chemical specificity between interfacial modifiers and surfaces (e.g., alkanethiols on noble metals and silanes on oxides), the use of complex instrumentation and limitations of substrate size and shape (Langmuir-Blodgett deposition), or the need for multistep procedures for implementation (layer-by-layer assembly and genetically engineered surface-binding peptides).Development of simple and versatile strategies for surface modification of multiple classes of materials has proven challenging, and few generalized methods for accomplishing this have been previously reported (10). Our approach is inspired by the adhesive proteins secreted by mussels for attachment to wet surfaces (11). Mussels are promiscuous fouling organisms and have been shown to attach to virtually all types of inorganic and organic surfaces (12), including classically adhesion-resistant materials such as poly(tetrafluoroethylene) (PTFE) (Fig. 1A). Clues to mussels' adhesive versatility may lie in the amino acid composition of proteins found near the plaque-substrate interface (Fig. 1, B to D), which are rich in 3,4-dihydroxy-Lphenylalanine (DOPA) and lysine amino acids (13). In addition to participating in reactions

The glue proteins secreted by marine mussels bind strongly to virtually all inorganic and organic surfaces in aqueous environments in which most adhesives function poorly. Studies of these functionally unique proteins have revealed the presence of the unusual amino acid 3,4-dihydroxy-L-phenylalanine (dopa), which is formed by posttranslational modification of tyrosine. However, the detailed binding mechanisms of dopa remain unknown, and the chemical basis for mussels' ability to adhere to both inorganic and organic surfaces has never been fully explained. Herein, we report a single-molecule study of the substrate and oxidationdependent adhesive properties of dopa. Atomic force microscopy (AFM) measurements of a single dopa residue contacting a wet metal oxide surface reveal a surprisingly high strength yet fully reversible, noncovalent interaction. The magnitude of the bond dissociation energy as well as the inability to observe this interaction with tyrosine suggests that dopa is critical to adhesion and that the binding mechanism is not hydrogen bond formation. Oxidation of dopa, as occurs during curing of the secreted mussel glue, dramatically reduces the strength of the interaction to metal oxide but results in high strength irreversible covalent bond formation to an organic surface. A new picture of the interfacial adhesive role of dopa emerges from these studies, in which dopa exploits a remarkable combination of high strength and chemical multifunctionality to accomplish adhesion to substrates of widely varying composition from organic to metallic.3,4-dihydroxylphenylalanine ͉ atomic force microscopy ͉ mussel adhesive protein N umerous living creatures rely on physical adhesion to biotic and abiotic objects for essential activities, such as movement, protection, and self-defense (1-3). From a purely functional point of view, bioadhesion can be of two major types: temporary and permanent. A characteristic example of a temporary bioadhesive strategy is given by the specialized foot hairs used by geckos for climbing sheer surfaces (1). A classic example of permanent bioadhesion is exemplified by mussels, (4) which secrete holdfasts essential for stability within the tidal marine environment. The remarkable features of mussel adhesion include the ability to achieve long-lasting adhesion in a wet environment (3) and adherence to virtually all types of inorganic and organic surfaces (5). The adhesive apparatus of the mussel consists of a series of byssal threads that tether the organism to a substrate (Fig. 1A). At least five specialized adhesive protein subtypes known to contain 3,4-dihydroxy-L-phenylalanine (dopa) at concentrations ranging from a few mol % to 27 mol % (Fig. 1B) are found within the distal adhesive pad of the widely studied blue mussel, Mytilus edulis (6). The highest dopa content occurs in M. edulis foot protein (Mefp)-3 (21 mol %) and Mefp-5 (27 mol %) (7, 8), both of which are localized near the interface between the adhesive pad and the substrate (Fig. 1C).The role of dopa in mussel adhesi...

The adhesive strategy of the gecko relies on foot pads composed of specialized keratinous foot-hairs called setae, which are subdivided into terminal spatulae of approximately 200 nm (ref. 1). Contact between the gecko foot and an opposing surface generates adhesive forces that are sufficient to allow the gecko to cling onto vertical and even inverted surfaces. Although strong, the adhesion is temporary, permitting rapid detachment and reattachment of the gecko foot during locomotion. Researchers have attempted to capture these properties of gecko adhesive in synthetic mimics with nanoscale surface features reminiscent of setae; however, maintenance of adhesive performance over many cycles has been elusive, and gecko adhesion is greatly diminished upon full immersion in water. Here we report a hybrid biologically inspired adhesive consisting of an array of nanofabricated polymer pillars coated with a thin layer of a synthetic polymer that mimics the wet adhesive proteins found in mussel holdfasts. Wet adhesion of the nanostructured polymer pillar arrays increased nearly 15-fold when coated with mussel-mimetic polymer. The system maintains its adhesive performance for over a thousand contact cycles in both dry and wet environments. This hybrid adhesive, which combines the salient design elements of both gecko and mussel adhesives, should be useful for reversible attachment to a variety of surfaces in any environment.

A new surface bioconjugation strategy is presented. A polydopamine surface coating provides chemical activation on material surfaces, is resistant to hydrolysis, and offers selectivity in coupling of biomolecules via nucleophilic groups through simple pH control. Control of orientation of immobilized biomolecules may be possible using terminally modified DNA or His‐containing proteins.

Polydopamine is one of the simplest and most versatile approaches to functionalizing material surfaces, having been inspired by the adhesive nature of catechols and amines in mussel adhesive proteins. Since its first report in 2007, a decade of studies on polydopamine molecular structure, deposition conditions, and physicochemical properties have ensued. During this time, potential uses of polydopamine coatings have expanded in many unforeseen directions, seemingly only limited by the creativity of researchers seeking simple solutions to manipulating surface chemistry. In this review, we describe the current state of the art in polydopamine coating methods, describe efforts underway to uncover and tailor the complex structure and chemical properties of polydopamine, and identify emerging trends and needs in polydopamine research, including the use of dopamine analogs, nitrogen-free polyphenolic precursors, and improvement of coating mechanical properties.