Mussel-inspired surface chemistry for multifunctional coatings

Abstract

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.

Fig. 1

(A) Photograph of a mussel attached to commercial PTFE. (B and C) Schematic illustrations of the interfacial location of Mefp-5 and a simplified molecular representation of characteristic amine and catechol groups. (D) The amino acid sequence of Mefp-5 (13, 34). (E) Dopamine contains both amine and catechol functional groups found in Mefp-5 and was used as a molecular building block for polymer coatings. (F) A schematic illustration of thin film deposition of polydopamine by dip-coating an object in an alkaline dopamine solution. (G) Thickness evolution of polydopamine coating on Si as measured by AFM of patterned surfaces. (H) XPS characterization of 25 different polydopamine-coated surfaces. The bar graph represents the intensity of characteristic substrate signal before (hatched) and after (solid) coating by polydopamine. The intensity of the unmodified substrate signal is in each case normalized to 100%. Substrates with characteristic XPS signals indistinguishable from the polydopamine signal are marked by “N.A.” The blue circles represent the N/C after polydopamine coating (details of XPS data analysis are available in fig. S1 and table S2).

Fig. 2

Polydopamine-assisted electroless metallization of substrates. (A to C) Electroless copper deposition on polydopamine-coated nitrocellulose film (A), coin (B), and three-dimensional plastic object (C). (D) Schematic representation of electroless metallization of photoresist-patterned surfaces coated with polydopamine. Photoresist (blue) was removed before silver metallization (left) or after copper metallization (right). (E and F) Scanning electron microscopy images showing micropatterns of silver on Si (E) and copper on a glass substrate (F).

Fig. 3

Polydopamine-assisted grafting of various organic molecules. (A) Schematic illustration of alkanethiol monolayer (top right) and PEG polymer (bottom right) grafting on polydopamine-coated surfaces. (B) Pictures of water droplets on several unmodified (left), polydopamine-coated (middle), and alkanethiol-grafted (right) substrates. Substrates investigated include organic polymers [PTFE, PC, and nitrocellulose (NC)], metal oxides (SiO₂ and TiO₂), and noble metals (Cu and Au). Contact angle values are shown in table S1. (C) NIH 3T3 fibroblast cell adhesion to unmodified glass (“Bare”) and OEG6-terminated alkanethiol monolayer formed on polydopamine-coated glass. Error bars indicate SD. (D to F) Total internal reflection fluorescence (TIRF) microscopy of Cy3-conjugated Enigma homolog protein adsorption to mPEG-NH₂–grafted polydopamine-coated glass (48-hour exposure to protein solution) (D), bare glass (30-min exposure) (E), and mPEG-silane immobilized on bare glass (48-hour exposure) (F). (G) NIH 3T3 fibroblast cell adhesion to bare surfaces (black) and to polydopamine-coated surfaces after grafting with mPEG-SH (red) (prenormalized data are available in table S3). Error bars indicate SD.

Fig. 4

Polydopamine-assisted grafting of a biomacromolecule for biospecific cell interaction. (A) Representative scheme for HA conjugation to polydopamine-coated surfaces. (B) Adhesion of M07e cells on polydopamine-coated PS increases with the HA solution concentration used during grafting. Error bars indicate SD. (C) Bioactive HA ad-layers were formed on polydopamine-coated glass, tissue-culture PS, and indium tin oxide (ITO), as demonstrated by attachment of M07e cells (red bars). Competition with soluble HA (blue bar) confirmed that cell adhesion was due to grafted HA. Error bars indicate SD. (D to F) Polydopamine-modified PS grafted with HA (0.5 mg of HA per milliliter of 10 mM tris, pH 8.0) retains bioactivity during long-term culture with M07e cells. Images taken after normal-force centrifugation show almost 100% attachment of expanding M07e cells at days 2 [2760 ± 390 cells/cm² (D)] and 4 [5940 ± 660 cells/cm² (E)]. In the absence of HA, the polydopamine-coated surface supported similar levels of M07e cell expansion at day 4 but did not support cell adhesion [610 ± 630 cells/cm² (F)].