Phillip B. Messersmith

Single-molecule mechanics of mussel adhesion.

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 oxidation-dependent 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.

A reversible wet/dry adhesive inspired by mussels and geckos

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.

pH-induced metal-ligand cross-links inspired by mussel yield self-healing polymer networks with near-covalent elastic moduli

Growing evidence supports a critical role of metal-ligand coordination in many attributes of biological materials including adhesion, self-assembly, toughness, and hardness without mineralization [Rubin DJ, Miserez A, Waite JH (2010) Advances in Insect Physiology: Insect Integument and Color, eds Jérôme C, Stephen JS (Academic Press, London), pp 75–133]. Coordination between Fe and catechol ligands has recently been correlated to the hardness and high extensibility of the cuticle of mussel byssal threads and proposed to endow self-healing properties [Harrington MJ, Masic A, Holten-Andersen N, Waite JH, Fratzl P (2010) Science 328:216–220]. Inspired by the pH jump experienced by proteins during maturation of a mussel byssus secretion, we have developed a simple method to control catechol-Fe3+ interpolymer cross-linking via pH. The resonance Raman signature of catechol-Fe3+ cross-linked polymer gels at high pH was similar to that from native mussel thread cuticle and the gels displayed elastic moduli (G′) that approach covalently cross-linked gels as well as self-healing properties.

Peptidomimetic polymers for antifouling surfaces

Exposure of therapeutic and diagnostic medical devices to biological fluids is often accompanied by interfacial adsorption of proteins, cells, and microorganisms. Biofouling of surfaces can lead to compromised device performance or increased cost and in some cases may be life-threatening to the patient. Although numerous antifouling polymer coatings have enjoyed short-term success in preventing protein and cell adsorption on surfaces, none have proven ideal for conferring long-term biofouling resistance. Here we describe a new biomimetic antifouling N-substituted glycine polymer (peptoid) containing a C-terminal peptide anchor derived from residues found in mussel adhesive proteins for robust attachment of the polymer onto surfaces. The methoxyethyl side chain of the peptoid portion of the polymer was chosen for its chemical resemblance to the repeat unit of the known antifouling polymer poly(ethylene glycol) (PEG), whereas the composition of the 5-mer anchoring peptide was chosen to directly mimic the DOPA- and Lys-rich sequence of a known mussel adhesive protein. Surfaces modified with this biomimetic peptide-peptoid conjugate exhibited dramatic reduction of serum protein adsorption and resistance to mammalian cell attachment for over 5 months in an in vitro assay. These new synthetic peptide based antifouling polymers may provide long-term control of surface biofouling in the physiologic, marine, and industrial environments.

Colorless Multifunctional Coatings Inspired by Polyphenols Found in Tea, Chocolate, and Wine

Polyphenolic compounds present in tea, red wine, and chocolate form thin adherent polyphenol films on substrates through spontaneous adsorption from solution. From this observation emerged a versatile and comprehensive approach to surface modification of a variety of solid, porous, and nanoparticulate substrates composed of metals, ceramics, and polymers (see picture; ROS=reactive oxygen species).

Catechol Polymers for pH-Responsive, Targeted Drug Delivery to Cancer Cells

A novel cell-targeting, pH-sensitive polymeric carrier was employed in this study for delivery of the anticancer drug bortezomib (BTZ) to cancer cells. Our strategy is based on facile conjugation of BTZ to catechol-containing polymeric carriers that are designed to be taken up selectively by cancer cells through cell surface receptor-mediated mechanisms. The polymer used as a building block in this study was poly(ethylene glycol), which was chosen for its ability to reduce nonspecific interactions with proteins and cells. The catechol moiety was exploited for its ability to bind and release borate-containing therapeutics such as BTZ in a pH-dependent manner. In acidic environments, such as in cancer tissue or the subcellular endosome, BTZ dissociates from the polymer-bound catechol groups to liberate the free drug, which inhibits proteasome function. A cancer-cell-targeting ligand, biotin, was presented on the polymer carriers to facilitate targeted entry of drug-loaded polymer carriers into cancer cells. Our study demonstrated that the cancer-targeting drug–polymer conjugates dramatically enhanced cellular uptake, proteasome inhibition, and cytotoxicity toward breast carcinoma cells in comparison with nontargeting drug–polymer conjugates. The pH-sensitive catechol–boronate binding mechanism provides a chemoselective approach for controlling the release of BTZ in targeted cancer cells, establishing a concept that may be applied in the future toward other boronic acid-containing therapeutics to treat a broad range of diseases.

Biological performance of mussel-inspired adhesive in extrahepatic islet transplantation.

There is significant need for effective medical adhesives that function reliably on wet tissue surfaces with minimal inflammatory insult. To address these performance characteristics, we have generated a synthetic adhesive biomaterial inspired by the protein glues of marine mussels. In-vivo performance was interrogated in a murine model of extrahepatic syngeneic islet transplantation, as an alternative to standard portal administration. The adhesive precursor polymer consisted of a branched poly(ethylene glycol) (PEG) core, whose endgroups were derivatized with catechol, a functional group abundant in mussel adhesive proteins. Under oxidizing conditions, adhesive hydrogels formed in less than 1 min from catechol-derivatized PEG (cPEG) solutions. Upon implantation, the cPEG adhesive elicited minimal acute or chronic inflammatory response in C57BL6 mice, and maintained an intact interface with supporting tissue for up to one year. In-situ cPEG adhesive formation was shown to efficiently immobilize transplanted islets at the epididymal fat pad and external liver surfaces, permitting normoglycemic recovery and graft revascularization. These findings establish the use of synthetic, biologically-inspired adhesives for islet transplantation at extrahepatic sites.

Bioinspired antifouling polymers

Controlling biointerfacial phenomena is crucial to the success of many biomedical technologies. For applications in biosensing, diagnostics, and medical devices, precise control of interactions between material surfaces and the biological milieu is an important but elusive goal 1 . Emerging strategies for manipulating the biological response to medical materials seek to take advantage of specific interactions between designed surfaces, biomolecules, and cells 2 . Limiting nonspecific interaction of cells, proteins, and microorganisms with material surfaces is critical, since these interactions can prove highly problematic for device efficacy and safety. Thus, a central research focus continues to be the development of versatile, convenient, and cost-effective strategies for rendering surfaces resistant to fouling by proteins, cells, and bacteria.

pH responsive self-healing hydrogels formed by boronate-catechol complexation

Here we report the synthesis and characterization of pH-responsive, self-healing hydrogels based on boronate-catechol complexation.

Norepinephrine: Material-Independent, Multifunctional Surface Modification Reagent

A facile approach for material-independent surface modification using norepinephrine was investigated. pH-induced oxidative polymerization of norepinephrine forms adherent films on vastly different types of material surfaces of noble metals, metal oxides, semiconductors, ceramics, shape-memory alloys, and synthetic polymers. Secondary biochemical functionalizations such as immobilization of proteins and growth of biodegradable polyester on the poly(norepinephrine) films were demonstrated.