Polydopamine Surface Chemistry: A Decade of Discovery

Hexamethylenediamine-Mediated Polydopamine Film Deposition: Inhibition by Resorcinol as a Strategy for Mapping Quinone Targeting Mechanisms

Hexamethylenediamine (HMDA) and other long chain aliphatic diamines can induce substrate-independent polymer film deposition from dopamine and several other catechols substrates at relatively low concentrations, however the mechanism of the diamine-promoted effect has remained little understood. Herein, we report data indicating that: (a) film deposition from 1 mM HMDA and dopamine is not affected by chemical oxidation with periodate but is markedly inhibited by resorcinol, which also prevents PDA film formation at 10 mM monomer concentration in the absence of HMDA; (b) N-acetylation of HMDA completely inhibits the effect on PDA film formation; (c) HMDA enables surface functionalization with 1 mM 5,6-dihydroxyindole (DHI) polymerization at pH 9.0 in a resorcinol-inhibitable manner. Structural investigation of the polymers produced from dopamine and DHI in the presence of HMDA using solid state 13C and 15N NMR and MALDI-MS suggested formation of covalent cross linked structures. It is concluded that HMDA enhances polydopamine adhesion by acting both on dopamine quinone and downstream, e.g., via covalent coupling with DHI. These results provide new insights into the mechanisms of PDA adhesion and disclose resorcinol as a new potent tool for targeting/mapping quinone intermediates and for controlling polymer growth.

Tropoelastin-Coated Tendon Biomimetic Scaffolds Promote Stem Cell Tenogenic Commitment and Deposition of Elastin-Rich Matrix

Tendon tissue engineering strategies that recreate the biophysical and biochemical native microenvironment have a greater potential to achieve regeneration. Here, we developed tendon biomimetic scaffolds using mechanically competent yarns of poly-ε-caprolactone, chitosan, and cellulose nanocrystals to recreate the inherent tendon hierarchy from a nano-to-macro scale. These were then coated with tropoelastin (TROPO) through polydopamine (PDA) linking, to mimic the native extracellular matrix (ECM) composition and elasticity. Both PDA and TROPO coatings decreased surface stiffness without masking the underlying substrate. We found that human adipose-derived stem cells (hASCs) seeded onto these TROPO biomimetic scaffolds more rapidly acquired their spindle-shape morphology and high aspect ratio characteristic of tenocytes. Immunocytochemistry shows that the PDA and TROPO-coated surfaces boosted differentiation of hASCs toward the tenogenic lineage, with sustained expression of the tendon-related markers scleraxis and tenomodulin up to 21 days of culture. Furthermore, these surfaces enabled the deposition of a tendon-like ECM, supported by the expression of collagens type I and III, tenascin, and decorin. Gene expression analysis revealed a downregulation of osteogenic and fibrosis markers in the presence of TROPO when compared with the control groups, suggesting proper ECM deposition. Remarkably, differentiated cells exposed to TROPO acquired an elastogenic profile due to the evident elastin synthesis and deposition, contributing to the formation of a more mimetic matrix in comparison with the PDA-coated and uncoated conditions. In summary, our biomimetic substrates combining biophysical and biological cues modulate stem cell behavior potentiating their long-term tenogenic commitment and the production of an elastin-rich ECM.

4-(3-Aminopropyl)-benzene-1,2-diol: An Improved Material-Independent Surface-Coating Reagent Compared to Dopamine

Dopamine surface chemistry has been of great interest because of its universal coating property and ability to transform nonadhesive molecules into adhesive molecules. Catechol oxidation and intramolecular cyclization underlie the unique property of dopamine (DA) surface chemistry and provide clues for developing new surface modification reagents such as norepinephrine, 5-pyrogallol-2-aminoethane, and perfluorinated DA derivatives. Based on these inspiring properties, a fast and universal surface chemistry technique using 4-(3-aminopropyl)-benzene-1,2-diol (3-catecholpropanamine, CPA) is reported herein. A single carbon insertion in the aliphatic chain of DA gives rise to the significantly accelerated intermolecular assembly and surface coating of CPA. The effect of CPA conjugation on an anticoagulant polysaccharide coating is also investigated. The use of CPA instead of DA to make polysaccharide coating materials improves the coating rate, while maintaining excellent antiplatelet performance on the coated surface.

Precise cell behaviors manipulation through light-responsive nano-regulators: recent advance and perspective

Nanotechnology-assisted spatiotemporal manipulation of biological events holds great promise in advancing the practice of precision medicine in healthcare systems. The progress in internal and/or external stimuli-responsive nanoplatforms for highly specific cellular regulations and theranostic controls offer potential clinical translations of the revolutionized nanomedicine. To successfully implement this new paradigm, the emerging light-responsive nanoregulators with unparalleled precise cell functions manipulation have gained intensive attention, providing UV-Vis light-triggered photocleavage or photoisomerization studies, as well as near-infrared (NIR) light-mediated deep-tissue applications for stimulating cellular signal cascades and treatment of mortal diseases. This review discusses current developments of light-activatable nanoplatforms for modulations of various cellular events including neuromodulations, stem cell monitoring, immunomanipulation, cancer therapy, and other biological target intervention. In summary, the propagation of light-controlled nanomedicine would place a bright prospect for future medicine.

Safety and photochemotherapeutic application of poly(γ-glutamic acid)-based biopolymeric nanoparticle

The safety of nanomaterials, a crucial consideration for clinical translation, is enhanced by using building blocks that are biologically nontoxic. Here, we used poly(γ-glutamic acid) (γ-PGA) and dopamine as building blocks of polymeric nanomaterials for carrying hydrophobic anticancer drugs. The introduction of phenylalanine onto γ-PGA enabled the resulting amphiphilic derivative of γ-PGA acid to self-assemble in the presence of the anticancer drug paclitaxel (PTX) to form PTX-encapsulated micelles. The surfaces of PTX-loaded micelles were then coated with polymerized dopamine (PDA). The PDA-coated, amphiphilic γ-PGA-based micelles (AM) carrying PTX (PDA/AM/P) exerted near-infrared-responsive photothermal effects. Near-infrared irradiation of cancer cells treated with PDA/AM/P nanoparticles produced a greater anticancer effect than that observed in other treatment groups, indicating a synergistic effect. Intravenous administration of PDA/AM/P completely ablated tumors and prevented their recurrence. Notably, the in vivo safety profile of PDA/AM/P nanoparticles allowed PTX to be delivered at a 3.6-fold higher dose than was possible with PTX solubilized in surfactant, and circumvented the side effects of the surfactant. These results support the multifunctional potential of PDA/AM for the delivery of various hydrophobic drugs and imaging dyes for safe translation of nanomaterials into the clinic.

Electrochemical Co-deposition of Polydopamine/Hyaluronic Acid for Anti-biofouling Bioelectrodes

Bioelectrodes are key components of electronic devices that efficiently mediate electrical signals in biological systems. However, conventional bioelectrodes often undergo biofouling associated with non-specific proteins and cell adhesion on the electrode surfaces, which leads to seriously degraded electrical and/or electrochemical properties. Hence, a facile and effective method to modify the surface of bioelectrodes is required to introduce anti-biofouling properties and improve performance. Here, we report an electrochemical surface modification of a bioelectrode via co-deposition of hyaluronic acid (HA) and polydopamine (PDA). The electrochemical polymerization and deposition of PDA offered simple and effective incorporation of highly hydrophilic and anti-fouling HA to the electrode surfaces, with no substantial increase in impedance. HA-incorporated PDA (PDA/HA)-modified electrodes displayed significant resistance to non-specific protein adsorption and the adhesion of fibroblasts. In addition, 4-week subcutaneous implantation studies revealed that the modified electrodes attenuated scar tissue formation compared with that induced by unmodified bare electrodes. This simple and effective electrochemical surface modification could be further employed for various implantable bioelectrodes (e.g., prosthetics and biosensors) and could extend their bioelectronic applications.

Hierarchically Patterned Polydopamine-Containing Membranes for Periodontal Tissue Engineering

Periodontitis is a common chronic inflammatory disease that affects tooth-supporting tissues. We engineer a multifunctional periodontal membrane for the guided tissue regeneration of lost periodontal tissues. The major drawback of current periodontal membranes is the lack of tissue regeneration properties. Here, a series of nanofibrous membranes based on poly(ε-caprolactone) with tunable biochemical and biophysical properties were developed for periodontal tissue regeneration. The engineered membranes were surface coated using biomimetic polydopamine to promote the adhesion of therapeutic proteins and cells. We demonstrate successful cellular localization on the surface of the engineered membrane by morphological patterning. Polydopamine accelerates osteogenic differentiation of dental-derived stem cells by promoting hydroxyapatite mineralization. Such multiscale designs can mimic the complex extracellular environment of periodontal tissue and serve as functional tissue constructs for periodontal regeneration. In a periodontal defect model in rats, our engineered periodontal membrane successfully promoted the regeneration of periodontal tissue and bone repair. Altogether, our data demonstrate that our biomimetic membranes have potential as protein/cell delivery platforms for periodontal tissue engineering.

Direct Evidence for the Critical Role of 5,6-Dihydroxyindole in Polydopamine Deposition and Aggregation

The definitive role of the intermediate 5,6-dihydroxyindole (DHI) in the formation of polydopamine (PDA) coatings from aqueous dopamine (DA) has not been clearly elucidated and remains highly controversial. Our foray into this debate as reported in this study agrees with some reported assertions that DHI-based coatings are not synonymous with PDA coatings. Our conclusion arises from a systematic comparison of the components and properties of DHI-based coatings and PDA coatings. In addition, through careful copolymerization studies of DA and DHI, our studies reported herein unequivocally suggest that both DA and DHI are partial building blocks for PDA formation. Our results also provide additional evidence of the critical role of DHI in controlling the thickness of PDA coatings, through competitive events between PDA aggregation in solutions and deposition onto substrates. These findings highlight the complex interplay between both DHI and uncyclized DA moieties in the formation of adhesive catechol/amine materials.

Novel Mussel-Inspired Universal Surface Functionalization Strategy: Protein-Based Coating with Residue-Specific Post-Translational Modification in Vivo

Surface functionalization can effectively endow materials with desirable properties, promoting the performance between the material and environment, with extensive applications. However, a universal and straightforward surface functionalization method with biocompatibility is scarce. In this study, with synthetic biology strategy, recombinant mussel plaque protein with a zwitterionic peptide inspired by molecular chaperone was engineered through post-translational modification, in which 3,4-dihydroxyphenylalanine was residue-specifically obtained efficiently from tyrosine with tyrosinase coexpressed in vivo. The rational designed chimeric protein coating in this work could successfully anchor to various substrates and exhibit excellent antifouling performance in resisting protein adsorption, cell attachment, and bacterial adhesion with eminent biocompatibility.

Enhanced Adhesion of Carbon Nanotubes by Dopamine Modification

According to the fact that gecko-inspired vertically aligned carbon nanotubes (VA-CNTs) exhibit ultrastrong adhesion, dopamine is utilized to make a modification to this traditional biomimetic material. The composite material is tested for adhesion performance under different environmental conditions by an atomic force microscope. The adhesion force of the modified VA-CNTs does not show obvious fluctuation during the gradual heating process; however, the material gains improved adhesion when increasing the ambient humidity. In addition, the modified CNTs show a stronger adhesion force than the original CNTs in their performance tests. The dopamine polymer has a good combination with CNTs, which is responsible for the aforementioned excellent performance. Overall, this modification method is simple, convenient, efficient, and environmentally friendly, which all indicates a promising future in its application. The modified CNTs are expected to be used for super-adhesion in harsh environments, as well as in the field of microelectronics.

Polydopamine-assisted surface modification for orthopaedic implants

Along with the massive use of implants in orthopaedic surgeries in recent few decades, there has been a tremendous demand for the surface modification of the implants to avoid surgery failure and improve their function. Polydopamine (PDA), being able to adhere to almost all kinds of substrates and possessing copious functional groups for covalently immobilizing biomolecules and anchoring metal ions, has been widely used for surface modification of materials since its discovery in the last decade. PDA and its derivatives can be used for the surface modification of orthopaedic implants to modulate cellular responses, including cell spreading, migration, proliferation, and differentiation, and may thereby enhance the function of existing implants. In addition, the osseointegration and antimicrobial properties of orthopaedic implants may also be improved by PDA-based coatings. The aim of this review is to provide a brief overview of current advances of surface modification technologies for orthopaedic implants using PDA and its derivatives as a medium. Given the versatility of PDA-based adhesion, such PDA-assisted surface modification technologies will certainly benefit the development of new orthopaedic implants.

THE TRANSLATIONAL POTENTIAL OF THIS ARTICLE

Surface treatments of orthopaedic implants, which are normally inert materials, are essential for their performance in vivo. This review summarizes recent advances in the surface modification of orthopaedic implants using facile and highly versatile techniques based on the use of polydopamine (PDA) and its derivatives.

Role of polydopamine's redox-activity on its pro-oxidant, radical-scavenging, and antimicrobial activities

Polydopamine (PDA) is a bioinspired material and coating that offers diverse functional activities (e.g., photothermal, antioxidant, and antimicrobial) for a broad range of applications. Although PDA is reported to be redox active, the association between PDA's redox state and its functional performance has been difficult to discern because of PDA's complex structure and limitations in methods to characterize redox-based functions. Here, we use an electrochemical reverse engineering approach to confirm that PDA is redox-active and can repeatedly accept and donate electrons. We observed that the electron-donating ability of PDA offers the detrimental pro-oxidant effect of donating electrons to O₂ to generate reactive oxygen species (ROS) or, alternatively, the beneficial antioxidant effect of quenching oxidative free radicals. Importantly, PDA's electron-donating ability depends on its redox state and is strongly influenced by external factors including metal ion binding as well as near-infrared (NIR) irradiation. Furthermore, we demonstrated that PDA possesses redox state-dependent antimicrobial properties in vitro and in vivo. We envision that clarification of PDA's redox activity will enable better understanding of PDA's context-dependent properties (e.g., antioxidant and pro-oxidant) and provide new insights for further applications of PDA. STATEMENT OF SIGNIFICANCE: We believe this is the first report to characterize the redox activities of polydopamine (PDA) and to relate these redox activities to functional properties important for various proposed applications of PDA. We observed that polydopamine nanoparticles 1) are redox-active; 2) can repeatedly donate and accept electrons; 3) can accept electrons from reducing agents (e.g., ascorbate), donate electrons to O₂ to generate ROS, and donate electrons to free radicals to quench them; 4) have redox state-dependent electron-donating abilities that are strongly influenced by metal ion binding as well as NIR irradiation; and 5) have redox state-dependent antimicrobial activities.

Surface Modification of 3D Printed PLA Objects by Fused Deposition Modeling: A Review

Polylactic acid (PLA) filaments are very popular as a thermoplastic source used in the 3D printing field by the “Fused Deposition Modeling” method in the last decade. The PLA market is expected to reach 5.2 billion US dollars in 2020 for all of its industrial uses. On the other hand, 3D printing is an expanding technology that has a large economic potential in many industries where PLA is one of the main choices as the source polymer due to its ease of printing, environmentally friendly nature, glossiness and multicolor appearance properties. In this review, we first reported the chemical structure, production methods, general properties, and present market of the PLA. Then, the chemical modification possibilities of PLA and its use in 3D printers, present drawbacks, and the surface modification methods of PLA polymers in many different fields were discussed. Specifically, the 3D printing method where the PLA filaments are used in the extrusion-based 3D printing technologies is reviewed in this article. Many methods have been proposed for the permanent surface modifications of the PLA where covalent attachments were formed such as alkaline surface hydrolysis, atom transfer polymerization, photografting by UV light, plasma treatment, and chemical reactions after plasma treatment. Some of these methods can be applied for surface modifications of PLA objects obtained by 3D printing for better performance in biomedical uses and other fields. Some recent publications reporting the surface modification of 3D printed PLA objects were also discussed.

Polydopamine-Stabilized Aluminum Nanocrystals: Aqueous Stability and Benzo[a]pyrene Detection

Aluminum nanocrystals have emerged as an earth-abundant material for plasmonics applications. Al nanocrystals readily oxidize in aqueous-based solutions, however, transforming into highly stratified γ-AlOOH nanoparticles with a 700% increase in surface area in a matter of minutes. Here we show that by functionalizing Al nanocrystals with the bioinspired polymer polydopamine, their stability in aqueous media is dramatically increased, maintaining their integrity in aqueous solution for over 2 weeks with no discernible structural changes. Polydopamine functionalization also provides a molecular capture layer that enables the capture of polycyclic aromatic hydrocarbon pollutants in H₂O samples and their detection by surface-enhanced Raman spectroscopy, when polydopamine-stabilized Al nanocrystal aggregates are used as substrates. This approach was used to detect a prime carcinogenic H₂O pollutant, benzo[a]pyrene with a sensitivity in the sub part-per-billion range.

Material-Independent Surface Chemistry beyond Polydopamine Coating

Various methods have been developed in surface chemistry to control interface properties of a solid material. A selection rule among surface chemistries is compatibility between a surface functionalization tool and a target material. For example, alkanethiol deposition on noble metal surfaces, widely known as the formation of a self-assembled monolayer (SAM), cannot be performed on oxide material surfaces. One must choose organosilane molecules to functionalize oxide surfaces. Thus, the surface chemistry strictly depends on the properties of the surface. Polydopamine coating is now generally accepted as the first toolbox for functionalization of virtually any material surface. Layer-by-layer (LbL) assembly is a widely used method to modify properties of versatile surfaces, including organic materials, metal oxides, and noble metals, along with polydopamine coating. On flat solid substrates, the two chemistries of polydopamine coating and LbL assembly provide similar levels of surface modifications. However, there are additional distinct features in polydopamine. First, polydopamine coating is effective for two- or three-dimensional porous materials such as metal-organic frameworks (MOFs), synthetic polyolefin membranes, and others because small-sized dopamine (MW = 153.18 u) and its oxidized oligomers are readily attached onto narrow-spaced surfaces without exhibiting steric hindrance. In contrast, polymers used in LbL assembly are slow in diffusion because of steric hindrance due to their high molecular weight. Second, it is applicable to structurally nonflat surfaces showing special wettability such as superhydrophobicity or superoleophobicity. Third, a nonconducting, insulating polydopamine layer can be converted to be a conducting layer by pyrolysis. The product after pyrolysis is a N-doped graphene-like material that is useful for graphene or carbon nanotube-containing composites. Fourth, it is a suitable method for engineering the surface properties of various composite materials. The surface properties of participating components in composite materials can be unified by polydopamine coating with a simple one-step process. Fifth, a polydopamine layer exhibits intrinsic chemical reactivity by the presence of catecholquinone moieties and catechol radical species on surfaces. Nucleophiles such as amine and thiolate spontaneously react with the functionalized layer. Applications of polydopamine coating are exponentially growing and include cell culture/patterning, microfluidics, antimicrobial surfaces, tissue engineering, drug delivery systems, photothermal therapy, immobilization of photocatalysts, Li-ion battery membranes, Li-sulfur battery cathode materials, oil/water separation, water detoxification, organocatalysts, membrane separation technologies, carbonization, and others. In this Account, we describe various polydopamine coating methods and then introduce a number of chemical derivatives of dopamine that will open further development of material-independent surface chemistry.

Mussel-Inspired Self-Healing Coatings Based on Polydopamine-Coated Nanocontainers for Corrosion Protection

The mussel-inspired properties of dopamine have attracted immense scientific interest for surface modification of nanoparticles due to the high potential of dopamine functional groups to increase the adhesion of nanoparticles to flat surfaces. Here, we report for the first time a novel type of inhibitor-loaded nanocontainer using polydopamine (PDA) as a pH-sensitive gatekeeper for mesoporous silica nanoparticles (MSNs). The encapsulated inhibitor (benzotriazole) was loaded into MSNs at neutral pH, demonstrating fast release in an acidic environment. The self-healing effect of water-borne alkyd coatings doped with nanocontainers was achieved by both on-demand release of benzotriazole during the corrosion process and formation of the complexes between the dopamine functional groups and iron oxides, thus providing dual self-healing protection for the mild steel substrate. The coatings were characterized by electrochemical impedance spectroscopy, visual observations, and confocal Raman microscopy. In all cases, the coatings with embedded benzotriazole-loaded MSNs with PDA-decorated outer surfaces demonstrated superior self-healing effects on the damaged areas. We anticipate that dopamine-based multifunctional gatekeepers can find application potential not only in intelligent self-healing anticorrosive coatings but also in drug delivery, antimicrobial protection, and other fields.

Development of Cu-Modified PVC and PU for Catalytic Generation of Nitric Oxide

Nitric oxide (NO) generating surfaces are potentially promising for improving haemocompatibility of blood-contacting biomaterials. In the present report, Cu-modified poly(vinyl chloride) (PVC) and polyurethane (PU) were prepared via polydopamine (pDA)-assisted chelation. The copper content on the PVC and PU modified surfaces, assessed by inductively coupled plasma - optical emission spectrometry (ICP-OES), were about 3.86 and 6.04 nmol·cm−2, respectively. The Fourier-transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) data suggest that copper is attached to the polymer surface through complex formation with pDA. The cumulative leaching of copper from modified PVC and PU during the five day incubation in phosphate buffered saline (PBS), measured by inductively coupled plasma mass spectrometry (ICP-MS), was about 50.7 ppb and 48 ppb, respectively which is within its physiological level. Modified polymers were tested for their ability to catalytically generate NO by decomposing of endogenous S-nitrosothiol (GSNO). The obtained data show that Cu-modified PVC and PU exhibited the capacity to generate physiological levels of NO which could be a foundation for developing new biocompatible materials with NO-based therapeutics.

Tailoring the Performance of Organic Solvent Nanofiltration Membranes with Biophenol Coatings

This study reports a systematic investigation of fine-tuning the filtration performance of nanofiltration membranes with biophenol coatings to produce solvent-resistant membranes with 390-1550 g mol-1 molecular weight cutoff (MWCO) and 0.5-40 L m-2 h-1 bar-1 permeance. Six kinds of inexpensive, commercial biophenols (dopamine, tannic acid, vanillyl alcohol, eugenol, morin, and quercetin) were subjected to identical oxidant-promoted polymerization to coat six kinds of loose asymmetric membrane supports: polyimide (PI), polyacrylonitrile (PAN), polysulfone (PSf), polyvinylidene difluoride (PVDF), polybenzimidazole (PBI), and polydimethylsiloxane (PDMS). The coatings were characterized by Fourier-transform infrared spectroscopy (FTIR), and the morphologies were characterized by scanning electron microscopy (SEM) and atomic force microscopy (AFM). The long-term stability of 42 membranes were tested in 12 organic solvents, including emerging green solvents MeTHF and Cyrene. The biophenol coatings led to tighter membranes with a decrease in MWCO of 12-79% at a penalty of a 22-92% permeance decrease in acetone.

Cu nanoparticles decorated WS2 nanosheets as a lubricant additive for enhanced tribological performance

Tungsten disulfide–polydopamine–copper (WS₂–PDA–Cu) nanocomposites were first prepared by a green and effective biomimetic strategy and then used as a lubricant additive in polyalkylene glycol (PAG). The biomimetic strategy is inspired by the adhesive proteins in mussels. WS₂ nanosheets were decorated by uniformly dispersed Cu nanoparticles (Cu NPs). The WS₂–PDA–Cu nanocomposites with good dispersion stability, showed better friction reducing and anti-wear properties than WS₂, Cu NPs and WS₂–Cu dispersed in PAG base oil. The average friction coefficient and wear volume were reduced by 33.56% and 97.95%, respectively, at 150 °C under a load of 100 N for the optimal concentration of 0.9 wt%. The lubrication mechanism was discussed.

Tungsten disulfide–polydopamine–copper (WS₂–PDA–Cu) nanocomposites were first prepared by a green and effective biomimetic strategy and then used as a lubricant additive in polyalkylene glycol (PAG).

Silicic Acid-Mediated Formation of Tannic Acid Nanocoatings

Tannic acid (TA) adheres to a broad variety of different materials and forms versatile surface coatings for technical and biological applications. In mild alkaline conditions, autoxidation processes occur and a firm monolayer is formed. Up to now, thicker coatings are obtained in only a cross-linked multilayer fashion. This study presents an alternative method to form continuous TA coatings using orthosilicic acid (Si_aq). Adsorption kinetics and physical properties of TA coatings in the presence of Si_aq were determined using a quartz-crystal microbalance and nanoplasmonic spectroscopy. An in situ TA layer thickness of 200 nm was obtained after 24 h in solutions supplemented with 80 μM Si_aq. Dry-state measurements indicated a highly hydrated layer in situ. Furthermore, chemical analysis by Fourier transform infrared spectroscopy revealed possible complexation of TA by Si_aq, whereas UV-vis spectroscopy did not indicate an interaction of Si_aq in the autoxidation process of TA. Investigation of additional metalloid ions showed that germanic acid was also able to initiate a continuous coating formation of TA, whereas boric acid prevented the polymerization process. In comparison to that of TA, the coating formation of pyrogallol (PG) and gallic acid (GA) was not affected by Si_aq. PG formed continuous coatings also without Si_aq, whereas GA formed only a monolayer in the presence of Si_aq. However, Si_aq induced a continuous layer formation of ellagic acid. These results indicate the specific importance of orthosilicic acid in the coating formation of polyphenolic molecules with multiple ortho-dihydroxy groups and open new possibilities to deposit TA on interfaces.

Polydopamine-mediated covalent functionalization of collagen on a titanium alloy to promote biocompatibility with soft tissues

The clinical success of a titanium (Ti) percutaneous implant requires the integration with soft tissues to form a biological seal, which effectively combats marsupialization, premigration and infection after implantation. However, the bioinert surface of Ti or its alloys prevents the material from sufficient biological sealing and limits the application of Ti or its alloys as percutaneous implants. In this study, we achieved a collagen coating to bioactivate the surface of Ti-6Al-4V. In order to enable covalent functionalization, we first deposited a polydopamine (PDA) coating on Ti-6Al-4V based on dopamine self-polymerization and then immobilized collagen chains on PDA. Compared with physical absorption, such a chemical bonding method through mussel-inspired chemistry showed better stability of the coating. Meanwhile, the cellular tests in vitro indicated that collagen functionalization on the Ti-6Al-4V surface showed better adhesion of human foreskin fibroblasts (HFFs) and human immortal keratinocytes (HaCaTs). The subcutaneous implantation tests in rats indicated that the collagen modification attenuated soft tissue response and improved tissue compatibility compared with either pure Ti-6Al-4V or merely PDA coated samples. The facile bioinspired approach enables a persistent modification of metals by macromolecules under aqueous environments, and the PDA-collagen coated titanium alloy is worthy of further investigation as a percutaneous implant.

Hyperbranched Zwitterionic Polymer-Functionalized Underwater Superoleophobic Microfiltration Membranes for Oil-in-Water Emulsion Separation

Inspired by mussel adhesion chemistry, a kind of hydrophilic poly(vinylidene fluoride) (PVDF) microfiltration membrane with underwater superoleophobicity was prepared using thiolated hyperbranched zwitterionic poly(sulfobetaine methacrylate) (HPS) as a nanoscale surface modifier. The HPS was first synthesized via reversible addition fragmentation chain transfer (RAFT) copolymerization and followed by sulfonation reaction and then coated onto polydopamine (PD) adhesive PVDF membranes via thiol-mediated Michael addition reaction. The successful and uniform coating of HPS onto the membrane surface was demonstrated by X-ray photoelectron spectroscopy and by using an energy dispersive X-ray detector. The surface micro-nano morphology and increased roughness of the PD/HPS-modified (M-PD/HPS) membrane were also investigated by using a field emission scanning electron microscope and an atomic force microscope. The M-PD/HPS membrane could be wetted completely by water, and the underwater oil contact angles were about 160°, indicating the M-PD/HPS membrane has excellent hydrophilicity and underwater superoleophobicity. Compared with the pure PVDF membrane, the M-PD/HPS membrane for hexane-in-water emulsion separation exhibited an enhanced water filtration flux of 10 707 L m^-2 h^-1 (0.1 MPa), and the oil rejection ratio was above 99.9%. Besides, the excellent antifouling ability and recyclable properties of the M-PD/HPS membranes would make them suitable for long-time use. Thus, the approach of mussel adhesion chemistry employing the RAFT-mediated nanosized hyperbranched zwitterionic polymers as postmodification reagents showed a good application prospect in purification of oily waste water and oil recovery.

Polydopamine Coating Enhances Mucopenetration and Cell Uptake of Nanoparticles

Mucus is an endogenous viscoelastic biopolymer barrier that limits the entry of foreign pathogens and therapeutic carriers to the underlying mucosal cells. This could be overcome with a hydrophilic and nonpositively charged carrier surface that minimizes interactions with the mucin glycoprotein fibers. Although PEGylation remains an attractive surface strategy to enhance mucopenetration, cell uptake of PEGylated nanoparticles (NPs) often remains poor. Here, we demonstrated polydopamine (PDA) coating to enhance both mucopenetration and cell uptake of NPs. PDA was polymerized on carboxylated polystyrene (PS) NPs to form a PDA coating, and the resulting PS-PDA achieved a similar level of mucopenetration as our PEGylated PS (PS-PEG) positive control in three separate studies: NP-mucin interaction test, transwell assay, and multiple particle tracking. Compared to water, the diffusions of PS-PDA and PS-PEG in reconstituted mucus solution were only 3.5 and 2.4 times slower, respectively, whereas the diffusion of bare PS was slowed by up to 250 times. However, the uptake of PS-PDA (61.2 ± 6.1%) was almost three times higher than PS-PEG (24.6 ± 5.4%) in T24 cells, which were used as a model for underlying mucosal cells. Our results showed a novel unreported functionality of PDA coating in enhancing both mucopenetration and cell uptake of NPs for mucosal drug delivery applications, not possible with conventional PEGylation strategies.

Iron-Chelated Polydopamine Decorated Doxorubicin-Loaded Nanodevices for Reactive Oxygen Species Enhanced Cancer Combination Therapy

Combination therapy which enhances efficacy and reduces toxicity, has been increasingly applied as a promising strategy for cancer therapy. Here, a reactive oxygen species (ROS) that enhanced combination chemotherapy nanodevices was fabricated based on the Fe-chelated polydopamine (PDA) nanoparticles (NPs). The structure was characterized by dynamic light scattering-autosizer, transmission electron microscopy, energy dispersive spectroscopy, and Fourier-transform infrared (FT-IR) spectrophotometer. The in vitro drug release profile triggered by low intracellular pH indicated that the system demonstrated controlled therapeutic activity. In vitro cell uptake studies showed that doxorubicin (DOX)-loaded Fe-PDA/ folic acid (FA)- polyethylene glycol (DOX@Fe-PDA/FA-PEG) had a strong uptake capacity and can be rapidly internalized by MCF-7 cells. The in vitro experiments demonstrated that DOX@Fe-PDA/FA-PEG triggered the intracellular ROS overproduction, thereby enhancing its therapeutic effect on breast cancer. In summary, this experiment demonstrated the novel DOX-loaded composite NPs used as a potential targeted nanocarrier for breast cancer treatment, which could be a promising therapeutic strategy against breast cancer.

Oxidation-Mediated, Zwitterionic Polydopamine Coatings for Marine Antifouling Applications

We synthesized a zwitterionic dopamine derivative ( ZW-DOPA) containing both catechol and amine groups, and we demonstrated an excellent marine antifouling surface by controlling the oxidation of ZW-DOPA. The oxidation was mediated by the deprotonation of catechol or the addition of an oxidant (ammonium persulfate (AP) or sodium periodate (NaIO₄)). The oxidation and subsequent molecular transformation of ZW-DOPA was investigated over time by UV-vis spectroscopy. Among the different oxidation conditions tested, NaIO₄-induced ZW-DOPA coating was the most efficient and successfully formed on various substrates, such as titanium dioxide, stainless steel, and nylon. Compared with uncoated substrates, ZW-DOPA-coated substrates showed high resistance to marine diatom adhesion. Considering the ease of use and substrate independence of the ZW-DOPA coating, this method shows promise as a basis for inhibiting marine fouling on a variety of substrates used in the marine industry and aquatic environments.

Rapid Mussel-Inspired Surface Zwitteration for Enhanced Antifouling and Antibacterial Properties

Mussel-inspired dopamine chemistry has increasingly been used for surface modification due to its simplicity, versatility, and strong reactivity for secondary functionalization with amine or thiol containing molecules. In this work, we demonstrate a facile surface modification technique using dopamine chemistry to prepare a zwitterionic polymer coating with both antifouling and antimicrobial property. Catechol containing adhesive monomer dopamine methacrylamide (DMA) was copolymerized with bioinspired zwitterionic 2-methacryloyloxyethyl phosphorylcholine (MPC) monomer, and the synthesized copolymers were covalently grafted onto the amino (-NH₂) rich polyethylenimine (PEI)/polydopamine (PDA) codeposited surface to obtain a stable antifouling surface. The resulting surface was later used for in situ deposition of antimicrobial silver nanoparticles (AgNPs), facilitated by the presence of catechol groups of the coating. The modified surface was characterized using X-ray photoelectron spectroscopy (XPS), water contact angle measurements, and atomic force microscopy (AFM). This dual functional coating significantly reduced the adhesion of both Gram-negative Escherichia coli and Gram-positive Staphylococcus aureus bacteria and showed excellent resistance to bovine serum albumin (BSA) adsorption. This bioinspired and efficient surface modification strategy with dual functional coating promises its potential application in implantable biomedical devices.

Universal nanothin silk coatings via controlled spidroin self-assembly

Robust, biocompatible, and facile coatings are promising for improving the in vivo performance of medical implants and devices. Here, we demonstrate the formation of nanothin silk coatings by leveraging the biomimetic self-assembly of eADF4(C16), an amphiphilic recombinant protein based on the Araneus diadematus dragline spidroin ADF4. These coatings result from concurrent adsorption and supramolecular assembly of eADF4(C16) induced by KH2PO4, thereby providing a mild one-pot coating strategy in which the coating rate can be controlled by protein and KH2PO4 concentration. The thickness of the coatings ranges from 2-30 nm depending on the time immersed in the aqueous coating solution. Coatings can be formed on hydrophobic and hydrophilic substrates regardless of surface chemistry and without requiring specialized surface activation. Moreover, coatings appear to be stable through vigorous rinsing and prolonged agitation in water. Grazing incidence wide angle X-ray scattering, single-molecule force spectroscopy, and Congo red staining techniques confirm the formation of β-sheet nanocrystals within the eADF4(C16) coating, which contributes to the cohesive and adhesive stability of the material. Coatings are exceptionally smooth in the dry state and are hydrophilic regardless of substrate hydrophobicity. Under aqueous conditions, nanothin silk coatings exhibit the properties of a hydrogel material.

Direct Evidence for the Polymeric Nature of Polydopamine

Inspired by the adhesive proteins of mussels, polydopamine (pDA) has emerged as one of the most widely employed materials for surface functionalization. Despite numerous attempts at characterization, little consensus has emerged regarding whether pDA is a covalent polymer or a noncovalent aggregate of low molecular weight species. Here, we employed single-molecule force spectroscopy (SMFS) to characterize pDA films. Retraction of a pDA-coated cantilever from an oxide surface shows the characteristic features of a polymer with contour lengths of up to 200 nm. pDA polymers are generally weakly bound to the surface through much of their contour length, with occasional "sticky" points. Our findings represent the first direct evidence for the polymeric nature of pDA and provide a foundation upon which to better understand and tailor its physicochemical properties.

Degradation of phthalic acid esters (PAEs) by an enzyme mimic and its application in the degradation of intracellular DEHP

An enzyme mimic inspired by serine proteases was developed for the degradation of PAEs and applied in the hydrolysis of intracellular DEHP.

Unravelling the polydopamine mystery: is the end in sight?

Mass spectroscopic studies using deuterium-labeling reveal a novel structure for polydopamine, deriving from dopaminochrome and uncyclized dopamine.

Stable and self-healable LbL coating with antibiofilm efficacy based on alkylated polyethyleneimine micelles

An antibacterial and self-healing coating was fabricated via LbL assembly based on N-decyl PEI (DPEI) micelles.

Polydopamine-coated Au nanorods for targeted fluorescent cell imaging and photothermal therapy

Au nanorods (AuNRs) have attracted a great interest as a platform for constructing various composite core/shell nanoparticles for theranostics applications. However, the development of robust methods for coating AuNRs with a biocompatible shell of high loading capacity and with functional groups still remains challenging. Here, we coated AuNRs with a polydopamine (PDA) shell and functionalized AuNR-PDA particles with folic acid and rhodamine 123 (R123) to fabricate AuNR-PDA-R123-folate nanocomposites. To the best of our knowledge, such AuNR-PDA-based composites combining fluorescent imaging and plasmonic phothothermal abilities have not been reported previously. The multifunctional nanoparticles were stable in cell buffer, nontoxic and suitable for targeted fluorescent imaging and photothermal therapy of cancer cells. We demonstrate the enhanced accumulation of folate-functionalized nanoparticles in folate-positive HeLa cells in contrast to the folate-negative HEK 293 cells using fluorescent microscopy. The replacement of folic acid with polyethylene glycol (PEG) leads to a decrease in nanoparticle uptake by both folate-positive and folate-negative cells. We performed NIR light-mediated targeted phototherapy using AuNR-PDA-R123-folate and obtained a remarkable cancer cell killing efficiency in vitro in comparison with only weak-efficient nontargeted PEGylated nanoparticles. Our work illustrates that AuNR-PDA could be a promising nanoplatform for multifunctional tumor theranostics in the future.